Vulcanization, also called crosslinking, is the chemical change in which linear rubber molecules become a three-dimensional network. Before cure, the raw rubber is soft and flows easily; after cure, its elasticity, strength, heat resistance and chemical resistance improve markedly, which is what makes rubber products useful.

Route 1: reaction with a crosslinking agent

Sulfur and its relatives, or organic compounds such as resins and amines, act as crosslinkers that react with the polymer chains and tie them together. This is the traditional way to vulcanize rubber.

Route 2: reaction started by free radicals

An initiator first generates free radicals, and the radicals then join the chains together. The usual example is peroxide crosslinking; radiation crosslinking belongs here as well. The peroxide cure of silicone rubber is of this type.

Route 3: reaction between reactive groups

Polar groups on the chains react with one another, with a metal oxide added as the crosslinking agent.

Catalysts and co-agents

To raise crosslinking efficiency or adjust the reaction rate, a formulation sometimes also includes catalysts or crosslinking co-agents. Crosslinking chemistry is complex, and the same rubber cured with a different system can end up with a different network and different properties. The cure system should therefore be chosen according to the rubber type and how the product will be used.