Thermoplastic polyurethane* is a fully thermoplastic elastomer. It is elastic and melt-processable. TPU can be extruded, injection molded, and compression molded. Because TPU is a linear segmented block copolymer made up of hard and soft segments, it allows for a considerable number of physical property combinations. This diversity makes TPU an extremely flexible material, adaptable to dozens of uses. For even greater utility, the molecular weight, ratio, and chemical type of the hard and soft segments can be varied. The hard segment is either aromatic or aliphatic.
Aromatic segments can be made up of organic compounds containing aromatic rings with six carbons as their backbones. Isocyanates are built in such a way as to have toluene, benzene or other aromatic solvents as some of their possible precursors. The precursors are used to produce MDI—(MDI is a blend of 2,2’, 2,4’ and 4,4’ methylene diphenyl diisocyanates) which is one of thermoplastic’s key constituents. They also have to have at least two linear OH functionals. Such compounds are typified by benzene and its derivatives and are based on isocyanates. Wet environments generally require a polyether based TPU. Generally speaking, thermoplastic TPU remains stable when it comes into contact with greases, lubricants and test oils even at high temperatures up to 100°c and over a period of several weeks. However, some oil-based fluids may damage TPU, so compatibility testing is recommended. For example, oil and hydrocarbons resistance often demands a polyester-based TPU.
When combined with polymer blends, TPU contributes outstanding benefits. If TPU is combined with PVC, it will improve the compression set, abrasion resistance, and low temperature flexibility. When added to polycarbonate or ABS, a TPU with a nominal flexural modulus of 18,000 makes a compound with flexural modulus values as high as 150,000 psi.
TPU’s versatility results in high resilience, good compression set, and resistance to impacts, abrasions and tears, weathering, and hydrocarbons. TPU is flexible without the use of plasticizers with a broad range of hardness’ and high elasticity. It bridges the material gap between rubbers and plastics. Additionally, TPU can be used both as a hard rubber and a soft engineering thermoplastic. It can be sterilized or welded, is easily processed, and can be colored, painted, and die cut.
Some grades exhibit optical clarity, hydrolytic stability, flame retardance and anti-static properties. Other thermoplastic polyurethanes can be compounded when greater structural strength is needed. These materials are based simply on isocyanates connected by diols (a chemical compound containing two hydroxyl groups) or triols (a chemical compound containing three hydroxyl groups.) In this case, the soft segments act more as the hard segments around which their connecting diols/triols can twist and stretch to only a very limited degree.
Ozone is a powerful oxidizing agent that can affect some elastomers but TPU is generally ozone resistant and meets VDE 472-805 requirements. Also, there are numerous standards that document how the material should behave in a fire. One of the most important is UL (Underwriters Laboratories) 94. TPU is not flame-retardant but is generally classified HB or slow burn on a horizontal specimen. It can, however, with the right additives be made into a 94 V-O rated material. As for exposure to UV radiation, TPU can yellow. In cases where the TPU will be exposed to sunlight, it is recommended that an aliphatic TPU be used as these will not yellow or degrade when used outdoors.
TPU is used in hundreds of products including in-line skates, mining screens, sporting goods, swim fins and goggles, cattle tags, drive belts, inflatable rafts, fire hose liner, footwear, caster wheels and auto-body side molding.
*Formerly sold as Isoplast® by DOW™.
Craftech Industries supplies TPU fasteners and other parts to several industries where strength, chemical resistance and high resistance to UV radiation are critical.
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