Q.Discuss the main purpose of vulcanisation of rubber.

Raw (unvulcanised) natural rubber has several practical drawbacks: it is soft and tacky (sticky), has relatively low tensile strength, is not very resistant to oxidation and organic solvents, becomes soft/plastic above about 335 K and brittle below about 283 K — so its useful temperature range is narrow and its mechanical properties are poor.

Purpose of vulcanisation: to chemically modify raw rubber into a much stronger, more elastic, and more temperature-stable material suitable for practical use (tyres, footwear, hoses, etc.). This is achieved by heating a mixture of raw rubber with sulfur (typically 3–5% for flexible rubber; along with additives such as zinc oxide) at a temperature of about 373–415 K.

During vulcanisation, sulfur atoms form cross-links (-S-S- bridges) between adjacent polyisoprene chains at their reactive double-bond sites. These cross-links:

• Restrict the chains from permanently slipping past each other under stress, giving true elastic recovery after stretching.
• Greatly increase tensile strength and hardness.
• Improve resistance to oxidation, abrasion and organic solvents. …

First list raw rubber's shortcomings, then explain the vulcanisation process (heating with sulfur) and how the resul …

• Describing vulcanisation only as 'making rubber stronger' without explaining the sulfur cross-linking mechanism.
• Forgetting to mention the narrow usable temperature range of raw rubber as one of the problems vulcanisation solves. …