RUBBER

elastic substance obtained from the exudations of certain tropical plants (natural rubber) or derived from petroleum and natural gas (synthetic rubber). Because of its elasticity, resilience, and toughness, rubber is the basic constituent of the tires used in automotive vehicles, aircraft, and bicycles. The same properties make it useful for machine belting and hoses of all kinds. Rubber is also used in electrical insulation, and because it is waterproof, it is a favoured material for shoe soles. Natural rubber was first scientifically described by C.-M. de la Condamine and Franois Fresneau of France following an expedition to South America in 1735. The English chemist Joseph Priestley gave it the name rubber in 1770 when he found it could be used to rub out pencil marks. Its major commercial success came only after the vulcanization (q.v.) process was invented by Charles Goodyear in 1839. Natural rubber is produced from a wide variety of plants, but predominantly from Hevea brasiliensis (family Euphorbiaceae), a tall softwood tree originating in Brazil. Hevea trees descended from seedlings transplanted to South and Southeast Asia now produce most of the modern world's natural rubber. The largest producing countries are Indonesia, Malaysia, Thailand, India, Sri Lanka, and China. The latex found in the inner bark of H. brasiliensis is obtained by tappingcutting or shaving the bark with a sharp knifeand collecting the latex in cups. Solid rubber is coagulated from the fluid by the addition of chemicals, such as formic acid, that cause the rubber to form curds on the surface of the liquid. The curds can then be pressed between rollers to remove excess moisture and to form sheets. The sheets are commonly packed in bales for shipping. Rubber is also commonly transported in the form of a concentrated latex. Despite the competition of synthetic rubber, natural rubber continues to hold an important place; its resistance to heat buildup makes it valuable for tires used on racing cars, trucks, buses, and airplanes. Research on the chemistry of natural rubber led in the 19th century to the isolation of isoprene, the chemical compound from which natural rubber is polymerized. Polymerization, the process by which long chainlike molecules are built up from smaller molecules, attracted continued research in the early 20th century. During World War I, German scientists produced a crude synthetic rubber, and during the 1920s and '30s several polymerizing processes were developed in Germany, the Soviet Union, Britain, and the United States. During World War II huge quantities of synthetic rubber, mostly based on polymers of butadiene, were produced. By the early 1960s synthetic rubbers had overtaken natural rubber in quantity produced. Among the most important synthetics are polybutadiene; styrene-butadiene copolymer; polychloroprene (neoprene); the polysulfides (Thiokol); the isobutylene-isoprene copolymers (butyl rubbers); and the polysiloxanes (silicone rubbers). Synthetic rubbers, like natural rubbers, can be toughened by vulcanization and improved and modified for special purposes by reinforcement with other materials. See also elastomer.