Question

why sulphur is added to rubber

Answer 1

To make it hard
You know na ki natural rubber is very sof and hence we cant able to make applications from it

Answer 2

Sulfur introduces extensive cross-linking within the polymeric framework of rubber causing it to become flexible yet with high tensile strength.

The cross-linking reaction between sulfur and rubber occurs mainly at the carbon-carbon double bonds (C=C) in large excess of sulfur. Often accelerators are added to speed up the vulcanization.