THE EFFECT OF A LIQUID ISOTHERMAL BATH IN THE THREADLINE ON THE STRUCTURE AND PROPERTIES OF POLY(ETHYLENE-TEREPHTHALATE) FIBERS

The process of melt-spinning poly(ethylene terephthalate) (PET) filame nt at high speeds was modified through the inclusion of a liquid isoth ermal bath (LIB) in the spinline. A wide range of positions, temperatu res, and depths associated with the operation of the LIB were utilized in this study. The structural characteristics and mechanical properti es of the as-spun fibers were characterized by birefringence, wide-ang le X-ray diffraction (WAXD), infrared spectroscopy, and tensile testin g. Experimental results showed that the structure and mechanical prope rties of the as-spun fibers were significantly influenced by the LIB o perating conditions. The as-spun fibers prepared under optimum LIB con ditions exhibit high birefringence and excellent mechanical properties . Results suggest the development of a critical value of threadline st ress that is determined primarily by LIB depth and take-up velocity. B elow this critical value, raising of LIB temperature, LIB depth, and t ake-up velocity resulted in increases of the apparent crystallite size , sample crystallinity, and both the crystalline and amorphous orienta tion. As would be expected, the mechanical properties of the fiber sam ples were improved in a corresponding manner. Above this critical stre ss value, molecular chains in the amorphous phase are stretched tautly , but the crystal growth process is restricted, resulting in a decreas e in crystallite size and crystallinity, as well as a continued increa se in mechanical properties. The fiber properties were also found to b e very responsive to the relative location of the LIB. A unique struct ure, believed never before obtained in a one-step high-speed PET melt- spinning process, has been achieved that combines high amorphous orien tation, low crystallinity, and high tenacity. (C) 1995 John Wiley and Sons, Inc.