edited bv
products of chemistry
GEORGE B. KAUFFMAN Calfornia State Univenitf, Frasna Fresno. CA 93740
Elastomers Ill. Thermoplastic Elastomers Raymond B. seymourl University of Southern Mississippi, Hattiesburg, MS 39406 George B. Kauffman CaliforniaState University, Fresno, Fresno, CA 93740 Before 1920 the only available elastomer was natural rubber (NR) ( I ) . However. several svnthetic elastomers (synthetic rubherfSK,, have been intr:,duced since the late 1920's (2,.The bridce between XR and SR was ~rovidedbv the introduction of thermoplastic elastomers ~TPE's).A; described in this article, TPE's are readily moldable and do not require the addition of vulcanization agents for the formation of chemical cross-links between polymer chains. Instead, the hard domains in TPE's serve as polymer crosslinks, which prevent slippage of polymer chains a t ordinary temperatures. Cross-Linking the Chains in Elastomers Vulcanized Rubber As explained in our previous papers on elastomers (rubbers) (1,2),both NR and SR have limited utility unless the linear polymer chains are joined by occasional cross-links. As demonstrated by Charles Goodyear (1800-1860) a century and a half ago, NR can be cross-linked or vulcanized by heating with 1-3% sulfur. (As demonstrated by his brother Nelson, a hard nonelastic thermoset plastic called ebonite is produced when NR is heated with 23-35% sulfur.) Improving Vulcanized Rubber During the century following Goodyear's discovery of the vulcanization process, considerable effortwas expended to effect various improvements. .The time of vulcanization was shortened by using small amounts of organic accelerators. The resistance of vulcanized NR to the effects of weathering was improved by adding small amounts of stabilizers. .The strength and wear resistance of vulcanized NR were also improved by adding relatively large amounts of carbon
science, which provided an understanding of the concept of rubber elasticity (ref 3, Chapter 8). Staudinger summarized his numerous investigations and conclusions in a book published in 1932 and reprinted in 1960 (4). Few chemists of the 1920's, other than Staudin~er,recognized the existence of giant molecules (macromo~ecules). Staudinger maintained that macromolecules were a special class of organic substances with high melt viscosities (5).He also described elastomers as coils or spirals like bedsprings. He maintained that the property of elasticity depends on the extensibility of these spirals, which are now called random coils. Rubber Elasticity Johan Rudolf Katz (188&1938) amplified Staudinger's "spiral concept" using X-ray spectroscopy to show that stretched rubber is crystalline (6). It is now assumed that unstretched, amorphous rubber molecules exist as random flexible coils that have a high conformational entropy or disorder due to the many possible shapes available to them. The extent of this disorder decreases as the polymer chain is stretched to a rodlike shape having only one conformation, that is, an entropy of zero. This is a reversible process, and there is a strong tendency for the molecules to return to their high state of disorder, that is, random coils. If unwlcanized rubber is stretched, the chains slip by each other, and the polymer flows like a glacier. However, occasional cross-links allow the essential conformational changes to occur but prevent uncontrolled flow or slippage. When polymer scientists accepted these concepts, they also recognized that linear polymer chains with alternating domains of soft elastic and hard elastic could behave much like cross-linked elastomers. This distinctive feature of these chains determines some of the properties of thermoplastic elastomers.
Ha&.
However, because little was known about the relationship between the structure and the properties of rubber, most of the discoveries were empirical and based on the cross-linking process discovered by Goodyear in 1838 (3). A Unifying Principle of Polymer Science Staudinger's Contributions
Fortunately, 1953 Nobel laureate Hermann Staudinger (1881-19651, who is now recognized as the "father of polymer chemistry", proposed a unifying principle of polymer
Thermoplastic Elastomers Their Development The development of thermoplastic elastomers (TPE's) in the early 1960's bridged the gap separating elastomers and thermoplastics (7-9).Although TPE's have not displaced elastomers in pneumatic tires yet, they are being widely used in many other elastomeric applications. TPE's require few additives and are readily injection-molded in faster time cycles than NR or SR. In addition, scrap from 'Deceased, November 15,1991: Volume 69 Number 12 December 1992
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Weak
. ..
Fluid
T
Tg or Tm
Sofl
of Hard Phase
Rubbery Phase
Figure 1. Sketch of a thermoplastic elastomer, SBS, which is the block copolymer of hard, glassy polystyrene and soft, flexiblepolybutadiene styrene. TPE's can be reused, and the molded articles may be heator solven&sealed.
T
Temperature
Tg Of
Figure 2. Modulus-temperature relationships in TPE's. TPE Blends
Thermoplastic Olefins Another widely used TPE consists of a blend of (11)
Concepts
The most widely used TPE is styrene-butadiene rubber (SBR),which is a block copolymer consisting of polymers of styrene--butadiene (SB) and polymers of styrene (S), with the formula SBS. (Also see Fig. 1.) styrene H2C=CHC6H, butadiene
H2C=CHCH=CH2
The hard, glassy, polystyrene domains in SBS act as physical cross-links connecting the soft flexible polybutadiene domains. (A block copolymer is a copolymer in which the same monomer units occur in relatively long alternate sequences in the chain). The following is an abbreviated structural formula for the styrene-butadiene-styrene block copolymer (S,B.S.).
The moduli (stiffnesses) of these TPE's, which are sold under the trade names of Kraton (Shell)and Stereon (Firestone), increase as the size of the polystyrene segment increases. Because the polystyrene domain is a thermoplastic, it limits the use of this TPE to temperatures below the glass transition temperature (T,) of polystyrene (95'C). The T, is the temperature at which the flexible polymer chain becomes brittle or glass-like. The Tgof the polybutadiene domain is -90°C. More than 150,000 metric tons of this TPE were produced in the United States in 1989. The worldwide production was more than 300,000 metric tons. Because the soft domains in SBS are unsaturated, this TPE is not resistant to weathering. However, the hydrogenated copolymer is saturated and is weather-resistant. The saturated TPE, which is actually a block copolymer of styrene-butyleue and styrene, has T, values similar to those cited for SBS. As shown in Figure 2, the optimum service temperature for TPE's is between the T, values for the soft and hard domains (10,II).
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a copolymer of ethylene (H2C=CH2) ' propylene (H2C=CH(CH3)) (In isotadic polymers, substituents on the asymmetriccarbon atoms have the same, rather than a random, configuration in relation to the main chain.) Although polypropylene and polyethylene are hard and nonelastic thermoplastics, the random copolymer of ethylene and propylene (EPM) is a soft elastomer with a T, of -60 OC. The melting point (T,) of the polypropylene hard segment is 165 T. These thermoplastic polyolefin elastomers (TPO's) are produced under the trade names of Santoprene, Somel, and Geolast. The utility of these TPE's has been increased by blending the components in a reactor, by cross-linking, and by using halogenated butyl rubber in place of EPM. More than 300 million pounds of TPO's were produced in the United States in 1989. Other TPE's Polyurethanes The original TPE's were thermoplastic polyurethanes (TPL''6,. These urethane-ester block copolymers were called Vulkollan by Adolf Baver r1902-1982, and Adiorene by Du Pant ( 1 2 ) . - ~ h e segment in the pibneer TPU, which had a T, of -60 "C, has been replaced in many cases by a polyether segment with a Tgof -40 'C. The T, of the hard segment is 190 'C. The T, is related to the size of the polyurethane domain (13). More than 75 million tons of TPU's were produced in the United States in 1989.
gaskets and sealants wire and cable coatings
Production of Various Thermoplastic Elastomers (TPE's) by Year and Countries of Origin (x lo3 Tons) 1986
1991
1996
110 106 20
145 123 30
192 146 42
47 62 16
83 77 23
121 79 34
25 33 7
31 43 11
40 54 18
13 5 4
22 8 8
34 14 13
8 2 38
19 3 47
33 6 58
biomedical applications TPE's are also being used for
SBS
USA ECC
Japan TPO's USA ECC
Japan Polyurethanes USA ECC
Japan Copolyesters USA ECC
Jaoan Other TPE's USA ECC
Japan Total TPE's USA ECC
Japan
Copolyesters Copolyester TPE's, which are related to TPU's, consist of two domains (14): A domain of hard, crystalline polyester Tg= 185--220 .C A domain of a soft, amorphous polyePter T,= -40 to -65 OC
These TPE's are sold under the trade names of Hytrel, Pelprene, Lomax, Armite, and Gaflex. Thirty-five million pounds of these copolyester TPE's were produced in the United States in 1989. Polyesteramides Polyesteramides (PEA'S) are produced by the condensation of (15) An aromatic diisocyanate (Ar(NC0)2) 'A polyester with terminal carbovyl gmups (GCOOH) T,of the sol%domains: -28 to -50 OC Tmof the hard domains: 230-275 OC
(dependingon the dicarboxylie acid extender used) Applications of TPE's
Industrial Products TPU's are used for casting solid pneumatic tires and for filling the carcasses of ordinary tires. Cordless TPU tractor tires are being used, and tires for passenger cars are being produced by Polyair Maschinebau GmbH in Austria (16). TPU's and other TPE's are used in many applicaions (17). molded shoe soles and ski boots mechanical goods automotive parts
..
trays syringes and catheters gloves
Also, extremely thin pinhole-free film can be cast from homogeneous TPE melts or from dispersions of TPE's in organic solvents. Production TPE's, which are relatively new, account for only 2% of the total production of elastomers. However, they represent 8% of the dollar value of the elastomer market. and their annual growth rate is more than twice that ofconventional elastomers. The worldwide production of TPE's in 1986 and predicted production f i r 1991 and 1996 are shown in the table. Theoretical Importance TPE's not only fdl a need in the marketplace, but they also help to demonstrate the theory of rubber elasticity. Because they are both plastic and elastomeric, TPE's will help unify macromolecular concepts. Few persons were able to recognize the structure-property relationships of NR until they were explained by Staudinger in the 1920's. Only then was it understood why Joseph Priestley (1733-1804) used NR instead of bread crumbs for rubbing out pencil marks or why John Wesley Hyatt's (1837-1920) mixture of cellulose nitrate and camphor (Celluloid) in 1870 was a moldable flexible plastic (19). Structure-Properly Relationships These concepts were elaborated by
Wallace Hume Carothen (1896-1937) (ref 3, Chapter 12; ref 18) Peter J. W. Debye (1884-1966) .Paul J. Flory (1910-1985) H e r m a n F. Mark (1895-1992) and other eminent polymer scientists. Their work has revealed that the differences among elastomers, plastics, and fibers are due primarily to the strength of their intermolecular forces, which attract the polymer chains to each other. In addition to the weak London forces present in all polymers, plastics might also have stronger dipolar forces. Fibers usually have a variety of special properties. They have regular molecular strudures. They are crystalline. They have very strong intermolecular hydmgen bonds. Summary Goodyear (20) restricted the slippages of polymer chains by introducing a few cross-links by heating NR with 1-2% sulfur. The gap between thermoplastics and vulcanized NR has now been bridged by the development of TPE's in which there are no chemical cross-links. We should expect future discoveries of equal importance as polymer chemists apply the present knowledge of the structurs-property relationships of polymers (21,212). Literature Cited 1. %&an, G. B.; Seymour,R. B. J. Chem. Edue 1990,67,422. 2. %&an, G. B.: Seymour,R. B. J. Chem. Edue 1991,68,217. 3. Piono~rsinPolymer&&nee: S%ymour.R.B.: Mark, H.F;Paulig, L.:Marvel, C. S.; Stahl. G. A,. Eds.; Kluwu Academic: Dmdreeht, The Netherlands, 1989:Chapter 21.
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4. Staudinger, H. Die hoehmolekulorpn oganlschm Volbindungpn: Springer-Vedag: Berlin, 1932,1960. 5. Staudinger, H.Ben 1920.53.1074. 6. Kat~.,J.R. Nafuruiss. 1925.13. 410. 7. Thermoplo*ic Ehstomers: A Comprehnsiue npuiem; ~ ~ g N.g R.; ~ ~. d d G.;~ Schmeder.. H. E... Eds.:. Word Universitv: New York. 1988. 8. Waker, B. M.Handbmk ofThormpiosticElostomers;VanNostrandRemho1d:New York,1979. 9. Holden, G. In Encyclopsdio of Polymer Science a n d Technology; Kmschwitz, J. L.. Ed.; Wiley: New York. 1986:Vol. 5. l a . Holden, G. hRvbber Technology. 3rded.; Morton,M., Ed.; VanNoshand Reinhold: New Yor*,1887; Chaoter 16. r a n d ~echnol.1988, 61,747. 11. ~ d nM. . M. a d . ~ u b b r cham.
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Journal of Chemical Education
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12. Seymour, R. B. InEqclopedrO dPhysico1 Scronce and T~chno1ogy;Academic:New York,1987; Vol 11. 13. Chen, 2. S.;Yang, W P : Maeosko, C. M. Rubber Chem. andl&hnol. 1988,61,86. 14. Witsiepe. W K. U.S. Pat. 3 651 014, 1972. , 15. Farissey, W J.; Rauach, K WEloslomrlcs lm8,120IT), 22. 16. Woods. G. TholCIPolvurothoneaBooIr:Wdev: New York. 1987. 17. Toensmeier, P. A. ModemPlastics 1988,65(1), 14. G. B, J. Chem. Educ. 1988.65,803; CHEMTECH 1988, 18,725. 18. &&an, 19. Seymour,R. B.: Kauffman,G. B. J C h m . Educ. 194%,69,311. 20. Kauf6nan.G.B. Educ. C h . 1994,26,167. 21. Seymour, R. 8.; Carraher. Jr, C. E. Sfruchrm-Pmpsdy Relotionships in Polymm; Pien-: New Y d . 1981. 22. Seymour, R. B.; C a n a h ~ rJr, , C. E. Gionl Molecul~s;Wiley: New York, 1989
