Abstract
Natural rubber chemically consists essentially of cis-1,4-polyisoprene that is produced as a latex, primarily by the tropical tree Hevea brasiliensis. Natural rubber is of special interest in most developing countries where the climate is suitable for its cultivation, since it constitutes a valuable indigenous, renewable, resource. This natural polymer, and the analogous synthetic materials, can be crosslinked (vulcanized) with varying percentages of sulfur to produce materials ranging in physical properties from true elastomers to high modulus rigid materials (hard rubber). The use of high concentrations of sulfur raises the glass transition temperature (Tg) from well below 0°C to as high as 100°C. In practice, the hard rubbers will contain a minimum of 20–23 parts per hundred of rubber (phr) of sulfur1 and usually 30–50 phr. Flexible elastomers are made with lower 2–5 phr of sulfur.
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