|
Elastomeric alloys: This class of thermoplastic
elastomers consists of mixtures of two or more polymers that have
received a proprietary treatment to give them properties significantly
superior to those of simple blends of the same constituents. The two
types of commercial elastomeric alloys are melt-processible rubbers (MPRs)
and thermoplastic vulcanizates (TPVs). MPRs have a single-phase; TPVs
have two phases.
Thermoplastic vulcanizates are essentially a fine dispersion of highly
vulcanized rubber in a continuous phase of a polyolefin. Critical to the
properties of a TPV are the degree of vulcanization of the rubber and
the fineness of its dispersion. The crosslinking and fine dispersion of
the rubber phase gives a TPV high tensile strength (1,100 to 3,900 psi),
high elongation (375 to 600%), resistance to compression and tension set,
oil resistance, and resistance to flex fatigue. TPVs have excellent
resistance to attack by polar fluids and fair-to-good resistance to
hydrocarbon fluids. Maximum service temperature is 275°F.
Elastomeric alloys are available in the 55A to 50D hardness range, with
ultimate tensile strengths ranging from 800 to 4,000 psi. Specific
gravity of MPRs is 1.2 to 1.3; TPV's range is 0.9 to 1.0.
In 1981, Monsanto Chemical Co. commercialized a line of TPVs, called
Santoprene, based on EPDM rubber and polypropylene, designed to compete
with thermoset rubbers in the middle performance range. In 1985, the
company introduced a second TPV, Geolast, based on polypropylene and
nitrile rubber. This TPV alloy was designed to provide greater oil
resistance than that of the EPDM-based material. The nitrile-based TPV
provides a thermoplastic replacement for thermoset nitrile and neoprene
because oil resistance of the materials is comparable.
The MPR product line, called Alcryn, was introduced in 1985 by Du Pont
Co. It is a single-phase material, which gives it a stress-strain
behavior similar to that of conventional thermoset rubbers. MPRs are
plasticized alloys of partially crosslinked ethylene interpolymers and
chlorinated polyolefins. These materials have excellent oil and weather
resistance. Maximum recommended service temperature is 275°F.
Alcryn is available in black and colorable grades, in hardnesses from
55A to 80A. Unlike other TPEs, it can be processed on rubber equipment
as well as on conventional thermoplastic equipment. Several
injection-molding grades are now available. Commercial applications of
elastomeric alloys include automotive protective boots, hose covering,
electrical insulation, seals, gaskets, medical tubing and syringe
plungers, architectural glazing seals, and roofing sheet.
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 guide to polymer processing technology,
materials, additives & equipment -
Life insurance