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Thermoplastic elastomers (TPEs) have two big advantages over the
conventional thermoset (vulcanized) types -- processing ease and speed.
Other compelling reasons for considering the TPEs are recyclability of
scrap, lower energy costs for processing, and the availability of
standard, uniform grades (not available in thermosets). This last
advantage is particularly important to multinational corporations.
The TPEs are molded or extruded on standard plastics-processing
equipment in considerably shorter cycle times than those required for
compression or transfer molding of conventional rubbers. They are made
by copolymerizing two or more monomers, using either block or graft
polymerization techniques. One of the monomers develops the hard, or
crystalline, segment that functions as a thermally stable component (which
softens and flows under shear, as opposed to the chemical crosslinks
between polymeric chains in a conventional, thermosetting rubber); the
other monomer develops the soft, or amorphous segment, which contributes
the rubbery characteristic.
Properties can be controlled by varying the ratio of the monomers and
the lengths of the hard and soft segments. Block techniques create
long-chain molecules that have various sequences, or blocks, of hard and
soft segments; graft methods involve grafting one polymer chain to
another as branches. Graft techniques offer more possibilities to vary
the copolymer because both the backbone monomer and the grafted branches
can be rubbery, glassy hard, or somewhere between. In general,
environmental and fluid resistance are totally predictable.
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.
The polyamide TPEs are low-density, high-elongation materials with good
solvent and abrasion resistance. They are expected to fill specialty
needs in automotive, sports, medical, and electric-electronic equipment.
The
elastomeric alloys are based on olefins but their proprietary
manufacturing methods give them higher properties than the conventional
thermoplastic olefins. They are designed to replace thermoset rubbers
such as EPDM, nitrile, and neoprene. |