Rubber, Synthetic
Natural rubbers, before vulcanization, tend to be sticky and soft at high temperatures, while at low temperatures they are brittle and stiff, making them difficult to process. Because of these properties as well as the difficulties associated with obtaining adequate and affordable supplies of natural rubber, the search for natural rubber substitutes began.
In 1906 Farbenfabriken of Elderfeld began to search for a viable production process. Fritz Hofmann was appointed head of the research group. He attempted to polymerize isoprene, which was known to be a component of rubber. Convinced that the purity of the isoprene was paramount, he spent two years researching methods of producing pure isoprene. Finally he developed a six-step process. He next attempted to polymerize the pure isoprene. None of the techniques discussed in the scientific journals of the time were successful. Ultimately Hofmann simply heated the isoprene in an autoclave. Various viscous liquids were used in the heating process to emulsify the rubber. The product was comparable to natural rubber in toughness and elasticity and was much less sticky. But the mixture had to be shaken at 140°F (60°C) for several weeks. Because of the long production time and high cost, the process was abandoned as impractical.
Once World War I started and German access to natural rubber supplies was cut off, Farbenfabriken developed two grades of synthetic rubber, both made from 2,3-dimethyl butadiene. The softer grade was formed by pretreating the monomers (single units) with oxygen and polymerizing them at 149°F (65°C). The firmer grade used an initiator (usually preformed rubber) and was polymerized at 86°F (30°C). Despite the initiators both reactions took several weeks to complete.
After the war, the price of natural rubber fell below the cost of production for synthetic rubber. It was not until natural rubber producers raised the price from $.17/lb. to $1.21/lb. that interest in synthetic rubbers resumed. Buna rubber was made of butadiene (Bu) by sodium polymerization. Buna S was a copolymer of butadiene and styrene; Buna N used butadiene and acrylonitrile as copolymers. Both were developed by Hermann Staudinger (1881-1965) and his research team, and patented in 1929. Both were more resistant to oil, gasoline, and aging than natural rubber.
Neoprene is another synthetic rubber discovered in the pre-World War II years. Father Julius Nieuwland (1878-1936) of Notre Dame University shared his research with Wallace Carothers (1896-1937) of Du Pont. Father Nieuwland was researching the polymerization of acetylene, and his research showed strong similarities between the structure of polymerized acetylene and natural rubber.
Arnold Collins, a research chemist on Carother's team, purified a sample of Nieuwland's acetylene and produced a small amount of a liquid which, when left out over a weekend, formed an elastomeric (rubber-like) solid. After additional research, a reaction converting acetylene to vinyl acetylene, using copper chloride as a catalyst and hydrogen chloride as an additive, was patented. The product of the reaction was 2-chlorobutadiene, called chloroprene because of its similarity to isoprene. Du Pont renamed it neoprene and began to market it in 1930. It was more expensive than natural rubber but had an even greater resistance to oil, gasoline, and ozone.
Thiokol Chemical Corporation produced a synthetic rubber from ethylene dichloride and sodium polysulfide. It was developed by J. C. Patrick, Thiokol's president, and marketed in 1929. Although it emitted an unpleasant odor, it found use in fuel tank linings for aircraft and for windshield seals for cars.
GR-S (government rubber, styrene type) came on the market during World War II, when there were again restricted supplies of natural rubber.
Studies by the German scientist Karl Ziegler (1898-1973) and by the Italian Giulio Natta (1903-1979) showed that organometallic compounds greatly increased the speed of some reactions. By using organometallic catalysts, scientists were able to prepare copolymers of styrene and butadiene very quickly, with reactions completing in less than twenty-four hours. GR-S rubbers are now commonly called SBRs (styrene-butadiene rubbers). Ziegler's catalyst system led to the development of ethylene-propylene rubber in 1960. Du Pont succeeded in crosslinking the ethylene-propylene rubber by adding small amounts of a third monomer (termonomer) in 1961; by 1967, Uniroyal, Copolymer Corporation, and Jersey Standard were producing ethylene-propylene rubber using a termonomer patented by Du Pont. Ethylene-propylene polymers (EPMs) are most often used in abrasion-resistant applications.
Today there are many synthetic rubbers on the market. Silicone rubbers are linear polymers (polymers whose molecules are arranged in long chain-like structures) derived from dimethyl silicone. They are difficult to process but are stable over a wide range of temperatures -130-601°F (-90-316°C). They are used in wire and cable insulation and gasket applications.
Besides these synthetic rubbers, many styrene-based polymers have been used in sporting goods. Thermoplastic elastomers are currently the fastest reacting and most economical synthetic rubbers available.
Most developments in the rubber industry since 1955 have been more characterized by technology transfer rather than innovation. The major West German synthetic rubber producer Chemische Werk Hüls decided to abandon its own obsolete technology and purchase a GR-S plant from Firestone in 1954. Synthetic rubber production began in Great Britain and Italy in 1957, in France and Japan two years later, and in Brazil in 1962. By the early 1970s, Japan had become a major producer of SBR as well as of all-cis polybutadiene and polyisoprene.
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