crosslinking

A compound is often cured by subjecting it to a combination of heat and pressure for sufficient time for crosslinking to take place. When a polymer is crosslinked, the long carbon backbones are connected together to form a network.

(a)

Uncured polymer systems consist of randomly orientated carbon chains

In a cross-linked (vulcanised or cured) material these carbon backbones are lightly (b) or strongly (c) linked to form a cohesive elastomeric material. 

Note: in real systems, the long chains may be entangled which will also form localised restrictions to movement.

Before crosslinking, the compound can be regarded as a visco-elastic material i.e. a material that flows but that also exhibits an ability to return to a former shape (elasticity).

The crosslinked material is primarily elastic with its stiffness determined by the polymer type, the filler materials and the crosslink density. Although prolonged exposure to stress fields may induce creep, crosslinked materials are not regarded as being fluids.

The cure system is added to the formulation to promote the crosslinking. Sufficient curative (traditionally sulphur or peroxide, but now extended to include other curatives for high performance systems) is added to ensure the correct degree of crosslinking.  

Accelerants are added to increase the rate of cure for a given process temperature, saving both time and energy although this is usually offset by the cost of the additional ingredients. 

Retardants are added to delay the onset of cure (the point at which curing begins) to provide time for process steps, such as filling a hot mould, to take place whilst the compound is still in an uncured state.