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

Abstract

In order to investigate the phenomenon of elongation occurred in heated P. E. T. fibers under a constant length after hot drawing, the effects of draw ratio and drawing temperature on longitudial and thermal stress changes for successive heating and cooling at a constant rate were discussed. And also the fine structure of specimens, quenched at 0°C from transformation point of longitudial change-temperature curves were studied by X-ray, density and birefringence. The results obtained are as follows:1) The low stretched specimens at 70° or 80°C in water and subsequently passed the thermal history of heating and cooling show a-axis orientation in X-ray diffraction patterns, and the length of fiber may becomes slightly longer than that of the original, because of superposing thermal creep effects on the spontaneous elongation beyond a temperature range of maximum shrinkage. On the other hand, the specimens high stretched at 70°C (draw ratio: 3.8) shows a fiber-like orientation in X-ray pattern as well as the one at 60°C (draw ratio: 2.0). Both of these show a larger thermal contractive stress, The shrinkage at a range of 85_??_90°C as well as the abnormal elongation in the vicinity of 115°C become smaller. And the specimen stretched at 70°C (draw ratio: 2.5) shows an intermediate orientation in X-ray pattern, and the orientation of its crystal may have preferentially a fiber-like orientation, On the other hand, the molecular chains in the amorphous phase do not orient so much and accordingly the maximum thermal contractive stress becomes smaller. The thermal shrinkage becomes larger, while the abnormal elongation smaller due to the unstable, structure.2) The drawn specimen at 90°C (draw ratio: 2.0) shows negative birefringence and a-axis orientation in X-ray pattern. While both the hot shrinkage and contractive thermo-stress of this sample become smaller, the, spontaneous elongation does not occur so much.3) The contribution of crystalline birefringence Δnc•X for a-axis oriented fiber on the temperature range of cold crystallization shows negative, and for c-axis oriented fiber strongly positive. On next heating and cooling processes, absolute values of Δnc•X may increase with closed regularity of crystals. On the other hand, the amorphous birefringence Δna shows the minimum values on the temperature range of maximum shrinkage, and on the heating and cooling processes values of Δna only increase in spite of rising and lowering temperature.