A physicochemical change resulting from cross-linking of the unsaturated hydrocarbon chain of polyisoprene (rubber) with sulfur, usually with application of heat. The precise mechanism that produces the network structure of the cross-linked molecules is still not completely known. Sulfur is also used with unsaturated types of synthetic rubbers; some types require use of peroxides, metallic oxides, chlorinated quinones, or nitrobenzenes. Natural rubber can be vulcanized with selenium, organic peroxides, and quinone derivatives but these have limited industrial use; high-energy radiation curing is an important innovation. Vulcanization can be effected with sulfur alone in high percentage, but the time required is too long to be economical and the properties obtained are poor. Inorganic accelerators and metallic oxides (usually zinc) are essential for satisfactory cure. Organic accelerators, introduced in the early 1920s, notably shortened vulcanization time.Three factors affect the properties of a vulcanizate: (1) the percentage of sulfur and accelerator used, (2) the temperature, and (3) the time of cure. Sulfur is usually from 1 to 3%; with strong acceleration the time can be as short as three minutes at high temperature (150C). Vulcanization can also occur at room temperature with specific formulations (self-curing cements).Vulcanization was discovered in 1846 by Charles Goodyear in the U.S. and simultaneously by Thomas Hancock in England. Its overall effect is to convert rubber hydrocarbon from a soft, tacky, thermoplastic to a strong, temperature-stable thermoset having unique elastic modulus and yield properties.See Rubber; Rubber, Synthetic.
