Thermoplastic polyurethane rubbers and Spandex fibres

When polyols, di-isocyanates and glycols are reacted together as described in the previous section they do in fact tend to produce block copolymers as can be seen from the following reaction mechanism  

Similar reactions occur when an amine is used instead of a glycol as chain extender  

With these polymers hard blocks with TgS well above normal ambient temperature are separated by soft blocks which in the mass are rubbery in nature. This is very reminiscent of the SBS triblock elastomers discussed in Chapter 11 and even more closely related to the polyether-ester thermoplastic elastomers of the Hytrel type described in Chapter 25. 

Several materials designated as thermoplastic polyurethanes have been introduced onto the market but many of them are slightly cross-linked and this may be increased permanently by a post-curing operation after shaping. One product may, however, be regarded as truly thermoplastic (Estane by Goodrich). 

The thermoplastic rubbers have properties similar to those of the cast polyurethane rubbers but, because of the absence of covalent cross-links, have rather higher values for compression set, a common problem with thermoplastic rubbers. Their main uses are for seals, bushes, convoluted bellows and bearings. 

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