27—Relaxation of Stress in Stretched Cellulose Fibres

Abstract

An apparatus is described for making stress—relaxation tests on yarns or monofils at controlled humidity and temperature. It incorporates an optical lever for measuring the tension, and observations can be made from 1/20 second after the start of an experiment up to several weeks. Cotton, flax, viscose, and cellulose acetate yarns were rapidly stretched at 65 per cent r.h. and 25°C, and the decay of tension followed for 24 hours. Further tests were made in which the specimens were held stretched for 15 minutes and relaxed for two hours, repeatedly, until mechanically conditioned. The results are expressed graphically as stress against log (time) and are in good agreement with those of other workers, over the limited time range which they have used. The fraction of the initial stress which decays is quite large. Thus taking the stress at one second after extending as a reference value, the decay of stress from 1/10 second to a day is approximately 30 to 50 per cent for cotton, 42 to 83 per cent for viscose rayon, and 33 to 85 per cent for cellulose acetate, depending on the imposed extension. The data for cotton and flax have been analysed according to the Nutting power law relating stress, strain, and time. Reaction rate theory, applied to the stress relaxation of viscose rayon and cellulose acetate, gave activation energies of about 26 kcal per mole for viscose rayon and 27 kcal per mole for cellulose acetate, for extensions from 2 to 20 per cent. From the slope of the stress—log (time) curves between 1/10 and 1 second, the ‘internal friction’ was calculated for the viscose rayon and cellulose acetate and the values obtained agree well with those calculated on the same materials from vibration experiments at a frequency of 50 to 100 c/s. In appendices, it is shown that (i) high extensions with certain yarns which have not been mechanically conditioned will produce an appreciable rise in temperature and (ii) the duration of stretching is in the region of 1 to 10 milliseconds.