Polyurethanes

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.

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.