Rubber

Rubber is obtained in two basic forms, natural rubber and synthetic rubber. Natural rubber is the sap of the particular species of trees called Hevea brasiliensis, see References 3 and 4, which is sticky when at tropical temperature, reasonably hard at low temperatures and oxidises when exposed to the atmosphere. Natural rubber is a naturally occurring compound of carbon and hydrogen, and is of little use as a basic material. It is therefore mixed with other chemical compounds, filler materials such as carbon black and then vulcanised to produce 'vulcanised rubber' or more generally called simply 'rubber'. The vulcanising process requires sulphur to be added and the application of heat and pressure. The molecules of rubber are formed in long chains. Individual chains are not bonded to adjacent chains; hence the chains can slide alongside each other with little resistance to movement. This gives processed rubber the ability to recover without permanent deformation. Natural rubber does not necessarily recover to its original shape, since its stability depends on its ambient temperature. Vulcanising or 'curing' causes the sulphur to cross-bond adjacent chains, which stiffens the material thereby making it more useful. By increasing the sulphur content or extending the vulcanising time, or a combination of both functions, the rubber becomes progressively harder with higher tensile strength. Increasing additives such as carbon black can reduce the dielectric strength, thereby making the rubber a poorer insulator. Carbon by itself is of course a conductor.

Synthetic rubbers are also composed of carbon and hydrogen molecules, but they are combined by manufacturing processes. A synthetic rubber, which closely resembles natural rubber, is
polyisoprene, which has the same chemical composition.


source  : Handbook of Electrical Engineering: For Practitioners in the Oil, Gas and Petrochemical Industry. Alan L. Sheldrake  2003 John Wiley & Sons, Ltd ISBN: 0-471-49631-6