In order for a rubbery polymer to attain an effectively high elastic state, it is necessary to lightly crosslink the highly flexible polymer molecules to prevent them from slipping past each other on application of a stress. In the rubber industry this process is known as ‘vulcanization’. Vulcanization increases the retractile force and reduces the amount of permanent deformation remaining after removal of the deforming force by the insertion of crosslinks between polymer chains forming a crosslinked molecular network.
The crosslink may be a group of sulfur atoms in a short chain, a single sulfur atom, carbon-to-carbon bond, a polyvalent organic radical, an ionic cluster, or a polyvalent metal ion. The process is usually carried out by heating the rubber (mixed with vulcanizing agents) in a mold under pressure.