The influence of resin characteristics on the high speed melt spinning of isotactic polypropylene. II. On‐line studies of diameter, birefringence, and temperature profiles

An investigation was carried out of the high speed melt spinning of three polypropylene resins with melt flow indices of 12, 35, and 300. On‐line measurements were made of diameter, birefringence, and temperature as a function of distance from the spinneret for a range of spinning conditions for each polymer. A plateau (decrease of cooling rate) in the temperature profile was associated with the occurrence of crystallization in the spinline. The position of this plateau correlated with a rapid rise in the birefringence profile and a rapid decrease in the rate of drawdown in the diameter profile. The temperature and birefringence profiles were used to determine the temperature and position on the spinline at which the onset of crystallization occurred. It was found that the position and temperature of crystallization onset varied considerably with changes in take‐up velocity, extrusion temperature, and resin melt index (weight average molecular weight). The crystallization onset occurred nearer the spinneret and at higher temperatures with (1) an increase of take‐up velocity, (2) a decrease of extrusion temperature, or (3) a decrease of resin melt flow index. An analysis was carried out to estimate the rate of stress development with distance along the spinline; the results were also used to estimate the stress at the onset of crystallization for each spinning condition. It was concluded that the observed behavior could be attributed to the role of spinline stress in producing molecular orientation and consequent increase of crystallization rate.