Polymerization methods
Synthetic elastomers are produced on an industrial scale in either solution or emulsion polymerization methods. (Solution polymerization and emulsion polymerization are described in the article chemistry of industrial polymers.) Polymers made in solution generally have more linear molecules (that is, less branching of side chains from the main polymer chain), and they also have a narrower distribution of molecular weight (that is, greater length) and flow more easily. In addition, the placement of the monomer units in the polymer molecule can be controlled more precisely when polymerization is conducted in solution. The monomer or monomers are dissolved in a hydrocarbon solvent, usually hexane or cyclohexane, and polymerized, using an organometallic catalyst such as butyllithium.
In emulsion polymerization, the monomer (or monomers) is emulsified in water with a suitable soap (e.g., sodium stearate) employed as a surfactant, and a water-soluble free-radical catalyst (e.g., potassium persulfate, peroxides, a redox system) is added to induce polymerization. After polymerization has reached the desired level, the reaction is stopped by adding a radical inhibitor. About 10 percent of synthetic elastomer produced through emulsion techniques is sold as latex. The rest is coagulated with acidified brine, washed, dried, and pressed into 35-kg (77-pound) bales.
When emulsion polymerization of SBR is carried out “hot” (i.e., at 50 °C, or 120 °F), the polymer molecules are more branched. When polymerization is carried out “cold” (i.e., at 5 °C, or 40 °F), they are more linear and generally higher in molecular weight—features that improve the rolling resistance and wear resistance of tires. In some cases polymerization is continued in order to give products of such high molecular weight that they would normally be intractable. In these cases about 30 percent of a heavy oil is added before coagulation to yield “oil-extended” elastomers with superior wear resistance.
The rise of synthetic rubber
The origins of the elastomers forming the base of synthetic rubber can be traced to the first half of the 19th century, when attempts were made to elucidate the composition and structure of natural rubber with the eventual goal of reproducing the material. In 1838 the German F.C. Himly obtained a volatile distillate from the substance, and in 1860 the Englishman C. Greville Williams broke down rubber by distillation into three parts—oil, tar, and “spirit”—this last part being the more volatile fraction and the main constituent, which Williams named isoprene. The Frenchman Georges Bouchardat, with the aid of hydrogen chloride gas and prolonged distillation, converted isoprene to a rubberlike substance in 1875, and in 1882 another Briton, W.A. Tilden, produced isoprene by the destructive distillation of turpentine. Tilden also assigned isoprene the structural formula CH2=C(CH3)―CH=CH2.
The efforts outlined above were attempts to replicate natural rubber. It was only when the search for chemical equivalents to natural rubber was abandoned and comparable physical properties were emphasized that synthetic rubber came into being. The choice fell upon butadiene (CH2=CH―CH=CH2), a compound similar to isoprene, as the basis for a synthetic product. Several significant contributions came from Russia. In 1901 Ivan Kondakov discovered that dimethyl butadiene, when heated with potash, produced a rubberlike substance, and in 1910 S.V. Lebedev polymerized butadiene, which he obtained from ethyl alcohol. During World War I, Germany, under the stimulus of the blockade imposed by the Allies, began production of “methyl rubber” by using Kondakov’s process. This was an inferior substitute by present-day standards, and after the war German manufacturers returned to the cheaper and more satisfactory natural product. Research and experiments continued, however, and in 1926 the German G. Ebert succeeded in producing a sodium-polymerized rubber from butadiene. During the following decade this material evolved into various types of “buna” rubber (so called from the initial syllables of the two materials used to make them: butadiene and natrium [sodium]).
In the Soviet Union, production of polybutadiene by using Lebedev’s process was begun in 1932–33, using potatoes and limestone as raw materials. By 1940 the Soviet Union had the largest synthetic rubber industry in the world, producing more than 50,000 tons per year. In Germany, meanwhile, the first synthetic elastomer that could be used to replace natural rubber and make satisfactory tires was developed at I.G. Farben by Walter Bock and Eduard Tschunkur, who synthesized a rubbery copolymer of styrene and butadiene in 1929, using an emulsion process. The Germans referred to this rubber as Buna S; the British called it SBR, or styrene-butadiene rubber. Because styrene and butadiene can be made from petroleum, grain alcohol, or coal, SBR was in great demand during World War II. Immense amounts were made—as much as 100,000 tons per year in Germany and the Soviet Union. About 800,000 tons of SBR were produced per year in the United States, where it received the wartime designation GR-S (government rubber-styrene). During the war German chemical engineers perfected low-temperature, or “cold,” polymerization of SBR, producing a more uniform product.
Other important synthetic elastomers were discovered in the decades before World War II, though none was suitable for making tires. Among these were polysulfides, synthesized in the United States by Joseph Patrick in 1926 and commercialized after 1930 as oil-resistant thiokol rubbers; polychloroprene, discovered by Arnold Collins in 1931 and commercialized by the DuPont Company in 1932 as Duprene (later neoprene), a high-strength oil-resistant rubber; nitrile rubber (NBR), an oil-resistant copolymer of acrylonitrile and butadiene synthesized by Erich Konrad and Tschunkur in 1930 and known as Buna N in Germany; and butyl rubber (IIR), a copolymer of isoprene and isobutylene discovered in 1937 by the Americans R.M. Thomas and W.J. Sparks at Standard Oil Company (New Jersey).
After World War II, increasing sophistication in synthetic chemistry led to many new polymers and elastomers. In 1953–54 two chemists, Karl Ziegler of Germany and Giulio Natta of Italy, developed a family of organometallic catalysts that were able to control precisely the placing and arrangement of units along the polymer chain and thus produce regular (stereospecific) structures. With the use of such catalysts, isoprene was polymerized in such a manner that each unit in the chain was linked to its predecessor in a cis configuration, virtually identical to the structure of natural rubber. In this way virtually 100 percent cis-polyisoprene, “synthetic natural rubber,” was made. In 1961 the same type of catalyst with butadiene as the monomer was used to produce cis-1,4-polybutadiene, a rubber that was found to have excellent abrasion resistance, especially in tires subjected to severe service conditions.
Several other advances characterized the postwar years. For example, block copolymers, in which a long sequence of one chemical unit is followed in the same molecule by a long sequence of another, were made, using many different units and sequence lengths. New oil-resistant and heat-resistant elastomers were introduced, including the styrene-acrylonitrile copolymers, the polysulfides, and chlorinated and chlorosulfonated polyethylene. Control has been achieved, to some degree, over the wide range of molecular length found in most polymers, so narrow or broad distributions can be produced in many cases, with quite different viscous properties. In addition, polymers have been synthesized with branched molecules, either with many small branches along the main chain or with several long “arms” radiating from a central point, giving different flow properties and more facile cross-linking.
World consumption of synthetic rubber reached nine million tons in 1993. About 55 percent of all synthetic rubber produced is used in automobile tires.
