STUDIES ON THE THERMAL PROPERTIES OF HOT DRAWN P. E. T. FIBERS

(5) EFFECTS OF TIME-DEPENDENCE ON THE FINE STRUCTURAL CHANGE AT A CONSTANT TEMPERATURE

Abstract

Effects of time-dependence on the fine structural change annealed at a constant temperature (85_??_115°C) under either a less load or a constant length, regarding the heating process as its base were investigated by measurements of X-ray, density and birefringence. And the mechanism of crystallization and the effects of orientation for various stretched P. E. T. fibers are discussed. The results obtained are as follows:
1) The (density_??_logarithmic time) curves of the various stretched fibers under a free contraction are shown as a sigimoid, and the time, when it starts to make a greater density suddenly, shifts towards shorter time as both draw ratio and temperatures of immersion bath increase. Corresponding to the time-dependence on density change, the birefringence of higher stretched fibers of more than draw ratio: 2.5 increases positive value with time, but that of lower stretched ones less than draw ratio: 2.0 show negative value, and their absolute values decrease with lowered draw ratio. Applied the Avrami's equation to the (density)_??_(logarithmic time) curves, Avrami's index (n) is 3 or 4 for lower stretched fibers; n equals to 1 or 0<n<1 for higher stretched ones, and constants of crystallization rate (k) become larger with increased draw ratio.
2) The (density_??_logarithmic time) curves of various stretched fibers undeer a constant length may be similar to its behavior under a free contraction and the time started in making greater density suddenly shifts towards further shorter time as compared with heating under a free contraction. The changes of birefringence with time may be similar to its time-dependence under a free contraction, except for 2.0 of a draw ratio. The sample of a draw ratio: 2.0 does not show negative birefringence under a constant length even though the density becomes greater and show a weakly positive birefringence. The Avrami's index (n) calculated from the (density_??_logarithmic time) curves under a constant length, may be generally smaller than that under a free contraction, and furthermore the constants of crystllization rate larger. These results may be due to the contribution of the thermal shrinking strss occured during the heating under a constant length.
3) The conversion in the signs of birefringence (either positive or negative) for stretched fiber of a draw ratio: 2.0, due to the different heated conditions may be explained by considering the rotative orientation of the (100) plane.
4) When the density, birefringence and logarithmic time curves for same fibers at a different temperatures are compared, it is found that master curves can be made of superposition by lateral shift along the lomarithmic time scale within temperatures of this experiment, independent of heated conditions. And it is found eviently from these master curves that a different heated conditions may affect time-dependence on birefringence for the stretched fiber of a draw ratio: 2.0, that is, the absolute values increase with time. While birefringence changes the negative value under a free contraction and the positive minimum value under a constant length, corresponding to the time started in making a sudden greater density and thereon.