edited by GEORGE6.
KAUFFMAN
California State University. Frerno
Fresno. CA 93740
Elastomers I. Natural Rubber George 6. Kauffman California State University, Fresno, CA 93740 Raymond 6. S e y m w r University of Southern Mississippi, Hattiesburg, MS 39406 Rubber is one of nature's unique materials. The Indians of the New World recognized the uniqueness of this versatile elastomer (Fig. 1). The Spanish navigator and historian Gonzalo Fernindez de Oviedo y Valdes (1478-1557) was the first to describe the rubber halls used hv the Indians, and in the late 16th century Philip I1 of ~pain;shistorian, Antonio de Herrera v Tordesillas (1559-1625), wrote in his journals (1,Z):
Christopher Columbus, having again landed on the island of Hispsniola [Haiti], on his second voyage to the New World in 1493, set out to explore his new domain, search for gold and observe the natives, whom he called Indians, since he still believ'd he had reached the [East] Indies. In their villages he noted that one of their oastimes was a eame olaved with halls made of the gum of a tree, which tho' heavy would fly and bound better than those fill'd with Wind in Spain. Rubber was brought back t o Europe from the Amazon in 1735 by Charles Marie de La Condamine (1701-1774), a French mathematical geographer (3),but i t remained largely a mere museum curiosity. Although i t found limited use for waterproofing boots, shoes, and garments (4), because it hardened and cracked when cold and melted when warm, its potential was not realized until the American inventor Charles Goodvear (1800-1860) discovered the mocess known as vulckization (4-11). Rubber was known to the Maina Indians as "caoutchouc" (from can (wood) and o-chu (to flow or weep)) until 1770, when Joseph Priestley suggested the name "rubber" since by rubbing on paver it could be used to erase pencil marks, instead of thdp;eviously used bread crumbs (12-14). Sourcea of Natural Rubber ( 15-22) Although rubber, which is usually found as a colloidal emulsion in a white, milky fluid called latex, is widely distributed in the plant kingdom, a t one time more than 98%of the world's natural rubber was produced from the rubber tree. Heuea brasiliensis (order Eu~horbiales,familv Euphorbiaceae-the spurge family). he tree, qo&ng wiid t o a heieht of 120feet and known t o the Indians a s heue, is native to the Amazon Basin of Brazil (Fig. I), and rubber production was a Brazilian monopoly until June 14,1876, when Sir Henry A. Wickham (18461926), a British explorer and Amazon Basin planter, smuggled 70,000 seeds of the tree out of Brazil and brought them to England, where almost 30,000 of them were successfully germinated in Kew Gardens near 422
Journal of Chemical Education
London (17, 19). The young trees were shipped to various English colonies in the Far East and were cultivated on plantations in Ceylon (now Sri Lanka) and the Straits Settlements (now Malaysia). Today most of the world's 3 million tons' annual production of natural rubber is produced on plantations in Malaysia, Indonesia, Singapore, Thailand, and Sri Lanka. Rubber producers have constantly searched for alternas also been tive sources. and other rubber-bearine ~ l a n thave found that can becultivated outside t i e intertropicalzone to which Heuea trees are native. These belone. ~rimarilvto the family Compositae and include guayule ( ~ r ; ; t h e n i u iargentatum), a shrub grown along the U.S.-Mexican border, whose main disadvantage is its high gum and resin content; Scorzonera tau-saghyz, a plant similar to salsify growing in Turkestan, and the Russian dandelion (Taraxacum koksaghyz), which yields a latex with a 30% dry rubber content. These plants were used as sources of rubber during World War 11, and University of California scientists are currently exploring ways of utilizing guayule as a commercial source of rubber. More natural rubber is now obtained annually from the guayule shrub than from Heuea trees in Brazil, which is now
Figure 1. South Arnerlcan natives tapping rubber trees. collecting the latex. and smokino it over a fire. (From a c o.w. of ref 5. orinted on an india-rubber tossue and oound in ruboer coven.The only copy snsnt is in the Smothsonlsn Inst man. Washhgton. DC Photogaph horn ref 8. p 216)
an importer of natural rubber and a producer of SBR (styrene-butadiene) synthetic rubber. The plantation owners in Malaysia and Indonesia, which produces more than 90% of all natural rubher, are attempting to maintain their share of the world rubber market by cloning Heuea trees and by producing rubber derivatives at the source. The yield of rubber has been increased by cloning from 1,200 l b h a (10 X lo3m2)to more than 2,000 Ibha in Malaysia. Yields of 7,500 Ibha have been oroduced exoerimentallv. and it has been predicted that yields of 20,006 Ibha are iheoretically possible hv clonine and growth stimulation. Yields have also been increased b; the &e of stimulants such as Ethephon or ethryl (2-chloroethylphosphonic acid), which is applied to the bark below the tapping cut. Ethephon releases the growth stimulant ethylene in the presence of moisture. Chernlcal Constkutlon and Properties ( 16-22) Crude plantation rubber consists of the rubber hydrocarbon, for which Michael Faraday (1791-1867) established the emoirical formula CCHRin 1826 (23). alone with 2-4% ~ r o te& and 1-490 acetoneioluble miterial (resins, fatty acids, and sterols). In 1835 the Scottish chemist William Greeorv (1803-1858) distilled rubber and must have obtained crud; isoprene (2-methyl-1,3-butadiene,CH2=C(CH3)CH=CH2) (24). In 1838 the French physician and pharmacist Appolinaire Bouchardat (1806-1886) similarly obtained impure isoprene, which he called "cauchene", by distilling rubber (25). Pure isoprene was obtained from rubber distillate by the British chemist Charles Hanson Greville Williams (18291910), who gave it its present name, determined its vapor density and molecular formula, and showed that it polymerized to a rubberlike product (26), an observation leading to the view that rubber is a oolvrner of isoorene. Rubber is unsaturated; and for each'five carbon atoms it adds 1 mol of bromine or 1 mol of hydroaen chloride, showing the presence of one double bond for each isoprene unit. In 1904 Carl Dietrich Harries (1866-1923), Professor of Chemistry at the University of Kiel(27), began his extensive series of experiments on the determination of the position of the double bonds bv.~ ozonolvsis of unsaturated oreanic compounds. BYo~onizin~rubbbr, he obtained levulin; aldehyde and levulinic acid from the hvdrolvsis 1281.In 1910 - oroducts . the English chemist Samueishrowder Pickles (1878-1962) (29) proposed that rubber is a linear polymer of isoprene with the structure (30): ~~~~
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When the arrangement is trans, i.e., when the chain extensions are on opposite sides of the ethylene double bond, the polymer is a hard plastic:
The trans isomers occur naturally as gutta-percha (obtained from the leaves of hybrids of Palaquiurn, sapotaceous trees grown in Malaysia and the East Indies) and balata (similar to gutta-percha but obtained from Mimusops globosa, native to Panama and northern South America). Gutta-percha was the preferred wire and cable coating during the 19th century, and balata is the preferred golf ball cover of many of today's low handicap and professional golfers. The hydrocarbons of gutta-percha and balata are identical and have the same composition as natural rubber. However. unlike rubber. thev are hornv. Gutta-nercha exists in twokodifications: with an ident:ty periodbf 8.7 A (consistent with a trans confieuration with coolanar carbon atoms in the chain), and 8, wsh an identity pekod of 4.8 A (consistent with a trans configuration with the carbon atoms of the chain not in one lane, similar to natural rubber). The identity period (8.2 ) shown by the X-ray diffraction patterns for stretched rubber are consistent only with a cis configuration at the double bonds with some buckling so that the carbon atoms of the chain do not lie in a plane. The structures of the three related hydrocarbons are shown in Figure 2.
A,
1
82 ;
I
(a) Rubber
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~
(b) u-Gutta-percha
Rubber
I-4ai-I
Levulinic Aldehyde A detailed analysis of ozonolysis products accounted for 95% of the carbon content of the rubber molecule and showed that 90% of the isolated oroducts can be considered an derived from levulinic aldehyde in agreement with Pickles'~ostulatedstructure. According to one study, the molecul& weights of rubber moleculei range from 50,000 to 3,000,000 (with 60% of the molecules having molecular weights greater than 1,300,000). The repeating unit in natural rubber (cis-1,4-polyisoprene) is -CH-C(CH3)=CH-CH2-. The cis arrangement, in which the chain extensions are on the same side of the ethylene double bond, is essential for elasticity in polyisoprene
skeletal structure for cis-1,4-polyisoprene
(c) p~utta-perAha Figure 2. Structures of rubber and a-and B-guttapercha (ref 78, p 784).
The properties of rubber such as its elasticity are directly dependent on its chemical constitution. Long-range elasticity is a reversible process in which random coils of polymeric chains are uncoiled and the stretched chains become aligned and tend to form crystals. Stretched unpigmented gum rubber is opaque because of crystal formation. That a rubber band becomes warm when stretched can be demonstrated by pressing a newly stretched rubber band to one's lips. Since the arrangement of the uncoiled aligned chains is more ordered than the disordered unstretched random coils, the stretched rubber chains are said to have deVolume 67 Number 5 May 1990
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Figure 4. Rubber ~ o I B c u I ~(a) % ~ n v ~ l ~ ~ n l(b) z ev~lcenlzed d; but unstretched; (c) VUlCaniZed and swetohed (ref 16. p 782).
itv 4(h)). * (Fie. . . . . If onlv a few cross-links are resent, segments i f the molecules' prkcipal sections can hialigned a d eloneated considerahlv bv stretching hut cannot slip past one Lother. ~hermal~agiiation causes the return of these molecules to their orieinal random orientation when the tension is removed (compare Figs. 4(h) and (c)). Vulcanization was once thought to he a free-radical process, but it is currently considered to involve a polar mechanism. Cross-linking probably involves disulfide or polysulfide bonds between a-carbon atoms adjacent to the douhle honds rather than addition to the douhle bonds; addition of sulfur ~ ~ ~~- to - the douhle honds seems to result from formation of cyclic sulfur compounds. While as little as 0.3%sulfur results in vulcanization, soft rubber usually contains 1-3% sulfur, and hard ruhher or ebonite (developed by Goodyear's hrother Nelson) contains 23-35% sulfur. Ruhbers containing sulfur in percentages between-these ranges are unmanageable and useless. More than 35.000 tons of sulfur were consumed in the United States in 1988 in vulcanizing natural and svnthetic ruhher. Other vulcanizing agents such as metal oxides, organic peroxides, or aromatic nitro compounds may also he used. Exposure of thin sheets of ruhber to light causes enough cross-linking to produce an Bffect similar to vulcanization (17). In modern vulcanization temperatures of 140-180 'C are employed, and, in addition to sulfur, other additives are used. Accelerators such as 2-mercaptohenzothiazole (Captax) and the corresponding disulfide, sodium and zinc dimethyl-, diethyl-, and dihutyldithiocarbamates, and tetramethylthiuram disulfide (Tuads) allow the reaction to take place at lower temperatures in less time and, in some cases, with less sulfur. Organic accelerators are usually most effective in the presence of accelerator activators such as zinc oxide, whichisused withstearic acid toincrease its solubility in the rubber. Antioxidants, ex., secondary aromatic m i n e s such as N-phenyl-P-naphthylamine,prolong the life of rubber articles by lessening deterioration caused by atmospheric oxveen or ozone. The aeine of ruhher is caused hv autocata&&;utoxidation of the centers of unsaturation; resulting in the breakine of bonds and the lowerine- of the molecular weight. Reinforcing agents such as carbon black are used to increase the stiffness, tensile strength, and resistance to abrasion of the rubber. "Carbon white", a superfine amorphous silica powder, is reputed to he equally effective for this uurvose and does not add color. Fillers such as barium sulfate, calcium carbonate, or diatomaceous earth can be used as extenders to reduce the cost of articles, hut they decrease their strength. Softeners such as fatty acids, wood resins, pine oils, pine tar, or coal-tar byproducts like bitumen are also sometimes added. Colorine aeents or oiements must withstand vulcanization temp&at;res. For-high-temperature cures they are therefore limited to inorganic substances ~
Figure 3. An artist's conception of Charles Goodyear's discovery of vulcanlratlon (Imm Allen, H. m e House of Gocdyear: FiW Yeas of Men and lndustv Carday & Gross: Cleveland, OH. 1949; p 97).
creased entropy or disorder. This entropy increases when the tension of the stretched band is released, and the freshly unstretched hand will feel cool when pressed to the lips. If the stretched hand is cooled below its glass transition temperature (-70 "C), it will remain stretched, but this racked ruhher will snap back to its original length when the temperature is raised to above 70 OC. The stretched rubber will also contract when heat is applied. This decrease in length when a stretched rubber hadd is heated is the opposite of the effect noted in most other materials such as metals. Vulcanlzatlon (4-1 1) Since there are few if any cross-links in the chains of rubber molecules, natural rubber is a thermoplastic; it becomes soft and sticky in the summer and hard and brittle in the winter. It is also malodorous and is softened or dissolved by various solvents. These undesirable properties were overcome in 1839. when Charles Goodvear, after five years of ---constant experimenting, accidentally placed some rubber mixed with sulfur and litharge (lead monoxide, PbO) on a hot stove in Wohurn, Massachusetts (Fig. 3). This process converted ruhber into a tough, elastic, heat- and cold-stable substance suitable for tires and numerous other manufactured products and marked the birth of the modern ruhher industry. Subsequent discoveries have modified Goodyear's original procedure to some extent, hut today vulcanization remains essentially the same process that it was a century and a half ago. The reactions between rubber and sulfur are complex and ~~~-~~~~~~ still not completely understood, but in vulcanized ru-bher the sulfur is not dissolved or dispersed. Vulcanization is a chemical reaction between rubber and sulfur, resulting in crosslinks between the chains of rubber molecules, thus preventing slippage of the chains while retaining the desired elastic~
~
424
Journal of Chemical Education
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such as zinc oxide, titanium dioxide, iron oxide, and ultramarine. whereas for lower temoerature cures various oreanic " dyes can he used. Todav rubber is indisoensable to a varietv of industries and products, and our modern world, with i& multifaceted necessities and luxuries, would be unthinkable without it. Synthetic rubbers will be considered in Part I1 of this series. Literature Clted 1. Buehr, W.Rubber:NofurolandSynihelir:Marrow: New York. 1964 p7. 2, deHsrrers yTord~si1laa.A.T h e G ~ n e r a i H k t o r y o f f hVml CanfinenImdIdonds of Stevens. J.,Trans.: J. Batley: London, 1725. Amwioa.. ; 3. La Condamine, C. M. de. A Succinct Abridgement of a Voyage Mode Wilhin the InIondPorlsof South America; fromthe Constaof the South-Sea Lo the Cwafa of B r a d and Cuiono, damn the River of the Amoron: Withers: London, 1717. 4. Kauffman. G. B. The World& 11989.1i51. 294. 5. Goodyear,C. (el Gum-Elaafirondlts Voriely,~thnDetoilodAccountoflfa Appiicaond of the Dlarouav of Vuleonirotion; Published for the Author: Lions end USDB, New Haven, CT, 1855; Vol. 1; ihl The Agplicationr and Uses d Vulcanized Gum. Elasfie; vith D ~ s c r i p f i and o ~ Diracfion, for Monufocluring Purposes Published for the Author: New Haven, CT, 1853: Vol. 2. Both these vdumas, together with Heneoek, T Persoml Norrotiue of the O~ig
