Vulcanisation

Vulcanisation is the process of heating rubber with sulfur so that sulfur atoms form cross-links between neighbouring poly(isoprene) chains. These cross-links tie the chains together and greatly improve the properties of the rubber. Malay: pemvulkanan · Chinese: 硫化.

Unvulcanised natural rubber is soft and its chains can slide past one another, so it loses shape easily and becomes sticky when warm. During vulcanisation the sulfur atoms react at the carbon–carbon double bonds along the chains and build sulfur bridges from one chain to the next. Because the chains are now locked together at many points, the rubber becomes harder, stronger, more elastic and more resistant to heat and to chemicals. The more sulfur used, the more cross-links form and the harder the rubber becomes, which is how soft elastic bands and hard tyre rubber are made from the same starting material.

Example. Rubber for vehicle tyres is vulcanised by heating it with sulfur. The finished tyre rubber keeps its shape under load, springs back after being squashed, and does not soften on a hot road, all because of the sulfur cross-links formed during vulcanisation.

Confusion to avoid. Do not confuse vulcanisation with coagulation. Coagulation only clumps rubber particles into solid rubber using acid and forms no new bonds within the molecules. Vulcanisation forms actual sulfur cross-links between chains and changes the rubber’s properties. Coagulation makes rubber; vulcanisation improves it.

A frequent exam question asks you to compare vulcanised and unvulcanised rubber: vulcanised rubber is harder, stronger, more elastic and has a higher elasticity limit, and it withstands heat and solvents better, all because of the sulfur cross-links. Learning this comparison as a list of paired properties is a reliable way to score the marks.

Vulcanisation is the climax of the natural-rubber section, explaining why treated rubber outperforms raw rubber, and it is a favourite structured-question topic in SPM Chemistry.