Elastomers (rubbers) are special polymers that are very elastic. They are lightly cross-linked and amorphous with a glass transition temperature well below room temperature. They can be envisaged as one very large molecule of macroscopic size. The intermolecular forces between the polymer chains are rather weak. The crosslinks completely suppress irreversible flow but the chains are very flexible at temperatures above the glass transition, and a small force leads to a large deformation 1 Thus, elastomers have a low Young’s modulus and very high elongation at break when compared with other polymers. The term elastomer is often used interchangeably with the term rubber, although the latter is preferred when referring to vulcanized rubbers.
Elastomers can be classified into three broad groups: diene, non-diene, and thermoplastic elastomers. Diene elastomers are polymerized from monomers containing two sequential double bonds. Typical examples are polyisoprene, polybutadiene, and polychloroprene. Nondiene elastomers include, butyl rubber (polyisobutylene), polysiloxanes (silicone rubber), polyurethane (spandex), and fluoro-elastomers. Non-diene elastomers have no double bonds in the structure, and thus, crosslinking requires other methods than vulcanization such as addition of trifunctional monomers (condensation polymers), or addition of divinyl monomers (free radical polymerization), or copolymerization with small amounts of diene monomers like butadiene. Thermoplastic elastomers such as SIS and SBS block copolymers and certain urethanes are thermoplastic and contain rigid (hard) and soft (rubbery) repeat units. When cooled from the melt state to a temperature below the glass transition temperature, the hard blocks phase separate to form rigid domains that act as physical crosslinks for the elastomeric blocks.
Manufacturing elastomeric parts is achieved in one of three ways: injection molding, transfer molding, or compression molding. The choice of the molding process depends on various factors, including the shape and size of the parts, the required tolerance, as well as the quantity, type of elastomer, and raw material cost.
As with almost any material, selecting the right elastomeric product for the application requires consideration of many factors, including mechanical and physical service requirements, exposure to chemicals, operating temperature, service life, manufacturability of the parts, and raw material and manufacturing cost.
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