Elizabeth Hazeldine, of Rangi Ruru Girls? School, asks :-

What makes elastic stretchy?

Mike Christmas, a rubber chemist with Skellerup Industries, responded.

Elastic is made up from a number of ultra fine threads of natural rubber latex. This latex is obtained from the 'tapping' of rubber trees. Following the collection process, the latex is concentrated and mixed with various ingredients including sulphur. Once the latex thread has been formed, usually by extrusion, it is dried and cured (vulcanised) at elevated temperatures to give the properties required for its use.

Raw or uncured latex is an entanglement of hydrocarbon chains. It can be stretched, but will not retain its original shape. So for rubber to be a useful product, these chains need to be linked together to increase the strength of the material and to ensure that when elongated the chains `snap' back to their original configuration. Vulcanisation is the process whereby sulphur atoms form bridges between the rubber chains. This process was invented by Charles Goodyear in 1839, and requires heat for the reaction to proceed.

Elastic is stretchy because it is made from vulcanised natural rubber which can be elongated to many times its original length but will return to its original shape due to the presence of sulphur holding all the chains together in a defined network.

We should be very obliged to Mr Goodyear - if elastic could not return to its original shape, there would be many embarrassing situations worldwide!