Second Edition, Revised and ExDanded
edited by
ANI1 K. BHOWMICK Rubber Technology Centre Indian Institute of Technology Kharagpur, India
HOWARD L. STEPHENS The University of Akron Akron, Ohio
M A R C E L
95 D E K K E R
MARCEL DEKKER, INC.
NEWYORK BASEL
ISBN: 0-8247-0383-9 This book is printed on acid-free paper.
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Copyright 0 2001 by Marcel Dekker, Inc. All Rights Reserved. Neithcr this book nor any part may be reproduced or transmitted i n any form or by any mcans. electronic or mechanical, including photocopying, microfilming, and recording. or by any information storagc and retricval systcm. without permission in writing from the publisher. Current printing (last digit): 1 0 9 8 7 6 5 4 3 2 1
PRINTED IN THE UNITED STATES OF AMERICA
TO
Jatindra Mohan Bhowmick Hem Prova Bhowmick Kundakali Bhowmick Asmit Bhowmick Marian Stephens
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Preface
Exactly 10 years have glidedby since the first edition of Hardbook of Elastomers was published. Wherever we have traveled, we have heard good words about the book. It has been found to be useful for teaching, research, and business purposes. The overwhelming response from around the world prompted us to undertake a second edition. Considering the success of the first edition, the style of the book has not been changed. New chapters have been included and materials no longer in vogue have been deleted. Most of the chapters from the first edition have been updated with new information and technology, and only a few have been retained in their original form because no significant new developments have occurred. Readers’ suggestions have been incorporated in many places. We wish to thank all the authors for their fine contributions and sharing of their expertise. We are grateful to many rubber companies, polymer institutes, and research and development organizations around the world for valuable suggestions and assistance. We acknowledge our indebtedness to our family members, especially to Asmit Bhowmick, Dr. S. K. Biswas, and Ms. K. Biswas, for their patient understanding. Last,but not least, we are thankful to Russell Dekker, Chief Publishing Officer, and EricStannard, Production Editor, of Marcel Dekker, Inc., for their wholehearted support and guidance. We hope that the second edition of Handbook of Elastomers will be even more useful to our readers. Ani1 K. Bhownick Howard L. Stephens
V
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Contents ...
111
.vi
1. Guayule Rubber D. Mclrltyre, Ho~turclL. Stephens, W. W. Schlorwnn, Jr., crr~clAni1 K . Bhon*nlick
1
2. H e l w Natural Rubber A. H. Eng m t l E. L. Or1g
29
3.
Modified Natural Rubber Crispirl S. L. Brrkrr
61
4.
Chemical Modification of Synthetic Elastomers Dorltrltl N . Sch~rl:m t l A ~ ~ I ~ I I I0. L IPatil II~II
109
5. Liquid Rubber Douglcrs C. Ed1cwrd.s
133
6. Powdered Rubber Colirl W. EIWS
167
7. Rubber-Rubber Blends: Part I C. Miclwel Roltrrltl
197
8.
Rubber-Rubber Blends: Part 11. New Developments C. Micl~nelU o l c r d
227
9.
Short Fiber-Filled Rubber Composites Lloyd A. Goc.tt1L.r arlcl Willimu F. Cole
241
10. Thermoplastic Elastomeric Rubber-Plastic Blends Aubert Y. C o t m
265
11. Thermoplastic Styrenic Block Copolymers Geojfrey Holrlerl m r l C11rrrles U. Wilder
321 vii
viii
Contents
12. PolyesterThermoplasticElastomers:Part Rorleric P. Quirk unci Qizkuo Zhuo
I
353
13. PolyesterThermoplasticElastomers:Part H . M. J . C. Creerwers
I1
367
14. Thermoplastic Polyurethane Elastomers CIzarles S. Schollenberger
387
15. Thermoplastic Polyamide Elastomers Anil K. Bhowrnick
417
16. Ionomeric Thermoplastic Elastomers Kurnal K. Kar and Ani1 K. Bhowrnick
433
17. Miscellaneous Thermoplastic Elastomers Anil K. BhoMmick
479
18. Halogen-Containing Elastomers Daniel L. Hertz, Jr.
515
19. Tetrafluoroethylene-Propylene Rubber Gen Kojirna and Masayuki Saito
547
20. Carboxylated Rubber John R. Dunrl
561
21. Polyphosphazene Elastomers D. Frederick Lohr and Harold R. Penton
591
22. Advances in Silicone Rubber Technology: Part I, 1944-1986 Keith E. Polrnarzteer
605
23. Advances in Silicone Rubber Technology: Part 11, 1987-Present Jerome M. Klosowski
649
24. Acrylic-Based Elastomers Piero Anrlreussi und Arturo Carrano
659
25. Poly(propy1ene oxide) Elastomers Dotninic A. Berta ancl Edwin J. Vandenberg
683
26. Polyalkenylenes Adolf Drusler
697
27. Polytetrahydrofuran P. Dreyfuss
723
Contents
ix
28. Crosslinked Polyethylene Bllarot Dave'
735
29. Millable Polyurethane Elastomers Klalrss Knoerr and Uwe HofSlnann
753
30. Cast Polyurethane Elastomers Klaw Reeker
765
31. Polynorbornene Rubber Ani1 K. Bhonwick, C. Stein, and H o ~ ~ r L. r d Stephens
775
32. Nitrile and Hydrogenated Nitrile Rubber Saclzio Huycrslli
785
33. Diene-Based Elastomers Jcrdit E. Plrskas
817
34. Recycling of Rubber Willianl H.Klingerwnith and Krishna C. Bararntwl
835
35. EPDM Rubber Technology Richard Karpeles m r l Anthony V. Grossi
845
36. Isobutylene-Based Elastomers Neil F. Nert-rnnrt and James V. Flrsco
877
I n dex
909
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Contributors
Piero Andreussi EniChem, Milan. Italy Crispin S. L. Baker Tun Abdul Razak Research Centre, Brickendonbury. Hertford, England Krishna C. Baranwal AkronRubberDevelopmentLaboratory. DominicA.Berta
Inc., Akron, Ohio
Basell R&D Center, Elkton, Maryland
Ani1 K. Bhowmick India
RubberTechnology Centre, IndianInstitute of Technology.Kharagpur.
Arturo Carrano EniChem, Milan. Italy William F. Cole FlexsysAmerica L.P.. Akron. Ohio Aubert Y. Coran The Institute of Polymer Engineering, The University of Akron, Akron. Ohio H. M. J. C. Creemers DSM Engineering Plastics
BV, Sittard. The Netherlands
Bharat DavC ECC Productd3M Co.. Chelmsford.Massachusetts AdolfDraxler"
Degussa-Hnls AG. Marl, Germany
P. Dreyfuss Consultant.Midland,Michigan John R. Dunn J. R. Consulting, Sarnin. Ontario, Canada Douglas C. Edwards" PolysarLimited. Sarnia, Ontario, Canada A. H. Eng Rubber Research Institute of Malaysia, Malaysian Rubber Board. Kuala Lumpur. Malaysia Colin W. Evans? Consultant.Gateshead.England James V. Fusco" ExxonChemical Co., Baytown, Texas LloydA. Goettler$ Solutia. Inc.. Pensacola.Florida Anthony V. Grossi Crompton Corporation/UniroyalChemical Company. Inc.,Middlebury. Connecticut
xi
xii
Contributors
Sachio Hayashi
NipponZeonCo.,
Daniel L. Hertz, Jr. Uwe Hoffmann
Ltd., Tokyo. Japan
Seals Eastern, Inc., Red Bank, New Jersey
Rhein Chelnie Rheinau GmbH, Mannheim, Germany
Geoffrey Holden
HoldenPolymer Consulting, Incorporated,Prescott,Arizona
Kamal K. K a r
Rubber Technology Centre, Indian Institute of Technology, Kharagpur, India
Richard Karpeles Crompton CorporationNniroyal Connecticut William H. Klingensmith
AkronConsultingCo.,Akron,
Jerome M. Klosowski Dow Corning Klaus Knoerr Gen Kojima
ChemicalCompany.Inc.,Naugatuck, Ohio
Corporation,Midland,Michigan
Rhein Chemie Rheinau GmbH, Mannheim, Germany Asahi Glass Co., Ltd., Yokohama, Japan
D. Frederick Lohr* The D. McIntyre The
Firestone Tire and Rubber Company, Akron, Ohio
University of Akron, Akron, Ohio
Neil F. Newman" Exxon Chemical Co., E. L. Ong Rubber Malaysia
Research Institute of Malaysia, Malaysian Rubber Board, Kuala
Abhimanyu 0. Patil Jersey Harold R. Penton
ExxonMobilResearch
and Engineering Company, Annandale,New
Enterprises, Lady Lake, Florida
University of Western Ontario, London, Ontario, Canada
Judit E. Puskas Roderic P. Quirk Akron, Ohio
Maurice Morton Institute
of Polymer Science, The University of Akron,
Bayer AG, Leverkusen, Germany
C. Michael Roland Masayuki Saito
Lumpur,
EthylCorporation,BatonRouge,Louisiana
Keith E. Polmanteer Consultant, KEP
Klaus Recker
Baytown, Texas
NavalResearchLaboratory,
Washington, D.C.
Asahi Glass Co., Ltd.,Yokohama, Japan
W. W. Schloman, Jr. The University of Akron, Akron, Ohio Charles S. Schollenberger* PolyurethaneSpecialistand
Consultant, Hudson, Ohio
Donald N. Schulz ExxonMobil Research and Engineering Company, Annandale, New Jersey C. Stein* CdF Chimie
S.A.,Paris,France
Howard L. Stephens The Edwin J. Vandenberg Charles R. Wilder* Qizhuo Zhuo Ohio
* Retired
University of Akron, Akron, Ohio
Arizona State University,
Tempe, Arizona
PhillipsPetroleum Company, Bartlesville, Oklahoma
Maurice Morton Institute of Polymer Science, The University of Akron, Akron,
HANDBOOK OF ELASTOMERS
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Guayule Rubber D. Mclntyre, Howard L. Stephens, and W. W. Schloman, Jr. The University of Akron, Akron, Ohio
Ani1 K. Bhowmick Rubber Technology Centre, Indian lnstitute of Technology, Kharagpur, India
1. INTRODUCTION 1.l
Commercial Natural Rubbers
Although thousands of plant species contain rubberin small amounts,only a few speciesgenerate enough rubber to make them commercially attractive. In fact. only H e l m Drcrsiliensis and Pertheniun~argentaturn (guayule) have been used commercially in modern times and are likely to be used in the immediate future. Therefore. in this review these two natural rubbers will be called hevea rubber and guayule rubber. Other natural rubbers will be included in the discussion to giveabetterperspective on the nature of naturalrubbersandtheirprospects for future development. While guayule rubber is discussed here, the developments in the area of hevea rubber are treated in the next chapter.
1.2 Previous Reviews and Present Perspectives There have beenseveralreviews of hevea and guayule rubbers. The internationalscientific conferences on guayulerubber are particularlyinterestingandrelevantbuthavefrequently included overwhelming amounts of agronomic and economic material (McGinnies and Haase, 1975; Campos-Lopez. 1978;Gregg et al., 1983). Thelater conferences (Guayule Rubber Society, 1983, 1984, 1985) only have abstracts and are therefore less helpful for details of research. The International Symposium on Natural Rubber, 1980 (on hevea, guayule, and other rubbers), has some interesting comparisons of the two rubbers in its discussions. Several early authorative review articles on the whole subject of the viability and value of guayule rubber have been published (U.S. National Academy of Sciences, 1977; Campos-Lopez, 1978). A more easily accessible article on guayule rubber also appeared (Eagle, 1981), but it comprises an extensive literature review without either a critical evaluation of the underlying inconsistenciesof different workers or an attempted synthesis of the science and technology of guayule rubber. More recently, the Guayule Administrative Management Committeeand the U.S. Departmentof Agriculture (USDA) Cooperative State Research Service preparedacollectivereview of basicand applied research on guayule, including biochemistry (Benedict, 1991), processing (Wagner and Schloman. 1991), and rubber and coproduct utilization (Schloman and Wagner, 1991). 1
Mclntyre, et al.
2
In this small chapter we hope to present a current critical, scientific, and technological point of view. The remarks on the agronomicand economic aspects of the subject are tenuous and are included briefly only to suggest the challenge and uncertainty involved in discussing this important aspect of guayule rubber.
2. 2.1
STRUCTURE AND BIOGENESIS OF GUAYULE RUBBER Plant Morphology and Biogenesis of Rubber
Rubber is found in plant ductsinas the omnipresent milkweed or the tropical Heuea brasiliensis tree. The free-flowing milk from the tapped or broken duct contains the rubber as emulsified spheres in the cell fluids. Rubber is also found in single cells (parenchymal of cells) the ubiquitious goldenrod or guayule plants. Since there is no long duct to tap or break, each cell must be broken open by strong mechanical forces. Common childhood experience with goldenrod and milkweed neatly illustrates the technological problem in producing rubber from guayule and hevea. Milkweed. like hevea, immediately gives the sticky rubber-containing milk, while goldenrod, like guayule, does not appear to conta rubber. Fortunately, the Indians of northern Mexico and the southwestern United States found that by chewing the guayule plant they could spit out coagulated rubber. The extraction technology thus is more complicated for guayule than for hevea. Figure 1 shows an electron micrographof a cross sectionof a stem of the guayule plant. Note the dark cellular regions where the rubber is contained. The parenchymal cell, 30 p m X 30 pm X 30 pm, is full of spherical rubber droplets, as shown in Figure 2. Although the size of the mbber droplets appears to be approximately 1 pm, there are many droplets as small as 45 km is detrimental for natural rubber properties. Even 0.5%of these causes 30-40% reduction in tensile and fatigue-to-failure properties (Bhowmick etal., 1986a). Stress-strain isotherms athigh elongation, birefringence. and wide-angle x-ray diffraction have also been studied for guayule rubber blends.
2.4
Nonrubber Constituents
The nonrubber constituents in both guayule and hevea rubbers are primarily resins, residual plant tissue, and minerals. However, the amounts and types of constituents and their effects on physical and chemicalproperties can be quite different. For example,the principal residual plant tissue in hevea rubber is protein, which increases green strength, whereas the principal plant tissue in guayule rubber is ligneous and cellulosic cell wall material, which in ltrrge Jing~nenfs impairs ultimate physical properties. The mineral content of guayule is very low in all known guayule rubber extraction processes and is particularly low in processes that avoid water separation steps. Therefore there will be no further discussion here of rubber contaminants. The resins from the plant often remainin the rubber. Figures 3and 4 show the GPC curves of the tetrahydrofuran extractables from the totally ground-up branches of a guayule bush. The GPC trace of the high molecular weight rubber portion is given in Figure 3, while that of the resin is shown in Figure 4. The resin peak can be resolved into many small peaks by highpressure liquid chromatography or more extensive GPC.However, even in routine analyses the
a
Mclntyre, et al.
Fig. 4 GPC chromatogram of resin from fresh guayule shrub.
peaks can be separated into two main peaks: a low molecular weight peak approximating monoto triterpenes ( A ) and a high molecular weight peak approximating triglyceride molecular weight (B). Both the A and B curves can be conceived as resolvable into two major peaks. For the processing of guayule rubber it is imperative to control the total amount of A and B and specific compounds in A or B that change therubberpropertiesdrastically. Forexample. both the triglyceride and terpenoids plasticize lubber, yet some unsaturated fatty acid components increase thermal and oxidative degradation (Bhowmick et al., 1985). Again. large plant tissue fragments give inferior strength properties (Bhowmick et al.. 1984a). Apparently both the fine dirt ( c 4 5 p n ) and coarse dirt (B45 pm) contain a mesh of finer plant tissue tied together by rubber gel. The typical values allowed in commercial guayule rubber are approximately 0.2% for the plant tissue (dirt) and 3-4% for resin. The components of harvested guayule shrubs are given in Table 2.
Table 2 Components of Harvested Guayule Shrubs. c/c Moisture Rubber Resins Bagassc Leaves Cork Water-solubles Dirt and rocks
4-60 8-26' 5-25" so-SS" 15-20' 1-31' 10-12''
Variable
Guayule Rubber 2.5
9
Guayule RubberLatex
In its native form. guayule rubber exists in plant tissue as a latex (Hager et al., 1979; Goss et al., 1984; Backhaus. 1985: Backhaus et al.. 1991). The averagesize of rubber particles in latex from mature (3-year-old) plants is about 0.45-0.50 km. As isolated. guayule latex has a dry rubber content of about 40% (Schloman et al., 1996b). The coagulated rubber phase contains 8- 10%resin, although it is not clear whether these particular secondary metabolities are actually present in the rubberparticlesthemselvesprior to processing.Fromreported bulk viscosity measurements, it appears that latex rubber is indistinguishable from unfractionatedpolymer isolated by solvent extraction techniques (Schloman et al., 1987). Among the complement of proteins in guayule latex are a glycoprotein with a molecular weight of 5.2 X 10‘. possibly a subunit of a large rubber transferase complex, and two smaller proteins with molecular weights of 0.9 X IO‘ and 1.5 X 10‘ (Siler and Cornish, 1994a). Unlike Hevea latex (Slater, 1904).guayule latex does not elicit an immediate or Type I hypersensitivity reaction in human test subjects (Schrank et al., 1993). This is due to the absence of allergenic proteins analogous to those in Hevea latex (Siler and Cornish, 1994a; Siler et al., 1996). These allergens have molecular weights from 0.2 to 10 X 10‘ and include both soluble and particlebound proteins (Czuppon et al.. 1993; Beezhold et al., 1994). As a consequence, removal of soluble components by conventional washing techniques cannot render hevea latex freeof allergens. Extracts of finished goods such as latex gloves contain these same allergens (Hamilton et al., 1994).
3. 3.1
RUBBER-SEPARATION PROCESS General Methods of Processing Guayule Rubber
There are three major components to be separated in the bush: rubber, plant tissue, and resins. Fortunately. the plant tissue is not soluble in simple organic solvents or water. Also, there are solvents that will dissolve resin and not rubber (e.g., acetone), denoted S,,.,, and solvents that will dissolve both rubber and resins(e.g.. tetrahydrofuran and chlorinated hydrocarbons). denoted Thenthere are nonsolvents for rubber. NS,.,,,,, and for resins, NS,.,,. The individual diffusion coefficients for resin diffusion fl-orn pure rubber have been measured (Budiman and Mclntyre. 1984). In all processes. it is necessary to rupture the cellular structure of bark and woody tissue to gain access to guayule rubber, whether in the form of a latex or a coagulated bulk polymer. At the same time it is necessary to separate the rubber from various nonpolymeric secondary metabolities (resin). Commercially acceptable processing must also accommodate the inherent variability in the composition of the rubber in cultivated shrub. An additional consideration is the need to accommodate posthat-vest degradation of the rubber, either in intact shrub or in ground plant material (Black et al., 1986: Dierig et al., 1991 ). For the production of bulk rubber, three methods have been evaluated on a pilot scale: flotation. sequential extraction, and simultaneous extraction. In flotation processing (Fig. 5 ) . a dilutecausticsolution is used to coagulaterubber in the form of resinous “worms”(large aggregates of rubber). which are skinmed off and deresinated by washing with a polar organic solvent. typically acetone. Flotation process rubberwas produced in 1976- 1980 at a pilot facility in Saltillo.Mexico.operated by the Centre de lnvestigacion en Quimica Aplicada (CIQA) (Campos-Lopez et al., 1978; Foster and Compos-Lopez, 1982: Motomochi. 1983). Used successfully in the manufacture of aircraft. automobile, and truck tires, the rubber had acceptably high
Mclntyre, et
10
al.
aqueous alkali
Flotation
SHRUB
_____
RESINOUS RUBBER WORMS
Resin Extraction rubber non-solvent
1
RESIN SOLUTION rubber sol"ent
DERESiNATED RUBBER WORMS
- ---
~
BAGASSE
+
AQUEOUS EFFLUENT
Removal *""""""""""-""."
Filtration
Solvent Removal
RUBBER
Fig. 5 Flotation processing.
Mooneyviscosities but did not meet the FederalEmergency Management Agency (FEMA) specification for acetone extractables (4.0% maximum) (Wagner and Schloman, 1991). Sequential extraction involves the initial deresination of ground shrub with a polar solvent such as acetone. The resin-free plant tissue is then extracted with a nonpolar solvent such as hexane to remove rubber (Fig. 6). Flash evaporation of solvent with steam injection is necessary, with the water being removed from the rubber slurry viaa combination of dewatering operations of thermal or mechanical origin. Sequential extraction was evaluated using an oilseed extractor
Resin
SHRUB
- "1
I
Extraction
Extraction
WO?:K:SUE
,___""____
RUBBER
CEMENT
Filtration
1 RESIN SOLUTION
Solvent Removal
t RUBBER Fig. 6 Sequentialextraction.
RESIN
11
Guayule Rubber
rubber-resin solvent
SHRUB
,
_____________
Size Reduction
____._"_
Extraction RUBBER-RESIN MISCELLA
t
rubber non-solvent
SWOLLEN RUBBER
~
_______________
Filtration RESIN + LOW-MW RUBBER
Removal
RUBBER
BAGASSE
Fig. 7 Simultaneousextraction.
operated in a semi-batch mode at the Northern Regional Research Center of the USDA (Hamerstrand and Montgomery. 1984). Product quality was never reported. Simultaneous extraction (Fig. 7) addresses the weaknesses of both flotation and sequential extraction; the need for efficient deresination, theneed to control the bulk viscosity of the rubber, and theneed to minimizesolventstream contamination. In simultaneousextraction,ground shrub is extracted with a monophase solvent or solvent mixture (toluene, perchloroethylene, pentane, or pentane-acetone azeotrope) capable of removing both rubber and resin as a dilute solution or miscella (Cole et al.. 1987; Kay and Gutierrez, 1987; Wagner andParma,1988; Wagner et al., 1991). High molecular weight polymer is then coagulated from the miscella by addition of a polar solvent such as acetone, ethanol, or methanol (Beinor and Cole, 1986). The process used to isolate high molecular weight guayule rubber is associated with the formation of a resinous by-product containing, among other components. low molecular weight guayule rubber. The resin is dissolved in acetone. Further treatment with 90% ethanol results in the precipitation of low molecular weight guayule rubber. An alternative separation process involves the use of xylene with subsequent rubber precipitation by addition of ethanol. The latter process ensures high-purity, grey colored, tack-free rubber having a molecular weight of 40,000-50.000. The organic-soluble resins, separated into several fractions, the water-soluble resinous portion, and bagasse, a woody pulp containing lignins and cellulosics, are used for derivatization and development of value added materials. For the production of latex (Fig. 8),fresh shrub is cut and milled in water or other aqueous medium (Jones, 1948; Cornish, 1996). The dilute rubber dispersion that results is clarified and concentrated by centrifugation, creaming, or some combination of the two (Jones, 1949; Cornish, 1996; Schloman et al., 1996b). Overall process efficiency can be increased by recovering and using thedilutelatex serum in the dispersion step. Laticeswith 35-50% solidshavebeen produced in this way. While no continuous pilot-scale evaluations have been carried out, Jones (1 948) estimated that it may be possible to disperse 85% of the rubber available in the shrub and recover 90% of the dispersed rubber as latex product. Processing the residual plant tissue after latex removal has not been reported to date. Presumably, somevariation of solvent extraction could be applied to recover resin and undispersed rubber.
ification
12
Mclntyre, et al.
SHRUB
Concentration
Washing DILUTE RUBBER 1 4 SUSPENSION
_."_""_."_""_."".....""..~..."""".., '
RESINOUS BAGASSE
v LATEX CONCENTRATE Fig. 8 Production of guayule latcx.
3.2
AcceptanceSpecifications
The rubber specifications currently used for accepting guayule rubber are basically those of hevea rubber, with extra care being given to the amount of resin or its practical consequences: flow properties due to plasticization by resin and degradative properties due to the enhancement of lubber degradation by resins. The acceptance specification for bulk guayule rubber (FEMA. 1984) is based on the ASTM specification for grade 20 natural rubber (Table 3). A modified specification (Cole et al., 1991) was established for the rubber produced by BridgestoneFirestone, Inc., at its Sacaton, Arizona. pilot processing facility. No specification has been established for guayule latex.
Table 3 Specifications: Raw Guayule Rubber Compared
with Grade 20 Hevea Rubber
Guayule rubbcr FEMA
Modified Grade FEMA
20 NR ASTM
Property 5 0 . 2 0 50.20 Dirt, % 5 1.25 Ash, 9 50.80 Volatilc tnattcr. % 50.60 Nitrogen. % 50.008 Copper. 50.00 % 50.002 50.002 Manganese, 54.0 Acetone cxtract. 'P 230 Wallocc plasticity. P,, 240 Plasticity rctention, PR1 (Z-
'' N o spccificntlon. " Corrected fur antloxldant content
50.20 5 1.25 51
.so 1
50.008 54.0'' 2 30 240
5 I .00
50.80 50.60 50.008 15 I
235 240
13
Guayule Rubber Table 4 FeedstockShrub:Usablc Rubber and MeanPlasticity of Product Batches
Shrub cultivntor(s) AZ-IO1 (Sacaton) USDA composite (Sacaton) USDA composite (Marana) USDA composite (Salinas)
3.3
3.4
l.O 3.5 2
11.3-
38 -t 3 37 f 3 37 f 2 45 f 3
Recent Production History
The largest contemporary processing operation has been the 1987- 1990 production compaign carried out at the Sacaton. Arizona, prototype facility operated by Bridgestone/Firestone. Inc. (Cole et a l . , 1991). Extraction of rubber and resin employed acetone and pentane or hexane. Coagulation and fractionation were effected by the addition of acetone. Total rubber production amounted to 8.7 1 long tons. Atotal of 3.26 long tons of rubber meeting the TSR20 specification was delivered to various contractors for the production and evaluation of military aircraft tires. A total of 2.06 long tons meeting the FEMA specification was delivered to a contractor for the production of light truck tires. Feedstock for rubber production included cultivated shrub maintained at sites other than Sacaton, including Salinas, California, and Marana. Arizona. Table 4 provides a comparison of the rubber quality from these locations. A general observation was that the rubber product frequently had ash levels in excess of the ASTM maximum specification of 1.00%. It was determined that the most likely source of the ash was inorganic material contained within the shrub itself, rather than soil and dust on the surface of the plant material. Volatiles were also frequently higher than the ASTM maximum specification of 0.80%. Unlike hevea rubber volatiles, usually water or low nmlecular weight fatty acids, guayule rubber volatiles consist of entrained terpenes. Evaluations at Bridgestone/Firestone confirmed the suitability of higher-ash and higher-volatiles rubber for truck tire compounds (Valaitis et al.. 1992). Those batches meeting the ASTM specifications for ash and volatiles were selected for aircraft tire fabrication.
4.
COMPOUNDING AND PROCESSING OF GUAYULE RUBBER FOR ENDPRODUCT USES
Guayule rubber, (GA) has been used in both the resinous and deresinated form since the early 1900s. However, due to the high level (ca. 25%)of guayule resin components in guayule rubber, it was found that both the raw rubber and its vulcanizates were prone to rapid and accelerated oxidative degradation. This effect was probably due to the resin fatty acid compounds (i.e.,linoleic and linolenic) in the rubber, which are effective in promoting oxidative degradation of natural and other unsaturated rubbers (Bhowmick et al., 1985). Studies conducted with present-day forms of deresinated guayule rubber have shown that the amount of oxidative degradation is greatly reduced when the resin content is in the range of 5 phr or lower.
Mclntyre, et al.
14
4.1
Processing of Guayule Rubber and the Effect of Guayule Resin on Processing Characteristics
A comparative study of transient and steady-state shear viscosity, stress relaxation, and elonga-
tional stretching of hevea rubber, guayule rubber, and two synthetic polyisoprenes (IR) has been reported by Montes and White( 1982). For example, steady-state viscosity of the gum elastomers is shown in Figure 9. Hevea rubber has the highest viscosity and maximum relaxation time because of the presence of large amounts of gel and higher levels of long-chain branching. The effect of gel on natural rubber (NR) properties has been studied by means of novel experiments (Bhowmick et al.. 1 9 8 6 ~ )Gel . and nonrubber constituents have marked effects on the extrusion properties of guayulerubber (Montes andPonce-Velez, 1982). At 14OoC, gel increasesthe viscosity at low shear rates. The mixing of natural, guayule, and isoprene rubbers with EPC, FEF, and HAF black at different concentrations (30,50,and 70 phr)at 60 and 80°C was reported. The black incorporation time, optimum mixing time, and energy are lower for GR and IR than for NR (Ponce and Ramirez, 1981 ). Since the earlier studies conducted with GR did not indicate the resin content of the rubber, the poor properties reported for vulcanizates containing GR were due to the excessive amounts of resin present in the raw rubber. The current practice of removing or reducing the amount of resin to 5 phr has greatly improved the quality of the rubber and its vulcanizates. The plasticizing effect caused by the resins has been explained by Winkler and Stephens (1978), whose experiments involved various amounts of guayule resin in raw rubber mixes. In addition. the resin was effective i n reducing the gel content of both NR and GR, thus acting as an efficient chemical plasticizer. Thechanges occurring in plasticitymeasurements and gel content with the addition of resins as shown by mill mixing are given in Table 5. The reduction in molecular weight (lower plasticity values) is shown in Figure 10, where the reductions in extrusion time utilizing a Monsanto capillary rheometer for a natural rubber (SMR-5), CR. and styrene-butadiene rubber are compared. It is quite apparent that in all three elastomerstheaddition of guayule resin waseffective in reducingmolecularweightand is probably why GR processes more readilywith lower powerconsumption andfaster incorporation of fillers than NR. This effect has been reported by many authors, and perhaps the resin also functions as a homogenizing agent when fillers are used in rubber mixes. The data given in Table 6 illustrate the rapid breakdown of GR when compared with the other forms of NR. Ramirez and Ponce (1978) also reported this effect when they measured the power consunlption of pale crepe, smoked sheet, and guayule rubber (with added resin) when mixed at
?
O
HR
-
F.5 m 0
"
L
3
1
I
I
I
I
l
-4
-3
-2
-1
0
1
2
Fig. 9 Steady-state viscosity shear rate bchavior of cis-l ,4-polyisoprenesat 100°C.
Guayule Rubber
VI*
0 0 0 a em .- .an-
/ ]m
l-
.-i
V
h
v
m
'D
W ¶
3
.-S
h
e
m
h
0
U-
15
0
16
Mclntyre, et al.
Table 5 Effect of Milling on WallacePlasticity"
Resin (phr) passes
Mill
1
2
5
10
SMR-5 0
58
-
5
56
10
50 35 21 12
S4 41 49 27 33 16 19 10
25 50 100
-
43
49 43 29 17
45
9
9
23 14 9
-
-
-
44 38 37 25
33
15
16
13
10
9
36
GR 0 5 10 25 50 100 "
-
46 54 40 28
42 37 22 26
18 11
7
33 29 19 12 7
N o gel ohserved below the solid line.
80°C for set time periods. Their results, given in Table 7, also confirm that the addition of resin does reduce the power needed when compared to NR, especially when 6% resin was added. This produced a 24% reduction in power consumption under the conditions used. Consequently, guayule resin may find a market as a processing aid.
4.2
Rate and State of Vulcanization
The preparation of suitable vulcanizates with GR has always been questionable, since therubber collected from different sources was not thoroughly deresinated to the same extent or as well characterized as the elastomer prepared in the Mexican pilot plant. Using this elastomer,Winkler and Stephens (1978) conducted studies showing how vulcanization systems can be modified to produce adequate properties compared to NR. The results of these studies, indicating the compounds used and the vulcanization systems studied, are given in Tables 8 and 9 and illustrated in Figures 1 1 - 14. The ASTM and efficient vulcanization recipes were selected to determine whether or not the GR had the same vulcanization characteristics and physical properties as a high-quality NR. in this case SMR 5L.
Table 6 EffectofMilling
on MolecularWeight (g/mol)
Guayule rubber Four mill passes Ten mill passes
49 I ,000 254,000
Smoked sheet Pale crepe
472,000 382,000
614,000 321,000
So1trc.r: U.S. Department of Agriculture Technlcal Bullctin No. 1327: Research on Guayule. 1942- 1959.
17
Guayule Rubber Table 7 EnergyConsumptionin10-MinuteMixing Type of rubber Mixing speed (RPM) #1Pale crepe
Energy used
106.80 106.24 99.42 89.97 80.88 70.24 112.17 105.25 94.05 86.85 73.02 69.74 105.44
70 60 50
#l Smoked sheet
40 30 20 70 60 50
Guayule
Guayule
+ 2% resin
+ 6% resin
(watts)
40 30 20 70 60 50 40 30 20 70
100.00
90.88 82.56 71.60 72.56 90.32 87.62 82.53 73.06 68.43 53.12
60 50
40 30 20
Table 8 CompoundRecipes ASTM Semi-eff. Efficient vulcanization stock Black stock Gum Elastomer (NR, GR, or IR) Carbon black, N 330 Zinc oxide Stearic acid Polymerized 1.2-dihydro 2,2,4-trimethylquinoline Sulfur N-t-Butylbenzothiazolyl-2sulfenamide N-CycIohexyl-2benzothiazolylsulfenamide Tetramethylthiuram disulfide Total
vulcanization
100
100
100
0
35 5 2
0
5 2 -
2.25 0.70
109.95
3.5 2.5
-
2.0
2.25 0.70
-
144.95
0.25
100
0 3.5 2.5 2.0 1.2 -
2.2
0.8
1 .o
0.4
1 1 1.45
110.40
Mclntyre, et al.
18
Table 9 CuremeterVulcanizationData (140°C) ASTM
stock Gum Initial torque, dN-m Minimum torque, dN-m Maximum torque, dN-m t,(2), min t,(90), min Cure rate, d N - d m i n Reversion time, min
Efficient vulcanization vulcanization
Black stock
Semi-eff.
GR
NR
GR
NR
GR
NR
GR
NR
2.00
2.56 2.12 20.6 5 24.5 3.4 85
5.65 4.20 28.3
6.20 4.50 39.6 7 19 6.0 60
2.83 2.12 7.49
3.96 2.68 18.5
4.52 2.46 14.5
IO
8 21 .S
4.52 3.16 22.3 7 12 6.8 > 120
1.00
15.5 IO
31 I .7 94
11
25 2.8 55
24.5 1.1
3.4
> 120
> 120
10
16 4.8 35
TIME (minl
a-
ASTM BLACK STOCK I l C O ' C )
NR
GR
(B 0 0
I
1
I
20
I
40
I
ao
TIME (minl Fig. 11 Vulcanization characteristics of natural and guayule rubbers (a) in an ASTM gum stock and (b) in an ASTM black stock.
Guayule Rubber
19
1
I E V RECIPES GUM STOCKS (140°C) E
-
NR
Z
GR SEMI
U
W' 1 0 3,
GR
0. 0
1
l
40
80
120
TIME (min) Fig. 12 Vulcanization characteristics of natural and guayule rubbers with efficient vulcanization recipes.
A S T M G R BLOCKSTOCK (14Oy)
E
0
20
60
40
TIME (min) Fig. 13 Effect of recipe variations on the vulcanization rate of guayule rubber (ASTM black stock).
I E V RECIPES GUM GR
1
STOCKS (14Oy)
€ 1
z
+1SA
*2SA
t-
g
0 1 0
0
L 20
s
!
E
40
3
60
Time (min) Fig. 14 Effect of the addition of stearic acid on the vulcanization rate of guayule rubber.
Mclntyre, et al.
20
It is apparent that with the ASTM recipe, neither the gum nor the black GR stocks gave vulcanizationrates or torquevalues comparableto those of the NR stocks.However, both elastomers gave similarly shaped cure curves, indicating that the degree of cross-linking in the GR stocks was lower than that with NR at the same cure time. The two recipes represented as efficient and semi-efficientcure systems showeda definite increase in vulcanization rate with lengthy plateaus, that is, less than 2% reversion. The semiefficientrecipewasmoreefficient in producing a rapidvulcanizationtime with these gum stocks. Neither of the GR stocks was as tightly cross-linked as the NR stocks. The ASTM recipe utilizing GR when modified by using 4 phr of stearic acid in place of 2 phr and 1 phr of accelerator (TBBS) in place of 0.7 phr did not affect the scorch properties of the stock too greatly, but the additional accelerator noticeably reduced the 90% cure time. This indicates that in recipes of this type the cure rate of GR is more affected by the addition of accelerator than by the use of stearic acid, which functions as an accelerator-activator. It is known that GR generally requires additional stearic acid to obtain reasonable cure rates and physical properties. The efficient vulcanization recipe was used to show this effect with this well-characterized GR. It is apparent from the data that only 1 phr of additional stearic acid was necessary to increase the torque to about double that of the original recipe. Addition of 2 phr did not give results greatly different from those obtained with 1 phr.
4.3 Physical Properties of Vulcanitates Although studies by Spence and Boone (19271, Hauser and Le Beau (1943a.b. 1944), Morris et al. (1943, 1944), and Clark et al. (1945, 1956a,b) had examined the effects of various compounding ingredients and vulcanization techniques, none of these researchers worked with a well-characterized GR. Hopefully, with the use of modem technology and GPC for structure characterization, present-day studies may prove that extraction techniques are capable of producing a “standard” GR. Utilizing the ASTM recipe, physical properties were obtained both on unaged and aged black samples (14 days at 70°C) (Table 10). The 90% cure time was used for the sample preparation. Basically, the GR stock containing carbon black gave lower modulus, tensile strength, rebound, hardness, and tear strength than SMR. However, with the exception of tear strength, the properties would be sufficient for most industrial compounds. The differences in properties may have been due to the lower cross-link density obtained with the GR compound.
Table 10 VulcanizateProperties”(ConventionalRecipe
Cure time, t,(90), min Stress at 300% elongation, MPa Tensile strength, MPa Elongation, % Set at break, % Bashore rebound, 70 Shore A hardness Tear strength, kN/m Molecular weight between cross links, M,
A, 140°C)
GR
NR
25 7.24 ( + 120) 25.14 ( - 12) 635 ( -43) 14 40 54 (0) 31.15 13,000
19 12.21 ( + 100) 27.93 ( - 15) 490 ( - 39) 13 48 60 (0) 76.65 9,500
‘‘ Percent change after aging for 14 days at 70°C is gwen in parentheses.
Guayule Rubber
21
Table 11 VulcanizateProperties(EfficientVulcanizationRecipes,
140°C)
Efficient vulcanization GR
Cure time, t,(90), rnin Stress at 500% elongation, MPa 3.62 Tensile strength, MPa 620 Elongation, 8 34 Shore A hardness Bashore rebound, % Molecular weight between cross links, M,
24.5 1.38 9.48 690 31 59 16,800
12
Semi-eff. Vulcanization
NR
GR
21.5 5.00 1 1S21.21 5 680
16 1.72 11.38 720 32
37 62 13,900
NR
60
64
1 5,000
1 1,500
On aging, the percent changes in tensile properties were equivalent, indicating that GR was sufficiently stabilized to age at the same rate as SMR. The properties obtained with the gum stocks, using the efficient vulcanization systems (Table 1 l ) , showed the same trend. However, the tensile strength for GR was improved using the semi-efficient recipe. Again, without adjustments for increasing the fatty acid content of GR, the cross-link densities of the guayule compounds were lower, indicating a slower cure rate and lower tensile values. Processing and compounding studies were conducted on IR, NR. and GR in tank track padrecipes (Touchet, 1987). The guayulerubberwasproduced by TexasA&M University using a simultaneous extraction process. In general, the stock containing guayule rubber was indistinguishable from the NR stocks. While tensile strengths did not vary much among the various compounds, the guayule stock had higher 200% modulus and heat resistance. Aged flex fatigue was lower than that of either IR or NR stocks. The compounding properties of guayule latex reported by Scholam et al. ( 1 996b) are consistent with earlier work (Jones, 1948), indicating that the latex produces slowcuring, lowmodulus films. Unaged guayule films had a 500% modulus of 1.4 MPa after 60-minute prevulcanization at 65°C followed by 20 minutes at 104°C. In contrast, an equivalent Hevea film recipe had a 500% modulus of 5.1 MPa. Suitably aged to accommodate the slower cure rate, guayule film had a tensile strength ( 1 9 MPa) comparable to that of films produced from hevea lattices (2 1-22 MPa). The modulusof unaged guayule films canalso be increased by longer prevulcanization times and increased levels of zinc oxide and accelerator. The vulcanization properties of guayule latex are at least in part a consequence of having lower viscosity rubber [MLI + 4 (100°C) 5 691 and higher resin levels ( 2 8 % ) than hevea latex.
4.4
End Uses
Studies conducted utilizing ASTM and efficient vulcanization recipesfor determining vulcanization characteristics and physical properties of guayule rubber indicate that this elastomer can be utilized as a direct substitute for hevea natural rubber. Neglecting recipe modifications, guayule rubber does give physical properties similar to those obtained with natural rubber vulcanizates. If a “technical specified type” of guayule is commercially feasible, this form of rubber can become a direct substitute for all types of hevea rubber. For example, tests conducted by the U.S. Navy and other governmental agencies have found that guayule functionswell when used in aircraft tires and othermechanical goods. Blends with other synthetic rubbers and grafted copolymers could be used for many applications.
22
Mclntyre, et al.
Tires fabricated from rubber produced at the BridgestoneSirestone prototype processing facility were tested from July 1993 through December 1995 at the U.S. Army Yuma Proving Ground (Lucas, 1996). Two test sets were evaluated: one compounded with a 50 :50 blend of guayuleandhevearubbersand a secondwith 100% guayule rubber. Guayulestocks were substituted in all parts of the tires where hevea rubber would normally be used. The test tires were mounted on light trucks designated as commercial utility cargo vehicles. Tire performance was compared with three control, orbaseline, sets of commercial light truck tires. Guayule tires met or exceeded the performance of controls in ride handling, stability, evasive maneuver, and braking. The multiseason on-road endurance capabilities of the guayule tires were comparable to those of the baseline tires over 10,000 miles. Cured dipped film prepared from guayule latex yielded 0.16 mg of leachable protein per gram, 44% of the yield from a hevea film (Schloman et al., 1996b). Enzyme-linked immunosorbent assays of guayule film extracts confirmed the absence of immunogenic proteins. Guayule latex would not elicit a systematic Type I allergic reaction in individuals sensitized to hevea latex. Processing of guayule shrub provides mainly five coproduct fractions including high molecular weight rubber, low molecular weight rubber, organic-soluble resin, water-soluble resin, and bagasse. Although the guayule generates high-quality and high molecular weight natural rubber with properties comparable to Heveu brusiliensis, cultivation, harvesting, and processing costs are high. As a result, development of value-added materials from the coproduct fractions of guayule is necessary. The alkene character of low molecular weight guayule rubber affords the opportunity for chlorination, epoxidations. maleinization, cyclization, hydrogenation, and many other reactions. Thames et al. (1994) reported the chlorination of low molecular weight guayule rubber. They developed 100% solid coatings that cured rapidly with UV light, thus offering energy savings, ease of handling, and wide formulation lattitude. The addition of reactive groups to the polymer backbone of chlorinated rubber resulted in the formationof environmentally compliant coatings. Thus, chlorinated hydroxylated rubber was developed. This rubber (2.7 weight% hydroxyl) with polyol was reacted with isocyanate in a hydroxyllisocyanate ratio of 1.0/1.05- 1.10. The coating cured at room temperature confirmed toughness, high gloss, and resistance to water, organic solvents, and chemicals, as given in Table 12 (Thames et al. 1994). Additionally, chlorinated hydroxylated rubber was reacted with acrylol chloride to produce acrylated chlorinated rubber, a binder for use in wood fillers and clear finishes. Epoxidized rubber and maleinized rubber were also reported by the same authors. The resins seemed tooffer the most promiseof financial return in connection with producing deresinated rubber (Arid Lands studies, 1979). Guayule resin may find use as a peptizing agent for rubber, aiding to breakdown gel or high molecular weight fractions in the rubber, which are broken by mechanical shear. Work hasalso been carried out atCenter deInvestigacion en QuimicaAplicada to developvarnishes and adhesives. Possible other uses are pigment dispersors and tackifers for rubber. The use of guayule coproducts as extender/plasticizer for epoxy resin coatings was also investigated (Thames and Kaleem, 1991). The coatings were applied onto aluminum and cold rolled steel substrates. When compared with the unmodified epoxy coating, the formulations containing 10% guayule resins performed equallywell on treated metal substrates. In contrast, the films formed on nonheated steel and aluminum panels are strippable. These resins contains triglycerides of fatty acids, which when incorporated into epoxy formulations impart flexibility and plasticizationto the resultant coatings. Guayule resins have also been evaluated as a wood protectant against termites, fungi, and barnacles (Thames and Poole, 1992). Based on weight, bagasse is the largest by-product. In a typical harvested guayule shrub, bagasse constitutes 50-55% as compared to 8-26% of rubber and 5-15% of resin. Its importance
23
Guayule Rubber Table 12 Properties ofTwoComponent Polyurethane Coatings with Chlorinated
Hydroxylated Low Molecular Weight Guayule Rubber Property
Wet thickness Drying time touch to Set free Dust free Tack Solid ess Pencil strength, Tensile Wpsi Elongation at break n-lb direct, Impact Adhesion D-3359) (ASTM rub) MEK (double 8-hour spot tests Water NHJOH 10% NaOH 20% HZSOJ 5 = No effect; 4 = stain only: 3 lifted film; 1 = failure.
2 mils 15 min min 105 min 65.40 H
3.90 1270
120 5B 200
5 5
5 5 =
blistering; 2 =
as a source of fuel for guayule processing is indisputable. Direct combustion of bagasse gives a fuel value of 18200 kJ/kg. A gas containing olefins, hydrogen, and carbon monoxide is formed (Thames and Poole, 1992). Bagasse and guayule leaves mixed in a particular ratio and subjected to a pressure of 8000 pounds per square inch (psi) and temperature of 90- 110°C indicated possible use of these materials for insulation or wallboard (Arid Lands-Studies, 1979). Bagasse can also be used to provide cellulosic materials and is a source of fermentable sugars or fibers. Various cellulosic derivatives like cellulosic acetate, cellulose nitrates,and regenerated cellulosics have been prepared. It may also find use in paper, cardboard, pressed board manufacturing, or some other lower quality uses. Leaves constitute an excellent soil amendment, especially when composted. After parboiling, they can be compressed into a building board.
5. IMPLICATIONS OF COMMERCIAL GUAYULE PRODUCTION 5.1
Present
In its most recent manifestation, guayule research has involved cooperation among government agencies, academic institutions, and private industry. Various programs have emphasized integrating the science and economics of agricultural production, processing, and product development. Extensive testing has validated guayule rubber as a substitute for hevea rubber in tire applications (Bailey, 1995). Ongoing research indicates that guayule latex is a promising material for use in medical products. While a significant amount of the rubber and coproduct resin from
24
Mclntyre, et al.
the Bridgestone/Firestone production campaign still remains, the pilot facility that produced the material has been mothballed.
5.2 Future Guayule hasbeen characterized timeand again as a new industrial crop destined for commercialization. The Office of Technology Assessment of the U.S. Congress (1991) has concluded that economic, not technical, constraints will have the most profound impact on such development. Guayule rubber must be competitive withhevea andsynthetics in terms of cost as well as quality and performance. Weihe and Nivert (1983) predicted commercial viability for producing bulk rubber if the yield of rubber per acre exceeded 1000 lb, if the price of rubber reached the record highs of the 1970s andif guayule resin had a value greater than that of pine resin. More recently, Foster et al. (199 1) concluded that guayulegrowers would have toyield over 1300 lb of high-molecular weight rubber per acre per year to break even. Alternatively, the price of rubber would have to exceed $1 .00 per pound. The necessary increases in agricultural production will have to come from ongoing efforts to identify high-biomass, high-yield cultivars, as well as efforts to develop techniques for low-cost direct seeding. Chemistry, such as the use of amine bioregulators to stimulate biomass and rubber production, has had less of an impact in this area thanconventional plant breeding. In the Foster analysis, profitability was more affectedby the value of the resin and bagasse (residual woody tissue) rather than of the rubber. Process development may ultimately focus on products for niche markets rather than for the tire industry. Guayule rubber’s future could be in high-value applications such as the production of hypoallergenic dipped goods and industrial products. What seems of even more importance for the future of natural rubbers and fuels is the hope that intensive and continuing studies of guayule rubber biosynthesis and guayule rubber extraction in a commercially successful venture would lead to a host of new discoveries in bioregulation and plant extraction processes. If these trained scientists and new discoveries were then harnessed for future world polymer production and hydrocarbon fuels, the exploitation of land that is not useful for food production could be kept for the production of hydrocarbons to the benefit of human kind.
REFERENCES Angulo-Sanchez, J. L., Jimcnez-Valdez, L., and Campos-Lopez, E. (1981), J. Appl. Polvm. Sci. 26:lSl I . Angulo-Sanchez. J. L., Neira-Velazquez, G., and Jasso de Rodriguez, D.(1995). I t d . Crops Prod. 4 :113. Appleton, M. R., and van Standen, J. (1989), J. PIrrr~t.Physiol. 134524. Archer,B.L.,Bamard, D., Cockbain, E. G.,Dickenson, P. B.,andMcMullen, A. T. (1963), in The Chernistp U I I Physics ~ .f Rubber-like Substarms (L. Baternan, Ed.), Wiley. New York, Ch. 3. AridLandsStudies(1979), Report-A SociotechnicalSurvey of GuayuleRubberCommercialization, Office of Arid Lands Studies, University of Arizona, Tucson, Arizona and Midwest Research Instltute, Kansas City, MO, April 1979. Arreguin, B. (1978). in Gucryule: Reencuentro er] el Desierto (E. Campos-Lopez, Ed.), CONACYT, Mexico. p. 9s. Backhaus, R. A. (198S), Bot. Grrz. 144391, Backhaus, R.A., Cornish, K., Chen, S.-F., Huang, D.-S., and Bess, V. H. (1991).P / t ~ ~ t o c ~ ~ r 30:2493. rr~~i.~tr~ Backhaus, R. A.. and Nakayama, F. S. (1988). Rubher Chem Techno/. 61:78. Bailey, C. A. (1995), paper presented at the 1995 Spring National Meeting of the AIChE. Houston, TX.
Guayule Rubber
25
Beezhold. D. H., Sussman. G. L., Kostyl, D. A., and Chang, N. S. (1994), Clin. Exp. lmmurwl. 98:408. Beinor, R. T., and Cole. W. M. (1986). U.S. Pat. 4,623,713 (The Firestone Tire & Rubber Co., assignee). Benedict. C. R. (1983), in Biosyrlthesis of lsoprerroid Corrlpoutds, Vol. 2 (J. W.Porter and S. L. Spurgeon, Eds.), Wiley-Intersciencc, New York, pp. 355-369. Benedict, C. R., Madhavan, S., Greenblatt, G. A., Venkatachalam, K. V., and Foster, M. A. (1989), Plcu~t Physiol. 92:8 16. Benedict, C. R. (1991), in Guqwle Nuturd Rubber (J. W. Whitworth and E. E. Whitehead, Eds.), GAMCUSDAKSRS. TucsoI1, AZ, pp 93-106. Bhowmick. A. K., Rampalli. S.. Kasemsuwan, S., and McIntyre, D. (1984a). Proc. Fifth Arlrl. Cor!f:Gltnyuie R ~ d ~ hSocie/y. t~r Washington, DC. June 17-21, p. 94. Bhowmick, A. K., Manzur. A., and McIntyre, D. (l984b), unpublished observations. Bhowmick, A. K., Rampalli, S., and McIntyre, D. (1985). J. Appl. Polyrtl. Sci.. 30:2367. Bhowmick. A. K., Kasemsuwan, S., Oroz, M. A., Patt, J., Secger, R., MacArthur, A., and McIntyre, D. ( 1986a), Kc~rrtschrrk Gurwni Kurlststofle 39:1075. Bhowmlck, A. K., Kuo, C. C., Manzur, A., MacArthur, A., and McIntyre, D. ( 1986b), J. Mr~crorr~ol. Sei.Plrys. E d 25:283. Bhowmick, A. K., Cho, J., MacArthur, A., and McIntyre, D. (1986c), Polyrrler 2 7 1889. Bhowmick, A. K., Rampalli, S., Gallagher, K., and McIntyre, D. (1987), J. Appl. Polym. S r i 33:l 125. Black, L. T., Swanson, C. L., and Hamerstrand, G. E. (1986), Rubber Cllerr~.Teckr~ol.5Y: 123. Budiman, S., Chu. E., Secger, R., and McIntyre, D. (1981). Rubber World 184:26. Budiman. S.. and McIntyre, D. (1984), Rubber Cherrl. Techol., 57:352. 370. Campos-Lopez, E., and Angulo-Sanchez, J. L. (1976). J. P o l y r ~ Sci.. . Purl A - l 14:649. Campos-Lopez. E., and Palacios, J. (1976). J. Po/.vrr~.Sei.. Ptrrt A - l 14:1561. Campos-Lopez, E., Ed. ( 1978), Gurryle: retwc'uentro er1 el cle>.sierto,Proc. Intern. Conf. Guayuleat Saltillo, Coahuila. Mexico, August 1-5, 1977. Campos-Lopez, E., Neavez-Cnmacho, E., and Garcia, R. M. (1978). in Gucryule reerlcuerltro en el rlesierto (E. Campos-Lopez. Ed.). CONACYT, Mexico, p. 375. Clark, F. E., and Place, W. F. L. (1945). I r ~ d i c cRubber World 112:67. Clark, F. E., and Place, W. F. L. (1946a), l t l d i r i Rubber World, I /5:370. Clark. F. E.. and Place. W. F. L. (1946b), h r l . h g . C h m . 38:1026. Cole. W. M.. Fenskc, S. L., Serbin, D. J., Malani, S. R., Clark, F. J., and Beattie, J. L. ( 1987). U.S. Pat. 4,681,929 (The Firestone Tire & Ruber Co., assignee). Cole. W. M., Hilton, A. S.. Schloman. W. W., Jr., Compton, J. B., Dembek, J. A., Jr. (1991). Guayule Rubber Projcct: Final Report Prepared under Contract No 53-3142-7-6005, pp. 68-182. Cornish, K., and Backhaus. R. A. (1990), Ph~toche~rt~i.stt~v 29:3809. Cornish, K., Siler, D. J., and Grosjean, 0.-K. K. (l994), Pk~ltoc'hmzistt~\~ 35:1425. Cornish, K., and Siler, D. J. (l99S), J. Plrrlt. Physiol. 147:301. Cornish, K. ( 1996), U S . Pat. 5,580,942 (Secretary of Agriculture, assignee). Czuppon, A. B., Chen, Z.. Rennert, S., Engelke. T.. Meyer, H. E., Heber, M,, and Baur, X. (1993). J. Allergy Clir~.,Irr~rrrur~ol.92:690. Dierig, D. A., Thompson, A. E., and Ray, D. T. (1991). R u l h r Chertz. Techrlol. 64:211. Eagle, F. E. (1981 ), Rubber Cherr~.Tec1111ol. 54:662. Federal Emcrgency Management A g e ~ ~ ( 1984), y National Stockpile Purchase Specification. Rubber-Parthenium (Guayule), U.S. Department of Commerce Report P-48C-R. Foster. K. E., and Campos-Lopcz, E. (1982), Technology Assessment of the Commercialization of Mexican Guayule, Final Report, July, NSF Grant No. PRA 880 7458, and CONACYT, p. 73. Foster. K. E., Weight, N. G., and Fansler, S. F. (1991), Gutryule Natured R~rhberC~rt7rr1ercj~~/i~cr/i(1r~; A Scttle-Up Fetr.sibili/y Study. OALS, Tucson. AZ. Goss, R. A., Benedict, R. A., Keithly, J. H., Nessler, C. L., and Stipanovic, R. D. (1984). Plorlt Physiol. 74534. Gregg, E. C., Tipton,J. L.. and Huang, H. T., Eds. ( 1983). Proceedings of the Third International Guayule Confcrencc, Pasadena. CA. Guayule Rubber Society ( 1983). Fourth Annual Conference, Riverside, CA; (1984). Fifth Annual Conference, Washington, DC; ( 1985) Sixth Annual Conference, Tucson, AZ.
26
Mclntyre, et al.
Hager, T., MacArthur, A., McIntyre, D., and Seeger, R. (1979), Rubber Clfern. Techr~ol.52:693. Hamerstrand, G. E., and Montgomery, R. R. (1984), Rubber Chen~.Tecknol. 57344. Hamilton, R. C., Charous, B. L., Adkinson, N. F., Jr., and Yunginger, J. W. (1994), J. Lab. Clin. Med. 123594. Hauser, E. A., and Le Beau, D. S. (1943a), India Rubber World 106:447. Hauser, E. A., and Le Beau, D. S. (1943b), India Rubber World 107568. Hauser, E. A., and Le Beau, D. S. (1944), India Rubber World 108:37, 44. Ji, W., Benedict, C. R., and Foster, M. A. (1993), Plant. Physiol. 103:535. Jones, E. P. (1948). Ind. Eng. Chenl. 0 8 6 4 . Jones, E. P. (1949), U.S. Pat. 2,475,141 (Secretary of Agriculture, assignee). Kay, E. L., and Gutierrez, R. (1987), U.S. Pat. 4,684,715 (The Firestone Tire & Rubber Co., assignee). Lucas, W. (1996), SummaryTest Report for the Evaluation of the M1028, Commerclal Utility Cargo Vehicle, TECOM Project No 1-EG-095-000-028, YPG No 96-055. MacArthur, A., and McIntyre, D. (1983), in Proc. Third Intern. Guayule Cor$, Guayule Rubber Soc. (E. C. Gregg, J. L. Tipton, and H. T. Huang, Eds.), p. 309. McGinnies, W. G., and Haase, E. F., Eds. (1975), Proc. Intern. Conj On the Urilizatiorr of Guayule, Tucson, AZ. McIntyre, D. ( 1978), in Guayule: reencuentro C I I el desiertn (E. Campos-Lopez, Ed.), CONACYT,Mexico, p. 251. McIntyre, D., Shih, A.L., Savoca, J., Seeger, R.,and MacArthur, A. (1984),in Size Exclusion Clmrncttography, ACS Publ. 245 (T. Provder, Ed.), American Chemical Society, Washington, DC, p. 227. Montes, S. A., and White, J. L. (1982), Rubber Chern. Techno/. SS:1354. Montes, S. A., and Ponce-Velez, M. A. (1982), Rubber Chern. Teclrnol. 55:l. Morris, R. E., Barrett, A. E., Lew, W. B., and Werkenthin, T. A. (1943), 111dict Rubber World, pp. 109, 150, 192, 252. Moms, R. E., Barrett, A. E., Lew, W. B., and Werkenthin, T. A. (1944), India Rubber World, pp. 57, 63. Motomochi, B. (1983), Proc. Third Intern. Guayule Con$ Guayule Rubber Soc. (E. L. Gregg, J. L. Tipton, and H. T. Huang, Eds.), p. 89. d 5S:203. Nakayama, F. S., Cornish, K., and Schloman, W. W., Jr. (1995), J. Arid h r ~ Studies Office of Technology Assessment (1991), Agricultural Materials os lndusrricrl Rarcl Materictls, OTA-F476, USGPO, Washington, pp. 1-10, Ponce, M. A., and Ramirez, N. R. (1981). Rubber CIre~n.Techrlol. 5 4 2 1 I . Porter, L. S., and Stephens. H. L. (1979). Rubber Clrerrr. Techrlol. 52:361. Ramos de Valle, L. F., and Montelongo, M. (1978), Rubber Clfern. Tecllrrol. 51:863. Ramirez, R. R., and Ponce, M. A. (19781, in Guayule: Corlsejo N d n c r l de Cierlcirr Y Technologin. CIQA, Salitillo, Mexico. Reddy, A. R., and Das, V. S. R. (1988), J. Plant Physiol. 133:152. Schloman, W. W., Jr., Carrot, D. I., Jr., and Ray, D. T. (1986), J. Agric. Food Cherrl. 34683. Schloman, W. W., Jr., Ray, D. T., and Coates, W. (1987). paper presented at the 7th Annual Meeting of the Guayule Rubber Society, Annapolis, MD. Schloman, W. W., Jr., and Wagner, J. P. (1991). in Guuvule Naturcrl Rubber (J. W. Whitworth and E. E. Whitehead, Eds.), GAMC-USDNCSRS, Tucson. AZ. pp. 287-310. Schloman, W. W., Jr.. McIntyre, D., Hilton, A. S., and Beinor, R. T. (1996a). J. Appl. Polym. Sei. 60: 1015. Schloman, W. W.. Jr., W Y Z ~ O SF., ! ~ ,McIntyre, D., Cornish, K., and Siler, D. I. (1996b), Rubber Chert!. Technol. 69:215. Schrank, P. J., Carey, A. B., Simon, R. A., Ward, B., and Cornish, K. (1993), J. Allergy Clin. I ~ I I I I I U I I O ~ . 91:385. Sidhu, 0. P,,Ratti, N., and Behl, H. M. (1993). J. Agric. Food Chern. 41:1368. Siler, D. J., and Cornish, K. (1994a), Phytocl~ernistr~ 36:623. Siler, D. J., and Cornish, K. (1994b). I n d . Crops Prod. 2:307. Siler, D. J., Cornish, K., and Hamilton, R. G. (1996), J . Allergy Clin. ImfurroI. 98:895. 94:139. Slater, J. E. (1994) J. Allergy Clin. In~n~unol.
Guayule Rubber
27
Spence, D. and Boone, C. E. (1927), NBS Tech. Rep. No. 353. Subramanian, A. (1972). Rubber Chem. Technol. 48:346. Tanaka, H. (1985), private communication. Thames, S. F. and Kaleem, K. (1991), Biosource Technol. 35:185. Thames, S. F. and Poole, P. W. (1992), Polymeric materials from agriculture commodities, in ACS Symposium series 476: Emerging Technologies f o r Materials and Chernicals from Biomass. pp. 274-297. Thames, S. F., Poole, P. W., He, Z. A., and Copeland, J. K. (1994), Synthesis, characterization, derivation and application of guayule coproducts, in ACS Symposium series 575; Polyners fromAgricultural Coproducts. pp. 223-239. Touchet, P. (1987), in Elastomers andRubber Technology: Sagamore Army MaterialsConfererlce Proceedings, Vol. 32 (R. E. Singler and C. A. Byrne, Eds.), USGPO, Washington, pp. 535-545. U.S. National Academy of Sciences (1977), Guayule, An AlternativeSource of Natural Rubber, U.S. Natl. Acad. Sci., Washington, DC. Valaitis, J. K., Kern, W. J., Schloman, W. W. Jr., and Hilton, A. S. (1992), in New Industrial Crops crnd Products: Proceedings of the First International Conferenceon New Industrial Crops and Products, Riverside, Cali$, 1990 (H. H. Naqvi, A. Estilai, and I. P. Ting, Eds.), AAIC, Riverside, CA, pp. 131-134. Wagner, J. P., Engler, C. R., Parma, D. G., and Lusas, E. W. (1988), Po1ym.-Plast. Technol. Eng. 2 7 155. Wagner, J. P., and Parma, D. G. (1988), Po1ym.-Plast. Technol. Eng. 27335. Wagner, J. P., and Parma, D. G. (1989), Polvm.-Plast. Technol. Eng. 28:753. Wagner, J. P,, and Parma, D. G., and Benedict, C. R. (1991). Po1ym.-Plast. Technol. Eng. 30:473. Wagner, J. P,, and Schloman, W. W., Jr. (1991), in Guayule Natural Rubber (J. W. Whitworth and E. E. Whitehead, Eds.), GAMC-USDAKSRS, Tucson, AZ, pp. 261-286. Weihe, D. L., and Nivert, J. J. (1983), in Proc. Third Intern. Guayule Conk (E. C. Gregg, J. L. Tipton, and H. T. Huang, eds.), Guayule Rubber Society, Riverside, CA, p. 115. Winkler, D. S., Schostarez, H., and Stephens, H. L. (1978a), in Guayule Consejo Nacional de Ciencia Y Technologic/. CIQA, Saltillo, Mexico, Ch. 15. Winkler, D. S., and Stephens, H. L. (1978b), in Guuyule, Consejo Nacional de Ciencia Y Technologia, Ch. 17.
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Hevea Natural Rubber A.
H. Eng and E. L. Ong
Rubber Research lnstitute of Malaysia, Malaysian Rubber Board, Kuala Lurnpur, Malaysia
1. INTRODUCTION 1.l
Naturally Occurring Polyisoprene
Natural cis- 1.4-polyisoprene occurs in over 2000 species of higher plants, the most well known of which is natural rubber from Helva brasiliensis. Other plants, such as guayule (Partheniuru argentaturn),Russian dandelion (T~rrasacu~~z kok-scrghyz), goldenrod (Solidago rdtissir~~a), Jelutong (Dyera retusa), and fungal genera such as Lnctarius tvolerrlus have also been known to producecis-1,4-polyisoprene.Only a relatively few plantspeciessuchas balata (Marrru.scq)s balata) and Gutta percha ( P ~ l a q ~ r i gutta) ~ r r ~ ~produce gutta or trrrns- 1,4-polyisoprene. Chicle (Achras sapota) is known to produce a mixture of tram- 1,4-polyisoprene and cis- 1.4-polyisoprene in a ratio of about 1 : 4 (Schlesinger and Leeper, 1951; Stavely et al., 1961. Archer and Audley,1973). Despite the various possible sources of naturally occurring rubbers. natural rubber from Het9ea brasilierlsis remains the most widely used. Much effort has been made to replace natural rubber with synthetic analogs in various applications, such as the discovery of the Ziegler-Natta catalyst for theproduction of syntheticcis-polyisoprene in 1956. This, however, has never been achieved, and today natural rubber accounts for about 40% of the total rubber consumed worldwide.Withthegrowingconcern for thehugequantities of toxicwastegeneratedand energy consumed by the synthetic rubber industries, the consumption of natural rubber (Wan A. Rahaman, 1994; Jones, 1994), a more environment friendly and sustainable raw material, is expected to increase in the years to come.
2. STRUCTURE AND BIOGENESIS OF NATURAL RUBBER 2.1 Natural RubberLatex Cis- 1,4-polyisoprene rubber occurs in the H. brmi1iensi.s tree as minute particles, which form the dispersion phase of a milky fluid or latex. Latex vessels have been found in all organs of the tree. However. their density varies. with the lowest occurring in the wood and highest in the secondary phloem. where the vessels occur as a series of rings concentric with the cambium (Gomez and Moir, 1979). It is this part of the rubber tree that produces the latex initials. The 29
Eng and Ong
30
sequence of these rings indicates the developmental stages of these vessels, ranging from the youngest laticifers in the ring next to the cambium to senescent and disintegrating vessels in the outer bark. Upon tapping, or with a puncture made into the phloem with a sharp object, many vessels are severed and the latex that flows out is a mixture of the latices from vessels of different developmental stages. When atree of anyone clone or seeding is first tapped, it produces an unstable latex with a high dry rubber content (DRC). As the tapping is continued with a regular tapping system, the latex stability increases and the DRC falls to a steady level, which can vary between25 and 45%, depending on the nature of the planting material. Changes in DRC can also be brought about by other factors, such as the tapping system, seasonal variation, and yield stimulation (Wiltshire, 1934;M o m s and Sekhar, 1959;Resing, 1955;Abraham et al., 1971). For example, a full spiral cut gives a lower DRC than a half spiral cut, and alternate daily tapping results in a higher DRC than daily tapping. Higher DRCis also observedfor latex obtained from high-panel tapping compared with that from low-panel tapping (Heusser and Holder, 1931).Furthermore, intensive tapping or drastic tapping causes marked decreases in DRC. 2.2
Rubber Particles
The rubber particles range in size from about 50 A to about 30,000A (3 pm). Exceptionally, particles up to 5 or 6 pm in diameter are found. In young trees and potted plants the particles are spherical (Fig. l), but in mature trees the larger particles are often pear-shaped (Fig.2). The origin of the pear shape is mysterious; it is frequent in certain clones (e.g., Tjir 1 and PR 107) and rare in others (e.g., Pil. B84* and RRIM 526). The pear shape is visible under the light
Fig. 1 Osmium tetra-oxide stained rubber particles from young tree. (Magnification:
X 30,000)
Hevea Natural
Fig. 2 Osmium tetra-oxide stained rubber particles from mature trees. (Magnification:
31
X25,OOO)
microscope and was noted as long ago as 1911: the early literature is briefly reviewed by Southorn (1968). Attempts to measure the particle size distribution by light microscopy gave misleading results since many of theparticles lie beyond the limit of resolution. Usingelectron microscopy, van den Tempe1 (1952) found a maximum inthe size frequency curve at about loo0 A; in fact, the most numerous particle species is too small to be seen by light microscope. A subsequent study of latex fromemature trees of clone RRIM 600 also showed a unimodal curve with a maximum at 1000 A and a long tail in the large particle size range (Fig. 3). A multimodal distribution was found in latices from young potted plants. It has been calculated that a rubber particle with a diameter of 1000 A would contain several hundred moleculesof the hydrocarbon. The hydrocarbon is surrounded by a surface film of protein and lipids, including phospholipids. About 40% of the membrane proteins of the rubber particles were found to be proteolipids. They were hydrophobic proteins containing 70% nonpolar amino acids and were closely associated with phospholipids and glycolipids. Triglycerides, sterols, sterol esters, tocotrienols, and other lipids are also associated with the rubber particles. Their precise location is not known, but it has been suggested that the sterol esters are located inside the particles rather that at the surface. The surface film or envelope is visible in sections of osmium-stained rubber particles (Fig. 4) and is approximately 100 A thick. In permanganate-fixed preparationsthe hydrocarbon is oxidized and ashell remains, which may represent the original envelope (Fig. 5). The envelope carries a negative charge and confers colloidal stability on the rubber particles.
Eng and Ong
32
LATEX PARTICLE SIZE DISTRIBUTION (mean of 3 tappings)
10m - m
0
1
2000
4000
6000 PARTICLE SIZE IN 8 UNITS
a a m I I 8000
Fig. 3 Rubber particle size distribution in mature trees of clone RRlM 600.
Fig. 4 Section throughrubber partcles in mature latex vassel. (Magnification: X45,OOO)
Hewea Natural
Fig. 5 Section through rubber particles from latex fixed with permanganate. (Magnification:
2.3
33
X 9,OOO)
Biogenesis of NaturalRubber
Although research work on natural rubber has been carried out for more than a century, the fundamental question of whyplants produce rubber remains to be conclusively answered.Some researchers suggested that natural rubberis a by-productof the tree (Archer and Audley, 1981), while others reported that it is a stored energyfor the plant (Fournier and Tuong, 1961). These suggestions, however, have not been verified. Recently, different a proposal-that natural rubber acts as a radical scavenger (Tangpakdee and Tanaka, 1998b)"was made on the basis of the presence of oxidative degraded rubber sample found in untapped rubber trees. However, the presence of oxidative degraded rubber in the untapped rubber tree is not totally unexpected, because it is well known that natural rubber can gradually oxidize through radical process in latex state during long-term storage inside or outside the tree (Bloomfield, 1951). Therefore, the finding does not necessarily imply its role in the plant. The rubber biosynthesis starts from trans, trans-famesyl pyrophosphate or its derivative as the initiating species followed by addition of isopentenyl pyrophosphatein the cis configuration to form a two-trans and poly-cisisoprene structure (Eng et al., 1994b;Tanaka et al., 1996). The termination step probably involves the formation of a phospholipid complex (Eng et al., 1994a). 2.4 Molecular Structure of Natural Rubber Chemical Structure On the basis of NMR studies, the fundamental structure of natural rubber has been confirmed to be as follows:
Eng
34
CH3
l
l
R-CH2
H
H&
and
Ong
H
where R and CL are believed to be protein or amino acid and phospholipid, respectively (Eng et al., 1992, Tanaka et al., 1996, Tangpakdee and Tanaka, 1998a). The n-value is in the range of 600-3000 (Eng et al., 1994a). Abnormal Groups Apartfromthisbasicstructure,small amounts of nonisoprenegroups,whichareknownas abnormal groups, havebeen reported to be present on the main-chain molecule. These abnormal groups are very low in concentration, but they exert a strong influence on the properties of the polymer that distinguishes it from the synthetic analog. The abnormal groups reported to be on themain-chainmoleculeinclude epoxide (Burfield,1974), ester (Tanaka, 1984), aldehyde (Sekhar, 1960. Subramaniam, 1977), and lactone (Gregg and Macey, 1973). The presence of epoxide groups was suspected when a reduction of rubber molecular weight was observed after treating the hydrolyzed rubber with periodic acid (Burfield and Can, 1977). However, recent I3C-NMR studies confirmed that natural rubber contains no significant amount of such groups (Enget al., 1998a,b). Thepresence of ester groups in commercial natural rubber was first reported by Gregg and Macey (1 973). However, they attributed the infrared band at 1738 cm" in the spectra of commercial rubber to the presence of lactone groups. It was later confirmedthat the ester groups areassociated with fatty acids, which could be removed by transesterification with sodium methoxide (Tanaka, 1984). The fatty acids have been postulated to be located at the branching point of the rubber (Eng, 1994; Tangpakdee and Tanaka. 1998a). The existence of aldehyde groups was proposed because rubber-hydrazone was found when natural rubber was treated with 2.4-dinitrophenylhydrazine (Subramaniam, 1977). More recently, both aldehyde and ester groups were found to have a similar distribution in fractionated natural rubbers of different molecular weights (Eng et al., 1997). The concentration of these groups decreasedwithdecreasingmolecularweight of therubber,suggestingthataldehyde groups are not derived from oxidative degradation of the rubber. A drastic reduction in the aldehyde content was foundwhen the bonded fatty acids were removedfrom the rubber, indicating that aldehyde groups could be derived from oxidative degradation of olefinic groups in unsaturated fatty acids bonded to the rubber molecule (Eng et al., 1997).
Gel and Branching Natural rubber isolated from fresh field latex immediately after collection and dried at room temperature normally contains small amounts of rubber insoluble in rubber solvent, known as the gel phase. The gel content of commercial rubbers and rubbers from commercial latices can be as high as 70%. It is also a matter of common knowledge that the gel content varied with source, type of rubber. and with the polarity of the solvent used (Allen and Bristow, 1963). The process of gelationinlatex is acceleratedunderalkaline storage conditions(Gorton.1974). Addition of alcohol or acids could help to dissolve the rubber in the solvent (Bloomfield, 195 l).
Hevea Natural
35
The gel phase also containshigher nitrogen and mineral contentsthan the sol phase (Grechanovskii et al., 1987). This led to the postulation that rubber chains in the gel phase are linked up by proteins via hydrogen bonding. This is further supported by the observation that the gel phase in therubber from high-ammonialatexdecreased from 42.5 to 2.2%afterdeproteinization (Ichikawa et al., 1993). The gel fraction became solubilized after it was treated with sodium methoxide. The number-average molecular weight of the rubber chain that makes up the gel phase was found to be in the range of 5.5-8.3 X IOs (Tangpakdee and Tanaka, 1997). Based on these observations, it was suggested that branching and gel phase of natural rubber consist of two types crosslinks, i.e., one through association with protein at the initiating end and the other through phosphoric ester at the terminal end. The existence of branching in natural rubber is indicated by the higher Huggins constant value, K’ (Eng, 1994; Tangpakdee and Tanaka, 1998a) than in the linear polymer. The degree of branching in natural rubber has been quantitatively estimated using GPC viscometry and was found to increase with increasing molecular weight in the range of 1-6 branches per rubber molecule (Angulo-Sanchez and Caballero-mata, 1981; Fuller and Fulton, 1990). A similar result was also obtainedusing I3C-NMR (Eng et al., 1993).Whenextrapolated to zerodegree of branching, it was estimated that natural rubber molecules with molecular weight of 0.65 X 10’-1 X lo5 have no branching, i.e., are linear (Angulo-Sanchez and Caballeromata, 1981). Storage Hardening The progressive increase in Mooney viscosity of natural rubber on prolonged storage under ambient conditions has long been recognized (De Vries, 1927). This phenomenon is known as storage hardening of natural rubber. The increase in the viscosity upon storage is not a desirable property of natural rubber as raw material because this means change a in its processing behavior. However, thetechnologicalaspect of this has been overcome (Sekhar,1964),andconstant viscosity grade rubbers (CV grade) are now available on the market. On the other hand, the mechanism of storage hardening has yet to be conclusively explained (Burfield. 1986, 1989). It is generally agreed that the process involves certain cross-linking reactions of abnormal groups, most probably aldehyde groups in natural rubber (Sekhar, 1962; Subramaniam, 1976; Burfield, 1987). Although other reactions involving epoxide groups have also been postulated (Burfield, 1974; Burfield and Gan, 1977), the failure to detect the abnormal groups in the recent studies weakens this argument (Eng et al., 1998a,b). The characteristics of the process are: ( 1 ) hardening is accelerated under low-humidity conditions (Wood, 1952), (2) the process requires amino acids or proteins (Gregory and Tan, 1976), and(3) it can be inhibited by the additionof monocarbonyl reagent such ashydroxylamine, dimedone, or semicarbazide (Sekhar, 1961). Although storage hardening leads to the formation of gel in dry rubber. the process may involve a mechanism different from that of gelation of natural rubber in latex, because the former is accelerated under low-humidity conditions, whereas the latter proceeds under aqueous conditions (Burfield, 1989). Studies of natural rubber under accelerated storage hardening conditions revealed that the bimodal molecular weight distribution rubber gradually changed to unimodal, where the peak in the low molecular weight region slowly shifted to high molecular weight region (Li et al., 1997). Storage hardening was found to increase the plasticity retention index of natural rubber (Morris, 1991) and contributes to the high green strength of the elastomer (Fernandoand Perera, 1987). Molecular Weight and Molecular Weight Distribution Many factors such as clonal origin, the age of the rubber tree, weather, frequency of tapping, method of rubber isolation, and treatmentof the rubber sample beforeanalysis (e.g.,mastication,
36
I
lo4
Eng and Ong
I
lo5
I
I
lo6
lo7 Molecular welght
Fig. 6 Molecular weight distribution of naturalrubber.
heating) have been known to affect the molecular weight (MW) and molecular weight distribution (MWD) of natural rubber (NR). The effects of clonal variation on MW and MWD of natural rubber from fresh latex have been investigated by gel permeation chromatography (GPC) (Subramaniam, 1976). The MW of natural rubber has been found to be of either a distinctly bimodal distribution, wherethe peak height in the low molecular weight region is nearly equal or half of that in the high molecular weight region, or a unimodal distribution, with a shoulder i n the low MW region as shown in Figure 6. The MW is normally in the range of 104-107 with high MW and low MW peaks centered at 10" and lo5, respectively. The polydispersity of MW. MJM,,. is therefore wide, usually in the region of 2.5-10. Study of the MW of rubber obtained from rubber trees of different ages revealed that young rubber trees also produce rubber with bimodal distribution (Tangpakdee et al., 1996). However, in this case, the height of the low MW peak is greater than that of the high MW peak. As the age of the tree increases, the intensity of the peak at low MW decreases while that at high MW increases. The positions of both peaks remain unchanged despite the variation in tree age, indicating that the bimodal distribution is due tothe biosynthesis process in the rubber tree. A similar observation was alsoreported by Hager et al. (1979)in their studies on guayule rubber. The MWD of NR is also influenced by the frequency of tapping. If a mature tree is tapped for the first time, the rubber contains asmuch as 80% gel (Sekhar. 1962) and the soluble fraction contains mainly oxidizedrubber of low MW(Bloomfield,1951;Tangpakdee and Tanaka, 1998b). Because the tree is frequently being tapped, the gel content decreases and the MW in the soluble fraction increases accordingly (Sekhar, 1962). Mastication has long been known to be a way of breaking down the high MW fractions of natural rubber. Therefore. masticatedNR normally has a unimodalMWD. Heating at elevated temperature can causeoxidative degradation of the rubber double bond. The removal of moisture by heating the rubber at reduced pressure, on the other hand, accelerates the storage hardening process. Therefore, sample treatment of natural rubber can influence the MWD of NR. The actual molecular weight of NR is expected to be much higher than that obtained from GPC analysis because even rubber isolated from freshly tapped latex contains some high molecular weight insoluble microgel. which is normally filtered and discarded in the sample "
Hevea Natural Rubber
37
preparation. The development of the thermal field flow fractionation (ThFFF) technique has allowed the MWD of whole NR to be analyzed at a resolution higher than GPC without removing the microgel. In fact, the gel content could be estimated by analyzing the filtered and unfiltered samples with ThFFF (Leeand Molnar, 1995). Analysis of a commercial NR revealed that rubbers witha MW of 10' to 3 X lo7 arestar-shaped or branchedmolecules.Above 3 X lo', the rubbers are mostly in the form of microgel particles (Fulton and Groves, 1997). However, the analysis of gel fraction with ThFFF is complicated by the phenomenon of steric inversion, where larger microgel particles may be co-eluted with the soluble low molecular weight species at the beginning of the ThFFF separation.
2.5
Nonrubbers
Heveo latex as obtained from the tree consists not only of rubber hydrocarbon particles but also of nonrubbersubstances,includinglipids, proteins. carbohydrates,acids. amines, and some inorganic constituents. It is generally known that some of these nonrubbers can affectthe properties of latex concentrates and bulk rubber derived from the field latex. Most of the nonrubber compounds in natural rubber are either trapped, tenaciously held, or co-precipitated with the rubber during coagulation due to their poor solubility in the aqueous medium or strong entanglement with the rubber molecule. A typical composition of natural rubber is given in Table 1.
Lipids Natural rubber lipids, conlprised of neutral lipids, phospholipids, and glycolipids, make up the largest proportion of the nonrubber components. Water-insoluble lipids are expected to remain inthe dry rubberafternormallatex-processingconditions.Atypicallipid of wholenatural rubber consists of 54% neutrallipids, 33% glycolipids, and 14% phospholipids (Hasma and Subramanium, 1986; Ho et al., 1976). The amount of lipids isolated from rubber particles was found to vary from 1.3% for clone PR225 to 3.4% for clone PB 28/59. In contrast, the phospholipid and glycolipid contents do not vary significantly (Hasma, 1987, 1991). Neutral lipids are composed of more than 14 substances, including sterols, sterol esters, free fatty acids, fatty acid esters, wax esters, monoglycerides, diglycerides. triglycerides. and phenolic compounds. The distribution of these substances varied according to rubber clone. In the case of clone RRIM 501, 63% of the neutral lipids were triglycerides, of which 98% were
Table 1 Composition of Natural Rubber Percentage Component 93.1 hydrocarbon Rubber Lipids Proteins Carbohydrates Ash Others
by weight 3.4 2.2 0.2
38
Eng and Ong
Table 2 Composition of FreeFatty Acids in Natural Rubber ~
Acid
Caproic Myristic Palmitic Palmioleic Stearic Oleic Linoleic Linolen~c Furanoic
~
~~
Percentage by weight Trace 0.01 0.08 0.01 0.16 0.12 0.29 0.03 0.09
found to be furanoid or 10,13-epoxy-l l-n~ethyloctadeca-l0,12-dienoic acid (Hasma and Subramaniam. 1978). A typical free fatty acid composition of natural rubber is given in Table 2. Other acids include inorganic acids, such as hydrochloric, glycero-phosphoric, and phosphoric, and volatile organic acids. such as succinic, malic, acetic latic, and propionic. Three free tocotrienols (a-,6-. y-) and two tocotrienol esters (S-. y-), three sterols and their derivatives, three types of fatty alcohol acetates, have been identified in neutral lipids of natural rubber. The total level of tocotrienol in dry natural rubber is about 0.1% w/w rubber (Dunphy et al., 1965). Morimoto (1985) analyzed the acetone extract of commercial rubber and found four types of tocotrienols (a-,p-, 6-, y-) and three types of free fatty acids (stearic. arachic, behenic) in the sample. Fatty acids were also found in the rubber molecule at the chain terminal. which could be isolated by treating it with sodium methoxide. Glycolipids of natural rubber consist of esterified steryl glycoside (ESG), monogalactosyl diglyceride (MGDG), steryl glucoside (SG), and digalactosyl diglyceride (DGDG). The fatty acids of ESG, MGDG, and DGDG consist mainly of stearic oleic and linoleic acids (Hasma and Subramaniam, 1986). Phospholipids in H e w a rubber consist of phosphatidylcholine (or lecithin) as the main component (Altman and Kraay, 1940) and phosphatidyl ethanolamine, phosphatidyl inositol, and metal phosphatides.The acyl components of these phospholipids are mainly palmatic, stearic, oleic, and linoleic acids (Hasma and Subramaniam, 1986). Processing of natural rubber latex into dry rubber changes the composition of the lipids, especially polar lipids. and different processing methods result in different lipid compositions. In the presence of ammonia, free fatty acids can form soaps in latex. More than 95% of the higher fatty acids (HFA) soaps were found to be associated with the rubber phase of the latex. About 80-90% of the HFA soaps are in their free forms, but only 50% of the furanoic acid is in its free form (Jurado and Mayhan, 1986). Free fatty acids such as stearic and linolenic are activators of sulfur vulcanization. Free fatty acids. which serve as crystallization nuclei, have also been found to accelerate cold crystallization of natural rubber by a factor of about 5 at a temperature where the rate of crystallization is highest (i.e., - 25°C) (Gent. 1954). It was also found that free fatty acids and unsaturated methyl esters could accelerate the oxidation rate and chain scissionof deproteinized natural rubber(Arnold and Evans, 1991).Free tocotrienols arising from neutrallipids are themostimportant natural antioxidants of naturalrubber. The 0.1% tocotrienols (0.8% in lipid fractionof latex) together with phenoliccompounds fromthe unsapon-
Hevea Natural
39
ifiable fraction of tocotrienol esters are responsible for preventing the autoxidation of the raw rubber (Morimoto, 1985; Hasma and Alias, 1990).
Proteins, Amino Acids, and Other- Nitrogenous Substances Fresh latex contains about 2% of proteins, about 25% of which are absorbed on the rubber particle, 25% on the bottom fraction, and 50% on the serum fraction of centrifuged latex (Tata, 1980). The proteins include acidic/anionic and basic/cationic with isoelectrical point in the pH region of 3.5-9.5 and molecular weights of < l 4 to >l00 kDa (Hasma and Amir, 1997). The largest component of proteins in the serum is a-globulin (Archer and Cockbain, 1955). It is soluble in salt and can be coagulated by heat. Its isoelectrical pH of 4.55, which is similar to thepHvaluewherefreshlatex is coagulated, suggests that it is one of theproteins that is absorbed on the rubber particles and partly responsible for the colloidal stability of the latex (Archer et al., 1969). In the case of bottom fraction, 70% of the protein contentis hevein (Archer, 1960; Tata, 1976), a water-soluble, anionic protein with a molecular weight of 5 kDa. In addition, minorproteinssuch as hevaminesA and B (Archer,1976), somehighbasicproteins,and enzymes (Archer et al., 1963) are present. The presence of rubber elongation factor, a protein bound to rubber particles, has also been reported (Dennis and Light, 1989). The preparation of latex concentrate results in a loss of the amount of proteins and changes the composition of proteins,particularly very acidic (pH 3.5-4.6) andbasic (pH 8.0-9.5) proteins (Hasma and Amir, 1997). The level of these proteins was found to diminish on increasing storage period of the latex, so much so that 2- to 3-month-old latex contained mainly acidic proteins (pH 4.6-6.0). The proteins in high-ammonia latex concentrate have also been studied (Hasma, 1992). Substantial amounts of proteins were found to be strongly bound to the rubber particle, and they were only extractable by detergent or organic solvents. Compounding the high-ammonia latex that was less thana month old was found to affect the basic serum proteins, especiallyin the presence of zinc oxide. Heating the compounded latex at 70°C for 2 hours rendered the basic proteins and the very acidic proteins undetectable (Hasma and Amir, 1997). Free amino acids constitute about 0.1% of the latex, of which 80% is found in the serum fraction. The main amino acids in latex are glutamic acid and its amide, alanine, and aspartic acids (Ng, 1960; Brzozowska et al., 1974). Other nitrogenous components in the latex include methylamine, ethanolamine, tetramethylenediamine,pentamethylenediamine,stachydrine,trigonelline,nucleic acids, nucleotides (McMullen, 1960, 1962), and lipids such as lecithin and alkaloids (Archer, 1976). Proteins absorbed on the surface of latex particles have been shown (Resing, 1955; Yip, 1978) to play an important role in the stability of Hevea latex. A certain influence of seasonal changes on this parameter was also observed by Resing (1955) in a study of older clones. The presence of proteins in natural rubber increases its moisture content as well as water uptake of the dry rubber when immersed into water (Muniandy et al., 1988). Proteins have been found to increase the modulus, creep, and stress relaxation of natural rubber. The development of low-nitrogen natural rubber is desirable for engineering applications, requiring low creep and stress relaxation (Fuller et al., 1988). Morimoto ( I 985) reported that proteins or amino acids played a role in protecting natural rubber against thermal oxidative degradation. However, Hasma and Alias (1990) reported otherwise. Since these compounds have been shown to promote storage hardening of natural rubber, it is not surprising to observe a retention of tensile strength at elevated temperature. Atman (1 948) observed the accelerating effects of choline and ethanolamine on the vulcanization of rubber. Ethanolamine and arginine increase the torque modulus and reduce the scorch time and cure rate.
40
Eng and Ong
Table 3 Mineral inthe Ash Component of Natural Rubber Component MnO, CuO Chlorine, carbonic acid
so3 MgO Na20 CaO K20
P20s
Percentage by weight 0.00 1 0.7 1.4 6.2 8.9 16.4 23.3 43.0
Ash A typical composition of minerals in ash of natural rubber is given in Table 3 (Archer, 1963). These components vary according to the methods of latex coagulation. Approximately one third of the phosphorus in latex is found in the rubber hydrocarbon phase and the rest in the rubber hydrocarbon and nonrubber phases. Magnesium. on the other hand, is found mainly in the nonrubber fractions (C- and B-sera) (Yip and Chin, 1977).Both phosphorus and magnesium have been shown to affect latex stability. While phosphorus compounds exert a stabilizing effect, magnesium,in the form of the divalent cation, is destabilizing. It has been shown that although the ratio of phosphorus to magnesium is colrelated with the stability of latex to a certain extent, it is by no means the only influencing factor (Yip and Subramaniam, 1984).Nevertheless, in the processing of latex concentrate, diammonium hydrogen phosphate is sometimes added to precipitate the undesirable free magnesium ions to ensure better stability of the concentrate produced. Copper, manganese,and iron are the well-knownpro-oxidants of natural rubber,with copper being the most active (Barnard et al., 1963;Bateman and Sekhar, 1966).The normal method of determining copper content in natural rubber does not relate well to the PR1 value of the rubber, as observed by Alias and Chan (1980).This has been attributed to the inability of the analytical method to determine the actual amount of free copper from the total copper (Hasma and Alias, 1990) since only free copper can act as a catalyst in a thermal oxidation process of natural rubber (Shelton, 1972).The copper in natural rubber could be reduced by soaking the rubber in phosphoric acid or thiourea. Copper in fresh latex might complex with proteins and amino acids, and it will not impart any deleterious effects on the aging of natural rubber. However, when proteins and amino acid-copper complexes are attacked by microbial activities. free copper is released. This probably explains the general susceptibility of autocoagulatedrubber to thermaloxidativedegradation as comparedto normalacid-coagulated rubber (Hasma and Alias, 1990).
Inositols and Curbohydrates The most abundant polyolin latex is quebrachitol(1-~-2-O-methyl-( - )-chiro-inositol). It constitutes about 1% of fresh natural rubber latex (Rhodes and Wiltshire, 1931).M- and /-inositols have also been reported to be present in latex (Archer et al., 1963).The major glucid in the latex is sucrose. Small amounts of glucose, galacose, frutose, raffinose, and two pentoses have also beenidentified(Smith, 1953, 1954; Lowe, 1960;Tupy andResing, 1968).Quebrachitol has been reported to be a potential starting material for the synthesis of certain natural bioactive materials (Lau, 1996).
Hevea Natural Rubber
41
Volatile MatterWater is the major component in the volatile matter. Other volatile acids such as formic, acetic, and propionic have also been reported (Crafts et al., 1990). The water adsorption of natural rubber is due to thepresence of hydrophilicimpurities, mainly inorganicsaltsandproteins (Burfield et al., 1989). High volatile matter content can promote mold growth and causes undesirable odor of the rubber (Nadarajah et al., 1987).
3.
NATURAL RUBBERPROCESSING
The premium product of a rubber tree is latex. The by-product of tapping process is cuplump, which is actually the latex drip collected at alternate days after the collection of latex. Along with the cuplump. small amounts of treelace also combine with the cuplump. Under normal conditions latex contributes to about 80% of the output, while the cuplump andtreelace amounts to about 20%. Thus, the raw output for natural rubber processing can generally be classified as latex (liquid) and cuplump (solid). The types and grades of natural rubber processed depend greatly on the raw material input.
3.1 Classification of Rubber Processing Natural rubber is normally processed into either latexor dry rubber, depending on its application. Rubber products such as dipped goods, foam, and thread produced from latex, whereas other products (e.g., tires) are made from dry rubber. Different types and grades of commercial natural rubber are available in the market, and they summarized in Figure 7.
3.2 Technically Specified Natural Rubber The demand for technically specified rubber (TSP) in the form of block rubber has been overwhelming. Thus, most of the NR-producing countries have been converting their conventional rubber processing to TSR. The preparation of NR in block form has given a tremendous boost to the success of technical specifications for NR. Technical specification allows fordiversity of rubber-producing units of widely varying sizesto conform to important technical parameters, consistencyin quality, minimum space for storage area. cleanliness. and ease of handling. 3.3
Production of Block Rubber
The production of block rubber is basically the conversion of wet raw rubber into granular form by fast and continuous processing techniques. In its final form the dried crumb or the granule is compacted into blocks of solid rubber. Hence this presentation is known as block rubber. In processing field latex, thefollowing operations are involved: reception, bulking, chemical addition, coagulation, milling, size reduction, drying, baling, testing, grading, and packing. A combination of machinery such as crusher, crepers, hammermill, and shredders is used. Technicallyspecified NR in theblock form latex is as lightcoloredrubberstandard Malaysian rubber (SMR L), constant viscosity (SMR CV). SMRL production essentially focuses on color, with the addition of sodium metabisulfite at 0.04% dry rubber content (DRC). In CV production addition of hydroxylamine neutral sulfate at 0.15% DRC is necessary. The coagulation of field latex is done at field DRC.
42
Eng and Ong
NR
Latex
grade)
Latex Concentrate Conventional (=,LA) RSS, ADS, Pale Crepe
1
TSR S M R 10, SMR 20
Fig. 7
Cuplump (Field
TSR Specialty rubber S M R L, SP, MG, SMRCV DPNR
Speciality TSR S M R 10 CV, S M R 20 CV SMR GP (House grades)
General natural rubber types and grades
The popular grades of TSR produced from fieldgrade or combination in the form of block rubber is SMR 10 and SMR 20, SMR GP, SMR 10 CV, and SMR 20 CV. Selection and blending are necessary before further processing. The other steps involved are precleaning, initial size reduction, crepeing, intermediate size reduction, crepeing, and final size reduction. A combination of machinerylikeslabcutter,granulator,prebreaker,crepers,extrude,andshredder is generally used. The individual processors select different types and numbers of machines for the samepurpose with the idea of increasing productivity and meeting the required specifications. Processing methods have reduced processing timesto less than 24 hours. Besides technical specifications, deep bed drying at a temperature of 100-120°C has been a vital change. This allows the drying to be completed in less than 4 hours. The dried rubber biscuit of crumb is weighed and baled using a hydraulic press. 3.4 Specification The introduction of technical specifications was an important step in the development of the NR-processing industry. The specification parameters and their limits are changing features in thescheme. Advancements in the rubber product-manufacturingindustryandthe need for continuous improvement in the raw natural rubber-processing sector has necessitated the reexamination of the existing specification parameters and introduction of new grades and parameters, which may truly reflecttheprocessibilityandtechnologicalproperties ofNR (see Tables 4 and 5).
Hevea Natural Rubber
a c n
P
43
44
Eng and O n g
m
U
i
F
Hevea Natural Rubber
45
3.5 Conventional Types The conventional types of NR include the ribbed smoked sheet (RSS), air-dried sheet (ADS), and crepe rubbers. Among the conventional grades the production of RSS was popular until the emergence of block rubber-processing techniques. The rubber smoked sheet is the oldest method of processing latex grade rubber. This is still made in small or medium-sized rubber estates where the infrastructure for transportation of latex is lacking. Theprocessing of RSS passes through the stages of latex collection. bulking, reduction of field latex DRC to about 12.5%, coagulation, overnight maturation, milling the next day, and smoking (drying). The millingoperation allowsthe latex coagulumto besheetedaftermaturation. The sheeting process actually squeezes out serum present in the coagulum and reduces its thickness to about 3.0 mm by passingthe coagulum throughaset of four smooth rollersandfinally grooved rolls, lending a ribbed design to the final sheet. The ribbed pattern assists in increasing the surface area and improves the drying performance. The RSS is dried in a smokehouse, whereas the ADS is dried in a hot air chamber. A tunnel smokehouse is operatedatatemperature of 45-63°C. The dried sheets are visually examined andgraded.Visualgrading is seen as asetback for thetechnicalspecification of rubber. Visual grading is based on the recommendations provided by the International Rubber Quality and Packing Committee. Since there are no technical specifications, rubber is graded on the degree of dryness, contamination, virgin rubber. blisters, bubbles, oxidized rubber, transparency, color, tackiness, and mold growth.
3.6 Avoiding the Problems of Odor During Rubber Processing Cis-polyisoprene is both colorless and odorless. It is the 5% or so of nonrubbers that gives bale natural rubber, particularly field-grade material. its color and the characteristic smell, which those experienced in the rubber industry immediately recognize and generally accept. These nonrubbers, the inherent products of latex biosynthesis, also give natural rubber many of the characteristic advantages in processing and aging that have enabled it to resist successfully the challenge of synthetic polyisoprene, the development of which was once predicted to sound the death knell of the natural product. Nevertheless, it must be admitted that the odor given off during the processing of natural rubber may cause offense to both workers and people living close to factories where processing takes place. Methods of coagulation and subsequent conversion of the coagulum into bale rubber affect the smell of natural rubber. There appears to be no direct correlation between grade of rubber and odor, but field-grade material tends to have a stronger smell than rubber prepared by the deliberate, controlled coagulation of latex. The main constituents of the gases arelow molecular weight volatile fatty acids (Table 6), which can be effectively removed by water scrubbers with efficiencies of 92-99%. The drying process reduces much of the volatile fatty acid content of SMR, some 99% of the 0.8% (8000 ppm) of volatile matter allowableunder the SMR specification being moisture. However, other nonrubbers that constitute about 6% of the total weight of bale rubber comprise a range of chemical species,as described earlier. Whenthese nonrubbers are subjected to high-temperature processing conditions, such as mastication, the proteins break down to give amine derivatives, which can smell quite unpleasant. Certain fatty acids are also pungent, and when combined with amines, the resulting mixture produces an odor that can cause complaint. High-temperature mastication is recognized as one of the predominant factors causing workers
Eng and Ong
46
Table 6 Volatile Fatty Acids in DrierExhaustGases from SMR Factories
Concentration range (ppm)" ~~
Factory acidFatty
A
Acetic Isobutyric Isovaleric Valeric
459- 1438 40 1-707 726- 1239 793- 1471
~~~~~
Factory B 41 1-725 343-922 798- 1526 867-20 10
For elght (factory A ) and five (factory B ) sampling perlods of 2 hours.
"
in the rubber-manufacturing industry and membersof the public who live in the vicinityof rubber factories to express concern regarding the odor given off during the manufacturing process. Odor can be determined by olfactometry. This technique uses the human nose to detect very low concentrations of compounds that cause odor. Assessment of odor is very much a subjective matter. Olfactometry uses a panel containing several people who give a yes or no decision as to whether an odor can be detected or not; a positive result is obtained at the point at which 50% of panel members can detect an odor after its dilution with odor-free -
Cat
I
.
CH0
L -CH-CH2 -
I
HONH2HCI
- CH - CH2-
P205
l
& N Y , CHzClz
I -CH-CH-
I
Hg
Chemical Modification of Synthetic Elastomers
127
13. SURFACE MODIFICATIONS Polydienes have also been surface modified in solution. For example, syndiotactic 1.2 polybutadiene has been reacted on thesurface by photolyticaddition of thiols,as well astreatment with aqueous permanganate. Such surface treatment leads to changes in wettability (Carey and Ferguson, 1994).
14. SUMMARY This chapter surveys the chemical modification of unsaturated synthetic elastomers. Emphasis is placed on the use of chemical methods to introduce reactive functionalities along synthetic elastomer backbones. Special attentionis drawn to modifications of intrinsic scientific or technological interest. On the scienceside, new processes (e.g., phase-transfer catalysis) and new understandings (e.g., random vs. blocky modifications; modificationas an aid to characterization) are featured. On the technological side, commercially important modified synthetic elastomers (e.g., halobutyl) are covered.
REFERENCES Abdel-Razik, E. A. (1988), Po/yrrwr- 29(9):1704. Adams, H. E., Boutsicaris, S. P,, and Hnlasa, A. F. (1978). U.S. Pat. 4,129,699. AI-Malaika, S . (1990). CHEMTECH 20(6):366. Arshady, R. ( 1997). Desk Rqkr.rcwc.ac$F~rrlctiorlrr/ Po/ymers: Synthesis r r r d Applictrtiorrs. ACS. Washington, DC, p. 62 1. Ast, W.. Zott, C., and Kerber, R. ( 1 9 7 9 ~Mukrorrrol. Cl~err~. /80:3IS. Azuma, C., and MncKnight, W. J. (1977), J. Po/ym. Sei. Po/yrrr. Cherr~.Ed. 15:547. Baldwin. F. P,. Buckley, D. J., Kuntz. I., and Robinso, S. B. (1961), Ruhher P/nstic.s A,ge 42:SOO. Barantsevich, E. N., Breslen, L. S., Rabinerson, E. L., and Kalans, A. E. (1978), V w o k o r n o l Sordir~Ser. A 20: 1289. Bhattacharjee, S., Bhowmlck, A. K., and Avasthi, B. N.(1990~1).J. A/>/>/.P o / w . Sci. 41:1357. Bhattacharjee, S., Bhowmick, A. K., and Avasthi. B. N.(1992), J. Po/yrr~.Sei.. Purt A, Po/yrrr. Clreru. 30: 471. Bhattacharjee. S., Bhowmlck. A. K.. and Avasthi, B. N.(l990b), J. Po/yrrr. Prrrt A, Po/ynr. Chern. 30: 1961. Bhattacharjee, S.. Rajagopalan, P,, Bhowmlck, A. K. (1993a), J. A/>/>/.Po/yr. Sei. -16:1971. Bhattacharjee, S., Bhowmick, A. K., and Avasthi, B. N.(1993b). Pnlyrrrer 34:24. Blackshaw, G. C. (1978). in The Vmtlerbilf Rubher Htrrrr/l>ook (R. 0 . Babbit. Ed.), R. T. Vanderbilt Co., Norwalk, CT, pp. 102-127. Brosse. J. C., Soutif, J. C., and Pinazzi. C. ( 1 9 7 9 ~Mtrkrormrl. Chem. 180:2109. Bruzzone, M., and Carbonaro, A. ( 198S), J. P d y m Sei. Po/ym. Ckeno. Ed. 2.3: 139. Brydon. A., and Cameron, G. G. (1975). in Progress irr Pn/ymer Scicwce. Vol. 4 (A. D. Jenkins, Ed.). Pergamon Press, Oxford, England, Ch. 4. Brydson, J. A. (1978), Ruhher Cherrlistry, Applied Science Publishers, London, England. Ch. 7. Buchan. G. M,, Cameron, G. G.. and Christi, S. A. A. (1978). Mnkrortlol. Chew. 179: 1409. Buchan. G. M,, and Cameron, G. G. (1979). J. Cltern. Soc. Perkirt Trms. I, p. 783. Butler. G. B. (1980), I d . Eng. Cheru. Prod. Res. Dotr. 19512. Cameron. G. G., and Muir. R. B. (1976). J. Po/ynr. Sei. Po/yrrl. Lett. Ed. 14661. Cameron, G. G. (1980). in Mtrcrorr~olecrrlrrr-Cl~ernistry,Vol. 1 (A. D. Jenkins and J. F. Kennedy, Eds.), Royal Society of Chemistry. London, Ch. 16.
128
Schulz and Patil
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Liquid Rubber Douglas C. Edwards* Polysar Limited, Sarnra, Ontario, Canada
1. INTRODUCTION Conventional vulcanized elastomers, whether naturalor synthetic, consist of very long molecules connected into a continuous network by means of occasional crosslinks. High extensibility is possible because the chains are sufficiently flexible and mobile, at temperatures encountered during use, to accommodate large imposed strains without suffering local chain failure. The retractive force following deformation is essentially entropic in origin and depends on the freedom of the chains to undergo very facile thermal motion with respect to one another. Hence elastomers are based on polymeric molecules that at ambient temperatures are far above their glass transition temperatures and are also amorphous (in the unstrained state at least) and relatively free of highly polar or bulky side groups. In practice. the molecular weight between crosslinks requiredto provide a suitable balance between high extensibility, elastic recovery, and strength properties is in the order of 10,000. Consequently, the molecular weight of the polymer must necessarily be high. for otherwise the proportion of dangling chain ends, which cannot contribute to either strength or elasticity, will become excessive. In the case of butyl rubber gum vulcanizates, for example, high strength requires nlolecular weights of at least 100,000 (Flory. 1946). These fundamental considerations are of central importance when considering liquid polymers. The term “liquid” implies easy pourability at ordinary temperatures, and in general this means a molecular weight in the region below about 5000. The constraints therefore impose a requirement that the liquid polymer chains be linked end to end, during cure,so as to be capable of forming a finished network with few free ends and with appropriate average chain lengths between the crosslinking sites. The liquid polymer must carry reactive groups at the chain ends to provide a mechanism for the occurrence of chain extension. This is illustrated schen~atically in Figure 1. The term “telechelic” [from the Greek words telos (end) and chele (claw)] was proposed by Uraneck et al. (1960) to describe such terminally reactive polymers and is now in general use. This chapter will consider only telechelic polymers, since other liquid polymers are not capable of providing strong elastomeric products. In discussing network constraints, we have considered one of the two fundamental requirements necessary for high-performance elastomeric materials. The second fundamental factor is reinforcement. As a generalization, reinforcement involves the presence of a second, harder phase within the continuous elastomeric matrix, the interfacial regions providing a locus for
133
Edwards
134
A
0
Fig. 1 Schematiccomparison of conventional ( A ) and liquid elastomer (B) networks.
stress dissipation under conditions of deformation that would otherwise result in the initiation of catastrophic failure. In conventional vulcanized elastomers, reinforcement is provided by the incorporation of very finely divided, high-surface-areaparticulatefillers. The carbon blacks represent by far the most generally used class of materials. The presence of carbon black raises the tensile strengthof amorphous (non-stress-crystallizing) elastomers by a factor of 10 or more, with concurrent large increases in modulus, tear strength, and abrasion resistance. Reinforcement is therefore essential to the service performance of most rubber goods. Thedetailed mechanisms of carbon black reinforcement remain somewhat controversial, a principal issue being the extent to which chemical bonding across the interface is involved. Valuable reviews of this complex subject have been provided by several authors, including Dannenberg ( 1 985) and Kraus (1977). Reinforcement by non-black finely divided fillers, notably fumed or precipitated silicas, is also extensively practiced. This subject has been reviewed by Wagner (1976). The presence of very small, hard regions within a continuous elastomeric matrix can be established by other mechanisms. In particular. the polymer chains may contain blocks (long sequences of monomer units or alternating pairs) that are inherently glassy or crystalline at ambient temperatures. These blocks may coalesce with others in neighboring chains to form glassy, or crystalline, “domains” within the elastomeric matrix. In thermoplastic elastomers of the ABA type, in which A represents a “hard” block and B an elastomeric block, the domains formed by coalescence of the A components serve both as reinforcement sites and as effective crosslinks to establish and maintain the shape of the unstressed product. In this case, a hightemperatureprocessing step is necessary to permitformation of the “hard” domains while cooling in the desired shape. The role of this type of mechanism, and of reinforcement effects in general, must be considered as one of the fundamental elements affecting the performance of liquid-based elastomeric systems. This chapter first outlines the history and current status of the principal classes of liquid elastomer systems that have achieved commercial importance.Some of the fundamental requirements are then discussed that would be necessaryfor hydrocarbon-based liquid-polymer technol-
135
Liquid Rubber
ogy to displace conventional elastomers in major segments of the rubber industry. Finally, some of the additional themes in telechelic polymer research, past and present, are reviewed.
2. 2.1
CLASSES OF COMMERCIALLY ESTABLISHED LIQUID ELASTOMERS Polysulfides
Polysulfides were the first synthetic elastomers to be manufactured commerciallyin North America, being introduced in the late 1920s. Subsequent developmentsin polysulfide technology led, during the 194Os, to the first family of liquid telechelic elastomers. The route to these products is an example of one of the several generic techniques for telechelic polymer synthesis, namely, the scission of a preformed long-chain polymer into shorter chains by a mechanism that results in reactive groups at the severed chain ends. The basic chemistry of the polysulfides has been reviewed by Bertozzi (1968). The initial discovery of polysulfide elastomers was made in 1920 by J. C. Patrick, who treated ethylene dichloride with sodium polysulfide during a study of possible routes to ethylene glycol (see Whitby, 1954). Patrick’s interest in the unexpected product, a rubbery gum, was the first step on the path to the polysulfide industry. An initial patent was granted several years later (Patrick and Mnookin, 1927). The reaction of organic dihalides with sodium polysulfide leads to linear condensation polymers:
nCI-R-Cl
+
nNazS.
- - (RS.),-
+
2nNaCI
(1)
In practice, a substantial excess of sodium polysulfide is used in order to maintain reactive and groups. Propagation would otherwise terminate becauseof the competing reaction of hydroxyl ions to yield -RS,R’OH. The excess sodium polysulfide functions by solubilizing the inert terminals into the aqueous phase:
- RS.R’OH
+
NaS.Na
- - RS.Na
+
NaS.R’OH
(2)
The two principal variables are the nature of the R group and the value of X (referred to as the “rank” of the polysulfide). Early products employed ethylene dichloride, but these products were found to be inferior in odor and in low-temperature flexibility to polymers based on bis-2-chloroethyl ether. In this case, however. the process produces substantial quantities of asix-memberedether-thioethercyclicby-product. The preferred monomer is now bis(2chloroethyl) formal, which is obtained economically by the reaction of ethylene chlorohydrin with formaldehyde:
2CICHzCH2OH * HCHO
- CICHZCH?OCH~OCH~CHZCI * Hz0
(3)
Small proportions of a trifunctional monomer, 1,2,3-trichloropropane, may be introduced to provide branched polymers. The preparation of liquid telechelic polymers by the scission of pre-polymerized polysulfides was introduced much later (Patrick and Ferguson, 1945). The parent polymer is produced by addition of monomers (difunctional and trifunctional chlorides) to excess aqueous sodium polysulfide in the presence of colloidal magnesium hydroxide and a small amount of surfactant. The high molecular weight polymer forms as a latex. This is then treated with sodium sulfite and sodium sulfhydrate. Polysulfidic linkages in the polymer chains are reduced to disulfide,
Edwards
136
and these are cleaved to the desired extent to provide terminal mercaptan or -RSNa groups as indicated by the following equations:
-RSSSR-
+
-RSSR-
+
NaSSR-
+
- -RSSR-
Na2S03
NaSH
- -RSH
-
+
+
NazS203
NaSSR-
Na2S03 NaSR-
+
Na2S20s
Finally, the mixture is acidified. This destabilizes the latexand also converts theremaining “RSNa terminal groups in the product to “RSH. During the subsequent washing and recovery of the product, molecular weight randomization occurs by interchange reactions between the mercaptan and disulfide groups, as illustrated schematically by the following equilibrium:
-RSSR-
+
R’SH
+ -RSSR’-
+
-RSH
(7)
The curing of the telechelic liquid polymer to form an elastic network requires both chain extension and crosslinking mechanisms (Fig. l ) . As a generalization, the terminal reactivity of liquid elastomers is always used for chain extension and is usually involved in crosslinking reactions as well. The crosslinkingstep, however, does not in principledepend on terminal reactivity. The liquid polymer may be synthesized with branched structures already in place. Crosslinking reactions different from those used for chain extension may also be applied to reactive sites along the polymer backbone. In the case of polysulfides, branched structures are present in the liquid product due to the presence of a trichloride during the synthesis of the parent polymer. The terminal mercaptan groups maybe reacted with tri-or polyfunctional agents to produce crosslink sites if desired, but in principle this is not necessary for network formation. The chain extension is normally carried out by oxidation reactions to form disulfide linkages. Useful oxidizing agents include metal peroxidesandorganicoxidizing agents such as peroxides or p-quinone dioxime. The following reactions are representative:
2 -RSH
+
6 -RSH 2 -RSH
PbOz HON
+
- -RSSR-
PbO
0.0. ,, -
CaOz
+
+
H20
3 -RSSR-
- -RSSR-
+
Ca(OH)2
+
,
H~O N ,N . 2
(8) +
2HzO
(9)
(10)
Lead dioxide is commonly used in formulations to be cured at room temperature in compounds in which a dark color is acceptable and the toxicity of lead is not an impediment. MnO,, CaO,, and Z n 0 2 are also used. Curing is generally accelerated by alkaline additives-amines, inorganic bases-and may be retarded by the addition of acidic materials such as stearic acid. A small amount of moisture accelerates the metal peroxide cures. When using calcium peroxide, Seegman et al. (1961) observed that dry compositions are stable in storage for several months but cure slowly upon exposure to ambient humidity. One-component sealant compositions are formulated on this basis. Doughty (1962) incorporated barium hydroxide into compositions of this type as a desiccant during storage and as a cure accelerator following exposure. The moisture cure of one-component polysulfide sealants can be accelerated by the use of organic additives. One such system (Doughty and Christman, 1967) employedan N.N’-dialkyl amide in the presence of dibutylbutyl phosphonate. A superior system (Doughty and Christman, 1969), claimed to be capable of curing to a depth of about 6 mm in one day at 27°C and 80% relative humidity. is achieved using a mixture of ( I ) an oxidizing metal oxide such as calcium
137
Liquid Rubber Table 1 Properties of Liquid PolysulfidePolymers LP-3 1 Viscosity (25°C). P 950- 1550 Mercaptan content, % 1.0-15 Average mol. wt. 8000 Pour polnt, “C IO Crosslinklng agent. o/o 0.5 Specific gravlty (25°C) 1.31 Avg. Viscosity (4°C). P 7400 Avg. Viscosity (65”C), P 140 Stress-strain Properties Tensile strength. Mpn 2.50 300% modulus, Mpa 2.07 Elongation. % 600 48 Hardness. Shore A Rec~pe:LP-31. LP-32. LP-2, LP-l2 Base Compound: Liquid polymer 100 30 N774 black Curing Paste: PbOT 7.8 HP-40 (plasticner) 4.8 Stearic acid 0.1 Alumina (ALzO?) 0.2 Cire: 2 hr at 70” in closed mold, postcure 20 hr at 2 3 T , 50% RH after unmolding.
LP-2
LP-33
410-52s 1 .S-2.0 4000 7 2.0 l .29 3800 65
410-52s 1.5-2.0 4000 7 0.5 I .29 3800 65
2.82 2.41
2.07 1.45 930 50
5I O S0
45
Black
LP- 12 410-525 1
4000 7 0.2 1.29 3800 65
LP-3
LP-33
9.4- 14.4 5.9-7.7 1000 - 26 2.0 1.27 90 I .S
15-20 5.0-6.5 1000 - 23
0.5 1.27 165
2.1
2.07 2.07 1.38 1.03 900 275 48 Recipe: LP-33 and LP-3
2.59 1.38 700 34
Base Compound Liquld polymer N990 p-Quinonedioxirne (GMF) Diphenylguanldine (DPG) 4. Magnesium oxde Sulfur
Io0 20 6.67 0.67 15 0.50
Cure: 20 hr at 77°C In mold, postcure mlnimum 2 hr at 23°C and 50% RH after unmolding.
peroxide, (2) metalhydroxides such as Ba(OH)? andor oxides such as BaO, and (3) a sulfonamide of thetype RS02NHR’, in whichR is an aryl group and R’ is an alkyl or hydrogen. The properties of liquid polysulfide polymers presently manufactured in North America are illustrated in Table 1. The strengthpropertiesattained with compositions of thistype are low compared withthose of conventionalreinforcedelastomers. Their utility depends on the physical form of the unvulcanized compounds (fluid or low-viscosity paste) together with adhesion to substrates and the development of elastic properties following cure. Resistance of oilsandsolvents,togetherwithgoodweatheringproperties(resistance toozone and moisture) and a fairly broad range of service temperatures, from about - 50 to about 120°C, renderthemsuitable for a variety of applications. These includevarioustypes of sealants used in theaircraft,automotive,building construction, marine,and,particularly,insulated glassindustries.Fluid compositions areused as electricalpotting compounds and as rocket propellantbinders.Polysulfidescan also be used to imparttoughness to epoxyresins, with whichtheycanbeco-cured:
+
Edwards
138
2.2
Silicones
The silicone elastomers are discussed in detail in another chapter, and hence the present section will be limited to a brief account of that part of the technology that relates to telechelic liquids. The basic polymerization chemistry is an exampleof a second generic mechanism for establishing terminal reactivity, namely, systems in which the initiation and termination steps inherently produce distinctive and groups. Silicone elastomers,in the formof high molecular weight gums, were introduced commercially in the 1940s, General Electric and Dow Corning being the dominant suppliers in North America. The polymersare based. in general. on dichlorodimethylsilane as thepredominant monomer. Polymerization occurs viahydrolysis,followed by condensation according to the following simplified scheme:
n(CH3)zSIC1z nHIO +
- H 0 I(CH&SiOlnH
+
2nHCI
(12)
Early use of the silicone polymers did not involve the terminal silanol sites, and hence the chain ends were normally blocked with inactivegroups such as -Si(CH3)3. Thehigh molecular weight polymers were compounded conventionally,in rubber process equipment, andvulcanized by the use of peroxides (see Bueche, 1955). During the 1950s, room-temperature vulcanizing (RTV) silicone compositions were introduced for use in sealant applications. Initially these were limited to two-component systems supplied as liquids or pastesthat,followingmixingandapplication, would develop elastic properties at room temperature. In the late 1950s, more sophisticated one-component systems were developed in which atmospheric moisture catalyzes the cure (see Ceyzeriat,1957; Brunner. 1959; Brown and Hyde, 1960; Nitzsche and Wick, 1960). The following generalized reactions are representative:
-
-RSi(OR')3 HzO -RSi(OR')20H R'OH 2 "RSI(OR')~OH -RSI(OR')~-O-SI(OR')ZR+
-
(13)
+
+
Hz0
(14)
Crosslinking occurs by the same nlechanism,eachhydrolyticscission of the =Si " S i O H sites that will condensetoform =SiUSi= crosslinks with elimination of a molecule of water, The curing reaction thus propagates rapidly through the material following initial exposure to external moisture. The physical strength properties of sealant compositions derived from liquid silicones are low and similar to those obtained with polysulfides. The sealants are outstanding in adhesive properties, high- and low-temperature flexibility, color stability, and immunity to the effects of sunlight and weather. In addition to use in sealants, liquid silicone polymers are used principally in the fabrication of tlexible molds and in formed-in-place seals and gaskets. It is appropriate in this section to take note of the use of telechelic silicone polymers in fundamental studies of network structure. and the effects of network structure on physical properties. In principle, at least, telechelic polymers provide a means to prepare model networks in which the molecular weight between crosslinks and the number of chains that arejoined together at the crosslink sites can be manipulated as independent variables. Networks free of dangling ends are also possible in principle. These potentialities are of interest with respect to physical properties that might be attainablein optimized liquid-polymer systems. They are also of interest in fundamental studies relating to the nature of rubber elasticity. The achievement of truly model networks requires, of course, a very high degree of perfection in the materials used and in the methods of fabrication. Specifically, one must have polymers that are precisely difunctional; chain extension and crosslinking mechanisms that involve no side reactions; a perfect stoichio-
-"-c= bondsproducing
Liquid Rubber
139
metric match between the network-forming agents and the terminal reactive sites; and, most difficult of all, a crosslinking reaction with a “yield” of 100%. These are demanding requirements that may or may not be achievable; the subject will be discussed again in the context of terminally functional liquid polybutadienes. Silicone polymers have been used extensively for model network studies by J. E. Mark and coworkers at the University of Cincinnati. In these studies, silanol-terminated silicone polymers of known molecular weight were crosslinked using a tetrafunctional agent (tetraethylorthosilicate) in the presence of a catalyst (stannous 2-ethylhexanoate);
The reactions were carried out at room temperature for 2 days. vacuum being applied to remove the ethanol. Thenetworks were then extractedto remove material (a few weight percent) not incorporated in the network and attributed, in part at least (Llorente et al., 1981), to inert cyclic polydimethylsiloxane molecules, which are normally present in silicone polymers. In related studies, vinyl-terminated silicone polymers were crosslinked with agents of the type = SiH, using a platinum catalyst, for example: 4 -RSi(Ch)zCH=CH2
+
Si[OSi(CH3)2H j4
- Si[OSi(CH3)2-(CH2)2-Si(CH3)~R-~4
The use of polymers of the type [OSi(CH3)H-],, as crosslinking agents permits the preparation of networks with nlultifunctional crosslinking sites. Reaction time is 2 days at 70°C or 1 day at 95°C in the presence of a small amount of chloroplatinic acid (Llorente and Mark, 1980). While the strength properties of unreinforced polydimethylsiloxane networks are always very low by the standards of ordinary rubber technology. the relative behavior observed in the various model network systems is of considerable practical and theoretical interest. The ultimate strength obtained from networks prepared by endlinking is higher, for a given network density, than that obtained by randomcrosslinking(peroxide or radiation curing) of high molecular weight polymers (Anrady et al.. 1981). A second, more surprising observation is that superior strength properties can be obtained by the purposeful introduction of a population of relatively very short chains. Thus. a mixture of 90 mol% short chains (M,, 220) and I O mol% long chains (M,, 18,500), gave a substantially higher tensile strength at room temperature than other networks tested (Llorente et al., 1981). This result is interpreted (Mark, 1984) as arefutation of the “weakest link” theory of network rupture, whereby local failure is initiated when the shortest chains reach their full extension. This theory contains the assumption of “affine” displacement of network junctions (i.e., relative movement of junction sites proportional to that of the overall deformation). The present observations indicate that very “nonaffine” behavior occurs at high extensions, the network simply reapportioning strains among the longer and shorter chains until no further reapportionment is possible. Recent work by thisschool has touched upon the otherfundamentalrequirement for strength in amorphous elastomers-reinforcement. This study has involved a novel method of introducing particulate fillers into an elastomer network, namely, in situ silica formation via hydrolysis of tetraorthosilicate that has been swollen into the finished network. The hydrolysis may be carried out by immersion i n an aqueous solution of an amine (Ning et al., 1984) or simply by exposure to ambient humidity (Jiang and Mark, 1984).
Edwards
140
The particles form as spheres with an average diameter in the region of 25 nm. Since the particles are precipitatedinto a preformednetwork, they do not form clusters or reticulate structuressuch as arenormallyfound in silica-reinforced elastomers but remain as discrete spheres. Strength is increased severalfold compared to the unreinforced network.
2.3 Polyurethanes While the fundamentals of the polyurethane industry involve chain extension and crosslinking of liquid telechelic polymers, the complexity and scale of the technology has grown to an order of magnitude far beyond that represented by the other materials discussed in this chapter. The outlines of polyurethane technology will be sketched brietly, with particular reference to reinforcement mechanisms and to the relationship between urethane liquid elastomer technology and that of conventional rubber. For broader insights, the reader is referred to textbooks and general references (Saunders and Frisch, 1978; Ulrich, 1983). Early research on polyurethanes was conducted in Germany in the 1930s (Bayer, 1947), and commercial products were first introduced in the 1940s. The liquid precursors are either polyester or polyether diols. Polyesters were introduced initially, being prepared by, for example, the condensation of adipic acid with an excess of 1,4-butane diolto give the terminally functional polymer:
nHOOCRCOOH + mHOR'OH
- H/OR'OC(O)RC(O)-]),OR'OH
+
nHzO
(IS)
The polyethers were introduced in the 1950s and are made by polymerization of cyclic ethers (e.g., ethylene oxide, propylene oxide, tetrahydrofuran). with appropriate catalysts:
I nCH-CH2 \ /
0
OH4
aqu.
I HO(CH"CH2"O).H
The reaction of diisocyanates with diols produces chain extension:
2HOROH + OCNR'NCO
HOROC(0)-NHR'NHC(0)OROH
(20)
The -NHC(O)Ogroup is referred to as urethane. hence the generic term "polyurethanes." Crosslinking may be built into the polyether or polyester precursors by various mechanisms. The facile reaction of isocyanates with reactive hydrogen species leads to a very broad spectrum of polymeric materials and processes.With respect to liquid-polymer processes relating to rubber, this is best exemplified by the chemistry and technology of flexible foams. These are usually made by thereaction of a branchedpolyether diol withtoluenediisocyanatein the presence of water together with catalysts, silicone surfactants, and. if desired, a fluorocarbon blowingagent for low-densityproducts.Foaming is produced by thegeneration of carbon dioxide:
"ROC(0)NHR'NCO + Hz0 The primaryanlineterminal formation of urea moieties:
- -ROC(O)NHR'NHz
+
CO2
group is highlyreactivewithisocyanates,leading
(21 1 to the
Liquid
Rubber
141
Further reaction of isocyanate groups results in branching via formation of “biuret” linkages:
-NHC(O)NH-
+
+‘NCO
- “NHC(0)N-I
c=o I NH
I R” Crosslinking is thus introduced both in the preparation of the prepolymer and in the reactions of the terminal groups. Multifunctional isocyanates may also be used to increase crosslinking. This process lends itself ideally to continuousfoam rubber production; hencesoft polyurethane foams for furniture and mattresses have long since largely replaced rubber latex foam products. The inherently higher raw material costs are more than offset by process efficiency in this type of product. To return to the fundamentals of network formation and reinforcement, in the case of a polyurethane made from a long-chain triol and diisocyanate (i.e., a network of highly flexible chains with only occasional network junctions), the extensibility is high but the strength properties are very modest. In other words, strength is low in polyurethanes, as in other elastomeric networks, in the absence of a reinforcement mechanism. High strength properties are achieved in polyurethanes by the inclusion of both “hard” and “soft” segments in the polymer chains. The “hard” segments contain high proportions of diisocyanates reacted stoichiometrically with low molecular weight diols or diamines(such as butane diol or 3,3’-dichloro-4,4’-diamnodiphenylmethane, respectively) to give relatively inflexible chain sequences having regular structures and a strong capability for hydrogen bonding. These hard segments form domains within the continuous softer elastomeric matrix, providing reinforcement. The detailed nature of the hard domain structure is complex andvariable. depending on both chemical composition and process history. The morphology and temperature behavior of the domains isinfluenced by an interplay of glass transition temperatures, crystallization effects, and hydrogen bonding. A concise review of this subject has been provided by Redman (1978). While a detailed discussionof polyurethane technology is beyond the scope of this chapter, mention should be made of the possible development of polyurethane processes for automotive tire manufacture. If fundamental property and cost factors should permit the liquid fabricationof tires of a quality comparable to today’shighly engineered radials, then a gradual but nevertheless revolutionary change in the tire industry would be anticipated. This subject gained prominence in 1970, when publications were issued by the Firestone Tire and Rubber Company (Alliger et al., 1971) on experimental cast tires for automobiles. In subsequent years, much publicity has been generated by LIM International S.A., formerly PolyairMaschinenbau GmbH(Marshall,1982). Ideally.fromthestandpoint of production economics such tires would comprise a single polyurethane composition injected into a mold andcuredrapidly,thefinishedtire containing no mechanicalreinforcementotherthanthe beadwires. During the past 15 years or so, research and development efforts on liquid injection molded or cast tires have continued throughout the industrial world, as evidenced by a very extensive patent literature. The fundamental challenges are associated with certain thermal and mechanical properties that are characteristic of polyurethanes. The urethane linkageitself is thermally reversible, and hence failure of the primary polyurethane chains will occur at sufficiently elevated temperatures. The reinforcementmechanism is also thermallysensitive in principle, since it depends on the presence of domains that must remain above their glass transition temperatures
142
Edwards
and/ormeltingpointsduringservice.Gradual creep underload, even at relatively moderate temperatures, is also characteristic of this class of materials. These factors have resulted in experimental tires that exhibit gradual “growth” under the combined influences of inflation pressure and heat and that can suffer catastrophic damage to the tread region when subjected to panic stops or other circumstances that produce local overheating. Recent efforts have therefore been directed toward more complex tire structures, in which the polyurethane components are integrated with other functional elements. For example, Rossi (1982) described a tire in which a soft polyurethane tread portion is first spin cast and partially cured and then a reinforcing belt is stapled to the inner surface. A harder polyurethane carcass is now spin cast so as to enclose the reinforcing belt between the tread and carcass components. Rau and Just (1983) described the introduction of a fabric reinforcement connecting the beads before the beadhbric assembly is positioned over the inner core of the mold. The net result is to produce a finished geometry of cord reinforcement somewhat similar in principle to that of a belted radial tire. Cesar et al. (1980) described the use of metal hoops running from bead to bead, together with a provision for positioning a crown reinforcement (belt) between the hoops and the tread region, such that the reinforcingmembers are positioned precisely withinthe liquid or paste material prior to cure. Schmidt (1978) described a three-step operation in which the carcass is first molded, then a cord reinforcement is wound around the circumference so as to provide structural support in the manner of a belt, and the tread component is then added by casting or injection molding. Bead-to-bead winding to provide radial reinforcement has also been described (Schmidt and Kubica, 1978). Many other combinations of fabricationprocessesandmaterials.includingthe use of conventional rubber vulcanizates to perform the tread function (,road holding and wear resistance), have been and are being considered. The ultimateresults of theseeffortscannot. of course, be forecast with certainty. From present perspectives, however, it seems most unlikely that a single isotropic composition can provide a balance of properties capable of matching the performance standards of current high-quality tires. If this is true, then the ideal of fabricating a tire from a single liquid-rubber composition in a single molding operation is not attainable. The direction of technology development in this area hasbeen toward multicomponent structures and increasingly complex fabrication techniques. This trend is in opposition to the economic advantage that liquid-processing methods might otherwise provide. The question of whether or not an economic compromise can be reached is unlikely to be answered in the near future.
2.4 Terminally Reactive Butadlene-Based Polymers Terminally Hylroxvlrtecl Polybutadienes
The preparation of terminally hydroxylated polybutadienes can be accomplished by a number of procedures. Hsieh (1959) described a preparation based on “living” anionic polymerization techniques, this being anotherof the generic methods for telechelic polymer synthesis. A difunctional organolithium catalyst in an inert, dry hydrocarbon solvent medium is employed. The number of polymer chains formed is equal to the number of initiating molecules. If all of the monomer is charged initially, so that all chains begin growing at essentially the same time, the resulting polymer chain lengths are all nearly equal. Polymerization proceeds until all the monomer is exhausted, the chain ends remaining reactive toward the addition of new monomer, if it is introduced. This behavior accounts for the term “living” polymerization, as originally introduced by Szwarc et al. (1956). As applied to butadiene, this may be represented as follows:
Liquid Rubber
LIRLI
+
143
2nCH2=CH"CH=CH2 LjlCH2-CH=CH-CH21.RICH2-cH=cH-cH2~"L~
-
(24)
To form the terminal hydroxyl groups, ethylene oxide is added, followed by acid hydrolysis:
-RLi
+
CH2-CH2 \ / 0
- -R-CH2-CH2-OLi
-. HX
-R"CH2"CH20H
+
LiX (25)
A product of this type was developed by the Phillips Petroleum Company under the trade name Butarez HT. Because of the narrow molecular weight distributions providedby this type of polymerization, the terminally functional products are of interest with respect to model network studies. Morton and Rubic (1977), working with polyisoprene diols crosslinked with p p ' , p"-triphenylmethanetriisocyanate, concluded that strength properties are enhanced by having uniform chain lengths between crosslink sites and that a maximum in gum tensile strength for such networks is reached at molecular weights in the region of 6000. A less elegant, but more economical, route to terminally hydroxylated polybutadiene arose during the late 1950s and early 1960s. beginning with an investigation of the free radical polymerization of butadiene in solution using hydrogen peroxide as the initiator (Burke et al.. 1959). The object of the early work appears to have been simply the preparation of liquid butadiene polymers or copolymers by a clean and fundamentally economic solution process. The reaction mediumwasa common solvent for the monomer and aqueous hydrogenperoxidecatalyst, isopropanol being suitablefor this purpose. Polymerizations were conducted for periods of about 2 hours at temperatures in the region of 1 20"C, yielding essentially water-white. odorless liquid products. Although the technical literature contains little basic information on polymerizations of this type. the predominant mechanisms are essentially as follows:
2HO* HO. + CH2=CH-CH=CH2 2HOR-• HOR ROH
H202
(26) (27) (28) This reaction scheme is, of course, an oversimplification. Although termination by combination [ Eq. (28)] is normally predominant in solution free radical polymerizations of butadiene. chain transfer to solvent. initiator, polymer, and monomer all occur as side reactions. The decomposition of hydrogen peroxide to HOO- and H' is also a significant side reaction. although the indicated path [Eq. (26)] is favored by high temperature (Pinazzi et al.. 1973). French workers have studied this reaction as applied to several monomers and have provided chain transfer data (Brosse et al., 1978).The products are found to contain low molecular weight oligomers (monofunctional polymers) as well as high polymers with two or more hydroxyl groups. In addition to hydroxy end groups, stnall amounts of other oxygenatedspecies, including aldehydes, ketones, carboxylic acids, hydroperoxides, and epoxides, have been detected (Brosse et al., 1982). The commercial development of this technology was pursued in the United States by the Sinclair Oil Corporation and subsequently by A R C 0 Chemical Company. Reaction of the hydroxy-functional polymer, containing an average of 2.1-2.2 hydroxyl groups per chain, with isocyanates was demonstrated in the early 1960s (Verdol and Ryan, 1966).Chain extension via condensation with formaldehyde was also described (Isaacson and Young, 1966). As with all terminally reactive polymers, subsequent reactions may be carried out to change the nature of the end groups. Thus, terminally hydroxylated polybutadiene may be esterified, for example, with an acrylic acid. to give acrylate end groups that can take part efficiently in subsequent free
-.
-
HO"CH2"CH=CH"CH2*
144
Edwards
radical polymerizations (Ryan and Thompson, 1968). Terminally halogenated products, curable with amines at room temperature, can be synthesized via reaction with excess dry hydrogen halide (Edwards and Wunder, 1968). Japanese workers have conducted extensive studies on the curing and reinforcement of this type of polymer. Curing with esters of phosphoric acid was explored as a possible route to enhance flameresistance (Minoura et al.. 1977).The cure is catalyzed by 2,3,6-tris(dimethylaminomethy1)phenol (DMP-30):
R' in this instance is phenyl. The cure occurs at elevated temperatures, e.g., in several hours at 130°C. The crosslinks decompose by hydrolysis upon exposure to water. A thorough investigation of the effects of reinforcing and nonreinforcing fillers has also been reported (Yamashita et al., 1978). Good strength properties were observed at optimum levels of diphenylmethane diisocyanate (about 16.5 phr) using N330 black or precipitated silica. Coarse fillers gave less reinforcement, analogous to conventional rubber. Curing of hydroxyl-terminated polymers by metal chelation has been described (Kambara andAotani,1970). The terminal group is converted by postreaction to -C(O)CH?C(0)"CH3. On heating with metal alkoxides (e.g., aluminum, magnesium, calcium, titanium), chain extension and crosslinking occur rapidly in one step. At the present time, two grades of hydroxytelechelic polybutadiene are produced by the A R C 0 Chemical Companyunder the tradenames Poly bd R-45HT and R-45M. These polymers have number-average molecular weights of about 2800 and hydroxyl functionalities (moles OH per mole polymer) of about 2.3 and 2.5, respectively. As with other terminally hydroxylated polymers, the curing chemistry of practical value is that of the polyurethane industry. Simple chain extension and crosslinkingwithdiisocyanatesproducessoftvulcanizateswithlittle strength. This type of system can be useful in electrical potting compounds and in imparting elastic properties to asphalt compositions used as caulks,mastics, or jointsealers in the construction industry. Soft vulcanizates with good low-temperature properties are also suitable for use as rocket propellant binders. To achieve high strength properties from initially pourable compositions, provision must be made for theincorporation of "hard" domains in the vulcanizate. Thisis illustrated in Table 2, which refers to the use of increasing amounts of a polyfunctional liquid isocynanate in conjunction with a suitable low molecular weight diol, N,N-bis-(2-hydroxypropyl) aniline. A progressive increase in modulus and strength properties, to levels in the order of 20 times those of the simple network, may be noted. Polyurethanesbased on polybutadiene diols are notable for excellentlow-temperature properties, compatibility with low-cost hydrocarbon oils, anddegree a of resistance to hydrolysis claimed to be much superiorto that of polyether-based or (particularly) polyester-based polyurethanes. These properties lead to applications in electrical insulation, waterproofing membranes or coatings, and liquid-castable general-purpose rubber goods. Terminally Reactive Liquid Nitriles
The use of a functional free radical initiator in solution polymerizations of butadiene and butadiene copolymers, particularly butadiene-acrylonitrile, has been developed commercially by the B. F. Goodrich Company. Carboxyl-terminated polymers may be prepared using azodicyanovaleric acid (Siebert, 1964):
Liquid Rubber
145
Table 2 Properties of ARCO Poly bd R-45 HT Urethanc Compositions
Formulations Poly bd R-4SHT, Isonol 100 Catalyst T- 12" drops CAO- 14' g Isonate 143L," g Equivalent ratio, Poly bd/Isonol 100 Vulcanizate properties" Tensile strength, Mpa 200% modulus, Mpa Elongation, c/o Hardness, Shore A Shore D "
I
2
3
100
100
100
4 0. I O 12.76
1.2
4
5
6
2.22 4 0.10 15.45
4.45 4 0.10 19.14
IO0 8.89 4 0. I O 25.53
100 1 1.85
4 0. I O 29.78
100 17.78 4 0.10 38.29
411
211
1/1
314
6.2 2.6 238 7s
8.2 5.3 244 82
2.6I .7 -
101
151
53
56
1.3 195 62
7
8
100
1 00
26.67 4 0. I O S I .05
35.56 4 0.10 63.81
I I2
I /3
114
13.9 10.3 300
18.6 14.7 325
24.0 19.4 297
-
43
-
51
-
53
N,N-bis-(2-Hydroxypropyl) aniline (UpJohn Co.).
'' Dibutyltin dilaurate.
' Antioxidant (Sherex Chemical Co.). '' Polyfunctional liquid isocyanate (Upjohn Co.). '' Press cure 30 min at 80°C. Postcurc 64 hr at 49°C. S O I ~ - CCourtesy ~: of ARCO Chemical Co., Philadelphia. Pennsylvania.
This is analogous to the use of hydrogen peroxide as described above, and the same generalizations with respect to chain transfer side reactions are applicable here. The reaction is carried out in f-butyl alcohol, this solvent being chosen because of a relatively low degree of chain transfer to solvent. Reaction temperature is typically 80°C. The terminal functionality may be converted to hydroxyl by postreaction with ethylene oxide in the presence of a catalytic amount of a tertiary aliphathic amine (Siebert, 196th):
Alternatively, the carboxyl groups may be reacted with an excess of a diol, such as 1.4butane diol, in the presence of an acid catalyst such as p-toluene sulfonic acid (Siebert. 1968b):
146
Edwards
-R--COOH + H O - - R ' - o H R " ~ ~ H - R - ~ ( ~ ) -+ ~H~~' O ~~ (34) Vinyl-terminated polymers having highly reactive double bonds may be obtained by reaction of the terminally carboxylated polymers with glycidyl acrylate or methacrylate in the presence of a tertiary amine catalyst. This reaction may be conducted in acetone or toluene solution at temperatures above 90°C (Skillcorn, 1972):
-RCOOH
CHz-CH-O-C(O)-CH=CH2
+
\
/
.O'
R',N 4
-R-C(0)-OCHz-CH(OH)-OC(O)-C~=~~z
(35)
The preparation of amine-terminated polymers, again fromthe carboxyl-terminated precursors, is described by Riew ( 1 976). This is preferably carried out using an amine with mixed functionality and containing no more than one primary amine site. The preferential reaction of the primary amine minimizes chain extension:
-RCOOH
n
+
NHzR'N
NH
W
- -RC(O)-ONHR'N
q
U
H
+ H20
(36)
This basic technology has led to a family of commercial products. These include carboxylterminated polybutadiene (CTB), suitable for use as a rocket propellant binder, as well as nitrile copolymers with carboxyl (CTBN), hydroxyl (HTBN), vinyl (VTBN), or amine (ATBN)terminal groups. The nitrile contents range from approximately I O to 25% by weight. The carboxyl- and amine-terminated products have been developed principally as modifiers for epoxy resin structural adhesives, coatings, and composites (Siebert, 1984). In the case of the carboxyl functionality, the properties of the epoxy products are dependent on the type of curing system and the sequence of reactions in a complex manner, since the carboxyl groups may react with amine catalysts as well as with the epoxy resin monomers. In some studies this has been simplified by prereacting the carboxyl-terminated polymer with a diepoxide, such as the commonly used diglycidyl ether of bisphenol A, so as to form a prepolymer with epoxy and groups:
- RCOOH
+
CH~-CH"R'-CH"CH~ \ / \ /
As the epoxy cure proceeds, a rubber-rich component separates out as discrete domains in the order of a few micrometers in diameter, these domains being grafted to the continuous epoxy resin matrix.This type of morphology represents a general methodfor imparting increased impactstrengthandtoughness to resinousmaterialswithoutexcessivelosses in modulus or tensile strength. Table 3 shows data for a simplified system of this kind, chosen to illustrate the principles involved. In this case the ingredients were mixed and cured together without a prereaction step. As the proportion of rubbery component increases, a phase inversion occurs in the region of 5060 elastomer/epoxy. At higher rubber contents the elastomeric phase is continuous. This combination of materials therefore provides a spectrum of compositions ranging from rubbertoughened plastics to resin-reinforced castable elastomers. A comprehensive key to the literature on epoxy formulations for adhesives has been provided by Drake and Siebert (1984). The amine-terminated polymers have alsobeen developed primarily for use in conjunction with epoxy resins (Riew, 1981). In this case the amine functionality is reactive with epoxies at
147
Liquid Rubber Table 3
Properties of CBTNEpoxy Resin Compositions 1
Formulations DGEBA" Hycar CTBN 1300x8'' Piperidine Physical properties' Tensile strcngth. Mpa Tensile modulus, Gpa Elongation, 76 Fracture surface energy, kJ/m' Gardner impact, J Heat distortion temp., "C '' Diglycidyl ether of hisphenol A.
100 -
5 65.8 2.8 4.8 0.18
6 80
2
3
4
5
100
100 10
100 15
100
S 5
5
S
62.8 2.5 4.6 2.63 8 76
58.4 2.3 6.2 3.33 8 74
51.4 2.1
8.9 4.73 8 71
20 5 47.2 2.2 12.0 3.33 25 69
-
" CAN
content 18 wt%, functionality 1 .X, M,, 3600. ' Cure 16 hrat 120°C. Sorrrce: Courtesy of the B. F. Goodrlch Co.. Akron. Ohlo
room temperature. The reaction products, unlike the case of carboxyl-terminated polymers, do not contain ester groups and are therefore more stable toward hydrolysis. Once again a variety of compositions, rangingfromtoughenedplastics to reinforced elastomers, is possible. The toughened epoxy compositions are particularly useful as adhesives. The vinyl-terminated polymers are designed principally for use in unsaturated polyester glass-reinforced bulk molding, or sheet molding. compounds. Thehighly reactive terminal double bond can participate efficiently in the styrene-polyester polymerization process that occurs during cure. The nitrile polymer appears in the product as rubber-rich spherical domains. The domain sizes are relatively large (10- 15 pm); nevertheless, improvements in resistance to internal cracking during flexure, and in impact damage resistance, are observed, with little sacrifice in strength or modulus properties (McGarry et al., 1977). In general,theterminallyfunctionalnitrileliquid polymers have evolvedprimarilyas additives, to confer toughness to thermosetting resin compositions, with which they are chemically compatible.
3. MODEL STUDIES USING TERMINALLY FUNCTIONAL POLYBUTADIENE A question of both practical and theoretical interest is whether or not it is possible to achieve conventionally reinforced elastomeric networkscomparable in physical propertiesto high-quality general-purpose rubber vulcanizates, startingfrom liquid telechelic precursors. The processing of natural rubberand of the various high molecular weight synthetics requires very heavy machinery because of the viscosities involved. Indeed,one can only marvelat the temerity of those pioneers, such as Thomas Hancock, who undertook the bulk mixing of a substance as intractable as raw natural rubber. Conceptually. the mixing and shaping of many rubber articles could be carried out much more economically, at least with respect to the capital cost of process equipment, if low-viscosity materials could be used.
Edwards
148
As discussed above, the question resolves itself into two fundamental issues: ( I ) whether it is possible to achieve full network development, that is, networks with as few, or fewer,loose ends than those of conventional elastomers, fromliquid precursors; and ( 2 ) whether it is possible to achieve a full degree of reinforcement, using carbon black in particular, without the very high shearing forces that are brought to bear on the carbon agglomerates during conventional rubber mixing processes. At firstglancemanyrubbertechnologistsmight expect both of these questions to be answerable in the affirmative. However, demonstrationsof high strength i n conventionally reinforced liquid-derived vulcanizates have been rare, and high strength does not in fact appear to beattainablewithmany of thebutadiene-basedliquid polymers that have been tested. The difficulty of achieving full network development in many systems led, in the case of one detailed study, to a conclusion that network development cannot proceed much beyond the gel point becausetheunreacted groups are prevented by theexistingnetwork from encountering one another within a reasonable time (French et al., 1970). With respect to carbon black reinforcement, many mechanisms for possible chemical reactions at the carbon surface have been postulated. It could well be supposed that reinforcement might not develop fully in the absence of the elevatedtemperatures, high-energy mastication processes, and free radical curing procedures involved in conventional rubber manufacture. These questions have been studied using, as the model polymer, a liquid polybutadiene having terminal allylic bromide groups (Edwards, 1975). This polymer was prepared by a free radical emulsion polymerization process (Buckler et al., 1965), using carbon tetrabromide as a chain transfer agent.The normal effect of efficient chain transfer agents in emulsion polymerizations is to produce monofunctional oligomers, that is, short chainswith nonidentical end groups. With carbon tetrabromide, the telomerization occurs during the early part of the reaction, such that the carbon tetrabromide is consumed during approximately the first 10% conversion of the butadiene to form telomers of the type Br(Bd),,,CBr3, in which m is a small number and Bd represents the butenylene radical. The low molecular weight telomer then functions as a chain transfer agent throughout the remainder of the polymerization, which may be carried to essentially complete conversion of the butadiene (Beaton, 1971):
-
-R* + CBrJ(Bd),Br -R& + *CBrz(Bd),Br Br(Bd),CBrZ* + nCHz=CH-CH=CH2 Br(Bd)mCBrplCHp-CH=CH-cHz~n.
(39)
Br(Bd),CBrzI CH~-CH=CH--CHZ-~~* + Br(Bd),CBr3 -Br( Bd)mCBrnlCH~-CH=CH-CHzIn& + Br(Bd),CBrZ* (product)
(40)
-
(38)
The resulting polymer has a reactive (allylic) bromide at both ends of the chain and a relativelynonreactive(nonallylic)dibrornide site locatednear oneend. In practicethere is also a minor amount of telomer remaining in the product, as well as nonpolynleric emulsion polymerization residues. To make suitable model polymers, these materials may be removed by dissolving the crude product in a hydrocarbon solvent, centriguging, and precipitating the telechelic polymer from the clear solution with excess acetone. The allylic bromide can be reacted with a tertiary amine to form the quaternary salt:
-RBr
+
NR',
- -RNR'3@Bro
(41)
For network studies, a tetrafunctional curing agent was prepared by methylation of triethylene tetramine. This agent was observed, as expected, to produce insoluble networks.
Liquid
Rubber
149
Regarding network formation, one should be able to demonstrateconsistency between the physical and chemical characteristics of the polymer, as measured by independent methods, and the stoichiometricbehavior of thepolymer-curativesystem. In other words,the amount of curative observed to give the highest state of crosslinking should be stoichiometrically equivalent to theconcentration of reactive sites based on molecular weight measurements and difunctionality. In the present instance this was done using five polymer samples having different molecular weights. The number-average molecular weights of the polymers were determined by a combination of ultracentrifugation and gel permeation chromatography (GPC). The ultracentriguge measurements provided a value for the weight-average molecular weight in each case. Thesevalues were used to calibrate the GPC data so as to provide the optimum fit between calculated and observed molecular weight values. The polymer samples, after mixing with various loadings of methylated triethylene tetramine (MTETA) and curing for 7 days at room temperature were swollen to equilibrium in benzene for determination of network density. The values of network density were calculated by the method of Flory and Rehner (Flory, 1950) using a value of 0.39 (determined by stressstrain measurements on representative swollen specimens) for the solvent-polymer interaction parameter and assuming tetrafunctional crosslinks. This reaction proceeds at a moderate rate at room temperature and does not involve side reactions or by-products. It is therefore experimentally convenient for model studies. In considering model systems, one must be careful to ensure that the expected, or desired, chemical processes actually occur. One cannot safely assume that the polymer is truly difunctional and that the reactions involved in chain extension and crosslinking proceedto completion. In order to have confidence in the model, one must observe physical behavior that conforms very closely to what is predicted. The simplest case is that of chain extension using a difunctional reagent. The change from a liquid to a soluble, but high molecular weight, solid polymer is unequivocal proof of effective difunctionality in bothcomponents andof suitable reaction conditions. No amount of indirect information based on the quantitative measurement of functional groups and number-average molecular weights can provide a degree of confidence to compare with the demonstration of practical chain extension. In the present instance a sample of the polymer was mixed, in an early experiment, with a difunctional tertiary amine prepared by the methylation of 1,6-hexanediamine. The viscous liquid polymer became an elastic solid within a few hours at room temperature. After 3 days, a portion was placed in a solvent and was observed to dissolve completely. The intrinsic viscosity was measured as 1.8 dL/g (in toluene at 30°C), representing a molecular weight in the same order of magnitude as that of conventional elastomers. Effective chain extension was thus unequivocally demonstrated. With respect to polymer characterization, the absolute value of network density is not significant, since it is not known at this stage to what extent the curing reaction has approached completion. What is important is the location of the maximum in network density. since this should correspond stoichiometrically to the reactive sites in the polymer and thus provide a chemical measure of molecular weight. The data resulting from this comparison are given in Table 4. The columns of interest are thefinaltwo, in whichthenumber-averagemolecularweightsderivedindependently from physical measurements and chemical measurements (curingbehavior) are compared. The agreement is seen to be very satisfactory, although the physical measurements are consistently higher by a few percent. This would suggest a true polymer functionality slightly in excess of 2. Table 4 also includes a column showingthe highest observed value of M, (number-average molecular weight between crosslinks) following the 7-day cures.If the crosslinking reaction had proceeded to 100% “yield,” these values shouldequal thenumber-average molecular weights of
Edwards
150
Table 4 PolymerCharacterizationData
Min. ViscositySp. gr. Polymer (25°C) A
0.92
B
0.93 0.94 0.95 0.96
C D E
M I,
M,
(25"C), P
M,M,
40,000 14,000 3,100 730 300
3.1 3.0 2.4
140
2.1
Chem. (7Phys. d, 25°C) 14,000
2.2
27,000
9,200
1 3,000 10,000 5.000
6,300 5.360 3,860
13,000 8,900 5,800 5,000 3,600
the parent polymers. The actual values are much higher, indicating that the cure was far from complete under these conditions. Since the extent to which full cure is achievable is a major factor in the consideration of model polymer systems, further experiments were conducted using a product of intermediate molecular weight (polymer C ) . Figure 2 shows V, (volume fraction of polymer in the swollen vulcanizate at equilibrium) as a function of curative loading and time of cure at 25°C. The samples were kept under nitrogen, small portions being removed for swelling determinations at the indicated time intervals. The maximum state of crosslinking occurred at 2.0 phr of the
.20
-
.18-
-a
16
-
14
-
c .12a, N
C
g
.lo-
Y
>' .08 .06
-
'O4I .02 I
1
1.6
1.8
1
I
2.0 2.2 MTETA (phr)
l
1
2.4
2.6
Fig. 2 Vulcanization of polymer C gumcompounds at 25°C.
Liquid Rubber
151
X
curative. Figure 3 shows a plot of calculated M, values against time for the sample containing the optimum concentration of crosslinker. The value of M, is seen to approach M,,, the numberaverage molecular weight of the polymer, after approximately 12 weeks at 25°C. In separate measurements it was shown that the crosslinking reaction proceeds in two successive rate stages, a rapid rate up to the gel point (about 3 hr at room temperature) and a much slower rate during the approach to full cure. This is attributed to the inhibition of chain movements following the initial formation of a continuous network. It is tempting to conclude from this experience that the Flory-Rehner expression, with a single value for the interaction parameter, is quantitatively exact as applied to real systems and that the polymer and curing chemistry used in this model study were both quantitatively perfect. However, there is circularity in any such statement,because the observed agreementcould arise, as well. from fortuitous inexactness on both sides of the equation. Nevertheless. it is proper to conclude that the formation of a network very similar to that of conventional rubber vulcanizates is possible by this type of process and that the slowing of cure following the gel point, while very pronounced, does not prevent a subsequent approach to full network development. To determine whether carbon black reinforcement comparable to conventional rubber can be realized, mixtures containing N 1 19 black were prepared. These mixtures were made at room temperature using a three-roll laboratory paint mill, several passes being required. The product of such a process is no longer a pourable liquid but rather a smooth, glossy paste that may be spread or pumped very readily and that retains its shape for an indefinite period under small gravitational stresses. In rheological terms, the behavior is pseudoplastic; viscosity decreases as shear rate increases. Detailed data on liquid polymer systems of this kind have been provided (Rivin and True, 1973). At very high shear rates. viscosity is similar to that of the unfilled liquid. Thixotropy is also observed. The development of tensile strength and modulus upon curing at room temperature is illustrated in Figure4. The tensile strength reaches valuesin the region of 20 MPa, with modulus values above 10 MPa. These strengthvaluesarehigh for apolybutadiene, as evidenced by comparison withconventionalcis-polybutadieneandmixed-structure(butyllithium-polymerized) linear polybutadianes, mixed with the same loading of carbon black and vulcanized by conventional procedures (Edwards, 1975).
152
Edwards
1
1
1
L
2
4
6
8
Tlme at 25°C (Weeks)
Fig. 4 Development of stress-strainproperties carbonate, 5: MTETA, 2.
a t 25°C. Polymer C, 100: N119 black, 40: basic lead
The samestudy included tests using finelydivided esterified silica as the reinforcing agent. As shown in Figure 5, high strength properties are attainable in this case as well. Functionalization of the esterified silica with tertiary amine groups, so as to vary the degree of polymer-filler bonding from zero to highvalues, was includedinthiswork. The results showed thatlow degrees of bonding provide an increase in modulus with little effect on tensile strength, while high degrees of bonding result in excessivelosses in bothextensibilityandstrength. These combined observations are of interest with respect to postulated mechanisms for the reinforcement of rubber by particulate fillers. Carbon black appears to be fully effective in the absence of any history of heating above room temperature, high-energy mastication during mixing, or
15
c
0
1
l
I
1
1
10
20
30
40
50
Flller (phr)
Fig. 5 Reinforcing behavior of esterified silica. Cure 48 hr at 60°C. 2.4 phr MTETA.
Liquid Rubber
153
heat or free radical processes during cure. Fumedsilica, rendered as chemically inert as possible by esterification, also provides high degrees of reinforcement. Thus, chemical bonding to the filler surface, while probably desirablein small amounts forthe optimization of technical properties, is not fundamental to the reinforcement of rubber. With respect to liquid polymer systems intended for use with conventional reinforcing agents, the combined results show unequivocally that the two essential requirements-a high degree of network development together with reinforcementby carbon black-are fundamentally attainable.
4.
PRACTICAL CONSIDERATIONS AFFECTING THE DEVELOPMENT OF TELECHELIC POLYMERS AS GENERAL-PURPOSE ELASTOMERS
As we have seen, the only successful displacement of a conventional elastomer by a telechelic polymer system in a high-volume application has been that of natural rubber latex foam by polyurethane foam.Since both foamprocesses employ liquids (the naturalrubberbeing in latex form initially),displacement of solidrubber is qualified even in thisinstance.Having demonstrated that fundamental factors donot preclude the liquid fabrication of materials closely analogous to conventional rubber, it is appropriate now to consider other factors that bear upon technical and economic progress in this direction. A first consideration is the need for a "universal" curing technology comparable to the sulfur curing of natural and synthetic elastomers. Many practical compositions, particularly in the tire sector, involve mixtures of elastomers for optimized cost and property balances, and strong adhesion between components of differing composition. Compatible,although not necessarily identical, curing systemsare needed in all such compositearticles. This suggests a requirement that various types of butadiene, butadiene-styrene, isoprene, ethylene-propylene, and isobutylene polymers, as a minimum, must be made available at low cost in liquid telechelic form, probably with a common functionality such as hydroxyl. A universal chain extension system, if not diisocyanate chemistry itself. should have some of the same features: economy of scale, efficient and rate-adjustable reactivity, and a reaction mechanism that results in chain extension without the production of objectionableby-products.Athermalstabilitybetter than that of urethanelinkages is also desirable, as well as freedom from toxicological or environmental concerns. A second requirement is for economical mixing processes. This has been studied extensively by workers at the Rubber and Plastics Research Association (RAPRA) of Great Britain. Various procedures using conventional bulk mixers (Z-blade, Brabender) were tested in early trials (Pyne, 1970) but did not produce dispersions equivalent to those obtained with a threeroll paint mill. Further experience with paint mill mixing (Daniel et al., 1972) suggested that the energy requirements for dispersion of carbon black might not be less than those associated with conventional rubber. Concern was also expressed regarding both the physical properties attainable with the hydroxyl-, carboxyl-, and bromine-terminated polymers investigated and the projected costs of these products as compared to conventional rubber. A continuous-mixing device was developed by RAPRA to facilitate the dispersion of carbon black in liquid polymers (Humpidge et al., 1972. 1973). This provided for a bulk premixing stage to disperse the carbon black pellets followed by an intensive mixing stage employing principles similar to those of paint milling to produce the final dispersion. A review of this subject several years later (Lee et al., 1978) indicated that the mixing power required. using the RAPRA mixer, is similar to that of conventional rubber mixing and that the limiting factor is the work required to break down the agglomerates of carbon black.
Edwards
154
Subsequent processing, on the other hand. is greatly simplified in operations such as injection molding, “since the forces required to make the compounded pastes flow are some 30 times smaller than those required for solidrubber compounds.” Injection molding of a tread compound over a conventional tire carcass, without distoriton, was demonstrated.The samereview stressed the practical difficulties involved in achieving degreesof chain extension necessary for satisfactory product performance, as well as the high costs of existing liquid polymers. An alternative method of achieving carbon black dispersions in liquid polybutadienes has been described by Japanese workers (Inomata et al., 1975). The carbon black is dispersed in water using a conventional high-speed homogenizer. Upon mixing of this dispersion with the liquid polymer. transfer of the dispersed carbon black into the organic medium occurs immediately. The water, containing very little residual black, is drained off. and the polymer dispersion is then dried. Physicalpropertiesatleastasgood as thoseobtained by paintmillingwere achieved. It was also shown (Yamawakiet al.. 1974) that very highly loaded black concentrates, suitable for blending with additional polymer to provide the desired final loading, canbe prepared by this general method. Satisfactory carbon black dispersions arealso attainable using conventional internal mixers by first mixing very highly loaded concentrates in which the viscosity is sufficient to permit the necessary input of work and then diluting with fresh polymer (Masuko et al., 1974). High vulcanizate strength properties, comparable to conventional SBR tread compounds, have been claimed using a fabrication process whereby the chain extension and crosslinking steps are conductedseparately and sequentially (de Zarauz, 1975). The compositionswere based on terminally hydrocylated polymers (butadiene-styrene) compoundedwith both a diisocyanate and a conventional sulfur curing system. The mixture was heated successively at two temperatures, the first (ca. 100°C) sufficient for chain extension to occur and the second (ca. 160°C) suitable for sulfur crosslinking. The use of entirely separate mechanisms for chain extension and crosslinking is an interesting approach. providing enhanced flexibility in fabrication techniques and finished network design. There has been relatively little activity in the field of liquid polymers for general-purpose rubber displacement during the 1980s. Although fundamental feasibility has been demonstrated, and many of the basic incentives remain as they were, the practical and economic impediments are formidable. There wasa strong commitment to further research and development along these lines in the former SovietUnion (Fedjukin, 1984). but in general it appearslikely from the perspectives of today that most liquid elastomer work will continue to be focused on relatively high-price. small-volume specialty applications.
5. ADDITIONAL THEMES IN TELECHELIC ELASTOMER RESEARCH AND DEVELOPMENT This section surveys other elements of research that have already led. or may lead i n future, to practical developments in the liquid elastomer field. For convenience the subject matter is discussed under the general classes of preparative chemistry involved.
5.1
FreeRadicalPolymerizations
The preparation of carboxyl-terminated polybutadienes by a solution free radical process was developed during the 1960s (Berenbaum et al., 1961; Hoffman and Gobran, 1973). I n this case the preferred initiator is glutaric acid peroxide. The polymerization may be carried out in acetone solution at temperatures between about 75 and 130°C.
155
Liquid Rubber
A
IHOOC-(CHZ)J-CO)~ II
0 HOOC-(CH2)3COO'
2 HOOC-(CHZ)JCOO*
HOOC"(CH2)3*
+
CO2
(43)
Reaction (43) proceeds to the extent of about 70%. Thepolymerization is initiated predominantly by HOOC-(CH2)I. radicals. The key to effective difunctionality, as in all butadienesolutionfreeradicalprocesses of thiskind, is that termination is predominantly by mutual termination of growing chains. This process was developed by Thiokol Chemical Corporation to produce a polymer. HC-434. having a molecular weightof about 3600 anddesigned primarily for use in rocket propellant compositions vulcanized under mild conditions using tris-(2-methyl aziridiny1)phosphine oxide as the curative. This system was favored for many years owing to compatibility with the propellant mixture and adequate adhesion to the propellant components and the casing as well as excellent low-temperature properties. Reinforcement with carbon black and cure with an epoxy resin were shown to result in fairly high strength properties (Hoffman and Gobran, 1973). The use of such symmetrical free radical initiators, of the types ROOR or RN=NR, has been studied extensively by numerous workers since about 1960. Polymerizations using 4,4'azo-bis-(4-cyano-r?-pentanol) and 4,4'-azo-bis-(4-cyanovaleric acid) were discussed in a series of papers by Samuel F. Reed. Jr., during the period 1971- 1973. Hydroxyl-terminated polymers based on butadiene, isoprene, and chloroprene were prepared that had molecular weights in the order of a few thousands and functionalities (by chemical and molecular weight analysis) in the region of 2. Chloroprene produced higher molecular weights and higher yields, under nominally equivalent conditions, than isopreneor butadiene (Reed, 197 1). Copolymers of these three monomers with p-chlorostyrene, using both carboxy and hydroxy functional azo initiators, were prepared i n dioxane solution (Reed, 1972a). Copolymers of butadiene with chloroethyl acrylate and chloroethyl methacrylate, with both types of initiator, were also described (Reed, 1973). Russian workers have reported the preparationof bromine-terminated polybutadiene using 4,4'-azo-bis(4-cyano-l-bromo-n-pentane)in acetone solution (Barantsevich et al., 1973). The liquid products are curable with polyfunctional amines. Terminally carboxylated polybutadiene may be prepared in an aqueous emulsion system using cyclohexanone peroxide in the presence of ferrous ion (Allen, 1963):
H0
0-0
OH
00
-
Fe"
2 H0
0.4 HOC(CH2)s'
0
II
0
(44)
In a typical polymerization, water, benzene, ferrous sulfate, and an emulsifier are mixed and then cooled to 0°C. Cyclohexanone peroxide in THF is added dropwise during a 3-hour period. Molecular weight is controlled by the rate of addition of the initiator. Products having mixed carboxyl and hydroxyl functionality wereobserved under different reaction conditions (Quinn, 1968) with the same initiator. Preparation of butadiene with nonallylic terminal hydroxyl groups may also be carried out by solution free radical polymerization using r-butyl hydroxyethyl peroxide as the initiator (Gaylord, 1973):
Edwards
156
Initiation is believed to be predominantly by the HOCH,CH20. radical. To obtain satisfactory products, the reaction must be carriedout in a solvent, isopropanol or xylene being suitable. Typical conditions are 2 hours at 120°C. Mutual termination provides the difunctional product. Functionalities above 2 are found when the polymerization is conducted in a solvent, but only about 0.6 in the absence of a solvent. Terminally carboxylated copolymers of vinylidene fluoride and perfluoropropylene have been made using a peroxide initiator of the type
in which R , is a perfluoroalkane group. The initiator is soluble in the mixed monomers, and polymerization is conducted atabout 25-30°C. In practice the initiator may beprepared in aqueous solution in the same reaction vessel via reaction of hydrogen peroxide with acid chloride precursor in the presence of sodium hydroxide. The peroxide then transfers to the fluorocarbon phase, where polymerization occurs. The product, a viscous liquid, is curable to a tough rubber when mixed with pentaerythritol and heated (Rice and Sandberg, 1965). The copolymerization of butadiene with ferrocenyl methacrylate (Baldwin and Reed, 1969) or vinyl ferrocene (Reed, 1972b) may becarriedout in solution using azo-bis(2-methyl-5hydroxyvaleronitrile) as initiator.Yields of about 50% are observed in 72 hours at 67°C in dioxane. The introduction of iron into the liquid polymer is claimed to be useful in solid rocket propellant binders, the iron catalyzing increased burning ratesin the case of ammonium perchlorate-aluminum powder propellant. Another class of free radical polymerization processes involves the use of chain transfer agents of the type RSSR in emulsion systems. The chain transfer process occurs as follows: -*
+
RSSR
- -SR
+
RS'
(46)
If R contains a functional gorup, and if the polymerization conditions are such that a very high molecular weight product wouldbe formed in the absence of a chain transfer agent, the resulting product is difunctional for practical purposes. A study of the chain transfer coefficients of various disulfides was conducted during the 1950s (&stanza et al., 1955). The use of diisopropyl xanthogen disulfide in emulsion polymerizations of butadiene, followed by hydrolysis to yield terminal mercaptan groups. developed shortly thereafter (Byrd, 1958):
s
s
S
S
The resulting terminal groups were hydrolyzed with KOH in this instance. No11 and McCarthy ( I 966) polymerized butadiene (100 pbw) in an emulsion system with diisopropyl xanthogen disulfide (8 pbw) to 75% conversion and then heatedthe product at ISWC to produce terminal mercaptan groups by pyrolysis:
Liquid Rubber
157
Branching of the polymer via addition of thiol to double bonds occurs as a side reaction. This process may be refined by using small amounts of emulsifier and a persulfate initiation system, such that an acceptable product is obtained by direct drying of the latex to yield the finished product (Csontos, 1972). Mercaptan-terminated SBR polymers of high molecular weight provide excellent tensile properties when conventionally compounded and cured with a peroxide (Uraneck et al., 1969). In this case the conversion of the xanthate end groups was carried out in the latex by hydrolysis, using ammonia plus ethylenediamine at 70°C. Russian workers (Fokina et al., 1971) have provided data on the molecular weights and molecular weight distributions of liquid emulsion polymers as a function of conversion and of the concentration of diisopropyl xanthogen disulfide. For their purposes the end groups were hydrolyzed in benzeneethanol with excess ammonia. Another example of this general class of polymerizations is the photo-polymerization of chloroprene in emulsion with 4-hydroxybutyl xanthogen disulfide as the chain transfer agent (Takeshita.1974).
s s
S I1 II HO(CH2)rCSSC(CH2)40H-SC(CHa)rOH
S
”
-.
II
II
+
+
HO(CH2)LS.
(49)
Ultraviolet irradiation of the stirred latex, using a mercury lamp, for example, for16 hours at room temperature, yielded a liquid product capable of chain extension to an elastic solid with a diisocyanate. Since polychloroprene tends to crystallize on standing, pourability may not be retained. Preparation of aminotelechelic polymers in aqueous solution using a TiCI3/NH20Hredox system has been reported (Rubio et al., 198 1). NH,.radicals are formed,initiating polymerization. For the case of methyl acrylate, functionalities close to 2 were reported for polymers prepared under carefully defined conditions. The system is complicated by precipitation of the growing polymer from the aqueous initiation medium. The preparation of carboxy-terminated hydrocarbon polymers by the electrolysis of dicarboxylic acids has been claimed (Mersereau. 1969). This may occur by successive combinations of free radicals generated at the anode (Kolbe reaction):
RCO$
g RCOO- -R.
+
C02
This method is claimed to be capable of producing, for example, dicarboxy poly(isobuty1eneco-methylene) from dimethylglutaric acid, or dicarboxy polyethylene from linear alkyl diacids.
5.2 Cationic Polymerizations Pioneering work of a very comprehensive nature in the field of telechelic polyisobutylenes made by cationic polymerization has been reported by J. P. Kennedy and coworkers at the University of Akron (Kennedy, 1984). This involves the use of cocatalysts that serve both as initiators and chain transfer agents (“inifers”). The further terms “unifer,” “binifer.” and “trinifer” have been coined to describe agents with the corresponding functionalities. The case of a difunctional agent (binifer) may be taken as illustrative (Kennedy and Smith, 1980). The polymerization may conveniently by conducted in dry methylene chloride at tempera-
Edwards
158
tures from about - 30 to - 70°C. A preferred binifer isl'-CI(CHi)~C"C~,HJ"C(CH~),C1, used in conjunction with BC13. The essential steps in the reaction may be represented as follows:
-
ClRCl + ClRoBClP Cl&CL'+ C H ~ = C ( C H J )ClR[ ~ CH~C(CHJ)~-]~%C~~~ C~R~CHZC(CHJ)~-~~OBCIP+ ClRCl CIR~CHZC(CHJ)Z-]~CI + C I&C14' C~R[CHZC(CHJ)Z-]~CI + BC13 BCI~OR~CH~C(CHJ)Z-]~CI
-
-
(51)
(52) (53) (54)
Propagation and chain transfer then proceed. as before, to produce the product Cb"-R"CI. in which both chain ends are equal. Termination without chain transfer produces the same terminal structure for the case:
-CH~C(CH,)ZOBCI~~-CH2C(CH&CI
+
BC13
(55)
It is critical to the process that the rateof chain transfer to monomer be negligible compared to the rate of chain transfer to binifer. The aboveconditions satisfy this requirementto the extent that essentially difunctional products are obtained. Given the terminal unsaturation and the chemical inertness of the poly-isobutylene chain. numerous other terminal functionalities including amine, vinyl, phenol. hydroxyl, and epoxy can be prepared by appropriatesyntheticmethods (Kennedy, 1984). Dehydrochlorination to provide terminal unsaturation, followed by sulfonation with acetyl sulfate. produces terminal sulfonic acid groups of the structure:
-C"CHZSOJH II CHz When applied to a trifunctional "star" polymer derived from a "trinifer," the effect is to produce an "endless" thermoplastic rubber via association of the sulfonic acid end groups or their metal salts. The physical properties of materials such as thermoplastic elastomers are
fairlygood (Bagrodia andWilkes. 1985). The generalstudy of moleculardesigns, based on these elegant caticnic polymerization techniques. is a subject of intensive ongoing research by Kennedy and coworkers. An unusual cationic polymerization of isobutylene, using a molecular sieve (Linde Type 5A) to provide a polymer with terminal unsaturation, has been claimed in a patient specification (Miller, 1969). Here it is speculated that the sieve functions as an initiator by hydride removal due to incompletely neutralized calcium or aluminate ions on the surface:
The second terminal unsaturated group is presumed to arise by normal termination processes. A conversion of 34% was observed in 4 hours at 0°C when 25 g of the dried sieve was stirred with 62 mL of isobutylene. A functionality of 2. l , based on NMR analysis and ozonolysis (no molecular weight decrease). was indicated. Another unusual cationic process involving a dibromide chain transfer agent has been described (Ver Strate and Baldwin. 1977). These authors polymerized 4-methylpentene-l with aluminum chloride at - 60°C in the presence of I ,3-dibromo-3-methylbutane. A liquid product having, by analysis, approximately two bromine a t o m per chain, was obtained. Dehydrobromination followed by ozonolysis gave results consistent with terminal functionality. The unsatu-
Liquid Rubber
159
5.3 Condensation Polymerizations This section takes note of a few condensation reactions that differ generically from those, such as polysulfides and polyesters, already discussed. Terminallyunsaturated monomers canbe condensed with hydrogensulfide to prepare thiol- or vinyl-terminated products (Erickson, 1965). For example.
0 II
0
It CHz=CHCO(CH2)rOCCH=CH2
-
+
H2S
0 0 (57) II II HSICHZCH~CO(CH~)~OCCH~CH~S--]~H
The reaction may be carried out in pyridine solution at room temperature with diisopropylamine as a catalyst. Vinyl-terminated polymers are prepared by using reduced amounts of hydrogen sulfide, about 95% of stoichiometric equivalence. Dithiols may be copolymerized with vinyl acetylene under ultraviolet irradiation to produce thiol-terminated alternating polymers (Oswald. 1970): HRSH (excess)
+
CHGC-CH,
- HSIRSCH-CH-S”),RSH I
CH3 This reaction may be carried out in the absence of solvent at about 15°C. With excess methyl acetylene, terminally unsaturated polymers are prepared. Polyoxyalkylene diols may be condensed with a mixture ofmercapto-alkanoic and thiodialkanoic acids to produce terminal mercaptan liquid polymers(Jones and Marrs, 1972), represented schematically as follows:
n HOOCRSCOOH 2 HOOCR’SH -HSR’C(O)O~C(O)RSRC(O)O-O-]~C(O)R’SH H20 (59) A product of this type (PM polymer) was introduced for use in sealant compositions by the Phillips Petroleum Company. A process based on the addition reaction of dithiols with terminally unsaturated precursors has been described (Singh et al., 1981). the following steps being representative: nHO-OH
+
+
+
nB H S ( C H Z ) Z S ( C H ~ ) (excess) ~SH +
- polymeric dithiol
+
2H20 (62)
Edwards
160
Reaction (60) may be conducted at about 50°C with a free radical catalyst. Reaction (61) may be carried out under nitrogen at 150- 180°C with triphenyl phosphite as a catalyst. The final polymer (62) may be prepared in about 16 hours at 70°C in the presence of t-butyl perbenzoate. Afamily of terminallyreactivepolyetherhhioetherliquids has been introduced by Products Research and Development Corporation. The terminal groups may be either thiol or hydroxyl. The products are used principally as sealants for insulated glass. The hydroxyl-terminated type is claimed to provide polyurethanes with excellent solvent and water resistance.
5.4
Ring-Opening Polymerizations
The polymerization of propylene oxide, tetrahydrofuran, etc., to prepare diols is one of the established routes to polyurethane building blocks,as already discussed. A terminally unsaturated liquid polymer of epichlorohydrin has been disclosed more recently (Hsu, 1979).Polymerization of epichlorohydrin in methylene chloride solution at 50°C. with (C2Hs)30PF(, as catalyst, in the presence of hydroxyethyl acrylate or methaclylate as “chain transfer” agents, is stated to produce liquid polymers with terminal acrylate or methacrylate groups suitable for modification of unsaturated polyester compositions or for photopolymerization.
5.5
Chain Cleavage
The cleavage of high molecular weight polymers to terminally functional liquids was the first generic process for telechelic polymer formation, as described earlier for polysulfides. Among other possible processes of this general type, most attention has been given to the cleavage of unsaturated elastomers by ozonolysis. In principle this is particularly applicable to polymers, such as butyl rubber, in which there are occasional unsaturated linkages in an otherwise inert polymer chain. The chain ends of the liquid products of ozonolysis will vary in composition depending onthe starting materials and the detailsof the ozonization conditions. As a generalization, the endgroups may be mixtures of oxygenatedspecies that can be fullyoxidized(to carboxyl) or reduced (to hydroxyl) by appropriate procedures. Butyl rubber (isobutylene-co-isoprene) may be conveniently ozonized in cold hexane solution using an ozonized oxygen stream. The liquid product, redissolved in diethyl eher, may be treated with LiAIH., at room temperature to produce a liquid polyisobutylene diol curable with diisocyanates (Jones and Marvel. 1964: Manton and Brock, 1965). Nagakawa and Rudy ( 1966)copolymerized isobutylene with small amounts of butadiene or isoprene. The butadiene copolymer was ozonizedin carbon tetrachloride solution and subsequently treated with excess fuming nitric acid to complete the oxidation process. In another example using the isoprene copolymer, the final oxidation was carried out in dioxane solution with sodium hypobromite. The terminally carboxylated products were curable with tris-(2-methyl aziridinyl) phosphine oxide. A useful comonomer for isobutylene in this type of system is 1,3-pentadiene (piperylene). This monomer copolymerizes more readily than butadiene and provides a symmetrical double bond that simplifies the subsequent generation of closely similar and groups. Ozonization to ternlinally carboxylated polyisobutylene followed by modification with ethylene imine to form terminal amine or hydroxyl has been described (Minckler and Watchung, 1970):
Liquid Rubber -RCOOH
161
+
CHz"CH2 "RCO(CH2)2NHz \ / II
N
0
A "+
-RCNH(CH2)20H II
0
(64)
H
In this case the ozonolysis may be conducted in hexane solution at about 4°C for about 5 hours in the presence of pyridine. The solution may be heated to complete the cleavage reaction. The ethylene iminepostmodification may be conducted in hexane/THF solution and heated to 100°C in order to complete reaction (64). The product is curable with diisocyanates. Ethylene-propylene terpolymers with suitable diolefins can be used in a similar manner. For example, tenninally hydroxylated liquid ethylene-propylene has been described by Greene and Soh1 (1973). A terpolymer containing butadiene may be ozonized in a mixture of carbon tetrachloride and ethanol at a temperature below 0°C. Under these low-temperature conditions the reactions are believed to be as follows:
Both of these end groupsmay then be reduced to -CHlOH by addition of sodium borohydride in ethanol. maintaining the temperature at about - 10°C. The recovered liquid polymer product is curable with diisocyanates. Ozonolysis of saturated ethylene/propylene rubber in carbon tetrachloride at 25°C using ozonized air has been claimed to yield liquid polymers having primarily carboxyl end groups (Meyer, 1973). Rhein and Ingham (1975) prepared unsaturated ethylene/propylene lubber elastomers by first brominating the polymer and then dehydrobrominating. The unsaturated product was then cleaved by ozonolysis (ozonized oxygenin carbon tetrachloride or heptane solution). The ozonized liquid products were reduced to terminally hydroxylated derivatives. with various reducing agents being used. Ozone degradation of saturated polymers (polyisobutylene, ethylene-propylene, amorphouspolypropylene) was also observed in this work, and the reduced products exhibited a considerable degree of curability with diisocyanates. Analysis indicated the presence of some hydroxyl functionality along the polymer chains aswell as at the chain ends. Themaximum proportion of polymer isolubilized during the curing process was in the region of 90% in these cases. Cleavage procedures other than ozonolysis may also be noted. Isoprene and, less readily, butadiene-styrene can be copolymerized with sulfur in emulsion. The polymeric products. containing di- and polysulfide linkages in the backbone. can be reductively cleaved while still in emulsion (Costanza, 1963a). using zinc dust together with solvents. followed by hydrochloric acid. Alternatively, the dried polymer can be swollen with a solvent and treated with LiAIHJ (Costanza, 1963b). Costanza reacted the product of polyisoprene cleavage with epichlorohydrin to provide terminal groups of the structure:
-SCH~-CH"CHZ \ /
0
This provided curability with diamines in a manner considered suitable for potting compounds. Postmodification with glycidyl acrylate. for the same purpose, was also demonstrated. High molecular weight polyethers may be cleaved into liquid diols by hydrolysis (Reegen and Frisch, 1964). In the case of stereoregular poly-propylene oxide, the product is claimed to
162
Edwards
be superior to conventional poly-propylene oxide diols as a polyurethane building block. The degradation may be carried out in water plus isopropanol under reflux conditions (e.g., 80°C for 40 hr) in the presence of sulfuric acid.
5.6 Miscellaneous Curing Reactions The scope for postmodification of end groups and for the exploration of novel curing systems is, of course, very broad. It is appropriate here to take note of a few examples that have not otherwise been touched upon. Terminally hydroxylated polymers suchas polyether diols have been converted to terminal primary amines by reaction with ammonia (Palmquist and Jones, 1966). Such a product was designed to form an elastic,adhesivebinder to provide a reflectivesurfacecomposition on bicycle tire sidewalls, by curing with an epoxy resin. Curing of carboxylated liquid nitriles with metal oxides has also been studied (Matsuda and Minoura. 1979). The gum vulcanizates are generally low in tensile strength, about l MPa or less. A 2-to- 1 MO/carboxyl ratio is superior to a 1-to- 1 ratio, and hence the preferred mechanism appears to be pairing or clustering of terminal ionic sites of the type -RC(O)OMOH. Such products are reprocessable at elevated temperatures. When polymers havingterminalacrylate groups, such asdiacrylate or dimethacrylate esters of polyethylene or polypropyleneglycols,aremixed with an organicperoxideanda tertiary amine. they remain stable in the presence of oxygen (air). but they polymerize rapidly when oxygen is excluded, for example, by applying the mixture to a threaded bolt and then installing the bolt. This principle is well established as a means of locking bolts into position more firmly than is possible without the polymerized bonding agent (see, e.g., Bosworth et al., 1963). The reaction of di- or polyfunctional thiols with terminally unsaturated polymers under gamma or electron beam irradiation has been studied extensively (Kehr and Wazolek, 1970). These authors prepared terminally unsaturated products from available precursors by numerous methods, including. for example. the reaction of isocyanate-terminated prepolymers with allyl alcohol. Cure of a liquid film with,for example,pentaerythritol-tetrakis-(P-mercaptopropionate) occurs in a few seconds during passage under an electron beam.
6. CONCLUDING REMARKS From a research standpoint, the subject of terminally reactive liquid elastomers is both broad and deep, embracing all classes of polymerization technology and, in the manipulation of end groups and design of practical curing and reinforcement systems, an extremely wide spectrum of synthetic possibilities. This chapter. which is necessarily a very brief treatment of the field, has outlined the technology of commercially useful products and has given some attention to the question of whether future liquid polymer technology may lead to a significant displacement of high molecular weight elastomers in conventionally reinforced vulcanizates. Since it has been amply demonstrated that there is no fundamental impediment to such a development. one must entertain the possibility that it may occur in the course of time. However, such a change in the fundamentals of the rubber industry would seem to require some form of technology unification that would permit the economic manufacture,compounding, and vulcanization of a broad variety of materials analogous to the general-purpose and specialty elastomers that we now use in high molecular weight form. From today’s perspective, such a development does not seem likely. The refinement andfurther development oftelechelic liquids for specialty purposeswill continue,
Liquid Rubber
163
however, to be one of the most interesting and challenging areas of research in the elastomers field.
ACKNOWLEDGMENT I wish to thank my colleagues for their assistancc and Polysar Limited for permission to publish.
REFERENCES Allen, H. C. (1963). US. Pat. 3,440,292 (issued 1969, Sec. of the A m y , U.S.A.). Alliger, G., Smith, W. A., and Smith. F. M. ( l97 l). Rubber World f64(3):5 I . [Also R1thber World f 61(5): 29 ( 1970)]. Anrady, A. L., Llorente, M. A., Sharaf, M. A., Rahalkar, R. R., and Mark, J. E. (1981). J . App/. Pniynl. Sci. 26: 1829. Bagrodia, S., and Wilkes, G . L. (1985). ACS Spring Meeting. Rubber Division, Los Angeles, Paper No. 3. Baldwin, M. G., and Reed, S. F. (1969), U.S. Pat. 3,847,882 (issued 1974, Sec. of thc Army, U.S.A.). Barantsevich, E. N., Pronin, B. N., Bclov, I. B., Kalaus, A. E., Beresneva, N. K.B., and Troitsky, A. P. (1973), Ger. Pat. 2.444.65 I (issued 1975). Bayer, 0. (1947), Allgew.. Cltern. AS9:257. Beaton, J. ( 197 1 ), Br. Polym. J. 3:129. Berenbaum, M. B., Bullcnko, R. H., Gobran, R. H., and Hofman, R. F. (1961 ), U S . Pat. 3,235,589 (issued 1966, Thiokol Chem. Corp.). Bcrtozzi, E. R. (1968), Rubber Cllenl. Teckrtol. 41: 114. Bosworth, P,, Yysc, B., and Swire, J. (1963), Brit. Pat. 1,090,753 (issued 1963, Borden Chem. Co., U.K.). Brosse. J.-C., Legcay, G.. Lenaln, J.-C., Bonnier, M,, and Pinazzi, C. (1978). McIkromol. Cller~l.f79:79. Brosse. J.-C., Bonnier, M,, and Legeay, G. (1982), Makrormd. Clleru. fK3:303. Brown. L. B.,and Hyde, J. F. (1960), U.S. Pat. 3,170,894 (issued 1965. Dow Corning). Brunner, L. B. (1959). U.S. Pat. 3,077,466 (issued 1963, Dow Coming). Buckler, E. J., Edwards, D. C., Wunder, R. H.. and Beaton, J. (1965), U.S.Pat. 3,506,742 (issued 1970, Polymer Corp.). Buechc, A. M. (1955). RuOher Cham. Technol. 28:865. Burke, 0. W., Kizer, J. A. A., and Davis, P. (1959), Can. Pat. 772,708 (issued 1967). See also U.S.Pat. 3,673,168 (issued 1972). Byrd, N. R. ( 1958), U S . Pat. 3,047,544 (issued 1962, Goodyear). Cesar, J. P., Gouttbessis, J., and Schneider, A. (1980). U S . Pat. 4,476,908 (issued Oct. 1984. Michelin). Ceyzeriat, L. (1957), US. Pat. 3,133,891 (issued 1964, Rhone-Poulcnc). Costanza, A. J., Coleman, R. W., Pierson, R. M,, Marvel, C. S.. and King. C. (1955). J. Poiym. Sei. 27: 3 19. Costanza. A. J. (1963a), U S . Pat. 3,338,874 (issued 1967, Goodyear). Costanza, A. J. (1963b), U.S. Pat. 3,338,875 (issued 1967, Goodyear). Csontos, A. A. (1972), Can. Pat. 954,998 (issued 1974, B. F. Goodrich Co.). Daniel, T. J., Needham, A., and Pyne, J. R. (1972), Trcrrls. Inst. Rubber I d . 6:253. Dannenberg, E. M. (1985). Progr. Rubber P1o.stic.s Techrtol. I: 13. de Zarauz, Y. (1975), Can. Pat. 1,021,889 (issued 1977, Michelin). Doughty, J. I. (1962), U.S. Pat. 3,912,696 (issued 1975, 3M). Doughty, J. I., and Chnstman, P. G. (1967), U.S.Pat. 3,645,956 (issued 1972, 3M). Doughty, J. I., and Christman, P. G. (1969). U.S. Pat. 3,654,241 (isycd 1972, 3M). Drakc, R. S., and Siebert, A. R. (1984). in Adltesive Chemistry (Leing-Huang Lee, Ed.), Plenum, New York, p. 643.
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Edwards, D. C., and Wunder, H. R. (1968). U.S. Pat. 3,551,402 (issued 1970, Polymer Corp.). Edwards. D. C. (1975). Rubber Chern. Tec/rrzo/.48:202. Erickson. J. G. (1965). U.S. Pat. 3,415,764 (issued 1968, 3M). Fedjulun, D. L. (1984). Proc. Infer.Rubber Cor$, Moscow, 1984. Plenary lecture: Prospects of Processing Developments in the Production of Mechanical Rubber Goods. Flory, P. J. (1946), I r z d Eng. Chenr. 38(4):417. Flory, P. J. (1950). J. Clrent Phvs. 18:108. Fokina. T. A., Apukhtina, N. P.. Klebanskii, A. L., Paviova, L. V., and Pikhtengol’ts, V. S. (1971), Po/yrrz. Sei. U.S.S.R. 13:2216. Frcnch, D. M,, Strecker, R. A. H., and Tompa, A. S. (1970), J. App/. Po/yrr~.S i . 14599. G:lylord, N. G. (1973), U.S. Pat. 3,959,244 (issued 1976, Dart Industries). Greene, R. N., and Sohl, E. (1973), U.S. Pat. 3,857,8263 (issued 1974, DuPont). Hoffman, R. F., and Gobran, R. H. (l973), Rubber Chern. Techrzol. 46: 139. Hsieh, H. L. (1959), U.S. Pat. 3,175,997 (issued 1965, Phillips). Hsu, C. C. (1979), U S . Pat. 4,256,910 (issued 1981, B. F. Goodrich Co.). Humpidge, R. T., Morrell, S. H., and Nelms, R. P. (1972), Rubber World 66:47. 46: 148. Humpidge, R. T., Mathews, D., Morrell, S. H., and Pyne, J. R. (1973), Rubber Clzerrz. Tec/zr~o/. Inomata, J.. Michishima, S., Hino, S.. and Igarashi, S. (1975), Proc. Inter. Rubber Cor$. Tokyo (Oct. 1975). Society of the Rubber Industry, Japan, p. 277. Isaacson, H. V., and Young, D. W. (1966). U.S. Pat. 3,392,118 (issued 1968, Sinclair Research Inc.). Jiang, C. Y., and Mark, J. E. (1984). Colloid Polwz. Sei. 262758. Jones, E. B., and Marvel, C. S. (1964), J. Pdvrn. Sei., A2:5313. Jones, F. B., and Marrs, 0. L. (1972), U S . Pat. 3,817,936 (issued 1974, Phillips). Co.). Kambara, S., and Aotani, S. (1970). Can. Pat. 908,897 (issucd 1972, Japan Synthetic Rubber Kehr, C. L., and Wazolek, W. R. (1970). U.S. Pat. 3,725,228 (issued 1973, W. R. Gracc & Co.). Kennedy, J. P,.and Smith, R. A. (1960). J . Pdyrn. Sei. P d y t z . Ckern. Ed. 18:1523. Kennedy, J. P. ( 1984), J . Ap/d. Polyrtz. Sei. Appl. Polyrzz. S>wp. 39:21. Kraus, G. (1977), Arrgew. Mdwortrol. Chern. 60/61:215. Lee, T. C. P,, Morrcll, S. H., and Willoughby, B. G. (1978). Kcrutsch. Gurrzrrzi Kurrsr. 3/:723. Llorentc, M. A., and Mark, J. E. (1980), Mrtcrornolecules 13:681. Llorentc, M. A., Anrady, A. L., and Mark, J. E. ( 198 l), J. Polym. Sei. Po/ym. Ph>x Ed. f9:621. McGarry, F. J., Rowe. E. H., and Riew, C. K. (1977), Section 16-C, 32nd Ann. Tech. Conf., Reinforced Plastics/Composites Institute, The Society of the Plastics Industry Inc., New York. Manton, J. E., and Brock, D. J. (1965), Can. Pat. 792,805 (issued 1968, Polymer Corp.). Mark, J. E. (1984), ACS Spring Meeting, Rubber Division, Indianapolis, Paper No. IO. Marshall, S. ( 1982), Rubber Plrstic News, Dec. 20, p. 6. Masuko, T., Yanagida. O., and Yamamoto, S. (1974), U.S. Pat. 4,098,715 (issued 1978, Mitsubishi Chem. Ind. Ltd.). Matsuda, H., and Minoura. Y. (1979), J . A p p / . Po/yrtr. Sei. 24:81 I . Mersereau. J. M. (1969), U.S. Pat. 3,616,313 (issued 1971, Uniroyal Inc.). Meyer, J. M. (197.7). U.S. Pat. 3,910,990 (issued 1974, DuPont). Miller, J. A. (1969), U.S. Pat. 3,634,383 (issued 1972, NASA). Mincklcr, L. S . , and Watchung, N. J. (1970), U.S. Pat. 3,678,013 (issued 1972, Esso). Minoura, Y.. Yamashita, S., Kojiya, S., Okamoto, H., Yamaguchi, H., Matsuo, T., Sakata, M., and Okada, M. ( 1977), Irzt. Polvnz. Sei. Techrznl. 4( 1 l):T/l4. Morton, M,, and Rubio, D. C. (1977), International Rubber Conference, Brighton, U.S., Paper No. 15. Nagakawa, T. W., and Rudy, T. P. (1966). U.S. Pat. 3,427,351 (issued 1969, United Aircraft Corp.). . Sei. 293209. Ning, N. P,. Tang, M. Y., Jiang, C. Y., and Mark, J. E. (1984), J. A p / ~ l Po!\arz. Nitzsche, S., and Wick, M. (1960), U.S. Pat. 3,065,194 (issued 1962, Wacker-Chcmie G.m.H.). Noll. R. F., and McCathy, W. J. (1966), Brit. Pat. 1.139,655 (issued 1969, B. F. Goodrich Co.). Oswald, A. A. (1970), U.S. Pat. 3,717,618 (issucd 1973, Esso Research and Enginecring). Palmquist, P. V., and Jones, N. (1966). U.S. Pat. 3,449,201 (issued 1969, 3M). Patrick, J. C., and Mnookin. N. M. (1927). Brit. Pat. 302,270 (issued 1930).
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Patrick, J. C.. and Ferguson, H.R. (l94S), U.S. Pat. 2,466,963 (issued 1949, Thiokol Corp.). Pinazzi, C., Legeay, G., and Brosse, J.-C. (1973). J. P o / y n ~Sei. . Syn~p.42:1 1. Pyne, J. R. ( 1970). J. Inst. Ruhher I u d . 4:207; also Ruhber C / I ~ Techr~ol. ~I. 44350 (1971 ). Quinn, E. J. (1968); U.S. Pat. 3,668,243 (issued 1972, Uniroyal Inc.). Rau, H. J., and Just, G. (1983), U.S. Pat. 4,453,993 (issued 1984, Bayer). Redman,R. P. (l978), in Deve/opnwrzt.s in Po/yurethctrres, Vol. 1 (J. M. Buist, Ed.), Applied Science Publishers, London, Ch. 3. Reed, S. F. (1971), J. P()/yttz. Sci. A - l 92029. Reed, S . F. (1972~1). U.S. Pat. 3,813,306 (issued 1974, Sec. of the Army, U.S.A.). Reed, S . F. (1972b), J. P d y m Sei. P d y m . Chen~.Ed. A - / , /0:2025. Reed, S. F. (1973). J. Po/yrn. Sci. Polytn. Chern. Ed. 11:1435. Reegen, S. L., and Frisch, K. C. (1964). U.S. Pat. 3,395,185 (issued 1968, Wyandotte Chem. Corp.). Rhein. R. A.. and Ingham, J. D. (1975). Polyrrler 16:799. Rice, D. E., and Sandberg, C. L. (1965), U.S. Pat. 3,438,953 (issued 1969, 3M). Riew. C. K. (1976), U.S. Pat. 4,133,957 (issued 1979, B. F. Goodrich). Riew, C. K. (1981), Ruhher Cherr~.Techrlol. 81:374. (See also Hycar Reactive Liquid Polymcrs, Tech. Bull. AB-9 and AB-16, B. F. Goodrich Chemical Group, 1983.) Rivin, D., and True, R. G. (1973). Ruhher Clrem. Tec1utnl. 46:161. Rossi, R. K. (1982), Eur. Pat. App1.,091,391 A (Goodyear). Rubio. S., Scrre, B., Slcdz. J., Schue, F., and Chapelet-Letourneux, G. (1981). P d y r w r 22:519. Ryan, P. W.. and Thompson, R. E. (1968), U.S. Pat. 3,652,520 (issued 1972, ARCO). Saunders, J. H., and Frisch, K. C.(1978), Po/yrrrr,t/ltrrlrs: Chemistry N I I ~ Techrlology. R. E. Krieger, Mclbournc, Florida. Schmidt, 0. (1978). U S . Pat. 4,259,129 (issued 1981, LIM International). Schmidt, O., and Kubica, W. (1978), U.S. Pat. 4,277,295 (issued 1981. LIM Internationnl). Seegman. I. P., Morris, L., and Mallard, P. A. (1961), U S . Pat. 3,225,017 (issued 1965, Products Research Co.). Siebert, A. R. (1964), U.S. Pat. 3,285,949 (issued 1956. B. F. Goodrich). Siebert, A. R. (1968a). U.S. Pat. 3,551,471 (issued 1970, B. F. Goodrich). Siebert. A. R. (1968b). U.S. Pat. 3,551,472 (issucd 1970, B. F. Goodrich). Siebert, A. R. (1984). ACS Adv. Chem. Series. No. 208 p. 179. Singh, H., Hutt. J. W., and Williams, M. E. (1981). U.S. Pat. 4,366,307 (issued 1982, products Research and Chem. Corp.). Skillcorn, D. E. (1972), U.S. Pat. 3,910,992 (issued 1975, B. F. Goodrich). Szwarc, M,, Levy, M., and Milkovich, R. (1956), J. Am, C/~enl.Soc. 78:2656. Takeshita, T. (1974). U.S. Pat. 3,900,379 (issued 1975, DuPont). Ulrich, H. (1983). in K i r k - O f h e r Erlc)lc./opdirr c$ C/wrrliccl/ Tec/rr~o/(,gy3rd d . , Vol. 23, Wiley, New York, p. 576. Uraneck, C. A.. Hsieh. H. L., and Buck, 0. G. (1960). J. po/vn~. Se;. 46:535. SC;. 1 3 : 149. Urancck, C. A.. Hsich, H. L., and Sonnenfeld, R. J. (1969), J. App/. Vcrdol, J. A., and Ryan, P. W. (1966). U S . Pat. 3,427,366 (issued 1969, Sinclair Rcsearch Inc.). Vcr Strate, G., and Baldwin, F. P. (1977), U.S. Pat. 4,278,822 (issued 1981, Exxon). Wagner, M. P. ( 1976), Ruhber C h e r ~T~ d. u w l . 49703. Whitby, G. S. (1954), Syrlthelic. Ru/~ber,Wiley, New York, p. 892. Yamashita, S.. Minoura, Y., Okamoto, H., Nukui, T., and Monmoto, E. (1978). h t . Po/ynr. Sei. Tec/lrlo/. 5(7):T/100. Yarnawakl, S., Masuko, T., Yanagida, 0.. :tnd Yamarnoto, S. (1974): Jpn. Pat. 75,1 10,443 (published 1975, Mitsubishi Chem. Ind.). I'O/~W~,
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Powdered Rubber Colin W. Evans* Consultant, Gateshead,England
1. INTRODUCTION Powdered lubber is, as its name implies, rubber in powdered form. Strictly speaking, the particle size is approximately 1 mm, although, as discussed later, the technologyalso includes particulate rubber up to approximately 10 mm particle size. Since volume production using powdered and particulate rubbers has existed since the mid-l970s, the following outstanding advantages over olderbale processing methods have been confirmed:
1. 2. 3. 4. 5. 6. 7. 8.
9. 10. 11.
12. 13.
Bale cutting can be eliminated. Much shorter mixing cycles are possible, with either internal mixers or the openmill route. Less power is consumed. Less plant maintenance is needed per kilogram mixed. Processing plants need not be so capital-intensive. Better ultimate dispersion is possible with powdered rubber. More rapidly accelerated compounds can be mixed. Considerably less heat memoryis retained when some of the more difficultpolymers such as polychloroprenes are processed, and hence there is less scorch. Blends can be fed directly to the extruders and molding presses, thus eliminating the internal mixer and/or open-milling operations. There is no need for massive premastication and masterbatching of many rubbers. Mixing and dump temperatures are considerably lower. Because of ease of mechanizing, factory controls are simpler. Better and cleaner environmental conditions can be maintained.
The only serious negative aspect of the use of powdered rubbers is that of the grinding premium, but even this is being reduced now that the use of the powdered material is growing, and in any case the savings of labor and energy compared with existing technology more than justifies its use. It would be very unwise not to consider even partial usage of powdered rubber in presentday mill-room areas.
* Deceased. 167
168
2.
Evans
CONVENTIONALMIXING
For approximately 150 years, the rubber industry has mixed and compounded rubber in very robust and heavy equipment. Plastic polymers, on the otherhand, need much lighter engineering equipment. It is strange that theconventionalandtraditionalmethod of processingwithinrubber factory mill rooms, over many decades, has been to (a) thoroughly masticate, or rather “knock to pieces,” the structure of the polymers, eitheron mills or in internal mixers, then (b) incorporate fillers, softeners, antioxidants, accelerators, curatives, and many other ingredients, in an endeavor to correct the damagealready done. and then(c) build the ruptured chains together by crosslinking. The energy consumed in doing this is out of all proportion to the actual needs, and it is not really surprising that many of thestrangeprocessingvariablesandproblems that occur without warning in the factory, and then disappear just as quickly, can still remain unsolved mysteries. When powdered elastomers became available. it was decided to study whether products with satisfactory properties could be made on equipmentsimilar to that used to process plastics (E. I. duPont, 1972). A comparison of existingrubber-processing techniques with powder systems will therefore be initially discussed. Mixers used for rubber compounds can be categorized into three general types: 1. Open mill 2 . Internal,e.g.,Banbury or Intermix 3. Continuous, high-speed
The cooling of the mixers is of paramount importance, and much development work has been performed by the makers of mixers to make the current faster mixing cycles possible.
2.1
Open-MillMixers
Internal mixers have high outputs, but for hoses, open-mill mixing is still practiced and will continue to be, because of the small production runs of certain products. Also, very necessary, good dispersion and freedom from contamination cannot be guaranteed with internal mixers. The open-mill mixer masticates the polymers until an even and smooth band is formed around the frontroller. The fillers and oils areadded alternately, followed by any small additions and finally the vulcanizing materials. During the whole operation, cutting and blending by hand rolling are carried out. As the powders drop into the mill tray, they are swept to the front by the operator and added back into the mill nip. The mill tray is usually slightly sloping to help the operator, and a vibratory mechanism that continuously returns powder to the operator, thus saving physical effort, is very useful. The best dispersion and blending of compounds containing mixed polymers is obtained by breaking down each polymer individually and then blending them together while still hot. The addition of the fillers, etc., then follows in the usual way. In order to help the breakdown, special peptizers may be added during mastication,and in the case of polychloroprenes, retarders and other processing aids may be added very early in the mixing cycle. For the processing of butyl rubbers, it is desirable to have the mills so positioned as to be able to work safely on the back roll due to the affinity of this polymer for the faster roll. In large user factories, it is possible tokeep butylrubberconfined to itsownline,making it practicable to adjust the mill gear ratios so that the rubber is banded around the front roller.
Powdered Rubber
169
2.2 Internal Mixers With internal mixers, the ideal setup is to have twomills in the chain. The first is used to remove the heat from the compound rapidly prior to the addition of the vulcanizing ingredients on the second mill. This also has the advantage of keeping the internal mixer free of curing materials, with far less tendency to scorch problems. The high initial capital cost of such a system is more than justified by the ease of subsequent processing. Indeed, many factories are installed in this manner. As with open-mill mixing cycles, the general rule is to masticate in the mixer, and when the polymer has reached the desired state the additives are mixed in and dispersed. The batch is then dropped and passed through the first mill several times, and then the curing agents are added on the second mill. However, mixing procedures, whetheron open-mill or internal mixers, are peculiar to particular factories.
Processing Techniques The following techniques are widely used: Direct Mixing Process. The compound is mixed and is then fed directly to an extruder or calender. This processrequirestheaccurateplanning of the mixer cycle andsubsequent operations and has fairly widespread use. It is necessary to have tight quality controlof curing and dispersion. Theinitial mastication is extremely important because of subsequent “nerviness,” as there is no maturing time in the cycle. Indirect Mixing Process. In this cycle, the compound is mixed, slabbed off, and stored. The curing materials may be added before slabbing or after maturing in storage, depending on the particular compound. This system and open-mill mixers give the best compound for processing. Premastication. In certain instances, particularly in compounds where there is a fairly high hydrocarbon content, it is necessary to premasticate the polymers, slab off and cool, and then mix in the normal way with this premasticated material. Alternatively, if the compound contains a fairly high filler content, either black or mineral, a masterbatch may first be mixed, slabbed, and cooled and then final-mixed. A masterbatch is a mixture of polymer and filler, with the filler content as high as 50%. Oil Extension. In the case of naturalrubber compounds, it is possible to oil-extent without undue loss of subsequent processing or physical properties by selecting a suitable oil and preblending it with the requisite carbon black. The “carboil” so produced is added to the internal mixer with the rubber, right at the beginning of the cycle,and the wholeis then masticated together. This techniquepreventsunduechainscission due to mastication,andtheresultant plasticity so obtained is very satisfactory. The addition of dihydrazine sulfate to the compound also helps subsequent processing of such oil-extended natural rubber compounds (Evans,1979). Dump Mixing. When the compound contains a fairlyhighproportion of filler, it is sometimes difficult to get the rotors of the mixer to “bite,” and in such cases it is normal to literally dump the wholeof the ingredients (rubbers and fillers) into the mixer together and then carry on with the cycle. Upside-Down Mixing. This technique consists of adding the powders to the mixer first, followed by the polymer. This not only produces a satisfactory mixed material but also makes it possible to mix certain difficult polychloroprene recipes, which hitherto had to be mixed on open mills because of scorch and/or sticking problems. Seeding. This is another extremelyusefulmixingtechnique (Nye, 1943). Developed during World War I1 because of shortages of natural rubber and other hydrocarbons, it consists of adding a small portion of the previous batch of the same material, and allowing the new
Evans
170
batch to “seed” on it during mixing. This is particularly effective in mixing compounds of extremely low hydrocarbon content. By selecting the correct mixing procedure, coupled with accelerator and curing systems, it is now possible to speed operations by processing the whole batch in the mixer. Additionally, by masterbatching certain of the more difficult accelerators, it is also possible to add these and blend away rapidly without precure, right at the end of the internal mixing sequence. BNtch Size
Wear on both the rotors and shells of the mixers must be compensated for by increasing the batch weights slightly from time to time. Otherwise various mixing difficulties, such as poor dispersion and mastication, occur. Cycles Two techniques are used in an internal mixer. One is to mix to a fixed time cycle and ignore the ultimate temperature reached, and the other is to mix to a fixed temperature rise on mastication and ignore the total time. Both of these systems economically produce uniform material. and in fact it may be necessary to operate both systems (in different machines, of course) to suit the particular recipe. It is current practice to mix as nearly continuously as is practicable. Furthermore, the use of high-speed rotors is increasing. This considerably reduces the mixing time but increases the temperature.Because of this,a lot of work has been carriedout on efficientwater-cooling systems and on methods for the rapid discharge of the mixed compound, such as the use of drop doors. Synthetic Materids The use of magnesium oxide in stick form in chloroprene rubber (CR) recipes helps dispersion at the critical stage of mixing, with a reduction in mixing time and temperature rise (Evans, 1979). This reduces scorch tendencies. Scorchis the term used to describe incipient vulcanization of a rubber compound. CR is a polymer that because of its heat memory develops at each stage of processing an additive and accumulative heat history. Thus, in highly loaded compounds, the basic scorch properties are aggravated. Because powdered rubber compounds, as has been shown, both mix and process at lower temperatures, this accumulative heat buildup is considerably reduced, and hence safer processing characteristics are conferred to the mix.
2.3
Continuous and Semicontinuous Mixers
Before the mid-1960s there had been very little change in the methods or the equipment used within the mixingrooms of the rubber industry for over 100 years. Over the past decade, systems approaching the ultimate goal of continuous mixing (Ellwood, 1978) have been introduced, with varying degrees of success, as many are much more costly than established methods. Because of the various polymers used, the mixing equipment must be very strong and robust to endure the very high loads and stresses developed during processing. The future pattern of continuous mixing could, however, depend very much on the success of the resurrection of the so-called newer technologies of liquid rubber processing and also of powdered rubbers (Morrall, 1973). Over the years, these technologies have had limited success,
Powdered Rubber
171
mainly due to deficiencies in materials, methods, and processing machinery, but they currently stand a much greater chance of success. The rubber industry has always been faced with the physical difficulty of breaking down the material before the fillers and other ingredients can be incorporated and blended. There is a continual search for an easy and cheaper way of achieving this. Latex technology (Murphy and Twiss, 1930) hasbeen examined, as it appears to be a comparatively easy way of obtaining dispersion and good mixing with the minimum of shear and hence lower power consumption. The difficulties caused by the presence of water led to the use of solvents, with the attendant problems of fire and toxicity. Finally, rubber was liquefied by melting (Morrell, 1978), thus producing depolymerized rubber, with a viscosity of about 500 P at 20°C. In the 1930s, fillers were added to this material on rubber mills, and excellent electrical insulation material was produced. More recently, low molecular weight SBR materials have been produced, with viscosities of around 500 P at 25°C (Mees, 1985). The present difficulty is still to obtain dispersion. of the carbon black in particular, while at the some time retaining a pourable material. Another filler is nylon fiber, around 6 mm in length, but in both cases so far the flex resistance is inferior to that of solid rubber. Fulthermore, while it is possible to disperse in Z-blade mixers, the black in particular remains in large aggregates even after prolonged mixing.
3.
POWDEREDPOLYMERTECHNOLOGY
The plastics industry has been using thermoplastic polymers in powder form for at least 40 years. The first of these in any quantity was poly(viny1 chloride) (PVC), which was available as a white, freeflowing resin of approximately 1 mm particle size. It is therefore surprising that this technology has only very recently become of interest to the rubber industry. despite the fact that powdered nitrile rubber has been available for some 20 years. This was first produced by B. F. Goodrich (Goshorn et al.. 1969; Whittington and Woods, 197 1; Woods and Krasky, 1973; Woods andWhittington,1973; Woods et al.,1973;Whittington, 1974) in theUnited States, basically for blending with PVC as a dry and solid plasticizer. Much of the current work with nitrile materials in powder form has been attributed to and published by Goodrich. In addition to Goodrich, the Bayer Company in Germany (Bayer A.G.), has been very active not only with nitriles but also with polychloroprene rubbers, and a complete range of this latter polymer is available in powder form. Nitrile polymers are now also available in powder form from the majority of the major manufacturers. Many of these manufacturers incorporate the “powdering” operation into their mainline streams duringtheproduction of the basicpolymer. It is also possible to produce powder from the existing and finishedbales. and this is currently being carried out in the United Kingdom (Eagles, 1973;Whally,1973; Honday, 1974;B. P. Chemicals, 1985; Wood, 1985). Indeed, not only nitriles, but also powdered natural rubber, polychloroprene, SBR, EPDM, and most other common polymers can equally as easily be made by this method. The process uses primarygranulation,followed by reduction to a true powder form in attritionmillsandthe minimum addition of antitack and partitioning agents to prevent sticking in storage. Other routes to prevent sticking and partitioning include the use of freeze- and spraydrying and freeze-grinding, precipitation, and coagulation in-line. Of course, the method used depends very much on the particular polymer and the proprietary method of the producer concerned. It is therefore sufficient to say that the operation is carried in the presenceof a partitioning agent, which could be a talc, whiting, carbon black, starch and starch xanthate, or other similar
Evans
172
material. The amount of partitioning agent used depends on the polymer and on the method of powder production. At the 1974 National Rubber Conference in Munich, it was revealed that powdered masterbatch from the polymerization plant was now possible, using virtually any elastomer, with any filler, in any proportion (Nordsiek, 1974). This is in addition to the development of nitrile/ SRF black masterbatch as revealed at the I.R.I. National Rubber Conference in May 1974 at Black-pool (Evans, 1974). In the field of natural rubber, the latter was on display at the 1972 I.R.I. International Rubber Meeting at Brighton (Pike, 1972) and has since been used in bulk with satisfactory results. Additionally, at least one other large natural rubber producer is currently investigating the economics of launching a granulated natural rubber on the market (Thompson, 1973). Very recently, powdered reclaim has been made available in pilot quantities. The earliest references to powdered natural rubber are in Dunlop patents (1929).
3.1
Effect of Type and Quantity of Partitioner
A partitioning agent is required for powdered elastomers i n order to make them free flowing
and also capable of being transported and stored in stacked bags without compacting. Five partitioning agents were examined, using NBR, CR, natural rubber (NR), and SBR rubbers with quantities of 2 112, 5, 7 1/2, and 10% partitioner. These were magnesium silicate,
Table 1 Effect of Type and Quantity of Partitioner Versus Time" Magncsium silicate
Calcium silicate Silica Calcium carbonate Starch
Time (mo) Time (mo) Polymer
NBR
Percent partitioner
2 '/. S 7
CR
IO 2 '/. S 7 '/. 10
NR
2 '/. S 7 10
SBR
2
S 7% IO
I
3
2 1
2 1 1
1
I 1 I 1 1
2 I I 1 1 I 1 1
(mo)
6 3 2 2 I 2
1
2 I 1
I 3 2 2 1 2 I 2 I
1
3 1
1 1 1 1 1
3 2 2 2 3
2 I 1 I 1
2
1 3 I
2 I 1 1 2 1
1 1
1 1 1
6
1
1 3 2 2 I 2 2
1
1
1
1
1
3
6
1
3 1 2 3 3 2 1 2 2 3 2 1 1 2 3 1 1 1 1 2 2 1 1 2 3 1 1 1 1 3 I 1 1 1 3 1 1 I 1 3 3 2 3 3 3 2 1 2 2 3 2 2 2 2 3 2 1 1 1 3 3 1 2 3 3 3 1 1 3 3 2 1 1 2 3 1 1 1 2 3
3
Time (mo)
6
1 -
-
"
3
-
-
-
-
-
-
-
"
"
-
-
"
-
-
"
"
"
3 3 3 2 2 3 3 3 2 3 3 3 3 3 3 3 3
6
3 3 3 -
Key: 1. Free flowlng: 2, slight corupacted. hut easily broken down wlth light finger pressure; 3, compacted. not easily broken down. Source: Evans and Thesis. I98 1.
l'
173
Powdered Rubber Table 2 Effect of Magnesium Silicate at 25’&5, 75”. and 10% Levels with NBR Powder
92
1
2 3 4
Average Std. dev.
I12 I09 13
130 131 126 122 127 4
135 132 123 128 130 5
133 125 125 119
126 6
101 103 107 114 106 6
150
I60 170 I59 160 8
200 200 215 195 203 9
185 190
215 205 195
170
190
185
201 II
183 9
155 155 165 150 156 6
calcium silicate, silica, calcium carbonate. and starch, in the particle size range pass 200 mesh. The visual effect of compacting and free-flow properties was examined in 25 kg bags of material after transportation and storage.Stacking of thebagswas fourdeep, andthematerialwas examined in the bag at the base of each stack of post pallets (Table l ) . Transportation included examination of truck container loads. Table 1 shows the immediate unsuitability of starchand calcium carbonate,andthese materials were therefore discarded. Thesuitability of the silicate materials is clearly shown, and the effect of quantities between 2% and 10% shows that the best results are obtained at approximately 5% concentration, before a fall in physical properties is discernible (Tables 2-10). At the end of these tests one 18-ton container of CR partitioned with 5% magnesium silicate was sent by boat to South America and returned. This was to test transportation through the tropics. It was found to be satisfactory on return. At this early stage of the work, it should be mentioned that there is a definite pattern shown of greater consistency attained by the use of powdered materials compared with the bale rubber controls, and furthermore in general there are higher tensile and elongation properties (both at the same time) with the powdered rubber recipes. The achievement of simultaneous improvement in tensile and elongationproperties is amostunusualphenomenon in rubbercompounding technology and can only be attributed to the fact that better dispersion coupled with less chain scission of the polymers (due to mastication) must be present. This is a major advantage of powdered technology, as subsequent results will show.
Table 3 Effect of Magnesium Silicate at 2 k . 5 , 7%, and 10% Levels with CRPowder at break (c/o)
Tensile strength (kglcm’) Elongation Control CR 1
2 3 4
Average Std. dev.
1 l4 1 10 100
I25 112 10
2’/2%
5%
7Y1%
10%
125
114 114 121 106 114 6
110
102 93 102 106
110
114 116 116 6
120 114 102 112 8
Control 5%2Y2%
101
220 248 235 236 235
6
11
240 235 230 242 237 5
235 235 235 235 235 0
75”8
10%
220 225 220 210 219 6
200 200 205 190 199 6
Evans
174
1 2 3 4 Average Std. dev.
7s 62 87 87 88 II
9s 95 9s 9s 9s 0
1 2 3 4 Average Std. dev.
92 124 I06 112
125 12s 132 120 126
109 13
S
95 95 91 96 2
90 84 96 88 90 S
80 84 78 80 81 3
S40 S70 ss0 S78 560
122 12s 126 125 125 2
I10 120 122 129 120 8
105 102 104 101 103 2
150 160 170 1 S9 160 8
101
18
S70 S60 S70 S80 S70 8
S70 S70 S70 S70 S70 0
S40 SS0 S60 S45 S49 9
S35
190 190
190 195 195 195 194
180 175 180 180 179
3
3
IS0 150 160 160 1.55 6
190
200 193 S
S40
S35 S30 S35 4
Table 6 Effect of Calcium Silicate at 2%, S, 7 k , and 10% Levels with CR Powder Tensile strength (kgkm') CR
Control
2Y2%
SQ
7Y2%
Elongation at hreak (%!) Control
2'/2%
5%
7Y2%
10%
108 105 10.5 106
220 248 23s 236 235 11
230 230 235 240 234 6
235 235 230 245 236 6
190 200 200
2
240 245 240 235 240 4
10%
~
I 2 3
1 l4 1IO 100
IIS
118
115 115
115 11.5
4
12s I12 IO
114
112
115 0
115
Average Std. dev.
2
114 114
118 118 115 2
104
180 193 10
0
Powdered Rubber Table 7
175
Effect of Calcium Silicatc at 2Y2, S, 7Y2, and 10% Lcvcls with NR-SBR Powdcr Blend ~_______
Tensile strcngth (kglcm') NR-SBR 1
2 3 4 Averagc Std. dev.
Elongation at break (76)
Control
2Y2%
5%
7
%$10%
Control
2Y'%
S 9
7s I02 87 87 88 II
97 96 92 94 95 2
95 95 98 90 95 3
89 89 94 91 91 2
78 82 76 83 80 3
S40 S70 ss0 578 560
560 580 600 S80 S80 16
S80 580 S70 570 S75 0
18
7?'"/. 570 S70 S90 560 S73 13
IO%
S60 560
SS0 S60 558 S
Table 8 Effcct of Amorphous Silica at 2Y2, S, 7Y2, and 10% Levels with NBR Powder Tensile strcngth NBR
Control
2Y?%,
(kglcm')
5%
7Y>%
break Elongation at 10%
Control
(%)
5%
2Y2c/r
~~
124
I 2 3 4 Average Std. dev.
7Y2%
10%
~~
92 106 I12 109
13
140 122 130 125 130 7
130
12.5
1 50
100
141
130
120 130 9
10.5 110
125 4
104 4
I60 I70 1S9 160 8
195184 195 195 192 190 176 198 180 195 190 195
7
2
145 178
1.55 150
179 179 3
IS0 150
4
Table 9 Effect of Amorphous Silica at 2%, S, 7Y2, and 10% Levels with CR Powder Tcnsilc strength (kglcm')
2Y2%
Control CR
Elongation at break 5%
7Y29
10%
Control
2Y2%
~~~
1 2 3 4 Averxge Std. dcv.
114 1IO
117 116
116 116
100
115 110 115
114 l15 108
I25 112 IO
3
115 I
114
114
110 100
118
114
116 116 2
108
6
(76)
5%
7Y2%
10%
245 240 240 240 241 3
225 230 230 230 4
200 205 220 210 209 0
~~
235 220 248 23s 236 235 11
240 240 240 250 243 S
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176
Table 10 Effect of Amorphous Silica
Tensile strength NR-SBR Control ~
87
I 2 3 4
Average Std. dev.
2 ! 4 5, 7 k . and 10% Levels with NR-SBR Powder Blend
at
Elongation a t break
(kgkm')
(%)
2%%
5%
71/:c/(
10%
Control
2v2%
58
7'/,%
10%
98 98 108 104 102 5
102
99 S40 98 104 102
90 90 X7 94 90 3
570 S50 578 560
580 600 600 590 593
570 575 570 580 574
18
10
490 595 590 590 591 3
550 545 550 530 544 9
~~
75 I02 87 88 II
106 102 100
103 3
101 3
S
3.2 Measurement of Dispersion
In any rubber compound, it is essential that good dispersionof all of the compounding ingredients be achieved, in order to optimize the properties of the particular recipe. The normally accepted methods for decidingthecorrect degree of dispersion in a compounded recipeincludethe measurement of tensile strength (TS) and elongation at break (EB) at various states of cure. The plateau in the plot of the tensile product(TS X EB) gives the optimum state of vulcanization. Modulus and, less frequently, tear strength, hardness. and compression set may be used as added criteria. These tests give a reasonably accurate measure but unfortunately take rather a long time to carry out and therefore are not ideally suited as routine quality control checks in the high-speed context of powder blending and mixing techniques. Other methods are those developed by Dannenberg ( 1 970) and Leigh-Dugmore (1 948) using visual and transmitted light standards to compare the torn surfaces. These are very good and quite accurate, although once again requiring time and a fair degree of skill of assessment during the fixing of the rating. Electrical conductivity and other microscopic techniques have also been developed (Leigh-Dugmore, 1948), but for obvious reasons very few routine control laboratories have these facilities. A rapid and accurate system is thus required to determine the degree of mixing in production batches. It was decided to use as a basis the TSO test. In its original form this was a test for state of cure although it is not in the current specification (ASTM D 1329-79). I n 1941 the T50 test was used by Callenders Cables (now B.I.C.C.) for routine determination of the state of cure of natural rubber used in electrical cables. Considerable work on other polymers (e.g., Smith, 1978) is cited in the ASTM specifications. The T50 value is the temperature at which 50% recovery of the specimen has taken place after a cured dumbbell sample has been stretchedby 1 OO%, frozen in a batch of acetone-alcoholCO? mixture, and allowed to warm up. This is a very rapid means of determining the state of cure of natural rubber compositions and many other polymers, including SBR, NBR, and CR. As TSO is a measure of extensibility, it will be influenced by the amount of crosslinking, just as in glassy polymers the T, is affected by introduction of crosslinks (Gordon, 1963). The T50 value may therefore be used either as a measure of extensibility or to determine the state of cure. T50 values are clearly discernible, and the reproducibility is good. In using this technique, it has been shown in practice (irrespective of the measurement method of dispersion) that if the smallest ingredients by weight in the compound-namely, the curatives, activators, and accelerators (organic and inorganic)-are correctly dispersed, then the state of cure will be correct. If, therefore, the small items are well dispersed, it may be assumed that the bulk fillers, including the blacks and inorganic materials, are equally well dispersed.
Powdered Rubber
177
Three basic compounds of NBR, CR, and NWSBR were used. Carefully check-weighed batchesweremixedunder ideal andsupervisedconditions by internal-mixerandopen-mill techniques (Evans, 1969, 1979). The controls were mixed from bale and powder, and the T50 values were determined and compared with Cabot rating values. Good agreement was achieved.
3.3
Preblend Mixing
Just as with PVC and other thermoplastics, the preblending and compounding process for powdered rubber mixing consists of blending together all the ingredients in the recipe in an intensive rapid mixer.Suitablemachinesforthisoperationincludethosemanufactured by Werner & Fleiderer, Fielder, Papenmeier, Henschel, Lodige, and Drosna (Evans, 1974). These machines have been designed to enable intensive mixing to be carried out to gain the full effect of the mixing action. The equipment is jacketed so that additional heating or cooling may be used as necessary, depending upon the polymer and the recipe. Aerodynamically shaped mixing impellers achieve thorough dispersion and rapid frictional heating by causing powder particles to collide with each other in a turbulent flow pattern above the impeller blades and outof contact with themetal surfaces. The machines are emptiedquickly, usually by means of pneumaticallyoperateddischargevalvesanda chute to mills,internal mixers. storage containers, or other units depending upon the next process stage. With powdered rubber formulations, however, since recorded discharge temperatures are below 50"C, it is not necessary to use an aftercooler, as is the case with thermoplastic recipes. Neither has it been found necessary to carry out any major recipe changes, as are used for baled rubbers. when using the powdered system, but upside-down mixing techniques-i.e.. all ingredients in the mixer first, with the powdered polymer on top-give the best results. The following system has been used throughout this work, using a 300 L capacity Fielder mixer:
0 speed. slow Start, 1 min Switch to high speed. 2% rnin Switch back to slow speed. 3 min Add etc.oils, 3% rnin Continue slow speed. at 4 min stop. Such a cycle produces a free-flowing powdered compound that is suitable for final mixing on a roll mill or in an internal mixer or for direct powder processing. Generally speaking, a batch weight of between 45 and 90 kg is ideal for the 300 mixers, and between these limits. uniform mixes are produced batch after batch. Whenunderloaded, i.e., less than 45 kgbatches,there is insufficientmaterial to fully engage the impellers; and if overloaded, i.e., in excess of 90 kg batches, overflowing occurs. In both cases there is inadequate mixing and consequently poor dispersion. Throughout this work, the preblended powdered recipes are compared with the relevant baled-rubber control recipe mixed in an internal mixer by conventional rubber-processing techniques (Evans, 1974). Because of the considerably higher throughputs that are possible using powdered rubber preblend techniques. it became very obvious at the outset that more rapid testing control techniques were necessary. It is for this reason that the modified T50 test was developed and used for dispersion measurement.
Evans
178
4.
4.1
EXTRUSION, INJECTION MOLDING, AND TRANSFER MOLDING OF POWDEREDRUBBERPREBLENDEDCOMPOUNDS Extrusion
A study was designed to eliminate the necessity of using baled elastomer, mixed either on an open rubber mill or an internal mixer prior to extrusion (Evans, 1969). Preblend powder compounds were prepared in the Fielder intensive rapid mixer, and comparisons were made with similar control bale rubber compounds after extrusion. Several extruder manufacturers (Farrel-Bridge Ltd.,Troester A. G., Werner and Pfleiderer. Francis-Shaw Ltd., Stewart-Belling Inc.) have already developed machines capable of direct processing. The feeding may be either via feed hoppers, with or without vibration depending on the polymer and formulation, by suction (Meyer Mashinen, Howe-Richardson), or by tube conveyor system (Floveyer Ltd.). depending on conditions. lnitial work here commenced by direct feeding a 47’ in. PVC scroll extruder at 25 rpm ( 15 : 1 ) with Fielder-blended CR premix. This proved to be a complete failure in that tremendous heat was generated and premature cure and thermal degradation took place. At first this was considered to be due to the back pressure, which had been deliberately induced by the fitting of both a baffle plate and gauze behind the spider and dies. Upon removing the dies and screw and then very carefully extracting the powder premix. it was seen that at only approximately 25 mm behind the diehad any consolidation occurred, the remainder still being in powder form. The continued turning of the screw had therefore produced the scorch conditions. despite cooling water on both screw and barrel. This unsuccessful experiment and the subsequentscrewexaminationneverthelessproduced the solution to the problems. It was observed that consolidation of the premix, but not fusion, had occurred in the immediate die area. If. therefore, consolidation was achieved by compaction of the powdered premix by a “pill-making” technique, and if the pellets so made were fed to the extruder, it was considered that direct extrusion should then be possible. Indeed, this proved to be the case. and a very good and smooth extrudate was produced. The experiment was repeated several times,not only with CR preblend but also with NBR and NWSBR mixtures, and the same good extrudates were obtained. Processing temperatures were cooler than when conventional compounds wereused, and both TS0 and other physical tests (takenfromthe extrudate) showed that good dispersion properties had been achieved. Having established the fact that, with consolidated and pelletized preblend, direct extrusion is possible, an improvement in consolidation technique was considered to be necessary on a continuous basis. It was ascertained that the bulk density of the preblend is approximately three times the relative density, so a compaction factor of 3 was decided upon for the continuous compactor. This initially was located above the extruder throat. The feed hopper was filled with preblend material via vacuum transfer equipment; the preblend was then continuously compacted and fed directly into the extruder. However, there were processing difficulties in that the preblend did not feed evenly and consistently into the compactor due to “bridging,” and hence voids were being formed within the preblend in the feed hopper. This could not be eliminated even by using vibration techniques on the feed hopper. Erratic feed to the extruder was present, and this in turn produced surging in the extruder and uneven extrudate dimensions. It was therefore decided to operatethe hopperkompactor as a separate unit a shortdistanceawayfromthe extruder and fit it with a twin-die setup to produce continuous strips, which in turn could be detected visually immediately prior to feeding to the extruder.
179
Powdered Rubber F E E D HOPPER
7 J
I POWDER PREBLEND
SCREW CONVEYOR COMPACTION ZONE
Fig. 1 Horizontalcompactor.
Despite improvement in processing techniques, the bridging was not completely eliminated, and, in addition, rather more heat than was considered desirable in producing the strips became apparent. Compactor die temperatures on occasion in excess of I 10°C were developed. It wasthereforedecided to water cool the die and lowercompactionzones.thusachieving satisfactory temperatures of 45-55°C within the material. I n view of the continued bridging tendencies, it was also decided to change from compaction by vertical format to horizontal compaction. This achieved the necessary satisfactory processing system, with all other properties remaining good (see Fig. 1 ). A completely new concept i n mixedextruder design (Fig. 2) has also been investigated. This machine has the capability of continuously mixing and extruding the powder preblend. (This work was carried out on a prototype machine by courtesy of Farrel-Bridge Laboratories. Rochdale.) The results showed that satisfactoly physical properties were developed during the extrusion of the NBR. CR, and NR/SBR compacted preblends and at a compaction ratio of 3/ 1 (bulk density/relative density). However, the ultimate development is to direct extrude from preblend without compaction. Discussion with Farrel-Bridge enabled them to develop a mixing extruder. which they designated MVX. Work also carried out at Farrel-Bridge (Fig. 2) showed that by theuse of this MVX machine it was possible to direct mix and extrude direct from preblend without the compaction operation. The results were equally satisfactory and comparable with those already reported and again show that results using powderedmaterialaremoreconsistent than the baled rubber control.
4.2
InjectionandTransferMolding
Early attempts to mold preblended compounds direct from the Fielder mixer were unsuccessful, even in ordinary compression molds, until compacting by pelletization had taken place. This was caused by entrapped air and spillage of powder due to movement when closing the press.
180
Evans
Fig. 2 Farrel Bridge MVX mixing and venting extruder. This machine operates in four stages, indicated by circled numbers: ( I ) Feed-a compacting, pressurizing, feeding device, with adjustable pressure. (2) Mix-a high-shear twin-rotor mixing section with separate dc driver motor. The chambers and rotors are designed to give intimate shearing, smearing, and blending to all particles within the mixing chamber. care being taken to avoid “dead spots” and short-circuit paths to ensure uniformity of output. ( 3 ) Vent-a venting and transfer port located at the rear of the screw arranged to vacuum vent the mixed material before it passes through to the extrusion screw. (4) Extrude-a precision high-pressure extruder with separate dc drive. This section can be fitted with different screws to suit polymer viscositics. but one screw is able to pump a wide range of polymers. All four distinct operations are automatically synchronized and are controlled by the speed of the extruder screw. (From Farrel Bridge, Ltd., Rochdale, Lancashirc, United Kingdom.)
a satisfactory molding was However, when pills (pellets) were fed to the compression mold, obtained. The work was carried out with NBR and CR control compounds. By continuing the exercise along the same technical course as for direct extrusion, continuous compaction, it is also possible to feed the continuous strips via a screw linked to either the injection or transfer molding press. Further work in the design of machinery includes a mixer screw linked to an injection mold (Werner and Pfleiderer A. G., 1975; Dehnen, 1985). By the use of a mixer screw coupled to an injection mold, an experiment completed at Bayer, West Germany, showed that the NBR recipe in powder preblend form could be mixed and injection molded direct. The complete time was 2 minutes, cure temperature 175”C, shot weight 100 g.
Powdered
Rubber
181
The technology of injection and transfer molding, with respect to compounding, hasparallels in extrusion technology. The results of compression molding show that with the use of compaction techniques the compound properties developed from preblends are similar to those shown for compacted powdered preblend extrusion. This parallel produces similar properties when using compacted strips in transfer and injection molds.
5. 5.1
EFFECT OF POWDER TECHNOLOGY ON MIXING CYCLE TIMES, POWER CONSUMPTION, AND PLANT MAINTENANCE COSTS CycleTimes
An internally mixed compound, by directprocess,takesapproximately 10 minutes to mix, depending upon the formulation and the polymer. Some high-plasticity hose compounds take 12 minutes to mix (in two passes) by the twostage or masterbatch technique (Evans, 1979). This applies in particular to theNBRcontrolrecipe,which must bemixed in two stages by the internal mixer route because of its high plasticity and high cured hardness. It was therefore decided to examine the effect of replacing the polymer in bale form by an equal weight in powder form, all the other ingredients and the batch weight remaining the same. The controls forthe CR and SBR-NR blend can be mixed satisfactorily without two-stage techniques, and therefore only single-stage compounding was used for them. The NBR preblend wasfinal-mixed by open-milltechniques (Evans,1979) in I O minutes,whereas the normal technique of bale mastication requires 60 minutes for final open-mill mixing. In the case of high-plasticity NBR compounds, it is necessary to use a two-stage mixing technique. It was found that it is possible not only to eliminate this two-stage operation but also to produce compounds in one mixing stage either with a preblend or with powder. A reduction in mixing cycle time in theinternalmixer of between9and I O minutes or an increasein production of between 400 and 600% is thus obtained. With CR and NR/SBR rubbers twostage mixing is not necessary, but increases in production of between 333 and 500% are shown to be possible. In open-mill mixing of NBR, a production increase of 600% is possible. In all cases, dispersion is excellent, and yet again the greater reproducibility of results and improved physical vulcanizate characteristics are very evident. The use of shorter mixingcycles with particulates has been discussed. In a little moredetail, a normal industrial products typeof rubber compound, by direct process, takes approximately 10 minutes to mix in, say, a No. 3D Banbury, depending upon formulation and polymer. Some mixes can take up to 12 minutes and even longer if two-stage or masterbatch processes have to be used. As a general statement, not only can masterbatching be eliminated by the use of particulate rubbers, but the mixing cycle can be drastically reduced. Satisfactory mixes using preblended particulates can be dropped from the mixer in a mixing time of 2 minutes and in as little time as 3 minutes using nonpreblended material and with very satisfactory physical and processing properties (Tables 1l and 12). It can thus be seen that internal mixer outputs can be increased at least threefold by the direct mixing of powdered polymer weight-for-weight with bales polymer, or at least fourfold with preblends of the whole mix via the intensive mixer initially. The physical properties are also atleast equal to, or an improvement on, those of the controls.Physicalproperties are improved in both TS and EB, both at the same time. This isa common occurrencewith powdered rubber compounds and has been confirmed by many workers (Goshorn et al., 1969;Whittington and Woods, 1971; Woods and Krosky, 1973; Woods and Whittington, 1973: Whitlington et al., 1974; Woods, 1976; Evans, 1978a,b; Smith, 1978).
182
Evans
Table 11 Effcct of Powdcred Elastomer (NBR) on MixingCycle Timcs lntcrnnl mixet
I 2 3 4 Avg. Std. dcv. "
-
-15 0.07
92 124 106 112 109 13
1.50 I60 170 159 160 8
- 15 - 15 -1.5
-15 -1.5 0
118
17.5
I l6 120 118 118
180 18.5 180 180
2
4
- 15 -1.5 -15 -15 -15
0
I15
170
-
180 180 180
-
-
-
1 l7
-
-
-
-
-
-
117 116 116 1
175 6
-
-
See Table 2.
5.2
Energy Savings
The fact that more batches can be mixed in u n i t time, as shown by the work dealing with the effect of powdered rubber on mixing cycle times, proves that an energy saving is achieved by the use of powdered rubber recipes. It has also been shown that there is a significant reduction in the maximum current used, and this is also reflected in lower recorded dump temperatures, becauselessenergyhasbeen expended and hencelesswork done. This is a very desirable phenomenon. particularly in the case of CR rubbers, which have a heat memory. Lower scorch tendencies are therefore imparted to the compound. Work carried out on laboratory-scale equipment (Whittington et al., 1974) showed that energy savings were possible, and this was also reviewed by Doak (1974). Thiswork investigated the possibility of saving energy on factory-scale, mass production equipment. Electric current and compound temperatures were recorded using the bale control and powdered polymer i n the same recipe. with both internal mixer and open-mill mixing techniques (Evans. 1979).
Table 12 Effcct of Powdcred Elastomer (CR) on Mixing Cycle Times Internal mixer
I 2 3 4 Avg. Std. dev.
- 22.5
0.06 22.5 0.06
114
II O I 10 125 112 IO
220 248 235 236 23s II
-22.5 - 22.5 - 22.5 - 22.5 -22.5 0
112 I 13 I13 I13 113 0
235 240 240 240 239 3
-22.5 - 22.5 - 22.5 - 22.5 -22.5 0
1 14
245 23s 240 240 240
0
4
114 114 114 1 14
See Table 2 - 22.5 0.06
1 14 1 IO 100
125 112 IO
220 248 235 236 23s II
Powdered Rubber
183
Traditionally, the rubber industry has used very heavy and robust mixing equipment in the mill-room areas. This has been necessary because of the need, right from the early origins of the industry, to masticate the natural rubber to the right viscosity prior to the addition of the other compounding ingredients. Obviously, there has therefore been a very high energy usage. Very regretfully. this energy has always been there irrespective of cost, although until the early 1970s it was usually relatively inexpensive. The costof the energy used within a mixedcompound has in general been a very small percentage of the total mixed cost, but the time. although long overdue. is now very opportune for examination of this cost, not only because of the present high price of electricity but also because of the certainty of future shortages and the real need for energy conservation. Without doubt, considerable energy saving lies in the use of powdered and particulate rubbers in the manufacture of hose and cable (Evans, 1978a,b, 1980a,b). The use of polymers i n this form has been rather slow to gain acceptance but is now gaining momentum worldwide, especially in Europe. It has already been shown (Evans, 1978a,b) that, because of the smaller particle size of particulate rubbers. more batchesof compound canbe mixed per hourusing less energy. Perhaps one of the most graphic demonstrations of the value of powdered rubber technology was seen in England during the energy crisis in 1974, when a 3-day week had to be operated with an allowance of 65% of normal power requirements. By the use of powdered rubber rather than baled rubber. 95% of the normal 5-day bale output was achieved without exceeding the 65% energy restriction in 3 days of operation. Another very advantageous. but perhaps not too obvious, property of using particulates via internal mixers is linked very closely with the fact that less power consumption is needed because of the physical form of the polymer as presented to the mixing machines. As a result of this, lower dump temperatures from the internal mixers are achieved. and hence there are fewer fumes on discharge. This, then. is an aid to better environmental conditions. another very important current topic in the industry. It is occasionally argued that the energy savings achieved is less than claimed because of the grinding operation used in some cases forobtaining the particulate material, which obviously has to be taken into account. However, there are manufacturing routes available that do not involve the drying and baling and subsequent grinding of the coagulum at the later stage but rather use to advantage the small particle size already present. This currently does not apply to all polymers. but it is confidently anticipated that in the future this route will be widely followed. So-called friable bales. which are an intermediate stage between full bale and particulates. are already available. and these are also energy savers in the mixing processes. Closely allied with cleanliness. energy savings. and the coagulum route is the so-called polyblack process (B. P. Chemicals Ltd., 1985). which involves the introduction of carbon black in wet form to NBR latex, thus producing a very clean black masterbatch in friable and clean crumb form.which is then capable of processing either through the internal mixer route or other direct particulate route. It can thus be seen that grinding is not always necessary, but if it has to be used. then a much smaller sized particle starting point than a bale is possible, with a very obvious reduction in grinding energy. It is the final grinding operation, i.e., from particulate to true powder (1 mm). that adds time, cost, and energy to the “powdering,” so unless a direct process route using true powder form is to be operated, particulates up to 10 mm in size should be considered in conventional internal mixing equipment. Furthermore, one big advantage of one new machine (MVX. Farrel-Bridge Ltd.. shown i n Fig. 2) (seealso Evans, 1978a.b; Smith. 1978)is that it is capable of directprocessing
Evans
184 Table 13 OperatiodEnergy Usage (Bale) Operation Bale cutting Banbury, stage 1 Banbury, stage 2 Open mill Cracker mill Warm-up mill Hot feed extruder Cold feed extruder Calendering
kWhikg
0.32 0.97 0.97 9.02 0.52 0.52 0.39 0.64 0.77
preblended particulate polymerin many instances, with considerably better processing characteristics of the mixed stock and also improved physical properties when vulcanized. Returning to the use of polyblack NBR, apart from cleanliness, it is possible in the case of hose compounds and those used elsewhere to completely eliminate the two-stage mixing operation, which has previously been essential because of the basic hardness of the stocks with regard to viscosity, and thus there is a considerable energy-saving potential here, once again with improved physical properties. Work within the mill-room areas has shown considerable energy savings achieved by the use of powdered and particulate rubber (up to approximately 10 mm). Typical energy usages and savings are shown in Tables 13- 16 for each operation in bale and particulate form and in typical extrusion routes. It must, of course, be realized that many of these results have been obtained from very high Mooney hydraulic and other hose compounds, and hence some of the energy values quoted could well be higher thanin other branches of the rubber industry. Irrespective of the type of compound used, the comparisons of like with like are valid and show givings by the use of particulates. Tables of each processing operation should be compiled for both bale and particulate over. say, at least one typical week’s production and the total kilowatthours of electricity used set
Table 14 OperatiodEnergy Usage (Powder or Particulatey Operation
2.26
Intensive mixing Compacting Milling Blend 0.52 (Banbury) Blend (mill) MVX Direct extrusion Direct injection molding l‘
1-6 mm.
kWhkg 0.06 0.13 0.06
0.64 0.90 0.32
185
Powdered Rubber Table 15 Energy Usage Bale route
ing
Bale
Banbury, stage (1) Banbury, stage (2) Mill 1 Mill 2, strip Cold feed extruder Total
kWh/kg 0.32 0.97 0.97 0.52 0.52 0.64 3.94
against the total kilograms of compound processed. It is then a simple matter to obtain a very accurate kilowatthour/kilogram reading for each operation and under the conditions prevailing in each particular factory and operation.
5.3
PlantMaintenanceCosts
As has been mentioned, it is a fact that more batches may be mixed in unit time; as a simple example, it is possible to eliminate at least one shift from the three normally operated and still achieve the same volume of output. Thus there is an immediate maintenance saving of 33%, plus many other fringe benefits. All mill room operations should be closely examined to see where savings of time, etc., can be achieved, with the obvious ultimate savings in plant maintenance costs per unit output.
6. CONTINUOUSPRODUCTION This system was developed from the successful introduction of the Fielder preblend via the internal-mixer route or by the use of the horizontal compactor unit. It consists of automatically weighing the recipe and transporting this through the Floveyor (Floveyer Ltd.) into a hopper holding tone situated immediately above the Fielder mixer. The batch is then transferred to the Fielder, mixed, and emptied into a large holding container (1 ton capacity). When filled, this container is transferred to and immediately above the compactor unit, and the compacted strip
Table 16 EnergyComparisons (Powder) PowderParticulate route
feed
Intensive mixing 0.06 Banbury Mill I Mill 2 MVX Cold 0.64 Total
Banbury 0.06 0.52 0.52 0.52
MVX Strip
MVX Direct 0.06
-
-
-
-
-
0.64
0.64
0.64 -
-
-
1.34
0.70
186
Evans
is then fed by belt conveyor to one end of an open mill, with the nip setting slightly out of parallel. This allowsthe compound, asit is masticated. to blend along the mill. prior to continuous strip cuttingand water cooling. The strip is then stored and matured. priorto feeding theextruders. Compound continuously produced by this method gives excellent results. Results show that exceptional reproducibility was obtained when several tons of NBR and CR preblends were processed by the continuous production system. Theseresults are even more remarkable when compared with the conventionally mixed controls and with the rheographs taken from normal baled Banbury batches.
7. POLYMER BLENDS (NBR-SBR) It is everyday practice in the rubber industry to blend various elastomers by both open-mill and internal mixer techniques. However, it is sometinles impossible to obtain satisfactory dispersion. Such a caseoccurs in the hose industry with regard to NBR-SBR hydraulic hose lining compound. where it is necessary to produce high-plasticity, high-hardness materials. NBR-SBR blends are generally used to allow controlled oil swelling in hydraulic oils, which enables assembled hoses to remain coupled. Because of different viscosities between the two polymers. good dispersion cannot be guaranteed even with two-stage internal mixing or open-mill mixing, and this involves very time-consuming and expensive processing techniques in the mill room. It was therefore decided to use the powder preblend mixing technique in the Fielder blender using the NBR control recipe and replacing 25,50, and 75% of NBR, respectively, by SBR. Theresults achieved were satisfactory. Despite the practical mixing difficulty experienced with baled NBR and SBR blends, the results of various blends of NBR and SBR powders quite clearly show that excellent dispersion has been obtained and that new and consistent T50 values have been achieved.
8. ADHESIVESANDDOUGHS These materials are manufactured by thoroughly masticating the polymer prior to final mixing. and then sheeting the compounded mix to approximately 1 mm thick. The sheet is then placed in a rectangular metal box and covered with the appropriate solvent for that polymer. Layers of polymer and solvent are alternated until the bin is filled. and a lid is then placed in position. This operation is known as laying down. The solvent is then allowed to swell the compound for approximately 96 hours. after which the swollen mass is cut with a spade and transferred to a Z-blade mixer for final mixing with more solvent until the desired consistency is achieved. An adhesive containing 20% dry solids by weight is used within the hydraulic hose industry. Experiments using powder preblend with solvent showed that the laying-down procedure could be eliminated and the pl-eblend could be placed at once with the solvent in the Z-blade mixer and completed to a 20V~solids solution within 5 hours. The commercial solvents used for adhesive and doughs are toluol for NBR and CR. and for NR-SBR. The effect of toluol on the NBR control and powdered preblend is shown in Figure 3 . The results obtained show that it is possible not only to eliminate the very time-consuming and power-wasteful operation of laying down but also to achieve a perfectly satisfactory 20% dry solids adhesive in 5 hours instead of 96 hours by using the NBR preblend. This is of great practical importance in the adhesives and rubber-spreading and rubber proofing industries.
187
Powdered Rubber
Fig. 3
9.
Dissolved solids versustimeforpowder
and bale
ENVIRONMENTALCONSIDERATIONS
The mixing and mill-room areas of rubber factories are dusty, and this is not desirable. Indeed, the 1976 U.K. Health and Safety at Work Act sets maximum threshold limits for the various materials used i n rubber formulations. The use of powdered rubber preblends enables all the materials to be bulk handled in a closed system (Fig. 4). thus making it possible for extremely clean and dust-free working conditions. The definition of the separating point between powdered and granulated rubber is I mm (British Standards Inst., London 2955). (1976). The rubber powder may not in itself be a dust explosion hazard. but some of the ingredients. such as finely divided sulfur. are potential hazards if mishandled (Davies, 1976). The guiding principle must be that at no time may a hazardous material be dispersed as an explosive dust cloud in the mixer. This can be avoided by ensuring that inert components are dispersed first. thus rendering inert the atmosphere inside the mixer. Many chemical products, including sulfur, are available in forms that have been treated to render them free-flowing and non-dust-forming. A restriction on the use of potentially hazardous materials to dust-free forms only is a useful extra safeguard in powder processing, but not an alternative. There is some hazard increase in storage over baled rubbers, but this increase is significant only in unsprinkled premises. The use of powdered rubber in a totally enclosed metal storage and conveying system(Fig. 4) should give a reduction in hazard over conventional storage, handling, and processing of baled rubber. Adequate exhaust systems are therefore necessary. Such systems carry no additional insurance premiums. A laboratory test for the assessment of dust in solid rubber chemicals has been developed by Hill and Robinson (1978). This gives an indication of the dust hazard as a possible nuisance,
Evans
188
FIELDER HANDLING SYSTEM
Fig. 4 Fielder closed powder-handlingsystem.
not only as a potential explosion propagator but also in general factory environmental cleanliness and improved working conditions. Other work carried out in Holland (Goshorn and Ciago, 1974) has shown that it was not possible to produce an explosion in eitherastationarydust-airmixture or flowingdust-air mixtures or at various powder concentrations. Early work by Manley and Hampson (1975)with regard to the flammability of vulcanized NBR has shown that the use of dry-blended powdered NBR gives a material with lower flammability. both before and after immersion in various hydraulic fluids, than does the use of conventional baled product. 10.
ECONOMICS OF POWDERED RUBBER SYSTEMS
The cost of a powdered rubber system depends upon the actual process used and the premium charged for the particular polymer. Furthermore, as the process flow lines in Figure 5 show, the actual product process determines which operation can be eliminated. This in turn depends upon the plant and operation process employed. Costs related to production rates have been discussed by Schultz (1973), and figures have been produced by Woods and Whittington ( 1973). derived from a conversion cost equation. These items have also been reviewed by Doak (1974).
189
Powdered Rubber Weighing of ingredients Weighing
of ingredients
i x e rI n t e n s i v em i x e ri n t e n s i v e
and/or Banbury Mill Powdered compound direct
Preparation
J Calendering
1 Extruding/molding/adhcsives (a)
Fig. 5 (a) Normal or powderroutc equipment.
i Extrusion Molding Continuous vulcanization Adhesives (b)
via convcntionalequipment. (b) Powderroutebypassing
heavy
When deciding upon which operation can be omitted from a process, each factory location and layout must be examined to decide which points are relevant to its own particular mode of operation, and the various advantages must be set against the powdered premium cost of the polymer. This latter cost has been progressively reduced as commercial use of the powder has increased. It should be remembered, however, that this premium applies only to the elastomer content of the recipe, unlike the custom mixing premium, which is charged on every kilogram of compound produced. “Custom mixing” is the term used in the rubber industry when the compound is bought from a supplier whose only business is to mix material to a customer’s recipe. Powders are cheaper than bales: 1. Where normal bale custom-mixed material is being used (when powder is mixed inhouse). 2. Where internalmixing is already at capacityand extra mixingequipment is being considered. 3. Where masterbatching is used and either two-, three-.or four-stage mixing is practiced. since one or more stages can be eliminated. 4. Where it is necessary to carry out operations subsequent to mixing, e.g., strip preparation for extruders. These operations may be eliminated, and the strips cut directly from the mixing mill.
In addition, the use of powder permits the use of internal, rather than open-mill. or allows openmill mixers to be operated at higher outputs.
Evans
190
The use of particulate rubber obviously means that bale cutting is no longer required. More and more equipment is being used in the nonure section to transform bales to powder or particulate at the beginning of the in-house factory process. The premium point for powder is approximately at break-even when the elimination of some processing operations leads to a reduction in manpower and when eliminating operator shifts by increasing internal mixer cycle speeds and also by increasing open mill mixer cycle speeds. The elimination of conventional mixing methods (e.g., internal mixers or open-mill techniques) makes savings possible in: 1. Direct feed to cold-feed extruders by powder compressing or compaction 2.Directfeed to transferandotherinjection-moldingmachines 3. The use of pills forcompressionmolding 4. The rapid manufacture of adhesives 5. The rapid preparation of doughs for spreading and proofing operations
Other,lessapparent.areas processing include:
of savings that have been shown to be present in factory
1. Energy. 2. Maintenance, due to (a) extra output in the same time but without high-speed rotor techniques, (b) elimination of operator shifts. 3. Less waste by production of technically improved compounds, because of (a) better dispersion and hence less batch-to-batch variation. (b) consistently lower dump temperatures and hence less scorch tendency, (c) less powder loss in closed systems. 4. Dilution of the compound is possible because of improved physical properties of the powdered vulcanizates. 5. Blending of polymers is more consistent; dilution again is possible. 6. Some blends are possible that are not commercially viable using other techniques.
Environmental conditions and cleansing
costs are also improved because:
l . Lower dump temperatures produce fewer mixing fumes and less contamination. 2. The mill-room area is much cleaner because of the enclosed powder-handling systems.
In an attempt to quantify the potential savings of powdered rubber technology, it has been found necessary to generalize, rather than be specific. because of the diversity of existing processes and the wide differences in premiums charged for the powdering operation. For example, powdered reclaim rubber carries no premium, but powdered natural rubber can be as high as E120 per ton. Also. depending upon the supplier, NBR premiums are in the range of E20 to 70 per ton. However, there are several common factors peculiar to any one factory: 1. Laborcost 2. Fixed costs related to capital,rent, etc. 3. Variable costs includingutilities, etc. 4. Material costs
Material costs are included in common factors because the same recipe, on a weight-for-weight basis, is used for either the bale or the powder route. and the difference in cost therefore becomes a straight addition of the powdering premium onto the hydrocarbon content of the recipe. Thus, taking the first three costs listed as the basic costs. it is possible to derive an equation:
191
Powdered Rubber
Conversion costkg =
labor cost fixed kg
If production mixing rates are included,
Conversion cost/kg =
costs kg
+
costs + variable kg
the equation becomes
+
labor cost/hr fixed costshr mixing rate, k g h r
costs + variable kg
(,This equation assumes that the variable costs are relatively unaffected by production rates. which is commercially true.) In typicalrubberprocesses.twoitemsareknown: (I)the manninglevelsand ( 2 ) the capital cost of the equipment being used. At this point. it is necessary to prepare line drawings of the plant and equipment being used for the particular operation, up to the vulcanization stage. From this, the manning levels for each piece of manufacturing equipment can be summed, and also the capital cost of all the equipment in use can be calculated. Individual plant costs for the various types of equipment are also required. Plant Cost Iterlls
Banbury 3D size Banbury 1 ID size 60" mill ( 2 per Banbury) 84" mill ( 2 per Banbury) 60" three-bowl calender 120 mm cold feed extruder Intensive rapid mixer Compactor unit Floveyor MVX machine
+
mills Banbury Calender Extruder line Intensive mixer, compactor, etc. Therefore, from the last equation and introducing manning levels and capital costs, equation becomes
Conversion c o d k g =
+
manpower fixed costs mixing rate, k g h r
the derived
cost + variable kg
In order to quantify the manpower andfixed costs. together with the variable costs. the following items must be considered for the actual factory location being studied. These will vary from factory to factory, from company to company, and from area to area.
192
Evans
Typical Output Rates (Internal Mixer) Single-stage Banbury mixed,
10-min cycle
6 6
Size 3D: Size 1 1D:
X X
150 Ib = 900 Ibkr = 409 k g h r 450 lb = 2700 l b k r = 1227 k g k r
Two-stages Banbury mixed Size 3D
only:
= 205 k g h r
450 lbhr
Factors Wage rate Overheads Depreciation Maintenance Variable costs (energy, etc.)
W X
Y z C
hour) per (pence (2w, 100%) i.e., hour)per (pence hour)per (pence hour)per (pence
These factors should be converted to unit time, i.e., cost per hour, and the values substituted in the equation Conversion cost/kg =
M + F + V
R
where M = manpower ( W X number of workers) F = fixed costs(x y z) V = variable costs (e) R = mixingrate (kghr)
+ +
For this exercise, at Dunlop (Gateshead), the calculations were based upon depreciation of the plant over 10 years and used the 1976 actual processing costs as follows:
maintenance room Mill E59357 E35,247 energy roomMill 3D output (kg/hr) rate 410
If the Bunbury routeenergy comparisons are compared with thosederived by the use of powdered rubber recipes, it is shown that considerable savings have been achieved, and at today's energy cost levels the savings are even higher. When the powder premium is taken into consideration, the technology is comnlercially viable. up to a powder premium of approximately E60 per ton at a hydrocarbon content level of 50%, dependent, of course. on the Banbury size in use. Furthermore. by "in-house grinding," initial costings indicate thatthe grinding premium will be virtually eliminated when the percentage of partitioning agent is taken into account. This has been confimled by Ellwood (1981). The costs have also been compared with those possible from a Banbury 1 ID, although it is not always desirable to use this larger machine for all recipes for technical reasons such as temperature in hose compounds.
Powdered Rubber
193
If theeconomics ofplant conversion were ignored. thereis nodoubt that particulate rubbers could be used at once, and in all branches of the rubber industry, as a major source of energy savings. Some day, if energy becomes so precious that it musf De c o n s e n t d then once again particulate rubbers will be part of the answer to compound mixing. In the meantime, however, the premium cost of powdered/particulate rubber cannot be ignored, and it is therefore essential that all aspects of the operating process be closely examined to determine which existing stages can be eliminated or up. To establish this. flowcharts should be drawn of the current methods in use and compared with the system that could be used with the powder route (Evans, 1978a. b). If this exercise is carried out for each particular process, and the savings (if any) set against the powder/particulate premium of the polymer. it is then quickly apparent which route should be taken. In the majority of cases, the scales will be tipped in favor of particulates. Wheelans (1981) confirmed the viability of powderedparticulate rubber technology and stated that “financial advantages are critically dependent upon whether the processing advantages exceed the premium on powdered rubber or the costs of granulating.” However, work by Ellwood (198 la) showedthat by the use of in-house granulation, in line with an MVX machine, the cost of particulating the rubber is virtually eliminated or is only a nominal amount in the overall context, and this should open the door to the greater and wider usage of particulate technology within the rubber industry and additionally if those areas where advantage costs are only marginal.
11. CONCLUSIONS The use of powdered and particulate rubber has made it possible to completely eliminate the bale-cutting operation in the mill room of some hose and cableplants, as well as to saveconsiderable amounts of energy. Much greater production rates are possible from existing mixing machines. and two-stage mixingcan be eliminated, particularly for high-viscosity NBR compounds. As less energy is used, the processing temperatures are much cooler, and this not only reduces scorch tendencies but also makes it possible to use more rapidly accelerated compounds and thus increase the speed of vulcanizing cycles in some areas. Also. with CR rubbers there is considerablylessheatmemoryretained by thepolymer.producingmoreeasilyprocessable compounds. Direct extrusion and injection molding of the preblend has been shown to be possible. thus making possible the elimination of heavy and high-capital-cost machines such as internal mixers. This also makes the labor cost factor of the compound much more attractive. It has also been shown that technically improved compounds are produced by the powder route. particularly in thedispersioncharacteristics.Indeed,much closer reproducibility is achieved and better physical propertiesshown, such asincreased tensile strengths and elongation at break. The ASTM (T50 modified) test has been adapted as a process and quality control test for dispersion, and thus production control has been made easier. Because of the easier handling of the polymer in powder systems, there is less process loss. This not only reduces the wasteof materials and energy but also improves the environment, which remains much cleaner than when the old conventional systems are used. Factors delaying the introduction of powderedrubberaretherestrictedavailability of some polymers and the premium charged for manufacturing the powder. These two points are related because unless thedemand is created the premiumwill not be reduced. andif the premium is not reduced, sales will be restricted (Hanmond, 1977).
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Finally, with the work currently being done by the rubber machinery manufacturers with continuous mixing equipment and mixer screws for extruders and injection-molding machines, the prospects look very good for powdered rubber, and there is no reason why the forecast should not be ultimately improved.
REFERENCES Dannenherg, E. M. (1980). Cabot Torn Rating Chart. Cabot Corporation, Inc., Cambridge. Mass. Davics, R. (1986), Fire and Explosion Hazards in the U.K. Rubber Industry, Insurance Technical Bureau. Ref. R106, January issue. Doak, N. (1974), Eur. Rubber J., October, p. 60, Dunlop Ltd. (1929), Brit. Pats. 327,451 and 338,975. E. I. Du Pont de Ncmours. Inc. (l972), Wilmington, DE, Delphi Study. Future Rubber Processing. Eaglcs, A. E. (1973). Mernbers J., Rubber and Plastics Res. Assoc., Shawbury, Salop, U.K., April. Ellwood, H. (198 l a ) , paper F6, RUBBERCON, Harrogatc, U.K. Ellwood. H. (1981h).Farrel-Bridge Ltd. (198 l), MVX Machine, Sales Leaflet MVX R79, Rochdale,Lancs.. U.K. Evans, C.W.(1969), Europeancompoundingtechnlques, presented at RubberDivision,A.C.S.,Los Angclcs, CA. Evans, C. W. (1974), IRI National Confcrencc, Blackpool, U.K. Evans, C. W.(1 978a). Powdered u r d Pnrticultrte Rubber Technology, Applied Sclence Publishers, London. Evans, C. W. (1978b). Powdered rubber technology, Int. Rubber Conf.. Kiev, USSR. Evans, C. W. (1979). in Hose Tecknoloe,: 2nd ed. (Stiffener, D.S.C., Ed.), Applied Science Publisher, London, p. 45. Evans, C. W. (1980a). The effect of powdered rubber processing in energy, SGF May Meeting, Swcden. Evans, C. W. (1980b), Encrgy Symposium, ACS Rubber Div., Spring Meeting, Las Vegas, NV. Farrel-Bridge Ltd. (1974). Rochdale, Lanc., U.K., literature. Fernyhough, 1. (1985). B. P. Chemicals Ltd., Barry Glamorgan, Walcs, private communication. Francis Shaw Ltd. (1974), Clayton, Manchester, manufacturer. Gordon, M. (1963), H i g h Po/vrners, 2nd ed.. Iliffe, London, p. 38. Goshorn, T. R., and Ciago, N. V. (1974). Amhem, Holland, private communication. Goshorn, T. R., Jorgenson, A. H., and Woods, M. E. ( 1969), Rubber World, January, p. 66. Hammond, R. (1977), Delphi study update, E. I. Du Pont de Nemours Inc., Wilmington, DG. Hill, P., and Robinson, J. C. (1978), IC1 Organics Division, Blackley MKR. Holiday. G. J. (1974), G. J. Holiday (Plastics) Ltd., Tclford, Salop, U.K. Howe Richardson (1974), Howe Richardson Ltd., England, System Designers. Farrel Bridge Ltd. (l974), Internal Mixers, Farrel Bridge Ltd., Rochdale, Lancs.. and Francis Shaw Ltd., Clayton, Manchcster, U.K. Lehnen, J. P. (1973), Bayer A. G., Levcrkusen, West Germany, prlvate communication. Lehncn, J. P. (1985). Bayer A. G., Leverkusen, West Germany, private communication. Manley, R. T., and Hampson. F. W. (1975). J. Appl. Po/ym. Sci. 19:2347. Mees, F.. (1983, Firestone Tire and Rubber Co., Inc., Akron, Ohio, private communication. Morrell, S. M. (1973),Rubber and Plastm Research Association, Shawbury, Salop.U.K.. private communication. Murphy, E. A.. and Twiss, D. S. (1930), Dunlop Rubber Co., Ltd., Brit. Pat. 327,451. Nye, H. (1943), British Insulated Calenders Cables Ltd.. Leigh, Lanc., U.K., private communications. Nordsiek, K. (1974), National Conference, Munich, FDR Powdered Filler Masterbatches. Pike, M. (1972). Harrison Crossfield Ltd., London, private communication. Schultz, S. (1973). Gurnrni Ahest. Kunst. ?6:258. Smith, L. P. (1978). European mixing techniques, Eltr.storneric.s. Thompson, C. W. (1973), Guthrie Estates Ltd., London, private communication. Whally, V., and Morrell, S. M. (1973, RAPRA, MernhPrs J., February.
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Whcclans, M. A. (1981 ). paper F.5, RUBBERCON 8, Harrogate. U.K. Whittmgton. W. H.. Woods, M. E,, and Holman, P. R. (1974). paper presented at the Rubber ACS Meeting, Toronto. Whitting, W. H., Woods, M. E.. and Holman, P. R. (1974). paper presented at the Rubber ACS Meeting, Toronto. Wood, N. (1985). Goodycar Chemicals Inc., Paris, France, private communication. Woods. A. E. (1976). Southern Rubber Group Meeting, Houston, TX, February. Woods, M. E., and Krosky, R. P. (1973). R d h r r Age, April, p. 33. Woods, M. E., Morsek, R. J., and Whittington, W. A. (1973), Rubber World. June, p. 42. Woods, M. E., and Whittington, W. A. (1973), paper presented at the Rubber Division, ACS Meeting, Detroit. MI.
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/ Rubber-Rubber Blends: Part
I
C. Michael Roland Naval Research Laboratory, Washington, D.C.
1. INTRODUCTION
This chapter discusses the thermodynamic and processing considerations involved in preparing rubber-rubber blends and reviews recent developments in the analysis and performance of such materials. Since it has been recognized for some time that the prospects are limitedfor continued synthesis of new polymeric materials with practical utility, efforts to developblends for diverse applications have continued to burgeon. Reviews of rubber blends were published by Corish and Powell in 1974 and Roland in 1989. The focus in this chapter is on more recent advances. With more than half of domestic rubber consumption going into tires. and considering that all majortire components with theexception of the tread ply can andhavebeensuccessfully formulated using rubber blends, it is obviousthat tires represent the major applicationof rubberrubber blends. The properties of blends, however, are described here in general terms, with specific applications undoubtedly suggesting themselves.
2.
MORPHOLOGY
2.1 Thermodynamics of Polymer Compatibility
The morphology of a blend is a function of the nature of the blend components (both their mutual compatibility and the rheological properties of the rubbers) and of the method employed, to produce the blend. It is necessary to distinguish between compatible blends and ones that are truly miscible. The former are homogeneous on a macroscopic scale but miscible only in a technological sense; that is, the rubbers can be mixed together and vulcanized to give a useful product. Truly miscible rubber blends, on the other hand, are those in which the free energy of mixing is negative so that the morphology is homogeneous on a segmental level; that is, the molecular coils interpenetrate. Miscible rubber blends are very rare because of the high molecular weight of elastomers. According to Robeson ( 19821, only about200 casesof miscible blends have been reported among all polymer mixtures, includingeven block copolymers and polyelectrolyte complexes. Thereason for this can be seen from consideration of the requirements for miscibility (McMaster, 1973; Patterson and Robard, 1978). The change in free energy upon mixing must be negative, AG
=
AH - TAS
(1) 197
Roland
198
and, in order that small composition fluctuations do not lead to spinodal decomposition. the second derivative of the free energy change with respect to some measure of composition must be greater than zero. The entropic term, TAS, includes both the usual combinatorial entropy of mixing. which depends upon the number of molecules in the system and is therefore small for the molecular weights associated with rubbery polymers, anda negative (dernixing) contribution. which results from the loss in volume upon mixing of two polymers (Patterson, 1969). This reduction in volume is due to the differences in the free volume of the polymers prior to their blending. The contraction increases the free energy of mixing by reducing the available space and thus the number of ways in which the polymer segments can be arranged. This negative contribution to the entropy of mixing provides a driving force for polymer-polymer immiscibility. As a result. miscible rubber-rubber blends are expected only if the free energy change is made negative by virtue of an exothermic heat of mixing, AH. This statement is strictly true only for polymers of infinite molecular weight (Roland. 1987).Absent some specific interaction between blend components (e.g., hydrogenbonding).heats of mixing of polymerpairs are endothermic. and accordingly literature surveys reveal few examples of miscible rubber-rubber blends (Krause. 1972). An interesting exception to this can be found in the case ofcopolymers. Blends containing these are sometimes found to be miscible even when their corresponding homopolymer blends are not. Although there are no specific interactions in these cases, Paul and Barlow (1984) have suggested that a net mixing exotherm can exist due to dilution of the more unfavorable unlike monomer-monomer contacts when at least one of the components of a blend is a copolymer.
2.2
Miscible Rubber Blends
Examples of rubber-rubberblendsreported to be miscibleincludestyrene-butadienerubber (SBRs) of differentstyrenelevels (Livingston and Rongone, 1967).nitrile-butadienerubber (NBRs) of different acrylonitrile contents (Bartenev and Kongarov. 1963). SBR and butadiene rubber (BR) (Marsh et al., 1968). and natural rubber (NR) with vinyl BR (Ueda et al., 1985; Trask and Roland. 1988). It is not surprising that a copolymer would in some cases be miscible with another copolymerof slightly different composition. Moreover,the seemingly contradictory findings regarding the homogeneity of some blends (Walters and Keyte, 1965; Yoshimula and Fujimoto, 1969) are likely attributable to differences in microstructure of the various materials employed in thesestudies.Anotable example is the blend of NR withBR. While the 1.4polybutadiene-NR blend is heterogeneous (Marsh et al., 1967),highvinyl BR andisoprene rubber (IR) are miscible. as evidenced by their spontaneous interdiffusion (Roland, 1987). This is due to a fortuitons rear equivalence of the polarizabilities of the respective chain subunits (Tomlin and Roland. 1992). Although the incidence of miscible rubber-rubber blends is not high. there are advantages to such a morphology, including greater mechanical integrity than multiphase systems and an enhancement of tensile properties due to the contraction in volume and correspondingly greater number of chains per unit cross-sectionalarea(Friedet al.. 1979). Also, the presence of a morphology that is i n thermodynamic equilibrium can minimize its dependence on mix conditions, along with minimizing the likelihood of changes in the morphology during postmixing operations. In addition to equilibrium considerations regarding rubber blend morphology. it must be recognized that the slow diffusion of macromolecules makes possible the preparation of blend vulcanizates in which the morphology is. however, by no means at equilibrium. Three possibilities exist:
Rubber-Rubber Blends I
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The polymers are miscible but exhibit heterogeneity because the mixing process was of insufficient duration to allow the attainment of equilibrium prior to crosslinking. Since few elastomer blends exhibit miscibility, this circumstance is perhaps only of academic interest. 2 . The elastomer pair is not miscible, but vigorousmechanicalmixing overcomes the small potential barrier to unlike segmental interactions and so gives rise to a morphology that is homogeneous, at least down to the level accessible by standard analytical technique. Althoughdiffusion of elastomer molecules,particularly at the elevated temperatures of mechanical mixing, would permit demixing of chains over a scale greater than the coil size (ca. 100 A), the inhibition of these reptative motions either from bonding with adjacent filler particlesor due tothe presence of long-chain branching might result in a stable, albeit nonequilibrium. morphology. 3 . The temperature at which blending or curing is carried out may be greater than the lower critical solution temperature (LCST) of the polymer pair. The phase separation of miscible polymers at elevated temperatures results principally from an increase in the entropy loss associated with the volume changes accompanying mixing (Sanchez, 1982). (The combinatorial entropy contribution, which favors mixing, also increases with temperature but, as discussed above, this is negligible for high molecular weight materials.) 1.
Polymer blends that are miscible will usually exhibit LCST behavior. Although observations of upper critical solution temperatures have been reported [for example, in an SBR-BR blend by Inoue et al. (l985)], Sanchez (1982) has argued that, absent attractive interactions of a specific magnitude. UCST is not possible i n high molecular weight materials.
2.3 Multiphase Rubber Blends Although in the great majority of elastomer blends thecomponents are not molecularly dispersed, they may be referred to as compatible if some technically advantageous combination or compromise of properties can be realized from the blend. It would perhaps be more appropriate to reserve the term “compatible” to describe systems that do not spontaneously demix on a macroscopic scale, but, in fact, the retarded diffusion of macromolecules makes this unlikely. even in polymer blends in which the unlike interactions are strongly repulsive. The morphology of these “compatible” rubberblends is dependent upon the mixingprocedureandrheological properties of the blend components as well as thermodynamic considerations. This structurecan be adispersion of onecomponent in a continuous matrix of the other, or thephases can be co-continuous. CO-continuity implies that an interpenetrating polymer network (IPN) exists. Although an IPN can be intentionally produced during synthesis (Sperling and Friedman, 1969)or by controlledmixingtechniques [e.g., latex blendingas described by FrischandKlempner (1970)], with conventionalrubber-mixingtechniques equal-volume fractions and equalviscosities of the components will favor co-continuity (Avergopoulos et al., 1976;Gergenet al.,1985), ascanbe seen in Figure I . While from thestructure of IPNs one canexpect that propertiessuch as modulus will be additive with regard to thecomponents’moduli,the unique morphology of IPNs andtheirpotential for exceptionalultimatepropertieshave led to expandedresearchactivitiesaimed at exploiting their commercial utility (Sperling, 1981). Although only a few of the patented IPN materials involverubber-rubbermixtures (Clark, 1970; Lohr and Kang, 1975;Falcetta et al.,1975), it seems likely that the morphology of many of therubberblendscurrently in commercial use consists of interpenetratingpolymernetworks.
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Roland
\
8’ 64TORQUE RAT IO EPDM PBD
2I .8 I . 6 - EPDM .4 -- CONT.
0
0.25
0.50
W E I G H FT R A C T I O N
0.75
1 .o
PBD
Fig. 1 Phase continuity of EPDM-BR blendsas a function of compositionandtherelativeBrabender mixing torques of the components. (From Nelson et al., 1977.)
A blend morphology wherein one component is dispersed within a continuum of the other has received by far the greatest attention in the scientific literature. The continuous phase in these materials is invariably found to be the rubber of lower viscosity, provided it is present at a sufficiently high concentration (Avgeropoulos et al., 1976). Thisobservation is at least plausible, sincethe more fluidcomponent can readily encapsulate the more viscous phase.The capacity for a component tobe highly extended without fracturing underthe conditions of mixing probably increases the likelihood of that component existing as a continuous phase. From consideration of the interfacial tension between two phases, it has been argued that the phase with the larger normal stress function will form the dispersed particles (Van Oene, 1972). Confirmation of this prediction is lacking, although measurement of normal stresses, especially at high shear rates, is difficult. During the mixing of rubber blends, the dispersed domains are deformed during passage through the high-shear regions of the mixing vessel and, under the proper circumstances. will fracture to produce smaller particles. Simultaneously, these flowing particles collide and often coalesce toform largerdispersed domains. The blendmorphologyobtainedrepresentsthis competition between dispersion of the rubber particlesand their flow-induced coalescence (Tokita, 1977; Roland and Bohm, 1983). Attempts to predict the morphology of rubber blends from consideration of the competition between breakup and coalescence have beenmade by assuming an energy criterion for particle fracture (Tokita, 1977; Bohm, 1980). In fact, breakup is more related to the stress level exerted on the particle by the flowing matrix and how effectively this stress can sustain particle deformation. The number of particles produced upon breakup also is a strong function of the stress level as well as depending upon the relative viscosities of the components (Grace, 1982). Experimental studies of particle deformation and breakup invariably focus on single drops in a dilute suspension. In the concentrated systems usually employed in more practical situations, particles are surrounded by their neighbors. This shielding makes it more difficult to disperse domains in practice than results from the more idealized situations studied in the laboratory would suggest. Characterizationof the material properties is, moreover, usually based on low-strain andor steady-state data, whereasthe fracture of fluid particles clearly
Rubber-Rubber Blends I
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involves high deformations and transient behavior. For these reasons, along with the complexity of the factors governing thecoalescence process, the theoretical predictionof blend morphologies is a formidable task. Manystudies,bothexperimental(Bentley, 1985) andtheoretical(Barthes-Bieseland Acrivos, 1973), have focused on thedispersion of fluid particlesin a fluid medium. The minimum stress necessary to break upa suspended droplet hasbeen shown tobe lowest when the viscosities of the two phases approach one another under the prevailing conditions of temperature and deformation rate (Nelson, et al., 1977). Although shear flow dominates in the mixers utilized in the rubber industry today, extensional flow fields are more effective for the dispersion of particles (Roland and Nguyen, 1988). This is due to their continual stretching of the droplets, whereas the vorticity inherentin shear flow causes rotation of suspended particles. Consequently the particles alternately experience extension and compression (Torza et al., 1972; Pipkin and Tanner,1977). The flow-induced coalescence of the dispersed domains requires their collision, removal of the intervening film through its drainage and fracture, and finally molecular interdiffusion between the droplets. Coalescence has been found to be very extensive both when the viscosity of thesuspendedparticles is much lower thanthat of the continuous phase (Rolandet al., 1986) and when the viscosities are comparable (Fig. 2). As would be expected, a more viscous continuous phase reduces the rate of coalescence, although, interestingly, Roland and Bohm (1984) found that an increased rate of shearing increased the fraction of interparticle collisions that resulted in coalescence. Thenet result of flow-induced coalescence in sheared rubber blends is that the ultimate particle sizeis thereby limited. It has long been recognized that a continuation of the mixing process will, after some point, no longer result in a finer dispersion (Rehner and Wei, 1969). This is generally due to the attainment of a steady-state competition between the particle breakup and coalescence processes, although the ability of the high-shear regions of the mixing vessel to further fracture the dispersed particles can also be a limiting factor.
INVARIANT x
~
0.70
20
40
60
-
MI L L PASSES Fig. 2 The reduction in small-angle neutron scattering invariant accompanying mill mixing of a BR-CR blend. The decrease in scattering intensity results from the loss of isotopic purity in the BR domains due to their flow-induced coalescence. The arrow indicates the value corresponding to complete homogenization of the particles after extensive multiple coalescence. (From Roland and Bohm, 1984.)
Roland
202
3. ANALYTICALMETHODS FOR BLENDCHARACTERIZATION Various techniquesexist for study of the thermodynamics of polymer-polymerblends,and considerable attention has been focused on predicting or assessing miscibility. The great bulk of polymer blends possess a heterogeneous morphology by virtue of their immiscibility. Since the multiphase structure producedin many practical applications is not necessarily representative of any equilibriumcondition, it is often usefulto determine the nature of the morphology obtained under particular conditions. Various analytical techniques available for the characterization of blends are described in this section, with an emphasis given to newer developments in the field. 3.1
Electron Microscopy
The most straightforward method of examining the structure of multiphase polymeric systems is direct observation in the electron microscope (EM). The principal difficulty is in ensuring that sufficient contrast exists when the electron densities of the rubber components are similar. When a difference in unsaturation exists, staining techniques (e.g., with Os04) have long been successfully employed. Of particular interest for elastomer blends is the ebonite method (Smith and Andries, 1974), in which the preferential reaction of one of the rubber phases with sulfur and zinc effects a large increase in its electron density. Advantage can also be taken of the differential capacity for swelling in a particular solvent in order to obtain phase contrast. As described by Marsh et al. (1967). the blend sample is immersed in the solvent, stretched, and subsequently observed in the electron microscope after evaporation of the solvent. The phase that was more swollen will have been more thinned out upon stretching. To avoid the distortion in zone sizes and shapes encountered with the differential swelling method, advantage can be taken of the differing susceptibilities to pyrolysis of the rubbers in a blend (Hess and Chirico, 1977).Differentialpyrolysisselectivelyremoves one of the rubbers,causingits domains to becomemoretransmissive in theelectronmicroscope.Rolandetal. (1985) havedescribed several approaches for obtaining electron micrographs of the transient structure that may arise in multicomponent rubbers as a result of deformation. Recent advances in digital image analysis have facilitated the rapid obtaining of particlesize data from electron micrographs (Sax and Ottino, 1985). The micrographic image is converted by a video camera into an array in which each element represents the corresponding optical density of a small section of the original image. Spatial resolution can be as fine as about I O pm. An associated computer extracts from this array the desired particle-size distribution and statistics. With resolution limits as low as a few angstroms (Kruse, 1973), in principle the electron microscope can be used to probe rubber-rubber blends of the finest dispersion. In practice, the need to obtain thin sections and the problem of contrast limit its range of usefulness.
3.2 SolutionBehavior Since the retarded diffusion of polymers in the solid state makes it difficult to attain a condition of true thermodynamic equilibrium, the behavior of polymer mixtures in solution has often been utilized in efforts to assess miscibility. Phase separation of a polymer pair in a common solvent is usually indicative of their immiscibility, although when a sufficiently large difference exists in therespectivepolymer-solventinteractionparameters, phase separation can occur in solutions of miscible polymers (Patterson, 1982). It is even more common to find instances of misciblesolutionsinvolvingpolymerpairs that in the absence of solventexistas a phaseseparated blend. Asan example, Braun and Rehage(1 985) found that blends of BR and polypen-
Rubber-Rubber Blends I
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tenamer form a miscible solutionin toluene, while electron micrographs clearly indicated heterogeneity in films cast from this solution. As discussed earlier, a negative enthalpy of mixing is usually requiredfor polymer-polymer miscibility. An indication of the magnitude of this interaction, but not its sign, can be extracted from determination of solubility parameters, 6, of the respective blend components. Defined as the square root of the cohesive energy density of the polymer, 6 can be obtained from measurement of the interaction parameter, xI2, which is the ratio of the noncombinatorial free energy of interaction in the polymer-solvent system to the available thermal energy (Hildebrand et al., 1970),
where V is the molar volume, and the empirical binary coefficient I I 2 is often taken to be zero. Various methods are available to measure the polymer-solvent interaction parameter (Orwoll, 1977), includingprediction from intrinsicviscosities(Kok and Rudin, 1982). The polymerpolymer interaction parameter. can then be deduced from the respective solubility parameters for the two polymers, using
where
It can be shown that a negative value of x 2 3 can only result from a nonnegligible value of (Paul and Barlow, 1984). It isobvious that the used in thismethodmustcorrectlyinclude the various contributions (dispersion forces. hydrogen bonding, etc.) to the interaction of the polymer pair, which may require xI2determinations in a variety of solvents. A more useful approach to obtaining polymer-polymer interaction parameters from polymer-solvent interactions is the useof gas-liquid chromatography (Su and Patterson, 1977). Retention volumes of gas-phase components on solid phases composed of the polymeric materials of interest (the respective pure components and their mixtures) provide a measurement of xli.The polymer-polymer interaction parameter can then be calculated according to
where +2 and +3 are the volume fractions of the polymers in the mixture. A related approach is the determination of from the uptake of vapor by the polymer blend (Kwei et al., 1974). While determination of polymer-polymer interaction parameters can be of value as an indication of potential miscibility or in comparing the relative compatibilityof a series of blends, it must be recognized that it is of limited utility in describing the structure of blends encountered in practical processing operations, where decidedly nonequilibrium conditions may prevail. Shutilin (l982), for example, found that solubility parameter differences did not correlate with the apparent relative compatibility of various blends of NR, BR, and SBR.
3.3 Glass Transitions A popular methodof adducing thedegree of homogeneity in polymer blendsis from measurement of the temperatures of transition from rubbery to glassy behavior. This canbe accomplished with
204
Roland
a variety of methods, including dilatometry, nuclear magnetic resonance, dielectric response, differential scanning calorimetry, differential thermal analysis, radiothermalluminescence, and dynamic mechanicalmeasurements. The observation of distincttransitionscorresponding to each component of the blend indicates that a multiphase structure exists. The appearance of a single transition cannot, however, be taken as unambiguousevidence ofmiscibility.If the respective T,s are close (ca. 10"C), they may appear as a single, broad transition. The appearance of one or more transitions in dynamic mechanical testing can also be affected by the frequency employed. Ramosand Cohen ( 1977). for example, observed a single glass transitionin heterogeneous BR-IR blends at 1 10 Hz, while at a lower test frequency both transitions were in evidence. Fried ( 1983) has discussed improving the resolution in thermal analysis by selectively annealing one of the components orevaluating derivatives of the usual heat capacity measurements. When the domain size is sufficiently small [Kaplan (1976) suggests 150 Al. the thermal or mechanical response becomes no longer sensitive bo heterogeneity. For example, when the domains of a NR-cis-polypentenamer were 50- I00 A i n diameter, torsion pendulum measurements revealed only a single. intermediate glass transition (Braun and Rehage, 1985). There is also evidence suggesting that when two rubbers are close i n structure, such as NR and BR (Bauer and Dudley, 1977) orBR and SBR (Inoue et al., l985), a broad or interconnected interface region, developed particularly upon vulcanization, can cause disappearance of the expected distinct glass transitions. The exact temperature of the individual glass transitions i n a multiphase blend will be shifted somewhat from the values for the pure components. This is a mechanical effect and does not necessarily indicate anything about phase interactions (Dickie, 1979). Changes i n the magnitude of the damping have been used to assess the state of cure of the phases in a rubber blend (Husonet al., 1984). Sinceobtaining a satisfactory network structure in both components of a rubber blend can be difficult when there are differences in the extent of unsaturation of the rubbers. this information can be of practical value. There have been recent developments in the use of pulsed NMR to investigate the heterogeneity of blends (Nishi, 1978; Miller et al.. 1990; Roland et al., 1993). Sincethe spatial resolution is limited by spin diffusion distances, this techniqueis comparable to more conventional methods for the detection of small domains. Since the motion of a carbon atom in a polymer chain will be more rapid when the segments are in a rubbery environment than when the surroundings are rigid. measurement of spin-spin relaxation times at temperatures intermediate to the respective T,s can be used as a probe of the extent of interfacial mixing.
3.4
Elastic Scattering
The irradiation of matter usually gives rise to scattering of a portion of the incident intensity, where both the energy and propagation vector of the scattered waves may differ from that of theincidentradiation.Whiletheenergydependence of the scattering is related to dynamic processes in the scatteringmedium,theangledependence of theelasticscatteringprovides morphological information. Elastic light, x-ray, and neutron scatteringall result from heterogeneities in the structure of the irradiated material. In a homogeneous system, thermal fluctuations in density and composition are responsible for the scattering. with the extrapolated zero-angle intensityprovidingameasure of polymer-polymerinteractionparameters (Wendorff, 1980; Murrayet al., 1985). In multiphasepolymerblends, the angledependence of thescattering reflects the size and spatial distribution of the phases. and so it can be usefully applied to the study of the morphology of rubber blends. There is an extensive literature devoted to methods of analyzing scattering data from nlultiphase systems [see, for example, Glatter and Kratky (1982) and Higgins and Stein (1978)l. Thedifferences among light, x-ray, andneutron scattering have to do with the nature of the material heterogeneities as well as with their size.
Rubber-Rubber Blends I
205
The scattering of visible light, whose wavelengths are thousands of angstroms, results from inducedpolarization in themolecular electron cloud. Contrast,therefore,results from variations in refractive index of the sample over distances of a few thousand angstroms and greater, with larger heterogeneities amplifying the scattering at the smaller angles. The application of light scatteringto solid-state polymer blendsis not as popular as its use in the characterization of dilute polymer solutions, due to difficulties in the former with multiple scattering and sample transparency. In the area of elastomer blends, much of the light-scattering work has focused on block copolymers, including, for example, blends of styrene-isoprene copolymers with polyisoprene (Moritani et al., 1970). Light scattering has also been used to monitor the broadening of the interface in BR-SBR blends during vulcanization (Inoue et al., 1985). The wavelengths of x-rays (e.g., 1 S 4 A for Cu K,) are a thousand-fold less than that of visible radiation, so that whereas light scattering arises from coupled electrons as described in terms of a molecular polarizability, the scattering of x-rays is simply related to the number density of electrons. The variations in electron density that give rise to this x-ray scattering can extend spatially from a few to roughly a thousand angstroms, beyond which the scattering angles become toosmall to be experimentally accessible. The application of x-ray scattering to polymer blends is well established and often complements light-scattering results. This is particularly true in two-phase systems in which the particle sizes extend over a broad range. In Figure 3 is displayed the particle-size distribution determined from small-angle x-ray measurements from a BR-CR blend, along with results obtained from analysis of electron micrographs (Roland and Bohm, 1984). Although thereare limitations associated with extracting particle-size distributions both from micrographs and from scattering curves. it can be seen that the different techniques are in reasonable agreement. A major advancement in the investigation of polymer blends has been the development of small-angle neutron-scattering techniques. The wavelengths of the neutrons used in these experiments range from 2 to 20 A, so the sizes of structures probed are comparable to those in
A
TEM
- CURVE
FITTING INTEGRAL TRANSFORM
0
300
600,
900
I200
RADIUS (A)
Fig. 3 BR particle size distribution in a well-mixed CR blend as inferred from (A)analysis of electron micrographs, ( 0 )Fourier inversionof small-angle x-ray scattering data, and( - ) the log-normal distribution function whose adjustable paramcters gave a best fit to the experimental x-ray scattering curves. (From Roland and Bohm, 1984.)
206
Roland
small-angle x-ray studies. The scattering contrast, however, is associated with short-range neutron-nucleus interaction. This interaction is different for different elements aswell as for different isotopes. The great advantage of neutron scattering is that by isotopic substitution, particularly of deuterium for hydrogen, scattering heterogeneities canbe selectively introduced into a sample without appreciable change in the thermodynamic properties of the material. A fraction of the chains of one component of a polymer blend can be labeled in this manner. so that not only can domain sizes in the blend be measured (as with small-angle x-ray scattering), but the dimensions of the polymer chains of the labeled component can also be determined. There has to date been limited application of small-angle neutron scattering to rubberrubber blends. Kirste and Lehnen (1 976) measured the increase in coil size of high molecular weight,polydimethylsiloxanewhen it was blended with lower molecular weight PDMS. Roland and Bohm (1984) used neutron scattering to follow the coalescence of BR domains dispersed in a CR matrix. It seems likely that significant advances in analysis of the structure of multiphase rubber systems can be realized from further application of neutron-scattering techniques. Although the intimate mixing associated with miscible rubber blends might be expected to preclude formation of a crystalline lattice by any crystallizable component of the blend, the principal effect of miscible blending is on the crystallization rate. Since, for example, it has been reported (Ghijsels, 1977) that the heat of fusion of BR reflects the extent of its blending with SBR, investigation of crystallinity in certain rubber blends may prove useful. Riga (1978) has demonstrated how the angle and breadth of the amorphous halo in the wide-angle diffraction pattern from various polymers can be used to distinguish homopolymer blends from the corresponding copolymers. The effect of blending on the crystallization of natural rubber has been studied by Zenel and Roland ( 1992a) and by Tomlin and Roland (1993).
4.
PREPARATION OF RUBBERBLENDS
The objective ofan industrialmixingprocess is production of materialhavingthedesired properties. For multiphase polymer systems, this does not necessarily correspond to any equilibrium or steady-state morphology, although such a structure could probablybe most reproducibly generated. Only when the benefits in terms of a rubber’s performance warrant the additional expenditure of effort is any “ultimate” mode of dispersion likely to be pursued. Moreover. many physical properties of rubber blends are insensitiveto the details of the two-phase structure. Rubber-rubber blends can be prepared by a variety of methods, including during synthesis. by latex or solution blending, and by conventional mechanical mixing. The discussion in the following sections is limited to mechanical mixing, which represents the most widely practiced procedure. Descriptions of other methods of preparing blends can be found in the literature. with references contained in the review articles of Corish and Powell (1974) and McDonel et al. (1978). The intluenceof mixing conditions on the distribution of the various compounding ingredients of a rubber formulationwill also be described herein.The effect of blend morphology on properties will be taken up in Section 5. which is concerned with the individual properties of rubber blends.
4.1
Mixing Equipment
The principal functions of the mixing operation in preparing rubber-rubber blends are essentially identical to those associated with incorporating fillers, curatives,etc., intoa rubber stock; distriburilqr nli.ving, in which the composition of the blend is made uniform throughout the batch, and disprrsi\’e m i x i q , in which the initially macroscopic components are broken up into finer do-
Rubber-Rubber Blends I
207
mains. While the throughput advantages of continuous mixing are well recognized. commercial rubber operations usually employ batch mixing. This is in large part due to the need to supply a continuous mixer with particulates or free-flowing materials, whereas rubberis usually obtained in large bales. Accordingly, the extruders used in the rubber industry are most often employed in an auxiliarycapacity to improvedistributivemixing and to shape therubber.Dispersive mixing is carried out in internal mixers such as the Farrel Banbury or, for small batches, often on open-roll mills. Although developments in the design of rubber mixers were not expressly carried out to improverubber-rubber blending. the optimal distribution and dispersion of carbon black and the other ingredients of a rubber formulation are controlled by the same mechanisms pertinent to the blending of rubbers. In the design of internal mixers these developments include increases in the number of wings, or nogs, on the mixer rotors to increase the quantity of highshear areas inside the mixer, and allowing the rotors to intermesh (Fig. 4) so that dispersive shearing occurs not only between the rotor tip and the walls, but also between the rotors themselves (Freakley, 1985). Dispersive mixing requires high stresses; accordingly, it is often important to efficiently cool the rubber in an internal mixer in order to maintain a high viscosity. Of course, precise temperature control in general requires the capability for efficient removal of heat. The cooling
(b)
Fig. 4
Internalmixer with (a) tangential and (b) intermeshing rotors.
Roland
208
systems of internal mixers have evolved over the years. The most effective method is to drill passages that allow flow of the cooling water close to the inner surfaces of the mixing chamber. Along with their use in the transfer and forming of rubber stocks, extruders can also contribute to distributive mixing. Increased demands for the screw and barrel of the extruder to not only convey the material to a die but also mix it have led to modifications in screw design. The principal aim is to promote homogenization of the mix by the exchange of material between planes. Representative approaches include the use of pins in the barrel to divert and divide the flow of material (Freakley, 1985) and the use of the cavity transfer mixer, whichhas hemispherical cavities in thescrewand barrel to promote exchange of material (Hindmarch and Gale, 1983). The resulting diversion and reorientation of the flow stream improves the distributive blending efficiency of the extruder. In recognition of the efficiencies of automated production systems, there has been continued interest in the rubber industry in developing continuous mixing methods. The plastics industry has further evolved toward this goal, and advancementsin that industry may facilitate development of the continuous processing of rubber. The problem associated with feeding acontinuous rubber mixer has been mentioned. There is the additional constraint of obtaining acceptable dispersion. which, with regard to filler-rubber, if not rubber-rubber, mixing, is a most demanding requirement. In order for an extruder to perform the dispersive functions of an internal mixer, special mixing screws must be employed(Plastics Compounding, 1981, 1984). Thesecan contain dams so that the material is forced through tight clearances with the wall and is thereby subjected to large shearing stresses. Twin-screw extruders can also be used where shearing forces are developed by employing various mixing elements. Other continuous mixers include the Farrel continuous mixer,which is an extruder containingBanbury-typerotors to provideintensive mixing: the Berstorff planetary gear extruder, in which the primary screw is surrounded by an array of planetary spindles; the Buss-Condux extruder. which enlploys a single rotating and reciprocating screw along with teeth running axially inside the barrel: and the Sterling Transfermix, where the variationin the rotor diameter along its length causes material to be transferred from the rotor to concentric stator rings attached to the barrel. Although the various designs lead to good distributive mixing and acceptable dispersive mixing,thelatter is beingobtainedalmoststrictlythrough shear flow. For example, if one compares the carbon black dispersion obtained in various mixers as a function of the energy of mixing, it can be seen that various designs all fall on the same general curve (Fig. 5). In the Mooney viscometer this is known to be shear flow, indicating that the stresses developed in the various mixers are evidently arising from the same type of flow field. Although it is one of the most common, shear flow is by no means the most effective dispersive flow field. In general, a flow can have both rotational and extensional components. The strength of a flow field can be described by consideration of the ratio of the magnitude of the strain tensor to that of the vorticity tensor (Fuller and Leal, 1981). This ratio is given by the quantity I + P I - P
where S = - 1 corresponds to pure rotation and (3 = 0 ;S simple shear flow. It has long been recognized that dispersion is most effectively accomplished with pure straining (P = 1) flow fields. Nevertheless, withthe exception of the drawing off of material from a calender or extruder and the converging of tlow paths into adie orthrough the nipof a roll mill or mixer, commercial mixingreliespredominantly on shearing flows andthe development of shearingstresses to achieve dispersion.
209
Rubber-Rubber Blends I
+ m
“ . . . . J ” -
6oo
200 400 JOULES PER GRAM
Fig. 5 Carbon black dispersion index measured for an SBR stock with 63 phr N339 a s a function of the energy of mixing using the ( ) Mooney viscometer, (a)Brabender Plasticorder, ( 0 )Buss-Condux reciprocating screw kneader, and ( A ) Berstorff planetary gear extruder. (Courtesy of Ulmer et al.. 1985.)
+
4.2
Distribution of Compounding Ingredients
A major difficultyin obtaining acceptable vulcanizate propertiesfrom a blend is that of developing a satisfactory network structure in each of the rubber phases. Due to the higher solubility of sulfur in unsaturated elastomers (Van Amerongen, 1964), along with the greater affinity of many accelerators for more polar rubbers(Gardiner, 1968),the crosslink densityof the respective rubbers in the blend can differ significantly, leading to less than optimal physical properties. Leblanc (1982) has suggested that initially thecurvatives will locate within thecontinuous phase. In fact, the curatives probably make first contact with the lower-viscosity phase, since it tends to occupy the outer regions of the flowing rubber mass, where the shear rates during mixing are highest. Of course, this lower-viscosity component does tend to become the continuous phase, as discussed above. Generally, the details of the mixing scheme can have some effect on the initial distribution of curatives. The curatives will then diffuse into the elastomer component in whichtheirsolubility is highest. Since the levels of sulfurandacceleratortypically employed are below the solubility limits, diffusion of curatives into a given phase is usually not prevented by saturation of that phase; hence, this curative migration often results in a cure imbalance. In addition, if the rate of vulcanization varies considerably between the elastomers of the blend, depletion of the curatives in the faster-curing component can cause continued curative migration into this phase (Bhowmick and De, 1980).Leblanc (1982) demonstrated that preblending of curatives into the respective elastomers at their optimal concentrations prior to blending of the rubbers can improve the blend crosslink distribution, although the usual practice is to incorporate the curative last in order to avoid prevulcanization (scorch) problems. Altera-
Roland
210
tions in blend physical properties can sometimes be realized from the use of very short, hightemperature cure cycles (Bhowmickand De, 1980).In these instances the distribution of curatives obtained during the mixing stage will be more critical, since rapid reaction of the curatives can reduce the extent of curative migration. Approaches to overcome these imbalances in curative distribution include the chemical modification of accelerators so that the respective solubilities in the components of a rubber blend will be more equal (Mastromatteo et al.. 1971) and the direct attachment of curatives to the elastomer with which the solubility is lower (Hashimoto et al., 1976).Obviously the problems encountered with curative imbalance are particularly significant when the components of a blend are most dissimilar, such as blends of EPDM or butyl rubber with dienes or nitrile rubber. Along with the desirability of having a balanced crosslink density in the various rubber phases, there also exists the requirement that for mechanical integrity the phases must bechemically bound to one another; that is, linkages must exist across the phase boundaries. The extent of interfacial crosslinks is sensitive to both the rate of vulcanization and the specific cure systems employed (Woods and Mass, 1975). Hashimoto and coworkers ( 1970) have touted polysulfidic linkages as most useful for interfacial crosslinking of EPDM-SBR blends, while Zapp (1973) found that interfacialbonding in chlorobutyl-BRmixturescorrelatedwith a high degree of monosulfidic linkages. Related to the subject of interfacial bonding is the fact that if a third, small-molecule component that is soluble with both polymers is presentintheir blend, the interface will be richer than the bulk phases in this component. This results from a reduction in energetically unfavorable interactions between dissimilar polymer chains at the interface by virtue of the presence of the third component (Hefland and Tagami, 1972). In principle, this affect could promote accumulation of curative at the interface. A somewhat analogous method of promoting interfacial connectivity is by incorporation into the blend of a block or graft copolymer containing segments identical to, orat least miscible with, each of the rubber phases (Paul, 1978). Provided the block lengths are sufficiently long, the copolymer additive will preferentially locate at the interface in a configuration whereby it is intimately mingled with each phase. Because of their greater ability to favorably configure themselves,block copolymers aresuperior to grafts in this regard.Withthis approach, the interfacial bonds correspond to the covalent backbone of the copolymer molecule, the respective segments of which are dissolved in theseparatephases.While the use of thesepolymeric compatibilizers is largely confined to blends of rubber with glassy (or crystalline)materials, the application to rubber-rubber blends may be feasible when the added cost of the block or graft copolymer is warranted by improvement in ultimate properties. Paddock (1973) has patented such a system based on EPDM and SBR. The distribution of fillers and various processing aids in a multicomponent rubber stock can also be nonuniform, with a resulting influence on properties. Extensive investigations in this area have demonstrated the preferential takeup of carbon black by certain rubbers, with carbon black affinity decreasing in the order (Callen et al., 1971 ). BR > SBR
> NR > EPDM > IIR
During mechanical mixing of carbon black with unsaturated elastomers, sufficient interaction, primarily chemisorption, occurs to prevent any subsequent transfer of the black. If the method of mixing is less vigorous (e.g., solution blending) or involves more saturated rubbers, the carbon black can transfer to phases with which it is more compatible. As discussed in more detail below, the nonuniform distribution of carbon black can influence various properties. Although the significance is perhaps not as great, the oils,resins,andvariousrubber chemicals used in a rubber compound can have differing affinities for the phases of a blend. Both their nonuniform distribution and postmixing migration have been observed.
21 1
Rubber-Rubber Blends I
5.
PROPERTIES OF RUBBERBLENDS
Other than obtaining a lower-cost material, the only motivation for blending rubbersis to improve performance. either the ease with which parts can be manufactured from the stock or the properties the part exhibits in end use. This section discusses certainaspects of the behavior of polymer blends and how they may be intluenced by the details of the blend morphology.
5.1
Rheology
Rubbers are often blended to obtain a better-processing material. This improvement may consist of lowering the stock viscosity or producing a material that is less prone to fracture or crumbling when subjected to flow. Normal stress functions and the related phenomena of die swell and shrinkage can also be altered by blending. Qualitatively. the expectation is that the processing behavior of the blend will be intermediate between that of the components. I n fact. however, polymer blends can often display anomalous rheological properties. The viscosity of a blend may exhibit a minimum and/or maximum as a function of the composition. In this regard, the behavior of rubber blends has been found to follow no simple trends (Fig. 6). A blend viscosity greater than the mean of the components' viscosities can result from the reduction in free volume accompanying miscible polymer mixing. It has been claimed that in a multiphase polymer blend there is an increased fractional free volume. which serves to reduce the viscosity (Lipatov et al., 1981). while the additional energy dissipated into dispersed particles when the continuous phase is sheared can contribute to an elevation in the resistance
30 S EC"
70C
600 ui m A
15 SEC"
I
W
g 50C 2 4OC I l
I
20
I
I
40
I
WT.
I
60 %
I
I
80
I
I
100
BR
Fig. 6 Capillary extrusion force a s a function of BR-NR blcnd composition at different nominal shear ratcs. (From Folt and Smith, 1973.)
212
Roland
50-
U E M U R A 8. TAKAYANAGI
30 -
0.0
0.2
0.4
0.6
0.8
COMPOSITION
1.0
Fig. 7 Viscosity as a function of blend composition calculated according
tO
Eqs. (7)-( IO).
to flow (Chaffey and Mason, 1966). Theoretical treatments are available that attempt to predict the viscosity of blends. Various derived expressions are given below and plotted in Figure 7. From Uemura and Takayanagi (1966):
From Hashin (1 964):
From Heitmiller et al. (1964):
In these expressions for the blend viscosity, q, the subscripts 1 and 2 refer to the respective phases of volume fraction or weight fraction W, and IJ. is Poisson’s ratio. Colby (1 989) observed distinct terminal relaxations in the mechanical spectra of miscible polymer blends. The components’ entanglement molecular weights, and hence the magnitude of the plateau modulus, can be influenced by blending (Roland, 1988; Roovers and Toporowski, 1992; Arendtet al., 1994). An important effect in miscible blends is theso-callednematic interaction, whereby orientation of neighboring chains induces orientation of a given chain(Sotta et al., 1987; Erman et al., 1988; Kornfield et al., 1989; Zemel and Roland, 1992b).
+
Rubber-Rubber Blends I
213
The morphology of a blend can rearrange to better accommodate theapplied stresses. This principle of minimum energy dissipation(Everage, 1973) underlies the often-encountered sheathcore configuration. Since in the vicinity of a wall of the containing vessel the velocity gradients tend to be highest, while at the core of a flowing polymeric material (through a pipe, on a roll mill, etc.) there is often plug flow, the lower-viscosity component will tend to accumulate toward the surface of the polymer mass. The result is a blend viscosity that can in the limit be as low as that of thelower-viscosity component. Van Oene (1978) has suggested that theinternal circulation occurring in the dispersed particles of a sheared blend may also contribute toviscosity minima, the secondary flowgiving rise to “drag reduction.” Incorporation of only a few percent of EPDM was found to significantly reduce the viscosity of a fluoroelastomer, and vice versa (Shih, 1976). This was attributed to the plating out of the minor component onto the wall of the viscometer, giving rise to interfacial slippage. A similar phenomenon has been observed in SBR containing less than 2% by volume of polydimethylsiloxene (Roland and Nguyan, 1988). The lubricity of thelatter effects a reduction in the apparentviscosity of theblend during processing. A complicating factor in predicting the rheology of industrially interesting rubber blends is the presence of a third phase of inextensible filler. A nonuniform distribution of carbon black, for example, canmodifytheprocessingbehavior of the individual components to differing degrees. In particular, saturated rubbers will not suffer the viscosity elevation experienced by unsaturated rubbers, with which the carbon black preferentially reacts. In a study in which both components had a high affinity for carbon black, Lee (1981) found the viscosity of BR-SBR blends to be independent of the location of the carbon black in the blend. This distribution did, however, alter the elastic properties. Generally speaking, the preferential incorporation of carbon black into one of the phases will cause a change in their relative viscosities. Such a viscosity differential can be expected to promote a sheath-coreconfiguration. As was noted,thiscanlead to a lower thanexpected blend viscosity. The expectations with respect to die swell and related phenomena are not as straightforward, since the coremay suffer the same magnitude of extensional flow as the sheath, unlike the case of shear deformation. Sircar et al. (1974) found that the die swell could be correlated to some extent with the location of the carbon black. Optimal reduction in die swell could in some instances be brought about by incorporating the black into the gum rubber whose die swell was found to be most reduced by the presence of filler.
5.2
Modulus
Two factors to be considered in estimating the modulus of an elastomerblend are the individual component moduli and the nature of the blend morphology. One influence on the stress-strain response of the components is the distribution of curatives and the resulting blend network structure. Factors governing this distribution are discussed in an earlier section. Another important aspect is the glass transition temperatures of the elastomers comprising the blend. When a blend is subjected to deformation at a temperature intermediate between the respective component T,s, it is generally found that the stress atlow strains is close tothat expected for the stiffer component while the extensibility nevertheless approaches that of the softer material (Newman, 1973). This isthe increased toughness underlying the rubber modification of plastics. The upper and lower bounds on blend modulus are given by parallel coupling,
Roland
214
and series coupling,
respectively (Nielsen, 1974). Various theoretical formulations [see, for example. Chen (1973)] predict intermediate behavior.The moduli of a seriesof EPDM-BR blends were foundby Nelson and coworkers (1977) to be well described by Eq. ( I 1 ). The crosslinking in that case was accomplished by electron beam irradiation to avoid complications with curative distribution. In general, the extent of radiationcrosslinking in agiven component will not be significantly affected by the specific natureof the other component,at least for blends of the typical hydrocarbon rubbers. In principle, however, the penetrating power and crosslinking efficiency of the radiation can be influenced by the electron density and atomicnumbers of the blend constituents [see Bohm and Tveekrem (1982)]. The nature of the blend morphology can also affect the stress-strain response of the blend. This is most marked when the blends are miscible. Kleiner et al. (1979) have reported that in such a situation the blend modulus can exceed the upper bound given by the parallel model. They have modified Eq. (1 1) by introducing an empirical interaction term, t 1 2 .
where El, is themodulusmeasured blend modulus is given by
for a blend i n which
$l
=
$?
=
0.5. Accordingly,the
This increasedmodulus can be attributed in part to thereduction in volume accompanying miscible blending and the consequent greater density of chains per cross-sectional area. When heterogeneous morphology exists in rubber-tubber blends, the effect of the details of this structure are not particularly significant. While the expectationwould be for the continuous phase tohave more influence, the stress-strain responseof EPDM-BR blends, at high concentrations of both elastomers at least, was found to be unaffected by change in the BR domains from continuous to discrete (Nelson et al., 1977). When one component of the blend is present as discrete particles, in blends of NR with SBR or BR (Walters and Keyte, 1965) and EPDM with BR (Weissert and Avgeropoulos, 1977). there was no effect of domain size on modulus. The stress-opticalcoefficient,which is a modulus characterizing the microscopicdeformation whereas the more usual mechanical modulus refers to the macroscopic strains, has been likewise found to be insensitive to phase separation, although its magnitude can be related to the degree of phase interaction (Bauer, 1982). In elastomer blends reinforced with carbon black, the distribution of filler can profoundly influence the modulus. Displayed in Figure 8 is the dynamic shear modulus measured for an elastomer blend in which the carbon black distribution was systematically varied. It can be seen that, particularly at the lower strains where the carbon black network structure dominates the stiffness properties, an increase in the nonuniformity of this filler distribution results in a lower stock modulus. In terms of the models described by Eqs. ( I 1 ) and (12), this indicates that the transfer of a portion of the carbon black from one phase would lower its modulus proportionally more than the increase in modulus of the phase with the higher carbon black concentration. The effect of carbon black distribution on modulus is thus related to the nonlinear dependence of modulus on carbon black loading.
215
Rubber-Rubber Blends I
54G' (MPa)
32-
01
Io-2
I
1
-
IO" IO0 S T R A I N ?A)
IO'
Fig. 8 Dynamic shcar modulus measured for an SBR with 105 phr carbonblack in which 0 , 20, and 30%. respectively, of the polymer was added as gum rubber during a second stage after incorpornt~onof thc carbon black into the initial ruhbcr. (From Nguycn, 198 1.)
5.3
Transport Properties
The transport properties of polymer blends are of interest for the practical application of blends in air retention, vapor resistance. permselectivity. etc.,as well as for the insight into the morphology of the blend that can be gained from study of the penetration of small molecules into the structure. The passage of vapor through a rubbery material entails dissolution of the gas into the rubber, molecular diffusion, and subsequent evaporation of the gas from the other side of the specimen. The kinetics of this process can usually be described as Fickian. whereby the concentration of the vapor in this rubber is proportional to the external pressure (Henry's law) and the flux of the gas isproportional to its concentration gradient (Fick's law). Thepermeability coefficient P is thereby expressed as the product of the proportionality constants. P = KD
(15)
where K is the Henry's law solubility coefficient and D is the diffusion constant. The diffusion constant can vary with penetrant concentration, while at higher pressures K may become pressure-dependent. In miscible polymer blends the permeability is often found to be described by an empirical relation.
Miscible blends (Nielsen. 1978) as well as certain of those with a heterogeneous structure [for example, rubber-modified polyethylene. as reported by Pieski (1960)] can in some instances exhibit synergistic permeability behavior. Most rubber-mbber blends are heterogeneous. with a permeability that is intermediate between that of the components. Attempts to model the transport properties of blends use some formulation of parallel and series models as their basis. If the continuous phase is the more permeable,a parallel configuration representsthe limiting behavior, with the dispersed phase effecting a more tortuous pathof the penetrant. The series model serves
Roland
216
as the limiting case when the dispersed phase exhibits the greater permeability. Robeson et al. (1978) have combined these extremes to obtain an expression for the permeability given by
P = X;,P,
P1 P2
[
+ XhP2
1
+ 2P, - +?(PI - P?) + 2P, + +?(PI - P?) P1 + 2P3 - 2+,(P, - P l j j P , + 2P2 + +I(P? - PI)
[
where X, represents the fraction of the composition in which component I is the continuous phase, and X h corresponds to a continuous phase of component 2. The description of such cocontinuity is limited by the restriction that
It can be seen that a composition range in which the permeability data are described by Eq. (17) with XA = X” can be taken as an indication of phase inversion. Expressions such as Eq. (17) are based on considerations of the dispersed phase as spherical in shape. More extended structures, particularly those lying in a stacked or lamellar configuration, can lead to reduced permeability due to the more tortuous path that must be taken by penetrants. The most extensive study of the permeability of rubber blends was the early work of Barrier (1955). Some of his data for blends of natural rubber with selected synthetic elastomers are depicted in Figure 9. While the preceding discussion was concerned primarily with mass transport properties (permeability and sorption), the electrical conductivity of rubbers is a transport property that also has some practicalimportance. In elastomers, conductive or semiconductiveproperties
1
20
1
40 WT
I
60
80
rbo
% NR
Fig. 9 The relative air permeability of various elastomer blends, with that of NR taken a s 100. (From the data of Barrier, 1955.)
217
Rubber-Rubber Blends I
CllR
0 2 VOL
0.E
I .o
FRACTION, 2nd POLYMER
Fig. 10 Volumc clcctrical conductivity of blends of CR with (A)chlorinated butyl, (0) nitrilc. and (0) natural rubber. (From Sircar, 198 1.)
result from the presence of carbon black. Indeed, electrical conductivity is a well-established probe of the extent of microdispersion of the carbon black. Blending of elastomers with different affinities for black provides an opportunity to control the state of aggregation and connectivity and thereby influence the electrical conductivity. Displayed in Figure 10 is the electrical conductivity measured by Sircar (1981) for blends of CR with various rubbers. It can be observed that conductivities can be achieved in these blends that exceed those of the pure components. This is due to increased agglomeration of carbon black in these immiscible blends. Carbon black tends to redistribute when mixed into blends, particularly when it has a low affinity for one of the phases. This can result in an accumulation of carbon black at the interface (Marsh et al., 1968) and consequently higher electrical conductivity. Blends of rubbers with similar affinity for carbon black (e.g., SBR-NR) do not exhibit this synergism.
5.4 Adhesion and Tack Acceptable levels of both the (cocure) adhesion and the autoadhesion (or tack) of rubber stocks can often be obtained only through the blending of rubbers. The fact that many synthetic elastomers (e.g., SBR, BR, EPDM)are very deficient in this latter property results in their often being used in combination with natural rubber. The autoadhesion of a blend is unlike many properties in that it is strictly a surface phenomenon; separation of two layers of green stock will typically deformmaterialonlyaboutamillimeter into thebulk of thelayers.Accordingly,thetack performance of ablendreflectsonlythecompositionatthesurface. This providesforthe possibility of obtaining an elevated level of tack without necessarily requiring the use of a large amount of high-tack rubber in the blend. Whether this is realized in practice depends upon the method by which the blend is prepared as well as the rheological characteristics of the blend constituents. Morrissey (1971) measured the tack of a series of blends of NR with various synthetic rubbers and found that the level of tack paralleled the NR content. Hamed (1960) similarly
218
Roland
Table 1 Autoadhesion of NR-SBR Blends Composition (phr)
70 40
NR
SBR
IBMA-SBR"
60 30 30 20
40
-
-
70
-
40
30 40 30
Autoadhesion (Jlrn')
1400 300 I 050 670 0
reported that blends of NR and SBR exhibited autoadhesion that increased with NR concentration; however. a maximum was observed when the NR was 80% of the total polymer. This synergism was attributed to the optimal green strength of such a composition. The high density of chain entanglements i n SBR in combination with the crystallization at high strain of the NR results in a material of greatest energy to rupture. In general, of course. green strength will affect autoadhesion only when the latter is limited by the energy required for rupture. More usually the ability of the plied surfaces to fuse together is the controllingvariable in tack measurements(RolandandBohm, 1985). When BR wasblended with agraft copolymer of isopropylazodicarboxylate and BR. which possesses very high autoadhesion, high tack in the blend stocks was obtained only when the copolymer rubber constituted the continuous phase (Roland et al.. 1985). The behavior described in these examples suggests surface compositions that must at least approximately reflect that of the bulk. When the components of a blend differ widely in viscosity, this may no longer be the case. It is worth noting in this regard that the autoadhesion of the blend is not necessarily an indication of the autoadhesion of the pure components. For example. displayed in Table 1 is the tack measured for a variety of blends of NR. SBR. and a terpolymer consisting of styrene, butadiene. and 3 mol% of N-isobutoxymethylacrylamide (IBMA). When only a small fraction of NR is present in blends with SBR alone, the tack is low. Replacing a portion of the SBR with the IBMA-SBR terpolymer effects a large increase in autoadhesion; nevertheless, blends of SBR and IBMA-SBR without natural rubber have negligible autoadhesion. The IBMA-SBR itself is devoid of tack, yet its presence in SBR-NR blends promotes high autoadhesion. This seeming paradox results fromlack of correspondence between the surface and bulk composition in blends containing the three elastomers. During mixing at elevated temperatures. IBMA-SBR undergoes a condensation reaction leading to coupling of the IBMA moieties. This crosslinking markedly increases the viscosity of the SBR phase. so that during milling to produce the test sheets it locates in the core of the sheets. where the deformationis largely plug flow. The natural rubber constitutes most of the surface phase, so that its high autoadhesive capacity is most fully taken advantage of by incorporating the IBMA-SBR into the blend. While natural rubber is usually selected to impart autoadhesion to a blend, because of both its superior performance in this regard and its general utility and low cost, other elastomers can. of course. be employed for this purpose. Barager (1983) has reported that i n blends with chlorobutyl rubber. for example. polychloroprene produces a larger increase in tack than does natural rubber. Bettercocureadhesion Inay also be obtained by the blending of rubbers.A common example is the adhering of highly unsaturated rubbers to stocks of low unsaturation. Maksimova
Blends
Rubber-Rubber
I
219
and Shvarts (1984) reported that good adhesion could be obtained between blends of IR. BR, and chlorinatedbutyl rubber and blends of EPDM and butyl rubber only if the levelof chlorinated butyl rubber exceeded 75%. A reduction in the level of chlorinated butyl rubber and an increase in polyisoprene, on the other hand, gave, as expected, superior adhesion to SBR. Thenlagnitude of the adhesion in allcases could be influenced by the nature of the cure system. Barager (1983) found that the adhesion of epichlorohydrin rubber to unsaturated rubbers could be accomplished by blendingtheepichlorohydrinwith25-50phr of polychloroprene.Althoughfewstudies investigate the surface composition. it undoubtedly has a large role in determining the adhesion obtained with rubber blend stocks. This adhesion for pure rubber stocks has been shown to depend upon the number and length of the interphase, or interconnecting, strands (Bhowmick and Gent, 1984).
5.5 Hysteresis Low hysteresis may be obtained in a rubber stock by a variety of methods (e.g., reduced carbon black loading or higher crosslink density). which, however. are accompanied by a sacrifice of other aspects of performance, in particular the ultimate properties. Blending of elastomers, on the other hand, affords a means to achieve lower hysteresis with a more tolerable compromise of other properties. The hysteresis of a blend is often found to be lower than the weighted average of the components. This effect is most notable in filled systems in which the carbon black distribution is nonuniform. The phase with the lower carbon black loading will have both reduced modulus and lower hysteresis. Particularly when this softer phase is the continuous phase, a low blend hysteresis is observed (Hess and Chirico, 1977). The origin of this effect can be examined by introducing a nonuniform distribution of carbon black into a single-component stock by delaying addition of a portion of the polymer until after the carbon black has been well dispersed in the initial portion (Nguyen, 198 1 ). The hysteresis measured on a series of compounds prepared by thismethod is displayed in Figure 11, alongwiththeappropriately weighted sum of thehysteresisgenerated in twostocks with the full carbon black loading
0 20r
t 0
IO 20 RUBBER V O L . W O ) ADDED I N 2 n d STAGE
Fig. 11 The ratio of the dynamic loss and elastic moduli of an SBR with 105 phr carbon black, in which a portion of thc rubber was added afterthe black was mixed into the stock. The solid curve wascalculated by assuming lincar additivity of thc hystcrcsis measured for the corresponding filled and gum compounds. (From Nguyen. 198 1.)
220
Roland
0.201
l
I
20
0 VOL
C A R BBOLNA C K
J
40
(N-339)
Fig. 12 Thc phase anglc measurcd for SBR ;IS ;I function of carbon black loading. At the higher filler levels the mix quality becomes progrcssivcly poorer. (From Nguyen, l98 1.)
with without carbon black, respectively. It can be seen that most of the hysteresis reduction accompanyinganonuniformdistribution of carbonblackcan be attributed to thenonlinear relationship between hysteresis and carbon black loading, particularly at very high loadings (see Fig. 12). Another development in the area of hysteresis reduction is the finding of Keller (1973) that the addition of a small amount of chlorobutyl rubber (which itself is relatively hysteretic) to NR-BR blends resulted in tire treads with lower rolling resistance. Afragon et al. (1980)have patented a specific blend ratio of these rubbers to constitute a tread stock in which the decrease in hysteresis is not accompanied by the expected reduction in wet traction. Evidently the blend of a resilient rubberwith the halobutyl rubber produces a material with the appropriate frequency dependence of its energy loss, that is. high resilience at the low deformation rates associated with rolling and high hysteresis at the higher rates accompanying skidding on wet surfaces. Hirakawa and Ahagon ( 1982) have also reported that by introducing a nonuniform distribution of carbon black into these blends. further reductions in rolling loss can be attained.
5.6
Failure Properties
An improvement in tear strength. cut growth resistance, fatigue life. and ozone cracking resistance can all be realized from the blending of elastomers. including attainment of a level of performance that exceeds that of either pure component. Synergism in the strength properties of miscible blends might be expected from a potentially greaterchain density, but heterogeneous blends can likewise exhibit markedly superior ultimate properties. An importantaspect of rubber-rubberblends is the nature of theinterphasebonding. Although even the presence of voids (or a completely unbound dispersed phase) can toughen a material by reducing the stress concentration through blunting of the crack tip. the mechanical integrity of an intercrosslinked morphology will usually lead to superior performance. In blends of SBR and chlorobutyl, for example, a threefold increase in fatigue life was obtained by the introduction of interphase crosslinking (Bauer, 1982). Similarly, providing for interfacial coupling improves the tensile strength of EPDM-silicone rubber blends (Mitchell, 1985). Therelative
Rubber-Rubber Blends I
221
magnitude of the interfacial bonding compared tothe cohesive strengthof the rubbers themselves can influence the blend performance. Hamed (1982) has demonstrated that when the bonding is weak enough to promote deviation in the direction of crack propagation, blends of EPDM and BR exhibit greater tear strength than either pure component. When, on the other hand, the interfacial bonding is sufficiently strong, the crack is not deviated but proceeds through the particle. In this case the tear strength of the blend is found to be intermediate between those of the individual rubbers. Blends of cis-l. 4-BR and syndiotactic 1.2-BR prepared in a proprietary fashion are reputed to have exceptional resistance to tearing and cracking. There is no chemical bonding between the polybutadienes, and it has been suggested that an interpenetration of the phases is the origin of the property improvements (Buckler et al., 1982). When scrap rubber is blended with another elastomer, it is usually found that tensile strength and fatigue life are poor, due primarily to an absence of interphase crosslinking. When additional carbon black is incorporated into the blend, however. this interfacial adhesion increases along with substantial improvement in ultimate properties (Phadke et al., 1984). In multiphase rubber blends, the ultimate properties will to a large extent reflect those of the continuous phase. Low-temperature fatigue performancein particular will be radically altered by a phase inversionif the temperature is intermediate between the component T,s. The continuous phase must be elastic if the blend is to remain flexible. In general, a stronger continuous phase will give rise to a stronger blend. For example, when a nonuniform distribution of carbon black is present,greatertearresistance is found for NR-BR and NR-SBR blends when the reinforcingfiller is depositedprincipally in the continuous phase (Hess andChirico,1977). Similar effects on the cut growth resistance of rubber blends have been reported (Lee. 1982). When co-continuity exists in NR blends. the expectation is that greater strength will be obtained when the reinforcing filleris present in the second component, since the ability of NR to crystallize upon extension confers a measure of self-reinforcement that is lacking in rubbers such as BR and SBR. Blending of unsaturated rubbers with, for example, EPDM is a long-recognized method of obtainingresistance to ozone crackingwithoutthe use of staining or expensive antiozonants. Matthew ( 1984) has reported that in heterogeneous blends of this type a balanced distribution of filler is necessary for the greatest ozone resistance. The tendency for the continuous phase of a blend to be the lower-viscosity material, and moreoverforthe high deformations existing on the surface of aflowingmass to cause the surface to be richer in the lower-viscosity component, can influence the performance of a blend subjected to flexure. The component of the blend vulcanizate that was less viscous during the processing will be subjected to the large surface strains, while the more viscous rubber will be located more in proximity to the neutral axis. where strains are minimal. By controlling the relative viscosities of the component rubbers. improvements in tlex performance can in this manner be realized. It was pointed out in Section 5.2 that when the test temperature is intermediate between the T,s of the blend components, the modulus is largely influenced by the glassy phase. while the ultimate elongation may remain close to that of the rubbery material. A similar increase in toughness. or fracture energy. can be achieved with miscible blends of high and low molecular weight elastomers in which the network is achieved by end-linking the chains. These can be identical chemically. For example, when low. molecular weight(/)/.Po/yrr~.Sci. 21:1323. Araki and White ( 1997), Fluid Mecl~crnics.In press. Auchter, J. F. (1981). papcr presented at a meeting of the Rubber Division, ACS, October. Avgeropoulos. G. N., Weissert, F. C.. Biddison, P. H., and Boehm, G. G.A. (1976). Ruhher Clzern. Techno/. 49:93. Badum (1942), U.S. Pat. 2,297,194. Barentsen, W. M,, Heikens, D. and Piet, P. (1974). P o / y r ~ e r15:l 19. Bhowmick, A. K., and Inoue, T. (1993), J. Ap,u/. Polyru. Sci. 49: 1893. Bhowmick, A. K.. Chibn, T., and hove. T. (1993), J. App/, Po/yru. Sci. 50:2055. Bucknall, C. B. (1977), Torcgkerled Pltrstics. Applied Sciencc Publishcrs, London. Callan, J. E., Hess, W. M., and Scott, C. E. ( l 9 7 l ) , Rubber Chern. Techrlol. 44:814. Campbell, D. S., Elliot, D. J., and Wheelans, M. A. (1978), N R Techno/.9:21. Carman, C. J., Batiuk. M., and Herman. R. M. (1977), U.S. Pat. 4,046,840, Sept. 6. Chen, C. C., and White, J. L. (1993). P d y m En,?. Sci. 33:923. Choudhury, N. R., and Bhowmick, A. K. (1989). J. App/. P o / w . Sci. 38:1091. Chung, O., and Corm, A. Y. (1996). a paper presented and amecting of thc RubberDivision,ACS. Octobcr 8- 1 1. Chung, 0.. and Corm, A. Y. ( 1997). Rrrhlwr Cllerr~.Techno/. 70:781 Cimmino. S.. Dorazio. L., Greco, R., Magho, G., Malinconico, M., Mancarella, C., Martuscelli, E., Palumbo, R., and Ragosta. G. ( 1984). Po/ym. Enc?. Sci. 24:48. Coran, A. Y.. and Patel, R. ( 1976), J. App/, P o / y n . Sci. 20:3005. Corm, A. Y., and Patel, R. (1978). U.S. Pat. 4,104,210, Aug. 1. Coran, A. Y., and Patel, R. ( 1 9 8 0 ~ )U.S. . Pat. 4,183,876, Jan. 15. Coran, A. Y., and Patel, R. ( 1980a), Ruhher C/tem. Techno/. 53:141. Coran, A. Y., and Patel, R. ( 1980b). Rubher Chctn. T d ~ n o l53:78 . 1. Coran, A. Y., and Patel. R. ( 198 I a). Rlrhhrr Chcwl. Tecltnol. 54:9 1. Corm, A. Y., and Patcl, R. ( l981 b), Rrthhrr Chcm Techno/.54892.
318
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Coran, A. Y., and Patel, R. ( 1 9 8 1 ~ ) U.S. . Pat. 4,271.049, June 2. Coran. A . Y.. and Patcl. R. ( l982), U.S. Pat. 4.355,139. Oct. 19. Coran. A . Y., and Patcl. R. (1983a). Rttbbrr Clzrrrz. T e h o l . 56910. C o r m A . Y., and Patel. R. (198%). Rubher Chrnt. Techzol. 56:1045. Cora% A. Y.,a11d Pntcl. R. (1985). Rubher Chrrrz. Trchrrol. 58:xxx. Coran. A. Y., Das, B.. and Patcl, R. P. (1978). U.S. Pat. 4,130,535. Dec. 19. Coran, A. Y., Patel, R., and Williams, D. ( 1982a). Ruhber CIzprrt. Trchrzol. 55:116. Cora% A . Y., Patcl, R., and Williams, D. (1982b). Rtthher Churn. T e c h d . 55: 1063. Coran. A. Y., Patcl. R., and Williams-Hcadd. D. (1985). R~thDerCl~rrrr.Teclrrwl. 58: Corish. P. J.. and Powell, B. D. (1974), Ruhhrr Chrrrz. Trcltrtol. 4748 I . Crocker, G. I. ( 1969), Rttbhrr C h m . Tdzrzol. 42:30. Danesi. S.. and Porter, R. ( 1978), Po/yr)wr /9:448. Davies, W. E. A. (197 l), J. Phys. ( D ) 4318. Dunn, J. R. ( 1976), R~thhrrC l ~ e rTrchrrol. ~. 49978. Einstcin, A.. (1905), Arw. P I y . /9:549. Elemans. P. H. ( 1989). Modelling of the processing of incompatible polymcr blends, dissertation, Eindhoven University. Elcmendorp. J. J. (1985), Po/wrz. O r g . Sei. 25: 104 I . Elcmendorp, J. J. ( 1986). A study on polymer blending morphology, dissertation. Eindhoven Univcrsity. Ellul, M. ( 1998). Rrrhher Chrrrz. T r c h o l . 71:244. Emmctt, R. A. (1944). h d . D I ~ Cllern. . 36:730. Endo. S.. Min. K.. White, J. L., and Kyu, T. (1986), Po/yrz~.B I R . Sci. 26:45. Fischcr. W. K. (1973). U.S. Pat. 3,758.643, Sept. 1 I . Flory. P. J. ( 1953). Prirzciples ofPo/yrrzrr C/wmi.s/ry,Corncll University Prcss, Ithaca, NY, pp. 568, 576. Gnylord. N.G. ( 1975), Ad~v.Chrrrz. Srr. /42:76. Gesncr. B. D. ( 1969), Errc:\c/oprdict ofPo!\vrt. Sci. ttrttl Trchrzol.. Vol. 10 (Mark. H. F., and Gaylord, N. G. Eds.). Intcrscicncc Publishcrs, Wiley 6t Sons. Inc., New York, p. 694. Gent. A. N., and Lindley. P. B. (19%). Pmc. R. Soc. (Lorldnrt) A 2493195. Gent. A. N.. and Wong. C. (199 I ) , J. M~trer.Sci. 26:3392. Gcssler. A. M. (1962). U.S. Pat. 3,037,954, June 5 . Ghinm. F.. and Whitc, J. L. (1991). Polyrrz. Eng. Sci. 3/:76. Giller, A. ( 1966), Krtut. Gurwri K m s t s t . 19:188. Goettler, L. A., Richwine. J. R., and Willc, F. J. ( 1982). Ruhhcv Chenr. T c c h r d . 55:1448. Gotoh, K. (1970), in Pdyrrrrr Blerrds. Nikkan Kogyo Shinbun-sha, Tokyo. p. 109. Grace, H. P. (1982). Chcwz. Ertg. Corrzrrzwz. 14:225. Guth, E. J. (1945). J . AppI. Phys. 16:20. Hamcd, G. R. (1982), Rtthhrr C/rrrrz. Trclzrrol. 55:15 I . Han. P. K.. and Whitc, J. L. (l99S), Ruhlwr C/zrrrz. Tc.c./trzo/.68:728. and Shtrikmm, S. J. (1963), Mrch. Phys. Solids / I : 127. Hashin, Z.. Hclfand. E., and Sapse, A. M. (1975). J. Cltrrrr. Phys. 62: 1327. Holden, G.. and Milkovich. R. (1965). U.S. Pat. 3.265.766. Ide. F. and Hascgnwa, A. ( 1974). J . Appl. Pdyrrz. Sci. 18:963. Jha. A., and Bhowmick, A. K. (1997). Rubhcv Clzcwz. T e c h o l . 70:798. Jordhamo, G. M,, Manson, J. A., and Sperling, L. H. ( 1986), Po/.vrzz. Gig. Sei. 26517. Kolfoglou, N. K. ( 1983a). J. Mtrcrorrznl. Sci. -Phys. B22:343. Kalfoglou. N. K. (1983b). J. Mttcrorrtol. Sei. -Phy.s. B22:363. Kerncr, E. H. (1956), Proc. Phys. Soc. 69808. Kresgc, E. N.( 1978), Po/yrrwr BImtls, Vol. 2 (Paul, D. R. and Newmnn, S. Eds.), Acadcmlc Press, New York. p. 293. Kresge. E. N. ( 1984). J . A ~ / J / Po/yrrz. . Sei.: Appl. Pdyrrr. Syrrtp. 3Y:37. Laokijcharoen and Coran ( 1996),a paper presented and a meeting of the Rubber Division, ACS, Louisville. KY. October 8- 1 I . Liang, B. R., White, J. L., Spruiel, J. E., and Goswam, B. C. (1983). J. AppI. Pdyrrt. Sei. 28:201 I .
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319
Lindsay, G. A., Singleton, C. J., Carman, C. J.. and Smith, R. W. (1979), Multiph~sePolyrt~ers.(Cooper, S. L. and Estcs, G. M., Eds.), American Chemical Society, Washington, DC, p. 367. Matsuo, M,, Nozaki, C., and Jyo, T. (1969). P o l y n ~ B. 7 g . Sei. 9(3):197. 2: 1 12. Mikami, T., Cox, R. G., and Mason, S. G. (1975), I n / . J. Mulfiphcrse Min. K., White, J. L., and Fellcrs, J. F. ( 1984), Polyrn. Ens. Sei. 24: 1327. Minoura. M. Veda, Mizunuma, S., and Oba, M. (l969), J. Appl. P o l w l . Sci. 13:1625. Mooney, M. J. ( 195 1 ). J. Colloid Sci. 6:162. Morris, H. L. ( 1 9 7 7 ~U.S. Pat. 4,031,169, June 21. Morris. H.L.(1979). in Hmdbonk nf Tl~er~nr~pl~rstic Eltrstorner.~(Walker, B. M,, Ed.), van Nostrand Rclnhold, New York, p. 5. Neb, R. G., and Chen. A. T. (1996), in Tlrermnplmtic~Ek~~.ston~ers. 2nd ed. (Holden. G.,Lcgg, N. R., Quirk, R., and Schroeder. H. E., Eds.), Hanser, New York. p. 191. Nielsen, L. E. (1974), Rl~rol.Act(/. /3:86. Nielson, L. E. (195 I), Rev. Sei. Instr. 22:690. O’Connor, G. E., and Fath, M. A. (1981), Ruhber Wnrld, December. O’Connor, G. E., and Fath, M. A. ( 1982), R~thherWorld, January. Polvrners. (Cooper. S. L., and Estes, G. M., Eds.), Olabisi, 0.. and Famam, A. G. (1979). Mrdfi[~l7~1.s~ American Chemical Soc., Washington, DC, p. 559. Olabisi, O., Robeson, L. M,, and Shaw, M. T. (1979).Pol~lnlrr-Pol~nler Miscibili~.Academic Press, New York, pp. 277, 321. Ouhadi, T., Shen, K. S., and Abdou-Sabet, S . (1996), SPE ANTEC Tech. Papers 42:3376. Patterson, H. T., Hu, K. H., and Grindstaff, T. H. ( 1971 ), J. Polyn7. Sci. C (34):3I. Paul, D. R. (1978). Polyrl7er Blends. Vol. 2 (Paul, D. R., and Newman, S.. Eds.), Academic Press, New York, p. 35. Polvmers (Cooper S . L. and Estcs, G. M,, Eds.), American Paul. D. R. and Barlow, J. W. (1979),M~rltiphr~se Chemical Society, Washington, DC, p. 315. Paul, D. R., and Barlow, J. W. (1980). J . Mac.ron~ol.Sei.-Rev. Mocrornol. Chem. C18:109. Payne, M. P,, Wang, D. S. T., Patel, R. P,, and Sasn, M. M. (1990). a paper presented at a Kubber Division, ACS Meeting, Washington, DC, Oct. 10-12. Paync, M. T.. and Rader, C. P. (1992). in Elrrstorr~erT e c h o l o g y Harldbook, CRC Prcss, Boca Raton, FL, Chap. 14. Puydak, R. C. and Hazelton D. R. (1988), P h i . Ens. 4437. Rwei, S. P,, and Manas-Zloczower (1990), P n l w l . E q . Sci. 30( 12):701. Schollenberger. C. S., Scott, H., and Moore, G. R. (1958). Rubher World 137549. Polym. Bull. 26:341. Scott, C. E., and Macosko, C. W. ( 1 9 9 1 ~ Smallwood. H. M. (1944). J. Appl. Pl7y.s. /5:758. Stchling, F. C., Huff, T., Speed, C. S.. and Wissler, G. (1981), J . Appl. Polvn~.Sei. 26:2693. Takayanagi, M. (1962), P/tr.sric.s 13:I . Takayanagi, M., Harlma, H., and Iwala, Y. (1963). Mern. Soc. B I ~K.y u s l ~ uUr7iv. 23( 1):l. Taylor, G. I. (1932). Proc.. Roy. Snc. A138:41. Taylor, G. I. (1934). Proc. Roy. Soc. A146:501. Thelamon, C. (1963). Rubber Clwrr~.Techrzol. 36:268. Tsai, H. Y., and Min. K. (1997), Polyrl~.Ens. Sci. Tsai, W. S . ( 1 9 5 8 ~Formulasfor the clasticproperties of fibers-reinforcedcomposites, A. D. 834851. June. Vander Meer. S. (1943), Rev. Gerf. Cnourch. P l m . 20:230. Wagner, H. L., and Flory. P. J. (1952). J. Am. C l ~ e mSoc. . 74:195. of Tlwrtrlnp/a.sticElnston~ers, 2nd ed., Van Walker,B. M,, and Rader, C. P. Eds. (1988), Hrrr~dl~ook Nostrand Reinhold, New York. White, J. L.. and Min. K. (1985). in Po/.vrrrer Bletuls m7d Mixtures (Walsh, D. J.,Higgins. J. S., and Maconnachie, A., eds.). NATO AS I Series No. 89, Martinus Nijhoff. Wragg, R. T.. Yardley, J. F., and Nightengale A. F. (1981), Kaursch. G u n ~ n Kur~stst. ~i 34:657. F l o ~ l
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Thermoplastic Styrenic Block Copolymers Geoffrey Holden Holden Polymer Consulting, Incorporated, Prescott, Arizona
Charles R. Wilder* Phillips Petroleunl Company, Bartlesville, Oklahorna
1. INTRODUCTION Synthetic rubber has a long and illustrious history. However, it wasn’t until World War I1 that synthetic rubbers became the“workhorses” of the industry that they are today. With the shortage of natural rubber it became a necessity for the rubber industry to develop a substitute. With a combined effort that reached across company boundaries, emulsion-polymerized styrene-butadiene rubber in today’s form was created. This copolymer,which was to be known as emulsion SBR, became the basic polymer for the tire industry as well as many other segments of the rubber industry. Butadiene and styrene have been combined in many ways to make rubbery copolymers. The emulsion SBR copolymers had a random distribution of the two monomers. Modification of emulsion SBR is mainly possible through changes in molecular weight and butadienektyrene ratios. Other changes in the polymer were possible through the use of different polymerization temperatures,initiatorsystems.andfinishingtechniques. In more recent years,styrene-butadiene-based copolymers have been produced in solution polymerization systems that have allowedmuchmore control.Someexamples aremicrostructure (cis, frans, and vinyl ratios), molecular configuration, molecular weight distribution, and combination of comonomers. The discovery and development of the new polymerization systems have led to greater versatility in the preparation of elastomers. These systems havebeen applied to several monomers. principally styrene. butadiene, and isoprene. Developments worthy of note are selectively structured polyisoprenes (synthetic natural rubber and balata), polybutadienes (polymers of high cis, trans, and vinyl content), and random copolymers containing styrene. Probably the most interesting polymers to come fromthe technology of solution polymerization are the styrenic block copolymers as described by Haws ( 1974). Legge (1985). and Holden and Legge ( 1996). Polybutadiene-polystyrenetapered diblock copolymers havebeen produced since the early 1960s and have been described by Crouch and Short ( 1961) and Railsback et al. (1964). These block copolymers havebeen used to blend with other polymers to provide blends with improved
322
Holden and Wilder
-
-
Polystyrene Domain Elastomer Mid Segment
Fig. 1 Morphology of styrenic block copolymers.
properties, including combinations with rubbery polymers and plastics. They also found use as thesolepolymer in vulcanizedrubber compounds. The applications of thesetaperedblock copolymers have been described by Kraus and Railsback (1974). Other interesting block copolymers are thosewith multiple blocksof polystyrene connected by segments that are essentially rubbery in nature. Polymers of this type exhibit high strength and elastomeric characteristics without theuse of vulcanization chemicals-i.e., they are thermoplastic elastomers. Their thermoplastic nature can be explained by a simple example. Consider an S-E-S structure. with the S segments blocks of polystyrene separated from each other by an elastomeric segment designated asE. If the elastomer is the main constituent, the polymer should have a morphology similar to that shown in Fig. 1. Here, the polystyrene end segments form separate spherical regions, i.e., domains, dispersed in a continuous elastomer phase. Most of the copolymer molecules have their polystyrene end segments in different domains. At room temperature, these polystyrene domains are hard and act as physical crosslinks, tying the elastomeric midsegments together in a three-dimensional network. In some ways. this is similar to the network formed by vulcanizing conventional rubbers using sulfur crosslinks. The difference is that in thermoplastic elastomers. the domains lose their strength when the material is heated or dissolved in solvents. This allows the polymer or its solution to flow. When the material is cooled down orthe solvent is evaporated, the domains harden and the network regainsits original integrity. Branched block copolymers with the structure (S-E),,x (where x is a junction point with a functionality of n) have similar properties. However, block copolymers with only one hard segment (e.g.. S-E or E-S-E) have quite different properties. The elastomer phase cannot form a continuous interlinked network since only one end of each elastomer segment is attached to the hard domains. These polymers are not thermoplastic elastomers, but are weaker materials similar to wnwlcunized synthetic rubbers. The structure of styrenic block copolymers, without associationinto domains, is illustrated in Fig. 2. Figure 3 illustratesthese samepolymers withthestyreneblocksassociated into domains.
-
Thermoplastic Styrenic Block Copolymers
L I NEAR
RAD I AL
f
323
POLYSTYRENE
(TRI-CHAIN)
(TETRA-CHAIN)
Fig. 2 Linearandradial
LINEAR
5
polystyrene-polybutadiene block copolymers.
S-8-S
W
Fig. 3 Structure of linearandradial the polystyrene blocks.
polystyrene-polybutadietle blockcopolymersafternssocintion
of
propylene chloride)
Holden and Wilder
324
Table 1 Comparison of Thermoplastic Elastomers with Conventional Plastics and Rubbers
Thermosetting
T
Rigid Phenol-formaldehyde Urea-formaldehyde High-density polyethylene Flexible Highly filled and/or highly vulcanized Poly(viny1
Rubbery
rubbers
Vulcanized rubbers (NR, SBR, IR, etc.)
Low-density polyethylene EVA Plasticized PVC Thermoplastic Elastomers
The use of styrenic block copolymers has significantly increased since they were first produced about 35 years ago. An article by Reisch ( 1996) estimated their worldwide annual consumptionatabout 500,000 metric tons per year in1995, and thiswasexpected to rise to about 700,000 metric tons per year by 2000. In commercial applications. four elastomeric midsegments have been used-polybutadiene, polyisoprene, poly(ethy1ene-butylene), and poly(ethylene-propylene). The correspondingblock copolymers will be referred to as S-B-S. S-I-S, S-EB-S, and S-EP-S. The properties of thermoplastic elastomers in relation to other polymers are summarized in Table 1 . This table classifies all polymers using two characteristics-how they are processed (as thermosets or as thermoplastics) and the physical properties (rigid, flexible, or rubbery) of the final product. All commercial polymers used for molding, extrusion, etc.. fit into one of the six resulting classifications-the thermoplasticelastomersarethenewest. Their outstanding advantage can be summarized in a single phrase: they allow rubberlike articles to be produced using the rapid processing techniques developedby the thermoplastics industry. They have many of the physical properties of rubbers, e.g., softness, flexibility, and resilience. However. they achieve their properties by a physical process (solidification) compared tothe chemical process (crosslinking) in vulcanized rubbers. Two bookshavecovered this subject in detail. T/~errnoplasticElastonwrs (Holdenet al., 1996) concentrates mostly on the scientific aspects of these polymers, while Hunclhok of Thermoplastic Elrrstornrra (Walker and Rader, 1988) concentrates on their end uses.
2.
HISTORY
Development of homogeneous anionic polymerization systems in the 1950s provided new capabilities for controlling the course of polymerization, which in turn changed the synthetic rubber industry. Articles by Szwarc in Natlrre (1956) and the Jour-rlul cfthe Arr~ericanCherl~icrrlSociety (Szwarc etal., 1956) described the discoverythat the polymerization of styrene in tetrahydrofuran initiated by sodium naphthalene complex produced what he called a “living” polymer. After the initial volumeof styrene had been consumed, the red color of the polystyryl ion still remained. indicating that the reaction had not been terminated. The living polystyrene added not only additional styrene nlonomer to increase molecular weight. but other monomers as well.
Thermoplastic Styrenic Block Copolymers
325
Szwarc recognized the potential for producingblock copolymers by thismethod and succeeded in preparing block copolymers of styrene and isoprene using a sodium naphthalene complex. which produces a dianionic species:
3SSSSSSSS-
+
Isoprene Monomer
e- IIIIIIIIIIII-SSSSS-Illlllllllll
Lithium metal and lithium alkyls also initiate polymerization of styrene and dienes but produce mono-anions (Stravely, 1956; Foreman, 1969). Porter (1957) showed that S-B-S block copolymers could be produced using lithium alkyl initiator. Later Holden and Milkovich ( 1962) produced both S-B-S and S-I-S using lithium alkyl initiator and showed that the products were thermoplastic elastomers. Morton and Ells( 1962) found that during polymerization of butadiene and styrene in benzene, the values of the reactivity ratios were such that butadiene polymerizes preferentially. The addition of butadiene to a butadiene end is more likely than the addition of styrene to a butadiene end. Data from a Phillips Petroleum Company patent (1958) describing the polymerizations of styrene. butadiene. and a 25/75 styrene-butadiene mixture in cyclohexane show how the amount of boundstyrenevaries with conversion (Fig. 4). Very little styrene monomer is polymerized into the polymer chain during the early stages of polymerization of a styrene-butadiene mixture. As the butadiene monomer is depleted, more styrene monomer is added to the polymer chain, and a section of the molecule becomes tapered. The tapered section, which contains both butadiene and styrene units, is short. Polymers prepared in these systems differ little from pure block copolymers polymerized by sequential polymerization of two monomers. The first commercial production of styrenic block copolymers began in 1963. when Phillips Petroleum Company introduced solution-polymerized tapered S-B materials under the Solprene trade name. As noted above, these were not thermoplastic elastomers but were used in blends and conventional vulcanizates. In 1965 Shell Chemical Company introduced a series of S-B-S and S-I-S block copolymers. These were thermoplastic elastomers and initially had the trademark Thermolastic. which was later changed to Kraton. Some time later Phillips produced branched (S-B)x,, and (S-I)x,,block copolymers that were also thermoplastic elastomers.
BOUND SNRENE. X 24
-
20
-
16 -
12 8 4
J
I ( I I ( J I I l ( l 10 20 30 40 50 60 70 80 90 100 CONVERSION, X Fig. 4 Bound (i.e.. polymcrizcd) styrene versus conversm durlngpolymcrizntlon of of styrenc and butndicnc monomcrs.
;I
25/75 mixture
Holden and Wilder
326
A chapter by Holden and Leggein a recent book (Holden et al.. 1996) and a paper presented by Legge (1985) at a meeting of the ACS Rubber Division describe much of the history of the development of Shell's thermoplastic elastomers.
3.
MANUFACTURE
Styrenic block copolymers are usually produced commercially in polymerization systems that employ a butyllithium initiator in a hydrocarbon solvent. Since the reaction involves anionic species, there are no termination reactions from interaction of polymer chains as there are in free radical systems. Initiation is very rapid, and since the polymer chains grow at the same rate. the molecular weightof the polymer(M,,)is determined by the amount of initiator employed:
M" = weiaht of monomer moles of initiator If oxygen, moisture, and other reactive nlaterials are rigorously excluded, the individual polymer anions persist indefinitely. After polymerization is complete, the reaction product can be deactivated by the addition of a protonating species such as alcohol. Solvent is removed by steam-stripping or devolatilizing in an extruder or other equipment. The finished products are marketed as crumb, bale, pellets, or powder.
3.1
S-B and S-l Diblock Copolymers
The simple diblock copolymers can be produced by two routes, either by sequential polymerization or by mixing monomers having different reaction rates. In the case of S-B diblock copolymers: Seque~~tial polstnerixztior~:
Styrene monomer SSSSSSSS-
+ initiator + SSSSSSSS-
+ butadiene monomer + SSSSSSSBBBBBBBBB-
Diflerentiul reaction rates:
Styrene monomer BBBBBBSB-
+ butadiene monomer + initiator +- BBBBBBSB-
+ styrene monomer + BBBBBBSBSSSSS-
Sequential polymerization produces pure block copolymers. since the second monomer is added only after the firstone has been completely polymerized. Tapered block copolymers are produced by polymerization of a mixture of monomers that have differential reaction rates. Processes to produce tapered block copolymers have been described in both Phillips (1058) and Firestone (1966) patents.
3.2
S-B-S and S-I-S Triblock Copolymers
Triblock copolymers of the S-B-S or S-I-S type are produced commercially by sequential polymerization. by coupling of S-B or S-I anions. or by multifunctional initiators. In the case of S-B-S triblock copolymers:
327
Thermoplastic Styrenic Block Copolymers
S c c / r / l ~ r / l r t rP/ o l ~ ~ / r r c ~ / ~ r , t r r i c , r r
Styrene monomer
+ initiator
SSSSSSS-
+ D
SSSSSS- + butadiene monomer ”+ SSSSSSBBBBBBBBBSSSSSSBBBBBBBB’ + styrene monomer + SSSSSSBBBBBBBBBSSSSSS-
-
I
1,2 I -CH,-CH=CH-CH2-CH,-CH1,4
‘‘
I
I
CH
II CH,
Polvbutadiene
E
‘
B
I
---> -CH2-CH,-CH2-CH2-CH,-CHl 1
I I Polv(ethv1ene-butvlene)
Holden and Wilder
328
In the same way, S-EP-S block copolymers can be produced by hydrogenating S-I-S precursors (Bhattacharya et al., 1993) and have similar properties to S-EB-S analogs. A more complete review of anionic polymerization is given in a book by Hsieh and Quirk ( 1996). Styrenic block copolymers are commercially available as diblock copolymers, triblock copolymers, diblockhriblock mixtures, and branched copolymers. They aremarketed as (l) pure (nonextended or neat), (2) oil-extended. or (3) fully compounded products. Trade namesof some manufacturers are given in Table 2 .
4.
CLASSIFICATIONANDSTRUCTURE
In styrenic block copolymers, several structural variations
4.1
are possible.
MolecularWeight
Compared to similar homopolymers. the melt viscosities of styrenic block copolymers are very high and unusually sensitiveto molecular weight. Both these effects are caused by the persistence of the two-phase domain structure in the melt and the extra energy required to disrupt it during flow. For these reasons the pure styrenic block copolymers are often difficult to process. and in practical use compounded products are always used (see later).
4.2
PolystyreneSegmentContent
The hardness of these styrenic block copolymers depends on the ratio of the hard polystyrene phase to the softer elastomer phase and so can be varied within quite wide limits. As the ratio of the S to B segments in an S-B-S block copolymer is increased, the phase morphology changes from a dispersion of spheres of S in a continuous phase of B to a dispersion of rods of S in a continuous phase of B. As the proportion of S increases still further, a lamellar or “sandwich” structure is formed in which both S and B are continuous (see Fig. S ) . If the proportion of S is increased still further, the effect is reversed in that S now becomes disperse and B continuous. As the polystyrene phase predominates, the block copolymergets harder and stifferuntil eventually it becomes a clear flexible thermoplastic (e.g., Phillips K-Resin).
4.3
Diblock Content
Many styrenic block copolymers contain significant amounts of diblock. This is usually the result of incomplete coupling during production. The diblock makes the product softer, weaker, and less viscous. For some purposes (mostly adhesives and sealants) this diblock content is desirable, and materials with up to 80% diblock are produced commercially.
4.4
ElastomerSegments
Analogous S-B-S, S-1-S, S-EB-S. and S-EP-S block copolymers have somewhat different properties (Table 3). The differences in therelativestiffness of thesepolynlersarerelated to the difference in the degree of entanglements i n the three types of elastomer segment.Poly(ethy1enebutylene) and poly(ethy1ene-propylene) are very similar and will be considered together. They are the most highly entangled and so have the most effective crosslinks per unit volume of polymer, thus giving them the highest modulus. In contrast, polyisoprene is the least entangled and so S-I-S block copolymers arethe softest of the three types.All these differences are reflected
Thermoplastic Styrenic Block Copolymers
”
a
a W.
W
8 m 8-mmmm 8mm 8 m m m m
x
C
E
L
U
m
m
e,
I
.-U
c,
C
x m
5 .-
n
x
F
329
330
Holden and Wilder
A
Spheres
A Cylinders
A. B Lamellae
B
B
Cylinders
Spheres
Decreasing B-Content Fig. 5 Morphology changes with composition in A-B-A block copolymers.
in the end uses. S-B-S polymers are often used to make lower-cost products where stability is not critical (e.g., footwear).S-I-S analogs are softerand stickier and are mostly used in adhesives. S-EB-S and S-EP-S copolymers are the hardest of the three and themost resistant to degradation. Thus they are used where high stability is required (e.g., autoniotive parts and wire insulation).
5.
PHYSICALPROPERTIES
Properties of styrenicblock copolymers result from the number.length. and type of block segments. When two incompatible homopolymers are mixed, phase separation occurs, with one polymer usually becoming the dispersed phase and the other the continuous phase. We see a similar effect i n styrenic block copolymers. This presence of the two-phase structure in styrenediene copolymers has been verified by electron microscope studies reported by many workers, including Vanzo (1966). Hendus et al. (1967), Bradford and Vanzo (196X), Meier (l969), and Bi andFetters (1975). Very small areas, or domains, of polystyrenedispersed in arubbery matrix can be seen. The domains are so small that they do not refract light; hence the block copolymers are transparent, although a blend of the same homopolymers would be translucent or opaque. This two-phase system gives a structure with the polystyrene segments associating into hard "domains" connected by flexible elastomer chains. In triblock copolymers, this results in an elastomeric network being formed without the necessity of forming crosslinks by conventional sulfur/accelerator curing systems. Styrenic triblock copolymers havehigh tensile strength, resilience, and coefficient of friction. When heated, the polystyrene domains soften and the polymer will flow under pressure. Similarly, in a suitable solvent (e.g., toluene) both the end and center segments dissolve to give low viscosity solutions. When the solvent is evaporated the network reforms and the polymer regains its original properties.
Table 3 Comparison of S-B-S. S-I-S. S-EB-S, and S-EP-S Block Copolymers Relative stiffness Relative S-B-S S-I-S S-EB-S and S-EP-S
cost
1
1
0.5 2
1.3 2 to 2.5
Stability Degradation Moderatc Moderate Excellent
product Crosslinking Chain scission Chain scission
Thermoplastic Styrenic Block Copolymers
Besides high strength,elasticity.resilience,andthermoplasticity.thestyrenic copolymers have other advantages:
331
triblock
They are elastomeric. Their scrap can be recycled. They have high tensile strength at moderate temperature. They can be prepared or compounded to give a wide range of product properties. They possess a high coefficient of friction. Vulcanization is not needed, and so there are no vulcanization residues. They have no color. They are thermoplastic. They have good low-temperature properties. Their die swell is low. Although the styrenic block copolymers have all the advantages listed above, there are limitations. Tensile strengthand hardness decrease as temperature increases, with strength and hardness becoming low as the softening point of the polystyrene phase is approached.
5.1 TensileStrength The morphology of styrenic block copolymers accounts for ( I ) the decidedly different tensile properties of S-B copolymers compared to those of S-B-S analogs and (2) themoresubtle differences between the properties of the radial (S-B),,x copolymers and those of allalogous S-B-S linear copolymers. S-B copolymers are clusters of molecules formed by association of the polystyrene segments with polybutadiene chains extending from a central polystyrene domain. One end of each polybutadiene chain is firmly attached to the polystyrene domain. However, the remaining part of each polybutadiene chain is attached to the other chains only by weak attractive forces and by chain entanglements. Thus, uncured S-B copolymers have little tensile strength and must be cured to provide the properties usually associated with rubbery products. In contrast to the S-B copolymers, the S-B-S copolymersof either the linear or radial type exhibit tensile properties in the uncured state that are typical of good-quality vulcanized rubber. The unusually high strength of these polymers results from the extensive network created as the polystyrene domains tie down both ends of the polybutadiene chains. In addition to the domains acting as effective crosslinks. permanent chainentanglements areformed in the rubbery portion of the block copolymer. In the case of radial (S-B),,xblock copolymers. the site at which the coupling agent connects the individual polymer segments is also an effective crosslink. An increase in molecular weight resultsin an increase in the service temperatureof styrenic block copolymers. An example of this is for S-B-S block copolymers, shown in Fig. 6. Morton et al. (1969) reported that the stress-strain properties of S-B-S block copolymers containing 20-40 wt% polystyrene and with molecular weights in the range of 60,000-150,000 show little effect of molecular weight when compared at constant composition. Holden et al. (1968, 1969) reported similar results on three S-B-S copolymers with about 27% polystyrene content and molecular weights ranging from 73,000 to 150,000. In contrast. Zelinski and Childers (1 968) found that at higher molecular weights, increasing molecular weight causes an increase in tensilestrength. These apparentcontradictions may beresolved asfollows. Below some critical molecular weight for the polystyrene segments.an increase in molecular weight improves tensileproperties.However, once thiscriticalmolecularweight has been exceeded, further increases have negligible effects. Styrenic blockcopolymers having the best balanceof rubber-like properties usually contain 15-40 wt% polystyrene. Below this range the tensile strength is reduced. Above this range, the
332
Holden and Wilder
TENSILE STRENGTH, M Pa
MOLECULAR
30
20
l0
27 49 TEST TEMPERATURE.
60 C
Fig. 6 The influcncc of molccular weight and temperature on thc tensile strength of S-B-S block copolymers.
block copolymers have yield pointsandexhibitcolddrawproperties.Blockcopolynlers of the S-EB-S typehavebeendescribedandcharacterized by Gergen(1985, 1996). One area of particularinterest is the comparison of tensilepropertiesexhibited by the S-B-S and SEB-S block copolymers. Of the two, the S-EB-S exhibits higher modulus, which is attributable to its higher association energy, greater degree of elastomer chain entanglement, and absence of substantialinterfacevolume. The elongation of the S-EB-S block copolymer is lower than that of the S-B-S equivalentbecausethe S-EB-S hasthesmaller contour length. The rate of tensile loss with increase in temperature is much less for the S-EB-Scopolymer than for the S-B-S copolymer. This difference in the temperature-tensile strength relationship allows the S-EB-S block copolymer to be used forapplications in which similarS-B-S block copolymers would not besuitable.
5.2 Hardness Hardness of styrenic block copolymers isstrongly dependent onpolystyrene content. Thiseffect is shown in Table 4 for a series of S-B-S block copolymers.
5.3 Melt Flow Melt flow of S-B and S-B-S block copolymers has been studied by Kraus and Gruver (1967) and Holden et al. (1968, 1969). They reported the melt viscosities of block copolymers to be much higher than those of either the corresponding homopolymers orthe corresponding random copolymers. The higher viscosities are attributed to the persistence of the network in the melt. Van der Bie and Vlig (1969) reported that the effect of temperature on melt viscosity is much more pronounced at low shear rates. Processing under low-shear conditions should be done at
333
Thermoplastic Styrenic Block Copolymers Table 4 Effectof Styrene Content ontheHardnessof Copolymers Molecular Shore weight wt% Styrene
S-B-S Block A hardness
13 21.5 39 53 65 80
93,000 7 3,000 84,000 72,000 73,000 83,000
41 63 89 92 96 98
the upper end of the temperature range. High-shear processing, such as injection molding, can be done over a wider temperature range.
5.4 Stress Softening Repeated extension of conventionally crosslinked or cured elastomers leads to lower stresses at the same extension. This stress softening is also observed with S-B-S block copolymers. Zelinski and Childers (1968), Childers and Kraus (1967), and Holden et al. (1968, 1969) all reported that S-B-S block copolymers with styrene content above 30% exhibit yield points and stress softening. With increasing styrene content, interconnections between domains increase, and the yield point occurs as this network is broken. In block copolymers that do not exhibit yield points, stress softeningoccurs when polystyrene segments are pulled out of the domains. Stress softening of S-B-S block copolymers is reversible; if the sample isrelaxed or annealed at elevated temperatures, the original stress-strain properties are regained.
5.5 Glass Transition Temperature Styrene-butadiene random copolymers exhibit single glass transition temperatures intermediate between those of polystyrene and polybutadiene. In contrast, block copolymers of polybutadiene and polystyrene with sufficiently long blocks exhibit two glass transition temperatures that are close to the glass transition temperatures of the individual homopolymers (see Fig. 7). 5.6
Environmental Resistance
Environmental resistance of polymers can be subdividedinto three classes: resistanceto degradation produced by oxygen, ozone, and UV light. In styrenic block copolymers, resistance to all three types of attack mostly depends onthe elastomer segment. Thus,for those that haveunsaturated midblocks, their stability is similar to that of polybutadiene or polyisoprene. In contrast, those with poly(ethy1ene-butylene)or poly(ethy1ene-propylene)midblocks show the better stability associated with EPR or EPDM rubbers. Hindered phenols in combination with thiodipropionate esters are usefulantioxidants.Zincdibutyl dithiocarbamate improves high-temperature properties. Ozone resistance of the unsaturated styrenic block copolymers can be improved by compounding with ozone-resistant polymers such as EVA or LDPE. UV resistance of styrenic block copolymers can be improved by addition of stabilizers such as benzotriazoles andor by compounding with pigments such as carbon black or titanium dioxide. Application of these technologies will be discussed in Section 6.
334
Holden and Wilder
r ses " "
\
l60
200
\
"" 200
240
S6R
3 20
360
L
1
Ternperohre, K'
Fig. 7 Viscous damping of an S-B-S block copolymcr andan SBR random copolymer.
5.7 AbrasionandFlexResistance The abrasion and flex resistance of the compounded styrenic block copolymers is sufficient for many applications. An example of a suitable use would be in shoe soles. A balance of resistance and processability can be obtained by the incorporation of compounding ingredients such as polystyrene, oils, and filler. Flex resistance is better at lower temperatures; this is a complete contrast to most competitive materials. In compounds based on S-B-S and (S-B),x block copolymers, those with higher molecular weight polystyrene segments show the best flex resistance.
5.8 Coefficient of Friction The frictional properties of compounds based on S-B-S and (S-B),,x block copolymers are in the same range as vulcanized rubbers such as natural rubber and emulsion SBR. Their values are about 50% higher than those of flexible plastics of similar hardness (Holden, 1973).
5.9 Shear Resistance in Adhesives Higher styrene content, higher molecular weight, and branching all improve the resistance of the styrenic block copolymers used in adhesives to failure under shear. Higher molecular weights and styrene contents increase viscosity; however, branching (at constant arm length) has little effect on this property. At similar formulation viscosity there is an increase in shear resistance as the structure changes from linear (di-branched) to tri-branched to tetra-branched. This effect was shown by Man's et al. (1973), and more details are given in Table 5.
5.10
PeelStrength in AdhesiveFormulations
Peel strengths follow the same general trends as shear strengths. The best peel strengths are obtained from styrenicblock copolymers with relativelyhighmolecularweightpolystyrene
335
Thermoplastic Styrenic Block Copolymers Table 5 S-B-S and (S-B),,XBlock Polymers at Equal Formulation Viscosity (7000 Butadiene-
Styrene Copolymers) Formulation resistance viscosity Shear Molecular structure weight Chain ~~
(Pa5)
(hours to fail a t 90°C)
84,000 136,000
1 .S2 1.S8
IX2.000
I .x2
I .o 2.4 2.8
~~
Linear Trichain Tetrachain
segments. However, this increase in molecular weight or styrene content may be limited by the desired tack and viscosity.
6. APPLICATIONS Likemostconventionalvulcanized rubbers-and unlikemost thermoplastics-the styrenic block copolymers arenever used commercially as purematerials. To achievetheparticular requirements for each end use, they are compounded with other polymers, oils, resins, fillers, etc. In almost all cases, the final products contain less than 50% of the block copolymer. Thus, a study of their end uses is in effect a study of how they are blended to achieve the properties needed for the particular application. Before discussing the end usesin detail, it is important to consider how the added materials are distributed with respect to the two phases in the block copolymer. For any additive, there are four possibilities. It can go intothe elastomerphase. I n this case theadditiveincreasestherelative volume of the elastomer phaseand so makes the product softer. The addition can also change the glass transition temperature of the elastomer phase. This in turn affects such properties as tack and low-temperature flexibility. 2. It can go into the polystyrene phase. In this case the additive increases the relative volume of the polystyrene phase and makes the product harder. The glass transition temperature of the additive should be similar to or greater than that of polystyrene (100°C). If not, its addition will reduce the high-temperature performance of the final product. 3. It canform a separatephase.Unless the molecularweight of theadditive is much less than that of either type of segment in the block copolymer, this is the most likely outcome. Thus, only oils and low molecular weight resins are compatible with either of the existing two phases. Higher molecular weight(i. e., polymeric)materials generally form a separate third phase. This polymeric third phase is usually co-continuous with the block copolymer. Therefore, it confers someof its own properties(e.g., higher upper service temperature, improved solvent resistance) on the final blend. 4. It cango into both phases.Additives that do so areusuallyavoidedbecause they reduce the degree of separation of the two phases and so weaken the product.
1.
Blending with such a widerange of materials gives the styrenic block copolymers an exceptional variety of end uses, three of which are major:
I . Formed goods, i.e., replacements for vulcanizedrubberarticles 2. Adhesives,sealants,andcoatings 3. Polymer andasphaltmodification
Wilder
336 Table 6 CompoundingStyrenicBlockCopolymers
Effect on Oil/Solvent resistance Processability Hardness Component Oils
Polystyrene Polypropylene
Cost
Decreases Increases Increases
Improves Improves Improves especially
-
Improves
Decreases Decreases Decreases
-
Improves resistance
with S-EB-S
Slight increase Slight Filler
Other
-
density Increases Decreases
improvement
6.1 Formed Goods (Replacements for Vulcanized Rubber) This end use has been described in several books (Halper and Holden, 1988; Holden, 1996.)The products are usually manufactured by machinery used to process conventional thermoplastics. Examples are injection molding, blow molding, blown film, and profile extrusions. S-I-S block copolymers are not used very much in this application. However, there are many applications for compounded products based on S-B-S, (S-B),,x, S-EB-S, and S-EP-S block copolymers. A list of some compounding ingredients and their effects on the properties of the products is given in Table 6. Products that are as soft as 5 Shore A have been described by Deisler (1991). Others are as hard as 55 Shore D. Large amounts of compounding ingredients can be added. The final products often contain as little as 25% of the styrenic block copolymers. From an economic point of view, this is most important. For example, it enables compounds based on S-EB-S block copolymers to compete with those basedon polypropylene/EPDM or EPR blends, although the pure S-EB-S copolymer is much more expensive than the polypropylene. the EPDM, or the EPR. Polystyrene is often used as a compounding ingredient for S-B-Sand (S-B),,xblock copolymers. It actsasaprocessingaid and makes the productsstiffer.Napthenicmineraloils are excellent processing aids but make the products softer. Inert fillers such as calcium carbonate, talc. and clays are often used in these compounds. They haveonly a small effect on most physical properties but reduce costs. In some casesup to 200 parts (per 100 parts of the block copolymer) are used in compounds intended for footwear applications. Reinforcing fillers such as carbon black are not suitable for this application. Large quantities of such fillers make the final product stiff and difficult to process. Resins can adjust hardness and increase melt flow and adhesion. A combination of resins can be used to achieve a desired balance of properties but may reduce the rubbery feel of the product. The influence of resins in a variety of applications has been described by Haws and Wright (19761, Halper and Holden (1988), and Holden (1996). Addition of process oils to thermoplastic S-B-S block copolymers generally increases melt flow and flex life. Conversely, tensile strength, hardness. and abrasion resistance are reduced. Naphthenic oils have a minimum influence on tensile strength and hardness. In contrast, ester plasticizers and aromatic oils soften the polystyrene block of the S-B-S block copolymers and greatly reduce properties, and so they are not used. Only low levels of paraffinic oils should be
Thermoplastic Styrenic Block Copolymers
337
used, as these oils are less compatible with the polybutadiene phase and tend to bleed from the compound. S-EB-S and S-EP-S block copolymers can be similarly compounded. In this case polypropylene is the preferred polymeric additive, actingin two different waysto improve the properties of the compounds. First, it improves processability. Second, when the compounds are processed under highshear, the polypropylene and the block copolymedoil mixture form two co-continuous phases.Polypropylene is insolubleandhasahighcrystalmeltingpoint ( - 165°C). The COcontinuous polypropylene phase significantly improves both the solvent resistance and upper service temperatureof these compounds. Another advantageof S-EB-S and S-EP-S block copolymers is that because of their lower midsegment solubility parameter, they are very compatible with paraffinic mineral oils. Large amounts of these oils can be added without bleedout. and this allows very soft compounds to be produced. Napthenic oils are less compatible with the EB or EP center segments, and oils with high aromatic contents are not used. Blends of these block copolymers with mineral oils and polypropylene are transparent. This is probably because therefractive index of an S-EB-Soil or S-EP-S/oilmixturealmostexactlymatches that of crystalline polypropylene. Blends with silicone oils are used in some medical applications. Thesame inert fillers used in theS-B-S-based compoundsare also usedwiththe S-EB-S and S-EP-S analogs. In addition, barium or strontium sulfate fillers give very dense compounds for sound-deadening applications. In another development, fire retardants can be added. These compounds will qualify under many current regulations. Compounds based on styrenic block copolymers must be protected against oxidative degradation and, in some cases, against sunlight. Hindered phenols are effective antioxidants and are often used in combinationwiththiodipropionatesynergists.Benzotriazolesare effective UV stabilizers and are often used in combination with hindered amines. If the product does not have to be clear, titanium dioxide or carbon black pigments give very effective protection against sunlight. Phase modification of S-EB-S block copolymer by different additives and its effect on morphology, mechanical, and dynamic mechanical properties have been reported by Ghosh et al. (19%). Compounding techniques are relatively simple and standard. There is one important generalization-the processing equipment should be heated to a temperature at least 20°C above either the melting point of the polymeric additive or the glass transition temperature of the polystyrene segments, whichever is greater. The use of cold mills, etc. can result in polymer breakdown. Not only is this unnecessary, but it is also detrimental to the properties of the final product. S-B-S, S-EB-S, and S-EP-S block copolymers with high molecular weight polystyrene segments and/or high styrene contents are very difficult to process as pure polymers. Versions with oil contents from about 25% to about 5 0 8 are commercially available. These are easier to process, and of course more oil can be added during mixing. Extruders, internal mixers, open roll mills, or dry-blendingmethodscan be used to mix styrenicblock copolymers with the various compounding ingredients. Extruder. Mixing Unfilled or lightly filled compounds can be made on a single screwextruder fitted with a mixing screw. The length/diameter ratio should be at least 24: I . If large amounts of fillers or fire retardants are to be added, these can be dispersed on either a twin screw extruder or a closed intensive mixer that discharges into an extruder. BmDuty Mixirlg Internal mixers such as Banburys permit fast and efficient processing. A preheated mixer chamber will aid incorporation if large amounts of filler or oil are to be added. If large amounts of
Holden and Wilder
338
oil are to be used, it should be added in increments. Addition of the first portion of oil early in the mixing cycle may be beneficial to flux the ingredients. If a high-melting polymer such as polystyrene or propylene is to be added, it should be melted before the oil is added. To obtain good fluxingandpropermixingandblending,temperaturecontrol is important. The mixer discharge temperature should not exceed 160°C for S-B-S-based compounds and 200°C for S-EB-S-based compounds. A suggested Banbury mixing cycle for a high fillerhigh oil S-B-S compound, using a mixer preheated to 65-9S°C, is as follows:
0 min: Add all ingredients except oil.* 2-3 min: Scrape and add one fourth of the oil. When thepower load increases, add another increment of oil, and continue to add oil in this manner until all the oil is dispersed. 145-160°C: Discharge and sheet off on a roll mill with roll temperatures of 85-105°C. Trialbatches or small-scaleproductionrunscan be madeusingasmallbatch-typeclosed intensive mixer. Mixing times of about 5 minutes are usually adequate. After this mixing, the hot product is passed to a heated two-roll mill. When it has banded, it is cut off, allowed to cool, and then granulated. The granulator blades must be sharp and clearances minimized.
Open Mill Mi.xing Mixing on an open roll mill requires roll temperatures of about 100°C for S-B-S-based compounds and 170°C for S-EB-S compounds containing polypropylene. Smooth banding will not take place if roll temperatures are too cool, while the compound will stick to the rolls if they are too warm.
Dry Blending Dry blending is defined as the mechanical mixing of the components of a compound to give an even dispersion throughout the mixture but without causing the mixing temperature to rise above the melting temperature of any of the ingredients. This mixing procedure works best if the materials to be blended have about the same particle size. It is particularly desirable to have all polymeric material about the same particle size. Absorbed oil tends to cover the surface of the polymer particles and should bind the coating of fines, stabilizers, and powdered resins. The advantages of dry blending in comparison to other mixing techniques are that it: 1. Eliminatesinternalmixers 2 . Eliminates pelletizing by feeding the dry blend directlyto theplastic fabrication equipment 3. Reducesmixingtime 4. Reduces mixingcost
This technique requires that all the polymers in the blend are free-flowing powders. The polymers can bepurchased in this form, orthey can be groundon-site i n commercially available equipment. A typical dry-blend mix cycle for a Welex 8M mixer with a water-cooled jacket is: 0 min: Add all ingredients, except any resins melting below mixer speed of 90 Hz (1500 rpm).
110°C. Mix for 30
~
* For very high oil and filler levels,
a small amount of oil may be added at 0 min to wet the filler.
S
with
Thermoplastic Styrenic Block Copolymers
339
0.5 min:Reducespeed to 20 Hz (1200 rpm) and begin gradual oil addition. An extra mixing time of 45-60 s is suggested for 40-50 phr of oil. 1.5 min: Mix at 90 Hz ( 1 500 rpm) until the compound appears to be free-flowing. If powdered resins are to be added, add them after the mixture is free-flowing and mix an additional 30 S at 33-42 Hz (2000--2500 rpm). 4-6 min: Discharge the batch. The temperature of the compound should be less than 65°C to prevent softening and agglomeration of the ingredients. Pelleti:ir~gr r t ~ Chopping l
Compounded stock that is to be used i n plastics-processing equipment must be in a form that will feed to the machines. Dry-blended compounds are satisfactory in the form in which they are discharged from the dry blender. Compounded stock mixed in an internal mixer is either pelletized or chopped for ease of feeding into the processingequipment. Onemethod of pelletizing is to strip-feed the product into an extruder and then extrude through a multistrand die. After extrusion, the compound is fed to a pelletizer. This can be either a strand-cuttingor an underwater face-cutting system. If the first type is used, it is important to remember that rubbery compounds must be cut rather than shattered. Thus, the blades of the cutter should be sharp and the clearance between the fixed and the rotating blades should be minimized. With this type of pelletizer. the strands must be thoroughly cooled before they enter the cutter. A chilled water bath can be used to increase production rates. The processing and property advantages that give the compounds their value are: Ease of molding Temperature resistance Low cost Low density Softness Paintability Bonds to other polymers These compounds usually have relatively high surface friction. Ejection of the molded parts can be difficult. especially with softer products. Use of a release agent or a suitable coating on the mold makes for easier ejection. If possible the mold should be designed so that the ejection of the part can be air-assisted. Tapering the sides of the mold is also helpful, as isthe use of stripper rings. Small-diameter ejection pins should not be used, since they tend to deform the molded part rather than eject it. Ground scrap from molding is reusable and can be blended with virgin product. Grinding is quite easy if the conditions required for successful pelletization are met, i.e., if the grinder blades are kept sharp and the clearances minimized. Many end users prefer precompounded products, and numerous specialized grades have been developed. Some examples are products designed to make milk tubing. shoe soles, sound deadening parts, wire insulation, and tlexible automotive parts (Holden and Speer, 1988). After priming, the parts can be coated with flexible paints. If a compound contains a homopolymer (e.g., polystyrene or polypropylene), it will adhere well when insert-molded or extruded against this homopolymer oragainst other compoundscontaining it. This allowsthe production of parts having a rigid structure supporting a soft, flexible outer surface, as described by Holden and Sun (1991). The physical properties of compounded thermoplastic elastomers based 011 both styrenic block copolymers are affected by both the processing conditions and the processing equipment. Thus, it is most important to make test samples under conditions and on equipment similar to
Wilder340
and
Holden
Table 7 TypicalOperatingConditions for Injection Molding S-B-S-
01
S-EB-S-Based Compounds on Reciprocating Screw Machincs ~~
Mold type Cavities Shot weight, oz. Cylindcr temperatures,"F ("C) Feed zone Center zone Front zone Nozzle Mold temperature, "F ("C) Injection pressure. PSI" High Low Injection time, sec Injection rate Hold time, sec Clamp time, sec Screw rpm Back pressure, psi Cycle time, sec "
~~~
S-B-S-based compounds
S-EB-S-bascd compounds
2 Plate I 2
2 Plate I
2
1 IS (80) 350 (17s) 380 (195) 390 (200) 1s (25)
700
so0
700 S00
3
1
Moderate
Fast
S0
S 7 40 50
20
1s
S IO 30
These pressures are typ~calof those used wlth the more viscous compounds
those that will be used in production. Misleading results will be obtained if, for example, prototype parts or test pieces are compression molded when the actual products will be made by extrusion or injection molding. Processing conditions for these compounds have been discussed in some detail in Shell Chemical Company Technical Bulletin SC:455-96 (1996). Generally, compounds based on SB-S block copolymers are processed under conditions suitable for polystyrene. Those based on S-EB-S block copolymers are processed under conditions suitable for polypropylene. Typical conditions for injecting molding these compounds on reciprocating screw machines are given in Table 7. Various compounds have been developed for the production of blown and extruded (slot cast) film, including heat-shrinkable films. These were discussed in Shell Chemical Company Technical Bulletin SC: 1 105-90 (1990). These films are based on both S-B-S and S-EB-S block copolymers and can be very soft and tlexible. They also exhibit low hysteresis and low tensile set. A significant advantage is that they can be used in contact with skin or with certain foods. One unusual application is the use of solutions of S-EB-S/oil blends to replace natural rubber latex in the manufacture of dipped articles such as surgeon's gloves. This is described in a patent granted to Buddenhagen et al. ( 1 992). These blends have two advantages over natural rubber. First, they are more resistant to attack by oxygen or ozone. Second, natural rubber latex contains proteins that can produce dangerous allergic reactions in some people. These proteins do not occur in the extremely pure S-EB-S block copolymer.
341
Thermoplastic Styrenic Block Copolymers Table 8 Resins Ued to Formulate Adhesives, Sealants, etc., from Styrenic Block Copolymers Resin
compatibility
Polymerized CS resins (synthetic polyterpenes) Hvdromnated rosin esters Saturated hydrocarbon resins Naphthenic oils Paraffinic oils Low molecular weight polybutenes Aromatic I
-
Segment
I B EB I. B EB EB S
I = Compatible with polyisoprene segments; B = compatible with polybutadiene segments: EB = compatible with poly(ethy1ene-butylene) or poly(ethy1ene-propylene) segments; S = compatible with polystyrene segments.
6.2.
Adhesives,Sealants,andCoatings
These are very important applications for styrenic block copolymers and probably the fastest growing. They are often used in solution. In this case, some of their advantages are: 1. 2. 3. 4. 5.
Polymersare directly soluble-no millingnecessary. Solutiontimes are shorter-greater output. Solutionsare more uniform-better control. Solution viscosity is lower-higher solids level, lower solvent cost. Solutions are stable-longer shelflife.
Again, the products are always compounded, and the subject has been extensively covered by St. Clair (1982),Harlan et al. (1989), and Shell Chemical Company TechnicalBulletin SC: 19892 ( 1 992). As previously mentioned, the effectsof the various compounding ingredients depend on the region of the phase structure with which they associate. Since fourelastomers (B, I, EB, and EP) are used in these block copolymers, each has particular resins andor oils with which it is most compatible. Table 8 gives details of the various resins and oils suitable for use with each elastomer as well as with the polystyrene phase. Ingredients that go into both phases are not used, since they make the phases in the block copolymer more compatible with each other. This makes the product weaker. Polymers that form a separate phase (e.g., polypropylene) are used in some hot melt applications (see below). These polymers stiffen the products and improve upper service temperature. Fillers can also be added to reduce cost. The products can be applied eitherfrom solutions or as hot melts. The high tensile strength and ready solubility of styrenic block copolymers are important advantages in solvent cements and mastics. The existence of two separate and essentially incompatible segments in the same molecule should be taken into account when the styrenic block copolymers are used in solution. Aromatic solvents dissolve both the elastomer and the polystyrene. Thus they are good solvents for styrenic block copolymers. Aliphatic solvents dissolve only the elastomer blocks and hence will dissolve only those block copolymers whose styrene content is low. Blends of aliphatic and aromatic or polar solvents can be used. The necessary amount of polar solvent depends on the molecular weight and polystyrene content of the copolymer. In general, suitable solvents have solubility parameters between those of polystyrene and the elastomer (B, I, EB, or EP)
342
Holden and Wilder
Table 9 SolubilityParameters of Polymers and Solvents rl-Pentnne"." rl-Hcxane"." Poly(ethylcne-propylene) Poly(ethy1ene-butylene) Polyisoprcne Cyclohexane Polybut ;I d'lene Methyl isobutyl ketone Amyl acetate Toluene Xylcne Ethyl acetate Methyl ethyl ketone" Polystyrene Benzene 1,1,2.2-Tetl.achlorocthat~e Acetone"," lsopropyl alcohol""
7.0 7.4 7.7 7.8 8.I 8.2 8.4 8.4 8.5 8.9 9.0 9.1 9.I 9.I 9.2 9.7 10.0 11.9
(see Table 9). The molecular weights of these block copolymers are relatively low (typically 15O.OOO), and so the solutions can be made at high solids content. Some details of the solution behavior of styrenic block copolymers are given in Shell Chemical Company Technical Bulletin SC:72-85 (1985). Styrenic block copolymers have made possible the development of rubber-based hot-melt adhesives. This developmenthas been most importantto formulators faced with increased solvent cost and government restrictions 011 the release of solvents into the atmosphere. Low molecular weight additives such as hydrocarbon oils, resins, and polyisobutylenes may be used to lower the melt viscosity. They usually improve tack but may have a harmful effect on peel and shear strengths. In hot-melt applications the molten resins and/or oils may be regarded as taking the place of the solvents. Application rates of hot-melt products are usually faster than those of solventbased analogs. This is because the time for a product to cool is much less than the time for a solvent to evaporate. Some of the advantages and disadvantages of hot-melt and solvent-based applications are summarized in Table IO. The high temperatures necessary for mixing hot-melt adhesives require that the mixture be protectedfromoxidation during processing.Ablanket of carbon dioxide or nitrogen, i n combination with antioxidants, is useful in preventing degradation. S-I-S block copolymers are often preferred over S-B-S equivalents for hot-melt adhesives, because they are more tacky and are less likely to form gel during processing. Partial replacement of S-I-S with S-B in hot-melt adhesive formulations can give lower-cost compounds. These block copolymer blends retain good adhesive properties and adequate stability. Presslrre-Seilsiti\!e Adhesives
This is probably the largest single end use for styrenic block copolymers. These adhesives are usually applied as hot melts. Solvent application is also possible and takes advantage of the low
ore
343
Thermoplastic Styrenic Block Copolymers Table 10 Solvent-BascdversusHot-MeltApplication
Disadvantages ~~
Advantages
~
basedSolvent
Low Fl:mmability viscosity Toxicity High solids equipment Simple
Air pollution Drying time
Safcty Hot melt No pollution Fast set-up
Dcgradation
solution viscosity of these copolymers. The end uses include various kinds of tapes and labels as well as adhesive fasteners such as diaper tabs. The mechanism by which the resins and the styrenic block copolymers combine to give tacky products has been described by Kraus et al. (1977),Chu and Class (1985), and Halper and Holden (1988).According to this theory. the resins have two functions. First, they mix with the elastomer phase in the styrenic block copolymer and so soften the product. This softening allows the adhesive to conform to the substrate. This is considered as the “bonding” stage of adhesion and is relatively slow. The second stage is the removal of the adhesive from the substrate. This is the “disbonding” stage of adhesion and is much faster. Here the function of the resins is to adjust the glass transition temperature of the elastomer phase (i.e., the mixtureof the resins withthe midsegment of the styrenic block copolymer). This causes the adhesive to stiffen up and so resist removal from the substrate. Tack is maximized when the calculated glass transition temperature for the elastomer phase is about - 15°C. This is a good starting point for making trial formulations intended for room-temperature service. Since a soft product is necessary to form the adhesive bond. softer styrenic block copolymers are used to formulate pressure-sensitive adhesives. These soft styrenic block copolymers usually have low polystyrene contents and may contain significant amounts of diblock (i.e., S-I, S-B. S-EB. or S-EP). The diblock is non-load-bearing. It. and the resins in the elastomer phase. weaken and soften the adhesive. The weakening can be tolerated as long as it does not cause cohesive failure of the adhesive during service. Adhesiveshavebeendescribed by Erikson (1986) that can be crosslinkedafterbeing applied to the tape. This improves the solvent resistance of the adhesive. which is important for applications such ;IS masking tapes. AsserllDIy Aclllesives
In this application S-B-S and S-EB-S block copolymers are preferred. Again. hot-melt application is more usual than application from solution. Tack is not important (it may even be undesirable). and so harder products are satisfactory. These adhesives are usually formulated to contain two types of resins. One type of resin is compatible with the polystyrene phase. and the other (and possibly oil) is compatible with the elastomer phase. The relative proportions of these resins determines the softness of the adhesive. Thetotal amount added determines the viscosity of the final product. Constrlrctiorl Aclhe.si\v>s There is a growing use of mastics in construction projects for adhering wall panels and plywood subflooring. The S-B-S block copolymers areparticularly well suited for this application because
344
Holden and Wilder
Table 11 ConstructionAdhesive,Typical Formulation*
styrene S-B-S 30% polymer, Aliphatic resin, softening 105°C point Styrenic resin, softening point 140°C carbonateCalcium filler tioxidant Phenolic Low molecular weight epoxy resin Toluene VM&P Super naphtha
* Parts by
100 100 100 350 2 3.5 80 350
weight.
of their high uncured strength. These mastics often contain a high proportion of resin and clay. Resins with low residual unsaturation are suggested for better aging properties. Solvents are mostly naphtha with enough toluene added to dissolve the copolymer. A typical construction adhesive formulation is shown in Table 11.
Sealants This application is dominated by S-EB-S block copolymers. Both hot-melt and solvent-based applications are important. Hot-melt sealants are often applied by robotics. They canbe processed as foamed products. Often they are used as formed-in-place gaskets. In contrast. the solventbased products are mostly used in the building industry, where they are applied on-site. They can be used both in the initial construction and in subsequent maintenance and repair. Diblock copolymers are often part of both hot-melt and solvent-based sealants. They either reduce the viscosities of the hot-melt products or allow the solvent-based products to be formulated at higher solids content. The diblock copolymers can also reduce the strength of the sealant to the point where it fails cohesively during peel. This is a requirement in many sealant specifications. Again, both types of resins are often used, together with oils. If clarity is not a requirement; large amounts of fillers such as calcium carbonates can be added. Suggested startingformulations for both hot melt and solvent-based sealants have been published by Holden ( 1982) and Holden and Chin (1986). Coatings The most important application for coatingsbased on styrenic block copolymers is the chemical milling of metals. A protective film is first applied to the whole surface of the metal sheet. This is then selectively taken off the areas from which metal is to be removed. The assembly is then immersed in an etchant bath, which dissolves away the unprotected metal. The two metals most commonly processed in this way are aluminum andtitanium. Aluminum isetched under alkaline conditions. It can be protected by coatings based on S-B-S block copolymers. Titanium,however. is etched by strongly oxidizing acids that attack S-B-S block copolymers. Therefore. coatings based on S-EB-S block copolymers must be used for this application. Both types of protective coatings are probably formulated with the usual resins, fillers, etc. Details of the compositions have not been published. Oil Gels Styrenic block copolymers are very compatible with mineral oils. Blends with as little as 5% of an S-EB-S block copolymer (the remainder being 90% mineral oil and 5% wax) have been
Thermoplastic Styrenic Block Copolymers
345
described by Mitchell and Sabia (1980). These are used in cable-filling compounds, which fill the voids in “bundled” telephone cables and prevent water seepage. Other potential applications i n toys, hand-exercising grips, etc. are covered in patents granted to Chen (1983, 1986, 1993). In Shell Chemical Company Technical Bulletin SC: 1102-89 (1989), diblock copolymers such as S-EB and S-EP are suggested for use as gelling and antibleed agents in greases.
6.3
Blends with Thermoplastics, Thermosets, or Other Polymeric Materials
The styrenic block copolymers are technologically compatible with a surprisingly wide range of other polymeric materials. They give blends with improved properties when compared to the originalpolymers.Impactstrength usually is the most obvious improvement.Othersinclude tear strength. stress crack resistance. low temperature flexibility, and elongation. Thermoplastic and thermoset polymers can be modified in this way, as can asphalts and waxes. B1etd.Yn-it11 Tller7llol,l~lstic.r
Styrenic block copolymers have several advantages in this application. The other elastomers that can be blended with thermoplastics (e.g., SBR, EPDM, and EPR) can normally be used only in the unvulcanized state-the vulcanized products cannot be dispersed.Since unvulcanized elastomers are soft and weak, they reduce the strength of the blends. Therefore, only limited amounts can be added. In contrast. styrenic block copolymers are much stronger, even though they are unvulcanized. and so unlimited amounts can be added without reducing the strength of the product. Blending is usually carried out in the processing equipment (injection molders, extruders. etc.). Thisis an easy process if a styrenic block copolymer with low viscosity is used. The styrenic block copolymer forms a separate phase and so does not change the T, or T,,, of the thermoplastic into which it is blended. Thus, these blends retain the upper service temperature of the original thermoplastic. Shell Chemical Company Technical Bulletin SC: 165-93 (1993) describesthe use of styrenic block copolymers to modifythreelarge-volumethermoplastics-polystyrene, polypropylene, and polyethylene (both high and low density). Because of their lower price, S-B-S block copolymers are most commonly used with these thermoplastics. I n polystyrene there are two important applications: one is restoring the impact resistance that is lost when flame retardants are mixed into high impact polystyrene, the other upgrading highimpact polystyrene to a super high-impact product (Fig. 8). Polypropylene has very poorimpactresistanceat low temperatures. This also canbe improved by adding styrenic block copolymers. Impact improvement of any polymer usually results in a loss of clarity. This is because the added elastomeric polymer forms a separate phase with a different refractive index. However, blends of S-EB-S and S-EP-S with polypropylene are about as transparent as pure polypropylene, probably because of a match in refractive indices. As described in a patent granted to Holden and Hansen (l990), blends of LLDPE and S-EB-S with polypropylene also retain the clarity of pure polypropylene and show improved impact resistance (Fig. 9). Blends with polyethylene are mostly used to make blown film, where they have improved impact resistance and tear strength. Both S-B-S and S-EB-S block copolymers are blended with poly(phenyiene oxide) to improve its impactresistance.Shell Chemical Company Technical Bulletin SC: 1432-93 ( 1993) describesthe use of maleated S-EB-S block copolymers as impact modifiers for more polar thermoplastics such as polyamides. Another application is the use of styrenic block copolymers to make useful blends from otherwise incompatible thermoplastics. For example, polystyrene is completely incompatible
Holden and Wilder
346
t Impact Strength
_____)
25
5
Rubbery Volume Fraction, % Fig. 8 Impact resistance of blends of an S-B-S copolymer with high-impact polystyrene.
with polyethyleneor polypropylene. Blends of this type form a two-phase system with virtually no adhesion between the phases. Thus, when articles made from them are stressed, cracks easily develop along the phase boundaries and the productsfail at low elongations. Addition of a low molecular weightS-B-S or S-EB-S block-copolymerconverts the blendsto more ductile materials, as described by Paul (1996.) Similar results were reported on blends of poly(pheny1ene oxide) with polypropylene compatibilized using S-EB-S and S-EP. Also, polyamides were compatibilized with polyolefins by the use of functionalized S-EB-S block copolymers. In another example, polystyrene was compatibilized with ABS by the addition of S-B-S or S-EB-S. This allowed mixed scrap from coextruded sheet to be recycled (see Table 12). Blends with Thermosets Sheet molding compounds (SMC) are thermoset compositionscontaining unsaturated polyesters, styrene monomer, chopped fiberglass, and fillers. They are cured to give rigid parts that are often used in automobile exteriors. Special types of styrenic block copolymers have been developed as modifiers for these compositionsand are described in Shell Chemical Company Technical Bulletin SC: 1216-91 (1991). They give the final products improved surface appearance and better impact resistance. In an entirely different application, Arkles (1983) describes the useofS-EB-S block copolymers in blends withsilicone rubbers. These contain either vinyl or silicon hydride functional groups. The silicone rubbers containing the vinyl groups are pelletized separately from those containing the silicon hydride groups. When melted and mixed together in the processing equipment, the two groups react underthe influence of a platinum catalyst. This gives an interpenetrating network of the vulcanized silicone rubber and thestyrenic block copolymer.The products are useful in medical applications.
347
Thermoplastic Styrenic Block Copolymers 1000
140
130
100 Gardner Impact, inIb a t -10°C
120
Flexural
Modulus, psi x l o 3 110
10
100
1
90
0
5
10
'la S-EB-S / LLDPE
Fig. 9 Impactresistanceandstiffnessofa copolymer polypropylene.
15
in Blend 50/50 mixture of S-EB-S/LLDPE blended with Random
Asphalt Blends The styrenic block copolymer content of these blends is usually less than 20%. Even as little as 3% can significantly change the properties of asphalts. The styrenic block copolymers make theblendsmoretlexible(especially at low temperatures)andincreasetheirsofteningpoint. They decrease the penetration and reduce the tendency to flow at high service temperatures, such as those encountered in roofing and paving applications. They also increase the stiffness, tensile strength. ductility, and elastic recovery of the final products. Melt viscosities remain low, and so the blends are still easy to apply. The effects vary with the amount of styrenic block copolymeradded. At low concentrations,thisstyrenicblock copolymeris dispersed in the asphalt.Astheblock copolymer concentration is increased to about 5%, an interconnected
Table 12 Impact Strength of HIPS/ABSBlends Composition* HIPS ABS S-B-S S-EB-S impact strength Dart impact (ft-lb/in.)
100
-
90
-
100
10
-
-
-
180
-polyestersincludephenolsand some chlorinated hydrocarbons such as chloroform and 1.1.2.3-tetrachloroethane (Witsiepe, 1973). m-Cresol is reported to be a useful solvent for measurement of dilute-solution viscosity, osmotic pressure determinations, and size exclusion chromatographic analyses (Witsiepe. 1973). No significant gel fraction is reported for these materials based on their complete solubility in m-cresol. None of the properties evaluated for them shows any indication of significant long-chain branching (Witsiepe. 1973).
Quirk and Zhuo
358
3.4 Blends Some properties of blends of 4GT/PTMG with pure 4GT homopolymer and with poly(viny1 chloride) have been reported (Wells, 1979). Blends of softer poly(ester-ether) copolymers (33 and 58 wt% 4GT) with poly(buty1ene terephthalate) (4GT) exhibited more flexibility at low temperatures but were stiffer at room temperature thanthe corresponding 4GT/PTMG copolymer with the same composition. Blends of 4GT/PTMG (33% 4GT) with flexible compositions of poly(viny1 chloride) exhibited considerable improvement in tensile and tear strength as well as better low-temperature flexibility and impact strength (Crawford and Witsiepe. 1973; Thomas et al., 1987). Blends of polyester with a dissimilar polymer such as polypropylene (Blakely and Seymour. 1992), polyacetal (Gergen, 1989),and alternating co-olefin copolymer (Gergen, 1989) were also reported to improve processing and toughen the base polymer.
4. 4.1
MORPHOLOGY Electron Microscopy, X-Ray Diffraction, and Light Scattering
The morphologies of PTMG/4GT-type polymers have been examined by transmission electron microscopy, x-ray diffraction,andsmall-anglelightscattering(Cella.1973; Seymour et al.. 1975; Lilaonitkul and Cooper, 1977). All of this experimental evidence is in accord with the expected two-phase morphological structurefor segmented poly(ester-ether) copolymers. A simple model for these copolymers is presented in Figure l , showing both crystalline and amorphous domains. The most direct evidence for the two-phase morphology of these copolymers has been obtained by electron microscopy. For example. a transmission electron micrograph of a thin film of PTMG( 1000)/4GTcopolymer is shown in Figure 2 (Cella. 1973). The polymerfilm was cast froma 1o/o solution of the polymer in 1.1,2,2-tetrachloroethaneand stained with phosphotungstic acid, which is preferentially absorbed by the elastomeric phase. Thus. thealight regions correspond to the hard-segment domains. These domains are approximately I00 A in thickness and up to several thousand A in length. Wide-angle x-ray diffraction patterns for a drawn fiber of the copolymer are shown i n Figure 3 (Cella.1973). By comparison with the diffractionpattern for the homopolymer of poly(tetramethy1ene terephthalate), 4GT, it was concluded that the hard segments in the thermoplastic elastomer crystallize in the same way as 4GT. More detailed morphology characterization has been obtained from small-angle light scattering (SALS) of these copolymers (Seymour et al., 1975; Lilaonitkul and Cooper, 1977). Although the data from electron microscopy and x-ray diffraction were interpreted in terms of a two-phase structure of randomly oriented lamellar hard-segment domains (Cella, 1973).the lowangle light-scattering patterns are consistentwith a spherulitic morphology(Seymour et al., 1975; Lilaonitkul and Cooper, 1977). Typical scattering patterns are shown in Figure 4 (Lilaonitkul and Cooper, 1977). It is important to note that these patterns. characteristic of well-developed spherulitic structures, were observed across a broad range of composition. In a l l cases the 4GT hard blocks crystallize into a lamellar structure that forms the skeleton ofthe spherulitic structure. and it is proposed that the interradial regions contain amorphous polyester and the amorphous PTMG segments (seeSec.4.2)(Seymour et al., 1975;LilaonitkulandCooper. 1977).The morphological models capable of producing the three different spherulitic superstructures (see Fig. 4 ) are shown in Figure 5 (Lilaonitkul and Cooper, 1977). It is noteworthy that all of the different spherulitic morphologies displayed by the block copolymers have also been observed for the poly(tetran1ethylene terephthalate) (4GT) homopolymer.
Polyester Thermoplastic Elastomers
359
Fig. 1 Schematic diagramof the morphology of poly(ether-b-ester) thermoplastic elastomers: (l. tetramethyleneterphthalatehard-phasesegment; (m),poly(oxytetramethy1ene)diolterephthalate soft segment. (From Cella, 1977.)
The explanation for the fractionation of the hard segments between the crystalline and amorphous phases isa matter of some controversy. Wegner et al. (1978) postulated a nucleation process in which only hard segments with the most frequent hard segment length are able to crystallize while both shorter and longer sequences were rejected to the amorphous region. Wegner’s model excludedthe possibility that chainfolding will occur. However,other evidence strongly suggests that chain-folded lamellae can develop. The first evidence of chain folding in 4GT/PTMG was provided by Seymour and coworkers (1975). More recently, based on the small-angle neutron-scattering studies of the chain conformations of hard and soft segments as well as the whole chain in bulk polymer samples, Cooper, Miller, and coworkers (Cooper and Miller, 1985; Miller et al.. 1985; Cooper et al., 1988) proposed that the polyester hard segments can chain fold to adjacent cells at room temperature witha repeat distance of three chain units. There is now a general agreement in the literature that the overall morphology of a polyetherpolyester is that of a two-phase system consisting of a pure 4GT crystalline phase and an amorphous phase. However, clean phase separation between the polyether and the polyester segments does not occur (Adams et al., 1996). The amorphous phase contains a substantially homogenous mixture of polyether soft segments and 4GT hard segments rejected from the crystalline phase (Adams et al., 1996).
360
,
Quirk and Zhuo
Fig. 2 Transmission electron micrograph of a poly(ether-h-ester) film cast from 1,l ,2,2-tetrachloroethane solution and stained with phosphotungstic acid. (From Cella, 1973.)
4.2 Thermal Analysis Further evidence for the two-phase nature of these segmented poly(ether-ester)copolymers can be deduced fromthermal analysis of these materials. Theyexhibit a glasstransition temperature for the elastomeric phase and a melting endotherm for the crystalline domains. Typical differential scanning calorimetricthermograms for two poly(ether-ester)copolymers are shown in Figure 6. Representative values ofT,, T,, and calculated percent crystallinity are summarized inTable 2 (Lilaonitkul and Cooper, 1977). The T, for the homopolymerof poly(oxytetramethy1ene)diol is - 88°C (Yoshida et al., 1973). The observed glass transition temperature for the poly(etherester) copolymers depends on the weight fraction of the hard segments in the polymer. The Gordon-Taylor equation (Gordon and Taylor, 1952):
accurately modelsthe T, behavior of these samples provided that the crystallinepolyester component is not included in the definition of the hard-segment composition. The Gordon-Taylor equation also predicts the glass transition temperature for compatible copolymers, where k is the ratio of the difference in thermal expansioncoefficientsof the two polymers T, andis the glass transition temperature of a copolymer consisting of weight fractions W I and W2 of monomers 1 and 2, which have homopolymer glass transition temperatures Tgl and Tg2. respectively. It has been proposed that part of this dependence on composition results from incomplete phase separation; thus, the compositionof the amorphous phase wouldinclude a moleculardispersion of some of the hard segments within the elastomeric phase (Cella, 1973; Seymour et al., 1975). However, this interpretation has been questioned by Lilaonitkul and Cooper (1977) based on the large difference in calculated solubility parameters for the two polymers:8.4 for poly(tetramethylene oxide) and 10.4 for poly(tetramethy1ene terephthalate), respectively. Other factors to
stic
Polyester
361
(b)
Fig. 3 X-raydiffractionpatterns of (a) drawn fiber of a poly(ether-b-ester); (b) drawn specimen of poly(tetramethy1ene terephthalate). (From Cella, 1973.)
be considered are the expected decrease in the length of unrestricted elastomeric chain segments, the increased number of crystalline tie points, and the greater reinforcement of the amorphous phase by the on-average longer polyester segments (Cella, 1973; Seymour et al., 1975). As expected, increasing hard-segment content increases the observed melting point, which approaches the value for the 4GT homopolymer (222°C) (Lilaonitkul and Cooper, 1977). This increase in meltingpoint is accompaniedby an increase in the percentcrystallinity. The percent crystallinity was calculated from the observed value of the heat of fusion, AHf, determined from the area under the melting endotherm curve, assuming that 3 1.4 kJ/mol corresponds to 100% crystallinity (Lilaonikul and Cooper, 1977). A number of studies have shown that copolymer melting points increase with increasing polyether block molecular weights. This has been explained by assuming that increasing the polyether chain length at constant weight percent 4GT forces the 4GT units to form longer blocks with higher melting points (Wolfe, 1983). It would also be expected that the compatibilityof the two types of segments woulddecrease with increasing molecular weight (Meier, 1969). While there is general agreement on the morphology of 4GT/PTMG cooled rapidly to ambient temperature, the morphology of materials annealed at elevated temperature is more controversial and is the subject of ongoing research. The long period spacing,L, defined as the average distance between crystalline domains including the thickness of the crystal plus the amorphous region between crystals, has been found to increase regularly withincreasing temper-
362
Quirk and Zhuo
(Cl
Fig. 4 Small-angle light-scattering patterns of spherulitic structures in PTMEG/4GT segmented copolymers: (a) Type I; (b) Type II; (c) Type 111. (From Lilaonitkul and Cooper, 1977.)
(C)
(dl
Fig. 5 Quadrants of schematic spherulite models: (a) Type I, lamellar principal axis at 45" to radical direction; (b) Type I, lamellar principal axis in radial direction, molecular chains tilted at 45" to lamellar surface; (c) Type11, lamellar principal axis in radial direction; (d) Type 11, lamellar principal axis perpendicular to radial direction. (From Lilaonitkul and Cooper, 1977.)
363
Polyester Thermoplastic Elastomers
I -100
r.
-50
I
A
150 Temperature, ' c
Fig. 6 Typicaldifferentialscanningcalorimetryscans 58% 4GT; B, 33% 4GT. (From Cella, 1977.)
1
I
200
250
for several poly(ether-b-ester) copolymers: A,
Table 2 Thermal Characterization of 4GT/PTMEG(1000) Poly(ether-ester) Copolymcrs
content. "C 33.3
GT wt% DSC T,, T,,,, "C Crystallinity, 28.6 r/c22.9
33
11.5 11.5
50 - 59 214 189
57
63
55 209 I96
-51 200
-
76 -
33 40.7
84
-9 42.8
DSc. differential scannmg colormetry: GT refers to ethylene terephthalate content (see Table 1 for definition of GT).
364
Quirk and Zhuo
atureduringtheannealingprocess at elevatedtemperature or on crystallizationatelevated temperature. along with an increase in the volume of crystalline phase in the sample. Early workers believed that this increase in L was largely due to thickening of the crystalline lamellae (Buck et al., 1974). Bandara and Droescher (1983) confirmed the increase in L and found that the melting point was also increased after annealing. They suggested that the increase in melting point after annealing is mainly due to refinement of the crystal structure and the new growth in the lateral direction of the polymer crystal without an increase of lamellar thickness in the chain direction. More recently, Apostolov and Fakirov (Apostolov and Fakirov. 1992; Fakirov et al.. 1990. 1992), working with 4GT/PEOT, confirmed the increase in both L and crystallinity but found by other means that the lamellar thickness is relatively unaffected by the annealing process. They also showed that the increase in L is proportional to the square root of the molecular weight of the long-chain diol and thus to itsunperturbed end-to-end distance. This suggeststhat the increase i n L is in the amorphous phase rather than in the crystalline phase.
REFERENCES Adams. R. K., Hocschcle. G. K.. and Witsiepe, W. K. (1996), Thermoplastic polycther ester elastomers, in T/~errr~o)plrr.stic Elostorrler.7, 2nd ed. (Holden. G., Leggc, N. R., Quirk, R., and Schrocder, H. E., eds.), Hanscr/Gardner Publications, Inc., Cincinnati, OH. p. 192. Allport, D.. and Janes. W. H., Eds. (1973), Block Copo/ymer.s, Halsted Press, New York. Apostolov. A. A.. and Fakirov. S. (1992). J. Mtrcrorrwl. Sci. Phy. R3/(3):329. Bandara. U,. and Droescher. M. (1983), Colloid Polym 26/:26. Blakcly. D. M,, and Seymour. R. W. (l992), U.S. Pat. 5.1 18,760 (to Eastman Kodak). ( 1986), Br. Plastics K u h h r r 6:7 Buck. W. H., Cella, R. J., Gladding. E. K., and Wolfe, J. R. (1974). J. P o l w . Sci. S w p . 48347. Cella. R. J. (1973), J . P d y u . Sc;. Symp. 42:727. Cella, R . J. ( 1977). i n D ~ c y o p e t l i uof’Poly~rlerScier~c.ec7rrd Techr~ology,Suppl. Vol. 2, Wilcy, New York, p. 485. . 35:190-194. Chang, S.-J., Chang, F.-C. and Tsai, H.-B. (1995), Polyrrl. B I ~ Sci. Cooper, S. L., and Miller, J. A. ( 1985), Ruhher Cllrm. Techno/.58:899. Cooper. S. L., Miller, J. A., and Homan, J. G. ( 1988), J . A/)/)/.Clyst. 21:692. Crawford. R. W.. and Witsiepe. W. K. (l973), U.S. P a t . 3,7 18,715 (to DuPont). Du Pont ( 1986).De.rip~Htrrldhok, Hytrel Bull. E-52083. E. 1. du Pont de Nemours & Company, Wilmington, DE. Du Pont ( 1986). Hytrel Will Clltrrlge the Wuy You 7 h i d Ahour Kuhhe,; Hytrel Bull. E-53634, E. I. du Pont de Nemours & Company, Wilmington, DE. Du Pont (1986). Grrlerd Guide to Products m t i Propcrric~s,Hytrel Bull. E-80913, E. I. du Pont de Ncmours & Company, Wilmington, DE. Fakirov. S., Apostolov, A. A., and Fakirov, C. (1992). h / . J. f o l y r m v i c Mtrter. 18:s1. Fakirov. S., Apostolov, A. A., Boescke, P,, and Zachtnann, H. G. (1990). J. Mtrcrotlwl. Sei. Phys. 829(4): 379. Frensdorff. H. K. ( 197 1 ), Mtrc,rorl~olecules4369. Gergen, W. P. (1989). U.S. Pat. 4,818,798 (to Shell Oil Co. USA). Gordon. M,. and Taylor. J. S . ( 1952). J. App/. Cheru. 2:493. Grcene, R. N. (1992), U.S. Pat. 5,116,937 (to DuPont). Higashiynma, A., Yamamoto. Y., Chujo, R., and Wu, M. (1992), Polyrrz. J. 24:1345. Hoeschele, G. K. (1973). Gcr. Pat. 2,263,046 (to DuPont). Hoeschele, G. K. (1974), Chinrirc 28544. . Hoeschele, G. K., and Witsiepe. W. K. ( 1973). Aqew. Mtckrotnol. C / I ~29(30):267. Hoeschele, G. K. (1980). U S . Pat. 4,185,003 (to DuPont). Hoeschele. G. K., McGirk, R. H., and Health. R. (1992). U S . Pat. 5,120.822 (to DuPont).
Polyester Thermoplastic Elastomers
365
Holden, G.. Lcgge, N. R., Quirk, R., and Schroeder, H. E., Eds. ( 1 9 9 6 ~Tlwrrloplastic Eltrstor?zer.s, 2nd ed., HanserEardner Publications, Inc, Cincinnati, OH, p. 192. Khan, F. A. (1986), Br. Plnst. Rubber 9:32. I. Lilaonitkul, A., and Cooper. S. L. ( 1977), Rubber Cllern. Tecl111ol50: Lloyd, I. R. (1982), in Developrnerlts ill Rubber Techrdogy9Vol. 3, Tller~nopltrsticElnstorners (A. Whelan and K. S. Lee. Eds.), Applied Science Pub., London, England, p. 183. Manuel H. J. and Gaymans, R. J., (1993). Polymer, 34(3),636-641. McCready, R. J. (1985), U.S. Pat. 4,544.734 (to General Electric). Meier, D. J. (1969). J. folyrrz. Sei., Pt. C 26:8 I . Miler, J. A., McKenna, J. M,, Pruckmayr, G., Epperson, J. E., and Cooper, S. L. (1985), Macrorrlolecules 18:1727. Morton. M,, Ed. (1987), Rubber Techrlology, 3rd ed., Van Nostrand Reinhold, New York. Noshay, A., and McGrath, J. E. ( 1977). Block Copolwlers. Overview mtl Critictrl Survey Academic Press, New York. Ryan, J. D. ( 198 1), Org. Cont. Plnst. Cllerll. 44387. Seymour, R. W.. Overton, J. R., and Corley, L. S. (1975). Macromolecules 8:331. Takanawo, Y., Okino, I., and Nakatani, Y. (1994), U.S. Pat. 5,331,066 (to Kanagafuchi Kagaku Kogyo Kabushiki Kaisha) Tamura, S., Matsuki, T., Kuwata, J., and Ishii, H. (l990), Jpn. Pat. 02,269.1 18 (to DuPont-Toray). Thomas, S., Gupta, B. R., and De. S. K. (1987). J. Vir1.d. Techrd. 9(2):71. van Berkel, R. W.M,. Borggreve, R. J. M,,van der Sluijs, C. L., and Buning, G. H. W. (l997),in Handbook of T1~e~rr~o~~~ltr.sric.s. (0.Olabisi, Ed.), Marcel Dekker, New York, p. 397. Wegner, G., Fujii, T., Meyer, W., and Lieser, G. (1978). Angew. Makronlol. Cllem. 74:295. Walch, E., and Gaymans, R. J. (l994), Polymer .?5(3):636-641. E1a.storrler.s(B. M.Walker, Ed.), Van Nostrand Reinhold, Wells, S. C. (1979), in Htrrlt~l?ookofTllerrrlo/,la.stic Ncw York, p. 103. , 2:42. Whclan. T., and Goff. J. (1985). Br. P l t ~ s t i cRuhberirld. Whitlock. K. H. (1973), Ned. Rubberirztl. 34:l. Witsiepe, W. K. (1972), Poly171.Prepr. Am. Cllern. Soc., Div. Polym. Cllem. 13(1):588. Witsiepe, W. K. (1973), in Polyrrlerizutiorl Re.rrctior1.sand New Polymers (Adv. Chem. Ser., No. 129) (N. Platzer, Ed.), American Chemical Society, Washington, DC, p. 39. Wolfe, J. R., Jr. (1973), U.S. Pat. 3,775,373 (to DuPont). Wolfe, J. R., Jr. (1977), Rubber Cller?l.Techrlol. 50:688. Wolfe, J. R., Jr. (1983), in Block Copo1.vnrer.s. Science trnd Techrwlogg (D. J. Meier, Ed.), MM1 Press Symp. Ser., Harwood Academic Pub., MM1 Press, New York, p. 145. Yoshida, S., Suga, H., and Seki, S. (1973). Polyrrl. J. 5:25.
This Page Intentionally Left Blank
13 Polyester Thermoplastic Elastomers: Part II H. M. J. C. Creemers DSM Engineering Plastics SV, Sittard, The Netherlands
1. COMMERCIALCOPOLYESTERELASTOMERICGRADES 1.l
Introduction
Polyester thermoplastic elastomers (TPE-Es or COPEs) have been on the market for about 20 years. The total market volume produced in 1994 was around 35 kton, of which Western Europe accommodated for - l 1 kton, the United States 20 kton, and the rest of the world, including Japan, -4 kton. The expected growth figures (average annual increase) were for Europe 7% and for the United States 3%. The biggest producer of COPEs is the company that developed and first marketed the product. du Pont. which produced Hytrel. Other producers. more or less in order of available production capacity. are:
Company
Product
du Pont
Hytrel Arnitel Ritetlex Pibiflex Pclprene Ecdcl Kopel Skypel
DSM Hoechst-Cclanese Enichem Toyobo Enstman Chcm. Kolon Sunkyong Ind.
The most important market segments for COPE are automotive, electrical (mainly wit-e and cable), hoses and tubes, mechanical goods, and footwear. A new market segment for these products is films with breathability properties. COPEs are available to the market mostly in pellet form. For all commercial COPEs the normal hardness range is between 40 and 72 Shore D. There are a few exceptions for special applications with lower (Hytrel 3078. Shore D30) and higher hardnesses (Hytrel 8238. 82D = 104R Rockwell hardness). COPEs or TPE-Es are situated at the higher hardness end of the TPE family, as are copolyamides (COPAs) and polyester-based thermoplastic polyurethane elastomers (TPUs). The
367
Creemers
368
-res
Temperature range
L
-70 to 16OoC -
-
-50 to 12OoC .
-
-60 to 12OoC
-50 to 110°C -40 to 80°C
I
I fuel r a s l s t a n o e . - Oil pain+abllity,creep,
COPE / TPE-E
mechanlcalpmpdes
Abrasion. X-linkable,
I
ESTER-TYPE TPU ETHER-TYPE TPU
smulding Hydrolysis, chemlcal
~ W H E R AMIMS PA 11/12] mechanlcal properties Low cost
'
High Rexlblllty. X-linkable, low
ti
Cost
ShoreA 40 50 60 70 80
Shore D
30
40
50
60
70
80
90
Fig. 1 Range of hardness per TPE family.
lower-hardnessrange (Shore A 40-80) consists of styrenic block copolymers (SBCs or S(E)BS), thermoplastic polyolefins (TPOs), and thermoplastic vulcanisates (TPVs) (Fig. 1). Figure 1 shows the common available hardness ranges of the different TPEs versus the temperature ranges at which the materials can be used. The important features of the TPEs are also given in this figure. Another method of comparing the different TPEs is shown in Fig. 2, where so-called continuous use temperature is plotted against the Vicat,indicating temperature stability under load, an important characteristicfor demanding engineering applications. Clearly
Fig. 2 TPE types: thennomechanical stability.
Polyester Thermoplastic Elastomers II
369
Soft-segmcnt properties
Hard-segment properties
Flex fatigue Low-temperaturc properties Impact strength Hydrolysis resistance
Mechanical strength UV/Ozone Oxidation resistance Oil/Chemical resistance
Effects of increasing hardness
Effccts of decreasing hardness
Mechanical strength UV and oxidation resistancc ChemicallOil resistance Permeability (lower)
Hydrolysis resistancc Flcxibility Impact strength Low-temperature properties
In theirnaturalstate, segmented poly(ether-ester)materials are opaque, creamywhite solids. Ecdel materials from Eastman have high clarity when processed under the correct conditions. These block polymers have a different polyester block PCT (basedon cyclohexane dirnethanol) rather than PBT. Teijin has developed a product called Nouvelan. in which the hard segment is based on polybutylene naphthalate (PBN) rather than terephthalate (4300 series) and the soft segment on an aromatic polyether (4100 series). Teijin is claiming for these materials better hydrolysis and thermal resistance (4300 series) and better chemical resistance and vibration damping (4100 series), respectively. However, most of the TPE-Es or COPES have PBT as the hard segment and PTMEG. PPG, or PEG as soft segments. 1.2 The Hytrel Family The different grades of Hytrel are designated using four digits, the first two of which represent the durometer D hardness. The third digit has no official significance, but relates to viscosity. The fourth digit represents the type of antioxidant (0-5, discoloring; 6-9, nondiscoloring). There are two main groups of products depending on the type of soft segment used: Standard grades.The mosteconomicalgradesofferthebestbalance of costand performance. These grades range in Shore D hardness from 35 to 82. 2. High-performance grades. These provide an extra measure of performance and service life in applications where properties such as abrasion resistance and tear strength are critical. They range in Shore D hardness from 40 to 72. 1.
In addition, Hytrel has some special grades, e.g., flame-retardant. blow molding, improved heat aging, and high fuel permeation-resistant. DuPont has developed some special grades for film applications (HTR 817 1, HTR 8206).
370
Creemers
1.3 The Arnitel Family Polyester thermoplastic elastomers manufactured by DSM Engineering Plastics B.V. aredistributed under the trade name Arnitel. There are three groups of Arnitels based on different soft segment formulations. Arnitel E is based on PTMEG, Arnitel P on modified PPG, and Arnitel U is a polyester-ester elastomer specially developed for cable applications. The following example of an Arnitel code illustrates the meaningsof the different constituents: EM400-G6 where: E = Arnitel type (E, P, or U) M = L, low viscosity; M, medium viscosity; B, high viscosity for blow molding application 40 = hardness (Shore D) 0 = commercialserialnumber G = additives: G, glass fiber-reinforced; V, flame-retardant; L. UV stabilized; M, mineral-filled; H, heat stabilized 6 = amount of filler or reinforcement ( X 5 % )
2.
ENGINEERING PROPERTIES
We will elucidate the engineering properties of TPE-Es using the properties of the different Arnitel grades. Arnitel combines the performance characteristics of elastomers with the processing features of a thermoplastic. Its noteworthy properties include: High load-bearing capability Excellent flexural fatigue endurance Good thermal stability High impact strength even at low temperature Good resistance to chemicals and weathering High tear and abrasion resistance High moisture vapor permeability Ease of processing A list of standard and specialized properties IS0 14910-2.
2.1
of COPEs is given in the International Standard
Properties of COPEs
The mechanical and electrical properties of the Arnitel product portfolio, as an example of a typical COPE range, is shown in Table 2. The chemical resistance and chemical properties of some softer representatives of the Arnitel COPE range is given in Tables 3 and 4.
2.2
Processing of COPEs
Material Handling COPEs like Arnitel are supplied predriedin moisture-proof bags at a moisture content sufficiently low to permit immediate processing for most applications.When exposed to air,Arnitel granules
371
Polyester Thermoplastic Elastomers II Table 1 Comparison of Properties of DifferentArnitcls Property profile
Stability oxidative
uv hydrolytic Low-temperature Impact properties Tear strength Chemical resistnncc Oil resistnncc Wear
Arnitcl E
Arnitcl P
Arnitel U
+
+ + +++ +++ + + +
+++ +++ + + +++ ++ +++
++ ++t +++ +++ ++ +
+++
+
+++
+ . good: + + . very good: + + + . exccllenr.
absorb moisture. At the high temperatures encountered during processing. even small quantities of absorbed moisture (e.g.. 0.02%) in the Arnitel granules can cause degradation during processing. This can result in varying molecular weights leading to a decrease in mechanical performance. For this reason, it is important to limit the moisture content of the granules as much as possible. The following precautions should be t’‘I k en: room to adaptslowly to the Allowmaterial that has been stored in arelativelycold temperature in the processing room. Do not open the packages until the machine is heated and ready for production. Always feed the entire contents of one or more bags into the hopper and close the hooper tightly immediately. Do not refill the hopper until there is room for the entire contents of a bag. Always try to refill the hopper to the top. Ensure that the hopper is not larger than necessary i n order to limit residence time of the material. Granules that have been exposed to ambiant air for too long must be assumed to have picked up moisture. These granules can be dried in a circulation oven with hot, predried air or in a rotary vacuum drier. The recommended drying conditions are as shown i n Table 5. Materials dried in this way will reabsorb moisture quickly during cooling. Therefore one of the following procedures must be adopted: Leave the hot. dried granules to cool in a sealed moisture- and airtight package. After cooling these to room temperature, these granules can be processed in a similar way as directly delivered Arnitel. If sealing equipment is not available. the hot and dry granules should be transferredimmediately to the hopper and the lid of the hopper closed tightly. If the temperature of the granules does not go below XO’C, then the amount of moisture absorbed will not be excessive. Rapid cooling of the granules can be prevented by insulating the hopper or by using a hopper dryer set to IOO’C.
Injectiorl Molding I n principle, Arnitel can be processed on all standard injection-molding machines plasticization. A plunger machine is not recommendable.
with screw
372
Creemers
Table 2 Properties forDifferent AmitelGrades Polyether cstcrs Properties
Units
Physiccrl properties Relative density Melting point Coefficient of linear thermal exp. Deflection temperature under load Vicat softening temperature: at 10 N at 50 N Moisture absorption: equilibrium in air equilibrium in water Flammability Mechanical properties Tensile modulus Tensile stress: at 5% elongation at 10% elongation at 50% elongation Tensile strength Elongation at break Izod impact strength: unnotched, at + 2 3 T unnotched, at - 30°C notched, at 23°C notched, at - 30°C Hardness Shore D
+
Electricrrl properties Dielectric strength Volume resistivity Surface resistivity Dielectric constant (E' ): at 50 Hz at 1 MHz Dissipation factor (tan 8): at 50 Hz at 1 MHz Tracking resistance: CTI CTI( M) Source: From Ref. 4.
EM400 EM460 EL550 EL630 EL740 PL380 1.12 195 220
1.16 I85 160
1 .20
"C pm/m.K
202 I 50
1.23 213 140
I .27 22 1 I 10
1.16 I97 150
"C
-
-
1 10
I IS
I20
-
"C "C
130
1 50
50
I80 85
200 1 15
205 150
145
-
r/C -
0.30 0.75 HB
0.30 0.70 HB
0.20 0.65 HB
0.20 0.60 HB
0.45 0.60 HB
0.40 7.0 HB
MPa
5s
1 10
220
375
900
60
MPa MPa MPa MPa
4.0 5 .4 8.4 17 700
7. I 9.0 11.4 21 800
13.2 15.7 16.6 32 600
20.2 23 22.0 40 600
26.9 33.5 26.8 4s 360
3.5 5.2 8.5 15 450
NB NB NB NB 38
NB NB NB NB 45
NB NB NB 20 55
NB NB NB 4 63
NB 200 9 4 74
NB NB NB NB 38
-
76
8
kJ/m' kJ/m' H/m' kJ/m' -
-
-
-
-
-
-
-
IO'" 10''
10"
10'4
10'1'
IO'"
I1
S x 10" > 10'5
10'4
10"'
10'"
IO'"
-
4.1 4.0
-
-
4.0
3.8 3.4
__
4.4
3.3
4.7 4.4
x 10' x 10"
10 170
-
-
350
400
3.8 350
300
310 350
-
600 600
600 600
600 600
600 600
600 600
600 600
MV/m bl.cm
-
ermoplastic Polyester
373
It
Polyester esters
PL720 PL580
UM55 UL740 UL.550
UM55 1
1-V
methods Test
1.23 218 I10
1.28 223 90
1.25 200 160
I.27 217 40
1.28 205
1.26 195
IS0 R 1183 ASTM D21 l7
-
-
-
120
80
120
-
-
205 95
210 I 55
I80 80
205 I 50
-
-
-
-
0.40 2.6 HB
0.25 0.85 HB
0.25 0.75 HB
0.15
0.40
0.16
0.35 HB
-
-
-
-
u194
27 5
920
I90
1.150
310
I80
I S 0 527 IS0 527
17.0 19.5 21 .o 22 300
24.9 30.9 28.2 37 300
13.4 15.8 17.8 35 500
31.1 36.0 27.7 38 350
8.2 13.9 17.6 30 600
7.0 11.0 14.0 31 500
NB NB NB 25 58
NB NB 17
NB NB NB 4.0 55
NB NB 13 6 75
-
-
-
-
-
-
55
55
14 3 x 101"
-
10'4
10'4
10
72 -
-
-
> 1016 > 10'"
10'4 10'4
> 10Ih >l014
-
I S 0 75 I S 0 306
-
1.8
,
14.6 X IO'" -
4.0
-
-
-
3.3
3.9
3.0
5.03 4.02
5.43 4.45
400
-
-
-
1 50
110
350
500
200
470
590
600 600
600 575
600 600
600 450
-
-
-
-
I S 0 306/A IS0 306/B
IS0 527 IS0 527 IS0 180 I S 0 180/1C I S 0 180/1C IS0180/1A IS0 180/1A I S 0 868 IEC 243 IEC 93 IEC 93 IEC 247
IEC 247
IEC112
Creemers
374
Table 3 Chemical Rcslstance of Polycstcr Thermoplastic Elastomcrs of 40 and 46 Shorc D Hardness
Chemical rcsistancc (6 weeks at 23°C)
5% Acetic acid 100% Acctic acid 10% Sodium hydroxyde 50% Sodium hydroxyde Sca watcr 25% SO2 solution 30% Sulfuric acid Acetone Ethanol Ethylacetate Mcthanal Tetrachloromethane Xylenc Gasolinelmcthanol (85/15) Keroscnc Gasolinc Two-stokc gasolinc Isooctalle Isooctane/toluene (70/30) Antar LVC Antar LVR 30 ASTM oil No. 1 ASTM oil No. 3 Avcat Dcrd 2498-7 Crude oil Dicso KN 10323 Donax HB break fluid Esso turho oil 2380 Esso turho oil 2389 Skydrol LD Skydrol 500 B
Weight incrcase, Tensile strength, 9 % rctcntion
E h g a t i o n at break, % retention
EM 400
EM 460
EM 400
EM 460
EM 400
EM 460
I .6 130 - 0.06 - 0.02 0.26 1.S - 0.04 28 17 47 13 170 86 54 16 33
1.4 68
-
__
-
-
31 IIO I 10 95 98 97 72
57 I10 I IS
53
77
100 100
10s 100
83
97 97 95 88
-
-
-
64 -
79
81
-
-
91 -
54 66
60 70 7s
75 80 81
-
-
-
-
94 I00 94 81 96 I 00 98 99 97 96 105
0.01 - 0.0 1
0.30 1.4 0.0 I 19
9.5 28 -
97 48 35 8.5
37 48 57
9s -
100
19
-
h5 -
-
86
-
5 16
100
105
-
-
83 77
-
-
__
1.6
0.5 I 6.6 9.5 10 11.5 13 4.5 7.2 18 20
105
92 73 I 00 I 10 I 05 95
I00 89 80
-
9 27
15
17 19
20 -
13 18
32 37
-
-
100 94
98 92 96 95 88
-
80
-
99 92 78 77
105
93 90 94 87
IO0 90 88
95 97 85 83
Notes
96 -
100 95 ~
100 98
96 90
Cylinder. For optimal processing of Arnitel. the residence time of the material in the cylinder diameter should be such that the product weight is within a range of approximately 40-70% of the maximum shot capacity. The heating elements should have sufficient heating capacity and the temperature should be accurately controlled to avoid large melt temperature fluctuations. Generally, but i n particular for glass-filled grades, a high injection rate is required for a good product quality. Screw. The geometry of the screw determines the transport behavior and the degree of plasticization of the granules. Standard three-zone screws with an L/D ratio from 17 to 23 and a thread depth ratio of approximately 1 :2 yield excellent results. Conical progressive screws (as used for PVC) are not suitable. To avoid backflow of the melt during injection and holding pressure, the screw should be equipped with a nonreturn valve.
Polyester Thermoplastic Elastomers II Table 4
375
Chemlcal Properties of Polyester Thermoplastic Elastomers of 40 and 46 Shore D Hardness r/c
Weight increase Chemical resistance at 100°C for 6 weeks
EM 400
Water Karafol BU, I cm'/litcr Neo Disher AX, 0.5% ASTM oil No. 1 ASTM oil No. 2 ASTM oil No. 3 Crude oil Esso turbo oil 23x0 Esso turbo oil 2389 Skydrol LD Skydrol S00 B At vnrious temperatures and immersion times Sulfuric ~ d 30% . Sulfuric acid. 45% Water pH5 Water pH8 ATF oil Kexosenc Klubcr Oppnnol Optilnol Peanut oil Soybean oil Sunflower oil
EM 460
Tensile strength, Elongation 8 retention EM 400
EM 460
at break, 76 rctcntion EM 400
7s
72
-
-
-
EM 460
-
100 86 96 94 88 -l -I
7s S0 66 65
I os S2 43 S8
-
-
-
81
1os
97
100
I 0s
-
90
91 1 10
-
93 93 92
-
86 -
70 80 68 70 -I
I os 97 9s 95
102 17 -
-
-
-
Notes
1 1 11 11
wk/70"C wW70"C wW90"C wW90"C -
4 wW60"C 3 W!i/lI0"C 1 wW85"C 3 ww1 10°C 6 wW60"C 6 wWh0"C 6 wW60"C
Nozzle. Arnitel is preferably processed on decompression-controlled machines with an open nozzle. With ;I short nozzle anda wide bore (3 mm or more), frictional heating and pressure losses are thus minimized. Particularly with glass fiber-reinforced and tlame-retardant grades, injection molding problems can thus be avoided. Nozzles that can be closed (hydraulically, if possible) may be used as well, provided they are equipped with an effective. precision-controlled nozzle heating system. It is advisable to withdraw the nozzle from the mold after the injectiotdholding pressure phase to prevent it from cooling down LIIKIUIY. Hopper. The hopper should be equiped with a tightly closing lid. which should be kept closed during processing to keep the granules dry and free from dust. SOM. Aspects of Mold Design Good mold design is essentialforoptimalinjectionmolding and. consequently, for ahighquality product. In designing molds for the processing COPES like Arnitel, the following points should be observed.
376
Creemers
Table 5 Recommended Drying Conditions for Different Arnitel Grades Depending on Hardness Amitel grades Base grade
Drying conditions
Hardness, shore D
Time (hr)
Temperature ("C)
28 38 S8 12 40 46
3 3 6 6
120 120 120 120
10
100 100 1I O 110 110
P P P
P E E E
ss
E E E
58 63 14
U U
ss 14
IO 10
8 8 6 3 6
120 I20 120
Gating Systems. All familiar gating systems-cone, pinpoint, tunnel, film, fan,and ring gates-may be used. Externally heated hot runner and semi-hot runner systems also qualify, but they require efficient heating and very accurate temperature control to avoid freezing or overheating of the material. Gate Locations. Gate locations should be chosen with care to minimize deformation or warpage of the product due to anisotropic shrinkage behavior; this particularly holds for glassfiber-reinforced Arnitel grades. The gate should preferably be located on the thickest section of the product and in such a position that the product fills as evenly as possible. Dimensions of Runners and Gates. The cross section of the runners should preferably be trapezoidal. Recommended runner and gate dimensions for various wall thickness are given in Table 6 and Figure 3. For products with a wall thickness exceeding 3-5 mm a full sprue gate with a diameter of approximately three fourths of the largest wall thickness is to be preferred. A short sprue cone with a taper of at least 1" 30 is recommended. Venting. Special attention should be given to effective mold venting. Venting is effected by vents (dimensions 1.5 X 0.02 mm) provided in the mold faces or via existing small channels such as around ejector pins and cores. Vents should be located in the mold at the end of the flow paths.
-
Table 6 Recommended Runner and Gate Dimensions Versus Wall Thickness Wall thickness (mm)
0.7- 1.2 1.2-3.0 3.0-5.0 >5.0
Gate diametedlength (mm) 0.7- 1.0/0.8- 1 0.8-2.0/0.8-1 1.5-3.YO.9- 1 3.5-6.0/0.8- I
377
Polyester Thermoplastic Elastomers II
. .
I o
1
2
3
4
5
6
Fig. 3 Recommended runner dimenuons.
Ejection. Molded products are removed from the mold by means of ejector pins, plates, or rings. The designand number of ejectors isdictated by product design and stiffness. Ejection must not cause damage or deformation. In view of Arnitel’s flexibility, particularly the softer types. the part of the product i n contact with the ejector should be under uniform load. It follows that a fairly large ejector face is required. Cooling. The cooling system is an important part of the mold and needs to be configured with scrupulous care. To prevent warpage and long cycle times, the product must be cooled down rapidly and uniformly. hjection Molclitlg Corditions
Cylinder Temperatures. In accordance with theirrespectivemeltingpoints,Arnitel grades are processed in the 220-260°C range (harder grades at higher temperatures; soft grades at low temperatures). A rising temperature profile will normally yield the best results. Because it depends on injection molding grade, type of machine, and product to be injection-molded, there is no generally applicable optimum temperature. The residence time of the material in the cylinder is an important processing parameter. To avoid thermal degradation of the melt as a result of prolonged residence times, it is best to observe the lower limit of the recommended temperature range. Too high melt temperaturesshould be avoidedbecausethermaldegradationadversely affectsmechanicalproperties. Cylinder temperaturesettingsshouldbe in the range of 200-250°C.
378
Creemers
Mold Temperature. Forthin-walledproductsa mold temperature of 50°C is recommended. Thick-walled products can be moldedat 20°C. Higher mold temperatures improve flow but add to cycle time. Injection Pressure. Injection pressure is primarily determined by the wall thickness of the product.the tlow pathlength,andtheflowbehavior of the injectionmolding grade. In general. Arnitel grades have excellent flow properties. Injection pressure should be sufficiently high to obtain uniform mold filling. Injection Rate. To avoid premature freezing during injection. fast filling of the mold is recommended. A relatively high injection rate is usually possible. but a moderate rate may be necessary in certain conditions. Holding Pressure and Time of Follow-Up Pressure. Shortly before the mold is completely filled, the injection pressure is usually stepped down to the holding pressure. which in most cases is 40-70% lower than the injection pressure. During the holding pressure phase, the volume shrinkage of the cooling melt is compensated. Holding pressure should therefore be set high enough to prevent sink marks. Excessively high holding pressures should be avoided since they may cause residual stresses in the product or visible burning. Holding pressure should be sustaineduntil the gate freezes.The appropriate time of followup pressure is best determined by weighing the product. Sink marks of shrinkage voids indicate that the time of follow-up pressure has been too short. Time of follow-up pressure should be prolonged proportionately as wall thickness and gate dimension increase. Back Pressure and Screw Speed. In general. back pressure and screw speed should be set as low as possible to avoid excessive heat generation through friction and the reduction of glass fiber length in reinforced grades. Back pressure promotes homogenity of the melt. It should be set just high enough to ensure that the melt is free of air bubbles. that the screw plasticizes evenly, and that the product weight is constant. In practice. a back pressure of approximately 3-6 bar has been found to be sufficient. The screw speed should be such that plasticizing time remains just within the cooling time. Low screw speeds (30-100 rpm) will limit both heat generation through shear and the reduction of glass fiber length. Clamping Pressure. Theclamping pressure is matched to the injectionpressureand the projected surface of the product. Arnitel’s low relative viscosity and good flow properties render a high clamping pressure necessary to prevent flashing forination. Metering. The screw metering rate should be so controlled that. during holding pressure. there remains a sufficiently large buffer of molten material in front of the screw to serve as afterfilling material. A small buffer of 2-5 m m is recommended, since a large buffer might lead to loss of pressure and to prolonged residence of the melt in the cylinder. CoolingKycle Time. The cycle time is primarily determined by the injectiothfter pressure time and the cooling time. The nucleating agent. crystallization accelerator, and glass fiber reinforcement. if any. bring aboutrapidcrystallization of Arnitel grades.Thecooling time depends on wall thickness and shape of the product. For this reason the cycle time varies from approximately 9 seconds for products with a wall thickness from 0.8- I .5 n m to approximately 40 seconds for products with a wall thickness of 5-6 mm. Mold Shr-inkqe Mold shrinkage of Arnitel moldings is intluenced by many factors. such a s product design, wall thickness. mold temperature. cross section of gating. type of gate. dwell pressure. and holding time. Wall thickness and mold temperature have the greatest intluence. Mold shrinkage increases with wall thickness and mold temperature. It decreases with higher dwell pressure and longer
Polyester Thermoplastic Elastomers It
379
holding time. The difference between shrinkage in the flow direction and shrinkage across the tlow direction is relatively low for Arnitel.
Extrusion Arnitel is as easy to extrude as other polymers. Good results are obtained with most conventional single screw extruders. Extruder Barrel. Extruder barrels that are suitable for the use of PA, PVC. and polyolefins are usually also suitable for Arnitel. Barrels with axial grooves and intense cooling of the intake zone are not suitable for the extrusion of Arnitel. If an intake zone with relatively short and shallow grooves (< 1 mm) is used. good results can be obtained only if intensive cooling is avoided. Screw Design. For Arnitel extrusion, good results are obtained with conventional single screw extruders equipped with a three-zone screw. Length-to-diameter ratios of 24 or higher provide the best melt quality. The clearance between screw tlights and barrel should be small: 0.08-0.10 mm for extruders up to 45 mm in screw diameter 0.1-0.15 mm for larger extruders
The compression ratios should be between 2.4 and 3.2, as determined by the depth of the feed section divided by the depth of the metering section. The depth of the channel in both the feed and metering sections is important; if the feed channel is too deep and not sufficiently long, particularly with largediameterscrews,poor feeding and loss of output can result. If the metering channel is too deep, insufficient pressure will be built up, resulting in lower output, particularly with low viscous grades. A metering channel which is tooshallow can result in overheating of the melt. due tohigh shear, particularly at high viscous types. Many factors have to be taken into consideration in selecting the correct screw design. To give some idea of screw design successfullyused i n the extrusion of Arnitel, the characteristic design parameters and approximate values are listed in Table 7. It should be added that certain designs of barrier screws have been found to be effective in extruding Amitel. Power Requirements. The twomainfunctions of an extruder are (1 ) to produce a homogeneous melt from solid material and ( 2 ) to convey material from the feedhopper to the die so that a constant and stable flow of material is delivered to the die opening at a constant pressure. The energy required for melting is supplied by two sources: the heater bands and the electric motor driving the screw. Because Arnitel has a high heat capacity and a high heat of
Table 7 CharacteristicDeslgnParameters Screw length Pitch Extruder diameter mm Length of section: D section Feed Compression sectlon Meterlng Dsection Channel depth: mm sectionFeed Metering sectmn mm
for Arnitels
24-27 1D
D
30
45
60
90
7-10 4-6
7-10 4-6 8-11
7-10 4-6
7-10 4-6
8-11
8-11
2.5 6.5
3.5 8
4 IO
8-11
2 5
Creemers
380
Table 8 RecommendedProcessingTemperaturesfor
Arnitcl Grades
Melt flow indcx Processing temperatures at: grades Arnitel P-X5975 (28D) PM380 PM580 EM400 EM460 EM550 EM630 EM740 UM55 1 UM552 UM552-V UM740
Melting point ("C) 207 218 218 195 185
202 213 22 1 200 195 200 217
220°C (dg/min) (dg/min) (dg/min)
230°C
240°C
13 6
25 38 7
18
7 18
6 7
Maximum Minimum ("C) 220 225 230 205 195 215 225 230 210 210 210 230
("C) 255 260 265 240 240 260 260 260 260 260 260 260
fusion, high engine power is needed to attain the high temperature required for extrusion. One of the two heating zones situated directly downstream from the hopper should have a power of 4-5 Wkm'. For the remaining zone a power of 1.5 Wkm' is sufficient. For extruder start-up, an engine power of approximately 0.3 kW per kg output is required, after which an engine power of 0.15-0.2 kW per kg output is sufficient. Heating elements and thermocouples should be installed at strategic positions to avoid overheating of the melt. All positions that are not directly heated should be properly insulated to avoid cold spots, which result in an undesirable flow and varying material properties. To ensure that all residual polymer is thoroughly melted before start-up, a brakerplate (head clamp) is usually used. This is an area where a large amount of heat can be lost to the surrounding air; thus, the heater design in this zone is crucial. Processing Temperatures. Depending ongrade, processingtemperaturesfor Arnitel range from 200 to 250°C. The optimum temperature profile depends largely on the grade and its application. Some guidelines are given. along with other important information, in Table 8. The actual temperature of the molten polymer in the extruder should be between the given maximum and minimum temperatures. Temporary Shutdown. When the extrusion process is interrupted for less than 30 minutes. no specialmeasures are required. The normaltemperaturesettingscan be maintained. When the production is resumed. the extruder should be purged until the residual material has been replaced. Use of Regrind. The excellent heat stability of the Arnitel melt permitsthe use of regrind provided the material was properly processed during the initial extrusion. Depending on the demands to be met in service by the products. up to 20% regrind can be used. It is recommended that the scrap be chopped into granules approximately the same size as the original pellets. The regrind must be blended with virgin polymer and dried to ensure uniform quality. Afrertreatnrent
Coating. Arnitel is easily coated provided that no silicone-containing mold release agents or other products with an adverse effect on adhesion were used during the injection-molding
Polyester Thermoplastic Elastomers II
381
process. No special adhesion promotors are necessary. Paint system suppliers will be glad to tell you whether the tlexibility of the coating you have selected matches the hardness-or rather stiffness-of the Arnitel grade used. Metallizing. The vacuum-metallizing method is the best metallizing procedure for Arnitel. In connection with the low flexibility of the metal film, it is best not to use soft Arnitel grades and always to run a test first. Printing. Arnitel is easy to print.Polyesterprintingfilm,whichpermitsthe use of standard equipment, is a relatively simple method. offering a wide choice of coatings adapted to thespecific properties of the end product. So-called laser printingis also possible with Arnitel. Scfety Aspects Arnitel is anonreactive,nonvolatile,harmlessrawmaterial. No specialsafetymeasures are necessary during storage (but exposure to moisture must of course be avoided for quality reasons). Upon opening of the package and during the processing of unreinforced or mineral reinforced grades, a slight odor will be perceptible near the injection-molding machines, which will usually be discharged by the room ventilation system customary for production areas. Although no special precautions are necessary,the use of safety glasses near the processing machines is recommended. If any part of the skin shouldcome into contact with molten polymer, at once cool the affected part in running cold water and keep it submerged for several minutes at least. Local fume extraction is recommended in the processing of Arnitel, particularly if the recommended temperatures and/or residence times are exceeded, since there is always the risk of hazardous volatile components escaping that are highly flammable (e.g., tetrahydrofurane) and could irritatethe respiratory tract. Localfume extraction is indispensable in cases ofoverheating, such as during a stop with a filled cylinder or during the cleaning of machine parts in an oven.
3. APPLICATIONS Commercial available COPEs generally range in hardness from 40 to 80. shore D. This means that the applications are very diversified. Materials with a hardness of Shore D > 60 find their applications in the field of impact-modified engineering plastic (e.g.. connectors and optical fiber-coatings). A hardness range between 40 and 60 Shore D (the most important part) finds its applications in the rubberlike applications such as (CVJ) boots and bellows, tubes and hoses, airbag covers, etc. The few materials with a hardness lower than 40 are finding applications in the softtouch area. likekeypadsandgrips,and in the filmandnonwovenarea. The most important properties for each of the segments is given in Table 9. Another division can be made in injection molding, blow molding (CVJ boots) and extrusion applications. Typical examples of extrusion applications for COPEs are: Tube and hoses: Hydraulic hoses Convoluted tubes Profile and belting: Conveyer belts Automotive belts HardSoft combinations for water-tight sealings
Creemers
382
Table 9 Important Properties for Diffcrent Hardness Segments of Thermoplastic Elastomcrs ~
Elastic fibers and film
~
~~~~
Rubhcr-like
Impact-modified EP
60 Impact rcsistance
Vapor tr;msmission
Low modulus Low temp. propertics Cont. use temperature Chemical resistancc Flex fatigue
Cont. use tcmpcrnturc HDT Chemical resistancc Proccssability Crecp
Wire and cable: For halogen-free Arnitel U’s automotive cables. class T3 (CUT 125°C) and T4 (CUT 150°C) Telecommunication cables Coiled wire jacketing Fibers and nonwovens Various textile applications Monofilament: Textile applications Flexible woven tubing Film and coating: Durable and disposable laminates with textile and/or nonwovens Medical applications such as surgical drapes and gowns. wound coverings. and transdern u l patches (most Arnitel grades are USP VI approved) Construction (e.g., roof and wall membranes) Food packaging (several Arnitel grades have FDA and BGA approvals) Diaper backsheet Divided over the hardness range. these applications can be classified as shown in Table 10. Looking at the usage in tonnage, automotive applications are oneofthe biggest segments (25%). Typical examples are boots, bellows, airducts. airbag covers, body plugs, and shock absorption applications. Another 25% of the applications may be generalized as hosesand tubes, like painting hoses, hydraulic hoses, umbilicals. convoluted tubes, mandrels, etc. About 12.5% of the applications is found in the sport and footwear area, e.g.. cushions, shoe studs, binders. windsurf mast socks. ski-boot parts. etc. The rest is divided over wire and cable insulation. film, nonwovens, and miscellaneous applications like special brushes. watch streps. soft touch grips. etc.
3.1 SomeDetails of COPE Applications Autornoti\v> Applicrttior1.s
I n the automotive market the trend is moving away from thermoset rubbers like EPDM. chlorinated PE. etc. to TPEs. This replacement is not only based on performance, but are also a result of the introduction of new processing techniques allowing more complicated shapes. There are new possibilities of using rigid/soft conlbinations in one processstep. In the past. when formulat-
on grade
Polyester Thermoplastic Elastomers II
383
Arnitcl WireBase ProfileTube Hardness. grade
P P P P E E
E E E E U U
Shore D
and hose
and belting
and Fiber and cnblc nonwovcns
Film Monofilnmcnts
and sheet
28 38 S8 72 40 46
ss S8 63 74 55 74
ing air ducts, thermoset EPDM was used in the flexible parts, while polyamide was used in the rigid parts. In the case of dirty ducts. combinations PBT and COPE are preferred and are under study (oil resistance). CVJ (constant velocity joint) boots in frontwheel-driven cars are another example of an interestingmarket.where i n thiscase COPEs arereplacingthermosetelastomersbased on performance (temperature resistance. cold and hot; abrasion resistance, grease resistance). The use of more aggressive greases by the automotive industry is bringing COPEs in a good position for this application. This trend started in the United States and continued into Europe. In Japan this changeover is still small, mainly due to the big price difference i n favor of rubbers. Airbag covers is a new, safety-driven. application. Originally only installed on the drivers side. today airbags areinstalled on both frontsides and even in the back of the car. Thepolymeric materials used in the manufacture of airbag covers are TPVs. thermoplastic copolyesters. and SBS materials. Intercompetition between the different TPEs is, in fact. happening. Depending on the requirements. a choice among the TPEs has to be made. Soji-Tolrcl?
Because of their rubber-like behavior. TPEs in general are well suited for so-called soft-touch applications like watchstraps and grips for tooling. For price performance reasons TPOs and TPVs perform very well. Film Applicatiotls
COPEs are certainly not only aiming a t replacement or vulcanized rubber application. One of the characteristic examples is the use of TPE-E in elastic and breathable film. The polyether segment (the soft segment) gives COPE a certain permeability for vapor. The longer the soft polyether segment, the higher the moisture vapor transmission. Also. the kind of polyether used in the soft segment has an intluence. The lower the number of carbon atoms between the ether groups. the higher transmission for moisture vapor. Polypropylene glycol (soft segment in Armi-
Creemers
384
tel P) is better in this respect than polytetramethylene glycol (in Amitel E). COPE films are elastic and water-repellant, but moisture vapor transmission can be adjusted. This makes COPE film very well suited for application in sport clothes, in general in applicationswherebody comfort is important.
3.2 Arnitel Approvals, Recognitions, and Registrations Arnitel has been approved. recognized, or registered by several institutes and organizations. These approvals are necessary to operate successfully in today's markets.
General TOSCA (Toxic Substances Act: USA). Arnitel E and P are Tosca registrated (include. DSL-Canada) under the following CAS numbers: Arnitel P CAS nr 648 11-37-6; Arnitel E CAS nr. 37282-12-5. For Arnitel U, a so-called polymer exemption document has been filed (P-case nr. Y-95-92). This registration is important for imports into the United States. UL Recognition. Underwriters Laboratories Inc. (UL), an independent, noncommercial institute in the United States, tests materials according to standards which they partly develop themselves. The test results, which provide as independent opinion about a product for customers, are listed on a yellow card, company by company. and collected once a year in a yellow book. UL recognition is especially important in the E and E application area. The Arnitel materials given in Table I 1 have UL recognition.
Food and Medical Approvals EEC Food Approval. Food approval is necessary for end products in contact with foods. For Europe therequirements are laid down in national and supra-nationallaws (EEC Commission
Table 11 AnlitelMaterials with UL Recognition
Grade
Color
Min. Thk mm
RTI "C EL UL94
PL460-S 130 130 BK 130 94V-0 1.60 PL720-S All 1.S0 94V-0 UL 550 All 0.75 160 94HB 120160 94HB 1.50 150 120 160 94HB 3.00 All UM550 0.75 16094HB 1.50 94HB 16094HB 3.00 UM55 l All 0.75 160 94HB 150120160 94HB 1.50 3.00 160 94HB UMS52 All150 120 0.75 16094HB 1.50120160 94HB 3.00 94HB UMSS2-V NC 1.50 94V-2
HA1 CTI D495 HVTR
HWI WO1 W1 -
-
-
-
-
1
-
0 0 1 0 0
-
-
0
S
0
1 0 0 1
-
-
-
I S0
4 3 2 4 3 2 4 3 2 4 3 2
0 0
S0
-
-
130 130130
120
160
IS0 IS0
120 120 120 120
150 150 150 IS0
120
150 150
160
50
120 SO
-
-
0
5
0
-
-
-
-
-
-
S
0 -
-
-
0
5
0
-
-
-
0 -
-
Polyester Thermoplastic Elastomers II
385
Directive 90/128/EEC plus corrections and amendments). For the EEC, Arnitel E and P are in compliance with the regulationsfordryfoodstuffsand for nonfattyfoodapplicationsup to 12 1°C. FDA Food Approval. Arnitel E grades are also in compliance with the code of Federal Regulation,issued by the Food and DrugAdministration (FDA) 21 CFR 177.2600(rubber articles for repeated use) in the United States-so-called FDA approval. Arnitel E may be used in contact with foods containing not more than 8% alcohol and limited in its use in contact with food at temperatures not exceeding 150°F. Arnitel E is also approved under 177.1590 (polyester elastomers), but is restricted to dry food with nonfat or oil at the surface. 3A Sanitary Standard-US Department of Agriculture. Arnitel grades EM460 and EM550 are in compliance with the requirements of the 3A Sanitary Standard for multiple use Plastic Materials used as Product Contact Surface for Diary Equipment. This approval is important for the application of COPES in transport belts for dairy products. U.S. Pharmacopoeia Class VI. Arnitel EM400,EM460,EM550, PM580.and P28D have been tested and approved according to U.S. Pharmacopoeia (USP) ClassVI. This approval is necessary for medical applications.
REFERENCES Amitel, TPE-E Typical Properties of Thermoplastic elastomer grades (DSM), Edition 05/93. Arnitel Guidelines for the injection moulding of thermoplastic elastomer TPE-E (Ed. 2/97). Arnitcl Guidelines for thc extrusion of polyester elastomers (TPE-E) (Ed. 3/97). Creemers, H. M. J. C. ( 1996), Thermoplastische Elastomere (TPE). Ku/uf.sfoflc,86: 1845- 185 1. Dupont-Hytrel Engineering Thermoplastic Elastomers. General Guide to products and properties. Edition 03.96 (H 23287). Guldemond, C. P. (1996). Thermoplastic Elastomers Market Review, lecture a t Thcrrnoplastic Elostomer Course at University of Twenic, The Netherlands.
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14 Thermoplastic Polyurethane Elastomers Charles S. Schollenberger* Polyurethane Specialist and Consultant, H ~ d s o n ,Ohio
1. BACKGROUND Rubberlike elastonler systems that have achieved commercial importance through the years are largely aI11oIphol1s. gumlike masses of polymer chains with low glass transition tenlperatures (T,) whose interchiin attractive forces are relatively weak and delocalized (randomly distributed) along the polymer chains. To be useful for mechanical applications. such polymer chains must be joined together in lateral fashion by covalent chemical crosslinks (CC) in the process known a s vulcanization or curing. 0. Bayer and his chemists. first at the Main Scientific Laboratory of I. G. Farbenindustrie beginningabout 1937, lateratFarbenfabrikenBayerandthen other laboratoriesworldwide, subscribed to and applied the classical CC vulcanization concept in explaining and exploiting the excellent mechanical properties of the extant elastomeric polyurethanes (Bayer, 1947; Bayer et a l . . 1950; Bayer and Muller. l960), including the castable liquid types. However, it was subsequently recognisedthat very high-level, useful nlechanical properties can be obtained in amorphous, low-T, polyurethane elastomer systems that are essentially linearly structured, thermoplastic. soluble, and therefore devoid of chemical crosslinks (Schollenberger et al.. 1958, 1962). This phenomenon was attributed to tiepoints among the linear polyurethane chains that are reversible with heat or solvation, and the term “virtual crosslink” (VC), i.e., “crosslinked in effect but not in fact.” was applied to such tiepoints (Schollenberger et al., 1958, 1962). Virtual crosslinks are a consequence of polymer chain structure. and the recognition and exploitation of this concept (Schollenberger et al., 1958. 1962; Schollenberger. 1959) resulted in the invention, development, and commercialization of thermoplastic polyurethane elastomers (TPUs), the first members of a unique polymer class, thermoplastic elastomers, which has since assumed major commercial importance (Dreyfuss et al., l98 1 : Hepburn, 1982). Published views bearing on the nature of virtual crosslinks in TPUs include hydrogen bonding among urethane-group hydrogen atoms and carbonyl groups (Boyarchuk et al., 1965: Cooper and Tobolsky, 1966; Nakayama et a l . , 1969). urethaneaggregationtendency due to hydrogen bonding (Boyarchuk et al., 1965). and the enhancement of these phenomena in urethane-rich TPU chainsegments (Cooper and Tobolsky,1966), whichproducesheterophase polymer morphology.
387
Schollenberger
388
2.
INTRODUCTION
Thermoplastic polyurethane elastomers largely possessthe same outstanding mechanical properties as other types of solid polyurethane elastomers. such as the castable and injectable liquids and millable gums. These property levels usually enable less material (TPU) to be used in a given application than would be the case with a nonurethane material, and this feature gives TPU an edge in many instances. Relative to other important types of polyurethane, the plastic, highly polar nature of TPU allows the use of a different set of processing methods that prove to be the stock-in-trade of the plastics fabricating industry. These include milling, Banbury mixing, calendering, extruding, molding [injection, compression, transfer, centrifugal (powder). blow], and solution processing. Further assets of TPU are its ability to be fabricated into finished elastomer products without the need for curing (crosslinking) them, which makes it possible to reprocess TPU scrap formed during product manufacture. In contrast to mostliquid-processingpolyurethane systems, TPU can be compounded extensively with additives and loaded heavily with pigments. But the need for compounding, e.g., plasticization with its problems of migration, evaporation, etc., or pigment reinforcement is often precluded by the ability to tailor the desired mechanical and chemical properties into TPUs during their chemical synthesis. This has considerable value from an aesthetic standpoint, often enabling transparency, light color. etc., and also from a physiological standpoint. because of the superior purity that can be attained, which might not be achieved with conventional elastomers that require compounding. With so many favorable features, it is little wonder that TPUs have become so attractive to polymer producers and so well received by plastics fabricators. It is estimated that the worldwide production and use of TPU in 1985 reached a level of 40 million (Mobay Chemical Co. 1985 estimate) to 55 million pounds (B. F. Goodrich 1985 estimate). Fromatechnicalstandpoint, the linear,tractablenature of TPUshas allowedtheir investigation by numerousaccomplishedscientistsinternationally. in solution, in the melt, and inthesolid form.This hasgreatlyaided in theirscientificstudy, thereby helping to providea clearer understanding of thebehavior of TPUs and of a l l polyurethanesand thermoplasticelastomers.
3. SCOPE AND CONTENT OF CHAPTER This chapter deals with several aspects of thermoplastic polyurethane elastomers, but there is much more published information on the subject than could be cited and discussed here. What has been attempted is to provide a path through the wealth of information on TPUs, leading from their inception, progressively through the science and technology that have attended their development, to the uses and markets for these remarkable materials. In the process. the topics covered include TPU chemistry: reaction components and structure effects; polymerization processes; chain structure. organization, and behavior; morphology and thermal responses; molecular weight effects; chemical crosslinking effects; environmental stability (to hydrolysis, ultraviolet and gamma radiation, and microbes);compounding. processing, and applications; commercial polymers; and market volume. Hopefully, the path will be considered to have been sure and direct. If rather narrow.
389
Thermoplastic Polyurethane Elastomers
4.
CHEMISTRY
Polyurethane isocyanate chemistryis a very broad fieldthat embraces a large numberof chemical reactions, including the many reactions of isocyanates with active hydrogen compounds, with other isocyanate groups. with other unsaturated compounds, etc. (Saunders and Frisch, 1962). Only the more important isocyanate reactions in TPU formation are covered here. Thesynthesis of TPU components is not discussed. 4.1
Urethane Group Formation
The basic chemical reaction involved in making any type of polyurethane, including TPU, is urethane group formation. Thisis almost always accomplished by causing an organic isocyanate group (-N===C"V) to react with an alcoholic hydroxyl group ("OH), as seen in Eq. (1). RNCO Isocyanate
+
R'OH Alcohol
- RNHCOOR' .- Urethane
+
AH Heat
As seen in the equation urethane formation is a reversible equilibrium reaction.The equilibrium state liesfar tothe right sideof the equationat normal temperatures, andat room temperature there is negligible urethane dissociation. However, the dissociation increases with increasing temperature, and the converse is also true: TPU melt polymerization studies in a small torque rheometer have made this clear (Schollenberger et al., 1982) (see Fig. I). Studies also show that the thermal stability of urethanes varies considerably depending on their structure. For example, those from tertiary alcohols decompose readily at temperatures as low as 50°C. In contrast, urethanes from primary and secondary alcoholsmay undergo changes only slowly at 150-200°C. The presence of other reactants and of catalysts can also influence the stability of urethanes (Saunders and Frisch, 1962).
4.2
Urea Group Formation
Equation (2) shows the isocyanate group reacting with the carboxyl the urea group.
Isocyanate
+ 2R'C02H
Acid
-
RNH-CO-NHR Disubstituted urea
+
group (,’-diisocyanate)”
H IMethylene ?MD1
bis(4-cyclohexyl isocyanate)”
pPDI
p-Phenylene diisocyanate“
NDI
1 ,SNaphthylene diisocyanatc“
pXDldiisocyanate“
p-Xylylene
OCN~CHz-@~cO
OCN QNCO
oc OCNCHG
’‘ Aromatic. h c
Cycloaliphatic Aliphatic.
Thermoplastic polyurethanes based on polyether glycols are considerably more hydrolysis- and fungus-resistant than thosebased on polyesterglycols (excepting polycarbonate glycols). Table 3 lists the types of macroglycols that could be used to make TPU elastomers and provides some specific examples.
5.3
Chain Extenders
The chain-extender component of TPU elastomers is again a relatively small difunctional molecule, usually a glycol, of molecular weight 100-350, about the same size as the diisocyanate component. In the TPU polymerization,thechainextender and somediisocyanatereact to produce urethane-rich hard segments that comprise regions of strong hydrogen bonding in the TPU polymer chains. The chain extenders used in TPU also make a pronounced structural contribution to polymer physical properties (Schollenberger, 1969). Some of the same generalizations as were discussed for the diisocyanate component apply to the chain extender; the chain extender should produce hard segments and urethane groups that associate and pack well. Short, open-chain structures free of appendages favor this, while chain extenders having compact, symmetrical, cyclic nuclei produce harder, higher-modulus TPU. Table 4 lists the types, with examples, of some TPU chain extenders that have achieved commercial importance.
-
6.
POLYMERIZATION PROCESSES
Basically,thereare two polymerisationmethods by which TPU may be prepared:thetwostep (“prepolymer”) process and the one-step (“one-shot”) process. The former involves the
393
Thermoplastic Polyurethane Elastomers Table 3 Some Commcrcial Macroxlycols Used to Make TPU Elastomers Abbreviation PTAd
Common chemical name
Structure
HOf(CH2~OCO(CH2tqC00]-(CH2~OH
Poly(tctramethy1ene adipatc) glycol"
n
€-cnprolactollc) PCL glycol" PHC
Poly(hexamethy1cnecarbonate) glycol'
PTMO
Poly(oxytetran1ethylcnc) glycol'l
PPG
1,2-oxypropylenc) Poly( glycol''
Table 4 Some Commercial Chain Extenders Structure
Abbreviation namc
chemical
Useful in TPU Elastomers
Common
I ,4-BDO
1.4-Butanediol"
CHDM
1.4-Cyclohcxanedimethanol" p-Xylylenc glycol'
HQEE
I ,4-Bis(2-hydroxycthoxy) benzene''
Schollenberger
394
preparation of a low molecularweight,linear.isocyanate-terminatedprepolymer[Eq. followed by its chain extension to a high molecular weight linear polynler (Eq. (&)l. nOCN-R-NCO Diisocyanate
+
(6a)l
~-
HO-R'-OH Macroglycol
OCN-R-(NHCO-O-R'-OCONH-R-)NCO
+
(n
- 2)OCN-R-NCO
Prepolymer Unreacted diisocyanate
l
(n
- 1)HO"R""OH Chain extender
ftNHCO"O-R"O-CONHR~~NHCOO-R"-OCONHR)-+
n-l x
In the first step [Eq. (6a)], the dry macroglycol and (excess) diisocyanate react in urethane link formation [Eq. ( l ) ] to produce isocyanate-terminated linear chains, which remain relatively low i n molecularweightand melt viscosity,thusfacilitatingsubsequentmixing with chain extender. The diisocyanate-macroglycol chain segments (singly underlined) comprise the ut-ethane-sparse soft segments in the TPU chains. In the second step [Eq. (6b)], added dry chain extender reacts with prepolytner terminal isocyanate groups in further urethane link formation to couple the prepolymer molecules and produce a high-molecular-weight TPU elastomer. If the prepolymer of Eq. (6a) contains free diisocyanate component, which can be achieved by raising the diisocyanate/macroglycol molar ratioto >2/1 i n the initial charge orby supplemental diisocyanate addition to the preformed prepolymer, a quasi-prepolymer results, whose reaction with equivalent omounts of chain extender [Eq. (6b)l produces TPU elastomers that are harder and of higher modulus due to their increased content of urethane-rich hard segment. In Eq. (6b). when the isocyanate content of the prepolytner and the hydroxyl content of the chain extender are charged to be equivalent. or when the latter is a bit greater. linear TPU results. In effect, the process has produced linear polyurethane chains that can be segmented, i.e.. of the +AB+,, structure, to any desired degree. But if somewhat less than an equivalent amount of the chain extender is charged with the prepolymer. then the excess isocyanate of the prepolymer can ultimately react with TPU-chain urethane groups in forming allophanate links [Eq. (4)1, providing branches on the TPU chains or chemical crosslinks among them. Such regulation of component stoichiometry allows the production of chemically crosslinked TPU if desired, e.g.. like Mobay's Texin. In the "one-shot" (single-step, random-melt-polymerization condition) process all of the TPU components are mixed together at one time. Here the alternating soft (singly underlined) and hard (doubly underlined) segments joined end-to-end through urethane linkages [Eq. (7)]. nOCN-R-NCO Diisocyanate
+
HO-R'-OH Macroglycol Chain
+
(n
- 1)HO-R'"- OH extender
As in Eq. (6b), the stoichiometric balance of isocyanatehydroxyl groups charged determines whethertheproduct will be alinearlystructured TPU (NCO/OH 5 1.0) as pictured. or an allophanate-branched and/or crosslinked TPU (NCO/OH > 1 .0).
Thermoplastic Polyurethane Elastomers
395
Just as the prepolylner process. TPU hardness and modulus can be increased by inci-easing the diisocyanate and the (balancing) chain-extender levels charged in the polynlerization, which, i n effect, produces more and longer diisocyanate-chain extender hard segments in the polymer. ,411 example of the one-shot batch process for preparing TPU is provided in the fOllOWing. A mixture of 1.70 mol of dry poly(tetramethy1ene adipate) glycol (M,,, 849; acid no.. 0.89) and 1.22 mol of dry 1.4-butanediol is mechanically stirred at I 10°C with a spiral ribbon stirrer in a 4-liter resin kettle heated with a Glas-col mantle. After about 10 min of such mixing, 2.92 mol of diphenylmethane-~,p’-diisocyanateis added cleanly in one portion. Vigorous stirring is continued for 1 min, and the reaction mixture is then poured into a lubricated I-gal can, which is promptly closed with a friction-fitting lid. The sealed can is placed in a 140°C oven for 3.5 hr to complete the polymerization and then cooled, and the snappy elastomeric TPU product is removed. It has 85 degrees Shore A hardness, mills satisfactorily at 225°F on a plastics mill, dissolves in N,N-dimethylformamide,and has excellentphysicalproperties (Schollenberger, 1959). Other details of TPU preparation and continuous commercial processes have been published (Saunders and Piggot, 1965; Bartel et al., 1971; Rausch and McClellan, 1972; Quiring et al., 198 I ) . The two latter references involve the use of a twin-screw extruder-reactor. An elegant way to study TPU melt polymerization is to conduct it in a small torque rheometer.where it is possible to followthereaction full termwhilecontinuouslyrelating; polymerizate viscosity to reaction temperature and time. Such studies of the one-shot polymerization process have been done using a C.W. Brabender Plasticorder torque rheometer as reactor (Schollenberger et al.. 1982). Polymerization variables investigated included the effects on the course and degree of polymerization of reaction temperature and time, macroglycol acid number, antioxidant. TPU composition, catalyst, shortstop, and reactant imbalance. Figure 1 shows the plot of typical results from this study when identical recipes were polymerized at 202°C (Polymers 6 and 7) and at ”222°C (Polymer 8).
7. CHAINSTRUCTURE,ORGANIZATION, ANDBEHAVIOR The urethane linkages in TPU chainsprovide ample opportunity for interchain hydrogen bonding. This occurs to different degrees depending on the groups involved, for example, between the urethane hydrogen atoms of one chain and the urethane and ester carbonyl groups or the ether oxygen atoms of adjacent chains (Boyarchuket al., 1965; Seymour et al., 1970). In polyurethane elastomer chains, urethane-urethane association through hydrogen bonding,with attendant ordering (aggregation) of the urethane units thereby (Boyarchuk et al., 1965), and the same greatly intensified process in segmented polyurethane elastomer chains (Cooper and Tobolsky, 1966) would seem to account for the elastic nature of TPU. The foregoing interpretations can be drawn together and stated as the “virtual crosslink” principle, which underlies the behavior of TPU: TPU elastomerpolymerization produces linear, segmented, polymer primary chains comprising alternating urethane-sparse, low-T, soft segments (diisocyanate-macroglycol linear reaction products) (A) and urethane-rich. high-T, hard segments (diisocyanate chain-extender linear reaction products) (B), which are connected end to end through urethane linkages. TPU chains thus have the structure (AB),,. These chains exhibit the customary, rather delocalized, weak van der Waals association forces for each other, and they are long enough to tangle with each other. But in addition, due to their high concentration of urethane groups, hydrogen bonding is particularly strong among the TPU-chain hard segments, and the association of aromatic T electrons when present supple-
396
Schollenberger
0
I
2
3
4
5
6
7
8
9
1 0 1 1 1 2 1 3 1 4 1 5 1 6
REACTION TIME(mlnutes) Fig. 1 Antioxidant-stabilized thermoplastic urethane melt polymerization viscosity-time-temperature relations. (O), Polymer 6; (X), Polymer 7, (A), Polymer 8.
1I
A
or SOLVENT
lb)
Fig. 2 TPU e1;lstomer chainstructure,organization,andbehavior:(a), network of polymer primary chains; (h), polymer primary chains.
virtually crosslinkedextended
Thermoplastic Polyurethane Elastomers
397
ments this attraction. Consequently, the hard segments associate to form aggregates (domains) in thesoft-segmentmatrix,which also becomesmoreself-associatedintheprocess. These phenomena give rise to detectable heterophase morphology in TPU. The aggregated hard segments accordingly tie the TPU chainstogether at localized points in lateral fashion and extend them linearly, producing a giant networkof chains andthus elasticity in TPU. However, although these crosslinks and extension links are effective under practical use conditions, they are reversible with heat and with solvation. This permits thermoforming and solution applications of TPU. As a consequence. this type of TPU linkage has been called a“virtualcrosslink” (VC)(Schollenberger et al., 1958,1962)andthenetworkproduced a “virtual network,” for they are crosslinks and networks “in effect but not i n fact.” The foregoing types of TPU chain structure, organization, and behavior are depicted schematically in Figure 2.
8. MORPHOLOGYANDTHERMALRESPONSES The return to the VC state on cooling after VC thermal disruption in a heated TPU sample has been shown to be a morphology-related, time-dependent, phase-separation phenomenon that is concurrent with and explains the redevelopment of mechanical properties on cooling and aging the heated sample (Schollenberger andDinbergs, 1975, 1978; Wilkeset al. 1975). Comparisons of VC network thermal disruption in VC versus CC-VC TPU (covalent-crosslinked TPU) show that the covalent crosslinks of the latter system impede but do not prevent the redevelopment of virtual crosslinks on cooling and aging (Schollenberger and Dinbergs, 1975, 1978; Ophir and Wilkes,1979). The hard-segment aggregates (domains) in TPU have been studied from several standpoints. In unmanipulated (unoriented. unannealed) samples of many useful TPU compositions, the hard segments do not registerwell, if atall, as crystallineregions in wide-angle x-ray diffraction studies (Cooperand Tobolsky, 1966; Wilkesand Yusek, 1973; Wilkes and Wildnauer, 1975; Wilkes et al., 1975). Inadequate crystallite size has been proposed to explain this or the difficulty in achieving a good crystalline lattice due to the impeding action of hydrogen bonding on segment mobility (Schollenbergerand Dinbergs.1975,1978;Schollenberger, 1979). The term “paracrystalline” (Hosemann andBagchi.1962; Hosemann, 1975)has been applied to thesemicrystallinestate of aggregated hard segments in unmanipulated,N-monosubstituted polyurethane elastomer samples (Bonart, 1968). In any case, small-angle x-ray scattering measurements clearly show the presence, size, and separation of hard-segment aggregates in relaxed TPU samples (Bonart, 1968; Bonart and Muller. 1974; Wilkes and Emerson, 1976; Ophir and Wilkes 1979, 1980). Thermal analysis by differential scanning calorimetry (DSC) shows their presence, thermal disruption (melting), and reaggregation (crystallization) on cooling, as can be seen in Figure 3 (Schollenberger, 1979). The TPU sample tested (at lO”C/min) in Figure 3 wasinjectionmoldedfrom a highurethane-content injection-molding polymer that contains MDI/PTAd soft segments and MDI/l, 4-BD0 hard segments. On heating note the -30.5”C T, value for the soft segment at C and hard-segment thermal disruption (melting) at 175.5-204.5”C between D and H, with a heat of fusion, A H,-,of 3.20 mcal/mg. Then on cooling from 250°C note the hard-segment reaggregation (crystallization) exotherm at 145.0-105.0”C between J and L, with a heat of crystallization, A H,. of 3.1 1 mcal/mg. The annealing and orientation of TPU samplescan develop and improve their crystallinity, making it easier to detect and to study both soft- and hard-segment domains by wide-angle xray diffraction. Such treatment (manipulation) has led to much useful information about the
Schollenberger
398
C -30.5"
I\ I
SAMPLE TEMPERATURE, "c
-
Fig. 3 DSC thermogram of an injection-moldingTPU.
crystalline character of the domains, including hard-segment packing. hydrogen bonding.conformation, density. and size (Blackwell and Lee, 1983; Born et al., 1984). Both the structure and the sizeof hard segments are factors in their effectiveness as virtual crosslinks in TPU. Good component symmetry, compactness, lack of appendages, rigidity, and long segmentsarefavorablefactors(Schollenberger, 1969; Seefriedet al., 1975). But it has been shown that hydrogen bonding is, i n fact, r w t a prerequisite for effective hard segments by the demonstration that certain N-disubstituted poly(ether-urethane)TPUs whose urethane groups bear no hydrogen, precluding hydrogen bonding, nevertheless exhibit elasticity and good strength (Hanell, 1969). In this type of TPU, whose structure is shown i n Figure 4, the hard segments
X
Fig. 4
Non-hydrogeu-bondiugTPUstructure.(FromHarrell,
1969.)
Thermoplastic Polyurethane Elastomers
399
again associate to form aggregates (VCs) in the soft-segment matrix, but these hard-segment aggregateshaveconsiderablecrystallinity (Harrell, 1969; Samuels andWilkes, 197 I , 1973; Allegreza et al.. 1974). possibly due to the absence of hydrogen bonding to impede segment ordering within their aggregates. Studies have indicated that hard segments of only one unit length (MW 228) can produce effective virtual crosslinks in TPU (Harrell. 1969; and Sanwels and Wilkes, 1973). So now we see that crystallinity alone, as well as hydrogen bonding, can produce virtual crosslinks. In addition. the VCprinciple has been extended to includeglassy (amorphous) hardsegment aggregates (VCs), as in the segmented, triblock A-B-A (e.g., polystyrene-polybutadienepolystyrene) type of thermoplastic elastomers and ionomers where the VCs are ionic (electrostatic).
9. MOLECULARWEIGHT EFFECTS Due tothe linearity of their primarychains (seeFig. 2). TPU molecular weights can be determined and related to physical properties (Schollenberger andDinbergs. 1973, 1974. 1979a.b; Scholleaberger 1979). This was done using gel permeation chrornotography for a series of TPUs (M, 48,000-367,000) all made according to the same recipe from MDI, PTAd,and 1,4-BDO (SchollenbergerandDinbergs, 1979a.b. Schollenberger 1979).The interestingresults of the study show that in many cases the change (enhancement or impairment) in TPU characteristics and properties with increasing polymer weight-average molecular weight(M,) continued to a certain M, level (in the range 100,000-200.000) and then tended to level off at an inflection molecular weight (IMW). The physical explanations advanced to account for IMW were the achievementof polymer chain average lengths that (1 ) favor a "balled" (less extended)chain configuration. (2) produce a virtual network of chains (due to increase in VC sites and entanglement opportunities per chain) that is unresponsive to furtherchainlengthincrease,and (3) have a chainend (free volume) content whose further reduction by molecular weight increase does not affect polymer internal (segment) mobility (and thus polymer morphology development) or polymer density. Results showed that as TPU weight-average molecular weight increased, the following polymer characteristics and properties increased (approximate IMWvalues givenin parentheses): specific gravity ( I 80.000), processing temperature by dynamic extrusion rheometer T2 value (200,000). T, value ( 1 60,000). tensile strength ( 1 45.000), abrasion resistance (Taber) and lowtemperature modulus Gehman Tloo( 1 12,000). and Clash-Berg T , ( 134,000). In addition, as TPUweight-average molecular weight (M,) increased, the following polymer characteristics and properties decreased: melt index (200,000), hysteresis (125,000). extension set ( 130,000). stress relaxation and flex life (deMattia) ( 1 80,000. with a maximum flex life at 100,000). TPU hardness varied little with M,, and neither hardness nor 300% modulus showed systematic dependence on MW. In contrast with the number-average molecular weight (M,,) versus intrinsic viscosity[q]relationships noted in an earlier study (Schollenberger and Dinbergs, 1973, 1974). the foregoing study showed no IMW in the MWversus [q]plot, but only increasing [q]with increasing Mw. Figure 5 shows the stress-strain plot for the TPU series wherein the individual polymers had the following M, values: A, 47,900; B. 60, 100; C. 100,200; D, 1 17,100; E. 182,800; F, 190,500; G, 248,700; H. 289,100; 1.35 1,200:J, 366,800 (Schollenberger and Dinbergs, 1979a,b). In Figure 5, the stress-strain curves steepen with increasing M, for polymers A-J. Sample tensile strength increases with MWto a maximum level in the group E-H. An increase in overall
-
400
Schollenberger
polymermoduluswithincreasing MW is quite obvious in Figure 5 . reflecting the effect of increasingly moreextensive chain networks, which alsoaccounts forthe progressively decreasing ultimate elongation of the polymer samples with increasing chain length MW.These relationships are attributed to the greater number of chain entanglements and the greater number of virtual crosslinks per chain in the longer TPU chains, both contributing to a more extensive and thus tougher and less extensible network of polymer chains. 10. CHEMICAL CROSSLINKING EFFECTS It would not seem that the covalent crosslinking (CC) of TPU would be of interest for these materials, so touted for their advantages as therrnoplastic elastomers. But the usefulness of TPU
401
Thermoplastic Polyurethane Elastomers
--CH,&
-CHPeroxidL -CH-CH-
~.
-CH
2
-
Methylenepositions in polyurethane chains chains
.
-CH_ _ _ _ L '
Freeradical positions in polyurethane chains
I
Carbon-carbon crosslinked polyurethane
Fig. 6 Covalent chemical crosslinking of TPU by organic peroxide.
can sometimes be increased by superposing a CC network on the existing VC network. This Can be done via free radical sources such as organic peroxides, which can be incorporated into ordinary TPU on a rubber or plasticsfriction roll mill (Schollenberger andDinbergs, 1975. 1978) or during TPU polymerization (Schollenberger and Dinbergs, 1981). Here peroxide decomposition tenlperatures must be compatible with TPU processing temperatures. A peroxide found useful for theseapproaches is a,a'-(bis-t-buty1peroxy)diisopropylbenzene supplied as Percadox 14 (Akzo Chemie; half-life at 100°C. 1000 hr; at 160°C, 0.32 hr) (Schollenberger and Dinbergs, 198 I). Solution mixing of TPU with peroxides is another option, allowing gentler mixing conditions. The peroxide-TPU compound, which may also contain a co-agent and other additives. is stable until the peroxide is activated, e.g., thermally, as during extrusion or injection molding, to provide polyurethane parts having both thermally reversible VC and thermally stable CC networks. The free radical crosslinking of TPU is considered to involve the generation of free radical positions in the polyurethane chains and the coupling of these positions to form carbon-to-carbon covalent chemical crosslinks as depicted in Figure 6. Sites suggested as likely points for free radical formation in polyurethane chains are the a-methylene positions in the adipyl moieties of adipate-based polymers (Urs, 1962: Weisfeld et al., 1962) and the methylene group between the two phenyl rings in the diurethane bridge structure of MDI-based polymers (Bayer and Muller, 1960). Thus, one can capitalize on the familiar high-speed thermoplastic processability of TPU while producing polyurethane products with enhanced properties, including greater resistance to solvation effects, stress relaxation, heat distortion, permanent set, etc. The CC-VC TPUsalso show higher modulus values but reduced extensibility, tear strength, low-temperature flexibility, and flex life-all expected consequences of a higher degree of polymer chain crosslinking. Figure 7 shows the stress relaxation behavior at 25 and 100°C of a (1.30) MDVPTAd (MW 1000)/1,4-BDOTPU in the uncured (V) and peroxide-cured (C) states.All, samples were preconditioned relaxed at test temperature for 10 min just before testing. At 25"C, the VC polyurethane (V) showed 50% stress relaxation in 1 hr versus 35% for the CC-VC polyurethane (C), showing the relativelyweak virtual crosslinking in this lowurethane-content TPU and the positive effect of covalent crosslinking. AtIOO'C, the VC polyurethane (V) melted and does not appear in Figure 7, whereas, the CC-VC polyurethane (C), withitsvirtualcrosslinksthermallydispelledattesttemperature but its thermally stable covalent crosslinks still intact and effective, showed only 15% stress relaxation. Virtual crosslink networkdevelopment in aged VC and CC-VC samples has been compared in stress relaxation tests (Schollenberger and Dinbergs, 1975,1978) and by other methods (stressstrain, wide-angle x-ray diffraction. differential scanning calorimetry) (Ophir and Wilkes, 1979).
402
Schollenberger
loo
-
90
-
a W
z
e 80v)
a a g ro-I
U
z
25"
2 a
z 600
S 50 -
V = VC POLY URETHANE C = %-VC POLYURETHANE
Fig. 7
\
X
25"
Stress relaxation (continuous, 20% extension).
Results show that virtual crosslink development progressed during long-term aging in both VC and CC-VC samples, but it was impeded by covalent crosslinking in the CC-VC TPU.
11. ENVIRONMENTALSTABILITY AND STABILIZATION Polyurethanes, including TPUs, although highly useful and serviceable materials,do havelimitations. Some of these are related to their instability in certain environments such as in the presence of water, ultraviolet radiation, microorganisms, and nitrogen dioxide. Here they occasionally must be stabilized with additives or by polymer chemical structural changes. On the other hand, polyurethanes show excellent resistance to certain other environments such as gamma radiation, ozone, andhydrocarbon oils and fuels. Atthis point TPU environmental stability and stabilization will be reviewed. 11.l
Hydrolysis
When carboxylate polyester-based TPUs such as those based on poly(tetramethy1ene adipate) glycol (see Table 3 ) are prepared, a few of the chains may terminate in unreacted carboxyl groups (-C02H) introduced with the macroglycol component. And when carboxylate polyesterbased TPUs hydrolyze, chain cleavage occurs at the carboxylate ("COO-) linkage, producing some new chains that bear terminal carboxyl groups. The carboxyl groups from both sources autocatalyze further TPU hydrolysis and accelerate this degradation process.
403
Thermoplastic Polyurethane Elastomers
\lW0
1,m0
0
I
I
I
I
2
3
I 4 IMMERSION TIME
I 5
n
6
IN 709: W4TER (WEEKS)
I
7
I
e
,0%
0-
9
Fig. 8 Effcct of added polycarhodilmidc on TPU hydrolysis stability.
Polycarbodiimides of the structure
n prove to be very effective hydrolysisstabilizers for poly(ester-urethane)s.Theyfunction by reacting with carboxyl groups asthey are generated, neutralizingthem and sinlultaneously mending cleaved chains in the hydrolyzing polymer. Figure 8 shows the effectiveness of 2.0 phr of a polycarbodiimide, Stabaxol P (Mobay, Bayer). in stabilizing an ordinary TPU against hydrolysis in 70°C water (Schollenberger and Stewart, 1971a, b. 1973a). Polyepoxides are also used as polyurethane hydrolysis stabilizers due to their ability to react with and neutralize carboxyl groups and to thereby also concurrently mend cleaved TPU chains (Kaiserman and Singh. 1976). Lactone polyester-based TPUs such as those made from poly(ecapro1actone) glycol (see Table 3 ) behave much like the carboxylate polyester-based TPUs with respect to hydrolysis stability and stabilization.In fact, the macroglycolester linkage in PCL is the carboxylate linkage, but the macroglycol is made by lactone polymerization rather than by dicarboxylic acid-glycol condensation. Carbonate polyester-based TPUs such as those based on. poly(hexan1ethylene carbonate) glycol (see Table 3 ) have excellent hydrolysis resistance (Muller, 1970). Hydrolysis of their ester linkage, the carbonate link ( - ~ O - O - ) , produces terminal hydroxyl groups on the cleaved chains and carbon dioxide gas. but no carboxyl ends to autocatalyze further hydrolysis (Schollenberger and Stewart, 19733).
Schollenberger
404
Polyether-based TPUs, such as those based on poly(oxytetran1ethylene) glycol (see Table 3), have outstandinghydrolysisstability, since the ether linkage of themacroglycol is very difficult to hydrolyze. In fact, the urethane linkage becomes the most susceptible linkage in regard to hydrolysis in poly(ether-urethane)s. Another significant factor in determining TPU hydrolysis stability is the degree of hydrophobic character and thepermeability to water of the TPU chains (Muller,1970; Schollenberger and Stewart 1971a. b; Schollenberger and Stewart, 1973; Dieter et al., 1974). Thus, the more hydrophobic (due, e.g., tohigh hardness) the TPU is, the less water it will absorb and the more hydrolysis resistant it will be.
11.2 Ultraviolet Radiation (Schollenberger and Dinbergs, 1961; Schollenberger; and Stewart, 1972, 1973b, 1976a,b) Perhaps the greatest factor in the terrestrial weathering of TPU is ultraviolet radiation in the wavelength range 330-410 nm. This energy, from incident solar radiation, initiates an autoxidative degradation processin TPU that can chemically crosslinkthe chains extensively, embrittling and insolubilizing the TPU, particularly aromatic urethane TPUs. In addition, aromatic urethane TPUs develop a pronounced yellow-to-brown color in the process, whereas aliphatic urethane TPUs are color-stable. The UV degradation process is believed to generate quinone-imide structures in aromatic urethanes having proquinoid structures [Eq. (g)] that can be followed by active hydrogen moiety addition to the quinone-imide that would crosslink TPU chains.
Quinone
Urethane
Imide +
H,O
Autoxidation of Urethane to Quinone-Imide A different view holds that the aromatic urethane UV degradation process involves photoFries rearrangements of the type seen in Eq. (9) to explain discoloring and crosslinking of TPU chains.
+o
NH -CO,R
___,
CO2 R
-Aminobenzoate o-ArninobenzoateUrethane then 2NH2
Amino group
0 2 --
-N=N-Azo group
+
H 0 2
Equations (8) and (9) are not possible in aliphatic urethane TPUs, which are nonquinoid structures. Both mechanisms are compatible with the fact that aliphatic urethane TPUs show better UV stability than aromatic urethane TPUs. There is little doubt that some hydroperoxidation of the TPU chains also occurs in the UV-initiated autoxidation process regardless of the structure of their urethane groups. In view
Thermoplastic Polyurethane Elastomers
405
of the nature of the TPU UV degradation process, it is not surprising that a combination of a UV absorber, 2-(2’-hydroxy-3’,5’-di-r-amylphenyl)benzotriazole(Tinuvin 328, Ciba-Geigy), and an antioxidant, tetrakis[methylene-3-(3’,5’-di-r-butyl-4’-hydroxyphenyl) propionatelmethane (Irganox I O I O , Ciba-Geigy), proves to be a particularly effective UV stabilization system for TPU (Schollenberger and Stewart, 1976a,b). In addition, low levels of carbon black alone, acting as a UV screen, are extremely effective UV stabilizers for TPU (Schollenberger and Dinbergs, 1961). 11.3 Microbial Attack
Some polyurethanes, including TPUs, are subject to microbiological attack (B. Pat., 1958, 1972; DiPinto, 1963; Kanaval et al., 1966; Darby and Kaplan, 1968; Kaplan et al., 1968; Elmer, 1970; Huang, 198 1 ). Both poly(ester-urethane)s and poly(ether-urethane)s will support microbiological growth, but only the poly(ester-urethanels are degraded by it. Fungi are commonly involved, and damagetakes the form of surface cracks (“fungus cracks”) that penetrate the sample. These cracks usually eventually proliferate and become obvious to the unaided eye in stressed areas (bends, folds. etc.) of infected samples. They areusually not attended by polymer discoloration. in contrast to microbial attack involving soil exposure, which often discolors the TPU. Factors favoring microbiological attack on TPU are those that favor microbial infection and growth. These include outdoor exposures above, on, or below ground to damp, warm, dark environments. If these conditions are met indoors, microbiological attack can also occur there. Various chemical compounds that find use as microbicides in polyurethanes, including TPU. include Cunilate DOP (copper-8-quinolinolate) and Fungitol 11 [N-(trichloromethylthio)phthalimide], both from Nuodex Products Division, Heyden Newport Chemical Corp.; Vinyzene SB-I [ 10, IO’-bis(phenoxyarsine) from Ventron Div., MortonlThiokol Chemical Corp.]; TMTD (tetramethylthiuram disulfide); TBTO [bistri(n-butyltin) oxide];chlorosulfonyl (sulfinyl pyridine) compounds; and carbodiimides. Troysan Polyphase AF-I (3-iodo-2-propynyl butyl carbamate, Troy Chemical Corp.) might be especially suited for use in polyurethanes, including TPU, due to its urethane structure.
11.4 Gamma Radiation Early studies (Harrington, 1957, 1958, 1959; Schollenberger et al., 1960) comparing the gammaradiation resistance of many types of high molecular weight materials showed polyurethanes to be among the most radiation-resistant of polymers and polyurethanes the most radiation-resistant elastomer tested (Bauman). The tendency is for polyurethanes, including TPU, to both chaincleave and (predominantly) crosslink during gamma radiation, but the latter appreciably less than other elastomers, so original properties are better retained. As a consequences. TPUs have found use in applications involving nuclear radiation. 11.5 Other
Polyurethanes, including TPUs, undergo thermal autoxidation (Singh et al.. 1966; Fabris, 1976). This was clearly seen in oxygen uptake experiments with a poly(ester-urethane) TPU at 130°C, which also showed the antioxidant DPPD (diphenyl-p-phenylenediamine) to be highly effective in suppressing oxygen absorption. Surprisingly, EPC carbon black was not far behind in effectiveness (Schollenberger and Dinbergs, 196 1 ). Poly(ether-urethane)s thermally autoxidize more readily than poly(ester-urethane)s.
406
Schollenberger
Polyurethanesundergosimultaneouscrosslinkingandchainscission when exposedto nitrogen dioxide (NO?),with the latter predominant in long exposures. The degradation is accelerated by air in mixtures with NO?. Losses in polyurethane strength accompany the degradation (Jellinek and Wang. 1973; Jellinek et al.. 1974). Tests on atypicalpoly(ester-urethane) TPU showed it to haveexcellentresistance to degradation by ozone (Schollenberger and Dinbergs, 1961 ). The polar nature of polyurethanes, including TPU and especially poly(ester-urethane)s, renders them notably resistant to aliphatic hydrocarbon oils and fuels (Schollenberger and Dinbergs. 1975, 1978).
12. COMPOUNDING Compounding (Schollenberger, 1969; Schollenberger and Esarove. 197 I ) is practiced to stabilize, aid in the processing of, extend, reinforce, or plasticize TPU. Stabilizatiotl was just discussed: it involves only minor amounts of additives ( 1 -5 phr), as does processing. However, the other cases usually involve large amounts of additives (10-100 phr). TPU compounds can be mixed on plastics or rubber mills, in a Banbury mixer, in situ during TPU polymerization, or in solution. Additives should be dry, and the best are neutral, stable materials that are not reactive with the forming or finished TPU either when they are added or later. The TPU polymer itself should be dry for the mixing operation. TPU processing aids are added as release agents for calendering and molding operations and to increase extrusion rates and reduce blocking tendencies in extrusion compounds. They includelubricantssuch as powderedpolyolefins(e.g., low molecularweight polyethylene), synthetic waxes (e.g.. Ross Wax stearamide types), and natural waxes (e.g., bees wax). Extendersareadded to TPU primarily to lower thecost of articlesmade from TPU. Secondary effects usually include increased compound hardness, modulus, and tear strength, and sometimes improved processability. On the other hand, tensile strength usually declines: at the 25-phr level, extenders may reduce TPU tensile strength by 20-309’0. Extenders useful in TPU include carbon black and silica (preferred because they reduce tensile strength the least), whiting, clay. and others. Mixtures with other polymers are possible and provide some interesting and useful blends. Examples of blendable polymers are vinyl polymers (e.g., PVC), vinyl copolymers, copolymer nylons, ABS, polycarbonates, polyolefins (limited amounts), and a wide range of elastomers. The effect on TPU properties is dependent on the blending polymer. In general, the hardness. modulus, and elongation move toward that of the blending polymer with increasing amounts of it. The effect on tensile strength depends on the degree of compatibility of the blending polymer with TPU. Table 5 shows some property changes i n a typical TPU [MDIPT Ad (MW 1000)/1. 4BD01 due to the addition of 100 parts of a vinyl polymer, a phenoxy, and a nitrocellulose per 100 parts of the TPU. The TPU andblendingpolymerwere mixed in methyl ethylketone solution, and test pieces were cut from 3- to 5-mil-thick film cast from the solution. The term “reinforcing agent” originated in the rubber industry, where gum rubbers have low strengthbeforeand after cure (covalent chemical crosslinking) but very good strength, abrasion resistance, tear strength, etc.. after mixing with the preferred reinforcing agent, carbon black, and cure. Such a type and degree of improvement is not seen in TPUs, which, due to virtual crosslinking, are very strong to begin with and are not further strengthened in the established sense by adding “reinforcing agents,” which act more like fillers in TPU.
407
Thermoplastic Polyurethane Elastomers Table 5 Properties of Somc TPU-Plastic Polymcr Blends 300% Modulus
Tcnsile strength Addcd plastic
psi
MPn
None Vinyl“ Phenoxy” Nitroccllulosc‘
5300 6200 7300 8100
36.5 42.7 50.3 55.8
Elong. ( 8 )
730 350 400
< 100
Graves tear strength
psi
MPa
pli
kN/m
450 5 100 6400
3.1
35.2 44.1
-
-
200 400 570 620
3.5.0 70.0 99.8 108.6
Table 6 shows the effect of several levels of EPC carbon black on some properties of a typical TPU [MDIPTAd (MW 1000)/1. 4-BDO]. Mixing was done on a rubber mill. and test samples were 75-mil-thick microdumbbells (Schollenberger, 1969). The preferred method of regulating TPU properties (e.g.. of reinforcing or softening them) is to tailor the polymerchemical structure during TPUsynthesis. This has been termed “chemical compounding.” As discussed earlier, there are numerous ways to do this, including the adjustment of urethane content upward for harder. higher-modulus. tougher polymers and downward for softer. lower-modulus. more soluble polymers. The use of longer. less regular macroglycols is another route to softer TPUs. Dibenzoates, phthalates. and organic phosphates have been used to plasticize TPU. but in the writer’s experience the phosphates tried have degraded any polyurethane. including TPU. in which they have been incorporated. It can be appreciated that plasticization (solvation) of TPU could weaken virtual crosslinks and the virtual network in the polymer. This, together with the migratory. fugitive nature of many plasticizers. recommends a skeptical attitude with respect to plasticization as ;I means of softening TPUs in the place of “chemical compounding.”
13. PROCESSING Thermoplastic polyurethanes can be processed by many methods that are familiar to the plastics and rubber industries. addingto the attractiveness and acceptance of these materials. They include
Table 6 Effect of EPC Carbon Black on TPU Propcrties ~
300% Modulus
Black Tensile strength
0 5 10 25 50
~~
Shore hardness (deg.)
Elot~g.
COIIC.
(phr)
Graves strength tear
psi
MPa
(%v)
5200
35.9 35.2 36.9 22.8 22.4
630 630 630 470 320
5190
5350 3300 3250
psi
MPa
pli
1000 1300 1700 2600 3200
6.9 9.0 11.7 18.6 22.0
420 470 550 640 760
kN/m
55 74 82 96 73 112 133
A
88 90 92 95 98
C
59 62 85
Schollenberger
408
extrusion, molding [compression, injection, blow, transfer, centrifugal (powders)], calendering, coating (extrusion, melt, transfer, solution), film blowing, sealing (heat, solvent), etc. Many widely used plastic materials including TPUs absorb some moisture from the air. and, although supplied dry as granules or pellets, TPUs and their compounds are best dried at 105°C for 2 hr in a tray or a dehumidifying hopper-dryer before processing from open bags (B. F. Goodrich. 1984). The abbreviated discussion that follows is intended to give the reader some feel for the conditions employed in processing typical commercialTPUs and is limited to extrusion. injection molding. and calendering.
13.1Extrusion
(B. F. Goodrich,1984)
The effective extruder barrel length should be at least 24 times its internal diameter (L/D = 24/1), but higher L/D ratioshavebeenusedsuccessfully. Screwdesign shouldincorporate the following principles: single-stage, constant-taper construction; 3/1 compression ratio; long transition section (30-40% of screw length); long meter section (40-50% of screw length); hard chrome. pinhole-free, polished surface; cored for temperature control; screw-barrel linear clearance, 0.003-0.005 inch. In the extrusion process the proper melt temperature will range between 165 and 200°C depending upon the particular equipment used, the compound selection, and other processing parameters. Harder TPU compounds will require higher processing temperatures than the softer compounds.
13.2 Injection Molding
(B. F. Goodrich, 1985a)
Although all types of machines have been used successfully, a reciprocating screw machine is preferred for injection molding Estane TPU because of the speed, control. and melt uniformity it provides. A shot weight of 60-75% of barrel capacity is recommended, but shot sizes as low as 30-35% can be accommodated withadjustments. The besttype of screw is a"generalpurpose" screw with a compression ratio of211 to 311 and a 60" included angle tip with an antibackflow mechanism (ball-check or sliding-ring type). It should rotate at about 40-50 rpm. A straight, open nozzle with a reverse-taper tipis recommended. Any type of mold that incorporates good thermoplastic design principlesis satisfactory for Estane TPU.Two-plate, three-plate, and hot runner molds have all been used successfully for a variety of large and small parts. The mold must also be adequately cored for cooling to 10-44°C to provide optimum cycles. The recommended stock temperature for injection molding is 192-210°C but varies with the Estane TPU. Temperatures to 232°C are satisfactory for molding large parts. It is recommended that injection-molded TPU parts be held for 48 hr before exposure to elevated temperature (such as the 122°C that would be encountered in a paint-curing oven) to relieve molding stress and avoid heat distortion of the parts. Table 7 summarizes injection-molding information for a typical poly(ester-urethane) TPU, Estane 58206, which has a Shore A hardness of 85 degrees (B. F. Goodrich. 1985a).
13.3 Calendering TPU can be calendered (Schollenberger, 1969; B. F. Goodrich. 1964) in much the same way as other thermoplastic polymers adapted to this form of processing. In general they are readied for calendering by mastication in an internal mixer (e.g., a Banbury mixer) or on a plastics or rubber mill at stock temperature of 140-170°C. The high frictional heat generated during TPU mastication enables production mixing equipment to be set lower, at 100- 120°C. When lubri-
-
409
Thermoplastic Polyurethane Elastomers
Table 7 Injection-MoldingConditionsforEstane
BUITI temp., "C Rear Mid Front Nozzle Melt temp.. "C Mold temp., "C Fill rate Screw rate. rpm Back pressure, psi/MPa Injection pressure, psi/MPa Molding pressure, psi/MPa Mold shrinkage," in./in. or cm/cm
58206 TPU 177 188 199
204 203 10-32 Slow to modcrate 20-50 50/0.345 3000-8000/20.7-55.1 2000-5000/13.8-34.5 0.0 I2
cant levels above 1% are used. somewhat higher equipment temperatures are required. Calender roll temperaturesare usually 120- 150°C depending on sheet thicknessand compounding ingredients. Calendered sheet in the thickness range 3-60 mils (0.008-0.152 cm) has been produced. Close control of roll and stock temperatures is necessary to calender the thinner gauges, while steady feed and stock temperatures are important in getting uniform heavy gauge sheets. Table 8 lists the conditions that were used to calender a typical TPU in an "inverted L" calender configuration.
-
14. APPLICATIONS
The ease of processing TPUs by numerous familiar methods; their outstanding physical properties. which enable effective use in thin cross sections; the aesthetics they allow in products; their persisting novelty; and their proven value and acceptance have all combined to generate a great number of applications for TPUs, and the number continues to grow. The followinglist gives some idea of the versatility of TPUs in their applications.
Table 8 ConditionsforCalendering TPU Compound Stock tempcraturc. "C Banbury mixer (at drop) Preparation mill Calender temperature, "C Offset roll Top roll Middle Bottom roll
;I
Typical
168-177 17 I 124 130
135 140
41 0
Schollenberger
Arrtorwti\*e: Fascia, sight shields, bumpers. wing nuts Petrochemicrrl: “Pigs,” liners, jackets. hose, vapor barriers (storage tanks) Grrrphic r u t s : Printing rolls Apl~c~rel: Coats. jackets. rainwear, handbags Footnwr: High-fashion boots, shoe uppers, soles. heels. heel plates, ski boots Sports: Golf ball covers. golf club and racquet grips. footballs and soccer balls. rollerskate wheels. skateboard wheels, life jackets. life rafts Medicrrl: Catheters, prostheses,bloodbags.tubing.surgicaldrapes.disposablegloves. suction bulbs, hypothermia pads. lapidous pads (light, thin. inflatable. anti-bed-sore pads) Hygiene: Disposable diapers (elastic leg bands) Agriculture: Cattle ear tags Furniture: Upholstery Aerosprce: Deicer boots, erosion protection (rain. hail. sand). fuel tanks, window gaskets (satellites). aircraft interior parts (arm rests. floor track inserts), weather ballons. logging ballons lr~t1~rstrir~Iyrotl~rct.s: Chute liners. pipe liners, conveyor belts (including food-grade),cleats. powertransmissionbelts,seals.gaskets, hose (jackets and liners for garden and fire hoses) tubing. rolls. film, sheet. profile extrusions. sewer rehabilitation sleeves, adhesives (solution), adhesives (film forfusing emblems togarments, wear patches), caulks.paintsandcoatings, pipe thread caps, silent gears, flexiblebacking pads (sanders). water sprinklers (cutoffunit). oil pouches, pipeline wraps. bellows(spring, gasoline pump nozzle),shopping carts (wheels,bearing housing). washing machines (ball hinge). tarpaulins. coated paper, coated metal and foil E/Pctronic.s: Magnetic tape binders. w/c jackets, retractile cords. cablejackets (audio. lowtemperature, underwater, underground. power). fiber optics inner and outer jackets. computers, communications. connectors, plugs Energy: Cable jackets (mining, geophysical, nuclear radiation) TrNnsl’ortcrtion: Covers. dunnage bags, OTR vehicles (bushings. bearings) Militc1r;y: Gas masks. capes. footwear. drop bags (potable water).collapsible storage tanks Marine.: Ship repair (retrofit conduit)
Table 9 Commercial TPU Suppliers United States Company
Albis
Foreign Product
Company
B. F. Goodrich Estane Dcsmopan Bayer Dow BASF Mohay Texin Polyurethane Nippon Ind. American Cyanamid Cyanaprenc, Dainippon Cytor Ink CYC Chem. K. J. Quiun Q-ThaIIe Nippon Elastollan Ind. Morthane Morton-Thiokol Plastlcs (UK) Hookcr
Product Elastollan Paroprene
Pandex Elastollan Jectothane
P D
v)
F D 0 Y
Table 10 TPU Product Comparison Chart BFG estane Polyester Property
ASTM method
Units
58206
Shore hardness Specific gravity Ult. tensile str.
D2240 D 792 D 412
deg g/cm3 MPa
85A 1.20 45
-
psi
Tensile stress At 100% elong.
D 412
At 300% elong. Ult. elongation Compression set" (22 hr at 70°C) Flex modulus at 23°C Vicat soft. temp. (method B) Taber abrasion CS 17 wheel, I -kg load HI8 wheel, I-kg load H22 wheel, I-kg load Tear resist., die C Mold shrinkageh Price (list, T. 1. qty)
-
-
-
-
D 412 D 395 D 790 -
D 1525
Dl044 D 624
-
-
MPa psi MPa psi 9c
7c
MPa psi "C "F mgll OOO cycles
-
kN/m pli idin. $/lb
6500 -
8.5 800 10 1500 550 64
-
85 I85 3 36 -
86 500 0.012 2.13
f
4
Polyether
58137 70D 1.23 40 5800
-
22 3200 33 4800 440 SO 33 1 48,000 149 300
12
119
-
228 1.300 0.005 2.11
58300 80A 1.13
32
58810
6400
-
9
-
76 169 I
(D
m 9ON42D 1.15 44
4600 4.8 700 6.9 1000 700 78
D 3
53 $
1300 17 2400 590 64
111
232
-
-
36 -
70 400 0.016 2.70
70
88 500 0.014 2.75
2 .A
Table 10 continued Dow (Upjohn) Pellethane Polyester
2 102-8SA
Property ~
~~
~~~
Mobay texin Pol yether
2355-65D
-
2
ro
Polyester
2 103-80A
2103-90A
480A
80A ? 5 1.13 41 6000
9OAi47D 1.14 43 6250
86A 1.20 40 5800
6 800 12 I675 550 25-40
I1 1530 24 3430 475 25-40
-
-
902B
~
87A k 5
Shore hardness Specific gravity Ult. tensile str.
63D k 4 1.22 41 5900
1.18
43 6300
Tensile stress At 100% elong.
22 3200 31 4500 450 50 269 39,000
8 1100
At 3 0 0 4 elong.
15
2100 600 30
Ult. elongation Compression set:' (22 hr at 70°C) Flex modulus at 23°C Vicat soft. temp. (method B) Taber abrasion CS17 wheel, I-kg load HI8 wheel, 1-kg load H22 wheel. 1-kg load Tear resist., die C
-
5 700
37 5400
11
-
1600 520
55 55 80oO 91
-
Mold shrinkageh Price (list. T/i. qty)
I96 2.7 -
15 245
50 88 500
I400
-
-
2.42
20 83 475 2.53
2.04
~
.' BFG samples unannealed: Dow (Upjohn) and Mobay samples annealed 16 hr at 240°F. Mold shrinkage determined on 0.125 x 3 x 6 molded plaques, Actual shrinkage will vary with part size and design Part thickness. in. 80A 55D
' Mold shrinkage values for Pellethane:
0.125
0.250 >0.250
0.01 1-0.015 0.015-0.020 0.020-0.030
'0.120-in. wall thickness
0.008-0.01 1
0.010-0015 0.0 15-0.020
10 95 540 -
2.50
75D 1.21 41-48 6000-7000
94 535 0.008" 2.19
150- 175 1068 155,000 I82 360
-
2.19
Thermoplastic Polyurethane Elastomers
413
15. COMMERCIAL POLYMERS ANDTHEIR PROPERTIES There are several commercial suppliers of thermoplastic polyurethanes. A fairly complete list of these appears in Table 9. Table 10 lists the physical properties of several typical commercial TPU polymers to provide the reader with some idea of the rangeof property levels available. However, there are many other intermediate polymers whose properties lie between the extremes appearing in Table
10.
REFERENCES Allegreza, A. E., Jr.. Seymour, R. W., Ng. H. N., and Cooper, S. L. (1974). P o / w e r /5:433. Bartel, G. F., Klnwittcr, M,, and Denker, E. (197 I ). U.S. Pat. 3,620,680, to Die Kunststoffburo Orzabruck, Dr. Rester, Nov. 16. Baumnn, R. G., unpublished, B. F. Goodrich Res. Ctr., Brecksvillc, OH. Bayer. 0. (1947),.Arl,qew.Chrrr~.59(9):257. Bayer, 0.. and Mullcr, E. (1960), A r ~ ~ q Chrrrz. e ~ ~ . 72(24):934. Bayer, 0..Muller, E., Peterscn, S., Piepenbrink, H. F., and Windcmuth, E. (1950), Ruhher Cllrrrl. T r c h o l . 23(4):812. Blackwell, J.. and Lce. C. D. (1983), J. Po/yrn. Sci.-Phqs. 212169. Bonart, R. (1968),.J. Mrrcmrrrol. Sci.-Phys. B2( 1):l 15. Bonart, R., and Muller, E. H. (197.4). J . Mlrcrorlrol. Sci.-P/ry.s. B/0:345. Born, L., Crone, J.. Hespe, H., Mullcr, E. H., and Wolf, K. H. (1984), J. PO/WI.Sci.. Phys. Ed. 22: 163. Boyarchuk, Y. M,,Rappaport, L. Y., Nikitin, V. N., and Apukhtina. N. P. (1965), Po/yrr~.Sei. USSR 7: 859. British Patcnt (1958). 707.575, to Farbenfahriken Bayer, July 2. British Patent (1972). 1,274,145, to Mobay Chemical Co., May 10. Cooper, S. L., and Tobolsky, A. V. ( 1966), J. A/>/)/.PO/JWI.Sei. /0:1837. Darby, R. T., and Kaplan, A. M.( 1968), App/. Micro/io/. f6(16):900. Dieter. J. A.. Frisch, K . C.. Shanafelt. G. K., and Devanney, M. T. (1974). Ku/&r Atgo /06(7):49. DiPinto, J. G. (1963), Tech. Bull., Fungus Resistance of Adiprene L-100, E. I. Dupont de Nemours and Co., Mar. 22. Dreyfuss. P,. Fctters. L. J., and Hansen, D. R. ( 1961), RLt/>berCkurtI. Techrlol. 5 4 1): 18 1, Elmer, 0. C. (1970). U.S. P a t . 3,513.433, to the General Tire and Rubber Co.. Sept. 29. Fabris, H. J. ( 1976), Adv. Urethrrrle Sci. Trchrlol. 4 8 9 . B. F. Goodrich (1964). Tcch. Bull.. Estane Sheet and Film (Nov.). B. F. Goodrich. B. F. Goodrich (1984). Estnnc Tech, Bull. ES-20, B. F. Goodrich Chem. Group. B. F. Goodrich (1985a), Estane Tech. Bull. ES-1 I , B. F. Goodrich Chcm. Group. B. F. Goodrich (1985b). Estane Tech. Bull., Product Comparison Data, B. F. Goodrich Chem. Group. B. F. Goodrich (198Sc), Estane Markcting Group private communication, (September. B. F. Goodrich Chem. Group. Harrell, L. L.. Jr. (1969), Mrrcrorrlo/rcrr/r.s2(6):607. Harrington, R. (1957), R d h r Age (N.Y.) 82(3):461. Harrington. R. (1958). Ruhher Age (N.Y.) 82(6):1003. Harrington, R. (1959). Ruhbrr Age (N. Y.) 85(6):963. Hepburn, C. ( 1982). Polytrrrthrrrlr E/tr,storrwrs. Applied Science Pub., London. Hoseman, R. ( 1975),J. Po/yrrl. Sei. S w p . 50:265. Hosemm R., and Bagchi, S. N. (1962). Direct Ar~cr/y,si,sof'D(ff'r~tior1 /?>l Mtrtter, Narth-Holland publ., Amsterdam. Huang. S . J. (1981). Biodegradation of polyurcthancs. Lecture, Polymer Conf. Ser., UIliv, Detroit, JuIlc, . Cherrl. Ed. 1/:3227. Jellinek, H. H. G., and Wang. T. J. Y. (1973). J. Po/yrrt. S C ~Po/SVI.
Schollenberger
414
Jellinek. H. H. G., Martin, F., and Wegner, H. (1974), J. Appl. Polym. Sei. /(1(6):1773. Knisernwn. S., and Singh, A. (1976), Br. Pat. 1,425,529, to the American Cyanamid Co., Feb. 18. Kanaval. G. A., Koons, P. A., and Lauer, R. E. ( 1966), Rlt/Awr Chprrr. Trc/v~o/. 39(4):1338. Kaplan, A. M., Darby, R. T.. Greenberger, M., and Rogers, M. R. (1968), L h v / o p . / d . Mj(.r.o/>jo/.9:201. Kogon, I. C. ( 1958), J. Org. Cherrr. 23:1594. Kogon. 1. C. (1959). J. Org. Chrm. 2493. Mobya Chem. Co. ( 1968). An engineering handbook for texin urethane elastolnertc matertals. technical bulletin. Mobay Chem. CO. (1985). Textn Marketing Group, private communication. September. Muller, E. (1970). Arrgrw. Mnkrorrlol. Chrrrl. /4(203):7.5. Nakayama. K., 1110. T.. and Matsaburo, I. (1969). J. Mocron~ol.Sci. Chrrn. A 3 ( 5 ) :1005. Ophir. Z. H., and Wilkes, G. L. (1979). in M~tlriphctsrPolyrnrrs S. L. Cooper and G. M. Estes, Eds.), American Chemical Society, Washington. DC, p. 53. Ophir. Z. H., and Wilkes. G. L. (1980), J. Po/w. Sei. Polym. P h y . Ed. /8:1469. Quiring. B., Niedcrdellmann. G., Goyert, W., and Wagner, H. (1981). U.S. Pat. 4,245,081, to Baycr A. G.. Jan. 13. Rausch. K. W.. Jr., and McClellan, T. R. (l972), U.S. Pat. 3,642,964. to the Upjohn Co., Feb. 1.5. Snmuels, S. L.. and Wilkes, G. L. ( 1971a). Polyrr~.Prep., D;\,, Polyr~r.C / ~ I Am. . , C / ~ r mSoc., , 12, No. 2694. Samuels, S. L., and Wilkes, G. L. (1971). J. Polyrr~.Sci. BY:761. Samuels. S. L.. and Wilkes, G. L. ( 1973). J. Poly~rr.Sci. Syrrrp. 43: 149. Saundcrs, J. H., and Frisch, K. C. (1962). Poly~rrrthcrrles:Cltrrristry c t r d Trclrrwlo,qy. Part I, Chrrr~istry. Wiley-Intersciellce, New York. Saunders. J . H.. and Piggot, K. A. (1965). U.S. Pat. 3,214,411, to the Mobay Chemical Co., Oct. 26. Saytgh. A. A. R.. Ulrtch, H., and Fanissey, W. J. (1972). in Corlderlsrttiorl M o r t o r r w ~ s(J. K. Stillc, Ed.), Wiley, New York, Ch. 5. Schollenbergcr. C. S. (1959). U.S. Pat. 2,871,218, to the B.F. Goodrtch Co., Jan. 27. Schollenberger, C. S. (1969), in Polyurrrhrrrw T c d ~ r ~ o l o g(P. y F. Bruins, Ed.), Wiley-Interscience, New York. p. 197. Schollenbcrger. C . S. (1979). in Mlrltiphcrse Polyrwrs (S. L. Cooper and G. M. Estes, Eds.). American Chemical Society, Washington, DC. Ch. S . Schollenhcrger. C. S., and Dinbergs, K. (1961), SPE Trtrus. l(l):3l. Schollenbergcr. C. S., and Dinbergs, K. (1973), J . EI~~stoplrr.st.5:222. Schollenhcrger, C. S., and Dinbergs, K.(1974), U r r t l ~ ~Sci. r ~ cT r c h o l . k36. Schollcnberger. C. S.. and Dinbergs, K. ( 1975). J. Ektst. P h t . 7:6.5. Schollenberger, C. S.. and Dinbergs, K. (1978), Ad\,. Urrthtrr~eSei. Trchrwl. 6:60. Schollcnherger. C. S . , and Dinbergs, K. (1979a), J. Elast. Pltrsr. 1 1 5 8 . Schollenbcrger, C. S.. and Dinbergs, K. (1979b). A d , . U W I I I L ISci. I I ~ Trc/rrrol. 7 1. Schollenberger. C. S.. and Dinbergs, K. (1981). U.S. Pat. 4,255.552, to the B.F. Goodrich Co., Mar. IO. Schollenhcrger. C. S.. and Esarovc, D. (1971), 111 Tlw Scirrrcr crrrrl Trchrrolo~yyof Polyrrrrr Films, Vol. I1 (0.J. Sweetmg, Ed.). Wiley-Interscience, New York, p. 487. Schollenhergcr. C . S., Scott, H., and Moore. G. R. (IO%), Kublwr World 137(4):549. Schollcnhergcr. C. S.. Pappas, L. G., Park, J. C.. and Vickroy, V. V., Jr. (1960), Krrbhrr World /42(6): Ad19.
81.
Schollenbergcr. Schollenhergcr. Schollenberger. Schollenbergcr, Schollenbcrger. Schollenherger, Schollenberger, Schollenherger. Schollenhcrger.
C. S,, Scott. H.. and Moore, G. R. ( 1962), Rrtbhrr Cllcw. Trchrfol.35(3):742. C. S., Dinbergs, K.. and Stewart, F. D. (1982). Krthhrr Chew. Trchrrol. 54( 1 ): 137. C. S . , and Stewart, F. D. (197 la), J. E/o.sfop/rr.st. 3:28. C. S., and Stewart, F. D. ( 1971h), Arbs. UrrfIfctwSci. Trchrlol. 1:65. C. S.. and Stewart. F. D. (1972), J. Eltrstopltrst. 4294. C. S.. and Stewart. F. D. ( 1973a), A ~ ~ y c wMerkrorrlol. '. Clwru. 29/-?0:413. C. S., and Stewart, F. D. ( 1973h), Ad\,. U r r f h r r Sci. i"cc/~~~ol. 2:71. C. S., and Stewart, F. D. (1976a). J. Elrrst. Plrrst. 8:1 I . C. S., and Stewart. F. D. ( 1976b). Adv. Urrthctrw Sci. Trc.l~r~ol. 4:68.
Thermoplastic Polyurethane Elastomers
41 5
Seefried, C. G., Jr., Koleske, J. V., and Critchfield, F. E. (1975). J. A/>/>/.Po/yrr~.Sci. (Cherrr.) 19( 12): 3185. Seymour, R. W., Estes, G. M,, and Cooper, S. L. (1970), Mmrorrrol. 3(5):579. Siefken. W. ( 1948), Arrrr. C/rem. 562(2):75. Singh, A., Weissbcin. L.. and Mollica, J. C. ( 1966). Rrthher A , p (Dcc.):77. Urs, S. V. (1962), O r d . B r g . Prod. Res. Dr~vlop./ ( 3 ) :199. Weisfcld. L. B.. Little. J. R., and Wolstenholme, W. E. (1962), J. Po/yrrr. Sci. 56:455. Wilkcs. C. E., and Yusek, C. E. (1973). J. Mrrcrorrrol. Sci.-Phys. B7( 1 ):157. Wilkcs, G. L., and Emerson, J. A. (1976). J. A/)/)/. Phys. 47(10):4261. Wilkes, G. L., and Wildnauer, R. (1973, J. A/>/>/.Phys. 46:4148. Wilkes, G. L., Bagrodin, S., Humphrics, W.. and Wildncucr, R. ( 1975). J . Po/yrrr. Sci. Po/yrrr. let^. Ed. /3:321.
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Thermoplastic Polyamide Elastomers
Thermoplastic polyamide elastomers consist of a regular linear chain of rigid polyamide segments interspaced with flexible polyether segments. The general formula may be written as
HO-
Ls
C -P A - C - 0 - P E - 0
II 0
1
In
-H
where PA representsthe polyamide segmentsand PE thepolyether segments. These are basically segmented block copolymers having the general structure (AB),,. The hard segments may be based on partially aromatic polyamide or aliphatic polyamide. Polyester amide (PEA) andpolyether ester amide (PEEA) are prepared from the aromatic polyamide, while in the polyether block amide (PEBAX or VESTAMID)the hard segments are derived from aliphatic polyamide. In both PEEA andPEBA, thesoftsegments of aliphaticpolyethersarelinked to the hard segments by an ester group. All thermoplastic polyamides are characterized by excellent toughness and tlexibility at low temperature in the absence of plasticizer, low density, high dimensional stability. ease of conventionalthermoplasticprocessing, good chemical resistance. good environmentalstress cracking resistance, good elastic recovery, and good dynamic properties. The properties could be varied by variation of the block length and nature of the block (Bornschegel et a l . . 1985; Legge et al.. 1987). In this chapter, the polyether block amide (PEBAX or VESTAMID) will be discussed first.
1. POLYETHER BLOCK AMIDE
1.1 Grades Vestamid grades (CREANOVA Engg. Plastics) are represented by E40-S3, E47-S3. E47-S1, E55-S3. E62-S3, E62-S1, X4442. X7375, EX9200, E50-R2, etc. All these commercial grades are designated by a series of letters and numbers: ( 1 ) E and two digits: Nylon-12 elastomer in which the two digits indicate Shore D hardness: (2) S and digit: heat-stabilized S 1 and heat and light-stabilized S3; (3) R and digit: permanently antistatic grade in which the digit indicates the average surface resistivity to the power of 10. E(X) indicates products under development. 417
418
Bhowmick
Similarly, the following PEBAX from ATOCHEM are available: 2533SN01, 3533SN01. 4033SN01, 5533SN01, 6333SNO1,7033SN01, 2533SD01, 3533SD01,4033SD01, 4033SN70, 5533SN70. 401 1MAOO1, 5562MAOl. MX1502. MX1503, MX1057, MX1058,MX1059, MX1060, MX1084. The first two digits indicate Shore D hardness, the second two digits are used for the series, S and N represent applications (S for all uses, M for molding, and E for extrusion) and stabilization (A for no additive, N for UV stabilized, D for UV. stabilized and mold release additive, and T for heat stabilized), respectively. The last two digits represent fillers and formula code, respectively.
1.2
Preparation of Polyamide Elastomers
Several research groups studied the reaction of dicarboxylic polyamide blocks polyether blocks
with diamine
HOOC-PA-COOH+H2N-PE-NH2+H 0 €1
n
or diamine polyamide blocks with dicarboxylic polyether blocks in the molten state (ICI, 1965, Morsanto, 1969; Toray. 197 1).
H ? N - P A - N H z + H O O C -P E - C O O H + HN - P A - N - C - P E - CO H
‘A :1 li
An
The reaction of poly(oxyethy1ene) a,w-bis chloroformate with adipyl chloride and piperazine in solution to give a polyamide polyether block copolymer with urethane linkages between the blocks was reported (DuPont, 1968). Studies on the polyesterification reaction in the melt between a dicarboxylic polyamide and a polyether diol have been discussed (BASF, 1968; Unilever Emery, 1973). Thefirst patent covered the reaction of a dicarboxylic acid polyamide based on caprolactum and poly(oxyethylene) dihydroxy polyether blocks at 250°C with paratoluene sulfonic acid as a catalyst. The second patent described the reaction of a C-36 fatty acid dimer and a diamine with a polyoxyethylene dihydroxy polyether blocks without catalyst at 250°C. These two products were low in molecularweightand were used as waxy additives in textile fiber formulations to provide antistatic properties.The use of a particular catalyst family,Ti (OR)4,discovered by ATOCHEM, was the appropriate way to produce a high molecular weight polyamide polyether block copolymer with ester linkages. This catalyst appears to modify the con~patibilitybetween the diacid polyamideanddihydroxypolyethersegmentsandallowspolymerization in a homogeneous phase, which was not possible with any other catalysts. A study of the kinetics of polyesterification was done by Deleens, and the first patent was applied for in 1974 (Deleens et al., 1974). A wide studyof the combinations between different polyamide blocks and polyether blocks was then made. In addition, research on catalysts, polymerization processes, formulations, and applications was also undertaken (Deleens et al., 1976. 1977). Dicarboxylic polyamide blocks were produced by the reaction of polyamide precursors with a dicarboxylic acid chain limiter.
419
Thermoplastic Polyamide Elastomers
The reaction was achieved at high temperature under pressure. The molecular weight of the polyamide block was controlled by the amount of chain limiter. The polyamide precursors can be selected from the following: Amino acids (aminoundecanoic acid. aminododecanoic acid, etc.) Lactams (caprolactam. lauryllactam, etc.) Dicarboxylic acids (adipic acid, azeloic acid. dodecanedioic acid. etc.) The polyether blocks are produced by two different reactions: Anionic polymerization of ethylene oxide and propylene oxide for polyoxyethylene dihydroxy and polyoxypropylene dihydroxy polyether blocks Cationic polymerization of tetrahydrofuran for polyoxytetramethylene dihydsoxy polyether blocks
1.3 Structure The structure of thermoplastic polyamide elastomer compriseslinear and regularchains of polyamide segments and flexible polyether segments. The chemical structure (see above) can be varied by manipulating the following: the nature of the polyamide blocks. the nature of the polyether blocks. the length of the polyamide and the polyether blocks, and the mass relationship between the polyamide and the polyether blocks. Table 1 gives an idea of the block length of thermoplastic polyamide elastomer obtained from nylon- 12 and poly(tetramethy1ene ether) glycol. Since polyamide and polyether segments are not miscible, Vestamid or PEBAX presents a biphasic structure where each segment offers its own properties to the polymer. Figure 1 shows the atomic force microscopy (AFM) structure of a typical polymer. White regions or "hills" are polyamide blocks whose typical dimensions are 400-800 nm. The nature of the polyamide
Table 1 M,, of PTHF and PA Segments, Hardness. and Melting Point for Differcnt Segmented Polynmldes"
Samples
DSC melting point ("C )
Shore D hardness
174 171
68 62
MI174Ol~ MII4I,XI MIiZOO M1115110
M H I IIHI "I,
MSl40Il Ms2cnn, .' In the structure shown
166 1 S6 IS1 IS2 1S0 I S3
PTHF segment
PA scgment
M,,
X
M,,
35 30
I400 2000
14 14 14 14 14 9 19
7400
S3
1000 1000 1000 1000 1000 650
ss 47
40
28
4100
2300 I S00 1 100 1200 1100 I100
YI +
Y2
37 21 11 7
S 6
S S
I n the Introductlon. k = 4 (polytetrahydrofuran): I = II (laurol:rcturn); m = 11 (dodecanoic acid): 11 = 7 - 10: x Indicates the length of the soft block. while y I + y2 Indicates the length of the hard block. The total molecular wclght of the samples estmated by the terminal group tltration was between 31.000 and 76.000. h Here Mkl denotes v:lrl;ltlon of hard block molecular welght at a constant soft segment molecular welght of 1000. But M Sdenotes varlatlon of soft segmentmolecular welght at a constant hard block molecular welght of I 100. The numbers such a s 7400 or OS0 occurring In the prefix of M , , or M S denotes the corresponding h x d block or soft segment molecular welght.
420
Bhowmick
Fig. 1 Atomic force micrographs of typical Vestarmd polymers. (a) M H ~ polymer; ~w (b) MHI100 polymer; (c) Mssso polymer; and (d) Mszm polymer.
block influences the melting point of the polymer, the specific gravity, and the chemical resistance. The nature of the polyether block influences the glass transition temperature, the hydrophilic properties, and the antistatic properties. 1.4 Structure-PropertyRelationship Effects of block lengthof the hard and thesoft segments of the polyamide thermoplastic elastomers are shown below. Figure 2 illustrates the variation of tan8 with temperature for a series
421
Thermoplastic Polyamide Elastomers
8
l
I
-150
-100
I
-50
I
I
I
0
50
100
150
50
100
150
T E M P E R A T U R E ('C )
( 4
I 10.75
t
t
-.. -M H 1500 -+-
-1 50 (b)
M H l100
-1 00
- 50
0
TEMPERATURE('C)
Fig. 2 Dynamic mechanical properties of PEBA of varying block lengths: modulus curve. (From Ghosh et al., 1998a.)
(a) tan6 curve; (b) storage
Bhowmick
422
of polymers having the hard segment molecular weights of 7400, 4100, 2300, and 1100 at a constant soft segment molecularweight (Mn = 1000) (Ghosh etal..1998). Three distinct dispersion regions labeled as a, fi, and y descending from higher to lower temperatures are observed. For MH7JO0 polymer (see Table 1 for designation), these transitions occur at 27, - 65, and - 1 13°C. respectively. The a transition reflects the onset of motion of large chain segments caused by the breaking of intermolecular bridging in the amorphous region of the hard block. The (3 dispersion is due to the combined relaxation of the soft amorphous polyether segments and local segmental motionof the amide groups in theamorphous region, which arenot hydrogen bonded to the other amide groups. They transition occurs because of the cooperative movement of the methylene groups. As the hard block molecular weight decreases at a constant soft block molecular weight, the a peak gradually shifts towards lower temperature and tan6 peak height increases. The fi peak also becomes lessconspicuous. They peak at - 1 13°C does not change. E’ decreases asthe molecular weightof the hard block decreases, especially in the high-temperature region. Similarly, in the series Msbso, MslJo(), and Mslo()opolymers (Table l ) , the soft segment n~olecularweight increases at a constant hard block molecular weight of 1 100. The a and p peaks merge together to give one single transition occurring at about - 32, - 47, and - 47°C respectively. for these three polymers. Log E’ at 25°C has the highest value of 9.52 in the case of Mshso polymers. E’ decreases with increase in molecular weight of the soft segment. Figure3 depicts the variation of stress-strainpropertieswith the change in molecular weight of the hard and the soft segments. The tensile strength, 300% modulus, and Young’s modulus decrease with the decrease in molecular weight of the hard block. The values for Mill 1 0 0 polymer with lowest hard blockmolecularweightare 22.9, 11.5. and 32.9 MPa, whichare
ELONGATION
(O/O)
Fig. 3 Stress-strnin properties of thermoplastic polyarnidc clastomers. (From Ghost1 ct al., I998n.)
423
Thermoplastic Polyamide Elastomers
-3 ll -200
Fig. 4
I
I
I
450
-200
-50
I
I
I
0 50 100 TEMPERATURE ("C)
l
150
l
1
200
Dielectric properties of PEBA of varying block lengths. (From Ghosh et
250
d.,
1998b.)
similar to those of the vulcanized rubbers. The hard polyamide blocks are responsible for the good mechanical strength of the polymer, and their decrease in molecular weight lowers the mechanical properties. The crystallinity also decreases with decreasing hard block molecular weight. In theseries of polymers MS(,5o,MsI4(H),and Mszooo.thepolymerhasthelowest soft segment molecular weight and exhibits the highest tensile strength (32.3 MPa). The 300% nlodulus decreases with increase in soft segment molecular weight and attains a minimum value of 7.3 MPa for Ms2000polymer with highest soft segment molecular weight.The M,174lH, polymer having the highest hard block molecular weight exhibits the highest hysteresis loss, hysteresis loss ratio, and set values of 22.1 X IO" J/m3, 0.95 and 78%, respectively. With the decrease in molecular weight of hard block, these parameters decrease and attain minimum values in the case of MI[, polymer. Mshso polymer with the lowest soft segment molecular weight exhibits the highest set of 69%. The effects of molecular weight variation of the blocks on dielectric properties have been studied (Ghosh et al., 1998b). The decrease in molecular weight of the hard block lowers the a- and@-transitiontemperaturesatconstantsoftsegmentmolecularweight (Fig. 4). The a transition gradually becomes less conspicuous for low hard block molecular weight polymers. The y-transition temperature remains unaltered. Values of activation energy for the a,@,and y transitions decrease with decrease in the hard block molecular weight. The activation energy records the highest value for the a transition. However, the activation energy shows marginal
424
C
y$ ZE E
l
l
A
l z gA z
A
I ILgg
N
c c c
4 4 4
C
C
C
8 4 4 4
- c c c
$3riDD
044D m c c c
0444 Q c c c
Bhowmick
Thermoplastic Polyamide Elastomers 425
Bhowmick
426
increase with increaseinsoftsegmentmolecularweightforthe p transition. The dielectric constant of the polyether block amide polymers decreases with decrease in hard block molecular weight at 1000 Hz at 100°C. An increase in soft segment molecular weight from 1400 to 2000 lowers the dielectric constant value. The shear viscosity of the thermoplastic polyamide elastomers decreases with increasing shear stress. An increase in soft segment molecular weight or a decrease in hard block molecular weight decreases the shear viscosity in general. The die swell at a fixed temperature and shear rate decreases with decrease in hard block molecular weight and increases with increase in soft segment molecular weight (Ghosh et al., 1999). The adhesion strength between two aluminum sheets joined by polyether block amide increases with either decrease in hard block molecular weight at a constant soft segment molecular weight of 1000 or with increase in soft segment molecular weight at a constant hard block molecular weight of 1100 (Ghosh et al., 2000). Thermal aging of the joints has a significant effect on their adhesion strength. As the aging time is increased, the joint strength increases.
1.5
Properties
The properties of variousVestamidgradesare shown in Table 2 (Creanova Engg. Plastics, 1998). The density may vary from 1 .0 1 to 1.14 and is among the lowest of any thermoplastic elastomers. It has low water absorption, which gives dimensional stability and consistent mechanical andelectricalproperties.Depending on the grade, the meltingpointvaries from 120 to 210°C. The vitreous transition phase is always approximately -60°C. As shown in Fig. 5, the polyether block amide bridges the gap between thermoplastic and rubbers on Shore hardness. which can vary from 60A to 75D. although certain grades are not marketed at present. The hardness varies relatively little with temperature between -40 and 80°C. It retains almost ail of its flexibility at low temperatures and maintains its excellent mechanical properties down to -60°C. The behavior under stress varies as a function of the polyether content, as discussed
POLYETHER BLOCK AMIDES I
1
I
I I
THERMOPLASTIC POLYURETHANE
I
1
I POLYETHER BLOCK I
I
I I
I
,
Thermoplastic Polyamide Elastomers
427
earlier. The polyether content gives the product its flexibility and its more or less elastomeric character. The strengthatlowdeformation is high-higher than that of most thermoplastic elastomerswiththe same hardness.allowingthickness to be reduced in many applications. Thermoplastic elastomeric polyamide has good resistance to tearing and abrasion.The resistance increases with the hardness of the grade. Compared to most other thermoplastic elastomers, its abrasion resistance is high even in contact with highly abrasive media, making it suitable for athletic footwear. The hysteresis values of PEBA polymers are lower than most thermoplastic elastomers or vulcanized rubber with equivalent hardness. The high resistance to cyclic flexing, even at low temperatures, is one reason why it is chosen for the soles of football and ski boots, transmission belts, gear trains, etc. The thermal and thermo-mechanical properties depend on the length of the blocks and the type of polyamide. For example, the melting point of PEBAX grades 2533-401 1 varies from 133 to 204"C, with a latent heat of fusion between 1.2 and 6.3 cal/g (Elf Atochem, 1998). Similarly, thermal conductivity of the same grades lie between 0.26 and 0.29 W/m"C at 30°C. The linear coefficient of expansion is of the order of 20-25.1 X lo5 cm/cm/"C in the temperature range between 30 and 0°C. Deformation temperature under load indicates that at constant stress, more flexible grades distort at lower temperatures. The volume resistivity varies from 10'j to IO3 L! cm'/cm, as shown in Table 2. It offers excellent resistance to tracking. The standard VDE 0303Part 6 test indicates that this elastomer leads to no corrosion, even under extremely humid conditions. Antistatic and semiconductive materials can be produced from this elastomer by introducing carbon black. The resistance to aging in dry heat depends on the grade. The rigid grades withstand dry heat better than the flexible grades. The rigid grades also have better UV resistance than the flexible grades. Gas permeability decreases with increase in the hardness. Typical PEBAX 6333 and 2533 grades of 120 km thickness have gas permeability values of 3 1 and 150 (oxygen), 420 and 2600 (carbon dioxide), 5 and 170 (nitrogen), and 46 and 235 (helium), respectively (all units in 10- l o cm3 mm/cm'.s.cm of Hg) at 23°C. Certain grades can be made permeable to water vapor. Table 3 reports the resistance to various solvents and chemicals.
1.6 Processing PEBA can be processed using the following techniques: Injection molding Extrusion Thermoforming Coating
No hazardous degradation products are generated by processing of PEBA resins (Elf Atochem, 1998). However, special precautions must be taken in drying the materials. PEBA grades are generally supplied as pellets in moisture-proof packaging ready for use. The drying conditions are 4 hours at 80°C (PEBA with Shore D 2 40) or 6 hours at 70°C (PEBA with Shore D < 40) in a propelled air oven, dehydrated hot air oven, or vacuum oven. The rheological properties, as discussed earlier, are important for understanding processing. Melt flow index, which corresponds to the quantity of material at 235°C that can flow in 10 minutes through a 2 mm line when 1 kg of mass is applied, gives a measure of the viscosity. MFI values lying between 5.5 and 12 g/10 minutes have been reported for PEBAX grades. Melt viscosity, which is a function of shearing speed and temperature, indicates that a high molding temperature increases flow capability. Unlike melt flow index and melt viscosity, tlow lengths may be obtained by injecting PEBA via an Archimedes or reciprocating screw into a mold with rectangularsection.Duringmolding, the dimensions of thegating may be determined. For
428
Bhowmick
Table 3 Resistance of PEBAX to Solvents ~~
Test conditions days
PEBAX grade"
(J)
("C)
7033
6333 4033
5533
10% acidsulfuric 10% caustic soda
l l 7
A A A A A A A A A A A A A B A A B A A A A A A A A A C B A B B B
A
chloride SO% zinc Boiling "Skip" detergent (30 g/L) Caustic potash (34" Baume) Lockeed H55 ASTM No 1 oil ASTM No I oil ASTM No 3 oil ASTM No 3 oil Ethanol Propanol Butanol Isooctane 4-Star petrol M l5 fuel Kerosene Paraffin Fuel B Fuel B Fuel C Benzene Acetone glycol Ethylene Methylethylketone chloride Methylene Trichloroethylene Perchloroethylene FREON 1 I FREON R 22 FREON R 502
23 23 23
A A A A A A B A A A A A B B A B B A A A B B A A A A C B B B C B
Chemical
water
l l 7
l 3
l 3
l l 7 7 7
l 2
l 7 7 2 2
l l l 7
l 7 7
l l l
100
95 l0 121 IO0 121 100
121 23 23 23 23 23 50 23 23 23
so 50 23 23 23 23 23 23 23 23 45 45
A A A A A A A A A A A A B A A B A A A A A A A A A C B A B B B
A A A A A A C A A C B A B C A B C B A B B B B A A B C C B B C C
3533
2533
A A A A A A C B C C C B C C A C C B B C C C C A A C C C C C C C
A A A A A A C B C C C C C C A C C C B C C C C A A C C C C C C C
A = little or n o effcct: B = moderate cffect; C = severe effect. (Reprinted f r o m Elf Atochem wlth permission.)
example, in the case of large components, the size of the gate may be increased to broaden the range of temperature and pressure available and to facilitate processing. Itljectiotl Molding
The equipment used for polyamideinjectionmolding is suitableforPEBAprocessing.For flexible grades, mold designs suitable for polyurethanes (particularly the cavity feed) may be used. The choice of injection temperature depends primarily on the length of time the material stays in the sleeve. If the cycle is short, a high injection temperature should be used. Depending on the grade, recommended temperature varies from 160 to 280°C. In some cases, an injection temperature of 300°C is reported (Elf Atochem, 1998). Higher temperatureresults in lower
Thermoplastic Polyamide Elastomers
429
pressures within the component during cooling. Injection speeds are chosen according to the lowest flow sections, which cause the highest shearing speeds. PEBA should be molded in cold molds (20-40°C). which help in the release from molds. The right molding temperature controls the component’s finish. dimensional stability, and shrinkage. An injection pressure in the range of 500-800 bars for nonreinforced grades has been recommended. Holding pressure is applied when the mold cavity is full to compensate for material shrinkage during cooling. Extrusion
PEBA can be extruded using the same types of machines and screws used for polyamide. The screws used for PVC or PE can be used for flexible grades. The recommended temperature for extrusion of PEBAX 2533-7033 lies between 160 and 230”C, depending on the grade. Assembly
There are a few ways of assembling PEBA to other materials: Insert molding Bonding Welding The process parameters canbe adjusted by optimizing adhesion between the material and inserts. The oven-molding technique is used for athletic shoe soles (Elf Atochem, 1998). Other Methods
Blown film can be made from PEBA. Blown extrusion film (25 Km thick) has been made on a Kaufmann line (D = 64 mm, L/D = 28, die = 150 mm, width = 500 mm) at a temperature of 160- 180°C and a screw speed of 30 rpm, drawing speed of 20 m/min and blow rate of 2. I , and a die temperature of 175°C. Cast film of 25 p m thickness has been made on the ERWEPA extruder (D = 90 mm, L/D = 32) at 96 rpm at 200-250°C at a speed of 100 m/min. Similarly, PEBA can be coated onto other substrates in the same machine (D = 60 mm) at a lower speed andscrewrpm. PEBA can be recycled by incorporating IO- 15% of the materialinto new granules of the same grade. Compounding with mineral or organic pigments or incorporation of liquid dyes or masterbatches has also been reported.
1.7 Applications The special properties of PEBA make it suitable for a range of applications, for example, in sports, fashion, medical, automotive, electrical and electronic industries, household appliances, machine tools, agriculture, toys, etc. (Fig. 6). The properties of breathable films from polyether block amides are shown in Table 4.
2.
PEEA AND PEABLOCK COPOLYMERS
PEEA and PEA block copolymers have been synthesized by the condensation of aromatic diisocyanate, [4,4’-methylene bis(phenylisocyanate),(MDI)] with dicarboxylic acids and a carboxylic acid-terminated polyester or polyether prepolymer with a Mn of 500-5000 (Chen et al., 1978). The homogeneous polymerization is carried out at elevated temperature in a polar solvent that is nonreactive with isocynates. The dicarboxylic acid serves as the hard segment chain extender
430
Bhowmick . . .
-
, .
.
.””~ ‘j-
“6
Fig. 6 Typical applications of thermoplastic polyamide elastomer: (a) sports shoes; (b) toys. (From Elf Atochem with permission.)
and forms the amide hard segment with the MDI. The carboxylic acid-terminated prepolymer forms the soft segment matrix-ester for PEA and ether for PEEA polymers. The amide content of the elastomer and hence crystallinity can be changed by varying the amounts and types of dicarboxylic acid chain extenders in the formulation or by changing the molecular weight of the polyester or polyether soft segments. As discussed earlier, physical properties of PEA and PEEA segmented block copolymers are influenced bythe chemical composition of the hard and soft segments andtheir respective length (Deleenset al., 1987).Generally,hard segmentchemical composition affects the polymer melting point, degree of phase separation, and mechanical strength. Similarly,the soft segment chemical compositioninfluences hydrolytic stability, chemical and solvent resistance, thermooxidative stability, and low-temperatureflexibility. The molec-
Thermoplastic Polyamide Elastomers Table 4
431
Pronerties of Breathable Film from PEBAX
Hardness, shore D Melting point. "C Density USP Class VI Water absorption, r/r EquiIibriun1"20"C, 65% RH 24 hr i n water MVTR, g/m2/24hr (permeability to water) ASTM E96 BW (38°C/S0% RH) 12 pm 25 pm 50 p m ASTM E96 E (38"C/90%RH) 12 p m 25 p m 50 pm
MX 1205
MV 1041
MV 3000
MV 1074
40 I47 I .01 Yes
60 170
35 1 58
40 1sx
1.04
I .02
Yes
Yes
1.07 Yes
0.5 I .2
0.9 12
I .0 28
48
3,000 1,800 1,400
l 8.000 12.000 7,000
28,000 22.000 18,000
30.000 35.000 2 l ,000
3.000 1,800 1.200
3.s00 2,700 1,800
4.500 3,300 2,200
4.800 4.300 3.600
1 ..I
ular weight of the hard segment influences also polymer melting point, thermal stability. and low-temperature flexibility. These thermoplasticelastomers (TPEs) have highermechanicalpropertiesand moduli than those of many otherTPEs in the same hardnessrange.They also retain thesetensile properties at higher temperature. PEEA and PEA elastomers are very resistant to long-term dry heat agingeven at 150°C. The abrasion resistance, tear strength, and compression set are excellent and comparable with other segmented block copolymers like TPVs. Theyexhibit good insulating and adhesive properties. High-temperature tensile properties. dry heat aging, humid aging, chemical and solvent resistance, tear strength. abrasion resistance, compression set. flex properties, adhesive, weatherability, electrical properties, processing characteristics. and applications have been discussed in detail by Nelb et al. (1987). They can be melt processed on injection molding. blow molding, and extrusion equipment like other TPEs.
ACKNOWLEDGMENTS The author is thankful to Dr. J . Lohmar of Creanova Spezial Chemie GmbH, Marl, Germany, and Dr. Y. Aubert of Elf Atochem, Paris. France. for providing the technical literature on the subject.
REFERENCES Bornschlepl, E., Goldbach, G.. and Meyer, K. (1985). Prog. Co//oir/.P o / y w v Sci. 7/: I 19. BASF, (1968), U.K. Pat. 1 , l 10,394. Chen, A. T., Farrissey Jr., W. J., and Nelb, R. G. ( 1 978), U.S. Pat. 4,129.7 IS (The Dow Chemical Company). Creanova Spezial Chemic GmbH (1978), Germany, technical literature.
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Bhowmick
Deleens. G., Foy. P,, and Jungblut, C. (1974), French Pat. 2,273,021. Deelens. G., Guerin, B., and Poulain, C. (1976), French Pat. 2,359,879. Deleens, G.. Ferlampin, J., and Gonnet, M. (1977), French Pat. 2,401.947. Deleens. G., Foy, P., and Marechal. E. (1977), Eur. Po/. J. /3:337,343,351. Deleens. G. ( 1987) in T/fer~,lol,/os/ic. E/o.storl/er.s-A Conqm/wrl.siw R e ~ i e (N. ~ . R. Legge. G. Holden. and H. E. Schroeder, Eds.). Hanser Pub., Munich. p. 215. Du Pont de Nemours (1968). U.K. Pat. 1,098,475, Elf Atochem ( 1998). Paris, France, technical literature. Ghosh. S.. Kahstgir, D., and Bhowmick, A. K. (l998a), P o / w w r 39:3967. Ghosh, S., Khastgir, D., and Bhowmick, A. K. (1998b), Po/yruer P o / y w r . Cortrpos. 5:323. Ghosh, S., Khastgir, D.. Bhattacharyyu, A. K., and Bhowmick, A. K. ( 1999). J. A/I/J/.Po/ytl7. Sci. 7/:1739. Ghosh, S., Khastgir, D., and Bhowmick. A. K. (2000),J . Adlwsiorl Sci. Techno/. /4:529. IC1 (1965), U.K. Pat. 1,108,812. Legge N. R., Holden G., and Schroeder H. E.,Eds., ( 1987), T/fcf.ff/ol~/~/.s/I'(. E/o.storrwrs-A CorlqJreherl.si,~e Rr\iew, Hanser Pub., Munich, 1987. Monsanto, (1969), U.S. Pat. 3,454,534. Nelb, R. G., Chen. A. T., and Onder, K. ( 1987). in Thermop/rt.stic Elo.stnmer.s-A C ~ ~ ~ f f / ~ r e / fRe e~ f~. si iml ,~~ , (N. R. Leggc. G. Holden, and H. E. Schroeder, Eds.). Hanser Pub., Munich, p. 197. Toray (1971). French Pat. 1.603,901. Unilever Emery French Pat. (1973). 2,178,205.
16 lonomeric Thermoplastic Elastomers Kamal K. Kar and Ani1 K. Bhowmick Rubber Technology Centre, Indian Institute of Technology, Kharagpur, lndia
1. INTRODUCTION Ionic polymersare a specialclass of polymeric materials havinga hydrocarbon backbone containing pendant acid groups. These are then neutralized partially or fully to form salts. Members of one family of these ionic polymers where the salt group content is very high (i.e., where every monomer repeatunit has a pendant salt groups) aregenerally water soluble. Suchpolymers are called polyelectrolytes. Other classes of ionic polymers in which the polymers backbone is highly polar contain fewionic pendant groups and also behave as polyelectrolytes. The combination of low ionic content and low-polarity backbone results in ionomers that show high extensibility and low permanent set: ionic elastomers. Ionic thermoplastic elastomers are another class of ionic elastomers in which the properties of vulcanized rubber are combined with the ease of processing of thermoplastics. These polymers contain upto 10 mol% ionic groups. However, they have a number of practical advantages over conventional rubbers: 1. Theyrequire no vulcanizationand little compounding. 2. They are amenable to methods of thermoplastic processing like blow molding, thermoforming, injection molding, heat welding. etc., unsuitable for conventional rubber; they have short mixing and processing cycles and low energy consumption. 3. Theirscrapcan be recycled. 4. Their properties can be easily manipulated by changing the ratio of components.
The disadvantages associated with ionic thermoplastic elastomers are that they soften/melt at elevatedtemperatureand showcreep behavior on extendeduse. They deteriorate in the presence of water, which is not the case forthermoplastic elastomers. A typical ionic thermoplastic elastomer is shown in Fig. I . The ratio of n/m is usually on the order of 10-100. The presence of this level of salt groups combined chemically with a nonpolar backbone with covalent bonds has a dramatic influence on polymer properties and applications. The properties of ionic thermoplastic elastomers depend on the ionic interactions of the polymer backbone with the pendant groups. The degree of ionic interaction depends on the following variables: 1. Types of polymers, i.e., elastomer or plastic 2 . Chemical structure of thepolymers 3. Level of ionicfunctionality (ionic content) (0-1076)
433
434
Kar and Bhowmick
F”
ICH3C H 3 I fCf-12
142-ffCH2-C X L - v q - - C H 2 - C H 2
V C H 2 C
% L= HCH3
\ “ C H 3
$03- N
‘*
Fig. 1 Schematic representation of an ionic thermoplastic elastomer (sodium sulfanated EPDM).
4. Degree of neutralization (0-100%) 5. Types of ionic moiety. i.e., carboxylate, sulfonate, phosphate, etc. 6. Types of cation, i.e., monovalent, divalent, multivalent, amine, etc. With this range of variables, the spectrum of polymer properties and applications in extremely broad, which bridgesthe gap betweenconventionalrubbervulcanizatesandthermoplastics. Ionic polymers have been discussed in detail in several reviews and books (Holliday, 1975; Eisenberg, 1980; MacKnight andEarnest, 198 1 ; Chakraborty et al. 1982; MacKnight and Lundberg, 1984,1987;Rees,1987;TantandWilkes.1988;Mauritz,1988;FitzgeraldandWeiss, 1988;Lundberg,1989;Haraand Sauer, 1994), butthere has beenlittlediscussion on ionic thermoplastic elastomers. This chapter will briefly discuss the salient features of various ionic thermoplastic elastomers.
2.
CLASSIFICATION OF IONICTHERMOPLASTICELASTOMERS
At the present time there is no standard method for the classification of ionic thermoplastic elastomers (ITPEs). They can be grouped on the basis of the following variables: Type of cations Nature of ionic groups Method of preparation Example of commercial and experimental ITPEs are given in Table 1
3. SYNTHESIS OF IONICTHERMOPLASTICELASTOMERS There are two techniques for the preparation of ITPEs. The first is copolymerization of a low level of functionalized monomer with an olefinic unsaturated monomer (Farbenindustric, 1932; Semon, 1946;Bryant,1970;Ress,1966,1968; Longworth, 1975).Directfunctionalization of apreformedpolymer is asecondroute(Gilbert,1965;Makowskietal.. 197th; Saidt et al., 1980a;Makowski and Lundberg,1978b,1980a;Fitzgerald andWeiss, 1988).Carboxylated elastomers are prepared by free radical polymerization, solution polymerization, emulsion polymerization, and grafting procedures on preformed polymers. Typically 6% carboxylic monomer is incorporated into the polymer to maintain elastomeric properties. But the vast majority of commercially available carboxylated elastomers are synthesized by emulsion polymerization in acidic medium because the free acid copolymerizes much more rapidly than the neutral salt and the low solubility of the monomer salt in the hydrocarbon phase prevents significant monomer
435
lonomeric Thermoplastic Elastomers Table 1 Comtncrcial and Experimental ITPEs Polymcr systctn
T K I ~n;mc K Applications Mnnufncturer
Commercial Sulfonatcd ethylene propylcnc dicnc tcrpolymer Goodrich Telechelic polyhutadicnc Goodrich Butadicne acrylic acid copolytncr
Ionic elnstomcr Hycar Hycar
Uniroyal
surlyn
dl1 Pont
Ethylcnc mcthacrylic acid copolymer Experimentd Quatcmary phosphonium sulfonated ethylene propyletlc dicnc tetpolymcr Meleatcd cthylenc propylcnc dicnc tcrpolytner Tekchelic polyisohutylcnc sulfonated ionomer Thioglycolated polypentenamer Phosphonatcd polypentenamer Carboxylatcd polypentenamer Sulfonated polypcntcnamcr Polyurethane ionomcr Sulfonated styrene (cthylcnc-cobutylene) styrcnc trihlock copolymcr Poly (butadiene-co-sodium styrene sulfonate) 2-Butyl styrcnc sulfonate isoprene copolymcr Copolymer of ethylcnc vinyl acetate and organic acid Blcnd of zinc salts of malcated EPDM and ethylcne methacrylic acid copolymer
Thermoplastic clastomcr Spccinlty uscs High green strcngth elastomer Modified thermoplastic Model ionomer
Modcl ionotner Model ionomer Model ionomer Model ionomer Model 1onon1cr Model ionomer Model ionomcr Model ionomer
Model ionomct Model ionomc~ Elvax
du Pont
Model ionomcr Modcl ionomct
incorporation. The polymerization is carried out at 30-50°C. A typical formulation for preparation ofcarboxylated elastomeris given as 100 parts butadiene (or combination with other monomers), 100 parts deionized water, 5 parts nlethacrylic acid, 1 part sodium alkyl aryl polyether sulfate, and 0.4 part potassium persulfate (Jenkins and Duck, 1975). The resulting elastomers are neutralized to the desired degree by metal hydroxide, metal acetates or sinlilar salts. On the other hand. the direct functionalization of a preformed polymer is carried out i n a homogeneous solution, using a sulfonating agent, (i.e., acetyl sulfate). I n the next section, the salient features of synthesis of various ITPEs are discussed.
3.1
Metal Sulfonated EPDM
The preparation of metal sulfonated EPDMs consists of two steps: sulfonation of EPDM and neutralization of the free acid form of sulfonated EPDM (Makowski et.al., 1980).Typical diene
Kar and Bhowmick
436
Table 2 Specifications ofStartingEPDMs ~
-
~~~
viscosity
content
sourcc
(wt%)
(100°C)
(wt%)
Commercial 40 Extruder 20 breakdown Extruder breakdown Direct 17 synthesis Direct synthesis Commercial
52 52
Designation V-2504 CR-2504 CR-709A E-S5 E-70
~
Ethylene ENB content
Mooncy
S
68
20
55 70 70
20
5 7.5 4.4 4.7
-
-
S o ~ r r w :Makowski et al.. 1980
monomers that can be sulfonated include5-ethylidene-2-norbornene (ENB), endo-dicyclopentadiene (DCPD), and 1,4 hexadiene. The unique features of these diene monomers are that only one of the two unsaturated bonds is consumed during polymerization. The specifications of EPDMs used are presented in Table 2. These materials vary primarily with respect to Mooney viscosity, a measure of molecular weight. and are fully amorphous. The EPDM designated as V-2504 is a commercial elastomer. CR-2504 is thermal and shear degraded by an extruder. A higher ethylene content EPDM, CR709A, is generated similarly through extruder. The EPDMs designated as E-55 and E-70 are synthesized directly. All of the EPDMs contain ENB as the third monomer. The free acid form of sulfonated EPDM is prepared from EPDM by using sulfonating agent in a manner similar to that described by O’Farrell and Serniuk ( 1974) and Makowski et al. (1978b). First, EPDM is dissolved in an aliphatic hydrocarbon. namely hexane or heptane, to make a concentration of 50-100 g/L. The sulfonating agent (O’Farrell and Serniuk. 1974; and Gilbert, 1965) or SO3 complex of triethyl phosphate dioxane and tetrahydrofuran (Canter, 1972) is added at room temperature. After 30 minutes a chain-terminating agent (i.e., isopropanol or alcohol) is added to terminatesulfonationreaction. At thispoint, 2-2’ methylene-bis (4methyl-6-tert butylphenol) antioxidant is added. The polymeric sulfonic acidis separated through steam stripping of the resultant solution. The resultant polymeric mass is washed with water in a Waring blender. The crumb is filtered and then dewatered by banding on a rubber mill at about 50°C to a water level of 2-5% or is dried in a laboratory aeromatic fluid bed dryer at 70°C to a water level of 0.2%. Sulfur content is determined by direct sulfur analysis (ASTMD-1552). The acid content of the sulfonated EPDM is determined through the titration of a solution of the sample in 95 mL toluene and 5 mL methanol with 0.1 N NaOH. A95% conversion of sulfuric acid to polymer sulfonic acid is typical (Fig. 2). Neutralization of sulfonated EPDM is carried out in one of two ways: neutralization of theisolatedpolymericacid or directneutralization, i.e., after sulfonation. In thefirstcase. polymer is redissolved in a mixture of toluene/methanol or hexane/alcohol to make a concentration of 50- 100 g/L. A solution of metal acetate in water or water/methanol is added to neutralize thepolymer. After 30 minutes of agitation,theneutralizedsulfonated EPDM is isolated by solvent flashing in boiling water. The wet polymer is dried on a rubber mill at 100°C or in a laboratory aeromatic fluid bed dryer at 100°C. In the second procedure (direct neutralization), after sulfonation but before drying, the resultant solution is treated with an alcohol or wateralcohol solution or an aqueous solution of metal hydroxide or acetate. The neutralized polymer is isolated and dried by the same procedure.
437
lonomeric Thermoplastic Elastomers
N CH c 0 3
f C H 2 -CH 2
F
CH2-CHCH3t
c -CH
n
j’
3
I
Fig. 2 Synthesis of sodium sulfonated EPDM.
The sulfonation of EPDM is alsocarried out in a single screw extruder (Saidt et al., 1980b). It is prepared by injecting acetic anhydride with sulfuric acid into the barrel along withan EPDM. The barrel temperature is 100°C. The residence time varies from 5 to 10 minutes depending on the degree of conversion. The resulting acid is neutralized by the addition of metal stearate, which is pumped directly into the product stream as it exits the extruder. The advantages of using an extruder are shorter processing time and the absence of a solvent. 3.2 Quaternary Phosphonium Sulfonated EPDM Brenner and Oswald (1980) have synthesized quaternary phosphonium sulfonated EPDM containing 1 mol% sulfonation over a wide range of chain length in the counterions. The free acid form of sulfonated EPDM is prepared from commerical EPDM by using acetyl sulfate in a manner similar to that described in the section on metal sulfonated EPDM. The acid form of sulfonated EPDM is dissolved in a mixture of 95% toluene and 5% methanol. The resulting solution is neutralized fully with solution of various quaternary hydroxide at room temperature. The neutralized sulfonated polymer is recovered from solution by either steam stripping or by precipitation in methanol. Samples are dried in vacuum for 3 days at 50°C.
3.3
Zinc Maleated EPDM
Zinc maleated EPDM is another ITPE (Uniroyal Technical Information Bulletin, 1982). There are two grades of maleated EPDM. One has a low ethylene content (ethylene/propylene ratio 55/45) and the other a high ethylene content (ethylene/propylene ratio 75/25). Zinc maleated EPDM is prepared by the addition of zinc oxide into maleated EPDM. The mixing processes
438
Kar and Bhowmick
depend on the ethylene content of nlaleated EPDM. The mixing of zinc oxide into maleated EPDM of low ethylene content is done on an open two-roll mill. The mixing of zinc oxide into maleated EPDM containing of high ethylene content is done in a Brabender plasticorder at a temperature of 70°C and at a rotor speed of 60 r p m .
3.4 Telechelic Carboxylated Elastomers The synthesis of telechelic carboxylated elastomers by free radical-initiated polymerization and anionic polymerization has been reported by Reed (1971) and Schulz et al. (1981 ). The first routeprovidespolymers of broadermolecularweightdistribution.Reactiontemperature is 70- 130°C. This process is utilized for copolymerization of 1.3-butadiene and acrylonitrile. The typical free radical initiator is 4.4‘ azobis (4-cyano-pentanoic acid). The selection of solvent is important to minimize chain transfer to the solvent. Typically t-butyl alcohol is preferred. Tetrahydrofuran and acetone are also used. The liquid polymers are recovered by solvent stripping. The neutralization of these carboxyl-terminated polymers are done by suitable alkoxide in toluene under vacuum (Broze et al.. 1981). The secondroute offers polynlers of relativelynarrowmolecularweightdistribution: 1500-6000. But in this case a large amount of organometallic compound is required. These ionomers are prepared in the following three molecular architectures: linear monofunctional.lineardifunctional,andthree-armstartrifunctional (Kennedy and Storey, 1982; Bagrodia et al., 1983a, 1983b, 1985; Mohajer et al.. 1984; Tant et al., 1985), which are obtained by the use of unifier (monocumyl chloride/BCI3),binifer (p-dicumyl chloride/BCI3),and trinifer (tricumyl chloride/BC13) initiator. respectively. Sulfonation is canied out i n hexane solution at room temperature.
3.5 2-Butyl Styrene Sulfonate Isoprene Copolymer 2-Butyl styrene sulfonate isoprene copolymer is prepared by emulsion copolymerization (Saidt andLenz.1980).Asolution of 1.6 mL of polyoxyethylene,0.25 mL of 14% solution of 1dodecanethiol in benzene. 0.25 mL of diisopropyl benzene hydroperoxide, and 1 mL of 2-butyl styrene sulfonate in 25 mL of isoprene is added to a mixture containing 25 mL of activator solution. 0.15 g of sodium pyrophosphate decahydrate, and 5 mL of distilled water. Polymerization reaction is conducted at 25°C for 10 hours at 9 psi pure nitrogen atmosphere. The polymerization reaction is terminated at the desired conversion by the addition of a solution containing 0.22 g of 2.6-di-t-butyl-4-methyl phenol (antioxidant), 0.015g of hydroquinone (inhibitor), 1.86 mL of methanol, 4.7 mL of distilled water, and 0.094 g of emulsifier. The ester copolymers are hydrolyzed to ionomers by the dropwise addition of a 10% solution of ester copolymerin toluene to 15 mL of 5 N solution of NaOH in methanol.
3.6 Poly(butadiene-co-sodium styrene sulfonate)
Poly(butadiene-co-sodiumstyrene sulfonate) is prepared by free radical copolymerization (Weiss et al., 1980). Butadiene monomer is injected into a solution containing sodium styrene sulfonate. sodium lauryl sulfate (surfactants). triethylene tetramine (redox initiator), dodecanethiol (chain transfer agent), sodiumpyrophosphate decahydrate (buffer). and water.At the end of the reaction, a methanolic solution of hydroquinone (inhibitor) and 2,2’-methylene-bis (4-methyl-6-t-butyl phenol) (antioxidant)is added to the reaction mixtures. The polymer is precipitated in methanol and washed with water.
lonomeric Thermoplastic Elastomers
3.7
439
Poly(ethy1ene-co-methacrylic acid) lonomer
The poly(ethy1ene-co-methacrylic acid) ionomersare marketed by du Pont as surlyn. The copolymer is prepared by free radical polymerization (Rees, 1966. 1968; Langworth. 1975). Typically 3-6 mol% methacrylic acidis incorporated into the polymer. The acid copolymers are neutralized on a two-roll mill at 150-200°C by the addition of NaOH or other bases. 3.8
Substituted Polypentenamer lonomers
Sanui et al. ( 1974a). Azuma and MacKnight ( 1978), Rahrig and MacKnight(1980a). and Tanaka and MacKnight (1979) have synthesized several new ionomers based on polypentenamer. These new ionomers are thioglycolated polypentenamer, phosphonated polypentenamer. carboxylated polypentenamer, and sulfonated polypentenamer. All ionomers are prepared from polypentenamer, which is a linear elastomer having very few or no vinyl groups and a relatively narrow molecular weight distribution (M,v/M,, = 1 .g). Thioglycolate, phosphonate, sulfonate. and carboxylate groups are incorporated up to 19 mol% based on the polypentenamer (i.e.. 3.8/100 backbone carbon). Ester form of thioglycolated polypentenamers (PPS) is prepared by adding methyl thioglycolate to the double bonds of polypentenamers via a free radical reaction (Sanui et al., 1974a). Hydrolysis reactions are conducted to prepare the corresponding acid (PPSH). Various salts are prepared by neutralization with various bases. Phosphonateside groups areincorporatedinto the polypentenamer by the freeradical addition of dimethyl phosphite to synthesize dimethyl ester of phosphonated polypentenamer (PP-PO) (Azumaand MacKnight, 1978). Thecorresponding acid derivative(PPPOH) is prepared by bubbling HCI gas through a dilute solutionof the ester. The sodium salt. PP-PONa.is prepared by treatment with methanolic sodium hydroxide solution. Carboxylic acid groups are introduced into the polypentenamer chain by a carbene addition of ethyl diazoacetate to give ethyl ester form (PP-COOEt) of carboxylated polypentenamer. Acidandsaltderivatives are prepared by hydrolysisandtreatmentwithmethanolicsodium hydroxide solution. Sulfonatedpolypentenamer is synthesized by reactinga complex of sulfur trioxide to diethylphosphatewithpolypentenamer. The reactionmixture is precipitated into ;L sodium hydroxide solution. which converts the polymer directly to the sodium salt. PPS03Na. 3.9
Sulfonated Styrene(ethy1ene-co-butylene)Styrene Triblock lonomer
Weiss et al. (1990) have synthesized sodium and zinc salt of sulfonated styrene (ethylene-cobutylene) styrene triblock ionomer. The starting material is a hydrogenated triblock copolymer of styrene and butadiene with a rubber midblock and polystyrene end blocks. After hydrogenation, the midblock is converted to a random copolymer of ethylene and butylene. the ethylene segments arising from 1,4 addition of butadiene and butylene groups from 1,2 addition. Ethyl sulfate is used to sulfonate the block copolymer in 1.2-dichloroethane solution at a temperature of 50°C using the procedure developed by Makowski et a l . (1975). The sulfonic acid form of the functionalized polymer is recovered by steam stripping. The neutralization reaction is carried out in toluene or toluene/methanol solution using the appropriate metal hydroxide or acetate.
3.10 Polyurethane lonomer Polyurethaneionomersareobtained from polyurethanethermoplastic elastomers, which are composed of short. alternating blocks of hard and soft segments. Polyurethane is prepared from
Kar and Bhowmick
440
a prepolymer obtained from glycol (poly(oxytetramethy1ene glycol) and diisocyanate (methylene-bis (4-phenyl isocyanate)) and a diisocyanate in the presence of a tertiary amine containing diol extender (N-methyldiethanolamine).Then polyurethane is transformed into azwitterionomer by dissolving it in dimethyl acetamide and addingan appropriate amount of y-propane sulfone (Hwang et al., 1981). A ring opening reaction occurs on the sulfone, resulting in the formation of a quaternary ammonium ion closely linked to a sulfonyl anion. Miller et al. (1983) have converted this zwitterionomer to an ionomer by reacting it with metal acetate.
3.1 1 Ionic Thermoplastic Elastomeric Blends and Some Recent lonomers Ionic thermoplastic elastomeric blends prepared by blending of rubbers and plastics in the right proportions have not received wide attention. Duvdevani et al. (1982) have prepared a ionic thermoplastic elastomeric blends from sulfonated EPDM and PP. Another blend was developed from sulfonated EPDM and poly(styrene-co-4-vinyl pyridine) by Peiffer et al. (1986). Blends based on polyurethane ionomers with polyacrylonitrile have been studied by Oh et al. (1994). Datta et al. (1996) have prepared a ionic thermoplastic elastomer by blending of zinc maleated EPDM and zinc saltof ethylene methacrylic acid copolymer. Blends are prepared ina Brabender Plasticorder at 170°C at a rotor speed of 60 rpm. Fitzerald and Weiss (1988) and Hara and Sauer (1998) have reviewed blends of ionomers. DeSarkar et al. (2000) prepared ionomeric hydrogenated styrene-butadiene rubber, and Chakraborty etal. (2000) reported ionic polychloroprene rubber.
4.
STRUCTURE
ITPEs contain boundion and free counterion. The bound ion is covalently bonded to the polymer network, as a carboxyl group in butadiene acrylic acid anda sulfonyl group in sulfonated EPDM. In contrast to bound ion, the counterion is free to move. But its actual mobility depends on the strength of the ionic bond, the nature of the polymer backbone, the temperature,and the presence of other additives in the polymer matrix. The ion pairs without a hydrocarbon layer aggregate to form multiples. A single ion pair can be represented as a small multiplet, and the size of the multiplet depends on its geometry:ionpairs,triplets,quartets, etc. The maximum size of a multiplet is eight ion pairs. The multiplet is completely coated by a hydrocarbon layer.Therefore, it is impossible for another multiplet to come closer than the distance of the thickness of a hydrocarbon chain. A loose association of multiplets is called a cluster. A cluster consists a central core having a multiplet of maximum size surrounded at a distance by other multiples of various sizes. This association is favored by the electrostatic interactions between multiplets and opposed by forces arising from the elastic nature of the backbone chains. Composition of clusters strongly depends onthe polarity of polymer matrix, ionic functionality, and temperature. At low ion content, multiples are favored only in a low polarity matrix. Cluster formation is favored with increasing ion content. With an increase in matrix polarity. a higher content of ionic groups isrequired in order to favor cluster formation. Asthe polarity of the matrix increases, the degree of ionic functionality required for cluster formation increases substantially. Several questions arise as to the state of aggregation of the ionic bonds: 1. What is the critical concentration for the formationof multiplets and clusters in ITPEs? 2. Are the multiplets and clusters uniformly distributed in space? 3. If they are multiplets or clusters, how large are they?
lonomeric Thermoplastic Elastomers
441
Various analytical techniques and a numberof models have been developed to answer the above questions. The analytical methods of characterizing the structure of ionomers include infrared spectroscopy,far-infraredspectroscopy.Ramanspectroscopy. Mossbaur spectroscopy, x-ray scattering. small-angle x-ray scattering, electron spin resonance spectroscopy, fluorescencespectroscopy,transmissionelectronmicroscopy, dynamic mechanicalthermalanalysis.dielectric thermal analysis. differential scanning calorimetry, etc. A number of different models i.e., these of BonottoandBonner (1968:~ 1968b),Eisenberg (l970), LongworthandVaughan (196821, 1968b), Holliday (1975). Marx et al. ( 1973a), Binsbergen and Kroon (1973), MacKnight et al. (1974), Meyer andPineri (1978). andYarussoand Cooper (1983), have been developed to determine the structure of ionomers. The following factors are involved in each model (Holliday, 1975): Upon cluster formation. work is undertaken to stretch the segments of polymer chain between ionic groups fromthe distance corresponding to random dispersed multiplets to the distance corresponding to higher clusters, which will be further apart. 2. Electrostaticenergy is released when multipletsaggregate. 3. The cluster is not infinitely stable, and above sometemperature, Tc, it will decompose. At this temperature. the electrostatic and elastic forces balance each other. 4. Some ring formation will take place between sequential ion pairs incorporated in the same cluster. 1.
No single model. however, can adequately describe the wide range of ionomer structures. Most systems are intermediate between the homogeneous aggregate and the phase-separated cluster, depending on backbone polarity and ionic functionality. The first attempt to develop a quantitative theory to answer the above questions was made by Eisenberg. Among the various models. those of Eisenberg, MacKnight et al.. and Yarusso and Cooper are worth noting.
5. ATTEMPTS TO DEVELOP QUANTITATIVE THEORIES FOR THE STRUCTURE OF IONIC THERMOPLASTIC ELASTOMERS 5.1
TheEisenbergModel (1970)
A molecular energy based theory of microphase formation in ionomers has been proposed by Eisenberg (1970). He chose a salt solution in a media of low dielectric constant. His model is based on eight assumptions: 1.
2.
3.
4. 5.
6.
The cation and anion are separated from each other by a distance corresponding to their ionic radius. Steric properties are used to calculate the largest number of ion pairs, which can group together without the presence of any intervening hydrocarbon. Energetic considerations are invoked to argue the formation of larger entities, which are composedof ion pairs separated from eachother by a hydrocarbon skin consisting of a portion of the backbone to which the acid groups are chemically attached. Work is performed to stretch the polymer chains during the formation of a cluster. The chain lengths between ionic groups are Gaussian in nature. The driving force for an ion pair aggregation or multiplet-multiplet condensation is an electrostatic interaction. Electrostaticenergy is released when multipletsaggregate toform acluster. The amount of released energy depends on the geometry of clusters and the dielectric constant of the medium.
442
Kar and Bhowmick
7. One-half of all the sequential ion pairsincorporated within the same clusteryields "rings." 8. The clusters will break down at some critical temperature. Tc., at which the rubberelastic forces and electrostatic forces are in equilibrium. These assumptions lead to the followingexpressions for concentration of ion pairs per cm' (C). number of ion pairs within a stable cluster (n), radius of multiplets (rIl1). averageintercluster distance (r,). and work required to separate an ion pair into dissociated ions for single charged ions (W):
(1)
r,,,
h p
= -
S,,
where. C, p. N. M,. n, MC., 1. k. TV, h'. h,)'. k'. K, eo, e, r. up. S,,,. rill. W. MO, n,,. and r, are the concentration of bound ions per cm', macroscopic polymer density. Avagadro's number. average molecular weight of the chain between ion groups. number of ion pairs within a stable cluster. molecular weight between ionic groups, length of the C-C bond, Boltzmann's constant. critical temperature above which the cluster becomes unstable, mean square end-to-enddistance of the free chain. mean square end-to-end distance for a corresponding freely joined chain. fraction of the electrostatic energy released upon formation of an ion pair from isolated ions for the particular ionic aggregate geometry. dielectric constant of the medium, permittivity of free space. electronic charge. center-to-center distance of positive and negative charges in a contact ion pair. volume of the ion pair. area of the hydrocarbon chain i n contact with the surface of the multipletsphere.radius of thetnultiplet.workrequired to separate an ion pair into dissociated ions. molecular weight per chain repeat unit, number of ion pairs per multiplet. and average intercluster distance, respectively. Quantitative calculations of Eisenberg's model are difficult. Values of n. r,. and rlllare reported for various cluster geometries. For a sodium salt of ethylene methacrylic acid copolymer containing 4.5 mol% acid, model predictedr, ranged from 44 to 95 However. the experirnental measured r, value is around 83 On this basis. the model is adjudged reasonable.
A.
A.
5.2 The MacKnight, Taggart, and Stein Model (Core-Shell model) (1974) MacKnight et al. (1974) have elucidated an entirely different origin for the ionic peak in ionomers. This model is based on the radial distribution function (RDF) and the analysis of lowangle x-ray scattering peak using the theories of Porod and Guinier. The RDF is the Fourier transform of the angular dependence of the scattered x-ray intensities. The intensity I (in electron units) of x-ray scattered by the amorphous medium is given by: Ill1
Ill1
443
lonomeric Thermoplastic Elastomers
where
Here, 28. h, N I , ml, n,. fi, IillC, rI, pi, ( 7 ) and el, ( m ) are scattering angle. wavelength of the radiation. total number of structural units contributing to the scattering intensity, number of different kinds of atoms present in the structural unit, number of i-type atoms in the structural units, the atomic scattering factorof the i-type atoms, Comptonscattering factor of i-type atoms, distance, the number of j-type atoms per unit volume at a distance r from a given i-type atom, and the average number of j-type atoms per unit volume, respectively. RDF is represented as:
5
4 m l [ D ( r 1 )- Do] =
-
Si(S) sin (Sri) dS
where
cc Ill1
111,
x
Si(S) = IT
r l sin Sr,
1 - 1
ni ni KiKj[pij(r,)- pi,(m)]drl
,-I
Here, Si(S), K,, K,. and D(rl)are the interference intensity, effective electron numbers for a t o m of type i(which is equal to the atomic number of the i-type atom), effective electron numbers for atoms of type j (which is equal to the atomic number of j-type atom), and superposition of RDFs for each kind of atom, respectively. MacKnight et al. (1974) adapted the experimental RDF difference curve to a hard sphere model in which number of particles per sphere and radius of the sphere are varied to give the best fit. A best fit is obtained for a cluster of radius 8 A and number of particles per cluster 48. The two peaks between 3 and 9 A in the RDF are due to the internal cluster structure. The peaks in RDF arise from the arrangement of multiplets within the cluster. RDF analysis indicates that ionic peak does not arise from interference between scattering centers, as assumed by both Marx et al. (1973) and Binsbergen and G o o n (1973). The size of the cluster radius is well beyond Eiserberg's multiplet limit. MacKnight et al. (1974) have predicted the radius of the cluster to be in the order of 3 to 4 in the dry state and somewhat larger than this in the wet state from the Porod analysis based on Eq. (IO) and other assumptions; i.e., the discrete phase is composed of monodisperse spheres and the volume fraction of the dispersed phase is 0.05 based on RDF analysis.
A
where Jc
Q
SfUS.)d S ;
= 0
2 sin H S . = ___ h
+
Here S/Vl, 4, and +?, and are interfacial area per unit volumeof the dispersed phase, volume fraction of two phases, and scattering invariant. respectively. However, comparable radius. i.e., 8-10 A. is observed from Guinier analysis. in which volume fraction of clusters (+l) is calculated from Eq. (13) (MacKnight et al.. 1974):
444
Kar and Bhowrnick
SS'(I' - 1)dS $1
=
l, x
S+S'I.' dS
where I' is the calculating intensity for the Guinier approximation. I' is obtained from Eq. (14) (MacKnight et. al., 1974):
US
k )
= I(O)exp[ - (4/3)aS;R']
(14)
where I(0) is the extrapolated scattering intensity at zero angle and R is the radius of gyration. These deviations in results are due to the particle size dispersity, nonuniform electron density of phases, possible lackof sharp phase boundary, and the presence of interference effects between particles. Based on this analysis, MacKnight et al. ( 1974) have proposed a shell-core model and a lamellar shell-core model for clusters. In the shell-core model, a cluster of 8- 10 A in radius is shielded from surrounding matrix ions by a shell of hydrocarbon chain. The surrounding matrix of ions, which cannot approach the cluster more closely than the outside of the hydrocarbon shell. is attracted to the cluster by the electrostatic force in the case of coordinate metal ions. The distance between the cluster and the matrix ions is on the order of 20 A, which is the origin of the ionic peak. 5.3 The Yarusso and Cooper (Modified Hard-Sphere Model) (1983)
The simple hard-sphere model involvesthe assumption of spherical particles that have nointeraction other than impenetrability. This model is incapable of predicting the observed intensity upturn near zero angle, the large difference between the calculated values, and the experimental values of functional groups in aggregates and provides a poor fit to the experimentaldata. Yarussoand Cooper (1983) proposed a model called the modifiedhard-spheremodel. This model is based on the following assumptions: The closest distance between the aggregate is 2 RCA, where RCAis greater than R (radius of the aggregate). 2. Each ionic aggregate is coated by alayer of hydrocarbonmaterialwhoseelectron density is same as the matrix.
I.
Yarusso and Cooper (1983) calculated the intensity of x-ray scattered, I(S) by the equation:
where
(b(SR) = 3
sin SR - SR cos SR (SR).'
Here, I,. V, p,, and E areintensity of x-ray scattered by thesingle electron, volume of the sample, difference of electron density between the spheres and the matrix. and constant very
lonomeric Thermoplastic Elastomers
close to one, respectively. The model provides a better existing model.
445
fit to the experimental data than any
6. PROPERTIES The presence of metal carboxylate ormetal sulfonate groups or other groups pendent in a polymer chain has a strong effect on polymer properties such as mechanical properties, glass transition temperature. the rubbery modulus above the glass transition temperature, dynamic mechanical behaviour. relaxation behavior, melt rheology. dielectric properties, thermal properties. electrical properties. optical properties, polymer solution behavior. etc. These properties depend on the morphology of ionomers. The morphology of ionomers is characterized by electron microscopy, infrared spectroscopy. Raman spectroscopy, Massbauer spectroscopy, nuclear magnetic resonance spectroscopy, x-ray diffraction, nuclear scattering, and electron spin resonance spectroscopy. A discussion and a review on ionomer properties are given by Holliday (1975), Eisenberg (1980), MacKnight and Earnest (1981). Rees (1987). MacKnight and Lundberg (1987), Tant and Wilkes ( 1988), Mauritz ( 1988). Fitzgerald and Weiss (1988), Lundberg ( 1989). and Hara and Sauer ( 1 988). In this section, the effects of ion content, counterion type, degree ofneutralization, aging and thermal treatment, plasticizer and other additives. and blend composition on various properties of ionic thermoplastic elastomers are discussed. 6.1
Infrared Spectroscopy
Infrared, Fourier transform infrared, and far-infrared spectroscopies are widely used to investigateion-ioninteraction and domainformation in carboxylatedandsulfonatedelastomersat variouslevels of neutralization with differentcations (Rees. 1964;MacKnightet al.. 196th; Otoaka and Kwei, 1968b: Uemura et al., 197 I : Tsatsas et al., 1971; Eisenberg and King, 1977; Brozoski et al., 1984a, 1984b; Agarwal et al., 1987; Coleman et al. 1990). Infrared investigation has been conducted on a sulfonated EPDM system. where the sulfonate group is neutralized with various monovalent and divalent cations to understand ion-ion interactions. Table 3 lists the vibrational bands observed in metal sulfonated EPDM (Agarwal et al., 1987). The base EPDM polymer shows bands due to atactic polypropylene and amorphous ethylene units. Two bands are observed in the region from 1750 to 1700 cm", at 1725 cm-' and 1715-' cm-' for Li',Ba'+,Mg",Zn", and Pb'+, and at 1720 cm" and 1700 cm-' for the NH4+salt. These bands are assigned to acetic acid, which is liberated upon reaction of the metal acetate with the sulfonic acid. The Zn'+.Pb" and NH4+ salts do not show any band in the region of 1650 to 1550 cm-', whereas Li' and ME'+ show at 1620 cm" due to cation-bound water of hydration, and Ba'" shows at 1580 cm" due to free water. The band at -1250 cm" is due to a -CH2 wagging motion. The intensity of this band is dependent on the polar end group and the conformation of the -CH2 group. The band at 1 100 cm-' is due to the longchain fatty acid. The band at 731 cm-' is characteristic of crystalline polyethylene segments containingthe tmrrr7s sequence of themethylenegroup. The1050 cm" band is due to the symmetrical stretching of the -SO3- group. Theband at 610-615 cm-' is due to C-S stretching of the polymer -SO>- band. The bands appear at 1 190. 1 155, and I 162 cm-' for Li+, Mg'+ and NH4'. indicatingasymmetricalattachment to the SO3- chain. In thecase of B$+. the interaction with the sulfonate ion appears to beasymmetrical,sincewell-definedbands are observed at 1 192 and 1 155 cm-'. There is some splitting in the case of Zn" and Pb'+, indicating an asymmetrical bonding. The sharp band at 731 cm" for Ba". a small amount in Zn'+. and
Kar and Bhowmick
446
Table 3 Vibrational Bands (cm"
)
1730 1 720
I728
1725
I725
I720
1718 1620 -
-
17 18 -
1700 -
I255
I155 1200 (S11) 1 15 0
1255 I 175 (Sh) I150 (Sh)
I255
-
1135 1115
I135
1728 1718 1 580 1255 I192 I155 I135 1115 I 100 1052 1030 73 I 61 0
1620 -
125s I190 113s 1118
I100 1065 103x 73 I 615 SorrrcY.
of Sulfonated EPDM Salts
-
1155 I135 1115 1 100 1050
1030 73 I 608
1115 1100
I 055 1030 731 (Sh) 60X
1450
I100 1045 (Sh) 1020 73 I 605
1162 -
111s 1100
1052 1025 7 31 610
AgLlrwal et a i , . 1987.
various degrees intheothersaltssuggest that ionicaggregationdue to thesulfonatecation interaction brings aboutan order rearrangement of the polymer chains, forming extended regions in which the methylene groups are in a f r a m configuration. Infrared spectra of ethylene-methacrylic acid containing 4. I mol% methacrylic acid and its ionomer over a range of neutralization from 0 to 78% at room temperature (MacKnight et al.. 1968a) have been investigated. The 2650 cm" band is characteristics of hydrogen bonding (hydrogen-bondedhydroxyl). Thereis unionizedcarbonyl at 1700 cm" and asynmetrical stretching of the carboxylate ion at 1560 cm". The 1560 cm" band increases with degree of neutralization.Investigation of the temperature dependence of the relativeintensities of the bands at 1700 cm" (hydrogen-bonded carbonylstretchingvibration)and 3540 cm" (free hydroxyl stretching vibration) evident in the acid copolymer gives a dissociation constant for the carboxyl-dimer association. The dissociation constant, K,,. is defined as:
K''
=
["COOH]' [(-COOH)2]
These infraredstudies have been extended to use the infrareddichroism to characterize the structural features. which are responsible for (Y relaxation (MacKnight et al.. 1968a: Uemura et al.. 197 1 ). The infrared dichroism is related to the orientation of a molecular chain by:
where ,f: is the molecular orientation function of the ith molecular segment. defined as: ,f: = [3 < cos2 0> ave (21)- 1]/2
D, is the dichroism. defined as
where A , I and A , are the absorbances for radiation polarized parallel and perpendicular to the
lonomeric Thermoplastic Elastomers
447
stretching direction. and C, is a constant related to the angle between the stretching axis and the transition moment. The orientation function (Uemura et al., 1971) and dichroic ratio (MacKnight et al.. 196th) have been calculated for various bands characteristic of the hydrocarbon segments (1470 cm". CH, bending: 720 cm". CHI rocking).unionizedacid (1700 cm". hydrogen-bonded C = O), and ionized carboxyl (1560 cm". carboxylate ion). The 720 cm" band shows large perpendicular dichroism and little of the hydrogen-bonded carbonyl (1700 cm") and carboxylate (1560 cm"). But the orientation functions of the above bands are found to increase with the degree of stretching. Infrared spectra of ethylene methacrylic ionomers are strongly dependent on annealing conditions. the presence of moisture. as well as coordinating tendency of metal ion (Brozoski et al., 1984a. 1984b; Coleman et al.. 1990). Far infrared spectroscopy is also applied to investigate domain formation in ionic polymer. The spectra covers a range of 33-800 cm". A well-defined band in the region below 600 cm" is observed in all salts. which is not present in acid forms of the copolymer. This band shifts from 450 ? S cm-' for the Li ionomer to 230 k 5 cm" for Na". 180 k 3 cm" for K'. and 135 ? 3 c n - ' for CS'. This band is attributed to the cation motion in the anionic fields of the polymers. The intensity of the peak is related to the cation properties andis assigned to perturbed skeletal motions of a neighboring polymer segment.
6.2 NuclearMagneticResonance Nuclear magnetic resonance is widely used in the study of relaxation phenomena. the extent of aggregation of metal ions. and phase transition of ionomers. Read et al. ( 1969) have measured relaxation time of copolymer derived from ethylene methacrylic acid copolymer containing 4.1 mol% methacrylic acid and its 53% ionized sodium salt at a radiofrequency of 30 MHz. T I , the spin-lattice relaxation time, and TIC.the spin-lattice relaxation time in the rotating frame, are rates at which the nuclear spins exchange energy with other modes of motion under certain conditions and are measured as a function of temperature in the study of phase transition. The n1inima in the TI curve at 0 and - 100°C are identified as y and 6 relaxation. The TI, data is represented by two separate relaxation times a t each temperature. which are tentatively assigned to nuclei in the amorphous and crystalline regions of the polymer. Four minima are observed in each of the TI, curves. These are assigned as a,p. y. and 6 relaxations. Similar behavior is observed for TI and TI, in unionized copolymer. The broad-line NMR technique gives some idea about the extent of aggregation of the metal ions in ionomer. Otocka and Davis (1969) examined NMR linewidth of ethylene acrylic acid copolymer (4.9 mole% acrylic acid) and its lithium ionotner (fully neutralized) over;I range of temperatures measured by both proton and lithium-7 magnetic resonance spectroscopy. In all cases. NMR line width narrows into two stagesidentified by y and p transition. The difference of the line narrowing observed in these two techniques indicatesthat nuclei are not well dispersed throughout the matrix. They are segregated to some extent. 6.3 X-RayDiffraction Wide-angle and small-angle x-ray scattering results elucidate the state of ionic aggregation in ionicpolymer.Wilson et al. (1968) compares diffraction scans of branchedpolyethylene.a copolymer of ethylene, and methacrylic acid. and its ionomer prepared by fully neutralizing with sodium. Polyethylene-like crystallinity is observed in all three samples. arising from the orthorhombic polyethylene unit. This is characterized by the presence of 1 10 and 200 peaks. The percent crystallinity is calculated from the ratio of the areas of 1 10 and 200 peaks to the
Kar and Bhowmick
448
total area. The acid copolymer and ionomer show less crystallinity than the parent polyethylene. The ionomer contains a new peak at approximately 20 = 4", referred to as the ionic peak. This is a common feature of all ionomers regardless of the presence or absence of backbone crystallinity and the natureof the backbone. In addition to this, the ionic peak has the following characteristics (MacKnight et al., 1974):
1. The ionic peak is observed above a certain ion content. 2. The ionic peak appears in all ionomers regardless of the nature of the cations being present with lithium,including all alkalimetals,as well asheavymetals,divalent cation, trivalent cation, quaternary ammonium ion, etc. 3. The nature of thecation influences thelocation of the ionic peak as well asthe magnitude of ionic peak. The ionic peak is observed at a low angle for cesiumcation compared to lithium cation at the same ion concentration. Similarly, the magnitude of the ionic peak of cesium is several thousand-fold greater than that of lithium. 4. The ionic peak is relatively insensitive to temperature. Its peak persistsat a temperature of even 300°C (Wilson et al., 1968). 5. The ionic peak shows no evidence of orientation in cold draw samples. 6. The ionic peak is moved to a lower angle or destroyed when the ionomer is saturated with water. The scattering profile in the vicinityof the ionic peakin the water-saturated ionomer is different from that of the parent acid copolymer. 7. The magnitude and position of the ionic peak depend on the amount of acid present in the parent copolymer and on the degree of neutralization (Wilson et al.. 1968). The low-angle x-ray scattering measurementsof a series of cesium ionomer obtained from ethylene methacrylic acid copolymer have been extended to a level of 28 = 0.01 radian (Delf and MacKnight, 1969). Theresults were also comparedwith low-density polyethylene. A steady decrease in intensity with increasing Bragg angle is observed in low-density polyethylene and the copolymer of ethynene-methacrylic acid. But the ionomer shows a strong peak at 28 = 0.02 radian. corresponding to a periodicity of 83A. This is due to the presence of aggregates containing cesium ions. Thispeak is insensitive to annealing. Thecorresponding lithium ionomer does not show any peak, which is due to the poor scattering of lithium. An extensive study of x-ray scattering has been done on ionomer prepared from butadiene methacrylic acid copolymer and ethylene methacrylic acid copolymer (Marx et al.. 1973a; Marx and Cooper, 1973).Variabilityincludestheeffect of plasticizersincludingwater,methanol. formic acid, acetic acid and methacrylic acid, degree of neutralization and acid content (up to 7 mol%). The results are interpreted according to the following equation: dBrkIgI! = c(V'
f-1)"3
(23)
where dH,k,gg, V', c, and .f" are the spacing correspondingto the measured Braggangle, constant, volume per carboxyl group (calculated from composition), and number of carboxyl groups per scattering site, respectively. The values of f - ' are increased from 2 for copolymers containing 2 mol% acid to 3 for composition between 3 and 5 mol% and 4 for compositions of 5-7 mol%. The number is not dependent on degree of neutralization. They do not observe the low angle peak as observed by Delf and MacKnight (1969). They conclude that the existence o f a secondary low angle peak outside the main peak in the wide angle is indicative of regularity in the spacing between the scattering centres.
Neutron Scattering Small-angle neutron scattering(SANS) hasgreat importance in the investigation of polymer morphology. Oneofthe most impressive accomplishmentis the measurementofdimensions ofa single
lonomeric Thermoplastic Elastomers
449
chain in bulk or the dimensions of ionic clusters. Several SANS studieshave been done on both deuterium-labeled and unlabeled ionomers (Mayer and Pineri. 1978; Roche et al., 1980; Earnest et al.. 1982). Contrast is achieved by adding measured amount o f D 2 0 to the samples. There is no evidence of a scatteringmaximum in thecase of adry sample. But the SANS peak becomesdetectable when small amounts of D,O are added to the sample. This can be explained by the measurements of neutron contrast factors shownin Table 4 calculated from the following equation:
where b, and b2 arethescatteringlengthspermolecular units and V I and v2 are the molar volumes of these molecular units. Table 4 reveals that the differences between neutron scattering contrast factorsof polypentenamer chains and cesium sulfonategroupsaresmall. But there is asubstantialdifference between D 2 0 and polypentenamer. Similar scattering curves are obtained for the 5.5% and 12% ionomers. The Bragg spacing of the SANS ionic peak observed at low D 2 0 concentration is the same as the SAXS peak for dry samples of cesium sulfonated polypentenamer. The SANS ionic peak moves to a low angle with increasing D 2 0 concentration in the sample (above a D20/S03- ratio of 6).
6.4
MossbauerSpectroscopy
MOssbauer spectroscopy is applied to several families of ionomers, such as the ferric salt of poly(butadiene-CO-styrene-CO-4-vinylpyridine)(Meyer and Pineri, 1978) and Nafion. to identify the various types of ionic aggregates. The appearance of a hyperfine spectrum at the expense of the doublet D1 is characteristic of magnetically ordered clustered complexes of radius 30 A with supermagnetic behavior. The supermagnetic behavior is confirmed by the existence of a residual thermal magnetization. At a temperature of -245°C there is only doublet Dl 1 and hyperfine spectrum (SH). The second doublet (D1 l ) , i.e.. the second component of the spectrum, has three characteristics: 1. The large quadruple splitting indicates a very asymmetrical environment. 2 . It appears at a temperature of - 33°C. near to the glass transition temperature, 3. The evolution of D l 1 with an applied magnetic field is characteristic of a zero spin.
Table 4 Scattering Lengths and Neutron Scattcrinp Contrast Factors (K,,) for Chemical Units in lonomer Chcmical unit -(CH?)j-CHSH-(CH?)j-CHS-
l
S0~"CS' -(CH:)j-CHSHD20 "CH: CH:D20
b , X 10'' (cm)
0.004 I 0.0 165
0.004 1 0.096 - 0.0071 0.096
K,,
X
10" (cm-2) 1S 4
84 106
Kar and Bhowmick
450
These lead to theconclusion that these complexes are dimerized with an antiferromagnetic coupling. At a temperature of -269°C. there appears a third doublet (Dl 11) in addition to second doublet and hyperfine spectrum.
6.5 Electron Spin Resonance Spectroscopy Electronspinresonancespectroscopy is used to studytheinteractionsbetween cations and ionomers. The spectrum of electronspinresonance is sensitive to the local environment of paramagnetic ions and depends on the interactions between the electrons and nuclear spins of ions and ligands. An electron spin resonance spectrum of butadiene methacrylic acid iononer neutralized with zinc (95%) and copper (5%) and containing 9% acid group at 25°C shows that when copper is used the degeneracy of five 3d levels is removed in the presence of a crystal field (Pineri et al., 1975). The spectrum is a characteristic of isolated Cu2+.It clearly shows the presence of hyperfine structure. The Lande factors, g, and g , and hyperfine interaction parameters. A;, and A;, are 2.056. 2.282. 146 Oe, and 23 Oe, respectively. This is attributed to the presence of the RCOO- Cu'+ -0OCR group.
,,
6.6
Raman Spectroscopy
Raman spectroscopy is also ernployed on a series of ionomers based on ethyl acrylate-co-sodium acrylate and poly(styrene-co-p-carboxy styrene) to characterize the degree of ionic aggregation (Neppel et al., 1979a, 1979b. 1981).
6.7 Electron Microscopy Various experimental studies have shown that strong structural changes occur in a variety of systemsuponneutralization of polymeracids.Electronmicroscopy is the best technique to demonstrate the presence of ionic regions in ionomer systems. Electron microscopy studies of sulfonated EPDM havebeen done by Handlin et al. (1981). Sulfonated EPDM contains doublebonds. The bondsare strained by osmium tetraoxide. Strained sections show the presence of ionic domains. Most of these ionic domains are spherical in shape and less than 3 nm in diameter. An examination of iononlers from butadiene methacrylic acid copolymer was done by electron microscopy (Marx et al., 1971). A grainy appearance was observed in the ionomers but not the free acid. The granular size was found to be considerably smaller than the ethylene ionomers. varying from 13 to 26 W. The distribution and size of ionic clusters in a butadiene-styrene-4-vinyl pyridine terpolymer-crosslinked by coordination of the pyridine groups with ferric chloride have been examined by electron microscopy (Meyer and Pineri, 1978). Many heterogeneities of high electron density were visible. No such heterogeneities are observed in the uncoordinated polymers. Large domains are due to the super position of smaller ones. Diameters vary from 50 to 1000 A. Transmission electron microscopy and surface replication electron microscopy have been carried out on ethylene methacrylic acid ionomer to elucidate the ionomer morphology (Davis et al., 1968; Langworth, 1975; Handlin et al., 1981). In the acid copolymer, the lamellas typical of crystalline morphology of polyethylene are clearly observed.These lamellas are further organized into spherulitic structures. But the ionomer shows no evidenceof such structures exhibiting two major features.
lonomeric Thermoplastic Elastomers
451
I.
Spherical regions approximately 1 k m in diameter appear: secondary electron imaging of a gold coated film shows that these are surface features, which are not present in the highly transparent bulk polymer. 2. Irregular electron dense features of about 2-20 nm proposed by Davis et al. ( 1968) are ionic domains. These are randomly distributed throughout the film. Another study showed thatthe acid form exhibits spherulitic morphology and the rubidium salt shows no such spherulitic structure, rather presents an irregular granular structureof diameter about 100
A.
7. MECHANICALPROPERTIES The high tensile strength of ITPEs compared to base polymers is attributed to their ability to relieve local stresses by an ion exchange mechanism. ITPEs in general show low permanent set even at considerable levels of stress relaxation and creep. The creep andstress relaxation behaviors are explained by the exchange mechanisms between time-dependent crosslinks. However, the creep recovery indicates that some of the crosslinking sites are very stable. These stable crosslinking sites are expected to be larger aggregates of clusters of ionic groups and to be stable at high temperatures. Mechanical properties of metal sulfonated EPDMs have been systematically studied by Amassetal. (1972), Rees and Reinhardt (1976). Makowski et al. (1980), andKurianetal. ( 1995). The effect of sulfonate content on tensile strength for zinc sulfonated EPDM at room temperature is shown in Fig. 3 (Makowski et al.. 1980). Tensile strength begins to develop at about IO- 15 mEq of sulfonate per l00 g of polymer. Remarkable tensile strength is obtained at 30 mEq of sulfonate per I00 g of polymer (equivalent to 1 mol% sulfonate) and attributed to the association of the ionic groups. Figure 3 also shows that tensile strength depends on the base polymer. Highest tensile strength is observed in E-70 (see Sec. 3). This is directly attributed to the combined effects of the ethylene crystallinity and ionic association. E-55 and CR-709-A behave similarly. The CR-2504, which is a fully amorphous copolymer, deviates from the behavior of other polymers. This can be explained on the basis of a less even distribution of sulfonate groups within the polymer backbone. Comparative data for mechanical properties and rheological properties for nine different cations are given in Table 5 (Makowski et al.. 1980). The rheological properties will be discussed in detail. The high degree of ionic association of these various metal sulfonated EPDMs are clear in their low elongation. These low elongations decrease the tensile strength. But zinc and lead systems show high tensile strength and elongation. The mechanical properties of sulfonated EPDMs dependon the ethylene content (Makowski et al., 1980). The tensile strength increases appreciably with increase in ion content in all series of ionomers. It also increases with the ethylenecontent of the ionic thermoplasticelastomer at any given ion content. Makowski and Lundberg (1980b) studied the plasticization effect of a large number of metal stearates and stearic acid on mechanical properties of various metal sulfonated EPDMs. The tensile strength of zinc sulfonated EPDM showslittle change with stearic acidconcentration. On the other hand, barium and magnesium sulfonates exhibit improvement in tensile properties. Although stearic acid is beneficial to tensile strength at room temperature, it shows a deleterious effect at high temperature, even at 70°C. The effects of different kinds of metal stearate (i.e., zinc, barium, and magnesium) on tensile properties of barium. magnesium. and zinc sulfonated EPDMs are given in Table 6 (Makowski and Lundberg, 1980b).
Kar and Bhowmick
7000 ._. 6000
[ E-70
W a "
."R 5000Y
I
5 r E &OOOIY
0 ( CR-709-A )
Li
!3000' v)
Z W
I[R-2504)
l-
20001000 a-
0
10 20 30 40 50 60 SULFONATE CONTENT, meq./100 POLYMER
Fig. 3 Effect of sulfonatecontent and EPDM on tensile strength at 25°C (metalcation:zinc).(From Makowski ct al., 1980.)
Table 5 Effect of Cations on Physical and Rheological Properties" 70°C
(poise Metal
Hg Mg
ca
CO
Li Ba Na Ph Zn
Melt Apparent viscosityh shear at X 10')
fracture rate (sec" )
-
-
55.0 53.2 52.3 51.5 50.8 50.6 32.8 12.0
800 75
ZnSt,
(psi)
85 470 >800 450 370 370 750
0.0 1 0.6 1.0 0.02 0.02 I .9 4.0 0.2 0.04 0.05 8.2
Base polymer: CR-2.501. Sulfonate content: 33 mEq/100 g polymer. Source: Makowskl and Lundbcrg. 1980b. "
Little improvement in tensile properties is observed with barium and magnesium stearates. On the other hand, the best tensile property is observed from a zinc stearate plasticized zinc sulfonated EPDM. The effect of zinc stearate concentration on the tensile properties of zinc, barium, and magnesiumsulfonated EPDMs at roomtemperature and 70°C wasreported by Makowski and Lundberg (1980b). Substantial improvements were observed for every cation. But zincsulfonated EPDM shows the best properties.Zincsulfonated EPDM showshigher tensile strength compared to barium and magnesium EPDMs. Retention of tensile properties is maximunl for zinc due to its stability. Brenner and Oswald ( 1980) studied systematically the influenceof the structure of quaternary phosphonium counterious on physical properties and melt flow rate of sulfonated EPDM containing I mol% sulfonation. The physical properties and melt flow rates are given in Table 7 (Brenner and Oswald, 1980). Table 7 demonstrates the effect of chain length of n-alkyl substituents, number of higher alkyl substituents, and mono- and divalent quaternary ionson physical properties. Tensile properties decrease with increasing length of n-alkyl substituents and with an increasing number of higher alkyl substituents on the charged central atom. The addition of long chain further diminishes the available space near the central atom. This results in a much higher degree of steric hindrance in the region near the central atom as well as making it difficult for anions to come close. Of course. the magnitude of the effects of adding long chains depends strongly on the length of the short chain originally present in the polymer and on the length of the added long chain.Increasinglength of theoriginal short chain decreases the effect of theadded chain lengths. The physical properties of the ionomer depend not only on the structure of the single counterion but also on the interaction between counterion-anion ion pairs. The anion from a
Kar and Bhowmick
454
Table 7 Physical and Rheological Quatcrnary Phosphonium Ions
Propcrties of Sulfonated EPDM" Neutralized with Various
Tcnsile
modulus strengthh Quatcrnary Samplc no. I 2 3 4 5 6 7 X 9 10 11
12 13 14
15 16
phosphomum ions
Initial
(kPa) (kPa) S40 300 290 260 300 290 300 520 390 I80 550 310 210 I85 I60 700
1700 I100 970 940 1000 I000 1000 I 500 1250 700 1500 1100 YO0
750 560 1700
flow Mclt rate (150"C, 12.5 kg of I 10 min)
0.09 0.01 0.03 0.22 0.01 0.08 0.0 I 0.02 0.2s
0.003
first ion pair, in attempting close approach to the cation of a second ion pair. must contain not only the substituents of that quaternary ion but also the anion i n the second ion pair. At the same time, the second anionis trying to minimize its distance from its own associated counterion and thereby is also dragging its covalently attached polymer backbone chain close to the central charged atom. In addition, the first anion must contain the substituents of its quaternary counterion in the first ion pair. to which it is closely held. as well as with its appended polymer backbone chain. which it must drag alone. Thus, physical properties of ionomers depend on the strength of the interaction of the ion pairs.Similarly, the relative strength of the ionomer samples depends on the monovalent and divalent counterions. But it is very difficult tomake any generalizations. Aging at ambient temperature near the glass transition temperature for longer periods of time or thermal treatment at elevated temperatureshave little effect on the mechanical properties of metal sulfonated EPDMs (1 mol%) (Duvdevani et al., 1986). The modulus of an ionomer containing 50 phr zinc stearate increases about 35% at an aging time of 100 hours. Only the stress-strain response is slightly altered at 230 hours of aging. The copolymerization of methacrylic acid with 1,3-butadiene gives a tougher, less elastic elastomer. more thermoplastic in nature than base polymer (Brown, 1963; Jenkins and Duck, 1975). The advantages of carboxyl incorporation are increased hardness, higher green strength. better milling properties. good resistance to hydrocarbon solvents, higher temperature limit of elasticity. and superior adhesion properties. The main disadvantages are a greater tendency to swell or react in aqueous ammoniacal or alkaline solutions and increased oxidative attack leading to degradation and crosslinking. These propertiesincrease with increasingmethacrylicacid content, as shown by an increase in tensile strength. The rubbery properties of the copolymer lessen above 40% acid content. Theincorporation of a suitable divalent metal oxide in a butadieue-methacrylic acid copolymer containing less
lonomeric Thermoplastic Elastomers
455
than 40% methacrylic acid leads to a rubber of very high green strength. This divalent metal oxide neutralizesthe acid group andproducesastableneutralsalt, -OOC-Zn"-COO-. But neutralization of this carboxylated rubber with monovalent salts provides a weaker networkthan that obtained with divalent salts. For example, the tensile strength and elongation at break of butadiene-methacrylicacid copolymer having 10% methacrylicacid change to 11.7 M N h ' and 900% from 0.7 MN/m' and 1600% after treatment with NaOH followed by heating and to 41.4 MN/m' and 400% after zinc oxide treatment (Jenkins and Duck, 1975).It can be crosslinked by salt of diamine. salt of polyamine. polyalcohol, polyepoxy resin, and carbodiimides. Polyalcohol and polyepoxy are used for covalent crosslinking, but the physical properties are inferior compared to sulfur vulcanizates. It shows very poor compression set and high stress relaxation at elevated temperature. However, a combination of epoxy and metal oxide gives excellent hightemperature properties. As for the raw polymer. tensile strength of the cured polymer increases with acid content i n the copolymer (Jenkins and Duck, 1975). The thermoplastic nature of butadiene-methacrylic acid copolymers of varying acid content neutralized by lithium have been observed by Otocka and Eirich ( 1968). The effect of zinc oxide concentration on butadiene-acrylonitrile-methacrylic acid copolymer (56:33: I O ) has been studied by Brown ( 1957). Theoretically the amount of metal oxide or salt necessary to obtain the best properties should be proportional to the amount of carboxyl groups present in the polymer. But in actual practice. twice the calculated amount of metal oxide is required to obtain optimum mechanical properties. Only a portion of the oxide is used in effective chemical crosslinking; the rest remains as a mixture of some free oxide, some -0OCM-OH unlinked metal bonds, some intramolecular bonds, and some -00C-Zn2'-C00- bonds (Brown, 1975). The disadvantagesof oxide-cured butadiene-acrylonitrile-methacrylic acid ionomer are poor compression set. high stress relaxation, low flex resistance, low hysteresis loss, very low fluidity at high temperature. tendency to scorch during compounding, and poor mold flow of the compounded rubber. These properties are a result of the lack of stable crosslinking. The tendency to scorch during compounding can be avoided by using retarder (e.g.. phthalic anhydride) or by late addition of metal oxide in the mixing cycle. The mechanical properties of butadiene-styrene-methacrylic acid copolymer containing I .5 wt% carboxyl group with various divalent metal oxides and hydroxides have been discussed by Dolgoplosk et a l . (1959). They found that metal hydroxides give better physical properties than metal oxides. except magnesium oxide. Neutralization of these carboxylated rubbers with monovalentsaltsgivesaweakernetworkthan that obtainedfromdivalentsalts. This weak network is destroyed above 100°C. The lack of stablecrosslinks i n the metal oxide-cured vulcanizates is evident in the stress relaxation experiment. The unmodifiedtelechelicpolyisobutylene (PIB) polymer is aviscous liquid atroom temperature. while the sulfonated telechelic polyisobutylene ionomers (SPIB)exhibit properties of typical ITPEs. SPIB shows low permanent set, low hysteresis loss. and high tensile strength. It can be extended upto 1000% (Bagrodia et al., 1983a).Mechanical properties of SPIB ionomers depend onthe molecular weight.SPIB ionomershaving number average molecular weightabove 1 1,000 show good tensile strength andhigh elongation at break.The enhancementof mechanical properties with increased molecular weight is attributed to the formation of a end-linked pseudonetwork by the association of physical crosslinks of the ionic groups at the chain ends (Bagrodia et al.. 1983a). Unmodified SPIB doesnot show any strain-induced crystallization.However, above 600% elongation. a sharp diffraction pattern is observed by wide-angle x-ray diffraction in Ca-SPIB ionomer. The presence of terminal ionic groups and long-range coulombic interactions are responsible for maintaining molecular orientation with strain (Mohajer et al., 1982; Bagrodia et al., 1982). The strain-induced crystallites assist in maintaining low permanent set.
Kar and Bhowmick
456 Table 8 Influence of Cations on Ethylene Methacrylic Acid Ionomer Properties
Condition Anion Cross-linking agent (Wt%) Property Melt index ( I O g/min) Yield point (MPa) Elongation (%) Ultimate tensile strength (MPa) Stiffness (MPa) Visual transparency
Acid
Na'
Li+
BaZ+
Mg'
-
CHjO4.80
OH2.80
OH9.60
CH3COO8.40
5.80
0.03
0.12
0.19
0.12
6.10 553.00 23.40
13.20 330.00 35.80
13.10 317.00 33.90
13.40 370.00 33.90
15.00 326.00 40.40
13.20 3 13.00 29.70
7.10 347.00 22.00
68.90
190.30 Clear
206.80 Clear
223.40 Clear
164.10 Clear
208.00 Clear
103.40 Clear
-
Hazy
t
Zn"
AI3+
CH3COO12.80
CH3COO14.00
0.09
0.25
Soltrce: Rees. 1966.
Ca-SPIB ionomer shows higherelongation than K-SPIBionomer.Mechanicalproperties of SPIB ionomers neutralized with lanthanide series elements (cerium,lanthanum, etc.) are comparable with potassium and calcium ionomers and depend on cation valence andexcess neutralizing agent (Tant et al., 1986). The influence of various metal cations on mechanical properties of ethylene methacrylic acid ionomer is given in Table 8 (Rees, 1966). Monovalent,divalent,andpolyvalent cations enhance physicalproperties.Mechanical properties depend on the degree of neutralization. Table 9 demonstrates the effect of degree of neutralization on mechanicalpropertiesand melt index of ethylene-acrylicacid copolymer neutralized with various alkali metals (Rees, 1987). The modulus increases with degree of neutralization up to 30% and then decreases. Stiffness increases with neutralization to a plateau at about 40%. and tensile strength increases up to 77% neutralization for most cations (Rees, 1987). The effectsof aging in nitrogen atmosphere at ambient temperature on mechanical properties of ethylene-methacrylic acid ionomers neutralized by zinc and a combination of zinc and organic amine, such as 1,3-bis(amino methyl) cyclohexane (BAC), have been investigated by Hirasawa et al. (1989). Stiffness increases with aging time. The increase in stiffness is high in the case of fully neutralized ionomer. After 38 days of aging, the percent increase in stiffness is about 28% for the acid, 56% for the 0.6 zinc ionomer, 81 % for the 0.6 zinc and 0.4 BAC ionomer, and 130% for the 0.2 zinc and 0.97BAC ionomer. In another study of ethylene-methacrylic acid ionomers neutralized with various typesof counterions at various degree ofneutralization,asignificantincrease in both modulus and yield stress was observed (Hirasawa et al., 1991). But the effect of ageing on large strain properties is different. There was little change in the value of tensile strength and elongation at break in the prolonged aging time. The stiffness and yield point of diamine salts of ethylene methacrylic acid ionomer are increased substantially, but the tensile strength and melt viscosity remain constant ( R e a , 1987). A relationship between diamine chain length, acid content, and these properties has been established (Rees, 1987). Short diamines are effective only at high acid levels, while long diamines such as decamethylene diamine are unsuitable in all acid-containing polymers. But the diamine ionomers of ethylene methacrylic acid are not commercially viable due to oxidative instability.
Table 9 Properties of Ethylene-Acrylic Acid Copolymer Salts 4 Neutralized Starting material, 14.84 acrylic acid wt Sodium salt
Potassium salt
Lithium salt
" Infrared measurements Source: Rees. 1987.
0
12.0 30.0 47.5 66.0 8.0 25.0 51.0 63.0 12.0 28.5 52.5 67.5
Melt index, 43.25 psi (dg min-') 67 12.2 3.9 1 .o 0.3 16.30 4.50 2.70 0.57 18.7 5.2 1.4 0.2
Melt index, 432.5 psi (dg min-') 2.570 256.0 92.0 30.0 7.6 360 110 49 15 442.0 116.0 38.0 5.4
Secant modulus 14 extension (psi)
Tensile strength (ultimate) (psi)
Elongation at break (%)
Density (g/cc)
7000
2150
470
0.949
33,400 48,600 42,500 39,700 29,300 52,600 49,500 44,800 26.300 48,900 48,500 36,900
3,150 4,000 4,600 4.800 3,050 3.700 4,450 5,000 3,150 3,850 4, I00 4.600
420 330 310 280 470 410 370 390 410 350 260 250
0.9568 0.9586 0.9603 0.9633 0.9588 0.9626 0.9684 0.9750 0.95 16 0.95 10 0.9493 0.9446
Kar and Bhowmick
458
Table 10 Mechanical Properties of Ionomers Derived from Terpolymer Precursors Composition by weight Methacrylic index Melt Tensile modulus Strength Elongation Cation acid acetate Ethylene Vinyl 70 70 70 65 65 65
20 2s
10
-
10
Nai Mg?'
10 10 IO 10
Na' Mg' '
(g110 min)
(MPa) (MPa)
9.00 1.20 0.04 12.50 1.70 0.007
13.80 56.00 37.44 12.90 18.50 37.40
(%)
10.3 43.4 45.0 9.6 30.9 37. I
530 410 280 610 410 300
The mechanical properties of a large number of ionomers derived fromethylene dicarboxylic acid have also been reported by Rees (1987). Tensile strength increases with neutralization when the acids contain adjacent carboxyls. The polymers do not increase significantly in modulus on neutralization.althoughthetensilestrengthincreases.Withnonadjacent carboxyls, as in itaconic acid. the expected modulus increase is observed. Rees (1987) has reported a series of ionomers from ethylene vinyl acetate methacrylic acidterpolymersynthesized by Wolff. A very interestingproperty, good tensilestrength, is obtained, as shown in Table IO. A report from E.I. du Pont de Nemours shows another interesting type of ionomer derived from terpolymers of ethylene, vinyl acetate, and an organic acid. These were supplied with the trade name Elvax and have the advantages of superior oil and grease resistance and improved adhesion to polar substances. Their important properties are shown i n Table 1 I . The effect of mixed peroxide and metal oxide vulcanizates on the mechanical properties of carboxylic nitrile elastomer have been discussedby Brown (1963). Compression set drastically decreases and modulus increases in salt and peroxide crosslinking vulcanizates. On the other hand, tensile strength and tear strength dramatically decrease. The best properties are obtained from mixed peroxide and metal oxide vulcanizates compared to pure metal oxide vulcanizates. The effect of a mixed crosslinking system in carboxylated nitrile rubber has been reviewed by Chakraborty et al. (1982).
8.
DYNAMICMECHANICALPROPERTIES
The dynamic mechanical properties of ionomer systems provide definite evidence that the salt forms of these systems are dramatically different from the acid forms and the parent polymers. Specifically,theneutralized forms of thesematerialsdisplayarubbery plateau in modulus temperature curves that is not present in the base polymers. In this section, the storage modulus, loss modulus, loss tangent, stress relaxation behavior, and glass transition temperature of individual ionomers are highlighted. The data of storage modulus for three different monovalent cations, i.e., lithium. cesium, and ammonium, and one bivalent cation, i.e., barium, at different temperatures was reported by
lonomeric Thermoplastic Elastomers
459
Table 11 TypicalProperties of Ethylcne Vinyl Acetate h o m e r . '
Mclt indcx" Vinyl acetate, % Acid number' Density ( ASTM D I 505 ). 23°C kg/m.' (g/cm.') Tensile strength (ASTM D 1708)," MPa (psi) Elongation at break, (ASTM D 1708)," ?+ Elastic (tcnsilc) modulus (ASTM D 1708),'1~'MPa (psi) Hardncss, Shore A-2 (Durometer, 10 sec, ASTM D 2240) Softening point, ring and ball, (ASTM E 28), "C ("F) Cloud point in paraffin wax,' "C ("F) ~~
Elvax 4260
Elvax 4310
Elvax 4320
Elvax 4355
5.0-7.0 27.0-29.0 4-8 955 (0.955)
420-580 24.0-26.0 4-8 945 (0.945)
125-175 24.0-26.0 4-8 947 (0.947)
5.0-7.0 24.0-26.0 4-8 952 (0.952)
19 (2700)
2.0 (300)
5.2 (750)
19 (2800)
IO00 10 (1500)
600 6.2 (900)
900 8.3 ( 1200)
1000 I4 (2000)
80
68
72
83
158 (316)
83 (181)
91 (195)
151 (304)
99 (210)
88 ( 190)
88 (190)
88 (190)
~~
These data are presented as a general description of properties and are not Intended to he used for design spccificatlons. l' dg/min (ASTM D 1238, modified). Milligrams potasslum hydroxlcie per gram polymer. 100 > 100 3.5 3 0.1
strength Ultimate 1,500 1,500 1,700 12,000 160,000
(psi)
506
Bhowmick
Table 21 Oil Resistance of ALCRYN Melt ProcessableRubber: Typical Properties
ASTM Oil No. 1 Immersion 7 days at 100°C Tensile strength. MPn 100'2 Modulus. MPa Elonption at break, % Hardness. Shore A durometer Volume change. 'A ASTM Oil No. 3 Immersion 7 days at 100°C Tensile strength. MPa 100% Modulus. MPa Elongation a t break. Hardness Shore A durometer Volume change. (2 Orlginal Properties Tensile strength, MPa 100% Modulus. MPn Elongatloll at break. r/r Hardness, Shore A durometer
R1201 B 60A
R1201 B 70A
R1201 B 80A
13.9
15.4
4.0 335 66 - IO
5.9 280 73 -9
15.9 9.2 205 84
11.2 3.0 290 53
12.3 6.5
+ 10
10.7 4.6 205 60 +II
12.1 3.7 325 62
13. I 4.5 205 60
13.4 7.2 210 78
- 11
180
68 +7
10.3 Processing ALCRYN'K' is a unique plastic to process because it never melts. but it does soften enough above 300°F that high shear will reduce its viscosity drastically. The molding grades have no crystalline melting point and are essentially amorphous. ALCRYNIR' can be injection molded. extruded. and blow molded in faster. higher-productivity cycles than rubber. Conventional plastic equipment. particularly that used for polyvinyl chloride, is satisfactory. Rubber equipment may need to be modified.
10.4 Applications ALCRYN'"' offers high value i n use for many applications now served by vulcanized rubbers, thermoplastic elastomers, and flexible thermoplastics. The applications can be divided into the following groups: ( 1 ) automotive, ( 2 ) architectural, ( 3 ) industrial. (4) wire and cables. and (5) tools and appliances. Some of the applications suggested include gasoline cap seal. antifreeze testing lid. truck bumpers. door latch. truck hub seal. taillight housing, sidelight window lace, in-line fuse housing. fuel tank gaskets. seatbelt duct. copper belt cores. gasoline splash guard, reducing couplings, bulkhead seal. splash goggles. instrument gauge enclosures. full face mask. pipe couplings and adapter. window weather stripping. tool handle cores, suction cups, palm sander. friction rubber. tool grips. and flashlight handle.
11. HIGH-TEMPERATURETHERMOPLASTICELASTOMERS FROM RUBBER PLASTIC BLENDS Most of the thermoplastic elastomers prepared so far from rubber-plastic blends have poor hightemperatureproperties. For example, commercialblendsbased on EPDM-PP, NBR-PP, and
Miscellaneous Thermoplastic Elastomers
507
Alcryn melt processable rubber have a maximum operating temperature of 150°C. Thermoplastic elastomers could be made by using plastic of high melting point, thereby enhancingthe operating temperature.However, many high-meltingplastics like polyamidespresent some interesting processing problems, somewhat like those involved in perfluorocarbon resins. Also, many rubbers like silicone, which can withstand high temperature, do not have an appropriate match of hard segments. The problems are still greater when chemical and oil resistance are demanded from such TPEs. This section will highlight a few recent developments in this area. Jha and Bhowmick (1997a) reported the preparation of thermoplastic elastomeric reactive blends of nylon-6 and acrylate rubber, their characterization, and the influence of interaction between nylon-6 and ACM on the mechanical and dynamic mechanical properties, rheology, swelling, and thermal degradation of the blends. It has been observed that during melt blending of nylon-6 and ACM in a Brabender Plasticorder at 220°C the mixing torque increases after the initial softening period, indicating the occurrence of interfacial reaction between nylon-6 and ACM at the processing temperature (Fig. 14). The increment of torque value is maximal atthecomposition of 55/45 (w/w) nylon-6/ACM,suggesting that the maximumamount of reaction occurs at that proportion. The solubility measurement of the blends in formic acid (a solvent for nylon phase) has revealed a maximum amount of graft formation near 50/50 (w/w) ratio. The dynamically vulcanized blends display higher amounts of graft formation relative to unvulcanized blends. IR spectroscopic analysis has shown a reduction in the intensities of the peaks corresponding to epoxy groups of ACM as well as carboxylic acid end groups of nylon6, suggesting a chemical reaction between the two at the processing condition. NMR analysis of 40/60 (w/w) nylon-6/ACM blend has suggested 75% consumption of epoxy groups of ACM during the reaction (Jha and Bhowmick, 1999a). Based upon IR and NMR analyses, a probable mechanism of reaction between nylon-6 and ACM at the processing temperature has been pro-
60 50/50 BLEND (DV)
Fig. 14 Torque-time chart at 220°C for nylon,ACM (-) andnylon:ACM (S0:SO w/w) blendboth unvulcanized and dynamically vulcanized (---) with 0.5 phr HMDC in a Brabander Plastlcorder.
508
Bhowrnick
t
Fig. 15 Dynamic mcchanical spectra of ny1on:ACM (40:60 w/w)blcnd a t various molding times.
posed. In the dynamic mechanical analysis. it has been observed that the tan 6,,,;,, (maximum value of tan6 at the transition) as well as Tg (glass transition temperature) of the bulk rubber phasearedecreasedwith the level of interactionbetween the twophases, followed by the appearance of a secondary transition at a higher temperature region (1 7-22°C) for 40/60 (w/w) nylon-6/ACM blend due to the formation of graftedACM chains(Fig. 15). In thecase of dynamically vulcanized blends. the tan6 peaks corresponding to the rubber phase are broadened and shifted to slightly higher temperatures ascompared tothe blends withoutdynamic vulcanization. With increasing level of interaction, the storage modulus of 40/60 (w/w) nylon-6/ACM blend at room temperature (25°C) increases from 1.26 X to 4.36 X MPa. The observed storage moduli of the blends at 50°C are close to those obtained from Kerner’s hard matrix-soft filler model, suggesting the formation of nylon-6 as the continuous matrix, which has beenfurtherconfirmed from themorphologystudies(Fig. 16). The interactionbetween nylon-6 and ACM also increases the Young’s modulus of 40/60 (w/w) blend from 20 MPa to 37 MPa and the hardness of the blend from 35 to 48 Shore D. Also, the tensile strength and the elongation at break increase appreciably with the level of the reaction (Table 22). Dynamic vulcanization of the blends results in a slight reduction in hardness and Young’s modulus, but the tensile strength and the elongation at break increase significantly.
ermoplasticMiscellaneous
509
NYLON-6 MATRIX Fig. 16 Morphology of ny1on:ACM (50:50 w/w)blend.
The blends are pseudoplastic in nature and an increase in shear rate decreases the viscosity and increases the extrudate swell of the blends (Jha and Bhowmick, 1997b). The viscosity of the blends displays positive deviation from the average values, suggesting the reactive nature of the blend components. The viscosity increases with increasing degree of crosslinking of the rubber phase. In the case of dynamically vulcanized blends, the interparticle interaction is low, and hence the effect of concentration of the rubber phase on the viscosity is found to be low compared to those without dynamic vulcanization. The activation energy of the melt flow of the dynamically vulcanized 40/60 (w/w) nylond/ACM blend varies in the range of 8-15 kcaV
Table 22 Mechanical Properties of Nylon-6/ACM Blends With and Without Dynamic Vulcanizationn Weight percent
(“/.l 40 45 50 55 60
of plastic Tensile strength Elongation at break Young’s modulus Hardness (ma) (%) (10)b 12 13 (13) 12 (15) 19 (16) 17 (20)
(122) 96
90 (142) 92 (140) 120 (116) 100 (150)
48
(30)
Wa) 37 41(35) 45 (43) 62 (56) 71 (68)
Vulcanned with 0.5 phr of W C . Value In parentheses Indicates the properties corresponding to the dynamically vulcanned blends.
(Shore D)
(44) 50 (47) 55 (51) 57 (54) 60 (57)
Bhowmick
51 0
mol and decreases with increasing shear rate. The morphology study of the extrudate suggests rupture of the ACM phase at a high shear rate in the case of uncrosslinked blends. whereas the morphology of the dynamically vulcanized blends is stable against shear stress. The blends are found to be reprocessable at 240°C without any appreciable degradation of either phase, which suggests its applicability as a thermoplastic elastomer. The swelling behavior of nylon-6/ACM blends in various solvents and oil and the effect of blend ratio, dynamic vulcanization of the ACM phase, and the interaction between the two phases on the extent of swelling of the blends in different solvents and oils have been examined (Jha and Bhowmick, 1998). The swelling of the dispersed ACM particles in nylon-6 matrix is greatly constrained compared to the free swellingof crosslinked ACM rubber i n the same solvent. This is due to the constraints imposed by the least swellable continuous phase (i.e., hard nylon6 matrix) and also due to formation of a reduced mobility zone in the ACM phase by grafting reaction at the interface. To evaluate the constraints imposed by the nylon-6 matrix alone. a simple thermodynamic model based on the modified Flory-Huggins equation has been applied to this system, which could explain the data over a certain region. With increasing the extent of reaction between the two phases. both the rate and the extent of swelling of 40/60 (w/w) nylon-6/ACM blend decrease progressively. Also, the increase in the crosslink density of the rubber phase substantially improves the solvent resistance of the blends. The fuel resistance of the 40/60 (w/w) nyIon-6/ACM (dynamically vulcanized) blend at 25°C and oil resistance (in ASTM oil #3) at 150°C are found to be excellent. The mechanical properties of the thermoplastic elastomeric 40/60 (w/w) blend (dynamically cured) do not deteriorate to a significant extent when the samples are aged at different temperatures ( 150-200°C) and times ( 1 -7 days) (Fig. 17). This implies excellent heat-resistant properties of the blends. The DMTA results of the aged sample suggest that during aging. the
16
170
Ageing a t 1 5 O O C
6 0
1
2
3
1,
5
6
7
Number o f d a y s o f ageing Fig. 17 Aging behavior of ny1on:ACM thermoplastic vulcmizates at 150°C.
Miscellaneous Thermoplastic Elastomers
51 1
bonds between nylon-6 and ACM break down predominantly. The FTIR studies of the aged samples indicate the formation of imide linkages on nylon4 chains through a thermal oxidation process. The results of the investigation on the effects of various fillers and plasticizers on the key performance of thermoplastic elastomeric blends based on nylon-6 and ACM have been described (Jha and Bhowmick, 1999b). It is concluded that the addition of carbon black and clay reduces the extent of reaction between nylon-6 and acrylate rubber. while silica interacts with ACM chainsthrough covalent bond formation. which increasesthe overall polymer-filler interaction in the blends. The viscosity of the filled blends is found to be higher than that of the control, unfilled blend.However.addition of ester plasticizerlowerstheviscosityandimproves the processability. The fillers do not change the glass transition temperature of the ACM phase, but the Tg of the nylon4 phase is reduced in the filled blends, probably due to a decrease in its percent crystallinity. However, a substantial improvement in the damping properties of the blends in the service temperature rmge (25-175°C) is revealed from the DMTA results. Mechanical properties of the blends are greatly improved with the addition of a lower amount of carbon black (i.e., 10-20 phr) and a higher percentage of silica (30 phr). The extensibility of the blends is increased by 50% with the addition of silica due to higher polymer-filler interaction. Also, the elastic recovery of the blend is improved in the case of filled samples; the improvement in the overall integrity of the blends is probably due to the formation of a co-continuous morphology, which is evident from the optical micrographs of the blends. However. the volume swell in ASTM oil #3 at 150°C of the blends is well below IO%, which suggests its excellent hot oil resistance. An attempthas also been made to enhance the thermalstabilityandhigh-temperature resistance of EPDM/PP TPEby incorporating nylon, whichf o r m a continuous matrix (Venkataswanly and Payne, 1990). The same group has prepared a variety of rubber-plastic conlpositions that impart thermoplastic elastomeric properties (Venkataswamy, 1998). It has been reported (see Chapter 10) that the TPVs prepared from the blends of nitrile rubber (NBR/PP) provide very good heat and oil resistance with improved mechanical properties by thepresence of small amount of in situ formed NBR-PP block copolynler duringmeltblending. It has also been observed that the compatibilized NBR/PP TPVs can be mixed with EPDM/PP TPVsto give compositions of both excellent mechanical properties andoil resistance. Coran and Patel ( 1982) reported on TPVs based on reactive blends ofpolyamide and chlorinated polyethylene (CPE), which show excellent mechanical properties and hot oil resistance (Table 23). This was attributed to the chemical bond formation between the rubber and plastic molecules during melt-blending. Technological conlpatibilization of nylon/NBR through the formation of nylon-NBR graft copolymer by the use of phenolic resin curative hasalso been shown to improve both mechanical integrity and hot oil resistance of the above blends. Structure development and reactive processing of nylon/HNBR blends in the presence of compatibilizers have been thoroughly discussed (Bhowmick and Inoue. 1993). It has been demonstrated that mixing time and temperature, addition and amount of vulcanizing agent, and nature of compatibilizer influence the particle size of the dispersed domains. The blends of EPDM and polybutylene terephthalate (PBT) have been studied recently as thermoplastic elastomers, both with and without dynamic vulcanization. by Moffett and Dekkers( 1992). In situ formation of EPDM-PBT block copolymer by the addition of maleated EPDM was necessary to reduce high surface energy betweenEPDM and PBT. The structure of the blends made from epoxy functionalized EPDM and PBT controls the mechanicaland dynamic mechanicalproperties(Vongpanishet al., 1994). Patel (19%) highlightedhigh-temperaturestable,lowsolventswelling TPVs comprising po1y:lmide resin and crosslinked acrylate rubber. Jha et al. ( 1997b) reported TPEs prepared by reactive blending of polyethyleneterephthalate (PET) andacrylaterubber (ACM) throughtrans-esterification
Bhowmick
512
Table 23 Properties of ThermoplasticVulcanizates 6 number
Stock5
4
3
Ingredients, parts by wt. Nylon 6, 6-6, 6-10' Nylon 6-9" CPE rubber' MgOd Lead stearate, stabilizer Epoxide stabilizer" rmPhenylenebismaleimide' Trimethylolpropane triacrylate' 2,5-Dimethyl-2,5-bis(t-butyl-peroxy)hexane (90% active)" Temperatures Mixer oil bath temp., "C Molding temp., "C Properties U", MPa u I l H ) r ME1 8.5 E". o/r E,, p/o
Hardness, D scale 70-hour volume swell, 125"C, r ASTM No. 3 oil, 7
49 26
1
2 40
-
60
60
-
-
I .2 -
0.60.6
160 180 14.7 8.2 340 45 40 38
40
-
21.8 310 46
40
40
-
-
60
-
-
-
-
60 6 1.2 3
-
-
4.8 -
6
-
1.2
1.2
2.4
160 180
180 210
19.5 13.817.917.2 10.0 6.5 270 350 45 45 46 35 -
-
1.2 0.6
-
3 -
-
-
180 210
210 250
12.4 280 35 56
100
-
-
40 60 6 l .2 3
40 60
59 50 42
1.2 0.6
210 250 17.3 15.9 160 59 59 23
zytel8 63(du-Pont): 506 nylon 6. 3 1 6 nylon 6-6, 10% nylon 6-10 terpolymer. VydyneB 60H (Monsanto). CPE CM 0342 (Dow). " MagliteB D: (Merck). DrapexO 6.8, epoxlde stabilizer (Argus). ' HVA-2 curatlve (du Pont). 8 SR351 (Sartomer). " L-101 (LucidoI). h
reaction between the blend components. Thermoplastic elastomers made from PBT/ACM prepared by the same authors show a tensile strength of 4 MPa and hardness of 35 Shore D (Jha and Bhowmick, 1999e).
ACKNOWLEDGMENT The technical data taken from the product literature of various companies and contained herein are guides to the use of their product. The advice is based on tests and information believed to be reliable, but users should not rely upon it absolutely for specific applications. It is given and accepted at the user's risk, and confirmation of its validity and suitability in particular cases should be obtained independently. The companies make noguarantee of the results and assume no obligation or liability in connection with their advice. This publication is not to be taken as a license to operate under or a recommendation to infringe any patent. Theauthoracknowledges the assistancereceived from his students in typesettingthe manuscript.
Miscellaneous Thermoplastic Elastomers
513
REFERENCES Advanced Polymer Alloys, Technical Literature on ALCRYN'"', 1999. Ashitaka, H., Kusuki, Y.. Asano, Y . .Yamanioto, S., Ueno. H., and Nagasoka, A. (19833). J. Po/yrrl. Sci. Po/yrr?. Cken1. Et/. 21: 1 1 1 1. Ashitaka, H., Ishikawa,H., Ueno, H., and Nagasakn, A. (198%). J. Po/yrrl. Sci. Po/yrr~.Clrrrrl. Ed. 21: 1863. Banik, I., Dutta, S., Chaki, T. K., and Bhowrnick, A. K. (1999). Po/yrrler 0 4 4 7 . Bayer A. G. (1993). M m l t n l j b r the Rlrhher Irldustr?. T. Kempermann, S. Koch and J. Summer. Eds. Bayer Publications, Leverkusen, Germany. Bayer A. G. (1999), Technical Literature, Leverkusen, Germany. Bhagawan S. S. (1987). Ph.D. thesis, Indian Institute of Technology, Kharagpur. Bhowmick. A. K., (1999) Polymer Processing Society Meeting, Bangkok, Thailand. Bhowmick. A. K., and Inoue, T. (1993). J. App/. P o / w . Sei. 491893. Bhowmick. A. K., and Mangarai, D. (1994), in Ruhher Prot/ucts Mttrrufitcturirl,q T d l r l o l o g y (A. K. Bhowmick, M. M. Hall, and H. Benarey, Eds.), Marcel Dekker, New York. Bhowmick, A. K., Kuo. C. C., Manzur, A., MacArthur. A., and McIntyre, D. ( 1986). J. M t r c w r r ~ o l .Sei.P h y . B25(3):283. Chattapadhyay, S., Chak, T. K., and Bhowmick A. K. ( 1999). Rnditrt. P h y . Cl~errl.( i n press). Chen, H., Guest, M. J., Chum, S., Hiltner. A., and Baer, E. ( 1998), J. A/)/)/. P o / y r ~Sci. . 70: 109. Coran, A. Y.,and Patel, R. (1982), U.S. Pat.4,355,139 (Oct. 19). De Sarkar, M,, De, P. P,, and Bhowmick, A. K. (1999), P o / w ~ e 40:1201. r DuPont Dow Elastomers, (1999). Product information literature. Freeport, Texas. Elliott, D. J. (199O), in Therrrqdtrstic E/tr.storrwr.s ,frorrl Rlrhher-P/osric.s Bler~tls( S . K. De and A. K. Bhowmick. Eds.), Ellis Horwood, London. Rlthhrrs (A. Whelan Gilby, G. W. (1982). in De\v/oprrlerlt.s irl Ruhher TechrIo/o,qv.Vol. 3, T/~errr~o/,/rr.stic. and K. S. Lee, Eds.), Applied Science Pub., New York. Hoshino, S., Yamamoto, S.. and Asano, Y. (1985). Int. Seminar Elastomers, Itoh. Shizuoka, Japan. Oct. 20-22. Japan Synthetic Rubber Co. Ltd. (1984). Jpn. Pat. 58,168.640: Chrrrl. Abstr. 100:176, 245 f. Japan Synthetic Rubber Co. Ltd. (1985), Jpn. Pat. 59,155,478; Chrrrl. Ahstr. 102:800-905. Japan Synthetic Rubber Co. Ltd., (1999) technical literature. Tokyo, Japan. Jha, A., and Bhowmick, A. K. ( 1997a), RLhber Chcwr. Techr~oI.70:798. Jha, A., and Bhowmick, A. K. (1997b). Po/ynwr 17:4337. Jha, A., and Bhowmick, A. K. (1998). J. A p p / . Po/yrrr. Sci. 6P23.7 I . Jha, A., Bhowmick, A. K., Fujitsuku, R. and Inoue, T. ( 1999a). J. Adh. Sei. T ~ I ~ I 13, o / 649. . Jha, A., and Bhowmick, A. K. (1999b). J. AppL Po/yrn. Sei. 74, 1490. Jha, A., and Bhowmick, A. K. ( 1 9 9 9 ~ )J.. App/. Po/yrn. Sei. ( i n press). Jha, A., Bhnttacharya, A. K., and Bhowmick, A. K. (1997), Polymer Networks and Blends, 7. 177. Kali Ray, A., Jha, A., and Bhowmick, A. K. (1997). J. Eltrstnrrwrs P/tr.stic.s 29:201. Kannan, S., Mathew, N. M,, Nando. G. B., and Bhowmick, A. K., (1995). P/tr.stic,y R ~ t / ) / wCorllp~.y;tesr Proc. AppL 24: 149. Kiji, J. ( 1983), Ar~gew.Mncrornol. Chrrrl. I f 1:53. Kole, S., Santra, R., Samantaray, B. K., Tripathy, D.K.. Nando, G. B., and Bhowmick, A. K. (199.5). Po/yrner Net,t*orks Blerlds 5:I 5 I . Moffett, A. J., and Dekkcrs M. E. (1992). Po/yrrwr E r ~ gSei. 32:l. Naskar, A. K., Bhowmick, A. K., and De, S. K. (1999). Po/yrrl. G I ~ Sei. . (in press). Nevatia, P., Banerjee. T. S., Dutta, B., and Bhowmick, A. K. (1995). Report on thermoplastic elastomers from blends of waste rubber-waste plastics. Patel, R. (1998), U.S. Pat. 5,591,798. Roy Choudhwory, N. R., and Bhowmick, A. K. (1989), J. AppL Po/yrr~.Sei. 38:1091. Roy Choudhwory, N. R., and Bhowmick, A. K. (1990), J. Atlh. Sei. 32: 1. Santra, R., Nando, G. B., and Bhowmick, A. K. (1993). J. AppI. P o / y I . Sci. 49: 1 145.
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Sen Majumdar, P., and Bhowmick, A. K. (1998), Rcrditrrion Ph.v.sics Cl~cwz.53:63. Sen, A. K., Mukherji, B., Bhattacharya, A. S., Sanghi, L. K., De. P. P,, and Bhowmick, A. K. (1990), Tlwnmchem. Acttr 157:45. Sen, A. K., Bhattacharya. A. S., De, P. P,, and Bhowmlck, A. K. (1993). J. 7'herr). A d . 39887. Takcuchi, Y., Sekimoto, X., and Abe, M. (1974), ACS Div. Org. Coatings-Plast. Chem. Paper 34:122. Thomas, S., Kunakose, B., Gupta, B. R., and De, S. K. (1986). PIcrst. Rubber Pruc. Appl. 6 9 3 . Venkataswamy, K. (1998), Symposlum on Dynamic Vulcanixates, Polymeric Materials Science and Engineerlng Div., fall ACS meeting, Boston. Venkataswamy, K.. and Payne, M. T. (1990). Symposium on TPE, ANTEC, Dallas, Texas. Vongpanish, P,, Bhowmick, A. K., and Inoue, T. (1994). P1rt.sfic.s Rubbur Conll'"sites-Proc.. Appl. 21: 109. Wang, S., Zhang, X., and Zhao, D. (1984). Hecheng Xiarlgiicro Gongye 7215; Che~n.Absfr. I 0 I : l 12213.
18 Halogen-Containing Elastomers Daniel L. Hertz, Jr. Seals Eastern, Inc., Red Bank, New )ersey
1. INTRODUCTION This chapter tells the story of halogen-containing elastomers. As synthetic rubbers, they range from non-oil-resistant to the most oil-resistant elastomers that can be developed. Consider first the breadth of the category: Halobutyl elastomers Chlorinated polyethylene Chlorosulfonated polyethylene Epichlorohydrin elastomers Polychloroprene (neoprene) elastomers Fluorosilicone elastomers Hydrofluorocarbon elastomers Pertluoroelastomers Now consider some of the unique properties theseelastomers offer asa result of halogen substitution: Versatility of crosslinking chemistry due to ease of halogen displacement Increased oil resistance Improved solvent resistance Enhanced thermal stability Increased flame retardancy or nonflammability 1.l
HalogenReactivity
The halogens (group VI1 elements) are reactive in the order of fluorine (F2) > chlorine (Cl?) > bromine (Br?) > iodine ( l ? ) .The high reactivity is due to their basic electronic structure: they are one electron shyof an argononlic (inert) species. The alkanehlkeneseries have hydrogens that are readily abstracted and replaced by halogens, the process initiated by heat or ultraviolet light. The relative ease of hydrogen replacement follows the classic route: 3" > 2" > 1" > CH3-H. 515
516
Hertz, Jr.
Since the halogens are electronegative elements, they create polar bonds, which in turn create polar molecules.
1.2
Polar Molecules
Polar molecules (structures distinguished by the presence of halogen, oxygen, or nitrogen atoms), in sufficient quantity, create oil (hydrocarbon) resistance. Hydrocarbons are essentially nonpolar and are therefore repelled by polar molecules.
1.3
Polar Bond Strength
Bond strengths, or, more correctly, bond dissociation energies (BDE as measured by (AH) (H/ m) for methane and substituted methane, are revealing: ~~
CH3-H CHj-F CHj"C1 CH3-Br CH>-I
435.14 45 I .87 35 1.46 292.88 234.30
The difference in bond dissociationenergies establishesbasic parameters for crosslink site availability and elastomer thermal stability.
1.4
Crosslinking Mechanisms
The relative ease of a halogen displacement (elimination) through various reactions utilized to create a crosslink site. Typical examples follow.
is often
Polychloroprene Typically, in chloroprene polymerization, about 1 '/. mol% of the monomer rearranges to form a 1,2addition with the chlorine atom in an allylicposition. This is a very easy chlorine to displace, creating a crosslink site. Cltlorobutyl When isobutylene-isoprene rubber is chlorinated, the isoprene backbone segment rearranges, forming several structures. predominantly a pendant 1.2 addition with the chlorine in the allylic position. This cure site is essentially the same one utilized for crosslinking polychloroprene. Cltlorasulfmated Polyethylene A tertiary amine readily reacts with the sulfonyl chloride group. This creates a reactive sulfene, ultimately leaving a double-bond site suitable for crosslinking.
Epichlorohylrin Elastomers The chlorine onthe chloromethyl group is readily displaced by a strong base, typicallya diamine, creating a reactive site. The diamine serves a dual role: it is an electron donor (nucleophile) to displace the chlorine and subsequently serves as a difunctional crosslink.
Halogen-Containing Elastomers
517
Hydrofluorocarbon Elastomers There are twodistinct halogen-elimination reactions that subsequently develop crosslinking sites in hydrofluorocarbon elastomers:
1. E2 mechanism: the simultaneous departure of a hydrogen and the adjacent fluorine atominitiated by a base. This is theprobableroute for creatingcrosslinksitesin vinylidine fluoride (VF2)-containing elastomers. 2. E l mechanism:normally operates without a base. DuPont patentsindicatebromine or iodine-containing C? and C., fluoro-olefinswereexcellent cure sitemonomers. These sites readily undergo attackby peroxide-generated radicals, which in turn create free radical sites on the elastomeric backbone or chain terminal groups. In the case of the perfluoroelastomer, the monomer appears tobe a perfluorophenoxyvinyl ether.
2.
HALOBUTYL ELASTOMERS
2.1 Introduction The history, chemistry, and compounding of halobutyl elastomers are described in a separate chapter, so we will only briefly comment on the vulcanization process.
2.2 Vulcanization Halobutyl elastomers can be vulcanized with a wide variety of curing systems. As mentioned in the introductory discussion, the bond dissociation energy (BDE) is lower for bromine, so bromobutyl tends to cure more rapidly. Timar and Edwards (1979) describe a curing study using retention of tensile product as a practical approximation of the retained cohesive energy. Common to all cure systems is zinc oxide, which readily dehalogenates the halogen, creating a zinc halide (a strong Lewis acid). After dehalogenation, the crosslinking can be achieved by any number of crosslinkers, for example, Zinc oxide (alone) Quinone complex Dithiocarbamate and magnesium oxide Thiuram Thiuram and magnesium oxide Phenol-formaldehyde resin (SP- 1045) Peroxide-bismaleimide Bismaleimide These systems were consideredof interest for high-temperature service. For less rigorous applications, crosslinking combinations using sulfur or sulfur donorscan be considered. Two typically used for covulcanization are sulfur-MBTS-TMTD and sulfur-CBS-TMTD.For zinc oxide-free cure systems, consider using a diamine, such as hexamethylenediamine carbamate. 3. CHLORINATED POLYETHYLENE ELASTOMERS
3.1 Introduction MaynardandJohnson (1963) note that the development of severalsaturatedpolymerswith elastomeric character prior to 1941 freed us from assuming that therubberystatecould be
Hertz, Jr.
518
Table 1 Producers of ChlorinatedPolycthylcne
name Trade Company Country
ostaprenO Hocchst Hoechst Germany Dow United States Osaka Japan owa Japan Hungary
Location
Plaquemine, CPEO
tons/yr mctric Capacity
LA Arnagnsaki
N.A.
6,000 40,000' 2,400
N.A.
duplicated only with the natural rubber structure. Their references (McQueen 1940; McAlevey etal.. 1947; Brookset al., 1953) aretheearliest ones to chemicalderivatives of the newly discovered polyethylene. Dow Chemical Co. is currently the only domestic producer offering two grades of 36 and 42 wt% chlorine. World production facilities for chlorinated polyethylene production are listed in Table 1. Label capacity for all plants worldwide appears to be far in excess of current consumption (author's estimate).
3.2 Applications Chlorinatedpolyethylenesare used in extruded goods such as hose (tubing and covers) for hydraulic oil and hydrocarbonserviceandoil-resistant cable jacketing; ascalenderedstock. liners, and membranes; and for molded mechanical goods, including seals. The growthpotentialcould be large, if only due to acurrently low usagepercentage compared to other elastomers.
3.3 Nomenclature and Basic Properties Nomenclature and basic properties of chlorinated polyethylene are summarized
in Table 2.
3.4 Characteristics of Chlorinated Polyethylene Chlorinated polyethylene is available in two grades based on weight-percent chlorine: (1) 36% chlorine, 35 and 80 Mooney viscosity, and (2) 42% chlorine, 80 Mooney viscosity. The higher Mooney materials allow higher plasticizer and/or filler loadings for extruded and calendered goods. The chlorination process, as described by Sollberger and Carpenter (1974), is initiated by free radical chlorination of Ziegler-process high-density polyethylene in an aqueous slurry. In the chlorination process, the larger-diameter chlorine atoms randomly replace the far smaller hydrogen atoms. The size discrepancy and random location aspect of the chlorine substitutions destroy the effectiveness of the dispersion (London) forces that originally created the highly crystalline structure. Chlorinated polyethylene is now essentially a high glass transition (T,) structure with a strong dipole force at each chlorine-carbon bond. On drying, the resulting product is a powder (due to dipole forces), making it unique as an uncompounded elastomer. The incorporation of a thermodynamicallysolublepolarplasticizer destroys the dipole attractionbetween the interchains, and the chlorinated polyethylene powder becomes a truly rubbery elastomer.
519
Halogen-Containing Elastomers Table 2 Nomenclature and Properties of Chlorinated
Polyethylene Elastomers Common name ASTMname ASTM D 14 18 designation IUPAC trivial name IUPAC structure-based name SAEJ200/ASTM D2000 Spccific gravity Durometer rangc Tensile strength (max), MPa Elongation (max), 5% Glass transition temp. (T,),“ K
Chlorinated polyethylene Chloro-polyethylene CM Not applicable Not applicable CE 1.16-1.25 50-YO
20 350 26 I
Considering that the basicraw material, polyethylene, is a series of ethylene (C,) structures, Oswald and Kuber (1963) present an interesting observation: chlorinated polyethylene can (and should) be considered as a terpolymer consisting of ( I ) ethylene, (2) vinyl chloride, and (3) 1. l-dichloroethylene (vinylidene chloride). Keep these three “monomers” in mind, as we will dwell on their individual contributions in terms of crosslinking, aging, and fluid resistance during subsequent discussions. Increasing the weight-percent chlorine from 36 to 42 decreasesthe swell in lighter hydrocarbons but does not enhance upper temperature limits (Normand and Johnson, 1975). Normand and Johnson( 1 975) alsoreview theeffects of various other weight-percent chlorinated polyethylenes. Sollberger and Carpenter (1974)and Dow Chemical literature should beread for additional background on the effects of varying the weight-percent of chlorine in polyethylene. Heat resistance and low-temperature tlexibility are best reviewed separately i n light of the elastomer being treated as a terpolymer. Heat resistance, i.e., ISO’C, is advertised as a strong selling point by the domestic supplier. Consider that part of the polymer chain is a number of vinyl chloride segments. The welldocumented heat instability of PVC is clearly established; Blanchard (1973) uses compounding guidelines with plasticizersandheatstabilizerstypically used forPVCcompounding.Guy and Sollberger ( 1970) describe a method of deliberately introducing backbone unsaturation by treatment at high temperature with zinc oxide. Guy is utilizing the zinc oxide to dehalogenate backbone sites that have chlorine atoms on adjacent carbons (geminal dihalides). Guy also notes the possibility of “hazard due to occasional rapid decomposition.” Dehalogenation at the vinyl chloride sites is activated by zinc and magnesium and, to a lesser extent, by copper, iron, and nickel. Low-temperature properties are improved by plasticizers. DOS (di-2-ethylhexyl sebacate) gives the greatest improvement in flexibility, with a trialkyl trimellitate offering the best overall balance. Epoxidized soybean oil would be the plasticizer of choice for the highest temperature stability. Chlorinated polyethylene elastomers should be used only in nonelectrolytes (hydrocarbon service). The ease of dehydrochlorination at the I , 1 -dichloroethylene site in a basic environment should always be considered. Crosslinking of chlorinated polyethylene can be achieved at each of the three specific monomers:
Hertz, Jr.
520
Ethylene site: A peroxide is used to abstract a hydrogen and a coagent to effect the crosslink (triazine, bismaleimide, or dimethacrylate). 2. Vinyl chloride site: Organic accelerator combinations of an amine-thiadiazole serve as a crosslink.The chlorine is best scavenged by a lead complex orepoxidzed soybean oil plasticizer. Avoid zinc and magnesium oxides. 3. 1, I-Dichloroethylene site: Same as mechanism 2, except that themechanism is a dehydrohalogenation-a reaction specific to a geminal dihalide. 1.
3.5
Compound Technology
Typical chlorinated polyethylene model recipes might consist of the ingredients listed below. PeroxideCure Chlorinated polyethylene Filler (reinforcing agent) Plasticizer (peroxide compatible) Stabilizer (halogen acceptor) Peroxide Coagent Antioxidant (peroxide compatible) 2. Amine-ThiadiazoleCure Chlorinated polyethylene Filler (reinforcing agent) Plasticizer Stabilizer (halogen acceptor) Amine-type accelerator (nucleophile) Dimercaptothiadiazole derivative (crosslinker) 1.
Mixing Mixing can be done on both open-mill and internal mixers. The high-Mooney elastomers have rapid heat-buildup tendencies and should be treated accordingly. Process Aids Process aids, both proprietary and generic, are widely available. Typically, materials such as paraffin, low molecular weight polyethylene, and waxes are commonly utilized. More complex and expensive process aids are offered; however, there is no outstanding candidate. Fillers Fillerstypically are furnace-typeblacksbecause of theirantioxidanttendencies (Blanchard, 1973). The finer particle sizes, N500-N300, are reported by Rose and Coffey ( 1 982) to offer the best balance of properties. Plasticizers
Plasticizers, as noted earlier, are a necessity. A good understanding of plasticizer structure and plasticizer theory is key to successful compounding. The book by Sears and Darby (1982) is particularly recommended because of the PVCsimilarity of chlorinated polyethylene. Plasticizers such as those based on epoxidized soybean oil serve a dual function: plasticizing and halogen
Halogen-Containing Elastomers
3.6
521
ChlorinatedPolyethyleneElastomerBlends
Blends of chlorinated polyethylene with other elastomers are noted in various reports. Considering the use of PVC-elastomer blends and the time needed to develop them, it is not likely that this application will create a major market.
4.
4.1
CHLOROSULFONATEDPOLYETHYLENE(CHLOROSULFONYLPOLYETHYLENE) ELASTOMERS Introduction
Maynard and Johnson (1963) developa very broad background on theearly history and development of chlorosulfonated polyethylene, first introduced by DuPont in 1955. The development was intended(like chlorinated polyethylene) to utilize theadvantage of the saturated polyethylene backbone and concurrently develop a more universal group of crosslinking sites. Dupuis ( l 982), in his excellent review. points out the basic differences between chlorosulfonated polyethylene (CSM) and chlorinated polyethylene (CM). Using polyethylene as a raw material, the DuPont process begins by solvating the polyethylene in chlorinated solvents and treating with chlorine and sulfur dioxide. The chlorosulfonation process creates two modifications (Keeley, 1959): The chlorine atoms break up the regularity of the polyethylene chain structure so that crystallization is no longer possible, thus imparting an elastomeric character to the polymer. 2 . The sulfonyl chloride groups provide sites for crosslinking. 1.
By manufacturingviaasolutionprocess,theheterogeneityassociated with theslurry process (used for chlorinated polyethylene) is avoided and homogeneous polymer results. World production facilities for chlorosulfonated polyethylene are listed in Table 3.
4.2
Applications
Chlorosulfonated polyethylene, with its inherent oil and heat resistance at relatively low cost, has a broad range of volume applications. Automotive tubing and electrical insulation are major markets. Others include construction, specifically single-membrane roofing for service more rigorous thanEPDM can handle; pond liner membranes for water and liquid storage;and mechan-
Table 3 ProductionFacilities for ChlorosulfonatedPolyethylene Trade Company Country United States Japan United Kingdom Source: IISRP, 1997.
tons/yr DuPont Dow Tosoh DuPont Dow
HypalonB TOSO-CSMB HypalonB
Beaumont, TX Shinnanyo Londonderry. Northern Ireland
Capacity metric 34,500 2.000 18,100
522
Hertz, Jr.
Table 4 Nomenclature and Properties of Chlorosulfonated Polyethylene Elastomers Common name ASTM name ASTM D 1418 designation IUPAC trivial name IUPAC structure-based name SAEJ200/ASTM D2000 Specific gravity Durometer range Tensile strength (max), MPa Elongation (max), % Glass transition temp. (T,)," K
Chlorosulfonated polyethylene Chloro-sulfonyl-polyethylene CSM Not applicable Not applicable CE 1.08-1.27 45-95 28 500 274
.' From Brandrup and Immergut. 1975.
ical molded goods, includingsealsand gaskets. Coloredelectrical cable jacketing is avery visible application. Growth potential appears to be in line with the U.S. gross national product (GNP). The absence of major competition indicates a stable and modest growth pattern.
4.3
Nomenclature and Basic Properties
The nomenclatureandbasicproperties Table 4.
4.4
of chlorosulfonatedpolyethylene
are summarizedin
Characteristics of Chlorosulfonated Polyethylene
The elastomeric chlorosulfonated polyethylenes can be made from branched, free radical (lowdensity), or linear(Ziegler process) polyethylenes. This affordsagroup of elastomerswith viscosities ranging from very low to very high suitable for paint coatings to highly abrasionresistant hose covers. Dupuis (1982) interrelates viscosity, weight-percent chlorine, and type (branched, linear) of the various Hypalonsm. Chlorine content can range from 24 to 438, with the optimal content of 3 0 8 for branched (Hypalona 20)and 35% for linear (HypalonB40) chlorosulfonated polyethylene. Chlorosulfonation, as noted earlier, is performed by solvating the polyethylenein a mixture of carbon tetrachloride and chloroform. Chlorine and sulfur dioxideare added, and the process, a chain reaction (Jones, 1964), is initiated by a free radical generator or ultraviolet light (Reed reaction). The effect of increasing weight-percent chlorine is predictable; note the volume increase in ASTM No. 3 oil, aged 70 hr at 121°C (Dupuis, 1982):
% Chlorlne Volume 5% increase
CSM (24%) 86%
+
CSM (35%) 38%
+
CSM (43%) 13%
+
Halogen-Containing
523
The upper temperature limit based on 1000-hr serviceability is 135°C. Dupuis (1982) cautions that this value is very dependent on compound quality, i.e., filler level, plasticizer volatility, and crosslink stability. Low-temperature performance is characterized as usually good, but the “rubbery response” or rebound would by nature always be sluggish (time-dependent). Fluid resistance is indicated by oil swelling (see tabulation above). Swell characteristics in nonelectrolytes are typicallysimilar to polychloroprene,exceptthathydrocarbon swell is dependent on weight-percent chlorine, as previously noted. Service in aqueous and nonaqueous electrolytes should be viewed with caution. The presence of metal chlorides as a by-product of crosslinking leads to potential problems as outlined by Briggs et al. ( 1 963). Crosslinking chemistry for chlorosulfonated polyethylenes is understandably very broad. In addition to the mechanisms outlined in Section 3, the chlorosulfonyl group introduces another dimension. An article by Devlin and Folk ( 1 984) has very interesting observations based on spectroscopic techniques for those seriously interestedin crosslinking chemistry. Peroxide cures, reviewed by Honsberg (1983), represent the latest concepts in crosslinking chlorosulfonated polyethylene elastomers. Maximum thermal stability as measured by stress relaxation appears to be represented by bismaleimide crosslinks. Haaf and Johnson (197 1 ) have written a very comprehensive paper covering this technology, which should be reviewed by those concerned with long-term stress-relaxation consequences.
4.5 Compound Technology Chlorosulfonated polyethylene elastomers can be compounded with a broad variety of filler, plasticizers,andcrosslinkingsystems. The very highmolecularweightlinearpolyethylenes utilized for HypalonB 45 allow formulations that require no crosslinking. Such compositions might be usedfor cable jacketing, roofing membranes, andpond liners. Examples of two formulations from Dupuis ( 1982) are detailed below.
1. Thiuram-SulfurCure Vulcanization Chlorosulfonated polyethylene (Hypalon” Filler (clay) Plasticizer (aromatic oil) oxide)(magnesium Stabilizer (pentaerythritol) Stabilizer Process aid (paraffin) Accelerator (TMTD) Crosslinker (sulfur)
2.
100.0 80.0 25.0 4.0 3.0 3.0 2.0 1.o
45)
100.0 50.0 35.0 4.0
UncuredApplications
Chlorosulfonated polyethylene (HypalonB Filler Pigment (titanium dioxide) acceptor) (halogen Stabilizer Processing aids glycol Polyethylene Stearamide
carbonate) (calcium
40)
2.0 I .o
Hertz, Jr.
524
Mixirzg
Mixing can be done on both open-mill and internal mixers. The high-Mooney elastomers have rapid heat-buildup tendencies and should be treated accordingly.
Plocess Aids Process aids, both proprietary and generic, are widely available. Typically, materials such as paraffin, low molecular weight polyethylene, and waxes are utilized. More complex and expensive process aids are offered; however, there is no outstanding candidate.
Fillrrs Filler effects in chlorosulfonated polyethylene are similar to those in polychloroprene. The high molecular weight of the elastomer categorizes it as a strain-crystallizing elastomer, so the essential reinforcing effects of fine particle carbon blacks are not required. Nonblack formulations are very popular. both for aesthetic appeal and for electrical properties. The range of nonblack fillers that can be utilized should be selected based on suppliers’ recommendations (see also Briggs et al., 1963).
P1nstici:er.s Plasticizers are utilized i n almost all formulations, and the guidelines presented in Section 3 are a good starting point. Epoxidized soybean oil plasticizers are utilized in white or colored stocks. Maynard and Johnson (1963) note that, as in PVC, these plasticizers react with the HCI byproduct to create a water-insoluble, high molecular weight chlorohydrin. Aromatic hydrocarbon plasticizers are widely used in nonperoxide cures. Diester plasticizers (phthalates. adipates, and sebacates) are plasticizers of choice for peroxidecures. They are also used for developing improved low-temperature properties.
4.6
Chlorosulfonated Polyethylene Blends
Blends of chlorosulfonated polyethylene with both chloroprene and nitrile rubber have been noted in suppliers’ literature. In each case, improved resistance to ozone attack was the goal. As with any blend ofelastomers, the result is often difficult to reproduce due to phase incompatibility and varying cure rates between the two elastomers.
5. 5.1
EPICHLOROHYDRIN ELASTOMERSCHEMISTRYAND MARKETS Introduction
Vandenberg ( 1 983) discusses the development ofepichlorohydrin elastomers in a unique context. In theprocess of developing crystallinepolymersfrompolarmonomers,Vandenbergnoted that n particular catalyst polymerized epichlorohydrin to a rubbely, predominantly amorphous polymer. The amorphouspolyepichlorohydrin (a polyalkylene oxide) was categorized as a homopolymer (CO). Subsequent work resulted in the development of a 1/1 copolymer of epichlorohydrin and ethylene oxide (ECO) that was also primarily amorphous with only small amounts of
525
Halogen-Containing Elastomers Table 5 Production Facilities for Epichlorohydrin Elastomer Country United States Japan
Company
Trade name
Location
Capacity metric tondyr
Zeon Chemicals Nippon Zeon Osaka Soda
HydrinB GechronB EpichlomerB
Hattiesburg, MS Tokuyama Kurashiki
9,900 1.500 1,200"
'' IISRP estimate. Source: IISRP. 1997
crystallinity. The moderate cost and good oil and gasoline resistance of these two products, coupled with their excellent low-temperature flexibility, made them particularly attractiveto the automotive industry. Hercules subsequently licensed the technology to B. F. Goodrich in the early 1960s and recently (1 986) has decided to exit the business, with Zeon Chemical now the domestic supplier. World production facilities for epichlorohydrin elastomer production are listed in Table 5.
5.2 Applications
The combination of fuel resistance, air aging, broad temperature range, and cost has assured a largemarket in the automobile industry.Hose,tubing, seals gaskets,andcoatedfabrics are major applications. Rubber-covered rolls, oil-field specialties, and industrial products are noted by Kyllingstad (1982) as substantial markets. The very good low-temperature properties have led to special military applications such as oxygen mask hose and large gaskets for fuel-transfer systems. Growth potential appears to track U.S. gross national product (GNP) statistics, with automotive production a key indicator. The exit of Hercules as a major supplier appears to be in line with a slow-to-average growth situation.
5.3
Nomenclature and Basic Properties
Nomenclature and basic properties are summarized in Table 6.
5.4 Characteristics of Epichlorohydrin Elastomers Epichlorohydrin elastomers are available as both a homopolymer [poly(epichlorohydrin)] and a copolymer [poly(epichlorohydrin-co-ethyleneoxide)]. There is also a terpolymer (Oetzel and Scheer, 1978), essentially a copolymer with a cure-site monomer to allow greater freedom in crosslinking chemistry. Within the various classes, thereare a range of molecular weights (Mooney values) available for specific compounding. Monorners for Polymerizntion
Monomers employedin epichlorohydrin elastomer production are characterized as cyclic ethers. Three examples are ( I ) epichlorohydrin (chloromethyl oxirane):
Hertz, Jr.
526 Table 6 Nomenclature and PropertiesofEpichlorohydrinElastomers
Common name ASTM name
Epichlorohydrin polymer Polychloromethyl oxirane
ASTM D141 8 designation IUPAC triv~alname SAEJ200/ASTM 02000 Specific gravity Durometer range Tensile strength ( m a x ) , MPa Elongation (max). 7!Gloss transition temp. (TS),"K
CO Poly (epichlorohydrin) CH 1.36 30-40
Epichlorohydrin copolymer Ethylene oxide (oxirane) and chloromethyl oxirane ECO Poly (epichlorohydrin-co-ethylenc oxide) CE 1.27 40-90
18
17
350 2s I
400 22 I
0
l\ CH2 -CH-CH2 C l (2) ethylene oxide (oxirane):
0
l\ CH2 - CH2 and (3) allyl glycidyl ether (cure-site monomer):
0
l \ CH, -CH-CH -0-CH 2
2
-CH=CH
2
Polymerization of the monomer(s) is viaacationicsolutionprocess using amodified aluminum alkyl-water catalyst. Molecular weights of the copolymer were very high, and it was necessary to deliberatelylowerthem to aid in processing (Vandenberg, 1983). There is no branching or gel formation during the process. The homopolymer (CO) is simply described as a saturated polyether with the polar aspect created by the chloromethyl side group. The 38% chlorine content develops the fuel resistance and promotes flame retardancy. The copolymer (ECO) has a lower chlorine content (about 26%). It has improved low-temperature flexibility contributed by the ethylene oxide monomer and higher fuel swell due to lower chlorine content. As noted earlier,the 1/1 copolymer (ECO) hadexcellentlow-temperatureproperties, with a glass transition temperature (T,) below -40°C compared to the homopolymer (CO) value of - 20°C. The terpolymer cure-site monomer allows a broad application of peroxide, peroxide/ coagent, and sulfur-cure mechanisms. Heat Resistunw and Tlwr-rml Stability The ease of halogen displacement by a nucleophile and heat is a recurring topic in this chapter. The very polar chloromethyl group common to the CO and ECO elastomers not only creates
Halogen-Containing Elastomers
527
the basicoil resistance but is also the crosslinking sitein these elastomers. Duringthe crosslinking process it assumed that there is one crosslink for every 130-200 constitutional repeating units (CRU). This would leave the bulk of the chlorine atoms on the chloromethyl groups vulnerable to dehalogenation. It was initially assumed that dehalogenation and the subsequent formation of HCI caused the rapid degradation of early formulations. Yatnada et al. (1973) and Nakamura et al. (1974) provedconclusively that aging occurs in twosteps: ( 1 ) oxidativedegradation initiating at a beta hydrogen following the thermal decomposition mechanismfor alkylene oxides (Dulog, 1966),and (2) subsequent formation of a hydroperoxide, creating a chloroketone structure that decomposes, yielding HCI. The mechanism(s)of protection for long-term heat stability dictate both an antioxidant (step 1 ) and an HCI acceptor (step 2). Typicalantioxidantsare metal dithiocarbamates: nickel dibutyldithiocarbatnate (NBC), nickel diisobutyldithiocarbamate (NIBC), and nickel dimethyldithiocarbamate (NMC). Typical HCI acceptorsare red lead oxide andmagnesium oxide. Zinc oxide andzinc stearate should be avoided. as they become strong Lewis acids and promote rapid elastomer breakdown. Low-temperature properties of the copolymer (ECO) are particularly good, as was previously noted. The ether (oxygen)linkage in the backbone is highly mobile, much like the siloxy linkage in the silicone rubber backbone. Long-term aging characteristics are a function of operating temperature. The antioxidant and HCI acceptor ingredients are essentially sacrificial. On depletion of these agents. there is typically a reversion to lower molecular weight materials. Fluid resistance values in terms of the automotive environment are available from the suppliers. whose bulletins are very encouraging with respect to long-term utilization of CO and ECO elastomers. A major problem is with the sour (peroxidized) gasoline, which causes an attack similar to that described above as step 2 of the aging process. Specific compounding, to be described below, can minimize sour gasoline attack but cannot completely stop it (Mori and Nakamura, 1984). Aqueous and nonaqueous electrolytes should be avoided, as they promote nucleophilic attack on the chloromethyl group. causing rapid breakdown. The crosslinking of CO and ECO elastomers can proceed by several mechanisms, each utilizing nucleophilic displacement of the chlorine from the chloromethyl group using: Ethylenethiourea (nucleophile and crosslinker) Red lead oxide (acid acceptor) 2. Amine accelerator (nucleophile) Thiadiazole complex (crosslinker) Barium carbonate (acid acceptor) Magnesium oxide (acid acceptor) 3. Diphenyl guanidine(nucleophile) 2,4,6-Trimercapto-s-triazine (crosslinker) Magnesium oxide (acid acceptor) l.
The terpolymer may be crosslinked through the reactive backbone by the following cure systems:
double bond that is pendant to the
Peroxidekoagent, peroxide Sulfur and organic accelerators 2,4,6-Trimercapto-s-triazine and organic accelerators (Mori and Nakamura, 1984) The various crosslinking mechanisms are discussed in detail by Mori and Nakamura Kyllingstad (1982), Oetzel and Scheer, (1978), and Nakamura et al. (1974).
( 1984).
528
5.5
Hertz, Jr.
Compound Technology
Some typical CO and ECO formulations utilizing the different cure mechanisms follow: 1.
For-mulcttion N Homopolymer (CO) 100.0 Process aid I .0 Red lead oxide (Pb304) 5.0 Filler 40.0 Ethylene thiourea 1.2 NBC 1 .o
Copolymer (ECO) Process aid Calcium carbonate (shelf-life improver) Filler 2.4.6-Trimercapto-S-triazine Cyclohexylthiophthalimide(cure retarder) NBC
100.0 2.0 5.0 40.0 0.9 I .0 I .0
Terpolymer (ECO) 100.0 Process aid 2.0 Magnesium oxide 3.0 Calcium carbonate 5.0 Filler 40.0 Peroxide-DBPH 2.0 Trimethylolpropane-trimethacrylate 3.0
Mixirlg
Mixing can be can-ied out on both open-mill and internal mixers. The high-Mooney elastomers have rapid heat-buildup tendencies and should be treated accordingly. Process Aids
Process aids are critical for mill release. An incorrect selection can interfere with cure rate or promote rapid aging. The elastomer suppliershould be consulted for the latest recommendation based on the type of elastomer, cure system, and service requirements. Fillers
Fillers are typically furnace-type blacks with the reinforcing effects predictable (Kyllingstad, 1982). N500 types are used in plasticizer-free formulations. N700 types are used to offset the
Halogen-Containing
5.6
529
Epichlorohydrin Elastomer Blends
Blends with other elastomers do not appear in the literature. There is little evidence that blend would offer any improvements over the properties offered by either elastomer.
6.
6.1
POLYCHLOROPRENE RUBBER Introduction
Collins (1973),in his Charles GoodyearMedal Address, has a very brief but concise story about the discovery of polychloroprene. As with many famous discoveries. it was preceded by a series of events that were pieced together by several dedicated perceptive scientists. in turn creating the product. As background, theexperiences of World War 1 and the rapid growthof the automobile led many to the belief that a synthetic replacement for natural rubber was imperative. The basic diene structure of natural rubber was accurately proposed by W. A. Tilden in 1892 (Fisher, 1957). Duplication of the diene molecule was assumed to be a necessity as the basis of any truly elastic synthetic rubber, with the early work using butadiene (a gas). The intractability of the rubber developed from sodium metal-catalyzed butadiene discouraged the process, and other reactive dienes were sought out. Work by Father Nieuwland at Notre Dame on divinylacetylene. reported in 1925. was noted by Dr. E. K. Bolton, DuPont's research director. Arnold Collins. whose background was in coatings, joined the group as a resins expert. During separation of the various isomer fractions by distillation in April of 1930, it was found that one spontaneously polymerized. This fraction was analyzed as a monovinylacetylene that had become chlorinated by hydrogen chloride (partof the catalyst composition). Larger quantities weremade by emulsion polymerization techniques developed by Ira Williams, and formal announcement of the discovery was made on November 2, 1931, at an ACS Rubber Division meeting. After factory trials in 193 1. the polychloroprene rubber was offered for sale in 1932 under the trade name DuPrene. Worldproductionfacilities for polychloropreneelastomerarelisted in Table 7. Label capacity for all plants worldwide is estimated to be in excess of 502,000 metric tons.
6.2 Applications Markets forpolychloroprene, as evidenced by production capacity, arefar ranging. Typical highvolume applications are industrial and automotive hose, construction, vee-belts, tires, molded goods,footwear, caulking andglazing, conveyer belts. wireand cable insulation. andadhe-
Hertz, Jr.
530
Table 7 ProductionFacilitiesforPolychloropreneElastomer ~~
~~
Country ~
United States France Germany Japan
People's Republic of China United Kingdom CIS
Company ~
~
Location tondyr Capacity metric ~~
discontinued 1998 DuPont Dow EniChem Bayer AG Denki Kagaku Kogyo DuPont-Showa Denko TOSOH
DuPont Dow V I 0 Raznoimport
Louisville Champagnier Dormagen Omi Kawasaki Shinnanyo Changshou Datong Quingdao Londonderry, Ireland Erevan
136,000 40,000 68,000 48,000 20,000 20,000 10,000' 5,000' 5,000' 33,000 40,000'
.' IISRP estlmate. Source: X R P . 1997.
sives. The distribution of applications is well balanced over six primary industries (Graham, 1982):
Automotive Construction Machinery Apparel Appliances industrialMisc.
208 15% 15%
15% 5Yo 30%
Growth potential appears to track gross national product (GNP) statistics, as indicated by plant label capacity, and is not much greater for 1997 figures than for 1986. 6.3
Nomenclature and Basic Properties
The nomenclature and basic properties of polychloroprene elastomers are summarized in Table 8. 6.4
Characteristics of Polychloroprene Elastomers
After the discoveryof chloroprene rubber, poly(1 -chloro- 1 -butenylene), various potential routes to develop the monomer were considered (Johnson, 1976). The acetylene route was originally utilized, and butadiene subsequently became the monomer precursor of choice for safety and other considerations. As with any four-carbon (C.,) structure, the chlorination created a mixture of isomers and analogs that required separation; Johnson (1976) should be referred to for this discussion. Basically,there are two distinctclasses of polychloropreneelastomers: copolymers of
531
Halogen-Containing Elastomers Table 8 Nomenclaturc and Properties of Polychloroprene
Elastomers Common name
ASTMname ASTM D I41 8 designation IUPAC trivial name IUPAC structure-based names SAEJ200/ASTM D2000 Specific gravity Durometer range Tensile strength (mnx). MPa Elongation (max), c/r Glass transition temp. (T3,),‘l K
Neoprene Chloroprene CR Poly(ch1oroprene) Poly( 1-chloro- I-butcnylene) BC. BE I .?S
30-95 22 600
233
From Brandrup and lmlnergut 1975
chloroprene and sulfur (G series) and Polymers and copolymers of chloroprene nlonomers (W series) Monomers and Polynxrixttion The complexity of monomers, isomers, analogs, and comonomers utilized in the production of polychloroprene elastomers, although relevant,are beyond the purview of this chapter. Johnson ( 1 976) and Hargreaves ( 1 968) are recommended as background reading. Polymerization of the monomer(s) is by a free radical emulsion process. The need for molecular-weight control was identified early, and the copolymerization with sulfur allowed cleavable sites in the chain for molecular-weight control (G series). Subsequent developments i n emulsion polymerization led to the development of the sulfurless W series, with mercaptans utilized for molecular-weight control serving as chain transfer agents. A typical emulsion polymerization recipe for a sulfur-modified (G series) polychloroprene is detailed by Gintz (1968). The effect of copolymerization of the various monomers along with their concentration and sequencing can lead to many specific, useful properties. Some examples are:
G Series: The sulfur actually copolymerizes in the backbone chain in multiple sequences of sulfur ranging from two to six atoms. The sulfur linkages, from a mechanical viewpoint, are highly mobile but thermally very weak. This gives the G series outstanding flexibility but poor resistance under high stress-strain relaxationconditions. W Series: These have far better aging characteristicsdue to the absence of sulfur-backbone linkages and aremore suitable for nondynamic applications (see Murray and Thompson, 1963. and Johnson, 1976). Elastomer microstructure is unique. Consider the following. Polyisoprene derives many properties from the high(99% + ) degree of backbone uniformity of head-tail cis units. Polychloroprene is also highly regular in structure but consists primarily of trans units. Considering natural rubber, if all units were tram, the material would be categorized as gutta-percha or balata, a very stiff, thermoplastic rubber with a higher specific gravity than natural rubber. (The higher specific gravity results from the more uniform structure of the chain, which allows closer “packing.”) Although polychloroprene is predominantly trans, there is sufficient cis to disturb the backbone symmetry and maintain a rubbery state (Murray and Thompson, 1963). The ability to develop increasing and decreasing cis-trans relationships by polymerization is the basis for polychloroprene-base adhesives.
532
Hertz, Jr.
Heat resistance of polychloroprene is superior to that of polyisoprene, but the vulnerability of the backbone double bond cannot be obviated. Conventional approaches using antioxidants are a necessity, and suppliers should be consulted the for latest concepts ofantioxidant protection. Murray and Thompson (1963) deal specifically with compounding for heat resistance, noting that the W series are superior to the G series for such service. Polychloroprenes seem suitable for long-tern1 heat resistance in applications below 100°C. Low-temperature properties of polychloroprene have to be considered in two different aspects: crystallization andoperating close to the glass-transition temperatureof polychloroprene (Murray and Thompson, 1963). The high percentage of tram units creates the inherent potential of crystallization (100-fold increase in stiffness). The reaction is time-dependent and appears most commonly in the vicinity of - 12°C; it is completely reversible on warming. The polar nature of polychloroprene allows the use of a broad spectrum of plasticizers. Again, the reader should review the specific low-temperature properties desired (impact, torsion, or tension) and utilize the supplier’s technical support. Long-term aging characteristics are excellent for polychloroprene elastomers that have been compounded specifically for such applications. The chlorine atom not only tends to shield the trans double bond from ozoneattack but also servesto make polychloroprene rubber incapable of supporting combustion. Serviceabilityis directly relatedto correct selectionof age-resisters (antioxidants,antiozonants),plasticizerstability,andfiller-volumerelationships.Highly extended formulations that use large ratios of plasticizers and fillers should not be considered candidates for such service. Fluid resistance should be considered in terms of nonelectrolytes (nonpolar fluids) and aqueouslnonaqueous electrolytes (polar fluids). Nonelectrolytesare typically hydrocarbons ranging from gases to liquids to solids. Lower molecular weight aliphatic hydrocarbons cause high swell, decreasing as molecular weight increases. Aromatic hydrocarbons should be avoided. Lubricating oils cause intermediate swelling as a function of their aniline points. Halogenated hydrocarbons cause excessive swell and should be avoided. Fluid service guides from suppliers should be consulted if the electrolytes are specific cases. Water has a strong affinity for metal oxides utilized in the cure mechanism as well as for fillers (Briggs et al., 1963; Murray and Thompson, 1963). Specificformulations aresuggested if polychloroprene properties are required in such service. Crosslinking in polychloroprene rubber is a function of the particular type. Common to both the W and G series is the specific crosslink site, a 1,2 addition created by a rearrangement of the chloroprene molecule. As a result of the 1,2 polymerization, 1.5% of the chlorine is in an allylic position and is easily displaced by a nucleophile. The controlled number of crosslink sites (1,2 additions) prevents overcuring, a valuable feature when a molded component is subjected to a mechanically demanding application (Murray and Thompson, 1963). Curingproceeds through a somewhat controversial mechanism (Hargreaves, 1968),but there is reasonable agreement that crosslink sites are created by the allylic chlorine displacement. G series elastomers may be crosslinked by using zinc oxide and magnesium oxide alone. The basic environment probably displaces the chlorine, which reacts with the zinc oxide to become zinc chloride, a powerful Lewis acid. The magnesium oxide reacts with waterto become a hydroxide. A PO~YSUIfidic segment in the elastomer backbone matures to a lower order of rank, donating sulfur to serve as a crosslink. W series mechanisms (homopolymers) typically utilize an organic accelerator, normally a difunctional amine (nucleophile), which activates the reaction in the presence of heat. The accelerator (diamine) becomes the crosslink, creating 2 moles of water (scavenged by the magnesium oxide) for each crosslink generated. The range of crosslinking systems is quite extensive (Johnson, 1976).
Halogen-Containing Elastomers
6.5
533
Compound Technology
Typical polychloroprene formulations for the two classes are listed below. 1.
G Series Polychloroprene 100.0 oxide Zinc 5 .O Magnesium oxide 4.0 Antioxidant 2.0 Process aid 0.5 Carbon black 0-200 0-70 Plasticizer
2.
W Series Polychloroprene Zinc oxide Magnesium oxide Antioxidant Process aid Carbon black Plasticizer Organic accelerator
100.0 5 .O
4.0 2.0 0.5 0-200 0-70 0.25- 1 .O
Mixing Mixing can be done on both open-mill and internal mixers. The high-Mooney elastomers have rapid heat-buildup tendencies and should be treated accordingly. Process Aids Process aids, both proprietary and generic, are widely available. Typically, materials such as paraffin, low molecular weight polyethylene, and waxes are utilized. More complex andexpensive process aids are offered; however, there is no one unique product. Fillers
Fillers typicallyare the N990 thermal blacks.These blacks offer reasonable reinforcing characteristics with the economy of higher loadings and lower attendant hardness increases. Furnace blacks ranging from N 100 to N700 typesare utilized to developspecific propertiesfor mechanical applications. Nonblack fillers should be used with great caution if the environment is humid or wet. Briggs et al. (1 963) should be reviewed before considering nonblack fillers. In addition, aqueous testing should be performed at the pH anticipated in actual service. Some nonblack fillers and compounding ingredients are very specific in some instances to pH variation. Nonblack fillers, although widely used, should be considered in light of the application. Suppliers’ literature is of great value and should be studied. Plasticizers Plasticizers are utilized in almost all formulations. Aromatic oils (forhigh levels) and naphthenic oils (for low and medium levels) are most commonly used. There are distinct solubility limits
Hertz, Jr.
534
for hydrocarbon oils in polychloroprene elastomers, and suppliers’ recommendations should be used as guidelines.
Polychloroprene Rubber Blends
6.6
Blends ofpolychloroprene with other elastomers have potential problems due tophase incompatibility. The literature illustrates the use of elastomers such as SBR, nitrile (NBR), and butyl in blends for specific applications.There doesnot appear tobe any indication that elastomer blends offer overall improvement to either material.
7.
FLUOROSILICONE ELASTOMERS
7.1
Chemistry and Markets History
The addition of fluorine into a polyalkylsilsiloxane (silicone) system created an elastomer with a high degree of solvent resistance and excellent low-temperature capabilities. LS-53, originally developed by Dow Corning Corporationunderagovernmentcontract.wasthefirst of the tluorosilicone family. Pierce (1970) briefly describes its chemistry and commercial uses along with high- and low-temperature response. The original LS-53 appears to have been named LS (lowswell). 53 (1953) based on a paper by Pierce (1953). Currentlytherearethreemajor suppliers: Dow Coming Corp., General Electric Company, and Shinetsu Chemical Co. World production facilities for tluorosilicone rubber are listed in Table 9. Label capacity for all producers worldwide is estimated to be in the range of 650-700 metric tons per year (author’s estimate).
7.2 Applications The original market, primarily O-ring seals, has expanded greatly. Current high-volume applications also include shaft seals and gaskets, molded goods, duct hose, and covers. Other volume applications include wire and cable, insulation, electrical connector inserts, and North Slope oilfield service. The long-termfluorosiliconegrowthpotentialappears to be in excess of 10% a year. Increasingly aggressive automotive fuels and higher engine operating temperatures are creating major markets for fluorosilicones. 7.3 Nomenclature and Basic Properties The nomenclature and basic properties IO.
of fluorosilicone elastomers are summarized
Table 9 Production Facilities for Fluorosilicone Rubber Country United States Japan
Company
Trade name
Plant location
Dow Corning General Electric Shinetsu
LS FSE FE
Midland, MI Waterford, NY Isobe
in Table
535
Halogen-Containing Elastomers Table 10 Nomenclature and Properties of FluorosiliconeElastomers ~~
~
Common name ASTM name ASTM D1118 designatton IUPAC trivial name IUPAC structure-based mtne SAEJ200/ASTM D2000 Specific gravity Durometer grange Tensile strength ( n u x ) . MPa Elongation (max). %, Glass transttion temp. (T?)." K
Fluorosilicone Fluorosilicone FVMQ Poly(methyltrifluoropropylsiloxnne) Poly(oxymethyl-3,3.3-triflu~~ropropylsilylet~e~ FK 1.4 40-80 10 400 < 193
.' From Brandrup ;und Immcrgut. 1975.
7.4 Characteristics of Fluorosilicone Elastomers Fluorosilicone elastomers as specified by MIL-R-25988 are available ;IS ( 1 ) general-purpose. (2) high-strength general-purpose. and (3) high-modulus. increased temperature resistant. Razzano and Simpson (1976) describetheprocessforpreparation of arange of tluorosilicone copolymers. Typically, tluorosilicone elastomers are a copolymer of 90 mol% tritluoropropylsiloxy and I O mol% din1ethylsiloxy monomers.
Mononwrs ar~dPo1yrrreri:atiou Monomers currently utilized for commercial tluorosilicone elastomer production are cyclic alkyl trifluoropropyl trisiloxane, (CFICH,CH,SiCH30)3, cyclicdimethyltrisiloxane, (CH3 SiCH30)3,and cure-site monomers. A specific cure site is developed by incorporating 0.2 mol% of methylvinyl siloxane. The highly reactive vinyl site allows a wide latitude in the peroxide crosslinker selection. The basic hydrocarbon resistance is imparted by the polar tritluoro (-CF.IO"'
2.8 24
#I
socs 1 .ss S1
I .4 17 360 >IO'"
7.8 23
ties and electricalpropertieswithout any ingredients. such asvulcanizingagents,co-agents. promoters. and fillers. From these data. the TFE-P elastomer is observed to be the most heat-resistant elastomer among the elastomeric insulating materials. Blends of AFLAS with other elastomers likeacrylic elastomer provide the customer with an economically reasonable elastomer. which has a well-balanced combination of heat and oil resistance at lower cost. Blends of AFLAS with thermoplastic resins are of interest in terms of polymer alloys. which are expected to have intermediate properties between elastomer and thermoplastic resin. Polyethylene and ethylene-tetrafluoroethylene copolymers are especially of interest since they are easily blended with AFLAS dipolymer in a wide range of blend ratio by means of heatprocessing mixers, such as extruders and kneaders. These blends can be extruded like a thermoplastic resin and cured by EB irradiation to yield a soft elastic resin. As described above, TFE-P elastomer has been successively developed to be provided with versatile types and gradesas well as various application technologies.all of which now allow the elastomer a worldwide established market appreciation as a distinguished high-performance elastomer to be used in variety of applications where harsh conditions and special requirements rule out the use of other elastomers.
REFERENCES Apothekcr, D., and Krusic, P. J. (1980).U.S. Pat. 4,214.060 (to E. I. Du Pont de Nemours and Company). Grootaert, W. M,, Kolb. R. E., and Worm, A. T. ( 1990). Ruhher C / w m 7 e h o l . 6 3 5 16. Kojima, G.. and Hisasuc, M. (1981),M w ~ o ~ ~ MC/rertr. J / . 182: 1429. Kojima, G.. and Kojima, H. ( 1977), R ~ h h e rCherrr. T e h w l . 50(2):403. Kojima. G., and Tabata, Y.(1972), J . Mrccrorlrol. Sci.-C/~eru.A6(3):417. Kojima. G . . and Wachi, H. ( 1978). K u b b ~ ~Cherr~. r T e c h o l . 5/(5):940. Kojima, G., and Wachi, H. (IOXS),Int. Rubber Conf., Kyoto, Oct. 15-18, Paper No. 16A18. Kojima, G., Wachi. H.. Ishigre, K.. and Tabata. Y.( 1976), J. Polyrr. Sci. Po/yru. Ed. 1 4 6 ) : I3 17. Miwa, T., Kaneko. T.. and Saito, M. (1996). The 9th Seminar on Elastomers, Kobe. Dec. S-6, Paper No. A-X. Tatemoto. M,, and Morita, S. (1982). U.S. Pat. 4,361,678 (to Daikin Industries. Ltd.).
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20 Carboxylated Rubber John R. Dunn 1. R. consulting,
Sarnia, Ontario, Canada
1. INTRODUCTION
2.
HISTORICAL
The early work on carboxylated elastomers was authoritatively reviewed by H. P. Brown (1957). Subsequently, several further reviews have been presented (Bryant, 1970; Jenkins and Duck, 1975; Longworth, 1983; MacKnight and Lundberg, 1983; Shaheen and Grimm, 1985). Jones and Smith (1985) compared the properties of carboxylated hydrogenated NBR with those of carboxylated NBR. The firstpreparation of a carboxylic elastomer, acopolymer of butadiene and acrylic acid, was recorded in a French patent published in June 1933 and assigned to I. G. Farbenindustrie (1933). In the ensuing years several more patents were issued describing the introduction of carboxyl groups by emulsion polymerization, including one assigned to B. F. Goodrich Co. by Semon (1946) that describes the preparation of CO-and terpolymers. This includes butadiene and isoprene among the dienes and acrylonitrile among the third monomers. As Brown (1957) notes, nothing remarkable was recorded about the vulcanizate properties of these polymers. It was claimed that solvent resistance was better than that of the corresponding uncarboxylated polymers. The first commercial carboxylated elastomer, a butadiene-styrene-acrylic acid terpolymer latex, was introduced byB. F. Goodrich in 1949 under the trade name HYCAR 1571 (Jenkins and Duck, 1975). Brown (1957) realized the role of carboxyl groups in crosslinking reactions in 1950, and a series of patentsresulted. In September 1954,Brown and Duke (1954) reportedthat two carboxylated NBR latices were commercially available. They noted that high strengths could be obtained in vulcanizates of gum stocks and latex films. Equivalent cures could be obtained 561
562
Dunn
without sulfur by the use of polyvalent metal oxides or salts. At the time, dry rubbers were not available. Even in 1975. Jenkins and Duck ( 1975) noted that latices were used in far greater volume than dry rubbers because of the tendency for compounds containing metallic oxides to scorch. The use of coated zinc oxide and of zinc peroxide masterbatch, or of modified polymers, to reduce scorch is described in the papers by Bryant ( 1970) and Shaheen and Grimm (1985). This topic will be discussed in a later section. As a result of improvements in scorch resistance, carboxylated NBR is now being used in avariety of applications,which will be discussed subsequently. While emulsion polymerization is the most common route to carboxylated elastomers, the modification of noncarboxylic elastomers to introduce carboxyl groups has also been studied and is described in the earlier reviews. So far as is known, none of the commercially available carboxylated elastomers are produced by polymer modification.
3. 3.1
PREPARATION OF CARBOXYLIC RUBBERS Emulsion Polymerization
The majority of carboxylated elastomers are produced by emulsion polymerization at temperatures ranging from about 50 to 60°C. Many emulsifying agents have been investigated and are suitable, but the use of an acidic system is regarded as essential. Marvel et al. (1952) showed that butadiene could not be copolymerized with methacrylic acid in the Mutual GR-S recipe, presumably because the acids were converted to water-soluble salts. Coagulation must also be carried out using reagents that ensure that the carboxyl group remains acidic,andacids, or blends of salts and acids, fulfill this function satisfactorily. Jalics ( I 984) claims that emulsion copolymerization of half-esters of carboxylic acids may be carried out in basic media. The halfester moiety is said to renderthecarboxyl compound hydrophobic. The ability to produce carboxylated elastomers in a basic medium would indeed offer an advantage, since polymerization in an acidic medium requires the use of stainless steel vessels and piping. The preparation of carboxylated elastomers by emulsion polymerization hasbeen described in some detail by Jenkins and Duck (1975). They suggest the following as a typical recipe:
persulfate
0.2 ulfate
Monomcrs Sodium alkylarylpolyether sulfate Potassium Water
100.0 1.0
0.3 188.0
As in many emulsion polymerization recipes, tertiary dodecyl mercaptan is recommended as a modifier to control molecular weight. HCI (or methanolic HCI) is suggested as coagulant. Brown and Gibbs (1955) quoted the following as a typical recipe:
Monomers Dodeeylaminc (90% neutralized with HCI) Aluminum chloride Potassium Sulfole Water
100.0 5.0 0.2
200.0
Carboxylated
Rubber
563
The recommended polymerization conditions were 6-25 hours at 30-50°C for 75-90% conversion. This is a somewhat lower temperature than the 60°C quoted in earlier patents (Brown. 1957). Polymerprocessabilityandpropertiesareaffected to someextent by polymerization temperature. The amount of carboxylic acids in the polymer and their distribution depend on the type of acid, the ratio of monomers charged, whether or not they are charged incrementally. and the degree of conversion to polymer. The efficiency of incorporating the acid has been said to depend on its relative solubility in the hydrocarbon and water phases. Acrylic acid is more soluble in the aqueous phase, and only half of that charged is polymerized. Methacrylic acid is about five times as soluble in the hydrocarbon than in the water phase. Hence it is efficiently incorporated into the polymer. Marvel et al. (1955) described the preparation of copolynlers of butadiene with 15-20 parts of acrylic acid. They used asodiumalkyl aryl polyethersulfate (Triton X-301) as ~ l n emulsifier in order tobroaden the range of monomer composition. They used azobisisobutyronitrile as initiator andrl-decyl mercaptan as the modifier. Dolgoplosket al.(195Ya) used decomposition of isopropyl benzene hydroperoxide byFe’+ salts, in the presence of dihydroxymaleic acid, to initiate polymerization at 5°C of various monomers (includingisoprene) with methacrylic acid. The preparation of crosslinked carboxylated NBR and SBR containing methacrylic acid. a chloroethyl methacrylate, and111- andp-diisopropylbenzene and crosslinker has been described by Ivanova and coworkers (1970). Polymerization was carried out at 30°C at pH 3.0. Despite the activity on polymerization of carboxylated rubbers in emulsion i n the 195Os, the topic has remained of interest, and the terpolymerization of butadiene, acrylonitrile, and methacrylic acid was investigated further by Jerman and Janovic (1984). Polymerization was carried out at 50°C using potassium persulfate as initiator and sodium dioctylsulfosuccinate as emulsifier. When polymerizations were stopped at low conversions, the experimental and theoretical copolymerization data for the terpolymers were in good agreement. Although there was no point of true azeotropic composition, where the composition of the polymer at all degrees of conversion corresponded with the monomer composition, a “pseudo-azeotropic” region was recognized. Okubo et al. ( 1 987) described the localization of carboxyl groups at the particle surface during the polymerization of styrenehutyl acrylate/methacrylic acid. A terpolymer“seed” emulsion was prepared at low pH. and then polymerizationof styrene andbutyl acrylate was continued at high pH. The resulting particles in the emulsion had carboxyl groups located predominantly at the surface. Kalinina et al. (1996) claimed that copolymerization of methacrylic acid with butadiene, styrene, and/or acrylonitrile resulted in a localization of carboxyl groups in the surface layers if conversion of the main monomers was high at the moment of addition of the methacrylic acid. Introduction of methacrylicacid at 56% conversion of the main monomers resulted in improved resistance of the latices to mechanical effects. When methacrylic acid was replaced by acrylic acid, the degree of conversion at which the acid was introduced did not appreciably affect properties. Okubo (1990) patented a process for producing hollow carboxylated latex particles.
3.2 Preparation by Polymer Modification Carboxylated elastomers have also been produced by grafting unsaturated acids onto polymers. but the products of such reaction do not appear to have become commercially significant. Prior to 1950 such reactions had been conducted primarily with natural rubber, using either grafting in solution or grafting by mechanically induced reaction on the mill. This work includes reaction of maleic anhydride with rubber and its subsequent hydrolysis to carboxyl groups. Cuneen et al.
Dunn
564
( 1 960) described the graftingof thioglycolic esters ontonatural rubber latexusing hydroperoxide
initiators and the subsequent formation of the acid by hydrolysis. Jenkins and Duck ( 1975) described the grafting of thioglycolic acid onto polybutadiene. A 6% solution of polybutadiene, treated with benzoyl peroxide and thioglycolic acid for 24 hours at 50°C, was said to give 85% incorporation of the acid. The peroxide constituent could also be produced in situ by blowing air through a mixture of a thiol acid and rubber in toluene. The same authors describe grafting p-mercaptopropionic acid to polybutadiene on atwo-roll mill by milling in thepresence of benzoyl peroxide for 38 minutes at 38°C. Sanui et al. ( I 974) introduced from > 1 to about 16 mol% of thioglycolic acid derivatives into polypentenamer. They used free radical addition reactions and subsequently hydrogenated the polymers.Tanaka and MacKnight (1979) prepared carboxylated three-membered ring derivatives of a polypentenamer by carbene addition of ethyl diazoacetate using a copper catalyst. Again the products were fully hydrogenated.This method of preparation introduced 5-10 mol% of the three-membered rings. The carboxyl groups couldbe hydrolyzed to the acid or neutralized to form salts. Reaction conditions could be chosen to prevent backbone degradation and crosslinking during modification. The preparation of cyclized polydiene rubbers containing pendant photosensitive p-unsaturatedcarboxylicacids,suitable for useasphotoresists, has been described by Azuma et al. (1980). In a typical reaction, 10 g of cinnamic or acrylic acid was added with stirring to 0.5 g of polydiene in 25 cm3of chlorobenzene containing 0.12 g ofp-toluenesulfonic acid. The reaction was terminated by addition of triethylamine. The process for polymerization of ethylene propylene elastomers is not suitable for the preparation of carboxylated EPM or EPDM. The comment made by Jenkins and Duck ( l 975) that it has proved difficult to graft carboxyl-containing reagents onto these rubbers is still true. They drew attention to the observations of Gaylord et al. (l972), who injected a live styrene maleic anhydride copolymer into an extruder containing ethylene propylene rubber. The living copolymer reacted with the rubber, and a carboxylated terpolymer was formed on hydrolysis. Joshi (1979) has described the chlorocarboxylation of polyethylene. EPDM. and other rubbers. generally at l 10°C, withchlorineandmaleic anhydride (M) andsimilarmaterials to givea carboxylated elastomer. The key reactions were given as
+ Cl.
P-H
-
P.
+ HCI
is polymer and P . is polymer radical.
where P-H P.
-
+ c12
P"CI (chlorination)
+
+ c1
1;ISl
P. M-P-M. (carboxyl grafting with maleic anhydride and
-
P-M.
+P
P-".
+ P-".
hl250”C) melt polymerization of highly purified hexachlorocyclotriphosphazene trimer. Careful selection of time and temperature is necessary to avoid the formation of a crosslinked matrix. More recent studies have demonstrated the effectiveness of various acids and organometallic compounds as catalysts for the preparation of the chloropolymer (Hagnauer, 198 1 ). The catalyzed polymerizations can generally be carried out at temperatures less than 250°C. and conversions to gel-free chloropolymer are typically 70% and higher. Polydichlorophosphazene obtained from the uncatalyzed thermal polymerization is characterized by high weight-average molecular weights (Mw,2 X 10”) and a fairly wide molecular weight distribution ( M J M , , , 5-10). Catalyzed polymerizations have been foundto yield chloropolymers with slightly lower M , but significantly narrower molecular weight distribution (MJM,,,
l-
3 m
1.0
l-
OI
Lu ll-
z W
V U n W
0.5
C 0
200
400
-
600
800
DPC
Fig. 6 Effect ofvinyl Polmanteer, 1981.)
content in polymer and concentration oft-butyl
Fig. 7 Hydrosilation rcaction. (From Polmanteer, 1981.)
perbenzoate on DP,. (From
615
Advances in Silicone Rubber Technology, Part I, 1944-1986
H0
-
]-I!
iy3-[0
OSI CH3 - OH
+
ETO
CH3 X Y
x
=
100
-
l
200
Y.3-10
H WHERE:
ET
=
-
H
C R O S S L I N K E D RUBBER
+
H
c -c
H
H
ETOH
Fig. 8 Condensation crosslinking chemistry. (From Polmanteer,
1981.)
shown in Fig. 8). This cure system was filed with the U S . Patent Office in 1954, but the patent was not issued to Polmanteer until 1960 (Polmanteer, 1960). This is a simple condensation reaction catalyzed with fatty acid salts of tin, lead, cobalt, etc., giving a siloxane linkage and ethanol. Thus, the silicate, being polyfunctional, can react with several chain ends to tie the network together. The catalyst necessary for this reaction is normally not added until it is desired to have the crosslinking start; hence this system is referred to as a two-part system. Cure does not require atmospheric moisture,hence deepsection cures are possible.However, it is preferable for the alcohol by-product formed to rapidly diffuse out of the system so as not to retard the crosslinking reactions via silanol-ethanol reactions. One-Part Systems One-part curesystems canbe placed in one packageandremain non-crosslinked. When the material is exposed to a reactant, normally from the atmosphere, a sequence of reactions take place leading to the formation of crosslinks. Water in the atmosphere is the most common reactant of one-part RTVs. One-part cure systems that depend on atmospheric water to start the sequenceof reactions leading to crosslink formation have several points in common. Atmospheric water first reacts with hydrolyzable groups attached to silicon (Fig. 9). Once hydrolysis begins, the silanol formed can condense with either another silanol group or a hydrolyzable group on silicon, thus forming a S i U S i linkage and a condensation byproduct(Fig. IO). In mostinstancestheseby-products are acidic or basic enough to actas catalysts for the crosslinking (Si+Si-forming) reaction. These one-part systems must be packaged in water-impernleable containers to prevent crosslinking within the package. The two
S i O Y + HOH + S i O H
+ HOY
where O Y = hydrolyzable groups
Fig. 9 Hydrolysisreaction.(FromPolrnanteer,1981.)
616
Polmanteer
HOSE 3 i O H + or -+ S i - O - S i G YO S 6
+
HOH or HOY
Fig. 10 Condensation reaction. (From Polmanteer,1981.)
Name Acyloxy
Group
,OC-R (e.g.. R = CH3) 0
Oxime
-ON=C,/R 1 (e.g.. R 1 . R 2 = alkyl)
Alkoxy
-OR (e.g.. R =
Amlno
-NH,-, R3-P,where p = 1,2.3 R = alkyl or cycloalkyl
R2
CH,,etc.)
Fig. 11 Examples of hydrolyzable groups attached to silicon. (From Polmanteer, 1981 .)
generalized reactions just described can be applied to a wide variety of specific hydrolyzable groups. Examples of suitable hydrolyzable groups are shown in Figure 11. The reactivity of hydrolyzable groups attached to silicon is influenced by several factors such as the number of OY groups per silicon, number and kind of R groups also attached to silicon, and the substituents on the hydrolyzable group itself. For example, oximes (OX) are shown in Figure 12. Similar effects are observed with other hydrolyzable groups. To formulate a one-part sealant, it is normally best to attach the hydrolyzable groups to both ends of each polymer molecule. Using acetoxy groups as an example, under anhydrous conditions an excess of methyltriacetoxysilane can be mixed with a dihydroxy-ended polydimethylsiloxane to give a methyldiacetoxysilane group on each end of the polymer molecules. Fillers can alsobe added and thematerial packaged in watertightcontainers (Fig. 13). Depending on the specific hydrolyzable group incorporated, catalysts can also be added to assist in the condensation reactions. Although a specific catalyst may be applicable to only a certain type of RTV, examples of some of the catalysts include fatty acid salts of metals such as tin and lead, amines, amine salts, titanates, and aluminates.
2.5
Formulating Methods
Typical silicone rubber compositions are prepared by adding reinforcing silica filler, extending fillers,pigments, special additives (e.g., heatstabilityimprovers, flame retardants,handling property improvers, etc.), and a vulcanizing agent. If the surface of the reinforcing silica filler has not been modifiedby pretreatment to reduce polymer-filler interaction,a hydroxyl-containing low molecular weight silicone plasticizer may be used to prevent or minimize polymer filler interaction, thus allowing good shelf-aging characteristics (e.g., no crepehardening). The plasti-
617
Advances in Silicone Rubber Technology, Part I, 1944-1986
--
2.
.-
.->
U
m Q,
Si
VISI (OX),
(0x14
[r
RSi (OX),
PhSl (OX),
2
R2Si (OX)?
MeSl (OX),
B w
R,Si (OX),
0
C .v) m
V
EIS1 (OX), V
l
-SION=C(Me)2 =StON=CMeEl ~SION=C (El)?
Fig. 12 Order of reactivity of oximes. (From Polmanteer, 1981.)
cizer is usually dispersed in the polymer prior to addition of the reinforcing filler. With the exception of plasticizer.thematerials are added to thesiliconepolymer in the order listed above. Fillers should be added in small enough increments to avoid the formation of large filler agglomerates, and enough mixing time should be used to obtain good uniform dispersion of ingredients. This must be determined by the formulator and depends on the specific mixing equipment and the quantity of the composition being formulated. The type of mixing equipment suitable for formulating high-consistency silicone rubber includes two-roll mills, Banbury mixers, dough mixers, and continuous mixers. In the case of thetwo-rollmills,the roll speed ratioshould be in therange of 1.2/1 to 1.4/1. The lowerconsistency RTVs and LSRs are formulated using equipment capable of mixing and dispensing low- to medium-viscosity materials. Because of the highly competitive market and the manufacturing complexity of formulating these low-consistency RTV and LSR products,the compounding and packaging of these products is proprietary with the basic suppliers.
H
+ CH3Si(OCCH3)3 (excess)+ 0
CH3 (CH3C0)2Si-0
+ C H J S ~ ( O C C H J+) ~
0
0
0
Trace HOCCH
0 ( 2 ) Add Filler Pa*ge
(3)
R T V Compound
R T V Compound (packaged)
+ HOH +Crosslinked Elastomer + CHJCOH t 0
Fig. 13 Example of preparingaone-part
RTV. (From Polmanteer, 1981.)
618
2.6
Polmanteer Fabrication Methods
It is good practice to freshen a silicone rubber stock prior to fabricating into the desired product form for use. However, there are many stocks that are designed to not require freshening prior to fabrication. Freshening is done by remilling on a two-roll mill. Silicone rubber has good flow propertiesunderpressureeven at ambienttemperatures. This property is responsible for its success in manyconventionalfabricationmethods.For example, thesefabricationmethods include compression molding, transfer molding, blow molding, extrusion, calendering, dispersion coating on fabrics and organic polymerfilms, andmandrel wrapping techniques. Fabrication details are extensive, and space does not permit a detailed discussion here. However, Lynch (1978), in his book on silicone rubber fabrication, describes details of various fabrication methods as they pertain to silicone rubber.The chemistry of the various vulcanizing systemswas discussed earlier. but nothing was said about the mechanics of accomplishing vulcanization. This subject is paramount for a well-fabricated rubber part. For heat-vulcanized compositions, the method of applying heat to the fabricated shape becomes an important factor that must be integrated intothefabrication system. In moldingoperations such as compression,transfer, blow, and injection molding, both heat and pressure are applied via the heated walls of the mold. Extrusionsarecuredusinga variety of alternativemethodsincluding CV (continuous steam vulcanization), HAV (continuous hot-air vulcanization), and FBV (continuous fluid-bed vulcanization). The method used is chosen based on the applicability to the total fabrication process and the incremental cost added to the product being produced. Dispersioncoating procedures use a dispersion coating tower, which usually has three separate heating zones. The first zone has thetemperature set to drive off solvent below the vulcanization temperature, the second has its temperature setto accomplish vulcanization, and the third hasits set to driveoff vulcanization by-product volatiles. Mandrel wrapped fabricated parts are usually vulcanized in a steam autoclave.
2.7 General Properties and Uses The unusual combination of properties exhibited by silicone rubber has provided the basis for significant growth in the marketplace, with many new uses continually being unveiled since its 1944 introduction. The unique combination of properties were listed in the second paragraph of Section 1, and the inherent characteristics of silicone polymers that are primarily responsible for their properties was given in the third paragraph. By way of emphasis, these inherent characteristics include strong chain bonds, backbone chain flexibility, ease of rotation of the organic side chain groups, low inter- and intramolecular forces, and inorganic/organic makeup.
Polyn~erRheological Properties Polmanteer (1981) showed that the type of organic groups attached to silicon have a marked effect on rheological properties. For example, in Figure 14 the viscosity as a function of shear stress for silicone homopolymers is shown. This type of plot depicts the relative newtonian character of the homopolymers (e.g., Newtonian character increases with decrease in slope-a Newtonian fluid has a slope of zero). In this homopolymer series, Newtoniancharacter increases in the order of increase in size of the second group attached to silicon (e.g. methyl, propyl, trifluoropropylmethyl, and phenyl). The first group attached to each silicon atom is a methyl group. It may also be seen in Figure 15 that viscosity increases with the size of the second group for a given degree of polymerization, E. The linear plot of viscosity versus temperature in Figure 16 shows the dramatic effect of the particular R groups on the viscosity-temperature relationships. Figure16 clearly demonstrates
Advances in Silicone Rubber Technology, Part I, 1944-1986
619
Fig. 14 Apparent viscosity as a function of shear stress. (From Polmanteer, 1981.)
the small effect of temperature on the viscosity of polydimethylsiloxane, which is the most commonly used polymer in silicone rubber. Energy of activation for viscous flow values, E,,,,,, numerically define thesensitivity of viscosity to temperature. The measured E,,,,, values in kilojoules per mole for the homopolymers were 14.2 for polydimethylsiloxane, 18.0 for polymethylpropylsiloxane, 33.0 for poly(1nethyl-3,3,3-trifluoropropyl)siloxane,and 49.8 for polymethylphenylsiloxane. The molecular weight range of homopolymers studied did not appear to change the E,.,,, values. The values for the energy of activation for viscous flow give a good index for the level of inter- and intramolecular attraction forces. Low-Tentperuture Properties One of the salient characteristics of silicone rubber that make it stand out among all types of elastomers is its ability to remain flexible at very low temperatures. The factors that cause an
620
Polmanteer
1 05
1o4
10)
10’ m
(F, PrMeSiO)
v (PrMeSiO)
(Me,SiO) 10
0
1
2
DP, x
1
1
1
l
3
4
5
6
103
Fig. 15 Log Newtonian viscosity as a function of degree of polymerization. (From Polmanteer, 198I .)
elastomer to become stiff at some particular temperature are crystallization, nearnessto theglass transition, or a superposition of the two phenomena. Both of these phenomena can be affected by changing the makeup of the polymer. The crystallization temperature, T,, can be lowered and/or eliminated by the random inclusion of bulky side groups. If these bulky groups possess alargerintergroup or intermolecularforceconstant than methyl groups,theglasstransition temperature, T,, will be increased by an amount that depends on the molar concentration of these bulky groups. Polmanteer and Hunter (1959), using a Gehman cold-flex apparatus, showed how the random inclusionof phenylmethylsiloxane mer units changed the equilibrium stiffening temperature (see Fig. 17). The influence of the same polymer compositions on T, is shown in Figure 18. T, for MQ- and VMQ-based silicone rubber compositions is - 123°C while for the PVMQ extreme low-temperature compositions, it is slightly higher at about - 114°C because of the increase in intermolecular forces. The incipient crystallization temperature for MQ- and
621
Advances in Silicone Rubber Technology, Part I, 1944-1986
.96 .88
.eo .?Q .64
.56 Y)
4
0-
z -
.4a
0
c
.40
32 .24
.l€
.08
t 60 40
+
-T
1
70
50 "C
80
- TEMPERATURE
Fig. 16 Newtonian viscosity as n function of temperature. (From Polmanteer, 1981.)
VMQ-based silicone rubber varies somewhat with experimental test methods and compositional variables but is approximately - 54°C. This is lower than the equilibrium crystallization temperature, which approaches the T,,, value (-40°C).
Liquid Media Resistance Solubilityparameter, a,, valuesgive a good indication of whether an elastomer containing sizable amounts of a given siloxane unit will be resistant to a particular solvent or oil. For example, if the 6, of the solvent or oil is close to that of the elastomer, considerable swell is anticipated. The 6, values for some of the moreimportant siloxane units are givenin
622
Polmanteer 0
-2c V W
5
-40
k
a
EK
-60
5lU
zZ
-80
W
U-
-100
lv)
-120
I I 10 3020
-140
I
I
I 50
40
I
I
60
70
I 80
l 90
1
M % PHENYLMETHYLSILOXANE
Fig. 17 Stiffening temperature as a function of phenylmethylsiloxane molar content. (From Polmanteer. 1981.)
0
40
20
M
60
80
100
% PHENYLMETHYLSILOXANE
Fig. 18 Effect ofphenylmethylsiloxane content on glass transition temperature. (From PoImantecr, 198 1 .)
623
Advances in Silicone Rubber Technology, Part I, 1944-1986 Table 3 Solubility Parameters of Scvcrnl Siloxane Unlts Unit
6,
(CH1)2Si0 (CH3)?Si(/)./)’-ChH,)Si(CH1)~0 (CH3 NCCH2CHI(CH3)Si0 F3CCH2CH2(CH3)Si0
7.5 8.9 9.0 9.0 9.6
Source;
Polmnntcer. 198 I .
Table 3. Chemical-. solvent-, and oil-resistance data are given in Table 4.
for VMQ and FVMQ silicone elastomers
Su$ace Energy Properties The surface energy of a polymeric surface provides an excellent guide as to the ability of other materials to adhere to the surface. An organic material with a surface energy or surface tension higher than that of the substrate surface will not adhere to that surface. Silicone elastomers have lower surface energy values than most organic materials, including most foods. Consequently, the goodreleaseproperties of silicone elastomers allowtheir use in many food-processing
Table 4 Chemical and Oil Resistance of Silicone Rubber Volume change (9) Chemical or mcdia
WMU)
Acid (7 days at 24°C) 10% Hydrochloric Hydrogen chloride 10% Sulfuric 10% Nitric Alkali (7 days at 24°C) 10% Sodium hydroxide 50% Sodium hydroxide Solvent (24 hr at 24°C) Acetone Ethyl alcohol Xylene JP 4 fuel Butyl acetate Oil ASTM No. 3 (7 days at 149°C) Turbo Oil 15 (Mil L-7808)(1 day at 177°C) Dimethylsiloxane, 500 CS.(14 days at 205°C)
Fluorosilicone rubber (FVMQ) +l +8 Nil +l
Nil
+ 180 + 180 +S
+ 20 + 10 >IS0 +6
+8 Nil
Siliconc mbher
f 3
+ IS +S +8 Nil +B
+ IS +B
>IS0
> 150 > IS0 + 2 0 to +S0 30 Swells, deteriorates
Polmanteer
624
Table 5 Surface Energy Values of Polymers Polymer Polydimethylsiloxanc Polytrifluoropropylmethylsiloxanc Polyphenylmethylsiloxane
Polystyrene Poly(viny1 chloride) Polyethylene Poly(viny1 alcohol) Poly(viny1idene chloride) Polyacrylamide Polyacrylate Poly(ethy1ene terephthalate) Poly(methy1 methacrylate) Polytctrafluorocthylene Polyhexafluoropropylcne Polytrifluorocthylene Poly(viny1idene fluoride) Wool
Starch Cellulose (regenerated) Amylose
21 -22 21-22 26 33-35 39 31 37 40 35-40 35 43 33-44 18.5 16.2-17.1 22 25 45 39 44 37
operations and in the manufacture of thermoplastic polymeric end products such as films. Many applications utilizing the low surface energy of silicone elastomers also take advantage of their other properties such as good high-temperature resistance. The surface energy values given in Table 5 demonstrate how various polymers compare. It can be noted that the only polymers having lower surface energy than polydimethylsiloxane are some of the highly fluorinated polymers such aspolytetrafluoroethylene. The low surface energy of polydimethyl-siloxane is maintained even when reinforced with silica. The low surface energy of silicone elastomers suggests applications as both abhesives and adhesives, depending on how they are used with materials having other surface energy values.
Physiological Inertness Properties The use of silicone elastomers as body prostheses began in about 1956. with the result that these elastomers are generallyconsideredphysiologically inert. However,eachapplicationhasits associated boundary conditions that determine the results for a specific use. One of the very successful implant uses is the replacement of diseased finger joints with silicone rubber joints.
Pertneability Properties Siliconeelastomers are based on polymers having low intermolecularforcesandrelatively unhindered single bonds that link the alternate silicon and oxygen backbone chain atoms together. These facts combine to provide a polymer of a higher than normal amount of free volume and a high degree of chain mobility. These characteristics explain why it behaves like a rather open
625
Advances in Silicone Rubber Technology, Part I, 1944-1986 Table 6 ContinuousExposureTemperature as a Function of Service Life Temperature ("C)
15,000 7,500 2,000 100-300 0.25-0.50
150
200 260 316 37 1 Solrrcet
Service lifc (hr)
Courtesy o f Dow Cornmg Corporation. Mid-
land. MI.
screen to gases. For example, a silicone elastomer based on polydimethylsiloxane is 25 and 429 times more permeable to oxygen than natural rubber and butyl rubber. respectively.
Therrwal StabiliQ Properties Silicone rubber has long been recognized as the rubber of choice for high-temperature service uses. For example, the data in Table 6 are for samples formulated to enhance high-temperature stability. The samples were exposed at the indicated temperature continuously until the elongation decreased to 50%, with the time necessary for this to occur considered the service life under the oven-aging conditions. Actual service conditions vary with time and make accelerated test results difficult to use directly in predicting actual life expectancy. Fortunately. the continuous exposure service times are usually very conservative, with actual service times being longer in those caseswhere service conditionsconsist of high-temperature air. When the high temperatures accompany contaminants such as oils, sulfur oxides, and nitrogen oxides. the service life can be shortened.
Electrical Properties The electrical properties of silicone rubber are very good in general but can be varied by the type and amount of compounding ingredients used in the composition. Since electrical properties can be varied by compounding ingredients, special electrical application compounds have been developed for such applications as wire and cable insulation and rubber insulating tapes. Table 7 lists the range of electrical properties typical for silicone rubber.
Table 7 ElectricalProperties Test Electric strength (1/4-in. electrodes, rapid rise on specimens 1/16 in. thick), V/mil Dielectric Dissipation Volume resistivity, ohm-cm Insulation ohms resistance,
Results of 500 V/sec,
450-600 2.9-3.6 0.0005-0.2 8 x 10'3-2 x 10'5 1
x
10'2-1
x
10'3
Polmanteer
626
Mechanical Pwyerties and Uses The applications for silicone elastomers cover a multiplicity of areas. Both the number of uses for silicone elastomers and the volume used are increasing at a good rate as a result of rapid advancements in all technology areas. As a consequence of these rapid changes in technology, many new silicone elastomer applications have been born that require a high level of performance under severe conditions. In every instance the reason for selecting a silicone elastomeris related to its uniqueproperties, which allow it to be functional in theindicatedapplication,while organic elastomers fail for various reasons such as poor thermal stability, poor low-temperature flexibility, poor ozone resistance, low gas transmissibility, poor weatherability,or lack of physiological inertness. Examples of the types of commercial silicone elastomers available from theDow Corning Corporation are listed in Tables 8 and 9 with their property profiles and some of their major uses. Similar products are also available from other manufacturers. These elastomers are sold both in a form containing the vulcanizing agent and in an uncatalyzed form. The uncatalyzed compounds have improved shelf stability and permit more flexibility in fabrication methods by appropriate selection of special-purpose peroxides. The test specimens used for property measurement were preparedby vulcanizing and oven-curingthat rubber according to the specific recommendations for each particular stock. The general-purpose elastomers listed in Table 8 satisfy applications in a wide variety of areas. The extreme high temperature classification represents a group of elastomers specially designed for long life at high temperatures. These materials normally remain flexible for up to about 300 hours at 3 15°C. The high strength classification elastomers are based on phenylcontaining low-temperature polymers. These polymers were the first to be designed to give high strength and Die B tear values as well as improved low-temperature properties. Most of the wire and cable compounds are unique in that the raw stock does not require mill freshening before being fabricated, and they are supplied in hat form for direct continuous feeding to an extruder. The elastomers typical of those in the extreme low temperature classification in Table 9 are lower in strength than those in the high strength classification. However, they exhibit somewhat more resilience, which is important in some applications. Rubber in the low compression set classification give, exceptionally good compression set, even up to 250°C as shown. These materials. although showing lower Die Btear values, are more resilient and havemore resistance to tear initiation than some of the higher-tear compounds. Elastomersclassified as fuel, oil, and solvent resistant are based upon 3,3,3-trifluoropropylmethyl-substituted polumers. Compounds in the no postcure classification may be used in applications after vulcanization and do not require the usual oven postcure. These compounds are more reversion resistant than many of the materials represented in the previous classifications. It should also be pointed out that the compressionsetvalues are in many cases asgoodas or better than thoseformany of the compounds that require oven postcures.
3. 3.1
ADVANCES IN SILICONERUBBER Introductory Remarks
Advances in silicone rubber have been continuous since the first commercial silicone rubber becameavailable in 1944. They includeadvances in all aspects of siliconerubbersuchas technology,fabrication,andproductmarketing. In addition, new useshavedeveloped,and economics have continually improved to benefit the silicone rubber supplier. the fabricator, and the end user.
Advances in Silicone Rubber Technology, Part I, 1944-1986 627
628
Polmanteer
In some instances, various types of advances have combined and are jointly responsible for the growth in the silicone rubber market. The limited number of examples singled out for discussion here represent the key advances in silicone rubber. but are a small percentage of the actual total. This is reflected in the thousands of patents that have been issued worldwide in this field as well as the very large number of technical and commercial papers published. It is impossible to mentionherealltheindividualsand companies who have contributed to the progress in this field. This section will list advances in chronological order as first choice. However, in some instances it will be more convenient to keepthe type of advance suchas "new synthetic silicas" under one heading and simply list the year of pertinent events. Uses listed in this section are not all-inclusive, but instead are meant as representative examples.
3.2 Tensile Strength The first commercial advances in silicone rubber were primarily related to improvements in tensile strength. Hunter (1964) and later Warrick ( 1976) discussed these i n some detail. The stepwise increase in tensile strength is depicted in Figure 19 from Warrick (1976) in his Charles Goodyear Medal address of that year. Brietly, the principal contributions responsible for the tensile strength advances were polymerization improvements starting from polymer gels to high molecular weight linear polymers and copolymers, improved reinforcement with small-particlesize amorphous precipitated and/or fume silicas compared to the originally used larger-particlesize metal oxide fillers (e.g.. TiOz, ZnO, etc.), development of methods to retard or eliminate "crepe hardening" of amorphous silica-reinforced compositions (e.g., low molecular weight hydroxyl-containing silicone fluids-work in 1950 by Konkle, McHard, and Polmanteer with patent issued much later in 1959), and introduction of vinyl groups to the polymer first reported by Marsden (1948) but not commercially used until the early 1950s.
629
Advances in Silicone Rubber Technology, Part I, 1944-1986
16
Tensi 1 e MPa
I
12
a
I 44
48
52
56
60
64
YEAR
Fig. 19 Tensilc strengthimprovemcnt step curvc. (From Warrick, 1976.)
Figure 19 provides a historical review of improvements in tensile strength that produced satisfactory range for many applications. Once a desirable tensile strength range was achieved, attention was focusedon other properties needing improvement, suchas tear strength, toughness, and flammability resistance.
21
3.3 Room-TemperatureVulcanization The first commercial RTVs were sold in 1954 and used the crosslinking chemistry shown in Figure 8. During subsequent years many specific new leaving groups were identified with the chemistry depicted in a generalized manner in Figures 9- 13. Hydrosilation chemistry, discussed in Section 2.4. is also occasionally used in RTV products. The hydrosilationreaction is an addition reaction rather than the condensation reaction used in other RTVs and is often used where by-products cannot be tolerated. Advances in RTVs include significant improvements in their mechanical property profile such that the mechanical propertiesof special high-strength and tough RTV products are comparable to those of high-consistency silicone rubber. Interested readers should contact silicone rubber manufacturers for a listing of specific products and attendant properties. FMQ-based RTVs were developed by Dow Corning Corporation in the late 1950s and exhibit resistance to many solvents and oils in addition to having other properties typical of RTVs. RTVs in general embrace an interesting combination of properties that make them very desirable for a very large number of applications. These special properties include ( I ) a consistency range from flowable liquids to soft pastes; (2) excellent thermal stability characteristic of high-consistency silicone rubber; (3) good adherence to most surfaces without the application of pressure; (4) The ability to replicate fine detail such as record grooves, newspaper pictures and print, wood grain, and leather detail; (5) curability without heat, as the name RTV implies; and (6) all the properties of silicone rubber discussed in Section 1 of this chapter. The above combination of properties make silicone RTVs particularly well suited to construction industry uses such as sealants, adhesives. or protective coatings with masonry, metal, wood, plastic, and glass substrates for outdoor and/or indoor locations. RTVs have found applications in seals for
630
Polmanteer
automotive, appliance, and lighting areas.A particularly sizable use is as fot-med-in-place gaskets (FIPG), wherein a thin bead of a single-component RTV silicone adhesive sealant is applied to one of the surfaces, which is then pressed with the mating surface to provide an excellent seal within a few minutes.The characteristics of RTVs makethem very good mold-making materials for such things as furniture, art objects, and many other items. RTV applications are limited only by the imagination and innovative ability of those using them.
3.4 Copolymers (VMQ; PMQ and PVMQ; FMQ and FVMQ) The first patent that teaches the use of vinyl groups i n silicone polymers was issued to Marsden ( 1948) and assigned to General Electric. As so often is the case, work was also being done in competitive silicone-manufacturing companies. The use of vinyl groups was very important, since it significantly improved the peroxide crosslinking efficiency shown in Figure 6 (Sec. 2.4). One of the benefits was the much improved compression set of VMQ compared to that of MQ silicone rubber (e.g., from >20 to The ligands still present on the 6-valentWatomhave been omitted in theschematic illustration. This would explain the formation of the exclusively cyclic oligomers of the polyalkenylenes. If acyclic monoolefins are added to the catalyst, they act as molecular weight modifiers that also influence the proportionof cyclic macromolecules. In these reactions, the modifiers are consumed and undergo addition at the ends of the newly formed chains (Dall'Asta, 1974); see Eqs. ( 5 ) and (6). When the reptation movement of the linear polymer (deGennes, 1971)and the associated, relatively restricted "action radius" of the oscillating active chain ends carrying the metallacarbene groups are taken into account.
m
Cleavage of macrocycles
R1
+ CH= CH
I I
R2
CHR2 RICH
" " " " "
"
+ R,CH=CH.R2
cham
Degradatron by linear sclsslon
Draxler
700
it is obvious that the intramolecular metathesis leads to the formation of relatively small, i.e.. oligomeric, rings. The formation of macrocrings in the polymer range, which must be assumed in the case of polyoctenylene, is more difficult to explain, a possible route being ring closure by means of a metathesis reaction between two linear polymers with active ends. If a
I mear
CH2-CHy"H2-CH =CH -CH2-CH2-CH2
CYCllC
CH
CH
I1
II
CH
CH
linear polymer is just at the point of passing through a cyclic molecule, a catenane is formed. The reptation of linear polymers in the longitudinal direction has been studied for some years. In the case of cyclic molecules, this movement must necessarily take place in the peripheral direction. This ensuresthat the macrorings are open topermit linear polymers to "pass through" in this way. The structural details and mechanisms of formation in this field are difficult to explain and will remain a subject of research for polymer chemists and polymer physicists for a fairly long time. Regarding the theory of catenane formation, see also Jacobson (1984). Polyoctenylenes have a bimodal molecular weight distribution curve. GPC diagrams (Fig. 1) have an oligomer fraction in the first maximum that can be extracted with isopropanol or acetone up to a molecular weight of about 1100. With increasing mean molecular weight, these
100-
I (M)/%.
iiw
-
80-
M, = J
60-
LO-
Molec. weight Fig. 1 Vestcnamer 8012, not extracted.
= 120000
x
6100 117m11g
701
Polyalkenylenes
9 IPA-Ext
1
x
[%l
X
15-
10-
5X
Fig. 2 Isopropanol extract versus viscosity number J of polyoctenylenes.
extraction values (Fig. 2) therefore also increase as a result of smaller amounts of modifier. The scatter of the values is due to different catalysts and reaction procedures. Another peculiarity of the polyalkenylenes is the way in which their crystallization properties vary with their microstructure. The polyoctenylenes obtained always possess double bonds that have cis and trans configurations. The cisltruns ratio is substantiallyconstant over all
-20 100
1
10 90
1
Z’O
$0 70
LO
50 50
60
710
30
80
9‘0 10
*
I d 0 YO t r a n s 0 % CIS
Fig. 3 Polyoctenylene melting temperature (T,,,) versus cis-rrms proportion.
Draxler
702
molecular weight fractions of a polyoctenylene. Both the cis double bonds and the truns double bonds (the latter to a much greater extent than the former) in sequential order form crystallites that have defined melting temperatures (T,ll).By extrapolation, it is possible to calculate the T,,, for both 100% cis- and 100% trans polyoctenylenes (Gianotti and Capizzi, 1970; Gianotti et al.. 1976); the relevant values are about 38 and 79"C, respectively. The lowest T,,, of - 8°C was determined by Giannotti for a polyoctenylene containing about 35% of the trans component and 65% of the cis component. Even this product had crystalline properties (Fig. 3). Figure 4 shows the DSC graph of a polyocetenylene containing 80% truns double bonds (Vestenamer 80 12). Norbornene polymers crystallize only when they contain a predominant amount of the cis component. This is due to a syndiotactic monomer arrangement in the chain, which, occurs only for the cis configuration of the double bond. Truns polynorbornene is amorphous (Ivin and Saegusa,1984). High crystallinity. as obtained in the case of polyoctenylene where the trum content is greater than 7.570, leads to high hardness and relatively high tensile strength of the uncrosslinked polymer at room temperature. Its stress-strain diagram resembles that of a thermoplastic, for example. a polyethylene. The molecular structure of the polyalkenylenes prepared by metathesis shows an exact head-to-tail addition of the monomers. Polyoctenylenes therefore have an ideal polymethylene structure with a double bond in the chain exactly after every eighth carbon atom, regardless of whether the polymer is linear or cyclic. This explains their high chain mobility and their low glass transition temperature (T,) (see Fig. 5). Depending on the amount of modifier, very different molecular weights (M,, ) can be obtained. A four-digit combination whose first two digits indicate the tmns content in percent and whose last two digits denote the J value (mL/g) times 10" as an integer has proven useful for designating the various polyoctenylenes. Depending on whether the t t m s content or the cis content predominates in the polyoctenylene. the substances are referred to as a TOR or a COR (trans- or cis-polyoctenylene rubber). HUIS AG in Marl (Germany) currently produces two relatively low polymer TOR grades (see Table 1). Both grades are supplied in the form of granules. They have an extremely low Mooney value (ML, , at 100°C = -5). The polyoctenylenes have a molecular structure that makes them relatively stable to UV radiation and oxygen (Draxler, 1983a). When stored in hot air at 100"C, both Vestenamer grades remain virtually unchanged after as long as 24 hours. even without the addition of a stabilizer. However, the commercial product always contains a small amount of a phenolic nonstaining stabilizer approved foruse with foodstuffs. The stability to hot air also implies high resistance to degradation (Draxler, 1980). Polynorbornenes have a different molecular structure.
.,
CH
CH =
CH 7
CH,-
/
CH2
As a result of the metathetic polymerization. C5 rings (1,3-cyclopentylene groups) remain in the polymer chain:
703
Polyalkenylenes
t 0
Fig. 4
L O 80
20
60 Tempero!ure,OC
DSCgraph for Vestenamer 8012.
J '
LOO [mLlgI
300 -
200-
100-
0
io
2'0
io
io
5'0
io
io
Fig. 5 Dependence of T, on viscosity number J and
io
e
do rbo sc
I T ~ O I Scontent
trans
In polyoctenylenes.
Table 1 Basic Parameters of VestenamerTypes Crystallinity Trans numbercontent Vestenamer 8012 Vestenamer 6213
T,,, ("C)
22°C
(%)
Viscosity J (mL/g)
80 62
120 130
55 28
30 8
(%)
M
\I
100.000 110,000
T, ("C) - 75 - 80
Gel content (%)
H Test N Adhesion level, 863 793 APR (max. = 9) 8.3 8.2 Water diffusion in the rubber (100% RH, 40°C) 0.85 0.85 H 2 0 in original, % Increase of H 2 0 (%) after 1 day 0.24 0.29 10 days 0.14 0.88 20 days 1.28 1.Ol 40 days 1.71 I .44 80 days 2.38 2.03 160 2.25 Humidity test on CPT samples, cord adhesion (N) of three cords 0 day 195 220 195 I O days 130 20 l 60 205 245 40 days 140 80 days 103 I35 Salt spray (5% NaCI) on CPT samples, cord adhesion (N) of three cords 220 0 day 1 95 I wcek 180 280 4 weeks 130 200 Monsanto fatigue test. Number of cycles to failure X 10" 68% elongation X 1358 3049 1.66 S 1.27 89% elongation I364 X 934 1.44 S 1.10 101% elongation X 132 1230 1.40 ss 1.15 126% elongation X 295 322 S 1.30 1.73 .' Steelcord No. 3472. 7 X 4 X 0.175 + I . h Equal amounts of antloxidants. formaldehyde and resorcm donors and accelerators Source: Courtesy of Bekaert. Zwevegem. Belgium.
Polyalkenylenes
721
There is also alarge variety of blend components forpolyoctenylenes in the hose sector, including polyblends based on NBR-PVC. It is envisaged that the improved covulcanization of various rubbers will be utilized in the hose sector. There are also good reasons for using polyoctenylenes in calendered rubber compounds, as shown in Sections 3 and 5. Other improvements in the quality of the articles include the easier penetration in the fabric during lamination of fabrics with hard rubber compounds, as a result of the reduction in viscosity (heavy tarpaulins, air beds, etc.). Vestenamer is also used for reducing the roll sticking of CR compounds. which strongly tend to scorchduringprewarming or on the calender. The inclusion of 5-1095 of TOR is sufficient for this purpose. Furthermore, hardening of CR and CSM as a result of hot-air aging is reduced by adding TOR. In no case does polyoctenylene have an adverse effect on the ozone resistance. The Vestenamergrades are therefore used in fabric coatings that areresistant to chemicals and are waterproof, in container linings, printing blankets, protective aprons. etc. In special cases, it may also be possible to provide better solutions to covulcanization problems in laminates. The same applies to the adhesion to polyester fabrics. When blended with TOR, ebonite linings of NR, SBR, or NBR exhibit higher elongation values and notched impact strengths as well as greater resistance to ozone and to chlorine. The Martens value (softening temperature) of ebonites is not reduced by TOR. These properties are also of interest for the production of ebonite moldings. The producers of rubber coatings for rollers for various industrial purposes began using TOR at a remarkably early stage. In particular, Vestenamer blends made it possible to process NBR grades much more uniformly and easily. The reduction in the rejection rate is particularly advantageous in the roll sector. For reasons relating to oil swelling, an upper limit of 15-20% of TOR is generally maintained in NBR blends. If necessary, Hycar 1312 may also be used in addition to Vestenamer. TOR is used in rubber coatings for textile and printing rollers, as well as on rollers for the paper and steel industries. An interesting possibility is the use of compounds containing larger amounts (up to40%) of TOR to provide rubber coatings on fairly small, very hard rollers by injection molding. This applies in particular to rice-husking rollers and to typewriter rollers. Because of the compression set at room temperature, only the less crystallineVestenamer 62 13 grade is suitable for the latter application.
7 . SUMMARY The metathetic polymerization of cyclic olefins led to new unsaturated polymers that are used as vulcanizable rubbers. In particular, the polymerization of cyclooctene can in principle be used to produce not only linear polymers but also cyclic ones and is very relevant in rubber technology. All knowledge to date indicates that the oligomeric macrocyclics also formed are to be regarded as extenders comparable with mineral oils but have no other technical function. The polymer fraction of the polyoctenylenes was long regarded as being purely linear. Since the postulation of the presence of a macrocyclic fraction within the polymer in 1980 on the basis of the interpretation of particular technological effects, initial positive analytical results in this very difficult and inaccessiblearea of polymerchemistryandphysicsare now also available. It is very probable that the polymeric macrocyclics contain substantial proportions of catenanes. With regard to rubber technology. the macrostructures and the molecular weight of the polyoctenylenes, as well as their crystallinity based on their microstructure, are of particular importance. These three polymer parameters are each associated with advantageous and dis-
722
Draxler
advantageous trends as far as the rubber processor is concerned. However, the possibilities of utilizing the advantageous effects with a suitable choice of polymer and suitable compounding are so fascinatingand varied that the polyoctenylenes are used even todayincontinuously increasing amounts in all areas of production in the rubber industry with the exception of latex processing. This applies in particular to tire production.
REFERENCES Arlie, J. P., Chauvin, Y., Commereuc, D., and Soufflet. J. P. (1974), Makrornol. Cltern. /75:861. Calderon, N., Chen, H. Y., and Scott, K. W. (1967a), Tetrahedron Lett., 3327. Calderon, N., Ofstead, E. A., and Judy, W. A. (1967b), J. Polyn~.Sci. A-/(5):2209. Dall’Asta, G. (1974). Rubber Chern. Tecknol. 4751 1. Dall’Asta, G., Mazzanti, G., Natta, G., and Porri, L. (l962), Makrornol. Chem. 56:224. de,Gennes, P. G. (1971). J. C h m . Phys. 55572. Draxler, A. (1980), presented at the ikt 80, Nuremberg. Reprint (1981) in Kautsch. Gunznti Kunst.st. 3 4 ( 3 ) : ._ 185. Reprint (1983) in Elastomerics //5(2):16. Draxler, A. ( 1 983a). Kuutsck. Gunmi Kunstst. ,36(12):1037. Draxler, A. (1983b), U.S. Pat. 4,551,392 (to Huls AG). Eleuterio, M. C. (1957), U.S. Pat. 3,074,918 (to du Pont). Gianotti, G., and Capizzi, A. (1970), Eur. Polwn. J. 6:743. Gianotti, G., Capizzi, A., and Del Giudice, L. (1976), Rubber Cllem. Techno/. 49:170. Graulich, W. (1974). Chirnia 28:534. Gr,aulich, W., Swodenk, W., and Theisen, D. (l972), Hydrocarbon Proc., 71 ... Gunther, P., Haas, F., Marwede, G., Nutzel, K., Oberkirch, W., Pampus, C. Schon, N., and Witte, J. (1970), J. AngeM,. Markrorn. Cltern. /4:87. Haas, P,,Nutzel, K., Pampus, G. and Theisen, D. (1970), Rubber Cltern. Technol. 43: 11 16. Herisson, J. L., and Chauvin, Y. (1970), Makronrol. Cllern. /4/:16l. Hocker, H., Reimann, W., Reif, L., and Riebel, K. (1980). J. Mol. Cutcrl. 8:191. Holtrup, W., and Hammel, R. (1985), Che~n.Z. 7/8:267. Holtrup, W., Kupper, W., Meckstroth, W., Meyer, H. H., and Nordsiek, K. H.(1974), Der Lichthogen /75:86 1. Ivin, K. J., Lavertin, D. T., and Rooney, J. (1977). Mukromol. Chern. 178:1545. Ivin, K. J., and Saegusa, T. (l984), Ring Opening Polymerization, Elsevier, New York. Jacobson, H. (1984), Mucrornolecules 17705. Kupper, F. W., and Streck, R. (1973). J. Orgunomet. Chern. 55:C75. Kupper, F. W., and Streck, R. (1974), Makrornol. Chetn. /75:2005. Lohmar, J. (1985). presented at the ikt 85, Stuttgart. Michelotti, F. W., and Keaveney, W. P. (1965), J. Polyn. Sci. A3:895. Natta, G.. Dall’Asta, G., and Mazzanti, G. (1964), Angew. Chem 3:723. Natta, G.. Dall’Asta, G., Bassi, J., and Carella, G. (1966). Mukrond. Cltern. 91237. Ofstead, E. A., and Calderon, N. (1972), Mukrornol. Chem. 15421. Patton, P. A., and McCarthy, T. J. (1985), presented at the ACS Meeting, Miami, Florida. Porri, L., Rossi, R., Diversi, P,, and Lucherini, A. (1974), Makronwl. Cltern. /75:3097. Reif, L. (1978), diploma thesis, University of Mainz, Germany. Reif, L. (1983), thesis, University of Bayreuth, Germany. Riebel, K. (l976), thesis, University of Mainz, Germany. Scott, K. W., Calderon, N., Ofstead, E. A., Judy, W. A., and Ward, J. P. (1969), A h . Chem. Ser. 91:399. Streck, R. (1979), cl ten^ Z. 99397. Truett, W. L., Johnson, D. R., Robinson, J. M,, and Montague, B. A. (1960) J. Am. Cltenl. Soc. 82:2337. Usami, T., Gotoh, Y., and Takayama, S. (1985), Eur. Po/ynz. J. 21:885. Wasserman, E., Ben Efraim, D. A., and Wolovsky, R. (l970), J. Am. Chem. Soc. 92:2132, 3286. Witte, J., and Hoffmann, M. (1978), Makrornol. Chern. /79:641.
Polytetrahydrofuran P. Dreyfuss Consultant, Midland, Michigan
1. INTRODUCTION Polytetrahydrofuran (PTHF) is an elastomer with the repeat unit [--CHlCH2CHlCH20-],,. It cannot be prepared usefully by the free radical methods commonly used to prepare many commercial polymers. Instead, cationic ring-opening polymerization methods must be used (Dreyfuss, 1982; Inoue and Aida, 1984; Penczek et al., 1985; Dreyfuss et al., 1989; Pruckmayr et al., 1996). PTHF wasfirstprepared in the late 1930s by Meelwein (1 939) and coworkers, who continued their polymerization studies for several decades thereafter (Meerwein et al., 1960). Worldwide interest in PTHF polymers and copolymers started after WorldWar 11. It was demonstrated that the high molecular weight material can have properties comparable to those of a good general-purpose elastomer, but its production is too expensive to competein such applications. (In 1996, for example, general-purpose rubbers were selling for about $1 per kilogram, while PTHF cost $3.50-4.20 per kilogram.) Fortuitously,the nature of the polymerization makes it easy to produce PTHF in the form of low molecular weight glycols, which are well suited for the preparation of polyurethanes and polyester thermoplastic elastomers with outstanding enough properties to justify the relatively high cost of the specialty elastomer. In these applications PTHF is valued as a precursor leading to products with outstanding hydrolytic stability at elevated temperatures, high fungal resistance, superior abrasion resistance, excellent resiliency, and very desirable dynamic properties. In 1996 PTHF was produced as a glycol at a rate in excess of I10,OOO metric tons per year worldwide. Current producers, in order of overall world production capacity. include du Pont,BASF, Q 0 Chemicals (asubsidiary of Great Lakes Chemicals), Hodagaya, Mitsubishi, Sanyo, and Asahi. Both du Pont and BASF announced plans for construction of new PTHF facilities to come on stream in 1997 or shortly thereafter (Pruckmayr, et al. 1996). Polytetrahydrofuran is the name mostoftenused,and it will be usedthroughoutthis chapter. Alternative names used elsewhere include poly(tetramethy1ene oxide), polyoxytetramethylene, and polytetrahydrofurane. Chenzicul Abstrucrs lists PTHF under the entries “furan, tetrahydro, polymer, homopolymer,” (C4H,0), (24979-97-3), and “poly(oxy-l,4-butanediyl),” (C4HxO),, (26913-43-9). Polymers with known end groups have still other entries. For example, the a,w-glycolis listedas “poly(oxy- 1,bbutanediyl,a-hydro-o-hydroxy,”(C4HxO),,H10 (25 190-06-1). Alternative names for the glycol are poly(tetramethy1ene ether) glycol, poly( 1,4723
Dreyfuss
724
oxybutylene)glycol, and a-hydro-w-hydroxypoly(oxytetramethy1ene).All names refer to PTHF with the same repeat unit and the same linear backbone.
2.
POLYMER PREPARATION
The synthesis of PTHF has been exhaustively reviewed (e.g., Saegusa and Kobayashi, 1973; Dreyfuss, 1982; Inoue and Aida, 1984; Pruckmayr et al. 1996). Only the most salient features of its preparation will be discussed here. Readers are referred to the earlier reviews and the references contained therein for further detail. PTHF is prepared from tetrahydrofuran (THF), the only important source of the polymer, by ring-opening polymerization of its tertiary oxoniumion [Eq. (1 )l. The first stepin the polymerization is thegeneration of a stable tertiaryoxoniumion.Pure,dryreagents in a dry, inert atmosphere or high-vacuum conditions are essential for a controlled polymerization (Plesch, 1963; Dreyfuss, 1982).
initiation)
THF THF
C
+0-R X'
3
>> ROfCH2CH2CH2CH2)0+ a '
M
X'
4
/
l
depropa .gation depropagation propagation
propagation
,-"'\
R0
2.1
Counterions
A complex counterion must be associated with the oxoniumion to prevent its immediatecollapse. With a simple anion like Cl-, for example, an alkyl halide and an alkyl ether are formed:
725
Polytetrahydrofuran
C
>
+OR Cl’
ROCH~CH~CHZCH~C~
The most stable counterions are complex ions such as PF6-, AsF6-, and SbF6-. SbC16supports polymerization,butthechlorineligandcanreactwith the oxonium ion,liberating SbCIS, which is able to reinitiate THF polymerization. BF4- also supports polymerization, but it, too, can react slowly with THF. Since BF3 alone, exceptin very large concentrations, cannot reinitiate THF polymerization, termination of polymerization occurs. S03CF3-, S03F-,C104-, and Nafion Resin ion (NfS03-) also stabilize oxonium ions very effectively, but polymerization rates are reduced when these counterions are used because the growingoxonium ion equilibrates with a nearly unreactive ester:
R-O(”J
2.2
CF3S03’
RO( CH2)40S02CF3
Temperature
The temperature of polymerization must also be carefully selected. Rates of initiation and propagation become impractically slow much below 0°C. In addition, the polymerization of THF is an equilibrium process with a significant rate of depropagation. Conversion to polymer falls off rapidly as the temperature approaches 83 ? 2 ° C the ceiling temperature for THFpolymerization (Dreyfuss and Dreyfuss, 1966; Dreyfuss, 1982).
2.3 Kinetics The details of the kinetics of the polymerization of THF havebeen worked out (Penczek, 1979; Dreyfuss, 1982). The principal conclusions are With “stable” complex anions, such as PF6-, SbF6-, and AsF(,-, the polymerization is “living” under normal polymerization conditions. This means that the polymerizations are free from significant spontaneous termination and transfer reactions, polymers with narrow molecular weight distributions can be prepared, and sequential block copolymerizations are possible. When initiation is fast, kinetics of polymerization in bulk can be closely approximated by the expression:
where k, is the specific rate constant of propagation, t is time, I, is the initiator concentration at t = 0, and [M,,], [M,], and [M,] are the monomer concentrations at time = 0, equilibrium, and t, respectively. k,, at 25°C is about 0.046 L/mol/sec in the presence of stable “complex” anions in 8.0 M monomer in CC14 solvent. Withless stable conlplex counterions like SbCI6- or BF4-, atmosttemperaturesthe influence of transfer and termination reactions, respectively, must be taken into account (Vofsi and Tobolsky, 1965). THF does not behave ideally in solution,and the equilibrium monomer concentration varies with both solvent and temperature. Kinetics of THF polymerization in solution fit Eq. (4) provided the equilibrium monomer concentration is determined for the conditions used.
726
Dreyfuss
With anions that can form esters from the growing oxonium ions [Eq. (3)], the kinetics of propagation are dominated by the rate of propagation of the macro ions. With any given anion, the proportion of macro ion compared to macro esters varies with the solvent-monomer mixture and must be determined independently before a kinetic analysis can be made. The macro esters can be considered to be in a state of “temporary termination.” When the proportion of macro ions is known and initiation is sufficiently fast, Eq. (4) is satisfied.
2.4
Initiation
The initiator determines the head group of a THF polymer, except in cases where the head group is capable of furtherreaction with another species in thesystem(e.g.,hydroxyl or tertiary oxonium ion head groups). The chemistry of the initiation of THF polymerization has been described in detail (Dreyfuss 1982, 1983). Several methods of generation to tertiary oxonium ion are known, including the following: 1. Direct alkylation or acylation of the oxygen by exchange or addition 2 . In situ generation of the oxonium ion from a variety of combinations suchas epichlorohydrin (ECH) or reactive halide and Lewis acid (e.g., ECH + BF3, CH3CH2CI FeC13), or reactive halide and metal salt (e.g., CH3COCI AgSbF(,), or sometimes from Lewis acid alone (e.g., PFS but not BF3) 3. Direct addition of strong protonic acid (e.g., HS03F) or of in situ generated strong protonic acid (by e.g.,PhN2+PF6-, Ph2Br+PF(,-)followed by addition to THFto form an intermediate oxonium ion, which is slowly converted to a tertiary oxonium ion in the next reaction with THF
+
2.5
+
Transfer
Transfer to counterion has already been described. Transfer reactions can also occur with added small molecules such as ethers, formals, anhydrides, and acid chlorides. Reaction with small molecules is an effective way to control polymer molecular weight. For example, if a strong acidsuch as NfS03H is used in combinationwithacetic anhydride, adiacetate of PTHF, CH3CO2CH?CH~CH~CH?-(OCH7CH2CH~CH~),,”OCH,CH~CH~CH7_O2CCH3, is formed. Subsequent hydrolysis or alcoholysis of such a diacetate is oneway of producing a low molecular weight PTHF glycol (du Pont, 1979). It is a much more reliable method than initiation with strong protonic acid followed by termination with water, because hydroxyl end groups that are present during polymerization can interact with the growing oxonium ion to form unwanted high molecular weight PTHF (Pruckmayr and Wu, 1978b). The oxygen atomsin the polymer backbone can also react with the growing oxonium ion. The principal result of such reactions is a much more rapid progression toward a normal molecular weight distribution than would be expected if only propagation and depropagation reactions occurred. A minor result is the formation of up to 3% of product in the form of macrocyclics (McKenna et al., 1977; Pruckmayr and Wu, 1978a). 2.6
Termination
It is necessary to terminate most THF polymerizations in order to produce a stable product with controlled end groups. (Otherwise the PTHF might depolymerize on drying at elevated temperatures and/or end groups will form from something adventitiously present.) The beauty of being able to terminate PTHF polymerizations at will is that almost any desired end group
Polytetrahydrofuran
727
can be introduced. As stated above, termination with water leadsto hydroxyl end groups;termination with polystyryl anion gives block copolymer: and termination with any number of UVor NMR-active groups (e.g.. phenoxide, triphenylphosphine, I3C-labeled methoxide) leads to products in which the number of active end groups at any time in the polymerization can be quantitatively determined (Dreyfuss. 1982).
2.7
Commercial Polymers
Only low molecular weightPTHF in the form of either glycols or glycol derivatives (isocyanates) is produced on a large scale commercially. The major producers were listed in the introduction. Du Pont, the largest producer, markets ~ I Y C O I worldwide S under the tradename Terethane,while BASF and Q 0 Chemicals market them as PolyPTHF and Polymeg, respectively. Commercial PTHF isocyanates are called Adiprenes by du Pont and Vibrathanes by Uniroyal. 3M marketed PTHF amines for a time, but the product did nothave a large market and has been discontinued. Several large-scale processes for the preparation of the glycols initiate the polymerization of THF with HS03F at low temperatures. The exothermic polymerization that ensues results in PTHF chains with sulfate ester end groups (Pruckmayr and Wu, 1978b):
THF
FSOj
In the subsequent product workup, the sulfates are hydrolyzed. Water extraction removes both the acid and most water-soluble short polyether chains, and the polydispersity is reduced from the theoreticalvalue of 2 to 1.6 or less, depending on theoverallmolecularweight. After neutralization and vacuum-drying, the product is filtered hot and loaded into drums or tank cars under nitrogen. The glycols are hygroscopic and must be stored in tightly closed containers and subsequently transferred under nitrogen (Pruckmayr et al., 1996). (Moisture present in the glycols would participate in later reactions and could detrimentally alter the properties of the polyurethanes and polyesters prepared from the glycols at a later stage.)PTHF is sensitive to oxidation, and 300- 1000 ppm of 2.6-di-tert-4-hydroxytolueneis normally added commercially to prevent peroxide formation. THF polymerization can be initiated by many strongly acidic substances. but not many of them produce the required bifunctional polyether glycol with a minimum of by-products. Many polymerization processes have been patented, but in 1996 only a few of them appear to be developed or under development forcommercial production (Pruckmayr et al., 1996).Examples of such large-scale processes include acetic anhydride with either acid montmorrillonite clay or strongperfluorosulfonic acid resin initiators.The product in these processes is a diacetate, which must be subjected to alkaline hydrolysis or hydrogenolysis. Another exampleis the Asahi Chemical Industries process in which PTHF glycol is produced directly by using heteropoly acids such as phosphomolybdic or phosphotungstic acid in the presence of a small amount of water.
3.
PHYSICAL PROPERTIES
Some typical properties of PTHF and its glycol are given in Tables 1 and 2 , respectively. More detailed discussion of the properties can be found in Dreyfuss ( I 982). in Dreyfuss et al. (1994), and in the references therein, especially in the product literature.
728
Dreyfuss
Table 1 Selected Properties of PTHF Property Melting temperature, "C Glass transition temperature, "C Density Amorphous (25"C), g/cm3 Crystalline 300% modulus, Mpa High to low mol. wt. Tensile strength, MPa High mol. wt. High to low mol. wt. Cured Elongation at break, % High mol. wt. High to low mol. wt. Cured Modulus of elasticity, Mpa Shore A hardness Thermal expansion coefficient, deg" Compressibility, kPa" Internal pressure, Mpa AC,, (at Tg), J/(mol . K ) Rapidly cooled Annealed Coefficient of expansion (dV,/dT), cm3/(& . K) Refractive index (at 20°C) dddc, mL/g (THF, A = 546 km)' Dielectric constant k, (at 25°C) Solubility parameter, (J/cm')"'
Value
43, 58-60' - 86 0.975 1.07- 1.08 1.6- 14.3" 29.0 27.6-4 1.4~' 16.8-38.3" 820 300-600;' 400-740h 97.0 95 4-7 x 1 0 - 4 4-10 X IO" 28 1 19.4 15.8 7.3 x 10-4 1.48 0.064 5.0 17.3-17.6
'' Depends o n molecular welght. h c
Depends on curlng system. dn/dc values for other solvents can be found In Dreyfuss (1982). Dreyfuss and
Dreyfuss (1982) and in the onglnal references contamed thereln. Source; Dreyfuss, 1982.
Infrared, Raman, NMR, and x-ray spectroscopy (Bunn and Holmes, 1958; Imada et al., 1965; Vainstein etal., 1969) have established that PTHF isa linear polymer with a planar zigzag conformation. Its melting temperature is above room temperature but well below the boiling point of water, and polymers of all molecular weights crystallize readily at or below room temperature. The glass transition temperature of PTHF is low, - 86°C. High molecular weight (>100,000 &/mol) polymers have properties comparable to those of general purpose rubbers. while very high molecular weight (> 1,000.000 g/mol) polymer behaves like a plastic. When molten, these high molecular weight polymers have good tack and good green strength. When crystallized, low molecular weight (650-5000 g/mol) PTHF is a tack-free wax. Manufacturers of the glycol settheir own specifications for hydroxyl number, melt viscosity, water content,color, melting point, etc. The PTHFglycols are strictly difunctional andallow
rathane
729
Polytetrahydrofuran Table 2 Selected Properties of Terathane Polyether Glycols
Property Hydroxyl Viscosity at 40"C, rnPa . S (1450 = cP) 28-40 Melting point, "C Color, APHA Refractive index, 1 1 , ~ ~ ' 1.464 Heat of fusion. kJ/kg Water content, wt% Ash, wt% Iron, pprn Peroxide, as HzOl, ppm
Flash point, TOC, "C
107-118
9501.465
260-320 25-33 < 10 1,46390.4 OR
l "CH
A $ - C H Z
OR
XLPE
+ CH30H
Catalytic
condensation OR l
-Ssi-O--Si-Cfi
2 1
2-Si(OR)20H
I
2
OR CH -2
"1I2O
< 'S
>
3. COMPOUNDINGAND MIXING OF POLYETHYLENE In a review article on compounding crosslinked polyethylene, Carlson ( 1960) pointed out that low-density polyethylene polymers and copolymers have the lowest crystallization temperature and thus they are readily processed in rubber-processing equipment. In early 1940, laboratory experiments on crosslinking polyethylene with peroxides had been carried out: however. a practical method was developed only in the mid-1950s. A. Gilbert and F. Precopio of General Electric successfullyused dicumyl peroxide to crosslink polyethylene. Improved heat stability and better volatility of dicumyl peroxide enabled it to be dispersed into polyethylene above its melting temperature in a practical processing operation such as mixing, molding, and extrusion without decomposition of peroxide. The fabricated product can be successfully cured at temperatures above the peroxide decomposition temperature to fully crosslink the polymer (Martens, 1978). Low-densitypolyethylene,ethylene vinyl acetate (EVA), and ethylene ethylacrylate (EEA) copolymers are the base polymersused most extensively i n compounding XLPE products. e.g., wire and cable insulations. Polyethylene (homopolymer) exhibitssuperior electrical properties and is used for higher voltage cables, e.g.. 15 kV and above. However,its molecular weight and melt index must be selected for efficient crosslinking. Lower-voltage cables use copolymers. In XLPE, the choice of base polymer, antioxidant. fillers. and crosslinking agents can have a great effect on end-product properties. The selection of compounding ingredients is very importantandmakes it possible to design a compound with specificproperties for theend product.
740
Dave
3.1 Crosslinking Agents Processing and propertiesof XLPE compoundsdepend on the type and level of peroxide selected. Some peroxides offer processing advantages: for example, higher processing temperatures can be used without decomposing the peroxide during mixing and other processing operations such as molding and extrusion. The level of peroxide affects the degree of crosslinking. In general, the higher the peroxide level, the greater the crosslink density, provided it is cured at the same temperature, pressure, and time. Reactive monomers are used as coagents to improve the rate and degree of crosslinking. Commonly used additives are triallyl cyanurate (TAC), triallyl isocyanurate (TAIC), and trimethylolpropane trimethacrylate (TMPTMA), and a wide selection of similar acrylic monomers. For crosslinking polyethylene by electron beam irradiation or gamma rays. It is not necessary to use free radical initiators. However, for economic reasons and faster processing. free radical initiators, e.g., TAC, TAIC and TMPTMA, are used. 3.2
Antioxidants
Antioxidants are important additivesin crosslinked polyethylene. These additives provide stabilization against oxidative degradation, especially at elevated temperatures. Selection of antioxidant type and level is very important in designing compounds, anda balance between antioxidant and crosslinking agent must be made to achieve the optimum processing and product characteristics. Without antioxidants, the polymer would rapidly degrade, leading to embrittlement of the compound. Antioxidants have a significant effect on crosslinking efficiency. Since saturated polymers such as polyethylene are more difficult to crosslink than unsaturated rubbers, interference by reactive ingredients is of greater importance. Therefore, it is essential to use antioxidants that are relatively less reactive as free radical acceptors and preferably ones that are eitherneutral or alkaline. 3.3
Fillers
Polyethylene has a much higher tolerance for particulate fillers when it is crosslinked than in the uncrosslinked state. This property allows the use of fillers for improvement of mechanical, chemical, and,in some cases,electrical properties of XLPE products. Relatively, nonreinforcing fillers have been found useful in XLPE. In most applications for XLPE, good extrudability and electrical properties are required. For black filler formulations, nonacidic medium thermal black or furnace blacks are used, especiallyin peroxide cures, where they do not interfere with peroxide crosslinking. In wire and cable applications for black insulations, low loading of carbon black is preferred, since the dielectric loss is directlyproportional to carbon blackloading.Black insulations are used only for 600 V to 1 kV cables. This voltage range is usually unshielded, and carbon black provides good protection from UV radiation. Semiconducting XLPE wire and cable shielding conlpounds use high-structure blacks, e.g., acetylene black or furnace black. Such blacks provide the necessary carbon-to-carbon linkage within the polymer matrixto impart semiconductive properties. Inorganic mineral fillers such as clay, calciumcarbonate, silica, talcs, and hydrated alumina have been used in combination or alone and with carbon black to improve the properties of XLPE. The development of ultimate product properties depends on the level and type of filler. Such fillers improve thermomechanical properties, e.g., deformation resistance and cut through resistance at elevated temperatures. Mineral fillers improve dielectric strengthin cables. Considerable work has been done on nonblack pigments for use with XLPE. Peroxide curing in the
Crosslinked Polyethylene
741
presence of mineral fillers is more difficult than with carbon black. Reinforcement of the polymer can be enhanced by modifying the filler/polymer interface through the use of filler treatments that result in a modification of the bond between filler and polymer. Silicone compoundshaving alkoxysilicone groups and a reactive organic group, e.g., vinyl structure, may react with the polymer during crosslinking at the filler surface. Thisresults in a strongchemical bond across the mineral filler/polymer interface.This surface modification rendersthe filler surface hydrophobic, resulting in lowmoisturepickupandstable wet electricalpropertiesundervoltagestress at elevated temperature. Crosslinkable polyethylene formulations may be readily handled in conventional rubberprocessing equipment. The mixing of formulations containing peroxides is primarily done in two steps. The first step is the thorough mixing and dispersion of all the ingredients except the peroxide. This is done at temperatures well abovethepolymermeltingpointandperoxide decomposition temperatures. The second step is the mixing and dispersion of peroxide in the compound. This is done by keeping the mixing temperature below the peroxide (dicumyl peroxide) decomposition temperature but above the polymer melt temperature (230-270°F). Mixing of XLPE formulations is carried out in an internal or intensive-type mixer. e.g., a Banbury mixer. In order to achieve uniform dispersion of all ingredients in the finished compound, it is essential that the mixing equipment be equipped withappropriateandaccurate temperature control instrumentation,e.g.. process water or steam inlet-outlet temperature monitor and stock temperature-monitoring device. The most commonly used process for mixing XLPE compounds is shown in the flow diagram of Figure 1. It is essential that the mixing temperature be controlled so as not to decompose the peroxide or other additives during the mixing and forming process. In the case of radiation-crosslinkable compounds, a one-pass mix can be used. since such formulations do not requireperoxides. However, quite often reactiveliquid nlonorners are addedto such compounds, and it is necessary to keep the mixing temperature below the flash point temperature of the monomers. Mixing temperature range for radiation-crosslinkable polyethylene compounds is 280-32OoF, depending on the properties of the monomers used.
L L P0LYME.S
2?P?
0>!
TWO R C L L MILL
Y ? i ? +F F?
e(
CCQ!L.
3 Ic E?.
31CE3
-Jc COr.!?nU:;C
Fig. 1 Proccss flow diagram for compounding and mixing XLPE compounds (*e.g.. a Banbury mixer, ;I registered tradcmnrk of Farrcll Corp.).
Banbury being
Dave
742
Moisture-cured polyethylene (Currat, 1983; Bullen. 1983) compounds require the blending of two components prior to forming the product. Presently, two basic processes are available for moisture-cured crosslinked polyethylene products. Sioplas is a two-step process in which two masterbatch compounds are prepared separately by mixing in extruders or internal mixers. This two-part system is then melt blended and extruded in one process to manufacture the end product. Monosil, on the other hand, is a one step process in which all the ingredients are mixed in an extruder and the product is extruded. A detailed description of these two processes is given in Section 4.4.
4. 4.1
PROCESSING Product-Forming Processes
Major cases of XLPE compounds are in wire and cable industry applications. Almost 85-90% of products made from XLPE compounds are extruded products. Other types of forming processes include injection molding, compression molding. ratational molding, etc.
4.2
Continuous Vulcanization
All applications of wire and cable insulations, sheaths, tapes. tubes, etc., are extruded products. This process involves melt extrusion of XLPE compoundsbelow the decomposition temperature of crosslinking additives, mainly dicumyl peroxide. In the case of peroxide-cured compounds, curing of the product is done in-line under high-pressure steam in a process called continuous vulcanization (CV). In this process the product is directly extruded from the forming die into a high steam pressure chamber or tube. A process flow diagram for steam CV is presented in Figure 2. Several other processes of continuous vulcanization have also been made commercial. In some. steam is replaced by unpressurized or pressurized liquid heat transfer medium, e.g., molten salts (eutectic mixture) or high-temperature heat transfer water-soluble oils. e.g., Ucon oils. In another case, the steam or liquid is replaced by hot dry nitrogen gas under pressure. All of these processes are very efficient in producing a variety of large-diameter heavy-duty cables where uniform crosslinked insulation or jacket is required. Since the curing of peroxide-based XLPE is both time and temperature dependent. the heat transfer medium under pressure ensures that the XLPE insulation or jacket will be cured uniformly throughout the cross section of the cable. The rate or line speed will depend on the size of the cable.
Fig. 2 Centinuous vulcanization of XLPE wirc and cable coatings.
743
Crosslinked Polyethylene
4.3
Radiation Crosslinking
In the case of crosslinking by electron beam irradiation. the extrusion and crosslinkingor curing are done in two separate steps. Product is first extruded or formed by any one of the forming processes and is then crosslinked in a separate operation during which it is exposed to electron beam irradiation. Typically, extruded products like wire and cable, tubings, tapes and sheaths are extruded in long lengths. In a separate setup, these extruded products are irradiated by being passed under an electron beam, often several times. to achieve the required crosslink density. A typical set up for electron beam irradiation system is shown in Figure 3.
V-
CABLE & PIPE
W I R E & TUBING
PLASTIC F I L M
Fig. 3 Electron beamcrosslinking of XLPE products: wire andcable,tubing, stock, web, etc. (From Becker et al., 1979).
plasticand rubber flat
744
Dave
The development of crosslink density in the radiation-crosslinked products depends the following processing parameters:
on
Dose Beam energy Beam current Beam power, i.e., beam energy X beam current Material characteristics controlling the degree of crosslinking are Material response to irradiation Material density Material thickness The relationship between thickness in thousands and megavolts of power determines the useful electron beam penetration characteristics. This relationship is graphed in Figure 4.
Fig. 4
Electron beam penetration curve. (From Morganstern,
1974.)
745
Crosslinked Polyethylene
4.4
Moisture Cure Processes
There are two processes available for moisture curing of XLPE compounds: the Sioplas process and the Monosil process. The chemistry of crosslinking in both cases is identical. They differ only in the process itself.
Sioplas Process The Sioplas process is executed in two stages (Fig. 5): 1. Polyethylene is mixed with an organic silicone derivative called silane in the presence of an antioxidant and a peroxide. Chemical interactionoccurs at some elevated temper-
I
I
LPPE PS30XIPJt
I
I
* CROSSLINKABLE CZAFTED CCI.I?CIU.'i!l
cA:,:.lT
::/ 3
S I O P L A SM O I S T . C U R A B L SH I X T U R S (BLEND)
I
"
100 1 10
6 3 0.5 170 85 420 420 350 15
35 55 -2 10 27
IIR
VMQ
CSM
I40 12 130 150 50 50 7 -5 0.5 135 80 400 400 350 30 60 35 - 2.5 25 7
240 20 120 250 10 10 0.5 2 - 0.2 90 15 300 275 150 10 3 7 - 0.5 5 0.5
260 4 40 150 4 4
85 4 65 120 40 45 1.2 5 0.5 85 150 600 350 430 70 80 20 - 0.5 4 0.5
I
8
v
250 60 200 250
SBR
40 -
30O -80
I
I
-20 BrittlenessTemperature -40
I
0
('C )
Test Conditions: *Fuel Cfor 48 hours at 40c Fig. 10 Volume change vs. brittleness temperature:
(e),
HNBR; (+), NBR; (@), ECO.
1 15 1
270 150 300 300 240 7 10 25 30 2 0.5
Hayashi
804
Methanol /Fuel C ratio (%) Fig. 11 Alcohol contained fuel resistance of FKM (+), FMVQ (B), and NBR (A).
has improved gas impermeability,as shown inTable 18 (1,34,60). Table 19 (61) shows solvent impermeability of various rubbers. Figure 13 shows strain-stress curves of NBR measured at various temperatures. At a temperature under the glass transition, the curve is like a resinthat has a yield point.At temperatures over the glass transition, it has a typical rubber-like S curve. At hightemperatures, tensile strength and elongation are low (1, 62).
Fig. 12 Lubricating oil additive resistance tensile strength at break after 168 hours immersed in ASTM #2 oil, including various additives, at 150°C.
nd
805
Nitrile Table 17 Lubricating Oil Additives Used in the Evaluation No. A- 1 B- 1
Type of additive -
c-1 2 3 4 D- 1 2 3 4 E- 1 2 F- 1 G- 1 2 H- 1 2 3 4 5 I- 1 2 3 4
Dispersant
Detergent
Antioxidant Viscosity index improver Antiwear agent
Extreme pressure additive
Additive package
Main chemical composition
Concentration"
Original tensile strength ASTM #2 oil with no additive Polyalkenyl succinimide Polyalkenyl succinimideborate Polyalkenyl succinic ester Polyalkenyl succinimide/succinic ester Calcium sulfonate-Basicity 24 Calcium sulfonate-Basicity 300 Magnesium sulfonate-Basicity 400 Calcium phenate-Basicity 205 Primary dialkyl zinc dithiophosphate Secondary dialkyl zinc dithiophosphate Polyalkyl methacrylate Olefin sulfide for gear oil Olefin sulfide (Technical grade) Dialkyl phosphoric ester Dialkyl phosphoric ester Zinc dibutyl dithiocarbamate Molybdenum compound Lead naphthenate Package #l for gear oil Package #2 for gear oil Package for automobile engine oil Package for ATF (Dexiron IID) oil
-
10 IO 10 10 IO IO IO 10 5 5 10
IO IO 10
2 1
0.3 IO 10 IO 10
10
g/100 cc of ASTM #L oil.
Table 18 Gas Permeability" ofNBR and Other Rubbers Rubber
Temp. ("C)
NR BR SBR NBR 27 % NBR 39% CR
IIR
T I O cm'/sec/atm
25 50 25 50 25 50 25 50 25 50 25 50 25 50 25
H-2
0-2
N-2
37 91 32 77 31 74 12 34 5.4 17
18 47 14 36 13 35 2.9 10.5 0.7
6.1 19
10
28 5.5 17 1.2
3.5 3 IO
1 4 6.2
4.9 14
4.8 14
0.8 3.6 0.18 1.1
0.9 3.5 0.25 1.3
CO-2 IO0 22 1 105 200 94 195 23 68 5 .l 22 19 56 3.9 14 2.4
C H 4
He
24 52 -
16 43 2.4 10 -
2.5 IO -
17 42 9.3 23 5.2 14 6 -
6.4 17
Hayashi
806
Table 19 Effect of Temperature on Various Rubbers Solubility of liquid in rubber (rnLlrnL) Test liquid ("C)
and temp. Permeability (kghm')
Specific permeability days (kg.m/h.rn')14
1 day
STYRENE RUBBER Di-isobutylene 25.0 54.4 82.2 SR-6 25.0 54.4 82.2 Methyl ethyl ketone 25 .O 54.4 82.2 Benzene 25.0 54.4 82.2 Ethyl acetate 25.0 54.4 82.2 Methyl alcohol 25.0 54.4 82.2 Carbon tetrachloride 25.0 54.4 82.2
9.12E 1.36E 1.60E
+ 00
+ 01 + 01 3.51E + 01 5.59E + 01 7.18E + 01 l.lOE + 01 1.95E + 01 3.12E 01
+
6.98E - 01 1.00E + 00 1.17E + 00
I .42 1S O 1.67
1.38 1.48 2.28
+ 00
+ 00 + 00
2.40 2.37 2.47
2.37 2.40 3.82
8.15E - 01 1.36E + 00 2.24E + 00
0.88 0.99 1.10
0.85 0.98 1.19
2.568 3.97E 5.458
5.358 9.04E 1.02E
+ 01
3.938 6.04E 7.71E
+ 00
+ 00 + 00
3.05 2.9 1 2.89
2.88 3.02 3.63
1.21E 2.538 3S7E
+ 01
+ 01
+ 01
9.38E - 01 1.95E + 00 2.72E + 00
0.93 1.02 1.11
0.88 1.oo 1.29
5.85E - 02 6.47E - 01 1.30E 00
4.31E - 03 4.62E - 02 9.73E - 02
0.02 0.05
0.05 0.16
-
-
+ 00 + 00 + 00
2.36 3.33 3.36
3.20 3.37 4.73
+ 01 + 02
+
3.34E 5.258 7.45E
+ 01 + 01
+ 01
2.38E 3.97E 5.70E
PARACRIL 18 Di-isobutylene 25.0 54.4 82.2 SR-6 25.0 54.4 82.2 Methyl ethyl ketone 25.0 54.4 82.2 Benzene 25.0 54.4 82.2
2.24E - 01 9.31E - 01 2.61E + 00
2.398 - 02 6.79E - 02 1.96E - 01
0.30 0.43 0.49
0.34 0.69 0.44
7.12E 1.69E 2.62E
+ 00 + 01 + 01
5.40E - 01 1.22E + 00 1.91E + 00
1.07 I .09 1.20
1.03
3.75E 5.73E 8.30E
+ 01 + 01 + 01
6.888 1.09E 1.52E
+ 00 + 01 + 01
2.32 1.89 2.02
2.04 2.07 2.74
4.41E 5.73E 1.02E
+ 01 + 01
8.01E 1.22E 1.84E
+ 00 + 01
2.43 2.40 2.36
2.41 2.44 2.64
+ 02
+ 01
?
1.29
kghm’)
807
Nitrile and Hydrogenated Nitrile Rubber Table 19 Continued
Solubility of liquid in rubber (mL/mL) Test liquid and temp. (“C) Ethyl acetate 25.0 54.4 82.2 Methyl alcohol 25.0 54.4 82.2 Carbon tetrachloride 25.0 54.4 82.2
Permeability
2.34E 3.62E 4.488
+ 01 + 01 + 01
2.668 - 01 1.96E 00 7.97E 00
+ + 1.08E + 01 2.03E + 01 2.63E + 01
Specific permeability (kg,m/h,m’)
1 day
14 days
1.65 1S 9 I .60
1.62 1.65 1.81
4.978 - 02 3.58E - 01 1.43E 00
0.14 0.18
0.14 0.15
-
-
+ 00 + 00 + 00
1.72 1.65 1.63
l .66 1.64 1.86
4.49E 6.78E 8.46E
+ 00 + 00 + 00 +
2.04E 3.94E 4.84E
PARACRIL 35 Di-isobutylene 25.0 54.4 82.2 SR-6 25.0 54.4 82.2 Methyl ethyl ketone 25.0 54.4 82.2 Benzene 25.0 54.4 82.2 Ethyl acetate 25.0 54.4 82.2 Methyl alcohol 25.0 54.4 82.2 Carbon tetrachloride 25.0 54.4 82.2
Negligible 6.48E - 02 9.66E - 01
Negligible 1.29E - 02 1.71E - 01
0.07 0.22 0.27
0.18 0.20 0.23
2.58E 4.63E 6.59E
+ 00 + 00 + 00
4.91E - 01 8.72E - 01 1.19E 00
0.68 0.70
0.66 0.70
-
-
3.92E 5.48E 7.07E
+ 01
+ 00 + 01 + 01 6.56E + 00 8.53E + 00 1.11E + 01
2.39 2.42 2.61
2.5 1 2.70 3.38
2.22 2.16 2.14
2.15 2.18 2.41
1.73 1.63 1.61
1.69 1.70 1.92
9.66E - 02 1.74E - 01 3.59E - 01
0.18 0.23
0.17 0.22
2.40E - 01 4.08E - 01 7.55E - 01
1.06 1.06 1.09
1.03 1.04 1.11
0.02 0.05 0.08
0.04 0.06
+ 01 + 01 3.55E + 01 1.51E + 01 5.87E + 01 2.21E + 01 3.03E + 01 3.85E + 01 1.31E + 00 2.31E + 00 4.92E + 00 3.41E + 00 5.75E + 00 1.06E + 01
+
7.56E 1.03E 1.31E
1.64E 2.17E 2.77E
+ 00 + 00 + 00
THIOKOL Di-isobutylene 25.0 54.4 82.2
Negligible Negligible Negligible
Negligible Negligible Negligible
.l
(continued)
Hayashi
808
Table 19 Continued Solubility of liquid in rubber (mL/mL) Test liquid and temp. Permeability (“C) SR-6 25.0 54.4 82.2 Methyl ethyl ketone 25.0 54.4 82.2 Benzene 25.0 54.4 82.2 Ethyl acetate 25.0 54.4 82.2 Methyl alcohol 25.0 54.4 82.2 Carbon tetrachloride 25.0 54.4 82.2
(kghd)
Specific permeability (kgmhm’)
1 day
14 days
Negligible 4.87E - 01 1.31E 00
Negligible 5.14E - 02 1.04E - 01
0.16 0.22 0.27
0.19 0.22 0.37
+ 00 + 01
3.32E - 01 5.378 - 01 8.88E - 01
0.60 0.65 0.77
0.61 0.70 0.77
8.60E + 00 1.60E + 01 2.92E 01
6.50E - 01 1.20E + 00 2.17E 00
1.22 1.29 1.46
1.25 1.41 2.3 1
+ 00 + 00
+ 00
1.64E - 01 4.25E - 01 6.27E - 01
0.48 0.50 0.53
0.48 0.52 0.58
Negligible 4.45E - 01 2.14E 00
Negligible 3.26E - 02 1.55E - 01
0.06 0.09
0.07 0.09
-
-
6.93E - 01 1.95E 00 3.56E 00
1.25E - 01 3.94E - 01 7.14E - 01
0.36 0.64 0.75
0.54 0.65
+ 4.40F + 00 5.28E 1.22E
+
2.11E 5.76E 8.25E
+
+ +
+
1
NEOPRENE Di-isobutylene 25.0 54.4 82.2 SR-6 25.0 54.4 82.2 Methyl ethyl ketone 25.0 54.4 82.2 Benzene 25.0 54.4 82.2
Negligible l.lOE + 00 2.23E + 00
Negligible 2.39E - 01 4.68E - 01
0. I O 0.48 0.57
0.25 0.47 0.57
7.48E + 00 1.25E + 01 1.72E 01
1.66E 2.78E 3.33E
+ 00 + 00
1S 4 1.84 2.06
1S 9 1.90 2.41
1.39 1.41 1.71
1.42 1.52 2.02
2.94 3.59 3.61
2.98 3.75 4.12
1.06E 1.55E 2.31E
+ 01 + 01 + 01
+ 00 2.44E + 00 3.91E + 00 5.33E + 00
2.61E 4.79E 6.19E
+ 01 + 01 + 01
5.85E + 00 1.03E + 01 1.41E + 01
+
809
Nitrile and Hydrogenated Nitrile Rubber Table 19 Continued ~
~~~
~
Solubility of liquid in rubber (mL/mL) Test liquid and temp.
Permeability (kghm’ )
(“C)
Ethyl acetate 25.0 54.4 82.2 Methyl alcohol 25 .O 54.4 82.2 Carbon tetrachloride 25.0 54.4 82.2
Specific permeability (kg.m/hm’)
1 day
14 days
+ 00 + 01
+ 00 + 00 + 00
I .25
+ 01
1.31E 3.26E 2.85E
1.15 1.24
1.20 1.16 1.27
1.21E - 01
2.87E
-
0.02 0.09
0.04 0.26
-
-
3.09 3.20 3.24
3.44 3.69 3.41
5.628 1.43E 1.27E
h
h
h
h
+
1.75E 01 2.40E + 01 h
3.62E 5.13E
02
+ 00
+ 00
h
Samples dissolved. be measured due to leakage.
” Could not
- 30°C
40
0°C
0
100
200
300
Elongation (%) Fig. 13 NBR stress-strain curves at various temperature.
400 600
500
Hayashi
810
ACM Peroxide Cured HNBR Sulfur Cured H N B R NBR CR 10’
10’
I 0‘
Service Life* (hour) *Service life based on the time to lose 80% of elongationafter aging in air.
Fig. 14 Servicetemperature and service life.
There are some indexes for the heat resistance, tensile strength, elongation, the product of strength and elongation, and compression setof rubber. These aredifferent for different kinds of rubber product (18, 63, 64). For example, we used the temperature at which the change in elongation is 80% after 1000 hours. For CR cured by ethylene thiourea (ETU), the temperature change is 100°C. NBR cured by sulfur is 106°C. HNBR cured by sulfur is 126°C. HNBR cured by peroxide is 150°C. ACM cured by ammonium benzoate is 159°C as shown in Figure 14 (65).
7. APPLICATIONS OF NBRANDHNBR NBR, XNBR and HNBR are widely used in industrial products for their oil resistance, solvent resistance, and chemical resistance. Their function is sealing and delivering oil, fuel, water, and chemicals in the automotive, aerospace, chemical, food,machinery, oil-drilling, marine, railroad, textile, and printing industries (2, 7). 7.1
AutomotiveUses
NBR and HNBR are mainly used for automobiles. Automotive elastomersare classified by type of heat resistance and class of oil resistance (66,67). Figure 15 demonstrates the elastomers classified by the Society of Rubber Specification Committee (CARS)in the Society of Automotive Engineers (SAE), adding some candidate elastomers that might be registered in the near future (22,66-71). NBR is classified BF, BG, BK at 100°C application and CH at 125°C. HNBR is classified as a DH and will be proposed to be DK.
7.2 Hoses and Tubes NBR is typically used as the liner in hoses reinforced by fabrics or steel wire and covered by metal, textile,or weather-resistant rubbers. NBRis used for fuel,transmission, brake, and steering
811
Nitrile and Hydrogenated Nitrile Rubber Volume Swell(%)in ASTM # 3 Oil ~ ~ ' 1 4120 0 l o o 80 60 40 30 20 IO I
I
I
I
I
I
I
I
I
I
J -
n-
e
'G
x
2
-
FE-
U)
3 c4
c-
Sm
B A
-
mMQ
- 225 - 200 - l75
ACM HSN
D -
-.a
- 250
VMQ
EPDM GPCO
€CO
CR
NBR
I1R
- 275
FKM
-
- 125
- loo -
SBR NR I
I
I
I
A
B
C
D
I
E
I
F
I
1
G
H
I
I
p
I
70
A
K
Oil Resistance (Class)
* No Rquircmcnt Fig. 15 Classification on SAE J 200.
hoses for automobiles. In industrial applications, hydraulic hose; delivery hose for crude oil, heavy oil, fuels, acid,and alkali; and dairy hoses are also made of NBR. HNBR hasbeen adopted for fuel hose, as shown in Figure 16 (72). HBNR has also been used for power steering and air conditioning hoses for longer life (59. 73, 74). 7.3 Seals,Packing,and O-rings
Various elastomers are used for sealing of oils, water, fuel, and chemicals. One of the main applications of NBR and HNBR is in seals and gasketsbecause of theirresistance to oils, lubricants, and greases (2, 7, 58). Typical specifications for seals and O-rings in Japan are listed on JIS B2401-1 grades A and B, B2402 grade B of seals and JIS K6380 of industrial packing BI, BII, BIII materials (75-77). HNBR is used in high-pressure and heat-resistance applications (9). 7.4
Rolls
NBR covered rolls are widely used in the paper, dyeing, textile, fabric, leather, steel, printing, chemical, and polymer processing industries (7). NBR rolls are resistant to oils, surface-active agents, dyes, ink, solvent, acid, alkali, and so on. Polyurethane rubber is widely used for highload roller applications.However, HNBRand HNBR blended with zinc methacrylate are recently being used in heavy-duty roller applications such as paper, steel, and textile rollers because of their long life at high temperature (28, 59). 7.5 Belts NBR and XNBR are widely used in conveyor belts carrying ore, coke, sand, and oil sand and in flat belts for conveying paper moneyor train tickets. NBR blended with PVC is often employed for textile and food applications requiring ozone resistance (7). Synchronous belts, which trans-
812
Hayashi
Immersion Time (day)
Conditions Immersion Media : Fuel B containing 1% lauroyl peroxide. It was replaced twice a week. Immersion Temperature : 60%
Fig. 16 Oxidized fuel resistance: (O), HNBR 1020; (+), NBR; ( X ) , ECO.
Ho
t 0
5
10
I5
20
Vehicle Running Distance (1000km) Fig. 17 Belt running test.
25
30
813
Nitrile and Hydrogenated Nitrile Rubber
Tensile Strength in Liquid Phase
HNBR 1020
Conditions Gas:
T
W
-A-
EPDM
-A-
FKM-GFFKM-E -INBR
0
I
I
I
24
72
I68
XNBR
-+-
-*-
HP CO:
5% 20% "%I
Liquid:
oil
9-55 4% AmineB 1% 150C 6.9MPa
HX)
Exposure Time(hours)
Fig. 18 Sour cnvironmcnt test for tensile strength
in liquid phase.
mit power from the crankshaft to camshaft in automobiles, are mainly made of HNBR having well-balanced properties of high modulus at high temperature, abrasion resistance. flexibility, and oil resistance, as shown in Figure 17 (74. 78. 79). 7.6
MiscellaneousApplications
Nitrile rubbers including NBR, XNBR. and HNBR are widely used for products such as pump valves, diaphragms, bushings, grommets, adhesives (80, 81), brake shoes (82) clutch facings sponges, isolation insulators. (83) cables (84) and so on. Oil-drilling devices such as blowout preventors, downhole packers, drill pipe protectors, snake pumps, accumulators, pump stators, and rotary drilling hoses are made of conventional NBR. HNBR is applied in products that require resistance to hydrogen sulfide, steam, methane gas at high pressure, corrosion inhibition in deep wells, and extrusion resistance.Figure 18 shows that HNBR demonstrates good resistance to sulfidemixtures(85-87). Examples of HNBRlatexapplications are gaskets.paints and adhesives to metal, ceramics, and textiles (80, 88).
REFERENCES 1. Komuro K. (1973), Gorrru Kogpo Bi/rrm, 3rd ed., The Society of Rubber Industry. 2. Komuro, K. Todani, Y. and Matsukawa, J. (1976), Gouseigorrlu Kctkougijutsu Zensyo Nitorirugnrw
Taiseisya. 3. Scil. D. A. and Wolf, F. R. ( 1987), in Rubher Tc.c/rr~o/ogy. 3rd ed. (M. Morton, Ed.), Van Nostrand Reinhold. Ncw York; p. 22. 4. ASTM D-22 Sub-Commlttee Mceting, 1990. S . Hofman, W. ( 1964), Kuhher Chern. Techlo/. 3 7 1. 6. JPX8-35641B, 88-35642B, 86-41922B.
814
Hayashi
7. Furuyo. M. and Komuro, K. (1963, Application c u d Processirrg of NER. 1963. 8. Uchida, I. (1972), J. Soc. Rubber Ind. J p . 45:1057. 9. Sugli, N. (1990), J. Soc. Rubber Ir~d.Jpn. (Nihorl Cornu Kyokaishi) 63322. lOa.Kubo, Y . Proc. 1111.Rubber Conf. Kyoto, Japan, Oct. 15-18 1985, p. 32. I0b.Bhattacharya. S . Avasthi B. N. and Bhowmick, A. K. (l992), J. Polwrl. Sci.-Po/vrn. Clzern. 32471. 10c.Bhattacharya. S. Avasthi, B. N. and Bhowmick, A. K. (1991), h r l . B i g . Chenl. 30:1086. 1 1. International Institute of Synthetic Rubber Producers (1992). TISRP Elustorrwr Marzucrl. 12. Urabe, S. (1988), Polymer Digest ( T o k y ) 40:65. 13. Weir, J. and Burkey, R. C. (1989), Rubber Plasr. News. Feb. 16. 14. Meyer, G. E. Kavchol, R. W. and Naples, F. J. (1993), Rubber Clwrr~.Tuchrlol. 46:106. 15. Kotani, T. and Teramoto, T. (1980), J. Soc. Ruhher I t d . J p . 53:350. 16. Mori, Y. and Nishihata, S. (1985). J. Soc. Rubber I r d . Jpn. 58:158. 17. Asai, H. (1985). J. Soc. Rubber- I r d . J p . 58:133 (1985). 18. Inoue, T. and Takemura, Y. (1979), in Kouseirm Errrsutorrw r ~ oK r d ~ r t u(J. Furukawa, Ed.), Taiseisya, p. 167. 19. Abrams, W. J. (1962), Rubber Age 91:255. 20. Zeon Chcmicals Europe, Technical Report. 21. Ishihara, M. Toya, T. Hayashi, S. and Oyama M. (lO91), TheSociety of RubberIndustry,Japan, Erasutoma Touronkai. Dec. 5 . 22. Hashimoto, K. (l992), The Society of Rubber Industry, Japan, Haiteku Semina, Tokyo, Feb. 18. 23. Zetpol Brochure, Nippon Zeon. 24. Bayer Japan. Technical Report. 25. Zeoforte Brochure. Nippon Zeon. 26. Klingender, R. C., Oyama, M. and Saito, Y. (1989). paper no. 62 presented at a meeting of the Rubber Division, American Chemical Society, Mexlco City. Mexico, May 9-12, 1989; abstract in R14bher Cherrz. Techrlol. 62:77. 27a.Klingender, R. C. (1990). Rubber Roller Group Meeting, Ncw Orleans, LA, Jan. 31. 27b.Thawamani, P. and Amie, A. Bhowmick, L. (1992). Ruhher Clfern. Techrzol. 65:31. 28. Nishimura, K. (1991). Pol$le 2851. 29. Dunn, J. R. Coulthard, D. C. and Pfisterer H. A. (1978). Rubher Cl~err~. Techrlol. 51:38. 30. Fukuda, H. Nippon Zeon Technical Report. 3 1 , Zetpol Technical Note, Nippon Zeon Technical Report. 32. Hashimoto, K. and Todani, Y (l988), in Hcrrrdhook qfE/cl.storrwr.s (Bhowmick. A. K. and Stephens, H. L. Eds.), Mercel Decker Inc., New York. 33. Oyama, M. and Watanabe. N. (1984), Polvtrz. Frierds 22:202. 34. Zetpol Technical Report, Nippon Zeon. 35. Takeuchi, T. and Murase. H. (1965), Koukrrshi 68:2505. 36. Kondo. A. Ohtani, H. Kasuge, Y. Tsuge, S. Kubo, T. Asada, N. Inuki, H. and Yoshioka, A. (1988). Mcrcrornolecules 21:29 1. 37. Marchall, A. J. Jobe, D. T. and Taylor, C. (1990). Rubber 63:244. 38. Wood, L. A. et a l . (1943), Rubber 16:244. 39. Rebizova, V. G. Bartenev, G. M. and Kosenkova, A. S . (1966), Sol'. Ruhher Tdzrzol. 25( 11):20. 40. Bekkedhl. N. and Scott, R. B. ( 1943). Ruhher Clleru. Techrlo/. 16:310. 41. Copeland, L. E. (1983). J. Appl. Plys. 19445. 42. Tagcr, A. A. et a l . (1988). Colloid J. USSR 17373. 43. Moson, P. (l9S9), Trms. Frtmcloy Soc. 55:146. 44. Marris, R. E. James, R. R. and Guyton, C. W. (1956). Ruhhur Age 78:725. 45. Nishizawa. H. (1973), J. Soc. Rubber I t d . Jpu. 46:688. 46. Scheehan, C. J. and Bisso, A. L. (1966), Ruhher Clfern. Teclmol. 39149. 47. Ushold. R. E. and Finlay, J. B. (1974). Appl. Polvn~.Svrrfp. 25:205. 48. Oppclt, 0. Schuster, H. Thormer, J. and Brdcn, R. (1977), DOS, 2539132. 49. Weinstcin. A. H. (1984), Ruhber 57203. 50. Hashimoto. K. Watanabe, Oyama, M. and Todani, Y. (1985), paper presented at a meeting of the Rubber Division. American Chemical Society, Los Angeles, CA, April 24.
Nitrile and Hydrogenated Nitrile Rubber
815
Hayashi, S. Sakakida, H. Oyama, M. and Nakagawa, T. (1991), Rubber 64:534. Kubo, Y. (1990), S e k i y t Gtrkknishi 33:193. Nippon Zeon, Technical Report. Hashimoto, K. Watanabc. N. Oyama, M. and Todani, (1984). Y. Swedish Institution of Rubber Tcchnology Gothenburg, Sweden, May 17- 18. 5 5 . Maeda, M. (1960). Po/wr. Friends 3:383. 56. Kubo, Y. (1986), J. Soc Rubber 6 1 d . Jprr. 69:442. 57. Nippon Zeon (1981). Taiyugomu no Atarashiitenkai, Kobunshi Gijutsu Kenkyukai, March 18. 58. Matsuda, J. (l985), J. Soc. Ruhhrr Ind. J ~ I58: . 148. 59. Klingender, R. C. Watanabc, N. Hashimoto, K. and Oyama, M. (l989), paper no. 38 presented at a meeting of the Rubber Division, American Chemical Society, Cincinatti, Ohio, Oct. 18-21, 1988; abstract in Rubber Cherll. Techrlol. 62:165. 60. Amcrongen, G. J. ( 1950), J. Polvnl. Sci. 5:307. 61. Mueller, W. J. (1957), Rrthher Agr 81:982. 62. Ecker,R. (1960), Kmttsch. Gummi. 297. 63. Walter, G. (1976). Rubber 49:775. 64. Maeda, A. and Hashimoto, K. (1981). Taiyugomu no Taikyusci, Kobunshi Gijutsu Kcnkyukai. 65. Hashimoto, K. Watanabe, N. Oyama, M. and Todant, Y. (1984). Swedish Rubber Conf., May 21. 66. SAEHandbook, 1990. 67. HS K 6403, 1981. Y. (1988),SAE Tech. Paper880026. 68. Hashimoto, K. Oyama, M. Watanabe, N. Komatsu, K. and Todani, 69. Sugimoto, T. (1986). Polvjile 23:33. 70. Macda, A. Hashimoto, K. and Inagami, M. (1987). SAE Tech. Paper 870194. 71. Eggers, R. E. (1991). Rubber World 204(3):24. 72. Fukushima, H. and Oyama, M. (1983), Polyrrl. Frier~ds2/:487. . 10):650. 73. Tsuzuki, Y. (1989). J. Soc. Rubber I n d . J ~ H62( 74. JP84- 184235A. 75. JIS B 2401. 76. JIS B 2402. 77. JIS K 6380. 78. Hashimoto, K. Oyama, M. Watanabe, N. and Todani, Y. (1986). paper No. 5 presented at a meeting of the Rubber Division. American Chemical Society, Clevcland Ohto, October 1-4. 1985; abstract in Rubher Cllerrr. Techno/.59:16 1. 79. Bradford, W. G. and Klingender, R. C. (1991). Elostomerics 123(8):10. 80. Nakao, K. (1987), Polj$/e 42:16. 81. Sato. M. (1987). J. Soc. Rubber I d . Jprl. 60:69. 82. Bahnhousc, J. (1990). Zcon Chem. Inc., unpublished. 83. Iwasaki, Y. Ohyama, T. and Hashimoto, K. (1992), paper no. 30 presentcd at ;I meeting of the Rubber Division, Amcrican Chemical Society, Lounsville, KY, May 19-22. 1992; abstract in Rubber Clwnl. Tec.lrrlo/.65:856. 84. Thormcr, J. Mirza, J. and Buding, H. (1984), PR1 Ruhhrr Cor$, Eirr1~inghm,UK, March 12-15. 85. Hashimoto, K, Watanabe, N. and Yoshioka, A. (1983), paper presented at a meeting of the Rubber Division, American Chemical Society, Houston, TX, October 25-29. 86. Watanabe,N.Kyker, G. S. andHashimoto,K. (1989), paperno. 3 presented at ;I meetingof the Rubber Division, Amcrican Chemical Society, Dallas. TX, April 19-22, abstract in Rubber Cllenl. Techno/.61:717. 87. Klingender, R. C. Hashimoto, K. Kubo, Y. Oyama, M. Todani, Y. and Watanabe, N. (]%g), Energy Rubber Group, Dallas, TX. 88. Kubo, Y. Mori, 0. Ohura, K. and Hisaki, H. (1990). paper no. 29 presented at a meeting of the Rubber Division. American Chcmical Society, Washington DC, Oct. 9-12. 89. Komuro, K. and Fukushima, H. (1983), Eng. Mttter. 31: 104. 90. Todani, Y. and Fukushima, H. (1988), Intern. Cornbust. Etlgirle 23:25. 91. Mirza, J. Leibbrandt, F. and Thoermer, J. (1987). SAE Tech. Paper 870193.
5 1. 52. 53. 54.
This Page Intentionally Left Blank
33 Diene-Based Elastomers judit E. Puskas University of Western Ontario, London, Ontario, Canada
1. INTRODUCTION Diene-based elastomers comprise the majority of the over 15 million tons of total elastomer consumption in the world. Approximately 35% of the total global elastomer consumption is natural rubber (NR), while the remaining 65% is synthetic rubber (SR). The major application of elastomers is in car tires-60% of the global rubber consumption represent. tire manufacturing. The building block of natural rubber is isoprene, a conjugated diene:
H
H
The driving force for the development of SR was the supply shortage World Wars. The first SR was derived from 2,3-dimethyl-butadiene:
CH,
I
of NR during the two
H
I
The building blockor monomer,polymerized. becomes theelastomer. The elastomer willbecome a crosslinked rubber by incorporating the polymer chains into a network structure. In this socalled curing or vulcanizing process the double bonds of the polymer chain serve as curing sites. In diene-based elastomers each repeat unit has a potential curing site; in diene-containing copolymer elastomers such as styrene-butadiene, the diene sequences will provide the potential crosslinkingsites. The crosslinkdensity will be determined by the fraction of thesites that actually participated in the crosslinking process. In the literature the terms “elastomer” and “rubber” are often used interchangeably. The first tires produced commerciallyfrom poly(2,3-butadiene) were used by the Emperor Wilhelm I1 of Germany on his car. The properties of this rubber did not measure up to NR (for 817
Puskas
818
instance. the maximum speed the emperor could drive his car was 40 k m h , due to excessive heat generation), which further inspired the search for a possible synthetic replacement of NR. During World War 11, SBR (styrene-butadiene copolymer rubber) wasdeveloped. Today, there are more than 20 different kinds of elastomers, most developed after World War 11. It is interesting, however, that no human-made elastomer canmatch the combination of properties NR gives us, and the exact chemical structure of NR is still unknown. The fieldof diene-based elastomers isvery large. Several reviews and encyclopedia chapters have been published on this subject. The science and technology of elastomers and tire manufacturing are perceived to be mature, with incremental developments. However, we have witnessed major changes in the past decade ortwo. These changeswere driven by environmental concerns and restrictions (e.g., green tire, emission control in the polymerization processes), demands for tire performance improvements, and, last but not least, advances in polymer and catalyst synthesis (Quirk et al., 1996; Taube and Sylvester, 1996a). The versatility of the living anionic polymerization process (Szwarc, 1956; Hshieh and Quirk, 1996) continues to open up new synthetic routes to custom-made polymers. The development of new living polymerization processes [cationic (Kennedy et al., 1990; Kaszas et al., 1990, 1992) and radical (Georges et al., 1993, 1994)] opens up entirely new avenues for elastomer synthesis. This chapter will give an overview and update on the latest developments in the field of diene-based elastomers. The high-volume commodity elastomers-butadiene (BR), styrene-butadiene (SBR), and isoprene (IR) rubbers-will be discussed in more detail. Butyl (IIR) will be covered briefly. EPDM and NBR rubbers and block copolymers (polybutadiene-polystyrene,polyisoprene-polystyrene, and polyisobutylene-polystyrene)will be covered in separate chapters.
2.
2.1
DIENE-BASEDELASTOMERS Butadiene Rubbers
Butadiene rubbers are made of 1,3-butadiene, a conjugated diene:
H
I
I
H
When this diene is polymerized, a polymer containing double bonds in its main chain is formed. The configuration of the double bonds and the substituents attached to the double bond can be cis-1,4 (A), trans-1,4 (B), and vinyl (or 1,2 incorporation, C): H H H H
\ / H
/c=c
A
\
-C
!/
\ / H
/c=c\
H
C H\q H
B
-c
/"H H
C
Diene-Based Elastomers
a19
The vinyl incorporation, C, can be atactic, with random D and L spatial configuration, or with regulated spatial configuration. The isotactic structure has the vinyl groups all in either D or L configuration,whilethesyndiotacticstructurehasalternating D and L configuration. The microstructure has a profound effect on the physical properties of the polymer. The manufacturing conditions will determine the microstructure of the BR (Stephens, 1989). High-cis, hightrans, and syndiotactic polybutadiene are produced by Ziegler-Natta (ionic-coordination) polymerization, while radical and anionic polymerization producemixed microstructures. In general, polybutadienes are linear polymers, but recent advances in analytical techniques have revealed that some polybutadienes, believed to be linear, contain branched fractions. A perfectly linear, perfectly1,4-polybutadienewasproduced in the laboratory by acyclic diene metathesis (ADMET) polymerization (Ne1 et al., 1989). Branching is considered to be an advantage; longchain branching is believed to improve processability (Kozlov et al., 1991). High-cis Polybutadienes High-cis polybutadiene is a very important commodity elastomer, mostly used in tire manufacturing. It is also used as the rubber component in the manufacture of high impact polystyrene (HIPS) and technical goods (including golf balls). The glass transition temperature, T,, of cis1,4-polybutadiene is low and varies with the microstructure. High-cis polybutadiene contains about 93-98% cis-1,4 structures, with T, reported in the - 103 to - 109°C range, depending on the trans-1,4 and vinyl content (Laflair, 1993). Other reported values are - 106°C for pure cis (Bahary et al., 1967), - 102°C for “high” (not specified)-cis-1,4 (Trick, 1960), and -95°C for 98-9976 cis-1,4-polybutadiene (Baccaredda, et al., 1960).High-cis polybutadiene has good low-temperature properties, high resilience and low hysteresis, and good tear strength and abrasionresistance. It crystallizesuponstretching,givingthepolymerhightensilestrength; the melting point, T,,,, of the crystalline domains is 2°C (Stephens, 1989). While the tensile properties of this elastomer are good, it has a lower “green strength” (strength of the unvulcanized elastomer) and tack than NR. Depending on the manufacturing process, high-cis polybutadienes contain more or less branched fractions. The occurrence of branching has been verified by size exclusion chromatography (SEC) coupled with viscometry and sedimentation (Kozlov et al., 1991). While it is not easy to identify the presence of small branched fractions or the degree and nature of branching, the effects of branching can easily be observed by dynamic mechanical and stress relaxation tests. It is suggested that branched polymers areeasier to process, but linear polymers havebetter mechanical properties. Long-chain branching reduces cold flow or “creep” characteristic of highlylinearpolymerssuch as high-cis polybutadienes. The occurrence of branching during polymerization and the structure-property relationships in branched polymers are not fully understood, and presently there is a great interest in researching this area (Gnanou, 1996; Fetters et al., 1996). High-cis polybutadiene is produced by Ziegler-Natta catalysis in a solution process. Commercially important catalysts are based on cobalt, titanium, nickel, and neodymium (lanthanide) compounds (Laflair, 1993). Cobalt-based catalysts produce polymers with varying degrees of branching and a medium molecular weight distribution. The titanium-catalyzed polymers also have medium molecular weight distribution and a lower degree of branching. The nickel-catalyzed elastomers have a broad distribution and higher degree of branching. It was reported that at high conversion nickel-catalyzed polybutadienes exhibit bimodal molecular weight distribution (Schroeder et al., 1992). The neodymium-catalyzed polymers have very broad distribution and are highly linear. These polymers have a good balance of properties, with the exception of high cold flow and poor extrusion.
820
Puskas
High-trans Polybutudienes High-trans polybutadiene has been of relatively little practical importance due to its high crystallinity. However, it has been used as a blend with NR, polyisoprene, and styrene-butadiene rubber (SBR) fortire building (Haynes, 1988). It has two melting transitions, with valuesdepending on the trans content (97 and 145°C for 99-100% and 50 and 175°C for 90-99%) (Dreyfuss, 1996). The T, was reported to be - 107°C for 100% trans (Bahary et al., 1967) and - 83°C for 94% trans (Dainton et al., 1962), indicating good low-temperature properties. It is produced by Ziegler-Natta polymerization; the stereoregularity will be influenced by the reaction conditions such as the nature of the ligand a n d o r the solvent. The mechanism of stereoregulation was discussed recently in a study comparing allyl-nickel and allyl-lanthanide catalysts (Taube et al., 1995). A recent patent disclosed a copper-based catalyst for the synthesis of truns-1,4polybutadiene (Seki et al., 1996). Vinyl Polybutadietzes The Ziegler-Natta-type high-cis and high-trans polybutadienes contain a small amount of vinyl structures (0-5%), while emulsion BR can be produced with 7-25% vinyl content (Stephens, 1989).The vinyl content of polybutadienes produced by anionic polymerization canvary between 10 and nearly loo%, demonstratingthe versatility of this process (Hsieh and Quirk, 1996).The vinyl content has a profound effect on the properties of the polymer; for instance, as the vinyl content is changing between 0 and 100%. the T, of the polymer changes between - 100 and 5°C (HalasaandMassie, 1993). Alow-vinyl (IO-30%) polybutadienewith a T, of -70 to - 85°C has good wearand fuel economy but poor traction. while a high-vinyl (80-95%) polymer with a T, of - 10 to - 30°C has good traction and fuel economy but poor wear properties. The optimum was found at about 70% vinyl with a T, of - 40°C (Halasa and Massie, 1993). Recent developments in anionic polymerization techniques put new life into vinyl polybutadienes. For instance. the versatility of this process allows the controlled termination of the chain ends. The end groups were shown to have a profound impact in tire performance by influencing the fillerpolymer interaction in spite of their small concentration in the polymer chain. For instance, the introduction of bis(4,4’-dimethylamino)benzophenone and N-methyl-Zpyrrolidone end goups improved tire performance (Nagata etal., 1987; Kawanaka, 1989).The importanceof end group functionalization further increased with the introduction of silica and silica-carbon black “composite fillers” (“green tire”), in which the improvement of filler-polymer interaction is crucial (Wang and Wolff. 1991; Okel and Waddell, 1994). Functional groups can also be introduced by the use of functional anionic initiators; developments in this area were reviewed recently (Quirk et al., 1996). Anionic polybutadienes are essentially linear with narrow molecular weight distribution ( M J M , , = 1 in batch; M,/M,, = 2 in a continuous process), which results in poor processability. This can be improved by introducing branching into the polymer chain (Tsutsumi et al., 1990; Sierra et al., 1995). Linking agents such as tin tetrachloride can be used to make star polymers; in addition, the tin was shown to improve carbon black-polymer interaction, thereby improving tire performance. Low-Vinyl Polybutadienes In this section polymers with around 10-25% vinyl content will be discussed. These elastomers are produced with anionic (living) or emulsionpolymerization and are also called low- or medium-cis polybutadienes. A typical anionic polybutadiene has about 31% cis, 53% trans, and 10% vinyl content, with a reported T, of -93°C (Laflair, 1993; Brydson, 1988). This rubber is mostly used to produce HIPS; it dissolves readily in the styrene and produces rubber domains of the right size distribution to arrest crack propagation in polystyrene. The viscosity of the polybutadiene dissolved in the styrene is very important and can be regulated by controlling the molecular weight and the molecular weight distribution
Diene-Based Elastomers
821
or introducing long-chain branching into the polybutadiene. It was shown that narrow molecular weight distribution gives a better core-shell structure in high-gloss HIPS; interestingly, the use of star-branched polybutadienes reportedly ledto nonuniform structures (Toyoshima et al., 1997). Branching also reduces the coldflow of this elastomer and improves processability.The standard anionic polybutadiene has poor processability and wear. The anionic living polymerization of butadiene is attractive because it is well understood (Hsieh and Quirk, 1996). This process has no termination or other side reactions. and the molecular weight of the polymer can easily be controlled by the initiator/monomer ratio or by deliberately added terminating agents (Puskas, 1993). It is a very versatile process that can produce tailor-made structures, including branched and end-functionalized polymers (Fetters et al., 1996; Quirk et al., 1996; Fathi et al., 1996). The interest in these specialty elastomers is steadily growing. A typical emulsion butadiene has about 17% vinyl, 70% trans-, and 13% cis-1,4 structures (de Decker et al., 1965). Due to its mixed microstructure, the T, of this polymer was reported to be - 80°C. It also has a higher gel content than other BRs. As a rule, the emulsion polymerization must be terminated at about 60% conversion to avoid excessive gel formation. Emulsion low (medium)-vinyl polybutadiene is used for tire manufacturing and rubber-toughening of plastics. Tire-grade emulsion butadienes usually are not available commercially-tire manufacturers produce these rubbers for captive usage (Obrecht, 1993). The most important use of emulsion low(medium)-vinyl polybutadiene is the production of ABS (styrene-acrylonitrile toughened with butadiene rubber) (Dinges, 1979; Echte, 1989) and other composite materials. Medium-Vinyl Polybutadienes Medium-vinylpolybutadienescontainabout35-55% vinyl structuresand have T, values in therange of -70 to - 50°C (Haynes, 1988). These elastomers were found to have properties similar to SBR; good wear and wet-skid properties, low hysteresis and rolling resistance. They are also produced by anionic polymerization. but in the presence of polar additives. The versatility of the anionic living processallows the production of tailor-made medium-vinyl polybutadienes. Medium-vinyl polybutadiene is used in tire manufacturing. alone or as a blend (Laflair, 1993; Halasa and Massie, 1993). High-Vinyl Polybutadienes Atactic. High-vinylatactic (amorphous) polybutadiene with nearly 100% vinyl content was first produced by anionic polymerization in the presence of polaradditives (Halasa et al., 1981).High-vinylatacticpolybutadienesgeneratedinterest recently due totheir low hysteresis and good rolling resistance together with improved wet grip. Optimum properties were foundaround 70%vinyl content, with a reportedT, of - 40°C (Halasa and Massie 1993). Theseelastomers can also be tailor-made to producebranched or other architectures, further improving their physical properties and processability. High-vinyl polybutadienes are used in winter tires and tires with lower rolling resistance. Syndioractic. This vinyl polybutadiene is produced by Ziegler-Natta polymerization and was thefirst example of a syndiotacticpolymer (Natta and Corradini, 1956). The physical properties of this elastomer are determined by the degree of crystallinity and molecular weight. The melting point of syndiotacticpolybutadiene is reported as 156°C (Stephens,1989), but melting points as high as 220°C were reported (Ashitaka et al., 1983; Halasa and Massie, 1993) and - 28°C (Laflair, 1993) were reported. Polymers with various melting points canbe prepared using different polymerization conditions(Halasa andMassie, 1993; Dreyfuss, 1996). The industrially moreimportanttypes have high (T,,, = 190-260°C) or low (T,,, = 70-90°C)melting transitions. Commercially, syndiotactic polybutadiene is produced with 15-30% crystallinity so it can be processed (Laflair, 1993). It is used as a highly permeable film in food packaging. Isotactic. Isotactic 1,2-polybutadiene was produced by Ziegler-Natta polymerization and has a melting point of 126°C. Natta (1965) described the properties of 99% isotactic polybutadiene. This polymer has not generated commercial or scientific interest.
Puskas
822
2.2
Isoprene Rubbers
Isoprene is a methyl-substituted butadiene. When polymerized, it can also form cis-(A) and trans-1,4 (B) enchainment. However, because of the methyl substituent. the 1,2-(C) and 3,4(D) enchainments are different, so polyisoprene has more microstructure variation than polybutadiene:
-C
/c=c\
C -
B
A H
H
l
l
H3C\;
C "
-C / S H '
H C
D
The 3,4structure (D) is a dialkyl-substituted olefin, therefore based on olefin chemistry(Morrison and Boyd, 1992) it would be expected tobe thermodynamically more stable than the monoalkylsubstituted 1,2 structure (C).Indeed, vinyl polyisoprenes mostly have 3,4 enchainment (Morton, 1983). Similarly to that discussed for polybutadiene, the microstructure has a profound effect on the properties of the polymer. Comparison of natural and synthetic cis-polyisoprenes shows, however, that microstructure is not the only determining factor. Natural rubber is almost 100% cis-polyisoprene, but synthetic high-cis-polyisoprene has inferior properties (e.g., green strength and tack) (Schoenburg et al., 1977; Chakravarty et al.). The reason for this is still unknown. The steady availability of NR since World War I1 severely hindered developments in synthetic polyisoprene production in the developed countries. However, the former Eastern block countries developed a substantial high-cis-polyisoprene capacity,as the lack of convertible currency necessary to buy NR forced them to substitute IR for NR. Since the fall of the Berlin wall these countries are marketing synthetic polyisoprene as a NR substitute. However, in a free-market economy the future of high-cis-polyisoprene production is questionable. Vinyl polyisoprenes have very little commercial importance; they are used mostly as processing aides for NR. In vinyl polyisoprenes, the spatial arrangement of vinyl groups can be atactic, syndiotactic, or isotactic.
Diene-Based Elastomers
823
High-cis Polyisoprenes NR is almost 100%cis-polyisoprene. Polyisoprene with very high cis content (97%) (Van, 1966) can be produced by titanium-based Ziegler-Natta polymerization, but anionic polymerization can also producepolymers with up to 96%cis content (with 4% 3,4-vinyl enchainment) (Morton, 1983). The so-called “low-cis” anionicpolyisoprene still has about 92% cis content (Duck and Locke, 1977), but its properties are worse than the high-cis Ziegler-Natta IRs. High-cispolyisoprene can also be obtained by neodymium-based catalysts (Laflair, 1993). High-trans Polyisoprenes High-trans polyisoprene is also produced by Ziegler-Natta polymerization. It is a crystalline polymer with low meltingpoint. Its properties are similar to natural 100%trans-l ,4-polyisoprene (guttapercha or balata). It is used in golfball covers and splinting/prosthetic devices. Its high price limits its use as specialty polymer. Vinyl Polyisoprenes These polymers have 3,4 enchainment, probably due to the higher stability compared to the 1,2 structure. They can be amorphous orcrystalline. The preparation of amorphous 3,4-polyisoprene by anionic or Ziegler-Natta polymerizations have been reported (Ziegler, 1936; Duch andGrant, 1964; Natta et al., 1964; Halasa et al., 1981). In the presence of polar solvents, anionic polymerization can yield almost 100% 3,4-vinyl-polyisoprenes,with some 1,2content possible (Morton, 1983). Crystalline 3,4-polyisoprene was first prepared using organometallic catalysts (Sun and Wang, 1988; Qiu et al., 1989). Thestructure and properties of vinyl polyisoprenes with 70-81 % 3,4 structure and 30-19% cis content were investigated in more detail (Brock and Hakathorn, 1972). These polymers had no 1,2 content, and long sequences of head-to-tail units were said to be responsible for its crystallization. The T, of these polymers was measured to be around 8”C, and they exhibited several melting transitions.It was speculated that the spatial arrangement of the vinyl groups is syndiotactic (Halasa and Hsu, 1996). Pure isotactic vinyl (3,4)-polyisoprene has not been prepared yet, possibly due to the steric hinderance of the methyl side groups.
3.
DIENE-CONTAINING COPOLYMER ELASTOMERS
The most important representatives of this group are the styrene-butadiene rubbers (SBR). On a volume basis, these are the most important synthetic elastomers. They are mostly used in tire manufacturing and the production of industrial rubber goods. Otherimportant diene-containing copolymers are poly(styrene-co-isoprene-co-butadiene)terpolymer (SIBR) and poly(isobuty1ene-co-isoprene) (butyl rubber, IIR). The following section will discuss the above-mentioned copolymers. The specialty rubbers poly(butadiene- 1,3-co-isoprene) and poly(butadiene- 1&COpentadiene- 1,3) will be mentioned briefly. Poly(acrylonitri1e-co-butadiene-1,3)(nitrile rubber, NBR) will be covered in a separate chapter.
3.1 Styrene-ButadieneRubbers SBR was first made in 1929 by an emulsion polymerization process. The resulting material (Buna-S) was inferior compared to NR, but the development of this commercial process was a fundamental milestone in the history of rubber. Thesolution process was developedin the 1950s.
Puskas
824
The structure of these polymers can be characterized by the styrene content, by the microstructure of the polybutadiene segments (cis,trans, and vinyl), and by the sequence distribution of the styrene and butadiene units (random or blocky). A typical emulsion SBR (E-SBR) has 23.5% styrene, 14% cis-1,4-, 67% trans-1,4-, and 19% vinyl (1,2-) polybutadiene, with random distribution. Note that the vinyl, cis, and trans contents add up to 100%; another convention used in the literature reports styrene + cis + trans vinyl = 100% (Niziolek, 1997). E-SBR is prepared by radical initiation in a “hot” (50°C) or a “cold” (5”C, redox initiation) process. This elastomer has a broad molecular weight distribution and a high degree of branching, so it processes better than solution SBR. Theemulsifier residues increase the hysteresis of the rubber (KernandFutarama, 1987) but improveprocessabilityandtearstrength. Some E-SBRs are crosslinked by introducing divinylbenzene into the process. Emulsion SBR represents 40% of the market today becauseof its excellent balance of properties and cost performance. It is mainly used in replacement tire manufacturing (Lambert, 1993; Niziolek, 1997). In contrast, solution SBR is used mainly in new tires. The solution process uses anionic initiators (living polymerization) and is very versatile; the composition and microstructure of the polymer can be controlled, functional and end groups can be introduced, and the molecular architecture can be varied. The styrene sequence distribution can be varied from completely random to blocky (Zelinski and Childers, 1968;Hsieh andGaze, 1970;Antkowiaket al., 1972;Butonand Futamara, 1974, Tanaka et al., 1983; Bywater, 1985). In nonpolar solvents the butadiene polymerizes to high conversion before the styrene starts to react, leading to a blocky structure; the typical vinyl content under these conditions is 10%. In polar solvents or in the presence of polar compounds called “randomizers,” butadiene and styrene polymerize with nearly equal rates, yielding random sequence distribution. These randomizers also increase the vinyl content, which can go up to 90% (Oberster et al., 1973). For random SBR, with increasing vinyl content and increasing styrene content the T, increases, which negatively impacts the low-temperature properties of the rubber. However, high-performance tires are made with SBR with a styrene content as high as 40% and vinyl content up to 70% (Nentwig,1993). As mentioned in Section 2.1, the versatility of the living anionic polymerization allows the controlled termination of the chain ends. The end groups were shown to have profound impact on tire performance by influencing the fillerpolymer interaction in spite of their small concentration in the polymer chain (Nagata et al., 1987; Kawanaka, 1989). The importance of this filler-polymer interaction is magnified in the case of silica and silica-carbon black composite fillers (“green tire”)(Wang and Wolff, 1991; Wang et. al., 1997: Okel and Waddell, 1994). Functional groups can also be introduced by the use of functional anionic initiators; developments in this area were reviewed recently (Quirk et al., 1996). Similarly to anionic polybutadienes, solution SBRs are essentially linear with narrow molecular weight distribution (Mw/M,, = I in batch; M,/M,, = 2 in a continuous process). The processability of SBRs can also improved by branching (Tsutsumi et al., 1990; Sierra et al., 1995). Tin tetrachloride used as a linking agent to produce branched SBRs led to improved carbon black-polymer interaction and tireperformance, similar to that discussed in the polybutadiene section. Solution SBR technology continues to improve, and because of the incredible versatility of the anionic living polymerization process, the skyis the limit. Continuous original tire performance improvements will drive further developments. The cost factor will probably maintain E-SBR’s position as the workhorse of the replacement tire industry.
+
3.2 Poly(styrene-co-isoprene-co-butadiene)Terpolymer The development of SIBR, also called “integral rubber,” was based on the wide modal loss factor concept (Nordsiek and Kiepert, 1985). This can be achieved by using blends such as polybutadiene. natural rubber, or synthetic polyisoprene and SBR, but immiscibility in these
Diene-Based Elastomers
825
blends leads to problems and the effects of blend morphology on the physical properties of the final cured rubber are not fully understood. Halasa and coworkers (Halasa, 1997) pioneered the development of SIBR, a terpolymer prepared by anionic polymerization. By taking advantage of the versatility of living anionic polymerization and reaction engineering principles, these researchers achieved simultaneous controlof polymer composition, microstructure, and sequence distribution. The polymer chain is segmented, with different T,s for the individual segments. Thus the position, height, and breadth of the loss factor-temperature correlation can be varied, resulting in complex viscoelastic properties and optimum rolling resistance/wet grip/wear resistance combinations. This is a truly magnificent example of macromolecular engineering, allowing tailor-made elastomers for specific tire applications. In addition, morphologies previously reported only in SBS or SIS plastic-elastic block copolymers were obtained in the new SIBRs. This opened up new avenues in investigating the relationships between phase morphology and the physical properties of cured rubber. The commercial production of SIBR started in 1991 (Nentwig. 1993; Marwede et al.. 1993). 3.3
Isobutylene-IsopreneRubbers
Butylrubber (IIR) is the copolymer of isobutylene with 1-3% isoprene. The isopreneunits supply the curing sites. The first butyl-type rubber contained butadiene instead of isoprene, and it was the first example of “low-functionality” rubber (Thomas and Sparks, 1944; Thomas. 1969). According to this principle, only a small number of curing sites is necessary to obtain good physical properties, as opposed to the traditional diene rubbers with a curing site for each repeat unit. The isoprene is incorporated ina rruns-l,4 configuration. A small fraction (0.1-0.3%) of other structures, which were thought to be 1,2 units (Chu and Vukov, 1985), were proven to be branching points (White etal., 1995). The presence of branchinghimodal molecular weight distribution was claimed to improve the processability of the rubber (Duvdevani, 1989; Puskas and Kaszas 1993). IIR has the lowest permeabilityto air ormoisture of all elastomers, combined with high damping and good oxidative and chemical resistance (Kennedy, 1975). It is used in tire-relatedapplications (inner tubes,tire-curingbladders,innerliner blends) and as abase polymer for halobutyl production (Duffy and Wilson, 1993; Kaszas et al., 1996). The market share of IIR itself is decreasing, with little incentive for product development. With the advent of living carbocationic polymerization, block copolymers of isobutylene and styrene and styrene derivatives-butyl-like polymers that do not need chemical crosslinking-were made for the firsttime (Kennedy et al., 1990;PuskasandKaszas, 1996). The development of these new materials is still only on a pilot scale (Kaszas et al., 1992). The livingcopolymerization of isobutylene with isoprene and 2,4-dimethyl-1,3-pentadienehas been achieved (Kaszas et al., 1992).
3.4
SpecialtyDiene-ContainingCopolymerElastomers
The specialty butadiene-isoprene and butadiene-piperylene copolymers were discussed in the earlier edition of this book (Haynes, 1988). They are used in specialty tire applications.
4.
RECENTDEVELOPMENTS IN DIENE-CONTAININGELASTOMERS
The information published during the period 1995-1997 concerning catalyst and initiator development is almost overwhelming. The former Soviet Union and China seem to be very active, especially in catalyst and process development.
826
4.1
Puskas
Emulsion Polymerization
Not much activity was found in this area. A new copper(I1)-enolate redox initiating system for BR and SBR production was reported, with the option for termination by various functional groups (Harwood and Goodrich, 1995). Another report discussed the use of aqueous emulsion of organicperoxides for E-SBR (Tauraet al., 1997).Amethod of short-stopping with the suppression of nitrosamine formation was reported (Maestri and Lo, 1995). An interesting report discusses the suspension polymerization of dienes with transition metal catalysts in diluents of high specific gravity (Kimura et al., 1996).
4.2
SolutionPolymerization
New Catalysts and Initiators
Ziegler-Natta and Related Catalysts Publicationandpatentingactivity in thisarea testifies to renewed interest in this field. The following new and modified catalysts were found in the literature: Ni-based catalystwith increased activity (Wang and Wang, 1997);Ni naphthenatealuminum triisobutyl-borontrifluoride etherate complex (Zhu, 1996); Ni-acetylacetonate with methylaluminoxane as cocatalysts (Endo etal., 1996); new dicationic Ni complex (Ni-phosphine) (Engel Gerbase et al., 1996); Ni-naphthenate with triisobutyl aluminum and aluminum chloride (Xu et al., 1995); highly active allyl Ni-based catalysts for cis-1,4-polyisoprene (Novak and Deming, 1995); dibromo-bis(tripheny1phosphine)cobalt-magnesium chloride-dimethoxydiphenylsilane-triethylaluminum catalysts in the presence of ethylene (Takeuchi et al., 1995); triisobutylaluminum-dichloroiodotitanium,to which isopropylbenzene hydroperoxide is added prior to the deactivation of the catalyst (Grachev et al., 1996); polynuclear Nd-AI dimetallic complex (Dong et al., 1995, 1997); titanium(1V)-halide/triethylaluminum neodymium or praseodyrnium complex (Aksenov 19951); neodymium versatate with diisobutylalane, trichlorotriethyldialuminum alcohols, and aluminoxanes(Sylvester and Vernaleken, 1995); halogenated complexes of lanthanides, neodymium toluene ethylchloroaluminum (Garbassi et al., 1995); neodymium tributylphosphate/triisobutylaluminum(Iovu et al., 1997); neodymium-triisobutylaluminumwith a complexing agent (lovu, 1997a); isopropyl alcohol solution of rare earth chlorides with the reaction product of water and triisobutylaluminum in the presence of piperylene (Bodrova et al., 1995), catalyst formed by the oxidative addition of hydrocarbons to lanthanides (Dolgoplosk, 1996); and neodymiumbutoxide/triisobutylaluminum (Biagini et al., 1995a).It was reported that the neodyn~ium-phosphonate/triisobutyIaluminum/ethylaluminumsesquichloride system yields quasiliving polymerization of isoprene (Liu et. al., 1995). Other interestingcatalyst systems includealkyl-iron complexes(Xiaet al., 1997a,b; 1996a); a molybdenum-aluminum colloid (Xia et. al., 1996b); transition metals (e.g., vanadium) with aluminoxane (Igai et al., 1997); toluene solution of triisobutyl aluminum with aluminum chloride (Panfilova et al., 1996); molybdenum trichloride dioctanoate with diisobutyl (methylphenoxy)aluminum (Xia et. al., 1 9 9 6 ~ )and ; a tungsten/aluminum alkyl catalyst for SBR (Song et al., 1995). These catalysts produce cis configurations. For trans configurations an irradiationadsorbing copper cyclohexanedicarboxylate catalyst was reported (Seki et al., 1996). Metallocene Catalysts Dienes are believed to bepoisonsformetallocene-based catalysts. However, the following metallocene or similar catalysts were found in the literature for diene polymerization: monocyclopentadienyl vanadium, niobium, tantalum with aluminoxane and borate cocatalysts(Tsujimoto et al., 1996); metalloceneA4AOcomplexes for the polymerization of isoprene and butadiene (Endo, 1997); lanthanoceneh4AO for butadiene, isoprene, and styrene(Cuiet al., 1997); group IV metal (zirconium,titanium) complexes with MAO for low-temperature butadiene and isoprene polymerizations (Longo et al., 1996); mono- and bis-
+
Diene-Based Elastomers
827
cyclopentadienyl compounds of vanadium and titanium with MAO (Ricci et al., 1996, 1996a); and the metallocene TiBzMAO for butadiene and isoprene polymerizations (Huang and Tion, 1996). The copolymerization of isobutylene with isoprene was reported with the metallocenelike cp-TiMe3/B(Cc,FS)3catalyst system (Barsan and Baird, 1995). Supported Catalysts There is great interest in supported (both inorganic and polymersupported) heterogeneous catalysis. For diene polymerization, the following work was reported during the review period: MgC1,-supported cobalt-based catalyst with trimethylaluminum (Takeuchi et al., 1996); poly(acry1amide-styrene)-supported metal complexes (e.g., neodymiumtrichloride) (Zheng et al. 1996; Zheng, 1997);poly(styrene-4-vinylpyridine)-supportedneodymium (Li et al., 1997); and poly(styrene-2-methylsu1finyl)ethyl methacrylate-supported rare earth catalysts (Li et al., 1995). An interesting report discusses diene polymerizations catalyzed by neodymium salts supported on homogeneous fullerenes (C60/70) (Chen et al., 1995). The area of supported Ziegler-Natta and lanthanide catalysts for diene polymerizations was reviewed (Ran, 1996; Yu and Li, 1996). The copolymerization of isobutylene with isoprene using poly(biphenylaminoethy1styrene)-supported TiC14/Et2AlClwas reported, butlowmolecularweight polymer formed (Ran, 1996). Anionic (Living) Initiators There is great activity in the field of functionalized and other specialized anionic initiators. The following reports were of interest: (tert-butyldimethyl siloxy) alkyllithium-protected initiator (Sutton and Schwindeman, 1997); composite modifiers for anionic living polymerizations (Pan et al., 1997); potassium salts of ethoxylated alcohols (Yudin et al., 1996); dipiperidinoethane-based initiators (Wang et al., 1996); a new class of ether-functionalized initiators (e.g., dimethylethoxi propyllithium) producing star-branched and heterotelechelic polymers (Schwindeman et al., 1997, 1997a); butyldimethylsilyloxypropyllithium functionalized initiator producing star polymers by coupling with silicon tetrachloride (Letchford et al., 1997); multifunctional anionic initiators from the reaction of divinylsilanes with alkyllithiums (Chamberlain et al., 1997); new ether modifiers (tetrahydrofurfuryl-based) for isoprene polymerization (Halasa and Hsu, 1996a); functionalized amine (e.g., dimethylpropylamine) initiators (Engel et al., 1996); anionic soluble organosodium catalyst (Arest-Yakubovich et al., 1995); hydrocarbon-soluble initiators for SBR polymerizations (Kitamura et al., 1997); hydrocarbon-soluble lithioamine, lithium amide, amino-substituted aryllithium, and trimethyltetrahydroazepine hexamethyleneimine lithium initiators for diene rubber production with reduced hysteresis (Lawson, 1996, 1996a, 1996b, 1997); multifunctional organic alkali metal initiators (Zhang et al., 1997); lithium cycloamino-magnesiate initiator for diene rubbers with reduced hysteresis (Antkowiak and Hall, 1996); divinylbenzene-butyllithium adduct for multifunctional initiation (Lutz et al., 1996); and triorganotin lithium initiator (Hergenrother et al., 1995). In this lastworkSn-NMR evidence was found for 1,2 initiation (Hergenrother etal., 1995a). Reports on star polymers include silane-coupled SBR stars for silica tires (Toba et al., 1997) and star polymers by efficient anionic living coupling (Ono et al., 1995). The status of lithium polymers in China has been reviewed (Li et al., 1995; Sun, 1997). Process Research and Development The number of publications in process research and development indicate a renewed interest in this area. The reports listed cover the following aspects: effect of process conditions on the structure and properties of cis- and trans- 1,4 polydienes (Cai, 1995); catalyst-monomer interaction in cobalt naphtenoate/isobutylaluminum chloride/water catalyst system in the low temperature polymerization of cis-1,4-polybutadiene (Smimova et al., 1996, 1997); the effect of water in Ni-naphtenate/isobutylaluminum chloride-catalyzed diene polymerizations (Xu et al., 1997); the effect of using a preformed cobalt salt/alkylaluminum chloride in low-temperature diene
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polymerization (Glebova et al., 1996); effect of additives (octanol, butylacetate) in Ni-boron trifluoride etherate-catalyzed polymerizations (Zhu, 1996a); the effect of modifiers in CO-catalyzed polymerizations (Krivoshein et al., 1995); control in CO-naphtenoate-diisobutylaluminum chloride-water-catalyzed low temperature diene polymerization (Smirnova et al., 1996); cis1,4 BR with less gel (Suzuki et al., 1997);the effect of titanium trichloride purity anddeactivation problems in diene polymerizations (Costa et al., 1997); the nature of active centers and pecularities in CO-catalyzeddiene polymerizations (Sharacv, 1996); the effect of solvent in aluminoxanebased neodymium-carboxylate-catalyzed polymerizations (Wilson, 1996); the effect of Nd/ halide ratio and halide type in Nd-catalyzed butadiene polymerizations (Wilson and Jenkins, 1995); titanium tetrachloride-alkylaluminum chloride-catalyzed process (Aksenov, 1995b); the effect of alkylaluminums and alkylaluminum chlorides on Nd-catalyzed polymerizations (Wilson, 1995); kinetics, molecularweight distribution, andchainend structure in Nd-catalyzed polybutadiene (Nickaf et al., 1995); the effect of diisopropyl xantogenathe disulfide/catalyst ratio andprocess control in Ziegler-Natta-catalyzed continuous isoprene polymerizations (Abramzon et al., 1995); polymerization kinetics in Nd-di(isopropoxy) chloride/triethylaluminum-catalyzed diene polymerization (Cai et al., 1995); effect of parameters on Nd-tris(2ethylhexanoate)/diisobutylaluminum chloride/triisobutylaluminum-catalyzed diene polymerizations (Gehrke et al., 1996): effect of reaction time on the molecular weight distribution of Ndcatalyzed diene polymerization (Oehme et al., 1997); Nd-based catalyst preparation (Biagini et al., 1995); kinetics of lanthanide-catalyzed isoprene polymerization (Dimonie et al., 1995); the efficiency of titanium-magnesium/triisobutylaluminum catalyst in diene polymerization (Mushina et al., 1996). Direct extraction is discussed as an improved preparation of Nd-naphthenate (Yang et al., 1996). An interesting report discusses a process for cis-1,4-polyisoprene production using a multizone tubular prereactor (Minsker et al., 1996). In anionic polymerization technology, less process research isreported. The process control is discussed in continuous anionic polymerization (Konovalenko et al., 1996). The effect of feed distribution in anionic continuous diene copolymerization with divinylbenzene is discussed (Aksenov et al., 1995b). The effect of polar additives (ethoxylated sodium or aluminum alcoholates) on thecontinuous anionic copolymerization of a butadiene-styrene with divinylbenzene is reported (Shalganova et al., 1996). Termination and long-chain branching was modeled in the anionic copolymerization of butadiene with divinylbenzene (Fathi et al., 1996). It is interesting to mention the development of a butyl process using liquid CO? (Baade et al., 1996). reduced fouling in the butyl process (Baade et al., 1996a), and an olefin metathesis process for degelling diene polymerization reactors (Oziomek, 1995). 4.3
Gas-Phase Polymerization
One of the most important developments of the past few years was the gas-phase polymerization of diene elastomers. The gas phase, rare earth allyl/aluminoxane-catalyzed polymerization of butadieneyielded a high-cis product (Taube et al., 1996). In a recent report, a neodymium catalyst was used in the gas-phase laboratory polymerization of polybutadiene (Sun et al., 1997). For gas-phase butadiene polymerization, silica gel-supported rare earth alcoholates (Reichert et al., 1996) and activated charcoal-supported Nd-catalysts (Buysch et al., 1995) were reported.
ACKNOWLEDGMENTS The author would like to acknowledge the contribution of Rosemary O’Donnell and her colleagues (Technology Department, Rubber Division, Bayer Inc., Canada) for the literature search.
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Nel, J. G., Wagener, K. B., and Boncella, J. M. (1989), Po/ymer Preprints 30(2):130. Nentwig, W. (1993). Ulltncrrfrf'S Encycloperli~rV23. Synthetic Rubher, VCH Publishers. Inc., Weinheim. Nickaf, J. B., Burford, R. P., and Chaplin, R. P. (1995), J. Po/ynz. Sci.. Part A: Po/vrn. Chern. 33:1125. Niziolek, A. (1997), Paper #P, ACS Rubber Division Meeting, Anaheim, CA. Nordsiek, K. H., and Kiepert, K. M. (1985). Ktrursch. Gunuui Kurzstst. 38: 178. Novak, B., and Deming, T. J. (1995), U.S. Pat. 539581 I (U. California). Oberster, A. E., Bouton, C., and Valaitis, J. K. (l973), Angew. Mcrkromol. Clzern. 29/30:291. Obrecht, W. (1993), in Ullrrztrtzn 'S Encyc~lopuediaV23. Synthetic R14hber; VCH Publishers, Inc., Weinheim. Oehme, A., Gebauer, U,, Gehrke, K., and Lechner, M. D. (1997). Ktrut.wh. Gumnli Kumtst. 50(2):82. Okel, T. A., and Waddell. W. H. (1994). Ruhher C l m l . Techlo/. 67:217. Ono, T., Kanamori, T., Ito, K., Betsusho, K., and Kubota, K. (1995). Jpn. Pat. JP 95196729 (Japan Synth. Rubber Co. Ltd.). Oziomek,J.,Hergenrother, W. L.,Hamm, D. R., and Bouton, T.C.(1995). US Pat. US 5446102 (Bridgestone Co.) Pan, Z., Yang, .l., and Gu, M. (1997). Heckerzg Xicrngjitro Gongye 20(2): 105. Panfilova, Z. P., Korneev, N. N., Govorov, N. N., Tomashevskij, M. V., Zolotarev, V. L., Kropacheva, E. N., Smirnova, L. V. (1996). Russian Pat. RU 2057756. Puskas, J. E. (1993), Makronzol. Clrerrl., T e o n S i n d . 2:141. Puskas, J. E., and Kaszas, G. (1993), U.S. Pat. 5,194,538 (Bayer AG). Puskas, J. E., and KZIW~S, G. (1996), Rubber C/rem. Techno/. 69444. Qiu, Z. W., Chen, X., Sun B., Zhoul, Z., and Wan, F. ( 1989). J. Macrorrzol. Sci., Chenl. A25: 127. Quirk, R. P,, Jang, S. H., and Kim, J. (1996). Rubher Chem. Techrwl. 69444. Ran. R. (1996). The Po/yrrreric Motrricrls Encylopedicr (J. Salamone, Ed.). CRC Press, Inc., Boca Raton FL. Ran, R.,Pittman, C. U. (l996), Met-Corzttrirzirfg Po/ym. Muter. (Proc. Int. Symp., C. U. Pittman Ed.), Plenum, New York, p. 241. Reichert, K.-H., Marquardt, P,, Eberstein, C. Garmatter, B., and Sylvester. G. (1996), World Patent WO 963 1 543 (Bayer AG). Ricci. G., Bosisio, C., and Porri, L. (1996). Mtrcnmol. Rapid Convmrn. 17( 11 ):78 I . Ricci, G., Panagia, A., and Porri, L. (1996a), P o l y r w r 37363. Saengcr, J., Tefehne, C., Lay, R., and Gronski. W. (1966), P o / w . h / / . 36:19. Schoenburg, E., Marsh, H., Walters, S., and Saltman. W. (1977). Rubber Cllern. Techno/. 52527. Schroedcr, K., Gehrke, K., Schmitz, G., and Lechner, M. D. (l992), Makrornol. Chetrz., Rapid Cotrztrturz. 1357 1. Schwindeman, J. A., Kamienski, C. W., and Morrison, R. C. ( 1 9 9 7 ~U.S. Pat. 5654371 (FMC Corp.). Schwindeman, J. A., Letchford, R. J. Kamienski, C. W., and Quirk, R. P. (1997a), World Pat. WO 9705174 (FMC Corp.). Seki, K., Fujiwara, M., Kamaike, K., Mori, K., and Kajiwara, A. (1996), Jpn. Pat. JP 9621781 1. Shalganova, V. G., Yudin, V. P., Semenova, N. M,,Mistyukova, L. N., Markova, Z. N., Zudina, N. N., Stankevich, V. V., and Ivanova, T. P. (1996), Soviet Pat. SU 788672. Sharaev, 0. K., Glebova,N. N., Markevich. I. N., Bondarenko, G. N., and Tinyakova, E. I. (l996), Vysokormd. Soedin. Ser. A. Ser. B. 38(3):447. Sierra, C. A., Galan, C., Gomez Fatou, J. M., and Ruiz Santa Quiteria, V. (1995), Ruhher Chertr. Techrzol. 68:259. Smirnova, L. V.. Tikhomirova, I. N., Kropacheva, F. N., and Zolotarev, V. L. (1996), Vysokortfol. Soediff. Ser. A. Se,: B. 38(3):458. Smirnova, L. V., Saraev, V. V., Cherkasov, V. K., Tikhomirova, I. N., and Kropacheva, E. N. (1997), R14.s.s. J. Koorcl. Chenf. 23(5):332. Song, J., Fan, H., Chen, D., Zhong, C., and Tang, X. (1995), Heckerzg Xirrngjirro G o r f p e 18:233. Stephens, H. L. (1989), P o / ~ v wHorltlbnok, r j r l f ed. (J. Bandrupt and E. H. Immergut, Eds.), John Wiley & Sons, New York. Sun, J., Eberstein, C., and Reichert, K.-H. (1997), J . Appl. Polynf. Sci. 64:203. Sun, Y. (1997), Hechertg Xictngjicro Gongye 2054.
Diene-Based Elastomers
833
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This Page Intentionally Left Blank
Recycling of Rubber William
H. Klingensmith
Akron Consulting Co., Akron, Ohio
Krishna C. Baranwal Akron Rubber Development Laboratory, Inc., Akron, Ohio
1. INTRODUCTION
The interest in recycling of rubber has increased in the last decade. This has been driven by the concern about the effect of scrap tires and rubber products on the environment. Many tire companies, trade associations, government agencies, and private recycling firms have expanded their efforts in solving the problem. In the United States an estimated 270 million tires a year are scrapped. It is estimated that over 800 million used tires are stockpiled in various tire piles all over the United States. Large piles are reported in Ohio, California, and Texas. In addition, it is estimated that over 350 million pounds of rubber are scrapped from the production of nontire goods in the form of runners, trim, and pads. Many landfills are closing to scrap rubbers. Numerous small, medium, and large rubber companies are trying to find a way to deal with their scrap and are looking for ways to reuse it.
2. TIRE-DERIVEDFUEL It was estimated that over 172 million (64%)of the total number of tires would be used as tirederived fuel (TDF) in the United States in 1997, representing the single largest use of scrap tires in the US. The data in Table 1 summarize the total utilization of scrap tires in the United States for 1997 (1). TDF is expected to continue to grow as more and more cement plants, power-generation facilities, and pulp and paper plants continue to expand their use of scrap tires for fuel. The purpose of this chapter is to review the technical uses of scrap tires; the discussion of TDF is beyond its scope. A more thorough review of TDF can be found in Ref. 1.
3. OVERVIEW OF SCRAP TIRE USE With the efforts of so many companies and recycling firms trying to develop new uses for scrap tires, the results have been many new andexpanded uses. Table 2 summarizes the market demand for size-reduced rubber. 835
836Baranwal
and
Klingensmith
Table 1 Reported and Estimated Market Demand for Scrap Tires by Market Segment (millions of tires, except totals) Market segment
1998"
Tire-derived fuel Cement klins Pulp and paper mills Utility boilers Dedicated tire to energy Industrial boilers Resource recovery facilities Lime kilns Copper smelters Iron cupola foundries Total fuel Products Size-reduced rubber CISIP products Civil engineering Pyrolysis Agricultural Export Miscellaneous uses Totals Annual generation Scrap tire markets as % of total generation
I996
1997" 45.5 35 29.5 15 20.5 6
53 37 32 15
23 8 2
1
0 0 152.5 12.5 8 IO 0 2.5 15
I .5 202,000,000 266,003,000 75.9
1 1
I72 15 8 14 Unknown 2.5 15 1.5 228,000,000" 270,000,000" 84
Estimated. Source: Ret'. 1.
Table 2 U.S. Market Demand (lb.) for Size-Reduced Rubber from Tires (Ibs) 1998" Pneumatic tires Friction materials Molded and extruded goods Rubberplastic-bound products Athletic and recreation Asphalt products Total Estimated. Source: Ref. 2.
1996 48 million 8 million 18 million
140 million 8.5 million 524 million
24 million 168 million 400 million
50 million 200 million 582.5 million
58 39 36 IO 25 IO 3 1 4 186 18 8 18 Unknown 2.5 15 1.5 249,000,000" 275,000,O0Ot 90
ntaineer Mercury
Recycling of Rubber
4.
a37
AUTOMOTIVEINDUSTRY’SRECYCLINGEFFORTS
The automotive industry is a major user of rubber products. As such, it also is a major generator of used and scrap rubber parts. Through the Vehicle Recycling Partnership, a precompetitive cooperative effortbetween Ford, General Motors, and Chrysler, work is being done toestablish the most efficient way to dismantle and reuse automotive components. Eachof the automotive companies is also pursuing the use of recycled rubber in the components they purchase. Ford has been noticeably active in their tire efforts and are working closely with the tire companies to reach a goal of 10% recycle content. A recent announcement by Ford reports that 1.2 million Continental General tires on the F-series trucks containing recycled rubber will be in service. In addition, 100,000 recycled content tires from Michelin are projected for the Ford Windstar. The recycled rubber content is reported at 5%, mostly in the tread area (3). In the nontire area, the following are reported as using recycled rubber:
Ford Ford
Lincoln Navigator
Air reflectors Fender insulators
GM
Not disclosed (report over 1 10 parts) Not disclosed
Baffles Sound barriers
Chrysler
deflectors Water Splash seals
Many of the nontire components containing recycled rubber are in static applications. Much work is underway looking at using recycled rubber in more dynamic applications such as body seals, bushings, and gaskets. The goal is to utilize 25% postconsumer scrap in the automotive nontire components and 10% recycle content in tires (R. Pett personal communication). Dr. Robert Pett of Ford reports that the automotive industry’s goals are to have no more than 15% of the vehicles retired from service go to landfills by 2002, and this is to be reduced to 5% by 2015 (3). The automotive andtransportation industry is the biggest consumer of rubber goods, using an estimated 70-75% of all rubber articles produced. Experiments to use ground scrap tires in roads have proven useful in some areas. The product is known as crumb rubber modifier (CRM). California,Arizona, Texas, and Florida use CRM in their asphalt roads. Effortsto makethis a national standard through legislation were tried in 1991. A bill called the Intermodal Surface Transportation Efficiency Act (ISTEA) required incorporating 20 pounds per ton of CRM to portions of roads being paved. Because of the increased cost involved, many states resisted, and in 1995 the act was repealed. 5.
RECYCLINGMETHODS
At the present time the major methods of producing recycled rubber are reclaiming, ambient grinding, cryogenic grinding, and wet or solution grinding. Reclaiming of silicone and butyl rubbers is common, and the resulting recycled products are useful for cost reduction and improved processing when added to virgin compounds.
5.1
Reclaiming
In the past large volumes of reclaimed tires and tubes were used in the manufacture of tires and mechanical goods. Reclaiming is done by grinding the used rubber articles into small pieces
Klingensmith and Baranwal
838
and mixing it with a reclaiming agent, usually a thiol derivative, or exposing the material to steam digestion. The reclaim rubber can be added to rubber compounds at concentrations of 5-35%. The reclaim rubber improves processingandextrusionquality.However, it lowers tensile strength, tear, abrasion resistance, and green strength. The need for high green strength, abrasion resistance, and consistency in the manufacture of radial tires will lead to the decline and eventual elimination of reclaim rubber from themanufacture of tires. It is still used in small quantities in footwear, mats, solid tires, and low-end mechanical goods.
5.2 Ambient Versus Cryogenic Grinding Vulcanized scrap rubber is first reduced to a 2“ X 2“ or I ” X 1” chip. This can then be further reduced using ambient ground mills or frozen and “smashed” or ground into fine particles while frozen using cryogenic grinding. Below is a brief comparison of the two methods. Ambient Grinding
The ambient process often uses a conventional high-powered rubber mill set at close nip, and the vulcanized rubber is seared and ground into a small particles. Itis common to produce10-30 mesh materials using this relatively inexpensive method to produce relatively large crumbs. Typical yields are 2000-2200 pounds per hour for 10-20 mesh and 1200 pounds per hour for 30-40 mesh. The finer the desired particle, the longer the rubber is let run on or in the mill. In addition multiple grinds can be used to reduce the particle size. The lower practical limit for the process is the production of 40 mesh material. Any fiber and extraneous material must be removed using air separation or an air table. Steel is separated using a magnetic separator. The resulting material is fairly clean. The process produces a material with an irregular jagged particle shape. In addition, the process generates a significant amount of heat in the rubber during processing. Excess heat can degrade the rubber, and, if not cooled properly, combustion can occur upon storage. Cryogenic Grinding
Cryogenic grinding usually starts with chips or a fine crumb. This is cooled using a chiller. The rubber is put through a mill white frozen. This is often a paddle type mill. The Best Practice on Cryogenic Grinding covers this process in detail. The final product is a range of particle sizes, which are sorted and either used as is or passed on and further size reduction performed, e.g., using a wet-grind method. A typical process generates 4000-6000 pounds perhour. Typical sizes are 60-100 mesh. The cryogenic process produces fairly smooth fracture surfaces. Little or no heat is generated in the process. This results in almost no degradation of the rubber. In addition the most
Table 3 AmbientVersusCryogenicallyGroundRubbers
Physical property Ambient Cryogenic ground ground gravity
Specific Particle shape Fiber content Steel content cost
Irregular
Regular
0.5% 0.1%
Nil
Comparable
Nil Comparable
Recycling of Rubber
839
Table 4 Particle Size Distribution for Two 60MeshGround
Rubbers Amount retained Ambient 10-12
30 mesh 40 mesh
60 mesh 80 mesh 100 mesh Pan
(%)
2
15 60-75 15
5 5-10
Cryogenic
(5%)
2 35-40
35-40
20 2- 10
significant feature of the process is that almost all fiber or steel is liberated from the rubber, resulting in a high yield of usable product and little loss of rubber. The price of liquid nitrogen has come down significantly recently, and cryogenically ground rubber can compete on a large scale with ambient ground products (4). Table 3 compares the properties and benefits of ambient and cryogenically ground rubbers. Table 4 shows the particle size distribution for two typical 60 mesh ground rubbers. One was prepared ambient and the other cryogenically. Table 5 shows the properties of rubber compounds containing ambientground rubber and cryogenically ground rubber.
6.
SOLUTION OR WET GRINDING
Micromills or micromilling, also called the wet process, is apatentedgrindingprocess for ultrafine grinding. It reduces particle sizeby grinding in a liquid medium, usually water. Grinding is performed between two closely spaced grinding wheels. A certificate of analysis for a typical lot of wet-ground rubber, Ultrafine 80 from Rouse Rubber, is shown in Figure 1. A review of scrap tire processing was published by Astafan in Tire Technology Intemational '95 ( 5 ) . In addition, microwave (6), ultrasonic (7), chemical devulcanization (8) microbial degradation, and mechanical shear have been used to produce recycled rubber. Ultrasonic and chemical devulcanization are discussed in detail in Tire Technology International '96 (9). 7.
SURFACE TREATMENT AND ADDITIVES FOR PRODUCING RECYCLED RUBBER
Numerous methodshavebeenand are being used to modifythesurface or composition of recycled rubber to make it more compatible or useful. These include halogenation liquid polymers (10): thermoplastic polymers, homogenizing agents (1 l), and wetting agents. These are too extensive to cover here, but they are reviewed in detail in the Clean Washington Center's (CWC) Best Practices Manual (4). Many other processes are also being studied to enhance the value of ground scrap rubber.
8. TESTINGSTANDARDS Recently we summarized the quality, testing, and handling issues for dealing with scrap tires and rubber for the CWC Best Practices Manual (4). In December 1996 ASTM published two
840
Klingensmith and Baranwal
Table 5 Properties of Different Rubber Compounds The following data show the effects of a 20 mesh, ambient-ground rubber compounded into an SBR 1502 compound. The ground rubber was evaluated at 17, 33, and SO% levels. The compound recipe is as follows: Ingredient
Level, phr
SBR1502 Zinc oxide Stearic acid TMQ N660 Carbon black Aromatic oil Sulfur MBTS TMTD
100.0
5.0 1.o 2.0 90.0 50.0 2.0 1.o 0.5
The 20 mesh crumb was added at 17, 33, and 50%. The properties of the materials are as follows: 0% Ground Mooney Viscosity, MU Rheometer max torque, Ibf inch tc90, min. Tensile strength, psi Ultimate elongation, %
40 59 2.5 1470 330
17% Ground 34
61 47 2.4 1IS0 270330
33% Ground
50% Ground
91 33
111
1.8
870 300
2.0 560
The following data shows the effect of concentration and particle size of a cryogenically ground rubber on an EPDM compound: CryOfine ground rubber used at 10% levels (except control at 0%) Control Tensile strength, psi 1290 Ultimate elongation, % 100% modulus, psi 300% modulus, psi 1220 Hardness, Shore A Die C tear, ppi 175
1410 410 535 1180 1220 73 193
40 Mesh 330 490 1230 70
60 Mesh
80 Mesh
100 Mesh
1430 340 530 1230 70 173
1470
400 490
1440 380 480
70 171
71 172
CryOfine ground rubber used at 2070 levels (except control at 0%) Control Tensile strength, psi 1230 Ultimate elongation, % 100% modulus, psi 300% modulus, psi 1220 Hardness, Shore A Die C tear, ppi
1410 1410 410 535 1180 13 181193
40 Mesh 1460 320 450 72 165 178
60 Mesh 1360 390
so0 1200 1300 70 163
80 Mesh
100 Mesh
390 460
390 460 1 l60 68
69
841
Recycling of Rubber
Rouse Rubber Industries, Inc. P.O. BOX 820369 VICKSBURG, MS 39182-0369 TELEPHONE: 601-636-7141 FAX: 601-636-1181
CUSTOMER: R: ORDER HIPPED: DATE SHIPPED: WEIGHT
2113/98 I O Ib
Stock:
GF-80
SIEVE ANALYSIS: SCREEN: 30M 40M 60M 80M 1OOM 200M PAN
% PASSING
% RETAINED
0 0 0 5 6 44 45
0
MOISTURE:
0.67
TGA ANALYSIS: ACETONE EXTRACTION: ASH: CARBONBLACK RHC:
13.59 7.963 30.21 48.24
Sample. W-80
0
0 6 6 39 49
TGA
Slzt. 6 . 7 3 3 0 mg Method: rubber. t e s t s
F i l e . O.\TA\rGA\GATA\RUNO219.g01 Operator: TGA TA Team Hun O a t e : 20-Feb-9a 01.03
n
13.39% a c e t u m c n t r a c : (0.9149mg)
80
In
-I
-==7 f l/ I l\
4 8 . 2 4 % RHC (3.2481119)
I
I
30.26 carbon black 17 O d m o \
'
l-
L " L~ IPJb:,.I, lI . ' r 00
10
20
30
40
Time (mln)
Fig. 1 Certificate of analysis.
50
(0.5361mg) 60
7 0 -10
Universal V 1 108 TA I n s t r u m e n t s
Klingensmith and Baranwal
842
documents: ASTM D-5603-96 (12) and ASTM D-5644-96 (13). In late 1997 the Chicago Board of Trade (CBOT) also published a document that includes definitions of terms and particle size specifications of recycled rubber for buying and selling materials (14). Thus, there are specifications available for recycled rubber that vendors and customers should use to ensure material quality.
9.
MATERIALSTORAGE
Recycling of rubber from whole tires into chipsand crumbs can generate heat up to 220-240°F. At these temperatures and in the presence of oxygen, spontaneous combustion can occur. Also, the presence of iron catalyzes oxidation of natural rubber. To minimize any such problem, material should be cooled with air or water before storage or shipping. Make sure the material temperature is below 200°F. Avoid iron, store at ambient temperature, and not in metal sheds or warehouses at both vendor’s and customer’s location.
10. MOISTURECONTENT
The current accepted level of moisture is 1% (ASTM D-5603-96). Typically, however, it is less than 1%. Too much moisture can cause caking andmay inhibit free flowin processing. Anticaking agents suchas calcium carbonate can be used. Moisture build-upcan lead to acidic conditions giving slower cure rates in compounds. Therefore, recycled crumb rubber should be stored in a cool and dry place. Moisture content is determined by heating a weighed amount of sample at 125°C for one hour, cooling and weighing again. The difference in sample weight is the heat loss (ASTM D1509-95).
11.
BULKDENSITY
Because of the particulate nature of crumb rubber, it is rather difficult to measure specific gravity of crumb rubber. Bulk density measurement may be more appropriate. There is no bulk density specification for crumb rubber; however, ASTM D-15 13 for carbon black can be used. Our recommendation is that bulk density be part of material specification and a range of values be agreed upon between vendor and customer. Another way of determining specific gravity of crumb rubber may be done by making solid sheets or pieces by passing crumb rubber through a tight mill nip and measuring density of compressed pieces.
12. SAMPLINGAND QA TESTING
In crumb manufacturing plants, two samples, each about 125 g, are taken from each skid (about 1000 kg) at the timeof bagging. At the customer’s site, two samples per truckload are tested for percent moisture and bulk density.For ash,carbon black, acetone extract, and rubber hydrocarbon content, testing is doneoncea day per shipmentaccording to ASTMD-5603-96.Specific frequency and sampling procedures may be agreed upon between vendor and the customer. For ambient ground materials, moisture content and bulk density measurements are made on every
Recycling of Rubber
a43
skid because of possible moisture content variations in feedstock. Vendors should send material specification conformance data along with shipment to customers.
13. CHEMICAL ANALYSIS AND MATERIAL SPECIFICATIONS
ASTM D-5603-96 lists specificationsfor acetone extractable, ash, moisture, carbon black, natural rubber, and rubber hydrocarbon contents for recycled rubber. As mentioned in this document, these chemical tests are done according to ASTM D-297. This specification also lists maximum metal content of 0.1% and fiber content of 0.5% in whole tire crumb. Fiber and metal contents in tread buffing should be zero. In production of recycled rubber steel wire pieces are separated by magnetic separator. Fibers are separated by use of vibrating screen table and “vacuuming” off the “fabric balls” from the top of the screen. To determineiron content, a preweighedamount of recycled rubber is spread on a nonmagnetic flat surface. A small magnet is used to go over the material, which should pick up steel pieces. The weight of the material thus picked up is obtained. However, for very small particles of crumb rubber (c100 mesh), the magnet may pick up recycled rubber particles as well. In that case atomic absorption (AA) should be used to determine amounts of iron.
14. PARTICLE SIZE AND DISTRIBUTION
Particle size and distribution of recycled cured rubber particulates are determined by the RoTap method as described in ASTM 5644-96. Six sieves are used in this mechanical shaker. The first two screens are defined in the above document for 10, 20, 30, 40, 60, 80, and 100 mesh particle size designations (see Table 6). The remaining four screens are to be decided upon by the vendor and customer. About 100 g of crumb rubber are weighed and put on the top pan with a cover and five other screens. After a fixed time of running the shaker, materials in each panareweighedandplotted as afunction of screensizegivingparticlesizedistributions. Vibrators and sieves areavailable from most scientific suppliers. This technique works well for
Table 6 Recycled Rubber Product Deslgnation Nominal product designation
Example ASTM D 5603 designation”
Zero screen (km)
Percent retained on zero screen
10 Mesh 20 Mesh 30 Mesh 40 Mesh 60 Mesh 80 Mesh 100 Mesh
Class 10-X Class 20-X Class 30-X Class 40-X Class 60-X Class 80-X Class 100-X
2360 (8 mesh) 1180 (16 mesh) 850 (20 mesh) 600 (30 mesh) 300 (50 mesh) 250 (60 mesh) 180 (80 mesh)
0 0 0 0 0 0 0
Size designation screen (km)
2000 (10 mesh) 850 (20 mesh) 600 (30 mesh) 425 (40 mesh) 250 (60 mesh) 180 (80 mesh) 150 (100 mesh)
Maximum percent retained on designation screen
5 5 10 10 10 10
10
“When specifylng materials replace the X with the proper parent materlal grade deslgnation code. For example Class 30-2 would indicate a 600 k m (30 mesh) product made from Grade 2 material. car. truck, and bus tread rubber. Class 100-6 would indicate a 150 k m (100 mesh) product made from Grade S material, nontlre rubber.
a44
Klingensmith and Baranwal
coarser particles (>S0 mesh). For 80 mesh and finer, small balls are formed as a result of particle agglomeration on screens giving higher “apparent” particle sizes than they really are. Several other techniques for determining finer particle sizes are being evaluated by Dr. Baranwal in our laboratory. We have developed an ultrasonic technique where a small quantity of crumb rubber is put in a nonsolvent liquid exposed to low levels of ultrasonic energy. The resulting dispersion is put on a glass slide and dried. Using an image analysis software program, particle size distribution is obtained. Our experience is that this technique works well even with small particles, i.e., up to 1-2 pm. Some of the other commercially available techniques are those from Coulter Corporation (Miami, Florida), Malvern Instruments Limited (Southborough, Massachusetts), Particle Sizing Systems (Langhorn, Pennsylvania), and Elcan Industries, Inc. (New Rochelle, New York).
REFERENCES 1. Scrap Tire Use / Disposal Study (1996), Scrap Tire Management Council. 2. Scrap Tire Recovery, An Analysis of Alternatives. ( 1 9 9 8 ~published by Goodyear Tire and Rubber Company. Akron, Ohio. s, 23, 1998. pp. 22-23. 3. Ruhher rrnd Plostics N ~ M ~February 4. Best Prrrcticrs it1 Scrtrp Tires & R&!wr Recycliug. Clean Washington Center, June 1997. 5 . Scrap Tire Processing in the US, Tire Technology International 95, by Charles Astafan, 1995. 6. U.S. Pat. 4,104,205. Microwave Devulcanization of Rubber, August 1, 1978. 7. U.S. Pat. 5,258,413. Ultrasonic Devulcanization, November 1993. 8. European Pat. Application EP 0690 901 A l , Application No. 95301399.2, filed 03.03.1995. 9. Ultrasonic Devulcanization of Tire Compounds and De Link Concept Tire Technology International 1996, pp. 82-84, 87-88. IO. U.S. Pat. 4,481,335 Rubber Compositions and Methods, issued to Fred Stark, Nov. 6, 1984. 1 I . U.S. Pat. 5.510,419, Polymer Modified Surface, issued to Burgoyne. Fisher, and July, April 23, 1996. 12. ASTM D-5603-96. Standard Classification for Rubber Compounding Materials-Recycled Vulcanizatc Particulate Rubber. 13.ASTMD-5644-96,StandardTestMethodforRubberCompounding Materials-Determination of Particle Size Distribution of Recycled Vulcanizate Particulate Rubber. 14. Chicago Board of Trade, Crumb Rubber (Tire or Non-Tire), CrumbRubber Grades Definitions, 1997.
EPDM Rubber Technology Richard Karpeles Crompton Corporation/Uniroyd/ Chemical Company, Inc., Naugatuck, Connecticut
Anthony V. Crossi Crompton Corporation/Uniroya/ Chemical Company, Inc., Middlebury, Connecticut
1. INTRODUCTION 1.l
Nomenclature and Structure
EPDM is the designation agreedupon by ASTM and IISRP for ethylene-propylene rubber, where “E“ and “P” stand for ethyleneand propylene, respectively. “D” designates the nonconjugated diene that provides a site of unsaturation for sulfur vulcanization, and “M” refers to the polymethylene saturated backbone of the polymer, i.e., CH2-(CH2),,-CH2(Fig. 1). Several good reviews on EPDM rubber have been published by Baldwin and Ver Strate (1972). Ver Strate ( 1986), and Allen and Easterbrook (1987).
1.2 EPDM Weather Resistance The saturated backbone of EPDM is the main structural feature that provides this rubber with its excellent weather and chemical resistance. Sites of unsaturation in a polymer are the primary point of attack for oxidants. Oxidative cleavage of double bonds contained in the backbone of a polymer. such as a diene rubber, will reduce polymer molecular weight and result in loss of desirable physical properties such as tensile, modulus, tear, oil swell, etc. In EPDM the sites of unsaturationarependant to thebackbone,andoxidation of the double bond will not affect ultimatephysicalpropertiessignificantly.Commercial EPDM containsbetweenzeroand IO wt% nonconjugated diene and therefore contains significantly less unsaturation than styrenebutadiene rubber, butadiene rubber, polyisoprene, etc.
1.3 Interaction with Oils and Carbon Black EPDM is a nonpolar substrate. It has excellent compatibility with aromatic, naphthenic, and paraffinic mineral oils. BecauseEPDM’s level of unsaturation is lowerthan thatof diene rubbers, EPDM interactslessstrongly with carbonblackthan diene rubbers. This results in a very 845
846
Karpeles and Grossi
CH 3 Fig. 1 Structurc of EPDM containing ENB termonomcr.
reasonable compounded Mooney viscosity and good physical and mechanical properties, even at very high filler loading.This makes EPDM an economical rubber for many common applications.
1.4
HighMolecularWeightEPDM
For years EPDM was available only as a high molecular weight rubber ranging from 300,000 to greater than 1,000.000 daltons as measured by get permeation chromatography (GPC) using polystyrene (PS) standards. All molecular weights referenced in this chapter will be PS equivalent. GPC of high molecular weight EPDM is carried out at 150°C using ortho-dichlorobenzene or trichlorobenzene as solvent. Care must be taken when comparing data in patents and open literature, where both polystyrene and polyethylene equivalent molecular weights are reported. For linear polymers in orrho-dichlorobenzene, the polyethylene equivalent molecular weights are a factor of approximately 2 less than polystyrene equivalent molecular weights. This factor decreases as branching increases. High molecular weight EPDM is used primarily in construction and automotive markets. Included in constructionapplicationsaresingle-ply roof sheeting, doorandwindowseals, spacers, wire and cable sheathing and insulation. and hoses and seals for water systems. Diverse automotive application areas include radiator hoses. sponge and dense weather seals, wire and cable sheathing and insulation, thermoplastic elastomer and thermoplastic vulcanizate bumpers and interior surfaces, engine oil thickeners and dispersant-thickeners, tire inner tubes, tire sidewalls, seals (gaskets and O-rings), air ducts, bellows, shock mounts, and belts. Although used primarily in construction and automotive market areas. EPDM also finds application in diverse markets, such as medical devices, oil well and pipeline applications, rolls, sporting equipment, impact modification of engineering plastics by functionalized EPDM. etc. Jebens and Kaufman (1996) have published a marketing research report on ethylene-propylene elastomers. Recent advances in process engineering have now extended the molecular weight range of the EPDM family to cover a continuum of molecular weights, including traditional high molecular weight products, intermediate molecular weight products ranging between 80.000 and 300,000 daltons.andlowmolecularweightmaterialswithmolecularweightsup to 80,000 daltons.
1.5
Low MolecularWeightEPDM
In addition to many of the traditionalrubberapplications. EPDM products of intermediate molecular weight provide advantages in molded and extruded applications, as components of
EPDM Rubber Technology
thermoplastic elastomers, as lube oil thickeners, and as reactive plasticizers weight rubber compounds. 1.6
847
for high molecular
Very Low Molecular Weight EPDM
Lowmolecularweightproductsrangingfrom liquid oligomers to polymers with molecular weights up to 80,000 daltons are alsocommercially available in both copolymer and terpolymer grades from Uniroyal Chemical under the tradenameTrileneBand copolymer grades from Mitsuiunder the trade name Lucant@. These productsaremadewithbothmetallocene and traditional Ziegler-Natta catalysis. The GPC of very low molecular weight EPDM is run at low temperature (e.g., 35°C) in tetrahydrofuran. These unique very low molecular weight products find use as reactive plasticizers, encapsulants, viscosity modifiers, synthetic oil components, adhesives, and sealants.
2. 2.1
EPDM PHYSICAL FORM Bales
EPDM is most commonly supplied in solid rectangular bales, which can weigh between 25 and 34 kg (55 to 75 Ib), and can vary in density from 0.5 to 0.86 gkc. Bales are individually wrapped in an ethylene vinylacetate or a polyethylene film. The film-wrapped bales are packaged in a multitude of ways, including reusable and nonreusable cardboard containers on wood pallets, reusable aluminum containers,high-densitypolyethylenestretchwrapping on woodpallets, wood boxes. etc. To obtain faster mixing, bales can be supplied in a low-density, “friable” form. These friable bales break apart more easily in internal mixers allowingfor faster carbon black dispersion Friable bales are usually reserved for EPDM grades with high ethylene content. because they contain enough crystallinity and green strength for bale shape retention. Polymers with low ethylene content are supplied in dense bales because their lack of crystallinity and low green strength will cause the polymer to flow and coalesce. resulting in nonuniform bale densification.
2.2
Pellets
A small number of product grades are supplied in pellet form. These products are sold into the
wire and cable and thermoplastics markets where continuous extruder feeding is required. Pelletized products also tend to have a high ethylene content to avoid coalescence of the pellets into a solid mass. Even with a high ethylene content, EPDM pellets must be treated with an antiagglomeration agent, such a s polyethylene dust, to keep them free-tlowing.
2.3
Crumb Rubber
A limited number of manufacturers offer granulated crumb rubber as an alternative product form but this product form is very difficult to store. Due to the irregular shape of the granulate, the granulated EPDM coalesces quickly and densifies.
2.4 Latex No EPDM manufacturers offer product in a latex form. EPDM latex stability tends to be poor, and upon storage the solidswill rise. This process, called creaming, can be reversed by agitation.
Karpeles and Grossi
848
As withall latexes, EPDM latex is susceptible to biological attack. EPDM latex finds applications in coatings and in blends with other latexes.
2.5
Extender Oils
EPDM grades at the high molecular weight end of the spectrum (i.e., >600,000 daltons) are routinely extended with 50- 100 parts of mineral oil per 100 parts of rubber to lower the raw polymer’s viscosity. The reduction in viscosity of high molecular weight EPDM grades resulting from oil extension benefits both the manufacturer, by providing easier polymer finishing in the back end of the manufacturing process, and the end user, by providing improved processing (easier and/or faster mixing). Many different types of extender oils are used, but there is a clear trend toward the use of paraffinic oils (especially white oils) and away from use of aromatic oils (due to toxicological concerns) and naphthenic oils, which are usually darker in color than the paraffinic oils. Additional benefits to the use of white oils are improved stability toward exposure to both sunlight and tluorescent light and improved raw polymer color for colored (non-carbon black containing) end-use applications.
3. EPDMHEATSTABILITY EPDM is manufactured containing phenolic antioxidants to ensurestorage stability. The antioxidants are nonstaining for automotive applications in proximity to painted surfaces. When stored in a cool, dark environment, EPDM should have a long shelf life. Exposure to heat (Baranova et al. 1970),light, or chemical agentswill reduce the polymer’s shelf life. In general, raw EPDM rubber is stable at elevated temperatures up to 150°C for short periods of time. During the heat-induced EPDM degradation process, both chain scission and crosslinking occur (Saha Deuri and Bhowmick, 1987). This dynamicprocess can be studied using rheological measurements such as complexviscosity and tangent delta (the ratio of the viscous to the elastic modulus). The value of tangent delta is very sensitive to small increases in branching, which simultaneously decreases the viscous modulus and increases the elastic modulus. Tangent delta also reveals the increase in the viscous component due to chain scission. With phenolic antioxidants, it can be observed that early in the heat aging process, tangent delta does not change significantly, but there is evidence that chain scission is occurring by the tackiness of the polymer surface. It must therefore be concluded that during this time period, chain scission and crosslinking are occurring simultaneously. Later in the degradation process, however, crosslinking dominates and the viscosity rises and tangent delta drops. Addition of phosphite co-antioxidant appears to shift the balance between the degradation pathwaysto favor crosslinking, eliminating the early tackiness noted when using phenolics alone. Figure 2 shows the effect of heat aging at 121°C on EPDM’s viscosity and tangent delta. The presence of transition metal impurities greatly affects the stability of EPDM. Vanadium, iron, and other transition metals are pro-oxidants that catalyze degradation.
4. 4.1
EPDMLIGHTSTABILITY Sunlight
All EPDM is susceptible to degradation due to exposure to sunlight. Although EPDM grades containing ENB termonomer exhibit the greatest sensitivity to sunlight, DCPD- and 1,4-IID-containing terpolymers and ethylene-propylene copolymers also exhibitsensitivity to sunlight expo-
849
EPDM Rubber Technology
1,01E*06
8,iOEtOS
5
.-b VI
0 0
p5 6.10Et05
.U vi
5s c-!
0""
4'i0E+05
-p
F
U
2,10E*05
1,WEt04
0
50
100
200
150
3W
250
350
4W
450
500
Hours at 125OC
Fig. 2 Dynamic viscosity and tangent delta of heat-aged EPDM.
sure. Lightexposure results in formation of hydroperoxides (De Paoli, 1988; De Paoli and Duek, 1990; De Paoli et al., 1990; Chmela et al., 1996) that ultimately cause surface gelation via crosslinking. When the raw polymer additionally contains extender oils, sensitivity to sunlight (i.e., the potential for gelation)is even greater. Tangent delta, again,is a useful tool for determining the presence of surface gelation (see Fig. 3).
1
-
75 phr Oil Exlended High ENB
VI
.
Grade
I% 0,95
1 V-
5
0~9
:
X 0.85
W
-
8
W
d .
Non-all Extended Hlgh ENB
2-
0,8
Grade
0,75
___~
~-
0,7 0
1
2
3
4
Hours of Sunlight Exposure
Fig. 3 Effcct of sunlight exposure on tangent delta of EPDM.
5
6
7
Karpeles and Grossi
850 1.2 Non-Oil Extended High EN6 Grade
II
75 phr 011Extended Hgh ENB Grade
0 0
5
10
15
20
25
Days of Fluorescent Light Exposure
Fig. 4 Fluorescent light exposure effect on EPDM tangent delta
4.2
FluorescentLight
Surface gelation also occurs due to fluorescent light exposure of oil-free and oil-extended grades of EPDM containing ENB (see Figs. 4.5). The rate of surface gelation occurs in the following order: ENB grades containing aromatic extender oils > ENB grades containing naphthenic oils > ENB grades containing paraffinic nonwhite oils > ENB grades containing paraffinic white oils > oil-free ENB grades. In general, DCPD- and 1,4 HD-containing polymers and ethylenepropylene copolymers donot exhibit sensitivity to fluorescent light, even when oils are present.
45
Days Exposure to Fluorescent Light
Fig. 5 Branching Gel formation of light-exposed EPDM.
EPDM Rubber Technology
851
5. EPDMSTRUCTURE,COMPOSITION, AND PROPERTIES As discussed previously, the versatility of EPDM rubber arises from: 1. Its unique combination of weather and heat resistance due to its saturated polymethylene backbone 2 . Its reasonable compounded cost, because its high molecular weight allows for high extendibility with inexpensive oils and fillers 3. The structural diversity that can be designed into the polymer by manufacturers.
Controllable structural propertiesinclude molecular weight (MW), molecular weight distribution (MWD), diene type and content, level of branching, ethylene/propylene monomer ratio, monomer distribution along the polymerchain, and the homogeneityor heterogeneity of different polymer chains. EPDM structural properties are influenced by a variety of factors in the polymerization process, which will be discussed in the following section. 6. THE EFFECT OF ETHYLENE AND PROPYLENE CONTENT ON EPDM
PROPERTIES The character of EPDM changes greatly based on the ratio of ethylene to propylene in the polymer. An ideally alternating ethylene-propylene copolymer would contain 40 wt% ethylene (50 mol%) and 60 wt% propylene (50 mol%). Polymers of this composition are amorphous. Commercial polymers, however, generally contain between50 and 80 wt%ethylene. The boundary values of ethylene were chosen for practical reasons. Above 75% ethylene, EPDM is extremely hard and difficult to mix in internal mixers. Below 50% ethylene, traditional vanadiumbased Ziegler-Natta catalysts have difficulty incorporating propylene at an acceptable commercial production rate. These catalysts exhibit significantly higher reactivity toward ethylene than toward propylene. Manufacture of propylene-rich EPDM grades is therefore slower and more costly. As additional carbon atoms are added to the length of the alpha-olefin chain, the ZieglerNatta catalyst’s reactivity toward the alpha-olefin decreases. The effect of polymer structure on low-temperature properties (Kontos and Slichter, 1962; Martini and Milani, 1986; Avella et al. 1987; Mahlke,1987)and the relationshipbetweenglasstransitiontemperatureandpolymer composition (Baldwin and Ver Strate, 1972) have been reported. 6.1
Effect of Ethylene Content on Crystallinity
Low Ethylene Content Polymers with ethylene contents at the low end of the commercial range, i.e., containing 50-55 wt% ethylene, are totally amorphous and exhibit no ethylenecrystallinity above their low glass transition temperature (Fig. 6) as observed by differential scanning calorimetry (DSc). They are soft and pliable and have excellent low-temperature flexibility and compression set, but they cannot accept high levels of fillers. Between 56 and 62% ethylene, EPDM containslonger and/or morefrequentethylene sequences and exhibits a below room temperature melt transition as seen by DSc. This lowtemperature crystallinity only influences the most demanding low-temperature applications. Interinediate Ethylene Content Products with intermediate ethylene content, (63-67%) will contain both below room temperature crystallinity and a small amount of higher-temperature crystallinity between 40 and 60°C.
Karpeles and Grossi
852
45
40
-
35
€/P = 75/25 L
E
g
a
25
EIP = 68/32
L 20 CI
g
15
€/P = 51/49
I 10
5 0
m
-61 -71 -81 -91
-51
4 1-11 -21 -31
-1 19 9
29
39
49
59
69
79
89
99
1W138 128 119
148
Temperature ("C) Fig. 6 DSC thcrmograrns of EPDM with varying ethylene/propylene ratios.
High Ethylene Content High-ethylene polymers (6840%) exhibit high green strength, high vulcanizate tensile strength and toughness at room temperature, and they can accept high filler loading. These products, however. will have inferior low-temperature properties due to significant crystalline melt transitions, both below room temperature and at 40-60°C. EPDM produced by conventional vanadium-based Ziegler-Natta catalysis generally does not have any melt transitions above 60°C and does not contain any lamellar crystals indicative of true polyethylene crystallinity. Polymers produced with titanium-based catalysts can contain melt transitions above 60°C.
6.2 AnalyticalTechniques The monomer composition of EPDM is measured by Fourier transform infrared spectroscopy (FTIR) using a transparent thin film of rubber (Noordermeer, 1996). The test methodology is described in ASTM D3900 (1994). The infrared instrument is calibrated using standard EPM polymers whose composition has been determined by nuclear magnetic resonance (NMR) as described by DiMartino and Kelchtermans (1995). The EPDM industry has standardized this test method, but not the reporting format. Reporting options include: wt% Ethylene 100%) 2. wt%Ethylene 3 . mol%Ethylene
+
1.
wt% Propylene = 100% (thediene content is additional to the
+ wt%Propylene + wt% Diene = 100% + mol%Propylene = 100% (thediene content is additional to the
100%)
4. mol% Ethylene
+ mol%Propylene + mol%Diene
=
100%
When comparing EPDM grades made by multiple manufacturers, the method of compositional reporting must be considered.
853
EPDM Rubber Technology
3 ENB
DCPD
1,4-HD
Fig. 7 Commercialdienes used for EPDM.
7. DIENES The choice of diene heavily influencesEPDM properties and structure. Three dienes are currently usedinthemanufacture ofEPDM: 5-ethylidene-2-norbornene (ENB), dicyclopentadiene (DCPD), and 1P-hexadiene (HD). Their structures are shown in Figure 7. Cyclic dienes suchas ENB and DCPD influence thelow-temperature properties of EPDM by increasing the glass transition temperature (Tg) due to their rigid structure, but they also reducecrystallinity by breakingup ethylenesequences. VerStrate (1972) reported that Tg increases 0.8"C for every wt% ENBin the polymer, up to a maximum of 10%.Linear, nonconjugated dienes similarly reduce ethylene crystallinity by breaking up long ethylene sequences and increase Tg to a lesser extent than cyclic dienes.
7.1 ENB ENB termonomer is the most widely used by EPDM manufacturers because ENB is a fastcuring diene with a sulfur cure system due to the six allylic hydrogens on carbon atoms adjacent to the olefinic bond (Baldwin et al., 1970). The allylic hydrogens are the sites of attack of the cure system. ENB is a bicyclic, nonconjugated diene that incorporates effectively into EPDM during the polymerization process.This occursbecause the double bond contained in the bicyclo[2.2.l.]heptene (norbornene) portion of the ENB structure places a strain on the ring system. This strain is eliminated when the monomer is incorporated into the polymer backbone and the double bond is eliminated. The second, noncyclic double bond is available for crosslinking. Use of ENB termonomer results in a product with low to moderate long-chain branching. Branching arises during thepolymerizationprocess from cationicsidereaction of ENB's double bond outside of the ring structure. This branching reaction is easily controlled to achieve specific levels of branching, which provides the desired processability.
7.2 DCPD EPDM grades containing DCPD termonomer are not as popular because DCPD is slower curing than ENB. It contains only three allylic hydrogens on carbon atomsadjacent to the double bond that can participate in vulcanization. DCPD contains two cyclic double bonds, and like ENB, the double bond at the 2 position of the bicyclo[2.2.1 .]heptene portion of the molecule incorporates easily into the polymer backbone. The second double bond in the attached five-member ring is available for vulcanization. Unlike ENB, however, the second double bond in DCPD can participate in the Ziegler-Natta polymerization, resulting in a highly branched polymer with
a54
Karpeles and Grossi
broad MWD. DCPD that has participated in branching is not available for crosslinking, further reducing the polymer’s cure rate. Moreover, polymers with broad MWD are generally slower curing.
7.3 1,4-HD
1,4-HD is a linear nonconjugated diene. It has one terminal and one internal double bond. The terminal double bond is incorporated into the polymer backbone, and the internal double bond is available for vulcanization. Although the internal double bond has five hydrogens on adjacent carbons, it is much slower curing than ENB. One possible explanation reported in the patent literature is that up to 25% of the 1,4-HD isomerizes to yield a saturated cyclic structure (U.S. 3,467,633, 1969)that cannot take part in sulfur-based crosslinking. 1,4-HDtermonomer is essentially a long-chain alpha-olefin and therefore is less reactive towards the Ziegler Natta catalyst. It also does not have the elimination of ring strain as a driving force for reaction like ENB or DCPD, 1,4-HD inherently provides a linear polymer structure with narrow MWD because the internal double bond is inactive toward either Ziegler Nattd catalysts or acid-catalyzed cationic branching.
7.4 AnalyticalTechniques Diene content can be determined by either high-temperature refractive index (HTRI), FTIR of thick polymer films, or NMR spectroscopy. A standardized test method has been adopted based on athickfilminfraredtechnique (Noordermeer,1996). Although not practical for routine quality control testing, NMR spectroscopy is employed to certify the reference standards used to createtheFTIRcalibrationcurves. The FTIRtechniqueprovidessignificantlyimproved (lower) standard deviation than the HTRI test.
7.5
Branching
Branching in EPDM can be measured indirectly by the “branching index,” the logarithm of the ratio of the zero-shear viscosity of an EPDM to that of a linear copolymer having the same intrinsic viscosity (Beardsley and Tomlinson, 1990). Tangent delta, the ratio of the viscous to elasticmodulus, is also an indicator of branching (BeardsleyandHo, 1984), but itis also influenced by changes in MWD. In general, the lower the tangent delta, the greater the branching/ MWD. Branching greatly influences the viscosity of compounded EPDM. Branched or broad MWD polymers are more non-Newtonian and hence are lower in compounded viscosity. The ratio of the raw polymer’s Mooney viscosity to the compounded Mooney viscosity provides information on the relative branching of different grades of EPDM rubber with the same raw Mooney viscosity. A lower ratio indicates a lower level of branching.
8.
RHEOLOGICAL PROPERTIES AS RELATED TO EPDM STRUCTURE
EPDM structure impacts on rheological properties like tangent delta and dynamic viscosity. Dynamic testing is very sensitive to small differences in structure. This is illustrated in Table
855
EPDM Rubber Technology Table 1 Polymer Properties for Four Widely Differing EPDM Polymers Polymer A
Polymer B
Polymer C
63 53/47 0 4.3 I .9 2.2
62 57/43 2.0 ENB 5.0 1.7
60 52/48 2.0 ENB 5.7 I .7 3.3
+
MLl 4 at 125°C E P , Wt. Ratio Diene content, o/o MW ( X lo-'), (PS equiv.) Mn ( X MwlMn 3.0
Polymer D
66 56/44 3.0 DCPD 4.6 l .2 3.7
1 by a series of four polymers (A-D) varying greatly in branching and MWD (Beardsley and Wortman, 1997). 8.1
DynamicViscosity vs. Frequency
Polymer A is a linear ethylene-propylene copolymerwith narrow MWD. Polymers B and C are ENB terpolymers with an intermediate level of branching but are differentiated by Polymer C's slightly broader molecular weight distribution. Polymer D is a highly branched DCPD terpolymer. All four polymers have Mooney viscosities (ML1 + 4 at 125°C) between 60 and 66. Data in Figure 8, gathered on an RPA-2000 at 1OO"C, show the dynamic viscosity over a range of frequencies for each polymer. The data illustrate the effect of structural differences on dynamic mechanical properties. At low shear rates, the polymers are ranked according to their level of branching. Thus, linear Polymer A has the lowest viscosity and branched Polymer D has the highest viscosity at the lowest shear rate. The curves cross overnear the Mooney viscosity shear rate. This is expected since the polymers have similar Mooney viscosities. At the highest shear rates, the polymers are ranked in order of their number average molecular weights.
+A
(linear EPM)
-C
( 2.0% ENB, 3.3 Mw/Mn)
D ( 3.0% DCPD, 3.7Mw/Mn) 0
1
1
2
5
10
21
52
105
209
Frequency in radianslsecond
Fig. 8 Dynamic viscosity vs. frequency for EPDM polymers varying in branching and MWD
Karpeles and Grossi
856
-x-
A (linear EPM)
-C-
B ( 2% ENB, 3.0 MwlMn) ( 2% €NB. 3.3 MwlMn)
-+C
D ( 3% DCPD. 3.7 MwlMn)
9 .
0.0
' 0
1
10
100
1000
Frequency radiandsecond
Fig. 9 Tangentdelta vs frequency for EPDM.
The effect of MWD is shown by comparison of Polymers B and C. Due to its broader MWD. Polymer C has a slightly lower viscosity at higher shear rates than Polymer B. 8.2 TangentDeltaVersusFrequency
Figure 9 shows the relationship between tangent delta and frequency for the four polymers. Tangent delta differentiates the polymers more dramatically than the dynamic viscosity. The linear Polymer A has a very high tangent delta at low shear rates, which indicates that polymer flow during storage will occur without rigid packaging. The branched Polymer D has the lowest tangentdelta from the lowest frequency to the crossover point,indicating a high degree of elasticity. As with dynamic viscosity, the tangent delta value is heavily influenced by MW at high shear rates. Polymer A, which has the highest MW, has the lowest tangent delta, and Polymer D, which has the lowest MW, has the highest tangent delta at the highest shear rates. Polymer C has a broader molecular weight distribution than Polymer B. It shows a lower tangent delta at lowshear rates thanPolymer B, indicating increased elasticitydue to the presence of a higher MW fraction.
9.
EPDM MANUFACTURING PROCESSES
First commercialized in 1962, the nameplate worldwide production capacity for EPDMcurrently stands at 920,000 metric tons per year. Table 2 lists the major worldwide manufacturers of EPDM and their respective product trade names. Solution-based manufacturing processes are utilized for approximately 85% of this capacity. The remaining 15% of the EPDM production capacity utilizes a slurry phase polymerization
857
EPDM Rubber Technology Table 2 MajorWorldwideManufacturers of EPDM
Company
Trade name
Uniroyal Chemical Exxon DSM (Copolymer, Nitriflex, DSM-Idemitsu) DuPont-Dow Enichem B ay er
Royalene Vistalon
Sumitomo Mitsui
Esprcnc Mitsui EPDM JSREPDM Herlene Kumo EPDM
JSR Herdillia Unimers Kumo
Keltan Nordel
Dutral Buna
process.Most of thisexistingcapacity is based on Ziegler-Nattachemistry. New capacity, totaling 90,000 MTA, was recently brought onstream by DuPont-Dow based on a metallocene solution process. Future capacity of 90,000 MTA has been announced by Union Carbide based on a gas phase process. Process descriptionsof solution and slurry phase manufacturing facilities have been extensively analyzed and reviewed. An excellent report of early patents, economics, and process flow diagrams for solution and slurry phase processes was provided in SRI Report 4B (1981). The information was updated in SRI Report 4C (1990) to include an assessment of the gas phase EPDM process in comparison to the solution and slurry processes.
9.1
The Solution Process
In the traditional solution process (U.S. 8,341,503, 1967; SRI Report 4B, 1981: SRI Report 4C, 1990) chilled monomers and solvent, vanadium catalyst, aluminum cocatalyst, and polymerization modifiers are fed into the polymerization reactor. Chilling monomers and solvent aids in removing heat from the exothermicpolymerization process. The reaction is carried out between 40 and 80°C. Temperatures above 80°C are not utilized due to the temperature instability of vanadium-based Ziegler-Natta catalysts. When a vanadium catalyst species comes in contact with an aluminum cocatalyst, the vanadium catalyst is reduced from its original vanadium (IV) or (V) oxidation state to the vanadium (111) oxidation state, which is the active oxidation state for EPDMpolymerization. The developingpolymer chains aresoluble in the hydrocarbon solvent and forma“cement.” This polymer cement can vary from4 to 15% “solids”(polymer) depending on the molecular weight of the polymer and the temperature of the polymerization system. After polymerization, the reaction is terminated, monomers are removed, and the cement is washed to remove metals left over from the catalyst system. Solvent is then removed via one of two approaches:
1. Steam flocculation, to give an aqueous slurry of polymer crumb. The wet crumb is dewatered, dried, baled, weighed, checked for metal impurities, film-wrapped, and packaged. 2 . Direct solvent evaporation by mechanical means, providing dry polymer that is extruded and pelletized or baled. Similarly, the EPDM is then weighed, checked for metal impurities, and packaged.
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9.2TheSlurry
and
Karpeles
Process
In the slurry process(SRI Report 4B, l981 ;Galli et al., 1985; SRI Report 4C, 1990) the polymerization occurs in liquidpropylenemonomer. Thedeveloping polymer is not soluble in the polymerization medium. A casolvent is used to swell the polymer particles to aid in washing out the catalyst residues. After polymerization, the reaction is terminated and the co-solvent and the propylene are removed via steam flocculation to give an aqueous slurry of polymer crumb. The back end of the slurry EPDM plant looks very much like that of a solution plant. Simplified slurry processes have been developed for the production of EPM and are described by Galli et al. (1985).
9.3TheGasPhaseProcess Process development work toward the commercial application of gas phase processes for EPDM is currently being carried out. No commercial terpolymer products are currently manufactured via a gas phase polymerization process. U.S. patent 4,7 10.538 (1987) describes a process in which chilled gaseous monomers, catalyst, and “inert filler” are injected into a fluidized bed reactor. The growing polymer particles, by design, become coared with inert filler to prevent particle agglomeration. The coated particles are removed from the reactor, and unreacted monomer is removed with a purge stream of hot inert gas or steam (U.S. 5,05 1,546, 1991). The particulate is then packaged.
10. ZIEGLER-NATA CATALYSTS AND COCATALYSTS 10.1Ziegler-NattaCatalysts The subject of Ziegler-Natta catalysis has been extensively reviewed by Boor ( 1 979), Kaminsky and Sinn (1980), and Chandrasekhar et al. (1988). Most manufacturers utilize vanadium-based Ziegler-Natta catalysts for the production of EPDM rubber (Brett et al., 1971; Baldwin and Ver Strate, 1972). Titanium (Ti)-based catalysts are only utilized for the production of EPR (Galli et al., 1985), because they are not effective in incorporating dienes into the polymer. Moreover, titanium catalysts tend to produce more crystalline polymer than their vanadium counterparts. Vanadium catalysts cited in the literature include vanadium oxytrichloride (V0Cl3), vanadium tetrachloride (VC14), vanadium acetylacetonate (V[AcAcI3), and vanadates (VOCI,[OR]3.,). The above catalysts are used in conjunction with aluminum based cocatalysts to form the active Ziegler-Natta catalyst species. No simple correlation can be made between the vanadium catalyst type and the polymer structure produced.The molecular weight of the resultant polymer is directly dependent on the amount of catalyst used in the polymerization, i.e., using higher quantities of catalyst will lower MW, and conversely, use of lower levels of catalyst will raise MW. MW and MWD are measured by a size exclusion chromatography technique known as gel permeation chromatography (GPC).
10.2 Cocatalysts Aluminum (AI) cocatalysts are required to activate (reduce) the vanadium (IV) or vanadium (V) catalyst to vanadium (111), the active state for EPDM polymerization. Common aluminum cocatalysts include diethylaluminum chloride (DEAC) [(CH3-CH2),AICI], ethylaluminum sesquichloride(EASC) [(CH3-CH2)3A12C13],andethylaluminum dichloride(EADC) [(CH3CH?)AICI?].
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10.3 CocatalystKatalyst Ratio Effect on Polymer Structure The ratio of AI to V used in the polymerization system controls the solubility of the catalyst system. Changes in catalyst solubility affects monomer incorporation, branching, and molecular weight distribution (Brett et al., 1971). At high A I N ratio, the catalyst is homogeneous, soluble, and “single-sited.’’ Soluble catalyst provides random monomer distribution, a narrow molecular weight distribution,lower levels of branching, and a faster curerate. For a givenethylene content, the crystallinity of a polymer made at a high ratio will be lower than one made at a low ratio. EPDM made by this technology is often excellent in mixing but poor in mill processing. At low A I N ratio, the catalystis made up of both soluble and insoluble components and is multisited. The multisite catalyst produces a broad molecular weight distribution polymer with a more heterogeneous intramolecular monomer and diene composition, a slower cure rate, and a lower cure state. Polymer chain composition varies with the MW of each individual chain. EPDM polymers madewith lowA W ratio exhibit excellentmill processing due totheir broad molecular weight distribution.
10.4 Cocatalyst Effect on Branching and Rheological Properties Polymer structure is influenced heavily by combining the catalyst of choice with a variety of cocatalysts.Cocatalysts vary in theiracidity (Baldwin and Ver Strate, 1972) based on their chlorine-to-aluminum ratio. The higher the CVAI ratio, the higher the acidity. For the above three cocatalysts, the acidity and the Cl/AI ratio decrease in the following order: EADC (CI/Al = 2) > EASC (CI/Al = 1.5) > DEAC (CI/AI = l ) ) . Use of more basiccocatalystslike DEAC provide a linear but heterogeneous polymer structure with blocky ethylene sequences and polymer chain compositions that vary with MW. Use of a very acidic cocatalyst such as EADC with ENB-based terpolymers canproduce amorphouspolymers and high levels of branching (Baldwin and Ver Strate, 1972). Thus, the cocatalyst can be a valuable tool in modifying EPDM structure, i.e., branching, to fit the customer’s processing needs. EPDM manufacturers choose cocatalyst and catalyst types to tailor the product’s structure. Table 3, showstheproperties of threepolymersthat are similarinterms of Mooney viscosity and polymercomposition (Beardsley and Wortman,1997). The polymerswere prepared with three different cocatalyst systems, each designed to impart varying levels of branching in the polymer through changes in the catalyst system’s acidity.
Table 3 Polymer Properties for Three Similar EPDM Polymers Prepared with Catalysts of Varying Acidity Polymer E
Polymer F
High 67 60140 8.5 4.3 1.6 2.6
Intermediate 67
Polymer G ~~
Relative catalyst acidity MLl 4 at 125°C E P Wt. Ratio60140 ENB content, c/o MW ( X lo-’) (PS equiv.) Mn ( X IO-’) Mw/Mn2.5
+
8.7 4.2 1.6
Low 65 60140 8.7 4.3 1.9 2.3
Karpeles and Grossi
860
-E
(Acidic Calalysl)
"... F (Intendlate)
(LeastAadic)
-G
01
10
1
FrequencyIn radiandsecond
Fig. 10 Dynamic viscosity vs. frequency for different catalyst systems of varying acidity.
Figure 10 illustrates the dynamic viscosity vs. frequency profiles obtainedon a rheometrics dynamic spectrometer at 150°C. The behavior of linear Polymer G, prepared with the more basiccatalystsystem,closelyresembles that of linear Polymer A discussed in theprevious section. Branched Polymer E, prepared with the acidic catalyst system, correlates well with the behavior of branched Polymer D. The catalyst of intermediate acidity produces polymer whose
10
1
Frequency in radiandsecond
Fig. 11 Tangent delta vs. frequency for different catalyst systems.
100
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viscosity falls between that of Polymers E and G, produced by the acidic and low-acidity catalysts. Figure 1 1 shows the tangent delta versus frequency results for Polymers E through G. The tangent delta curve of Polymer G, prepared with the more basic catalyst system, again correlates well with that of linear Polymer A. Similarly the tangent delta of branched Polymer E prepared with the most acidic catalyst system, resembles the resultsfor Polymer D.As expected, Polymer E, made by the catalyst with intermediate acidity, falls between the two extremes. Dynamic mechanical testing indicates very clearly that more acidic catalyst systems produce increased polymer branching. Since EPDM producers employ different catalyst and cocatalyst systems, their polymers tend to vary slightly in terms of branching level. The variation is, however, normally not as severe as between the three polymers presented in this section.
11. POLYMERIZATIONADDITIVES
Polymerization additives are utilized to control the polymerization and the polymer structure. The patent literature describes the use of chain-transfer agents, bases, activators (oxidants), and branching agents. 11.l
Chain-TransferAgents
The most common chain-transfer agent used to control MW and MWD is hydrogen (Condit et al., 1963; U S . 3,051,690, 1962). Hydrogen acts selectively to reduce or eliminate thepolymer’s high molecular weight fractionand narrow the MWDby terminating the polymer chain’s growth. Use of diethylzinc in place of hydrogen has been reported (Belgium 720,059, 1969). 11.2 Bases The addition of bases, such as ammonia, pyridine, ethers, etc.,serve to eliminate cationic branching ( U S . 3,242,149. 1966) causedby catalyst acidity. Bases can be used in lieu of using a more basic co-catalyst system.This technique will produce linear products with a concurrent molecular weight distribution broadening. 11.3 Activators(Oxidants)
Catalyst activatorscan be utilized with vanadium-based Ziegler-Natta catalysts to “reactivate,” or oxidize, vanadium in the vanadium(I1) oxidation state, which is inactive for EPDM polymerization, to vanadium(III), the active EPDM polymerization oxidation state. Activators include chlorinated hydrocarbons (U.S. 3,349,064, 1967; British 1,020,808, 1966; U S . 4,181,790, 1980; U.S. 4,36 1,686, 1982), such as hexachlorocyclopentadiene,ethyl trichloroacetate, chlorinated malonates, butyl perchloro-crotonate, etc., and nitro compounds ( U S . 3,441,546, 1969). The use of these agents results in increased catalyst productivity (measured by pounds EPDM produced per pound of catalyst). 11.4
BranchingAgents
Nonconjugated olefin branching agents (Christman and Keim, 1968) can be used to create long chain branches and tobroaden MWD. Branching occurs because both doublebonds in the agent
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are very active toward polymerization. Examples of branching agents include 1,5-hexadiene, 1,7-octadiene, vinyl norbornene, and methylene norbornene. With ENB-containing polymers, cationic initiators (Kautt and Kuehne, 1984) can be utilized to branch the polymer, but this technique is less controllable than use of the above dienes. Branching agents are especially necessary for EPDM based on 1.4-HD termonomer, which would otherwise be very linear and have a narrow MWD. Moreover, when metallocene systems are utilized with ENB, DCPD, or 1.4-HD, the resulting polymers are inherently linear, with a narrow MWD. Toobtain the desired rheological properties, some quantity of branching is required.
12. METALLOCENE CATALYST AND COCATALYST SYSTEMS 12.1 Catalysts Metallocene catalysts (Kashiwa et al., 1992; Hamielec and Soares, 1995; Huang and Rempel, 1995) are structurally distinct from Ziegler-Natta catalysts because they contain either one or two five-carbon aromatic cyclopentadienyl (Cp) rings coordinated to a Group 4btransition metal such as titanium, zirconium, or hafnium. The Cp rings can be simple structures with a hydrogen attached to each carbon or intricate structures substituted with complex organic groups. The two Cp rings can be bridged (connected), most commonly by carbon or silicon, or unbridged. All metallocenes are single-site catalysts. Each catalyst molecule has a specific structure, which results in only one type of active center. Single-site catalysts produce inherently linear polymers with a most probable polydispersity (MWD) of 2.0.
12.2 Cocatalysts Metallocene catalysts require unique cocatalysts for activation and initiation of polymerization. Two classes of cocatalysts are used: (1) MAO(methylaluminoxane), a reactionproduct of trimethyl aluminum and water, and (2) boranes, such as tris perfluorophenyl borane, trityl- and tetrakis-perfluorophenyl borate, and dimethylphenylamino tetrakis perfluorophenyl borate. Manymetallocenestructures are capable of producing EPM or lowmolecularweight EPDM, but onlyalimitednumber of metallocenestructures are capable of producinghigh molecular weight EPDM. Thereis significant interest in metallocene systems for EPDMbecause of the catalysts’ stability at higher temperatures, their very high productivity, and their high reactivity toward ethylene, propylene, and higher alpha-olefins.
13.MODIFIEDEPDM EPDM and EPM can be modified with a variety of monomers or inorganic agents. The primary uses for modified EPDM are as dispersant viscosity modifiersfor lubricants; in impact modification of plastics,suchaspolypropylene,polyethylene, polyamide, polycarbonate,PET, PBT, PVC, ABS, and SAN; and as compatibilizers for polarhonpolar polymer blends.
1. Chlorinationandbromination:Chlorinationhas been heavilyinvestigatedasa way to impart oil resistance to EPDM. Mitsui (U.S. 4,764,562, 1988), Showa Denko (EP 0268457, 1988), JSR, and Sumitomo Chemical have patent coverage in this area. Production costs for chlorination are a major hurdle to commercialization. Bromination is reported to provide faster cure and higher tensile strength in cured EPDM (Hashim et al., 1995; Kohjiya et al., 1995).
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2.Maleation:Maleicanhydride-modified EPDM(Greco etal.1987; U.S. 4,010.223, U.S. 4,506,056, 1995); has found wide use in the lubricants industry for the manufacture of dispersant-grade viscosity modifiers. The reactive anhydride moiety serves as an anchor for many polar materials. A second application areais the impact modification of engineered plastics such as Nylon 6, Nylon 66, PET, and PBT. Commercial products based on maleic anhydride modification are available from Uniroyal Cheniical under the trade name RoyaltuP (Constable et al., 1997) and from Exxon under the trade name
[email protected] of dibutyl maleate in place of malcic anhydride has also been reported (Sen et al., 1991). 3. N-Vinylpyrrolidone and C-vinylpyridine:Modification of EPDM with unsaturated pyridines and pyrrolidones are reported (U.S. 4.146,489, 1979) for the manufacture of dispersant-grade viscosity modifiers. 4. Vinyl silanes: No commercial products are available based on organosilane-modified EPDMs, but their preparation has been patented (U.S. 4,340.689. 1982). 5. Sulfonation,chlorosulfonation. and ionomers: The preparation.characterization.and uses of sulfonated and chlorosulfonated EPDM have been reviewed (Earnest and MacKnight, 1980; Lundberg and MacKnight. 1984). The use of zinc-sulfonated EPDM has been reported for creating ionic thermoplastic elastomers. (De et al., 1994; Bhattacharya et al.. 1995). 6. Antioxidants:Graftingantioxidants (Scott, 1987, 1989; Devore andHahnfeld, 1993) onto EPDMprovides a nonmigrating, nonextractable antioxidantfor lubricant applications and applications in contact with tluids. 7.Styrene-acrylonitrile:Styrene-acrylonitrile (SAN)grafts on EPDMare used for weather resistant applications and plastics impact modification (U.S. 3,538,190, 1970; U.S. 3,538,191, 1970; U.S. 3,657,395, 1972; U.S. 3,671,608, 1972; U.S. 3,683,050. 1972; U.S. 3376,727, 1975; Hamann et al.. 1989; Motomatsu, 1989). SAN grafted material is commercially available from Uniroyal Chemical under the Royaltuf@ trade name.
14. INDUSTRIAL APPLICATIONS AND
USE OF EPDM
14.1 Introduction Industrial applications and uses of EPDM were reviewed by Grossi and Karpeles (1996). The proper selection of polymer and curatives for the various applications utilizing EPDM rubber is critical. It is important that the choice of polymer and compound components be consistent with the processing requirements and desired final product properties (Chodha. 1994). Several characteristics of EPDM rubber make it the polymer of choice for a variety of applications (Cheremisinoff, 1992; Suryanarayanan, 1992; Umeda, 1995). Table 4 sunlrnarizes many of the attributes inherentin EPDM polymers, and Table 5 summarizes many of the features of EPDM polymers that result from their structural characteristics. EPDM polymers are very nonpolar, and, unlike many other rubbers, EPDM has a saturated polymer backbone. Yet, while the backbone of EPDM rubber is saturated, these polymers contain diene termonomers such as ENB, DCPD, or 1,4-HD. allowing for curing using sulfur or sulfur donors. EPDM polymers can also be cured by peroxides (Endstra and Wreesman, 1993; Hellendorn, 1995a. 1995b) when sulfur cure is not acceptable. The polymer is an excellent choice for outdoor applications when good ozone and weather resistance is needed. In severe environments where excellent heat and oxidation resistance is needed, such as in under-the-hood automotive applications where high temperatures are common, EPDM rubber is a very good choice. EPDM polymers also have
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Table 4 Attributes of EPDM Saturated polymer backbone Diene termonomer for sulfur curing Versatility in polymer structure possible Ethylene/Propylene ratio Diene type and amount Molecular weight Molecular weight distribution Branched/Linear structure Economical cost
excellent low-temperature flexibility and high resiliency. They also have very good resistance to water and aqueous solutions and other polar fluids. The lack of inherent polarity (discussed in detail in Section I) also provides for excellent nonconductive electrical properties. Other structural features can be varied, which can have a great effect on the polymer’s properties, which in turn can effect serviceability, processability. and cure characteristics. For example, the ratio of ethylene to propylene, the diene type and amount, the MW and the MWD, and the branching(or linearity) of the polymer areall characteristics that can be varied. Increasing the ratio of ethylene topropylene can improve the modulus or cold green strengthof the polymer by introducingmore“crystallinity.”However,low-temperatureproperties are sacrificed in high-ethylene polymers. lncreasing the MW of a polymer can improve the hot and cold green strength (Stella, 1994) and allow for higher filler loading, but it can make the compound more difficult to mix unless processing aids are used. High molecular weight polymers also provide increased vulcanizate tensile strength. The molecular weight distribution can be varied to affect processing-narrow to allow for fast extrusion, or broad to improve milling and calendering. The diene type and amount can affect the cure rate of the polymer and can also have an effect on properties such as compression set and aging. It is the proper balance of these characteristics that must be taken into consideration when choosing a polymer for a particular application. Examples of how altering the polymer structure can effect properties are shown in Table 6. Some of the common elastomeric applications for EPDM rubber are summarized in Table 7. These include (but are not limited to) hose, automotive weather seals, roof sheeting, wire and cable, plastic modification, tires and tubes, gaskets and seals, and diaphragms. A few of these will be discussed in more detail, providing some general guidelines for polymer and cure system selection.
Table 5 Features of EPDMPolymers Ozone and weather resistance Heat and oxidation resistance Polar fluid resistance Water and aqueous solution resistance Low-temperature flexibility High resilience Excellent electrical properties
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Table 6 Polymer Structure Versus Effect on Property High MW
Narrow MWD
Broad MWD High ethylene content
Increasing diene content
Increased green strength Potential for poorer mixing Increased vulcanizate tensilehear strength Poor calenderability Slower extrusion (higher temperature required) Higher loading possible Increased extrusion rate Smooth extrusion surface Lower die swell Improved low-temperature properties Increased cure rate and state of cure Faster mixing Higher green strength Improved mill handling and calendering Increased cold green strength More difficult to mix Increased tensile strength of vulcanizate Higher filler/oil loading possible Poorer low-temperature properties Higher hardness Decreased scorch safety Faster rate of cure Improved compression set of vulcanizate Increased modulus of vulcanizate Decreased elongation and heat aging of vulcanizate
14.2 Hose EPDM isused in many types of automotive hose, including automotive heater and radiator hose (Keller and Mills, 1991), air-conditioning hose, air-emission hose, crankcase vent hose, brake hose, and tubing. It is also commonly used for nonautomotive applications such as hydraulic hose tube andcover, utility hose, inlet and drain hosesfor appliances, garden hose, and industrial air and water hose. In choosing a polymer for hose, service conditions and processing must be considered. In general, most hose is extruded and subsequently reinforced, so polymers that provide good
Table 7 Some Uses of EPDM Rubber Hose Sponge and weather seals Sheeting/Roofing Wire and cable Plastic modification Seals and gaskets Diaphragms Tires and tubes
and Grossi
866 Table 8 Hose: General Polymer, Processing, and Cure Requirements ~~
Automotive coolant hose
Appliance, industrial, and garden hose
Polyrner Requirements Lower ethylene content for improved low-temperature properties Medium ENB for fast cure High molecular weight for green strength Narrow MWD for fast extrusion Cure Requirements Peroxide for improved heat aging Process Extrusion
Pnlvnwr Requirer?~ents
High ethylene for high filler/oil loading and green strength High molecular weight for freen strength and high filler/oil loading Medium ENB for fast cure Cure Requirenuwts Sulfur/Sulfur donor Process Extrusion
cold and warm green strength and good extrudability are preferred. High-ethylene polymers (cold green strength) and high molecular weight polymers (warm green strength) provide good shape retention, while a high ethylene content and narrow molecular weight distribution allow for fast extrusion. These polymers, with high molecular weight and high ethylene content, also allow for higher filler (improved cost) and oil loading (improved cost and mixing). Medium ENB polymers utilizing a sulfur cure are generally favored to provide fast cure and good heat aging characteristics. Heat aging can be improved as needed by using a sulfur donor system or a peroxide cure system. Compounds for appliance, industrial, and garden hose applications are generally highly loaded for cost-effectiveness. They use high molecular weight, high-ethylene polymers and a sulfur/sulfur donor cure system. On the other hand, automotive coolant hose typically has more stringent physical property and aging requirements. Filler and oil loadings are lowerto accommodate those requirements. Polymers with lower ethylene content are used to meet special lowtemperature and compression set requirements, and peroxide cures are used for stringent heataging requirements. Table 8 summarizes the general selection criteria for hose applications. Tables 9, IO, and 11 provide examples of typical formulations for a heater hose, radiator hose, and industrial garden hose, respectively.
Table 9 HeaterHose Formulation EPDM
100
EA' = 15/25 ENB = 5 (medium) ML, I 4 at 125°C = 70 MWD = narrow Activator Reinforcement Process aid, extender Antioxidant Improve ratehate of cure Curekrosslinkforimproved heataging
+
Zinc oxide Carbon black N-650 Paraffinic oil Styrenated diphenyl amine TMPT (trimethylolpropane triacrylate) Peroxide
5
160 120 1 2
IO
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EPDM Rubber Technology
Table 10 RadiatorHoseFormulation ~
~~
EPDM 1
50
E/P = 75/25 ENB = 5 (medium) ML I 4 at 125°C = 70 MWD = narrow E/P = 56/44 ENB = 5 (medium) ML I 4 at 125°C = 75 MWD = medium Activator Reinforcing Less reinforcing Process aid, extender Antioxidant Antioxidant Ultrafast accelerator Sulfur donor/accelerator Sulfur donor/accelerator Accelerator Cure/crosslinking
+
EPDM 2
50
+
Zinc oxide Carbon black, N 650 Carbon black, N 762 Paraffin oil Styrenated diphenyl amine ZMTI (zinc-2-mercaptotoluinedazole) ZMDC (zinc dimethyl dithiocarbamate) TMTD (tetramethyl thiuram disulfide) DTDM (dithiodimorpholine) ZBDC (zinc dibutyldithiocarbamate) Sulfur
5 13 85 12
I .5 I .5 3 3 2 3 0.5
14.3 Closed Cell Sponge Weather Seals Closed cell automotive sponge weather seals are processed by extrusion and typically cured using a microwave oven, hot air oven, liquid salt bath, or fluidized bed (Burbank et al., 1995). Curing is very fast, typicallytaking 1.5-3.5 minutesat 150-200°C(300-40OoF), depending on the equipment used. For microwave curing, in the precure phase of curing (no expansion) approximately 30% of total cure must occur in the first 30 seconds. Enough modulus must be developed prior to the decomposition of the chemical blowing agent so as to provide sufficient
Table 11 GardenHoseFormulation EPDM
100
EIP = 75/25 ENB = 5 (medium) ML I 4 at 125°C = 70 MWD = narrow Activator Activator, process aid Reinforcing black Low reinforcing filler Plasticizer Process aid Process aid Accelerator Sulfur donor/accelerator Accelerator Cure, vulcanization
+
Zinc oxide Stearic acid Carbon black, N-650 Clay Naphthenic oil Polyethylene glycol Paraffin wax Dibenzthiazyl disulfide TMTD (tetramethylthiuram disulfide) ZMDC (zinc dimethyldithiocarbamate) Sulfur
5 I 260 200 2 10 2 5 I .0 1.S 1.0
2.0
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Table 12 Closed-CellSpongeWeatherSeals:GeneralPolymer, Processing. and Cure Requirements Polymer requirernerlts High ENB (>8%) for ultra-fast cure High molecular weight andor high ethylene for green strength Narrow molecular weight distribution for fadsmooth extrusions Cure reqrrirernerzts Sulfur cure Ultra-fast accelerators, e.g., thiazoles, dithiocarbamates. and thiurams Carbon black required for microwave receptivity Chemical blowing agent for gas formation Processing Extrusion Microwave cure Hot-air cure LCM (salt bath) cure Fluidized beds
cell wall strength to contain the gas pressure developed i n the second phase of the cure. In the expansion phaseof curing, alsolasting about30 seconds, the chemical blowing agent decomposes completely and the rapid cure continues until the extrudate reaches close to 100% of cure. In the final phase, the cure is completed, volatile decomposition products are driven off and the normalization of the sponge occurs. The total average cure time for this technique is about 1.5 minutes. Because very fast curing is required, high-ENB polymers (>8%) are used. The choice of other polymer characteristics, such as percent ethylene, molecular weight, and molecular weight distribution. are determined by the final required sponge properties and the method of curing to be used. Forexample, high molecular weight polymersor high-ethylene polymers are generally used for LCM curing in order to provide better green strength to control stretching and shape distortion. Curing of closed-cell sponge is typically done with sulfur and the use of accelerators such as thiazoles, dithiocarbamates, and thiurams to provide the required ultrafast curing. Sufficient heat will not be generated for microwave curing EPDM unless receptive promoting ingredients such as carbon black are added, since EPDM is very nonpolar. The most common types of chemical blowing agents used are azodicarbonamides, which decomposeto give off large amounts of nitrogen gas.Table 12 summarizes thegeneralselectioncriteriaforclosed-cell sponge weather seals. Table 13 provides an example of a typical microwave-cured, closed-cell, extruded sponge weather seal formulation.
14.4 Wire and Cable EPDM is used to produce wire andcable forboth low-voltage and mediundhigh-voltage applications. Someexamples of eachtypeareshown in Table 14. Thechoice of EPDMdepends upon the application and differs considerably between low-voltage and mediundhigh-voltage applications. Low-voltage wire and cable compound is generally highly filled. For this reason. highethylene, high molecular weight polymers, with a narrow MWD. that provide for good green
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Table 13 Closed-Cell Extruded Sponge WeatherSeals,MicrowaveCureFormulation I00
Zinc oxide Stearic acid Carbon black. N-660 Carbon black, N-550 Whiting Paraffinic oil Brown Facticc Accelercrfiort Mercnptohenzyl thi;mle TEDC (tellcnuln diethyldithiocarbamate) ZBDC (zinc dihutyldithiocarbamatc) Dipentamethylene thiutxm tetrasulfide Sulfur 9 p m ADZ (azodicnrbonamide)
4 1 1 IO 20 Filler 30 80
IS I .S 0.7 1.5 I .2 2 l .S
E/P = 57/43 ENB = 8.5 (high) MWD = medium Activatorcure/CBA) (for Activator. process aid Lower reinforcing Reinforcing and nucleating agent Plasticizer Extender, resiliency Ultra-fast accelerator Ultra-fast accelcrator Ultra-fast accelerator, sulfur donor Cure/Crosslink Gas fonnation
strength and extendibility are typically used. Wire and cable is processed by extrusion, so oils and fillers are added to aid in processing to compensate for the narrow MWD of the polymer. Because of the more stringent electrical requirements in mediudhigh-voltage wire and cable applications. much lower compound loading with oils and fillers can be used. For this reason. low molecular weight polymers with broad MWD that aid in processing are generally used. Polymers with high ethylenecontent are often used where high electricalbreakdown strength is needed, such as i n single conductor cables. Low-ethylene, amorphous polymers are used for multiple conductor cables where less distortion is tolerated. Both low-voltage and mediutdhigh-voltage cable require low levels of ionic, nonpolar constituents for wet electrical stability. All are peroxide cured by CV steam, salt bath, or hot dry gas under pressure at temperatures between 175 and 200°C (350-400°F). Since sulfur curing is not acceptable for wire and cable. the level and type of termonomer in the polymer is not important. Table IS sunlrnarizes the general selection criteria for wire and cable. Tables 16 and 17 provide examples of two types of wire and cable formulations.
Table 14 Wire and Cnblc Applications Low-voltage applications High-voltage applications UL flexible cord Submersible pump cables Appliance wire Automotive ignition cable Insulation Track resistawe insulators Welding cable insulation
Industrial powcr cable Utility power cables URD cable
Karpeles and Grossi
870 Table 15 Wire and Cable: General Polymer, Processing,
and Cure Requirements
h-voltage applications Low-voltage Polymer requircvnents High MW (high Mooney) High ethylene Narrow MWD Low ionic polar constituents Highly fillcd for low cost
Process requirenlents
Extrusion Oilslfillers to aid processing
Curing requirements Peroxide 350°C CV steam, salt bath, or hot dry gas
14.5
Polymer requirements Low MW (low Mooney) High ethylene for single conductor cable
Low ethylene for multiple conductor cable Broad MWD Low ionic polar constituents Low filler for better electrical properties Process requirements Extrusion Broad molecular weight distribution polymer to aid processing Curing requirernents Peroxide 350°C CV steam, salt bath, or hot dry gas
Roof Sheeting
EPDM compounds for sheeting are mixed in internal mixers using fast mix cycles and subsequently calendered into several plieson multiroll calenders.Curing istypically done in autoclaves with cure times of 4-6 hours at temperatures of 130-160°C (270-320°F). Important cured and uncured properties must be taken into consideration when choosing the proper EPDM polymer (Gish, 1995). Important uncured properties include good tack, good greenstrength, and good mixingandcalenderingcharacteristics.Importantcuredproperties include moderately high tensile strength and high tear strength, excellent weathering resistance and heat aging, bondability, and good low-temperature properties.Typically, low-ENB polymers
Table 16 FlexibleCordLow-VoltageFormulation EPDM
100
Calcined clay TMQ (polymerized 1,2-dihydro-2,2,4trimethylquinoline) Paraffinic oil Paraffin wax Vinyl
200
Zinc oxide TMPT (trimethylolpropane triacrylate) Peroxide
1
70 5
5 2 7.5
EIP = 75/25 ML 1 4 at 125°C = 60 MWD = narrow Filler Antioxidant
+
Process aid Process aid Coupling agent, improves interaction of polymedfiller Acid acceptor CO-agent, peroxide activator crosslinking Cure,
871
EPDM Rubber Technology Table 17 Medium-Voltage Insulation 100
EPDM
Lead oxide Zinc oxide Silane-treated clay Vinyl silane TMQ (polymerized 1,2-dihydro-2,2,4trimethylquinoline) Peroxide
EIP = 75/25 ML 1 4 at125 = 25 MWD = broad 5 scavenger Ion Acid 5 acceptor 60 Filler, treated to enhance electrical properties 1.5 Coupling agent, polymer/filler 2 Antioxidant 3
+
Curelcrosslinking
with medium ethylene content are used to provide the good green strength, tensile and tear strength, and resistance to weathering and heat aging. A moderately broad molecular weight distribution provides a combination of good mixing and calendering of the compound while also providing good overall cured properties. Sheeting compounds are typically sulfur cured and contain carbon blacks for reinforcement. High-viscosity, low-volatility oils are used to aid in the processing of the compound. Table 18 summarizes the general selection criteria for sheeting compounds. Table 19 provides a typical roof-sheeting compound. 14.6
Mechanical Goods and Other Applications
EPDM can be used in a varietyof molded and extruded mechanical goods. For example, EPDM is an excellent choice for automotive brake components where good ozone resistance and heat resistance, low stress relaxation, and resistance to nonmineral oil hydraulic fluids are required. In addition, EPDM is used in conveyor belting (both skim and cover), bridge bearing pads, dock fenders and bumpers, window gaskets, grommets, bushings, and seals. EPDM has
Table 18 SheetinglRoofing:GeneralPolymer,Processing,and Cure Requirements Polvrnrr requirenwzt.7 Low ENB for good weathering resistance Medium ethylene for green strength Moderately broad molecular weight distribution for good mixing, calendering, and overall cured properties Cure requirer~wnts Sulfur cure Autoclave 130-160°C (270-320°F) 4-6 hours Processing Internal mixers Calenders High-viscosity, low-volatility oils to aid processing
872
Karpeles and Grossi
Table 19 RoofSheetingFormulation
EPDM Zinc oxidc Stearic acid Paraffinic oil Carbon black (C.&.. N650) TBBS (N-ferf-butyl-benzothiazolesulfenamide) TMTD (tetramethylthiuramdisulfide) TETD (tetraethylthiuramdisulfide) Sulfur
EIP = 60140 ENB = 2 (low) MWD = broad 5 1 95 I25 2
0.5 0.5 0.7
Activator Activator, process aid p1.dstlclzcr .’: Reinforcement Primary accelerator accelcrator Secondnry accelcrator Secondary Vu1c:unization
also been found to be effective in gaskets and seals for water systems requiring good chloramine resistance.
14.7 Very Low Molecular Weight EPDM Reactive Plasticizers Very low molecular weight liquid EPDM is used as a processing aid that can react into the polymer matrix during peroxide or sulfur cure, rendering it nonextractable and nonvolatile as opposed to processing oils (Cesareet al., 1987; Cesare, 1995). Thisuse of low molecular weight EPDM hasadvantages in molding.extrusion,andcalenderingoperationsand is particularly important in applicationswhere the part comes in contact with fluids.which can extract a conventional process aid and cause premature failure. In severe applications. these “reactive plasticizers” are nonvolatile and offer advantages i n heat aging compared to conventional process oils. Some examples ofwhere these types of EPDM are used for processing andpel-fomxulce advantages include automotive brake cups and hose ( U S . 5,445.191, 1995). automotive heater hose, industrial hose, adhesives. sealants. and waterproofing membranes.
15. CONCLUSIONS EPDM polymers are used for a multitude of applications due to the many unique features of this class of polymers. The exact typeof EPDM used and cure systememployed for any particular application will depend on the processing requirements and service requirements of the enduse product. Many structural characteristics can be varied in the EPDM polymer to accommodate the many varying requirements. Specialty-grade EPDM can be used in combination with conventional EPDM or other polymers for enhanced properties in demanding applications.
ACKNOWLEDGMENT The authorswould like to acknowledge Dr. Ken Beardsley and Mr. Gerard Rioux for the rheological measurements and Dr. Ali Mohammadi for the thermal analysesused in this chapter (Uniroyal Chemical Company. Polymer Physics Laboratory. Naugatuck CT): Mr. Bill Wortman. Mr. Joe Longo. Mr. Manfred Stegmeier, and Mr. Dan Janczak for conducting lab polymerizations and
EPDM Rubber Technology
873
aging experiments (Uniroyal Chemical Company, Royalene R&D, Naugatuck CT): and Mr. Vern Vanis. Mr. Thomas Jablonowski, Mr. Donald Tredmnick, Mr. Arturo Maldonado, and Mr. Charanjit Chodha (UniroyalChemical Company, Royalene Technical Sales Service, Naugatuck, CT) for their input and assistance in preparing the applications portion of this chapter.
ABBREVIATIONS ABS ADZ A1 ASTM BR CBA
c1 CP CV DCPD DEAC DPA DSC DTDM EADC EASC ENB E/P EPDM EPM EPR
FTIR GPC 1,4-HD IR IISRP MAO MBT MBTS ML Mn MW MwIMn MW MWD PBT PET PEG PPM PS PVC
acrylonitrile butadiene styrene azodicarbonamide aluminum Association of Standards and Testing Materials butadiene rubber chemical blowing agent chlorine cyclopentadienyl ring continuous vulcanization dicyclopentadiene diethyl aluminum chloride diphenylamine differential scanning calorimetry dithiodimorpholine ethyl aluminum dichloride ethyl aluminum sesquichloride ethylidene norbornene ethylene/propylene ratio ethylene-propylene-dienerubber ethylene-propylene rubber ethylene-propylene rubber Fourier transform infrared spectroscopy gel permeation chromatography 1.4-hexad’lene polyisoprene International Institute of Synthetic Rubber Producers methylaluminoxane 2-mercaptobenzothiazole benzothiazyl disulfide Mooney viscosityflarge rotor number-average molecular weight weight-average molecular weight molecular weight distribution molecular weight molecular weight distribution polybutylene terephthalate polyethylene terephthalate polyethylene glycol parts per million polystyrene polyvinyl chloride
874
RIS SAN SBR TBBS TEDC TETD TMPT TMQ TMTD TPE TPV Ti UL URD V ZBDC ZMDC ZMTI
Karpeles and Grossi
radians per second styrene acrylonitrile styrene butadiene rubber N-tert-butyl-benzothiazolesulfenamide tellerium diethyldithiocarbamate tetraethylthiuram disulfide trimethylopropane triacrylate polymerized 1,2-dihydro-2,2,4-trimetylquinoline tetramethylthiuram disulfide thermoplastic elastomer thermoplastic vulcanizate titanium Underwriters Laboratories underground residential distribution cable vanadium zinc dibutyldithiocarbamate zinc dimethyldithiocarbamate zinc 2-mercaptotoluimidazole
REFERENCES Allen. R., and Easterbrook, E. K. (1987), in Rubber Teckrlology, 3rd ed. (Morton, M,, Ed.), Van Nostrand Rcinhold, New York, pp. 260-283. ASTM D3900 (1994), Standard Test Methods for Rubber. Avella, M,. Greco, P,, and Malinconico, M. (1987). in Po[yrners at Low Ternpercrturr, Proceedings of the confercnce held in London, pp. 5/1-5/10. Baldwin, F. P,, and Ver Strate, G. (1972), Rubber Chetn. T e c h r d 15(3):709-881. Baldwin, F. P,. Borzel, P,. Cohen. C. A., Makowski, H. S. and Van de Castle, J. F. (1970). Rtrhher C l l m ~ . T/>I. . 24(5):285-292. Boor, J. ( 1979), Zirlgler-Nrrtfa Crrtrr/wt.s m d Po/yrrlrrizatiorls. Academic Press, New York. Brett, T. J., Easterbrook, E. K., Lovelcss, F. C., and Matthcws, D. N. (1971).XXIII Infernntiorml Congress of Pure & Applied Cl~er~isfrv: Macromolecular preprints (Boston), IUPAC. British 1,020,808, 1966 to Hercules Powder Company. Burbank. F., Fredinnick, D., and Tyler, R. (1993, Continuous mixing of EPDM automotive weatherseals, Presented at Meeting of the Rubber Division, American Chemical Society. Cleveland, OH. Cesane, F., Matthews, D. N., and Paeglis, A. (1987), Rubber Plastic Ne\vs, Dec. 28. Cesane, F. (1995), Use of liquid EPDM in natural rubber to improve static ozone resistance and as a reactiveplasticizer i n otherelastomers,Presentedto the AssociationFrancaisdesIngenieurs et Cadres de Caoutchouc et des Plastiques, Paris, France. Chandrasekhar, V., Stvaram, S., and Srinivasan, P. R. (l988), Rcccnt dcvelopments in Ziegler-Natta catalysts for olefin polymerization and their processes. Ir~dicrr~ J. T e c h t d , 2653-82.
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Cheremismoff, N.P. (1992), Spotlight on EPDM Elastomers,Polv~n.Plasf. Techno/. Eng. 31(7,8):714-744. Chmela, S., Lacoste, J. Pilnebowski, J. F., and Tessedre, G. (1996). Polym. Degmd. St& 53(2):207-215. Chodha, C. (1994). FPDM technology and application, Presented at the SPOGS Seminar, Czech Republic. Christmar, D. I., and Kem G. I. (1968), Mrrcmnlolecules 1(4):358-363. Condit, P,, Frics, B., and Hottmar, A. (1963), J. Pol.vrner Sci. Part C 4:109-126. Constable, R., Roberts, D., and Flinruvengada, S . (1997), Advances In maleated polyolefins for plastic applications, Polynwr Eng. Sci. 37(8):1421- 1426. De, P. P,, De, S. K., Manoj, N. R., and Peiffer, D. G. (1994). J. Appl. Polyrner Sci. 53(3):361-370. De Paoh, Marco-A.( 1988), The photo-oxidation of EPDM rubber Part l-Kinetics of oxygen consumption, Polymer Drgrccd. Stubility, 2/:277-283. De Paoh, Marco-A, and Duek, F. R. ( 1990), The photo-oxidation of EPDM rubber: Part 111-Mechanistic aspects and stabilization, Polvrner Degrrtd. Stability 30:283-292. De Paoli, Marco-A, Duek, E. R., Guzzo, M,, Juliano, V. F., and Kascheres, C. (1990). Thephoto-oxidation of EPDM rubber Part 11-The photo-initiation process, Polvnler Degmd. Stability 28:235-248. Devore, D. D., and Hahnield, J. N. (1993), Polyn. Degrnd. Stclh. 39(2):241-249. DiMartino, S., and Kelchtermans, M. (1995), J. Appl. Poly. Sci. 56(13):1781-1787. Sci. Macromol. Rev., 16:41-122. Earnest, T. R., and MacKnight, W. J. (1980), J. Pol~w?er Endstra, W. C., and Wreesman, C. T. (1993), Peroxidecrosslinking of EPDM rubbers, in Elmtorner Technology Handbook, (Cheremasnoff N. P., ed.) CRC Press, Boca Raton, FL, pp. 495-518. Showa, E EP026845 (1988) Denko, Chlorinated EP in aqueous suspension. Galii, P,, Milani, U,, and Seaghoth, F. (1985). USRP 26th Annual Mtg. San Francisco, CA. Gish, B. (1995). Performance classification system for polymer based roofing membranes. Presented at a Meeting of the Rubber Division, American Chemical Society, Philadelphia, Paper No. 36, p. 3 1. Greco, R., Maglio, C., and Musko, P. V. (1987), J. Appl. Polymer Sci. 33:2513-2527. Grossa, A. V., and Karpetes, R. (1996). An introduction to the synthesis, structure, properties and uses of EPDM rubber.Presented at a Meetingof the Rubber Divislon, American Chemical Society, Montreal, Queber, Canada, Paper No. K. Hamann, B., Klodt, R. D., and Runge, J. (1989), Plnste Kcrut. 36(6):188-193. Hamelec, A., and Soares, J. ( 1995). Polytn. Reclcr. Eng., 3(2):, p. 13 1. Hashim, A. S., Ikeda, Y., Kohjiya, S. and Yoon, J. (1995), Rub. Chem. Tech. 68(5), pp. 824-35. Hellendom. R (1995a), Peroxide crosslinking of EPDM rubbers. I. Rubber blend preparation. In, Polyrn. Sci. Tech~lol.22(3):95-103. Hellendom, R. (1995b), Peroxide crosslinking of EPDM rubbers. 11. Processing of stocks, Plasp Kuuc. 32(3):70-73. Huang, J., and Rempel, G. (1995), Prog. Polynl. Sci. 20:459. Jebens, A., and Kautman, S. (1996), CEH Marketing Research Report: Ethylene Propylene Elastonwr.y. SRI International, Menlo Park, CA. Kameisky, W., and Sinn, H. (1984), Advcrnces Orgccnonwtallic C/leln. 18:99-149. Kashiwa, N., Kioka, M,, and Tsutsu, T. (1992), Karninsky catalysts, Petrotech. 15(2):138-142. Kabu, J., and Kuehne, J. K. (1984), Ktrutsch Curnmi Kunstst. 37(2):101-104. Kelier, R. C., and Mills, T. A. (1991). Evolution of ethylene-propylene radiator Prose technology. Kaol, N.Gu~nrniKunst 4 4 I l): 1032-1038. Kohjiya, S., Tsukahara, Y., and Yoon, J. (1995). Pol~vrn.Plrrst. Tech. Eng. 34(4):581-98. Kontos, I. G. and Slichler, W. P. (1962). Relaxationphenomena in homopolymers and copolymers of ethylene and propylene. J. Polymer Sci. 61:61-68. Lundberg, R., and MacKnight, W. I. (1984), Plastomerlc monomers. Rub. Chetn. Tech. 57(3):652-653. Mahlke, D. ( 1987). Verhalten von EPDM bei Tiefen Temperatwen. Kmtsch. Gumrni Kurutst 40:93 1-934. Martini, E., and Milani, F. (1986), Correlation between structure and low temperature properties of EPM and EPDM elastomers, Int. Rubber Conference 86: Proceedings. pp. 198-235. Motomatsu, K. (1989), Putasuchikkusu En 334):164-168. Noordermeer, J. W. M. (1996), Standardization of EPDM characterization testsfor quality-control purposes, Kuutsch. Gurnnli Kunstsf. 49(7/8), pp. 52 1-53 1. Saha Deuri, A., and Bhowmick, A. K. (1987), J. Appl. Polyrn. Sci. 342205.
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876
Scott, G. (1987), Po/vn1. Degrud. Stab. 19( 1):43-50. Scott, G. (1989),Modification of polyolefins by antioxidants and stabilizers, Mtrkrort~ol chert^. Mtrcrorr~ol. S ~ I ~ I28159-7 P. I. Sen, A. K., Mukherjee, B., Bhattacharya, A. S., De P. P., and Bhowmick, A. K. (l99l),Angewandie Chenl. 191:15.
Stella, G. (1994), The molecular weight determines the performance of EPDM rubber. I d Cornu. 38(9): 14-21. SRI Report 4B. Ethylene-Propylene Perpolymer Rubber, 198I . SRI Report 4C, Ethylene-Propylene Copolymer and Terpolymer Rubbers, 1990. Suryanarayanan, B. (1992). Recent developments in EPDM elastomer technology. Chern. Eng. Work1 27(12):141-145. Umeda, I. (1995), Recent trends of the application of ethylene-propylene elastomers (EPDM) and their future development, Porirna Duijesurn 47(6):47-57. U.S. 3,051,690 to Hercu!es Powder Company (1962). U.S. 3,242,149 to Montecation (1966). U.S. 3,341,503 to Uniroyal (1967). U.S. 3,349,064 to Hercules Powder Company (1967). U.S. 3,441,546 to Uniroyal (1969). U.S. 3,467,633 to DuPont (1969). U.S. 3,538,190 (1970); U.S., 3,538,191 (1970); U.S. 3,657,395 (1971); to CoPolymer Corp., process to make AES U.S. 3,671,608 (1972); U S . 3,683,050 (1972); U.S. 3,876,727 (1975); to CoPolymer C o y , AES product. U.S. 4,010,223 to DuPont, Maleic anhydride modified polymers. U S . 4,146,489 to Rohm and Haas (1979), bolyolciin graft copolymers. U.S. 4,181,790 to Huels (1980). U.S. 4,340,689 to Copolymer Corp. (l982), method of grafting EPM and EPDM. U.S. 4,361,686 to Huels (1982). U.S. 4,506,056 to Gaylord Research Institute ( 1995), Maleic anhydride modified polymers and process for preparation thereof. U.S. 4,710,538 to Union Carbide (1987), process for the production of sticky polymer. U.S. 4,764,562 to Mitsui, chlorinated EPDM in CC14 solution (1988). U.S. 5,05 1,546 to Union Carbide (1991), process for removing dienes from EPDM resins U.S. 5,445,191 to General Motors Corporation (1995). Ver Strale, G. (1986). GlcvclopediLI of Polvn~rrScience m t l Engir~eerir~g. Vol. 6. John Wiley & Sons. New York, pp. 522-564.
36 Isobutylene-Based Elastomers Neil F. Newman* and James V. Fusco* Exxon Chemical Co., Baytown, Texas
1. INTRODUCTION AND HISTORY Isobutylene-based elastomers of commercial importance include homopolymers, copolymersof isobutylene with isoprene, and halogenated isobutylene-isoprene and isobutylene-p-methylstyrene copolymers. They owe their commercial success to a unique set of properties, including exceptionallylowpermeability to gases, excellent vibration damping, andgood-to-excellent heat, chemical, ozone, and oxidation resistance. Low molecular weight homopolymersof isobutylene were first prepared in 1873 by Gorianov and Butlerov (I). In the early 1930s, the German company I. G. Farben produced a higher molecular weight homopolymer using boron trifluoride catalyst and low temperatures. Standard Oil Development Company (now Exxon Research and Engineering Co.) quickly advanced this work with the production of rubbery homopolymers via still lower temperatures and purified ingredients. Further development at Standard Oil, principally conducted by R. M. Thomas and W. J. Sparks, focused on copolymerizing isobutylene with dienes in an effort to make a vulcanizable elastomer. Their work achieved fruition in 1937 (2-6) with the invention ofbutyl rubber. copolymers of circa 97% isobutylene with up to 3% of isoprene. Butyl rubber represented the first of the limited-functionality elastomers. The low level of olefinic functionality gave this elastomer chemical,heat, and ozone resistance much superior to that of the highly unsaturated general purpose rubbers. Commercialization of butyl rubber was accelerated by the events of World War 11, and the polymer was first manufactured in a large-scale plant in 1943. Butyl rubber achieved its major success as the elastomer of choice for tire inner tubes, a position it retains today. Halogenation of butyl rubber was first performed commercially at B. F. Goodrich (7-1 1 ) in the mid-1950s. Their product. which was made via bulk batch bromination of butyl rubber. proved difficult to manufacture and was withdrawn from the market in 1969. Solution halogenation processes,whicharemuchmorecontrollable.were developed during thistimeperiod. Chlorobutyl was developedby Exxon Chemical Company(12- 15) and introduced commercially in 1961. Solution Bromobutyl was commercialized by Polysar (now Bayer) ( 16) in 197 1 and by Exxon Chemical Company in 1980.
* Retired. 877
Newman and Fusco
878
Halogenation increases the reactivity of cure sites and introduces additional pathways for vulcanization. Accordingly. halobutyls are more covulcanizable with general purpose rubbers than is butyl rubber. This cure activity, combined with low permeability to gases, led to the development of tubeless tires employing halobutyl innerliners.This remains the largestcommercial application for halobutyls. Butyl and halobutylshavingimprovedprocessing and low die swell were introduced commercially by Exxon Chemical Company (17, 18) in 1989. These processability improvements are achieved via the addition of a branching agent to the polymerization so that a portion of the polymer is present as highly branched star-like structures. Hence, the products have been labeled “star-branched (halo) butyls.” The newest member of the isobutylene-based elastomer family is the brominated copolymer of isobutylene with pam-methylstyrene. This product has a fully saturated backbone and so is completely resistant to ozone and highly heat stable, while retainingthe desirable attributes of the other isobutylene-based elastomers. The bromine is present as benzylic bromide, a versatile and reactive functional group that covulcanizes well with general purpose rubbers and lends itself to further polymer modifications. This rubber, trademarked as “Exxpro Elastomers,” was commercialized by Exxon Chemical Company in 1995 (19, 20).
2. 2.1
HYDROCARBONISOBUTYLENE-BASEDELASTOMERS Polymerization Chemistry
The isobutylene-based elastomers are formed via a cationic polymerization mechanism ( 2 1, 2 2 ) initiated by Lewis acids, optionally activated with Bronsted acids or alkyl halides. Typical Lewis acids include BF3, AlC13, and (C2H5)A1C12.Typical activators include HCI. H 2 0 , and f-butyl chloride. Representative initiation reactions for Lewis acid/Bronsted acid are: AICI,
+ HCI
. dAIC16
AICI, H+ + CH,=C(CH,),
H’
“--,
CH,-C’AICI,‘
I
CH3
Initiation reactions are followed by a chain of propagation reactions in which monomer unitsadd to the carbenium ion end of the growingpolymer. These reactionsare very fast, highly exothermic, and affected by the reaction temperature. solvent polarity. and nature of the counterion.
CH,
CH3
Propagation proceeds until either a chain-transfer or termination reaction occurs. Chain transfer occurs if the carbenium ion of the chain reacts with another species, the chain-transfer agent. in a way that terminates the growing chain and starts a new one. Isobutylene itself acts as a chain-transfer agent via proton transfer to generate an olefinic chain end and initiate a new chain.
Isobutylene-Based Elastomers
879
The averagemolecularweight of thepolymer chains depends on therelativerates of propagation and chain transfer.The activation energy of chain transfer is generally much greater than that of propagation, so molecular weight (MW) is highly dependent on temperature (23, 24), and lower temperatures give higher molecular weights.In commercial practice, temperatures below approximately - 90°C are required to produce polymers of MW suitable for typical rubber applications. Termination reactions leadto discontinuance of the propagation without immediate generation of a new chain. This can be caused by reaction of the carbenium ion with nucleophiles, including the anion of the propagating ion pair or adventitious electron-rich species, usually oxygenates, which may be present as impurities. A representative example of the latter is:
--CH2-C+
I
AICI;
+
ROH
+ Polymer-OR +
H'AICI,
CH3
where R = H or alkyl. For the star-branched butyls, a styrene-butadiene-styrene block copolymer is added to the polymerization to serve as a termination agent for multiple propagating chains. Termination occurs when the growing chains react with olefinic functions on the polybutadiene blocks (17).
2.2
Polymer Structure
Polyisohrtylene Polyisobutylene is a linear amorphous polymer. At high molecular weight, it can crystallize when extended. The crystallites have a helical conformation with a repeating length of eight monomer units (25). It is the only member of the isobutylene-based elastomer family to so crystallize, dueto the absence of comonomer.The gem-dimethylfunctionality on alternate chain carbon atoms causes sufficient crowding to force the bond angles away from the normal tetrahedral 109.5"to approximately 123" (26,27). Polyisobutylene has a glass transition temperature of approximately - 70°C (28). The molecular weights of commercial polyisobutylenes range from approximately 30,000 to approximately 5 million. They usually have one olefinic chain end due to chain transfer with an isobutylene molecule. High-MW polyisobutylenes generally have the narrowest molecular weight distribution (MWD) of the isobutylene-based elastomers. with M,/M,, slightly above 2.0.
Butyl Rlrbher Isoprene is incorporated into butyl rubber via 1,4-polymerization, with the polymer chain in the trarls configuration (29):
Newman and Fusco
880
For commercial grades of butyl rubber, the ratio of n :m ranges approximately from 40 : 1 to 200 :1. Because the isoprene content is low and the reactivity ratio for the monomers is near unity (4), the unsaturation is randomly distributed along the chain. Commercialelastomers have molecular weights of approximately 150,000 (M,) and 450,000 ( M w ) . The MWD, as MwN,, is typically 2.5-5. Star-Branched Butyl
The star-branched butyls have a bimodal MWD. One mode consists of normal linear chains, as described above. The second mode consists of branches of normal chains connected through the SBS polymeric branching agent added to the polymerization. Because of this bimodality, the polymer andits compounds give improved processingrelative to conventional butyl rubber. The amount of branching agent is chosen to give the best balanceof processability and properties. In commercial star-branched butyl, approximately 87% of the weight is in normal chains and approximately 13% in the star polymer. Figure 1 illustrates the overall structure, and Figure 2 shows MWD curves for conventional and star-branched products of similar bulk (Mooney) viscosity.
Poly(isobuty1ene-co-para-methylstyrene) Pura-methylstyrene (PMS) copolymerizes through its vinyl functionality:
This hydrocarbon polymer, sometimes called XP-50, is not sold commercially but is brominated at the benzylic positionto give the commercial bromo-(isobutylene-co-paru-methylstyrene)elas-
CH2&,
,CH3
+
?H3
CHyC-CH-CHz
CH3
Isobutylene
+
Styrene Isoprene I
Cationic Polymerlzatlon
BUTYL CHAINS
- 87% Fig. 1 Structure of star-branched butyl (SBB).
\ Block Copolymer
Isobutylene-Based Elastomers
881
Fig. 2 Molecular weight distribution of SB BIIR vs. BIIR.
tomers denoted as BIMS by the IISRP. Commercial products have ratios of x:y ranging from 16: 1 to 40: 1. Carbon- 13 NMR studies have shown that the p-methylstyrene preferentially polymerizes with itself. For example, a polymer with x: y of 37 was found (30) to have about half of the PMSasindividualmersflanked by isobutylenes, i.e., -BSB-, andabout half as diads, i.e.,-BSSB-units. A random copolymer at this low a concentration of PMS would have virtually all of the mers as individual units. This implies that the reactivity-ratio product for these two monomers is significantly greater than unity. Poly(isobuty1ene-co-para-methylstyrene)hasmolecularweightssimilar to thosegiven above for butyl rubber. Its MWD tends to be narrower and is typically in the range of 2.2-3.5.
2.3 PolymerProperties Physical Properties
The most conlmercially important propertiesof the isobutylene-based elastomers are low permeability to gases and high mechanical damping. These properties. as well as their high density for hydrocarbon elastomers of 0.92 g/cc, are rooted in the gem-dimethyl groups on alternate carbon atoms of the long polyisobutylene chain segments. Thisfunctionality causes densepacking along the chain and low chain mobility. This combination has been directly linked to the unique permeability characteristics of these elastomers (3 I ) . Comparative diffusivities of several gases for butyl and natural rubbers are shown in Table 1 (32). The practical manifestation of low permeability is the much better air retention of butyl rubber innertubes relative to NR tubes. This is clearly shown in Table 2 (33) for inner tubes on cars driven at 97 k d h for 161 k d d a y . The high damping ability of isobutylene-based elastomers is caused by the crowding due to the gem-dimethyl groups. This crowding imposes rotational restrictions and high internal friction because of the energy dissipated in moving the gem-dimethylgroups around each other's interference. High internal friction increases the loss modulus, G", because a high proportion of imposedenergy is expended nonrecoverablyintothemovement of the dimethylgroups. Correspondingly, the storage modulus,G', is decreased because only alow proportion of imposed energy goes into elastically recoverable distortions of the polymer backbone. (See Ref. 33 for
ber
Newman and Fusco
882
Table 1 Diffusivity of Gases in ButylandNaturalRubbers (25°C cm% X 10")
Butyl
Gas Helium Hydrogen Oxygen Nitrogen dioxideCarbon
21.6 10.2 1S 8 1.10
5.93 1.52
0.08 1 0.045 0.058
1.10
further discussion on this subject.) The high damping of butyl rubbers leads to their use in shock-, vibration-, and sound-absorption applications. Compared to other hydrocarbon elastomers, the isobutylene family has a unique combination of high molecular weight between entanglements and low tendency to crystallize on extension (35) resulting ina low plateau modulus.Therefore, they have relatively low uncured strength but high tack and self-adhesion, leading to commercial application in adhesives, caulks, and sealants. Uncured strength, at short time scales, is markedly improved for the star-branched butyls in which entanglements have been sharply increased via the long-chain branches of the star molecules. Chetnical Properties
Isobutylene-based elastomers show solubility characteristics consistent with amorphous hydrocarbon polymers. They are very soluble in hydrocarbon solvents, with alicyclics being the most solvating, paraffinics next, and aromatics least. They are also soluble in chlorohydrocarbons and tetrahydrofuran. They are essentially insoluble in polar liquids such as acetone, ether, dioxane, and lower alcohols. The relationship between intrinsic viscosity[q]and molecular weightfor linear isobutylene polymers in diisobutyleneat 20°C follows (36, 37), and data have beentabulated for other solvents (38): (MW > ca. lo4)
[q] = (3.6 X 10-4)(MW0.64)
(MW < ca. 10")
[q] = (1.78 X 10")(MWO.JhS)
The olefinic groups in butyl rubber undergo reactions typicalof hindered olefins such as halogenation and oxidation. The first of these is the route to commercial halobutyls. Oxidation is slow and is inhibited in commercial products by addition of antioxidants such as hindered phenols (39). Oxidation of the butyls results in degradation of molecular weight.
Table 2 Air Loss of Inner Tubes in Driving
Tests (Wa) Air pressure loss
Original elastomer tube pressure Inner 3.4 Butyl rubber
Natural rubber
1 week 193 193
28
1 month 13.8 114
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The presence of olefinic functionality and the associated allylic hydrogen atoms allow butyl rubber to be vulcanized with curatives similar to those used in general purpose rubbers. Higher levels of ultra-accelerators, such as thiurams or dithiocarbamates, are usually required because of the low functionality levelin butyls compared to otherhydrocarbon elastomers. Other types of crosslinking agents used for butyl rubber includep-quinone dioxime and polymethylolphenol resins (40). Cures of the latter type give carbon-carbon crosslinks and, therefore, the most heat-stable butyl vulcanizates.
2.4
Manufacturing
The bulk of the world production of isobutylene-based elastomers is made in a low-temperature slurry process using methyl chloride as the polymerization medium and aluminum chloride as initiator. One Russian plant uses a solution process (41) with an alkane solvent andan alkylaluminum halide initiator. Figure 3 shows an overview of the slurry process for the complete family of isobutylenebased elastomers. The hydrocarbon members of the family (PIB, IIR, star-branched IIR, and the precursor of BIMS) pass directly from the polymerization sectionof the plant to the finishing section without going through the halogenation process. The heart of the polymerization section is the reactor itself.This reactor is of sophisticated design because it must operate at - 100°C to - 90°C while removing the considerable heat of reaction-820 J/gm. This is accomplished via a vertical reactor configuration, which combines the technology of stirred reactors with that of shell-and-tube heat exchangers. Refrigeration is accomplished by boiling ethylene in the shell portion of the reactor interior. Polymerization is conducted in the tubes. In practice, an axialflowmixeratthebottom of thereactor is used to circulatethe polymerizing mixture through the tubes. A mixture of purified monomers dissolved in methyl chloride is introduced to the bottom of the reactor. Catalyst, also dissolved in methyl choride is added in a separate stream, as is the SBS branching agent when star-branched products are desired. Turbulentflow of these solutions gives very rapid contact and the polymerizing mixture is forced upward through the tubes and eventually overflows the reactor. The growing polymer chains are insoluble in methyl chloride at the reactor temperatures and quickly form a milky
Butyl. Vx, XP-50 MeCl Monomers Catalyst @ MeCl
Hexane (Exxpro)
Antloxldant Hexane
or
DISSOLVER XP-50
Fig. 3 Manufacturingoverview.
"2'
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slurry of fine particles. The concentration is kept sufficiently lowso that the suspension is always maintained at low bulk viscosity and turbulent flow. The suspension does not agglomerate within the bulk because the rubber particles arewell below their glass-transition temperature. However, this process is prone to slow fouling at the tube surfaces. This is handled in commercial plants through the installation of multiple reactors. Reactors may then be sequenced through periods of polymerization and periods of cleanout while maintaining continuous plant production. Monomer conversion to polymer is restricted to obtainpolymer of satisfactorilyhigh MW. In practice, isoprene conversion is typically 45-85% and isobutylene conversion 75-95%, depending on the desired composition of butyl rubber (42). When PMS is the comonomer, its conversion is similar to that of isobutylene. Consequently, significant amounts of monomers, as well as methyl chloride, need to be separated from the polymer. This is done by contacting the slurry with either hot hexane for butyl that is to be halogenated or hot water for polymers that are to be finished nonhalogenated. The hexane contacting is known as the solvent-replacement process because it directly replaces one solvent (methyl chloride) with another, hexane, without an intermediate finishing step. This is the process used for most of the world supply of halobutyls. although some is manufactured by finishing the butyl and redissolving it in hexane. The halogenation portion of the process is discussed later in this chapter. For polymers that are finished unhalogenated, the polymer slurry, containing 30% rubber, overflows the polymerization reactor and is transferred to a stirred flash tank containing water at 55-70°C. The slurry contacts steam and hot water in the transfer nozzle. Slurry aid. typically zinc stearate or calcium stearate, is added at that point to control crumb size and prevent agglomeration of the crumb particles.Antioxidant may also beadded.Most of the methyl chloride andresidual monomers flash overhead at this stage. The remaining solvent and monomers are removed in a second stripper stage maintained under vacuum. The combined overhead streams are dried, separated, and recycled. The polymer (-10% rubber in water) is now ready to be dried. This is typically accomplished in a three-stage finishing procedure. Firstthe aqueous slurry is transferred onto a screen, through which most of the water passes while the wet polymer crumb, containing 50% water, is retained. Second, the wet crumb is passed through a squeezing device, such as an extruder. which presses most of the entrained water from the rubber and reduces the moisture content to 5-1096. Third, the wet rubber is put through a heated drying extruder, which has a peak temperature of 150-200°C. This high temperature. combined with a sudden release of the internal pressure at the extruder outlet, flashes off almost all of the remaining water. The dried rubber is cooled and conveyed, typically in a fluidized bed or airvey line, to the final finishing steps. For products that will be sold commercially, these consist of baling, wrapping, and boxing. Dense bales are formed by pressing carefully metered weights of rubber, typically 34 kg. into blocks.The bales are wrapped with polyethylene or EVA film and assembled into multibale boxes for the bulk of commercial sales.
2.5
Applications
Polyisobutylrrle
Polyisobutylene (PIB) cannot be vulcanized because it is a paraffinic elastomer. Its commercial uses include: Impact-strength blending with polyolefin plastics Uncured sheeting (e.g., roofing membranes)
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Isobutylene-Based Elastomers Table 3 Butyl RubberApplications:MarketShare sales rubber Application Butyl
(%)
71 8 7
Inner tubes Adhesives and sealants Tire curing bladders Shock absorbers Valves Cable insulation Health care Other
6 2
2 2 2
Adhesives, caulks, and sealants Wax modifier Pipe wrap and electrical tapes Chewing gum base Fluids modification and drag reduction Worldwide sales of PIB is approximately 20 ktonnes/year Butyl Rubber
Low permeability to air is key to the worldwide market for butyl and halobutyl, whose principal application is air retention in tires.Totalannualworldwidesales of butylrubberare -250 ktonnedyear, primarily as inner tubes for tires (Table 3). Table 4 gives formulationsfor typical butyl inner tubes. The standard compound is recommended for bias ply and heavy-duty tubes. The low-modulus compound, designed for radial tire applications and severe operating conditions, offers excellent splice life under demanding conditions. The bicycle tube offers improved processability via higher levels of high-structure black, while maintaining properties compatible with the lower demands of these small vehicles.
Table 4 ButylInner TubeFormulations (phr) Standard Butyl 268 N550 (FEF) black N660 (GPF) black N762 (SRF) black Paraffinic oil Escorez 1102 Stearic acid Zinc oxide Sulfur TMTDS MBT ZDEDC
100
Low Bicycle modulus IO0 10
100 40
50 22
30 22 3
70 25 1 -5 2 1
0.5
I 5 1.25 1.S 0.5
1
5 1.5 1.5 0.5 0.75
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Butyl rubber is the polymer of choice for tire-curing bladders because of its good heataging resistance and low permeability to curing media. For this very severe high-temperature duty, compounds aregenerally cured through a polymethylolphenol resin. This leads to carboncarbon crosslinks and the most thermally stable butyl vulcanizates. A typical curing bladder formulation is 100 butyl 268, 5 neoprene W, 50 N330 black, 5 castor oil, 5 zinc oxide, 10 polymethylolphenol resin. Neoprene is included to provide reactive chlorine, which combines with the zinc oxide to activate the resin cure. The excellent vibration and shock-damping of butyl leads to its use in applications such as automotive suspension bumpers (43) and body mounts. A typical body-mount compound is 100butyl268, 40 N330 black,15 N-990 black, 20 paraffinic oil, 5 zinc oxide, 1 sulfur,2 CdDEDC. 0.5 MBTS.
3. HALOBUTYLS Halogenatedelastomericpolymers have beenknown for sometime in thesyntheticrubber industry. The nature of these products, their synthesis, and properties are adequately covered in the chapter in this volume on halogenated elastomers. The halogenation of isobutylenehsoprene copolymers (previously described), commonly referred to as “halobutyls,” is detailedhere, since their compositions contain small amounts of halogen ( 5 2 . 2 wt%) to give extraordinary reactivity to enhance vulcanization latitude and rates. This enhancement provides co-vulcanization capability with high-unsaturation rubbers. Butyl rubbers, in general, cannot otherwise be co-vulcanized very easily in practical systems with the general purpose rubbers. Halogenation with elemental chlorine andor bromine at approximately a 1 : 1 molar ratio (moles of halide to moles of unsaturation) results in the cure and co-cure enhancements while preserving the many unique attributes of the basic butyl molecule.
3.1
Halogenation Reactions and Process Description
The synthesis of the halogenated butyl involves the reaction of a solution of butyl in an alkane (e.g.,hexane or pentane) in the “dark” withelementalhalogens at processtemperatures of 40-60°C. The target is to produce a halogenated butyl in which no more than one halogen per unsaturation site is introduced. The final product weight-percent halogen specification for the major halobutyl commercial grades can be summarized as follows: Chlorobutyl: 1.1-1.3 wt% Cl for a 1.9-2.0 mole YO unsaturation Bromobutyl: 1 .8-2.2 wt% Br for a 1.6- 1.7 mole % unsaturation The reaction with chlorine under the above conditions is very fast, essentially completed in 15 seconds or less, even at the low molar concentration of reactants. The bromine reaction is much slower-about five times slower than chlorination. In both systems, thorough mixing is a prerequisite to meet the synthesis targets.These reactions leading to the primary specification products have been well documented (44-49) (Fig. 4). The relatively fast reactions occur via an ionic mechanism.The halogen molecules are polarized at the olefinic sites undergoing heterolytic scission and consequent reaction. The resulting halogenated derivatives are shown in Figure 5. The predominant allylic halide structures are structure I1 (80-90%) and structure 111 (10-20%). Minor amounts (
