Development and Applicability of a Mathematical Model for the High Speed Melt Spinning of Crystallizable Polymers

A mathematical model was developed to describe the high speed melt spinning behavior of crystallizable polymers. This model included the effects of acceleration, gravity and friction on the kinematics of the process, temperature and molecular orientation on the crystallization kinetics of the polymer and temperature, molecular weight and crystallinity on the elongational viscosity of the material. Experimental online diameter, birefringence and temperature profiles were obtained for a 12000 Mn nylon-66 at 2.5 gm/min spun at take-up speeds ranging from 2800 to 6600 meters/minute. These profiles were qualitatively and reasonably quantitatively in agreement with the predicted profiles. They indicated that orientation induced crystallization occurs at spinning speeds greater than 4000 meters/minute. The experimental diameter and birefringence profiles were compare to those predicted by the model using Avrami indices of 3, 2 and 1. There was a small increase in the crystalline index at the lover speeds with decreasing index. The effect of the strain hardening was more significant at the higher speeds, this being shown by decreasing the exponent in the relationship for the crystallinity on the elongational viscosity.

The model was also applied to two polypropylenes of different molecular weight. There was very good agreement between the predicted and experimental diameter and birefringence profiles for both molecular weights. There was some differences in the temperature profile comparisons, but there was good agreement between the predicted and experimental temperatures and positions where the crystallization is first observed. The position and duration of the temperature plateaus were qualitatively in good agreement with those predicted.

The model developed in this study indicates that high spinning speeds provide a high stress environment which increases the molecular orientation within the fiber. It is this higher molecular orientation which is the driving force for rapid crystallization on the spinline. This rapid crystallization causes a strain hardening preventing any further drawdown in the fiber diameter and an abrupt rise in the birefringence. This behavior closely corresponds to the observed spinline profiles.