Jolene King, of Greymouth Junior High School, asks :-

How is elastic made?

Mike Christmas, an industrial chemist with Rubber Development Ltd, responded.

The raw material used in the production of elastic is natural rubber latex, which is the liquid sap tapped from rubber trees. Prior to thread production, a number of ingredients including sulphur is mixed in with the latex in a mixer.

The thread is formed by forcing the latex through a shaped die. This process is called `extrusion'. The shape of the die determines the cross sectional shape of the final product (very much the same way as cakes are decorated with icing).

The die is immersed in acetic acid which coagulates the latex, i.e. allows the latex to maintain its shape prior to the curing process. Up to 200 threads can be simultaneously extruded.

Coagulated threads are drawn out of the acid bath by a roller and washed in water to remove acid. At this stage the threads can be fed through a textile covered machine which produces single or multi fabric covered elastic depending on end use applications.

The threads are then vulcanised in an oven for about 24 hours at 70 degrees Celsius. Vulcanisation is the process whereby sulphur crosslinks with the carbon atoms in the polymer chain to produce an elastic network. The elastic is then ready for shipment to customers.