Vulcanised rubber
Have you heard of vulcanised rubber? Because of rubber's elasticity, it is hard to use it for some purposes. Vulcanising increases the hardness of rubber and reduces its elasticity.
Vulcanising
To vulcanise rubber, natural rubber is made to react with sulphur. Sulphur then forms cross bonds between the linear chains of the rubber.
Figure to be added
Vulcanised rubber is used to make tyres, tubes, battery cases and similar products.
Natural rubber
Linear chains that can slide past each other
Very stretchy; goes sticky when hot and brittle when cold
Vulcanised rubber
Chains cross-linked by sulphur
Harder and tougher; springs back into shape
Why the sulphur bridges help
In raw rubber the long chains are only tangled, so when you pull, they slowly slide apart and do not all come back; in heat they slide even more easily and the rubber goes soft and sticky. Sulphur joins neighbouring chains at the double bonds that polymerisation left in each unit. Now the chains can stretch and uncoil, but the bridges pull them back to where they were, and they cannot flow away when warm. The rubber becomes hard-wearing and keeps its shape — exactly what a tyre on a hot road needs. The more sulphur, the more links, and the harder the rubber.
Vulcanised rubber is still rubber
Vulcanising does not make a new polymer; it links the existing polyisoprene chains. That is also why a used tyre cannot simply be melted down and moulded again.
Sulphur cross-links rubber
Vulcanising reacts rubber with sulphur, cross-linking its chains: harder, less elastic, used for tyres, tubes and battery cases.