The hard segment can be either aromatic or aliphatic in nature. Synthetic polymers, like polyurethane, are produced by reacting monomers in a reaction vessel. Aromatic TPU are based on isocyanates such as MDI. Aliphatic TPU are based on isocyanates like H12 MDI, HDI and IPDI are light stable and offer excellent optical clarity . When these isocyanates are combined with various short chain diols they become the hard block. Normally it is aromatic, but where color and clarity retention during exposure to sunlight is a priority, an aliphatic hard segment is typically recommended. The soft segment can be either polyether or polyester type. Polyester-based TPU (mainly derived from adipic acid esters), Polyether-based TPUs (mainly based on tetrahydrofuran (THF) ethers). By altering the ratio of the rigid phase, it is possible to vary properties such as hardness, strength, rigidity, extensibility and low-temperature flexibility over a broad range. A distinction is drawn between polyether and polyester polyurethane as a function of the polyols employed. The choice affects the relative suitability for a given application. For use in wet environments, for example, a polyether-based TPU is preferred. When oil and hydrocarbon resistance are primary factors, a polyester-based TPU is the material of choice. Another polyester type, polycaprolactone, also provides oil and hydrocarbon resistance with improved hydrolytic stability.
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