RHEOOPTICAL INVESTIGATIONS OF HIGHLY CONCENTRATED POLYURETHANE SOLUTIONS FOR INDUSTRIAL PROCESSING INTO ELASTIC FIBERS

On a large industrial scale, segmented polyurethanes are processed fro m solution in the dry spinning process to produce textile fibres. Rheo -optical investigations of the flow behaviour of polyurethane solution s enable new material functions to be determined and provide important information for processing. Proportional increases with shear rate we re observed for the Row birefringence, Delta n', and the orientation, phi. The polymer segments were more easily aligned in the direction of the shear field in more concentrated solutions than in dilute solutio ns. The same tendency was observed for samples with differing molar ma sses. An ideal standardisability for the temperature (in a window of 2 0 K) was found over the entire range of shear rate and, hence, the cha nge in the Newtonian and non-Newtonian flow behaviour was also observe d to be completely identical. Using the stress-optical rule, it was po ssible to determine the first normal stress difference. The stress-opt ical coefficient, C, was 2.6.10(-9) Pa-1. The normal stress values lie in the range of accessible shear rates below the shear stress, but do , however, rapidly approach this value as the strain increases. Even a t a shear rate of 100 s(-1) the viscoelasticity of a 17 wt.-% polyuret hane solution is already significant. At a high shear rate the Weissen berg number, We, which is a measure of the viscoelasticity, has a cons tant limiting value that only depends on the power law exponent, n. It s values in the range of high shear rates mostly lie between 2 and 3 a nd are rarely (for the smallest n) above 3.