Vulcanization is activated by zinc oxide and stearic acid and the process is "accelerated" by the addition of small quantities of complex sulphur-based chemicals, typically sulphenamides which not only speed up the process, but also influence the properties of the vulcanizate, especially its resistance to ageing.

In its raw state rubber consists of long randomly kinked hydrocarbon chains which can slide past each other. Raw rubber is therefore plastic, weak and permanently deformable. The purpose of vulcanization is to chemically link the rubber chains together by "crosslinks" to form a three-dimensional network.

Two factors are very important in the vulcanization of rubber:

• The density of crosslinking (the frequency with which a rubber chain is linked to others)
• The nature of the crosslink.
  

There is a clear maximum in strength properties at a certain level of crosslinking but it is often advantageous to exceed this level to increase resilience and resistance to set. These latter properties improve with increasing hardness (or modulus), brought about by increasing crosslinking. However, increasing the hardness by means of raising the filler loading has a deleterious effect on set and resilience.

The most common vulcanizing agent is sulphur which is used with an accelerator and other auxiliary chemicals. In a conventional vulcanization system employing 2 to 3 parts of sulphur per 100 parts of rubber each crosslink formed consists of a chain of several sulphur atoms. However, a sulphur vulcanizate may have only one atom of sulphur in each crosslink in rubber vulcanized by an efficient vulcanization (EV) system which may use as little as 0.25 parts of sulphur. The differences in the physical properties conferred on natural rubber by these different systems should be noted.

Highly productive manufacturing techniques, such as injection moulding, demand the use of fast vulcanizing systems known as Efficient and Semi-efficient vulcanizing systems (EV and semi-EV systems) where sulphur donors replace some, or all, of the elemental sulphur used in "conventional" systems.

It is not possible to list all the chemicals used as accelerators, but some of the main groups used in association with natural rubber are: thiazoles; sulphenamides and guanidines.

Vulcanization may also be achieved with peroxides, urethanes and with high energy radiation.