Fibres and Crystalline Moulding Compounds

As previously mentioned the initial research on polyurethanes was directed towards the preparation of fibre-forming polymers. Many polyhydroxy compounds and many di-isocyanates were used and the melting points of some of the more linear aliphatic polyurethanes produced are given in Table 27.1. 

The polymer may be prepared by running the isocyanate into the glycol while the temperature is raised slowly to near 200°C. The reaction is exothermic and carried out under a blanket of nitrogen. The polymers produced have a molecular weight of 10000-15 000 and after filtration may be melt spun into fibres. 

Compared with nylon 66 fibres, the polyurethane fibres (known as Perlon U) have a tensile strength at the higher end of the range quoted for nylon 66, they are less prone to discolouration in air, are more resistant to acid conditions and they have a lower moisture absorption. On the debit side they are less easy to dye, are hard, wiry and harsh to handle and have too low a softening point for many applications. They are currently of little importance but have found some use in bristles, filler cloths, sieves and a few other miscellaneous applications. 

The properties of the polyurethane moulding compositions are also very similar to nylon 66. The greatest difference in properties is in water absorption, the 6,4-polyurethane absorbing only about of that of nylon 66 under comparable conditions. This results in better dimensional stability and a good retention of electrical insulation properties in conditions of high humidity. Resistance to sulphuric acid is somewhat bettter than with nylon 66 but both types of polymer are dissolved by phenols and formic acid. 

There is little call for these thermoplastics materials (marketed as Durethan U, Farbenfabriken Bayer) since they are about twice the price of nylons 66 and 6. Where a thermoplastic for light engineering purposes is required with a low water absorption, nylon 11, acetal resins and, in certain instances, polycarbonates are cheaper and, usually, at least as satisfactory. 

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