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. More than half of all rubber produced goes into automobile tires; the rest goes into mechanical parts such as mountings, gaskets, belts, and hoses, as well as consumer products such as shoes, clothing, furniture, and toys.
The main chemical constituents of rubber are elastomers, or “elastic polymers,” large chainlike molecules that can be stretched to great lengths and yet recover their original shape. The first common elastomer was polyisoprene, from which natural rubber is made. Formed in a living organism, natural rubber consists of solids suspended in a milky fluid, called latex, that circulates in the inner portions of the bark of many tropical and subtropical trees and shrubs, but predominantly Hevea brasiliensis, a tall softwood tree originating in Brazil. Natural rubber was first scientifically described by Charles-Marie de La Condamine and François 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 vulcanizationprocess was invented by Charles Goodyear in 1839.
Natural rubber continues to hold an important place in the market today; its resistance to heat buildup makes it valuable for tires used on racing cars, trucks, buses, and airplanes. Nevertheless, it constitutes less than half of the rubber produced commercially; the rest is rubber produced synthetically by means of chemical processes that were partly known in the 19th century but were not applied commercially until the second half of the 20th century, after World War II. Among the most important synthetic rubbers are butadiene rubber, styrene-butadiene rubber,neoprene, the polysulfide rubbers (thiokols), butyl rubber, and the silicones. Synthetic rubbers, like natural rubbers, can be toughened by vulcanization and improved and modified for special purposes by reinforcement with other materials.
Essential properties of the polymers used to produce the principal commercial rubbers are listed in the table.