Thermoplastic Elastomers

Nov. 15, 2002
Thermoplastic elastomers (TPEs) have two big advantages over the conventional thermoset (vulcanized) types -- processing ease and speed.

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.

Polyurethanes: The first major elastomers that could be processed by thermoplastic methods were the urethanes. Thermoplastic urethanes do not have quite the heat resistance and compression-set resistance of the thermoset types (see chapters on Thermoset rubber and Polyurethane, but most other properties are similar. They are available in a wide range of hardness grades and in a number of forms, from several manufacturers.

Urethanes are a reaction product of a diisocyanate and long and short chain polyether, polyester, or caprolactone glycols. The polyether types are slightly more expensive and have better hydrolytic stability and low-temperature flexibility than the polyester types.

Mechanical properties of the polyester types are generally higher, however. Caprolactones offer a good compromise between the polyether and polyester types. Abrasion resistance of the urethanes is outstanding among elastomers, low-temperature flexibility is good, oil resistance is excellent to 180°F, and load-bearing capability ranks with the best of the elastomers. Additives can improve dimensional stability or heat resistance, reduce friction, or increase flame retardancy, fungus resistance, or weatherability. Resistance of the polyester types to strong acids, organophosphorous esters, and steam is poor.

Urethane tubing is used for fuel lines, fluid devices, and parts requiring oxygen and ozone resistance. The excellent abrasion resistance of urethanes qualifies them for use in bumpers, gears, rollers, sprockets, cable jackets, chute linings, textile-machinery parts, casters, and solid tires. Other applications include gaskets, diaphragms, shaft couplings, vibration-damping components, conveyor belts, sheeting, bladders, keyboard covers, and films for packaging.

The most recently introduced commercial thermoplastic polyurethanes are polyether aliphatic diisocyanates based on 1,4-butane diol, HMDI, and polytetramethyl-ethylene diol. These lower molecular-weight materials have better color stability to UV radiation and hydrolysis than the conventional grades. The softer grades are used in medical applications (with suitable antioxidants) and as adhesives in security glazing for armored vehicles, prisons, banks, and in aircraft glazing. Other new grades are stabilized for use as wear layers for aircraft wings.

Copolyesters: These thermoplastic elastomers are generally tougher over a broader temperature range than the urethanes. Also, they are easier and more forgiving in processing. Several grades are produced by Du Pont (Hytrel), Hoechst-Celanese (Riteflex), and Eastman Chemical (Ecdel), ranging in hardness from 35 to 72 Shore D. These materials can be processed by injection molding, extrusion, rotational molding, flow molding, thermoforming, and melt casting. Powders are also available.

Copolyesters, which along with the urethanes, are high-priced elastoplastics, have excellent dynamic properties, high modulus, good elongation and tear strength, and good resistance to flex fatigue at both low and high temperatures. Brittle temperature is below -90°F, and modulus at -40°F is only slightly higher than at room temperature. Heat resistance to 300°F is good.

Resistance of the copolyesters to nonoxidizing acids, some aliphatic hydrocarbons, aromatic fuels, sour gases, alkaline solutions, hydraulic fluids, and hot oils is good to excellent. Thus, they compete with rubbers such as nitriles, epichlorohydrins, and polyacrylates. However, hot polar materials, strong mineral acids and bases, chlorinated solvents, phenols, and cresols degrade the polyesters. Weathering resistance is low but can be improved considerably by compounding UV stabilizers or carbon blacks with the resin.

Copolyester elastomers are not direct substitutes for rubber in existing designs. Rather, such parts must be redesigned to use the higher strength and modulus, and to operate within the elastic limit. Thinner sections can usually be used -- typically one-half to one-sixth that of a rubber part.

Applications of copolyester elastomers include hydraulic hose, fire hose, power-transmission belts, flexible couplings, diaphragms, gears, protective boots, seals, oil-field parts, sports-shoe soles, wire and cable insulation, fiber-optic jacketing, electrical connectors, fasteners, knobs, and bushings.

A copolyester-based thermoplastic elastomer, trademarked Lomod, was introduced by General Electric Plastics in 1985. In addition to general-purpose, flame-retardant and high-heat grades, specific grades have been developed for airdams, fascias, and filler panels with excellent impact resistance down to -40°F and capable of withstanding on-line painting. Lomod thermoplastic elastomers are also used in connectors, wire, cable, hose, tubing, and other applications.

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.

Olefins: Thermoplastic olefin (TPO) elastomers are available in several grades, having room-temperature hardnesses ranging from 60 Shore A to 60 Shore D. These materials, being based on polyolefins, have the lowest specific gravities of all thermoplastic elastomers. They are uncured or have low levels of crosslinking. Material cost is mid-range among the elastoplastics.

These elastomers remain flexible down to -60°F and are not brittle at -90°F. They are autoclavable and can be used at service temperatures as high as 275°F in air. The TPOs have good resistance to some acids, most bases, many organic materials, butyl alcohol, ethyl acetate, formaldehyde, and nitrobenzene. They are attacked by chlorinated hydrocarbon solvents. Compounds rated V-0 by UL 94 methods are available.

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.

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