Vulcanization

Rubber contains long, flexible polymer molecules. In sulfur vulcanization, a rubber compound is heated with sulfur so that chemical cross-links form between some of its chains. The links act like bridges in a loose molecular network: they limit the chains' ability to slide past one another while allowing the material to stretch and recover. The sulfur bridges can contain different numbers of sulfur atoms, and the reactions depend on the rubber formulation and curing conditions; the chemistry is more complex than a single universal reaction.

Compared with untreated rubber, a vulcanized product generally has greater tensile strength and resistance to abrasion and swelling, and remains elastic across a wider temperature range. These properties made rubber more dependable for products that must flex repeatedly or tolerate heat and wear, including tires, belts, hoses and seals. The final properties depend on the type of rubber and the compound's ingredients and cure, not on sulfur alone.

The U.S. inventor Charles Goodyear discovered sulfur-based vulcanization in 1839 and received U.S. Patent No. 3,633 in 1844. The English inventor and manufacturer Thomas Hancock patented a vulcanization process in Britain in 1843, before Goodyear's U.S. patent. Historians have debated how Hancock arrived at his process and whether it was influenced by Goodyear's work, so the history is better described as a development involving both men than as a simple single-inventor story.

Commercial vulcanization is a controlled manufacturing step. Sulfur, accelerators and activators may be combined with rubber and other additives, then cured under selected conditions to achieve the desired balance of strength, flexibility and durability. Sulfur is common, but it is not suitable for every elastomer: some synthetic rubbers are cured using alternatives such as metal oxides or organic peroxides.