Styrene copolymers: The styrenics are the lowest priced thermoplastic elastomers. They are block copolymers, produced with hard polystyrene segments interconnected with soft segments of a matrix such as polybutadiene, polyisoprene, ethylene-propylene, or ethylene-butylene. These elastomers are available from Shell (Kraton) in several molding and extrusion grades ranging in hardness from 28 to 95 Shore A.
Tensile strength of these materials is lower and elongation is higher than SBR or natural rubber, weather resistance is about the same. Other resistance characteristics can be improved by the addition of resins such as polypropylene or ethylene-vinyl acetate. The styrenic elastoplastics resist water, alcohols, and dilute alkalies and acids. They are soluble in, or are swelled by, strong acids, chlorinated solvents, esters, and ketones. One type has a service temperature limit of 150°F; another grade can be used to 250°F. Both have excellent low-temperature flexibility to -120°F.
Applications for the styrene-butadiene block copolymers include disposable medical products, food packaging, tubing, sheet, belting, mallet heads, and shoe soles. These materials are also used as sealants, hot-melt adhesives, coatings, and for wire and cable insulation.
The four oldest thermoplastic elastomer types are polyurethanes, polyester copolymers, styrene copolymers, and the olefinics. Mechanical properties of the first two types are generally higher than those of the last two. Dynamic properties, such as flex life are also generally better. Newest TPEs are three classes of high-performance materials. One is based on polyamide (nylon) chemistry; another, called elastomeric alloys, consists of polymer alloys of an olefinic resin and rubber. The third group, melt-processible rubbers, are sold by Du Pont under the Alcryn tradename.