Thermoplastic elastomers

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.