Natural and Synthetic Rubber

James Richard Fromm

Natural rubber is a polymer found in the sap of the rubber tree, Hevea brasilensis, originally native to Brazil. The rubber tree grows throughout the tropics and is cultivated in plantations, primarily in Southeast Asia; Malaysia and Indonesia are the most significant sources. The sap is collected, and on exposure to air and mild heat gives natural rubber. Chemically, this is a polymer of 2-methyl-1,3-butadiene (isoprene), CH2=C(CH3)-CH=CH2. The polymerization reaction is:

nCH2=C(CH3)-CH=CH2 rarrow.gif (63 bytes) -[CH2-C(CH3)=CH-CH2]n-.

Natural rubber has long been known, but became valuable only with the development of vulcanizing (heating in the presence of sulfur) developed by Charles Goodyear, which gives a much more rubbery and coherent substance.

The development of synthetic rubber was a German program for many years. Germany produced some 2500 tonnes of methyl rubber (polymer of 2,3-dimethyl-1,3-butadiene) during World War I, but returned to natural rubber at the end of the War. Specialty rubbers were produced and research was carried out in Germany and elsewhere prior to World War II, but the synthetic rubber industry of the world arose during World War II. At the present time over 75% of the rubber used in the United States is synthetic, while on a world basis about 65% of the rubber used is synthetic.

The most significant forms of synthetic rubber used today are the Buna S, Buna N, and synthetic natural rubbers developed in Germany. Other types (Thiokol, Neoprene) have somewhat different properties and specialty uses. Buna-S, by far the most significant, has been made since 1933 in Germany. It is a copolymer of 1,3-butadiene and styrene. The polymerization reaction is:

nCH2=CHCH=CH2 + nC6H5CH=CH2 rarrow.gif (63 bytes) -[CH2CH=CHCH2CH(C6H5)CH2]n-.

The Buna-N rubber, which is soil-resistant, is a copolymer of 1,3-butadiene and acrylonitrile. The polymerization reaction is:

nCH2=CHCH=CH2 + nCH2=CHCN rarrow.gif (63 bytes) -[CH2CH=CHCH2CHC=N]n-.

Like the Buna-S type, Buna-N rubber can be vulcanized.

True synthetic rubber, a polymer of isoprene, was achieved almost simultaneously in 1955 by several major rubber manufacturers using different catalysts. The process is a polymerization of isoprene itself and the product is virtually identical to the natural Hevea product. The feedstocks for the synthetic rubber industry are products of the petrochemical industry.

Rubber, either natural or synthetic, is normally vulcanized, most often with sulfur, although for some specialty uses other agents are used. The majority of rubber used is used for automobile tires. For this and similar applications a filler or reinforcing agent must be added, of which carbon black is by far the most common.

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Copyright 1997 James R. Fromm