Elastomeric alloys

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.