products of chemittry
edited by GEORGE B. KAUFFMAN
CaliforniaState University, Frerno Fresno, CA 93740
Elastomers II. Synthetic Rubbers George 6. Kauftrnan California State University, Fresno, CA 93740 Raymond 6. Seymour University of Southern Mississippi, Hattiesburg. MS 39406
Early Products ( 1.16-22,29,31-4 1) In 1879 Gustave Bouchardat (1842-1918), the son of the aforementioned Apollinaire Bouchardat, polymerized isoprene with hydrogen chloride and obtained an elastic mass that yielded isoprene on distillation (42). Ever since this discovery of what was the first, unsatisfactory synthetic ruhber. continual attempts were made tu develop commercial processes fur rubber sul~stitutes,especially by English and German chemists, who vied with each ot her in the quest. In 1884 William A. Tilden (1842-1926) prepared isoprene bv heatina I"crackinpl",dof turventineand itsisumers.and h; noted ;hat, on standing, this iaoprene sluwly polymerized to vellow, r u b h e n lumos (43).Althoueh the followina vear he succeeded in &anizing a sample i f turpentine-derived rubber, Tilden was unable to produce a usable svnthetic rubber. Harries and his collaboiators succeeded in getting isoprene t o form 1,5-dimethylcyclooctadiene ( C I O H ~ ~ ) , which further polymerized into a product that they considered to he ruhber (44). As we have seen, natural rubber is a ~ o l v m e rconsistine of reoeatine isovrene units. hut initial &cess in producing a &able suhstitnte was attained only when chemists abandoned their search for an exact chemical
Part I appeared on p 422 of the May 1990 issue. Part Ill will appear in a fuiure issue. References are numbered sequentially throughout
the three parts.
equivalent and instead sought to synthesize a polymer with the physical properties of natural rubber. The development of synthetic rubber was a slow process because it was virtually impossible to compete economically with natural ruhher, varying in price from $0.03 to $3.00 per pound, and because early synthetic products were not as satisfactory for most uses as natural rubber. Thus, in the past, synthetic rubbers were used only when shortages had been created either by wartime conditions or controlled markets. During World War I Germany was cut off from her supplies of Chile saltpeter and natural rubber by the British naval blockade. The first problem was ameliorated by the Haher-Ostwald process and the second by the development of methyl rubber. Methyl Rubber This somewhat inferior rubber was made in Germanv bv the polymerization of 3-methylisoprene (2.3-dimethyl:l,ihutadiene, CH,=C(CHXYCH I ) = C H ~obtained . from acetone) in the presence of sodium, which served as an anionic polymerization initiator, i.e., negatively charged organic ions initiate the volvmerization. Bv the end of hostilities. Germa- ~ ny wasproducihg 165 tonspe;month ofunreinforced methyl ruhber. which todav would be reearded as an inferior snhstitute. The first c o k t r y to insztute a full-scale synthetic ruhber industw was the U.S.S.R..which built a oilot nlant a t Leningrad in f930 and three factories in 19322933: A pro-
Volume 68 Number 3 March 1991
cess based on ethanol was used to produce the hutadiene needed to make this rubber. Thiokol The American synthetic rubber industry originated with two cases of serendipitous discovery. In 1922 Joseph C. Patrick (1892-1965), an independent inventor and physician, accidentally produced Thiokol. This polysulfide rubber (American government code, GR-P; for government ruhberpolysulfide), a condensation product of ethylene dichloride (CHCl=CHCI) and sodium tetrasulfide (Na2S4), was produced while trying to hydrolyze ethylene dichloride with various alkaline reagents to obtain ethylene glycol (1,2-ethanediol, HOCH2CH20H) t o be used as an antifreeze: +CH,CHZS,+ + 2NaC1 (2) n(CICH,CH,CL) + nNa,S,
H(CHZCH,S,)H (3) Thiokol A Commercial production of this weak, malodorous, but solvent-resistant elastomer began in 1930 a t Yardville, New Jersev. . Because i t is resistant to oil and organic solvents, i t is still used for gaskets; sealants for fuel ceils; sealer adhesive for machine comoonents; caulking.ship - decks and buildings; and paint spray, gasoline, oil suction, and discharge hose. Because it is unaffected by electric discharge and light, it is used for electrical insulation. Marketed as a solid, its tensile strength cannot compare with conventional rubbers, but it exhibits excellent resistance to weathering and permeability over a wide temperature range. A recent use is as a roomtemperature polymerizable liquid hinder (LP-3) made by the reduction of solid Thiokol. Thiokol LP-3 is mixed with ammonium percblorate to produce solid rocket fuels. Other types of Thiokol are Thiokol ST, made from &chloroethyl formal (CICH2CH20CH20CH2CH2C1)and sodium disulfide, and Thiokol FA, the most widely used product, a copolymer of ethylene dichloride and chloroethyl formal. The trade name Thiokol has now become generic. Neoprene The second American synthetic rubber was polychloroprene (American government code, GR-M; current code, CR-for chloroprene rubber) or neoprene, originally a trade name which has become generic. I t was first given the name Duprene. Duprene was prepared in 1931by Arnold Collins, a chemist in the research group of Wallace Hume Carothers (1896-1937). the discoverer of nylon (451, in the course of investigating the by products formed from divinylacetylene. I t was first manufactured commercially a t Deepwater Point, New Jersey by Du Pont, beginning in 1933 (46). In his studies of acetylene chemistry, Father Julius Arthur Nieuwland (1879-19313, professor of hotany (1904-1918) and of chemistry (1918-1936) a t the University of Notre Dame, prepared the explosive vinylacetylene by the action of copper(1) salts on acetylene: 2HCeCH
Carothers and Collins produced chloroprene (2-chloro-1,3butadiene) by adding hydrogen chloride to vinylacetylene: H,C=CH--C=CH
unstable 1,4-addition product
C1 There are several types of neoprenes. Type W neoprenes are prepared by the emulsion polymerization (37,41) of chloroprene with potassium peroxydisulfate ("potassium persulfate") (K2S208), while the molecular weight of type G neoprenes is controlled by addition of sulfur during emulsion 218
Journal of Chemical Education
polymerization. Neoprenes possess high tensile strength, high resilience, and excellent resistance to oxygen, ozone, chemicals, oil, heat, flame, and tearing. They are good general purpose rubbers, but because of their high cost they are limited to uses requiring special properties such as lining for oil-loading hose and reaction equipment, adhesive cements, hinder for rocket fuels, coatings for electrical wiring, extruded automobile parts, gaskets, seals, and protective clothing. The annual United States production of neoprenes was 86,000 metric tons in 1988. (One metric ton equals 1,000 kg or 2,204.62 pounds.) Butyl Rubber The third American synthetic rubber was hutyl rubber (American government code, GR-I-for government rubber-isohutylene; current code, IIR-for isobutylene-isoprene rubber). I t was synthesized in 1937 at Standard Oil Development Company (Esso, now Exxon) by Robert M. Thomas (190&1986) and William Joseph Sparks (19041976) (47) hy the copolymerization of a large amount of isobutvlene (2-methvl~rooene-1. ( C H.P.) ~ C = C H with ~ a .- . small amount of isoprene a t extremely low temperatures (-15I0C) with an aluminum chloride catalyst. The polymerization of isohutylene catio-nically catalyzed bv a Lewis acid yield3 a viscous to rubberlike product that is c&npletely sat&ated and cannot he vulcanized. However, if i t is copolymerized with a small amount (-5%) of a diene, one double bond remains for each molecule of diene. This is enough unsaturation to permit vulcanization, hut the polymer Fs almost completely saturated and hence highly resistant to oxidation by oxygen or ozone and to chemicals. Since to eases. it is used aslinings and i t is also hiehlvimoermeahle u . . carcasses for tubeless tires, especialiy for tractors and-other oversized vehicles: electric wire insulation: steam hose and other mechanical ;ubbergoods; and pond and reservoirsealants. It was first marketed in 1943. Theannual production of butyl rubber, which is essentially polyisobutylene, in the United States was about 200,000 metric tons in 1988. ~~
Buna Rubbers (39 In Germany Interessengemeinschaft Farhenindustrie A.G. (I.G. Farben) began to carry out research on synthetic rubber in 1926, and by 1929 it had produced a polyhutadiene rubber similar to that made in the U.S.S.R. Called "Buna" ("Bu" for hutadiene and "na" for sodium, the polymerization catalyst), it was never produced on an industrial scale but was soon replaced by two butadiene copolymers-Buna S (S for styrene-phenylethylene, C6H&H=CH2), discovered by Eduard Tschunkur and Walker Bock in the early 1930's (Ger. Pat. 570,980 (1933); U S . Pat. 1,938,731(1933)), and Buna N (N for nitrile-acrylonitrile, HG=CHCN), discovered by ErichKonrad and Tschunkur, also in the early 1930's (Ger. Pat. 658,980 (1933); U.S. Pat. 1,973,000 (1933)), I.G. Farhen first concentrated on Buna N, which showed promise as an oil-resistant ruhber since natural rubber tends to .. dis~olve ..---.- in -~~easoline.. mineral oil. and benzene. However. under pressure from the Nazi regime, the company built two large Buna S plants by 1940 and a third by 1943. Buna S (American government code, GR-S-for government rubber-styrene; current code, SBR-for styrene-butadiene rubber) is the major synthetic ruhber and today exceeds all others in consumption. The free-radical copolymerization of 75% hutadiene and 25% styrene by weight (a mole ratio of -6:l) is carried out with the monomers emulsified in water with a soap or synthetic dispersing agent, and hence the suffix "-na" is misleading since sodium metal (Na), if present, would react with water. Oxygen must he excluded, and hence a nitrogen atmosphere is used. Recnoae of-a-contractual arraneement with Esso. LG. Far-. ben also applied for and received-a US. patent. ~ h u during s World War 11, when the United States was cut off from the ~
Far Eastern rubher supply by the Japanese. American producersoftires wereable to use the reripes in theTschunkurBock patent to make thisgeneral-purpose rubber (GR-SJ for the American war effort (39.40). The worldwide production of this elastomer, now called SBR. exceeded that of natural rubber in the mid-1960's and is now producedat ahout twire the volumeofnatural rubber. The annual production of SBR (including carboxylated SBR) in the United States was about 1.3 million metric tons in 1988. GR-S is classified in two categories. depending on the .temperature of copolymerization. kegular GR-s rubbers are cooolvmerized a t 50°C. while cold GR-S rubbers are copolymerized a t as low a temperature as possible consistent with apractical reaction rate (5%). in order to keep cross-linking to a minimum. Cold GR-S (SBR) rubbers are used mainly for longer wearing automobile and truck tires. Since most synthetic rubbers generate more beat than natural rubber on flexing, natural rubber is used for heavy-duty truck tire casings, but GR-S (SBR) is used in the treads. Buna N (American government code, GR-N-for government rubber-nitrile; current code, NBR-for nitrile-hutadiene rubber) covers a wide range of copolymers of butadiene and acrylonitrile, with the latter ranging from 3356% in "standard" NBR and from 20-50% in other rubbers. Its repeating structure may be represented as -CH2CH=CHCH2CH2CH(CN)-, and copolymers using Ziegler-Natta catalysts have been developed. Produced by an emulsion process similar to that used for SBR rubber, . - ~ ~ossess& ~ oil resistance as its outstanding property. NRR This resistanceincreases with theacrylonitrilecontent-hut at the exoenseof decreased low-temperature flex~hility.Like SRH,NBR requires carbon black orother reinforcing agents to ewe it sat~sfactorvphysical propenles, but these add~uves t h a n that of conventional re&ce its cost, which is rubbers. I t was produced a t an annual rate of 70,000 metric tons in the United States in 1988. Applied to woven and unwoven fabrics, NBR makes them waterproof and improves the finish. I t also blends readily with polyvinyl chloride (PVC). High-acrylonitrile NBR is used for oil well parts, fuel tank liners, fuel hose, gaskets, packing oil seals, and hydraulic equipment. Medium-acrylonitrile --.- - - NBR is used for shoe soles, kitchen mats, sink topping, printing rolls, and general-purpose oil-resistant applications. Low-acrylonitrile NBR is used for low-temperatureflexible gaskets, grommets, and O-rings; adhesives; and hinder fuel is solid rocket propellants. A hydrogenated (saturated) NBR is also commerchlly available. Polylsoprene As we bave seen.. earlv - attempts to prepare satisfactory synthetic rubher from isoprene bere not &ccessful. While chemists have been unable to duplicate the polymerization process used to produce natural rubber (cis-1,4-polyisoprene) in Heuea trees, synthetic (98%) cis-1,4-polyisoprene (current code, IR-for isoprene rubber) was prepared in 1955 by American chemist Samuel Emmett Horne, Jr. (h. 1924)(ref 47, pp 232-234) by the stereospecific polymerization of isoprene, using a Ziegler catalyst, finely dispersed lithium or butyllithium, in a hydrocarbon solution in the absence of air and moisture. This product differs from natural ruhber onlv in that it contains a small amount of cis-1,2polyisoprene, but it is indistinguishable from natural rubber in ohvsical oro~erties.I t can be used to replace natural rudbei for n k t p u r p o s e s except when the ninruhher coustituents of natural rubber are advantageous. Under the trade names Coral rubber, Ameripol SN, and Natsyn, it was produced in the United States a t an annual rate of 50,000 metric tons in 1988. Horne later used a modified Ziegler catalyst t o produce trans-1,4-polyisoprene, a hard resin used for golf ball coven, whose structure is identical to that of balata and gutta-percha. ~
Since butadiene is more readily available and less expensive than isoprene, a polymer of hutadiene was sought for with the usual catalysts led many years, but to less useful mixed polymers. However, stereospecific polymerization in hvdrocarbon solvents with Zieeler - catalvsts such as the isobutylaluminum-titanium tetrachloride catalyst yields a rubberlike polymer that is almost exclusively cis-l&polybutadiene (current code, BR-for hutadiene rubher). Its high resistance to abrasion and crackinrc and low heat bkldup Lave led to its main use-in tire treads, especiallv in eiant tires, for which it is blended with natural or s ~ d r u b b e rFirst . produced in 1961, its produrtion reached third dace hv 1962 and was second to SHR hy 1963. It was produced a t -an annual rate of 440,000 metric tons in the United States in 1988. A polybutadiene that is almost 100% trans can be prepared with different catalysts and is used for golf hall covers. ~
Ethylene and Propylene Copolymers l v ~ r.o ~ v l ewhich u e . are proWhereas ~olvethvlene . " and .~ o .. duced by stkreospecific polymerization with ~ i e g l e r - ~ k t a catalysts, are hard, nonelastic solids, the copolymer of ethylene (CHz=CH2) and propylene (CH(CH$)=CHz) is a weather-resistant elastomer. I t is customary to add a small amount of a diene t o the reactants, thus producing sufficient unsaturation so that the product (common name,.EPDM) can be vulcanized. The annual production of EPDM in the United States was 225,000 metric tons in 1988. The ZieglerNattn catalvst that was used bv 1963 Nobel chemistw laureKarl &egler (1898-1973) t o produce polyeibylene IHDPE) consists of titanium tetrachloride (Ticla) and d ~ h y l & m i n u mchloride ((C2Hd2AlC1).
Speclaky Rubbers In addition to the general-purpose elastomers discussed above, 50,000 tons of specialty synthetic elastomers were produced in the United States in 1988. Among these are fluorocarbon elastomers (CFM), which are related to Teflon; silicone rubbers (SI), which bave outstanding beat resistance; polyacrylates (ACM), which are related to Lucite and Plexiglas; epichlorohydrin elastomers (ECO), which have excelient resistance to ozone; Hypalon, a saturated chlorosulfonated polyethylene elastomer that is extremely resistant to ozone and strong oxidizing agents; and polyurethanes (PU), which are related to the flexible foam used in upholstery and mattresses. Polvurethanes Polyurethane wassynthesized by Otto Bayer (1902-1982), who was attempting to produce a nylonlike fiber in the late e da versatile polymer that is being 1930's. The ~ u - ~ r o d u c is used for rigid and flexihle foams, bristles, impact-resistant coatings, snapback fibers, and automotive parts such as bumpers. These large parts are produced by a process called reactive injection molding (RIM), in which the polymerization takes place in the moldcavity. The RIMprocess is being used to produce wear-resistant automobile tires in Czechoslovakia, and PU tires produced by this technique should replace traditional rubber tires by the late 1990s. Rubber Latex Regardless of the polymerization techniques used or their molecular structure, natural and synthetic rubbers possess the unique propertiof long-range elasticity. Elastomers are used for the production of tires, hoses, belting, gaskets, sealants, footwear, adhesives, and in building construction. Vulcanized natural rubber latex is used for gloves, sheeting, and contraceptives. SBR and NBR are also produced as lattices, but these elastomers as well as most other synthetic elastomers are also produced in organic solvent solutions. Volume 68 Number 3 March 1991
Prior to the commercialization of svnthetic thermodastics, rubber derivatives, such as chlorinated rubber t~ar.lon), rubber hydrochloride (Pliofilm), and isomerized rubber (Pliolite) were used as coatings and films, but because of high costs, the use of these products has decreased. However, a thermoplastic product called Hevea Plus, which is produced by the copolymerization of methyl methacrylate and rubber in the latex. is commerciallv viable and can be used competitively for adhesives and thdrmoplastic elastomers. Epoxydized rubber (ENR) is readily produced by the addition of peroxyacetic acid to rubber latex. The elasticity and wet-skid resistance of tires made from ENR are superior to those made from natural rubber. ENR has been produced in a 2,000-ton capacity plant in Malaysia since 1988. This production facility can he expanded readily to meet demands as the consumption of this important rubber derivative increases. During World War I1 the development of the synthetic rubber industrv freed the world from denendence on natural rubber and itsUfluctuatingprices and kailability. As Plato first observed. "Necessitv is the mother of invention!' and within a remarkably shok period of time the United states develo~eda svnthetic rubber industrv. which soon was nroducing800,060 tons annually (39,40).-kfter the terminalion of hostilities the American synthetic rubber industrv declined sharply, but by the early 19509s,as superior andmore uniform synthetics became available, it experienced a renaissance. By the early 1960's the amount of synthetic rubber produced worldwide equaled that of natural rubber, and it has increased steadily ever since. Although natural rubber performs well for most uses, some of the newer synthetics are sunerior to i t for s~ecializednumoses. In the United States 75% of all elastomers used ini988 were based on synthetics. Tlre8
The manufacture of automobile tires consumes almost 70% of today's rubber, a use unforeseen by Charles Goodyear. Regardless of whether they are based on natural or synthetic rubber or blends of these elastomers, the end products are usually cross-linked (vulcanized) composites of rubber and additives. A typical rubber tire contains about 20 % extender oil, 35% carbon hlack reinforcement, 47% elastomer, and small amounts of zinc oxide (ZnO), stearic acid, stabilizers, wax, sulfur, and accelerators. Zinc oxide, stearic and accelerators such as 2-mercantoacid (C~~H.XCOOH). benzothiazse (captax) are used with the cross-linking'sulfur additive to control the rate and deeree of \,ulcanization. Wax and stabilizers are used to protect the tire tread from attack by sunlight, heat, and ozone. The present 50,000-mile tire has evolved from a solid rubber tire invented in the 1840's to tube-inflated fnhric -..or .cord reinforced tires to present-day radial tires in which the reinforcing filaments or wires are nlaced in a radial direction from head to bead. These continued improvements have resulted in longer wear, which results in a smaller number of tires producedannually. These changes have had an adverse effect on the major
Journal of Chemical Education
tire-producing firms. Firestone, Uniroyal (US. Rubber), and B.F. Goodrich have stopped tire production, and Goodyear now remains as the sole survivor of the major US. tire producers. Today's tire production facilities are distributed throughout the United States, No automobile tires are made in Akron, Ohio, which was once the rubber capital of the world. Today vulcanized rubber is widely reclaimed, especially from worn, discarded tires. Although an acid reclamation Drocess was develo~eddurine the late 19th centurv bv N. chapman ~ i t c h e l i ( 1 8 3 8 - 1 9 k ) most , vulcanized rihber is reclaimed by the alkali nrocess developed near the turn of the centuryby Arthur Hudson Marks i1874-1939) (4830). Both men were Americans. In the alkali process, tires or scrap rubber, either natural or synthetic, are shredded and heated with 5 4 % caustic soda (NaOH) solution to plasticize the rubher and dissolve the fabric, which is removed by washing (16,48). No sulfur is removed. but because the sulfur content is initiallv" low. there is still enough unsaturation to permit vulcanization. The price of reclaimed rubber is slightly less than half that of SBR, but it contains only about 50% rubber hydrocarbon. Reclaiming is not orofitable unless it costs nomore than half as much as virgin;ubber, and if the new ruhber is inexpensive synthetic, the market for reclaimed rubber suffers QO). bout 30,000 metric tons of reclaimed rubber were used in the United States in 1988.
Rubber, whether natural or one of the numerous synthetic elastomers, remains one of the world's most important technological products, being indispensable to a variety of basic industries. Although plastics have usurped some of the uses of this versatile elastomer and have made inroads into some of the markets previously monopolized by it, new applications and augmentation of traditional uses have assured a steady expansion and bright future for the rubber industry. Literature Clted
37. ~ l ~ ~ kD. i C.' ~~ m~v ,l s ~ a~n~ l y m r r i & n Theory end Practice: Wiley: New York. 1976. 38. Solo. R. Acru8 the High Technology Threshold: The Case o/ Synthetic Rubber: Norwood Editions: N u r u o d , PA, 1980. 39. Tutflc, Jr.. W. M. Techno1 Culture 1981.22.35. 40. Herbert. V.; Birio,A. SynthrdcRubbac A Piojerl That Hod ToSueceed, Greenwnod: Westport, CT. 1985. 41. Enrylupedio o/Srrencr and Technology, 2nd ed.: Wiley: New York. 1986: V o l t pp 151. 42. B0uchardst.G. Compt. rend. 1879.89.1117, 43. Tilden, W.A. J. Chem. S o c I884.A,410. 44. Harries,C.D.et el. Ann. 1911,383, 157: 1914,406,173:Z. ongslu. Cham. 1920.33,226. 45. Kauf1msn.G.B. J. Chsm.Educ. l988,85,M3. 46. Keu1fman.G. B.: Mas0n.S. W.: Sevmour.R. B. J.Chem.Educ. 1990.67.198, 47. Seymour, R. B.; Fisher, C. H. Pro/iies 01 Eminent American Chemists, Litsrvsn: Sydney,Australis. 1988: pp 448-452. 48. Ball, J. M. Racloimed Rubbe,: The l o r y o l on Am~riconRom Molrriol: Rubber Reelaimerr Association: New York. 1947. 49, Blow, C.M.; Hepburn. C. Rubber T~chnologyandMonulodure. 2nd ed.: Butteworth Scientific: London. 1981. 50. Stempel, G. H. Ref.9, pp315-316.