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Material hierarchy: Polymer - Engineering Thermoplastic
Though not a single family of materials, thermoplastic elastomers have a set of properties in common. They can be processed by conventional thermoplastic extrusion and molding techniques but function much like thermoset rubbers.
The earliest commercial TPEs, introduced in the 1950s, were the thermoplastic polyurethanes (TPUs). Since that time, other types of materials have been commercialized, among them styrenic block copolymers, copolyesters, olefin blends, and rubber olefin alloys.
TPUs are on the high end of the price and property range of thermoplastic elastomers. They can be processed easily at low melt temperatures (under 450°F) but must be thoroughly predried. Because of their natural adhesive properties, mold releases usually are required.
TPUs are extruded into hoses and tubing for automotive, medical, and electronics applications. Blown film is used for meat wrapping. Injection-molded TPU parts are used in auto exteriors, shoes and boots, drive wheels, and gears. TPUs can be blended with PVC styrenics, nylons, or polycarbonate to achieve a wide range of properties.
Styrenic block copolymers are the most commonly used TPEs. Their structure normally consists of a block of rigid styrene on each end with a rubbery phase in the center, such as styrene/butadiene/styrene (SBS), styrene/isoprene/styrene (SIS), styrene/ethylene-butylene/styrene (SEBS), and styrene/ethylene-propylene/styrene (SEPS).
Styrenic TPEs can be processed via conventional extrusion and molding techniques. With a hardness range from 28 to 95 Shore A, they can be made into a wide variety of products. Wire and cable jacketing, medical tubing and catheters, shoe soles, and flexible automotive parts are some examples.
Compatibility with many other polymers has led to the use of styrenic TPEs as impact modifiers and as compatibilizers for resin blends. They also are used as tie layers in coextruded products.
Olefinic TPEs, or TPOs as they are typically called, are blends of polypropylene with rubber (EPDM or EP) and polyethylene. They are characterized by high impact strength, low density, and good chemical resistance. TPOs can be easily processed by extrusion, injection, and blow molding.
Gas-phase polyolefin producers can make in-reactor TPOs with properties similar to those made by compounding. The in-reactor products offer cost advanatages for large-volume applications. The largest single market for TPOs is in automotive exterior parts. Paintable formulations are used to mold bumper fascia, air dams, and rub strips. Weatherable grades with molded-in accent colors are top-coated with a clear finish.
Olefin blends with vulcanized rubber (partially or fully crosslinked) form a special category of TPE. These materials can cover the broad property range from thermoplastic to thermoset rubber. They process easily in conventional extrusion and molding equipment and are replacing thermoset rubbers in applications such as hose and tubing, cable covering, gaskets and seals, and sheeting. Special grades are available for medical and food contact applications.
Recently, olefinic TPEs have been developed that use proprietary olefinic materials in place of vulcanized rubber as the soft phase. They are made by special blending and cross-linked methods.
Polyester TPEs are high-strength elastomeric materials. They are block copolymers with hard and soft segments providig strength and elasticity. The primary characteristic of polyester TPEs is their ability to withstand repeated flexing cycles. They retain their high impact strength at low temperatures and may have heat resistance up to 300°F. Polyester TPEs process well (they should be predried) by extrusion and molding. Applications include automotive parts (fascias, constant-velocity joint boots), sporting goods, and geomembrane sheeting.