On-line measurement of orientation development in the high-speed melt spinning process of polypropylene

摘要

In order to clarify the mechanisms of molecular orientation in high-speed melt spinning of polypropylene (PP), the change of birefringence along the spinline was analyzed through the 5'multaneous on-line measurement of the diameter and optical retardation profiles. The optical retardation was evaluated using an optical system with a rotating polarizer, polarizing filters, Quarter-wave plates and a He-Ne laser as the light source. The birefringence increased only slightly in the upstream portion of the spinline. On the other hand, there was a steep increase of birefringence, which became more significant with an increase in the take-up velocity, near the solidification point where crystallization appears to start. An additional increase of birefringence was notably observed in the downstream portion of the spinline where the thinning of the spinline was already completed. The as-spun PP fibers also showed a gradual increase of birefringence even after the spinning, indicating that the increase of birefringence after the solidification of the spinline was caused by the change in the super-structure of the fibers, not by the increase in tensile stress along the spinline because of the accumulation of the air-friction force. To analyze the applicability of the stress-optical law, tension and temperature profiles of the spinline were calculated based on the experimental results of the diameter profiles. The small increase of birefringence in the molten state was found to be due to the low stress-optical coefficient of PP. It was also found that at take-up velocities above 1 km/mm the steep increase of birefringence started at the position where the tensile stress of the spinline exceeded about 1 MPa. These results suggest that the orientation-induced crystallization of the PP spinline starts at this stress level and also that the development of birefringence of PP fibers in the high-speed spinning process is significantly affected by the crystallization.