Natural rubber is an elastic hydrocarbon polymer that naturally occurs as a milky colloidal suspension, or latex, in the sap of some plants. It can also be synthesized. The entropy model of rubber was developed in 1934 by Werner Kuhn.
The major commercial source of natural rubber latex is the Para rubber tree, Hevea brasiliensis (Euphorbiaceae). This is largely because it responds to wounding by producing more latex. Henry Wickham gathered thousands of seeds from Brazil in 1876 and they were germinated in Kew Gardens, England. The seedlings were sent to Colombo, Indonesia, Singapore and British Malaya. Malaya was later to become the biggest producer of rubber. Liberia and Nigeria are examples of African rubber-producing countries.
Other plants containing latex include figs (Ficus elastica), euphorbias, and the common dandelion. Although these have not been major sources of rubber, Germany attempted to use such sources during World War II when it was cut off from rubber supplies. These attempts were later supplanted by the development of synthetic rubber.
Synthetic rubbers are made by the polymerization of a single monomer or a mixture of monomers to produce polymers. These form part of a broad range of products extensively studied by polymer science and rubber technology. Examples are SBR, or styrene-butadiene rubber, BR or butadiene rubber CR or chloroprene rubber and EPDM (ethylene-propylene-diene rubber).
Current sources of rubber
Close to 21 million tons of rubber were produced in 2005 of which around 42% was natural. Today Asia is the main source of natural rubber, accounting for around 94% of output in 2005. The three largest producing countries (Indonesia, Malaysia and Thailand) together account for around 72% of all natural rubber production.
Hypoallergenic rubber can be made from Guayule.Early experiments in the development of synthetic rubber led to the invention of Silly Putty.
Natural rubber is often vulcanized, a process by which the rubber is heated and sulfur, peroxide or bisphenol are added to improve resilience and elasticity, and to prevent it from perishing. Vulcanization greatly improved the durability and utility of rubber from the 1830s on. The successful development of vulcanization is most closely associated with Charles Goodyear. Carbon black is often used as an additive to rubber to improve its strength, especially in vehicle tires.
Kottayam District of Kerala is the leader in rubber production among the states of India. The rubber plant is not a native plant of India. Dutch colonialists who also cultivated rubber in their plantations in Indonesia introduced the rubber plant to Kerala, India, because of its similar tropical climate.