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An elastomer is a polymer with viscoelasticity (having both viscosity and elasticity) and very weak inter-molecular forces, generally having low Young's modulus and high failure strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred when referring to vulcanisates. Each of the monomers which link to form the polymer is usually made of carbon, hydrogen, oxygen or silicon. Elastomers are amorphous polymers existing above their glass transition temperature, so that considerable segmental motion is possible. At ambient temperatures, rubbers are thus relatively soft (E~3MPa) and deformable. Their primary uses are for seals, adhesives and molded flexible parts. Application areas for different types of rubber are manifold and cover segments as diverse as tires, shoe soles, and dampening and insulating elements. The importance of rubbers can be judged from the fact that global revenues are forecast to rise to US$56 billion in 2020.
Contents
Rubber like solids with elastic properties are called elastomers. Polymer chains are held together in elastomers by weakest intermolecular forces. This weak binding forces permit the polymers to be stretched. Natural rubber, neoprene rubber, buna-s and buna-n are elastomers.
Background
Elastomers are usually thermosets (requiring vulcanization) but may also be thermoplastic (see thermoplastic elastomer). The long polymer chains cross-link during curing, i.e., vulcanizing. The molecular structure of elastomers can be imagined as a 'spaghetti and meatball' structure, with the meatballs signifying cross-links. The elasticity is derived from the ability of the long chains to reconfigure themselves to distribute an applied stress. The covalent cross-linkages ensure that the elastomer will return to its original configuration when the stress is removed. As a result of this extreme flexibility, elastomers can reversibly extend from 5-700%, depending on the specific material. Without the cross-linkages or with short, uneasily reconfigured chains, the applied stress would result in a permanent deformation.
Temperature effects are also present in the demonstrated elasticity of a polymer. Elastomers that have cooled to a glassy or crystalline phase will have less mobile chains, and consequentially less elasticity, than those manipulated at temperatures higher than the glass transition temperature of the polymer.
It is also possible for a polymer to exhibit elasticity that is not due to covalent cross-links, but instead for thermodynamic reasons.
Examples
Unsaturated rubbers that can be cured by sulfur vulcanization:
(Unsaturated rubbers can also be cured by non-sulfur vulcanization if desired).
Saturated rubbers that cannot be cured by sulfur vulcanization:
"The definitions are not authentic as the Rubber which is classified in World Customs Organisation Books in Chapter 40, where as the above definitions stating all rubber and different polymers in same chapter which is classified in Chapter 39 of the World Custom Organisation's Harmonised Commodity for Description and coding system. One should go through all differentiation while editing between Plastics and articles thereof and Rubber and articles thereof."
Various other types of elastomers: Various other types of 4S elastomers: