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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. |
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