Vulcanisation.

Vulcanisation.

The two disadvantages of rubber is that it becomes soft and sticky when heat and hard and solid when cold. In the 18th century these disadvantages were overcome, by the discovery of vulcanisation process accidently by Charles Goodyear.

Vulcanisation process is defined as the process which involves chemical reaction between unsaturated rubber (polyisoprene) and small amount of sulphur at high temperature in the presence of activators and accelerators, to form crosslinking in free rubber molecule chains together to form a 3D elastic network.

The vulcanisation process is also known as curing. The vulcanisation process and crosslinking reaction is shown in figure below.

vulcanised and unvulcanised network.

The vulcanisation process contains following principle stages:

1) Mixing of crude rubber with about 5-30% sulphur (crosslinking agent) and other additives such as accelerators, activators, antioxidants, color pigments etc.

2) Molding (Shaping) of the mixture. The rubber mixture must be shaped before heating because crosslinking makes shaping impossible.

3) The mixture is heated to 120-200°C. Increased temperature speed up the vulcanisation process resulting in fast and complete crosslinking. C-S bond replaces C-H bond linking chain polyisoprene molecules.

The curatives (Vulcanised) used for vulcanisation of rubber depends on the type of rubber, saturated or unsaturated. The sulphur is used as curative for vulcanisation of unsaturated rubbers. The saturated rubber cannot be vulcanised by sulphur. The common curatives used for vulcanisation of saturated rubber are peroxides, metal oxides, amines, isocyanates and urethanes.