Vulcanization is a kind of chemical reaction between sulphur, or another agent, and natural rubber. It makes cross-links between the molecular chains in the rubber. The reaction is not reversible.
The purpose of this is to harden the rubber so it does not melt or soften in relatively low temperatures. This hard rubber can keep its shape when it is used in applications where it might be warmed or cooled.
Images for kids
Schematic presentation of two strands (blue and green) of natural rubber after vulcanization with elemental sulfur.
"Vulcanization curve" showing the increase in viscosity of the polymeric material during crosslinking. The steepness of the curve is strongly affected by the nature of the accelerators and other additives.
An example of a silicone rubber keypad typical of LSR moulding