Handbook of Thermoset Plastics, 2nd Ed., Second Edition (Plastics & Elastomers)

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HANDBOOKOFTHERMOSET PLASTICS Second

Edition

Edited by

Sidney

H. Goodman

Raytheon Systems Company El Segundo, California

clw

NOYES PUBLICATIONS W&wood.

New Jersey.

U.S.A.

Copyright Q 1998 by Noyes Publications No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without permission in writing from the Publisher. Library of Congress Catalog Card Number: 98-3566 ISBN: O-8155-1421-2 Printed in the United States Published in the United States of America by Noyes Publications 369 Fairview Avenue, Westwood, New Jersey 07675 10987654321

Library of Congress

Cataloging-in-Publication

Data

Handbook of thermoset plastics / edited by Sidney H. Goodman. -- 2nd ed. p. cm. Includes bibliographical references and index. ISBN O-8155-1421-2 I. Goodman, Sidney H. 1. Thermosetting plastics. TPllSO.T55H36 1998 668.4’22--dc21 98-3566 CIP

DEDICATION

This new edition is dedicated: again to three marvelous people, Leslie, Susan and Stephen, for sticking with me one more time. to Alicia and Beth, for accepting me into their lives, andparticularly to Doris, for reminding me how dull life used to be.

V

Contributors

Sidney H. Goodman Raytheon Systems Company El Segundo, CA

Rodney F. Patterson Consultant Fountain Valley, CA

Christopher C. Ibeh Department of (Plastics) Engineering Technology Pit&burg State University Pittsburg, KS

Isao Shimoyama Consultant La Palma, CA Oscar C. Zaske Consultant Palos Verdes Estates, CA

Abraham L. Landis Consultant Burlingame, CA Kreisler S. Y. Lau Allied Signal AdvancedMicroelectronic Materials Sunnyvale, CA

xi

NOTICE TO the best of our knowledge the information in this publication is accurate; however the Publisher does not assume any responsibility or liability for the accuracy or completeness of, or consequences arising from, such information. This book is intended for informational purposes only. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the Publisher. Final determination of the suitability of any information or product for use contemplated by any user, and the manner of that use, is the sole responsibility of the user. We recommend that anyone intending to rely on any recommendation of materials or procedures mentioned in this publication should satisfy himself as to such suitability, and that he can meet all applicable safety and health standards.

Preface to the Second Edition

The issuance of a new edition of a technical reference text usually signifies that the material in the book has provided repeated value-added information to the reader/buyer. It is a benchmark for the editors, authors, and contributors that they in fact participated in a significant way to the progress of science, engineering, and technology. It is highly gratifying to get calls and letters from one’s peers, years after publication, addressing content, errata, further topics of interest, etc. All this despite the fact that the polymer and plastics field continues to advance at such a rapid pace that the written word is obsoleted on the day of public release. Additionally, new data and information is communicated so much faster and easier through electronic means that the need to “update” an old text seems a Sisyphean task. A new edition does however allow the correction of oversights and mistakes, the inclusion of new information and data and, as a result of time and tide, the contribution ofnew authors, their new perspectives and styles. Thus the text becomes refreshed, modernized, able to serve the readership’s needs for valuable information and understanding.

vi

Preface to the Second Edition

vii

We have not tried to make major changes for change sake. Many chapters, such as Polyesters and Allyls (thanks to Mr. D. Luh) have just had data upgrade. The chapters on Polymides and New Developments were combined by the two separate authors, Drs. Landis and Lau, into one overview of the most modem of thermoset materials. The untimely loss of Dr. Art Wooten prompted us to invite Dr. Christopher Ibeh to revisit the chapters on Phenolics and Aminos and craft them from an entirely new perspective. The chapter on Epoxies was embellished with new data through the help of Susan Oldham and Darryl Hicks both retired from active work in the field. Isao Shimoyama has added substantive new information on Urethanes that expands the understanding of the “how” and “why” of this technology. Rod Patterson was able to update the chapters on Silicones and Crosslinked Thermoplastics despite a near catastrophic accident from which he recovered marvelously. The bulk of the information, especially the introductory material remains essentially unchanged because the concepts are fundamental to the technology. Thus the value to the new engineer, technician, or scientist continues to be a summarial overview, a place to get started. To those practicing the trade, the new information we believe will augment that which is known and offer fresh concepts for consideration as you ponder the solution to the next problem or design. Again we thank the small army of support who helped package this project. George Narita and his staff at Noyes provided the leadership, prodding, and encouragement to get the job done. Beverly Godwin’s long hours, pleasant demeanor and superb editorial skills helped navigate the manuscript maze. Special thanks also to Blanche Petrovic. Her job was to make order out of the chaos that constituted the eclectic consortium of contributors into a readable, coherent, manuscript. The typists, family members, and colleagues who suffered us this foolishness, receive our most sincere gratitude. Calabasas, CA 1998 July, 1998

Sidney H. Goodman

Table of Contents

1. INTRODUCTION .............................................................................. 1 Sidney H. Goodman History ............................................................................................... 3 Definitions ......................................................................................... 5 Crossiinking and Curing ................................................................. 6 Influence of Time, Temperature, and Mass .................................. 9 Shelf Life and Pot Life ................................................................... 13 Curing ............................................................................................. 14 Staging ............................................................................................. 17 Stoichiometric Considerations ...................................................... 18 Prepolymerization and Adducting ................................................ 20 References ....................................................................................... 22 2. PHENOL-FORMALDEHYDE ....................................................... 23 Christopher C. Ibeh Introduction .................................................................................... 23

.. .

x111

xiv

Handbook of Thermoset Plastics

Raw Materials ................................................................................ 24 Phenol .............................................................................................. 25 Cumene Process for Making Phenol ............................................ .25 Raschig Process ........................................................................... .25 Dow Process ................................................................................. 26 Formaldehyde (CH,O) .................................................................. 26 .27 Hexamethylene Tetramine (Hexamine or “HEXA”) (CH,),N, ......... Resinification (Production) of Phenol-Formaldehyde Resins .... ..2 7 Reaction Chemistry ...................................................................... 27 Polymerization Process ................................................................. 28 Resole Phenolic Resins ................................................................. 28 Novolac Phenol-Formaldehyde Resins ........................................ .30 Differences Between Resole and Novolac Phenolic Resins ........ .32 .32 Properties of Phenolic Resins ...................................................... Fillers for Phenolic Resins ............................................................ 38 Processing Methods for Phenolic Resins ..................................... .42 Applications of Phenolic Resins ................................................... 44 Phenolic Resins in Plywood ......................................................... 44 .47 Other Composite Wood Products ................................................ .49 Phenolic Resins in Adhesive and Bonding Applications.. ........... Phenolic Resins as Insulation Materials ...................................... .49 Phenolic Resins in Friction Materials .......................................... 50 Bonded and Coated Abrasives ...................................................... 51 Phenolic Resins in Foundry and Shell Molding Applications ..... .52 Shell Molding Process .................................................................. 53 Cold Box Process .......................................................................... 54 Trends in Foundry and Shell Molding ......................................... .54 Phenolic Resins in Laminating Applications ................................ 55 Phenolic Resins in Molding Applications .................................... 57 Phenolic Resins in Coating Applications ...................................... 60 Phenolic Resins Trade Names and Manufacturers .................... .7O References ....................................................................................... 70

Contents

xv

3. AMINO AND FURAN RESINS ...................................................... 72 Christopher C. Ibeh Introduction .................................................................................... 72 Raw Materials ................................................................................ 73 Urea.. ............................................................................................ .73 .74 Melamine ..................................................................................... Furans ........................................................................................... 75 Amino Resins .................................................................................. 76 Chemistry and Resinification ......................................................... 76 Adhesive and Bonding Resins ...................................................... 78 Coating Resins ............................................................................. .79 Laminating Resins ........................................................................ .80 Amino Molding Resins .................................................................. 81 Furan Resins .................................................................................. 82 Chemistry and Resinification of Furan Resins .............................. 82 Properties of Amino and Furan Resins ...................................... 83 Applications of Amino and Furan Resins ...................................... 86 Adhesive and Bonding .................................................................. 86 Trade Names .................................................................................. 95 References ..................................................................................... 96 4. UNSATURATED POLYESTER AND VINYL ESTER RESINS.. .97 Oscar C. Zaske and Sidney H. Goodman Unsaturated Polyesters .................................................................. 97 History ......................................................................................... .97 Chemistry .................................................................................... 102 Processing ................................................................................... 106 Typical General Purpose (GP) Unsaturated Polyester Resin.. ... .107 Common Resin Synthesis Raw Materials ................................... 107 Copolymerization of Unsaturated Polyester Alkyds with Monomers ................................................................................ 108 Processing Equipment and Manufacturing ................................. 111 Unsaturated Polyester Resin Alkyd Properties ........................... 112 Styrenated Unsaturated Polyester Resin Liquid Properties ....... .113

xvi

Handbook of Thermoset Plastics Monomers Used in Unsaturated Polyesters ................................ 115 Unsaturated Polyester Properties and Chemical Composition ...116 General Purpose Resins .............................................................. 116 Molar Ratio of PA to MA ........................................................... 116 Flexibilization ............................................................................ .117 Isophthalic Resins ....................................................................... 118 Molecular Weight Comparisons ................................................. 118 Hydrolytic and Chemical Resistance .......................................... 119 119 Styrene Compatability ................................................................ 120 Flame Retardance ....................................................................... 122 ........................................................................ Vinyl Ester Resins .122 Chemistry ................................................................................... Basic Vinyl Ester Resin .............................................................. 123 History ....................................................................................... .124 Toughness and Chemical Resistance .......................................... 124 Vinyl Ester Resin Structure and Properties ................................ 125 Specialty Vinyl Ester Resins ...................................................... 125 Vinyl Ester Resins Overview ...................................................... 128 Typical Styrenated Vinyl Ester Resin Liquid Properties.. ......... .128 Typical Styrenated Vinyl Ester Cast Resin Properties .............. .128 Compounding of Unsaturated Polyester and Vinyl Ester Resins 129 .129 Overview .................................................................................... Curing Systems .......................................................................... .13 1 Ultraviolet Absorber ................................................................... 135 Thixotropic/Flow Control Agents ............................................... 136 Fillers .......................................................................................... 137 .14 1 Thickening Agents ..................................................................... 142 .................................................................. Fiber Reinforcements 142 Applicable Manufacturing Processes ......................................... Overview.. ................................................................................... 142 Hand Layup.. ............................................................................... 143 .144 Spray Layup ............................................................................... Resin Transfer Molding (RTM) .................................................. 145 Water Extended Polyester (WEP) .............................................. 146

Contents

xvii

147 Casting ........................................................................................ 153 Acrylic Backup ........................................................................... Matched Die Mat, Preform and Premix Molding ....................... 153 Pultrusion .................................................................................... 154 Sheet and Bulk Molding Compounds (SMC) and (BMC) ........ .158 Bulk Molding Compounds (BMC) ............................................. 160 Recent Developments ................................................................... 162 Foamed Polyester.. ...................................................................... 162 Urethane Hybrid Resins .............................................................. 162 Reduced Styrene Emission Resins .............................................. 162 Trade Names and Manufacturers of Unsaturated Polyester and Vinyl Esters ........................................................................ 164 References ..................................................................................... 166 5. ALLYLS .......................................................................................... Sidney H. Goodman Introduction .................................................................................. Chemistry ...................................................................................... Polymerization and Processing ................................................... Formulation .................................................................................. Properties ...................................................................................... Applications .................................................................................. References and Bibliography ...................................................... 6. EPOXY RESINS ............................................................................ Sidney H. Goodman Introduction .................................................................................. Resin Types ................................................................................... Diglycidyl Ether of Bisphenol A ................................................ Novolacs ..................................................................................... Peracid Resins ............................................................................ Hydautoin Resins ....................................................................... Other Types.. ..............................................................................

169 169 170 171 173 173 190 191 193 193 194 194 195 .200 .202

.203

xviii Handbook of Thermoset Plastics Curatives and Crosslinking Reactions ....................................... 208 Stoichiometry ............................................................................. .209 Alkaline Curing Agents ............................................................... 212 Lewis Bases ................................................................................ 212 Primary and Secondary Aliphatic Amines ................................. .213 Amine Adducts ........................................................................... 214 Cyclic Amines ............................................................................ .216 Aromatic Amines ........................................................................ 218 Polyamides .................................................................................. 221 Other Amines .............................................................................. 222 Acid Curing Agents .................................................................... 223 Lewis Acids ............................................................................... .223 Phenols ........................................................................................ 223 Organic Acids ............................................................................ .224 Cyclic Anhydrides ....................................................................... 225 Polysulfides and Mercaptans ...................................................... 230 Formulation Principles ............................................................... 232 Epoxy-Containing Reactive Diluents ........................................... 233 Resinous Modifiers ...................................................................... 239 Nonreactive Diluents ................................................................... 239 Fillers ........................................................................................... 239 Colorants and Dyes ..................................................................... 244 OtherAdditives ........................................................................... 244 Properties ..................................................................................... 249 Applications .................................................................................. 254 7. THERMOSET POLYURETHANES ........................................... 269 Isao Shimoyama

Introduction .................................................................................. 269 Environmental Regulation and its Impact on Polyurethane Technology ................................................................................. 270 Modification of Amines for Reaction with Isocyanates .......... ..27 9 Recent Development .................................................................... 280 Toxicological Profile .................................................................. 282

Contents

xix

Amines .......................................................................................... 284 JEFFAMINE@ (Polyoxyalkylendimine) ................................. 284 Water-Borne Polyurethanes ...................................................... 285 Other Two-Component Polyurethanes .................................... 288 Catalysts ....................................................................................... 296 Diisocyanates ............................................................................... 296 New Water Scavengers ............................................................. .297 Toxicological Profiles .................................................................. 298 Acknowledgment ......................................................................... 298 References ................................................................................... 299 Bibliography ................................................................................. 300 8. HIGH PERFORMANCE POLyDllDES AND RELATED THERMOSET POLYMERS; PAST AND PRESENT DEVELOPMENT, AND FUTURE RESEARCH DIRECTION 302 Abraham L. Landis and Kreisler S. . Lau

Historical Perspective .................................................................. 303 Polyimides from Condensation Reactions ................................. 307 Thermoplastic Polyimides ........................................................... 328 DuPont NR-150 Polyimides ...................................................... .329 General Electric Ultem@ Poly (ether-imides) ............................ .330 Amoco Torlon@ Poly (amide-imides) ........................................ .338 Ciba-Geigy Fully Ionized Indane-based Polyimides ................. .344 Soluble, Fully Imidized Fluorinated Polyimides ........................ .348 Addition-Curable Polyimides and Other Polymers.. ............. .349 Nadimide-Terminated Thermosetting Polyimides ................ .353 362 Modified Nadimide End-Groups .................................................. AFR 700B Development.. ........................................................... 363 Maleimide-Terminated Thermosetting Polyimides .............. .365 Cyanate-Terminated Thermosetting Polymers ...................... 371 Cyanate Ester SIPNs .................................................................. 375 Polyimide-Based Cyanate Esters ................................................ 378 Polycyanurates in Electronic Applications .................................. 378

xx


High Temperature Thermosetting Resins Based on 379 Phthalonitrile ............................................................................. Thermosetting Polymers ..................... .384 Acetylene-Terminated

Acetylene-Terminated Quinoxalines .......................................... 387 Acetylene-Terminated Sulfones ................................................ .3 89 Acetylene-Terminated Imide Oligomers ................................... .39 1 Isoimide Modification of Polyimides ........................................ .394 Cure Mechanism of Acetylene-Terminated Oligomers ............. .398 Propargyl-Terminated Oligomers .............................................. Thermosetting Polymers.. ............ Phenylethynyl-Terminated Applicability of Thermoset Isoimides/Imides to Resin Transfer Molding Processing ........................................ Relevance of Polyimides to RTM Processing ...........................

404 ,406 415

.418 Emerging Low-Viscosity SIFN Imide Blends ........................... .419

Application of High-Performance Polymers to Improve Galvanic Corrosion of Imide-Based Composites .................. .419 .420 Mechanism of Corrosion ........................................................... .422 Insulative Coatings .................................................................... .422 Conductive Polymer Blends ...................................................... .423 Imide Structural Analogs ........................................................... Application of High-Performance Polymers in Lightning Strike Protection Technology Using Nonmetallic Materials .......... ..42 4 .425 High-Temperature Resistant Coatings ....................................... ......................... .425 Conductive Paint and Adhesive Formulations.. Advanced Nonmetallic Conductive Materials ........................... .426 Future Demands in Ultrahigh Temperature Resistant 426 Polymers ................................................................................... Chemical Structures Suitable for Ultrahigh 428 Temperature Use .................................................................... Novel Cross-Linking Mechanisms for Stability at 435 Ultrahigh Temperatures ........................................................ .43 5 ............................................................................... Biphenylene .436 [2.2] Paracyclophane ................................................................. .437 Benzocyclobutene ......................................................................

Contents

xxi

........................................................................... 441 Diazine ....................................................................................... .441 Polymer-Ceramic Materials ...................................................... 442 Silicon Alkoxide-Derived Polymer Ceramic Materials.. ............. .446 Aluminum Phosphate and Silicate Refractory Materials.. .......... .447 Partially Stabilized Zirconia ........................................................ .449 Organically Modified Litidionite ................................................. .449 Clay-Polymer Nanocomposites .................................................. .449 Solgel-Derived Polyimide-Silica Nanocomposites ...................... .450 References ................................................................................... 451 Acenaphthylene

9. sILIcolWs

....................................................................................

468

Rodney F. Patterson

Introduction .................................................................................. 468 Silicone Fluids ............................................................................... 470 Dimethyl Types ......................................................................... .474 Methylphenyl Types .................................................................. .475 Other Fluid Types/Copolymers ................................................. .476 Silicone Rubbers ........................................................................... 478 Room-Temperature-Vulcanizing Silicones ................................ 479 One-Component Systems .......................................................... .479 Two-Component Systems .......................................................... .483 Condensation Cure ..................................................................... .484 Addition Cure ............................................................................ .485 Heat Cured Systems ..................................................................... 487 Compounding ............................................................................. .489 Curing ........................................................................................ .489 Silicone Laminates ....................................................................... 491 Government Specifications for Silicone Products.. .................. .494 References ..................................................................................... 497

xxii Handbook of Thermoset Plastics 10. CROSSLINKED THERMOPLASTICS .................................... 498 Rodney P. Patterson Introduction .................................................................................. 498 Crosslinking of Thermoplastics .................................................. 499 Effects of Crosslinking on Polymer ............................................ 504 Polyethylene .............................................................................. .504 Polyolefin Foams ....................................................................... .505 .5 10 Polypropylene ............................................................................ .5 12 Polyvinyl Chloride ..................................................................... Chemical Crosslinking ................................................................. 513 Polyethylene .............................................................................. .5 14 Polypropylene ............................................................................ .5 18 Rotational Molding ...................................................................... 519 Post-Irradiation Effects ............................................................... 525 Acrylates ....................................................................................... 534 Trade Names ................................................................................. 539 References ..................................................................................... 540 INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 542

1

Introduction Sidney H. Goodman

This book presents an overview of a major class of materials of construction: thermosetting plastics. Using the biological analogy, this class fits into the family of materials as shown in Figure l- 1.

Chemishy I

Organic Materials I Hi& Polvmers I

Bionolvmers

I

I

Plastics

Rubbers

I

I

Thermonlastics

Figure l-l : Relationship of thermosets in chemistry 1

I

Thermosets

2


One popular definition of thermosets is: ...a polymeric material which can be formed by the application of heat and pressure, but as a result of a chemical reaction, permanently crosslinks and cannot be reformed upon further application of heat and pressure. (Goodman and Schwartz, p 9) Another

more rigorous definition is found in Whittington’s

Dic-

tionary ofPlastics (p 239): Resin or plastic compounds which in their final state as finished articles are substantially infusible and insoluble. Thermosetting re&ns are often liquid at some stage in their manufacture or processing, which are cured by heat, catalysis, or other chemical means. After being fully cured, thermosets cannot be resoftened by heat. Some plastics which are normally thermoplastic can be made thermosetting by means of crosslinking with other materials. This leads to an interesting concept. All too often trade usage confers titles on classes of materials. These titles reflect a nomenclature or jargon that is fully comprehensible to those in the trade. Those new to the trade soon learn the meaning of the terms by association, osmosis, etc. At some point in the technology maturation, someone decides to establish a precise definition of the terms. The true definitions are quickly found to be elusive: no two practitioners define them exactly the same way: the definitions are not “scientitic” enough; more exceptions to the rule exist than examples of the rule; and on and on. The term “thermoset” or “thermosetting plastics” is a classic illustration of this phenomenon. This book is an attempt to collate and present the current practices and technology associated with a group of commercial polymeric materials called “Thermosets.” Everyone who works with these materials has an intuitive understanding of the types of plastics that fall into this category. We know, for example, that chemical crosslinking must occur in order for the resultant product to be called a thermoset. We know that the monomeric precursors may or may not be polymeric in and of themselves, will undergo

Introduction

3

reaction when the chemical kinetics are right; that these precursors are commonly called thermoset resins because they will participate in a crosslinking reaction. We also know that under the right conditions many of these resins can polymerize linearly and form a traditional thermoplastic polymer. Vulcanization is a form of crosslinking wherein a rubber is formed, yet rarely do technologists refer to rubber as a thermoset plastic. Biopolymers (amino acid/protein based) are known to crosslink (one theory suggests this as a root cause of aging) and we hardly think of animals as thermosetting plastics. This book then will be structured based on the commonly perceived “definitions” of thermosetting resins. Both definitions stated earlier remain valid and useful. This introductory chapter will include a series of basic terms and definitions that will be referred to throughout the individual chapters that follow. Many of the “definitions” will in fact be descriptions of the phenomena which best illustrate the sense of the terms, as opposed to a rigorous definition per se. That these explanations are “common usage” or “trade jargon,” that they are not scientifically precise, does not compromise or lessen their meaning or value.

HISTORY

Goodyear’s (and Hancock in England) discovery of the vulcanization of natural rubber in 1839 could be construed as the first successful commercial venture based on thermosetting polymers. The plastics industry dates the beginning of thermosetting plastics to the development by Leo Baekeland in 1909 of phenolics. In this instance, Baekeland not only produced the first synthetic crosslinked polymer, but as importantly, he discovered the molding process that enabled him to produce homogeneous useful articles of commerce. The Bakelite product line dominated plastics technology for years until the advent of alkyds in 1926 and the ammos in 1928. Table l-l lists a synopsis of the various historical milestones in thermosetting resin technology. Progress was made more often as a result of the economical commercialization of key precursor materials rather than as a conscientious result of a chemist’s

4


ability to tailor polymers for specific properties and characteristics. It must be remembered that the acceptance of Staudinger’s heretical concept of macromolecules was not universally accepted until the late 1920s and early 193Os, long after products made from polymeric materials had reached commercial maturity.

Table l-l : Historical Milestones of Thermosets* 1839 Goodyear discovered vulcanization of rubber. 1909 Baekeland granted his ‘Heat and Pressure’ patent for phenolic resins. 1926 Alkyd introduced. Aniline-formaldehyde introduced in U.S. 1928 Urea-formaldehyde introduced commercially. 193 1 Hyde began research on organo-silicon polymers. 1933 Ellis patented unsaturated polyester resins. 1935 Henkel made melamine-formaldehyde resins. 1937 Automatic compression molding introduced commercially. Polyurethanes first produced. 1938 Melamine introduced commercially. 1939 First patent (in Germany) on epoxy. 194 1 Urethane-polyester type-introduced in Germany. 1942 Dow Coming made silicone industrially. 1943 Castan’s patent issued on epoxy. 1946 Polyurethane elastomers introduced. 1947 Epoxy introduced commercially. 1954 Polyurethane introduced in U.S. 1957 Urethane-polyether type-introduced in U.S. 1964 Polyimides introduced as a fabricated product. *Extracted from SPE JOURNAL, 1967.

Introduction

5

DEFINITIONS

The broad classifications of plastics -

general purpose, engineering,

and specialty - applies to thermosets as well as thermoplastics. General purpose thermosets &e characterized by average (for thermosets) mechanical properties, lower resistance to temperature, higher coefficients of expansion, and low cost/commodity-like production and sales (tons/year). Engineering thermosets have higher mechanical properties and temperature resistance and they are perceived to be more durable. They are more expensive with a moderate production volume (pounds/year). Specialty thermosets are useful because of one or more highly specific and unusual property which offsets any lack of other “good” properties. They are usually very expensive and are produced in relatively small quantities (pounds/batch). Overlapping between the three categories often occurs a general purpose phenolic is often competitive with an engineering polyimide. The individual families of plastics in this book can be loosely classed as shown in Table I-2.

Table l-2: Categories of Thermosets General Purpose

Phenolics, aminos, polyesters

Engineering

Epoxy, polyurethane

Specialty

Silicones, allyls, high temperature thermosets, crosslinked thermoplastic

It is assumed that the reader has a reasonable understanding of the basic principles of polymer science and organic chemistry. These initial discussions therefore, are designed to highlight and review some of the basic concepts in order to establish the proper perspective for the material which follows.

6


CROSSLINKING AND CURING

A linearpolymer is a long continuous chain of carbon-carbon bonds with the remaining two valence bonds attached primarily to hydrogen or another relatively small hydrocarbon moiety. Figure l-2 shows a schematic representation of some linear polymer configurations.

Figure l-2: Simplified representation of various linear polymer configurations (Goodman & Schwartz, 1982).

A networkpolymer is formed as a result of the chemical interaction between linear polymer chains or the build-up Corn monomeric resinous

Introduction

7

reactants of a three-dimensional fish-net configuration [Figures 1-3(a) and 1.3(b)]. The process of interaction is called crosslinking and is the main distinguishing element of a thermosetting material. The “thermo” implies that the crosslinking proceeds through the influence of heat energy input, although, as will be seen in the individual chapters, much crosslinking occurs at room temperature (25 “C, 77 “F) and below. The “setting” term references the fact that an irreversible reaction has occurred on a macro scale. The network polymer formed has an “inhn.ite”molecular weight with chemical interconnects restricting long chain macromovement or slippage. Molecularfinctionality (i.e., number of reactive moieties per mole of reactant) dictates the potential for a crosslinking reaction. A total average httxtionality between reactant elements greater than two suggests the potential for crosslinking independent of mechanism. In other words, the bifunctional C=C, would, via an addition reaction, normally produce a linear polymer. If, however, other unsaturation is generated or remains in the formed linear chain, crosslinking can yet occur (Figure l-4).

Figure l-3: (a) Lightly crosslinked network polymer. (b) Highly crosslinked network polymer.

8


nC=C

standard linear addition polymer (polyethylene),

--t -(C-C)n-

nC=C-C=C

+ -(C-C=C-C)n

-

I’ --(C-C-C-C)-

crosslinked addition polymer

I C-C-C)-

-(C-

(polybutadiene)

I

Figure l-4: ization.

Linear chain formation and crosslinking via addition polymer-

Similarly for a condensation reaction, a tri- or polyfunctional will form a thermoset structure with a polyfunctional comonomer.

00

reactant

00

II II

II II standard linear condensation polymer (linear polyester)

HOCRCOH + HOR’OH -+--(OCRCOR’)-

00

00

II II

00

II II

II II

HOCRCOH + HOR’OH - --(OCRCOR’OCRCO)-- crosslinked condensation I

OH

I

OCRCO--

polymer (polyester)

II II 00 Figure l-5: Linear chain formation and crosslinking via condensation polymerization.

Introduction

INFLUENCE OF TIME, TEMPERATURE,

9

AND MASS

The temperature dependency of crosslinking reactions, for all intents and purposes, behaves in a traditional Arrhenius relationship. Thus ambient temperature strongly influences crosslinking rate. Since all commercial thermosetting reactions are exothermic, a mass effect also influences the rate of reaction. Monomer concentration effects are generally associated with stoichiometric balances between reactants as well as the normal free volume accessibility of each of the reactants to each other. Perceptually, it is easier to describe the events of crosslinking if we focus on the reaction between two low viscosity liquids. The principles, however, are valid whether the monomers are solids, liquids, gases, or mixtures thereof. Referring to Figure 1-6, we can track a polymerizing mixture of monomers by observing the viscosity change versus time at a given temperature. Beginning at b , the mixture has a viscosity no . The heat generated from the exothermic reaction produces a typical viscosity decrease ( TJ, ). As the molecular weight of the mass increases, the resultant mixed viscosity increase outpaces and quickly surpasses any reduction caused by heat. The molecular growth continues over time until a perceptible macroscopic gel-like “lump” can bc sensed. This is tge, the gelpoint, or more commonly, the gel time. From this point forward the viscosity goes to infinity, i.e., the polymeric mass becomes a macroscopic solid - a plastic. In some liquid systems the knee in the curve at the gel time is very hard to identity because the viscosity increase is very gradual over time. With solid molding powders, pressure and heat must be applied in order to generate a fluid condition so that the gel time can be determined. Usually a wooden probe is sufficient to detect the gel point with a good deal of accuracy (*OS minutes). Sophisticated equipment is available that automatically measures the gel point based on the length of flow of a molding powder, the increase in torque of an oscillating rubber-like mass, the change in dielectric constant of the crosslinking mass, as well as many others. However measured, accuracy less than minutes is rarely required.

10


t - TIME

Figure l-6: Viscosity vs time at constant temperature for a liquid thermo-

setting system.

The term most frequently used in the trade for this gelling phenomenon is curing. To cure a thermoset is to cause it to crosslink. Vulcanization is the rubber industry’s term for curing. Typically, the coreactant monomers are referred to as the resin and curing agent. The resin is the resinous monomer from which the family name is derived; e.g., an epoxy plastic is an epoxy resin that has been crosslinked. The curing agent is the coreactant and goes by many names: curative, curing agent, hardener, catalyst, “Part B,” etc. Any crosslinking reaction is influenced by temperature. As the ambient temperature increases, the rate of reaction increases. Additionally, these reactions are exothermic. Since all polymers are inherently thermal insulators, the exothermic heat cannot easily leave the curing mass and thus adds to the heat input for continued reaction. Figure l-7 demonstrates this effect. Curve 1 represents a normal room temperature cure similar to Figure 1-6. With added heat (T, > T, , Curve 2) the gel time decreases. Curve 3

Introduction

11

( T3 c T,) shows the effect of decreasing the temperature, i.e., tBel increases. Curve 4 (T, T, > T3 *z T,

t-

Figure l-7:

TIME

Influence of ambient cure temperature on the gel time of thermosets.

12


The mass effect on gelation parallels the temperature effect. As a crosshnking mass increases in size, the ability to transfer the exothermically generated heat away from the reaction site decreases significantly because of the thermally insulative nature of polymers. Curve A in Figure l-8 shows the typical gel profile for a given mass. Curve B represents the doubling of the mass, whereas curve C represents a halving of the mass (no Arrhenius relationship is suggested by doubling or halving). Curve D describes a condition where the mass is below some critical threshold size that arrests the crosslinking and generates an effective latency.

08

/ /

I f

I /

-

I

I

I

I

Mg C< MB = 2MA = 4MC

I

I

I L

I

I

‘g+

’ gelA

I

‘IWIC

t - TIME

Figure 1-8: Influence of mass on the gel time of thermosets.

Introduction

13

In practical terms, the temperature/mass dependency is very significant. For example, a 5-gallon mix of a urethane flooring varnish compound may gel in 20 to 30 minutes with an often violent exotherm. However, if the same mass is poured and spread over a cold floor within a few minutes of mixing, the gel time may extend itself to 4 to 8 hours. Similarly, an adhesive bonding two dissimilar metals will take longer to gel than if it is bonding two pieces of plastic: the metal acting as a heat sink, the plastic acting as an insulator.

SHELF LIFE AND POT LIFE

Shelf life is an arbitrary time for practical storage of a thermoset system. Shelf life derives from the storage concept; i.e., how long can a thermoset be left on the shelf before it becomes difficult or even impossible to use in the intended application? The term can refer to a one-can system (e.g., a phenolic molding compound must be molded within 1 year of compounding) or a two-can mix that must be set aside for a few hours before use. Shelf life is also used to describe the storage stability of unmixed components of a thermosetting resin system if there is some threat to their reactivity as a consequence of the storage. For example, some curing agents are very hygroscopic and will lose reactivity if airborne moisture were to penetrate the storage container. Working life, or pot life, is the available time to process a reacting thermosetting formula. Once the ambient cure temperature is reached and the crosslinking reaction begins, pot life describes the time available before the mixture becomes intractable or otherwise difficult to process. For example, the pot life of a coating is the time during which the viscosity remains low enough to allow for easy brushing or spraying. In a molding compound, the working life represents the amount of residence time available in the molding machine before the material must be injected into the mold in order to have trouble-free molding and/or a defect-free part.

14


CURING

The establishment of a sequence of time, temperature, and pressure needed to produce a thermoset part is the cure schedule. A simple example of one such schedule is that found on tubes of household epoxy glue. Here residence at room temperature for 16 to 24 hours under slight contact pressure clearly defines the conditions needed to affect a sufficient bond. An example of the other extreme, an extended highly specialized schedule established for a polybenzimidazole laminate is shown in Table l-3.

Table l-3: Typical Processing Schedule for a Polybenzimidazole Laminate Expose laminate prepreg in a press to 120°C with pressure increasing from contact to 200 psi. Increase temperature to 370°C and hold for 3 hours. Cool to lOO”C, remove from press and post cure under dry N, or vacuum as follows: 24 hours 24 hours 24 hours 24 hours 8 hours 3 hours

at at at at at at

315°C 345 “C 370°C 400°C 425 “C 370°C in air

In a multi-step curing sequence, the gel point most often occurs in the very early stages. From an efficient producibility point of view, the sooner a part can be handled atter forming the better. This frees up the more expensive molding equipment and allows the cure to reach completion (post-curing) in a relatively low cost environment (e.g., an oven). The criteria used to establish

thispoint aregenerallysuffkient “green”strengthof the part (canbe handled without deformation)and sufkient cum to miniize shrinkage(very little shrinkagestressor warpagewill set in beyondthis point). A cure scheduleis derivedby plotting the changein the plastics’ properties of interestagainsttime at temperatureand pressure. A sample curveis generatedlike the oneshownin Figure l-9. hactical considerations generallydictatethat the curetime be chosenat somefractional level of the ultimate properties. This is becausethe time scalecanoften be logarithmic. Thus90%of, say,ultimatetensilestrength,may be achievedin a few hoursat 25 “C. Therenmining 10% (oftennot neededfor use)may requiremonthsto yearsfor achievement

Figure 1-9: Mechanicalpropertyof a thermosettingpolymer vs time.

16


Where more than one plastics property is important the cure schedule must reflect a reasonable time-temperature-pressure relationship that will yield an optimized combination of properties. Figure l-10 describes such a circumstance. A thermoset formulator must be aware of all these considerations in his design of a useful compound. In addition, heat-up and cool-down rates, volatiles release, part design, and many other factors influence the cure sequence ultimately designated to produce a part. An example of a ramped cure schedule for a polyimide composite is shown in Figure 1- 11.

TENSILE

ULTIMATE

STRENGTH

ELONGATION

J

CURE TIME AT TEMPERATURE

Figure l-10: Optimization of cure schedule for thermosetting plastics. Optimum properties occur at point A.

Introduction

17

600 t

Figure l-l 1: Autoclave cure cycle used for typical polyimide composite.

(Delmonte)

STAGING

The influence of temperature on curing generates another practical production control. This is the concept of staging, The thermoset formula when first mixed (crosslinking has effectively not begun) called the “A-stage.” As time and thus crosshnking, progresses, the compound goes through the “Bstage.” This is a time line as shown Figure 1-12.

Time --t

__-----_-_--

P

A-stage

v B-stage

Figure 1-12: Staging time line for thermosets.

C-stage

18


Many thermosets can have the reaction arrested at any point along this line. The simplest way to do this is to drop the temperature sufficiently below the reaction temperature to a point of latency. Depending on conditions, this latency period can be quite long (not uncommonly 6 to 24 months at room temperature and lower). In addition, the polymer/monomer mixture at this point may well change its physical state to a more useful form. For example, a B-staged phenolic molding powder will harden and embrittle compared to the A-stage. It can be fiangibly crushed into a non-sticking, free-flowing powder that is stable at room temperature for 12 or more months. Epoxy resins can be coated while liquid on a polymeric carrier film, B-staged to a fixed degree of tackiness, and stored under refrigeration for 6 to 12 months. This provides a useful tape-supported adhesive that only requires application to a substrate and subsequent heating for bonding. The C-stage represents the fully crosslinked part in its final configuration,

STOICHIOMETRIC CONSIDERATIONS

One of the major responsibilities of the thermoset resin chemist is to balance the coreactants stoichiometrically. He does this by establishing a mix ratio, the weight to weight proportion of the resins and curatives. In theory, each functional group in each monomer must react on a l/l molar basis. The final crosslinked plastic should have no residual reactive sites if all reactants have been properly proportioned and subjected to optimum cure conditions. In reality, many considerations drive the polymerization process away from the ideal. To begin with, as the molecular weight of the polymerizing mass increases, it becomes sterically less possible for reacting species to come together and react. Side reactions and chain stopping contaminants may reduce the calculated number of reactive sites. Although molecular movement never truly ceases in a crosslinked mass, the time span for “complete cure,” i.e., to reach ultimate properties, may be logarithmic (see earlier discussion on cure schedules).

Introduction

19

In practice, a resin formulator will calculate a theoretical stoichiometry for his intended formula. Then he will prepare samples under a given set of cure conditions and test for the change in selected properties of the resultant plastic versus change in stoichiometric ratio. This can be a long, cumbersome and expensive process. One short cut, common to the epoxy chemist, is to run a Soxhlet extraction in acetone. A curve like the one shown in Figure 1- 13 is obtained.

Figure 1-13: Optimizing mix-ratio using a Soxhlet apparatus.

20


Obviously that mix ratio which creates the least weight loss should represent the maximum integrity of the cured part, i.e., the best properties. Once established, the formulator can narrow his choice of mix ratios to those near this optimum and reduce the testing required to pinpoint the exact ratio that will provide the properties he is seeking. Examples of representative stoichiometric calculations can be found in the individual chapters on polyurethanes and epoxies. Experience has demonstrated that it is not at all uncommon for mix ratios to depart from l/l molar by as much as 20 to 30%. The responsibility for insuring that the stoichiometric balance is maintained varies among thermoset types. The phenolic chemist is concerned during the initial manufacture of a polymer. The user need only add heat and pressure to get a part. The polyester chemist establishes the balance when he makes the base resin. He then adds his crosslinking monomer and the user catalyzes the mix to affect the cure. The epoxy and urethane chemist, on the other hand, may not only do as the others, but may also design a system that requires the end user to mix the reactants in the correct ratio. As will be described in later chapters, this may impose a significant constraint on the user to insure that the predetermined mix ratio tolerance is maintained.

PREPOLYMERIZATION

AND ADDUCTING

Prepolymerization is a method of increasing the molecular weight of a forming polymer to some intermediate value. Prepolymerization is often confused with B-staging. During B-staging the polymer formation is arrested at some practical intermediate point. It is a random process which yields various molecular weight moieties and some crosslinking. A prepolymer is normally formed under precisely controlled conditions to yield a stable polymer of specific molecular weight and configuration, most oflen without any crosslinking. For example, an isocyanate will be coreacted with a glycol at a stoichiometric ratio much greater than l/l. A urethane prepolymer will be formed with sufticient residual isocyanate to further react in a curing environment.

Introduction

21

There are many reasons for generating prepolymers. Among the more prevalent are: to increase the viscosity of monomer, to decrease toxicity and/or reactivity for control of gel time and exotherm, and to balance the mix ratio of a formulated system (compensate for the addition of additives and fillers). When a monomeric resin is “capped” with a coreactant, the process is called adducting. The technique proceeds via the following schematic sequence. Monomer A has 2 functional groups, F,: F&F, Monomer B has 3 functional groups, F,: F,R’F,

In a direct polymerization the two monomers will react as in equation (l), i.e., a crosslinked polymer.

(1)

3nF,RF,

+ 2nF,R’F,

--t -(RF,F,R’)-

I FB

I

F,F,R-

If F, and F, are olefmic bonds then the adducted moiety contains F,F,, i.e., C-C bonds, resulting from the typical addition reaction. If F, is a carboxyl group, say, and F, is an amine, then a condensation reaction will occur yielding an amide, F,F, = CONH-, and H,O. The adduction process proceeds as in Equation (2).

(2)

3F,RF,

+ lF,R’F, I 83

-t F,RF,F,R’FnF,RF, I Wad,

The crosslinking is completed by stoichiometrically reacting the resulting pendant FAgroups with more F,-containing reactant. The net result, equation (3), is a polymer with essentially the same crosslinked structure as with a standard crosslinking, equation (1).

22


(3)

mF,RF,FnR’F,F,RF, I FJP*

+ mF,R’F,

+

I 53

The reasons for adducting are essentially polymerization.

-(RF,F,R’)mI

F,F,R-

the same as for pre-

BIBLIOGRAPHY

Delmonte, J., Technology of Carbon and Graphite Fiber Composites, Van Nostrand Reinhold Co., New York (1981). Morton, M., Introduction to Rubber Technology, Reinhold Publishing Corp., New York (1964). Schwartz, S.S. and Goodman, S.H., Plastics Materials and Processes, Van Nostrand Reinhold Co., New York (1982). Severs, E.T., Rheology of Polymers, Reinhold Publishing Co., New York (1967). Whittington, L.R., Whittington’s Dictionary ofPlastics, Technomic Publishing Co., Stamford, Connecticut (1968).

Phenol-Formaldehyde

Resins

Christopher C. Ibeh

INTRODUCTION

Phenol-formaldehyde resins (phenolic resins), the first thermosetting plastics, are considered to be the first truly synthetic commercially available plastic resins. Unlike celluloids, the first man-made plastic resins, phenolic resins are made from purely synthetic materials. Phenol-formaldehyde resins are formed by the chemical reaction between phenols and formaldehyde solutions (formalin). Work in the area of phenols and fomraldehydes began prior to the 20th century with Adolf Bayerr’] in 1872 and LosekamL21in 1889. The first commercially available phenolic resin, called Laccain, was introduced by Blumert21 in 1902 as a substitute for shellac; it was not a commercial success 13]The use of phenolic resins was popularized by the “heat and pressure” patents[3q of Dr. Leo H. Baekeland in 1907; hence, he is known as the “father of phenolic resins.” Today, some of the most popular phenolic resins bear the trade name “Bakelite” in reference to the company (General Bake-lite Company) he formed in 19 10. Bakelite Corporation became a subsidiary of Union CarbideI in 1939. Baekeland’s patents involved the use of (1) High pressure and a closed mold to overcome the problems of bubble fomration and the evolution of gases and steam. 23

24

Handbook of Therrnoset Plastics

(2) Fillers, such as wood flour, to overcome the problem of the brittleness of cured phenolic resin. These inventions made it possible, using phenol-formaldehyde resins, to cost effectively produce molded parts. Prior to Baekeland’s inventions, it was necessary to use low-temperature processing methods to suppress the evolution of steam and gases. These methods caused bubble formation and expensive long production cycles. Also, the use of fillers enhances properties and lowers the cost of cured resin. Over the years, phenolic resins have competed favorably with newer plastic resins. In 1993, phenol-formaldehyde resins had a consumption volume of 3.07 billion pounds, ranking second only to polyurethane (3.476 billion pounds) among thermosets. In 1983, the American Chemical Society organized a symposiumt’l entitled “Phenolics Revisited, 75 Years Later” in Washington to showcase the importance of phenolic resins. Today, phenolics find usage in many areas, such as insulation, electrical devices, automotive parts, and adhesives.

RAW MATERIALS

Phenol-formaldehyde resins are general-purpose thermosets formed mainly by the polycondensation reaction between phenols and formaldehyde solutions. The three major raw materials for making phenolic resins are: (1) Phenol C,H,OH. (2) Formaldehyde CH,O. (3) Hexamethylene Tetramine (CH,),N,. Other phenols, such as cresols (monomethyl-phenols), xylenols (dimethyl-phenols), resorcinol (m-dihydroxybenzene), and alkylated-phenols, can be used in place of phenol, but their usage is limited due to the availability of less expensive synthetic phenol. Resorcinol-based resin can be room-temperature cured and used in laminated beams for churches, boat keels, etc. Alkylated phenols, such as p-tertiary butyl phenol and p-tertiary octyl phenol,

arc used, respcctivcly, as tackiliers in pressure scnsitivc tapes and automobile tires.

PHENOL

Phenol is primarily obtained from the fractional distillation of coal tar and various synthetic proccsscs. There are at least six known commercial synthetic processes for making phenol, of which the four most common are Cumene, Raschig, Dow, and Sulfonation. The Sulfonation process, although popular at one time, is no longer in use.

Cumene Process for Making Phenol

In the cumene process, commercialized in 1952 by H. Hook, the reaction between benzene and propylene with an aluminum chloride catalyst yields isopropyl benzene (cumene). Cumene oxidizes to hydroperoside, which is broken down by acidification into phenol and acetone:

C,H, + CH,=CHCH, benzene propylene

0, C,H&H(CH,), cumene

+

AICI, _) catalyst

C,H,CH(CH,), cumene

acid C,H, COOH + cumene hydroperoside

C,H,OH Phenol

+

CH,COCH, Acetone

Raschig Process The Raschig process passes benzene, hydrogen, and air over a heated copper catalyst at 200’ to 300°C. The intermediate product is chlorobenzene

26

Hondbook of Thermoset Plastics

and water in the gaseous state. The water hydrolyses the chlorobenzene, passed over hot silica catalyst at 5OO”C, to phenol and HCl.

when

CdFe C,H, + benzene

HCI +-&O, hydrogenchloride

C,H,Cl + Hz0 chlorowater benzene

_) catalyst + 200”-300°C

C,H,Cl + H,O water chlorobenzene

SiO, + catalyst + 5oo”c

C,H,OH phenol

+ HCl hydrogen chloride

Dow Process The Dow process, established in 1920, uses the direct vigorous reaction of chlorobenzene and sodium hydroside solution at 300°C and 4000 psi:

C,H,Cl chloro benzene

+

Formaldehyde

NaOH sodium hydroxide

300°C + 4000 psi

C,H,OH phenol

+

NaCl sodium chloride

(CH 20)

Formaldehyde is produced by the controlled catalytic oxidation of methyl alcohol (methanol). The result is the dehydrogenation of methanol to formaldehyde. In the process, a mixture of methanol vapor and air is passed over a heated copper oxide catalyst at 300°C to 6OO’C to produce a mixture of formaldehyde and water. The product is a 37% solution formaldehyde that is subsequently enriched to a 40% solution known as formalin:

CH,OH + +O, methanol

300”-600°C + cue

HCHO + formaldehyde

H,O water

Impurities in the formed formalin include formic acid as a side product and residual methanol. The residual methanol scrvcs as a stabilizer during storage, whcrcas the formic acid is rcmovcd. This process was commcrcializcd as the Formox process in the late 1950s by the Rcichhold Chemical, Inc.

Hexamethylene

Tetramine (Hexamine or “HEXA”) (CH *) 6N .,

HEXA is produced by passing ammonia gas into a 30% formaldehyde solution at room temperature:

6CH,O formaldehyde

+

4NH, ammonia

+

(CH,),N, HEXA

+

6H,O water vapor

RESINIFICATION (PRODUCTION) of PHENOL-FORMALDEHYDE RESINS

Reaction Chemistry

The reaction nxxhanism between phenol and formaldehyde is not yet completely known. It is known, however, that the reaction is initiated by the activation of the benzene ring by the hydroxyl group such that a methylol group (CH,OH) joins the benzene nucleus at the ortho and para positions. The reaction produces a resole if an alkaline catalyst is used and a novolac if an acid catalyst is used. From the work of Baekeland and Lebacht’l , three basic stages are recognized in the phenol-formaldehyde reaction: A-stage or resole. (I) B-stag-c or resitol. (2) C-stage or resite. (3)

At the A-stage, the initial condensation products are mainly alcohols. The resin at this point is thermoplastic and soluble in inorganic solvents. At the B-stage, thcrc is a higher degree of condensation and some crosslinking, with a consequent incrcasc in molecular weight and viscosity, and a dcxxcasc in solubility. The resin is not fully cured; it is soft and fusible when hot but hard and brittle when cold. At the C-stage, the degree of polymerization and crosslinking is very high, and there is almost a complete cure. The resin is infusible and insoluble.

Polymerization

Process

Phenolics are produced by the polycondensation reaction between phenols and formalin (40% aqueous solution of formaldehyde). The trifunctional phenol reacts with the difunctional formaldehyde, resulting in a threedimensional matrix when the reaction is carried out beyond the gel point. After the gel point, further curing results in a thermoset. The ready-to-be-used resin is processed to just before the gel point during the polymerization process. Based on this, there are two main types of phenolic resins: Resoles or one-stage resins. (1) Novolacs or two-stage resins. (2)

Resole Phenolic Resins

Resole phenolic resins are produced by polymerizing (heating) a 1: 1 (or more) molar ratio of phenol and formalin in a reactor vessel (Figure 2- 1) in the presence of an alkaline catalyst such as ammonia, sodium carbonate, or sodium hydroxide at 100°C for about I hour. It is usual to have 1.1 to 1.5 mole of the formaldehyde for each mole of phenol. Polymerization is stopped short of the gel point by cooling (Bstaging). The product at this point is an intermediate resole phenol-formaldehyde resin. If a solid product is desired, the intermediate is dried by hcating under a vacuum for 3 to 4 hours to prevent heat hardening.

Phenol-Formaldehyde Resins

Wafer

out

29

Vent

---_--u A

\‘J

Te fqxroture :_

mdicotor

Water in

Figure 2-l : Schematic of typical reactor vessel for the bulk polymerization of phenol-formaldehyde resins[81[g1

30


Resole phenol-formaldehyde resin is a water soluble methyl01 (CH,OH) bearing thermoplastic. The curing process to the final thermoset material can be initiated by just heating the resole in a mold above its gel point. The resole resins formed have reactive methyl01 and hydroxyl groups. When heated, resoles form larger molecules with methylene crosslinks without the use or addition of a curing agent. The phenol-formaldehyde resin resinification reaction is a typical polycondensation reaction since water is given off as a by-product.

OH

OH

bheno 1

Resole

OH

Phenolic

OH

Resin

IMethylolBearing)

Resinifkation

Reaction of Resole-type Phenol-Formaldehyde

Resin

Resole phenolic resins have a short shelf life: less than 1 year (less than 60 days in most cases). They are referred to as one-step phenolics because they do not require curing agents, only heat. Casting resins, bonding resins, and resins for laminating paper and wood are made in this way.

Novolac Phenol-Formaldehyde

Resins

The polymerization of phenol-formaldchydc novolac resin is carried out in the presence of an acid catalyst such as oxalic acid, sulfuric acid, hydrochloric acid, formic acid, and aromatic sulfuric acids. Sulfuric and oxalic acids

arc the two most commonly used. The gel point of the curt is dclibcrately dclaycd by using a phctlol-fonnaldchydc fccdstock that is dclicicnt in formaldehydc. It is customary to use a phenol-fonnaldchydc ratio of 1:0.8 in the first stage (prepolymerizalion). Polymerization is carried out by heating the mislurc for 2 to 4 hours at rctlux, with water removal at temperatures as high as 160°C (Figure 2- 1). The resultant low-molecular-\~~eight molten intermediate is cooled. The glassy material is carefully crushed and blcndcd with hesamethylenc tctramine (HEXA) (in powder form) to produce a molding compound. The HEXA is the second part or hardcncr; hcncc, the blended resins arc rcfcrrcd to as two-step resins or novolac phenolic resins. Upon heating the novolac resins to about 165°C in a mold, the HEXA decomposes to provide the fomlaldehyde necessary for the final curing. The addition ofHEXA gives the resin a final working phenol-fomlaldehyde molar ratio of 1: 1S. Novolac phenolic resins have an infinite shelf life under normal storage conditions.

OH

Phenol

Novolac

Phenolic

Resin

(Non-MethylolBearing)

Resinification Reaction of Novolac-type

Phenol-Formaldehyde

resin

A schematic of a reactor vessel used for the typical bulk polymerization process in the resinification of phenol-formaldehyde resins as shown in Figure 2-l. The sleanljacket provides the temperature condition needed for the poly-

condensation reaction of phenol and formalin. The condcnscr facilitates the condensation and removal of water vapor resulting from the phenol/formalin rcaclion. The removal of waler from the syslem cnhanccs the phcnol/formalin reaction and causes formation of more phenol-formaldehyde resin. If the water was not removed, the formation of phenol-formaldehyde resin would bc suppressed.

Differences Between Resole and Novoiac Phenolic Resins

The differences between resole and novolac phenolic resins are: Resole phenolic resins are produced by using alkaline catalysts, (1) whereas novolac resins are produced via acidic catalysts. Resole phenolic resins are produced via B-staging while novolac (2) resins are made by prepolymerization. Resole phenolic resins are methylol-bearing resins while novolacs are (3) non-methylol-bearing resins. Resoles typically have a shelf life of less than 1 year (less than 60 (4) days), whereas novolacs have an infinite shelf life. Resoles split off water when they cure while novolacs give off am(5) monia when they cure. Novolac resins are twice more dimensionally stable than resoles. (6) Resoles are typically casting and bonding resins, whereas molding (7) compounds are made from novolacs. (Of course, some molding compounds can be made from resoles.) Resoles are usually liquids while novolacs are solids. (8)

Properties of Phenolic Resins

Phenolic resins are typically opaque and range from pale amber and dark brown to black in color. Of course, some resoles are light in color prior to processing. The dark color of phenolic resins limits their application to a narrower market niche. Phenolic resins are available in flakes, films, and liquid and powder forms.

Phenolic resins arc considcrcd to be gcncral-purpose thermosets though they can bc compounded into cnginecring structural materials. They cost about $0.60 to X0.85 per pound u31t2Glt2’l depending on make and grade. Phenolics arc the second most used thcrmosets, with an annual consumption volume of 3.07 billion pounds (in 1993). Phenolic resins (and thcrmosets in general) arc brittle without tillers. It is customary to use tillers and other additives to achieve their desired properties and characteristics. Hence, fomn.rlation is the essence. Table 2- 1 presents the typical properties of different fomls of phenolic resins, and Table 2-2 gives the parts per hundred resin (phr) composition of different formulations for phenolic molding resins. The effect of tillers on phenolic resin properties can be seen Cramthe data in Table 2- 1. For example, the compressive strength of unfilled phcnolic molding resin is about 10,000 to 30,000 psi compared to 26,000 to 70,000 psi for glass-filled phenolic molding resins. Phcnolic resins exhibit a high degree of property variance due to physical and chemical variation in composition. To discuss the properties of the various forms of phenolic resins based on their end uses or processing fomls such as casting, adhesives and bonding, coating, laminating, and molding resins would be cumbersome and virtually impossible. Although phenolic molding resins have only about 6% of the phenolic resin market, they are used in a wide spectrum of products that show the essence of the properties and characteristics of phenolics. There are several types of phenolic molding resinst31t’01: 1. General-purpose grade (wood flour filled). 2. Non-bleeding grade (resole based, glass filled). 3. Heat-resistant grade (mineral filled, mica). 4. Impact grade (cellulose, rubber, glass, fabric filled). 5. Special or electrical grade (mica, glass filled). Phenolic molding resins arc generally made of two-stage novolacs, although some are made of resoles.

Tahb _..“.W

3-l - Phmmlir m

m

1.

..W..“.._

Properties

R&n _._I...

Pmnm+i~&l-l’) .

.”

y-.

.SWU

Casting Resin Untilled

Molding Resin Untilled

Molding Resin Cellulose Filled

Molding Resin Woodflour Filled

Molding Resin Glass Fiber Filled

Tensile strength (at break) (psi)

s,ooo-9,000

7,000-8,000

s.ooo-9,000

5,000-9,000

7,000-l 8,000

Impact strength izod (notched) @lb/in.)

0.24-0.4

0.2-0.36

0.2-0.6

0.24-0.6

0.5-18

Compressive strength (at rupture) (psi)

12.000-15.000

1o.ooo-30,000

22,000-3 I .ooo

25000-3 1.000

26,000-70,000

Flexural strength (at yield) (psi)

11.000-17.000

11 .ooo-14,000

7,000-14,000

7,000-14,000

15.000-60,000

Elongation (at break) (%)

1.5-2.0

--

0.4-0.8

0.4-0.8

0.1-0.2

Hardness (Rockwell)

M93-120

M124-128

E64-95

MlOO-115

E54-101

Heat deflection temperature (at 264 psi) ( “F)

165-175

_____I

300-350

300-370

350600

Table 2-l: Phenolic ResinProperties’“~” I

Propaiies

continued

Casting Resin UIltilled

Molding Resin Unfdled

Molding Resin Celluloss Filled

Molding Resin Woodflour Filled

Molding Resin Glass Fiber Filled

Coetficient of linear thermal expansion lOa (k/in. “C)

1,7cG1,800

640-1.520

3045

3045

8-2 1

Thermal conductivity IO’ (calisec cm “C)

3-5

-...--

-.--..

4.8

R-14

Linear mold shrinkage (h/in.)

O-0.01

n-m

0.w40.006

0.004-0.009

0.001-0.004

Dielccttic strength (V/mm1

250400

200.350

300.380

260-400

140-4cu

Dissipation factor (at60Hz) (‘=I KHz)

0.10-0.15

0.c.5-o.10

* 0.04J3.20

0.03.0.3

0.01-0.1

Water absorption (24 H) (%)

0.142

0.05-0.9

0.3-1.5

0.03-1.2

0.2-0.4

Specific gravity

1.236-1.320

1.25-1.30

1.37-1.46

1.30-1.35

1.69-2.0

*Conversion factors: 1 psi = 0.006895 MF’a

Table 2-2: Typical Formulations (phr) of Phenolic Molding Resins Composition

(phr)’

Gmeml-purpose

Grade

High-impact Grade

Electrical (Insulation)

Grads

(Pipe)

NOVOl~ClF%ill

100

100

100

loo

Fabric

--

150

-

-

-...

--

40

--

.--

-_

120

-

2.0

2.0

2.0

5.0

...-

--

-

5.0

2.0

3.0

2.0

-

I --

l 10.0

Fiber

shreds glass

Mica Limclmaenesium

oxide

Clay Magnesium

steamte

Coating

Grade

(lubricant) I Plasticizer Colorants

I -(dyes or

4.0

l -_ 3.0

3.0

pigments) Alcohol ‘phr

= parts per hundred

.-. resin

--

--.

73.0 (iron 55.00

I

oxide)

The properties of phcnolic molding resins include: Ease of molding. Phcnolic resins can be easily molded into (1) intricate shapes and to finished dimensions with littlc or no extra finishing touches. Very good dimensional accuracy and stability. Phenolic mol(2) ding resins have very good dimensional accuracy and stability at normal atmospheric conditions. Their linear mold shrinkage is 0 to 0.0 1 in./in. and their tilled compounds have a coefficient of linear thermal expansion of about 8-45s 10” in/in. “C. Typically, molding compounds based on novolacs are twice more dimensionally stable than resole-based molding compounds. Novolac-based compounds are more stable because they give off ammonia upon curing, while resolebased compounds give off water. The water molecule is larger than the ammonia molecule. The glass-tilled resins are more stable than the cellulose-filled types. The lower the cellulose content, the more stable is the resin. High creep resistance. The high dimensional stability ofphe(3) nolic resins is complemented by their high creep resistance”” Data fiornMateria1.sEngineering, Manual 249, September 1974 indicate that phenolic resins, in comparison with engineering thermoplastics such as polycarbonate, polyacetal, and polyphenylene oxide, have very high creep resistance. When tested at 2000 psi, 73’F, and over 400 hours, the maximum total strain (%) for phenolic resin is less than 0.1 compared to more than 0.4 for polycarbonate, more than 0.6 for polyphenylene oxide, and more than 1.4 for polyacetal. High deformation resistance under load. Phenolic resins ha(4) ve high resistance to defomration under load. Flexural and compressive strengths for general-purpose phenolic molding resin are 7,000 to 14,000 psi and 25,000 to 3 1,000 psi, respectively. Good heat resistance. Phenolic molding resins have low the(5) rmal conductivities and are good heat insulation materials. The thermal conductivity of general-purpose phenolic resin

(6)

(7)

(8)

(9)

(10)

is about 4-8x IO-” cnllscc cm.‘T. Its heat dcflcction temperature (HDT) is about 300 to 370°F. Good electrical resistance. Phcnolic molding resins arc good electrical insulation materials, with a dielectric strength of 260 to 400 V/mm and a dissipation factor of 0.03 to 0.3 for gcncral-purpose grades. They are most suitable for low voltage (260 to 400 V/mm) electrical insulation. Good chemical resistance. Phenolic molding resins arc rcsistam to common solvents, weak alkalis, weak acids, hydrocarbons, and dctcrgcnts, but are attacked by strong alkalis and concentrated oxidizing acids. Low water absorption. Water absorption of phenolic molding compounds is only about 0.03 to 1.75%. The resolebased molding resins are more water resistant. Unpreheated preforms (loose powder form) tend to absorb more water under humid conditions and will lose moisture and stiffen in low moisture or dry conditions such as in winter. It is recommended that phenolics be stored at room tempcraturc and 50 to 60% relative humidity. Good weather resistance. Phenolics can be used outdoors for short periods, but prolonged outdoor exposure to ultravioletlight and heat can cause failure. Good machining qualities. Machining qualities are fair to good for molding compounds but are excellent for casting resins.

Fillers for Phenolic Resins

The properties of phcnolic resins can be enhanced by the use of fillers and additives; the type of tiller used depends on the application(s) for which the material is formulated. The fillers typically used with phenolic resins arc: Wood flour. This is made from pine and spruce wood chips (1) that have been chemically treated to remove the lignin. Wood flour, the least expensive of the tillers. has highvolume usage and provides good flow and appearance. low-

(2) (3)

(4)

(5)

(6)

heat conductivity, good lensilc strength, and poor impact slrcnglh. Ccllulosc (cotton flock). It is typically used for good impact slrcngth. Mineral lillcrs (asbestos, mica, and clay). Asbestos is no longer used as a tiller bccausc it is a suspecled carcinogen. Mica is complex potassium/aluminum silicates (K,AL,)made from the minerals muscovite and (AL,Si@,J(GW, phlogopite. It has good electrical and heat insulation properties and dimensional stability. It does not wet well and sticks in the mold; it needs to be used with a wetting agent such as sodium stearate, sodium lauiyl sulfate, sodium lignin sulfonate, or silicone coupling agents. The wetting agent reduces interfacial tension between filler and resin. Mica dust is toxic and must be handled with care, The auto industry uses mica as an inexpensive substitute for fiberglass in polypropylene and phenolic parts. Clay is a form of aluminum silicate used to enhance heat and chemical resistance, dimensional stability, and viscosity control. Its dust is toxic and must be handled with care. Silica (SiOJ. It is natural crystalline sand, quartz, and diatomaceous earth, although synthetic non crystalline, amorphous forms exist. It is used for its abrasive, electrical and heat insulation properties, viscosity enhancement, and shrinkage and crack reduction. Graphite fiber. Produced from the pyrolysis of stretched polyacrylonitrile or rayon, it has more than 99% carbon content (less than 97% carbon content = carbon fiber). It is used to improve strength and frictional properties (lubrication). It is relatively expensive (Figures 2-2 and 2-3). Glass fiber (fiber glass). It has a composition of 55% silica, 20% CaMg Oxides, 14% alumina, 10% borates, and 1% sodium and potassium oxides. It is used to improve impact strength, heat, and electrical insulation properties.

Figure 2-2: Hercules chopped graphite fibers for compression and injection molding. (Courtesy of Hercules Advanced Materials & System Company.)

I’henol-l;ormnld~hyde Resins

4I

Figure 2-3: Hercules continuous graphite fibers for pre-pregging and filament

winding applications. (Courtesy of Hercules Advanced Materials & System Company.)

42


(7)

(8)

69

(10) (11)

Talc. This mineral filler is a form of hydrated magnesium silicate (3 Mg0.4SiO,.H,O) used to enhance stiffness, creep resistance, and heat resistance. Fabric fibers (sisal, rag fibers). Sisal is a form of natural fiber from Africa and Haiti used in rope making. Rag fiber is dcrivcd from textile materials. These tillers improve impact strength and produce a rough surface when machined, properties necessary or desirable in “no-slip” surfaces as in pulley belts and ropes. Synthetic fibers (orlon, nylon, teflon). These tillers provide improved impact and tensile strengths and vibration dampening. Paper. This filler is typically used for electrical insulation properties. Polyaramid fibers (aromatic polyamide fibers). Two popular types of polyaramid fibers: Kevlar and Nomex, are trade names of DuPont. Kevlar is an organic reinforcing fiber with high tensile strength and modulus used mainly in advanced composites. Nomex is valued mainly for its heat and electrical properties. Aramid/phenolic composites are used extensively in aerospace interiors owing to their high strength-toweight ratios and heat resistance.

A combination of fillers is commonly used to achieve optimum property enhancement. Fillers also have a direct effect on the shrinkage of molded phenolic parts. Polyaramid tibers give the least mold shrinkage; synthetic fibers give the most. There is also a direct correlation between mold shrinkage and the coefficient of linear expansion (Table 2- 1). Fillers are also used to reduce cost and exothenn.

Processing Methods for Phenolic Resins

The three major processing methods are compression molding, injection molding, and transfer molding. Other fonns of processing, such as runnerless injection compression @UC)and casting, are becoming popular.

l’hct~~~i-f~~rm~ldeh~~deRcsin.v

43

(RIC is a rcgistcrcd trademark of Durcs.) Compression molding is the original processing method for phenolic resins per Leo Baekeland’s %.x1 and pressure” patent. Still the most popular method in use today, produces the strongest, most dimensionally stable products because less damage is done to the Gbrous fillers and shrinkage is low. Warpagc is less because pressure is applied evcnly, and internal stresses arc minimized. The molds are infrared or radio-frequency heated and the correct amount of resin is put into the cavity. The molding is carried out under prcssurc. Compression molding cannot handle delicate inserts, and flashing is common. Compression molding temperature and pressure conditions for phenolic resins are typically in the range of 290”400’F and 2,000 to 20,000 psi, respectively. Transfer molding is popular for applications involving delicate inserts. This process involves using a separate heated transfer charger from which a preheated, preweighed resin is pushed into the mold by an auxiliary ram or plunger. Transfer molding is also used for parts with both thin- and thick-walled sections. of phenolic resin is similar to that of thennoInjection molding t161[‘71 plastic resins. Technological advances, such as availability of glass-filled, injection molding grade phenolic resins and use of screw (reciprocating) injeclion molding process, have made injection molding of phenolic parts economically feasible and desirable. In screw injection molding, the material is preheated, plasticized via frictional heat, conveyed, and injected into a closed mold by the screw action and movement. Direct and automatic feeding of the material to the mold eliminates the prefomling steps (storage and pre-heating), thereby reducing process cycle time and increasing productivity. The higher mold temperatures (320”-380”F), high frictional heat, and faster injection speed of the process require adequate venting. Processing pressures and material temperatures for the screw injection molding process of phenolic resins are 10,000 to 20,000 psi and 220”-240”F, respectively. RIC molding t’‘1is a new process invented by Durez. RIC combines the direct and automatic feeding action of injection molding and the dimensional stability of compression molding. The low pressure injection process introduces the material into a partially open mold (l/4 to l/2 in.), and the compression portion of the process takes place with the closing of the mold. This combination results in such advantages as speed (faster process cycle time) and the dimensional stability of molded parts. Also, because the mold

is open during “mold lill,” venting and mold staining problems are reduced. The runncrlcss system makes for lower scrap level and material savings. Casting of phcnolic parts makes for east for processing and costeffectiveness, cspccially for profile shakes such as bowling balls, billiard balls, cutlery handles, and specialty items.

Applications of Phenolic Resins 12113i117v23i

Trends in the consumption of phenolic resins are presented in Table 2-3. There are live major areas of phenolic resins application: plywood, adhesive and bonding, laminates, molded parts, and coatings. Plywood and laminates are part of the adhesive and bonding market, but the large volume usage of phenolic resins in plywood (49% of the phenolic resin market) and laminates (6%) warrant their consideration as distinct areas of application of phenolic resins.

Phenolic Resins in Plywood 121P~f1eJ

Plywood belongs to a class of materials referred to as composite wood materials or wood resin boards. The other wood products included in this category are particle board, fiber board, wafer board, and strand board. Phenolic resins serve as the bonding material in these products. Table 2-4 presents the typical resin formulations for the adhesives used for composite wood materials. These composite products other than plywood (particle board, fiber board, wafer board, and strand board) are considered part of the adhesive and bonding market and are treated as such under Fibrous and Granulated Wood (Table 2-5). Together, these fibrous and granulated wood products account for 15.5% of the phenolic resins market (compared to 49% for plywood). In 1993, 1.55 billion pounds of phenolic resins went into the plywood market.

Table 2-3: Trends in Phenolic Resins C )nsumptiol [ Year 1 Adhesives & 1 Laminates Plywood 1983 % of total 1984 1985 1986 t 1987 1988 1989 1990 1991 1992 1993

Bonding 590 26.5 630 25.18 755 28.6 740 27.8 1 769 25.57 870 28.5 806 28.52 859 29.24 751 28.20 843 28.51 916 29.82

134 6.0 149 5.96 142 5.38 167 6.27 I 193 6.43 213 6.98 246 8.71 197 6.71 157 5.90 176 5.95 173 5.63

1160 52.0 1318 52.68 1255 47.5 1270 47.41 1557 51.78 1582 51.82 1415 50.07 1515 51.57 1320 49.57 1465 49.54 1515 49.33

_I!E&% Composites 200 9.0 238 9.51 209 7.92 195 7.33 197 6.55 198 6.49 176 6.23 168 5.72 160 6.0 173 5.85 163 5.32

(1061b/yr)f1 Protective Coatings 20 0.9 22 0.88 23 0.87 22 0.83 23 0.76 21 0.69 13 0.46 17 0.58 16 0.60 17 0.57 13 0.43

i 16 0.7 21 0.84 21 0.80 18 0.68 18 5.99 22 0.72 19 0.67 21 0.71 17 0.64 18 0.61 30 0.98

I 110 4.9 124 4.95 235 8.9 250 9.38 250 8.32 147 4.8 151 5.34 161 5.47 242 9.09 265 8.96 260 1 8.49

Table 2-4: Typical Formulations for Adhesives Used in Composite Wood Products

* phr = parts per hundred resin

Insulation

414

423

346

385

366

Total

806

859

751

843

916

* Data not available

I’henol-l;ornlnldeh))de

Resins

47

The USCof phcnolic resins in plywood was introduced into the U.S. in 1932 with the importation of the Goldschmidt glue line (“Tego lilm”) from Germany. Tego film is a thin sheet of paper impregnated with phenolic resin. Typically, the thin sheets of paper are placed between layers of veneer (wood), and the layers of veneer arc hot pressed at temperatures of loo”- 150°C and pressures of 700 to 6000 psi. The use of phenolic resins (in Tego film) in plywood was a response to the demand for exterior weather-proof plywood. Prior to the introduction of Tego film, the bonding materials for plywood were casein glue, peanut meal, and soybean meal, which are strictly for interior use. The adhesive material for exterior grade plywood is resole-type phenolic resin with a swell-resistant tiller such as coconut shell. Other fillers used arc oat hulls, wheat flour, and chalk. Wheat flour and chalk are mainly used as extcnders for adhesiveness. The extenders are also used to reduce cost and porosity. Sodium hydroxide serves as a solubilizer in phenolic resin in water. Phenolic resins compete very favorably with urea-formaldehyde bonded plywood, and most of the plywood made today is phenolic resins based. The lack of homogeneity in wood composition is a problem in the production of plywood. The degree of penetration or saturation of the wood by the resin determines the level of bonding. It is customary for plywood manufacturers to carry out tests and experimentation to determine the right resin formulation for each wood or veneer system. Phenolic resins-based plywood find application in the making of interiors and parts for automobiles, boats, ships, trucks trains, machines, and tool handles. Some of the luxury cars, such as Mercedes and Cadillacs, use plywood for dashboard decoration.

Other Composite Wood Products

The consumption of phenolic resins-based composite wood materials (fibrous and granulated wood) has increased dramatically, with a 1993 consumption volume of 475 million pounds compared to 269 million pounds in 1989. This reflects an increase from 9.5% to 15.5% of the phenolic resins market. Particle Board. This composite wood product, commercially introduced in the USA in 1941 consists essentially of about 10% resin and 90% wood chips (saw dust, plywood scraps, planer shavings). The percentages

vary dcpcnding on the number of layers of the product. The resin, which wets and binds the wood material, improves the rclativc humidity of the wood-resin blend from about 5% to about 15%. The blend is prcprcssed at room tempcrature and 145 to 140 psi, and ultimately comprcsscd at I 60”-220°C and 290 to 590 psi, dcpcnding on the desired density of the product. The steam released during the process acts as the heat transfer medium (steam shocking). The boards produced are postcured by hot stacking them in storage. The strength of the boards produced depends on the type and structure of the wood chips used and the resin formulation. Particle boards with higher resin contents have higher bonding strength. Particle board is predominantly a ureaformaldehyde-based product, with phenolic resins-based particle board products accounting for only about 10% of the particle board market. Particle board is used in the construction of furniture core, floor underlayers, prefabricated housing, freight cars, and ships. Fungicides and insecticides such as tributyl tin oxide are added to the resin formulation to enhance the resistance of particle board to fungi and insects. Wafer Board. This material is a type of particle board with large wood chip sizes of up to 0.008 m thick, 0.00 15 m wide, and 0.0075 m high. Phenolic resin is the predominant bonding material. Wafer board is used mainly used for exterior structural applications. Fiber Board. This material is a paper-based phenolic resins bonded composite material used for heat and sound insulation. The paper sheets are derived from lignocellulosic fibers and generally are more than 1.5 mm thick. The paper sheets are typically self-bonding, but adding phenolic resin reduces water absorption and swelling, and improves strength. Fiber boards can be classified according to their density. Those with densities of 0.02 to 0.40 g/cm’ are known as noncompressed fiber boards; those with densities in the range of 0.60 to 1.45 g/cm’ are classified as compressed fiber boards. Examples of noncompressed fiber board are semi-rigid insulation board (0.02 to 0.15 g/cm”) and rigid insulation board (0.15 to 0.40 g/cm’). Examples of compressed fiber boards are medium-density fiber board (MDF, 0.60 to 0.80 gIcm3), high-density fiber board or hard board (HDF, 0.90 to 1.20 g/cm3), and special dens&d hard board (1.20 to 1.45 g/cm3). The American Plywood Association has done a lot of research in the area of fiber board manufacturing. Strand Board. This material is also a phenolic resin-based composite board product, that is used extensively in exterior and structural applications.

I’llcnol-l;ormnldeI?),de Resins

49

Phenolic Resins in Adhesive and Bonding Applications

Phenolic resins totalling 9 16 million pounds went into the adhesive and bonding markets in 1993 (Table 2-3), making the adhcsivc and bonding market the second largest outlet for phenolic resins. It is arguable that the adhesive and bonding market includes the composite wood and laminating applications because phenolic resin is the bonding material in these applications. Table 2-5 indicates that phenolic resins in adhesive and bonding applications include: (1) Insulation materials. (2) Fibrous and granulated wood. (3) Foundry and shell molding. (4) Friction materials. (5) Bonded and coated abrasives.

Phenolic Resins as Insulation Materials

The low thermal conductivities of plastics in general and phenolic resins in particular make these resins good heat insulators. The thermal conductivity of phenolic resins ranges from 3-15~10-~ caI/cm.°C (Table 2-l). It also accounts for the strong competition that phenolic resins are experiencing in the insulation market from other plastics such as polyurethanes and polystyrenes. Phenolic resins totaling 366 million pounds were used as insulation materials in 1993, reflecting an 11.6% decrease from the 1989 consumption volume of 4 14 million pounds. The 366 million pounds of phenolic resins consumed in the insulation market in 1993 represent a respectable 12% share of the total phenolic resin market (Table 2-5). Three major forms of phenolic resins-based insulation materials are in use: mineral fibers, phenolic foams, and fiber mats. The mineral fibers and phenolic foams, used for heat insulation, are typically aqueous, low-solids content, and resole-based. The fiber mats (textile) are novolac based (with about 10% HEXA as the curing agent) and used mainly for sound or acoustical insulation. Glass wool (fiber glass) and rock wool are the most common mineral fibers.

The production of mineral fiber insulation involves spraying the hot mineral tibcrs with an aqueous resole solution and heating the resole-soaked fibers at about 200°C to cure the resin. The resin content of mineral fiber insulation is typically less than lo%, with the libcr accounting for 90% or mom of the insulation material composition. The production of phenolic resin foam requires the use of blowing agents and water-soluble surfactants to control the size of foam cells. These phenolic resin insulation materials have characteristic flame resistance, low smoke generation, low thermal conductivity, and high temperature resistance. Phenolic resin foams are good heat insulators up to 120°C, whereas glass wool and rock wool are good insulators up to 260°C and 385 OC, respectively. The added superior moisture resistance qualities of glass wool, in spite of its higher cost, make it the preferred material for low-temperature home and residential insulation where moisture permeation is a constant problem. The higher operating temperature of rock wool makes it the material of choice for industrial-type insulation, such as for pipes, reactor vessels, and boilers. About 80% of phenolic resin insulation is glass wool based, and 10% is rock wool based. Some phenolic resin foams and mineral fibers are used in sound insulation, but the predominant choice material for sound insulation is fiber mat. Phenolic resins fiber mats are used for reduction and control of sound in automobiles, offices, auditoriums, and industrial plants.

PHENOLIC

RESINS IN FRHXION

MATERIALS

‘2”3’

Friction materials refer to brake linings and clutch facings. Grinding wheels, cut-off wheels, emery paper, scrub and scouring pads, and fiber discs, typically classified as bonded and coated abrasives, are also friction materials. The consumption volume of phenolic resins as friction materials during 1993 was 73 million pounds (including 26 million pounds for bonded and coated abrasives) or 2.45% of the total phenolic resins market. Brake linings are used especially in the transportation, automotive, aircraft, railroad, and trucking industries. Other heavy users of brake linings are the drilling (oil, water wells) rigs and construction (cranes, fork lifts) industries. While the Society of Automotive Engineers has standard test methods

I’hcnol-l;ortnnldeh)~de Nesins

51

for brake linings, most brake lining manufacturing operations arc customized lo tit the specific end usage rcquircmcnts. “The family car rcquircs a relatively soll lining for easy brake control, quiet action and good friction. The taxi cab or bus rcquircs long lasting linings. A heavy duty truck requires csccllent heat resistance and long life. “t3] Hence the composition in friction materials varies. Typically, the phcnolic resin used is tither resole or novolac based but is rubber modified for elasticity and toughness. Asbestos had been the choice rcinfarcing tiller (300 phr) but was discontinued because it is a suspected carcinogen. Mica, talc, or fiber glass are the choice fillers for replacing asbestos. Graphite is used as a lubricant (40 phr), and powdered metals (200 phr) are used to improve thermal conductivity. Wear resistant additives, such as cashew nut shell liquid plus formaldehyde cure reaction in pulverized form, are used to the tune of 50 phr.

Bonded and Coated Abrasives

Grinding wheel and emery paper are the major outlets for phenolic resins in bonded and coated abrasives. Leo Baekeland introduced the first phenolic resin-bonded grinding wheel in 1909. Today, phenolic resin-bonded grinding wheels are the most popular type of grinding wheel. They have replaced ceramic wheels to a large extent, mainly due to the enhanced performance of phenolic resins. Phenolic resin bond is more thermal, water, and mechanical shock resistant than other bonding materials such as powdered clay and rubber. Also, these grinding wheels have higher tensile strength and can operate at higher speeds and remove metal more efficiently. Contrary to the popular impression, grinding wheels are used more in the industry than in home workshops. Steel manufacturing and fabricating plants are the biggest users of grinding wheels. A grinding wheel ” 18 inches in diameter and 0.1 inch thick can slice a 1 inch bar of steel in a few seconds leaving a mirrored finish.” [31 A grinding wheel consists of three parts: the grit or abrasive grain, bonding material, and fillers. The two most commonly used grits for these wheels are synthetic fused alumina made from hydrated aluminum oxide (bauxite, AI,O,.H,O) and silicon carbide (SIC) obtained from the high-temperature (2000°C) reaction of silica (sand) and coke in an electric furnace. The grit has a grain size of 20

IAm to 3 mm. Alumina-based wheels, being tougher, arc used for grinding high-stren@h products such as steel. Silicon carbide is harder and is used for grinding hard and brittle materials such as glass, ceramics, stone, and cast iron. The bonding material is alkaline catalyst-cured liquid resole-based phcnolic resin. Sometimes a combination of liquid resole-based and powder form novolac-based phenolic resins is used to enhance performance. Lowsolids, low-viscosity phenohc is preferred for improved shelf life. Toughness and elasticity can be enhanced by using modified phenolic resins. Phenolic resins can be modified for low flow by being co-curing with epoxy resin or polyvinylbutyral (interpenetrating network resins IPNs). Common fillers for grinding wheels are a1un~inun~oxide, iron oxide, silicates, and chalk. (The use of asbestos as a filler has been discontinued.) Typical formulation for grinding wheel is as follows: resole phenolic resin l/m 100 parts per hundred resin (phr), novolac phenolic resin l/m 250 phr, grit l/m 1500 phr, tiller l/m 200 phr, and curing agent/accelerator (calcium or magnesium oxides) l/m 15 phr. Emery paper is a polishing material that has largely replaced animal hide glue-bonded sandpaper. Emery is an impure corundum (natural occurring alumina - Al,O,) mixed with iron oxide that serves as the grit or abrasive material in emery paper. The substrate or backing material is rubber or acrylic-modified paper, and the bonding material is phenolic resin (resole or novollac-based). Sometimes, combinations of phenolic resins and animal glue are used. Emery paper is used primarily for wet-polishing automobile body coatings.

Phenolic Resins in Foundry and Shell Molding Applications

[221131120-221

Foundry and shell molding applications account for about 3% of the total phenolic resins market, with about 80 million pounds consumption volume per year. Phenolic resins and blends are used as the binding material for sand-based lost core molds. Lost core molding and pemranent mold processes are two major ways to cast metals. Permanent molds, usually made of metals, ceramics, or graphite are used in mass production die casting of low melting metal alloys. Lost core molding is used for the precision molding of novelty items, such as jewelry, and specialty products, such as titanium-based

jet turbine blades and parts or products that have metal-free volunm as part The automotive, skcl, construction, and machine parts industries are major users of lost core molding. The engine block, with its cylinder holes and the cylinder head water jacket section, is a good candidate for lost core molding. Lost core molds arc typically made of wax or plaster of paris and sand. The sand-based lost core mold uses phenolic resins and other binder materials such as polyurethane, urea-based resins, oils, starch, clays, and gypsum. The inorganic binders, gypsum (cement), and clays do not meet the dimensional stability, high-strength, and high-speed production rates requirements for metallic castings. This and other post-mold processing requirements, such as finishing, have made phenolic resins and blends the preferred binding materials for sand-based lost core molds. Commonly, phenolic resin blends are phenolic resin/polyurethane and phenol/UF resins. The sand core bonded with inorganic binds tends to produce parts with poor finishing, and post-mold machining is very high. Four major lost-core molding processes are in use: shell molding, cold box, hot box, and no-bake.

oftheir profile.

Shell Molding Process

J. Croning invented the shell molding (Croning) process in Germany in 1944 and introduced it into the U.S. in 1949. The process uses novolactype phenolic resin as a binder. In the process, the resin-coated sand is put into a mold of the desired shape or heated metal pattern at about 270°C. The mold is usually treated with a mold release agent (usually a silicone type) before the resin-coated sand is put into it. The melting and fusion of the resin binds the sand grains and fomls a solid shell. The desired thickness of the shell mold is achieved by controlling the cure rate of the phenolic resin/blend, mold temperature, and residence or cure time. The automobile industry favors the shell molding process for the manufacture of crank shafts, cam shafts, and engine valves.

54


Cold Box Process

The cold box process popularized by “The Ashland Process” uses a two-component thermosetting hybrid system of phenolic resin (IsoCure I) and polyurethane (IsoCure II) as the binder. The IsoCure binder with amine catalyst, such as triethylamine (TEA) or dimethylethylamine (DMEA), produces a flowable sand mix that is easily blown or shot into the core box. The sand/ phenolic resin/polyurethane (sand plus IsoCure) mix cures at room temperature to produce the core or mold, depending on the particular application. The room temperature cure of the cold box process results in energy savings and warpage- and shrinkage-free casting. Also, the cores or molds ejected from the box or pattern, respectively, are almost completely cured and dimensionally accurate, so core and pattern scrap is reduced.

Trends in Foundry and Shell Molding

The laboratory operations of Pittsburg State University’s Kansas Foundty and Manufacturing Technology program epitomize the current trends in the foundry and shell molding industry. The foundry program, one of the 26 Foundry Educational Foundation afliliatcd programs in the U.S., uses a blend of some of the most current technologies and traditional foundry and shell-molding processes in investment casting activities. Professor Russell Rosmait, who teaches the foundry courses, has investment casting activities that use plastics-based (wax) prototypes produced by a CAD-driven Stratasys (fused deposition modeling) equipment as the core or pattern for making molds. Rosemait’s laboratory also has a Palmer mixing machine, which has provisions for metering and mixing the two-component (phenolic resin and polyurethane) binder system, and a WSF Industries, Inc.-certified steam-type autoclave. This Pittsburg State University laboratory uses aluminum, iron, steel, or bronze to produce parts such as the University’s nameplate, bookends, and Stratasys-designed parts (engine parts, piston, clutch parts).


55

Phenolic Resins in Laminating Applications

The 1993 consumption volume of 125 million pounds for phenolic resins in laminating applications (Table 2-G), when compared to the 1989 figure of 246 million pounds or the 1983 figure of 134 million pounds, reflects the gradual but steady loss of the competitive edge of phenolic resins in the plastics laminating market. This loss of market share represents the strong presence of engineering thermosets such as polyurethanes and epoxy resins.

Table 2-6: Consumption

Patterns for Phenolic Resins in Laminating Markets


Phenolic resins in laminating applications are used mostly in the building, furniture, electrical, and electronics industries. Specifically, laminates are used in printed circuit boards, laminated tubes and rods, decorative laminates, filters, separators, and molded parts. A laminate essentially consists of a substrate (support material) and an impregnating material. In phenolic-resin-based laminates, the impregnating material is phenolic resin or phenolic resin blends, and the substrate is typically a cellulose-based fiber such as cotton fiber or paper fiber. Two paper-

substrate types arc common, cspccially in clcctrical laminates: cotton linter paper and KratI paper. Cotton linter paper is easier to impregnate and has bcttcr clcctrical propcrtics and punchability. Kraft paper is less cspensive and has greater mcehanical strength Cotton fiber, used in high-strength laminates, is stronger than paper. The substrate is generally impregnated in two steps (cotton linter paper requires only one step). The first step uses low-molecular-weight phenolic resin (about 300), and the second uses medium molecular weight resins. It is easier for the low-molecular-weight resins to penetrate the pores of the substrate. The resin formulation varies, depending on the end use of the laminate. Laminates for electrical applications need phenolic resins of low inorganic ions content. Hence, only deionized water must be used in such applications. The phenolic resin-impregnated substrate is cut to size, stacked to achieve the right thickness, and cured in a press at about 170”- 190°C and 2000 psi. Typically, paper based electrical laminates are used mainly for making the circuit board in printed circuit boards for radio and other electronic equipment. The laminate circuit board is completely covered with copper for ease of printing the conductor network. Thus, the laminate board serves as a support and insulation in printed circuit boards. These boards are also used for switchboards, switch gear insulating washers, and aircraft pulley wheels. Laminated tubes and rods are made from both paper and cotton fibers and are typically used to insulate machine parts, mainly because of their high dielectric strength. They also have a very high strength-to-weight ratio. Their other applications include winding support for transformers, bobbins, and guide rolls for paper and textile machines. The tube shapes are produced by core molding; the core is removed after molding, and the tube laminate is cut to size via machining. Cotton fiber laminates have high strength, heat resistance, excellent moisture barrier, and very good resistance to lubricants, acids, weak alkalis, and solvents. They are commonly used in construction and electrical insulation applications. Wheel bearings and switch-based plates, bushings, and gears are examples of products made with cotton-fiber laminates. Some glassphenolic resin grades are available, such as Westinghouse Electric Corporation’s Micarta resins used as gaskets and seals, which offer electrical, moisture, and temperature resistance.


57

Decorative or high-pressure laminates, the most popular type of phenolic resin-based laminates, find major application in the furniture industry. “Fomrica,” a trade name for a decorative laminate produced by Formica Corporation, a subsidiary of American Cyanamid Company, is used for chemical scratch- and heat-resistant surfaces. Other forms of decorative laminates made by other companies abound, especially for counter tops and wood veneers. Decorative laminates in general are used for lamination of particle boards and wood materials used in furniture construction. The other major phenolic resins-based lamination products are filters and battery separators. Oil fuel and air filterst21[31t24] for the automotive and building industries arc made f;om tiller-free paper impregnated with phenolic resin. The resin fomrulation determines such properties as strength, fuel, chemical, swelling resistance, and permeability. The formulation should not reduce the porosity or filtering ability of the filter. Typically, the phcnolic resin is the novolac type, and the resin content is about 20% to 30%. In the making of filter paper, the resin impregnation rate and cure temperatures are controlled to achieve partial cure. The preimpregnatcd substrate (paper) is cut to size and folded. The partially cured, folded preimpregnate is completely cured in an oven at about 180°C. Bursting pressure for filters is about 30 to 45 psi. Battery separators arc used mainly for the separation and electrical insulation of the electrodes in automobile batteries. Separators are either paper or sintered poly vinyl chloride (PVC) or polyethylene fiber sheets based. Impregnation is via low-molecular-weight, alkaline-catalyst-cured (resole) phenolic resin. The impregnated substrate is corrugated to form ribs and then cut to a size of about 0.00 18 to 0.006 m. Typical characteristics of separators are porosity (to allow current flow) and high oxidation resistance.

Phenolic Resins in Molding Applications

About 160 to 165 million pounds of phenolic resins arc used annually in molded parts and products, accounting for about 5.5% of the total phenolic resins market (Table 2-7). Though they represent a relatively small part of the phenolic resins market, phenolic molding compounds arc used in many applications, such as in the appliances, closures, electrical, housewares, and transp-

58

Handbook ojThertnoset I’lostics

ortation industries. This versatility is due to ease of molding into intricate parts and products and inherent dimensional stability and accuracy.

Table 2-7: Consumption Patterns for Phenolic Resins in Molding

Transportation

19

17

15

15

17

Other

5

5

4

4

*

Total

176

168

160

167

163


The appliances, closures, and housewares industries prefer the use of resole-based phenolic molding compounds in the making of household-type products because (unlike novolacs which give off ammonia) resoles give off water upon curing and are odor Gee. Some of the numerous resole-based applications are bottle caps, knobs, utensil handles, stove tops, toaster components, refrigerator switch boxes, hermetically sealed switches, steam irons, and sterilizablehospital equipment. The absence of ammonia prevents the problem of corrosion from nitrous acids that could be produced by the presence of an electric arc in switches.


59

The two-stage novolac-based phenolic molding compounds are more dimensionally stable than resoles and are used for high strength and nonhousehold applications. Owing to their shock resistance, impact grades of phenolic molding compounds are used for automotive and industrial pulleys, electrical switch gears and switch blocks, fuse holders, and motor housings and frames. Electrical grades of phenolic molding compounds, because of their high dielectric strength, are used for automotive ignition parts (distributor caps, coil tops, and rotors), wiring device parts, circuit breakers, commutators (unidirectional current device connected to the electric motor or generator), brush holders, and electronic connectors. Heat resistant grades are used for appliance thermal barriers such as stove tops, thermostat bases, automotive ashtrays, switch cases, and terminal blocks. Other under-the-hood automotive applications of phenolic molding compounds are resistant to automotive fluids, gasoline, coolants, lubricants, and salt solutions. These include disc brake caliper pistons, thermostat housings, water pumps, water pump impellers, throttle bodies, pulleys, rocker arm covers (engine block covers), and solenoids. The disc brake caliper piston needs to have a low coefficient of thermal expansion to help maintain the relative tit between the piston outside diameter and the caliper inside diameter over a wider temperature range (Figures 2-4 to 2-11). Current trends in phenolic resins consumption include the use of phenolic resins in rubber compounding and tackifying applications, and in structural composites. Occidental Chemical Corporation and Durez Division, among others, produces four types of rubber modifying phenolic resins: (1) Thermoplastic phenolic tackifters and processing aids (2) Phenolic reinforcing resins (3) Phenolic reinforcing and tackifying resins (4) Phenolic resins for bonding rubber compounds to fabric Some of Durez’s products include tires, drive belts, shoe and boot soles, rubber hoses, and rubber gaskets. American Cyanamid Company produces modified, toughened phenolic resins for glass fiber, aramid fiber (Kevlar, Nomex) and graphite composites suitable for aircraft interior and ablative applications. Moderate tack and drape, exellent flame resistance, low smoke generation, non toxicity, and high temperature resistance are typical characteristics of these resins. American

60

Handbook of ‘I’hertnosetPlastics

Cyanamid’s modified phenolic resin suitable for nylon and aramid composite helmets has high toughness and ballistic impact resistance. The nose cone on the large cxtemal fuel tank that supplies fuel (liquid hydrogen and oxygen) to the space shuttle main engine is made of phenolic resin/carbon fiber composite. The nose cone protects the system from thermal failure because it can withstand temperatures up to 1000°F (537.8”C) and loads up to 50 g mass during its 8.5 minutes of critical operation. (The nose cone, previously made of metal with a protective foam coating, is produced for NASA by Martin Marietta Manned Space Systems, New Orleans LA.) The phenolic resin/carbon Iiber composite (prepregnate) used in the nose cone was supplied by Cylec Industries, West Peterson, N.J.“31

Phenolic Resins in Coating Applications

1211411251

The very good properties and characteristics that make phenolic resins good adhesives and molding compound as also make them a very good protective, environmental, high temperature, and anti-corrosion coating for a variety of materials, such as aluminum, bronze, iron, and magnesium. Phenolic coating resins have good wetting and adhesive properties, and very good chemical and abrasion resistance. The baking step in coating production involves a crosslinking process. Crosslinking makes the coating insoluble, strong, and resistant to exposure to chemicals, solvents (except alkalies), and hot water. It also makes phenolic coating resins tasteless and odorless. Phenolic coating resins are good electrical insulators. Dielectric strcngth for phcnolic coating resins is about 500 V/mm; dissipation factor and water absorption are very low. Phenolic coating resins have good thermal resistance with a continuous-use temperature of 145°C and can withstand high temperatures up to 350 “C for short periods. Phenolic coating resins exhibit flexibility and compatibility with other resins, such as polyurethanes, epoxies, alhyds, and polyvinyl butyryl, and can be easily modified to suit various applications. Also, phenolic resins arc sterilizable and can be used for food applications where sterilization is a Food and Drug Administration requirement.

I’henol-l’ornlaldeIl)?),deResins

61

Major coating applications arc as protective coatings, undercoats, and primers for automotives; metal containers and pipes; and industrial equipment. Examples of specific applications of phenolic resins, such as coatings, are in heat exchangers, pipelines, boiler pipes, reaction vessels, storage tanks, brine tanks, solvent containers, food containers, railroad cars, beer and wine tanks, beer cans, pail and drum linings, water cans, rotors, blower fans and ducts in heating and air conditioning systems, boats, ship, wood finishes, and paper. Because of their versatility, phenolic coating resins, can be applied by most available coating technologies, such as dip and spray (pneumatic and electrostatic) coating in solutions, high solids, and powder forms. Georgia Pacific Resins, Inc. and other plastics companies offer a variety of grades of coating resins. A particular coating application can have more than one resin type, for example, a rail car could have an epoxy primer, a modified phenolic undercoat, and a polyurethane finish.

62

Handbook of Themoser Plastics

Occidental Chemical Corporation, Durez Division, phenolic resins in under-the hood, automotive applications. (Courtesy of Occidental Chemical Corporation.) Figure 2-4:

Phenol-Fortnnldehyde Resins

63

Figure 2-5: Thermostat housing of an automobile engine made from phenolic resins. (Courtesy of Occidental Chemical Corporation.)

64

Hmdbook of Thermoset Plastics

,. “a,

Figure 2-6: Throttle body of an automobile engine made of phenolic resins. (Courtesy of Occidental Chemical Corporation.)


65

Figure 2-7: Water pump of an automobile engine made from phenolic resins.

(Courtesy of Occidental Chemical Corporation.)

6ti

Handbook of Thermoset Plmtics

Figure 2-8: Water pump impeller made from phenolic resins. (Courtesy of

Occidental Chemical Corporation.)

Phenol-Formaldehyde

Resins

67

Figure 2-9: Rockerarm cover of an automobile engine made from phenolic resins. (Courtesy of Occidental Chemical Corporation.)

68


Figure 2-10: A one-step phenolic molding compound from Rogers Corp. was

selected for lead insulation in these audio cable connectors for SwitchcrafP. The compound resists creep and has excellent electrical properties. (Courtesy of the Rogers Corporation.)


69

Figure 2-l 1: Variety of phenolic resins-based (paper, linen, canvas) laminate chips compared to light colored epoxy-glass laminate chips. The comparable mechanical and electrical insulation properties, but lower costs of the phenolic laminates, make them choice materials. (courtesy of Accurate Plastics, Inc.)

70


PHENOLIC RESINS TRADE NAMES AND MANUFACTURERS

Arochem ................................................. Bakelite .................................................. Beckacite ................................................ Catacol ................................................... Durez ...................................................... Genal ...................................................... Micarta ................................................... Plenco ..................................................... Plyophen ................................................ Resinox .................................................. UCAR ....................................................

Ashland Chemical Co. Georgia Pacific Reichhold Chemical, Inc. Ashland Chemical Co. Durez Plastics Division General Electric Co. Westinghouse Electric Co. Plastics Engineering Co. Reichhold Chemicals, Inc. Mosanto Co. Georgia Pacific

REFERENCES

1. 2. 3.

4. 5. 6. 7.

8. 9. 10.

Bayer, A., Bev. 5:25 (1872) Knopf, A., Sheib, W., Chemistry and Applications of Phenolic Resins, p. 2, Springer-Verlag, New York (1979) Wooten, A.L., Phenolic Resins, Forest Products Utilization Laboratory, Mississippi State University, Handbook of Thermoset Plastics, First Edition, Noyes Publication, 1986, p. 40 Baekeland, L.H., U.S. Patent 942,699; July 13, 1907 Baekeland, L.H., Journal of Ind. and Eng. Chem., 1: 149- 16 1 (1909) Baekeland, L.H., Journal of Ind. and Eng. Chem., 6:90 (19 14) Abstracts of Papers, 186th ACS National Meeting, Washington, D.C., August 28 to September 2,1983, American Chemical Society, 1155 16th Street, N. W., Washington, D.C. 20036 Rodriguez, F., Principles of Polymer Systems, Hemisphere Publishing Corporation, McGraw Hill Company, New York, Second Edition Miles, D.C., B&on, J.H., Polymer Technology, Chemical Publishing, New York, 196 1, p. 44 Keegan, J.F., Introduction to Phenolics, Durez Plastics Division, Hooker Chemical Corporation, North Tonawanda, New York

Phenol-Formaldehyde 11. 12. 13. 14. 1s. 16. 17.

18. 19. 20.

21. 22. 23. 24. 25. 26. 27.

Resins

71

Carwell, T.S., Phenoplasts,Their Structure, Properties and Chemical Tech. nology, Interscience, New York, N.Y., 1947, p. 10 Modem Plastics Encyclopedia, McGraw Hill Publications Cbmda, M., Roy, S., Plastics Technology Handbook, Marcel Dekker, Inc., New York, N.Y., 10016, 1987 Edition Richardson, T.L., Industrial Plastics: Theory and Applications, Delmar Publishers, Inc., Second Edition, 1989 Schwartz, S.S.,Goodman, S.H., Plastics Materials and Processes, Van Nostrand Reinhold Company, New York, p. 308 Harrington, H.J., Phenolic, Modem Plastics Encyclopedia, 1986-1987, p 30-32 Knipple, R.P., Durez Engineering Materials -Molding Methods, Durez Division, Occidental Chemical Corporation, North Tomwanda, N.Y., 14120-0535 Modem Plastics, January Issues (I 984.1994) Brautigam, D.P., A New Automated Cold Shell Molding Process, Westran Corporation, Muskegon, Michigan (AFS Transactions 81.62) Langer, H..., Dunna Vant, W.R., New Cold Box Process Is Developed, Research and Development Division, Ashland Chemical Corporation, Dublin, Ohio (Foundry Management and Technology, Februy, 1982) Monroe, R.W., The Use of Iron Oxides in No-Bake Bonded Sand Molds, Research Division, Steel Founders Society of America Foundty Educational Foundation Directory, 1993-l 994,484 E. Northwest Highway, Des Plains, IL 60016-2202 “Composite Nose Cone,” High Performance Composites, .Ianualy/ February, 1994, page 5. Watts, G.F., Vozella, A.F., “Phenolic Resin Technology,” Applications in the Manufacturing ofFiller Paper, TAPPI, Vol. 61, No. 3, March, 1978 “Pmtective Coatings Resins,” Georgia-Pacific Resins, Inc., Georgia Pacific CoToration, Decatur, Georgia 30035 Plastics News, March 3 1, 1997 Issue Ibeh, C.C., Graham, G., “Simulation Techniques for In-Lab Cost-Effective Optimization of Thermosetting Polymeric Materials Processes,” American Society for engineering Education, Midwest Section, 32nd Annual Conference proceedings, April 2-4, 1997

Amino and Furan Resins Christopher C. Ibeh

INTRODUCTION

The amino and finan resins are grouped together primarily because they are versions of phenolic resins that complement and supplement phenolic resins. They are also relatively low-volume consumption resins. The lightcolored aminos were originally introduced to expand the market share of phenolic-type products. Phenolic resins occur only in dark, opaque colors and can therefore be used only to manufacture dark-colored products. Amino resins are thermosetting plastic materials that are produced by the reaction between amino group (NH&bearing compounds (such as aniline, guanamines, urea, melamines, thiourea, ethylene urea, and sulfonamide) and formaldehyde. The two most popular and viable aminos, urea-formaldehyde and melamine-formaldehyde resins, are the ammos of interest in this chapter. Furan resins, on the other hand, are produced by the reaction between phenols and fumns [such as fLrf%ral(aldehyde), and finfural derivatives]. They are used in place of formaldehyde in the conventional production of phenolic resins. Work with urea and urea-formaldehyde resins may have begun as early as the 1880s in France and Germany by Einhorn, Holzer, and Goldschmidt et al. H. John and F. Pollack introduced them commercially into the U.S. 72

Amino and Furan Resins

73

in 1928.[‘-‘1 Companies manufacturing urea-formaldehyde resins include American Cyanamid, Reichhold Chemicals, and Allied Corp. The melamine-formaldehyde resins were brought into the market in 1935. In addition to their light-coloredness, they exhibit enhanced water and heat resistance. These resins have completely replaced thiourea-formaldehyde resins, which were also produced in the 1930s. American Cyanamid Company is the major producer of melamine-formaldehyde resins having begun the first commercial manufacture in the U.S. in 1939. Other companies producing these resins are Fiberite Corp., Ciba-Geigy Corp., and Allied Corp. Furan resins are relatively recent inventions. The Rufurals and furfmal derivatives on which they are based were first produced by Quaker Oats Company after World War I. Other companies producing furan resins include Furan Division of Ashland Chemical and M&T Chemical. Ashland Chemical also produces urea and melamine-formaldehyde resins.

RAW MATERIALS

Urea (NH,CONH,) and formaldehyde (CH,O) are the raw materials of urea-formaldehyde resins. Melamine (C,N,(NH,),) and formaldehyde are the raw materials for melamine-formaldehyde resins. For fixan resins, the raw materials are fixf%ral and its derivatives and phenol.

Urea

Urea, a white crystalline solid, is a diamide of carbonic acid. Wohler’s work in 1824 in urea synthesis marks a cornerstone in the connection between the chemistry of living and inanimate matter. Wohler’s process for synthesizing urea involved the molecular rearrangement of ammonium cyanate.c61 Urea is synthesized from the high pressure reaction between carbon dioxide and ammonia:

74


2NH,

+

CO,

Ammonia + carbon dioxide

NH,2COHN, + H,O 150-200°C _) 1400-1500 psi +

Urea

+ Water

Melamine

Melamine is the triamide of cyanuric acid (2,4,6-triamino-1,3,5-triazinc). In the Ciba Geigy process patented in 1936, calcium cyanamide is converted to cyanamide, and the cyanamide is reacted with ammonia under pressure to yield melaminers] (Figure 3-l).

Amino and Fur-anResins

75

Furans

The term furans refers to &rt%ral(aldehydes) and furfnral derivatives such as fM%ral alcohol, furan, tetrahydroIman, and tetrahydrofurfural alcoh01[‘1[*1 (Figure 3-2). Furmtal is a thermally stable, amber-colored aldehyde that occurs in liquid form. It is synthesized by the acid hydrolysis and steam distillation of agricultural byproducts such as corn cobs, rice hulls, oat hulls, and sugar cane bagase. Furtural alcohol (FA) is a pale yellow, water soluble liquid produced by the hydrogenation of fnrfnral. Furan is a colorless, cyclic, dienic ether produced from the decarbonylation of I%rI$ralusing a noble metal catalyst such as palladium on charcoal. Tetrahydrotumn (THF) is a colorless, cyclic ether produced from the hydrogenation of I&m. Tetrahydrofbrfixyl alcohol (THFA) is a colorless, water-miscible, biodegradable primary alcohol produced commercially by the vapor-phase hydrogenation of tnrhuyl alcohol using a nickel or copper catalyst. FA, THF, and THFA are registered trademarks of QO Chemicals, Inc., a subsidiary of Great Lakes Chemical Corporation, West Lafayette, IN.

Furfural

Furfuryl Alcoho 1

Furan

Figure 3-2: Structure of Furans

TetrahydroFurfuryl Alcoho 1

76


AMINO RESINS

Chemistry and Resinification

The resinification process for ureaformaldehyde occurs in two main stages, addition or methylolation and condensation. In the methylolation step, urea and formaldehyde are reacted under controlled conditions using an alkaline catalyst. Typically, the methylolation step is carried out at a pH of about 8.0. The methylolation stage usually results ts in a mixture of metbylolated species, monomethylol urea, and dimethylol urea and trimethylol urea. Urea-Formaldehyde

NH,CONH,

+

urea

+

Resins.

CH,O

pH=8 +

monomethylol urea

formaldehyde

NH,CONHCH,OH + CH,O

NH,CONHCH,OH

pH=8 _)

HOCHmCONHCH,OH dimethylol urea

HOCH@lXONHCH,OH + CH,OH pH=8.0 +

(HOCH,),NCONHCH,OH trimethylol urea

The urea methylolation reaction is controlled such that 1 mole of urea combines with 2 moles of formaldehyde to yield dimethylol urea predominantly. t’-‘] The second phase of the resinification process involves the condensation of the methylolated species in the presence of an acid catalyst, which when carried to completion, results in a fully cured, infusible resin with methylene linkages. The condensation reaction proceeds to a predetermined


77

end point, and the resin intermediate is cooled. The resin intermediate is stabilized by adjusting the pH to about 7.0 to 8.0. The condensation of the methylolated species is equivalent to 2 moles of urea reacting with 1 mole of formaldehyde to form urea-formaldehyde resin. +

2NH,CONH,

+ CH,O

urea

+ formaldehyde

NH,CONH-CH,-NHCONH, + H,O urea-formaldehyde

+ water

Melamine-Formaldehyde Resins. The reaction of melamine and formaldehyde occurs more readily and completely than that of urea and formaldehyde. Up to 6 moles of formaldehyde can be combined with 1 mole of melamine to produce hexamethylol melamine. The trimethylol melamine is most common, however. The melamine-formaldehyde resins are more water and heat resistant than the urea-formaldehyde resins (Figures 3-3a and 3-3b). The final form of the amino resin produced depends on such factors as reaction temperature, pH control, reactant ratio, and degree of polymerization. These factors are varied to achieve the forms suitable for various enduses such as: (1) Adhesives and bonding resins. (2) Crosslinking agents for coating resins. (3) Laminating resins. (4) Molding resins.

r-4’\‘N II

I

+ 3cH20

rw2Af \NH2 Melamine

-->

N’94

” d,MiCH2oH

MiCH20H~C,N~

Trimethylol-Melamine

Figure 3-3a: Structure of Trimethylol Melamine

78

Handbook of E’zermoset Plastics

cH2oH

cH2oH

‘N’ I

N-l2

I

c

,4-I I

II

+-

N’+N

-’

II

-\

clt2oH/ Melamine

1

/cH2oH

N/C\//CNN N

VliH2OH

Hexamethylol+elamine

Figure 3-3b: Structure of Hexamethylol Melamine

Adhesive and Bonding Resins

Urea-formaldehyde and melamine-formaldehyde resins, usually in the liquid or spray-dried forms, are used as adhesives. Though the melamine-formaldehyde resins are more water and heat resistant and give more durable adhesives and bonding resins than the urea-formaldehyde resins, their higher costs limit their use. Their durability and water-resistant characteristics promote the use of melamine-formaldehyde resins in outdoor and marine applications. The light colored amino resins are attractive for decorative plywood veneers without encountering the associated problem of discoloration caused by resin bleedthrough. Typically, the adhesive and bonding resins have urea:formaldehyde ratios of 1:1.5 to 2.0 and a melamine:formaldehyde ratio of 1:3.0. The reactions are carried out at a pH of 7.5 to 8.0 and at reflux for up to 8 hours until 50 to 60% solid composition is attained. The pH is lowered as viscosity is increased; the reaction is then stopped and the resin is stabilized using caustic


79

soda by raising the pH to 8.0. The typical formulation of adhesive resins is about 15% resin, woodtlour, pecan, and walnut shells are the common fillers. Acid catalysts are favored in adhesive and bonding amino resins. The processing or pressing conditions for amino resins are typically 70’ and 200 psi (cold pressing) for up to 24 hours. Melamine-formaldehyde resins can be cured or pressed without a catalyst but only at a higher processing temperature (hot pressing). Amino resin adhesives are typically applied in the bonding of wood. The bonding strength of aminos is most effective with heat and pressure for wood particles of the 40 to 80 mesh range. Hot pressing causes the amino resins to seep through the pores of the wood core and polymerize (crosslink) inside the wood. This crosslinking binds the wood together, resulting in a structure that is stronger and more moisture resistant than the original wood. Some amino resin-based adhesives are made from blends of urea and melamine resins. The American Cyanamid Company’s Melurac resin is a co-spray dried melamine-urea-formaldehyde adhesive in a free-flowing powdered form designed for exterior waterproofing applications.

Coating Resins

Amino resins serve as crosslinking agents for hydroxyl, carboxyl, and amide functional polymers such as acrylics, polyesters, epoxies, and alkyds. Liquid amino coating resins are produced by reacting the initial methylolated species, dimethylol urea and hexamethylol-melamine, with either n-butanol or methanol. This step results in an amino resin that is more soluble in and compatible with the coating resins. The increased compatibility enhances the ether exchange reaction between the amino resins and the reactive sites of the coating polymers to produce coating films with a very high degree of crosslinking. Urea-formaldehyde coating resins cure more rapidly but have lower moisture resistance than melamine-formaldehyde coating resins. In general, melamine-based coating resins have better overall performance, but, again, their higher costs limit their use. It is common to use a combination of urea/ melamine-based resins to achieve the right balance of properties, costs, and performance.

80


Beetle, Cymel, and Melmac are trade names of American Cyanamid Company’s liquid coating resins.t’“l The Beetle grades are butylated and iso-butylated urea-formaldehyde resins with low-temperature cure characteristics, very good substrateiintercoat adhesiveness, and low cost. These urea-formaldehyde coating resins are compatible with hydroxyå polymers such as amine-catalyzed epoxy resins, oil-alkyd resins, epoxy-ester resins, cellulosics, and conversion varnishes. The Cymel grades are either methylated or butylated melamine-formaldehyde resins with UV-resistance, chemical resistance, exterior durability, fast-cure characteristics, and very good adhesiveness. These melamine-formaldehyde coating resins exhibit compatibility for a wide range of resin types with thio, hydroxyl, carboxyl, and amide functional groups such as alkyd and polyester resins, epoxy resins, acrylics,vinyl polymers, and cellulosics. (Compatible cellulosics include ethyl cellulose, hydroxyethyl cellulose, nitrocellulose, and carboxylated cell&se derivatives.) They are also good wetting and dispersing agents for carbon black and organic pigments. Some Cymel resins require the presence of strong acid catalysts for effectiveness and a high degree of crosslinking. P-toluene sulfonic acid is the most popular catalyst used with Cymel resins. The other catalysts are dodecylbenzene sulfonic acid, oxalic acid, maleic acid, hexamic acid, and metal salts. Metal salts like magnesium bromide (MgBr,), aluminum nitrate (Al(NO,),), and zinc nitrate (Zn(NO,),) are used to achieve hardness and solvent resistance, but they cause discoloration and low gloss.

Laminating Resins

Amino laminating resins are predominantly melamine-formaldehyde resins based. Typically, 1 mole of melamine reacts with 2 moles of formaldehyde at a pH of 8 to 10 to achieve a 50 to 65% solids resin. Catalysts and plasticizers are usually added to enhance cure and flexibility. Melamine-formaldehyde laminating resins have characteristic hardness, clarity, stain resistance, and UV-resistance. Spray drying is sometimes used to achieve long shelf life. The methylolated melamine-formaldehyde resins form stable cationic colloids in the presence of such acids as carboxylics. The colloidal melamine


81

resins impregnate and form strong ionic bonds with cellulose fibers (paper) in water dispersions, [‘Iwith a consequent increase in wet tensile strength. The degree of impregnation is enhanced by using water dispersions containing 0.5 to 1.Opercent of alcohol as a surfactant. The alcohol surfactant reduces the surface tension of the resin solution and increases fiber wettability. Typical colloid composition is of the melamine resin-acid-water ratio of 1: 1:6.5 by weight. Saturation of the fiber material with the resin typically involves the free-turning-roll-pulling of fiber material through a resin solution bath. The resin-saturated web is then drawn to the dryer. The resin concentration, pulling speed, and residence time of the fiber in the bath influence the rate of impregnation. The dryer and drying process are an integral part of the laminating process. The drying process helps to evaporate the resin solvent and enhance the degree of resin polymerization.

Amino Molding Resins

Granule and powder forms of urea-formaldehyde and melamine-formaldehyde resins are used in molding resins. Their characteristic clarity promotes their use in a variety of colored products. Amino molding compounds are commonly formulated with fillers for strength and dimensional stability. Chemically purified alpha cellulose fibers are the most popular fillers for amino molding resins. The other tillers are talc, mica, glass fibers, chopped cotton flock, and wood flour. Common mole ratios of urea or melamine to formaldehyde in amino resins are 2:3 and 3:4. The resinification process is carried beyond the point of water solubility, and then the resin-tiller mixture is heated at controlled humidity conditions. The resin-to-filler ratio, filler type, catalyst type, and degree of polymerization are varied to achieve different molding properties. Compression transfer methods for processing and injection (screw and cold manifold) molding are the major amino molding resins. Molding temperatures are 260°-340’F for urea-formaldehyde resins and 260°-360’F for melamine-formaldehyde resins. Compression molding pressures of 2000 to 8000 psi are common for amino resins. Processing is enhanced by the presence of an acid catalyst such as phthalic anhydride and an inhibitor such as

82


hexamethylenetetramine (“HEXA”). Small amounts of the inhibitor help to stabilize the molding resin during storage and prior to molding, and to control the cure rate during molding.

FURAN RESINS

Chemistry and Resinification of Furan Resins Hw1[81

Furfuryl alcohol-based resins are the most important industrial furan resins in terms of usage and volume.[‘I The final cross-linked products exhibit outstanding properties and characteristics. Furfural replaces formaldehyde in the conventional production of phenolic resins. It reacts easily with phenol in the presence of an alkaline catalyst to form a novolac phenol-furfural resin. (Novolac phenolic resin requires an acid catalyst.) Furfuryl alcohol readily resinilies or homopolymerizes in the presence of an acid catalyst [such as mineral acids, organic acids, Lewis acids (boron halides, e.g., BF,), and acyl halides] to produce liquid linear chains (oligomers). These chains consist primarily of dimers and trimers that have methylene linkages between the furan rings. The process essentially is a methylolation involving the condensation of the methyl01group of one furfury alcohol molecule with another molecule at the fifth position (Figure 3-4). The furfury alcohol resinification process is highly exothermic; the necessary temperature control is accomplished by cooling via either reflux or an external cooling fluid. The process is carried to a predetermined viscosity end point, and the reaction is stopped by adjusting the pH to between 5 and 8. The resulting liquid resin has a shelf life of more than 6 months. Furfuiyl alcohol also undergoes copolymerization with aldehydes such as formaldehyde and furfural, and with phenols and urea in the presence of an aldehyde.


83

Figure 3-4: Resinification Reaction of Furfuryl Alcohol These furfnryl alcohol resins cross link (cure) in the presence of a strong acid catalyst via condensation. The terminal methyl01 group of one linear chain (Figure 3-4) joins with the methylene bridge of another chain to form a three-dimensional network structure (Figure 3-5).

Figure 3-5: Crosslinking of Fmliuyl Alcohol to Form 3-D Network Structure

PROPERTIES OF AMINO AND FURAN RESINS The amino and fiuan resins, previously mentioned, were originally introduced to complement the phenolic resins and, as such, have comparable, but sometimes better, properties than the phenolic resins Table 3- 1. The characteristic light colors of these resins imply that they can be used in various colored products. Ammo resins are generally stronger than phenolic resins. Cellulosefilled ammo resins have a tensile strength of about 5000 to 13,000 psi compared to 5000 to 9000 psi for cellulose-filled phenolic resins. Melamine-formaldehyde resins have higher water and heat resistances than either phenolic resins or urea-formaldehyde resins.

84


Figure 3-6: Light colored amino resins-based coasters compared to dark colored phenolic resins-based coasters. The coasters were compression molded by students of Professor Robert Susnik’splastic processing laboratory class at Pit&burg State University, Pittsburg, Kansas. The coasters were tested for arc resistance using a Beckman arc tester housed in the plastics testing laboratory. Melamine-formaldehyde resin has higher arc resistance than phenol-formaldehyde and urea-formaldehyde resins.


85

Figure 3-7: Fabricated items made from particleboard. Particleboard has superior structural strength and is somewhat less expensive than mediumdensity fiberboard. (Courtesy of Weyerhaeuser)

86


Applications

of Amino and Furan Resins p1[21[511’-111

Trends in the consumption of amino resins are presented in Tables 3-2 through 3-5. The five major areas of amino resins application are adhesive and bonding, coatings, molded parts, plywood, and laminates. Adhesive and bonding is the largest market for amino and furan resins. Another major use of furan resins is as binders in core moldings and friction materials.

Adhesive and Bonding

Amino resins totaling 1.44 billion pounds were consumed in 1993 through adhesive and bonding applications (Table 3-2), mainly fibrous and granulated wood products (composite wood materials other than plywood). The major amino and fnran resins used to bond wood products are urea-formaldehyde resin, melamine-formaldehyde resin, melamine-urea copolymer resins, and turf&y1 alcohol-modified urea-formaldehyde resins. Composite wood materials or composition boards, such as fiberboard, particleboard, waferboard, and oriented strandboard (OSB), account for more than 70% of the amino and furan resins adhesive and bonding market. Other uses include boat hulls, flush doors, fwniture, bag seam pastes, glass and mineral fiber mats, foundry sand cores (lost cores and molds), coated abrasive paper (emery), orthopedic casts and bandages, urea-formaldehyde foams, furan-polymer concrete, and general assembly bonding. American Cyanamid’s Melurac-400 resin achieves high-frequency bonding of truck and railroad flooring, laminated timber bonds, and millwork. It is also used for water-proof bonding of exterior doors and curved plywood.


87

Table 3-l : Properties of Amino (Urea, Melamine, Furan) Molding Compounds Melamine formaldehyde

Urea ” .m

.-

_

ti %

ASTM test method

Alpha cellulo¶efilled

CellUlO6efilled

Glass fiberreinlorced

Thermoset

Thermosel

rhrrrrosrl

Processtog lemperalure range. ‘F IC = compressaon: T = Iranrler: E = ex,rus8on,

C. 275.350 I 290-320 1 270-300

C 260-370 I ZOO-340 T. 300

c 260.350

I =m,ec,~on:

2-6

Properties 1

Melhog lemperal”re.

‘C. 1, (cryslalllnr)

g ._

1, (amorphous) 2.

2: : 0

3

Molding pressure range. IO’ p s I.

2.20

6-20

&

4

Compressson ,ilteo

2 2-3 0

2.1-3

5

Mold (knear) shrmkage, r./ln.

0955

0.006-0.014

0.005-0.015

0001-0006

6

Tensile strength at break. p s.i.

Rockwelt

0765

t.4110.120

t.4115.125

Ml I5

Shore/Barcol

02240/ 02563 0696

22-36

40.45

15-26

0646

260-290

350-390

-___ 37,.400

I

5. IO

,%-I”. Iheck specmw,“, I5

16

Hard”.%,

Coel 01 knear thermal exyanrton. 10-e I” A” /‘C -

z

E

I?

Oalleclton ,emprral”,a under tla.uraI lo.,*. “F

264 p I I

-___

G

66pst

0646

f. I6

lhe,m.al conduclwly. WC -cm z-“C

10.‘can -cm /

c,,r--

7 10

6.5-10

__10.11 5

88


Table 3-l : Properties of Amino (Urea, Melamine, Furan) Molding Compounds (Continued)

Reprinted by permission of Modem Plastics Encyclopedia, McGraw-Hill, Inc.


89


90

Table 3-3: Trends in Amino (Urea-Formaldehyde and MelamineFormaldehyde) Coating Resins Consumption (106 Ib/yr)1511”1

Amino Coating Resins

I

Year

Protective 35

84 90

~Paper Treatment

Textile Treatment

33

I 31

87

I24

c

103

I 1987

111 111 100 104

57

I 1991

92

55

I34 I24

1992

138

50

19

t- 1993

147

__

I __

Medium density fiberboard (MDF) and particleboard are the largest application areas for the adhesive and bonding market. (For more detailed information concerning the composition of MDF, particleboard, and other composition boards, refer to the preceding section on Adhesive and Bonding Resins and the Chapter 2, Phenol-Formaldehyde Resins.) Particleboard is wood bas-


91

ed, whereas fiberboard is paper based, but both find major use in interior applications, mainly due to the low moisture resistance of urea-formaldehyde resin. Outdoor application types are generally based on melamine-type amino resins. The waferboard and oriented strandboard markets are dominated by phenolic resin-type adhesives, designed mainly for exterior (outdoor) and structural applications. Polymer concrete typically has a composition of about 10% resin and 90% aggregate. The aggregate is made up of 50% pea gravel, 35% fine sand, and 15% fly ash. Due to its chemical resistance, it is mainly used for manholes, road repairs, seamless flooring, and corrosion-resistant bricks.

Table 3-4: Trends in Urea-Formaldehyde

Molding Resins Consumption

1989

49.3

3.3

1.6

54.2

1990

50.7

3.8

0.8

55.3

1991

49.6

3.4

0.8

53.8

1992

52

3.4

1.2

56.6

1993

54

3

1.5

58.5

92


Table 3-5: Trends in Melamine Molding Resins Consumption

Furfkyl alcohol resin binder is used primarily for foundry sand cores and molds. Coatings. Amino resin coatings have three principal applications: protective coatings, paper treatment, and textile treatment (Table 3-3). The protective coating applications predominate, with about 47 million pounds of amino resins used in 1993. Protective coatings. Alkylated (butanolated and methylolated) amino resins are used mainly as crosslinking agents for protective coating resins such as acrylics, alkyds, epoxies, and polyesters. Butanolated amino


93

resins dominate the coating market, but methylolated melamine-formaldehyde resins are preferred for moisture-resistant, chemical-resistant, high-solids, outdoor coating systems in automotive topcoats, beverage cans, appliances, metal decorating, and prefabricated metals. Urea-formaldehyde resin-based coatings are typically used for the indoor coating of metals and wood. Ureaformaldehyde resins are also used, for instance, in carborundum-based abrasive, and fiber glass insulation coatings. Floor finishes based on urea-formaldehyde/epoxy copolymers are also common. Textile treatment and coating. Wool, cotton, and other cellulosic textiles resist creasing by being impregnated with, typically, low molecular weight, highly methylolated resins (especially dimethylolated resins). About a 10 to 15% resin solid solution, with aluminum acetate or formic acid added, is used to impregnate alkaline fabric. The impregnated fabric is squeezed and pressed to twice its dry weight, and then dried and cured at about 140- 160 oC for 2 minutes. Textile treatment and coating enhance strength, minimize shrinkage, and impart chlorine resistance, abrasion and wear resistance, mildew proofing, and wash and wear (permanent press) characteristics. Paper treatment and coating. Enhancing strength is the major reason for impregnating paper with resin. Sulfate pulp paper, Krafi paper, and unbleached sulfite cellulose paper are impregnated with amino resins typically at a pH of about 4 to 5. Urea-formaldehyde is the most commonly used amino resin, but melamine-based resins are used more for unbleached sulfite cellulose paper. Resin-impregnated Kraft paper is used for making bags, printing paper, and towel paper. Other paper products include shrinkage-free sheets from urea-formaldehyde-impregnated cellulose pulp and water-proof corrugated cardboard. Laminating. Plywood, which by definition of being a sandwich-type construction is a laminate but is never categorized as such, commands more usage of amino resins than all other laminates combined. Sixty-one million pounds of amino resins were used in plywood applications in 1993 compared to 38 million pounds for other laminates. These amino mainly urea-formaldehyde based resins, are typically used for interior applications. The use of amino resins in plywood is relatively small compared to phenolic resins. The resins, with 1.55 billion pounds of phenolic resins used in plywood in 1993, dominated the market. Phenolic resins-based plywood is used for exterior applications, hence its domination of the plywood market.

94

Handbook of l%ermoset Plastics

Melamine resins, which dominate the amino resins laminates market are used more for decorative than for electrical and industrial purposes. Amino-m&s laminates are paper, cloth, veneers (wood) and glass (cloth and mat) based, sandwich-type constructions with high percentages amino resins. Stacks of heat-cured sheets are pressed together to produce laminates having the desired chara&ristics. Decorative laminates are used mainly for furniture construction kitchen counter tops, cabinets, and vertical wall surfaces. Industrial laminates are used for printed circuit boards, electrical panels, welding torch electrode insulators, and electrical switch gears. Other laminate applications include light reflectors and di&sers, refrigerator breaker strips, and name plates. Molding. Eighty million pounds of amino molding compounds were used in 1993 (Table 3-2). About 73% were urea-type resins (Tables 3-4 and 3-5), due mainly to the higher costs of melamine resins (in spite of their better properties). The major areas of amino molding resins application are electrical, closures, housewares, buttons, and sanitaryware. Alpha celhtlose-filledmelamine molding resins are used to make houseware such as dinnerware (dishes, cups), ash trays, utensil handles, knobs, appliance components, control buttons, and sanitaryware such as toilet seats and bowl, and shaver housing. Alpha cellulose-filled urea molding resins are used to make electrical wiring devices, such as circuit breakers, receptacles, electric blanket control housings, toothpaste tube housing, and knob handles. Woodflour-tilled melamine molding resins are used for industrial electricalparts and military specifications. Glass- and mineral-filled melamine resins are also used for military specifications. Miscellaneous. Filled urea resins, which can develop a dielectric strength of up to 1500 V/mm and electrical resistances of up to lOI ohm/cm, are used for electrical insulation. Urea formaldehyde foam is used as artificial soil to grow plants and grass (plastoponics l/m introduced by Baumamr in 1967).r’21Urea foam is also used in road construction projects to protect loose soil by growing grass in it such as for freeway abutments.


95

Modified urea resins are being used in glass mat shingles to replace tarpaper shingles. Glass mat shingles have higher resistance than tarpaper shingles and an improved fire risk insurance rating. Melamine resins-based plywood and particleboard forms are used for concrete pouring. Melamine resins-impregnated slag cotton is used in acoustic tiles for sound and fire-resistant insulation. Furfuryl alcohol resin is used as a nonreactive diluent for epoxy resins. It is also the bonding material for corrosion-resistant fiberglass reinforced plastic, which has excellent resistance to heat distortion and flame spread. This plastic is used in process piping, underground sewer, tanks, vats, ducts, and reaction vessels. Furfuryl alcohol-impregnated graphite is used in nuclear reactors owing to its low permeability. Furfmal and fin-fural-phenolic molding compounds are used for TV cabinets because of their long flow and chemical resistance characteristics.

Trade Name Beetle Chem-Rez Cymel Fabrez Melamine Melatine Mehnac Mehuac Permalite Plaskon Plaspreg QuaCorr

TRADE NAMES Type of Resin U/F FlNail U/F

U/FandM/F Fl.lIaIl Furan

U/F - urea-formaldehyde M/F - melamine-formaldehyde

Company. American Cyanamid Ashland Chemical American Cyanamid Reichold Chemicals Fiberite Ciba-Geigy Corp. American Cyanamid American Cyanamid Ciba-Geigy Corp. Allied Corp. FuraneDivM&TChemical QO Chemicals, Inc.

96


REFERENCES

1. 2. 3.

8.

9. 10. 11. 12.

Blais, F.J., Amino Resins, Reinhold Publishing Corporation, New York, p 3, (1959). Meyer, B., Urea-Formaldehyde Resins, Addison-Wesley Publishing Company, Inc., Reading, MA, p 4, (1979). Dick, S.J., Compounding Materialsfor the Polymer Industries: A Concise Guide to Polymers, Rubbers, Adhesives, and Coatings, Noyes Publications, Park Ridge, NJ, p 3, (1984). Richardson, T.L., Industrial Plastics: Theory and Applications, 2nd edition, Delmar Publishers, Inc., Albany, NY, p 2 10, ( 1989). Wooten, A.L., Urea, Melamine, and Furan Resins, Forest Products Utilization Laboratory, Mississippi State University, (1986). Gilman, H., Organic Chemistry: An Advanced Treatise, Vol. I, 2nd edition, John Wiley and Sons, Inc. New York, p 967, (1947). McKillip, W.J., Furan andDerivatives, QO Chemicals, Inc., West Lafayette, IN (reprint f?om Ulhnan’s Encyclopedia oflndustrial Chemistry, Vol. Al 2) VCH, New York, N.Y. p 119, (1989). Othmer, R., Furan Derivatives, QO Chemicals, Inc., West Lafayette, IN (reprint from Encyclopedia of Chemical Technology, Vol. II, 3rd edition, John Wiley & Sons, Inc. 1980), p 499, New York Schupp, R.J., Amino,Modem Plastics Encyclopedia, McGraw-Hill, Inc., New York, N.Y. p 17, (1986-1987). Cymel and Beetle-Conventional Butylated Amino Resins, American Cyanamid Company, Charlotte, NC. U.S. Resins Sales by Process and Market, Modem Plastics, January issues p 57-67, (1984-1994). Meyer, B., Urea-Formaldehyde Resins, Adison-Wesley Publishing Company, Inc., Reading, MA, p 207, (1979).

Unsaturated Polyester and Vinyl Ester Resins Oscar C. Zaske SidneydH. Goodman

UNSATURATED POLYESTERS

History

The laboratory preparation of polyesters probably first occurred in 1847 with Berzelius cooking a saturated polyester from tartaric acid and glycerine.~1 The earliest record of chemical work with unsaturated polyesters is the study of glycol maleates by Vorlander in 1894.[*] Polymer chemistry as a science did not really develop until the first half of the twentieth century. The 1920s witnessed the brilliant and pioneering work of Wallace Carothers which included studies on polyesters among which were unsaturated types prepared from ethylene glycol and unsaturated acids and anhydrides such as fumaric acid and maleic anhydride.r31 It was soon discovered that unsaturated polyesters, although possessing reactive double bond unsaturation, were sluggish in reacting with themselves or homopolymerizing. Although responsive to catalysis with peroxide catalysts, relatively high temperatures and rather long times were required 97

98


to obtain a complete curing reaction. These resins were also totally unlike our present day low viscosity liquid unsaturated polyester (UP) resins in that they were either solids or very high viscosity, relatively immobile liquids. The key which made possible the modem unsaturated polyester resins of today was the discovery by Carlton Ellis that the addition of liquid unsaturated monomers such as monomeric styrene gave mixtures which would copolymerize at rates twenty to thirty times faster than the homopolymerization rate for unsaturated polyesters by themselves. Further, if a low viscosity liquid monomer such as styrene were used, the resultant mixture could be an easily handled liquid that could be readily cast or molded without the need for high molding pressures. Styrene monomer, although an optimum monomeric diluent for unsaturated polyesters, was in this time period still a relatively costly chemical. The economics of unsaturated polyesters based on such high cost styrene would have, very likely, retarded any broad based applications of UP. The second World War provided the last two developments which propelled UP into the applications and economic position they occupy today. Early during the war, it was found that styrenated polyester could yield high strength, low weight structures when reinforced with glass fibers. The fabrication of such structures could be accomplished using very low molding pressures making it possible to mold very large structures, if necessary, in relatively light weight, low cost tooling. It was also found that such fiber glass reinforced UP composites had excellent electrical properties including low loss factor properties which gave them relatively high transparency to the radar beams which were to play an ever increasingly important role as the war progressed. Concurrently, styrene monomer became increasingly available and cheaper as styrene monomer moved from the status of a lab chemical to that of a chemical commodity. This was a result of large styrene plants being built with U.S. Government assistance to provide the styrene necessary for the production of styrene-butadiene synthetic rubber. Prior to the entry of the United States into World War II, the U.S. Government foresaw the development of natural rubber shortages due to the spread of the war into the Far East and established The Rubber Reserve Corporation to stockpile natural rubber and pursue synthetic rubber research and development. This research work resulted in the development of GR-S (Government Rubber Styrene) rubber, a styrene-butadiene copolymer. By the end of the war, five styrene production

99

Unsaturated Polyester and Vinyl Ester Resins

plants had been built with government financing to supply the styrene for the production of GR-S rubber. These styrene plants, by 1955, were all sold to private industry.t41 The war years saw the production techniques for making polyester fiber glass radomes developed to a fine art. With the end of the war, commercial development proceeded rapidly with materials and molding methods research moving in all directions. Resin types proliferated, applications multiplied, and new raw materials for resin manufacture became available. Hosts of compounding materials, such as fillers, pigments, reinforcements, light stabilizers, curing catalysts, and promoters were introduced for use with unsaturated polyesters. Consumption patterns for unsaturated polyesters were estimated in a recent Jan. 1997 article inModem Plastics magazine, “Uniform Properties for a Common Market ” (p. 49ff) to be as follows:

‘able4-l a: Polyester Unsaturatec

Pattern of Consumption Million lb.

Market 1995b

1996

Reinforced polyester Molded, Filament-wound pultruded, hand layup, etc. _*’

Sheet, flat and corrugated

1109

Total reinforced Polyester Non-reinforced polyester surface coating

I

24

1116 24

Other Total non-reinforced

polyester

Export Grand total a: Resin only

1569

I

1579

b: 1995 figures revised in light of new information

100


Similarly, the use of unsaturated polyester in Western Europe and Japan is shown in Tables 4-lb and 4- lc.

Table 4-1 b: Unsaturated Polyester Million lb.

Market 1995’

1996

SMC/BMC

127

125

Filament winding

64

64

Pultrusion

24

26

367

363

Resin transfer molding

24

24

Sheet, flat and corrugated

79

79

Other

84

73

769

754

Non-reinforced resin Polymer concrete/synthetic marble

97

92

Surface coatings

31

29

Other

51

48

Total non-reinforced

179

169

Grand Total

948

923

Composite

Hand, spray lay-upb

Total composites

a: Some data changes in light of new information. b: Including gelcoats

101


Table 4-l c: UI saturated and Reinforced Polyester Million lb. 1996 1995

Market raturated

Polyester

1 Non RP use Domestic Total

437

1 439

88

88

525

527

Export

rrced

Polyester

Marine and ship

Miscellaneous uses

I

88

1

93

Similar data for vinyl ester resins is difficult to obtain or estimate but some market surveys have estimated volume to be in the neighborhood of twenty to thirty million pounds per year. The history of vinyl ester resins development is mainly chemical and will be covered in the section on the chemistry of vinyl ester resins later in the chapter.

102

Handbook of Thermoset Plnstics

Chemistry

General Concepts: Unsaturated polyesters are condensation polymers formed by the reaction of polyols and polycarboxylic acids with oletinic unsaturation being contributed by one of the reactants, usually the acid. The polyols and polycarboxylic acids used are usually difimctional alcohols (glycols), and difimctional acids such as phthalic and maleic. Water is produced as the by-product of the esteritication reaction and is removed from the reaction mass as soon as it is formed to drive the polyesteritication reaction to completion. All of the materials used must be at lcast difunctional to make the polyesterification reaction possible. Unsaturated polyesters differ from saturated polyesters such as the polyethylene terephthalate which constitutes the polyester films and fibers of commerce in that acids or glycols having bond unsaturation are included in the formula to provide reactive olefinic unsaturation in the unsaturated polyester alhyd. This reactive unsaturation can then be used to form thermosetting crosslinked polymers with monomers such as styrene and methyl methacrylate which contain olefinic double bonds. The term “alkyd” originating from the early days of coating resins technology is often also used for polyester resins capable of crosslinking with themselves or copolymerizing with monomers. Functionality: As noted above the acids and polyols used to make unsaturated polyesters must be at least difunctional. The fundamental work underlying this understanding was done on the types of alhyds used for protective coatings. A prominent worker in coating alhyds, R.H. Kienle, proposed the functionality theoryt5-‘1in papers during the 1930s. This theory simply states that monomer molecules must have at least two reactive groups to be able to form a polymer. Monofunctional reactants such as ethyl alcohol and acetic acid can react (esterify) to form an ester but are incapable of forming a polymer (polyester). Two difunctional reactants, such as propylene glycol (dihydroxy functionality) and maleic acid (dicarboxylic functionality) can esterify with the esterification reaction continuing to form a long chain polyester which can contain many repeating units of the basic mer, propylene glycol maleate. Polyesterification Reaction: Polyesterification is the most important reaction in the preparation of unsaturated polyesters. Side reactions also take place. These have been enumerated by E.E. Parker [*Ias:

103


1. 2. 3. 4.

Isomerization of maleate to fumarate. Addition of glycol to maleate and fumarate double bonds. Oxidative destruction of double bonds. Loss of glycol.

The general chemistry of unsaturated polyesters can be illustrated by the following representation of the synthesis of a general purpose propylene glycol, maleic anhydride, phthalic anhydride polyester.

HO-a,-w-m,

> (204-232’C)

Propylene

H

”

Glycol

Hslrlc

Anhydride

7”) 0 0 74 CH-CH,-0-cpt=CH-~-O-cH-cn,-o-c-

Phthsllc

::0

0

Anhydrldc

-1 coon +

HOH

IE

”

1:1

nA:PA-PG

GP

Unsaturated

Polyamter

Alkyd

Isomerization: The esterified maleic, is shown as the trans isomer, fumarate, since under the conditions of esterification more than 90% of the maleate ester isomerizes from the cis maleate isomer to the trans fumarate isomer.

0 CH-; 0

CH-COOH II COOH-CH

CH-C II 0 cis

trans

104


The tram3isomer is a lower energy level, less strained structure than the cis isomer configuration and the isomerization of maleate to fumarate occurs readily in the processing of most unsaturated polyesters. This is of great commercial importance because maleic anhydride is the least costly source for unsaturation in unsaturated polyesters. Fortunately, fumarate unsaturation is much more reactive in crosslinking reactions than maleate unsaturation and is the isomer preferred for practical reactivity levels. Fumarate ester in the form of ethyl fumarate has been cited as being up to forty times more reactive with vinyl monomers than ethyl maleate.[gl The polyols used in the manufacture of unsaturated polyesters are generally dihydric alcohols or glycols. Propylene, ethylene, dipropylene and diethylene glycols are frequently used members of the glycol family. Use of secondary glycols such as propylene glycol in the unsaturated polyester cook will favor a high isomer&&ion of maleate to fumarate. Isomerization will be less with the use of primary glycols such as ethylene glycol. V. Szmercsanyi, Marcos, and Zahran investigated the effect of different glycols on the maleate-fumarate isomerization in unsaturated polyesters. Table 4-2 shows the isomerization variations incidental to the use of different glycols they found.[lol

Table 4-2: Isomerization vs Glycol Type Reaction Temperature Glycol 1,2-Propylene Ethylene Diethylene 1,6-Hexamethylene

Constant = 356 “F ( 180 “C)

Type Secondary Primary Primary Primary

Isomerization, 96 64 53 36

%

The extent of maleate to fumarate isomerization also depends on reaction or cooking temperature and time, higher temperatures and longer times generally favoring greater isomerization.


105

V. Sanercsanyi et al, in their investigation of the kinetics of maleatel%marateisomer&&on, measured the percent isomerization in the cooking of poly(propylene glycol maleate) at temperature ranging from 22 1-356°F (1051SOYZ). These investigators found a direct dependence of isomerization on reaction temperature as shown in Table 4-3.

Table 4-3: Isomerization Reaction Temperature, 221 257 284 356

(105) (125) (140) (180)

“F (“C)

vs Reaction Temperature Isomerization

to Fumarate,

%

-34 -56 -75 -96

From an overall standpoint it was concluded that the rate of isomerization was mainly a function of the glycol structure and the temperature of the condensation since after a time, with all glycols and temperatures, the isomerization approached a constant value. Polyesterification Reaction Speed: The speed of the polyesterification reaction can be increased in several ways all of which effect more efficient removal of the water produced as a by-product of the reaction. Unsaturated polyesters are cooked under a blanket of inert gas to minimize oxidative degradation at cook temperatures. Inert gas is also usually introduced below the surface of the cook (sparging) to increase the liquid/gas interfacial area for mass transfer of water from the cook and to assist with the agitation. Increasing the inert gas sparge rate or agitator speed, by increasing mass transfer area, increases the rate of water removal and hence reaction rate. The use of vacuum can also increase reaction rate by increasing the partial pressure of the by-product water. The reaction can also be accelerated by the introduction of esterification catalysts. Among these are mineral acids such as sulfuric, aryl sulfonic acids such as p-toluene sulfonic acid, tin compounds such as dibutyl tin oxide and titanates such as tetrabutyl titanate.

106


Most esterification catalysts introduce disadvantages such as darker colored resins, haze in the resin or the necessity for their removal Corn the resin on completion of reaction. In unsaturated polyesters some of the tin compounds such as dibutyl tin oxide and butyl stannoic acid offer the best compromise between reaction acceleration and undesirable side effects.

Processing

It is usually necessary to use a stoichiometric excess of glycol in cooking unsaturated polyesters because of some glycol loss along with the reaction water and decomposition of some of the glycol. This glycol excess can oflen be reduced with the use of some of the more, efficient esterification catalysts. On completion of the polyesterification reaction the unsaturated polyester alkyd can be discharged from the reaction kettle as a molten mass into drums or onto flaking equipment such as a water cooled stainless steel belt or drum or it can be mixed with a liquid monomer such as styrene to give the liquid resin which is the most commonly used form of unsaturated polyesters. As noted previously, the homopolymerization rates of unsaturated polyesters are very sluggish and they are usually used in combination with unsaturated monomers, the most common of which, by far, is monomeric styrene. In the manufacture of a typical styrenated unsaturated polyester the molten unsaturated polyester alkyd, on completion of the cook, is transferred from the cooking kettle to a thinning or styrenation tank containing styrene monomer where the alkyd and styrene are blended with vigorous agitation. Since the mixture of unsaturated polyester alkyd and monomer are very coreactive, polymerization inhibitors must be present in the styrene or alkyd before thinning to prevent copolymerization from occurring during the thinning operation. Hydroquinone or substituted hydroquinones such as tohrhydroquinone or monotertiary butyl hydroquinone are commonly used.


Typical General Purpose (GP) Unsaturated

107

Polyester Resin

Table 4-4 shows the reactor charge for a typical GP polyester resin.

Table 4-4: Reactor Charge for a Typical GP Polyester Resin

Material Phthalic Anhydride

Propylene Glycol Maleic Anhydride Hydroquinone Styrene Monomer Total materials to make 1 lb. of thinned resin at 60% NVM (40% styrene content)

Mols Pounds

1.5 2.7 1.0

222.0 205.2 98.0

Pounds Per Pound of Resin @60% NVM”

0.2774 0.2564 0.1225 0.0001 0.4000

1.0564

Reaction water = approximately 45 pounds. Alkyd cook yield = approximately 480.2 lbs. = approximately 91.43%. Overall thinned resin yield = approximately 94.66%. *Nonvolatile Matter.

Common Resin Synthesis Raw Materials

Common raw materials used for cooking unsaturated GP and specialty polyesters are shown in Table 4-5.

108


Table 4-5: Common Raw Materials for Polyesters Contributes Glycols: Propylene Glycol (PG) Ethylene Glycol (EG) Dipropylene Glycol (DPG) Diethylene Glycol (DEG) Neopentyl Glycol (NPG) Trimethylpentanediol (TMPD) Cyclohexane Dimethanol (CHDM) Propoqlated Bisphenol A (PBPA) Hydrogenated Bisphenol A (HBPA) Dibromoneopentyl Glycol (DBNPG) Acids: Phthalic Anhydride (PA) Maleic Anhydride (MA) Adipic Acid (AA) Isophthalic Acid (IPA) Terephthalic Acid (TPA) Fumaric Acid (FA) Glutaric Acid Dimer Acids Azelaic Acid Chlorendic Acid Tetrabromophthalic

Anhydride

Tetrachlorophthalic

Anhydride

Endomethylenetetrahydrophthalic Anhydride

Copolymerization

of Unsaturated

Low cost, styrene compatibility Low cost, rigidity Flexibility, toughness Flexibility, toughness UV, water and chemical resistance Water and chemical resistance Electrical properties Water and chemical resistance Water and chemical resistance Flame retardance Low cost, styrene compatibility Lowest cost unsaturation Flexibility, toughness Toughness, water and chemical resistance Higher heat deflection point Maximum reactive unsaturation Flexibility, toughness Flexibility, toughness _ Flexibility, toughness Flame retardance, chemical resistance Flame retardance, chemical resistance Flame retardance, chemical resistance Air drying properties

Polyester Alkyds with Monomers

Unsaturated polyesters copolymerize with monomers having olefmic unsaturation much more rapidly than they homopolymerize so most ununsat-

109


urated polyesters are used as mixtures with reactive, usually liquid, monomers. Of such monomers, styrene is by far the most used monomer. Commercially the term unsaturated polyester usually refers to the combination of an unsaturated polyester alkyd with styrene monomer. With number average molecular weights usually in the range of 800 to 3,000, unsaturated polyester alkyds are not high polymers but rather may be considered to be reactive low molecular weight prepolymers. Copolymerization with an unsaturated monomer such as styrene in a subsequent curing reaction forms the three-dimensionally crosslinked polymer that is a cured unsaturated polyester resin. Unless noted otherwise, subsequent reference to polyester resin will refer to the liquid resin made by the solution of unsaturated polyester alkyd in styrene monomer. This type of resin, being a liquid system, offers all of the advantages of liquids such as ease of handling with little or no pressure being required for mixing with other ingredients or casting and molding operations. Styrene is an ideal monomer in most respects for use in polyester resins because it is low in cost, can give low viscosity resins at reasonable monomer levels, and copolymerizes readily with unsaturated polyester albd in either room temperature or elevated temperature curing systems. The copolymerization chemistry of unsaturated polyester alkyds and unsaturated monomers is usually initiated by free radicals generated by the decomposition of peroxides, azo compounds, or the generation of free radicals by the use of medium to high energy radiation such as ultraviolet light or electron beams. With styrene as the monomer, the copolymerization reaction involves the addition of styrene monomer across the fumarateimaleate double bonds in the unsaturated polyester alkyd chains as pictorially depicted below.

PG-FA-PG-PA-PG-FA-PG-PA-PG-FA-PG-PA-PG-FA-PG-PA I

I

I

STYRENE

STYRENE

STYRENE

I

I

I

I

STYRENE

I

PG-FA-PG-PA-PG-FA-PG-PA-PG-FA-PG-PA-PG-FA-PG-PA PG-FA PG-PA

Propylene Glycol Fumarate in alkyd chain Propylene Glycol Phthalate in alkyd chain.

110


The above is a simplified schematic; in commercial resins more, styrene is employed resulting in a ratio of styrene double bonds to alkyd double bond greater than one. Workers such as Harnan, Funke and Gilch early found that the number of fumarate double bonds participating in the cross-linking reaction with styrene increases with increasing styrene content in the tmsatmated polyester resin. ~‘1 In an unsaturated polyester prepared from 3.6 mols of FA, 2.4 mols of AA, and 6.6 mols of 1.6 hexane diol it was found that maximum cure occurmd at a ratio of about 1.5 to 2.0 styrene double bonds per fumarate bond. Lower styrene contents resulted in no reaction of an appreciable number of the unsaturated polyester alkyd double bonds.[‘*] Typical general purpose resins have styrene to alkyd double bond ratios ranging from two to three or even higher. This means, of course, that some of the styrene crosslinks in the cured resins can have more than one styrene molecule forming the crosslink and that some styrene molecules may simply add a branch on an alkyd chain formed from two or three styrene molecules which have no attachment to another alkyd chain. It has been claimed that a small amount of low molecular weight polystyrene can be formed under some conditions. [I31It has also been claimed, on the basis of degradative and spectroscopic studies, sol-gel analysis and dynamic measurements that in addition to styrene-fumarate copolymerization, a fumaratefumarate cross-linking reaction occurs at low styrene concentrations.[‘41 In the polymerization or cure of a typical styrene containing unsaturated polyester the liquid resin reaches gel-like consistency (gel point) when less than 5% of the original carbon to carbon double bonds have been converted and a physically rigid state is achieved when only 50% of the double bonds of the unsaturated polyester alkyd and siyrene have been used UP.[‘~] Cure of the resin system, of course, normally continues on beyond the 50% double bond conversion point to a fairly high degree of conversion, usually above 90 to 95%. The curing behavior of an unsaturated polyester resin can be illustrated by the time versus exotherm temperature curve generated by the SPI (Society of The Plastics Industry) Gel Time Test, a very useful tool for assessing the curing rates and exothermic characteristics of a resin.


111

Processing Equipment and Manufacturing

Typically unsaturated polyesters are cooked in 304 stainless steel kettles from 1,000 to 5,000 gallon capacities. The kettles are equipped for heating via internal coils and heat transfer fluids. The same coils and fluids, cooled, are also used for cooling the kettle contents when necessary. The kettles are equipped with heavy duty agitators. The reaction water is taken off the kettle through either packed distillation columns or partial condensers to more efficiently separate the glycol which comes off with the reaction water so the glycol can be returned to the kettle. Coming off with the reaction water are reaction by-products from glycol decomposition such as aldehydes which are more volatile. These reaction by-products, together with the reaction water and small nonrecoverable amounts of the glycols and acids used in the charge, must then be dealt with in conformance to regulations that regulate the handling and disposal of industrial wastes. The cooking operation begins with charging raw materials into the kettle by the use, usually, of a jacketed weigh hopper for the materials handled as liquids such as molten PA, molten MA, PG, EG, DEG, and DPG. Nonliquid materials such as AA and IPA can manually be “bagged” in via the kettle charging port or by handling equipment for powdered and granular materials. The liquid glycols are usually charged first followed by the granular and then the molten materials such as PA and MA. Agitation can be started once a liquid “heel” has been established in the kettle. The kettle is then slowly brought up to temperature at a programmed rate which is designed to allow for the significant exothermic heat released when the MA and PA anhydride rings open on reacting with the glycols in the charge. Gnce the kettle contents have been brought up to cooking temperature the contents are sampled periodically during the cook for the measurement of acid number and process viscosity. Cooking is continued until the desired acid number and process viscosity are achieved in the resin batch. A deviation from the desired acid number-viscosity curve can usually be detected early enough in the cook so that the resin batch can be brought back to the correct acid number-viscosity relationship by the addition of a small, calculated amount of extra glycol or acid.

112

Handbook of l’hermoset Plastics

Typical resin cooking temperatures are in the range of 400 o to 45 0 QF (204” to 232°C). Cooking times range from 8 hours to 28 hours depending on the raw materials used, the degree of condensation or molecular weight desired, and the cooking temperature employed. Mixing the completed unsaturated polyester alkyd with a liquid monomer, usually styrene, is accomplished by transferring the finished molten alkyd to a thinning tank previously charged with styrene containing polymerization inhibitors to prevent copolymerization of the unsaturated polyester alkyd and styrene. The molten alkyd is added gradually with vigorous agitation in the thinning tank. The thinning tank, also usually made of 304 stainless steel, is equipped with cooling coils through which cooling water is circulated to keep the thinning tank contents below a safe temperature, usually 180” to 200°F (82” to 93°C). The alkyd must be transferred to the thinning tank at a rate at which the sensible heat being added by the alkyd does not exceed the ability of the cooling system to keep the tank contents at a safe temperature level. Thinning tanks, naturally, must be larger than their corresponding cooking kettles. On completion of the thinning operation, the thinned unsaturated polyester is cooled down to or near room temperature and further compounded to make the various types of unsaturated polyester resins of commerce.

Unsaturated Polyester Resin Alkyd Properties

The 100% alkyd from a typical general purpose resin cook is a pale yellow solid at room temperature having the general properties shown in Table 4-6. The preceding alkyd properties would change as functions of the raw materials used and the molecular weight to which the resin is cooked. Substitution of either glycols such as diethylene glycol in place of propylene glycol and adipic acid in place of PA can give an alkyd that is a viscous liquid at room temperature rather than a solid resin. Cooking to a much higher molecular weight, as is often done with resins using isophthalic acid in place of phthalic anhydride, can raise the melt-


113

pointofthealkyd,forexample,towellover200” to240”F(104” to 116°C). Employing halogenated intermediates in place of phthalic to build in tire retardance can increase the specific gravity to 1.3 to 1.5 depending on the amount of halogen, such as chlorine or bromine, introduced into the alhyd. Although some of the unsaturated polyester alhyd produced is sold as alkyd for specialized uses such as prepreg or molding compounds, most of the unsaturated polyester alkyd is sold and used as a solution in styrene monomer.

Table 4-6: GP Unsaturated Alkyd Properties

Specific gravity Solubility Melting point (Durran Mercury Method) Acid number Viscosity of 60% solution in methyl Cellosolve Molecular weights by gel permeation chromatography: Number average Weight average Z average Dispersity

Styrenated Unsaturated

1.13-1.15 Insoluble water, soluble ketones and aromatic solvents 140” - 170°F (60” - 77°C) 30-40 G-H (Gardner Holdt)

900 2,400 6,800 2.7

Polyester Resin Liquid Properties

Liquid unsaturated polyester resins range in viscosity from thin 50 centipoise liquids to quite viscous fluids with viscosities of 4,000 to 6,000 centipoises and higher. Liquid resin colors can range from a very pale yellow to dark amber. These basic resin colors can further be affected by the presence of color contributing additives such as curing promoters.

114


Typical liquid resin properties obtained by the styrenation of a typical general purpose unsaturated polyester alkyd are shown in Table 4-7.

Table 4-7: Styrenated GP Polyester Liquid Resin Properties

Styrene content, by weight Viscosity, centipoise Specific gravity Color SPI gel time Peak exotherm time Peak exotherm temperature Flash point (tag closed cup)

32%

1,100 1.14 Pale,yellow 5-7 minutes 6-8 minutes 340°-3600F(1710-1820C) 87”- 95’F (3 lo- 35°C)

As mentioned previously, styrene is by the far the most widely used monomer employed for making liquid unsaturated polyester. With the use of styrene a reactive liquid resin can be created which is relatively low in cost compared to other reactive liquid resin systems such as epoxies and polyurethanes. Further, the styrenated unsaturated polyester can be made to a very wide range of viscosities and can be made compatible with curing methods ranging from ambient temperature cures all the way to high temperature curing conditions. The styrenated unsaturated polyester resins do not present the allergy and high toxicity problems associated with some liquid reactive epoxy and urethane systems. On the other hand styrene monomer is to an extent volatile and the use of unsaturated polyester resins must be carried out in a manner that will keep the styrene in the air and workplace below the levels rccommended/mandated by advisory or regulatory bodies (50 to 100 ppm depending The sfyrene monomer in an unsaturated polyester results on the agency ).[16171 in the liquid resin having a flash point below 100°F (37.8”C) and such rmsaturated polyesters are classified as flammable liquids which require a Department of Transportation (DOT) Red Label. Other less volatile monomers having olefinic unsaturation, such as vinyl toluene or p-methylstyrene,


115

can be used in place of styrene to increase the flash point of the resin or to reduce monomer emission levels. General purpose styrene containing polyester resins of this type find uses in compounding resins for glass fiber reinforced composites such as boats and other marine applications, translucent fiber glass reinforced building panels, cast synthetic stones and general purpose fiber glass reinforced molded articles.

Monomers Used in Unsaturated

Polyesters

Monomers other than styrene used in unsaturated polyesters are shown with their common abbreviations in Table 4-8.

Table 4-8: Nonstyrene

Monomers Used in Polyesters

Monomer

Application

Methyl methacrylate (MMA) Butyl acrylate (BA) Butyl methacrylate (BMA) Alpha methyl styrene (AMS) Vinyl toluene (VT) Para-methyl siyrene (PMS) Diallyl phthalate (DAP) Diallyl isophthalate (DAIP) Octyl acrylamide (OAA) Trimethylol propane triacrylate (TMPT) Triallyl cyanurate (TAC) Triallyl isocyanurate (TAIC) Diallyl maleate (DAM) Diallyl tetrabromophthalate

Enhanced weather resistance Enhanced weather resistance Enhanced weather resistance “Cooler” cure, reduced exotherm Less volatility, higher flash point Less volatility, higher flash point Very low volatility, prepregs Very low volatility, prepregs Solid monomer, molding compounds UV and electron beam cures High heat deflection High heat deflection High heat deflection Fire retardance

Handbook of l’hermosel Pfas~ics

116

Unsaturated Polyester Properties and Chemical Composition

The physical properties of cast cured unsaturated polyester will largely depend on the raw materials used for manufacture and to a lesser extent on the degree of condensation or molecular weight to which the alkyd is cooked.

General Purpose Resins

Rigid general purpose (GP) resins are cooked using chiefly propylene glycol (l,Zpropanediol), maleic anhydride and phthalic anhydride. The molar ratio of phthalic anhydride to maleic anhydride will usually range from 1: 1 to 2: 1. Rigid GP resins of this type can be used for most types of casting, molding, and laminating. These are the so-called GP ortho resins of commerce, so named for the ortho isomer of phthalic used in their manufacture. (Phthalic acid can exist in three isomeric forms: the ortho isomer is always referred to as phthalic acid or anhydride, the meta isomer as isophthalic and the para isomer as terephthalic).

0 0

0

-COOH

-CooH

0

-COOH

ortho Phthalic Acid

-coot4

Meta Isophtbalic Acid

00

-coon

-COOH

Para Terephthahc Acid

Molar Ratio of PA to MA

The molar ratio of PA to MA will affect such cast resin properties as cured hardness, tensile elongation, heat deflection point or softening temperature, cast resin refractive index, and reactivity. Increasing unsaturation, by


117

using more MA, will increase cured hardness, heat deflection point, and reactivity. Increased PA on the other hand will give resins with higher refractive index and decreased reactivity.

Flexibilization

Greater tensile elongation, flexibility, and toughness can be built into unsaturated polyester resins by substitution of ether glycols such as dipropylene or diethylene glycol for part or all of the propylene glycol and by using flexibilizing long chain aliphatic acids such as adipic acid for all or part of the phthalic used in the GP resin formula.

OH OH I I CH, -CH-CH, -O-CH, -CH-CH,

HO-CH, -CH, -O-CH, -CH, -OH Diethylene Glycol

Dipropylene Glycol

COOH-CH, -CH, -CH, -CH, -COOH Adipic Acid

Such flexibilized resins will be softer when cured and will have their heat deflection points reduced in proportion to the amount of flexibilization employed. Since the flexibilization is introduced into the alkyd “backbone” of the resin the flexibilization in the cured, crosslinked resin is permanent unlike that which might be created by the addition of a fugitive external plasticizer such as dibutyl phthalate. A drawback of flexibilization is reduced hydrolytic stability.

118


lsophthalic Resins

Resins made with phthalic anhydride can be practically cooked up to only a relatively modest molecular weight before decarboxylation by loss of PA places a practical limit on the extent of condensation. Ortho-phthalic based unsaturated polyester alhyds will generally be condensed to (by GPC) 800 to 1,000 number average molecular weights. Substitution of the metaphthalic isomer, isophthalic acid, for the ortho PA is the most common commercial route to cooking higher molecular weight unsaturated polyester resins. Resins with (by GPC) number average molecular weights of 1,500 to 2,000 can be easily made. The higher molecular weight isophthalic resins can give higher physical properties with respect to toughness, heat deflection point, hydrolytic stability, and chemical resistance compared to their lower molecular weight ortho-phthalic cousins.

Molecular Weight Comparisons

A typical GP ortho-phthalic resin compares in molecular weight characteristicsto a high molecular weight isophthalic resin as shown in Table 4-9 below when tested by size esclusion chromatography (Gel Permeation Chromatography).

Table 4-9: Molecular Weights of Ortho- and Isophthalic UP* Number average molecular weight (MW,) Weight average molecular weight (MW,) Z average molecular weight Dispersity (MWJMW,,)

GP Ortho 910 2,430 6,800 2.7

High MW Iso 1,520 10,160 26,550 6.7

*(Run on Waters Associates Liquid Chromatograph1’81 equipped with Varian Associates(“l Micropak TSK lOOOH and 300H Columns at 2 ml/min, tetrahydrotian carrier solvent with data reduction by a Waters Data Module microprocessor.)


119

GPC is an excellent characterization technique for determining the molecular weight characteristics of unsaturated polyester resins and is especially useful if calibration data is available from an absolute correlative detector such as a Low Angle Laser Light Scattering (LALLS) detector. GPC can give considerably more data than the process viscosity and thinned resin viscosity measurements which also are a measure of molecular weight. Extensive literature is available on the application of GPC to polymer characterization.t20~221

Hydrolytic Stability and Chemical Resistance

Significant improvement in hydrolytic stability, chemical resistance, and resistance to yellowing on exposure to ultraviolet radiation can be obtained by substituting neopentyl glycol (NPG) for propylene glycol as follows:

HO-CH,

-C-CH,-OH

Neopentyl Glycol (NPG) Neopentyl glycol confers these advantages in both ortho- and isophthalic unsaturated polyester resins. NPG, because of its linear primary glycol structure, will tend to give a more linear alkyd structure. This can result in alkyd-styrene incompatibility problems in high maleic content resins or if the alkyd structure is made more linear by the use of significant amounts of a straight chain acid such as adipic in the formulation.

Styrene Compatibility

The styrene compatibility limitations of such NPG formulations can be greatly reduced or eliminated by reducing the stereoregularity of the alkyd

120


chain by replacing part of the NPG with a secondary glycol such as propylene glycol. For the same styrene compatibility reasons, ethylene glycol is very seldom used as the sole glycol in an unsaturated polyester. OH-CH,

-CH,

-OH

Ethylene Glycol (EG) Cyclohexanedimethanol, a cycloaliphatic primary glycol, will also tend to give stereoregular alkyds if cooked with fumaric acid alone. With proper alkyd formulation, resins having good electrical properties can be made with this diol. H,OH

0

H,OH

Cyclohexanedimethenol (CHDM)

Flame Retardance

Flame retardance can be built into unsaturated polyesters by the use of halogenated intermediates in the cook charge. The halogens used are either chlorine or bromine. Part or all of the saturated acid in the unsaturated polyester cook can be replaced with tetrabromophthalic anhydride (TBPA), tetrachlorophthalic anhydride (TCPA), or chlorendic acid (CA). 5’

TBPA

TCPA

coon

::O@;oo, 2, CA


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Bromine can also be built into the unsaturated polyester backbone by the use of dibromopentyl glycol as the diol in the cook. CH, Br

I HO-CH,

-C-CH,

-OH

I

CH, Br Dibromoneopentyl glycol Improved chemical resistance can also be obtained by the use of aromatic and cycloaliphatic diols such as propoxylated bisphenol A and hydrogenated bisphenol A in place of the conventional diols such as propylene glycol.

Propoxylated bisphenol A An equilibrium is reached in the propoxylation of bisphenol A in the formation of primary and secondary hydroxyl groups. According to Kerle, Connolly, and Rosenfeld the ratio of primary to secondary hydroxyls is 15:85.[23] The same investigators also claimed that the percentage of para, para isomer in the commercial bisphenol A used could vary from 97.15 to 100.00% depending on the source.

“o_(y;-J-o” 3

Hydrogenated bisphenol A

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Unsaturated polyesters can give fairly good chemical resistance to aqueous acids and salts but are tierable to attack by high pH alkaline media on the unsaturated polyester alkyd ester linkages. Use of the bisphenol A derived diols introduces considerable steric shielding of the ester linkages and can extend the use of unsaturated polyesters into higher pH alkaline environments. 2,2,4-Trimethyl-1,3-pentandiol (TMPD) can also give improved hydrolytic and chemical resistance. CH, I

CH, I

HOCH,-C-CH-CH-CH, CH3 bH TMPD

The methyl groups of this glycol sterically shield ester linkages formed by its hydroxyl groups from hydrolytic and chemical attack. The same steric shielding makes it a bit more difficult to synthesize resins with TMPD and also reduces curing reactivity somewhat. These problems have been addressed by the manufacturer of the glycol with the development of techniques for formulating and synthesizing TMPD resins and room temperature curing promoter systems for resins made with this glyco1.t241TMPD resins’ cured specific gravities tend to be lower than those of conventional resins.

VINYL ESTER RESINS

Chemistry

Vinyl esters are the reaction products of epoxy resins with ethylenically unsaturated carboxylic acids. Simple diepoxide resins, such as the diglycidyl ether of bisphenol A or bisphenol A extended higher molecular weight


123

homologues thereof, can be used as well as brominated analogues for flame retardance or epoxy novolac resins for special properties such as higher heat resistance. Vinyl ester resins typically have terminal unsaturation except for some special types such as those designed for thickening with Croup II metal oxides and hydroxides in the manufacture of sheet molding compound (SMC). The terminal unsaturation will react to give crosslinking either by homopolymerization of the vinyl ester resin with itself or by copolymerization with unsaturated monomers such as styrene. The most common vinyl esters are made by esterifying a diepoxide resin with a monocarboxylic unsaturated acid such as methacrylic acid or acrylic acid. Such epoxy methacrylates or epoxy acrylates can be used in free radical curing reactions alone or can be dissolved in unsaturated monomers such as styrene to give liquid resins which can be used very much like styrenated unsaturated polyester resins.

Basic Vinyl Ester Resin

A basic vinyl ester preparation employing a simple diglycidyl ether of bisphenol A epoxy resin and methacrylic acid can be represented as:

250-300°F (121-149X)

e~..c.,,~-~~~,,“.a., +“O-E;,., CH,

*Benzyl trimethyl ammonium chloride catalyst

124


Generally, onium salts, tertiary amines and phosphines are effective catalysts for the acid-epoxide reaction which proceeds readily at only moderately elevated temperatures. Methacrylic acid is most commonly used for vinyl ester resins intended for composites applications while acrylic acid is favored for resins intended for application in coatings. The use of other unsaturated acids such as crotonic and cinnamic acids have been reported.[251

History

Some of the earliest vinyl ester resins resulted from efforts by the U.S. Commerce Department to find a better dental adhesive with which to bond acrylic dental prostheses to teeth. RL. Bowen esteritied glycidyl methacrylate and acrylate with bisphenol A in these efforts.[261[27] Unfortunately, these resins were too reactive to afford a practical working life and only further vinyl ester development succeeded in providing commercially useful dental bonding resins. Other workers, such as Fekete and his associates, worked on resins intended for use in electrical insulation and chemically resistant composites.r281r2g1

Toughness and Chemical Resistance

Vinyl ester resins offer toughness and chemical resistance properties which are generally superior to unsaturated polyesters. The epoxy resin backbone used in making vinyl ester resins confers toughness and greater tensile elongation properties to these resins. The molecular weight of the vinyl ester resin can be varied by the choice of the epoxy backbone employed. For the most common vinyl ester resins used for composites two mols of the diglycidyl ether of b&phenol A are chain extended with one mol of bisphenol A to form the epoxy backbone. In this manner, molecular weight and backbone structure dependent properties such as tensile strength and elongation, heat deflection point and reactivity can be varied for different applications.


125

Vinyl Ester Resin Structure and Properties

The superior chemical resistance (compared to unsaturated polyesters) of vinyl ester resins is in part due to the absence of ester linkages in the epoxy backbone in those sites where the polymer units are connected with phenyl ether linkages. These latter moieties are much more resistant than ester linkages to degradation in many chemical environments and especially in high pH alkaline situations. The ester linkages in a vinyl ester resin are present only at the end of the molecule which minimizes the number of ester linkages that can be chemically attacked. Further, if the vinyl ester resin molecule is terminated with methacrylate groups the spatially large methyl group pendant on the methacrylate group sterically shields the ester linkage from chemical attack. With the vinyl unsaturation present on the ends of the molecule, vinyl ester resins can be made to be very reactive. They have the ability to cure rapidly with fast green strength development either as homopolymers or as copolymers with monomers such as styrene.t251

Specialty Vinyl Ester Resins

Specialty vinyl ester resins can be made based on the use of epoxy novolacs (epoxy resins based on phenol formaldehyde novolacs) for the epoxy resinbackbone. Heatdeflectionpointsof270”to300”F(l32” to 149”C)can be achieved while still maintaining excellent chemical resistance. Flame retardant vinyl ester resins can be made by using diepoxide resins based on the use of tetrabromobisphenol A. Vinyl Ester Resin Thickening for SMC: Conventional vinyl ester resins do not respond well to the divalent Group II metal oxides and hydroxides used for thickening sheet molding compounds (SMC). Good thickening response can be obtained by introducing carboxyl group functionality on the vinyl ester molecule. RJ. Jackson described acid modification of a vinyl ester resin by replacing part of the monocarboxylic methacrylic acid with a dicarboxylic acid, maleic acid.t301An example from Jackson’s patent illustrates such a formula change and the improvement in thickening response. This is shown in Table 4- 10 below.

126


Thickening responses of the two resins were quite different on having 2.5 PHR (parts per hundred of resin) of magnesium oxide thickener dispersed in them. Table 4- 11 shows the thickening of acid modified vinyl ester resin. Flame Retardant Vinyl Esters: Flame retardant vinyl ester resins are usually made using an epoxy backbone incorporating tetrabromobisphenol A in place of bisphenol A.

Table 4-10: Maleic Acid Moditied Vinyl Ester SMC Resin Standard Vinyl Ester

Acid Modified Vinyl Ester

Stage I:* Epoxy Resin (DGEBA)* * Bisphenol A Trimethyl ammonium chloride

795 237 0.825

795 237 0.825

Stage II: * Glacial methacrylic acid Hydroquinone

195.2 0.45

171.3 -

Trimethyl ammonium chloride Maleic acid Styrene

6.6 970

6.6 32.25 970

*Stage I was processed for 1 hour at 340’F (17 1“C). Stage II was processed for 3 hours at 24O“F (116°C) with a nitrogen-air sparge. **DGEBA: Av. MW 350, equiv. wt. about 170- 190.

Table 4-l 1: Thickening of Acid Modified Vinyl Ester Resin Brookfield Viscosity, cps Standard Vinyl Ester Acid Modified Vinyl Ester

Viscosity Initial After 24 hours A&r 48 hours Atter 7 days

500 500 500 500

1,230 85,000 2.15 x lo6 4.00 x 1 O6


127

One Step Vinyl Ester: An early novel “one step” approach to the preparation of vinyl ester type resins was described by C.A. May, who using a kettle charge of bisphenol A, epichlorohydrin and methacrylic acid, first reacted the epichlorohydrin with the methacrylic acid and then, adding sodium hydroxide, reacted the bisphenol A with the methacrylated epichlorohydrin residue.t39 Other variants were explored by May. Among these was the modification of a vinyl ester resin by reacting the secondary hydroxyls of the resin with isocyanate to give a urethane modified vinyl ester resin.v2] Rubber Modified Vinyl Ester: More impact resistant vinyl ester resins were made by D.J. Najvar by replacing up to 20% of the unsaturated monocarboxylic acid, such as methacrylic acid, with a functionally equivalent amount of a liquid carboxy terminated polydiene rubber.[331 As an example the following were reacted to an exotherm of 374 “F (19OC) as shown in Table 4-12.

Table 4-12: Rubber Modified Vinyl Ester Resin Synthesis Parts by Weight

DGEBA epoxy equiv. wt. (EEW) 189 (Dow 33 1) Bisphenol A t-B@1 phosphonium acetate

455 154 0.5

This gave a polyepoxide resin with EEW ca 600 This was then reacted at 248” to 266°F (120” to 130°C) with: Methacrylic acid DMP-30 catalyst B.F. Goodrich Hycar CTBN (COOH terminated butadiene acrylonitrile rubber with 2.5% COOH Hydroquinone The above was reacted to 1.15% COOH content.

76 1.2

228 0.17

128


Vinyl Ester Resins Overview

The epoxy backbone and methacrylic/acrylic acid used to make vinyl esters make them significantly more costly than conventional unsaturated polyester resins. Vinyl esters, however, do offer advantages over unsaturated polyester resins which have been very well summarized by Anderson and Messick:[34] I.

Excellent reactivity due to terminal vinyl unsaturation in either homopolymerization or copolymerization reactions.

2. With methacrylate termination, increased hydrolysis resistance due to ester linkage shielding by the methacrylate methyl group. 3. 35 to 50% fewer hydrolysis prone ester linkages than conventional unsaturated polyester resins. 4. Better wetting and bonding to glass reinforcements due to secondary hydroxyls on the vinyl ester resin molecule. 5. hnproved elongation and toughness conferred by the epoxy resin backbone whose ether linkages give superior acid resistance.

Typical Styrenated Vinyl Ester Resin Liquid Properties

Table 4-13 shows the typical styrenated vinyl ester resin liquid properties.

Typical Styrenated Vinyl Ester Cast Resin Properties

Table 4- 14 shows the typical styrenated vinyl ester cast resin properties.


129

Table 4-13: Styrenated Vinyl Ester Resin liquid Properties Styrene content, % NVM, % Viscosity, cps, 77°F (25°C) Color, Gardner Wt. per gallon, lbs. Flash point,, tag open cup, “F (“C) Reactivity Gel time at 77°F (25°C) min.* SPI gel time, min. *Promoted with 0.5 PHR 12% cobalt octoate MEK peroxide catalyst.

so-45 50-55 80-600 2-3 8.6 - 8.7 95 (35) 20-25 10-19 and catalyzed with 1.O PHR

Table 4-14: Physical Properties of Cast Vinyl Ester Resin Tensile strength, psi, 77’F (25°C) Tensile modulus, psi, 77’F (26°C) Tensile elongation, % Flexural strength, psi, 77 “F 25 “C) Flexural modulus, psi, 77 “F((25”C) Heat deflection temperature, “F (“C) Notched Izod impact - ft. lbs./in. of notch Barcol hardness

COMPOUNDING OF UNSATURATED VINYL ESTER RESINS

1o,ooo- 11,000 0.36-0.44 x lo6 5.3-5.8 18,000- 19,000 0.45-0.46 x lo6 190-2 10 (88-99) 0.5-0.55 40-50

POLYESTER

AND

Overview

Most commercial unsaturated polyester resins contain styrene as the crosshking monomer. Some exceptions are resins for:

130


Structural panels which may also contain some acrylic monomer for improved outdoor weathering. Mine bolt resins which usually contain enough vinyl toluene in addition to styrene to raise the flashpoint to over 100°F (37.792). Resins for the manr.&actureof prepreg and molding compounds which may be sold as monomer-free 100% unsaturated polyester alkyd or as monomer-free alkyd dissolved in acetone. The liquid styrenated resins which form the volume of commerce can be compounded for use with: Curing Systems Promoters for room temperature cures. Promoters for elevated temperature cures. Thixotropic Agents To give flow control and prevent, sagging in vertical lamination and coating. Fillers To reduce cost, reduce curing shrinkage and impart special properties such as flame retardance. Pigments For coloration. Thickening Agents To give the compound thickening necessary for sheet and bulk molding compounds (SMC and BMC). Fiber Reinforcements To give high strength composites as in laminates, SMC, and BMC. Wetting Agents Facilitating wetout of fillers and reinforcements. Bubble Release Agents To enhance air bubble release in laminating or casting.


131

Internal Mold Release Agents Catalyst Indicators Which indicate catalyst addition by a color change.

Most of the compounding techniques and materials used with polyesters can also be used in the compounding of vinyl ester resins. Vinyl esters, being more costly than polyesters, are generally not used in as broad a range of applications as polyesters. Vinyl esters owe most of their applications to their superior chemical resistance properties and higher physical properties of composites made with them.

Curing Systems

Styrenated unsaturated polyester resins can be cured by either room temperature (RT) or heat curing methods. Other monomers such as vinyl toluene, methyl methacrylate, and para-methylstyrene can also be used, generally with styrene, to give room temperature or heat curing systems. Diallylphthalate or isophthalate monomers do not respond well in RT cures and are generally only used in such heat curing applications as prepregs and molding compounds. Vinyl esters can also be cured by either room temperature or elevated temperature curing routines, with styrene being the most common monomer in use. Room Temperature (RT) Curing Systems: RT curing systems are usually based on the use of transition metal soaps as the primary promoters and ketone peroxides as the catalysts. Cobalt soaps such as cobalt naphthenate, octoate, or neodecanoate are the most popular metallic promoters providing RT curing resins which give good curing behavior and package life. Other primary metallic promoters that have been used are compounds of manganese, vanadium, tin, calcium, and barium. Vanadium can give very fast curing RT systems with ketone peroxide catalysts but imparts a pronounced yellow color to the cured resin with the

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additional drawback of shelf life problems with the promoted resin unless special stabilization methods are used. Manganese is primarily useful with cumene hydroperoxide which is attractive as a lower cost catalyst. Calcium and barium have been cited as being able to speed up the cure of promoted systems employing very low cobalt concentrations to minimize cured resin color. An example of a cobalt single promoted RT curing resin is shown in the following Table 4- 15.

Table 4-15: Single Promoted Polyester Resin Formulation Parts by Weight

Polyesterresin containing 40% styrene Cobalt octoate 12% (containing 12% cobalt as metal)

100 0.25

On catalyzation with 1 PHR of MEK peroxide catalyst (Lupersol DDM-9, a trademark ofLucid Pennwalt Corp.) reactivity at 77°F (25°C) would be: 50 gram resin sample Gel time* Peak temperature interval* * Peak exotherm temperature***

14-20 min. 2530 min. 240”-270”F(116”-132°C)

*Time from catalyst addition to gelation. **Time from gelation to maximum temperature in the curing resin mass. ***Maximum temperature reached in the resin mass.

Many commercial resins are double promoted in that they contain not only metallic primary promoters but also secondary promoters which can: 1. 2.

Speed up the cure after gelation. Help stabilize the gel time drift on aging that often occurs with single cobalt promoted resins.


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Although cobalt compounds are the major primary promoters, there are many different types of secondary promoters in use in commercial RT curing resins. Some common secondary promoters are shown below in Table 4- 16.

Table 4 -16 (Secondary Promoters) Tertiary Amines Diethylaniline Dimethylaniline Dimethylethanolamine Phenyldiethanolamine Dimethyl-p-toluidine Acetamides N,N-dimethylacetoacetamide mono-N-methylacetamide Acetoacetates Methyl acetoacetate Ethyl acetoacetate Quatemary Salts Benzylttimethylammonium chloride C,, - C,, quatemary ammonium chlorides Triphenylsulfonium chloride

As an example, conversion of the single promoted resin shown earlier to a double promoted system could be accomplished by the addition of 0.25 PHR of diethylaniline. The reactivity characteristics using the same catalyzation are shown in Table 4- 17 below.

Table 4-17: Double Promoted Polyester Reactivity Gel time Peak temperature interval Peak exotherm temperature

S- 12 min. 8-12 min. 290”-320°F (143”-160°C)

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Benzoyl Peroxide Catalyzed RT Cures: Fast RT curing systems can also be obtained by the use of aromatic tertiary amines such as diethylaniline and dimethylaniline as promoters and an acyl peroxide such as benzoyl peroxide for the catalyst. These systems are most often used by the user of unpromoted resins who will very oflen employ them in a “2 pot method” where half of the resin is promoted with the amine promoter and the other half of the resin is catalyzed with the benzoyl peroxide. Equal parts of promoted and catalyzed resin portions are then mixed at point of use. This approach suffers from limited pot life of the catalyzed resin portion. It has the advantage that it is much less sensitive to inhibition by moisture than the cobalt promoted ketone peroxide systems. Commercial Prepromoted Resins: Most commercially available resins prepromoted for RT cure employ cobalt-based single and double promoter systems designed for use with ketone peroxides. Resins of this type can be easily catalyzed by the addition of liquid ketone peroxide either by batchwise addition and mixing or by the convenient catalyst injection equipment available for spray-layup, casting and laminating equipment. Catalysts for RT Cobalt Promoted Resins: Methyl ethyl ketone peroxide is probably the most widely used catalyst for the room temperature curing of prepromoted polyester and vinyl ester resins. This peroxide catalyst is not a pure compound and may contain varying ratios of the peroxide and its dimer as well as hydrogen peroxide and free water. Pentanedione peroxide catalyst is sometimes used in conjunction with MEK peroxide catalyst or alone when faster room temperature curing systems are necessary. Many varieties of MEK peroxide catalysts are commercially available as well as related catalyst compounds. Vinyl ester resins are more commonly sensitive to the per-oxide dimer content than polyesters. The cobalt promoted resins are sufficiently stable for adequate package life and with proper compounding can have minimal variation in curing behavior as they age after manufacture. Heat Curing Systems: Heat curing systems are used in processes such as matched die molding, building panel lamination, press molding, and pultrusion. Organic peroxides having higher decomposition temperatures, such as t-butyl peroctoate, benzoyl peroxide, and t-butyl perbenzoate, are used. Handling Catalysts and Promoters: Catalysts andpromoters must never be allowed to directly contact each other because a violent reaction


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can occur which may result in five, explosion, or injury. Promoters, if used, must be thoroughly mixed into the resin before the addition of any catalyst. Catalysts andpromoters should be stored and handled in a manner which will eliminate the possibility of accidental contact. Good housekeeping is especially important with catalysts andpromoters. Catalyst suppliers can supply excellent instructions on this subject.

Ultraviolet Absorbers

Cured unsaturated polyester resins are quite susceptible to degradative attack by the ultraviolet radiant energy in daylight and sunlight. Such degradation results in discoloration of the resin to a yellow and in severe cases a brownish coloration. Some halogenated unsaturated polyester resins such as those based on tetrachlorophthalic and tetrabromophthalic anhydrides are more W sensitive than nonhalogenated types. Ultraviolet light absorbers can be added to unsaturated polyester resins to greatly improve the resistance to W degradation. Types of UV Absorbers: W absorbers most commonly used in unsaturated polyester resins fall into two classes as shown in Table 4- 18. W absorbers are typically used in polyester resins at levels ranging from 0.1 to 1.0 PHR. The higher levels would be typical in the more W susceptible resin systems such as the halogenated types.

Table 4-18: Ultraviolet Absorbers Chemical Type

Substituted benzophenones Substituted benzotriazoles

Commercial

Examples

Cyasorb UV-9 (American Cyanamid) Uvinol400 (Ciba-Geigy) Tinuvin P (Ciba-Geigy) Tinuvin 327 Tinuvin 328 Cyasorb UV-54 11

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ThixotropidFlow

Control Agents

In some processes in which polyester resins are used, such as open mold hand layup, spray layup, or gel coating, it is essential that the resin does not drain down from inclined or vertical surfaces before gelation takes place. The desired behavior is much like that in a high grade drip-resistant paint which will exhibit minimal downward flow or sagging on application to a vertical surface. Thixotropic agents can be dispersed in polyester resins to develop the necessary thixotropic or psuedoplastic rheology. With ideal thixotropic properties, the polyester resin would have a storage viscosity high enough to keep the thixotropic agent in perfect suspension and yet become a relatively low viscosity liquid on the application of liquid shearing forces so it can be readily handled, sprayed, and impregnated into reinforcements. The thixotropic “false” viscosity should then quickly redevelop to prevent drainage from a coating or from reinforcement. Not surprisingly, real life thixotropic resin systems approach but do not completely achieve such ideal behavior. Thixotropic agents commonly impart these characteristics by the formation of shear labile hydrogen bonds between their own particles to form microchains between their particles and the resin. Being shear labile, the thixotropic viscosity generating three dimensional structure that is formed is readily broken on exposure to liquid shearing forces such as occur on mixing, spraying, or impregnating reinforcements. The hydrogen bonds quickly assert themselves on removal of shear with regeneration of the higher thixotropic viscosity. Examples of these thixotropic agents are shown below in Table 419.

Table 4-19: Thixotropic

Type Pyrogenic silicas Precipitated silicas Modified bentonite clays Hydrogenated castor oil

Agents

Typical Commercial Examples Aerosil200 (Degussa Corp.) Cabosil PTG (Cabot Corp.) Sylox TX (Davison Chemical Division) Claytone PS (Southern Clay Products Co.) Thixin E (Baker Castor Oil Co.)


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Excellent dispersion of the thixotropic agent is usually necessary to obtain optimum thixotropy and maintenance of this property as the resin ages. Thixotropic agent levels of 0.4 to 1.OPHR are commonly used in laminating resins while gel coats will generally require higher levels such as 1.5 to 2.0 PI-N. Small (0.1 to 0.3 PHR) additions of polar hydroxylic additives such as simple glycols, glycerin, or surfactants such as Tween 20 (Atlas Chemical Industries) are often used in conjunction with silica thixotropic agents to obtain enhanced thixotropic behavior. Thixotropic resin viscosities are commonly measured using rotational multispeed viscometers such as the Brookfield Synchroelectric Viscometer.[“] Viscosities are usually measured at a higher and lower rotational speed such as 50 and 5 RPM and 20 and 2 RPM. A thixotropic resin will give a higher viscosity reading at the lower RPM than at the higher RPM. The low RPM viscosity reading divided by the high RPM viscosity reading is called the Thixotropic Index.

Fillers Liquid polyester resins can accept relatively large filler loadings. Most of the fillers used are inorganic and most are of mineral origin. The addition of mineral fillers to polyester resins usually produces the following effects. 1. Viscosity of the liquid resin system is increased. 2. Curing shrinkage is decreased. 3. Peak exotherm on curing is decreased since the filler acts as a heat sink. 4. Specific gravity of the resin-filler mixture is higher than pure resin if a filler denser than resin is used. 5. Cured hardness of the resin-filler mixture is increased (except with cellular fillers). 7. Tensile elongation is decreased. 8. Impact strengths are decreased. 9. Curing behavior may be affected. Gel time may be accelerated or retarded. 10. Raw material cost of the tiller-resin composite is usually lower on a weight or volume basis than the pure cured resin.

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Specialized low density fillers can also be used to reduce the specific gravity of the resin-filler composite. Hollow, very small diameter, glass or ceramic microballoons may be used for this purpose. Plastic microballoons made from polyvinylidene chloride, epoxy, or phenolic resin have also been used for achieving lower density composites. Some Common Filler Applications in Resins: Common filler applications are shown in Table 4-20.

Table 4-20: Filler Applications Typical Applications Calcium carbonates

Clays Talcs Alumina trihydrate

Dolomites Glass and ceramic microballoons Phenolic microballoons Glass microspheres Nepheline syenite Silica sand

Synthetic marble, SMC, BMC, matched die molding, building panels, autobody putty, mine bolt grouts Molding compounds, SMC, BMC Autobody putty, gel coats Synthetic onyx, flame retardant SMC, BMC and molding compounds, flame retardant construction composites SMC, BMC, matched die molding Synthetic marble, bowling ball cores, low density SMC, deep submergence vehicles Deep submergence vehicles SMC, gel coats Building panels Polymer concrete

Filler Dispersion and Mixing Equipment: In compounding polyester resins with fillers care must be taken to completely wet out the fillers with resin. Adequate mixing equipment must be used. Medium shear mixers of the Cowles or Myers type are effective for medium to high filler loading mixing. Dough mixers of the Hobart type have also been used, especially in synthetic marble and onyx manufacture.


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High tiller content mixes which must also incorporate reinforcing fibers such as BMC compounds are most commonly made using double arm sigma blade mixers. Hollow fillers such as inorganic and organic microballoons require special care in mixing because the relatively thin walls of the microballoons can be fractured by mixing action that is too vigorous. Microballoon types will vary in their sensitivity in this respect. Suppliers and their literature should be carefully. consulted for mixing recommendations and cautions. Order of Mixing and Dispersion: The order of addition of ingredients in tilled compositions will vary somewhat with the application. With heat curing systems where a long catalyzed pot life can be expected, the polyester resin is often first mixed with the catalyst. Typical of these are tilled systems for: 1. 2. 3.

SMC BMC Filled systems for matched die molding with glass mat and glass fiber preforms.

On thorough dispersion of the catalyst in the resin, the rest of the additives such as internal mold release, wetting/bubble release agents, pigments, etc., are added. A variant on this sequence can occur with shrink controlled SMC and BMC where the shrink control/low profile agent may precede the catalyst. On mixing of the preceding ingredients, the filler is added slowly and thoroughly dispersed. Propeller-type mixers can be used for low filler content systems such as those for mat and preform molding where 40 or so PHR of filler is used. SMC mixes, which are termed pastes and on the average contain 150 PHR filler, require more powerful mixers of the Cowles or Myers type. Calcium Carbonate Fillers: Calcium carbonate fillers of either the ground limestone type or the precipitated chalk type are the most widely used fillers for the preparation of filled compositions for mat and preform molding and SMC and BMC. The calcium carbonate fillers are low in cost and generally have low oil absorption numbers which allow considerable freedom in filler loading levels while keeping the viscosity of the filled system at a usable level. Calcium carbonate fillers have minimal effect on the curing properties of the filled systems.

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One of the largest applications for calcium carbonate fillers is in the manufacture of synthetic or, as the industry prefers to call it, cultured marble. The carbonate filler levels used here are quite high, ranging from 300 to 400 PHR. Relatively coarse particle size fillers ranging from 30 to 100 mesh are used to achieve such high filler loading levels. Two compounding sequences are common in the use of calcium carbonate fillers in marble. In the first sequence, resin is catalyzed and then mixed with the filler. The common production methods depend on room temperature curing systems so pot life on catalyst addition is very short. A second sequence, called the “master batch” method, mixes the filler with the resin first in larger batch amounts than with the first sequence. Portions of this “master mix” are then catalyzed just before use as required. Clay Type Fillers: Clay type fillers are also used even though they generally have higher oil absorption numbers and tend to increase mix viscosity more than the calcium carbonates at equivalent filler loading levels. The incorporation of clay fillers can give good electrical properties, enhance flow in molding, and being more chemically inert than carbonates enhance chemical resistance. Clays also find use in pigmented gel coats. Talc Fillers: Talc filers generally have high oil absorption numbers and consequently give high viscosities on addition to resin. Auto body putties used for auto body repair commonly contain some talc filler. Talcs are also used in pigmented gel coats. Alumina Trihydrate Fillers (ATH): Among the earliest applications for ATH in polyesters was the improvement of electrical tracking resistance and flame retardancy.[361 Currently, ATH is used largely for imparting flame retardance to polyester composites and, because its refractive index approaches that of cured polyester resin, for the manufacture of synthetic/cultured onyx. The similarity between the refractive indices of ATH and the cured polyester resins permits the manufacture of ATH tilled cultured onyx composites which have the depth of translucence of natural onyx. Special resins which on cure give almost colorless castings have been developed for the manufacture of cultured onyx. The flame retardance conferred by ATH is due to the energy absorption that occurs as three water molecules are liberated from each molecule of ATH at combustion temperatures. By the use of 100 PHR of ATH GP laminating resins can give glass fiber reinforced laminates which can yield moderate


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flame retardant properties. ATH can enhance the flame retardance of flame retardant halogenated resin systems to give more cost effective systems.[37401 Development of flame retardance by the use of ATH can also give lower smoke densities than flame retardant systems which depend on the use of halogenated resins a10ne.[4’1 SMC and BMC employ filler loading levels of from 150 to over 200 parts of filler per 100 parts of resin. Partial or complete replacement of these tillers with ATH can give such molding compounds good flame retardance without the use of halogenated flame retardants. Zaske, Wang, and Wuh investigated the effects of different ATH fillers on the thickening of SMC paste and came to the conclusion that the major factors affecting thickening were the particle size distribution of the ATH and the water content in the paste.r421 Pigments and Colorants: Very often pigments and colors are added to polyester resins in the form of dispersions rather than as pure dry colors. The use of dispersions is convenient and permits the use of low shear mixing equipment such as propeller mixers with the assurance that the color will be excellently dispersed in the resin mix. Pigment dispersions are prepared by suppliers from pure colors and special pigment grinding vehicles using high shear dispersion equipment such as three-roll mills and sandmills. Early pigment dispersions employed low molecular weight saturated polyesters as vehicles. The pigment grinding vehicles used in modem dispersions are often special liquid unsaturated polyesters which contain no styrene or other diluent monomer and which being unsaturated, can crosslink with styrenated polyester during the curing process. Inorganic pigments such as titanium oxides and organic pigments such as the phthalocyanine pigments are used as well as dyes. Care must be taken in using colorants in polyester resins to ensure that the pigments or dyes will not interfere with proper cure of the resin. With peroxide catalyzed systems, the colorants must also be able to resist the oxidizing action of the peroxide.

Thickening Agents

Thickening agents are an integral part of SMC and BMC molding technology. The earliest and still most widely used thickening system employs

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Group II metal oxides and hydroxides to thicken reinforced polyester molding compounds so that the compounds are essentially tack-free for handling purposes and will also have the rheology necessary for high quality molding. Sheet molding compound, when properly made, has been likened to leather in its uncured state. The most commonly used metal oxides and hydroxides are those of magnesium and calcium. Early workers in the field explored this type of thickening extensively and also developed an extensive patent literature.[43-501 Within recent years, nonmetal oxide/hydroxide SMC thickening systems have been developed.~‘1 Thickening in these systems depends on a urethane reaction which by increasing the molecular weight of the polyester resin in the SMC produces the desired thickening of the molding compound. Later work focused on the influence of other factors on the thickening reaction and approaches for better thickening control in manufacturing.t52‘581

Fiber Reinforcements

Unreinforced polyesters in cast form have, like many pure cast thermoset resins, only moderate physical properties. Excellent physical properties including good toughness and impact qualities can be obtained by the use of high strength reinforcing fibers to make polyester composites. Glass fibers in the form of woven cloths, chopped glass mats, multifilament forms such as rovings and chopped rovings are the most common reinforcement used in polyester composites. Sisal, polyester, polyamide, and polyaramid fibers are also used in the form of chopped fibers, filaments, and woven reinforcements.

APPLICABLE

MANUFACTURING

PROCESSES

Overview

Most of the manufacturing processes employing unsaturated polyesters and vinyl esters depend on the liquid nature of these resins, the viscos-


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ities of which can be readily varied by increasing or decreasing the content of a monomer such as styrene and by changing the molecular weight of the unsaturated polyester alkyd. The viscosity changes as styrene content is varied is shown for a GP orthophthalic resin and a high molecular weight isophthalic resin in Figure 4- 1.

Hand Layup

Hand layup was one of the first ways in which polyester resins were used. The process is very simple and consists of impregnating fiber reinforcement such as fiber glass mat or woven cloth with catalyzed resin in a female mold. Ditferent types of reinforcement forms such as woven fiber glass roving ,

--

JM

Figure 4-1: Viscosity Vs styrene content for ortho- and isopolyesters.

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and knitted fabric, may also be used alone or in conjunction with each other. This process usually employs room temperature curing resins which are prepromoted and require only the addition of catalyst, usually a ketone peroxide such as methyl ethyl ketone peroxide. This process gives a high grade surface on the mold side. Most often, after application of mold release to the mold surface, a pigmented gel coat is applied to the mold and allowed to gel and partially cure before the lamination with reinforcement and resin. The room temperature gel coats are usually applied by spray equipment using either small batches of catalyzed gel coat or by the use of catalyst injector guns that inject and blend the requisite amount of catalyst into the gel coat as it is sprayed on the mold. 15 to 20 mil thick (cured) gel coats are common. The hand layup process gives molded parts with only one good surface. Precise thickness control in the molding is also difticult to achieve. Resins for hand layup are usually thixotropic to minimize resin drainage Corn vertical sections of the layup. Room temperature curing systems are ahnost always used. All types of styrene containing polyesters can be compounded for hand layup work. Replacement of part of the styrene with methyl methacrylate to enhance weathering or parametbyl styrene to reduce monomer emissions is also possible. Styrene containing vinyl ester resins can also be promoted and compounded for use in room temperature hand layup molding. Hand layup is often used with vinyl esters because the large pieces of chemically resistant equipment for which they are largely intended are most easily fabricated by this molding method.

Spray Layup

Spray layup, a later development than hand syrup, became possible with the development of catalyst injection spray equipment and the fitting of small roving choppers on the spray guns themselves. These developments enabled the simultaneous application of catalyzed resin and chopped fiber glass roving to molds. The chopped reinforcement is partially wet out during


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the spraying. Wet out and air removal is completed by “rolling out” the layup with special hand rollers. As with hand layup, room temperature curing resins are used together with pigmented gel coats. Similarly, the molding has only one good surface and precise control of thickness in the mold is difficult.

Resin Transfer Molding (RTM)

RTM is a modern version of an old molding method called the “Marco Method” developed by I. Muskat (Marco Chemicals) around 1950. The method employs a male and a female mold. The reinforcement being used is placed between the two molds. A gel coat may be used on either one or both mold halves. After clamping the mold together with the provision of a tight seal around the mold periphery, catalyzed resin is forced into the mold by a pump until resin is seen exiting from a transparent tube placed on a vent located at the highest point in the mold. The resin “transfer” and air removal is assisted in some operations by attaching a vacuum source to the vent line. The process generally employs room temperature curing resins. Both polyesters and vinyl esters can be used. Considerably stronger mold construction is required for RTM than for hand or spray layup because pressure and perhaps vacuum are used to force the resin into the mold and through the reinforcement. Low viscosity resins are necessary to keep the pressure requirements moderate and to facilitate wetting out the reinforcement. Resins having low to medium exotherm temperatures are preferred since the curing polyester composite is totally enclosed and insulated by the mold halves which are often resin composites themselves. Excessive exotherm can cause mold damage or reduce useful mold life. Figure 4-2 shows electrostatic precipitator collector plates made by the RTM process using a vinyl ester resin reinforced with continuous strand mat and unidirectional roving over a balsa wood core and surfaced with polyester fiber veil. These collector plates are continuously exposed to wet high voltage service.

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Figure 4-2: RTM molded electrostatic collector plates. Manufacturer: FluidIonic Systems, Dresser Industries. Molder: Sundt Products. Photo courtesy of Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, New York, NY.

Water Extended Polyester (WEP)

Water-in-oil unsaturated polyester emulsions on cure can result in cellular polyester composites in which all the cells are filled with water.t*‘] WEP resins oflen will have hydrophilic groups in the architecture of the alkyd backbone and employ emulsifying agents that will act in conjunction with this chemistry to allow the resin to form emulsions with water in which the resin is the continuous phase and the water is the discontinuous phase.


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Cured polyester resins are not completely impermeable to water so WEP composites will have a tendency to lose water on aging which can cause shrinkage of the composite unless special stabilizers in the water such as hygroscopic agents or water soluble polymers are used. Special WEPs have been formulated so as to more readily lose water as a route to the production of reduced density polyester foams. This approach has fallen from favor as a method for making low density polyester composites since the cost of driving out the water has escalated with the rising cost of energy. WEP is limited in strength since the water, unlike a solid filler, contributes nothing toward strength or rigidity. WEP can be a serviceable composite for articles such as flower pots and lamp bases where strength and dimensional stability are not especially critical.

Casting

Liquid polyester resins lend themselves very well to casting operations. Some typical applications include giftware and art objects, cultured marble and onyx, polymer concrete, and monolithic flooring. Giftware and art objects are usually made using special polyester resins that can cure to a clear colorless casting. This type of resin is made using highly proprietary formulations and methods. Methyl ethyl ketone peroxide catalyst is usually employed in a room temperature cure. The resin can be cast uncolored or can be colored by the use of peroxide resistant dyes or pigments. Glass, polypropylene, metal, and other materials have been used for molds. Elastomeric molds can permit the casting of artifacts with negative draft and undercuts. Silicone rubber has been the most popular elastomer system used for this purpose. Very large art pieces, weighing hundreds of pounds, that have achieved international recognition have been cast by artists such as Dwayne Valentine of Venice, California. Figure 4-3 illustrates a smaller art piece executed in clear resin by Caryl Craig. Cultured marble and onyx, from a beginning in the early 196Os, have grown to be one of the major applications for unsaturated polyesters. These synthetic stone products are cast into either flat stock or complete bathroom items such as full size bathtubs and unilavs (bathroom counter top complete

148


with one or mom washbowls). The flat stock can be also used for tops and for wall applications such as tub surrounds. The pigmentation and appearance duplicate the appearance of the finest onyx and marble with the advantage of greater durability with rcspcct to such properties as stain resistance. Figures 4-4,4-5, and 4-6 illustrate three applications of cultured onyx sanitaryware: a unilav, a luxury tub installation, and a toilet and bidet. The polyester resins which dominate this application are carefully compounded to be compatible with the mineral fillers employed to give color and appearance which, with proper pigmentation, can faithfully reproduce the appearance of the natural stones. The resins must also give composites which will give the very long term service durability required of sanitaryware.

Figure 4-3: Clear unsaturated polyester art casting. Artist: Caryl Craig. Photo courtesy of Silmar Div. Sohio Engineered Materials Co.


149

Figure 4-4: Polyester cultured onyx cast one piece lavatory top (unilav). Photo courtesy of Gruber Systems, Inc., Valencia, CA. m .- _.‘F! /

Figure 4-5: Polyester cultured onyx cast bathtub and tub surround. Photo courtesy of Gruber Systems, Inc., Valencia, CA.

150


Figure 4-6: Polyester cultured onyx commode and bidet. Photo courtesy of: Gruber Systems, Inc., Valencia, CA.

Calcium carbonate, in the form of ground limestone, is used to make cultured marble. 30 and 80 mesh are popular grades and are usually used as a mixture of two mesh sizes. A typical resin-filler mix for cultured marble is shown in the following Table 4-2 1.

Table 4-21: Cultured Marble Matrix Formulation Pounds Polyester marble resin 30 mesh calcium carbonate filler 80 mesh calcium carbonate filler Methyl ethyl ketone peroxide catalyst Titanium dioxide white pigment ground color to suit Other colors to simulate marble veining to suit

100 200 100 0.4-0.8


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A common compounding sequence will mix all of the ingredients except the veining colors. The mix, usually called the “matrix,” can then have the veining pigments only partially mixed into the matrix to give, on casting and cure, the pigmented veining typical of natural marble. Large rotary mixers such as Hobart dough mixers have been used for this purpose. Other scraped wall type mixers of the Myers and Cowles types have also been successfully used. Recently, continuous matrix mixing machines have been introduced for cultured marble and onyx production. Two machines of this type, the German Respecta machine and the American Venus machine, have been installed in some domestic marble/onyx production plants recently. A feature common to both machines is the ability to heat the matrix above room temperature before discharge into the molds. Cultured onyx production is very similar to the production of cultured marble. The major differences are that it is necessary to use resins that can cure without the development of any color and that the refractive indices of the f&r systems are close to those of the cured resins. The most popular filler for onyx production is alumina trihydrate followed by powdered glass fiits. Alumina trihydrate filler is sometimes used in conjunction with glass frit fillers. A typical onyx mix is shown in Table 4-22.

Table 4-22: Cultured Onyx Matrix Formulation

Polyesteronyx resin Alumina trihydrate filler Methyl ethyl ketone peroxide catalyst

Pounds 100 200 l-2

Titania ground color to suit Veining colors to suit

Cultured marble and onyx mixes are relatively high filled systems which must be pourable with adequate entrapped air release and correct room temperature curing characteristics. The high filler levels can exaggerate the reactivity differences of various catalysts and the effects of resin, filler, and shop temperatures on the curing behavior. A number of papers and technical presentations exploring these effects have been presented by workers such as

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Hrouda, Husain, and Zaske at the Annual and Regional Meetings of the Culturul Marble In.stit~te.[~‘~~ Polymer concrete,a more recentdevelopment,fti found favor in Europe. With the correcttillers suchas silica sandand chemicallyresistant resins, chemically m&ant mortars and aggregatefilled polymer concrete mixcaem beformulated.Continuousmixers suchasthe Respcctaequipment havebeensuccessfullyusedin this applicationalthoughbatchmining equipmentcanalsobeused. Vinyl esterresinscanbe usedfor maximum chemical resistancein thoseenvironmentsnot suitablefor polyesters. Polyesterandvinyl esterresinshavebeenvery successfullyapplied iu cartmooolithicflooring. This typeof flooringgivesa seamless,wearresistantfloor whichcanhaveconsiderablechemicalresistanceand which is easily cksaned.A monolithic castlaboratoryfloor is shownin Figure4-7.

Figure 4-17:Cast monolithic polyesterlaboratoryfloor. Photo courtesyof SihnarDiv. Sohio EngineeredMaterials Co., Hawthorne,CA.


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Acrylic Backup

Fiber glass laminate backup of vacuum formed acrylic articles such as spas and sanitaryware has become a growing application for unsaturated polyesters. The vacuum formed acrylic shape in effect becomes the service surface of the molded artifact and also serves as the mold for the application of the backup of polyester and chopped glass fiber which is usually applied by the chopper gun spray layup method. Development of good adhesion between the acrylic and the polyester fiber glass backup laminate is crucial to the success of this fabrication technique. A number of papers presented by A.H. Horner et al before the Annual Conferences of the SPI Reinforced Plastics Composites Institute have explored the performance, economics, and recent developments in acrylic faced composites. [61431Thixotropic resins, prepromoted for room temperature curing, which are specially formulated to maximize adhesion, are usually used.

Matched Die Mat, Preform and Premix Molding

Both polyester and vinyl ester resins can be used in these processes which all employ matched male and female dies, usually made of metal, mounted in hydraulic molding presses. Heat cures are normally employed using such catalysts as benzoyl peroxide, tertiary butyl peroctoate and tertiary butyl perbenzoate. In production, semipositive telescoping dies are often used with flame hardened shear edges. Molding is usually to die stops. Molding temperatures can range from 225 o to 250°F for mat and preform to 275 o to 300 “F for premix. Molding pressures are relatively low in the range of 100 to 300 psi. Press cycles can range from 2 minutes or so to longer periods depending on the thickness of the part, the mold materials, and the size of the part being molded. Prototype molding has often been successfully carried out using inexpensive methods such as Kirksite dies. These, however, are not suitable for sustained production. Mat molding, as the term implies, employs fiber glass mat for the reinforcement. Mat reinforcement, being planar, is limited to flat parts, parts

154

Handbook of ntermoset Plastics

with curvature in only one plane, and parts which are essentially flat with only limited compound curved areas such as cafeteria trays. Preform molding gets around the limitations of mat by the use of fiber glass preforms in which chopped glass fibers have been formed into a mat having the shape of the part to be molded. This process is applicable to small parts and also very large parts such as outboard motor boat hulls and auto body parts. Both mat and preform molding employ orthophthalic and isophthalic polyesters and, to a more limited degree, vinyl ester resins. The resin molding compositions are often filled with mineral tillers such as calcium carbonates at levels of 30 to 65 PHR. Reinforcement levels are often at 35 to 40% by weight in the molded composite. Premix molding is based on the use of a bulk molding compound made from liquid resin, filler and chopped reinforcement all combined in suitable mixers which are designed, as much as possible, to not fracture and thereby degrade the reinforcing fibers. Due to the limitations of the mixing processes, fiber lengths are usually limited to lengths shorter than can be used in mat or preform molding. For the same reasons, reinforcing fiber levels are usually lower in premix compounds. The shorter fiber lengths and lower levels of reinforcing fiber limit the physical properties of premix moldings to lower levels than can be obtained by either mat or preform molding. Resins intended for these molding processes must have good hot strength to permit part removal from the dies without damage. The resins must also permit complete flow to the mold without pregelation and must then quickly cure for fast molding cycles. The overall combination of characteristics of isophthalic polyesters have made them favorites for press molding with mat, preform, or premix.

Pultrusion

Both polyester and vinyl ester resins are successfully used in pultrusion operations. Pultruded composites will typically contain high reinforcement levels and are made by continuously pulling reinforcement through a resin impregnating bath and then through one or more heated dies the interior of which have the shape desired in the cured composite. The pultrusion pro-


155

cessplaces several concummt demands on resins. The resins must quickly and very thoroughly wet the reinforcement, even when containing tiller, and must very rapidly develop a high enough green strength at die temperatures to avoid excessive pulling loads and to prevent surface cosmetic defects commonly called “sloughing.” Rapid curing, high heat deflection point resins are essential for the achievement of satisfactory pulling speeds. Isophthalic polyesters are probably the most widely used resins for pultrusion. Pultruded composites, due to their generally high reinforcement levels, usually have exceptionally high physical properties parallel to the direction of pultrusion. Physical properties in a direction normal to the direction of pultrusion are usually lower. Figures 4-8,4-9, and 4-10 illustrate a number of pultruded composite applications. Figure 4-8 shows wind turbine blades pultruded using a vinyl ester resin reinforced with continuous strand glass mat and rovings. The blades are 9 inches wide x 11 feet 3 inches long.

Figure 4-S: Pultrudedvinyl ester wind turbine blades. Manufacturer: Bergey Windpower Co. Molder: Morrison Molded Fiberglass Co. Photo courtesy of Reinforced Plastics/Composite Institute, The Society of the Plastics Industry, Inc., New York, NY.

156


Figure 4-9 illustrates an interesting application of pultrusion in molding window sash and frame components. These were molded using non yellowing light stabilized polyester resin reinforced with glass fiber roving and continuous strand mat at a 50% reinforcement level.

Figure 4-9: F?&mded polyester window sash and frame components. Manufacturer and molder: Omniglas Corp. Photo courtesy of Reinforced Plastics Composites Institute, The Society of the Plastics Industry, New York, NY.

Figures 4-10 and 4-11 picture an unusual pultrusion application in the manufacture of light weight, flexible poles and spreaders for the support of camouflage netting. Made for the U.S. Army, these composites are made using unsaturated polyester resin with glass fiber roving and mat reinforcement. An important plus for the composite is nondetectability by radar.


157

Figure 4-10: Pultruded polyester military camouflage netting support. User: U.S. Army Troop Support Command. Molder: The Pultrusion Corp. Photo courtesy of Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, New York, NY.

Figure 4-11: CamoufIage netting supported by pultruded polyester support system. User: U.S. Army Troop Command. Molder: The Pultrusion Corp. Photo courtesy of Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, New York, NY.

158


Sheet and Bulk Molding Compounds (SMC and BMC)

Polyester or vinyl ester resins can be used to make SMC. Proper thickening with the usual thickening agents is an essential resin characteristic if the SMC is to be ready to mold, i.e., mature, in a reasonable time and to remain moldable long enough that it can be all used up under normal production schedules. Long moldability is especially important in SMC that is intended for use in remotely located satellite plants or SMC intended for outside sales. As previously mentioned, the most widely used thickening systems depend on the reaction of Group II metal oxides and hydroxides with carboxylic functionality on the resin. Carboxylic functionality on the shrink control or “low profile” resin used with the polyester or vinyl ester can also contribute to the thickening reaction. The thickening reaction results in the formation of a certain amount of chain extended high molecular weight fractions which can then greatly increase the viscosity of the SMC by entanglement and hydrogen bonding. For resins to be satisfactory for SMC, they must also develop suflicient incompatibility with the shrink control additive resin on gelation and cure. This is necessary so that the shrink control additive properly precipitates as a second, monomer solvated, phase. The resultant expansion during cure can offset the curing shrinkage of the polyester or vinyl ester resin. A good SMC resin should also have good hot strength to minimize damage to the molded part on ejection and removal from the die. Molding temperatures for SMC generally range from 275” to 300°F (135” to 149°C). SMC is commonly made on special continuous machines designed for this purpose. Usually an SMC paste made from the molding resin, low profile resin, filler, mold release, catalyst, thickener, and pigment is fed onto a continuous web of carrier film. Chopped fiber glass roving, often one inch in length, is dropped into the paste layer as it passes the glass chopping station on the machine. The film carrying the paste layer and the chopped glass is then continuously combined with a top carrier film also having a layer of SMC paste. The SMC, now encased between two layers of film, on passing between a number of rolls on the SMC machine is kneaded and squeezed to complete the impregnation of the chopped glass by the SMC paste. A take-off stand at the end of the machine rolls up the SMC which is then taken off the machine in rolls of convenient size for transfer to a “maturing” area where the rolls are


159

stored until moldable. SMC made on equipment of this type is generally made in weights of 12 to 24 ounces per square foot. Heavier weight SMC is often more readily made on “TMC” or thick molding compound machines which are of different design than ordinary SMC machines. Highly unsaturated isophthalic and propylene glycol maleate polyester resins are probably the most often used resins for SMC molding. A typical low shrink SMC formulation employing a high reactivity isophthalic polyester resin is shown in Table 4-23. The SMC paste is then combined on an SMC machine with 1 inch length chopped roving to a 25 to 30% glass content in the finished SMC.

Table 4-23: Isophthalic Resin SMC Formulation SMC Paste lsophthalic polyester Magnesium oxide thickener Calcium carbonate filler* Low profile additive** Zinc stearate mold release Tertiary butyl peroctoate catalyst Tertiary butyl perbenzoate catalyst Pigment dispersion: to suit

Pounds 100 1 150 54 3 0.5 0.6

*Camelwite (a trademark of Harry T. Campbell and Sons), Snowflake (a trademark of Thompson, Weimnan and Co.). **LP-40A (a trademark of Union Carbide Corp.).

SMC is widely used now for molding automotive body parts such as grille opening panels, truck lids and hoods. Business machine housings are another growing application as are sections from which satellite reception TV antennas are assembled. There are a diversity of other applications such as for swimming pool filter tanks, seating and snowmobile body parts.

160

Handbook of Xhermoset Plastics

Bulk Molding Compound (BMC)

BMC can be consideredto be a premix compound which incorporates shrink control additive polymer and which may also contain some thickener. As with premix, filler levels are higher and reinforcing fiber lengths and level are lower than those used in mat or preform molding and also SMC. BMC finds large application in items such as electrical parts and housings which have complicated features such as ribs and bosses and may incorporate inserts. Other applications include dinnerware and housings for small tools and appliances. Some applications of BMC are shown in Figures 4- 12 and 4- 13. Figure 4-12 shows disposable frozen dinner plates made by compression molding using a glass fiber reinforced BMC formulated for microwave applications.

Figure 4-12: Polyester BMC disposable dinner plates. User: Campbell Soup Co. Molder: Premix, Inc. Photo courtesy of Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc., New York, NY.


161

Figure 4-13 shows sewing machine structure and parts molded by injection molding using a flame retardant fiber glass reinforced polyester BMC. High stifl%essand dimensional stability together with the flame retardance required to meet the requirements of Underwriters’ Laboratories are important features of these parts.

Figure 4-13: Polyester BMC sewing machine components. Manufacturer:

The Singer Co. Molder: The Singer Co. and The Glastic Co. Photo courtesy of: Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc., New York, NY.

162


RECENT DEVELOPMENTS

Foamed Polyester

The preparation of cured foamed polyesters has been attempted, generally with indifferent success, since the resins have been available commercially. A recently developed foaming catalyst, Luperfoam 329 (a trademark of the Lucidol-Pennwalt Corp.), shows promise for the preparation of medium to high density polyester foams. Foams as low as 15 pounds per cubic foot have been claimed with the use of this foaming system.[641 Luperhoam 329 is an aqueous solution containing tertiary butylhydrazinium chloride and ferric chloride. This foaming catalyst is used at 1.25 to 1.5 PHR levels together with methyl ethyl ketone peroxide catalyst at a minimal 3 PHR level. It is reported that it is necessary to have a cobalt promoter in the polyester. The presence of even trace amounts of copper in the resin is deemed to be detrimental. 1.0 to 1.5 PHR of surfactants such as DC-193 (Dow Corning Corp.) or LK-22 1 (Air Products Corp.) are recommended to control cell size and promote a smooth foam rise. Auxiliary fluorocarbon blowing agents can be used to attain lower density foams.

Urethane Hybrid Resins

The preparation of urethane hybrid resins from unsaturated isophthalit polyester polyols and diisocyanates has been described by H.R. Edwards of the Amoco Chemicals Corp. The same worker has also described the use of such hybrids in casting compounds, RIM (Reaction Injection Molding) and SMC. Superior processing and physical properties were claimed in many instances.r6s-661

Reduced Styrene Emission Resins

Increased regulatory attention to styrene monomer levels in the workplace and pressures for reduction of the allowable levels have prompted


-

-!

-

Table 4-24: Physical Properties Unsaturated Polyesters

-f

-! -ii -B

-t -E

-B -i -I -

-a -H -9

1

-

1

-B

-1 -E -1 -

i.,

P -

163

164


development work on reduced emission resins and also directed attention to shop practices which can reduce styrene emission. The reduced emission or suppressed resins generally depend on the formation of a film on the surface of the resin. Effective film formers develop a thin surface barrier through which styrene vapor escapes more slowly than from the surface of ordinary resin. The film formers generally have limited styrene solubility and “kick out” on the surface of the resin as styrene evaporates from the surface. Paraffin waxes will function in this manner but like many effective similar materials can give problems with interlaminar bond strength on secondary lamination P. Nylander[“] and M. J. Duffey[681have described styrene suppressed resin systems. Most resin manufacturers now can offer reduced emission resins. Interlaminar bond strengths should be checked with such resins to ensure that no problems exist.

TRADE NAMES & MANUFACTURERS OF UNSATURATED POLYESTER AND VINYL ESTERS Trade Name

Product

AquaShield AlillUX

Polyester resin

I

Structural RIM resin

Aropol

Unsaturated polyester resins

Manufacturer Advance Coatings Co. Ashland Chemical Co. Composite

I Polvmer Div. Ashland Chemical Co. Composite

I Polymer Div. Aropol WEP

Water-extended polyester resin

Arotech

Ashland Chemical Co. Composite

I Polymer Div.

I Structural resin


I Polymer Div.

I Arotran

RTM body panel resin

Ashland Chemical Co. Composite Polymer Div.

Auac

Thermoset polyester

Reichhold Chemicals, Inc.

Atry1

Tbermoset polvester/automotive

Owens Coming

Core-Bond

Polyester-based syntactic adhesives

ATC Chemical Corp.

CoreLyn

Polyester molding compound

Bulk Molding Compounds Inc.

16

Unsaturated Polyester and Vinyl Ester Resins TRADE NAMES & MANUFACWRERS OF UNSATURATED POLYESTER AND VINYL ESTERS (Continued) Trade Name

Manufacturer

Product

resins

&RaYn

Vinyl ester

hnerplastic

cybond

SMC adhesive

CYTEC Industries Inc.

cyglas

Thermoset polyester molding camp.


Derakane

Vinyl ester resins

Dow Plastics

Dielectric

Polyester molding compounds

Industrial Dielectrics Inc.

Dion

Unsaturated

Reichhold Chemicals, Inc.

polyester

I Vinyl ester resins

Corp.

I Shell Chemical Co.

Glaskyd

Thermos& alkyd molding compound


GriPP

Reinforced polyester putty

Idaho Chemical Industries

H-100

Thermos& polyester sheet

Homalite

Hetron


Polyester resins

I Div.

I -sd

llxrmoset polyester

Aristech Chemical Corp.

Polaris

High-performance polyester resins

A&tech Chemical Corp.

Poly-Bond

Polyester-based

Polycor

Gel coats

Cook Composites & Polymers

Polylite

Polyester resins

Reichhold Chemicals,

premi-GlaS

Thermoset polyester-based compounds

PKZlli-Ject

Thermoset polyester injection molding compounds

Premix. Inc

Resipol

Polyester molding compound

Raschig Corp.

Stnpol

Polyester resins

Cook Composites & Polymers

Vibrin

Polyester and vinyl ester resins

Owens Coming

thermoset

syntactic adhesives

molding

ATC Chemical Corp.

Inc.

Premix, Inc.

166


REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

22. 23. 24.

Boenig, H.V., Unsaturated Polyesters, Elsevier, New York (1964). Boenig, H.V., Unsaturated Polyesters, Elsevier, New York (1964). Bjorksten, J.,Po&esters And Their Applications, Reinhold, New York (1956). Morton, M., Rubber Technology, Van Nostrand Reinhold, New York (1973). Kienle, R.H., Ind. Eng. Chem., 22,590 (1930). Kienle, R.H., Ind. Eng. Chem., 55,229T (1936). Bjorksten, J.,Polyesters And Their Applications, Reinhold, New York (1956). Parker, E.E.,Mod. Plastics, 36, 135 (June 1959). Mayo, F.R., Lewis, F.M. and Walling, C., J. Am. Chem. Sot., 70, 1529 (1948). Vanso-Sz~ercsanyi, I., Marcos, L.K. and Zahran, A.A., J. Applied Polymer Science, 10,5 13-522 (1948). Haman, K., Funke, W. and Gilch, R., Angew. Chemie., 71,596 (1959). Boenig, H.V., Unsaturated Polyesters, Elsevier, New York (1964). 164 Hayes, B.T., Read, W.J. and Vaugham, L. V., Chem. and Znd., London, 1165 (1957). Cook, W.D. and Delatycki, D.,J. Macromol. Chem., A12(5), 769-787 (1978). Dennnler, K. and Schlag, J., Kunstoffe, 57,566-672 (1967). Ofke of Safety and Health Administration, U.S. Department of Welfare. American Conference of Governmental and Industrial hygienists. Waters Associates, 34 Maple St., Milford, Massachusetts. Varian Associates, 2700 Mitchell Dr., Walnut Creek, California. Schotte, G. and Meijerink, N.L., British Polymer J., 133-139 (June 1977). Scheuing, D.R., Spectra-Physics Chromatography Review, V6, No. 1, l4 (January 1980) (Pub. by Spectra-Physics Corp., 2905 Stender Way, Santa Clara, California). Hagnauer, G.L., Anal. Chem., V54, No. 5,265R-276R (April 1982). Kerle, E.J., Connolly, W.J. and Rosenfeld, I., 29th Annual SPI RPC Conference, Paper 11 G (1974). Synthesis and Cure of Unsaturated Polyesters Based on TMPD Glycol for RP Applications, Public. No. N-176 (June 1974) (Pub. by Eastman Chem. Prods., Kingsport, Tennessee).

Unsaturated

25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39.

40.

41. 42. 43. 44. 45. 46. 47. 48. 49. 50.

Polyester and Vinyl Ester Resins

167

Pritchard, G., et al, Thermosetting Resins for Reinforced Plastics,Applied Science Publishers, London (1982). Bowen,R.L.,U.S. Patent 3,066,112 (1962). Bowen, R.L., U.S. Patent 3,179,623 (1965). Fekete, F., Keenan, P.J. and Plant, W.J., (U.S. Patent 3,22 1,043 (1965). Fekete, F., Keenan, P.J. and Plant, W.J., U.S. Patent 3,256,226 (1966). Jackson, R.J., Ed., Epoxy Resin Technology-Developments Since 1979, Noyes Data Corp., 228 (1982). May, C.A., U.S. Patent 3,345,401 (1967). May, C.A., U.S. Patent 3,3 73,221 (I @)66). Najvar, D.J., U.S. Patent 3,892,819 (1975). Anderson T.F. and Messick, V. 13., Developments In Reinforced Plastics-l, Ed. by G. Pritchard, Applied Science Publishers Ltd., London (1980). Brookfield Engineering Corp., 240 Cushing, Stoughton, Massachusetts. Bautista, T.D., Polymer-Plastics TechnologyEngineering, 18(2), 179-207 (1982). Bonsignore, P.V. and Manhart, J.H., Aluminum Hydroxide-Fire Retardant Additive and Filler for Plastics, 29th Annual SPI RPC Conference (1974). Connolly, W.J. and Thornton, A.M., Alumina Trihydrate Filler In Polyester Systems, Mod. Plastics (October 1965). Sprow, T.K., Connolly, W.J. and Kerle, E.J., Filled Polyester Spray-Up Systems Offering Improved Fire Hazard Classification, 28th Annual SPI RPC Conference (1973). Wampner, F.D., Alumina Trihydrate Average Particle Size Effect on the Flammability and Physical Properties of Fiberglass Reinforced Plastics and Bulk Molding Compounds, 3 1st Annual SPI RPC Conference (1976). Keating, J.Z., Flame and Smoke Management in Polyester Systems, 32nd Annual SPI RPC Conference (1977). Zaske, O.C., Wang, M. and Wuh, J., 40th Annual SPI RPC Conference, Paper 8F (1985). Frillete, V., U.S. Patent 2,568,331 (1951). Fisk, C.F., U.S. Patent 2,628,209 (1953). Schnell, Raichle, Prater and Bruhne, U.S. Patent 3,390,205 (1968). Jemigan, J.W., U.S. Patent 3,446,259 (1969). Fekete, F., Keenan, R.J. and Plant, W.J., U.S. Patent 3,256,226 (1966). Fekete, F., Keenan, R.J. and Plant, W. J., U.S. Patent 3,301,743 (1967). Doyle, T. and Fekete, F., U.S. Patent 3,3 17,465 (1967). Fekete, F., 27th Annual SPI RPC Conference, Paper 12D (1972).

168


51.

Fermrini, J., Longnecker, D.M., Shah, N.N., Feltzin, J. and Greth, G.G., 33rd Annual SPI RPC Conference, Paper 9D (1978). Burns, R., Gandhi, K.S., Hankin, A.G. and Lynskey, B.M., Plastics and Polymers @-it.), 228-235 (December 1975). Zaske, O.C., Fintehnann, C. and Wuh, J., 36th Annual SPI RPC Conference, Paper 23A (1981). Homer, AH., Zaske, O.C. and Brill, R., 37th Annual SPI RPC Conference, Paper 1B (1982). Horner, A.H. and Brill, R., 39th Annual SPI RPC Conference, Paper 8A (1984). Homer, A.H. and Brill, R., 40th Annual SPI RPC Conference, Paper 16D (1985). Zaske, O.C. and Hrouda, G., The Effect of Different Commercial Ketone Peroxides on Cultured Marble Matrix Gel and Demold Times, Annual Meeting of the Cultured Marble Institute (September 22, 1975). Hussain, K. and Zaske, O.C., The Effect of Ambient Temperature on the Viscosity, Gel and Demold Time of a Typical Marble Matrix, National Cultured Marble Institute Meeting (February 23, 1979). Zaske, O.C., Interactions in Marble and Onyx Matrixes, Regional Cultured Marble Institute Meeting (August 17, 1979). Zaske, O.C. and Husain, K., Gel Coat and Matrix Resin Chemistry Response to Water Boil Tests, National Cultured Marble Institute Meeting (February 28, 1980). Homer, A.H. and Church, S.L., 28th Annual SPI RPC Conference, Paper 7C (1973). Homer, A.H. and Church, S.L., 29th Annual SPI RPC Conference, Paper 7E (1974). Homer, A.H., 30th Annual SPI RPC Conference, Paper 3B (1975). Luperfoam 329 Technical Bulletin, Lucid01 Div., Pennwalt Corp. Edwards, H.R., 39th Annual SPI RPC Conference, Paper 8C (1984). Hybrid Resins, Technical Bulletin IP-77, Amoco Chemicals Corp. Nylander, P., 34th Annual SPI RPC Conference, Paper 6B (1979). D&Q, M. J., 34th Annual SPI RPC Conference, Paper 6D (1979).

52. 53. 54. 55. 56. 57.

58.

59. 60.

61. 62. 63. 64. 65. 66. 67. 68.

5 Allvls Sidney H. Goodman

The need for materials to resist the extraordinary conditions imposed by the fledgling aerospace industry in the 1950s encouraged polymer chemists to evolve a family of plastics based on diallyl phthalate (DAP). These plastics were characterized by excellent chemical resistance, low electrical loss, excellent weathering, very low mold shrinkage and good dimensional stability. For 20 years, DAP compounds were among the highest priced insulating materials. In the 197Os, improvements in competitive plastics (epoxies, polyesters, polyurethanes, and many new engineering thermoplastics) significantly reduced the market for DAPs. At one point in the late 197Os, there was only one supplier of monomer in the world and no more than two suppliers of compound in the U.S. During the last few years, new lower cost DAP-based compounds were produced. More economical molding processes based on traditional compression molding and advanced injection molding techniques were also established. Thus new incentives for market growth were established and DAPs remain a commercially viable plastics family. 169

170


A thermoplastic allyl, ally1diglycol carbonate, is a colorless, optically transparent resin that is often included within the ally1 family. However, discussion of this plastic is not germane to this book and the reader is referred to the bibliography (e.g., Thomas or Sare) for further detail.

CHEMISTRY

Diallyl phthalate monomers are made from propylene and phthalic anhydride as follows:

C’z Cn3Cn~Cti2 _c

reduc Ct$-Cl+-CH2 ;I

Propylene

__)

CH2=CtiCH2 on

;I

sliyl alcohol

dlchloropropans

2Cn2=cncn20n

t

a

P

()

phlhallc

c’ 6

anhydrlde

-0

a diallyl

FI

c-o-cn2-cn=cn2

t-o-cn2-wcn2 0

phthalale

Depending on the choice of anhydride, a series of ally1monomers can be created. Two of the most significant commercial importance are diallyl phthalate (diallyl orthophthalate, DAP) and diallyl isophthalate (DAIP). Diallyl fbmarates and maleates have found use as highly reactive trifunctional monomers containing two kinds of polymerizable double bonds. Diallyl chlorendate is used to impart flame retardance to molding compositions. Other ally1 monomers that have found application include ally1methacrylate (crosslinking agent in unsaturated polyesters and monomer intermediate) and triallyl cyanurate (crosslinking of unsaturated polyesters).

171

Allyls

POLYMERIZATION

AND PROCESSING

Polymerization and crosslinking of the ally& occur via a peroxide induced addition through the ally1unsaturation.

a ;-o-c-c= -0

0

0

0

&&-c

c

nO

ye-c

0

=c

&c-c-o-& c-&c-o-c-

a

1

;

C-0

a& -i d-

Typical catalysts include t-butyl perbenzoate, benzoyl peroxide, or dicumyl peroxide at levels of 2-3 phr. Dimeric peroxyethers and esters have also been used. Allylic homopolymerization is very slow at room temperature, catalyzed formulas being stable for over a year with hardly any resin advancement. Oncethetemperature exceeds 150°C (300°F) the cure rate proceeds very rapidly. Because of the vinyl-type addition reaction mechanism, no volatiles are generated during cure. Typical cure temperatures vary within the 135 o177°C (275”-350°F) range. For moldings, mold residence times are around 0.5-4.0 minutes at pressures of 500-8,000 psi for compression molding and 2,000- 10,000 psi for transfer molding. Laminates made using matched-die moldings or vacuum bag techniques require cure schedules of 20-35 minutes at 93”-177°C (200”-350°F). Shrinkage of allylic monomers during cure is only 12% v/v with prepolymer shrinkage less than 1% (see Figure 5-l).

172


Figure 5- 1: Volume resistance decrease of several thermosetting materials at 70°C and 100% RH. (GL indicates glass filler, and Min-GL indicates mineral and glass tiller.) (Harper). Allylic compounds generally fill crevices and completely surround inserts in complicated molds because of their good flow characteristics. Molds with small dratt angles can be used without hampering release. Chrome-plated molds are recommended, although polished steel has been used. The very low viscosity of the monomers is useful when admixed as a crosslinking agent in unsaturated polyesters. The overall compound viscosity is reduced and more tillers and additives can be incorporated. For many homopolymeric molding and laminating applications, the use of prepolymers have been found more useful. The prepolymers are syrupy to solid, linear, internally cyclized thermoplastic structures containing unreacted allylic groups spaced at regular intervals along the polymer chain. By using prepolymers, better

AllyIs

173

handling of molding powders and laminate prepregs is obtained, along with increased control of flow and exotherm.

FORMULATION

Ally1compounds typically contain some monomer (for viscosity and reactivity control), catalyst, fillers, pigments and processing aids. Fibrous and mineral tillers comprise the bulk of tillers used. Glass fibers produce moldings with the best all around properties. Long glass fibers provide greater impact strength. Glass fibers also combine to give the highest shock and arc resistance in DAP compounds. Acrylic fibers yield the best electrical properties particularly under high humidity exposure. Polyester fibers give impact resistance and strength in thin sections. Nylon filled systems provide high durability, i.e., the best resistance to abuse. Cellulosic and other fibrous mineral fillers generally are used to reduce cost or for some specialized property enhancement. Asbestos was a popular filler to improve impact strength at low cost (however electrical properties decreased). This filler has been effectively eliminated from use because of its carcinogenic effects. Table 5- 1 shows comparative properties of various fiber-filled DAP systems. Particulate fillers include calcium carbonate and silicate, treated clays, and barium sulfate (barytes). These are added primarily to reduce cost and control flow.

PROPERTIES

Table 5-2 presents a general summary of ally1properties. As can be seen these plastics demonstrate high dimensional stability, excellent heat and chemical resistance, and superb electrical properties particularly under extreme temperature and humidity conditions. Since there are no corrosive volatiles released during cure, they will not attack metallic inserts nor support galvanic

Table 5-l: Typical Properties of Several Diallyl Phthalate Molding Compounds with Various Fillers (Harper Property

Orlon

Dacron

Lwz? Glass

Asbestos

Short Glass

Short Glass*

Tensile strength, psi.. ....

6,000

5,000

10,000

5,500

7,000

7,000

Compressive strength, psi

25,000

25,000

25,000

25,000

25,000

28,000

Flexural strength, psi.. ..

10,000

11,500

16,000

9,600

12,000

12,000

Flexural modulus psi X 106.....

0.71

0.64

1.3

1.2

1.2

1.3

Impact strength, Izod, &-lb/in of notch.....

1.2

4.5

6.0

0.4

0.6

0.5

Hardness, Rockwell M.....

108

108

100

100

105

110

Specific gravity at 25 “C..

1.31-1.45

1.39-1.62

1.55-1.70

1.55-1.65

1.6-1.8

1.65-1.75

Dielectric constant: At 1 kHz..... At 1 MHz

3.7-4.0 3.3-3.6

3.79 3.4

4.2 4.2

............. 4.5-6.0

4.4 4.4

4.1 3.4

Table 5-l : Typical Properties of Several Diallyl Phthr ate Molding Compounds with Various Fillers (Harper) (Con nuedj Property

Orlon

Dacron

Long glass

Asbestos

Short glass

Short glass*

Dissipation factor: At 1 kHz..... At 1 MHz.....

0.020-.025 0.015-0.02

0.008 0.012

0.004-.006 0.008

0.04-0.08 0.04-0.08

0.006 0.008

0.004 0.008

0.002

0.006

0.003

0.003

0.0007

0.001

0.0007

0.0002

392

325

400

500+

350-400

350-400

350-400

450

Heat-deflection temperature, oF.. .... Isoatrc;rstancc,---

1265

1290

I 300-500

I

*Based on diallyl isophthalate

350-400

2

176


corrosion in the presence of moisture. Compounds based on DAIP will withstand higher temperatures, are slightly stronger, and are easier to mold than those based on DAP. Table 5-2: General Summary of DAP Properties

Physical Extremely good dimensional stability. Almost no post-mold shrinkage. Chemically inert, gives off no corrosive vapors. Mechanical Excellent strength in compression. Excellent impact resistance. Exceptional thin wall strength. Excellent for applications subjected to: sudden extreme jolts, severe stresses. Electrical Retains high insulation resistance at elevated temperatures. Performance virtually unaffected by high ambient humidity. Thermal Outstanding thermal stability at normal use temperatures. No decomposition below 150°C (300°F). Eliminates fouling of metal contacts caused by condensation of decomposition products. Chemical Resistant to solvents, acids, and alkalies. Fungus proof.

The resistance of DAPs to humidity is demonstrated in Figures 5-2 and 5-3. Figure 5-2 shows that the insulation resistance of allyls does not deteriorate even after 4,000 hours exposure to 70 “C (160 “F) and 95% RH. Only polybutylene terephthalate (PBT) is better. Figure 5-3 shows that DAP and DAIP retain a much higher percentage of tensile strength after 2,880 hours exposure to 70°C (160 “F) and 100% RH than either PBT or polyphenylene sulfide (PPS). Dielectric strength values for DAP are good up to 190°C (374°F) and in excess of 205°C (400°F) for DAIP. Figures 5-4 and 5-5 show the effect of frequency and temperature on the dielectric constant and dissip-

Allyls

177

ation factor of unfilled DAP. DAP materials are capable of withstanding extensive radiation exposure. Compounds have withstood 104-1012rad dosages of gamma radiation without breakdown.

PETlSODAP@

IO0

1000

2Ocu

3ca4cOo

HOURS 6.37O”C,95% RH

Figure 5-2: Insulation resistance vs exposure to high humidity. (Dalton and Landi).

178


1440

720 HOURS

HOURS

HOURS

2200

@ 7O”C,lOO%

HOURS

2880

HOURS

RH

Figure 5-3: Retention of tensile strength after exposure to high humidity.

(Dalton and Landi). 3.9N, 3.8- ‘.,

3.1 60

I ld

IO’

1$

2X10618x106Id

Frequency, cps

Figure 5.4: Effect of frequency and temperature on the dielectric constant of

unfilled diallyl phthalate. (Harper).

AllyIs

179

Temperature, T

Figure 5-5: Effect of frequency and temperature on the dissipation factor of unfilled diallyl phthalate. (Harper).

Tables 5-3 through 5-7 are property listings for a variety of commercial DAP compounds. No recommendations or preference by the author is intended or implied. Rather the data is included to provide up-to-date information only. The reader is advised to contact individual suppliers for specific assistance and most recent developments.

(panuquo3) 30s 051 lO’/lIO’ lW6.f

30s 0t1 910’/600’ 6'E/l't

30s 9ow 3aw %lw 3ow s-m 081 SE1 OS1 SE1 SE1 0t1 SZI 81O'lEIO‘ LIO'1010'61O'/Slo' flO'IS10'61O'/SlO'PlO'/EIO'120'1910 2'PIp't

9HV6 L62/LOI SZE SZP

OSE/O01 SLE 00s

OLE/O6E

)9E/O8E 08ElOOt

8H P6 192186 SZP szs SZ’O 000’0E OOSL )Ix 11 000’81 /OOO’SI 0’1 ““I;$

SZ’O 000’82 0006 901 x 6’1 000’02 /000’91 0’1 ‘OO”;M4’

p’z

5’2 lepue,g

OIEI

Oim

ssWl voqs

ssel9 voqs

oq1’0

relnuelg osl

SE’0 OOO’EZ 0059 901 s 8’1 OOO’EI /000’01 01'0 foo’-S;m;

E’Z lelnuel3 oql’o le,au!fl ssel3 Uoqs

OK-1

6'E/l't OWSLE

OK/O1 SLE 00s Of’0 000’82 0008 901 s 8’1 000’81 /OOO’SI 0'1 EOO’-IOU 08’1 t.2 jepuelg

8'UE't OO~/OOP

8HV6 222~SIl szr szs SZ’O OOO’LZ OOSS 901 x 0’2 OOO’E 1 /000’01 SP'O 9oo’-coo’ 06’1 1’2 lepuelg osl

oqvo ssel3 voqs

le*aulw

l'c/z't 06UOOt

612/011 s22 SZE

L'E/E't OBUOOP

EHP6 002 OOE

9'uz'r OS&/SLE

ELEI06 ot2 OK

T'W'C OSEISLE

Ott/L8 s22 062

OP.0 OS’0 OV’O or0 000’12 000’62 ooo’s2 ooo’tz 000s OOSL ooss 0059 901 x C’l 901 s 1’1 901 x 1’0 901 s 9’1 OOO’SI OOO’E I OOO’PI 000’21 /000’01 /OOO’I I 10006 St'0 S9'0 St‘0 Olo'-LOO'900'~COO'ZIO’-010‘ 08.1 OP.1 EL'1 /000’21 sr'o 900’~EOO’ 99’1 P’Z lelnuelg oq1’0 !ejautw

OZE-I NIOS-Z NOW1

92 jelnuelg oqvo uollN lelautw

E’2 lelnuel3 oq1’0

lejautw

NWIOS-I NIOS-I

O’f xlnuelg oqvo

uol’o

SOS-I

.8/I~U!Vti in '3% '(m3/NW h!l!vJJWj j, amleladwal asn snontyuo3 j, ampadwal uo!pallag $ ‘uogdjosqy la)eM (Isd) qpual]S amsaldwo3 (isd) q@uaqS apual (tsd) snlnpow pnwalj (tsd) q@ual)S lemralj (L/3('wql 'II) wdv WI w/u! (dwo$ allqiyqg (IwM? 4!neJ!J 3!1!3ads lovej

9ln9

lllJOj (dV0)u!w giawacuop!ap

3OVWl

Table 5-3: DAP Diallyl Phthalate Molding Materials (Rogers Corp.)

(Continued)

z-530

3-l-530 3-2-530

GRADE

3-2-520F 3-l-540 3-l-525F1366FR l-530 3-l-501N

Reinforcement

Long Long Short Long Short Short Long Glass Glass Glass Glass Glass Glass Glass Mineral Ortho Is0 Is0 Ortho Drtho Ortho IS0 Ortho Ortho Coarse Coarse Granular Granular Granular Granular Granular Coarse Coarse Granular Granular Granular Granular 3.5 3.5 3.5 3.5 2.0 2.4 2.2 2.3 2.4

Resin (DAP) Form BulkFactor Specific Gravity (g/cm? Shrinkage (Comp)in/in lzodImpact (It. lb./in.) (C/T) Flexural Strength (psi) Flexural Modulus (psi) Tensile Strength (psi) Compressive Strength (psi) WaterAbsorption, % Deflection Temperature OF Continuous UseTemperature OF Flammability (IGN/Burn). sec. UL Rating l/8' Dielectric Strength 60Hr,SVSS, wet(vpm) Dielectric Constant 1 khr/lmhz,wet Dissipation Factor 1 khzllmhz.wet ArcResistance (sec.) MILM14G Type

Mineral

Short Glass

1.83 .003..006 0.60 11,ooo/ 14,000 1.7 x 106 6500 25,000 0.30

1.76 1.76 1.87 1.74 1.73 1.90 2.08 1.86 .0015-,004 .0015-,004 .0015-,003 .0015-,003 .0015..004 .OOlS-,004 .OOl-,003 .oo15..ooi 1.1 3.0-7.0 3.0-7.0 3.0.7.0 3.0-7.0 1.0 0.70 1.0 17.000/ 17.000/ 18,000/ 18.000/ 14.000/ 13.000/ 16.000/ 16.000/ 21,000 21.000 17,000 16,000 19,000 20,000 20,000 20,000 1.4 x 10’ 2.4x 10L 1.8x lo6 1.7 x 10’ 18x10' 1.4x 10' 1.6x 10' 1.7.x101 8500 8500 7500 8000 9000 8500 7500 7500 27,000 28.000 26.000 27.000 25,000 26,000 28,000 28,000 0.35 0.35 0.35 0.25 0.25 0.25 0.25 0.35

325 225 106123

525 425 119140 94v.o

400 300 159132 94v-0

450 350 110140 94v-0

400 350 120120 94v-0

500 400 951275

550 450 951339

.wo

550 450 110140

4001375

3751350

3701350

4001375

4001375

4001350

4001350

4001350

400/350

4.3/3.7

4.113.9

4.414.3

4.3/4.1

4.413.9

4.114.0

3.813.6

4.213.9

4.113.9

.013/.016 135

.010/.013 150

MDG

SDG-F

.011/.015 .010/.016 180 130 SOG-F

Testmethods usedareaccordmg toA S.T.M.. Fed.Std 406.or Mrhtary Specihcations.

.009/.017 .011/.019.010/.019 140 135 135 S0G.F GOIGDI-30

400 110140 94v-0

.010/.018.010/.019 130 135 GOI.3OF GDI-30F

182

Handbook of Therrnoset Plastics

rable 5-4:

DAP Molding

Materials

Properties

(Cosmic

Plastics,

Inc+)

DIALLYI, ORTHO PHTHALATE Cam9amadNasba

D69

FIBU

RESIN Da

LONG GLASS FIBER

FnmS

Flakes

Dmdard(s) SMC

(RWW Da

or Phme RetardanKP)

cbwity

Bulk Factor PSI

IFlakes

IFlakes

s

F

1.62

1.82

1.77

6

6

6 500~8Oca

s

5x@8000

500-8000

Mddiq

Temperah~~~9

275-350

275-350

275-393

Molh

s*

0.001-o.M)4

0.001-0.004

0.001-0.004 _wo

Mokiing Resrute

infm.

Heat Dklortion Temperature “F

_w

500

Contiwous Heat Resistance -‘F

350-m

3YJ-400

350-400

Dimensional

0.01

0.01

0.01

sthlily

K Inax.

lll~EXDZJGOn10’1oC

2.4

Fh.me Resktancc:

Ignition Tie

sc(min)

Bumingtimesedmax~ Water Absorpbon: 48 bts. @ 50 “C % Immct Strew&

Ft-lblin.

FkxudS~: complwiw TensikSrqtb: An Resisti:

notch &odj

PSI PSI

SW: PSI

Seconds

Didecbic Strqtb:volts/mil.

Step by Step

DRY

I90

2.4

2.4

1 155

190

3oo

30

285

0.23

0.25

0.25

4-10

610

4-10

uooo-17.m

17.m

17P.m

29,wo

29,ooo

29,M)o

7,lxwo,OOO

7,000.10,000

7,cnw10.000

180

180

180

400

400

374

1

AllyIs

183

Table 5-4: DAP Molding Materials Properties (Continued)

I

I

DULLVL

ORTHO PRTHALATE RESIN -J D33

CommundNumbw Fu*r

StadwJ(s) specitic

01 Flame Retardant(F) cbwity

Bulk Factor MoldinS PTessurc PSI Mdditq

Temperaturr

OF

MoldinRshrinkaneinh.

I Heat

BRORT GLASS FIBER

I

FOrmS

Distortion Temaaahln

ContinuousHeat Resistance Dimensiod lltennal

OF

Stability % max.

Exp=ansicm IO” / OC

Flame Resistance:

Ignition Tie seqmin) Bumingtimes+nax)

WataAbsqtionz lmpsct

4Shts.@50~%

Strqth:

FlexId

Ft-lb./in. notch @.od)

Strewh:

TensikStrqth:

PSI

F

180

1.82

2.5

2.5

500-8ooo

5OO-Sca

275-350

275-350

0.001-0.004

0.001-0.004

350-400

0.01

0.01

1.2

1.6

90

110

300

60

0.2

0.2

0.5-1.2

OS-I.2 13,Olxl I25,oca

7,000. IO,000

Seconds

Didecbic

Streqth:volWmil.

Dklectk

Breakdowx

Step by Step

Kv Step by Step

Dissipation

IKCUMC

Factor IKCXMC

145

DRY

3S5

3SO

WET

340

xi0

DRY

62

62

WET)60

164

DRY 14.2I4.0

I4a4.0

m

14.Y4.2

DRY WET

Stnfkx Resistance:

me&mu

AS IS

30Days@1W%RH.@15S°F Volume Resistallce

mqtobms

~30Daw@100%R.H.@1Y3°F Water AbstmctConduetnnce106 MHOSICM CetiiSable

to Military Specificstions

&Typeoesign Flammability

RatinS UL-94 l/8”

7,000-10,caO

145

I

DielecbicConstant:

I

lxm

350-400

I2S.lm

PSI

Arc Resistena

Gmnuk3

S

13,000

PSI

comptessives~

oranules

l5cul

OF

D72

ASIS

14X4.2 O.OC&O.O13

I0.0I2/0.015

0.007/0.013 I0.011/0.015

lOso plus

IO’” plus

10,tmO

10,000

10’” plus

I1o.cal I

10”

plus

I 10.m I

MIL-M-I&

MIL-M-14G

SDG

SDGF

HB

vo

I

184


DIALLYL

Stpnaprds)

01

Flame RetardanU~

s

ORTHO PHTHALATE

RESIN

s

Ts

spcci6cGrwify

1.60

I .52

1.74

Bulk Factor

6

6

2.3

5OwwJO

500-8,~

5Oa-wYJ

275.350

275-350

275-350

Molding Plesswe PSI Moldinn Temanhlrr

Surfoee Resisti:

OF

me@ms

AS IS

30 Days @ 100% R.H. @ 158 OF V&me

Rcsism:

mmhms

W&I AbslmcIconductancelO4 c!MiIiable

IO MiliIaly

sp.elkaIions

&lLpeDesign Flanumbilay Ratag lJL-94 IiS

ASK

MHOSJCM

IOlD plus

IO’O plus

IU

lO,@Xl

lOto plus

IO’” Dlus

I

I

I I

MIL-M-14G

MIL-M-14G

MIL-M-140

SDI-30

SDI-5

MIX3 HB

Allyls

Table 5-4: DAP Moldin

~1

Flexumlstrengm: compfessive

Tensile Stren@h:

Materials Pro erties Continued)

PSI

strmgth:

PSI

PSI

Arc Resistance: Seconds Diekctdc S~:volts/mil.

step by step

Dielecbic Breakdown: Kv Slep by Step

17,000

I 7$Qo

M,@JO

wm

7,000-10,Oml

7,Ow1o,caO

IS0

IS0

DRY

475

410

WET

420

DRY 164

30Dnys@100%R.H.@lSB°F V&me

5O@Jo

Resistance: megohms

xv00 10’” plus

30,000

30,000

Water Abstract Conductancclo-6 MHOWCM

30

30

Catitiable to Mi

MIL-M-14G MIL-P-19833

MIL-M-14C MIL-p-19833

=tdTypeD=kn

GDI-MF

GDI-M

Flammability Ratiaq UL-94 IIS”

vo

HB

lOO%R.H.@

ls8OF

Specibstions

ASIS

406 164

IO’O plus

3ODays@

185

I

186


Table 5-4: DAP Molding Materials Properties (Continued) DIALLYL

META PHTHALATE

RESIN

(Hl~b ikat Rnlstamt) Wl

K77 SM

K43

Glass Fiber

Mbrnl

hdea

~chd.3s

1 Pow&l

F

Is

Is

SpeciticGravity

1.78

1.70

I .67

Bulk Factor

2.5

2.5

2.3

sco-8,ooo

~,~

500-8,ooo

lls-350

275-350

17s3M

0.001-0.004

0.0016.004

0.003-0.007 500

Molding Pressure PSI Molding Tempaahrre

OF

Moldmn shrinkape in.iin.

Heat Distortion

Temperature OF

500

ml

ContinuousHeat Resistance -‘F

425-m

42s500

42sakl

Dim-

0.01

0.01

0.08 4.6

Stability % max.

Themml Expansion 10’ I T Flame Resistance:

Water Absomtion:

Cu6linbie dT~Lk@l

to Miitnly

I.5

2.5

Ignition Time sec(minl

I IO

90

105

Buming~sec(mW

59

MO

280

0.2

0.2

0.4

ML-M-140

MIL-M-IG

SDGF

SD0

MDG

“f-l

“rl

UT4

spcci6cations

I

187

Table 5-5: Plaskon @ DAP Long-Glass-Filled Molding Compounds Typical Molded Properties and Molding Parameters (Plaskon Electronic Materials Inc., subsidiary of Rohm and Haas Co.)

PROPERTY

52-20-30

52-40-40

FS-4 -

orlho flake 6

ortho (lake 6

flake 6

llske 6

15,ooo

15,000

15.000

1.3 4.0

1.3 3.9

1.4 3.6

15.000 1.4

FED-STD.406 ASTM D 790

3.4

FED-STD-406

63

63

65

65

ASTM

130 350

133 350

136 350

145 340

4.2 0.016

4.3 0.016

4.0 0014

4.1 0.015

FED-STO-406 FED-STD-406 FED-STD-406 FED-STD-406

ST0

FR

STD

FR

no

>90/500 7’260

>5OQ ,260

> 375 >,W

,460 ,240

yes “0

ye* “0

ye= WI

Ve* yes

ye= Vflb

2slO 5Ow

25nO w30 4wO auto

2500 Mw 4000 a”,0

25(10 5030 4Oca ato

2500 5000 4coO a”,0

0.0025 o.ooo5

0 0025 OCW7

0 0025 OooO7

00025 0.0005

a% 0.0025 o.ooo7

Method

ASTM

D1895

FED-STD-405 ASTM D970 FEDSTDMG

it

1.93

,500 ,260

Twl

meta

ortno Qrallnulw

2.3

150 350 4.5 0.016

737070’ --

1.91

FED-STDdOS FED-STD406 FED-STDdC6 FED-STD*Ob

“I_ 94 ASTM D 2553 “L 746-A

ASTM 0792 FEDSTO-

Allyls

189

Table 5-7: Plaskon @ DAP Mineral-Filled Molding Compounds Typical Molded Properties and Molding Parameters (Plaskon Electronic Materials Inc., subsidiary of Rohm and Haas Co.) PROPERTY

61.01 CAFR

115 CAF

ortho granular

granular

FeF

Tast method

Raw malerlrl Resin type Physical lorm

ortho

meta granular

Mechanical Bulk factor Flexural strength.

2.3 10.000 1.2 0.4

psi

Flexural modulus. psi x 106 Impact strength. R-lb/in. notch Barcol hardness

55

2.6 10,000 1 .o 0.4 50

2.3 10,500 1.3 0.4 57

ASTM

D 1965

FED-STD.406 ASTM 0 790 FED-STD-406 ASTM D 2563

Electrlcat Arc resistance. set Dielectric strength.vlmtl !&p-by-step Dielectric constant. at 1 MHz Dissipalion factor. at 1 MHz

135 375 4.3 0.037

138 400 4.5 0.026

175 350 4.1 0.035

FR >90/30 -

24

23 -

1.75

1.67

1.66

MIL-M-14

UL94 ASTM D 2663 UL 746A

Other propertles Specific

gravity

0.55

Water absorption, % Heat distortion temperature HOT, OF ‘C MIL-M-14 ceriification”

300.350°F 150.170°C

0.65

0.50

300-35O’F 16&17O’C

300350°F 150170°C

MDG

MDG

MDG

2500 5000 4000

2500 5000 4cQO

2500 5006

auto

auto

au10

0.0050

0.0065 0.0005

0.0035 0.0007

PROCESSING -___ PARAMETERS Molding pressure. compression transfer injection

psi

Preforming procedure’ Mold shrinkage -compression. in./in Post mold shrinkage,

‘V-0

grade

** in Im

available

0.0007

m btack

“Military cerliticacion requires Individual batch lesting *“auto =automaltc Note: All Plaskon mineral are asbestostree.

tilled

DAP molding

compounds

ASTM D 762 FEDSTD-lOG ASTM D 646 ASTM D 646 MIL-M-14

190


APPLICATIONS

The major use of DAP compounds is in the electrical/electronic industry. Connectom for electronic communications, computers and aerospace systems consume a large volume of molding compounds. Insulators, potentiometers, circuit boards, potting vessels, trim pots, coil forms, switches, and TV components represent other end-uses of allylics (see Figure 5-6).

Figure 5-6: Electronic connectors, switches and other devices molded from DAP. (Photo courtesy of Rogers Corp.).

Sealants have been made based on ally1resins. They are used in the vacuum impregnation of metal castings and in ceramic compositions. DAP prepolymers are used for improved surface laminates, plywood, hardboard, and particle board. They are usually applied as an overlay by means of a resin-treated non-woven acrylic fabric. ln addition tubing, ducting, radomes, junction boxes, aircraft and missile parts find wide use of reinforced DAP.

Allyls

191

Allylic monomers, principally DAP, are used in the crosslinking of unsaturated polyesters and alkyds. They are found in preform or mat binders, laminating prepregs or in wet lay-up formulas as well as in rope, granular and premix gunk molding compounds. The DAP content of these systems usually varies between lo-15%. DAP is preferred over styrene, especially in large moldings, because of its low vapor pressure at molding temperatures [around 2.4 mm Hg @ 150°C (300”F)l. This low volatility allows for higher molding temperatures which translates into faster molding cycles. In addition, mold shrinkage is lowered. The DAP, when mixed in styrenic-based formulations, extends the shelf life also. DAP polyesters have good mechanical and electrical properties in the 70°C (160°F) range. In formulating a DAP-based polyester, the proportion of DAP and glycol strongly influences the high temperature properties of the compound. DAP polyesters have been reported with temperature resistance values approaching those of triallyl cyanurate polyesters. Laminates of DAP polyesters have been prepared with flexural strengths of 66,600 psi at room temperature and 27,500 psi at 260 “C (500°F). The use of DAP increases the cost of a polyester system and trade-offs must be made against the value of the improved properties obtained.

TRADE NAMES

cosmic Dapex Plaskon

Monomers, polymers and compounds Compounds Compounds

Cosmic Plastics Rogers Corp. Plaskon Electronic Materials, Inc.

REFERENCES AND BIBLIOGRAPHY

Allied Chemical Co., Plaskon Polyester Resins Premix Molding, Bulletin 85 l-36. Diallyl Phthalate Mineral Filled, Bulletin 612-100 (October 1978). High Performance Molding Compounds -DAP, Bulletins 6 12- 10 1 and 2 (July 1978).

192

Handbook of Thermose t Plastics

Beacham, I-H-L,Diallyl Phthalate Resin and Monomer, Plastics Design & Processing, pp 20-23 (April 1967). Beacham, H.H. and Johnston, C.W., How to Formulate Heat Resistant DAP Polyesters, Plastics Technology, pp 44-46 (May 1963). Cosmic Plastics, Inc., Data & Property Sheet (1984), updated 1996 Dalton, J.L. and Landi, V.R., Resistance of Diallyl Phthalate and Other Engineering Plastics to Demanding End Use Conditions, Private communication, Rogers Corp. (1983). DuBois, J.H. and John, F.W., Plastics, 5th Ed., Van Nostrand Reinhold, New York, pp 38-39 (1974). Harper, C.A., Ed., HandbookofMaterials & Pmcesses for Electronics, McGraw-Hill Book Co., Inc., New York, pp 1-18 and 1-19 (1970). Hayes, W.A. Jr., A Case for Thermosets vs. Thermoplastics, Private communication, Rogers Corp., (September 1983). Landi, V.R., Long Term Test Data Helps Connector Material Choice, Reprint from Electronic Packaging& Production (May 1983). Luh, C.H., A New Look at DAP for Electronics Insulation, Znsulatiopt/Circuits (October 1981). Pixley, D. and Richards, P., Thermoset or Thermoplastic? Reprint from Plastics Design Forum Focus Issue (April 198 1). Powers, P.O. and Brother, G.H., The Chemistry of Plastics, in Handbook ofPlastics, by Simonds, H.R. et al., 2nd Ed., D. Van Nostrand Co., Inc., Princeton, NJ, p 1054 (1955). Rogers Corp., Diallyl Phthalate Molding Materials, Bulletin 54208 (1981). Sare, E. J., Allyl, in Modem Plastics Encyclopedia, Vol60, No. 1OA, McGraw-Hill Inc., New York, p 18 (1983-4). Schwartz, S.S. and Goodman, S.H., Plastics Materials & Processes, Van Nostrand Reinhold, New York, pp 339-345 (1982). Thomas, J.L., Allyl, in Modem Plastics Encyclopedia, Vol58, No. lOA, McGrawHill Inc., New York, pp lo-12 (1981-2).

6 Epoxy Resins Sidney H. Goodman

INTRODUCTION

In the late 193Os, Dr. Pierre Castan in Switzerland and Dr. S.O. Greenlee in the United States synthesized the first resinous reaction products of bisphenol A and epichlorohydrin. These materials were characterized by terminal epoxide groups and were the germination of the epoxy family of plastics. The commercial production and introduction of this family occurred in 1947. New types of epoxies proliferated from the 1950s through the 1970s with at least 25 distinct types available by the late 1960s. The generic term epoxy (epoxide in Europe) is now understood to mean the base (thermoplastic, uncured) resins as well as the resultant crosslinked (thermoset, cured) plastic. Chemically, an epoxy resin contains more than one a -epoxy group situated terminally, cyclicly, or internally in a molecule which can be converted to a solid through a thermosetting reaction. The a-epoxy, or 1,2-epoxy, is the most common type of functional moiety. Ethylene oxide,

0

I-I*

&I, 193

Epoxy Resins

195

The basic commercial version of this resin is the one having a molecular weight of 380. Purified versions (n = essentially 0) have molecular weights as low as 344. Higher molecular weight versions (n = l- 10) have been produced by reducing the amount of epichlorohydrin and reacting under more alkaline conditions. Tables 6-1 through 6-5 list some commercial grades of these resins. Changes in the base resin structure have been made to adjust final plastics properties. Higher reactivity, greater crosslink density, higher temperature, and chemical resistance are obtained by using novolac and some types of peracid epoxies.

Novolacs

Novolacs are epoxidized phenol-formaldehyde or substituted phenolformaldehyde resins

-CH

‘-cH’ W 0 I

AH,

‘--CH CH, 0 I

AH, '-CH W

7

196


Table 6-1: Standard Undiluted BIS Resins

*Gardner-Holdt. **Special vacuum-casting

resin characterized by rapid foam breakdown under vacuum.

NOTE: Standard undiluted resins for all general purposes requiring performance up to 400°F. Ahphatic polyamines or polyamides satisfactory up to approximately 230°F. Anhydrides, such as phthalic, satisfactory to approximately 200°F. Aromatic amines and anhydrides satisfactory to 400 “F. The anhydrides are effective viscosity reducers to permit higher tiller loading.

Table 6-2: Lowest Viscosity Resins (Courtesy of Dow Chemical Co.) Products

EEW

Viscosity -cps @ 25oc

Color”

D.E.R. 332

172-176

4,000-6,000

75 (APHA)

Epon 825

175-180

5,000-6,500

1 max

Araldite GY6004

179-196

S,OOO-6,500

1 max

*Gardner-Holdt NOTE: The low equivalent weight resins are virtually pure diglycidyl ethers of Bisphenol A. They are the lowest viscosity undiluted Bisphenol A resins available. They are so pure; however, that they crystallize during storage. The crystals melt on warming above 125’F.

197

Epoxy Resins

They tit all the general uses of D.E.R. 330 or D.E.R. 331 resins with the following advantages: 1. 2. 3. 4. 5. 6. 7.

Increased HDT Lower viscosity More chemical uniformity Longer pot life with most curing agents Better wetting of glass reinforcements Very pale color Better electrical properties

Table 6-3: High-Viscosity Resins

*Gardner-Holdt ** at 70% NV in DOWANOL DB glycol ether solvent. NOTE: The lower EEW resins in this series have the same general properties as D.E.R. 331, except for viscosity. The higher EEW resins are very viscous liquids, finding their primary use in coatings or adhesive systems where solvents may be used to reduce viscosity. As EEW increases, pot life is shorter, HDT decreases, elongation, and adhesion improve.

and exotherms

decrease;

impact,


198

Table 6-4: Low Melting Solid Resins (Courtesy of Dow Chemical Co.) I

I

I

I

EEW

Viscosity*

Color**

Durran’s SP oc

D.E.R. 661

500-560

G-J

1

75-85

Epon 1OOlF

525-550

G-I

1

Epotuf 37-00 1

475-575

G-J

2 max

Araldite GT707 1 1

450-530

Products

1

D-G

1

2

1

65-75

*Gardner-Holdt at 40% NV in DOWANOL DB at 25 OC. **Gardner at 40% NV in DOWANOL DB at 25 “C. NOTE:

Primary uses in amine cured protective coatings and for prepreg glass cloth for electrical laminates. D.E.R. 661 resin modified with polyamines or polyamides is used where high chemically resistant performance is required coupled with a room temperature or low-bake application. Blends of ketone solvents (MEK or MIBK) with aromatics (xylene or toluene) are generally suitable for thinning these systems. Higher boiling solvents, such as glycol ethers, can be used in amounts of 5 to 15% to improve flow and film surface properties, Systems 0fD.E.R. 661 resin can be used on all substrates-metal, wood, glass, masonry by all applications--brushing, spraying, dipping, etc. Coatings end uses include pipe and drum linings, maintenance tinishes, and marine finishes.

Table 6-5: High Molecular Weight Solid Resins (Courtes J of Dow Chemical Co.) I I Products Viscosity* Durran’s EEW Color* * SP OC I D.E.R. 667

1,600-2,000

Epon 1007F

1,700-2,300

Araldite GT6097

2,000-2,500

Araldite GT7097

1,667-2,000

W-Y

3

113-123

Epoxy Resins

199

Table 6-5: High Molecular Weight Solid Resins (Continued)

(Courtesy of Dow Chemical Co.) EEW

Viscosity*

Color* *

Durran’s SP oc

Epotuf 37-006

1,650-2,000

x-z

3

115-130

Epotuf 37-007

2,000-2,500

Y-Z,

3

115-130

Products

*Gardner-Holdt at 40% in DOWANOL DB at 25 ‘C **Gardner at 40% NV in DOWANOL DB at 2.5 OC NOTE: Optimum epoxy coating can be obtained by modifying D.E.R. 667 resin with urea, melamine formaldehyde, or phenolic resins. These systems in the blended solution form have excellent pot life and can be stored for several months without noticeable viscosity change. To cure the coating, high bakes of 300 O-400 OF from 15 to 30 minutes are required. Phenolic modiied systems require the maximum bake schedule for complete cure. The addition of 1% phosphoric acid will catalyze the cure at somewhat lower temperatures. Ketones and aromatic solvents are used to thin D.E.R. 667. End-use applications include tank and drum linings, wire enamels, collapsible tube coatings, and metal furniture finishes.

The number of glycidyl groups per molecule per resin is a function of the number of available phenolic hydroxyls in the precursor novolac, the extent of reaction, and the extent of chain extension of the lowest molecular species during synthesis. Table 6-6 describes a number of commercial novolac resins.

Table 6-6: Epoxy Novolac Resins (Courtesy of Dow Chemical Co.) EEW

172-179 t

I D.E.N. 438-EK85

176-181t 176-181 176-181t

Color*

Viscosity

-cps at 25

OC

Durran’s SP oc

Solvent/%NV

l,lOO-1,700**

3

-I100

600- 1,600

2

MEW85

2

-/loo

2

Acetone185

20,000-50,000 500-1~00

200


Table 6-6: Epoxy Novolac Resins (Continued..)

1

I

2

I 4000-10,000**

I

3

1

4,000-10,000

1

3

I 35,000-70,000

I

2

D.E.N. 438MK75

176-181t

I

D.E.N. 439

191-210

D.E.N. 439-EK85 Araldite EPNl138

191-2lOt

176-181

200-600

I

I

Dun-an’s SP oc

ProdUCtS

Solvent/%NV

I

b

I K/75

I

Araldite EPN1138 A-85 Araldite EPNl139 *Gardner **At 125 “F ***85% in MEK t On solids NOTE: The multi-functional epoxy novolacs have greater heat and chemical resistance than Bisphenol A-derived resins when cured with appropriate hardeners.

Peracid Resins

Of the peracid resins the cyclic types contribute to higher crosslink densities. These resins have lower viscosities and color compared to novolac and DGEBA types.

0 -Li +:c+~!o” +

;c’o‘c’,

0

perbenzoic

acid

olefin

benzoic acid

epoxy

Epoxy Resins

201

Such a typical resin is illustrated by the structure

3,4epoxycyclohexylmethyl-3,4epoxycyclohexane

carboxylate

A series of peracid based resins are also made for modification of standard resin systems. They alter such properties as cure rate, flexibility, and heat deflection temperature. These resins are acyclic aliphatic resins such as epoxidized soya, linseed oils, or polybutadiene. Table 6-7 lists commercial types of peracid epoxies.

Table 6-7: Peracid Epoxies Kourtesv of Dow Chemical Co.) I

ERL 422 1

Viiosity @ 25T

EEW

Products

131-143

-cps

Color *

4

350-450

Epoxy cyclohexyl methyl epoxy cyclohexane carboxylate

+ ERL 4299

205_216

ERL 4234

133-154

Araldite CY-179

131-143

I

1

Bis (3,4epoxy cyclohexyl) adipate

7,00017,000**

2

Epoxy cyclohexyl spiroepoxy cyclohexane dioxide

350450

1

I Cycloaliphatic diepoxide

I

3

I Cycloaliphatic diepoxide

I

1

Cycloaliphatic diepoxide

Araldite CY-184 Araldite CY-192 Gardner max **At 1OO°F

I Composition

L

202 Handbook of i’lermoset Plastics

Hydantoin Resins

ln recent years, the hydantoin resins have shown greater popularity for increasing temperature resistance and improving mechanical properties, particularly in structural composites. Numerous structural modifications are feasible with hydantoin resins, as shown in Figure 6.1.

II 0

Rich

III

0

‘CAR,

0

z-NyN,x,NyN-CH2&iH2 0

0

Figure 6-l: Hydantoin epoxy resin structures. R, and R, can be alkyl groups such as methyl, ethyl and pentamethylene; X can be methylene, bis-hydroxyethyl esters of various chain lengths, or urethane or urea groups.

This type of epoxy has presented toxicity problems. At least one hydantoin based product is being supplied, however, for commercial applications, but it requires special handling precautions.

204

Handbook of Tf3ermoset Plastics

organic acids, such as dime&d

aliphatic

fatty acids;

diacid glycidyl

esters where n = 0, 2,4,

5 and 8

and modified bisphenols, such as bisphenol F.

Also, a whole series of resins is based on elastomeric modification. The first of this series used carboxy-terminated polybutadiene/acrylonitrile (CTBN) liquid elastomers. These telechelic polymers are macromolecular diacids that coreact easily to become part of cross-linked structure. The incorporation of such elastomers as modifiers in resin formulations is a very effective way to toughen thermosetting matrices, thus significantly improving impact and fracture strengths of the cured multiphased networks. However, these improvements are achieved at the expense of lowering Tg’s of the cured resin. Incorporation of certain oligomeric or polymeric thermoplastics, such as polysulfones, polyethersulfones, polymides, or polyetherimides, will also enhance fracture strengths but without sacrificing Tg’s or other desirable properties. Further studies with diaminodiphenyl sulfone (DDS) cured systems demonstrate that by incorporating both a low level of CTBN (5 phr) and a practical level of thermoplastic polysulfone (20 phr), the Gacture toughness of the cured product exhibits an improvement of 300% in fracture energy over the epoxy/DDS control, The presence of 3 phr CTBN does not depress the Tg and renders the epoxy network more toughen-able by the polysulfone.

HCCC f (CH2 - CH = CH - CHZ)~- (CH2 - 7” & j- COOH n CN

CTBN

205

Epoxy Resins

Polvsulfone Amine terminated elastomers of this same type were introduced in the mid 1970s. Although they can also be adducted to DGEBA-type resins, they are more frequently combined with curing agents. Table 6-8 lists some commercial versions of flexible epoxies.

Table 6-8: Flexible Epoxy Resins (Courtesvof Dow Chemical Co.) ~ ~Products

Color*

EEW

Viscosity -cps at 25oc

D.E.R. 736

175-205

30-60

Polyglycol

D.E.R. 732

305-335

55-100

Polyglycol

Epon 87 1

390-470

400-900

12 max

Epon 872

650-750

1,500-3,800

10 max

Epon 872-x-75

625-700

2,000-2,800

6

75%NV in xylene

Araldite GY508

400-455

2,000-5,000

5

Polyglycol

Epotuf37-151

450-550

30,000-70,000

4

Heloxy 505

550-650

300-500

8

Type

*Gardner

Flame retardancy is often introduced into epoxy systems with halogen- or phosphorus-based additives. Some resins have, however, been provided which have these constituents prereacted in the resin. Chlorinated and brominated versions of DGEBA are the most common.

206


New developments in resin synthesis continues beyond the more traditional types. Among them is a polytiurctional resin based on triyphenylhydroxy methane designed for high and low temperature (-195Oto 2OO”C,3 19” to 392°F) resistance.

A resin having a low-temperature cure with high-temperature properties is triglycidyl p-aminophenol.

Resins having a high resistance to weather degradation and attack by biological organisms have substantial amounts of fluorine in the backbone structure,

where R, = C,F, or C,F,,.

Epoxy Resins

207

Epoxy-silicone hybrid resins have been developed for use in the molding of microelectronic packages. A typical structure is

Table 6-9: Silicone Epoxy ASTM Test Method

Properties

Melt flow (gm.110 min.)

I

D1238

Molding and Encapsulating Compound

I

I Thermos&

Melting temperature, ‘C T, (crystalline) T, (amorphous)

c:350

Processing temperature range, “F. (C=compression; T=transfer, I=injection, E=extrusion) Molding pressure range, lo3 p.s.i.

I

Mold (linear) shrinkage, in./m.

I

Tensile strength at break, p.s.i.

I

I

0.4-l .o

I

D955

1

0.005-0.006

1

D638

I

500-8,000

I

60

Elongation at break, %

D638

Tensile yield strength, psi.

D638

Compressive strength (rupture or yield) p.s.i.

D695

28,000

Flexural strength (rupture or yield), p.s.i.

D790

17,000

208


Table 6-9: Silicone Epoxy (Continued)

Izod impact, ft.-lb./in. of notch (M-in. thick specimen)

Molding and Encapsulating Comwund

AST Test Method

Properties

0.3

D256A

I

I

I D2240

A68-95

Coef. of linear thermal expansion, 1U6 in./in./ “C.

D696

30-200

Thermal conductivity, lOA cal.-cm./sec.- cm.* OC

Cl77

16

Specific gravity

D792

1.2-1.84

Water absorption 24 hr. (l/S-in. thick specimen), %

D570

0.2

Dielectric strength (1 B-in. thick specimen), short time, v./mil

D149

246-500

Hardness Shore

CURATIVES AND CROSSLINKING REACTIONS

The conversion of epoxy resins from the thermoplastic state to tough, hard, thermoset solids can occur via a variety of crosslinking mechanisms. Epoxies can catalytically homopolymerize or form a heteropolymer by coreacting through their functional epoxide groups with different curatives. In epoxy technology, curatives are most frequently called curing agents. Often

Epoxy Resins

209

the term hardener, activator, or catalyst are applied to specific types of curing agents. It is advisable to distinguish clearly between true catalytic curing agents that participate in the crosslinking via the traditional chemical concept of catalysis, and multifunctional crosslinking agents that become chemically bound in the final three-dimensional structure. The latter, therefore, can strongly influence the properties of the end plastic. Too often this fact is overlooked or not understood, causing non-optimized formulations to be used under inappropriate circumstances. Consultation with established epoxy formulating chemists is rigorously advised before indiscriminate changes in formula or cure conditions are made to effect property changes.

Stoichiometry

In the same vein, attention must be paid to the stoichiometric relationships between curing agents and resins. Catalytic curatives are added at relatively low levels (0 to 5 parts per hundred of resin, phr). Because their behavior during cure is truly catalytic, the application principles that apply to other catalytic polymerizations (e.g., with polyester resins) are the same with epoxies. On the other hand, multifunctional coreactants require that the user address the stoichiometric balance between the reacting species. An epoxy formulator will often establish the correct reactive ratio and supply the system accordingly. Many users, however, develop individualistic recipes and must therefore be able to calculate and optimize the proportions of curatives and resins. Au example of a simple stoichiometric calculation for DGEBA and a typical polyamine is shown in Table 6- 10.

210


Table 6-10: Example of a Stoichiometric Resin: Amine Curative:

Calculation

DGEBA Triethylene Tetramine (TETA)

Molecular weight of amine: 6 carbons 4 nitrogens 18 hydrogens

=6x12= = 4x14= =18x1=

72 56 18 ____

= 146 Molecular weight There are 6 amine hydrogens functionally reactive (bolded) with an epoxy group. Therefore 146 grams/mol ____________________ = 24.3 grams/equivalent 6 equivalents/mol Thus, 24.3 grams of TETA are used per equivalent of epoxy. If the DGEBA has an equivalent weight of 190 (380 g/mol/2 eq./mol), then 24.3 grams of TETA are used with19OgramsofDGEBA,or24.3/190 = 12.8gramsofTETAperhundredgrams of DGEBA.

Often a commercial curing agent’s chemical structure is kept proprietary or the amount of reactive fimctional group is ambiguous. In such cases, the vendor provides an amine or active hydrogen equivalent from which an appropriate mix ratio can be calculated. It is also important when performing stoichiometric balances to be aware of reactive groups that may be bifunctional (e.g., anhydride, olefm). Experience has determined that a precise stoicbiometric balance does not always produce a cured resin system having optimized properties. Consequently, a formulator will run experiments to establish the variance of properties of interest witb mix ratio. Figure 6-2 shows such a variance. Note that the optimum level of TETA is about 12.5 phr, almost exactly the theoretical value calculated in Table 6- 10. It is not uncommon, however, for the mix ratio to depart by 80 to 110% of theoretical. The selected and optimized ratio will subsequently be published in data sheets or on package labels

Epoxy Resins

211

for the user’s convenience or the vendor will prepackage the resin and curing agent in an appropriate volumetric or weight proportion.

PHR OF CURING

AGENT

Figure 6-2: Effect of concentration of DETA and TETA on deflection temperatures of DGEBA, (From Handbook of Epoxy Resins by Lee and Neville. Copyright 1967 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Co.)

Earlier, the ambiguousness of establishing an accurate equivalency was mentioned. Such a situation arises, for example, with aminopolyamides whose structure is too complex to determine just how many hydrogens will coreact with oxirane rings. Table 6- 11 shows the variance of hardness obtained with a wide spread of curing agent levels. The final use ratio is selected, in these cases, based on the best combination of desired cured-resin properties. Epoxy curing agents can be divided into two major classes: alkaline and acidic. The alkaline class includes Lewis bases, primary and secondary amines and amides, and other nitrogen-containing compounds.

212


Table 6-l 1: Variation of Hardness with Mix-Ratio (Skiest) I

I

W,eight Ratio Versamid 125/ERL-2795

Shore Hardness Durometer A

Barcol Hardness ImpressorModel GYZJ-935

40160

60-65

-

50/50

20-25

-

60140

-

90

65135

-

85

70130

-

50

75125

-

30

80120

-

5

ALKALINE CURING AGENTS

Lewis Bases

Lewis bases contain an atom with an unshared electron in its outer orbital. The main types of Lewis bases used in epoxy resin chemistry are tertiary amines. They catalyze epoxy polymerization if some hydroxyl containing molecules are present.

0

a3 N

+,cl”-.i, 2

-

--+

R,N’+

-C”,-L&-CH*CH_ I 0-

A I -CH,-CHI O-

Epoxy Resins

213

Primarily used in adhesives, laminating, and coatings, the tertiary amines are widely used as accelerators for acid anhydride and aromatic amine curing agents. They are rarely used at more than 1.5 phr unless they are used for low-temperature curing of epoxy adhesive formulas, in which case they may be used as high as 15 phr. Excess tertiary amine does degrade cured-resin properties. Many of the popular tertiary amines contain hydroxyl groups for enhanced reactivity. Examples of tertiary amines include tris-dimethylaminomethylphenol (DMP30), dimethylaminoethanol (S-I) and diethylaminoethanol (S-2), benzyldimethylamine, c1-methylbenzyldimethylamine, and triethyl- and trimethylamine. Cured systems behave similarly to aliphatic amines in large masses. The more steric hindrance of the nitrogen, the less reactive the catalyst. A tertiary amine salt (DMP-30 tri-2-ethyl hexoate) has been used in electrical applications because of improved resistance to humidity and good metal adhesion. Chemical resistance, however, is poor, and all properties drop rapidly with increasing temperature.

Primary and Secondary Aliphatic Amines

The epoxy-primary amine reaction is:

RNH,

/O\_CH

+ CH

2

OH -R’

-+R::CH,dHR’

The epoxy-secondary amine reaction is:

H

R :CH,

E

HR’+CH,-

PH

,CH,CH-R’

IO\ CH-R’

---b R N \ CH2Y -R’ OH

214


The hydroxyls formed are further reactive; however, the tertiary amine is generally too sterically hindered to contribute much to cure. Aliphatic amines constitute the largest group of epoxy curing agents. They can be used as is or adducted to modify volatility, toxicity, reactivity, and stoichiometry. They are characterized by short pot lives and high exotheims. They are skin sensitizers, and some can cause respiratory difficulties. Aliphatic amine curing agents can add to the yellowing of epoxy thermosets when exposed to ultraviolet light during weathering. In thin films, the lower amines may also exhibit a whitening or hazy appearance on the film surface caused by the reaction of primary amine groups with atmospheric carbon dioxide to form incompatible amine carbonates. Primary amines can be used as latent curing agents via reaction with a ketone (MEK, MIBK) to form the ketimine derivative which is reasonably stable in admixture with epoxy resin. When exposed to atmospheric moisture in thin lilms, the ketimine readily hydrolyzes to regenerate the amine and ketone. The amine proceeds to cure the resin, and the volatile ketone vaporizes from the film. The most common aliphatic polyamines are those that belong to the following homologous series: diethylene triamine (DETA), triethylenetetramine (TETA), and tetraethylene pentamine (TEPA). Typical properties are shown in Table 6- 12. Systems cured with these three curing agents generally have similar properties, particularly electrical and chemical resistance. Another common aliphatic amine, diethylaminopropylamine (DEAPA), provides more pot life than the DETA types and even requires some heat to complete the cure. DEAPA cured resins are softer with lower heat deflection temperatures. Trimethyl hexamethylene diamine (TMD) is another useful aliphatic diamine. Compared to other aliphatic amine curing agents, it is characterized by longer pot life and improved light stability, flexibility, and chemical resistance in cured products. TMD is often used in combination with other amine curing agents.

Amine Adducts

Adduction of DETA-type curing agents reduces their volatility, alters the reaction rate, and/or increases the mix ratios. The oldest adduct is the product

Epoxy Resins

215

of DETA and DGEBA. It provides a shorter cure time because the adduct is a partially reacted substance with added hydroxyls. Thus, less reactant in a more accelerated reaction is required to reach the gel point.

Table 6-12: Comparative Mechanical Properties of DETA and TETA Cured Epoxy Castings at 25 OC

Flexural modulus, X lo” psi

5.0-5.4

4.4-4.9


I

16,500

I

16,300

I

Tensile strength, psi

I

11,400

I

11,400

I

Ultimate elongation, %

I

5.5

I

4.4

I

Izod impact strength, f&lb/in. of notch

I

0.4

I

0.4

I

Hardness, Rockwell M

99-10s

1

106

Other types of adducts are based on ethylene and propylene oxides and alkylene polyamines. Examples are N-(2-hydroxypropyl)-ethylenediamine and ethylene oxide/DETA. These adducts tend to be more hygroscopic, calling for careful storage. They are not recommended for casting applications but to perform well in laminating and patching kits. Amine terminated polyglycols appeared in the early 1970s to introduce flexibility into the three-dimensional crosslinked structure of epoxies. A series of bi-, tri-, and tetrafunctional polyoxypropylene amines was commercialized. While providing for increased flexibility, they also increased pot life and decreased viscosity of formulations without the attendant age hardening that accompanies the use of nonreactive flexibilizers.

216


Cyclic Amines

Some applications require only intermediate cured-resin properties between the aliphatic and aromatic amines. The cycloaliphatic amines fill this gap. The four major cycloaliphatics are piper&line, N-aminoethylpiperazine (AEP), menthanediamine, and m-xylylenediamine (MXDA). Piperidine,

“N/(CH’)4, ‘@Hz), 7 has one active hydrogen for reaction; however, the resultant tertiary amine has suflicient catalytic strength to promote continued polymerization of epoxy. It provides long pot life and lower exotherm with other properties equivalent to the aliphatic polyamines. Its toxicity has, however, made it the subject of governmental restrictions which have significantly limited its use. N-aminoethylpiperazine,

NJcH2’\~~

H2N(CH2)

*

‘bq’

contributes improved impact strength when compared to the DETA series of hardeners. Although gel time and exotherm are also comparable, a postcure of 38’ to 66°C (100’ to HOOF)is required for complete cure. Menthanediamine, W

I

t&C,

H2N -

0

-C NH,

I

W

Epoxy Resins

217

makes processing easier through reduced viscosity of resin mixtures. It improves temperature resistance as compared to the aliphatics. Its properties are not as good, however, as the aromatics. m-Xylylenediamine,

yields the same properties as menthanediamine but contributes hardly any color to formulations. It is popularly used in so-called “water-white” castings. Table 6- 13 shows some general properties of the cycloaliphatic amines. Isophorone diamine (IPD) is a unique type of curing agent in that its structure contains both a primary cycloaliphatic amine group and a primary aliphatic amine group. Isophorone Diamine (IPD)

CH3 NH2 CH3

IsoPhorone

CH2NH2

Diamine (IPD)

Because the aliphatic amine group (methylene bridged) is considerably more reactive than the ring attached group, this material lends itself well to Bstaging applications. Complete cure with IPD under ambient temperature is achieved through use of an accelerator (a phenol, tert. amine salt, salicylic acid) or in combination with another amine such as trimethylhexylene diamine.

218


Table 6-13: Mechanical Properties of Cycloaliphatic Amine Cured Epoxy Resins (Bruins)

Heat deflection


7,000-9,500

9,000

9,000

10,600


6.0-8.5

8.8

2.9

6.7

Iz.od impact strength, &lb/in. of notch

0.3-0.5

1.0-l .2

0.3-0.4

Rockwell M hardness

90-96

95-105

105

Aromatic Amines

Aromaticaminesgenerally contribute the best properties of the amine cured epoxies. Specifically, they increase temperature and chemical resistance, extend pot life (although exotherms remain high), and always require heat for cure. In addition, in recent years many aromatics have become increasingly scrutinized and regulated by government agencies because of their potential health hazards. In some instances, it has been guilt by association

Epoxy Resins

219

because of their structural resemblance to aniline-based suspect carcinogens. One formulator has marketed an adducted aromatic amine that is reported to minimize toxicity with minimal loss in properties. In any case, the reader is advised to consult with vendors regarding the current hazard status and proper handling procedures before use. The three major aromatics are as follows: m-phenylenediamine (MPDA),

0 0

-NH2 -NH2

4,4’-methylenedianiline (MDA),

H,N -@Hz-@NH1

and 4,4’-diaminodiphenyl sulfone (DADS),

Because of their reduced reactivity, these materials lend themselves to formulations which are easily B-staged. Typically, these B-stage systems are stable at room temperature for months. Molding compounds, tape adhesives, and laminating prepregs are key applications for such formulas. MPDA contributes cured resin temperature resistance of 150’ to 177 “C (300’ to 350’F) as compared to 93Oto 107OC(200” to 225OF) for aliphatics. It is a skin and clothing stainer and must be melted into resins. MDA’s properties are somewhat less than MPDA, however, because its polarity is less; it contributes improved dielectric constant and loss factor. DADS yields

220


the highest heat deflection temperature of the aromatic amines. With proper selection of resin, systems resistant to 204°C (400’F) are common. A series of eutectic blends of MPDA and MDA has been commercialized, many blends with proprietary additives to retard crystallization. This tends to maintain hardener liquidity and allows one to mix resin and hardener at lower temperatures. This step then extends pot life and improves handling. Table 6- 14 lists typical properties of aromatic amine cured epoxy.

Table 6-14: Properties of Aromatic Amine Cured Epoxy Resins

(Bruins) Property

MPDA

MDA

DDS

Eutectic

Heat deflection temp., “C

150

144

190

145

Flexural modulus, psi x 10”

4.0

3.9

Flexural strength, psi

15,500

17,500


10,500

10,500


8,000

8,100

8,550

8,000


3.0

4.4

3.3

4.8

Izod impact strength, ft-lb/in. of notch

0.2-0.3

0.3-0.5

Rockwell M hardness

I 108

1106

4.4 17,900

16,400 10,500

0.5

III0

105-l 10

Aromatic amine adducts have also been prepared for the coatings industry. MPDA, MDA, and DADS have been adducted with styrene oxide, phenyl glycidyl ether, cresyl glycidyl ether, and low molecular weight DGEBA. Accelerators such as phenols or organic acids, (e.g., salicylic) are added to the adduct to promote ambient temperature cures. The adducts are usually dark in color and high in viscosity. They tend to produce brittle films unless non-

Epoxy Resins

221

reactive diluents such as dibutyl phthalate or benzoyl alcohol are added to reduce viscosity and flexibilize the formulation. A series of composites has been formulated using new novel bisimide amines as the epoxy curative. These curing agents have the following structure.

Typical amines used include DADS and MDA in the Ar structure. The increased aromaticity creates higher temperature and improved moisture resistance over current epoxy/graphite systems.

Polyamides

The polyamides used to cure epoxies are, in fact, aminopolyamides. The literature also refers to them as amidopolyamines. They are fundamentally dimerized or polymerized fatty acids that have been coreacted with various aliphatic amines such as ethylenediamine, DETA, TETA, and TEPA,

(W) 5 CH, where R = other dimer units and amine units.

222


The resultant molecules are very large and contain varying levels of primary and secondary amine hydrogens, reactive amide, and carboxyl groups -all of which can contribute to epoxy curing. Establishment of mix ratios is thus more a function of property selection than of stoichiometric balance (see the earlier discussion on Stoichiometry). Formulations made from these polyamides are the bases of “userfriendly” systems, because the tolerances on mix ratio are very broad. Although the resultant properties do vary, these systems find application in uses that do not require optimized, highly specific properties; for example, in twotube household glues where ease of mixing volumetrically is more important than maximum shear strength. The aminopolyamides introduce considerably reduced volatility and dermatitic potential, increased flexibility and impact strength, and water resistance (even to the point of effecting underwater cure). They have poor chemical resistance and low heat deflection temperature. Principal applications are coatings and adhesives with lesser use in laminates and castings.

Other Amines

Other amine-containing curatives fall into the catalytic class of curing agents. Dicydiandiamide has been long popular for use in stable one-can systems.

This material is used in catalytic quantities even though its breakdown products have been shown to participate in coreactive crosslinking. Various imidazoles have been used for similar applications. Typical are 2-ethyl, 3-methyl, and 2-ethyl-4-methyl imidazoles. N II Cti,CH,~ 1;

$HCH, ’ /CHZ

Epoxy Resins

223

Heat is required for full cure of these systems, and the final resin exhibits high temperature, electrical, and chemical resistance.

ACID CURING AGENTS

The acidic class of epoxy curing agents includes Lewis acids, phenols, organic acids, carboxylic acid anhydrides, and thiols.

Lewis Acids

Lewis acids contain empty orbitals in the atomic outer shell. Metal halides, like zinc, aluminum and ferric chlorides, and adducted BF, compounds (e.g., BF,-monoethylamine or BF,-etherate) are the most commonly used to cure epoxies. Most Lewis acids are latent catalysts used in heat-curing stable one-can systems with room temperature shelf lives of up to one year. Although the electrical properties are good, metallic corrosion can occur from decomposition by-products, thus precluding them from numerous insulation and encapsulation applications. Trifluorometbane sulfonic acid (triflic acid) and its salts have been used to catalyze hydroxyl/epoxy reactions for coatings applications. Blocking these acids with selected amines provides extended shelf stability. Heating unblocks the compound and allows the catalysis of the polymerization to proceed. Diethylammonium triflate has cocured epoxies with polyols, phenolics, and aminoplasts, as well as homopolymerized DGEBA. As a result, new one-component, very-high-solids epoxy coatings have been commercialized.

Phenols

Phenols will react with epoxies; however, they are seldom used as the sole curing agents. They perform much better as reactive accelerators for other curing agents.

224


Phenols can etherif) epoxies as follows:

Typical phenols are phenol-formaldehyde resoles and novolacs and substituted phenols.

Organic Acids

Organic acids are infrequently used alone as curing agents. The reaction mechanism is the key to the utility of the acid anhydrides. The esteritication proceeds as follows:

0 \ /O\ / --&OH +,c -c,

0

3

-rocP”I I

The alcoholic hydroxyl which is formed can etherify as described for the phenols. Both the esteritication and etheritication reaction are temperature dependent. High temperatures promote ester formation; low temperatures promote ether formation. Steric considerations, such as position of the oxirane ring and nature of the carboxyl group, will also influence the course of these reactions. Tertiary amines tend to enhance esteritication and retard etheritication. When organic acids are used, they act as accelerators like the phenols. Typical acids include dimerized and trimerized fatty acids, phthalic, oxalic and maleic acid, and carboxy-terminated polyesters.

Epoxy Resins

225

Cyclic Anhydrides

The cyclic anhydrides have been used most successtklly with epoxy resins. Ring opening is effected by the presence of active hydrogens present as hydroxyls or water, or by a Lewis base.

A

C OR

s

Anhydrides are the second largest curatives for epoxies and are especially suited for electrical insulation applications. While they are not shin sensitizing, their vapors can be irritating. The liquid anhydrides are easily blended into epoxy resins. The solid anhydrides, on the other hand, need heat and extremely good mixing for proper blending. Formulations have low viscosity, long pot life, and low exotherm. They have higher temperature resistance than the aliphatic amines, although not as good as some of the aromatic amines. Elevated temperature cure and postcuring are generally required. Because of the competing esteritication and etheritication reactions, cure schedules are often detailed and relatively complex. Consideration must be given to gel timekmperature, postcure time/temperature, presence or absence and type of accelerator, amount of hydroxyl groups present, anhydride/epoxy (A/E) ratio and stoichiometry, and amount of free acid. Tertiaryamines are the most favored accelerators, typical ones being BDMA and DMP30, which promote esteritication, as mentioned earlier. Acid accelerators, like BF, complexes, phenols, and dibasic acids, promote

226


etheriflcation. These considerations strongly influence the optimization of the A/E ratio and thus the cured resin properties. Phthalic anhydride (PA),

the least expensive anhydride, is used where formulation cost is of primary importance and overall performance is secondary. It is used in laminates, castings, and pottings and provides medium-range heat deflection temperatures. PA sublimes readily and must be reacted quickly with resin. It yields low exotherms in the production of large castings. Hexahydrophthalic anhydride (HHPA),

R

cc C\

0

C'

1:

is a low melting solid providing good general-purpose properties. In electrical encapsulation and filament winding, it adds resilience without significant loss in mechanical properties. It does not sublime like PA, and epoxy mixtures have lower viscosities combined with long pot life, low exotherm, and very light color. Nadic methyl anhydride (NMA),

Epoxy Resins

227

is also used in electrical laminating and filament winding. It is a liquid easily blended into resins. Cured products have light color, excellent arc resistance, and high heat deflection temperature. Dodecenylsuccinic anhydride (DDSA),

CH,(CH,)I,

CH -i, I H,C-C

0 / II

is another easily mixed liquid anhydride. The dodecenyl group contributes added flexibility and impact resistance to systems. In addition, this anhydride yields the most outstanding electrical resistance properties of this class of curatives. It has a high equivalent weight, so to optimize cost vs properties, it is frequently admixed with other anhydrides. Tetrahydrophthalic and maleic anhydrides (THPA and MA) are primarily used in anhydride blends. The THPA can cause darkening of cured resins, but contributes to lower cost while yielding properties similar to HHPA. MA by itself produces very brittle systems. In blends, however, it contributes to compressive strength with some loss in tensile and flexural strength. Pyromellitic dianhydride (PMDA),

is a high melting solid of limited solubility in epoxies. Blending with other anhydrides is common in order to facilitate incorporation into formulations. PMDA/MA blends have generated heat deflection temperatures of 250°C

228


(480’F). This curing agent is one of the earliest dianhydrides developed to maxim.& temperature resistance by significantly increasing crosslink density. Tensile and flexural strengths are reduced as a result but electrical properties are maintained. Two other high melting solid anhydrides that provide high-temperature-resistant epoxy systems are trimellitic anhydride (TMA),

and benzophenonetetracarboxyhc dianhydride (BTDA)

They have found wide use in molding powders and prepreg for laminating. Heat deflection temperatures of 200’ to 300°C (392’ to 572°F) are common. Chlorendic anhydride,

Cl Cl

CD I

CI-

Cl

::

/C

'0

-Cl

\/

Cl

C

a

229

Epoxy Resins

is the major halogenated anhydride for incorporation of flame resistance into cured systems. It contains 57% chlorine, yet holds good electrical and mechanical properties to its heat deflection temperature [2OO”C(392’F)]. Another anhydride curing agent of the aliphatic type that has achieved some degree of commercial use is maleinized polybutadiene. This product is formed by the adduction reaction of maleic anhydride with a liquid polybutadiene containing a high level of terminal unsaturation. Thus, anhydride groups are positioned along the poly-BD chain, yielding a polyanhydride which, in admixture with an epoxy resin, produces a high degree of cross-linking.

‘c=

cA

C

Such curing agents are characterized by low-cure temperatures ( 120’ to 130 “C) with shorter cure times and enhanced toughness properties (i.e., higher impact resistance while maintaining or improving hardness and Tg properties). These two package systems yield initial mix viscosities in the 800 to 2000 cps range and workable pot lives up to 3 to 4 hours, depending on operating temperature. RTM injection, composite fabrication, and filament winding are among established uses for these systems. Many of the solid anhydrides can be blended into eutectic liquids for improved mixing into resin formulas. The eutectics may be liquid at room or near-room temperature. Examples include a 70/30 mix of chlorendic and HI-WA,a 75/25 NMA/THPA, and a 50/50 DDSA/HHPA, all of which have melting points below 25OC (77’F). Table 6-15 describes some generalized properties of various anhydride-cured epoxies. Figure 6-3 compares the relative reactivity of anhydride curing agents to those of the other curing agents described here.


230

Table 6-15: Mechanical and Electrical Properties of Anhydride Hardened Epoxy Resins

stm@h, psi

8,100

11,8CO

11,400

10,wo

3,670

12,ca

Ultimate elongation %

4.5

4.8

7.4

2.5

0.9

2.6

Imd impact stmgth & lb/in of notch

0.3-0.4

0.46

0.3-0.4

0.48

0.34

100

105

111

109

111

Rockwell M hardness Dielectric constant 601106 cps

3.112.8

4.oL3.5

4.0/3.5

3.1513.0

3.731334

3N3.0

Dissipation factor 60/106 cps

.OOl-.Ol

sKJ11.02

.007/.02

.002/.02

.007/.026

.OQ3/.02

Polysulfides and Mercaptans

Several liquid polystide polymers have been available for curing and modifying epoxies for many years. They have the general structure, HS-(-C,H,OCH,OC,H,S-S-),

-C,H,0CH20C2H4-SH

232


These end groups are mercaptan terminated and end groups are sufficiently acidic to create a gel but generally not strong enough to complete cure. Consequently, they are added as reactive modifiers to other curing agent formulas. They impart impact resistance and toughness, increased flexibility, and reduced shrinkage. In the 1970’s, new accelerated polymercaptans, extremely fast (seconds to minutes) room temperature curatives, were commercialized. Prior to that, some BF, adducts could gel epoxies in seconds at 25°C (77°F). They were not active, however, at lower temperatures, and the properties were not very good. The new polymercaptans can cure at temperatures as low as -40°C (-40”F), allowing them to be used in adhesive formulations requiring rapid cure, such as bonding of highway markers (where traffic must be allowed to resume in a matter of hours) and repair and patch kits (so-called Sminute systems).

FORMULATION PRINCIPLES

Epoxy resins have achieved their commercial success due in no small way to their amenability to a variety of formulating techniques. Not only can different resins and curatives be brought together to achieve property goals, but many additives, reactive and nonreactive, can be included for further optimization. The epoxy resin formulator’s skill lies in his/her ability to effect the appropriate handling, curing, processing, and property tradeoffs needed by deftly manipulating the plethora of potential recipe ingredients. Table 6- 16 describes a typical epoxy formulation. By adjusting the type and quantity of ingredients in the table, the formulator can create systems that vary from stable free-flowing molding powders to highly viscous caulks and adhesives to clear, water-thin castable liquids. With the conclusion of our summary of resins, curatives and catalysts, we next briefly review the remaining ingredients.

Epoxy Resins

233

Table 6-16: A Typical Epoxy Formula Curative Side* Resin Side Curing agent(s) Epoxy resin(s) Catalysts and Accelerators Epoxide-containing reactive diluents Non reactive diluents and resinous modifiers** Fillers (reinforcing and/or non-reinforced)* * Colorants (pigments and dyes)** Rheological additives (thixotropes, viscosity suppressants)** Property promoters (wetting agents, adhesion promoters, flame retardant additives)** Processing aids (deaerating agents, mold release agents)** *In one-can systems, both sides are combined together and are stored under shelf -stable conditions until ready for use. **hIay be commonly added to either or both sides provided no interfering reactions can take place.

Epoxy-Containing

Reactive Diluents

Epoxy-containing reactive (diluents are either low viscosity epoxy resins or monoepoxides. They are called reactive because the epoxide moieties will react with the curing agents and their presence must be accounted for in the stoichiometric analysis. Table 6- 17 lists typical monoepoxides. Tables 618 and 6- 19 list commercial DGEBA resins that have been prediluted with reactive diluents.

Table 6-17: Commercial Epoxy-Containing

Reactive Diluents”

234


Table 6-17: Commercial Epoxy-Containing Reactive Diluents” (Continued)

1.4~Butanediol

Dialvcidvl Ether

CH3 H2C-CH-CH2-0-CH2-C-CH2-0-CH2CH-CH2

I CH3 Neopentvl Glvcol Dialycidvl Ether

0-CH2-CH-CH2

Cg’-‘l9

-6%

Nonvl Phenol’Glvcidvl

Ether

CH3 - (CH2)3 - CH - CH2 - 0 - CH2 CH - CH2

2-Ethvlhexvl

Glvcidvl Ether

*From Handbook ofEpoxy Resins by Lee and Neville. Copyright 1967 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Co.

Epoxy Resins

Table 6-l 7: Commercial Epoxy-Containing (Continued)

CH2-CHCH2-0-CH2

235

Reactive Diluents”

CH2 -0-CH2CH-CH2

Cyclohexane

Dimethanol

Dialvcidyl Ether

CH2-0-CH2-CH-CH2 I /“\ CH-CH2

CH3-C-CH2-0-CH2

/“\

I CH2-0-CH2

Trimethvlol

CH-CH2

Ethane Trialvcidvl

U-l 2 -0-CH

Ether

2 -Ci?kH,

/“\ H3C-CH2-C-CH2-0-CH2-CH-CH2

:H 2 -0-CH

Trimethvlol

2 -C&Z,,

ProDane Trialvcidyl

Ether

*From I&ndbc& of Epoxy Resins by Lee and Neville. Copyright 1967 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Hook Co. Updated Material Courtesy of Dow Chemical Co.

236

Handbook of lhermoset Plastics

Table 6-18: Lowest-Viscosity Diluted Resins (Courtesy of Dow Chemical Co.) Products

D.E.R. 324

EEW

Viiosity -cps at 25 OC

Color*

197-206

600-800

3

I AralditeGY506 I AralditeGY507 I AralditeGY509

1 172-185

1

I I

I I

I Epon 815

1 175-195

1

500-700

1

1 185-200

1

500-700

1

I Epon 813

185-192 189-200

500-700 500-700 500-700

1

I I

Dilueut

Akyl glycidyl ether

I CGE I Epoxide 7

I I I

1

1 BGE-12%

1

7

1 CGE

1

1 BGE-12%

3 1

Epon 8132

195-215

500-700

1

Epon 828/Heloxy 8 blend

Epotuf 37-130

175-195

500-700

3

BGE-12%

Epotuf 37-137

185-200

500-700

3

CGE

I

* Gardner. NOTE: These resins are similar to the D.E.R. 331 resin diluted with a reactive diluent to reduce viscosity. This permits easier laminating and/or higher filler loading for cost reduction. The resins are suitable for most general purposes where we temperatures will not exceed 200 OF and are generally used with polyamine or polyamide hardeners.

Table 6-19: Medium-Viscosity Diluted Resin S (Courtesy of Dow Chemical Co.) I Pmducts

I EEW

I

AralditeGY502 Epon 824 Epon 8131 Eoon 8161

I I I I

232-250 192-204 265 210

I

Viiosity Color* I -cps at 25 OC I

I I I

I

2,100-3,600 4,000-7,000 1,225 2.200

I 3 I lmax ~~ I

cross

linking

i-

RI-NC=0

___-_

linear

urea

>

urethane

>

I

R”

The advantages of polyureas are chemical resistance, a wide range of flexibility from very soft to very hard, weather resistance, and high thermal stability up to 350°F.t4] The chemical resistance of various liquids is presented in Table 7- 1. 0

R-NH2 + RI-NC=0 @rimary amine) (iswymate)

II

______> R _ NH _C- NH _ R’ (fat)

@=4

Formulators were hindered by the idea that reactions between the isocyanates and the primary amines were so fast that the system had no commercial value and that this line of endeavor was to be avoided. The appearance of new amines, new uses for existing amines, and the mechanical application systemst41have rendered that idea obsolete. The quick reaction time and excellent properties of polyurea are a commercial reality. Furthermore, the liquids are 100% solid; the application has no VOC. The reaction between the amines and the isocyanate requires no catalyst. When applied as a film, it does not sag. The product can be sprayed and poured without the


272

Table 7-l: Chemical Resistance SDot Testing

A A A C NR IC IC

. ._. A A B A NR IA IB

IR I--

I-

IC

IC

_____________ Gasoline, unleaded

Hexane Hot tub wate? Hydraulic oil JEFFAMINE@D400 Me:thanol 5%methanol/gasoline Motor oil Propylene cart onate _~~... Sodium hydroxide 50% 25% 10% 5%

Sutiric acid, conc.4 Sulfuric acid a 50%

A B A NR IA 1A IR IC I--

IR

A A B A NR IA B B C

B A A A NO

B A A A NO

B A A A NO

B A A A NO

NO

NO

NO

NO

B A C A

B A B A

B B 10% B A 5% B Vinegar (5% acetic acid) A A A Water Zode descriime chemical’s effect on elastomer: A - No visibli damage B - Liile visible damage C - Some swelling, discoloration, cracking NO -Not recommended ‘Trademark of the Clorox Company ‘All samples darkend ‘Brominated water 4Lessthan 24-hr exposure

I

Thermoset Polyurethanes

273

need for polluting solvents! These accomplishments are made possible by a dispensing unit that blends the amines and diisocyanate prepolymers, or straight diisocyanate, at a 1 to 1.05 stoichiometric mix ratio (i.e., slight excess of the diisocyanate) in a self-cleaning mixing head with very low residency time. The equipment controls the temperature, as required, from room to elevated temperatures. Increasing processing temperatures and pressures improve flowability and the mixing of products passing through the unit (Figures 7-l and 7-2). Even though the reaction time is in seconds, coatings are being sprayed or poured onto prepared substrates to produce an amazing variety of products using this method. Table 7-2 shows the potential uses of sprayed polyureas. The reaction time is consistent and the liquid products do not deteriorate, in contrast to polyurethane systems that require the presence of a catalyst that loses its reactivity over time.t5] Because the amine-to-isocyanate reaction takes precedent over an isocyanate-to-water reaction that produces a urethane foam, claims have been made that polyurea sprayed onto water surfaces forms a solid film or skin. t41Perhaps it can be used as a quick cover on water to prevent evaporation. Another possible use is reinforcing a sandbagged wall with a coating of in-place polyurea film to protect against the ravages of water erosion. A well-known fact in polyurethane chemistry is that replacing aromatic diisocyanates with aliphatic diisocyanates can produce good, light stability polymers that are more durable for exterior applications than the aromatic isocyanates.t61 Polyureas are especially suitable for long-lasting exterior coatings with their resistance to weathering and deterioration by oxygen, especially cracking and embrittlement.81 There is a good potential for use as a roof coating. While the cost of materials may be a factor, the longevity of the applied product may favor its use. Examples of ingredients used in the polyurea spray application are given in Table 7-3 and 7-4. The dispensing gun has been used successfully in reaction injection molding (RIM) applications with equipment modification and slower formulations. The high temperature used seems to slow the reaction time of the amine and isocyanate.t’l

274


Figure 7-1: Gusmer Spray Gun


Figure 7-2: Gusmer Proportioning Unit

275

276


Table 7-2: Potential Use of Spray Polyurea

Since Texaco Chemical Company introduced the concept of polyurea spray elastomers in early 1989,a large number of innovative companies have been evaluating these exciting materials. Severalof these companies have now successfully commercialized their own versions of these polyurea elastomers. Others are in the final stages of their evaluations and expect to begin commercial use soon. Presented here is a partial list of the potential nonconfidential uses for polyurea sprayed elastomers as reported by the companies who have been evaluating these of potential users of these products. This list is presented in order to stimulate the thiig unique polymers. No guarantee of suitabiity in any specific application is implied or intended.

1

IPotential Uses Protective coatings Abrasion-resistant coatings Coatings for swimming pools and accessories Pipe coatings Holding-pond linears Flooring Coatings for expanded polystyrene Roof and deck coatings Soil stabilization Construction coatings Grouting Potting and encapsulation Asbestos encapsulation Courtesy: Texaco Chemical Company

Industrial coatings Highway striping Wire bundles Automobile undercoating/soundprooting Lowcost molds Spray-molded parts Automotive gravel protection coatings Highway and bridge overlays Conformal coatings Temporary building repairs PVC mplacements Glass-reinforced polyester replacements


I

I

Table 7-3: Aliphatic Spray Polyurea Elastomers i-1 j-2 1 -3

I-4

Isocyanate Quasi-prepolymer m-TMXDI@ TciX?AAmTc@ Resin Blends JEFFAMINE* T-5000 JEFFAMINE@T-3000 JEFFAMB JE@’ D-2000 JEFFAMINE@T-403 .TEFFAMNF,@ -l-I-230 _-___-.-.-__

Index Iso / Res volume ratio Iso / Res weight ratio Elastomer Physical Properties Tensile strength, psi Elongation, % t Tear strength. nli Shore A Hardness I osec 1osec She3re D Hardness

I

I-I

277

62.5 26.3 17.4 ______ _____ ______ ______1I 26.3 1 ~~~~ I 26.1 ~--~ 1 37.5 i 31.6 1 26.1

1 I 13.6 ~-~1 42.4

1 -----

I

I

20.3

-_._

I

I 1

1 ‘U-I4

I317

I

1.00 GO 1~--.oo l...00 I 1.05 1I 1.07

i.05 1.00 I 1.10

G5 1.00 I 1.10

1

I

1

I

I

546 I319 Ill1 I

109 1 1319I260 I

57 48

82 73

ltTij&t+ Ref NES6650-53

--__-_

1 15 II

I

osec psi 1Modulus, psi

I

1307

1

I

~---

956

323 267 -____ -----

I

~---

I

I 1649 __ ._

Ill1 I 334 --_-_____

r

278


Table 7-4: Slower Polyurea Spray Formula Slower Systems Formulation B-Component (weight%) JEFFAMINE@ T-5000 JEFFAMINE@ T-4000 JEFFAMINE@ T-2000 Unilink 4200 A-Component (weight %) Isonate 143L THANOL SF-5505 A/B MIX Ratio By volume By weight Elastomer Properties Glass Content, % Gel time. set Surface tack-free time, set Hardness, Shore D 0 set 10 set Tensile strength, psi Elongation, % Tear strength, pli Flexural modulus, psi @ 77°F @ 158°F @ -2O’F Abrasion resistance, lOOOg., 1000 rev., H- 18 wheels Izod Impact, notched, ft. - lbs. MVT, perms at 30 dry mils Water absorption, wt. % gain deference No. 6325-4 1

18.66 18.66 7.46 55.22 60 40 1.011.o 1.03/l .oo 41-1 0 45 90

41-2 6 ---____

56 50 1715 147 453

44 39 2067 5 607

45620 13100 103080

98520 36270 170640

170 mg 7.67 0.014 1.74

_____ 8.38 _-_-_ -_--_


279

REACTION INJECTION MOLDING (RIM) PROCESS

General Motors Corporation used the polyurethane RIM process to produce vertical body panels for the 1984 Pontiac Fiero and other models.[41 Its all-plastic skin construction used more than 45 pounds of glass-fiberreinforced and filled-polyurethane elastomers. RIM products were used from bumper fascia to integral skin parts. In 1988,65% of the cars made in the United States had polyurethane bumper systems. In 1987, cancellation of Fiero production dealt a blow to RIM polyurethanes. Today, polyurethane use in the RIM process is a commercial reality. The industry expects to make a comeback.

MODIFICATION NATES

OF AMINES FOR REACTION

WITH ISOCYA-

Efforts have been made in the past to extend the short reaction time of the primary amines using isocyanates. Methylene dianiline (MDA) is thought to be the ideal curing agent for isocyanates, but it reacts with the isocyanates so quickly that early formulators avoided its use. The classic example of altering molecules to allow longer use time is to insert a chlorine molecules onto the aromatic rings adjacent to the amines to deflect the reactivity of the amines. The result is MBOCA from MDA: MDA

+

2 Cl

-------

>

methylene

his ortho chloro

aniline

(MBOCA) The addition of a chlorine molecule on the aromatic ring slowed the reac&ty, making the amine commercially viable. The disadvantage was that MBOCA had to be liquefied, either by melting at 100” C or being dissolved in a solvent before being incorporated with a diisocyanate prepolymer. Using the melted MBGCA also required the heating of the prepolymers to 100°C so that the lower temperature of the prepolymer would not recrystallize the melted MBOCA and prevent it from participating fully in the reaction. A further obstacle to MBOCA use was the addition of the chlorine molecules

280


onto the MDA causing MBOCA to become a greater health hazard from an environmental point of view. It was placed on OSHA’s chemical warning list.

RECENT DEVELOPMENT

R-NH

+

RI-NC=0

----_ >

linear urea

I

R” secondary amine

isocyanate

urea

Earlier isocyanate reaction with secondary amines was determined to be very sluggish at room temperatures and required heating for the reaction to take place. This situation presented a drawback for field use; it was therefore decided that there was little potential for commercial use; hence, not much effort was directed to the development of secondary amine curatives. Recently, a company t91has succeeded in replacing one of the hydrogens with an alkyl group of both primary amine nitrogen of MDA (solid at room temperature).

UNILINKe and the phenylene amines structures below are secondary amines which are easy-to-handle liquids at room temperatures. They readily blend with any polyamines and polyols. With the use of a novel new accelerator they provide controllable reactions with diisocyanate prepolymers at room temperatures. The UN&INK@4200 structure is:t’“l


281

H Cd2 I/ \ RI. N-C c -c-c C- N-R1 \\ 4 \\ // c-c H C-C H I/ \ I C-N-R, R,-N -C $ // c-c

When the modified and compounded amines’ reactivity (with the use of the novel catalyst) is compared with MBOCA to cure isocyanate prepolymers at lOO”C,it has twice the pot life allowing for larger castings. Because there is only a single hydrogen at the amine site (secondary amine), the polyurethane created by the sole use of the alkyl-substituted amines produces thermoplastic products softer than MBOCA. The secondary alkyl group functions as an internal plasticizer. Crosslinking triIYmctiona1 polyols are typically added to produce thermoset compounds. Although these secondary amines are not a “drop in” replacement for MBOCA, but a complement, they can be compounded to give similar properties.t’] These secon dary amine curatives react with all isocyanate prepolymer molecules made from MDI, TDI, and aliphatic diisocyanates to form linear molecules at room temperatures. Linear polymers are formed by the elimination of the hydrogen on the amine, which normally would provide for cross linking. The secondary amines reacting with slower reacting aliphatic diisocyanates allow additional pot life. Cross linking molecules, such as Quadrole (tetrapropoxylated ethylenediamine), typically used at 30% (by equivalent) levels, are required to obtain optimum polymer properties. These products do not require the usual catalysts, although the usual organometallic catalysts accelerate the polyol crosslinker with the isocyanate more than the reaction of the aromatic secondary diamine with isocyanates. I91 Organic acids such as oleic or stearic work well in accelerating the secondary diamine with isocyanates, particularly

282


at room temperature. Other useful catalysts are organobismuth, organomercuty and dibutyl tin dilaurate. The hardness values of Shore A of 18 to Shore D of 70 can be obtained using these a&y-substituted amines. Obtaining a low Shore A reading does not require the addition of plasticizers. Compression set values as low as 3% have also been produced with these amines. Tensile strength values of 1400 to 6100 psi, with elongation of 400% for so&r polymers to about 250% for the harder polymers, are possible. Adjusting the Quadrole level can influence tensile strength, elongation, tear strength, and compression set without significantly affecting modulus and hardness.

Toxicological Profilef”1

Suitable precautions are necessary in handling the material. It is non toxic by dermal application but is moderately toxic by oral administration. It is not an eye irritant, but is a moderate skin irritant upon prolonged or repeated contact. The product is not mutagenic. Permissible exposure limits have not been established. From a formulator’sview point, the secondary amines provide a new and important element in the polyurethane field. They give formulators a wide variety of options and great versatility in designing new products. Formulators can obtain properties similar to those derived by using MBOCA, but at room temperature. NF 1500 Iiom Cal Polymer (Fig 7-3) is one of the newer products on the market based on this new curative approach. Table 7-4 also presents an example of formulation based on the new substituted curative.


283

Figure 7-3: CalPolymer NF 1500 Special Product Specification and Packaging Calthane NF 1500 Specialis a two part, non-tilled urethane elastomer. It is based on MDI, and MOCA fi-ee. Calthane NF 1500 Special is the toughest, solvent f&e, room tempemture cured coating (and adhesive) available today. Shoes soles coated with only 2 mils. Of Calthane NF 1500 Special will outwear the uppers under normal wear. As a castable automobile tin+ Calthane NF 1500 Special would require 100,000 miles before it would wear out. Calthane NF 1500 Special is also a fantastic adhesive, giving good initial bond strength characteristic. It is a natural for composite applications. Calthane NF 1500 Special “A” is a low viscosity, MD1 based prepolymer aoecific gravity at 77-F is 1.22. Calthane NF 1500 Special ‘3” is a medium viscosity resin system: specific gravity at 77’F is 1.05. whenmixedas -W 11x3-t“A”, with 5 parts of “B”, by weight or volume, the resultant, room temperatu~ cured elastomer has these properties.

Hardness

I

Tensile

88 shore A f 3

I

6,000 psi I

Elongation

I 560%

Tear Strength

1 267

Cure Cycle

I

psi, DIEC

one week at room temperature or 1 overnight at 160”F

I

284


AMINES

JEFFAMINES@(Polyoxyalkylendimine)

JEFFAMNES@ (Table 7-5) in polyurea spray application are an interesting linear d&nine with a backbone of polyether. Molecular weight ranges from 230 to 5000. One would expect the higher molecular weight JeRunines to contribute flexibility to the produced polyurea polymers similar to the effect polyether polyols have on flexible polyurethane foams. The lower molecular weight Jeffamines are expected to produce harder polymers.

Table 7-5: JEFFAMINEP Functionality

Approximate Molecular Weight

JEFFAMBlE@T-5000

3

5000

JEFFAMJNE@D-4000

2

4000

JEFFAMINE@T-3000

3

3000

JEFFAMINE? D-2000

2

2000

JEFFAMINE@T-403

3

400

JEFFAMlNE@D-230

2

230

Thermoset PO&urethanes

285

WATER-BORNE POLYURETHANES

Environmental and legislative pressures have driven the urethane industry to mod@ its polyurethane formulations in order to achieve a more efficient way of utilizing technology and, if necessary, to go outside the normal boundaries of commonly practiced urethane technology to manufacture products. The formulator’s original guide to polyurethane compounding was to avoid reactions (6), (7) and (8): 0

II (6)

R -N=C=O + H2() Lmymate water

________R

_

NH_

C_ OH Cmbamic acid

0

(7)

II R-NH-C-OH ~.&amic acid

______----a CO2 +

R-NH2 amine

carbon dioxide gas

0

II

(8)

R-NH2 amine

+ R'_N=C=O____-_> R-NH-C _ NH _ R Isocy&ulate

-

Previously, formulators were cautioned to take all possible steps to avoid introducing water into two-component polyurethane systems. In reaction (6), unwanted water reacts with the isocyanate to produce carbamic acid. In reaction (7), the carbamic acid decomposes into carbon dioxide gas and amine. In reaction (8), the amine immediately reacts with more isocyanates. Usually, the gases are trapped in the polymerizing urethane mass. The amines immediately react with isocyanate and produce ureas. This throws the formulation off balance, adversely alters the quality of the formed product, and destroys the shelf life. The idea of avoiding all water contact for one- or two-component polyurethane coating systems has been discarded. Now, there are water-based

286

Handbook of Dzermoset Plastics

or water-dispersed one- or two-component polyurethane polymer coatings. Industry now considers polyurethanes a premium material for coatings and has established a demand for them. Polyurethanes coatings are more resistant to acids, bases, pigeon excrement, acid rain, and industrial and automotive pollutants than many conventional coatings.tr2] Additional properties favoring polyurethanes are abrasion resistance, solvent resistance, light stability, and excellent weatherability. The result has been of various water-borne polymer types exhibiting a wide range of use and properties that can frequently match and exceed those provided by conventional solvent-borne systems.tL31 Two major classes of water-borne polymers are in use: aqueous polymer dispersion prepared by emulsion polymerization and dispersion of preformed polymers in water. Examples of the first class are aqueous polyurethanes. Examples of the second, produced by free radical polymerization, include acrylic copolymers, vinyl acrylics, styrene acrylics, and styrenebutadiene. Water-borne resins do not form true solutions, but rather colloidal solutions or dispersions of different particle size. Typical resin is rather hydrophobic and solvent soluble, but it is not water soluble. The urethane the~ryt’~] is based on the fact that aliphatic diisocyanates blended into water with dispersing aids react very sluggishly, and initiation of the reaction is very slow. After 4 hours of heating the mass to 24”C, only 5% of the isocyanate was consumed. At 20 hours at 24OC, only 25% had been consumed. Prepolymers of aliphatic diisocyanates dispersed in water at low temperatures remained stable for relatively long periods in aqueous two-phase systems. Water-borne polyurethane systems generally begin as prepolymers. The prepolymer systems that provide lower viscosity are carefully selected to avoid using solvents. The system is usually one pack, and users do not have The new dispersion technology allows for to handle free isocyanates. t11t13rt141 preparation of high molecular weight polymer dispersion having the characteristic properties of polyurethanes. Earlier experiments to disperse the prepolymers with surfactants under high shear mixing resulted in large particle sizes, poor colloids, and low shelf stability. The excess surfactant contributed to water sensivity of the applied film. At present, surfactants are avoided. Procedures used today instead involve incorporating a monomer in the backbone of the prepolymer In one system, the isocyanate-capwhich renders it water dispersible. t11t141t151


287

ped prepolymer is synthesized with diisocyanate, polyols, and dimethylolproprionic acid (DMPA) as the stabilizing diol agent. The prepolymer can be either isocyanate or hydroxyl terminated. Upon neutralization of the carboxyl group, the prepolymer is dispersed in water where spontaneous particle formation oc~urs.[~~~(Colloidal solution or dispersion of different particle sizes occurs). A minimum critical concentration of 0.25m mole/g DMPA is required for particle formation. Ino reasing the concentration of DMPA increases translucency, indicating a decrease in particle sizes. Triethyl amine and hydrazine monohydrate are used as neutralizing bases. Stabilization arises solely from ionized carboxylic groups, which are relatively inactive with nucleophiles.[11t121[151 The bulk of the hydrophobic materials is inside the emulsion particles. At this point, a choice is offered to either produce a single-component or twocomponent system where one component is isocyanate terminated and the other is hydroxyl terminated. Currently, a one-pack system is favored. Functional chain terminators are added at this time.[11[L31 By varying the molecular structure of the chain terminator, a dispersion with increased functionality can be made from linear prepolymers. The colloidal properties of the polyurethane dispersions have broad particle size distributions. The particles have openness and are swollen with water[‘l as a result of the synthesis procedure used. An interesting and important feature of polyurethane dispersion is that, although the particles are substantially fluffy and laden with water, the resulting film does not suffer from water sensitivity. In fact, extremely waterresistant films can be obtained through caret3 design of the polymer backbone. Their hydrated, open, and fluffy morphology give polyurethane particles the ability to harden at lower temperatures. The amount of solvent used in the prepolymer determines the openness of the particles.[‘] Optimum properties were obtained when the -NC0 to -OH ratios were 2.0 or greater to 1, presumably due to the reaction with water. Initial drawdown of waterborne unpigmented polyurethane systems are opaque white but become transparent as water evaporates quickly from the filmnl in 20 to 60 minutes, depending on the film thickness.[131 In formulating surface coatings, the dispersions are compounded with surface active agents, wetting agents, defoamers, flow control agents, antifoams, and suspending aids. The advantages of water-borne polyurethane systems are:

288


(1) Low VOC formations that surpass the requirements of air quality regulators, (2) Film cures at room temperatures, (3) Increased functionality of the prepolymer, (4) F’repolymerviscosities and dispersion characteristics that can be manipulated, and (5) Improvement through cure.

The claim is made that this system produces polyurethane coating qualities that compare favorably with solvent-borne aliphatic diisocyanate polyurethane coatings.t11t131 The limitation to this approach is that drying conditions significantly affect the final coating. Low temperatures and high humidity produce lower quality films, probably due to slow water evaporation and the time needed for the water to react with the isocyanate.

OTHER TWO-COMPONENT

POLYURETHANES

The regulatory requirements to lower VOC content had caused a demand for replacement polymers for currently available systems. The new sets of polyuMhane polymers offer the opportunity to replace next-generation products with improved properties that the original never had. Improvements can be made without changing the basic performance parameter if the ingredients are chosen carefully. As always, cost may be a factor, but the longevity of the polyurethane that delays recoating time and improved chemical and scuffresistance and weatherability will overcome this problem. Industry now recognizes versatility of the polyurethanes. When the word “polyurethane” is used, it generally implies “quality”. Solvent-based, twocomponent-based products allow applications to work year-round in lessthan-ideal conditions. These products can be used during cold periods because they can cure at very low temperatures.t161 The two-component system can mean that the system is composed of a matched component A and component B. In the moisture-cured system, the

Thermoset Poljwethanes

289

component A prepolymer is isocyanate terminated; atmospheric moisture is the second component. The result is a one-component system. Recently, a manufacturer has developed an intriguing new set of polyurethane prepolymers[171with low-end viscosities that facilitate the designing of new improved substitution products. Raw material suppliers have greatly simplified the choice of materials to use for the components. These products are versatile enough to use as coatings as well as cast solids. Formulators can pick their desired prepolymers for investigation from the list on Table 7-6 and 7-7. More and more of the recent popular available polyurethane formulations ate based on the use of aliphatic diisocyanates for their light stability and nonambering values, as well as for their toughness, flexibility, fast cure times, and lower temperature applications. Other features imparted in the coatings are outstanding chemical resistance to atmospheric and industrial fallout such as acid rain, sulfuric acid, or hydrochloric acid. Also, there is resistance to chipping; and scratch resistance to softer plastics, paints, and furniture.[121 Table 7-6 presents a variety of prepolymets and the basic components of the prepolymers, along with the required technical data for investigation. Air Products’ two-component, high-solid polyurethane and polyurea coating formulations meet VOC requirements. Their literature describes 80% nonvolatile, aliphatic diisocyanate prepolymers coatings cured with amines dispersed in xylene or MIAK (5methyl-2-hexanone) solvents with a pot life of up to 1 hour. The product guide certainly evokes the imagination of the formulators and facilitates in developing new high-performance products. Computer friendly published data sheets from suppliers have simplified formulation, compounding to a point where computers can perform the initial formulation investigations, saving valuable time by avoiding false statt~.[~~Exxon Chemical Company offers CO-ACT, a computer reformulation service. The service is based on a computer program that uses solubility parameters and other physical characteristics to provide information on formulation cost, density, surface tension, viscosity, dry time, and resin/solvent compatibility throughout the drying process.[141

290

Handbook of i%ermoset Plastics

Table 7-6: Air Products Prepolymers TDI-Polyether Prepolymer Airthane

PWOba

1

PET-75D PET-70D

TDI

1 PTMEG

1 499

1 8.3

PET-95A

TDI

1 PTMEG

1 674

i 6.2

PPT-95A

TDI

I PPG

I 686

I 6.1

I PPG

I 500

I 8.4

PC-500 PC-350

TDI

PPG

3.9

I

PCG475A TDI 2.7 I TDI-Polyether kpolymers TDI- lolyetherplepolymers offer excellent price/performance adval tages inmoisturecure, indoor-pigmented coating or in other nonweatherable plycol (PPG) are our most costeffectiveversions. Polytetmmethyleneether golycol (PTMEG) version, on the other hand, offer better tear and tensile performance and may demonstrate improvements in chemical resistance. Please see the following data sheets for viscosity/temperatur and viscosity/ percent solids relationships for these products.

291


Table 7-6: Air Products Prepolymers (Continued) I

I

lPDI-Polyether Pnzpolymers Airthane Prepolymer

Isocyanate

Primary Backbone

Equiv. Wt.

Nominal % N~CO

Average Functionality

APC-722

IPDI

PTMEG

722

5.8

2.0

APC-504

IPDI

PTMEG

504

8.3

2.6

PTMEG

317

13.3

2.9

PPG

1225

3.4

2.0

, Ax-3 17 APC-1225

IPDI

I

I rPDI-Polyether Prepolymers IPDI-polyether-basedpepolymers combine the advantages of polyether systems with the reactivity control of isophorone Mate.

I

IPDI-Polyester Precolvmers

[PDI-Polyester Prepolymers IPDI-Polyester-based prepolymem are ideal for weatherable coating applications. Using these prepolymers in two-component formulations pro-vides more even mix ratios and allows for the development of lower VOC formulations.

292


Table 7-7: Air Products Extended Prepolymer List IAIRTHANE Prepolylll PET-75Der PET-70D

I Description TDI-PTMEG TDI-PTMEG

I Nominal % NC0 9.0 8.0

I

Elastomer Durometer DiamineCured Trial-Cured 75D I70D

A_____

TDI-PPG PPT-80A TDI-PPG PSThOD TDI-polyester PST-90A TDI-polyester PST-80A TDI-polwster AIRTHANE@Prepolymers “Perfect” prcpolymem ~0.1% residual TDI OSHA and DOT nonhazardous

6.0 “3.5 6.4 4.2 3.1

l

l

l

l

l

l

95AbOD 80A 60D 90A 80A

70A 60A 55A

Len gpotlife Faster demold Superior dynamic performance in the final elastomer

I


293

Table 7-7: Air Products Extended Prepolymer List (Continued) CYANAPRENE 1 &lvnler Irn 207,n

1 Nominal

Elastomer Durometer

I

I

1080

t incm

I

Tn

l

I

Tn

.Y.”

I 1nsn ____ 31hn --._

I sinn _I””

-_-

Irn

t A-70M

,“1.

““I

I

4.2 90A 55A A-9QM TDI-polyether D-SQM TDI-polyether 5.0 95AkSOD 60A D-55 TDI-polyether 5.4 55D 65A 70A D-6 TDI-nolvether 5.8 60D I 70D I 75A 1 D-7 I TDkolvether I 6.7 I CYANAPRENF? Prepolymers l Broad range of TDI-polyether and TDI-polyester products l Used to ~IWJZhigh-performance elastomers that exhibit excellent strength, resilience and abrasion resistance l Low residual TDI (~0.7%) l Easy to melt “QM” TDI-polyester prepolymer l Lower viscosity polyether-based products l Uniform batch-to-batch consistency

294


Table 7-7: Air Products Extended Prepolymer List (continued) SMEISMS Prepolymer

Description

Nominal %NCO

Elastomer Durometer Dial-Cured

SME-75A

MDI-PTMEG

4.0

75A

I SME3OA

I MDI-PTMEG

I 5.1

I 80A

SME-90A

MDI-PTMEG

7.3

90A

SME-95A

MDI-PTMEG

8.4

95A

SMS-80A

MDI-polyester

5.0

80A

SMS-85A

MDI-polyester

6.5

85A

SMS-90A

MDI-polyester

8.0

90A

SMS-95A

I MDI-polyester

I 10.1

1 95A

SMEISMS Prepolymers l MD1 products with polyether and polyester backbones l Non-TDI, non-MOCA high-petiormance elastomers

l l

Quasi-Prepolymer

Description

Nominal % NC0

QE164

MTD-PTMEG

16.4

QE230

MTD-PTMEG

23.0

QE162

MTDPPG

16.2

QSA164

MTD-polyester

16.4

QSB164

MTD-polyester

16.4

QSBZOO

MTD-polyester

Quasi- and standard prepolymer systems Technology for cast and spray elastomer systems

295


Table 7-7: Air Products Extended Pre Nominal % NC0

m

I XPE-10

3.6

I TDI-PTMEG

6.2 7.6

90A

I

I-

F-k-l

POLATHANE@ Prepolymers POLATHANE XPE SYSTEMS? l Designed for specialized-perfor lance elastomers l FDA-approved resin and suring genrt l High tensile strength, high n&o md when used wi h POLACURE”

STE-83A

TDI-PTMEG

3.0

83A

STE-90A

1 TDI-PTMEG

1 4.3

90A

740M curative

I -

I 95A/50D 73D 85A POLATHANE STE/STS Prepolymers Designed for standard-perftormance elastomers l Included both polyether and polyester-based TDI prepolymers l

ULTRACAST” Prepolymers 0 Exceptional dynamic performance . Outstanding hydrolytic stability

t

I-

I I 55A

I I

296


CATALYSTS

Some of the recent polyurethane reaction catalysts or reaction facilitators are: Dimethvl Tin Dichloride: This catalyst is more effective in H,, (1) MDItlz] [Bis(4&ocyanato cyclohexyl) methane diisocyanate or Desmodur W@, or PICM] reactions and has lower activation temperatures than dibutyl tin dihuate. Dabco 120@Dibutylin mercaptide: This catalyst from Air Products (2) is used in moisture-cured two-component coating systems. Tin Octoate and Diazobicvclooctane: Tin octoate and diazabicyclo(3) octoate, a tertiary amine, are used as synergistic catalysts. Oleic or Steric Acids, Organobismuth, and Organomercurv: Oleic (4) acid or steric acid facilitates the reaction of alkyl-substituted secondary amines.t91 Organobismuth (Coscat 83’@)and organomercury (&cure 26”) are also effective. The relative order of reactivity with the majority of catalyst on rate of cure of isocyanates i@: MDI>TDI>HDI>IPDI

DIISOCYANATES

Currently used diisocyanates are: TDI Toluene diisocyanate (aromatic) MD1 Diphenyl methane diisocyanate (aromatic) Hydrogenated MD1 ( aliphatic) H&DI HDI Hexane diisocyanate (aliphatic) IPDI Isophrone diisocyanate (aliphatic) Meta-tetramethylxylene diisocyanate (aliphatic) TMXDI TMDI Trimethylhexamethylene diisocyanate (aliphatic) TDI, MDI, IPDI, HDI and HJ4DI am manufactured on a multimillion-pound scale and currently account for virtually all polyurethane products.[121 TMXDI, a close relative of TDI in structure, has been favored lately. The


297

isocyanates ofTMXD1 are separated from the aromatic ring by carbon chains, which results in it being classified as an aliphatic diisocyanate. The structure, interestingly, is an intermediate between aliphatic and aromatic classification. ‘I‘MXDI

meta-tetramethylxylene

NCO

diisocyanate

12

:

NCO

NCO NCO

American Cyanamid Company. Venture Chemical Division, One Cyanamid Source: Plaza Wayne. NJ 07470

TMDI

A straight chain aliphatic diisocyanate is a new product, and is manufactured by HULS AMERICA, INC.

New Water

Scavengerd”]

The common practice is to use para tohtene sulfony-I isocyanate (TSl)t’*t, molecular sieves, and other calcium sulfates or zeolites to physically absorb water from the polyurethane systems. The problem is that TSI has safe-handling problems. and zeolites physically absorb and retain the moisture that may cause problems later. The zeolites also have a tendency to lessen gloss. Recently. a new type of water scavenger has been developed. It reacts with water and unlike the zeolites that absorb water within their crystalline structure. It is relatively easy to use. It could be used to dry wet polyols, solvents, pigments, and basic urethane raw materials. It is a low molecular weight ketone-based oxazolidine. It is (4-ethly 2 methyl-2(3 methyl butyl)I,3 oxadolidine) prepared by stoichiometrically mixing 5-methyl-2-hexanone (MIAK) into 2-amino-l -butanol at ambient temperatures, heated. refluxed to remove water, and purified by vacuum distillation. The product has been shown to perform efficiently and successfully as moisture scavengers in moisture-sensitive urethane systems. It has little or no effect on film proper-

298


ties. The Oxazolidine Structure is 4-ethyl 2 methyl-2(3 methylbutyl>l,3 oxazolidim+*! C*% V-----V

Oxazolidines, along with ketimines, aldimines, and enamines, have been used in the past as moisture-activated blocking amines.t121

Toxicological Profile

Generally, these compounds show good toxicity characteristics.

CONCLUSION

Urethane technology has been significantly advanced by the recent developments. The introduction of new chemical molecules, additives, and processing equipment will facilitate the introduction of new urethane formulations. These results should not only improve existing formulations but could have many future applications.

ACKNOWLEDGMENT

The author is grateful to Mr. John Milon, President of Cal Polymers, Inc., for his encouragement and support.


299

REFERENCES

1. Satguru, R., and McMahon, J., and Coogan, R.G., Aqueous PolyurethanePolymer Colloids With Unusual Colloidal, Morphological, And Application Characteristics. Angus Chemical Company, Vo165 # 830 Journal ofcoatings Technology (March 1994) 2. Bassner, S.L., Air Products and Chemicals Inc., Experimental Design Use of the Formulation of High-Solids Polyurethane Coatings, American Paint Coatings Journal, (August 17 & 24,1992) 3. AirthaneRPolyurethane Prepolymer for Coatings, Air Products and Chemicals., Inc. 140-9348. 4. Bushman, E.F., Polyurea- An Entrepreneur’sDream, PlasticTrends (April/March 1989) 5. Polyurea Spray Formulations Based on Aliphatic Isocyanates and Chain Extenders. Technical Bulletin, Texaco Chemical Company. 6. Thankachan, C., Chemistry of Curing Reactions: Room-Temperature Cure Coatings Based on Epoxies and Urethanes, Journal of Coatings Technology 6 1 No 769 (February 1989) 7. Primeaux, D.J., Anglin, K.C., Processing Polyurea Elastomer Systems - A spray Equipment Approach, Texaco Chemical Company 8. Verbanic,C., Urethanes Face Challenges, Chemical Business (November 1988) 9. House, D.W., Scott, and R.V. t Baumann, W.M., The Use of Aromatic Secondary Diamine Based Curing Agents in Polyurethane Cast Elastomers, Polyurethanes 88

10. UOP Technical bulletin UNILINK R4200, UOP inc. 11. Eagle, G., Ren Plastics, Polyurethane Sealants Challenge Vinyl Plastisols in Automotive Field, Plastics Engineering, p 29 (July 1980) 12. Potter, TA and Williams, JL, Coatings Based on Polyurethane Chemistry: An overview and recent developments. Journal of Coatings Technology, ~0159, No 749 (June 1987) 13. Jacobs PB., and Yu, P.C., Miles Inc., Two-Component Waterborne Polyurethane Coatings, 65 #822 Journal of Coatings Technology, (July 1993) 14. Modern Paint and Coatings page 3, (June 1994) 15. Blank, W.J. Polurethanes as Reactive Cosolvents In Water-Borne Coatings, Journal of Coatings Technology 6 1 No. 777 (October 1989) 16. Kramer, J., Bassner, S., Polyurethane Prepolymers for Moisture-Cure Primers, Modern Paint and Coatings (June 1994)

300


17. Air Products literature packet. Specialty Products for Industrial Coatings. 18. Robinson, G.N., and Alderman, JF., and Johnson, T.L., New Oxazolidine-Based Moisture Scavenger for Polyurethane Coating Systems, Journal of Coatings Technology 65 Vol.820 (May 1993)

BIBLIOGRAPHY

1. Duzey, R.H., Weigel, J.E., Niax Polyether in Urethane Coatings, Union Carbide corps. 2. Wilson, J.B., Isocyanate Adhesive as Binders for Composition Board, Adhesive Age, p 41 (May 1981) 3. Plastics Technology, p 22 (August 1980) 4. Hirosawas, F.N., U.S. Patent 3,265,669, (August 9, 1966); assigned to Furane Plastics, Inc. 5. Hagen, E.L., MDI-Prepolymer Systems, Uniroyal Chemical. 6. Taller, R.A., Coe, J.A., Polylactone Polyols Expand Scope of Urethanes,Union Carbide Corp., Modern Paint and Coatings, p 58 (March 1975). 7. Plastics Technology, p 29 (January 1985) 8. von Hassell, A., For Urethanes, Flourishing R & D in Materials and Additives, Plastics Technology, p 75 (January 1985) 9. Wilwerth, L.O., K.J. Quinn & Co., Inc., Aliphatic Polyurethanes; Has Their Time Come? Plastics Engineering, p 25 (January 1984) 10. New Urethanes Take Hard Line, Chemical Week, p 445 (March 20,1955) 11. Currier, V., Jefferson Chemicals, How to Compound High-Density Urethane Foams, Plastics Technology, p 35 (February 1981) 12. Anagnostou, T., Wyanndotte., Synthesis of Blocked MD1 Adducts, Journal of Coating Technology, p 35 (February 198 1) 13. Isophrone diisocyanate IPDI., Veba-Chemie Ag 14. Castor Oils and Chemical Derivatives., Baker Castor Oil Co. (1962). 15. Ramasri, M., and Srinivasa,R., and Sampathkumaran, P.S., and Sirshalkar,M.M, Polyurethane Coating System For Cathodic Electrodeposition, Journal of Coatings Technology 61 No.777(0ctober 1989) Urethane 16. Yang, CP, and Lee, LT., Effects of Different Polyol-Terminated Prepolymers on the Properties of Their Corresponding Crosslinked films, Journal of Coatings Technology, vol.59 #753 (October 1987)


301

17. Novel Technologies to Meet High-Performance Coatings Needs, Air Products & Chemical, Inc. (1983) 18. Potential Uses for Polyurea Spray Elastomers, Texaco Chemical Co., (1988)

High-Performance Polyimides and Related Thermoset Polymers: Past and Present Development, and Future Research Directions

Abraham L. Landis and Kreisler S. Y. Lau

302

High-Performance

HISTORICAL

Polyimides and Thermoset Polymers

303

PERSPECTIVE

After World War II, rapid expansion of the aerospace industry created a need for new materials for the new high technology which it generated. Many of these requirements could not be achieved with metallic and existing plastic materials. There was a need for materials which were lightweight, oxidatively and thermally stable, that had good mechanical properties and could operate in space environments. During the last 40 years, many new polymers have been discovered which had unusual high-temperature properties. Relatively few achieved commercial success, however, mainly because of their high potential cost and difficulty in being fabricated into useful forms. One class of polymers, the polyimides, was successfully introduced as commercial materials in the early 1960s by E. I. Du Pont de Nemours and Company.111t21 The continued success of these polymers was due to the availability of inexpensive starting material and the ability to tailor these polymers to a variety of end uses. Polyimides, prepared from a variety of dianhydride and diamine monomers, are characterized by repeating imide structural units (I) in the polymer backbone. This structure contributes to the exceptional thermal and oxidative High thermo-oxidative stability has been achieved using stability of polyimides. aromatic structures for R and R’ (II) (Figure 8.1). Evidently, it is necessary to have the imide structure along with the aromatic moieties, because wholly aromatic polymers such as polyphenylenes are not as thermo-oxidatively resistant as polyimides. The combination of the aromatic structure and the imide structure results in a polymer with a high glass transition temperature (Ts) and high thermo-oxidative stability. These aromatic polyimides can be processed either as thermosetting or thermoplastic resins, depending on the polymer resin’s processing characteristics or the process required to transform the prepolymer resin into the final form or shape. These polymers are broken down conveniently into three categories, depending on their processing characteristics. One of these categories includes thermoplastic polyimides. Because their high glass transition temperatures overlap the glass transition temperatures of some thermosetting polyimides, they will be discussed in conjunction with the thermosetting polyimides, which represent an important commercial advancement in polyimide technology.

304 Handbook of Thermoset Plastics

II Figure

8.1. Aromatic characteristics in polyimides thermo-oxidative stability.

contribute

to their

The polyimides in this first category are processed via precursors that undergo condensation reactions to form the final product. The precursors are usually the amic acid or the amic ester intermediate. They are tractable, soluble in common organic solvents, and generally applied as lacquers. Upon conversion to the imide through curing, the polymer is rendered insoluble and intractable. Figure 8.2 illustrates the thermal cure reaction: The cyclodehydration step requires temperatures greater than 14O’C (284’F). Generally, temperatures up to 3OO’C (572’F) are used to ensure complete cychzation. It is also possible to effect cyclization by chemical dehydration with reagents such as aliphatic acid anhydrides, ketenes, and strong Lewis acids such as phosphorus trichloride. During transformation from the amic acid to the imide by these reagents, the intermediacy of the relatively thermally unstable isoimide is suggested. When N,N-dicyclohexylcarbodiimide (DCC) is used as the dehydrating agent, the polyamic acid is transformed into polyisoimide in greater than 80% conversion. Trifluoroacetic anhydride generally converts polyamic acid to a mixture of isoimide and imide, the isoimide content ranging from 50 to 70%. As

High-Performance


305

will be discussed later, the commercial isoimide oligomer product, Thermid@ IP600, has been successfully produced via DCC dehydration at National Starch and Chemical Corporation.

X=OHorOR

A

-H,OorROH

v

*N-R,-N~~R~-(J$-R~~ 0

Figure

8.2. Conversion

0

of polyamic acid to polyimide type thermal curing reaction.

0

via the condensation-

The polyimides in the second category owe their processibility to their thermoplasticity above their glass transition temperature. They are processed much the same way as other thermoplastics, except at much higher temperatures. Thermoplasticity of polyimides is attained through modification of the polymer backbone or by the attachment of pendent side chains. In some cases, these polyimides have sufficient solubility in select solvents to permit the formulation of lacquers so they can be used as coatings or laminating resins. The third category of polyimides owes its processibility to the use of low molecular weight soluble and fusible short-chain oligomers, which have homopolymerizable end-groups. This arrangement permits the polymer chain to grow by addition-type chemical reactions that do not generate volatile by-products.


The most successful reactive prepolymers in this category are based on bismaleimides (III) and acetylene-terminated polyimides (IV) (Figure 8.3).

c 0

0

NO-%

I

0

\-NJ

1

rN

0

r

Iv

Figure 8.3. Illustrative examples of thermoset polyimides with reactive end-groups.

These prepolymers undergo homopolymerization upon heating. To obtain a processible prepolymer, it is necessary that they do not homopolymerize below their melting point and that they have reasonable solubility in a number of solvents that can be used as a lacquer for prepregs. Thus, cross-linked polyimides have been made by thermal polymerization of bismaleimides, preferably by heating them above their melting points in the presence of free-radical catalysts such as dicumyl peroxide and also from acetylene-terminated polyimides by thermal polymerization. In more recent years, phenylethynylated imide prepolymers have also been studied (referred to a later section).

High-Performance

POLYIMIDES


FROM CONDENSATION

307

REACTIONS

The most important polyimides of this first group are based on the monomer pyromellitic dianhydride (PMDA) (V). This monomer, manufactured by Du Pont Company, is obtained by the vapor-phase oxidation of durene (1,2,4,5tetramethylbenzene), using a supported vanadium oxide catalyst. The reaction of PMDA with certain aromatic amines, for example, m-phenylendiamine (MPDA) (VI), benzidine, and bis(4aminophenyl) ether (4,4’-ODA) (VII), produces polymers having a high degree of oxidative and thermal stability. Figure 8.4 depicts important commodity monomers for polyimide commercialization.

V

VI

Figure 8.4. Important monomers

VII

for polyimide

production.

The synthesis is carried out generally as a two-stage process,t31 that involves preparation of the intermediate polyamic acid prepolymer and conversion of the prepolymer to polyimide under a specific cure cycle. The imidization process has been studied using infrared spectroscopy. In the temperature range of 220”-250°C (428-482’F), complete imidization takes place. Higher temperatures appear to cause cross-linking by the formation of a interchain network. Russian workers have reported that, at the higher temperatures, the structurization depends The Du Pont on reversible cleavage of -CO-N- bondst41 and their recombination. Company has marketed a number of polyimide products based on PMDA (V) and the aromatic amine 4,4’-ODA (VII), namely Kapton@ (supplied as a film), Vespel@ (supplied as sintered parts), and Pyre-M.L.@ (supplied as a lacquer of the polyamic acid prepolymer). The Kapton@ tihn is supplied in several forms: Type H is the uncoated polyimide film. Figures 8.5 and 8.6 show two major applications of this film. Type

308 Handbook of Thermoset Plastics V is similar to type H but with a superior dimensional stability, and type F which is a Type H film coated on one or both sides with a Teflon fluorinated ethylene propylene (FEP) resin to provide a moisture barrier and to enhance chemical resistance. Typical properties of these tihns are presented in Tables 8.1 through 8.6.t51 Vespel@ is supplied as sintered parts and formulated with various fillers. The various commercially available VespeF compositions are described in Table 8.7.t61 Typical properties associated with these compositions are shown in Tables 8.8 through 8.12.t61 These property data are for reference only. Because service conditions may differ from laboratory test conditions, users of Vespel@ parts and shapes should independently evaluate the suitability of these parts using their own test procedures. The properties of Pyralina’ are presented in Tables 8.13 and 8.14.[51

Figure 8.5. Kapton@ cover on solar panel in the spaceship Discovery. (Courtesy E. I. Du Pont de Nemours and Company)

High-Performance


309

In recent years, Sheldahl, Inc. has been involved in manufacturing a coated material for the photovoltaic solar cell array assembly of NASA’s Space Station Freedom.[‘l The material selected was a special Kapton@ film designed to withstand oxygen plasma etching. This special Kapton@ tihn was prepared by applying a low earth orbit (LEO), atomic oxygen resistant (AOR) thin tihn to Du Pont’s Type H Kapton@ film.

Figure 8.6. Kapton@ printed circuit in disk drive (Courtesy E. I. du Pont de Nemours and Company)

3 10 Handbook of Thermoset Plastics

Table 8.1. Physical Properties Physical Properties

(1 mil) of Kapton@ Type H FilmR

r

Typical Properties

Test Method

-195V

23’C

2oo”c

35,000

25,000

17,000

ASTM D882-64T

Yield point at 3%, psi

-

10,000

6,000

ASTM D882-64T

Stress to produce 5% elongation, psi

-

13,000

8,500

ASTM D882-64T

2

70

90

ASTM D882-64T

510,000

430,000

260,000

ASTM D882-64T

-

Du Pont pneumati impact test

-

ASTM D2 176-63’

Ultimate tensile strength, psi

Ultimate elongation, % Tensile modulus, psi Impact strength, j/mm

-

Folding endurance MIT

-

Tear strengthpropagating (Elmendorf), g

-

8

-

ASTM D1922-6 1’

Tear strength-initial (Graver), g

-

510

-

ASTM D1922-61’

Density, g/cm3

-

1.42

-

ASTM D1505-63’

Coefficient of friction, Kinetic (film to film)

-

0.42

ASTM D1894 63

Refractive index (Becke line)

-

1.78

Encyl. Diet. of Phys., Vol; Ave. ? samples, elongate1 at 5%, 7%, 10%

Poisson’s ratio

-

0.34

23

10,000 cycles

(Reprinted by permission

-

of Du Pont Company)

High-Performance


Table 8.2. Typical Properties Property

Typical Values

Dielectric strength 2 mil 3 mil 5 mil Dielectric constant 2 mil 3 mil 5 mil

of Kapton@ Type V Film[51 Test Conditions

Test Method

5,400 V/mil 4,600 V/mil 3,600 V/mil

60 Hz

3.6

1kHz

ASTM D 149-64

0.0025 0.0025 0.0025

1kHz

ASTM D 149-64

8x 1Ols ohm-cm 5~10’~ ohm-cm lx10L5 ohm-cm

125 v

ASTM D257

ASTM D 149-64

l/4-in. electrode

3.7

3.7

Dissipation factor 2 mil 3 mil 5 mil Volume resistivity 2 mil 3 mil 5 mil


Table 8.3. Typical Electrical Property

2 mil 3 mil 5 mil Dissipation factor 1 mil 2 mil 3 mil

7,000 5,400 4,600 3,600

of Du Pont Company)

Properties of Kapton@ Type H Film151

Typical Values

Dielectric strength 1 mil 2 mil 3 mil 5 mil Dielectric constant 1 mil

V/mil V/mil V/mil V/mil

3.5

Test Conditions

Test Method

60 Hz l/4-in. electrode

ASTM D149-61

1kHz

ASTM D150-59T

1kHz

ASTM D150-59T

125 Volt

ASTM D257-6 1

3.6 3.7 3.7 0.0025 0.0025

5 mil Volume Resistivity 1 mil 2 mil 3 mil 5 mil

311

0.0025 0.0027 lx10150hm-cm 8~lO’~ohm-cm 5x10150hm-cm 1x10150hm-cm

(Reprinted by permission of Du Pont Company)


Table 8.4. Typical Electrical

Properties

Film Type* 120F616

Property

of Kapton@ Type F FilmI

Film Type* 150F019

Film Type* 250F029

Dielectric strength Total volts Volwnil

7,500 6,800

6,300 4,200

Dielectric constant

2.8

3.0

-

Dissipation factor

0.0022

0.0014

-

Volume Resistivity ohm-cm at 23OC ohm-cm at 200°C

1.5x1016 5x10’4

10’8 10’6

4,500

7x10’7

*Film engineering: 1.2-mil nominal thickness; 0.1 -mil Teflon@ FEP; I-mil Kapton@ Type H, 120F616 150F019 250F029

0. I-mil Teflon@ FEP 1.5-mils nominal thickness, O.O-mil Teflon@ FEP; 1-mil Kapton@ Type H, 0.5-mil Teflon@ FEP 25-mils nominal thickness, O.O-mil Teflon@ FEP; 2-mil KaptonB Type H; 0.5 mil-Teflon@ FEP (Reprinted by permission

of Du Pont Company)

High-Performance


Table 8.5. Mechanical

Properties

313

of Kapton@ Type F FilmI

Typical Values Property Film Type 120F616*

Film Type 150F019’

24,000 16,000

17,000 11,000

25,000 16,000

9,000 5,500

7,300 4,000

10,000 8,000

12,500 7,500

9,000 5,500

-

65 95

75 85

-

415,000 215,000

320,000 173,000

-

Impact strength at 23“C, Kg cm/mil

6.0

4.6

-

Tear strength-propagating Elmendorf, g/mil

10

13.5

750

435

80

57

Ultimate tensile strength, psi 23°C 2oo”c Yield point at 32 psi 23°C 2oo”c Stress at 5% elongation, 23°C 200°C Ultimate elongation, 23°C 2oo”c

Film Type 250F029*

psi

%

Tensile modulus, psi 23“C 200°C

Tear strength-initial g/mil

80

12

(graves),

Weight, % polyimide

73

Weight, % FEP

20

43

27

Density gm/cm3

1.53

1.67

1.57

Film engineering: 120F616

1.2-mil nominal thickness; H, 0.1 -mil Teflon@ FEP

150F019

1.5-mil nominal thickness, O.O-mil Teflon@ FEP; 1-mil Kapton@ Type H, 0.5-mil Teflon@ FEP

250F029

25-mil nominal thickness, O.O-mil Teflon@ FEP; 2-mil Kapton@ Type H; 0.5-mil Teflon@ FEP (Reprinted by permission

0.1 -mil Teflon@ FEP; 1-mil Kapton@ Type

of Du Pont Company)


Table 8.6: Gas Permeability

Gas

of Kapton@ Type H Film~51

(cc/100inz)(24hr)(atm/mil)

Carbon dioxide

Test Method

45

Hydrogen

250

Nitrogen

at 23°C

6

oxygen

25

Helium

415

Water vapor

5.4 g/( 1OOin*)(24 hr)/min (Reprinted by permission

Table 8.7. Composition

Resin Designation

ASTM D14340

Description

ASTM E96-63

of Du Pont Company)

of Vespel@ Polyimide161

Characteristics

SP-2

Unfilled base resin

Provides maximum physical strength, elongation, and toughness and best electrical and thermal insulation.

SP-21

15% by weight graphite filler

Graphite added to provide low wear and friction for bearings, thrust washers, and dynamic seals.

SP-22

40% by weight graphite filler

Same as SP-21 for wear and friction plus improved dimensional and oxidative stability. It has the lowest coefficient of thermal expansion.

SP-211

15% by weight graphite and 10% by weight Teflon@ fluorocarbon resin fillers

Has the lowest coefficient of friction over wide range of operating conditions. Also has lowest wear rate up to 300°F.

SP-3

15% by weight molybdenum disulfide

MoS, added to provide lubrication for seals and bearings in vacuum and dry environments.


of Du Pont Company)

High-Performance Table 8.8. Mechanical


315

Properties

of Vespel@ SP Polyimide

rem.,

ASTM Method

SP-1

SP-21

SP-22

Tensile strength, ultimate, ksi

73 500

D1708 or E8

12.5 6.0

9.5 5.5

7.5 3.4

6.5 3.5

8.2 -

Elongation, ultimate, %

73 500

D1708 or E8

7.5 7.0

4.5 2.5

3.0 2.5

3.5 3.0

4.0

Flexural strength, ultimate, ksi

73 500

D790

19.0 11.0

16.0 9.0

14.0 8.0

10.0 5.0

Flexural modulus, ksi

73 500

D790

Property

Compressive stress, ksi at 1% strain at 10% strain at 0.1% offset Compressive modulus, ksi Axial fatigue endurance limit, ksi at 103cycles

OF

500 250

500 370

3.6 19.3 7.4

73

D695

4.2 19.3 6.6

350

4.6 16.3 6.6 420

8.1 3.8 6.1 2.4

6.7 3.3 4.7 2.4

Flexural fatigue endurance limit, ksi at 103cycles at 107cycles

73 73

9.5 6.5

9.5 6.5

Shear strength, ksi

73

D732

13.0

11.2

Impact strength Izod, notched, J/m

73

D256

80.0

42.7

Impact strength Izod, unnotched, J/m

73

D256

Poisson’s ratio

73

Properties are non

SP-211

500 200

SP-3

-

73

73 500 73 500

at 107cycles

700 100

Resins*@]

.ection (Reprinted by permission

501 0.41

3.0 14.8 5.4 175

300

-

-

-

-

-

-

-

-

-

-

-

-

427 0.41

-

-

of Du Pont Company)


Table 8.9. Wear and Friction Properties* of Vespel@ SP Polyimide Resinsf61

SP-1

Property

SP-21

17-85

6.3

Friction Coefficient** PV=0.875MPa m/s PV=3.5MPa m/s

0.29 -

0.24 0.12

0.20 0.09

0.12 0.08

0.25 0.17

In Vacuum

-

-

-

-

0.03

Static in Air

0.35

0.30

0.27

0.20

-

Wear Rate, M/l 0%

*Properties are non-directional **Steady state, unlubricated in air. (Reprinted by permission of Du Pont Company)

Table 8.10. Electrical Properties of Vespel@ SP Polyimide Resins at 23’%Y1

Property

ASTM Method

Dielectric Constant at lO*Hz at 104Hz at 106HZ

D150

Dissipation Constant at lO*Hz at 104Hz at 106Hz

D150

Dielectric Strength Short time 0.002 m thick, MV/m

SP-1

SP-21

3.62 3.64 3.55

13.53 13.28 13.41

0.0018 0.0036 0.0034

0.0053 0.0067 0.0106

D149

22

9.84

Volume Resistivity, ohm-cm

D257

10’4-10’5

10’*-10’3

Surface Resistivity, ohm

D257

IO’S_10’6


-

High-Performance

Polyimides and rJzermoset Polymers

317

Table 8.11. Thermal and Electrical Properties of Vespel@ SP Polyimide Resin@] Property

Thermal Coefficient of Linear Expansion, u in/inN

Temp.

ASTM

OC

method

23-300 -62-23

D696

Thermal Conductivity, W/ml”K

40

Specific Heat, BTU/lb/“F JiKgPC

0.27 1130

EC

,5:

-

SP-1

SP-21

SP-22

SP-211

54 45

49 34

38 -

-

0.35

0.87

1.73

54

0.76

-

,f:,O.l.l,;;,

-,-

(Reprinted by permission of Du Pont Company) Table 8.12. Other Select Properties of Vespel@ SP Polyimide Resin#l Property

ASTM Method

Water Absorption, % 24 hrs at 23’C 48 hrs at 50°C

SP-1

SP-21

SP-22

0.24 0.72

0.19 0.57

0.14 0.42

0.21 0.62

-

1.0-1.3

0.8-1.1

-

-

-

1.43

1.51

1.65

1.55

1.60

45-58 82-94

32-44 68-78

15-40 69-79

5-25 -

40-55

92-102 D2863

53

49

D570

Equilibrium, -50% Specific Gravity

D792

Hardness Rockwell “E” Rockwell “M”

D785

Limited Oxygen Index, %

SP-211 SP3

-


-

-


Table 8.13. Typical Lacquer Properties

Resin Property

PI-2540

of Pyralin@ Coatings*151

PI-2545

PI-2550

PI-2555

14.5

14

25

10

Viscosity (LVP#3 at 12 rpm) Poise Pascal sec.

50-70 5-7

9-13 0.9-l .3

50-70 5-7

12-16 1.2-1.6

Weight per gal, lb per liter, kg

8.80 1.06

8.78 1.06

8.95 1.08

8.80 1.06

Solution Density, g/cc

1.06

1.04

1.05

1.06

Solvent

NMP/aromatic hydrocarbon

NMP/aromatic hydrocarbon

NMP/acetone

NMP/aromati hydrocarbon

Flash Point, “C Filtration, pm

64 0.1 absolute

64 0.2 absolute

-7 0.1 nominal

64 0.2 absolute

Resin Solids (2 gm, 2 hr at 2ooq %

*Typical properties; not to be used for specification (Reprinted by permission

purposes

of Du Pont Company)

High-Performance


Table 8.14. Properties

of Pyralin@ Polyimide

Property Physical Tensile strength, ksi Elongation, % Density, gm/cc Refractive index (Beck Line) Flexibility Thermal Melting Point Weight Loss at 3 16°C in air after Final decomposition temperature, “C Coefficient of thermal expansion Coefficient of thermal conductivity, cal/(cm)(sec)(oC) Flammability Specific Heat, cal/gmK Electrical Dissipation factor (1 kHz) Dielectric strength, V/mil Volume resistivity, ohm-cm Surface resistivity, ohm Dielectric Constant (1 kHz)

319

Film151

PI-2540 (PI-2545)

PI-2550 (PI-2555)

17 (1.17x10* Pascal) 25 1.4 1.78 180’ bend, no cracks

19 (1.31x10* Pascal) 10 1.39 1.70 180” bend, no cracks

None

None

300 hr, % 560 2.0x 10-?"C 37x10-5

44 560 4.ox1o-5Pc -

Self-extinguishing 0.26

Self-extinguishing 0.26

0.002 4,000 10’6 10’5 3.5

0.002 4,000 10’6 10’5 3.5


Polyimides based on the dianhydride, 3,3’,4,4’-benzophenonetetracarboxylic diaubydride (BTDA) (VIII) and meta-phenylenediamine (MPDA) (VI) have been marketed by Monsanto Company as the Skybond 700 seriesPrl and by American Cyanamid Company as FM-34 lacquers (Figure 8.7). The Skybond@ 700 lacquer consists of a solution of monomer reactants of the diester, diacid, and diarnine. Solvents such as ethanol, butanol, ethylene glycol, and N-methylpyrrolidine (NMP) are used as solvents for these monomers. Skybond@ 700 is specifically designed for structural, electrical, and specialty applications where extended temperatures up to 371°C (700’F) are required. These resins are


useful for preparing prepregs for laminates that can be molded by either press or vacuum bag techniques. Mechanical properties of Skybond@ 700 are shown in Table 8.15.1*1 The properties were obtained after a sixteen-hour postcure cycle which included a final postcure for four hours at 371°C (700°F) for developing adequate initial hot strength at 37 1“C (700°F). Table 8.16 tabulates the electrical properties of Skybond@ laminates.[81 These properties were measured on laminates hot press molded to keep the void content low.

o)JyTJJo

H*N,A,NH2

u

0

0

VIII

VI

Figure 8.7. Precursor monomers to commercial skybond and FM-34 polyimides

FM@‘-36 is a modified polyimide supplied as a supported film with a lightweight glass cloth.191 It is suitable for bonding metal to metal, composites and various sandwich structures; it is serviceable over a temperature range of -55°C (-67’F) to 288°C (550’F). Typical tensile shear properties of FM@-36 adhesive are shown in Table 8.17.191 FM@-34-18 adhesives are arsenic-free versions of FMa’-34 polyimide adhesive film. This adhesive is available as a supportive and unsupportive film and as a paste. These adhesives are noted for strength retention after long-term exposure to temperatures of -55’C (-67°F) to 371°C (700°F) in both metal and composite constructions. The mechanical properties of this adhesive are shown in Table 8.1 8.1101 Recently, General Electric Company introduced a polyimidelllI that also has the silicon-oxygen linkage in the polymer backbone. Known as silicone polyimides (WI), they show promise for use as passivation coating and interlayer dielectrics for electronic applications. Unlike conventional polyimides, the SiPI possess excellent inherent adhesive

High-Performance

properties


321

that eliminate the necessity for primers. The adhesive properties seem to be

insensitive to moisture.


Properties of Laminate Polyimide[*l

Property

Utilizing

Skybond@ 700

High TemperatureHigh Pressure

Vacuum Bag

75-85 45-60 20-35 3.0

76-83.5 22-32 20-24

V-O at ‘& in.***

V-O al /,6 in.*’

V-O at I,6 in. **,

Thermal, cont. Coefficient of thermal expansion (0” to 300’F) mold direction, in/in“F

Ultem 6200

Ultem 6202

Flammability Oxygen index, % Vertical bum (UL Bulletin 94)** NBS smoke, flame mode (0.060 in.)

-

E662

5 70

Ds at 4 min D MAx at 20 min

-

Electrical

4 70

-

Dielectric strength (l/,&n.), in oil, V/mil

D149

750

*

580

530

Dielectric constant at 1 kHz, 50% RH

D150

3.0

3.1

3.1

3.1

D150

0.001

0.001

0.001

0.001

Volume resistivity (1/16in.), ohm-cm

D257

1.0x10’;

l.OxlO’7

1.0x10’

1.Ox10’7

Arc resistance,

D495

127

Specific gravity

D792

1.29

1.35

1.43

1.42

Mold shrinkage, in.iin.

D955

5-7)x10

*

*

5-7)x10-

Water absorption at 24 hr, 73”F, %

D570

0.28

0.24

0.22

0.22

Dissipation Factor at 1 kHz, 50% RH, 73°F

sec.

-

140

Other

* Testing in progress **This rating is not intended to reflect hazards presented by this or any other material under actual fire conditions. *** General Electric Company test data


Amoco Torlon@ Poly(amide-imides) Another route to fusible and processible polyimides is to introduce the amide moiety into the polyimide chain. These poly(amide-imide) polymers are commercially available as Amoco’s Torlon@‘. ChemicalIy, Torlon@ is a polymer from the reaction of Amoco’s trimellitic anhydride and aromatic diamines. The polymer is depicted by structure XIll (Figure 8.14).

Figure 8.14. Chemical

structure

of Torlon@,

a Poly(amide-imide).

The polymer chain comprises amide alternating with imide linkages which results in a thermoplastic polymer. The versatility and processibility has made these resins attractive materials. Figure 8.15 shows parts fabricated from Torlon@ which take advantage of this material’s excellent wear and temperature resistance. Torlon@ has even been used to make a plastic gasoline engine (see Figure 8.16). The resulting plastic engine delivers surprising performance and reduced weight of the 2-liter, four-cylinder engine to about 168 pounds, about 200 pounds less than an all-metal engine. The Torlon@ polymer was chosen because of its strength, toughness, and temperature resistance. It would not be surprising if the plastic engine parts used in this demonstration engine may appear in passenger cars before long. Various grades of Torlon@ are available for a number of engineering applications. The grades and their application are described in Table 8.26.t*61 The mechanical properties are shown in Table 8.27,t161 electrical properties in Table 8.28,p6] and thermal and general properties in Table 8.29.[i6]

High-Peformance


Figure 8.15. Torlon parts are long lasting in wear-intensive applications. (Courtesy Amoco Chemical Corporation).

339


Figure 8.16. The 2-liter, dual-overhead cam plastic engine developed by Polimoter Research, Inc. utilizes Torlon@,an injection moldable thermoplastic made by the Amoco Chemicals Corporation. Weighing in at 175 pounds and developing 3 118 brake horsepower at 9,500 rpm, the engine is 200 pounds lighter than its steel counterpart. (Courtesy Amoco Chemicals Corporation).

High-Performance


Table 8.26. Grades of Commercially Grade Code Number

341

Available Torlon@[161

Description

TORLON 4000T

This powdered form of the resin is used for compression molding. It is the base resin used to produce most other grades of TORLON.

TORLON 42031,

This grade contains TORLON 4000T plus 3 % TiO, and 0.5 % of pTFE compounded and pelletized. It is used primarily for injection molding. pFTE is added to enhance moldability and mold release. 4203L is used where elongation and impact resistance are important considerations.

TORLON 430 1

This grade contains TORLON plus 12 % graphite powder and 3 % pTEF compounded and pelletized. It was designed to reduce the coefficient of mction and to increase its wear resistance in lubricated and non-lubricated bearing applications.

TORLON 4275

This grade contains TORLON 4000T plus 20 % graphite powder and 3 % pTFE compounded and pelletized. It was specifically designed to provide maximum wear resistance and a lower coefficient of friction.

TORLON 4347

This grade contains 12 % graphite powder and 8 % pTFE compounded and pelletized. This new material is designed for the lowest coefficient of friction and should be usell for bearings under high load or surfaces subject to reciprocating motion.

TORLON 5030

This grade contains TORLON 4000T plus 30 % glass fiber and one % pTFE compounded and pelletized. It is used primarily in applications requiring high modulus, low shrinkage, and high creep resistance to deformation at high temperatures, or good electrical properties.

TORLON 7 130

This grade contains TORLON 4000T plus 30 % graphite fibers and 1% pTFE compounded and pelletized. The most metal-like of all TORLON resins, this grade was specifically developed as a structural metal replacement in the aerospace industry. Incorporation of graphite fibers resulted in a material with a high modulus that is highly resistant to creep and has the highest fatigue resistance available in a TORLON resin.

TORLON 9040

This grade is the first in a new series of TORLON resins made possible by developments in the basic polymer chemistry. This version contains 40 % glass and carries a volume tag of $5.40 per pound. Even higher heat resistance has been achieved with a minor loss of impact strength. Ground forms of 4203,5030, and 4301 are also available for compression molders.

‘ORLON 5430

This grade is compounded with 30 % glass fibers. It offers superior resistance to creep and excellent stifthess at elevated temperatures, as well as improved processibility.


of Amoco Chemicals Corporation)


Table 8.27. Mechanical Properties of Torlon@ Resin Compositiod61 -Testinl Temp, OF

-

-

503(1 5430

713tl 9044

-

-

29.5 29.7 27.7 23.1 19.0 16.3 12.9

22.8 29.4 12.8 15.7

4203 and 4301 12031 --

427:

-321 73 275 450

31.5 27.8 16.9 9.5

23.7 16.3 10.6

1818 22.C 16.3 8.1

-321 73 275 450

6 15 21 22

7 20 17

3 7 15 17

9 21 15

4 7 15 12

12 10 8

7.0

9.5

Il.3

8.7

15.6

15.8

-321 73 275 450

41.0 34.9 24.8 17.1

31.2 13.5 16.2

29.0 $0.2 L2.4 15.8

L7.0 LO.5 14.3

54.4 18.3 15.9 !6.2

39.7 29.7 20.7

-321 73 275 450

11.4 7.3 5.6 5.2

10.0 7.9 7.2

13.9 10.6 8.1 7.4

9.1 6.4 6.2

!0.4 17.0 15.5 14.3

Compressive Strength nsi, ASTM D695

73

32.1

21.7

17.8

18.3

Shear Strength, ksi 4STM D732

73

18.5

16.1

11.1

73

2.7 20.0

1.2 7.6 --

1.6 4.7

Property

Tensile Strength, ksi ASTM D1708

Tensile Elongation, ASTM D1708

29.7 15.1 7.8

25.5 24.0 19.8

%

Tensile Modulus, msi ASTM D 1708 Flexural Strength, ksi ASTM D790

Flexural Modulus, msi ASTM D790

[zod Impact, ft lbs/in 4STM D256 notched unnotched

4347

73

-


-

-

3 6 14 11

4 8.3 7.7

3.2

-

15.0 io.7 17.6 !5.2

49.0 39.5 22.8

16.7 14.2 13.2

15.7 !2.8 !7.2 !2.8

21.1 20.9 18.3

34.8

34.1

32.7

-

11.5

20.1

19.1

17.3

-

1.3 -

1.5 9.5 -

1.3 -

0.9 6.4 -

1.8

-

-

-

of Amoco Chemicals Corporation)

-

-

High-Performance


343

Table 8.28. Electrical Properties of Torlon@ Resin Compositions[‘@ Property

4203L

4301

4275

4347

5053

5430

Dielectric constant at lo3 Hz at lo6 Hz

4.2 3.9

6.0 5.4

7.3 6.6

6.8 6.0

4.4 6.5

4.2 4.0

Dissipation factor at lo3 Hz at lo6 Hz

0.026 0.03 1

0.037 0.042

0.059 0.063

0.037 0.071

0.022 0.023

0.020 0.036

Volume resistivity, ohm-in, D257

8x10’6

8~10’~

3~10’~

3~10’~ 6~10’~ 2~10’~

Surface resistivity, ohm, D257

5x10’8

8x10”

4x10”

1~10’~ 1x10’* 3x10”

Dielectric strength, V/mil

580

-

-

-

835

650

Table 8.29. Thermal and General Pro erties of Torlon@ Resin CompositionsI i?1 4203 Pverty

and 4301 4275 4347 5030 5430 7130 904 4203L 532

534

536

532

539

534

540

536

Coeff’cient of linear thermal expansion 1P6 in/in, “F, D696

17

14

14

15

9

10

6

7

Thermal conductivity BtuinIhr , ft*, OF9Cl17

1.8

3.7

-

-

Flammability Underwriters Lab 94

94vo

94vo

94vo

Limiting oxygen index %, D2863

45

44

45

46

51

48

48

50

Density, g/cc, D792

1.38

1.43

1.44

1.44

1.56

1.62

1.42

1.6C

86

72

70

66

94

93

94

107

0.33

0.28

0.33

0.17

0.24

0.31

0.26

0.21

Deflection temperature OF at 264 psi, D646

Hardness Rockwell E, D792 Water absorption, % D57

2.5

-

-

-

94vo 94vo 94vo 94vo 94vt

(Reprinted by permission of Amoco Chemicals Corporation)


Ciba-Geigy

Fully Imidized Indane-Based

Polyimides

Ciba-Geigy Corporation markets the fully imidized polyimide XU 218 (XIV) as a powder (Figure 8.17). It has high solubility in volatile solvents such as methylene chloride, and tetrahydrofiuan, as well as in solvents such as Nmethylpyrrolidine. It is an excellent film former with a relatively high glass transition temperature [Ts= 32O’C (608”F)]. Films made fromthispolyimide have excellent chemical resistance. Exposure in 5N sodium hydroxide for 48 hours produced only surface pitting. Under similar conditions, Kapton@ film lost its integrity entirely. Select properties are shown in Tables 8.30, 8.31, and 8.32.[“1 Properties of the XU2 18 film with Kapton@ H film are compared in Table 8.33. This polyimide may be useful for various electronic applications requiring films and conformal coating.

XIV

Figure 8.17. Chemical structure of Ciba-Geigy’s

XU2 18 polyimide.

Table 8.30. Thermal Stability and Aging of 1 mil XU 218 Polyimide Films1171

Temperature, OC

Environment

200 225 250 250 300

Air Air Air Nitrogen Vacuum

Hours to Embrittlement >2000 925 250 >2500 ~2500

(Reprinted by permission of Ciba-Geigy Corporation)

Resin

High-Performance


Table 8.31. Electrical Properties of XU 218 Polyimides Property

Temperature, OC

Dielectric constant, Hz 100 Hz 1,000 Hz 1,000 Hz 1,000 Hz 1,000 Hz 1 x lo6 Hz Dissipation factor, Hz 100 Hz 1,000 Hz 1,000 Hz 1,000 Hz 1,000 Hz 1x106Hz

ResW71

Value

25 25 60 100 150 25

3.4 3.3 3.0 3.0 3.0 3.0

25 25 60 100 150 25

0.0061 0.0026 0.0039 0.0061 0.0073 0.0091

Dielectric strength, kV/mil

-

5.5

Volume resistivity, ohm-cm

25

4.4 x 10’6


of Ciba-Geigy Corporation)

Table 8.32. Tensile Properties

Property

l’ensile strength Machine direction

Cross-machine direction

Temperature, OC

345

of XU-218 Polyimide

Tensile Strength, ksi Yield Break

Tensile Modulus ksi

Film*ll’l % Elongation Yield

Break

25 100 150 204 260

14.4 11.9 9.1 5.9

22.5 18.4 17.8 14.4 8.8

560 370 330 363 270

8.8 7.7 6.4 3.9

17 30 41 47 38

25 100 150 204 260

11.7 9.0 7.0 4.7

14.6 11.1 8.3 6.4 4.3

460 340 273 275 260

8.0 6.8 5.4 3.4

10 32 49 51 69

*0.5-mil film uniaxially stretched from 1.O-mil film oriented in machine direction. (Reprinted by permission

of Ciba-Geigy Corporation)


Table 8.33. Comparison

of Select Properties of H Film (Du Pont) and XU218 (Ciba-Geigy)

Room Temperature Properties

Du Pont H Film

Ultimate tensile strength, psi

25,000

22,500

Tensile modulus, psi

430,000

560,000

Dielectric constant at 1,000 Hz

3.5

3.3

Dissipation factor at 1,000 Hz

0.0027

0.0026

Ciba-Geigy XU-218

Researchers at the Technische Universitit Miinchen have been interested in the properties of poly(ether-ketones) based on the same 1,1,3-trimethyl-3phenylindane structural unit as Ciba-Geigy’s XU 2 18 polyimides.[l *I The standard method of producing 1,1,3-trimethyl-3-phenylindane is the acid-catalyzed dimerization of a-methylstyrene. Their recent interest in polyimides prompted the functionalization of a-methylstyrene, via a sequential Friedel-Crafts acylationl sulfonation-nucleophilic aromatic substitution procedure, to yield the required diamine monomers XVa and XVb[191 (Figure 8.18).

XVa:

Y=CO

xvb:

Y=SO,

Figure 8.18. Two novel diamine monomers phenylindane.

based on 1,1,3-Trimethyl-3-

High-Performance


347

These d&nines were allowed to react with 6FDA and BTDA to yield four high-temperature, stable poly(ether-imides), XVIa-XVId (Figure 8.19), which can be cast into flexible, transparent films from common solvents such as N,Ndimethylacetamide. The physical characterization of these poly(ether-imides) are summarized in Table 8.34.

XVIa

XVIb

XVId

Figure

8.19. Soluble, high-temperature, stable poly(ether-imides) 1,1,3-trimethyl-3-phenylindane.

based on



of Indane-containing

M g.mk;

Poly(ether-imides)[191

M g.mY

TP?’

T*

Td,

oc**

Y

Dianhydride

XVIa

co

BTDA

-

232

463

XVIb

so*

BTDA

-

251

450

XVIC

co

6FDA

28,000

810,000

241

481,510t

XVId

so2

6FDA

23,000 t 1,200,000

257

461

Polymer

M,and M, determined by GPC in THF *Tg from DSC at 20Wmin

Soluble, Fully Imidized Fluorinated

“Td from 5% wt loss in air at 1OIUmin tTd from 5% wt loss in nitrogen at 1OK/min

Polyimides

Many fluorinated thermoplastic polyimides have recently become available. The tbermo-oxidative stability ofpolyimides containing the hexafluoroisopropylidene (6F) group has been well known.[201 The 6F groups impart to polymers enhanced processibility, improved fracture toughness,[211 and moisture resistance while maintaining excellent oxidative stability.[**l A related flexible (12F) system[231 is potentially applicable as connecting units for high-temperature chemical building blocks (Figure 8.20). The 3F connecting group has been used to optimize cost, processibility, and mechanical properties.1241-[271 The presence of fluorine substituents in the polymers also improves their processibility in terms of solvent solubility, melt characteristics, and other properties such as low dielectric constant, low moisture adsorptivity, and optical transparency. Fluorinated polyimides are important to the microelectronic industry because of their unique electrical properties. They are the topic of many other It is sufficient to mention here the R&D reviews pertaining to their applications. effort in this class of polyimides originated in the unique dianhydride monomer 6FDA. The American Hoechst Company marketed 6FDA and their series of SixeP Polyimides during the 1980s. The best known is Sixef@-44, made from 6FDA and 2,2-bis(4-aminophenyl)hexafluoropropane. A related soluble, fluorinated polyimide is Hughes Aircraft Company’s 6F33. Other fluorinated diamines

High-Performance


349

are: 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoro-propane (BDAF),[28] 2,2-bis(4aminophenyl)- 1-phenyl-2-trifluoroethane,[251-[271 and 2,2’-bis(trifluoromethyl)-4,4’diaminobiphenyl (PFMB)[291[301(Figure 8.21). Recent effort in the design and application of low dielectric polyimides that are based on 6FDA are summarized in Table 8 .35 .[311-[341

CFs

dF

2,2-Bis (3,4_dicarboxypbenyl)hexafluoropropane

3

l,l-Bis (3,4-dicarboxyphenyl)-l-phenyl-

Dianhydride

2,2,24rifluoroethane

WDA)

Dimhydride

(3mA)

1,3-bis-[2-(3,4_dicarboxyphenyl)hexatluoropropyl]benzene

Dianhydride

(12FDA)

Figure 8.20. Fluorinated

ADDITION-CURABLE

dianhydrides based on special chemical connecting groups of 3F, 6F, and 12F.

POLYIMIDES

AND OTHER POLYMERS

As discussed in previous sections, commercialization of a product is the cuhnination of many years of persistent effort in developmental and application research efforts. After the synthetic feasibility of the process has been established, the research resin is evaluated for its potential in engineering applications. Optimization of reaction parameters ensues to produce an efficient method for large-scale preparations. As more materials become available, further testing and more extensive evaluation can be pursued.

High-Performance


Table 8.35. Examples of 6FDA-based

Diamine

Polyimide Dielectrics

Research Organization

Reference

BDAF

Raychem Coporation, Menlo Park, CA

31

BAPB

Raychem Coporation, Menlo Park, CA

31

13FDAPB

DuPont Wilmington,

32 DE

DAOFB, DATFB

IBM Yorktown Heights, NY

33

PFMB

Nippon Telegraph & Telephone Company, Japan

34

2,2-Bis[4-(4_nminophenoxyphenyl)]bexafluoropropane (BDAF)

2,2’-Bis(trifluoromethyl)4,4’-diaminobiphenyl (PFMB)

Bis[4-(4-amiaophenoxy)]biphenyl (BAPB)

HZN \

4,4’-Diamino-2,2’,3,3’,5,5’,6,6’ochfluorobipbenyl (DAOFB) F

F

,

-

CH 2-C(CF

r)r-CF

&TF +F

D H2N

1,3-Diamino-5-[2,2-bis(trifluorometbyl)3,3,4,4,5,5,5heptnfluoro]pentylbenzene (13FDAPB)

s

/

Hz 42

F

-

‘-NH, F

1,4-Diamino-2,3,5,6,-te~afluorobenzene (DATFB)

351

352


Thermosetting polymers have long been in demand in producing composites for the fabrication of high-performance aerospace engineering structural components. The success of epoxy resins as the most developed and most widely used polymer matrix materials rests on their excellent mechanical properties and processing characteristics. In fact, all new classes of matrix resins are measured against the epoxies in evaluating their performance. Most epoxy resins are not usable at temperatures beyond 177°C (350’F). During the heydays of the aerospace industry in the past two decades, there has been a continuous demand for new classes of high-performance polymers as matrix resins and adhesives that have to survive for longer and longer times at higher and higher temperatures. Furthermore, the microelectronic industry has a need for an adhesive that must withstand processing temperatures exceeding 400°C (752°F) for a few hours in an inert atmosphere. Although the quantity requirement in the microelectronic industry is, in general, not critical, more material demands are foreseeable in the application of such dimensionally stable high-temperature polymer materials as structural matrix resins. The search for thermally stable polymers in the past two decades has led to the conclusion that thermal stability and processibility are antithetical in nature. Although many classes of high-temperature resistant polymers are in theory capable of reaching end use temperatures of 300’ to 4OO’C (472” to 752”F), they are too rigid in nature and therefore too intractable to be useful. The research in hightemperature polymers has always aimed at an optimal balance between processibility and high-temperature stability. Polyimides have been known for their high-performance and potential as advanced structural resins suitable for aerospace applications. As discussed in earlier sections, polyimides generally are processed by conversion of the amic acid prepolymer to the imidized structure via a condensation reaction, which presents formidable processing problems. They are difficult to use as molding compounds. For the fabrication of composite structures, very high molding pressures and a carefully controlled post-cure process are required to allow dissipation of volatile byproducts from the condensation process. To alleviate these problems, thermosetting, fusible, and soluble short-chain polyimide oligomers were developed which undergo thermal cure to polyimides via homopolymerization of the reactive endgroups. Ideally, these end-groups homopolymerize into thermo-oxidatively resistant structures without the evolution of volatile by-products.

High-Pe$ormance Polyimides and Thermoset Polymers

353

Several classes of addition-curable polyimides have been developed which undergo chain extension and cure reactions at the reactive termini of shorter and therefore more processible oligomers to yield cross-linked thermoset matrix resins applicable to advanced fiber composites. These addition-curable polymers also remove the major processing problem associated with the evolution of volatiles. Several classes of thermosetting polyimides have attained various stages of commercialization.

NADIMIDE-TERMINATED

THERMOSETTING

POLYIMIDES

In the 196Os, TRW developed bismaleimide polymers marketed by CibaGeigy Corporation, known as P13N polyimides. These difficult to process polymers developed only a small market and were eventually discontinued. The P13N polyimides were based on nadimide-terminated imide oligomers that underwent a reverse Diels-Alder reaction at elevated temperatures of 275’ to 350°C (527’F to 662’F) and then addition to form polymers. The polymerization of the P 13N polyimide oligomers is shown in Figure 8.22. Investigations at the NASA-Lewis and NASA-Langley Research Centers have been instrumental in the development of other nadimide-terminated addition-type polyimides to replace P13N. The requirement for high-temperature polymers with enhanced processibility has been met by such novel resins as in-situ polymerization of monomer reactant (PMR), LARC-160, and LARC-13.[351 The PMR technology was the subject of a recent review by its originator.[361 The development of the first-generation PMR matrix resins has culminated in various The key features of the PMR composite forms of commercial applications. technology are the availability of monomers, good solubility of monomer reactants in low boiling alcohols, processibility of the resins, and, ultimately, excellent retention of properties at elevated temperatures. The versatility of the PMR approach is demonstrated with the tailoring ofprocessing characteristics and properties by simple variations on the chemical nature of either the diester acid or the aromatic d&nine, or both, and the stoichiometric proportion of the monomer reactants. In the PMR approach the reinforcing fibers are impregnated with a solution derived from a mixture of monomers in low boiling alcohols. At room temperature, the monomers are essentially unreactive, but at elevated temperatures they react to form a

354


thermcxkzlatively stable polyimide resin. The solution of monomers consists of a high solid content (70-90%) of a dialkyl ester of an aromatic tenacarboxylic acid, an aromatic diamine, and a monoalkyl ester of 5-norbomene-2,3dicarboxylic acid (NE)(XWl) (Fignre 8.23).

Figure 8.22. Polymerization

of P13N polyimide.

Various modifications of the PMR polyimides were prepared. The fast generation of PMR resin was designated PMR-15 and the second generation PMR-II. Thus, PMR used (NE), the dimethylester of 3,3’,4,4’benzophenontetracarboxylic acid (BTDE)(XVIII) and 4,4’-methylenedianihne (MDA). The second generation of PMR resins used the perfluorinated isopropylidene analogue of BTDE known as 6FDE (XIX) and phenylenediamine1371[381(Figure 8.24). The success of the second generation PMR resins depends on the supply of the precursor to 6FDE, namely 6FDA. A number of systems have been


355

investigated and modifications of the reactants have shown the ability of the PMR approach to produce tailor-made resins with varying degrees of flow.

COOH Norborn-5-ene-2&dicarboxylic methyl ester (NE) XVII

Figure 8.23. End-capping

3,3’,4,4’-Benzophenone Tetracarboxylic Dimethyl Ester (BTDE)

agent used in PMR chemistry.

2,2-Bis(3,4_dicarboxypheny hexafluoropropane Dimethyl (6FDE)

XVIII

Figure 8.24. First- and second-generation PMR use tetracarboxylic dimethyl esters BIDE and 6FDE, respectively.

acid

PMR system LARC- 160, developed at NASA-Langley Research Center, uses the monomers BIDE and NE, and the polyaromatic amine Jeffamine AP-22 (XX), which has an average molecular weight of 23439 (Figure 8.25). The LARC 160


resin is readily processed and gives excellent mechanical properties, but is a brittle system and has marginal thermo-oxidative stability at 3 16°C (600’F).

Jeffamine AP-22

Figure

8.25. Polyaromatic

amine, Jaffamine AP-22, for LARC- 160

The thermo-oxidative stability of the original PMR resin has been improved. Progress in developing this 6FME-based PMR was slow due to an earlier lack of a commercial source for the corresponding dianhydride, 6FDA. This development is facilitated by the production of 6FDA at American Hoechst and Hitachi Chemicals (Japan). The room temperature properties of PMR-15 neat resin are shown in Table 8.36.[40J Some typical properties for graphite PMR composites are shown in Table 8.37.1401 The excellent retention on interlaminar shear strength of graphite fiber/PMR- 15 composites as a function of air aging and testing at 600’F is shown in Table 8 .38 .[35l

Table 8.36. Room Temperature

Properties

of PMR-15 Neat Resin1401

Property

I

Value

Tensile strength, 1O3x psi

8.1

Tensile modulus, 1O6x psi

0.47

Compressive yield strength, 1O3x psi

16.5

Compressive strength, 1O3x psi

27.2

Thermal coefficient of expansion, 10e6x inlinl”F

28.0

Reprinted by permission of Van Nostrand Reinhold Company

High-Performance Polyimides and Thermoset Polymers


of HT-WPMR-15

Property

Composites*1401

4(klO-10)

6(90)

357

8(0,+10,0, -lO,lO,O, +lo,o)

13[MO, )(O)O +401

Tensile strength, 1O3psi

180

9.74

150

191

124

Tensile modulus,

21.7

1.15

21.1

19.0

14.0

Compressive strength, 1O6psi

13.5

34

Flexural strength,

1O3psi

206

16.35

230

197

145

Flexural modulus,

1O6psi

17.6

1.07

16.8

19.0

9.3

Short-beam.interlaminar shear, 1O3psi

16.0

-

-

-

-

Miniature Izod impact energy, in-lb

15.2

1.8

-

Coefficient of thermal expansion, 10.6 inlinl”F

0

14.5

-

1O6psi

*Fiber volume, 55% by volume;

-

-

-

tested along 0“ ply direction

Reprinted by permission

of Van Nostrand Reinhold Company

Table 8.38. Interlaminar Shear Strength of Graphite Fiber/PMR-15 Composites as a Function of Air Aging and Testing at 600°FP51 I Time, hr

0

400

Interlaminar Shear Strength, lo3 rsi, Fiber Rein lrced Celion 6K (58%) 1 HTS-2 (62%) Thornel Fortatil3 I 7.3 6.5 5.4 6.5 I I 7.7 6.5 6.1 6.5

700

-

-

6.5

-

800

7.7

6.2

6.3

6.1

1200

7.3

5.7

5.5

4.8

4.8

1400

7.0

5.6

I I


4.8

I I

of Dr. T. T. Serafini)

1


As the requirements for high-temperature adhesives coincide with those for structural matrix resins, the candidate high-temperature matrix resins are also screened for their suitability as high-temperature adhesives. The LARC-13 system was developed as a high-temperature adhesive for bonding large areas. Previous adhesive systems frequently yielded poor results because large volumes of volatiles are usually given off during the cure/bonding process for large areas. The LARC-13 system is an addition-type cure that results in the evolution of few volatiles. Its unique polymer chain incorporates meta-linkages in the oligomer chains (XXI) (Figure 8.26).

XXI Figure

8.26. Chemical structure of LARC-13 adhesive.

The oligomer has a relatively low softening temperature and can be processed at 266” to 3 16°C (5 12” to 600°F), properties that make it attractive for bonding metals. Based on a structure-property relationship study on 14il LARC- 13 was shown to exhibit addition polyimides at NASA-Langley, the highest lap-shear among nadimide-end-capped polyimide resins (Table 8.39) and was further developed as a structural adhesive for specialized bonding applications.14*l Besides its high lap-shear strength, LARC-13 also has a high degree of flow during cure and is easily processible by conventional autoclave techniques. Its high cross-linking density provides dimensional stability during use at elevated temperatures in excess of its glass transition temperature of 270°C (5 1B’F). Figure 8.27 shows the chemistry of LARC-13 adhesive. Research at Langley on elastomer toughening of the inherently brittle LARC- 13 has produced some noteworthy salient features:1421 (1) a compromise exists between toughness improvement and elevated-temperature adhesive strength, (2) novel high-tem-

High-Pe@ormance Polyimides and Thermoset Polymers

359

perature resistant elastomers are needed, and (3) a 50-50 bimodal distribution of long- and short-chained elastomers in LARC-13 can contribute to a significant improvement in adhesive properties.

Table 8.39. Adhesive Properties of Nadimide End-capped Polyimide Resins

Z

Amine Structure

0 II

I

I I

Amine Isomer

Titanium Adherends Lap Shear Strength, psi (mPa)*

3,3’

2,800

(19)

-c-

LARC- 13

P -cn

4,4’

600

( 4)

393

2,500

(17)

I

-O-

4,4’

1,300 ( 9)

II

0 II

3,3’

2,100

4,4’

1,300 ( 9)

3,3’

3,000

4,4’

1,300 ( 9)

(14)

-c0 II

II

II II

~~

(21)

(Reprinted by permission of Reference 42) A further investigation was conducted to find a cross-linking end group that would improve the thermo-oxidative stability of the LARC-13 adhesive. Table 8.40 shows that the acetylene-terminated LARC- 13-based material has 40% higher adhesive lap shear strength than LARC- 13 itself after aging 1,000 hours at elevated temperatures.


y==J+=J=

+

0

1 hb

+

OO

Amic Acid Prepolymer

I

18CC

@N-AR

fpAoGN

-AR{N+3

0

” Imide Prepolymer

3owc

i CROSS-LINKED POLYIMIDE

Figure 8.27. Chemistry of LARC- 13 adhesive.


361

A NASA-sponsored study by Boeing Aerospace Company on the effects of surface treatment of titanium on the lap shear strengths of titanium bonded with LARC-13 shows a wide range of values, with the best values being obtained with chromic acid anodized treatment [2,900 psi at room temperature and 200 psi at 450’F (232”C)] and the poorest with Pasa-Jell 107 [850 psi at room temperature and 580 psi at 450’F (232”C)].[43J Table 8.40. Lap Shear Strengths of Titanium/Addition

LSS Unaged Sample, psi (MPa)

LSS after 1,000 hours at 450’F (232”(Z), psi (MPa)

Oligomer End Group RT

Polyimide Bonds

450’F (232°C)

RT

450’F (232°C)

Nadic

3,200

(22)

2,600

(18)

2,600

(18)

2,000

(14)

Ethynyl

2,900

(20)

2,500

(17)

2,500

(17)

2,800

(19)

N-Propargyl

3,100

(21)

2,800

(19)

800

(6)

1,000

(7)


of Reference 42)

LARC- 13 and its modified versions thus represent some of the state-ofthe-art polymeric materials suitable for applications as both structural matrix resins and high temperature adhesives. Owing to their high cross-link density, they are able to perform for short terms at temperatures up to 1,112’F (600°C) where linear systems fail thermoplastically. On the other hand, further improvements are needed to enhance their thermo-oxidative stability and long-term performance at elevated temperatures. As experience in the field of high-temperature resistant polymers shows, successful development of polymer systems possessing both ultrahigh thermal and thermo-oxidative stabilities while maintaining good processibility and a high level of toughness will represent a major technological breakthrough. A likely answer lies in extending the systematic resin toughening studies carried out at NASA-Langley to beyond the realm of organic polymers.


Modified Nadimide

End-Groups

Ciba-Geigy researchers have evaluated the benefits of using modified nadimide end-caps in tbermoset polyimides (Figure 8.28). Prepolymers were made at MW 5000 to ensure reasonable solvent solubility (i.e., 35% concentration or higher) and melt processibilityP1 Laminate properties were obtained from cured polyimide materials using these modified nadimide end-caps, compared significantly better with polyimide materials without end-caps in thermomechanical properties such as Ts and modulus retention under hot, wet conditions. Generally, polyimides with the dinadimide end-capper cure to higher Ts matrices and are suitable for applications in the 260“ to 290°C range. Data for the allylic nadimideterminated polyimides indicate a useful continuous operating range of 232’ to 260°C.

AR-NH-

Allylic Nadimide End-capped Polymer Dinadimide End-capped Polymer

Figure 8.28. Modified nadimide

end-groups

for thermoset polyimides.

The use of the dinadimide end-group originated in earlier work at Boeing Aerospace Company. STAR was initially developed as an experimental resin to increase the Ts of polyimides several hundred degrees above the cure temperature, without post curing and without increasing the cross-link density. The approach centers around end-capping the oligomer with a dinadimide endcapping agent that reacts in two distinct steps. The first step joins the ends of two oligomers, and the second step involves the second nadimide in the end-group moiety, now proximal, to form a double strand linkage that stiffens the chain and raises the T,.


363

This chain rigidizing (reinforcing) concept via intramolecular linking is similar to earlier exploratory work[45l at Hughes supported by the Air Force on thermal intramolecular cyclization (IMC) curing of 2,2’-bis(phenylethynyl)biphenyl units along the polymer main-chain structure (Figure 8.29). The intramolecular cycloaddition chain-rigidization concept has been successfully used in polyphenylquinoxaline (PPQ) research at the Air Force Materials Laboratory.1461

A

*

-\\/ /\-/\/ Y\/

Figure 8.29. Intramolecular cycloaddition as a mechanism of chain rigidization of polymers containing the 2,2’-Bis(phenylethynyl)biphenyl units. AFR7OOB Development In a continuing effort to improve the PMR technology in processibility and thermo-oxidative resistance, the Air Force has sponsored R&D of the AFR700B resin in recent years. The cure chemistry of AFR7OOB resembles that of PMR resins. The use of 6FDE aims at improving thermo-oxidative resistance and processibility of the resin. The stoichiometric offset of the various reactant monomers affords an oligomeric molecular weight of 4,382 to allow easy solubility in methanol solvent. hnidization of the amic acid prepolytner occurs at around 205” to 220°C (400’ to 43O’F) and the imide prepolymer undergoes cross-linking at 343” to 371°C (650” to 700°F)14711481 (Figure 8.30). The originator of 371°C (700°F) polyimides recently summarized its development.[48l The different polyimides based on the PMR concept are detailed in Table 8.41. The improvement in processibility has been demonstrated by excellent ultrasonic C-scan results on quartz-supported AFR700A and AFR700B. Ts values obtained for AFR700 materials exceed 400°C (752°F). Isothermal aging showed that quartz/AFR700B laminates are superior to comparable laminates made from PMR-15 and AFR700A. Retention of mechanical properties after 371 “C (700“F) aging in air for 100 hours is excellent.1481


Table 8.41. Formulations

365

of AFR700B and Other PMR Polyimidesf481

~ Monomers

Monomer Ratio

Oligomer Molecular Weight

10: 10: 1

5,000

1.83

10: 8: 2: 1

4,900

1.88

Aliphatic Content, %

RX-I

BTDE/MDAlNE

Rx-2

BTDE/MDA/PPD

Rx-3

6FDE/PPDA/NE

9: 9: 1

4,800

1.92

RX-4 (AFR700A)

6FDE/PPDA/NE

7: 7: 1

3,800

2.42

RY- 1 (AFR700B)

6FDE/PPDA/NE

8: 9: 1

4,400

2.10

PMR-15

BTDE/MDA/NE

2.1: 3.1: 2

1,500

PMR-II-30

6FDEL’PDA.NE

5: 6: 2

3,000

A/NE


MALEIMIDE-TERMINATED

12.2 6.13

of Dr. T. T. Serafini)

THERMOSETTING

POLYIMIDES

Another group of thermosetting polyimides curable by addition reactions is the N-substituted bismaleimides (BMIs). By choosing the appropriate moieties in the polymer chain, it is possible to prepare very soluble and fkble oligomers. These BMI prepolymers can be heated above their melting point in the presence of a free radical catalyst, such as dicumyl peroxide, to polymerize into a cross-linked polyimide resin. Rhbne-Poulenc Company introduced the Kerimid@ resins which are based on mixtures of bismaleimide monomers and aromatic diamines.[491 Such mixtures produce linear chain extension via a Michael addition reaction. Cross-linked poly(bismaleimides) were formed from these bismaleimide oligomers. Improved high-temperature properties could be obtained from these resins if nonstoichiometric mixtures of aromatic diarnines and bismaleimide were used. Two types of reactions are postulated.[501 One type is the Michael addition to form linear polymers as depicted in Figure 8.3 1. The second ty-pe is free radical cure of the terminal double bonds leading to cross-linking.


A melt processible bismaleimide copolymer (Kerimid 353) was developed using a ternary mixture of aliphatic and aromatic bismaleimides.[5~l-[531 Further developments of bismaleimide technology using a combination of free radical cure and diamine addition had yielded new types of processible bismaleimide resins, the Kinels and Kerimid@ 601 series, which are suitable as molding and laminating resins, respectively.[541 Recent copolymer&ion studies have demonstrated that bismaleimide monomers or prepolymers can best be used as cross-linking agents to yield products with high glass transition temperatures (Figure 8.32).ts51

0

G3 N-R,-N

0

1

+

H,N-R-NH

z

------w

EN-R-k;-R+

0

Figure

8.31. Chain extension

POLY(AMIDE-IMIDE

of bismaleimides

via Michael addition.

+ 1 part

3 parts

2-ethylimidazole (catalyst) * 2 hr, 1909c 4 hr, 240°C

Figure

8.32. Application

CROSS-LINKED

RESIN

Tg z- 3WC

of bismaleimides

as cross-linking

agent.

Typical molding parameters of these resins are presented in Table 8.42. Thus, Kerimid@ FE70003 coated on graphite or glass cloth fibers can be molded using vacuum bag techniques. Properties of the neat resin are listed in Table 8.43. The low viscosity and reasonable gel times are very important parameters which make these resins easy to process. Tables 8.44 through 8.46 show typical thermal mechanical and electrical properties for Kerimid@’ 601/l 8 1E glass cloth laminates.


Table 8.42. Kerimid@ FE 70003 Modified Bismaleimides

Processing Impregnation

367

Resinl”]

Reinforced from glass cloth, graphite, etc., can be coated from melted resin (90°C) or from a lacquer in methylene chloride or methylethyl ketone.

Molding

Can be molded by vacuum bag technique. The cure takes place at 200” to 250°C at pressures of 75 to 150 psi. It is advisable to postcure the parts 12 to 24 hours at 250°C

Properties of Composites

Seven-plie graphite fabric at 215g/m* with 42% resin content and molded and cured as indicated above

Flexural strength, lo3 x psi Flexural modulus, 1O6x psi

98.6 (20°C 68°F) 0.77 (20°C, 68°F)

(Reprinted by permission of Rhone-Poulenc, Inc.)

Table 8.43. Kerimid@ FE 70003 Modified Bismaleimide Properties of Neat ResinIs

Property Density, g/cc

Value 1.2

Viscosity, cps 60°C 7o”c 80°C 90°C 100°C

27,000 4,500 1,200 500 450

Gel Time, min 15o”c 180°C

45 20

Note: Solubility:

Resin Select

Solutions of 50% in weight in methylene chloride or methvlethvl ketone.

368


Table 8.44. Electrical Properties of Kermid@ 601/181E Glass Cloth Laminate as a Result of Aging15’l

Condition

Dielectric Strength, kV/mm

Volume Resistivity, ohm x cm

Dielectric Constant, 1kHz

Dissipation Factor, 1 kHz

D159

D1.50

ASTM method

D149

D257

Initial

25

6~10’~

4.5

0.012

24 hr. in water

20

1.5x1013

5.4

0.016

1000 hr. at 355’F

>16.5

-

1000 hr. at 390°F

>16.5

-

-

1000 hr. at 430°F

12

-

-

2000 hr. at 480’F

-

2.2x1015

-

-

10000 hr. at 355’F

-

-

5.5

-

10000 hr. at 390’F

-

-

5.5

10000 hrs at 430’F

-

-

4.7

-

-

(Reprinted by permission of Rh6ne-Poulenc Inc.)

Several years ago Ciba-Geigy marketed a two-component bismaleimide system XU292 that, when combined and cured, is suitable for high temperature advanced composites and adhesives applications. The system is based on 4,4’bis(maleimidophenyl)methane (III) and 3,3’-diallyl Bisphenol A (xxII)[~*I (Figure 8.33). Upon combining the two components with continuous stirring to 120” to 150°C (248’ to 302”F), a clear homogeneous solution is obtained The resulting liquid can be used either as a casting resin for the preparation of prepregs or as an adhesive. The twocomponent system provides flexibility to ensure the optimum formulation for the prepreg. One formulation of the neat resin by Ciba-Geigy[5gI reports a room temperature tensile strength of 13.6 ksi, a modulus of 564 ksi, an elongation of 3.0%, a flexural slrengtb of 26.8 ksi, and a flexural modulus of 580 ksi. At 204°C (4OO”F),these values drop to 104 ksi for tensile strength and 394 ksi for the modulus. The Ts obtained from the TMA penetration method is 282°C (540°F).

High-Performance

Polyimides

and Thermoset Polymers

Table 8.45. Thermal Aging and Mechanical Properties 181E Glass Cloth 18-ply Laminate15’l

369

of Kerimid@ 601/

-I Flexural Modulus, DSIx 103 77OF

390°F

Weight Loss, %

Initial 1000 2000 5000 8000 10000

71 67 67 67 59 59

57 59 59 57 52 47

3850 3600 3800 3850 3500 3850

3150 3350 3350 3400 3300 3350

0.25 0.30 0.5 0.9 1.4

390

Initial 1000 2000 5000 8000 10000

71 66 66 57 46 37

57 56 54 54 37 30

3850 3650 3650 3650 3300 3600

3150 3400 3400 3350 3150 3000

0.4 0.5 1.1 1.9 2.7

430

Initial 1000 2000 3000 5000 8000 10000

71 64 64 59 44 19 17

57 56 54 51 37 16 12

3850 3800 3700 3600 3300 2800 1800

3150 3400 3200 3200 3050 2600 2450

Initial 1000 2000 3000

71 57 50 26

57 47

3850 3300 3100 2600

3150 3150 2950 2600

180

480

;:

G 0.9 1.4 2.8 5.4 7.7 4.0 5.7 8.3

The specimens were taken from laminates prepared in the following conditions: Impregnation bath: Fiberglass fabric: Resin content in the prepreg: Rate of flow of the prepreg: Laminate thickness: Curing conditions:

Resin content: Specific gravity: Barcol Hardness:

Solution of Kermid@601 at 45% in NMP Continuous filament yam, satin weave of the 18 1 type with an aminosilane finish 30 to 35% 30 to 40% Stack of 18 plies; (a) Under 2 10 psi pressure at 480°F and postcuring for 48 hours at 390”F, or (b) Curing under 2 10 psi at 390’F and postcuring for 24 hours at 480’F. 22 to 24% 1.94 ‘70.

(Reprinted by permission of RhBne-Poulenc Inc.)


Table 8.46. Selected Properties of Kerimid@ 601/181E Glass Cloth Laminateds71 Property

ASTM

Approximate Value

Flexural strength, psi x 1O3 77’F (25°C) 390’F (200°C) 480“F (25O’C)

70 60 50

Flexural modulus, psi x 1O3 77’F ( 2Y’C) 390°F (2OO’C) 480’F (25O’C)

D790

Tensile strength, psi x 1O3 77°F (25’C)

D638

Compressive strength, psi x 1O3 77’F (25’C)

D695

Delaminating strength, psi 77“F (25°C)

D2355

Izod Impact Strength, fi x lb/in 77°F (25’C) Notched Unnotched

D256

4000 3800 3200

50

50

2150

13 15

(Reprinted by permission of RhGn*Poulenc Inc.)

Figure 8.33. Components of Ciba-Geigy XU292 BMI resin (Matrimid@’5292). Compounds III and XXII, now known as the Matrimid@ 5292 system (111=5292A and XXII=5292B), complements the polyimide Matrimid@ 5218. Other similar bismaleimide (BMI) systems have also been developed as Ciba’s


371

BMI family of high-performance resins. A notable new BMI is RD85-101, that was synthesized from 1,1,3-trimethyl-3-phenylindane and maleic anhydride (Figure 8.34). Formulated with common BMI co-curing agents such as ally1 phenols (e.g., Matrimid@’ 5292B) or aromatic diamines, RD85-101 shows outstanding thermomechanical and hygrothermal performance. Table 8.47 shows some exemplary physical and thermomechanical data of this resin system.1601

RD85-101

Figure 8.34. RD85-10 1, BMI based on 1,1,3-trimethyl-3-phenylindane.

CYANATE-TERMINATED

THERMOSETTING

POLYMERS

Cyanate esters are stable one-component resins that are storageable in their crystalline form for over 1 year at 25°C (77°F) in sealed containers. Thermal curing forms three-dimensional networks of oxygen-inked triazine rings (Figure 8.35)Pl The monomers or prepolymers are cured with or without transition metal catalysts and active hydrogen co-catalysts (~0.15% of formulation by weight). While optimum cured-state properties for the homopolymer based on Bisphenol A (Ciba Arocy B) are typically achieved after 1 to 2 hours at 177°C (350°F) with a post-cure at 250°C (482“F), early work with uncatalyzed resin formulated with thermoplastics showed that cures can be accomplished in 30 minutes at 250” to 270°C (482“ to 5 18°F).1621 Cyanate functional resins are chemically compatible with epoxy resins. They co-cure via a combination of co-reaction and catalytic polyetherification.1631 Dicyanates modified with epoxy resins represent one of the best routes to lower the cure temperatures while improving the compatibility with graphite/epoxy adhere&. High conversions can be achieved without transition metal catalysts. Alternatively, the blending with epoxy resin can improve wettability.


Table 8.47. Cured Neat Resin Thermomechanical Data of RD85-lOl/Matrimid 5292B Formulations1601

Property 25OC

Test Temperature 177v

232°C

Flexural strength, ksi System I System II

17.5 17.5

15.9 16.2 (204°C)

13.2

Flexural modulus, ksi System I System II

531 502

428 383 (204’C)

348

Tensile strength, ksi System I System II

11.5 8.8

(2ZC)

Tensile modulus, ksi System I System II

539 547

396 (204°C)

Tensile elongation, % System I System II

1.8

DMA modulus (dry), ksi (System II) DMA modulus (hot/wet)*, ksi (System II) Te, “C (TMA), System II Water Pick-up*, System II

1.8 512 463

-

-

(2ZC) 402 (15OOC) 336 (15OOC) 298 2.6%

Note: Cure condition: 1 hr/l 80°C + 2 hrl200”C + 6 hrl25O”C *Immersion in 160°F water for 48 hours System I = 1: 1 molar ratio (RD85-lOl/Matrimid 5292B) System II = 1:0.87 molar ratio (RD85-lOl/Matrimid 5292B)

334 (25O’C) (25O’C)

374


Cyanate esters also exhibit very low moisture absorption (0.5 to 2.5% immersed in water at 1OO’C) when compared to epoxies and other thermosets or thermoplastics. To maintain good adhesive processibility and mechanical properties requires an ability to keep the adhesive dry, which is made easier with a resin having a low equilibrium moisture absorption. Polycyanurates are known to have excellent adhesive properties. WI Partly responsible for the excellent adhesion is the low degree of shrinkage in processing from the uncured to cured state. Volume change with cyclotrimerization produces the expected shrinkage up to gelation (60 to 65% conversion), but an unexpected expansion occurs with continued curing (Figure 8.36).La] Low shrinkage translates to reduced thermal stress between the adhesive patch and the substrate.

40

60

100

% Conversion

Figure 8.36. Specific volume change in uncatalyzed Arocy RTX-366 during cure. A wide product range of cyanate ester resins is commercially available from such suppliers as Allied Signal, Ciba, Dow, Hoechst ,and YLA to allow flexibility in matrix and adhesive formulation. These cyanate esters range from


375

those based on Bisphenol A and low molecular weight novalacs [Dow Chemical XU 71787 series, YLA RS-3 (Figure 8.37)], to those based on Matrimid@,[651 RTX 366 (Figure 8.38), and the 6F analog of Bisphenol A produced by Hoechst.

Figure 8.37. Polycyclic-containing

polycyanate resins I?om Dow and YLA.

Figure 8.38. RTX 366 dicyanate monomer.

The Dow XU 71787 polycyanurates perform up to 177’C (350’F) and offer zero cure shrinkage, low moisture absorption, low dielectric constant, and low dielectric 10ss.[~~l A comparable choice is the YLA RS-3 toughened polycyanurate.[‘j71 Both the Dow and YLA polycyanurate materials are based on dicyclopentadiene. The polycyclic structures in the cured polycyanurate matrix are resposnsible for the material’s low moisture absorption and low dielectric constant/loss (Table 8.48).

Cyanate Ester SIPNs A semi-interpenetrating polymer network (SIPN) is defined as a combination of one thermoplastic polymer and one thermoset polymer in network form, one of that is synthesized and/or crosslinked in the immediate presence of the other. Formulation of cyanate esters with thermoplastics would provide the proper rheology for good wet out and adhesion in the bonded joint


Table 8.48. Typical Properties of Neat Polycyanurate

Property

Dow xu-71787

Tg, “C

265 (untoughened)

Castings

YLA RS-3 (Toughened)

254

254 (toughened)* Density, g/cm3

1.19

Viscosity, cps

1,50~3,000 (70°C) 20-40 (13OOC)

1.194 -

CTE, p in/in “F

28

G,, fracture toughness, in.-lb/in.

0.4 (untoughened) 2.8 (toughened)*

2.1

Moisture absorption after 14 day water boil, %

1.20

1.45

Tensile strength, ksi

10.1

11.6

Tensile modulus, ksi

470

430

Tensile strain to break, %

4.0

4.9

Dielectric constant

2.8 (1 MHz)

Loss Tangent

0.002

44 (per “C)

2.67 (2-18 GHz) 0.005

*Toughened with 10% elastomeric additive Cyanate esters blended with thermoplastics usually develop excellent tack and drape. The aromatic nature of the monomer backbone in RTX-366 imparts thermal stability to polymers cured at temperatures as low as 12 1“C and reduces moisture absorption (0.6% at 100% saturation).[681 Other features of RTX 366 are its low melting (68°C) and the viscosity (8,000 cps at 2YC) of its monomer. Thermoplastics of interest include polyarylsulfone (Figure 8.39). Although unmodified cyanate esters possess twice the fracture toughness of multifunctional epoxies, they are still highly cross-linked materials which are prone to embrittlement. Formation of SIPNs with compatible thermoplastics

High-PerjTormance Polyimides and Thermoset Polymers

377

improves their fracture toughness. Several high-T, (170” to 22O’C) thermoplastics are soluble in dicyanate monomers and phase separates during cure. Thermoplastic resins with reactive hydroxyl end-groups develop smaller co-continuous thermoplastic-rich domains (usually ~1 micron in diameter) than similar thermoplastics terminated with nonreactive end-groups. This permits formulation of melt processable blends achieving dramatic improvements in fracture toughness (G,,) (Figure 8.40).16211691The free hydroxyl groups also serve as a co-catalyst for homopolymerization. Several noteworthy commercial thermoplastics with hydroxyl termination are polyarylsulfones (i.e., ICI’s Victrex 5003, Amoco’s Radel A, and BASF’s PES) and copolyesters (Vitel PE307).

Figure

0

l A

00.0

8.39. Polyarylsulfone.

Polyethersulfone Copolyester(CPE) Polysulfone(PS)

5.0

(PES)

10.0

15.0

20.0

% Thermoplastic

Figure

8.40. Effect of thermoplastic resin concentration of Arocy B castings.

on fracture toughness

378


Polyimide-Based

Cyanate Esters

During the late 1980s and early 199Os, several aerospace research groups including Lockheed and Rohr, focused on improving the thermo-oxidative stability, mechanical strength, and durability of cyanates ester resins through application of acetylene- and phenylethynyl-terminated imidelisoimide technologies, PMR resins, BMIs, silicate-based polymer-cerarnics,1701-[841 and polyimide-based cyanate ester resins. The general strategy for producing polyimide-based cyanate ester resins is a two-step process: end-capping of an anhydride-terminated imide oligomer with 3- or 4-aminophenol, followed by the standard cyanate ester formation between the hydroxy-terminated oligomer and cyanogen bromide.Ig51 An interpenetrating polymer network (IPN) and a semi-interpenetrating polymer network (SIPN) can be formed by blending the thermosetting and thermplastic polyisoimides/polyimides with the compatible liquid dicyanate monomer AroCy L- 10 (Figure 8.4 1). The dicyanate monomer thermally cures to form a cross-linked network, interpenetrating with the network formed by the thermosetting polyimide (hence, IPN) or imbibing the linear thermoplastic polyimide strands to give an SIPN.

Figure 8.41. Low viscocity dicyanate monomer

Polycyanurates

in Electronic

Arocy L-10.

Applications

As mentioned earlier, cyanate ester materials possess outstanding electrical properties, such as low dielectric constant and high dielectric breakdown strength, in addition to their excellent thermomechanical properties such as high Ts, adhesion to metals, chemical resistance, and dimensional stability. The microelectronic industry has used various polycyanurates in printed wiring circuit boards, thin cards, and multichip modules, and for encapsulation.lg61

High-Per$ormance Polyimides and Thermoset Polymers

379

To alleviate the brittleness and microcracking problems of cyanate esters, SIPNs have been found to be useful. Recently, IBM researchers focused on fluorinated hydroxy-terminated polysulfone thermoplastics and a fluorinated dicyanate monomer (Figure 8.42).lg7] Fracture toughness, as indicative of the Krc and G,, values, have been significantly improved.

+

t Cross-Linked Network

Figure 8.42. A toughened

HIGH-TEMPERATURE PHTHALONITRILE

fluorinated

THERMOSETTING

polycyanurate

network.

RESINS BASED ON

The literature on high-performance polymers contains references to phthalonitrile resins extending back more than 15 years. Most recent activities focus on the feasibility study of optimizing phthalonitrile resins as prepreg resins at Merlin Technologies (San Carlos, CA), sponsored by the Naval Research Laboratory.Ig81

380


The synthesis and evaluation of phthalonitrile resins have been a key materials interest at the Naval Research Laboratory, summarized in 46 publications and patents in the past 15 years.[8gl-[‘351 For example, phthalocyanine polymers having high thermal and oxidation resistance, good toughness, and low water absorptivity have been prepared from various bisphthalonitriles, that in turn, were synthesized by nucleophilic displacement of the nitro group of nitrophthalonitrile with aromatic bisphenolstg91 or via metal-mediated coupling of 4-iodophthalonitrile and iodofluoroalkanes.[901 Generally, the reactants are heated at 250°C, giving a brown solid in 2 hours, that is postcured at 250°C for 6 hours and 3 15°C for 16 hours. The fluorinated polyphthalocyanine materials have ~1% water absorption under standard soak tests. The thermo-oxidative properties of phthalonitrile-based polymers can be exemplified by the aromatic diether-linked phthalonitrile resin, 4,4’-bis(3,4dicyanophenoxy)biphenyl with 1,3-bis(3-aminophenoxy)benzene (APB). This resin is easily processed from the melt of the monomer in a controlled manner as a function of the amine content and processing conditions. The resulting polymer shows an excellent retention of mechanical properties followinginert atmosphere post-cure to 375°C and oxidative aging at 315°C for 100 hours. Results are reported for the effects of cure, post-cure, and aging conditions on the tensile and fracture properties of these polymers.[*OOl Phthalonitrile adhesives, prepared from 4,4’-bis(3,4-dicyanophenoxy)biphenyl and 1,3-bis(3-aminophenoxy)benzene curing agent, can withstand temperatures of 200” to 55O’C and are useful for bonding various substrates, especially metals. These adhesives were tested with aluminum substrates showing shear strength at room temperature and at 2OO”C, of 1,450 and 1,070 psi, respectively.Lg71 Polymerization of the phthalonitrile resin occurs through the cyan0 groups via an addition mechanism to afford heterocyclic cross-linked products with high T, and thermo-oxidative stability. Polymerization can be easily controlled by the concentration of the curative and the cure temperature[135] in one step to gellation or to any desired level of viscosity and quenched. Such B-staged prepolymers can be kept indefinitely at ambient temperatures. The breadth of the studies is shown by the representative references summarized in Table 8.49.


381

Table 8.49. Select Phthalonitrile Resins Studied at Naval Research Laboratory Phthalonitrile Precursor

High-Temperature Resin

Authors

Reference

p,p’-bis(3,4dicyanophenoxy)biphenyl

Highly aromatic polyphthalocyanines

Keller, Griffith (1981)

89

1,3-Bis(3,4dicyanophenyl)perfiuoropropane

Polyphthalocyanines with perfluorinated linkages


90

Fluorinated dumbbell phthalonitriles

Highly crosslinked polyphthalocyanines


92

(Perfluoroalkyl)phthalonitiles

Monomeric and polymeric phthalocyanines


94

bis(3,4-dicyanopehnyl-Z tetrafluoroethoxy)- 1,5pertluoropentane

Polyphthalocyanine with improved water repellency


91

m-andp[KOC(CF,)J,C,H, with tetrafluorophthalonitrile

Highly fluorinated polyphthalocyanines


94

perfluoroalkylcopper 4-iodophthalonitrile

Fluorinated polyphthalocyanines (SnCl, cure)

Keller, Griftith (1979)

95


Highly aromatic polyphthalocyanines (tosic acid cure)

Keller (1992)

96


Highly aromatic polyphthalocyanine adhesives (amine cure)

Keller, Roland (1982)

97


Highly aromatic (Tg 300°C) polyphthalocyanines (tosic acid cure)

Keller (1992)

98

4-(3-aminophenoxy) phthalonitrile with BTDA

Imide-containing polyphthalocyanines

Keller (1993)

99


Highly aromatic polyphthalocyanines (Amine APB cure)

Warzel, Keller (1993)

100

with


Table 8.49. (Continued) Phthalonitrile Precursor


Authors

Reference

Phthalonitriles with phosphazenes

Flame-retardant aromatic polyphthalocyanines (Amine cure)

Keller (1992)

101

4aminophthalonitrileend-capped oligomer of BTDA and APB

Imide-containing polyphthalocyanines (phosphazene cure)

Keller (1991)

102

4,4’-bis(3,4,dicyanothiophenoxy)biphenyl

Highly aromatic polyphthalocyanines with electrical conductivity (MDA cure, pyrolysis at 9OOOC)

Keller, Price (199 1)

103

4,4’-bis(3,4dicyanothiophenoxy)Ibiphenyl

Highly aromatic polyphthalocyanines with electrical conductivity (MDA cure, pyrolysis at 9OOOC)

Keller, Price (1990)

104

4arninophthalonitrile-

Keller (1990)

105

BTDA and APB


Q-aminophthalonitrileend-capped ,oligomer of BTDA and APB


Keller (1991)

106

4-(3- or 4-aminophenoxy)1ohthalonitrile and BTDA

Keller Void-free polyphthalocyanine (1990) compositeswith ether and imide linkages

107

1l-aminophthalonitrilet:nd-capped faligomer of 6FDA and uomatic amines

Keller High-temperature fluorinatedhnide-conlaining (1990) polyphthalocyanines (phosphazene cure)

108

Is,p’-bis(3,4(dicyanophenoxy)biphenyl


Keller, Moonay (1989)

111

Io,p’-bis(3,4(licyanophenoxy)biphenyl


Keller (1988)

113

I

,end-capped ,oligomer of

I

1


383

Table 8.49. (Continued)

Phthalonitrile Precursor


Authors

Reference

2,2-Bis[4-(3,4dicyanophenoxy)phenyl] hexafluoropropane

Highly aromatic fluorinated polyphthalocyanines (Amine APB cure)

Keller (1989)

114

4-(3-Aminophenoxy) phthalonitrile with BTDA


Keller (1989)

115

2,2-Bis[4-(3,4dicyanophenoxy)phenyl] hexafluoropropane

Highly aromatic fluorinated polyphthalocyanine as molding compounds (Amine APB cure)

Keller (1987)

116


Polyphthalocyanines with electrical conductivity (Amine APB cure, then pyrolysis)

Keller (1986)

120

Bisphenol A phthalonitrile and Bisphenol A diglycidyl ether

Highly aromatic polyphthalocyanines (Amine MDA cure)

Keller (1985)

121


Polyphthalocyanines with electrical conductivity (Amine MDA cure, then pyrolysis to 700°C)

Keller (1985)

122

3,CDicyanophenylterminated oligomeric

Highly aromatic

Keller

124


ACETYLENE-TERMINATED

THERMOSEITING

POLYMERS

As a result of the development of novel synthetic methods for attaching ethynyl and phenylethynyl groups on aromatic systems, more and more monomers, oligomers, and prepolymers containing terminal ethynyl and phenylethynyl groups are available as basic units for a new class of addition curable resins. Similar to the development of nadimide-terminated polyimides and bismaleimides, the development of ethynyl-terminated, also called acetylene-terminated (AT) polyimides and other heteroaromatic polymers, has the important feature of an addition cure mechanism that provides chain extension and cross-linking without the evolution of volatile by-products. Many of the cured resins have good solvent and moisture resistance, and exhibit outstanding physical and mechanical properties. A comprehensive review has summar ized the research activities conducted on the development of AT monomers, oligomers, andpolymers.[1361 The AT polyimides have been commercial products for over a decade. It is sufficient here to highlight key features in the development of this novel class of addition thermoset resins. Attachment of ethynyl groups to aromatic and heteroaromatic nuclei favors the application of organometallic chemistry and, inparticular, organopalladium chemistry (Fig. 8.43). The availability of a variety of organopalladium catalysts, the catalytic nature of the palladium complexes for synthetic applications, the simple experimental procedure, and the high yield of reaction product are the favorable factors. An aromatic halide and a terminal acetylene compound can undergo coupling under the catalysis of palladium[0].[1371 In principle, for the placement of an ethynyl group on an aromatic structure, a terminal acetylenic compound would be required having the protective R group easily removable after the palladium-catalyzed coupling reaction has been effected.

x

+

H

-cd+R

L

X=Br,I Figure 8.43. Palladium-catalyzed

ethynylation.


3 85

In the synthesis of ethynylated aromatic compounds,two key terminal acetylenes have been widely used (Fig. 8.44). Both ethynyltrimethylsilane (variation A) and 2-methyl-3-butyn-2-01 (variation B) undergo facile palladium-catalyzed coupling with haloaromatics to give internal acetylene compounds. Removal of the trimethylsilyl group or the acetonyl group generates the corresponding terminal acetylenes.

P H-crc-_Si-C& CN3 A

Figure 8.44. Ethynyl compounds

Cb H-PC-

c -OH

C&3 B used in palladium-catalyzed reactions.

ethynylation

The highly basic reaction medium required in variation B to remove the acetonyl group precludes compounds containing base-sensitive functional groups. The removal of the trimethylsilyl group (variation A) requires only mild conditions.11381 Variation A, therefore, has specific application in the ethynylation reactions of base-sensitive compounds. As stated earlier, the most important characteristic of resins capable of addition cure is that no volatile by-products are released. Void-free components that can be produced with these thermoset resins are critical to large dimensional structural use. A variety of aromatic and heteroaromatic oligomers having terminal ethynyl groups has been prepared and evaluated. Some of them have the potential of meeting all the processing criteria of epoxies but are more thermally stable and moisture resistant. Acetylene-terminated (AT) resins can be used in a wide range of processing temperatures and conditions.1 is91 The Airforce Materials Laboratory has made significant contributions to the synthesis and evaluation of acetylene-terminated reactive oligomers and polymers. While resins with flexible backbones, such as acetyIene-terminated 2,2-bis (4-phenoxyphenyl)propane (ATB) [1401[1411 exhibit tack and drape characteristics, others with high molecular weight and rigid backbones possess only a short melt state, such as BATQ[1421 (Fig. 8.45). Good mechanical properties are obtainable in cured AT systems, even those systems with more flexible backbones, such as ATB (Table 8.50).


BATQ

ATQ Figure 8.45. Acetylene-terminated

resins (Air Force Materials Laboratory).

Table 8.50. ATB Neat Resin Mechanical Properties

Property

RT

Elongation at break Tensile strength

100°C (212°F)

177°C (350°F)

Dry

Wet

Dry

Wet

3.4

3.1

4.4

3.4

5.9

6.9

67.9

58.4

57.2

49.4

39.7

37.9

Dry

Wet

rensile modulus, GPa

2.40

2.34

1.93

2.18

1.54

1.53

Shear modulus, GPa

0.85

-

0.73

-

0.61

-

(Reprinted by permission of Refs. 140 and 141)

High-Per$ormance Polyimides and Thermoset Polymers Acetylene-Terminated

387

Quinoxalines

Earlier studies with thermoplastic polyphenylquinoxalines showed that they were difficult to process, requiring temperatures in excess of 3 16°C (600’F). Subsequently, the AT-oligomers were developed, They exhibited excellent potential as moisture-resistant adhesives and matrix materials.[‘431-[1451 The fully cured AT resin, acetylene-terminated quinoxaline (ATQ), yielded a glass transition temperature of 32 1“C (610”F).[1461 Tensile strength data obtained for this resin at both room temperature and 232°C (450’F) are summarized in Table 8.5 1. Mechanical properties at both temperatures are not affected by aging at 93.3”C (200°F) and 95% humidity environment. The ATQ resin has also been evaluated as a matrix in graphite composites. The resulting composite showed good flexural strength and modulus. Table 8.51 Mechanical Properties of Cured Neat Resin of ATQ

Temperature

Tensile Strength, MPa Dry

Wet

Room Temperature

98 (14)

98 (14)

450“F (232°C)

27 (3.8)

32 (4.5)


of References

140 and 14 1)

It is noteworthy, that the ease of processibility of these AT-oligomers is proportional to the oligomer chain length. On the other hand, thermo-oxidative stability and adhesive strength of the cured ATQ resin increase with longer linear segments in the oligomer. The cured ATQ resins are thermo-oxidatively less stable than the corresponding parent linear polymer. These observations have proven to be universal among AT po1ymers.r 14’1 It has been suggested that the thermooxidative stability of the AT resins is related to the large percentage of polyene and enyne structures formed in the resin during cure. Solid-state carbon-13 NMR[14*l has demonstrated that only a small proportion of the acetylene terminations of AT resins undergoes trimerization to yield thermal and thermo-oxidatively stable benzene rings. Several acetylene-terminated quinoxalines containing only phenyl and quinoxaline units were synthesized specifically to measure their thermo-oxidative

388 Handbook of Thermoset Plastics stability under isothermal aging conditions. Three systems, A, B, C, each being an isomeric mixture, were synthesized and cured prior to isothermal agingt1491 (Fig. 8.46). All three systems showed poor thermo-oxidative stability at 37 1“C (700’F). The cured acetylene linkages apparently were the predominant site of thermooxidative degradation. The rate of weight loss at 371°C (700’F) was directly proportional to the number of cured acetylene linkages (Table 8.52). These results demonstrate that thermally cured AT resins do not have long-term 37 1“C (700’F) utility. A new cure mechanism must be sought for high temperature resins in order to achieve 37 1“C (700°F) durability.

Structure B

Structure A

1. R,=R1=ethynyl, Rs=Rd=H 2. R1=R4=ethynyl, Rz=Ra=H R2 = R4 = ethynyl 3. Rl=R3=H,

1. RI= ethynyi, R2 = H 2. R1=H, R2 = ethynyl

Structure C 1. RI= R3 = ethynyl, 2. RI= RI = ethynyl, 3. Rl=R4=H, Figure 8.46. Acetylene-terminated thermo-oxidative

R2=R4=H R2=R1=H R2= R3 = ethynyl

quinoxalines designed for comparative stability measurements.


389

Acetylene-terminated diluents, such as BA-DAB-BA, AA-BA and diethynyl-bisphenoxybenzene (ATEB), were developed to improve the processibility of BATQs so that they would become more amenable to melt processing. These diluents all plasticize the BATQs to significantly lower bulk viscosity. They then co-react with the BATQ oligomers to become part of the thermoset network.[1501 The BATQ most extensively studied was the BATQ-H (Fig. 8.47).

Table 8.52. Correlation of Cured Acetylene Linkages With Thermo-oxidative Stability

Ethynylated aromatic

Time Required to

% Ethynyl

Thermosetting

Give 90% Weight

Polyimide

Loss in System, hr.

System

A

30

15.2

B

50

9.4

C

85

3.2

(Reprinted

Acetylene-Terminated

by permission

Component

in

of Ref. 149)

Sulfones

The acetylene-terminated sulfone (ATS) has been studied extensively (Fig. 8.48). Depending on the method of synthesis, different forms of ATS have been obtained. Low molecular weight vinyl ether oligomers were present to the extent of 15 to 2 1%. The oligomers are responsible for the ATS’ appearance as a tacky resin that can be melt impregnable. Neat resin mechanical properties have been determined for the ATS resins (Table 8.53).


39 1

Table 8.53 ATS Neat Resin Mechanical Properties

(Reprinted by permission of Ref. 136) ATS has been evaluated as a matrix resin in graphite fiber-reinforced composites. Although judged as brittle, the resin yielded composites that retain mechanical properties well at 177“C (350°F) before and after high humidity exposure.[1511[1V

Successful experiments have been conducted with semi-interpenetrating polymer network (SIPN) blends. The objective is to achieve blends of resin materials that can retain all good properties originally ascribed to each neat material, while at the same time reducing undesirable properties. ATS has been evaluated as a reactive diluent with thermoplastic polysulfones. The high cross-linking density inherent in ATS can help improve the Ts of the thermoplastic polysulfone, while the brittleness associated with the high cross-linking density of ATS can be compensated for by the presence of the thermoplastics. To this end, the polysulfone RadeF (Union Carbide), after being modified with various amounts of ATS, showed a significant improvement in its processibility. After cure, the resin blend showed a higher Ts (24’C versus 220°C for neat Radel@ resin) and enhance solvent resistance.[1531 Similar results were obtained for blends of ATS with UDEL@ (Union Carbide). Ethynyl-terminated sulfone oligomers other than ATS have also been prepared and used to modify properties of linear polysulfones such as UDEL@ P1700. While the toughness and thermoform-ability indigenous to the thermoplastic are preserved, the presence of cross-linked ethynyl-terminated sulfone oligomers improves solvent resistance, especially when under load.[1361

Acetylene-Terminated

Imide Oligomers

Acetylene-terminated imide oligomers were also developed processibility. The terminal ethynyl groups are capable of undergoing

for better thermally

392


induced addition reactions, yielding a complex cross-linked network.l147l In the late 1960’s and early 1970’s, chemists at Hughes Aircraft Company under AFWAL sponsorship developed unique, fusible and tractible acetylene-terminated polyimides that undergo addition-type homopolymerization by virtue of the terminal ethynyl (acetylene) groups. The monomer systemused for these oligomers was designed so that the prepolymer would melt before undergoing homopolymerization. The prepolymer was synthesized from benzophenonetetracarboxylic dianhydride (BTDA) (VIII), 3-aminophenylacetylene (APA )(Xx111), and 1,3-bis(3aminophenoxy)benzene (APB) (XXIV) (FIG.8-49). The prepolymer is shown by structure (XXV) where n is generally one, two or three (Fig. 8.50). Higher values of n reduce prepolymers’ solubility in solvents such as Nmethylpyrrolidinone (NMP). Their proportionally higher melting temperatures are often close to the homopolymerization temperatures of about 22O’C (428’F) to 275’C (527oF).

Figure 8.49. Constituents of Thermid@ resin (formerly Hughes Aircraft Company I-R600 resin).

High-Peflormance


393

xxv Figure 8.50. Chemical structure of Thermid@ 600 resin. A significant development in acetylene-terminated imide oligomers was the commercialization of the Hughes HR600 resins as Thermid 6OO’s’ by Gulf Chemicals. Since 1983, various forms of’lhermid 600@ have been supplied by the National Starch and Chemical Corporation. Although neat resin, composite, and adhesive properties[r541[1551ofthe HR600 resins (Tables 8.54 and 8.55) are indicative of their suitability as high-temperature resistant matrix resins and adhesives, their poor flow characteristics and insolubility in common solvents make them difficult to process. Table 8.54. Typical Properties of WR600

Tensile strength, Mpa

96.5

Tensile modulus, Gpa

3.79

Elongation, %

2.6

Flexural strength, Mpa

124

Flexural modulus, Gpa Tensile shear strengths, Mpa

4.48 22.1

room temp.

13.1

(2320C,4500F)

8.3

(260”C,50O”F)

(Reprinted by permission of References 154 and 155) Thermida MC-600 is a pre-imidizedmolding powder; Thermid@ LR-600, an amic acid form with 50% solids in NMP solvent; and Thermid@ AI-600, an amine and ester monomeric mixture with 75% solids in ethanol. The properties of Thermida MC600 neat resins are shown in Table 8.56.

394


Table 8.55. Properties of Unidirectional HT-S Graphite Fiber Laminates Made With Thermid 600@Resin

Test Condition

Flexural Strength, GPa (ASTM D-760)

Interlaminar Shear Strength, MPa (ASTM D-2344

Room temperature

1.28

83.4

2oo”c (after 500 hr at 200°C in air)

1.17

60.0

288’C (after 500 hr. at 288“C in air)

0.99

51.0

316’C [after 500 fir. at 3 16OCin air)

1.04

41.4

(Reprinted by permission of Reference 136)

The Thermid@Research Institute generated data using heat-cleaned 112 glass cloth as an adhesive carrier at a 70% level of impregnation. The adhesive was formulated with 5% hydroquinone plus primer of a 10% solids of Thermid@ LR-600 in methylethyl ketone. Curing for 500 hours at 288°C (550°F) gave a lap shear value of 2,600 psi and 500 hours at 3 16°C (600”F), a value of 2,500 psi, All the specimens were heat soaked at the test temperature for 30 minutes before testing.

Isohnlde Modification of Polyimides To improve the processibility of these acetylene-terminated oligomers, Hughes Aircraft Company chemists synthesized acetylene-terminated isoimides using the same monomers as for the imide form.1156111571These isoimides are isomeric to the imides, are more soluble in common solvents, and are more processible. The isoimide converts to its imide form upon heating (Fig. 8.51). In contrast to the condensation closure of amic acid to imide, the isomerization of isoimide to imide yields no volatiles.


395

Table 8.56. Thermid@ MC-600 Neat Resin Properties Resin Property Physical Flexural strength, psi Flexural modulus, psi Tensile strength, psi Tensile modulus, psi Tensile elongation at failure,% Compressive strength, psi Hardness, Barcol Shore D Water absorption, % wt, gain (1000 hr at 95% R H) Electrical Dielectric constant at 10.0 MHz at 9.0 GHz at 12,0 GHz Loss tangent at 10.0 MHz at 9.0 GHz at 12.0 GHz Thermal Thermal expansion coefficient, 10M5 in./in. OF(73’F to 572’F) % wt. loss during 600°F aging After 500 hr After 1000 hr Flexural strength, psi Initial 70°F 600°F After 1000 hr of 600°F aging 70°F (% retention) 600°F

Value 19,000 650,000 12,000 570,000 2 25,000 53 91 1

5.38 3.13 3.12 0.0006 0.0068 0.0048

4.42 2.89 4.04

18,600 4,000 13,400 (72) 2,600 (62)

(Reprinted by permission of National Starch and Chemical Corporation)


Amic acid

Isoimide

Figure 8.51. Chemistry of isoimide modification.

The isoimide form of Thermid@ 600 can be conveniently synthesized via the dehydration of the amic acid by N,N’-dicyclohexylcarbodiimide (DCC).1771-18*l This technology is the basis of the National Starch Thermid IP-600 thermosetting resin. Although other dehydrating agents, such as trifluoroacetic anhydride and ethyl chloroformate, are also workable, the DCC method is more universally applicable.14sl The DCC conversion of polyamic acid to polyisoimide has recently been repeated by Japanese researchers.[r5*l The more processible Thermid@ IP-600 isoimide is supplied in the powder form. Developmental quantities of higher molecular weight polyimides (IP 603 to IP 650) are also available. The Thermid@ IP-600 series is characterized by a lower melting temperature, longer gel time and good solubility in low boiling solvents, retention of strength following high-temperature exposure and superior processibility in vacuum bag/autoclave composite structure fabrication in comparison with Thermid@. These key attributes resolve many processing problems encountered with conventional polyimides. The higher values for the degree of polymerization (DP) for the Thermid@ IP-600 series give resins with excellent film-forming capabilities, lower glass transition temperature, and improved strain capability, all with little change in tensile strength (Fig.8.52). These materials have substantially increased processibility by virtue of drastically reduced melting points and prolonged gel time. Soluble and tractable isoimide oligomers have been prepared with degrees of polymerization (DPs) up to 25. By contrast, the Thermid 600@ imide oligomers become insoluble and intractable at DP >3. At elevated temperatures, the ethynyl groups undergo the usual chain extension and cross-linking reactions while the isoimide functions rearrange to the amides. Adhesive and composite properties obtained with cured acetylene-

High-Perjkmance


397

terminated isoimide oligomers are compatible to those of cured HR600 imide oligomers.[1561[1571The high DP materials are excellent film formers and thus are useful for coating applications. The properties of Thermid@ IP-600 neat resins are presented in Table 8.57. Isoimide modification of acetyleneterminated polyimide precursors offers the potential of improving processibility of otherwise difficult-to-process hightemperature polyimides. Especially important is the observation that the technology can be applied without compromising the thermo-mechanical properties of the

0

0

Figure 8.52. Thermid@ lP-600 isoimide oligomer. Table 8.57. Thermid@ IP-600 Neat Resin Propertie@‘]

I

Property

I

Temp., OF

1

Value -

Tensile strength, lo3 psi

78 600

8.3 4.5

Modulus, 106 psi

78 600

0.75 0.20

Elongation; %

78 600

1.4 4.0

Ta after post-cure in air,“F 8 hr at 700’F 15 hr at 750°F 4 hr at 750’F

572 662 626


final products. Previously, the concept of producing thermally stable addition curable polymers was severely limited by the intractability of the oligomers. With the introduction of isoimide modification to polyimide precursors, the processing window is significantly broadened. Some commercially available diamines and dianhydride monomers, which previously formed intractable polyimide prepolymers, become useful candidates for processible resin systems. In addition to the melting point and gel time improvements, the isoimides impart to the precursors increased solubility in such common low-boiling, noninteracting solvents as tetrahydrofuran. This is contrasted with state-of-theart polyimide precursors that are soluble only in strong aprotic solvents, i.e., Nmethylpyrrolidinone (NMP). The latter class of solvents is very difficult to remove during processing and its use often results in void formation in cured composites. New families of processible high-temperature resistant polymers for long-term >700°F (37 1“C) utility could be obtained through isoimide modification, leading to hybrid polyisoimides containing polyheterocyclic structural units.

Cure Mechanism of Acetylene-Terminated

Oligomers

Acetylene-terminated oligomers have been of continued interest in the development of more processible high-temperature resistant composite resins. The final product properties obtained from these cross-linked materials are directly related to the structure of the 3-dimensional network. A better understanding of the reaction mechanism of the thermal cure of the terminal ethynyl units would allow better design of the cure schedule and optimization of the final composite properties. There has been much speculation as to the cure mechanisms of acetyleneterminated thermosetting oligomers. Although low molecular weight model compounds were found to undergo some trimerization to form aromatic rings, the homopolymerization of the higher molecular weight acetylene-terminated polyimides is more comp1icated.t isal The initial products of thermal curing of terminal ethynyl groups are postulated to be ene-yne type structures (i.e., Straus,[*601 Glaser,[*611and Sabourin dimerst1621) from the coupling of terminal ethynyl functionalities. Condensed aromatic compounds were isolated from thermal reactions of monoethynylated compounds. The preliminary report of this interesting work also suggested that monofimctional (one terminal ethynyl group) model compounds


399

form condensed aromatic structures (benzenes and naphthalenes) from the initial Straus and Glaser dimer products, but difimctional compounds tend to produce polyene chains in preponderance, based on FTWNMR techniques and thermal kinetics (Fig. 8.53).1163111641 An elegant solid-state 13C cross-polarization magic-angle spimiing(CPMAS) nuclear magnetic resonance (NMR) spectrometric study has been carried out at IBM Ahnaden Research Center (San Jose, California) on ethynyl-terminated BTDAAPB isoimide oligomers which have been selectively labelled using i3C labelled precursorsti651 (Fig. 8.54). The selectively labelled resin specimens were cured in the identical way as the unlabelled (control) samples and subsequently studied by the NMR method. Differential spectra showing only the selective label were obtained by subtracting the resonances due to the natural abundance of the 13C nuclei. In addition, delayed decoupling experiments were performed to allow protonated carbon nuclei to relax, thereby distinguishing them from unprotonated carbon nuclei. The results showed that the carbonyl function on BTDA remained unchanged in the cured product. The isoimide/imide carboxyl carbons underwent the isomerization reaction in the expected manner. The solid-state cure products, due to the ethynyl termini were found to comprise aromatic structures, polycyclic aromatic structures, backbone addition and bridge structures. The aromatic structures were interpreted as trimerization products via intermediate Straus and Glaser coupling products, in agreement with results of the French group.I*631t1641The po 1ycyclic aromatic structures, backbone addition, and bridge structures were logical deductions from 13C spectrometric analysis. Table 8.58 summarizes the correlation of 13C spectral results and expected products. Results from NASA-Langley’s study of thermally induced addition of ethynyl-terminated imide oligomers to bismaleimides supported findings from previous mechanistic studies of the thermal cure of ethynyl-terminated thermosetting polymers.P66111671Such an addition reaction approach yields high-performance thermoplastics with processibility, toughness, and thermo-oxidative stability (Fig. 8.55). The Diels-Alder addition mechanism has been substantiated by model compound reactions both in solution and in the solid state. All reaction products were identified by conventional anayltical techniques, including NMR, FTIR, UV-visible, mass spectrometry, andHPLC. Solution reactions of the model compounds N-(3-ethynylphenyl)phthalimide and N-phenylmaleimide yielded a Diels-Alder product but no coupling products due to the ethynyl groups (Fig. 8.56). Another dienophile,

400 Handbook of Thermmet Plastics

Figure 8.53. Elucidation of thermal cure Mechanism for terminal ethynyl groups?~~]


40 I

benzoquinone, also underwent Diels-Alder reaction with N-(3 -ethynylphenyl) phthalimide, but in a 1 to-2 ratio. In the solid state at 214°C (417”F), the reaction was complicated by the volatility ofN-phenylmaleimide and a less volatile analog N-(4-phenoxy)phenylmaleimide was used. The isolated reaction products were interpreted to have been formed by Diels-Alder reaction of an intermediate Straus coupling product ofN-(3-ethynylphenyl)phthalimide with the dienophile (Fig. 8.57).

* = Carbon-13 Label Figure 8.54. Carbon- 13 labelled monomers for synthesis of acetyleneterminated BTDA-APB isoimide oligomers.

C=CH

A

-RI\

z

Figure 8.55. Addition-type terminated

&

thermoplastics formation imide and a bismaleimide.

0

N, 0

R2-

from an ethynyl-

402 Handbook of Thermoset Plastics Table 8.58. Correlation of Carbon-13 CP-MAS NMR Results With Ethynyl Cure Products

High-Performance


403

Table 8.58 (Continued) Reaction

Further prodcts

Mechanism

cyclization

2

*

Iegradation

and Products

of coupling

Model

Compound

Fluoranthene

r3C Chemical Shift, ppm Cl: 125-130 protonated c2: 129 protonated

Observability Cured Material Y

+$

Y

of coupling

product

“3 of I-Phenylazulene

C2: 117.6

N

protonated

,&

-$I Ar

Diels-Alder

addition

Pyrene

C2: 138 unprotonated

Y

Tram-Stilbene Cl & c2:

protonaed 127-131

Y

None

Aliphatic

k

Friedel-Crafts

addition

secondary bridging product +ield-Crafts product

from

carbons 50 to 70

range

Y in

il

404

Handbook

of Thermoset

N

y

Plastics

-

0

12,4-Trichlorohcnxenc 24 hr, reflux

Figure 8.56. Solution reaction between N-(3-ethynylphenyl)-phthalimide phenyhnaleimide

PROPARGYLTERMIN

and N-

ATEDOLIGOMERS

Resembling ethynyl and phenylethynyl end groups, propargyl-terminated oligomers represent yet another thermosetting resin family. The synthetic approach to the easily accessible propargyl-terminated Bisphenol A (PTBA) is identical to that for diglycidyl ether of Bisphenol A. Preliminary results indicate that PTBA,could be useful as high-performance materials[‘681[1691(Fig. 8.58). The resin which can be cured to a T, of 360°C exhibits tensile strength, modulus and elongation of 14.9 ksi, 660 msi, and 2%. Fracture toughness, K,, is 387 (25”(Z), 401 (100”G),and347(177°C)psi.ino.


405

Figure 8.57. Solid-state reaction illustrating the Diels-Alder reaction mechanism and the intermediacy

of a Straus dimer.

406


Figure 8.58. Propargyl-terminated

PHENYLETHYNYL-

oligomer PTBA.

TERMINATED THERMOSETTING

POLYMERS

The high-performance thermoset polymers of intense interest include various acetylene-terminated (AT) arylene ether,[‘70j arylene ether-ketone, and imide/isoimide oligomers,[771_[821[1561[1571 many of which were reviewed earlier.[1361 Recently, attention has focused on their corresponding phennylethynyl-terminated (PET) o1igomers.t 171~+178jOf special relevance are the results on PET&ride oligomers.[~721-[~751[‘771~~7~1 The phenylethynyl end-capping agent via the palladium-catalyzed coupling reaction was briefly studied at National Starch.[1731 Palladium-catalyzed coupling reactions are a key feature in acetylene chemistry. Figure 8.59 shows the synthesis of the phenylethynyl end-capping agent 3-(phenylethynyl)aniline (PEA), illustrating the palladium-catalyzed phenylethynylation reaction. An alternative method uses commercial 3arninophenylacetylene as the,starting materia1.[179j[180j 3-(Phenylethynyl)aniline can be routinely prepared for use in end-capping imide and isoimide oligomers. PET-polyisoimides/polyimides and thermoplastic polyisoimides form compatible blends that thermally cure to SIPNs with Ts above 300°C which is desirable for advanced matrix resin and adhesive applications to hot areas on airframe and gas turbine engines.

(Ph,PhPdCI*

PP~,

4N

+

W

b

Br

Figure 8.59. Synthesis of 3 (phenylethynyl)aniljne via palladium-catalyzed phenylethynylation of 1-bromo-3-nitrobenzene (National Starch and Chemical Corporation).

High-Performance


407

Recent results in synthetic chemistry have revealed a more cost-effective and efficient end-capping agent, 4-(phenylethynyl)phthalic, anhydride (PEPA) (Fig.8.60),11**l which logically replaces PEA. Switching to PEPA fromPEA as the end-capping agent for polyisoimides necessitates a subtle modification in the small stoichiometric offset during synthesis, now requiring a slight preponderance of the diamine monomer to generate amine-terminated polymer strands to be end-capped with PEPA. NASA scientists have demonstrated the potential of phenyletbynylterminated imide materials as high-temperature adhesives.[*75111771t17ElOf note is the phenylethynyl-terminated OPDA-3,4’-ODA imide oligomer, PETI, which is based on the homopolymer poly(OPDA-3 ,C-0DA)imide known as LARCTM-IA. The end-capping agent, 4-(3- or 4-aminophenoxy)-4’-phenylethynyl-benzophenone (APEB), resembles 3-(phenylethynyl)aniline but with an additional “ether-ketone” connection between the phenylethynyl and amino terminals (Fig.8.61).

o-

CmCXi bPdClp (LzPPb)

+

EtrN

I-(Phenylethynyl)Phthalic Anhydride

Figure 8.60. Synthesis of the end-capping agent, 4-(phenyl-ethynyl)phthalic anhydride.[t*tl

408


Figure 8.61. Phenylethynyl-end-capping

agent 4-(3- or 4-amimophenoxy)-4’phenylethynyl-benzophenone (APEB).

The stoichiometric offset ratios for oligomerizationn can be adjusted to yield PET1 oligomers with an Mn of 3,000,6,000, and 9,000 g/mol (Fig. 8.62). The potential of these PET materials has been substantiated by the exceptionally high adhesive strengths (Table 8.59).

I

lsoquloollne (cat.), m-cresol. A

Figure 8.62. Synthesis of PET1 oligomers. The uncertain future availability of the crucial dianhydride OPDA for the NASA phenylethynyl-terminated polyimides has prompted the search for similar PETI oligomers with the commercially more available dianhydrides, such as PMDA, BTDA, and 3,3’,4,4’-biphenyltetracarboxylic dianhydride (BPDA). The optimal formulation LARCTM-PETI-4, which exhibits excellent processibility, was chosen for scale-up and finther evaluation. PET14 comprises the end-capper 4-(3aminophenoxy)-4’phenylethynylbenzophenone (3-APEB), the d&hydride BPDA, and a 85: 15 diamine ratio of3,4’-ODA and APB, with an average M,, of 5000 gA401 (Fig.8.63). It cures to a

High-Performance

Polyimides and T%ermoset Polymers

tough material with a Ts of 251°C and has been fabricated into composites adhesives with excellent mechanical properties (Table 8.60).

409 and

Table 8.59. Fabrication Conditions and Adhesive Properties of PhenyletbynylTerminated Imide Oligomers, PETI117sI

Oligomer Mn, g/m01

Molding Pressure psi

Exposure Conditions

Test Temperature, “C

Shear Strength, psi

3000 3000 3000 3000 3000 9000 9000 9000 9000 9000

15 25 50 50 50 15 25 50 50 50

None None None 48 hr soak* None None None None 48 hr soak* None

23 23 23 23 177 23 23 23 23 177

4730 5270 5270 5410 3860 4930 5460 6440 6370 5000

‘Chevron HyJet hydraulic fluid Lockheed and Rohr researchers1 1821previously demonstrated that SIPNs can be formed with phenylethynyl-terminated (PET) isoimide/imide oligomers, in lieu of AT-isoimidelimide oligomers, and can thermally cure to materials with high T, values in the 300°C range. A final Ts exceeding 3OO’C is desirable for materials used in supersonic civilian aircraft, currently being pursued under the NASAsponsored High Speed Civil Transport (HSCT) program. Phenylethynyl-terminated isoimide oligomers have comparable low melt viscosity as acetylene-terminated isoimides. The synthesis of various: PET-polyisoimides based on benzophenone-3,3’,4,4’-tetracarboxylic dianhydride (BTDA) and isophthaloyl bisphthalic dianhydride (IPDA) has been demonstrated. Figure 8.64 shows the structural difference of the AT- and PET- oligomers of the illustrative (IPDA-MPDA) isoimide system. PET-isoimide oligomers have a low melt viscosity comparable to AT-isoimides (Figs. 8.65 and 8.66) Model compound studies showed that the PET-imides cure to give higher Ts products. The AT- and PET-isoimide oligomers have melt viscosities in the loo

4 10 Handbook

of Thermoset Plastics

to lower 10’ poise range and are potentially useful for RTM applications. Further reduction in the monomer Aweight couldA bring the melt viscosities as low as 10-l poise, which will match current RTM-able epoxy resins.


Property

Properties of PETI-

25°C (77=‘F)

Imide[1771

177°C (350’F)

Films Tensile strength at yield, ksi Tensile strength at break, ksi Tensile modulus, ksi Elongation, %

16.8 16.8 470 5.3

11.0 10.0 385 7.5

Moldings K,,, psi-in.‘” G,,, in.-lb/in.2

3430 25

-

Composites Flexural strength, ksi Fluxural modulus, msi Short beam shear strength, ksi

268 23 16.4

190 22 10.2

5700

4400

6100

4500

Adhesives Lap shear strength, psi (25psi processing pressure) Lap shear strength, psi (100 psi processing pressure)

The differential scanning calorimetric (DSC) thermogram of PET-(IPDAMPDA) isoimide oligomer shows two distinct reaction exothem at 233°C and 372°C (Fig. 8.67). DSC characteristics ofother PET-is, oimides are similar. Anexample comes from the National Starch study (Fig. 8.68).t*73] The processing temperature for RTM has to be selected judiciously to achieve low resin viscosity, but not too high as to inflict penalties of shorter pot life. Increasing the injection pressure can dislocate fiber placement within the mold and increase the cost of molds and clamping presses designed economically for low-pressure operation. As a result, an upper temperature limit to RTM processing is dictated by the resin chemistry.

High-Performance


The cure characteristics of these isoimides window available with these resins.

illustrate

411

the wide RTM processing

oc&_f&o +(BPDA

APB (15)

3,4’-ODA (85)

Y +

0

0 8 IAPEB

Cf

Q-Q

End-Capper

0

E

PETI-,

Figure 8.63. Formulation

lmlde

of PETI-4/ imide oligomer.

Acetylene-terminated (IPDA-MPDA) isoimide

Acetylene-terminated (IPDA-MPDA) isoimide Figure 8.64. End-capped (IPDA-MPDA)

isoimide oligomers.

cat

-0

412


llt . Id

t .

. .

t .

. l*

3

Figure 8.65. AT-(IPDA-MPDA)

1

. . . . .

.

isoimide oligomer viscosity profile.

Candidate high-performance polyisoimides and polyimides suitable for hightemperature composite applications include various combinations of the readily available BTDA, 6FDA, BPDA, and IPDA monomers and select diamines. Fluorinated backbones impart enhanced thermo-oxidative stability and processibility to the resulting polymers. 6FDA is the most commonly used fluorine-containing dianhydride. Some of the notable fluorinated diamine monomers include Ethyl’s 2,2-bis[4-(4-aminophenoxy)phenyl] -hexafluoropropane (BDAF) (Fig. 8.21), and NASA-Lewis’ 2,2-bis(4-aminophenyl)l -phenyl-2-trifluoroethane (Fig.8.69). Workable candidates for RTM process applications can be attained from low molecular weight oligomeric versions of these polyisoimides and polyimides, end-capped with phenylethynyl groups.

High-Performance


4 13

Other potential PET-imide/isoimides can be derived from polymer backbones based on DuPont’s Avimid-N, K-III, and K-IIIB; NASA-Lewis’ PMRII-30 andnewervariations (PMR-11-50, VCAP-75, APR-700B, andEMTL-8-T); OPDAbased polyimides (previously Oxychem, currently Imitec); and Upilex polyimides(Ube). As discussed above, NASA-Langley’s current focus is on the OPDA- and BPDA-based polyimides, namely, the PETI resins, whose exceptionally high lap-shear strengths has been demonstrated.[1751[1771

Figure 8.66. PET-(IPDA-MPDA)

isoimide oIigomer viscosity profile.


4.q

I

Ic

-

a

*

I

Temperature (InOC)

Figure 8.67. DSC ofphenylethynyl-terminatedIPDA-MPDA

isoimide

z.4

2.0 ..

CURE

L6 ‘.

I.2

I.

0.a ..

01

.a.

..

:

,: u

:

: ”

:

: m

:

: w

:

: Ran

:

: 010

:

: : : em m

:

: YO

:

: 44s

:

: 44,

Temperature inT

Figure 8.68. DSC ofphenylethynyl-terminated

BTDA-APB isoimide

High-Per$ormance

Polyimides

and Thermoset

Polyisoimide made from 2,2-bis[4-(4+minophenoxy)phenyl]

Polymers

4 15

hexafluoropropane (BDAF)

Polyisoimide made from 2,Zbis(4-aminophenyl)l-phenyl-2-trifluoroethane

Figure 8.69. Other relevant polyimide

precursors.

APPLICABILITY OF THERMOSET ISOIMIDEWIMIDES TRANSFER MOLDING PROCESSING

TO RESIN

Reduction in processing and manufacturing cost is a major directive to enhance the use of composike techno\ogy in aircraft primary structures ancIprotective housings and cowls for the turbine engines, as well as in hot regions of the aircraft, such as the fakings. Expedient and effective processing shortens production cycles, simplifies production procedures, accommodates complex parts fabrication, and enhances the performance of composite structures. RTh4 is recognized as a process that provides the technical advantages listed below:t1831

416

Handbook of Thermoset Plastics 1) Energy savings 2) Low emissions 3) Fast part production 4) Relatively

low tooling cost

5) Encapsulation

of ribs and inserts

9 Short tooling lead times 7) No air entrapment 8) All sides of part smooth 9) All sides can be gel-coated 10) Close dimensional

tolerances maintained

11) Versatility RTM was almost exclusively considered a process specific for the highly processible polyester resins. The ever expanding requirements of the end user and constant development of new resin formulations to fit these requirements have seen epoxies and even bismaleimides become available and be adapted to the RTM process. Similar RTM processing approaches for high-temperature (550” to 800°F) imide-based composites are logical extensions. Advanced composites suitable for high-temperature durable airframe and engine applications are produced from highperformance polymer materials with a high level of fracture toughness, high modulus and strength, and environmental durability. State-of-the-art technologies indicate that high-performance polymers meeting the high-temperature requiremenrs are mostiy poiymi&s. Low molecular weight acetylene-terminated (AT-) and phenylethynylterminated (PET-) isoimide oligomers of the BTDA-APB, IPDA-MPDA, and BTDADAB constituencies have melt viscosities in the IO0 to the low IO’ range, which approach the desired viscosity range of 10-l to loo for RTM applications. The low viscosity exhibited by liquid cyanate ester resins suggests that SIPNs incorporating cyanate-terminated imide and isoimide oligomers could achieve low melt viscosities to allow hot-melt prepregging and RTM applications Industrial Effort in RTM-able Aerospace Resins. Resin transfer molding has been used successfully for manufacturing fiber-glass parts for many years. The main manufacturing advantages are the reduction of cycle time, repeatable producibility in high volumes, and accommodation

High-Peformance


4 17

of complex parts via preform placement. Advanced resins of higher use temperatures which have been applied to RTM processes to mold aerospace components are epoxies, bismaleimides (BMIs), andpolystyrylpyridines (PSP).11841 The liquid monomeric dicyanate AroCy L-10 resin has a 2S’C (77°F) viscosity of 140 MPa/s (cps) or 1.40 poise and achieves the cured T, of approximately 259% (498”F), typical of the polycyanurates. It has been applied as a lowviscosity reactive solvent for compatible high-temperature thermoplastics such as polyimides, polysulfones, and polyarylates.[ig51 In the 260°-37 1“C (500-700’F) service range, commercial choices are limited to liquid crystal polymers, polybenzimidazoles (PBIs) and the polyimides (inclusive of BMIs).[ig61 A proprietary BMI-based RTM-able resin, FE-70,000, has been developed for transfer molding of electronic parts and for encapsulation of electronic elements by Nippon Polyimide Company (a Mitsui Petrochemical Company and RhBne-Poulenc S.A. joint venture).[is71 BP demonstrated their RTM-able BMI resins with Toray T300B fibers in a complex flywheel component.[ ls81 Processing of Compimide@ BMI resins (Technochemie, GmbH) by RTM processing compares favorably to other processing techniques such as wet- and tow-preg filament winding and low pressure autoclave molding.1 is91 DSM’s Desbimid BMI resin is RTM-able and has been successfully applied to manufacturing the rear cowling beam of the Fokker 50 airplane.t1901 Most of the attention has been given to the issues of materials availability, processing technology, automation, producibility,[i9il and economic advantages over metals. Applications have been successful in airframes and automobile bodies.[i92l The thermal expansion resin transfer molding technique (TERTM) combines the technologies of epoxy RTM and rigid polyimide-based foam core prefabrication with a selective mold heating system, expanding the foam to varying densities in different regions. 11931-[19slBeyond these efforts, little has been done to RTM polyimide resins. Increased understanding of interfacial interaction and improved materials processing techniques potentially yield a better interface, which leads to better composite performance and durability. Optimization of interfacial interactions would facilitate fiber impregnation and total wet-out during resin injection. Process control and engineering methods are available to optimize the processes.[1961-[1981 Many focus on dielectric spectroscopic methods as the most versatile approach to monitor cure.

418


Relevance of Polyimides to RTM Processing Polyimides are known to be useful over a broad range of temperatures, even to beyond 700°F for hundreds of hours. Isothermal weight loss studies at 700°F in air indicated that polyimides fare significantly better than other processible nitrogen-containing polyheterocyclic polymers. Their well-defined chemistry and a vast literature database allow many possibilities for structural modification. Many polyimide systems have demonstrated usefulness as structural matrices, adhesives, and coatings. The inherent poor processibility of polyimides is due to their high viscosity, volatilization (in the amic acid form) during cure, and insolubility in common low-boiling solvents. Polyimides are, however, also amenable to isoimide modification to enhance their processibility.1771-Is*l Low molecular weight polyisoimides end-capped with reactive groups are potentially useful as high-temperature resistant RTM-able resins because of their qualifying attributes, such as low-melt viscosity, volatile-free cure, and thermooxidative stability. AT-isoimideslimides have been used widely as high-temperature composite matrices and structural adhesives. They impart higher thermooxidative stability and toughness than BMIs to laminates and adhesive joints.l199l12ool More development is needed to reduce further the melt viscosity to conventional RTM requirement of about 1 poise. In principle, low molecular weight thermosetting polyisoimides, with their low viscosity in the melt stage, can enable efficient penetration between the fiber filaments prior to resin advancement and cure. These isoimides undergo isomer&&on at 250°C to the imide without liberation ofvolatiles. The isoimidization process is easily adaptable to commercial polyimides to reduce viscosity and improve solubility.1771-[821 In addition, low molecular weight polyisoimides and polyimides, end-capped with reactive phenylethynyl and cyanato end-groups, have been shown to possess low-melt viscosity during processing. This approach therefore greatly expands the pool of candidate resins for selection as RTM-able high-temperature resistant materials. In principle, an IPN blending approach that involves these isoimides and a liquid polycyanurate precursor, such as the AroCy L- 10 dicyanate (140 cps at 25’C), would produce RTMable resins (Fig. 8.41). Similar IPNs with low viscosities and low-melt temperatures have been made by blending low molecular weight AT-isoimide oligomers with BMIs. Recently, Lockheed and Rohr Industries’ collaborative studies have shown that these resulting IPNs are significantly more tough than BMIs due probably to the chainextending reaction of the AT oligomers. Phenylethynyl-terminated (PET) oligo-


4 19

mers, in place of the AT oligomers, would result in IPNs with higher Ts’s. Complementary studies using AT-imide oligomers and BMIs were conducted at NASALangley.12011

Emerging Low-viscosity SIPN hide

Blends

Dow Chemical’s perfluorocyclobutane (PFCB) poly(aryl ether) (Fig. 8.70) and poly(ary1 etherimide) are innovative in terms of the the polymers’ thermooxidative stability, potential long-term durability, and low moisture adsorbance. The step-growth polymerization mechanism to allow molecular weight advancement of the prepolymer to any specified viscosity for processing would facilitate processing. This step-growth polymerization also produces no volatile by-products. High molecular weight polymers, poly(bipheny1 perfluoro-cyclobutyl ether), and poly( 1 , 1,l -triphenylethane perfluorocyclobutyl ether), were obtained from their corresponding aryl trifluorovinyl ether monomers.l202l An optimal exploitation of this step-growth technique is to form an SIPN. The step-growth mechanism advances the molecular weight from that of the (trifluorovinyl) aryl ether or -aryl etherimide monomer to a thermoplastic (MWcontrolled). Introduction of this step-growth concept of molecular weight build-up can potentially improve the processing rheology of all SIPN blends. A SIPN blend can be formulated by mixing the (trifluorovinyl) aryl etherimide monomer with a thermoset component, such as an AT-imidelisoimide or a PETimidelisoimide. Because the initial mixture is of low MWs, the viscosity can be maintained low for easy control during processing (either prepregging or scrim impregnation). The thermally induced cyclodimerization takes place at 140°C. The MW of the resin can be advanced to a desired viscosity. The final curing of the prepreg or impregnated scrim placed at the bondline during consolidation completes the process. The design of such a low-viscosity SIPN system is shown in Fig. 8.7 1.

APPLICATIONOFHIGH-PERFORMANCEPOLYMERSTOIMPROVE GALVANICCORROSIONRESISTANCEOFIMlDEBASEDCOMPOSITES Imide-based composites show evidence of galvanic corrosion in the form of nucleophilic attack of alkali on the imide structures of polyimide and bismaleimide (BMI) composites. The existence of a galvanic corrosion cell under appropriate conditions between carbon fibers and aluminum surfaces is well

420


known.t203112041 The General Dynamic experiment consisted of a graphite/BMI specimen in a tin can with 3.5 weight percent sodium chloride solution and JP5 jet fuel. The can was heated to 60°C and corrosive attack of the composite occurred within 24 hours. The greatest rate of degradation occurred when the temperature and the amount of exposed graphite fibers were increased.[205l Among different metals, only aluminum exhibited cathodic control corrosion.

”

F,C =CF -0

F,C=CF-0

Figure 8.70. Perfluorocyclobutane

poly(ary1 ethers).

Mechanism of Corrosion Carbon and aluminum become “electrodes” in the presence of an aqueous electrolyte. An EMF of approximately 800 mV exists between carbon and alumina. When the carbon fiber is coated with an insulating polymer (such as a polyimide or bismaleimide), the carbon electrode is effectively insolated, thus avoiding the creation of a corrosion cell. Most polymers are, however, susceptible to water


Thermoplastic component

0

0

Thermoset component (as imide or isoimide) Figure 8.71. Design for a low-viscosity

SIPN polymer blend.

42 1

422


adsorption and gradually behave like a “solid electrolyte,” concentrating and transporting ions. If the carbon and aluminum touch, the circuit is closed and electrochemical decomposition of aluminum occurs. Even if the carbon and aluminum do not touch initially, some of the carbon may slough off with time, gradually forming a bridge through the polymer to complete the corrosion cell. Hydrogen and hydroxide ions are formed at the carbon electrode, and aluminum ions are produced at the counter-electrode. Highly soluble aluminum hydroxide is the product. The elevated pH due to the hydroxide ion concentration causes the polymer to degrade, exposing more of the carbon surface for further destruction. Such catastrophic corrosion can be prevented by using several approaches. The corrosion cell can be rendered inactive by removing of any one of the components needed to complete the electrochemical cell. Insulative Coatings Isolation of the anode can be achieved by a noninteractive coating on the carbon tibers. The use of hydrophobic polymers may isolate the anode and/or avoid the formation of a solid electrolyte. Such polymers are, however, prohibitively expensive, and the necessary 100% surface coverage may not be achieved easily. Alternatively, the surface of the cathode may be rendered inactive by the deposition of a stable oxide layer that eliminates the high EMF. Aluminum, in fact, forms an inactive oxide layer in an acidic environment, but this layer is destabilized at high pH (neutral to basic electrolyte). IR&D results from Lockheed and Sandia National laboratory in conductive polymers, such as polyaniline, suggest a potentially useful and cost-effective approach to offset the BMI corrosion problem. Conductive Polymer Blends Certain emeraldine salts may be blended with polyimides to render the latter conductive.[2061 Addition of protonic acids to polyaniline free base forms emeraldine salts. The proton is mobile. While concentrated solutions of the salts have the green color of the protonated form (emeraldine), the dilute solutions have the blue color of polyaniline free base. High temperature also causes reversion of emeraldine to the free base form. The emeraldine salt can be blended with the BMI or the polyimide matrix of the composite. Its presence provides a proton source that may shift the equilibrium


423

potential and avoid the production of hydroxide ions, which chemically degrade the BMI matrix. A high hydrogen ion concentration may also stabilize the inactive oxide layer on the aluminum. Both conditions effectively shut off the corrosion cell. The emeraldine salt may at least extend the useful life of the imide-based composite by neutraliziig the hydroxide ion, thus preventing fast decomposition of the polymer. To test the effectiveness of polyaniline as an anti-corrosion agent, a carbon fabric can be treated with a BMI resin, such as Ciba Geigy’s Matrimid, and another with a blend of Matrimid and polyaniline tosylate. Pieces of treated fabric are placed in separate containers, each with a 3% aqueous sodium hydroxide solution along with a piece of aluminum. The two materials are connected by a conducting wire. The temperature, pH, and open-circuit voltage are monitored periodically. The corrosion current is monitored if possible. Indicators of a positive anti-corrosion response include the absence of an increased pH, decreased cell potential, and decreased cell current over the time of the test. Ideally, the cell current should drop to zero. Visually, there should be very little, if any, decomposition ofthe polyimide matrix.

hide

Structural Analogs

Imide analogs are five-membered heterocyclic amides that mimic structurally the imide ring, but nevertheless follow similar synthetic schemes as for &ides. Structurally, they do not have the usual arrangement of adjacent amide linkages that are susceptible to base-catalyzed hydrolysis. Polyimidines~2071-[2101 and polybenzodipyrrolediones121ilIrirl are representative heat-resistant imide analogs (Fig. 8.72). For example, the polyimidine does not contain adjacent amide linkages (such as in the case of imides). If hydrolysis were to occur at the sole amide bond, the entire polymer chain would not be scissioned. Polyimidines can be easily made from readily available starting materials as light-colored polymers, soluble in common organic solvents. An on-going research project at PolyComp Technologies (Del Mar, California), supported by the Naval Air Warfare Center, includes selecting some appropriate molecular-weight-controlled thermoplastic version of these imide analogs (based on a balance of processibility and fracture toughness) to be blended with a low molecular weight AT- or PET-oligomer based on the same imide analog structure to give a processible SIPN coating system.

424


AR

A BTDA-Based Polyimidine

A BTDA-Based Polybenzodipyrroledioae

Figure 8.72. Heat-resistant

imide analog polymers.

APPLICATIONOFHIGH-PERFORMANCEPOLYMERSIN LIGHTNINGSlMKEPROTECTIONTECHNOLOGYUSING NONMETALLICMATERL4JS Recent development of conductive polymers results in improved environmental stability, thermal stability, processibility, and level of conductivity. Switching devices, EM1 shielding, and static electricity dissipation are made possible with nonmetallic conductive polymers. The electrical current capacity of


425

lightning attachment and the accompanying shock wave generated by the ionized air column require very rapid dispersal of the I*R electrical heating lest severe damage be sustained. Epoxy composite materials cannot resist the pyrolysis induced by the undissipated I*R heating. Lightning strike protection of composite structures involves such issues as thermal resistance, environmental resistance, repairability, and Mil Spec requirements. Until high-conductivity, nonmetallic materials are available, nonmetallic conductive materials can provide protection as sacrificial layers. While lightning strike damages cannot be avoided, they can be reduced to a less severe level. Some approaches are under evaluation at various aerospace companies.

High-Temperature

Resistant Coatings

The most straightforward approach is the replacement of epoxy materials with more thermal and thermo-oxidative resistant polymers, such as high-temperature epoxies, bismaleimides (BMIs), and high T, polyimides and polyisoimide materials that can perform long term (106 W-r cm-l, that compare well with metallic conductivities. They are lightweight, noncorrosive, and enviromnentally stable.[2141 Polymer layers of interconnecting carbon frameworks and fullerene C,, and C,c molecular spheroids can potentially address the issues of conductivity and current-carrying capacity, equipotential current density, loadbearing ability, noncorrosiveness, and thermal/environmental stability.

FUTUREDEMANDSINULTRAHIGHTEMPERATURERESISTANT POLYMERS Projected requirements for fnture high-performance jet engines and aircraft structures for civilian and military uses, such as NASA’s High Speed Civil Transport (HSCT) initiative and the Air Force Advanced Tactical Fighter (ATF) program, will necessitate extensive use of advanced composite materials and hightemperature stmctural adhesives. These requirements include high specific strength and stiffhess at very high temperatures. Many advanced jet engine and select airframe components will have to withstand temperature surges to 37 1‘C (700’F) and above. For advanced air-to-an tactical missiles and air-launched stand-off missiles in military applications, composite airframe structures capable of maintaining strength for short periods (minutes) at 538°C (1,OOO’F)and above will be needed. Although extended exposure of the skin of airframe structures at the Mach 2 range (HSCT) of cruise speed does not require sustained performance at higher temperatures than 177°C (350’F), the consideration of long-term durability dictates the need for thermo-oxidative stabile materials in the 3 16” to 37 1“C (600’ to

High-Pe$ormance


427

700’F)range. Other needs for composites and adhesives capable of performing at these high temperatures include structures for extended range cruise missiles, specialty materials for stealth applications, and, potentially, space-plane structures. The critical need for an easily processible resin capable of meeting these performance parameters cannot be satisfied using state-of-the-art high-temperature materials. Although polymers capable of such performance exist, none of them can be processed practically. Examples include polybenzimidazoles (PBls), polyphenylquinoxalines (PPQs), polybenzothiazoles (PBTs), and polybisbenzimidazobenzophenanthrolines (BBBs). The persistent problem is that the solubility, flow, gel time, and melting temperature of such linear and branched heteroaromatic polymer molecules are inadequate to achieve suitable processibility. Development of polymer systems possessing both ultrahigh thermal stability and good processibility while maintaining a high level of toughness will represent a major technological breakthrough. Most of the state-of-the-art high-temperature composite resin and adhesive systems are imide based. The multifaceted engineering applications of various forms of polyimides are well known. High-temperature resistance, in general, results from a high degree of aromaticity and cross-linking in the polymer structure.12151 The thermal stability of the imide ring is a major factor that would allow the use of aromaticpolyimides as 371°C (700’F) resins. An ideal connecting linkage for the aromatic structural components in high-performance polymers must possess high thermo-oxidative resistance and good chemical resistance, comparable to the aromatics. The linkage must also facilitate processibility by state-of-the-art techniques and promote high polymer formation either directly, via efficient reactions actually taking place at the group, or indirectly, by activation of another functional group. The hexafluoroisopropylidene (6F) group has been used to connect aromatic rings in macromolecules, providing high temperature and chemical resistance, and enhancing processing properties. Incorporation of the 6F groups in the polymer backbone has been a key feature in the development of 371 ‘C (700’F) polymers that possess good processibility. As stated earlier, new cross-linking and chain extension techniques are required so that the ultimate structure, after thermal cross-linking, will be able to withstand long-term exposure at 371°C (700’F) in air. At present, this crucial requirement in achieving this stability has not been met. Some of the organic

428


chemical techniques under study by various research groups are thermal and catalyzed biphenylene opening, and catalyzed acetylene trimerization. The potential 371°C (700’F) polymers thus have two characteristics in common: high thermal stability of the monomers and stability of the groups or bonds linking the monomers in the polymers. Recent advancement in ceramic technology has provided the possibility of designing refractory materials at low temperatures. It represents a viable entry to a new processing technique whereby polymer-ceramic composite materials can be made via blending techniques to achieve light weight, dimensional stability, and environmental resistance. It is interesting to note that the structural material Quazite’@ was molded from inorganic and organic polymer mixtures (vide in&).

CHEMICAL STRUCTURES TEMPERATURE USE

SUITABLE FORULTRAHIGH

Improved thermal stability in polymers can be achieved by incorporating rigid heterocyclic units, inparticular ladder structures,[2161 along the polymer chain. Chain scission in ladder structures is less likely to occur by a single bond-breaking reaction. Many of the potential 371 “C (700°F) polymers have heterocyclic and/or ladder structures. Some well known examples are 1,4,5,?3naphthaleneteiracarboxylic dianhydride-based poly[bis(benzimidazo-benzophenanthrolines)], e.g., BTP,12171 BBB,[218H220]BBL,lul] BBL-DBS,[2221TAP-BB,[2231andbis(naphthalenedicarboxylic anhydride)-based poly[bis(benzimi-dazobenzoisoquinolinones)] (BBQ).[2241 An important aspect of the BBB and BBL (ladder) polymers is that they show weight retention of58% and 87%, respectively, after a 500-hour isothermal aging at 371°C (700°F)[2251(Fig. 8.73). High molecular weight polybenzothiazoles show excellent thermal stability via isothermal thermogravimetric (ITGA) analyses. These polymers at 3 16°C (600°F) for 200 hours have shown negligible weight 1oss[2261[2271 (Fig. 8.74). Structures containing trigonal nitrogens have been known to impart It is noteworthy that outstanding thermal stability to polymer systems. poly(benzimidazoquinazolines) (PlQs) showed excellent retention of thermo-oxidative and thermomechanical properties after 200 hours of exposure to air at 37 1“C (700°F)[228H2301(Fig. 8.75).

High-Performance

Polyimides

and Thermoset

Polymers

429

The phthalocyanine structure is recognized as one of the most thermally and thetrno-oxidatively stable organic structures known. It has been incorporated in the synthesis of thermally stable hybrid oligomers12311 and polymers, e.g., benzimidazoles.1232112331 The most relevant are phthalocyanine tetraamines and benzophenonetetracarboxylic dianhydride (BTDA)1234l and those derived from metal phthalocyanine tetraamines and pyromellitic dianhydride (PMDA).[235] Because of the apparent ease of synthesis of the phthalocyanine derivatives and the exceptional thermo-oxidative stability of the derived polymers, these phthalocyanine polyimides can be considered as potentially useful 37 1“C (700°F) resins. Development of the relevant phthalonitrile

L

resins was discussed in an earlier section (vide sup-u).

Jn BBL

(X-CR)

TAP-BB

(X = N)

-n

BBLDBS

BBQ

BTP

AR, AR’ = Aryl

Figure 8.73. Fused ring structures for high temperature

stability.

430


Figure 8.74. Chemical structure of polybenzothiazole.

Figure 8.75. PIQ polymers.

A parallel and apparently quite promising development in phthalocyaninebased polymers is the silicon-phthalocyanine polymers (Fig. 8.76). They have been readily synthesized in large quantities, dissolved in trifluoromethanesulfonic acid, and wet spun into fibers (either inpure form or as blends withKe~l~).[*~~1[*~~1 This class


43 1

of potential ultrahigh temperature resistant polymers is especially attractive because of its structural similarities relative to high-temperature silane-type coupling agents (needed for adhesive bonding) and silicate glassy networks produced by low-temperature, high-tech ceramic processing, which will find ultrahigh temperature utility. These stacked phthalocyanine polymers can also be synthesized with germanium or tin as the metallic center. The neat, wet-spun fibers can be doped with iodine to conductivities of 1 to 2 R-l cm-‘, while the Kevlar-metal phthalocyanine blends reach conductivities of 0.01 to 0.1 Q-t cm-l. Incorporation of adamantane units in polyimides[2381-[2421(Fig. 8.77) is also expected to enhance processibihty without compromising their thermal and themrooxidative stabilities. Comparable studies, albeit less extensive, have also been carried out with biadamantane-based polymers.[ 243)[244) An adamantane-based polybenzoxazolef 24slhas been developed as a useful 450°C material. Similar incorporation of adamantane units in polyimides is expected to yield processible resins for potential 37 1“C (700°F) applications. Recent literature also suggests that extremely high thermal stability can be attained by incorporating dicarbadodecaborane (carborane) ~ts.Wl

Because thermo-oxidative stability (reflected by isothermal aging) is importaut to the design of a high-performance polymer, consideration must be given to the presence of excessive aromatic hydrogens. Aromatic hydrogen atoms have been shown to be surprisingly labile at high temperatures and participate extensively in degradative and cross-linking reactions, thus decreasing the long-term thermo-oxidative stability of the polymer. The anhydride precursor pyrazine-tetracarboxylic dianhydride (PTDA), which does not contain hydrogen, was shown to impart unusual thertno-oxidative stability (25 hours at 4OO’C in air) to polyimides containing this structural moiety[247](Fig. 8.78). A similar heterocyclic anhydride precursor, hexaazatriphenylene trianhydride (HTTA), without hydrogens in its structure also produces polyimides with high-temperature resistance up to 7OO’C in air.[2481[2491 The addition of H’M’A as a cross-linking agent augments the reaction of pyromellitic dianhydride with aminophenyl ether. Even when the amount of HTTA added reached 30% of the total anhydride content, films could be cast and postcured thermally to provide polyimide films with good thermal stabilities and a high degree of cross-linking. Polymerization using HTTA as the only anhydride component followed by imidization, afforded a deep red polyimide film. Thermogravimetric analysis in argon revealed that 60% of this sample remained at 850”C.[2so]-[252]

432


> -H,O

-

OH

M = Si

Figure 8.76. Silicon-phthalocyanine

polymer.

High-Peformance

HO

Polyimides

and Thermoset

Polymers

433

-g&O” JQ-gglgO” HO

Figure 8.77. Starting materials for adamantane-based

PTDA

polymers.

PTDA-DATD

Figure 8.78. Hydrogen-free

heterocyclic

polymer.

HTTA derives from hexaazatriphenylenehexanitrile [HT(CN),] which was prepared in analytically pure form in an 81% yield on a large scale from hexaoxocyclohexane octahydrate and (Z)-2,3-diaminobut-2-en- 1,Cdinitrile for 2 hours in refluxing acetic acid.t 2521[2531 The conversions of HT(CN), to the corresponding hexaamide, hexaacid, hexaester derivatives, and HTTA were then accomplished (Fig. 8.79).t2541 In stmnnary, imide-based hybrid polymers incorporating the various classes of high-temperature-resistant structures are potential 37 1“C (700’F) resin materials if good processibility could be achieved, The introduction of fluorine-containing monomers, particularly those containing the 6F and 3F groups, have been shown to yield polymers with increased processibility while maintaining excellent thennooxidative stability and improved moisture resistance.[221 Many 6F- and 3F-containing monomers are readily available (vide s~pru).[~~1[~~~~


435

The fundamental properties of the 6F-monomers and the polymers in which they are incorporated have indicated improved thermo-oxidative stability (c.f. DuPont’s NR-150@, which incorporatesthe6F-containingdianhydride 6FDA). The application of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafIuoro-propane (BDAF) (Fig. 8.21) inpolyimide coatings for 37 1“C (700’F) exposure is also illustrative.12561The hydrophobic nature of the hexafluoroisopropylidene group is expected to enhance the moisture resistance of the resin systems of which it is a part. Furthermore, the flexible nature ofthe 6F-linkage is conducive to improving fracture toughness.[i511 NOVELCROS!+LlNKINGMECHANIS~FoRSTABJLITYATULTRAHIGH TEMPERATURES The harsh environmental conditions at 37 1“C (700°F) in air pose severe limitations on the choice of a suitable cross-linking and/or chain extension mechanism. The ultimate structure after thermal cross-linking must be able to withstand long-term exposure up to 371T (700°F) in air. It has been demonstrated that ethynyl and phenylethynyl end-groups on polyphenyl-quinoxalines undergo thermally induced chain extension and cross-linking. In the case of ethynyl endcapped polymers, the final cured resins have less thermo-oxidative stability than the corresponding polymers which do not contain the ethynyl groups.12571t2581The thermo-oxidative instability is ascribed to the presence of nonaromatic end products resulting from the ethynyl groups undergoing thermally induced cross-linking. Biphenylene An attractive cross-linking mechanism presents itself in the thermally induced ring opening of biphenylene to produce dimeric and polymeric products via diradical intermediates12501-12541(Fig. 8.80). Such a cross-linking mechanism for high-temperature polymers containing these biphenylene units along the polymer chain provides thermally stable cross-links and yields no volatile by-products. Bisbiphenylene compounds having the corresponding skeletal structures to the respective imide, quinoline, and phenylene oxide polymers have been synthesized. The presence of the bisbiphenylene cross-linking agent in the polymer initially plasticizes the polymer. Upon curing, this plasticizer forms a cross-linked network with the polymers.l263l The addition of an organometallic reagent to catalyze the cross-linking reaction did not seem to affect the ultimate thermal and thermooxidative stability of the resins. Although the mechanism looks promising, certain

436


technical problems persist. For example, the control over completeness in ring opening reaction is lacking. Furthermore, it is not certain that the recombination of the radical species generated necessarily produces thermally stable products. More extensive studies are necessary. A recent extension of the biphenylene recombination cure mechanism is the application of the thermal (or nickelcatalyzed) cycloadas the cure mechanisml266l (Fig. 8.81). dition of biphenylene to acetylenet 2641[2651

Figure

8.80. Cross-linking

mechanism based on biphenylene recombination.

ring opening and

Organometallic complex-catalyzed trimerization,[26~ intramolecular cyclization,[2711[2721 and polymerization1 2611of many acetylenic compounds often leads to well-defined, highly condensed aromatic structures. Of particular interest are complexes based on cobalt, nickel, palladium, rhodium, and, most recently, tungsten, tin, niobium, and tantalum. The catalyzed reactions are often facile and proceed at relatively low temperatures. In the light of the generally accepted catalytic effect of organometallic compounds in the reactions of aromatic acetylene compounds, the addition of organometallic reagents is expected to catalyze trimerization specifically and at the same time lower the temperature required for the reaction. Candidate reagents for achieving this include rhodium[I] carbonyl, nickel[O] carbonyl, and palladium[II] benzonitrile complexes. The ultimate product after the acetylene cure would have thermally stable crosslinks. The inherent high-temperature instability in thermally cured acetylene-terminated resins is due to the low percentage of stable aromatic and/or heterocyclic ring structures (vi& supru). [2.2]Paracyclophane Scientists at NASA-Lewis evaluated [2.2]paracyclophane as a high-temperature, stable, cross-linking unit for polyimide resins with a 425°C (797°F) use potential.12711t2721The objective is to develop a better end-capping agent than the nadic group for the processing of the PMR-II-50 resin.


437

o”o 83 0

+

Figure

8.81. Thermal Cycloaddition

of Biphenylene

to Acetylene

The CYCAP resins have been reported[2721 to have excellent thermooxidative stability, high glass transition temperatures, and excellent processibility. They form void-free consolidated disks and generate no volatiles during consolidation. The N-CYCAP resin comprises a chloroform solution of 4-amino[2.2]paracyclophane, the dianhydride 6FDA, and p-phenylenediamine (PPDA). Figure 8.82 depicts the idealized polymer structure. The resin itself processes similar to PMR polyimides.t35~ Laminate testing results indicated that N-CYCAP is comparable to PMR-II-50, both being good performers at 343°C (650°F). PMR-II-50 matrix composites are, however, more thermo-oxidatively stable thanN-CYCAP at 371’C (700’F) (Table 8.61). Mechanical strength measurements at 371°C (7OO’F) do not indicate significant differences inperformance between N-CYCAP and PMR-II50 composites (Table 8.62). Benzocyclobutene Benzocyclobutene-terminated imide oligomers have been developed as a close counterpart to biphenylene-terminated resins, with an expected advantage of a lower cure temperature (250°C versus 380 to 400°C for biphenylene materia1s).t2731[2741 The thermally induced electrocyclic ring opening of benzocyclobutene is well referenced.[451 The highly reactive o-quinodimethane intermediate either self-reacts or undergoes Diels-Alder reactions with dienophiles. Bis(benzocyclobutene)imide monomers were prepared from 4aminobenzocyclobutene and several dianhydrides. The cured samples from these materials lost only 7 to 10% of the original weights during isothermal ageing at 600°F for 200 hours[273l (Fig. 8.83).

438

Handbook

of Thermoset

Plastics

A series of aromatic imide AB-monomers containing both benzocyclobutene and acetylene end-groups was prepared to evaluate the potential of a single reactant polymerization technique (Fig. 8.84). Among the compounds evaluated, only N-(4-benzocyclobutenyl)-4-(phenylethynyl)phthalimide gave a clean single exotherm by differential scanning calorimetry, indicating the expected benzocyclobutene ring opening and Diels-Alder reaction.12741 Dow Chemical Company is developing bis(benzocyclobutene) materials for their potential as easily processible matrix resins for high-performance advanced composites. The bis(benzocyclobutene) diketone monomer, DK-bis-BCB (Fig. 8.85), has a T, at 340°C and can be processed by (RTM) techniques. The graphite composites obtained from it have excellent 274°C (525’F) hot/wet properties and are potentially useful at service temperatures of 260°C (SOO’F) or above.[2VPW

AB-monomers having both BCB and maleimide termini have also been studied.12771-12791Besides the advantage of being single components for polymerization, the AB monomers also resolve the observed difficulty in improving fracture toughness of bis(benzocyclobutenes) via reaction with bisolefm-type dienophiles. As summarized in Table 8.63, the residual compression strength after impact (CAI) measured for Celion@ G30-500 8HS fabric/AB-BCB was 332 MPa (48.1 ksi), which is comparable to the typical value of 300 MPa (43.5 ksi) for thermoplastic composites.

Figure 8.82. Idealized Structure ofN-CYCAP (NASA-Lewis).

Polyimide


439

Table 8.61. 371’C Isothermal Weight Loss Comparison Between N-CYCAP and PMR-II-50 on T40R and G40-700 Fibers* Resin

Fiber

% Weight Loss (500 hr)

% Weight Loss (750 hr)

-

N-CYCAP

T40R

15.7* 1.7

N-CYCAP

G40-700

10.0 f 0.7

N-CYCAP (25% MPDA)

T40R

ll.OhO.2

N-CYCAP

G40-700

11.3 f 0.5

T40R

8.9 f 0.5

G40-700

8.0 f 0.4

(25%MPDA)

PMR-II-50 PMR-II-50 T _-:__r__ (IL II .-i-l_. LIZ” *~,arruruires piy, ~24

x

,1u.10

cry

were

25.4 f 0.5 29.3 f 1.3

23.2 f 0.4

testes in i-am7 riowmg air.

All fibers used were unsized 12k tow.

Table 8.62. Preliminary Mechanical Properties of N-CYCAP and PMR-II-SO Composites

Resin

Fiber

r

Fluxural Strength Isi) (I 25°C 371°C

T71

(PS

25’C

T 371°C

N-CYCAP

T40R

155*3

25& 1

7.6 f 0.3

3.1 f 0.3

N-CYCAP

T650142

201*8

24*8

12* 0.2

4.7 f 0.2

N-CYCAPI 25% MPDA

T40R

154*4

27&2

6.8 f 0.3

3.5 f 0.2

N-CYCAP/25%MPDA

T650142

185 f 12

22*2

11.8*0.7

3.5 f 0.5

PMR-II-50

T40R

185 f 14

29& 1

15.5 f 0.6

4.3 f 0.2

PMR-II-50

T650f42

170*9

25*6

8.1 f 0.4

3.8 f 0.:

1

Note: 1. Flexural strength (ASTM D790-81) 2. All fibers were unsized and 12k tow 3. Interlaminar shear strength (ASTM D2433) 4. Composites were post-cured at 385°C (16 hr, in air)

440


f

t

CFt R+

CFI

0

CFs

Figure 8.83. Bis(benzocyclobutene)imide

Oligomers

R--CC

CeHs

-,,.ao~ -

0

Figure 8.84. Benzocyclobutene-Ethynyl-Terminated

AB-Imides

High-Pe$ormance


44 1

b DK-bis-BCB

Figure 8.85. BCB monomers

AB-BCB

for processing RTM composites.

Acenaphthylene Acenaphthylenyl-terminated phenylquinoxahne (PPQ) and aroylene (e.g., aryl ether-ketone, aryl ether) oligomers have been evaluated as alternative highperformance thermosetting resins to BMIs to overcome the latter’s moisture sensitivity and improve their thero-oxidative stabilityt2801 (Fig. 8.86). The acenaphthylenyl oligomers are derived from facile conversion of lowcost acenaphthene starting material. Their ease of polymerization is comparable to that of BMIs. The known chemistry of thermal reactions of acenaphthylene to yield high-temperature, stable, fused ring systems (Fig. 8.87) suggests that acenaphthylenyl-terminated oligomers can cure to thermo-oxidatively stable materials.

Diazine Two interesting diazine compounds,[2811[2821capable of undergoing 1,3dipolar addition reactions with various dipolarophiles, can also be considered in the context of chain extension and thermal cure for acetylene-terminated, nadimideterminated, and propargyl-terminated polymers. In particular, the tetrakis(trifluoromethy1) derivative, i.e., hexafluoroacetone-azine, forms a 1: 1 copolymer with norbomadiene at ambient temperature in quantitative yields1282I (Fig. 8.88). It also reacts similarly with diallyl resorcinol,[2831 diallyl a,walkylenedisilanes,t2841 a,w-alkylenedienes,[ 2851and nadimide-terminated oligomers.t2861 The reaction between hexafluoroacetone azine and bisnadimides (mole ratio 1: 1) yieldedpolymers with a number-average molecular weight of 1700 to 5700, depending on the reactants. It is interesting that maleimide-terminated oligomers do not undergo chain extension with hexafluoroacetone+uinezine.[2851A spontaneous rearrangement of the primary product, a 1: 1 adduct, to 1-H-pyrazolines.


Reinforcing polymers with a high-modulus, high-temperature resistant and a compatible inorganic second phase results in organic-inorganic polymer networks (OIPNs) with increased ablation resistance, modulus, and a higher level of thermal and thermooxidative resistance. Multicomponent systems that involve both organic and inorganic materials are noteworthy. The inorganic components include a multitude of silicates (zeolite, litidionite, clay), aluminum phosphates, alumina, titania, and zirconia. Although these components are naturally occurring, many have been synthesized in a controlled environment via solgel polymerization techniques. Silicate-based OIPNs are generally available from solgel polymerization of tetraallcoxysilanes in the presence of the organic components.

Table 8.63. Mechanical Properties of Benzocyclobutene Composites

Mechanical Property

DK-Bis-BCB AS4 (8HS)

DK-Bis-BCB Celion G30-500 (8HS)

AB-BCB Celion G30-500 (8HS)

1lSO(171) 680 (98)

1140(165) 800(116)

1311(190) -

58.5 (8.5) 54.3 (7.9)

63.2 (9.2) 64.3 (9.3)

68.3 (9.9) -

329 (47.7) 279 (40.5)

293 (42.4) 202 (29.6)

Flexural strength, MPa (ksi)* 2YC (77’F) 274T (525’F) Flexural modulus, GPa (msi)* 25T (77’F) 274°C (525°F) Gpen hole compressive strength, MPa (ksi)* * 25T (77’F) 177°C (350’F) Compressive strength

High-Pe$ormance

R=

-0-,


-,

-CR

2--,

-so

443

,-

Figure 8.86. Acenaphthylenyl-terminated aroylene and phenylquinoxaline oligomers. This interdisciplinary materials research explores the synergism of polymer technology and ceramic technology, and is actively pursued at Frauenhofer Institut ftir Silicatforschung (Wtirzbnrg, Germany), Virginia Tech (Blacksburg, Virginia), and the University of Arizona (Tucson, Arizona). Preliminary results suggest that polymer-ceramic materials are applicable as high-temperature structural matrices, adhesives, and ablative resistant coatings. The shrinkage associated with solgel-derived ceramics can be mitigated by the organic components.[287H2891.

444


Figure 8.87. Fused aromatic systems resulting from thermal reactions of acenaphthylene

The proper choice of the polymer resin and the inorganic component can lead to materials with tailorable adhesive, optical, laser damage resistant, and ablative resistant properties. The conventional precursor for solgel polymerization is tetraethoxysilane. Modification of the inorganic component of the OIPNs can be carried out by incorporation of diarylsiloxy units during solgel polymerization. The presence of such units strongly contributes to the thermoplasticity and solvent solubility of the OIPN materials.1288112g0112g11 Careful control of reaction conditions and the order of mixing during OIPNpreparation are vital to the control ofthe microstructural,

High-Per$ormance Pobimides and Thermoset Polymers

445

mechanical, and optical properties of the final product. Toward this end, applications of the solgel process to designing polymer-ceramic materials for hot-melt adhesives and contact lens with improved oxygen permeability have been real~Cd.[2911[294

OIPNs open the possibility of merging organic materials with inorganic materials. Processible materials to service the thermal regime of 371 “C (700’F) to 1.oc)O°C( 1.832”F1 can he a realitv.

R2

+

-r

A

na

R,-C’C-H

a. &=&=CF, b. R,=C& &=H

,‘%

F&\ ,c=N-N==e,

W

CF, +

Figure 8.88. Polymerization of norbomadiene

and hexafluoroacetone-azure.

446


Silicon Alkoxide-Derived Polymer Ceramic Materials The low-temperature aspect in preparing and processing silicon alkoxidebased glass materials[2931-[295~allows reinforcement with polymer fibers and preparation ofpolymer-ceramic composite materials based on otherwise hard-to-process high-temperature polymers. Siloxane and inorganic silicon chemistries afford a unique opportunity to modify polyimides with alkali-free polysilicates while retaining the polyimides’ processibility during processing and cure. Researchers at Hughes Aircraft Company have previously established their beneficial qualities, such as improved structural integrity (modulus and toughness) and optical properties, of inorganic components incorporated into organic polymer resins.t2961 Thermid’” 600 polyimides, similar acetylene-terminated resins, and/or 6F-containing polyimides have been used as the organic components. Compatibility of organic and inorganic components has been demonstrated in organosilicates and organoaltiosilicates.[741 The ultimate resin ultrastructure is single phase, i.e., there is no discernible boundary going from the organic component to the inorganic. Thin films can be formed from organic polyimides and silicon acetates.[2971 The resulting silicate-containing polyimide film (1 to 2 micron) resisted high temperatures (~530°C or 986°F) in air. Under the same conditions, the organic polyimide resin underwent precipitous weight loss. “Ceramer” materials were obtained at Virginia Polytechnic Insitute[2981[2991 as poly(dimethylsiloxane) oligomers were incorporated into silicate glass network during low-temperature solgel polymerization (Fig. 8.89). The choice of siloxane oligomers for this study was based on the high thermally stability characteristic of these materials and that a direct silanol coupling reaction is possible between the oligomers and tetraalkoxysilane. The final ceramic obtained was thermal stable up to 500°C (932’F). The fact that only low molecular weight polydimethylsiloxane materials (MW = 1.7 x 103) were used suggests that even higher level of thermal stability can be achieved with high molecular weight polymeric dimethylsiloxanes. At the ultrahigh molecular weight range (MW>106), the enhanced polymer entanglement further enhances mechanical and dimensional stability. Electron microscopy[3001 of similar silicate-reinforced organic materials indicated that the silicious particular precipitates were not agglomerated and had an average particle size of 200 A. Mechanical testing showed that an increase in ultimate tensile strength was observed with increasing amount of the inorganic component.

High-Performance Polyimides and ntermoset Polymers

447

An elaborate study of the interplay of several solgel reactions of tetraalkoxysilane, mixed silicon alkoxide and titanium alkoxide, diphenyldialkoxysilane, methylvinyldialkoxysilane, and subsequent thermal curing has led to the development of an optimized hot-melt sealant formulation of 5 Si(OCH,CH,),, 60 (C,H,)$i(OH),, 5 Ti(OCH,CH,),, and 30 (CH,=CH)(CH,)Si(OCH$H&.~2g1j (Fig. 8.90).

OCH ,

I

H,CO

X”,

+

HO

W”3

”

OCH s

Ceramer

Figure 8.89. Formation of a ceramer material. Aluminum Phosphate and Silicate Refractory Materials The possibility of preparing refractory materials at very low temperatures has been demonstrated, and these materials have been shown to be stable well beyond 37 1°C (700’F). For example, aluminum phosphate-based glass material is refractory up to 1,6OO”C, at which point aluminum phosphate begins to decompose. The processing is typically carried out at low temperature, and heat treatment requires only temperatures as low as 1OO”C.t30’1 Physical blending of an inorganic (ceramic) precursor and a high-temperature polymer has been studied in the development of a high-temperature adhesive-sealant composition.Po2j The composition comprised a mixture of an aluminous cement and a poly(amic acid) precursor. A rectangular glass fiber sheet was formed into a cylinder and the edges were joined with the adhesive system. After drying for 3 hours at 204’C (400°F) and heating for 3 hours at 426°C (800°F), the joint was shown to have good cohesive and adhesive strength.

448 Handbook of Thermoset Plastics OR

OR I

R10 +

OR -4-0,

OR

OR

&-OR

&OR

+

+

OR

R’“SI(OR)

h

-w

Hct,H

10

+

H&=CH

-Si

I

I +

HO -SII

I

I I

I-

C”lX

w

I

I

-N-OH

I

-

-O-L-O-Si-OAH

I

-.%-OH I

I

-St-o -ssi-0

OH w

&-OR

AR

-SI-CH,

-n-o

-0

+

RO

-nI

->

cure

cure

I

I

-SI-CH,-CH,-CH2-SI-

I

I

I

I

I

-81-O-S-

*

I

-s-o-nI

I

I

+

H,O

+

Hz0

I

Figure 8.90. Interplay of reactions in solgel processing. Commercially available Qua&e@ is an improved structural material molded from 95% inorganic material[303l and 5% high-performance organic polymers. The process, which involves controlled mixing, molding, and curing, is proprietary to Quazite Corporation. The ultimate structure is an intertwined cross-linked network, monolithic in nature and having tailorable characteristics. Quazite@ possesses the formability of fiberglass, twice the bending strength of and more abrasion resistance than granite, excellent impermeability to liquids and chemical resistance equal to titanium-clad steel. It could also be fiber-reinforced, polished, or gel-coated. A 20-foot x 1 ‘/,-inch-thick Quazite@ panel is able to withstand a 50,000-A, 10,000-V arc. Temperatures above 1,094” C (2,OOO”F) were contained. Chemically, it resisted temperatures up to 232°C (450“F) and pressures to 500 psi in a highly concentrated, corrosive salt solution.

High-Pe$ormance


449

Partially Stabilized Zirconia The utilization of ceramic materials in matrix resins and structural adhesives is another beneficial feature. Chemical compounds that undergo expansive phase transformations, such as the one occurring in partially stabilized zirconium oxide (PSZ), can contribute to the toughening of the structural adhesive matrix. The PSZ is a matrix of cubic zirconium oxide containing 20% to 50% of the metastable tetragonal form. As the adhesive matrix sustains a crack, the crack tip induces the metastable tetragonal particles to transform to the monoclinic form with a net volume increase. Such a volume increase exerts a compressive stress to the crack tip, thus halting fiuther propagation. While PSZ is known to impart toughening to alumina-based ceramics, other silicate-based expansive toughening agents, such as calcium silicate, can perform well in silicon-derived ceramic matxices.[3041 Organically Modified Litidionite An inherently fibrous siloxane polymer having pendent trimethylsilyl (Me3Si, Me=CH,) groups was prepared from synthetic litidionite using a Lentztype extraction-substitution process.[ 3051The polymer was prepared from litidionite by treatment with a mixture of chlorotrimethylsilane, water, and an organic solvent for a period of days at room temperature. The polymer had a waxy texture and was insoluble in a wide range of organic solvents. The IR spectra of the polymer showed trimethylsilyl (SiMe,), silanol (SiOH), and siloxane (S&O-Si) bands, while its Si 2~~‘~ x-ray photoelectron (ESCA) spectrum showed overlapping SiO, and SiOC, peaks. Spectra data showed the presence of a small amount of potassium but no sodium or chlorine. The polymer was further characterized by electron microscopy, elemental analysis, differential thermal analysis (DTA), differential scanning calorimetry (DSC), and x ray powder diffiactometry. Many fibers, often with diameters of 40 to 60 A, were seen in the micrographs obtained.[3061 Litidionite was prepared from a Na-K-Cl silicate glass by devitrification or from a 2: 1: 1:8 mole ratio mixture of CuO, Na&O,, K2CO3, and silica by sintering at 765’C (1,409“F) for several daysJ3071 Clay-Polymer Nanocomposites The mechanical and thermal properties of clay-polymer nanocomposites often are superior to the polymer itself.1 3081Intercalation of the polyimide derived

450


from pyromelltitic dianhydride (PMDA) and 4,4’oxydianiline (4,4’-ODA) in the galleries of extended-chain monoalkylammonium montmorillonite indicates that the polyimide forms a monolayer, with the aromatic nuclei parallel to the clay layers.[3091 Polyimide films with the incorporation of 2.0 weight % and 5.0 weight % of clay decrease carbon dioxide transmission to 50% and 20%, respectively. Solgel-Derived

Polyimide-Silica

Nanocomposites

Based on previous work on epoxy-silica and polyethersulfone-silica13i01 and solgel polymerization techniques, [2871-[2901[3111 polyimide-silica nanocomposites have been attained from solgel polymerization of tetraethoxysilane (TEOS) in the presence of poly(PMDA-4,4’-0DA)amic acid in N,N-dimethylacetamide. Macrophase separation was observed due to early separation of phases as a consequence of solubility limitation.1 3121 As solgel polymerization of tetramethoxysilane (TMOS) in the presence ofpoly(PMDA-4,4’-0DA)amic acid and triethylamine in methanol was carried out, better solubility was achieved and much different morphology of the resulting nanocomposite was observed.13131 The solgel synthesis of silica structures has resulted in the formation of interconnected globular 0.2 and 0.07 micron microstructures at drying temperatures of 60°C and 20°C respectively. That the globular microstructure was formed during drying of the solgel product and not as a consequence of the 800°C firing of the polyimidesilica tihn was demonstrated by observing the same globular structure by SEM of a polyimide-silica tihn etched with hydrazine hydrate to remove the organic polymer. In theory, these inorganic films of interconnected silica globules of uniform size can be stacked and compacted at moderate to high pressures to yield ceramic sheets of controlled thicknesses. Similar polyimide films containing a homogeneous dispersion of silica particles were prepared via similar solgel processes.[314l Analogous to the earlier Hughes work, an alkoxysilane-terminated organic monomer has been transformed via the solgel process into a network polymer.1 3151 Application of this organicinorganic polymer network concept has led to the formation of an nonlinearly optically active silica-polyimide nanocomposite. The polyamic acid used in this research was Skybond 705, Monsanto’s poly(BTDA-MDA)amic acid.13161 The interdisciplinary research in polymer-ceramic materials has the potential of offering a novel solution to the quest for high-temperature (>371”C or >700”F) processible polymer resins, particularly thermosetting resins.


45 1

Related to the development of pohner-ceramic materials is their potential in improving compressive modulus in polymeric materials. Conceivably, enhancement of inherent compressive modulus of the polymer also can be achieved through integration of star-shaped polymers as second components.[3171t3’81 The feasibility of such an approach to polymer blending warrants attention.

1. E. I. DuPont deNemours andCompany,FrenchDemande Australian Patent 58,424 (1960)

1,239,491(1960);

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189. Stenzenberger, H. D., Romar, W., Herzog, M., Konig, P., and Fear, K., Proceedings, SAMPE (European Chapter) Conference, July 11-13, 1989, Birmingham, United Kingdom, p, 277 190. Dane, L. M., and Brouwer, R., Proceedings, 33rd SAMPE Symposium Exhibition, Mar. 7-l 0, 1988, Anaheim, California, p. 12 17 191. Wadsworth, M., Proceedings, Sot. Aeronaut. Eng., GeneralAviation Meeting and Exposition, Apr. 1 I-13, 1989, Wichita, Kansas. 192. Margolis, J. M., Chemical EngineeringProgress,

h

Aircraft

83( 12), 30 (1987)

193. Ware, M., Proceedings, SPI Reinforced Plastics/Composites Institute, 40th Annual Conference, Jan. 28-Feb. I, 1985, Atlanta, Georgia, Paper 18-A, p. 627 194. “Cuts Cost ofAdvanced

Composites,“Modern

Plastics Int., 15(4), 24 (1985)

195. “Sandwich Structures by Resin Transfer Moulding,“Materials 109(2),31(1992)

Engineering,

1%. Michaeli, W., Kotts, R., Mitcherling, J., Mullerferli, G., Rosenbaum, U., Specker, O., Stoger, M., Starke, J., andMahlke, M., GummiFas. Kunststofi, 43(6),315(1990) 197. Kranbuehl, D., Eichinger, D., Hamilton, T., Levy, D., Reyzer, M., Kingsley, P., Hart, S., Loos, A., and Long, E. R., Sot. Plastics Eng., ANTEC ‘90 Conference, May 7-l 1, 1990, Dallas, Texas, p. 912 198. Keller, L. B., and Dominski, M., Sot. Manufactur. Eng. Conference, January 19-20, 1993, Pasadena, California 199. Leung, C. L., Lau, K. S. Y., and Landis, A. L., Amer. Chem. Sot., Polymer Preprints, 33(l), 509 (1992) 200. Leung, C. L., Ghaffarian, R., and Lau, K. S. Y., Amer. Chem. Sot., Polymer Preprints, 37(l), 000 (1996) 201. Pater, R. H., Proceedings, High Temple Workshop ZX, Pasadena, California (January31-February2,1989) 202. Babb, D. A., Ezzell, B. R., Clement, K. S., Richey, W. F.,Kennedy, A. P., and Frye, D. R., Amer. Chem. Sot., PolymerPreprints, 34(l), 413 (1993) 203. Beavers, J. A., Kock, G. H., and Berry, W. E., MCIC Report MCIC-86-50, Metals & Ceramics Information Center, a DOD Information and Analysis Center, Columbus, Ohio

High-Performance

Polyimides

and

Thermoset

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204. Aylor, D. M., Ferrara, R. J., and Kain, R. M., Metal Performance,

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23(7), 37

(19W 205. Rommel, M. L., Postyn, A. S., and Dyer, T. A., SAMPE Journal, 29(2), 19 (1993) 206. Preto-Clement, S., and Cameron, R. E., Proceedings, 4th IntenationalSAMPE Electronics Conference, Albuquerque, New Mexico, pp. 367 (June 12-14, 1990) 207. Cassidy, P. E., and Syrinek, A., J. Polym. Sci., Polym. Chem. Ed., 14, 1485 (1976) 208. Cassidy, P. E., and Lee, F. W., J. Polym. Sci., Polym. Chem. Ed., 14, 15 19 (1976) 209. Cassidy, P. E., and Doctor, S. V., J. Polym. Sci., Polym. Chem. Ed., 18, 69 (1980) 210. Cassidy, P. E., Lohr, R. A., Kutac, A., andDoctor, Polymer Preprints, 19(2), 46 (1978)

S. V., Amer. Chem. Sot.,

211. Ueda, M., Takahashi, T., and Imai, Y., J. Polym. Sci., Polym. Chem. Ed., 14, 591(1976) 212. Imai, Y., Ueda, M., andTakahashi, 2391(1976)

T.,J. Polym. Sci., Polym. Chem. Ed., 14,

213. Chung, W. C., Proceedings, Mat. Res. Sot. Symposium, Massachusetts, p. 379 (Nov. 27-30,1989) 214. Sugimoto, I., andMiyake,

S., SyntheticMetals,

Vol. 171, Boston,

46,93 (1992)

215. Arnold, C. Jr.,J. Polym. Sci., Macromol. Rev., 14,379 (1979) 216. Bailey, W. J., andFeinberg, 229(1967)

B. D., Amer. Chem. Sot., PolymerPreprints,

217. Evers, R., Amer. Chem. Sot., PolymerPreprints,

8,

12(l), 240 (1971)

218. Coleman, J., and Van Deusen, R. L., Air Force Technical Report, AFML-TR69-289 (Mar. 1970) 219. Van Deusen, R. L., Goins, 0. K., and Sicree, A. J., J. Polym. Sci., A-l, 1776 (1968) 220. Waddella, W. H., and Younes, U. E., Amer. Chem. Sot., Polymer Preprints, 21(l), 195(1980)

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221. Arnold, F. E., Amer. Chem. Sot., PolymerPreprints,

12(2), 179 (1971)

222. Sicree, A. J., and Arnold, F. E., Air Force Technical Report, AFML-TR-73256(Feb. 1974) 223. Gerber, A. H., J. Polym. Sci., Polym. Chem. Ed., 11,1703 (1973) 224. Jedlinski, Z. J., Kewalski, B., and Gaik, U., Macromolecules, 225. Arnold, F. E., and Van Deusen, R. L., Macromolecules, 226. Wolfe, J. F., and Arnold, F. E., Macromolecules,

16,522 (1983)

2,497 (1969)

14,909 (1981)

227. Wolfe, J. F., Loo, B. H., and Arnold, F. E., Macromolecules,

14,9 15 (198 1)

228. Milligan, R. J., Delano, C. B., and Aponyi, T. J., J. Macromol. AlO, 1467 (1976) 229. Korshak, V. V., and Rusanov, A. L., Izv. Akad. NaukSSR, (1970) 230. Korshak, V. V., and Rusanov, Chem.,C21,275(1982)

A. L., J. Macromol.

Sci. Chem.,

Ser. Khim., 1917

Sci.-Rev.

Macromol.

231. Achar, B. N., Fohlen, G. M., and Parker, J. A.,J. Polym. Sci., Polym. Chem. Ed.,20,269(1982) 232. Achar, B. N., Fohlen, G. M., and Parker, J. A., J. Polym. Sci., Polym. Chem. Ed.,20,1785(1982) 233. Achar, B. N., Fohlen, G. M., and Parker, J. A., J. Polym. Sci., Polym. Chem. Ed., 20,2073 (1982) 234. Achar, B. N., Fohlen, G. M., and Parker, J. A., J. Polym. Sci., Polym. Chem. Ed., 20,2773 (1982) 235. Achar, B. N., Fohlen, G. M., and Parker, J. A., J. Polym. Sci., Polym. Chem. Ed.,20,2781(1982) 271. Sutter, J. K., Waters, J. F., and Schuerman, M. A., Amer. Chem. Sot., Polymer Preprints, 33(l), 366 (1992) 272. Sutter, J. K., Waters, J. F., and Schuerrnan, M. A., Amer. Chem. Sot., Polymer Preprints, 32(2), 203 (1991) 273. Tan, L. S., and Arnold, F. E., Amer. Chem. Sot., Pobmer Preprints, 26(2), 176 (1985) 274. Tan, L. S., and Arnold, F. E., Amer. Chem. Sot., Polymer Preprints, 26(2), 178 (1985)

High-Perjormance Polyimides and Thermoset Polymers 275. Lee, W. M., Laman, S. A., McGee, R. L., andHoule, Symp. Exhibition, pp. 1207(Apr. 1%18,199l) 276. Hergenrother, P. M., and Rogalski, Preprints, 33(l), 354 (1992)

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S., 36th Intemat. SAMPE

M. E., Amer. Chem. Sot., Polymer

277. Tan, L. S., Soloski, E. J., and Arnold, F. E., Amer. Chem. Sot., Polymer Preprints, 27(l), 240 (1986) 278. Bishop, M. T., Bruza, K. J., Laman, S. A., Lee, W. M., and Woo, E. P., Amer. Chem. Sot., Polymer Preprints, 33(l), 362 (1992) 279. Bruza, K. J., Bell, K. A., Bishop, M. T., and Woo, E. P., Amer. Chem. Sot., PolymerPreprints, 35(l), 373 (1994) 280. Rusanov, A. L., Bulycheva, E. G., and Bocharov, Polymer Preprints, 33(l), 9 16 ( 1992) 281. Wagner-Jauregg,

S. S., Amer. Chem. Sot.,

T., Synthesis, 349 (1976)

282. Armstrong, S. E., and Tipping, A. E., J. Chem. Sot., Perkins Transactions 1411(1975) 283. Nuyken, O., Maier, G., andBurger,

K., Makromolekulare

Chemie, 191,2455

K., Makromolekulare

Chemie, 189,2245

I,

(1990) 284. Nuyken, O., Maier, G., andBurger, (1988) 285

Nuyken, O., Maier, G., and Burger, K., Makromolekulare (1989)

Chemie, 190,623

286

Nuyken, O., Maier, G., Burger, K., and Serra i Albet, A., Makromolekulare Chemie, 190,1953 (1989)

287. Schmidt, H. K., Mat. Res. Sot. Symp. Proc. 32,327 (1984) 288. Schmidt, H. K., and Seiferling, B., Mat. Res. Sot. Symp. Proc., 73,739 (1986) 289. Schmidt, H. K.,J. NoncrystallineSolids,

73,681-91(1985)

290. Schmidt, H. K., Ttinker, G., and Scholze, H., German patent DP 30 11761,20 (Mar. 1980) 291. Schmidt, H. K., Scholze, H.,and Ttinker, G., J. NoncrystallineSolids,

80,557

(1986) 292

Schmidt, H. K., and Philipp, G., Glass: Current Issues, Volume 92, p. 580, (Wright & Dupuy, eds.), NATO ASI( 1985)

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293. Yoldas, B. E.,J. Mat. Sci., 12,1203 ( 1977) 294. Yoldas, B. E., J. Mat. Sci., 14,1843 (1979) 295. Yoldas, B. E., J. Noncrystalline 2%.

Solids, 5 1, 105 (1982)

Dougherty,T. K., Landis, A. L., and Lau, K. S. Y ., US Patent 4,93 5,479 (June 19,199O); ApplicationUS 73,025 (July 14,1987)

297. Ogura, K., andNakaboh,H.,U.S. Patent4,528,216 (July9,1985) Electric Industry Company, LimitedTokyo, Japan

issuedtooki

298. Wilkes, G. L., Orler, B. F., and Huang, H. H., Amer. Chem. Sot., Polymer Preprints, 26(2), 300 (1985) 299. Huang, H. H., Glaser, R. H., and Wilkes, G. L., Amer. Chem. Sot., Div. Polym. Chem., Preprints, 28(l), 434 (1987) 300. Mark, J. E., Jiang, C. Y., andTang, M. Y.,J. Appl. Polym. Sci, 29,3209 (1984) 301. Birchall, J. D., andKelly,

A., ScientificAmerican,

248(5), 104 (1983)

302. Holloway, J. G., Barth, II. W., andFahey, D. M., U.S. Patent 3,990,409 (issued 7 Dec. 1976); Chem. Abstr., 86,91245y (1976) 303. Available from Quazite Corporation,

Houston, Texas.

304. Sanders, H. J., Chem. Eng. News, p. 26 (July 9,1984) 305. Hefter, J., and Kenney, M. E., Amer. Chem. Sot., Symp. Ser., 194 (Soluble Silic.),319(1982) 306. Hefter, J., andKenney,

M. E., J. Amer. Chem. Sot., 103,5929 (1981)

307. Hefter, J., andKemrey,

M. E., Znorg. Chem., 21,281O (1982)

308. Usuki,A.,Kojima, Y., Kawasumi, M., Okada, A., Fukushima, Y., Km-au&i, T., and Kamigaito, O., J. Mat. Research, 8,1179 (1993) 309

Lan, T., Kaviratna, P. D., and Pinnavaia, Preprints, 35(l), 823 (1994)

310. Yamanaka, Y., andInoue,

T. J., Amer. Chem. Sot., Polymer

T., Polymer, 30,662 (1989)

3 11. Morikawa, A., Iyoku, Y., Kakimoto, M., and Imai, Y., J. Mat. Chem., 2,679 (1992) 312. Morikawa, A., Iyoku, Y., Kakimoto, M., andhnai,

Y., Polym. J., 6,107 (1992)

313. Kakimoto, M., Iyoku, Y., Morikawa, A.,Yamaguchi, Chem. Sot., Polymer Preprints, 35(l), 393 (1994)

H., and Imai, Y., Amer.

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and Thermoset

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4309(1992) 316. Martunmkakul,

S., Chen, J. I., Jeng, R. J., Sengupta, S., Kumar, J., and Tripathy, S. K., Amer. Chem. Sot., PolymerPreprints, 34(l), 711(1993)

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of Massachusetts,

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(1987) 318. Dickstein,

W. H., and Lillya, C. P., Amer. Chem. Sot., Polymer Preprints,

28(1),290(1987)

9 Silicones Rodney F. Patterson

INTRODUCTION

Silicones are a class of synthetic compounds, the molecules of which consist of polymer chains of alternately connected silicon and oxygen atoms. They are found in a variety of applications with requirements ranging from long life at elevated temperatures to fluidity at low temperatures. They are true polymers because their basic silicon-oxygen linkage, the “me?‘, is repeated along the polymer chain backbone many hundreds to thousands of times. Their Si-0 siloxane structure makes them chemically different, however, from the typical organic polymer materials whose backbone chain consists of all carbon atoms linked together. The silicone bond linkage is similar to the repeating inorganic structure found naturally in silicate minerals such as quartz, glass and sand, hence, the outstanding high-temperature properties of silicone rubber. This chemical makeup also accounts for the general inertness of silicone rubber products toward many deteriorating influences, such as ozone, chemicals, weathering, and radiation. Another contributor to this chemical inertness is the lack of double bonds in the molecular chain, a state called saturation. When a polymer is unsaturated, i.e., contains double bonds in its backbone chain (as do most organic rubbers), it can be attacked by ions or radicals such as ozone, 0,. 468

Silicones

469

Ozone can open up or cleave one of the two electron pairs of the double bond, a process known as oxidative cleavage, and, thereby weaken the polymer. Still another contributor to the inertness of silicones is their high covalent bond (electron pair bond) energy. The bond energy of its Si-0 linkage is 106.0kcal/mole, which is much higher than the 84.9 k&mole bond energy of the carbon-to-carbon bond of other organic rubbers. This difference in bond energies accounts for the superior stability of silicone rubbers. In general, they have much better heat resistance, electrical insulation, chemical stability, etc., than organic rubbers. Up to this point, we have represented the repeating unit or “me? of the silicone polymer as “S&O”to show that it is repeating. Each silicon atom, like carbon in its same column in the Periodic Table, has a normal valency of four, and, themfore, can connect to four adjacent atoms. It therefore connects not only to the two adjacent oxygen atoms but also to two other atoms and whatever atoms they correct to, forming two pendants coming off the main chain. This structure is represented as [R,SiO], where “n” is the number of times that the R,SiO structure is repeated. The two “R’s” represent the two pendant groups extending from the silicon atoms along the polymer chain. The “R” group or pendant is typically methyl (CH,), phenyl (C,H,,), propyl (C,H,), or a combination thereof. The polymer chains are connected to one another (to an adjacent chain or chains) by covalent bonds and so are crosslinked into a threedimensional structure. All of the crosslinking takes place at the ends of the polymer chains. The reactive groups at the ends are either hydroxyl (-OH) or vinyl (-CH=CH,) groups. They react with the crosslinking agent and sometimes a catalyst to give a cured elastomer. Let us look Gtrtherat the silicon atom as compared to the carbon atom concerning their roles and differences in polymer backbones. The silicon atom is below carbon in its column in the Periodic Table, indicating that it is less electronegative than carbon. This means that its bonds with carbon and oxygen are less covalent and partly ionic. This polar nature of the bonds, together with the larger size of the silicon atom, may accotmt, in part, for the great freedom of motion and flexibility of the -Si-O-Si- bond. As may be expected, it was found that very free rotation of methyl and bulkier alkyl and atyl groups about the silicon-oxygzn bond persisted even at low temperatures. The very free rotation about bonds attached to silicon, including the Si-0 bond, is a contributing factor to the unusual properties of the -Si-O-Si-

470


chains. Also, the premise of weak intermolecular forces between the “polysiloxane” chains, as they am called, is used [‘Ias a partial explanation for the viscosity characteristics of silicone fluids, as well as their low freezing points, remarkably low second-order transition temperatures (i.e., Tg’s), and very low boiling points. As we have said, silicones are polymers consisting of silicon and oxygen atoms covalently bonded as linear long chains of inorganic siloxanes (-Si-0-Si-). Their properties can be varied by introducing organic side groups, i.e., “pendant”, on the silicon atom. The phenyl group, for example, provides enhanced low-temperature properties. For another example, the trifluoropropyl group provides excellent fuel and solvent resista.nce.t*] The form in which the polymers can be produced ranges Tom fluids to rubbers to lubricants, and to hard and soft coatings and resins. This chapter first takes up silicone fluids that include polishes, release agents, surfactants, and dielectric fluids. (Silicone fluids are the lowest in molecular weight among the silicone polymers.) Silicone rubbers are taken up next with consideration of both room-temperature vulcanizing (RTV) and heat curing systems. Our coverage of RTVs includes both onecomponent and two-component systems with the different cure chemistries of each, followed by a separate discussion of silicone laminates.

SILICONE FLUIDS

Commercially available silicone fluids are principally linear dimethylpolysiloxanes and methyphenylpolysiloxanes. Each of these is discussed in some detail later in this section. First, the effect of changes in silicone fluid molecular weight, temperature, viscosity, and compressibility is diSCUSSed. When silicone fluids

are made into a polymer, they differ according to the degree of condensation cure and viscosity attained; this, of course, relates to the molecular weight attained. As one might expect, there is an algebraic relationship between the two. For molecular weights greater than 2,500, viscosity q, in centistokes is given by the expression log q= 1.00 +O.O123M where M is molecular weight. An illustration of how physical

Silicones

471

properties vary with viscosity (and molecular weight) is given in Table 9- 1 for methylpolysiloxane fluids.[*ITable 9-l shows that viscosity can vary between 5 and 2,500,OOOcSt. The physical properties of the fluids exhibiting a viscosity of 300 cSt or greater vary only slightly if at all. Fluids with a viscosity of 20 cSt or greater exert a vapor pressure of about 0.01 mm mercury at 2OOOC.In addition, their flash point is greater than 200°C to greater than 3OOOCfor fluids with a viscosity of 200 cSt or greater. These properties are obviously well suited to various high-vacuum and electrical applications. Also, the methylpolysiloxane fluids are characterized by low freezing points and low viscosity-temperature coefficient constants.[‘l More detailed electricalproperties of one of the most commonly used commercial products of the methylpolysiloxane type, are presented in Table 9-2. The table shows the properties of the fluid varying hardly at all for all viscosities at and above 100 cSt. Table 9-3 compams the effect of fluid type on the change of viscosity with temperature. Over the temperature range of -25°C to 120°C, the silicone fluid changes 16 fold and the mineral oil changes 1000 fold. In relative terms, silicone fluids may be characterized as having a comparatively flat temperatureviscosity slope versus that of organic fluids. This property is of prime importance when choosing a coolant because it allows pumping pressures and rates of fluid flow to remain more constant over a wide range of tempemmres.[*ITable 9-l shows that the viscosity-temperature coefficient increases only slightly with an increase in the viscosity of the starting fluid; the less viscous product exhibits a smaller change than the more viscous product. To better understand the use of certain silicone fluids as hydraulic and dash pot damping fluids, a discussion of their rheological behavior is in order. Fluids with a viscosity of 1000 cSt or less are characterized as having Newtonian behavior for shear rates (as in hard stirring) up to about 10,000 WC-‘. For fluids with a viscosity greater than about 1000 cSt, Newtonian behavior is exhibited below a certain level of shear rate; the higher the initial viscosity of the fluid, the lower the level of shear rate for the onset of “pseudoplastic” behavior. In other words, them is a critical value of shear rate beyond which the behavior is described as “pseudoplastic” (apparent viscosity is less than the initial viscosity extrapolated to a zero gradient). This change is reversible. The resistance of dimethyl silicone fluids, in particular to intense and prolonged shearing, finds application for these materials as

50

0.59

0.959

280

-55

20.7

1x10’*

1.05~10”

2.8

15

100

0.60

0.965

>300

-55

20.9

1x10’2

0.95~10~~

2.8

16

300

0.62

0.970

>300

-50

21.1

1x10-2

0.95x10”

2.8

16

500

I 0.62

I 0.970

I >300

I -50

I 21.1

I 1x10-*

I 0.95~10”

I 2.8

1 16

1000

0.62

0.970

>300

-50

21.1

1x10’2

0.95~10’~

2.8

16

5000 to 2,500,OOO

0.62

0.973

>300

-45

21.1

1x10”

0.95~105

2.8

18

Viscosity/temperaturecoeficient = 1 - (viscosity at 99”C/viscosity at 38’C) At 200°C Volume coefficient of expansion between 25”and 100°C Between 0.5 and 100 kHz

(3) (4) Data taken from Reference 1. Reprintedwith permissionof Rhone-Poulene h.

96KUO

osv tlo1 xc0 tro1 x0’s

MOOO’O SKCO'O

LLZ LL’Z

L

96000’0

osv WOIx ID t,OI x 072

ZOOOO’O 8oooO'O

9L‘Z 9L’Z

96ooo’O 00 ,101 x I ‘0 NOI x o’z

96ooo’O 0-a t,OI x I.0 WOIx 0’1

%OOO.O ot3 HOI x 1‘0 tro1XO'I

po100’0 OZP *loI x 1'0 trO1XO'I

Lo100

ozv

ozt HOI x 1'0 HOI XO'I

s1ooo‘o s1Owo

MOOO'O SCN@'O

ZIXWO SCCWO

ZOO000 SWOO'O

ZOCWO 01ooo'0

WOI x 1'0 ,I01 x 0’1

S9'Z 59'2

9L’Z 9L’Z

ZHhlO’O 51000'0

S91

9IE

01

0%

474


Table 9-3:

Effect of Fluid and Temperature

on Viscosity

Viseosit~. cSt. at Oil TWX

-25°C

25°C

m

Rhodorsil oil 47v 100

350

100

22

Mineral oil

5000

100

5

Data taken fkom Reference 1. Reprinted with permissiin of RhonePoulenc Inc.

hydraulic and damping fluidst’l Silicone fluids of differing viscosities may be blended to obtain a fluid of some intermediate, nonstandard viscosity. The fluids selected for blending should be those with viscosities closest to the desired intermediate viscosity.t’] Because the blend will have a broader molecular weight distribution than the starting materials, the physical properties of the blend may be different, except for viscosity at a low rate of shear. The Dow Coming Corporation, refening to its Dow Coming 200 dielectric fluid described in Table 9-2, has stated the following in its literature about the fluid’s various insulating uses: “Dow Coming 200 dielectric fluid is also employed as a liquid dielectric in transformers, capacitors, filter networks, and high voltage power supplies; as a damping fluid in the dash pots of time delay relays; as a coating for glass and ceramic electronic parts to maintain high surface resistivity; and as a lubricant for plastic surfaces. In addition, dielectric fluid of all viscosities is used as a moisture repellent lubricant for clocks, timers, switches, television tuner contacts, and other electronic devices.“t3f

Dimethyl Types

Dimethylsilicone fluids can be used over a temperature range of -60°C to 200°C at atmospheric pressure without risk of gelling. Moreover, in the absence of air, these products with stand exposure at 250°C for several

Silicones

475

hundreds of hourst’l Dimethylsilicone fluids are insoluble in water, low molecular weight alcohols and glycols, and higher molecular weight hydrocarbons such as petroleum, vegetable oils and fatty acids. They are soluble in hydnxarbons (hexane, heptane, benzene, xylene), including chlorinated hydrocarbons and higher molecular weight ketones, such as methyl ethyl ketone. Solubility is, in fbct, a function of the viscosity of the products. Low-viscosity fluids may provide limited solubility in solvents in which high-viscosity oils are completely insoluble. An unusual feature of dimethylsilicone fluids is their high compressibility. Table 9-4 shows that a 100 cSt fluid may be compressed about 15% at 3,500 kg/cm2 without becoming solid. Compressibility decreases as the viscosity of the fluid increases. The compmasibii of silicone fluids is greater than the degree of compression of mineral oil. This is probably related to the freedom of rotation of the substituents about the silicon atom.[*l

Table 9-4: Compressibility

t

of Dimethylpolysiloxanes

Reduction in Volume, %

Applied

oil 47v,

PlWSWe, K&m’

100 cst

oil 47v, 1000 cst

Mineral Oil

I

I

I 4.5

I 3.8

I 3.1

1,000

7.3

6.5

5.2

2,000

11.2

10.7

6.8

3,500

15.1

1 14.4

500

-

DatatakenCorn Refaence 1. Reprinted with permission of Rhone-Poulenc Inc.

I

I

Methylphenyl Types

Partial replacement of the methyl by bulkier phenyl groups imparts better protection of the Si-0-Si chain through stearic hindrance. This repla-

476


cement results in less susceptibility to attack of the backbone by oxidizing agents and improvements in the thermal stability of the polymer, as well as compatibility with slightly polar organic groups. The bulkier groups interfere with the Mm of rotation of the C-Si bond about the Si-0-Si bond. Table 9-5 shows that these effects are reflected in increased viscosity-temperature coeficients and higher freezing points for methylphenylpolysiloxanes as compared to the methyl derivates of the same viscosity. Compressibility of the phenyl derivatives is also less than the methhylpolysiloxanes. For example, at an applied pressure of 1,000 kg/cm2, the 125 cSt and the 500 cSt fluids compress about 5.2 volume %, and 4.2 volume %, respectively.[‘] The 125 cSt fluid is stable to oxidation and radiati0n.u’ The fluid is suggested for use between -50 to 250°C and is not affected a&r 1,000 hours heating at 250°C in air, except that contact with lead should be avoided. Lead is a catalyst that would alter the physical characteristics of the silicone fluid should contact occur. In contrast to the methylpolysiloxane fluids which offer relatively poor resistance to radiation, the methyl phenyl-polysiloxane materials withstand irradiation of 150 Megarads at ambient temperature. They am used as a heat transfer medium for metal treatment baths, a dielectric coolant and lubricant for plastic gear systems, among others. Silicone fluids can be prepared containing substituents that are attached to the phenyl group and impart reactivity to the fluid. This also improves bondability or attraction. For example, methylchlorophenyl polysiloxanes are used in lubrication applications under severe conditions.[‘l The chlorine group enables a chemical bond to be effected between the lubricant and the metal surfaces. The chemical bond is stable at high temperature, and the phenyl group confers resistance to high-temperature degradation. The fluid is recommended for use in steel/steel lubrication and in hydraulic systems operated at high or low temperatures.[‘l Other Fluid Types/Copolymers

Controlled chemical reactions can provide copolymers of polysiloxanes with organic intermediates. Copolymers may be obtained

Table 9-5: Approximate Physical Properties of 25OC of Methylphenylpolysiloxane (Rhodorsil Oils)

Fluids

Viscosity cst

VTC”’

Specific Gravity

Flash Point “C

Freezing Point “C

SUrfaCe Tension DYNES/CM

VCE Vapor Pressure, cm3 mm. Hg (2) cm-’“C

Dielectric Gonstant

Dielectric Strength, kV/mm

125

0.76

1.065

300

-50

24.5

1X10-2

0.75X105

2.9

14

0.77x10”

2.95

14

500 (1) (2) (3) (4)

0.86 1.103 300 -22 28.5 40x10-2 Viscosity/temperature coef%icent = 1 - (viscosity at 99”C/viscosity at 38°C) At 200°C Volume coefficient of expan: on between 25” and 100°C Between 0.5 and 100 kHz

Data taken from Reference 1. Reprinted with permission of Rhone-Poulenc Inc.

478


which are polycondensates of ethylene oxide and/or propylene oxide and polysiloxanes (Figure 9-l).

Figure 9-1: Schematic representation of polysiloxanes and polycondensates of ethylene oxide and/or propylene oxide. (Data taken from Reference 1. Reprinted with permission of RhonePoulenc Inc.)

This part-silicone, part-organic structure exhibits very unusual surface tension properties in different media and is used, for example, to control the cell structure of urethane foams. The low-surface-tension characteristicof silicone fluids is translated into consumer markets such as car and fbrnitum polishes and into such industrial applications as a mold release agent for plastics, metals, and elastomerst’l

SILICONE RUBBERS

Silicone rubbers are elastomers based on high molecular weight linear polymers, generally polydimethysiloxanes, which also may be modified with functional groups. They are available in the form of liquid or paste consistencies as room-temperature-vulcanizing (RTV) sealants, adhesives, potting, and encapsulating compounds, and also as gums, bases, and compounded stocks (dough-like) for fabricating heat-cured rubber products. The RTV silicone rubbers are discussed first followed by the heat curing silicone rubbers. The discussion of the RTV silicones will be divided between those that am supplied as one-component systems and those that are supplied as two-component systems, differing as they do in their cure chemistries.[4]

Silicones

ROOM-TEMPERATURE-VULCANIZING

479

SILICONES

One-Component Systems

One-component silicone rubbers use moisture in the air to hydrolyze a functional group and provide sites for the formation of a network (crosslinked) structure of the Si-0-Si bonds. When the crosslinking is completed, each molecular chain of Si-0-Si bonds is connected/crosslinked to an adjacent molecular chain (at the provided-for sites) such that all the molecular chains are connected to one another by covalent bonds. This The silicone structure, of course, is what makes it a thermos& one-component systems are unique, however, in that their cure to a crosslinked structure is initiated by an external air-borne substance (moisture), which means that the degree of cure obtained is dependent on the silicone’s cross-sectional thickness. A thickness of l/4 inch is considered to be a limit because cure time (to crosslink) increases with thickness as measured from the exposed surface or edge inward. Thus, where broad surf&es are to be mated with the silicone adhesive/sealant, it should be applied in a thin ribbon or bead (less-than-l/4-inch-wide ribbon or less-than l/4-inch-diameter bead), around the edge of the surface to be bonded. For applications where section depth must exceed l/4 inch, two-component RTV silicone rubber compounds are recommended.t41 Generally, there are considered to be two different variations of the moisture vapor cum chemistry utilized by the one-component silicone rubbers, nominally called acetoxy and alkoxy. I41Their designations refer to the cure by-products generated during the curing process, namely acetic acid and methyl alcohol, respectively. Two other lesser known cure chemistries are called oxime and acetamide, respectively. The oxime cure chemistry/by-product is said by Dow Coming to be best for solvent dispersions as a spray or a dip coating. It requires a primer for a reliable bond to all substrates. ~1 The acetamide cure chemistry, on the other hand, does not require any primer for an excellent and reliable bond to a wide variety of substrates. It makes cured silicones with very low modulus, i.e., very high elongation together with low tensile strength. It is therefore best where the cured silicone is used to seal joints that must have wide mov-

480


ement.[”

Acetoxy and alkoxy cure chemistries, while similar in that each requires atmospheric moisture to effect a cure, differ in cure speed, rate of evaporation of cum by-products, and in other ways such as, but not limited to, odor, cur&on potential, tack-free time (maximum work life), and adhesion. Table 9-6 presents key differences between the two cure chemistries.

Table 94

Effixt of Hydrolyzable Cure Chemi ‘try Type on Properties of One-Component Silicone R lbbers AlXtoxY

I Task-tiee time, hr, 25 “C (77 OF),50% RH

l/4-1/2 I 24-72 I

Glass and ceramic

I Excellent

Aluminum

I Excellent

Comer

I

I

Plastics Polycarbonate

I Not AmSable I bod

ACryliC

PVC

Fair

Odor during cure

Acetic acid, pungent

Corrosion potential

Moderate during cure

Data taken from Refenznce 4. Reprinted with permission of ( enezal Electric Corp.

Silicones

481

The one-component system is limited to end uses exhibiting a thin cross section because the cure depends on the diffusion of moisture into the system and the release of acetic acid or alcohol for products using the acetoxy or alkoxy cure system, respectively. The cure process 141begins with the formation of a skin on the exposed surface and progresses inward through the material. At 25°C and 50% relative humidity, acetoxy products will form a tack&e skin in 10 to 45 minutes, Table 9-6 shows that alkoxy products will form a tack free skin in 2 to 4 hours. High temperature and high humidity acceleratethe cure process; low temperatures and low humidity slow the cure rate. Products using the acetoxy or alkoxy cure systems will usually be free of acetic acid or alcohol, respectively, within one to three days of application, with 7 days being more than adequate.t31 One-component RTV silicone rubbers are used in applications such as formed-in-place gasketing, sealants, adhesives, and other uses involving bonding to a variety of substrates. Table 9-7 shows typical properties of a general purpose alkoxy system. High-strength alkoxy systems are also available from silicone manufacturers. At only slightly higher, hardness they have roughly four times greater tear strength and three times greater peel strength. Acetoxy systems have lower reported peel strengths, but they are o&n preferred because of their much faster task free times when work must be moved to the next process station as soon as possible. Note that the physical properties in Table 9-7 are stated to have been taken after 3 days cure at 25OCand 50% relative humidity. This time period is adequate for handling purposes, including bonding, but physical properties develop as the RTV cure progresses, which may take several weeks. Note that the electrical properties are given after 7 day cure. Superior adhesion is obtained between the RTV silicone system and a number of metal, plastic and other substrates, as shown in Tables 9-6 and 9-7. In many instances, preparation of the surfaces for bonding involves wiping with a clean, oil&e rag wetted with a solvent such as mineral spirits, naphtha, or ketones, and then allowing the surface to dry thoroughly before applying the primer or the adhesive/sealant. In other instances, the surface may need to be abraded with the aid of a wire brush or a sandblast. After the residual dust is removed, a solvent wipe is recorntn~ded. It is also recommended that any solvent wipe&aning of a surface for bonding include wiping dry so that solvent is not allowed to just evaporate and leave residue in its place. For more exact surface preparation instructions, including primer

482

Handbook of rtrermoset Plastics

Table 9-7: Typical Properties of a General-Purpose Silicone Rubber As Supplied Color 1 Flow, sag or slump, in.

Alkoxy RTV I 1 Black. White

Extrusion rate (l/8 in. Orifice, 90 psi), grams/mitt.

1 0.1 1.51 300

Cure Characteristics - exposed to air, 77” F (25°C) and 50% RH Skinaver time, min. Working time, min. Task-free time, hr Cure time (118 in. Thickness), hr.

I I2Oto30 30 2to3 24

specific gravity

Physical Properties - cured 3 days at 77°F (25’C) and 50% RH Durometer hardness shore A, points Tensile strength, psi Elongation, percent Tear strength, die B, ppi Adhesion, lap shear (to glass and aluminum), psi Adhesion, peel stmngth, * lb/ii width

I

I

28 150 550 27 100 20

Electrical Properties - cured 7 days at 77 “F (25 “C) and 50% RH Volume resistivitv, ohm-cm I 4.7 x 10’4 Dielectric strength, V/mil 500 Dielectric constant at 100 Hz 3.6 Dielectric constant at 100 KHz 3.6 Dissipation factor at 100 Hz 1 0.0021 Dissipation factor at 100 KHz 1 0.0010 i * Laboratory tests and market tests have demonstrated superior adhesion to many subsnates, including : Plastic surfaces-acrylic, polycarbonate, polyvinylidene fluoride, polyvinyl chloride, polystymne, and acrylonitrile-butadiine-styrene Metals-milled aluminum, anodized aluminum, steel, galvanized steel, and stainless steel Other-glass, wood, cement, and painted surfaces Note: Test placement prior to general use is recommended. l

l

l

Data taken f?om Reference 5. Reprinted with permission of Dow Coming Corp.

Silicones

483

recommendations, the rnanufacmmr of the silicone adhesive/sealant should be consulted. Some primers are organoiimctional silane derivatives that are matched to and react with the substrate, leaving a modified surface that can bond (with the same reacted molecule) also to the silicone rubber.w The primers that ate usually used to make most nonsilicone surfaces bondable to silicones are the hydrolyzable silicone primers. They act in a way similar to the silane derivative primer. They react with the nonsilicone substrate and, with the other end of the same molecule, react with the silicone adhesive sealant when it is then applied! Directions for applying these primers, as well as the primers themselves, may be obtained from the supplier of the silicone adhesive-sealant being used.

Two Component Systems

Two-component RTV silicone rubber systems enjoy a major advantage over the one-component systems in that they do not require moisture to cure. Their ability to cure does not depend on being thin enough in cross section for the moisture in the air to penetrate. Their relative d&advantage,of course, is that their two components, the base and the curing agent, require skillful weighing out and thorough blending. The base portion consists of silanol terminated polymers that need to be mixed with a curing or crosslinking agent to effect a cure. They utilize either of two cure system chemistries, referred to as condensation cure and addition cure.t61 The condensation cure system consists of a base compound that may be mixed with any one of several interchangeable curing agents and in varying proportions, depending on the mixing methods employed (hand or machine) and the application and cure time required for the production cycle. Following addition of the curing agent, cure takes place at room temperature to form a moderate strength, durable, and resilient silicone rubber. Cured section depth of v&ally any thickness is possible with proper selection of the curing agent. Condensation cure RTV silicone rubber compounds are recommended for applications that am not completely sealed prior to full cure because moisture and an escape path for cure by-products are required to complete the cure. Addition cure products are generally suggested for completely sealed assemblies.

484


Condensation Cure

Separate interchangeable curing agents for condensation cure products are normally metallic soaps. Dibutyl tin dilaurate (DBT), the standard curing agent, is generally pref4 for most applications. DBT cures at moderate speeds. It is an essentially colorless liquid curing agent that is added in a very small amount (medicine dropper proportion) to the base compound It is easy to use, requires no sophisticated measuring equipment, and is recommended for hand mixing operations and small- volume applications. For hand or machine mixing, greater accuracy, and therefore improved control over the curing rate, can be achieved by using paste curing agents that are based on tin soaps as the active ingredients. Paste curing agents offer color contrast with base compounds for visual determination of thorough mixing. They are designed for a 1O:l (base compound to curing agent) mix ratio, suitable for automatic mixing equipment. Paste curing agents used with automatic mixing equipment minimize pot life problems, reduce material waste, save time and manpower, and are ideally suited for large-volume applications. Stannous tin octoate (STO), a clear, colorless liquid curing agent, provides a faster cure than DBT. It is recommended for use in small-volume applications and short production cycles where rapid production of cured parts is required. ST0 provides a RTV silicone rubber cure in approximately 30 minutes. While careful measuring is necessary to ensure proper cure characteristics, only a small portion of curing agent is added to the base compound and no sophisticated measuring equipment is needed. Because of the fast cure rate of STO, however, work time is reduced considerably, and the catalyzed material must be poured or applied immediately after curing agent addition and thorough mixing. Deep section cure greater than 25 mm (1 in.) is possible with ST0 curing agent but a specially formulated paste curing agent is usually preferable because it permits longer working time. Condensation cure products will cure in contact with virtually all types of materials without cure inhibition. When adhesion to non-silicones is desired however, a primer should be used. Condensation cum RTV silicone rubbers are available in a wide range of viscosities characterized as an easily pourable product of 12,000 cp to a

Silicones

485

pastelike product of 600,000 cp. Three examples of methylphenyl products that are offered for extremely low-temperature (-175°F) and hightemperatures (400’ to 500’F) range applications are given in Table 9-8. Some suggested uses for the products include potting materials for airborne electronic assemblies, aerospace materials for mechanical applications, thermal insulation ablative material, and ~ealants.[~l

Addition Cure

The addition cum systen@l consist of two-component RTV silicone rubber compounds supplies in prepackaged premeesured kits (typically 10: 1 base compound to curing agent ratio). Each base compound (A portion) has its own specific curing agent (B portion). Use of addition cure products thus eliminates the need for curing agent selection, ensures uniform quality, and allows simplified inventory control. Some addition cure RTV silicone rubber products offer high-strength properties, and all of them offer reliable noncorrosive, deep-section cure because there are no cure by-products. They readily cure in sections of unlimited depth, even in completely enclosed assemblies. Cum may be achieved at room temperature without exotherm or may be accelerated with heat. More rapid cure with elevated temperatures allows higher unit production in shorter cycle time and reduces storage space required during cure. Addition cure products, are, however,, susceptible to cure inhibition caused by the surface contaminants present in some materials. For example, a primer coating may be needed to minimize possible cure inhibition. Use of a primer is also recommended to obtain adhesion to nonsilicone materials. Addition cum silicone rubber products ate available as low viscosity systems which will flow freely in and around complex parts providing electrical insulation and shock resistance. Unfilled systems can cure to a soft transparent gel or to a tough transparent rubber (Table 9-9). They are useful where clear rubber is required such as solar cell potting (Figure 9-2), optical instrument applications, and windshield interlayers. Besides using Tables 9-8 and 9-9 to compare typical properties of condensation cure verses addition cure silicone rubber products, Table 9- 10 summarizes their different featwes. (Note: In Tables 9-8 and 9-9, viscosity

Table 9-8: Typical Properties of Condensation Cure Methylphenyl BTV Silicone Rubber Products

Iatataken kiorn Reference 6. Reprinted with permission of General Electric Corp.

Table 9-9: Typical Properties of Additional Cure Clear RTV Silicone Rubber Products

Data takenfirornReference 6. Reprinted with permission of General Electric Corp.

Silicones

487

Table 9-10: Features of Condensation Cure Systems Versus Addition Cure Systems in the Two-camp nent Silicone Rubber Family” Condensation Cure &stems

Addition Cure Svstems

1. Alcohol condensate released during cure 2. Can cure in deep sections ifhave moisture access and alcohol escape PathS 3. Virtually no cure inhibition, i.e., cures in contact with vhtually all types of materials

1. 2.

No by-products released Can cum in completely sealed assemblies

3.

Cure inhibition likely when cured in contact with certain mater&* - see applicable silicone supplier for details and recommendations for appropliatebarriercoatingtouse RTV base is usually supplied with the necessary curing agent in pre-measured kits of A and B components. Available in consistencies ranging from low viscosity, free-Rowing systems to re@ively high-viscosity pourable systems and soft gels Cure may be either room temp. cure or heat accelerated

4.

Choice of several curing agents allows adjustable work time and cure rate for a given RTV silicone base compound

4.

5.

Available in consistencies ranging nom easily poumble to paste

5.

6.

Need elevated temp. cure to allow volatiles to escape and prevent reversion Moderate strength; durable and resilient

6.

High tear strength and tough grades available, allow use in tlexible molds. Because tney use a ptatmum cataty$ addition cure silicones will not cure after coming in contact with vinyl plastics; synthetic and natural rubbers; sulfiu+ontaining mate&& such as polysuhides; tin soaps; certain epoxies containing strong amine cata@sts; some clays, woods, leathem tape adhesives and heatxured rubbers; and chlorinated substances such as neoprene. These materials form a strong complex with platinum thus deactivating it! 7.

7.

is given in centipoises not in centistokes.

HEAT CURED SYSTEMS

Raw or unvulti silicone rubber called gum stock, is supplied to parts manufacturers in a state ranging from a soft to a relatively stiff dough-

488


Figure 9-2: Potting of solar cells using RTV 655. (Reprinted by permission of General Electric Corp.)

like consistency. [*I Before fabrication, the gum stock generally needs to be compounded on a two-roll mill or in a Banbury mixer with vulcanizing agents, extending and reinforcing fillers and special additives to tailor the properties of the fmished products or to fabricate products that meet various industry specifications. Silicone rubber parts can be produced in a broad range of sizes and shapes using rubber processing techniques, such as compression, transfer or injection molding, extrusion, and calendering. Hence, large size production volumes are no problem; although the manufacturing equipment for the rubber parts is quite expensive. Silicone rubbers are grouped by polymer type and performance characteristics. Polymer classifications are based on the organic group side chains attached to the silicon-oxygen chain, methyl groups being either alone or in combination with vinyl, phenyl, and fluoride-containing groups.t91When classified by performance characteristics,[loIsilicone rubber is available in four basic types: general purpose (methyl or methyl and vinyl), high performance (methyl and vinyl), extreme low temperatures service (phenyl and methyl or phenyl, methyl and vinyl), and solvent-resistant rubbers (vinyl, methyl, and fluorine-containing groups).

Silicones

489

Compounding

Compounding provides a means of producing high quantity rubber products in an efficient manner. ~‘1 The approach can be divided into three categories: extending, blending, and modifying. Extending is the process of adding semi-remforcing fillers, such as diatomaceous earth or ground quartz, to a silicone rubber product to reduce cost, increase hardness, or increase fluid resistance. Extending fillers are often used together with reinforcing fillers. Reinforcing fillers are employed to make silicone rubber products with optimum physical ptoperties. Specific reinforcing fillers are often chosen for special applications: fume process (pyrogenic) silicas for the strongest vulcanizates and the best retention of electrical properties under wet conditions, wet process silicas for a low tendency to creep-harden, and carbon black for electrically conductive stocks. The enhancement of specific features of a formulation is carried out by blending and/or modZying.ullTwo or more silicone rubber gum stocks may be blended to provide a rubber with different processing characteristics or properties, such as “green” strength or lower compression set. Modifying is the process of improving an aspect of a formulation by incorporating small portions of the modifier into a rubber. Depending on the type used, the modifier can enhance specific traits such as flame retardancy, high temperature stability, internal mold release, or improved shelf life, among OtherS.

Curing

The curing of the compounded silicone rubber usually occurs in the presence of peroxide vulcanizing agents at elevated temperatures.r’21 Table 911 lists some typical curing agents that are recommended for various uses. Molding temperatures vary from about 100 to 180°C, depending on the method of processing used and the physical dimensions of the vulcanized product. In addition, the selection of a curing agent is related to the polymer type and desired properties of the finished product, among others. Specific applications require the use of air oven post cures.

490


1

Peroxide

rble 9-11: Typical

‘eroxide

bring Agei

Commer

Amount Added %

Typical molding Temperature

US?

50% active

1.2

104-132°C (220-270°F)

Hot air VulCanimtion

50% active

0.8

116-138“C (240-280°F)

Mokting steamcuring

DiCup

40%

1.0

4oc3

active

154-177°C (3 lo-36O’F)

Molding thick sections, bonding, steam curing

0.8

166-182’C (330-360°F)

Molding thick sections, bonding

Form

Cid

Bis (2,4 Dichkxobenzoyl) penKide

cadox

TS-50’

Recommended

Or

Luperco CST’ Benzoyl peroxide

cadox BS’ Or

Lupelw AST2 Di Cumyl peroxide

25 Dimethyl2,5 Di (t-butyl Peroxy) hexane

VarOX

50%

Or

active pasteor

Luperco 101~XL2

100% active paste

0.4

steamcuring

rs b&m& of and available tbm Noury Chemical Corporation, Route 78, Burt, New York 14028 ?mdemark of and ava&ble fbm Lucid01Division, Pennwalt Corporation, 1740 Military Road, Buf, New York 14240 Tnubmadr of and available Rem Hercules Powder Company, 9 10 Market Street, Wilmington, Delaware. 19899 ‘Trademark of and available fmm R.T.Vanderbilt Company, 30 WinfEld Street, E. Norwalk cOMCCtiCUt 06855 Data Taken ti Reference 12. Reprinted with permission of General Electric Corp.

Silicones

491

The peroxides are ideal curing agents because they are stable in the polysiloxane composition at room or moderate temperatures, becoming active crosslinking agents only above their decomposition temperature. They decompose quite rapidly at the decomposition temperature. The resulting free radicals activate some of the CH, groups by hydrogen radical removal and the mmlting methylene radicals, attached to silicon, can combine to form ethylene crosslinks. These products are generally fabricated by specialized rubber fabricators and are not generaliy used as adhesives. ~1

SILICONE LAMINATES

The properties of the finished silicone rubber depend on the type of gum, filler, curative, modifiers, and solvents used, if any. Solutions or solvent dispersions of silicone rubber are used in the fabrication of laminates comprised of sheets of silicone solid rubber reinforced with glass cloth; normally, both sides of the fiber glass are coated.[‘l The silicone coated fabrics are thin and tough, dimensionally stable and flexible. Their many applications include belting, vacuum blankets, press pads and diaphragms (Figure 9-3). If three plies of fiber glass are used, the laminates show the extra rigidity and breaking strength needed in extremely high-pressure applications. The properties of the laminate may be modified by varying the glass style and rubber formulation; special constructions include one-side coated and alternate base fabrics. The most outstanding property of silicone rubber is its great resistance to temperature extremes. [131When compared with many popular organic rubbers at room temperature, silicone rubber is relatively weak. Under normal operating conditions, however, at temperatures as high as 500’F and as low as - 150°F, silicone rubbers stay elastomeric and flexible. The estimated useIkl life of silicone rubber at elevated temperatures is shown in Table 9-12. Useful life is defined as the period of time during which the rubber retains an elongation of 50% or more. The results indicate that parts are expected to be serviceable for 10 to 20 years at 250°F. Extrapolation to nom-& operating temperatures indicates a very long life for silicone rubbers. The aging resistance of silicone rubber is superior to organic-based rubbers.

492


Figure 9-3: A multilayer layup showing COHRlastic@silicone rubber press pad material on both top and bottom. (Reprinted by permission of CHR Industries, Inc.)

Silicone polymers have inherently good electric insulating properties. They are nonconductive because of their chemical nature and, when compounded with proper fillers and additives, are used for a wide range of electricalinsulating applications. As a rule, rubber compounded for optimum retention of physical properties after heat aging will also show optimum retention of electrical after heat agingJ8J Silicone rubbers swell when immersed in various liquids, but solvent resistance usually improves as curing time or temperature increases.[8]The results indicate that the degree of swelling and the degree of cross linking are interrelated. Undoubtedly, swelling is related also to the difference between

Silicones

the cohesive energy densities of the rubber and the liquid. Table 9-12: Estimated Useful Like of Silicone Rubber at Elevated Temperatures Service Temperature

UsefUl Life*

250°F

10to20years

300°F

5to10years

400°F

2 to 5 years

500°F

3 months to 2 years

500”-600°F

1 week to 2 months

600” -700°F

6 hours to 1 week

700”-800°F

10 minutes to 1 week

800”-900°F ’ Retentionof 50% elongation.

2 to 10 minutes

Data taken fhm Reference 13. Reprinted with permission of General Electric Corp.

TRADE NAMES

493

494


GOVERNMENT

SPECIFICATIONS

FOR SILICONE PRODUCTS

Note. Some of the specifications listed have an associated Qualified Products List (QPL) showing each product and the manufacturer which the government has found to meet the associated specification. The QPL makes it easy to know what product to purchase when a particular specification is imposed, and the specification requirements make it easy to know the minimum or guaranteed properties to expect from a QPL-listed product. If a product is needed to meet a specification which has no QPL, a silicone product manufacturer will ofkn have a product which he is able to certifL as meeting the specification. In the following specification lists, a letter Q in parenthesis appears to the left of those that have a QPL, and a letter C in parenthesis for those that do not but may be expected to have certifiable products. The title of the specification is underlined; it may have been reworded for clarity. 1. Fluids (Q) Mil-B-46176, (C)W-D-1078, (C)W-I-% 117,

Brake Fluid, Silicone, Automotive Damning Fluid (Dimethvl nolysiloxane) Insulating Fluid, Electrical. Silicone

2. Varnishes (Q) MIL-I-24092, Comp. I, Grade CB,

Clear Baking Varnishes, DiD ProceSS

3. Primers

0 MIL-A-46 106, 0 MIL-A-46 146,

Primer (manufacturer will supply with the MIL-A-46 106 adhesive-sealant) Primer (manufacturer will supply with the MIL-A-46 146 adhesive-sealant)

4. Coatings (Q) MIL-I-46058, Type SR,

Insulating Comnound, Electrical (for Coating PC%)

Silicones

ME-A-46 106, Group I, Type IL Q ME-A-46146 Group I, Type II 0 ME-R-472 11, Type IV

495

0

Self-levelinp Liauid (Acetoxv) Self-leveling Liauid (Alkoxvl Rubber, Silicone, Room Temperature CWiIl~

5. Lubricants (Q) M&G-6032, Type I, bulk; Type II stick form,

Grease, Plug Valve

Molybdenum Disulfide Base 0 M&M-7866, Lubricant, Solid Film, Heat Cured (Q) MIL-L-8937, (Q) ME-L-460 10, Lubricant, Solid Film, Heat Cured 6. Heat Sink Compounds Silicone ComDound (Q) M&S-8660, (Q) m-C-2 1567, Compound Silicone, Soft Film Q ME-c-471 13 ComDound, Heat Sink Type I, 7. Grease-Like Lubricants Grease. Pneumatic Svstem O-ring (Q) M&G-4343, (Q) ME-L- 157 19, Grease (Hiah Temperature) Motor Ball Bearing (Q) ME-G-2761 7 Type Grease, .Aircraft and Instrument based on service temperature, (Q) ME-G-46886 Grease, Light Consistency Type I, (Q) M&G-46886, Grease, Medium Consistencv Type I& 8. Resins

496


Q M&R-25506,

Type I, general purpose Type II, radar purpose, Resin Silicone Low-pressure Laminating Form A, liquid or solid Form B, preimpregnating resin 9. Adhesive-Sealants, Acetoxy Type 0 MIL-A-46 106 Group No. based on application, Type I, Adhesives-Sealants One Comoonent Q TT-S-230, Sealing Compound, Elastomeric Tvue Type II, Class A, 0 TT-s-1543, Class A, Sealing Compound, Silicone Rubber Base 10. Adhesive-Sealants, Alkoxy Type Q MU-A-46146 Group No. based on application, Type I, 0 M&R-472 11 Type III

Adhesives-Sealants, m

Compound. Heat Sink

11. Rubber Stocks (Q) MIL-M-14 Moldinrr Comuound for Parts Type MS1 30, Rubber. Fabricated Products 8 M&R-3065, 0 ZZR-765 Rubber, Silicone Various classes and grades, 12. Electrical Insulating Paste (Q) MIL-S-8660,

Silicone ComDound

Silicones

13.

497

Potting Materials (Rubber Kits)

(Q) M&S-23586 Type, class, and grade based on viscosity, cure, and reversion resistance, Sealinn Compound, Electrical, Silicone Rubber (Q) M&I-81550 Insulating Compound, Type based on cure temperature, Electrical, Embedding, Reversion Resistant Silicone

REFERENCES

1. 2.

3. 4. 5. 6. 7.

Rhone-PoulencInc.,Technical Bulletin, Rhodorsil Oils X03-04 (April 1979). Dow Corning Publication Number MIDL-050, Proper Selection and Use of Silicone Adhesives Ensures Extended Life, presented at the Journal of Applied Polymer Science: Applied Polymer Symposium (1977) by David Crossan Dow Corning Publication Silicone Electical and Electronic Materials (1968). General Electric Technical Bulletin, The Sealers S-2H Rev 7/83 Dow Coming Technical Bulletin, Silastic@ 739 RTV Plastic Adhesive General Electrical Tech&al Bulletin, The Versatiles S-35C Rev 6/83 CHR Industries Technical Bulletin, COHRlastic@Silicone Rubber Products SR-2-7183

8. 9.

Dow Coming Teclmical Bulletin, Designing withSilastic@Silicone Rubber 17158A-79 American Society for Testing Materials ASTM D1418-81, Rubber and Rubber Latices-Nomenclature

10. Dow Corning Technical Bulletin, Information About Silicone Elastomers 1780A dated 4176. 11. Dow Corning Technical Bulletin, The SiZastic@ComnpoundingSystem 17-26479 12. General Electric Technical Bulletin, SiIph&?Elastomeric Systems SE6035, SE6075.

13.

General Electric Technical Bulletin, Silicones S-1E.

10 Crosslinked Thermoplastics Rodney F. Patterson

INTRODUCTION

Enhancement of properties is an underlying incentive for the commercial development of crosslinked thermoplastics. Crosslinking of polymers improves resistance to thermal degradation of physical properties and improves resistance to cracking effects by liquids and other harsh environments, as well as resistance to creep and cold flow, among other effects. This chapter deals with the crosslinking of primarily aliphatic polymers, an example of which are the olelins that include the polyethylenes and polypropylenes. These, plus polyvinyl chloride and acrylates, are discussed. High-intensity radiation from electron beams or ultraviolet sources has been used to initiate polymerization in thermoset systems of oligomers capped with reactive methacrylate (acrylic) groups or isocyanates. Using this crosslinking polymerization technique, films with low shrinkage and high adhesion properties have been used in such applications as pressure-sensitive adhesives, glass coatings, and dental enamels. On the other hand, for thermoplastic systems such as polyethylene, chemical or irradiation techniques have been used as the crosslinking technology; this is the recognized standard for manufacturing industrial materials such as cable 498

Crosslinked

Thermoplastics

499

coverings, cellular materials (foams), rotationally molded articles, and piping. These applications are discussed separately later in the chapter. For crystalline olefin polymers, the structure of the crystallite (regions where the long-chain molecules are closely aligned) and the connected/ crosslinked noncrystalline regions contribute to polymer properties, including density. For example, polyethylenes with densities in the range of 0.94 to 0.96 g/cc may be expected to contain 65 to 90% crystalline material (crystallite), while polyethylene with a density of about 0.92 g/cc commonly contains 50 to 60% crystalline material! The proportion of noncrystalline (amorphous) material is increased by high-energy radiation, an effect which is evidently due to the crosslinking of the molecules that occurs. Exposure on low-density polyethylene sufficient to cause crosslinking of about 10% of the carbon atoms gives a product that is entirely amorphous at room temperature. This loss of crystalline structure toward being 100% crosslinked is quite easy to accomplish because it takes only a few of the hundreds of carbon atoms in each chain to be crosslinked to an adjacent chain for the structure to be all crosslinked. Polyethylene, like other noncrosslinked crystalline thermoplastic macromolecules, melts over a wide temperature range; as the temperature rises, the proportion of amorphous material increases until all the crystalline regions are melted (disappear). High-density polyethylenes (with a relatively high crystallite proportion) melt at 125’ to 13 l”C, while lower density polyethylenes melt at 110” to 115°C (Figure 10-l). These results are expected. The presence of structural or stereochemical irregularities in macromolecules makes for a lower degree of crystallinity and an understandably lower melting point. The molecular weight spread in the molecular weight of a specimen is not expected to influence the final melting point or the melting range unless there is an appreciable proportion of molecules of molecular weight below 1500, which is not expected in commercial processes.

CROSSLINKING OF THERMOPLASTICS

Thermoplastics may be crosslinked using irradiation techniques and chemical agents. Their effect is to interconnect the long-chain molecules of

500


the thermoplastic by covalent bonds, with the results being much the same as when thermoset resins are cured or hardened by curing agents. Radiation chemical studiest”l on macromolecules (the word used originally to describe polymers) provided the first evidence for the formation of crosslinks, namely formation of insoluble gel (due to crosslinking), production of main chain unsaturation (producing some double covalent bonds in the polymer macromolecules), peroxide formation, and changes in physical properties, among other effects. 0.97 -

0.96 -

2 0.95 -

MELTING

POINT.

“C

Figure 10-l: Change of melting point with density of polyethylene homopolymers. (Data taken from Reference 5a. Reprinted with permission of Phillips Chemical Co.)

In the following discussion of crosslinking, the actions of radiation rather than the actions of chemical agents are discussed. This is done so as not to confuse the reader concerning these two crosslinking techniques. A later section covers chemical crosslinking. Initially, the radiation chemical studies on crystalline olefin polymers were carried out mostly on polyethylene. ~1 When polyethylene is irradiated with x-rays, gamma rays, high-energy electrons, or pile irradiation, a con-

Crosslinked

Thu-moplaslics

501

siderable change in physical properties can result from a limited degree of chemical change. Hydrogen gas is liberated together with paraffins, such as methane, ethane, and propane, in smaller amounts. Physically, the polymer becomes increasingly insoluble; at first, it becomes more flexible and transparent (as it is changing from crystalline to amorphous with crosslinking), but, after protracted radiation, it hardens and becomes brittle. The structural changes involved in this transformation occur in the absence of heat and chemicals. These crosslinking reactions, induced by radiation, can be closely controlled and cover an extremely wide range. Although some property studies have been made concerning the effect of irradiation at very high doses, most studies have concerned themselves with irradiation doses in the range of 0 to 150 Mrad. The reason for this is simply that the beneficial changes in properties occur at relatively low doses, and that increasing doses serve only to degrade some of the properties of olefin polymers. From a technological standpoint, therefore, the most importance dose range is 0 to 60 Mrad. The radiation allows controlled and limited crosslinking reactions to proceed in the solid state so that an object molded into its final shape can be irradiated subsequently and modified to give it the required properties.[‘l The two fimdamental actions of radiation are to ionize and to excite, and both ions and excited molecules can give free radicals[31 that lead to crosslinking. The first stepf41 in the production of chemical effects by highenergy radiation is the interaction between the electromagnetic field of the gamma ray and the electrons of the polymer to produce ionization or electronic excitation. The gamma rays give rise to fast electrons that have appreciable energy. As the energy required.to produce chemical change is only a few electron volts per molecule, a fast electron is capable of altering several thousand molecules and possibly producing a polymeric segment in an energy-rich state. The gamma ray photons lose part of their energy by ejecting electrons from sites along the macromolecule (Compton scattering). Interaction with outer electrons is the principal process that produces chemical change. As a result of this photon absorption process, the outer electrons are excited to higher energy levels or are ejected, thereby leaving ions in their place. The positive ions, being electron deficient, possess the properties of free radicals as well as of ions. The ionization should be considered as the removal of an electron from the molecule as a whole rather than from just one part. Consequently, the positive charges can be present

502

Handbook

anywhere

of Thermoser

Plastics

in the molecule, although it will tend to be localized at certain

positions more than others, as, for example, at electron deficient sites such as

double bonds. Nonetheless, the molecule is thereby ionized to act as an ion and so can give free radicals. Excited molecules can also be formed just by the high-energy particles themselves. t4] Fast particles excite molecules to the same levels as ultraviolet light, mainly the lowest allowed excited state, but slow electrons can excite to any level, and their excitation is of the whole molecule, not just one part of it. In fact, it does not even tend to be localized as is the case with gamma ray radiation. It can occur at a site quite remote from the track of the particle. The same phenomenon occurs in photochemistry where energy is absorbed by the chromophore, and yet reaction or emission of fluorescence can occur elsewhere in the molecule. Excited molecules, if they do not phosphoresce or fluoresce, can react in three ways. They can lose their energy by internal conversion to give a strongly vibrating lower electronic state. The energy of vibration may then be removed by collisions. Excited molecules can also decompose into free radicals, but unless the decomposition is an energetic one, there is, in condensed phases at any rate, the possibility that the radicals may recombine within the solvent cage (Franck-Rabiniwitch effect), leading to no net reaction. This effect should be especially marked with large molecules. Another mode of decomposition may be noted: decomposition to yield molecular products either directly by a unimolecular process or by reaction with other excited molecules. Finally, some excited molecules may react chemically like free radicals themselves. The role of ions and excited molecules in radiation chemistry is reasonably well understood in general terms, but in specific instances, especially in the irradiation of liquids and solids, it is very difftcult to sort out what is happening. Both ions and excited molecules can, however, give free radicals (as well as stable molecular products). The nature and reactions of the free radicals can be studied even though it is rarely possible to decide exactly how they were formed.t*“l As mentioned, hydrogen gas is liberated by irradiation. In order to produce hydrogen from polyethylene, the C-H bond must be broken. Since the chemical bond strength of the C-H bond is greater than the strength of the C-C bond, chain scission (of its C-C bonds) might be expected to occur more frequently than scission of the C-H bond. Despite this, experimental evi-

Crosslinked

Thermoplaslics

503

dencef31shows that chain scission is low for low-density polyethylene. We know that if a C-C bond is broken, the sigma bonding electrons are separated and the result is free radical formation. It would be expected, however, that the two chain fragments produced by the C-C separation are held firmly in the solid matrix, and, as a result, recombination of the two free radicals is likely (cage effect). Therefore, polyethylene is, as discussed later, a good candidate for radiation crosslinking. When a polymer contains double covalent bonds in its main chain, as does polybutadiene, no chain scission upon irradiation is detected.t31 This is obviously because only its C=C double bonds are being attacked, and they can use up some of the energy by converting to a C-C single bond. In the case of the polymer main chain containing carbon atoms, each of which is connected to four carbon atoms as in polyisobutylene or to three as in polypropylene (all single bonds), a considerable amount of chain scissions is produced by the irradiation. They would therefore not be good candidates for radiation crosslinking. Substances containing chlorine yield HCI, but chlorine or HCI cannot be liberated so long as hydrogen atoms are present. Therefore, instead of the chlorine pendant to the chain being broken by irradiation, it is the carbon bonds of the chain itself that are broken. This may explain why polyvinyl chloride is among the least resistant of all plastics to irradiation. Upon bmaking the C-H bond in polyethylene (and not breaking a C-C bond in the backbone main chain), an alkyl free radical of the structure -CH,CHCH,- results.t3~ It contains a now unshared electron where the hydrogen atom was previously connected. If two such free radicals are formed on neighboring or adjacent chains, a very probable reaction would be crosslinking by recombination of the free radicals. The evidence is that the crosslinks occur primar@ in the amorphous phase of polyethylene, as well as in the amorphous surface layers of the (nonamorphous) single crystals of polyethylene. When radiation produces liberation of hydrogen from polyethylene, it also produces unsaturation in the molecule.~2p~Three types of unsaturation are observed: vinylidene, terminal, and trans-vinylene. While trans-vinylene unsaturation is being produced in the molecule during irradiation, there is a concomitant decrease of vinyl and vinylidene unsaturation. A further change that occurs upon irradiation is the reduction of the pendant methylene groups present in the original polymer. Another change occurs when irradiation of polyethylene is carried out in air. In this case, crosslinking is inhibited and

oxidation takes place mostly on or near the surfaces. This oxygen effect is attributed to the formation of peroxides and hydroperoxides. The mechanism may involve the combination of oxygen with a free-radical to form a free radical of the type RO,; this free radical can then abstract a hydrogen to form hydroperoxide, which, in turn, can decompose into two free-radicals. (Other mechanisms are also possible.) Two aspects of oxidation should be emphasized.t31 Oxygen gas dissolves only in the amorphous regions of polymers and can therefore oxidize only within the amorphous phase of the surface. Secondly, the extent of oxidation per unit of radiation dose is dependent on the dose rate. The lower the dose rate the more time the oxygen has to diffuse into the plastic per unit of dose so that the net amount of oxidation for the same dose is much greater at low rates. Oxidation and crosslinking subsequent to irradiation have been observed.t31 The phenomena have been attributed to the persistence of free radicals in the polymers. Not only does oxidation occur during irradiation but it may continue for weeks after the irradiation if the alkyl free radicals are not annealed out by heating (in the case of polyethylene, to 100°C or higher after irradiation). The post-irradiation oxidation of polyethylene has been followed, using as a basis, the increase in the carbonyl infrared absorption band at 1725/cm even after 60 days of exposure to air at room temperature. Carbonyl formation may be accompanied by chain scission or increased crosslinking. A delayed crosslinking reaction was observed when a highdensity polyethylene was irradiated and then immediately heated above its crystalline melting point. The increased crosslinking was attributed to the migration of free-radical centers through the polyethylene (now amorphous) by random jumps of a hydrogen atom from an adjacent site to the free-radical site.

EFFECTS

OF CROSSLINKING

ON POLYMER

Polyethylene The effects of beta and gamma irradiation on the properties of highdensity polyethylene are given in Table 1O-l .t’d The data indicate that poly-

Crosslinked

Thermoplastics

505

mer crosslinking/irradiation is accompanied by an increase in tensile strength and hardness and a decrease in solubility. The table also indicates that, at 132°C unirradiated polyethylene exhibits insignificant properties. Beta irradiation, may then provide material with properties adequate for an application even at 132°C. Irradiation also increases resistance to environmental stress cracking.

Polyolefin Foams

Table 10-2 shows typical properties of a series of radiation crosslinked closed-cell polyethylene foam.t”l The foams, ranging in density from 1.5 to 15 Ib/fi3, are characterized by excellent mechanical, thermal, and chemical properties, together with a fine-cell structure and an exceptionally smooth surface; they are available in thicknesses from l/32 in. to more than 1 in.. A crosslinked polyethylene foam sheet with an integral skin is also available in the same range of densities and thicknesses. The skin offers increased abrasion resistance without reducing the foam’s flexibility. Table IO-2 shows VOLARA foam products with a Type E designation which Voltek literature describes as a crosslinked polyethylene copolymer foam especially formulated to provide more flexibility and resilience than their standard grade of Type A designation. Polypropylene foam is also available. The crosslinked polyethylene and polypropylene foams are resilient cushioning materials. In terms of compression set resistance per ASTM D-395, they even outperform the foam industrystandard, silicone foam. Most applications are based on this property together with some other quality such as buoyancy, shock absorption, thermal or electrical insulation, vibration dampening, and moisture protection. Automotive applications include gasketing, sun visors, and insulating liner for air-conditioner housing and carpet backing. Recreation and sport uses are based on protection against repeated shock from relatively high stresses (Figure 1O-2). The foams also find uses in medical products because they add comfort to orthopedic braces and cervical collars, for example. Applications of the crosslinked polyolefin foams are very diverse because these materials can be combined with others using proven plastics industry methods but with variations because of the foamed materials.t6b1 However, the foams, like the polyolefin plastics, have a low-energy surface

506


Table 10-l: Effects of Gamma and Beta Irradiation on Properties of Marlex@High Density Polyethylene Typical Properties 4560 (31.4) 1477(10.8) ~745(5.13)

Tensile sbmgth, psi (ma)

-,

Shore D

Density, s/an’ Solubility, tetdin 266°F (13OOC)

XlON

20

20

1x1@

20

3xw

24

-

6x10’

110

40

MO’

700

350

3x10’

350

350

IXW

1

-

*Measured by various laboratories **Environmental stress cracking in Igepal CO-630 at 122°F (50°C). Data taken from Reference Sa. Reprinted with permissiin of Phillips Chemical Company.

Table 10-2: Typical Properties of Closed-Cell, Radiation Crosslinked Polyethylene Foam camF?dwtNomd Compressive >ensity, SW wff (pi)at50% Deneuion

VaARA 1.5A

Maximum Compressial (%o$lLk) ASTM Lx395

Temile

Elongation (%bBreak) Ash4 Is1564

SW Et4 D-1564

TearResistam (lb/in) ASTMD-624

I’hemdStability K% LiIPm Fats ShilIkAfh 3&m

M

C

M

C

M

C

180”

215”

1.5

11-14

15

38

25

121

101

8

6

2.5

8.5

VOLARA 2.4.

2

12-16

16

50

41

138

114

11

8

1.5

3.0

VOLAU 2h4F

2

12-16

30

48

36

78

62

13

8

1.5

2.2

VOLARA 2E

2

11-15

21

60

48

250

250

11

10

3.6

20.0

VOLARA 2E!z

2

lo.15

25

35

29

190

2co

6

5

14.0

50.0

VoLaA 4A

4

19-24

12

100

82

174

148

22

18

1.2

2.8

Wars zz? SufaccMZU ASTM D-1667

0.25

0.04

0.25

0.04

0.30

0.04

Table 10-2: Typical Properties of Closed-Cell, Radiation Crosslinked Polyethylene

mtinued)

(’

zit

Normal =Ompressive Density, St=&

I

lb/A)

(pi)at50% Deflection

M

25-31

I

148

9max

Elongation (% to B=W ASIN D-IS64

C

M

124

220

I 22-27

8mzx

60-80

14max

7sloo

15 max.

1823

4vailable in formula

Temile Strength (psi) ASTM D-1564

Manmum compression Set (% Gig Thick) ASTM D395

2omax

a containing tire Fardant

addi

es.

Data taken Corn Reference 6a Reprinted with permission from Voltek, Inc.

I I

C 176

Foam

C‘rosslinked Thermoplastics

509

Figure 10-Z: Crosslinked polyoletin foam in sport applications. (Reprinted by permission of Voltek, Inc.)

that is not easily wet by laminating adhesives. The most common way to improve wettability and adhesion is corona ion plasma treatment. A similar effect can be obtained by flame treating the polyolefin foam priorto lamination/bonding. Then, using heat and pressure; the foam may be laminated to itself, to urethane foam, to polyolefin films, and to fabrics made from natural fibers. Just before joining at the laminating nip, each substrate is preheated using a gas flame or electric heaters. To bond to a fabric, film, or other substrate, a heat reactivation method can be used. This technique employs a thermoplastic film or preapplied coating which, after heating and applying pressure, bonds to another substrate without the need to evaporate off water or carrier solvent. Examples of adhesive films include low density urethane foam, solid polyethylene, and ethylene-vinyl acetate films. These materials melt completely and become a solid continuous layer in the final laminate. For substrates that cannot be bonded using heat and/or pressure, adhesives may be used. According to the manufacturer, adhesives are available which

510

Hundhook

of Thermoset

P1astic.t

will bond polyolefin foam without pretreating the foam. Other adhesives require pretreatment.

The radiation crosslinked polyolefin foams can be shaped by compression molding alone or in combination with other foams of different color and density, fabrics, films, foils, non-wovens, solid plastics and cellulosics.t6c~ By combining the crosslinked foams with other materials in whatever way, a wide variety of products with desired properties and appearances can be tailored to specific end use requirements. While the combining is usually done by lamination prior to molding, materials can be bonded together in the mold. The choice is based on the cost and ease of laminating various materials. For example, a two-way stretch nylon fabric should be laminated before molding, whereas a rigid high-density polyethylene plastic insert would be heated and then placed in the mold between layers of hot foam.

Polypropylene

The use of stabilizers that limit the resin degradation of and therefore permit the sterilization of biomedical products by gamma irradiation may broaden the range of disposable products made from polypropylene. The sterilization of polypropylene biomedical products by gamma irradiation results in severe resin degradation. 1’1 Table 10-3 shows the changes in molecular weight of polypropylene and polystyrene samples after gamma irradiation was carried out by a Cobalt-60 source at dosages to 2.5 Mrad at a 0.5 Mradihr rate. The results show that, for polystyrene, weight average and number average molecular weights and molecular weight distribution were nearly unchanged. For polypropylene, however, the data show the shift from high to low molecular weights for unirradiated versus irradiated polyethylene, clearly indicating that severe chain scission has occurred. During gamma irradiation of polypropylene in air, random freeradical formation is believed to occur followed by oxidative propagation of radicals.t8t The incorporation [‘Iof stabilizers into polypropylene, such as a hindered amine or phenol, suppresses oxidation and inhibits the deterioration of the polypropylene properties, thereby rendering it more resistant to irradiation. This stabilizer action was found to be accompanied by a reduction in stabilizer content during irradiation and the subsequent storage period. The

~‘rosslinked Thermoplaslics

51 I

reduction was attributed to the stabilizer’s possible mechanisms, namely that of radical scavenger, chain transfer agent, or peroxide decomposer. The conclusion was that, regardless of the mechanism, stabilizers play a sacrificial role in protecting polypropylene from further degradation following irradiation.

Table 10-3: Changes in Molecular Weight of Polypropylene Polystrene After Gam Ia Irradiation t 2.5 Megarads Samples

IJnirradiated

h-radiated

Molecular weight distribution

4.0 x lo5 I.1 x 105 3.7

9.4 x 104 1.2 x IO4 8.0

Polystyrene* * Wei ht average molecular weight Numgb, r average molecular weight Molecular weight distribution

2.6 x IO’ 1.5 x IO5 1.8

2.6 x IO5 1.6 x IO5 1.7

Polypropylene* Weight average molecular weight Number average molecular weight

and

*Hercules Prefax 650 I **Dow Styron 685D

The progressive deterioration during storage of polypropylene articles following gamma irradiation has been attributed to the presence of hydroperoxide groups. These groups are known to be thermally unstable and are considered a potential source of storage oxidation together with “trapped” peroxyl radicals. Trapped peroxyl radicals are thought to be in crosslinked (amorphous), solvent accessible (thereby not trapped) domains. It has been suggestedt9]that incorporation of the stabilizer/additive increases the internal free volume in the amorphous phase of the polymer, thus enhancing main chain mobility and accelerating main chain recombination; this holds true both during and following irradiation.

512

Hundbook

of Thermoset

Plusrics

Polyvinyl Chloride

In polyvinyl chloride (PVC), carbon-chlorine and carbon-hydrogen bonds (both off the carbon in the main chain) are susceptible to cleavage by ionizing radiation, producing free radical sites on the polymer backbone,tiOli.e. the main chain. During irradiation, these radicals would initiate dehydrochlorination and small amounts of main chain scission and crosslinking. This dehydrochlorination proceeds via chain reaction to produce conjugated double bonds. These now-unsaturated structures absorb in the ultraviolet-visible region, thereby discoloring the polyvinyl chloride. This discoloration, of course, may accompany some embrittlement (crosslinking) and cracking (chain scission) of the PVC article. A number of insulations and coatings used in high-temperature environments are produced by the radiation crosslinking of PVC resin.ttol A typical formulation for a radiation-curable coating includes a base resin (a PVC), a crosslinking sensitizer (e.g., polyfimctional monomer), and a plasticizer. A typical resin crosslinking sensitizer must be capable of being crosslinked by irradiation as well as incorporating the PVC resin molecules into the network via grafting reactions. Polyfunctional monomers that have been shown to act as such for polyvinyl chloride include ally1 esters, dimethacrylates,trimethacrylates, triallyl isocyanurate, divinyl benzene, and triacryla&s. The polyfimctional methacrylates and acrylates were found to possess the greatest crosslinking sensitivity. The plasticizer component is used to obtain the required physical properties for a specific coating apphcation. In one study, the system of polyvinyl chloride blended with trimethylolpropane trimethacrylate (TMPTh44) and diundecylphthalate (DUP) was selected as a representative example of such radiation-curable coatings. In the absence of a plasticizer, there was an initial preference for TMPTMA homopolymerization, a&r which PVC molecules were bound into the network.[‘OlIncreasing the irradiation temperature primarily increased all the reaction rates equally. Dehydrochlorination of the PVC did, however, begin to compete with the grafting and crosslinking mechanisms at 80°C. Post irradiation thermal treatment was shown to alter the physical properties of the irradiatedblends markedly. This alteration was caused by the reactions of residual monomer molecules and unreacted double bonds in the crosslinked network TMPTMA does not use all the available double bonds to form the

C’rosslinked Thermoplaslics

513

network. Over a wide range of blend compositions, the crosslinking rate was found to be proportional to the TMPTMA concentration.[‘Or As the TMPTMA concentration decreased, soluble graft copolymers were produced in addition to insoluble networks. In the blends, the free-radical sites are scavenged by TMPTMA, which grafts to PVC and initiates incorporation into the network via structures like PVC-(TMPTMAX and PVC- (TMTMA),-PVC. Blends with no TMPTMA (pure PVC or PVCDUP blends) showed no significant gel formation. The introduction of a DUP plasticizer component into the mixtures enhanced (I ) TMPTMA homopolymerization, (2) TMPTMA grafting, (3) PVC ctosslinking (at low dose), and (4) reactivity of double bonds.1 “1 These effects are interrelated. For example, TMPTMA grafting produced precursors for PVC crosslinking. Among the parameters that determine the chemical kinetics of the system are the concentration and mobility of the reactants. The competition between reactions (l), (2), and (3) above are determined by the reactivity and mobility of the double bonds. The ease with which PVC, TMPTMA and free radicals can diffuse through the matrix and react together constantly changes throughout the reaction. With PVC/ TMPTMA blends, the medium changes from a flexible PVC resin (plasticized by the monomer) to a three-dimensional network (strong and brittle). With DUP present, the blends remain flexible a&r irradiation; on a molecular level, this means that the mobility of the reactive species remains high. Until high conversions were reached, the mobility of the reactive sites was enhanced by the presence of DUP, and the double bonds were accessible for reaction. These results show that, in the manufacture of crosslinked coatings, the inclusion of DUP would result in energy efficiency, double-bond efftciency, and a more stable product. tr”l The increase in mobility (due to DUP) produced faster crosslinking and therefore required less irradiation (less energy). A typical dose required was 4 Mrad.

CHEMICAL CROSSLINKING

Chemical crosslinking of saturated (all single covalent bonds) polymers, such as polyethylene, polypropylene and polyvinyl chloride, cons-

514

Hundhook of Thermosel P laslics

ists of forming bonds between the polymer chains using organic peroxides, in general, as a source of freeradicals. Crosslinking with free- radicals involves hydrogen abstraction to produce a free-radical initiator site on the polymer molecule. It would be expected that the resultant recombination of adjacent free-radical sites (between adjacent polymer chains) would produce a crosslinked material with enhanced thermal stability. Polyethylenes, modified with organic peroxides, are used in such applications as rotational moldingISbl and piping for hot water use.t”l Organic peroxides are useful as free-radical initiators because they are stable compounds until heated and their decomposition rate (to free radicals) is temperature dependent. [“I Because, in many instances, peroxide decomposition follows first-order kinetics, decomposition rate at a particular temperature is usually expressed as half-life. Half-life is defined as the time necessary to decompose one-half the amount of peroxide originally present. For example, dicumyl peroxide, which can be used to chemically crosslink polyethylene, has a half-life of 23 hours at 1 15”C, 1.8 hours at 130°C and 0.3 hour at 145’C. The melting point of low-density polyethylene ranges from 110” to 115°C. After mixing or milling of the polymer and peroxide, the mixture may be shaped and then heated to induce the decomposition of the peroxide initiator with consequent crosslinking of the polymer in the molten state (above the polymer’s melting point). (Of course, the polymer is not molten for long as the crosslinking begins immediately to solidify it.) Bearing in mind the oxygen effect leading to carbon groups in an oxidized state, the structure of the chemically crosslinked polymer most probably is more complex than simply a crosslinked parafftn.

Polyethylene

Polyethylene crosslinked with dicumyl peroxide exhibits two separate but overlapping regions of dielectric loss between - 150” to -50” C. Polyethylene also shows three regions of dielectric loss centered around 60”, 0” and 100°C:alpha-, beta-, and gamma-loss regions respectively.t’31 These regions essentially result from carbonyl dipoles produced by adventitious or deliberate oxidation. Whereas both the beta and gamma losses originate from the amorphous phase, the alpha region is associated with the crystalline phase.

C’rosslinked Thermoplastics

515

The dielectric loss regions can easily be located by dielectric or capacitance measuring instruments. At a relatively high dielectric loss, the dipoles present become relatively slow and less able to move in response to an alternating current (AC) field being imposed. Obviously, the reason has to do with the contribution of the dipoles to crosslinking or otherwise to rigidizing the molecular structure. This information contributes to how widespread is the use of crosslinked polyethylene as insulation for underground electric cable and to its growing acceptance in foamed form as microwave insulation. When dry cured, the polyethylene noted above contains approximately equal amounts of the by-products of the dicumyl peroxide initiator, namely acetophenone and 2-phenyl-2-propanol. Both of these molecules are polar and would be expected to be located in the amorphous phase and hence to give rise to beta or gamma losses but with activation energies different from those of oxidized polyethylene. The major loss effects that occur at the lower temperatures are attributed to the individual loss peaks of these two major by-products of the crosslinking agent. Analog materials prepared by Yang, et al.tr3] from linear low-density polyethylene, by blending in either the acetophenone or the 2-phenyl-2-propanol, confirm the hypothesis. The concentration of dicumyl peroxide used to modify a low-density polyethylene (number-average molecular weight = 32,000 ) affects the static and dynamic mechanical properties of the chemically crosslinked polymer. Kunettn41teported on studies carried out at levels of 0.5 to 2.5 weight percent to determine the changes in structure of crosslinked polyethylene. The results of his dynamic mechanical property testing in shear, using a torsional pendulum, showed that the maximum value of the logarithmic shear decrement occurred in the vicinity of 47°C (the so-called alpha relaxation point) in the uncrosslinked and the crosslinked polyethylene. At 27”C, values for the storage modulus of crosslinked polyethylene were below the values of the unmodified polymer; storage modulus decreased slightly as the peroxide concentration decreased to about 2%. The same correlation was observed at 80°C. At 87”C, however, the storage modulus of the crosslinked polymer was greater than for the noncrosslinked polyethylene; the storage modulus decreased slightly with increasing peroxide concentration. Kunert concluded that a storage modulus at 27°C is probably affected mainly by crystallinity because this modulus exhibited higher values for the uncrosslinked polyethylene than for the crosslinked material where crystallinity is not a factor. At 87”C, however, which is near the melting point of polyethylene crystallites,

516

Handbook of Kbermoset Plastics

the storage modulus of the uncrosslinked polyethylene shows lower values than the crosslinked material. This is mainly due to an increased stiffness of the polyethylene network (crosslinking) at this temperature, as well as near elimination of the crystallites. The Weissenberg rheogoniometer was used to test the crosslinked polyethylene as a function of frequency over four decades at room temperature.h4’1 The storage modulus increases as a function of frequency between 0.005 and 5 Hz. A plot of storage modulus as a function of peroxide concentration showed that, at constant frequency, the characteristic feature of the plot was the appearance of two maxima for storage modulus occurring at peroxide concentrations of 0.5 and 2% (Figure 1O-3). A resonance method in the frequency range of 100 to 900 Hz gave similar results. The absence of any maximum of storage modulus plotted against the peroxide concentration when testing with the torsion pendulum was attributed to the effect of a large deformation that probably exceeded the range of linear viscoelasticity of this material. Thus the storage modulus represents, in the torsional method, the averaged resultant properties of different phases in the crosslinked polyethylene specimen. On the other hand, the appearance of some maxima with the Weissenberg and resonance methods was taken to indicate that the deformations are at least half those of the torsional method and are probably within the range of linear viscoelasticity. Thus the mechanical properties of different phases are not averaged. Two rigid structures in crosslinked polyethylene are proposed. The first maximum is presumably related to the appearance of crystallites (excessive crystallinity) and the second maximum is probably caused by a uniform rigid polyethylene network. The structure attributed to excessive crystallinity is presumed to occur at a low concentration of peroxide (-2%); the very regular and perfect polyethylene network is presumed to occur at the high peroxide concentration. The results of static mechanical property testing showed that, as the peroxide content increased, Young’s modulus and stress at yield point decreased while percent elongation at the yield point increased. A plot of ultimate tensile strength exhibited a maximum in the vicinity of 0.5% peroxide concentration. Elongation at break exhibited a similar maximum. Kunert’s conclusion was that Young’s modulus depends on the amount of rigid amorphous phase in crosslinked polyethylene. As the amount of peroxide is increased, the amount of soft phase increases. Kunert considers the soft phase may act as a plasticizer for the hard phase. He postulates that

Crosslinked

G' =

10’

Thermoplastics

517

1

(dynkm’) 20

Dicumyl

18 -

6

0

Peroxide

a = 0.005

0.5

1D Concentration

1.5

2.0

Hz

2 5 (‘I.1

of Peroxide

Figure 10-3: The storage modulus G’ of crosslinkes polyethylene as a

function of dicumyl peroxide concentration measured in a Weissenberg rheogoniometer. (Data taken from Reference 14. Reprinted with permission of John Wiley & Sons, Inc.)

ultimate tensile strength depends on the amount of crystallites. Except for this, static mechanical properties, and especially Young’s modulus, depend on the amount of hard, amorphous structures in the crosslinked polyethylene. At the other end of the spectrum, L.emand Hanr’sl reported on studies in which the highest peroxide concentration was 0.1 weight percent. In this range, little, if any insoluble gel particles were formed between the particular virgin resins and dicumyl peroxide. A comparison of the molecular weight distribution curves of a low-density polyethylene (number-average molecular weight = 7,500) before and after reacting with peroxide showed that the modified material has more high molecular weight portions compared to the starting resin; both number-average and weight-average molecular weight increase with increasing amounts of dicumyl peroxide. In addition, the modified low-density polyethylene has more low-molecular-weight portions

compared to the virgin resin. The results show that, from a mechanistic viewpoint, dicumyl peroxide added to the polymer and extended the polymer chains (especially, the long chain molecules) giving rise to larger molecules. There was evidence also that the degree of long-chain branching is increased in the presence of dicumyl peroxide free-radicals. The molecular weight distribution curve of a high-density polyethylene (number-average molecular weight = 15,300) showed that, after reacting with dicumyl peroxide, the entire curve of the modified resin was shifted toward the right-hand side, indicating that polymer chains are extended in the presence of peroxide free-radicals and little dissociation of C-C bonds has taken place.t’51 The little degradation of high-density polyethylene observed compared to low-density polyethylene, was explained by the fact that, when subjected to dicumyl peroxide free-radicals, the tertiary carbon atoms present in the long- and short-chain branching of low-density polyethylene are very susceptible to degradation compared to those in the straight chains. The rheological properties of the modified polyethylene were determined with a Weissenberg rheogoniometer. Plots of first normal stress difference versus shear stress for virgin and modified low-density and highdensity polyethylene show that the resin becomes more elastic as the amount of dicumyl peroxide is increased. The results showed that if the concentration of dicumyl peroxide is still low enough not to yield insoluble gel particles, modified resins are obtained which may be considered to have resulted from chain extension rather than crosslinking.t”l

Polypropylene

Radicals produced by peroxide decomposition may abstract hydrogen at any site along the chain. If unsaturation is present, hydrogen abstraction will likely occur t?om a carbon group alpha to a double bond. In the absence of on allylic group, abstraction is expected at the site of ternary hydrogen. Tertiary hydrogen atoms characterize polypropylene and are found at branch points in polyethylene. Hydrogen abstraction of a tertiary hydrogen forms free-radicals that tend to undergo chain scission at the expense of crosslinking. In contrast to polythylene, radical formation in polypropylene is

Crosslinked

Thermoplustics

519

accompanied by degradation and crosslinking. The mechanism of degradation

reaction of polypropylene is generally explained by fragmentation of tertiary alkyl macroradicals, while the crosslinking is explained by a combination of secondary alkyl macroradicals.[ial The decreased efftciency of peroxide crosslinking of copolymers of ethylene and propylene was accounted for in the same way. In order to counteract this inherent low crosslinking tendency of polypropylene, monomers containing several olefinic bonds may be incorporated into the polypropylene so that the overall system is provided with a very much increased number of crosslinked sites. In one study,n61pentaerythritol triallyl ether was evaluated as a “coagent” in chemical crosslinking of isotactic polypropylene with the aid of organic peroxide. Looking at the structure of the peroxides used in this study, it seems that the most efficient crosslinking initiators formed benzoyloxy or phenyl radicals upon decomposition. On the other hand, initiators giving alkyl radicals were inefftcient.

ROTATIONAL

MOLDING

Powder process techniques for the making of hollow parts of polyethylene evolved into iotational molding.15bIIn this method, a split metal mold is used. A cold mold is filled with a powdered resin, then the mold is placed in an oven and rotated simultaneously.about two perpendicular axes. During this stage, a uniform layer of resin is deposited on the mold. After sufftcient time has elapsed to properly fuse the resin, and while it is still rotating the mold is cooled. After cooling, the part is removed from the mold and more resin is added to start the cycle again. The absence of positive pressure in the powder rotational process places certain limits on the type of resin that can be used. The higher molecular weight resins used for blow molding and sometimes injection molding cannot be rotationally molded because they will not flow out in the absence of pressure to form a homogeneous, void-free part. The polyethylene resins used in powder processes generally range from as low as 3 to 5 to as high as 70 melt index. (If molecular weight distribution is considered constant, the lower the molecular weight of the resin, the higher the melt

520

Handbook of Thermosct P1lastic.v

index.) The resins with the 3 melt index have better stress cracking resistance, impact strength, and resistance to creep than the high melt index resins but are also more difticult to mold. Recall that a low melt index polymer is one that has low flow rate at the test temperature. Parts with hardto-fill areas would require a slightly higher melt index for good moldability. Because of the somewhat limited stress cracking resistance and creep resistance of the powdered polyethylene resins, parts fabricated from these resins may be limited to the severity of service to which they can be subjected. On the other hand, tanks made in the rotational molding process using crosslinkable high-density resins have been used in a variety of applications, including the handling of corrosive chemicals. Tanks varying in sizes from a few to thousands of gallons are being used in agriculture and industrial applications. t171Crosslinking gives products that have excellent resistance to stress cracking and chemical attack, excellent impact strength, weathering characteristics and overall toughness. Although the crosslinked product goes through a crystalline melt point at a temperature similar to uncrosslinked resin, it has sufftcient melt strength to support itself at temperatures up to about 210°C. Smaller molded products that are not subjected to a load will not deform even at temperature at which uncrosslinked high-density polyethylene will melt and flow. Crosslinkable resins, e.g. Marlex@ CL-50 and CL-100, generally require lower oven temperatures and slightly longer heating cycle times than normally used with conventional high-density polyethylene resins.t5b1In most cases, an oven temperature between 550°F and 625°F should be used. Temperatures below 550°F can be used quite. satisfactorily, but longer cycle times are required. Temperatures above 625°F tend to generate too rapid a decomposition of the crosslinking agent. This can cause bubbles in the wall, blow holes through the wall, pock marks on the surface, rough inside surface, or over-pressuring of the mold if adequate venting is not provided. Table 1O-4 shows the nominal physical properties of two rotational molding, crosslinkable high-density polyethylene resins. When tested in a bent strip test (ASTM D1693), both Marlexe CL-l 00 and CL-50 have outstanding environmental stress crackiig resistance (ESCR). The bent strip test is used for ESC characterization because it is presumed to be representative of the stresses and strains encountered in use. In this test, ten polymer samples (1 ‘/z in. x % in. x l/s in.) are bent 180” and immersed in a stress cracking agent. Each bar contains a longitudinal slit % in. long x 21100 in.

Crosslinked

521

Thermoplastics

Table 10-4: Nominal Physical Properties of Marlex@ CL-100 and CL-50 High-density Polyethylene Resins MAHl.liX”‘Rk:SIN NUMBER?

**CL-SO Natural

*CL-l00

ASTM 131248 ClaGiicalion

TypeIV(l) ClassA cate@y l

Type IV(I) ClassA

No

NO

0.939-0942’

0 937-0.940’

>200 >I75

>ICOO >I000

Tensile Strength@ Yeild, ASTM D63B Type IV Specimen 2 in. (5Omm)per min. psi (MW

2600(16)

2600(16)

Elongbm, ASTM D638 Type IV Specimen, 2-in (SOnun)per min.,%

400

450

Flexual Modulus,ASTM D790,psi (MPa)

1IOM (758)

ICOM(689)

Via Sotkning Tempahre, ASTM D1525,“F (“C)

-255 (- 124)

-260(-127)

Mesh Size

35

35

Meets Food and DrugAdministrationRegulation In. I520 for Food P?lckagjng

Natural,white,and standard colors

megory 1

NOMINAL PI IYSICAL PROPERTIES Density,ASTM D 1505,&m’ ROTATIONAL MOLDED PROPERTIES EnvinmmentalStressCracking Resistance,ASTM Dl693, ConditionA, F,, hr

1w/0 lgepal 10%Isepal

bAvailablein either35 meshpowderor pellets. **Available in 35 meshpow&x only (1) (2) (3) (4)

ASTM Classificationfor ‘Type” on basetin Spe&mm moldedin accordanoe with ProcedureCofASTM Dl928. De&y of nahualcamslinkedprodud. Dataobtainedusinga gs operatede&n&m pltiometer basedon a &sign by CanadianIndustriesLtd with a die havinganorificediameIerofO.VK925 “(0.049 mm) anda landlengthof0.176 “(4.48mm).

Datatakenfrom Reference 17.Repinted with pamissionof PhilipsChemicalCo.

deep down the center of the upper face. Resistance is defined as the length of time needed for five of the ten bars to show visible signs of cracking perpendicular to the slit. In the ASTM D1693 test, CL-l 00 has an F50 value greater than 1,000 hours. With a properly crosslinked CL- 100 sample, not a single specimen has failed in this test. MarlexeCL-50 has a nominal ESCR F50 valve of 200 hours. When properly molded, most CL-50 parts will have ESCR in excess of this. A more serve test under ASTM Dl693 uses a 10% solution of the stress cracking agent; the crosslinkable resins have similar values with this test as they do with less severe 100% solution. Table 1O-4 shows this comparison. Long-term hoop stress testing of the crosslinkable resins indicates that they are superior to other rotational molding resins and equal to highdensity polyethylene extrusion grade pipe resins. Both CL-l 00 and CL-50 rotational molded samples of 2-in-diameter pipe, 0.150-in. walls were used for long-term hoop stress testing at both 80”and 140°F. One set of CL-100 test samples at 1,750 psi hoop stress and lower has gone more than 50,000 hours without failure. Before testing, it was anticipated that the 1700-psi sample would fail at approximately 100 hours. Because failing did not occur design hoop stress could not be determined; however, it does indicate that a well crosslinked sample will have excellent long-term stress at both 80” and 140°F. The CL-50 data on the 0.150-in. wall part indicated its hoop stress would be superior to conventional HDPE pipe resins.t’*l When properly cured, the crosslinked part has exceptional impact and overall toughness at both room and low temperatures. Good impact can be developed even at -20” and -40°F. The impact trait has been deritonstrated both by dart impact test and part drop tests. Tanks filled with liquid have dropped 30 feet without failure at mom and low temperatures (-20°F). A few noncrosslinked rotational molding resins might have similar dart impact but will not give the part drop performance of the crosslinable resins. Another demonstration of the toughness of the crosslinked part is its ability to withstand repeated drop impacts of 30 feet without failure. Even parts that have been creased on previous drops can withstand repeated 30-foot drops.t’81 With plastics in general, it is difftcult to correlate nominal physical properties with part performances. This is even more difftcult when comparing nominal physical properties and part performance of crosslinkable to noncmsslinable polyethylene. With the exception of environmental stress cracking resistance (ESCR), the nominal properties of the crosslinkable resins

C~hsslinked

‘I’hcv-nroplustics

523

give little indication of the performance that crosslinked parts exhibit. In the early development stage of crosslinkable HDPE, special testst’81were developed to illustrate and give a better understanding of what could be expected of crosslinked parts. These tests illustrated such properties as ESCR, longterm hoop stress, gasoline resistance, impact resistance, and overall toughness. For one test, a 2-gallon jerry can (portable gasoline container) was 75% filled with gasoline, sealed, and placed in a 130°F room. At 130”F, the fuel has a vapor pressure of 7.25 psi. The crosslinked jerry can had a nominal wall thickness of N in. and weighed 800 grams. This container underwent testing for 3 years and did not fail. Similar tests run on the same container molded from noncrosslinked resins available at that time had failures which varied from less than 1 hour to a maximum of 4 days. Some currently available resins would be expected to give better performance but still would not equal the crosslinkable resins. Drop impact tests of molded containers were used to evaluate part impact strength and overall toughness. Containers varying in size from 2 to 3,000 gallons have been drop tested. Containers up to 80 gallons and filled with water or antifreeze solutions have been dropped from 30 feet at temperatures of 80°F. Larger containers containing only 50 gallons of water have been dropped 30 feet without failure. The capacity of the lift used for drop tests limited the volume of water used in the larger containers. A ‘/-in. wall, 3,OOOgallontank filled with water was dropped 10 feet without failure. This same tank filled with sand passed a similar drop impact test. For another drop, a 50-gallon tractor fuel tank was filled with water and dropped 30 feet. The part had a nominal wall thickness of 0.200 in. and weighed 30 pounds. The part and water had a total weight of 450 pounds. After the drop, no evidence of the deformation which occurred on impact could be detected.tr*l Low-temperature impact testing is one of three quality control procedures for indicating the level of crosslinking in production parts. The others are percent gel and the bent strip test. These two tests are discussed later. Dart impact at -20°F is considered to be the most critical and comprehensive quality control test for parts molded from CL- 100 and CL50.t5’l As the compounds reach increasing degrees of crosslinking, the last property to reach maximum values is low-temperature impact strength. Room temperature impact strength for crosslinable resins can be misleading because values am high even for poorly crosslinked parts. Critical physical properties, such as stress crack resistance, and percent elongation, develop their maxi-

524

Handbook of ‘I’hermo.ve[Plastics

mum values when low-temperature impact strength of the molded part is equivalent

to its room temperature impact strength.

According to the

manufacturer, particular attention should be given to failure patterns

of impacted samples. Improperly crosslinked parts will crack or shatter when impacted. Such brittle type failures at -20°F have consistently been an indication of improper curing. When properly crosslinked specimens do fail, they exhibit ductile failures at temperatures of -20°F or higher. Ductile failures appear as a puncture through the specimen and show that the tensile strength of the material was exceeded. Should impact properties be poor, a progressive increase in heating time and/or oven temperature is needed until impact properties improve. The percent gel test is another method to indicate crosslinking levels in molded CL-I 00 and CL-50 parts. fscl Refluxing ethylbenzene extracts the non-crosslinked portion of a specimen. The remaining gel insolubles are largely crosslinked polyethylene yielding a quantitative measure for degree of cure. Normally a high degree of crosslinking (optimum crosslinking) is indicated by gel levels of 85 to 90%, but this can vary f I O%, depending on wall thickness and molding conditions. The effect of wall thickness on percent gel is a phenomenon contrary to expected values. Wall thickness of less than l/B in. can produce 80 to 85% gel and be well crosslinked, while a part in a ‘/-in. wall may produce percent gels consistently above 90%. When wall thickness exceeds ‘/4in., percent gel should be tested in the inner surface of the wall to ensure that the results are not unrealistically high and misleading as to whether there was good crosslinking throughout the wall. Other variations observed in this test are caused by the amount of surface area of the gel sample exposed to the refluxing ethylbenzene. Another consideration in interpreting percent gel data is its relationship to impact properties. A high percent gel is usually achieved before full development of maximum low-temperature dart impact. Because low-temperature dart impact is the last property to be improved in CL-100 and CL-50, it should be included with a gel test for the best quality control of production. A bend test is a quick test for a rough estimate of the degree of cr_ue.[“lThis method of determining whether the interior surface of rotational molded CL-100 parts is properly cured provides a means of checking for degree of cross-linking shortly after the part is molded. Due to the heat differential between the interior and exterior surfaces of a rotational molded part, the interior surface cures later than the exterior surface. The difference in cure means the interior portion of the part crosslinks last, so that stressing

the interior portion provides some indication of crosslinking. Conclusions horn this test should be verified periodically by low-temperature impact tests. The producetISclconcludes that the best production quality control test for crosslinking in Marlexe CL-100 and CL-50 is low-temperature dart impact at -20°F or lower. Percent gel is also a good method but requires more time and expense than low-temperature impact testing. The bend test may also be used but should be related to percent gel or dart impact testing. During initial molding trials for a new production part, low-temperature impact levels and/or percent gel of proposed test areas should be correlated with acceptable performance of the overall part.

POST-IRRADIATION

EFFECTS

One of the most interesting and novel features of crystalline olefin polymers is the irradiation-induced elastic memory phenomenon.t2bl When these polymers are crosslinked as, for example, into heat shrinkable tubing product, they behave as typical thermoplastics below their crystalline melting range and as elastomers, i.e., crosslinked rubbers, above their crystalline melting range. This is possible only because of their crosslinked structure. It is therefore possible to deform, e.g., stretch, such as product in its amorphous state (above the crystalline melting range) and freeze/cool the product in the deformed state. It will then ‘remain in the deformed state (having been cooled below its crystalline melting range) until the material is heated above the crystalline melting range, whereupon it will return precisely to its crosslinked geometry. The action of this heat to relax the deformed, e.g., stretched, material is what makes it able to return to its original, undeformed or unstretched state. This phenomenon can be illustrated by simply taking a rubber band (crosslinked, of course), hanging a weight on it to stretch/deform it, and then heating it until it returns to its original unstretched/undeformed geometry. A further explanation of the forces involved is given as follows. When crosslinked polyethylene structure is heated above its ctystalline melting point, the crystalline structure is destroyed and a rubbery (still crosslinked) amorphous material results. tzbl Now in this state the material can be deformed by a force and will return to its original dimensions upon

526


removal of the force. So now if the material is deformed while hot, the molecules will distort elastically. If the force is removed, the molecules will return to their original, lower free energy state. If, however, the polymer ( i.e., crosslinked molecules) is cooled in the elastically distorted state, the material will crystallize (below the crystalline region) and remain in the deformed/ distorted state. The molecules remain in this distorted state because the total crystalline forces have gteater strength that the forces due to the relatively few crosslinks, and the molecules cannot relax to any extent until the crystallites are remelted. The deformed condition is the form in which heat-shrinkable tubing is supplied to users. t19]The user/render is referred to Chart lo- 1 for a summary of engineering considerations to take when using heat-shrinkable tubing.

Chart 10-l: Engineering Considerations When Using Heat- Shrinkable

Tubing. (Date taken from Reference 19).

1.

2.

3.

All heat-shrinktubing changes dimension longitudinally as well as diame-trically when shrunk. This may affect one’s selection of material and/or shrink ratio offered. When shrinking over long cables, it might help to shrink both ends in place before proceeding to prevent/restrict longitudinal shrinkage. The more relatively noncrystalline (amorphous) tubings tend to shrink over a wide temperature range, starting as low as lOOoF. The more crystalline materials such as the polyolefins exhibit a narrow shrink temperature range based on the crystalline melt points; they shrink quickly when raised to the necessary temperature. The wall thickness on heat-shrinkabletubing increases when the tubing is shrunk. The wall thickness at any stage of recovered inside diameter, e.g., at the inside diameter needed to tightly shrink down to and encircle a particular diameter cable, can be calculated as follows: d, = I.D. with/at unrestricted/full recovery w, = wall thickness with/at unrestricted/full recovery d, = I.D. at which the wall thickness is to be determined w, = wall thickness at the recovered I.D. to be determined

Crosslinked Thermoplastics

For thin-wall tubing, this can be approximated by

w2 = w,-

527 d, d2

Neither formula takes longitudinal change into account. Note that a shrinkable tubing’s unrestricted/fully recovered inside diameter and wall thickness are always given by the tubing manmacmrer and in the applicable MIL specification. 4. Most plastics are notch sensitive, so care should be exercised to have smooth cut ends (because irregular edges may induce splitting during the shrinking process.) 5. Flame retardant tends to make polyolefins opaque. Therefore, clear polyolefins are not flame retarded. KynaP is a good substitute when clear non-flammable tubing is needed. (The MIL spec slash sheet for Kynar@shrink sleeving is MILI-23053/8.) 6. Dual-wall tubings offer some interesting design possibilities. The inner wall melts and flows around the encased part. The inner wall can be supplied adhesive lined so it not only encases but also adheres to the part. There are adhesives available which adhere well to polyolefins and other materials. These tubings offer excellent moisture seals. 7. When covering rollers, the best method is to use a hot air gun, and starting at one end, rotate the roll while the tubing shrinks, chasing the “shrink shoulder” down the roll to the other end. 8. When shrinking any tubing, care must be taken to heat it uniformly around the circumference. If a localized area becomes completely shrunk before the remainder, it will leave a chill mark (which is nearly impossible to remove) spoiling the looks of the job. (Raychem Corp. recommends the use of a proper reflector to help avoid this problem. See Reference 19.) 9. Hot air is the most commonly used method of shrinking tubing, especially thin tubing over non-metallic parts. Care must be exercised to see that the air blast does not move the tubing out of place. 10. Oven heating is a very good method of batch processing. It usually provides a tight covering. In the case of electrical components being covered, oven heating also tends to drive out moisture. 11. A gas torch is olten used when the tubing is large and has a heavy wall. It can be used very successfully in production for small parts in lieu of a hot air blast. A soft flame should be used so avoid burn or soot deposit on the parts. 12. It&red heat is a hworite production method because it can be easily controlled, is very fast, and has no air blast to disturb the tubing’s location on the part. Since it can be too t&t, care must be exercised to not damage the assembly. Color will have a marked effect - the darker colors absorb more heat than the light, i.e., clear tubing over black cable is nearly impossible to shrink with infrared heat.

528

Handbook of llermoset Plastics

Although the initial work in the field of elastic memory was carried out with polyethylene, other polymers exhibit this phenomenon after aeratetion. Table 10-5 provides a listing of various heat-shrinkable tubing products and their typical applications and properties after shrinking.[lg~ The applications include the insulation of a variety of electrical and electronic components including wines,lugs, terminals and connectors. In practice the tubing is supplied in the form of an expanded diameter ranging from 3/64 in. to 5 in.. The user heats the tubing, melting the crystallites in the molecular structure. This allows the crosslinked material to return to its original shape, exhibiting its elastic memory. Table 10-5 shows that the shrink ratio varies among the products approximately from 1.75: 1 to 3: 1. After cooling the crystallites reform and hold the tubing in the original/recovered form. Upon subsequent reheating, no further change in shape will take place unless a mechanical force is applied. The heat needed to melt the crystallites of a heat-shrinkable crosslinked thermoplastic is, in certain instances, also sufftcient to melt solder. This has led to the development of solder-sleeve devices consisting of fluxed solder and thermoplastic inserts at each end.[*‘l When placed over a cable shield and heated, the solder melts and flows, connecting the ground lead and shield (Figure 10-4). The outer sleeve shrinks and the thermoplastic inserts me16encasing the termination to provide a sealed termination. Other devices are available for a connector that takes removable contacts. The contacts may terminate coaxial cables, shielded wires and twisted wire pairs. In addition, a mctangular, multicavity, heat-shrinkable device is available for permanent splicing of flat conductor cable to flat conductor cable or round wires.

C‘rosslinked Thermoplastic.~

529

heavy dual wall, adhesive tubing flame retarded

istmt,flamerztaded

530

Handbook of’Thermmet Plastics

Table 10-5: Heat-shrinkable

Insul: tion and Encapsulation

Tubings

(Contir Pdti Flexible PolynletilS

RNF-100 TYPI

identiiicatmn:

RNF- IO0 Type2

Transparent mvering~ for components such as resistors, cafrxitors and cables where marking must be nmtccted and remain ledble.

I--

F RT- IO2

RP4OQ

Semitiljd Polyoletins

CRN TypelandType2

terminal and component insulation.

Coverim for cables and comwnents wkre excellent flexibility and outstanding flame

21

insuhtmn

Especdly

I 1

I

effective for low temperahire

Repair sleeving for hamess jackets; bigb shrink ratio to fit over connector and recover tightly on

I

11 lnsthtion

and strain reliefof soldered or aimpxl

Partiahriy suited for automated application systems to insulate and strain relieve crimped w soldered terminals. Furnished in cut cieces.

I

Encapsulation of components, splices, terminations, requiring moishxe resistance. Medunical pmteaionandshkkmtios~highas6to1. Insulates and seals elechica.l splices, bimetallic joints and components from nkture and corrosion TO&I owering for delicate axnponents. Environmental pot&ion for a wide variety of elechical compownt.3, including wire splks and harness hakouts.

I

Ttamwzent insulation, mechanical txntection of

Insuhtion of splices and terminations in aimtaft and mass transit markets; cable and wire ihtification

I

Insulation and covering terminals. kindles.

I

of cables, components,

Insulation and abrasion protection of wire buwks and cable hrunaes Cable and harness tx&xtion

Viton (FlouroeltiOrIW

requiring maximum

I

C‘rosslinked Thermoplastics Table 10-5: Heat-shrinkable

Tubings

(-31 t”221)

(1.2to50.8) ‘I, to 4 (6.4 to 101.6)

1.75:1

Black

z”mN

-70 to 121 (-94 to 250)

SFR (Silicone)

75 to 180 (-103 I” 356)

‘/,to2 (6.4 to 50.8)

1.75:1

Bkk

Viton (Fl&St-

-40 I” 200 (-U) to 392)

‘Is, to 2 (4.7 to 50.8)

2: 1

Black

Elastomers

Specific minimum

Insulation and Encapsulation

531

quirements and testmethod5 are givenin applicable

Them&it

specifkati”ns.

532

Hundhook

of Thermoset Plastics

Table 10-5: Heat-shrinkable

Insulation and Encapsulation

Tubings

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