Rubber, Synthetic

synthetic polymers that, like natural rubber, can be converted to vulcanized rubber.

All synthetic rubbers are usually classified as general-purpose or special-purpose (see Table 1). General-purpose rubbers are used in the production of articles in which the basic property of vulcanized rubbers—that is, high elasticity at ordinary temperatures—is desirable (for example, tires, conveyer belts, and footwear). Special-purpose rubbers are used in producing articles that should be resistant to the action of solvents, oils, oxygen, ozone, heat, and frost—that is, able to retain high-elastic properties over a broad range of temperatures. The classification of synthetic rubbers according to use is to a degree arbitrary, since many rubbers possess a range of properties that make them suitable as both general-purpose and special-purpose material. On the other hand, special demands are sometimes placed on certain general-purpose articles, such as in the production of frost-resistant tires and oil-resistant and gasoline-resistant rubber footwear. Polymers called thermoelastic plastics have been developed that combine the properties of elastomers and thermoplastic polymers and that therefore can be processed into rubber articles while bypassing the vulcanization stage. The special groups of synthetic rubber include aqueous dispersions of rubber (latexes), liquid rubbers (oligomers, which harden with the formation of rubberlike materials), and filled rubbers (mixtures of synthetic rubbers with fillers and plasticizers and produced in the manufacture of synthetic rubbers).

The most widely used methods of producing synthetic rubbers are emulsion and stereospecific polymerization. Polymerization makes it possible to regulate the molecular weight of the rubbers. In processing the synthetic rubbers polymerization makes it possible to do without the energy-intensive stage of mastication. The production processes (in the majority of instances these are continuous) also include the stages of separating the rubber from the dispersions or solutions (for example, by coagulation or sedimentation), removing the residues of catalysts, emulsifiers, and other impurities from the rubber, drying, briquetting, and packaging. The most important monomers in the synthesis of rubbers —butadiene, isoprene, and styrene—are obtained primarily from casinghead-gas by-products and cracking gases. For example, butadiene can be obtained by the catalytic dehydrogenation of //-butane. In addition to these monomers, acrylonitrile, fluoro-olefins, and certain silicone compounds can also be used.

The successful solution to the problem of the industrial synthesis of rubber is among the most important scientific and technical achievements of the 20th century. Rubber was synthesized for the first time in the world on a large industrial scale in the USSR in 1932, using a method developed by S. V. Lebedev— that is, the polymerization with metallic sodium of 1, 3-butadiene obtained from ethyl alcohol to produce SKB sodium-butadiene rubber. The industrial production of butadiene-styrene rubbers was organized in Germany in 1938, and the large-scale production of synthetic rubbers was started in the United States in 1942. By 1972 more than 20 countries were producing synthetic rubbers. The USSR holds one of the leading places in terms of the production volume of synthetic rubbers.

World production of synthetic rubbers is growing rapidly. Whereas in 1950 synthetic rubbers constituted about 22 percent of the total production volume of all rubber in the capitalist

Table 1. Major industrial synthetic rubbers
Name of rubbers and their Soviet abbreviations	Chemical composition	Special properties
General-purpose rubbers
Butadiene, SKD..........	Cis-1, 4-polybutadiene	–
Styrene-butadiene (a-methyl-styrene), SKS (SKMS).....	Copolymers of butadiene and styrene (α-methylstyrene)	–
Isoprene, SKI...........	Cis-1, 4-polyisoprene	–
Ethylene-propylene, SKEP.............
SKEPT
Butyle rubber, BK..........	Copolymers of isobutylene and small quantities of isoprene	Impermeability to gases, weather resistance
Chloroprene (Nairit)..........	Polychloroprene	Satisfactory oil resistance and gasoline resistance
Special-purpose rubbers
Butadiene nitrile, SKN.......	Copolymers of butadiene and acrylonitrile	Oil resistance, gasoline resistance
Polysulfide (Thiokol)........	Polysulfides	Oil resistance, gasoline resistance
Silicone, SKT...........	Polyorganic siloxanes	Heat and frost resistance, high electric-isolating properties, physiological inertness
Fluorine-containing rubbers, SKF	Copolymers fluoro-olefins	Heat, oil, weather, and fire resistance, resistance to attack by aggressive media
Urethane, SKU..........	Polyurethanes	Great strength in stretching and wear
Chlorosulfonated polyethylene KhSPE.............	Polyethylene containing Chlorosulfonated groups	Weather resistance, heat resistance, durability

countries, in 1960 the figure was approximately 48 percent, and by 1973 it had risen to approximately 63 percent (that is, about 5.9 million tons of synthetic rubber and about 3.5 million tons of natural rubber). The intensive growth in the output of synthetic rubbers is explained by the significantly lower prime cost of production of the most widely used general-purpose rubbers (in particular, butadiene-styrene) in comparison to the prime cost of production of natural rubber, as well as by the impossibility of using natural rubber in certain special-purpose articles, such as heat, oil, and gasoline resistant items. The development of butadiene and isoprene stereoregular synthetic rubbers has also led to a relative decline in the demand for natural rubber, since the former are competitive with natural rubber in the production of certain tires (for example, for passenger cars).

There are around 50, 000 listed processed-rubber products manufactured from synthetic rubbers. The largest consumer of synthetic rubbers is the tire industry, which accounts for more than 50 percent of the total volume of synthetic rubbers consumed. Technical progress in various areas of industry has confronted the synthetic rubber industry with the task of developing rubbers that will combine high heat resistance, resistance to ionizing radiation, and oil and gasoline resistance. This problem in particular can be solved by synthesizing rubbers from monomers that contain inorganic elements, such as boron, phosphorus, nitrogen, fluorine, and silicon.