Relationship of Polypropylene Fiber Orientation as Determined by Optical Birefringence and Sonic Velocity Measurements

Abstract

Sonic velocity and optical birefringence measurements were made on yarns prepared from various polypropylene polymers that were extruded and cooled under conditions leading to the formation of yarns possessing different crystalline structures. Yarns produced from relatively-low-temperature melts were of the predominantly monoclinic crystalline structure in contrast to the paracrystalline structure of yarns produced from high-temperature melts. The basic relationships of melt temperature, molecular weight degradation, and development of spuming orientation are briefly discussed. A correla tion of sonic modulus with birefringence for undrawn yarns was found for yarns with a given molecular weight and crystalline structure. Orientation of the varns by stretch ing brought about the simultaneous increase of both the sonic modulus and the bire fringence for those yarns possessing the paracrystalline structure. Yarns of the mono clinic crystallme structure were observed to exhibit a temporary decrease in sonic modulus, although birefringence continued to increase. The minimum in the sonic modulus curve is presumed to be a result of reduction of crystallinity due to cleavage of crystalline zones resulting from stretching or of the conversion of the crystal form to the paracrystalline form.