Encyclopedia of materials, parts, and finishes, Second Edition

C ND N IO SE Materials, Parts, and Finishes O OF • • ENC A Y OPE D L C I T I ED Mel Schwar tz CRC PR E S S...

Author: Mel M. Schwartz

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C

ND

N

IO

SE

Materials, Parts, and Finishes O

OF •

• ENC

A

Y

OPE D L C I

T I ED

Mel Schwar tz

CRC PR E S S Boca Raton London New York Washington, D.C.

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Library of Congress Cataloging-in-Publication Data Schwartz, Mel M. Encyclopedia of materials, parts, and finishes / by Mel Schwartz.—2nd ed. p. cm. ISBN 1-56676-661-3 1. Smart materials—Encyclopedia. I. Title. TA418.9.S62 S39 2002 620.1'18—dc21

2002019220

This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publisher. The consent of CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from CRC Press LLC for such copying. Direct all inquiries to CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe.

Visit the CRC Press Web site at www.crcpress.com © 2002 by CRC Press LLC No claim to original U.S. Government works International Standard Book Number 1-56676-661-3 Library of Congress Card Number 2002019220 Printed in the United States of America 1 2 3 4 5 6 7 8 9 0 Printed on acid-free paper

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Preface This encyclopedia represents an update of existing materials and presents new materials that have been invented or changed, either by new processes or by an innovative technique. The encyclopedia covers basic materials such as rubber and wood. This two-volumes-in-one includes two decades of the process of materials; the process/fabrication selection has been hindered by new and unusual demands from all quarters. No change in this trend is expected in the foreseeable future. This trend has been visible in several industries — aerospace, automotive, electronic, space, computers, chemical, and oil — and in many other commercial endeavors. Metals (wrought, cast, forged, powder), plastics (thermoplastics/thermosets), composites, structural ceramics, and coatings are continually finding new applications in the above industries. The trend toward combining high strength and light weight is evident in fiber/particle/whiskerreinforced composites. This encyclopedia/handbook covers not only these matrix composites (metallic, plastic, ceramic, and intermetallic), but also other materials of the future — nano and functionally graded structures, fullarenes, plastics (PEEK, PES, etc.), smart piezoelectric materials, shape memory alloys, and ceramics. Higher processing temperatures as well as more resistant and effective high-temperature materials have attracted the attention of engineers, scientists, and materials workers in many industries. Engines now operate more efficiently at temperatures higher than those attainable with the materials of the past. For example, interest in 2000°F (1093°C) turbine engines has brought more hightemperature, high-strength ceramics into development and use. The use of a vacuum environment has improved many materials not only in their initial production and processing, i.e., steels, but also eventually in their fabrication. For example, a vacuum environment in brazing and welding and in hot isostatic pressing removes voids and consolidates material structures. New environmental regulations by government agencies (the Environmental Protection Agency, the Occupational Safety and Health Administration, etc.) have sent the technologist back to the drawing board and laboratory to design and develop new and better materials and processes that are not potential health hazards, and many of these new material substitutes are included in this revised edition. Finally, political diplomacy, rather than economics and regulation, could well be the most important factor in materials supply in the near future. The major supply of many critical raw materials and supplies for the processes needed to sustain the future economies of many nations lies in the hands of a few small nations. Consequently, there is no guarantee of a steady supply of these strategic materials, and we must continually innovate and explore new sources of materials development (ocean floor and space).

© 2002 by CRC Press LLC

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Editor Mel M. Schwartz is a consultant to the vast field of materials and processes. He is editor of the Journal of Advanced Materials and editor-in-chief of the Smart Materials Encyclopedia. Schwartz received his bachelor of arts degree from Temple University, his master’s degree from Drexel University, and is currently working in the doctorate program at the University of Sarasota. His professional experience includes a career in metallurgy, manufacturing research, and development and metals processing at the U.S. Bureau of Mines, U.S. Chemical Corp., Martin-Marietta Corp., Rohr Industries, and Sikorsky Aircraft, from which he retired in 1999. Awards and honors include Inventor of the Year for Martin-Marietta, the Jud Hall Award (Society of Manufacturing Engineers), the first G. Lubin Award (Society for the Advancement of Materials and Processing Engineers), and Engineer of the Year in Connecticut (1973). He is an elected Fellow for the Society for the Advancement of Materials and Processing Engineers and American Society for Materials International, and sits on several peer-review committees; as well, he is a member of numerous national and international societies. Schwartz has written 14 books and over 100 technical papers and articles and has given company in-house courses and numerous seminars around the world.


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Dedication To Carolyn, Anne-Marie, and Perry whose enormous courage, will, and determined spirit are overwhelming. Mel Schwartz


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A ABRASIVE An abrasive is defined as a material of extreme hardness that is used to shape other materials by a grinding or abrading action. Abrasive materials may be used as loose grains, as grinding wheels, or as coatings on cloth or paper. They may be formed into ceramic cutting tools that are used for machining metal in the same way that ordinary machine tools are used. Because of their superior hardness and refractory properties, they have advantages in speed of operation, depth of cut, and smoothness of finish. Abrasive products are used for cleaning and machining all types of metal, for grinding and polishing glass, for grinding logs to paper pulp, for cutting metals, glass, and cement, and for manufacturing many miscellaneous products such as brake linings and nonslip floor tile.

ABRASIVE MATERIALS These may be classified in two groups, the natural and the synthetic (manufactured). The latter are by far the more extensively used, but in some specific applications natural materials still dominate. The important natural abrasives are diamond (the hardest known material), corundum (a relatively pure, natural aluminum oxide, Al2O3), and emery (a less-pure Al2O3 with considerable amounts of iron). Other natural abrasives are garnet, an aluminosilicate mineral; feldspar, used in household cleansers; calcined clay; lime; chalk; and silica, SiO2, in its many forms — sandstone, sand (for grinding plate glass), flint, and diatomite. The synthetic abrasive materials are silicon carbide SiC, aluminum oxide Al2O3, titanium carbide TiC, and boron carbide B4C. The


synthesis of diamond puts this material in the category of manufactured abrasives. There are other carbides, as well as nitrides and cermets, which can be classified as abrasives but their use is special and limited. Various grades of each type of synthetic abrasive are distinguishable by properties such as color, toughness, and friability. These differences are caused by variation in purity of materials and methods of processing. The sized abrasive may be used as loose grains, as coatings on paper or cloth to make sandpaper and emery cloth, or as grains for bonding into wheels.

ABRASIVE WHEELS A variety of bonds is used in making abrasive wheels: vitrified or ceramic, essentially a glass or glass-plus crystals; sodium silicate; rubber; resinoid; shellac; and oxychloride. Each type of bond has its advantages. The more rigid ceramic bond is better for precision-grinding operations, and the tougher, resilient bonds, such as resinoid or rubber, are better for snagging and cutting operations. Ceramic-bonded wheels are made by mixing the graded abrasive and binder, pressing to general size and shape, firing, and truing or finishing by grinding to exact dimensions. Grinding wheels are specified by abrasive type, grain size (grit), grade or hardness, and bond type. The term hardness as applied to a wheel refers to its behavior in use and not to the hardness of the abrasive material itself. Literally thousands of types of wheels are made with different combinations of characteristics, not to mention the multitude of sizes and shapes available; therefore, selecting the best grinding wheel for a given job is not simple.

A


A

ABS PLASTICS ABS plastics are a family of opaque thermoplastic resins formed by copolymerizing acrylonitrile, butadiene, and styrene (ABS) monomers. ABS plastics are primarily notable for especially high impact strengths coupled with high rigidity or modulus. Consisting of particles of a rubberlike toughener suspended in a continuous phase of styreneacrylonitrile (SAN) copolymer, ABS resins are hard, rigid, and tough, even at low temperatures. Various grades of these amorphous, medium-priced thermoplastics are available offering different levels of impact strength, heat resistance, flame retardance, and platability. Most natural ABS resins are translucent to opaque, but they are also produced in transparent grades, and they can be pigmented to almost any color. Grades are available for injection molding, extrusion, blow molding, foam molding, and thermoforming. Molding and extrusion grades provide surface finishes ranging from satin to high gloss. Some ABS grades are designed specifically for electroplating. Their molecular structure is such that the plating process is rapid, easily controlled, and economical. Compounding of some ABS grades with other resins produces special properties. For example, ABS is alloyed with polycarbonate to provide a better balance of heat resistance and impact properties at an intermediate cost. Deflection temperature is improved by the polycarbonate, molding ease by the ABS. Other ABS resins are used to modify rigid polyvinyl chloride (PVC) for use in pipe, sheeting, and molded parts. Reinforced grades containing glass fibers, to 40%, are also available. Related to ABS is SAN, a copolymer of styrene and acrylonitrile (no butadiene) that is hard, rigid, transparent, and characterized by excellent chemical resistance, dimensional stability, and ease of processing. SAN resins are usually processed by injection molding, but extrusion, injection-blow molding, and compression molding are also used. They can also be thermoformed, provided that no post-mold trimming is necessary. The triangle in Figure A.1 illustrates the properties and characteristics that each constituent acrylonitrile, butadiene, and styrene


Acrylonitrile

Chemical resistance Heat stability Tensile strength Aging resistance

ABS

Toughness Impact strength Low temperature properties Butadiene

Gloss Processibility Rigidity Styrene

FIGURE A.1 Properties and characteristics of acrylonitrile, butadiene, and styrene.

contributes to the final product. Polymerization of these materials produces the ABS terpolymer, a two-phase system consisting of a continuous matrix of styrene-acrylonitrile copolymer and a dispersed phase of butadiene rubber particles. Properties are varied principally by adjusting the proportions in which the materials are combined and by altering the molecular weight of the SAN.

PROPERTIES The unique combinations of excellent impact strength with high mechanical strength and rigidity plus good long-term, load-carrying ability or creep resistance are characteristic of the ABS plastics family. In addition, all types of ABS plastics exhibit outstanding dimensional stability, good chemical and heat resistance, surface hardness, and light weight (low specific gravity), Table A.1. These materials yield plastically at high stresses, so ultimate elongation is seldom significant in design; a part usually can be bent beyond its elastic limit without breaking, although it does stress-whiten. Although not generally considered flexible, ABS parts have enough spring to accommodate snap-fit assembly requirements. The individual commercially available ABS polymers span a wide range of mechanical properties. Most suppliers differentiate types on the

ASTM or UL Test

Standard ABS Grades Property

D792 D792

Specific gravity Specific volume (in.3/lb)

D638 D638 D638 D790 D790 D256

Tensile strength (psi) Elongation (%) Tensile modulus (105 psi) Flexural strength (psi) Flexural modulus (105 psi) Impact strength, Izod (ft-lb/in. of notch) Hardness, Rockwell R

D785 D696 D648

UL94

Coefficient of thermal expansion (10–5) in./in.-°F Deflection temperatureb (°F) At 264 psi At 66 psi Flammability rating

High Impact

Superhigh Impact

Special-Purpose ABS Grades

Medium Impact

High Heat

Flame Retardant

Clear

Expandable

Plating

SAN Grades

1.19–1.22 —

1.05 26

0.55–0.85 —

1.05–1.07 26

1.07–1.08 26

5,500–10,000 5–25 3.2–3.7 9,000–12,250 3.0–3.4 4.0–13.0

5,800–6,300 25–75 3.0–3.3 10,500 3.4–3.9 2.5–4.0

3,000–4,000 — 1.0–2.5 3,000–8,000 1.4–2.8 —

5,500–6,600 — 3–3.8 8,700–11,500 3.0–3.8 5.0–7.0

9,000–12,000 1–4 4.5–5.6 14,000–17,000 5.5 0.35–0.50

90–117

100–105

60–70a

103–109

M85

3.7–4.6

4.6

4.9

4.7–5.3

3.0

180–220 198–238 V-0 to V-1c

168 180–185 HB

160 185 HB–V-0

189 214 HB

210 — HB

400+ 20–60

400 120–130

— —

— —

— —

1.01–1.05 27

1.02–1.05 27

1.04–1.06 28

Physical 1.04–1.06 28

6,000 5–20 3.3 10,500 3.4 6.5

5,000–6,300 5–70 2.0–3.4 6,000–11,500 2.0–3.5 7.0–8.0

6,000–7,500 5–25 3.6–3.8 11,500 3.6–4.0 4.0–5.5

Mechanical 6,000–7,500 3–20 3.0–4.0 10,000–13,000 3.1–4.0 2.3–6.0

103

69–105

107

5.3

5.6

4.6

Thermal 3.9–5.1

188 203 HB

192 208 HB

184 201 HB

220–240 230–245 HB

111

Electrical D149 D495 a b c

Dielectric strength (V/mil) Short time, 1/8-in. thk Arc resistance (s)

400 89

350–500 50–85

350–500 50–85

Density has a marked effect. Unannealed. 0.060-in.-thick samples.

ASTM = American Society for Testing and Materials; UL = Underwriters’ Laboratories. Source: Mach. Design Basics Eng. Design, June, p. 674, 1993. With permission.


350–500 50–85


TABLE A.1 Properties of ABS and SAN


A

basis of impact strength and fabrication method (extrusion or molding). Some compounds feature one particularly exceptional property, such as high heat deflection temperature, abrasion resistance, or dimensional stability. Impact properties of ABS are exceptionally good at room temperature and, with special grades, at temperatures as low as –40°C. Because of its plastic yield at high strain rates, impact failure of ABS is ductile rather than brittle. Also, the skin effect, which in other thermoplastics accounts for a lower impact resistance in thick sections than in thin ones, is not pronounced in ABS materials. A long-term tensile design stress of 6.8 to 10.3 MPa (at 22.8°C) is recommended for most grades. General-purpose ABS grades are adequate for some outdoor applications, but prolonged exposure to sunlight causes color change and reduces surface gloss, impact strength, and ductility. Less affected are tensile strength, flexural strength, hardness, and elastic modulus. Pigmenting the resins black, laminating with opaque acrylic sheet, and applying certain coating systems provide weathering resistance. For maximum color and gloss retention, a compatible coating of opaque, weather-resistant polyurethane can be used on molded parts. For weatherable sheet applications, ABS resins can be coextruded with a compatible weather-resistant polymer on the outside surface. ABS resins are stable in warm environments and can be decorated with durable coatings that require baking at temperatures to 71°C for 30–60 min. Heat-resistant grades can be used for short periods at temperatures to 110°C in light load applications. Low moisture absorption contributes to the dimensional stability of molded ABS parts. Molded ABS parts are almost completely unaffected by water, salts, most inorganic acids, food acids, and alkalies, but much depends on time, temperature, and especially stress level. Food and Drug Administration (FDA) acceptance depends to some extent on the pigmentation system used. The resins are soluble in esters and ketones, and they soften or swell in some chlorinated hydrocarbons, aromatics, and aldehydes. Properties of SAN resins are controlled primarily through acrylonitrile content and by


adjusting the molecular weight of the copolymer. Increasing both improves physical properties, at a slight penalty in processing ease. Properties of the resins can also be enhanced by controlling orientation during molding. Tensile and impact strength, barrier properties, and solvent resistance are improved by this control. Special grades of SAN are available with improved ultraviolet (UV) stability, vapor-barrier characteristics, and weatherability. The barrier resins — designed for the blown-bottle market — are also tougher and have greater solvent resistance than the standard grades.

FABRICATION

AND

FORMS

ABS plastics are readily formed by the various methods of fabricating thermoplastic materials extrusion, injection molding, blow molding, calendering, and vacuum forming. Molded products may be machined, riveted, punched, sheared, cemented, laminated, embossed, or painted. Although the ABS plastics process easily and exhibit excellent moldability, they are generally more difficult flowing than the modified styrenes and higher processing temperatures are used. The surface appearance of molded articles is excellent and buffing may not be necessary. Moldings The need for impact resistance and high mechanical properties in injection-molded parts has created a large use for ABS materials. Advances in resin technology coupled with improved machinery and molding techniques have opened the door to ABS resins. Large complex shapes can be readily molded in ABS today. Pipe The ABS plastics as a whole are popular for extrusion and they offer a great deal for this type of forming. The outstanding contribution is their ability to be formed easily and to hold dimension and shape. In addition, very good extrusion rates are obtainable. Because ABS materials are processed at stock temperatures of 400 to 500°F, it is generally necessary to preheat and dry the material prior to extrusion.


The largest single ABS end product is plastic pipe, where the advantages of high longterm mechanical strength, toughness, wide service temperature range, chemical resistance, and ease of joining by solvent welding are used. Sheet ABS sheet is manufactured by calendering or extrusion and molded articles are subsequently vacuum-formed. The hot strength of the ABS materials coupled with the ability to be drawn excessively without forming thin spots or losing embossing have made them popular with fabricators. The excellent mechanical strengths, formability, and chemical resistance, particularly to fluorocarbons, are largely responsible for the rapid increase in the use of ABS.

APPLICATIONS Molded ABS products are used in both protective and decorative applications. Examples include safety helmets, camper tops, automotive instrument panels and other interior components, pipe fittings, home-security devices and housings for small appliances, communications equipment, and business machines. Chrome-plated ABS has replaced die-cast metals in plumbing hardware and automobile grilles, wheel covers, and mirror housings. Typical products vacuum-formed from extruded ABS sheet are refrigerator liners, luggage shells, tote trays, mower shrouds, boat hulls, and large components for recreational vehicles. Extruded shapes include weather seals, glass beading, refrigerator breaker strips, conduit, and pipe for drainwaste-vent (DWV) systems. Pipe and fittings comprise one of the largest single application areas for ABS. Typical applications for molded SAN copolymers include instrument lenses, vacuumcleaner and humidifier parts, medical syringes, battery cases, refrigerator compartments, foodmixer bowls, computer reels, chair shells, and dishwasher-safe houseware products.

ACETAL PLASTICS Acetals are independent structural units or a part of certain biological and commercial


polymers, and acetal resins are highly crystalline plastics based on formaldehyde polymerization technology. These engineering resins are strong, rigid, and have good moisture, heat, and solvent resistance. Acetals were specially developed to compete with zinc and aluminum castings. The natural acetal resin is translucent white and can be readily colored with a high sparkle and brilliance. There are two basic types — homopolymer (Delrin) and copolymer (Celcon). In general, the homopolymers are harder, more rigid, have higher tensile flexural and fatigue strength, but lower elongation; however, they have higher melting points. Some high-molecular-weight homopolymer grades are extremely tough and have higher elongation than the copolymers. Homopolymer grades are available that are modified for improved hydrolysis resistance to 82°C, similar to copolymer materials. The copolymers remain stable in long-term, high-temperature service and offer exceptional resistance to the effects of immersion in water at high temperatures. Neither type resists strong acids, and the copolymer is virtually unaffected by strong bases. Both types are available in a wide range of melt-flow grades, but the copolymers process more easily and faster than the conventional homopolymer grades. Both the homopolymers and copolymers are available in several unmodified and glassfiber-reinforced injection-molding grades. Both are available in polytetrafluoroethylene (PTFE) or silicone-filled grades, and the homopolymer is available in chemically lubricated low-friction formulations. The acetals are also available in extruded rod and slab form for machined parts. Property data listed in Table A.2 apply to the generalpurpose injection-molding and extrusion grade of Delrin 500 and to Celcon M90. Acetals are among the strongest and stiffest of the thermoplastics. Their tensile strength ranges from 54.4 to 92.5 MPa, tensile modulus is about 3400 MPa, and fatigue strength at room temperature is about 34 MPa. Acetals are also among the best in creep resistance. This combined with low moisture absorption (less than 0.4%) gives them excellent dimensional stability. They are useful for continuous service up to about 104°C.

A


A

TABLE A.2 Properties of Acetals ASTM or UL Test

D792 D792 D570

D638

D638 D638 D790 D790

D256

D671 D785

Property

Copolymer

Physical Specific gravity Specific volume (in.3/lb) Water absorption, 24 h, 1/8-in. thk (%) Mechanical *Tensile strength (psi) At 73°F At 160°F *Elongation (%) *Tensile modulus (105 psi) Flexural strength (psi) Flexural modulus (105 psi) At 73°F At 160°F Impact strength, Izod (ft-lb/in.) Notched Unnotched Fatigue endurance limit, 107 cycles (psi) Hardness, Rockwell M

Homopolymer

1.41 19.7 0.22

1.42 19.5 0.25

8,800 5,000 60 4.1 13,000

10,000 6,900 40 5.2 14,100

3.75 1.80

4.10 2.30

1.3 20 3,300 80

1.4 24 4,300 94

5.5 1.6

8.9 2.6

8.5

10.0

230 316 HB

277 342 HB

2,100 — 500

3,000 2,000 500

3.7 3.7

3.7 3.7

0.001 0.006

0.001 0.005

1014 240 (burns)

1015 220 (burns, no tracking)

0.35 0.15

0.3 0.15

Thermal C177

D696 D648

UL94

Thermal conductivity (10–4 cal-cm/s-cm2-°C) Btu-in./hr-ft2-°F) Coefficient of thermal expansion –40 to +185°C (10–5 in./in.·°C) Deflection temp (°F) At 264 psi At 66 psi Flammability rating Electrical

D149

D150

D150

D257 D495

Dielectric strength Short time (V/mil) 5 mils 20 mils 90 mils Dielectric constant At 1 kHz At 1 MHz Dissipation factor At 1 kHz At 1 MHz Volume resistivity (ohm-cm) At 73°F, 50% RH Arc resistance (s) 120 mils Frictional

—

Coefficient of friction Self Against steel

* At 0.2 in./min loading rate. Source: Mach. Design Basics Eng. Design, June, p. 676, 1993. With permission.



Injection-molding powders and extrusion powders are the most frequently used forms of the material. Sheets, rods, tubes, and pipe are also available. Colorability is excellent. The range of desirable design properties and processing techniques provides outstanding design freedom in the areas (1) style (color, shape, surface texture and decoration), (2) weight reduction, (3) assembly techniques, and (4) one-piece multifunctional parts (e.g., combined gear, cam, bearing, and shaft).

ACETAL HOMOPOLYMERS The homopolymers are available in several viscosity ranges that meet a variety of processing and end-use needs. The higher-viscosity materials are generally used for extrusions and for molded parts requiring maximum toughness; the lower-viscosity grades are used for injection molding. Elastomer-modified grades offer greatly improved toughness. Properties Acetal homopolymer resins have high tensile strength, stiffness, resilience, fatigue endurance, and moderate toughness under repeated impact. Some tough grades can deliver up to 7 times greater toughness than unmodified acetal in Izod impact tests and up to 30 times greater toughness as measured by Gardner impact tests (Table A.2). Homopolymer acetals have high resistance to organic solvents, excellent dimensional stability, a low coefficient of friction, and outstanding abrasion resistance among thermoplastics. The general-purpose resins can be used over a wide range of environmental conditions; special, UV-stabilized grades are recommended for applications requiring long-term exposure to weathering. However, prolonged exposure to strong acids and bases outside the range of pH 4 to 9 is not recommended. Acetal homopolymer has the highest fatigue endurance of any unfilled commercial thermoplastic. Under completely reversed tensile and compressive stress, and with 100% relative humidity (at 73°F), fatigue endurance limit is 30.9 MPa at 106 cycles. Resistance to creep is excellent. Moisture, lubricants, and solvents


including gasoline and gasohol have little effect on this property, which is important in parts incorporating self-threading screws or interference fits. The low friction and good wear resistance of acetals against metals make these resins suitable for use in cams and gears having internal bearings. The coefficient of friction (nonlubricated) on steel, in a rotating thrust washer test, is 0.1 to 0.3, depending on pressure; little variation occurs from 22.8 to 121°C. For even lower friction and wear, PTFE-fiber-filled and chemically lubricated formulations are available. Properties of low moisture absorption, excellent creep resistance, and high deflection temperature suit acetal homopolymer for closetolerance, high-performance parts. Applications Automotive applications of acetal homopolymer resins include fuel-system and seat-belt components, steering columns, window-support brackets, and handles. Typical plumbing applications that have replaced brass or zinc components are showerheads, ball cocks, faucet cartridges, and various fittings. Consumer items include quality toys, garden sprayers, stereo cassette parts, butane lighter bodies, zippers, and telephone components. Industrial applications of acetal homopolymer include couplings, pump impellers, conveyor plates, gears, sprockets, and springs.

ACETAL COPOLYMERS The copolymers have an excellent balance of properties and processing characteristics. Melt temperature can range from 182 to 232°C with little effect on part strength. UV-resistant grades (also available in colors), glass-reinforced grades, low-wear grades, and impactmodified grades are standard. Also available are electroplatable and dimensionally stable, low-warpage grades. Properties Acetal copolymers have high tensile and flexural strength, fatigue resistance, and hardness. Lubricity is excellent. They retain much of their toughness through a broad temperature range

A


A

and are among the most creep resistant of the crystalline thermoplastics. Moisture absorption is low, permitting molded parts to serve reliably in environments involving humidity changes. Good electrical properties, combined with high mechanical strength and an Underwriters’ Laboratories (UL) electrical rating of 100°C, qualify these materials for electrical applications requiring long-term stability. Acetal copolymers have excellent resistance to chemicals and solvents. For example, specimens immersed for 12 months at room temperature in various inorganic solutions were unaffected except by strong mineral acids — sulfuric, nitric, and hydrochloric. Continuous contact is not recommended with strong oxidizing agents such as aqueous solutions containing high concentrations of hypochlorite ions. Solutions of 10% ammonium hydroxide and 10% sodium chloride discolor samples in prolonged immersion, but physical and mechanical properties are not significantly changed. Most organic reagents tested have no effect, nor do mineral oil, motor oil, or brake fluids. Resistance to strong alkalies is exceptionally good; specimens immersed in boiling 50% sodium hydroxide solution and other strong bases for many months show no property changes. Strength of acetal copolymer is only slightly reduced after aging for 1 year in air at 116°C. Impact strength holds constant for the first 6 months, and falls off about one-third during the next 6-month period. Aging in air at 82°C for 2 years has little or no effect on properties, and immersion for 1 year in 82°C water leaves most properties virtually unchanged. Samples tested in boiling water retain nearly original tensile strength after 9 months. The creep–modulus curve of acetal copolymer under load shows a linear decrease on a log-log scale, typical of many plastics. Acetal springs lose over 50% of spring force after 1000 h and 60% in 10,000 h. The same spring loses 66% of its force after 100,000 h (about 11 years) under load. Plastic springs are best used in applications where they generate a force at a specified deflection for limited time but otherwise remain relaxed. Ideally, springs should undergo occasional deflections where they have time to recover, at less than 50% design


strain. Recovery time should be at least equal to time under load. Applications Industrial and automotive applications of acetal copolymer include gears, cams, bushings, clips, lugs, door handles, window cranks, housings, and seat-belt components. Plumbing products such as valves, valve stems, pumps, faucets, and impellers utilize the lubricity and corrosion and hot water resistance of the copolymer. Mechanical components that require dimensional stability, such as watch gears, conveyor links, aerosols, and mechanical pen and pencil parts, are other uses. Applications for the FDAapproved grades include milk pumps, coffee spigots, filter housings, and food conveyors. Parts that require greater load-bearing stability at elevated temperatures, such as cams, gears, television tuner arms, and automotive underhood components, are molded from glass-fiberreinforced grades. More costly acetal copolymer has excellent load-bearing characteristics for long-lasting plastic springs. To boost resin performance, engineers use fillers, reinforcing fibers, and additives. Although there are automotive uses for large fiber-reinforced composite leaf springs, unfilled resins are the better candidates for small springs. Glass fibers increase stiffness and strength, but they also limit deflection. And impact modifiers reduce modulus and make plastics more flexible but decrease creep resistance.

ACETAL RESINS Processing Acetals Acetal resin can be molded in standard injection molding equipment at conventional production rates. The processing temperature is around 204°C. Satisfactory performance has been demonstrated in full-automatic injection machines using multicavity molds. Successful commercial moldings point up the ability of the material to be molded to form large-area parts with thin sections, heavy parts with thick sections, parts requiring glossy surfaces or different surface textures, parts requiring close tolerances, parts with undercuts for snap fits, parts requiring


metal inserts, and parts requiring no flash. It can also be extruded as rod, tubing, sheeting, jacketing, wire coating, or shapes on standard commercial equipment. Extrusion temperatures are in the range of 199 to 204°C. Generally the same equipment and techniques for blow molding other thermoplastics work with acetal resin. Both thin-walled and thick-walled containers (aerosol type) can be produced in many shapes and surface textures. Various sheet-forming techniques including vacuum, pressure, and matched-mold have been successfully used with acetal resins. Fabrication Acetal resin is easy to machine (equal to or better than free-cutting brass) on standard production machine shop equipment. It can be sawed, drilled, turned, milled, shaped, reamed, threaded and tapped, blanked and punched, filed, sanded, and polished. The material is easy to join and offers wide latitude in the choice of fast, economical methods of assembly. Integral bonds of acetal-toacetal can be formed by welding with a heated metal surface, hot gas, hot wire, or spin-welding techniques. High-strength joints result from standard mechanical joining methods such as snap fits, interference or press fits, rivets, nailing, heading, threads, or self-tapping screws. Where low joint strengths are acceptable, several commercial adhesives can be used for bonding acetal to itself and other substrates. Acetal resin can be painted successfully with certain commercial paints and lacquers, using ordinary spraying equipment and a special surface treatment or followed by a baked top coat. Successful first-surface metallizing has been accomplished with conventional equipment and standard techniques for application of such coatings. Direct printing, process printing, and roll-leaf stamping (hot stamping) can be used for printing on acetal resin. Baking at elevated temperatures is required for good adhesion of the ink in direct and screen-process printing. In hot stamping, the heated die provides the elevated temperature. Printing produced by these processes resists abrasion and lifting by cellophane adhesive tape.


ACETYLENE Acetylene is a colorless, flammable gas with a garlic-like odor. Under compressed conditions, it is highly explosive; however, it can be safely compressed and stored in high-pressure cylinders if the cylinders are lined with absorbent material soaked with acetone. Users are cautioned not to discharge acetylene at pressures exceeding 15 psig (103 kPa), as noted by the red line on acetylene pressure gauges. With its intense heat and controllability, the oxyacetylene flame can be used for many different welding and cutting operations including hardfacing, brazing, beveling, gouging, and scarfing. The heating capability of acetylene also can be utilized in the bending, straightening, forming, hardening, softening, and strengthening of metals.

ACRYLIC PLASTICS The most widely used acrylic plastics are based on polymers of methyl methacrylate. This primary constituent may be modified by copolymerizing or blending with other acrylic monomers or modifiers to obtain a variety of properties. Although acrylic polymers based on monomers other than methyl methacrylate have been investigated, they are not as important as commercial plastics and are generally confined to uses in fibers, rubbers, motor oil additives, and other special products.

STANDARD ACRYLICS Poly(methyl methacrylate), the polymerized methyl ester of methacrylic acid, is thermoplastic. The method of polymerization may be varied to achieve specific physical properties, or the monomer may be combined with other components. Sheet materials may be prepared by casting the monomer in bulk. Suspension polymerization of the monomeric ester may be used to prepare molding powders. Conventional poly(methyl methacrylate) is amorphous; however, reports have been published of methyl methacrylate polymers of regular configuration, which are susceptible to crystallization. Both the amorphous and crystalline forms of such crystallization-susceptible

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polymers possess physical properties that are different from those of the conventional polymer, and suggest new applications. Service Properties Acrylic thermoplastics are known for their outstanding weatherability. They are available in cast sheet, rod, and tube; extruded sheet and film; and compounds for injection molding and extrusion. They are also characterized by good impact strength, formability, and excellent resistance to sunlight, weather, and most chemicals. Maximum service temperature of heatresistant grades is about 200°F. Standard grades are rated as slow burning, but a special selfextinguishing grade of sheet is available. Although acrylic plastic weighs less than half as much as glass, it has many times greater impact resistance. As a thermal insulator, it is approximately 20% better than glass. It is tasteless and odorless. When poly(methyl methacrylate) is manufactured with scrupulous care, excellent optical properties are obtained. Light transmission is 92%; colorants produce a full spectrum of transparent, translucent, or opaque colors. Most colors can be formulated for long-term outdoor durability. Acrylics are normally formulated to filter UV energy in the 360-nm and lower band. Other formulations are opaque to UV light or provide reduced UV transmission; infrared light transmission is 92% at wavelengths up to 1100 millimicrons, failing irregularly to 0% at 2200 millimicrons; scattering effect is practically nil; refractive index is 1.49 to 1.50; critical angle is 42°; dispersion 0.008. Because of its excellent transparency and favorable index of refraction, acrylic plastic is often used in the manufacture of optical lenses. Superior dimensional stability makes it practicable to produce precision lenses by injection molding techniques. In chemical resistance, poly(methyl methacrylate) is virtually unaffected by water, alkalies, weak acids, most inorganic solutions, mineral and animal oils, and low concentrations of alcohol. Oxidizing acids affect the material only in high concentrations. It is also virtually unaffected by paraffinic and olefinic hydrocarbons, amines, alkyl monohalides, and esters containing more than ten carbon atoms. Lower esters,


aromatic hydrocarbons, phenols, aryl halides, aliphatic acids, and alkyl polyhalides usually have a solvent action. Acrylic sheet and moldings are attacked, however, by chlorinated and aromatic hydrocarbons, esters, and ketones. Mechanical properties of acrylics are high for short-term loading. However, for long-term service, tensile stresses must be limited to 1500 psi to avoid crazing or surface cracking. The moderate impact resistance of standard formulations is maintained even under conditions of extreme cold. High-impact grades have considerably higher impact strength than standard grades at room temperature, but impact strength decreases as temperature drops. Special formulations ensure compliance with UL standards for bullet resistance. Although acrylic plastics are among the most scratch resistant of the thermoplastics, normal maintenance and cleaning operations can scratch and abrade them. Special abrasion-resistant sheet is available that has the same optical and impact properties as standard grades. Toughness of acrylic sheet, as measured by resistance to crack propagation, can be improved severalfold by inducing molecular orientation during forming. Jet aircraft cabin windows, for example, are made from oriented acrylic sheet. Transparency, gloss, and dimensional stability of acrylics are virtually unaffected by years of exposure to the elements, salt spray, or corrosive atmospheres. These materials withstand exposure to light from fluorescent lamps without darkening or deteriorating. They ultimately discolor, however, when exposed to high-intensity UV light below 265 nm. Special formulations resist UV emission from light sources such as mercury-vapor and sodiumvapor lamps. Product Forms Cell-cast sheet is produced in several sizes and thicknesses. The largest sheets available are 120 × 144 in., in thicknesses from 0.030 to 4.25 in. Continuous-cast material is supplied as flat sheet to 1/2 in. thick, in widths to 9 ft. Acrylic sheet cast by the continuous process (between stainless steel belts) is more uniform in thickness than cell-cast sheet. Cell-cast sheet, on the other hand, which is cast between glass plates,


has superior optical properties and surface quality. Also, cell-cast sheet is available in a greater variety of colors and compositions. Cast acrylic sheet is supplied in general-purpose grades and in UV-absorbing, mirrored, super-thermoformable, and cementable grades, and with various surface finishes. Sheets are available in transparent, translucent, and opaque colors. Acrylic film is available in 2-, 3-, and 6-mil thicknesses, in clear form and in colors. It is supplied in rolls to 60 in. wide, principally for use as a protective laminated cover over other plastic materials. Injection-molding and extrusion compounds are available in both standard and highmolecular-weight grades. Property differences between the two formulations are principally in flow and heat resistance. Higher-molecularweight resins have lower melt-flow rates and greater hot strength during processing. Lowermolecular-weight grades flow more readily and are designed for making complex parts in hardto-fill molds; see Table A.3. Fabrication Characteristics When heated to a pliable state, acrylic sheet can be formed to almost any shape. The forming operation is usually carried out at about 290 to 340°F. Aircraft canopies, for example, are usually made by differential air pressure, either with or without molds. Such canopies have been made from (1) monolithic sheet stock, (2) laminates of two layers of acrylic, bonded by a layer of polyvinyl butyral, and (3) stretched monolithic sheet. Irregular shapes, such as sign faces, lighting fixtures, or boxes, can be made by positive pressure-forming, using molds. Residual strains caused by forming are minimized by annealing, which also brings cemented joints to full strength. Cementing can be readily accomplished by using either solvent or polymerizable cements. Acrylic plastic can be sawed, drilled, and machined like wood or soft metals. Saws should be hollow ground or have set teeth. Slow feed and coolant will prevent overheating. Drilling can be done with conventional metal-cutting drills. Routing requires high-speed cutters to prevent chipping. Finished parts can be sanded, and sanded surfaces


can be polished with a high-speed buffing wheel. Cleaning should be by soap or detergent and water, not by solvent-type cleaners. Acrylic molding powder may be used for injection, extrusion, or compression molding. The material is available in several grades, with a varying balance of flow characteristics and heat resistance. Acrylics give molded parts of excellent dimensional stability. Precise contours and sharp angles, important in such applications as lenses, are achieved without difficulty, and this accuracy of molding can be maintained throughout large production runs. Since dirt, lint, and dust detract from the excellent clarity of acrylics, careful handling and storage of the molding powder are extremely important. Applications In merchandising, acrylic sheet has become the major sign material for internally lighted faces and letters, particularly for outdoor use where resistance to sunlight and weathering is important. In addition, acrylics are used for counter dividers, display fixtures and cases, transparent demonstration models of household appliances and industrial machines, and vending machine cases. The ability of acrylics to resist breakage and corrosion, and to transmit and diffuse light efficiently has led to many industrial and architectural applications. Industrial window glazing, safety shields, inspection windows, machine covers, and pump components are some of the uses commonly found in plants and factories. Acrylics are employed to good advantage as the diffusing medium in lighting fixtures and large luminous ceiling areas. Dome skylights formed from acrylic sheet are an increasingly popular means of admitting daylight to industrial, commercial, and public buildings and even to private homes. Shower enclosures and deeply formed components such as tub–shower units, which are subsequently backed with glass-fiber-reinforced polyester and decorated partitions, are other typical applications. A large volume of the material is used for curved and flat windshields on pleasure boats, both inboard and outboard types.

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TABLE A.3 Properties of Acrylics Molding Grade

ASTM Test

Property

D792 D792 D570

Specific gravity Specific volume (in.3/lb) Water absorption, 24 h, 1/8-in. thk (%)

D638 D638 D638 D790 D790 D256

Tensile strength (psi) Elongation (%) Tensile modulus (105 psi) Flexural strength (psi) Flexural modulus (105 psi) Impact strength, Izod (ft-lb/in. of notch) Hardness, Rockwell

D785

D696 D648

Coefficient of thermal expansion (10–5) in./in.-°C Deflection temperature (°F) At 264 psi At 66 psi

Cast Sheet

Standard

High Impact

1.19 23.3 0.3

1.15–1.17 24.1 0.3

10,500 5 4.3 16,000 4.5 0.4

5,400–7,000 50 2.2–8.2 7,000–10,500 0.65–2.5 0.6–1.2

M95

R99–M68

3.6

3.8

198 214

170–190 187

500

500

383–450

3.3 2.5

3.3 2.3

3.9 2.5–3.0

0.04 0.02–0.03

0.04 0.02–0.03

— 0.01–0.02

>1017 No track

>1017 No track

>1015 No track

1.49 92

1.49 90

Physical 1.19 23.3 0.2

Mechanical 10,500 5 4.5 16,500 4.5 0.4 M100–102 Thermal 3.9

200–215 225 Electrical

D149

Dielectric strength (V/mil) Short time, 1/8-in. thk

D150

Dielectric constant At 1 kHz At 1 MHz Dissipation factor At 1 kHz At 1 MHz Volume resistivity (ohm-cm) At 73°F, 50% RH

D150

D257 D495

Arc resistance (s)

D542 D1003

Refractive index Transmittance (%)

Optical 1.49 92

Source: Mach. Design Basics Eng. Design, June, p. 678, 1993. With permission.

Acrylic sheet is the standard transparent material for aircraft canopies, windows, instrument panels, and searchlight and landing light covers. To meet the increasingly severe service requirements of pressurized jet aircraft, new grades of acrylic have been developed that have


improved resistance to heat and crazing. The stretching technique has made possible enhanced resistance to both crazing and shattering. Large sheets, edge-lighted, are used as radar plotting boards in shipboard and groundcontrol stations.


In molded form, acrylics are used extensively for automotive parts, such as taillight and stoplight lenses, medallions, dials, instrument panels, and signal lights. The beauty and durability of molded acrylic products have led to their wide use for nameplates, control knobs, dials, and handles on all types of home appliances. Acrylic molding powder is also used for the manufacture of pen and pencil barrels, hairbrush backs, watch and jewelry cases, and other accessories. Large-section moldings, such as covers for fluorescent street lights, coin-operated phonograph panels, and fruit juice dispenser bowls, are being molded from acrylic powder. The extrusion of acrylic sheet from molding powder is particularly effective in the production of thin sheeting for use in such applications as signs, lighting, glazing, and partitions. The transparency, strength, light weight, and edge-lighting characteristics of acrylics have led to applications in the fields of hospital equipment, medical examination instruments, and orthopedic devices. The use of acrylic polymers in the preparation of dentures is an established practice. Contact lenses are also made of acrylics. The embedment of normal and pathological tissues in acrylic for preservation and instructional use is an accepted technique. This has been extended to include embedment of industrial machine parts, as sales aids, and the preparation of various types of home decorative articles.

HIGH-IMPACT ACRYLICS High-impact acrylic molding powder is used in large-volume, general use. It is used where toughness greater than that found in the standard acrylics is desired. Other advantages include resistance to staining, high surface gloss, dimensional stability, chemical resistance, and stiffness, and they provide the same transparency and weatherability as the conventional acrylics. High-impact acrylic is off-white and nearly opaque in its natural state and can be produced in a wide range of opaque colors. Several grades are available to meet requirements for different combinations of properties. Various members of the family have Izod impact strengths from


about 0.5 to as high as 4 ft-lb/in. notch. Other mechanical properties are similar to those of conventional acrylics. High-impact acrylics are used for hard service applications, such as women’s thin-style shoe heels and housings, ranging from electric razors to outboard motors, piano and organ keys, and beverage vending machine housings and canisters — in short, applications where toughness, chemical resistance, dimensional stability, stiffness, resistance to staining, lack of unpleasant odor or taste, and high surface gloss are required.

ADHESIVES These are materials capable of fastening two other materials together by means of surface attachment. The words glue, mucilage, mastic, and cement are synonymous with adhesive. In a generic sense, the word adhesive implies any material capable of fastening by surface attachment, and thus will include inorganic materials such as portland cement and solders such as Wood’s metal. In a practical sense, however, adhesive implies the broad set of materials composed of organic compounds, mainly polymeric, that can be used to fasten two materials together. The materials being fastened together by the adhesive are the adherends, and an adhesive joint or adhesive bond is the resulting assembly. Adhesion is the physical attraction of the surface of one material for the surface of another. From an industrial manufacturing standpoint, the advent of the stealth aircraft and all the structural adhesive bonding it entails has drawn widespread attention to the real capabilities of adhesives. Structural bonding uses adhesives to join load-bearing assemblies. Most often, the assemblies are also subject to severe service conditions. Such adhesives, regardless of chemistry, generally have the following properties: • Tensile strengths in the 1500 to 4500 psi range • Very high impact and peel strength • Service temperature ranges of about –65 to 3500°F

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If these types of working conditions are expected, then one should give special consideration to proper adhesive selection and durability testing.

THEORIES The phenomenon of adhesion has been described by many theories. The most widely accepted and investigated is the wettability–adsorption theory. Basically, this theory states that for maximum adhesion the adhesive must come into complete intimate contact with the surface of the adherend. That is, the adhesive must completely wet the adherend. This wetting is considered to be maximized when the intermolecular forces are the same forces as are normally considered in intermolecular interactions such as the van der Waals, dipole–dipole, dipole–induced dipole, and electrostatic interactions. Of these, the van der Waals force is considered the most important. The formation of chemical bonds at the interface is not considered to be of primary importance for achieving maximum wetting, but in many cases it is considered important in achieving durable adhesive bonds. If the situation is such that the adhesive completely wets the adherend, the strength of the adhesive joint depends on the design of the joint, the physical properties of the adherends, and, most importantly, the physical properties of the adhesive.

PARAMETERS Innumerable adhesives and adhesive formulations are available today. The selection of the proper type for a specific application can only be made after a complete evaluation of the design, the service requirements, production feasibility, and cost considerations. Usually such selection is best left up to adhesive suppliers. Once they have been given the complete details of the application they are in the best position to select both the type and specific adhesive formulation. Types and Forms Adhesives have been in use since ancient times and are even mentioned in the Bible. The first adhesives were of natural origin; for example,


bitumen, fish oil, and tree resins. In more modern times, adhesives were still derived from natural products but were processed before use. These modern natural adhesives include animal-derived (such as blood, gelatin, and casein), vegetable-derived (such as soybean oil and wheat flour), and forest-derived (pine resins and cellulose derivatives) products. Forms include liquid, paste, powder, and dry film. The commercial adhesives include pastes, glues, pyroxylin cements, rubber cements, latex cement, special cements of chlorinated rubber, synthetic rubbers, or synthetic resins, and the natural mucilages. Characteristics Adhesives are characterized by degree of tack (or stickiness), by strength of bond after setting or drying, by rapidity of bonding, and by durability. The strength of bond is inherent in the character of the adhesive itself, particularly in its ability to adhere intimately to the surface to be bonded. Adhesives prepared from organic products are in general subject to disintegration on exposure. The life of an adhesive usually depends on the stability of the ingredient that gives the holding power, although otherwise good cements of synthetic materials may disintegrate by the oxidation of fillers or materials used to increase tack. Plasticizers usually reduce adhesion. Some fillers such as mineral fibers or walnut-shell flour increase the thixotropy and the strength, while some such as starch increase the tack but also increase the tendency to disintegrate.

CLASSIFICATION Adhesives can be grouped into five classifications based on chemical composition. These are summarized in Table A.4. Natural These include vegetable- and animal-based adhesives and natural gums. They are inexpensive, easy to apply, and have a long shelf life. They develop tack quickly, but provide only low-strength joints. Most are water soluble. They are supplied as liquids or as dry powders to be mixed with water.

Natural

Thermoplastic

Thermosetting

Elastomeric

Alloysa

Casein, blood albumin, hide, bone, fish, starch (plain and modified); rosin, shellac, asphalt; inorganic (sodium silicate, litharge-glycerin) Liquid, powder By vehicle (water emulsion is most common but many types are solvent dispersions)

Polyvinyl acetate, polyvinyl alcohol, acrylic, cellulose nitrate, asphalt, oleo-resin

Phenolic, resorcinol, phenolresorcinol, epoxy, epoxyphenolic, urea, melamine, alkyd

Liquid, some dry film By vehicle (most are solvent dispersions or water emulsions)

Liquid, but all forms common By cure requirements (heat and/or pressure most common but some are catalyst types)

Natural rubber, reclaim rubber, butadiene-styrene (GR-S), neoprene, acrylonitrile-butadiene (Buna-N), silicone Liquid, some film By cure requirements (all are common); also by vehicle (most are solvent dispersions or water emulsions)

Bond characteristics

Wide range, but generally low strength; good resistance to heat, chemicals; generally poor moisture resistance

Good to 150–200°F; poor creep strength; fair peel strength

Good to 200–500°F; good creep strength; fair peel strength

Good to 150–400°F; never melt completely; low strength; high flexibility

Major type of useb

Household, general purpose, quick set, long shelf life

Unstressed joints; designs with caps, overlaps, stiffeners

Stressed joints at slightly elevated temp

Unstressed joints on lightweight materials; joints in flexure

Materials most commonly bonded

Wood (furniture), paper, cork, liners, packaging (food), textiles, some metals and plastics; industrial uses giving way to other groups

Formulation range covers all materials, but emphasis on nonmetallics—esp wood, leather, cork, paper, etc.

Epoxy-phenolics for structural uses of most materials; others mainly for wood; alkyds for laminations; most epoxies are modified (alloys)

Few used “straight” for rubber, fabric, foil, paper, leather, plastics, films; also as tapes; most modified with synthetic resins

Phenolic-polyvinyl butyral, phenolic-polyvinyl formal, phenolic-neoprene rubber, phenolic-nitrile rubber, modified epoxy Liquid, paste, film By cure requirements (usually heat and pressure except some epoxy types); by vehicle (most are solvent dispersions or 100% solids); and by type of adherends or end-service conditions Balanced combination of properties of other chemical groups depending on formulation; generally higher strength over wider temp range Where highest and strictest endservice conditions must be met; sometimes regardless of cost, as military uses Metals, ceramics, glass, thermosetting plastics; nature of adherends often not as vital as design or end-service conditions (i.e., high strength, temp)

Types within group

Most used form Common further classifications

a

“Alloy,” as used here, refers to formulations containing resins from two or more different chemical groups. There are also formulations that benefit from compounding two resin types from the same chemical group (e.g., epoxy-phenolic). b Although some uses of the “nonalloyed” adhesives absorb a large percentage of the quantity of adhesives sold, the uses are narrow in scope; from the standpoint of diversified applications, by far the most important use of any group is the forming of adhesive alloys.



TABLE A.4 Adhesives Classified by Chemical Composition


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Casein-latex type is an exception. It consists of combinations of casein with either natural or synthetic rubber latex. It is used to bond metal to wood for panel construction and to join laminated plastics and linoleum to wood and metal. Except for this type, most natural adhesives are used for bonding paper, cardboard, foil, and light wood. Synthetic Polymer. The greatest growth in the development and use of organic compound-based adhesives came with the application of synthetically derived organic polymers. Broadly, these materials can be divided into two types: thermoplastics and thermosets. Thermoplastic adhesives become soft or liquid upon heating and are also soluble. Thermoset adhesives cure upon heating and then become solid and insoluble. Those adhesives that cure under ambient conditions by appropriate choice of chemistry are also considered thermosets. An example of a thermoplastic adhesive is a hot-melt adhesive. A well-known hot-melt adhesive in use since the Middle Ages is sealing wax. Modern hot-melt adhesives are composed of polymers such as polyamides, polyesters, ethylene-vinyl acetate copolymers, and polyethylene. Modern hot melts are heavily compounded with wax and other materials. Another widely used thermoplastic adhesive is polyvinyl acetate, which is applied from an emulsion. Thermoplastic Adhesives They can be softened or melted by heating and hardened by cooling. They are based on thermoplastic resins (including asphalt and oleoresin adhesives) dissolved in solvent or emulsified in water. Most of them become brittle at subzero temperatures and may not be used under stress at temperatures much above 150°F. As they are relatively soft materials, thermoplastic adhesives have poor creep strength. Although lower in strength than all but natural adhesives and suitable only for noncritical service, they are also lower in cost than most adhesives. They are also odorless and tasteless and can be made fungus resistant. Pressure Sensitive. Pressure-sensitive adhesives are mostly thermoplastic in nature and exhibit an important property known as


tack. That is, pressure-sensitive adhesives exhibit a measurable adhesive strength with only a mild applied pressure. Pressure-sensitive adhesives are derived from elastomeric materials, such as polybutadiene or polyisoprene. Thermosetting Adhesives Based on thermosetting resins, they soften with heat only long enough for the cure to initiate. Once cured, they become relatively infusible up to their decomposition temperature. Although most such adhesives do not decompose at temperatures below 500°F, some are useful only to about 150°F. Different chemical types have different curing requirements. Some are supplied as two-part adhesives and mixed before use at room temperature; some require heat or pressure to bond. As a group, these adhesives provide stronger bonds than natural, thermoplastic, or elastomeric adhesives. Creep strength is good and peel strength is fair. Generally, bonds are brittle and have little resilience and low impact strength. Elastomeric Adhesives Based on natural and synthetic rubbers, elastomeric adhesives are available as solvent dispersions, latexes, or water dispersions. They are primarily used as compounds that have been modified with resins to form some of the adhesive “alloys” discussed below. They are similar to thermoplastics in that they soften with heat, but never melt completely. They generally provide high flexibility and low strength, and without resin modifiers, are used to bond paper and similar materials. Alloy Adhesives This term refers to adhesives compounded from resins of two or more different chemical families, e.g., thermosetting and thermoplastic, or thermosetting and elastomeric. In such adhesives the performance benefits of two or more types of resins can be combined. For example, thermosetting resins are plasticized by a second resin resulting in improved toughness, flexibility, and impact resistance.


STRUCTURAL ADHESIVES Structural adhesives are, in general, of the alloy or thermosetting type and have the property of fastening adherends that are structural materials (such as metals and wood) for long periods of time even when the adhesive joint is under load. Phenolic-based structural adhesives were among the first structural adhesives to be developed and used. The most widely used structural adhesives are based on epoxy resins. Epoxy resin structural adhesives will cure at ambient or elevated temperatures, depending on the type of curative. Urethanes, generated by isocyanate-diol reactions, are also used as structural adhesives. Acrylic monomers have also been utilized as structural adhesives. These acrylic adhesives use an ambient-temperature surface-activated free radical cure. A special type of acrylic adhesive, based on cyanoacrylates (so-called superglue), is a structural adhesive that utilizes an anionic polymerization for its cure. Acrylic adhesives are known for their high strength and extremely rapid cure. Structural adhesives with resistance to high temperature (in excess of 390°F, or 200°C) for long times can be generated from ladder polymers such as polyimides and polyphenyl quinoxalines. Three of the most commonly used adhesives are the modified epoxies, neoprene-phenolics, and vinyl formal-phenolics. Modified epoxy adhesives are thermosetting and may be of either the room-temperature-curing type, which cure by addition of a chemical activator, or the heat-curing type. They have high strength and resist temperature up to nearly 500°F (260°C). A primary advantage of the epoxies is that they are 100% solids, and there is no problem of solvent evaporation after joining impervious surfaces. Other advantages include high shear strengths, rigidity, excellent self-filleting characteristics, and excellent wetting of metal and glass surfaces. Disadvantages include low peel strength, lack of flexibility, and inability to withstand high impact. Neoprene-phenolic adhesives are alloys characterized by excellent peel strength, but lower shear strength than modified epoxies. They are moderately priced, offer good flexibility and


vibration absorption, and have good adhesion to most metals and plastics. Neoprene-phenolics are solvent types, but special two-part chemically curing types are sometimes used to obtain specific properties. Vinyl formal-phenolic adhesives are alloys whose properties fall between those of modified epoxies and the thermoset-elastomer types. Vinyl formal-phenolics have good shear, peel, fatigue, and creep strengths and good resistance to heat, although they soften somewhat at elevated temperatures. They are supplied as solvent dispersions in solution or in film form. In the film form the adhesive is coated on both sides of a reinforcing fabric. Sometimes it is prepared by mixing a liquid phenolic resin with vinyl formal powder just prior to use. Other Adhesives/Cements Paste adhesives are usually water solutions of starches or dextrins, sometimes mixed with gums, resins, or glue to add strength, and containing antioxidants. They are the cheapest of the adhesives, but deteriorate on exposure unless made with chemically altered starches. They are widely employed for the adhesion of paper and paperboard. Much of the so-called vegetable glue is tapioca paste. It is used for the cheaper plywoods, postage stamps, envelopes, and labeling. It has a quick tack, and is valued for pastes for automatic box-making machines. Latex pastes of the rub-off type are used for such purposes as photographic mounting, as they do not shrink the paper as do the starch pastes. Glues are usually water solutions of animal gelatin, and the only difference between animal glues and edible gelatin is in the degree of purity. Hide and bone glues are marketed as dry flake, but fish glue is liquid. Mucilages are light vegetable glues, generally from water-soluble gums. Rubber cements for paper bonding are simple solutions of rubber in a chemical solvent. They are like the latex pastes in that the excess can be rubbed off the paper. Stronger rubber cements are usually compounded with resins, gums, or synthetics. An infinite variety of these cements is possible, and they are all waterproof with good initial bond, but they are subject to

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deterioration on exposure, as the rubber is uncured. This type of cement is also made from synthetic rubbers that are self-curing. Curing cements are rubber compounds to be cured by heat and pressure or by chemical curing agents. When cured, they are stronger, give better adhesion to metal surfaces, and have longer life. Latex cements are solvent solutions of rubber latex. They provide excellent tack and give strong bonds to paper, leather, and fabric, but they are subject to rapid disintegration unless cured. In general, natural rubber has the highest cohesive strength of the rubbers, with rapid initial tack and high bond strength. It also is odorless. Neoprene has the highest cohesive strength of the synthetic rubbers, but it requires tackifiers. Graphite–sulfur rubber (styrene–butadiene) is high in specific adhesion for quick bonding, but has low strength. Reclaimed rubber may be used in cements, but it has low initial tack and needs tackifiers. Pyroxylin cements may be merely solutions of nitrocellulose in chemical solvents, or they may be compounded with resins, or plasticized with gums or synthetics. They dry by the evaporation of the solvent and have little initial tack, but because of their ability to adhere to almost any type of surface they are called household cements. Cellulose acetate may also be used. These cements are used for bonding the soles of women’s shoes. The bonding strength is about 10 lb/in.2 (0.07 MPa), or equivalent to the adhesive strength of the outer fibers of the leather to be bonded. For hot-press lamination of wood the plastic cement is sometimes marketed in the form of thin sheet. Polyvinyl acetate-crotonic acid copolymer resin is used as a hot-dip adhesive for book and magazine binding. It is soluble in alkali solutions, and thus the trim is reusable. Polyvinyl alcohol, with fillers of clay and starch, is used for paperboard containers. Vinyl emulsions are much used as adhesives for laminates. Epoxy resin cements give good adhesion to almost any material and are heat-resistant to about 400°F (204°C). An epoxy resin will give a steel-to-steel bond of 3100 lb/in.2 (22 MPa), and an aluminum-to-aluminum bond to 3800 lb/in.2 (26 MPa).


Some pressure-sensitive adhesives are mixtures of a phenolic resin and a nitrile rubber in a solvent, but adhesive tapes are made with a wide variety of rubber or resin compounds. Furan cements, usually made with furfuralalcohol resins, are strong and highly resistant to chemicals. They are valued for bonding acidresistant brick and tile. Acrylic adhesives are solutions of rubberbased polymers in methacrylate monomers. They are two-component systems and have characteristics similar to those of epoxy and urethane adhesives. They bond rapidly at room temperature, and adhesion is not greatly affected by oily or poorly prepared surfaces. Other advantages are low shrinkage during cure, high peel and shear strength, excellent impact resistance, and good elevated temperature properties. They can be used to bond a great variety of materials, such as wood, glass, aluminum, brass, copper, steel, most plastics, and dissimilar metals. Ultraviolet cure adhesives are anaerobic structural adhesives formulated specifically for glass bonding applications. The adhesive remains liquid after application until ultraviolet light triggers the curing mechanism. A ceramic adhesive developed by the Air Force for bonding stainless steel to resist heat to 1500°F (816°C) is made with a porcelain enamel frit, iron oxide, and stainless steel powder. It is applied to both parts and fired at 1750°F (954°C), giving a shear strength of 1500 lb/in.2 (10 Mpa) in the bond. But ceramic cements that require firing are generally classed with ordinary adhesives. Wash-away adhesives are used for holding lenses, electronic crystal wafers, or other small parts for grinding and polishing operations. They are based on acrylic or other low-melting thermoplastic resins. They can be removed with a solvent or by heating. Electrically conductive adhesives are made by adding metallic fillers, such as gold, silver, nickel, copper, or carbon powder. Most conductive adhesives are epoxy-based systems, because of their excellent adhesion to metallic and nonmetallic surfaces. Silicones and polyimides are also frequently the base in adhesives used in bonding conductive gaskets to housings for electromagnetic and radio-frequency interference applications.


Properties An important property for a structural adhesive is resistance to fracture (toughness). Thermoplastics, because they are not cured, can deform under load and exhibit resistance to fracture. As a class, thermosets are quite brittle, and thermoset adhesives are modified by elastomers to increase their resistance to fracture. Applications Hot-melt adhesives are used for the manufacture of corrugated paper, in packaging, in carpeting, in bookbinding, and in shoe manufacture. Pressure-sensitive adhesives are most widely used in the form of coatings on tapes. These pressure-sensitive adhesive tapes have numerous applications, from electrical tape to surgical tape. Structural adhesives are applied in the form of liquids, pastes, or 100% adhesive films. Epoxy liquids and pastes are very widely used adhesive materials, having application in many assembly operations ranging from general industrial to automotive to aerospace vehicle construction. Solid-film structural adhesives are used widely in aircraft construction. Acrylic adhesives are used in thread-locking operations and in small-assembly operations such as electronics manufacture, which require rapid cure times. The largest-volume use of adhesives is in plywood and other timber products manufacture. Adhesives for wood bonding range from the natural products (such as blood or casein) to the very durable phenolic-based adhesives.

ALKYDS Several types of alkyds exist. Alkyd coatings are used for such diverse applications as air-drying water emulsion wall paints and baked enamels for automobiles and appliances. The properties of oil-modified alkyd coatings depend on the specific oil used as well as the percentage of oil in the composition. In general, they are comparatively low in cost and have excellent color retention, durability, and flexibility, but only fair drying speed, chemical resistance, heat resistance, and salt spray resistance. The oil-modified alkyds can © 2002 by CRC Press LLC

be further modified with other resins to produce resin-modified alkyds. Alkyd resins are a group of thermosetting synthetic resins known chemically as hydroxycarboxylic resins, of which the one produced from phthalic anhydride and glycerol is representative. They are made by the esterification of a polybasic acid with a polyhydric alcohol, and have the characteristics of homogeneity and solubility that make them especially suitable for coatings and finishes, plastic molding compounds, calking compounds, adhesives, and plasticizers for other resins. The resins have high adhesion to metals; are transparent, easily colored, tough, flexible, and heat and chemical resistant; and have good dielectric strength. Alkyd plastic molding compounds are composed of a polyester resin and usually a diallyl phthalate monomer plus various inorganic fillers, depending on the desired properties. The raw material is produced in three forms — granular, putty, and glass-fiber-reinforced. As a class, the alkyds have excellent heat resistance up to about 150°C, high stiffness, and moderate tensile and impact strength. Their low moisture absorption combined with good dielectric strength makes them particularly suitable for electronic and electrical hardware, such as switch-gear, insulators, and parts for motor controllers and automotive ignition systems. They are easily molded at low pressures and cure rapidly. Alkyds are part of the group of materials that includes bulk-molding compounds (BMCs) and sheet-molding compounds (SMCs). They are processed by compression, transfer, or injection molding. Fast molding cycles at low pressure make alkyds easier to mold than many other thermosets. They represent the introduction to the thermosetting plastics industry of the concept of low-pressure, high-speed molding. Typical properties are shown in Table A.5. Alkyds are furnished in granular compounds, extruded ropes or logs, bulk-molding compound, flake, and putty-like sheets. Except for the putty grades, which may be used for encapsulation, these compounds contain fibrous reinforcement. Generally, the fiber reinforcement in rope and logs is longer than that in granular compounds and shorter than that in flake compounds. Thus, strength of

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TABLE A.5 Properties of Alkyds Filler ASTM Test

Property

Mineral

D792 D570

Physical Specific gravity Water absorption, 24 h, 1/8-in. thk (%)

1.60–2.30 0.05–0.50

D638 D638 D790 D790 D256 D785

Mechanical Tensile strength (psi) Tensile modulus (105 psi) Flexural strength (psi) Flexural modulus (105 psi) Impact strength, Izod (ft-lb/in. of notch) Hardness, Rockwell

3,000–9,000 5–30 6,000–17,000 20 0.3–0.5 E98

Glass

2.0–2.3 0.03–0.5

4,000–9,500 20–28 8,500–26,000 20 0.5–16 E95

Thermal C177 D696 D648

12.2–25

Thermal conductivity (10–4 cal-cm/s-cm2-°C) Coefficient of thermal expansion (10–5 in./in.-°C) Deflection temperature (°F) At 264 psi

15–25

2–5

1.5–3.3

350–500

400–500

350–450

250–530

5.5–6.0

—

0.02–0.04

—

1013–1015 180+

— 180+

Electrical D149 D150 D150 D257 D495

Dielectric strength, (V/mil) Short time, 1/8-in. thk Dielectric constant At 1 kHz Dissipation factor At 1 kHz Volume resistivity (ohm-cm) At 73°F, 50% RH Arc resistance (s)


these materials is between those of granular and flake compounds. Because the fillers are opaque and the resins are amber, translucent colors are not possible. Opaque, light shades can be produced in most colors, however. Molded alkyd parts resist weak acids, organic solvents, and hydrocarbons such as alcohol and fatty acids; they are attacked by alkalies. Depending on the properties desired in the finished compound, the fillers used are clay, asbestos, fibrous glass, or combinations of these materials. The resulting alkyd compounds are characterized in their molding behavior by the following significant features: (1) no liberation


of volatiles during the cure, (2) extremely soft flow, and (3) fast cure at molding temperatures. Although the general characteristics of fast cure and low-pressure requirements are common to all alkyd compounds, they may be divided into three different groups that are easily discernible by the physical form in which they are manufactured. 1. Granular types, which have mineral or modified mineral filters, providing superior dielectric properties and heat resistance


2. Putty types, which are quite soft and particularly well suited for low-pressure molding 3. Glass fiber-reinforced types, which have superior mechanical strengths For each of these distinct types a more detailed description follows.

GRANULAR TYPES The physical form of materials in this group is that of a free-flowing powder. Thus, these materials readily lend themselves to conventional molding practices such as volumetric loading, preforming, and high-speed automatic operations. The outstanding properties of parts molded from this group of compounds are high dielectric strength at elevated temperatures, high arc resistance, excellent dimensional stability, and high heat resistance. Compounds are available within this group that are self-extinguishing and certain recently developed types display exceptional retention of insulating properties under high humidity conditions. These materials have found extensive use as high-grade electrical insulation, especially in the electronics field. One of the major electronic applications for alkyd compounds is in the construction of vacuum tube bases, where the high dry insulation resistance of the material is particularly useful in keeping the electrical leakage between pins to a minimum. In the television industry, tuner segments are frequently molded from granular alkyd compound since electrical and dimensional stability is necessary to prevent calibration shift in the tuner circuits. Also, the granular alkyds have received considerable usage in automotive ignition systems where retention of good dielectric characteristics at elevated temperatures is vitally important.

PUTTY TYPES This group contains materials that are furnished in soft, puttylike sheets. They are characterized by very low pressure molding requirements (less than 800 psi), and are used in molding around delicate inserts and in solving special loading problems. Molders customarily extrude these materials into a ribbon of a specific size,


which is then cut into preforms before molding. Whereas granular alkyds are rather diversified in their various applications, putty has found widespread use in one major application: molded encapsulation of small electronic components, such as mica, polyester film, and paper capacitors; deposited carbon resistors; small coils; and transformers. The purpose here is to insulate the components electrically, as well as to seal out moisture. Use of alkyds has become especially popular because of their excellent electrical and thermal properties, which result in high functional efficiency of the unit in a minimum space, coupled with low-pressure molding requirements, which prevent distortion of the subassembly during molding.

GLASS- FIBER-REINFORCED TYPES This type of alkyd molding compound is used in a large number of applications requiring high mechanical strength as well as electrical insulating properties. Glass-fiber-reinforced alkyds can be either compression or plunger molded permitting a wide variety of types of applications, ranging from large circuit breaker housings to extremely delicate electronic components.

OTHER TYPES OF ALKYD MOLDING COMPOUNDS Halogen and/or phosphorus-bearing alkyd molding compounds with antimony trioxide added provide improved flame resistance. Other flame-resistant compounds are available that do not contain halogenated resins. Many grades are UL-rated at 94V-0 in sections under 1/16 in. Flammability ratings depend on specific formulations, however, and can vary from 94HB to V-0. Flammability ratings also vary with section thickness. Glass- and asbestos-filled compounds have better heat resistance than the cellulose-modified types. Depending on type, alkyds can be used continuously to 350°F and, for short periods, to 450°F. Alkyd molding compounds retain their dimensional stability and electrical and mechanical properties over a wide temperature range.

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MOLDING CHARACTERISTICS Although full realization of the advantages of molding alkyds is best attained through the use of high-speed, lightweight equipment, nearly all modern compression presses are suitable for use with these materials. Since these compounds are quite fast curing, the press utilized in molding them should be capable of applying full pressure within approximately 6 to 8 s after the mold has been charged. In selecting a press to operate a specific mold for alkyds, the following rule should prove useful: for average draws, the press should furnish about 1500 psi over the projected area of the cavity and lands for molding granular alkyds; about 800 psi for alkyd putty; and about 2000 psi for glass-reinforced alkyd. Alkyd parts are in successful production in positive, semipositive, and flash molds. In general, the positive and semipositive types are recommended to obtain uniformly dense parts with lowest shrinkage. However, flash molds are frequently used with alkyd putty because of its low bulk factor. In any case, hardened, chromiumplated steel molds are recommended. The resin characteristics of alkyd molding compounds are such that the material goes through a very low viscosity phase momentarily when heat and pressure are applied. This low viscosity phase makes possible the complete filling of the mold at pressures much lower than those required for other thermosets. Under ordinary conditions, alkyd materials have good release characteristics, and no lubrication is necessary to ensure ejection from the mold.

APPLICATIONS High-impact grades of alkyd compounds (with high glass content) are used in military switchgear, electrical terminal strips, and relay and transformer housings and bases. Mineral-filled grades, which can be modified with cellulose to reduce specific gravity and cost, are used in automotive ignition parts, radio and television components, switch-gear, and small appliance housings. Alkyds with all-mineral fillers have high moisture resistance and are particularly suited for electronic components. Grades are available that can withstand the temperatures of vapor-phase soldering. © 2002 by CRC Press LLC

ALLOY An alloy is a metal product containing two or more elements as a solid solution, as an intermetallic compound, or as a mixture of metallic phases. Except for native copper and gold, the first metals of technological importance were alloys. Bronze, an alloy of copper and tin, is appreciably harder than copper. This quality made bronze so important an alloy that it left a permanent imprint on the civilization of several millennia ago now known as the Bronze Age. Alloys are used because they have specific properties or production characteristics that are more attractive than those of the pure, elemental metals. For example, some alloys possess high strength, others have low melting points, others are refractory with high melting temperatures, some are especially resistant to corrosion, and others have desirable magnetic, thermal, or electrical properties. These characteristics arise from both the internal and the electronic structure of the alloy. In recent years, the term plastic alloy also has been applied to plastics. Metal alloys are more specifically described with reference to the major element by weight, which is also called the base metal or parent metal. Thus, the terms aluminum alloy, copper alloy, etc. Elements present in lesser quantities are called alloying elements. When one or more alloying elements are present in substantial quantity or, regardless of their amount, have a pronounced effect on the alloy, they, too, may be reflected in generic designations. Metal alloys are also often designated by trade names or by trade association or society designations. Among the more common of the latter are the three-digit designations for the major families of stainless steels and the fourdigit ones for aluminum alloys. Structurally there are two kinds of metal alloys — single phase and multiphase. Singlephase alloys are composed of crystals with the same type of structure. They are formed by “dissolving” together different elements to produce a solid solution. The crystal structure of a solid solution is normally that of the base metal. In contrast to single-phase alloys, multiphase alloys are mixtures rather than solid solutions. They are composed of aggregates of


two or more different phases. The individual phases making up the alloy are different from one another in their composition or structure. Solder, in which the metals lead and tin are present as a mechanical mixture of two separate phases, is an example of the simplest kind of multiphase alloy. In contrast, steel is a complex alloy composed of different phases, some of which are solid solutions. Multiphase alloys far outnumber single-phase alloys in the industrial material field, chiefly because they provide greater property flexibility. Thus, properties of multiphase alloys are dependent upon many factors, including the composition of the individual phases, the relative amounts of the different phases, and the positions of the various phases relative to one another. When two different thermoplastic resins are blended, a plastic alloy is obtained. Alloying permits resin polymers to be blended that cannot be polymerized. Not all plastics are amenable to alloying. Only resins that are compatible with each other — those that have similar melt traits — can be successfully blended.

TYPES

OF

ALLOYS

Bearing Alloys These alloys are used for metals that encounter sliding contact under pressure with another surface; the steel of a rotating shaft is a common example. Most bearing alloys contain particles of a hard intermetallic cornpound that resists wear. These particles, however, are embedded in a matrix of softer material that adjusts to the hard particles so that the shaft is uniformly loaded over the total surface. The most familiar bearing alloy is babbitt metal, which contains 83 to 91% tin (Sn); the remainder is made up of equal parts of antimony (Sb) and copper (Cu), which form hard particles of the compounds SbSn and CuSn in a soft tin matrix. Other bearing alloys are based on cadmium (Cd), copper, or silver (Ag). For example, an alloy of 70% copper and 30% lead (Pb) is used extensively for heavily loaded bearings. Bearings made by powder metallurgy techniques are widely used. These techniques are valuable because they permit the combination of materials that are incompatible as liquids,


for example, bronze and graphite. Powder techniques also permit controlled porosity within the bearings so that they can be saturated with oil before being used, the so-called oilless bearings. Corrosion-Resisting Alloys Certain alloys resist corrosion because they are noble metals. Among these alloys are the precious metal alloys, which will be discussed separately. Other alloys resist corrosion because a protective film develops on the metal surface. This passive film is an oxide that separates the metal from the corrosive environment. Stainless steels and aluminum alloys exemplify metals with this type of protection. Stainless steels are iron alloys containing more than 12% chromium (Cr). Steels with 18% Cr and 8% nickel (Ni) are the best known and possess a high degree of resistance to many corrosive environments. Aluminum (Al) alloys gain their corrosion-deterring characteristics by the formation of a very thin surface layer of aluminum oxide (Al2O3), which is inert to many environmental liquids. This layer is intentionally thickened in commercial anodizing processes to give a more permanent Al2O3 coating. Monel, an alloy of approximately 70% nickel and 30% copper, is a well-known corrosion-resisting alloy that also has high strength. Another nickel-base alloy is Inconel, which contains 14% chromium and 6% iron (Fe). The bronzes, alloys of copper and tin, also may be considered to be corrosion resisting. Dental Alloys Amalgams are predominantly alloys of silver and mercury, but they may contain minor amounts of tin, copper, and zinc for hardening purposes, for example, 33% silver, 52% mercury, 12% tin, 2% copper, and less than 1% zinc. Liquid mercury is added to a powder of a precursor alloy of the other metals. After compaction, the mercury diffuses into the silverbase metal to give a completely solid alloy. Gold-base dental alloys are preferred over pure gold because gold is relatively soft. The most common dental gold alloy contains gold (80 to 90%), silver (3 to 12%), and copper (2 to 4%). For higher strengths and hardnesses, palladium

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and platinum (up to 3%) are added, and the copper and silver are increased so that the gold content drops to 60 to 70%. Vitallium, an alloy of cobalt (65%), chromium (5%), molybdenum (3%), and nickel (3%), and other corrosionresistant alloys are used for bridgework and special applications. Die-Casting Alloys These alloys have melting temperatures low enough so that in the liquid form they can be injected under pressure into steel dies. Such castings are used for automotive parts and for office and household appliances that have moderately complex shapes. This processing procedure eliminates the need for expensive machining and forming operations. Most die castings are made from zinc-base or aluminum-base alloys. Magnesium-base alloys also find some application when weight reduction is paramount. Low-melting alloys of lead and tin are not common because they lack the necessary strength for the above applications. A common zinc-base alloy contains approximately 4% aluminum and up to 1% copper. These additions provide a second phase in the metal to give added strength. The alloy must be free of even minor amounts (less than 100 ppm) of impurities such as lead, cadmium, or tin, because impurities increase the rate of corrosion. Common aluminum-base alloys contain 5 to 12% silicon, which introduces hard-silicon particles into the tough aluminum matrix. Unlike zincbase alloys, aluminum-base alloys cannot be electroplated; however, they may be burnished or coated with enamel or lacquer. Advances in high-temperature die-mold materials have focused attention on the diecasting of copper-base and iron-base alloys. However, the high casting temperatures introduce costly production requirements, which must be justified on the basis of reduced machining costs.

called a eutectic alloy. A typical eutectic alloy is formed by combining 28.1% of copper with 71.9% of silver. A homogeneous liquid of this composition on slow cooling freezes to form a mixture of particles of nearly pure copper embedded in a matrix (background) of nearly pure silver. The advantageous mechanical properties inherent in composite materials such as plywood composed of sheets or lamellae of wood bonded together and fiberglass in which glass fibers are used to reinforce a plastic matrix have been known for many years. Attention is being given to eutectic alloys because they are basically natural composite materials. This is particularly true when they are directionally solidified to yield structures with parallel plates of the two phases (lamellar structure) or long fibers of one phase embedded in the other phase (fibrous structure). Directionally solidified eutectic alloys are being given serious consideration for use in fabricating jet engine turbine blades. For this purpose eutectic alloys that freeze to form tantalum carbide (TaC) fibers in a matrix of a cobalt-rich alloy have been heavily studied. Fusible Alloys These alloys generally have melting temperatures below that of tin (450°F, or 232°C), and in some cases as low as 120°F (50°C). Using eutectic compositions of metals such as lead, cadmium, bismuth, tin, antimony, and indium achieves these low melting temperatures. These alloys are used for many purposes, for example, in fusible elements in automatic sprinklers, forming and stretching dies, filler for thinwalled tubing that is being bent, and anchoring dies, punches, and parts being machined. Alloys rich in bismuth were formerly used for type metal because these low-melting metals exhibited a slight expansion on solidification, thus replicating the font perfectly for printing and publication.

Eutectic Alloys In certain alloy systems a liquid of a fixed composition freezes to form a mixture of two basically different solids or phases. An alloy that undergoes this type of solidification process is


High-Temperature Alloys Energy conversion is more efficient at high temperatures than at low; thus the need in powergenerating plants, jet engines, and gas turbines


for metals that have high strengths at high temperatures is obvious. In addition to having strength, these alloys must resist oxidation by fuel–air mixtures and by steam vapor. At temperatures up to about 1380°F (750°C), the austenitic stainless steels (18% Cr–8% Ni) serve well. An additional 180°F (100°C) may be realized if the steels also contain 3% molybdenum. Both nickel-base and copper-base alloys, commonly categorized as superalloys, may serve useful functions up to 2000°F (1100°C). Nichrome, a nickel-base alloy containing 12 to 15% chromium and 25% iron, is a fairly simple superalloy. More sophisticated alloys invariably contain five, six, or more components; for example, an alloy called René-41 contains approximately 19% Cr, 1.5% Al, 3% Ti, 11% Co, 10% Mo, 3% Fe, 0.1% C, 0.005% B, and the balance Ni. Other alloys are equally complex. The major contributor to strength in these alloys is the solution-precipitate phase of Ni3 (TiAl). It provides strength because it is coherent with the nickel-rich phase. Cobalt-base superalloy may be even more complex and generally contain carbon, which combines with the tungsten (W) and chromium to produce carbides that serve as the strengthening agent. In general, the cobalt-base superalloys are more resistant to oxidation than the nickel-base alloys are, but they are not as strong. Molybdenum-base alloys have exceptionally high strength at high temperatures, but their brittleness at lower temperatures and their poor oxidation resistance at high temperatures have limited their use. However, coatings permit the use of such alloys in an oxidizing atmosphere, and they are finding increased application. A group of materials called cermets, which are mixtures of metals and compounds such as oxides and carbides, have high strength at high temperatures, and although their ductility is low, they have been found to be usable. One of the better-known cermets consists of a mixture of TiC and nickel, the nickel acting as a binder or cement for the carbide. Joining Alloys Metals are bonded by three principal procedures: welding, brazing, and soldering. Welded joints melt the contact region of the adjacent metal; thus, the filler material is chosen to


approximate the composition of the parts being joined. Brazing and soldering alloys are chosen to provide filler metal with an appreciably lower melting point than that of the joined parts. Typically, brazing alloys melt above 750°F (400°C) whereas solders melt at lower temperatures. A 57% Cu–42% Zn–1% Sn brass is a generalpurpose alloy for brazing steel and many nonferrous metals. A Si–Al eutectic alloy is used for brazing aluminum, and an aluminum-containing magnesium eutectic alloy brazes magnesium parts. The most common solders are based on Pb–Sn alloys. The prevalent 60% Sn–40% Pb solder is eutectic in composition and is used extensively for electrical circuit production, in which temperature limitations are critical. A 35% Sn–65% Pb alloy has a range of solidification and is thus preferred as a wiping solder by plumbers. Light-Metal Alloys Aluminum and magnesium, with densities of 2.7 and 1.75 g/cm3), respectively, are the bases for most of the light-metal alloys. Titanium (4.5 g/cm3) may also be regarded as a light-metal alloy if comparisons are made with metals such as steel and copper. Aluminum and magnesium must be hardened to receive extensive application. Age-hardening processes are used for this purpose. Typical alloys are 90% Al-10% Mg, 95% Al–5% Cu, and 90% Mg–10% Al. Ternary (three element) and more complex alloys are very important light-metal alloys because of their better properties. The Al–Zn–Mg system of alloys, used extensively in aircraft applications, is a prime example of one such alloy system. Low-Expansion Alloys This group of alloys includes Invar (64% Fe–36% Ni), the dimensions of which do not vary over the atmospheric temperature range. It has special applications in watches and other temperature-sensitive devices. Glass-to-metal seals for electronic and related devices require a matching of the thermal-expansion characteristics of the two materials. Kovar (54% Fe–29% Ni–17% Co) is widely used because its expansion is low enough to match that of glass.

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Magnetic Alloys Soft and hard magnetic materials involve two distinct categories of alloys. The former consists of materials used for magnetic cores of transformers and motors, and must be magnetized and demagnetized easily. For AC applications, silicon–ferrite is commonly used. This is an alloy of iron containing as much as 5% silicon. The silicon has little influence on the magnetic properties of the iron, but it increases the electric resistance appreciably and thereby decreases the core loss by induced currents. A higher magnetic permeability, and therefore greater transformer efficiency, is achieved if these silicon steels are grain-oriented so that the crystal axes are closely aligned with the magnetic field. Permalloy (78.5% Ni–21.5% Fe) and some comparable cobalt-base alloys have very high permeabilities at low field strengths, and thus are used in the communications industry. Ceramic ferrites, although not strictly alloys, are widely used in high-frequency applications because of their low electrical conductivity and negligible induced energy losses in the magnetic field. Permanent or hard magnets may be made from steels that are mechanically hardened, either by deformation or by quenching. Some precipitation-hardening, iron-base alloys are widely used for magnets. Typical of these are the Alnicos, for example, Alnico-4 (55% Fe–28% Ni–12% Al–5% Co). Since these alloys cannot be forged, they must be produced in the form of castings. Hard magnets are being produced from alloys of cobalt and the rare earth type of metals. The compound RCo5, where R is samarium (Sm), lanthanum (La), cerium (Ce), and so on, has extremely high coercivity. Precious-Metal Alloys In addition to their use in coins and jewelry, precious metals such as silver, gold, and the heavier platinum (Pt) metals are used extensively in electrical devices in which contact resistances must remain low, in catalytic applications to aid chemical reactions, and in temperature-measuring devices such as resistance thermometers and thermocouples. The unit of alloy impurity is commonly expressed in karats,


when each karat is 1/24 part. The most common precious-metal alloy is sterling silver (92.5% Ag, with the remainder being unspecified, but usually copper). The copper is very beneficial in that it makes the alloy harder and stronger than pure silver. Yellow gold is an Au–Ag–Cu alloy with approximately a 2:1:1 ratio. White gold is an alloy that ranges from 10 to 18 karats, the remainder being additions of nickel, silver, or zinc, which change the color from yellow to white. The alloy 87% platinum–13% rhodium (Rh), when joined with pure platinum, provides a widely used thermocouple for temperature measurements in the 1830 to 3000°F (1000 to 1650°C) temperature range. Shape Memory Alloys These alloys have a very interesting and desirable property. In a typical case, a metallic object of a given shape is cooled from a given temperature T1, to a lower temperature T2, where it is deformed to change its shape. Upon reheating from T2 to T1, the shape change accomplished at T2 is recovered so that the object returns to its original configuration. This thermoelastic property of the shape memory alloys is associated with the fact that they undergo a martensitic phase transformation (that is, a reversible change in crystal structure that does not involve diffusion) when they are cooled or heated between T1 and T2. For a number of years the shape memory materials were essentially scientific curiosities. Among the first alloys shown to possess these properties was one of gold alloyed with 47.5% cadmium. Considerable attention has been given to an alloy of nickel and titanium known as Nitinol. The interest in shape memory alloys has increased because it has been realized that these alloys are capable of being employed in a number of useful applications. One example is for thermostats; another is for couplings on hydraulic lines or electrical circuits. The thermoelastic properties can also be used, at least in principle, to construct heat engines that will operate over a small temperature differential and will thus be of interest in the area of energy conversion.


Thermocouple Alloys These include Chromel, containing 90% Ni and 10% Cr, and Alumel, containing 94% Ni, 2% Al, 3% Cr, and 1% Si. These two alloys together form the widely used Chromel–Alumel thermocouple, which can measure temperatures up to 2200°F (1204°C). Another common thermocouple alloy is Constantan, consisting of 45% Ni and 55% Cu. It is used to form iron-Constantan and copperConstantan couples, used at lower temperatures. For precise temperature measurements and for measuring temperatures up to 3000°F (1650°C), thermocouples are used in which one metal is platinum and the other metal is platinum plus either 10 or 13% rhodium. Prosthetic Alloys Prosthetic alloys are alloys used in internal prostheses, that is, surgical implants such as artificial hips and knees. External prostheses are devices that are worn by patients outside the body; alloy selection criteria are different from those for internal prostheses. In the United States, surgeons use about 250,000 artificial hips and knees and about 30,000 dental implants per year. Alloy selection criteria for surgical implants can be stringent primarily because of biomechanical and chemical aspects of the service environment. Mechanically, the properties and shape of an implant must meet anticipated functional demands; for example, hip joint replacements are routinely subjected to cyclic forces that can be several times body weight. Therefore, intrinsic mechanical properties of an alloy, for example, elastic modulus, yield strength, fatigue strength, ultimate tensile strength, and wear resistance, must all be considered. Similarly, because the pH and ionic conditions within a living organism define a relatively hostile corrosion environment for metals, corrosion properties are an important consideration. Corrosion must be avoided not only because of alloy deterioration but also because of the possible physiological effects of harmful or even cytotoxic corrosion products that may be released into the body. (Study of the biological effects of biomaterials is a broad


subject in itself, often referred to as biocompatibility.) The corrosion resistance of all modern alloys stems primarily from strongly adherent and passivating surface oxides, such as TiO2 on titanium-based alloys and Cr2O3 on cobaltbase alloys. The most widely used prosthetic alloys therefore include high-strength, corrosionresistant ferrous, cobalt-base, or titanium-base alloys. Examples include cold-worked stainless steel; cast Vitallium, a wrought alloy of cobalt, nickel, chromium, molybdenum, and titanium; titanium alloyed with aluminum and vanadium; and commercial-purity titanium. Specifications for nominal alloy compositions are designated by the American Society for Testing and Materials (ASTM). Prosthetic alloys have a range of properties. Some are easier than others to fabricate into the complicated shapes dictated by anatomical constraints. Fabrication techniques include investment casting (solidifying molten metal in a mold), forging (forming metal by deformation), machining (forming by machine-shop processes, including computer-aided design and manufacturing), and hot isostatic pressing (compacting fine powders of alloy into desired shapes under heat and pressure). Cobalt-base alloys are difficult to machine and are therefore usually made by casting or hot isostatic pressing. Some newer implant designs are porous coated; that is, they are made from the standard ASTM alloys but are coated with alloy beads or mesh applied to the surface by sintering or other methods. The rationale for such coatings is implant fixation by bone ingrowth. Some alloys are modified by nitriding or ion-implantation of surface layers of enhanced surface properties. A key point is that prosthetic alloys of identical composition can differ substantially in terms of structure and properties, depending on fabrication history. For example, the fatigue strength approximately triples for hot isostatically pressing vs. as-cast Co–Cr–Mo alloy, primarily because of a much smaller grain size in the microstructure of the former. No single alloy is vastly superior to all others; existing prosthetic alloys have all been used in successful and, indeed, unsuccessful implant designs. Alloy selection is only one determinant of performance of the implanted device.

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Superconducting Alloys Superconductors are materials that have zero resistance to the flow of electric current at low temperatures. There are more than 6000 elements, alloys, and compounds that are known superconductors. This remarkable property of zero resistance offers unprecedented technological advances such as the generation of intense magnetic fields. Realization of these new technologies requires development of specifically designed superconducting alloys and composite conductors. An alloy of niobium and titanium (NbTi) has a great number of applications in superconductivity; it becomes superconducting at 9.5 K (critical superconducting temperature, Tc). This alloy is preferred because of its ductility and its ability to carry large amounts of current at high magnetic fields, represented by Jc(H) (where Jc is the critical current and H is a given magnetic field), and still retain its superconducting properties. Brittle compounds with intrinsically superior superconducting properties are also being developed for magnet applications. The most promising of these are compounds of niobium and strontium (Nb3Sn), vanadium and gallium (V3Ga), niobium and germanium (Nb3Ge), and niobium and aluminum (Nb3Al), which have higher Tc (15 to 23 K) and higher Jc (H) than NbTi. Superconducting materials possess other unique properties such as magnetic flux quantization and magnetic-field-modulated supercurrent flow between two slightly separated superconductors. These properties form the basis for electronic applications of superconductivity such as high-speed computers or ultrasensitive magnetometers. Development of these applications began using lead or niobium (Tc of 7 and 9 K) in bulk form, but the emphasis then was transferred to materials deposited in thin-film form. PbIn and PbAu alloys are more desirable than pure lead films, as they are more stable. Improved vacuum deposition systems eventually led to the use of pure niobium films as they, in turn, were more stable than lead alloy films. Advances in thin-film synthesis techniques led to the use of the refractory compound niobium nitride (NbN) in electronic applications. This


compound is very stable and possesses a higher Tc (15 K) than either lead or niobium. Novel high-temperature superconducting materials have revolutionary impact on superconductivity and its applications. These materials are ceramic, copper-oxide-based materials that contain at least four and as many as six elements. Typical examples are yttrium–barium–copper–oxygen (Tc 93 K), bismuth–strontium–calcium–copper–oxygen (Tc 110K), and thallium–barium–calcium–copper (Tc 125 K). These materials become superconducting at such high temperatures that refrigeration is simpler, more dependable, and less expensive. Much research and development has been done to improve the technologically important properties such as Jc (H), chemical and mechanical stability, and device-compatible processing procedures. It is anticipated that the new compounds will have a significant impact in the growing field of superconductivity.

ALLOY STRUCTURES Metals in actual commercial use are almost exclusively alloys, and not pure metals, since it is possible for the designer to realize an extensive variety of physical properties in the product by varying the metallic composition of the alloy. As a case in point, commercially pure or cast iron is very brittle because of the small amount of carbon impurity always present, whereas the steels are much more ductile, with greater strength and better corrosion properties. In general, the highly purified single crystal of a metal is very soft and malleable, with high electrical conductivity, whereas the alloy is usually harder and may have a much lower conductivity. The conductivity will vary with the degree of order of the alloy, and the hardness will vary with the particular heat treatment used. The basic knowledge of structural properties of alloys is still in large part empirical, and indeed, it will probably never be possible to derive formulas that will predict which metals to mix in a certain proportion and with a certain heat treatment to yield a specified property or set of properties. However, a set of rules exists that describes the qualitative behavior of certain groups of alloys. These rules are statements


concerning the relative sizes of constituent atoms, for alloy formation, and concerning what kinds of phases to expect in terms of the valence of the constituent atoms. The rules were discovered in a strictly empirical way, and for the most part, the present theoretical understanding of alloys consists of rudimentary theories that describe how the rules arise from the basic principles of physics. These rules were proposed by W. Hume-Rothery concerning the binary substitutional alloys and phase diagrams.

ALLYLICS (DIALLYL PHTHALATE PLASTICS) Allylics are thermosetting materials developed since World War II. The most important of these are diallyl phthalate (DAP) and diallyl isophthalate (DAIP), which are currently available in the form of monomers and prepolymers (resins). Both DAP and DAIP are readily converted to thermoset molding compounds and resins for preimpregnated glass cloth and paper. Allyls are also used as cross-linking agents for unsaturated polyesters. DAP resin is the first all-allylic polymer commercially available as a dry, free-flowing white powder. Chemically, DAP is a relatively linear partially polymerized resin that softens and flows under heat and pressure (as in molding and laminating), and cross-links to a threedimensional insoluble thermoset resin during curing.

PROPERTIES In preparing the resin, DAP is polymerized to a point where almost all the change in specific gravity has taken place. Final cure, therefore, produces very little additional shrinkage. In fact, DAP is cured by polymerization without water formation. The molded material, depending on the filler, has a tensile strength from 30 to 48 MPa, a compressive strength up to 210 MPa, a Rockwell hardness to M108, dielectric strength to 16.9 × 106 V/m, and heat resistance to 232°C. Allylic resins enjoy certain specific advantages over other plastics, which make them of interest in various special applications. Allylics


exhibit superior electrical properties under severe temperature and humidity conditions. These good electrical properties (insulation resistance, low loss factor, arc resistance, etc.) are retained despite repeated exposure to high heat and humidity. DAP resin is resistant to 155 to 180°C temperatures, and the DAIP resin is good for continuous exposures up to 206 to 232°C temperatures. Allylic resins exhibit excellent post-mold dimensional stability, low moisture absorption, good resistance to solvents, acids, alkalis, weathering, and wet and dry abrasion. They are chemically stable, have good surface finish, mold well around metal inserts and can be formulated in pastel colors with excellent color retention at high temperatures. DAP resin currently finds major use in (1) molding and (2) industrial and decorative laminates. Both applications utilize the desirable combination of low shrinkage, absence of volatiles, and superior electrical and physical properties common to DAP.

MOLDING COMPOUNDS Compounds based on allyl prepolymers are reinforced with fibers (glass, polyester, or acrylic) and filled with particulate materials to improve properties. Glass fiber imparts maximum mechanical properties, acrylic fiber provides the best electrical properties, and polyester fiber improves impact resistance and strength in thin sections. Compounds can be made in a wide range of colors because the resin is essentially colorless; see Table A.6. Prepregs (preimpregnated glass cloth) based on allyl prepolymers can be formulated for short cure cycles. They contain no toxic additives, and they offer long storage stability and ease of handling and fabrication. Properties such as flame resistance can be incorporated. The allyl prepolymers contribute excellent chemical resistance and good electrical properties. Other molding powders are compounded of DAP resin, DAP monomer, and various fillers like asbestos, Orlon, Dacron, cellulose, glass, and other fibers. Inert fillers used include ground quartz and clays, calcium carbonate, and talc. Allyl moldings have low mold shrinkage and post-mold shrinkage — attributed to their

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TABLE A.6 Properties of DAP Molding Compounds Filler ASTM Test

Property

D792 D792 D570

Specific gravity Specific volume (in. 3/lb) Water absorption, 24 h, 1/8-in. thk (%)

D638 D790 D790 D256

Tensile strength (psi) Flexural strength (psi) Flexural modulus (105 psi) Impact strength, Izod (ft-lb/in. of notch) Hardness, Rockwell M

D785

Long Glass

Short Glass

Arc-Track Resistant

Physical 1.39–1.42 19.96–19.54 0.2

1.70–1.90 17.90–16.32 0.05–0.2

1.6–1.8 17.34–15.42 0.05–0.2

1.87 14.84 0.14

Mechanical 5,000 11,500–12,500 6.4 4.5–12

9,000 18,000 16 6.0

7,000 16,000 17 0.8

7,000–10,000 24,000 19 3.6

105–110

105–110

112

—

14–16

14–15

15–17

—

2.0–3.0

2.0–3.0

23–27

290

450

420

>572

400

385

400

400

0.008

0.004–0.006

0.006

0.003–0.008

3.6

4.2

4.4

4.1–4.5

2–3 × 1015 125

2–3 × 1015 140

2–3 × 1015 135

1016 125–180

— —

Stat/Dyn 0.14/0.13 0.20/0.19

— —

Polyester

108 Thermal

C177 D696 D648

Thermal conductivity (10–4 cal-cm/s-cm2-°C) Coefficient of thermal expansion (10–5 in./in.-°C) Deflection temperature (°F) At 264 psi

Electrical D149 D150 D150 D257 D495

Dielectric strength, (V/mil) Step by step, 1/8-in. thk Dielectric constant At 1 kHz Dissipation factor At 1 kHz Volume resistivity (ohm-cm) At 73°F, 50% RH Arc resistance (s)

Frictional —


— —


nearly complete addition reaction in the mold — and have excellent stability under prolonged or cyclic heat exposure. Advantages of allyl systems over polyesters are freedom from styrene odor low toxicity, low evaporation losses during evacuation cycles, no subsequent oozing or bleed-out, and long-term retention of electrical-insulation characteristics.


APPLICATIONS Uses of such DAP molding compounds are largely for electrical and electronic parts, connectors, resistors, panels, switches, and insulators. Other applications for molding compounds include appliance handles, control knobs, dinnerware, and cooking equipment.


Decorative laminates containing DAP resin can be made from glass cloth (or other woven and nonwoven materials), glass mat, or paper. Such laminates may be bonded directly to a variety of rigid surfaces at lower pressures (50 to 300 psi) than generally required for other plastic laminates. A short hot-to-hot cycle is employed, and press platens are always held at curing temperatures. DAP laminates can, therefore, be used to give a permanent finish to highgrade wood veneers (with a clear overlay sheet) or to upgrade low-cost core materials (by means of a patterned sheet). Allyl prepolymers are particularly suited for critical electronic components that serve in severe environmental conditions. Chemical inertness qualifies the resins for molded pump impellers and other chemical-processing equipment. Their ability to withstand steam environments permits uses in sterilizing and hot-water equipment. Because of their excellent flow characteristics, DAP compounds are used for parts requiring extreme dimensional accuracy. Modified resin systems are used for encapsulation of electronic devices such as semiconductors and as sealants for metal castings. A major application area for allyl compounds is electrical connectors, used in communications, computer, and aerospace systems. The high thermal resistance of these materials permits their use in vapor-phase soldering operations. Uses for prepolymers include arc-trackresistant compounds for switchgear and television components. Other representative uses are for insulators, encapsulating shells, potentiometer components, circuit boards, junction boxes, and housings. DAP and DAIP prepregs are used to make lightweight, intricate parts such as radomes, printed-circuit boards, tubing, ducting, and aircraft parts. Another use is in copper-clad laminates for high-performance printed-circuit boards.

ALUMINA The oxide of aluminum is Al2O3. The natural crystalline mineral is called corundum, but the synthetic crystals used for abrasives are designated usually as aluminum oxide or marketed under trade names. For other uses and as a


powder it is generally called alumina. It is widely distributed in nature in combination with silica and other minerals, and is an important constituent of the clays for making porcelain, bricks, pottery, and refractories. The crushed and graded crystals of alumina when pure are nearly colorless, but the fine powder is white. Off colors are due to impurities. American aluminum oxide used for abrasives is at least 99.5% pure, in nearly colorless crystals melting at 2050°C. The chief uses for alumina are for the production of aluminum metal and for abrasives, but it is also used for ceramics, refractories, pigments, catalyst carriers, and in chemicals. Aluminum oxide crystals are normally hexagonal, and are minute in size. For abrasives, the grain sizes are usually from 100 to 600 mesh. The larger grain sizes are made up of many crystals, unlike the single-crystal large grains of SiC. The specific gravity is about 3.95, and the hardness is up to 2000 Knoop. There are two kinds of ultrafine alumina abrasive powder. Type A is alpha alumina with hexagonal crystals with particle size of 0.3 µm, density 4.0, and hardness 9 Mohs, and Type B is gamma alumina with cubic crystals with particle size under 0.1 µm, specific gravity of 3.6, and a hardness 8. Type A cuts faster, but Type B gives a finer finish. At high temperatures gamma alumina transforms to the alpha crystal. The aluminum oxide most frequently used for refractories is the beta alumina in hexagonal crystals heat-stabilized with sodium. Activated alumina is partly dehydrated alumina trihydrate, which has a strong affinity for moisture or gases and is used for dehydrating organic solvents, and hydrated alumina is alumina trihydrate. Al2O3 · 3H2O is used as a catalyst carrier. Activated alumina F-1 is a porous form of alumina, Al2O3, used for drying gases or liquids and is also used as a catalyst for many chemical processes. Alumina ceramics are the most widely used oxide-type ceramic, chiefly because Al2O3 is plentiful, relatively low in cost, and equal to or better than most oxides in mechanical properties. Density can be varied over a wide range, as can purity — down to about 90% Al2O3 — to meet specific application requirements.

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Al2O3 ceramics are the hardest, strongest, and stiffest of the oxides. They are also outstanding in electrical resistivity and dielectric strength, are resistant to a wide variety of chemicals, and are unaffected by air, water vapor, and sulfurous atmospheres. However, with a melting point of only 2037°C, they are relatively low in refractoriness, and at 1371°C retain only about 10% of room-temperature strength. Besides wide use as electrical insulators and chemical and aerospace applications, the high hardness and close dimensional tolerance capability of alumina make this ceramic suitable for such abrasion-resistant parts as textile guides, pump plungers, chute linings, discharge orifices, dies, and bearings. Alumina Al-200, which is used for highfrequency insulators, gives a molded product with a tensile strength of 172 MPa, compressive strength of 2000 MPa, and specific gravity of 3.36. The coefficient of thermal expansion is half that of steel, and the hardness about that of sapphire. Alumina AD-995 is a dense vacuum-tight ceramic for high-temperature electronic use. It is 99.5% Al2O3 with no SiO2. The hardness is Rockwell N80, and dielectric constant 9.27. The maximum working temperature is 1760°C, and at 1093°C it has a flexural strength of 200 MPa. Other alumina products have found their way in the casting of hollow jet engine cores. These cores are then incorporated in molds into which eutectic superalloys are poured to form the turbine blades. Alumina balls are available in sizes from 0.6 to 1.9 cm for reactor and catalytic beds. They are usually 99% alumina, with high resistance to heat and chemicals. Alumina fibers in the form of short linear crystals, called sapphire whiskers, have high strength up to 1375 MPa for use as a filler in plastics to increase heat resistance and dielectric properties. Continuous single-crystal sapphire (alumina filaments) have unusual physical properties: high tensile strength (over 2069 MPa) and modulus of elasticity of 448.2 to 482.7 GPa. The filaments are especially needed for use in metal composites at elevated temperatures and in highly corrosive environments. An unusual method for producing single-crystal fibers in lieu of a crystal growing machine is the floating zone fiber-drawing


process. The fibers are produced directly from a molten ceramic without using a crucible. FP, a polycrystalline alumina (Al2O3) fiber, has been developed. The material has greater than 99% purity, and a melting point of 2045°C, which makes it attractive for use with hightemperature metal-matrix composite (MMC) processing techniques. Thanks to a mechanism, currently not explainable by the developer of FP fibers (Du Pont), a silica coating results in an increase in the tensile strength of the filaments to 1896 MPa even though the coating is approximately 0.25 µm thick and the modulus does not change. Fiber FP has been demonstrated as a reinforcement in magnesium, aluminum, lead, copper, and zinc, with emphasis to date on aluminum and magnesium materials. Fumed alumina powder of submicrometer size is made by flame reduction of aluminum chloride. It is used in coatings and for plastics reinforcement and in the production of ferrite ceramic magnets. Aluminum oxide film, or alumina film, used as a supporting material in ionizing tubes, is a strong, transparent sheet made by oxidizing aluminum foil, rubbing off the oxide on one side, and dissolving the foil in an acid solution to leave the oxide film on the other side. It is transparent to electrons. Alumina bubble brick is a lightweight refractory brick for kiln lining, made by passing molten alumina in front of an airjet, producing small hollow bubbles which are then pressed into bricks and shapes. The foam has a density of 448.5 kg/m3 and porosity of 85%. The thermal conductivity at 1093°C is 0.002 W/(cm2)(°C).

ALUMINIDES True metals include the alkali and alkaline earth metals, beryllium, magnesium, copper, silver, gold, and the transition elements. These metals exhibit those characteristics generally associated with the metallic state. The B subgroups comprise the remaining metallic elements. These elements exhibit complex structures and significant departures from typically metallic properties. Aluminum, although considered under the B subgroup metals, is somewhat anomalous in that it exhibits many characteristics of a true metal.


The alloys of a true metal and a B subgroup element are very complex, because their components differ electrochemically. This difference gives rise to a stronger tendency toward definite chemical combination than to solid solution. Discrete geometrically ordered structures usually result. Such alloys are also termed electron compounds. The aluminides are phases in such alloys or compounds. A substantial number of beta, gamma, and epsilon phases have been observed in electron compounds, but few have been isolated and evaluated. The development of intermetallic alloys into useful and practical structural materials remains, despite recent successes, a major scientific and engineering challenge. As with many new and advanced materials, hope and the promise of major breakthroughs in the near future have kept a very active and resilient fraction of the metallurgical community focused on intermetallic alloys. Compared to conventional aerospace materials, aluminides of titanium, nickel, iron, niobium, etc., with various compositions offer attractive properties for potential structural applications. The combination of good hightemperature strength and creep capability, improved high-temperature environmental resistance, and relatively low density makes this general class of materials good candidates to replace more conventional titanium alloys and, in some instances, nickel-base superalloys. Moreover, titanium aluminide matrix composites appear to have the potential to surpass the monolithic titanium aluminides in a number of important property areas, and fabrication into composite form may be a partial solution to some of the current shortcomings attributed to monolithic titanium aluminides. The material classes include both monolithic and continuous fiber composite materials based on the intermetallic composition Ti3Al (α2-phase) and monolithic alloys based on the intermetallic composition TiAl (γ-phase). In their monolithic form, and as a matrix material for continuous fiber composites, titanium aluminides are important candidates to fill a need in the intermediate-temperature regime of 600 to 1000°C. Before these materials can become flightworthy, however, they must demonstrate


reliable mechanical behavior over the range of anticipated service conditions. The β and γ phases that are found to exist in the Mo–Al alloy system are generally considered to correspond to the compositions MoAl5 and MoA12, respectively. Powder metallurgy techniques have proved feasible for the production of alloys of molybdenum and aluminum, provided care is taken to employ raw materials of high purity (99% +). As the temperature of the compact is raised, a strong exothermic reaction occurs at about 640°C causing a rapid rise in temperature to above 960°C in a matter of seconds. Bloating occurs, transforming the compact into a porous mass. Complete alloying, however, is accomplished. This porous, friable mass can be subsequently finely comminuted, repressed, and sintered (or hot-pressed) to form a useful body quite uniform in composition. Vacuum sintering at 1300°C for 1 h at 0.04 µm produces clean, oxide-free metal throughout. Wet comminution prevents caking of the powder, and a pyrophoric powder can be produced by prolonged milling. Hot pressing is a highly successful means of forming bodies of molybdenum and aluminum previously reacted as mentioned above. Graphite dies are employed to which resistance heating techniques are applicable. A parting compound is required since aluminum is highly reactive with carbon causing sticking to the die walls. Hot-pressed small bars exhibit modulus of rupture strengths ranging from 40,000 to 50,000 psi at room temperature, decreasing to 38,000 to 40,000 psi at 1040°C. Room temperature resistance to fuming nitric acids is excellent. As has been recognized for some time, ordered intermetallic compounds have a number of properties that make them intrinsically more appealing than other metallic systems for hightemperature use. The primary requirements for high-temperature structural intermetallics, as with any high-temperature structural material, are that they (1) have a high melting point, (2) possess some degree of resistance to environmental degradation, (3) maintain structural and chemical stability at high temperatures, and (4) retain high specific mechanical properties at elevated temperatures whether they are intended as

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HfRe2 3000

Melting Temperature (Celsius)

A

monolithic components or as reinforcing fibers or matrix in composite structures. Melting point is a useful first approximation of the high-temperature performance of a material, as various high-temperature mechanical properties (e.g., strength and creep resistance) are limited by thermally assisted or diffusional processes and thus tend to scale with the melting point of the material. Therefore, the intermetallics can be crudely ranked in terms of their melting points to indicate their future applicability as high-temperature structural materials. As may be seen in Figure A.2, metallic materials (intermetallics or otherwise) that are currently in use or being studied melt at temperatures much lower than 1650°C. If these materials are discounted from consideration, the remaining intermetallics in Figure A.2 may be roughly divided into two groups: those that fall in the temperature range just above 1650°C and those whose melting points extend to much higher temperatures.

2500

Re2 Zr Ir3Zr

ZrRu 2000

Nb3 Al HfAl Cr2 Nb

1500

Nb2 Be17 ZrBe13 TiBe12

HfAl2 ,NiAl TlAl

Superalloys and Nl 3 Al (1300 - 1400)

FIGURE A.2 Melting points of various intermetallic compounds relative to superalloys. (From Schwartz, M., Emerging Technology, Technomics, 19. With permission.)


This second group of intermetallic compounds (IMCs) belongs to a group of intermetallics that are predicted on the basis of the Engel-Brewer phase stability theory. There are several techniques that have been developed and used to improve the toughness of intermetallics as well as intermetallic compounds: • Crystal structure modification (macroalloying) • Microalloying • Control of grain size or shape • Reinforcement by ductile fibers or particles • Control of substructure Table A.7 includes the above major categories, however, the use of hydrostatic pressure and suppression of environment should also be cited. Additions of chromium and manganese have induced appreciable compressive ductility and modest improvements in bend ductility of Al3Ti, but significant tensile ductility remains unattainable. The fracture toughness of Ti3Al alloys also can be markedly improved by a control of composition, microstructure, and processing techniques. However, the maximum benefits are obtained at about 400°C. Microstructural control has proved to be a particularly effective means of ductilizing TiAl and Ti3Al. It is now generally accepted that lamellar microstructures in TiAl, consisting of alternating γ- and α2-plates, provide the highest ductility. The interest in aluminides has covered the high-melting-point phases in metallic systems with aluminum. Ordered intermetallics constitute a unique class of metallic materials that form long-rangeordered crystal structures (Figure A.3) below a critical temperature that is generally referred to as the critical ordering temperature (Tc). These intermetallics usually exist in relatively narrow compositional ranges around simple stoichiometric ratios. The search for new high-temperature structural materials has stimulated much interest in ordered intermetallics. Recent interest has been


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TABLE A.7 Toughness and Ductility Improvements Microalloying B in Ni3Al, Ni3Si, PdIn Be in Ni3Al Fe, Mo, Ga in NiAl Ag in Ni3Al Macroalloying Fe in Co3V Mn, V, Cr in TiAl Nb in Ti3Al Mn, Cr in Al3Ti Pd in Ni3Al Grain Size Refinement NiAl Hydrostatic Pressure Ni3Al Martensite Transformation Fe in NiAl Composites (Fibers or Tubes) NiAl/304SS Al3Ta/Al2O3 MoSi2/Nb-1Zr Composites (Ductile Particles) Nb in TiAl Fe, Mn in Ni3Al Nb in MoSi2

Al Ni Ni3Al a

Al Fe, Ni NiAl, FeAl b

Source: Schwartz, M., Emerging Technology, Technomics. With permission.

focused on aluminides of nickel, iron, and titanium. These aluminides possess many attributes that make them attractive for hightemperature structural applications. They contain enough aluminum to form, in oxidizing environments, thin films of aluminide oxides that often are compact and protective. They have low densities, relatively high melting points, and good high-temperature strength properties. For example, Ni 3 Al shows an increase rather than a decrease in yield strength with increasing temperatures. The aluminides of interest are described in Table A.8. In the range of 14 to 34% aluminum by weight occur the two intermetallic phases Ni3Al and NiAl. The alloys are prepared by powder metallurgy and by casting techniques. Compacts of NiAl + 5% Ni, produced by powder techniques, exhibit room-temperature modulus of rupture values of 144,000 psi, and heat shock © 2002 by CRC Press LLC

Al Fe c

Fe3Al

FIGURE A.3 The crystal structure of nickel and iron aluminides: (a) LI2, (b) B2, (c) DO3. (From Schwartz, M., Emerging Technology, Technomics, 19. With permission.)

resistance is considered excellent. Good resistance to red and white fuming nitric acids is obtained. The cast alloys of nickel and aluminum exhibit an increasing exothermic character with increasing aluminum content. Alloys with the exception of the 34% aluminum alloy and NiAl (31.5% Al) produce sound castings free of excessive porosity. A room-temperature tensile strength of approximately 49,000 psi is exhibited by the Ni3Al compound with about 5%


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TABLE A.8 Properties of Nickel and Iron Aluminides

Alloy

Crystal Structurea

Critical Ordering Temp. (°C)

Ni3Al NiAl Fe3Al

L12 (ordered fcc) B2 (ordered bcc) D03 (ordered bcc) B2 (ordered bcc) B2 (ordered bcc)

1390 1640 540 760 1250

FeAl a

Melting Point (°C)

Material Density (g/cm3)

Young’s Modulus (Gpa)

1390 1640 1540 1540 1250

7.50 5.86 6.72

178 294 141

5.56

260

fcc = face-centered cubic; bcc = body-centered cubic.

Source: Schwartz, M., Emerging Technology, Technomics. With permission.

elongation. The 25% Al (NiAl) alloy has a room temperature tensile strength of 24,000 psi. The 17.5% aluminum alloy that contained a mixture of the phases Ni3Al and NiAl exhibits the best strength properties. Room-temperature tensile strength is approximately 80,000 psi with about 2% elongation, while at 815°C the tensile strength is about 50,000 psi. This alloy can be rolled at 1315°C, and possesses good thermal shock and oxidation resistance. Impact resistance is fair. The 100-h stress-to-rupture strength at 734°C is 14,000 psi, but, it should be noted, creep rates are high. Ni3Al is an intermetallic phase that forms at the nickel-rich end of the Ni–Al system and has a crystal structure (Figure A.3). Ni3Al is the most important strengthening constituent of nickel-base superalloys. This is because the aluminide possesses an excellent elevated-temperature strength in addition to good oxidation resistance. Unlike conventional materials, Ni3Al and its alloys show a yield anomaly; that is, its yield strength increases rather than decreases with increasing temperature. Boron has been found to be most effective in improving the tensile ductility of Ni3Al (1900°F water quenched. f As cast. g Equivalent wrought grades are given for comparison only; the ACI designations, generally included in ASTM A743 and A297, are used to specify the cast stainless steel grades. h Annealed. b



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molecules with three hydroxyl groups for each glucose unit. Cellulose is the most abundant of the nonprotein natural organic products. It is highly resistant to attack by the common microorganisms. However, the enzyme cellulase digests it easily, and this substance is used for making paper pulp, for clarifying beer and citrus juices, and for the production of citric acid and other chemicals from cellulose. Cellulose is a white powder insoluble in water, sodium hydroxide, or alcohol, but it is dissolved by sulfuric acid. One of the simplest forms of cellulose used industrially is regenerated cellulose, in which the chemical composition of the finished product is similar to that of the original cellulose. It is made from wood or cotton pulp digested in a caustic solution. Cellophane is a regenerated cellulose in thin sheets for wrapping and other special uses include windings on wire and cable.

CELLULOSE PLASTICS For plastics, pure cellulose from wood pulp or cotton linters (pieces too short for textile use) is reacted with acids or alkalis and alkyl halides to produce a basic flake. Depending upon the reactants, any one of four esters of cellulose (acetate, propionate, acetate butyrate, or nitrate) or a cellulose ether (ethyl cellulose) may result. The basic flake is used for producing both solvent cast films and molding powders. Ethyl cellulose plastics are thermoplastic and are noted for their ease of molding, light weight, and good dielectric strength, 15 to 20.5 × 106 V/m, and retention of flexibility over a wide range of temperature from –57 to 66°C, the softening point. They are the toughest, the lightest, and have the lowest water absorption of the cellulosic plastics. But they are softer and lower in strength than cellulose-acetate plastics. Typical ethyl cellulose applications include football helmets, equipment housings, refrigerator parts, and luggage. For molding powders, the flake is then compounded with plasticizers, pigments, and sometimes other additives. At this stage of manufacture, the plastics producer is able to adjust hardness, toughness, flow, and other


processing characteristics and properties. In general, these qualities are spoken of together as flow grades. The flow of a cellulose plastic is determined by the temperature at which a specific amount of the material will flow through a standard orifice under a specified pressure. Manufacturers offer cellulosic molding materials in a large number of standard flow grades, and, for an application requiring a nonstandard combination of properties, are often able to tailor a compound to fit. Cellulose can be made into a film (cellophane) or into a fiber (rayon), but it must be chemically modified to produce a thermoplastic material. Cellulosics are synthethic plastics, but they are not synthethic polymers; see Table C.6. Because the cellulosics can be compounded with many different plasticizers in widely varying concentrations, property ranges are broad. These materials are normally specified by flow, defined in American Society for Testing and Materials (ASTM) D569, which is controlled by plasticizer content. Hard flows (low plasticizer content) are relatively hard, rigid, and strong. Soft flows (higher plasticizer content) are tough, but less hard, less rigid, and less strong. They also process at lower temperatures. Thus, within available property ranges listed, no one formulation can provide all properties to the maximum degree. Most commonly used formulations are in the middle flow ranges. Molded cellulosic parts can be used in service over broad temperature ranges and are particularly tough at very low temperatures. Ethyl cellulose is outstanding in this respect. These materials have low specific heat and low thermal conductivity — characteristics that give them a pleasant feel. Dimensional stability of butyrate, propionate, and ethyl cellulose is excellent. Plasticizers used in these materials do not evaporate significantly and are virtually immune to extraction by water. Water absorption (which causes dimensional change) is also low, with that of ethyl cellulose the lowest. The plasticizers in acetate are not as permanent as those in other plastics, however, and water absorption of this material is slightly higher. Butyrate and propionate are highly resistant to water and most aqueous solutions except strong acids and strong bases. They


TABLE C.6 Properties of Cellulosics ASTM or UL Test

Property

D792 D792 D570


D638 D638 D790 D790 D256

Tensile strength (psi) Tensile modulus (105 psi) Flexural strength (psi) Flexural modulus (105 psi) Impact strength, Izod (ft-lb/in. of notch) Hardness, Rockwell R

D785

Cellulose Propionate

Cellulose Acetate Butyrate

Ethyl Cellulose

1.16–1.24 23.4–22.4 1.2–2.8

1.15–1.22 24.1–22.7 0.9–2.2

1.09–1.17 25.5–23.6 0.8–1.8

1400–6200 0.5–2.0 1800–9250 0.9–3.0 1.1–9.1

3000–4800 2.2–2.5 4700–6800 — 3.0–8.0

To 115

To 112

79–106

4–8

4–8

4–8

3.8–7.0

8–16

11–17

11–17

10–20

111–195 120–209 V–2, HB

111–228 147–250 HB

113–202 130–227 HB

115–190 170–180 —

250–600

300–500

250–400

350–500

3.2–7.0

3.3–4.0

3.4–6.4

3.0–4.1

0.01–0.10

0.01–0.05

0.01–0.04

0.002–0.020

1010–1014 50–310

1012–1016 175–190

1011–1015 —

1012–1014 60–80

Optical 1.46–1.50 80–92

1.46–1.49 80–92

1.46–1.49 80–92

— —

Cellulose Acetate Physical 1.22–1.34 22.7–20.6 1.7–4.5

Mechanical 2200–6900 1400–7200 0.65–4.0 0.6–2.15 2500–10,400 1700–10,600 1.2–3.6 1.15–3.7 1.0–7.3 1.0–10.3 To 122 Thermal

C177 D696

D648

UL94

Thermal conductivity (10–4 cal-cm/s-cm2-°C) Coefficient of thermal expansion (10–5 in./in.-°C) Deflection temperature (°F) At 264 psi At 66 psi Flammability rating

Electrical

D495

Dielectric strengtha (V/mil) Short time, 1/8-in. thk Dielectric constant At 1 kHz Dissipation factor At 1 kHz Volume resistivity (Ω-cm) At 73°F, 50% RH Arc resistance (s)

D542 D1003

Refractive Index Transmittanceb (%)

D149 D150 D150 D257

a b

At 500V/s rate of rise. For 1/8-in. thick specimen.



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resist nonpolar materials such as aliphatic hydrocarbons and ethers, but they swell or dissolve in low-molecular-weight polar compounds such as alcohols, esters, and ketones, as well as in aromatic and chlorinated hydrocarbons. Acetate is slightly less resistant than butyrate and propionate to water and aqueous solutions, and slightly more resistant to organic materials. Ethyl cellulose dissolves in all the common solvents for this polymer, as well as in such solvents as cyclohexane and diethyl ether. Like the cellulose esters, ethyl cellulose is highly resistant to water. Although unprotected cellulosics are generally not suitable for continuous outdoor use, special formulations of butyrate and propionate are available for such service. Acetate and ethyl cellulose are not recommended for outdoor use.

APPLICATIONS Acetate applications include extruded and cast film and sheet for packaging and thermoforming. Cellulose Acetate Cellulose acetate is an amber-colored, transparent material made by the reaction of cellulose and acetic acid or acetic anhydride in the presence of sulfuric acid. It is thermoplastic and easily molded. The molded parts or sheets are tough, easily machined, and resistant to oils and many chemicals. In coatings and lacquers the material is adhesive, tough, and resilient, and does not discolor easily. Cellulose acetate fiber for rayons can be made in fine filaments that are strong and flexible, nonflammable, mildew proof, and easily dyed. Standard cellulose acetate for molding is marketed in flake form. In practical use, cellulose acetate moldings exhibit toughness superior to most other general-purpose plastics. Flame-resistant formulations are currently specified for small appliance housings and for other uses requiring this property. Uses for cellulose acetate molding materials include toys, buttons, knobs, and other parts where the combination of toughness and clear transparency is a requirement. Extruded film and sheet of cellulose acetate packaging materials maintain their properties over long periods. Here also the toughness of © 2002 by CRC Press LLC

the material is advantageously used in blister packages, skin packs, window boxes, and overwraps. It is a breathing wrap and is solvent and heat sealable. Large end uses for cellulose acetate films and sheets include photographic film base, protective cover sheets for notebook pages and documents, index tabs, sound recording tape, as well as the laminating of book covers. The grease resistance of cellulose acetate sheet allows its use in packaging industrial parts with enclosed oil for protection. For eyeglass frames, cellulose acetate is the material in widest current use. Because fashion requires varied and sometimes novel effects, sheets of clear, pearlescent, and colored cellulose acetate are laminated to make special sheets from which optical frames are fabricated. The electrical properties of cellulosic films combined with their easy bonding, good aging, and available flame resistance bring about their specification for a broad range of electrical applications. Among these are as insulations for capacitors; communications cable; oil windings; in miniaturized components (where circuits may be vacuum metallized); and as fuse windows. Cellulose triacetate is widely used as a solvent cast film of excellent physical properties and good dimensional stability. Used as photographic film base and for other critical dimensional work such as graphic arts, cellulose triacetate is not moldable. Cellulose Propionate Cellulose propionate, commonly called “CP” or propionate, is made by the same general method as cellulose acetate, but propionic acid is used in the reaction. Propionate offers several advantages over cellulose acetate for many applications. Because it is “internally” plasticized by the longer-chain propionate radical, it requires less plasticizer than is required for cellulose acetate of equivalent toughness. Cellulose propionate absorbs much less moisture from the air and is thus more dimensionally stable than cellulose acetate. Because of better dimensional stability, cellulose propionate is often selected where metal inserts and close tolerances are specified.


Largest-volume uses for cellulose propionate are as industrial parts (automotive steering wheels, armrests, and knobs, etc.), telephones, toys, findings, ladies’ shoe heels, pen and pencil barrels, and toothbrushes. Cellulose Acetate Butyrate Commonly called butyrate or CAB, it is somewhat tougher and has lower moisture absorption and a higher softening point than acetate. CAB is made by the esterification of cellulose with acetic acid and butyric acid in the presense of a catalyst. It is particularly valued for coatings, insulating types, varnishes, and lacquers. Special formulations with good weathering characteristics plus transparency are used for outdoor applications such as signs, light globes, and lawn sprinklers. Clear sheets of butyrate are available for vacuum-forming applications. Other typical uses include transparent dial covers, television screen shields, tool handles, and typewriter keys. Extruded pipe is used for electric conduits, pneumatic tubing, and low-pressure waste lines. Cellulose acetate butyrate also is used for cable coverings and coatings. It is more soluble than cellulose acetate and more miscible with gums. It forms durable and flexible films. A liquid cellulose acetate butyrate is used for glossy lacquers, chemical-resistant fabric coatings, and wire-screen windows. It transmits ultraviolet light without yellowing or hazing and is weather-resistant. Cellulose Acetate Propionate This substance is similar to butyrate in both cost and properties. Some grades have slightly higher strength and modulus of elasticity. Propionate has better molding characteristics, but lower weatherability than butyrate. Molded parts include steering wheels, fuel filter bowls, and appliance housings. Transparent sheeting is used for blister packaging and food containers. Cellulose Nitrate Cellulose nitrates are materials made by treating cellulose with a mixture of nitric and sulfuric acids, washing free of acid, bleaching, stabilizing, and dehydrating. For sheets, rods,


and tubes it is mixed with plasticizers and pigments and rolled or drawn to the shape desired. The lower nitrates are very inflammable, but they do not explode like the high nitrates, and they are the ones used for plastics, rayons, and lacquers, although their use for clothing fabrics is restricted by law. The names cellulose nitrate and pyroxylin are used for the compounds of lower nitration, and the term nitrocellulose is used for the explosives. Cellulose nitrate is the toughest of the thermoplastics. It has a specific gravity of 1.35 to 1.45, tensile strength of 41 to 52 MPa, elongation 30 to 50%, compressive strength 137 to 206 MPa, Brinell hardness 8 to 11, and dielectric strength 9.9 to 21.7 × 106 V/m. The softening point is 71°C, and it is easy to mold and easy to machine. It also is readily dyed to any color. It is not light stable, and is therefore no longer used for laminated glass. It is resistant to many chemicals, but has the disadvantage that it is inflammable. The molding is limited to pressing from flat shapes. Among thermoplastics, it is remarkable for toughness. For many applications today, however, cellulose nitrate is not practical because of serious property shortcomings: heat sensitivity, poor outdoor aging, and very rapid burning. Cellulose nitrate cannot be injectionmolded or extruded by the nonsolvent process because it is unable to withstand the temperatures these processes require. It is sold as films, sheets, rods, or tubes, from which end products may then be fabricated. Cellulose nitrate yellows with age; if continuously exposed to direct sunlight, it yellows faster and the surface cracks. Its rapid burning must be considered for each potential application to avoid unnecessary hazard. The outstanding toughness properties of cellulose nitrate lead to its continuing use in such applications as optical frames, shoe eyelets, ping pong balls, and pen barrels.

CENTRIFUGAL CASTINGS Centrifugal castings can be produced economically and with excellent soundness. They are used in the automotive, aviation, chemical, and process industries for a variety of parts having

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a hollow, cylindrical form or for sections or segments obtainable from such a form. There are three modifications of centrifugal casting: (1) true centrifugal casting, (2) semicentrifugal casting, and (3) centrifuging. 1. True centrifugal casting is used for the production of cylindrical parts. The mold is rotated, usually in a horizontal plane, and the molten metal is held against the wall by centrifugal force until it solidifies. 2. Semicentrifugal casting is used for disk- and wheel-shaped parts. The mold is spun on a vertical axis, the metal is introduced at the center of the mold, and centrifugal force throws the metal to the periphery. 3. Centrifuging is used to produce irregular-shaped pieces. The method differs from static casting only in that the mold is rotated. Mold cavities are fastened at the periphery of a revolving turntable, the metal is introduced at the center, and thrown into the molds through radial ingates. The nature of the centrifugal casting process assures a dense, homogeneous cast structure free from porosity. Because the metal solidifies in a spinning mold under centrifugal force, it tends to be forced against the mold wall while impurities, such as sand, slag, and gases, are forced toward the inside of the tube. Another advantage of centrifugal casting is that recovery can run as high as 90% of the metal poured. Certain types of castings are produced in centrifugal casting machines. There are essentially two types of those machines — the horizontal type that rotates about a horizonal axis and the vertical type that rotates about a vertical axis. In general, horizontal machines are used to make pipe, tubes, bushings, cylinder sleeves, and other cylindrical or tubular castings that are simple in shape. Castings that are not cylindrical, or even symmetrical, can be made using vertical centrifugal casting machines.


FERROUS CASTINGS Centrifugal castings can be made of many of the ferrous metals — cast irons, carbon and low-alloy steels, and duplex metals. Mechanical Properties Regardless of alloy content, the tensile properties of irons cast centrifugally are reported to be higher than those of static castings produced from the same heat. Hydrostatic tests of cylinder liners produced by both methods show that centrifugally cast liners withstand about 20% more pressure than statically cast liners. Freedom from directionality is one of the advantages that centrifugal castings have over forgings. Properties of longitudinal and tangential specimens of several stainless grades are substantially equal. Shapes, Sizes, Tolerances The external contours of centrifugal castings are not limited to circular forms. The contours can be elliptical, hexagonal, or fluted, for example. However, the nature of the true centrifugal casting process limits the bore to a circular cross section. Iron and steel centrifugally cast tubes and cylinders are produced commercially with diameters ranging from 28.6 to 1500 mm, wall thickness of 0.25 to 102 mm, and in lengths up to 14.30 m. Generally it is impractical to produce castings with the a ratio of the outside diameter to the inside diameter greater than about 4 to 1. The upper limit in size is governed by the cost of the massive equipment required to produce heavy castings. As-cast tolerances for centrifugal eastings are about the same as those for static castings. For example, tolerances on the outside diameter of centrifugally cast gray iron pipe range from 0.3 mm for 76 mm diameter to ±0.6 mm. for 1.2 m diameter. Inside-diameter tolerances are greater, because they depend not only on the mold diameter, but also on the quantity of metal cast; the latter varies from one casting to another. These tolerances are generally about 50% greater than those on outside diameters. Casting tolerances depend to some extent also on the shrinkage allowance for the metal being cast.


The figures given above apply to castings to be used in the unmachined state. For castings requiring machining, it is customary to allow 2.35 to 3.2 mm on small castings and up to 6.4 mm on larger castings. If the end use requires a sliding fit, broader tolerances are generally specified to permit additional machining on the inside surface. Cast Irons Large tonnages of gray iron are cast centrifugally. The relatively low pouring temperatures and good fluidity of the common grades make them readily adaptable to the process. Various alloy grades that yield pearlitic, acicular, and chill irons are also used. In addition, specialty iron alloys such as “Ni-Hard” and “Ni-Resist,” have been cast successfully. Carbon and Low-Alloy Steels Centrifugal castings are produced from carbon steels having carbon contents ranging from 0.05 to 0.90%. Practically all of the AISI (American Iron and Steel Institute) standard low-alloy grades have also been cast. Small-diameter centrifugally cast tubing in the usual carbon steel grades is not competitive in price with mechanical tubing having normal wall thicknesses. However, centrifugally cast tubing is less expensive than statically cast material. High-Alloy Steels Most of the AISI stainless and heat-resisting grades can be cast centrifugally. A particular advantage of the process is its use in producing tubes and cylinders from alloy compositions that are difficult to pierce and to forge or roll. The excellent ductility resulting in the stainless alloys from centrifugal casting makes it possible to reduce the rough cast tubes to smaller-diameter tubing by hot- or cold-working methods. For example, billets of 18-8 stainless steel, 114.5 mm outside diameter by 16 mm wall, have been redated to 27-gauge capillary tubing without difficulty.


Duplex Metals Centrifugal castings with one metal on the outside and another on the inside are also in commercial production. Combinations of hard and soft cast iron, carbon steel, and stainless steel have been produced successfully. Duplex metal parts have been centrifugally cast by two methods. In one, the internal member of the pair is cast within a shell of the other. This method has been used to produce aircraft brake drums by centrifugally casting an iron liner into a steel shell. In the second method, both sections of the casting are produced centrifugally; the metal that is to form the outer portion of the combination is poured into the mold and solidified and the second metal is introduced before the first has cooled. The major limitation of this method is that the solidification temperature of the second metal poured must be the same or lower than that of the first. This method is said to form a strongly bonded duplex casting. The possibilities of this duplex method for producing tubing for corrosion-resistant applications and chemical pressure service have been developed.

NONFERROUS CASTINGS Nonferrous centrifugal castings are produced from copper alloys, nickel alloys, and tin- and lead-base bearing metals. Only limited application of the process is made to light metals because it is questionable whether any property improvement is achieved; for example, differences in density between aluminum and its normal impurities are smaller than in the heavy metals and consequently separation of the oxides, a major advantage of the process, is not so successful. Shapes, Sizes, Tolerances As with ferrous alloys, the external shapes of nonferrous centrifugal castings can be elliptical, hexagonal, or fluted, as well as round. However, the greatest overall tonnage of nonferrous castings is produced in plain or semiplain cylinders. The inside diameter of the casting is limited to a straight bore or one that can

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be machined to the required contour with minimum machining cost. Nonferrous castings are produced commercially in outside diameter ranging from about 25.4 mm to 1.8 m and in lengths up to 8.1 m. Weights of individual castings range from 0.2268 to 27300 kg. Although tolerances on as-cast parts are about the same as those for sand castings, most centrifugal castings are finished by machining. An advantage of centrifugal casting is that normally only a small machining allowance is required; this allowance varies from as little as 1.53 mm on small castings to 6.4 mm on the outside diameter of large-diameter castings. A slightly larger machining allowance is required on the bore to permit removal of dross and other impurities that segregate in this area. Copper Alloys A wide range of copper casting alloys is used in the production of centrifugal castings. The alloys include the plain brasses, leaded brasses and bronzes, tin bronzes, aluminum bronzes, silicon bronzes, manganese bronzes, nickel silvers, and beryllium copper. The ASTM lists 32 copper alloys for centrifugal casting; in addition, there are a number of proprietary compositions that are regularly produced by centrifugal casting. Most of these alloys can be cast without difficulty. Some trouble with segregation has been reported in casting the high leaded (over 10% lead) alloys. However, alloys containing up to 20% lead are being cast by some foundries; the requirements are (1) rapid chilling to prevent excessive lead segregation and (2) close control of speed. The mechanical properties of centrifugally cast copper alloys vary with the composition and are affected by the mold material used. Centrifugal castings produced in chill molds have higher mechanical properties than those obtained by casting in sand molds. However, centrifugal castings made in sand molds have properties about 10% higher than those obtained on equivalent sections of castings produced in static sand molds. (Castings produced in centrifugal chill molds have properties 20 to


40% higher than those produced in static sand molds.) Nickel Alloys Centrifugal castings of nickel 210, 213, and 305; “Monel” alloys 410, 505, and 506; and “Inconel” alloys 610 and 705 are commercially available in cylindrical tubes. Centrifugal castings are also produced from the heat-resisting alloys 60% nickel–12% chromium and 66% nickel–17% chromium. These alloys should behave like other materials and show improved density with accompanying improvement in mechanical properties. The nickel alloys are employed for service under severe corrosion, abrasion, and galling conditions. Bearing Metals Centrifugal casting is a standard method of producing lined bearings. Steel cylinders, after being cleaned, pickled, and tinned, are rotated while tin- or lead-base bearing alloys are cast into them. The composite cylinder is then cut lengthwise, machined, and finished into split bearings.

CERAMIC FIBERS Alumina-silica (Al2O3–SiO2) fibers, frequently referred to as ceramic fibers, are formed by subjecting a molten stream to a fiberizing force. Such force may be developed by high-velocity gas jets or rotors or intricate combinations of these. The molten stream is produced by melting high-purity Al2O3 and SiO2, plus suitable fluxing agents, and then pouring this melt through an orifice. The jet or rotor atomizes the molten stream and attenuates the small particles into fine fibers as supercooling occurs. The resulting fibrous material is a versatile high-temperature insulation for continuous service in the 538 to 1260°C range. It thus bridges the gap between conventional inorganic fiber insulating materials (e.g., asbestos, mineral wool, and glass) and insulating refractories. Al2O3–SiO2 fibers have a maximum continuous use temperature of 1093 to 1260°C, and a melting point of over 1760°C. If the fiber is exposed to temperature in excess of 1093°C for


extended periods of time, a phenomenon called devitrification occurs. This is a change in the orientation of the molecular structure of the material from the amorphous state (random orientation) to the crystalline state (definitely arranged pattern). Insulating properties are not affected by this phase change but the material becomes more brittle. Most ceramic fibers have an Al2O3 content from 40 to 60%, and an SiO2 content from 40 to 60%. Also contained in the fibers are from 1.5 to 7% oxides of sodium, boron, magnesium, calcium, titanium, zirconium, and iron. Fibers as formed resemble a cottonlike mass with individual fiber length varying from short to 254 mm, and diameters from less than 1 to 10 µm. Larger-diameter fibers are produced for specific applications. In all processes, some unfiberized particles are formed that have diameters up to 40 µm. Low density, excellent thermal shock resistance, and very low thermal conductivity are the properties of Al2O3–SiO2 fibers that make them an excellent high-temperature insulating material. Available in a variety of forms, ceramic fiber is in ever-increasing demand due to higher and higher temperatures now found in industrial and research processes.

APPLICATIONS Ceramic fibers were originally developed for application in insulating jet engines. Now, this is only one of numerous uses for this material. It can be found in aircraft and missile applications where a high-temperature insulating medium is necessary to withstand the searing heat developed by rockets and supersonic aircraft. Employed as a thermal-balance and pressure-distribution material, ceramic fiber in the form of paper has made possible the efficient brazing of metallic honeycomb-sandwich structures. Successful trials have been conducted in aluminum processing where this versatile product in paper or molded form has been used to transport molten metal with very little heat loss. Such fibrous bodies are particularly useful in these applications because they are not readily wet by molten aluminum.


Industrial furnace manufacturers utilize lightweight ceramic fiber insulation between firebrick and the furnace shell. It is also used for “hot topping,” heating element cushions, and as expansion joint packing to reduce heat loss and maintain uniform furnace temperatures. Use of this new fiber as combustion chamber liners in oil-fired home heating units has materially improved heat-transfer efficiencies. The low heat capacity and light weight, compared to previously used firebrick, improve furnace performance and offer both customer and manufacturer many benefits.

SIC FIBERS These fibers, capable of withstanding temperatures to about 1200°C, are manufactured from a polymer precursor. The polymer is spun into a fine thread, then pyrolized to form a 15-µm ceramic fiber consisting of fine SiC crystallites and an amorphous phase. An advantage of the process is that it uses technology developed for commercial fiber products such as nylon and polyester. Two commercial SiC fiber products are the Ube Industries Tyranno fiber and the Nippon Carbon Nicalon fiber.

CERAMIC-MATRIX COMPOSITE The class of materials known as ceramic-matrix composites, or CMCs, shows considerable promise for providing fracture-toughness values similar to those for metals such as cast iron. Two kinds of damage-tolerant ceramic–ceramic composites have been developed. One incorporates a continuous reinforcing phase, such as a fiber; the other, a discontinuous reinforcement, such as whiskers. The major difference between the two is in their failure behavior. Continuousfiber-reinforced materials do not fail catastrophically. After matrix failure, the fiber can still support a load. A fibrous failure is similar to that which occurs in wood. Incorporating whiskers into a ceramic matrix improves resistance to crack growth, making the composite less sensitive to flaws. These materials are commonly described as

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being flaw tolerant. However, once a crack begins to propagate, failure is catastrophic. Of particular importance to the technology of toughened ceramics has been the development of high-temperature SiC reinforcements.

SIC FILAMENTS SiC filaments are prepared by chemical vapor disposition. A thick layer of SiC is deposited on a thin fiber substrate of tungsten or carbon. Diameter of the final product is about 140 µm. Although developed initially to reinforce aluminum and titanium matrices, SiC filaments have since been used as reinforcement in Si3N4.

SIC WHISKERS SiC whiskers consist of a fine (0.5–5 µm-diameter) single-crystal structure in lengths to 100 µm. The material is strong (to 15.9 GPa) and is stable at temperatures to 1800°C. Whiskers can be produced by heating SiO2 and carbon sources with a metal catalyst in the proper environments. Although these materials are relatively new, at least one successful commercial product is already being marketed. An SiC-whisker-reinforced Al2O3 cutting-tool material is being used to machine nickel-based superalloys. In addition, considerable interest has been generated in reinforcing other matrices such as mullite, SiC, and Si3N4 for possible applications in automotive and aerospace industries.

DIMOX PROCESS CMCs are steadily moving from the laboratory to initial commercial applications. For example, engineers are currently evaluating these materials for use in the hot gas zones of gas turbine engines, because ceramics are known for their strength and favorable creep behavior at high temperatures. Advanced ceramics, for example, can potentially be used at temperatures 204 to 482°C above the maximum operating temperature for superalloys. Until recently, however, there has been more evaluation than implementation of advanced ceramics for various reasons. Monolithic or single-component ceramics, for example, lack the required damage tolerance and toughness. Engine designers are put off by the


potential of ceramic material for catastrophic, brittle failures. Although many CMCs have greater toughness, they are also difficult to process by traditional methods, and may not have the needed long-term high-temperature resistance. A relatively new method for producing CMCs developed by Lanxide Corp. promises to overcome the limitations of other ceramic technologies. Called the DIMOX directed metal oxidation process, it is based on the reaction of a molten metal with an oxidant, usually a gas, to form the ceramic matrix. Unlike the sintering process, in which ceramic powders and fillers are consolidated under heat, directed metal oxidation grows the ceramic matrix material around the reinforcements. Examples of ceramic matrices that can be produced by the DIMOX directed metal oxidation process include Al2O3, A12Ti5, AlN, TiN, ZrN, TiC, and ZrC. Filler materials can be anything chemically compatible with the ceramic, parent metal, and growth atmosphere. The first step in the process involves making a shaped preform of the filler material. Preforms consisting of particles are fabricated with traditional ceramic-forming techniques, while fiber preforms are made by weaving, braiding, or laying up woven cloth. Next, the preform is put in contact with the parent metal alloy. A gas-permeable growth barrier is applied to the surfaces of this assembly to limit its shape and size. The assembly, supported in a suitable refractory container, is then heated in a furnace. For aluminum systems, temperatures typically range from 899 to 1149°C. The parent metal reacts with the surrounding gas atmosphere to grow the ceramic reaction product through and around the filler to form a CMC. Capillary action within the growing ceramic matrix continues to supply molten alloy to the growth front. There, the reaction continues until the growing matrix reaches the barrier. At this point, growth stops, and the part is cooled to ambient temperature. To recover the part, the growth barrier and any residual parent metal are removed. However, some of the parent metal (5 to 15% by volume) remains within the final composite in micron-sized interconnected channels.


Traditional ceramic processes use sintering or hot pressing to make a solid CMC out of ceramic powders and filler. Part size and shapes are limited by equipment size and the shrinkage that occurs during densification of the powders can make sintering unfeasible. Larger parts pose the biggest shrinkage problem. Advantages of the directed metal oxidation process include no shrinkage because matrix formation occurs by a growth process. As a result, tolerance control and large part fabrication can be easier with directed metal oxidation. In addition, the growth process forms a matrix whose grain boundaries are free of impurities or sintering aids. Traditional methods often incorporate these additives, which reduce high-temperature properties. And cost comparisons show the newer process is a promising replacement for traditional methods.

CERAMICS Ceramics are inorganic, nonmetallic materials processed or consolidated at high temperature. Ceramics, one of the three major material families, are crystalline compounds of metallic and nonmetallic elements. The ceramic family is large and varied, including such materials as refractories, glass, brick, cement and plaster, abrasives, sanitaryware, dinnerware, artware, porcelain enamel, ferroelectrics, ferrites, and dielectric insulators. There are other materials that, strictly speaking, are not ceramics, but that nevertheless are often included in this family. These are carbon and graphite, mica, and asbestos. Also, intermetallic compounds, such as aluminides and beryllides, which are classified as metals, and cermets, which are mixtures of metals and ceramics, are usually thought of as ceramic materials because of similar physical characteristics to certain ceramics. Ceramic materials can be subdivided into traditional and advanced ceramics. Traditional ceramics include clay-base materials such as brick, tile, sanitaryware, dinnerware, clay pipe, and electrical porcelain. Common-usage glass, cement, abrasives, and refractories are also important classes of traditional ceramics. Advanced materials technology is often cited as an “enabling” technology, enabling engineers to design and build advanced systems


for applications in fields such as aerospace, automotive, and electronics. Advanced ceramics are tailored to have premium properties through application of advanced materials science and technology to control composition and internal structure. Examples of advanced ceramic materials are Si3N4, SiC, toughened ZrO2, ZrO2-toughened Al2O3, AlN3, PbMg niobate, PbLa titanate, SiC-whisker-reinforced Al2O3, carbon-fiber-reinforced glass ceramic, SiC-fiber-reinforced SiC, and high-temperature superconductors. Advanced ceramics can be viewed as a class of the broader field of advanced materials, which can be divided into ceramics, metals, polymers, composites, and electronic materials. There is considerable overlap among these classes of materials. Advanced ceramics can be subdivided into structural and electronic ceramics based on primary function or application. Optical and magnetic materials are usually included in the electronic classification. Structural applications include engine components, cutting tools, bearings, valves, wear- and corrosion-resistant parts, heat exchangers, fibers and whiskers, and biological implants. The electronic-magneticoptic functions include electronic substrates, electronic packages, capacitors, transducers, magnets, waveguides, lamp envelopes, displays, sensors, and ceramic superconductors. Thermal insulation, membranes, and filters are important advanced ceramic product areas that do not fit well into either the structural or the electronic class of advanced ceramics. Advanced ceramics are differentiated from traditional ceramics such as brick and porcelain by their higher strength, higher operating temperatures, improved toughness, and tailorable properties. Also known as engineered ceramics, these materials are replacing metals in applications where reduced density and higher melting points can increase efficiency and speed of operation. The nature of the bond between ceramic particles helps differentiate engineering ceramics from conventional ceramics. Most particles within an engineering ceramic are self-bonded, that is, joined at grain boundaries by the same energy-equilibrium mechanism that bonds metal grains together. In contrast, most nonengineering ceramic particles are joined by a so-called ceramic bond, which is a

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weaker, mechanical linking or interlocking of particles. Generally, impurities in nonengineering ceramics prevent the particles from selfbonding. A broad range of metallic and nonmetallic elements are the primary ingredients in ceramic materials. Some of the common metals are aluminum, silicon, magnesium, beryllium, titanium, and boron. Nonmetallic elements with which they are commonly combined are O2, carbon, or N2. Ceramics can be either simple, one-phase materials composed of one compound, or multiphase, consisting of a combination of two or more compounds. Two of the most common are single oxide ceramics, such as alumina (Al2O3) and magnesia (MgO), and mixed oxide ceramics, such as cordierite (magnesia alumina silica) and forsterite (magnesia silica). Other newer ceramic compounds include borides, nitrides, carbides, and silicides. Macrostructurally, there are essentially three types of ceramics: crystalline bodies with a glassy matrix; crystalline bodies, sometimes referred to as holocrystalline; and glasses. The specific gravities of ceramics range roughly from 2 to 3. As a class, ceramics are low-tensile-strength, relatively brittle materials. A few have strengths above 172 MPa, but most have less than that. Ceramics are notable for the wide difference between their tensile and compressive strengths. They are normally much stronger under compressive loading than in tension. It is not unusual for a compressive strength to be five to ten times that of the tensile strength. Tensile strength varies considerably depending on composition and porosity. One of the major distinguishing characteristics of ceramics, as compared to metals, is their almost total absence of ductility. They fail in a brittle fashion. Lack of ductility is also reflected in low impact strength, although impact strength depends to a large extent on the shape of the part. Parts with thin or sharp edges or curves and with notches have considerably lower impact resistance than those with thick edges and gently curving contours. Ceramics are the most rigid of all materials. A majority of them are stiffer than most metals, and the modulus of elasticity in tension of a number of types runs as high as 0.3 to 0.4 million MPa compared with 0.2 million MPa


for steel. In general, they are considerably harder than most other materials, making them especially useful as wear-resistant parts and for abrasives and cutting tools. Ceramics have the highest known melting points of materials. Hafnium and TaC, for example, have melting points slightly above 3870°C, compared to 3424°C for tungsten. The more conventional ceramic types, such as Al2O3, melt at temperatures above 1927°C, which is still considerably higher than the melting point of all commonly used metals. Thermal conductivities of ceramic materials fall between those of metals and polymers. However, thermal conductivity varies widely among ceramics. A two-order magnitude of variation is possible between different types, or even between different grades of the same ceramic. Compared to metals and plastics, the thermal expansion of ceramics is relatively low, although like thermal conductivity it varies widely between different types and grades. Because the compressive strengths of ceramic materials are five to ten times greater than tensile strength, and because of relatively low heat conductivity, ceramics have fairly low thermal-shock resistance. However, in a number of ceramics, the low thermal expansion coefficient succeeds in counteracting to a considerable degree the effects of thermal conductivity and tensile-compressive-strength differences. Practically all ceramic materials have excellent chemical resistance, and are relatively inert to all chemicals except hydrofluoric acid and, to some extent, hot caustic solutions. Organic solvents do not affect them. Their high surface hardness tends to prevent breakdown by abrasion, thereby retarding chemical attack. All technical ceramics will withstand prolonged heating at a minimum of 999°C. Therefore, atmospheres, gases, and chemicals cannot penetrate the material surface and produce internal reactions that are normally accelerated by heat. Unlike metals, ceramics have relatively few free electrons and therefore are essentially nonconductive and considered to be dielectric. In general, dielectrical strengths, which range between 7.8 × 106 and 13.8 × 106 V/m, are lower than those of plastics. Electrical resistivity of many ceramics decreases rather than increases


with an increase in impurities, and is markedly affected by temperature.

FABRICATION PROCESSES A wide variety of processes are used to fabricate ceramics. The process chosen for a particular product is based on the material, shape, complexity, property requirements, and cost. Ceramic fabrication processes can be divided into four generic categories: powder, vapor, chemical, and melt processes. Powder Processes Traditional clay-base ceramics and most refractories are fabricated by powder processes as are the majority of advanced ceramics. Powder processing involves a number of sequential steps. These are preparation of the starting powders, forming the desired shape (green forming), removal of water and organics, heating with or without application of pressure to densify the powder, and finishing. Vapor Processes The primary vapor processes used to fabricate ceramics are chemical vapor deposition (CVD) and sputtering. Vapor processes have been finding an increasing number of applications. CVD involves bringing gases containing the atoms to make up the ceramic into contact with a heated surface, where the gases react to form a coating. This process is used to apply ceramic coatings to metal and tungsten carbide (WC) cutting tools as well as to apply a wide variety of other coatings for wear, electronic, and corrosion applications. CVD can also be used to form monolithic ceramics by building up thick coatings. A form of CVD known as chemical vapor infiltration (CVI) has been developed to infiltrate and coat the surfaces of fibers in woven preforms. Several variations of sputtering and other vacuum-coating processes can be used to form coatings of ceramic materials. The most common process is reactive sputtering, used to form coatings such as TiN on tool steel.


Chemical Processes A number of different chemical processes are used to fabricate advanced ceramics. The CVD process described above as a vapor process is also a chemical process. Two other chemical processes finding increasing application in advanced ceramics are polymer pyrolysis and sol-gel technology. Melt Processes These are used to manufacture glass, to fusecast refractories for use in furnace linings, and to grow single crystals. Thermal spraying can also be classified as a melt process. In this process a plasma-spray gun is used to apply ceramic coatings by melting and spraying powders onto a substrate.

METAL OXIDE CERAMICS Although most metals form at least one chemical compound with O2, only a few oxides are useful as the principal constituent of a ceramic. And of these, only three are used in their fairly pure form as engineering ceramics: Al2O3, BeO, and ZrO2. The natural alloying element in the Al2O3 system is SiO2. However, Al2O3s can be alloyed with chromium (which forms a second phase with the Al2O3 and strengthens the ceramic) or with various oxides of silicon, magnesium, or calcium. Al2O3s serve well at temperatures as high as 1925°C provided they are not exposed to thermal shock, impact, or highly corrosive atmospheres. Above 2038°C, strength of Al2O3 drops. Consequently, many applications are in steady-state, high-temperature environments, but not where abrupt temperature changes would cause failure from thermal shock. Al2O3s have good creep resistance up to about 816°C above which other ceramics perform better. In addition, Al2O3s are susceptible to corrosion from strong acids, steam, and sodium. See Aluminum. BeO ceramics are efficient heat dissipaters and excellent electrical insulators. They are used in electrical and electronics applications, such as microelectric substrates, transistor bases, and resistor cores. BeO has excellent

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thermal shock resistance (some grades can withstand 816°C/s changes), a very low coefficient of thermal expansion (CTE), and a high thermal conductivity. It is expensive, however, and is an allergen to which some persons are sensitive. See Beryllium. ZrO2 is used primarily for its extreme inertness to most metals. ZrO2 ceramics retain strength nearly up to their melting point — well over 2205°C, the highest of all ceramics. Applications for fused or sintered ZrO2 include crucibles and furnace bricks. See Zirconium. Transformation-toughened ZrO2 ceramics are among the strongest and toughest ceramics made. These materials are of three main types: Mg-PSZ (ZrO2 partially stabilized with MgO), Y-TZP (Y2O3 stabilized tetragonal ZrO2 polycrystals), and ZTA (ZrO2-toughened Al2O3). Applications of Mg-PSZ ceramics are principally in low- and moderate-temperature abrasive and corrosive environments — pump and valve parts, seals, bushings, impellers, and knife blades. Y-TZP ceramics (stronger than Mg-PSZ but less flaw tolerant) are used for pump and valve components requiring wear and corrosion resistance in room-temperature service. ZTA ceramics, which have lower density, better thermal shock resistance, and lower cost than the other two, are used in transportation equipment where they need to withstand corrosion, erosion, abrasion, and thermal shock. Many engineering ceramics have multioxide crystalline phases. An especially useful one is cordierite (MgO–Al2O3–SiO3), which is used in cellular ceramic form as a support for a washcoat and catalyst in catalytic converters in automobile emissions systems. Its low CTE is a necessary property for resistance to thermal fracture.

GLASS CERAMICS Glass ceramics are formed from molten glass and subsequently crystallized by heat treatment. They are composed of several oxides that form complex, multiphase microstructures. Glass ceramics do not have the strength-limiting porosity of conventional sintered ceramics. Properties can be tailored by control of the crystalline structure in the host glass matrix. Major applications are cooking vessels, tableware,


smooth cooktops, and various technical products such as radomes. The three common glass ceramics, Li–Al–SiO 3 (LAS, or beta spodumene), Mg–Al–SiO 3 (MAS, or cordierite), and Al–SiO3 (AS, or aluminous keatite), are stable at high temperatures, have near-zero CTEs, and resist various forms of high-temperature corrosion, especially oxidation. LAS and AS have essentially no measurable thermal expansion up to 427°C. The high SiO2 content of LAS is responsible for the low thermal expansion, but the SiO 2 also decreases strength. LAS is attacked by sulfur and sodium. MAS is stronger and more corrosion resistant than LAS. A multiphased version of this material, MAS with AlTiO3, has good corrosion resistance up to 1093°C. AS, produced by leaching lithium out of LAS particles prior to forming, is both strong and corrosion resistant. It has been used, for example, in an experimental rotating regenerator for a turbine engine. A proprietary ceramic (Macor) called machinable glass ceramic (MGC), is about as strong as Al2O3. It also has many of the hightemperature and electrical properties of the glass ceramics. The main virtue of this material is that it can be machined with conventional tools. It is available in bars, or it can be roughformed, then finish-machined. Machined parts do not require firing. A similar glass ceramic is based on chemically machinable glass that, in its initial state, is photosensitive. After the glass is sensitized by light to create a pattern, it is chemically machined (etched) to form the desired article. The part can then be used in its glassy state, or it can be fired to convert it to a glass ceramic. This material/process combination is used where precision tolerances are required and where a close match to thermal expansion characteristics of metals is needed. Typical applications are sliders for disk-memory read/write heads, wire guides for dot-matrix printers, cell sheets for gas-discharge displays, and substrates for thick-film and thin-film metallization. Another ceramic-like material, glass-bonded mica, the moldable/machinable ceramic, is also called a “ceramoplastic” because its properties


are similar to those of ceramics, but it can be machined and molded like a plastic material. A glass/mica mixture is pressed into a preform, heated to make the glass flow, then transfer- or compression-molded to the desired shape. The material is also formed into sheets and rods that can be machined with conventional carbide tooling. No firing is required after machining. The thermal-expansion coefficient of glassbonded mica is close to that of many metals. This property, along with its extremely low shrinkage during molding, allows metal inserts to be molded into the material and also ensures close dimensional tolerances. Molding tolerances as close as ±0.01 mm can be held. Continuous service temperatures for glass-bonded mica range from cryogenic to 371 or 704°C depending upon material grade.

CERIUM A chemical element, cerium (Ce) is the most abundant metallic element of the rare earth group in the periodic table. Cerium occurs mixed with other rare earths in many minerals, particularly monazite and blastnasite, and is found among the products of the fission of uranium, thorium, plutonium. Ceric oxide, CeO2, is the oxide usually obtained when cerium salts of volatile acids are heated. CeO2 is an almost white powder that is insoluble in most acids, although it can be dissolved in H2SO4 or other acids when a reducing agent is present. The metal is an iron-gray color and it oxidizes readily in air, forming a gray crust of oxide. Misch metal, an alloy of cerium, is used in the manufacture of lighter flints. Cerium has the interesting property that, at very low temperatures or when subjected to high pressures, it exhibits a face-centered cubic form, which is diamagnetic and 18% denser than the common form.

results from a small amount of mutual or partial solubility. Some systems, however, such as the metal oxides, exhibit poor bonding between phases and require additions to serve as bonding agents. Cermet parts are produced by powder-metallurgy (P/M) techniques. They have a wide range of properties, depending on the composition and relative volumes of the metal and ceramic constituents. Some cermets are also produced by impregnating a porous ceramic structure with a metallic matrix binder. Cermets can also be used in powder form as coatings. The powdered mixture is sprayed through an acetylene flame, and it fuses to the base material; see Table C.7. Although a great variety of cermets have been produced on a small scale, only a few types have significant commercial use. These fall into two main groups: oxide-base and carbide-base cermets. Other important types include the TiC-base cermets, Al2O3-base cermets, and UO2 cermets specially developed for nuclear reactors.

TABLE C.7 Representative Components of Cermets Class Oxides

Carbides

Borides

CERMETS A composite material made up of ceramic particles (or grains) dispersed in a metal matrix. Particle size is greater than 1 µm, and the volume fraction is over 25% and can go as high as 90%. Bonding between the constituents


Silicides Nitrides

Ceramic

Metal Addition

Al2O3

Al, Be, Co, Co–Cr, Fe, stainless steel Cr Al, Be, Co, Fe, Mg Cr, Si Zr Zr, Al, stainless steel Ag, Si, Co, Cr Mo, W, Fe, Ni, Co, Inconel, Hastelloy, stainless steel, Vitallium Co Ni, Si Ni Fe, Ni, Co Ni Ni, Co, Pt, Fe, Cr Ni

Cr2O3 MgO SiO2 ZrO2 UO2 SiC TiC

WC Cr3C2 Cr3B2 TiB2 ZrB2 MoSi2 TiN

Source: McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 3, McGraw-Hill, New York, 483. With permission.

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The most common type of oxide-base cermets contains Al2O3 ceramic particles (ranging from 30 to 70% volume fraction) and a chromium or chromium-alloy matrix. In general, oxide-base cermets have specific gravities between 4.5 and 9.0, and tensile strengths ranging frorn 144 to 268 MPa. Their modulus of elasticity ranges between 0.25 and 0.34 million MPa, and their hardness range is A70 to 90 on the Rockwell scale. The outstanding characteristic of oxide-base cermets is that the metal or ceramic can be either the particle or the matrix constituent. The 6MgO–94Cr cermets reverse the roles of the oxide and chromium, i.e., the magnesium is added to improve the fabrication and performance of the chromium. Chromium is not ductile at room temperature. Adding MgO not only permits press-forging at room temperature but also increases oxidation resistance to five times that of pure chromium. Of the cermets, the oxide-base alloys are probably the simplest to fabricate. Normal P/M or ceramic techniques can be used to form shapes, but these materials can also be machined or forged. The oxide-base cermets are used as a tool material for high-speed cutting of difficult-to-machine materials. Other uses include thermocoupleprotection tubes, molten-metal-processing equipment parts, mechanical seals, gas-turbine flame holders (resistance to flame erosion), and flow control pins (because of Cr–Al2O3’s resistance to wetting and erosion by many molten metals and to thermal shock). There are three major groups of carbidebase cermets: tungsten, chromium, and titanium. Each of these groups is made up of a variety of compositional types or grades. WC cermets contain up to about 30% cobalt as the matrix binder. They are the heaviest type of cermet (specific gravity is 11 to 15). Their outstanding properties include high rigidity, compressive strength, hardness, and abrasion resistance. Their modulus of elasticity ranges between 0.45 and 0.65 million MPa, and they have a Rockwell hardness of about A90. Structural uses of WC–Co cermets include wiredrawing dies, precision rolls, gauges, and valve parts. Higher-impact grades can be applied where die steels were formerly needed to withstand inpact loading. Combined with superior abrasion resistance, the higher impact strength


results in die-life improvements as high, in some cases, as 5000 to 7000%. Most TiC cermets have nickel or nickel alloys as the metallic matrix, which results in high-temperature resistance. They have relatively low density combined with high stiffness and strength at temperatures above 1204°C. Typical properties are specific gravity, 5.5 to 7.3; tensile strength, 517 to 1068 MPa; modulus of elasticity, 0.25 to 0.38 million MPa; and Rockwell hardness, A70 to 90. Typical uses are integral turbine wheels, hot-upsetting anvils, hot-spinning tools, thermocouple protection tubes, gas-turbine nozzle vanes and buckets, torch tips, hot-mill-roll guides, valves, and valve seats. CrC cermets contain from 80 to 90% CrC, with the balance being either nickel or nickel alloys. Their tensile strength is about 241 MPa, and they have a tensile modulus of about 0.34 to 0.39 million MPa. Their Rockwell hardness is about A88. They have superior resistance to oxidation, excellent corrosion resistance, and relatively low density (specific gravity is 7.0). Their high rigidity and abrasion resistance make them suitable for gauges, oil-well check valves, valve liners, spray nozzles, bearing seal rings, and pump rotors. Other cermets are berium–carbonate–nickel and tungsten-thoria, which are used in higher-power pulse magnetrons. Some proprietary compositions are used as friction materials. In brake applications, they combine the thermal conductivity and toughness of metals with the hardness and refractory properties of ceramics. UO2 cermets have been developed for use in nuclear reactors. Cermets play an important role in sandwich-plate fuel elements, and the finished element is a siliconized SiC with a core containing UO2. Control rods have been fabricated from B4C–stainless steel and rare earth oxides–stainless steel. Other cermets developed for use in nuclear equipment include Cr–Al 2 O 3 cermets, Ni–MgO cermets, and Fe–ZrC cermets. Nonmagnetic compositions can be formulated for use where magnetic materials cannot be tolerated.

INTERACTIONS The reactions taking place between the metallic and ceramic components during fabrication of


cermets may be briefly classified and described as follows: 1. Heterogeneous mixtures with no chemical reaction between the components, characterized by a mechanical interlocking of the components without formation of a new phase, no penetration of the metallic component into the ceramic component, and vice versa, and no alteration of either component (example, MgO–Ni). 2. Surface reaction resulting in the formation of a new phase as an interfacial layer that is not soluble in the component materials. The thickness of this layer depends on the diffusion rate, temperature, and time of the reaction (example, Al2O3–Be). 3. Complete reaction between the components, resulting in the formation of a solid solution characterized by a polyatomic structure of the ceramic and the metallic component (example, TiC–Ni). 4. Penetration along grain boundaries without the formation of interfacial layers (example, Al2O3–Mo).

BONDING BEHAVIOR One important factor in the selection of metallic and ceramic components in cermets is their bonding behavior. Bonding may be by surface interaction or by bulk interaction. In cermets of the oxide-metal type, for example, investigators differentiate among three forms of surface interaction: macrowetting, solid wetting, and wetting assisted by direct lattice fit.

COMBINATIONS One distinguishes basically between four different combinations of metal and ceramic components: (1) the formation of continuous interlocking phases of the metallic and ceramic components, (2) the dispersion of the metallic component in the ceramic matrix, (3) the dispersion of the ceramic component in the


metallic matrix, and (4) the interaction between the metallic and ceramic components.

APPLICATIONS Aside from the high-temperature applications in turbine buckets, nozzle vanes, and impellers for auxiliary power turbines, there is a wide variety of applications for cermets based on various other properties. One of the most successful applications for the TiC-base cermets is in elements of temperature sensing and controlling thermostats where their oxidation resistance together with their low coefficient of thermal expansion as compared with nickel-base alloys are the important properties. Their ability to be welded directly to the alloys is also important. The TiC-base cermets are also used for bearings and thrust runners in liquid metal pumps, hot flash trimming and hot spinning tools, hot rod mill guides, antifriction and sleeve-type bearings, hot glass pinch jaws, rotary seals for hot gases, oil well valve balls, etc.

CESIUM A chemical element, cesium (symbol Cs) is the heaviest of the alkali metals in group I. It is a soft, light, very low melting temperature metal. It is the most reactive of the alkali metals and indeed is the most electropositive and the most reactive of all the elements. Cesium oxidizes easily in the air, ignites at ordinary temperatures, and decomposes water with explosive violence. It can be contained in vacuum, inert gas, or anhydrous liquid hydrocarbons protected from O2 and air. The specific gravity is 1.903, melting point 28.5°C, and boiling point 670°C. It is used in low-voltage tubes to scavenge the last traces of air. It is usually marketed in the form of its compounds such as cesium nitrate, CsNO3, cesium fluoride, CsF, or cesium carbonate, Cs2CO3. In the form of cesium chloride, CsCl, it is used on the filaments of radio tubes to increase sensitivity. It interacts with the thorium of the filament to produce positive tons. In photoelectric cells CsCl is used for a photosensitive deposit on the cathode, since cesium releases its outer electron under the action of ordinary light, and its color sensitivity is higher than that of other alkali

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metals. The high-voltage rectifying tube for changing AC to DC has cesium metal coated on the nickel cathode, and has cesium vapor for current carrying. The cesium metal gives off a copious flow of electrons and is continuously renewed from the vapor. Cesium vapor is also used in the infrared signaling lamp as it produces infrared waves without visible light. Cesium salts have been used medicinally as antishock agents after administration of arsenic drugs. Cesium metal is generaly made by thermochemical processes. The carbonate can be reduced by metallic magnesium, or the chloride can be reduced by CaC. Metallic cesium volatilizes from the reaction mixture and is collected by cooling the vapor.

CHEMICAL MILLED PARTS Chemical milling is the process of producing metal parts to predetermined dimensions by chemical removal of metal from the surface. Acid or alkaline, pickling, or etching baths have been formulated to remove metal uniformly from surfaces without excessive etching, roughening, or grain boundary attack. Simple immersion of a metal part will result in uniform removal from all surfaces exposed to the chemical solution. Selective milling is accomplished by use of a mask to protect the areas where no metal is to be removed. By such means optimum strength per unit of construction weight is achieved. Nonuniform milling can be done by the protective masking procedure or by programmed withdrawal of the part from the milling bath. Complex milling is done by multiple masking and milling or withdrawal steps.

on all surfaces contacted by the etching solution. The solution will easily mill inside and reentrant surfaces as well as thin metal parts or parts that are multiple racked. The method does not require elaborate fixturing or precision setups, and parts are just as easily milled after forming as in the flat. Job lots and salvage are treated, as well as production runs. Maximum weight reduction is possible through a process of masking, milling, measuring, and remilling with steps repeated as necessary. Planned processing is the key to production of integrally stiffened structures milled so that optimum support of stresses is attained without the use of stiffening by attachment, welding, or riveting. A level of ability comparable to that required for electroplating is necessary to produce chemically milled parts. Planned processing, solution control, and developed skill in masking and handling of the work are requisite to success. Periods to train personnel, however, are relatively short as compared to training for other precision metal-removal processes. Tooling requirements are simple. Chemicals, tanks, racks, templates, a hoist, hangers, and a few special hand and measuring tools are required. Although chemical milling skill can be acquired without extensive training, it is not feasible to expect to produce the extremes of complexity and precision without an accumulation of considerable experience. However, a number of organizations are available that will produce engineering quality parts on a job shop basis. The processes are well established, commercially, either in or out of the plant.

SPECIFIC ETCHANTS NEEDED VERSATILITY OFFERED The aircraft industry, as an example, utilizes production chemical milling for weight reduction of large parts by means of precise etching. The process is the most economical means of removal of metal from large areas, nonplaner surfaces, or complex shapes. A further advantage is that metal is just as easily removed from fully hardened as from annealed parts. The advantages of chemical milling result from the fact that metal removal takes place


It is anticipated that any metal or alloy can be chemically milled. On the other hand, it does take time to develop a specific process and only those metals can be milled for which an etchant has been developed, tested, and made available. Aluminum alloys have been milled for many years. Steel, stainless steel, nickel alloys, titanium alloys, and superalloys have been milled commercially and a great number of other metals and alloys have been milled experimentally or on a small commercial scale.


It is advantageous to be able to mill a metal without changing the heat-treated condition or temper, as can be done chemically. Defective or nonhomogeneous metal, however, can respond unfavorably. Porous castings will develop holes during milling and mechanically or thermally stressed parts will change in shape as stressed metal is removed. Good-quality metal and controlled heat treating, tempering, and stress relieving are essential to uniformity and reproducibility.

PROCESS CHARACTERISTICS Almost any metal size or shape can be milled; limitations are imposed only by extreme conditions such as complex shapes with inverted pockets that will trap gases released during milling, or very thin metal foil that is too flimsy to handle. Shapes can be milled that are completely impractical to machine. For example, the inside of a bent pipe could easily be reduced in section by chemical removal of metal. This possibility is used to advantage to reduce weight on many difficult-to-machine areas such as webs of forgings or walls of tubing. Thin sections are produced by milling when alternate machining methods are excessively costly and the optimum in design demands thin metal shapes that are beyond commercial casting, drawing, or extruding capabilities. Surface roughness is often reduced during milling from a rough-machined, cast, or forged surface to a semimatte finish. The milled finish may vary from about 30 to 250 µin., depending on the original finish, the alloy, and the etchant. In some instances the production of an attractive finish reduces finishing steps and is a cost advantage. So-called etching that takes place during milling often causes a brightened finish and etchants have been developed that do not result in a loss of mechanical properties.

COMPLEMENT MACHINING Chemical milling has flourished in the aircraft industry where paring away of every ounce of weight is important. It has spread to instrument industries where weight or balance of working parts is important to the forces


required to initiate and sustain motion. It has also become a factor in the design of modern weight-limited portable equipment. A realistic appraisal of the limitations and advantages of the process is essential to optimum designs. The best designs result from complementing mechanical, thermal, and chemical processes. Chemical milling is not a substitute for mechanical methods but, rather, is more likely as an alternative where machining is difficult or economically unfeasible. It does not compete with low-cost, mass production mechanical methods but, rather, is successful where other methods are limited due to the configuration of the part.

TOLERANCES It is good design practice to allow a complex shape to be manufactured by the most economical combinations of mechanical and chemical means. To allow this, print tolerances must reflect allowances that are necessary to apply chemical milling. Chemical milling will produce less well-defined cuts, radii, and surface finishes. The tolerance of a milled cut will vary with the depth of the cut. For 2.5-mm cut, a tolerance in depth of cut of ±0.10 m is commercial. This must be allowed in addition to the original sheet tolerance. Line definition (deviation from a straight line) is usually ±0.8 mm. Unmilled lands between two milled areas should be 0.004 mm minimum. Greater precision can be had at a premium price. In general, milling rates are about 0.0004 mm/s and depth of cut is controlled by the immersion time. Cuts up to 12.7 mm are not unrealistic although costs should be investigated before designs are made that are dependent on deep cuts.

LIMITATIONS There are limitations to the process. Deep cuts on opposite sides of a part should not be taken simultaneously. One side can be milled at a time but it is less costly to design for one cut rather than two. Complex parts can be made by step milling or by programmed removal of parts to produce tapers. In general, step milling is less expensive and more reliable. Chemical milling

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engineers should be consulted relative to the feasibility and cost of complex design. Very close tolerance parts can be produced by milling, checking, masking, and remilling but such a multiple-step process could be more costly than machining.

CHLORINATED POLYETHER Chlorinated polyether is a thermoplastic resin used in the manufacture of process equipment. Chemically, it is a chlorinated polyether of high molecular weight, crystalline in character, and is extremely resistant to thermal degradation at molding and extrusion temperatures. It possesses a unique combination of mechanical, electrical, and chemical-resistant properties, and can be molded in conventional injection and extrusion equipment.

spectrum of corrosion resistance second only to certain of the fluorocarbons. In steel construction of chemical processing equipment chlorinated polyether liners or coatings on steel substrates provide the combination of protection against corrosion plus structural strength of metal. Electrical Properties Along with the mechanical capabilities and chemical resistance, chlorinated polyether has good dielectric properties. Loss factors are somewhat higher than those of polystyrenes, fluorocarbons, and polyethylenes, but are lower than many other thermoplastics. Dielectric strength is high and electrical values show a high degree of consistency over a range of frequencies and temperatures.

FABRICATION PROPERTIES Chlorinated polyether provides a balance of properties to meet severe operating requirements. It is second only to the fluorocarbons in chemical and heat resistance and is suitable for high-temperature corrosion service. Mechanical Properties A major difference between chlorinated polyether and other thermoplastics is its ability to maintain its mechanical strength properties at elevated temperatures. Heat distortion temperatures are above those usually found in thermoplastics and dimensional stability is exceptional even under the adverse conditions found in chemical plant operations. Resistance to creep is significantly high and in sharp contrast to the lower values of other corrosion-resistant thermoplastics. Water absorption is negligible, assuring no change in molded shapes between wet and dry environments. Chemical Properties Chlorinated polyether offers resistance to more than 300 chemicals and chemical reagents, at temperatures up to 121°C and higher, depending on environmental conditions. It has a


The material is available as a molding powder for injection-molding and extrusion applications. It can also be obtained in stock shapes such as sheet, rods, tubes, or pipe, and blocks for use in lining tanks and other equipment, and for machining gears, plugs, etc. In the form of a finely divided powder it is used in a variety of different coating processes. The material can be injection-molded by conventional procedures and equipment. Molding cycles are comparable to those of other thermoplastics. Rods, sheet, tubes, pipe, blocks, and wire coatings can be readily extruded on conventional equipment and by normal production techniques. Parts can be machined from blocks, rods, and tubes on conventional metalworking equipment. Sheet can be used to convert carbon steel tanks into vessels capable of handling highly corrosive liquids at elevated temperatures. Using a conventional adhesive system and hot gas welding, sheet can be adhered to sandblasted metal surfaces. Coatings of finely divided powder can be applied by several coating processes and offer chemical processors an effective and economical means for corrosion control. Using the fluidized bed process, pretreated, preheated metal parts are dipped in an air-suspended bed of


finely divided powder to produce coatings, which after baking are tough, pinhole free, and highly resistant to abrasion and chemical attack. Parts clad by this process are protected against corrosion both internally and externally.

USES Complete anticorrosive systems are available with chlorinated polyether, and lined or coated components, including pipe and fittings, tanks and processing vessels, valves, pumps, and meters. Rigid uniform pipe extruded from solid material is available in sizes ranging from 12.7 to 50.8 mm in either Schedule 40 or 80, and in lengths up to 6 m. This pipe can be used with injection-molded fittings with socket or threaded connections. Lined tanks and vessels are useful in obtaining maximum corrosion and abrasion resistance in a broad range of chemical exposure conditions. Storage tanks, as well as processing vessels protected with this impervious barrier, offer a reasonably priced solution to many processing requirements. A number of valve constructions can be readily obtained from leading valve manufacturers. Solid injection-molded ball valves, coated diaphragm, and plug valves are among the variety available. Also available are diaphragm valves with solid chlorinated polyether bodies.

CHLORINATED POLYETHYLENE This family of elastomers is produced by the random chlorination of high-density polyethylene. Because of the high degree of chemical saturation of the polymer chain, the most desirable properties are obtained by cross-linking with the use of peroxides or by radiation. Sulfur donor cure systems are available that produce vulcanizates with only minor performance losses compared to that of peroxide cures. However, the free radical cross-linking by means of peroxides is most commonly used and permits easy and safe processing, with outstanding shelf stability and optimum cured properties.


Chlorinated polyethylene elastomers are used in automotive hose applications, premium hydraulic hose, chemical hose, tubing, belting, sheet packing, foams, wire and cable, and in a variety of molded products. Properties include excellent ozone and weather resistance, heat resistance to 149°C (to 177°C in many types of oil), dynamic flexing resistance, and good abrasion resistance.

CHLOROSULFONATED POLYETHYLENE This material, more commonly known as Hypalon, can be compounded to have an excellent combination of properties including virtually total resistance to ozone and excellent resistance to abrasion, weather, heat, flame, oxidizing chemicals, and crack growth. In addition, the material has low moisture absorption, good dielectric properties, and can be made in a wide range of colors because it does not require carbon black for reinforcement. Resistance to oil is similar to that of neoprene. Low-temperature flexibility is fair at –40°C. The material is made by reacting polyethylene with chlorine and SO2 to yield chlorosulfonated polyethylene. The reaction changes the thermoplastic polyethylene into a synthetic elastomer that can be compounded and vulcanized. The basic polyethylene contributes chemical inertness, resistance to damage by moisture, and good dielectric strength. Inclusion of chlorine in the polymer increases its resistance to flame (makes it self-extinguishing) and contributes to its oil and weather resistance.

SELECTION Hypalon is a special-purpose rubber, not particularly recommended for dynamic applications. The elastomer is produced in various types, with generally similar properties. The design engineer can best rely on the rubber formulator to select the appropriate type for a given application, based on the nature of the part, the properties required, the exposure, and the performance necessary for successful use. In combination with properly selected compounding ingredients, the polymer can be extruded, molded, or calendered. In addition, it

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can be dissolved to form solutions suitable for protective or decorative coatings. Initially used in pump and tank linings, tubing, and comparable applications where chemical resistance was of prime importance, this synthetic rubber is now finding many uses where its weatherability, its colorability, its heat, ozone, and abrasion resistance, and its electrical properties are of importance. Included are jacketing and insulation for utility distribution cable, control cable for atomic reactors, automotive primary and ignition wire, and linemen’s blankets. Among heavy-duty applications are conveyor belts for high-temperature use and industrial rolls exposed to heat, chemicals, or abrasion. Interior, exterior, and underhood parts for cars and commercial vehicles are an increasingly important area of use. Representative automotive applications are headliners, window seals, spark plug boots, and tractor seat coverings. Chlorosulfonated polyethylene is used in a variety of mechanical goods, such as V-belts, motor mounts, O-rings, seals, and gaskets, as well as in consumer products like shoe soles and garden hose. It is also used in white sidewalls on automobile tires. In solution, it is used for fluid-applied roofing systems and pool liners, for masonry coatings, and various protective-coating applications. It can also be extruded as a protective and decorative veneer for such products as sealing and glazing strips.

CHROMIUM An elementary metal, chromium (symbol Cr) is used in stainless steels, heat-resistant alloys, high-strength alloy steels, electrical-resistance alloys, wear-resistant and decorative electroplating, and, in its compounds, for pigments, chemicals, and refractories. The specific gravity is 6.92, melting point 1510°C, and boiling point 2200°C. The color is silvery white with a bluish tinge. It is an extremely hard metal; the electrodeposited plates have a hardness of 9 Mohs. It is resistant to oxidation, is inert to HNO3, but dissolves in HCl and slowly in H2SO4. At temperatures above 816°C, it is subject to intergranular corrosion.


Chromium occurs in nature only in combination. Its chief ore is chromite, from which it is obtained by reduction and electrolysis. It is marketed for use principally in the form of master alloys with iron or copper. Most pure chromium is used for alloying purposes such as the production of Ni–Cr or other nonferrous alloys where the use of the cheaper ferrochrome grades of metal is not possible. In metallurgical operations such as the production of low-alloy and stainless steels, the chromium is added in the form of ferrochrome, an electric-arc furnace product that is the form in which most chromium is consumed.

USES Its bright color and resistance to corrosion make chromium highly desirable for plating plumbing fixtures, automobile radiators and bumpers, and other decorative pieces. Unfortunately, chrome plating is difficult and expensive. It must be done by electrolytic reduction of dichromate in H2SO4 solution. It is customary, therefore, to first plate the object with copper, then with nickel, and finally, with chromium.

ALLOYS In alloys with iron, nickel, and other metals, chromium has many desirable properties. Chrome steel is hard and strong and resists corrosion to a marked degree. Stainless steel contains roughly 18% chromium and 8% nickel. Some chrome steels can be hardened by heat treatment and find use in cutlery; still others are used in jet engines. Nichrome and chromel consist largely of nickel and chromium; they have low electrical conductivity and resist corrosion, even at red heat, so they are used for heating coils in space heaters, toasters, and similar devices. Other important alloys are Hastelloy C — chromium, molybdenum, tungsten, iron, nickel — used in chemical equipment that is in contact with HCl, oxidizing acids, and hypochlorite. Stellite — cobalt, chromium, nickel, carbon, tungsten (or molybdenum) — noted for its hardness and abrasion resistance at high temperatures, is used for lathes and engine valves, and Inconel — chromium, iron, nickel — is used in heat


treating and in corrosion-resistant equipment in the chemical industry. Biological Aspects Chromium is essential to life. A deficiency (in rats and monkeys) has been shown to impair glucose tolerance, decrease glycogen reserve, and inhibit the utilization of amino acids. It has also been found that inclusion of chromium in the diet of humans sometimes, but not always, improves glucose tolerance. On the other hand, chromates and dichromates are severe irritants to the skin and mucous membranes, so workers who handle large amounts of these materials must be protected against dusts and mists. Continued breathing of the dusts finally leads to ulceration and perforation of the nasal septum. Contact of cuts or abrasions with chromate may lead to serious ulceration. Even on normal skin, dermatitis fequently results.

CHROMIUM ALLOYS AND STEELS CHROMIUM COPPER A name applied to master alloys of copper with chromium used in the foundry for introducing chromium into nonferrous alloys or to Cu–Cr alloys, or chromium copper alloys, which are high-copper alloys. A Cr–Cu master alloy. Electromet chromium copper, contains 8 to 11% chromium, 88 to 90% copper, and a maximum of 1% iron and 0.50% silicon. Wrought chromium copper alloys are designated C18200, C18400, and C18500, and contain 0.4 to 1.0% chromium. C18200 also contains as much as 0.10% iron, 0.10% silicon, and 0.05% lead. C18400 contains as much as 0.15% iron and 0.10% silicon, and several other elements in small quantities. C18500 is iron-free, and contains as much as 0.015% lead, and several other elements in small quantities. Soft, thus ductile, in the solution-treated condition, these alloys are readily cold-worked and can be subsequently precipitation-hardened. Depending on such treatments, tensile properties range from 241 to 482 MPa ultimate strength, 103 to 427 MPa yield strength, and


15 to 42% in elongation. Electrical conductivity ranges from 40 to 85% that of copper. Chromium copper alloys are used for resistancewelding electrodes, cable connectors, and electrical parts.

CR–MO STEEL This is any alloy steel containing chromium and molybdenum as key alloying elements. However, the term usually refers specifically to steels in the American Iron and Steel Institute (AISI) 41XX series, which contain only 0.030 to 1.20% chromium and 0.08 to 0.35% molybdenum. Chromium imparts oxidation and corrosion resistance, hardenability, and high-temperature strength. Molybdenum also increases strength, controls hardenability, and reduces the tendency to temper embrittlement. AISI 4130 steel, which contains 0.30% carbon, and 4140 (0.40%) are probably the most common and can provide tensile strengths well above 1379 MPa. Many other steels have greater chromium and/or molybdenum content, including high-pressure boiler steels, most tool steels, and stainless steels. Croloy 2, which is used for boiler tubes for high-pressure superheated steam, contains 2% chromium and 0.50% max molybdenum, and is for temperatures to 621°C, and Croloy 5, which has 5% chromium and 0.50% max molybdenum, is for temperatures to 649°C and higher pressures as well as Croloy 7, which has 7% chromium and 0.50% molybdenum.

CHROMIUM STEELS Any steel containing chromium as the predominating alloying element may be termed chromium steel, but the name usually refers to the hard, wear-resisting steels that derive the property chiefly from the chromium content. Straight chromium steels refer to low-alloy steels in the AISI 50XX, 51XX, and 61XX series. Chromium combines with the carbon of steel to form a hard CrC, and it restricts graphitization. When other carbide-forming elements are present, double or complex carbides are formed. Chromium refines the structure, provides deep-hardening, increases the elastic limit, and gives a slight red-hardness so that the

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steels retain their hardness at more elevated temperatures. Chromium steels have great resistance to wear. They also withstand quenching in oil or water without much deformation. Up to about 2% chromium may be included in tool steels to add hardness, wear resistance, and nondeforming qualities. When the chromium is high, the carbon may be much higher than in ordinary steels without making the steel brittle. Steels with 12 to 17% chromium and about 2.5% carbon have remarkable wear-resisting qualities and are used for cold-forming dies for hard metals, for broaches, and for rolls. However, chromium narrows the hardening range of steels unless balanced with nickel. Such steels also work-harden rapidly unless modified with other elements. The high-chromium steels are corrosion resistant and heat resistant but are not to be confused with the high-chromium stainless steels that are low in carbon, although the non-nickel 4XX stainless steels are very definitely chromium steels. Thus, the term is indefinite but may be restricted to the high-chromium steels used for dies, and to those with lower chromium used for wear-resistant parts such as ball bearings. Chromium steels are not especially corrosion resistant unless the chromium content is at least 4%. Plain chromium steels with more than 10% chromium are corrosion resistant even at elevated temperatures and are in the class of stainless steels, but are difficult to weld because of the formation of hard brittle martensite along the weld. Chromium steels with about 1% chromium are used for gears, shafts, and bearings. One of the most widely used bearing steels is AISI 52100, which contains 1.3 to 1.6% chromium. Many other chromium steels have greater chromium content and, often, appreciable amounts of other alloying elements. They are used mainly for applications requiring corrosion, heat, and/or wear resistance.

CR–V STEELS Alloy steel containing a small amount of chromium and vanadium, the latter having the effect of intensifying the action of the chromium and the manganese in the steel and controlling grain


growth. It also aids in formation of carbides, hardening the alloy, and in increasing ductility by the deoxidizing effect. The amount of vanadium is usually 0.15 to 0.25%. These steels are valued where a combination of strength and ductility is desired. They resemble those with chromium alone, with the advantage of the homogenizing influence of the vanadium. A Cr–V steel with 0.92% chromium, 0.20% vanadium, and 0.25% carbon has a tensile strength up to 689 MPa, and when heattreated has a strength up to 1034 MPa and elongation 16%. Cr–V steels are used for such parts as crankshafts, propeller shafts, and locomotive frames. High-carbon Cr–V steels are the mildalloy tool steels of high strength, toughness, and fatigue resistance. The chromium content is usually about 0.80%, with 0.20% vanadium, and with carbon up to 1%.

CLAD METALS Cladding means the strong, permanent bonding of two or more metals or alloys metallurgically bonded together to combine the characteristic properties of each in composite form. Copperclad steel, for example, is used to combine the electrical and thermal characteristics of copper with the strength of steel. A great variety of metals and alloys can be combined in two or more layers, and they are available in many forms, including sheet, strip, plate, tubing, wire, and rivets for application in electrical and electronic products, chemical-processing equipment, and decorative trim, including auto trim; see Figure C.1.

CLADDING PROCESSES In the process a clad metal sheet is made by bonding or welding a thick facing to a slab of base metal; the composite plate is then rolled to the desired thickness. The relative thickness of the layers does not change during rolling. Cladding thickness is usually specified as a percentage of the total thickness, commonly 10%. Other cladding techniques, including a vacuum brazing process, have been developed. The pack rolling process is still the most widely used, however.


sheet or strip (clad on one side or both sides)

bar or wire (any size) tubing (clad inside or outside)

inlay

stripe

edgelay

FIGURE C.1 Types of cladding. (From McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 3, McGraw-Hill, New York, 738. With permission.)

ALLOYS Generally speaking, the choice of alloys used in cladding is dictated by end use requirements such as corrosion, abrasion, or strength. Cladding supplies a combination of desired properties not found in any one metal. A base metal can be selected for cost or structural properties, and another metal added for surface protection or some special property such as electrical conductivity. Thickness of the cladding can be made much heavier and more durable than obtainable by electroplating. Combinations The following clad materials are in common use: • Stainless steel on steel. Provides corrosion resistance and attractive surface at low cost for food display cases, chemical-processing equipment, sterilizers, and decorative trim. • Stainless steel on copper. Combines surface protection and high thermal conductivity for pots and pans, and for heat exchangers for chemical processes. • Copper on aluminum. Reduces cost of electrical conductors and saves copper on appliance wiring.


• Copper on steel. Adds electrical conductivity and corrosion resistance needed in immersion heaters and electrical switch parts; facilitates soldering. • Nickel or Monel on steel. Provides resistance to corrosion and erosion for furnace parts, blowers, chemical equipment, toys, brush ferrules, and many mechanical parts in industrial and business machines; more durable than electroplating. • Titanium on steel. Supplies hightemperature corrosion resistance. Bonding requires a thin sheet of vanadium between titanium and steel. • Bronze on copper. Usually clad on both sides, for current-carrying springs and switch blades; combines good electrical conductivity and good spring properties. • Silver on copper. Provides oxidation resistance to surface of conductors, for high-frequency electrical coils, conductors, and braiding. • Silver on bronze or nickel. Adds current-carrying capacity to low-conductivity spring material; cladding sometimes is in form of stripes or inlays with silver areas serving as built-in electrical contacts. • Gold on copper. Supplies chemical resistance to a low-cost base metal for chemical processing equipment. • Gold on nickel or brass. Adds chemical resistance to a stronger base metal than copper; also used for jewelry, wristbands, and watchcases.

APPLICATIONS Gold-filled jewelry has long been made by the cladding process: the surface is gold, the base metal bronze or brass with the cladding thickness usually 5%. The process is used to add corrosion resistance to steel and to add electrical or thermal conductivity, or good bearing properties, to strong metals. One of the first industrial applications was the use of a nickel-clad steel plate for a railroad tank car to transport

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caustic soda; stainless clad steels are used for food and pharmaceutical equipment. Corrosionresistant pure aluminum is clad to a strong duralumin base, and many other combinations of metals are widely used in cladding; there is also a technique for cladding titanium to steel for jet engine parts. Today’s coinage uses clad metals as a replacement for rare silver. Dimes and quarters have been minted from composite sheet consisting of a copper core with Cu–Ni facing. The proportion of core and facing used duplicates the weight and electrical conductivity of silver so the composite coins are acceptable in vending machines. A three-metal composite sole plate for domestic steam irons provides a thin layer of stainless steel on the outside to resist wear and corrosion. A thick core of aluminum contributes thermal conductivity and reduces weight, and a thin zinc layer on the inside aids in bonding the sole plate to the body of the iron during casting. Clad metals have been applied in nuclear power reactor pressure vessels in submarines as well as in civilian power plants. Other applications where use of clad is increasing are in such fields as fertilizer, chemicals, mining, food processing, and even seagoing wine tankers. Producers see a market for clad metal curtain wall building panels, and even stainless clad bus and automobile bumpers.

CLAY Clay is composed of naturally occurring sediments that are produced by chemical actions resulting during the weathering of rocks. Often clay is the general term used to identify all earths that form a paste with water and harden when heated. The primary clays are those located in their place of formation. Secondary clays are those that have been moved after formation to other locations by natural forces, such as water, wind, and ice. Most clays are composed chiefly of SiO2 and Al2O3. Clays are used for making pottery, tiles, brick, and pipes, but more particularly the better grade of clays are used for pottery and molded articles not including the fireclays and fine porcelain clays. Kaolins are the purest forms of clay.


The fineness of the grain of a clay influences not only its plasticity but also such properties as drying performance, drying shrinkage, warping, and tensile, transverse, and bonding strength. For example, the greater the proportion of fine material, the slower the drying rate, the greater the shrinkage, and the greater the tendency to warping and cracking during this stage. Clays with a high fines content usually are mixed with coarser materials to avoid these problems. For two clays with different degrees of plasticity, the more plastic one will require more water to make it workable, and water loss during drying will be more gradual because of its more extensive capillary system. The high-plasticity clay also will shrink more and will be more likely to crack. The most important clays in the pottery industry are the ball clays and china clays (kaolin).

COMMERCIAL CLAY Commercial clays, or clays utilized as raw material in manufacturing, are among the most important nonmetallic mineral resources. The value of clays is related to their mineralogical and chemical composition, particularly the clay mineral constituents kaolinite, montmorillonite, illite, chlorite, and attapulgite. The presence of minor amounts of mineral or soluble salt impurities in clays can restrict their use. The more common mineral impurities are quartz, mica, carbonates, iron oxides and sulfides, and feldspar. In addition, many clays contain some organic material.

MINING

AND

PROCESSING

Almost all the commercial clays are mined by open-pit methods, with overburden-to-clay ratios ranging as high as 10 to 1. The overburden is removed by motorized scrapers, bulldozers, shovels, or draglines. The clay is won with draglines, shovels, or bucket loaders, and transported to the processing plants by truck, rail, aerial trainways, or belt conveyors, or as slurry in pipelines. The clay is processed dry or, in some cases, wet. The dry process usually consists of crushing, drying, and pulverizing. The clay is


crushed to egg or fist size or smaller and dried usually in rotary driers. After drying, it is pulverized to a specified mesh size such as 90% retained on a 200-mesh screen with the largest particle passing a 30-mesh screen. In other cases, the material may have to be pulverized to 99.9% finer than 325 mesh. The material is shipped in bulk or in bags. All clays are produced by this method.

PROPERTIES Most clays become plastic when mixed with varying proportions of water. Plasticity of a materal can be defined as the ability of the material to undergo permanent deformation in any direction without rupture under a stress beyond that of elastic yielding. Clays range from those that are very plastic, called fat clay, to those that are barely plastic, called lean clay. The type of clay mineral, particle size and shape, organic matter, soluble salts, adsorbed ions, and the amount and type of nonclay minerals all affect the plastic properties of a clay. Strength Green strength and dry strength properties are very important because most structural clay products are handled at least once and must be strong enough to maintain shape. Green strength is the strength of the clay material in the wet, plastic state. Dry strength is the strength of the clay after it has been dried. Shrinkage Both drying and firing shrinkages are important properties of clay used for structural clay products. Shrinkage is the loss in volume of a clay when it dries or when it is fired. Drying shrinkage is dependent on the water content, the character of the clay minerals, and the particle size of the constituents. Drying shrinkage is high in most very plastic clays and tends to produce cracking and warping. It is low in sandy clays or clays of low plasticity and tends to produce a weak, porous body.


Color Color is important in most structural clay products, particularly the maintenance of uniform color. The color of a product is influenced by the state of oxidation of iron, the state of division of the Fe minerals, the firing temperature and degree of vitrification, the proportion of Al2O3, lime, and MgO in the clay material, and the composition of the fire gases during the burning operation. Uses All types of clay and shale are used in the structural products industry but, in general, the clays that are used are considered to be relatively low grade. Clays that are used for conduit tile, glazed tile, and sewer pipe are underclays and shales that contain large proportions of kaolinite and illite. Clays used for brick and drain tile must be plastic enough to be shaped. In addition, color and vitrification range are very important. For common brick, drain tile, and terra-cotta, shales and surface clays are usually suitable, but for high-quality face bricks, shales and underclays are used.

COATINGS Plastic, metal, or ceramic coatings can be applied to the surface of a material in a variety of ways to achieve desired properties. Coatings improve appearance, corrosion resistance, abrasion resistance, and electrical or optical properties. They can be applied by wet or dry techniques, with simple or complex equipment. The choices are almost limitless because almost any coating material offers some degree of protection as long as it retains its integrity. If it provides a continuous barrier between the substrate and the environment, even a thin, decorative coating can do the job in a relatively dry and mild environment.

METAL COATINGS Many new materials have been developed, but steel remains the principal construction material for automobiles, appliances, and industrial machinery. Because of the vulnerability of steel

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to attack by aggressive chemical environments or even from simple atmospheric oxidation, coatings are necessary to provide various degrees of protection. They range from hotdipped and electroplated metals to tough polymers and flame-sprayed ceramics. In general, corrosive environments contain more than one active material, and the coating must resist penetration by a combination of oxidizers, solvents, or both. Thus, the best barrier is one that resists “broadband” corrosion. Physical integrity of the coating is as important as its chemical barrier properties in many applications. For instance, coatings on impellers that mix abrasive slurries can be abraded quickly; coatings on pipe joints will cold-flow away from a loaded area if the creep rate is not low; and coatings on flanges and support brackets can be chipped or penetrated during assembly if impact strength is inadequate. Selecting the best coating for an application requires evaluating all effects of the specific environment, including thermal and mechanical conditions. Zinc One of the most common and inexpensive protection methods for steel is provided by zinc. Zinc-coated, or galvanized, steel is produced by various hot-dipping techniques, but more steel companies have moved into electrogalvanizing so they can provide both. Oxidation protection of steel by zinc operates in two ways — first as a barrier coating, then as a sacrificial coating. If the zinc coating is scratched or penetrated, it continues to provide protection by galvanic action until the zinc layer is depleted. This sacrificial action also prevents corrosion around punched holes and at cut edges. The grades of zinc-coated steel commercialized in recent years have been designed to overcome the drawbacks of traditional galvanized steel, which has been difficult to weld and to paint to a smooth finish. The newer materials are intended specifically for stamped automotive components, which are usually joined by spot welding and which require a smooth, Class A painted finish.


Aluminum Two types of aluminium-coated steel are produced, each a different kind of corrosion protection. Type I has a hot-dipped Al–Si coating to provide resistance to both heat and corrosion. Type 2 has a hot-dipped coating of commercially pure aluminium, which provides excellent durability and protection from atmospheric corrosion. Both grades are usually used unpainted. Type 1 aluminium-coated steel resists heat scaling to 677°C and has excellent heat reflectivity to 482°C. Nominal aluminium-alloy coating is about 1 mil on each side. The sheet is supplied with a soft, satiny finish. Typical applications include reflectors and housings for industrial heater panels, interior panels and heat exchangers for residential furnaces, microwave ovens, automobile and truck muffler systems, heat shields for catalytic converters, and pollution-control equipment. Type 2 aluminized steel, with an aluminum coating of about 1.5 mil on each side, resists atmospheric corrosion and is claimed to outlast zinc-coated sheet in industrial environments by as much as five to one. Typical applications are industrial and commercial roofing and siding, drying ovens, silo roofs, and housings for outdoor lighting fixtures and air conditioners. Electroplating Use of protective electroplated metals has changed in recent years, mainly because of rulings by the Environmental Protection Agency (EPA). Cyanide plating solutions and cadmium and lead-bearing finishes are severely restricted or banned entirely. Chromium and nickel platings are much in use, however, applied both by conventional electroplating techniques and by new, more efficient methods such as fast rate electrodeposition (FRED), which has also been used successfully to deposit stainless steel on ferrous substrates. Functional chromium, or “hard chrome,” plating is used for antigalling and low-friction characteristics as well as for corrosion protection. These platings are usually applied without copper or nickel underplates in thicknesses from about 0.3 to 2 mil. Hard-chrome plating


is recommended for use in saline environments to protect ferrous components. Nickel platings, in thickness from 0.12 to 3 mil, are used in food-handling equipment, on wear surfaces in packaging machinery, and for cladding in reaction vessels. Electroless nickel plating, in contrast to conventional electroplating, operates chemically instead of using an electric current to deposit metal. The electroless process deposits a uniform coating regardless of substrate shape, overcoming a major drawback of electroplating — the difficulty of uniformly plating irregularly shaped components. Conforming anodes and complex fixturing are unnecessary in the electroless process. Deposit thickness is controlled simply by controlling immersion time. The deposition process is autocatalytic, producing thicknesses from 0.1 to 5 mil. Proprietary electroless-plating systems contain, in addition to nickel, elements such as phosphorus, boron, and/or thallium. A relatively new composition, called the polyalloy, features three or four elements in the bath. These products are claimed to provide superior wear resistance, hardness, and other properties, compared with those of generic electrolessplating methods. One polyalloy contains nickel, thallium, and boron. Originally developed for aircraft gas turbine engines, it offers excellent wear resistance. Comparative tests show that relative wear for a polyalloy-coated part is significantly less than that for hard chromium and Ni–P coatings. In general, Ni–B coatings are nodular. As coating thickness increases, nodule size also increases. Because the columnar structure of the coating flexes as the substrate moves, Ni–B resists chipping and wear. Adhesion quality depends on factors such as substrate material, part preparation, and contamination. Although it is excellent for tool steels, stainless steel, high-performance nickeland cobalt-base alloys, and titanium, a few metal substrates are not compatible. These include metals with high zinc or molybdenum content, aluminum, magnesium, and tungsten carbide (WC). Modifications can, however, eliminate this incompatibility. Another trend in composite electroless plating appears to be toward codeposition of


particulate matter within a metal matrix. These coatings are commercially available with just a few types of particulates — diamond, SiC, Al2O3, and polytetrafluoroethylene (PTFE) — with diamond heading the list in popularity. These coatings can be applied to most metals, including iron, carbon steel, cast iron, aluminum alloys, copper, brass, bronze, stainless steel, and high-alloy steels. Conversion Coatings Electroless platings are more accurately described as conversion coatings, because they produce a protective layer or film on the metal surface by means of a chemical reaction. Another conversion process, the black oxide finish, has been making progress in applications ranging from fasteners to aerospace. Black oxide is gaining in popularity because it provides corrosion resistance and aesthetic appeal without changing part dimensions. On a chemical level, black oxiding occurs when the Fe within the surface of the steel reacts to form magnetite (Fe3O4). Processors use inorganic blackening solutions to produce the reaction. Oxidizing salts are first dissolved in water, then boiled and held at 138 to 140°C. The product surface is cleaned in an alkaline soak and then rinsed before immersion in the blackening solution. After a second rinse, the finish is sealed with rust preventatives, which can produce finishes that vary from slightly oily to hard and dry. Black oxiding produces a microporous surface that readily bonds with a topcoat. For example, a supplemental oil topcoat can be added to boost salt-spray resistance to the same level as that of zinc plate with a clear chrome coating (100 to 200 h). Black oxide can be used with mild steel, stainless steel, brass, bronze, and copper. As long as parts are scale free and do not require pickling, the finish will not produce H2 embrittlement or change part dimensions. Operating temperatures range from cryogenic to 538°C. Chromate conversion coatings are formed by the chemical reaction that takes place when certain metals are brought in contact with acidified aqueous solutions containing basically water-soluble chromium compounds in

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addition to other active radicals. Although the majority of the coatings are formed by simple immersion, a similar type of coating can be formed by an electrolytic method. Protective chromate conversion coatings are available for zinc and zinc alloys, cadmium, aluminum and aluminum alloys, copper and copper alloys, silver, magnesium and magnesium alloys. The appearance and protective value of the coatings depends on the base metal and on the treatment used. Chromate conversion coatings both protect metals against corrosion and provide decorative appeal. They also have the characteristics of low electrical resistance, excellent bonding characteristics with organic finishes, and can be applied easily and economically. For these reasons the coatings have developed rapidly, and they are now one of the most commonly used finishing systems. They are particularly applicable where metal is subjected to storage environments such as high humidity, salt, and marine conditions. The greatest majority of chromate conversion coatings are supplied as proprietary materials and processes. These are available usually as liquid concentrates or powdered compounds that are mixed with water. In the case of the powdered compounds, they are often adjusted with additions of acid for normal operation. Chromate conversion coatings are formed immersing the metal in an aqueous acidified chromate solution consisting substantially of chromic acid or water-soluble salts of H2CrO3 together with various catalysts or activators. The chromate solutions, which contain either organic or inorganic active radicals or both, must be acid and must be operated within a prescribed pH range. Maximum corrosion protection is obtained by using drab or dark bronze coatings on zinc and cadmium surfaces, and yellow to browncolored coatings on the other metals. Lighter iridescent yellow type coatings generally provide medium protection, and the clear-bright type coatings, produced either in one dip or by leaching, provide the least protection. Chromate conversion coatings provide maximum corrosion protection in salt spray or marine types of environment, and in high


humidity such as encountered in storage, particularly where stale air with entrapped water may be present. They also provide excellent protection against tarnishing, staining, and finger marking, or other conditions that normally produce surface oxidation. Olive drab type coatings are widely used on military equipment because of their high degree of corrosion protection coupled with a nonreflective surface. Iridescent yellow coatings are widely used for corrosion protection where appearance is not a deciding factor. The clear-bright chemically polishing type coatings for zinc and cadmium have been widely used to simulate nickel and chromium electroplate and are primarily used for decorative appeal rather than corrosion protection. Where additional corrosion protection or abrasion resistance is desired, these clear coatings act as an excellent base for a subsequent clear organic finish. Heavy olive drab and yellow coatings for zinc, cadmium, and aluminum can be dyed various colors. Generally speaking, the dyed colors are used for identification purposes only since they are not lightfast and will fade upon exposure to direct sunlight or other sources of ultraviolet. Because of their low electrical resistance, chromate conversion coatings are widely used for electronics equipment. Surface resistance depends on the type and thickness of the film deposited, the pressure exerted at the contact, and the nature of the contact. Low-resistance coatings are particularly important on aluminum, silver, magnesium, and copper surfaces. Chromate conversion coatings can also be soldered and welded. A chromate coating on aluminum, for example, facilitates heliarc welding. Because of the slight increase in electrical resistance, an adjustment in current (depending upon the thickness of the coating) must be made to satisfactorily spot-weld. Soldering, using rosin fluxes, can be performed on cadmium-plated surfaces that have been treated with clear bright chromate conversion coatings. Clear, bright coatings on zinc-plate surfaces and colored coatings on both zinc and cadmium necessitate the use of an acid flux or removal of the film by an increase in soldering iron


temperature, which burns through the coating, or by mechanical abrasion, which removes the film and provides a clean metal surface for the soldered joint. Most chromate conversion treatments are applied by simple immersion in an acidified chromate solution. Because no electrical contacts need be made during immersion, the coatings can be applied by rack, bulk, or strip line operation. Under special situations, swabbing or brush coating can be used where small areas must be coated, as in a touch-up operation. Chromate conversion coatings can also be applied by an electrolytic method in which the electrolyte is composed essentially of watersoluble chromium compounds and other radicals operated at neutral or slightly alkaline pH. This type of application is limited primarily to rack-type operation. In general, processing can be placed in two categories: (1) over freshly electroplated surfaces; and (2) over electroplated surfaces that have been aged or oxidized, or other metal surfaces such as zinc die castings, wrought metals, or hot-dipped surfaces. Sputtering Formerly used primarily to produce integratedcircuit components, sputtering has moved on to large, production-line jobs such as “plating” of automotive trim parts. The process deposits thin, adherent films, usually of metal, in a plasma environment on virtually any substrate. Sputtering offers several advantages to automotive manufacturers for an economical replacement for conventional chrome plating. Sputtering lines are less expensive to set up and operate than plating systems. And because sputtered coatings are uniform as well as thin, less coating material is required to produce an acceptable finish. Also, pollution controls are unnecessary because the process does not produce any effluents. Finally, sputtering requires less energy than conventional plating systems. Chrome coating of plastics and metals is only one application for sputtering. The technique is not limited to depositing metal films. PTFE has successfully been sputtered on metal, glass, paper, and wood surfaces. In another application, cattle bone was sputtered


on metallic prosthetic devices for use as hipbone replacements. The sputtered bone film promotes bone growth and attachment to living bone. Sputtering is the only deposition method that does not depend on melting points and vapor pressures of refractory compounds such as carbides, nitrides, silicides, and borides. As a result, films of these materials can be sputtered directly onto surfaces without altering substrate properties. Much sputtering has been aimed at producing solid-film lubricants and hard, wear-resistant refractory compounds. NASA is interested in these tribological applications because coatings can be sputter-deposited without a binder, with strong adherence, and with controlled thickness on curved and complex-shaped surfaces such as gears and bearing retainers, races, and balls. Also, because sputtering is not limited by thermodynamic criteria (unlike most conventional processes that involve heat input), film properties can be tailored in ways not available with other deposition methods. Most research on sputtered solid-lubricant films has been done with MoS2. Other films that have been sputtered are WC, TiN, PbO2, gold, silver, tin, lead, indium, cadmium, PTFE, and polyimide (PI). Of these coatings, the gold-colored TiN coatings are most prominent. TiN coatings are changing both the appearance and performance of high-speed steel metal-cutting tools. Life of TiN-coated tools, according to producers’ claims, increases by as much as tenfold, metal-removal rates can be doubled, and more regrinds are possible before a tool is discarded or rebuilt. Sputter Coating Process The SCX™ sputter coating process, a proprietary, computer-aided process developed by Engelhard-CLAL, Carteret, NJ, a producer of high-purity materials, enables the coating of base or refractory metals with precious metals. The source of the coating material can be almost any metallic composition. A major benefit of sputtering is the ability to deposit alloys or compounds that cannot be mechanically worked or alloyed as is required in the cladding process. By fabricating a segmented target comprising of two or more individual elements, a

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deposition can be made that is a uniformly dispersed “alloy” of the constituents. SCX sputtering is conducted at low temperatures (1014 120–180

>1014 150–190

1012–1017 45–120

1013–1016 150–300

1015 150–180


dissimilar materials can be supplied either as one- or two-part systems. One-part systems require heat for curing; two-part systems usually cure at room temperature, but properties are improved when the materials are heat-cured. Some epoxy adhesive systems can withstand temperatures to 232°C, although properties at such temperatures are considerably lower than at room temperature.


APPLICATIONS • Casting resins are primarily used for potting or encapsulating electrical or electronic equipment. The excellent adhesion and extremely low shrinkage of epoxies, coupled with their high dielectric properties, provide a

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well-sealed, voidless, well-insulated component. Foaming resins have also been used for electronic potting. Liquid resins are also formulated with a variety of fillers, such as metal powder, to provide effective patching and repair putties or pastes. Such compounds can be used to patch both metal and plastic surfaces. Liquid resin systems are used to produce low-pressure reinforced laminates and moldings, high-pressure industrial thermosetting laminates (NEMA Grade G-10 glass clothbase, as well as paper-based laminates for electrical uses), and filament wound shapes. Epoxy laminates are also widely used in plastic tools. They are used for drilling, checking, and locating fixtures, where dimensional accuracy is critical. They are also used to provide durable surfaces for metalforming tools, such as draw dies for short-run production. Filament wound structures are used for such applications as rocket-motor booster cases, pressure vessels, and chemical tanks and pipe. Molding compounds provide the performance characteristics of epoxy resins with the automated speed and economy of compression and transfer molding. They are being used primarily for electrical components.

HANDLING

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RESIN SELECTION

Because the cure of an epoxy is brought about by a chemical reaction, and not by a simple process of solvent evaporation, it is quite important that users recognize at the outset that handling (particularly for the two-component, lowtemperature curing compounds) requires more care than that of older materials. However, because this is, and probably will be, a problem for some time to come, procedures have been developed that keep difficulties in handling to a minimum. One simple and inexpensive way of eliminating frequent weighing is to use cal-


ibrated mixing containers where it is necessary only to fill to the first calibration with the base, to the second with the hardener, mix, and apply. Formulators meet this problem by supplying the compounds packaged in preweighed containers, while equipment manufacturers use automatic mixers and dispensers. It is possible through the use of these machines to eject preweighed and premixed shots of compound as the operator requires. One-component systems, as the name indicates, eliminate problems of weighing, mixing, and pot life, and are subdivided into solids, most often “B” staged epoxy powders, and liquids containing latent hardeners such as boron trifloride, or anhydride/solvent solutions. Powders have been used to some extent as adhesives and in potting, but are employed most widely for the fluidized-bed coating method or in compression and transfer molding. The epoxy fluidized-bed approach provides a relatively easy way of applying encapsulant coatings in uniform thickness over contours but is not suitable for impregnation, or for coating units that cannot be heated. In resin selection, epoxy compounds can be categorized (although there are exceptions) as (1) two-component liquids that will cure at temperatures from 21.1 to 60°C, but that have short pot lives; (2) two-component liquid systems requiring cure at temperatures up to 177°C, but that offer longer pot lives and improved operational characteristics; and (3) one-component liquids and powders that reduce handling problems, offer optimum operating characteristics, but that require higher curing temperatures (in the range of 177 to 204°C). With respect to possible methods of application, with the exception of the powders, epoxies are supplied for use by spray, brush, spatula, roller coat, knife coat, dipping, filament winding, laminating, and casting. All these except casting are self-descriptive and this is simply the method of producing a defined shape by pouring a compound into a mold where it cures and from whence it can later be removed. On the other hand, if an object has been placed in the mold and epoxy cast around it, the process is called potting.

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ETCHING MATERIALS

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These are chemicals, usually acids, employed for cutting into, or etching, the surface of metals, glass, or other material. In the metal industries they are called etchants. The usual method of etching is to coat the surface with a wax, asphalt, or other substance not acted upon by the acid; cut the design through with a sharp instrument; and then allow the acid to corrode or dissolve the exposed parts. For etching steel, a 25% solution of H2SO4 in water or an FeCl3 solution may be used. For etching stainless steels a solution of FeCl3 and HCl in water is used. For high-speed steels, brass, or nickel, a mixture of HNO3 and HCl acids in water solution is used, or nickel may be etched with a 45% solution of H2SO4. Copper may be etched with a solution of HcrO3 acid. Brass and nickel may be etched with an acid solution of FeCl3 and KClO3. For red brasses, deep etching is done with concentrated HNO3 acid mixed with 10% HCl acid, with the latter added to keep the SnO2 in solution and thus retain a surface exposed to the action of the acid. For etching aluminum a 9% solution of CuCl3 in 1% acetic acid, or a 20% solution of FeCl3 may be used, followed by a wash with strong HNO3 acid. NaOH, NH4OH, or any alkaline solutions are also used for etching aluminum. Zinc is preferably etched with weak HNO 3 acid, but requires a frequent renewal of the acid. Strong acid is not used because of the heat generated, which destroys the wax coating. A 5% solution of HNO 3 acid will remove 0.005 cm of zinc/min, compared with the removal of over 0.013 cm/min in most metal-etching processes. Glass is etched with HF acid or with white acid. White acid is a mixture of HF acid and ammonium bifluoride, a white crystalline material of the composition (NH4)FHF. The process in which the metal is removed chemically to give the desired finish as a substitute for mechanical machining is called chemical machining.

ETHER Ether is the common name for ethyl ether, or diethyl ether, a highly volatile, colorless liquid of the composition (C2H5)2O made from ethyl © 2002 by CRC Press LLC

alcohol. It is used as a solvent for fats, greases, resins, and nitrocellulose, and in medicine as an anesthetic. The specific gravity is 0.720, boiling point 34.2°C, and freezing point –116°C. Its vapor is heavier than air and is explosive. Recently, the promotion for the production and use of cleaner-burning fuels was announced. As a result, methyl tertiary butyl ether (MTBF) became a very important petrochemical. MTBF is the most widely produced ether for oxygenates. It is commonly produced by the dehydrogenation of isobutane and the subsequent reaction of isobutylene with methanol.

ETHYLENE Ethylene, also called ethane, is a colorless, inflammable gas, CH2:CH2, produced in the cracking of petroleum. Ethylene liquefies at –68.2°C. Ethylene is the largest-volume organic chemical produced today, and is the basic building block of the petrochemical industry. Polymerization of ethylene is its largest use. When ethylene is reacted in the presence of transition metal catalysts, such as Mo2O5 or Cr2O3, at high pressures, it forms low-density polyethylene (LDPE); at lower pressures, high-density polyethylene (HDPE) is produced. Recently, low pressures have been employed for making a new variant, linear low-density polyethylene (LLDPE). Ethylene is now used to produce ethyl alcohol, acrylic acid, and styrene, and it is the basis for many types of reactive chemicals. Trichloroethylene is a colorless liquid of pleasant odor of the composition CHCl:CCl2, also known as westrosol. Its boiling point is 87°C and its specific gravity 1.471. It is insoluble in water and is unattacked by dilute acids and alkalis. It is non flammable and is less toxic than tetrachlorethane. Trichloroethylene is a powerful solvent for fats, waxes, resins, rubber, and other organic substances, and is employed for the extraction of oils and fats, for cleaning fabrics, and for degreasing metals preparatory to plating. The freezing point is –88°C, and it is also used as a refrigerant. It is also used in soaps employed in the textile industry for degreasing.


ETHYLENE GLYCOL This substance, also known as glycol and ethylene alcohol, is a colorless syrupy liquid CH2OHCH2OH, with a sweetish taste, very soluble in water. It has a low freezing point, –25°C, and is much used as an antifreeze in automobiles. A 25% solution has a freezing point of –20.5°C, without appreciably lowering the boiling point of the water. It has the advantage over alcohol that it does not boil away easily, and permits the operation of the engines at much higher temperatures than with water, giving greater fuel efficiency. Ethylene glycol is also used for the manufacture of acrylonitrile fibers, and as a solvent for nitrocellulose. It is highly toxic in contact with the skin.

ETHYLENE-PROPYLENE ELASTOMER Ethylene-propylene elastomer is a completely saturated copolymer made by solution polymerization. The remarkable properties of the material include exceptional ozone resistance, excellent electrical properties, good high- (149 to 163°C) and low-temperature properties, good stress–strain characteristics, and resistance to chemicals, light, and other types of aging. Ethylene-propylene rubber has required a peroxide or peroxide-sulfur modified curing system. Sulfur improves the peroxide curing efficiency and assists in chemical cross-linking of the polymer chains, thereby imparting better physical properties to the vulcanizate. Some plasticizers used in other rubbers are not suitable for ethylene-propylene rubber. Most acceptable plasticizers are saturated materials of relatively low polarity such as paraffinic hydrocarbon oils and waxes.

EXTRUDED METALS Extrusion is the forcing of solid metal through a suitably shaped orifice under compressive forces. Extrusion is somewhat analogous to squeezing toothpaste through a tube, although some cold extrusion processes more nearly resemble forging, which also deforms metals by application of compressive forces. Most metals can be extruded, although the process


may not be economically feasible for highstrength alloys.

HOT EXTRUSION The most widely used method for producing extruded shapes is the direct, hot extrusion process. In this process, a heated billet of metal is placed in a cylindrical chamber and then compressed by a hydraulically operated ram; see Figure E.2. The opposite end of the cylinder contains a die having an orifice of the desired shape; as this die opening is the path of least resistance for the billet under pressure, the metal “squirts” out of the opening as a continuous bar with the same cross-sectional shape as the opening. By using two sets of dies, stepped extrusions can be made. The small section is extruded to the desired length, the small split die is replaced by the large die, and the large section is then extruded. The most outstanding feature of the extrusion process is its ability to produce a wide variety of section configurations. Structural shapes can be extruded that have complex nonuniform and nonsymmetrical sections that would be difficult or impossible to roll. In many instances, extrusions can replace bulky assemblies made up by joining, welding, or riveting rolled structural shapes, or sections previously machined from bar, plate, or pipe. An extrusion die is relatively simple to make and inexpensive when compared to a pair of rolls or a set of forging dies. The low cost of dies and the short lead time for die changes cylinder

die ram

billet extrusion

dummy block FIGURE E.2 Schematic representation of the direct extrusion process (hot). (From McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 7, McGraw-Hill, New York, 1997, 651. With permission.)

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make it possible to extrude small quantities more economically than by most other methods. Lubricants are used to minimize friction and protect the die surfaces. Graphite is a common lubricant for nonferrous alloys, whereas for hot extrusion of steel, glass is an excellent lubricant. Indirect, or inverted, extrusion was developed to overcome such difficulties as surface friction and entrainment of surface oxide of direct extrusion. In the indirect process the ram is hollow, the die opening is in the dummy block, and the opposite end of the cylinder is closed. As the ram advances, the billet does not move as in the case of direct extrusion, and the metal is extruded backward through the die and the hollow ram. However, the process is not very popular because the hollow ram is weaker, resulting in lower machine capacity; troublefree operation requires that the extruded product be straight and not hit the inside of the ram.

COLD EXTRUSION The extrusion of cold metal is variously termed cold pressing, cold forging, cold extrusion forging, extrusion pressing, and impact extrusion. The term cold extrusion has become popular in the steel fabrication industry, while impact extrusion is more widely used in the nonferrous field. The original process (identified as impact extrusion) consists of a punch (generally moving at high velocity) striking a blank (or slug) of the metal to be extruded, which has been placed in the cavity of a die. Clearance is left between the punch and the die walls; as the punch comes in contact with the blank, the metal has nowhere to go except through the annular opening between punch and die. The punch moves a distance that is controlled by a press setting. This distance determines the base thickness of the finished part. The process is particularly adaptable to the production of thinwalled, tubular-shaped parts with thick bottoms, such as toothpaste tubes. Advantages of cold extrusion are high strength because of severe strain-hardening, good finish and dimensional accuracy, and economy due to fewer operations and minimum of machining required.


METALS EXTRUDED Extrusion can be used to fabricate practically all structural metals and alloys. Among the more common metals extruded on a commercial or semi-commercial basis are alloys in the following metal systems: Magnesium Aluminum Brass Copper Titanium Zirconium Beryllium Nickel

Carbon and alloy steels Stainless steel Iron superalloys Nickel superalloys Columbium or Niobium Molybdenum Tantalum Tungsten

These materials span a range of working temperatures from about 316 to 2205°C, in approximately the order shown above. The wide range of extrusion temperatures gives rise to the major differences in processing that center around such variables as extrusion lubricants, die materials, die design, billet preparation, and extrusion speed. Present tool materials are capable of maintaining adequate strength and wear resistance for extrusion at temperatures only slightly higher than 538°C. At higher temperatures, lubricants are necessary not only to reduce friction but to insulate and protect the tooling surface from overheating. Also, the speed of extrusion must be more rapid to avoid prolonged contact between the tools and the hot billet. Thus, the extrusion method for magnesium and aluminum is quite different from that for the other metal systems. The Light Alloys Magnesium and aluminum are extruded at temperatures below 538°C with no lubrication and flat, sharp-cornered dies. Deformation of the billet occurs by shear flow, which is from within the billet so that the surface skin of the billet is retained in the container as discard. This type of turbulent flow is possible because of the ability of these materials to form sound welds when severely deformed, but requires comparatively slow pressing speeds, often less than 1.66 m/min. With clean tools and no lubricants there are no contaminants present to cause internal


defects or laminations, and several sections can be extruded at one time by using multihole dies. Precise dimensional control is attained with ordinary hot-work tool-steel dies, which last for hundreds of extrusions. Other Metals With higher-temperature materials, e.g., titanium, steels, refractory metals, it is necessary to use lubricants and die designs so that deformation occurs by uniform flow. In this case, the surface of the billet becomes the surface of the extrusion; otherwise, laminations and inclusions could occur. Graphitic lubricants are suitable for producing relatively short lengths at temperatures up to about 1093°C if the operation is performed at high speeds. The Ugine–Sejournet process in which molten glass serves as a lubricant is most widely used for high-temperature extrusion. Because of the insulating as well as lubricating properties of glass, overheating of tools does not occur and die life is increased. For titanium and steels, dies are usually made of tungsten hot-work tool steels. Ceramic coatings (Al2O3 or ZrO2) on the dies are necessary at the temperatures required for refractory metals. Pressing speeds are usually in the range of 8.33 to 33.32 m/min.

SHAPE, SURFACE, LIMITATIONS

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TOLERANCE

Extruded shapes are generally classified by configuration and include rod, bar, tube, and hollow, semihollow, and solid shape. Although many asymmetrical shapes can be produced, probably the most important factor in the extrudability of a shape is symmetry. Hollow and semihollow shapes cost more than solid shapes and usually cannot be extruded with as thin sections. Semihollow shapes with long thin voids should be avoided. For best extrudability the length-to-width ratio of partially enclosed voids, channels, or grooves should not exceed 3:1 for aluminum and magnesium, 2:1 for brass, or 1:1 for copper, titanium, and steels. Wall thickness surrounding the voids should be as uniform as possible. The size and weight of extruded shapes are limited both by the section configuration and © 2002 by CRC Press LLC

by the material properties. The maximum size that can be extruded on a press of given capacity is determined by the circumscribing circle, which is the smallest circle that will enclose the shape. The circumscribing circle size controls the die size, which in turn is limited by the press size. Thickness limitations are related to the size of the cross section as well as the type of material. As a rule, thicker sections are required with increased section size. Sharp corners and edges are usually possible with aluminum and magnesium alloys, but 0.38-mm corner and fillet radii are preferred. Minimum fillet radii of 3.2 mm for steel and 4.5 mm for titanium are suggested by most extruders. Typical minimum corner radii are 0.8 mm for steel and 1.5 mm for titanium. Smooth surfaces with finishes better than 30 µin. rms are readily attainable in aluminum and magnesium alloys. High-temperature alloys are characteristically rougher; an extruded finish of 125 µin. rms is generally considered acceptable for most steels and titanium alloys. Improved surface finishes can be produced by a cold-draw finishing operation. Although extruded shapes minimize and often eliminate the need for machining, they do not possess the dimensional accuracy of machined parts. The tolerances of any given dimension vary somewhat depending on the size and type of shape, and the relative location of the dimension. Detailed standard tolerances covering straightness, flatness, twist, and crosssectional dimensions such as section thickness, angles, contours, and corner and fillet radii have been established for magnesium, aluminum, copper, and brass by most extruders and are published in handbooks. Standard tolerances also have been established for steels and titanium alloys in simple sections, but in many instances these are subject to mill inquiry.

EXTRUDED PLASTICS Extrusion is a process for making articles of constant cross section, called “continuous shapes,” by forcing softened material through a hole approximating the desired shape. With plastics the process is carried out by one of two methods: ram extrusion or screw extrusion.

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E

In ram extrusion, the softened mass fills a cylinder to which the die—the shaped hole—is attached at one end. A closely fitting piston, the ram, enters the cylinder and pushes the mass through the die at pressures ranging up to 68 MPa. The product, or extrudate, is cooled or otherwise hardened shortly after leaving the die. Subsequent handling depends on the material and the shape. Ram extrusion is used chiefly for extruding TFE fluorocarbon (tetrafluoroethylene) resin (Teflon), which is damaged by the shearing action of screw extrusion, and for cellulose nitrate, whose extreme heat sensitivity and inflammability make screw extrusion dangerous. Screw extrusion, by far the more economical and commercially important process, centers around the screw extruder. This consists of a heavy cylindrical barrel inside which turns a motor-driven screw, or worm. The screw is essentially a thick shaft with a helical blade, or flight, wrapped around it. At the rear end of the barrel, a feed hopper admits cold plastic particles that normally fall into the screw channel by gravity. As the screw rotates, the particles are dragged forward by frictional action between screw, plastic, and barrel. Electric band heaters on the outside of the barrel heat the plastic, which is further heated by the frictional action of the screw. Soon, the particles coalesce into a voidless mass that softens further to become a melt. This plastic melt is very viscous


(a million times as viscous as water), so considerable pressure, on the order of 3.4 to 68 MPa, must be developed to force it through the die at the front of the extruder at economical rates.

EXTRUDABLE MATERIALS All thermoplastics can be extruded by either ram or screw extrusion. Today, high-viscosity grades of type 66 nylon and other nylons are available and their extrusion presents no special difficulties. In extruding rigid polyvinyl chloride (PVC), extreme care must be taken not to overheat the resin since thermal decomposition, once started, snowballs. This simply means being careful to avoid extreme temperatures everywhere and to streamline meticulously all passages through which the melt must pass. To a lesser degree, CFE fluorocarbon (trifluorochloroethylene) resin (Kel-F et al.), cellulosics, nylons, and acetal (polyoxymethylene) are similarly heat sensitive. Some thermosets can be extruded provided they are formulated to flow at temperatures safely below the curing temperatures. The process has been used to make pipe and structural shapes, to coat wire with thermosetting compositions, and to prepare “rope” and pellets for compression molding. The extrusion of rubbers closely resembles plastics extrusion.

TX66613_frame_F(6) Page 223 Wednesday, March 13, 2002 11:23 AM

F FABRICS, NONWOVEN BONDED Although there are several types of fabrics that are not woven, the term nonwoven fabrics is recognized in the textile trade as applying to those materials composed of a fibrous web held together with a bonding agent to obtain fabriclike qualities. These fabrics may be of a uniform, close-bonded fibrous structure or of a foraminous unitary construction. They may be formed by processing on modifications either of textile type machines or papermaking equipment. In either case, the fibers as laid up in the basic web prior to bonding or to postforming may be oriented in one or more prescribed directions, or may be distributed in a completely random fashion. They are secured in place by suitable adhesives incorporated in the web. The application of these adhesives may be controlled to coat and bond the fibers completely, or to bond them only in selected areas, or at points of individual fiber contact. Nonwovens may be thick or thin and of either low or high density. The conditions under which nonwoven fabrics are manufactured and the possible combinations of fibers and adhesives permit the production of structures offering a wide range of physical and chemical properties.

PRODUCTION METHODS Production of a nonwoven fabric may be divided into two basic steps: (1) formation of the web and (2) bonding the web. The most widely used means for forming the web is a series of cotton cards feeding to a common conveyor belt to build up a unidirectional composite web of the desired weight. The number of cards per line will depend on the maximum


weight of the product to be produced. Each card in the line may be geared to produce webs ranging from some 35 grains up to 100 grains per yard at speeds of 66.66 m/min down to 15 m/min for the heavier material. Material from these lines is usually limited to 101.6-cm widths, with strength favoring the machine direction over the cross machine direction in ratios from 3:1 to as much as 20:1. Where wider material of heavier weight, or when the strength balanced in the machine and cross machine directions is required, a web production line consisting of a single breaker and a finisher garnett equipped with a cross lapper may be used to advantage. The most versatile type of web for nonwoven fabrics is that produced by the air disposition of precarded fibers that are collected with a minimum of orientation as a uniform mat. In contrast to nonwoven fabrics made from webs that have been dry-processed on modified textile equipment are those produced from wetlayup webs using papermaking machines. Such webs usually depend on adhesive additives or postbonding to impart the necessary physical properties. Once the web has been formed, by either the dry or wet layup process, it may be further modified by techniques such as needle punching, aeration, or impingement with gaseous, liquid, or other means, to produce a patterned configuration of desired characteristics. Nonwovens of such postformed webs may be characterized by added resistance to delamination, superior drape, flexibility, porosity, abrasion, and flame resistance or other desirable properties.

TYPES

OF

FIBERS

In the production of nonwoven fabrics most every type of natural and anthropogenic fiber

F


F

can be used. Price, equipment, and quality, as well as chemical and physical requirements of the product, govern the particular fiber used as well as the bonding agent. The use of more virgin first-quality fiber, especially the anthropogenic cellulosics, is preferred, in everything from diapers to casket liners. Rayon is the predominant fiber used for both utility as well as aesthetic appeal. It is made in a wide variety of descriptions to fit the different manufacturing methods and end-use requirements. The finer deniers give the best tensile, tear, and bursting strength values. For special applications calling for particular chemical or electrical resistance, more use of the expensive synthetic fibers such as Acrilan, nylon, or Dacron may be warranted.

are sifted into the fiber web as formed. They are used especially in the low-density, highbulk nonwovens where wetting by the binder or the application of pressure might cause excessive matting and compression of the material. Bonding is effected by heating either with or without the use of pressure. Thermoplastic fibers. The thermoplastic fiber binders have the advantage of constituting an integrated structural part of the fiber web that forms the fabric. To bond the web they may be activated by solvents or by heat and pressure. By regulation of the amount of heat and pressure as well as the amount of thermoplastic fiber present, a wide variety of characteristics may be built into these nonwoven fabrics.

APPLICATIONS BONDING AGENTS Properties of nonwoven fabrics are as dependent on the bonding agent as they are on the fiber that forms the foundation of the material. Both are selected with the end use in mind, and each must be compatible with the other. Bonding agents may be grouped into three broad classifications: (1) liquid dispersions, (2) powdered adhesives, and (3) thermoplastic fibers. Liquid dispersions. Liquid dispersions are the most extensive type used. Among these are polyvinyl alcohol, generally used as a preliminary binder or where high strength and permanence are not essential; polyvinyl acetate for good strength and flexibility where freedom from odor and taste are important; polyvinyl chloride (PVC) for good wet and dry strength, and toughness; synthetic latices of butadiene–acrylonitrile or butadiene–styrene for good adhesive and elastic properties where strength and a high degree of permanence are more important than color, stability, and odor; the acrylics for good strength, soft “hand,” color stability, and permanence. These dispersions are applied (1) by spraying, generally used for low-density materials, (2) by saturator, for denser, more durable material, and (3) by printing, usually for selective bonding of localized areas in soft absorbent products. Powdered adhesives. These are usually of thermosetting or thermoplastic resin types and


Construction and performance of nonwoven fabrics have not been standardized. They are usually constructed to fit a particular end-use requirement or are built around particular specifications. Industrial products for which nonwoven fabrics have been used include acoustical curtains; artificial leather and chamois; automotive plumpers; backing for adhesive tapes; base for vinyl and rubber coatings; bagging; buffing wheels; cable and wire wrappings; electrical tapes; filters for air, gases, and liquids; insulation; laminate reinforcements; polishing and wiping cloths; and wall coverings.

FABRICS, WOVEN By far the greatest volume of textile materials is used in consumer textiles, such as apparel. But textiles are extremely versatile materials, which have been applied to a large number of engineered uses, e.g., thermal, acoustical, and electrical insulation; padding and packaging; barrier applications; filtration, both dry and wet; upholstery and seating; reinforcing for plastics or rubber; and various mechanical uses such as fire hose jackets, tenting, tarpaulins, parachutes, and marine lines. Textiles are highly complex materials. Their properties depend not only on the fiber but on the form in which it is used — whether the form be a felt, a bonded fabric, a woven or


knit fabric, or cordage. Properties such as heat, chemical, and weather resistance depend primarily on the type of fiber used; properties such as mechanical strength, thermal transmission, and air or liquid permeability depend both on the fiber and the textile form. The versatility of textiles stems from (1) the wide range of fibers that can be used, and (2) the range of complicated textile structures that can be formed from the fibers. There are two important factors that should be considered in discussing textile needs with textile suppliers:

openness of the weave, of course, can be varied to any desired degree. In twill weave fabrics, a sharp diagonal line is produced by the warp yarn crossing over two or more filling yarns. Satin weave fabrics are characterized by regularly spaced interlacings at wide intervals. This weave produces a porous fabric with a smooth surface. Satins woven of cotton are called sateen. In the leno weave fabrics, the warp yarns are twisted and the filling yarns are threaded through the twist openings. This weave is used for meshed fabrics and nets. Cordage

1. The types of finishes that can be applied to the finished textile product can substantially alter or modify the stability, “hand,” and/or durability of the textile. 2. Combining of textiles with other materials, such as resins or rubber, either by impregnation or by coating, will substantially alter performance characteristics of the final composite.

TEXTILE CONSTRUCTIONS Textile engineering materials can be classified generally as (1) nonwoven fabrics, including both felts and bonded fabrics; (2) woven or knit fabrics, and (3) cordage. Nonwoven fabrics are discussed in a separate article. Woven and Knit Fabrics Woven fabrics and knit fabrics are composed of webs of fiber yarns. The yarns may be of either filament (continuous) or staple (short) fibers. In knit fabrics, the yarns are fastened to each other by interlocking loops to form the web. In woven fabrics, the yarns are interlaced at right angles to each other to produce the web. The lengthwise yarns are called the warp, and the crosswise ones are the filling (or woof) yarns. The many variations of woven fabrics can be grouped into four basic weaves. In the plain weave fabric, each filling yarn alternates up and under successive warp yarns. With a plain weave, the most yarn interlacings per square inch can be obtained for maximum density, “cover,” and impermeability. The tightness or


The term cordage includes all types of threads, twine, rope, and hawser. Essentially all cordage consists of fibers twisted together, plied, and in many cases cabled to produce essentially continuous strands of desired cross section and strength. In addition to the type of fiber used, the most important determinants of the end properties of cordage are the type and degree of twist employed. The two major types of twist are (1) cable twist, in which the direction of twisting is alternated in each successive operation, i.e., singles may be “S” twisted, plies “Z” twisted, and cables “S” twisted (a yarn or cord has an “S” twist if, when held in a vertical position, the spirals conform in direction of slope to the central portion of the letter “S,” and a “Z” twist if the spirals conform in direction of slope to the central portion of the letter “Z”), and (2) hawser twist, in which the singles, plies, and cables are twisted “SSZ” or “ZZS.” Hawser twist generally provides higher strength and resilience.

SPECIFICATIONS Textile specifications contain two important types of information: (1) descriptive information and (2) service property requirements. Specifications that physically describe the textile fabric usually include (1) width, in inches, (2) weight, usually in ounces per square yard, (3) type of weave, such as twill, broken twill, leno, or satin, (4) thread count, both in warp and filling (e.g., 68 × 44 denotes 68 warp yarns/in. and 44 filling yarns/in.), (5) type of

F


F

fiber and whether the yarn is to be filament or staple, (6) crimp, in percent, (7) twist per inch, and (8) yarn number both for warp and fill. Yarn number designations are somewhat complex, as they have been developed in a relatively unorganized fashion over the years, and different systems are used in different types of fibers. (Filament yarns are usually stated simply in denier, which is the weight in grams of 9000 m of yarn.) Essentially, yarn numbers provide a measure of weight per unit length, or length per unit weight. A typical yarn designation on a specification may appear as “210 (denier)/1 × 20/2 (cotton system).” This means that (1) the warp yarn is 210-denier single yarn, and (2) the filling yarn contains 2 plies, each of which is a 20 singles yarn (determined by the cotton numbering system). A number of fabric-designation systems have been formalized by tradition. For example, sheetings, drills, twills, jeans, broken twills, and sateens are designated only by width in inches, number of linear yards per pound, and number of warp and filling threads per inch. “Specs” for equivalent synthetic fabrics also include fiber type, whether staple or filament.

FASTENERS Mechanical fasteners are among the most common components in fabricated products. Thus, fasteners are extremely important from both a manufacturing and a product standpoint. The proper selection of fasteners is necessary to provide the most value for the manufacturer as well as for the consumer. The most common types of fasteners can be grouped into the following categories: bolts/screws, studs, pins, nuts, rivets, and holes with or without threads tapped into them.

FASTENER TYPES

AND

MATERIALS SELECTION

Bolts and screws, two names for externally threaded fasteners, are certainly well-recognized fastener types. Like all fasteners, they are also a highly engineered method of joining. Figure F .1 is a schematic drawing of the significant features of a bolt, showing the tensile


Bracket

Boss

FIGURE F.1 Features of a bolt. (From Adv. Mater. Proc., 154(4), 49, 1998. with permission.)

force on the bolt and the compressive loading of the joint members. The ability of a bolt to translate rotational motion into a clamping force has made it the standard fastener. The common bolt is also an ideal fastener from a reusability standpoint. Its helical thread form enables it to attach, detach, then reattach joint members an almost infinite number of times. The simple tools required are another reason that bolts have become so common. Bolts provide an excellent ratio of performance to cost, but during both the design phase and installation, they require more consideration than other fastener types to achieve an optimized joint. Many different materials types are available for fasteners. Standard metric bolts and nuts are made of steels with varying chemical compositions. A rivet is a non-reusable fastener that is deformed to provide a mechanical clinching of joint members. Both aluminum and steel may be fabricated into rivets. In fact, the stem and body of a rivet can be easily made with two different materials. Low-carbon steel alloys in the AISI/SAE 1XXX series may be selected when cost is the most important factor, or when a higher-strength rivet is needed. Typical tensile strength values for joints made with steel rivets are 5 to 10 kN, which is about a third to half the value of equivalently sized bolts. An aluminum


rivet usually has about half to two thirds the strength of steel rivets. However, the shear resistance of both types of rivets can equal their tensile resistance, and can even exceed the value for a similar sized bolt. The typical aluminum alloys for rivets in automotive applications are AA 5052, 5056, 7075, 7178, and 2014. These alloys provide good strength, high corrosion resistance (without the need for additional corrosion protection), and light weight. Some rivets are made of thermoplastics, namely, polyamide 6 (PA6), polyamide 66 (PA66), and polyoxymethylene (POM). These are semicrystalline polymers with high strength, low mass, and resistance to automotive fluids and salt environments. As a result, they are also ideal for the myriad pins and clips that connect plastic components. Polyamides are also coated onto bolts and nuts and the plastic is added to reduce the allowance between the mating metallic threads, making them more resistant to vibrational loosening. Fluoropolymers are commonly added to the threaded region of weld nuts prior to installation, because of their low coefficient of friction. Finally, polymer adhesives are often added to bolts. These bolts resist vibrational loosening through mechanical means, such as thermoplastic additions or locally deformed features. In fact, the application of adhesives is the only method that provides long-term resistance to loosening. A final material being considered for fasteners is titanium. The mechanical properties of titanium are rather high. If low density is also required, as it is for automotive structures, then titanium appears to be the only current material that can challenge traditional steel alloys. The workhorse alloy titanium, 6% aluminum, 4% vanadium (Ti–6Al–4V) could definitely be chosen for fastener applications. The advantages of titanium are its extremely high strength-to-weight ratio and its almost complete imperviousness to corrosive attack by chloride ions. This means that a lower-mass fastener can be made, one that does not require the expensive and time-consuming application of corrosion-resistant coatings. Many mechanical joints in aerospace structures


currently rely on titanium alloys, including Ti–6Al–4V. If the raw material costs of titanium can be reduced, it will become a viable alternative material for automotive applications, with fasteners at the top of the list.

FELDSPAR Feldspars (Al2O3 and SiO4 tetrahedra) constitute 60% of the outer 13 to 17 km of the earth’s crust and are the most common mineral in crystalline rocks. Feldspars are aluminum silicates of potassium, sodium, and calcium. The importance of the many feldspars that occur so widely in igneous, metamorphic, and some sedimentary rocks cannot be underestimated, especially from the viewpoint of a petrologist attempting to unravel Earth history. With weathering, feldspars form commercially important clay materials. Economically. feldspars are valued as raw material for the ceramic and glass industries, as fluxes in iron smelting, and as constituents of scouring powders. Occasionally, their luster or colors qualify them as semiprecious gemstones. Some decorative building and monument stones are predominantly composed of weather-resistant feldspars. Knowledge of the composition of a feldspar and its crystal structure is indispensable to an understanding of its properties. However, it is the distribution of the aluminum and silicon atoms among the available tetrahedral sites in each chemical species that is essential to a complete classification scheme, and is of great importance in unraveling clues to the crystallization and thermal history of many igneous and metamorphic rocks.

FELTS Classes of felts in use as engineering materials are wool and synthetic fiber felts. Wool felt is a fabric obtained as a result of the interlocking of wool fibers under suitable combinations of mechanical work, chemical action, moisture, and heat, alone or in combination with other fibers.

F


Synthetic fiber felt is a fabric obtained as a result of interlocking of synthetic fibers by mechanical action.

PROPERTIES

F

Neither wool nor synthetic fiber felts require binders and exist as 100% fibrous materials. Felts generally exhibit the same chemical properties as do the fibers of which the felt is composed. Wool felts are characterized by excellent resistance to acids but are damaged by exposure to strong alkalies. They exhibit remarkable resistance to atmospheric aging and are as a class probably the most inert of all nonmetallic engineering materials with respect to nonaqueous liquids, oils, or solvents. Wool felts are not generally recommended for stressed dry uses at temperatures in excess of 82°C, because of changes in physical properties, but are used as dry spacers and gaskets in many applications up to 149°C, and the use of the materials as oil wicks and lubricating system components at ambient temperatures up to 149°C is common. Synthetic fiber felts are available in virtually all classes of fiber composition including regenerated cellulose, cellulose acetate, cellulose triacetate; polyamide, polyester, acrylic, modacrylic, olefin, and TFE fluorocarbon (tetrafluorethylene). The variety of types available provides a virtually infinite range of physical and chemical properties for application beyond the natural versatility of wool fiber felts. Outstanding among the properties of this class of engineering material are chemical, solvent, thermal, and biological stability as well as low moisture absorption, quick drying after aqueous wetting, abrasion resistance, and frictional and dielectrical features.

FORMS Wool felts are produced as “sheet” stock in a standard 91.4 × 91.4 cm size in thickness ranging from 1.6 to 76 mm. There are a number of density classifications based on the weight for a 914 × 914-mm sheet in 25.4-mm thickness from 5.4 to 14.4 kg with the weight for any given thickness in the density class proportioned to the weight per square meter at the 25.4-mm thickness. “Roll” felts are produced © 2002 by CRC Press LLC

in either 1522- or 1830-mm widths in lengths up to 160 m and standard thicknesses range from 0.8 to 25.4 mm. Synthetic fiber felt widths are available from 600 to 1830 mm in thicknesses from 0.8 to 19 mm. Both wool and synthetic fiber felts are nonforming and nonraveling and thus provide considerable ease of cutting and fabricating. In addition, wool felt lends itself to most grinding, cutting, shaping, extruding, and other machining operations so that special shaped parts such as polishing laps, round wicking, ink rollers, and others are produced.

APPLICATIONS The structural elasticity of felts as a class makes these materials suitable for molding and forming and parts of these types are produced to provide special shaped gaskets, seals, fillers, instrument covers, and the like. In many cases these shaped parts are stabilized through the use of resinous or rubber impregnants and this modified class of felt is finding ever-increasing use as flat stock for special gasketing, sealing, and other applications. Combinations of felt and plastic and elastomer sheet materials are also produced for application where resilience plus nonpermeability is desired as in sealing and frictional uses. Use of felt as an engineering material covers a broad spectrum of application. Major contributing properties include resilience, mechanical, thermal, and acoustic energy absorption; high porosity-to-weight ratio; resistance to aging; thermal and chemical stability; high effective surface area per unit volume; and solvent resistance. Applied uses include wet and dry filtration, thermal and acoustical insulation, vibration isolation, impact absorption, cushioning and packaging, polishing, frictional surfacing, liquid absorption and reservoirs, wicking, gasketing, sealing, and percussion mechanical dampening.

FERRITE A ferrite is any of the class of magnetic oxides. Typically, the ferrites have a crystal structure that has more than one type of site for the


cations. Usually the magnetic moments of the metal ions on sites of one type are parallel to each other, and antiparallel to the moments on at least one site of another type. Thus, ferrites exhibit ferromagnetism.

sintering. The last step completes the chemical reaction to the desired magnetic structure and effects homogenization, densification, and grain growth of the compact. It is perhaps the most critical step in optimizing the magnetic properties of commercial ferrites.

COMMERCIAL TYPES There are three important classes of commercial ferrites. One class has the spinal structure. The second class of commercially important ferrites has the garnet structure. Yttrium-based garnets are used in microwave devices. Thin monocrystalline films of complex garnets have been developed for bubble domain memory devices. The third class of ferrites has a hexagonal structure of the magnetoplumbite type. Because of their large magnetocrystalline anisotropy, the hexagonal ferrites develop high coercivity and are an important member of the permanent magnet family.

PROPERTIES The important intrinsic parameters of a ferrite are the saturation magnetization, Curie temperature, and magnetocrystalline (K1) and magnetostrictive (Ωx) anisotropies. These properties are determined by the choice of the cations and their distribution in the various sites. In addition to the intrinsic magnetic parameters, microstructure plays an equally important role in determining device properties. Thus, grain size, porosity, chemical homogeneity, and foreign inclusions dictate in part such technical properties as permeability, line width, remanence, and coercivity in polycrystalline ceramics. In garnet films for bubble domain device applications, the film must essentially be free of all defects such as inclusions, growth pits, and dislocations.

PREPARATION Polycrystalline ferrites are most economically prepared by ceramic techniques. Component oxides or carbonates are mixed, calcined at elevated temperatures for partial compound formation, and then granulated by ball milling. Dispersants, plasticizers, and lubricants are added, and the resultant slurry is spray-dried, followed by pressing to desired shape and


FERRITE DEVICES These are electrical devices whose principle of operation is based on the use and properties of ferrites, which are magnetic oxides. Ferrite devices are divided into two categories, depending on whether the ferrite is magnetically soft (low coercivity) or hard (high coercivity). Soft ferrites are used primarily as transformers, inductors, and recording heads, and in microwave devices. Since the electrical resistivity of soft ferrites is typically 106 to 1011 times that of metals, ferrite components have much lower eddy current losses and hence are used at frequencies generally above about 10 kHz. Hard ferrites are used in permanent-magnet motors, loudspeakers, and holding devices, and as storage media in magnetic recording devices.

CHEMISTRY

AND

CRYSTAL STRUCTURE

Soft ferrite devices are spinels with the general formula of MFe2O4, in which M is a divalent metal ion. The commercially practical ferrites are those in which the divalent ion represents one or more magnesium, manganese, iron, cobalt, nickel, copper, zinc, and cadmium ions. The trivalent iron ion may also be substituted by other trivalent ions such as aluminum. The compositions are carefully adjusted to optimize the device requirements, such as permeability, loss, ferromagnetic resonance line width, and so forth. The ferromagnetic garnets have the general formula of M3Fe5O12, in which M is a rare earth or ytrrium ion. Single-crystal garnet films form the basis of bubble domain device technology. Bulk garnets have applications in microwave devices. Hard ferrites for permanent-magnetic device applications have the hexagonal magneto-plumbite structure, with the general formula MFe12O19, where M is usually barium or strontium.

F


The material of choice for magnetic recording is δ–Fe2O3, which has a spinel structure. With the exception of some single crystals used in recording heads and special microwave applications, and particulates used as storage media in magnetic recording, all ferrites are prepared in polycrystalline form by ceramic techniques.

APPLICATIONS

F

Applications of ferrites may be divided into nonmicrowave, microwave, and magnetic recording applications. Further, the nonmicrowave applications may be divided into categories determined by the magnetic properties based on the B–H behavior, that is, the variation of the magnetic induction or flux density B with magnetic field strength H. The categories are linear B–H, with low flux density, and nonlinear B–H, with medium to high flux density. The highly nonlinear B–H, with a square or rectangular hysteresis loop, was once exploited in computer memory cores. Linear B–H Devices In the linear region, the most important devices are high-quality inductors, particularly those used in filters in frequency-division multiplex telecommunications systems and low-power wideband and pulse transformers. Virtually all such devices are made of either MnZn ferrite or NiZn ferrite, although predominantly the former. Nonlinear B–H Devices The largest usage of ferrite measured in terms of material weight is in the nonlinear B–H range, and is found in the form of deflecting yokes and flyback transformers for television receivers. Again, MnZn and NiZn ferrites dominate the use in these devices. A rapidly growing use of ferrites is in the power area, where ferrite transformers are extensively used in switched mode (AC to DC) and converter mode (DC to DC) power supplies. Such power supplies are widely used in various computer peripheral equipment and private exchange telephone systems.


Microwave devices. Microwave devices make use of the reciprocal propagation characteristics of ferrites close to or at a gyromagnetic resonance frequency in the range of 1 to 100 GHz. The most important of such devices are isolators and circulators. The garnets have highly desirable, small, ferromagnetic-resonance linewidths, particularly in single-crystal form. Magnetic recording devices. Vast amounts of audio and video information and digital data from computers are stored in magnetic tapes and disks. Here magnetic recording materials function as hard magnetic materials. The most widely used particles in magnetic recording are δ-Fe2O3 and co-modified δ-Fe2O3. The basic δ-Fe2O3 is generally used in audio recording, while the higher-coercivity Co-δ-Fe2O3 dominates video recording.

FERROALLOYS Ferroalloys compose an important group of metallic raw materials required for the steel industry. Ferroalloys are the principal source of such additions as silicon and manganese, which are required for even the simplest plain-carbon steels, and chromium, vanadium, tungsten, To, and molybdenum, which are used in both lowand high-alloy steels. Also included are many other more complex alloys. Ferroalloys are unique in that they are brittle and otherwise unsuited for any service application, but they are important as the most economical source of these elements for use in the manufacture of the engineering alloys. These same elements can also be obtained, at much greater cost in most cases, as essentially pure metals. The ferroalloys contain significant amounts of iron and usually have a lower melting range than the pure metals and are therefore dissolved by the molten steel more readily than the pure metal. In other cases, the other elements in the ferroalloy serve to protect the critical element against oxidation during solution and thereby give higher recoveries. Ferroalloys are used both as deoxidizers and as a specified addition to give particular properties to the steel. Many ferroalloys contain combinations of two or more desirable alloy additions, and well over 100 commercial grades and combinations


TABLE F.1 Analysis of Typical Ferroalloys (wt%) Type of Ferroalloya Ferromanganese Standard Medium carbon Low carbon Ferrosilicon 50% regular 75% regular Ferrochromium High carbon Low carbon SM low carbon Ferromolybdenum High carbon Ferrovanadium High carbon Ferrotitanium Low carbon

Mn

Si

78–82 80–85 80–85

1.25 1.25–2.5 1.25–7.0

— —

47–52 73–78

C

Cr

Mo

Al

Ti

7.5 1–3b 0.75

F

0.15 0.15

4–6

1–2 0.3–1.0 4–6

4.5–6.0b 0.03–2.0 1.25

—

1.5

2.5

—

55–70

—

13.0

3.5

—

—

1.5

—

—

3–5

0.1

—

—

6–10

38–43

—

V

67–70 68–71 62–65

30–40

a

In all cases the balance is iron, with the exception of minor impurities. The latter are usually specified, such as 0.10% max P. In several specified grades within this range.

b

Source: McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 7, McGraw-Hill, New York, 62. With permission.

are available. Although of less general importance, other sources of these elements for steelmaking are metallic nickel, nickel, SiC, Mo2O5, and even misch metal (a mixture of rare earths). Analyses of a few typical ferroalloys are given in Table F.1. The three ferroalloys that account for the major tonnage in this class are the various grades of silicon, manganese, and chromium. For example, 5.9 kg of manganese is used on the average in the United States for every ton of open-hearth steel produced. Elements supplied as ferroalloys are among the most difficult metals to reduce from ore.

FERROCHROMIUM A high-chromium iron master alloy used for adding chromium to irons and steel, ferrochromium is also called ferrochrome. It is made from chromite ore by smelting with lime, silica, or fluorspar in an electric furnace. High-carbonferrochrome is used for making tool steels, ballbearing steels, and other alloy steels. It melts


at about 1250°C. Low-carbon-ferrochrome is used for making stainless steels and acid-resistant steels. Simplex ferrochrome comes in pellet form and is used for making low-carbon stainless steels. Low-carbon ferrochrome is also preferred for alloy steel mixtures where much scrap is used because it keeps down the carbon and inhibits the formation of hard chromium carbides. The various grades of ferrochromium are also marketed as high-N2 ferrochrome, and for use in making high-chromium cast steels where the N2 refines the grain and increases the strength. Foundry-grade ferrochrome is used for making cast irons, as well as for ladle additions to cast iron to give uniform structure and increase the strength and hardness.

FERROMANGANESE This is a master alloy of manganese and iron used for deoxidizing steels, and for adding manganese to iron and steel alloys and bronzes. Manganese is the common deoxidizer and cleanser of steel, forming oxides and sulfides


F

that are carried off in the slag. Ferromanganese is made from the ores in either the blast furnace or the electric furnace. Spiegeleisen is a form of low-manganese ferromanganese and the German name, meaning mirror iron, is derived from the fact that the crystals of the fractured face shine like mirrors. Spiegeleisen has the advantage that it can be made from low-grade manganese ores, but the quantity needed to obtain the required proportion of manganese in the steel is so great that it must be premelted before adding to the steel. It was used for making irons and steels by the Bessemer process.

FERROPHOSPHORUS This substance is an iron containing a high percentage of phosphorus, used for adding phosphorus to steels. Small amounts of phosphorus are used in open-hearth steels to make them free-cutting, and phosphorus is also employed in tinplate steels to prevent the sheets from sticking together in annealing. Ferrophosphorus is made by melting phosphate rock together with the ore in making pig iron. There is also a master alloy, ferroselenium, for adding selenium to steels, especially stainless steels, to give free-machining qualities.

FERROSILICON This is a high-silicon master alloy used for making silicon steels, and for adding silicon to transformer irons and steels. It is made in the electric furnace by fusing quartz or silica with iron turnings and carbon. Silicon is often added to steels in combination alloys with deoxidizers or other alloying elements. Ferrosilicon aluminum is a more effective deoxidizer for steel than aluminum alone. It is also used for adding silicon to aluminum casting alloys. The alloy serves as a deoxidizer, fluxes the slag inclusions, and also controls the grain size of the steel.

FERROTITANIUM A master alloy of titanium with iron, ferrotitanium is used as a purifying agent for irons and steel owing to the great affinity of titanium for O2 and N2 at temperatures above 800°C. The © 2002 by CRC Press LLC

value of the alloy is as a cleanser, and little or no titanium remains in the steel unless the percentage is gauged to leave a residue. The ferrocarbon titanium is made from ilmenite in the electric furnace, and the carbon-free alloy is made by reduction of the ore with aluminum. Ferrotitanium comes in lumps, crushed, or screened, and it is used for ladle additions for cleansing steel. Low-carbon ferrotitanium is used as a deoxidizer and as a carbide stabilizer in high-chromium steels. Graphidox improves the fluidity of steel, increases machinability, and adds a small amount of titanium to increase the yield strength, and the Grainal alloys which are used to control alloy steels and have various compositions. MnTi is used as a deoxidizer for high-grade steels and for nonferrous alloys. NiTi is used for hard nonferrous alloys and columbium is used for adding columbium to steel. It has an exothermic reaction that prevents chilling of the molten metal.

FERROUS P/M Specifying powder-metallurgy (P/M) parts and their consolidation process used to be a simple process: Design the part, select the metal powders and lubricants that provide the required properties, compact the powders into a briquette, and sinter the briquette into its finished form. Through this procedure, millions of parts have been produced for applications ranging from automobiles to appliances and from business to farm and garden machines. However, the needs of industries have changed significantly. Removing weight from all products has risen to primary importance. Energy, tooling, and materials costs now figure prominently in parts design, and productivity has emerged as the new watchword With these changes have come changes in P/M technology. Through the many manufacturing processes, improvements have been made in the powders themselves — improvements such as lower levels of inclusions and higher compressibility. In addition to conventional iron and steel metals, the list of available powders has been expanded to include new classes of tool steel, as well as materials such


as cermets and alloys of titanium, nickel, and aluminum. Accompanying these developments has been the growth of new consolidation technologies. As a result, design engineers need current information on which P/M technologies are viable, cost-effective, and production-effective, and which have potentially wide application. Although P/M is used to fabricate parts from just about any metal, the most commonly used metals are the iron-base alloys. Low-density iron P/M parts (5.6 to 6.0 g/cm3), with a typical tensile strength of 108.8 MPa are usually used in bearing applications. Copper is commonly added to improve both strength and bearing properties. Alloy-steel powders are sometimes hot-forged to high or nearly theoretical density to form parts with improved mechanical properties that, when heat-treated, may have tensile strengths to 1156 MPa. Powder forging (P/F) is now established as a serious contender for parts formerly made as wrought forgings or machined from mill forms. Iron P/M or sintered Fe–Cu alloy strength can be varied by adjusting density, carbon content (up to 15%), or all three to satisfy specific design requirements. The mechanical properties of ferrous powder parts can be considerably improved by impregnating or infiltrating them with any one of a number of different materials, both metallic and nonmetallic, such as oil, wax, resins, copper, lead, and babbit. Low-density P/M parts are used in bearing applications because they provide porosity for oil storage. Impregnating sintered-metal bearings with oil usually eliminates the need for relubrication. For higher-strength needs, alloyed (frequently prealloyed nickel–molybdenum–iron) iron, compacted to a higher density, is used. When carbon or other alloying elements are mixed with the iron powders and densities exceed 6.2 g/cm3, the parts are considered to be steel rather than iron. As carbon content is increased up to 1%, the strength of steel P/M parts increases, just as the strength of wrought steel increases with higher carbon content. Ferrous-base P/M parts can range in size from about 2.5 mm thick and 3.2 mm in diameter to 50.8 mm thick and over 612 mm in diameter. Because they can be mass produced


at relatively low cost, iron-base P/M parts find a wide variety of uses in such high-volume products as appliances, business machines, power tools, and automobiles. Typical parts are gears, bearings, rotors, valves, valve plates, cams, levers, ratchets, and sprockets. Additional applications can be accommodated by sealing the pores in iron P/M parts. The sealing materials used are copper, polyesters, and anaerobics; each requires a different processing system to impregnate the parts. Impregnation of sintered P/M parts is done for any of several reasons: • To serve in pressure-tight applications • To improve surface finish (impregnated parts are platable) • To improve machinability • To improve corrosion resistance Although high precision has been achieved in P/M parts for many years, their application was once restricted because of mechanical property limitations. Now, however, mechanical properties can be increased in steel P/M parts by hot forging in closed dies. Properties of P/M parts forged to 100% theoretical density in production conditions are claimed to be equal, and sometimes superior, to those of wrought steels of similar composition.

FIBER-REINFORCED GLASS Fiber-reinforced glass (FRG) composites are glass or glass ceramic matrices reinforced with long fibers of carbon or SiC. These composites are lighter than steel but just as strong as many steel grades, and can resist higher temperatures. They also have outstanding resistance to impact, thermal shock, and wear, and can be formulated to control thermal and electrical conductivity. With proper tooling, operations such as drilling, grinding, and turning can be completed in half the time required for nonreinforced glass. Currently, FRG components are primarily used for handling hot glass or molten aluminum during manufacturing operations. FRG is also under test as an engineering material in a variety of markets, including the aerospace, automotive, and semiconductor industries.

F


PROPERTIES

F

Glass and glass ceramics are versatile materials because of such characteristics as high chemical resistance and special electrical behavior, but fragility under tensile stress has limited their structural applications. However, the addition of continuous fibers of carbon and SiC into glass produces a material that withstands very high mechanical stresses and loads. As a result, it is now possible to manufacture glass matrix composites that are capable of structural roles (Table F.2). The reinforcement process provides an increase in both admissible maximum stress and ultimate elongation: brittle fracture is replaced by an almost ductile behavior. These properties, together with others, make FRG composites advantageous as a material in the machine-building industry. For example, low density and high modulus of elasticity allow extremely strong and stiff structures. Another important characteristic of longfiber-reinforced glasses is that their mechanical properties are largely independent of the condition at the surface. This means that they can be drilled, even near edges, and joined with other parts by means of screws and bolts. In addition, FRG composites have very high resistance to temperature changes, low coefficient of thermal expansion, high specific heat capacity, and good chemical resistance. They also exhibit good tribological and wear properties. However, because of the great variety of tribological and tribo-mechanical processes and applications, each service environment must be evaluated before selecting an FRG composite.

FIBER EFFECTS Performance of FRG composites under various operating conditions depends primarily on the type of fiber, the amount of fiber, and its orientation within the matrix. It also depends on the operating environment and the duration or cycle of the specific application. Fiber Types Carbon fibers are stable up to >2000°C under inert atmospheres, but few applications require


such performance. Moreover, at those temperature levels, the matrix itself becomes soft. In air, carbon fibers remain stable to about 450°C. This temperature limit also applies to composites containing carbon fibers, unless the fibers have been completely sealed. By contrast, SiC fibers are stable in air up to about 1200°C. In this case, the heat resistance of the matrix is the limiting factor. Therefore, SiC FRG products can be considered stable to 500°C if the matrix consists of the alkali–borosilicate–glass Duran. On the other hand, if an alkali-free aluminosilicate glass is the matrix, the SiC-fiber-containing composite is stable up to 750°C. When quenched from 350 to 20°C, most non-reinforced glasses break. However, SiC FRG is capable of withstanding a 60-fold quench (thermal shock) from 550°C to ambient temperatures. Furthermore, the product has exhibited good fracture toughness at temperatures as low as –200°C. Amount of Fiber Performance at high temperatures and resistance to thermal shock can be improved by increasing the percentage of fiber within the composite. However, fiber is the chief cost factor in producing FRG. Therefore, performance advantages must continue to outweigh costs as fiber count is increased. Fiber Orientation Depending on the arrangement of the fibers, the properties of a FRG composite can be either isotropic or anisotropic. For example, fibers may be arranged so that heat conductivity is low in one direction, but high in the perpendicular direction. Absolute values of h e a t c o n d u c t iv i t y r a n g e f r o m 1 . 7 t o ~25 W/m · K or half the value of steel. As in high-temperature performance, heat conductivity depends on the direction, type, and amount of fiber in the composite. Relatively high conductivity values result when the composite contains carbon fibers aligned longitudinally in the direction of heat travel; otherwise, the material may work as a heat barrier.

Property Density, g/cm3 Bending strength, MPa (3-point bending) Young’s modulus, Gpa Max. strain, % Work of fracture, J/m2 Coefficient of thermal expansion, ppm/K Thermal conductivity, W/(K· m) Thermal shock resistance, ∆K Max. application temperature, °C Short time (seconds) Long time (hours) a b c

SiC-Reinforced Duran Glass, Fibers Oriented 0/90

Carbon-Reinforced Duran Glass, Fibers Oriented 0/90

SiC-Reinforced 8252 Aluminosilicate

SiC-Reinforced Machinable Glass Ceramic, Fibers Oriented 0/90

Unreinforced Duran Glass

Stainless Steel 1.4301/1.4304a

2.5

1.9–2.2

2.5

2.5

2.2

8

1.8–2.5

450 110 0.5–1 3.5 × 104 2–4

450 110 0.5–1 3.5 × 104 2–4

400–500 100 1

Mean 600 190–210 15

40 10 0.1

c

c

2–4

30–50 63 0.1 1.2 × 102 3.3

15–18

4–8

1.5–3

1.5–3

1.1

20

50–150

>530

500 130 0.5–1 4 × 104 0–5 at 0° 5–15 at 90° 1.5 at 0° 1.5 at 90° >450

>730

>900

>100

c

c

600 550

500 450

800 750

1000 900

600 530

650 550

550 450

1.5–3

b

1.4301 (Fe/Cr18/Ni10) and 1.4304 (Fe/Cr18/Ni10/Mo3) are material numbers for two types of stainless steel, used in, i.e., medical or vacuum applications. Values not measured until now. Values for those properties are not known (here) or not meaningful for those materials.


Carbon Steel


TABLE F.2 Properties of FRG Composites vs. Unreinforced Glass and Various Steel Grades


Similarly, the electrical resistance of composites has been measured at room temperature in a composite 40% fiber by volume. In the fiber direction, specific electrical resistance for the SiC product was 10 Ω·cm, while for the carbon product, it was 0.01 Ω·cm. Normal to the fibers, the specific electrical resistance may be several orders of magnitude higher.

POTENTIAL APPLICATIONS

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FRG components are used as replacements for asbestos and other materials for handling hot glass during glass manufacturing operations. These include pushers, grips, transporters, and takeout pads as well as other parts in contact with hot glowing glass. It is an application characterized by extreme conditions in terms of heat and thermal shock, in which FRG composites have a great advantage over traditional materials. Within the metals industry, FRG pads act as thermal buffers in the handling of molten aluminum and its alloys. In this application, they form insulation pads between the melt vessel and supporting structural work, thereby minimizing heat conductivity away from the melt. Other applications being exploited are the replacement for fragile glass substrates that hold silicon wafers during chemical vapor deposition (CVD) coating processes. Required properties include high fracture toughness for higher yields, a low coefficient of thermal expansion, and high stiffness. Automotive applications include piston inserts, valve-control components, and other parts subjected to thermal and mechanical shock. FRG may also be applied in products for the protection of areas that must be insulated from heat, or where fracture toughness is needed. The tribological and wear properties of FRG suggest applications as bearings and seals in pump manufacturing, and for off-road industrial vehicle and equipment brakes. Other potential applications include aerospace components with glass ceramic matrices for temperatures above 1000°C; scanning-mirror substrates for space and missile systems; and protective inserts for parts requiring high impact resistance. As FRG manufacturing technology


advances and costs are reduced, the composites would be suitable for the construction of safes and strong rooms, and as armor in vehicles. FRG components are being produced in several configurations, which include plates, disks, or rings that have a maximum dimension (length, width, or diameter) of 400 mm, and thickness of 50 mm. The minimum thickness of unprocessed forms is 0.5 mm. Semifinished material can be reprocessed into end products with complex geometric forms.

FIBER-REINFORCED PLASTICS Fiber-reinforced plastics (FRPs) comprise a broad group of composite materials composed of fibers embedded in a plastic resin matrix. In general, they have relatively high strength-toweight ratios and excellent corrosion resistance compared to metals. They can be formed economically into virtually any shape and size. In size, FRP products range from tiny electronic components to large boat hulls. Between these extremes, there are a wide variety of FRP gears, bearings, housings, and parts used in all product industries. FRPs are composed of three major components — matrix, fiber, and bonding agent. The plastic resin serves as the matrix in which are embedded the fibers. Adherence between matrix and fibers is achieved by a bonding agent or binder, sometimes called a coupling agent. Most plastics, both thermosets and thermoplastics, can be the matrix material. In addition to these three major components, a wide variety of additives — fillers, catalysts, inhibitors, stabilizers, pigments, and fire retardants — can be used to fit specific application needs.

FIBERS Glass is by far the most-used fiber in FRPs. Glass-fiber-reinforced plastics are often referred to as GFRP or GRP. Asbestos fiber has some use, but is largely limited in applications where maximum thermal insulation or fire resistance is required. Other fibrous materials used as reinforcements are paper, sisal, cotton, nylon, and Kevlar. For high-performance parts and components, more costly fibers, such as boron, carbon, and graphite, can be specified.


The standard glass fiber used in GRP is a borosilicate type, known as E-glass (E-Gl). The fibers are spun as single glass filaments with diameters ranging from 0.01 to 0.03 mm. These filaments, collected into strands, usually around 200 per strand, are manufactured into many forms of reinforcement. The E-Gl fibers have a tensile strength of 3447 MPa. Another glass fiber, known as S-G1, is higher in strength, but because of its higher cost its use is limited to advanced, high-performance applications. In general, in reinforced thermoplastics, glass content runs between 20 and 40%; with thermosets it runs as high as 80% in the case of filamentwound structures. There are a number of standard forms in which glass fiber is produced and applied in GRP. 1. Continuous strands of glass supplied either as twisted, single-end strands (yarn) or as untwisted multistrands (continuous) roving 2. Fabrics woven from yarns in a variety of type, weights, and widths 3. Woven rovings — continuous rovings worked into a coarse, heavy, drapable fabric 4. Chopped strands made from either continuous or spun roving cut into 3.2- to 12.7-mm lengths 5. Reinforcing mats made of either chopped at random or continuous strand laid down in a random pattern 6. Surfacing mats composed of continuous glass filaments in random patterns

RESINS Although a number of different plastic resins are used as the matrix for reinforced plastics, thermosetting polyester resins are the most common. The combination of polyester and glass provides a good balance of mechanical properties as well as corrosion resistance, low cost, and good dimensional stability. In addition, curing can be done at room temperature without pressure, thus making for low processing-equipment costs.


Polyesters are also available as casting resins, both in water-extended formulations for low-cost castings, and in compounds filled with ground wood or pecan-shell flour for furniture components. Low-profile molding resins are mixtures of polyester resins, thermoplastic polymers, and glass-fiber reinforcements. These are used to mold parts with smooth surfaces that can be painted without the need for prior sanding. For high-volume production, special sheetmolding compounds (SMC) are available in continuous-sheet form. Resin mixtures of thermoplastics with polyesters have been developed to produce high-quality surfaces in the finished molding. These polyester resins are available for use as coatings for curing by ultraviolet radiation. These 100%-solids materials cure in a matter of seconds and release no solvents. Although some styrene may be lost upon exposure to ultraviolet radiation, the amount is small. Prepregs are partially cured thermoset resin-coated reinforcing fabrics in roll or sheet. The prepregs (short for preimpregnated) can be laid or wrapped in place and then fully cured by heat. Other glass-reinforced thermosets include phenolics and epoxies. GR (glass-reinforced) phenolics are noted for their low cost and good overall performance in low-strength applications. Because of their good electrical resistivity and low water absorption, they are widely used for electrical housings, circuit boards, and gears. Since epoxies are more expensive than polyesters and phenolics, GR epoxies are limited to high-performance parts where their excellent strength, thermal stability, chemical resistance, and dielectric strength are required. Initially, GRP materials were largely limited to thermosetting plastics. Today, however, more than 1000 different types and grades of reinforced thermoplastics or GR+P are commercially available. Leaders in volume are nylon and the styrenes. Unlike thermosetting resins, Gr+P parts can be made in standard injection-molding machines. The resin can be supplied as pellets containing chopped glass fibers. As a general rule, a GR+P with chopped fibers at least doubles the tensile strength and stiffness of the plastic. Glass-reinforced

F


PROCESSING METHODS

through an orifice into the heated cavity of a closed matched metal mold. It is the major method for forming reinforced thermoplastics and is used for thermoplastic-modified thermosetting BMCs.

Matched Metal Die Molding

Hand Layup

This is the most efficient and economical method for mass-producing high-strength parts. Parts are press-molded in matched male and female molds at pressures of 1380 to 2068 MPa and at heats of 113 to 127°C. Four main forms of thermosetting resin reinforcement are used:

This is the simplest of all methods of forming thermosetting composites. It is best employed for quantities under 1000, for prototypes and sample runs, for extremely large parts, and for larger volume where model changes are frequent, as in boats. In hand layup, only one mold is used, usually female, which can be made of low-cost wood or plaster. Duplicate molds are inexpensive. The reinforcing mat or fabric is cut to fit, laid in the mold, and saturated with resin by hand, using a brush, roller, or spray gun. Layers are built up to the required thickness; then the laminate is cured to permanent hardness, generally at room temperature.

thermoplastics are also produced as sheet materials for forming on metal-stamping equipment and compression-molding machines.

F

1. Chopped fiber preforms, shaped like the part, are saturated with resin at the mold. They are best for deepdraw, compound curvature parts. 2. Flat mat, saturated with resin at the mold, is used for shallow parts with simple curvature. 3. SMC, a preimpregnated material, has advantages for parts with varying thickness. SMCs consist of polyester resin, long glass fibers (to 5.08 cm), a catalyst, and other additives. They are supplied in rolls, sandwiched between polyethylene carrier films. Uniformity of SMC materials is closely controlled, making these materials especially suited for automated production. Structural SMCs contain up to 65% glass in continuous, as well as random, fiber orientation. This compares with 20 to 35% glass, in random orientation only, of conventional SMC. 4. Bulk molding compound (BMC), a premix of polyester resin, short glass fibers (3.2 to 6.4 mm long), filler, catalyst, and other additives for specific properties. BMC is supplied in bulk form or as extruded rope for ease of handling and it is used for parts similar to castings. Injection Molding In this high-volume process, a mix of short fibers and resin is forced by a screw or plunger © 2002 by CRC Press LLC

Spray-Up Like hand layup, the spray-up method uses a single mold, but it can introduce a degree of automation. This method is good for complex thermoset moldings, and its portable equipment eases on-site fabrication and repair. Short lengths of reinforcement and resin are projected by a specially designed spray gun so they are deposited simultaneously on the surface of the mold. Cure is usually accomplished by a catalyst in the resin at room temperature. Filament Winding This method produces moldings with the highest strength-to-weight ratio of any reinforced thermoset because of its high glass-to-resin ratio. It is generally limited to surfaces of revolution — round, oval, tapered, or rectangular — but it can achieve a high degree of automation. Continuous fiber strands are wound on a suitably shaped mandrel or core and precisely positioned in predetermined patterns. The mandrel may be left in place permanently or removed after cure. The strands may be preimpregnated or the resin may be applied during or after winding. Heat is used to effect final cure.


Centrifugal Casting This is another method of producing round, oval, tapered, or rectangular parts. It offers low labor and tooling costs, uniform wall thicknesses, and good inner and outer surfaces. Chopped fibers and resin are placed inside a mandrel and uniformly distributed as the mandrel is rotated inside an oven. Continuous Laminating Continuous laminating is the most economical method of producing flat and corrugated panels, glazing, and similar products in large volume. Reinforcing mat or fabric is impregnated with resin, run through laminating rolls between cellophane sheets to control thickness and resin content, and then cured in a heating zone. Pultrusion Pultrusion produces shapes with high unidirectional strength such as “I” beams, flat stock for building siding, fishing rods, and shafts for golf clubs. Continuous fiber strands, combined with mat or woven fibers for cross strength, are impregnated with resin and pulled through a long heated steel die. The die shapes the product and controls resin content.

PROPERTIES The mechanical properties of fiber composites are dependent on a number of complex factors. Two of the dominating ones in GRPs are the length of fibers and the glass content by weight. In general, strength increases with fiber length. For example, reinforcing a thermoplastic with chopped glass fibers at least doubles the strength of the plastic, whereas long-glass fiber reinforced the plastics thermoplastics exhibit increases of 300 and 400%. Also heat distortion temperatures usually increase by about 37.8°C, and impact strengths are appreciably increased. Similarly, as a general rule, an increase in glass content results in the following property changes: • Tensile and impact strength increase. • Modulus of elasticity increases.


• Heat deflection temperature increases, sometimes as much as 149°C. • Creep decreases and dimensional stability increases. • Thermal expansion decreases. Properties of thermoset polyesters are so dependent on type, compounding, and processing method that a complete listing covering all combinations would be almost impossible. However, typical strength ranges obtainable in parts fabricated from various forms of polyester/glass compounds and processed by several methods are listed in Table F.3.

FIBERS By definition, a fiber has a length at least 100 times its diameter or width, and its length must be at least 0.5 cm. Length also determines whether a fiber is classified as staple or filament. Filaments are long and/or continuous fibers. Staple fibers are relatively short, and, in practical applications, range from under 2.5 to 15.2 cm long (except for rope, where the fibers can run to several centimeters). Of the natural fibers, only silk exists in filament form; synthetics are produced as both staple and filaments. The internal, microscopic structure of fibers is basically no different from that of other polymeric materials. Each fiber is composed of an aggregate of thousands of polymer molecules. However, in contrast to bulk plastic forms, the polymers in fibers are generally longer and aligned linearly, more or less parallel to the fiber axis. Thus, fibers are generally more crystalline than are bulk forms. Also in contrast to bulk forms, fibers are not used alone, but either in assemblies or aggregates such as yarn or textiles or as a constituent with other materials, such as in composites. Also, compared with other materials, the properties and behavior of both fibers and textile forms are more critically dependent on their geometry. Hence, fibers are sometimes characterized as tiny microscopic beams, and, as such, their structural properties are dependent on such factors as cross-sectional area and shape, and length. The cross-sectional shape and diameter of fibers vary widely. Glass,

F


TABLE F.3 Properties of FRP Parts Process and Reinforcement

F

Glass Fiber (% by wt)

Specific Gravity

Density (g/cm3)

Tensile Strength (MPa)

Flexural Strength (MPa)

Flexural Modulus (MPa)

30–45 30–65

1.4–1.6 1.5–1.7

1.35–1.56 1.45–1.64

62–126 207–385

110–193 207–455

56 69–77

30–50

1.4–1.6

1.35–1.56

62–126

110–193

51

35–50 15–35 15–30

1.5–1.7 1.8–21 1.7–2.1

1.45–1.64 1.75–2.02 1.64–2.02

241–207 27–69 56–140

69–276 60–140 126–207

91–126 98–140 98–140

20–30

1.5–1.7

1.45–1.64

83–140

152–265

91–97

Contact Molding Layup, mat Layup, woven roving Spray-up Compression Molding Preform, mat BMC SMC Cold Press Molding Preform, mat


nylon, Dynel, and Dacron, for example, are essentially circular. Some other synthetics are oval, and others are irregular and serrated round. Cotton fibers are round tubes, and silk is triangular. Fiber diameters range from about 0.01 to 0.04 mm. Because of the irregular cross section of many fibers, it is common practice to specify diameter or cross-sectional area in terms of fineness, which is defined as a weight-to-length or linear density relationship. One exception is wool, which is graded in micrometers. The common measure of linear density is the denier, which is the weight in grams of a 9000-m length of fiber. Another measure is the tex, which is defined as grams per 1 km. A millitex is the number of grams per 1000 km. Of course, the linear density, or denier, is also directly related to fiber density. This is expressed as the denier/density value, commonly referred to as denier per unit density, which represents the equivalent denier for a fiber with the same cross-sectional area and a density of 1. The cross-sectional diameter or area generally has a major influence on fiber and textile properties. It affects, for example, yarn packing, weave tightness, fabric stiffness, fabric thickness and weight, and cost relationships. Similarly, the cross-sectional shape affects yarn packing, stiffness, and twisting characteristics.


It also affects the surface area, which in turn determines the fiber contact area, air permeability, and other properties. For efficient production, a number of filaments are pulled simultaneously from several orifices in the bushing. These filaments (usually numbering about 204) are collected into a bundle, called a “strand,” at a gathering device, where a “size” is applied to the filament surfaces. The strand is then wound into a forming package called a “cake.” From this cake, shippable forms of fibrous glass are produced.

FIBROUS GLASS The primary engineering benefits of glass fibers are their (1) inorganic nature, which makes them highly inert; (2) high strength-to-weight ratio; (3) nonflammability; and (4) resistance to heat, fungi, and rotting. Glass fibers are produced in both filament and staple form. Their major engineering uses are (1) thermal and/or acoustical insulation and (2) as reinforcements, primarily for plastics.

TYPES The largest volume of glass fibers used for engineering applications are so-called “E” type, made from a lime-Al2O3 borosilicate glass that is relatively soda-free. Although its initial


strength at the bushing may be about 2758 to 3447 MPa, surface damage to fibers (both mechanical damage in handling and effects of moisture) reduces usable strength to 1034 to 1380 MPa. But at 1380 MPa tensile strength, the relatively low density of glass (0.092 lb/ft3) produces a strength-to-weight ratio of about 5,511,800 cm, superior to that of a 3060-MPa tensile strength steel. Modulus of E-glass fibers is about 68,666 MPa. Although essentially unaffected by low temperatures, E-glass is limited to a maximum continuous operating temperature of about 316°C. Other specialized types of glass (primarily used in specialty reinforced plastics applications) include:

glass flows. Continuous filaments are then drawn from the molten stream. During the early stages of cooling, the stream is attenuated into filaments by being pulled at very high speeds — usually ranging from 25 to 50 m/s. For efficient production, a number of filaments are pulled simultaneously from several orifices in the bushing. These filaments (usually numbering about 204) are collected into a bundle, called a “strand,” at a gathering device where a “size” is applied to the filament surfaces. The strand is then wound into a forming package called a “cake.” From this cake, shippable forms of fibrous glass are produced.

1. High silica, leached glass fiber — Fibers with silica content of 96 to 99% are produced by leaching glass fibers. Such fibers provide excellent heat resistance, but relatively low strengths. They are usually used in short-fiber form for molding compounds. 2. Silica or quartz fiber — Fibers of pure silica provide optimum heat resistance (to about 93°C), although strength is somewhat lower than that of conventional E-glass. 3. High modulus fibers — Fibers of a beryllia-containing glass have been developed (primarily for filament winding use) with modulus of about 109,866 to 123,599 MPA.

In general, fibrous glass insulation is available in densities ranging from 0.5 to 12 lb/ft3. Maximum operating temperature is about 316 to 1093°C, depending on type of glass. It provides high sound absorption, relatively high tensile strength, and resistance to moisture, fire, rotting, and fungi and bacteria growth. It is available in either flexible or rigid form. The excellent insulating properties of fibrous glass are due to the large pockets of air between the fibers. These air pockets take up considerable volume. Fibrous glass is not affected by low temperatures and has been used satisfactorily at temperatures as low as – 212°C. Heat resistance depends on type of glass: borosilicate glass is generally limited to operating temperatures of 316 to 538°C; high silica glasses are capable of operating at 999°C; silica (quartz) fibers are usable up to 1093°C. The heat resistance of bonded insulations is normally limited by the heat resistance of the binder (maximum, about 232 to 316°C).

PRODUCTION METHODS Most fibrous glass is produced either by air, steam, or flame blowing or by mechanical pulling or drawing. Blowing produces relatively short staple fibers; mechanical drawing produces continuous monofilaments. In blowing, steam or air jets impinge upon and break up molten streams of glass, forming fibers. The type of fiber produced depends on the pressure of the steam or air and the temperature and viscosity of the molten glass. In the mechanical drawing process, the molten glass is fed into a “bushing,” which contains a number of orifices through which the


INSULATION

FORMS Following are the various forms in which fibrous glass is used in reinforced plastics: 1. Rovings consist of a number of strands (usually 60) gathered together from cake packages and wound on a tube to form a cylindrical package. Rovings have very little or

F


2.

F 3.

4.

5.

6.

no twist. They are used either to provide completely unidirectional strength characteristics, such as in filament winding, or are chopped into predetermined lengths for preformmatched metal or spray molding. Chopped strand consists of strands that have been cut into short lengths (usually 12.7 to 50.8 mm) in a manner similar to chopped roving, for use in preform-matched metal or spray molding, or to make molding compounds. It is the least expensive form of fibrous-glass reinforcement. Milled fibers are produced from continuous strands that are hammermilled into small modules of filamented glass (nominal lengths of 0.8 to 3.2 mm). Largely used for filler reinforcement in casting resins and in resin adhesives, they provide greater body and dimensional stability. Yarns are twisted from either filaments or staple fibers on standard textile equipment. Although primarily an intermediate form from which woven fabrics are made, yarns are used for making rod stock, and for some very high strength, unidirectionally reinforced shapes. A common form in which yarn is available is the “warp beam,” where many parallel yarns are wrapped on a mandrel. Nonwoven mats are available both as reinforcing mats and as surfacing or overlay mats. Reinforcing mats are made of either chopped strands or swirled continuous strands laid down in a random pattern. Strands are held together by resinous binders. In laminates, mats provide relatively low strength levels, but strengths are isotropic. Surfacing or overlay mats are both thin mats of staple monofilaments. They provide practically no reinforcing, but serve to stabilize the surface resin coat, providing better appearance. Woven fabrics and rovings provide the highest strength characteristics to reinforced plastic laminates (except


for filament-wound structures), although strengths are orthotropic. A wide variety of fabrics and weaves are available both in woven yarns and woven rovings. Probably the most common types used are plain, basket, crowfoot satin, long shaft satin, unidirectional, and leno weaves.

FILAMENT-WOUND REINFORCED PLASTICS The true fiberglass filament-wound structure may be more appropriately termed a resinbonded filament-wound structure because it comprises approximately 80% glass fibers by weight and 20% bonding resin. Fibers are generally oriented to resist the principal stresses and the resin protects while secondarily supporting the fiber system. Filament winding is well adapted to the fabrication of internal pressure vessels; it has also performed well under external pressure and can be designed to function efficiently as a column or beam. This structure has the tensile strength of moderately heattreated alloy steel and one quarter of the weight.

THE WINDING PROCESS Bands of parallel glass filaments (usually in the form of roving) are wound over a mandrel following a precise pattern in such a way that subsequent bands lie adjacent, progressively covering the mandrel in successive layers, thus generating a shell structure. Liquid resin is simultaneously applied, generally by passing the filament band through a bath of catalyzed resin. Tension generates a running load between the curved work surface and filament band which forces out air and excess resin and allows each successive layer ultimately to rest on solid material while the remaining interstices are filled with resin. Precision of filament placement plus tension and viscosity control are primary controlling factors in the attainment of high fiber content, which is generally desired for high strength. Preimpregnation of the fiber strands is also used as a means of applying the resin binder. Such prepregs must fuse on contact with the


work to accomplish a bond so the fundamental relationships remain the same. The fiber bands are parallel strands only, because a cross weave or other structural filler would not bear primary loads and would preclude the true maintenance of equal tension on fibers in the band or roving when winding over crowned surfaces. Tension serves only the purpose of accomplishing high fiber content and cannot be considered as accomplishing any prestressing. This is primarily true because the dry strength of glass fibers, as wound, is only about one quarter of the ultimate resin consolidated strength. Also, unless some structural component is to remain within the wound shell (rather than a removable mandrel), there is no member against which a prestress can be maintained.

RANGE

AND

ACCURACY

Large or small structures are easily fabricated by adhering to the basic principles. Winding precision is important as is the relation of filament tension, resin viscosity, and radius of curvature of the filament path on the work surface. Small tubes have been made down to 0.65 cm in diameter, and there appear to be no fixed limitations in either direction. Wall thicknesses may be several centimeters or more because the normal bonding resins contain no volatile components and the glass content is so high that there is little danger of the exothermic heat becoming excessive. Dimensional control in winding depends upon mandrel accuracy as well as both material and process control. The bands of filaments are generally 0.13 mm thick, and a full layer requires coverage by both right- and left-hand helices making the layer thickness 0.25 mm. Glass fiber thickness is subject to some variation, and resin content variation will also affect thickness. Wall thickness can generally be held to ±5%. Length and diameter are easily held to 1/10 of 1% and less, because there is little resin shrinkage or wound-in strain due to winding tension. Machining may be accomplished by carbide tools or grinding techniques. Tolerances can be held as closely as in metals. The inner surface as-wound against a good steel mandrel can have a finish of approximately 30 µin., and normal machining or grinding will produce a


40- to 60-µin. finish. Cutting of surface fibers in machining does not weaken the structure.

COMPONENT MATERIALS There are three primary materials in this composite: the glass fiber, fiber finish, and the bonding resin. Glass fibers are continuous and each “end” contains 204 monofilaments approximately 0.01 mm in diameter. These ends are plied together without twist to form a strand of “roving”; 12-end, equally tensioned, roving is generally used for the winding of high-performance structures. The fiber finish generally includes compounds with a chemical affinity for the glass surface and the bonding resin. These are called coupling agents. Other functional components are lubricants and “film formers” for the generation of strand integrity. Both improve handling properties but do not contribute to the performance of filament structures. The resin binder materials are generally liquid at room temperatures or are wound hot for liquid integration of the system. Best results are obtained with strong tough resins such as the epoxies. Polyesters have also been extensively used (primarily for radomes where electrical properties are critical) and any bonding resin should be free of polymerization products of a volatile nature, which would have to escape through the cured structure. Chemical resistance and electrical properties of the constituents are usually critical in selecting the proper resin, as applications of the filament-wound structure are found in both the electrical and chemical industries.

FLAME-SPRAYED COATINGS PROCESSES Flame-spraying methods are used to produce coatings of a wide variety of heat-fusible materials, including metals, metallic compounds, and ceramics. There are three basic flame-spray processes. Wire-Type Guns These guns are used to produce coatings of metals, alloys, and, in some cases, ceramics.

F


F

With this type of equipment wire or rod from 0.76 to 4.7 mm in diameter is fed axially through the center of a fuel gas/O2 flame at a controlled rate. The flame is surrounded by an annular blast of air, which imparts high velocity to the burning gases and provided the kinetic energy needed to atomize the metal as it melts. Acetylene is the most commonly used fuel gas, although propane, natural gas, and H2 are also used. Wire-type guns are the most widely used, because of their versatility and ease of operation. Handheld guns are used to coat large areas such as bridges, ship hulls, and tanks. Machinemounted guns are used principally for machine element applications, as in surfacing rolls or salvaging journals by building up worn areas. In many production applications one or more electronically controlled guns are operated and cycled automatically by a central console. Powder-Type Guns A variety of materials that cannot be readily produced in the form of wire or rod utilize this type of gun. The guns are also used for spraying low-melting metals that are readily available in powder form. With the powder gun, metal or ceramic powders are fed axially through a fuel gas/O2 flame at a controlled rate. The powders are entrained in a carrier gas, which may be air, acetylene, or O2. Because the powders are finely divided and dispersed as they enter the flame, further atomization is not required and annular air blast is not needed. The powder supply for guns of this type may be a reservoir connected to the gun by a powder tube or hose, or it may be a cannister attached directly to the gun. Powder guns are used for flame-spraying a wide variety of ceramics, and for coating with “self-fluxing” alloys, which are subsequently fused to the base material. They have also been used for many years to apply zinc and aluminum coatings for corrosion prevention.

dissociation and ionization occur. The ionized gases form a conductive path within a watercooled nozzle, so that an arc of considerable length is maintained. The gases most commonly used are N2, H2, and argon. Temperature of the plasma flame depends on the type and volume of gas used, the size of the nozzle, and the amount of current used. For flame-spraying purposes temperature ranges of 5482.4 to 8232.4°C are generally employed, although much higher temperatures may be attained if desired. Plasma flame-spray guns usually operate at 20 to 40 kW, using 47 to 141 l/min of gas. In addition to their high-temperature capabilities, plasma guns have other advantages. Extremely high velocities are possible, and favorable environmental conditions can be obtained by proper selection of gases. Spraying within a controlled atmosphere chamber permits the production of oxide-free coatings. With the plasma gun, metal or ceramic powders are fed into the flame at a point downstream from the actual arc path. Current, gas flow, and powder flow must be adjusted for different coating materials, many of which would otherwise be completely vaporized.

SURFACE PREPARATION Regardless of the flame-spray method used or the type of coating material being applied, some sort of surface preparation is usually required. Bond to the base is often largely mechanical, and, in general, the greater the degree of surface roughness, the better the bond. Thin coatings require less elaborate preparation than thick coatings, and some coating materials require much more thorough preparation than other materials. Bonding methods used include abrasive blasting, rough threading, molybdenum bonding coats, heating, or combinations of these steps. Abrasive blasting is probably the most widely used.

Plasma Guns

FINISHING METHODS

The plasma flame is produced by passing suitable gases through a confined arc, where

Flame-sprayed deposits may be finished by machining or by grinding, depending on the



hardness of the particular coating material. Sintered carbide tools are generally used for machining because even the softer materials contain some amount of abrasive oxides that may cause rapid wear of tool steel. For those materials that must be finished by grinding, specific wheel recommendations are available from flame-spray equipment manufacturers. Flame-sprayed coatings may require sealing, depending on the type of coating and service requirements. A wide variety of impregnants are used to reduce porosity, enhance physical properties, or to improve friction characteristics.

FLINT Flint is SiO2 and is a black, gray, or brown cryptocrystalline variety of quartz. In the United States, ceramists often employ the term flint to include other siliceous minerals in addition to true flint. Calcined and ground flint is used in pottery to reduce shrinkage in drying and firing and to give the body a certain rigidity. Flint is employed in the manufacture of whiteware, such as fine earthenware, bone china, and porcelain.

FLUIDIZED-BED COATINGS The fluidized-bed process is used to apply organic coatings to parts by first preheating the parts and then immersing them in a tank of finely divided plastic powders which are held in a suspended state by a rising current of air. In this suspended state, the powders behave and feel like a fluid. The method produces an extremely uniform, high-quality, fusion bond coating that offers many technical and economic advantages. Fusion bond coatings are generally applied to metal parts, although other substrates have been successfully coated. The major fields of application are electrical, chemical, and mechanical equipment as well as household appliances. The process is now being used in every major industrial country in the world for applying many different plastics to a variety of parts.


COATING APPLICATION Objects to be coated are preheated in an oven to a temperature above the melting point of the plastic coating material. The preheat temperature depends on the type of plastic used, the thickness of the coating to be applied, and the mass of the article to be coated. After preheating, the parts are immersed with suitable motion in the fluidized bed. Air used to fluidize the plastic powders enters the tank through a specially designed porous plate located at the bottom of the unit. When the powder particles contact the heated part, they fuse and adhere to the surface, forming a continuous and extremely uniform coating. In many cases, the part is postheated to coalesce the coating completely and improve its appearance. Thickness of the coating depends on the temperature of the part surface and how long it is immersed in the fluidized bed.

COATING TYPES Cellulosic Cellulosic fluidized-bed coatings are noted for their all-around combination of properties and are especially popular for decorative/protective applications. They combine good impact and abrasion resistance with outstanding electrical insulation. The coatings have excellent weathering properties, salt spray resistance, high gloss, and can be made in an almost unlimited range of colors. They can be solvent-etched to provide a satin finish and heat-embossed for additional effects. The economy of cellulosic coatings combined with their excellent appearance and durability are particularly useful for such applications as indoor and outdoor furniture, kitchen fixtures, home and marine hardware, metal stampings, fan guards, and sporting goods. Major uses of cellulosic fusion bond finishes are coated transformer tanks and covers, reclosure tanks and covers, outdoor electrical equipment housings, and many pole line hardware parts.

F


Vinyl

F

Vinyl fluidized-bed coatings have a good combination of chemical resistance, decorative appeal, flexibility, toughness, and low-frequency insulating properties. They have excellent salt-spray resistance, outstanding outdoor weathering characteristics, and can be used for general-purpose electrical insulation. The vinyl fusion bond coatings are claimed to have better uniformity and edge coverage than plasticol coatings. Fusion bond vinyl coatings have been especially successful on wire goods for applications such as dishwasher racks, washing-machine parts, and refrigerator shelves. They are also being used on wire furniture and hardware, and in industrial applications such as bus bars, pump impellers, transformer tanks and covers, auto battery brackets, conveyor rollers, and other material-handling equipment. Cast iron, die castings, and expanded metal parts can be readily coated. Epoxy Both thermoplastic and thermosetting materials can be applied. Epoxy coatings have a smooth, hard surface and exceptionally good electricalinsulation properties over a wide temperature range. They are available in rigid and semiflexible variations with different combinations of electrical, physical, and chemical properties. Epoxy coatings on electrical motor laminations provide good dielectric strength and uniform coverage over sharp edges. When properly applied, epoxy coatings have good impact resistance and do not sacrifice toughness for surface hardness. Other electrical applications include torroidal cores, wound coils, encapsulated printed circuit boards, bus bars, watt hour meter coils, resistors, and capacitors. Nylon The combination of properties that have made the polyamide plastics unique for molded and extruded parts are obtainable in coatings, and nylon used as a solution coating technique applied by the fluidized-bed process offers many advantages to the design engineer.


Nylon fusion bond finishes combine a decorative, smooth, and glossy appearance with low surface friction and excellent bearing and wear properties. They minimize scratching and cut down undesirable noise. The frictional heat developed on nylon is dissipated more rapidly when used as a coating over a conductive metal surface than when used as a solid plastics member. Coated metal parts offer increased dimensional stability. Because of the unique properties of nylon-coated metal parts, many users of the process are able to reduce the number of metal component parts at substantial savings. By an additional immersion of the heated and coated part into a fluid bed of whirling molybdenum sulfide or graphite, an impregnated nylon surface can be produced with unusual bearing, frictional, and wear characteristics. Nylon fusion bonds are effectively used in machine shop fixtures, modern indoor furniture to simulate a wrought iron finish, aircraft instrument panels, ball-joint suspensions, collars, guards, and slide valves used in textile and farm equipment, knitting machine parts, switch box cover panels, tractor control handles, and radar and calculator component parts. Polyethylene Polyethylene coatings combine low water absorption and excellent chemical resistance with good electrical insulation properties. They can be applied successfully in thicknesses of 10 to 60 mils by the fluidized-bed process. The primary uses for polyethylene coatings are for protecting chemical processing equipment and on food-handling equipment. Typical chemical applications include pipe and fittings, pump and motor housings, valves, battery hold-downs, fans, and electroplating jigs. Chlorinated Polyether This fluidized-bed coating has an excellent combination of mechanical, chemical, thermal, and electrical properties. It has good resistance to wear and abrasion. It provides good electrical insulation even under high humidity and high temperature conditions, and has very low moisture absorption. It can be used continuously at 121°C and even up to 149°C.


Chlorinated polyether coatings have excellent chemical resistance and are widely used to coat equipment for the chemical industry such as valves, pipe and pipe fittings, pump housings and impellers, electroplating jigs and fixtures, cams, and bushings. However, chlorinated polyether coatings should not be used in contact with some chlorinated organic solvents, or with fuming sulfuric and nitric acids.

FLUOROCARBONS These are any of the organic compounds in which all of the hydrogen atoms attached to a carbon atom have been replaced by fluorine. Fluorocarbons are usually gases or liquids at room temperature, depending on the number of carbon atoms in the molecule. A major use of gaseous fluorocarbons is in radiation-induced etching processes for the microelectronics industry; the most common one is tetrafluoromethane. Liquid fluorocarbons possess a unique combination of properties that has led to their use as inert fluids for cooling of electronic devices and soldering. Solubility of gases in fluorocarbons has also been used to advantage. For example, they have been used in biological cultures requiring O2, and as liquid barrier filters for purifying air. Fluorocarbons may be made part hydrocarbon and part fluorocarbon, or may contain chlorine. The fluorocarbons used as plastic resins may contain as much as 65% fluorine and also chlorine, but are very stable. Liquid fluorocarbons are used as heat-transfer agents, hydraulic fluids, and fire extinguishers. Benzene-base fluorocarbons are used for solvents, dielectric fluids, lubricants, and for making dyes, germicides, and drugs. Synthetic lubricants of the fluorine type consist of solid particles of a fluorine polymer in a high-molecular-weight fluorocarbon liquid. Chlorine reacts with fluorocarbons to form chlorofluorocarbons, commonly referred to as CFCs. CFC 11 is used as a foam-blowing agent, and CFC 12 is employed as a refrigerant. CFC 113 is a degreasant in semiconductor manufacturing. Because they are strong depletants of stratospheric ozone, the use of CFCs as aerosol propellants has been banned in the United States since 1978, and is being phased out in Europe. Alternatives to


CFCs are being sought for other applications by partially substituting the chlorine with other elements. CFC 22, which has 95% less ozonedepleting capacity than CFC 12, is a potential candidate to replace CFC 12.

FLUOROPLASTICS Also termed fluoropolymers, fluorocarbon resins, and fluorine plastics, fluoroplastics are a group of high-performance, high-price engineering plastics. They are composed basically of linear polymers in which some or all of the hydrogen atoms are replaced with fluorine, and are characterized by relatively high crystallinity and molecular weight. All fluoroplastics are natural white and have a waxy feel. They range from semirigid to flexible. As a class, they rank among the best of the plastics in chemical resistance and elevated-temperature performance. Their maximum service temperature ranges up to about 260°C. They also have excellent frictional properties and cannot be wet by many liquids. Their dielectric strength is high and is relatively insensitive to temperature and power frequency. Mechanical properties, including tensile creep and fatigue strength, are only fair, although impact strength is relatively high.

PTFE, FEP,

AND

PFA

There are three major classes of fluoroplastics. In order of decreasing fluorine replacement of hydrogen, they are fluorocarbons, chlorotrifluoroethylene, and fluorohydrocarbons. There are two fluorocarbon types: tetrafluoroethylene (PTFE or TFE) and fluorinated ethylene propylene (FEP). PTFE is the most widely used fluoroplastic. It has the highest useful service temperature, 260°C, and chemical resistance. Their high melt viscosity prevents PTFE resins from being processed by conventional extrusion and molding techniques. Instead, molding resins are processed by press-andsinter methods similar to those of powder metallurgy or by lubricated extrusion and sintering. All other fluoroplastics are melt processible by techniques commonly used with other thermoplastics. PTFE resins are opaque, crystalline, and malleable. When heated above 341°C, however,

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F

they are transparent, amorphous, relatively intractable, and they fracture if severely deformed. They return to their original state when cooled. The chief advantage of FEP is its low melt viscosity, which permits it to be conventionally molded. FEP resins offer nearly all of the desirable properties of PTFE, except thermal stability. Maximum recommended service temperature for these resins is lower by about 37.8°C. Perfluoroalkoxy (PFA) fluorocarbon resins are easier to process than FEP and have higher mechanical properties at elevated temperatures. Service temperature capabilities are the same as those of PTFE. PTFE resins are supplied as granular molding powders for compression molding or ram extrusion, as powders for lubricated extrusion, and as aqueous dispersions for dip coating and impregnating. FEP and PFA resins are supplied in pellet form for melt extrusion and molding. FEP resin is also available as an aqueous dispersion. Teflon is a tetrafluoroethylene of specific gravity up to 2.3. The tensile strength is up to 23.5 MPa, elongation 250 to 350%, dielectric strength 39.4 × 106 V/m, and melting point 312°C. It is water resistant and highly chemical resistant. Teflon S is a liquid resin of 22% solids, sprayed by conventional methods and curable at low temperatures. It gives a hard, abrasion-resistant coating for such uses as conveyors and chutes. Its temperature service range is up to 204°C. Teflon fiber is the plastic in extruded monofilament, down to 0.03 cm in diameter, oriented to give high strength. It is used for heat- and chemical-resistant filters. Teflon tubing is also made in fine sizes down to 0.25 cm in diameter with wall thickness of 0.03 cm. Teflon 41-X is a collodial water dispersion of negatively charged particles of Teflon, used for coating metal parts by electrodeposition. Teflon FEP is fluorinated ethylenepropylene in thin film, down to 0.001 cm thick, for capacitors and coil insulation. The 0.003-cm film has a dielectric strength of 126 × 106 V/m, tensile strength of 20 MPa, and elongation of 250%.


Properties Outstanding characteristics of the fluoroplastics are chemical inertness, high- and low-temperature stability, excellent electrical properties, and low friction. However, the resins are fairly soft and resistance to wear and creep is low. These characteristics are improved by compounding the resins with inorganic fibers or particulate materials. For example, the poor wear resistance of PTFE as a bearing material is overcome by adding glass fiber, carbon, bronze, or metallic oxide. Wear resistance is improved by as much as 1000 times, and the friction coefficient increases only slightly. As a result, the wear resistance of filled PTFE is superior, in its operating range, to that of any other plastic bearing material and is equaled only by some forms of carbon. The static coefficient of friction for PTFE resins decreases with increasing load. Thus, PTFE bearing surfaces do not seize, even under extremely high loads. Sliding speed has a marked effect on friction characteristics of unreinforced PTFE resins; temperature has very little effect. PTFE resins have an unusual thermal expansion characteristic. A transition at 18°C produces a volume increase of over 1%. Thus, a machined part, produced within tolerances at a temperature on either side of this transition zone, will change dimensionally if heated or cooled through the zone. Electrical properties of PTFE, FEP, and FPA are excellent, and they remain stable over a wide range of frequency and environmental conditions. Dielectric constant, for example, is 2.1 from 60 to 109 Hz. Heat-aging tests at 300°C for 6 months show no change in this value. Dissipation factor of PTFE remains below 0.0003 up to 108 Hz. The factor for FEP and PFA resins is below 0.001 over the same range. Dielectric strength and surface arc resistance of fluorocarbon resins are high and do not vary with temperature or thermal aging (Table F.4).

CTFE

OR

CFE

Chlorotrifluoroethylene (CTFE or CFE) is stronger and stiffer than the fluorocarbons and


TABLE F.4 Properties of Fluoroplastics ASTM or UL Test

Property

PTFE

FEP

PFA

D792 D792 D570


2.13–2.24 13–12.3 1018 >180

>1018 >180

2 × 1014 50–70

2.5 × 1016 3360

1016 75

1.350

Optical 1.344

1.350

1.42

1.435

1.403

—

>95

>95

>90

>90

—

0.214

0.14

—

0.400

Frictional —

a

Coefficient of friction Against steel (100 psi, 10 fpm)

0.050

0.330

Crystalline compound. Below and above 135°F.



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F

has better creep resistance. Like FEP and unlike PTFE, it can be molded by conventional methods. Sensitivity to processing conditions is greater in CTFE resins than in most polymers. Molding and extruding operations require accurate temperature control, flow channel streamlining, and high pressure because of the high melt viscosity of these materials. With too little heat, the plastic is unworkable; too much heat degrades the polymer. Degradation begins at about 274°C. Because of the lower temperatures involved in compression molding, this process produces CTFE parts with the best properties. Thin parts such as films and coil forms must be made from partially degraded resin. The degree of degradation is directly related to the reduction in viscosity necessary to process a part. Although normal, partial degradation does not greatly affect properties, seriously degraded CTFE becomes highly crystalline, and physical properties are reduced. Extended usage above 121°C also increases crystallinity. CTFE plastic is often compounded with various fillers. When plasticized with lowmolecular-weight CTFE oils, it becomes a soft, extensible, easily shaped material. Filled with glass fiber, CTFE is harder, more brittle, and has better high-temperature properties. Properties CTFE plastics are characterized by chemical inertness, thermal stability, and good electrical properties, and are usable from 400 to –400°C. Nothing adheres readily to these materials, and they absorb practically no moisture. CTFE components do not carbonize or support combustion. Up to thicknesses of about 3.2 mm, CTFE plastics can be made optically clear. Ultraviolet absorption is very low, which contributes to its good weatherability. Compared with PTFE, FEP, and PFA fluorocarbon resins, CTFE materials are harder, more resistant to creep, and less permeable; they have lower melting points, higher coefficients of friction, and are less resistant to swelling by solvents than the other fluorocarbons. Tensile strength of CTFE moldings is moderate, compressive strength is high, and the


material has good resistance to abrasion and cold flow. CTFE plastic has the lowest permeability to moisture vapor of any plastic. It is also impermeable to many liquids and gases, particularly in thin sections.

PVF2

AND

PVF

The fluorohydrocarbons are of two kinds: polyvinylidene fluoride (PVF2) and polyvinyl fluoride (PVF). Although similar to the other fluoroplastics, they have somewhat lower heat resistance and considerably higher tensile and compressive strength. Except for PTFE, the fluoroplastics can be formed by molding, extruding, and other conventional methods. However, processing must be carefully controlled. Because PTFE cannot exist in a true molten state, it cannot be conventionally molded. The common method of fabrication is by compacting the resin in powder form and then sintering. PVF2, the toughest of the fluoroplastic resins, is available as pellets for extrusion and molding and as powders and dispersions for corrosion-resistant coatings. This high-molecular-weight homopolymer has excellent resistance to stress fatigue, abrasion, and to cold flow. Although insulating properties and chemical inertness of PVDF are not as good as those of the fully fluorinated polymers, PTFE and FEP, the balance of properties available in PVDF qualifies this resin for many engineering applications. It can be used over the temperature range from –73 to 149°C and has excellent resistance to abrasion. PVDF can be used with halogens, acids, bases, and strong oxidizing agents, but it is not recommended for use in contact with ketones, esters, amines, and some organic acids. Although electrical properties of PVDF are not as good as those of other fluoroplastics, it is widely used to insulate wire and cable in computer and other electrical and electronic equipment. Heat-shrinkable tubing of PVDF is used as a protective cover on resistors and diodes, as an encapsulant over soldered joints. Valves, piping, and other solid and lined components are typical applications of PVDF in chemical-processing equipment. It is the only fluoroplastic available in rigid pipe form.


Woven cloth made from PVDF monofilament is used for chemical filtration applications. A significant application area for PVDF materials is as a protective coating for metal panels used in outdoor service. Blended with pigments, the resin is applied, usually by coilcoating equipment, to aluminum or galvanized steel. The coil is subsequently formed into panels for industrial and commercial buildings. A recently developed capability of PVDF film is based on the unique piezoelectric characteristics of the film in its so-called beta phase. Beta-phase PVDF is produced from ultrapure film by stretching it as it emerges from the extruder. Both surfaces are then metallized, and the material is subjected to a high voltage to polarize the atomic structure. When compressed or stretched, polarized PVDF generates a voltage from one metallized surface to the other, proportional to the induced strain. Infrared light on one of the surfaces has the same effect. Conversely, a voltage applied between metallized surfaces expands or contracts the material, depending on the polarity of the voltage.

PFA, ECTFE,

AND

ETFE

The following three fluoroplastics are melt processible. Perfluoroalkoxy (PFA) can be injection-molded, extruded, and rotationally molded. Compared to FEP, PFA has slightly greater mechanical properties at temperatures over 150°C and can be used up to 260°C. Ethylene-chlorotrifluoroethylene (ECTFE) copolymer resins also are melt processible with a melting point of 240°C. Their mechanical properties — strength, wear resistance, and creep resistance, in particular — are much greater than those of PTFE, FEP, and PFA, but their upper temperature limit is about 165°C. ECTFE also has excellent property retention at cryogenic temperatures. Ethylene-tetrafluoroethylene (ETFE) copolymer resin is another melt-processible fluoroplastic with a melting point of 270°C. It is an impact-resistant, tough material that can be used at temperatures ranging from cryogenic up to about 179°C. One of the advantages of the coploymers of ethylene and TFE — called modified ETFE —


and of ethylene and CTFE — called ECTFE — compared with PTFE and CTFE is their ease of processing. Unlike their predecessors, they can be processed by conventional thermoplastic techniques. Various grades can be made into film or sheet, into a monofilament, or used as a powder coating; all grades can be heat-sealed or welded. Although these resins have lower heat resistance than PTFE or CTFE, they offer a combination of properties and processibility that is unattainable in the predecessor resins. Maximum service temperature for no-load applications is in the range of 149 to 199°C for ETFE and ECTFE, compared with 199°C for CTFE and 288°C for PTFE. Glass reinforcement increases these values by 10°C. Both tensile strength and toughness of these resins are higher than those of the other fluoropolymers; they are rated “no break” in notched Izod tests. The modulus of ECTFE is higher than that of ETFE up to about 100°C; above 150°C, ETFE has a higher modulus. Deflection temperature of both resins is similar, with ECTFE slightly higher (116°C, compared with 104°C, at 0.44 MPa, and 77°C compared with 71°C at 1820 MPa). Hardness of ETFE is Rockwell R50; that of ECTFE is R93; see Table F.4. The limiting oxygen index (LOI) of ETFE is 31; that of ETCFE is 60. (LOIs of PTFE, FEP, and CTFE are over 95.) As with other fluoroplastics, these resins are compatible with most chemicals, even at high temperatures. ETFE is not attacked by most solvents to temperatures as high as 199°C. ECTFE is similar to 121°C, but is attacked by chlorinated solvents at higher temperatures. ETFE has better chemical stress-crack resistance. Applications for these resins include wire and cable insulation, chemical-resistant linings and molded parts, laboratoryware, and molded electrostructural parts.

FLUORSPAR Also called fluorite, fluorspar is a crystalline or massive granular mineral of the composition CaF2, used as a flux in the making of steel, for making hydrofluoric acid, in opalescent glass, in ceramic enamels, for snaking artificial cryolite, as a binder for vitreous abrasive wheels,

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F

and in the production of white cement. It is a better flux for steel than limestone, making a fluid slag, and freeing the iron of sulfur and phosphorus. About 2.5 kg of fluorspar is used per ton of basic open-hearth steel. Acid spar is a grade used in making hydrofluoric acid. It is also used for making refrigerants, plastics, and chemicals, and for aluminum reduction. Optical fluorspar is the highest grade but is not common. Fluoride crystals for optical lenses are grown artificially from acid-grade fluorspar. Pure calcium fluoride, Ca2F6, is a colorless crystalline powder used for etching glass, in enamels, and for reducing friction in machine bearings. It is also used for ceramic parts resistant to hydrofluoric acid and most other acids. Calcium fluorite has silicon in the molecule and is a crystalline powder used for enamels. The clear rhombic fluoride crystals used for transforming electric energy into light are lead fluoride, PbF2.

FLUX Flux is a substance added to a refractory material to aid in its fusion. A secondary action of a flux, which may also be a primary reason for its use, is as a reducing agent to deoxidize impurities. Any material that lowers the melting temperature of another material or mixture of materials is a flux. Fluxing substances may occur as natural impurities in a raw material. Thus, the alkali content of a clay will flux the clay. In other cases, fluxes are separate raw materials. Example: use of feldspar to flux a mixture of clays and flint. An auxiliary flux is a third component that may make the primary flux more effective. Thus, addition of 2% dolomite, talc, or fluorspar to a whiteware mixture that contains 25% feldspar will produce a substantial decrease in vitrification temperature. The auxiliary constituent may be incapable of producing the same result (or too expensive to use) as the sole flux. Compounds of alkali metals (sodium, potassium, and lithium) are popular fluxes for clay bodies. Compounds of alkaline earth metals (calcium, magnesium, and, to a lesser extent, barium and strontium) are common auxiliary fluxes. However, they also may be primary fluxes for such products as low-loss


dielectrics. Lead and boron compounds are important fluxes for glasses, glazes, and enamels. And premelted glasses or frits may be used to flux clay or other bodies. The term flux also may be used to specify a low-melting glass used in decorating glass products or an overglaze for clayware. Pigments are mixed with the powdered glass flux and then applied to the object to produce a vitrifiable coating at temperatures 4400 (subl.)

>6500 (subl.)

>3450 (subl.)

Alumina 3722

Specific gravity Crystal

2.25

2.25

3.18

3.96

Body

2.10

1.6–2.0

1.5–2.7

2.6–3.9

Hardness, DPHa

30

20

1100

2800

Modulus of rupture (room temp.), 1000 psib

16||

2.2||c 1–20

38

7.3|

1.8|

4.17||

4.5–12||

0.43|

1.5–4|

1.37

4.3

199|d

120–360

130

20–30

At room temp.

1.7 × 1013||b

10–3c

At 900°F

2.3 × 1010||

Coefficient of thermal expansion (avg at 70–1800°F)/°F × 106b Thermal conductivity (room temp.), Btu-in./ft2 h/°Fb

105||d

Electrical resistance, Ω-cm

Dielectric constant a b c

N

d

4.1–4.8

—

1013

1016

1013

1012

9.4

12.3

Diamond pyramid hardness. || = Parallel to molding pressure; | = perpendicular. Varies widely with type of graphite. At 570°F.

in chlorine up to 704°C. However, at 982°C it is attacked rapidly. Like commercial graphite, hot-pressed boron nitride is anisotropic. Thermal expansion parallel to the direction of pressing is ten times that in the perpendicular direction. The ratio for modulus of rupture is 2:1. Although boron nitride resembles graphite in many respects, it differs uniquely in electrical characteristics, having high resistivity and high dielectric strength even at elevated temperatures (see Table N.4). This feature, combined with easy machinability has led to extensive use in high-temperature electronics. Boron nitride is available as –325-mesh powder. A cubic form of boron nitride (Borazon) similar to diamond in hardness and structure has been synthesized by the high-temperature, high-pressure process for making synthetic diamonds. Any uses it may find as a substitute for


diamonds will depend on its greatly superior oxidation resistance. Cutting tool materials like mixed ceramics or CBN cutting tools are already available for hard machining. To identify the proper cutting tool material, one must analyze the application. In case of cutting interruptions, CBN cutting tools will be the appropriate choice. Continuous cuts allow the use of mixed ceramics or coated mixed ceramics for better efficiency. When producing a gear wheel, for example, turning with a CBN-tipped insert reduces the cost per wheel by more than 60%, compared to grinding. At the same time, disposal costs for grinding sludge vanish, because hard turning does not require coolant. Silicon Nitride Silicon nitride is most easily prepared by direct reaction of nitrogen at about 1316°C with

TX66613_frame_N(13) Page 467 Wednesday, March 13, 2002 11:35 AM

finely divided elemental silicon (≤150 mesh), either as loose powder or as a slip-cast or otherwise preformed part. Conversion of silicon particles to the nitride Si3N4 is accompanied by the growth of a felt of interlocking needles in the void space between particles. Despite an overall porosity of 15 to 25%, silicon nitride bodies are effectively impervious in many applications because of the microscopic size of the pores. Although silicon nitride is not machinable in its final form except by grinding, the partially converted body can be machined by conventional methods after which conversion can be completed without dimensional change. Silicon nitride is indefinitely resistant to air oxidation up to 1649°C, but begins to sublime at about 1925°C. It is not attacked by chlorine at 899°C or hydrogen sulfide at 982°C nor by the common acids. Because of a low coefficient of thermal expansion, resistance to thermal shock is relatively good. Uncoated and coated silicon nitride cutting tools dominate the high-performance end of gray cast iron machining. They typically offer metal removal rates at least three times higher than coated carbide grades. The newly developed cutting tool combines a 6% cobalt substrate with a 10-µm-thick, medium-temperature TiCN/Al2O3/TiN coating. Medium-temperature chemical vapor desposition TiCN coatings show a reduced tendency for the forming of eta-phase at the interface between coating and carbide substrate. Recently developed silicon nitride cutting tools have a substantially improved fracture resistance. Because of their insufficient chemical wear resistance, however, they have a limited use in machining nodular cast irons, mainly in areas of severe cutting interruptions at higher speeds (>400 m/min). Titanium and Zirconium Nitride Titanium and zirconium nitrides for use in refractory bodies are most conveniently prepared by treating the corresponding metal hydrides with ammonia at 1000°C. Sintered TiN can be heated to a bright red heat with only superficial oxidation, and then


plunged into water without cracking; ZrN is less resistant to oxidation. Combination coatings involving both chemical vapor deposition and physical vapor deposition technologies provide the wear-resistance advantages of chemical vapor deposition TiCN coatings with the compressive residual stress advantages of physical vapor deposition TiN coatings. The net result is improved wear and chipping resistance. These coatings increase the speed capabilities of carbide cutting tools in titanium turning by a factor of 2. Eliminating coolants can turn an easy-tomachine material into a difficult drilling problem, when using standard cutting tools. The introduction of TiAlN coatings represents a significant step toward dry drilling. The goal in all dry machining is to develop cutting tools with higher resistance to thermal load and fatigue. Cermet tools may be one of the most suitable materials for these applications.

NITRIDING STEELS Nitriding steels are alloy steels (low- and medium-carbon steels with combinations of chromium and aluminum or nickel, chromium, and aluminum) designed particularly for optimum results when they are subjected to the nitriding operation. The composition is such that the required microstructure for optimum nitriding is produced after heat treatment. Nitrided parts made from nitriding steels have extremely high surface hardnesses of about 92 to 95 Rockwell N scale, wear resistance, and resistance to certain types of corrosion.

PROCESSING

AND

APPLICATIONS

Nitriding consists of exposing steel parts to gaseous ammonia at about 538°C to form metallic nitrides at the surface. The hardest coatings are obtained with aluminum-bearing steels. Nitriding of stainless steel is known as Malcomizing. After nitriding, these steels have extremely high surface hardnesses of about 92 to 95 Rockwell N. The nitride layer also has considerable resistance to corrosion from alkalies, the atmosphere, crude oil, natural gas, combustion products, tap water, and still salt water. Nitrided

N


parts usually grow about 0.003 to 0.005 cm during nitriding. The growth can be removed by grinding or lapping, which also removes the brittle surface layer. Most uses of nitrided steels are based on resistance to wear. The steels can also be used at temperatures as high as 538°C for long periods without softening. The slick, hard, and tough nitrided surface also resists seizing, galling, and spalling. Typical applications are cylinder liners and barrels for aircraft engines, bushings, shafts, spindles and thread guides, cams, rolls, piston pins, rubber and paper-mill product rolls, special oil tool equipment, bearings, rollers, etc.

FABRICATION

N

The fabrication characteristics of nitriding steels are basically the same as those of other steels of similar alloy content. They can be drilled, broached, tapped, milled, sawed, or ground. Light feeds and depth of cuts are recommended. Welding is done with rod or wire of similar composition. Flash welding is permissible. If very heavy cuts are involved, they usually are made prior to heat treatment. Normal machining is done on heat-treated material, and is followed by a stress-relieving treatment of not less than 37.8°C above the nitriding temperature, before finish machining or grinding. It is essential that in machining, sufficient removal be allowed to remove all decarburization from the surface prior to nitriding. The surface also must be clean and free of any surface contamination. Because nitriding is a low-temperature treatment, little or no warpage is encountered. If it is necessary to straighten because of residual stress, the part should be heated to 538 to 593°C to prevent surface cracking. Nitrided parts normally grow about 0.03 to 0.05 mm during nitriding. This may be removed by grinding or lapping. This also has the advantage of removing a brittle layer on the surface and exposes a slightly harder layer immediately beneath it. This operation, however, will reduce corrosion resistance to a large degree. Nitriding steels are available in all standard steel forms. They can be purchased heat-treated or annealed to desired physical properties.


Most uses of nitriding steels are based on resistance to wear. An outstanding property is that these steels can be heated to as high as 538°C for long periods without softening. The slick, hard surface produced also makes it ideal to prevent seizing, galling, and spalling, and it is not readily attacked by combustion products.

NITRILE RUBBER The nitriles are copolymers of butadiene and acrylonitrile, used primarily for applications requiring resistance to petroleum oils and gasoline. Resistance to aromatic hydrocarbons is better than that of neoprene but not as good as that of polysulfide. Nitrile butyl rubber (NBR) has excellent resistance to mineral and vegetable oils, but relatively poor resistance to the swelling action of oxygenated solvents such as acetone, methyl ethyl ketone, and other ketones. It has good resistance to acids and bases except those having strong oxidizing effects. Resistance to heat aging is good, often a key advantage over natural rubber. With higher acrylonitrile content, the solvent resistance of an NBR compound is increased but low-temperature flexibility is decreased. Low-temperature resistance is inferior to that of natural rubber, and although NBR can be compounded to give improved performance in this area, the gain is usually at the expense of oil and solvent resistance. As with SBR, this material does not crystallize on stretching, and reinforcing materials are required to obtain high strength. With compounding, nitrile rubbers can provide a good balance of low creep, good resilience, low permanent set, and good abrasion resistance. Tear resistance is inferior to that of natural rubber, and electrical insulation is lower. NBR is used instead of natural rubber where increased resistance to petroleum oils, gasoline, or aromatic hydrocarbons is required. Uses of NBR include carburetor and fuel-pump diaphragms and aircraft hoses and gaskets. In many of these applications, the nitriles compete with polysulfides and neoprenes. Essentially the same techniques of emulsion polymerization employed in manufacture of general-purpose synthetic rubber may be used for


production of nitrile polymers. Nitrile rubbers are supplied in various physical forms including sheet, crumb, powder, and liquid. The sheet is the most widely used type, with the other varieties offered for specialty applications.

BLENDS An outstanding feature of nitrile rubber is its compatibility with many different types of resins permitting it to be easily blended with them. In combination with phenolic resins it provides adhesives with especially high strengths. Other resins used include resorcinol formaldehyde, urea formaldehyde, alkyd, epoxy, and polyvinyl chloride (to produce Type 2 rigid PVC). Both slab- and crumb-type nitrile rubber are used in this type of application, with the crumb type directly soluble and of special interest to adhesive manufacturers who do not have rubber-mixing equipment. Nitrile rubber–phenolic resin solvent solutions are used in shoe sole-attaching adhesives, for structural bonding in aircraft, adhering automotive brake lining to brake shoes, and many other industrial applications. The powder-type rubbers were also developed for blending with phenolic resins, primarily for the manufacture of improved impact phenolic molding powders. The liquid nitrile polymer finds use as a tackifier and nonextractable plasticizer in molded rubber parts, cements, friction, and calendered stocks. Both the liquid and powder are of interest as curing-type plasticizers in vinyl plastisols. Nitrile rubber–PVC blends of various types are used in many other fields including cable jacket, retractable cord, abrasion-resistant shoe soles, industrial face masks, boat bumpers, and fuel lines.

COMPOSITION The ratio of butadiene to acrylonitrile in the commercially available rubbers ranges from a low of about 20% to as high as 50% acrylonitrile. The various grades are usually referred to as high, medium-high, medium-low, and low acrylonitrile content.


The high acrylonitrile polymers are used in applications requiring maximum resistance to aromatic fuels, oils, and solvents. This would include oil well parts, fuel-cell liners, fuel hose, and other similar applications. The low acrylonitrile grade finds use in those areas requiring good flexibility at very low temperatures where oil resistance is of secondary importance. The medium types are most widely used and are satisfactory for all oilresistant applications between these two extremes. Typical applications include conveyor belts, flexible couplings, soles, heels, floor mats, printing blankets, rubber rollers, sealing strips, aerosol bomb gaskets, milking inflations, seals, diaphragms, O-rings, packings, hose, washing machine parts, valves, and grinding wheels. These established uses give only a slight idea of products that are made of nitrile rubbers. Physical properties of cured nitrile rubber parts are directly related to the ratio of butadiene and acrylonitrile in the polymer, as indicated below: As acrylonitrile content increases: 1. 2. 3. 4. 5. 6.

Oil and solvent resistance improve. Tensile strength increases. Hardness increases. Abrasion resistance improves. Gas impermeability improves. Heat resistance improves.

As acrylonitrile content decreases: 1. Low-temperature resistance improves. 2. Resilience increases. 3. Plasticizer compatibility increases.

PROPERTIES The polymer with the highest acrylonitrile content produces the highest tensile strength and hardness; it also exhibits the best resistance to fuels and oils. As the percentage of acrylonitrile decreases, there is a corresponding decrease in resistance to fuels and oils; at the same time low-temperature flexibility characteristics are improved. Resiliency also increases. The lowest acrylonitrile polymer exhibits only moderate resistance to swelling in aromatic fluids but

N


remains flexible at very low temperatures in the range of –57 to –62°C. Thus, properly compounded nitrile polymers will provide high tensile strength, excellent resistance to abrasion, low compression set, very good aging under severe operating conditions, and excellent resistance to a wide range of fuels, oils, and solvents. They are practically unaffected by alkaline solutions, saturated salt solutions, and aliphatic hydrocarbons, both saturated and unsaturated. They are affected little by fatty acids found in vegetable fats and oils or by aliphatic alcohols, glycols, or glycerols. Nitrile rubber is not recommended, generally, for use in the presence of strong oxidizing agents, ketones, acetates, and a few other chemicals.

NITROCARBURIZING

N

Salt bath nitrocarburizing is a thermochemical process for improving the properties of ferrous metals. However, some tools and other highalloy steels are susceptible to reductions in core hardness after standard nitrocarburizing. To prevent such losses, a low-temperature salt bath nitrocarburizing process has been developed. With treatment temperatures as low as 480°C, this process not only maintains core hardness, but also can sometimes increase core hardness.

PROCESSING During salt bath nitrocarburizmg, the part is immersed in a vessel of molten salt. Nitrogen and carbon in the salt react with the iron on the surface, forming a compound layer with an underlying diffusion zone. The compound layer consists of iron nitrides, chromium nitrides, or other such compounds, depending on the alloying elements in the steel, and small amounts of carbides. Ranging in depth from 2.5 to 20 µm, the compound layer provides improvements in wear and corrosion resistance, as well as in service behavior and hot strength. Hardness of the compound layer, measured on a cross section, ranges from 700 HV on unalloyed steels, up to 1600 HV on high chromium steels. Note


that this layer is formed from the base metal and is an integral part of it, and is therefore not a coating. The diffusion zone can extend as deep as 1 mm, depending on the steel. This diffusion zone causes an increase in rotatingbending strength and rolling fatigue strength as well as pressure loadability. Salt bath nitrocarburizing may be applied to a wide range of ferrous metals, from lowcarbon to tool steels, cast iron to stainless steels. Specifically, the process: • Improves wear and corrosion resistance • Reduces or eliminates galling and seizing • Increases fatigue strength • Raises surface hardness • Provides highly predictable, repeatable results • Performs consistently, even with varying contours and thicknesses within the same part or load • Maintains dimensional integrity • Shortens cycle times • Offers flexibility and ease of operation Conventional treatment temperatures are in the range of 580°C, but for highly alloyed steels as well as stainless and tool steels, this temperature can cause a reduction in core hardness. The above benefits, derived both from the nitrogen and carbon diffused into the metal surface, as well as the processing in a liquid bath, are often necessary for applications in which a reduction in core hardness is not acceptable. For this reason, a new low-temperature process was developed. The low temperature process normally takes place at 480°C, although it can operate at 480 to 520°C. This process has specific advantages: • Core hardness and tensile strength are maintained in the tempered condition. • Very thin compound layers can be formed. • Distortion is extremely low.


• Formation of a compound layer on high-speed steels can be suppressed. • Hardness of surface and diffusion layers can be customized. This low-temperature process is beneficial for high-alloy steels such as stainless, tool, die, and high-speed steels (see Table N.5).

TABLE N.5 Suitable Steels for Low-Temperature Nitrocarburizing Steel

Application

D2 D3 AISI 420 H11 H13 HNV 3 17 4 PH HSS 36 HSS M2

Cold-work tool steel Cold-work tool steel Cold-work tool steel Die-cast tools, machine pistons Die-cast tools, pistons, extrusion dies Valves (martensitic) Planetary gears, press tools High-speed steel drill bits High-speed steel drill bits

Source: Adv. Mater. Proc., 154(3), 40–43, 1998. With permission.

NITROGEN Nitrogen is an element (symbol N) that at ordinary temperatures is an odorless and colorless gas. The atmosphere contains 78% nitrogen in the free state. It is nonpoisonous and does not support combustion. Nitrogen is often called an inert gas, and is used for some inert atmospheres for metal treating and in lightbulbs to prevent arcing, but it is not chemically inert. It is a necessary element in animal and plant life, and is a constituent of many useful compounds. Lightning forms small amounts of nitric oxide from the air, which is converted into nitric acid and nitrates, and bacteria continuously convert atmospheric nitrogen into nitrates. Nitrogen combines with many metals to form hard nitrides useful as wear-resistant metals. Small amounts of nitrogen in steels inhibit grain growth at high temperatures, and also increase the strength of some steels. It is also used to produce a hard surface on steels.


APPLICATIONS Because of the importance of nitrogen compounds in agriculture and chemical industry, much of the industrial interest in elementary nitrogen has been in processes for converting elemental nitrogen into nitrogen compounds. The principal methods for doing this are the direct synthesis of ammonia from nitrogen and hydrogen, the electric arc process, which involves the direct combination of N2 and O2 to nitric oxide, and the cyanamide process.

NODULAR CAST IRON Nodular cast irons, such as GGG 40, GGG 50, and GGG 60, have become popular for parts such as housings, wheel parts, crankshafts, and camshafts. These metals offer higher strength and toughness than other cast irons, a result of spherical inclusions of carbon in the metal matrix. Generally easy to machine, GGG 40 irons with higher ferrite content tend to produce built-up edges on the cutting tool. For materials such as GGG 60 and higher, abrasiveness increases as the pearlite content increases, which can result in rapid insert wear. These nodular iron grades present unique machining characteristics.

NONMAGNETIC STEELS These are steel and iron alloys used where magnetic effects cannot be tolerated. Manganese steel containing 14% manganese is nonmagnetic and casts readily but is not machinable. Nickel steels and iron–nickel alloys containing high nickel content are nonmagnetic. Many mills regularly produce nonmagnetic steels containing from 20 to 30% nickel. Manganese–nickel steels and manganese–nickel–chromium steels are nonmagnetic and may be formulated to combine desirable features of the nickel and manganese steels. One nonmagnetic steel with a composition of 10.5 to 12.5% manganese, 7 to 8% nickel, and 0.25 to 0.40% carbon has low magnetic permeability and low eddy-current loss, can be machined readily, and work-hardens only slightly. The tensile strength is 551 to 758 MPa, elongation 25 to 50%, and specific gravity 8.02. It is austenitic and cannot be hardened. The 18-8

N


austenitic chromium–nickel steels are also nonmagnetic. A nonmagnetic alloy used for watch gears and escapement wheels is not a steel but is a copper–nickel–manganese alloy containing 60% copper, 20% nickel, and 20% manganese. It is very hard, but can be machined with diamond tools.

NONWOVEN FABRICS

N

In the most general sense, nonwoven fabrics are fibrous-sheet materials consisting of fibers mechanically bonded together by interlocking or entanglement, by fusion, or by an adhesive. They are characterized by the absence of any patterned interlooping or interlacing of the yarns. In the textile trade, the terms nonwovens and bonded fabrics are applied to fabrics composed of a fibrous web held together by a bonding agent, as distinguished from felts, in which the fibers are interlocked mechanically without the use of a bonding agent. There are three major kinds of nonwovens based on the method of manufacture. Dry-laid nonwovens are produced by textile machines. The web of fibers is formed by mechanical or air-laying techniques, and bonding is accomplished by fusion-bonding the fibers or by the use of adhesives or needle punching. Either natural or synthetic fibers, usually 2.5 to 7.6 cm in length, are used. Wet-laid nonwovens are made on modified papermaking equipment. Either synthetic fibers or combinations of synthetic fibers and wood pulp can be used. The fibers are often much shorter than those used in dry-laid fabrics, ranging from 0.64 to 1.27 cm in length. Bonding is usually accomplished by a fibrous binder or an adhesive. Wet-laid nonwovens can also be produced as composites, for example, tissue-paper laminates bonded to a reinforcing substrate of scrim. Spin-bonded nonwovens are produced by allowing the filaments emerging from the fiber-producing extruder to form into a random web, which is then usually thermally bonded. These nonwovens are limited commercially to thermoplastic synthetics such as nylons, polyesters, and polyolefins. They have exceptional strength because the filaments are continuous and bonded to each other without an auxiliary


bonding agent. Fibers in nonwovens can be arranged in a great variety of configurations that are basically variations of three patterns: parallel or unidirectional, crossed, and random. The parallel pattern provides maximum strength in the direction of fiber alignment, but relatively low strength in other directions. Cross-laid patterns (like wovens) have maximum strength in the directions of the fiber alignments and less strength in other directions. Random nonwovens have relatively uniform strength in all directions.

NYLON PLASTICS Although nylon polymers are most familiar in fiber form, their combination of excellent chemical and mechanical properties, plus their ability to be molded and extruded into precise forms, have permitted their use in a wide variety of nontextile applications. These range from hammerheads, gears, and rifle stocks to miniature coil forms and delicately colored personal products.

CHARACTERISTICS Nylons are a group of polyamide resins that are long-chain polymeric amides in which the amide groups form an integral part of the main polymer chain, and that have the characteristic that when formed into a filament the structural elements are oriented in the direction of the axis. Nylon was originally developed as a textile fiber, and high tensile strengths, above 344 MPa, are obtainable in the fibers and films. But this high strength is not obtained in the molded or extruded resins because of the lack of oriented stretching. When nylon powder that has been precipitated from solution is pressed and sintered, the parts have high crystallinity and very high compressive strength, but they are not as tough as molded nylon.

PRODUCTION Nylons are produced from the polymerization of a dibasic acid and a diamine. The most common one of the group is that obtained by the reaction of adipic acid with hexamethylenediamine.


More specifically, nylons may be made by the amidation of diamines with dibasic acids, for example, hexamethylenediamine plus adipic acid (Type 6/6 nylon), or hexamethylenediamine and sebasic acid (Type 6/10 nylon). They can also be made by the polymerization of amino acids or their derivatives, for example, polycaprolactam (Type 6 nylon) and polymerized 11aminoun-decanoic acid (Type 11 nylon).

TYPES Nylon 6/6 is the most widely used of the nylon plastics because of its overall balance of properties. The second most widely used of the nylon family is nylon 6. Type 6/6 nylon resins have higher heat resistance, abrasion resistance, strength, stiffness, and hardness than type 6 nylons. The type 6 nylon resins are tougher and more flexible than type 6/6 nylons, and they have a wider processing window. Nylon 6/12 absorbs less moisture and, therefore, maintains both mechanical and electrical properties better in high-humidity environments. But the reduced moisture sensitivity is accompanied by lower strength, lower stiffness, lower use temperatures, and higher cost. Nylons 11 and 12 have lower moisture absorption combined with superior resistance to fuels, hydraulic oils, and most automotive fluids. The melting points of nylon 11 and 12 (180 to 185°C) are the lowest of the commercial polyamides. These two polyamides are often combined with plasticizers to generate a flexible, tough material suitable for tubing extrusion. Recently, nylon 12/12 was introduced with a slightly higher use temperature while maintaining good fuel resistance. Nylon 6/6T resins have low moisture absorption and they are much stronger, stiffer, tougher, fatigue resistant, and more heat resistant than type 6/6 nylons. The Ultramid type 6/6T resins also have better resistance to hot oils and fats than type 6/6 nylons. Reinforced grades of type 6/6T nylon resins also are available. Nylon 4/6 is the latest version of the shortrepeat-unit polyamides. Its melting point of 295°C is 12.2°C above that for nylon 6/6 and is the highest in the polyamide family. The inherent molecular symmetry of nylon 4/6


results in self-nucleation, rapid crystal growth, and, thus, a higher level of crystallinity. This higher level of crystallinity leads to faster setup and, hence, faster injection-molding cycles, up to 30% faster than for 6/6. Nylon 4/6 absorbs more water than nylon; however, its dimensional stability is similar to nylon 6/6 due to its high crystallinity. Higher crystallinity has a major effect on nearly all properties leading to higher strength, higher stiffness, high heat-deflection temperature (HDT), high fatigue resistance, high wear resistance, and high creep resistance. Semicrystalline polymers maintain useful properties above the glass transition in contrast to amorphous polymers, which transform into a viscous mass. Nylon 4/6, with its unusually high crystallinity, maintains a higher level of performance at elevated temperatures. The HDT for reinforced nylon 4/6 is 545°C. Nylon resin is available in a wide range of reinforcement levels, filler types, toughening agents, stabilizers, and flame-retardant additives. Newer flame retardants can provide good flammability ratings (UL 94V-0) while maintaining acceptable electrical properties. Toughening technology has reduced notch sensitivity providing notched Izod values over 15 ft-lb/in.; see Table N.6.

PROPERTIES Property comparisons among commercial grades of nylon vary widely because so many formulations are available. In general, however, nylons have excellent fatigue resistance, low coefficient of friction, good toughness (depending on degree of crystallinity), and they resist a wide spectrum of fuels, oils, and chemicals. They are inert to biological attack, and have adequate electrical properties for most voltages and frequencies. The crystalline structure of nylons, which can be controlled to some degree in processing, affects stiffness, strength, and heat resistance. Low crystallinity imparts greater toughness, elongation, and impact resistance, but at the sacrifice of tensile strength and stiffness. Nylons 6/6, 6/6T, and 4/6 have the lowest permeability of the nylons by gasoline, mineral oil, and fluorocarbon refrigerants. Nylon 6/12

N


TABLE N.6 Properties of Nylons (dry as molded) ASTM or UL Test

D79 D570

Property

Specific gravity Specific volume (in.3/lb) Water absorption, 24 h, 1/8 in. thick (%)

D638 D638 D790

Tensile strength (psi) Elongation (%) Flexural modulus (Kpsi)

D2117 D696

Melting point (crystalline) (°F) Coefficient of thermal expansion (1E-05 in./in./F) Deflection temperature (°F), at 264 psi Flammability rating

D648 UL94

D150 D257

N

Type

Dielectric constant, 75°F, at 1 kHz Volume resistivity (Ω-cm) at 73°F, DAM

4/6

6/6

6

6/12

11

Cast 6

Physical 1.13 1.14 23.5 24.3 2.3 1.5

1.14 24.3 1.6

1.07 25.9 0.4

1.04 26.6 0.4

1.15 24.1 1.6

Mechanical 14000 12000 30 60 460 440

11500 100 420

8800 150 150

8600 300 150

11000 15–50 400

Thermal 663 4.2

509 4.4

428 4.5

419 5.0

374 5.1

419 5.0

240

190

152

150

180

140

V-2

V-2

V-2

-2

—

HB

3.7 1E + 13

4.0 —

Electrical 4.0 3.9 1E + 15 1E + 16

3.8 1E + 15

4.0 1E + 12


and 6/6T are used where lower moisture absorption (and better dimensional stability) is needed. All nylons absorb moisture from the environment; however, type 6/6T nylon has much lower moisture absorption than any other type of nylon resin. Moisture absorption leads to dimensional and property changes dependent upon the equilibrium level absorbed. At elevated temperatures, the moisture equilibrium level decreased above 88°C, nylon begins to dry out by a combination of internal diffusion and surface volatile emission. Extended exposure to temperatures above 121°C will reduce moisture content to about 0.1% (similar to dryas-molded content). Nylons are sensitive to ultraviolet radiation. Weatherability will be reduced unless ultraviolet stabilizers are incorporated into the formulation. Carbon black is the most commonly used ultraviolet stabilizer. Carbon black lowers the ductility and toughness as trade-off for ultraviolet stability.


Nylons have good resistance to creep and cold flow compared with many less rigid thermoplastics. Creep resistance is better at higher levels of crystallinity as demonstrated in nylon 4/6. Creep can be calculated from long-term apparent modulus under load data.

PROCESSING Nylons are generally fabricated by injection molding or by extrusion. Precise, intricate shapes of a variety of colors can be molded with little or no finishing required. These can often replace an assembly of several metal parts. Thus, even where nylons cost more on a per-volume basis than the common die-casting metals, economies in finishing and assembly often result in lower ultimate costs. Tubing and rod stock manufacture, plus the coating of wire and cable, are the major forms of nylon extrusion. Film and relatively complex cross sections are also made, but in less volume. In general, tubing, film, and other unsupported


shapes require higher melt viscosity than is desirable for injection molding. Most manufacturers of nylon supply these high-viscosity grades.

APPLICATIONS Nylons are usually specified because of their combination of properties. Gears, bearings, cams, clutch facings, and similar mechanical parts require their strength, stiffness, low coefficient of friction, and resistance to fatigue and abrasion. In cases where oiling or greasing is apt to be neglected, as in home appliances, or is undesirable from a contamination standpoint, as in textile and food-handling machinery, nylon parts usually perform satisfactorily without any lubrication whatsoever. Designers often utilize the mechanical properties of nylons, plus one or more characteristics of particular value. For example, the nylon housing for an electric drill must be tough, stiff, dimensionally stable, and resistant to commonly encountered lubricants and solvents. However, its electrical nonconductivity and safety are the critical advantages. Similarly, one manufacturer makes an entire rifle stock, plus many of the moving parts of the rifle, out of nylon. It is far lighter and tougher than wood and provides moving surfaces that do not need lubrication. The ability of nylon to be molded into precise sections thus permits custom-quality guns to be mass-produced. By utilizing different properties, marine electrical stuffing tubes of nylon capitalize on the durability, lightness, resistance to corrosion, and cost advantage of the resin over machined brass. Washing machine mixing valves and valves for the dispensing of hot beverages require the mechanical properties of nylon and its excellent resistance to the effects of hot water. The coating of wire and cable construction is the most important extrusion application of nylon. Although it is most commonly used as a jacket over a primary insulator such as polyvinyl chloride or polyethylene to impart resistance to lubricants, abrasion, and to the effects of high temperature, its electrical properties are adequate for low-voltage uses. Exercise bikes used in health clubs undergo about 10 h of use per day. Further, to


be competitive in today’s hot exercise-equipment marketplace one must design bikes to last from 5 to 7 years. The jump on the competition is a newly designed thermoplastic composite pillow block, the structure that supports the drive shaft of the bike. The material selected was a long-glass-fiber-reinforced nylon 6/6 structural composite. When compared to conventional short-fiber-reinforced thermoplastics, the nylon 6/6 structural composites have enhanced mechanical properties, while remaining lightweight. As an added bonus, this combination of properties reduces the weight of the pillow block from 3.68 to 0.69 g. A nylon foot brace for kayakers has become the standard of excellence. The brace consists of a rail screwed into the bottom of the boat, and a foot pad that slides up and down the rail to fit the kayaker’s leg. Today, most kayak makers feature the brace as standard equipment. It consists of glass-reinforced, nylon 6, injectionmolded resins. The nylon combines high-strength, stiffness, and heat-deflection characteristics, while extending the retention of these properties at high temperatures, and a lower cost than competitive materials. A new sports car features a lightweight, high-performance nylon air-intake manifold with a molded-in fuel rail. The automaker claims the integrated air-fuel module with two fuel passages into the body of the intake manifold breaks the mold when it comes to underthe-hood components. It uses a nylon manifold/fuel rail using the fusible-core, injectionmolding (FCIM) technique. The manifold, with its molded-in fuel rail, weighs about 3.68 g. That is nearly 50% less than a similar design based on pressure-cast aluminum would weigh. The smooth inner walls of the injection-molded manifold increase airflow. This, combined with the low thermal conductivity of the nylon manifold, helps improve engine performance up to 5% more than that of an aluminum counterpart. And the nylon manifold insulates the air inside from engine heat, allowing high-density air to flow into the engine. The specially formulated 6/6 nylon resists hot engine temperatures and attacks from oil, gasoline, and battery vapors. It also produces a manifold that better withstands

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engine vibration stresses, while reducing engine noise. As a wire insulation, nylon is valued for its toughness and solvent resistance. Nylon fibers are strong, tough, and elastic, and have high gloss. The finer fibers are easily spun into yarns for weaving or knitting either alone or in blends with other fibers, and they can be crimped and heat-set. For making carpets, nylon staple fiber, lofted or wrinkled, is used to give the carpet a bulky texture resembling wool. Tire cord, made from nylon 6 of high molecular weight, has the yarn drawn to four or five times its original length to orient the polymer and give one-half

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twist per inch. Nylon film is made in thicknesses down to 0.005 cm for heat-sealed wrapping, especially for food products where tight impermeable enclosures are needed. Nylon sheet, for gaskets and laminated facings, comes transparent or in colors in thicknesses from 0.013 to 0.152 cm. Nylon monofilament is used for brushes, surgical sutures, tennis strings, and fishing lines. Filament and fiber, when stretched, have a low specific gravity down to 1.068, and the tensile strength may be well above 344 MPa. Nylon fibers made by condensation with oxalic acid esters have high resistance to fatigue when wet.

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O OLEFIN COPOLYMERS The principal olefin copolymers are the polyallomers, ionomers, and ethylene copolymers. The polyallomers, which are highly crystalline, can be formulated to provide high stiffness and medium impact strength; moderately high stiffness and high impact strength; or extra-high impact strength. Polyallomers, with their unusually high resistance to flexing fatigue, have “hinge” properties better than those of polypropylenes. They have the characteristic milky color of polyolefins, are softer than polypropylene, but have greater abrasion resistance. Commonly injection-molded, extruded, and thermoformed, polyallomers are used for such items as typewriter cases, snap clasps, threaded container closures, embossed luggage shells, and food containers. Ionomers are nonrigid plastics characterized by low density, transparency, and toughness. Unlike polyethylenes, density and properties are not crystalline dependent. Their flexibility, resilience, and high molecular weight combine to provide high abrasion resistance. They have outstanding low-temperature flexural properties but upper temperature use is limited to 71°C. Resistance to attack from organic solvents and stress cracking chemicals is high. Ionomers have high melt strength for thermoforming and extrusion-coating, and a broad temperature range for blow molding and injection molding. Representative ionomer parts include injection-molded containers, housewares, tool handles, and closures; extruded film, sheet, electrical insulation, and tubing; blow-molded containers and packaging. There are four commercial ethylene copolymers, of which ethylene vinyl acetate (EVA) and ethylene ethyl acrylate (EEA) are the most common; Table O.1 shows typical properties of some olefin copolymers.


EVA copolymers approach elastomers in flexibility and softness, although they are processed like other thermoplastics. Many of their properties are density dependent, but in a different way from that of polyethylenes. Softening temperature and modulus of elasticity decrease as density increases, which is contrary to the behavior of polyethylene. Similarly, the transparency of EVA increases with density to a maximum that is higher than that of polyethylenes, which become opaque when density increases above around 0.935 g/cc. Although the electrical properties of EVA are not as good as those of low-density polyethylene, they are competitive with vinyl and elastomers normally used for electrical products. The major limitation of EVA plastics is their relatively low resistance to heat and solvents; the Vicat softening point is 64°C. EVA copolymers can be injection, blow, compression, transfer, and rotationally molded; they can also be extruded. Molded parts include appliance bumpers and a variety of seals, gaskets, and bushings. Extruded tubing is used in beverage vending machines and for hoses for air-operated tools and paint spray equipment. EEA is similar to EVA in its density–property relationships. It is also generally similar to EVA in high-temperature resistance, and like EVA it is not resistant to aliphatic and aromatic hydrocarbons or their chlorinated versions. However, EEA is superior to EVA in environmental stress cracking and resistance to ultraviolet radiation. Similar to EVA, most of the applications of EEA are related to the outstanding flexibility and toughness of the plastic. Typical uses are household products such as trash cans, dishwasher trays, flexible hose and water pipe, and film packaging. Two other ethylene copolymers are ethylene hexene (EH) and ethylene butene (EB). Compared with the other two, these copolymers

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TABLE O.1 Typical Properties of Some Olefin Copolymers

Specific gravity Tensile strength, 1000 psi Elongation, % Hardness, Phase D Imp. strength, ft-lb/in. notch Softening point, Vicat, F Dielectric strength, V/mil

Polyallomer

Ionomer

EVA

0.898–0.905 3–4.5 350 — 1.5 250–275 500–650

0.94 3–5 450 60 9–14 162 1000

0.94 0.5–1.0 650 35 — 147 525

have greater high-temperature resistance; their useful service range being between 66 and 88°C. They are also stronger and stiffer and, therefore, less flexible than EVA and EEA. In general, EH and EB are more resistant to chemicals and solvents than the other two, but their resistance to environmental stress cracking is not as good.

OPTICAL FIBERS

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These are flexible transparent fiber devices, sometimes called light guides, used for either image or information transmission, in which light is propagated by total internal reflection. In its simplest form, the optical fiber or lightguide consists of a core of material with a refractive index higher than the surrounding cladding. The optical fiber properties and requirements for image transfer, in which information is continuously transmitted over relatively short distances, are quite different from those for information transmission, where typically digital encoding of information into on–off pulses of light (on = 1; off = 0) is used to transmit audio, video, or data over much longer distances at high bit rates. Another application for optical fibers is in sensors, where a change in light transmission properties is used to sense or detect a change in some property, such as temperature, pressure, or magnetic field.

FIBER DESIGNS There are three basic types of optical fibers (Figure O.1). Propagation in these lightguides is most easily understood by ray optics, © 2002 by CRC Press LLC

although the wave or modal description must be used for an exact description. In a multimode, stepped-refractive-index-profile fiber (part a of the figure), the number of rays or modes of light that are guided, and thus the amount of light power coupled into the lightguide, is determined by the core size and the core-cladding refractive index difference. Such fibers, used for conventional image transfer, are limited to short distances for information transmission due to pulse broadening. An initially sharp pulse made up of many modes broadens as it travels long distances in the fiber, because high-angle modes have a longer distance to travel relative to the low-angle modes. This limits the bit rate and distance because it determines how closely input pulses can be spaced without overlap at the output end. At the detector, the presence or absence of a pulse of light in a given time slot determines whether this bit of information is a zero or one. index cross section profile

light path

input pulse

output pulse

(a)

(b)

(c)

FIGURE O.1 Types of optical fiber designs. (a) Multimode, stepped-refractive-index profile. (b) Multimode, graded-index profile. (c) Singlemode, stepped-index. Graded index is possible. (From McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 12, 431. With permission.)


A graded-index multimode fiber (part b), where the core refractive index varies across the core diameter, is used to minimize pulse broadening due to intermodal dispersion. Since light travels more slowly in the high-index region of the fiber relative to the low-index region, significant equalization of the transit time for the various modes can be achieved to reduce pulse broadening. This type of fiber is suitable for intermediate-distance, intermediate-bit-rate transmission systems. For both fiber types, light from a laser or light-emitting diode can be effectively coupled into the fiber. A single-mode fiber (part c) is designed with a core diameter and refractive index distribution such that only one fundamental mode is guided, thus eliminating intermodal pulsebroadening effects. Material and waveguide dispersion effects cause some pulse broadening, which increases with the spectral width of the light source. These fibers are best suited for use with a laser source to couple light efficiently into the small core of the lightguide and to enable information transmission over long distances at very high bit rates. The specific fiber design and the ability to manufacture it with controlled refractive index and dimensions determine its ultimate bandwidth or information-carrying capacity.

ATTENUATION The attenuation or loss of light intensity is an important property of the lightguide because it limits the achievable transmission distance, and is caused by light absorption and scattering. Every material has some fundamental absorption due to the atoms or molecules composing it. In addition, the presence of other elements as impurities can cause strong absorption of light at specific wavelengths. Fluctuations in a material on a molecular scale cause intrinsic Rayleigh scattering of light. In actual fiber devices, fiber-core-diameter variations or the presence of defects such as bubbles can cause additional scattering light loss. Optical fibers based on silica glass have an intrinsic transmission window at near-infrared wavelengths with extremely low losses. Such fibers are used with solid-state lasers and lightemitting diodes for information transmission,


especially for long distance (greater than 1 km). Plastic fibers exhibit much higher intrinsic as well as total losses, and are more commonly used for image transmission, illumination, or very short-distance data links. Many other fiber properties are also important and their specification and control are dictated by the particular application. Good mechanical properties are essential for handling: plastic fibers are ductile, whereas glass fibers, intrinsically brittle, are coated with a protective plastic to preserve their strength. Glass fibers have much better chemical durability and can operate at higher temperatures than plastics.

ORGANIC COATINGS Organic coatings are chiefly additive type finishes that find use on almost all types of materials. They can be monolithic consisting simply of one layer, or coat, or they can be composed of two or more layers. The total thickness of coating systems varies widely. Some run less than 1 mil thick. Others go as high as 10 and 15 mils thick. Generally, by definition, coatings that are more than 10 mils thick are referred to as linings, films, or mastics. To function as a protective barrier against corrosion and oxidation, organic coatings depend principally on their chemical inertness and impermeability. In addition, however, some coatings provide protection with the use of inhibiting pigments that have a passivating action, particularly on metal surfaces. Also, some coatings contain metallic pigments that give electrochemical protection to metals.

COATING APPLICATION

AND

DRYING

Organic coatings are commonly applied by the following methods: brush, spray, dip, roller, flow coat, knife, tumbling, silk screen, and electrostatic means. Of these, application by brushing is the slowest; all the others are production methods. Organic coatings dry, or cure, by one or more of the following mechanisms: (1) evaporation or loss of solvent, (2) oxidation, and (3) polymerization. After application of the coating, the volatile ingredients, which are almost

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always present in at least a small amount, evaporate. Some finishes, such as lacquers, dry completely by evaporation of solvents. After evaporation, coatings that do dry solely by evaporation are still in a semifluid state, and depend on oxidation or polymerization, or a combination of both, to convert to their final form. Drying by oxidation, which is usually done at room temperature, is the slowest of the three methods. Polymerization, which involves polymer chain forming mechanism, can be done at normal or at elevated temperatures. Polymerization is speeded by use of heat. Radiation curing involving the use of an electron beam to polymerize the coating in a few seconds has found production usage.

production finishing are of two types: air-dry types and baking-types. The air-dry types have drying oil vehicle bases and are usually referred to as paints. They may or may not be modified with resins. They are not used as extensively as the baking-type primers, which have resin or varnish vehicle bases and dry chiefly by polymerization. Some primers, known as flash primers, are applied by spraying, and dry by solvent evaporation within 10 min. In practically all primers, the pigments impart most of the anticorrosion properties to the primer and, along with the vehicle, determine its compatibility and adherence with the base metal.

COATING TYPES

These are fillers, surfacers, and sealers. They can be applied either before or after the primer, but more often after the primer and sometimes after the surfacer coat. Their function is to fill in large irregularities in the surface or local imperfections. They are usually puttylike substances, and a variety of materials are used. Their chief characteristics are (1) must harden with a minimum of shrinkage; (2) must have good adhesion; (3) must have good sanding properties; and (4) must work smoothly and easily. Surfacers are often similar to primers; they usually have the same composition as the priming coat, except that more pigment is present. Surfacers are applied over the priming coat to cover all minor irregularities in the surface. Sealers as a rule are used either over the fillers or surfacers. The chief function of sealers is to fill up the pores of the undercoat to avoid “striking in” of the finish coats. This filling-in of the porous surfacer or sealer also tends to strengthen the entire coating system. The sealers when used over surfacers are usually formulated with the same type pigment and vehicle as used in the final coat.

AND

SYSTEMS

Coating Composition

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An organic coating is made up of two principal components: a vehicle and a pigment. The vehicle is always there. It contains the filmforming ingredients that enable the coating to convert from a mobile liquid to a solid film. It also acts as a carrier and suspending agent for the pigment. Pigments, which may or may not be present, are the coloring agents and, in addition, contribute a number of other important properties. Organic coatings are commonly divided into about a half dozen broad categories based on the types and combinations of vehicle and pigment used in their formulation. They are paints, enamels, varnishes, lacquers, dispersion coatings, emulsion coatings, and latex coatings. However, with the complexity in modern formulations, distinctions between these various types are often difficult to make. As mentioned earlier, organic finish systems are frequently composed of more than one layer or coat. These various layers are commonly classified as primers, intermediate coats, and finish coats.

Intermediate Coats

Finish Coats Primers These are the first coatings placed on the surface (except for fillers, in some cases). Where chemical pretreatments are used, primer coats may often be unnecessary. Primers for industrial or


Finish or top coats are usually the decorative or functional part of a paint system. However, they often also have a protective function. The primer coats may require protection against the service conditions, because although the pigments used


in primers are satisfactory for corrosion protection of the metal, they are frequently not satisfactory as top coats. Their color retention upon weathering or their physical durability may be poor. There are also one-coat applications where the finish coats are applied directly to the base material surface, and, therefore, provide the sole protective medium.

VEHICLES Vehicles are composed of film-forming materials and various other ingredients, including thinners (volatile solvents), which control viscosity, flow, and film thickness, and driers, which facilitate application and improve drying qualities. The main concern is chiefly the film-forming part of the vehicle, because it is that part of the vehicle that to a large extent determines the quality and character of an organic finish. It determines the possible ways in which the finish can be applied and how the “wet” finish will dry to a hard film; it provides for adhesion to the metal surface; and it usually influences the durability of the finish. Vehicles can be divided into three main types: (1) oil, (2) resin, and (3) varnish. The simplest and among the oldest vehicles are the straight drying oil types. Resins, as a class, can serve as vehicles in their own right, or can be used with drying oils to make varnish-type vehicles. Varnish vehicles are composed of resins and either drying or nondrying oils, together with required amounts of thinners and driers. They are often used alone as a full-fledged organic finish. Drying Oils Vehicles consisting of oil only are used to a limited extent in industrial finishes. Linseed oil is probably the most widely used oil. There are a number of different kinds that differ in rate of drying, and in such properties as water resistance, color, and hardness. Tung oil or China wood oil, when property treated, excels all other drying oils in speed of drying, hardening, and water resistance. Oiticica oil is similar to tung oil in many of its properties. Dehydrated castor oil dries better


than linseed oil, but slower than tung oil. Some of its advantages are good color and color retention, and flexibility. The oils from some fish are also used as drying oils. If processed properly, they dry reasonably well and have little odor. They are often used in combination with other oils. Perilla oil is quite similar in properties to fast-drying linseed oil. Its use is largely dependent upon its price and availability. Soybean oil is the slowest drying in the drying oil classes, and is usually used in combination with some faster-drying oil such as linseed oil. Resins Although both natural and synthetic resins can serve as organic coating vehicles, today the natural types, such as rosin, have been largely replaced by plastic resins. Nearly all the plastic resins — both thermosets and thermoplastics as well as many elastomets — can be used as film formers, and frequently two or more kinds are combined to give the set of properties desired. Typical thermoplastics used in vehicles are acrylics, acetates, butyrates, and vinyls. Commonly used thermosets for vehicles include phenolics, alkyds, melamines, ureas, and epoxies.

PIGMENTS Pigments are the second of the two principal components that make up most organic finishes. They contribute a number of important characteristics to a coating. They, first of all, serve a decorative function. The choice of color and shades of color by use of one or combinations of pigments is practically unlimited. Closely associated with color is the hiding power function or their ability to obscure the surface of the material being finished. In many primers the principal function of pigments is to prevent corrosion of the base metal. In other cases they may be added to counteract the destructive action of ultraviolet light rays. Pigments also help give body and good flow characteristics to the finish. And, finally, some pigments may give to organic coatings what is termed package stability — that is, they keep the coating material in usable condition in the container until ready for use.

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Pigments can be conveniently divided into three classes as follows: (1) white hiding pigments, (2) colored pigments, and (3) extender or inert pigments. White pigments are used not only in white paints and enamels, but also in making white bases for the tinted and light shades. Colored pigments furnish the finish with both opacity and color. They may be used by themselves to form solid colors, or in combination with whites to produce tints, and often provide rust-inhibitive properties. For example, red lead, certain lead chromates, zinc chromates, and blue lead are used in iron and steel primers as rust inhibiters. There are two general classes of colored pigments — earth colors, which are very stable and are not readily affected by acids and alkalies, heat, light, and moisture, and chemical colors, which are produced under controlled conditions by chemical reaction. The metallic pigments can also be included within this class. Aluminum powder is perhaps the best known. The chief functions of extender pigments are to help control consistency, gloss, smoothness, and filling qualities, and leveling and check resistance. Thus, particle size and shape, oil absorption, and flatting power are important selection considerations. Extender pigments are for the most part chemically inactive. They usually have little or no hiding power.

ENAMELS By definition, enamels are an intimate dispersion of pigments in a varnish or a resin vehicle, or in a combination of both. Enamels may dry by oxidation at room temperatures and/or by polymerization at room or elevated temperatures. They vary widely in composition, in color and appearance, and in properties, and are available in all colors and shades. Although they generally give a high-gloss finish, there are some that give a semigloss or eggshell finish and still others that give a flat finish. Enamels as a class are hard and tough and offer good mar and abrasion resistance. They can be formulated to resist attack of most commonly encountered chemical agents and corrosive atmospheres. Because of their wide range of useful properties, enamels are probably the most widely


used organic coating in industry. One of their largest fields of use is for coating household appliances — washing machines, stoves, kitchen cabinets, and the like. A large portion of refrigerators, for example, are finished with synthetic baking enamels. These appliance enamels are usually white, and therefore must have a high degree of color and gloss retention when subjected to light and heat. Other products finished with enamels include automotive products, railway equipment, office equipment, toys and sport supplies, industrial equipment, and novelties.

LACQUERS The word lacquer comes from lac resin, which is the base of common shellac. Lac resin dissolved in alcohol was one of the first lacquers and has been in use for many centuries. Nowadays, shellac is called spirit lacquer. It is only one of several different kinds of lacquers; these, except for spirit lacquer, are named after the chief film-forming ingredient. The most common ones are cellulose acetate, cellulose acetate butyrate, ethyl cellulose, vinyl, and nitrocellulose. A distinguishing characteristic of lacquers is that they dry by evaporation of the solvents or thinners in which the vehicle is dissolved. This is in contrast to oils, varnishes, or resin base finishes, which are converted to a hard film chiefly through oxidation or polymerization. Because many modern lacquers have high resin content, the gap between lacquer and synthetic-type varnishes diminishes until finally one has modified synthetic air-drying varnishes. They may dry chiefly by oxidation or polymerization. Lacquers normally dry hard and dust-free in a very few minutes at room temperature. In production line work, forced drying is often used. It is possible, therefore, to do a multicoat job without having to lose time between coats. Because of the speed of drying and the fact that they are permanently soluble in the solvents used for application, lacquers usually are not applied by brush. Spray application or dipping are the usual procedures. Lacquers can be either clear and transparent or pigmented, and their color range is practi-


cally unlimited. Lacquers in themselves have good color retention, but sometimes the added pigments, modifying resins, and plasticizers may adversely affect this property. They are hard and mar resistant. Inherently, they lack good adhesion to metal, but modern lacquer formulations have greatly improved their adhesion properties. Lacquers can be made to be resistant to a large variety of chemicals, including water and moisture, alcohol, gasoline, vegetable, animal and mineral oils, and mild acids and alkalies. Because of the volatile solvents, lacquers are inflammable in storage and during application, and this sometimes limits their application. Because of their fast drying speeds, lacquers find wide application in the protection and decoration of products that can be dipped, sprayed, roller-coated, or flow-coated. They are especially advantageous for coating metal hardware and fixtures, toys, and other articles that, because of volume production, must dry hard enough to handle and pack in a short period of time. Lacquers are widely used in automobile finishing and especially for refinishing autos and commercial vehicles where fast drying without baking equipment is a requirement. Lacquers also compete with enamels for coating metal stampings and castings, including die castings.

VARNISHES Varnishes consist of thermosetting resins and either drying or nondrying oils. They are clear and unpigmented and can be used alone as a coating. However, their major use in industrial finishing is as a vehicle to which pigments are added, thus formng other types of organic coatings. The drying mechanisms of varnishes all follow the same general pattern. First, any volatile solvents that are present evaporate; then, drying by oxidation and/or polymerization takes place, depending on the nature of the resin and oil. At high temperatures, of course, there is more tendency to polymerize. So varnishes can be formulated for either air or bake drying. Varnishes may be applied by brushing or by any of the production methods.


It is evident that with the large variety of raw materials to choose from and the unlimited number of combinations possible that varnishes have an extensive range of properties and characteristics. They range from almost clear white to a deep gold; they are transparent, lacking any appreciable amount of opacity. Japan, a hard-baked black-looking varnish, is an exception. It is opaque, due to carbon and carbonaceous material being present. There are some distinctions in properties between oil-modified alkyd varnishes and the other types. In general, oil-modified alkyds have better gloss and color retention and better resistance to weathering. They form a harder, tougher, more durable film and dry faster. On the other hand, they have less alkali resistance than the other varnishes. In such things as adhesion and rust inhibitiveness there is no distinctive difference. The major use of varnishes, as coatings in their own right, is for food containers, closures such as bottle caps, and bandings of various kinds. Another large application is as a clear finish coat over lithographic coatings.

PAINTS The word paints is sometimes used broadly to refer to all types of organic coatings. However, by definition a paint is a dispersion of a pigment or pigments in drying oil vehicle. They find little use these days as industrial finishes. Their principal use is for primers. Paints dry by oxidation at room temperature. Compared to enamels and lacquers their drying rate is slow; they are relatively soft and tend to chalk with age.

OTHER ORGANIC COATINGS Dispersion and Emulsion Coatings In recent years these coatings have become known as water-base paints or coatings because many of them consist essentially of finely divided ingredients, including plastic resins, fillers, and pigments, suspended in water. An organic media may also be involved. There are three types of water-base coatings. Emulsions, or latexes, are aqueous dispersions of highmolecular-weight resins. Strictly speaking,

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latex coatings are dispersions of resins in water, whereas emulsion coatings are suspensions of an oil phase in water. Emulsion and latex coatings are clear to milky in appearance, have low gloss, excellent resistance to weathering, and good impact resistance. Chemical and stain resistance varies with composition. Dispersion coatings consist of ultrafine fine, insoluble resin particles present as a colloidal dispersion in an aqueous medium. They are clear or nearly clear. Weathering properties, toughness, and gloss are roughly equal to those of conventional solvent paints. Water-soluble types, which contain lowmolecular-weight resins, are clear finishes and they can be formulated to have high gloss, fair to good chemical and weathering resistance, and high toughness. Of the three types, they handle and flow most like conventional solvent coatings. Plastic Powder Coatings

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Several different methods have been developed to apply plastic powder coatings. In the most popular process — fluidized bed — parts are preheated and then immersed in a tank of finely divided plastic powders, which are held in a suspended state by a rising current of air. When the powder particles contact the heated part, they fuse and adhere to the surface, forming a continuous, uniform coating. Another process, electrostatic spraying, works on the principle that oppositely charged materials attract each other. Powder is fed through a gun, where an electrostatic charge is applied opposite that applied to the part to be coated. When the charged particles leave the gun, they are attracted to the part where they cling until fused together as a plastic coating. Other powder application methods include flock coating, flow coating, flame and plasma spraying, and a cloud chamber technique. Although many different plastic powders can be applied by the above techniques, vinyl, epoxy, and nylon are most often used. Vinyl and epoxy provide good corrosion and weather resistance as well as good electrical insulation. Nylon is used chiefly for its outstanding wear and abrasion resistance. Other plastics frequently used in powder coating include chlori© 2002 by CRC Press LLC

nated polyether, polycarbonate, acetal, cellulosics, acrylic, and fluorocarbons. Hot Melt Coatings These consist of thermoplastic materials that solidify on the metal surface from the molten state. The plastic either is applied in solid form and then melted and flowed over the surface or is applied molten by spraying or flow coating. Since no solvent is involved, thick single coats are possible. Bituminous coatings are also commonly applied by the hot melt process. Lining and Sheeting Sheet, film, and tapes of various plastics and elastomers cemented to material shapes and parts are used to provide corrosion and abrasion resistance. Thicknesses usually range from 3.2 to 12.7 mm. The most widely used materials are polyvinyl chloride, polyethylene butadiene–styrene rubber, and neoprene. Specialty Finishes An almost infinite number of specialty or novelty finishes are available. Most of them are really lacquers or enamels to which special ingredients have been added or which are processed in some unique way to give the effects desired. One of the most common types are those giving a roughened or wrinkle appearance, which is obtained by use of high percentages of driers causing wrinkling when the finish is baked. Another group of specialty finishes gives a crystalline effect. They are enamels in which impurities are purposely introduced during the baking process by retaining the products of combustion in the oven while the coating dries. The wrinkle and crystalline finishes are widely used on instrument panels, office equipment, and a variety of other industrial and consumer products. Other unusual finishes are obtained by adding special ingredients to lacquers to give them a stringy or “veiled” appearance when applied by spraying. The application of the silk-screen process to organic finishing of metals has also resulted in unique finishes with multicolored effects.


ORGANOMETALLIC COMPOUNDS

C

MLn

C

C

C

M+ M

These are members of a broad class of compounds whose structures contain both carbon (C) and a metal (M). Although not a required characteristic of organometallic compounds, the nature of the formal carbon–metal bond can be of the covalent type. Figure O.2 depicts carbon–metal formal bonds. The term organometallic chemistry is essentially synonymous with organotransition metal chemistry; it is associated with a specific portion of the periodic table ranging from groups 3 through 11, and also includes the lanthanides. This particular area has experienced exceptional growth since the mid-1970s largely because of the continuous discovery of novel structures as elucidated mainly by x-ray crystallographic analyses; the importance of catalytic processes in the chemical industry; and the development of synthetic methods based on transition metals that focus on carbon–carbon bond constructions. From the perspective of inorganic chemistry, organometallics afford seemingly endless opportunities for structural variations due to changes in the metal coordination number, alterations in ligand–metal attachments, mixed-metal cluster formation, and so forth. From the viewpoint of organic chemistry, organometallics allow for manipulations in the functional groups that in unique ways often result in rapid and efficient elaborations of carbon frameworks for which no comparable direct pathway using nontransition organometallic compounds exists. As one moves across the periodic table, the metals that have found application include: • • • • • • • • • • •

Titanium Zirconium Chromium Molybdenum Tungsten Ruthenium Osmium Cobalt Rhodium Palladium Copper


(a)

(b)

(c)

FIGURE O.2 Carbon–metal formal bonds. (From McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 12, McGraw-Hill, New York, 574. With permission.)

Of interest to the ceramic industry are those organometallics, the hydroxide-free alkoxides, that can be used for the vapor-phase synthesis of hard ceramic oxide coatings, films, or freestanding bodies. Very fine particulate oxides also can be formed from these chemicals. Compounds now available include aluminum isoproproxide, aluminum hexafluoroisoproproxide, lithium hexafluoroisoproproxide, sodium hexafluoroisoproproxide, zirconium hexafluoroisoproproxide, and zirconium tertiary amyloxide.

ORGANOSOL COATINGS Organosol coatings are coatings in which the resin (usually polyvinyl chloride) is suspended rather than dissolved in an organic fluid. The dispersion technique permits the use of highmolecular-weight, relatively insoluble resins without the use of expensive solvents. In organosols, the fluid, or dispersant, consists of plasticizers together with a blend of inexpensive volatile diluents selected to give the desired fluidity, speed of fusion, and physical properties. The dispersant provides little or no solvating action on the resin particles until a critical temperature is reached at which point the resin is dissolved in the dispersant to form a singlephase solid solution. Since a portion of the liquid is made up of volatile diluents, the fusion process results in a proportional shrinkage. (Nonvolatile dispersions using only plasticizer dispersants are termed plastisols.) Organosols are available with a wide range of flow characteristics and consequently may be formulated for application by any of the conventional techniques. Because they contain substantial quantities of volatile diluents, their thickness is limited to 10 to 12 mils per coat. Single-coat applications of greater thickness blister during bake as a result of trapped solvents.

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Baking is generally accomplished in two stages. The volatiles are removed in the first stage at temperatures of from 93 to 121°C, but fusion does not occur until a temperature of 149 to 191°C is reached. The fusion stage accomplishes the union of the discrete vinyl particles into a single-phase solid. In addition to polyvinyl chloride, the organosol technique can be used with acrylonitrile-vinyl and polychlorotrifluoroethylene resins. A balance must be achieved in the baking operation between the removal of the volatiles and the solvation of the resins. Rapid heating results in solvent blistering, whereas the reverse causes a mud-cracking effect.

APPLICATION METHODS Organosol coatings can be applied by several methods. Spread Coating

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The bulk of the organosols is applied by spreadcoating methods to fabrics and paper. Several plants are using spread coaters for the application of organosols and plastisols to strip steel to provide materials competitive with the light metals and plastics. Two basic processes are available: the knife coater and the roll coater. Knife coating is simple and fast but the product lacks the uniformity afforded by roller coating. Fusion is accomplished in a tunnel oven and embossing rolls may be employed at the oven exit to impart a texture or pattern to the hot gelled coating. Strand Coating Wires or filaments may be coated with organosols by first passing the strand through a dip tank, then through a wiping die to set the thickness. Fusion is accomplished in a drying tower. As many as nine to ten passes may be required to build a thickness of 20 mils. Dip Coating One of the major problems encountered in dip coating with organosols is the tendency of dried organosol to fall back into the dip tank and cause coating rejects. For this reason much of the dip coating is done with modified plastisols © 2002 by CRC Press LLC

rather than organosols. A dipping formulation must have low viscosity with high yield value to prevent sags and drains. The rate at which the article is withdrawn from the dip tank is a determining factor in the thickness and quality of deposit. Withdrawal rates generally range between 1.7 to 5.5 mm/s. Special techniques such as inversion of the dipped article just prior to fusion may be employed to alleviate the drip problem when it is particularly troublesome. Spray Coating Organosols are readily handled in either suction or pressure spray equipment. For productionline spraying, pressure systems afford rapid delivery and are generally preferred. There is an increasing tendency to use electronic spray processes for handling organosols. A number of electrostatic processes are available including several “hand guns.”

PROPERTIES Organosols have the characteristic vinyl properties of toughness and moisture resistance. However, although the coatings possess good electrical resistance and are frequently used as secondary insulation to reduce shock hazards, they seldom meet the needs of primary wire insulation. Adhesive primers are required to bond the materials to metals and other dense substrates. Prolonged exposure to temperatures greater than 93°C causes thermal degradation, which is generally evidenced by a gradual darkening.

USES Vinyl organosols have found their widest usage in the coating of paper and fabric stock where their ease of handling has permitted the use of simplified, low-cost application equipment. Where fabric coating strike-through is to be avoided, as in open weave, thixotropic plastisols are employed rather than organosols. Closer weaves require some penetration. Textured finishes for metals are a growing use and offer competition to the vinyl laminates and other decorative finishes. Typical applications and related properties of organosols are shown in Table O.2.


TABLE O.2 Applications of Organosol Coatings Application

Related Properties

Automotive interiors: station-wagon flooring, roof liners, dashboards, cowling, kick plates

Ease of application; uniformity of color and texture; resistance to gasoline, grease, and polishes; resistance to impact damage Durability: resistance to abrasion and scuffing; cleanability; resistance to moisture and detergents Aesthetic qualities: novelty of appearance; durability: resistance to abrasion and scratching; resistance to moisture and staining Durability: resistance to abrasion, scuffing, and chipping; resistance to chemicals and perspiration; sounddeadening qualities Cleanability: resistance to moisture and detergents; aesthetic qualities: durability: resistance to abrasion and scuffing; sound-deadening qualities Durability: resistance to abrasion, impact, and scuffing; resistance to moisture and staining Durability: toughness, resistance to abrasion and scuffing; aesthetic qualities Ease of application: economy and simplicity of application equipment; resistance to abrasion and tearing; resistance to moisture and staining

Commercial vehicles: seat backs, trim, interior paneling, luggage racks, sill plates Appliance finishes: television and radio cabinets, slide projectors, refrigerator panels Business machines: typewriters, calculators, electronic computers, laboratory instruments Architectural applications: paneling, partitions, shower stalls, elevator doors, bathroom wall sections Office furniture: desks, file cabinets, showcases, counters, waste baskets, chair finishes Luggage Paper and fabrics: floor and wall covering, place mats, bottle-cap liners, containers for food packaging, bandage dressings, upholstery fabrics, safety clothing, glove coatings Glass coatings: perfume bottles, bleach and chemical reagent bottles, photo flash bulbs

OSMIUM A platinum-group metal, symbol Os, osmium is noted for its high hardness, about 400 Brinell. The heaviest known metal, it has a high specific gravity, 22.65, and a high melting point, 2698°C. The boiling point is about 5468°C. Osmium has a close-packed hexagonal crystal structure, and forms solid-solution alloys with platinum, having more than double the hardening power of iridium in platinum. However, it is seldom used to replace iridium as a hardener except for fountain-pen tips where the alloy is called osmiridium. Osmium is not affected by the common acids, and is not dissolved by aqua regia. It is practically unworkable, and its chief use is as a catalyst.

USES Osmium tetraoxide, a commercially available yellow solid (melting point 40°C) is used


Resiliency, feel, cohesion strength; resistance to alcohol, moisture, and chemicals; ease of application

commercially as a stain for tissue in microscopy. It is poisonous and attacks the eyes. Osmium metal is catalytically active, but it is not commonly used for this purpose because of its high price. Osmium and its alloys are hard and resistant to corrosion and wear (particularly to rubbing wear). Alloyed with other platinum metals, osmium has been used in needles for record players, fountain-pen tips, and mechanical parts.

OSPREY SPRAYFORMING Copper alloy semifinished products made by the Osprey sprayforming process feature the ability to combine properties such as high strength, good corrosion resistance, and excellent machinability. In the Osprey process, powder particles are liquefied and sprayed by an inert gas onto a substrate, where they are shaped into a billet. Because the droplets solidify very rapidly, the

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microstructure is much finer than that of conventionally cast material. For example, the high yield strength and low elastic modulus of Cu–14% Sn alloy (with a small lead addition) make this alloy (BO5) ideal for spring applications. Its elastic modulus is 80 to 90 GPa, yield strength is 800 to 950 MPa, and ultimate tensile strength is 900 to 1000 MPa, with hardness of 240 to 280 HV. For the fabrication of contact elements by stamping and bending, alloy Cu–15% Ni–8% Sn (CN8) would be a good choice. It has elastic modulus of 110 to 120 GPa, yield strength of 800 to 1300 MPa, ultimate tensile strength of 900 to 1400 MPa, and hardness of 240 to 400 HV.

OXIDE CERAMICS

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Oxide ceramics can be divided into two groups — single oxides that contain one metallic element, and mixed or complex oxides that contain two or more elements. Examples include alumina, beryllia, magnesia, zirconia, and thoria. As a class they are low in cost compared to other technical ceramics, except for thoria and beryllia. Each of them can be produced in a variety of compositions, porosity, and microstructure, to meet specific property requirements. Oxide ceramic parts are produced by slip casting or pressing or extrusion and then fired at about 1800°C. They are more difficult to fabricate than other types of ceramics, because of the usual requirement to obtain a high-density body with minimum distortion and dimensional error, except in the case of porous bodies for use as thermal insulation. Powder pressing produces bodies with the lowest porosity and highest strength, because of the high pressures and the small amount of binder required.

SINGLE OXIDES Aluminum Oxide (Alumina) Alumina is the most widely used oxide, chiefly because it is plentiful, relatively low in cost, and equal to or better than most oxides in mechanical properties. Density can be varied over a wide range, as can purity — down to about 90% alumina — to meet specific


application requirements. Alumina ceramics are the hardest, strongest, and stiffest of the oxides. They are also outstanding in electrical resistivity, dielectric strength, are resistant to a wide variety of chemicals, and are unaffected by air, water vapor, and sulfurous atmospheres. However, with a melting point of only 2039°C, they are relatively low in refractoriness, and at 1371°C retain only about 10% of room-temperature strength. In addition to its wide use as electrical insulators and its chemical and aerospace applications, the high hardness and close dimensional tolerance capability of alumina make this ceramic suitable for such abrasion-resistant parts as textile guides, pump plungers, chute linings, discharge orifices, dies, and bearings. Beryllium Oxide (Beryllia) Beryllia is noted for its high thermal conductivity, which is about ten times that of a dense alumina (at 499°C), three times that of steel, and second only to that of the high-conductivity metals (silver, gold, and copper). It also has high strength and good dielectric properties. However, beryllia is costly and is difficult to work with. Above 1649°C it reacts with water to form a volatile hydroxide. Also, because beryllia dust and particles are toxic, special handling precautions are required. The combination of strength, rigidity, and dimensional stability make beryllia suitable for use in gyroscopes; and because of high thermal conductivity, it is widely used for transistors, resistors, and substrate cooling in electronic equipment. Magnesium Oxide (Magnesia) Magnesia is not as widely useful as alumina and beryllia. It is not as strong, and because of high thermal expansion, it is susceptible to thermal shock. Although it has better high-temperature oxidation resistance than alumina, it is less stable in contact with most metals at temperatures above 1705°C in reducing atmospheres or in a vacuum. Zirconium Oxide (Zirconia) There are several types of zirconia: a pure (monoclinic) oxide and a stabilized (cubic)


form, and a number of variations such as yttria- and magnesia-stabilized zirconia and nuclear grades. Stabilized zirconia has a high melting point, about 2760°C, low thermal conductivity, and is generally unaffected by oxidizing and reducing atmospheres and most chemicals. Yttria- and magnesia-stabilized zirconias are widely used for equipment and vessels in contact with liquid metals. Monoclinic nuclear zirconia is used for nuclear fuel elements, reactor hardware, and related applications where high purity (99.7%) is needed. Zirconia has the distinction of being an electrical insulator at low temperatures, gradually becoming a conductor as temperatures increase. Thorium Oxide (Thoria) Thoria, the most chemically stable oxide ceramic, is only attacked by some earth alkali metals under some conditions. It has the highest melting point (3315°C) of the oxide ceramics. Like beryllia, it is costly. Also, it has high thermal expansion and poor thermal shock resistance.

MIXED OXIDES Except for zircon the principal mixed oxides are composed of various combinations of magnesia, alumina, and silica.

2. Low porosity bodies, developed principally for use as furnace refractory brick. 3. Vitrified bodies used for exposed electrical devices that are subjected to thermal variations. Forsterite (2MgO·SiO2) This mixed oxide has high thermal shock resistance, but good electrical properties and good mechanical strength. It is somewhat difficult to form and requires grinding to meet close tolerances. Steatite Steatites are noted for their excellent electrical properties and low cost. They are easily formed and fired at relatively low temperatures. However, compositions containing little or no clay or plastic material present fabricating problems because of a narrow firing range. Steatite parts are vacuum-tight, can be readily bonded to other materials, and can be glazed or ground to high-quality surfaces. Zircon (ZrO2 ·SiO2) This mixed oxide provides ceramics with strength, low thermal expansion, and relatively high thermal conductivity and thermal endurance. Its high thermal endurance is used to advantage in various porous-type ceramics.

Cordierite (2MgO·2Al2O3 ·5SiO2) Cordierite is most widely used in extruded form for insulators in such parts as heating elements and thermocouples. It has low thermal expansion, excellent resistance to thermal shock, and good dielectric strength. There are three traditional groups of cordierite ceramics: 1. Porous bodies that have relatively little mechanical strength due to limited crystalline intergrowth and absence of ceramic bond. With long thermal endurance and low thermal expansion, they are used for radiant elements in furnaces, resistor tubes, and rheostat parts. © 2002 by CRC Press LLC

OXIDE COATINGS The black oxide finish on steel is one of the most widely used black or blue-black finishes. Some of the advantages of this type of finish are (1) attractive black color; (2) no dimensional changes; (3) corrosion resistance, depending on the final finish dip used; (4) nongalling surface; (5) no flaking, chipping, or peeling because the finish becomes an integral part of the metal surface; (6) lubricating qualities due to its ability to absorb and adsorb the final oil or wax dips; (7) ease and economy of application; and (8) nonelectrolytic solutions and a minimum of plain steel tank equipment required.

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The black oxide finish produced on steel is composed essentially of the black oxide of iron (Fe3O4), and is considered by many to be a combination of FeO and Fe2O3. It can be produced by several methods: the browning process, carbonia process, heat treatment, and the aqueous alkali-nitrate process. Each of these processes produces a black oxide of iron finish, although the finish produced by each particular process differs in some characteristics. The chemical dip aqueous alkali-nitrate process is the most widely used to apply a black oxide finish on steel.

AQUEOUS ALKALI-NITRATE PROCESS

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In this process a blackening solution is used that is highly alkaline and that also contains strong oxidizing chemicals. Refinements such as penetrants and rectifiers are also used to promote ease of operation, faster blackening, and trouble-free processing. At specific concentrations and boiling temperatures, these solutions will react with the iron in the steel to form the black oxide of iron (Fe3O4). Because the reaction is directly with the iron in the steel, the finish becomes an integral part of the metal itself and, therefore, cannot flake, chip, or peel. For all practical purposes, there are also no dimensional changes. In transforming iron to black iron oxide, it is correct to assume that there is a change in volume. However, because of the highly alkaline nature of the blackening solution and because of the operating temperature, the blackening solution will dissolve a small amount of iron. Therefore, the amount of iron lost in this manner is compensated for by the buildup in volume from the change of iron to iron oxide — resulting in, for all intents and purposes, no dimensional changes. Extremely close measurements have shown that the actual change amounts to a buildup of only about 5 millionths of an inch. There are types and conditions of steel and many different products that may require special procedures or additional steps. It is important that only steel or steel alloys be immersed in the solution because other metals such as copper, zinc, cadmium, and aluminum will contaminate it. Many improvements have been incorporated into some proprietary


black oxide salts and the latest one will rectify approximately 50 times more contaminants than heretofore. This particular product causes the contaminants to boil to the top, from which they can be skimmed, or dragged out and rinsed away during processing. After a black oxide blackening solution has been mixed, there is a “breaking-in” period that can run from 24 to 48 h, depending on the volume of blackening solution and the amount of work being processed. During this period, if it occurs, erratic blackening may be encountered, resulting in some work being blackened and some remaining partially or totally unblackened. Chemical black oxide finishes today are being used on a wide variety of consumer and military parts. Some of the most important applications are guns, firearms and components, metal stampings, toys, screws, spark plugs, machine parts, screw machine products, typewriter and calculating machine parts, auto accessories and parts, tools, gauges, and textile machinery parts. A black oxide finish can normally be used for indoor or semioutdoor applications on metal parts or fabrications that require an economical attractive finish, nominal corrosion resistance, and, in many cases, where “dimensional changes” cannot be tolerated.

HEAT-TREATMENT METHODS These methods can be divided into three classes: (1) oven or furnace heating; (2) molten salt bath immersion; and (3) steam heat procss. Oven or Furnace Method The parts are heated to a temperature of 316 to 371°C at which temperature the metal surface is oxidized to a bluish-black color. The shade of color depends on the temperature and the analyses of the steel. Molten Salt Bath Method The oxide finish can be obtained in several ways, depending on the manufacturing or processing requirements: 1. In a molten salt bath composed essentially of nitrate salts maintained


at 316 to 371°C, the pieces are first cleaned of any oils, greases, or objectionable oxides and then immersed in the molten bath. They will take on a blue-black finish, after which they are quenched in clean water and given a final oil dip. 2. In a molten nitrate bath in an austempering operation, the pieces are heated to the hardening temperature in a neutral salt hardening bath, after which they are quenched in the molten nitrate bath at 316 to 371°C. They are then removed, cleaned, and given a final oil dip. Steam Process In this method, the steel is placed in a retort and heated to a minimum temperature of 316°C. The retort is then purged with steam. Under these conditions a black oxide is formed on the metal surface.

BROWNING PROCESS The browning process is commonly known as a rusting process. The pieces are first thoroughly cleaned and then swabbed with an acidic solution. After drying in a dry atmosphere at approximately 77°C, the pieces are then placed in an oven with an atmosphere of 100% humidity and temperature at around 77°C for about 1.5 h. They then become quite rusty and the surface is rubbed down to remove the loose rust. This procedure is carried out three or four times, after which the surface will have taken on a bluish-black finish. The pieces are then given a final oil dip.

CARBONIA PROCESS To apply a black oxide finish by this method, the pieces are placed in a rotary furnace heated to around 316 to 371°C. Charred bone or other carbonaceous material is placed in the furnace along with a thick oil known as carbonia oil. The floor or cover of the furnace is occasionally opened and closed to allow circulation of air. After the parts have been treated for approximately 4 h, they are removed and immersed in


oil. This finish is essentially a black oxide of iron, but because the metal surface is in contact with carbonaceous material and oil, some black carbon penetrates into the metal surface.

PROCESSES

FOR

NONFERROUS METALS

Following are typical methods for applying black oxide coatings to nonferrous metals: 1. Stainless steel: aqueous alkali-nitrate and molten dichromate methods. In the aqueous alkali-nitrate method a solution is made up by using approximately 2.07 to 2.3 kg of blackening salt mixture to make up a gallon of blackening solution, which is operated at a boiling point of 124 to 127°C. The parts, after cleaning and acid pickling, are immersed in the boiling solution and will take on a black color. They are then given a rust-preventive oil dip. In the molten dichromate method a molten bath of sodium dichromate or a mixture of sodium and potassium dichromates is used at a molten temperature of 316 to 399°C. The parts are immersed in the molten bath until they take on a blue or blue-black color, after which they are removed and cooled in oil or water. They are then cleaned to remove the salt and oil (if cooled in oil), after which they are given a dip in a clean, rust-preventive oil. 2. Zinc, zinc plate, zinc-base die castings, and cadmium plate: hot molybdate blackening method, chromate and black dye method, and the black nickel plate method. 3. Copper and copper alloys (brasses and bronzes): alkali-chlorite aqueous solution method, cuprammonium carbonate method, anodic oxidation method, and aryl-sulfone monochloramide-sodium hydroxide method. 4. Aluminum and aluminum alloys: anodize and dye method.

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OXYGEN

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An abundant element, oxygen constitutes about 89% of all water, 33% of the Earth’s crust, and 21% of the atmosphere. It combines readily with most of the other elements, forming their oxides. It is a colorless and odorless gas and can be produced easily by the electrolysis of water, which produces both oxygen and hydrogen, or by chilling air below –184°C, which produces both oxygen and nitrogen. The specific gravity of oxygen is 1.1056. It liquefies at –113°C at 59 atm. Liquid oxygen is a pale-blue, transparent, mobile liquid. As a gas, oxygen occupies 862 times as much space as the liquid. Oxygen is one of the most useful of the elements. Oxygen is separated from air by liquefaction and fractional distillation. The chief uses of oxygen in order of their importance are (1) smelting, refining, and fabrication of steel and other metals; (2) manufacture of chemical products by controlled oxidation; (3) rocket propulsion; (4) biological life support and medicine; and (5) mining, production, and fabrication of stone and glass products. Uncombined gaseous oxygen usually exists in the form of diatomic molecules, O2, but oxygen also exists in a unique triatomic form, O3, called ozone.

PRODUCTION

AND

DISTRIBUTION

Oxygen is produced on a large scale by liquefaction and fractional distillation of air. Minor quantities are produced by electrolysis of water with simultaneous production of hydrogen, which is usually the primary objective. Lowerpurity oxygen can be produced by the pressure swing adsorption (PSA) process: nitrogen is preferentially adsorbed, increasing oxygen content of the remaining stream to over 90%. Plants with output up to 20 tons (18 metric tons) per day have been built, but the process is mainly attractive for very small units. Tens of thousands of these produce oxygen-enriched breathing air for home treatment of chronic pulmonary deficiencies. Traditional methods of preparing oxygen often demonstrated in school chemistry courses are heating potassium chlorate and heating mercuric oxide (Priestley’s original method).


When oxygen is needed in laboratories, however, it is usually obtained from compressedgas cylinders. Oxygen is commonly distributed in three ways: (1) most oxygen is piped directly to users, (2) about 10% is liquefied for transportation and storage in insulated tanks, and (3) about 1% is compressed to high pressure more than 200 MPa for transport in steel cylinders or tube bundles. Oxygen pipelines are usually short because the raw material for air separation is readily available. In industrial areas a single large plant may supply a dozen consumers through a network of pipelines. For smaller or intermittent uses or for rocket engines, oxygen is produced and distributed as a liquid. In liquid form oxygen is about one third heavier than water. So long as it is kept at low temperature, the liquid can be stored, transported, pumped, or handled much as any other liquid. To keep heat away from this very cold liquid, the storage and transport tanks use the best possible insulating techniques.

USES Oxygen is widely used in a variety of applications. While the fraction of oxygen present in the atmosphere is sufficient for many purposes, higher concentrations are necessary to improve some processes. Metallurgical Uses Oxygen is a component used in the metallurgical processes of smelting, refining, welding, cutting, and surface conditioning. Smelting Smelting of ore in the blast furnace involves the combustion of about 1 ton (0.9 metric ton) of oxygen for each ton of metal produced. When air is used, 3.5 tons (3.2 metric tons) of nitrogen accompany each ton (0.9 metric ton) of oxygen and must be compressed, heated, and blown into the furnace. A large amount of heat is lost with the exhaust gases, which also carry powdered ore and coke away as dust and limit the capacity of the furnace. By removing some or all of the nitrogen, the furnace capacity can be increased,


less expensive fuels can replace some coke, and fuels can be used more efficiently. Metal Refining In refining copper and in making steel from pig iron various impurities such as carbon, sulfur, and phosphorus must be removed from the metal by oxidation. If air is blown through the molten metal, as in the Bessemer converter, nitrogen is picked up, limiting the product quality. Nitrogen also carries away a great deal of the heat produced by the oxidation process. Better-quality steel and copper can be produced injecting pure oxygen into the molten metal until the impurities are completely removed. Oxygen injection can be utilized in the open hearth or electric furnaces. However, steelmaking equipment has been developed that depends entirely on high-purity oxygen. All the heat for the furnace operation is supplied by oxidation of carbon and other impurities. The technique is called the basic oxygen process. Reheat furnaces. With specially designed burners, oxygen can be utilized without raising flame temperature. With substantial fuel saving and less pollution, furnace temperature is more easily achieved and controlled. Welding, Cutting, and Surface Conditioning The high-temperature flame of the oxyacetylene torch can be used in welding steel, although most welding is done by the electric arc process. In cutting, the point of the steel at which the cutting is to start is first heated by an oxygen-acetylene flame. A powerful jet of oxygen is then turned on. The oxygen burns some of the iron in the steel to iron oxide, and the heat of this combustion melts more iron; the molten iron is blown out of the kerf by the force of the jet. By feeding powdered iron into the oxygen stream, this cutting process can be extended to alloys such as stainless steel, which are not readily cut by oxygen alone, and to completely noncombustible materials such as concrete. Steel ingots normally have oxide inclusions and other defects at the outer surface. After preliminary rolling, the steel in slab or billet form has the surface skin removed to eliminate these defects. This can be most easily


accomplished by scarfing. Streams of oxygen from many nozzles are played on all sides of the billet at once. The oxygen burns off the surface defects and some of the steel in a spectacular shower of sparks. The billet is then ready for further rolling. Oxygen scarfing, also known as skinning, became a standard practice in most steel mills. Hydrometallurgy The mechanism by which metal values are leached from ores may involve oxidation. Oxygen gas is dissolved in the leach fluid to extract uranium from deeply buried ore deposits. Similarly copper is obtained from previously discarded mine waste. Chemical Syntheses Several syntheses in the chemical industry involve oxygen. Partial Oxidation of Hydrocarbons When natural gas or fuel oil is burned, the heat of combustion first cracks the hydrocarbon molecules into fragments. These fragments usually encounter oxygen molecules within a few hundredths of a second and are oxidized to water and carbon dioxide. However, if the supply of oxygen is carefully controlled and the passage of material through the combustion zone is very rapid, it is possible to freeze the reaction at various stages of completion. In this manner natural gas (mostly methane, CH4) can be converted to acetylene (C2H2), ethylene (C2H4), or propylene (C3H6). Ethylene (C2H4), in turn, can be partially oxidized to ethylene oxide (CH2CH2O). Syngas Production Reaction of carbon or hydrocarbons with oxygen and steam yields a mixture of carbon monoxide (CO) and hydrogen (H2), that is, syngas. By use of suitable catalysts, syngas can be recombined to form various organic compounds such as methanol (CH3OH), octane (C8H16), and many others. In the presence of other catalysts, carbon monoxide can combine with steam to form more hydrogen and carbon dioxide. After removal of the carhon dioxide, the hydrogen can be used for chemical reactions, such as the

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manufacture of ammonia (NH3), hydrogenation of fats, and hydrocracking of petroleum. Fuel Synthesis Conversion of solid fuels to liquid or gaseous hydrocarbons requires addition of hydrogen. This normally involves combustion of some carbon to create high temperatures. Steam reacts with hot carbon to produce hydrogen. Use of oxygen for combustion yields higherpurity fuel gas or higher yields of liquids. In similar manner, use of oxygen for in-place combustion of coal or heavy oil may improve yields and product quality. Manufacture of Pigments Both titanium dioxide white and carbon black are useful primarily because of the characteristics of their small particles. The size, shape, and surface activity of these particles govern the ability of the material to perform properly as a pigment, bulking agent, or stiffener when blended into other materials. Formation of titanium dioxide or carbon in a flame process produces very fine, useful particles. Carefully controlled addition of oxygen to such burner operations can improve yield and quality of the product.

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Liquid Fuel Rockets In rocket engines, liquid oxygen is used as an oxidizer either with kerosine or liquid hydrogen fuels. While fluorine could theoretically provide somewhat improved performance in terms of specific impulse, oxygen is very nearly as good, is much cheaper, and is easier to handle. Solid-fueled rockets, based on hydrocarbon polymers that contain sufficient oxidizer to effect self-combustion, dominate the shortrange military uses. Liquid-fueled rockets are expected to remain dominant in space work until the full development of nuclear propulsion. The Saturn-Apollo launch vehicle has a fully loaded weight of about 3000 tons (2700 metric tons) of which more than 2000 tons (1800 metric tons) is liquid oxygen. Most of the liquid oxygen consumed by the aerospace industry has been used in the development and proof-testing of rocket engines mounted in static test stands. The usage of oxygen in this


testing has been in excess of 1000 tons (900 metric tons) per day. Biological Applications Oxygen is a fundamental part of many biological processes. A few are described below. Aerospace and Diving Oxygen is necessary for life support of animals of this planet. Whenever humans desire to live or work in environments low or deficient in oxygen, it is necessary to carry oxygen along to supplement or substitute for the available atmosphere. High-altitude military aircraft normally provide oxygen for the aviators. Commercial transports carry oxygen for emergency use in case of failure of the cabin pressurizing system. Astronauts must of course carry their entire breathing gas requirements with them, which becomes one of the larger load requirements for any extended mission. Divers in shallow water are able to have air transmitted to them from the surface. However, for deeper diving the special breathing gases frequently are carried to the ocean bottom in special diving bells. Medicine In medical applications, patients breathe air enriched with oxygen. This is usually done to reduce the work of heart and lungs during the course of infectious disease, during or after major surgery, or in recovery from heart attack. Oxygen enrichment may be required to permit even moderate activity if lung function is chronically impaired, as in emphysema. Breathing oxygen at pressure up to 3 atm (300 kilopascals) absolute can increase dissolved oxygen content of body fluids, improving supply to tissues if circulation is impaired. Treatment of Biological Pollutants Experiments have demonstrated that eutrophication of ponds and lakes can be stopped, even reversed, by injecting oxygen into deeper water. The value of lakes for fishing and other water recreations can be preserved. Addition of oxygen to biological treatment equipment increases capacity and reduces power requirements of sewage treatment plants. Oxygen is sometimes pumped directly into sewer lines, rivers, or


streams overloaded with biochemical contamination. With extra oxygen, naturally occurring bacteria may decompose wastes without further treatment. Stone, Clay, and Glass Industries Oxygen has a place in these industries as described below. Glass Manufacture and Fabrication The glass industry uses large quantities of oxygen in the manufacture and shaping of glass. Oxygen additions raise the combustion temperature in the furnace, speeding up and improving control over the melting of glass and its raw materials. Oxygen is used in the burners that heat glass for blowing, shaping, and flame-polishing rough edges. Mining and Quarrying An oxygen-kerosine burner can be used to heat and shape some types of stone. Granite and similar rocks expand when heated rapidly by such a burner so that the surface cracks loose, or spalls. The hot combustion gases blow the fine chips of rocks away, presenting a fresh surface, which is rapidly heated, continuing the process. In this manner the extremely hard taconite iron ore can be pierced for blast holes more


effectively than by conventional drilling methods. Granite for construction and decorative purpose can be quarried by special burners equipped to cut channels through the rock. Slabs of granite can be cut to desired dimension and given an even and pleasing surface using still other burner designs. A rock surface fouled with paint or tarry materials can easily be cleaned by this technique. Artists have used flame shaping to produce statuary. Cement and Kiln Operations In most kiln-type operations, such as manufacture of cement, roasting or sintering ore, and production of refractories, the essential reactions take place at rather high temperatures. When enough heat is provided to carry out the desired reaction, there is more than enough heat to raise the temperature of the fresh feed. Much heat is wasted at lower temperatures where it is not useful to the process. By using oxygen instead of air, the flame temperature is raised and much more heat is available for the hightemperature reaction from a given amount of fuel. Extensive tests have shown that large increases in capacity and reductions in fuel consumption are possible. However, certain changes in equipment are needed to achieve all the potential benefits.

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P PAINT Paint is a term used since the dawn of history to designate cosmetics, marking chalks and pastes, tempera plaster, and colored fluids applied to surfaces for artistic, decorative, or weatherproofing purposes. The term survives today as a general marketing designation for decorative and protective formulations used in architectural, commercial, and industrial applications, with such diverse materials as lacquer, varnish, baking finishes, and specialty coating systems covered in a single category. There is a general definition that suits most products: paint is a fluid, with viscosity, drying time, and flowing properties dictated by formulation, normally consisting of a vehicle or binder, a pigment, a solvent or thinner, and a drier, which may be applied in relatively thin layers and which changes to a solid in time. The change to a solid may or may not be reversible, and may occur by evaporation of the solvent, by chemical reaction, or by a combination of the two. Paint is a general name sometimes used broadly to refer to all types of organic coatings. However, by definition, paint refers to a solution of a pigment in water, oil, or organic solvent, used to cover wood or metal articles either for protection or for appearance. Solutions of gums or resins, known as varnishes, are not paints, although their application is usually termed painting. Enamels and lacquers, in the general sense, are under the classification of paints, but specifically the true paints do not contain gums or resins. Stain is a varnish containing enough pigment or dye to alter the appearance or tone of wood in imitation of another wood, or to equalize the color in wood. It is usually a dye rather than a paint.


In modern technology, paint is classified in three major categories because of differing performance requirements: architectural paints, commercial finishes, and industrial coatings. A fourth category, artistic media, now admits inks, cements, pastes, dyes, plastics, semisolids, and conventional pigmented oils as acceptable materials.

ARCHITECTURAL PAINTS These are air-drying materials applied by brush or spray to architectural and structural surfaces and forms for decorative and protective purposes. Materials are classified by formulation type as solvent thinned and water thinned. Solvent-Thinned Paints The drying mechanism of solvent-thinned paints predominantly may be by solvent evaporation, oxidation, or a combination of the two, and paints in this classification are subdivided accordingly. Solvent-thinned paints that dry essentially by solvent evaporation rely on a fairly hard resin as the vehicle. Resins include shellac, cellulose derivatives, acrylic resins, vinyl resins, and bitumens. Shellac is usually dissolved in alcohol and is commonly used as shellac varnish. Paints based on nitrocellulose or other cellulose derivatives are usually called lacquers. Paints derived from acrylic and vinyl resins usually require a solvent such as ketone, and their architectural applications are limited. However, addition to the formulation of agents that result in emulsion polymerization has produced products with extensive architectural use. Bitumens or asphalts of petroleum or coal tar derivation are most often used in roofing and

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waterproofing applications where heavy layers are required and opportunity for renewal may be limited. In paints that dry by oxidation, the vehicle is usually an oil or an oil-based varnish. These usually contain driers to accelerate drying of the oil. Paints based essentially on linseed oil with suitable pigments such as titanium dioxide and zinc oxide extenders once were the conventional exterior house paints. However, the successful development of polyvinyl acetate and acrylic emulsion types of paint reached the point where these materials became dominant in the exterior house paint market. Water-Thinned Paints

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This group of paints may be subdivided into those in which the vehicle is dissolved in water and those in which it is dispersed in emulsion form. Paints with water-soluble vehicles include the calcimines, in which the vehicle is glue, and casein paints, in which the vehicle is casein or soybean protein. These paints are water-sensitive and have only limited use. Synthetic resins soluble to water and treatments rendering drying oils soluble in water are relatively recent developments. Water evaporates from paints having these materials in their formulation, and further chemical change, either oxidation or heat polymerization, converts the vehicle so that the film no longer is water-sensitive. These paints have a limited market because of the widespread utilization of latex emulsions. Nearly any solvent-thinned paint may be emulsified by the addition of a suitable emulsifier and adequate agitation. The use of fugitive emulsifiers and of vehicles especially processed for use in emulsion paints has produced materials with excellent water resistance, color retention, and durability when cured. Materials formed by emulsion polymerization are described as a latex, and products are called latex paints. The most common latexes are made from a copolymer of butadiene and styrene, from polyvinyl acetate, and from acrylic resin. Properties and performance differ slightly among these paints, but as a group they dominate the architectural market.


ENAMEL PAINTS Enamel paint is an intimate dispersion of pigments in either a varnish or a resin vehicle, or in a combination of both. Enamels may dry by oxidation at room temperature and/or by polymerization at room or elevated temperatures. They vary widely in composition, in color and appearance, and in properties. Although they generally give a high-gloss finish, some give a semigloss or eggshell finish and still others give a flat finish. Enamels as a class are hard and tough and offer good mar and abrasion resistance. They can be formulated to resist attack by the most commonly encountered chemical agents and corrosive atmospheres, and have good weathering characteristics. Because of their wide range of useful properties, enamels are probably the most widely used organic coating in industry. One of their largest areas of use is as coatings for household appliances — washing machines, stoves, kitchen cabinets, and the like. A large proportion of refrigerators, for example, are finished with synthetic baked enamels. These appliance enamels are usually white, and therefore must have a high degree of color and gloss retention when subjected to light and heat. Other products finished with enamels include automotive products; railway, office, sports, and industrial equipment; toys; and novelties.

HOUSE PAINTS House paint for outside work consists of highgrade pigment and linseed oil, with a small percentage of a thinner and drier. The volatile thinner in paints is for ease of application, the drying oil determines the character of the film, the drier is to speed the drying rate, and the pigment gives color and hiding power. Part or all of the oil may be replaced by a synthetic resin. Many of the newer house paints are water-based paints. Paints are marketed in many grades, some containing pigments extended with silica, talc, barytes, gypsum, or other material; fish oil or inferior semidrying oils in place of linseed oil; and mineral oils in place of turpentine. Metal paints contain basic pigments such as red lead,


ground in linseed oil, and should not contain sulfur compounds. Red lead is a rust inhibitor, and is a good primer paint for iron and steel, although it is now largely replaced by chromate primers. White lead has a plasticizing effect that increases adhesion. It is stable and not subject to flaking. Between some pigments and the vehicle there is a reaction that results in progressive hardening of the film with consequent flaking or chalking, or there may be a development of water-soluble compounds. Linseed oil reacts with some basic pigments, giving chalking and flaking. Fading of a paint is usually from chalking. The composition of paints is based on relative volumes because the weights of pigments vary greatly, although the custom is to specify pounds of dry pigment per gallon of oil.

BITUMINOUS PAINTS Bituminous paints are usually coal tar or asphalt in mineral spirits, used for the protection of piping and tanks, and for waterproofing concrete. For line pipe heavy pitch coatings are applied hot, but a bitumen primer is first applied cold. The bituminous paints also have poor solvent resistance.

COMMERCIAL FINISHES These include air-drying or baking-cured materials applied by brush, spray, or magnetic agglomeration to kitchen and laundry appliances, automobile, machinery, and furniture and used as highway marking materials. Paints in this group are subdivided by their drying mechanism. Air-Drying Finishes These materials once were the conventional factory-applied finishes. In the finishing of furniture, lacquers, varnishes, and shellac are still extensively used, but epoxy and unsaturated polyester materials have been adopted in recent years. Automobile manufacturers used lacquers and air-drying alkyd enamels until the adoption of baked acrylic and urea finishes. Epoxy, urethane, and polyester resins, converted at room temperature with a suitable catalyst, are beginning to replace conventional solvent-thinned paints as machinery finishes. The marking of


center lines on highways and other painted areas for the control of traffic requires a finish that dries rapidly, adheres well to both asphalt and concrete, and resists abrasion and staining. Solvent-thinned materials especially formulated for this service from alkyds, modified rubbers, and other resins now are extensively used, but some latex formulations have been tried with success. Baking Finishes Urea and melamine resins polymerize by heat and are used in baking finishes where extreme hardness, chemical resistance, and color retention are required as on kitchen and laundry appliances. Baked acrylic resin formulations now dominate the automobile finish market. Certain phenolic resins are converted by heat to produce finishes with excellent water and chemical resistance.

INDUSTRIAL COATINGS Industrial coatings are subdivided by their intended service: corrosion-resistant coatings, high temperature coatings, and coatings for immersion service. Materials in each subdivision are applied in a system that usually requires a base coat or primer, an intermediate coat or coats, and a top or finish coat. Corrosion-Resistant Coatings These are materials generally inert when cured to deterioration by acidic, alkaline, or other corrosive substances and applied in a system as a protective layer over steel or other substrates susceptible to corrosion attack. The base, or prime, coat in the corrosionresistance system is applied to dry surfaces prepared by abrasive blasting or other methods to a specified degree of cleanliness and toughness. The prime coat provides adhesion to the substrate for the entire coating system: adherence is by mechanical anchorage, chemical reaction, or a combination of the two. Prime coat materials once were predominately a red lead pigment dispersed in linseed oil and cured by oxidation. Today, the prime coat material commonly specified for application to steel surfaces is a dispersion of zinc in a suitable

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inorganic or organic vehicle. Although these zinc-rich primers now dominate the market where protection of steel is concerned, prime coat materials generically similar to subsequent coats in the system are specified when protection of nonferrous or nonmetallic substrates from corrosion attack is required. An intermediate coat in the corrosion-resistant system is not always required. When used, intermediate coat materials usually are highbuild layers of the same generic type specified for the top or surface coat, and are applied only to increase the dry-film thickness of the protective area in places where airborne corrosive fumes, particulates, or droplets are heavily concentrated or where splash and spillage of corrosive fluids are relatively frequent. A top or surface coat in the corrosion-resistant system may be selected from a variety of formulations with vehicles that include phenolic resins, chlorinated rubber, coal tar and epoxy combination, epoxy resin cured from a solvent solution with polyfunctional amines, polyamide resins, vinyl resin in solvent solution, elastomers, polyesters, and polyurethanes. Top coat materials are required to have good scaling properties; high resistance to corrosive deterioration, oxidation, erosion, and ultraviolet degradation; and relative freedom, when cured, from pinholes, blisters, and crevices. Color retention is a desirable but not a mandatory requirement. Selection of materials able to withstand corrosive attack in a given environment and to maintain cohesion and integrity among the various coats in system has become a technological specialty. High-Temperature Coatings These are materials used to alleviate or prevent corrosion, thermal shock, fatigue, oxide sublimation, and embrittlement of metals at high temperature. Formulation of high-temperature coatings is a relatively recent development in the art. Materials problems encountered since World War II in high-temperature chemical reactions, production of steel by the oxygen lance process, aerodynamic heating to complex high-speed aircraft, and aerospace vehicle launching and


reentry have been alleviated to an extensive degree by the results of research toward finding high-strength resistant metals and cements and toward producing coatings serviceable at high temperatures. Research continues as higher and higher temperatures are required by technological developments and radiation effects in outer space are better understood. Available high-temperature coatings include inorganic zinc dispersed in a suitable vehicle, serviceable to 400°C; a phosphate bonding system in which ceramic fillers are mixed with an aqueous solution of monoaluminum phosphate, applied by spraying, brushing, or dipping, and serviceable to 1538°C after curing at 204°C; and a “ceramic gold” coating used on jet engine shrouds and having a temperature limit near 538°C. Ablative formulations that absorb heat through melting, sublimation decomposition, and vaporization, or that expand upon heating to form a foamlike insulation that replenishes itself until the coating is depleted, employ silicone rubber or silicone resins, or polyamide and tetrafluoroethylene polymers to provide shortterm heat protection from 147 to 538°C. Coatings for Immersion Service Included here are materials used to coat or line interior surfaces of vessels of containers holding or storing corrosive fluids, pipelines in which corrosive fluids are the flowing medium, and hoppers or bins conveying or holding abrasive or corrosive pellets or particulates. These coatings are usually applied in relatively high-build systems to carefully cleaned and prepared surfaces. Air-drying formulations are used when extensive coverage is required and where polymerization by automatic heating apparatus is impractical. Small vessels, pipe spools, and assembly components may be protected by coatings cured by oven baking. Coatings for immersion service may be selected from a variety of formulations that include asphalt, chemically cured coal tar, thermoplastic coal tar, epoxy-furans, amine-cured epoxies, fluorocarbons, furfuryl alcohol resins, neoprene, baked unmodified phenolics, unsaturated polyesters, polyether resins, low-density polyethylene, chlorosulfonated polyethylene,


polyvinyl chloride plastisols, resinous cements, rubber, and urethanes, among other vehicles. Selection and specification of coatings for immersion service has become a technological specialty.

PAINT REMOVERS Paint removers, for removing old paint from surfaces before refinishing, are either strong chemical solvents or strong caustic solutions. In general, the more effective they are in removing the paint quickly, the more damaging they are likely to be to the wood or other organic material base. The hiding power of a paint is measured by the quantity that must be applied to a given area of a black and white background to obtain nearly uniform complete hiding. The hiding power is largely in the pigment, but when some fillers of practically no hiding power alone, such as silica, are ground to microfine particle size, they may increase the hiding power greatly. Paint making is a highly developed art, and the variables are so many and the possibilities for altering the characteristics by slight changes in the combinations are so great that the procurement specifications for paints are usually by usage requirements rather than by composition.

PALLADIUM A rare metal, palladium (symbol Pd) is found in the ores of platinum. It resembles platinum, but is slightly harder and lighter in weight and has a more beautiful silvery luster. It is only half as plentiful but is less costly. The specific gravity is 12.10 and the melting point is 1552°C. Annealed, the metal has a hardness of Brinell 40 and a tensile strength of 186 MPa. It is highly resistant to corrosion and to attack by acids, but, like gold, it is dissolved in aqua regia. It alloys readily with gold and is used in some white golds. It alloys in all proportions with platinum and the alloys are harder than either constituent.

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Palladium is soft and ductile and can be fabricated into wire and sheet. The metal forms duc© 2002 by CRC Press LLC

tile alloys with a broad range of elements. Palladium is not tarnished by dry or moist air at ordinary temperatures. At temperatures from 350 to 790°C a thin protective oxide forms in air, but at temperatures from 790°C this film decomposes by oxygen loss, leaving the bright metal. In the presence of industrial sulfur-containing gases a slight brownish tarnish develops; however, alloying palladium with small amounts of iridium or rhodium prevents this action. At room temperature, palladium is resistant to nonoxidizing acids such as sulfuric acid, hydrochloric acid, hydrofluoric acid, and acetic acid but the metal is attacked by nitric acid. Palladium is also attacked by moist chlorine and bromine.

USES The major applications of palladium are in the electronics industry, where it is used as an alloy with silver for electrical contacts or in pastes in miniature solid-state devices and in integrated circuits. Palladium is widely used in dentistry as a substitute for gold. Other consumer applications are in automobile exhaust catalysts and jewelry. The palladium–silver–gold alloys offer a series of noble brazing materials covering a wide range of melting temperatures. A palladium–silver alloy can be used as a diffusion septum for the separation of hydrogen from gas mixtures. Palladium supported on carbon or alumina is used as a catalyst for hydrogenation and dehydrogenation in both liquid- and gas-phase reactions. Palladium finds widespread use in catalysis because it is frequently very active under ambient conditions and it can yield very high selectivities. Palladium catalyzes the reaction of hydrogen with oxygen to give water. Palladium also catalyzes isomerization and fragmentation reactions. Palladium alloys are also used for instrument parts and wires, dental plates, and fountain-pen nibs. Palladium is valued for electroplating because it has a fine white color that is resistant to tarnishing even in sulfur atmospheres. Although palladium has low electric conductivity, 16% that of copper, it is valued for

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its resistance to oxidation and corrosion. Palladium-rich alloys are widely used for lowvoltage electric contacts. Palladium–silver alloys, with 30 to 50% silver, for relay contacts, have 3 to 5% the conductivity of copper. A palladium–silver alloy with 25% silver is used as a catalyst in powder or wire-mesh form. A palladium–copper alloy for sliding contacts has 40% copper with a conductivity 5% that of copper. Many of the palladium salts, such as sodium palladium chloride, Na2PdCl4, are easily reduced to the metal by hydrogen or carbon monoxide, and are used in coatings and electroplating.

PAPER

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Paper is a flexible web or mat of fibers isolated from wood or other plants materials by the operation of pulping. Nonwovens are webs or mats made from synthetic polymers, such as high-strength polyethylene fibers, that substitute for paper in large envelopes and tote bags. Paper is made with additives to control the process and modify the properties of the final product. The fibers may be whitened by bleaching, and the fibers are prepared for papermaking by the process of refining. Stock preparation involves removal of dirt from the fiber slurry and mixing of various additives to the pulp prior to papermaking. Papermaking is accomplished by applying a dilute slurry of fibers in water to a continuous wire or screen; the rest of the machine removes water from the fiber mat. The steps can be demonstrated by laboratory handsheet making, which is used for process control. Although there is no distinct line to be drawn between papers and paperboard, paper is usually considered to be less than 0.15 mm thick. Most all fibrous sheets over 0.30 mm thick are considered to be board. In the borderline range of 0.15 to 0.30 mm, most are considered to be papers, although some are classified as board. Although paper has numerous specialized uses in products as diverse as cigarettes, capacitors, and countertops (resin-impregnated laminates), it is principally used in packaging (~50%), printing (~40%), and sanitary (~7%) applications. Paper was manufactured entirely


by hand before the development of the continuous paper machine; this development allowed the U.S. production of paper to increase by a factor of 10 during the 19th century and by another factor of 50 during the 20th century. In 1960, the global production of paper was 70 million tons (50% by the United States); in 1990 it was 210 million tons (30% by the United States). The annual per capita paper use in the United States is 300 kg, and about 40% is recovered for reuse. Material of basis weight greater than 200 g/m2 is classified as paperboard; lighter material is called paper. Production by weight is about equal for these two classes. Paperboard is used in corrugated boxes; corrugated material consists of top and bottom layers of paperboard called linerboard, separated by fluted corrugating paper. Paperboard also includes chipboard (a solid material used in many cold-cereal boxes, shoe boxes, and the backs of paper tablets) and food containers. Mechanical pulp is used in newsprint, catalog, and other short-lived papers; they are only moderately white, and yellow quickly with age because the lignin is not removed. A mild bleaching treatment (called brightening) with hydrogen peroxide or sodium dithionite (or both) masks some of the color of the lignin without lignin removal. Paper made with mechanical pulp and coated with clay to improve brightness and gloss is used in 70% of magazines and catalogs, and in some enamel grades. Bleached chemical pulps are used in higher grades of printing papers used for xerography, typing paper, tablets, and envelopes; these papers are termed uncoated wood-free (meaning free of mechanical pulp). Coated wood-free papers are of high to very high grade and are used in applications such as high-quality magazines and annual reports; they are coated with calcium carbonate, clay, or titanium dioxide. Like wood, paper is a hygroscopic material; that is, it absorbs water from, and also releases water into, the air. It has an equilibrium moisture content of about 7 to 9% at room temperature and 50% relative humidity. In low humidities, paper is brittle; in high humidities, it has poor strength properties.


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The major attribute of paper is its extreme versatility. A wide range of end properties can be obtained by control of the variables in (1) original selection of the type and size of fiber, (2) the various pulp processing methods, (3) the actual web-forming operation, and (4) the treatments that can be applied after the paper has been produced. Papers have been specifically developed for a number of engineered applications. These include gasketing; electrical, thermal, acoustical, and vibration insulation; liquid and air filtration; composite structural assemblies; simulated leathers and backing materials; cord or twine; and as yarns for paper textiles.

WOOD Wood, a diverse, variable material, is the source of about 90% of the plant fiber used globally to make paper. Straw, grasses, canes, bast, seed hairs, and reeds are used to make pulp, and in many regards their pulp is similar to wood pulp. Fibers are tubular elements of plants and contain cellulose as the principal constituent. Softwoods (gymnosperms) have fibers that are about 3 to 5 mm long, while in hardwoods (angiosperms) they are about 0.8 to 1.6 mm long. In both cases, the length is typically about 100 times the width. Softwoods are used in papers such as linerboard where strength is the principal intent. Hardwoods are used in papers such as tissue and printing to contribute to smoothness. Many papers also include some softwood pulp for strength and some hardwood pulp for smoothness. Wood consists of three major components: cellulose, hemicellulose, and lignin. The first two are white polysaccharides of high molecular weight that are desirable in paper. Cotton is over 98% cellulose, while wood is about 45%. Hemicellulose, although not water soluble, is similar to starch. The hydroxyl groups of these materials allow fibers to be held together in paper by hydrogen bonding. Adhesives are not required to form paper, but some starch is usually used and has a similar effect to the hemicelluloses in helping the fibers bond together. Lignin makes up about 25 to 35% of softwoods


and 18 to 25% of hardwoods. It is concentrated between fibers.

PAPER PRODUCTION Most papers are made from crude fibrous wood pulps. Types of Pulp The type of pulp to a large extent determines the type of paper produced. Pulps are generally classified as mechanical or chemical wood pulps. Mechanical wood pulps produced by mechanical processes include: 1. Ground wood, which is used in a number of papers where absorbency, bulk, opacity, and compressibility are primary requirements, and permanence and strength are secondary. 2. Defibrated pulps, which are used for insulating board, hardboard, or roofing felts where good felting properties are required. 3. Exploded pulps, used for building and insulation hardboards, or so-called “wood composition materials.” Chemical wood pulps are produced by “cooking” the fibrous material in various chemicals to provide certain characteristics. They include: 1. Sulfite pulps, used in the bleached or unbleached state for papers ranging from very soft or weak to strong grades. There are about 12 grades of sulfite pulps. 2. Neutral sulfite or monosulfite pulps, used for strong papers for bags, wrappings, and envelopes. 3. Sulfate or kraft pulps, providing high strength, fair cleanliness and, in some instances, high absorbency. Such pulps are used for strong grades of unbleached, semibleached, or bleached paper (called kraft paper) and board.

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4. Soda pulps, used principally in combination with bleached sulfite or bleached sulfate pulp for book-printing papers. 5. Semichemical pulps, used for specialty boards, corrugating papers, glassine and greaseproof papers, test liners, and insulating boards and wallboards. 6. Screenings, used principally for coarse grades of paper and board, such as millwrapper, and as a substitute for chipboard, corrugating papers, and insulation board. Papermaking

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The two basic types of papermaking machines are the Fourdrinier and the cylinder machines; the Fourdrinier machine is the most commonly used. In Fourdrinier papermaking, the pulp, mixed to a consistency of 97.5 to 99.5% water, is fed continuously to Fourdrinier, which consists of an endless belt of fine mesh screen called the “wire.” As the pulp web travels along the wire, water drains from it into suction boxes. As it leaves the wire (at about 83% water) it passes through presses, and usually a variety of other types of equipment, such as dryers and calender rolls, depending on the type of paper produced. Cylinder machine papermaking differs from Fourdrinier in that the web of pulp is formed on a cylindrical mold surface instead of a continuous wire covered with fine wire cloth, which revolves in a vat of paper stock or pulp. The felt conveyor carries the resulting web to the press and dryers. The cylinder machine is used to produce a greater variety of paper thicknesses, ranging from the thinnest tissue to the thickest building board. Bleaching Chemical pulp for printing paper has 3 to 6% lignin, which gives the pulp a brown color. This lignin is removed with bleaching chemicals in four to eight stages. Each stage consists of a pump to mix the bleaching agent with the pulp, © 2002 by CRC Press LLC

a retention tower to allow the chemical to react with the pulp for 30 min to several hours, and a washing unit to remove the solubilized lignin and residual chemicals from the pulp. Usually an oxidizing material is followed by a stage of alkali extraction, because lignin becomes more soluble at high pH. A common bleaching sequence involves elemental chlorine, alkali extraction, and, finally, chlorine dioxide. Sodium hypochlorite, also found in household liquid bleach, is sometimes used. There is some pressure for the bleaching process to be elemental chlorine free. This is possible by using oxygen as the first bleaching chemical, chlorine dioxide in place of chlorine, and hydrogen peroxide as a bleaching agent.

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In the broadest classification, there are three basic types of papers: cellulose fiber, inorganic fiber, and synthetic organic fiber papers. Cellulose Fiber Papers These papers, made from wood pulp, constitute by far the largest number of papers produced. A great many of the engineering papers are produced from kraft or sulfate pulps. The term kraft is used broadly today for all types of sulfate papers, although it is primarily descriptive of the basic grades of unbleached sulfate papers, where strength is the chief factor, and cleanliness and color are secondary. By various treatments, kraft can be altered to produce various grades of condenser, insulating, and sheathing papers. Other types of vegetable fibers used to produce papers include: 1. Rope, used for strong, pliable papers, such as those required in cable insulation, gasketing, bags, abrasive papers, and pattern papers. 2. Jute, used for papers possessing excellent strength and durability. 3. Bagasse, used for paper for wallboard and insulation, usually where strength is not a primary requirement. 4. Esparto, used for high-grade book or printing papers. A number of other types of pulps are also used for these papers.


Inorganic Fiber Papers There are three major types of papers made from inorganic fibers: 1. Asbestos is the most widely used inorganic fiber for papers. Asbestos papers are nonflammable, resistant to elevated temperatures, and have good thermal insulating characteristics. They are available with or without binders and can be used for electrical insulation or for high-temperature reinforced plastics. 2. Fibrous glass can be used to produce porous and nonhydrating papers. Such papers are used for filtration and thermal and electrical insulation, and are available with or without binders. High-purity silica glass papers are also available for hightemperature applications. 3. Ceramic fiber (aluminum silicate) papers provide good resistance to high temperatures, low thermal conductivity, good dielectric properties, and can be produced with good filtering characteristics. Synthetic Organic Fiber Papers A great deal of research has been carried out on the use of such synthetic textile fibers as nylon, polyester, and acrylic fibers in papers. Some of the earliest appear highly promising for electrical insulating uses. Others appear promising for chemical or mechanical applications. They are most commonly combined with other fibers in a paper, primarily to add strength.

PAPER TREATMENTS Papers can be impregnated or saturated, coated, laminated, or mechanically treated. The major treatments used are covered and indicate the extent of treatments available. Impregnation or Saturation Impregnation or saturation can be carried out either at the beater stage in the processing of the pulp, or after the paper web bas been


formed. Beater saturation permits saturation of nonporous or adsorbent papers, whereas papers saturated after manufacture must be of the absorbent type to permit complete impregnation by the saturant. Papers can be saturated or impregnated with almost any known resin or binder. Probably the most commonly used are asphalt for moisture resistance; waxes for moisture vapor and water resistance; phenolic resins for strength and rigidity; melamine and certain ureas for wet strength (not to be confused with moisture resistance); rubber latexes, both natural and synthetic, for resilience, flexibility, strength, and moisture resistance; epoxy or silicone resins for dielectric characteristics or dielectric characteristics at elevated temperatures; and ammonium salts, or other materials for flameproofing. A number of proprietary beater saturated papers are currently available. They are used primarily for gasketing, filtration, simulated leathers, and backing materials. Most of these consist of cellulose or asbestos fibers blended with natural or synthetic rubbers. In some cases cork is added to the blend for increased compressibility. Another type of proprietary beater saturated paper consists of leather fibers blended with rubber latexes. Coatings Papers may be coated either by the paper manufacturer or by converters. Coating materials, which also impregnate the paper to a greater or lesser degree, include practically every known resin or binder and pigment used in the paint industry. Coatings can be applied in solvent or water solutions, water emulsions, hot melts, and extrusion coatings, or in the form of plastisols or organisols. The most important properties provided by coatings are (1) gas and water vapor resistance, (2) water, liquid, and grease resistance, (3) flexibility, (4) heat sealability, (5) chemical resistance, (6) scuff resistance, (7) dielectric properties, (8) structural strength, (9) mold resistance, (10) avoidance of fiber contamination, and (11) protection of printing. Coating materials range from the older asphalts, waxes, starches, casein, shellac, and

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natural gums, to the newer polyethylenes, vinyl copolymers, acrylics, polystyrenes, alkyds, polyamides, cellulosics, and natural or synthetic rubbers.

paper are reported to be a high degree of toughness, combined with a smooth surface, and a high resistance to tearing or punching. Twisting

Laminations Paper can be laminated to other papers or to other films to provide a variety of composite structures. Probably the most common types of paper laminates are those composed of layers of paper laminated with asphalt to provide moisture resistance and strength. Simple laminations of paper can be so oriented that overall characteristics of the composite are isotropic. Laminating paper with plastics or other types of films or with metal foils will, in many cases, combine the desirable properties of the film or foil with those of the paper. Scrim is a mat of fibers, usually laminated as a “core” material between two faces of paper. It is usually used to provide strength but can also provide bulk for cushioning, or a degree of “hand” to the composite material. Mechanical Treatments

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Several mechanical treatments can be applied to papers to provide particular special properties. Crimping, which can be done either on the paper web or on the individual fibers of the paper, essentially adds stretch or extensibility. Crimping the paper web results in crepe paper with improved strength, stretch, bulk, and conformability and with texture similar to that of cloth. The creping process usually consists of “crowding” the paper into small pleats or folds with a “doctor.” Typical range of elongation or stretch obtainable is 20 to 300%. Cross-creping can provide controllable stretch in directions perpendicular to each other, further improving drapability. A high degree of stretch, conformability, and flexibility is produced by a patented process that differs from creping in that the individual fibers in the paper web are crimped, rather than the web itself. The amount of stretch is variable, but about 10% stretch in the machine direction seems to be optimum for most industrial applications. The major advantages of this type of


Twisting is used to convert paper to twine or yarns. Such yarns have substantially higher strength than the paper from which they are made. Twisting papers are usually sulfate papers, either bleached or unbleached. High tensile strength is required in the machine direction, and the heavier-weight papers should usually be soft and pliable. Treatments to impart such characteristics as wear and moisture resistance can be applied during or after the spinning operation. Embossing and Other Techniques Decorative papers can be produced by embossing in a variety of patterns. Embossing does not usually improve strength significantly. Embossing or “dimpling” in certain patterns, followed by lamination, can produce composites with added strength as well as bulk and thermal insulation. Other mechanical methods include (1) shredding for bulk or padding, (2) pleating, used as a forming aid and for strength in paper cups and plates, (3) die cutting and punching, and (4) molding, which consists of compressing the wet pulp web in a mold to form a finished shape, such as an egg crate.

APPLICATIONS As an engineering material, paper has several important applications. As filtration material, paper can be used either as a labyrinth barrier material to guide the fluid or gas to be filtered or, more commonly, as the filtering medium itself. Paper is used for filtering automotive air and oil, and air in room air conditioners, as well as for industrial plant filtration, filtering liquids in tea bags, in addition to machine-cooling oil and domestic hot water filters. Papers are used for both light- and heavyduty gaskets, for such applications as high- and


low-pressure steam and water, high- and lowtemperature oil, aromatic and nonaromatic fuel systems, and for sealing both rough and machined surfaces. Electrical insulation represents one of the largest engineering uses of papers. For electrical uses, special types include coil papers or layer insulation, cable paper or turn insulation, capacitor papers, condenser papers, and hightemperature, inorganic insulating papers. A large and growing use for paper is in structural sandwich materials, where papers are impregnated with a resin such as phenolic and formed in the shape of a honeycomb. The honeycomb is used as a core between facing sheets of a variety of materials including paperboard, reinforced plastics, and aluminum. Another large-volume application of papers is as backing material for decorative films or other surfacing materials. These provide bulk and depth to the product in simulating leather or fabrics.

PARTICULATES Particulates are solids or liquids in a subdivided state. Because of this subdivision, particulates exhibit special characteristics that are negligible in the bulk material. Normally, particulates will exist only in the presence of another continuous phase, which may influence the properties of the particulates. A particulate may comprise several phases. They can be categorized into particulate systems that relate them to commonly recognized designations. Fine-particle technology deals with particulate systems in which the particulate phase is subject to change or motion. Particulate dispersions in solids have limited and specialized properties and are conventionally treated in disciplines other than fine-particle technology. The universe is made up of particles, ranging in size from the huge masses in outer space — such as galaxies, stars, and planets — to the known minute building blocks of matter — molecules, atoms, protons, neutrons, electrons, neutrinos, and so on. Fine-particle technology is concerned with those particles that are tangible to human senses, yet small compared to the human environment — particles that are


larger than molecules but smaller than gravel. Fine particles are in abundance in nature (as in rain, soil, sand, minerals, dust, pollen, bacteria, and viruses) and in industry (as in paint pigments, insecticides, powdered milk, soap, powder, cosmetics, and inks). Particulates are involved in such undesirable forms as fumes, fly ash, dust, and smog and in military strategy in the form of signal flares, biological and chemical warfare, explosives, and rocket fuels.

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There are many cases where a particulate is either a necessary or an inadvertent intermediary in an operation. Areas that may be involved in processing particulates may be classified as follows: Size Reduction Mechanical (starting with bulk material) Grinding Atomization Emulsification Physicochemical (conversion to molecular dispersion) Phase change (spray drying, condensation) Chemical reaction Size Enlargement (agglomeration, compaction) Pelletizing Briqueting Nodulizing Sintering Separation or Classification Ore beneficiation Protein shift Deposition (collection, removal) Coating or Encapsulation Handling Powders Gas suspensions (pneumatic conveying) Liquid suspensions (non-newtonian fluids) End products in which particulate properties themselves are utilized include the following:

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Mass or Heat Transfer Agents (used in fluidized beds) Recording (memory) Agents Electrostatic printing powders and toners (for optical images) Magnetic recording media (for electronic images) Coating Agents (paints) Nucleating Agents Control Agents (for servomechanisms) Electric fluids Magnetic fluids Charge Carriers (used in propulsion, magnetohydrodynamics) Chemical Reagents Pesticides, fertilizers Fuels (coal, oil) Soap powders Drugs Explosives Food Products

CHARACTERIZATION The processing or use of particulates will usually involve one or more of the following characteristics:

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Physical Size (and size distribution) Shape Density Packing or concentration Chemical Composition Surface character Physiochemical (including adhesive, cohesive) Mechanical or Dynamic Inertial Diffusional Fluid drag Dilute suspensions (Stokes’ law, Cunningham factor, drag coefficient) Concentrated suspensions (hindered settling, rheology, fluidization) Optical (scattering, transmission, absorption)


Refractive index (including absorption) Reflectivity Electrical Conductivity Charge Magnetic Thermal Insulation (conductivity, absorptivity) Thermophoresis Many of the characteristics of particulates are influenced to a major extent by the particle size. For this reason, particle size has been accepted as a primary basis for characterizing particulates. However, with anything but homogeneous spherical particles, the measured “particle size” is not necessarily a unique property of the particulate but may be influenced by the technique used. Consequently, it is important that the techniques used for size analysis be closely allied to the utilization phenomenon for which the analysis is desired.

PARTICLE SIZE Size is generally expressed in terms of some representative, average, or effective dimension of the particle. The most widely used unit of particle size is the micrometer (µm), equal to 0.001 mm. Another common method is to designate the screen mesh that has an aperture corresponding to the particle size. The screen mesh normally refers to the number of screen openings per unit length or area; several screen standards are in general use; the two most common in the United States are the U.S. Standard and the Tyler Standard Screen Scales. Particulate systems are often complex. Primary particulates may exist as loosely adhering (as by van der Waals forces) particles called flocs or as strongly adhering (as by chemical bonds) particulates called agglomerates. Primary particles are those whose size can only be reduced by the forceful shearing of crystalline or molecular bonds. Figure P.1 shows states of dispersion of particulates. The double arrows imply reversibility with application of light shearing forces.


crystallites or substructure

discrete particles agglomerates

primary particles

strong bonds weak bonds

flocs

flocculated agglomerates

flocculated primary particles

powder

weak bonds collection of agglomerates

collection of flocs

FIGURE P.1 States of dispersion of particulates. The double arrows imply reversibility with application of light shearing forces. (From McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 13, McGraw-Hill, 168. With permission.)

PASTES Conductor, resistor, dielectric, seal glass, polymer, and soldering compositions are available in paste or ink form. They are used to produce hybrid circuits, networks, and ceramic capacitors. The materials are often called thick-film compositions.

TYPES Conductor pastes consist of metallic elements and binders suspended in an organic vehicle. Primarily, precious metals such as gold, platinum, palladium, silver, copper, and nickel are used singularly or in combination as the conductive element. The adhesion mechanism to the substrate is provided by either a frit bond, reactive bond, or mixed bond. Important properties of conductor pastes include wire bondability, conductivity, solderability, solder leach resistance, and line definition. © 2002 by CRC Press LLC

Thick-film resistor pastes are composed of a combination of glass frit, metal, and oxides. These pastes are used in microcircuits, voltage dividers, resistor networks, chip resistors, and potentiometers. Dielectric compounds are used as insulators for the fabrication of multilayer circuits, crossovers, or as protective coverings. Solder pastes are one of the more common component attach products. They consist of finely divided solder powders of all common alloys of tin, lead, silver, gold, etc., suspended in a vehicle–flux system. The fluxes may be nonactivated or completely activated. The most popular is RMA (rosin, mildly activated).

PERMANENT MOLD CASTINGS Permanent mold casting is performed in a mold, generally made of metal, that is not destroyed by removing the casting. Several types of casting can be included in the description: pressure die casting, centrifugal casting, and gravity die casting.

ADVANTAGES

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Gravity die castings are dense and fine grained and can be made with better surfaces and to closer tolerances than sand castings. Tolerances are wider than for pressure die castings and plaster mold castings, but narrower than for sand castings. Production rates are lower than those obtainable by die casting. The process stands somewhere between sand and die casting with respect to possible complexity, dimensional accuracy, mold or die casts, etc. For many parts it provides an attractive compromise when the ultimate — whether in complexity of one-piece construction, narrow tolerances, or ultrahigh production rates — need not be met.

CASTING ALLOYS Lead-, zinc-, aluminum-, magnesium-, and copper-base alloys, as well as gray cast iron, can be cast by permanent molding. Less than 1% of total gray iron production is permanent mold cast, 5 to 7% of copper and magnesium alloy, and up to 40% of total aluminum casting

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production. Here is a breakdown of problems to expect: Gray iron. Mechanical properties depend on thickness. Aluminum. Gates must be made larger to take the low specific gravity of aluminum into account. Magnesium. Extreme caution is needed when removing metal from the furnace. Ladles must be kept at red heat to exclude moisture and prevent an explosion. Copper. Aluminum bronze is the most popular permanent-mold-cast copperbase alloy. Turbulence must be prevented in the die cavity to reduce oxidation of the molten metal during solidification.

PERMEABILITY ALLOYS

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This is a general name for a group of nickel–iron alloys with special magnetic properties. These soft magnetic materials possess a magnetic susceptibility much greater than iron. An early alloy was composed of 78.5% nickel and 21.5% iron. It also contained about 0.37% cobalt, 0.1% copper, 0.04% carbon, 0.03% silicon, and 0.22% manganese. It is produced sometimes with chromium or molybdenum, under the name of Permalloy, and is used in magnetic cores for apparatus that operates on feeble electric currents, and in the loading of submarine cables. It has very little magnetic hysteresis.

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Supermalloy for transformers contains 79% nickel, 15% iron, 5% molybdenum, and 0.5% manganese, with total carbon, silicon, and sulfur kept below 0.5%. It is melted in vacuum, and poured in an inert atmosphere. It can be rolled as thin as 0.00064 cm. The alloy has an initial permeability 500 times that of iron. Supermendur contains 49% iron, 49% cobalt, and 2% vanadium. It is highly malleable, and has very high permeability with low hysteresis loss at high flux density. Duraperm


is a high-flux magnetic alloy containing 84.5% iron, 9.5% silicon, and 6% aluminum. Perminvar is an alloy containing 45% nickel and 25% cobalt, intended to give a constant magnetic permeability for variable magnetic fields. Ametal is a nickel–iron alloy containing 44% nickel and a small amount of copper. It is used in transformers and loudspeakers to give nondistortion characteristics when magnetized. Alfenol contains no nickel, but has 16% aluminum and 84% iron. It is brittle and cannot be rolled cold, but can be rolled into thin sheets at a temperature of 575°C. It is lighter than other permeability alloys, and has superior characteristics for transformer cores and taperecorder heads. A modification of this alloy, called Thermenol, contains 3.3% molybdenum without change in the single-phase solid solution of the binary alloy. The permeability and coercive force are varied by heat treatment. At 18% aluminum, the alloy is practically paramagnetic. The annealed alloy with 17.2% aluminum has constant permeability. Aluminum–iron alloys with 13 to 17% aluminum are produced in sheet form for transformers and relays. They have magnetic properties equal to the 50–50 nickel–iron alloys and to the silicon–iron alloys, and they maintain their magnetic characteristics under changes in ambient temperature. Iron–nickel permeability alloys are used as loading in cable by wrapping layers around the full length of the cable. When nickel–copper alloys are used, they are employed as a core for the cable. Magnetostrictive alloys are iron–nickel alloys that will resonate when the frequency of the applied current corresponds to the natural frequency of the alloy. They are used in radios to control the frequency of the oscillating circuit. Magnetostriction is the stress that occurs in a magnetic material when the induction changes. In transducers it transforms electromagnetic energy into mechanical energy. Temperature-compensator alloys are iron–nickel alloys with about 30% nickel. They are fully magnetic at –29°C but lose their magnetic permeability in proportion to rise in temperature, until, at about 54°C, they are nonmagnetic. Upon cooling they regain permeability at the same rate. They are used in shunts in


electrical instruments to compensate for errors due to temperature changes in the magnets.

PEWTER Pewter is a very old name for tin–lead alloys used for dishes and ornamental articles; the term now refers to the use rather than to the composition of the alloy. Tin was the original base metal of the alloy, the ancient Roman pewter having about 70% tin and 30% lead, although iron and other elements were present as impurities. Pewter, or latten ware, of the 16th-century contained as much as 90% tin, and a strong and hard English pewter contained 91% tin and 9% antimony. This alloy is easily cold-rolled and spun, and can be hardened by long annealing at 225°C and quenching in cold water and tempering at 110°C. Pewter is now likely to contain lead and antimony, and very much less tin; when the proportion of tin is less than about 65%, the alloys are unsuited for vessels to contain food products, because of the separation of the poisonous lead. Early pewter, with high lead content, darkened with age. With less than 35% lead, pewter was used for decanters, mugs, tankards, bowls, dishes, candlesticks, and canisters. The lead remained in solid solution with the tin so that the alloy was resistant to the weak acids in foods. Addition of copper increases ductility; addition of antimony increases hardness. Pewter high in tin (91% tin and 9% antimony or antimony and copper, for example) has been used for ceremonial objects, such as religious communion plates and chalices, and for cruets, civic symbolic cups, and flagons.

Phenol has broad biocidal properties, and dilute aqueous solutions have long been used as an antiseptic. At higher concentrations it causes severe skin burns; it is a violent systemic poison. It is a valuable chemical raw material for the production of plastics, dyes, pharmaceuticals, syntans, and other products.

PROPERTIES Phenol melts at about 43°C and boils at 183°C. The pure grades have melting points of 39, 39.5, and 40°C. The technical grades contain 82 to 84% and 90 to 92% phenol. The crystallization point is given as 40.41°C. The specific gravity is 1.066. It dissolves in most organic solvents. By melting the crystals and adding water, liquid phenol is produced, which remains liquid at ordinary temperatures. Phenol has the unusual property of penetrating living tissues and forming a valuable antiseptic. It is also used industrially in cutting oils and compounds and in tanneries. The value of other disinfectants and antiseptics is usually measured by comparison with phenol.

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Phenol is one of the most versatile industrial organic chemicals. It is the starting point for many diverse products used in the home and industry. A partial list includes nylon, epoxy resins, surface active agents, synthetic detergents, plasticizers, antioxidants, lube oil additives, phenolic resins (with formaldehyde, furfural, and so on), cyclohexanol, adipic acid, polyurethanes, aspirin, dyes, wood preservatives, herbicides, drugs, fungicides, gasoline additives, inhibitors, explosives, and pesticides.

PHENOL Phenol is the simplest member of a class of organic compounds possessing a hydroxyl group attached to a benzene ring or to a more complex aromatic ring system. Also known as carbolic acid or monohydroxybenzene, phenol is a colorless to white crystalline material of sweet odor, having the composition C6H5 OH, obtained from the distillation of coal tar and as a by-product of coke ovens.


PHENOL–FORMALDEHYDE RESIN This is a synthetic resin, commonly known as phenolic, made by the reaction of phenol and formaldehyde, and employed as a molding material for the making of mechanical and electrical parts. It was the earliest type of hard, thermoset synthetic resins, and its favorable combination of strength, chemical resistance, electrical properties, glossy finish, and nonstrategic abundance

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of low-cost raw materials has maintained the resin, with its many modifications and variations, as one of the most widely employed groups of plastics for a variety of products. The resins are also used for laminating, coatings, and casting resins. Phenolic resins are used most extensively as thermosetting plastic materials, as there are only a few uses as thermoplastics. The polymer is composed of carbon, hydrogen, oxygen, and sometimes nitrogen. Its molecular weight varies from a very low value during its early state of formation to almost infinity in its final state of cure. The chemical configuration, in the thermoset state, is usually represented by a threedimensional network in which the phenolic nuclei are linked by methylene groups. The completely cross-linked network requires three methylene groups to two phenolic groups. A lesser degree of cross-linking is attainable either by varying the proportions of the ingredients or by blocking some of the reactive positions of the phenolic nucleus by other groups, such as methyl, butyl, etc. Reactivity can be enhanced by increasing the hydroxyl groups on the phenolic nuclei, for example, by the use of resorcinol.

flock, and are formulated for noncritical functional requirements. They provide a balance of moderately good mechanical and electrical properties, and are generally suitable in temperatures up to 149°C. Impact-resistant grades are higher in cost. They are designed for use in electrical and structural components subject to impact loads. The fillers are usually paper, chopped fabric, or glass fibers. Electrical grades, with mineral fillers, have high electrical resistivity plus good arc resistance, and they retain their resistivity under high-temperature and high-humidity conditions. Heat-resistant grades are usually mineral- or glass-filled compounds that retain their mechanical properties in the 190 to 260°C temperature range. Special-purpose grades are formulated for service applications requiring exceptional resistance to chemicals or water, or combinations of conditions such as impact loading and a chemical environment. The chemical-resistant grades, for example, are inert to most common solvents and weak acids, and their alkali resistance is good. Nonbleeding grades are compounded specially for use in container closures and for cosmetic cases.

FILLERS CHARACTERISTICS

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The outstanding characteristics of phenolics are good electrical properties, very rigid set, good tensile strength, excellent heat resistance, good rigidity at elevated temperature, good aging properties; also, good resistance to water, organic solvents, weak bases, and weak acids. All these characteristics are coupled with relatively low cost. Phenolics are used in applications that differ widely in nature. For example, wood is impregnated to make “impreg” and “compreg”; paper is treated to make battery separators and oil and air filters; specific chemical radicals can be added to the molecule to make an ionexchange material. Phenolics are also widely used in protective coating. The hundreds of different phenolic molding compounds can be divided into six groups on the basis of major performance characteristics. General-purpose phenolics are low-cost compounds with fillers such as wood flour and


Proper balance of fillers is important, because too large a quantity may produce brittleness. Organic fillers absorb the resin and tend to brittleness and reduced flexural strength, although organic fibers and fabrics generally give high impact strength. Wood flour is the most usual filler for general-service products, but prepared compounds may have mineral powders, mica, asbestos, organic fibers, or macerated fabrics, or mixtures of organic and mineral materials. Bakelite was the original name for phenol plastics, but trade names now usually cover a range of different plastics, and the types and grades are designated by numbers. The specific gravity of filled phenol plastics may be as high as 1.70. The natural color is amber, and, as the resin tends to discolor, it is usually pigmented with dark colors. Normal phenol resin cures to single-carbon methylene groups between the phenolic groups, and the molded part tends to be brittle. Thus, many of the innumerable variations of phenol are now


used to produce the resins, and modern phenol resins may also be blended or cross-linked with other resins to give higher mechanical and electrical characteristics. Furfural is frequently blended with the formaldehyde to give better flow, lower specific gravity, and reduced cost. The alkylated phenols give higher physical properties. Phenol resins may also be cast and then hardened by heating. The cast resins usually have a higher percentage of formaldehyde and do not have fillers. They are poured in syrupy state in lead molds and hardened in a slow oven. Some of the uses for phenolic resins are for making precisely molded articles, such as telephone parts, for manufacturing strong and durable laminated boards, or for impregnating fabrics, wood, or paper. Phenolic resins are also widely used as adhesives, as the binder for grinding wheels, as thermal insulation panels, as ion-exchange resins, and in paints and varnishes. Molding Compounds The largest single use for phenolic resins is in molding compounds. To make these products, either one- or two-stage resins are compounded with fillers, lubricants, dyes, plasticizers, etc. Wood flour is used as an inexpensive reinforcing agent in the general-purpose type of compounds. Cotton flock, chopped fabric, and sisal and glass fibers are used to improve strength characteristics; mineral fillers such as asbestos and mica are used where improvements in dimensional stability, heat resistance, or electrical properties are desired. The compounds are usually produced in granular, macerated, or nodular forms, depending on type of filler used. Since the color of the base resin is not stable to light, molding compounds are commonly produced only in dark colors such as black and brown. Molding compounds are usually processed in hardened steel molds and molds can be designed to operate using the compression, transfer, or plunger-molding techniques, depending on the design of the article to be fabricated. Molded parts can be drilled, tapped, or machined. About one third of all phenolic resins produced is processed into parts by molding. Compression and transfer molding are the principal


processes used but they can also be extruded and injection molded. Molded phenolic parts are used in bottle caps, automotive ignition and engine components, electrical wiring devices, washing machine agitators, pump impellers, electronic tubes and components, utensil handles, and a multitude of other products. Adhesives The thermosetting nature and good water- and fungus-resistant qualities of phenolic resins make them ideal for adhesive applications. Almost all exterior-grade plywood is phenolic resin bonded. This constitutes the second largest market for phenolics. The essential ingredient in many metal-to-metal, and metal-to-plastic adhesives is a phenolic resin. One-step phenol–formaldehyde resins are used predominantly for hot-pressed plywood. Special resorcin–formaldehyde resins curing at room temperature are employed for fabricating laminated timber. Laminates The third largest use for phenolic resins is in the manufacture of laminated materials. Many variations of paper, from cheap kraft to highquality alpha cellulose, in addition to asbestos, cotton, linen, nylon, and glass fabrics are the most commonly used reinforcing filler sheets. The laminate is formed by combining under heat 177°C and pressure 3.4 to 14 MPa multiple layers of the various reinforcing sheet; after saturation with phenolic resin, generally of the one-step type dissolved in alcohol. Paper laminates are used most extensively in the electrical and decorative fields. A large number of the laminates for the electrical industry are of the punching grade, making it possible to fabricate all kinds of small parts in a punch press. The laminate used for decorative purposes usually contains a surface sheet of melamine resin-treated paper for providing unlimited color or design configurations. Other fillers are used for special applications where superior dimensional stability, or water, fire, or chemical resistance, or extra strength is required.

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Casting Resins Plastic parts can also be manufactured by pouring resin into molds and heat-curing without pressure. Two basic grades of casting resins are manufactured commercially. Both are of the one-stage type and are cured under neutral to strongly acidic conditions depending on the application. The first grade is manufactured for its variegated color and artistic possibilities and is used primarily in the cutlery and decorative field. Since this type of cast material is noted for ease of machining, it is well adapted for small production runs or where machined prototypes are desired. The second grade of casting resins includes all those modified by fillers and reinforcing agents. Designed primarily to have low-shrinkage characteristics during cure, they are usually set with a strong acid catalyst to obtain a low temperature set. Uses include containers, jigs, fixtures, and metal-forming dies.

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Coated abrasives are manufactured by bonding abrasive grit to paper, fabric, or fiberboard by means of phenolic resins of the liquid one-stage type. Sander disks and belts are common applications. Brake linings and clutch facings are made by bonding asbestos, fillers, metal shavings, and friction modifiers with phenolic resin, usually performed by a mixing, forming, and baking operation. Most resins for these applications are specially formulated, but simple two-stage resins are sometimes used. Formulation for each friction lining or clutch face must be determined carefully. Wood composition products of all descriptions are manufactured by hot-pressing sawdust, wood chips, or wood flour containing 8 to 20% resin. Two-step resins are most frequently used in composition boards. Onestep resins are employed for special applications where the yellow color or the slight ammonia odor of two-step resins is undesirable.

Bonding Agents

FOUNDRY USE

Phenolic resins are noted for their excellent bond strength characteristics under elevated temperature conditions, and thus are used in such applications as thermal and acoustical insulation, grinding wheels, coated abrasives, brake linings, and clutch facings. Glass wool insulation is manufactured by spraying watersoluble one-stage resins on the glass fibers as they are formed. Heat given off by the fibers as they cool is sufficient to set the resin. Where organic fibers are used, finely pulverized, single-stage resins are distributed between the fibers, either by a mixing or a dusting operation, followed by an oven treatment. Grinding wheels bonded with phenolic resin are commonly known as resinoid-bonded wheels. By combining a liquid single-stage resin and a powdered two-stage resin, the material can be evenly distributed with abrasive grit and fillers so that the mixture can be pressed into wheels and baked in ovens. Some wheels are also hot-pressed and cured directly in a press. Resinoid-bonded wheels are used primarily where the application requires a bond of exceptional strength such as in cut-off and snagging wheels.

A relatively new application holding great promise for phenolic resins is in the shell mold process for the foundry industry. The basic principle involves binding sand grains with resin. The process is equally well adapted to making cores and is satisfactory for practically all metals, including magnesium and high-chrome alloys. The process has many intriguing possibilities. Molds thus made can be reproduced in exact composition, detail, and size; they are rigid and, having no affinity for water, can be racked and stored indefinitely, with or without cores. Castings from these molds have excellent surface finish and detail and can be held to close dimensional tolerances. The process can be completely mechanized, yields more castings per ton of melt than sand casting, simplifies cleaning of castings, minimizes problems with sand control and handling, and is a relatively clean operation. It is revolutionizing the foundry industry. Consumption of resin in the foundry industry has grown rapidly, and conceivably this could become the largest single outlet for phenolics. However, markets for phenolic resins in



plywood adhesive, insulation, and wood composition board applications have also expanded.

PHOSGENE The common name for carbonyl chloride, COCl2, a colorless, poisonous gas made by the action of chlorine on carbon monoxide. It was used as a poison war gas. But it is now used in the manufacture of metal chlorides and anhydrides, pharmaceuticals, perfumes, isocyanate resins, and for blending in synthetic rubbers. It liquefies at 7.6°C, and solidifies at –118°C. It is decomposed by water. When chloroform is exposed to light and air, it decomposes into phosgene. One part in 10,000 parts of air is a toxic poison, causing pulmonary edema. For chemical warfare it is compressed into a liquid in shells. Because of its toxicity, most phosgene is produced and employed immediately in captive applications. The biggest use of the material is for toluene diisocyanate (TDI), which is then reacted into polyurethane resins for foams, elastomers, and coatings. About 0.9 metric ton of phosgene is consumed to make a metric ton of polymethylene polyphenylisocyanate, also used for making polyurethane resins for rigid foams. Polycarbonate manufacturers require 0.42 metric ton phosgene per ton of product resin. Polycarbonate is used for making breakresistant housings, signs, glazings, and electrical tools. Phosgene also is a reactant for the isocyanates that are used in pesticides, and the di- and polyisocyanates are adhesives, coatings, and elastomers.

PHOSPHATE CONVERSION COATINGS Phosphate coatings have been used commercially for approximately 50 years. They are used on iron, steel, zinc, and aluminum surfaces to increase corrosion protection, provide a base for paint, reduce wear on bearing parts, and aid in the cold forming and extrusion of metals. Phosphate coatings are formed by chemically reacting a clean metal surface with an aqueous solution of a soluble metal phosphate of zinc, iron, or manganese, accelerating agents,


and free phosphoric acid. For example, when steel is treated, the surface is converted into a crystalline coating consisting of secondary and tertiary phosphates, adherent to and integral with the base metal. Properties of the coating such as size, weight, and uniformity of the crystals are influenced by many factors, for example, composition and concentration of the phosphating solution, temperature and processing time, type of metal, and the condition of its surface due to previous treatment. The performance of a phosphate coating depends largely on the unique properties of the coating, which is integrally bound to the base metal and acts as a nonmetallic, adsorptive layer to hold a subsequent finish of oil or wax, paint, or lubricant. Heavy phosphate coatings are normally used in conjunction with an oil or wax for corrosion resistance. The combination of the coating with the oil film gives a synergistic effect, which affords much greater protection than that obtained by the sum of the two taken separately. The stable, nonmetallic, nonreactive phosphate coating provides an excellent base for paint. It is chemically combined with the metal surface, which results in increased adsorption of paint and materially reduces electrochemical corrosion normally occurring between the paint film and the metal. The oil-absorptive phosphate coatings are useful in holding and maintaining a continuous oil film between metal-to-metal moving parts. They also permit rapid break-in of new bearing surfaces. Even after the coatings have been worn away, the controlled etched condition of the metal surface continues to hold the oil film between the moving parts. The ability of a wellanchored coating to hold a soap or oil-type lubricant is used in the cold forming and drawing of metals.

PROCESSES Most phosphate coatings are formed from heated solutions following a hot cleaning cycle. Effective coatings, both the zinc phosphate and the iron phosphate types, are now produced by the relatively cold system with up to 70% savings in heating costs.

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All steel must be thoroughly cleaned prior to phosphate coating to remove grease, oil, rust, and undesirable soils on the steel surface, which prevent or alter the formation of a satisfactory phosphate coating and interfere with paint adhesion. The cleaner used to remove oily soil prior to the formation of the zinc phosphate coating is a light-duty, mildly alkaline material especially formulated to function effectively at low temperature. The cleaner that is best suited for use in connection with the low-temperature iron phosphate coating process may be either a light-duty, low-foaming, mildly acidic mixture, or the above mild alkaline cleaner, depending upon the conditions of operation. Proper formulation of the phosphating solutions allows the coatings to form rapidly at low temperature on the cleaned steel surface. Simple, on-the-job, chemical controls enable the operator to adjust the addition of coating chemicals to the requirements of the steel being treated. Following the coating operation, a cold water rinse is used to remove excess coating chemicals. The flow of water through the rinse is regulated with the rate of production so that contamination of the main body of the rinse is minimized. An acidified rinse containing hexavalent chromium compounds follows the water rinse. This lapse has the specific effect of enhancing the corrosion resistance of the coating. An oven dry-off to remove surface moisture completes the process. Typical products being treated by the cold phosphate system are automotive body and sheet metal parts, refrigerator cabinets, office furniture, lighting fixtures, commercial air conditioners, home heating equipment, home laundries, kitchen cabinets, desk and filing cabinets, steel drums, and window sash. There are advantages of the cold phosphate system over conventional methods that require higher temperature operation: 1. Direct heat savings — up to 70% 2. Less heat-up time 3. Less maintenance due to decreased load on heating coils, steam traps, etc. 4. Reduced downtime; maintenance personnel can enter the units immediately after shutdown to make adjustments or repairs


5. Increased worker comfort near the installation 6. Reduced use of water through decreased evaporation 7. Elimination of exhaust fans

APPLICATIONS Phosphate conversion coatings have a wide range of application. Several examples follow. Base for Plastisol Coatings The domestic appliance industry has pointed toward the use of plastisol (polyvinyl chloride) coatings as a replacement for more costly porcelain enamel. During this period several major manufacturers of domestic dishwashers have standardized on plastisol films for coating tubs, lids, dish racks, etc. The metal preparation method meeting this requirement was a zinc phosphate coating system. The fabricated parts are cleaned in a conditioned cleaner, phosphated, and rinsed thoroughly. The rinse procedure includes an acidified chromic-phosphoric acid rinse followed by a closely controlled deionized water rinse and thorough drying to remove surface moisture. This treatment produces a continuous, uniform, fine-grained coating of approximately 150 to 250 mg of zinc phosphate coating per square foot of surface area treated. After the steel has been zinc phosphate coated, an especially formulated primer is applied to a very thin and closely controlled film build and followed by the subsequent application of 12 to 15 mils of plastisol with an intermediate and final baking operation. Bond for Vinyl Coatings Vinyl films of 5 to 15 mils thickness are applied to both steel and aluminum sheets by lamination of calendered and decorated films or by spray or roller coating. For the vinyl laminate as with the plastisol application, the base metal must be chemically treated to provide the necessary bond for laminate to metal surface. A controlled, accelerated iron phosphate treatment followed by an acidified chromate rinse has proved to be best for the preparation of steel for vinyl lamination to sheet or coil on a


continuous line. This method is now being used by several fabricators and rollers of steel. Aluminum can also be finished with vinyl laminates and plastisols. Chromate conversion coatings of the “gold” oxide type provide excellent adhesion of the vinyl film as well as excellent corrosion resistance on the unprotected surface. Metal preparation for both steel and aluminum is usually done by spray application in five-stage equipment in the following sequence: 1. Alkali clean — 30 s (smutty steel may require brush scrubbing) 2. Water rinse — 10 s 3. Phosphate coating — 10 s 4. Water rinse — 5 s 5. Acidified chromate rinse — 5 s The same sequence and time cycles are used for aluminum except that an appropriate chromate conversion coating solution is in stage 3. Consequently, installations have been engineered to prepare both steel and aluminum for subsequent application of vinyl laminates by providing interchangeable coating solution tanks at that stage, thereby providing efficient and versatile “in-line” operation for this new decorative treatment. Bolt Making Phosphate coatings are also finding wide acceptance in the fastener industry as applied to rod to facilitate the forming of bolts. The coating is produced from a dilute, especially accelerated, zinc acid phosphate solution that reacts chemically with the surface of the rod to form an insoluble nonmetallic phosphate coating integral with the surface of the rod. The coating forms a porous bond to carry the extruding and heading lubricant and prevent metal-to-metal contact in subsequent heading operations. The result is longer tool life, increased percentage of reduction, and improved surface appearance of the finished product. The methods of application vary from the conventional pickle-house immersion method to the in-line strand processes. In the immersion method, the coils of scaled rods are dipped in


the treating solutions in the following sequence of operations: 1. Acid pickle — 10 min to 2.5 h 2. Water rinse-cold, overflowing or spray 3. Hot water rinse 4. Phosphate coating — 5 to 15 min, 71 to 93°C 5. Cold water rinse 6. Neutralizing rinse — lime, etc. 7. Bake dry It has been found that a dip in a highstrength soap solution improves die life and reduces knockout pressures. The soap solution follows the neutralizing rinse, or in some instances, it is combined with the neutralizing rinse. The in-line strand process eliminates intermediate handling from scaled rod to drawn wire and the necessity of acid disposal. The coils are handled continuously. Line speeds up to ~300 million m/s are required with this process so that the phosphate solution must deposit the desired heavy coating in about 15 s. Even at this extremely short processing time, a line speed up to ~300 million m/s requires a special coating tank. Compact prerinse and neutralizing rinse stages are incorporated in this unit from which it is possible to obtain better than 750 mg/ft2 of coating in 15 s by use of specially formulated phosphate solutions adapted to strand processing. Wire produced by the strand method of processing produces wire superior in quality to that produced by conventional cleaning and drawing methods and does so at lower cost. As a consequence of the better quality, phosphate-base lubricated material will produce close tolerance bolts with full heads and sharp shoulders. Wire Drawing Phosphate coatings are also gaining acceptance as a lubricant carrier in the wire industry to permit increased drawing speeds and prolonged die life. This is particularly true in connection with both the dry and wet drawing of high carbon wire. Other advantages that the zinc phosphate coatings afford to the wire industry are

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increased corrosion resistance after drawing and closer dimensional tolerances. Improved dimensional tolerance is a major advantage in forming springs from spring wire. The zinc phosphate coating may be applied by immersion in the conventional pickle house installation, or by the newly developed fast coating, continuous strand method. The continuous strand method fits in well with fast, in-line travel of the wire. This adaptability and the lower labor costs involved warrant the recommendation of strand phosphate lines in wire processing. Extrusion

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Another important development is the use of zinc phosphate coatings as an aid in the rapidly developing field of cold extrusion of steel and aluminum. In this application the phosphate coating solution is formulated to deposit a considerably heavier zinc phosphate coating than heretofore mentioned. The coating is then chemically reacted with a soap-base lubricant to form a water-insoluble lubricating film. The use of the zinc phosphate coating is considered a basic requirement in the cold extrusion field where the pressure exerted by the tools on the steel being formed may be in excess of 2040 MPa. The phosphate and lubricant coating withstands the high unit pressure and temperatures developed in this type of cold forming. At the same time, it maintains the required separating film between the tools and workpiece being extruded to prevent scoring, galling, and tool breakage. A specially formulated phosphate coating solution produces a zinc phosphate coating on aluminum and is gaining wide acceptance as an aid in the cold extrusion of the heat-treatable alloys of this metal.

PHOSPHOR BRONZE This, a copper-base alloys with low phosphorus content, originally called steel bronze, was 928 bronze deoxidized with phosphorus and cast in an iron mold. It is now any bronze deoxidized by the addition of phosphorus to the molten metal. It may or may not contain residual phosphorus in the final state. Ordinary bronze


frequently contains cuprous oxide formed by the oxidation of the copper during fusion. By the addition of phosphorus, a powerful reducing agent, a complete reduction of the oxide takes place. Phosphor bronzes have excellent mechanical and cold-working properties and low coefficient of friction, making them suitable for springs, diaphragms, bearing plates, and fasteners. In some environments, such as salt water, they are superior to copper. At present, there are 18 standard wrought phosphor bronzes, designated C50100 to C54800. Tin, which ranges from as much as 0.8 to 1% depending on the alloy, is the principal alloying element, although leaded alloys may contain as much lead (4 to 6%, for example) as tin. Phosphorus content is typically on the order of 0.1 to 0.35%, zinc 0 to 0.3% (1.5 to 4.5% in C54400), iron 0 to 0.1%, and lead 0 to 0.05% (0.8 to 6% in leaded alloys). The principal alloys were formerly known by letter designations representing nominal tin content: phosphor bronze A, 5% tin (C51000); phosphor bronze B, 4.75% tin (C53200); phosphor bronze C, 8% tin (C52100); phosphor bronze D, 10% tin (C52400); and phosphor bronze E, 1.25% tin (C50500). Phosphor bronze E, almost 99% copper, is one of the leanest of these bronzes in the way of alloying ingredients and is used for electrical contacts, pole-line hardware, and flexible tubing. Its electrical conductivity is about half that of copper and it is readily formed, soldered, brazed, and flash-welded. Thin flat products have tensile yield strengths ranging from about 83 MPa in the annealed condition to 517 MPa in the extra spring temper. More highly alloyed C54400 (4% tin, 4% lead, and 3% zinc, nominally) is about one fifth as electrically conductive as copper, has good forming characteristics, and 80% the machinability of C36000, a free-machining brass. Its ultimate tensile strength ranges from about 331 MPa in the annealed condition to 690 MPa in the extra-spring temper. Uses include bearings, bushings, gear shafts, valve components, and screw-machine products.

PHOSPHOR COPPER An alloy of phosphorus and copper, phosphor copper was used instead of pure phosphorus for


deoxidizing brass and bronze, and for adding phosphorus in making phosphor bronze. It comes in 5, 10, and 15% grades and is added directly to the molten metal. It serves as a powerful deoxidizer, and the phosphorus also hardens the bronze. Even slight additions of phosphorus to copper or bronze increase fatigue strength. Phosphor copper is made by forcing cakes of phosphorus into molten copper and holding until the reaction ceases. Phosphorus is soluble in copper up to 8.27%, forming Cu3P, which has a melting point of about 707°C. A 10% phosphor copper melts at 850°C and a 15% at about 1022°C. Alloys richer than 15% are unstable. Phosphor copper is marketed in notched slabs or in shot. Phosphor tin is a master alloy of tin and phosphorus used for adding to molten bronze in the making of phosphor bronze. It usually contains up to 5% phosphorus and should not contain lead. It has an appearance like antimony, with large glittering crystals, and is marketed in slabs.

PHOSPHORIC ACID Also known as orthophosphoric acid, phosphoric acid is colorless, syrupy liquid of the composition H3PO4 used for pickling and rustproofing metals, for the manufacture of phosphates, pyrotechnics, and fertilizers, as a latex coagulant, as a textile mordant, as an acidulating agent in jellies and beverages, and as a clarifying agent in sugar syrup. The specific gravity is 1.65, melting point 73.6°C, and it is soluble in water. The usual grades are 90, 85, 75%, technical 50%, and dilute 10%. As a cleanser for metals, phosphoric acid produces a light etch on steel, aluminum, or zinc, which aids paint adhesion. Deoxidine is a phosphoric acid cleanser for metals. Nielite D is phosphoric acid with a rust inhibitor, used as a nonfuming pickling acid for steel. Albrite is available in 75, 80, and 85% concentrations in food and electronic grades, both high-purity specifications. DAB and Phosbrite are called Bright Dip grades, for cleaning applications. Phosphoric anhydride, or phosphorus pentoxide, P2 O5, is a white, water-soluble powder used as a dehydrating agent and also as an opalizer for glass. It is also used as a catalyst in asphalt coatings


to prevent softening at elevated temperatures and brittleness at low temperatures.

PHOSPHORUS A chemical element, symbol P, phosphorus forms the basis of a very large number of compounds; the most important class is phosphates. For every form of life, phosphates play an essential role in all energy-transfer processes such as metabolism, photosynthesis, nerve function, and muscle action. The nucleic acids which, among other things, make up the hereditary material (the chromosomes) are phosphates, as are a number of coenzymes. Animal skeletons consist of a calcium phosphate.

USES About 90% of the total phosphorus (in all of its chemical forms) used in the United States goes into fertilizers. Commercial phosphorus is obtained from phosphate rock by reduction in the electric furnace with carbon, or from bones by burning and treating with sulfuric acid. Phosphate rock occurs in the form of land pebbles and as hard rock. The superphosphate used for fertilizers is made by treating phosphate minerals with concentrated sulfuric acid. It is not a simple compound, but may be a mixture of calcium acid phosphate, CaHPO4, and calcium sulfate. Nitrophosphate for fertilizer is made by acidulating phosphate rock with a mixture of nitric and phosphoric acids, or with nitric acid and then ammoniation and addition of potassium or ammonium sulfate. Other important uses are as binders for detergents, nutrient supplements for animal feeds, water softeners, additives for foods and pharmaceuticals, coating agents for metal-surface treatment, additives in metallurgy, plasticizers, insecticides, and additives for petroleum products. Except for the last four items, these uses involve phosphates. Sodium tripolyphosphate is the major compound used in building synthetic detergents to achieve improved cleaning, primarily by dispersing inorganic soil and softening the water. The average phosphate-containing household

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detergent produced in the United States for washing clothes consists of about 40% by weight of sodium tripolyphosphate, Na5P3O10. This compound is used extensively in water softening, as are other members of the homologous series of chain phosphates. The largevolume usage of phosphates in detergent building has led to unwanted growth of algae in inland waters (lakes and rivers) into which the dirty dishwashers are discharged. As a result of this fertilizing action, phosphates are considered water pollutants in those areas where such discharges occur, and in some areas phosphates have been eliminated from detergents by law. For reasonably fast-flowing rivers that discharge directly into the ocean, phosphates are not a problem. An interesting water-softening application is found in “threshold treatment” in which tiny traces of a chain phosphate (much less than would be used in sequestering) are used to prevent the formation of pipe scale from hard waters. The application is related to the dispersing action of the phosphates, because traces of phosphate absorb on the growing surface of the pipe scale as it begins to form, and this inhibits its further growth. A major pharmaceutical use of phosphates is in toothpastes, in which dicalcium phosphate is the most popular polishing agent. Monocalcium phosphate and sodium acid pyrophosphate, Na2H2P2O (the pyrophosphate is the second member of the phosphate family), are employed as leavening agents in cake mixes, refrigerated biscuits, self-rising flour, and baking powder. Automobile bodies, for example, are generally phosphatized before they are painted to prevent rusting in use. Orthophosphate esters find wide use as plasticizers that have flameproofing properties and as gasoline and oil additives. The phosphorus compound of major biological importance is adenosinetriphosphate (ATP), which is an ester of the salt sodium tripolyphosphate, widely employed in detergents and water-softening compounds. Practically every reaction in metabolism and photosynthesis involves the hydrolysis of this tripolyphosphate to its pyrophosphate derivative, called adenosinediphosphate (ADP).


Phosphorus is an essential element in the human body; a normal person has more than a pound of it in the system, but it can be taken into the system only in certain compounds. Nerve gases used in chemical warfare contain phosphorus, which combines with and inactivates the choline sterase enzyme of the brain. This enzyme controls the supply of the hormone that transmits nerve impulses, and when it is inactivated the excess hormone causes paralysis of the nerves and cuts off breathing. Organic phosphates are widely used in the food, textile, and chemical industries. Other phosphates are used as a plasticizer in plastics and as an antifoaming agent in paper coatings and textile sizings. They are also employed for scale and corrosion control, ore flotation, pigment dispersion, and detergents. Flour and other foodstuffs are fortified with ferric phosphate, FePO4.2H2O. Iron phosphate is used as an extender in paints. Tricalcium phosphate, Ca3(PO4)2, is used as an anticaking agent in salt, sugar, and other food products and to provide a source of phosphorus. The tricalcium phosphate, used in toothpastes as a polishing agent and to reduce the staining of chlorophyll, is a fine white powder. Dicalcium phosphate, used in animal feeds, is precipitated from the bones used for making gelatin, but is also made by treating lime with phosphoric acid made from phosphate rock. Diammonium phosphate, (NH4)2HPO4, is a mildly alkaline, white crystalline powder used in ammoniated dentifrices, for pH control in bakery products, in making phosphors, to prevent afterglow in matches, and for flameproofing paper.

FORMS There are two common forms of phosphorus, yellow and red. The former, also called white phosphorus, P4, is a light-yellow waxlike solid, phosphorescent in the dark and exceedingly poisonous. Its specific gravity is 1.83 and it melts at 44°C. It is used for smoke screens in warfare and for rat poisons and matches. Yellow phosphorus is produced directly from phosphate rock in the electric furnace. It is cast in cakes of 0.45 to 1.36 kg each. Red phosphorus is a reddish-brown amorphous powder, with a specific gravity of 2.20 and a melting point of


725°C. Red phosphorus is made by holding white phosphorus at its boiling point for several hours in a reaction vessel. Both forms ignite easily. Amorphous phosphorus, or crystalline black phosphorus, is made by heating white phosphorus for extended periods. It resembles graphite, and is less reactive than the red or white forms, which can ignite spontaneously in air. Black phosphorus is made by this process. Phosphorus sulfide, P4S3, may be used instead of white phosphorus in making matches. Phosphorus pentasulfide, P2S5, is a canary-yellow powder of specific gravity 1.30, or solid of specific gravity 2.0, containing 27.8% phosphorus, used in making oil additives and insecticides. It is decomposed by water.

PHOTOGRAPHIC MATERIALS These are the light-sensitive recording materials of photography, that is, photographic films, plates, and papers. They consist primarily of a support of plastic sheeting, glass, or paper, respectively, and a thin, light-sensitive layer, commonly called the emulsion, in which the image will be formed and stored. The material will usually embody additional layers to enhance its photographic or physical properties.

SUPPORTS Film support, for many years made mostly of flammable cellulose nitrate, is now exclusively made of slow-burning “safety” materials, usually cellulose triacetate or polyester terephthalate, which are manufactured to provide thin, flexible, transparent, colorless, optically uniform, tear-resistant sheeting. Polyester supports, which offer added advantages of toughness and dimensional stability, are widely used for films intended for technical applications. Film supports usually range in thickness from 0.06 to 0.23 mm and are made in rolls up to 1.5 m wide and 1800 m long. Glass is the predominant substrate for photographic plates, although methacrylate sheet, fused quartz, and other rigid materials are sometimes used. Plate supports are selected for optical clarity and flatness. Thickness, ranging usually from 1 to 6 mm, is increased with plate size as needed to resist breakage and retain


flatness. The edges of some plates are specially ground to facilitate precise registration. Photographic paper is made from bleached wood pulp of high α-cellulose content, free from ground wood and chemical impurities. It is often coated with a suspension of barium sulfate in gelatin for improved reflectance and may be calendered for high smoothness. Fluorescent brighteners may be added to increase the appearance of whiteness. Many paper supports are coated on both sides with waterrepellent synthetic polymers to preclude wetting of the paper fibers during processing. This treatment hastens drying after processing and provides improved dimensional stability and flatness.

EMULSIONS Most emulsions are basically a suspension of silver halide crystals in gelatin. The crystals, ranging in size from 2.0 to less than 0.05 µm, are formed by precipitation by mixing a solution of silver nitrate with a solution containing one or more soluble halides in the presence of a protective colloid. The salts used in these emulsions are chlorides, bromides, and iodides. During manufacture, the emulsion is ripened to control crystal size and structure. Chemicals are added in small but significant amounts to control speed, image tone, contrast, spectral sensitivity, keeping qualities, fog, and hardness; to facilitate uniform coating; and, in the case of color films and papers, to participate in the eventual formation of dye instead of metallic silver images upon development. The gelatin, sometimes modified by the addition of synthetic polymers, is more than a simple vehicle for the silver halide crystals. It interacts with the silver halide crystals during manufacture, exposure, and processing and contributes to the stability of the latent image. After being coated on a support, the emulsion is chilled so that it will set, then dried to a specific moisture content. Many films receive more than one high-sensitive coating, with individual layers as thin as 1.0 µm. Overall thickness of the coatings may range from 5 to 25 µm, depending on the product. Most x-ray films are sensitized on both sides, and some black-andwhite films are double coated on one side.

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Color films and papers are coated with at least three emulsion layers and sometimes six or more plus filter and barrier layers. A thin, nonsensitized gelatin layer is commonly placed over film emulsions to protect against abrasion during handling. A thicker gelatin layer is coated on the back of most sheet films and some roll films to counteract the tendency to curl, which is caused by the effect of changes in relative humidity on the gelatin emulsion. Certain films are treated to reduce electrification by friction because static discharges can expose the emulsion. The emulsion coatings on photographic papers are generally thinner and more highly hardened than those on film products. Another class of silver-based emulsions relies on silver-behenate compounds. These materials require roughly ten times more exposure than silver halide emulsions having comparable image-structure properties (resolving power, granularity); are less versatile in terms of contrast, maximum density, and spectral sensitivity; and are less stable both before exposure and after development. However, they have the distinct advantage of being processed through the application of heat (typically at 116 to 127°C) rather than a sequence of wet chemicals. Hence, products of this type are called Dry Silver films and papers.

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PIEZOELECTRICITY Piezoelectricity is electricity, or electric polarity, resulting from the application of mechanical pressure on a dielectric crystal. The application of a mechanical stress produces in certain dielectric (electrically nonconducting) crystals an electric polarization (electric dipole moment per cubic meter) that is proportional to this stress. If the crystal is isolated, this polarization manifests itself as a voltage across the crystal, and if the crystal is short-circuited, a flow of charge can be observed during loading. Conversely, application of a voltage between certain faces of the crystal produces a mechanical distortion of the material. This reciprocal relationship is referred to as the piezoelectric effect. The phenomenon of generation of a voltage under mechanical stress is referred to as the direct piezoelectric effect, and the mechanical © 2002 by CRC Press LLC

strain produced in the crystal under electric stress is called the converse piezoelectric effect. Piezoelectric materials are used extensively in transducers for converting a mechanical strain into an electrical signal. Such devices include microphones, phonograph pickups, vibration-sensing elements, and the like. The converse effect, in which a mechanical output is derived from an electrical signal input, is also widely used in such devices as sonic and ultrasonic transducers, headphones, loudspeakers, and cutting heads for disk recording. Both the direct and converse effects are employed in devices in which the mechanical resonance frequency of the crystal is of importance. Such devices include electric wave filters and frequency-control elements in electronic oscillator circuits.

PIG IRON Pig iron is the iron produced from the first smelting of the ore. The melt of the blast furnace is run off into rectangular molds, forming, when cold, ingots called pigs. Pig iron contains small percentages of silicon, sulfur, manganese, and phosphorus, besides carbon. It is useful only for resmelting to make cast iron or wrought iron. Pig iron is either sand-cast or machine-cast. When it is sand-cast, it has sand adhering and fused into the surface, giving more slag in the melting. Machine-cast pig iron is cast in steel forms and has a fine-grained chilled structure, with lower melting point. Pig irons are classified as Bessemer or nonBessemer, according to whether the phosphorus content is below or above 0.10%. There are six general grades of pig iron: low-phosphorus pig iron, with less than 0.03%, used for making steel for steel castings and for crucible steelmaking; Bessemer pig iron, with less than, 0.10% phosphorus, used for Bessemer steel and for acid open-hearth steel; malleable pig iron, with less than 0.20%, used for making malleable iron; foundry pig iron, with from 0.5 to 1%, for cast iron; basic pig iron, with less than 1%, and low-silicon, less than 1%, for basic openhearth steel; and basic Bessemer, with from 2 to 3%, used for making steel by the basic Bessemer process employed in England.


Because silicon is likely to dissolve the basic furnace lining, it is kept as low as possible, 0.70 to 0.90%, with sulfur not usually over 0.095%. Pig irons are also specified on the basis of other elements, especially sulfur. The sulfur may be from 0.04 to 0.10%, but high-sulfur pig iron cannot be used for the best castings. The manganese content is usually from 0.60 to 1%. Most of the iron for steelmaking is now not cast but is carried directly to the steel mill in car ladles. It is called direct metal.

PIGMENT (MATERIAL) A finely divided material, pigment contributes to optical and other properties of paint, finishes, and coatings. Pigments are insoluble in the coating material, whereas dyes dissolve in and color the coating. Pigments are mechanically mixed with the coating and are deposited when the coating dries. Their physical properties generally are not changed by incorporation in and deposition from the vehicle. Pigments may be classified according to composition (inorganic or organic) or by source (natural or synthetic). However, the most useful classification is by color (white, transparent, or colored) and by function.

WHITE PIGMENTS These pigments are essentially transparent to visible light. Because of the difference in refractive index between the pigment particles and the vehicles, white pigments refract the light from a multitude of surfaces and return a substantial portion in the direction of illumination without significant change in the spectral composition of the light. The common white pigments are titanium dioxide, derived from titanium ores; white lead, from corrosion of metallic lead; zinc oxide, from burning of zinc metal; and lithopone, a mixture of zinc sulfide and barium sulfate. Pure zinc sulfide and antimony oxide are less commonly used. Titanium dioxide may be crystallized in the rutile or anatase form, depending on the method of production. It may be further modified by surface treatment to control the rate of chalking and other properties. Rutile titanium dioxide has a higher refractive index than anatase and therefore higher hiding power, but it has a


somewhat yellow color. Anatase titanium dioxide provides a purer white. White lead pigments are the oldest of white pigments and were used extensively to provide excellent hiding power, flexibility, and durability to interior and exterior paints and enamels. Consumer protection rulings have all but removed white lead paints from the market, because leaded paint particles were ingested by children, with toxic effects. Zinc oxide and lithopone pigments were extensively used in paint formulation, but have been superseded by titanium dioxide. Pure zinc oxide pigment is rarely used. Antimony oxide pigment is used chiefly in certain fire-retardant paints.

TRANSPARENT PIGMENTS The refractive indexes of these pigments are very close to the index of the paint vehicle (about 1.54). They are used to provide bulk, control setting, and contribute to the hardness, durability, and abrasion resistance of the paint film. Because they are commonly used to add bulk to other pigments, they are called extenders. Most transparent pigments are natural minerals reduced to pigment particle size. Among the most commonly used transparent pigments are calcium carbonate (ground limestone, whiting, or chalk), magnesium silicate, bentonite clay, silica, or barites (barium sulfate). Transparent pigments often constitute a substantial portion of a protective coating.

COLORED PIGMENTS These pigments are available in a wide variety of colors and properties, depending upon the end use. Several hundred have been used; the following are the most common. Red. Iron oxides, often classified by color, include Indian red, Spanish red, Persian Gulf red, and Venetian red, a mixture of iron oxide and calcium red sulfate. Other red pigments include cadmium red (cadmium selenide) and organic reds, which are usually coal tar derivatives either precipitated in pigment form (toners)

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or deposited on a transparent pigment (lakes). Organic reds include toluidines and lithols. Orange. Chrome orange (basic lead chromate), molybdatee orange (lead chromate-molydate), and various organic toners and lakes are the most common orange pigments. Brown. Browns are nearly always iron oxides, although certain lakes and toners are used for special purposes. Yellow. These pigments include natural iron oxides such as ocher or sienna, or synthetic iron oxides, which are stronger and brighter, such as chrome yellow (normal lead chromate) and cadmium yellow (cadmium sulfide), and organic toners and lakes such as Hansa yellow and benzidene yellow. Green. The most important green pigments are chrome green, a mixture of chrome yellow and Prussian blue; chromium oxide, duller but more permanent; phthalocyanine green, an organic pigment containing copper; and various other organic toners or lakes, often precipitated with phosphotungstic or phosphomolybdic acid. Blue. The blue pigments include Prussian blue (ferric ferrocyanide, sometimes called milori or Chinese blue, depending on the shade); ultramarine, an inorganic pigment made by fusing soda sulfur and other materials under controlled conditions; phthalocyanine blue, an organic pigment containing copper; and numerous organic toners and lakes. Purple and Violet. These are nearly all organic toners or lakes. Manganese phosphate is a very weak, inorganic purple pigment. Black. The vast majority of black pigments consist of finely divided carbon–carbon black, lampblack, and bone black — usually obtained by allowing a smoky flame to impinge on a cold surface. Black iron oxide and certain organic pigments are used where special properties are required.


SPECIAL PIGMENTS Anticorrosive pigments are used to prevent the formation or spread of rust on iron when the metal is exposed by a break in the coating. The most common are red lead, an oxide of lead, and zinc yellow or zinc chromate, a basic chromate of zinc. Other colored chromates are sometimes used. The color of red leads fades rapidly, and the anticorrosive chromates are usually very weak in tinting strength. Metallic lead is sometimes used for anticorrosive paint. Metallic pigments are small, usually flat particles of metal, prepared for dispersal in coatings. Aluminum is most commonly used because it leafs and forms a smooth, metallic film. The flakes are sometimes colored. Bronze, copper, lead, nickel, stainless steel, and silver appear occasionally. Zinc dust, or powdered zinc, is used more often because of its excellent adhesion to galvanized iron than because of its appearance. Luminous pigments radiate visible light when exposed to ultraviolet light. Phosphorescent pigments continue to glow for a period after the exciting light has been removed; these are usually sulfides of zinc and other materials, with small amounts of additives that control the phosphorescent properties. Fluorescent pigments lose luminosity as soon as the exciting light is removed; these pigments may be sulfides, although many organic pigments have this property. Other specialized pigments include pigments that change color at some predetermined temperature, used to indicate hot areas on motors; pigments that give a pearly appearance; and pigments that conduct electricity for printed circuits. Coarse materials such as pumice are often added when a nonslippery coating is required. Glass beads give a very high degree of refractivity in the direction of illumination and are often used in center-line paints or for signs where night visibility is required. Intumescent pigments puff up under heat, giving a fire-resistant coating.


PLASMA-ARC COATINGS In this process, a flow of gas, such as argon, is directed through the nozzle of a device called the plasma arc torch. When a high-current electric arc is struck within the torch between a negative tungsten electrode and the positive water-cooled copper nozzle, electrical and aerodynamical effects force the arc through the nozzle, which concentrates and stabilizes it. A substantial portion of the gas flows through the arc and is heated to temperatures as high as 16,649°C and accelerated to supersonic speeds to form an ionized gas jet called plasma. A cool layer of gas next to the nozzle wall effectively insulates the torch from the tremendous heating effect of the arc column. Particles of refractory coating material, introduced into the plasma in either powder or wire form, are melted and accelerated to high velocity. When these molten particles strike the workpiece, they impact to form a dense, highpurity coating. Sprays of cold carbon dioxide gas, played on the workpiece, keep it from overheating during the process and protect the purity of the coating from air oxidation.

CHARACTERISTICS The primary advantage of the process is its ability to combine the bulk properties of a base material with the surface properties of a refractory material. Furthermore, the application of the thin, tenacious coatings can be limited to the specific areas of the base material where a coating is needed, and warpage or distortion of precision parts is eliminated because of the low base material temperature maintained during coating. Whether as-coated or finished, the refractory coatings have extremely good resistance to wear, abrasion, and corrosion and erosion, even under the adverse conditions of high temperature, high load, and lack of lubrication and cooling. When ground and lapped, the coatings give superior performance under conditions of fretting corrosion. Finished coating, when mated with proper materials, have generally lower coefficients of friction than most metal-to-metal combinations. This ratio is also true at elevated temperatures. The coatings have a porosity of


less than 1% and an as-coated surface finish of approximately 150 µin. rms (root mean square), which can be finished down to better than 1 µin. rms.

FABRICATION Coatings can be applied in practically any desired thickness. But only areas that allow the particles free access will be coated evenly. This limitation excludes narrow holes, blind cavities, and deep V-shaped grooves. All corners and edges should be rounded by a minimum 0.38 mm rad or have a minimum chamfer of 0.38 mm by 45° to prevent weak spots. Several types of parts that can be coated: 1. 2. 3. 4. 5.

Long external cylindrical parts Short external cylindrical parts Internal diameters Rectangular flat surfaces Circular flat surfaces

Since plasma-arc coatings can be deposited in practically any desired thickness, it is also possible to fabricate parts by this method. The required thickness is built up on a mandrel formed to the desired internal shape of the finished part, and the mandrel is then removed chemically from the part with acid or caustic. This method allows intricate shapes to be made of materials that are normally difficult to fabricate. But, as with flame spraying, only areas that allow the particles of coating material sufficient access will be plated evenly.

MATERIALS Almost any base material can be coated, even certain reinforced plastics, and any known inorganic solid which will melt without decomposition can be used as a coating material. Many basic coatings have already been established including tantalum, palladium, platinum, molybdenum, tungsten, alumina, zirconium diboride, and oxide, and three combinations of tungsten with additives to improve its properties. These additives are zirconia, chromium and alumina. Other coatings include the refractory metals such as columbium; some of the refractory

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metal compounds such as the borides of tungsten, columbium, tantalum, titanium, and chromium; the refractory carbides of columbium, hafnium, tantalum, zirconium, titanium, tungsten, and vanadium; the refractory oxides of thorium, hafnium, magnesium, cerium, and aluminum; and other pure metals such as aluminum, copper, nickel, chromium, and boron.

PROPERTIES The properties of the coatings or parts made from all of these materials are equivalent to those of the pure materials themselves.

PLASTER MOLD CASTINGS Plaster mold casting is primarily used for producing parts in quantities that are too small to justify the use of permanent molds, yet large enough to outweigh the machining costs of sand castings. The process is noted for its ability to produce parts with high dimensional accuracy, smooth and intricate surfaces, and low porosity. On the other hand, it is limited to nonferrous metals (aluminum and copper alloys) and relatively small parts. Also production times are relatively high because the molds take relatively long to make and are not reusable.

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THE PROCESS The plaster used for molding generally consists of water mixtures of gypsum or plaster of paris (calcium sulfate) and strengthening binders such as asbestos, magnesium silicate, silicate flour, and others. Impurities such as salts and section thickness are about 40 to 60 mils and bosses and undercuts can be incorporated into the design.

APPLICATIONS Plaster mold castings are usually used for medium-production applications, and their cost falls between sand castings and permanent mold castings. Typical parts where the process has been used include gears, ratchet teeth, cams, handles, small housings, pistons, wing nuts, locks, valves, hand tools, and radar parts for aircraft, railroad, household, and electrical uses. © 2002 by CRC Press LLC

PLASTER OF PARIS The material (calcined gypsum), CaSO40.5H2O, is a white, gray, or pinkish-colored powder prepared by heating gypsum (CaSO42H2O) to remove 75% of its water of crystallization. When mixed with water and allowed to rehydrate to the dihydrate (CaSO42H2O), there is no apparent action at first, but soon a slight stiffening takes place and shortly after that it “sets” to a solid mass. As set progresses, the mass begins to heat and expand, and final set is not reached until the evolution of heat has ceased and expansion is complete. Through changes in the manufacturing process, the time of set can be varied widely (from a few minutes to many hours), and linear setting expansion also is controllable from 0.05 to 2.0%. The normal linear setting expansion of pottery plasters is –0.20% in all directions if the cast is unconfined, but under conditions of confinement, all the setting expansion may take place in one direction only.

APPLICATIONS Plasters are used in a variety of ceramic industry applications: 1. In a limited way, as chemical additives to glazes, supplying neutral, slightly soluble calcium and sulfate sulfur. 2. As a glass batching material to replace part or all of the salt cake when combined with soda ash in proper proportions. Here, use of plaster eliminates saltwater scumming, retaining the desirable fluxing property of salt cake. 3. As a bedding and leveling agent in grinding and polishing plate glass, plaster cements the glass to the grinding bed during the operation while also being easy to remove from the glass surface. 4. Optical glass mounting. Used to retain optical glass, lenses, prisms, and oculars in position while surfaces are formed to the desired curves by grinding and polishing.


5. Model making. Used in the ceramic industry generally for preparing original models. 6. Metal mold making. When suitably compounded with refractory substances, molds for the casting of nonferrous alloys such as white metal, brass, aluminum alloys, etc. are made with plaster. 7. Low-density insulation. Used to provide green strength to mixtures of clays, nonplastic refractories, and organics. 8. Potter mold and die making. This use constitutes the principal ceramic application of plasters.

PLASTIC ALLOYS AND BLENDS Plastics, like metals, can be alloyed. And like metal alloys, the resulting materials have different, and often better, properties than those of the base materials making up the alloys. These alloys consist of two thermoplastics compounded into a single resin. The two polymers must be melt-compatible. Some polymers are naturally compatible; others require the use of compatibilizing agents. The purpose of alloying polymers is to achieve a combination of properties not available in any single resin. There are a great many alloys available, and the list continues to grow. At present, some of the more widely used alloys are ABS/polycarbonate, ABS/polyurethane, polyvinyl chloride (PVC)/acrylic, PVC/CPE (chlorinated polyethylene), polyphenylene oxide (PPO)/polystyrene, nylon/ABS, PPO/PBT thermoplastic polyester, polycarbonate/PBT thermoplastic polyester, polycarbonate/ASA, and polysulfone/ABS. The plastics most widely used in alloys today are polyvinyl chloride (PVC), ABS, and polycarbonate. These three plastics can be combined with each other or with other types of polymers.

ABS ABS, in addition to its use with polycarbonate, can also be alloyed with polyurethane. ABS–polycarbonate alloys extend the


exceptionally high impact strength of carbonate plastics to section thicknesses over 0.16 cm. ABS–polyurethane alloys combine the excellent abrasion resistance and toughness of the urethanes with the lower cost and rigidity of ABS. The materials can be injection molded into large parts but cannot be extruded. Typical applications for which they are suitable include such parts as wheel treads, pulleys, low load gears, gaskets, automotive grilles, and bumper assemblies. ABS is also being successfully combined with PVC and is available commercially in several grades. One of the established grades provides self-extinguishing properties, thus eliminating the need for intumescent (nonburning) coatings in present ABS applications, such as power tool housings, where self-extinguishing materials are required. A second grade possesses an impact strength about 30% higher than general-purpose ABS. This improvement, plus its ability to be readily molded, has resulted in its use for automobile grilles.

PVC ABS–PVC alloys are available commercially in several grades. Two of the established grades are described above. ABS–PVC alloys also can be produced in sheet form. The sheet materials have improved hot strength, which allows deeper draws than are possible with standard rubber-modified PVC base sheet. They also are nonfogging when exposed to the heat of sunlight. Some properties of ABS–PVC alloys are lower than those of the base resins. Rigidity, in general, is somewhat lower, and tensile strength is more or less dependent on the type and amount of ABS in the alloy. Another sheet material, an alloy of about 80% PVC and the rest acrylic plastic, combines the nonburning properties, chemical resistance, and toughness of vinyl plastics with the rigidity and deep drawing merits of the acrylics. The PVC–acrylic alloy approaches some metals in its ability to withstand repeated blows. Because of its unusually high rigidity, sheets ranging in thickness from 1.5 to 0.5 cm can be formed into thin-walled, deeply drawn parts.

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PVC is also alloyed with CPE to gain materials with improved outdoor weathering or to obtain better low-temperature flexibility. The PVC–CPE alloy applications include wire and cable jacketing, extruded and molded shapes, and film sheeting. Acrylic-base alloys with a polybutadiene additive have also been developed, chiefly for blow-molded products. The acrylic content can range from 50 to 95%, depending on the application. Besides blowmolded bottles, the alloys are suitable for thermoformed products such as tubs, trays, and blister pods. The material is rigid and tough and has good heat-distortion resistance up to 82°C.

FORMS

Another group of plastics, PPO, can be blended with polystyrene to produce a PPO–polystyrene alloy with improved processing traits and lower cost than nonalloyed PPO. The addition of polystyrene reduces tensile strength and heat deflection temperature somewhat and increases thermal expansion.

Industrial thermosetting laminates are available in the form of sheet, rod, and rolled or molded tubing. Impregnating resins commonly used are phenolic, polyester, melamine, epoxy, and silicone. The base material, or reinforcement, is usually one of the following: paper, woven cotton or linen, asbestos, glass cloth, or glass mat. Laminating resins may be marketed under one trade name by the resin producer and other names by the molders of the laminate. Paraplex P resins, for example, comprise a series of polyester solutions in monomeric styrene that can be blended with other resins to give varied qualities. But Panelyte refers to the laminates that are made with phenolic, melamine, silicone, or other resin, for a variety of applications.

PLASTIC LAMINATES

PLASTIC POWDER COATINGS

These are resin-impregnated paper or fabric, produced under heat and high pressure; they are also referred to as high-pressure plastic laminates. Two major categories are decorative thermosetting laminates and industrial thermosetting laminates. Most of the decorative thermosetting laminates are a paper base, and are known generically as papreg. Decorative laminates are usually composed of a combination of phenolic- and melamine-impregnated sheets of paper. The final properties of the laminate are related directly to the properties of the paper from which the laminate is made. Early laminates were designated by trade names, such as Bakelite, Textolite, Micarta, Condensite, Dilecto, Phenolite, Haveg, Spauldite, Synthane, and Formica. These are designated as various types of laminates with a decorative facing layer for such uses as tabletops. Trade names now usually include a number or symbol to describe the type and grade. Textolite, for example, embraces more than 70 categories of laminates subdivided into use-specification grades, all produced in many sizes and

Although many different plastic powders can be applied as coatings, vinyl, epoxy, and nylon are most often used. Vinyl and epoxy provide good corrosion and weather resistance as well as good electrical insulation. Nylon is used chiefly for its outstanding wear and abrasion resistance. Other plastics frequently used in powder coating include chlorinated polyethers, polycarbonates, acetals, cellulosics, acrylics, and fluorocarbons. Several different methods have been developed to apply these coatings. In the most popular process, fluidized bed, parts are preheated and then immersed in a tank of finely divided plastic powders, which are held in a suspended state by a rising current of air. When the powder particles contact the heated part, they fuse and adhere to the surface, forming a continuous, uniform coating. Another process, electrostatic spraying, works on the principle that oppositely charged materials attract each other. Powder is fed through a gun, which applies an electrostatic charge opposite to that applied to the part to be coated. When the charged particles leave

PPO

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thicknesses. Textolite 11711 is an electronic laminate for such uses as multilayer circuit boards. It is made with polyphenolene oxide resin, and may have a copper or aluminum cladding.



the gun, they are attracted to the part where they cling until fused together as a plastic coating. Other powder application methods include flock and flow coating, flame and plasma spraying, and a cloud-chamber technique.

the two broad categories of engineering and commodity plastics. Some major characteristics of plastics that distinguish them from other materials, particularly metals:

PLASTICS

1. They are essentially noncrystalline in structure. 2. They are nonconductors of electricity and are relatively low in heat conductance. 3. They are, with some important exceptions, resistant to chemical and corrosive environments. 4. They have relatively low softening temperatures. 5. They are readily formed into complex shapes. 6. They exhibit viscoelastic behavior — that is, after an applied load is removed, plastics tend to continue to exhibit strain or deformation with time.

Plastics are a major group of materials that are primarily noncrystalline hydrocarbon substances composed of large molecular chains whose major element is carbon. The three terms — plastics, polymers, and resins — are sometimes used interchangeably to identify these materials. However, the term plastics has now come to be the commonly used designation. The first commercial plastic, Celluloid, was developed in 1868 to replace ivory for billiard balls. Phenolic plastics, developed by Baekeland and named Bakelite after him, were introduced around the turn of the century. A plastic material, as defined by the Society of the Plastics Industry, is “any one of a large group of materials consisting wholly or in part of combinations of carbon with oxygen, hydrogen, nitrogen, and other organic and inorganic elements which, while solid in the finished state, at some stage in its manufacture is made liquid, and thus capable of being formed into various shapes, most usually through the application, either singly or together, of heat and pressure.” There are two basic types of plastics based on intermolecular bonding. Thermoplastics, because of little or no cross-bonding between molecules, soften when heated and harden when cooled, no matter how often the process is repeated. Thermosets, on the other hand, have strong, intermolecular bonding. Therefore, once the plastic is set into permanent shape under heat and pressure, reheating will not soften it. Within these major classes, plastics are commonly classified on the basis of base monomers. There are over two dozen such monomer families or groups. Plastics are also sometimes classified roughly into three stiffness categories: rigid, flexible, and elastic. Another method of classification is by the “level” of performance or the general area of application, using such categories as engineering, general-purpose, and specialty plastics, or


Polymers can be built of one, two, or even three different monomers, and are termed homopolymers, copolymers, and terpolymers, respectively. Their geometrical form can be linear or branched. Linear or unbranched polymers are composed of monomers linked endto-end to form a molecular chain that is like a simple string of beads or a piece of spaghetti. Branched polymers have side chains of molecules attached to the main linear polymer. These branches can be composed either of the basic linear monomer or of a different one. If the side molecules are arranged randomly, the polymer is atactic; if they branch out on one side of the linear chain in the same plane, the polymer is isotactic; and if they alternate from one side to the other, the polymer is syndiotactic. Plastics are produced in a variety of different forms. Most common are plastic moldings, which range in size from 2 cm to several meters. Thermoplastics, such as polyvinyl chloride (PVC) and polyethylene, are widely used in the form of plastic film and plastic sheeting. The term film is used for thicknesses up to and including 0.25 cm, while sheeting refers to thicknesses over that.

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Both thermosetting and thermoplastic materials are used as plastic coatings on metal, wood, paper, fabric, leather, glass, concrete, ceramics, or other plastics. There are many coating processes, including knife or spread coating, spraying, roller coating, dipping, brushing, calendering, and the fluidized-bed process. Thermosetting plastics are used in high-pressure laminates to hold together the reinforcing materials that comprise the body of the finished product. The reinforcing materials may be cloth, paper, wood, or glass fibers. The end product may be plain flat sheets, or decorative sheets as in countertops, rods, tubes, or formed shapes.

PLASTICS ADDITIVES Almost all plastics contain one or more additive materials to improve their physical properties, processing characteristics, or to reduce costs. There is a wide range of additives for use with plastics, including antimicrobials, antistatic agents, clarifiers, colorants, fillers, flame retardants, foaming agents, heat stabilizers, impact modifiers, light stabilizers, lubricants, moldrelease agents, odorants, plasticizers, reinforcements, and smoke retardants.

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improve properties such as flexibility. Plasticizers are usually liquids that have high boiling points, such as certain phthalates. Substances that are themselves polymers of low molecular weight, such as polyesters, are also used as plasticizers.

STABILIZERS Stabilizers are added to plastics to help prevent breakdown or deterioration during molding or when the polymer is exposed to sunlight, heat, oxygen, ozone, or combinations of these. Thus there is a wide range of compounds, each designated for a specific function. Stabilizers can be metal compounds, based on tin, lead, cadmium, barium, and others. And phenols and amines are added antioxidants that protect the plastic by diverting the oxidation reactions to themselves.

CATALYSTS Catalysts, by controlling the rate and extent of the polymerization process in the resin, allow the curing cycle to be tailored to the processing requirements of the application. Catalysts also affect the shelf life of the plastics. Both metallic and organic chemical compounds are used as catalysts.

FILLERS Fillers are probably the most common of the additives. They are usually used to either provide bulk or modify certain properties. Generally, they are inert and thus do not react chemically with the resin during processing. The fillers are often cheap and serve to reduce costs by increasing bulk. For example, wood flour, a common low-cost filler, sometimes makes up 50% of a plastic compound. Other typical fillers are chopped fabrics, asbestos, talc, gypsum, and milled glass. Besides lowering costs, fillers can improve properties. For example, asbestos increases heat resistance, and cotton fibers improve toughness.

PLASTICIZERS Plasticizers are added to plastics compounds either to improve flow during processing by reducing the glass transition temperature or to


COLORANTS Colorants, added to plastics for decorative purposes, come in a wide variety of pigments and dyestuffs. The traditional colorants are metalbase pigments such as cadmium, lead, and selenium. More recently, liquid colorants, composed of dispersions of pigments in a liquid, have been developed.

FLAME RETARDANTS Flame retardants are added to plastic products that must meet fire-retardant requirements, because polymer resins are generally flammable, except for such notable exceptions as PVC. In general, the function of fire retardants is limited to the spread of fire. They do not normally increase heat resistance or prevent the plastic from charring or melting. Some fireretardant additives include compounds


containing chlorine or bromine, phosphateester compounds, antimony thrioxide, alumina trihydrate, and zinc borate.

REINFORCED MATERIALS Reinforcement materials in plastics are not normally considered additives. Usually in fiber or mat form, they are used primarily to improve mechanical properties, particularly strength. Although asbestos and some other materials are used, glass fibers are the predominant reinforcement for plastics.

PLASTICS PROCESSING Plastics processing includes those methods and techniques used to convert plastics materials in the form of pellets, granules, powders, sheets, fluids, or preforms into formed shapes or parts. The plastic materials may contain a variety of additives that influence the properties as well as the processibility of the plastics. After forming, the part may be subjected to a variety of ancillary operations such as welding, adhesive bonding, and surface decorating (painting, metallizing). As with other materials of construction, processing of plastics is but one step in the normal design-to-finished-part sequence. The choice of process is influenced by economic considerations, number and size of finished parts, and complexity of postfinishing operations, as well as the adaptability of the plastics to the process.

INJECTION MOLDING This process consists of heating and homogenizing plastics granules in a cylinder until they are sufficiently fluid to allow for pressure injection into a relatively cold mold where they solidify and take the shape of the mold cavity. For thermoplastics, no chemical changes occur within the plastic, and consequently the process is repeatable. Injection molding of thermosetting resins differs primarily in that the cylinder heating is designed to homogenize and preheat the reactive materials, and the mold is heated to complete the chemical cross-linking reaction


to form an intractable solid. Solid particles, in the form of pellets or granules, constitute the main feed for injection moldable plastics. The major advantages of the injection molding process are the speed of production, minimal requirements for postmolding operations, and simultaneous multipart molding. The development of reaction injection molding (RIM) allowed the rapid molding of liquid materials. In these processes, cold or warm, two highly reactive, low-molecular weight, low-viscosity resin systems are first injected into a mixing head and from there into a heated mold, where the reaction to a solid is completed. Polymerization and cross-linking occur in the mold. This process has proved particularly effective for high-speed molding of such materials as polyurethanes, epoxies, polyesters, and nylons.

EXTRUSION In this process, plastic pellets or granules are fluidized, homogenized, and continuously formed. Products made this way include tubing, pipe, sheet, wire and substrate coatings, and profile shapes. The process is used to form very long shapes or a large number of small shapes that can be cut from the long shapes. The homogenizing capability of extruders is used for plastics blending and compounding. Pellets used for other processing methods, such as injection molding, are made by chopping long filaments of extruded plastic.

BLOW MOLDING This process consists of forming a tube (called a parison) and introducing air or other gas to cause the tube to expand into a free-blown hollow object or against a mold for forming into a hollow object with a definite size and shape. The parison is traditionally made by extrusion, although injection molded tubes have gained prominence because they do not require postfinishing, have better dimensional tolerances and wall thicknesses, and can be made unsymmetrical and in higher volume production.

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THERMOFORMING Thermoforming is the forming of plastic sheets into parts through the application of heat and pressure. The pressure can be obtained through use of pneumatics (air) or compression (tooling) or vacuum. Tooling for this process is the most inexpensive compared to other plastic processes, accounting for the popularity of the method. It can also accommodate very large parts as well as small parts, which are useful in low-cost prototype fabrication.

ROTATIONAL MOLDING In this process, finely ground powders are heated in a rotating mold until melting or fusion occurs. If liquid materials, such as vinyl plastisols, are used, the process is often called slush molding. The melted or fused resin uniformly coats the inner surface of the mold. When cooled, a hollow finished part is removed. The processes require relatively inexpensive tooling, are scrap-free, and are adaptable to large, double-walled, hollow parts that are strain-free and of uniform thickness. The processes can be performed by relatively unskilled labor. On the other hand, the finely ground plastics powders are more expensive than pellets or sheet, thin-

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FIGURE P.2A Three types of compression molds. (a) Flash-type, (b) positive, (c) semipositive. (From McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 14, McGraw-Hill, New York, 43. With permission.)

(a)

(b)

walled parts cannot be easily made, and the process is not suited for large production runs of small parts.

COMPRESSION

AND

TRANSFER MOLDING

Compression molding is one of the oldest molding techniques and consists of charging a plastics powder or preformed plug into a mold cavity, closing a mating mold half, and applying pressure to compress, heat, and cause flow of the plastic to conform to the cavity shape. The process is primarily used for thermosets, and consequently the mold is heated to accelerate the chemical cross-linking. Transfer molding is an adaptation of compression molding in that the molding powder or preform is charged to a separate preheating chamber and, when appropriately fluidized, injected into a closed mold. The process predates, yet closely parallels, the early techniques of ram injection molding of thermoplastics. It is most used for thermosets, and is somewhat faster than compression molding. In addition, parts are more uniform and more dimensionally accurate than those made by compression molding. See Figures P.2A and P.2B showing compression molding and transfer molding.

(a)

(b)

(c)

(c)

FIGURE P.2B Transfer molding. (a) In the molding cycle, material is first placed in the transfer pot. (b) It is then forced through an orifice into the closed mold. (c) When the mold opens, the cull and sprue are removed as a unit, and the part is lifted out of the cavity by ejector pins. (From McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 14, McGraw-Hill, New York, 43. With permission.) © 2002 by CRC Press LLC

TX66613_frame_P(15) Page 533 Friday, March 22, 2002 8:36 AM

FOAM PROCESSES Foamed plastics materials have achieved a high degree of importance in the plastics industry. Foams can be made in a range from soft and flexible to hard and rigid. There are three types of cellular plastics: blown (expanded matrix, such as a natural sponge), syntactic (the encapsulation of hollow organic or inorganic microspheres in the matrix), and structural (dense outer skin surrounding a foamed core). There are seven basic processes used to generate plastics foams. They include the incorporation of a chemical blowing agent that generates gas (through thermal decomposition) in the polymer liquid or melt; gas injection into the melt, which expands during pressure relief; generation of gas as a by-product of a chemical condensation reaction during cross-linking; volatization of a low-boiling liquid (for example, Freon) through the exothermic heat of reaction; mechanical dispersion of air by mechanical means (whipped cream); incorporation of nonchemical gas-liberating agents (adsorbed gas on finely divided carbon) into the resin mix, which is released by heating; and expansion of small beads of thermoplastic resin containing a blowing agent through the external application of heat. Structural foam differs from other foams in that the part is produced with a hard integral skin on the outer surfaces and a cellular core in the interior. They are made by injection-molding liquefied resins containing chemical blowing agents. The initial high injection pressure causes the skin to solidify against the mold surface without undergoing expansion. The subsequent reduction in pressure allows the remaining material to expand and fill the mold. Coinjection (sandwich) molding permits injection molding of parts containing a thermoplastic core within an integral skin of another thermoplastic material. When the core is foam, an advanced form of structural foam is produced.

REINFORCED PLASTICS/COMPOSITES These are plastics whose mechanical properties are significantly improved because of the inclusion of fibrous reinforcements. The wide variety of resins and reinforcements that constitute this


group of materials led to the more generalized description “composites.” Composites consist of two main components, the fibrous material in various physical forms and the fluidized resin, which will convert to a solid. There are fiber-reinforced thermoplastic materials, and these are typically processed in standard thermoplastic processing equipment. The first step in any composite fabrication procedure is the impregnation of the reinforcement with the resin. The simplest method is to pass the reinforcement through a resin bath and use the wet impregnate directly. For easier handling and storage, the impregnated reinforcement can be subjected to heat to remove impregnating solvents or advance the resin cure to a slightly tacky or dry state. The composite in this form is called a prepreg. This B-stage condition allows the composite to be handled, yet the cross-linking reaction has not proceeded so far as to preclude final flow and conversion to a homogeneous part, when further heat or pressure is applied. Premixes, often called bulk molding compounds, are mixtures of resin, inert fillers, reinforcements, and other formulation additives that form a puttylike rope, sheet, or preformed shape. Converting these various forms of composite precursors to final part shape is achieved in a number of ways. Hand layup techniques entail an open mold onto which the impregnated reinforcement or prepreg is applied layer by layer until the desired thicknesses and contours are achieved; see part a of Figure P.3 depicting techniques for producing reinforced plastics and composites. The thermoset resin is then allowed to harden (cure). Often the entire configuration will be enclosed in a transparent sealed bag (vacuum bag) so that a vacuum can be applied to remove unwanted volatile ingredients and entrained air for improved densification of the composite (part b of Figure P.3). External heat may be applied to accelerate the process. Often a bagged laminate will be inserted into an autoclave so that the synergistic effects of heat, vacuum, and pressure can be obtained. At times, a specially designed spray apparatus is used that simultaneously mixes and applies a coating of resin and chopped reinforcement to a mold surface (part c). This

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cellophane

resin

press ram mold

mold

press platen

steam cores

resin-glass layup

molding

female mold

reinforcement molded part

mold

resin male mold

during layup

preform

after the cure

guide pins stops

(a)

before closing

to vacuum

to vacuum

clamps

glassresin layup

gasket

after closing press platen

(d)

molded part flexible bag

resin

moving winding head

mold before vacuum applied after vacuum applied (b)

metering/glass chopping gun and spray unit molded part catalyst and additives

longitudinal fiber

hoop fiber

mold

glass fiber

rotating mandrel (e) resin

(c)

FIGURE P.3 Techniques for producing reinforced plastics and composites. (a) Hand lay-up technique for reinforced thermosets; (b) vacuum bag molding method; (c) spray-up method; (d) matched metal die molding; (e) filament winding. (From McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 14, McGrawHill, New York, 45. With permission.)

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technique is particularly useful for large structures such as boat hulls and truck cabs, covering complex shapes as readily as simple configurations. Matched die compression molding resembles normal compression molding, although the pressures are considerably lower (part d). Premix molding is essentially the same process, except that premix compounds are used. Pultrusion is a term coined to describe the process for the continuous extrusion of reinforced plastics profiles. Strands of reinforcement are drawn (pulled) through an impregnating tank, the forming die, and finally a curing area (radio-frequency exposure). Filament winding is a process in which the continuous strands of reinforcement are drawn through an impregnating bath and then wound around a mandrel to form the part (part e). This technique is most used for the formation of hollow objects such as chemical storage tanks or chemically resistant pipe. Advanced automated processes, such as ply cutting, tape laying and contouring, and ply lamination are providing improved parts © 2002 by CRC Press LLC

and reduced costs particularly in the aerospace industry.

CASTING

AND

ENCAPSULATION

Casting is a low-pressure process requiring nothing more than a container in the shape of the desired part. For thermoplastics, liquid monomer is poured into the mold and, with heat, allowed to polymerize in place to a solid mass. For vinyl plastisols, the liquid is fused with heat. Thermosets, usually composed of liquid resins with appropriate curatives and property-modifying additives, are poured into a heated mold in which the cross-linking reaction completes the conversion to a solid. Often a vacuum is applied to gasify the resultant part for improved homogeneity. Encapsulation and potting are terms for casting processes in which a unit or assembly is encased or unpregnated, respectively, with a liquid plastic, which is subsequently hardened by fusion or chemical reaction; Figure P.4 depicts low-pressure plastics processes. These


casting material

mold (a) casting material potting

coil housing or case

(b) encapsulation

coil

mold

(c) sealant coil case

(d)

FIGURE P.4 Low-pressure plastics processes: (a) casting, (b) potting, (c) encapsulation, and (d) sealing. (From McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 14, McGraw-Hill, New York, 44. With permission.)

processes are predominant in the electrical and electronic industries for the insulation and protection of components.

CALENDERING In the calendering process, a plastic is masticated between two rolls that squeeze it out into a film that then passes around one or more additional rolls before being stripped off as a continuous film. Fabric or paper may be fed through the latter rolls, so that they become impregnated with the plastic.

PLASTISOL COATINGS Vinyl plastisols, or pastes, as they are described in Europe, are suspensions of vinyl resin in nonvolatile oily liquids known as plasticizers. They vary in viscosity from a motor oil consistency to a puttylike dough. In the more viscous state, the plastisol is termed plastigel, while the more fluid materials, to © 2002 by CRC Press LLC

which volatile diluents have been added, are known as modified plastisols. Modified plastisols differ from organosols in the function of the volatile components. In organosols, the volatiles are used as resin dispersants, whereas in modified plastisols they serve as diluents to adjust fluidity and are generally present in small quantities. The polyvinyl chloride resin, resembling confectioner's sugar, is blended into a mixture of one or more plasticizers to form a suspension. This fluid remains essentially unchanged until heat is applied. During the heating process, the dispersion first sets or gels; this is followed by solution or fusion of the resin in the hot plasticizer to form a single-phase solid solution. Upon cooling, the coating assumes the properties of a tough, rubbery plastic. These plastisols have no adhesion to metals or dense, nonporous substrates and consequently require the use of adhesive primers for bonding. To a large extent, the nature of these primers determines the suitability of plastisol coatings for specific applications.

APPLICATION METHODS The fluidity or absence of fluidity in the liquid plastisol is sometimes deceiving. These materials are supplied at very high solids content and consequently exhibit non-Newtonian flow (viscosity varies with applied shear). While the viscosity cup is satisfactory for many paints and lacquers, and may even suffice for organosols, it can only serve to mislead the plastisol user. Viscosity of plastisols should be specified and measured using a viscosimeter capable of operating over a range of shear rates preferably within the area of use. Spread Coating Roller and knife coaters are the two major types of spread-coating equipment used for handling plastisols. Fabric, paper, and even strip steel are all being coated with this type of process. Compound viscosity characteristics, speed of coating, clearance between the web and the knife or roll, and type and angle of the knife are all factors in determining the quality of the coating. Heavy paper and fabric coatings, which will withstand folding and forming, may be applied to porous

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stock without danger of penetration or strike through. The momentary application of heat to the coated side of the stock will fuse the plastisol with a minimum of thermal action on the paper. Plastisol-coated strip steel is currently being produced by roller-coating processes for use by appliance and other manufacturers. Dip Coating Two dip-coating processes are available. In hot dipping, the object for coating is prebaked, prior to immersion in the plastisol. The heat content in the article serves to gel a deposit on the surface of the object. This gelled coating must then be fused by baking. Plastisol formulation and temperature, dipping rate, mass, shape, and heat content of the article to be coated all serve to determine the thickness and nature of deposit. Cold-dip processes permit the application of from 1 to 60 mils per coat without the necessity of a prebake operation. Cold dips lend themselves to conveyor line coating of products. These coatings have a high yield value and permit controlled film thicknesses without the presence of sags or drips to mar the appearance. Spray Coating

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Plastisols may be spray-applied through either pressure or suction guns, but generally the pressure equipment is preferred because it permits faster delivery with a minimal use of volatile diluents. Airless spray equipment is currently available that operates at fluid pressures in excess of 13.6 MPa and requires no atomizing air. This airless-spray process is reported to give extreme smoothness to highly thixotropic formulations.

PROPERTIES Although plastisols may be modified with slight additions of volatile diluents, their fluidity is mainly due to the presence of large quantities of plasticizer. Unlike the fluid phase of the organosol, which is largely volatile, the plasticizers remain behind after baking, as a portion of the fused film. Thus, the plastisol tends to be softer and more resilient than the organosol. Its low volatile content (or its absence altogether) permits wide baking latitude by eliminating the problem © 2002 by CRC Press LLC

of mud cracking and reducing the solvent entrapment tendency found in organosols. Film thicknesses may range from 2 to 250 mils per coat.

USES Plastisols, because of their ability to be applied readily in heavy thicknesses, found early success in the electroplating field as rack coatings. The plastisol serves as an insulation, confining current to the work being plated, and is resistant to chemical attack by plating solutions. The use of plastisols as linings for tanks, chemical equipment, and steel drums followed. In fabric coating, they have replaced solution coatings by eliminating the need for expensive solvents lost in the baking operation. Rubber has been replaced by vinyl plastisols and organosols as coatings for wire baskets because of their superior resistance to moisture and detergents. One of the most dramatic applications of plastisols is as a lining for kitchen dishwashers. The use of a plastisol lining permitted a lightweight tub design not possible with porcelain enameling in which firing resulted in buckling and warping of light-gauge steel. Plastisols also served to reduce scrap units since defects may be readily patched and repaired. The resistance to impact damage, etching, and enamel erosion are other factors that prompted manufacturers to select plastisols for this application. A list of applications and the properties related to the specific application follow: Application Industrial: Tool handles, stair treads, conveyor hooks, conveyor rollers, railings

Related Property Resiliency, thermal and electrical insulating qualities, resistance to abrasion Electrical: Bus bars, conduit boxes, Dielectric strength, battery clamps and cases, toggle electrical resistivity, switches, electroplating racks, and resistance to moisture plating barrels and chemicals Linings: Tanks, duct-work, pumps, Resistance to abrasion filter presses, centrifugal cleaners, and impact, resistance to dishwasher tubs, piping, drums, moisture and chemicals and shipping containers Wire Goods: Dish-drain baskets, Resistance to moisture, egg baskets, deep-freeze baskets, detergents, and staining, refrigerator shelves, record racks, resiliency clothes hangers Miscellaneous: Bottles and Resiliency and aesthetic glassware, glove coating, bobbyqualities, abrasion pin coatings resistance, softness


PLASTISOLS Plastisols are dispersions of high-molecularweight vinyl chloride polymer or copolymer resins in nonaqueous liquid plasticizers, which do not dissolve the resin at room temperature. Plastisols are converted from liquids to solids by fusing under heat, which causes the resin to dissolve in the plasticizers. There are many advantages in molding with plastisols each varying in importance according to the particular type of molding application. Vinyl plastisol is supplied as a liquid and consequently is easy to handle. The material requires no catalysts or curing agents to convert it to a solid, only moderate heat in the range of 149 to 204°C. Vinyl plastisol does not require a long baking cycle nor high pressures to fuse and shape it. Consequently, lightweight inexpensive molds are suitable for molding. It can be formulated to have a virtually indefinite shelf life. Plastisols are usually 100% solids and shrinkage from the mold is at an absolute minimum, thus assuring that the molded object is exact and consistent.

PROPERTIES Chemical and physical properties of plastisols can be varied throughout a wide range. This versatility makes plastisols adaptable to a multitude of end uses. The following general ranges indicate the properties that may be compounded into plastisols: Specific Gravity: 1.05 to 1.35 Tensile Strength: As required to 27.2 MPa Elongation: As required to 600% Flexibility: Good to a temperature as low as –55°C Hardness: From 10 to 100 on the Shore A Durometer Scale; up to 80 on the Shore D Durometer Scale Chemical Resistance: Outstanding to most acids, alkalies, detergents, oils, and solvents Heat Resistance: Can resist 107°C for as long as 2000 h and 232°C for over 2 h


Electrical Properties: Dielectric strength at a minimum of 400 v/mil in thicknesses of 3 mils and over Flammability: Slow burning to selfextinguishing Colors: All colors available including phosphorescent and fluorescent shades

MOLDING METHODS There are several different methods by which plastisols may be molded. Pour and Injection Two of the simplest methods are pour molding and low-pressure injection molding. The first method entails merely pouring plastisol into a cavity until it is filled and subsequently fusing the compound. This system is used in manufacturing products such as plastic doilies, sink stoppers, and display plaques. If the mold is closed, a low-pressure injection system such as a grease gun can be used to inject the liquid plastisol into the cavity. A low-pressure injection mold should be designed with bleeders at the extremities of the cavity to ensure complete filling of the mold, as well as to relieve the minor pressure on the mold surface caused by expansion during the heating. Laboratory models, novelties, and electrical harnesses are products that are commonly low-pressure injection-molded with vinyl plastisol. Heating sources used for molding plastisols vary according to the particular product under consideration. For shallow, open molds, such as those for plastic doilies, radiant heat would be satisfactory. When this type of heat is used, the material thickness should not be so great that the open surface exposed to the heat overfuses in the time taken for the temperature to reach the mold surface of the part. Conductive heat is another source for fusing plastisol, in particular when closed molds are employed. Immersing the mold in a hot bath or using cartridge heaters are two conductive heating methods. A more commonly used method of fusion is convection heat. The advantage of a convection oven, in particular a forced air type, is that the entire inner area of the oven, and consequently

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the entire surface of the mold, is maintained at a constant temperature, ensuring a more even heat transfer through the mold. Pour and low-pressure injection molds usually are made of aluminum, electroformed copper, brass, or steel. The thickness of the metal should be kept to a minimum for good heat transfer yet should be thick enough to withstand expansion pressure during fusing.

Another molding process, which is somewhat similar to the foregoing methods, is in-place molding. This method permanently attaches plastisol to another component during the fusing process. The combination serves an important functional purpose and usually eliminates the need for several assembly steps. In-place molding is most commonly used in forming seals and gaskets of various sorts. Gaskets are applied to vitrified clay pipe by pouring plastisol into special molds on the bell and spigot ends. Such gaskets compensate for inherent out-of-roundness of the pipe. Flowed in gaskets also are applied to bottle caps and jar lids.

Another method for molding hollow pieces is slush molding. In this process an open-end metal mold is heated to a temperature in the range of 149 to 204°C and then filled with plastisol. The plastisol is allowed to dwell until the desired thickness has gelled on the inner surface of the mold and then the remaining liquid in the mold is poured back into a reservoir for use again. The mold with the gelled inner coating is placed in an oven where the plastisol is fused. Upon cooling, the plastisol part is stripped from the mold, retaining the design of its inner surface on the exterior of the piece. Molds of electroformed copper or fine sand-cast aluminum are usually used for slush molding. The above process is generally known as the single-pour system of slush molding. For a mold of intricate detail, a two-pour method often is used. In this process, the mold is filled when it is cold, vibrated to remove bubbles, and then emptied, leaving a thin film of plastisol on the inner surface. In this way, the plastisol does not have a chance to gel before flowing into the mold extremities.

Dip Molding

Rotational Molding

When a hollow object is to be molded and the internal dimensions are of importance, many times a dip molding process is employed. The metal molds are shaped according to the interior design of the molded object. They are usually solid and are made of cast or machined aluminum, machined brass, steel, or ceramic. These molds are preheated to a temperature in the range of 149 to 204°C, dipped into the plastisol, and allowed to dwell until the proper thickness has gelled on the mold. To eliminate drips or sags, the mold is withdrawn at a rate that does not exceed the rate at which the liquid residue drains from the gelled coating. The thickness of this coating can be varied by altering the preheating time and temperature, as well as the dwell time. A dip molding system can easily be conveyorized. In such a system mandrels holding the molds would be conveyed through a preheat oven, dipping station, fusing oven, cooling station, and stripping station.

Completely enclosed hollow parts can be produced by rotational molding. A measured amount of plastisol is poured into one half of a two-piece mold. The mold is closed and rotated in two or more planes while being heated. During this rotation, the plastisol flows, gels, and fuses evenly over the interior walls of the mold. Molds for this operation are either electroformed copper or cast or machined aluminum, and the molds are arranged in clusters or “gangs” so that the maximum number of molds can be operated per spindle. In rotational molding it is possible to vary the thickness of the walls of the molded piece. One way this can be accomplished is by rotating the mold more in one plane than in another. A few familiar products manufactured by this process are toys and novelties such as dolls and beach balls, swimming pool floats, and artificial fruit.

In-Place Molding

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Slush Molding



Combinations In many cases several of the above molding methods are combined to produce a product made from plastisol. For example, vinyl foam products such as armrests, toys, or electrical harnesses are manufactured by first forming a tough vinyl skin by spraying, slush molding, or rotational molding. The interior then is formed by casting, low-pressure injection, or rotational molding a vinyl plastisol foam within the pregelled skin.

PLATINUM A whitish-gray metal, symbol Pt, platinum is more ductile than silver, gold, or copper, and is heavier than gold. The melting point is 1769°C, and the specific gravity is 21.45. The hardness of the annealed metal is 45 Brinell, and its tensile strength is 117 MPa; when hard-rolled, the Brinell hardness is 97 and tensile strength 234 MPa. Electrical conductivity is about 16% that of copper. The metal has a face-centered cubic lattice structure and it is very ductile and malleable. It is resistant to acids and alkalies, but dissolves in aqua regia. Platinum is widely used in jewelry, but because of its heat resistance and chemical resistance it is also valued for electric contacts and resistance wire, thermocouples, standard weights, and laboratory dishes. Generally too soft for use alone, it is almost always alloyed with harder metals of the same group, such as osmium, rhodium, and iridium. An important use of the metal, in the form of gauze, is as a catalyst. Platinum gauze is of high purity in standard meshes of 18 to 31/cm, with wire from 0.020 to 0.008 cm in diameter. Dental foil is 99.99% pure and of maximum softness. Platinum foil for other uses is made in thicknesses as thin as 0.0005 cm. Platinum powder comes in fine submesh particle size. It is made by chemical reduction and is at least 99.9% pure, with amorphous particles 0.3 to 3.5 µm in diameter. Platinum flake has the powder particles in the form of tiny laminar platelets that overlap in the coating film. Because of the high resistance of the metal to atmospheric corrosion even in sulfur environments, platinum coatings and electroplating are used on springs and other functioning parts


of instruments and electronic devices where precise operation is essential. Coatings are also produced by vapor deposition of platinum compounds; thin coatings, 0.0005 cm or less, are made by painting the surface with a solution of platinum powder in an organic vehicle and then firing to drive off the organic material, leaving an adherent coating of platinum metal. Platinum is sometimes used in glazes to obtain luster and metallic effects. Liquid bright platinum and liquid bright palladium (an element of the platinum group) are preparations used in metallic decorations. As platinum produces a better silver effect than silver itself and is less likely to tarnish, platinum is preferred to that metal. A luster produced from a strong solution of platinum chloride and spirits or oil of lavender upon firing gives a steely appearance which is nearly opaque. Another method consists of precipitating the metal from its solution in water by heating it with a solution of caustic soda and glucose. The metal is mixed with 5% bismuth subnitrate, applied to the ware by painting, and fired in a reducing atmosphere.

PLATINUM ALLOYS Platinum is alloyed to obtain greater hardness, strength, and electrical resistivity. Because most applications require freedom from corrosion, the other platinum metals are usually employed as alloying agents.

PLATINUM–IRIDIUM Iridium is the addition to platinum most often used to provide improved mechanical properties. It increases resistance to corrosion while the alloy retains its workability. Up to 20% iridium, the alloys are quite ductile. With higher iridium content fabrication becomes difficult. Platinum–iridium alloys are employed for instruments, magneto contacts, and jewelry. The alloys are hard, tough, and noncorrosive. An alloy of 95% platinum and 5% iridium, when hard-worked, has a Brinell hardness of 170; an alloy with 30% iridium has a hardness of 400. The 5 and 10% alloys are used for jewelry manufacture; the 25 and 30% alloys are employed for making surgical instruments. An

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alloy of 80% platinum and 20% iridium is used for magneto contact points, and the 90–10 alloy is widely used for electric contacts in industrial control devices. The addition of iridium does not alter the color of the platinum. The 5% alloy dissolves readily in aqua regia; the 30% alloy dissolves slowly.

PLATINUM–RHODIUM

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The addition of rhodium to platinum also provides improved mechanical properties to platinum and increases its resistance to corrosion. For applications at high temperatures, the platinum–rhodium alloys are preferred because of retention of good mechanical properties including good hot strength and very little tendency toward volatilization or oxidation. Platinum–rhodium alloys are used for thermocouples for temperatures above 1100°C. The standard thermocouple is platinum vs. platinum–10% rhodium. Other thermocouples for higher operating temperatures use platinum–rhodium alloys in both elements. The alloys of platinum–rhodium are widely used in the glass industry, particularly as glass-fiber extrusion bushings. Rhodium increases the high-temperature strength of platinum without reducing its resistance to oxidation. Platinum–rhodium gauze for use as a catalyst in producing nitric acid from ammonium contains 90% platinum and 10% rhodium.

PLATINUM–RUTHENIUM Alloying platinum with ruthenium has the most marked effect upon both hardness and resistivity. However, the limit of workability is reached at 15% ruthenium. The lower cost and the lower specific gravity of ruthenium offer an appreciable economic benefit as an alternate to other platinum alloys. Platinum–ruthenium alloy, with 10% ruthenium, has a melting point of 1800°C, and an electrical conductivity 4% that of copper.

PLATINUM–GOLD Platinum–gold alloys cover a wide range of compositions and provide distinct chemical and physical characteristics.


PLATINUM–COBALT

AND

PLATINUM–NICKEL

These alloys, with about 23% cobalt, are used for permanent magnets. Platinum–nickel alloys, with as much as 20% nickel, are noted for high strength. With 5% nickel, for example, tensile strength of the annealed alloy is about 621 MPa, and with 15% it increases to 896 MPa. Strength almost doubles with appreciable cold work.

PLATINUM–RHENIUM These alloys are efficient catalysts for reforming operations on aromatic compounds. The platinum alloys have lower electric conductivity than pure platinum, but are generally harder and more wear resistant, and have high melting points. A platinum–rhenium alloy with 10% rhenium has an electrical conductivity of only 5.5% that of copper compared with 16% for pure platinum. Its melting point is 1850°C, and the Rockwell T hardness of the cold-rolled metal is 91 compared with 78 for cold-rolled platinum.

PLATINUM–TUNGSTEN These alloys, with 2 to 8% tungsten, have been used for aircraft-engine spark plug electrodes, radar-tube grids, strain gauges, glow wires, switches, and heating elements. The tungsten markedly increases electrical resistivity while decreasing the temperature coefficient of resistivity. It also substantially increases tensile strength — to 896 MPa for platinum (8% tungsten alloy in the annealed condition) — and tensile strength more than doubles with appreciable cold work.

PLATINUM GROUP METALS The platinum group metals — ruthenium, rhodium, palladium, osmium, iridium, and platinum — are found in the second and third long period in Group VIII of the periodic table. Platinum and palladium are the most abundant of the group although all are generally found together. The outstanding characteristics of platinum, the most important member of the group, are its remarkable resistance to corrosion and chemical attack, high melting point, retention of mechanical strength, and resistance to


oxidation in air, even at very high temperatures. These qualities, together with the ability of the metal to greatly influence the rates of reaction in a large number of chemical processes, are the basis of nearly all its technical applications. The other five metals of the platinum group are also characterized by high melting points, good stability, and resistance to corrosion. Addition of these metals to platinum forms a series of alloys that provide a wide range of useful physical properties combined with the high resistance to corrosion that is characteristic of the parent metals.

PLATINUM (PT) When heated to redness, platinum softens and is easily worked. It is virtually nonoxidizable and is soluble only in liquids generating free chlorine, such as aqua regia. At red heat platinum is attacked by cyanides, hydroxides, sulfides, and phosphides. When heated in an atmosphere of chlorine, platinum volatizes and condenses as the crystalline chloride. Reduction of platinum chloride with zinc gives platinum black, which has a high adsorptive capacity for hydrogen. Platinum sponge is finely divided platinum.

PALLADIUM (PD) Palladium is silvery white, very ductile, and slightly harder than platinum. It is readily soluble in aqua regia and is attacked by boiling nitric and sulfuric acids. Palladium has the remarkable ability to occlude large quantities of hydrogen. When properly alloyed it can be used for the commercial separation and purification of hydrogen. Palladium and platinum can both be worked by normal metalworking processes.

IRIDIUM (IR) This is the most corrosion-resistant element known. It is a very hard, brittle, tin-colored metal with a melting point higher than that of platinum. It is soluble in aqua regia only when alloyed with sufficient platinum. Iridium has its greatest value in platinum alloys where it acts as a hardening agent. By itself it can be worked only with difficulty.


RHODIUM (RH) Rhodium serves an important role in high-temperature applications up to 1649°C. Platinum–rhodium thermocouple wire makes possible high-temperature measurement with great accuracy. Rhodium and rhodium alloys are used in furnace windings and in crucibles at temperatures too high for platinum. It is a very hard, white metal and is workable only under certain conditions, and then with difficulty. Applied to a base metal by electroplating, it forms a hard, wear-resistant, permanently brilliant surface. Solubility is slight even in aqua regia.

RUTHENIUM (RU) Ruthenium is hard and brittle with a silver-gray luster. Its tetraoxide is very volatile and poisonous. When alloyed with platinum, its effect on hardness and resistivity is the greatest of all the metals in the group. It is unworkable in the pure state.

OSMIUM (OS) This element has the highest specific gravity and melting point of the platinum metals. It oxidizes readily when heated in air to form a very volatile and poisonous tetraoxide. Application has been predominantly in the field of catalysis. As a metal it is also practically unworkable.

PLYWOOD Plywood is a term generally used to designate glued wood panels made up of layers, or plies, with the grain of one or more layers at an angle, usually 90°, with the grain of the other. The outside plies are called faces or face and back, the center plies are called the core, and the plies immediately below the face and back, laid at right angles to them, are called the crossbands. The core may be veneer, lumber, or various combinations of veneer and lumber; the total thickness may be less than 1.5 mm or more than 76 mm; the different plies may vary as to number, thickness, wood species. Also, the shape of the members may vary. The crossbands and their arrangement generally govern both the

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properties (particularly warping characteristics) and uses of all such constructions. Plywood is an outgrowth of the laminated wood known as veneer, which consists of an outside sheet of hardwood glued to a base of lower-cost wood. The term veneer actually refers only to the facing layer of selected wood, used for artistic effect or for economy in the use of expensive woods. Veneers are generally marketed in strip form in thicknesses of less than 0.32 cm in mahogany, oak, cedar, and other woods. The usual purpose of plywood now is not aesthetic but to obtain high strength with low weight. The term laminated wood generally means heavier laminates for special purposes, and such laminates usually contain a heavy impregnation of bonding resin that gives them more of the characteristics of the resin than of the wood.

COMPOSITION

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The composition of a plywood panel is generally dependent on the end use for which it is intended. The number of plywood constructions is almost endless when one considers the number of wood species available, the many thicknesses of wood veneers used in the outer plies or cores, the placement of the adjacent plies, the types of adhesives and their qualities, various manufacturing processes, and more technical variations. Conventional plywood generally consists of an odd number of plies with the grains of the alternate layers perpendicular to each other. The use of an odd number permits an arrangement that gives a substantially balanced effect; that is, when three plies are glued together with the grain of the outer two plies at right angles to that of the center ply, the stresses are balanced and the panel tends to remain flat with changes in moisture content. These forces may be similarly balanced with five, seven, or some other uneven number of plies. If only two plies are glued together with the grain of one ply at right angles to the other, each ply tends to distort the other when changes in moisture content occur; cupping will result. Low-cost plywoods may be bonded with starch pastes, animal glues, or casein, and are not water-resistant, but are useful for boxes and


for interior work. Waterproof plywood for paneling and general construction is now bonded with synthetic resins, but when the plies are heavily impregnated with the resin and the whole cured into a solid sheet, the material is known as a hardboard or as a laminated plastic rather than a plywood.

GRADES

AND

TYPES

Broadly speaking, two classes of plywood are available — hardwood and softwood. Most softwood plywood is composed of Douglas fir, but western hemlock, white fir, ponderosa pine, redwood, and other wood species are also used. Hardwood plywood is made of many wood species. Various grades and types of plywood are manufactured. “Grade” is determined by the quality of the veneer and “Type” by the moisture resistance of the glue line. For example, there are two types of Douglas fir plywood — interior and exterior. The interior type is expected to retain its form and strength properties when occasionally subjected to wetting and drying periods. It is commonly bonded with urea–formaldehyde resin adhesives. On the other hand, the exterior type is expected to retain its form and strength properties when subjected to cyclic wetting and drying and to be suitable for permanent exterior use. It is commonly bonded with hot-pressed phenolic-resin glues. For construction purposes, where plywood is employed because of its unit strength and nonwarping characteristics, the plies may be of a single type of wood and without a hardwood face. The Douglas Fir Plywood Association sets up four classes of construction plywood under general trade names. Plywall is plywood in wallboard grade; Plypanel is plywood in three standard grades for general uses; Plyscord is unsanded plywood with defects plugged and patched on one side; and Plyform is plywood in a grade for use in concrete forms. The bulk of commercial plywood comes within these classes; the variations are in the type of wood used, the type of bonding adhesive, or the finish. Etch wood, for example, is a paneling plywood with the face wire brushed to remove the soft fibers and leave the hard grain for two-tone finish. Paneling plywoods


with faces of mahogany, walnut, or other expensive wood have cores of lower-cost woods, but the woods of good physical qualities are usually chosen.

ENGINEERING PROPERTIES The mechanical and physical properties of plywood are dependent upon the particular construction employed. Plywood may be designed for beauty, durability, rigidity, strength, cost, or many other properties. With practically an unlimited variety of constructions to choose from, there is a wide range of differing characteristics in any given plywood panel. Most important among properties are the following. High Strength–Weight Ratio Perhaps the most notable feature of plywood is its high strength–weight ratio. Plywood is given special consideration whenever lightness and strength are desired. Plywood is widely used for concrete form work, floor underlayment, roof decks, siding, and many other applications because of its high strength–weight ratio. A comparison between birch plywood and other structural materials shows that its strength–weight ratio is 1.52 times that of 100,000-lb test heattreated steel and 1.36 times that of l0,000-lb test aluminum. Bending Properties A most desirable characteristic of plywood is its flatness but it can and will support substantial curvatures without appreciable loss of strength. Standard construction plywood can be bent or shaped to nominal radii and held in place with adhesives, nails, screws, or other fixing methods. The radius of curvature to which a panel can be formed varies with panel thickness and the species of wood employed in the panel construction. The arc of curvature is limited by the tension force in the outer plies of the convex perimeter and by the compression forces in the outer plies of the concave perimeter. Waterproofed plywood, soaked or steamed before bending, exhibits approximately 50% greater flexibility than panels bent when dry.


Resistance to Splitting Because plywood has no line of cleavage, it cannot split. This is an exceptional property when one considers its effect on fastening. The crisscross arrangement of wood plies in plywood construction develops extraordinary resistance to pull-through of nail or screw heads. Resistance to Impact The absence of a cleavage line has a pronounced effect on the impact resistance of plywood. Plywood will fracture only when the impact force is greater than the tensile strength of the wood fibers in the panel composition. Under an impact force, the side of the panel opposite the impact point will rupture along the long grain fiber followed by successive shattering of the various plies. Splintering usually does not take place because pressure is dispersed throughout the panel at the point of impact. Under similar conditions, solid lumber will show complete rupture. Beauty Plywood has certain intrinsic qualities that add much to any structure or construction in which it is used. Because of improved modern methods of manufacture, there is practically no limit to the decorative potential of plywood. The entire range of fine woods is at the designer’s disposal; they vary in shade from golden yellow to ebony, from pastels to reds and browns. The use of bleaches, toners, and stains in manufacturing procedures gives the designer even greater latitude with respect to design freedom. The fine woods employed in the manufacture of plywood offer warmth and charm to any decorative scheme because the surface of the wood variously absorbs, reflects, and refracts light rays, giving the wood pattern depth and making it restful to the eye. This phenomenon accounts for the play of color and pattern when a plywood panel is viewed from different angles.

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Dimensional Stability

Resistance to Borers

The absorption of water causes wood to swell and this movement is much greater across the grain than along the grain. The alternating layers of veneers in standard plywood construction inhibits this cross-grain movement because the cross-grain weakness is reinforced by the longgrain stability. Therefore, the dimensional stability of a plywood panel can be controlled by controlling its moisture content. In the field, this control can be achieved by applying coatings such as paints, lacquers, and sealers of various types.

Plywood panels are subject to attack by borers to the same extent as the wood species of which they are composed, but phenol–formaldehyde resin glue lines are fairly effective barriers against further penetration. Panels may also be treated with pentachlorophenol for increased resistance to these pests.

Thermal Insulating Qualities The thermal insulating qualities of plywood are the same as those of the wood of which it is composed. The use of plywood as an insulating material can be attributed to two factors: (1) The use of large sheets reduces the numbers of cracks and joints and thereby inhibits wind leakage; and (2) the resistance of plywood to moisture vapor transmission stabilizes the moisture content of the trapped air and maintains its insulating qualities.

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Fire Resistance Fire-resistant plywood is manufactured by impregnating the core stock with a salt solution, which, upon evaporation, leaves a salt deposit in the wood. Plywood or wood treated in this manner will not support combustion but will char when heated beyond the normal charring point of the wood. Nonimpregnated plywood can be made fire resistant by applying surface coatings such as intumescent paint and chemicals such as borax. An intumesent paint has a silicate of soda base that bubbles or intumesces in the presence of heat, thus forming a protective coating. At high temperatures, borax releases a gas such as carbon dioxide, which blankets the fire. It must be noted that fire-resistant coatings are effective only in direct ratio to their thickness. Highly resistant plywood can be manufactured by using an incombustible core such as asbestos.


Fatigue Resistance Plywood has the same resistance to fatigue as the wood of which it is composed.

FABRICATION The fact that almost anyone can use plywood has contributed greatly to its wide acceptance in many varied applications. The utilization of plywood does not require special tools, special skills, or safeguards, and practically anyone capable of handling a saw and hammer can make use of its inherent engineering properties. Since plywood does not exhibit the typical cross-grain weakness of lumber, it can very often be used in place of lumber for various applications. For example, 0.312-mm-thick plywood replaces conventional 0.750-mm sheathing; 0.250-mm plywood can be used for interior wall paneling without sheathing and 0.375-mm plywood can be used for shipping containers, furniture, and case goods instead of 0.750-mm lumber. The use of the thinner plywood reduces weight, bulkiness, and is less fatiguing for tradespeople to handle. The use of large plywood sheets instead of narrow boards also reduces the amount of cutting, fitting, and fastening involved in a particular job. Plywood is especially adaptable to the portable power-driven saws, drills, and automatic hammers normally used on production or construction jobs. Multiple cutting with band saws may be done with assurance because even the thinnest plywood has strength in all directions and the danger of splitting or chipping is reduced to a minimum. This quality is particularly important when fine fitting is required. Whenever plywood is employed in a structure, fewer and smaller fastenings can be specified because consideration need be given only


to the holding power of the fastener and the tensile strength of the fastening itself.

AVAILABLE FORMS, SIZES, SHAPES Plywood is available in practically any size, but the 1.2 × 2.4 m panel has become the standard production unit of the industry. Larger panels usually demand a price premium and are available on special order. Panels with continuous cores and faces can be produced in one piece up to 4 m long. Oversize panels up to 2.4 m in width and of unlimited length can be manufactured by scarf jointing. (A scarf is an angling joint, made either in veneers or plywood, where pieces are spliced or lapped together. The length of the scarf is usually 12 to 20 times the thickness. When properly made, scarf joints are as strong as the adjacent unspliced material.) Decorative plywood is now commercially available with a plant-applied finish. Prefinished wall paneling is supplied with the finish varying from offset printing to polyester film.

GENERAL FIELDS

OF

APPLICATION

There are many uses and applications for plywood in industry today. Owing to the wide diversity of plywood applications, only the more prominent ones are mentioned here: Architectural Aviation Boatbuilding Building construction Cabinet work Concrete forms Containers, cases Die boards Display Fixtures Floor underlayment Furniture Hampers Luggage Machine bases

Marine construction Mock-ups, models Paddles Panel boards Patterns Prefabrication Remodeling Sheathing Signs Sporting goods Table tops Toys Trays Truck floors, bodies Wall paneling

POLONIUM Polonium (symbol Po) is a rare metallic element belonging to the group of radioactive


metals, but emitting only alpha rays. The melting point of the metal is about 254°C. It is used in meteorological stations for measuring the electrical potential of the air. Polonium-plated metal in strip and rod forms has been employed as a static dissipator in textile-coating machines. The alpha rays ionize the air near the strip, making it a conductor and drawing off static electric charges. Polonium-210 is obtained by irradiating bismuth; 45 kg yields 1 g of polonium-210. It is used as a heat source for emergency auxiliary power such as in spacecraft. The metal is expensive, but can be produced in quantity from bismuth.

POLYACRYLATE RESIN Useful polymers can be obtained from a variety of acrylic monomers, such as acrylic and methacrylic acids, their salts, esters, and amides, and the corresponding nitriles. Polymethyl methacrylate, polyethyl acrylate, and a few other derivatives are the most widely used. Polymethyl methacrylate is a hard, transparent polymer with high optical clarity, high refractive index, and good resistance to the effects of light and aging. It and its copolymers are useful for lenses, signs, indirect lighting fixtures, transparent domes and skylights, dentures, and protective coatings. Solutions of polymethyl methacrylate and its copolymers are useful as lacquers. Aqueous latexes formed by the emulsion polymerization of methyl methacrylate with other monomers are useful as water-based paints and in the treating of textiles and leather. Polyethyl acrylate is a tough, somewhat rubbery product. The monomer is used mainly as a plasticizing or softening component of copolymers. Ethyl acrylate is usually produced by the dehydration and ethanolysis of ethylene cyanohydrin. Modified acrylic resins with high impact strengths can be prepared. Blends or “alloys” with polyvinyl chloride are used for thermoforming impact-resistant sheets. Methyl methacrylate is of interest as a polymerizable binder for sand or other aggregates, and as a polymerizable impregnant for concrete: usually a cross-linking acrylic monomer is also incorporated.

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Polymers of methyl acrylate or acrylamide are water-soluble and useful for sizes and finishes. Addition of polylauryl methacrylate to petroleum lubricating oil improves the flowing properties of the oil at low temperatures and the resistance to thinning at high temperatures.

POLYACRYLIC RUBBER

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The first types of polyacrylic rubbers were proposed as oxidation-resistant elastomeric materials. Chemically, they were polyethyl acrylate, and a copolymer of ethyl acrylate and 2-chloro ethyl vinyl ether. The development of polyacrylic rubbers was accelerated by the interest expressed throughout the automotive industry in the potential applications of this type of polymer in special types of seals. An effective seal for today’s modern lubricants must be resistant not only to the action of the lubricant but to increasingly severe temperature conditions. It must also resist attack of highly active chemical additives that are incorporated in the lubricant to protect it from deterioration at extreme temperature. Polyacrylic rubber compounds were developed to provide a rubber part that would function in applications where oils and/or temperatures as high as 204°C would be encountered. These were also very resistant to attack by sulfur-bearing chemical additives in the oil. These properties have resulted in general use of polyacrylic rubber compounds for automotive rubber parts as seals for automatic transmission fluids and extreme pressure lubricants. Polyacrylic rubber will prove most useful in fields where these special properties are used to the maximum. It is recommended for products such as automatic transmission seals, extreme pressure lubricant seals, searchlight gaskets, belting, rolls, tank linings, hose, O-rings and seals, white or pastel-colored rubber parts, solution coatings, and pigment binders on paper, textiles, and fibrous glass.

CURING A typical polyacrylic rubber, such as the copolymer of ethyl acrylate and chloroethyl vinyl ether, is supplied as a crude rubber in


the form of white sheets having a specific gravity of approximately 1.1. It may be mixed and processed according to conventional rubber practice. However, polyacrylic rubber is chemically saturated and cannot be cured in the same manner as conventional rubbers. Sulfur and sulfurbearing materials act as retarders of cure and function as a form of age resistor in most formulations. Polyacrylic rubber is cured with amines; “Trimene Base” and triethylene tetramine are most widely used. Aging properties may be altered by balancing the effect of the amine and the sulfur. Like other rubber polymers, reinforcing agents such as carbon black or certain white pigments are necessary to develop optimum physical properties in a polyacrylic rubber vulcanizate. Selection of pigments is more critical in that acidic materials, which would react with the basic amine curing systems, must be avoided. The SAF or FEF carbon blacks are most widely used, while hydrated silica or precipitated calcium silicate are recommended for light-colored stocks. Typical curing temperatures are from 143 to 166°C at cure times of 10 to 45 min depending on the thickness of the part. Polished, chromium-plated molds are recommended. For maximum overall physical properties, the cured parts should be tempered in an air oven for 24 h at 149°C.

FORMING To obtain smooth extrusions, more loading and lubrication are necessary than for molded goods, because of the inherent nerve of the polymer. Temperatures of 43°C in the barrel and 77°C on the die are recommended. Generally, those compounds that extrude well are also good calendering stocks. Suggested temperatures for calendering are in the range of 37.8 to 54°C. Higher temperatures will result in sticking of the stock to the rolls. Under optimum conditions, 15-mil films may be obtained. Polyacrylic rubber may be coated on nylon either by calendering or from solvent solution. It also has excellent adhesion to cotton and is often used as a solvent solution applied to


cotton duck to be used as belting. Solvents generally used include methylethyl ketone, toluene, xylene, or benzene. Polyacrylic rubber is most widely used in many types of seals because of its excellent resistance to sulfur-bearing oils and lubricants. In general, polyacrylic rubber vulcanizates are resistant to petroleum products and animal and vegetable fats and oils. They will swell in aromatic hydrocarbons, alcohols, and ketones. Polyacrylic rubber is not recommended for use in water, steam, ethylene glycol, or in alkaline media. Laboratory tests indicate that polyacrylic vulcanizates become stiff and brittle at a temperature of –23°C. But in actual service, these same polyacrylic rubbers have been found to provide satisfactory performance at engine start-up and operation in oil at temperatures as low as –40°C. For those applications requiring improvement in low-temperature brittleness by as much as –4.0°C and that can tolerate considerable sacrifice in overall chemical oil and heat resistance, a copolymer of butyl acrylate and acrylonitrile may be used.

POLYACRYLONITRILE RESINS The polyacrylonitrile resins are hard, horny, relatively insoluble, and high-melting materials. Polyacrylonitrile (polyvinyl cyanide) is used almost entirely in copolymers. The copolymers fall into three groups: fibers, plastics, and rubbers. The presence of acrylonitrile in a polymeric composition tends to increase its resistance to temperature, chemicals, impact, and flexing. Acrylonitrile is generally prepared by several methods, including the catalyzed addition of hydrogen cyanide to acetylene. The polymerization of acrylonitrile can be readily initiated by means of the conventional free-radical catalysts such as peroxides, by irradiation, or by the use of alkali metal catalysts. Although polymerization in bulk proceeds too rapidly to be commercially feasible, satisfactory control of a polymerization or copolymerization may be achieved in suspension and in emulsion, and in aqueous solutions from which the polymer precipitates. Copolymers containing acrylonitrile


may be fabricated in the manner of thermoplastic resins. The major use of acrylonitrile is in the form of fibers. By definition an acrylic fiber must contain at least 85% acrylonitrile: a modacrylic fiber may contain less than 35 to 85% acrylonitrile. The high strength; high softening temperature; resistance to aging, chemicals, water, and cleaning solvents; and the soft wool-like feel of fabrics have made the product popular for many uses such as sails, cordage, blankets, and various types of clothing. Commercial forms of the fiber probably are copolymers containing minor amounts of other vinyl derivatives, such as vinyl pyrrolidone, vinyl acetate, maleic anhydride, or acrylamide. The comonomers are included to produce specific effects, such as improvement of dyeing qualities. Copolymers of vinylidene chloride with small proportions of acrylonitrile are useful as tough, impermeable, and heat-sealable packaging films. Extensive use is made of copolymers of acrylonitrile with butadiene, often called NBR (formerly Buna N) rubbers, which contain 15% acrylonitrile. Minor amounts of other unsaturated esters, such as ethyl acrylate, which yield carboxyl groups on hydrolysis, may be incorporated to improve the curing properties. The NBR rubbers resist hydrocarbon solvents such as gasoline and abrasion, and in some cases show high flexibility at low temperatures. In the 1960s development of blends and interpolymers of acrylonitrile-containing resins and rubbers represented a significant advance in polymer technology. The products, usually called ABS resins, typically are made by blending acrylonitrile–styrene copolymers with a butadiene–acrylonitrile rubber, or by interpolymerizing polybutadiene with styrene and acrylonitrile. Specific properties depend on the proportions of the comonomer, on the degree of grattings, and on molecular weight. In general, the ABS resins combine the advantages of hardness and strength of the vinyl resin component with toughness and impact resistance of the rubbery component. Certain grades of the ABS resin are used for blending with brittle thermoplastic resins such as polyvinyl chloride to improve impact strength.

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The combination of low cost, good mechanical properties, and ease of fabrication by a variety of methods, including typical metalworking methods such as cold stamping, led to the rapid development of new uses for ABS resins. Applications include products requiring high impact strength, such as pipe, and sheets for structural uses, such as industrial duct work and components of automobile bodies. ABS resins are also used for housewares and appliances, because of their ability to be electroplated for decorative items in general.

POLYAMIDES

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These are horny, whitish, translucent, highmelting polymers. Polyamide resins can be essentially transparent and amorphous when their melts are quenched. On cold drawing and annealing, most become quite crystalline and translucent. However, some polyamides based on bulky repeating units are inherently amorphous. The polymers are used for fibers, bristles, bearings, gears, molded objects, coatings, and adhesives. The term nylon formerly referred specifically to synthetic polyamides as a class. Because of many applications in mechanical engineering, nylons are considered engineering plastics. Nylon-6.6 and nylon-6.10 are products of the condensation reaction of hexamethylenediamine (6 carbon atoms) with adipic acid (6 carbon atoms), and with sebacic acid (10 carbon atoms), respectively. Nylon-6.6, nylon-6.10, nylon-6.12, and nylon-6 are the most commonly used polyamides for general applications as molded or extruded parts; nylon-6.6 and nylon-6 find general application as fibers. As a group, nylons are strong and tough. Mechanical properties depend in detail on the degree and distribution of crystallinity, and may be varied by appropriate thermal treatment or by nucleation techniques. Because of their generally good mechanical properties and adaptability to both molding and extrusion, the nylons described above are often used for gears, bearings, and electrical mountings. Nylon bearings and gears perform quietly and need little or no lubrication. Sintering (powder metallurgy) processes are used to make articles


such as bearings and gears that have controlled porosity, thus permitting retention of oils or inks. Nylon resins are used extensively as filaments, bristles, wire insulation, appliance parts, and film. Properties can be modified by copolymerization. Reinforcement of nylons with glass fibers results in increased stiffness, lower creep, and improved resistance to elevated temperatures. Such formulations, which can be readily injection-molded, can often replace metals in certain applications. The use of molybdenum sulfide and polytetrafluoroethylene as fillers increases wear resistance considerably. For uses requiring impact resistance, nylons can be blended with a second, toughening phase. Other types of nylon are useful for specialty applications. Solubility may be increased by interference with the regularity and hence intermolecular packing. This may be accomplished by copolymerization, or by the introduction of branches on the amide nitrogen, for example, by treatment with formaldehyde. The latter type of resin may be subsequently cross-linked. Nylons incorporating aromatic structures, for example, based on isophthalic acid, are becoming more common for applications requiring resistance to very high temperatures.

POLYAMIDE-IMIDES Polyamide-imides are engineering thermoplastics characterized by excellent dimensional stability, high strength at high temperature, and good impact resistance. Molded parts can maintain structural integrity in continuous use at temperatures to 260°C. Polyamide-imide, produced and called Torlon, is available in several grades including a general-purpose, injection-molding grade; three polytetrafluoroethylene (PTFE)/graphite wear-resistant compounds; a 30% graphitefiber-reinforced grade; and a 30% glass-fiberreinforced grade. Additional grades are developed to meet special requirements. Torlon resins are moldable on screw-injection-molding machines. Molds must be heated to 218°C, and the barrel and nozzle should be capable of being heated to about 371°C. High injection speed and pressure (136 MPa or greater) are desirable. Developing optimum


physical properties in injection-molded or extruded parts requires postcuring through an extended, closely controlled temperature program gradually reaching 260°C. The specific time/temperature program depends on part configuration and thickness.

PROPERTIES Room-temperature tensile strength of unfilled polyamide-imide is about 190 MPa and compressive strength is 213 MPa. At 232°C, tensile strength is about 61.2 MPa — as strong as many engineering plastics at room temperature — and continued exposure at 260°C for up to 8000 h produces no significant decline in tensile properties. Flexural modulus of 36.5 MPa of the unfilled grade is increased, with graphite-fiber reinforcement, to 19,913 MPa. Retention of modulus at temperatures to 260°C is on the order of 80% for the reinforced grade. Creep resistance, even at high temperature and under load, is among the best of the thermoplastics; dimensional stability is extremely good. Polyamide-imide is extremely resistant to flame and has very low smoke generation. Reinforced grades have surpassed Federal Aviation Administration requirements for flammability, smoke density, and toxic gas emission. Radiation resistance of polyamide-imide is good; tensile strength drops only about 5% after exposure to 109 rad of gamma radiation. Chemical resistance is good; the resin is virtually unaffected by aliphatic and aromatic hydrocarbons, halogenated solvents, and most acid and base solutions. It is attacked, however, by some acids at high temperature, steam at high pressure and high temperature, and strong bases. Torlon moldings absorb moisture in humid environments or when immersed in water, but the rate is low, and the process is reversible. Parts can be restored to original dimensions by drying.

POLYARYLATES These high-heat-resistant thermoplastics are derived from aromatic dicarboxylic acids and diphenols. When molded, they become amorphous, providing a combination of toughness,


dimensional stability, high dielectric properties, and ultraviolet stability. Polyarylates have heat deflection temperatures up to 175°C at 264 lb/in.2. The resins can be injection-molded, extruded, and blow-molded, and sheet can be thermoformed. These resins also are blended with other engineering thermoplastics and reinforcements.

POLYARYLETHERKETONES (PAEK) A glass-fiber-reinforced (polyaryletherketone) semicrystalline polymer has been designed into the sensor housing for a new conductivity measuring cell. Conductivity measuring cells are used to determine the electrolytic conductivity of media in the food and pharmaceutical industries. The primary requirements for the cell are resistance to corrosive media and biocompatible surface quality to remain in compliance with U.S. and European hygienic requirements. The PAEK polymer is insoluble in all common solvents, and can be immersed for thousands of hours at temperatures in excess of 250°C in steam or high-pressure water environments without significant degradation. The addition of glass fibers increases the chemical resistance of the base resin as well as mechanical strength at elevated temperatures.

POLYBENZIMIDAZOLE Polybenzimidazole (PBI) was used in a variety of applications for the U.S. space program, including flight suits and other protective clothing, webbings, straps, and tethers. Research in raw materials and process development continued, along with applications development, during the 1960s and 1970s. In 1983, commercial production of PBI fiber commenced. Today, PBI fiber has been used successfully in firefighters’ gear, industrial protective clothing, fire-blocking layers for aircraft seats, braided pump packings, and other high-performance products. Research has continued to develop other forms of PBI. In addition to Celazole molded parts, these forms include polymer additives, films, fibrids, papers,

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microporous resin, sizing, and coatings; see Table P.1 (Properties of Polybenzimidazole). Scientists say the material has no known melting point and can withstand pressures of up to 394 MPa. It also has demonstrated resistance to steam at 343°C.

POLYBUTADIENE RUBBER Polybutadiene may be prepared in several ways to yield different products. The method of polymerization can have a marked effect on polymer structure, which in turn controls the properties of the polymer and thus its ultimate end use.

COMPOSITION When polybutadiene is made, the butadiene molecule may enter the polymer chain by either 1,4-addition or 1,2-addition. In 1,4-addition, the unsaturated bonds may be either of cis or of trans configuration. Polybutadienes containing a more or less random mixture of these polymer units can be prepared with alkali metal catalysts or with emulsion polymerization systems but these have not achieved commercial significance as general-purpose rubbers in the United States.

TABLE P.1 Properties of Polybenzimidazole ASTM Test

D638 D638 D638 D790 D790 D695 D695 D256

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Property Mechanical Tensile strength Elongation Tensile modulus Flexural strength Flexural modulus Compressive strength Compressive modulus Izod impact strength Notched Unnotched Poisson’s ratio

Celazole U-60, Unified

23,000 psi 3% 850 kpsi 32,000 psi 950 kpsi 58,000 psi 900 kpsi 0.5-ft-lb/in. 11 ft-lb/in. 0.34

Electrical Dielectric strength Volume resistivity Dissipation factor (10 kHz) Arc resistance

550 V/mil 8 × 1014 0.003 186 s

TMA

Thermal Heat deflection temperature Glass transition temperature Thermal conductivity (77°F) Coefficient of linear thermal expansion

815°F 800°F 2.8 BTU/in.h-ft2-°F 13 × 10–6 in./in.-°F

D785 D570

Specific gravity Hardness Water absorption

D149 D257 D150 D495

D648 DMA

Physical 1.3 115 Rockwell K 0.4%




In recent years, new catalysts have been developed that allow the structure of the polymer chain to be controlled. Polybutadienes containing in excess of 85% of either cis, trans, or vinyl unsaturation have been studied and, in the case of cis and trans polymers, produced commercially. It is also possible to prepare a number of other polymers with various combinations of these three component structures. Of current importance are those types commercially available, which are (1) polymers of high cis-1,4 content (more than 85%) with low trans and vinyl content, (2) polymers of more than 80% trans, low cis, and low vinyl content, and (3) polymers of intermediate cis content (approximately 40% cis, 50% trans) with low vinyl content.

HIGH-CIS POLYBUTADIENE Outstanding properties of high-cis polybutadiene are high resilience and high resistance to abrasion. The high resilience is indicative of low hysteresis loss under dynamic conditions, i.e., low heat rise in the polymer under rapid, repeated deformations. In this respect cispolybutadiene is similar to natural rubber. The combination of good hysteresis properties and resistance to abrasion makes this polymer attractive for use in tires, especially in heavyduty tires where heat generation is a problem. Another favorable factor, particularly for use in tire bodies, is that high-cis polybutadiene imparts good resistance to heat degradation under heavy loads in dynamic applications. Tensile strength (in reinforced stock) is lower than for natural rubber or styrene–butadiene rubber (SBR) but is adequate for many uses. Modulus depends on the degree of cross-linking but cis-polybutadiene generally requires relatively low stress to reach a given elongation (at slow deformation). Hardness may be varied depending on the compound formulation and is similar to that of SBR and natural rubbers. Ozone resistance is typical of unsaturated polymers and is inferior to that of saturated rubbers. Oil resistance is comparable to that of SBR and natural rubber. Permeability to gases is higher than that of most other rubbers, which may be either an advantage or a disadvantage, depending on the application. Freeze point is quite low;


cis-polybutadiene tread compounds do not become brittle until very low temperatures are reached, lower than –101°C. Abrasion resistance of the high-cis polybutadienes varies with the type of service and in nearly all cases is superior to that of natural rubber or SBR. The advantage for these polybutadienes in relation to natural rubber and SBR increases as the severity of the service increases. From 30% to as high as 100% improvement in wear resistance has been reported through the substitution of these polybutadienes for natural rubber to tire treads. Processing Polybutadiene rubbers in general are more difficult to process in conventional equipment than natural rubber, particularly with regard to milling and extrusion operations. The processing problems have been overcome in many cases by treatment of the polymer, changes in compounding formulations, or by blending with other rubbers. Blends of natural rubber and high-cis polybutadiene are particularly attractive. The presence of natural rubber alleviates the processing difficulties and improves tensile and tear properties of the polybutadiene while the latter improves abrasion resistance and complements the already good hysteresis properties of natural rubber. Processibility can also be improved by the use of higher levels of reinforcing fillers and oils than normally employed. Consideration must also be given to changes in quality, but extension with 35 to 70 phr oil is possible with retention of properties suitable for tires and many rubber goods. Even in blends with natural rubber, where processing is not a problem, increases in carbon black and oil content have proved practical and often desirable. Applications For most uses, vulcanization is necessary to develop the desired strength and elastic qualities. The polymer chain is unsaturated and can be readily vulcanized with sulfur (in conjunction with the usual activators) or with other cross-linking agents such as peroxides. With some exceptions, admixture of the rubber with

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a reinforcing pigment is required to obtain high strength. Antioxidants or antioxidants should be used for most applications. Lower than normal sulfur levels provide a better balance of properties in many instances, particularly with blends of high-cis polybutadiene and natural rubber. In large tires, the use of blends of high-cis polybutadiene and natural rubber in treads improves both abrasion resistance and resistance to tread groove cracking compared to natural rubber alone. Substitution of high-cis polybutadiene for a portion of the natural rubber in the tire body has improved resistance to blowout or other heat failures (in some cases to a remarkable degree). The high-cis polybutadienes are also used in increasing quantity in blends with SBR for the production of tires for passenger cars and small trucks. In this use, advantages include improved resistance to abrasion and cracking as well as adaptability to extension with large amounts of oil and carbon black. As indicated, properties of high-cis polybutadiene are well suited for tire use, and this is expected to be the major application. Its use should be considered in other areas where high resilience, resistance to abrasion, or low-temperature resistance is required. Sponge stocks, footwear, gaskets and seals, and conveyor belts are possible applications. In shoe heels high-cis polybutadiene may be used to provide good resilience and better abrasion resistance than realized with other commonly used rubbers. In shoe soles designed to be soft and resilient, high-cis polybutadiene has been used as a partial or total replacement for natural rubber to maintain high resilience, improve abrasion resistance, and to provide better resistance to crack growth. An important use for high-cis polybutadiene is in blends with other polymers to improve low-temperature properties. Replacement of 35% of an acrylonitrile (nitrile) rubber in a compound with cis-polybutadiene can reduce the brittleness failure temperature from –40°C to –55°C. Similarly, it is possible to reduce the brittle point of neoprene compounds by some –6.7°C with the substitution of cis-polybutadiene for one fourth of the neoprene rubber. Such substitutions usually reduce resistance to


swelling in hydrocarbons but this effect can be lessened by compounding with oil-resistant resins or by using cis-polybutadiene as a replacement for plasticizer in the compound rather than as a replacement for the polymer. In many cases, of course, oil resistance is not required and cis-polybutadiene may be blended with various polymers to give low-temperature properties approaching those of special arctic rubbers such as butadiene–styrene copolymers with low styrene content. Partial substitution of cis-polybutadiene for other rubbers used in light-colored or blackreinforced mechanical goods can provide a product that displays better snap. Also, resilience, abrasion resistance, and low-temperature properties are usually improved; tensile strength and tear strength may be reduced but show less decline after aging. High-cis polybutadienes have been used successfully as base components of caulking compounds and sealants. Another use is as a base polymer for graft polymerization of styrene to produce high-impact polystyrene. Commercially available cis-polybutadiene rubbers are supplied in talc form. The rubber contains a small amount of antioxidant to provide stability during storage.

HIGH-TRANS POLYBUTADIENE In contrast to the soft, rubbery nature of cispolybutadiene, polybutadiene of high trans content is a hard, horny material at room temperature. It is thermoplastic and thus can be molded without addition of vulcanization agents. The softening point, hardness, and tensile strength increase with increasing trans content. A polymer of approximately 90% trans content, for example, displays a softening point near 93°C, Shore A hardness of about 98, and tensile strength in excess of 6.8 MPa without addition of other ingredients. High-trans polybutadiene can be readily vulcanized with sulfur or peroxides. It remains a hard, partially thermoplastic material when lightly vulcanized but can be made rubbery by increasing the curative level. Tensile strength is increased by the addition of reinforcing pigments (carbon black, silica, or clay). Compounded in such a manner,


high-trans polybutadienes are characterized by high modulus, tensile strength, elongation, and hardness, by moderate resilience, and by excellent abrasion resistance. In vulcanized stocks, properties such as hardness, resilience, and heat buildup can be modified. Uses Applications include those where balata has been used such as golf ball covers and wire and cable coverings. Properties have also been demonstrated to be suitable for shoe soles (high hardness, good abrasion resistance), floor tile (high hardness, low compression set), gasket stocks, blown sponge compounds, and other molded or extruded items. High-trans polybutadiene requires processing temperatures above its softening point but is relatively easy to mix, mill, or extrude at these temperatures. Care should be taken to avoid scorch or precure when handling stocks containing curatives at elevated temperatures.

POLYBUTADIENE CONTENT

OF INTERMEDIATE CIS

This type of product has a structure of approximately 40% cis, 50% trans, and 10% (or less) vinyl content. The raw polymer is soft and somewhat waxlike in character. This polybutadiene displays high resilience, high dynamic modulus, and a low brittle point. Processing difficulties in milling and extrusion operations may be encountered and it is usually recommended that polybutadienes of intermediate cis content be used in blends with other rubbers. In tire treads substitution of 40% cis-polybutadiene for a portion of the natural rubber or SBR 1500 improves resilience and abrasion resistance and gives some reduction in operating temperature. Other applications include products where similar changes in properties to those above are desired or in products where improvements in low-temperature properties are desired.


LIQUID POLYMERS Liquid polybutadienes can be prepared in most of the systems used to make solid rubbers. Such polymers may be cross-linked with chemicals or solidified with heat. One type of liquid polybutadiene has been manufactured on a small scale utilizing sodium catalyst. Uses for this type polymer include coatings, binders, adhesives, potting agents, casting and laminating resin, or vulcanizable plasticizer for rubber.

POLYCARBONATES Polycarbonate resins offer a combination of properties that extends the usefulness and fields of application for thermoplastic materials. This relatively new plastic material is characterized by very high impact strength, superior heat resistance, and good electrical properties. In addition, the low water absorption, high heat distortion point, low and uniform mold shrinkage, and excellent creep resistance of the material result in especially good dimensional stability. Of value for many applications is the its transparency, shear strength, stain resistance, colorability and gloss, oil resistance, machinability, and maintenance of good properties over a broad temperature range from less than –73°C to 132 to 138°C. The fact that polycarbonate resin is self-extinguishing is important in many applications. Polycarbonates are amorphous engineering thermoplastics that offer exceptional toughness over a wide temperature range. The natural resins are water-clear and transparent. Polycarbonate resins are available in general-purpose molding and extrusion grades and in special grades that provide specific properties or processing characteristics. These include flame-retardant formulations as well as grades that meet Food and Drug Administration regulations for parts used in food-contact and medical applications. Other special grades are used for blow-molding, weather and UV-resistance, glass-reinforcement, EMI, RFI, ESD-shielding, and structural-foam applications. Polycarbonate is also available in extruded sheet and film.

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COMPOSITION The polycarbonate name is taken from the carbonate linkage that joins the organic units in the polymer. This is the first commercially useful thermoplastic material that incorporates the carbonate radical as an integral part of the main polymer chain. There are several ways polycarbonates can be made. One method involves a bifunctional phenol, bisphenol A, which combines with carbonyl chloride by splitting out hydrochloric acid to give a linear polymer consisting of bisphenol groups joined together by carbonate linkages. Bisphenol A, which is the condensation product of phenol and acetone, is the basic building block used also in the preparation of epoxy resins. Other members of the polycarbonate family may be made by using other phenols and other ketones to modify the isopropylidene group, or to replace this bridge entirely 1)'N' other radicals Subatttutiotr on the benzene ring offer further possibilities for variations.

PROPERTIES

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Polycarbonate is a linear, low-crystalline, transparent, high-molecular-weight plastic. It is generally considered to be the toughest of all plastics. In thin sections, up to about 0.478 cm, its impact strength is as high as 24 kg.m. In addition, polycarbonate is one of the hardest plastics. It also has good strength and rigidity, and, because of its high modulus of elasticity, is resistant to creep. These properties, along with its excellent electrical resistivity, are maintained over a temperature range of about –170 to 121°C (see Table P.2). It has negligible moisture absorption, but it also has poor solvent resistance and, in a stressed condition, will craze or crack when exposed to some chemicals. It is generally unaffected by greases, oils, and acids. Polycarbonate plastics are easily processed by extrusion, by injection, blow, and rotational molding, and by vacuum forming. They have very low and uniform mold shrinkage. With a white light transmission of almost 90% and high impact resistance, they are good glazing materials. They have more than 30 times the impact resistance of safety glass.


Other typical applications are safety shields and lenses. Besides glazing, the high impact strength of polycarbonate makes it useful for air-conditioner housings, filter bowls, portable tool housings, marine propellers, and housings for small appliances and food-dispensing machines. Humidity changes have little effect on dimensions or properties of molded parts. Even boiling water exposure does not change dimensions more than 0.30 mm/mm after parts are returned to room temperature. Creep resistance is excellent throughout a broad temperature range and is improved by a factor of 2 to 3 in glass-reinforced compounds. The insulating and other electrical characteristics of polycarbonate are excellent and almost unchanged by temperature and humidity conditions. One exception is arc resistance, which is lower than that of many other plastics. Polycarbonates are generally unaffected by greases, oils, and acids. Nevertheless, compatibility with specific substances in a service environment should be checked with the resin supplier. Water at room temperature has no effect, but continuous exposure in hot (65°C) water causes gradual embrittlement. The resins are soluble in chlorinated hydrocarbons and are attacked by most aromatic solvents, esters, and ketones, which cause crazing and cracking in stressed parts. Grades with improved chemical resistance are available, and special coating systems can be applied to provide additional chemical protection.

FABRICATION Polycarbonate resin has been molded in standard injection equipment using existing molds designed for nylon, polystyrene, acrylic, or other thermoplastic materials. Differences in mold shrinkage must be considered. And, in fabrication, the polycarbonate does have its own unique processing characteristics. Most important among these are the broad plastic range and high melt viscosity of the resin. Production runs in molds designed for nylon or acetal resin are not recommended. Like other amorphous polymers, polycarbonate resin has no precise melting point. It softens and begins to melt over a range from


TABLE P.2 Properties of Polycarbonates ASTM or UL Test

General Purpose

Property

Physical 1.2 23 0.15

D792 D792 D570


D638 D638 D790 D790 D256 D671 D785

Mechanical Tensile strength (psi) 9,000–10,500 Elongation (%) 110–125 Flexural strength (psi) 11,000–15,000 Flexural modulus (105 psi) 3.0–3.4 Impact strength, Izod (ft-lb/in. of notch) 12–16 Fatigue endurance limit, 107 cycles (psi) 1,000 Hardness, Rockwell M 62–70

C177 D696 D648

UL94

D149

Thermal conductivity (Btu-in./h-ft2-°F) Coefficient of thermal expansion (10–5 in./in.-°C) Deflection temperature (°F) At 264 psi At 66 psi Flammability rating

D495

Dielectric strength (V/mil) Short time, 1/8-in. thk Dielectric constant At 1 kHz Dissipation factor At 1 kHz Volume resistivity (Ω-cm) At 73°F, 50%RH Arc resistance

D542 D1003

Refractive index Transmittance (%)

—


D150 D150 D257

Thermal 1.35 6.6–7.0

260–270 280 HB, V-0 Electrical 380–400

High Flexural Modulus

20% Glass Reinforced

1.25 22.2 0.12

1.35 20.5 0.16

8,000–9,600 10–20 15,000 5.0 2 2,000 85

16,000 4–6 19,000 8.0 2 5,000 91

1.41 3.2

1.47 2.7

288 295 V-2, V-0

295 300 V-2, V-0

450

490

3.02

—

—

0.0021

—

—

>1016 10–120 Optical 1.586 85–89

>1016 5–120

>1016 5–120

— —

— —

— —

— —

Frictional

a

0.52 0.39

A + 1 MHz.


216 to 227°C. Optimum molding temperatures lie above 271°C. The most desirable range of cylinder temperatures for molding the resin is in the area of 275 to 316°C.


Mold design must take into consideration the high melt viscosity of the material. Large sprues, large full-round runners, and generous gates with short lands usually give best results.

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Tab gating is good for filling large thin sections. The gate to the tab should be large. In injection molding polycarbonate, the following conditions are desirable:

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1. Heated molds. Generally, hot water heat is adequate for heating molds, with typical mold temperatures ranging from 77 to 93°C. Molds for large areas, thin sections, or complex shapes or multiple cavity molds with long runners may require higher temperatures. Some molds have run best at 110 to 121°C. 2. Cylinder temperatures. The most usual cylinder temperature for molding polycarbonates is in the range of 275 to 316°C. Few parts require cylinder temperatures above 316°C. A heated nozzle and adequate mold temperature are helpful in keeping cylinder temperatures below 316°C. In most cases rear cylinder temperatures higher than front cylinder temperatures give best results. 3. Heated nozzle. In general, nozzle temperature equal to front cylinder temperature gives good results. 4. Adequate injection pressure. Injection pressures used in molding polycarbonate resin range from 80 to 204 MPa. Most usual range is in the 103 to 136 MPa range. Typical pressure setting is 3/4 to full pressure capacity of the press. 5. Fast fill time. For most parts molded, a fast ram travel time has been found desirable for thick as well as thin sections. For very thick sections, it is better to utilize somewhat slower ram speed. Polycarbonate must be well dried to obtain optimum properties in the molded part. For this reason the resin is packaged in sealed containers. Preheating pellets in the can to 121°C for 4 to 8 h and using of hopper heaters at 121°C are recommended for production operations to prevent moisture pickup. Although polycarbonates are fabricated primarily by injection molding, other fabricating © 2002 by CRC Press LLC

techniques may be used. Rod, tubing, shapes, film, and sheet may be extruded by conventional techniques. Films and coatings may be cast from solution. Parts can readily be machined from rod or standard shapes. Cementing, painting, metalizing, heat sealing, welding, machining operations, and other standard finishing operations may be employed. Film and sheet can be vacuum-formed or cold-formed.

APPLICATIONS The properties of polycarbonate resin make this new plastic suitable for a wide variety of applications. It is now being used in business machine parts, electrical and electronic parts, military components, and aircraft parts, and is finding increasing use in automotive, instrument, pump, appliance, communication equipment, and many other varied industrial and consumer applications. One of the applications is in molded coil forms, which take advantage of the electrical properties, heat and oxidation resistance, dimensional stability, and resistance to deformation under stress of the resin. A transparent plastic with the heat resistance, the dimensional stability, and the impact resistance of polycarbonate resin has created considerable interest for optical parts, such as outdoor lenses, instrument covers, lenses, and lighting devices. Housings make use of the impact resistance of the material and its attractive appearance and colorability. In many cases, also, heat resistance and dimensional stability are important. An interesting application area for plastic materials is the use of polycarbonate resin for fabricating fasteners of various types. Such uses as grommets, rivets, nails, staples, and nuts are in production or under evaluation. The ability of polycarbonate parts to be coldheaded has developed considerable interest in rivet applications. Terminal blocks, connectors, switch housings, and other electrical parts may advantageously be molded of polycarbonate resin to take advantage both of the electrical properties of the material and the unusual physical properties, which give strength and toughness to the parts over a range of temperatures. Because of


the heat resistance of polycarbonate, and the fact that it is self-extinguishing, molded parts can be used in current-carrying support applications. Another application is in bushings, cams, and gears. Here, dimensional stability is important as is the high impact strength of the resin. Good physical properties over a broad temperature range, low water absorption, resistance to deformation under load, and resistance to creep suggest its use for many applications of this type. However, the resin has a higher coefficient of friction and a lower fatigue endurance limit than do some other plastics used in these types of applications. For this reason, it should not be considered a general-purpose gear and bearing material, but might be considered for applications subject to light loading, or to heavier but intermittent loading. In most heart-bypass operations, the saphenous vein from a patient’s leg replaces blocked blood vessels in the heart. The separate surgical procedure, performed during the heart-bypass operation, involves removing the vein through a long incision. Following the surgery, patients frequently complain of ongoing leg pain, potentially leading to reduced mobility and delayed rehabilitation while the large incision heals. A new generation of surgical instruments not only makes the procedure less invasive, it helps speed the patient’s return to normal activities. The system uses endoscopic techniques to harvest the vein. This, in turn, requires smaller incisions. The potential benefits are less postoperative pain, fewer wound-healing complications, minimal scarring, and quicker recovery. The materials chosen for these applications have provided several significant benefits and, from the medical side, the materials were biocompatible. The materials resist chemicals and withstand gamma sterilization. The balloon mount is made with polycarbonate. On the orbital dissection cannula, the end piece embodies a polycarbonate resin.

POLYESTER FILM Polyester film is a transparent, flexible film, ranging from 0.15 to 14 mils in thickness, used as a product component, in industrial processes, and for packaging. The most widely used type


is produced from polyethylene terephthalate (i.e., Mylar). Polyester films based on other polymers or copolymers or manufactured by other methods are not identical, although they are similar in nature. It is the strongest of all plastic films and strength is probably the outstanding property. However, it is useful as an engineering material because of its combination of desirable physical, chemical, electrical, and thermal properties. For example, strength combined with heat resistance and electrical properties makes it a good material for motor slot liners. Polyester film is made by the condensation of terephthalic acid and ethylene glycol. The extremely thin film, 0.00063 to 0.0013 cm, used for capacitors and for insulation of motors and transformers, has a high dielectric strength, up to 236 × 106 V/m. It has a tensile strength of 137 MPa with elongation of 70%. It is highly resistant to chemicals, and has low water absorption. The material is thermoplastic, with a melting point at about 254°C. Polyester fibers are widely used in clothing fabrics. The textile fiber produced from dimethyl terephthalate is known as Dacron. For magnetic sound-recording tape, polyester tape has the molecules oriented by stretching to give high strength. The 0.013-cm tape has a breaking strength of 3.4 kg/0.64 cm of width. Electronic tape may also have a magnetic-powder coating on the polyester. But where high temperatures may be encountered, as in spacecraft, the magnetic coating is applied to metal tapes.

PROPERTIES Polyester film has excellent resistance to attack and penetration by solvents, greases, oils, and many of the commonly used electrical varnishes. At room temperature, permeability to such solvents as ethanol, ethyl acetate, carbon tetrachloride, hexane, benzene, acetone, and acetic acid is very low. It is degraded by some strong alkali compounds and embrittles under severe hydrolysis conditions. Moisture absorption is less than 0.8% after immersion for a week at 25°C. Water-vapor permeability is similar to that of polyethylene film and permeability to gases is very low. The

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film is not subject to fungus attack and copper corrosion is negligible. An outstanding feature of the film is the fact that good physical and mechanical properties are retained over a wide temperature range. Service temperature range is –60 to 150°C. The effect of temperature is relatively small between –20 and 80°C. No embrittlement occurs at temperatures as low as –60°C, and useful properties are retained up to 150 to 175°C. Tensile modulus drops off sharply at 80 to 90°C. Melting point is 250 to 255°C; thermal coefficient of expansion is 15 × 10–6 in./in./°F; and shrinkage at 150°C is 2 to 3%.

FABRICATION

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AND

FORMS

Polyester film can be printed, laminated, metallized, coated, embossed, and dyed. It can be slit into extremely narrow tapes (0.038 mm and narrower), and light gauges can be wound into spiral tubing. Heavy gauges can be formed by stamping or vacuum (thermo-)forming. Matte finishes can be applied, and adhesives for bonding the film to itself and practically any other material are available. Because of its desirable thermal characteristics, polyester film is not inherently heat sealable. However, some coated forms of the film can be heat-sealed, and satisfactory seals can be obtained on the standard film by the use of benzyl alcohol, heat, and pressure. Polyethylene terephthalate film is available in several different types: A. General-purpose and electrical film for wide variety of uses C. Special electrical applications requiring high insulation resistance D. Highly transparent film, minimum surface defects K. Coated with a polymer for heat sealability and outstanding gas and moisture impermeability HS. Shrinks uniformly about 30% when heated to approximately 100°C; after shrinking, it has substantially the same characteristics as the standard film T. A film with high tensile strength (available in some thin gauges) with


superior strength characteristics in the machine direction; designed for use in tapes requiring high-strength properties W. For outdoor applications; resistant to degradation by ultraviolet light

APPLICATIONS The range of properties outlined above has made polyester film functional in many totally different industrial applications and suggests its use in numerous other ways. The largest current user of the film is the electrical and electronic market, which uses it as slot liners in motors and as the dielectric for capacitors, replacing other materials that are less effective, bulkier, and more expensive. It is found in hundreds of wire and cable types, sometimes used primarily as an insulating material, sometimes for its mechanical and physical contributions to wire and cable construction. Reduced cost of materials and processing and improved cable performance result. Magnetic recording tapes for both audio and instrumentation uses are based on polyester film. In audio applications they contribute toughness, durability, and long play; for instrumentation tapes, the film ensures maximum reliability. The film has proved to be a highly successful new material for the textile industry since it can be used to produce metallic yarns that are nontarnishing and unusually strong. They can be run unsupported, knit, dyed at the boil, and either laundered or dry-cleaned. Yarns are made by laminating the film to both sides of aluminum foil or by laminating metallized and transparent film. The structure is then slit into the required yarn widths. As a surfacing material, polyester film is used on both flexible and rigid substrates for both protective and decorative purposes. Metallized, laminated to vinyl, and embossed, the film becomes interior trim for automobiles, for example. With a special coating, the film becomes a drafting material that is tougher and longerlasting than drafting cloth. It is used for mapmaking, templates, and other applications in which its dimensional stability becomes a significant factor.


Strength in thin sections makes the film advantageous for sheet protectors, card holders, sheet reinforcers, and similar stationery products. Pressure-sensitive adhesives make the properties of the film available for uses ranging from decorative trim to movie splicing. The weatherable form (Type W) has a life of 4 to 7 years. It is principally used in greenhouses, where it cuts construction costs by as much as two thirds because a simple, inexpensive structure suffices and maintenance costs are at a minimum. In the packaging field, polyester film serves in areas where other materials fail or have functional disadvantages. In window cartons it lasts longer and does not break as other materials do; its toughness permits transparent packaging of heavy items; and, coated with polyethylene, it has made possible the “heat-in-the-bag” method of frozen-food preparation. An optically clear form of polyester film in thicknesses of 4 to 7 mils is used as a base for the coating of light-sensitive emulsions in the manufacture of photographic film. The outstanding qualities of toughness and dimensional stability make this film especially well suited as a base for graphic arts films, motion picture film, engineering reproduction films, and microfilm. Other advantages include excellent storage and aging characteristics.

POLYESTER PLASTICS The materials are commonly called polyester resins but this simple name does not distinguish between at least two major classes of commercial materials. Also, the same name is used within the unsaturated class to designate both the cured and uncured state. These plastics may be defined by identifying the materials as “unsaturated polyester resins which, when cured, yield thermoset products as opposed to thermoplastic products.” The latter, as exemplified by Dacron and Videne, are saturated polyesters.

COMPOSITION Unsaturated polyester resins of commerce are composed of two major components, a linear, unsaturated polyester and a polymerizable monomer. The former is a condensation-type of


polymer prepared by esterification of an unsaturated dibasic acid with a glycol. Actually, most polyesters are made from two or more dibasic acids. The most commonly used unsaturated acids are maleic or fumaric with limited quantities of others used to provide special properties. The second acid does not contain reactive unsaturation. Phthalic anhydride is most commonly used but adipic acid and, more recently, isophthalic acid are employed. The properties of the final product can be varied widely, from flexible to rigid, by changing the ratios and components in the polyester portion of the resin. The polyesters vary from viscous liquids to hard, brittle solids but, with a few exceptions, are never sold in this form. Instead, the polyester is dissolved in the other major component, a polymerizable monomer. This is usually styrene; diallyl phthalate and vinyl toluene are used to a lesser extent. Other monomers are used for special applications. Polyester resins may contain numerous minor components such as light stabilizers and accelerators, but must contain inhibitory components so that storage stability is achieved. Otherwise, polymerization will take place at room temperature. The most widely used polyester resins are supplied with viscosities in the range of 300 to 5000 centipoise. They are clear liquids varying in color from nearly water-white to amber. They can be colored with certain common pigments, which are available ground in a vehicle for ease of dispersion. Inert, inorganic fillers such as clays, talcs, calcium carbonates, etc. are often added, usually by the fabricator, to reduce shrinkage and lower costs. Resins with thixotropic properties are also available.

CURING The liquid resins are cured by the use of peroxides with or without heat to form solid materials. During the cure the monomer copolymerizes with the double bonds of the unsaturated polyester. The resulting copolymer is thermoset and does not flow easily again under heat and pressure. Heat is evolved during the cure. This must be considered when thick sections are made. Also, a volume shrinkage occurs and the

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density increases to 1.20 to 1.30. The amount of shrinkage varies between 5 to 10% depending on monomer content and degree of unsaturation in the polyester. The most popular catalyst is benzoyl peroxide but methylethyl ketone peroxide, cumene hydroperoxide, and others find application.

REINFORCEMENT

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The development of this field of commercial resins owes a great deal to the commercial production of two other products: The first was the production of low-cost styrene for the synthetic rubber program. Other cheap monomers will work but not nearly as well. Styrene and its homologues have relatively high boiling points and fast polymerization rates. Both properties are important in this field. The second development was the commercial production of fibrous glass. Polyester resins are not widely used as cast materials nor are the physical properties of such castings particularly outstanding. The unique property of polyesters is their ability to change from a liquid to a hard solid in a very short time under the influence of a catalyst and heat. This property was not available in any of the earlier plastic materials. Polyesters flow easily in a mold with little or no pressure so that expensive, high-pressure molds are not required. Alternatively, very large parts can be made because the total pressure required to form the material is low. Glass fibers of fine diameter have high tensile strengths, good electrical resistance properties, and low specific gravity when compared to metals. When such fibers are used as reinforcement for polyester resins (like steel in concrete) the resulting product possesses greatly enhanced properties. Specific physical properties of the polyester resins may be increased by a factor varying from 2 to 10. Naturally the increase in physical strengths obtained will depend upon both the amount of glass fibers used and their form. Strengths approaching those of metals on an equal weight basis are obtained with some constructions.


FABRICATION TECHNIQUES Hand Layup This method involves the use of either male or female molds. Products requiring highest physical properties are made with glass cloth. The cloth may be precoated with resin but usually one ply is laid on or in the mold, coated with resin by brushing or spraying, and then a second ply of cloth laid on top of the first. The process is continued until the desired thickness is built up. Aircraft parts, such as radomes, usually require close tolerances on dimensions and resin-to-glass weight ratio. After the layup has been completed, pressure is applied either by covering the assembly with flexible, extensible blanket and drawing it down by vacuum or the mold is so made that a rubber bag can be contained above the part. This is then blown up to apply pressure on the laminate. Thereafter, the cure is accomplished by heating in an oven, by infrared lamps, or by heating means built into the mold. Corrugated Sheet Molding Glass-reinforced polyester sheets are sold in large volume and are made by a relatively simple intermittent or continuous method. The process consists of placing a resin-impregnated mat between cellophane sheets, rolling or squeegeeing out the air, placing the assembly between steel or aluminum molds of the desired corrugation, and curing the assembly in an oven. Matched Die Molding This method produces parts rapidly and generally of uniform quality. The molds used are somewhat similar to those employed in compression molding, usually of two-piece, mating construction. The process consists of two steps. A “preform” of glass fibers is prepared by collecting fibers on a screen, which has the shape of the finished article. Suction is used to hold the fibers on the screen; fibers are either blown at it or fall on it from a cutter. Commercial equipment is available for this operation. When the desired weight of glass fibers has been collected, a resinous binder is applied. The preform


and screen are then baked to cure the binder, after which the preform is ready for use. The second step is the actual molding. The preform is placed in the mold, the catalyzed resin poured on the preform in correct amount, the mold closed, and the cure effected by heat. Design of the mold is very important and some features are different from those in other molding fields. Molding cycles vary from 2 to 5 min at temperatures from 110 to 149°C. Trimming, sanding, and buffing are usually required at the flash line. Premix Molding This branch of the field provides parts for automotive and similar end uses. The parts are strictly functional and are usually pigmented black. The strength properties required are relatively low except that impact strength must be good. As the name infers, the unsaturated resin is first mixed with fillers, fibers, and catalyst to provide a nontacky compound. The mixers used are of the heavy-duty Day or Baker-Perkins type. The “premix” is usually extruded to provide a “rope” or strip of material easily handled at the press. The fibers used most extensively are cut sisal, but glass and asbestos are also used, and frequently all three are present in a compound. The fillers are clays, carbonates, and similar cheap inorganic materials. A typical premix will contain 38% catalyzed unsaturated resin, 12% total fiber, and 50% filler. However, rather wide variations in composition are practiced to obtain specific end use properties. The premix is molded at pressures of 103 to 350 MPa and at temperatures ranging from 121 to 154°C. Cure cycles are short, usually from 30 to 90 s. Again, the fact that the resin starts as a liquid makes possible the molding of intricate parts because of ease of flow in the mold. Heater housings for autos are the largest use, but housings of many types and electrical parts are produced in volume. This process provides a cheap but very serviceable molded material.

PROPERTIES The strengths obtainable in the finished product are of prime interest to the engineer. However,


the numerous forms of reinforcement materials and the variations possible in the polyester constituent present a whole spectrum of obtainable properties. As an example, products are available that will resist heat for long periods of time at temperatures varying from 66 to 177°C; but the higher the temperature, the more expensive the resin. In general, the commercial resins have good electrical properties and are resistant to dilute chemicals. Alkali resistance is poor, as is resistance to strong acids. The strength-to-weight ratio of polyester parts and their impact resistance are outstanding physical properties. The data in Table P.3 (General Properties of Polyesters) are intended to be illustrative of the properties obtainable by different fabrication techniques.

AVAILABLE FORMS Probably over 95% of the unsaturated polyester resins sold are liquids in the uncured state. However, certain types are available in solid or paste form for special uses. The cured resins are available in laminate form as corrugated sheeting, which is sold widely for partitions, windows, patio roofs, etc. Rod stock may be purchased for fishing rods and electrical applications. Paper and glass cloth laminates are sold to fabricators. The boat end use is making the material more familiar to the general public. However, a large part of the industrial production is concerned with custom-molded parts. These are perhaps best classified by industries rather than specific products. The aircraft industry uses substantial quantities but automotive end uses are larger. The chemical industry is using increasing amounts in fume ducts and corrosion-resistant containers. The electrical industry uses the material in laminate and molded forms and as an encapsulation medium. Furniture applications are growing. The machinery industry uses moldings as housings and guards; a recent volume use is motor boat shrouds. There are few fields that have not found the material useful in some application.

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TABLE P.3 General Properties of Polyesters Type of Reinforcement Glass content, % by wt Specific gravity range Rockwell Hardness Flexural strength, 1000 psi Tensile strength, 1000 psi Compressive strength, 1000 psi Flexural modulus, 106 psi Tensile modulus, 106 psi Impact, notched, ft-lb/in. notch Shear strength, 1000 psi Water absorption (24 h), % a

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None

Glass Cloth

Mat or Preform

Mata

Parallel Yarn or Roving

Premix

—

60–70

35–45

20–30

60–80

10–40b

1.20–1.30

1.7–1.9

1.5–1.6

1.4–1.5

1.7–1.95

1.6–1.9

M100–110 13–17

M100–110 40–85

M90–100 25–35

M85–95 15–25

M90–110 80–115

M55–75 5–20

8–12

30–55

15–25

10–15

70–100

3–6

18–23

20–45

17–28

20–25

50–75

10–16

0.50–0.60

2.0–3.0

1.0–1.8

0.8–1.5

3.0–6.0

0.8–1.2

0.45–0.55

1.8–3.0

0.8–1.6

0.7–1.4

3.0–6.0

0.8–1.2

0.17–0.25

15–30

10–20

6–10

—

1.5–3.0

—

15–25

12–18

8–12

—

—

0.15–0.25

0.10–0.20

0.2–0.5

0.2–0.4

0.15–0.30

0.3–1.0

Corrugated sheet-type laminate. b Total fiber content.

POLYESTER THERMOPLASTIC RESINS There are several types of melt-processible thermoplastics, including polybutylene terephthalate, polyethylene terephthalate, and aromatic copolyesters.

POLYBUTYLENE TEREPHTHALATE (PBT) This plastic material is made by the transesterification of dimethyl terephthalate with butanediol through a catalyzed melt polycondensation. These molding and extrusion resins have good resistance to chemicals, low moisture absorption, relatively high continuous-use temperature, and good electrical properties (track resistance and dielectric strength). PBT resin is sensitive to alkalies, oxidizing acids, aromatics, and strong bases. Various additives, fillers, and fiber reinforcements are used with PBT resins, in particular flame retardants, mineral fillers,


and glass fibers. PBT resins and compounds are used extensively in automotive, electrical-electronic, appliance, military, communications, and consumer product applications.

POLYETHYLENE TEREPHTHALATE (PET) This is a widely used thermoplastic packaging material. Beverage bottles and food trays for microwave and convection oven use are the most prominent applications. PET resins are made from ethylene glycol and either terephthalic acid or the dimethyl ester of terephthalic acid. Most uses for PET require the molecular structure of the material to be oriented. Orientation of PET significantly increases tensile strength and reduces gas permeability and water vapor transmission. For packaging uses, PET is processed by blow molding and sheet extrusion. Typical trade names are Cleartuf, Traytuf, Tenite, and Kodapak. Injection-molding grades of PET,


with fillers and/or reinforcements, also are available for making industrial products. Aromatic polyesters (liquid crystal polymers, or LCP) have high mechanical properties and heat resistance. Commercial grades are aromatic polyesters, which have a highly ordered or liquid crystalline structure in solution and molten states. A high degree of molecular orientation develops during processing and, hence, anistropy in properties. Typically, these meltprocessible resins can be molded or extruded to form products capable of use at temperatures over 260°C. Tensile strengths up to 240.3 MPa and flexural moduli up to 3586.3 MPa are reported for LCPs. Chemical resistance also is excellent. Trade names for LCPs include Vectra and Xydur Granlar. Applications are in chemical processing, electronic, medical, and automotive components.

POLYESTER THERMOSETTING RESINS These are a large group of synthetic resins produced by condensation of acids such as maleic, phthalic, or itaconic with an alcohol or glycol such as allyl alcohol or ethylene glycol to form an unsaturated polyester which, when polymerized, will give a cross-linked, three-dimensional molecular structure, which in turn will copolymerize with an unsaturated hydrocarbon, such as styrene or cyclopentadiene, to form a copolymer of complex structure of several monomers linked and cross-linked. At least one of the acids or alcohols of the first reaction must be unsaturated. The polyesters made with saturated acids and saturated hydroxy compounds are called alkyd resins, and these are largely limited to the production of protective coatings and are not copolymerized. The resins undergo polymerization during cure without liberation of water, and do not require high pressure for curing. Through the secondary stage of modification with hydrocarbons a very wide range of characteristics can be obtained. The most important use of the polyesters is as laminating and molding materials, especially for glass-fiber-reinforced plastic products. The resins have high strength, good chemical resistance, high adhesion, and


capacity to take bright colors. They are also used, without fillers, as casting resins, for filling and strengthening porous materials such as ceramics and plaster of paris articles, and for sealing the pores in metal castings. Some of the resins have great toughness, and are used to produce textile fibers and thin plastic sheet and film. Others of the resins are used with fillers to produce molding powders that cure at low pressure of 3 to 6 MPa with fast operating cycles.

POLYESTER LAMINATES These are usually made with a high proportion of glass-fiber mat or glass fabric, and highstrength reinforced moldings may also contain a high proportion of filler. A resin slurry may contain as high as 70% calcium carbonate or calcium sulfate, with only about 11% of glass fiber added, giving an impact strength of 165 MPa in the cured material. Bars and structural shapes of glass-fiber-reinforced polyester resins of high tensile and flexural strengths are made by having the glass fibers parallel in the direction of the extrusion. Rods and tubes are made by having the glass-fiber rovings carded under tension, then passing through an impregnating tank, an extruding die, and a heat-curing die. The rods contain 65% glass fiber and 35% resin. They have a flexural strength of 441 MPa and a Rockwell M hardness of 65. Physical properties of polyester moldings vary with the type of raw materials used and the type of reinforcing agents. A standard glassfiber-filled molding may have a specific gravity from 1.7 to 2.0, a tensile strength of 27 to 68 MPa with elongation of 16 to 20%, a flexural strength to 206 MPa, a dielectric strength to about 16 × 106 V/m, and a heat distortion temperature of 177 to 204°C. The moldings have good acid and alkali resistance. But, because an almost unlimited number of fatty-type acids are available from natural fatty oils or by synthesis from petroleum, and the possibilities of variation by combination with alcohols, glycols, and other materials are also unlimited, the polyesters form an ever-expanding group of plastics. Some of the polyester-type resins have rubberlike properties, with higher tensile strengths than the rubbers and superior resistance to

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oxidation. These resins have higher wear resistance and chemical resistance than GRS rubber. They are made by reacting adipic acid with ethylene glycol and propylene glycol and then adding diisocyanate to control the solidifying action. They can be processed like rubber, but solidify more rapidly.

POLYETHERETHERKETONES The latest developments in polyetheretherketone (PEEK) resin formulations let manufacturers take advantage of its wear and strength qualities for pump applications.

USES

P

Pumps with mating components made of cast iron, stainless steel, and bronze have long been a source of problems for manufacturers and end users. The abrasive quality of metals leads to significant wear in pump parts, which frequently results in failures from galling and seizing. As anyone working in a manufacturing environment knows, one’s worst nightmare can come true when a pump runs dry or a bearing fails, shutting down an entire assembly line or plant operation. And the costs are hardly trivial for repairing all of the pump components destroyed or carrying out periodic maintenance on the equipment to head off such a nightmare. Engineers are starting to replace metals with advanced plastics for pump components such as bushings, line shaft bearings, and case and impeller wear rings. The resins they use are based on PEEK chemistry and were initially developed for aerospace and defense applications. Using these engineered resins, pump manufacturers improve performance, boost output, and cut costs by taking advantage of better wear and friction qualities, mechanical strength, and the ability to resist chemicals. The chemical makeup of PEEK resins helps ease engineers’ concerns about galling and seizing and are safer to work with. Taking advantage of these qualities, along with an understanding of some basic design guidelines, opens new doors in pump performance and reliability.


TYPES, FABRICATION,

AND

PROPERTIES

Engineers often choose PEEK resin because of its overall balance of qualities, including hydrolysis resistance, dimensional stability, wear resistance, temperature stability, strength, and chemical resistance. However, switching from metal to plastic in pump parts requires engineers to change their mind-sets. PEEK can be injection-molded, compression-molded, or extruded into stock shapes, which are typically machined to final-part dimensions. Most pump components can be injection-molded as long as they do not require tight tolerances. The strict tolerances required for bushings and wear rings require parts to be machined from molded tube stock. Although manufacturers may be used to machining metals, speeds and feeds are much different for plastics, and the process generally calls for high-end carbide and diamond tooling. Machining operations vary depending on which fillers and reinforcements are used in each resin grade. Even though plastics can be machined more quickly than metals, it is not as easy to hold tolerance because plastics tend to spring when parted off. Tolerances in the range of ±0.05 mm can be held on parts with diameters up to 254 mm. The most common blends of PEEK resin used in pump applications are carbon filled and carbon-fiber reinforced. Carbon-filled PEEK compounds can be molded in solids or tubes up to 101.6 cm in diameter. Carbon-fiber reinforced resins, in contrast, are robotically wound and formed around a mandrel. This limits fiberreinforced resins to hollow shapes no smaller than 9.5 mm in diameter. Splitting hollow shapes is not recommended because the strength of the composite relies on the carbon fibers being molded and kept in tension. Another difference between carbon-filled and carbon-fiber-reinforced PEEK resins is how they react to temperature changes. Carbonfilled compounds have a radial thermal expansion rate of 16 × 10–6 in./in./°F, while that of fiber-reinforced resins is less than 3 × 10-6 in./in./°F. These two values fall on either side of the thermal expansion rate of carbon steel,


TABLE P.4 Properties of PEEK Composites

Specific gravity Temperature range, °F Tensile strength, psi Tensile modulus, psi Elongation, % Compressive strength, psi Flexural strength, psi Coefficient of friction Coefficient of thermal expansion from 73 ot 290°F, in./in.-°F Color

WR 300 Carbon-Filled PEEK

WR 525 Carbon-Fiber Reinforced PEEK

1.47 –100–300 17,875 1,560,000 1.41 15,000

1.60 –100–525 300,000 20,000,000 1.0 197,000

38,000 0.20–0.28 16 × 10–6

290,000 0.10–0.15 3000 — >2700 >2100 1600 1065 1500 1450 1238 706 1200 1062 942 530

— — — — — — — — — 5.3 5.7 6.9 — 4.5 5.3 5.5

Hole Mobilitya

Electron Mobilitya

Energy Band Gap at 300 K, ev

Light Mass, cm2/v-s

Heavy Mass, cm2/v-s

Light Mass, cm2/v-s

Heavy Mass, cm2/v-s

— 4.6 — — — 2.42 2.16 1.6 3.25 2.25 1.43 0.70 — 1.27 0.33 0.17

— — — — — — — — — — 8600–11,000 5000–40,000 — 4800–6800 33,000–40,000 78,000

— — — — — — — 180–230 — 120–300 1000 1000 — — — —

— — — — — — — — — — 3000 7000 — — 8000 12,000

— — 500–1000 — — — — 420–500 150–250 70–150 426–500 700–1200 — 150–200 450–500 750

Since InSb is the only III–V compound that has been prepared with an impurity concentration low enough that the characteristic lattice carrier mobility can be determined, the best experimental value is given followed by the theoretical estimate for higher-purity material. All mobility values are for 300 K.


TX66613_frame_S(17) Page 684 Wednesday, March 13, 2002 11:52 AM

TABLE S.2 III–V Compounds and Their Properties


TABLE S.3 Compound Semiconductors Formula I-V I-VI II-IV II-V II-VI

III-VI

IV-IV IV-VI V-VI AIBIIC2VI AIBIIIC2VI AIBVC2VI A3IBVC3VI A3IBVC4VI AIIBIVC2V Oxides

Typical Compounds KSb, K3Sb, CsSb, Cs3Sb, Cs3Bi Ag2S, Ag2Se, Cu2S, Cu2Te Mg2Si, Mg2Ge, Mg2Sn, Da2Si, Ca2Sn, Ca2Pb, MnSi2, CrSi2 ZnSb, CdSb, Mg3Sb2, Zn3As2, Cd3P2, Cd3As2 CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, MoTe2, RuTe2, MnTe2, BeS, MgS, CaS Al2S3, Ga2S3, Ga2Se3, Ga2Te3, In2Se3, In2Se3, In2Te3, GaS, GaSe, GaTe, InS, InSe, InTe SiC PbS, PbSe, PbTe, TiS2, GeTe Sb2S3, Sb2Se3, Sb2Te3, As2Se3, As2Te3, Bi2S3, Bi2Se3, Bi2Te3, Ce2S3, Gd2Se3 CuFeS2 C4AlS2, CuInS2, CuInSe2, CuInTe2, AgInSe2, AgInTe2, CuGaTe2 AgSbSe2, AgSbTe2, AgBiS2, AgBiSe2, AgBiTe2, AuSbTe2, Au(Sb,Bi)Te2 Cu3SbS3, Cu3AsS3 Cu3AsSe4 ZnSnAs2 SrO, BaO, MnO, NiO, Fe2O3, BaFe12O19, Al2O3, In2O3, TiO2, BeO, MgO, CaO, CdO, ZnO, SiO2, GeO2, ZnSiO3, MgWO3, CuO

milled, and granulated before being pressed or extruded into the required shape. The main firing cycle takes place at temperatures around 1250°C in a controlled atmosphere. As a result the parts are sintered into a dense, homogeneous ceramic. During sintering, the parts shrink some 20% in linear dimensions.

APPLICATIONS The technology and practical importance of semiconductor devices have been growing steadily in the past decade. The major applications of semiconductors can be divided into ten categories: diodes and transistors, luminescent devices, ferrites, special resistors, photovoltaic cells, infrared lenses and domes, thermoelectric devices, piezoelectric devices, xerography, and electron emission. The materials most


commonly used for these applications are listed in Table S.4. Indium antimonide, InSb, has a cubic crystal structure, and it is used for infrared detectors and for amplifiers in galvanomagnetic devices. Indium arsenide, InAs, also has a very high electron mobility, and is used in thermistors for heat-current conversion. It can be used to 816°C. Some materials can be used only for relatively low temperatures. Copper oxide and pure selenium have been much used in current rectifiers, but they are useful only at moderate temperatures, and they have the disadvantage of requiring much space. Indium phosphide, InP, has a mobility higher than that of germanium, and can be used in transistors above 316°C. Aluminum antimonide, AlSb, can be used at temperatures to 538°C. In lead selenide, PbSe, the mobility of the charge-carrying electrons decreases with rise in temperature, increasing resistivity. It is used in thermistors. Bismuth telluride, Bi2Te3, maintains its operating properties between –46 and 204°C, which is the most useful range for both heating and refrigeration. When doped as a p-type conductor it has a temperature difference of 601°C and an efficiency of 5.8%. When doped as an n-type conductor the temperature difference is lower, 232°C, but the efficiency within this range is more than doubled. Lead telluride, PbTe, has a higher efficiency, 13.5%, and temperature difference of 582°C, but it is not usable below 177°C, and is employed for conversion of the waste heat atomic reactors at about 371°C. Gallium arsenide has high electron mobility, and can be used as a semiconductor. Cadmium sulfide, CdS, is thus deposited as a semiconductor film for photovoltaic cells, or solar batteries, with film thickness of about 2 µm. When radioactive isotopes, instead of solar rays, are added to provide the activating agent, the unit is called an atomic battery, and the large area of transparent backing for the semiconductor is not needed. Manganese telluride, MnTe, with a temperature difference of 982°C, has also been used as a semiconductor. Many other materials can be used, and semiconductors with temperature differences at different gradients can be joined in series electrically to obtain a wider gradient,

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but the materials must have no diffusion at the junction. Cesium sulfide, CeS, has good stability and thermoelectric properties at temperatures to 1093°C, and has a high temperature difference, 1110°C. It can thus be used as a high-stage unit in conversion devices. High conversion efficiency is necessary for transducers, while a high

dielectric constant is desirable for capacitors. Low thermal conductivity makes it easier to maintain the temperature gradient, but for some uses high thermal conductivity is desirable. Silver–antimony–telluride, AgSbTe2, has a high energy-conversion efficiency for converting heat to electric current, and it has a very low thermal conductivity, about 1% that of germanium.

TABLE S.4 Semiconductor Applications and Materials Used (including some insulators) 1. Transistors, diodes, rectifiers, Transistors Diodes Switching diodes Varactor diodes Tunnel diodes Photodiodes Zenner diodes Microwave diodes Magnetodiodes Power rectifiers Varistors 2. Luminescent devices Electroluminescence Phosphers Lasers

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3. Ferrites Soft Permanent 4. Special resistors Thermistors Photoconductors Particle detectors Magnetoresistors Piezoresistors Cryosars Bokotrons Helicons Oscillators Chargistors 5. Photovoltaic and hall effect Photovoltaic cells Photoelectromagnetic cells Hall effect devices 6. Optical materials Infrared lenses and domes 7. Thermoelectric devices Generators Refrigerators 8. Piezoelectric devices 9. Xerography 10. Electron emission


and related devices Ge, Si, GaAs Ge, Si, GaAs Ge, Si, GaAs Ge, Si, GaAs, GaSb Ge, Si, GaAs Ge, Si, GaAs Ge, Si, GaAs InSb Cu2O, Se, Si, Ge SiC, Cu2O ZnS, ZnO ZnS, CdS, ZnO, ZnSiO3, MgWO3, SrWO4 Al2O3, CaF2, CaWO4, BaF2, SrMoO4, SrF2, LaF2, LaF3, As2S3, CaMoO4, KMgF3, (Ba,Mg)2P2O7 ZnO, MnO, NiO, Fe2O3 BaFe12O19 B, U3O8, Si, (NiMn)O2 Ge, Se, CdS, CdSe, GaAs, InSb, PbSe, PbTe CdS, diamond, Si, GaAs InSb, InAs Si, PbTe, BaSb Ge Ge Fe2S Ge, Si, InSb Ge, Si Se, Si, GaAs InSb, InAs InSb, InAs, GaAs Ge, Si, Se, Al2O3, As2S3, MgO, TiO2, SrTiO3 PbTe, Bi2Te3, ZnSb, GeTe, MnTe, CeS, Bi2Te3 Bi2Te3 BaTiO3, PbTiO3, (PbZr)TiO3, PbNbO2, CdS, GaAs Se, ZnO BaO, SrO


The semiconductor-type intermetals are also used in magnetic devices, since the ferroelectric phenomenon of heat conversion is the electrical analog of ferromagnetism. Chromium–manganese–antimonide is nonmagnetic below about 250°C and magnetic above the temperature. Various compounds have different critical temperatures. Below the critical temperature the distance between the atoms is less than that which determines the line-up of magnetic forces, but with increased temperature the atomic distance becomes greater and the forces swing into a magnetic pattern. Organic semiconductors fall into two major classes: well-defined substances, such as molecular crystals and crystalline complexes, isotatic and syndiotactic polymers; and disordered materials, such as atactic polymers and pyrolitic materials. Few of these materials have yet found commercial application. Amorphous silicon containing hydrogen is promising for use in solar cells because of its low cost and suitable electrical and optical properties.

SENSITIZING COMPOUNDS Supersensitizing compounds are metal salts in aqueous or organic solutions, which form an invisible film on the surface of glass and other ceramic surfaces. This film is not completely understood but is believed to be an ionizing or electronic effect that serves to initiate and hasten surface treatments such as silvering and plating. Supersensitizing refers to a second step involving the use of noble metal compounds, which further enhances the reduction properties of the metals about to be formed on the glass or ceramic surface. Sensitizing compounds include aluminum compounds (basic aluminum acetate, aluminum chloride, aluminum formoacetate, aluminum nitrate), barium salts, boron trichloride, cadmium compounds, iron sulfate, tin chloride, titanium sulfate, and triethanolamine titanate. Supersensitizing compounds include gold chloride, iridium salts, osmium compounds, palladium chloride, silver nitrate, and silver oxide.


SHAPE MEMORY ALLOYS These are a group of metallic materials that can return to some previously defined shape or size when subjected to the appropriate thermal procedure. That is, shape memory alloys (SMA) can be plastically deformed at some relatively low temperature and, upon exposure to some higher temperature, will return to their original shape. Materials that exhibit shape memory only upon heating are said to have one-way shape memory, whereas those that also undergo a change in shape upon recooling have a two-way memory. Typical materials that exhibit the shape memory effect include a number of copper alloy systems and the alloys of gold–cadmium, nickel–aluminum, and iron–platinum. A shape memory alloy may be further defined as one that yields a thermoelastic martensite, that is, a martensite phase that is crystallographically reversible. In this case, the alloy undergoes a martensitic transformation of a type that allows the alloy to be deformed by a twinning mechanism below the transformation temperature. A twinning mechanism is a herringbone structure exhibited by martensite during transformation. The deformation is then reversed when the twinned structure reverts upon heating to the parent phase.

TRANSFORMATION CHARACTERISTICS The martensite transformation that occurs in shape memory alloys yields a thermoelastic martensite and develops from a high-temperature austenite phase with long-range order. The martensite typically occurs as alternately sheared platelets, which are seen as a herringbone structure when viewed metallographically. The transformation, although a first-order phase change, does not occur at a single temperature but over a range of temperatures that is characteristic for each alloy system. There is a standard method of characterizing the transformation and naming each point in the cycle. Most of the transformation occurs over a relatively narrow temperature range, although the beginning and end of the transformation during heating and cooling actually extends over a much larger temperature range. The transformation also exhibits hysteresis in

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Heating for shape recovery

Cooling Austenite

Two-way shape memory Martensite (twinned)

Strain

Martensite (deformed)

FIGURE S.2 Microscopic view of shape memory process. (Aerospace Eng., April 2000, p. 26. With permission.)

that the transformation on heating and on cooling does not overlap. This transformation hysteresis varies with the alloy system.

THERMOMECHANICAL BEHAVIOR AND ALLOY PROPERTIES

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The mechanical properties of shape memory alloys vary greatly over the temperature range spanning their transformation. The only two alloy systems that have achieved any level of commercial exploitation are the nickel–titanium alloys and the copper-base alloys. Properties of the two systems are quite different. The nickel–titanium alloys have greater shape memory strain, tend to be much more thermally stable, have excellent corrosion resistance compared to the copper-base alloys, and have higher ductility. The copper-base alloys are much less expensive, can be melted and extruded in air with ease, and have a wider range of potential transformation temperatures. The two alloy systems thus have advantages and disadvantages that must be considered in a particular application.

MATERIALS The most common shape memory alloy material is Nitinol, an acronym for Ni (nickel)–Ti (titanium)–NOL (Naval Ordnance Laboratory). As suggested by the name, the material consists of approximately equal parts of nickel and titanium and was originally developed by the Naval Ordnance Laboratory.


Two different phases, martensite and austenite, are typically associated with the crystalline structure of shape memory alloys. The austenite phase is a highly ordered phase that occurs above a certain transition temperature. In this phase, the crystalline bonds must be at right angles with one another, as shown in the upper sketch in Figure S.2. The martensite phase occurs below the transition temperature, and right angle bonds are not required, as indicated by the twinned case in Figure S.2 (lower left). The lower right of the figure shows that strain is required to achieve a particular alignment, since the martensite phase is not in an ordered state. The term shape memory is derived from the fact that a shape memory alloy can recover its original shape when heated above a certain transition temperature. Before a shape memory alloy such as Nitinol can present shape memory behavior, it must first be trained. The training process is illustrated in the upper drawing of Figure S.3, in which the shape memory alloy is annealed while it is constrained in the shape that it is to memorize. From a microscopic view, this corresponds to the austenite phase. For the Nitinol wire, the annealing process corresponds to a contraction in the length of the wire. After annealing, the material is said to have a oneway shape memory. The Nitinol recovers its memorized shape when heated above the transition temperature and will remain in that shape upon cooling (lower left of Figure S.3). For the shape memory alloy to return to its initial shape, an external


Anneal at desired shape

Anneal training process

Initial shape (preheating martensite)

Final shape (austenite & post-heating martensite)

One-way shape memory

Martensite phase (cold)

Austenite phase (heated)

Two-way shape memory

FIGURE S.3 Macroscopic view of shape memory process. (Aerospace Eng., April 2000, p. 27. With permission.)

force must be applied to deform it. The two-way shape memory (lower right of Figure S.3) corresponds to a martensite phase to austenite phase by heating (shape recovery) followed by a mechanically produced deformation of the material upon cooling.

APPLICATIONS The manufacturing and the technology associated with both of the two commercial classes of shape memory alloys are quite different as are the performance characteristics. Therefore, in applications where a highly reliable product with a long fatigue life is desired, the nickel–titanium alloys are the exclusive materials of choice. Typical applications of this kind include electric switches and actuators. However, if high performance is not mandated and cost considerations are important, then the use of copper–zinc–aluminum shape memory alloys can be recommended. Typical applications of this kind include safety devices such as temperature fuses and fire alarms. Heating a shape memory alloy product to a temperature above some critical temperature is not recommended. The critical temperature for nickel–titanium is approximately 250°C, and for copper–zinc–aluminum approximately


90°C. Extended exposure to thermal environments above these critical temperatures results in an impaired memory function regardless of the magnitude of the load. Another technical consideration in the practical application of shape memory alloys pertains to fastening or joining these materials to conventional materials. This is a significant issue because shape memory alloys undergo expansions and contractions not encountered in traditional materials. Therefore, if shape memory alloys are welded or soldered to other materials, they can easily fail at the joint when subjected to repeated loading. Alloys of nickel–titanium and copper–zinc–aluminum can also be brazed by using silver filler metals; however, the brazed region can fail because of cyclic loading. It is therefore desirable to devise some other mechanism for joining shape memory alloys to traditional materials. Shape memory alloys cannot be plated or painted for similar reasons. Free Recovery In this case, a component fabricated from a shape memory alloy is deformed while martensitic, and the only function required of the shape memory is that the component return to its

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previous shape upon heating. A prime application is the blood-clot filter in which a nickel–titanium wire is shaped to anchor itself in a vein and catch passing clots. The part is chilled so it can be collapsed and inserted into the vein; then body heat is sufficient to return the part to its functional shape. Constrained Recovery The most successful example of this type of product is undoubtedly a hydraulic coupling. These fittings are manufactured as cylindrical sleeves slightly smaller than the metal tubing that they are to join. Their diameters are then expanded while martensitic and, upon warming to austenite, they shrink in diameter and strongly hold the tube ends. The tubes prevent the coupling from fully recovering its manufactrured shape, and the stresses created as the coupling attempts to do so are great enough to create a joint that, in many ways, is superior to a weld. Force Actuators

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In some applications, the shape memory component is designed to exert force over a considerable range of motion, often for many cycles. Such an application is the circuit-board edge connector. In this electrical connector system, the shape memory alloy component is used to force open a spring when the connector is heated. This allows force-free insertion or withdrawal of a circuit board in the connector. Upon cooling, the nickel–titanium actuator becomes weaker, and the spring easily deforms the actuator while it closes tightly on the circuit board and forms the connections. An example based on the same principle is a fire safety valve, which incorporates a copper–zinc–aluminum actuator designed to shut off toxic or flammable gas flow when fire occurs. Other Applications A number of applications are based on the pseudoelastic (or superelastic) property of shape memory alloys. Some eyeglass frames use superelastic nickel–titanium alloy to absorb large deformations without damage. Guide wires for steering catheters into vessels in the


body have been developed using wire fashioned of nickel–titanium alloy, which resists permanent deformation if bent severely. Arch wires for orthodontic correction also use this alloy. Shape memory alloys have found application in the field of robotics. The two main types of actuators for robots using these alloys are biased and differential. Biasing uses a coil spring to generate the bias force that opposes the unidirectional force of the shape memory alloy. In the differential type, the spring is replaced with another shape memory alloy, and the opposing forces control the actuation. A microrobot was developed with five degrees of freedom corresponding to the capabilities of the human fingers, wrist, elbow, and shoulder. The variety of robotic maneuvers and operations are coordinated by activating the nickel-titanium coils in the fingers and the wrist in addition to contraction and expansion of straight nickel–titanium wires in elbow and shoulders. Digital control techniques in which a current is modulated with pulse-width modulation are employed in all of the components to control their spatial positions and speeds of operation. In medical applications, in addition to mechanical characteristics, highly reliable biological and chemical characteristics are very important. The material must not be vulnerable to degradation, decomposition, dissolution, or corrosion in the organism, and must be biocompatible. Nickel–titanium shape memory alloys have also been employed in artificial joints such as in artificial hip joints. These alloys have also been used for bone plates, for marrow pins for healing bone fractures, and for connecting broken bones.

SHEET METAL FORMING This is the process of shaping thin sheets of metal (usually less than 6 mm) by applying pressure through male or female dies or both. Parts formed of sheet metal have such diverse geometries that it is difficult to classify them. In all sheet-forming processes, excluding shearing, the metal is subjected to primarily tensile or compressive stresses or both. Sheet forming is accomplished basically by processes such as stretching, bending, deep drawing, embossing,


bulging, flanging, roll forming, and spinning. In most of these operations there are no intentional major changes in the thickness of the sheet metal. There are certain basic considerations that are common to all sheet forming. Grain size of the metal is important in that too large a grain produces a rough appearance when formed, a condition known as orange peel. For general forming, an American Society for Testing and Materials (ASTM) no. 7 grain size (average grain diameter 32 µm) is recommended. Another type of surface irregularity observed in materials such as low carbon steel is the phenomenon of yield-point elongation that results in stretcher strains or Lueder’s bands, which are elongated depressions on the surface of the sheet. This is usually avoided by cold-rolling the original sheet with a reduction of only 1 to 2% (temper rolling). Since yield-point elongation reappears after some time, because of aging, the material should be formed within this time limit. Another defect is season cracking (stress cracking, stress corrosion cracking), which occurs when the formed part is in a corrosive environment for some time. The susceptibility of metals to season cracking depends on factors such as type of metal, degree of deformation, magnitude of residual stresses in the formed part, and environment. Anisotropy or directionality of the sheet metal is also important because the behavior of the material depends on the direction of deformation. Anisotropy is of two kinds: one in the direction of the sheet plane, and the other in the thickness direction. These aspects are important, particularly in deep drawing. Formability of sheet metals is of great interest, even though it is difficult to define this term because of the large number of variables involved. Failure in sheet forming usually occurs by localized necking or buckling or both, such as wrinkling or folding. For a simple tension-test specimen, the true (natural) necking strain is numerically equal to the strain-hardening exponent of the material: thus, for example, commercially pure annealed aluminum or common 304 stainless steel stretches more than cold-worked steel before it begins to neck. However, because of the complex stress systems in most forming operations, the maximum


strain before necking is difficult to determine, although some theoretical solutions are available for rather simple geometries. Considerable effort has been expended to simulate sheet-forming operations by simple tests. In addition to bend or tear tests, cupping tests have also been commonly used, such as the Swift, Olsen, and Erichsen tests. Although these tests are practical to perform and give some indication of the formability of the sheet metal, they generally cannot reproduce the exact conditions to be encountered in actual forming operations.

STRETCH FORMING In this process the sheet metal is clamped between jaws and stretched over a form block. The process is used in the aerospace industry to form large panels with varying curvatures. Stretch forming has the advantages of low die cost, small residual stresses, and virtual elimination of wrinkles in the formed part.

BENDING This is one of the most common processes in sheet forming. The part may be bent not only along a straight line, but also along a curved path (stretching, flanging). The minimum bend radius, measured to the inside surface of the bend, is important and determines the limit at which the material cracks either on the outer surface of the bend or at the edges of the part. This radius, which is usually expressed in terms of multiples of the sheet thickness, depends on the ductility of the material, width of the part, and its edge conditions. Springback in bending and other sheetforming operations is due to the elastic recovery of the metal after it is deformed. Determination of springback is usually done in actual tests. Compensation for springback in practice is generally accomplished by overbending the part; adjustable tools are sometimes used for this purpose. In addition to male and female dies used in most bending operations, the female die can be replaced by a rubber pad. In this way die cost is reduced and the bottom surface of the part is protected from scratches by a metal tool. The

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roll-forming process replaces the vertical motion of the dies by the rotary motion of rolls with various profiles. Each successive roll bends the strip a little further than the preceding roll. The process is economical for forming long sections in large quantities.

RUBBER FORMING

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Although many sheet-forming processes are carried out in a press with male and female dies usually made of metal, there are four basic processes that utilize rubber to replace one of the dies. Rubber is a very effective material because of its flexibility and low compressibility. In addition, it is low in cost, is easy to fabricate into desired shapes, has a generally low wear rate, and also protects the workpiece surface from damage. The simplest of these processes is the Guerin process. Auxiliary devices are also used in forming more complicated shapes. In the Verson–Wheelon process hydraulic pressure is confined in a rubber bag, the pressure being about five times greater than that in the Guerin process. For deeper draws the Marform process is used. This equipment is a packaged unit that can be installed easily into a hydraulic press. In deep drawing of critical parts the Hydroform process is quite suitable, where pressure in the dome is as high as 100 MPa. A particular advantage of this process is that the formed portions of the part travel with the punch, thus lowering tensile stresses, which can eventually cause failure. Bulging of tubular components, such as coffee pots, is also carried out with the use of a rubber pad placed inside the workpiece; the part is then expanded into a split female die for easy removal.

DEEP DRAWING A great variety of parts are formed by this process, the successful operation of which requires a careful control of factors such as blank-holder pressure, lubrication, clearance, material properties, and die geometry. Depending on many factors, the maximum ratio of blank diameter to punch diameter ranges from about 1.6 to 2.3. This process has been extensively studied, and the results show that two important material


properties for deep drawability are the strainhardening exponent and the strain ratio (anisotropy ratio) of the metal.

SPINNING This process forms parts with rotational symmetry over a mandrel with the use of a tool or roller. There are two basic types of spinning: conventional or manual spinning, and shear spinning. The conventional spinning process forms the material over a rotating mandrel with little or no change in the thickness of the original blank. Parts can be as large as 6 m in diameter. The operation may be carried out at room temperature or higher for materials with low ductility or greater thickness. Success in manual spinning depends largely on the skill of the operator. The process can be economically competitive with drawing; if a part can be made by both processes, spinning may be more economical than drawing for small quantities. In shear spinning (hydrospinning, floturning) the deformation is carried out with a roller in such a manner that the diameter of the original blank does not change but the thickness of the part decreases by an amount dependent on the mandrel angle. The spinnability of a metal is related to its tensile reduction of area. For metals with a reduction of area of 50% or greater, it is possible to spin a flat blank to a cone of an included angle of 3˚ in one operation. Shear spinning produces parts with various shapes (conical, curvilinear, and also tubular by tube spinning on a cylindrical mandrel) with good surface finish, close tolerances, and improved mechanical properties.

MISCELLANEOUS PROCESSES Many parts require one or more additional processes: some of these are described briefly here. Embossing consists of forming a pattern on the sheet by shallow drawing. Coining consists of putting impressions on the surface by a process that is essentially forging; the best example is the two faces of a coin. Coining pressures are quite high, and control of lubrication is essential to bring out all the fine detail in a design. Shearing is separation of the material by the cutting action of a pair


of sharp tools, similar to a pair of scissors. The clearance in shearing is important to obtaining a clean cut. A variety of operations based on shearing are punching, blanking, perforating, slitting, notching, and trimming. Die materials used are cast alloys, die steels, and cemented carbides for high-production work. Nonmetallic materials such as rubber, plastics, and hardwood are also used as die materials. The selection of the proper lubricant depends on many factors, such as die and workpiece materials, and severity of the operation. A great variety of lubricants are commercially available, such as drawing compounds, fatty acids, mineral oils, and soap solutions. Pressures in sheet-metal forming generally range between 7 and 55 MPa (normal to the plane of the sheet); most parts require about 10 MPa.

SHEET METAL PARTS Stamping and pressing make up a large family of metal-forming processes. Included in this group are blanking, pressing, stamping, and drawing, all of which are used to cut or form metal plate, sheet, and strip. The steps common to all stamping and pressing operations are the preparation of a flat blank and shearing or stretching the metal into a die to attain the desired shape. In drawing, the flat stock is either formed in a single operation, or progressive drawing steps may be needed to reach the final form. In spinning, flat disks are dished by a tool as they revolve on a lathe. Stamping involves placing the flat stock in a die and then striking it with a movable die or punch. Besides shaping the part, the dies can perform perforating, blanking, bending, and shearing operations. Almost all metals can be stamped. In general, stampings are limited to metal thicknesses of 9.5 mm or less. Pressing and drawing operations can be performed on cold metals up to 19 mm thick and up to about 89 mm on hot metals. In recent years many new press-forming and drawing techniques have been developed. A number of them make use of rubber pads, bags, and diaphragms as part of the die or forming elements. Some involve stretch forming


over dies. Others combine forming and heattreating operations. And still other methods, known as high energy rate forming, employ explosive, electrical, or magnetic energy to produce shock waves that form the material into the desired shape.

STAMPING Frequently, secondary operations, such as annealing in furnaces, trimming in lathes or rolls, brake bends, tapping, etc., make it difficult to define a part as a stamping. Such operations, as well as finishing operations, may cost more than comparatively fast and economical press operations. Sizes and Materials Presses and presslike machines are not necessarily limited to sheet-metal forming. They punch paper doilies and cut uppers for shoes. Multislide machines form round or flat wire; some presses impact-extrude aluminum, zinc, and steel into deep shells for toothpaste tubes or shell bodies, using slugs cut from bars, or cast for the purpose, or sometimes punched from sheets and plates, in bellmouth dies to burnish the edges. Presses forge from billets and they compress powdered metal and carbon into compacts. Types of Work Presses perform such operations as blanking (some blanks are made on shears) and cutting off; piercing (punching) holes, cutouts, or extruding holes; bending to almost any angle (this would include lance forming of tabs); embossing; or forming strengthening ribs or shallow pockets and hemming (bending edges up and then back flat on themselves). The edges of shells may be curled in or out. Coining changes thicknesses, for the raw material of stamping is almost always uniform. Related to blanking are trimming operations to remove excess stock and shaving, a slight removal of stock. Examples are the first step of broaching to obtain close tolerances, small teeth, and straight edges where the breakout on stampings is objectionable, or to improve edge appearance to about 125 µin. on

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thinner materials, and about 150 µin. on stock 2.3 mm and over.

Sheet molding compound (SMC) combined with matched-metal-die compression molding has been the most successful glass fiber composite in automotive exterior body panel applications. In the medium and heavy truck market, SMC is a clear winner. In the more aesthetically demanding car market, however, SMC remains a niche player relegated to medium-volume speciality vehicles. More recent successes point to a shift in focus to more functional and structural applications. The most successful applications address the total system needs and offer unmatched value in terms of high quality, dependability and cost effectiveness. The radiator support assembly is such a case. This illustrates that SMC can win and retain large volume applications resulting in profitable growth. SMC solutions are perhaps the best known and most developed and thus offer valuable lessons for all composites.

SMC applications fall into three major categories: functional, structural, and appearance. Recent gains in functional applications, such as oil pans and heat shields, have been impressive. The main driver of this achievement has been resin development. Both elevated temperature and oil resistance needs are now being met with polyester resins, which are less expensive than the vinyl esters previously employed. Structural applications offer deeper insights into the capabilities and limitations of the SMC. Two such applications, the radiator support assembly and cross car beam, both illustrate the capability of SMC for parts consolidation. Yet, the former leads to a low-cost product, whereas the latter remains a premium niche component. Some reasons behind this outcome can be explained from the total system perspective. Appearance or Class A applications are of importance to both growth and survival of SMC. The opportunity for SMC in this arena is huge and significant in-roads have been made. Yet, SMC has penetrated only a small fraction of this potential and only, to a significant degree, in the United States. The battle with steel in this arena has been examined using cost models.

SMC DEVELOPMENTS

STRUCTURAL APPLICATIONS

Compression molded SMC is perhaps the most talked about composite system for automotive applications. SMC is a versatile composite capable of satisfying structural and aesthetic needs at fast molding cycles, and it is suitable for large production volumes. SMC is an accepted and commercially proven material for automotive part manufacture, serving the role of composite ambassador to the automotive world. SMC offers capital efficiency vs. steel. Capital efficiency makes SMC a viable option for both styling differentiation and new capacity expansion. Because of longer cycles and more defects, however, the cost of low-density SMC remains high, relegating it to but a few niche applications. This illustrates that the tension resulting from simultaneous needs of the original equipment manufacturers (OEMs) for low cost and weight reduction cannot be resolved with higher-cost technology.

Structural applications attempt to exploit the design freedom and parts consolidation potential of SMC leading to integration of multiple functional roles. Integrated front-end system design, for the Ford Taurus and Mercury Sable, incorporates an upper and lower radiator support molded in SMC. The lower radiator support consolidates 22 steel parts into two SMC parts, resulting in a 14% cost reduction vs. steel. Another application for SMC is cross car beams, which are also known as supported instrument panels. Because of their use on light truck platforms, cross car beams consume significant amounts of SMC. The level of functional integration is as large as in the case of the radiator support but the end result is very different. The added complexity pushes the boundaries of SMC flow and moldability. To make matters worse, attempts have been made to use very low density SMC, resulting in even more molding difficulties. In the end, low cost

SHEET MOLDING COMPOUND

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has not been achieved, making both OEMs and molders unhappy. Perhaps a simpler design, incorporating a steel structural insert, might be more successful. These two examples illustrate that parts consolidation is a useful tool for achieving low cost; however, there are limits to this approach and often the increased complexity leads to higher cost than in an equivalent system of many specialized components.

SHELL MOLD CASTINGS Shell mold casting is a process that uses relatively thin-wall mold made by bonding silica or zircon sand with a thermosetting phenolic or urea resin. It has gained widespread use because it offers many advantages over conventional sand castings. Shell mold casting is a practical and economical way to meet the demand for weight reduction, thinner sections, and closer tolerances.

ADVANTAGES There are five basic advantages: 1. Lower costs. High production rates and fewer finishing operations result in a lower unit cost for applicable parts. 2. Closer tolerances. Shell castings have closer tolerances than sand castings. Draft allowances are also reduced. 3. Smoother surface finish. Shell molding provides an improved surface compared with sand casting (250 to 1000 µin. rms for sand, 125 to 250 for shell). 4. Less machining. The precision of shell molding reduces, and in many cases eliminates, machining or grinding operations. 5. Uniformity. Insulating properties of the molds produce casting surfaces free of chill and with a more uniform grain structure. Although a shell mold is more expensive than a green sand mold, the possibility of


reducing weight, minimizing machining, and eliminating cores often results in savings sufficient to offset the mold cost.

THE PROCESS The shell molding process can be broken down into five operations: 1. A match plate pattern is made from tool steel with dimensions calculated to allow for subsequent metal shrinkage. 2. The resin–sand mixture is applied to the metal pattern, which is then heated to 218 to 232°C. The hot pattern melts the resin, which flows between the grains of sand and binds them together. Thickness of the mold increases with time. After the desired thickness is reached, excess unbonded sand is poured from the pattern. 3. The pattern, with the soft shell adhering, is placed in an oven and heated to 566 to 649°C for 30 s to 1 min. This cures the shell and produces a hard, smooth mold that reproduces the pattern surface exactly. The shell is stripped from the pattern by ejector pins. The other half of the mold is produced in the same way. 4. Sprues and risers are opened and cores inserted to complete the cope and drag halves of the mold. 5. The two shell halves are glued together under pressure to form a tightly sealed mold that can be stacked and stored indefinitely.

SHELL MOLDED CORES Shell molded cores are an offshoot of the shell molding process. These cores have several advantages over the sand cores that they replace: the shell core usually costs less than the sand core; strength and rigidity permit handling without damage or distortion; sharp details, including threads, are accurately reproduced; core weight is reduced; cured cores are unaffected by moisture and can be stored for

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long periods of time; most shell cores are hollow and can function as vents during casting.

SILICA Silica, as the dioxide of silicon is commonly termed, is the principal constituent of the solid crust of the Earth. Consequently, it is a major ingredient of most of the nonmetallic, inorganic materials used in industry. Silica occurs in a number of allotropic forms, which have different properties and different uses. The principal modifications of silica are quartz, silica glass, cristobalite, and tridymite. The last two are sometimes combined under the term of inverted silica. The valuable physical properties of these four principal modifications lead to a wide variety of applications for silica in industry and technology.

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The common low-temperature form of silica, quartz, is strong and insoluble in water. Consequently, sandstone, which is composed of grains of quartz held together by a siliceous cement, is an excellent building stone. Huge quantities of silica stone are used as aggregate for concrete. One curious variety has a structure that renders the rock flexible and is known as itacolumite or flexible sandstone. Quartzite, in which the quartz grains of an original sandstone have recrystallized and grown to a compact mass, and vein quartz are too hard and difficult to shape for use as building stones. Rock crystal, as the enhedral quartz found in nature is termed, is often carved into ornaments of great beauty because of its perfect transparency and because of the high luster given it by polishing. The “crystal ball” of story and romance is a polished quartz sphere. Colored varieties of quartz are much valued as semiprecious stones. The purple amethyst, blue sapphire quartz, yellow citrine or false topaz, red or pink rose quartz, smoky quartz, and the dark brown morion are examples. Sandstone is also useful as an abrasive. It is used for grindstones and pulpstones. “Berea” grit from northern Ohio and novaculite from Arkansas are examples of coarse and finegrained natural stones that are still preferred for


some purposes to the artificial abrasives made from silicon carbide or aluminum oxide. The strength and hardness of various natural forms of silica leads to use for crushing and for grinding. Flint pebbles are used in ball mills for grinding all sorts of materials. Agate mortars are invaluable in the chemical laboratory for pulverizing minerals before analysis. Sandstone and quartzite are used for millstones and buhrstones to crush and grind grain, paint pigments, fertilizers, and many other products. Quartz sand, driven by compressed air, constitutes the useful sandblast for cleaning metal, decorating glass, and refacing stone buildings. Coated on paper, quartz grains provide the carpenter and cabinet maker with the familiar “sandpaper,” although much modern sandpaper is really coated with crushed glass containing only about 70% silica. The abrasive properties of quartz grains are also employed by locomotives to gain traction on steel rails, in billiard cue chalk, and in the chicken gizzard, where quartz grains are used as a kind of natural ball mill. The thermal properties of the various forms of silica also lead to important uses. Its high melting point (1710°C) makes it a good refractory, while its low cost and low thermal expansion have brought about a wide use in industrial furnaces, particularly for melting steel and glass. Properties In general, the thermal expansion of all of the common forms of silica is low at high temperatures. This makes silica refractories capable of withstanding sudden temperature changes and large thermal gradients very well in these hightemperature ranges. At lower temperatures, large volume changes, due to rapid inversions from the high-temperature crystalline forms of quartz, tridymite, and cristobalite to corresponding low-temperature forms, make these materials rather sensitive to sudden temperature changes. The low-temperature forms of quartz, tridymite, and cristobalite also have higher thermal expansions than the high-temperature forms and the amorphous form of silica. This form of silica, variously known as silica glass, vitrosil, and fused quartz, is thus the only form of silica that may be heated rapidly from room


temperature without fear of breakage. A further limitation on the thermal behavior of silica refractories is occasioned by the large volume increase that occurs as quartz changes slowly to tridymite or cristobalite at temperatures above 870°C. This may lead to swelling and warping of silica bricks if they are not first converted to the forms stable at high temperatures by prolonged firing. Because the volume changes that occur when tridymite changes from its high-temperature form to the forms stable at lower temperatures are less than the corresponding change for cristobalite, an effort is made to convert the silica refractory as completely to tridymite as is feasible before using it.

radio transmitters, radar equipment, and other electronic “timing” gear. This use and other electrical and optical uses of crystal quartz depend upon the symmetry of the material. Some experts state that low quartz is the most important and most familiar example of the trigonal enantiomorphous hemihedral or trigonal trapezohedral class of symmetry. This class is characterized by one axis of threefold symmetry with 3 axes of twofold symmetry perpendicular thereto and separated by angles of 120˚. This class has no plane of symmetry and no center of symmetry.

Uses

The transparency of crystal quartz, particularly to the short waves of the ultraviolet, makes it very useful in optical instruments. Quartz prisms are employed in spectographs for analysis of light waves varying in length from almost 5 µm in the infrared to almost 0.2 µm in the ultraviolet. Lenses made of crystal quartz are also useful over this range for photography and microscopy. Crystal quartz is more transparent than fused quartz for these purposes, but its birefringence introduces complications in design. Quartz also has the property of rotating the plane of vibration of polarized light traveling along its axis. This property is associated with its left- or right-handed character. Sugar solutions have the same optical rotating power and quartz plates or wedges are employed as standards in optical devices used for analyzing sugar solutions by means of this rotary power. Many other organic chemicals that have asymmetric right- or left-handed molecules can be studied and assayed by this optical means. The variation of this rotary power of quartz with wavelength or color of light makes it possible to construct monochromators capable of selecting light of a desired color from a white source with very large optical apertures.

Silica bricks are made from crushed quartzite rock, known as ganister, which is bonded with 1.5 to 3.0% of lime. They are molded smaller than the dimensions desired in the finished brick to allow for an expansion of about 3/8 in./ft as the quartz inverts to tridymite during firing. A firing schedule of about 20 days at 1450°C (about cone 16) is required. Study of the phase diagram of the silica-lime system shows why considerable quantities of lime may be used to bond the quartzite in silica bricks without loss of refractoriness. The lime is taken up in an immiscible lime-rich glass phase of which only a small amount is formed because of its high lime content. In use, silica bricks are characterized by retention of rigidity and load-bearing capacity to temperatures above 1600°C, without the slow yield characteristic of fireclay brick. If a cold silica brick is heated suddenly, it spalls and disintegrates owing to the sudden volume changes taking place at the high-low inversions of tridymite and cristobalite. If heated cautiously through this sensitive temperature region, silica brick is very resistant to temperature shock. The refractory properties of quartz sand are also employed in molds for cast iron. Huge quantities of sand, with varying amounts of clay impurity to bond the grains, are used for this purpose in the foundries of the world. Crystal quartz in piezoelectric oscillators have become indispensable for the control of


Optical Properties

AMORPHOUS SILICA The amorphous form of silica also has mechanical, optical, thermal, and electrical properties, which make it very useful in hundreds of technical applications. Vitreous silica is made by

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three different processes. The earliest process, which is still used, consists simply of melting quartz by application of high temperature. The oxyhydrogen flame was the first source of heat for this process and is still used to melt fragments of rock crystal that are combined to form rods and other shapes of silica glass. Electrical heat from graphite resistors is more commonly employed at present, however, since there is less trouble from volatilization of the silica. Large masses can be made by this technique and shaped into various useful forms although the very high temperature required for working silica glass makes this method of manufacture difficult and expensive. To obtain a clear transparent product, crystal must be melted. If sand is used, the product is white and opaque because of the numerous air bubbles trapped in the very viscous glass. It is impossible to heat silica glass hot enough to drive out the bubbles or, in glass parlance, to “fine” it. This opaque “vitrosil” made by fusing sand, however, is very useful for chemical apparatus, when the low thermal expansion and insolubility of the silica glass play a role. Recently, the Corning Glass Works developed a new process for making silica glass by hydrolysis of silicon tetrachloride in a flame. The resulting silica is deposited directly on a support in the form of transparent silica glass, which is more homogeneous and purer than the glass made by fusing quartz. Large pieces can be made by this process and the unusual homogeneity of the product makes it especially useful for optical parts and for sonic delay lines where striations in the older fused quartz are detrimental. The sonic delay lines are polygons of the silica glass in which acoustic waves travel on a long path, being reflected at the numerous polygonal faces. Finally, silica glass of 96% purity or better is made by an ingenious process invented by the Corning Glass Works. In this third process for making vitreous silica, an object is shaped first from a soft glass containing about 30% of borax and boric acid as fluxes. A suitable heat treatment causes a submicroscopic separation of two glass phases. One phase, which is composed chiefly of the fluxes, is then leached from the glass in a hot, dilute nitric acid bath. If the composition and heat treatment are exactly


right, the high silica phase is left as a porous “sponge” with the shape and size of the original soft glass article. This is carefully washed and dried and fired by heating to about 1200°C. In the firing step, the porous silica sponge shrinks to a dense transparent object of glass containing 96% SiO2 or more, which has the desirable properties of silica glass made by either of the other processes. The 4% of impurity in the glass made by this process consists chiefly of boric oxide. Because of its presence this glass is appreciably softer than pure silica glass. For corresponding viscosities the “Vycor” 96% silica glass requires about 100°C lower temperature. Silica glass made by any of these processes has many uses because of its unique combination of good properties. Fibers of silica glass have very high tensile strength and almost perfect elasticity. This makes them useful in constructing microbalances, electrometers, and similar instruments. Silica fiber in diameters as small as 0.000076 cm comes in random matted form or in rovings. Because of its small thermal expansion, fused silica is very resistant to sudden temperature changes. It is also a very hard glass so that it may be used in the laboratory for crucibles and combustion tubes to much better advantage than ordinary glass. Vitreous silica makes possible the construction of thermometers operating up to 1000°C. The small thermal expansion and durability of fused silica have led to its use in fabrication of standards of length. Fused silica plates, ground to optical flatness, are used in interferometers for measurement of thermal expansion. The fact that fused silica is an excellent insulator with little or no tendency to condense surface films of moisture makes it valuable in the construction of electrical apparatus. It has a very high dielectric strength and low dielectric loss. Silica glass also has very good transmission for visible and ultraviolet light. This makes it useful for the construction of mercury lamps and other optical equipment. In the mercury lamps the strength and heat resistance of silica glass make it possible to operate with high internal pressures, producing very high light intensities and great efficiency. The 96% silica glass


made by the Vycor process may be fired in a reducing atmosphere or in vacuum to modify and improve its light-transmitting properties. Silica glass is among the most chemically resistant of all glasses. This makes it particularly useful in the analytical laboratory where there is the added advantage that any contamination of contained solutions can only be by the one oxide, silica. Crucibles of fused silica glass may be used for pyrosulfate fusions. Condensers of fused silica are extremely useful for distilling acids (except hydrofluoric) and for preparation of extremely pure water. Another useful form of silica is obtained by dehydrating silicic acid. In this way a porous “gel” is obtained that has an enormous surface area and is capable of adsorbing various gases and vapors, particularly water vapor. Silica gel is used in dehumidifiers to remove water vapor from the air. It is also used as a catalyst and as a support for other catalysts. The porous 96% silica skeleton obtained in the Vycor process also has a very large surface and is useful as an adsorbent and drying agent. It has less capacity than silica gel, but better mechanical strength. A very finely divided form of silica known as silica soot is obtained by hydrolizing silicon tetrachloride in a flame without heating the product hot enough to consolidate it as a glass. This material is valuable as a thermal insulator and as a white filler for rubber and plastics.

OTHER USES The silica (actually ground silica) used in the pottery industry is called flint. The addition of flint affects warpage very little. In ceramic bodies, potters’ flint or pulverized quartz or sand is the constituent that reduces drying and burning shrinkage and assists promotion of refractoriness. Flint has an important bearing on the resistance of bodies to thermal and mechanical shock, because of the volume changes that accompany crystal transformation. In the unburned body, it lowers plasticity and workability, lowers shrinkage, and hastens drying. A coarse crystalline form of quartz, called macrocrystalline quartz, is more often used for potters’ flint than the cryptocrystalline form.


Silica is used in all glazes as the chief, and often the only, acid radical (RO2 group). It may be adjusted to regulate the melting temperature of the glaze. In common glazes, the ratio of silica to bases (RO group) is never less than 1:1 nor more than 3:1. By varying the relative proportions of the RO group and balancing the group against any desired silica content, the maturing temperature of a glaze may be quite closely controlled. In other words, the fusibility of the glazes used in the presence of equal proportions of fluxes depends on their relative silica contents. In porcelain enamels, it may be taken as a general rule that, other things remaining constant, the higher the percentage of silica, the higher will be the melting point of the enamel and the greater its acid resistance. Silica has a low coefficient of expansion and increasing it in an enamel lowers the coefficient of expansion of that enamel. One method of regulating an enamel coating is to increase the silica content when the enamel is inclined to split off in cooling. Silica in the form of flint or quartz is used in both ground-coat and cover-coat enamels, and it has the same effect in either type. The temperature required for melting an enamel is materially affected by the fineness of the silica. Cryolite, antimony, and tin oxide give their maximum value as opacifiers with minimum heat treatment in the smelter. The form and fineness of the silica should, therefore, be carefully watched and allowed for in compounding the batch. All forms of silica may be used with good results, but experience has shown that, in the same enamel, a smaller quantity of sand than of powdered quartz is necessary. Similarly, less sand than flint should be used, but the difference in this case is less than in the former. High SiO2 tends to harden the enamel. The lower limit established in the usual run of enamels is 1.1 equivalents. In the manufacture of semiconductors, monolithic circuits, and integrated circuits for the electronics industry, the use of fused quartz is widespread for plumbing and diffusion furnace muffles. The need to prevent product contamination makes this choice mandatory. Irish Refrasil is 98% silica and has a green color. It is used for ablative protective coatings. It resists temperatures to 1588°C. Silica flour,

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made by grinding sand, is used in paints, as a facing for sand molds, and for making flooring blocks. Silver bond silica is water-floated silica flour of 98.5% SiO2, ground to 325 mesh. In zinc and lead paints it gives a hard surface. Pulverized silica, made from crushed quartz, is used to replace tripoli as an abrasive. Ultrafine silica, a white powder having spherical particles of 4 to 25 µm, is made by burning silicon tetrachloride. It is used in rubber compounding, as a grease thickener, and as a flatting agent in paints. Aerosil is this material. A polymer-impregnated silica, Polysil, has twice the dielectric strength of porcelain as well as better strength. It is also cheaper to make, and its composition can be tailored to meet specific environmental and operating conditions. Silica aerogel is a fine, white, semitransparent silica powder; its grains have a honeycomb structure, giving extreme lightness. It weighs 40 kg/m3 and is used as an insulating material in the walls of refrigerators, as a filler in molding plastics, as a flatting agent in paints, as a bodying agent in printing inks, and as a reinforcement for rubber. It is produced by treating sand with caustic soda to form sodium silicate, and then treating with sulfuric acid to form a jellylike material called silica gel, which is washed and ground to a fine dry powder. It is also called synthetic silica. Silicon monoxide, SiO, does not occur naturally but is made by reducing silica with carbon in the electric furnace and condensing the vapor out of contact with the air. It is lighter than silica, having a specific gravity of 2.24, and is less soluble in acid. It is brown powder valued as a pigment for oil painting, as it takes up a higher percentage of oil than ochres or red lead. It combines chemically with the oil. Fumed silica is a fine translucent powder of the simple amorphous silica formula made by calcining ethyl silicate. It is used instead of carbon black in rubber compounding to make lightcolored products, and to coagulate oil slicks on water so that they can be burned off. It is often called white carbon, but the “white carbon black” called Cab-O-Sil, used for rubber, is a silica powder made from silicon tetrachloride. Cab-O-Sil EH5, a fumed colloidal form, is used as a thickener in resin coatings. The thermal


expansion of amorphous fused silica is only about one-eighth that of alumina. Refractory ceramic parts made from it can be heated to 1093°C and cooled rapidly to subzero temperatures without fracture.

SILICA MINERALS Silica (SiO2) occurs naturally in at least nine different varieties (polymorphs), which include tridymite (high-, middle-, and lowtemperature forms), cristobalite (high- and low-temperature forms), coesite, and stishovite, in addition to high (β) and low (α) quartz. These forms have distinctive crystallography and optical characteristics. The transformation between the various forms are of two types. Displacive transformations, such as inversions between high-temperature (β) and low-temperature (α) forms, result in a displacement or change in bond direction but involve no breakage of existing bonds between silicon and oxygen atoms. These transformations take place rapidly over a small temperature interval and are reversible. Reconstructive transformations, in contrast, involve disruption of existing bonds and subsequent formation of new ones. These changes are sluggish, thereby permitting a species to exist metastably outside its defined pressure-temperature stability field. Two examples of reconstructive transformations are tridymite = quartz and quartz = stishovite.

SILICIDES Silicides are a group of substances, usually compounds, comprising silicon in combination with one or more metallic elements. These hard, crystalline materials are closely related to intermetallic compounds and have, therefore, many of the physical and chemical characteristics and some of the mechanical properties of metals. Silicides are not natural products. They received but little attention prior to the development of the electrical furnace, which provided the first practical means of attaining and controlling the high temperatures generally required in their preparation.


COMPOSITION Although a majority of the metals react with silicon, many of the resulting silicides do not have the properties usually required in engineering materials. The silicides that appear most promising for practical utilization in engineering and structural applications are, with few exceptions, limited to those of the refractory, or high melting, metals of groups IV, V, and VI of the periodic table. Included in this category are the silicides of titanium, zirconium, hafnium, vanadium, columbium, tantalum, chromium, molybdenum, and tungsten. Silicides can be prepared by direct synthesis from the elements, by reduction of silica or silicon halogenides and the appropriate metal oxide or halogenide with silicon, carbon, aluminum, magnesium, hydrogen, etc., and by electrolysis of molten compounds. They are also obtained as by-products in many metallurgical processes. High-purity silicides of stoichiometric composition are difficult to prepare. The chemical composition of silicides cannot, in general, be predicted from a consideration of the customary valences of the elements. The zirconium–silicon system, for example, is reported to include the compounds Zr4Si, Zr2Si, Zr3Si2, Zr4Si3, Zr6Si5, ZrSi, and ZrSi2. The disilicide composition (MSi2) occurs in all of the refractory metal–silicon systems and probably will prove the most important, particularly in those applications requiring high temperature stability.

GENERAL PROPERTIES Silicides resemble silicon in their chemical properties with the degree of similarity roughly proportional to the silicon content. At normal temperatures the refractory metal-disilicides are inert to most ordinary chemical reagents. The compounds are not thermodynamically stable in the presence of oxygen and in the finely pulverized state they oxidize readily. However, in massive form they are oxidation resistant because of the formation of a protective surface layer of silica. Bodies of MoSi2 and WSi2 are highly resistant to oxidation even at temperatures approaching their melting points.


The physical and mechanical characteristics of silicides are, to a large extent, determined by the properties of the component metal. The refractory metal silicides have highly crystalline structures and moderate densities. Their melting points are intermediate to relatively high. They have low electrical resistance, high thermal conductivity, and fair thermal shock resistance. They have high hardness, high compressive strength, and moderate tensile strength at both room and elevated temperatures. Their elevated temperature stress-rupture and creep properties are good. Brittleness and low impact resistance are the most serious disadvantages of these materials. Excellent oxidation resistance has prompted detailed studies of molybdenum disilicide. Quantitative information on the properties of most silicides has been developed within the past two decades.

FABRICATION The general methods used for consolidating powders can be applied to the silicides. Highdensity parts are obtained by cold pressing and sintering and also by hot pressing. Slip casting and extrusion are convenient methods for preparing certain shapes and sizes. Casting in the molten state is difficult due to the partial decomposition of silicides at their melting points. Silicide coatings can be prepared by vapor deposition techniques. Dense, fully sintered silicide parts are extremely difficult to work. They can be cut using silicon carbide or diamond wheels. Grinding has shown some promise but it is a slow process. “Green” or presintered compacts can, however, be shaped by conventional methods.

AVAILABILITY Although the availability of silicides is gradually increasing, the varieties, quantities, and shapes are limited. Molybdenum disilicide is commercially available in powder form and as furnace heating elements, and certain shapes and sizes have been produced on a custom basis. Some of the other compositions can be obtained on special order. The small market that has developed for MoSi2 can be expected to

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stimulate general interest in the silicides and to foster their commercial development and production. The practical utilization of silicides, with few exceptions, notably MoSi2 furnace heating elements, has been implemented. Molybdenum disilicide is a structural material for gas turbine and missile components, which do not require high impact and thermal shock resistance. Igniter elements, thermocouple shields, gas probes, and nozzles are other potential applications. The high hardness of these materials has found usage in metalworking dies and tooling. Silicide coatings, prepared by vapor-phase deposition, afford excellent oxidation protection to molybdenum and tungsten. This method of providing oxidation resistance is versatile because fabrication can be completed before the coating is applied. Similar coatings can be produced on other materials such as graphite by using silicide powders. Silicide coatings are commercially available and are a major item in the present market for silicide products.

SILICON

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A metallic element (symbol Si), silicon is used chiefly in its combined state and is the most abundant solid element in the Earth’s crust (28%). The metal has been prepared by reducing the tetrachloride by hydrogen using a hot filament, or by aluminum, magnesium, or zinc. The fluoride or alkali fluorosilicates have been reduced with alkali metals or aluminum. Silica can be converted to metal by reduction in the electric furnace with carbon, silicon carbide, aluminum, or magnesium. The element has also been produced by the fusion electrolysis of silica in molten alkali oxide–sodium chloride–aluminum chloride baths. Extremely pure metal has been made by the treatment of silanes with hydrogen. It is a gray-white, brittle, metallic-appearing element, not readily attacked by acids except by a mixture of HF and HNO3. It is soluble in hot NaOH or KOH and is prepared in the pure crystalline form by reduction of fractionally distilled SiCl4. Silicon combines with many elements including boron, carbon, titanium, and zirco-


nium in the electric furnace. It readily dissolves in molten magnesium, copper, iron, and nickel to form silicides. Most oxides are reduced by silicon at high temperatures.

FABRICATION Silicon can be cast by melting in a vacuum furnace and cooling in vacuum by withdrawing the element from the heated zone. Single-crystal ingots have been prepared by drawing from the melt and by the “floating zone” technique. It is claimed that silicon exhibits some workability above 1000°C, but at room temperature it is very brittle. Metals can be coated with a silicon-rich layer by reducing the tetrachloride with hydrogen on the hot metal surface.

TYPES In very pure form silicon is an intrinsic semiconductor, although the extent of its semiconduction is greatly increased by the introduction of minute amounts of impurities. Pure silicon metal is used in transistors, rectifiers, and electronic devices. It is a semiconductor, and is superior to germanium for transistors as it will withstand temperatures to 149°C and will carry more power. Rectifiers made with silicon instead of selenium can be smaller, and will withstand higher temperatures. Its melting point when pure is about 1434°C, but it readily dissolves in molten metals. It is never found free in nature but, combined with oxygen, it forms silica, SiO2, one of the most common substances on Earth. Silicon can be obtained in three modifications.

FORM Amorphous silicon is a brown-colored powder with a specific gravity of 2.35. It is fusible and dissolves in molten metals. When heated in the air, it burns to form silica. Graphitoidal silicon consists of black glistening spangles, and is not easily oxidized and not attacked by the common acids, but is soluble in alkalies. Crystalline silicon is obtained in dark, steel-gray globules of crystals or six-sided pyramids of specific gravity 2.4. It is less reactive than the amorphous form, but is attacked by boiling water. All these forms are obtainable by chemical reduction. Silicon is


an important constituent of commercial metals. Molding sands are largely silica, and silicon carbides are used as abrasives. Commercial silicon is sold in the graphitoidal flake form, or as ferrosilicon, and silicon–copper. The latter forms are employed for adding silicon to iron and steels. Commercial refined silicon contains 97% pure silicon and less than 1% iron. It is used for adding silicon to aluminum alloys and for fluxing copper alloys. High-purity silicon metal, 99.95% pure, made in an arc furnace, is too expensive for common uses, but is employed for electronic devices and in making silicones. For electronic use, silicon must have extremely high purity, and the pure metal is a nonconductor with a resistivity of 300,000 Ω · cm. For semiconductor use it is “doped” with other atoms yielding electron activity for conducting current. Epitaxial silicon is higher purified silicon doped with exact amounts of impurities added to the crystal to give desired electronic properties.

PRINCIPAL COMPOUNDS Silicon is reported to form compounds with 64 of the 96 stable elements, and it probably forms silicides with 18 other elements. Besides the metal silicides, used in large quantities in metallurgy, silicon forms useful and important compounds with hydrogen, carbon, the halogen elements, nitrogen, oxygen, and sulfur. In addition, useful organosilicon derivatives have been prepared. Hydrides The hydrides of silicon are named silanes; the compound SiH4 is called monosilane, Si2H6 disilane, Si3H8 trisilane, and so on. Compounds in which oxygen atoms alternate with silicon atoms in the principal part of the structure are called siloxanes, and those with nitrogen between silicon atoms are called silazanes. All other covalent compounds of silicon are considered for the purpose of nomenclature to be derived from these silanes, and modified silanes and are named according to substituent groups and their placement along the principal siliconcontaining chain or ring.


USE Because of its inherent brittleness, there are no engineering applications of silicon. The chief use of highly purified metal is as a semiconductor in transistors, rectifiers, and solar cells. It may be fired with ceramic materials to form heat-resistant articles. Silicon can serve as an autoxidation catalyst and as an element in photocells. Mirrors for dental use are formed with a reflecting surface of silicon. The metal is also employed to prepare silicides and alloys, and to coat various materials. In commercial quantities it is also used as a starting material for the synthesis of silicones. By far the largest use of silicon is as compounds in the ceramic industry. It also is employed as an alloying element in ferrous metals, and is the basis of the family of chemicals known as silicones. An important application of silicon is in the electronics industry where it has been widely employed in the manufacture of crystal rectifiers and integrated circuits. Sufficiently pure silicon has been produced by carefully controlled zone refining and crystal growth to make possible its use as transistors. Since the energy gap in silicon is 1.1 eV, compared with 0.75 eV for germanium, silicon transistors may be operated at higher temperatures and power levels than those made of germanium.

SILICON BRONZE Silicon bronze is a family of wrought copperbase alloys (C64700 to C66100) and one cast copper alloy (C87200), the wrought alloys containing from 0.4 to 0.8% silicon (C64700) to 2.8 to 4.0% silicon (C65600), and the cast alloy 1.0 to 5.0%, along with other elements, usually lead, iron, and zinc. Other alloying elements may include manganese, aluminum, tin, nickel, chromium, and phosphorus. The most well-known alloys are probably silicon bronze C65100, or low-silicon bronze B, and silicon bronze C65500, or high-silicon bronze A, as they were formerly called. As these names imply, they differ mainly in silicon content: 0.8 to 2.0% and 2.8 to 3.8%, respectively, although the latter alloy also may contain as much as 0.6% nickel. C87200 contains at least 89% copper, 1.5%

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silicon, and as much as 5% zinc, 2.5% iron, 1.5% aluminum, 1.5% manganese, 1% tin, and 0.5% lead. Regardless of alloying ingredients, copper content is typically 90% or greater. Both of the common wrought alloys are quite ductile in the annealed condition, C65500 somewhat more ductile than C65100, and both can be appreciably strengthened by cold working. Annealed, tensile yield strengths are on the order of 103 to 172 MPa depending on mill form, with ultimate tensile strengths to about 414 MPa and elongations of 50 to 60%. Cold working can increase yield strength to as much as 483 MPa. Electrical conductivity is 12% for C65100 and 7% for C65500 relative to copper, and thermal conductivity is 57 and 36 W/m · K, respectively. The alloys are used for hydraulicfluid lines in aircraft, heat-exchanger tubing, marine hardware, bearing plates, and various fasteners. Silicon bronze C87200 is suitable for centrifugal, investment, and sand-, plaster-, and permanent-mold castings. As sand-cast, typical tensile properties are 379 MPa ultimate strength, 172 MPa yield strength, and 30% elongation. Hardness is Brinell 85, electrical conductivity 6%, and, relative to free-cutting brass, machinability is 40%. Uses include pump and valve parts, marine fittings, and bearings.

SILICON CARBIDE

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The reduction of silica with excess carbon under appropriate conditions gives silicon carbide (SiC), which crystallizes in a number of forms but is best known in the cubic-diamond form with spacing ao of 0.435 nm (compared with 0.356 nm for diamond). In the pure form, silicon carbide is green (α-hexagonal) or yellow (β-cubic), but the commercial product is black and has a bluish or greenish iridescence. The carbide is not easily oxidized by air except above 1000°C, and retains its physical strength up to this temperature. For these reasons it is a favorite structural refractory material for the ceramic arts. It also is extremely hard, with a Mohs hardness in excess of 9, and so has found wide application as an abrasive. Silicon carbide does not melt without decomposition at atmospheric pressure, but does melt at 2830°C at 3.5 MPa.


FORMS β-SiC (cubic) forms at 1400 to 1800°C and α-SiC (hexagonal) forms at temperatures >1800°C. SiC is used as an abrasive as loose powder, coated abrasive cloth and paper, wheels, and hones. It will withstand temperatures to its decomposing point of 2301°C and is valued as a refractory. It retains its strength at high temperatures, has low thermal expansion, and its heat conductivity is ten times that of fireclay. Silicon carbide is made by fusing sand and coke at a temperature above 2204°C. Unlike aluminum oxide, the crystals of silicon carbide are large, and they are crushed to make the small grains used as abrasives. They are harder than aluminum oxide, and because they fracture less easily, they are more suited for grinding hard cast irons and ceramics. The standard grain sizes are usually from 100 to 1000 mesh. The crystalline powder in grain sizes from 60 to 240 mesh is also used in lightning arrestors. Carborundum, Crystolon, and Carbolon are trade names for silicon carbide.

TYPES Three main types are produced commercially. Green SiC is an entirely new batch composition made from a sand and coke mixture, and is the highest purity of the three. Green is typically used for heating elements. Black SiC contains some free silicon and carbon and is less pure. A common use is as bonded SiC refractories. The third grade is metallurgical SiC, and is not very pure. It typically is used as a steel additive.

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Silicon carbide is manufactured in many complex bonded shapes, which are utilized for superrefractory purposes such as setter tile and kiln furniture, muffles, retorts and condensors, skid rails, hot cyclone liners, rocket nozzles and combustion chambers, and mechanical shaft seals. It is also used for erosion- and corrosionresistant uses such as check valves, orifices, slag blocks, aluminum diecasting machine parts, and sludge burner orifices. Electrical uses of SiC include lightning arrestors, heating elements, and nonlinear resistors.


Silicon carbide refractories are classified on the basis of the bonds used. Associated-type bonds are oxide or silica, clay, silicon oxynitride, and silicon nitride, as well as self-bonded. A process for joining high-temperatureresistant silicon carbide structural parts that have customized thermomechanical properties has been developed and the materials include SiC-based ceramics and composites reinforced by different fibers. The method begins with the application of a carbonaceous mixture to the joints. The mixture is cured at a temperature between 90 and 110°C. The joints are then locally infiltrated with molten silicon or with alloys of silicon and refractory metals. The molten metal reacts with the carbon in the joint to form silicon carbide and quantities of silicon and refractory disilicide phases that can be tailored by choosing the appropriate reactants. In mechanical tests, the joints were found to retain their strength at temperatures from ambient to 1370°C. The technique can also be used in the repair of such parts.

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SiC is used in the manufacture of grinding wheels and coated abrasives. Large tonnages are used in cutting granite with wire saws and as a metallurgical additive in the foundry and steel industries. Other uses are in the refractory and structural ceramic industries. As an abrasive, silicon carbide is best used on either very hard materials such as cemented carbide, granite, and glass, or on soft materials such as wood, leather, plastics, rubber, etc. Refrax Silicon Carbide and KT The first material is bonded with silicon nitride. It is used for hot-spray nozzles, heat-resistant parts, and for lining electrolytic cells for smelting aluminum. Silicon carbide KT is molded without a binder. It has 96.5% SiC with about 2.5% silica. The specific gravity is about 3.1, and it is impermeable to gases. It is made in rods, tubes, and molded shapes.


Silicon Carbide Foam This is a lightweight material made of selfbonded silicon carbide foamed into shapes. It is inert to hot chemicals and can be machined. Silicon Carbide Crystals These are used for semiconductors at temperatures above 343°C. As the cathode of electronic tubes instead of a hot-wire cathode, the crystals take less power and need no warm-up. Silicon Carbide Fibers SiC fiber is one of the most important fibers for high-temperature use. It has high strength and modulus and will withstand temperatures even under oxidizing conditions up to 1800°C, although the fibers show some deterioration in tensile strength and modulus properties at temperatures above 1200°C. It has advantages over carbon fibers for some uses, having greater resistance to oxidation at high temperatures, superior compressive strength, and greater electrical resistance. There are two forms of SiC fibers, neither of which is available commercially. One consists of a pyrolytic deposit (chemical vapor deposition) of SiC on an electrically conductive, usually carbon, continuous filament. Fiber diameter is about 140 µm. This technology has been used to make filaments with both graded and layered structures, including surface layers of carbon, which provide a toughness-enhancing parting layer in composites with a brittle matrix (silicon nitride, for example). The other form of filamentary SiC is still in the development stage. Fibers are extruded from sinterable SiC powder, and allowed to sinter during free fall from the extruder. Commercialization is not yet possible because the fibers produced to date are too large: 0.13 to 0.25 mm in diameter. There are two commercial processes for making continuous silicon carbide fibers: (1) by coating silicon carbide on either a tungsten or carbon filament by vapor deposition to produce a large filament (100 to 150 µm in diameter), or (2) by melt spinning an organic polymer containing silicon atoms as a precursor fiber followed by heating at an elevated temperature

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to produce a small filament (10 to 30 µm in diameter). Fibers from the two processes differ considerably from each other but both are used commercially. Improved composites of SiC fibers in Si/SiC matrices have been invented for use in applications in which there are requirements for materials that can resist oxidation at high temperatures in the presence of air and steam. Such applications are likely to include advanced aircraft engines and gas turbines. The need for the improved composites arises as follows: Although both the matrix and fiber components of older SiC/(Si/SiC) composites generally exhibit acceptably high resistance to oxidation, these composites become increasingly vulnerable to oxidation and consequent embrittlement whenever mechanical or thermomechanical loads become large enough to crack the matrices. Even the narrowest cracks become pathways for the diffusion of oxygen. Typically, to impart toughness to an SiC/(Si/SiC) composite, the SiC fibers are coated with a material that yields at high stress to allow some slippage between the fibers and matrix. If oxygen infiltrates through the cracks to the fiber coatings, then, at high temperature, the oxygen reacts with the coatings (and eventually with the fibers), causing undesired local bonding between fibers and the matrix and consequent loss of toughness. The improved composites incorporate matrix additives and fiber coatings that retard the infiltration of oxygen by reacting with oxygen in such a way as to seal cracks and fiber/matrix interfaces at high temperatures. These matrix additives and coating materials contain glass-forming elements — for example, boron and germanium. Boron is particularly suitable for use in fiber coatings because it can react with oxygen to form boron oxide, which can, in turn, interact with the silica formed by oxidation of the matrix and fiber materials to produce borosilicate glasses. Boron and SiB6 may prove to be the coating materials of choice because they do not introduce any elements beyond those needed to form borosilicate glasses. One way to fabricate an SiC/(Si/SiC) composite object is first to make a preform of silicon carbide fibers interspersed with a mixture of


silicon carbide and carbon particles, then infiltrate the preform with molten silicon. Boron can be incorporated by chemical vapor deposition onto the fibers prior to making the preform. Boron and germanium can be incorporated into the matrix by adding these elements to either the matrix or the molten silicon infiltrant. Inasmuch as the solubility of boron in silicon is limited, it may be necessary to add the boron via the preform in a typical case. Silicon Carbide Platelets Single crystals of α-phase hexagonal crystal structure and four size ranges currently are produced: –100, +200 mesh (100 to 300 µm in diameter, 5 to 15 µm thick); –200, +325 mesh (50 to 150 µm in diameter, 1 to 10 µm thick); –325 mesh (5 to 70 mm in diameter, 0.5 to 5 mm thick), and –400 mesh (3 to 30 µm in diameter, 0.5 to 3 µm thick). The finest size is a research product and additional development work is being conducted to produce an even smaller diameter platelet in the 0.5 µm range, which would be an ideal reinforcement material for ceramic-matrix composites. In addition to reinforcing ceramics, silicon carbide platelets also are used to increase the strength, wear resistance, and thermal shock performance of aluminum matrices, and to enhance the properties of polymeric matrices. Because platelets are very free-flowing, they can be processed in the same manner as particulates. Silicon Carbide Whiskers Whiskers as small as 7 µm in diameter can be made by a number of different processes. Although these whiskers have the disadvantage in some applications of not being in continuous filament form, they can be made with higher tensile strength and modulus values than continuous silicon carbide filament. Silicon carbide whiskers are single crystals of either α- or β-phase crystal structure. The SiC whiskers tend to exhibit a hexagonal, triangular, or rounded cross section and may contain stacking faults. SiC whiskers can be fabricated by the reaction of silicon and carbon to form a gaseous species that can be transported and reacted in


the vapor phase. This type of formation is referred to as a vapor-solid reaction. The reactions occur at temperatures greater than 1400°C and in an inert or nonoxidizing atmosphere. In addition, a catalyst is added to assure the formation of whiskers rather than particulate during the reaction. Although SiC whiskers can be coated with several different materials, such as carbon, to enhance their performance, the as-produced SiC whiskers generally contain a 5 to 30 SiO2 coating, which forms during synthesis. SiC whiskers are added to a variety of matrices to increase the toughness and hightemperature strength of these materials. The elastic modulus for SiC whiskers is 400 to 500 GPa and the tensile strength ranges from 1 to 5 GPa. A variety of ceramic matrices, such as Al2O3, Si3N4, MoSi2, AlN, mullite, cordierite, and glass ceramics, are combined with SiC whiskers to increase the overall mechanical properties of the resulting composite. For example, the wear resistance, toughness, and thermal shock of Al2O3 is increased by the addition of SiC whiskers. The resulting composite has been used for such applications as high-performance cutting tool inserts. The addition of SiC whiskers to an alumina matrix can double the fracture toughness of the resulting composite, depending on whisker content and processing conditions. SiC whiskers also can be combined with metals to increase the high-temperature strength of the material as well as provide a comparable substitution for heavier traditional materials, such as steel. Metal-matrix composites (MMCs) are being tested for such applications as piston ring grooves, cylinder block liners, brake calipers, and aerospace components. MMCs can be fabricated by infiltrating an SiC whisker preform with aluminum or by the addition of SiC whiskers to molten aluminum. Polymer-matrix composites combine the strength and impact resistance of polymers with the thermal conductivity fatigue and wear resistance of the whiskers. Whisker-reinforced polymers have strong potential to replace traditional plastics in automotive, aerospace, and recreational applications.


SILICON CAST IRON This is an acid-resistant cast iron containing a high percentage of silicon. When the amount of silicon in cast iron is above 10%, there is a notable increase in corrosion and acid resistance. The acid resistance is obtained from the compound Fe3Si, which contains 14.5% silicon. The usual amount of silicon in acid-resistant castings is from 12 to 15%. The alloy casts well but is hard and cannot be machined. These castings usually contain 0.75 to 0.85% carbon. A 14 to 14.5% silicon iron has a silverywhite structure, and is resistant to hot sulfuric acid, nitric acid, and organic acids. Silicon irons are also very wear resistant, and are valued for pump parts and for parts for chemical machinery.

SILICON COPPER An alloy of silicon and copper used for adding silicon to copper, brass, or bronze, silicon copper is also employed as a deoxidizer of copper and for making hard copper. Silicon alloys in almost any proportion with copper, and is the best commercial hardener of copper. A 50–50 alloy of silicon and copper is hard and extremely brittle and black in color. A 10% silicon, 90% copper alloy is as brittle as glass; in this proportion silicon copper is used for making the addition to molten copper to produce hard, sound copper-alloy castings of high strength. The resulting alloy is easy to cast in the foundry and does not dross. Silicon-copper grades in 5, 10, 15, and 20% silicon are also marketed. A 10% silicon-copper melts at 816°C; a 20% alloy melts at 623°C.

SILICON HALIDES Silicon tetrachloride, SiCl4, is perhaps the best known monomeric covalent compound of silicon. It is readily available commercially. It can be prepared by chlorinating elementary silicon, or by the action of chlorine on a mixture of silica with finely divided carbon, or by the chlorination of silicon carbide. It is a volatile liquid that fumes in moist air and hydrolyzes rapidly to silica and hydrochloric acid.

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Pure silica with very high surface area, produced by this method, is used as a reinforcing filler (white carbon black) in silicone rubber and as a thickening agent in organic solutions. Silicon tetrachloride reacts readily with alcohols and glycols, for example, to form the corresponding ethers, which may also be considered to be esters of silicic acid.

SILICON MANGANESE An alloy employed for adding manganese to steel, and also as a deoxidizer and scavenger of steel, silicon manganese usually contains 65 to 70% manganese and 12 to 25% silicon. It is graded according to the amount of carbon, generally 1, 2, and 2.5%. For making steels low in carbon and high in manganese, silicomanganese is more suitable than ferromanganese. A reverse alloy, called manganese–silicon, contains 73 to 78% silicon and 20 to 25% manganese, with 1.5% max iron and 0.25% max carbon. It is used for adding manganese and silicon to metals without the addition of iron. Still another alloy is called ferromanganese–silicon, containing 20 to 25% manganese, about 50% silicon, and 25 to 30% iron, with only about 0.50% or less carbon. This alloy has a low melting point, giving ready solubility in the metal.

SILICON NITRIDE

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Silicon nitride (Si3N4) dissociates in air at 1800°C and at 1850°C under 1 atm N2. There are two crystal structures: α (1400°C) and β (1400 to 1800°C), both hexagonal. Its hardness is approximately 2200 on the Knoop K100 scale; and it exhibits excellent corrosion and oxidation resistance over a wide temperature range. Typical applications are molten-metalcontacting parts, wear surfaces, special electrical insulator components, and metal forming dies. It is under evaluation for gas turbine and heat engine components as well as antifriction bearing members.

PROCESSING Pure silicon nitride powders are produced by several processes, including direct nitridation


of silicon, carbothermal reduction — C + SiO2 + N2 yields Si3N4 (gas atmosphere) — and chemical vapor deposition — 3 SiH4 + 4NH3 yields Si3N4 + 12H2. Reacting SiO2 with ammonia or silanes with ammonia will also produce silicon nitride powders. It is found that the highest-purity powders come from gas-phase reactions.

TYPES Sinterable/Hot Pressed/ Hot Isostatically Pressed Silicon Nitride These types are SSN, HPSN, and HIPSN, respectively. They are used mainly in higher performance applications. Powdered additives, known as sintering aids, are blended with the pure Si3N4 powder and allow densification to proceed via the liquid state. Pore-free bodies can be so produced by sintering or hot pressing. Of course, the properties of the material and dense pieces are dependent on the chemical nature of the sintering aids employed. Sinterable silicon nitrides are a more recent innovation, and allow more flexibility in shape fabrication than does HPSN. Highly complex shapes can be die-pressed or isostatically pressed. Densification can be performed by either sintering or hot isostatic pressing (HIP). Properties of the dense piece are dependent on the additives, but in general the strength below 1400°C, as well as oxidation resistance of HPSN and SSN, far exceed those properties for reactor bonded silicon nitride (RBSN). Reaction Bonded Silicon Nitride More common today is RBSN. Silicon powder is pressed, extruded, or cast into shape, then carefully nitrided in a N2 atmosphere at 1100 to 1400°C, so as to prevent an exothermic reaction, which might melt the pure silicon. The properties of RBSN are usually lower than those of HPSN or SSN, due mainly to the fact that bodies fabricated in this manner only reach 85% of the theoretical density of silicon nitride and no secondary phase between grains is present.


Silicon Nitride Fibers Si3N4 fibers have been prepared by reaction between silicon oxide and nitrogen in the presence of a reducing agent in an electrical resistance furnace at 1400°C. Silicon nitride short fibers are used in composites for specialty electrical parts, aircraft parts, and radomes (microwave windows). Silicon nitride whiskers have also been grown as a result of the chemical reaction between nitrogen and a mixture of silicon and silica.

SILICON OXIDES Silicon dioxide is perhaps best known as one of its crystalline modifications known as quartz, colorless crystals of which are also known as rhinestones and Glens Falls diamonds. Purple or lavender-colored quartz is called amethyst, the pink variery is rose quartz, and the yellow type, citrine. Because rock crystal has been collected and admired for thousands of years, large and perfectly formed natural crystals of quartz are now very rare. With the growth of radio broadcasting and the electronics industry, piezoelectric crystals cut from perfect specimens of quartz have been used in increasing quantities, to the point of scarcity of natural crystals. As the supply diminished, considerable effort was devoted to the problem of growing crystals of quartz by artificial means. Some success has been achieved by growing the crystals hydrothermally from a solution of silica glass in water containing an alkali or a fluoride. A number of natural noncrystalline varieties of silicon dioxide are also known, such as the hydrated silica known as opal and the dense unhydrated variety known as flint. Onyx and agate represent still other semiprecious forms.

SILICON STEEL All grades of steel contain some silicon and most of them contain from 0.10 to 0.35% as a residual of the silicon used as a deoxidizer. But from 3 to 5% silicon is sometimes added to increase the magnetic permeability, and larger amounts are added to obtain wear-resisting or


acid-resisting properties. Silicon deoxidizes steel, and up to 1.75% increases the elastic limit and impact resistance without loss of ductility. Silicon steels within this range are used for structural purposes and for springs, giving a tensile strength of about 517 MPa and 25% elongation. The structural silicon steels are ordinarily silicon–manganese steel, with the manganese above 0.50%. Low-carbon steels used as structural steels are made by careful control of carbon, manganese, and silicon and with special mill heat treatment. The value of silicon steel as a transformer steel occurs where silicon increases the electrical resistivity and also decreases the hysteresis loss, making silicon steel valuable for magnetic circuits where alternating current is used.

SILICONE RESINS Silicone resins are synthetic materials capable of cross-linking or polymerizing to form films, coatings, or molded shapes with outstanding resistance to high temperatures. They are a group of resinlike materials in which silicon takes the place of the carbon of the organic synthetic resins. Silicon is quadrivalent like carbon. But, while the carbon also has a valence of 2, silicon has only one valence of 4, and the angles of molecular formation are different. The two elements also differ in electronegativity, and silicon is an amphoteric element, with both acid and basic properties. The molecular formation of the silicones varies from that of the common plastics, and they are designated as inorganic plastics as distinct from the organic plastics made with carbon. In the long-chain organic synthetic resins the carbon atoms repeat themselves, attaching on two sides to other carbon atoms, while in the silicones the silicon atom alternates with an oxygen atom so that the silicon atoms are not tied to each other. The simple silane formed by silicon and hydrogen corresponding to methane, CH4, is also a gas, as is methane, and has the formula SiH4. But, in general, the silicones do not have the SiH radicals, but contain CH radicals as in the organic plastics.

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COMPOSITION Silicones are made by first reducing quartz rock (SiO2) to elemental silicon in an electric furnace, then preparing organochlorosilane monomers (RSiCl3, R2SiCl2, R3SiCl) from the silicon by one of several different methods. The monomers are then hydrolyzed into cross-linked polymers (resins) whose thermal stability is based on the same silicon–oxygen–silicon bonds found in quartz and glass. The properties of these resins will depend on the amount of crosslinking, and on the type of organic groups (R) included in the original monomer. Methyl, vinyl, and phenyl groups are among those used in making silicone resins.

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Silicone resins have, in general, more heat resistance than organic resins, have higher dielectric strength, and are highly water resistant. Like organic plastics, they can be compounded with plasticizers, fillers, and pigments. They are usually cured by heat. Because of the quartzlike structure, molded parts have exceptional thermal stability. Their maximum continuous-use service temperature is about 260°C. Special grades exceed this and go as high as 371 to 482°C. Their heat-deflection temperature for 1.8 MPa is 482°C. Their moisture absorption is low, and resistance to petroleum products and acids is good. Nonreinforced silicones have only moderate tensile and impact strength, but fillers and reinforcements provide substantial improvement. Because silicones are high in cost, they are premium plastics and are generally limited to critical or high-performance products such as high-temperature components in the aircraft, aerospace, and electronics fields. Common characteristics shared by most silicone resins are outstanding thermal stability, water repellency, general inertness, and electrical insulating properties. These properties, among others, have resulted in the use of silicone resins in the following fields: 1. Laminating (reinforced plastics), molding, foaming, and potting resins 2. Impregnating cloth coating, and wire varnishes for Class II (high-performance) electric motors and generators © 2002 by CRC Press LLC

3. Protective coating resins 4. Water repellents for textiles, leather, and masonry 5. Release agents for baking pans Laminating Resins Laminates made from silicone resin and glass cloth are lightweight, strong, heat-resistant materials used for both mechanical and dielectric applications. Silicone-glass laminates have low moisture absorption and low dielectric losses, and retain most of their physical and electrical properties for long periods at 260°C. Laminates may be separated into three groups according to the method of manufacture: high pressure, low pressure, and wet layup. High pressure. Silicone-glass laminates (industrial thermosetting laminates) have excellent electric strength and arc resistance, and are normally used as dielectric materials. To prepare high-pressure laminates, glass cloth is first impregnated by passing it through a solvent solution of silicone resin. The resin is dried of solvent and precured by passing the fabric through a curing tower. Laminates are prepared by laying up the proper number of plies of preimpregnated glass cloth and pressing them together at about 6.8 MPa and 177°C for about 1 h. They are then oven-cured at increasing temperatures, with the final cure at about 249°C. The resulting laminates can be drilled, sawed, punched, or ground into insulating components of almost any desired shape. Typical applications include transformer spacer bars and barrier sheets, slot sticks, panel boards, and coil bobbins. Low pressure. Silicone-glass laminates made by low-pressure reinforced plastics molding methods usually provide optimum flexural strength, e.g., about 272 MPa even after heat aging. They are used for mechanical applications such as radomes, aircraft ductwork, thermal barriers, covers for high-frequency equipment, and high-temperature missile parts. In making low-pressure laminates, glass cloth is first impregnated and laid up as described above. Since the required laminating pressure can be as low as 0.068 MPa, matched-metalmolding and bag-molding techniques can be used in laminating, making possible greater


variety in laminated shapes. Lamination should be after-cured as already described. Wet layup. Silicone-glass laminates can now be produced by wet layup techniques because of the solventless silicone resins recently developed. Such laminates can be cured without any pressure except that needed to hold the laminate together. This technique should prove especially useful in making prototype laminates, and in short production runs where expensive dies are not justified. Laminates are prepared by wrapping glass cloth around a form and spreading on catalyzed resin, repeating this process until the desired thickness is obtained. The laminate surface is then wrapped with a transparent film, and air bubbles are worked out. Laminates are cured at 149°C and, after the transparent film is removed, postcured at 204°C. Molding Compounds Silicone molding compounds consist of silicone resin, inorganic filler, and catalyst, which, when molded under heat and pressure, form thermosetting plastic parts. Molded parts retain exceptional physical and electrical properties at high temperatures, resist water and chemicals, and do not support combustion. Specification MILM-14E recognizes two distinctly different types of silicone molding compounds: type MSI-30 (glass-fiber filled) and type MSG (mineral filled). Requirements for both are as follows. Type MSI-30. Glass-filled molded parts have high strengths, which become greater the longer the fiber length of the glass filler. Where simple parts can be compression-molded from a continuous fiber length compound, strengths will be approximately twice as great. Properly cured glass-fiber-filled molded parts can be exposed continuously to temperatures as high as 371°C and intermittently as high as 538°C. The heat-distortion temperature after postcure is 482°C. Because of the flow characteristics of these fiber-filled compounds, their use is generally limited to compression molding. Type MSG. Mineral-filled compounds are free-flowing granular materials. They are suitable for transfer molding, and can be used in automatic preforming and molding machines. They are used to make complex parts that retain


their physical and electrical properties at temperatures above 260°C, but that do not require impact strength. Silicone molding compounds are excellent materials for making Class H electrical insulating components such as coil forms, slot wedges, and connector plugs. They have many potential applications in the aircraft, missile and electronic industries. Foaming Powders Silicone foaming powders are completely formulated, ready-to-use materials that produce heat-stable, nonflammable, low-density silicone foam structures when heated. Densities vary from 160 to 288 kg/m 3, compressive strengths from 0.68 to 2.23 MPa. Electrical properties are excellent, and water absorption after 24-h immersion is only 2.5%. The maximum continuous operating temperature of these foams is about 343°C. Foams are prepared by heating the powders to between 149 and 177°C for about 2 h. The powder can be foamed in place, or foamed into blocks and shaped with woodworking tools. Foams are normally after-cured to develop strength, but can often be cured in service. Silicone foams are being used in the aircraft and missile industries to provide lightweight thermal insulation and to protect delicate electronic equipment from thermal shock. They can also be bonded to silicone-glass laminates or metals to form heat- and moisture-resistant sandwich structures. Potting Resins Solventless silicone resins can be used for impregnating, encapsulating, and potting of electrical and electronic units. Properly catalyzed, filled, and cured, they form tough materials with good physical and electrical properties, and will withstand continuous temperatures of 204°C and intermittent temperatures above 260°C. Typical physical properties of cured resins include flexural strength of 48.6 MPa, compressive strength of 117 MPa, and water absorption of 0.04%. Before use, resins are catalyzed with dicumyl peroxide or ditertiary butyl peroxide.

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Resins can be simply poured in place, although vacuum impregnation is suggested where fine voids must be filled. Fillers such as glass beads or silica flour are added to extend the resin; their use increases physical strength and thermal conductivity, but decreases electrical properties. The resin is polymerized by heating it to about 149°C, and postcured, first at 204°C, then at the intended operating temperature if higher. Electrical Varnishes Silicone varnishes (solvent solutions of silicone resins) have made possible the new high-temperature classes of insulation for electrical motors and generators. Silicone insulating varnishes will withstand continuous operating temperatures at 177°C or higher. Electrical equipment that operates at higher temperatures makes possible motors, generators, and transformers that are much smaller and lighter, or equipment that delivers 25 to 50% more power from the same size and still has a much longer service life. The resinous silicone materials used in Class H electrical insulating systems include the following:

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1. Silicone bonding varnish for glassfiber-covered magnet wire. 2. Silicone varnishes for impregnating and bonding glass cloth, mica, and asbestos paper. Sheet insulations made of these heat-resistant materials are used as slot liners for electric motors and as phase insulation. 3. Silicone dipping varnish that impregnates, bonds, and seals all insulating components into an integrated system. Other silicone materials used in electrical equipment include silicone rubber lead wire, silicone-adhesive-backed glass tape, and temperature-resistant silicone bearing greases. Silicone insulated motors, generators, and transformers are now being produced by the leading electrical equipment manufacturers.


USES The wide range of structural variations of silicone resins makes it possible to tailor compositions for many kinds of applications. Lowmolecular-weight silanes containing amino or other functional groups are used as treating or coupling agents for glass fiber and other reinforcements to cause unsaturated polyesters and other resins to adhere better. The liquids, generally dimethyl silicones of relatively low molecular weight, have low surface tension, great wetting power and lubricity for metals, and very small change in viscosity with temperature. They are used as hydraulic fluids, as antifoaming agents, as treating and waterproofing agents for leather, textiles, and masonry, and in cosmetic preparations. The greases are particularly desired for applications requiring effective lubrication at very high and at very low temperatures. Silicone resins are used for coating applications in which thermal stability in the range 300 to 500°C is required. The dielectric properties of the polymers make them suitable for many electrical applications, particularly in electrical insulation that is exposed to high temperatures and as encapsulating materials for electronic devices. Silicone enamels and paints are more resistant to chemicals than most organic plastics, and when pigmented with mineral pigments will withstand temperatures up to 538°C. For lubricants the liquid silicones are compounded with graphite or metallic soaps and will operate between –46 and 260°C. The silicone liquids are stable at their boiling points, between 399 and 427°C, and have low vapor pressures, so that they are also used for hydraulic fluids and heat-transfer media. Silicone oils, used for lubrication and as insulating and hydraulic fluids, are methyl silicone polymers. They retain a stable viscosity at both high and low temperatures. As hydraulic fluids they permit smaller systems to operate at higher temperatures. In general, silicone oils are poor lubricants compared with petroleum oils, but they are used for high temperatures, 150 to 200°C, at low speeds and low loads. Silicone resins are blended with alkyd resins for use in outside paints, usually modified


with a drying oil. Silicone-alkyd resins are also used for baked finishes, combining the adhesiveness and flexibility of the alkyd with the heat resistance of the silicone. A phenyl ethyl silicone is used for impregnating glass-fiber cloth for electrical insulation and it has about double the insulating value of ordinary varnished cloth.

SILICONE RUBBER Silicone rubbers are a group of synthetic elastomers noted for their (1) resilience over a very wide temperature range, (2) outstanding resistance to ozone and weathering, and (3) excellent electrical properties.

COMPOSITION The basic silicone elastomer is a dimethyl polysiloxane. It consists of long chains of alternating silicon and oxygen atoms, with two methyl (–CH3) side chains attached to each silicon atom. By replacing a part of these methyl groups with other side chains, polymers with various desirable properties can be obtained. For example, where flexibility at temperatures lower than –57°C is desired, a polymer with about 10% of the methyl side chains replaced by phenyl groups (–C6H5) will provide compounds with brittle points below -101°C. Sidechain modification can also be used to produce elastomers with lower compression set, increased resistance to fuels, oils, or solvents, or to permit vulcanization at room temperature. Curing, or vulcanization, is the process of introducing cross-links at intervals between the long chains of the polymer. Silicone rubbers are usually cross-linked by free radical-generating curing agents, such as benzoylperoxide, which are activated by heat, or the cross-linking can be accomplished by high-energy radiation beams. Room temperature vulcanized compounds are cross-linked by the condensation reaction resulting from the action of metalorganic salts, such as zinc or tin octoates. Pure polymers upon cross-linking change from viscous liquids into elastic gels with very low tensile strength. To attain satisfactory tensile strength, reinforcing agents are necessary. Synthetic and natural silicas and metallic oxides are


commonly used for this purpose. In addition to the vulcanizing agents and reinforcing fillers described, other additives may be incorporated into silicone compounds to pigment the stock, to improve processing, or to reduce the compression set of certain types of silicone gum. Ordinary silicone rubber has the molecular group (H · CH2 · Si · CH2 · H) in a repeating chain connected with oxygen linkages, but in the nitrile-silicone rubber one of the end hydrogens of every fourth group in the repeating chain is replaced by a C:N radical. These polar nitrile groups give a low affinity for oils, and the rubber does not swell with oils and solvents. It retains strength and flexibility at temperatures from –73°C to above 260°C, and is used for such products as gaskets and chemical hose. As lubricants, silicones retain a nearly constant viscosity at varying temperatures. Fluorosilicones have fluoroalkyd groups substituted for some of the methyl groups attached to the siloxane polymer of dimethyl silicone. They are fluids, greases, and rubbers, incompatible with petroleum oils and insoluble in most solvents. The greases are the fluids thickened with lithium soap, or with a mineral filler.

TYPES Silicone-rubber compounds can be conveniently grouped into several major types according to characteristic properties. Typical properties of several types are shown in Table S.5. According to one such classification system, types are (1) general purpose, (2) extremely low temperature, (3) extremely high temperature, (4) low compression set, (5) high strength, (6) fluid resistant, (7) electrical, and (8) room temperature vulcanizing rubbers. 1. General-purpose compounds are available in Shore A hardnesses from 30 to 90, tensile strengths of 4.8 to 8.24 MPa, and ultimate elongations of 100 to 500%. Their service temperature range extends from –55 to 260°C, and they have good resistance to heat and oils, along with good electrical properties. Many of these compounds contain semireinforcing

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2.

3.

4.

5.

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6.

or extending fillers to lower their cost. Extremely low temperature compounds have brittle points near –118°C and are quite flexible at –84 to –90°C. Their physical properties are usually about the same as those of the general-purpose stocks, with some reduction in oil resistance. Extremely high temperature compounds are considered serviceable for over 70 h at 343°C and will withstand brief exposures at higher temperatures; for example, 4 to 5 hr at 371°C and 10 to 15 min at 399°C. In comparison, general-purpose compounds are limited to about 260°C for continuous service and 316°C for intermittent service. Low compression set compounds provide typical values of 10 to 20% compression set after 22 h at 149°C. They have improved resistance to petroleum oils and various hydraulic fluids, and are particularly suitable for use in O-rings and gaskets. High-strength compounds, in Shore A hardnesses of 25 to 70, provide tensile strengths from 0.82 to over 136 MPa and elongations from 400 to 700%, with tear strengths from 26,790 to 58,045 g/cm. Compounds of this class may operate over a service temperature range from –90 to 316°C. Excellent resistance to a wide range of fuels, lubricants, and hydraulic fluids is offered by compounds based on a silicone polymer with 50% of its side methyl groups substituted by trifluoropropyl groups. Physical properties are similar to properties of other types of silicone compounds. However, service temperature range is somewhat limited. Its low-temperature properties are about the same as those of the dimethyl polymer, with a brittle point around –68°C and an upper service temperature of around 260°C.


7. In general, silicone-rubber compounds have excellent electrical properties, which, along with their resistance to high temperatures, make them suitable for many electrical applications. With proper compounding, dielectric constant can be easily varied from about 2.7 to 5.0 or higher, while the power factor can be varied from 0.0005 up to 0.1 or higher. The volume resistivity of a typical silicone compound will be in the range 1014 to 1016 Ω-cm and its dielectric strength will be about 450 to 550 V/mil thickness (measured on a slab 2.0 mm). Resistance to corona is excellent and water absorption is low. In most cases, excellent electrical properties are retained over a wide temperature and frequency range. Compounds can also be prepared with very low resistivity, as low as about 10 Ω-cm, for special applications. Insulated tapes for cable-wrapping applications can be prepared from electrical-grade compounds with a partial cure or from a completely cured self-adhering silicone compound. 8. Room-temperature vulcanizing silicone rubbers are available to provide most of the performance characteristics of silicone rubbers in compounds that cure at room temperature. In addition to having excellent heat resistance, silicone rubbers retain their properties to a much greater extent at high temperatures than do most organic rubbers. For example, a silicone compound with a tensile strength at temperature of 1.36 MPa will have a tensile strength at 316°C of 0.48 MPa or over. Most organic rubbers, although their initial properties are much higher, will be virtually useless at 260°C (except for the fluoroelastomers). In applications where a silicone rubber part operates in low oxygen atmosphere, such as sealing on high-altitude aircraft, its heat resistance will be still further improved. In using silicone rubber at high temperatures, care must be taken to prevent reversion


TABLE S.5 Properties of Some Typical Silicone Rubbers

Hardness (Shore A) Tensile strength, psi Elongation, % Tear strength, lb/in. Compression set (22 h at 300°F), % Max service temp., °F Continuous Intermittent Low temp. flex., °F Volume swell (ASTM No. 1 Oil; 70 h at 300°F), % a b

General Purpose 50 ± 5 1000 400 80 20

Extreme Low Temp. 25 ± 5 1000 600 120 20

Extreme High Temp. 50 ± 5 1200 300 150 15

Low Compression Set 60 ± 5 900 130 60 10

High Strength 50 ± 5 2000 600 300 30

Fluorosilicone (General Purpose) 60 ± 5 950 225 75 15

RTVa 65 ± 5 750 110 40 13b

500 600 –65 7

500 600 –130 10

550 700 –130 +9

500 600 –65 +5

500 600 –130 +10

500 550 –65 +1

500 600 –65 +3

Room temperature vulcanizing silicone rubber. After additional postcure 24 h at 480°F.

or depolymerization, which may occur where a part is required to operate in an enclosed environment. Here again, where this problem cannot be eliminated by the design engineer, the silicone-rubber fabricator can produce compounds with relatively high resistance to reversion.

FABRICATION

AND

USES

In general, silicone rubbers may be handled on standard rubber-processing equipment. Their fabrication differs from that of organic rubbers chiefly in that uncured silicone compounds are softer and tackier and have much lower green strength. Also, an oven postcure in a circulating air oven is often required after vulcanization to obtain optimum properties. Silicone rubbers can be extruded, molded, calendered, sponged, and foamed. Since compounding of the rubber stock determines to a great extent the processing characteristics of the material, the fabricator should be consulted before the material is specified, to determine whether compromises are necessary to obtain the best combination of physical properties and the most desirable shape. Silicone-rubber compounds can also be applied to fabrics by calender coating, knife spreading, or solvent dispersion techniques. For certain applications, very soft or low durometer materials are required. Suitable for


such applications are low durometer solid silicone rubbers, closed or open cell expanded silicone rubbers (designated here as sponge and foam, respectively), and fibrous silicone rubber. Silicone-rubber sponge is available in molded sheets, extrusions, and simple molded shapes. As in the case of solid silicone rubber, improved resistance to fluids or abrasion can be obtained by bonding molded or extruded sponge to a fabric or plastic cover. Silicone foam rubber can be fabricated in heavy cross sections and complex shapes, and can be foamed and vulcanized either at ambient or elevated temperature. It is suitable for use where an extremely soft, low-density silicone material is required. Like sponge, it can be bonded to fabrics and plastics. For some applications a solid, low durometer material has advantages over sponge or foam. For example, it should probably be specified for gaskets or seals where the low compression set of foam, the compress-ion-deflection characteristics of sponge, and the higher tensile and tear strength of solid silicone rubbers must be combined. The last highly compressible material, fibrous silicone rubber, consists of hollow rubber fibers sprayed in a random manner and bonded into a low-density porous mat. Its properties

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include excellent compression set combined with good tear and tensile strength, and very high porosity. As manufactured at present, it is serviceable from –55°C to over 260°C, and is available in mats 3.2 mm thick and 228 mm wide. Silicone rubbers are most widely used in the aircraft, electrical, and automotive industries, although their unique properties have created many other applications. Specific examples would include seals for aircraft canopies or access doors, insulation for wire and cable, dielectric encapsulation of electronic equipment, and gaskets or O-rings for use in aircraft or automobile engines. An example of another field in which they are useful is the manufacture of stoppers for pharmaceutical vials, since silicone rubbers are tasteless, odorless, and nontoxic.

SILK

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Silk is the fibrous material in which the silkworm, or larva of the moth, envelops itself before passing into the chrysalis state. Silk is closely allied to cellulose and resembles wool in structure, but unlike wool it contains no sulfur. The natural silk is covered with a wax or silk glue, which is removed by scouring in manufacture, leaving the glossy fibroin, or raw-silk fiber. The fibroin consists largely of the amino acid alanine, CH3CH(NH2)CO2H, which can be synthesized from pyruvic acid. Silk fabrics are used mostly for fine garments, but are also valued for military powder bags because they burn without a sooty residue. The fiber is unwound from the cocoon and spun into threads. Each cocoon has from 1829 to 2743 m of thread. The chief silk-producing countries are China, Japan, India, Italy, and France. Floss silk is a soft silk yarn practically without twist, or is the loose waste silk produced by the worm when beginning to spin its cocoon. Satin is a heavy silk fabric with a close twill weave in which the fine warp threads appear on the surface and the weft threads are covered by the peculiar twill. Common satin is of eightleaf twill, the weft intersecting and binding down the warp at every eighth pick, but 16 to 20 twills are also made. In the best satins a fine quality of silk is used.


SILVER AND ALLOYS A white metal (symbol Ag), silver is very malleable and ductile, and is classed with the precious metals. It occurs in the native state, and also combined with sulfur and chlorine. Copper, lead, and zinc ores frequently contain silver; about 70% of the production of silver is a by-product of the refining of these metals. Silver is the whitest of all the metals and takes a high polish, but easily tarnishes in the air because of the formation of a silver sulfide. It has the highest electrical and heat conductivity: 108% IACS relative to 100% for the copper standard and about 422 W/m · K, respectively. Cold work reduces conductivity slightly. The specific gravity is 10.7, and the melting point is 962°C. When heated above the boiling point (2163°C), it passes off as a green vapor. It is soluble in nitric acid and in hot sulfuric acid. The tensile strength of cast silver is 282 MPa, with Brinell hardness 59. The metal is marketed on a troy-ounce value. Pure silver has the highest thermal and electrical conductivity of any metal, as well as the highest optical reflectivity. Next to gold it is the most ductile and most malleable of any metal. Silver can be hammered into sheet 0.01 mm thick or drawn out in wire so fine 120 m would weigh only 1 g. Classified as one of the most corrosion-resistant metals, silver, under ordinary conditions, will not be affected by caustics or corrosive elements, unless hydrogen sulfide is present, causing silver sulfide to form. Silver will dissolve rapidly in nitric acid and more slowly in hot concentrated sulfuric acid. Unless oxidizing agents are present, the action of diluted or cold solutions of sulfuric acid is negligible. Organic acids generally do not attack the metal and caustic alkalies have but a slight effect on pure silver. Although silver tarnishes quickly in the presence of sulfur and sulfur-bearing compounds, it oxidizes slowly in air and the oxide decomposes at a relatively low temperature.

CLASSIFICATION Silver is classified by grades in parts per thousand based on the silver content (impurities are reported in parts per hundred). Commercial


grades are Fine Silver and High Fine Silver. As ordinarily supplied, fine silver contains at least 999.0 parts silver per 1000, and may go as high as 999.3 parts per 1000. Any of the common base metals may be present, although copper is usually the major impurity. Any silver of higher purity than commercial fine contains its purity in its description, i.e., 999.7 High Fine Silver. The purest silver obtainable in quantity is 999.9 plus; the impurities are less than 0.01 part per 1000. Fine silver may also contain small percentages of oxygen or hydrogen; deoxidized silver is available for applications where these elements may be a detriment.

FABRICABILITY Silver can be cold-worked, extruded, rolled, swaged, and drawn. It can be cold-rolled or cold-drawn drastically between anneals, and can be annealed at relatively low temperatures. To prevent oxidation when casting by conventional methods, silver should be protected by a layer of charcoal or by melting under neutral or reducing gas. Deoxidation by adding lithium or phosphorus can be obtained leaving a residual content of 0.01% max. The excellent ductility of silver makes it readily workable hot or cold. Molten silver will absorb approximately 20 times its own volume of oxygen. Most of this oxygen is given up when the silver solidifies in cooling, but care should be taken in melting and casting because any oxygen left in the cast bars will cause cracking when they are fabricated and the castings may have blow holes. Galling, seizing of the tool, and surface tearing are problems encountered when machining fine silver. This can be somewhat alleviated by using material cold-worked as much as possible. Joining Fine silver can be soldered without difficulty using tin–lead solders. Boron–silver filler metal can be used in brazing, and welding can be done by resistance methods and by atomic hydrogen or inert-gas shielded arc processes. A range of 204 to 427°C is recommended for annealing, with best strength and ductility achieved


between 371 and 427°C. Little additional softening occurs at higher temperatures, which may induce welding of adjacent surfaces. The lighter the gauge, the lower should be the annealing temperature.

APPLICATIONS OF ALLOYS AND COMPOUNDS Because silver is a very soft metal, it is not normally used industrially in a pure state, but is alloyed with a hardener, usually copper. Sterling silver is the name given to a standard highgrade alloy containing a minimum of 925 parts in 1000 of silver. It is used for the best tableware, jewelry, and electrical contacts. This alloy of 7.5% copper work-hardens and requires annealing between roll passes. Silver can also be hardened by alloying with other elements. The standard types of commercial silver are fine silver, sterling silver, and coin silver. Fine silver is at least 99.9% pure, and is used for plating, making chemicals, and for parts produced by powder metallurgy. Coin silver is usually an alloy of 90% silver and 10% copper, but when actually used for coins the composition and weight of the coin are designated by law. Silver and gold are the only two metals that fulfill all the requirements for coinage. The socalled coins made from other metals are really official tokens, corresponding to paper money, and are not true coins. Coin silver has a Vickers hardness of 148 compared with a hardness of 76 for hard-rolled pure silver. It is also used for silverware, ornaments, plating, for alloying with gold, and for electric contacts. Silver is not an industrial metal in the ordinary sense. It derives its coinage value from its intrinsic aesthetic value for jewelry and plate, and in all civilized countries silver is a controlled metal. Silver powder, 99.9% purity, for use in coatings, integrated circuits, and other electrical and electronic applications, is produced in several forms. Amorphous powder is made by chemical reduction and comes in particle sizes of 0.9 to 15 µm. Powder made electrolytically is in dendritic crystals with particle sizes from 10 to 200 µm. Atomized powder has spherical particles and may be as fine as 400 mesh. Silver-clad powder for electric contacts is a copper powder coated with silver to economize on silver. Silver flake is in the form of laminar platelets and is

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particularly useful for conductive and reflective coatings and circuitry. The tiny flat plates are deposited in overlapping layers permitting a metal weight saving of as much as 30% without reduction in electrical properties. Nickel-coated silver powder, for contacts and other parts made by powder metallurgy, comes in grades with 1/4 , 1/2 , 1, and 2% nickel by weight. The porous silver comes in sheets in standard porosity grades from 2 to 55 µm. It is used for chemical filtering. Silver plating is sometimes done with a silver–tin alloy containing 20 to 40 parts silver and the remainder tin. It gives a plate having the appearance of silver but with better wear resistance. Silver plates have good reflectivity at high wavelengths, but reflectivity falls off at about 350 nm, and is zero at 3000, so that it is not used for heat reflectors. Silver-clad sheet, made of a cheaper nonferrous sheet with a coating of silver rolled on, is used for food-processing equipment. It is resistant to organic acids but not to products containing sulfur. Silver-clad steel, used for machinery bearings, shims, and reflectors, is made with pure silver bonded to the billet of steel and then rolled. For bearings, the silver is 0.025 to 0.889 cm thick, but for reflectors the silver is only 0.003 to 0.008 cm thick. Silverclad stainless steel is stainless-steel sheet with a thin layer of silver rolled on one side for electrical conductivity. Silver iodide is a pale-yellow powder of the composition AgI, best known for its use as a nucleating agent and for seeding rain clouds. Silver nitrate, formerly known as lunar caustic, is a colorless, crystalline, poisonous, and corrosive material of the composition AgNO3. It is used for silvering mirrors, for silver plating, in indelible inks, in medicine, and for making other silver chemicals. The high-purity material is made by dissolving silver in nitric acid, evaporating the solution, and crystallizing the nitrate, then re-dissolving the crystals in distilled water and recrystallizing. It is an active oxidizing agent. Silver chloride, AgCl, is a white granular powder used in silver-plating solutions. This salt of silver and other halogen compounds of silver, especially silver bromide, AgBr, are used for photographic plates and


films. Silver chloride is used in the preparation of yellow glazes, purple of Cassius, and silver lusters. A yellowish-silver luster is obtained by mixing silver chloride with three times its weight of clay and ochre and sufficient water to form a paste. Silver chloride crystals in sizes up to 4.5 kg are grown synthetically. The crystals are cubic, and can be heated and pressed into sheets. The specific gravity is 5.56, index of refraction 2.071, and melting point of 455°C. They are slightly soluble in water and soluble in alkalies. The crystals transmit more than 80% of the wavelengths from 50 to 200 µm. Silver sulfide, Ag2S, is a gray-black, heavy powder used for inlaying in metal work. It changes its crystal structure at about 179°C, with a drop in electrical resistivity, and is also used for self-resetting circuit breakers. Silver potassium cyanide, KAg(CN)2, is a white, crystalline, poisonous solid used for silver-plating solutions. Silver tungstate, Ag 2WO4, silver manganate, AgMnO4, and other silver compounds are produced in high-purity grades for electronic and chemical uses. Silver nitrate, AgNO3, with a melting point of 212°C, decomposes at 444°C and is soluble, corrosive, and poisonous. It is prepared by the action of nitric acid on metallic silver. Silver nitrate is the most convenient method of introducing silver into a glass; a solution of the compound is poured over the batch. The photosensitive halides used in photography, the cyanides used in electroplating, and most of the minor silver salts are prepared from silver nitrate. In advanced ceramic applications, silver is unsurpassed as a conductor of heat and electricity. Silver is used in conductive coatings for capacitors, printed wiring, and printed circuits on titanites, glass-bonded mica, steatite, alumina, porcelain, glass, and other ceramic bodies. These coatings also are used to metallize ceramic parts to serve as hermetically sealed enclosures, becoming integral sections of coils, transformers, semiconductors, and monolithic and integrated circuits. Two types of conductive coatings can be used on ceramic parts: those that are fired on and those that are baked on or air dried. The fired-on type contains, in addition to silver


powder, a finely divided low-melting glass powder, temporary organic binder, and liquid solvents in formulations with direct soldering properties and others suitable for electroplating, both possessing excellent adhesion and electrical conductivity. The baked-on and airdry types contain, in addition to silver powder, a permanent organic binder and liquid solvents. These preparations have somewhat less adhesion, electrical conductivity, and solderability than the fired-on type, but can be electroplated if desired. The air-dry type is used when it is not desirable to subject the base material to elevated firing temperatures. Any of the above silver compositions are available in a variety of vehicles suitable for application by squeegee, brushing, dipping, spraying, bonding wheel, roller coating, etc. Firing temperatures for direct-solder silver preparations range from 677 to 788°C. Silver compositions to be copper plated are fired at 1200 to 1250F. The firing cycle used with these temperatures will vary from 10 min to 6 h, depending on the time required to equalize the temperature of the furnace charge. A 62% Sn–36% Pb–2% Ag solder is generally used with the direct-solder silver compositions. It is recommended that this solder be used at a temperature of 213 to 219°C. Soldering to the plated silver coating is less critical and 50% Sn–50% Pb or 40% Sn–60% Pb, as well as other soft solders, are being used with good results. The air-dried silver compositions will, as the designation implies, air dry at room temperature in approximately 16 h. This drying time can be shortened by subjecting the coating to temperatures of 60 to 93°C for 10 to 30 min. The baked-on preparations must be cured at a minimum temperature of 149°C for 5 to 16 h. The time may be shortened to 1 h by raising the temperature to 301°C. The same soft solders and techniques as recommended for the fired-on coatings may be used for the electroplated air-dried and bakedon preparations. It is extremely difficult to solder to air-dried or baked-on coatings without first electroplating. The surface conductivity of the fired silver coating is far better than that of the air-dried or baked-on coating. Fired coatings have a surface


electrical square resistance of approximately 0.01 Ω while the surface electrical square resistance of air-dried or baked-on is about 1 Ω.

USUALLY ALLOYED Because pure silver is so soft, it is usually alloyed with other metals for strength and durability. The most common alloying metal is copper, which imparts hardness and strength without appreciably changing the desirable characteristics of silver. Sterling silver has applications in manufacturing processes. For example, sterling silver plus lithium has been used in the aircraft industry for brazing honeycomb sections. Other silver–copper alloys are coin silver, 90.0% silver, 10.0% copper, and the silver–copper eutectic, 72% silver, 28% copper. This latter alloy has the highest combination of strength, hardness, and electrical properties of any of the silver alloys. Silver braze filler metals are widely used for joining virtually all ferrous and nonferrous metals, with the exception of aluminum, magnesium, and some other lower-melting-point metals. Whereas pure silver melts at 960°C, silver alloys, with compositions of 10 to 85% silver (the alloying metals are copper, zinc, cadmium, and/or other base metals), have melting points of 618 to 960°C. These alloys have ductility and malleability and can be rolled into sheet or drawn into wire of very small diameter. They may be employed in all brazing processes and are generally free flowing when molten. Recommended joint clearances are 0.05 to 0.13 mm when used with flux. Whereas fluxes are usually required, zinc and cadmium-free alloys can be brazed in a vacuum or in reducing or inert atmospheres without flux. Joints made with silver brazing alloys are strong, ductile, and highly resistant to shock and vibration. With proper design there is no difficulty in obtaining joint strength equal to or greater than that of the metals joined. The strongest joints have but a few thousandths of an inch of the alloy as bonding material. Typical joints made with silver brazing alloys, giving the greatest degree of safety, are scarf, lap, and butt joints. For electrical contacts, silver is combined with a number of other metals, which increase hardness and reduce the tendency to sulfide

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tarnishing. Silver–cadmium, for example, is extensively used for contacts, with the cadmium ranging from 10 to 15%. The advantages of these alloys are resistance to sticking or welding, more uniform wear, and a decreased tendency for metal transfer. Alloys used for contact and spring purposes are the silver–magnesium–nickel series (99.5% silver), which are used where electrical contacts are to be joined by brazing without loss of hardness, in miniature electron tubes for spring clips where high thermal conductivity is essential, and for instruments and relay springs requiring good electrical conductivity at high temperatures. These are unique, oxidation-hardening alloys. Before being hardened, the silver–magnesium–nickel alloy can be worked by standard procedures. After hardening in an oxidizing atmosphere, the room-temperature tensile properties are similar to those of hard rolled sterling silver or coin silver. Gold and palladium are also combined with silver for contact use because they reduce welding and tarnishing and, to some extent, increase hardness. When certain base metals do not combine with silver by conventional methods, powder metal processes are employed. This is particularly true of silver–iron, silver–nickel, silver–graphite, silver–tungsten, etc. These alloys are used in electrical contacts because of the desirable conductivity of silver and the mechanical properties of the base metals. They can be pressed, sintered, and rolled into sheet and wire that is ductile and suitable for forming into contacts by heading or stamping operations. Other silver products are those produced chemically — powder, flake, oxide, nitrate, and paint. Silver powder and flake are composed of large amounts of silver with 0.03 or 0.04% copper and traces of lead, iron, and other volatiles.

SILVER PAINTS These are used as conductive coatings that are pigmented with metallic silver flake or powder and bonding agents that are specially selected for the type of base material to which they are applied. These coatings are used to make conductive surfaces on such materials as ceramics,


glass, quartz, mica, plastics, and paper, as well as on some metals. They are used for making printed circuits, resistor and capacitor terminals, and in miniature electrical instruments and equipment. Silver paints fall into two classifications: (1) fired-on types for base materials that can withstand temperatures in the 399 to 927°C range, and (2) air-dried or baked-on types for organic base materials that are dried at temperatures ranging from 21.1 to 427°C. The bonding agent in the fired-on type of coating is a powdered glass frit, whereas in the airdried or baked-on type of coating organic resins are used. The viscosity and drying rate of each type varies, depending on the method of application, such as spraying, dipping, brushing, roller coating, or screen stenciling.

SILVER-TYPE BATTERIES Because they are six times lighter and five times smaller than other batteries of similar capacity, silver–zinc batteries have found wide use in guided missiles, telemetering equipment, and guidance control circuits and mechanisms. Where longer life and ruggedness are more important than the weight, silver–cadmium rechargeable batteries are specified. Where seawater activation is required, silver chloride–magnesium couples are used. Another silver type of battery is the solid electrolyte type made with silver, silver iodide, and vanadium pentoxide. This battery, designed for low-current applications, weighs less than 1 oz and has almost unlimited shelf life.

SINGLE CRYSTALS In crystalline solids the atoms or molecules are stacked in a regular manner, forming a threedimensional pattern, which may be obtained by a three-dimensional repetition of a certain pattern unit called a unit cell. When the periodicity of the pattern extends throughout a certain piece of material, one speaks of a single crystal. A single crystal is formed by the growth of a crystal nucleus without secondary nucleation or impingement on other crystals.


GROWTH TECHNIQUES Among the most common methods of growing single crystals are those of P. Bridgman and J. Czochralski. In the Bridgman method the material is melted in a vertical cylindrical vessel that tapers conically to a point at the bottom. The vessel then is lowered slowly into a cold zone. Crystallization begins in the tip and continues usually by growth from the first formed nucleus. In the Czochralski method, a small single crystal (seed) is introduced into the surface of the melt and then drawn slowly upward into a cold zone. Single crystals of ultrahigh purity have been grown by zone melting. Single crystals are also often grown by bathing a seed with a supersaturated solution; the supersaturation is kept lower than is necessary for sensible nucleation. When grown from a melt, single crystals usually take the form of their container. Crystals grown from solution (gas, liquid, or solid) often have a well-defined form that reflects the symmetry of the unit cell.

anisotropy, with respect to a given property, exists depends on crystal symmetry. The structure-sensitive properties of crystals (for example, strength and diffusion coefficients) seem governed by internal defects, often on an atomic scale. Industry and government are developing one of the first accurate computer-model predictions of molten metals and molding materials used in casting. Currently, the computer information is being used to design and cast aircraft turbine blades. Similarly, another company is using the information from the computer models to improve the casting of automobile and lighttruck engine blocks. Cast-metal parts are used in 90% of all durable goods such as washing machines, refrigerators, stoves, lawn mowers, cars, boats, and aircraft. The goal of the partnership is to produce accurate models for all alloys used by the casting industry. The information can then be used by manufacturers to standardize metalmixing recipes, allowing more effective competition in the marketplace.

PHYSICAL PROPERTIES Ideally, single crystals are free from internal boundaries. They give rise to a characteristic xray diffraction pattern. For example, the Laue pattern of a single crystal consists of a single characteristic set of sharp intensity maxima. Many types of single crystal exhibit anisotropy, that is, a variation of some of their physical properties according to the direction along which they are measured. For example, the electrical resistivity of a randomly oriented aggregate of graphite crystallites is the same in all directions. The resistivity of a graphite single crystal is different, however, when measured along different crystal axes. This anisotropy exists for both structure-sensitive properties, which are not affected by imperfections (such a elastic coefficients). Anisotropy of a structure-sensitive property is described by a characteristic set of coefficients that can be combined to give the macroscopic property along any particular direction in the crystal. The number of necessary coefficients can often be reduced substantially by consideration of the crystal symmetry; whether


SINTER HARDENING Recognizing the great potential of sinter hardening, researchers continue to develp useful data that will help to attain benefits derived from this heat treatment. Improved properties can be achieved by effective control of material composition, density, section size, sintering temperature, and cooling rate. By controlling these variables a variety of microstructures and resultant properties are achievable, enabling particular powder metallurgy parts to favorably perform under specific service conditions. However, materials options, process flexibility, and application requirements demand a better understanding of process, microstructure, and mechanical property relationships to capitalize fully on the opportunity of sinter hardening. Sinter hardening refers to a process where the cooling rate experienced in the cooling zone of the sintering furnace is fast enough that a significant portion of the material matrix transforms to martensite. Interest in sinter hardening has grown because it offers good manufacturing economy by providing a one-step process and

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a unique combination of strength, toughness, and hardness. A variety of microstructures and properties can be obtained by varying both the alloy type and content as well as the postsintering cooling rate. By controlling the cooling rate, the microstructure can be manipulated to produce the required proportion of martensite, which will lead to desired mechanical properties. By understanding how the sintering conditions affect the microstructure, materials can be modeled to produce the final properties that are desired.

ALLOYING Alloying elements are used in cast, wrought, and P/M materials to promote hardenability and increase the mechanical strength of the parts. A graphical way of examining the effects of alloying elements on the final microstructure of a steel is by using the characteristic isothermal transformation (I-T) diagram. This indicates the time necessary for the isothermal transformation of phases in the material from start to finish, as well as the cooling time and temperature combinations needed to produce the final microstructure. A similar diagram, known as a continuous cooling (CCT) curve, also is frequently used to determine the variation in microstructure as a function of cooling rate.

MATERIALS

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By evaluating the materials and properties shown in Tables S.6 and S.7, the apparent hardness, ultimate tensile strength, yield strength, total elongation, and martensite content of the various materials are seen. As expected, increasing the cooling rate resulted in increased apparent hardness and strength values. On the whole, hardness values were increased between 2 to 10 HRC for a given material. As expected, in all materials, the percent of martensite present increased significantly with the increase in cooling rate. The effect of the increased martensite levels is apparent in the hardness values for each of the materials. The effect of the higher levels of martensite on tensile properties is less obvious. In several cases, materials with significantly lower percentages


of martensite and lower hardness values demonstrated higher tensile strengths. Some results show the following trends: 1. Materials with 2 wt% Ni and 0.5 wt% Graphite Admixed • Accelerated cooling resulted in increased strength and apparent hardness while decreasing elongation values only slightly. This result was the consequence of increased martensite content and finer pearlitic microstructures. In these materials, the martensite was the result of transformation of nickel-rich areas in the microstructure. • The increase in prealloyed alloy content from 0.85 wt% to 1.5 wt% Mo resulted in a larger increase in strength than the addition of 1.0 wt% admixed Cu. • Although the 0.85 wt% Mo materials exhibited higher percentages of martensite than identical chemistries based on the 1.5 wt% Mo prealloyed material, the higher molybdenum materials had higher apparent hardness and strength values. This surprising result was explained by the presence in the 1.5 wt% Mo-based material of significantly finer pearlite. 2. Materials with 2 wt% Cu and 0.9 wt% Graphite Admixed • As the cooling rate was increased for these materials, the apparent hardness increased. This was associated with higher martensite contents in the faster-cooled materials. Martensite contents of greater than 50% were found in all three base materials when accelerated cooling was utilized. • The materials with the highest apparent hardness values (0.5 wt% Ni, 1.5 wt% Mo prealloy) did not exhibit the highest tensile strength values. The highest UTS values were determined for the


fast-cooled version of the 0.85 wt% Mo prealloyed material. Retained austenite may be one potential cause for the fall off in strength for the Mo–Ni material.

contain one or more or any combination of binders, lubricants, and coupling agents. Dextrin. A starch derivative commonly called starch in the fiberglass industry. It is of the low viscosity variety when used in fiberglass sizes. Practically all the fiberglass yarn that has been woven has been sized with this starch size. In industrial fiberglass fabrics, the size is removed in a process called heat cleaning, whereas in decorative woven material the size is burned off in the coronizing operation.

SIZING AGENTS These coatings are applied to glass textile fibers in the forming operation. The sizes used may

TABLE S.6 Premix Compositions Prealloyed Additions Mix 1 2 3 4 5 6 7

Premix Additions

Base

Nickel (wt%)

Molybdenum (wt%)

Copper (wt%)

Nickel (wt%)

Graphite (wt%)

Ancorsteel 85 HP Ancorsteel 150 HP Ancorsteel 85 HP Ancorsteel 150 HP Ancorsteel 85 HP Ancorsteel 150 HP Ancorsteel 4600V

— — — — — — 1.85

0.85 1.50 0.85 1.50 0.85 1.50 0.55

— — 1.00 1.00 2.00 2.00 2.00

2.00 2.00 2.00 2.00 — — —

0.50 0.50 0.50 0.50 0.90 0.90 0.90

Source: Ind. Heating, May, p. 12, 1998. With permission.

TABLE S.7 Properties of Material Matrix

Mix 1 1 2 2 3 3 4 4 5 5 6 6 7 7

VARICOOL Setting (%)

Apparent Hardness (HRC)

0.2% Offset YS (psi × 103/Mpa)

UTS (psi × 103/Mpa)

Elg. (%)

Martensite Content (%)

50 100 50 100 50 100 50 100 50 100 50 100 50 100

6 9 12 16 7 11 14 19 21 30 25 35 35 37

66.5/459 70.2/484 80.5/555 87.0/600 71.6/494 78.5/541 89.7/618 98.1/676 95.2/656 112.6/776 102.7/708 114.7/791 102.4/706 106.3/732

90.5/624 97.1/669 103.5/714 110.3/760 98.1/676 107.9/744 114.3/688 122.4/844 109.5/755 135.9/937 132.0/910 127.1/876 118.9/820 117.9/813

2.4 2.3 1.6 1.5 2.0 1.9 1.5 1.4 1.1 1.2 1.5 1.0 1.1 0.9

7.3 20.8 10.1 11.8 23.0 38.8 15.7 20.2 22.5 66.3 29.8 60.1 71.9 95.5

Source: Ind. Heating, May, p. 12, 1998. With permission.


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Gelatin. Used in small amounts as a constituent of a standard glass fiber textile size. Together with dextrin (starch), the gelatin acts as a binder for the other materials formulated into a size for the fiberglass textile yarns. Polyvinyl Acetate. As a latex, it is used as a binder in a standard size formulation for continuous fiberglass yarn. Usually this continuous yarn is made into roving and a large package is usually made up of 60 ends of continuous yarn, although other end counts such as 30 ends, 20 ends, and 8 ends are also available. Roving is used primarily as reinforcement in reinforced plastics, and in preforming operations.

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A metallic element (symbol Na and atomic weight 23), sodium occurs naturally only in the form of its salts. The most important mineral containing sodium is the chloride, NaCl, which is common salt. It also occurs as the nitrate, Chile saltpeter, as a borate in borax, and as a fluoride and a sulfate. When pure, sodium is silvery white and ductile, and it melts at 97.8°C and boils at 882°C. The specific gravity is 0.97. It can be obtained in metallic form by the electrolysis of salt. When exposed to the air, it oxidizes rapidly, and it must therefore be kept in airtight containers. It has a high affinity for oxygen, and it decomposes water violently. It also combines directly with the halogens, and is a good reducing agent for the metal chlorides. Sodium is one of the best conductors of electricity and heat. The metal is a powerful desulfurizer of iron and steel even in combination. For this purpose it may be used in the form of soda ash pellets or in alloys. Desulfurizing alloys for brasses and bronzes are sodium–tin, with 95% tin and 5% sodium, or sodium–copper. Sodium–lead, used for adding sodium to alloys, contains 10% sodium, and is marketed as small spheroidal shot. It is also marketed as sodium marbles, which are spheres of pure sodium up to 2.54 cm in diameter coated with oil to reduce handling hazard. Sodium bricks contain 50% sodium metal powder dispersed in a paraffin binder. They can be handled in the air, and are a source of active sodium. Sodium in combination with potassium is used as a heat-exchange


fluid in reactors and high-temperature processing equipment. A sodium–potassium alloy, containing 56% sodium and 44% potassium, has a melting point of 19°C and a boiling point of 825°C. It is a silvery mobile liquid. High-surface sodium is sodium metal absorbed on common salt, alumina, or activated carbon to give a large surface area for use in the reduction of metals or in hydrocarbon refining. Common salt will adsorb up to 10% of its weight of sodium in a thin film on its surface, and this sodium is 100% available for chemical reaction. It is used in reducing titanium tetrachloride to titanium metal. Sodium vapor is used in electric lamps. When the vapor is used with a fused alumina tube it gives a golden-white color. A 400-W lamp produces 42,000 lumens and retains 85% of its efficiency after 6000 h.

INORGANIC REACTIONS Sodium reacts rapidly with water, and even with snow and ice, to give sodium hydroxide and hydrogen. The reaction liberates sufficient heat to melt the sodium and ignite the hydrogen. When exposed to air, freshly cut sodium metal loses its silvery appearance and becomes dull gray because of the formation of a coating of sodium oxide. Sodium probably oxidizes to the peroxide, Na2O2, which reacts with excess sodium present to give the monoxide, Na2O. When sodium reacts with oxygen at elevated temperatures, sodium superoxide, NaO2, is formed; this reacts with more sodium to form the peroxide. Sodium does not react with nitrogen, even at very high temperatures. Sodium and hydrogen react above about 200°C to form sodium hydroxide. This compound decomposes at about 400°C and cannot be melted. Sodium hydride can be formed by the direct reaction of hydrogen and molten sodium or by hydrogenating dispersions of sodium metal in hydrocarbons. Sodium reacts with carbon with difficulty, if at all, and this reaction may be said to have been adequately studied. At room temperature fluorine and sodium ignite, dry chlorine and sodium react slightly, bromine and sodium do not react, and iodine and sodium do not react. However, in the presence


of moisture or at elevated temperatures all reactions take place at very high rates. Sodium reacts with ammonia, forming sodium amide and liberating hydrogen. The reaction may be carried out between molten sodium and gaseous ammonia (–30°C) in the presence of catalysts of finely divided metals. Sodium reacts with ammonia in the presence of coke to form sodium cyanide. Carbon monoxide reacts with sodium, but the resulting carbonyl, NaCO, is stable only at liquid ammonia temperatures. At high temperatures sodium carbide and sodium carbonate are formed from carbon monoxide and sodium. The reactions of sodium with various metal halides to give the metal plus sodium chloride are very important. Thus, titanium tetrachloride is reduced to titanium metal. Similarly, the halides of zirconium, beryllium, and thorium can be reduced to the corresponding metals by sodium. The interaction between sodium and potassium chloride is used in the commercial production of potassium metal. Sodium hydroxide, NaOH, is also commonly known as caustic soda, and also as sodium hydrate. Lye is an old name used in some industries and in household uses. It readily absorbs water from the atmosphere and must be protected in storage and handling. It is corrosive to the skin and must be handled with extreme care to avoid caustic burns. Most sodium hydroxide is produced by the electrolysis of sodium chloride solutions in one of several types of electrolytic cells. An older proces is the soda-lime process whereby soda ash is converted to caustic soda.

ORGANIC REACTIONS Sodium does not react with paraffin hydrocarbons but does form additional compounds with naphthalene and other polycyclic aromatic compounds and with arylated alkenes. It reacts with acetylene, replacing the acetylenic hydrogens to form sodium acetylides. Sodium adds to dienes, the reaction which forms the basis of the buna synthetic rubber process.

PRINCIPAL COMPOUNDS Sodium compounds are widely used in industry, particularly sodium chloride, sodium hydrox© 2002 by CRC Press LLC

ide, and soda ash. Sodium bichromate, Na2Cr2O7 · 2H2O, a red crystalline powder, is used in leather tanning, textile dyeing, wood preservation, and in pigments. Sodium metavandate, NaVO3, is used as a corrosion inhibitor to protect some chemical-processing piping. It dissolves in hot water, and a small amount in the water forms a tough impervious coating of magnetic iron oxide on the walls of the pipe. Sodium iodide crystals are used as scintillation probes for the detection and analysis of nuclear energies. Sodium oxalate is used as an antienzyme to retard tooth decay. In the drug industry sodium is used to compound with pharmaceuticals to make them water-soluble salts. Sodium is a plentiful element, easily available, and is one of the most widely used. Sodium carbonate, Na2CO3, is best known under the name soda ash because sodiurn carbonate occurs in (and once was extracted from) plant ashes. Most sodium carbonate is produced by the Solvay or ammonia-soda process. In an initial reaction, salt is converted to sodium carbonate, which precipitates and is then separated. Some soda ash is made synthetically by the Solvay process although an increasing amount is obtained from lake brines. Commercial grades of soda ash are available as 48% (Na2O) light and dense and as 58% (Na2O) light and dense; light and dense refers to apparent bulk density. Ordinary 48 to 58% grades are available in either light or dense but contain NaCl, which may affect certain ceramic uses. A 48% special grade is available in granular and extra light forms; it contains Na2SO4. The material derived from natural sources is almost NaCl-free. About one half of the total American soda ash production is used as a fluxing ingredient by the glass industry. The quality of soda ash in glass batches varies with the type of glass being made. Sodium sulfate, Na2SO4, is also known in the anhydrous form as salt cake. The decahydrate, Na2SO4 · 10H2O, is known as glauber salt. Most sodium sulfate is produced synthetically as a by-product or coproduct in various industries. Sodium aluminate, Na 2 OAl 2 O3 , whose melting point is 1650°C, is soluble in water and sodium carbonate. Sodium aluminate has found

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use as a settling-up agent for acid-resistant enamel. It is prepared by heating together bauxite and slips. When used in this capacity, it affords easier control of the slip than can be obtained by the use of alum or sulfuric acid, because of its tendency to stabilize the mobility and yield values. Sodium aluminate is also used as a substitute for sodium silicate and sodium carbonate in pottery slips. Sodium antimonate (sodium meta-antimonate), Na2OSb2O50.5H2O, is a white powder insoluble in water and fruit acids. Sodium antimonate is extremely stable at high temperatures and does not decompose below 1427°C. It is usually made from antimony oxide, caustic soda, and sodium nitrate. Sodium antimonate is used as the principal opacifier in dry-process enamel frits for cast iron sanitaryware and in some of the acid-resistant enamel frits for sheet steel. It is used in cast iron enamels; sodium antimonate is generally recognized as being more desirable than antimony trioxide. Sodium cyanide is a salt of hydrocyanic acid of the composition NaCN, used for carbonizing steel for case hardening, for heat-treating baths, for electroplating, and for the extraction of gold and silver from their ores. For carburizing steel it is preferred to potassium cyanide because of its lower cost and its higher content of available carbon. It contains 53% CN, as compared with 40% in potassium cyanide. The nitrogen also aids in forming the hard case on the steel. The 30% grade of sodium cyanide, melting at 679°C, is used for heattreating baths instead of lead, but it forms a slight case on the steel. Sodium cyanide is very unstable, and on exposure to moist air liberates the highly poisonous hydrocyanic acid gas, HCN. For gold and silver extraction it easily combines with the metals, forming soluble double salts, NaAu(CN)2. Sodium cyanide is made by passing a stream of nitrogen gas over a hot mixture of sodium carbonate and carbon in the presence of a catalyst. It is a white crystalline powder, soluble in water. The white copper cyanide used in electroplating has the composition Cu2(CN)2, containing 70% copper. It melts at 474.5°C and is insoluble in water, but is soluble in sodium cyanide solution. Sodium ferrocyanide, or yellow prussiate of soda, is a lemonyellow crystalline solid of the composition


Na4Fe(CN)6 · 10H2O, used for carbonizing steel for case hardening. It is also employed in paints, in printing inks, and for the purification of organic acids; in minute quantities, it is used in salt to make it free-flowing. It is soluble in water. Calcium cyanide in powder or granulated forms is used as an insecticide. It liberates 25% of hydrocyanic acid gas. Sodium nitrate (soda niter), NaNO3, with a melting point of 208°C, decomposes at 380°C and is soluble. Sodium nitrate is used in enamel frits in quantities of 2 to 8%. It is highly important that sufficient nitrate be present in enamels to prevent reduction of any easily reducible compounds in the batch, especially lead or antimony compounds. The function of sodium nitrate in glass is to oxidize organic matter that may contaminate batch materials, to prevent reduction of some of the batch constituents, to help maintain colors, and to speed the melt. It is the lowest melting of all glassmaking materials. Common applications of sodium nitrate are to ensure the pink color of manganese oxide and to prevent reductions of lead in potash lead glasses. Sodium nitrite, NaNO2, is soluble in water. It is prepared from sodium nitrate by reduction with lead. Sodium nitrite has been used for some years as a mill addition, or as an addition after milling, to enamel ground coats to prevent rust while drying, and also as a setting-up agent. More recently, sodium nitrite has been used rather generally in cover coats to correct for tearing. Sodium phosphate, Na2HPO412H2O, has a melting point of 346°C and is soluble in water. Sodium phosphate has been recently added to glass batches, producing an opal glass of unusual properties. Three other forms of the phosphate are available — monobasic, tribasic, and pyrophosphate. The last is most adaptable because it melts at 970°C in the anhydrous form. It is derived by the fusion of disodium phosphate. Sodium silicate, Na2OxSiO2, is commonly made by melting sand and soda ash in a reverberatory furnace. Various proportions of the two ingredients are used and widely divergent characteristics result. The most alkaline liquid silicate made by this furnace process has a ratio of 1Na2O:1.6SiO2 and the most siliceous liquid grade has a ratio of 1Na2O:3.75SiO2.


USES The largest single use for sodium metal, accounting for about 60% of total production, is in the synthesis of tetraethyllead, an antiknock agent for automotive gasolines. A second major use is in the reduction of animal and vegetable oils to long-chain fatty alcohols; these alcohols are raw materials for detergent manufacture. This use has been decreasing in favor of production of such alcohols by high-pressure catalytic hydrogenation. Another major use is in the reduction of titanium and zirconium halides to the respective metals. Here the use of sodium is increasing at the expense of magnesium as the preferred reducing agent in such operations. Sodium metal is also used in making sodium hydride, sodium amide, and sodium cyanide. It is also used in the synthesis of “isosebacic acid.” The use of liquid sodium metal as a heat-transfer agent in nuclear reactors is also becoming increasingly important. Sodium chloride is used in the manufacture of sodium hydroxide, sodium carbonate, sodium sulfate, and sodium metal. In sodium sulfate manufacture, hydrogen chloride is the coproduct; in metallic sodium manufacture, chlorine gas is the coproduct. Rock salt is used in curing fish, in meat packing, in curing hides, and in making freezing mixtures. Food preparation, including canning and preserving, consumes much salt. Table salt accounts for only a small percentage of sodium chloride consumption, most of it going into the industrial uses outlined above. Sodium hydroxide is perhaps the most important industrial alkali. Its major use is in the manufacture of chemicals, about 30% attributed to this category. The next major use is the manufacture of cellulose film and rayon, both of which proceed through soda cellulose (the reaction product of sodium hydroxide and cellulose); this accounts for about 25% of the total caustic soda production. Soap manufacture, petroleum refining, and pulp and paper manufacture each account for a little less than 10% of total sodium hydroxide use. Sodium carbonate finds its major use in the glass industry, which takes about one third of total production. Approximately another third


goes into the manufacture of soap, detergents, and various cleansers. The manufacture of paper and textiles, nonferrous metals, and petroleum products accounts for much of the balance. The major consumer of sodium sulfate (salt cake) is the kraft pulp industry. Increasing quantities of sodium sulfate are used in the manufacture of flat glass. Other uses of salt cake are in detergents, ceramics, mineral stock feeds, and pharmaceuticals. In the area of biological activity, the sodium ion (Na+) is the main positive ion present in extracellular fluids and is essential for maintenance of the osmotic pressure and of the water and electrolytic balances of body fluids.

SOLDER ALLOYS These are alloys of two or more metals used for joining other metals together by surface adhesion without melting the base metals as in welding and without requiring as high a temperature as in brazing. However, there is often no definite temperature line between soldering alloys and brazing filler metals. A requirement for a true solder is that it have a lower melting point than the metals being joined and an affinity for, or be capable of uniting with, the metals to be joined.

TYPES

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The most common solder is called half-andhalf, plumbers’ solder, or ASTM (American Society for Testing and Materials) solder class 50A, and is composed of equal parts of lead and tin. It melts at 182°C. The density of this solder is 8802 kg/m3, the tensile strength is 39 MPa, and the electrical conductivity is 11% that of copper. SAE (Society of Automotive Engineers) solder No. 1 has 49.5 to 50.0% tin, 50% lead, 0.12% max antimony, and 0.08% max copper. It melts at 181°C. Much commercial half-and-half, however, usually contains larger proportions of lead and some antimony, with less tin. These mixtures have higher melting points, and solders with less than 50% tin have a wide melting range and do not solidify quickly. Sometimes a wide melting range is desired, in which case a wiping solder with 38

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to 45% of tin is used. A narrow-melting-range solder, melting at 183 to 185°C, ASTM solder class 60A, contains 60% tin and 40% lead. A 42% tin and 58% lead solder has a melting range of 183 to 231°C. Slicker solder is the best quality of plumbers’ solder, containing 63 to 66% tin and the balance lead. Solder alloys are available in a wide range of sizes and shapes, enabling users to select that one that best suits their application. Among these shapes are pig, slab, cake or ingot, bar, paste, ribbon or tape, segment or drop, powder, foil, sheet, solid wire, flux cored wire, and preforms. There are 11 major groups of solder alloys: Tin–antimony. Useful at moderately elevated operating temperatures, around 149°C, these solders have higher electrical conductivity than the tin–lead solders. They are recommended for use where lead contamination must be avoided. A 95% Sn–5% Sb alloy has a solidus of 235°C, a liquidus of 240°C, and a resulting pasty range of –11.1°C. Tin–lead. Constituting the largest group of all solders in use today, the tin–lead solders are used for joining a large variety of metals. Most are not satisfactory for use above 149°C under sustained load. Tin–antimony–lead. These may normally be used for the same applications as tin–lead alloys with the following exceptions: aluminum, zinc, or galvanized iron. In the presence of zinc, these solders form a brittle intermetallic compound of zinc and antimony. Tin–silver. These have advantages and limitations similar to those of tin–antimony solders. The tin silvers, however, are easier to apply with a rosin flux. Relatively high cost confines these solders to fine instrument work. Two standard compositions: 96.5% Sn–3.5% Ag, the eutectic; 95% Sn–5% Ag, with a solidus of 221°C and liquidus of 245°C. Tin–zinc. These are principally for soldering aluminum since they tend to minimize galvanic corrosion. Lead–silver. Tensile, creep, and shear strengths of these solders are usually satisfactory up to 177°C. Flow characteristics are rather poor and these solders are susceptible to humid atmospheric corrosion in storage. The use of a zinc chloride base flux is recommended


to produce a good joint on metals uncoated with solder. Cadmium–silver. The primary use of cadmium–silver solder is in applications where service temperature will be higher than permissible with lower melting solder. Improper use may lead to health hazards. The solder has a composition of 95% Cd–5% Ag. Solidus is 338°C and liquidus is 393°C. Indium–lead alloys. These are alkali-resistant solders. A solder with 50% lead and 50% indium melts at 182°C, and is very resistant to alkalies, but lead–tin solders with as little as 25% indium are resistant to alkaline solutions, have better wetting characteristics, and are strong. Indium solders are expensive. Adding 0.85% silver to a 40% tin soft solder gives equivalent wetting on copper alloys to a 63% tin solder, but the addition is not effective on low-tin solders. A gold–copper solder used for making high-vacuum seals and for brazing difficult metals such as iron–cobalt alloys contains 37.5% gold and 62.5% copper. Palladium–nickel. A palladium–nickel alloy with 40% nickel has a melting point about 1237°C. The brazing filler metals containing palladium are useful for a wide range of metals and metal to ceramic joints. The remaining four groups of Zn–Al, Cd–Zn, and solders containing bismuth and indium were covered earlier.

SILVER SOLDER Silver solder is high-melting-point solder employed for soldering joints where more than ordinary strength and, sometimes, electric conductivity are required. Most silver solders are copper–zinc brazing filler metals with the addition of silver. They may contain from 9 to 80% silver, and the color varies from brass yellow to silver white. Cadmium may also be added to lower the melting point. Silver solders do not necessarily contain zinc, and may be braze filler metals of silver and copper in proportions arranged to obtain the desired melting point and strength. A silver braze filler metal with a relatively low melting point contains 65% silver, 20% copper, and 15% zinc. It melts at 693°C, has a tensile strength of 447 MPa, and elongation 34%. The electrical conductivity is 21%


that of pure copper. A solder melting at 760°C contains 20% silver, 45% copper, and 35% zinc. ASTM silver solder No. 3 is this solder with 5% cadmium replacing an equal amount of the zinc. It is general-purpose solder. ASTM silver solder No. 5 contains 50% silver, 34% copper, and 16% zinc. It melts at 693°C, and is used for soldering electrical work and refrigeration equipment. Any tin present in silver solders makes them brittle; lead and iron make the solders difficult to work. Silver solders are malleable and ductile and have high strength. They are also corrosion resistant and are especially valuable for use in food machinery and apparatus where lead is objectionable. Small additions of lithium to silver solders increase fluidity and wetting properties, especially for brazing stainless steels or titanium. Sil-Fos is a phosphor–silver brazing solder with a melting point of 704°C. It contains 15% silver, 80% copper, and 5% phosphorus. Lap joints brazed with SilFos have a tensile strength of 206 MPa. The phosphorus in the alloy acts as a deoxidizer, and the solder requires little or no flux. It is used for brazing brass, bronze, and nickel alloys. Another grade, Easy solder, contains 65% silver, melts at 718°C, and is a color match for sterling silver. TL silver solder has only 9% silver and melts at 871°C. It is brass yellow in color, and is used for brazing nonferrous metals. Sterling silver solder, for brazing sterling silver, contains 92.8% silver, 7% copper, and 0.2% lithium. Flow temperature is 899°C. A lead–silver solder to replace tin solder contains 96% lead, 3% silver, and 1% indium. It melts at 310°C, spreads better than ordinary lead–silver solders, and gives a joint strength of 34 MPa. Silver–palladium alloys for hightemperature brazing contain from 5 to 30% palladium. With 30%, the melting point is about 1232°C. These alloys have exceptional melting and flow qualities and are used in electronic and spacecraft applications.

COLD SOLDERS Cold solder, used for filling cracks in metals, may be a mixture of a metal powder in a pyroxylin cement with or without a mineral filler, but


the strong cold solders are made with synthetic resins, usually epoxies, cured with catalysts, and with no solvents to cause shrinkage. The metal content may be as high as 80%. Devcon F, for repairing holes in castings, has 80% aluminum powder and 20% epoxy resin. It is heatcured at 66°C, giving high adhesion. Epoxyn solder is aluminum powder in an epoxy resin in the form of a putty for filling cracks or holes in sheet metal. It cures with a catalyst. The metal–epoxy mixtures give a shrinkage of less than 0.2%, and they can be machined and polished smooth.

LEAD-FREE SOLDER REPLACEMENTS In spite of the sustained efforts of researchers and technology leaders in packaging, to date there is no lead-free solder alloy that is a dropin replacement for tin–lead solder in assembly processes. Because tin–lead solder has been used for so long and is so much a part of the typical process engineer’s thinking, the quest for an affordable lead-free alloy replacement is facing mounting pessimism that the effort will be successful. On the other hand, optimism that adhesivetype solders will prove to be a serious alternative to metallurgical materials continues to grow. These polymer-based conductive adhesives are being used in various applications previously “reserved” for tin–lead solders. Regular production equipment and traditional assembly processes are producing high-quality assemblies with demonstrated long-term reliability using the new solders. And for some products, polymers are considered an enabling technology. One major market is the polyester-based flexible circuit market, particularly those built using polymer thick film. Polymer Solders Polymer solders are also known as “conductive adhesives,” or materials that provide the dual functions of electrical connection and mechanical bond. The adhesive components of a typical material are some form of polymer, i.e., longchain molecules widely used to produce structural products, which are also known for their excellent dielectric properties. Already used

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extensively as electrical insulators, as solders their necessary conductivity is accomplished by adding highly conductive fillers to the polymer binders. The most common polymer solders are silver-filled thermosetting epoxies supplied as one-part thixotropic pastes. Silver is used not only because it is usually cost-effective, but also for its unique conductive oxide. A blend of silver powder and flakes achieves high conductivity while maintaining good printability. Because the mechanical strength of the joint is provided by the polymer, the challenge in a formulation is to use the maximum metal loading without sacrificing the required strength. (Some polymer solders contain more than 80% metal filler by weight.) Polymer solders do not typically form metallurgical interfaces in the usual sense. Electrical integrity requires that the metal filler particles be in close contact to form a conductive path between the circuit trace and the component lead. Ideally, the silver flakes will overlap and smaller particles will fill in the gaps to form a conductive chain. In the past, polymer solders were successful only on circuits using precious-metal conductors. This was because junction resistance was seen to increase to unacceptable levels when ordinary printed circuit boards and components were joined with these materials. To solve the instability problem it was necessary to create stable, nonmetallurgical junctions with oxidizable surfaces. For example, one polymer-solder formulation, used on flexible circuits and recently optimized for rigid boards, provides junction stability between solder-coated and bare-copper surfaces via polymer shrinkage during curing to force irregular particles through the interface oxides. Advantages in using polymer solders include compatibility with a range of surfaces including some nonsolderable substrates; lowtemperature processing, resulting in lower thermal stress; no pre- or postclean requirements, thereby reducing equipment needs and cycle times, and lowering or eliminating the release of volatile organic compounds.


SOLDER MATERIALS FLUXES Fluxes range from very mild substances to those of extreme chemical activity. For centuries rosin, a pine product, has been known as an effective and practically harmless flux. It is used widely for electrical connections in which utmost reliability, freedom from corrosion, and absence of electrical leakage are essential. When less stringent requirements exist and when less carefully prepared surfaces are to be soldered, rosin is mixed with chemically active agents that aid materially in soldering. The rosin-type fluxes may be incorporated as the core of wire solders or dissolved in various solvents for direct application to joints prior to soldering. Inorganic salts are widely used where stronger fluxes are needed. Zinc chloride and ammonium chloride, separately or in combination, are most common. They may also be obtained as so-called acid-core solder wire or in petroleum jelly as paste flux. All of the salt-type fluxes leave residues after soldering that may be a corrosion hazard. Washing with ample water accomplished by brushing is generally wise.

SOL-GEL PROCESS This is a chemical synthesis technique for preparing gels, glasses, and ceramic powders. The synthesis of materials by the sol-gel process generally involves the use of metal alkoxides, which undergo hydrolysis and condensation polymerization reactions to yield gels. The production of glasses by the sol-gel method is an area that has important scientific and technological implications. For example, the sol-gel approach permits preparation of glasses at far lower temperatures than is possible by using conventional melting. It also makes possible synthesis of compositions that are difficult to obtain by conventional means because of problems associated with volatization, high melting temperatures, or crystallization. In addition, the sol-gel approach is a high-purity process that leads to excellent homogeneity. Finally, the sol-gel approach is adaptable to producing films and fibers as well as bulk


pieces, that is, monoliths (solid materials of macroscopic dimensions, at least a few millimeters on a side).

species. This sol-to-gel transition is irreversible, and there is little if any change in volume. Drying

GLASS FORMATION Formation of silica-based materials is the most widely studied system. However, an enormous range of multicomponent silicate glass compositions have also been prepared. The sol-gel process can ordinarily be divided into the following steps: forming a solution, gelation, drying, and densification.

The drying process involves the removal of the liquid phase; the gel transforms from an alcogel to a xerogel. Low-temperature evaporation is frequently employed, and there is considerable weight loss and shrinkage. The drying stage is a critical part of the sol-gel process. As evaporation occurs, drying stresses arise that can cause catastrophic cracking of bulk materials.

Hydrolysis and Condensation

Densification

In general, the processes of hydrolysis and condensation polymerization are difficult to separate. The hydrolysis of the alkoxide need not be complete before condensation starts; and in partially condensed silica, hydrolysis can still occur at unhydrolyzed sites. Several parameters have been shown to influence the hydrolysis and condensation polymerization reactions: these include the temperature, solution pH, the particular alkoxide precursor, the solvent, and the relative concentrations of each constituent. In addition, acids and bases catalyze the hydrolysis and condensation polymerization reactions; therefore, they are added to help control the rate and the extent of these reactions.

The final stage of the sol-gel process is densification. At this point the gel-to-glass conversion occurs and the gel achieves the properties of the glass. As the temperature increases, several processes occur, including elimination of residual water and organic substances, relaxation of the gel structure, and, ultimately, densification.

Microstructural Development The conditions under which hydrolysis and condensation occur have a profound effect on gel growth and morphology. These structural conditions greatly influence the processing of sol-gel glasses into various forms. It is well established, for example, that acid-catalyzed solutions with low water content (that is, conditions that produce linear polymers) offer the best type of solution for producing fibers. Gelation and Aging As the hydrolysis and condensation polymerization reactions continue, viscosity increases until the solution ceases to flow. The time required for gelation to occur is an important characteristic that is sensitive to the chemistry of the solution and the nature of the polymeric


APPLICATIONS The sol-gel process offers advantages for a broad spectrum of materials applications. The types of materials go well beyond silica and include inorganic compositions that possess specific properties such as ferroelectricity, electrochromism, or superconductivity. The most successful applications utilize the composition control, microstructure control, purity, and uniformity of the method combined with the ability to form various shapes at low temperatures. Films and coatings were the first commercial applications of the sol-gel process. The development of the sol-gel-based optical materials has also been quite successful, and applications include monoliths (lenses, prisms, lasers), fibers (waveguides), and a wide variety of optical films. Other important applications of solgel technology utilize controlled porosity and high surface area for catalyst supports, porous membranes, and thermal insulation.

SOYBEAN OIL Biodiesel fuel, a combination of natural oil or fat with an alcohol such as methanol or ethanol,

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could help the U.S. reduce air pollution and its dependence on imported oil. Soybean oil is the most commonly used feedstock in the U.S. Biodiesel works in all conventional diesel engines and can be distributed through the existing industry infrastructure. Using 100% biodiesel fuel reduces carbon dioxide emissions by more than 75% compared to petroleum diesel. Using a blend of 20% biodiesel reduces them by 15%. The fuel also produces less particulate, carbon monoxide, and sulfur emissions, all targeted as public health risks. On the downside, biodiesel is more expensive than petroleum diesel and it produces slightly higher amounts of nitrogen oxide, a pollutant.

SPACE PROCESSING The carrying out of various processes on materials aboard orbiting spacecraft is known as space processing. Until the space age, the Earth’s gravity had always been considered a constant in the fluid-flow equations that govern heat and mass transport in materials processing. When the space shuttle became operational in the early 1980s, the potential benefits of suppressing the acceleration of gravity in certain processes began to be seriously considered. There have been numerous flight opportunities for microgravity experimentation.

growth of crystals is not well understood, but it is generally accepted that unsteady growth conditions that can result from convective flows are harmful to crystal growth. Attempts to grow protein crystals in space produce mixed results. Sometimes no crystals or crystals that are inferior to those grown on Earth are produced. However, occasionally, the space-grown crystals are larger and better ordered than the best ever grown on Earth. In fact, the improvement in internal order obtained in proteins grown in reduced gravity can be so dramatic as to allow structure to be solved or refined to higher resolution than is possible by using the diffraction data from the best available Earth-grown crystals. There is still the question of why attempts to grow protein crystals in space produce superior results only part of the time. (It should be remembered that many unreported experiments on the ground are not successful either.) One possible explanation is that the growth process is developed and optimized on the ground before committing the experiment to flight. However, the conditions that are optimum under normal gravity may not take advantage of the microgravity environment. Therefore, it may be necessary to actually develop the optimal growth processes in space to improve the yield of protein crystal growth experiments there.

ELECTRONIC PROTEIN CRYSTAL GROWTH

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The importance of crystallography as a mechanism for determining three-dimensional structure of complex macromolecules has placed new demands on the ability to grow large (approximately 0.5 mm on a side), highly ordered crystals of a vast variety of biological macromolecules to obtain high-resolution x-ray diffraction data. The growth of protein crystals in reduced gravity has the potential advantages of (1) the ability to suspend the growing crystals in the growth solution to provide a more uniform growth environment and (2) the ability to reduce the convective mass transport so that growth can take place to a diffusion-control led environment. The effect of convection on the


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PHOTONIC MATERIALS

Single-crystalline materials suitable for electronic and photonic applications have received much attention as candidates for microgravity processing for several reasons. These are critical, high-value materials whose applications demand extreme control of composition, purity, and defects. Despite the rapid advances made in electronic materials, progress on many fronts is still limited by available materials. Gravitydriven flows certainly influence mass transport in growth processes. Even though the primary purpose of most of the space processing with these materials has been to gain insight and understanding that can be used in Earth-based processing, limited production of certain specialty materials in space is a possibility if it turns out that there is no Earth-based alternative.


Vapor Growth

Solution Growth

Several vapor crystal growth experiments on the shuttle produced some interesting results that are not at all understood. An example is the growth of unseeded germanium–selenium (GeSe) crystals by physical vapor transport using an inert noble gas as a buffer to a closed tube. When this is done on the ground, many small crystallites form a crust inside the growth ampoule at the cold end. Growth in space produces dramatically different results: The crystals apparently nucleate away from the walls and grow as thin platelets, which eventually become entwined with one another, forming a web that is loosely contained by the tube. Even more striking is the appearance of the surfaces of the space-grown crystals. The surfaces are mirrorlike and almost featureless, exhibiting only a few widely spaced growth terraces. By contrast, crystallites grown on the ground under identical thermal conditions have many pits and irregular, closely spaced growth terraces. Another example is the growth of Hg0.4Cd0.6Te by closed-tube chemical vapor deposition on mercury–cadmium–tellurium (HgCdTe) substrates using mercuric iodide (HgI2) as the transport agent. Again, considerable improvements in the space-grown samples are observed, relative to those grown on the ground, in terms of surface morphology, chemical microhomogeneity, and crystalline perfection. In the growth of thin films of copper phthalocyanine on copper substrates by physical vapor deposition, a dramatic difference is found in the appearance and morphology of the space-grown film as compared with the films produced on the ground. Scanning electron microscopy reveals a close-packed columnar structure for the space-grown films, roughly resembling a thick pile carpet. The groundgrown samples have a lower-density, randomly oriented structure that resembles a shag carpet. Mercuric iodide crystals grown during orbital flight by physical vapor transport exhibit sharp, well-formed facets indicating good internal order. This is confirmed by gamma-ray rocking curves, which are approximately one third the width of those taken on samples grown on the ground. Both electron and hole mobility are significantly enhanced in the flight crystals.

Triglycine sulfate crystals can be grown from solution during orbital flight by using a novel cooled sting method. Supersaturation is maintained by extracting heat through the seed mounted on a small heat pipe, which in turn is attached to a thermoelectric device. Growth under diffusion-controlled transport conditions may avoid liquid and gas inclusions, the most common type of defect in solution-grown crystals, which are believed to be caused by unsteady growth conditions resulting from convective flows. This growth technique has produced crystals of exceptional quality. The usual growth defects in the vicinity of the seed that form during the transition from dissolution to growth (the so-called ghost of the seed) are notably absent. High-resolution x-ray topographs taken with synchrotron radiation indicate a high degree of perfection. In pyroelectric detection for far-infrared radiation, the detectivity of the space-grown crystal is significantly higher than the seed crystal and the Q (ratio of energy stored to energy loss per cycle) is more than doubled.


METALLIC ALLOYS

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COMPOSITES

Processing of metallic alloys and composites in space has been carried out to study dendrite growth, monotectic alloys, liquid-phase sintering, and electrodeposition. Dendrite Growth The microgravity environment provides an excellent opportunity to carry out critical tests of fundamental theories of solidification without the complicating effects introduced by buoyancy-driven flows. For example, one investigator carried out a series of experiments to elucidate dendrite growth kinetics under wellcharacterized diffusion-controlled conditions in pure succinonitrile. This constituted a rigorous test of various nonlinear dynamical pattern formation theories, which provide the basis for the prediction of the microstructure and physical properties achieved in a solidification process. Comparison of dendrite tip velocities, measured as a function of undercooling over a range from 0.05 to 1.5°C with ground-based

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measurements, shows that effects convection are more significant at the smaller undercoolings and are still important up to undercoolings as large as 1.3 K. Even in microgravity, there is a slight departure in the data at the smallest undercooling, which is attributed to the residual acceleration of the spacecraft. These data also allow the determination of the scaling constant important in the selection of the dynamic operating state, which the present theories have been unable to provide. Monotectic Alloys Some of the first microgravity experiments in metallurgy were attempts to form fine dispersions in monotectic alloys, that is, alloy systems that have liquid-phase immiscibilities. Attempts to solidify such alloys from the melt in normal gravity always result in macroscopic segregation because the densities of the two liquid phases are invariably different. It was thought that this phase separation could be avoided in microgravity and intimate mixtures of the two phases would result that might have interesting and unusual properties. Liquid-Phase Sintering

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Composites formed by liquid-phase sintering have many commercial applications, from cutting tools to electrical switch contacts. Fine particles of the more refractory phase are mixed with particles of a lower-melting material, which, when melted, forms a matrix to bind the nonmelting particles together. The system is stabilized during the sintering process by using a large volume fraction (80 to 85%) of nonmelting particles to support the structure while the molten phase interpenetrates the intergranular spaces. Fortunately, for many applications it is desirable to have a large volume fraction of the more refractory phase. However, there are some applications where there is a requirement to increase the volume fraction of the matrix material to amplify its properties. Space processing can be used to prepare composites (such as tungsten particles in a copper–nickel matrix, cobalt particles in a copper matrix, and iron particles in a copper matrix) with host-material volume fractions ranging from 30 to 50%, to


increase the sintering time, and to provide valuable insight into evolution of pores and other defects that occur in sintered products produced on Earth. Electrodeposition Electrodeposition experiments in reduced gravity have produced some intriguing results. With higher current densities than can normally used on Earth, nickel with a nanocrystalline structure can be deposited on gold substrates. Attempts to duplicate this result in normal gravity by the use of convectively stable geometries and porous media have not been successful. It is speculated that the morphology of the hydrogen bubbles that form on the cathode in microgravity somehow promotes the formation of nickel hydride, which produces the nanocrystalline structure. Attempts have been made to codeposit diamond dust with copper, and small particles of Co2C3 with cobalt, to form cermets that would be extremely hard and wear resistant. A groundbased technique for depositing a bonelike hydroxyapatite coating on prosthetic implants, which is based on this work, has significantly better adhesion than currently available coatings. Another derivative of this work is a plating process using Cr(III), which poses significantly fewer environmental problems than the common Cr(VI) process.

SPINEL Spinel is any of a family of important AB2O4 oxide minerals, where A and B represent cations. Spinel minerals are widely distributed in the Earth, in meteorites, and in rocks from the moon. The ideal spinel formula is MgOAl2O3 or MgAl2O4. Spinel has a melting point of 2135°C and the mineral is found in small deposits. It is formed by solid-state reaction between MgO and A12O3 and is an excellent refractory showing high resistance to attack by slags, glass, etc. High-purity spinel is a chemically derived spinel powder made by the coprecipitation of magnesium and aluminum complex sulfates, with subsequent calcination to form the oxide compound. Purities range from 99.98 to


99.995%. The ceramic powders prepared by this process can be hot-pressed into transparent window materials with exceptional infrared transmission range.

t=0 0% LIQ

APPLICATIONS The major ceramic applications for spinels are the magnetic ferrospinels (ferrites), chromite brick, and spinel colors. Magnetic recording tape coated with α-Cr2O3 is a relatively recent development. It is also used as a porous protective coating in oxygen sensors for automotive emission controls. The material is available as fused spinel in special refractory applications and also in a special particle shape and distribution for flame and plasma-arc spraying. The magnetic spinels are of special importance because of the widespread interest and application of the ceramic ferrospinels (ferrites). Two classes of ferrospinels occur: magnetic and nonmagnetic. The magnetic are related to the inverse structure and the nonmagnetic to the normal structure.

100% LIQ v=50 m/s 30% v=70LIQ m/s t=0.01s Liquid layer

S+L Fragmentation of dendrites on impact? S+L Splatting of liquid droplets

FIGURE S.4 Atomization and deposition process for spray metal forming. (From NASA Tech. Briefs, 21(5), 81, 1997. With permission.)

successfully spray-formed. These billets are combined, in the downstream manufacturing process, with extrusion or forging, and then coupled with high-speed machining to produce components in final form.

SPRAY METAL FORMING Spray metal forming is a rapid solidification technology for producing semifinished tubes, billets, plates, and simple forms in a single integrated operation. In contrast to other powder metallurgy processes, spray metal forming offers the distinct advantage of skipping the intermediate steps of atomization and consolidation by atomizing and collecting the spray in the form of a billet in a single operation. Also, the elimination of powder handling reduces oxide content and enhances ductility.

THE PROCESS Spray forming involves converting a molten metal stream into a spray of droplets by highpressure gas atomization (Figure S.4). The droplets cool rapidly in flight and ideally arrive at a collector plate with just enough liquid content to spread and completely wet the surface. The metal then solidifies into an almost fully dense preform with a very fine, uniform microstructure. Steel, copper, nickel-based superalloys, and aluminum alloys have been


ADVANTAGES Rapid solidification processes such as spray metal forming offer some distinct advantages over conventional ingot metallurgy processing. Superior properties due to fine grain sizes; a fine, homogeneous distribution of second-phase precipitates; and the absence of macrosegregation result from cooling rates on the order of 103 to 105 K/s (gas atomization processes approach 106 K/s). The high cooling rate in spray forming is obtained by higher gas-tometal ratios. In certain alloy systems, a high volume fraction of fine (0.05 to 0.2 µm) intermetallic dispersoids may be obtained with high gas-to-metal ratios.

APPLICATIONS Many different aluminum alloys and SiC-particulate-reinforced aluminum metal-matrix composites have been processed. They include conventional alloys in the 2XXX, 3XXX, 5XXX, 6XXX, and 7XXX series; high-temperature and high-strength alloys; and high-silicon-content

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alloys. The nonconventional alloys have been developed specifically for spray forming or adapted from alloys developed for rapid solidification rate (RSR) processes such as gas atomization or melt spinning. These alloys have shown superior properties such as wear resistance, room- and high-temperature strength, and creep resistance. Alloys developed especially for spray forming, such as the ultrahigh zinc content alloys and 7050 aluminum with additional zinc, have been processed. Many of these alloys have been processed with the addition of SiC to improve the stiffness. These alloys have shown superior strength properties over conventionally processed material. Four principal alloy systems originally developed for RSR processing have been extensively spray-formed. These are Al–Fe–V–Si (FVS), Al–Fe–Ce–W (FCW), Al–Ce–Cr–Co (CCC), and Al–Ni–Co (NYC) alloys. To optimize mechanical properties, these alloys were spray-formed over a range of processing parameters. For ultrahigh-temperature aluminum alloys, the melt superheat, or pour temperature, gas-to-metal ratio, and the injection of SiC particulate, have the greatest effect on microstructure (e.g., droplet and dispersoid size and volume fraction) and resultant properties.

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Two government-sponsored programs have been completed using spray-formed material to produce components for use in Department of Defense vehicles. The first program produced track pins for advanced tracked ground combat vehicles using an ultrahigh-strength aluminum alloy. The second program developed the sprayforming processing parameters to produce an ultrahigh-temperature aluminum alloy for stator vanes in high-performance jet aircraft engines. The goal of the track pin program was to develop processing that produces an ultrahighstrength aluminum alloy that can replace the steel currently used in the manufactured pins for tracked ground combat vehicles. The objective was to replace the hollow steel pin with a solid aluminum pin that has similar properties. Several alloys were spray-formed, extruded, heat-treated, and tested to determine the tensile


strength, ductility, and modulus. The two alloys with the best combination of properties were selected for additional processing. To increase the stiffness, SiC was added during spray forming. The spray-formed ultrahigh-strength alloys have yield strengths in excess of 690 MPa and show good ductility even with the addition of SiC. Track pins have been produced and are being tested. The corrosion resistance of these materials is improved by spray forming. The use of the ultrahigh-strength aluminum would result in a weight reduction of over 204.3 kg. The NYC alloy shows promise for use in static parts of jet engines. The material properties at high temperatures provide an opportunity to replace heavier titanium parts with a lighterweight aluminum alloy resulting in substantial saving in life-cycle costs. The high-temperature aluminum alloy has been produced as extrusions and forgings. Machining contractors have developed the most efficient and economical processes to deliver a finished part. The alloy has been extruded and machined into the final component configuration and the alloy has good machining characteristics. Forging trials have shown that the material can be readily deformed offering increased material yield.

SPRING STEEL This is a term applied to any steel used for springs. The majority of' springs are made of steel, but brass, bronze, nickel silver, and phosphor bronze are used where their corrosion resistance or electric conductivity is desired. Carbon steels, with from 0.50 to 1.0% carbon, are much used, but vanadium and chromium–vanadium steels are also employed, especially for heavy car and locomotive springs. Special requirements for springs are that the steel be low in sulfur and phosphorus, and that the analysis be kept uniform. For flat or spiral springs that are not heat-treated after manufacture, hard-drawn or rolled steels are used. These may be tempered in the mill shape. Music wire is widely employed for making small spiral springs. A much-used straight-carbon spring steel has 1% carbon and 0.30 to 0.40% manganese, but becomes brittle when overstressed. ASTM (American Society for


Testing and Materials) carbon steel for flat springs has 0.70 to 0.80% carbon and 0.50 to 0.80% manganese, with 0.04% max each of sulfur and phosphorus. Motor springs are made of this steel rolled hard to a tensile strength of 1723 MPa. Watch spring steel, for mainsprings, has 1.15% carbon, 0.15 to 0.25% manganese, and in the hard-rolled condition, has an elastic limit above 2068 MPa.

SILICON STEELS These are used for springs and have high strength. These steels average about 0.40% carbon, 0.75% silicon, and 0.95% manganese, with or without copper, but the silicon may be as high as 2%. A steel, used for automobile leaf springs and recoil springs, contains 2% silicon, 0.75% manganese, and 0.60% carbon. The elastic limit is 689 to 2068 MPa, depending on drawing temperature, with hardness 250 to 600 Brinell.

MANGANESE STEELS These steels for automotive springs contain about 1.25% manganese and 0.40% carbon, or about 2% manganese and 0.45% carbon. When heat-treated, the latter has a tensile strength of 1378 MPa and 10% elongation. Part of the manganese may be replaced by silicon and the silicon–manganese steels have tensile strengths as high as 1861 MPa. The addition of chromium or other elements increases ductility and improves physical properties. Manganese steels are deep-hardening but are sensitive to overheating. The addition of chromium, vanadium, or molybdenum widens the hardening range.

FORMS Wire for coil springs ranges in carbon from 0.50 to 1.20%, and in sulfur from 0.028 to 0.029%. Bessemer wire contains too much sulfur for spring use. Cold working is the method for hardening the wire and for raising the tensile strength. The highest grades of wire are referred to as music wire. The second grade is called harddrawn spring wire. The latter is a less expensive basic open-hearth steel with manganese content


of 0.80 to 1.10%, and an ultimate strength up to 2068 MPa.

APPLICATIONS For jet-engine springs and other applications where resistance to high temperatures is required, stainless steel and high-alloy steels are used. But, while these may have the names and approximate compositions of standard stainless steels, for spring-wire use their manufacture is usually closely controlled. For example, when the carbon content is raised in high-chromium steels to obtain the needed spring qualities, the carbide tends to collect in the grain boundaries and cause intergranular corrosion unless small quantities of titanium, columbium, or other elements are added to immobilize the carbon. Types include Type 302 stainless steel of highly controlled analysis for coil springs. Alloy NS-355 is a stainless steel having a typical analysis of 15.64% chromium, 4.38% nickel, 2.68% molybdenum, 1% manganese, 0.32% silicon, 0.12% copper, with the carbon at 0.14%. The modulus of elasticity is 205,100 MPa at 27°C and 168,000 MPa at 427°C. 17-7 PH stainless steel has 17% chromium, 7% nickel, 1% aluminum, and 0.07% carbon. Wire has a tensile strength up to 2378 MPa. Spring wire for high-temperature coil springs may contain little or no iron. Alloy NS-25, for springs operating at 760°C, contains about 50% cobalt, 20% chromium, 15% tungsten, and 10% nickel, with not more than 0.15% carbon.

SPUN PARTS Metal spinning, essentially, involves forming flat sheet metal disks into seamless circular or cylindrical shapes. It is a useful processing technique when quantity does not warrant investment needed for draw dies.

THE PROCESS The first step in the spinning process is to produce a form to the exact shape of the inside contours of the part to be made. The form can be of wood or metal. This form is secured to the headstock of a lathe and the metal blank is,

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in turn, secured to the form. Manual spinning techniques exist, and mechanical spinning lathes usually can be set up to force the blank against the form mechanically. In addition to manual or power spinning, hot spinning is sometimes used either to anneal a spun part, eliminating the need to remove a partially formed blank from the lathe, or else to increase the plasticity of the metal being formed. In the latter category, some metals such as titanium or magnesium must be spun hot because their normal room-temperature crystal structure lacks ductility. Heavy parts (up to 122 mm in some cases) can also be spun with increased facility at elevated temperature.

SHAPES

AND

TOLERANCES

Basically, a component must be symmetrical about its axis to be adaptable to spinning. The three basic spinning shapes are the cone, hemisphere, and straight-sided cylinder. The shapes are listed in order of increasing difficulty to be formed by spinning. Available spinning equipment is the limiting factor in determining the size of parts. Parts can be made ranging in diameter from 25.4 mm to almost 3.6 m. Thickness ranges from 0.010 to 122 mm. Most commonly, spun parts range in thickness from 0.059 to 4.7 mm.

SPUTTER TEXTURING Texturing of a metal surface improves the bonding of surfacing material to the substrate or the

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attachment of parts to the textured piece. One of many applications is the texturing of the metal surface of medical hip implants. These devices require an irregular surface to stimulate bone attachment. Texturing of complex shapes can be improved with a sputter-etching method that uses temporarily attached ceramic particles. By controlling the size and distribution of the ceramic particles, the width and depth of the texture can be regulated. The first stage of the process (see Figure S.5) is the spraying or dripping of adhesive on the area to be textured. Microspheres of ceramic are forced into the adhesive and the area is heated. The part is then placed in a discharge chamber where it is bombarded with argon ions. This operation produces an etching on the surface of the part not covered by the ceramic spheres. The etch depth is controlled by voltage, current density, and sputtering duration. The adhesive, which is charred by the sputter-etch process, is removed with atomic oxygen in a plasma asher. The brushing away of the ceramic particles reveals a textured surface. See Figure S.5.

STAINLESS STEEL Stainless steel comprises a large and widely used family of iron–chromium alloys known for their corrosion resistance — notably their “nonrusting” quality. This ability to resist corrosion Argon Ions

Ceramic Microspheres

CERAMIC MICROSPHERES PART WITH SMOOTH SURFACE TO BE TEXTURED PRESSED INTO ADHESIVE

Adhesive

PART COATED WITH ADHESIVE

SPUTTER-ETCHING IN ARGON PLASMA

CERAMIC PARTICLES BRUSHED OFF

Charred Adhesive

ADHESIVE CHARRED BY HEATING

Bone

REMOVAL OF CHAR IN PLASMA ASHER

TEXTURED PART IN USE AS A BONE IMPLANT

FIGURE S.5 Sputtering technique for improving surface for bonding. (From Ind. Heating, January, p. 45, 2000. With permission.)



is attributable to a chromium-oxide surface film that forms in the presence of oxygen. The film is essentially insoluble, self-healing, and nonporous. A minimum chromium content of 12% is required for the formation of the film, and 18% is sufficient to resist even severe atmospheric corrosion. Chromium content, however, may range to about 30% and several other alloying elements, such as manganese, silicon, nickel, or molybdenum, are usually present. Most stainless steels are also resistant to marine atmospheres, fresh water, oxidation at elevated temperatures, and mild and oxidizing chemicals. Some are also resistant to salt water and reducing media. They are also quite heat resistant, some retaining useful strength to 981°C. And some retain sufficient toughness at cryogenic temperatures. Thus, stainless steels are used in a wide range of applications requiring some degree of corrosion and/or heat resistance, including auto and truck trim, chemicaland food-processing equipment, petroleumrefining equipment, furnace parts and heattreating hardware, marine components, architectural applications, cookware and housewares, pumps and valves, aircraft and aircraftengine components, springs, instruments, and fasteners. The 18-8 chromium–nickel steels were called super stainless steels in England to distinguish them from the plain chromium steels. Today, wrought stainless steels alone include some 70 standard compositions and many special compositions. They are categorized as austenitic, ferritic, martensitic, or precipitationhardening (PH) stainless steels, depending on their microstructure or, in the case of the PH, their hardening and strengthening mechanism. There are also many cast stainless steels having these metallurgical structures. They are also known as cast corrosion-resistant steels, cast heat-resistant steels, and cast corrosion- and heat-resistant steels. Several compositions are also available in powder form for the manufacture of stainless steel powder-metal parts. See Tables S.8 and S.9.

FABRICATION As with steels in general, the so-called wrought stainless steels come from the melting furnaces


TABLE S.8 Standard Designations for Corrosion-Resistant Castings Cast Alloy Designation CA–15 Ca–40 CB–30 CB–7Cu CC–50 CD–4MCu CE–30 CF–3 CF–8 CF–20 CF–3M CF–8M CF–12M CF–8C CF–16F CG–8M CH–20 CK–20 CN–7M

Wrought Alloy Typea 410 420 431 — 446 — — 304L 304 302 316L 316 316 347 303 317 309 310 —

a

Wrought alloy type numbers are listed only for the convenience of those who want to determine corresponding wrought and cast grades. Because the cast alloy chemical composition ranges are not the same as the wrought composition ranges, buyers should use cast alloy designations for proper identification of castings.

in the form of ingot or continuously cast slabs. Ingots require a roughing or primary hot working, which the other form commonly bypasses. All then go through fabricating and finishing operations such as welding, hot and cold forming, rolling, machining, spinning, and polishing. No stainless steel is excluded from any of the common industrial processes because of its special properties; yet all stainless steels require attention to certain modifications of technique. Hot Working Hot working is influenced by the fact that many of the stainless steels are heat-resisting alloys. They are stronger at elevated temperatures than ordinary steel. Therefore, they require greater roll and forge pressure, and perhaps less reductions per pass or per blow. The austenitic steels are particularly heat resistant.

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TABLE S.9 Physical Properties of Corrosion-Resistant Grades

Alloy Type

Density, lb/cu in.

Specific heat at 70°F, Btu/lb/°F

CA–15 CA–40 CB–30 CC–50 CD–4Mcu CE–30 CF–3 CF–8 CF–20 CF–3M CF–8M CF–12M CF–8C CF–16F CG–8M CH–20 CK–20 CN–7M

0.275 0.275 0.272 0.272 0.277 0.277 0.280 0.280 0.280 0.280 0.280 0.280 0.280 0.280 0.281 0.279 0.280 0.289

0.11 0.11 0.11 0.12 0.12 0.14 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.11

Thermal Conductivity at 212°F, Btu/h/ft2/°F

Thermal Expansion 70–1000°F, in./in./°F × 106

14.5 14.5 12.8 12.6 8.8 — 9.2 9.2 9.2 9.4 9.4 9.4 9.3 9.4 9.4 8.2 8.2 12.1

6.4 6.4 6.5 6.4 6.5 9.6 10.0 10.0 10.4 9.7 9.7 9.7 10.3 9.9 9.7 9.6 9.2 9.7

Mag Perm Ferromagnetic Ferromagnetic Ferromagnetic Ferromagnetic Ferromagnetic 1.5 1.0 to 2.0 1.0 to 2.0 1.01 1.5 to 2.5 1.5 to 2.5 1.5 to 2.5 1.2 to 1.8 1.0 to 2.0 1.5 to 2.5 1.71 1.02 1.01 to 1.10

Electrical Resistance µΩ-cm at 70°F 78 76 76 77 75 85 76 76 78 82 82 82 71 72 82 84 90 90

Source: ACI Data Sheets.

Welding

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Welding is influenced by another aspect of high-temperature resistance of these metals — the resistance to scaling. Oxidation during service at high temperatures does not become catastrophic with stainless steel because the steel immediately forms a hard and protective scale. But this, in turn, means that welding must be conducted under conditions that protect the metal from such reactions with the environment. This can be done with specially prepared coatings on electrodes, under cover of fluxes, or in vacuum; the first two techniques are particularly prominent. Inert-gas shielding also characterizes widely used processes among which at least a score are now numbered. As for weld cracking, care must be taken to prevent hydrogen absorption in the martensitic grades and martensite in the ferritic grades, whereas a small proportion of ferrite is almost a necessity in the austenitic grades. Metallurgical “phase balance” is an important aspect of welding the stainless steels because of these complications


from a two-phase structure. Thus, a minor austenite fraction in ferritic stainless can cause martensitic cracking, while a minor ferrite fraction in austenite can prevent hot cracking. However, the most dangerous aspect of welding austenitic stainless steel is the potential “sensitization” affecting subsequent corrosion. Machining and Forming These processes adapt to all grades, with these major precautions: First, the stainless steels are generally stronger and tougher than carbon steel, such that more power and rigidity are needed in tooling. Second, the powerful workhardening effect gives the austenitic grades the property of being instantaneously strengthened upon the first touch of the tool or pass of the roll. Machine tools must therefore bite surely and securely, with care taken not to “ride” the piece. Difficult forming operations warrant careful attention to variations in grade that are available, also in heat treatment, for


accomplishing end purposes without unnecessary work problems. Finishing These operations produce their best effects with stainless steels. No metal takes a more beautiful polish, and none holds it so long or so well. Stainlessness is not just skin-deep, but body through. And, of course, coatings are rendered entirely unnecessary.

STAINLESS STEEL (CAST) Cast stainless steels are divided into two classes: those intended primarily for uses requiring corrosion resistance and those intended mainly for uses requiring heat resistance. Both types are commonly known by the designations of the Alloy Casting Institute of the Steel Founders Society of America, and these designations generally begin with the letter C for those used mainly for corrosion resistance and with the letter H for those used primarily for heat resistance. All are basically iron–chromium or iron–chromium–nickel alloys, although they may also contain several other alloying ingredients, notably molybdenum in the heat-resistant type, and molybdenum, copper, and/or other elements in the corrosion-resistant type. The corrosion-resistant cast stainless steel type follows the general metallurgical classifications of the wrought stainless steels, that is, austenitic, ferritic, austenitic-ferritic, martensitic, and precipitation hardening. Specific alloys within each of these classifications are austenitic (CH-20, CK-20, CN-7M), ferritic (CB-30 and CC-50), austenitic-ferritic (CE-30, CF-3, CF-3A, CF-8, CF8A, CF-20, CF-3M, CF-3MA, CF-8M, CF-8C, CF-16F, and CG-8M), martensitic (CA-15, CA40, CA-15M, and CA-6NM), and precipitation hardening (CB-7Cu and CD-4MCu). The chromium content of these alloys may be as little as 11% or as much as 30%, depending on the alloy. The heat-resistant cast stainless steel types may contain as little as 9% chromium (Alloy HA), although most contain much greater amounts, as much as 32% in HL. Although nickel content rarely exceeds chromium content in the corrosion-resistant type, it


does in several heat-resistant types (HN, HP, HT, HU, HW, and HX). In fact, nickel is the major ingredient in HU, HW, and HX. Several of the heat-resistant types can be used at temperatures as high as 1149°C. C-series grades are used in valves, pumps, and fittings. H-series grades are used for furnace parts and turbine components.

APPLICATIONS Iron–chromium alloys containing from 11.5 to 30% chromium and iron–chromium–nickel alloys containing up to 30% chromium and 31% nickel are widely used in the cast form for industrial process equipment at temperatures from –257 to 649°C. The largest area of use is in the temperature range from room temperature to the boiling points of the materials handled. Typical stainless castings are pumps, valves, fittings, mixers, and similar equipment. Chemical industries employ them to resist nitric, sulfuric, phosphoric, and most organic acids, as well as many neutral and alkaline salt solutions. The pulp and paper industry is a large user of high alloy castings in digesters, filters, pumps, and other equipment for the manufacture of pulp. Fatty acids and other chemicals involved in soap-making processes are often handled by high alloy casting. Bleaching and dyeing operations in the textile industry require parts made from high alloys. These corrosionresistant alloys are also widely used in making synthetic textile fibers. Pumps and valves cast of various high alloy compositions find wide application in petroleum refining. Other fields of application are food and beverage processing and handling, plastics manufacture, preparation of pharmaceuticals, atomic-energy processes, and explosives manufacture. Increasing use is being made of cast stainless alloys for handling liquid gases at cryogenic temperatures.

STAINLESS STEEL (WROUGHT) Except for the precipitation-hardening (PH) stainless steels, wrought stainless steels are commonly designated by a three-digit numbering system of the American Iron and Steel Institute. Wrought austenitic stainless steels constitute the 2XX and 3XX series and the wrought

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ferritic stainless steels are part of the 4XX series. Wrought martensitic stainless steels belong either to the 4XX or 5XX series. Suffix letters, such as L for low carbon content or Se for selenium, are used to denote special compositional modifications. Cast stainless steels are commonly known by the designations of the Alloy Casting Institute of the Steel Founders Society of America, which begin with letters CA through CN and are followed by numbers or numbers and letters. Powder compositions are usually identified by the designations of the Metal Powder Industries Federation. Of the austenitic, ferritic, and martensitic families of wrought stainless steels, each has a general-purpose alloy. All of the others in the family are derivatives of the basic alloy, with compositions tailored for special properties. The stainless steel 3XX series has the largest number of alloys and stainless steel 302, a stainless “18-8” alloy, is the general-purpose one. Besides its 17 to 19% chromium and 8 to 10% nickel, it contains a maximum of 0.15% carbon, 2% manganese, 1% silicon, 0.4% phosphorus, and 0.03% sulfur. 302B is similar except for greater silicon (2 to 3%) to increase resistance to scaling. Stainless steels 303 and 303Se are also similar except for greater sulfur (0.15% minimum) and, optionally, 0.6% molybdenum in 303, and 0.06 maximum sulfur and 0.15 minimum selenium in 303Se. Both are more readily machinable than 302. 304 and 304L stainless steels are low-carbon (0.08% and 0.03 maximum, respectively) alternatives, intended to restrict carbide precipitation during welding and, thus, are preferred to 302 for applications requiring welding. They may also contain slightly more chromium and nickel. 304N is similar to 304 except for 0.10 to 0.16% nitrogen. The nitrogen provides greater strength than 302 at just a small sacrifice in ductility and a minimal effect on corrosion resistance. 305 has 0.12% maximum carbon but greater nickel (10.5 to 13%) to reduce the rate of work hardening for applications requiring severe forming operations. S30430, as designated by the Unified Numbering System, contains 0.08 maximum carbon, 17 to 19% chromium, 8 to 10% nickel, and 3 to 4% copper. It features a still lower rate of work hardening and is used for severe cold-heading operations. 308 contains


more chromium (19 to 21%) and nickel (10 to 12%) and, thus, is somewhat more corrosion and heat resistant. Although used for furnace parts and oil-refinery equipment, its principal use is for welding rods because its higher alloy content compensates for alloy content that may be reduced during welding. See Table S.10 (Wrought Stainless Steels). Stainless steels 309, 3095, 310, 310S, and 314 have still greater chromium and nickel contents. 309S and 310S are low-carbon (0.08% maximum) versions of 309 and 310 for applications requiring welding. They are also noted for high creep strength. Stainless steel 314, which like 309 and 310 contains 0.25% maximum carbon, also has greater silicon (1.5 to 3%), thus providing greater oxidation resistance. Because of the high silicon content, however, it is prone to embrittlement during prolonged exposure at temperatures of 649 to 816°C. This embrittlement, however, is only evident at room temperature and is not considered harmful unless the alloy is subject to shock loads. These alloys are widely used for heaters and heat exchangers, radiant tubes, and chemical and oil-refinery equipment. Stainless steels 316, 316L, 316F, 316N, 317, 317L, 321, and 329 are characterized by the addition of molybdenum, molybdenum and nitrogen (316N), or titanium (321). Stainless steel 316, with 16 to 18% chromium, 10 to 14% nickel, and 2 to 3% molybdenum, is more corrosion and creep resistant than 302- or 304type alloys. Type 316L is the low-carbon version for welding applications; 316F, because of its greater phosphorus and sulfur, is the “free-machining” version; and 316N contains a small amount of nitrogen for greater strength. Stainless steels 317 and 317L are slightly richer in chromium, nickel, and molybdenum and, thus, somewhat more corrosion and heat resistant. Like 316, they are used for processing equipment in the oil, chemical, food, paper, and pharmaceutical industries. Type 321 is titanium-stabilized to inhibit carbide precipitation and provide greater resistance to intergranular corrosion in welds. Type 329, a high-chromium (25 to 30%) low-nickel (3 to 6%) alloy with 1 to 2% molybdenum, is similar to 316 in general corrosion resistance but more resistant to stress corrosion. Stainless steel 330, a


high-nickel (34 to 37%), normal chromium (17 to 20%), 0.75 to 1.5% silicon, molybdenumfree alloy, combines good resistance to carburization, heat, and thermal shock. Stainless steels 347 and 348 are similar to 321 except for the use of columbium and tantalum instead of titanium for stabilization. Type 348 also contains a small amount (0.2%) of copper. Both have greater creep strength than 321 and they are used for welded components, radiant tubes, aircraft-engine exhaust manifolds, pressure vessels, and oil-refinery equipment. Stainless steel 384, with nominally 16% chromium and 18% nickel, is another lowwork-hardening alloy used for severe coldheading applications. The stainless steel 2XX series of austenitics comprises 201, 202, and 205. They are normal in chromium content (16 to 19%), but low in nickel (1 to 6%), high in manganese (5.5 to 15.5%), and with 0.12 to 0.25% carbon and some nitrogen. Types 201 and 202 have been called the low-nickel equivalents of 301 and 302, respectively. Type 202, with 17 to 19% chroinium, 7.5 to 10% manganese, 4 to 6% nickel, and a maximum of 1% silicon, 0.25% nitrogen, 0.15% carbon, 0.06% phosphorus, and 0.03% sulfur, is the general-purpose alloy. Type 201, which contains less nickel (3.5 to 5.5%) and manganese (5.5 to 7.5%), was prominent during the Korean war due to a nickel shortage. Type 205 has the least nickel (1 to 1.75%), and the most manganese (14 to 15.5%), carbon (0.12 to 0.25%), and nitrogen (0.32 to 0.40%) contents. It is said to be the low-nickel equivalent of 305 and has a low rate of work hardening that is useful for parts requiring severe forming operations. Like stainless steels in general, austenitic stainless steels have a density of 7750 to 8027 kg/m3. Unlike some other stainless steels, they are essentially nonmagnetic, although most alloys will become slightly magnetic with cold work. Their melting range is 1371 to 1454°C, specific heat at 0 to 100°C is about 502 J/kg · K, and electrical resistivity at room temperature ranges from 69 × 10–8 to 78 × 10–8 Ω · m. Types 309 and 310 have the highest resistivity, and 201 and 202 the lowest. Most are available in many mill forms and are quite ductile in the annealed condition,


tensile elongations ranging from 35 to 70%, depending on the alloy. Although most cannot be strengthened by heat treatment, they can be strengthened appreciably by cold work. In the annealed condition, the tensile yield strength of all the austenitics falls in the range of 207 to 552 MPa, with ultimate strengths in the range of 517 to 827 MPa. But cold-working 201 or 301 sheet just to the half-hard temper increases yield strength to 758 MPa and ultimate strength to at least 1034 MPa. Tensile modulus is typically 193 × 103 to 199 × 103 MPa and decreases slightly with severe cold work. As to high-temperature strength, even in the annealed condition most alloys have tensile yield strengths of at least 83 MPa at 815°C, and some (308, 310) about 138 MPa. Types 310 and 347 have the highest creep strength, or stress-rupture strength, at 538 to 649°C. Annealing temperatures range from 954 to 1149°C, initial forging temperatures range from 1093 to 1260°C, and their machinability index is typically 50 to 55, 65 for 303 and 303Se, relative to 100 for 1112 steel. Among the many specialty wrought austenitic stainless steels are a number of nitrogenstrengthened stainless steels: Nitronic 20, 32, 33, 40, 50, and 60; 18-18 Plus and Marinaloy HN and 22; and SAF 2205 and 253MA. Nitrogen, unlike carbon, has the advantage of increasing strength without markedly reducing ductility. Some of these alloys are twice as strong as the standard austenitics and also provide better resistance to certain environments. All are normal or higher than normal in chromium content. Some are also normal or higher than normal in nickel content, whereas others are low in nickel and, in the case of 18-18 Plus, nickel-free. Nitronic 20, a 23% chromium, 8% nickel, 2.5% manganese alloy, combines high resistance to oxidation and sulfidation and was developed for engine exhaust valves. Unlike austenitics in general, it is hardenable by heat treatment. Solution treating at 1177°C, water quenching, and aging at 760°C provide tensile strengths of 579 MPa yield and 979 MPa ultimate. SAF 2205, an extra-low-carbon (0.03%), 22% chromium, 5.5% nickel, 3% molybdenum alloy, is a ferritic-austenitic alloy with high resistance to chloride- and hydrogen-sulfideinduced stress corrosion, pitting in chloride

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Designation AISI or Co.

UNS

Yield Strength 70°F (103 psi)

Impact Strength, Izod (ft-lb)

Tensile Strength (103 psi)

Fatigue Endurance Limit (103 psi)

Creep Strength 0.001% at 1000°F (103 psi)

Thermal Conductivity 32–212°F (Btu-ft/h-ft2-°F)

Coefficient of Thermal Expansion 32–212°F (10–6in./in.-°C)

— 35 34 35 34 —

— — 17 17 17 —

9.4 9.4 9.4 9.4 9.4 —

9.7 9.4 9.6 9.6 9.6 —

70°F

–320°F

Elongation 70°F(%)

38 30 30 35 30 25

90 75 75 85 75 70

220 275 220 230 220 220

40 60 60 50 60 60

30

75

—

45

110

—

—

16

9.0

8.3

70°F

–320°F

Austenitic Grades 115 120 100 110 80 110 80 85(Se) 110 115 160a 110

202 301 302 303/303/SE 304 304L 309/ 309S 310/ 310S 316 316L 317 317L 321 347/ 348 AL-GX 254S MO 18-9LW

S20200 S30100 S30200 S30323 S30400 S30403 S30900 S30908 S31000 S31008 S31600 S31603 S31700 S31703 S32100 S34700 S34800 NO8366 S31254 —

30 30 25 35 30 35

75 75 70 85 80 90

150 185 — — — 205

50 60 60 50 5 45

110 110 — — — 110

90 110 — — — —

31.5 38 — — — 38

20 25 — — — —

8.2 9.4 — 9.4 8.3 9.3

8.8 8.8 — 8.8 9.2 9.3

30 40 44 30

75 90 95 78

200 — — —

50 45 35 65

110 — — —

110 — — —

39 — — —

17 — — —

9.3 7.9 — —

9.2 8.5 9.4 —

18Cr-2Ni-12Mn 21Cr-6Ni-9Mn 18-18 Plus 22Cr-13Ni-5Mn Nitronic 30 Nitronic 60

— S21904 S28200 S20910 — S21800

60 57 65 65 50 55

115 100 120 120 108 105

— 220 228 228 — 213

Nitrogen-Strengthened 55 230a 53 240a 60 240a 60 240a 56 240a 60 240a

52 49 50 50 — 37

— — — — — —

— 8.0 — — — —

9.0 6.3 — — 9.0 8.8


Grades — 115a 28a 28a 18a 160a


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TABLE S.10 Wrought Stainless Steels

S15500 S17400 S36200 S45000 S45500 S13800 S17700 —

85–185 85–185 108–182 117–184 115–235 82–215 40–175 100

120–200 120–200 125–188 194–196 145–250 130–235 130–265 150

240–260 240–260 — 130–250 — — — 250

405 409 430 446/ 18Cr-2Mog 18SR Sea-Cure Monit AL29-4C AL29-4-2

S40500 S40900 S43000 S44600 S18200 — S44660 S44635 S44735 S44800

25 30 30 40 75–80 65 75 75 75 85

60 55 65 75 80–87 85 90 90 90 95

— — — — — — — — — —

403/ 410 416/ 416 Se 440A 440C

S40300 S41000 S41600 S41623 S44002 S44004

Precipitation-Hardened 15–22 16–100 15–22 16–100a 16–21 6–80 13–25 18–105a 3–14 9–70a 12–22 24–120a 2–35 5–35a 8–50 64a

30 22 22 20 14–15 27 32 21 25 22

Grades 3–28 3–23 — 1–36 3–5a 2–30a 3–5a 57a

Ferritic Grades 20 — — — 35 — 2 — — — — — — — — — — — — —

73–60 73b–60c 95 75–78 — 100 82–110 63d

— — — — — — 77e 80

10.6 10.6 — — — — 9.5 8.2

6.0 6.0 — — — 5.9 6.0 9.2

— — 40 47 — — — — — —

8 — 8.5 6.4 — — — — — —

15.6 14.4 15.1 12.1 — — 11.4 9.9 — —

6.0 6.5 5.8 5.8 — 5.9 5.4 6.1 5.2 5.2

Martensitic Grades 40

75

—

35

100

—

40

9.2

14.4

5.5

40 60 65

75 105 110

— — —

30 20 14

100 2 2

— — —

40 40 40

9.2 — —

14.4 14.0 14.0

5.5 5.7 5.7

Note: Unless otherwise indicated, data are for annealed wrought bar. Properties of other mill forms may vary somewhat. a b c d

Charpy V-notch. For 108 cycles, condition H900, RT. At 600°F. For 108 cycles aged.

For 15 × 108 cycles, condition RH 950, RT. Stabilized low interstitial (sheet, strip). g Free-maching bar. e f


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15-5PH 17-4PH AM362 Custom 450 Custom 455 PH 13-8 Mo 17-7 PH A286


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environments, and intergranular corrosion in welded applications. The wrought ferritic stainless steels are magnetic and less ductile than the austenitics. Although some can be hardened slightly by heat treatment, they are generally not hardenable by heat treatment. All contain at least 10.5% chromium and, although the standard alloys are nickel-free, small amounts of nickel are common in the nonstandard ones. Among the standard alloys, stainless steel 430 is the generalpurpose alloy. It contains 16 to 18% chromium and a maximum of 0.12% carbon, 1% manganese, 1% silicon, 0.04% phosphorus, and 0.03% sulfur. Stainless steel 430F and 430FSe, the “free-machining” versions, contain more phosphorus (0.06% maximum) and sulfur (0.15% minimum in 430F, 0.06 maximum in 430FSe). 430FSe also contains 0.15% minimum selenium, and 0.6% molybdenum is an option for 430F. The other standard ferritics are stainless steels 405, 409, 429, 434, 436, 442, and 446. 405 and 409 are the lowest in carbon (0.08% maximum) and chromium (11.5 to 14.5% and 10.5 to 11.75%, respectively), the former containing 0.10 to 0.30% aluminum to prevent hardening on cooling from elevated temperatures, and the latter containing 0.75% maximum titanium. Type 429 is identical to 430 except for less chromium (14 to 16%) for better weldability. Types 434 and 436 are identical to 430 except for 0.75 to 1.25% molybdenum in the former and this amount of molybdenum plus 0.70% maximum columbium and tantalum in the latter; these additives improve corrosion resistance in specific environments. Types 442 to 446 are the highest in chromium (18 to 23% and 23 to 27%, respectively) for superior corrosion and oxidation resistance, and in carbon (0.20% maximum). Type 446 also contains more silicon (1.50% maximum). These standard alloys melt in the range of 1427 to 1532°C, thermal conductivities of 21 to 27 W/m · K at 100°C, and electrical resistivities of 59 to 67 µΩ · cm at 21°C. In the annealed condition, tensile yield strengths range from 241 to 276 MPa for 405 to as high as 414 MPa for 434, with ultimate strengths of 448 to 586 MPa and elongations of 20 to 33%. For 1% creep in 10,000 h at 538°C, 430 has a stress-rupture strength of 59 MPa. Typical


applications include automotive trim and exhaust components, chemical-processing equipment, furnace hardware and heat-treating fixtures, turbine blades, and molds for glass. Wrought martensitic stainless steels are also magnetic and, as they are hardenable by heat treatment, provide high strength. Of those in the stainless steel 4XX series, 410, which contains 11.5 to 13.0% chromium, is the general-purpose alloy. The others, 403, 414, 416, 416Se, 420, 420F, 422, 431, 440A, and 440C, have similar (403, 414) or more chromium (16 to 18% in the 440s). Most are nickel-free or, as in the case of 414, 422, and 431, low in nickel. Most of the alloys also contain molybdenum, usually less than 1%, plus the usual 1% or so maximum of manganese and silicon. Carbon content ranges from 0.15% maximum in 403 through 416 and 416Se, to 0.60 to 0.75% in 440A, and as much as 1.20% in 440C. Type 403 is the low-silicon (0.50% maximum) version of 410; 414 is a nickel (1.25 to 2.50%)modified version for better corrosion resistance. Types 416 and 416Se, which contain 12 to 14% chromium, also contain more than the usual sulfur or sulfur, phosphorus, and selenium to enhance machinability. Type 420 is richer in carbon for greater strength, and 420F has more sulfur and phosphorus for better machinability. Type 422, which contains the greatest variety of alloying elements, has 0.20 to 0.25% carbon, 11 to 13% chromium, low silicon (0.75% maximum), low phosphorus, and sulfur (0.025% maximum), 0.5 to 1.0% nickel, 0.75 to 1.25% of both molybdenum and tungsten, and 0.15 to 0.3% vanadium. This composition is intended to maximize toughness and strength at temperatures to 649°C. Type 431 is a higher-chromium (15 to 17%) nickel (1.25 to 2.50%) alloy for better corrosion resistance. The high-carbon, high-chromium 440 alloys combine considerable corrosion resistance with maximum hardness. The stainless steel 5XX series of wrought martensitic alloys — 501, 501A, 501B, 502, 503, and 504 — contain less chromium, ranging from 4 to 6% in 501 and 502, to 8 to 10% in 501 B and 504. All contain some molybdenum, usually less than 1%, and are nickel-free. Most of the 4XX alloys can provide yield strengths greater than 1034 MPa and some, such as the 440s, more than 1724 MPa. The


martensitic stainless steels, however, are less machinable than the austenitic and ferritic alloys and they are also less weldable. Forging temperatures range from 1038 to 1232°C. Most of the alloys are available in a wide range of mill forms and typical applications include turbine blades, springs, knife blades and cutlery, instruments, ball bearings, valves and pump parts, and heat exchangers. The wrought PH stainless steels are also called age-hardenable stainless steels. Three basic types are now available: austenitic, semiaustenitic, and martensitic. Regardless of the type, the final hardening mechanism is precipitation hardening, brought about by small amounts of one or more alloying elements, such as aluminum, titanium, copper, and, sometimes, molybdenum. Their principal advantages are high strength, toughness, corrosion resistance, and relatively simple heat treatment. Of the austenitic PH stainless steels, A-286 is the principal alloy. Also referred to as an ironbase superalloy, it contains about 15% chromium, 25% nickel, 2% titanium, 1.5% manganese, 1.3% molybdenum, 0.3% vanadium, 0.15% aluminum, 0.05% carbon, and 0.005% boron. It is widely used for aircraft turbine parts and high-strength fasteners. Heat treatment (solution treating at 981°C, water or oil quenching, aging at 718 to 732°C for 16 to 18 h and air cooling) provides an ultimate tensile strength of about 1035 MPa and a tensile yield strength of about 690 MPa, with 25% elongation and a Charpy impact strength of 87 J. The alloy retains considerable strength at high temperatures. At 649°C, for example, tensile yield strength is 607 MPa. The alloy also has good weldability and its corrosion resistance in most environments is similar to that of 3XX stainless steels. The semiaustenitic PH stainless steels are austenitic in the annealed or solution-treated condition and can be transformed to a martensitic structure by relatively simple thermal or thermomechanical treatments. They are available in all mill forms, although sheet and strip are the most common. True semiaustenitic PH stainless steels include PH 14–8 Mo, PH 15–7 Mo, and 17–7PH. AM-350 and AM-355 are also so classified, although they are said not truly to have a precipitation-hardening reaction.


The above PH steels are lowest in carbon content (0.04% nominally in PH 14-8Mo, 0.07% in the others). PH 14-8Mo also nominally contains 15.1% chromium, 8.2% nickel, 2.2% molybdenum, 1.2% aluminum, 0.02% manganese, 0.02% silicon, and 0.005% nitrogen. PH 15-7Mo contains 15.2% chromium, 7.1% nickel, 2.2% molybdenum, 1.2% aluminum, 0.50% manganese, 0.30% silicon, and 0.04% nitrogen. 17-7PH is similar to PH 15-7Mo except for 17% chromium and being molybdenum-free. AM-350 contains 16.5% chromium, 4.25% nickel, 2.75% molybdenum, 0.75% manganese, 0.35% silicon, 0.10% nitrogen, and 0.10% carbon. AM-355 has 15.5% chromium, 4.25% nickel, 2.75% molybdenum, 0.85% manganese, 0.35% silicon, 0.12% nitrogen, and 0.13% carbon. In the solution-heattreated condition in which these steels are supplied, they area readily formable. They then can be strengthened to various strength levels by conditioning the austenite, transformation to martensite, and precipitation hardening. One such procedure, for 17-7PH, involves heating at 760°C, air cooling to 16°C, then heating to 565°C and air-cooling to room temperature. In their heat-treated conditions, these steels encompass tensile yield strengths ranging from about 1241 MPa for AM-355 to 1793 MPa for PH 15-7Mo. After solution treatment, the martensitic PH stainless steels always have a martensitic structure at room temperature. These steels include the progenitor of the PH stainless steels, Stainless W, PH 13-8Mo, 15-5PH, 17-4PH, and Custom 455. Of these, PH 13-8Mo and Custom 455, which contain 11 to 13% chromium and about 8% nickel plus small amounts of other alloying elements, are the higher-strength alloys, providing tensile yield strengths of 1448 MPa and 1620 MPa, respectively, in bar form after heat treatment. The other alloys range from 15 to 17% in chromium and 4 to 6% in nickel, and typically have tensile yield strengths of 1207 to 1276 MPa in heat-treated bar form. They are used mainly in bar form and forgings, and only to a small extent in sheet. Age hardening, following high-temperature solution treating, is performed at 427 to 677°C.

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STAINLESS STEEL PRODUCTS Metal fibers are used for weaving into fabrics for arctic heating clothing, heated draperies, chemical-resistant fabrics, and reinforcement in plastics and metals. Stainless-steel yarn made from the fibers is woven into stainless-steel fabric that has good crease resistance and retains its physical properties to 427°C. The fiber may be blended with cotton or wool for static control, particularly for carpeting.

STEEL

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Steel is iron alloyed with small amounts of carbon, 2.5% maximum, but usually much less. The two broad categories are carbon steels and alloy steels, but they are further classified in terms of composition, deoxidation method, mill-finishing practice, product form, and principal characteristics. Carbon is the principal influencing element in carbon steels, although manganese, phosphorus, and sulfur are also present in small amounts, and these steels are further classified as low-carbon steels and sometimes referred to as mild steel (up to 0.30% carbon), medium-carbon steels (0.30 to 0.60%), and high-carbon steels (more than 0.60%). The greater the amount of carbon, the greater the strength and hardness, and the less ductility. Alloy steels are further classified as low-alloy steels, alloy steels, and high-alloy steels; those having as much as 5% alloy content are the most widely used. The most common designation systems for carbon and alloy steels are those of the American Iron and Steel Institute and the Society of Automotive Engineers, which follow a four- or five-digit numbering system based on the key element or elements, with the last two digits indicating carbon content in hundredths of a percent. Plain carbon steels (with 1% maximum manganese) are designated 10XX; resulfurized carbon steels, 11XX; resulfurized and rephosphorized carbon steels, 12XX; and plain carbon steels with 1 to 1.65% manganese, 15XX. Alloy steels include manganese steels (13XX), nickel steels (23XX and 25XX), nickel–chromium steels (31XX to 34XX), molybdenum steels (40XX and 44XX), chromium–molybdenum steels (41XX), nickel–chromium–molybdenum


steels (43XX, 47XX, and 81XX to 98XX), nickel–molybdenum steels (46XX and 48XX), chromium steels (50XX to 52XX), chromium–vanadium steels (61XX), tungsten–chromium steels (72XX), and silicon–manganese steels (92XX). The letter B following the first two digits designates boron steels and the letter L leaded steels. The suffix H is used to indicate steels produced to specific hardenability requirements. High-strength, low-alloy steels are commonly identified by a 9XX designation of the SAE, where the last two digits indicate minimum tensile yield strength in 1000 psi (6.8 MPa). In contrast to rimmed steels, which are not deoxidized, killed steels are deoxidized by the addition of deoxidizing elements, such as aluminum or silicon, in the ladle prior to ingot casting, thus, such terms as aluminum-killed steel. Deoxidation markedly improves the uniformity of the chemical composition and resulting mechanical properties of mill products. Semikilled steels are only partially deoxidized, thus intermediate in uniformity to rimmed and killed steels. Capped steels have a low-carbon steel rim characteristic of rimmed-steel ingot and central uniformity more characteristic of killed-steel ingot, and are well suited for coldforming operations. Steels are also classified as air-melted, vacuum-melted, or vacuum-degassed. Air-melted steels are produced by conventional melting methods, such as open hearth, basic oxygen, and electric furnace. Vacuum-melted steels are produced by induction vacuum melting and consumable electrode vacuum melting. Vacuum-degassed steels are air-melted steels that are vacuum processed before solidification. Vacuum processing reduces gas content, nonmetallic inclusions, and center porosity, and segregation. Such steels are more costly, but have better ductility and impact and fatigue strengths. Steel-mill products are reduced from ingot into such forms as blooms, billets, and slabs, which are then reduced to finished or semifinished shape by hot-working operations. If the final product is produced by hot working, the steel is known as hot-rolled steel. If the final product is shaped cold, the steel is known as cold-finished steel or, more specifically,


cold-rolled steel, or cold-drawn steel. Hotrolled mill products are usually limited to lowand medium-nonheat-treated carbon steels. They are the most economical steels, have good formability and weldability, and are widely used. Cold-finished steels, compared with hotrolled products, have greater strength and hardness, better surface finish, and less ductility. Wrought steels are also classified in terms of mill-product form, such as bar steels, sheet steels, and plate steels. Cast steels refer to those used for castings, and P/M (powder metal) steels refer to powder compositions used for P/M parts. Steels are also known by their key characteristic from the standpoint of application, such as electrical steels, corrosion-resistant stainless steels, low-temperature steels, high-temperature steels, boiler steels, pressurevessel steels, etc.

STEEL POWDER Steel powder is used mainly for the production of steel powder metal parts made by consolidating the powder under pressure and then sintering, and, to a limited extent, for steel-mill products, principally tool-steel bar products. For powder metal parts, the powder may be admixed for the desired composition or prealloyed, that is, each powder particle is of the desired composition. For mill products, prealloyed powder is used primarily. Steel powder is widely used to make small- to moderate-size powder metal parts, with compositions closely matching those of wrought steels. Among the more common are carbon steels, copper steels, nickel steels, nickel–molybdenum steels, and stainless steels.

STEEL WOOL Steel wool consists of long, fine fibers of steel used for abrading, chiefly for cleaning utensils and for polishing. It is made from low-carbon wire that has high tensile strength, usually having 0.10 to 0.20% carbon and 0.50 to 1% manganese. The wire is drawn over a track and shaved by a stationary knife bearing down on it, and may be made in a continuous piece as long as 30,480 m. Steel wool usually has three


edges but may have four or five, and strands of various types are mixed. There are nine standard grades of steel wool, the finest of which has no fibers greater than 0.0027 cm thick; the most commonly used grade has fibers that vary between 0.006 and 0.010 cm. Steel wool comes in batts, or in flat ribbon form on spools usually 10 cm wide. Stainless steel wool is also made, and copper wool is marketed for some cleaning operations.

STEREOLITHOGRAPHY Stereolithography uses a laser beam to convert a special photosensitive polymer from liquid to solid. The liquid polymer is held in a tank that also contains a platform that can be raised and lowered and onto which the part will be built. The platform is first positioned just below the surface of the liquid polymer, and the laser beam is rastered across the surface of the polymer to solidify a two-dimensional image of the bottom layer of the part. The platform is then lowered a small distance to allow a thin layer of liquid polymer to cover the solidified layer, and the laser beam is again rastered to solidify the next layer on top of and bonded to the initial layer. The platform is lowered again to form the third layer, and so on. When the final (top) layer of the part has been solidified, the platform is raised from the tank to drain away the liquid polymer from the finished part. Very complex shapes can be formed, including holes and even internal hollows (if a means is provided to drain out the unsolidified polymer).

PROBLEMS Overhangs and undercuts are a challenge to form by stereolithography because the lower layers of such features will not be connected to the main body of the part, and temporary supporting structures (which can later be removed) must be fabricated as the layers are built up. By loading the polymer with ceramic or metal powder, stereolithography can be used to fabricate greenware that can be debinded and sintered to form a finished part. Because this and the other rapid prototyping processes are rather slow, they would be

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appropriate production methods only for limited runs. By the same token, however, the avoidance by rapid prototyping of expensive tooling makes it all the more attractive for limited runs. Stereolithography is perhaps the most well-developed technique for rapid prototyping, and a number of vendors provide hardware, software, and special polymers to accomplish this technique. Some of the hardware is small enough that the term desktop manufacturing can legitimately be applied to it.

STRIPPABLE COATINGS

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Strippable coatings are those that are applied for temporary protection and that can be readily removed. They are composed of such resins as cellulosics, vinyl, acrylic, and polyethylene; they can be water based, solvent based, or hotmelt. The choice of base depends on the surface to be protected. Water-base grades are neutral to plastic and painted surfaces, whereas solventbase types affect those surfaces. Clear vinyl strippable coatings, perhaps the most widely used, are usually applied by spraying in thicknesses of 30 to 40 mils. Acrylic strippable coatings impart a clear, high-gloss, high-strength temporary film to metal parts. Polyethylene strippable coatings are relatively low cost and can be used on almost all surfaces except glass. Cellulosic strippable coatings are designed for hot-dip application. Film thicknesses range widely and can go as high as 200 mils. The mineral oil often present in these coatings exudes and coats the metal surface to protect it from corrosion over long periods.

TYPES Vinyl coatings like those described in MIL-C3254 specification were first developed for ships. These are called cocooning systems and are applied over chicken wire or a similar frame over the object to be protected. The interstices of the chicken wire are coated by a process known as webbing. This consists of spraying a specially designed vinyl coating in a web fashion so that it coats the interstices with a very thin, spiderweb-like, fragile covering. This in turn is coated with a material similar to an ordinary strip coat vinyl by spraying. A more rigid


protective coat of asphalt is then applied, following which coats of vinyl or aluminum enamel are applied. The advantages of this coating system are long life, ability to cover irregular surfaces, and easy removal. Disadvantages include a cumbersome structure, which is expensive. Strippable, Sprayable, Vinyl Coatings These materials are generally applied by spraying in thicknesses of 30 to 40 mils. Their tensile strength runs 3.4 MPa minimum with an elongation of 200%, minimum. These materials are designed to be strippable after years of protective service. They are suitable for bright steel, aluminum, painted surfaces, wood, etc. They can also be used to protect spray booths, for aircraft protection, and on tanks, trucks, ships, and similar equipment. Coatings can be produced in a translucent, colored effect or in a clear form so as to show any defects in the substrate. These coatings are generally sprayed from 1 to 2 mils thick and are designed for protection in covered storage or in the transportation or fabrication of tools. They can be readily removed even though film thickness is low. Ethyl Cellulose, Type I, and Cellulose Acetobutyrate, Type II These 100% coatings are designed for dip application in a hot-melt bath of 177°C. Thickness ranges from 100 to 200 mils, depending on the protection desired. The mineral oil generally present in these coatings exudes and coats the metal surface to keep it from corroding and in a strippable condition for long periods. Tensile strength of the coatings is about 2.06 MPa minimum, with an elongation of about 90% when originally made, and 70% when aged. The coatings are generally used on tools, steel and aluminum parts, and many other parts that can withstand the temperature of dipping. Variations of specification types can be formulated that do not exude oil, which can be objectionable in handling, particularly with electrical equipment. Pourable variations are also commercially available.


Some special types of ethyl cellulose strippable materials can be used on painted surfaces and are formulated so that these surfaces are not affected. They can be applied by spraying. Other specially designed strippable materials are also practical; some of them can be used for packaging.

STRONTIUM AND ALLOYS A chemical element (symbol Sr), strontium is the least abundant of the alkaline-earth metals. It has a melting point about 770°C, and it decomposes in water. The metal is obtained by electrolysis of the fused chloride, and small amounts are used for doping semiconductors. Its compounds have been used for deoxidizing nonferrous alloys and for desulfurizing steel. But the chief uses have been in signal flares to give a red light, and in hard, heat-resistant greases. Strontium-90, produced atomically, is used in ship-deck signs as it emits no dangerous gamma rays. It gives a bright sign, and the color can be varied with the content of zinc, but it is short-lived. Strontium is very reactive and used only in compounds.

COMPOUNDS Strontium nitrate is a yellowish-white crystalline powder, Sr(NO3)2, produced by roasting and leaching celestite and treating with nitric acid. The specific gravity is 2.96, the melting point is 645°C, and it is soluble in water. It gives a bright crimson flame, and is used in railwaysignal lights and in military flares. It is also a source of oxygen, pyrotechnics, as well as a precursor for ceramic powders. The strontium sulfate used as a brightening agent in paints is powdered celestite. Strontium sulfide, SrS, used in luminous paint, gives a blue-green glow, but it deteriorates rapidly unless sealed. Strontium carbonate, SrCO3, is used in pyrotechnics, ceramics, and ceramic permanent magnets for small motors. The development of glazes for low-temperature vitreous bodies can be materially aided through the use of strontia. The added fluidity provided by strontia when replacing calcium and/or barium should promote interface reaction, improve glaze fit, while offsetting the


slightly higher thermal expansion evidenced in some cases in the dinnerware glaze tested. Strontia additions to such glazes should materially increase glaze hardness and lower the solubility. Scratch resistance should be improved when replacements are made, especially at the expense of calcium and barium, which would be due, in part, to the earlier reaction of strontia enabling the glaze to clear with a minimum of pits. Strontium hydrate, Sr (OH)2 · 8 H2O, loses its water of crystallization at 100°C and melts at 375°C. It is used in making lubricating greases and as a stabilizer in plastics. Strontium fluoride is produced in single crystals for use as a laser material. When doped with samarium it gives an output wavelength around 650 nm. Strontium hexaboride, which is generally known as SrB6 , is stable to temperatures up to 2760°C, above which decomposition initiates. Possible uses for the material are energy sources when using the radioisotope, high-temperature insulation, nuclear reactor control rods, and control additives. Strontium titanate, SrTiO3, has a melting point of 2080°C. Methods of compounding are (1) from mixed strontium carbonate and titanium dioxide, (2) from mixed strontium oxalate and titanium dioxide, and (3) from strontium titanyl oxalate. Strontium titanate is a highdielectric-constant material (225 to 250), which at lower temperatures has a temperature coefficient of dielectric constant somewhat higher than that of calcium titanate. Strontium titanate can be used by itself or in combination with barium titanate in applications for capacitors and other parts. The power factor of strontium titanate is unusually high at low frequencies with a great improvement in power factor in the neighborhood of 1 MHz. The thermal expansion of strontium titanate is linear over a wide temperature range (100 to 700°C).

STRUCTURAL FOAM Extending the size capabilities of molded parts beyond the limits of conventional injection molding is one of the main advantages of structural foam molding. Whereas injection moldings are usually referred to in terms of ounces

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and inches, foam moldings usually involve pounds and feet, despite the fact that foam density is much lower. Parts weighing 22.7 kg are not uncommon by low-pressure structural foam methods, and some molders can produce 45.4kg parts in a single shot.

the solid, smooth skin against the mold surfaces. Then, the second material, a measured short shot containing a blowing agent, is injected to form the foamed part interior. The core material is usually a lower-cost resin than the skin material.

FOAM PROCESSING

High Pressure

Although structural foam parts are produced by several different methods, all systems disperse a gas into the polymer melt during processing, either by adding a chemical blowing agent to the compound or by inducing a gas directly into the melt. The gas creates the cellular core structure in the part. Regardless of the type or form of foaming agent used or when it is added to the melt, structural foam processes are classified as either low-pressure or high-pressure methods. Such a classification relates directly to size range, surface finish, economics, and properties of the molded part.

This is an expandable-mold, structural foam molding that is closer to conventional injection molding. The heating melt (with a blowing agent) is injected into the mold, creating cavity pressures of between 34 and 136 MPa. The mold is entirely filled, and the pressure prevents any foaming from occurring while the skin portion solidifies against the mold surfaces. At this point, the method departs from conventional injection molding. Mold pressure must be reduced and space provided to allow foaming to take place between the solid-skin surfaces. Depending on the type of equipment and size and configuration of the part, these two provisions are made either by withdrawing cores or by special press motions that partially open the mold halves.

Low Pressure

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Also known as short-shot, conventional structural foam processing methods are the most commonly used because they are the simplest and best suited for economical production of large, three-dimensional parts. In this process, a controlled mixture of resin and gas is injected into a mold creating a low cavity pressure — from 1.36 to 3.43 MPa. The mixture only partially fills the mold, and the bubbles of gas, having been at a higher pressure, expand immediately and fill the cavity. As the cells collapse against the mold surface, a solid skin of melt is formed over the rigid, foamed core. Skin thickness is controlled by amount of melt injected, mold temperature, type and amount of blowing agent, and temperature and pressure of the melt. With the use of multiple injection nozzles, extremely large parts can be molded; alternatively, several parts of varying sizes can be molded simultaneously in multicavity molds. Standard nominal wall thickness is 3.2 mm. Another process variation, coinjection, involves the separate injection of two compatible resins. First, a solid resin is injected to form


ADVANTAGES In addition to large-size capability and lowdensity structure, structural foam parts offer high stiffness-to-weight advantages. A 25% increase in wall thickness (over that of a solid section) can provide twice the rigidity — at equal weight — of a solid part. Strength-toweight ratios of foamed sections can be two to five times those of structural metals. Foamed parts made by any of the various methods are relatively stress-free because the foaming is done at a low pressure. For the same reason, sink marks do not occur in foamed parts behind ribs or at wall intersections. Tooling for low-pressure structural foam molding is generally less expensive than that for injection molding because the low pressure permits the use of lighter-weight mold materials. Tooling for high-pressure systems is more expensive than for conventional molding because of the special tooling motions involved to accommodate the foaming cycle.


LIMITATIONS Surface finish is the most apparent difference in low-pressure structural foam parts, compared with conventional moldings. Part surfaces have a characteristic swirl pattern caused by the blowing agent, some of which becomes trapped between the mold surface and the skin of the part. The swirl pattern is both visual and tactile; surface roughness can be as much as 1,000 µin. Parts that require smooth, finished surfaces require secondary operations, usually sanding, filling, and painting. The principal process variables that control the swirl pattern are mold temperature, melt temperature, injection rate, and the nature and concentration of the blowing agent. Control of these variables can produce foamed parts with surfaces that replicate any mold surface, smooth or patterned. But this improvement is not achieved without trade-offs: changes involve slower injection rates, heating and cooling of the mold, and other alterations that increase mold costs. Parts produced by gas counterpressure molding have a significantly reduced swirl pattern because the foaming gases are kept in solution until the solid skin is formed against mold surfaces. Surfaces of coinjected parts are comparable to those of solid, injection-molded parts. Surfaces of parts made by the high-pressure processes are comparable to those of injection-molded parts because the surface of the melt in contact with the mold solidifies while under pressure. But such parts have a “witness line,” which has the appearance of a wide parting line at the edges — where the mold opening made provision for foaming. Such areas may require touching up. Another limitation of high-pressure foam molding is part size and shape. The high pressures and the cost of tools limit these systems to much smaller parts than can be molded economically by low-pressure systems. Most highpressure parts are relatively flat.

STRUCTURAL MATERIALS These are construction materials that, because of their ability to withstand external forces, are considered in the design of a structural


framework. Materials used primarily for decoration, insulation, or other than structural purposes are not included in this group.

CLAY PRODUCTS The principal products in this class are the solid masonry units such as brick and the hollow masonry units such as clay tile or terra-cotta. Brick is the oldest of all artificial building materials. It is classified as face brick, common brick, and glazed brick. Face brick is used on the exterior of a wall and varies in color, texture, and mechanical perfection. Common brick consists of the kiln run of brick and is used principally as backup masonry behind whatever facing material is employed. It provides the necessary wall thickness and additional structural strength. Glazed brick is employed largely for interiors where beauty, ease of cleaning, and sanitation are primary considerations. Structural clay tiles are burned-clay masonry units having interior hollow spaces termed cells. Such tile is widely used because of its strength, light weight, and insulating and fire protection qualities. Its size varies with the intended use. Load-bearing tile is used in walls that support, in addition to their own weight, loads that frame into them, for example, floors and the roof. Tiles manufactured for use as partition walls, for furring, and for fireproofing steel beams and columns are classed as non-load-bearing tile. Special units are manufactured for floor construction: some are used with reinforced-concrete joists, and others with the steel beams in flat-arch and segmental-arch construction. Architectural terra-cotta is a burned-clay material used for decorative purposes. The shapes are molded either by hand in plaster-ofparis molds or by machine, using the stiff-mud process.

BUILDING STONES Building stones generally used are limestone, sandstone, granite, and marble. Until the advent of steel and concrete, stone was the most important building material. Its principal use now is as a decorative material because of its beauty, dignity, and durability.

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CONCRETE Concrete is a mixture of cement, mineral aggregate, and water, which, if combined in proper proportions, form a plastic mixture capable of being placed in forms and of hardening through the hydration of the cement.

WOOD

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The cellular structure of wood is largely responsible for its basic characteristics, unique among the common structural materials. The strength of wood depends on the thickness of the cell walls. Its tensile strength is generally greater than its compressive strength. The ratio of its strength to its stiffness is much higher than that of steel or concrete; therefore, it is important that deflection be carefully considered in the design of a wooden floor system. Laminated structural lumber is formed by gluing together two or more layers of wood with the grain of all layers parallel to the length of the member. Both laminated lumber and plywood make use of modern gluing techniques to produce a greatly improved product. The principal advantages derived from lamination are the ease with which large members are fabricated and the greater strength of built-up members. Laminated lumber is used for beams, columns, arch ribs, chord members, and other structural members. Plywood, while also laminated, is formed from three or more thin layers of wood that are cemented or bonded together, with the grain of the several layers alternately perpendicular and parallel to each other. Plywood is generally used as a replacement for sheathing or as form lumber for reinforced concrete structures. Both laminated structural lumber and plywood have the advantage of minimizing the effects of knots, shakes, and other lumber defects by preventing them from occurring in more than one lamination at a given cross section.

STRUCTURAL METALS Of importance in this group are the structural steels, steel castings, aluminum alloys, magnesium alloys, and cast and wrought iron. Steel castings are used for rocker bearings under the ends of large bridges. Shoes and


bearing plates are usually cast in carbon steel, but rollers are often cast in stainless steel. Aluminum alloys are strong, lightweight, and resistant to corrosion. The alloys most frequently used are comparable with the structural steels in strength. However, because aluminum alloys have a modulus of elasticity one third that of steel, the danger of local buckling is likely to determine the design of aluminum compression members. Magnesium alloys are produced as extruded shapes, rolled plate, and forgings. The principal structural applications are in aircraft, truck bodies, and portable scaffolding.

COMPOSITE MATERIALS These are engineered materials synthesized with two distinct phases and comprising a loadbearing material housed in a relatively weak protective matrix. The combination of two or more constituent materials yields a composite material with engineering properties superior to those of the constituents. The associated materials are termed polymer-matrix composites (PMCs), ceramic-matrix composites (CMCs), and metal-matrix composites (MMCs). The principal features of a fibrous composite material are the fibers, the matrix material, and the interface region between these two dissimilar materials. This class of structural material can be classified as metallic, ceramic, or polymeric, depending on the load-bearing or reinforcing material employed. The reinforcement may be particulates, whiskers, laminated fibers, or a woven fabric. These reinforcements are bonded together by the matrix, which distributes the loading between them. Generally, the reinforcement is a fibrous or particulate material, with the latter category permitting far superior structural properties to be achieved at the expense of more-challenging fabricating technologies and higher costs. There are numerous examples in nature where this type of microstructure, comprising a load-bearing structural phase housed in a protective matrix, is present. An example is a tree, where the trunk and branches comprise flexible cellulose fibers in a rigid lignin matrix. Development of the class of synthetic materials with this type of structure, such as fiberglass


composites and graphite-epoxy laminates, revolutionized the automotive, aerospace, and sporting goods industries. However, it should be noted that composite materials have been used for centuries; for example, bricks were manufactured in ancient Egypt that featured a composite material of clay and straw. The latest generation of advanced composite materials includes some of the lightest, strongest, stiffest, and most corrosion-resistant materials available to the engineering community. For example, there is a striking contrast of the magnitudes of the stiffness-to-weight ratio or the strength-to-weight ratio of the commercial metals relative to those of the advance composite materials. Whereas the specific stiffness of aluminum can be increased threefold by the addition of silicon carbide fibers to create a metal-matrix composite, the specific stiffness of graphite-epoxy, fiber-reinforced, polymeric materials can be over four times greater than the specific strength of steel. The ramifications of this comparison are immense, because lightweight, high-strength, high-stiffness structures can be fabricated with these advanced polymeric composite materials with a weight savings of approximately 50%. This class of designs translates into superior performance for diverse products such as those in the aerospace, defense, automotive, biomedical, and sporting goods industries.

SULFONE POLYMERS Sulfones are amorphous engineering thermoplastics noted for high heat-deflection temperatures and outstanding dimensional stability. These strong, rigid polymers are the only thermoplastics that remain transparent at service temperatures as high as 204°C. Three commercially important sulfonebased resins are: polysulfone (PSU), including Udel and Ultrason S; polyarylsulfone (PAS), including Radel; and polyethersulfone (PES), including Ultrason E. These materials are claimed to offer the highest performance profiles of any thermoplastics processible on conventional screw-injection and extrusion machinery. Processing temperatures, however, are higher than those of other thermoplastics; the sulfones are processed on equipment that


can generate and monitor stock temperatures in the range of 343 to 382°C.

PROPERTIES Heat resistance is the outstanding performance characteristic of the sulfones. Service temperature is limited by heat-deflection temperature, which ranges from 174 to 204°C. A high percentage of physical, mechanical, and electrical properties is maintained at elevated temperatures, within limits defined by the heat-deflection temperatures. The strength and stiffness of PSU and PES are virtually unaffected up to their glass-transition temperature. For example, the flexural modulus of molded parts remains above 2040 MPa at service temperatures as high as 160°C. Even after prolonged exposure to such temperatures, the resins do not discolor or degrade. Thermal stability and oxidation resistance are excellent at service temperatures well above 149°C. The continuous service temperature limit (CSTL) for PSU is 160°C, and 180°C for PES. With respect to flammability, PES is rated V-0 per UL 94, and PSU is rated at V-2 (Table S.11). Electrical insulating properties are generally in the midrange among those of other thermoplastics, and they change little after heat aging at the recommended service temperatures. Dissipation factor and dielectric constant — and thus, loss factor — are not affected significantly by increased temperature or frequency. Creep of the sulfones compared with that of other thermoplastics is exceptionally low at elevated temperatures and under continuous load. For example, creep at 99°C is less than that of acetal or heat-resistant ABS (acrylonitrile–butadiene–styrene) at room temperature. This excellent dimensional stability qualifies the sulfone resins for precision-molded parts. The hydrolytic stability of these resins makes them resistant to water absorption in aqueous acidic and alkaline environments. The combination of hydrolytic stability and heat resistance results in exceptional resistance to boiling water and steam, even under autoclave pressures and cyclic exposure of hot-to-cold and wet-to-dry. PES resins have excellent resistance to hot lubricants, engine fuels, and radiator fluids, and they are resistant to gasoline.

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The aromatic resins are also resistant to aqueous inorganic acids, organic acids, alkalies, aliphatic hydrocarbons, alcohols, and most cleaners and sterilizing agents. The sulfones also share a common drawback: they absorb ultraviolet rays, giving them poor weather resistance. Thus, they are not recommended for outdoor service unless they are painted, plated, or UV-stabilized.

SULFUR Sulfur (symbol S) is one of the most useful of the elements. Its occurrence in nature is little more than 1% that of aluminum, but it is easy

to extract and is relatively plentiful. In economics, it belongs to the group of “S” materials — salt, sulfur, steel, sugars, starches — whose consumption is a measure of the industrialization and the rate of industrial growth of a nation. Strict environmental laws are driving the production of sulfur recovered as a by-product of various industrial operations. It is also obtained by the distillation of iron pyrites, as a by-product of copper and other metal smelting, natural gas, and from gypsum. The sterri exported from Sicily for making sulfuric acid is broken rock rich in sulfur. Brimstone is an ancient name still in popular use for solid sulfur.

TABLE S.11 Properties of Sulfone Polymers ASTM Test

S

Property

Polysulfone

D792 D570

Specific gravity Water absorption, 24 h, 1/8-in. thk (%)

Physical 1.24 0.3

D638 D638 D638 D790 D790 D256 D785

Tensile strength (psi) Elongation at break (%) Tensile modulus (105 psi) Flexural strength (psi) Flexural modulus (105 psi) Impact strength, Izod (ft-lb/in. of notch) Hardness, Rockwell M

Mechanical 10,200 50–100 3.6 15,400 3.9 1.3 69

C177

Thermal conductivity (104 cal-cm/s-cm2-°C) Coefficient of thermal expansion (10–5in./in.-°F) Deflection temperature (°F) At 264 psi Oxygen index rating

D696 D648 D2863

Polyarylsulfone

Polyethersulfone

1.37 0.40

1.37 0.43

12,000 40 3.9 16,100 4.0 1.6 85

12,200 40–80 3.9 18,650 3.8 1.6 88

Thermal 6.2

—

3.2–4.4

3.1

2.7

3.1

345 30

400 33

398 34–38

425

383

400

3.07–3.03

3.51–3.54

3.5

0.0008–0.0034 5 × 1016

0.00171–0.00564 7.71 × 1016

0.001–0.0035 1017–1018

Electrical D149 D150 D150 D257

Dielectric strength (V/mil) Short time, (1/8-in. thk) Dielectric constant At 60 Hz to 1 MHz Dissipation factor At 60 Hz to 1 MHz Volume resistivity (Ω-cm)




Sulfur forms a crystalline mass of a paleyellow color, with a melting point of 111°C. It forms a ruby vapor at about 416°C. When melted and cast, it forms amorphous sulfur with a specific gravity of 1.955. The tensile strength is 1 MPa, and compressive strength is 22 MPa. Since ancient times it has been used as a lute for setting metals into stone. Sulfur also condenses into light flakes known as flowers of sulfur, and the hydrogen sulfide gas, H2S, separated from sour natural gas, yields a sulfur powder. Elemental sulfur is widely used for the synthesis of sulfur compounds. It reacts directly with virtually all elements except the noble gases. In addition to its use as a chemical intermediate, sulfur is used increasingly as a construction material. For example, sulfur-impregnated concrete is much more resistant to acid corrosion than is conventional concrete. Highways have been paved with high-sulfur asphalts.

PROPERTIES Sulfur has twice the atomic weight of oxygen but has many similar properties and has great affinity for most metals. The crystalline sulfur is orthorhombic, which converts to monoclinic crystals if cooled slowly from 120°C. This form remains stable below 120°C. When molten sulfur is cooled suddenly, it forms the amorphous sulfur, which has a ring molecular structure and is plastic, but converts gradually to the rhombic form. Sulfur has a wide variety of uses in all industries. The biggest outlet is for sulfuric acid, mainly for producing phosphate fertilizers.

USES Sulfur is used for making gunpowder and for vulcanizing rubber, but for most uses it is employed in compounds, especially as sulfuric acid or sulfur dioxide. Sulfur is used in glass as a colorant to produce golden yellows and ambers, and also with cadmium sulfide in selenium ruby glass. In sulfur amber glasses, the element is introduced as flowers of sulfur, cadmium sulfide, or sodium sulfide. Its manufacture necessitates several precautions.


COMPOUNDS Sulfur dioxide, or sulfurous acid anhydride, is a colorless gas of the composition SO2, used as a refrigerant, as a preservative, in bleaching, and for making other chemicals. It liquefies at about –10°C. As a refrigerant it has a condensing pressure of 23.5 kg at 30°C. The gas is toxic and has a pungent, suffocating odor, so that leaks are detected easily. It is corrosive to organic materials but does not attack copper or brass. The gas is soluble in water, forming sulfurous acid, H2SO3, a colorless liquid with suffocating fumes. The acid form is the usual method of use of the gas for bleaching.

SULFURIC ACID An oily, highly corrosive liquid of the composition H2SO4, sulfuric acid has a specific gravity of 1.841 and a boiling point of 330°C. It is miscible in water in all proportions, and the color is yellowish to brown according to the purity. It may be made by burning sulfur to the dioxide, oxidizing to the trioxide, and reacting with steam to form the acid. It is a strong acid, oxidizing organic materials and most metals. Sulfuric acid is used for pickling and cleaning metals, in electric batteries and plating baths, for making explosives and fertilizers, and for many other purposes. In the metal industries it is called dipping acid, and in the automotive trade it is called battery acid.

USES Sulfuric acid is used in the enameling industry for pickling purposes. The solutions vary in strength from 5 to 8%, although it is said that a 6% solution of sulfuric acid heated to 71 to 77°C will be the most effective in the pickling of sheet iron. In making up H2SO4 solutions, always add the acid to the water, and never the water to the acid, as the latter method may cause a violent reaction. Sulfuric acid also has been used as a mill addition for acid-resisting enamels.

COMPOUNDS Sulfur trioxide, or sulfuric anhydride, SO3, is the acid minus water. It is a colorless liquid

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boiling at 46°C, and forms sulfuric acid when mixed with water. Niter cake, which is sodium acid sulfate, NaHSO4, or sodium bisulfate, contains 30 to 35% available sulfuric acid and is used in hot solutions for pickling and cleaning metals. It comes in colorless crystals or white lumps, with a specific gravity of 2.435 and melting point 300°C. Sodium sulfate, or Glauber’s salt, is a white crystalline material of the composition Na2SO4 · 10H2O, used in making kraft paper, rayon, and glass. Salt cake, Na2SO4, is impure sodium sulfate used in the cooking liquor in making paper pulp from wood. It is also used in freezing mixtures. Sodium sulfite, Na2SO3 or Na2SO3 . 7H2O, is a white to tan crystalline powder very soluble in water but nonhygroscopic. Sodium sulfide, Na2S, is a pink flaky solid, used in tanneries for dehairing, and in the manufacture of dyes and pigments. The commercial product contains 60 to 62% Na2S, 3.5% NaCl, and other salts, and the balance water of crystallization.

SUPERALLOYS

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The term superalloy is broadly applied to ironbase, nickel-base, and cobalt-base alloys, often quite complex, which combine high-temperature mechanical properties and oxidation resistance to an unusual degree. Alloy requirements for turbosuperchargers and, later, the jet engine, largely provided the incentive for superalloy development. Because of their excellent high-temperature performance, they are also known as high-temperature, high-strength alloys. Their strength at high temperatures is usually measured in terms of stress-rupture strength or creep resistance. For high-stress applications, the iron-base alloys are generally limited to maximum service temperature of about 649°C, whereas the nickel- and cobalt-base alloys are used at temperatures to about 1093°C and higher. In general, the nickel alloys are stronger than the cobalt alloys at temperatures below 1093°C and the reverse is true at temperatures above 1093°C. Superalloys are probably best known for aircraft-turbine applications, although they are also used in steam and industrial turbines,


nuclear-power systems, and chemical- and petroleum-processing equipment. A great variety of cast and wrought alloys are available and, in recent years, considerable attention has been focused on the use of powder-metallurgy techniques as a means of attaining greater compositional uniformity and finer grain size.

STRENGTHENING MECHANISMS Superalloys of the nickel–chromium and iron–nickel–chromium types usually contain sufficient chromium to provide the needed oxidation resistance and are further strengthened by the addition of other elements. The strengthening mechanisms include solid solution, precipitation, and carbide hardening. Most of the superalloys combine at least two and frequently all three of the above mechanisms. Solid Solution This is accomplished by introducing elements having different atomic sizes than those of the matrix elements, to increase the lattice strain. In addition to chromium, needed also for oxidation resistance, the elements molybdenum, columbium, vanadium, cobalt, and tungsten are effective in varying degree as solid-solution strengtheners when added in proper balance. Superalloys of this type, such as 16-25-6, were used in gas turbines of older design for disks, employing “hot-cold work” to obtain the required yield strength in the hub area. However, hot-cold work is not effective as a means of getting high strength at temperatures much above about 538°C. Therefore, as gas turbine operating temperatures increased and turbine disk rim temperatures appreciably exceeded 538°C, other approaches were needed. Cobalt-base alloys are strengthened principally by solid solution hardening, usually combined with a dispersion of stable carbides. Precipitation Hardening Precipitation hardening is the method now employed to impart high strength at high temperatures to most of the superalloys used for critical components of aircraft gas turbines. This includes alloys ranging from the 25% nickel A-286 wheel alloy to such recent and


Carbide Hardening Carbides provide a major source of dispersedphase strengthening in cobalt-base alloys. These alloys do not respond to age hardening with aluminum and titanium since the gammaprime phase does not form unless substantial


Partial solution

Hardness Cu Curv eB rv e A

complex high-temperature nickel-base wrought and cast turbine blade alloys as “Udimet” 700, “Nimonic” alloy 115, and IN-100. The presence of aluminum and titanium, usually jointly, in a nickel–chromium base, with or without iron, imparts unique age-hardening characteristics through the precipitation of gamma prime phase {Ni3(Al,Ti)}. The outstanding difference between the previously used age-hardening systems, typified by duralumin or beryllium–copper, and those utilizing gamma prime hardening lies in the fact that an alloy of the latter type can be heated in service appreciably above the optimum aging temperature without permanent loss of strength. In the conventional critical dispersion type age-hardening alloys, strength can only be restored by a complete cycle of heat treatment involving a high-temperature solution treatment and reaging. Figure S.6 shows the way in which the two types of age-hardening systems differ, with the aluminum–titanium hardened alloy (curve B) regaining most of its initial hardness (and hightemperature properties) when the overheat is removed. However, an alloy typical of the other type of system (such as beryllium–copper or beryllium–nickel) overages and does not regain its hardness when the overheat is removed. It is this “reversible” aging behavior, along with high resistance to overaging (agglomeration of gamma-prime hardening phase), that has led to the widespread use of aluminum–titanium agehardened nickel-base alloys for first-stage turbine blades in commercial and advanced design jet engines. The somewhat less complex iron–nickel–chromium alloys, such as A-286 and “Incoloy” alloy 901, used for turbine disks, are similarly age-hardened, with Ni3Ti comprising most of the age-hardening component in these two alloys. Sufficient aluminum is also present in the gamma-prime precipitate to improve structural stability.

Re-precipitation

Return to Initial temp. Overaging Temp o Increased 50 F o

50 F increase at time X

Overaging

Return to original temp at time Y

X

Y Aging Time

Curve A = Critical dispersion type [(e.g. Be-Ni)] Curve B = Reversible hardening [(e.g. Ni3 (Al Ti)]

FIGURE S.6 Effect of aging time on hardness varies with type of age-hardening system.

amounts of nickel are present. Although gamma prime {Ni3(Al,Ti)} is the principal strengthener in the age-hardenable nickel-base alloys, important auxiliary strengthening can be obtained by precipitation of quite complex carbides. The nature of the carbides formed and their mode of distribution can usually be controlled by alloy formulation and heat treatment. Deoxidizers and Malleabilizers In addition to the major alloying elements, small but effective amounts of malleabilizers such as boron and zirconium must be present to neutralize the effects of impurities that adversely affect hot ductility. As little as 0.005% boron is highly effective in nickel-base alloys, and zirconium, in a concentration about ten times that of boron, is also useful. In air melting of nickel-base alloys, magnesium is also added to “fix” any sulfur picked up during melting. The high-temperature properties of the agehardenable alloys are governed to a large degree by the hardener (aluminum plus titanium) content. However, as the hardener increases, the temperature of incipient fusion decreases and the lower temperature limit of forgeability rises, because of increased high-temperature strength, until forging is no longer practical by conventional procedures. The use of cast turbine blade alloys to meet very high temperature requirements is a natural consequence. While other co-present elements and the amount of hot and cold workability required are

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also factors, it may be considered that alloys with up to 4% total hardener (aluminum plus titanium) are generally available in all common mill forms and are fabricable by conventional methods. As the hardener increases to about 8 or 9%, wrought alloys are still available, but in progressively fewer forms, ultimately being limited to small forgings such as turbine blades, at increasingly greater cost. The commercial cast turbine blade alloys contain from about 7 to 11% total hardener and are often chosen over forgings for reasons of cost or necessity or both.

COMPOSITIONS

AND

PROPERTIES

Although factors such as tensile properties, corrosion resistance, fatigue strength, expansion characteristics, etc. are important, the creeprupture characteristics are usually the prime requisite in the selection of a superalloy. Vacuum Melting

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Vacuum induction-melted superalloys are generally more ductile than air-melted material, permitting the use of higher aluminum plus titanium levels while still retaining adequate ductility. Vacuum induction melting accomplishes refining and also permits closer control of composition than does air melting. The vacuum arc (consumable electrode) method is widely used for superalloys, especially those employed for turbine disks. Often the electrode for the vacuum arc-melting charge is obtained from a vacuum induction melt, the end product thus combining the benefits of refining occurring in vacuum induction melting with the controlled solidification and sound ingot structure associated with the vacuum arc process. Some refining is also accomplished in the vacuum arc process, but to a lesser degree than in vacuum induction melting. Iron-Base Superalloys The iron-base superalloys include solid-solution alloys and precipitation-hardening (PH), or precipitation-strengthened, alloys. Solid-solution types are alloyed primarily with nickel (20 to 36%) and chromium (16 to 21%), although other elements are also present in lesser amounts. Superalloy 16-25-6, for example, the


alloy designation indicating its chromium, nickel, and molybdenum contents, respectively, also contains small amounts of manganese (1.35%), silicon (0.7%), nitrogen (0.15%), and carbon (0.06%). Superalloy 20Cb-3 contains 34% nickel, 20% chromium, 3.5% copper, 2.5% molybdenum, as much as 1% columbium, and 0.07% carbon. Incoloy 800, 801, and 802 contain slightly less nickel and slightly more chromium with small amounts of titanium, aluminum, and carbon. N-155, or Multimet, an early sheet alloy, contains about equal amounts of chromium, nickel, and cobalt (20% each), plus 3% molybdenum, 2.5% tungsten, 1% columbium, and small amounts of carbon, nitrogen, lanthanum, and zirconium. At 732°C, this alloy has a 1000-h stress-rupture strength of about 165 MPa. PH iron-base superalloys provide greater strengthening by precipitation of a nickel–aluminum–titanium phase. One such alloy, which may be the most well known of all iron-base superalloys, is A-286. It contains 26% nickel, 15% chromium, 2% titanium, 1.25% molybdenum, 0.3% vanadium, 0.2% aluminum, 0.04% carbon, and 0.005% boron. At room temperature, it has a tensile yield strength of about 690 MPa and a tensile modulus of 145 × 103 MPa. At 649°C, tensile yield strength declines only slightly, to 607 MPa, and its modulus is about the same or slightly greater. It has a 1000-h stress-rupture strength of about 145 MPa at 732°C. Other PH iron-base superalloys are Discoloy, Haynes 556 (whose chromium, nickel, cobalt, molybdenum, and tungsten content is similar to that N-155); Incoloy 903 and Pyromet CTX-1, which are virtually chromium-free but high in nickel (37 to 38%) and cobalt (15 to 16%); and V-57 and W-545, which contain about 14% chromium, 26 to 27% nickel, about 3% titanium, 1 to 1.5% molybdenum, plus aluminum, carbon, and boron. V-57 has a 1000-h stress-rupture strength of about 172 MPa at 732°C and greater tensile strength, but similar ductility, than A-286 at room and elevated temperatures. Nickel-Base Superalloys Nickel-base superalloys are solid-solution, precipitation, or oxide-dispersion strengthened. All


contain substantial amounts of chromium, 9 to 25%, which, combined with the nickel, accounts for their excellent high-temperature oxidation resistance. Other common alloying elements include molybdenum, tungsten, cobalt, iron, columbium, aluminum, and titanium. Typical solid-solution alloys include Hastelloy. X (22 to 23% chromium, 17 to 20% iron, 8 to 10% molybdenum, 0.5 to 2.5% cobalt, 2% aluminum, 0.2 to 1% tungsten, and 0.15% carbon); Inconel 600 (15.5% chromium, 8% iron, 0.25% copper maximum, 0.08% carbon); and Inconel 601, 604, 617, and 625, the latter containing 21.5% chromium, 9% molybdenum, 3.6% columbium, 2.5% iron, 0.2% titanium, 0.2% aluminum, and 0.05% carbon. At 732°C, wrought Hastelloy X (it is also available for castings) has a 1000-h stress-rupture strength of about 124 MPa, and has high oxidation resistance at temperatures to 1204°C. The precipitation-strengthened alloys, which are the most numerous, contain aluminum and titanium for the precipitation of a second strengthening phase, the intermetallic Ni3(A1,Ti) known as gamma prime (γ ′) or the intermetallic Ni3Cb known as gamma double prime (γ ″), during heat treatment. One such alloy, Inconel X-750 (15.5% chromium, 7% iron, 2.5% titanium, 1% columbium, 0.7% aluminum, 0.25% copper maximum, and 0.04% carbon), has more than twice the tensile yield strength of Inconel 600 at room temperature and nearly three times as much at 760°C. Its 1000-h stress-rupture strength at 760°C is in the range of 138 to 207 MPa. Still great tensile yield strength at room and elevated temperatures and a 172-MPa stress-rupture strength at 760°C are provided by Inconel 718 (19% chromium, 18.5% iron, 5.1% columbium, 3% molybdenum, 0.9% titanium, 0.5% aluminum, 0.15% copper maximum, 0.08% carbon maximum), a wrought alloy originally that also has been used for castings. Among the strongest alloys in terms of stress-rupture strength is the wrought or cast IN-100 (10% chromium, 15% cobalt, 5.5% aluminum, 4.7% titanium, 3% molybdenum, 1% vanadium, less than 0.6% iron, 0.15% carbon, 0.06% zirconium, 0.015% boron). Investment cast, it provides a 1000-h stress-rupture strength of 517 MPa at 760°C, 255 MPa at 871°C, and 103 MPa at 982°C.


Other precipitation-strengthened wrought alloys include Astroloy; D-979; IN 102; Inconel 706 and 751; M252; Nimonic 80A, 90, 95, 100, 105, 115, and 263; René 41, 95, and 100; Udimet 500, 520, 630, 700, and 710; Unitemp AF2-1DA; and Waspaloy. Other cast alloys, mainly investment-cast, include B-1900; IN738X; IN-792; Inconel 713C; M252; MAR-M 200, 246, 247, and 421; NX-188; René 77, 80, and 100; Udimet 500, 700, and 710; Waspaloy; and WAZ-20. A few of the cast alloys, such as MAR-M 200, are used to produce directionally solidified castings, that is, investment castings in which the grain runs only unidirectionally, as along the length of turbine blades. Eliminating transverse grains improves stress-rupture properties and fatigue resistance. Grain-free alloys, or single-crystal alloys, also have been cast, further improving high-temperature creep resistance. Regarding powder-metallurgy techniques, emphasis has been the use of prealloyed powder made by rapid solidification techniques (RST) and mechanical alloying (MA), a high-energy milling process using attrition mills or special ball mills. Dispersion-strengthened nickel alloys are alloys strengthened by a dispersed oxide phase, such as thoria, which markedly increases strength at very high temperatures but only moderately so at intermediate elevated temperatures, thus limiting applications. TDnickel, or thoria-dispersed nickel, was the first of such superalloys, and it was subsequently modified with about 20% chromium, TD-NiCr, for greater oxidation resistance. The recent MA 754 and MA 6000E alloys combine dispersion strengthening with yttria and gamma-prime strengthening. Cobalt-Base Superalloys Cobalt-base superalloys are for the most part solid-solution alloys, which, when aged, are strengthened by precipitation of carbide or intermetallic phases. Most contain 20 to 25% chromium, substantial nickel and tungsten and/or molybdenum, and other elements, such as iron, columbium, aluminum, or titanium. One of the most well known, L-605, or Haynes 25, is mainly a wrought alloy, although it is also used for castings. In wrought form, it contains

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20% chromium, 15% tungsten, 10% nickel, 3% iron, 1.5% manganese, and 0.1% carbon. At room temperature, it has a tensile yield strength of about 462 MPa, and at 871°C about 241 MPa. Its 1000-h stress-rupture strength at 815°C is 124 MPa. The more recent Haynes 188 (22% chromium, 22% nickel, 14.5% tungsten, 3% iron, 1.5% manganese, 0.9% lanthanum, 0.35% silicon, and 0.1% carbon), which was developed for aircraft-turbine sheet components, provides roughly similar strength and high oxidation resistance to about 1093°C. MP35N (35% nickel, 20 chromium, 10 molybdenum) is a work-hardening alloy used mainly for hightemperature corrosion-resistant fasteners. Another alloy, S-816, contains equal amounts of chromium and nickel (20% each), equal amounts of molybdenum, tungsten, columbium, and iron (4% each), and 0.38% carbon. Primarily a wrought alloy, although also used for castings, it has a 1000-h stress-rupture strength of 145 MPa at 815°C. Other casting alloys include AiResist 13, 213, and 215; Haynes 21 and 31, the latter also known as X40; Haynes 151; J-1650; MAR-M 302, 322, 509, and 918; V-36; and W1-52. Their chromium content ranges from 19% (AiResist 215) to 27% (Haynes 21) and some are nickel-free or low in nickel. Most contain substantial amounts of tungsten or tantalum, and various other alloying elements. Among the strongest in terms of 1000-h stress-rupture strength at 815°C are Haynes 21 and 31 — 290 MPa and 352 MPa, respectively.

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FABRICATION Forging and Hot Working Many of the commercial nickel-base age-hardenable superalloys can be forged or hot-worked with varying degrees of ease. As has been indicated, the top side of the forging temperature range is limited by such considerations as incipient fusion temperature, grain size requirements, tendency for “bursts,” etc., and the lower side by the stiffness and ductility of the alloy. The recommendations of the metal producer should be sought for optimum forging practice for a given alloy.


The hot extrusion process increases the gamut of superalloy compositions that can be hot-worked. By the use of a suitable sheath on the extrusion billet, otherwise unworkable alloys can be reduced to bar by hot extrusion. In some instances, mild steel has been used for a sheath while in others nickel–chromium alloys such as Nimonic alloy 75 and Inconel alloy 600 have been employed. Some of the very high-speed hot extrusion processes may be of value in hot-working the more refractory superalloys. Heat Treatment The conventional heat-treating equipment and fixtures generally suitable for nickel alloys and austenitic stainless steels are also applicable to the nickel-base high-temperature alloys. Nickel-base alloys are more susceptible to sulfur and lead embrittlement than iron-base alloys. It is therefore essential that all foreign material, such as grease, oil, cutting lubricants, marking paints, etc., be removed by suitable solvents, vapor degreasing, or other methods, before heat treatment. When fabricated parts made from thin sheet or strip of age-hardening alloys such as Incond alloy X-750 must be annealed during and after fabrication, it is desirable, especially in light gauges, to provide a protective atmosphere such as argon or dry hydrogen to lessen the possibility of surface depletion of the age-hardening elements. This precaution may not be as necessary in heavier sections, since the surface oxidation involves a much smaller proportion of the effective cross section. It is usually necessary after severe forming, or after welding, to apply a stress relief anneal (above 899°C) to assemblies fabricated from aluminum–titanium age-hardenable nickelbase alloys prior to aging. It is vitally important to heat the structure rapidly through the agehardening temperature range of 649 to 760°C (which is also the low ductility range) so that stress relief can be achieved before any appreciable aging takes place. This is conveniently done by charging into a furnace at or above the desired annealing temperature. It has been found at times that the efficacy of this procedure has been vitiated in large welded structures by


charging on to a cold car, resulting in a slower and nonuniform heating of the fabricated part when run into the hot furnace. Contrary to expectations, little difficulty has been encountered with distortion under the above rapid heating conditions. In fact, distortion of weldments of substantial size has been reported to be less than by conventional slow-heating heating methods. Forming All of the wrought nickel-base alloys available as sheet can be formed successfully into quite complex shapes involving much plastic flow. The lower-strength Inconel alloy 600 and Nimonic alloy 75 offer few problems. The highstrength age-hardening varieties, processed in the annealed condition, can be subjected to a surprising amount of cold work and deformation, provided sufficient power is available. Explosive forming has also been successfully employed on a number of nickel-base alloys. Machining All of the alloys discussed can be machined, the strongest and highest hardener content materials causing the most difficulty. The recommendations of the metal producer should be followed with respect to optimum condition of heat treatment, type of tool, speed and feed, cutting lubricant, etc. Wrought alloys of quite high hardener content, such as Inconel alloy 700 and Udimet 500, although difficult to handle, can be machined with reasonable facility using high-speed-steel tools of the tungsten–cobalt type, and cemented carbide tools of the tungsten–cobalt anal tungsten–tantalum–cobalt type. Various electroerosion processes have been successfully used on a number of the age-hardened superalloys and, at high hardener levels, may be necessary for some operations such as drilling. Welding Inconel alloy 600, Nimonic alloy 75, and other nickel-base alloys of the predominantly solid solution strengthened type offer no serious problems in welding. All of the common resistance


and fusion welding processes (except submerged arc) are regularly and successfully employed. In handling the wrought superalloys agehardened with gamma prime [Ni3(Al,Ti)], it is necessary to observe certain precautions. Material should be welded in the annealed condition to minimize the hazard of cracking in weld or parent metal. If the components to be joined have been severely worked or deformed they should be stress-relief-annealed before welding by charging into a hot furnace to ensure rapid heating to the stress-relieving temperature. Similarly, weldments should be stress-relieved before attempting to apply the 704°C age-hardening treatment. Where subassemblies must be joined in the age-hardened condition, the practice of “safe ending” with a compatible nonaging material prior to age hardening can be usefully employed. The final weldment joining the fully age-hardened components is then made on the “safe ends.” Such new welding processes as “short arc,” electron, and laser beam have come into increasing use and have been helpful in joining some of the very high hardener content alloys. Brazing The solid solution type chromium-containing alloys, such as Inconel alloy 600, are quite readily brazed, using techniques and brazing filler metals applicable to the austenitic stainless steels. Generally speaking, it is desirable to braze annealed (stress-free) material to avoid embrittlement by the molten braze metal. Where brazing filler metals are employed that melt above the stress-relieving temperature, a prior anneal is usually not needed. As with the stainless steels, dry hydrogen, argon, and helium atmospheres are used successfully; and vacuum brazing is also very successfully employed. The age-hardened nickel-base alloys containing titanium and aluminum are rather difficult to braze, unless some method of fluxing, solid or gaseous, is used. Alternatively, the common practice is to preplate the areas to be furnace brazed with 0.01 to 0.03 mm of nickel, which prevents the formation of aluminum or

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titanium oxide films and permits ready wetting by the brazing filler metal. Silver brazing filler metals can be used for lower temperature applications. However, since the nickel-base superalloys are usually employed for high-temperature applications, the higher melting point and stronger and more oxidationresistant brazing filler metals of the Ni–Cr–Si–B type are generally used. The silver–palladium–manganese and palladium–nickel filler metals also provide useful brazing materials for intermediate service temperatures.

SUPERCONDUCTIVITY

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Superconductivity is a phenomenon occurring in many electrical conductors, in which the electrons responsible for conduction undergo a collective transition into an ordered state with many unique and remarkable properties. These include the vanishing of resistance to the flow of electric current, the appearance of a large diamagnetism and other unusual magnetic effects, substantial alteration of many thermal properties, and the occurrence of quantum effects otherwise observable only at the atomic and subatomic level. The ability of certain materials, when cooled to extremely low or cryogenic temperatures, to conduct electricity with essentially zero resistance to DC current, and to AC current below certain critical high-frequency ranges, was found in the scientific community. This current is referred to as supercurrent, and is carried on the surface of the superconductor within a particular depth characteristic of the material. Until recently, superconductivity was only seen in materials at fantastically cold temperatures not exceeding 23 K (–250°C) in the intermetallic compound Nb3Ge. This meant that all superconductors had to be cooled with liquid helium, which is expensive and cumbersome to handle. Applications for superconductors were quite limited. The main use was for nuclear magnetic resonance scanners used by hospitals to examine soft tissue without surgery. Nuclear magnetic resonance scanners tap the intensely powerful magnetic fields that superconductors can be made to generate.


However, in August 1986, researchers discovered a compound of the metals lanthanum, barium, and copper, along with oxygen, that would superconduct at 35 K (–238°C). Following this discovery, researchers brought about a dramatic jump in Tc to 93 K (with an onset temperature of 98 K) by substituting yttrium for lanthanum, while roughly switching the +2 and +3 ion ratios.

SUPERCONDUCTORS Superconductors are solid crystalline materials whose electrical resistance drops significantly as temperature decreases. Until recently, temperatures approaching close to absolute zero (–272°C) were required for the resistivity to vanish. Some of the metals exhibiting superconductivity at near absolute zero include iridium, lead, mercury, columbium, tin, tantalum, vanadium, and many alloys and chemical compounds. Alloys considered among the best commercially available are lead–molybdenum–sulfur, columbium–tin, and columbium–titanium. In recent years, alloys and compounds have been developed that are superconductive at temperatures substantially above absolute zero. These include a compound of lanthanum, strontium, copper, and oxygen, which is superconductive at –240°C, and a barium–yttrium–copper oxide, which is superconductive at –183°C. A two-phase ceramic superconductor has been developed in which one of the phases is superconductive at –33°C. It is basically copper oxide containing barium and yttrium. Composition of the second phase is yet to be determined.

PROPERTIES

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PROCESSING

Because Jc is higher in single crystals than in polycrystalline materials, it can be increased by minimizing the presence of grain boundaries. This is done by single-crystal growth or techniques such as melt texturing, where grain boundaries become highly directionalized to allow for significantly less random disruption of current flow. Here, the grain boundaries become predominantly parallel to the copper–oxygen chains (the direction of current flow) in all the crystals. Consequently, the


microstructure consists of long, needlelike grains that are parallel and intermeshed. Single crystals have been grown epitaxially by sputtering (as well as by pulsed excimer laser deposition and electron beam epitaxy). Good deposition can be achieved when the material is reactively sputtered in an oxygen-containing environment from three separate metallic targets (yttrium, barium, copper) simultaneously. This is known as triode sputtering, and allows for tight control of stoichiometry. Following this, as with bulk samples, the films typically must be annealed in pure oxygen at –500°C. Samples also must be slow cooled to 300°C, as gravimetric analysis shows that the capability of the compound to absorb more oxygen into its crystal structure increases with decreased temperature down to 300°C. Annealing can be successfully accomplished below 300°C by use of an oxygen-ion bombardment. The greatest reproducibility of results in single-crystal formation has come when using single-crystal SrTiO3 as the epitaxy substrate. Some success has been found in depositing single crystals on single-crystal MgO, which has a much lower dielectric constant than SrTiO3

APPLICATIONS There are a number of practical applications of superconductivity. Powerful superconducting electromagnets guide elementary particles in particle accelerators, and they also provide the magnetic field needed for magnetic resonance imaging (MRI). Ultrasensitive superconducting circuits are used in medical studies of the human heart and brain and for a wide variety of physical science experiments. A completely superconducting prototype computer has even been built. Most superconductive applications that have been considered to have reasonable possibility of being achieved in the next few years involve thin-film deposition of these materials. Thin films clearly have the higher Jc advantage over bulk superconductors. Photovoltaic substances, for example, if interfaced with a superconductor, can act as signal detectors (e.g., infrared devices) because they will be sensitive to the most minute electrical fields.


One of the potential uses of the newer superconductors is for making more powerful and efficient electromagnets that could be used in trains to levitate them above their tracks, and thus make train speeds of hundreds of miles per hour possible.

SUPERPLASTIC FORMING This is a process for shaping superplastic materials, a unique class of crystalline materials that exhibit exceptionally high tensile ductility. Superplastic materials may be stretched in tension to elongations typically in excess of 200% and more commonly in the range of 400 to 2000%. There are rare reports of higher tensile elongations reaching as much as 8000%. The high ductility is obtained only for superplastic materials and requires both the temperature and rate of deformation (strain rate) to be within a limited range. The temperature and strain rate required depend on the specific material. A variety of forming processes can be used to shape these materials; most of the processes involve the use of gas pressure to induce the deformation under isothermal conditions at the suitable elevated temperature. The tools and dies used, as well as the superplastic material, are usually heated to the forming temperature. The forming capability and complexity of configurations producible by the processing methods of superplastic forming greatly exceed those possible with conventional sheet forming methods in which the materials typically exhibit 10 to 50% tensile elongation.

PROCESSES Superplastic forming typically utilizes a gas pressure differential across the superplastic sheet to induce the superplastic deformation and cause forming. Two processes have been developed: blow forming and movable-tool forming. Blow Forming Where gas pressure alone is used, the process is termed blow forming. Blow forming utilizes tooling heated to the superplastic temperature, and the gas pressure differential is usually

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applied according to a time-dependent schedule designed to maintain the average strain rate within the superplastic range. The tools and the superplastic sheet are heated to the same temperature, and the gas pressure is applied to cause a creeplike plastic stretching of the sheet that eventually contacts and takes the shape of the configuration die. Movable-Tool Forming For relatively deep shapes, forming methods involving movable tools combined with gaspressure forming may permit greater thinning control and reduced forming times as compared with the blow-forming method. One method is essentially the same as the blow-forming process, except that the die may be moved during the forming process. Another method uses a more complex sequence. The bubbleplate holds the superplastic sheet in place and prevents gas breakage. The plug-assisted forming method involves a movable die that is pushed into and stretches the superplastic sheet material, followed by the application of gas pressure on the same side of the sheet as the movable die. In snap-back forming, the sheet is first billowed by tree forming with gas pressure imposed on the movable-tool side of the sheet; the tool is then moved into the billowed sheet, and finally the gas pressure is imposed on the opposite side of the sheet to form the superplastic material onto the tool.

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Diffusion Bonding Diffusion bonding, also known as diffusion welding, is sometimes used in conjunction with superplastic forming to produce parts of complexity not possible with a single-sheet forming process. Diffusion bonding is a solid-state joining process in which two or more materials are pressed together under sufficient pressure and at a sufficiently high temperature to result in joining. In diffusion bonding, there is usually little permanent deformation in the bulk of the parts being joined, although local deformation does occur at the interfaces on a microscopic scale. Because interfacial contamination, such as oxidation, will interfere with the bonding


mechanisms, the process is usually conducted under an inert atmosphere, such as vacuum or inert gas. Some superplastic materials are ideally suited for processing by diffusion bonding, because they deform easily at the superplastic temperature and this temperature is consistent with that required for diffusion bonding. The most suitable alloys tend to have a high solubility for oxygen and nitrogen, so that these contaminants can be removed from the surface by diffusion into the base metal. For example, titanium alloys fall into this class and are readily diffusion bonded. Aluminum alloys form a very thin but tenacious oxide film and are therefore quite difficult to diffusion bond. For certain materials, and under conditions of proper processing, the diffusion-bond interfacial strength can be equal to that of the parent base material. It has been found that metals processed to fine grain size, as required for superplastic deformation, are the most suitable for diffusion bonding because they require lower bonding pressure than coarse-grained metal of the same alloy composition.

COMBINED METHODS The processing conditions for superplastic forming and diffusion bonding are similar, both requiring an elevated temperature and benefiting from the fine grain size. Consequently, a combined process of superplastic forming with diffusion bonding has been developed that can produce parts of greater complexity than singlesheet forming alone. The combined process of superplastic forming and diffusion bonding can involve multiple-sheet forming after localized diffusion bonding, producing expanded structures and sandwich configurations of various types. It is also possible to form a sheet superplastically onto, and diffusion bonded to, a separate piece of material thereby producing structural configurations much more like forgings than sheet metal structures. There are other joining methods that have also been utilized as alternatives to diffusion bonding, such as spot welding, and have been combined with superplastic forming to produce complex structures.


APPLICATIONS There are a number of commercial applications of superplastic forming and combined superplastic forming and diffusion bonding, including aerospace, architectural, ground transportation, and numerous miscellaneous uses. Examples are wing access panels in the Airbus A310 and A320, bathroom sinks in the Boeing 737, turbo-fan-engine-cooling duct components, external window frames in the space shuttle, front covers of slot machines, and architectural siding for buildings.

SUPERPOLYMERS Many plastics developed in recent years can maintain their mechanical, electrical, and chemical resistance properties at temperatures over 213°C for extended periods of time. Among these materials are polyimide, polysulfone, polyphenylene sulfide, polyarylsulfone, novaloc epoxy, aromatic polyester, and polyamide-imide. In addition to high-temperature resistance, they have in common high strength and modulus of elasticity, and excellent resistance to solvents, oils, and corrosive environments. They are also high in cost. Their major disadvantage is processing difficulty. Molding temperatures and pressures are extremely high compared to conventional plastics. Some of them, including polyimide and aromatic polyester, are not molded conventionally. Because they do not melt, the molding process is more of a sintering operation. One indication of the high-temperature resistance of' the superpolymers is their glass transition temperature of well over 260°C, as compared to less than 177°C for most conventional plastics. In the case of polyimides, the glass transition temperature is greater than 427°C and the material decomposes rather than softens when heated excessively. Aromatic polyester, a homopolymer also known as polyoxybenzoate, does not melt, but at 427°C can be made to flow in a nonviscous manner similar to metals. Thus, filled and unfilled forms and parts can be made by hot sintering, high-velocity forging, and plasma spraying. Notable properties are high thermal stability, good strength at 316°C, high thermal conductivity, good wear resistance, and extra-


high compressive strength. Aromatic polyesters have also been developed for injection and compression molding. They have long-term thermal stability and a strength of 20 MPa at 288°C. At room temperature polyimide is the stiffest of the group with a top modulus of elasticity of 51,675 MPa, followed by polyphenylene sulfide with a modulus of 33,072 MPa. Polyarysulfone has the best impact resistance of the superpolymers with an impact strength of 0.27 kg · m/cm (notch). Polyetherimide (PEI) is an amorphous thermoplastic that can be processed with conventional thermoplastic processing equipment. Its continuous-use temperature is 170°C and its deflection temperature is 200°C at 2 MPa. The polymer also has inherent flame resistance without use of additives. This feature, along with its resistance to food stains and cleaning agents, makes it suitable for aircraft panels and seat component parts. Tensile strength ranges from 103 to 165 MPa. Flexural modulus at room temperature is 3300 MPa. Polyimide (PI) foam is a spongy, lightweight, flame-resistant material that resists ignition up to 427°C and then only chars and decomposes. Some formulations result in harder materials that can be used as lightweight wallboard or floor panels while retaining fire resistance. Aromatic polyketones are high-performance thermoplastics, which include polyetheretherketone (PEEK), glass transition temperature of 143°C and melting point of 335°C; polyetherketone (PEK), glass transition temperature of 154°C; polyetherketoneketone (PEKK), glass transition temperature of 154°C and melting point of 335°C; polyaryletherketone (PAEK), glass transition temperature of 170°C and melting point of 380°C. Glass fiber reinforcement improves the strength, stiffness, and dimensional stability of these materials. In addition, there are various ketone-based copolymers.

SURFACE PIGMENTS (FOR BRICK) Brick has a long history of durability. Environmental issues such as energy consumption and waste disposal are increasing in importance,

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producing opportunities for brick usage to increase. Facing brick need no rendering, painting, or regular maintenance. They look good year after year and often century after century with no energy or product input. The brick can be recycled again and again, and at the end of their lives can be used as aggregate. Brick are the material of the future, and as demand for brick increases, so will demand for choice. The range of choices can be expanded by using surface pigments.

COMPOSITIONS

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Surface pigments can be classified into several groups. Pigment mixtures, which on firing develop their intrinsic color, fall into the largest category of stains. The second class includes pigments that react with the brick surface sand or grog to produce a specific shade of color. Usually the iron in the surface layers of the clay is the critical factor. There are also colors that have been stabilized by prior treatment. These are normally used where it is necessary to have the maximum volume of brick one uniform color. Ferrous and ferric oxides are the source of a large number of colors ranging from very bright yellows through reds and maroons to blacks. The tone of the red surface colors depends on the firing temperature, the compound from which the oxide was formed, and the kiln atmosphere. The presence of zinc or titanium compounds modifies the iron color in pigments and produces a range of buffs and yellows. The spectral range of brown pigments is extremely wide, ranging in shade from cream to dark farmhouse brown. Black pigments are mixtures of compounds of iron and manganese. Occasionally, cobalt oxide is added to intensify the blue-black quality of some surface stains. Normally, gray pigments are produced by diluting selected black stains with inert and reactive fillers. The dominant tone of any black pigment is apparent when it is diluted, and some satisfactory blacks show undesirable tones when used diluted as grays.


DRY

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WET APPLICATIONS

Dry There are several methods of applying pigments in dry form. Surface pigments are trickle-fed through a sieve and then brushed or lightly rolled into the brick surface. Surface effects are applied by vibrating the granules onto the column and rolling them into the surface. For surface pigments or frits added to aggregates, the aggregate (a sand, grog, or other suitable material) is mixed with the pigment or frit at a predetermined ratio. Wet Surface pigments can be added to water-based suspensions and applied as a spray, slurry, or engobe. For spraying, a ratio of one part by weight of pigment to four parts by weight of water is generally acceptable. This thorough wetting of the finely powdered pigment ensures a smooth suspension.

SYNTHETIC NATURAL RUBBER (ISOPRENE) Stereoregular polyisoprene and polybutadiene elastomers, high in cis-1,4 content, are of growing interest to the engineer, both because of engineering performance, and their competitive price. IR (cis-polyisoprene) has been called “synthetic natural rubber” because chemically and physically it is similar to Hevea. General properties and examples of end-use performance show it to be a satisfactory supplement to natural rubber in a wide variety of products. Molecular weight can be controlled within quite wide limits and linearity can be maintained even with the longest chains. Higher-molecular-weight materials have been satisfactorily extended with oil to yield compositions with a desirable combination of low cost and attractive properties. The development of IR latex is a noteworthy advance in latex technology. The low emulsifier level, stereoregularity of the polymer, large particle size, and low viscosity have not hitherto been available in a general-purpose synthetic latex. These properties, combined


with high gum strength and elongation, offer advantages for many latex applications.

VULCANIZATE PROPERTIES IR can be processed in a manner similar to that used for natural rubber. Vulcanization can be carried out by means of curatives commonly used with natural or SBR (styrene–butadiene rubber). Properties of gum vulcanizates are quite similar to those of natural rubber, although IR has somewhat lower modulus and higher extensibility. There are excellent hysteresis properties, low heat buildup and high resilience of both the IR and the polybutadiene tread vulcanizates, and IR is second only to natural rubber in tear strength. Processing and compounding procedures for oil-extended IR are similar to those for unextended polymer, except that lower curative levels are recommended for maximum tensile strength and flat curing characteristics. Properties of extended vulcanizates approach those of the non-extended materials.

APPLICATIONS Similarity of performance between IR and natural rubber has permitted use of IR as a supplement for natural rubber in uses such as tire treads, carcasses, and white sidewalls. In nontire uses the low ash content, light color, and good mold flow characteristics of IR are of particular advantage. Good electrical properties and low moisture absorption make it suitable for a number of electrical insulating uses. Parts molded in IR exhibit sharp definition and excellent color stability to light. The low cost and good performance of oilextended IR are promising for tire carcass compounds, molded mechanical goods, and footwear. In latex form, IR is the first synthetic that possesses an average particle size as large and a particle size distribution as broad as that of natural rubber latex. It is highly promising for a number of coating and dipping applications, as well as foaming.

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T TALC

TANTALUM AND ALLOYS

Talc is a hydrous magnesium silicate, with the composition 63.4% SiO2, 31.9% MgO, and 4.7% H2O when found in pure form. It is an extremely soft mineral with a Moh hardness of 1. Talc is a soft friable mineral of fine colloidal particles with a soapy feel. It has a composition of 4SiO2 · 3MgO · H2O and a specific gravity of 2.8. It is white whelf pure, but may be colored gray, green, brown, or red with impurities. Talc is now used for cosmetics, for paper coatings, as a filler for paints and plastics, and for molding into electrical insulators, heater parts, and chemical ware. The massive block material, called steatite talc, is cut into electrical insulators. It is also called lava talc. The more impure block talcs are used for firebox linings and will withstand temperatures to 927°C. Gritty varieties contain carbonate minerals and are in the class of soapstones. Varieties containing lime are used for making porcelain. Talc used in ceramics is usually mined, sorted, crushed, and milled to 95 to 99% –200 mesh.

Tantalum is a white lustrous metal (symbol Ta), resembling platinum. Tantalum is a high-density, ductile, refractory metal that exhibits exceptional corrosion resistance and good hightemperature strength over 1663°C. The annealed wrought metal in its pure form is easily worked and can be cold-worked in much the same manner as fully annealed mild steel. It is one of the most acid-resistant metals and is classed as a noble metal. Its specific gravity is 16.6, or about twice that of steel and, because of its high melting temperature (2996°C), it is called a refractory metal. In sheet form, it has a tensile yield strength of 345 MPa and is quite ductile. At very high temperatures, however, it absorbs oxygen, hydrogen, and nitrogen and becomes brittle. Its principal use is for electrolytic capacitors, but because of its resistance to many acids, including hydrochloric, nitric, and sulfuric, it is also widely used for chemical-processing equipment. It is attacked, however, by hydrofluoric acid, halogen gases at elevated temperatures, fuming sulfuric, and strong alkalies. Because of its heat resistance, tantalum is also used for heat shields, heating elements, vacuum-furnace parts, and special aerospace and nuclear applications. It is a common alloying element in superalloys. The metal is also used for prosthetic applications. Tantalum metal is used in the manufacture of capacitors for electronic equipment, including citizen band radios, smoke detectors, heart pacemakers, and automobiles. An extremely stable film of tantalum oxide acts as an insulator in the capacitor. It is also used for heat-transfer surfaces in chemical production equipment, especially where extraordinarily corrosive conditions

APPLICATIONS The major applications for talc are tile and hobbyware bodies, cordierite catalyst supports, kiln furniture, and electrical porcelains. There are minor applications in electronic packaging, sanitaryware, dinnerware, and glazes. Talc is used as a flux for high alumina ceramics, sanitaryware, and dinnerware. It is a low-cost source of magnesium in these applications and helps to produce less porous bodies at lower firing temperatures.


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exist. Its chemical inertness has led to dental and surgical applications. Tantalum forms alloys with a large number of metals. Of special importance is ferrotantalum, which is added to austenitic steels to reduce intergranular corrosion. Tantalum is used extensively in the chemical industry where its excellent fabrication and joining properties permit the application to acid-resistant heat exchangers, condensers, ductwork, chemical lines, and other chemical process equipment. Tantalum also finds use in the medical profession. Because of its nontoxic properties and immunity to body chemicals, tantalum is used for sutures, gauze, pins, and plates.

FABRICABILITY Hot Working High-purity tantalum sintered bar, cast ingot, and annealed wrought forms can be worked at room temperature, although the working of large ingots and billets is sometimes performed at elevated temperature to permit working within equipment strength capacity. Cast ingots, protected by canning or coating materials, have been forged and extruded at temperatures up to 1316°C. Cold-worked tantalum can be stress-relieved or annealed at a variety of time–temperature schedules depending upon stress level and chemical purity of the material. A temperature of at least 1204°C is generally used for full annealing while temperatures between 816 and 927°C can be used for stress relieving. Annealing atmosphere must be highpurity argon, helium, or, preferably, a vacuum of one tenth of a micron or less.

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Cold Working The excellent room-temperature ductility of stress-relieved and fully annealed tantalum makes the forming of tantalum comparatively simple. But the grain size of the material must be carefully considered for requirements where surface finish is of importance. The combination of the higher tensile strength and fair uniform elongation of stress-relieved tantalum sometimes makes it more satisfactory for drawing and forming than fully annealed material. It is very important to consider the temper


properties of the material in the design of forming tools. Joining Tantalum can be joined by electron beam, tungsten-inert gas (TIG), and spot and resistance welding, but must be carefully protected from the effects of oxidation during welding. Uncontaminated welds are ductile and usually can be worked at room temperature. Electron-beam-melted 90% tantalum–10% tungsten alloy can be formed and joined in a similar manner as pure tantalum provided that the higher strength and more rapid work-hardening characteristics of the alloy are considered.

ALLOYS Tantalum alloys, including tungsten and tungsten–hafnium compositions, such as Ta-10W, T-111 (8% tungsten, 2% hafnium), and T-222 (9.6% tungsten, 2.4% hafnium, and 0.01% carbon), are used for rocket-engine parts and special aerospace applications. The tensile yield strength of Ta-10W is about 1089 MPa at room temperature and 621 MPa at 871°C.

TANTALUM BERYLLIDES AND CARBIDES TANTALUM BERYLLIDE TaBe12 and TaBe17 are intermetallic compounds with good strengths at elevated temperatures. TaBe12 is tetragonal; density is 4.18 g/cm3; melting point is 1849°C; and coefficient of thermal expansion is 8.42 × 10–6/°C. TaBe17 is hexagonal; density is 5.05 g/cm3; melting point is 2045°C; and coefficient of thermal expansion is 8.72 × 10–6/°C. Both compounds can be formed by all of the known ceramic forming methods plus flame and plasma-arc spraying. The materials are subject to safety requirements for all beryllium compounds.

TANTALUM CARBIDE TaC and Ta2C are the two primary carbides. The ore tantalum carbide with a congruent melting point is TaC. It is dark-to-light brown in color with a metallic luster. Ta2C melts incongruently


and is gray with a metallic luster. Ta2C melts at 3400°C; the melting point of TaC has been reported to be as high as 4820°C. TaC burns in air with a bright flash and is only slightly soluble in acids. The tensile strength at room temperature is 13.6 to 27.2 MPa. Tantalum carbide is used in cemented-carbide cutting tools.

regarded as a primary process, and one or more secondary operations are necessary to complete the fabrication of a green part ready for firing.

APPLICATIONS The “classic” applications of tape casting are centered in the electronics industry and include the production of flat, smooth substrates for either thick-film or integrated circuitry, capacitors, dielectrics, and piezoelectric elements.

TAPE CASTING Tape casting is a familiar technique to most ceramic engineers. It has been widely used as a method for fabricating improved capacitors. Conceptually, the process is simple. First, a slurry is prepared that contains ceramic powder (or powders) suspended in a solution of polymers. The slurry is spread into a relatively thin liquid coating on a smooth surface. The solution is then removed, usually through evaporation, although absorption into a porous medium can also be used, and a dry film is formed that is a green ceramic powder compact, termed green-sheet. The tape usually has three qualitatively distinct phases: an inorganic powder, a continuous polymer matrix, and a porosity phase. (The powder is dispersed within the polymer matrix.) Each phase is important. In considering each of these phases, it is useful to recognize that the process to produce green-sheet is usually best

LAYERED MANUFACTURING Although the ability to mix materials and stack sheets has long been recognized in the electronics industry, a recent and important development has been the recognition that green-sheet can play an important role as a feedstock for rapid prototyping (RP), also known as solid freeform fabrication (SFF). Although much that is useful can be derived from the work done for electronic applications, there are a number of distinct requirements for rapid prototyping that produce additional constraints on green-sheet physical properties and, therefore, formulations. The field of rapid prototyping was developed in response to the high cost of tooling that is often an obstacle to implementing design changes or material substitution. All RP processes start with a CAD (computer-aided design) file (A in Figure T.1), computationally

THE CAM-LEM PROCESS C) Slice Cutting Laser

E) Lamination

A) Computer Model F) Firing D) Stacking

B) Contour Representation

FIGURE T.1 Schematic of the CAM-LEM process.


G) Finished Component

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approximate it by a series of closely spaced two-dimensional outlines (B in Figure T.1), then use a process to construct each outline sequentially, assemble them, and suitably postprocess the assemblage to yield the final part (G in Figure T.1). Frequently, the term layered manufacturing is also applied to these processes. This name is particularly appropriate since each process involves the fabrication of a three-dimensional object by automatic sequential stacking of appropriately contoured thin (pseudo-twodimensional) sections. By controlling x – y motions in each layer, an (in principle) arbitrary component can be built up. Each RP technology is distinguished by the process and machinery used to define each thin section and create the final stack.

RP

AND

CERAMICS

The application of solid freeform fabrication to engineering ceramics is motivated by a desire to take advantage of the striking advances in these materials over the last 20 years (such as transformation-toughened oxides, high-toughness silicon nitrides, and ceramic-matrix composites) as a result of government-funded research programs throughout the world. CAM-LEM

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The CAM-LEM process (computer-aided-manufacturing of laminated engineering materials) was developed specifically for production of ceramic parts. Among the ceramic formulations that have been used with CAM-LEM are alumina, silicon nitride, zirconia-toughened oxides, and PZT. The essence of the CAM-LEM is illustrated in steps C through F in Figure T.1: (C) feeding the green-sheet onto a movable platform; (D) creating a cut outline through relative motion of the table and laser; (E) automatic and selective extraction of the cut outline; and (F) addition to the build stack. Conversion to a dense ceramic article requires lamination of the greensheets followed by binder burnout and firing. The total time to produce a ceramic part is the sum of that required to produce the green part plus the firing time, which includes binder


burnout and sintering. The CAM-LEM process produces green parts that are compatible with conventional firing cycles for laminated tapecast parts.

TELLURIUM An elementary metal (symbol Te), tellurium is obtained as a steel-gray powder of 99% purity by the reduction of tellurium oxide, or tellurite, TeO2. The specific gravity is about 6.2 and the melting point is 450°C. The chief uses are in lead to harden and toughen the metal, and in rubber as an accelerator and toughener. Less than 0.1% tellurium in lead makes the metal more resistant to corrosion and acids, and gives a finer grain structure and higher endurance limit. Tellurium–lead pipe, with less than 0.1% tellurium, has a 75% greater resistance to hydraulic pressure than plain lead. Tellurium copper (C14500, C14510, and C14520) is a free-machining copper confining 0.3 to 0.7% tellurium. It machines 25% more easily than free-cutting brass. The tensile strength, annealed, is 206 MPa, and the electric conductivity is 98% that of copper. A tellurium bronze containing 1% tellurium and 1.5% tin has a tensile strength, annealed, of 275 MPa, and is free-machining. Tellurium is used in small amounts in some steels to make them freemachining without making the steel hot-short as do increased amounts of sulfur. But tellurium is objectionable for this purpose because inhalation of dust or fumes by workers causes garlic breath for days after exposure, although the material is not toxic. As a secondary vulcanizing agent with sulfur in rubber, tellurium in very small proportions, 0.5 to 1%, increases the tensile strength and aging qualities of the rubber. It is not as strong an accelerator as selenium, but gives greater heat resistance to the rubber. Tellurium is an important component of many thermoelectric devices, and such devices can be used for both power generation and cooling. The requirements of a good thermoelectric element are high thermoelectric power, low thermal conductivity, and low electrical resistivity. Lead telluride (PbTe), bismuth telluride (Bi2Te3), and silver antimony telluride meet these requirements better than any other currently known materials. By the addition of


various other elements these compounds can be made either p-type or n-type semiconductors.

OXIDES The oxides of tellurium are tellurium monoxide, TeO; tellurium dioxide, TeO2; and tellurium trioxide, TeO3. The monoxide is reported as a black, amorphous powder that is stable in dry air in the cold but that is oxidized in moist air to the dioxide. On being heated in vacuum, it apparently disproportionates into the dioxide and elemental tellurium. It can be formed by heating the mixed oxide TeSO3. The dioxide is the most stable oxide and is formed when tellurium is burned in air or oxygen or by oxidation of tellurium with cold nitric acid. It has two crystalline forms.

TEXTILE FIBERS Natural fibers constitute one of man’s oldest sources of building materials. There is evidence to indicate that weaving and probably spinning were not unknown to our Stone Age ancestors. It is important to realize that there is no such thing as a natural textile fiber, although today there are human-made textile fibers. There are only natural fibers that have been diverted from their original function by mankind for use in textiles. In man’s search for fibers that can be used to further our own ends, literally dozens of naturally occurring fibers have been investigated. Only 23 are readily recognized by most textile authorities as being of commercial importance, and one fiber alone, cotton, accounts for approximately 70% of the total fibers consumed by the world population for textile purposes. If, however, this listing of natural fibers is carefully reviewed, it will be found that all fibers can be grouped into six different types of spinnable fibers, each differing fundamentally with respect to molecular and morphological structure. The distinctive characteristics possessed by the fibers in these six groups are such that the groups may be subjectively described as cottonlike, linenlike, sisal-like, wool-like, silklike, and asbestos-like.


TEXTILE A textile is a material made mainly of natural or synthetic fibers. Modern textile products may be prepared from a number of combinations of fibers, yarns, plies, sheets, foams, furs, or leather. They are found in apparel, and household and commercial furnishings, articles, and industrial products. Materials made directly from plastic sheet or film, leather, fur, or film are not usually considered to be textiles. The term fabric may be defined as a thin, flexible material made of any combination of cloth, fiber, or polymer (film, sheet, or foams); cloth as a thin, flexible material made from yarns; yarn as a continuous strand of fibers; and fiber as a fine, rodlike object in which the length is greater than 100 times the diameter. The bulk of textile products is made from cloth. The natural progression from raw material to finished product requires the cultivation or manufacture of fibers; the twisting of fibers into yarns (spinning); the interlacing (weaving) or interlooping (knitting) of yams into cloth; and the finishing of cloth prior to sale. Spinning Processes The ease with which a fiber can be spun into yarn is dependent upon its flexibility, strength, surface friction, and length. Exceedingly stiff fibers or weak fibers break during spinning. Fibers that are very smooth and slick or fibers that are very short do not hold together. To varying degrees, the common natural fibers (wool, cotton, and linen) have the proper combinations of the above properties. The synthetic fibers are textured prior to use to improve their spinning properties by simulating the convolutions of the natural fibers. Natural and synthetic filament fibers, because of their great length, need not be twisted to make useful yarns. The properties of a yarn are influenced by the kind and quality of fiber, the amount of processing necessary to produce the required fineness, and the degree of twist. The purpose of the yarn determines the amount and kind of processing. The yarn number (yarn count) is an indication of the size of a yarn — the higher the number, the finer the yarn. The degree of twist is measured in turns per inch (tpi) and is

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varied from three to six times the square root of the yarn number for optimum performance. The conversion of staple fiber into yarn requires the following steps: picking (sorting, clearing, and blending), carding and combing (separating and aligning), drawing (reblending), drafting (reblended fibers are drawn out into a long strand), and spinning (drafted fibers are further attenuated and twisted into yarn).

THALLIUM A soft bluish-white metal (symbol Tl), thallium resembles lead but is not as malleable. The specific gravity is 11.85, and melting point 302°C. At about 316°C it ignites and burns with a green light. Electrical conductivity is low. It tarnishes in air, forming an oxide coating. It is attacked by nitric acid and by sulfuric acid. The metal has a tensile strength of 9 MPa and a Brinell hardness of 2. Thallium–mercury alloy, with 8.5% thallium, is liquid with a lower freezing point than mercury alone, –60°C, and is used in low-temperature switches. Thallium–lead alloys are corrosion resistant, and are used for plates on some chemical-equipment parts.

APPLICATIONS

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The major use for thallium is as a rodenticide and insecticide. The sulfate compound is most commonly employed for this application; therefore, the largest commercial sale of the element is in the form of the sulfate. Thallium sulfate is a heavy white crystalline powder, odorless, tasteless, and soluble in water. The advantage of this compound over many other rodenticides is that it is not detected by the rodent. Other commercially available thallium chemicals are thallous nitrate and thallic oxide. Further uses of thallium compounds are as follows: (1) thallium oxisulfide, employed in a photosensitive cell that has high sensitivity to wavelengths in the infrared range; (2) Thallium bromide-iodide crystals, which have a good range of infrared transmission and are used in infrared optical instruments; and (3) alkaline earth phosphors, which are activated by the addition of thallium. Other minor uses for thallium are in glasses with high indices of refraction, in the produc-


tion of tungsten lamps as an oxygen getter, and in high-density liquids used for separating precious stones from ores by flotation.

TOXICITY Thallium and thallium compounds are toxic to humans as well as other forms of animal life. Therefore, special care must be taken that thallium is not touched by persons handling it. Rubber gloves should be used in handling both the metal and its compounds. Proper precautions should be taken for adequate ventilation of all working areas.

THERMAL SPRAYING Thermal spray comprises a group of processes in which a heat source converts metallic or nonmetallic materials into a spray of molten or semimolten particles that are deposited onto a substrate. Any material that does not sublimate or decompose at temperatures close to its melting point can be applied by thermal spray, as long as it is available in wire or powder form. Thermal spray coatings offer practical and economical solutions to a variety of industrial problems. They are most commonly applied to resist wear, heat, oxidation, and corrosion; provide electrical conductivity or resistance; and restore worn or undersized dimensions. Although the coating techniques have been around for some time, ongoing improvements are leading to lower application costs and a better understanding of how these coatings work. When properly selected and applied, thermally sprayed coatings can reduce downtime, lower production costs, and improve production yields. Thermal spray is somewhat related to the welding process. In welding, the added material is actually fused to the base metal, forming a metallurgical bond, whereas a thermally sprayed coating generally adheres to the substrate through a mechanical bond. Nonetheless, some thermal spray processes are capable of achieving mechanical bond strengths that exceed 70 MPa. The basic thermal spray technologies include plasma spray, wire arc spray, flame


spray, detonation gun, and high-velocity oxygen fuel.

PLASMA SPRAY The plasma spray process requires a plasma gun or torch to generate an arc, which creates the plasma by ionizing a continuous flow of argon gas that is injected into the arc. The arc is struck between a water-cooled copper anode and a tungsten cathode. This type of process is also referred to as nontransferred arc spraying, because the arc is confined to the plasma gun. It is generally operated at energies in the neighborhood of 40 to 100 kW. The plasma is a conductive gas with an extremely high internal working temperature (around 10,000°C). However, little heat is transferred by the plasma, so the part being sprayed remains relatively cool. For example, the process temperature of an 8-kg part will stay around 100°C. Because of the high internal operating temperature, this process is ideally suited for spraying high-melting-point materials such as ceramics and refractory metals. The high heat of plasma causes a large increase in the volume of inert gas introduced, and this produces a high-speed gas jet that accelerates the molten particles and propels them toward the substrate at high velocities. High particle velocities result in dense coatings with high bond strengths. The plasma transferred arc (PTA) process is somewhat of a hybrid between plasma spraying and welding. In this process, an arc is struck between the nonconsumable electrode of the plasma torch and the workpiece itself. The feedstock, in the form of wire or powder, is introduced into the resulting external plasma. The material is melted and puddled onto the substrate, producing a metallurgical bond similar to welding, but with a lot less dilution. This process is capable of producing dense and smooth coatings, but it is not capable of applying ceramics.

WIRE ARC SPRAY The wire arc spray process, like plasma spraying, requires an electrical heat source to melt materials. In this case, the feedstock consists of


two conductive metal wires. These two wires act as electrodes that are continuously consumed as the tips are melted by heat from the electrical arc that is struck between them. An atomizing gas shears off the molten droplets and propels them toward the substrate. The atomizing gas is usually compressed air, but it can also be an inert gas such as nitrogen or argon. Compressed air causes oxidation of metal particles, resulting in a large amount of metal oxide in the coating. Because of this, the coating is harder and more difficult to machine than the source material of the coating. This can be a disadvantage because some coatings have to be ground. However, the increased hardness can also enhance wear resistance. In addition, the temperature of the arc far exceeds the melting point of the sprayed material, resulting in the formation of superheated particles. Consequently, localized metallurgical interactions or diffusion zones develop, which enable achievement of good cohesive and adhesive strengths. The wire-arc process also operates at higher spray rates than the other thermal spray processes. The spray rate, which is dependent on the applied current, makes this process relatively economical.

FLAME SPRAY In the flame spray process, powder or wire materials are melted through the release of chemical energy triggered by a combustion process. A fuel gas (or liquid) is burned in the presence of oxygen or compressed air. Acetylene fuel gas is most frequently selected, due to its high combustion temperature of 3100°C and low cost. Propane, hydrogen, MAPP, and natural gas are also common choices. The flame melts the feedstock, and also accelerates and propels the molten particles. Compressed shopair is also used to assist and boost the particle velocities. However, a compressed inert gas such as argon or nitrogen is preferred if oxidation is a concern. The setup of a flame spray system is relatively inexpensive and mobile. A basic setup requires only a flame spray torch, a supply of oxygen, and a fuel gas. To increase safety, the

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setup might have to be augmented with an enclosed spray booth and exhaust. Because its particle velocities are lower than those of the other thermal spray processes, flame spray coatings are usually of lower quality; they have higher porosity and lower cohesive and adhesive strengths. However, coating quality can be improved by a “spray-and-fuse” process. After the coating is applied by flame spray, the combustion process is repeated to raise the substrate temperature to the point at which the previously applied coating starts to melt. Fusing temperatures exceed 1040°C. The final coating is extremely dense and wellbonded by a metallurgical bond. A disadvantage of this technique is the high substrate temperature required and the possibility for deformation of the part.

DETONATION GUN

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The detonation gun (D-gun) process involves an intermittent series of explosions, which melt and propel the particles onto the substrate. Specifically, a spark plug ignites a mixture of powder and oxygen-acetylene gas in a barrel. After ignition, a detonation wave accelerates and heats the entrained powder particles. After each detonation, the barrel is purged with nitrogen gas, and the process is repeated several times per second. Coatings produced by the detonation gun process are of excellent quality. The particle velocities are high, so the coatings are dense and exhibit high bond strengths. The drawback is that the process is relatively expensive to operate. It also produces noise levels that can exceed 140 decibels, and requires special sound- and explosion-proof chambers.

HVOF SPRAY The high-velocity oxygen fuel (HVOF) thermal spray process is closely related to the flame spray process, except that combustion takes place in a small chamber rather than in ambient air. The HVOF combustion process generates a large volume of gas caused by the formation and thermal expansion of such exhaust gases as carbon dioxide and water vapor.


These gases must exit the chamber through a narrow barrel several inches long. Because of the extremely high pressure created in the combustion chamber, the gases exit the barrel at supersonic velocities, thereby accelerating the molten particles. Although the particles do not reach the speed at which the gases are traveling, they do reach very high velocities. Particle velocities of over 750 m/s have been measured. These high particle speeds, and subsequent high kinetic energy, translate into dense coatings with some of the highest bond strengths possible. Coating Characteristics The goal of all these thermal spray processes is to provide a functional coating that meets all of the necessary requirements; see Table T.1. The quality of a coating depends on the final function of the coating, and can be determined by evaluating a number of coating characteristics. Characteristics that can be evaluated to determine coating quality include: • Microstructure (porosity, unmelts, oxidation level) • Macrohardness (Rockwell B or C) and microhardness (Vickers or Knoop) • Bond strength (adhesive and cohesive) • Corrosion resistance • Wear resistance • Thermal shock resistance • Dielectric strength. Coating Selection Metal forming, paper and pulp, paper converting, printing (including offset and flexographic), chemical, petrochemical, textile, infrastructure, food processing, automotive, medical, power generation, and aerospace all take advantage of thermally sprayed coatings. For each application, the coating is selected to perform one or more functions. The five most encountered functions are wear resistance, heat and/or oxidation resistance, corrosion resistance, electrical conductivity or resistance, and the restoration of worn or undersized dimensions. Basically, coatings fall into three categories: metals/alloys, ceramics, and cermets. Almost every metal and alloy available can be sprayed


TABLE T.1 The Functions and Applications of Thermal Spray Coatings Function Wear resistance Adhesive wear Abrasive wear Surface fatigue wear

Erosion Heat resistance Oxidation resistance

Application

Coating

Bearings, piston rings, hydraulic press sleeves Guide bars, pump seals, concrete mixer screws Dead centers, cam followers, fan blades (jet engines), wear rings (land-based turbines) Slurry pumps, exhaust fans, dust collectors Burner cans/baskets (gas turbines), exhaust ducts Exhaust mufflers, heat treating fixtures, exhaust valve stems

Corrosion resistance

Pump parts, storage tanks, food handling equipment

Electrical conductivity

Electrical contacts, ground connectors Insulation for heater tubes, soldering tips Printing rolls, undersize bearings

Electrical resistance Restoration of dimensions

Chrome oxide, babbitt, carbon steel Tungsten carbide, alumina/titania, steel Tungsten carbide, copper–nickel–indium alloy, chromium carbide Tungsten carbide, Stellite (DeloroStellite Co.) Partially stabilized zirconia Aluminum, nickel–chromium alloy, Hastelloy (Haynes International Co.) Stainless steel (316), aluminum, Inconel (Inco Alloys International), Hastelloy Copper Alumina Carbon steel, stainless steel

Source: Adv. Mater. Proc., 154(6), p. 32, 1998. With permission.

in some form. Frequent choices include copper, tungsten, molybdenum, tin, aluminum, and zinc. Frequently sprayed alloys include steels (carbon and stainless), nickel/chromium, cobalt-base alloys, nickel-base alloys, bronzes, brass, and babbitts. Ceramic materials are usually metaloxide ceramics such as chromium oxide, aluminum oxide (also called alumina), alumina–titania composites, and stabilized zirconias. Cermets are coatings that combine a ceramic and a metal or alloy. Two examples include tungsten carbide (the ceramic constituent) in a cobalt matrix, and chromium carbide in a nickel–chromium matrix. Overall, thousands of different products and components are coated with great success. In addition, new applications are developed daily. Although it is not possible to provide examples of every application and what coating is best for the specific function, the Table T.1 illustrates the broad range of applications and industries that are served. © 2002 by CRC Press LLC

Metal B (-) Current

Tcold

Thot

Metal A (+)

FIGURE T.2 Schematic drawing of a thermocouple.

THERMOCOUPLES Thermocouples are the most common type of temperature sensor used and nearly 16% of all process instrumentation measures, indicates, or controls temperature. Thomas Johann Seebeck is credited with inventing the thermocouple in 1821. His experiment consisted of two dissimilar metal wires joined at the ends to form a loop with each end held at a different temperature; Figure T.2. Seebeck detected the induced current by the displacement of a compass needle that was near one of the wires. Further study revealed that the

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TABLE T.2 Standard Thermocouple Designations Letter Code

Conductor Material

J

Iron Constantan Chromel Alumel Copper Constantan Chromel Constantan Platinum–rhodium Platinum Platinum–rhodium Platinum Platinum–rhodium Platinum–rhodium Tungsten Tungsten–rhenium

K T E R S B W

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Color Code +white –red +yellow –red +blue –red +purple –red +green –red +black –red +gray –red +white –red

temperature gradient induced an electric current and when this circuit was broken at the center, an open circuit voltage was measured, i.e., the Seebeck electromotive force. The thermocouple is based on the concept that for small changes in temperature (Thot to Tcold), the voltage is proportional to the temperature difference. Operating environment and temperature are important considerations for picking the correct thermocouple. Table T.2 provides some practical guidelines for selection. When using a thermocouple, it is very important to understand that the measured voltage is developed along the entire length of the thermocouple. Steep temperature gradients should be avoided because any defect in the wire within the gradient will contribute a large error. Steep gradients may also induce recrystallization and grain growth, thus changing the calibration. In this regard, feeding thermocouples through insulation is critically important because deformation of the wires may produce recrystallization during operation.

THERMOFORMED PLASTIC SHEET Thermoplastic sheet forming consists of the following three steps; (1) a thermoplastic sheet or


Magnetic Yes No No Yes No No No No No No No No No No No No

Temperature Range 0–760°C 32–1400°F 0–1260°C 32–2300°F –150–370°C –300–700°F –150–870°C –300–1600°F 0–1480°C 32–2700°F 0–1480°C 32–2700°F 0–1700°C 32–3100°F 0–2300°C 32–4200°F

Environment Oxidizing or reducing Oxidizing Oxidizing or reducing Oxidizing Oxidizing or inert Oxidizing or inert Oxidizing Inert or vacuum Vacuum or inert

film is heated above its softening point; (2) the hot and pliable sheet is shaped along the contours of a mold, the necessary pressure being supplied by mechanical, hydraulic, or pneumatic force or by vacuum; and (3) the formed sheet is removed from the mold after being cooled below its softening point.

SHEET MATERIALS The following five groups of thermoplastic materials account for the major share of the thermoforming business: 1. Polystyrenes: High-impact polystyrene sheet, ABS (acrylonitrile–butadiene–styrene) sheet, biaxially oriented polystyrene film, and polystyrene foam 2. Acrylics: Cast and extruded acrylic sheet, and oriented acrylic film 3. Vinyls: Unplasticized rigid PVC, vinyl copolymers, and plasticized PVC sheeting 4. Polyolefins: Polyethylene, polypropylene, and their copolymer films 5. Cellulosics: Cellulose acetate, cellulose acetate butyrate, and ethyl cellulose sheet.


A sixth group of increasing importance is the linear polycondensation products, such as polycarbonates, polycaprolactam (type 6 nylon), polyhexamethylene adipamide, oriented polyethylene terephthalate (polyester), and polyoxymethylene (acetal) films.

GENERAL PROCESS CONSIDERATIONS Thermoplastic sheets soften between 121 and 232°C. It is important that the sheets be heated rapidly and uniformly to the optimum forming temperature. The fastest heating is brought about with infrared radiant heaters. Some thermoplastics cannot tolerate such intense heat and require convection heating in air-circulating ovens or conduction heating between platens. In a few instances, the sheets are formed “in line,” making use of the heat of extrusion.

Male section

Four basic forming methods and more than 20 modifications are known: Matched Mold Forming This process, in which the hot sheet is formed between a registering male and female mold section, employs mechanical or hydraulic pressure (Figure T.3a). It is used for corrugating flat rigid sheeting either “in line” or in a separate operation. For continuous longitudinal corrugation, the hot sheet is pulled through a matched mold with registering top and bottom teeth. Transverse corrugation is accomplished with matched top and bottom rolls or molds mounted on an endless conveyor belt or chains. For stationary molding operations, a rubber blanket, backed with a liquid or inflated by air,

Pressure

Female section

Vent holes

a

Clamp Sheet Riser

Male mold

b Clamp

Gasket

Stop block

Compressed air

c Female mold

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Vent holes

Female mold Clamp

Vacuum holes Vacuum

Vacuum table

Clamp

Male mold

Vacuum holes Vacuum

Vacuum box

d

FIGURE T.3 The four basic methods of thermoplastic sheet forming. (a) Matched mold forming; (b) slip forming; (c) air blowing; (d) vacuum forming.



frequently replaces the male section. Another modification is the plug and ring technique in which the ring acts as a stationary clamping device and the moving plug resembles the top portion of the male mold. Elastic and oriented thermoplastic sheets possess a plastic memory; i.e., they tend to draw tight against the force that stretches them. This property occasionally permits forming against a single mold only. Slip Forming A loosely clamped sheet is allowed to slip between the clamps and is “wiped” around a male mold (Figure T.3b). This process has been in use to avoid excessive thinning when forming articles with deep draws. Air Blowing A hot sheet is blown with preheated compressed air into a female mold (Figure T.3c). Variations of this process are free blowing without a mold into a bubble, plug-assist blowing in which a cored plug pushes the hot sheet ahead before blowing, and trapped sheet forming in which a clamping ring slides over the mold before applying the compressed air. The last process is employed in automatic roll-fed packaging machines with biaxially oriented films.

•

•

•

Vacuum Forming This is the most common sheet-forming process with many modifications (Figure T.3d). Modifications of vacuum forming:

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• In straight vacuum forming, the hot thermoplastic sheet is clamped tight to the top of a female mold or of a vacuum box that contains a male mold. Drawing sheets into a female mold results in excellent replica of fine details on the outer surface of the drawn articles. Straight vacuum forming has proved excellent for shallow draws; however, in articles with small radii or deeper dimensions, the corners and bottom are excessively thinned out. Employing


•

a female mold with multiple cavities is more economical than forming over a number of male molds, because it permits smaller spacing between cavities (without bridging), which in turn allows more pieces per sheet. Drawing sheets over a male mold produces articles with the thickest section on top. It is used for the production of three-dimensional geographical maps, because it gives a greater accuracy of registration due to restricted shrinkage. Free vacuum forming into a hemisphere, without a mold, is similar to free air blowing and is employed with acrylic sheets where perfect optical clarity has to be maintained. Vacuum snap-back forming makes use of the plastic memory of the sheet. The hot sheet is drawn by vacuum into an empty vacuum box, while a male mold on a plug is moved from the top into the box. The vacuum is released and the sheet, still hot and elastic, snaps back against the male mold and cools along the contours of the mold. This method is employed with ABS and plasticized vinyl sheets for the production of cases and luggage shells. Drape forming is a technique that allows deeper drawing. After clamping and heating, the sheet is mechanically stretched over a male mold, then formed by vacuum, which picks up the detailed contours of the mold. Acrylic and polystyrene sheets slide easily over the mold, whereas polyethylene sheets tend to freeze on contact with the mold, causing differences in thickness. To overcome the problem of thinning, vacuum plug-assist forming into a female mold has been developed. Its principle is to force a heated and clamped sheet into a female mold using a plug-assist before applying the vacuum. This technique may be considered a reverse of the snapback method.


• Vacuum air-slip forming represents a modification of drape forming and is designed to reduce thinning on deep-drawn articles. It consists of prestretching the sheet pneumatically prior to vacuum forming over a male mold. Prestretching is accomplished either with entrapped air by moving the male mold like a piston in the vacuum chamber or by compressed air. • There are at least three variations of the reverse-draw technique. All employ the principle of blowing a bubble of the hot plastic sheet and pushing a plug in reverse direction into the outside of the hot bubble. This accomplishes a folding operation permitting deeper draws than any other common practice. • The technique of reverse-draw with plug-assist consists of heating the clamped sheet, raising a female mold so that a sealed cavity is formed while a bubble is blown upward. The preheated plug-assist is lowered and pushes the sheet into the cavity. The final shaping is accomplished with vacuum. • A variation of this method is reverse-draw with air-cushion. The plug-assist is furnished with holes through which hot air is blown downward and pushes the hot sheet ahead of the plug-assist, minimizing mechanical contact. This technique is used in forming materials with sharp softening ranges and limited hot strength, such as polyethylene and polypropylene. • Reverse-draw on a plug uses a male mold on the plug to preserve the finish of the sheet.

DESIGN MOLDS

AND

CONSTRUCTION

OF

VACUUM

Depth of draw is a prime factor controlling the wall thickness of the formed article. During straight vacuum forming into a female mold,


the depth of draw should not exceed one half of the cavity width. For drape forming over a male mold, the height-to-width ratio should be 1:1 or less. With plug-assist, air slip, or one of the reverse-draw techniques, the ratio may exceed the 1:1 ratio. Proper air evacuation assists material flow in the desired direction and in uniform wall thickness. In general, deep corners require intensified evacuation. The diameter of vacuum holes should be 0.25 to 0.6 mm for polyethylene sheets, 0.6 to 1 mm for other thin-gauge materials, and may increase to 1.5 mm for heavier rigid materials. Sharp bends and corners should be avoided, because they result in excessive stress concentration and in reduction of strength. The forming cycle can be accelerated and maintained by the use of mold temperature controls. Molds for permanent use are cast from aluminum or magnesium alloy.

FINISHING After the article has been formed, it must be cooled, removed from the forming machine, and separated from the remainder of the sheet. Trimming of thin-walled articles can be carried out hot or cold, in the forming machine or after removal. Heavy-gauge articles should be trimmed only after cooling. Clicker dies, high dies, and Walker dies are frequently used. Decorating formed articles is generally accomplished by printing the flat sheet before the forming operation. Formed articles may also be spray-coated.

THERMOPLASTIC One definition of thermoplastic covers the type of decorating glass enamels or overglaze applied through a hot screen. The media is a wax composition that is heated and added to the enamel powder. On cooling, it forms a solid case that is broken into cubes. The cubes are heated and flow onto a resistance-heated metal screen. When applied on ware, the thermoplastic freezes immediately and other colors can be superimposed without a drying cycle. Thermoplastic permits wraparound decorations and reduces ware handling losses.

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Other thermoplastic materials are used in conjunction with the injection molding of technical ceramics such as spark plugs.

THERMOPLASTIC CONTINUOUS-FIBERREINFORCED MATERIALS These materials are tougher and withstand impacts better. They mold readily and can be recycled. They also have unlimited shelf life and emit no hazardous solvents during processing.

PROCESSING This continuous-fiber-reinforced thermoplastic (CFRTP) process weaves together strands of powder-resin-coated fibers to produce TowFlex fabrics. This is in contrast to other fabrics where raw reinforcement fibers are first woven then coated. Weaving individual coated strands makes the fabric highly drapable. This is because each strand within the fabric moves freely relative to adjacent strands. In addition, the strands remain flexible because they are not fully wet out prior to molding. Complete wet out of the fabric comes during the compressionmolding process (Figure T.4). CFRTP Basics Fiber-reinforced thermoplastics are widely used in injection molding. The vast majority of these products use short or chopped fibers. These fibers generally measure less than 6.4 mm and will randomly orient themselves

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FIBER CREEL RACKS

FLUIDIZED-BED POWDER-COATING CHAMBER

CHARGED RESIN POWDER

FIGURE T.4 Continuous-fiber construction.


during molding. Typical injection molded parts contain only 20 to 30% reinforcement fiber. Short, randomly oriented fibers in low percentages do not provide much reinforcement. And it is often difficult to mold such material into complex, large, or thick-walled parts without voids or knit lines. In comparison, parts molded from new CFRTPs often contain more than 60% reinforcement. Reinforcement fibers run continuously throughout the entire part in specified directions to help optimize strength and stiffness. CFRTPs contain continuous-reinforcement fiber filaments that are powder-coated with melt-fusible thermoplastic particles. The uniformly coated filaments are woven into fabrics or braid, formed into semirigid unidirectional tapes or ribbons, or laminated into panels. The resin particles wet out and consolidate quickly when compression molded. The first step in compression molding CFRTP fabric is to cut and assemble fabric plies. The plies create a preform that approximates the flat pattern shape of the molded part. Automated cutting equipment such as reciprocating knives or ultrasonic gear may be an option for complicated patterns manufactured in high volume. Steel-rule dies, electric rotary shears, or hand shears/scissors might work best for lower-volume applications. It is often useful to machine a simple preform fixture with a cavity or recess in the shape of the flat pattern. The assembly fixture helps keep the precut fabric plies in proper order, location, and orientation. This includes any partial plies needed to build up additional thickness in specific areas.

INFRARED OVEN

PULLER

TAKE-UP SYSTEM To weaving To tapes To molding compounds


Stacked plies are tack-welded ultrasonically or via a soldering iron before removal from the fixture. This helps keep them in the right orientation during handling and storage. The preforms have unlimited shelf life and can be produced in large quantities independent of the molding process or molds. They also need no additional layup labor before being compression-molded into finished parts. The fabric drapes easily and makes for a straightforward molding process that forms and shapes the flat preform. In contrast, thermoset preimpregnated materials generally require cutting and splicing to mold complex shapes. Molds Matched steel metal molds give the best surface finish and mold life when molding CFRTP parts. Nickel-plated aluminum molds are good for moderate volumes of material processing below 316°C. Steel molds are mandatory for higher-temperature molding of matrix resins such as polyphenylene sulfide or polyetherether ketone. Registration of the upper to lower mold halves takes place through either guide pins or the mold configuration itself. Generally, molds are designed to fully “bottom out” on the CFRTP material rather than on thickness stops. This helps maintain pressure on the material throughout the molding process. Thickness stops, however, are used to maintain flatness and help ensure that the mold does not “rock” during the process. They also establish a minimum part thickness. Stops should typically be 0.25 to 0.50 mm lower than the desired nominal part thickness. For relatively thin parts or plates of less than 3.81 mm quantity of fabric plies loaded into the mold primarily controls thickness. Thicker parts or plates use a specified preform weight along with the number of plies to control part thickness. Compression Molding CFRTP fabrics generally use three variations of the compression-molding process. All employ conventional equipment and do not need rapid closing speeds or excessive press tonnage. Single-press/heated-cooled platens — The platens in a single press are heated and cooled


to reach the right processing conditions. This approach applies best in situations requiring a variety of different parts in relatively low volumes that do not need a quick molding cycle. The molding process begins with loading of the preform into the lower mold half. Operators next install the upper mold half and load the complete mold into the press. It is then heated with pressures on the order of 68.6 to 343 kPa. Low pressure during heating helps ensure good heat transfer and initiates the forming of the preform. Full pressure, 0.68 to 5.44 MPa, comes during final compaction as the mold reaches its required processing temperature. Next the press platen is cooled to bring the mold and CFRTP part to the removal temperature. Hot/cold shuttle press — Hot/cold shuttle presses separate the basic segments of the compression-molding process — heating, cooling, and loading/unloading — for better efficiency. Separate heated and cooled platen presses apply pressure to the mold and CFRTP material. Cycle times are short because molds shuttle between preheated hot presses and precooled cooling presses. Cycle times are particularly fast for relatively thin parts and low-mass molds. Molds do not need individual heating and cooling systems, which helps reduce cost. Molds for deep-draw parts may need to use heated or cooled press platens or bolsters. These approximate the part shape and mount to the hot and cold presses. This approach helps keep heating and cooling sources close to the material to increase processing speed. The hot/cold shuttle-press approach is useful for combinations of different parts in moderate to high volumes. Here economics do not justify the expense of heating and cooling provisions in each mold. The hot press station is first preheated to 10 to 37.8°C higher than the desired mold temperature. A preform goes in the lower mold half, the upper mold half lowers into place, and the mold shuttles into the preheated press. Low pressure, 68.6 to 343 kPa, is applied as the mold heats for good heat transfer and to start preform shaping. Full pressure of 0.68 to 3.4 MPa or more forces final compaction as the mold reaches the processing temperature range. Next, the hot press opens, pressure releases, and the mold shuttles into the cold press station.

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Full pressure again applies as the mold and material cool enough for part removal. Single-press/heated-cooled molds — Here integrally heated and cooled molds mount directly onto press platens. Processing cycles for complex-shaped parts can be fast because mold heating and cooling systems can be close to the CFRTP. Individual molds are more expensive, because each mold must contain integral heating and cooling. Press platens must also have sufficient travel to open wide enough for easy preform loading and part removal. Otherwise molds would need to be removed from the press for part removal. The singlepress/heated-cooled mold approach is especially appropriate for large production runs of a specific part or plate. Manufacturers can amortize molds over a large part count and speedy processing helps drive cost down. It is also appropriate for large or deep molds where the shuttle process is impractical. The integrally heated and cooled mold halves are usually attached to the press platens to minimize handling. The mold temperature cycles between the processing and part-removal temperatures. The CFRTP preform loads into the mold and the press closes with low pressure, 68.6 to 343 kPa, for good heat transfer and to initiate preform shaping. The heated mold goes under full pressure, 0.68 to 3.4 MPa or more, for final compaction and forming. The mold and material then cool and the part is demolded. Cycle times can be under 5 min depending on the mold mass, part thickness, and part geometry.

APPLICATIONS

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There have been few commercial applications for CFRTP, however. One reason has been that early product forms of these materials were tougher to process and mold. The development of highly drapable, conformable CFRTP fabrics addresses such shortcomings. They are made from thermoplastics such as nylon, polypropylene, polyphenylene sulfide, polyetherimide, and polyetheretherketone with carbon, glass, or aramid reinforcement fibers. They can easily be molded into complex structural shapes and are ideally suited for production quantities of between 1000 and 50,000 parts annually.


A CFRTP fabric called RF6 is used to make an eight-dihedral-faced kayak paddle. The paddle surface is said not only to grab the water more effectively than conventional blades but also to release surface pressure at eight precise locations along the outside edge of the blade. This produces a blade that has zero flutter while providing more bite per square inch of surface area. The thin blade design was made possible by the high stiffness and impact resistance of the CFRTP material.

THERMOPLASTIC ELASTOMERS Thermoplastic elastomers (TPEs) are a group of polymeric materials having some characteristics of both plastics and elastomers. They are also called elastoplastics. Requiring no vulcanization or curing, they can be processed on standard plastics processing equipment. They are lightweight, resilient materials that perform well over a wide temperature range. There are a half-dozen different types of elastoplastics.

TYPES TPEs have been traditionally categorized into two classes: block copolymers, which include styrenics, copolyesters, polyurethanes, and polyamides; and thermoplastic/elastomer blends and alloys, comprised of thermoplastic polyolefins and thermoplastic vulcanizates. Conventional TPEs are considered twophase materials composed of a hard thermoplastic phase that is mechanically or chemically mixed with a soft elastomer phase. The resulting material shares the characteristics of both.

ADVANTAGES

AND

DRAWBACKS

TPEs give engineers several advantages over thermoset rubbers, including lower fabrication costs, faster processing times, little or no compounding, recyclable scrap, and processing by conventional thermoplastic equipment. In addition, the processing equipment consumes less energy and maintains tighter tolerances than that of rubber processing.


There are several drawbacks, however. TPEs have relatively low melting temperatures, making them unusable in high-temperature applications. They usually require drying before molding because they are hygroscopic, or moisture absorbing. Manufacturers must use TPEs for high-volume applications for it to be economical. And molders accustomed to working with rubber have little experience using TPE materials and equipment.

PROPERTIES As for material properties, TPEs come in durometers, or hardnesses, ranging from 3 Shore A to 70 Shore D, which roughly translates into going from very soft and flexible to semirigid. Although they have relatively low melt temperatures, TPEs resist continuous exposure to temperatures up to 135°C, with spikes up to 149°C. On the other end of the temperature spectrum, they remain flexible at temperatures down to –69°C. The polymers have outstanding dynamic fatigue resistance, good tear strength, and resist acids and alkalis, ultraviolet light, fuels and oils, and ozone, and maintain their grip in wet and dry conditions (Table T.3).

ADVANCEMENTS

AND

APPLICATIONS

The latest advancements in TPE formulations are resin blends that adhere directly to engineering thermoplastics without any special features and can be applied by insert molding (overmolding) and two-shot injection molding. The new formulations let designers mold grips onto a wider range of thermoplastics, particularly high-strength materials such as high-impact polystyrene, glass-filled nylon, and polycarbonate, which are commonly used in power tools, sporting goods, and electronics. New materials are also easier to mold in thin-wall sections, which is crucial for consumer electronics where weight is a concern. Consumer-electronics manufacturers can use TPEs for grip strips, for example, because new resins flow easily across long, thin channels in part molds. Engineers prefer to mold “softtouch” elastomers onto rigid substrates instead of using adhesives and mechanical locks; yet


most elastomers do not provide the necessary adhesion, compatibility, and durability, To answer these needs, a material system was developed consisting of a rigid and a flexible TPU formulation. The two materials — Estaloc thermoplastic and Estane elastomer — have similar chemical makeup. This chemical compatibility helps the two materials form a strong bond without adhesives, giving automotive manufacturers, for example, a “one-stop shop” for interior applications such as ignition bezels. Copolyether-ester thermoplastic elastomer applications include tubing and hose, V belts, couplings, oil-field parts, and jacketing for wire and cable. Their chief characteristic is toughness and impact resistance over a broad temperature range.

THERMOPLASTIC OLEFINS The olefinics, or TPOs, are produced in durometer hardnesses from 54A to 96A. Specialty flame-retardant and semiconductive grades are also available. The TPOs are used in autos for paintable body filler panels and air deflectors, and as sound-deadening materials in dieselpowered vehicles. The TPOs have room-temperature hardnesses ranging from 60 Shore A to 60 Shore D. These materials, as they are based on polyolefins, have the lowest specific gravities of all thermoplastic elastomers. They are uncured or have low levels of cross-linking. Material cost is midrange among the elastoplastics. These elastomers remain flexible down to –51°C and are not brittle at –68°C. They are autoclavable and can be used at service temperatures as high as 135°C in air. The TPOs have good resistance to some acids, most bases, many organic materials, butyl alcohol, ethyl acetate, formaldehyde, and nitrobenzene. They are attacked by chlorinated hydrocarbon solvents. Compounds rated V-0 by UL 94 methods are available.

THERMOPLASTIC POLYESTERS Known chemically as polybutylene terephthalate (PBT) and polyethylene terephthalate

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TABLE T.3 Engineering Properties of Elastomeric Materials Thermoset Rubbers Chlorosulfonated Polyethylene

TPV

Polychloroprene

EPDM

Specific gravity Durometer (Shore) Compression set Continuous service temperature, high (°F) Continuous service temperature, low (°F) Cycle time Recyclability

0.97 45A to 50D Excellent to good 275

1.4 50A to 90A Good to fair 250

1.2 40A to 90A Excellent to good 275

1.4 40A to 90A Fair 250

1.2 40A to 90A Good to fair 240

1.1 40A to 90A Fair 240

–81

–50

–65

–50

–40

–65

Excellent Yes

Fair No

Fair No

Fair No

Fair No

Fair No

Ozone Ultraviolet light Acids Alkalis Lubricating oils Gas permeability

Excellent Excellent to good Excellent Excellent Fair Fair

Good Good Good Good Fair Good to fair

Good Excellent to good Good Excellent Good Fair

Good to fair Fair Good to fair Good to fair Excellent Fair

Excellent Excellent to good Excellent Excellent Fair Excellent


Environmental Resistance Excellent Excellent to good Excellent Excellent Poor Fair

Nitrile

Butyl

Styrene–Butadiene Block Copolymer

Copolyester

Flexible Vinyl

Polyurethane

Specific gravity Durometer (Shore) Compression set Continuous service temperature, high (°F) Continuous service temperature, low (°F) Cycle time Recyclability

0.97 60A to 90A Fair 250

0.94 3A to 55D Good 210

1.2 40D to 70D Good to fair 230

1.2 60A to 90A Fair to poor 200

1.1 60A to 60D Good 250

–50

–50

–90

–30

–40

Excellent to good Yes

Good Yes



Fair to good Yes

Ozone Ultraviolet light Acids Alkalis Lubricating oils Gas permeability

Excellent Good Excellent Excellent Poor Fair

Excellent Good to fair Excellent Excellent Poor Fair

Good Good Good Good Fair Good to fair

Good Good Fair Poor Fair Good

Source: Mach. Design, March 10, pp. 174-175, 1998. With permission.


Environmental Resistance Excellent Excellent to good Poor Good to fair Fair Fair


Thermoplastics Thermoplastic Polyolefin Rubber (TPR)

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(PET), the thermoplastic-polyester molding compounds are crystalline, high-molecularweight polymers. They have an excellent balance of properties and processing characteristics and, because they crystallize rapidly and flow readily, mold cycles are short. In addition to several unreinforced molding resins, the polyesters are available in glass-reinforced grades. Unreinforced and glass-filled grades are available with UL flammability ratings of 94 HB and 5V.

PROPERTIES Thermoplastic polymers have excellent resistance to a broad range of chemicals at room temperature including aliphatic hydrocarbons, gasoline, carbon tetrachloride, perchloroethylene, oils, fats, alcohols, glycols, esters, ethers, and dilute acids and bases. They are attacked by strong acids and bases. High creep resistance and low-moisture absorption give the polyesters excellent dimensional stability. Equilibrium water absorption, after prolonged immersion at 22.75°C, ranges from 0.25 to 0.50% and, at 66°C, is 0.52 to 0.60%. Black-pigmented grades are recommended for maximum strength retention in outdoor uses.

THERMOPLASTIC POLYPROPYLENE

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With long-fiber-reinforced materials, thermoplastic composite extrusions reportedly offer properties superior to those of PVC and polypropylene — as well as more costly engineered thermoplastics like polycarbonates. This development is referred to as Very High Modulus Extrusion (VHME). Parts reinforced by long-fiber materials have been injection-molded for 15 years. But VHME thermoplastic is the first commercial application of long-fiber extrusions. The material consists of a long-fiber-based core for strength and impact resistance, sheathed by inner and outer layers of ABS, polypropylene, or weatherable PVC. To date, the fibers extruded have been limited to glass and carbon and the glass fibers are about 1.27 cm long. Quite a number of


thermoplastics can be used as the matrix in VHME work. They include polyurethane, polypropylene, ABS, and polycarbonate. The nylons or polyacetals have not been done because current equipment is not capable of processing at that high a temperature.

APPLICATIONS Recent applications for VHME include a commercial refrigeration system and the subfloor of a refrigerated semi-truck trailer. In the commercial refrigeration application, VHME was used to make a corner support to hold glass in a glass assembly. Engineers chose the material because it offered low thermal conductivity and enough strength to retain the glass. In the trailer application, the material replaced a heavy wood that tended to rot. Aside from its resistance to rot, VHME produced other benefits. It improved the thermal performance of the unit, and it also reduced the weight about 1 lb/lineal foot. Thus, on the semi-trailer it saved about 184 kg.

THERMOPLASTIC POLYURETHANES Thermoplastic polyurethane (TPU) is often the choice for critical tasks because it offers a broad range of high-performance properties. Moreover, it is known for reliability and has a long working life even in harsh end uses. TPUs routinely provide low-temperature flexibility, high abrasion and moisture resistance, and a long storage life. TPU is a thermoplastic elastomer with many of the same physical and mechanical properties of vulcanized rubber, but with the wider range of processing options common to other thermoplastic polymers. TPUs, like vulcanized rubber, have low hysteresis, high elongation, and good tensile strength. Processors can also tailor their durometer, or hardness, by varying their internal structure without adding special chemicals or plasticizers. Urethane chemistry is built around four of the most common elements: carbon, hydrogen, nitrogen, and oxygen. And it uses the molecular urethane linkage (NHCO2) to connect a series of block copolymers with alternating hard and soft segments. The ratio and molecular structure


TABLE T.4 Flexible Material Comparison

Flex resistance Low temperature Total strength Lamination ability Manufacturability Abrasion resistance Chemical resistance Soft hand feel Recyclability Biogradability Clean incineration Migration Ultraviolet resistance Moisture resistance

Natural Rubber

Synthetic Rubber

VG F G P P F F to G VG P P F G to VG F to G G

VG F to G G to VG G G to VG F G VG G to VG P G to VG G to VG G G

TPU PVC G to VG F F to G G to VG VG F to G F to G F to G VG P P P G to VG G to VG

PE/Metallocene

Ether

Ester

Aliphatic

G P F to G F F F E F G P G to VG G to VG G to VG G to VG

E E VG E E VG G to VG E E F E E F G to VG

E G to VG E E VG E G to VG VG E VG E E F F to G

E E G E VG G to VG G to VG E E F E E VG to E G to VG

Source: Mach. Design, January 13, pp. 111–112, 2000. With permission.

of these segments determine the specific properties of a TPU grade. The hard segments contain an isocyanate structure while the soft segments consist of different polyols. These polyol segments, either polyether or polyester, are used to distinguish different types of TPUs (Table T.4).

TYPES

AND

PROPERTIES

Polyether TPU provides a softer “feel” or drape than polyester, and is generally preferred where there is skin contact. Compared with polyesters, it offers better moisture vapor transmission rates (MVTR) and superior low-temperature properties. It is inherently stable when exposed to high humidity, and is naturally more resistant to fungus, mildew, and microbe attack. However, polyester TPUs have better abrasion resistance with higher tensile and tear strengths for a given durometer. The polyester version also stands up better to fuels and oils, has superior barrier properties, and does not age as fast thanks to better oxidation resistance. However, polyester TPUs will eventually break down in high humidity. TPUs are further identified by the chemical makeup of their isocyanate, or hard segment components. TPUs are classified into aliphatics (linear or branched chains) or aromatics (con© 2002 by CRC Press LLC

taining benzene rings). Aromatic TPUs are strong, general-purpose resins that resist attack by microbes, stand up well to chemicals, and are easily processed. An aesthetic drawback, however, is the tendency of aromatics to discolor, or yellow, when exposed to ultraviolet (UV) light or low-level gamma sterilization. The addition of UV stabilizers or absorbers can reduce this discoloration. Aliphatic urethanes, on the other hand, are inherently light stable and resist discoloration from UV exposure or gamma sterilization. They are also optically clear, which makes them suitable laminates for encapsulating glass and security glazings. From an environmental standpoint, use of TPU often makes sense. Urethane burns more cleanly than polyvinyl chloride (PVC) and other films that compete in the disposable medical-supply market. Using urethane helps avoid the toxic by-products resulting from incinerating disposables fabricated from PVC. In addition, TPU is also readily recycled.

THERMOPLASTIC STYRENES The styrenics are block copolymers, composed of polystyrene segments in a matrix of

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polybutadiene or polyisoprene. Lowest in cost of the elastoplastics, they are available in crumb grades and molding grades, and are produced in durometer hardnesses from 35A to 95A.

THERMOPLASTIC URETHANES Thermoplastic urethanes are of three types: polyester-urethane, polyether-urethane, and caproester-urethane. All three are linear polymeric materials, and therefore do not have the heat resistance and compression set of the cross-linked urethanes. They are produced chiefly in three durometer hardness grades — 55A, 80A, and 90A. The soft 80A grade is used where high flexibility is required, and the hard grade, 70D, is used for low-deflection loadbearing applications.

THERMOPLASTIC VULCANIZATES

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Thermoplastic vulcanizates (TPVs), a type of TPE, are more challenging to color because they consist of a blend of polypropylene and very fine EPDM rubber particles. The EPDM particles make it impossible to develop a clear resin. However, recent developments offer whiter and cleaner TPVs that can produce bright colors. This class of thermoplastic elastomers consists of mixtures of two or more polymers that have received a proprietary treatment to give them properties significantly superior to those of simple blends of the same constituents. The two types of commercial elastomeric alloys are melt-processible rubbers (MPRs) and TPVs. MPRs have a single phase; TPVs have two phases.

PROPERTIES Thermoplastic vulcanizates are essentially a fine dispersion of highly vulcanized rubber in a continuous phase of a polyolefin. Critical to the properties of a TPV are the degree of vulcanization of the rubber and the fineness of its dispersion. The cross-linking and fine dispersion of the rubber phase gives a TPV high tensile strength (7.55 to 26.78 MPa), high elongation (375 to 600%), resistance to compression


and tension set, oil resistance, and resistance to flex fatigue. TPVs have excellent resistance to attack by polar fluids and fair-to-good resistance to hydrocarbon fluids. Maximum service temperature is 135°C. Elastomeric alloys are available in the 55A to 50D hardness range, with ultimate tensile strengths ranging from 5.44 to 27.2 MPa. Specific gravity of MPRs is 1.2 to 1.3; the TPV range is 0.9 to 1.0.

USES In 1981, a line of TPVs, called Santoprene, was commercialized based on EPDM rubber and polypropylene, designed to compete with thermoset rubbers in the middle performance range. In 1985, a second TPV, Geolast, based on polypropylene and nitrile rubber was introduced. This TPV alloy was designed to provide greater oil resistance than that of the EPDM-based material. The nitrile-based TPV provides a thermoplastic replacement for thermoset nitrile and neoprene because oil resistance of the materials is comparable. The MPR product line, called Alcryn, was introduced in 1985. It is a single-phase material, which gives it a stress–strain behavior similar to that of conventional thermoset rubbers. MPRs are plasticized alloys of partially cross-linked ethylene interpolymers and chlorinated polyolefins.

THERMOSET PLASTICS THERMOSET COMPOSITES Thermoset matrix systems dominate the composites industry because of their reactive nature and ease of impregnation. They begin in a monomeric or oligomeric state, characterized by very low viscosity. This allows ready impregnation of fibers, complex shapes, and a means of achieving cross-linked networks in the cured part. The early high-performance thermoset-matrix materials were called advanced composites, differentiating them from the glass/polyester composites that were emerging commercially in the 1950s. The “advanced” term has come to denote, to most engineers, a resin-matrix material reinforced


with high-strength, high-modulus fibers of glass, carbon, aramid, or even boron, and usually laid up in layers to form an engineered component. More specifically, the term has come to apply principally to epoxy-resinmatrix materials reinforced with oriented, continuous fibers of carbon or of a combination of carbon and glass fibers, laid up in multilayer fashion to form extremely rigid, strong structures.

RESIN SYSTEMS More than 95% of thermoset composite parts are based on polyester and epoxy resins; of the two, polyester systems predominate in volume by far. Other thermoset resins used in reinforced form are phenolics, silicones, and polyimides. Polyesters They can be molded by any process used for thermosetting resins. They can be cured at room temperature and atmospheric pressure, or at temperatures to 177°C and under higher pressure. These resins offer a balance of low cost and ease of handling, along with good mechanical, electrical, and chemical properties, and dimensional stability. Polyesters can be compounded to be flexible and resilient, or hard and brittle, and to resist chemicals and weather. Halogenated (chlorinated or brominated) compounds are available for increased fire retardance. Low-profile (smoother surface) polyester compounds are made by adding thermoplastic resins to the compound. Polyesters are also available in ready-tomold resin/reinforcement forms — bulk-molding compound (BMC), and sheet-molding compound (SMC). BMC is a premixed material containing resin, filler, glass fibers, and various additives. It is supplied in a doughlike, bulk form and as extruded rope. SMC consists of resin, glass-fiber reinforcement, filler, and additives, processed in a continuous sheet form. Three types of SMC compounds are designated by Owens-Corning Fiberglas Corp. as random (SMC-R), directional (SMC-D), and continuous fiber (SMCC). SMC-R, the oldest and most versatile form,


incorporates short glass fibers (usually about 25.4 mm long) in a random fashion. Complex parts with bosses and ribs are easily molded from SMC-R because it flows readily in a mold. SMC-C contains continuous glass fibers oriented in one direction, and SMC-D, long fibers (203 to 305 mm long), also oriented in one direction. Moldings using SMC-C and SMC-D have significantly higher unidirectional strength but are limited to relatively simple shapes because the long glass fiber cannot stretch to conform to a shape. These two types of SMC are usually, but not always, used in combination with SMCR. Various combinations are available that contain a total of as much as 65% glass by weight. These materials are used for structural, loadbearing components. High-glass-content SMCs are also produced by PPG Industries, designated as XMC. These compounds contain up to 80% glass (or glass/carbon mixtures) as continuous fibers in an X pattern. Epoxies These are low-molecular-weight, syruplike liquids that are cured with hardeners to crosslinked thermoset structures that are hard and tough. Because the hardeners or curing agents become part of the finished structure, they are chosen to provide desired properties in the molded part. (This is in contrast to polyester formulations wherein the function of the catalyst is primarily to initiate cure.) Epoxies can also be formulated for room-temperature curing, but heat-curing produces higher properties. Epoxies have outstanding adhesive properties and are widely used in laminated structures. The cured resins have better resistance than polyesters to solvents and alkalies, but less resistance to acids. Electrical properties, thermal stability (to 288°C in some formulations), and wear resistance are excellent. Phenolics The oldest of the thermoset plastics have excellent insulating properties and resistance to moisture. Chemical resistance is good, except to strong acids and alkalies.

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Reinforced phenolics are processed principally by high-pressure methods — compression molding and continuous laminating — because volatiles are condensed during the molding process. Recently developed injection-moldable grades, however, have made the processing of phenolics competitive with thermoplastic molding in some applications. Silicones These have outstanding thermal stability, even in the range of 260 to 371°C. Water absorption is low, and dielectric properties are excellent. Chemical resistance (except to strong alkalies) is very good.

PROPERTIES Typical thermoset composites are brittle and have poor impact resistance. An impact may cause little visible surface damage but makes the part dramatically weaker. Thermoset resins need a chemical reaction, usually brought on by heat, to harden, or cure. The chemical reaction is irreversible; once cured, a thermoset material cannot be reprocessed or reformed. And molding cycle time for thermoset materials is largely determined by the curing time.

THIXOMAG

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Auto and aerospace parts producers, in particular, are constantly being challenged with finding new ways to provide state-of-the-art, lightweight parts. Casting is a huge part of this process. When thinking of the casting process, most envision the standard high-pressure die process that forms components from liquid metal. A recently introduced technique called Thixomag transforms metal blanks into an intermediate semisolid before casting. Aluminum and magnesium offer the attractive combination of high strength and low weight, but magnesium is particularly reactive at the melting point of 650°C in the presence of oxygen; it requires an inert atmosphere for stability. Casting this temperamental metal is difficult; however, this technology offers the


possibility to those who want to diversify products, magnesium parts included, without investing much money to provide a fireproof working environment. Speaking of money, there are cost concerns in switching to a different process. Fireproofing facilities is an expensive process; this is another reason companies are a little apprehensive when it comes to working with magnesium. With the traditional die casting machine, one cannot make parts with thixotropic properties. Because of this product, those who previously could not manufacture magnesium parts, now can. It is possible to reuse the cold-chamber casting machine and buy only the Thixomag module along with the induction heating power supply.

THE PROCESS To work with magnesium, a low casting temperature must be achieved. Thixomag can cast at 550°C. Instead of casting from a liquid state, the material is put into a thixotropic or semisolid state. Because the material does not need to be melted, lower temperatures suffice. This semisolid also allows the finished product to be free of porosity defects. As the material flows, it is thick enough to resist gas pockets. The thixotropic state is achieved by proper stirring during solidification. After the stirring process, the thixotropic material is then reheated by induction to the temperature interval between a solid and a liquid. Upon heating the rheocast ingot to the two-phase region (semisolid, semiliquid), the material flows under the application of shear. Induction is the only type of heating process possible to obtain the required thixotropic state, viscous enough so the product does not flow before injection, but not so solid that injection would be impossible. The present developments will allow work rates of several parts per minute, depending on the material used.

THIXOTROPIC PROCESSING THIXOFORMING A method of producing aluminum castings with performance characteristics said to meet or


exceed those of steel forgings with 65% less weight, and up to 25% lower cost, has been developed. During the process, marketed under the name Thixoforming, aluminum ingots are heated to the precise temperature at which the material begins to be transformed from the solid to the liquid state. In this form, it can be cast to make strong, lightweight parts that are cheaper and perform better than many forged steel parts. The critical technology enabling this process is the temperature control system, which is capable of maintaining a uniform temperature throughout the ingot to within ±1°C. It is critical to the process that the entire ingot be precisely in the thixotropic state (borderline liquid/solid). If the center is harder than the exterior, the part will have less beneficial metallurgical properties. The process has reportedly had very good success in the automotive industries.

THIXOTROPIC CASTING Ranging from setter tiles and saggers for use in the electronic ceramics industry, to furnace forehearth shapes for the container glass industry, to kiln-car furniture and structural ceramics, precast shapes offer significant benefits. A new generation of precast shapes has been developed using a thixotropic casting process. This process provides a smoother surface, more precise tolerances, greater strength, improved thermal shock resistance, higher heat tolerances, and more resistance to chemical attack than traditional refractory castable shapes. The Process Thixotropic casting uses a dispersion agent that allows the ceramic mix to flow when vibrated without requiring high water content. The mix, vibrated into a plaster of paris mold, deairs and consolidates without the use of a cement bonding agent. The finely crafted plaster of paris mold provides greater dimensional uniformity than common wooden or metal molds and results in a much smoother surface finish.


The crafting process for plaster molds generally involves four separate steps. The first step is to make an exact model of the end product. A master mold, which is a negative of the model, is then made. Then a case mold is made. The case mold is a replica of the original model and is made to protect the original from damage. From the case mold comes the durable working mold, which is a copy of the master mold and a negative of the case mold. The four-step process is in place to ensure consistency from mold to mold. Depending on the various manufacturing processes, each working mold is used to produce roughly 10 to 50 pieces. The working mold is then replaced with another, again to ensure manufacturing uniformity. All shapes produced by the thixotropic casting process are high-temperature fired between 1412 and 1524°C. This develops a ceramic bond that enables the products to withstand greater temperatures for a longer period of time. The fact that these shapes are made without a cement bond — they use a high-fired ceramic bond, instead — accounts for their high heat tolerance. Above 1371°C, cement-bonded refractories begin to soften, the result of glass phase formation. But firing the shapes to up to 1524°C results in a completely formed ceramic bond that will not readily soften in temperatures above 1371°C. Firing at high temperatures also has the advantage of adding stability to precast shapes. When you first fire a castable shape with a cement bond, a lot of mineralogical changes take place as the product heats up. It can change the overall size and shape of the product, which can lead to an inconsistent product for the customer. By firing the shapes to 1412°C and beyond, there will be no unexpected or unwanted mineralogical changes when first used. The changes that occur in firing are taken into consideration when the molds are designed. The result is that the product performs as it was designed. The firing process for precast fired shapes is a highly controlled operation. All shapes are fired in a 7-day cycle that includes a precise drying process. The periodic kilns that are used to dry and fire the shapes are temperature-controlled to within 20°, which is close tolerance

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for this type of manufacturing. Even the kiln cars are loaded to facilitate a specific airflow pattern that enhances strength and consistency. Applications Typical applications for precast kiln furniture shapes include saggers, setter plates, and pusher tiles. The most common use is with technical ceramic products that have to be fired at highertemperature areas — above 1301°C. Other applications include kiln furniture and structural ceramics. The process can handle very small pieces to the very large pieces like kiln furniture, kiln car shapes, posts, beams — even tracks. Other applications for precast shapes include glass contact parts in the container glass industry — stirrers, cover blocks, spouts, plungers, and tubes. These glass tank forehearth shapes enable glass manufacturers to maintain consistency in manufacturing their products. Shapes are also available with a variety of chemical compositions, depending on the requirements of the application. Compositions range from 50% alumina products to shapes with ultrahigh alumina and alumina/zirconia/silica content.

THORIUM

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A soft, ductile, silvery-white metal (symbol Th), thorium occurs in nature to about the same extent as lead but so widely disseminated in minute quantities difficult to extract that it is considered a rare metal. It was once valued for use in incandescent gas mantles in the form thorium nitrate, Th(NO3)4, but is now used chiefly for nuclear electronic applications. For many years thorium oxide has been incorporated in tungsten metal, which is used for electric light filaments; small amounts of the oxide have also been found to be useful in other metals and alloys. The oxide is employed in catalysts for the promotion of certain organic chemical reactions. Thorium oxide has special uses as a high-temperature ceramic material. Thoria-urania ceramics are used for reactorfuel elements. They are reinforced with colum-


bium or zirconium fibers to increase thermal conductivity and shock resistance. The metal or its oxide is employed in some electronic tubes, photocells, and special welding electrodes. The metal can serve as a getter in vacuum systems and in gas purification, and it is also used as a scavenger in some metals, Because of its high density, chemical reactivity, mediocre mechanical properties, and relatively high cost, thorium metal has no market value as a structural material. However, many alloys containing thorium metal have been studied in some detail and thorium does have important applications as an alloying agent in some structural metals. Perhaps the principal use for thorium metal, beyond its use in the nuclear field, is in magnesium technology. Approximately 3% thorium, added as an alloying ingredient, imparts to magnesium metal high-strength properties and creep resistance at elevated temperatures. The magnesium alloys containing thorium, because of their light weight and desirable strength properties, are being used in aircraft engines and in airframe construction. Thorium can be converted in a nuclear reactor to uranium-233, an atomic fuel. The system of thorium and uranium-233 gives promise of complete utilization of all thorium in the production of atomic power. The energy available from the world supply of thorium has been estimated as greater than the energy available from all of the world’s uranium, coal, and oil combined.

THYRISTORS Thyristors (semiconductor controlled rectifiers) made from silicon carbide have been fabricated and tested as prototypes of power-switching devices capable of operating at temperatures up to 350°C. The highest-voltage-rated of these thyristors are capable of blocking current at forward or reverse bias as large as 900 V, and can sustain forward current as large as 2 Å with a forward potential drop of –3.9 V. The highest-powerrated of these thyristors (which are also the highest-power-rated SiC thyristors reported thus far) can block current at a forward or reverse bias of 700 V and can sustain an “on” current of 6 Å at a forward potential drop of –3.67 V. The highestcurrent-rated of these thyristors can block


Anode Contact Gate Contact

SiO2 p+ 4H-SiC Anode Layer

4H-SiC Gate Layer p- 4H-SiC Voltage-Blocking Layer +

n 4H-SiC Buffer Layer

0.65 µm 11 µm 0.5 µm

n+ 4H-SiC Substrate Cathode Contact

FIGURE T.5 This cross section (not to scale) shows the npnp-layer structure of a representative thyristor of the present type. The n+-, p+-, n-, and p+-doped 4H-SiC layers are formed by epitaxy on the n+ 4H-SiC substrate, which is cut at an angle of 8° off axis. Note: Layer thicknesses are not to scale.

current at a forward or reverse bias of 400 V and can sustain an “on” current of 10 Å. These thyristors feature epitaxial n- and p-doped layers of 4H-SiC in the sequence npnp starting on the substrate; this structure (Figure T.5) stands in contrast to the pnpn structure of common silicon thyristors. The fabrication of the high-quality crystalline structures needed in these layers has been made possible by advances in growth of crystals, epitaxial growth of thin films, doping by both in situ and ion-implantation techniques, oxidation, formation of electrical contacts, and other techniques involved in the fabrication of electronic devices. The above npnp 4H-SiC thyristors have been found to exceed the speed of the fastest inverter-grade silicon thyristors. Two other important parameters for a thyristor are (1) the maximum rate of increase of forward applied voltage that can be applied before the thyristor latches on and (2) the time taken to achieve a high forward current density. The 4H-SiC thyristors show no turn-on even when forward bias was ramped up at a rate of 900 V/µs. Measurements in pulsed operation showed that it took between 3 and 5 ns for these devices to start carrying currents at densities of 2800 A/cm2.

thin, usually flat, square product. Structural tile used for load bearing may or may not be glazed; it may be cored horizontally or vertically. Two principal grades are manufactured: one for exposed masonry construction, and the other for unexposed construction. Among the forms of exposure is frost; tile for unexposed construction where temperatures drop below freezing is placed within the vapor barrier or otherwise projected by a facing, in contrast to roof tile. Structural tile with a ceramic glaze is used for facing. The same clay material that is molded and fired into structural tile is also made into pipe, glazed for sewer lines, or unglazed for drain tile. As a facing, clay products are formed into thin flat, curved, or embossed pieces, which are then glazed and burned. Commonly used on surfaces that are subject to water splash or that require frequent cleaning, such vitreous glazed wall tile is fireproof. Unglazed tile is laid as bathroom floor. By extension, any material formed into a size comparable to clay tile is called tile. Among the materials formed into tile are asphalt, cork, linoleum, vinyl, and porcelain.

TILE

A silvery-white lustrous metal (symbol Sn), with a bluish tinge, tin is soft and malleable, and can be rolled into foil as thin as 0.0051 cm. Tin melts at 232°C. Its specific gravity is 7.298, close to that of steel. Its tensile strength is

As a structural material, tile is a burned clay product in which the coring exceeds 25% of the gross volume; as a facing material, any © 2002 by CRC Press LLC

TIN AND ALLOYS

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27 MPa. Its hardness is slightly greater than that of lead, and its electric conductivity is about one seventh that of silver. It is resistant to atmospheric corrosion, but is dissolved in mineral acids. The cast metal has a crystalline structure, and the surface shows dendritic crystals when cast in a steel mold. It alloys readily with nearly all metals. Tin is a nontoxic, soft, and pliable metal adaptable to cold working such as rolling, extrusion, and spinning. It is highly fluid when molten and has a high boiling point, which facilitates its use as a coating for other metals. It can be electrodeposited readily on all common metals.

PROPERTIES

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Tin reacts with strong acids and strong bases but is relatively inert to nearly neutral solutions. In indoor and outdoor exposure it retains its white silvery color because of its resistance to corrosion. A thin film of stannic oxide is formed in air which provides surface protection. Two allotropic forms exist: white tin (β) and gray tin (α). Although the transformation temperature is 13.2°C, the change does not take place unless the metal is of high purity, and only when the exposure temperature is well below 0°C. Commercial grades of tin (99.8%) resist transformation because of the inhibiting effect of the small amounts of bismuth, antimony, lead, and silver present as impurities. Alloying elements such as copper, antimony, bismuth, cadmium, and silver increase its hardness. Tin tends rather easily to form hard, brittle intermetallic phases, which are often undesirable. It does not form wide solid solution ranges in other metals in general, and there are few elements that have appreciable solid solubility in tin. Simple eutectic systems, however, occur with bismuth, gallium, lead, thallium, and zinc. Tin is rarely used alone; rather, it is generally used as a coating for a baser metal or as a constituent of an alloy. The range of useful alloys is extensive and extremely important.

FORMS Tin can be obtained in a number of forms: granulated, mossy, fine powder, sheet, foil, and wire. © 2002 by CRC Press LLC

Tin-base alloys are available in many forms: solder can be obtained in 0.46-kg bars, solid and cored wire, powder, sheet, and foil; babbitt type metal and casting alloys in bars and ingots; bronze in ingot; continuously cast bars and shapes, sheet, and foil.

APPLICATIONS Full use is made of the ductility, surface smoothness, corrosion resistance, and hygienic qualities of tin in the form of foil, pipe, wire, and collapsible tubes. Tin foil is devoid of springiness and is ideal for wrapping food products, as liners for bottle caps, and for electrical condensers. Heavy-walled tin pipe and tin-lined copper pipe are used by the food and beverage industries for conveying distilled water, beer, and soft drink syrups. Tin wire is used for electrical fuses and for packing glands in pumps of food machinery. Collapsible tubes, made by impact extrusion from disks of pure tin, are used for pharmaceutical and food products. Additionally, tin is used in brasses, bronzes, and babbitts, and in soft solders. The most important use of tin is for tincoated steel containers (tin cans) used for preserving foods and beverages. Other important uses are solder alloys, bearing metals, bronzes, pewter, and miscellaneous industrial alloys. Tin chemicals, both inorganic and organic, find extensive use in the electroplating, ceramic, plastic, and agricultural industries. Tinplate manufacture is now largely a continuous electrolytic process with only a small percentage of production in hot-tinning machines. The coating thickness may be less than 0.01 mm. Heavily coated tinned steel sheet is used in making gas meters and automotive parts such as filters and air coolers. The electrical industry is a large user of tincoated steel and copper in the form of connectors, capacitor and condenser cans, and tinned copper wire. Many kinds of food-handling machinery, including holding tanks, mixers, separators, milk cans, pipes, and valves, are made of tinned steel, cast iron, copper, or brass. The tin coating may be applied by dipping in molten tin or by electrodeposition. The plating industry utilizes tin as anodes for the electrodeposition of pure tin and tin


alloy coatings. Plated tin, either as a matt or bright finish, provides easily solderable surfaces for steel, copper, or aluminum. Tin alloy coatings (tin–copper, tin–lead, tin–nickel, tin–zinc, tin–cadmium, tin–cobalt) have advantages over single metal plates. They are denser and harder, more corrosion-resistant, brighter or more easily buffed, and more protective to basis metals. Tin–copper (12% tin) has the appearance of 24-karat gold and, when lacquered, serves as an attractive finish for jewelry, trophies, wire goods, and hardwares. Tin–lead electroplates (40 to 65% tin) have excellent corrosion resistance and solderability, and are well adapted to the plating of printing circuits and electronic parts. Tin–zinc coatings (75% tin) are a good alternative coating for cadmium in particular applications, and they provide galvanic protection to steel in contact with aluminum. Tin–cadmium coatings (25% tin) are especially resistant to salt vapors, and have a number of applications in the aircraft industry and as a coating for fasteners. A tin–nickel coating (65% tin) finds use as an etchant resist in the manufacture of printed circuit boards, as well as an ornamental and highly corrosion-resistant finish for watch parts, scientific instruments, and power connectors. The tin–cobalt alloy (80% tin) has an appearance similar to a chromium deposit, and is used to plate fasteners, ancillary office equipment, hinges, kitchen utensils, hand tools, and tubular furniture.

STANNIC OXIDE (SNO2) SnO2 has by far its largest commercial outlet in the ceramic industry, where it is used either as a white pigment (i.e., opacifier) or as a constituent of colored pigments in the glazes applied to, for example, crockery, lavatoryware, and decorative wall tile. Tin oxide is an important constituent of ceramic stains for enamels, glazes, and bodies. Pink and maroon colors are obtained with tin oxide, chrotin oxide, and vanadium compounds. Tin oxide also is an important color stabilizer for some of the tin-bearing pink, gray, yellow, and blue coloring stains for glazes. This oxide was formerly an important opacifier for enamels on cast iron and sheet steel, although it has been replaced by substitute


materials such as antimony, zirconium, titanium, and other compounds. The substitution has been largely for economic reasons, as tin oxide is still recognized as the superior opacifier from the standpoint of quality for both glazes and enamels. In glass, stannic oxide is an important addition to cadmium–selenium and gold colors, especially reds. Stannous oxide, SnO, is a necessary ingredient in the development of copper ruby glass and is also used to produce black glasses. Stannous oxide is a black powder and is also a component of ruby-red and black glasses. Tin oxide, because of its resistance to solution in most glasses, especially those high in lead oxide, is being used for refractories for special applications, such as glass feeders, and conducting electrodes for electrical resistance melting of glass.

ALLOYS Soft Solders Soft solders constitute one of the most widely used and indispensable series of tin-containing alloys. Common solder is an alloy of tin and lead, usually containing 20 to 70% tin. It is made easily by melting the two metals together. With 63% tin, a eutectic alloy melting sharply at 169°C is formed. This is much used in the electrical industry. A more general-purpose solder, containing equal parts of tin and lead, has a melting range of 31°C. With less tin, the melting range is increased further, and wiping joints such as plumbers make can be produced. Leadfree solders for special uses include tin containing up to 5% of either silver or antimony for use at temperatures somewhat higher than those for tin–lead solders and tin–zinc base solders often used in soldering aluminum. Bronzes Bronzes are among the most ancient of alloys and still form an important group of structural metals. Of the true copper–tin bronzes, up to 10% tin is used in wrought phosphor bronzes, and from 5 to 10% tin in the most common cast bronzes. Many brasses, which are basically copper–zinc alloys, contain 0.75 to 1.0% tin for additional corrosion resistance in such wrought

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alloys as Admiralty Metal and Naval brass, and up to 49% tin in cast leaded brasses. Among special cast bronzes are bell metal, historically 20 to 24% tin for best tonal quality, and speculum, a white bronze containing 33% tin that gained fame for high reflectivity before glass mirrors were invented. Although soft, conformable, and corrosion resistant, the low mechanical strength of bronze must be boosted by bonding to steel, cast iron, or bronze backing materials. Pewters Pewter is an easily formed tin-base alloy that originally contained considerable lead. Thus, because Colonial pewter darkened and because of potential toxicity effects, its use was discouraged. Modern pewter is lead-free. The most favorable composition, Britannia Metal, contains about 7% antimony and 2% copper. This has desired hardness and luster retention, yet it can be readily cast, spun, and hammered. Alloys that contain from 1 to 8% antimony and 0.5 to 3% copper have excellent castability and workability. For spun pewter products, antimony content is usually below 7%, and pewter casting alloys contain 7.5% antimony and 0.5% copper. Because of the excellent drawing and spinning properties of tin, wrought parts are usually made from pewter that is first cast into slabs, then rolled into sheet.

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antimony. As with most tin-bearing alloys, these are used and remelted repeatedly with little loss of constituents. Tin adds fluidity, reduces brittleness, and gives a structure that reproduces fine detail. Flake and nodular gray iron castings are improved by adding 0.1% tin to give a fully pearlitic matrix with attendant higher hardness, heat stability, and improved strength and machinability. Tin is commonly an ingredient in costume jewelry, consisting of pewterlike alloys and bearing metal compositions often cast in rubber molds, in die castings hardened with antimony and copper for applications requiring close tolerances, thin walls, and bearing or nontoxic properties; and in low-melting alloys for safety appliances. The most common dental amalgam for filling teeth contains 12% tin. Die-Cast Tin Base Historically, these were the first materials to be die cast. Low melting point and extreme fluidity of these alloys produce sound, intricate castings inexpensively and with little wear on molds. Antimony, copper, and lead are the principal additions to tin in die-casting alloys. These alloys are mainly gravity or centrifugally die cast. Cast tin-alloy parts can be held to tolerances of 0.015 mm/mm, with wall thicknesses down to 0.79 mm. Shrinkage is negligible.

Babbitt/Bearing Metal

Fusible Tin

Babbitt or bearing metal for forming or lining a sleeve bearing is one of the most useful tin alloys. It is tin containing 4 to 8% each of copper and antimony to give compressive strength and a structure desired for good bearing properties. An advantage of this alloy is the ease with which castings can be made or bearing shells relined with simple equipment and under emergency conditions. Aluminum–tin alloys are used in bearing applications that require higher loads than can be handled with conventional babbitt alloys.

Melting temperatures for these alloys are usually below the solidus of eutectic-base tin–lead solders (184°C). Primary alloying elements include bismuth, lead, cadmium, and indium. Most of these alloys provide electrical or mechanical links in safety devices. Other applications include low-temperature solders, seals for glass and other heat-sensitive materials, foundry patterns, molds for low-volume production of plastic parts, internal support for tube bending, and localized thermal treatment of parts.

Type Metals

Tin Powders

Type metals are lead-base alloys containing 3 to 15% tin and a somewhat larger proportion of

Produced by atomization techniques, these powders are available in a number of mesh sizes.



They are used in the manufacture of powdermetallurgy parts, in tinning and solder pastes, and in the spray metallization of surfaces. In small amounts, tin is also combined with titanium, zirconium, and other metals to provide special properties. It is used as an alloy in nodular and gray irons to provide greater strength, increased and uniform hardness, and improved machinability. Tin–nickel and tin–zinc coatings are used in the braking systems of automobiles. The tin–nickel alloy is coated on disk-brake pistons because of its good resistance to wear and corrosion. Tin–zinc is used to plate master cylinders in automotive braking systems. Noncritical parts such as costume jewelry and small decorative items such as figurines can be made by casting pewter and other low-melting tin-base alloys in rubber molds. Tin diecasting alloys are suitable for low-strength precision parts and bearings for household appliances, engines, motors and generators, and gas turbines. These bearings perform well even at start-up and run-down periods of operation, at which times they carry a heavy, unidirectional load without the benefit of a fully formed hydrodynamic film. Other applications for tin-base die castings include parts for food-handling equipment, instruments, gas meters, and speedometers.

TINPLATE Tinplate is soft-steel plate containing a thin coating of pure tin on both sides. A large proportion of the tinplate used goes into the manufacture of food containers because of its resistance to the action of vegetable acids and its nonpoisonous character. It solders easily, and also is easier to work in dies than terneplate, so that it also is preferred over terneplate for making toys and other cheap articles in spite of a higher cost. Tinplate is made by the hot-dip process using palm oil as a flux, or by a continuous electroplating process.

PROCESSING

AND

USES

Tinplate manufacture in the United States is largely a continuous, high-speed electrolytic process, with less than 1% of production from


hot-tinning machines. Electrolytic tinplate can be produced from either alkaline or acid electrolytes. Tinplates have either tin on each steel surface or a differential tin-coating thickness. Hot-dipped tinplate is used for special corrosive packs, kitchen utensils, hardware items, and automotive parts. For other industrial applications, hot-dip tin coatings are applied to copper wire and sheet, as well as steel and cast iron parts. Examples are tinned copper and copper alloy strip for manufacture of electrical connectors and tinned food processing equipment. Hot-dip tin–lead (terne) coatings find service as coatings for gasoline tanks, roofing materials, electronic applications, radiator water tubes, and component leads.

TITANATES Titanates are compounds made by heating a mixture of an oxide or carbonate of a metal and titanium dioxide. High dielectric constants, high refractive indices, and ferroelectric properties contribute primarily to their commercial importance. Ferroelectricity may be described as the electric analogue of ferromagnetism. As a field is applied to a ferroelectric material, a nonlinear relationship between polarization and field (similar to the magnetization curve for iron) is observed. This increase in polarization is a function of the orientation of ferroelectric domains within the crystal. As these domains become aligned, a saturation point is reached. If the field is now removed, the domains tend to remain aligned and a finite value of polarization (called remanent polarization) can be measured. Extrapolation of the polarization at high field strength to zero field gives a somewhat higher value (spontaneous polarization). To eliminate the remanent polarization, the field must be applied in the opposite direction, and the field required to return to the original state is called the coercive field. On further increase in the electric field, polarization in the opposite direction is achieved. This behavior leads to a characteristic ferroelectric hysteresis loop as the field is alternated.

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TYPE

OF

MATERIALS

Titanium Dioxide Titanates, for the most part, are prepared by heating a mixture of the specific oxide or carbonate with titanium dioxide. Titanium dioxide has an exceptionally high refractive index for a white oxide (2.6 to 2.9 for the rutile form, 2.5 for anatase) and due to its high refractive index finds wide application as a white pigment of high reflectance for the opacification of paint, plastics, rubber, paper, and porcelain enamels. For electronics, polycrystalline (ceramic) titanium dioxide with its moderately high dielectric constant (~95) has been used as a capacitor; it does not show ferroelectric behavior. Titanium dioxide is normally an insulator, but in the oxygen deficient state where some of the Ti4+ sites are occupied by Ti3+ ions, it becomes an “n-type” semiconductor with conductivities in the range 1 to 10/Ω-cm. Barium Titanate

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Barium titanate crystals, BaTiO3, are made by die-pressing titanium dioxide and barium carbonate a sintering at high temperature. This crystal belongs to the class perovskite in which the closely packed lattice of barium and oxygen ions has a barium ion in each corner and an oxygen ion in the center of each face of a cube with the titanium ion in the center of the oxygen octahedron. For piezoelectric use the crystals are subjected to a high current, and they give a quick response to changes in pressure or electric current. They also store electric charges, and are used for capacitors. Glennite 103 is a piezoelectric ceramic molded from barium titanate modified with temperature stabilizers. Ceramic barium titanate can be made piezoelectric by applying a polarizing field of about 30 kV/cm at room temperatures. The remanent polarization after removal of the polarizing voltage is permanent unless the material is overheated or subjected to high reverse voltages. The advantages of ceramic barium titanate as a transducer lie in its mechanical strength, chemical durability, and ease of fabrication into virtually any shape desired. Barium titanate transducers, as ultrasonic generators, are used in various applications (emulsification,


mixing, cleaning, drilling); other applications include such things as phonograph pickups and accelerometers. Calcium Titanate Calcium titanate, CaTiO3, occurs in nature as the mineral perovskite. As a ceramic it has a room-temperature dielectric constant of about 160. It is frequently used as an addition to barium titanate or by itself as a temperature compensating capacitor. Single crystals have been grown by the flame fusion technique; calcium titanate crystals show a strong tendency toward twinning and, although the material is not ferroelectric, the twinning in the crystal shows a marked resemblance to the domain structure observed in barium titanate crystals. Strontium Titanate Strontium titanate, SrTiO3, has a cubic perovskite structure at room temperature. It has a dielectric constant of about 230 as a ceramic, and is commonly used as an additive to barium titanate to decrease the Curie temperature. By itself, it is used as temperature-compensating material because of its negative temperature characteristics. Strontium titanate has been used as a brilliant diamond-like gemstone and is a strontium mesotrititanate. Stones are made up to 4 karat. The refractive index is 2.412. It has a cubic crystal similar to the diamond but the crystal is opaque in the x-ray spectrum. Single crystals of strontium titanate have been grown by the flame fusion process and strontium titanate is essentially colorless. Because of its cubic structure, it is somewhat more satisfactory as an optical material than rutile (tetragonal). Strontium titanate crystals show a room-temperature dielectric constant of about 300 with a loss tangent of 0.0003. Magnesium Titanate Magnesium titanate, MgTiO3, crystallizes as an ilmenite rather than perovskite structure. It is not ferroelectric, and is used with titanium dioxide to form temperature-compensating capacitors. It has also been used as an addition agent to barium titanate.


Lead Titanate Lead titanate, PbTiO3, is used as a less costly substitute for titanium oxide. It is yellowish in color and has only 60% of the hiding power, but is very durable and protects steel from rust. Good ceramic specimens of lead titanate are somewhat difficult to prepare owing to the volatility of lead oxide at the firing temperature. Lead titanate is commonly used as a solid solution additive to increase the Curie temperature of barium titanate. By substitution of zirconium for titanium in lead titanate a solid solution, lead zirconium titanate, may be produced. Lead zirconate–lead titanate is a piezoelectric ceramic that can be used at higher temperatures than barium titanate. Miscellaneous Titanates The metatitanates of cadmium, manganese, iron, nickel, and cobalt all have an ilmenite rather than perovskite structure. None of these, as far as is known, is ferroelectric and they are not particularly important electrically other than perhaps as addition agents to barium titanate. Nickel titanate has been grown as a single crystal, and its use as a rectifier has been suggested after the addition of suitable impurities during growth. Bismuth stannate, Bi2(SnO3) · 5H2O, a crystalline powder that dehydrates at about 140°C, may be used with barium titanate in capacitors to increase stability at high temperatures. Butyl titanate is a yellow viscous liquid used in anticorrosion varnishes and for flameproofing fabrics. It is a condensation product of the tetrabutyl ester of ortho-titanic acid, and contains about 36% titanium dioxide. Titanate fibers can be used as reinforcement in thermoplastic moldings. The fibers, called Fybex, can also be used in plated plastics to reduce thermal expansion, warpage, and shrinkage. Titanate fibers in plastics also provide opacity.

TITANIUM AND ALLOYS A metallic element (symbol Ti), titanium occurs in a great variety of minerals. The chief commercial ores of titanium are rutile and ilmenite.


In rutile it occurs as an oxide. It is an abundant element but is difficult to reduce from the oxide. High-purity titanium (99.9%) has a melting point of about 1668°C, a density of 4.507 g/cm3, and tensile properties at room temperature of about 234 MPa ultimate strength, 138 MPa yield strength, and 54% elongation. It is paramagnetic and has low electrical conductivity and thermal expansion. The commercial metal is produced from sponge titanium, which is made by converting the oxide to titanium tetrachloride followed by reduction with molten magnesium. The metal can also be produced in dendritic crystals of 99.6% purity by electrolytic deposition from titanium carbide. Despite its high melting point, titanium reacts readily in copper and in other metals, and is much used for alloying and for deoxidizing. It is a more powerful deoxidizer of steel than silicon or manganese.

MELTING Because titanium has a great affinity for oxygen, nitrogen, and hydrogen at elevated temperatures, particularly when molten, all melting operations must be conducted in a vacuum and/or an inert-gas atmosphere. The last 2 years has seen a dramatic shift in titanium melting capacity. At least 181,818,181 kg of cold hearth melting capacity has been added to an industry dominated historically by vacuum arc remelting (VAR). Cold hearth melting is a more complex melting process that brings advantages, such as scrap recycling, shape casting (rectangular slabs), and unique alloying capability. Titanium presents special problems during melting because of its high reactivity with oxygen, nitrogen, and carbon. Exposure to these elements during melting causes severe embrittlement, even at low concentrations. This means that all titanium melting must occur in a vacuum or inert atmosphere (argon or helium). It also means that ceramic or graphite containers or liners are not permissible, limiting the design of melting containers to water-cooled copper vessels (Figure T.6a). Vacuum arc remelting (VAR) in watercooled copper crucibles quickly became the standard melting method (Figure T.6b). When discussing titanium cold hearth melting, one

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Feed Ram Valve Water Out Electrode Arc Cold-Wall Crucible

Liquid Solid

To High Vacuum System Crucible Cooling Water Valve

Hearth Ingot

Water In

(a)

Water-Cooled Withdrawal Ram

(b)

FIGURE T.6 (a) Schematic of cold hearth furnace. (b) Vacuum arc remelting furnace. (From Ind. Heating, January, p. 49, 2000. With permission.)

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must divide the subject into two major divisions: (1) the production of commercially pure (CP) titanium and (2) the production of titanium alloys. Cold hearth production of CP titanium is done principally using electron beam (EB) melting and occurs at very high melting rates, in some cases greater than 2300 kg/h. Cold hearth melting of CP titanium provides several advantages. First is the use of economical raw materials, such as machined turnings contaminated with tungsten carbide particles. The cold hearth allows the tungsten carbide particles to be removed efficiently by gravity separation. The second advantage, as mentioned earlier, is high production rates. A third advantage is the ability to produce rectangular slabs instead of round ingots. Although titanium alloys are successfully produced in both plasma and EB cold hearth processes, advantages for cold hearth melting titanium alloys are similarly impressive. As with CP, cold hearth melting of titanium alloys allows the use of many low-cost raw material forms with excellent yield. If EB melting holds an advantage for producing CP, plasma is simpler to use for titanium alloy melting. Because no alloy changes occur during plasma melting, chemistry control is relatively easy. Electron beam melting of alloys containing high vapor pressure components is more problematic. For a common alloy such as Ti–6% Al–4%V, as much as 30% of the aluminum content may evaporate during EB melting. Alloy control in this case requires accurate prediction and compensation for elemental losses, and then very precise process control to meet the predicted losses uniformly throughout the ingot. A very important technical justification for the use of cold hearth melting for titanium is in © 2002 by CRC Press LLC

the production of premium quality titanium alloys for aircraft turbine engine rotating parts. Inclusion defects contained in the titanium used to produce these parts can be detrimental, leading to low cycle fatigue cracking, engine failure, and even aircraft loss. In the past 10 years, the aircraft engine producers have increasingly specified the use of cold hearth melting for the most critical titanium parts. Cold hearth melting provides important mechanisms for the removal of inclusion-forming contaminants from titanium raw materials. High-density inclusion sources, such as tungsten carbide, tungsten, tantalum, and molybdenum, are easily removed by gravity separation (settling) in the cold hearth. By contrast, VAR provides virtually no removal of these defects. The titanium melting industry has undergone a revolution in melting technology with plasma and electron beam cold hearth melting replacing traditional VAR melting. All major U.S. titanium producers now partly or wholly own large-scale cold hearth melting facilities. Cold hearth melting provides both economic and technical advantages to the titanium melter. Inexpensive raw materials not usable in other melt methods are readily utilized with excellent yields at high production rates. Potentially damaging inclusion sources may be removed assuring high-quality titanium products for the most demanding and critical applications.

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PROPERTIES

Titanium is one of the few allotropic metals (steel is another); that is, it can exist in two different crystallographic forms. At room temperature, it has a close-packed hexagonal structure, designated as the alpha phase. At around 884°C, the alpha phase transforms to a bodycentered cubic structure, known as the beta phase, which is stable up to the melting point of titanium of about 1677°C. Alloying elements promote formation of one or the other of the two phases. Aluminum, for example, stabilizes the alpha phase; that is, it raises the alpha to the beta transformation temperature. Other alpha stabilizers are carbon, oxygen, and nitrogen. Beta stabilizers, such as copper, chromium, iron, molybdenum, and vanadium, lower


the transformation temperature, therefore allowing the beta phase to remain stable at lower temperatures, and even at room temperature. The mechanical properties of titanium are closely related to these allotropic phases. For example, the beta phase is much stronger, but more brittle than the alpha phase. Titanium alloys therefore can be usefully classified into three groups on the basis of allotropic phases: alpha, beta, and alpha-beta alloys. Titanium and its alloys have attractive engineering properties. They are about 40% lighter than steel and 60% heavier than aluminum. The combination of moderate weight and high strengths, up to 1378 MPa, gives titanium alloys the highest strength-to-weight ratios of any structural metal. Furthermore, this exceptional strength-to-weight ratio is maintained from –216°C up to 538°C. A second outstanding property of titanium materials is corrosion resistance. The presence of a thin, tough oxide surface film provides excellent resistance to atmospheric and sea environments as well as a wide range of chemicals, including chlorine and organics containing chlorides. As it is near the cathodic end of the galvanic series, titanium performs the function of a noble metal. Titanium and its alloys, however, can react pyrophorically in certain media. Explosive reactions can occur with fuming nitric acid containing less than 2% water or more than 6% nitrogen dioxide and, on impact, with liquid oxygen. Pyrophoric reactions also can occur in anhydrous liquid or gaseous chlorine, liquid bromine, hot gaseous fluorine, and oxygen-enriched atmospheres.

FABRICATION Fabrication is relatively difficult because of the susceptibility of titanium to hydrogen, oxygen, and nitrogen impurities, which cause embrittlement. Therefore, elevated-temperature processing, including welding, must be performed under special conditions that avoid diffusion of gases into the metal. Heat is usually required in postforming operations. Generally, titanium is welded by gas-tungsten arc or plasma-arc techniques. Metal inertgas processes can be used under special conditions. Thorough cleaning and shielding are essential because molten titanium reacts with nitrogen, oxygen, and hydrogen, and will © 2002 by CRC Press LLC

dissolve large quantities of these gases, which embrittles the metal. In all other respects, gastungsten arc welding of titanium is similar to that of stainless steel. Normally, a sound weld appears bright silver with no discoloration on the surface or along the heat-affected zone. Like stainless steel, titanium sheet and plate work harden significantly during forming. Minimum bend radius rules are nearly the same for both, although springback is greater for titanium. Commercially pure grades of heavy plate are cold-formed or, for more severe shapes, warm-formed at temperatures to about 427°C. Alloy grades can be formed at temperatures as high as 760°C in inert-gas atmospheres. Tube can be cold bent to radii three times the tube outside diameter, provided that both inside and outside surfaces of the bend are in tension at the point of bending. In some cases, tighter bends can be made. Despite their high strength, some alloys of titanium have superplastic characteristics in the range of 816 to 927°C. The alloy used for most superplastically formed parts is the standard Ti6Al-4V alloy. Several aircraft manufacturers are producing components formed by this method. Some applications involve assembly by diffusion bonding. Titanium plates or sheets can be sheared, punched, or perforated on standard equipment. Titanium and Ti–Pd alloy plates can be sheared subject to equipment limitations similar to those for stainless steel. The harder alloys are more difficult to shear, so thickness limitations are generally about two-thirds those for stainless steel. Titanium and its alloys can be machined and abrasive-ground; however, sharp tools and continuous feed are required to prevent work hardening. Tapping is difficult because of the metal galls. Coarse threads should be used where possible.

FORMS Commercially pure titanium and many of the titanium alloys are now available in most common wrought mill forms, such as plate, sheet, tubing, wire, extrusions, and forgings. Castings can also be produced in titanium and some of the alloys; investment casting and graphite-mold

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(rammed graphite) casting are the principal methods. Because of the highly reactive nature of titanium in the presence of such gases as oxygen, the casting must be done in a vacuum furnace. Because of their high strength-toweight ratio primarily, titanium and titanium alloys are widely used for aircraft structures requiring greater heat resistance than aluminum alloys. Because of their exceptional corrosion resistance, however, they (unalloyed titanium primarily) are also used for chemical-processing, desalination, and power-generation equipment, marine hardware, valve and pump parts, and prosthetic devices.

UNALLOYED

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ALLOY TYPES

There are five grades of commercially pure titanium, also called unalloyed titanium: ASTM (American Society for Testing and Materials) Grade 1, Grade 2, Grade 3, Grade 4, and Grade 7. They are distinguished by their impurity content, that is, the maximum amount of carbon, nitrogen, hydrogen, iron, and oxygen permitted. Regardless of grade, carbon and hydrogen contents are 0.10 and 0.015% maximum, respectively. Maximum nitrogen is 0.03%, except for 0.05% in Grades 3 and 4. Iron content ranges from as much as 0.20% in Grade 1, the most pure (99.5%) grade, to as much as 0.05% in Grade 4, the least pure (98.9%). Maximum oxygen ranges from 0.18% in Grade 1 to 0.40% in Grade 4. Grade 7, 99.1% pure based on maximum impurity content, is actually a series of alloys containing 0.12 to 0.25% palladium for improved corrosion resistance in hydrochloric, phosphoric, and sulfuric acid solutions. Palladium content has little effect on tensile properties, but impurity content, especially oxygen and iron, has an appreciable effect. Minimum tensile yield strengths range from 172 MPa for Grade 1 to 483 MPa for Grade 4. There are three principal types of titanium alloys: alpha or near-alpha alloys, alpha-beta alloys, and beta alloys. All are available in wrought form and some of each type for castings as well. In recent years, some also have become available in powder compositions for processing by hot isostatic pressing and other powder-metallurgy techniques. Titanium alpha


alloys typically contain aluminum and usually tin. Other alloying elements may include zirconium, molybdenum, and, less commonly, nitrogen, vanadium, columbium, tantalum, or silicon. Although they are generally not capable of being strengthened by heat treatment (some will respond slightly), they are more creep resistant at elevated temperature than the other two types, are also preferred for cryogenic applications, and are more weldable but less forgeable. Ti–5% Al–2% Sn, which is available in regular and EL1 grades (extra-low inertial) in wrought and cast forms, is the most widely used. In wrought and cast form, minimum tensile yield strengths range from 621 to 793 MPa and tensile modulus is on the order of 107,000 to 110,000 MPa. It has useful strength to about 482°C and is used for aircraft parts and chemical-processing equipment. The EL1 grade is noted for its superior toughness and is preferred for containment of liquid gases at cryogenic temperatures. Other alpha or near-alpha alloys and their performance benefits include Ti–8% Al–1% Mo–1% V (high creep strength to 482°C), Ti–6% Al–2% Sn–4% Zr–2% Mo (creep resistance and stress stability to 593°C, Ti–6% Al–2% Cb–1% Ta–0.8% Mo (toughness, strength weldability), and Ti–2.25% Al–11% Sn–5% Zr–1% Mo (high tensile strength—931 MPa yield, superior resistance to stress corrosion in hot salt media at 482°C. Another alpha alloy, Ti–0.3% Mo–0.8% Ni, also known as TiCode 12, is noted for its greater strength than commercially pure grades and equivalent or superior corrosion resistance, especially to crevice corrosion in hot salt solutions. Titanium alpha-beta alloys, which can be strengthened by solution heat treatment and aging, afford the opportunity of parts fabrication in the more ductile annealed condition and then can be heat-treated for maximum strength. Ti–6% Al–4% V, which is available in regular and EL1 grades, is the principal alloy, its production alone having accounted for about half of all titanium and titanium alloy production. In the annealed condition, tensile yield strength is about 896 MPa and 13% elongation. Solution treating and aging increase yield strength to about 1034 MPa. Yield strength decreases steadily with increasing temperature, to about 483 MPa at about 510°C for the aged alloy. At


454°C, aged bar has a 1000-h stress-rupture strength of about 345 MPa. Uses range from aircraft and aircraft-turbine parts to chemicalprocessing equipment, and marine hardware. The alloy is also the principal alloy used for superplastically formed, and superplastically formed and simultaneously diffusion-bonded parts. At 899 to 927°C and low strain rates, the alloy exhibits tensile elongations of 600 to 1000%, a temperature range also amenable to diffusion bonding the alloy. Following are other alpha-beta alloys and their noteworthy characteristics: Ti–6% Al–6% V–2% Sn: high strength to about 315°C but low toughness and fatigue resistance Ti–8% Mn: limited use for flat mill products, not weldable Ti–7% Al–4% Mo: a forging alloy mainly, but limited use; 1034 MPa yield strength in the aged condition Ti–6% Al–2% Sn–4% Zr–6% Mo: high strength, 1172 MPa yield strength, decreasing to about 759 MPa at 427°C; for structural applications at 400 to 540°C Ti–5% Al–2% Sn–2% Zr–4% Mo–4% Cr and Ti–6% Al–2% Sn–2% Zr–2% Mo–2% Cr: superior hardenability for thick-section forgings; high modulus — about 117,000 to 124,000 MPa, respectively; tensile yield strength of about 1138 MPa Ti–10% V–2% Fe–3% Al: best of the alloys in toughness at a yield strength of 896 MPa; can also be aged to a yield strength of about 1186 MPa; intended for use at temperatures to about 315°C Ti–3Al–2.5V: a tubing and fastener alloy primarily, moderate strength and ductility, weldable. Beta titanium alloys, fewest in number, are noted for their hardenability, good cold formability in the solution-treated condition, and high strength after aging. On the other hand, they are heavier than titanium and the other alloy types, their density ranging from about 4.84 g/cm3 for Ti–13% V–11% Cr–3% Al, Ti–8% Mo–8% V–2% Fe–3% Al, and Ti–3%


Al–8% V–6% Cr–4% Zr–4% Mo to 5.07 g/cm3 for Ti–11.5% Mo–6% Zr–4.5% Sn, which is also known as Beta III. They are also the least creep-resistant of the alloys. Ti–13% V–11% Cr–3% Al, a weldable alloy, can be aged to tensile yield strengths as high as 1345 MPa and retains considerable strength at temperatures to 315°C, but has limited stability at prolonged exposure to higher temperatures.

APPLICATIONS One of the chief uses of the metal has been in the form of titanium oxide as a white pigment. It is also valued as titanium carbide for hard facings and for cutting tools. Small percentages of titanium are added to steels and alloys to increase hardness and strength by the formation of carbides or oxides or, when nickel is present, by the formation of nickel titanide. The major portion of the commercial applications of titanium has been in the chemical processing industries in the form of reactors, vessels, and heat exchangers. The pulp and paper industry has used it in bleaching equipment primarily as chlorine dioxide mixers, while the electrochemical industry has utilized heating and cooling (tubing) coils and anodizing and plating racks. Pumps, valve, thermowells, and other miscellaneous items are additional examples of commercial applications of titanium (Table T.5).

TITANIUM CARBIDE A hard crystalline powder of the composition TiC, titanium carbide is made by reacting titanium dioxide and carbon black at temperatures above 1800°C. It is compacted with cobalt or nickel for use in cutting tools and for heatresistant parts. It is lighter in weight and less costly than tungsten carbide, but in cutting tools it is more brittle. When combined with tungsten carbide in sintered carbide tool materials, however, it reduces the tendency to cratering in the tool.

PROPERTIES TiC theoretically contains 20.05% carbon and is light metallic gray in color. It is chemically

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TABLE T.5 Typical Titanium Applications for Various Markets Market Gas turbine engines Airframes General chemical Organic/petrochemical Power plants Electrolysis Pulp/paper Water technology Metal recovery Energy extraction Medical Environmental Marine High-performance vehicles

Application Compressor blades, disks, ducts, cases Landing gear beams, wing structure, hydraulic tubing Heat exchangers, condensers, mixers, piping Strippers, reboilers, condensers, reactors Surface condensers, turbine blades Anodes for chlorine, chlorate, manganese dioxide; cathodes for copper, manganese; cathodic protection of bridges Bleach tanks, wet chlorine systems, drum washers Flash desalination, heat exchangers for desalination Plating of chromium, nickel, silver, gold, zinc, and galvanizing; hydrometallurgy of copper Logging tools, seals, springs, tubulars for sour gas and geothermal power Prosthetic implants, instruments Flue gas desulfurization, wet air oxidation of waste, incinerator stacks, nuclear waste Heat exchangers, piping systems, ball valves, sonsar masts Racing valves, springs, retainers, connecting rods

Source: McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 18, McGraw-Hill, New York, . With permission.

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stable, and is almost inert to hydrochloric and sulfuric acids. In oxidizing chemicals, such as aqua regia and nitric or hydrofluoric acids, TiC is readily soluble. It also dissolves in alkaline oxidizing melts. When heated in atmospheres containing nitrogen, nitride formation occurs above –1500°C. TiC is attacked by chlorine gas and is readily oxidized in air at elevated temperatures. The density of TiC is 4.94 g/cm3, Mohs hardness is 9+, microhardness is 3200 kg/mm2, and modulus of elasticity is 309,706 MPa. The modulus of rupture at room temperature has been reported as 499.8 to 843.2 MPa for materials sintered at 2600 to 3000°C. Hot modulus of rupture values are given as 107.78 to 116.96 MPa at 982°C and 54.4 to 63.92 MPa at 2200°C. The melting point of TiC is 3160°C, and electrical resistivity at room temperature is 180 to 250 µΩ-cm. It can be used as a conductor at high temperatures. Coefficient of thermal expansion between room temperature and 593°C is 4.12 × 10–6/°F. Thermal conductivity is 0.041 cal/cm · s/°C.

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A general-purpose cutting tool of this type contains about 82% tungsten carbide, 8% titanium © 2002 by CRC Press LLC

carbide, and 10% cobalt binder. Kentanium is titanium carbide in various grades with up to 40% of either cobalt or nickel as the binder, used for high-temperature, erosion-resistant parts. For highest oxidation resistance only about 5% cobalt binder is used. Other grades with 20% cobalt are used for parts where higher strength and shock resistance are needed, and where temperatures are below about 982°C. This material has a tensile strength of 310 MPa, compressive strength of 3789 MPa, and Rockwell hardness A90. Another for resistance to molten glass or aluminum has a binder of 20% nickel. A titanium carbide alloy for tool bits has 80% titanium carbide dispersed in a binder of 10% nickel and 10% molybdenum. The material has a hardness of Rockwell A93, and a dense, fine-grained structure. Ferro-Tic has the titanium carbide bonded with stainless steel. It has a hardness of Rockwell C55. Machinable carbide is titanium carbide in a matrix of FerroTic C tool steel. Titanium carbide tubing is produced in round or rectangular form 0.25 to 7.6 cm in diameter. It is made by vapor deposition of the carbide without a binder. The tubing has a hardness above 2000 Knoop and a melting point of 3249°C. Grown single crystals of titanium carbide have the composition TiC0. 94, with 19% carbon.


The melting point is 3250°C, density 4.93, arid Vickers hardness 3230. The material is finding new uses in cermet components such as jet engine blades and cemented carbide tool bits. Titanium carbide has a relatively low electrical resistivity (1 × 10–4) and can be used as a conductor of electricity, especially at high temperatures. Extreme hardness of titanium carbide makes it suitable for wear-resistant parts such as bearings, nozzles, cutting tools, etc. It also serves for special refractories under either neutral or reducing conditions.

TITANIUM DIBORIDE The material has a melting point of 2980°C, is stable in HCl and HF acid, but decomposes readily in alkali hydroxides, carbonates, and bisulfates. It reacts with hot H2SO4.

PROCESSING Sintered parts of titanium diboride, TiB2, are usually produced by either hot pressing, pressureless sintering, or hot isostatic pressing. Hot pressing of titanium diboride parts is conducted at temperatures >1800°C in vacuum or 1900°C in an inert atmosphere. Hot pressed parts generally have a final density of >99% of theoretical. Typical sintering aids used for hot pressed parts include iron, nickel, cobalt, carbon, tungsten, and tungsten carbide. Pressureless sintering of TiB2 is a less expensive method for producing net shape parts. Because of the high melting point of titanium diboride, sintering temperatures in excess of 2000°C often are necessary to promote sintering. Several different sintering aids have been developed to produce dense pressureless sintered parts by liquid-phase sintering. A combination of carbon and chromium, iron, or chromium carbide can be used as a sintering aid to produce pressureless sintered parts with a final density >95% of the theoretical density. Boron carbide also is added to inhibit grain growth during sintering. These sintering aids as well as atmospheric conditions can be used to lower the sintering temperature necessary for full densification.


PROPERTIES Typical mechanical properties for hot pressed titanium diboride include a flexural strength of 350 to 575 MPa, a hardness of 1800 to 2700 kg/mm2, and a fracture toughness of 5 to 7 MPa · m-1/2. The mechanical property values are dependent on the type of fabrication method used (pressureless sintering vs. hot pressing), the purity of the synthesized powder and the amount of porosity remaining in the finished part. The elastic modulus of titanium diboride can range from 510 to 575 GPa and the Poisson ratio is 0.18 to 0.20. Titanium diboride has a room-temperature electrical resistivity of 15 × 10–6 Ω-cm and a thermal conductivity of 25 Ω/mK.

USES Titanium diboride is used for a variety of structural applications including lightweight ceramic armor, nozzles, seals, wear parts, and cutting tool composites. Titanium diboride also has shown exceptional resistance to attack by molten metals, including molten aluminum, which makes it a useful material for such applications as metallizing boats, molten metal crucibles, and Hall–Heroult cell cathodes because of its intrinsic electrical conductivity. TiB2 can be combined with a variety of other nonoxide ceramic materials, such as silicon carbide (SiC) and titanium carbide (TiC), and oxide materials, such as alumina (Al2O3), to increase the mean strength and fracture toughness of the matrix material.

TITANIUM NITRIDE Titanium nitride whiskers (TiNw) are singlecrystal, acicular-shaped particles of titanium nitride that typically range in size from 0.3 to 1.0 µm in diameter with aspect ratios ranging from 5:1 to 50:1. TiNw have been produced using several different approaches including carbothermic reduction, laser synthesis, plasma synthesis, solid/solid and solid/gas combustion synthesis, and vapor phase reactions. TiNw are

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reported to form from both vapor–solid and vapor–liquid–solid mechanisms. TiNw purity can vary from near stoichiometric (nitrogen of 22.7 wt%) to solid solutions of TiCN and TiON where carbon or oxygen substitutes for nitrogen. TiNw typically form with smooth surfaces and are relatively free from internal defects, which limit the strength of other whiskers such as SiCw. Unlike SiCw, TiNw are electrically conductive and have a coefficient of thermal expansion that is closer to steel and intermetallic materials. TiNw also have been found to have increased stability with iron-containing metals, alloys, and intermetallic compounds. Potential applications include reinforcements in various materials including alumina and tungsten–carbide-base cutting tool inserts for machining ferrous alloys, iron, nickel, and titanium aluminides (intermetallics), iron, nickel, and titanium metals/alloys and polymers. Improved chemical resistance of TiNw with iron compounds makes it an excellent candidate for use as tool inserts for the machining of cast iron and tool steels. The reactivity of SiCw with iron compounds has limited alumina/SiCw cutting tool insert use primarily to superalloys. Alumina/TiNw composites have been shown to be machinable using electric discharge machining. Other areas of interest include stable reinforcements for iron-, nickel- and titanium-based intermetallic compounds and metals/alloys, and electrically conducting reinforcements for polymers to promote electrical conduction and charge dissipation. TiNw provide chemical stability and current carrying characteristics desirable in many applications.

TITANIUM OXIDE The white titanium dioxide, or titania, of the composition TiO2, is an important paint pigment. The best quality is produced from ilmenite, and is higher in price than many white pigments but has great hiding power and durability. Off-color pigments, with a light buff tone, are made by grinding rutile ore. The pigments have fine physical qualities and may be used wherever the color is not important. Titania is


also substituted for zinc oxide and lithopone in the manufacture of white rubber goods, and for paper filler. The specific gravity is about 4. Titania crystals are produced in the form of pale-yellow, single-crystal boules for making optical prisms and lenses for applications where the high refractive index is needed. The crystals are also used as electric semiconductors, and for gemstones. They have a higher refractive index than the diamond, and the cut stones are more brilliant but are much softer. The hardness is about 925 Knoop, and the melting point is 1825°C. The refractive index of the rutile form is 2.7 and that of the anatase is 2.5; the synthetic crystals have a refractive index of 2.616 vertically and 2.903 horizontally. Titanium oxide is a good refractory and electrical insulator. The finely ground material gives good plasticity without binders, and is molded to make resistors for electronic use. A micro sheet is titanium oxide in sheets as thin as 0.008 cm for use as a substitute for mica for electrical insulation where brittleness is not important. Titania-magnesia ceramics have been made in the form of extruded rods and plates and pressed parts.

USES Titanium dioxide is a most important ceramic finish coat for sheet metal products. The opacity of this enamel imparted by titanium dioxide has lowered film thickness of these finishes to the range of organic coatings while retaining the durability of porcelain. These enamels are selfopacified. That is, titanium dioxide is not dispersed as an insoluble suspension during smelting nor is it added at the mill. Rather, titanium dioxide is taken into solution during smelting of the batch and is held in supersaturated solution through fritting. Upon firing the enamel, titanium dioxide crystallizes or precipitates from the glassy matrix. Trimmers or trimmer condensers employing TiB2 bodies are used for minute adjustments of capacitance. Normally, the rotor consists of a TiO2 body. Parts are made with extreme accuracy, and are usually supplied in one of three temperature coefficient types. The base is a lowloss ceramic composition.


TABLE T.6 General Characteristics of Tool Steels AISI Type (quench)

Hardening Depth

Toughness

Wear Resistance

Decarb Resistance

Distortion in Heat Treatment

A2 A6 A8 D2 D3 H11 L2 (Water) L2 (Oil) L6 S1 S7 (Air) S7 (Oil) O2

Deep Deep Deep Deep Medium Deep Medium Medium Medium Medium Med, deep Med, deep Medium

Medium High High Low Low Highest High High High High High High Medium

Medium Low Low High High Low Low Low Low Low Low Low Medium

Medium Medium Medium Medium Medium Med, high High High High Low Medium Medium Medium

Low Lowest Lowest Lowest Medium Very low High Medium Low Medium Low Low Medium


Mechanical and physical properties of TiO2 include relatively low strength (MOR 123.5 to 150.9 MPa; tensile strength 40.8 to 54.4 MPa, low thermal conductivity (0.14 cal/cm/s/°C), and a coefficient of thermal expansion (for rutile) of 7 to 9 × 10–6/°C.

TOOL STEEL To develop their best properties, tool steels are always heat treated. Because the parts may distort during heat treatment, precision parts should be semifinished, heat-treated, then finished. Severe distortion is most likely to occur during liquid quenching, so an alloy should be selected that provides the needed mechanical properties with the least severe quench (Table T.6). Steels are used primarily for cutters in machining, shearing, sawing, punching, and trimming operations, and for dies, punches, and molds in cold- and hot-forming operations. Some are also occasionally used for nontool applications. Tool steels are primarily ingotcast wrought products, although some are now also powder-metal products. Regarding powder-metal products, there are two kinds: (1) mill products, mainly bar, produced by consolidating powder into “ingot” and reducing the ingot by conventional thermomechanical wrought © 2002 by CRC Press LLC

techniques, and (2) end products tools, produced directly from powder by pressing and sintering techniques. There are seven major families of tool steels as classified by the American Iron and Steel Institute: (1) high-speed tool steels, (2) hot-work tool steels, (3) cold-work tool steels, (4) shock-resisting tool steels, (5) mold steels, (6) special-purpose tool steels, and (7) water-hardening tool steels.

HIGH-SPEED TOOL STEELS These steels are subdivided into three principal groups or types: the molybdenum-type, designated M1 to M46; the tungsten-type (T1 to T15); and the intermediate molybdenumtype (M50 to M52). Virtually all M-types, which contain 3.75 to 9.5% molybdenum, also contain 1.5 to 6.75% tungsten, 3.75 to 4.25% chromium, 1 to 3.2% vanadium, and 0.85 to 1.3% carbon. M33 to M46 also contain 5 to 8.25% cobalt, and M6, 12% cobalt. The T-types, which are molybdenum-free, contain 12 to 18% tungsten, 4 to 4.5% chromium, 1 to 5% vanadium, and 0.75 to 1.5% carbon. Except for T1, which is cobalt-free, they also contain 5 to 12% cobalt. Both M50 and M52 contain 4% molybdenum and 4% chromium; the former also contain 0.85% carbon and 1% vanadium, the latter 0.9% carbon, 1.25% tungsten, and 2% vanadium.

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The molybdenum types are now by far the most widely used, and many of the T-types have M-type counterparts. All of the high-speed tool steels are similar in many respects. They all can be hardened to at least Rockwell C63, have fine grain size, and deep-hardening characteristics. Their most important feature is hot hardness: they all can retain a hardness of Rockwell C52 or more at 538°C. The M-types, as a group, are somewhat tougher than the T-type at equivalent hardness but otherwise mechanical properties of the two types are similar. Cobalt improves hot hardness, but at the expense of toughness. Wear resistance increases with increasing carbon and vanadium contents. The M-types have a greater tendency to decarburization and, thus, are more sensitive to heat treatment, especially austenitizing. Many of the T-types, however, are also sensitive in this respect, and they are hardened at somewhat higher temperatures. The single T-type that stands out today is T-15, which is rated as the best of all high-speed tool steels from the standpoint of hot hardness and wear resistance. Typical applications for both the Mtype and T-type include lathe tools, end mills, broaches, chasers, hobs, milling cutters, planar tools, punches, drills, reamers, routers, taps, and saws. The intermediate M-types are used for what somewhat similar cutting tools but, because of their lower alloy content, are limited to less-severe operating conditions.

HOT-WORK TOOL STEELS

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These steels are subdivided into three principal groups: (1) the chromium type (H10 to H19), (2) the tungsten type (H21 to H26), and (3) the molybdenum type (H42). All are medium-carbon (0.35 to 0.60%) grades. The chromium types contain 3.25 to 5.00% chromium and other carbide-forming elements, some of which, such as tungsten and molybdenum, also impart hot strength, and vanadium, which increases high-temperature wear resistance. The tungsten types, with 9 to 18% tungsten, also contain chromium, usually 2 to 4%, although H23 contains 12% of each element. Tungsten hot-work tool steels with higher contents of alloying elements are more heat resistant at elevated temperatures than H11 and H13 chromium hot-work steels but the higher


percentage also tends to make them more brittle in heat treating. The one molybdenum type, H42, contains slightly more tungsten (6%) than molybdenum (5%), and 4% chromium and 2% vanadium. These alloying elements (chromium, molybdenum, tungsten, and vanadium) make the steel more resistant to heat checking than tungsten hot-work steels. Also, their lower carbon content in relation to high-speed tool steels gives them a higher degree of toughness. Typical applications include dies for forging, die casting, extrusion, heading, trim, piercing and punching, and shear blades.

COLD-WORK TOOL STEELS There are also three major groups of cold-work tool steels: (1) high carbon (1.5 to 2.35%); high chromium (12), which are designated D2 to D7; (2) medium alloy air-hardening (A2 to A10), which may contain 0.5 to 2.25% carbon, 0 to 5.25% chromium, 1 to 1.5% molybdenum, 0 to 4.75% vanadium, 0 to 1.25% tungsten, and, in some cases, nickel, manganese or silicon, or nickel and manganese; and (3) oil-hardening types (O1 to O7). They are used mainly for cold-working operations, such as stamping dies, draw dies, and other forming tools as well as for shear blades, burnishing tools, and coining tools.

SHOCK-RESISTANT TOOL STEELS These steels (S1 to S7) are, as a class, the toughest, although some chromium-type hot-work grades, such as H10 to H13, are somewhat better in this respect. The S-types are mediumcarbon (0.45 to 0.55%) steels containing only 2.50% tungsten and 1.50% chromium (S1), only 3.25% chromium and 1.40% molybdenum (S7), or other combinations of elements, such as molybdenum and silicon, manganese and silicon, or molybdenum, manganese, and silicon. Typical uses include chisels, knockout pins, screwdriver blades, shear blades, punches, and riveting tools.

MOLD STEELS There are three principal mold steels: (1) P6, containing 0.10% carbon, 3.5% nickel, and


1.5% chromium; P20, 0.35% carbon, 1.7% chromium, and 0.40% molybdenum; and P21, 0.20% carbon, 4% nickel, and 1.2% aluminum. P6 is basically a carburizing steel produced to tool-steel quality. It is intended for hubbing — producing die cavities by pressing with a male plug — then carburizing, hardening, and tempering. P20 and P21 are deep-hardening steels and may be supplied in hardened condition. P21 may be carburized and hardened after machining. These steels are tough but low in wear resistance and moderate in hot hardness; P21 is best in this respect. All three are oil-hardening steels and they are used mainly for injection and compression molds for forming plastics, but they also have been used for die-casting dies.

SPECIAL-PURPOSE TOOL STEELS These steels include L2, containing 0.50 to 1.10% carbon, 1.00% chromium, and 0.20% vanadium; and L6, having 0.70% carbon, 1.5% nickel, 0.75% chromium, and, sometimes, 0.25% molybdenum. L2 is usually hardened by water quenching and L6, which is deeper hardening, by quenching in oil. They are relatively tough and easy to machine and are used for brake-forming dies, arbors, punches, taps, wrenches, and drills.

WATER-HARDENING TOOL STEELS The water-hardening tool steels include W1, which contains 0.60 to 1.40% carbon and no alloying elements; W2, with the same carbon range and 0.25% vanadium; and W5, having 1.10% carbon and 0.50% chromium. All are shallow-hardening and the least qualified of tool steels in terms of hot hardness. However, they can be surface-hardened to high hardness and, thus can provide high resistance to surface wear. They are the most readily machined tool steels. Applications include blanking dies, coldstriking dies, files, drills, countersinks, taps, reamers, and jewelry dies. Coatings To prolong tool life, tool-steel end products, such as mills, hobs, drills, reamers, punches, and dies, can be nitrided or coated in several ways. Oxide coatings, imparted by heating to


about 566°C in a steam atmosphere or by immersion in aqueous solutions of sodium hydroxide and sodium nitrite at 140°C, are not as effective as traditional nitriding, but do reduce friction and adhesion between the workpiece and tool. The thickness of the coating developed in the salt bath is typically less than 0.005 mm, and its nongalling tendency is especially useful for operations in which failure occurs this way. Hard-chromium plating to a thickness of 0.0025 to 0.0127 mm provides a hardness of DPH 950 to 1050 and is more effective than oxide coating, but the plate is brittle and, thus, not advisable for tools subject to shock loads. Its toughness may be improved somewhat without substantially reducing wear resistance by tempering at temperatures below 260°C, but higher tempering temperatures impair hardness, thus wear resistance, appreciably. An antiseize iron sulfide coating can be applied electrolytically at 191°C using a bath of sodium and potassium thiocyanate. Because of the low temperature, the tools can be coated in the fully hardened and tempered condition without affecting hardness. Tungsten carbide is another effective coating. One technique, called Rocklinizing, deposits 0.0025 to 0.0203 mm of the carbide using a vibrating arcing electrode of the material in a hand-held gun. Titanium carbide and titanium nitride are the latest coatings. The nitride, typically 0.008 mm thick, has stirred the greatest interest, although the carbide may have advantages for press tools subject to high pressure. In just the past few years, all sorts of tools, primarily cutters but also dies, have been titanium nitride-coated, which imparts a gold- or brasslike look. The coating can be applied by chemical vapor deposition (CVD) at 954 to 1066°C or by physical vapor deposition (PVD) at 482°C or less. Thus, the PVD process has an advantage in that the temperature involved may be within or below the tempering temperature of the tool steels so that the coating can be applied to fully hardened and tempered tools. Also, the risk of distortion during coating is less. Another method being used to prolong tool life is to subject the tools to a temperature of –196°C for about 30 h. The cryogenic treatment, which has been called Perm-O-Bond and

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Cryo-Tech, is said to rid the steel of any retained austenite — thus the improved tool life.

PROPERTIES Toughness Toughness in tool steels is best defined as the ability of a material to absorb energy without fracturing rather than the ability to deform plastically without breaking. Thus, a high elastic limit is required for best performance since large degrees of flow or deformation are rarely permissible in fine tools or dies. Hardness of a tool has considerable bearing on the toughness because the elastic limit increases with an increase in hardness. However, at very high hardness levels, increased notch sensitivity and brittleness are limiting factors. In general, lower carbon tool steels are tougher than higher carbon tool steels. However, shallow hardening carbon (W-1) or carbon–vanadium (W-2) tool steels with a hard case and soft core will have good toughness regardless of carbon content. The higher alloy steels will range between good and poor toughness depending upon hardness and alloy content. Abrasion Resistance

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Some tool steels exhibit better resistance to abrasion than others. Attempts to measure absolute abrasion resistance are not always consistent, but in general, abrasion resistance increases as the carbon and alloy contents increase. Carbon is an influential factor. Additions of certain alloying elements (chromium, tungsten, molybdenum, and vanadium) balanced with carbon have a marked effect on increasing the abrasion resistance by forming extremely hard carbides. Hardness Maximum attainable hardness is primarily dependent upon the carbon content, except possibly in the more highly alloyed tool steels. Tool steels are generally used somewhat below maximum hardness except for deep-drawing dies, forming dies, cutting tools, etc. Battering or impact tools are put in service at moderate hardness levels for improved toughness.


Hot Hardness The ability to retain hardness with increasing temperature is defined as hot hardness or red hardness. This characteristic is important in steels used for hot-working dies. Generally, as the alloy content of the steel is increased (particularly in chromium, tungsten, cobalt, molybdenum, and vanadium, which form stable carbides), the resistance to softening at elevated temperatures is improved. High-alloy tool steels with a properly balanced composition will retain high hardness up to 593°C. In the absence of other data, hardness after high-temperature tempering will indicate the hot hardness of a particular alloy.

HEAT TREATMENT Hardenability Carbon tool steels are classified as shallow hardening, i.e., when quenched in water from the hardening (austenitizing) temperature, they form a hardened case and a soft core. Increasing the alloy content increases the hardenability or depth of hardening of the case. A small increase in alloy content will result in a steel that will harden through the cross-sections when quenched in oil. If the increase in alloy content is great enough, the steels will harden throughout when quenched in still air. For large tool or die sections, a high-alloy tool steel should be selected if strength is to be developed throughout the section in the finished part. For carbon tool steels that are very shallow in hardening characteristics, the P/F test, Disc test, and PV test are methods for rating this characteristic. Oil-hardening tool steels of medium-alloy content are generally rated for hardenability by the Jominy End Quench test. Dimensional Changes during Heat Treatment Carbon tool steels are apt to distort because of the severity of the water quench required. In general, water-hardening steels distort more than oil hardening, and oil hardening distort more than air-hardening steels. Thus, if a tool or die is to be machined very close to final size before heat treatment and little or no grinding


is to be performed after treatment, an air-hardening tool steel would be the proper selection. Resistance to Decarburization During heat treatment, steels containing large amounts of silicon, molybdenum, and cobalt tend to lose carbon from the surface more rapidly than steels containing other alloying elements. Steels with extremely high carbon content are also susceptible to rapid decarburization. Extra precaution should be employed to provide a neutral atmosphere when heat treating these steels. Otherwise, danger of cracking during hardening will be present. Also, it would be necessary to allow a liberal grinding allowance for cleanup after heat treatment.

MACHINABILITY Since most tool steels, even in the annealed state, contain wear-resistant carbides, they are generally more difficult to machine than the open-hearth grades or low-alloy steels. In general, the machinability tends to decrease with increasing alloying content. Microstructure also has a marked effect on machinability. For best machinability, a spheroidal microstructure is preferred over pearlitic. The addition of small amounts of lead or sulfur to the steels to improve machinability has gained considerable acceptance in the tool steel industry. These free machining steels not only machine more easily but give a better surface finish than the regular grades. However, some caution is advised in applications involving transverse loading since lead or sulfur additions actually add longitudinal inclusions in the steel.

AVAILABLE FORMS Tool steels are available in billets, bars, rods, sheets, and coil. Special shapes can be furnished upon request. Generally, the material is furnished in the soft (or annealed) condition to facilitate machining. However, certain applications require that the steel be cold-drawn or prehardened to a specified hardness. A word of caution: Mill decarburization is generally present on all steel except that guaranteed by the producer to be decarburizationfree. It is important that all decarburized areas


be removed prior to heat treating or the tool or die may crack during hardening.

TUNGSTEN AND ALLOYS In many respects, tungsten (symbol W) is similar to molybdenum. The two metals have about the same electrical conductivity and resistivity, coefficient of thermal expansion, and about the same resistance to corrosion by mineral acids. Both have high strength at temperatures above 1093°C, but because the melting point of tungsten is higher, it retains significant strength at higher temperatures than molybdenum does. The elastic modulus for tungsten is about 25% higher than that of molybdenum, and its density is almost twice that of molybdenum. All commercial unalloyed tungsten is produced by powder-metallurgy methods; it is available as rod, wire, plate, sheet, and some forged shapes. For some special applications, vacuum-arc-melted tungsten can be produced, but it is expensive and limited to relatively small sections.

FABRICATION Fabrication is a multistep process that converts tungsten metal from the original massive state (bars or ingots) to a more useful shape (sheet, tube, wire) and, at the same time, improves its physical properties. The exact details of fabrication depend on the method used for consolidating the metal and the type of product desired. Arc- or electron-beam-melted tungsten normally is extruded or forged to increase its ductility, whereas powder-processed material, because of its finer-grained structure and smaller tendency to crack, is less likely to require this initial step. Tungsten is usually worked below its recrystallization temperature because the recrystallized metal tends to be brittle. Because increased working decreases the recrystallization temperature, successive lower temperatures are used in each fabrication step. Full-density wrought tungsten can be hotforged, swaged, extruded, rolled, and drawn as secondary fabrication steps used to produce the final shape. Working temperature is usually

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1100°C or above depending on the grain size and type of deformation. Sintered billets are forged, swaged, or rolled initially at temperatures in excess of 1400°C. Working temperature can be progressively lowered as the amount of work increases, but consideration must be given to equipment capacity because of the high strength of tungsten. Several tungsten alloys are produced by liquid-phase sintering of compacts of tungsten powder with binders of nickel–copper, iron–nickel, iron–copper, or nickel–cobalt–molybdenum combinations; tungsten usually comprises 85 to 95% of the alloy by weight. These alloys are often identified as heavy metals or machinable tungsten alloys. In compact forms, the alloys can be machined by turning, drilling, boring, milling, and shaping; they are not available in mill product forms because they are unable to be wrought at any temperature.

unsatisfactory for any weight-conserving application, and its aggressive reaction with oxygen limits its service at high temperatures. Welding is difficult because of the reactivity of tungsten with oxygen, and the presence of oxygen and other interstitials in the metal can make it very brittle at room temperature. Nonetheless, the special properties of tungsten are so beneficial that in many cases it has been worth the cost and effort to engineer around the problems. Tungsten retains a tensile strength of about 344 MPa at 1371°C, but because of its heavy weight is normally used in aircraft or missile parts only as coatings, usually sprayed on. It is also used for x-ray and gamma-ray shielding. Electroplates of tungsten or tungsten alloys give surface hardnesses to Vickers 700 or above.

PROPERTIES

Tungsten has a wide usage for alloy steels, magnets, heavy metals, electric contacts, rocket nozzles, and electronic applications. Tungsten resists oxidation at very high temperatures, and is not attacked by nitric, hydrofluoric, or sulfuric acid solutions. Flame-sprayed coatings are used for nozzles and other parts subject to heat erosion. Tungsten is usually added to iron and steel in the form of ferrotungsten, made by electricfurnace reduction of the oxide with iron or by reducing tungsten ores with carbon and silicon. Standard grades with 75 to 85% tungsten have melting points from 1760 to 1899°C. Tungsten powder is usually in sizes from 200 to 325 mesh, and may be had in a purity of 99.9%. Parts, rods, and sheet are made by powder metallurgy, and rolling and forging are done at high temperature. The tungsten powder is used for spray coatings for radiation shielding and for powdermetal parts. Tungsten wire is used for spark plugs and electronic devices. Tungsten wire as fine as 0.00046 cm is used in electronic hardware. Tungsten whiskers, which are extremely fine fibers, are used in copper alloys to add strength. Copper wire, which normally has a tensile strength of 206 MPa, will have a strength of 827 MPa when 35% of the wire is tungsten whiskers. Tungsten yarns are made up

Tungsten, element 74 on the periodic chart, has a melting point of approximately 3410°C, with values ranging between 3387 and 3422°C reported in the literature. This value easily makes it the highest-melting-point metal. It has the lowest coefficient of thermal expansion of all metals, and with a density of 19.25 g/cm3 it is one of the heaviest. It has the lowest vapor pressure of all metals, and high thermal and electrical conductivity. Single crystals of tungsten are elastically isotropic and have very high tensile and bulk moduli, but mechanical properties are strongly temperature dependent, with the yield strength and ultimate tensile strength decreasing significantly with increasing temperature. At elevated temperatures, tungsten reacts rapidly with oxygen, forming a series of oxides that have stoichiometries ranging between WO2 and WO3. The unique properties of tungsten make it the element of choice for such applications as filaments for incandescent lamps and x-ray tubes, electron sources for scanning and transmission electron microscopes, and connectors for circuit boards. Although these characteristics might suggest an even wider range of applications, several actually limit its utility. For example, the high density of tungsten makes it


APPLICATIONS


of fine fibers of the metal. The yarns are flexible and can be woven into fabrics. Continuous tungsten filaments, usually 10 to 15 µm in diameter, are used for reinforcement in metal, ceramic, and plastic composites. Finer filaments of tungsten are used as cores, or substrates, for boron filaments. The metal is also produced as arc-fused grown crystals, usually no larger than 0.952 cm in diameter and 25.4 cm long, and worked into rod, sheet, strip, and wire. Tungsten crystals, 99.9975% pure, are ductile even at very low temperatures, and wire as fine as 0.008 cm and strip as thin as 0.013 cm can be colddrawn and cold-rolled from the crystal. The crystal metal has nearly zero porosity and its electrical and heat conductivity are higher than ordinary tungsten. One tungsten–aluminum alloy is a chemical compound made by reducing tungsten hexachloride with molten aluminum. Tungsten wire is not used exclusively for lamp filaments. Because of its high melting temperature, tungsten can be heated to the point where it becomes a thermionic emitter of electrons, without losing its mechanical integrity. Consequently, tungsten filaments are often used as electron sources in scanning electron microscopes and transmission electron microscopes, and also as filaments in x-ray tubes. In x-ray tubes, electrons produced from the tungsten filament are accelerated so that they strike a tungsten or tungsten–rhenium anode, which emits the x-rays. Again, this application takes advantage of the high melting point of tungsten, since the energy of the electron beam required to generate x-rays is very high, and the spot where the beam hits the surfaces becomes very hot. In most tubes, the anode is rotated to limit the peak temperature and to allow for cooling. Finally, tungsten filaments of a much larger size are often selected as the heating elements in vacuum furnaces. Again, because of the high melting point of tungsten, these furnaces can achieve much higher temperatures than furnaces made with other heating elements. It is important to note that in vacuum furnaces, as well as all of the other applications, the tungsten is in a controlled environment that inhibits its oxidation.


For example, tungsten heavy alloys are materials in which tungsten powder is liquidphase sintered, usually with nickel–iron powders, to produce a composite material in which tungsten occupies about 95% of the volume. As the sintering process proceeds, the nickel–iron powder melts. Although the solubility of liquid nickel–iron in solid tungsten is small, solid tungsten readily dissolves in liquid nickel–iron. As the liquid wets the tungsten particles and dissolves part of the tungsten powder, the particles change shape, and internal pores are eliminated as the liquid flows into them. As processing continues, the particles coalesce and grow, producing a final product that is approximately 100% dense and has an optimized microstructure. One of the main products made by this method is kinetic energy penetrators of military armored vehicles. This application takes advantage of the high density of tungsten, and it has been found that the liquid sintered materials have better impact properties than pure tungsten made by traditional powder processing. Cutting tools and parts that must resist severe abrasion are often made of tungsten carbide. Tungsten carbide chips or inserts, with the cutting edges ground, are attached to the bodies of steel tools by brazing or by screws. The higher cutting speeds and longer tool life made feasible by the use of tungsten carbide tools are such that the inserts are discarded after one use. Tungsten compounds (5% of tungsten consumption) have a number of industrial applications. Calcium and magnesium tungstates are used as phosphors in fluorescent lights and television tubes. Sodium tungstate is employed in the fireproofing of fabrics and in the preparation of tungsten-containing dyes and pigments used in paints and printing inks. Compounds such as WO3 and WS2 are catalysts for various chemical processes in the petroleum industry. Both WS2 and WSe2 are dry, high-temperature lubricants. Other applications of tungsten compounds have been made in the glass, ceramics, and tanning industries. A completely new and different approach to produce bulk tungsten products from the powder-metallurgy process is through chemical vapor deposition (CVD), which provides a tungsten coating on a substrate.

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Tungsten hexafluoride is the most common tungsten source for CVD processing. This compound is a liquid at room temperature, but its vapor pressure is high enough that the vapor can be continuously extracted and passed across the part that is to be coated. WF6 + 3H2 → W + 6HF

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The reaction requires temperatures above approximately 300°C and a surface that causes the dissociation of molecular hydrogen into atomic hydrogen. Therefore, sections of a part may be selectively coated by having surfaces that either catalyze or prevent this reaction. One of the most important applications of this process has been in the electronics industry, in which tungsten vias are placed in integrated circuits. The vias are small metal plugs that connect one level of wiring to another in the circuit board. They are generally about 0.4 mm in diameter, with an aspect ratio of about 2.5. In future applications, the diameter may shrink to less than 0.1 mm, and have an aspect ratio greater than five. The metal for this application must have good electrical conductivity, must not react with the surrounding materials, must adhere to the wiring or silicon above or below the via, and must be deposited by a CVD reaction, as that is the only way to fill such small holes. The most common method in the electronics industry is blanket CVD. In this technology, an adhesion layer is first put down to make certain that the CVD tungsten will stick to the surface. This adhesion layer is often titanium nitride, TiN. Tungsten is deposited on top of this layer, covering the surface and filling the vias. After the CVD is complete, the tungsten on the entire surface is removed by chemicalmechanical polishing. This procedure leaves the vias filled, but cleans the surface of the unnecessary tungsten.

ALLOYS A large number of tungsten-based alloys have been developed. Binary and ternary alloys of molybdenum, niobium, and tantalum with tungsten are used as substitutes for the pure metal because of their superior mechanical properties.


Adding small amounts of other elements such as titanium, zirconium, hafnium, and carbon to these alloys improves their ductility. Tungsten–rhenium alloys possess excellent hightemperature strength and improved resistance to oxidation, but are difficult to fabricate. This problem is ameliorated somewhat by the addition of molybdenum, a common composition being W (40 at%)–Re (30%)–Mo (30%). The strengths of tungsten or tungsten–rhenium systems can be increased by small amounts of a dispersed second phase such as an oxide (ThO2,Ta2O5), carbide (HfC, TaC), or boride (HfB, ZrB). The so-called heavy alloys are three-component systems composed mainly of tungsten in combination with a nickel–copper or nickel–iron matrix. These materials are characterized by high density (17 to 19 g/cm3), hardness, and good thermal conductivity. Tungsten is used widely as a constituent in the alloys of other metals, since it generally enhances high-temperature strength. Several types of tool steels and some stainless steels contain tungsten. Heat-resistant alloys, also termed superalloys, are nickel-, cobalt-, or ironbase systems containing varying amounts (typically 1.5 to 25 wt%) of tungsten. Wear-resistant alloys having the trade name Stellites are composed mainly of cobalt, chromium, and tungsten. Cobalt–tungsten alloy, with 50% tungsten, gives a plate that retains a high hardness at red heat. Tungsten RhC is a tungsten–rhenium carbide alloy containing 4% rhenium carbide. It is used for parts requiring high strength and hardness at high temperatures. The alloy retains a tensile strength of 517 MPa at 1927°C.

TUNGSTEN CARBIDE Tungsten carbide is an iron-gray powder of minute cubical crystals with a Mohs hardness above 9.5 and a melting point of about 2982°C. It is produced by reacting a hydrocarbon vapor with tungsten at high temperature. The composition is WC, but at high heat it may decompose into W2C and carbon, and the carbide may be a mixture of the two forms. Other forms may also be produced, W3C and W3C4. Tungsten carbide is used chiefly for cutting tool bits and for heatand erosion-resistant parts and coatings.


One of the earliest of the American bonded tungsten carbides was Carboloy, which was used for cutting tools, gauges, drawing dies, and wear parts. The carbides are now often mixed carbides. Carboloy 608 contains 83% chromium carbide, 2% tungsten carbide, and 15% nickel binder. It is lighter in weight than tungsten carbide, is nonmagnetic, and has a hardness to Rockwell A93. It is used for wear-resistant parts, and resists oxidation to 1092°C. Titanium carbide is more fragile, but may be mixed with tungsten carbide to add hardness for dies. Kennametal K601 is used for seal rings and wear parts, and is a mixture of tantalum and tungsten carbides without a binder. It has a compressive strength of 4650 MPa, rupture strength of 689 MPa, and Rockwell hardness A94. Kennametal K501 is tungsten carbide with a platinum binder for parts subject to severe heat erosion. Tungsten carbide LW-1 is tungsten carbide with about 6% cobalt binder used for flamecoating metal parts to give high-temperature wear resistance. Deposited coatings have a Vickers hardness to 1450, and resist oxidation at 538°C. Tungsten carbide LW-1N, with 15% cobalt binder, has a much higher rupture strength, but the hardness is reduced to 1150.

TUNGSTEN STEEL Tungsten steel is any steel containing tungsten as the alloying element imparting the chief

characteristics to the steel. It is one of the oldest of the alloying elements in steel. Tungsten increases the hardness of steel, and gives it the property of red hardness, stabilizing the hard carbides at high temperatures. It also widens the hardening range of steel, and gives deep hardening. Very small quantities serve to produce a fine grain and raise the yield point. The tungsten forms a very hard carbide and an iron tungstite, and the strength of the steel is also increased, but it is brittle when the tungsten content is high. When large percentages of tungsten are used in steel, they must be supplemented by other carbide-forming elements. Tungsten steels, except the low-tungsten chromium–tungsten steels, are not suitable for construction, but they are widely used for cutting tools, because the tungsten forms hard abrasion-resistant particles in high-carbon steels. Tungsten also increases the acid resistance and corrosion resistance of steels. The steels are difficult to forge, and cannot be readily welded when tungsten exceeds 2%. Standard tungsten–chromium alloy steels 72XX contain 1.5 to 2% tungsten and 0.50 to 1% chromium. Many tool steels rely on tungsten as an alloying element, and it may range from 0.50 to 2.50% in coldwork and shock-resisting types to 9 to 18% in the hot-work type, and 12 to 20% in highspeed steels.

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U ULTRAHIGH-STRENGTH STEELS These are the highest-strength steels available. Arbitrarily, steels with tensile strengths of around 1378 MPa or higher are included in this category, and more than 100 alloy steels can be thus classified. They differ rather widely among themselves in composition or the way in which the ultrahigh strengths are achieved. Medium-carbon low-alloy steels were the initial ultrahigh-strength steels, and within this group, a chromium–molybdenum steel (4130) grade and a chromium–nickel–molybdenum steel (4340) grade were the first developed. These steels have yield strengths as high as 1654 MPa and tensile strengths approaching 2068 MPa. They are particularly useful for thick sections because they are moderately priced and have deep hardenability. Several types of stainless steels are capable of strengths above 1378 MPa, including a number of martensitic, coldrolled austenitic, and semiaustenitic grades. The typical martensitic grades are types 410, 420, and 431, as well as certain age-hardenable alloys. The cold-rolled austenitic stainless steels work-harden rapidly and can achieve 1241 MPa tensile yield strength and 1378 MPa ultimate strength. Semiaustenitic stainless steels can be heat-treated for use at yield strengths as high as 1516 MPa and ultimate strengths of 1620 MPa. Maraging steels contain 18 to 25% nickel plus substantial amounts of cobalt and molybdenum. Some newer grades contain somewhat less than 10% nickel and between 10 and 14% chromium. Because of the low-carbon (0.03% max) and nickel content, maraging steels are martensitic in the annealed condition, but are still readily formed, machined, and welded. By


a simple aging treatment at about 482°C, yield strengths of as high as 2068 and 2413 MPa are attainable, depending on specific composition. In this condition, although ductility is fairly low, the material is still far from being brittle. Among the strongest of plain carbon sheet steels are the low- and medium-carbon sheet grades called MarTinsite. Made by rapid water quenching after cold rolling, they provide tensile yield strengths to 1517 MPa but are quite limited in ductility. There are two types of ultrahigh-strength, low-carbon, hardenable steels. One, a chromium–nickel–molybdenum steel, named Astralloy, with 0.24% carbon is air-hardened to a yield strength of 1241 MPa in heavy sections when it is normalized and tempered at 260˚C. The other type is an iron–chromium–molybdenum–cobalt steel and is strengthened by a precipitation hardening and aging process to levels of up to 1654 MPa in yield strength. High-alloy quenched-and-tempered steels are another group that have extra-high strengths. They contain 9% nickel, 4% cobalt, and from 0.20 to 0.30% carbon, and develop yield strengths close to 2068 MPa and ultimate strengths of 2413 MPa. Another group in this high-alloy category resembles high-speed tool steels, but are modified to eliminate excess carbide, thus considerably improving ductility. These so-called matrix steels contain tungsten, molybdenum, chromium, vanadium, cobalt, and about 0.5% carbon. They can be heattreated to ultimate strengths of over 2757 MPa — the highest strength at present available in steels, except for heavily cold-worked high-carbon steel strips used for razor blades and drawn wire for musical instruments, both of which have tensile strengths as high as 4136 MPa.

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ULTRAVIOLET-CURABLE HOT-MELT ADHESIVES For years, ultraviolet (UV)-curable pressuresensitive adhesives (PSAs) have been recognized as a fixture alternative to solvent-borne products. The idea of achieving the solvent and heat resistance of an acrylic without facing the various safety and environmental ramifications has always been enticing to both PSA formulators and users. The promise of this technology has led to the development of a variety of adhesive technology platforms. Photoinitiators can now be purchased that offer much better thermal stability for improved pot life and coatability. Polymers have been developed that are much more chemically active, dramatically reducing the amount of photoinitiator required to achieve proper cure (and consequently the total cost of the adhesive). Thanks to the response of their suppliers, adhesive manufacturers are making UV-curable products that are more versatile than ever. Converting a pressure-sensitive hot-melt coating line over to UV-curing no longer demands growing accustomed to radically different adhesives.

PROPERTIES

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APPLICATIONS

Conventional hot-melt PSAs are widely used for tape and label applications. Their room-temperature performance is difficult to match with alternative chemistries. They possess an outstanding combination of high tack, peel, and shear and adhere well to wet or low-energy surfaces. In addition to the performance advantages, conventional hot melts possess some significant processing advantages because they require no solvent vehicle for application. The lack of a combustible solvent makes them safer and more environmentally friendly than any other adhesive. Because they require no drying, they can be applied more easily at high depositions for use on slick or rough surfaces. Finally, since they require no dryers, hot-melt coaters are generally more compact and lower in cost than liquid coaters.


Unfortunately, users of conventional hot melts can only enjoy these benefits over a limited range of conditions. The products are hobbled by poor resistance to solvents, plasticizers, and heat. This precludes their use in some industrial applications where their high roomtemperature peel and shear could make them otherwise well suited.

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FORMS

Traditional pressure-sensitive hot melts are formulated primarily with block copolymers and various tackifying resins. The cohesive strength of the product is largely determined by the block copolymer used. Some of the most common block copolymers used are styrenic triblock copolymers. These are long polymer chains with polystyrene molecules grouped together to form two end blocks surrounding one mid-block made of an elastomeric material. Frequently used triblock copolymers are styrene–isoprene–styrene (SIS) or styrene–butadiene–styrene (SBS).

FUTURE UV offers an outstanding combination of versatile performance and ease of use. The UVcurable hot melt is comparable to the solventborne acrylic. In addition, a significant performance advantage has been seen on low-energy surfaces. This makes the technology even more appealing because of the ever-increasing use of plastics. With new tools at their disposal, the performance of UV-curables is now up to any adhesive task required. Formulators have crafted newer and better adhesives capable of a variety of tasks. These products capture the traditional advantages of hot melts while meeting many of the standards of acrylics.

UNIFORM MAGNETIC HEATING Uniform magnetic heating (UMH) is a system by which electrical energy is converted to heat within metallic materials in a very efficient and flexible manner. Although this system and conventional induction heating both require


electrical coils to convert electricity to magnetic flux energy, the similarities stop there

UMH

VS. INDUCTION

The CoreFlux UMH system utilizes two coils that are permanently fixed around a C-shaped laminate core. Similar to induction systems, the coils convert electrical energy into magnetic flux energy. However, in conventional induction, the component to be heat treated is placed inside the coil. The energy is directly transferred to the part in the form of surface eddy currents, which are generated as the current flows around the component. The CoreFlux system transfers energy in a different way. Energy is transferred into a laminate core (similar to a transformer core) and channeled directly to the part in a linear manner. In this way, the magnetic flux energy is distributed throughout the entire part. Key to the technology is that the flux direction in the core oscillates at a user-defined frequency from 20 to 400 Hz. As a result, the polarity of the flux field changes at this defined frequency. Each time the polarity changes, heat is released throughout the component via “hysteresis loss.” Simply put, hysteresis loss is the energy released throughout the material as the magnetic domains in the microstructure are forced to realign continually with the alternating magnetic field. The effect of this phenomenon is the uniform production of heat throughout the component. Because of the inherent nature of this method, the core and surface temperatures show minimal thermal gradients throughout the entire heating process. With induction, localized overheating of components with holes and unusual characteristics is a common problem, but overheating is not typically a concern with UMH. In addition to the benefits of uniform through-heating, UMH also provides several other key advantages: • The coils are permanently fixed to the machine and require no maintenance or coil changeovers. • The same coils can run a wide range of processes and bring the benefit of


flexibility by running families of parts with no coil changes required. • Metallurgical results are typically significantly better than conventional systems. • The machine requires no water cooling, which results in lower facility costs, less maintenance, and no energy lost to heated water. • Energy efficiency can be as much as twice that of conventional induction. The power supply is basically a standard AC variable motor drive. This eliminates costly custom power supplies and the problems associated with their maintenance. It also greatly reduces overall capital equipment cost (Figure U.1).

APPLICATIONS Heating Press Dies One simple application of the technology that offers significant benefits is the heating of press dies. A variety of die shapes may be placed in

Main core Coil

Core extension Oscillating field

Part

Coil

Main core

FIGURE U.1 The component to be treated may be placed around the core extension, which provides a secondary field that causes very rapid and uniform heating with no part contact. UMH generates a uniform field from the inner core extension outward through the entire part. (From Adv. Mater. Proc., 154(5), 41–43, 1998. With permission.)

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a common machine that can heat the dies to the required temperature in a matter of minutes. As an alternative to oven preheating, the UMH process offers much faster, cleaner, and more efficient heating, and allows customers to change dies more quickly. Machines may also be mobile, so that one machine can service multiple press locations. Current size and shape capabilities range from very small to approximately 180 × 45 × 45 cm. Larger components may be accommodated with custom designs. Tempering Gears and Bearings

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One of the most-promising areas of application for this technology is the tempering of gears and bearings. Parts do not need to be rotated, and taller parts do not require scanning. For example, if a component has a round shape and an inner bore, the CoreFlux UMH technology can be applied by utilizing a core extension. The component is placed around the core extension, which provides a secondary field that causes very rapid and uniform heating with no part contact. UMH generates a uniform field throughout the component, from the inner core extension. On the other hand, if the part were induction tempered, the coil would be placed around the outside diameter, and the field would be generated from the outside inward to the core. Fortunately, the CoreFlux UMH process overcomes time-at-temperature and the skineffect phenomenon. Because of the uniformity of heating, the core of the gear comes to temperature at virtually the same rate as the teeth. Metallurgical results typically exceed expectations and return properties similar, and sometimes superior, to oven tempering. An additional benefit is the ability to run multiple parts around the same core extension. Depending on the actual geometry, parts can be stacked around the core extension with minimal effect on the total cycle time. Obviously, this can have a profound effect on production throughput and machine utilization. Hardening Gears and Bearings The same core extension approach described above may be utilized for higher-temperature


applications. Although the research and documentation related to hardening is less mature than the lower-temperature applications, the technology has again demonstrated substantial benefits over today’s alternatives. Targeted at through-hardening applications only, the technology offers the same flexibility as in tempering. In fact, UMH has potential for the design of an entire hardening and tempering line that could allow for the flexible running of components inside a large “family” grouping, with no setup changes. Heating Aluminum Billets The properties of aluminum make billets difficult to through-heat quickly and uniformly by conventional technology. With the CoreFlux UMH process, an aluminum billet may be placed directly on the insulated core cap, and the top core/coil assembly may then be lowered to make light contact with the billet. This clamping effect is utilized to create the most efficient transfer of energy into the billet, and to facilitate holding the billet in place during heating. Clamping pressure is adjustable to eliminate any marking or deformation. Capabilities have been documented and proved, and application development continues in the aluminum field, with preheating applications ranging from 370°C to semisolid temperatures. Although steel forging offers similar promise, development is still under way in this area to assure that the machine cores will endure long exposures to extreme forging temperatures. Shrink-Fit Applications Another simple through-heat application is preheating components for shrink fitting. Again, when compared with any other alternative available for lower-temperature through-heating, the CoreFlux UMH process is an improvement. Press Tempering Although a relatively new development, presstemper applications have recently drawn considerable attention. In cases where thin parts must be stacked and held flat during tempering or stress relieving, the CoreFlux process is


worth considering. The top coil and core assembly are typically lowered via pneumatic cylinders, and the clamping pressure is limited to as little as a pound. However, the pressure can be adjusted to provide more than a ton of damping force if necessary. If higher force is required, the machine can be customized. In addition, this system could allow for the pressure to be controlled as a function of temperature.

UNSATURATED POLYESTER RESIN The use of unsaturated polyester resins in structural applications is well documented. There are, however, significant quantities of unsaturated polyester resins used in specialist compounded products, which are more likely to be unreinforced. The most well known of these technologies are formulated gel-coats, a technology that has been changing rapidly in recent years with improvements in gloss retention, color retention, and volatile organic compound emissions. The introduction of granite effect coatings and solid surface material is a further example of the versatility of unsaturated polyester resins. Although these materials have been predominantly used for interior applications, their potential for exterior use on buildings provides exciting possibilities for a new and varied range of composite building materials providing stone effects at a fraction of the weight of conventional building materials. Other compounded resins that are especially important to the building and construction market are those with fire resistant characteristics. In addition, the improvements in smoke reduction from unsaturated polyester resin systems make such materials attractive for cladding applications. Combining the advantages of these resins with decorative coatings and sandwich construction provides the basis for structural, insulating components.

MARKETS The markets for reinforced plastics are frequently split into a number of generally accepted sectors, such as marine, land transport, building and construction, and chemical


containment. There are, of course, subdivisions in each sector, for example, powered pleasure boats, powered work boats, sailboats, and offshore applications in the marine market, but most of the discussion in the literature is about the use of fiber-reinforced composites in these market sectors and market subgroups. In general, unfilled resins with good mechanical properties are preferred, but there are, very often, requirements for compounded products to provide special characteristics to meet specific performance requirements. Obviously, compounded fire-resistant materials fall into such a category and are used to impart resistance to ignition, resistance to surface spread of flame, and, increasingly, reduction in emissions of smoke and toxic fumes. Although such materials are often highly filled, they are used with fiber reinforcement for the manufacture of structural and semistructural components. The importance of these resins and their developments together with two other important compounded unsaturated polyester resin-based products has been disclosed. These latter materials are not used in conjunction with fiber reinforcement but are usually simply filled or pigmented; they are gel-coats and are mainly used as “in-mold” coatings and solid surface materials for the manufacture of synthetic granitetype products. Resin concrete and repair putties are also large consumers of unsaturated polyester resin in non-fiber-reinforced compounds. Resistance to Fire The use of glass-fiber-reinforced plastics (GRP) in applications where fire resistance was particularly important was introduced into the building industry five decades ago. Generally, the structural performance of the material was not questioned for building applications because it had been well proven for the construction of boats. However, as with most plastic materials, its ability to perform under fire conditions was in question for use in buildings even though it had been documented that fires in buildings originate from the contents and in a vast majority of circumstances the structure does not contribute to loss of life.

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One of the most successful means to improve the resistance of plastics to fire is by the incorporation of fillers, which break down with heat to produce heavy vapors to prevent oxygen reaching the surface of the material and hence reduce the possibility of burning. The major problem associated with the high levels of filler required to render resins fire retardant is the increase in their viscosity, which results in handling difficulties when manufacturing structural components. The use of halogenated additives, which work synergistically with some fire-retardant fillers, help to overcome handling problems but result in the potential for toxic fume production under fire conditions. The availability of improved viscosity modifiers is now enabling resins filled with high levels of nontoxic fillers, such as alumina trihydrate, to be used to manufacture laminates containing reasonable levels of reinforcement to produce, at least, semistructural components. Such systems will meet the new International Maritime Organization (IMO) requirements for use on passenger ships. Under the test conditions, the material has to exhibit low surface spread of flame characteristics, low smoke emissions, and low emissions of carbon monoxide. Gel-Coat Protection In the early days of the GRP industry, the need for resin-rich surfaces was established to:

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• Improve the durability of components • Protect the laminate from the environment • Reduce fiber pattern • Provide a smooth aesthetic finish • Eliminate the need for painting As a result of these requirements, a market for ready-formulated coatings was established and gel-coat product ranges became established. The availability of quality “in-mold” coatings, such as gel-coats, to fabricators saves labor and wastage in the workshop and improves the quality of molded components. Gel-coats are available in brush and spray versions with a


variety of properties and performance characteristics to meet a range of needs. They must be applied carefully and correctly to avoid faults. Gel-Coat Developments Over the years the need for improved gloss and color retention in gel-coats has been recognized and developments in ultraviolet (UV) resistance and color fastness have resulted in a range of gel-coats that can be weathered under the severest tropical weather conditions without changes in appearance. Solid Surfaces Resins have often been used to bind together fillers and aggregates to produce materials such as resin concrete and synthetic cultured and onyx marble. For decorative surfaces a clear (translucent) gel-coat is used to improve the quality of the surface finish and remove the effects of surface porosity. Although the gelcoat used is usually based on good quality, water-resistant resins, the inferior quality of the backing systems often results in a material that is susceptible to crazing, cracking, poor water resistance, and poor thermal resistance. Because the gel-coat surface is too thin for repairs to be effectively carried out, the problems cannot be easily rectified. The monopoly of the acrylic-based solid surface material has been gradually eroded by the introduction of unsaturated polyester-based solid surface, which offer a much wider range of colors to provide improved customer choice. Raw materials and manufacturing processes have been designed to eliminate voids in polyester-based solid surfaces. Traditionally, solid surface materials have been used for the manufacture of kitchen surfaces, sinks, and bathroom units. However, there is increasing interest in more diverse applications such as furniture, table tops, tiles, paneling, cutlery, and pens. It is also possible to use the material as a 2- to 3-mm-thick coating for other materials and the granite effect finish is reviving interest in GRP for cladding for buildings. The unsaturated polyester resin-based material comprises of three components:


1. Chips. The colored fillers or chips, used to provide the granite effect, can be based on thermoplastic or thermoset materials. 2. Resins. Resin must be clear and near “water white” to allow the depth of color of the chips to be appreciated. The resin must also be resistant to elevated temperature, water, staining, UV light, and cigarette burns. Hence, typical formulations giving an acceptable level of performance are based on isophthalic acid and neopentyl glycol (NPG). 3. Fillers. Only alumina trihydrate can be used in addition to the colored “chips” because it is translucent. It also offers fire-retardant characteristics. Solid surface systems are nonreinforced and can be machined and cut with conventional woodworking equipment. Patterns can be routed in solid surface materials and cast resin “in laid” to provide a variety of customized finishes. It is important to ensure when manufacturing solid surface that the resin is formulated to accept high filler loading without air entrapment and will develop hardness rapidly. The final product must be resistant to chipping, cracking, hot-cold water cycling, “blushing,” and UV light. It must also be easy to machine for shaping and finishing.

FUTURE Unsaturated polyester resin-based compounded products provide a range of materials with tailored performance characteristics for a variety of markets. Gel-coats are essential for most applications for GRP, providing aesthetic finishes in the marine, transport, building, and construction markets. They have well-proven durability but improvements in gloss and color retention will ensure their position as the major coating for fiber-reinforced composite materials in the future. Fire-retardant resins with exceptionally low smoke production under fire conditions are


becoming a reality with unsaturated polyester resin-based systems. New standards are providing new challenges, which are being met successfully to ensure materials meet new requirements for surface spread of flame for materials for use in construction applications.

URANIUM An elementary metal (symbol U), uranium never occurs free in nature but is found chiefly as an oxide in the minerals pitchblende and carnotite where it is associated with radium. The metal has a specific gravity of 18.68 and atomic weight 238.2. The melting point is about 1133°C. It is hard but malleable, resembling nickel in color, but related to chromium, tungsten, and molybdenum. It is soluble in mineral acids. Uranium has three forms. The alpha phase, or orthorhombic crystal, is stable to 660˚C; the beta, or tetragonal, exists from 660 to 760˚C; and the gamma, or body-centered cubic, is from 760˚C to the melting point. The cast metal has a hardness of 80 to 100 Rockwell B, workhardening easily. The metal is alloyed with iron to make ferrouranium, used to impart special properties to steel. It increases the elastic limit and the tensile strength of steels, and is also a more powerful deoxidizer than vanadium. It will denitrogenize steel and has also carbideforming qualities. It has been used in highspeed steels in amounts of 0.05 to 5% to increase the strength and toughness, but because of its importance for atomic applications its use in steel is now limited to the byproduct nonradioactive isotope uranium-238.

USES Metallic uranium is used as a cathode in photoelectric tubes responsive to ultraviolet radiation. Uranium compounds, especially the uranium oxides, were used for making glazes in the ceramic industry and also for paint pigments. It produces a yellowish-green fluorescent glass, and a beautiful red with yellowish tinge is produced on pottery glazes. Uranium dioxide, UO2, is used in sintered forms as fuel for power reactors. It is chemically stable, and has a high melting point at about 2760˚C, but

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a low thermal conductivity. For fuel use the particles may be coated with about 0.003 cm of aluminum oxide. This coating is impervious to xenon and other radioactive isotopes so that only the useful power-providing rays can escape. These are not dangerous at a distance of about 15 cm, and thus less shielding is needed. For temperatures above 1260°C, a coating of pyrolitic graphite is used. Uranium has isotopes from 234 to 239, and uranium-235, with 92 protons and 143 neutrons, is the one valued for atomic work.

UREA Also called carbamide, urea is a colorless to white crystalline powder, NH2 · CO · NH2, best known for its use in plastics and fertilizers. The chemistry of urea and the carbamates is very complex, and a great variety of related products are produced. Urea is produced by combining ammonia and carbon dioxide, or from cyanamide, NH2 · C · N. It is a normal waste product of animal protein metabolism, and is the chief nitrogen constituent of urine. It was the first organic chemical ever synthesized commercially. It has a specific gravity of 1.323, and a melting point at 135°C.

TYPES

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The formula for urea may be considered to be O · C(NH2)2, and thus as an amide substitution in carbonic acid, O · C(OH)2, an acid that really exists only in its compounds. The urea-type plastics are called amino resins. The carbamates can also be considered as deriving from carbamic acid, NH2COOH, an aminoformic acid that likewise appears only in its compounds. The carbamates have the same structural formula as the bicarbonates, so that sodium carbamate has an NH2 group substituted for each OH group of the sodium bicarbonate. The urethanes, used for plastics and rubber, are alkyl carbamates made by reacting urea with an alcohol, or by reacting isocyanates with alcohols or carboxyl compounds. They are white powders of the composition NH2COOC2H5, melting at 50˚C. Isocyanates are esters of isocyanic acid, H · N · C · O, which does not appear independently. The dibasic diisocyanate is made from


a 36-carbon fatty acid. It reacts with compounds containing active hydrogen. With modified polyamines it forms polyurea resins, and with other diisocyanates it forms a wide range of urethanes. Tosyl isocyanate for producing urethane resins without a catalyst is toluene sulfonyl isocyanate. The sulfonyl group increases the reactivity. Methyl isocyanate, CH3NCO, known as MIC, is a colorless liquid with a specific gravity of 0.9599. It reacts with water. With a flash point of less than –6.6°C, it is flammable and a fire risk. It is a strong irritant and is highly toxic. One of its principal uses is as an intermediate in the production of pesticides. Urea is used with acid phosphates in fertilizers. It contains about 45% nitrogen and is one of the most efficient sources of nitrogen. Urea reacted with malonic esters produces malonyl urea, which is the barbituric acid that forms the basis for the many soporific compounds such as luminal, phenobarbital, and amytal. The malonic esters are made from acetic acid, and malonic acid derived from the esters is a solid of the composition CH2(COOH)2, which decomposes at about 160°C to yield acetic acid and carbon dioxide. For plastics manufacture, substitution on the sulfur atom in thiourea is easier than on the oxygen in urea. Thiourea, NH2 · CS · NH2, also called thiocarbamide, sulfourea, and sulfocarbamide, is a white, crystalline, water-soluble material of bitter taste, with a specific gravity of 1.405. It is used for making plastics and chemicals. On prolonged heating below its melting point, 182˚C, it changes to ammonium thiocyanate, or ammonium sulfocyanide, a white, crystalline, water-soluble powder of the composition NH4SCN, melting at 150˚C. This material is also used in making plastics, as a mordant in dyeing, to produce black nickel coatings, and as a weed killer. Permafresh, used to control shrinkage and give wash-and-wear properties to fabrics, is dimethylol urea, CO(NHCH2OH)2, which gives clear solutions in warm water. Urea-formaldehyde resins are made by condensing urea or thiourea with formaldehyde. They belong to the group known as aminoaldehyde resins made by the interaction of an amine and an aldehyde. An initial condensation


product is obtained that is soluble in water, and is used in coatings and adhesives. The final condensation product is insoluble in water and is highly chemical resistant. Molding is done with heat and pressure. The urea resins are noted for their transparency and ability to take translucent colors. Molded parts with cellulose filler have a specific gravity of about 1.50, tensile strength from 41 to 89 MPa, elongation 15%, compressive strength to 310 MPa, dielectric strength to 16 × 106 V/m, and heat distortion temperature to 138˚C. Rockwell hardness is about M 118. Urea resins are marketed under a wide variety of trade names. The Uformite resins are water-soluble thermosetting resins for adhesives and sizing. The Urac resins, and the Casco resins and Cascamite, are urea-formaldehyde. They are used as adhesives for plasterboard, plywood, and in wet-strength paper.

URETHANES Also termed polyurethanes, urethanes are a group of plastic materials based on polyether or polyester resin. The chemistry involved is the reaction of a diisocyanate with a hydroxylterminated polyester or polyether to form a higher-molecular-weight prepolymer, which in turn is chain-extended by adding difunctional compounds containing active hydrogens, such as water, glycols, diamines, or amino alcohols. The urethanes are block polymers capable of being formed by a literally indeterminate number of combinations of these compounds. The urethanes have excellent tensile strength and elongation, good ozone resistance, and good abrasion resistance. Combinations of hardness and elasticity unobtainable with other systems are possible in urethanes, ranging from Shore hardnesses of 15 to 30 on the “A” scale (printing rolls, potting compounds) through the 60 to 90 A scale for most industrial or mechanical goods applications, to the 70 to 85 Shore “D” scale. Urethanes are fairly resistant to many chemicals such as aliphatic solvents, alcohols, ether, certain fuels, and oils. They are attacked by hot water, polar solvents, and concentrated acids and bases.


URETHANE FOAMS Urethane foams are made by adding a compound that produces carbon dioxide or by reaction of a diisocyanate with a compound containing active hydrogen. Foams can be classified somewhat according to modulus as flexible, semiflexible or semirigid, and rigid. No sharp lines of demarcation have been set on these different classes as the gradation from the flexibles to the rigids is continuous. Densities of flexible foams range from about 16 kg/m3 at the lightest to 64 to 80 kg/m3 depending on the end use. Applications of flexible foams range from comfort cushioning of all types, e.g., mattresses, pillows, sofa seats, backs, and arms, automobile topper pads, and rug underlay, to clothing interliners for warmth at light weight. Flexible Types The techniques of manufacture of flexible urethane foam vary widely, from intermittent hand mixing to continuous machine operation, from prepolymer to one-shot techniques, from slabforming to molding, from stuffing to foamedin-place. Future applications envision the flexible foam not as a substitute for latex rubber foam or cotton, but as a new material of construction allowing for design of furniture, for example, that is essentially all foam with a simple cloth cover and a very simple metal-supporting framework. Rigid Types Densities from about 24 to 800 kg/m3 on the semirigid side have been produced with corresponding compression strengths again for particular end uses ranging from insulation to fully supporting structural members. The usefulness of the urethane system has been in the foamin-place principle using a host of containing wall materials. Applications in the more rigid foam field have been thermal insulation of all types (lowtemperature refrigeration ranging from liquid nitrogen temperatures up to the freezing point of water and high temperature insulation of steam pipes, oil lines, etc.); shock absorption such as packaging, crash pads, etc., where the

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higher hysteresis values produce either a better one-time high impact “crash” use or, more often, lower amplitude but higher frequency container end use; filtration (air, oil, etc., where a large surface-to-volume ratio is needed with a simple technique to produce a reusable filter to allow for its initially higher cost factor); structural (building applications of all kinds combining a good thermal as well as structural behavior, filling of building voids, and curtain walls are some basic applications); flotation (boats, buoys, and every other imaginable object afloat represents some possible application of urethane foams); and, finally, generalpurpose applications that include all other uses such as decorative applications. Rigid foams can be produced using a simple spray technique and a number of machines are sold on the market for this technique. Timeconsuming layup of foam is eliminated using this method. Insulation of walls, tanks, etc. are applications in use today. With the use of lowvapor-pressure isocyanates such as MDI (4,4′diphenylmethane diisocyanate), the potential irritant hazard during spraying is greatly lowered. Self-adhesion of the sprayed foam is a valuable asset of this type of system. Urethane foams offer advantages over many of the better-known foams such as latex foam rubber, polystyrene, and polyethylene, with the combination of excellent properties and lower installed costs. Depending on the application, a lower foam density can be used with similar load-bearing properties, also one having an extremely low thermal conductivity can be fabricated. The oil resistance, high-temperature resistance, good high-tensile properties, good permanence properties, resistance to mildew, resistance to flammability, and so on are in general the types of properties that, combined with foamed-in-place technology, put urethane foam far ahead of competitive materials.

OTHER URETHANES Thermoplastic polyurethanes (TPU) include two basic types: esters and ethers. Esters are tougher, but hydrolyze and degrade when soaked in water. There also are TPUs based on polycaprolactone, which while technically


being esters, have better resistance to hydrolysis. TPUs are used when a combination of toughness, flex resistance, weatherability, and lowtemperature properties are needed. These materials can be injection-molded, blow-molded, and extruded as profiles, sheet, and film. Further, TPUs are blended with other plastic resins, including polyvinyl chloride, ABS, acetal, SAN, and polycarbonate. Urethane elastomers are made with various isocyanates, the principal ones being TDI (tolylene diisocyanate) and MDI (4,4′-diphenylmethane diisocyanate), reacting with linear polyols of the polyester and polyether families. Various chain extenders, such as glycols, water, diamines, or aminoalcohols, are used in either a prepolymer or a one-shot type of system to form the long-chain polymer. Flexible urethane fibers, used for flexible garments, are more durable than ordinary rubber fibers or filaments, and are 30% lighter in weight. They are resistant to oils and to washing chemicals, and also have the advantage that they are white in color. Spandex fibers are stretchable fibers produced from a fiber-forming substance in which a long chain of synthetic molecules are composed of a segmented polyurethane. Stretch before break of these fibers is from 520 to 610%, compared to 760% for rubber. Recovery is not as good as in rubber. Spandex is white and dyeable. Resistance to chemicals is good but it is degraded by hypochlorides. There are six basic types of polyurethane coatings, or urethane coatings, as defined by the American Society for Testing and Materials (ASTM), Specification D16. Types 1, 2, 3, and 6 have long storage life and are formulated to cure by oxidation, by reaction with atmospheric moisture, or by heat. Types 4 and 5 are catalystcured and are used as coatings on leather and rubber and as fast-curing industrial product finishes. Urethane coatings have good weathering characteristics as well as high resistance to stains, water, and abrasion.

FABRICATION Urethane elastomers can be further characterized by the method of fabrication of the final article. Three principal types of fabrication are


possible: (1) casting technique where a liquid prepolymer or a liquid mixture of all initial components (one-shot) is cast into the final mold, allowed to “set” and harden, and is then removed for final cure; (2) millable gum technique where conventional rubber methods and equipment are used to mill the gum, add fillers, color, etc., and/or banbury, extrude, calender, and compression mold the final shaped item; (3) thermoplastic processing techniques where the resin can be calendered, extruded, and injection- or blow-molded on conventional plastic machinery in final form (an important benefit here is that scrap can be reground and reused in fabricating other parts). The choice of the proper method of fabrication largely depends on the economics of the process, because the properties of the final product may be about the same regardless of the method of fabrication. If a few large-volume items are needed, casting these into a single mold is usually more economical. However, if many thousands of small, intricate pieces are needed, usually injection molding is the preferred, more economical method of fabrication.

USES Applications of urethane elastomers have been developed where high abrasion resistance, good oil resistance, and good load-bearing capacity are of value, as in solid tires and wheels, especially of industrial trucks, the shoe industry, drive and belting applications, printing rolls, gasketing in oil, etc. Other applications include vibration dampening; for example, in hammer heads, air hammer handles, shock absorption underlays for heavy machinery, etc.; low coefficient of friction with the addition of molybdenum disulfide for self-lubricating uses as ball and socket joints, thrust bearings, leaf spring slide blocks, etc. In the electrical industry, cable jacketing and potting compounds are developing as important uses. Various systems of urethane elastomers with specific fillers have been developed into an important class of caulks and sealants, which is just beginning to take hold in applications such as concrete road-expansion joints, building caulking, and so on, in direct competition with such older materials as the


polysulfides but at a much lower price and superior properties. A host of other applications varies from adhesive bonding of fibers of all kinds to rocket fuel binders of the more exotic variety, which are becoming so important in the U.S. national defense picture. Therefore, it is imperative that design engineers understand fully the material they are using and how they intend to utilize it in the final piece of equipment. For example, one recommendation is to limit the use of urethanes to below 82˚C in water for continuous exposures. Dry uses can go somewhat higher, e.g., to 107˚C for certain systems. In oil, exposures can be up to 121˚C. Disregard of such limitations can result in failures, but the design engineer can eliminate these by the proper choice of material. On the other hand, the design engineer should choose the urethanes for their virtues, such as hardness and elasticity, where other materials such as natural and other synthetic rubbers may fail.

PROPERTIES The urethanes have excellent tensile strengths and elongation, good ozone resistance, and good abrasion resistance. Knowledge of these properties is mandatory for good engineering design. The greater load-bearing capacity of urethanes as compared to other elastomers is noteworthy, for it leads to smaller, less costly, lowerweight parts in equivalent applications. Tear strength is extremely high, which may be important in particular applications along with the very high tensile strengths. The high abrasion resistance has made possible driving parts for which no other materials could compete. However, in every such dynamic application, the engineer must design the part to allow for the higher hysteresis losses in the urethane. Whereas in some applications such as dampening, the higher hysteresis works to advantage, in others hysteresis will lead to part failure if the upper temperature limit is thereby exceeded. Redesign of the part (thinner walls, etc.) to allow for greater dissipation of the heat generated will permit the part to operate successfully. This has proved to be the case many times.

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Urethane elastomers generally have good low-temperature properties. The same hysteresis effect works in reverse here so that a part in dynamic use at temperatures as low as –51˚C, while stiff in static exposure, immediately generates enough heat in dynamic use to pass through its second-order transition and does not show any brittleness but becomes elastic and usable. By proper choice of the polyester or

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polyether molecular backbone, lower use temperatures (as low as –62°C) have been formulated in urethane elastomers. In addition to good mechanical properties, urethanes have good electrical properties, which suggest a number of applications. Oxygen, ozone, and corona resistances of this system are generally excellent.

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V VACUUM ASSIST MOLD PROCESSING The use of atmospheric pressure to hold closed molds together during injection was the early process of vacuum assisted resin injection (VARI). Adding vacuum has enabled resintransfer molding (RTM) to challenge compression molding and autoclaving systems capable of making the best high-performance composites. Vacuum is used in two ways. First, it mixes resin and hardener under a 91-Pa vacuum just prior to injection. Using an impeller, it agitates the mixture to drive air to the surface where the vacuum removes it. Degassing, which takes about 2 h per tank, also removes volatiles and low-molecular-weight by-products. Some companies place their RTM tools in a vacuum chamber rather than using a vacuum tool. The chamber creates a vacuum that does not vary, even as resin fills the tool. The hard vacuum pulls any air and water vapor off the preform and sucks resin into the mold. This combination of degassed resin and vacuum keeps voids under 3% and often better. This ensures consistently high structural integrity because voids concentrate stresses that initiate fractures and cause premature failure. It takes only a 2% increase in voids to drain interlaminar shear strength 20% and flexural modulus 10%. The process matches equivalent compression molding and autoclaving fiber volumes and voids. Compared with compression molding and autoclaving, VARI does not need to apply pressure over the entire skin surface to vanquish voids, simplifying cocuring. For example, the process can fabricate cores and reinforced skins in a single step rather than bonding them after fabrication.


More importantly, the process makes composites in fewer, more controllable steps. These resin injection and molding processes — RTM, VARI, vacuum resin-transfer molding (VRTM), and vacuum-assisted resin-transfer molding (VARTM) — are much simpler processes to use. Each process step is also independent and controllable. In VRTM, air evacuation depends on only the vacuum, and resin preparation depends on only the resin mixer. Preform production varies with automated fabric weaving and preform placement, whereas core manufacture depends on molding or a machining process. The cure depends on a programmable heat source. VRTM composites also show excellent resistance to water, solvents, and chemicals. This is largely a function of resin type and surface finish. Rough surfaces pitted with micropores trap water and chemicals and act as tiny reaction chambers that set in motion their own destruction. VRTM yields parts with less than 20-µin. rms (root mean square) porosity. VRTM can achieve this fine finish repeatedly on all surfaces, depending on the finish of the tool. For high-quality finishes, compression and autoclaving processes depend on uniform resin flow under pressure, which they cannot always maintain. The main attraction of VRTM, despite its competitive properties, remains cost, where it offers real advantages over compression molding and autoclaving. Another production process is low-cost VARTM infusion technology. VARTM is becoming a manufacturing method of choice because of its ability to produce fairly large structures out of the autoclave with the high quality usually associated with higher-priced processes.

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VACUUM CARBURIZING Heat treatment with gas quenching has already been an established heat-treatment process for two or three decades in the field of full hardening. At first, it was limited to the hardening of high-alloyed tool steels whose alloy structure enabled them to be hardened satisfactorily with a rather slow gas cooling rate. The enhanced quenching action achieved with gas pressures above 10 bar has allowed successful extension of gas quenching to the field of low alloyed tool steels, steels for hardening and tempering, antifriction-bearing steels, and case-hardening steels. The capability to carburize and gas-quench in vacuum furnace installations has provided the industry with a new, environmentally friendly case-hardening process.

THE PROCESS Like plasma carburizing, vacuum carburizing can also be performed in a vacuum furnace system. Vacuum carburizing can be succinctly described by the following key points: • Carburizing gas is propane. • Pressure ranges up to 20 mbar (absolute). • Temperature range is usually 900 to 1050°C, but higher temperatures are also possible.

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Once the charge has been heated to the carburizing temperature under a neutral atmosphere (vacuum or nitrogen), propane is admitted into the evacuated heating chamber. Propane very rapidly undergoes 100% dissociation into more stable hydrocarbons and hydrogen. Carbon is also released and diffuses through the surface of the steel or component. Vacuum carburizing is characterized by a high carbon mass flow rate, which carburizes the surface layer to near the carbon saturation limit within a short treatment time. In the subsequent diffusion phase, no more carburizing gas is fed in — rather the existing carbon diffuses farther into the steel in accordance with the diffusion law until the desired carbon profile has been attained.


PROCESS COMPARISONS In contrast to protective-gas carburizing, vacuum carburizing can be performed with substantially higher case carbon contents. The case carbon percentage is already over 1.3% after a short period of carburization and then is held at 1.4 to 1.5% at 930°C, which is about 0.2% higher than in protective-gas carburizing with a carbon level just below the sooting limit. The higher case carbon content in vacuum carburizing results in shortened treatment times, even at the same carburizing temperature. Raising the carburizing temperature results in a further considerable time savings. Vacuum carburizing systems readily permit a carburizing temperature of over 1050°C, although the heat treatment racks made of heatresistant cast steel (which are currently in use) are no longer usable at such high temperatures. With racks made of CFC (carbon-fiber-reinforced carbon), the limit is shifted to much higher temperatures. CFC material can only be used in an oxygen-free atmosphere such as that prevailing during vacuum carburizing. Of course, the carburizing action at the point of contact with the component must be taken into consideration. The grain growth of case-hardening steels also does not permit such high temperatures over a long period of time. The vacuum furnace offers a pearlitizing treatment to refine the grain. Despite the time cost for pearlitizing, the result in comparison to the time required in a multipurpose protective-gas chamber furnace is a time savings of about 4 h for the case hardening of a 25% Cr Mo 4 steel to a case depth (550 HV) of 1.7 mm.

DISTORTION Vacuum carburizing with gas quenching offers a potential for reduced parts distortion. A large number of experiments, conducted primarily on transmission parts, have shown that the scatter of the dimensional and shape changes after gas quenching is narrower than after oil quenching. For example, a clutch body (O.D., 84 mm; I.D., 50 mm; height, 15 mm; mass, 0.2 kg each) made of 16% Mn Cr 5 with a case depth (550 HV) of 0.4 to 0.8 mm was tested. The study


Oil Quenching

Quenching with helium at 20 bar

1.6

Case C in %

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 over -30 -29 to -30 -19 to -10 -9 to 0 0 to 9 10 to 19 20 to 29 Change of the run-out in µm

over 30

FIGURE V.1 Comparison of run-out between case hardening in the vacuum furnace with gas quenching and protective-gas carburizing with oil quenching. (From Ind. Heating, January, 54, 2000. With permission.)

evaluated so clutch bodies after case hardening in the vacuum furnace (quenching with helium at 20 bar) and, for comparison, 50 others were evaluated after case hardening in the protectivegas furnace (oil quenching). The radial run-out of the clutch bodies was measured in the soft and hard states. The difference is illustrated in Figure V.1.

ADVANTAGES Case hardening in vacuum heat-treatment systems with gas quenching offers the user many advantages in comparison to conventional protective-gas carburizing with oil quenching. Parts are clean and dry after treatment requiring no washers or management or disposal of liquid waste. Leidenfrost phenomenon is avoided and with more uniform quenching, distortion is minimized. Vacuum carburizing also allows for carburizing at up to 1000°C. As a protective atmosphere, vacuum can prevent case oxidation and eliminate toxic offgases. The vacuum carburizing process also provides a high carbon mass flow rate with low consumption of carburizing gas. With regard to productivity, the vacuum process can be integrated into a production line without the burdening requirements for fire-protection and fireextinguishing systems, excessive heat removal to the surroundings, or extensive exhaust gas handling. In determining the carbon mass flow rate, it becomes clear that in the first few minutes of carburizing in this process (up to 30 min) there is a very high carbon mass flow rate of up to


100 g/m2h. The case-hardening steel can be carburized up to its limit of solubility in the surface layer without any sooting occurring. The system technology also makes it possible to carburize at temperatures above 1000°C. These two factors result in an enormously shortened process duration. The carburization results are comparable with those of the protective-gas process with regard to case depth, case carbon content, and surface hardness. The advantages for component quality lie in reduced distortion. Investigations of various transmission parts have shown that the scatter of the dimensional and shape changes can be narrowed with gas quenching in comparison to oil quenching. The clean surface of the component and the absence of case oxidation after heat treatment are additional advantages of this technology.

VACUUM COATINGS The process of vacuum coating is used to modify a surface by evaporating a coating material under vacuum and condensing it on the surface. It is normally carried out under high vacuum conditions (at approximately 1 millionth of an atmosphere pressure). The material to be evaporated is heated until its vapor pressure appreciably exceeds the residual pressure within the vacuum system. Vacuum coating can be used for many applications. For example, optical lenses are coated with magnesium fluoride to a fraction of a wavelength to prevent glare and provide much

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better transmission of light and a more reliable optical system. The deposited film is extremely adherent and will withstand normal cleaning. Silicon monoxide is frequently used as an abrasion-resistant coating material. As deposited, it is soft and requires postheat treatment in air to convert it to silicon dioxide, which is transparent and extremely hard. It is frequently used to protect front surface mirrors and increases abrasion resistance by a factor of 5000 to 10,000, while maintaining equal or higher reflectivity. Similarly, titanium is sometimes used for coating and is subsequently oxidized to yield a titanium dioxide abrasion-resistant surface. By far the most common type of vacuum coating is the process of vacuum metallizing. In this process metal is evaporated and used as deposited without further treatment as opposed to the evaporation of compounds or materials that require posttreatment. Vacuum metallizing has generally been used as a decorative process whereby costume jewelry, toys, etc. are given a metallic sheen and are made highly reflective. The base material may be either plastic or metal. In either case, the part is frequently lacquered before metallizing to prevent the evolution of gas from the base and to provide a smooth surface without mechanical buffing. Because the metal deposit is only about 2 or 3 millionths of an inch thick, the smooth surface is necessary to give a specular reflection. When the metal is on the outside of the coated part, it is referred to as front surface. However, in applications where it is used on the back of a transparent plastic (e.g., dashboards and taillight assemblies on automobiles) it is referred to as a second surface coating. The advantage of second surface coating is provided by using the plastic as the exposed surface. Front surface coatings must generally be protected with a transparent lacquer overcoat (applied after metallizing) because the thin decorative coatings are not wear resistant in themselves. Aluminum is the most popular vacuummetallizing coating material for most applications. However, other metals may be used, such as zinc, cadmium, copper, silver, gold, or chromium. Of all these metals, aluminum has


the best general combination of reflectivity, conductivity, and stability in air. By adding color to the topcoat lacquer, the aluminum deposit may be made to appear like copper or gold as well as metallic sheens of blues, reds, yellow, etc. By using separate sources for each constituent, it is possible to deposit alloys as well as pure metals. The above applications are for parts produced by batch metallizing; i.e., the individual parts are mounted on racks inserted in the vacuum system and after the necessary vacuum and evaporation temperatures are obtained, the parts are rotated so that they are uniformly coated by the evaporating metal. In batch metallizing the aluminum is evaporated from tungsten filaments, which are heated by direct resistance. Because of this, the amount of aluminum that can be charged is limited and only thin coatings can be produced. Similarly, only small surfaces (a few square centimeters), such as can be exposed within a matter of seconds, can be metallized. When it is desirable to coat larger surfaces, e.g., rolls of flexible material, a semicontinuous metallizing process must be employed. For semicontinuous metallizing a roll of material is mounted in the vacuum chamber and unrolled under vacuum to coat either or both sides of the web, which is subsequently rewound in vacuum. This process is currently in use for coating rolls of plastic sheeting and paper. To coat continuously over a period of hours, it is necessary to have larger volumes of aluminum available for evaporation than can be held on resistance-heated tungsten filaments. Therefore, the aluminum is generally heated by induction in crucibles. Coating of rolls of materials provided one of the first functional applications for coatings that used the electrical conductivity of the metal deposited. This conductive layer was deposited on thin insulating layers of either paper or plastic and could be used for winding miniature condensers. The electrical conductivity is also used in the metallizing process itself as a means of measuring the amount of metal deposited. Since the conductivity is a function of the thickness of the metal, continuously measuring conductivity provides a control for the amount of metal deposited. Other functional uses of the


coating are based on its reflectivity (e.g., reflective insulation). Vacuum metallizing has recently been extended to include thick films, i.e., in the range of 1 to 3 mils. Such coatings serve as corrosionresistant barriers, particularly on high-tensilestrength steel exposed to marine atmospheres. Where the temperature requirements of steel are less than 260°C, cadmium deposits can be used. For temperatures in excess of this, aluminum shows much better protection and does not react with the base steel as cadmium does. Truly continuous operation is necessary for coating rolled steel. Here the rolls are unwound and rewound in air with the strip passing through seals into the vacuum chamber where it is coated. The metallizing of rolled stock allows separate control on each side of the web and the composition of the coating, as well as thickness, may be changed from one side to the other. There are several typical advantages of vacuum metallizing: 1. Close control of coating thickness and composition 2. Uniform deposits without buildup at sharp discontinuities 3. High coating rate Ultrahigh vacuum

4. Low coating costs in volume production 5. Long life of equipment since few moving parts There are also disadvantages of the process: 1. Part must be extremely clean. 2. Surfaces to be metallized must not evolve gas under vacuum. 3. Parts must not be temperature sensitive, i.e., must be stable to about 125°C. 4. Deposits form well only on a surface exposed to hot metal; reentrant angles are not well coated. The cost for metallizing in production lots for corrosion-resistant coatings is comparable to electroplating. Decorative metallizing is generally much less expensive than electroplating.

VACUUM PROCESSING Vacuum processing is used in many industrial applications. Some of these processes and their typical working pressure ranges are shown in Figure V.2. The application of vacuum technology is especially critical to the success of the various coating processes. High vacuum

Medium vacuum

Rough vacuum

Annealing of metals Degassing of metals Electron beam melting Electron beam welding Evaporation Sputtering of metals Casting of resins and lacquers Drying of plastics Drying of insulating papers Freeze-drying of bulk goods Freeze-drying of pharmaceutical products

FIGURE V.2 Pressure ranges for various industrial processes. (From Ind. Heating, September, 113, 2000. With permission.)


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Through the use of vacuum it is possible to create coatings with a high degree of uniform thickness ranging from several nanometers to more than 100 mm while still achieving very good reproducibility of the coating properties. Flat substrates, web and strip, as well as complex molded-plastic parts, can be coated with virtually no restrictions as to the substrate material. The variety of coating materials is also very large. In addition to metal and alloy coatings, layers may be produced from various chemical compounds or layers of different materials applied in sandwich form. A significant advantage of vacuum coating over other methods is that many special coating properties desired, such as structure, hardness, electrical conductivity, or refractive index, are obtained merely by selecting a specific coating method and the process parameters for a certain coating material. Deposition of thin films is used to change the surface properties of a base material or substrate. For example, optical properties such as transmission or reflection of lenses and other glass products can be adjusted by applying suitable coating layer systems. Metal coatings on plastic web produce conductive coatings for film capacitors. Polymer layers on metals enhance the corrosion resistance of the substrate.

through them. However, there are restrictions regarding the type of material to be heated. In some cases, it is not possible to achieve the necessary evaporator temperatures without significantly evaporating the source holder and thus contaminating the coating. Furthermore, chemical reactions between the holder and the material to be evaporated can occur resulting in either a reduction of the lifetime of the evaporator or contamination of the coating.

COATING SOURCES

Cathode Sputtering

In all vacuum coating methods, layers are formed by deposition of material from the gas phase. The coating material may be formed by physical processes such as evaporation and sputtering, or by chemical reaction. Therefore, a distinction is made between physical vapor deposition (PVD) and chemical vapor deposition (CVD).

In the cathode sputtering process, the target, a solid, is bombarded with high energy ions in a gas discharge. The impinging ions transfer their momentum to the atoms in the target material, knocking the atoms off. These displaced atoms — the sputtered particles — condense on the substrate facing the target. Compared to evaporated particles, sputtered particles have considerably higher kinetic energy. Therefore, the conditions for condensation and layer growth are very different in the two processes. Sputtered layers usually have higher adhesive strength and a denser coating structure than evaporated ones. Sputter cathodes are available in many different geometric shapes and sizes as well as electrical circuit configurations. What all sputter cathodes have in common is a large particle source area compared to evaporators, and the

Thermal Evaporators In the evaporation process, the material to be deposited is heated to a temperature high enough to reach a sufficiently high vapor pressure and the desired evaporation or condensation rate is set. The simplest sources used in evaporation consist of wire filaments, boats of sheet metal, or electrically conductive ceramics that are heated by passing an electrical current


Electron Beam Evaporators (Electron Guns) To evaporate coating material using an electron beam gun, the material, which is kept in a water-cooled crucible, is bombarded by a focused electron beam and thereby heated. Since the crucible remains cold, in principle, contamination of the coating by crucible material is avoided and a high degree of coating purity is achieved. With the focused electron beam, very high temperatures of the material to be evaporated can be obtained and thus very high evaporation rates. Consequently, highmelting point compounds such as oxides can be evaporated in addition to metals and alloys. By changing the power of the electron beam, the evaporation rate is easily and rapidly controlled.


capability to coat large substrates with a high degree of uniformity. In this type of process, metals and alloys of any composition, as well as oxides, can be used as coating materials. Chemical Vapor Deposition In contrast to physical vapor deposition methods, where the substance to be deposited is either solid or liquid, in chemical vapor deposition, the substance is already in the vapor phase when admitted to the vacuum system. To deposit it, the substance must be thermally excited, i.e., by means of appropriate high temperatures or with plasma. Generally, in this type of process, a large number of chemical reactions take place, some of which are taken advantage of to control the desired composition and properties of the coating. For example, by using silicon–hydrogen monomers, soft silicon–hydrogen polymer coatings, hard silicon coatings, or — by the addition of oxygen — quartz coatings can be created by controlling process parameters. Web Coating Metal-coated plastic webs and papers play an important role in food packaging. Another important area of application of metal-coated web is the production of film capacitors for electrical and electronics applications. Metal coating is carried out in vacuum web coating systems. The unit consists of two chambers, the winding chamber with the roll of web to be coated and the winding system, as well as the coating chamber, where the evaporators are located. The two chambers are sealed from each other, except for two slits through which the web runs. This makes it possible to pump high gas loads from the web roll using a relatively small pumping set. The pressure in the winding chamber may be more than a factor of 100 higher than the pressure simultaneously established in the coating chamber. During the coating process, the web, at a speed of more than 10 m/s, passes a group of evaporators consisting of ceramic boats from which aluminum is evaporated. To achieve the necessary aluminum coating thickness at these high web speeds, very high evaporation rates


are required. The evaporators must be run at temperatures in excess of 1400°C. Thermal radiation of the evaporators, together with the heat of condensation of the growing layer, yields a considerable thermal load for the web. With the help of cooled rollers, the foil is cooled during and after coating so that it is not damaged during coating and has cooled significantly prior to winding. During the entire coating process, the coating thickness is continuously monitored with an optical measuring system or by means of electrical resistance measurement devices. The measured values are compared with the coating thickness set points in the system, and the evaporator power is thus automatically controlled. Optical Coatings Vacuum coatings have a broad range of applications in production of ophthalmic optics, lenses for cameras, and other optical instruments as well as a wide variety of optical filters and special mirrors. To obtain the desired transmission of reflection properties, at least 3, but sometimes up to 50, coatings are applied to the glass or plastic substrates. The coating properties, such as thickness and refractive index of the individual coatings, must be controlled very precisely and matched to each other. Most of these coatings are produced using electron beam evaporators in single-chamber units. The evaporators are installed at the bottom of the chamber, usually with automatically operated crucibles, in which there are several different materials. The substrates are mounted on a rotating calotte above the evaporators. Application of suitable shielding, combined with relative movement between evaporators and substrates, results in a very high degree of coating uniformity. With the help of quartz coating thickness monitors and direct measurement of the attained optical properties of the coating system during coating, the coating process is fully controlled automatically. One of the key requirements of coatings is that they retain their properties under usual ambient conditions over long periods of time. This requires the production of dense coatings, into which neither oxygen nor water can penetrate. Using glass lenses, this is achieved by

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to backing pumps

Entrance chamber

Transfer chamber 2

Transfer chamber 1

Exit chamber

Sputter chamber

FIGURE V.3 Plant for coating glass panes — three-chamber in-line system with throughput up to 3,600,000 m2/year. (From Ind. Heating, September, 118, 2000. With permission.)

keeping the substrates at temperatures up to 300°C during coating by means of radiation heaters. However, plastic lenses, as those used in eyeglass optics, are not allowed to be heated above 80°C. To obtain dense, stable coatings these substrates are bombarded with argon ions from an ion source during coating. Through ion bombardment, the right amount of energy is applied to the growing layer so that the coated particles are arranged on the energetically most favorable lattice sites, without the substrate temperature reaching unacceptably high values. At the same time, oxygen can be added to the argon. The resulting oxygen ions are very reactive and ensure that the oxygen is included in the growing layer as desired. Glass Coating

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Coated glass plays a major role in a number of applications such as heat-reflecting coating systems on windowpanes to lower heating costs; solar protection coatings to reduce air-conditioning costs in countries with high-intensity solar radiation; coated car windows to reduce the heating-up of the interior; and mirrors used both in the furniture and the automobile industry. Most of these coatings are produced in large in-line vacuum systems such as that shown in Figure V.3. The individual glass panes are transported into an entrance chamber at atmospheric pressure. After the entrance valve is closed, the chamber is evacuated with a forepump set. As soon as the pressure is low enough, the valve to the evacuated transfer chamber can be opened. The glass pane is moved into the transfer chamber and from there at constant speed to the process chambers, where coating is carried out by


means of sputter cathodes. On the exit side, there is, in analogy to the entrance side, a transfer chamber in which the pane is held until it can be transferred out through the exit chamber. Most of the coatings consist of a stack of alternative layers of metal and oxide. Because the metal layers may not be contaminated with oxygen, the individual process stations have to be vacuum-isolated from each other and from the transfer stations. To avoid frequent and undesirable starting and stopping of the glass panes, the process chambers are vacuum-separated through so-called “slit locks,” i.e., constantly open slits combined with an intermediate chamber with its own vacuum pump. The gaps in the slits are kept as small as technically possible to minimize clearance and therefore conductance as the glass panes are transported through them. The pumping speed at the intermediate chamber is kept as high as possible to achieve a considerably lower pressure in the intermediate chamber than in the process chambers. This lower pressure greatly reduces the gas flow from a process chamber via the intermediate chamber to the adjacent process chamber. For very stringent separation requirements, it may be necessary to place several intermediate chambers between two process chambers. The glass coating process requires high gas flows for the sputter processes as well as low hydrocarbon concentration. Turbomolecular pumps are used almost exclusively because of their high pumping speed stability over time. While the transfer and process chambers are constantly evacuated, the entrance and exit chambers must be periodically vented and then evacuated again. Because of the large volumes of these chambers and the short cycle times, a combination of rotary vane pumps and Roots


pumps is typically used to provide the necessary pumping speed. Data Storage Disks Coatings for magnetic- or magneto-optic data storage media usually consist of several functional coatings that are applied to mechanically finished disks. Most disks must be coated on both sides, and there are substantially greater low particle contamination requirements as compared to glass coating. The sputter cathodes in the process stations are mounted on both sides of the carrier so that the front and back of the disk can be coated simultaneously. An entirely different concept is applied for coating of single disks. In this case, the different process stations are arranged in a circle in a vacuum chamber. The disks are transferred individually from a magazine to a star-shaped transport arm. The transport arm cycles one station farther after each process step and in this way transports to substrates from one process station to the next. During cycling, all processes are switched off and the stations are vacuum-linked to each other. As soon as the arm has reached the process position, the individual stations are separated from each other by closing seals. Each station is pumped by means of its own turbomolecular pump and the individual processes are started. By sealing off the process stations, excellent vacuum separation of the individual processes can be achieved. However, since the slowest process step determines the cycle interval, two process stations may have to be dedicated for particularly time-consuming processes.

VANADIUM AND ALLOYS An elementary metal (symbol V), vanadium is widely distributed, and is a pale-gray metal with a silvery luster. Its specific gravity is 6.02, and it melts at 1780°C. It does not oxidize in the air and is not attacked by hydrochloric or dilute sulfuric acid. It dissolves with a blue color in solutions of nitric acid. It is marketed as 99.5% pure, in cast ingots, machined ingots, and buttons. The as-cast metal has a tensile strength of 372 MPa, yield strength of 310 MPa, and elongation of 12%. Annealed sheet has a tensile


strength of 537 MPa, yield strength of 455 MPa, and elongation of 20%, and the cold-rolled sheet has a tensile strength of 827 MPa with elongation of 2%. Vanadium metal is expensive, but is used for special purposes such as for springs of high flexural strength and corrosion resistance. Commercially important as an oxidation catalyst, vanadium also is used in the production of ceramics and as a colorizing agent. Studies have demonstrated the biological occurrence of vanadium, especially in marine species; in mammals, vanadium has a pronounced effect on heart muscle contraction and renal function.

FABRICATION Hot Working Since vanadium oxidizes rapidly at hot-working temperatures, forming a molten oxide, it must be protected during heating. This is most easily accomplished by heating in an inert-gas atmosphere. Other common practices have been found less suitable. Vanadium ingots up to 152 mm in size have been successfully hot-worked, but the degree of contamination is a modifying factor. Generally, the procedures used in working alloy steels apply. In view of the difficulties involved in heating the metal, reheating is generally avoided and the starting temperature is a function of the amount of hot work to be accomplished and of the desired finishing temperature. Starting temperatures can range as high as 1260°C and the finishing temperatures is limited by the beginning of recrystallization. Straightening is performed between 371 and 427°C but not at room temperature. Cold Working Vanadium has excellent cold-working properties, provided its surfaces are uncontaminated. They are therefore machined clean by removing between 0.50 to 1 mm. Strip can be readily made from hot-rolled sections 31 × 152 mm in cross section, and 0.25 mm material has been produced without and 0.03 mm with intermediate annealing. Where

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incipient cracking is observed, vacuum annealing at 899°C becomes necessary. Extrusion is one of the most suitable fabricating methods for vanadium, since warm extrusion followed by cold rolling or drawing avoids hot working with the troublesome heating step. At temperatures below 538°C, tube blanks 50.8 mm outside diameter × 6.4 mm wall thickness have been produced from hot-rolled and turned bars as well as from ingots. Wires can be drawn from 9.5-mm-diameter stock down to 0.025 mm, especially after copper plating. Reductions are usually 10% per pass. In machining, vanadium resembles the more difficult stainless steels. Low speeds with light to moderate feed are used and very light finishing cuts at higher speeds are possible. Welding is not difficult but contamination of the metal must be avoided by shielding from air by means of an inert gas, i.e., argon.

USES

V

AND

APPLICATIONS

The greatest use of vanadium is for alloying. Ferrovanadium, for use in adding to steels, usually contains 30 to 40% vanadium, 3 to 6% carbon, and 8 to 15% silicon, with the balance iron, but may also be had with very low carbon and silicon. Vanadium–boron, for alloying steels, is marketed as a master alloy containing 40 to 45% vanadium, 8% boron, 5% titanium, 2.5% aluminum, and the balance iron, but the alloy may also be had with no titanium. VanAd alloy, for adding vanadium to titanium alloys, contains 75% vanadium and the balance titanium. It comes as fine crystals. The vanadium–columbium alloys containing 20 to 50% columbium, have a tensile strength above 689 MPa at 700°C, 482 MPa at 1000°C, and 275 MPa at 1200°C. Vanadium salts are used to color pottery and glass and as mordants in dyeing. Red cake, or crystalline vanadium oxide, is a reddishbrown material, containing about 85% vanadium pentoxide, V2O5, and 9% Na2O, used as a catalyst and for making vanadium compounds. Vanadium oxide is also used to produce yellow glass; the pigment known as vanadium–tin yellow is a mixture of vanadium pentoxide and tin oxide.


Vanadium is used in the cladding of fuel elements in nuclear reactors because it does not alloy with uranium and has good thermal conductivity as well as satisfactory thermal neutron cross section. Because the metal alloys with both titanium and steel, it has found application in providing a bond in the titanium-cladding of steel. Also, the good corrosion resistance of vanadium offers interesting possibilities for the future; it has excellent resistance to hydrochloric and sulfuric acids and resists aerated salt water very well. But its stability in caustic solutions is only fair and, in nitric acid, inadequate.

BORIDES, CARBIDES,

AND

OXIDES

Vanadium boride, VB, has a melting point of 2100°C with oxidation at 1000 to 1100°C; density 5.1 g/cm3; Mohs hardness 8 to 9; electrical resistivity 16 Ω-cm. It is also formed as VB2. Vanadium carbide, VC, has a density 5.81 g/cm3 and is silver gray in color. It is chemically very stable; among the cold acids, it is attacked only by HNO3. Below 499°C, Cl2 reacts with VC. It burns in oxygen or air, but is stable to 2500°C in nitrogen. VC is harder than corundum. Vanadium pentoxide, V2O5, has a melting point of 690°C and is slightly soluble in water. V2O5 is used by the ceramic industry as coloring agents producing various tints of yellow and greenish yellow. Vanadium pentoxide is an excellent flux and small amounts may be helpful in promoting vitrification of ceramic products. Vanadate glasses are relatively fusible when compared with other oxide types.

VANADIUM STEEL Vanadium was originally used in steel as a cleanser, but is now employed in small amounts, 0.15 to 0.25%, especially with a small quantity of chromium, as an alloying element to make strong, tough, and hard low-alloy steels. It increases the tensile strength without lowering the ductility, reduces grain growth, and increases the fatigue-resisting qualities of steels. Larger amounts are used in high-speed steels and in special steels. Vanadium is a powerful deoxidizer in steels, but is too expensive


for this purpose alone. Steels with 0.45 to 0.55% carbon and small amounts of vanadium are used for forgings, and cast steels for aircraft parts usually contain vanadium. In tool steels, vanadium widens the hardening range, and by the formation of double carbides with chromium makes hard and keen-edge die and cutter steels. All these steels are classed as chromium–vanadium steels. The carbon–vanadium steels for forgings and castings, without chromium, have slightly higher manganese. Vanadium steels require higher quenching temperatures than ordinary steels or nickel steels. Society of the Automotive Engineers (SAE) 6145 steel, with 0.18% vanadium and 1% chromium, has a fine grain structure and is used for gears. It has a tensile strength of 799 to 2013 MPa when heat-treated, with a Brinell hardness 248 to 566, depending on the temperature of drawing, and an elongation of 7 to 26%. In cast vanadium steels it is usual to have from 0.18 to 0.25% vanadium with 0.35 to 0.45% carbon. Such castings have a tensile strength of about 551 MPa and an elongation of 22%. A nickel–vanadium cast steel has much higher strength, but high-alloy steels with only small amounts of vanadium are not usually classed as vanadium steels.

VAPOR-DEPOSITED COATINGS These are thin single or multilayer coatings applied to base surfaces by deposition of the coating metal from its vapor phase. Most metals and even some nonmetals, such as siliconoxide, can be vapor-deposited. Vacuum-evaporated films or vacuum-metallized films were produced by vacuum evaporation. In addition to vacuum evaporation, vapor-deposited films can be produced by ion sputtering, chemical-vapor plating, and a glow-discharge process. The first two are discussed under vacuum processing. In the glow-discharge process, applicable only to polymer films, a gas discharge deposits and polymerizes the plastic film on the base material.


APPLICATIONS Vapor plating is not considered to be competitive with electroplating. Its chief use (present and future) is to apply coating materials that cannot be electroplated, or cannot be applied in a nonporous condition by other techniques. Such materials include titanium, zirconium, columbium, tantalum, molybdenum, and tungsten, and refractory compounds such as the transition metal carbides, nitrides, borides, and silicides. Vapor plating will also continue to be useful in the preparation of ultrahigh-purity metals and compounds for use in electronics applications and in alloy development. A few of the main commercial uses of vapor plating are as follows: 1. The application of high-chromium alloy coatings to iron and steel articles by the displacement-diffusion coating process (known as pack chromizing), for abrasion resistance and for protection from corrosion by food products, strong oxidizing acids, alkalies, salt solutions, and gaseous combustion products at temperatures up to about 800°C. 2. The application of molybdenum disilieide coatings to molybdenum by gas-phase siliconizing, for protection against air oxidation at temperatures between 800 and 1700°C. 3. The preparation of ultrahigh-purity titanium, zirconium, chromium, thorium, and silicon by iodide vapor decomposition processes. 4. The preparation of junction transistors by the controlled diffusion of boron from boron halide into the surface of silicon or germanium wafers. 5. The preparation of oriented graphite plates and shapes (pyrolytic graphite), by the high-temperature pyrolysis of hydrocarbon gases, for use in rocket and missile applications. In addition, the following coatings have been developed:

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1. Tantalum coatings on iron and steel for corrosion resistance 2. Vanadized, tungstenized, and molybdenized iron for wear resistance 3. Tungsten coatings on copper x-ray and cyclotron targets 4. Conductive metallic coatings on glass, porcelain, alundum, porous bodies, rubber, and plastics 5. Refractory metal coatings on copper wires 6. Oxidation-resistant carbide coatings on graphite tubes, nozzles, and vanes 7. Metallic coatings of all types on metallic and nonmetallic powders 8. Decorative, colored coatings on glass 9. High-purity boron, rhenium, vanadium, germanium, and aluminum The displacement-diffusion plating processes, such as pack-chromizing, can plate uniformly somewhat larger pieces and more complex shapes with inaccessible areas. Sheets and rod up 0.6 to 0.9 m in dimension have been coated, and no technical obstacles are seen to scaling up the processes to coat even larger pieces. The pack coating processes have the advantage of minimizing the problems of specimen support and warpage during plating. Plating uniformity varies somewhat with the particular process used, with the shape of the object being plated, and with the attention given to providing proper gas flow around the object. A variation in thickness of 10 to 25% is usually obtained. However, some coating processes can be made self-limiting so that the variation in coating thickness is much less than this range.

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DISADVANTAGES Vapor plating has the following disadvantages: 1. Relative instability and air and moisture sensitivity of most of the compounds used as plating agents 2. A tendency to produce nonuniform deposits due to unfavorable gas flow patterns around the work, or to uneven specimen temperature


3. Alteration of physical properties of the substrate due to the elevated processing temperatures 4. The possibility of poor coating quality arising from undesired side reactions in the plating process In general, the materials used as plating compounds in vapor plating are relatively unstable and easily decomposed by air and moisture, thus rendering them more expensive and more difficult to store and handle than the compounds used in other plating techniques. Also, some of the metal carbonyls, hydrides, and organometallic compounds are highly toxic, and some of the hydrides and metal alkyls inflame spontaneously upon contact with air. To develop optimum properties in deposits of many materials, the plating compounds, particularly the moisture-sensitive metal halides, must be purified and used without contamination from the atmosphere. This apparent disadvantage is sometimes put to good use, however, when intentional contamination of the coating atmosphere with nitrogen or moisture is used to produce harder deposits (e.g., of titanium or tantalum), or to reduce the codeposition of carbon (e.g., with molybdenum from the carbonyl). Nonuniform plating may result in all vaporplating processes, except the displacement-diffusion process, if consideration is not given to the gas-flow pattern around, or through, the article being coated. The shape factor may also have to be taken into account in selecting the method of heating the article, to avoid nonuniform deposition due to nonuniform heating of the part. These difficulties can be overcome in extreme cases by applying more than one coating and using a different direction of gas flow over the specimen for each application. If necessary, the displacement-diffusion type of coating process can be carried out at very low gas flow rates (since solid-state diffusion is the rate-controlling factor), and still produce uniform coatings. For this reason, this type of coating process is ideally suited for coating large, or highly irregular objects, or large numbers of small objects. The elevated processing temperatures required in vapor plating may produce undesired physical changes in the article, such as loss


of temper, grain growth, warping, dimensional change, or precipitation or solution of alloying constituents. However, in many instances a vapor-plating procedure can be selected that will avoid marked undesirable change. Undesirable side reactions in vapor-plating processes must be watched for and avoided. A particularly troublesome one in plating from metal halide vapors is the interaction of the base material and the halide vapor to form lowervalent halides, either of the base material or of the coating vapor. If the substrate temperature is too low, or the plating atmosphere too rich in plating vapor, these lower-valent halides will condense at the surface of the substrate, producing a plate underlaid or contaminated with halide salts. Such deposits are always poorly adherent, porous, and sensitive to moisture. Incomplete reduction or decomposition of the plating vapor alone can produce the same result. Contamination of this type is less likely to occur when plating inert base materials such as graphite, glass, and some ceramics. After having been plated in a hydrogen atmosphere, some metals such as tantalum, columbium, and titanium, with a strong affinity for hydrogen, must be vacuum-annealed, or at least cooled in an inert-gas atmosphere to avoid excessive hydrogen absorption and embrittlement. Also, carburization of the substrate may occur in processes employing the metal carbonyls to coat metals with a strong affinity for carbon, if the substrate temperature is too high. The metal of the deposit itself may be partially carburized in some cases, as when depositing molybdenum, chromium, and tungsten from their carbonyl vapors.

VINYL ACETATE ETHYLENE Since their introduction, vinyl acetate ethylene (VAE) copolymer emulsions have been a staple base for adhesive manufacturers. As the performance requirements within the packaging and construction markets have increased and diversified, so too has the use of these emulsions. First, VAE copolymer emulsions offer a tremendous balance between performance properties and ease of use. The internal plasticization of the vinyl acetate with ethylene gives


these emulsions adhesion to many difficult-toadhere substrates while the polyvinyl alcohol (PVOH) stabilization system provides for high wet tack, good setting speeds, and excellent machinability. Second, manufacturers of VAE emulsions have continued to advance the performance capabilities of these materials. Available today are functionalized VAE systems for adhesion to metalized surfaces, a range of glass transition temperatures for specific film properties, low volatile organic compound emulsions for sensitive food packaging applications, and higher solids technologies as an alternative to nonwaterbased systems. With these new VAE copolymer emulsions, adhesive compounders are better able to address the ever-changing needs of the adhesive industry. Last, these types of VAE emulsions are made even more versatile by their ability to be compounded with other raw materials and polymer systems. The additional formulations that can result from their compatibility with plasticizers, resins, fillers, humectants, surfactants, polyvinyl alcohol, etc. can offer various improvements in adhesion, tack, heat/cold resistances, flame retardancy, and range.

PROCESSING

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APPLICATIONS

The most recent advance in VAE emulsion technology has been the introduction of a PVOH stabilized, ultrahigh-solids copolymer emulsion that is polymerized at 72% solids and a 2000 cps viscosity. Its composition, structure, and colloidal properties provide faster setting speeds, higher wet tack, and improved adhesion to difficult-to-adhere substrates than was thought possible for VAE emulsions a few years ago. These performance features are allowing adhesive compounders to broaden greatly the applications utilizing waterborne technologies. Polyurethanes have been available as adhesives for quite some time and are commonly found in vacuum-forming and plastics-bonding operations within the automotive and footwear industries. During the last 5 years waterborne urethane chemistry has undergone a significant transformation from solvent-borne or high cosolvent-containing polymer systems to 100% waterborne systems.

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These aqueous polyurethane dispersions, like their solvent-borne counterparts, have some unique performance characteristics. They offer low heat reactivation temperatures, good adhesion to difficult-to-bond substrates, rapid green strength development, and high temperature heat resistance. However, they also have some significant drawbacks. They are low in solids, low in wet tack, slow drying, and relatively high in cost. The blending of polymers to improve adhesive properties is already widely done in the industry. With the commercially available aqueous polyurethane dispersions on the market today, the opportunity exists to enhance the performance of ultrahigh-solids VAE emulsions through blending because these technologies are so complementary. They are both 100% waterborne, and the characteristics of the ultrahigh-solids VAE emulsion can compensate for the disadvantages of urethane with its speed of set, wet tack, and minimal water content.

chloride, vinyl acetate, or vinylidene chloride, but may include plastics made from styrene and other chemicals. The term is generic for compounds of the basic formula RCH:CR′CR″. The simplest are the polyesters of vinyl alcohol, such as vinyl acetate. This resin is lightweight, with a specific gravity of 1.18, and is transparent, but it has poor molding qualities and its strength is no more than 34 MPa. But the vinyl halides, CH2:CHX, also polymerize readily to form vinylite resins, which mold well, have tensile strengths to 62 MPa, high dielectric strength, and high chemical resistance, and a widely useful range of resins is produced by copolymers of vinyl acetate and vinyl chloride. The possibility of variation in the vinyl resins by change of the monomer, copolymerization, and difference in compounding is so great that the term vinyl resin is almost meaningless when used alone. The resins are marketed under a continuously increasing number of trade names. In general, each resin is designed for specific uses, but is not limited to those uses.

FUTURE Through the blending of an ultrahigh-solids VAE emulsion with many of the commercially available aqueous polyurethane dispersions on the market today, adhesive compounders can create a new class of stable high-performance waterborne adhesives. Depending on the urethane grade selected and the level incorporated in the blend, the performance properties of the ultrahigh-solids VAE emulsion can be dramatically enhanced in several areas:

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• • • •

Cohesive strength Adhesion (vinyl) Heat sealability Cross-linker performance

VINYL RESINS AND PLASTICS These are a group of products varying from liquids to hard solids, made by the polymerization of ethylene derivatives, employed for finishes, coatings, and molding resins, or it can be made directly by reacting acetic acid with ethylene and oxygen. In general, the term vinyl designates plastics made by polymerizing vinyl


VINYL ALCOHOL Vinyl alcohol, CH2:CHOH, is a liquid boiling at 35.5°C. Polyvinyl alcohol is a white, odorless, tasteless powder which on drying from solutions forms a colorless and tough film. The material is used as a thickener for latex, in chewing gum, and for sizes and adhesives. It can be compounded with plasticizers and molded or extruded into tough and elastic products. Hydrolyzed polyvinyl alcohol has greater water resistance, higher adhesion, and its lower residual acetate gives lower foaming. Soluble film, for packaging detergents and other waterdispersible materials to eliminate the need of opening the package, is a clear polyvinyl alcohol film. Textile fibers are also made from polyvinyl alcohol, either water soluble or insolubilized with formaldehyde or another agent. Polyvinyl alcohol textile fiber is hot-drawn by a semimelt process and insolubilized after drawing. The fiber has a high degree of orientation and crystallinity, giving good strength and hot-water resistance. Vinyl alcohol reacted with an aldehyde and an acid catalyst produces a group of polymers known as vinyl acetal resins, and separately


designated by type names, as polyvinyl butyral and polyvinyl formal. The polyvinyl alcohols are called Solvars, and the polyvinyl acetates are called Gelvas. The vinyl ethers range from vinyl methyl ether, CH2:CHOCH3, to vinyl ethylhexyl ether, from soft compounds to hard resins. Vinyl ether is a liquid that polymerizes, or that can be reacted with hydroxyl groups to form acetal resins. Alkyl vinyl ethers are made by reacting acetylene with an alcohol under pressure, producing methyl vinyl ether, ethyl vinyl ether, or butyl vinyl ether. They have reactive double bonds that can be used to copolymerize with other vinyls to give a variety of physical properties. The polyvinyl formals, Formvars, are used in molding compounds, wire coatings, and impregnating compounds. They are one of the toughest of the thermoplastics.

PLASTISOL A plastisol is a vinyl resin dissolved in a plasticizer to make a pourable liquid without a volatile solvent for casting. The poured liquid is solidified by heating. Plastigels are plastisols to which a gelling agent has been added to increase viscosity. The polyvinyl acetals, Alvars, are used in lacquers, adhesives, and phonograph records. The transparent polyvinyl butyrals, Butvars, are used as interlayers in laminated glass. They are made by reacting polyvinyl alcohol with butyraldehyde, C3H7CHO. Vinal is a general name for vinyl butyral resin used for laminated glass.

VINYL ACETATE Vinyl acetate is a water-white mobile liquid with boiling point 70°C, usually shipped with a copper salt to prevent polymerization in transit. The composition is CH3:COO:CH:CH2. It may be polymerized in benzene and marketed in solution, or in water solution for use as an extender for rubber, and for adhesives and coatings. The higher the polymerization of the resin, the higher the softening point of the resin. The formula for polyvinyl acetate resin is given as (CH2:CHOOCCH3)x. It is a colorless, odorless thermoplastic with density of 1.189, unaffected by water, gasoline, or oils, but soluble in the lower alcohols, benzene, and chlorinated


hydrocarbons. Polyvinyl acetate resins are stable to light, transparent to ultraviolet light, and are valued for lacquers and coatings because of their high adhesion, durability, and ease of compounding with gums and resins. Resins of low molecular weight are used for coatings, and those of high molecular weight for molding. Vinyl acetate will copolymerize with maleic acrylonitrile or acrylic esters. With ethylene it produces a copolymer latex of superior toughness and abrasion resistance for coatings.

VINYL BENZOATE Vinyl benzoate is an oily liquid of the composition CH2:CHOOCC6H5, which can be polymerized to form resins with higher softening points than those of polyvinyl acetate, but that are more brittle at low temperatures. These resins, copolymerized with vinyl acetate, are used for water-repellent coatings. Vinyl crotonate, CH2:CHOOCCH:CHCH3, is a liquid of specific gravity of 0.9434. Its copolymers are brittle resins, but it is used as a cross-linking agent for other resins to raise the softening point and to increase abrasion resistance. Vinyl formate, CH2:CHOOCH, is a colorless liquid that polymerizes to form clear polyvinyl formate resins that are harder and more resistant to solvents than polyvinyl acetate. The monomer is also copolymerized with ethylene monomers to form resins for mixing in specialty rubbers. Methyl vinyl pyridine, (CH3)(CHCH2)C5H3N, is used in making resins, fibers, and oil-resistant rubbers. It is a colorless liquid boiling at 64.4°C. The active methyl groups give condensation reactions, and it will copolymerize with butadiene, styrene, or acrylonitrile. Polyvinyl carbazole, under the name of Luvican, is used as a mica substitute for high-frequency insulation. It is a brown resin, softening at 150°C.

VINYL CHLORIDE Vinyl chloride, CH2CHCl, also called ethenyl chloride and chloroethylene, produced by reacting ethylene with oxygen from the air and ethylene dichloride, is the basic material for the polyvinyl chloride resins. It is a gas. The plastic was produced originally for cable insulation and for tire tubes. The tensile strength of the

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plastic may vary from the flexible resins with about 20 MPa to the rigid resin with a tensile strength to 62 MPa and Shore hardness of 90. The dielectric strength is high, up to 52 × 106 V/m. It is resistant to acids and alkalies. Unplasticized polyvinyl chloride is used for rigid chemical-resistant pipe. Polyvinyl chloride sheet, unmodified, may have a tensile strength of 57 MPa, flexural strength 86 MPa, and a light transmission of 78%.

POLYVINYL CHLORIDE Polyvinyl chloride (PVC) is a thermoplastic polymer formed by the polymerization of vinyl chloride. Resins of different properties can be made by variations in polymerization techniques. These resins can be compounded with plasticizers, color, mineral filler, etc. and processed into usable forms, varying widely in physical and electrical properties, chemical resistance, and processing versatility in coloring and design. Compared with other thermoplastics of comparable cost, articles produced from the vinyl chloride plastics have outstanding chemical, flame, and abrasion resistance, tensile properties, and resistance to heat distortion. PVC homopolymer resins, the largest single type of vinyl chloride-containing plastics, are produced by several methods of polymerization:

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1. Suspension: The largest-volume method that produces resins for general-purpose use, processed by calendering, injection molding, extrusion, etc. 2. Mass or solution: Produces fine particle size resins used principally for calendering and solution coating. 3. Emulsion: Produces extremely fine particle size resins used for the preparation of liquid plastisols or organosols for use in slush molding, coatings, and foam. The two largest-volume members of the family of vinyl chloride polymers are the pure polyvinyl chloride or homopolymer resins and the vinyl chloride–vinyl acetate copolymers containing approximately 5 to 15% vinyl acetate.


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Rigid Vinyls Products made from rigid vinyls are perhaps of most interest in the engineering field. Rigid materials can be prepared by calendering, extrusion, injection molding, transfer molding, and solution casting processes. Rigid PVC products are available in sheets, films, rods, pipes, profiles, valves, nuts and bolts, etc. The products can be machined easily with woodand metalworking tools. Sheets or other forms can be conventionally welded by hot-air guns, using extruded welding rods of essentially the same composition as that of the sheet. The welded joints have strength equal to that of the base material. Rigid sheets can be thermoformed into many intricate shapes by several different thermoforming techniques such as vacuum forming, ring and plug forming, etc. Rigid pipe can be threaded and joined like steel pipe or sealed with adhesives in a manner similar to the sweating of copper pipe. Vinyl pipe is being used increasingly in waterworks, the petroleum industry, in natural gas distribution, irrigation, hazardous chemical application, and food processing. Rigid vinyl made from vinyl acetate–vinyl chloride copolymers is prominent in sheeting used for thermoforming for such items as maps, packaging, advertising displays, toys, etc. Rigids made from homopolymer vinyl chloride reins are used in heavier structural designs, for example: pipe, pipe valve, heavy panels, electrical ducting, window and door framing parts, architectural moldings, gutters, downspouts, automotive trim, etc. In these fields, rigid vinyls compete with aluminum and other metals. Rigid vinyl products made from homopolymer resins are available in two types: Type I, or unmodified PVC, is approximately 95% PVC and has outstanding chemical resistance but low impact strength; Type II PVC, containing 10 to 20% of a resinous or rubbery polymeric modifier, has improved impact strength but reduced chemical resistance. Flexible Vinyls Flexible vinyl products are produced by the same general methods used for rigid vinyl


products. Flexibility is achieved by the incorporation of plasticizers (mainly high-boiling organic esters) with the vinyl polymer. By proper choice of plasticizer type, flexible products can be obtained that excel in certain specific properties such as gasoline and oil resistance, low temperature flexibility, flame resistance, etc. The flexible sheeting and film can be fabricated by heat sealing to itself or other substrates by induction or high-frequency methods, solvent sealing, sewing, etc. Flexible vinyl film and sheeting find application in upholstery, packaging, agriculture, etc. The corrosion resistance of vinyl sheeting makes it ideal for a pipe wrap to prevent corrosion of underground installations. Flexible extrusions in many different shapes and forms have application as insulating and jacketing on electrical wire and cable, refrigerator gaskets, weather stripping, upholstery, and shoe welting. Injection-molded flexible vinyl products are used as shoes, electrical plugs, and insulation of various sorts. Abrasion and stain resistance, coupled with unlimited coloring and design possibilities, have made flexible vinyl flooring one of the largest items in the floor-covering field. The major revolution in vinyl flooring is the greater emphasis on the use of relatively low to very low molecular weight homopolymer resins in place of the more expensive vinyl chloride–vinyl acetate copolymers. Coatings Coatings based on PVC polymers and copolymers can be applied from solutions, latex, plastisols, or organosols. Plastisols are liquid dispersions of fine particle size emulsion PVC in plasticizers. Organosols are essentially the same as plastisols but contain a volatile liquid organic diluent to reduce viscosity and facilitate processing. In many coating operations, conventional paint-spraying equipment is used. In other techniques, articles can be dip-, knife-, or roller-coated. Plastisols and organosols are used extensively for dip coating of wire products such as household utensils and knife coating of fabrics and paper. Plastisol and organosol products can be varied from hard (rigidsols) to very soft (vinyl foam). Vinyl coatings are used for a


variety of applications requiring corrosion and/or abrasion resistance. Production coatings with vinyl plastics involve a fluidized-bed technique. A metallic object, heated to 204 to 260°C, is immersed in a bed of finely ground plastic which is “fluidized” by air entering the bottom of the container. Articles can be coated by this method to a thickness of 7 to 60 mils. Both rigid and plasticized vinyls can be applied by this method. Vinyl–Metal Laminates These products are made by direct lamination of preprocessed, embossed, and designed vinyl sheet to metal, or continuous plastisol coating of metal sheet followed by fusing or curing of the plastisol and subsequent embossing. In the former process, the vinyl can be laminated to both sides of the metal sheet. Steel, aluminum, magnesium, brass, and copper have been used. The hardness, elongation, general properties, and thickness of the vinyl can be modified within wide limits to meet particular needs. These laminates are dimensionally stable below 100°C and combine the chemical and flame resistance, decorative and design possibilities of vinyl with the rigidity, strength, and fabricating attributes of metals. The vinyl–metal laminates can be worked without rupture by many of the metalworking techniques, such as deep-drawing, crimping, stamping, punching, shearing, and reverse-bending. Disadvantages are inability to spot-weld, and lack of covering of metal edges, which is necessary where severe exposure conditions are encountered. Vinyl–metal laminates find use in appliance cabinets, machine housings, lawn and office furniture, automotive parts, luggage, chemical tanks, etc. The cost of these laminates is comparable to some lacquered metal surfaces.

VINYLIDENE CHLORIDE PLASTICS Vinylidene chloride plastics are derived from ethylene and chlorine polymerized to produce a thermoplastic with softening point of 116 to 138°C. The resins are noted for their toughness and resistance to water and chemicals. The molded resins have a specific gravity of 1.68 to

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1.75, tensile strength 27 to 48 MPa, and flexural strength of 103 to 117 MPa. Saran is the name of a vinylidene chloride plastic, extruded in the form of tubes for handling chemicals, brines, and solvents to temperatures as high as 135°C. It is also extruded into strands and woven into a box-weave material as a substitute for rattan for seating. Saran latex, a water dispersion of the plastic, is used for coating and impregnating fabrics. For coating food-packaging papers, it is waterproof and greaseproof, odorless and tasteless, and gives the papers a high gloss. Saran is also produced as a strong transparent film for packaging. Saran bristles for brushes are made in diameters from 0.025 to 0.051 cm. Applications

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The two largest-volume applications are in upholstery made from monofilaments and film for food packaging. Other uses are in window screening (monofilaments), paper and other coatings, pipe and pipe linings, and staple fiber. Saran pipe and saran-lined metal pipe are of interest to the engineering field. Saran-lined pipe is prepared by swaging an oversize metal pipe on an extruded saran tube. These products can be installed with ordinary piping tool. Fittings and valves lined with saran and flange joints with saran gaskets are available. Vinylidene chloride–acrylonitrile copolymer has applications as coatings for tank car and ship-hold linings. Lacquers of these polymers are used in cellophane coatings yielding a product with the low moisture vapor permeability of vinylidene chloride polymers plus the handling ease of cellophane. The lacquers are also used for paper coatings, dip coatings, and sprayed packaging. Vinylidene chloride copolymers in latex form are used in paper coatings and specialty paints.

VINYLIDENE FLUORIDE Vinylidene fluoride, CH 2 :CF 2 , has a high molecular weight, about 500,000. It is a hard, white thermoplastic resin with a slippery surface and has a high resistance to chemicals. It resists temperatures to 343°C, and does not


become brittle at low temperatures. It extrudes easily, and has been used for wire insulation, gaskets, seals, molded parts, and piping.

VOLATILE ORGANIC COMPOUND Many efforts have been under way regarding the biological control of volatile organic compound (VOC) emissions. Water-based and hot-melt adhesives and coatings have been developed and evaluated extensively, but they are not satisfactory for all applications and may require solvent-based cleaners and primers. Various developments have shown how biological treatment can reduce solvent levels in air emissions and focuses in particular on footwear production. It is, however, applicable to all industries using organic solvents. For example, the U.K. Environmental Protection Act 1990 sets a 5 metric tons/year adhesive solvent usage threshold, above which processes are subject to local authority air pollution control. This regulation apparently affects manufacturing plants producing as few as 5000 pairs of shoes per week. By June 1998 shoemakers and material suppliers were set to meet an emission limit of 50 mg/m3 measured as carbon, or a stringent mass emission control regime of 20 g/pair for footwear; similar controls are anticipated throughout Europe. Catalytic combustion has been shown to be technically effective, but involves high capital and running costs. For example, adsorption on activated carbon is an established technique, but apparently it is unsuitable for the mixed cocktail of solvents in footwear production. Biological treatment processes are expected to be less costly and more suitable for arresting emissions of moderate concentration at ambient temperature, as found in shoe factory exhausts. Biological treatment has been shown suitable for halting VOC emissions from the manufacturing industry with an on-site biological treatment unit set up to demonstrate VOC abatement.


PROCESS The process biologically breaks down VOCs into biomass by mineralization and utilization of the carbon; the by-products of breakdown are carbon dioxide and water vapor. Within the system, microorganisms grow as a biofilm on selected media where they produce enzymes to break down the VOC contaminants. Liquid is continuously recirculated to solubilize the solvents from the contaminated inlet gas. Water is sprayed evenly where the microorganisms oxidize the solvents, and monitoring of the recirculation liquid for determinants, including pH, nitrogen, phosphorus, suspended solids, and so on apparently allows for tighter process control and greater removal efficiency. In this system there is a large interface between gas and liquid phases so that the bed can capture the compounds of poor solubility that are present in the off-gases. Inputs to the process include mains water supply, electricity to run pumps, solenoid valves, and a programmable logic controller, as well as nutrients to maintain the biomass, including nitrogen, phosphate, sulfate, and trace elements that are added according to the carbon load to the reactor. Although this process was used at a footwear manufacturing plant, biotechnology also has application within other industries producing low-concentration mixed solvent waste streams, where recovery is inappropriate and the capital and operating costs of thermal and catalytic systems can be prohibitively expensive. Examples include printing, painting, laminating, metal and leather finishing, and furniture coating. In another example, the U.S. Navy is currently studying a new environmentally safe paint coating for use on its fleet of helicopters. The three-shade flat, haze gray coats were sprayed on a new Navy fleet combat support helicopter. What makes the coating environmentally safe is the absence of VOCs that contribute to air pollution. The new coating eliminates the use of chemicals targeted by the federal government for reduction or elimination. Normal Environmental Protection Agency (EPA) -compliant aircraft coatings currently used bT-fy contain about 3.5 lb of VOC/gallon, whereas this new paint has zero VOC.


The zero-VOC paint was developed by Deft Coatings, Inc. This zero-VOC coating also offers a significant weight benefit compared to the current paint and is nonflammable. The first zero-VOC-coated helicopter is currently undergoing evaluation by the Navy to ensure that the new coating meets stringent Military Standard requirements.

VULCANIZED FIBER Vulcanized fiber is a pure, dense, cellulosic material with good electrical insulating properties and high mechanical strength. It is half the weight of aluminum, easily machined and formed, and is used for parts such as for barriers, abrasive-disk backing, high-strength bobbin heads, materials-handling equipment, railroad-track insulation, and athletic guards.

FORMS Most manufacturers provide vulcanized fiber in the form of sheets, coils, tubes, and rods. Sheets are made in a thickness of 0.06 to 50.8 mm, approximately 1.2 × 2 m in size, or in rolls and coils from 0.06 to 2.3 mm thick. Tubes are made in the outside diameter range of 4.7 to 111.5 mm, and rods are produced 2.3 to 50.4 mm in diameter. Sheets can be machined and formed to produce a variety of useful shapes for insulating or shielding purposes. Sheets, tubes, and rods can be machined using standard practices for cutting, punching, tapping, milling, shaping, sanding, etc.

PROPERTIES Vulcanized fiber possesses a versatile combination of properties, making it a useful material for practically all fields. It has outstanding arc resistance, high structural strength per unit area, and can be formed and machined. In thin sections it possesses high tear strength, smoothness, and flexibility. In heavier thicknesses it resists repeated impact and has high tensile, flexural, and compressive strength. The material is unaffected by normal solvents, gasoline, and oils, and therefore is recommended for applications where a structural

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support is required in the presence of these materials. Moisture absorption is high and dimensional stability is affected by conditions of humidity when not protected by moisture-resistant coatings. Vulcanized fiber is produced in 13 basic grades and numerous special grades to meet specific application requirements.

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Vulcanized fiber serves as the insulating material in a signal block for railroad track insulations. At the end of a signal station, the rails are completely insulated from the next adjoining section to form what is termed a block. The two meeting rails and the coupling fixtures are insulated with formed parts made from vulcanized fiber. This junction is effective while absorbing the repeated impact from trains under all weather conditions. Vulcanized fiber offers durability, ease of fabricating, excellent wear characteristics, and lightness of weight for materials-handling and luggage applications. The materials-handling equipment resists scuffing, battering, denting, rusting, and other general wearing conditions and provides protection by its hardness and resilience. Formed pieces of vulcanized fiber offer outstanding service as arc barriers in circuit breakers. The arc-resistant properties of vulcanized fiber prevent a breakdown when the circuit breaker is subjected to an overload. The formed barrier is tested to take higher electrical loads than the maximum that can be produced by the circuit and, since repeated circuit breaks will not affect the performance of vulcanized fiber, the need for replacement is negligible. Peerless control tape is used for programming data, processing equipment, or automatic machining equipment. The special properties that give this material outstanding service life are high tensile strength, high tear strength, low stretch, and good abrasive resistance. Flame-resistant vulcanized fiber gives designers a structural material that can be used in those applications requiring a nonburning material and reduces fire hazards by containing a fire at its source. Flame-retardant parts serve


as barriers in electrical equipment, materialshandling equipment, and wastebaskets.

CHEMISTRY Vulcanized fiber is produced by the chemical action of zinc chloride solution on a saturating grade of absorbent paper when processed under heat and pressure. The action of the zinc chloride converts the cellulosic fibers to a dense, homogeneous structure producing a laminated material that is refined to a chemically pure form. Final processing consists of drying to the proper moisture content and applying the proper calender to give smoothness and uniformity of thickness.

GRADES The 13 basic grades are as follows: Electrical insulation grade. Primarily intended for electrical applications and others involving difficult bending or forming operations. It is sometimes referred to as “fishpaper.” Commercial grade. Considered to be the general-purpose grade, sometimes referred to as “mechanical and electrical grade.” It possesses good physical and electrical properties and fabricates well. Bone grade. Characterized by greater hardness and stiffness associated with higher specific gravity. It machines smoother with less tendency to separate the plies in the machining operations. Trunk and case grade. Conforms to the mechanical requirements of “commercial grade,” but has better bending qualities and smoother surface. Flexible grade. Made sufficiently soft by incorporating a plasticizer, it is suitable for gaskets, packings, and similar applications. It is not recommended for electrical use. Abrasive grade. Designed as the supporting base for abrasive grit for both disk and drum sanders. It has exceptional tear resistance, ply adhesion, resilience, and toughness.


White tag grade. It has smooth clean surfaces and can be printed or written on without danger of ink feathering. Bobbin grade. Used for the manufacture of textile bobbin heads. It punches well under proper conditions, but is firm enough to resist denting in use. It machines to a very smooth surface. Railroad grade. Used as railroad track joint, switch rods, and other insulating applications for track circuits. Hermetic grade. Used as electricmotor insulation in hermetically sealed refrigeration units. High purity and low methanol extract-

ables are essential because it is immersed in the refrigerant. White grade. Recommended for applications where whiteness and cleanliness are essential requirements. Shuttle grade. Designed for gluing to wood shuttles to withstand the repeated pounding received in textile power looms. Pattern grade. Made to provide maximum dimensional stability and minimum warpage for use as patterns in cutting cloth, leather, and similar materials.

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W WASH PRIMERS Wash primers are a special group of corrosioninhibitive coatings designed for use on clean metal surfaces. They are also known as “washcoat primers,” “metal conditioners,” and “etch primers.” The most widely utilized primers consist of a two-part system that is prepared at the point of use by simple mixing of specified proportions. The base grind portion contains a corrosion-inhibiting pigment, basic zinc chromate (also known as zinc tetroxy chromate), and a small amount of talc extender ground in an alcohol solution of polyvinyl butyral resin. The reducer portion consists of phosphoric acid, alcohol, and water. When these are mixed, a slow chemical reaction ensues, resulting in partial reduction of the chromate pigment. The life of the mixed primer is usually 8 to 12 h. Singlepackage primers are now in use.

WATCH A watch is a portable timepiece. Its operation may be described as mechanical, electromechanical, or electronic.

MECHANICAL WATCHES

AND

ELECTROMECHANICAL

In the mechanical watch a mainspring in the barrel stores operating energy; the user retightens the spring daily by means of the winding stem. The wheel train advances at five increments per second under control by the escapement. From there the dial train turns the minute and hour hands across the watch face. The momentarily engageable setting feature enables the user to position the hands in accordance with a primary clock. The wheel train of four


pairs, with an overall turns ratio of 1:40,000, reduces the high torque from the barrel to a low value controllable by the escapement, yet sufficient to drive the dial train. Among the variety of features incorporated into modern watches are self-winding mechanism, substitution of an electrochemical cell for the mechanical mainspring, several forms of electromechanical escapement in place of the balance-and-hairspring mechanism, and instead of hands over a dial, marked disks viewed through windows for readout, extended to days of the week in calendar watches. A significant improvement in accuracy of the electromechanical watch is provided by relocation of the time base mechanism from the output of the wheel train to the input. Instead of deriving power from a mainspring, this arrangement obtains power from a dry cell. In place of an escapement at the end of the wheel train to control its incremental advance, a tuning fork as a resonant element in an electronic oscillator reciprocates an index finger that rapidly ratchets against a fine-toothed index wheel at the input to the wheel train to initiate its advance. Through these actions the tuning fork with its drive circuit serves both as time base and as electrical-to-mechanical transducer, introducing power from the dry cell into the wheel train at an intermediate level of torque. In one style of this watch the tuning fork vibrates at 360 Hz; in a smaller style the fork resonates at 480 Hz.

ELECTRONIC WATCHES When solid-state electronic integrated circuits became available in quantity from production stimulated by digital computers, the all-electronic watch became a commercial reality. In it, a chain of binary dividers triggered from a

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crystal oscillator develops a train of second pulses. These pulses drive a digital counter or scaler, which develops minute and hour pulses to activate the digital display. An externally switched fast/slow capacitor in the crystal oscillator enables the user to set the readout in accordance with a standard time signal. The frequency of the crystal oscillator, typically in the tens of kilohertz, is chosen so that successive divisions by 2 produce the desired 1-s pulse rate. For example, an oscillator with a 65.536kHz crystal is followed by 16 binary dividers. Power may be supplied by mercury or silver oxide cells, which are replaced annually, or lithium batteries, which operate for up to 5 years. To extend operating life, a solar cell may charge a nickel–cadmium power cell while the watch is illuminated. One form of readout uses a light-emitting diode (LED). An assembly of these on a monolithic chip illuminates appropriate bars of a seven-segment display for each digit of the readout. The LED display is self-illuminating and can be read in the dark. Because illumination of the readout consumes most of the power in an electronic watch, a liquid crystal display (LCD) is used where low power consumption is a first consideration. The LCD readout depends for its indication on ambient illumination; the display is brighter in incident light. In it, glass plates confine a thin layer of liquid crystal. On the inside surface of the front plate a transparent metallic coating in the seven-segment pattern receives signals from the readout counter. A highly reflective metal coating on the inside surface of the back plate operates at ground potential. When a bipolar high-frequency pulse train energizes a segment, the electric field established through the liquid causes that region to become turbulent and thereby scatter incident light so that the segment appears diffusely illuminated against a specularly illuminated background.

WATER Water is a chemical compound with two atoms of hydrogen and one atom of oxygen in each of its molecules. It is formed by the direct reaction (1):


2H2 + O2 → 2H2O

(1)

of hydrogen with oxygen. The other compound of hydrogen and oxygen, hydrogen peroxide, readily decomposes to form water, reaction (2): 2H2O2 → 2H2O + O2

(2)

Water also is formed in the combustion of hydrogen-containing compounds, in the pyrolysis of hydrates, and in animal metabolism. Some properties of water are given in Table W.1.

GASEOUS STATE Water vapor consists of water molecules that move nearly independently of each other. The atoms are held together in the molecule by chemical bonds, which are very polar — the hydrogen end of each bond is electrically positive relative to the oxygen. When two molecules near each other are suitably oriented, the positive hydrogen of one molecule attracts the negative oxygen of the other, and while in this orientation, the repulsion of the like charges is comparatively small. The net attraction is strong enough to hold the molecules together in many circumstances and is called a hydrogen bond. When heated above 1200°C, water vapor dissociates appreciably to form hydrogen atoms and hydroxyl free radicals, reaction (3): H2O → H + OH

(3)

These products recombine completely to form water when the temperature is lowered. Water vapor also undergoes most of the chemical reactions of liquid water and, at very high concentrations, even shows some of the unusual solvent properties of liquid water. Above 374°C, water vapor may be compressed to any density without liquefying, and at a density as high as 0.4 g/cm3, it can dissolve appreciable quantities of salt. These conditions of high temperature and pressure are found in efficient steam power plants.


TABLE W.1 Properties of Water Property Freezing point Density of ice, 0°C Density of water, 0°C Heat of fusion Boiling point Heat of vaporization Critical temperature Critical pressure Specific electrical conductivity at 25°C Dielectric constant, 25°C

Value 0 °C 0.92 g/cm3 1.00 g/cm3 80 cal/g (335 J/g) 100°C 540 cal/g (2260 J/g) 347°C 217 atm (22.0 MPa) 1 × 10–7/Ω-cm 78

Source: McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 19, McGraw-Hill, New York, , 579. With permission.

SOLID STATE Ordinary ice consists of water molecules joined together by hydrogen bonds in a regular arrangement. This unusual feature is a result of the strong and directional hydrogen bonds taking precedence over all other intermolecular forces in determining the structure of the crystal. If the water molecules were rearranged to reduce the amount of empty space, their relative orientations would no longer be so well suited for hydrogen bonds. This rearrangement can be produced by compressing ice to pressures in excess of 14 MPa. Altogether, five different crystalline forms of solid water have been produced in this way, the form obtained depending upon the final pressure and temperature. They are all denser than water, and all revert to ordinary ice when the pressure is reduced.

LIQUID STATE The molecules in liquid water also are held together by hydrogen bonds. When ice melts, many of the hydrogen bonds are broken, and those that remain are not numerous enough to keep the molecules in a regular arrangement. Many of the unusual properties of liquid water may be understood in terms of the hydrogen bonds that remain. As water is heated from 0°C, it contracts until 4°C is reached and then begins


the expansion that is normally associated with increasing temperature. This phenomenon and the increase in density when ice melts both result from a breaking down of the open, hydrogen-bonded structure as the temperature is raised. The viscosity of water decreases tenfold as the temperature is raised from 0 to 100°C, and this also is associated with the decrease of icelike character of the water as the hydrogen bonds are disrupted by increasing thermal agitation. Even at 100°C, the hydrogen bonds influence the properties of water strongly, for it has a high boiling point and a high heat of vaporization compared with other substances of similar molecular weight.

PROPERTIES Pure water, either solid or liquid, is blue if viewed through a thickness of more than 2 m. The other colors often observed are due to impurities. Water is an excellent solvent for many substances, but particularly for those that dissociate to form ions. Its principal scientific and industrial use as a solvent is to furnish a medium for purifying such substances and for carrying out reactions between them. Among the substances that dissolve in water with little or no ionization and that are very soluble are ethanol and ammonia. These are examples of molecules that are able to form hydrogen bonds with water molecules, although, except for the hydrogen of the OH group in ethanol, it is the hydrogen of the water that makes the hydrogen bond. On the other hand, substances that cannot interact strongly with water, either by ionization or by hydrogen bonding, are only sparingly soluble in it. Examples of such substances are benzene, mercury, and phosphorus. Water is not a strong oxidizing agent, although it may enhance the oxidizing action of other oxidizing agents, notably oxygen. Examples of the oxidizing action of water itself are its reactions with the alkali and alkaline earth metals, even in the cold. Water is an even poorer reducing agent than oxidizing agent. One of the few substances that it reduces rapidly is fluorine.

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Water reacts with a variety of substances to form solid compounds in which the water molecule is intact, but in which it becomes a part of the structure of the solid. Such compounds are called hydrates, and are formed frequently with the evolution of considerable amounts of heat.

WATER-SOLUBLE PLASTICS Within the plastics industry, water-soluble materials offer a variety of desirable physical properties, yet retain the advantages inherent in a water system. These advantages include ease of handling, negligible solvent costs, low toxicity, and low flammability. There is no sharp dividing line between water-dispersible and water-soluble polymers. Many so-called water-soluble plastics form colloidal dispersions rather than true solutions. In this text, emulsions or dispersions of waterinsoluble polymers are not discussed (such as acrylics, polyvinyl acetate, styrene butadiene, polyvinyl butyral, etc.). The water-soluble plastics can be roughly divided into two general classes: thermoplastic resins and thermosetting resins.

THERMOPLASTIC RESINS

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These plastics are usually synthesized by addition polymerization techniques. That is, small units (or monomers) are joined together to develop the final molecular weight and polymer configuration. Rarely do these polymers develop into long straight chains; considerable branching often occurs. The molecular weight, chemistry of side groups, and extent of branching all determine the properties that are obtained. These plastics are available as white or light-colored powders or in solution. Films, moldings, and extrusions are also available based on some of the thermoplastic resins. Alkali-Soluble Polyvinyl Acetate Copolymers Polyvinyl acetate itself is water insoluble. However, copolymers are available in which vinyl acetate is copolymerized with an acidic


comonomer. Such products retain the organic solubility of polyvinyl acetate but are soluble in aqueous alkali. These polymers exhibit low viscosity in solution and deposit high gloss films that are water resistant, provided a volatile alkali such as ammonia is used. The use of a fixed alkali will result in a film with permanent water sensitivity. They generally possess good adhesion to cellulose and a wide variety of other surfaces. Major uses include loom-finish warp sizes for dope dyed yarns, repulpable adhesives or sizes for paper and board, conditioning agents for masonry prior to painting, protective coatings for metals, and leveling agent and film former in self-polishing waxes. Ethylene-Maleic Anhydride Copolymers High-molecular-weight polymers have been prepared by copolymerizing ethylene and maleic anhydride. These resins are available either in “linear” form or cross-linked with either anhydride, free acid, or amide-ammonium salt side chains. Major applications include general thickening and suspending in adhesives, agricultural chemicals, cleaning compounds, and ceramics. This resin is used as a thickener for latex and as a warp size for acetate filament. Polyacrylates Commercially important polymers are prepared by polymerizing either acrylic or methacrylic acid. Usually these products are neutralized with bases to the salt form. Solution viscosity increases during neutralization. Cast films are hard, transparent, colorless, and somewhat brittle. Polyacrylic acid itself is used as a warp size for nylon. The neutralized polymers (polyacrylates) are used in various coating and binding applications (ceramics, grinding wheels, etc.). Because of interesting solution properties, the polyacrylates are used as thickeners, flocculants, and sometimes as dispersants in applications such as ore processing, drilling muds, and oil recovery.


Polyethers Two different polymer types are covered under this heading: polyoxyethylene (includes polyethylene glycols) and polyvinyl methyl ether and copolymers. Polyoxyethylene (polyethylene glycol). These resins are available over a wide molecular weight range. Low-molecular-weight members are slightly viscous liquids, whereas the medium-molecular-weight types (1000 to 20,000) are waxy solids. Polymers up through this molecular-weight level are known as polyethylene glycols. Extremely high molecularweight (several thousand to several million) homologues are also available. All types are soluble in water and in some organic solvents. Applications for the liquid and waxy solids include lubricants (rubber molds, textile fibers, and metal), bases for cosmetic and pharmaceutical preparations, and chemical intermediates for further reaction. The very high molecularweight types are useful principally as thickeners in many application areas. Polyvinyl methyl ether. This unique family of vinyl polymers shows inverse solubility in that the resins precipitate out above 35°C. They do redissolve upon cooling and the addition of low-molecular-weight alcohols increases the solubility in water and raises the precipitation temperature. Higher homologues are available that are water insoluble and quite tacky. The products exhibit pressure-sensitive adhesiveness coupled with good cohesive strength and high wet tack. Copolymers are available that contain maleic anhydride to modify physical properties, particularly solubility or tolerance to water and organic solvents and ease of insolubilization. Major uses take advantage of properties such as pressure-sensitive characteristics (adhesives), tackiness (various latex systems), thickening active, heat sensitizing (latices for dip forming), and binding power (pigments).

tensile strength, adhesion, and flexibility. In addition, the polymer is resistant to oxidation and in film form is an excellent barrier for various gases. Certain types exhibit surface activity in solution and all types are soluble in both acid and alkaline media. The resins can be cross-linked by borax and numerous organic and inorganic agents to produce thickening or even insolubilization. In adhesives, polyvinyl alcohol contributes machinability, viscosity control, specific adhesion, and in some cases remoistenability. Other major uses include paper coating and sizing (for increased strengths, ink hold-out, and grease resistance), textile sizing, wrinkle-resistant finishes (wash-and-wear fabrics in conjunction with thermosetting resins), polyvinyl acetate emulsion polymerization (protective colloid), binder (for nonwoven ribbons, filters, etc.), film (release agent in polyester and epoxy molding and water-soluble packaging), cement additive (for improved strength, toughness, and adhesion), and photosensitive coating (in the graphic arts industry). Polyvinyl Pyrrolidone Polyvinyl pyrrolidone (PVP) exhibits good solubility in both water and various organic solvents. A nontoxic material and tacky substance when wet, the polymer is a dispersant, suspending agent, and an adhesive component for bonding difficult surfaces. Major uses include cosmetic preparations (hair sprays, etc.), tablet binding and coating, detoxifying of dyes, drug, and chemicals, beverage clarification, and specialty textile and paper applications involving sizing, dyeing, and printing. Copolymers are also available (like PVPvinyl acetate) that have some advantages over the homopolymers in heat sealability, pressuresensitive adhesiveness, and other properties.

Polyvinyl Alcohol

Polyacrylamide

These resins are available commercially in a wide range of types, which vary in viscosity and chemical composition. Polyvinyl alcohol exhibits good water solubility, high resistance to organic solvents, oils, and greases, high

These high-molecular-weight polymers are soluble in both cold and hot water and in selected organic solvents. The resin is an efficient thickener and by reaction can be changed in physical and chemical properties.


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In addition to general uses for water-soluble resins, these resins have shown an outstanding ability to flocculate fines and increase the filtration rate of slurries. Consequently, polyacrylamide is used in ore processing and in other such systems where dispersed materials are encountered. Styrene-Maleic Anhydride Copolymers of these two monomers are soluble in some organic solvents and alkaline water. Styrene-maleic anhydride resins produce viscous and stable aqueous solutions. This resin is a strong polyelectrolyte. It is used as a textile warp size, paper coating, and static-electricity conductor. The polymer is also used in alkaline latex systems as a protective colloid, emulsifier, pigment dispersant, and filming aid. Cellosic Derivatives Various commercial derivatives are prepared from alpha cellulose, which is obtained from several plant sources. One class of derivatives is the water-soluble ethers. These products produce viscous aqueous solutions. All have some resistance to organic solvents, are hygroscopic, and are difficult to insolubilize. Major industries that use these polymers as well as the water-soluble synthetic resins include food, pharmaceutical, cosmetic, textile, paper, petroleum (drilling muds), ceramic, paint, emulsion polymerization, and leather. Hydroxyethylcellulose

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This polymer is manufactured by reacting alkali cellulose with ethylene oxide. It can be watersoluble or only alkali-soluble depending on the extent of reaction. The alkali-soluble types possess the advantage of increased water resistance in deposited films. This polymer is somewhat intermediate in properties between methylcellulose and sodium carboxymethyl-cellulose. It is a protective colloid and relatively insensitive to the inclusion of multivalent ions in solution. The polymer is soluble in both hot and cold water, nonionic, but depolymerized by strong acids.


Water-soluble hydroxyethylcellulose is used in polyvinyl acetate emulsions as a stabilizer and in latex paints as a thickener and leveling agent. Methylcellulose Methylcellulose exhibits inverse water solubility in that it is more soluble at low temperatures than at high temperatures. It is nonionic in solution and is a very efficient thickener. A major use is in latex paints (both polyvinyl acetate and acrylic types). Methylcellulose thickens the paint and contributes to good brushing characteristics as well. Other uses include bulking in laxatives, and binding and thickening in cosmetics and pharmaceuticals. Sodium Carboxymethylcellulose This polymer is soluble in both hot and cold water. It exhibits good thickening action and suspending ability for particulates. Since solutions are ionic in character, they are somewhat sensitive to pH shifts and salt additions. Major uses include soil suspension in synthetic detergents and viscosity control in oil-well drilling muds.

THERMOSETTING RESINS A number of thermosetting resins are available in water solutions or in water-soluble form. These are principally the addition reaction products of formaldehyde with urea, phenolic, or melamine. Resorcinol and thiourea may also be reacted with formaldehyde to form watersoluble precondensates, although these have not attained the volume of the three main classes defined above. These resins develop high molecular weight by a condensation reaction. The properties may change as the reaction proceeds. A-stage resins are those in which the degree of polymerization is minor. In some cases the degree of polymerization is such that only dimers, trimers, or similar small units are prepared. These products are water soluble or at least can tolerate the addition of significant amounts of water. Should the reaction proceed further, the polymers enter an area roughly defined as B-stage, in which they will tolerate addition of


only small amounts of water but are soluble in certain organic solvents. Manufacturers of thermosetting resins carry the polymerization reaction to the A- or Bstage. Further reaction (to the C-stage) is carried out by consumers of these resins. As the condensation reaction proceeds and the molecular weight builds to form a rigid, three-dimensional system, the polymers reach a point where they will not dissolve in organic solvents and are then termed cured (or Cstaged). Further heating of the resin beyond this point may establish additional cross-links, but the physical properties do not change drastically. Once the resin has reached the C-stage, excessive heating leads to chemical breakdown of the material. Thermosetting water-soluble polymers are treated with heat and/or catalyst to advance the cure after deposition on a particular surface or within a particular structure. Thus, the watersolubility feature is important in that it allows easy manipulation without the cost and hazards of organic systems. However, water is an essential ingredient when cross-linking cellulose with low-molecular-weight thermosetting resins. The general characteristics of a fully cured, water-soluble, thermosetting resin are similar to those obtained by curing B-stage varnishes or molding compounds. In many areas, the water-soluble thermosetting resins compete with one another. Certain ones may be preferred in an industry or in a certain particular application because of specific properties or cost. Generally, these resins offer high-temperature stability and hardness coupled with water and solvent resistance. Resistance to either acids or bases can also be obtained. Cyclic Thermosetting Resins In this category are cyclic ethylene urea–formaldehyde resins and triazones, which can be obtained by cyclization of dimethylolurea with a primary amine (usually ethylamine) and then adding 2 mol of formaldehyde. These cyclic thermosetting resins were developed primarily for textile applications because they do not react or polymerize with themselves (as do other


water-soluble thermosetting resins) but do react with the hydroxyls in cellulose through the methylol groups. These resins are called cellulose reactants and are by far the largest class of resins used in the wash-and-wear treatment of fabrics. Such resins impart crease resistance, wrinkle recovery, stiffness, tensile strength, water repellency, and good resistance to yellowing by chlorine-containing bleaches. In addition there are increases in resiliency, dimensional stability, and permanent texturizing. Cyclic ethylene urea–formaldehyde resins have the advantage over triazone in better color stability, absence of odor, and scorch resistance. Triazone resins have gained prominence particularly because they are more resistant to the effects of chlorine and are somewhat cheaper. Triazones are used principally to develop washand-wear properties on white cotton. Melamine-Formaldehyde In general, melamine-formaldehyde (MF) resins are the most expensive of the water-soluble thermosetting types. They possess good color, lack of odor, high abrasion resistance, and high resistance to alkali. The major use is in decorating laminants for surfacing of wood, paper, and other products. Other uses include binding of rock wool and glass wool for thermal insulation, finishing of nylon for stiffness and resilience, and imparting dimensional stability to wool and some cellulosics. Phenol-Formaldehyde This very popular class of water-soluble thermosetting resins is intermediate in cost between the MF and urea–formaldehyde (UF) types. They possess good water resistance, toughness, and acid resistance although they are somewhat poorer than the MF resins in color, odor, and flame resistance. In most other physical and chemical properties they are superior to the UF types. Applications include laminates (including plywood and fabrics), grinding wheels, thermal insulation, battery separators, brake linings, and foundry uses.

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Urea–Formaldehyde These resins are the lowest cost as a class of the three general types but still cure to hard and somewhat brittle resins that have many desirable properties. The UF resins have an added plus in that they can cure at room temperature with suitable catalysts, whereas both the melamine-formaldehyde and phenol-formaldehyde types normally require temperatures in the neighborhood of 149°C to develop their full properties. The UF resins suffer somewhat in comparison with the other two types in poorer water resistance, less toughness, and poorer resistance to cyclical changes in temperature or water exposure. Urea–formaldehyde resins are used in plywood because of case of handling and lower temperature of cure, on paper for increased wet strength in air filters, and on certain rayon fabrics for improved stabilization and water resistance. The UF resins are used as insolubilizers for hydroxyl-containing polymers and in many of the general application areas for water-soluble, thermosetting resins.

WAX

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Wax is a general name for a variety of substances of animal and vegetable origin, which are fatty acids in combination with higher alcohols, instead of with glycerin as in fats and oils. They are usually harder than fats, less greasy, and more brittle, but when used alone do not mold as well. Chemically, the waxes differ from fats and oils in that they are composed of highmolecular-weight fatty acids with high-molecular-weight alcohols. The most familiar wax is beeswax from the honeybee, but commercial beeswax is usually greatly mixed or adulterated. Another animal wax is spermaceti from the sperm whale. Vegetable waxes include Japan wax, jojoba oil, candelilla, and carnauba wax. Mineral waxes include paraffin wax from petroleum, ozokerite, ceresin, and montan wax. The mineral waxes differ from the true waxes and are mixtures of saturated hydrocarbons. The animal and vegetable waxes are not plentiful materials, and are often blended with or replaced by hydrocarbon waxes or waxy synthetic resins. However, waxes can be made from © 2002 by CRC Press LLC

common oils and fats by splitting off the glycerin and reesterifying selected mixtures of the fatty acids with higher alcohols.

TYPES

AND

USES

Some plastics have wax characteristics, and may be used in polishes and coatings or for blending with waxes. Polyethylene waxes are light-colored, odorless solids of low molecular weight, up to about 6000. Mixed in solid waxes to the extent of 50%, and in liquid waxes up to 20%, they add gloss and durability and increase toughness. In emulsions they add stability. Waxes are employed in polishes, coatings, leather dressings, sizings, waterproofing for paper, candles, carbon paper, insulation, and varnishes. They are softer and have lower melting points than resins, are soluble in mineral spirits and in alcohol, and insoluble in water. Synthetic waxes are used in liquid floor waxes, temporary corrosion protection, release agents, and as a melting point booster. There is a micronized polyethylene wax that is a processing and performance additive for adhesives, coatings, color concentrates, cosmetics, inks, lubricants, paints, plastics, and rubber. It can also be constituted from low-molecular-weight homopolymer, oxidized homopolymer, or as a copolymer. Another is a methylene polymer used to blend with vegetable or paraffin waxes to increase the melting point, strength, and hardness. This is a mixture of terphenyls. It is a lightbuff, waxy solid, highly soluble in benzene, and with good resistance to heat, acids, and alkalies. It is used to blend with natural waxes in candles, coatings, and insulation. Waxes are not digestible, and the so-called edible waxes used as water-resistant coatings for cheese, meats, and dried fruits are not waxes, but are modified glycerides. One is a white, odorless, tasteless waxy solid melting at 40°C, and is an acetylated monoglyceride of fatty acids. Microcrystalline waxes are used for the vacuum impregnation of inorganic-filled, organic-bonded electrical insulation and coatings for ceramic capacitors and other electronic components. The wax is chosen because of its low moisture permeability. Wax emulsions have been widely used as binders for dry-press mixes and glaze suspensions.


A high-melting-point paraffin also will make an excellent binder for dry-press granules. The paraffin is melted, added to the body, and then thoroughly incorporated by means of a heated, mullertype mixer. Ordinary paraffin also can be used to bond ceramic parts to steel plates for attachment to magnetic chucks during grinding.

WEAR-RESISTANT STEEL Many types of steel have wear-resistant properties, but the term usually refers to high-carbon, high-alloy steels used for dies, tooling, and parts subject to abrasion and for wear-resistant castings. They are generally cast and ground to shape. They are mostly sold under trade names for specific purposes. The excess carbon of the steels is in spheroidal form rather than as graphite. One of the earlier materials of this kind for drawing and forming dies is Adamite. It is a chromium–nickel–iron alloy with up to 1.5% chromium, nickel equal to half that of the chromium, and from 1.5 to 3.5% carbon with silicon from 0.5 to 2%. The Brinell hardness ranges from 185 to 475 as cast, with tensile strengths to 861 MPa. The softer grades can be machined and then hardened, but the hard grades are finished by grinding. Others have about 13% chromium, 1.5% carbon, 1.1% molybdenum, 0.70% cobalt, 0.55% silicon, 0.50% manganese, and 0.40% nickel. They are used for blanking dies, forming dies, and cams. T15 tool steel, for extreme abrasion resistance in cutting tools, is classed as a super-high-speed steel. It has 13.5% tungsten, 4.5% chromium, 5% cobalt, 4.75% vanadium, 0.50% molybdenum, and 1.5% carbon. Its great hardness comes from the hard vanadium carbide and the complex tungsten–chromium carbides, and it has full red-hardness. The property of abrasion or wear resistance in steels generally comes from the hard carbides, and is thus inherent with proper heat treatment in many types of steel.

WELDING ALLOYS Welding alloys are usually in the form of rod, wire, or powder used for either electric or gas


welding, for building up surfaces, or for hardfacing surfaces. In the small sizes in continuous lengths, welding alloys are called welding wire. Nonferrous rods used for welding bronzes are usually referred to as brazing rods, as the metal to be welded is not fused when using them. Welding rods may be standard metals or special alloys, coated with a fluxing material or uncoated, and are normally in diameters from 0.239 to 0.635 cm. Compositions of standard welding rods follow the specifications of the American Welding Society. Molded carbon, in sizes from 0.318 to 2.54 cm in diameter, is also used for arc welding. Low-carbon steel rods for welding cast iron and steel contain less than 0.18% carbon. High-carbon rods produce a hard deposit that requires annealing, but these are also used for producing a hard filler. High-carbon rods, with 0.85 to 1.10% carbon, will give deposits with an initial hardness of 575 Brinell, whereas high-manganese rod deposits will be below 200 Brinell but will work-harden to above 500 Brinell. For high-production automatic welding operations, carbon-steel wire may have a thin coating of copper to ease operation and prevent spattering. Stainless steel rods are marketed in various compositions. There are welding rods that comprise a range of stainless steels with either titania-lime or straight-lime coatings. Stainless C is an 18-8 type of stainless steel with 3.5% molybdenum. Aluminum-weld is a 5% silicon aluminum rod for welding silicon–aluminum alloys, and the Tungweld rods, for hard surfacing, are steel tubes containing fine particles of tungsten carbide. Kennametal KT-200 has a core of tungsten carbide and a sheathing of steel. It gives coatings with a hardness of Rockwell C63. Chromang, for welding high-alloy steels, is an 18-8 stainless steel modified with 2.5 to 4% manganese. Welding rods with grades of high-manganese steel give hardnesses from 500 to 700 Brinell, and high-speed-steel rods are used for facing worn cutting tools; others are used for facing surfaces requiring extreme hardness and have the alloy granules in a soft steel tube. The welded deposit has a composition of 30% chromium, 8% cobalt, 8% molybdenum, 5% tungsten, 0.05% boron, and 0.20% carbon. There is a group of welding alloys made especially for welding machines. They are, in

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general, sintered tungsten or molybdenum carbides, combined with copper or silver, and are electrodes for spot welding rather than welding rods. Tungsten electrodes may be pure tungsten, thoriated tungsten, or zirconium tungsten, the latter two used for direct-current welding. Thoriated tungsten gives high arc stability, and the thoria also increases the machinability of the tungsten. Zirconium tungsten provides adhesion between the solid electrode and the molten metal to give uniformity in the weld. Thermit is a mixture of aluminum powder and iron oxide used for welding large sections of iron or steel or for filling large cavities. The process consists of the burning of the aluminum to react with the oxide, which frees the iron in molten form. To ignite the aluminum and start the reaction, a temperature of about 1538°C is required, which is reached with the aid of a gas torch or ignition powder, and the exothermic temperature is about 2538°C. Cast iron thermit, used for welding cast iron, is thermit with the addition of about 3% ferrosilicon and 20% steel punchings. Railroad thermit is thermit with additions of nickel, manganese, and steel. The Stellite hardfacing rods are cobaltbased alloys that retain hardness at red heat and are very corrosion resistant. The grades have tensile strengths to 723 MPa and hardnesses to Rockwell C52. Inco-Weld A is welding wire for stainless steels and for overlays. It contains 70% nickel, 16% chromium, 8% iron, 2% manganese, 3% titanium, and not more than 0.07% carbon. The annealed weld has a tensile strength of 551 MPa with elongation of 12%. Nickel welding rod is much used for cast iron, and the operation is brazing, with the base metal not melted. Nickel silver for brazing cast iron contains 46.5% copper, 43.4% zinc, 10% nickel, 0.10% silicon, and 0.02% phosphorus. The deposit matches the color of the iron. Colmonoy 23A is a nickel alloy welding powder for welding cast iron and filling blow holes in iron casting by torch application. It has a composition of 2.3% silicon, 1.25% boron, 0.10% carbon, not over 1.5% iron, and the balance nickel, with a melting point of 1066°C. For welding on large structures where no heat treatment of the weldment is possible, the welding rods must have balanced compositions with no


elements that form brittle compounds. Rockide rods are metal oxides for hard surfacing.

WETTING AGENTS These are chemicals used in making solutions, emulsions, or compounded mixtures, such as paints, inks, cosmetics, starch pastes, oil emulsions, dentifrices, and detergents, to reduce the surface tension and give greater ease of mixing and stability to the solution. In the food industries chemical wetting agents are added to the solutions for washing fruits and vegetables to produce a cleaner and bacteria-free product. Wetting agents are described in general as chemicals having a large hydrophilic group associated with a smaller hydrophilic group. Some liquids naturally wet pigments, oils, or waxes, but others require a proportion of a wetting agent to give mordant or wetting properties. Pine oil is a common wetting agent, but many are complex chemicals. They should be powerful enough not to be precipitated out of solutions in the form of salts, and they should be free of odor or any characteristic that would affect the solution. Aerosol wetting agents are in the form of liquids, waxy pellets, or freeflowing powders. There are other free-flowing powders, basically modified polyacrylates, that are soluble in water and less so in alcohol. There are sodium or ammonium dispersions of modified rosin, with 90% of the particles below 1 µm in size. Also there is a sodium lignosulfonate produced from lignin waste liquor. It is used for dye and pigment dispersion, oil-well drilling mud, ore flotation, and boiler feedwater treatment. Increasingly used by the ceramic industry is a popular type of poly-oxyethylene alkylate ether with a very high resistance to water hardness. Sulfonated types and carboxylates have moderate wetting properties and strong detergent and solubilizing tendencies.

WHISKERS Whiskers are very fine single-crystal fibers that range from 3 to 10 µm in diameter and have length-to-diameter ratios of from 50 to 10,000. Since they are single crystals, their strengths


approach the calculated theoretical strengths of the materials. Alumina whiskers, which have received the most attention, have tensile strengths up to 0.2 million MPa and a modulus of elasticity of 0.5 million MPa. Other whisker materials are silicon carbide, silicon nitride, magnesia, boron carbide, and beryllia.

WHITE BRASS White brass is a bearing metal that is actually outside of the range of the brasses, bronzes, or babbitt metals. It is used in various grades; the specification are tin, 65%; zinc, 28 to 30%; and copper, 3 to 6%. It is used for automobile bearings, and is close-grained, hard, and tough. It also casts well. A different alloy is known under the name of white brass in the cheap jewelry and novelty trade. It has no tin, small proportions of copper, and the remainder zinc. It is a high-zinc brass, and varies in color from silvery white to yellow, depending on the copper content. White nickel brass is a grade of nickel silver. The white brass used for castings where a white color is desired may contain up to 30% nickel. The 60:20:20 alloy is used for white plaque castings for buildings. The high-nickel brasses do not cast well unless they also contain lead. Those with 15 to 20% nickel and 2% lead are used for casting hardware and valves. White nickel alloy is a copper–nickel alloy containing some aluminum. White copper is a name sometimes used for copper–nickel alloy or nickel brass. Nickel brasses known as German silver are copper–nickel–zinc white alloys used as a base metal for plated silverware, for springs and contacts in electrical equipment, and for corrosion-resistant parts. The alloys are graded according to the nickel content. Extra-white metal, the highest grade, contains 50% copper, 30% nickel, and 20% zinc. The lower grade, called fifths, for plated goods, has a yellowish color. It contains 57% copper, 7% nickel, and 36% zinc. All of the early German silvers contained up to 2% iron, which increased the strength, hardness, and whiteness, but is not desirable in the alloys used for electrical work.


Some of the early English alloys also contained up to 2% tin, but tin embrittles alloys.

CAST IRON White cast iron solidifies with all its carbon in the combined state, mostly as iron carbide, F3C (cementite). White iron contains no free graphite as does gray iron, malleable iron, and ductile iron. White iron derives its name from the fact that it shows a bright white fracture on a freshly broken surface. The main use for white iron is as an intermediate product in the manufacture of malleable iron. In addition to this, white iron is made as an end product to serve specific applications that require a hard, abrasion-resistant material. White iron is very hard and resistant to wear, has a very high compressive strength, but has low resistance to impact, and is very difficult to machine. By the proper balancing of chemical composition and section size, an iron casting can be made to solidify completely white throughout its entire section. By modifying the balance and adjusting the cooling rate, the casting can be made to solidify with a layer of white iron at the surface backed up by a core of gray iron. Castings with such a duplex structure are called “chilled iron” castings. Castings of white iron and chilled iron find their main use in resistance to wear and abrasion. Typical applications include parts for crushers and grinders, grinding balls, coke and cinder chutes, shot-blasting nozzles and blades, parts for slurry pumps, car wheels, metalworking rolls, and grinding rolls. By using a fairly low silicon content, cast iron can be made to solidify white without the use of any additional alloy. Carbon contents are kept high (about 3.6%) when high hardness (575 Bhn) is desired. Such irons have very low toughness and a strength of about 240 MPa. For somewhat higher toughness and strength, at some sacrifice of hardness, the carbon content is lowered to about 2.8%. Unalloyed white and chilled irons have a structure composed of particles of massive iron carbide (Fe3C) in a matrix of fine pearlite. For highest hardness, strength, and toughness, the white iron is alloyed to

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produce a martensitic matrix surrounding particles of massive carbide.

GOLD White gold is the name of a class of jewelers’ white alloys used as substitutes for platinum. The name gives no idea of the relative value of the different grades, which vary widely. Gold and platinum may be alloyed together to make a white gold, but the usual alloys consist of from 20 to 50% nickel, with the balance gold. Nickel and zinc with gold may also be used for white golds. The best commercial grades of white gold are made by melting the gold with a white alloy prepared for this purpose. This alloy contains nickel, silver, palladium, and zinc. The 14-karat white gold contains 14 parts pure gold and 10 parts white alloy. A superior class of white gold is made of 90% gold and 10% palladium. High-strength white gold contains copper, nickel, and zinc with the gold. Such an alloy, containing 37.5% gold, 28% copper, 17.5% nickel, and 17% zinc, when aged by heat treatment, has a tensile strength of about 689 MPa and an elongation of 35%. It is used for making jewelry, has a fine, white color, and is easily worked into intricate shapes. Whitegold solder is made in many grades containing up to 12% nickel, up to 15% zinc, with usually also copper and silver, and from 30 to 80% gold. The melting points of eight grades range from 695 to 845°C.

METALS

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Although a great variety of combinations can be made with numerous metals to produce white or silvery alloys, the name usually refers to the lead–antimony–tin alloys employed for machine bearings, packings, and linings, to the low-melting-point alloys used for toys, ornaments, and fusible metals, and to the type metals. Slush castings, for ornamental articles and hollow parts, are made in a wide variety of soft white alloys, usually varying proportions of lead, tin, zinc, and antimony, depending on cost and the accuracy and finish desired. These castings are made by pouring the molten metal into a metal mold without a core, and immediately pouring the metal out, so that a thin shell of the


alloy solidifies against the metal of the mold and forms a hollow product. A number of white metals are specified by the American Society for Testing and Materials for bearing use. These vary in a wide range from 2 to 91% tin, 4.5 to 15% antimony, up to 90% lead, and up to 8% copper. The alloy containing 75% tin, 12% antimony, 10% lead, and 3% copper melts at 184°C, is poured at about 375°C, and has an ultimate compressive strength of 111 MPa and a Brinell hardness of 24. The alloy containing 10% tin, 15% antimony, and 75% lead melts at 240°C, and has a compressive strength of 108 MPa and a Brinell hardness of 22. The first of these two alloys contains copper–tin crystals; the second contains tin–antimony crystals. Society of Automotive Engineers (SAE) Alloy 18 is a cadmium–nickel alloy with also small amounts of silver, copper, tin, and zinc. A bismuth–lead alloy containing 58% bismuth and 42% lead melts at 123.5°C. It casts to exact size without shrinkage or expansion, and is used for master patterns and for sealing. Various high-tin or reverse bronzes have been used as corrosion-resistant metals, especially before the advent of the chromium, nickel, and aluminum alloys for this purpose. A white metal sheet now much used for making stamped and formed parts for costume jewelry and electronic parts is zinc with up to 1.5% copper and up to 0.5% titanium. The titanium with the copper prevents coarse-grain formation, raising the recrystallization temperature. The alloy weighs 22% less than copper, and it plates and solders easily.

WHITEWARE CERAMICS Technical whitewares include clays, porcelains, china, white stoneware, and steatites. The modern oxide ceramics would also be in this group. For technical use, these whitewares are usually vitrified (nonporous) or very nearly so. Most commonly the pieces are glazed. To produce white-bodied ceramics, the raw materials must be of superior quality and selection. The range of materials available is comparatively limited. Although clays are widely distributed over the Earth, large, uniform deposits of white burning clays are not common.


COMPOSITION

AND

TYPES

Most whiteware clays are basically kaolins and chemically are hydrous aluminum silicates. Ball clays are less pure than kaolins and contain free silica and small amounts of other contaminants. Kaolins are generally not highly plastic, whereas ball clays are very plastic. The plasticity derives from the physical form of the minute particles, which are colloidal in size. Ball clays are not as white-burning as kaolins and impart color to the fired body. Feldspars are used as fluxes to provide the alkaline oxides for the glassy phase surrounding the mullite (3Al2O3 · 2 SiO2) crystals forming the mass of the body. Porcelains, china, and stoneware are composed of clays, silica, and feldspar. Steatites are composed of talc (magnesium silicate) and clay. Minor variations are made to enhance special properties. The porcelains, china, and stoneware are composed of alumina (Al2O3), silica (SiO2), sodium and potassium oxides (Na2O and K2O), as well as calcia (CaO), zinc oxide (ZnO), zirconia (ZrO2 ), titania (TiO2 ), barium oxide (BaO), magnesia (MgO), and phosphoric oxide (P2O5). Some other oxides may be present as traces. Iron oxide is usually present in small amounts as an undesirable impurity. The oxides are supplied in kaolin, ball clays, quartz, feldspar, whiting, magnesia, and talc. Oxide ceramics that do not use clays may use only mixtures of refined oxides.

COMPOUNDING

AND

FORMING

The raw materials are intimately mixed, usually by ball milling, and then prepared for forming into ware by one of the methods listed below. For jiggering and simple mechanical pressing or extruding, the ball-milled slip is dewatered, filter pressed, deaired, and extruded into convenient size billets. For casting, the specific gravity is adjusted to give a good casting viscosity. For dry pressing, the body is dried, shredded, and powdered or spray dried into minute granules.


The pieces are formed as close to size before firing as practical, allowing for the shrinkage during firing. This shrinkage may run as high as 25% and must be very closely controlled to avoid loss in the firing of off-dimension pieces. The ware is formed in a number of ways, some of them unchanged for centuries or even thousands of years and others unknown 60 years ago. Following are the primary methods used: 1. Throwing or jiggering on mechanical potters wheel from plastic clay body 2. Casting in plaster of paris mold from liquid slurry or slip 3. Pressing from plastic clay with simple mold 4. Pressing from dry powder in metal dies 5. Isostatic pressing from dry powder in rubber sack with hydraulic pressure 6. Hot pressing with heated clay blank and mold 7. Simple extrusion through die 8. Extrusion with thermoplastic resin to the body (injection molding) Many pieces are formed by a combination of pressing or extrusion followed by mechanical shaping in lathes by special tools or dry grinding. After forming, pieces are bisque-fired, which drives out moisture and water of crystallization. Porcelain is not vitrified during the bisque firing. The porcelain bisque is dipped or sprayed with glaze and, in the second firing, the body and glaze mature or vitrify together. China is vitrified in the first firing. In china manufacture, the glaze is applied to the fired body and the glaze matures in the second firing, which is at a lower temperature than the initial firing. In some cases the glaze can be applied to an unfired piece and only one firing is needed. Today, most kilns are fired with natural or manufactured gas, fuel oil, or electricity. The round beehive kiln fired periodically has been largely replaced with continuous tunnel kilns, which may be built with movable cars moving from one end to the other on a straight track or

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as a circular tunnel with a moving floor. Periodic kilns are often used in specialized work where the volume does not justify use of a tunnel kiln. They are also more versatile. The firing of technical ceramics is performed under the most carefully controlled conditions possible. Both the temperature and the atmosphere must be known and controlled. The ware must be properly placed in the kilns or damage to the piece will result. Warpage, uneven firing, and cracking can easily occur. Maximum firing temperatures for unglazed refractory porcelain are usually about 1760°C. Laboratory chemical porcelain that is glazed is fired to 1454°C. Hotel, sanitary china, and electrical porcelain is usually fired about 1260°C. Steatite bodies mature at around 1260 to 1316°C.

WIRE CLOTH

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Stiff fabrics made of fine wire woven with plain loose weave, wire cloth is used for screens to protect windows, for guards, and for sieves and filters. Steel and iron wire may be used plain, painted, galvanized, or rustproofed, or various nonferrous metal wires are employed. It is usually put up in rolls in widths from 46 to 122 cm. Screen cloth is usually 12, 14, 16, and 18 mesh, but wire cloth in copper, brass, or Monel metal is made regularly in meshes from 4 to 100. The size of wire is usually from 0.023 to 0.165 cm in diameter. Wire cloth for fine filtering is made in very fine meshes. Mesh indicates the number of openings per inch, and has no reference to the diameter of wire. A 200-mesh cloth has 200 openings each way on a square inch, or 40,000 openings per square inch (6.4 cm2). Wire cloth as fine as 400 mesh, or 160,000 openings per square inch (6.4 cm2), is made by wedge-shaped weaving, although 250 wires of the size of 0.010 cm when placed parallel and in contact will fill the space of 2.5 cm. Very fine mesh wire cloth must be woven at an angle because the globular nature of most liquids will not permit passage of the liquid through microscopic square openings. One wire screen cloth, for filtering and screening, has elongated openings. One way the 0.0140-cm wire count is 200 per 2.5 cm, while the other way the 0.018-cm warp wire is 40 per 2.5 cm.


Wire fabrics for reentry parachutes are made of heat-resistant nickel–chromium alloys, and the wire is not larger than 0.013 cm in diameter to give flexibility to the cloth. Wire fabrics for ion engines to operate in cesium vapor at temperatures to 1316°C are made with tantalum, molybdenum, or tungsten wire, 0.008 to 0.015 cm in diameter, with a twill weave. Meshes to a fineness of 350 by 2300 can be obtained. Porosity uniformity is controlled by pressure calendering of the woven cloth, but for extremely fine meshes in wire cloth it is difficult to obtain the uniformity that can be obtained with porous sintered metals. High-manganese steel wire is used for rock screens. For window screening in tropical climates or in corrosive atmospheres, plastic filaments are sometimes substituted for the standard copper or steel wire. For example, Lumite screen cloth is woven of vinylidene chloride monofilament, 0.038 cm in diameter in 18 and 20 mesh.

WOOD For most purposes wood may be defined as the dense fibrous substance that makes up the greater part of a tree. It is found beneath the bark, and in the roots, stems, and branches of trees and shrubs. Of the three sources, the stem or trunk furnishes the bulk of raw material for lumber products. Wood is a renewable resource. It is grown just about everywhere and can be produced in any reasonable quantity needed for future consumption. Wood products and the management of forested lands are changing to meet modern conditions; hence, trees were grown to meet modern production requirements for size, quality, and quantity. For example, with the developments in the modern technique of gluing, the former use of extremely wide, thick, and excessively long lumber is no longer necessary. Laminated lumber and plywood have generally taken the place of these large boards and timbers. Not only are the raw materials for such products easier to grow and more economical to obtain than are solid timbers of comparable size, but the products are generally improved by the use of modern methods of fabricating. Although there are many species of wood, the commercially important types can be


grouped into two categories of about 25 for each group.

ANATOMY Wood is composed mostly of hollow, elongated, spindle-shaped cells that are arranged parallel to each other along the trunk of a tree. The characteristics of these fibrous cells and their arrangement affect strength properties, appearance, resistance to penetration by water and chemicals, resistance to decay, and many other properties. The combined concentric bands of light and dark areas constitute annual growth rings. The age of a tree may be determined by counting these rings at the stump. In temperate climates, trees often produce distinct growth layers. These increments are called growth rings or annual rings when associated with yearly growth; many tropical trees, however, lack growth rings. These rings vary in width according to environmental conditions. Where there is visible contrast within a single growth ring, the first-formed layer is called earlywood and the remainder latewood. The earlywood cells are usually larger and the cell walls thinner than the latewood cells. With the naked eye or a hand lens, earlywood is shown to be generally lighter in color than latewood. Because of the extreme structural variations in wood, there are many possibilities for selecting a species for a specific purpose. Some species (for example, spruce) combine light weight with relatively high stiffness and bending strength. Very heavy woods (for example, lignum vitae) are extremely hard and resistant to abrasion. A very light wood (such as balsa) has high thermal insulation value; hickory has extremely high short resistance; mahogany has excellent dimensional stability. Many mechanical properties of wood, such as bending strength, crushing strength, and hardness, depend on the density of wood; the heavier woods are generally stronger. Wood density is determined largely by the relative thickness of the cell wall and the proportions of thick- and thin-walled cells present. © 2002 by CRC Press LLC

HARDWOODS

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SOFTWOODS

The terminology used in the classification of trees is confusing, but because it has become general in usage, it is important for those who make or purchase products of wood to understand it. The terms hardwood and softwood have no direct application to hardness or softness of the materials. Basswood is a softer domestic species, yet the yellow pines, which are classed as softwood, are often much harder. Even balsa, a foreign species that everyone knows, the lightest and softest wood used in commerce, is classed as hardwood. For practical purposes, the hardwoods have broad leaves, whereas the softwoods have needlelike leaves. Trees (hardwoods) with broad leaves usually shed them at some time during the year, while the conifers (softwoods) retain a covering of the needlelike foliage throughout the year. There are quite a few exceptions to these criteria, but as users gain familiarity with the various woods, they will soon learn by experience into which group a species falls. Those who use wood must know something of the botanical classification because the lumber industry is also divided into two distinct groups. The methods of doing business and manufacturing and grading for quality differ from each other. Generally, the hardwoods are used for the manufacture of factory-made products, such as tools, furniture, flooring, instrument cases, etc. The largest market for softwoods is in the home construction field or for other building purposes. But there is no line of demarcation that is reliable, for hardwoods and softwoods are often interchangeable in use. The more important hardwoods are as follows: Alder Ash Aspen Basswood Beech Birch Cherry Chestnut Cottonwood Elm Hackberry Hickory and pecan

Holly Locust Magnolia Maple (hard and soft) Oak, red Oak, white Sweet or red gum Sycamore Tupelo or black gum Walnut, black Willow, black Yellow poplar

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The more important softwoods are the following: Cedar (several species) Cypress Douglas Fir (not a true fir) Firs (eastern and western) Hemlock (eastern and western) Larch Pine, eastern white Pine, jack Pine, lodgepole Pine, pitch

Pine, Ponderosa Pine, red Pine, southern yellow (several species) Pine, sugar Pine, Virginia Pine, western white Redwood Spruce, eastern Spruce, Englemann Spruce, Sitka Tamarack

Hardwood The horizontal plane of a block of hardwood (for example, oak or maple) corresponds to a minute portion of the top surface of a stump or end surface of a log. The vertical plane corresponds to a surface cut parallel to the radius and parallel to the wood rays. The vertical plane corresponds to a surface cut at right angles to the radius and the wood rays, or tangentially within the log. In hardwoods, these three major planes along which wood may be cut are known commonly as end-grain, quarter-sawed (edgegrain), and plain-sawed (flat-grain) surfaces. Softwood The rectangular units that make up the end grain of softwood are sections through long vertical cells called tracheids or fibers. Because softwoods do not contain vessel cells, the tracheids serve the dual function of transporting sap vertically and giving strength to the wood. Softwood fibers range from about 3 to 8 mm in length. Cell Walls and Composition

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The principal compound in mature wood cells is cellulose, a polysaccharide of repeating glucose molecules that may reach 4 µm in length. These cellulose molecules are arranged in an orderly manner into structures about 10 to 25 nm wide called microfibrils. This ordered arrangement in certain parts (micelles) gives the cell wall crystalline properties that can be observed in polarized light


with a light microscope. The microfibrils wind together like strands in a cable to form macrofibrils that measure about 0.5 µm in width and may reach 4 µm in length. These cables are as strong as an equivalent thickness of steel. Wood, regardless of the species, is composed of two principal materials: cellulose, which is about 70% of the volume, and lignin, nature’s glue for holding the cells and fibers together, which is from 20 to 28%. Residues in the form of minerals, waxes, tannins, oils, etc. compose the remainder. The residues, although small in volume, often provide a species with unusual properties. The oils in cypress are responsible for its renown as a decay-resistant wood. Aromatic oils provide many of the cedars with distinctive odors that make them valuable for clothing storage chests. Other chemicals provide resistance to water absorption, which is useful for constructing light, high-speed boats that are relatively free from increase in weight due to water absorption. Many of the chemical residues of wood can be removed by neutral solvents, such as water, alcohol, acetone, benzene, and ether. Some of them may be caused to migrate from one part of the wood to another.

STRUCTURE

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WOOD

The roots, stem, and branches of a tree increase in size by adding a new layer each year, just as the size of one’s hand is increased by putting on a glove. The layer or growth ring will vary in thickness due to the age of the tree, growing conditions, amount of foliage, and other factors. The growth ring is divided into two parts — spring wood and summer wood. The former is usually lighter in weight than the latter and is denser and stronger. Generally, a tree or portion of a tree with the most summer wood is stronger than one that has less. The thickness of the growth ring and the relative amounts of spring and summer wood have great effect on the appearance of wood and are often a deciding factor in the choice of furniture materials. Excessively thick growth rings usually provide a rather coarse-textured material, whereas narrow growth rings provide fine texture. However, in hardwoods a thick ring may


and usually does have more summer wood and is therefore the stronger of the two. The situation is somewhat different for the softwoods. Grades of Lumber Modern grading of lumber is the result of experience. Over a period of 90 years there has been a gradual evolution to meet the changes in industry. This trend will continue as long as wood products are used. The grading of hardwoods and softwoods is entirely different; the former are used almost entirely as raw materials for manufactured products. For them, basic grading is on the number of usable cuttings or pieces that can be cut from a board. For some grades these cuttings need only be sound, but for others they must be clear at least on one face. The cuttings must also be of a certain size. Hardwood Grading The grades of hardwood lumber from the highest quality to the lowest are “Firsts,” the topquality grade; the next are known as “Seconds.” These two grades are usually marketed as one and called “Firsts and Seconds”; the designation for the grade is FAS. The next lowest grade is “Selects,” followed by “No. 1 Common,” “No. 2 Common,” “No. 3A Common,” and “No. 3B Common.” Sometimes a grade is further differentiated, such as “FAS One Face,” which means that it is of a much higher grade on one face than on the other. A prefix “WHND” is also used sometimes. It means that wormholes are not to be considered as defects in evaluating cuttings nor as a reason for disqualifying them as might otherwise be done. Softwood Grading The theory of softwood lumber grading is probably somewhat more difficult for the layperson to understand than that used for the hardwoods. This is because many of the species are graded under separate association rulings, which are similar, but with some important differences. Furthermore, softwoods are divided into three general classes of products, each of which is graded under a different set of rules.


1. Yard lumber, which is used for building construction and other ordinary uses, is obtainable in “Finish Grades,” which are called “A,” “B,” “C,” and “D”; “Common Boards,” which are called “No. 1,” “No. 2,” “No. 3,” “No. 4,” and sometimes “No. 5”; and as “Common Dimension” in grades “No. 1,” “No. 2,” and “No. 3.” 2. Structural lumber is a relatively modern concept in the field of lumber grading. It is an engineered product, intended for use where definite strength requirements are specified. The allowable stresses designated for a piece of structural lumber depend upon the size, number, and placement of the defects. The relative position of a defect is of great importance; therefore, if the maximum strength of the piece is to be developed it must be used in its entirety. It cannot be remanufactured for width, thickness, or length. 3. Factory and shop lumber is the third general category. These grades are similar to those for hardwoods in that the lumber is graded by the number of usable cuttings that can be taken from a board, but here the resemblance ceases, for both the grade descriptions and nomenclature are different. The term “Factory and Shop” is descriptive of the uses for which the product is designed. Much of it is used for general millwork products, patterns, models, etc., and wherever it is necessary to cut up softwood lumber for the production of factory-made items.

WOOD CHEMICALS These are chemicals obtained from wood. The practice was carried out in the past, and continues wherever technical utility and economic conditions have combined to make it feasible. Woody plants comprise the greatest part of the organic materials produced by photosynthesis on a renewable basis, and were the precursors

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of the fossil coal deposits. Future shortages of the fossil hydrocarbons from which most organic chemicals are derived may result in the economic feasibility of the production of these chemicals from wood. Wood is a mixture of three natural polymers — cellulose, hemicelluloses, and lignin — in an approximate abundance of 50:25:25. In addition to these polymeric cell wall components, which make up the major portion of the wood, different species contain varying amounts and kinds of extraneous materials called extractives. Cellulose is a long-chain polymer of glucose that is embedded in an amorphous matrix of the hemicelluloses and lignin. Hemicelluloses are shorter or branched polymers of five- and sixcarbon sugars other than glucose. Lignin is a three-dimensional polymer formed of phenylpropane units. Thus the nature of the chemicals derived from wood depends on the wood component involved.

into the chemicals usually derived from petroleum. Processes for which technical feasibility has been demonstrated are shown in the illustration. Economic feasibility is influenced by fossil hydrocarbon cost and availability.

WOOD DEGRADATION This refers to decay of the components of wood. Despite its highly integrated matrix of cellulose, hemicellulose, and lignin, which gives wood superior strength properties and a marked resistance to chemical and microbial attack, a variety of organisms and processes are capable of degrading wood. The decay process is a continuum, often involving a number of organisms over many years. Wood degrading agents are both biotic and abiotic, and include heat, strong acids and bases, organic chemicals, mechanical wear, and sunlight (ultraviolet degradation). Engineering Design

Modern Processes

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Chemicals derived from wood at present include bark products, cellulose, cellulose esters, cellulose ethers, charcoal, dimethyl sulfoxide, ethyl alcohol, fatty acids, furfural, hemicellulose extracts, kraft lignin, lignin sulfonates, pine oil, rayons, rosin, sugars, tall oil, turpentine, and vanillin. Most of these are either direct products or byproducts of wood pulping, in which the lignin that cements the wood fibers together and stiffens them is dissolved away from the cellulose. High-purity chemical cellulose or dissolving pulp is the starting material for such polymeric cellulose derivatives as viscose rayon and cellophane (regenerated celluloses from the xanthate derivative in fiber or film form), cellulose esters such as the acetate and butyrate for fiber, film, and molding applications, and cellulose ethers such as carboxymethylcellulose, ethylcellulose, and hydroxyethylcellulose for use as gums. Potential Chemicals Considerable development effort has been devoted to the conversion of renewable biomass, of which wood is the major component,


This is the process of creating products, components, and structural systems with wood and wood-based materials. Wood engineering design applies concepts of engineering in the design of systems and products that must carry loads and perform in a safe and serviceable fashion. Common examples include structural systems such as buildings or electric power transmission structures, components such as trusses or prefabricated stressed-skin panels, and products such as furniture or pallets and containers. The design process considers the shape, size, physical and mechanical properties of the materials, type and size of the connections, and the type of system response needed to resist both stationary and moving (dynamic) loads, and function satisfactorily in the end-use environment. Wood is used in both light frame or heavy timber structures. Light frame structures consist of many relatively small wood elements such as lumber covered with a sheathing material such as plywood. The lumber and sheathing are connected to act together as a system in resisting loads; an example is a residential house wood floor system where the plywood is nailed to lumber bending members or joists. In this system, no one joist is heavily loaded because


hydrocarbons tannins rosin turpentine waxes phenolic acids fatty acids gums

ammonia methanol hydrocarbons

CO + H2

cellulose Gassification

gas tar charcoal

cellulose derivatives paper

Pyrolysis

Extraction

dimethyl sulfoxide alkali-lignin lignin sulfonates vanillin

and

lignin

Pulping

WOOD

Hydrolysis organic acids alcohols ethanol

Fermentation

sugars lignin

Reduction acid

xylitol sorbitol glycols ethylene oxygenated butadiene aliphatics

polymers

polymers

furfural levulinic acid

polymers

aromatic chemicals phenols

polymers

FIGURE W.1 Chemical pathways for obtaining chemicals from wood. (From McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 19, McGraw-Hill, New York, 579. With permission.)

the sheathing spreads the load out over many joists. Service factors such as deflection or vibration often govern the design of floor systems rather than strength. Light frame systems are often designed as diaphragms or shear walls to resist lateral forces resulting from wind or earthquake. In heavy timber construction, such as bridges or industrial buildings, there is less reliance on system action and, in general, large beams or columns carry more load transmitted through decking or panel assemblies. Strength, rather than deflection, often governs the selection of member size and connections. There are many variants of wood construction using poles, wood shells, folded plates, prefabricated panels, logs, and combinations with other materials. Engineered Wood Composites Wood composites are products composed of wood elements that have been glued together to make a different, more useful or more economical product than solid sawn wood. Plywood is a common example of a wood-composite


sheathing panel product where layers of veneer are glued together. Plywood is used as a sheathing material in light frame wood buildings, wood pallets, and containers to distribute the applied forces to beams of lumber or other materials. Shear strength, bending strength, and stiffness are the most important properties for these applications and may be engineered into the panel by adjusting the species and quality of veneer used in the manufacture.

PROCESSING Processing involves peeling, slicing, sawing, and chemically altering hardwoods and softwoods to form finished products such as boards or veneer; particles or chips for making paper, particle, or fiber products; and fuel. Most logs are converted to boards in a sawmill that consists of a large circular or band saw, a carriage that holds the log and moves past the saw, and small circular saws that remove excess bark and defects from the edges and ends of the boards. One method is to saw the log to boards with a single pass through several saw blades

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mounted on a single shaft (a gang saw). Sometimes, the outside of the log is converted to boards or chips until a rectangular center or cant remains. The cant is then processed to boards with a gang saw. Other steps taken may be drying and machining, including veneer cutting. Wood is also ground to fibers for hardboard, mediumdensity fiberboard, and paper products. It is sliced and flaked for particle-board products, including wafer boards and oriented strand boards. Whether made from waste products (sawdust, planer shavings, slabs, edgings) or roundwood, the individual particles generally exhibit the anisotropy and hygroscopicity of larger pieces of wood. The negative effects of these properties are minimized to the degree that the three wood directions (longitudinal, tangential, and radial) are distributed more or less randomly.

WOOD PRODUCTS

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Wood products are those products, such as veneer, plywood, laminate of products, particleboard, waferboard, pulp and paper, hardboard, and fiberboard, made from the stems and branches of coniferous (softwood) and deciduous (hardwood) tree species. The living portion of the tree is the region closest to the bark and is commonly referred to as sapwood; the dead portion of the tree is called heartwood. In many species, especially hardwoods, the heartwood changes color because of chemical changes in it. The heartwood of walnut, for example, is dark brown and the sapwood almost white. Wood is one of the strongest natural materials for its weight. A microscopic view reveals thousands of hollow-tubed fibers held together with a chemical called lignin. These hollowtubed fibers give wood its tremendous strength for its light weight. These fibers, after the lignin bonding material is removed, make paper. In addition to lignin, wood is composed of other chemicals, including cellulose and hemicellulose. This category includes lumber products, veneer, and structural plywood.


Lumber Most small log sawmills try to maximize value and yield by automating as much of the manufacturing process as possible. The basic process of cutting lumber involves producing as many rectangular pieces of lumber as possible from a round tapered log. There are only a few sawing solutions, out of millions possible, that will yield the most lumber from any given log or larger timber. Veneer and Structural Plywood Structural plywood is constructed from individual sheets of veneer, often with the grain of the veneer in perpendicular directions in alternating plies. The most common construction is three-, four-, and five-ply panels. The alternating plies give superior strength and dimensional stability.

COMPOSITE PRODUCTS These include laminated products made from lumber, particleboard, waferboard, and oriented strand board. Laminated Products Laminated products are composite products, made from lumber, parallel laminated veneer, and sometimes plywood, particleboard, or other fiber product. The most common types are the laminated beam products composed of individual pieces of lumber glued together with a phenol resorcinol-type adhesive. Laminated beams are constructed by placing high-quality straight-grained pieces of lumber on the top and bottom, where tension and compression stresses are the greatest, and lower-quality lumber in the center section, where these stresses are lower. Another form of composite beam is constructed from individual members made from parallel laminated veneer lumber. This type of lumber has the advantage that it can be made into any length, thus creating beams to span large sections. These beams can be made entirely of parallel laminated veneer lumber, or with the top and bottom flange made from parallel laminated veneer lumber, and the center


web made from plywood or flakeboard. Some of these products use solid lumber for the flange material and resemble an I beam. All structural laminated products have the advantage over lumber in that much of the natural variation due to defects is removed, and wood structural members can be made much larger than the typical 51 × 305 mm lumber product. Particleboard Nonstructural particleboard is another type of composite product that is usually made from sawdust or planer shavings. It is sometimes made from flaked roundwood. This type of particleboard is one of the most widely used forms of wood product. Often hidden from view, it is used as a substrate under hardwood veneer or plastic laminates. It is commonly used in furniture, cabinets, shelving, and paneling. Particleboard is made by drying, screening, and sorting the sawdust and planer shavings into different size classifications.

roundwood that is chipped, and recycled paper. The fibers in the chips must be separated from each other by mechanically grinding the fibers or chemically dissolving the lignin from them. The most common chemical processes are sulfite and sulfate (kraft). Following the pulping process, the fibers are washed to remove pulping chemicals or impurities. In some processes (for example, writing papers), the fibers are bleached. Dry, finished paper emerges from the end of this section, and it is placed in rolls for further manufacture into paper products. Hardboard Hardboard is a medium- to high-density wood fiber product made in sheets from 1.6 to 12.7 mm. Hardboard is used in furniture, cabinets, garage door panels, vinyl overlaid wall panels, and pegboard. It is made by either a wet or dry process. Medium-Density Fiberboard

Waferboard and Oriented Strand Board Waferboard and oriented strand board are structural panels made from flakes or strands, and are usually created from very small trees. Unlike nonstructural particleboard, waferboard is designed for use in applications similar to those of plywood. Waferboard and oriented strand board can have flakes or strands oriented in the same direction, thus giving the board greater strength in the long axis. The most common type of waferboard is made from randomly oriented flakes.

FIBER PRODUCTS The most common fiber products result from pulping processes that involve the chemical modification of wood chips, sawdust, and planer shavings. Such products include pulp and paper, hardboard, and fiberboard.

Medium-density fiberboard is used in many of the same applications where particleboard is used. It can be used in siding and is especially well suited to cabinet and door panels where edges are exposed. Unlike particleboard, which has a rough edge, medium-density fiberboard has a very fine edge that can be molded very well. Medium-density fiberboard is produced in much the same way that dry-processed hardboard is produced in its early stages. The chips are thermomechanically pulped or refined prior to forming into a dry mat. Following refining, medium-density fiberboard is produced in a fashion similar to that of particleboard. The dry pulp is sprayed with adhesive (usually ureaformaldehyde or phenol-formaldehyde) and formed into a dry mat prior to pressing in a multiopening hot press. After the panels have been formed, they are cooled and cut to smaller final product sizes prior to shipment.

OTHER ENGINEERED WOOD PRODUCTS Pulp and Paper Paper making begins with the pulping process. Pulp is made from wood chips created in the lumber manufacturing process, small


The list of processes and potential processes that are available for wood construction is almost endless. The laminating of wood can be extended to using species of one kind for the

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surfaces and another for the core. Also, other species having altogether different properties may be used in other places of the assembly. Decking for aircraft carriers is a good example: the surface of the decking must have high abrasion resistance, but great strength in bending is also required. The combination of several species in the same timber is therefore most effective. Barrels and other cylindrical containers are made of laminated wood staves, plywood, or veneer. Hogsheads for tobacco export are of solid staves or plywood. Wood can be treated to provide considerable fire resistance; it then can be destroyed by heat, but will not support combustion. Natural wood is not adversely affected by extremes of cold such as are encountered in high latitudes and is therefore much in use for shelters, tools, sporting equipment, and other products where these conditions exist. Wood is widely used for structures and products that must be nonmagnetic; hence, it is used for minesweepers and similar craft. Laminated wood has no peer for this type of product. Special treatments have been developed for use in power-line construction. These treatment materials must act as nonconductors as well as preservatives. Wood has excellent thermal insulating properties. It is used for refrigerated spaces, refrigerated delivery trucks, and in a great many other places where light weight and thermal insulation, combined with strength, are necessary. For example, specially treated milk containers are in widespread use in the dairy industry. Special forms of timbers are manufactured for boxcar decking. Decking of this type provides a medium for fastening cargo as well as furnishing other needed functions. In the textile industry, wood, both treated and in the natural form, is used for shuttles and bobbins, pulleys, and many other types of machinery. Wood, in a natural form, treated, or laminated, is used for water and other liquid conduits, cooling towers, and chemical containers.


PRESERVATION/PROTECTION There are several wood-preservation methods. The system used depends upon the service requirements. For example, a railroad tie, once in place, must withstand continuous exposure to the elements and almost every conceivable condition that promotes decay, until it is worn out some 20 to 30 years later. It is therefore necessary to provide this and similar items with maximum protection. The two general methods of treating are surface applications where the wood is exposed to the chemicals by dipping, soaking, or brushing, and treatments wherein the chemicals are forced into the wood through pressure. These latter treatments are used for the most critical applications. Pressure-treating operations must be conducted in plants with considerable equipment. It is a specialized business, and a large industry has arisen to take care of the many needs for highly treated products. The most effective way to prevent damage through exposure of untreated wood is to perform all major machining operations prior to treating. If this is impossible, the exposed parts should be given a surface treatment, by brushing or dipping. Unfortunately, neither of these treatments is as effective as the original pressure treating. All domestic species can be pressure-treated, but some of them require it less than others. The inherently decay-resistant species, such as cypress, redwood, some of the cedars, and white oak, need no treatment for most uses. But this applies only to the heartwood of any specie. Of the various chemicals used for woodpreservative treatment, with the pressure system, creosote is one of the oldest and best. However, it has an odor that is sometimes offensive and is therefore not generally suitable for manufactured items that may be used in contact with the body, or adjacent to food or enclosed places where the fumes may become objectionable. It can also be somewhat of a fire hazard and is sometimes not used for this reason. A third objection to creosote is the difficulty of painting over it. However, creosote is very effective against decay fungi and insect damage. There are several effective forms of copper salts. These can be painted over; they do not


contribute to fire hazard and are odorless. They are extensively used in the manufacture of boats and other marine appliances. A relatively new chemical for decay and insect prevention is pentatchloraphenol. This has most of the advantages of creosote, except that it is probably not as effective against termite damage and is more expensive, but it lacks most of the disadvantages and can be used in enclosed places. For food containers preservative treatments are not generally recommended unless the conditions are closely examined and there is assurance that the chemicals are approved under the existing laws and are in no way harmful to health. The second method of treatment is by one of the surface systems. Generally, the chemicals used for this purpose are almost the same as for pressure treating. However, they are often specially prepared and their carriers may differ to obtain greater natural penetration. Of the various methods of application dipping is the most efficient, for in this way all of the surface is exposed to the chemicals and a maximum amount of usable chemical is deposited, which may not be the case with brush or spray treatments. Often, heating the chemicals and cooling the pieces during dipping will increase penetration. However, long periods of soaking are not usually of much advantage. It is better to dip the wood parts for several minutes (sufficient time to assure that surface exposure is complete) and then pile the pieces closely together soon after they are removed from the chemical bath. Several days of natural absorption under these conditions will often provide a surprising amount of penetration. The success of this method depends to a considerable extent on the type of chemical used and particularly the carrier. In the millwork industry the surface method of application is of tremendous importance and is widely used for nearly all its products. Windows, storm sash, exterior trim, and most other products exposed to weather are effectively treated. In addition, the preservative chemicals are often combined with waxes or oils to provide a reasonable amount of dimensional stability.


PROPERTIES The physical and mechanical characteristics of wood are controlled by specific anatomy, moisture content, and to a lesser extent mineral and extractive content. The properties are also influenced by the directional nature of wood, which results in markedly different properties in the longitudinal, tangential, and radial directions or axes. Wood properties within a species vary greatly from tree to tree and within a single axis. The physical properties (other than appearance) are moisture content, shrinkage, density, permeability, and thermal and electrical properties. The mechanical properties of wood include elastic, strength, and vibration characteristics. These properties are dependent upon species, grain orientation, moisture content, loading rate, and size and location of natural characteristics such as knots. Because wood is an orthotropic material, it has unique and independent mechanical properties in each of three mutually perpendicular axes — longitudinal, radial, and tangential. This orthotropic nature of wood is interrupted by naturally occurring characteristics such as knots that, depending on size and location, can decrease the stiffness and strength of the wood.

WOOD-BASED FIBER AND PARTICLE MATERIALS Flat-formed board products can be classified into two groups: (1) Those primarily from fiber interfelted during manufacture with a predominantly natural bond, although extraneous material may be added to improve some property such as bond (or other strength property) and water resistance, and (2) those made from distinct fractions of wood with the primary bond produced by an added bonding material. Moldings, based on a composition of woodbased fiber or particle and binding material similar to those used for boards, have found increasing use. Production statistics for molded units of fiber and wood particle are difficult to obtain because the units are used in such widely different commodities as toilet seats, croquet balls, school desktops and seats, frames for luggage,

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and armrests, door panels, and other molded components for automobiles. The number of uses for these to moldings is increasing because they combine shape with adequate structural strength and durability for many uses. See Wood.

WOOD–METAL LAMINATES Wood–metal laminates constitute a composite panel construction in which the core material is made up of a wood or wood-derivative slab to which metal facing sheets are adhesively bonded. The core material is usually plywood or a composition board of wood fibers or chips compressed and bonded together to form a flat core slab. Balsa wood and insulation boards are frequently used as core materials where thermal insulation is a requirement. The metal facing sheets may be steel, aluminum, stainless steel, porcelain enameled metals, rigidized metals, or metals with decorative finishes. Light-gauge metals are usually used, ranging in thickness from 0.25 to 1.5 mm depending upon the specific strength, stiffness, and service requirements. Adhesives used in bonding the metal to the core material are selected to meet the desired service requirements. For most standard laminates the adhesive is water resistant and fungusproof so that the panels may be subjected to exterior as well its interior exposure. Continuous service temperatures may range from a minimum of –51°C to a maximum of 77°C, although specially prepared laminates are available for continuous use as high as 177°C.

GREATER STIFFNESS

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Wood–metal laminates are designed to utilize the best properties of each of the component parts providing a panel that is not only light in weight but also has good structural strength and high flexural rigidity. Because the metal facing sheets are supported by a core material of substantial thickness, smooth flat panels free from waves and buckles are obtained with lightgauge metals. Wood–metal laminates are limited in size only by the size of the press equipment and


commercial sizes of the metal sheets and core material available. The panels may be sawed to exact size from stock size sheets. Frequently, however, the facing sheets are fabricated prior to bonding to the core material to provide special edge details.

APPLICATIONS Wood–metal laminates are used extensively in the architectural building and transportation fields. Curtain wall panels, column enclosures, partition panels, facia, and soffit panels are typical applications. Truck and trailer bodies, shipping containers, and railroad car partition panels and doors are common uses of the laminate. In general, wherever light weight, combined with high rigidity and structural strength, is prerequisite, wood–metal laminates may be used to good advantage.

WOODS, IMPORTED There are over 100 different species of foreign woods that are imported into the United States. By far, the greatest number of these are used for decorative or nonengineering purposes — some are used for both. Imported woods to be considered here are all defined as imported species, except those that are also native to the United States. Thus, all Canadian kinds are excluded because the United States has every species of wood that grows in Canada. Mexico, on the other hand, supplies some tropical woods not found in the United States. There are pines from the highlands of Mexico and Central America that are imported in fairly sizable volume at times, but the engineering aspects of their utilization coincide directly with the Southern pine found in the United States; consequently, no attempt is made to describe these. No other softwoods (conifers) are imported for use as engineering materials. Thus, all types discussed here are categorized as hardwood (broadleaf) species. Even balsa is a hardwood, because the terms “hardwood” and “softwood” in lumber-industry parlance allude to the botanical classification rather than the actual hardness or softness of the wood.


There are over 13 kinds of tropical hardwoods known to be used for engineering products or purposes. In certain parts of the world there are vast expanses of untouched tropical forests, which, no doubt, contain several other hardwoods of potential engineering value. However, the woods in current use are typical imported woods and applications: 1. Refrigeration: balsa 2. Wharves and docks: Greenheart, ekki, jarrah, ironbark, apitong, angelique 3. Boat construction: Philippine mahogany, Central and South American mahogany, African mahogany, balsa, apitong, teak, iroko, ironbark, jarrah, lignum vitae 4. Tanks and vats: Philippine mahogany, apitong 5. Building construction: mahogany (all types), apitong, balsa, greenheart 6. Poles, piling: greenheart, ekki 7. Machinery: lignum vitae 8. Aircraft and missiles: balsa 9. Vehicles: apitong

WROUGHT IRON Wrought iron is commercially pure iron made by melting white cast iron and passing an oxidizing flame over it, leaving the iron in a porous condition, which is then rolled to unite it into one mass. As thus made, it has a fibrous structure, with fibers of slag through the iron in the direction of rolling. It is also made by the Aston process of shooting Bessemer iron into a ladle of molten slag. Modern wrought iron has a fine dispersion of silicate inclusions that interrupt the granular pattern and give it a fibrous nature. Structurally, wrought iron is a composite material; the base metal and the slag are in physical association, in contrast to the chemical or alloy relationship that generally exists between the constituents of other metals. The form and distribution of the iron silicate particles may be stringerlike, ribbonlike, or platelets. Practically, the physical effects of the incorporated iron silicate slag must be taken into consideration in bending and forming wrought iron pipe, plate, bars, and shapes, but


when properly handled — cold or hot — fabrication is accomplished without difficulty.

MECHANICAL PROPERTIES The value of wrought iron is in its corrosion resistance and ductility. It is used chiefly for rivets, staybolts, water pipes, tank plates, and forged work. Minimum specifications for ASTM wrought iron call for a tensile strength of 275 MPa, yield strength of 165 MPa, and elongation of 12%, with carbon not over 0.08%, but the physical properties are usually higher. Wrought iron 4D has only 0.02% carbon with 0.12% phosphorus, and the fine fibers are of a controlled composition of silicon, manganese, and phosphorus. This iron has a tensile strength of 330 MPa, elongation 14%, and Brinell hardness 105. Manganese wrought iron has 1% manganese for higher impact strength. Ordinary wrought iron with slag may contain frequent slag cracks, and the quality grades are now made by controlled additions of silicate, and with controlled working to obtain uniformity. But for tanks and plate work, ingot iron is now usually substituted. The Norway iron formerly much used for bolts and rivets was a Swedish charcoal iron brought to America in Norwegian ships. This iron, with as low as 0.02% carbon, and extremely low silicon, sulfur, and phosphorus, was valued for its great ductility and toughness and also for its permeability qualities for transformer cores. Commercial wrought iron is now usually ingot iron or fibered low-carbon steel. The tensile properties of wrought iron are largely those of ferrite plus the strengthening effect of any phosphorus content, which adds approximately 6.8 MPa for each 0.01% above 0.10% of contained phosphorus. Strength, elasticity, and ductility are affected to some degree by small variations in the metalloid content and in even greater degree by the amount of the incorporated slag and the character of its distribution. Nickel, molybdenum, copper, and phosphorus are added to wrought iron to increase yield and ultimate strengths without materially detracting from toughness as measured by elongation and reduction in area.

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FABRICATION Forging Wrought iron is an easy material to forge using any of the common methods. The temperature at which the best results are obtained lies in the range of 1149 to 1316°C. Ordinarily “flat and edge” working is essential for good results. Limited upsetting must be accomplished at “sweating to welding” temperatures. Bending Wrought-iron plates, bars, pipe, and structurals may be bent either hot or cold, depending on the severity of the operation, keeping in mind that bending involves the directional ductility of the material. Hot bending ordinarily is accomplished at a dull red heat (704 to 760°C) below the critical “red-short” range of wrought iron (871 to 927°C). The ductility available for hot bending is about twice that available for cold bending. Forming of flanged and dished heads is accomplished hot from special-forming, equal-property plate. Welding

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Wrought iron can be welded easily by any of the commonly used processes, such as forge welding, electric resistance welding, electric metallic arc welding, electric carbon arc welding, and gas or oxyacetylene welding. The iron silicate or slag included in wrought iron melts at a temperature below the fusion point of the iron-base metal, so that the melting of the slag gives the metal surface a greasy appearance. This should not be mistaken for actual fusion of the base metal; heating should be continued until the iron reaches the state of fusion. The siliceous slag content provides a self-fluxing action to the material during the welding operation. Threading The machinability or free-cutting characteristics of most ferrous metals are adversely influenced by either excessive hardness or softness. Wrought iron displays almost ideal hardness for good machinability, and the entrained silicate produces chips that crumble and clear the dies.


Standard threading equipment that incorporates minor variations in lip angle, lead, and clearance is usually satisfactory with wrought iron. Protective Coatings Wrought iron lends itself readily to such cleaning operations as pickling and sandblasting for the application of protective coatings. Where protective coatings such as paint or hot-dipped metallic coatings are to be applied, the coatings are found to adhere more firmly to wrought iron and a thicker coat will be attained compared with other wrought ferrous metals. This is because the natural surface of wrought iron is microscopically rougher than other metals after cleaning, thus providing a better anchorage for coatings. Corrosion Resistance The resistance of wrought iron to corrosion has been demonstrated by long years of service life in many applications. Some have attributed successful performance to the purity of the iron base, the presence of a considerable quantity of phosphorus or copper, freedom from segregation, to the presence of the inert slag fibers disseminated throughout the metal, or to combinations of such attributes. In actual service, the corrosion resistance of wrought iron has shown superior performance in such applications as radiant heating and snow-melting coils, skating-rink piping, condenser and heat-exchanger equipment, and other industrial and building piping services. Wrought iron has long been specified for steam condensate piping where dissolved oxygen and carbon dioxide present severe corrosion problems. Cooling water cycles of the once-through and open-recirculating variety are solved by the use of wrought-iron pipe.

APPLICATIONS Building Construction — Hot and cold potable water, soil, waste, vent, and downspout piping; radiant heating, snow melting, air-conditioning cooling and chilled-water lines; gas, fire protection, and soap lines; condensate


and steam returns, ice-rink and swimming-pool piping; underground service lines and electrical conduit. Industrial — Unfired heat exchangers, brine coils, condenser tubes, caustic soda, concentrated sulfuric acid, ammonia, and miscellaneous process lines; sprinkler systems, boiler feed and blowoff lines, condenser water piping, runner buckets, skimmer bars, smokestacks and standpipes, salt and water well pipe and casing. Public Works/Infrastructure — Bridge railings, fenders, blast plates, drainage lines and troughs, traffic signal conduit, sludge digestor heating coils, aeration tank piping, sewer outfall lines, large outside diameter intake and discharge lines, trash racks, weir plates, dam gates, pier-protection plates, sludge tanks and lines, dredge pipe.

Railroad and Marine — Tie spacer bars, diesel exhaust- and air-brake piping, ballast and brine protection plates, brine, cargo and washdown lines on ships, hull and deck plating, rudders, fire screens, breechings, tanker heating coils, car retarder and yard piping, spring bands, car charging lines, nipples, pontoons, car and switch deicers. Others — Gas collection hoods, staybolts, flue gas conductors, sulfur mining gut, air and transport lines, coalhandling equipment, chlorine, compressed air lines, distributor arms, cooling tower and spray pond piping. Wrought iron is available in the form of plates, sheets, bars, structurals, forging blooms and billets, rivets, chain, and a wide range of tubular products, including pipe, tubing and casing, electrical conduit, cold-drawn tubing, and welded fittings.

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X X-RAY PHOTO-ELECTRON SPECTROSCOPY Carbon black filled rubber parts may challenge many analytical methods because the compounded rubber may include up to 20 individual ingredients. Often, after mixing, it is difficult to extract ingredients from the polymer base because of their interactions with other ingredients such as carbon black. Testing of bonded rubber-to-metal parts amplifies the problems twofold. The bonding adhesive is also a multicomponent polymeric blend, much like the rubber. Also involved are contaminants associated with the substrate. For example, a stamped steel insert may be contaminated with hydrocarbons. If the metal is porous, oil may bleed out of the metal grain over time and undermine a bond system. Or, if the oil is present initially, it may prevent adhesive wet-out, thus inhibiting adhesion to the substrate.

DISADVANTAGES Some techniques such as reflectants Fourier transform infrared spectrometer (FTIR) and energy dispersive x-ray (EDS) would not be adequate for testing rubber bonded to metal. EDS, using a windowless system, does not have sufficient resolution for lower-weight elements.

THE PROCESS To gain the most definitive picture of failed bonded surfaces, scanning electron microscopy (SEM) and x-ray photo-electron spectroscopy (XPS or ESCA) methods have proved the most valuable. XPS is an excellent method for determining chemical composition of a surface. This method


penetrates the surface of the sample to a depth of only 50 Å. XPS measures the binding energies of the atoms present on the surface, yielding a very distinctive signature of the atomic species present. XPS is sensitive enough to generate signals that provide both a quantitative and qualitative picture of the surface composition over a wide elemental range. Computer analysis techniques convert these signals to atomic percentages to allow reconstruction of the various materials present.

CASE STUDY The following case study analyzes black rubber bonded to a steel part. The part was exposed to a hostile environment (hydrocarbon fluids at 150°C) for a relatively short period of time before failure occurred. Parts of this type are expected to last approximately 8 years; this part failed in under 6 months. Initial visual analysis indicated failure primarily in an adhesive mode. However, no apparent cause of the failure was easily identifiable. Gaining a clear understanding of the cause of the adhesive failure at this point was difficult because the part was saturated with hydrocarbon oil. The cross section of the remaining bonded area indicated that under normal conditions, the adhesive could not be distinguished from the rubber. This would indicate a high degree of commingling is necessary between rubber and adhesive to achieve a good bond. At this point the unbond was believed to be caused by an overbake of the adhesive. In this situation, the poor bond is due to premature cross-linking of the adhesive prior to the application of rubber. The cross-linking of the adhesive causes mechanisms for adhesion to be compromised and inhibits physical interaction.

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To confirm this, both a control part, with bond failure due to adhesive overbake, and the failed part were subjected to analysis by XPS. Only failed mating surfaces were examined by this method. Typical areas scanned by this method were 1 × 3 mm. It is not necessary to neutralize with the electron gun due to only minimal charging effects. Only a single sample was tested for each condition. The XPS analysis of the mating surfaces of the defective part showed the surfaces to be very similar in elemental composition. Values from the failed part, when compared to the control part, indicated the hydrocarbon fluid did not affect the analysis. This is most likely because the high vacuum necessary in this testing technique removed the hydrocarbon fluid from the surface. The trace elements present in analysis of both surfaces of both parts support the conclusion that the parts failed by the same mechanism. Test results support the idea that both samples failed in a boundary layer between the rubber and the adhesive. Trace elements, known not to be native to the rubber compound, indicate the source to be the adhesive. From the presence of adhesive components on all mating surfaces tested, we can effectively eliminate contamination as the cause of the unbond. The presence of the adhesive and the pulled-out material present on the failed part would indicate the part may have been weakly bonded. Finally, the evident lack of adhesive and rubber interspersion would indicate one or both components had achieved sufficient viscosity to resist flow. The combined evidence using XPS tends to support our initial observation that all adhesive had prematurely cross-linked. These data allow the cause to be correctly identified and will help produce a more robust part and processing in the future.

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X-RAY TECHNOLOGY As with medical x-ray instruments there are analytical x-ray instruments that can produce images of internal structures of objects that are opaque to visible light. There are instruments that can determine the chemical elemental composition of an object, that can identify the crys-


talline phases of a mixture of solids, and others that determine the complete atomic and molecular structure of a single crystal. The determination of particle size and structural information for fibers and polymers, and the study of stress, texture, and thin films are x-ray applications that are growing in importance.

CHARACTERISTICS X-RAYS

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GENERATION

OF

X-Ray Electromagnetic Spectrum X-rays are a form of electromagnetic radiation and have a wavelength, λ, much shorter than visible light. The center of the visible light spectrum has a wavelength of about 0.56 × 10–6m. The most commonly used methods for generating x-rays are the synchrotron and x-ray tubes. Synchrotron Radiation X-rays are produced when very energetic electrons traveling close to the speed of light are decelerated. In synchrotrons, electrons are accelerated with electromagnets while traveling along a linear path. Then they are inserted into a nearly circular path, which is maintained by bending magnets. X-Ray Tubes X-ray tubes are the most widely used source for the generation of x-rays. In these tubes, electrons are accelerated by a high electric potential (20 to 120 kV). These electrons strike the target (anode) of the tube and decelerate as they pass through the electron clouds of the atoms. This phenomenon produces a continuous spectrum similar, but much less intense, to that of the synchrotron. In addition, some highenergy electrons knock electrons out of the atomic orbitals of the atoms of the target material. When these orbitals are refilled by electrons, x-ray photons are generated. The resulting x-ray spectrum of intensity vs. wavelength has a series of peaks known as characteristic lines. The materials that are used as targets in x-ray tubes depend on the application.


APPLICATIONS X-ray applications can be placed into three categories: 1. X-ray radiography permits the imaging of the internal structure of an object (e.g., bones of a hand). It is based on the comparative observance of photons as they travel through the different materials making up the object. 2. X-ray fluorescence spectrometry consists of the measurement of the incoherent scattering of x-rays. It is used primarily to determine the elemental composition of a sample. 3. X-ray diffraction consists of the measurement of the coherent scattering of x-rays. X-ray diffraction is used to determine the identity of crystalline phases in a multiphase powder sample and the atomic and molecular structures of single crystals. It can also be used to determine structural details of polymers, fibers, thin films, and amorphous solids and to study stress, texture, and particle size. X-Ray Fluorescence Spectrometry X-ray fluorescence spectrometry is a technique for measuring the elemental composition of samples. The basis of the technique is the relationship between the wavelength or energy of the emitted incoherently scattered x-ray photons and the atomic number of the element. When an atom is bombarded with x-ray photons of sufficient energy, an inner-orbital electron may be displaced, leaving the atom in an excited state. The atom can return to the ground state by transference of a higher orbital electron into the vacancy (the resulting higher level vacancy is filled by an electron from a still higher level and so on). In so doing, the difference in energy between the electron ousted from the lower shell and the energy of the higher orbital electron is emitted as an x-ray photon. Each element produces a fluorescence


spectrum of intensity vs. wavelength that is characteristic of that element. X-Ray Radiography X-ray imaging tests are widely used to examine interior regions of metal castings, fusion welds, composite structures, and brazed components. Radiographic tests are made on pipeline welds, pressure vessels, nuclear fuel rods, and other critical materials and components that may contain three-dimensional voids, inclusions, gaps, or cracks. Since penetrating radiation tests depend upon the absorption properties of materials on x-ray photons, the tests can reveal changes in thickness and density and the presence of inclusions in the material. X-ray fluoroscopy is used for direct online examination. A fluorescent screen is used to convert x-ray photons into visible light photons. A television camera receives the visible image and displays it on a television screen. This type of system is used for security screening of carry-on luggage at airports. As in medical x-ray imaging, computerized tomography (CT) can reveal the details of the internal structure of complex objects. Many detectors are used to measure the transmittance of x-rays along many lines through the object. A computer uses this information to produce an image of the internal structure of a slice of the object.

XYLENE Xylenes and ethylbenzene (EB) are C8 aromatic isomers with the molecular formula C8H10. The xylenes consist of three isomers: o-xylene (OX), m-xylene (MX), and p-xylene (PX). These differ in the positions of the two methyl groups on the benzene ring structures.

USES The majority of xylenes, which are mostly produced by catalytic reforming or petroleum functions, are used in motor gasoline. The majority of the xylenes that are recovered for petrochemicals use are used to produce PX and OX. PX is the most important commercial isomer.

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XYLYLENE POLYMERS In a process capable of producing pinhole-free coatings of outstanding conformality and thickness uniformity through the unique chemistry of p-xylylene (PX), a substrate is exposed to a controlled atmosphere of pure gaseous monomer. The coating process is best described as a vapor deposition polymerization (VDP). The monomer molecule is thermally stable, but kinetically very reactive toward polymerization with other molecules of its kind. Although it is stable as a rarified gas, upon condensation it polymerizes spontaneously to produce a coating of high molecular weight, linear poly(p-xylylene) (PPX).

APPLICATIONS Because the parylenes are generally insoluble in most solvents, even at elevated temperatures, they cannot be used as solvent-based coatings; neither can they be cast as films nor spun as fibers from solution. The most important application of parylenes is as a conformal coating for printed wiring assemblies. These coatings provide excellent chemical resistance, and resistance to fungal attack. In addition, they exhibit stable dielectric properties over a wide range of temperatures. The use of parylenes as a hybrid circuit coating is based on much the same rationale as its use in circuit boards. A significant distinction lies in obtaining adhesion to the ceramic substrate material, the success of which determines the eventual performance of the coated part. Adhesion to the substrate must be

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achieved using adhesion promoters, such as the organosilanes. Parylenes are superior candidates for dielectrics in high-quality capacitors. Their dielectric constant and loss remain constant over a wide temperature range. The thermistor sensing probe of a disposable bathythermograph is coated with parylene. This instrument is used to chart the ocean water temperature as a function of depth. Parylene is used in the manufacture of highquality miniature stepping motors, such as those used in wristwatches, and as a coating for the ferrite cores of pulse transformers, magnetic tape-recording heads, and miniature inductors. Use of parylene in the medical field is linked to electronics, for example, as a protective conformal coating on pacemaker circuitry. As books age, the paper of their pages becomes brittle. A relatively thin coating of parylene can make these embrittled pages stronger. By separating the coating from the substrate after deposition, the unique coating features of parylenes, especially continuity and thickness control and uniformity, can be imparted to a freestanding film. Applications include optical beam splitters, a window for a micrometeoroid detector, a detector cathode for an x-ray streak camera, and windows for x-ray proportional counters. Parylenes can be used for contamination control, that is, securing small particles to prevent them from damaging a surface in a sealed unit; barrier coating; coating for corrosion control; and as dry lubricants.

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Y YARNS Yarns are assemblages or bundles of fibers twisted or laid together to form continuous strands. They are produced with either filaments or staple fibers. Single strands of yarns can be twisted together to form ply or plied yarns, and ply yarns in turn can be twisted together to form cabled yarn or cord. Important yarn characteristics related to behavior are fineness (diameter or linear density) and number of twists per unit length. The measure of fineness is commonly referred to as yarn number. Yarn numbering systems are somewhat complex, and they are different for different types of fibers. Essentially, they provide a measure of fineness in terms of weight per unit or length per unit weight. Cotton yarns are designated by numbers, or counts. The standard count of cotton is 840 yd/lb. Number 10 yarn is therefore 8400 yd/lb. A No. 80 sewing cotton is 80 × 840, or 67,200 yd/lb. Linen yarns are designated by the lea of 300 yd. A 10-count linen yarn is 10 × 300, or 3,000 yd/lb. The size or count of spun rayon yarns is on the same basis as cotton yarn. The size or count of rayon filament yarn is on the basis of the denier, the rayon denier being 450 m weighing 5 cg. If 450 m of yarn weighs 5 cg, it has a count of 1 denier. If it weighs 10 cg, it is No. 2 denier. Rayon yarns run from 15 denier, the finest, to 1200 denier, the coarsest. Reeled silk yarn counts are designated in deniers. The international denier for reeled silk is 500 m of yarn weighing 0.05 g. If 500 m weighs 1 g, the denier is No. 20. Spun silk count under the English system is the same as the cotton count. Under the French system the


count is designated by the number of skeins weighing 1 kg. The skein of silk is 1000 m. A ply yarn is one that has two or more yarns twisted together. A two-ply yarn has two separate yarns twisted together. The separate yarns may be of different materials, such as cotton and rayon. A six-ply yarn has six separate yarns. A ply yarn may have the different plies of different twists to give different effects. Ply yarns are stronger than single yarns of the same diameter. Tightly twisted yarns make strong, hard fabrics. Linen yarns are not twisted as tightly as cotton because the flux fiber is longer, stronger, and not as fuzzy as the cotton. Filament rayon yarn is yarn made from long, continuous rayon fibers, and it requires only slight twist. Fabrics made from filament yarn are called twalle. Monofilament is fiber heavy enough to be used alone as yarn, usually more than 15 denier. Tow consists of multifilament reject strands suitable for cutting into staple lengths for spinning. Spun rayon yarn is yarn made from staple fiber, which is rayon filament cut into standard short lengths.

YTTRIUM AND ALLOYS A chemical element, yttrium (symbol Y, atomic number 39, atomic weight 88.905) resembles the rare earth elements closely. The stable isotope yttrium-89 constitutes 100% of the natural element, which is always found associated with the rare earths and is frequently classified as one. Yttrium forms a white oxide, Y2O3, which dissolves in acid to form trivalent yttrium salts. Yttrium has become commercially important since 1964. Yttrium forms the matrix for the europlumactivated yttrium phosphors. These phosphors, when excited by electrons, emit a

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brilliant, clear-red light. The television industry uses these phosphors in manufacturing television screens. It is claimed that this phosphor gives better color reproduction and a much brighter screen than did the older non-rare-earth red phosphor. Also, the yttrium iron garnets, Y3Fe5O12, and other garnets have found important uses in radar and communication devices. They transmit shortwave energy with very small losses. Yttrium metal absorbs hydrogen, and in alloys up to a composition of YH2 they resemble metals very closely. In fact, in certain composition ranges, the alloy is a better conductor of electricity than the pure metal. The density of hydrogen near the YH2 composition is greater per cubic centimeter than it is in water or liquid hydrogen; therefore, such alloys make excellent potential moderators for nuclear reactors. Also, these alloys can be heated to a white heat (about 1260˚C) before the vapor pressure of hydrogen exceeds 1 atm (102 kPa), and therefore the moderator in the reactor can be operated at very high temperatures. Yttrium metal has a low nuclear cross section so it is also a potential structural material for reactors of the future. Yttrium is used commercially in the metal industry for alloy purposes and as a “getter” to remove oxygen and nonmetallic impurities in other metals. Radioactive yttrium isotopes have been used in attempts at treating cancer.

Yttrium is not a rare earth but always occurs with them in minerals because of similar general chemistry. Applications are in electrically conducting ceramics, refractories, insulators, phosphors, glass, special optical glasses, and other ceramics. White powder has cubic crystal structure and small amounts of dysprosium oxide, gadolinium oxide, and terbium oxide as impurities. Yttria can be compounded into polycrystalline as well as single-crystal garnets for use in microwave generation and detection devices. Such materials are important to microwave technology because they exhibit both good dielectric and magnetic properties, which can be controlled through compositional variations. Yttria-stabilized zirconia can be used to produce a high-quality diamond substitute for jewelry or a rugged sensor for measuring oxygen in automotive exhaust, depending on the method of fabrication. Nd:YAG single crystal rods find many applications as lasers in industry and in research. Y2O3 can be used (with scandium, lanthanum, and cesium oxides) with TiO2 bodies for better control of properties than experienced with alkaline earths. In combination with europium oxide, yttria is used to make the red phosphor in color television picture tubes. Combined with ZrO2, it makes good high-temperature refractories. It also is used in silicon nitride as a sintering aid.

YTTRIUM ALUMINUM GARNET Yttrium aluminum garnet (YAG) has the formula, Y3Al5O12; its crystals are capable of sustaining laser activity when doped with neodymium.

YTTRIUM OXIDE This oxide (Y2O3) has a melting point of 2685°C; a density of 5.03 g/cm3; is soluble in acids, but only slightly soluble in water.

Y


Y-TZP Yttria tetragonal zirconia polycrystal (Y-TZP) is a fine-grained ceramic used in special engineering applications that benefit from its high density, excellent wear resistance, and fine grain size, such as fiber-optic ferrules. Highpurity fine reactive coprecipitated zirconia powders containing 3 mol% yttria are used to produce Y-TZP ceramics.

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Z ZINC AND ALLOYS A bluish-white crystalline metal, zinc (symbol Zn) has a specific gravity of 7.13, melts at 420°C and boils at 906°C. The commercially pure metal has a tensile strength, cast, of about 62 MPa with elongation of 1%, and the rolled metal has a strength of 165 MPa with elongation of 35%. But small amounts of alloying elements harden and strengthen the metal, and it is seldom used alone. Zinc is seldom used alone except as a coating. In addition to its metal and alloy forms, zinc also extends the life of other materials such as steel (by hot dipping or electrogalvanizing), rubber and plastics (as an aging inhibitor), and wood (in paints). Zinc is also used to make brass, bronze, and nickel silver; die-casting alloys in plate, strip, and coil; foundry alloys; superplastic zinc; and activators and stabilizers for plastics. Additionally, zinc is used for electric batteries; for die castings; and in alloyed sheets for flashings, gutters, and stamped and formed parts. The metal is harder than tin, and an electrodeposited plate has a Vickers hardness of about 45. Zinc is also used for many chemicals.

PRODUCTION The metallurgy of zinc is dominated by the fact that its oxide is not reduced by carbon below the boiling point of the metal. A large fraction of the world’s zinc is still produced from relatively small horizontal retorts with one furnace (or bank) containing hundreds of such units. Other large, continuously operated vertical retorts have operated, with top charging of briquets of zinc oxide and bituminous coal, and metal tapping from an outside condenser.


Another continuous method involves electrothermic reduction, using a novel condenser in which the retort vapors are sucked through molten zinc. Neither horizontal or vertical retorts, electrothermic units, nor blast furnaces normally produce zinc of the extreme high purity required by much of the total market for zinc. Since 1935, a redistillation process has been used as the thermal means of meeting this demand. The principles of fractional distillation are utilized and zinc of 99.99+% purity is made. There is also an electrolytic method of producing metallic zinc. Because the selective flotation process made additional quantities of zinc concentrates available in localities where electric power is cheap, the production of electrolytic zinc was increased. In the electrolytic process, the zinc content of the roasted ore is leached out with dilute sulfuric acid. The zinc-bearing solution is filtered and purified and the zinc content recovered from the solution by electrolysis, using lead alloy anodes and sheet aluminum cathodes. Current passing through the electrolytic cell, from anode to cathode, deposits the metallic zinc on the cathodes from which it is stripped at regular intervals, melted and cast into slabs. Zinc so produced is 99.9+ or 99.99+% pure depending on need and the process control exercised.

COMPOUNDS

AND

ZINC FORMS

Zinc is always divalent in its compounds, except for some of those with other metals, which are classed as zinc alloys. Most of the more important zinc compounds are inorganic, since they are much more widely used than the organic zinc compounds.

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Z

The old name spelter, often applied to slab zinc, came from the name spailter used by Dutch traders for the zinc brought from China. Sterling spelter was 99.5% pure. Special high-grade zinc is distilled, with a purity of 99.99%, containing no more than 0.006% lead and 0.004 cadmium. High-grade zinc, used in alloys for die casting, is 99.9% pure, with 0.07 max lead. Brass special zinc is 99.10% pure, with 0.6 max lead and 0.5 max cadmium. Prime western zinc, used for galvanizing, contains 1.60 max lead and 0.08 max iron. Zinc crystals produced for electronic uses are 99.999% pure, metal. On exposure to the air, zinc becomes coated with a film of carbonate and is then very corrosion-resistant. Zinc foil comes in thicknesses from 0.003 to 0.015 cm. It is produced by electrodeposition on an aluminum drum cathode and stripping off on a collecting reel. But most of the zinc sheet contains a small amount of alloying elements to increase the physical properties. Slight amounts of copper and titanium reduce grain size in sheet zinc. In cast zinc the hexagonal columnar grain extends from the mold face to the surface or to other grains growing from another mold face, and even very slight additions of iron can control this grain growth. Aluminum is also much used in alloying zinc. In zinc used for galvanizing, a small addition of aluminum prevents formation of brittle alloy layer, increases ductility of the coating, and gives a smoother surface. Small additions of tin give bright spangled coatings. Zinc has 12 isotopes, but the natural material consists of 5 stable isotopes, of which nearly half is zinc-64. The stable isotope zinc–67, occurring to the extent of about 4% in natural zinc, is sensitive to tiny variations in transmitted energy, giving off electromagnetic radiations that permit high accuracy in measuring instruments. It measures gamma-ray vibrations with great sensitivity, and is used in the nuclear clock. Zinc powder, or zinc dust, is a fine gray powder of 97% minimum purity usually in 325-mesh particle size. It is used in pyrotechnics, in paints, as a reducing agent and catalyst, in rubbers as a secondary dispersing agent and to increase flexing, and to produce Sherardized steel.


In paints, zinc powder is easily wetted by oils. It keeps the zinc oxide in suspension, and also hardens the film. Mossy zinc, used to obtain color effects on face brick, is a spangly zinc powder made by pouring the molten metal into water. Feathered zinc is a fine grade of mossy zinc. Photoengraving zinc for printing plates is made from pure zinc with only a small amount of iron to reduce grain size and is alloyed with not more than 0.2% each of cadmium, manganese, and magnesium. Cathodic zinc, used in the form of small bars or plates fastened to the hulls of ships or to underground pipelines to reduce electrolytic corrosion, is zinc of 99.99% purity with iron less than 0.0014% to prevent polarization.

APPLICATIONS For many years, the greatest use of zinc has been to protect iron and steel against atmospheric corrosion. Because of the relatively high electropotential of zinc, it is anodic to iron. If zinc and iron or steel are electrically connected and are jointly exposed in most corrosive media, the steel will be protected while the zinc will be attacked preferentially and sacrificially. This, along with the fact that zinc corrodes far less rapidly than iron in most environments, forms the basis for one of the great fields of use of zinc — in galvanizing (by hot dipping or electrolytically), metallizing, sherardizing, in zinc pigmented paint systems, and as anodes in systems for cathodic protection. The six techniques are described below. Hot Dip Galvanizing Zinc alloys readily with iron. Therefore, steel articles, suitably cleaned, will be wet by molten zinc and will acquire uniform coatings of zinc the thickness of which will vary with time, temperature, and rate of withdrawal. Such coats are continuous and reasonably ductile. Ductility is improved considerably by the restriction of immersion time and by the addition of small amounts of aluminum to the galvanizing bath. Millions of tons of steel products are protected by zinc annually. The time before first rusting of the iron or steel base is proportional to the thickness of zinc coat which in turn is


subject to control — depending on product and processing — within a range from thin wiped coats on some products to as much as 0.20 mm on certain low-alloy steels allowed to acquire a full natural coat. The usefulness of zinc as a coating material comes from its dual ability to protect, first as a long-lasting sheath, and then sacrificially when the sheath finally is perforated.

Zinc-Pigmented Paints Evidence has accumulated to demonstrate that paints heavily pigmented with zinc dust perform similarly to zinc coats otherwise applied. Electrical contact must exist between the steel and the zinc-dust particles; consequently, special vehicles must be used and the steel surface must be clean. Zinc Anodes

Electrogalvanizing Zinc may be electrolytically deposited on essentially all iron and steel products. Wire and strip are commonly so treated as are many fabricated parts. Electrodeposited coats are ductile and uniform but normally are thinner and therefore find application in less rigorous service. Metallizing Zinc wire or powder is melted and sprayed on suitably grit-blasted steel surfaces — a growing use. Its virtues are flexibility in application and substantial thicknesses that may be applied. The method is particularly useful for renewal of heavy coatings on areas exposed to particularly critical corrosive conditions and the coating of parts too large for hot dipping. Although metallized coats may be somewhat porous, the sacrificial nature of zinc nevertheless makes them protective. Suitable pore sealants may be used as a part of a metallizing system. Sherardizing Zinc powder is packed loosely around clean parts to be sherardized in an airtight container. When sealed, heated to temperature near but below the zinc melting point, then slowly rotated, the zinc alloys with the steel forming a thin, abrasion-resistant, and uniform protective coating (0.4 to 1.8 g/cm2). Sherardizing is used commonly to coat small items such as nuts, bolts, and screws; an exception is tubular electrical conduit. Sherardized coats receive varnish, paints, and lacquers particularly well.


High-purity zinc, normally alloyed with small additions of aluminum, with or without cadmium, is cast or rolled into anodes that, when electrically bonded to bare or painted steel, will protect large areas from the corrosive attack of such environments as seawater. The advantages of zinc in this application include self-regulation (no more current is generated than is required), a minimum generation of hydrogen, and long life. This is a growing application for the protection of ship hulls, cargo tanks in ballast, piers, pilings, etc. General Comment Reference has been made to the importance of coating thickness — the heavier the coat, the longer the time before first rusting. All evidence at hand indicates that the amount of zinc in a coat is the controlling factor and the method of application is of secondary importance. Uniformity of coat and adhesion must be good. No data are known to demonstrate that common zinc impurities normally present in amounts to or slightly above specification limits have any significantly deleterious or beneficial influence on the ability of zinc to protect iron or steel against atmospheric corrosion. Although any grade of zinc may be used for galvanizing, Prime Western is the one most commonly employed. Die casting is a market for zinc that may soon become its largest market. These alloys melt readily, are highly fluid, and do not attack steel dies or equipment. When used under good temperature control and with good die design practices, casting surfaces are excellent and easily finished. Physical properties are good and dimensional stability excellent.

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Z

Alloy control within the specified limits ensures long life. Low aluminum results in decreased casting and mechanical properties and adversely affects the performance of plated coatings. High aluminum can lead to brittleness (an alloy eutectic forms at 5% aluminum). High copper content decreases dimensional stability. Iron as commonly encountered is not critical. Lead, tin, and cadmium, if present above specification limits, can lead to intercrystalline corrosion with objectionable growth and serious cracking or brittleness as a result. Magnesium minimizes the deleterious influence of lead, tin, or cadmium but at or near specification maximum decreases ductility and castability and can lead to objectionable hot shortness. Other impurities such as chromium and nickel, which may be encountered, are not critical. Zinc die castings are used by the automotive, truck, and bus industry for functional, decorative–functional, or decorative purposes. A majority is plated with copper–nickel–chromium in a variety of plating systems especially adapted to withstand severe service conditions. Other major outlets for zinc die casting include household appliances, business machines, machine tools, air-brake systems, and communication equipment. Even such nonstructural materials as cardboard can be zinc-coated by low-temperature flame spraying. Other important uses of zinc are in brass and zinc die-casting alloys, in zinc sheet and strip, in electrical dry cells, in making certain zinc compounds, and as a reducing agent in chemical preparations. A so-called tumble-plating process coats small metal parts by applying zinc powder to them with an adhesive, then tumbling them with glass beads to roll out the powder into a continuous coat of zinc. Rechargeable nickel-zinc batteries offer higher energy densities than conventional dry cells. Foamed zinc metal has been suggested for use in lightweight structures such as aircraft and spacecraft. Some other uses of zinc are in dry cells, roofing, lithographic plates, fuses, organ pipes, and wire coatings. Zinc is believed to be needed for normal growth and development of all living species, including humans; actually, life without zinc would be impossible. Zinc is a common element that is present in virtually every type of


human food, and zinc deficiency is therefore not considered to be a common problem in humans. Zinc is a trace element that is present in biological fluids at a concentration below 1 ppm (parts per million), and only a small amount (normally 2205°C). Zircon has excellent thermal properties and its thermal conductivity is 14.5 Btu/ft2/hr/°F/in. and coefficient of thermal expansion is 1.4 × 10–6. The extremely high thermal conductivity and chilling action of zircon makes it very useful in controlling directional solidification and shrinkage in heavy metal sections.

USES Zircon sand is used as refractory bedding material for heat-treating metal parts. It is used as a

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sealing medium for prevention of atmospheric leaks around doors and parts of heat-treating furnaces. Also, it is a high-qualiIty, uniform sandblasting medium for metal preparation prior to plating, enameling, or buffing. The heavy, rounded grains give consistent peening without stray digs or gouges to mar the finish. The tough, resilient grains resist breakdown and loss.

ZIRCONIA (ZIRCONIUM OXIDE) Zirconium oxide, ZrO2, is a white crystalline powder with a specific gravity of 5.7, hardness 6.5, and refractive index 2.2. When pure, its melting point is about 2760°C, and it is one of the most refractory of the ceramics. It is produced by reacting zircon sand and dolomite at 1371°C and leaching out the silicates. The material is used as fused or sintered ceramics and for crucibles and furnace bricks. From 4.5 to 6% of CaO or other oxide is added to convert the unstable monoclinic crystal to the stable cubic form with a lowered melting point. Zirconia is produced from the zirconium ores known as zircon and baddeleyite. The latter is a natural zirconium oxide. It is also called zirkite and Brazilite. Zircon is zirconium silicate, ZrO2 · SiO2, and comes chiefly from beach sands. The sands are also called zirkelite and zirconite, or merely zircon sand. The white zircon sand has a zirconia content of 62%, and contains less than 1% iron.

USES

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Fused zirconia, used as a refractory ceramic, has a melting point of 2549°C and a usable temperature to 2454°C. The Zinnorite fused zirconia is a powder that contains less than 0.8% silica and has a melting point of 2704°C. A sintered zirconia can have a density of 5.4, a tensile strength of 82 MPa, compressive strength of 1378 MPa, and Knoop hardness of 1100. Zircoa B is stabilized cubic zirconia used for making ceramics. Zircoa A is the pure monoclinic zirconia used as a pigment, as a catalyst, in glass, and as an opacifier in ceramic coatings. As an opacifier, zirconium compounds are used in glazes and porcelain enamels. Zirconium dioxide is an important constituent of


ceramic colors and an important component of lead–zirconate–titanate electronic ceramics. Pure zirconia also is used as an additive to enhance the properties of other oxide refractories. It is particularly advantageous when added to high-fired magnesia bodies and alumina bodies. It promotes sinterability and, with alumina, contributes to abrasive characteristics. Zirconia brick for lining electric furnaces has no more than 94% zirconia, with up to 5% calcium oxide as a stabilizer, and some silica. It melts at about 2371°C, but softens at about 1982°C. The IBC 4200 brick is zirconia with calcium and hafnium oxides for stabilizing. It withstands temperatures to 2316°C in oxidizing atmospheres and to 1849°C in reducing atmospheres. Zirconia foam is marketed in bricks and shapes for thermal insulation. With a porosity of 75% it has a flexural strength above 3 MPa and a compressive strength above 0.7 MPa. For use in crucibles, zirconia is insoluble in most metals except the alkali metals and titanium. It is resistant to most oxides, but with silica it forms ZrSiO4, and with titania it forms ZrTiO4. Because structural disintegration of zirconia refractories comes from crystal alteration, the phase changes are important considerations. The monoclinic material, with a specific gravity of 5.7, is stable to 1010°C and then inverts to the tetragonal crystal with a specific gravity of 6.1 and volume change of 7%. It reverts when the temperature again drops below 1010°C. The cubic material, with a specific gravity of 5.55, is stable at all temperatures to the melting point, which is not above 2649°C because of the contained stabilizers. A lime-stabilized zirconia refractory with a tensile strength of 138 MPa has a tensile strength of 68 MPa at 1299°C. Stabilized zirconia has a very low coefficient of expansion, and white-hot parts can be plunged into cold water without breaking. The thermal conductivity is only about one third that of magnesia. It is also resistant to acids and alkalies, and is a good electrical insulator. To prepare useful formed products from zirconium oxide, stabilizing agents such as lime, yttrium, or magnesia must be added to the zirconia, preferably during fusion, to convert the zirconia to the cubic form. Most commercial stabilized zirconia powders or products contain calcium oxide as the stabilizing agent.


The stabilized cubic form of zirconia undergoes no inversion during heating and cooling. Stabilized zirconia refractories are used where extremely high temperatures are required. Above 1649°C, in contact with carbon, zirconia is converted to zirconium carbide. Zirconia is of much interest as a construction material for nuclear energy applications because of its refractoriness, corrosion resistance, and low nuclear cross section. However, zirconia normally contains about 2% hafnia, which has a high nuclear cross section. The hafnia must be removed before the zirconia can be used in nuclear applications.

FORMS Zirconia is available in several distinct types. The most widely used form is stabilized in cubic crystal form by a small lime addition. This variety is essential to the fabrication of shapes because the so-called unstabilized, monoclinic zirconia undergoes a crystalline inversion on heating, which is accompanied by a disruptive volume change. Zirconia is not wetted by many metals and is therefore an excellent crucible material when slag is absent. It has been used very successfully for melting alloy steels and the noble metals. Zirconia refractories are rapidly finding application as setter plates for ferrite and titanate manufacture, and as matrix elements and wind tunnel liners for the aerospace industry.

OTHER TYPES Toughening mechanisms, by which a crack in a ceramic can be arrested, complement processing techniques that seek to eliminate crack-initiating imperfections. Transformation toughening relies on a change in crystal structure (from tetragonal to monoclinic) that zirconia or zirconium dioxide (ZrO2) grains undergo when they are subjected to stresses at a crack tip. Because the monoclinic grains have a slightly larger volume, they can “squeeze” a crack shut as they expand in the course of transformation. Because of the transformation toughening abilities of ZrO2, which impart higher fracture toughness, research interest in engine applications has been high. In order for ZrO2 to be used © 2002 by CRC Press LLC

in high-temperature, structural applications, it must be stabilized or partially stabilized to prevent a monoclinic–tetragonal phase change. Stabilization involves the addition of calcia, magnesia, or yttria followed by some form of heat treatment. PSZ ceramic, the toughest known ceramic, has been investigated for diesel-engine applications. PSZ is a transformation toughened material consisting of a cubic zirconia matrix with 20 to 50 vol% free tetragonal zirconia added in the matrix. The material is converted into the stabilized cubic crystal structure using oxide stabilizers (magnesia, calcia, yttria). The conversion is accomplished by sintering the doped zirconia at 1700°C. Magnesia-stabilized zirconia exhibits serrated plastic flow during compression at room temperature. The flow stress is strain rate sensitive. Several different grades are available for commercial use, and the properties of the material can be tailored to fit many applications. One typical PSZ used for applications requiring maximum thermal shock resistance has a four-point bend strength of 600 MPa; PSZ is used experimentally as heat engine components, such as cylinder liners, piston caps, and valve seats. Vanadium impurities from fuel oil can cause zirconia destabilization, and sodium, magnesium, and sulfur impurities can cause yttria to dissociate from yttria-stabilized zirconia. Another area of interest for PSZ is in bioceramics, where it has use in surgical implants. A new zirconia ceramic being developed, tetragonal zirconia polycrystal (TZP) doped with Y2O3, has the most impressive room-temperature mechanical properties of any zirconia ceramic. The commercial applications of TZP zirconia include scissors with TZP blades suitable for industrial use for cutting tough fiber fabrics, e.g., Kevlar, cables, and ceramic scalpels for surgical applications. One unique application is fish knives. The knife blades are Y-TZP and can be used when the delicate taste of raw fish would be tainted by slicing with knives with metal blades. Another zirconia ceramic-developed material is zirconia-toughened alumina (ZTA). ZTA zirconia is a composite polycrystalline ceramic containing ZrO2, as a dispersed phase (typically ~15 vol%). Close control of initial starting

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powder sizes and sintering schedules is thus necessary to attain the desired ZrO2 particle dimensions in the finished ceramic. Hence, the mechanical properties of the composite ZTA ceramics limit current commercial applications to cutting tools and ceramic scissors. PSZ is also finding application in the transformation toughening of metals used in the glass industry as orifices for glass fiber drawing. This material is termed zirconia grain-stabilized (ZGS) platinum. Clear zircon crystals are valued as gemstones because the high refractive index gives great brilliance. Zirconia fiber, used for high-temperature textiles, is produced from zirconia with about 5% lime for stabilization. The fiber is polycrystalline, has a melting point of 2593°C, and will withstand continuous temperatures above 1649°C. These fibers are as small as 3 to 10 µm and are made into fabrics for filter and fuel cell use. Zirconia fabrics are woven, knitted, or felted of short-length fibers and are flexible. Ultratemp adhesive, for high-heat applications, is zirconia powder in solution. At 593°C, it adheres strongly to metals and will withstand temperatures to 2427°C. Zircar is zirconia fiber compressed into sheets to a density of 320 kg/m3. It will withstand temperatures up to 2482°C and has low thermal conductivity. It is used for insulation and for high-temperature filtering.

ZIRCONIUM AND ALLOYS

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A silvery-white metal, zirconium (symbol Zr), has a specific gravity of 6.5 and a melting point of about 1850°C. It is more abundant than nickel, but is difficult to reduce to metallic form as it combines easily with oxygen, nitrogen, carbon, and silicon. The metal is obtained from zircon sand by reacting with carbon and then converting to the tetrachloride, which is reduced to a sponge metal for the further production of shapes. The ordinary sponge zirconium contains about 2.5% hafnium, which is closely related and difficult to separate. The commercial metal usually contains hafnium, but reactor-grade zirconium, for use in atomic work, is hafnium-free.


Commercially pure zirconium is not a highstrength metal, with a tensile strength of about 220 MPa, elongation 40%, and Brinell hardness 30, or about the same physical properties as pure iron. But it is valued for atomic-construction purposes because of its low neutron-capture cross section, thermal stability, and corrosion resistance. It is employed mostly in the form of alloys but may be had in 99.99% pure single-crystal rods, sheets, foil, and wire for superconductors, surgical implants, and vacuum-tube parts. The neutron cross section of zirconium is 0.18 barn, compared with 2.4 for iron and 4.5 for nickel. The cold-worked metal, with 50% reduction, has a tensile strength of about 545 MPa, with elongation of 18% and hardness of Brinell 95. The unalloyed metal is difficult to roll, and is usually worked at temperatures to 482°C. Although nontoxic, the metal is pyrophoric because of its heat-generating reaction with oxygen, necessitating special precautions in handling powder and fine chips resulting from machining operations. The metal has a close-packed hexagonal crystal structure, which changes at 862°C to a body-centered cubic structure that is stable to the melting point. At 300 to 400°C the metal absorbs hydrogen rapidly, and above 200°C it picks up oxygen. At about 400°C it picks up nitrogen, and at 800°C the absorption is rapid, increasing the volume and embrittling the metal. The metal is not attacked by nitric (except red fuming nitric), sulfuric, or hydrochloric acids, but is dissolved by hydrofluoric acid. Zirconium powder is very reactive, and for making sintered metals it is usually marketed as zirconium hydride, ZrH2, containing about 2% hydrogen, which is driven off when the powder is heated to 300°C. For making sintered parts, alloyed powders are also used. Zirconium copper (containing 35% zirconium), zirconium nickel (with 35 to 50% zirconium), and zirconium cobalt (with 50% zirconium), are marketed as powders of 200 to 300 mesh.

PROPERTIES In addition to resisting HCl at all concentrations and at temperatures above the boiling temperature, zirconium and its alloys also have


excellent resistance in sulfuric acid at temperatures above boiling and concentrations to 70%. Corrosion rate in nitric acid is less than 1 mil/year at temperatures above boiling and concentrations to 90%. The metals also resist most organics such as acetic acid and acetic anhydride as well as citric, lactic, tartaric, oxalic, tannic, and chlorinated organic acids. Relatively few metals besides zirconium can be used in chemical processes requiring alternate contact with strong acids and alkalies. However, zirconium has no resistance to hydrofluoric acid and is rapidly attacked, even at very low concentrations.

USES Small amounts of zirconium are used in many steels. It is a powerful deoxidizer, removes the nitrogen, and combines with the sulfur, reducing hot-shortness and giving ductility. Zirconium steels with small amounts of residual zirconium have a fine grain, and are shock resistant and fatigue resistant. In amounts above 0.15% the zirconium forms zirconium sulfide and improves the cutting quality of the steel. A noncrystalline metal that reportedly has twice the strength of steel and titanium, has been developed. The material, known as Vitrelloy, is an alloy composed of 61% zirconium, 12% titanium, 12% copper, 11% nickel, and 3% beryllium. Its yield strength is 1900 MPa, compared with 800 MPa for titanium alloy, Ti–6% Al–4% V, and 850 MPa for cast stainless steel. Fracture toughness is said to be 55 MPam1/2, the same as high-strength steel but half that of titanium. Its resistance to permanent deformation is said to be two to three times higher than that of conventional metals. The density of Vitrelloy is 6.1 g/cm3 between cast titanium at 4.5 g/cm3 and cast stainless steel at 7.8 g/cm3. The material is particularly recommended for aerospace applications because of its surface hardness of 50 HRC. Cast titanium and steel are both tested at 30 HRC. The beneficial properties of the alloy are ascribed to its noncrystalline structure. Because there are no patterns or grains within the structure, weak areas caused by grain boundaries are eliminated.


An advanced machinable ceramic that may be used to produce thermal shock-resistant components for aerospace, automotive, electrical, heat treating, metallurgical, petrochemical, and plastics applications up to 1550°C has been introduced. The new material (AremcoloxTM 502-1550) is based on the zirconium phosphate system (Ba1+xZr4P6-2xSi2xO24) and is especially unique because of its low coefficient of thermal expansion (CTE) of 0.5 × 10–6 in./in.°F. This characteristic sets the material apart from standard ceramic materials such as alumina and zirconia which have CTEs of 4.0 x 10-6 and 2.5 × 10–6, respectively. A low CTE ensures that as a component is thermally cycled the mechanical stress induced through expansion and contraction does not cause the part to crack. This feature enables engineers to adapt the material to high thermal shock applications, such as combustion and heater systems, that were not previously feasible. Additional properties and applications of the machinable ceramic include their use as molds, optical stands, microwave housings, engine parts, and applications in which high mechanical strength, hardness, and low porosity are required. A low-density version of the material (502-1550 LD) is recommended for use as brazing fixtures, induction heating liners, rocket nozzles, and high-temperature gauges, tooling, and structures. The material is easily machined using carbide tooling and no postfiring is required.

ALLOYS Zirconium alloys generally have only small amounts of alloying elements to add strength and resist hydrogen pickup. Zircoloy 2, for reactor structural parts, has 1.5% tin, 0.12% iron, 0.10% chromium, 0.05% nickel, and the balance zirconium. Tensile strength is 468 MPa, elongation 37%, and hardness Rockwell B89; at 316°C it retains a strength of 206 MPa. Zirconium alloys can be machined by conventional methods, but they have a tendency to gall and work-harden during machining. Consequently, tools with higher than normal clearance angles are needed to penetrate previously work-hardened surfaces. Results can

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be satisfactory, however, with cemented carbide or high-speed steel tools. Carbide tools usually provide better finishes and higher productivity. Mill products are available in four principal grades: 702, 704, 705, and 706. These metals can be formed, bent, and punched on standard shop equipment with a few modifications and special techniques. Grades 702 (unalloyed) and 704 (Zr–Sn–Cr–Fe alloy) sheet and strip can be bent on conventional press-brake or roll-forming equipment to a 5t bend radius at room temperature and to 3t at 200°C. Grades 705 and 706 (Zr–Cb alloys) can be bent to a 3t and 2.5t radius at room temperature and to about 1.5t at 200°C. Small amounts of zirconium in copper give age-hardening and increase the tensile strength. Copper alloys containing even small amounts of zirconium are called zirconium bronze. They pour more easily than bronzes with titanium, and they have good electric conductivity. Zirconium–copper master alloy for adding zirconium to brasses and bronzes is marketed in grades with 12.5 and 35% zirconium. A nickel–zirconium master alloy has 40 to 50% nickel, 25 to 30% zirconium, 10% aluminum, and up to 10% silicon and 5% iron. Zirconium–ferrosilicon, for alloying with steel, contains 9 to 12% zirconium, 40 to 47% silicon, 40 to 45% iron, and 0.20% max carbon, but other compositions are available for special uses. SMZ alloy, for making high-strength cast irons without leaving residual zirconium in the iron, has about 75% silicon, 7% manganese, 7% zirconium, and the balance iron. A typical zirconium copper for electrical use is Amzirc. It is oxygen-free copper with only 0.15% of zirconium added. At 400°C it has a conductivity of 37% IACS, tensile strength of 358 MPa, and elongation of 9%. The softening temperature is 580°C. Zirconium alloys with high zirconium content have few uses except for atomic applications. Zircoloy tubing is used to contain the uranium oxide fuel pellets in reactors because the zirconium does not have grain growth and deterioration from radiation. Zirconia ceramics are valued for electrical and high-temperature parts and refractory coatings. Zirconium oxide powder, for flame-sprayed coatings, comes in


either hexagonal or cubic crystal forms. Zirconium silicate, ZrSi2, comes as a tetragonal crystal powder. Its melting point is about 1649°C and hardness about 1000 Knoop. Zirconium Beryllides Intermetallic compounds, ZrBe13 and Zr2Be17, have good strengths at elevated temperatures. ZrBe13 is cubic, density 2.72 g/cm3, melting point 1925°C; Zr2Be17 is hexagonal, density 3.08 g/cm3, melting point 1983°C; parts can be formed by all ceramic-forming methods plus flame and plasma-arc spraying. Materials are subject to safety requirements for all beryllium compounds. These intermetallics, because of their greater densities (BeO = 1.85 g/cm3), contain more beryllium atoms per unit volume than beryllia, a decided advantage for compact, beryllium-moderated nuclear reactors. Zirconium Carbide Zirconium carbide, ZrC2, is produced by heating zirconia with carbon at about 2000°C. The cubic crystalline powder has a hardness of Knoop 2090, and a melting point of 3540°C. The powder is used as an abrasive and for hotpressing into heat-resistant and abrasion-resistant parts. Zirconium Diboride Zirconium diboride (ZrB2) has a density of 6.09 g/cm3 and a hexagonal (AlB2) crystal structure with a melting point of 3040°C. Zirconium diboride is oxidation resistant at temperatures

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