Retraction of polypropylene tape

Retraction measurements have been made on 50 μm thick polypropylene tapes drawn on a yarn line under a variety of conditions. Small samples were annealed in silicone oil for different periods at temperatures up to the melting point. The retraction data were found to be described by a relation of the form R(%) = A + B log t, where A and B are constant for a given temperature, A being the intercept on the retraction axis for t = 1. As the temperature is increased towards the melting point, A increases, slowly at first and then much more rapidly. On the other hand, B reaches a maximum at a temperature T_k, and then decreases as the temperature is increased further. T_k corresponds to the point at which A, which represents a virtually instantaneous retraction as the sample reaches temperature, begins to increase rapidly. T_k promises to be useful in that it represents the susceptibility of a given polymer to retraction and corresponds fairly closely to the original draw temperature if the tape has not been heat set. D.s.c. analysis has demonstrated that the mechanism responsible for the instantaneous component of retraction is the partial melting out of crystallites. The small logarithmic retraction which is dominant below T_k is seen as resulting from a thermally activated process within the crystallites. The approach has enabled the effect of pre-anneals and heating rates to be understood and has also led to a general prediction of retraction behaviour under conditions of continuous heating.