The prediction of orientation and crystallinity in drawn poly(ethylene terephthalate)

Abstract

This paper describes the prediction of molecular orientation and fractional crystallinity in polymer products formed by deformation processes. The calculations are carried out for poly(ethylene terephthalate) articles such as film and stretch-blow moulded bottles formed just above the glass transition temperature. They are based on the interpretation of realtime X-ray diffraction patterns obtained during and after drawing at speeds comparable with industrial processing 1 , where crystallization occurs only after drawing has ceased. A parametric study reproduces trends observed in many experimental uniaxial draw studies, such as the increase in orientation and crystallinity with draw ratio, draw rate and decreasing temperature. The study also demonstrates the increase of crystallinity from annealing or less rapid cooling from the draw temperature. The prediction method has also been applied to the first draw stage of a biaxial film line, where temperature variations through the thickness and in the machine direction lead to orientation and crystallinity non-uniformity through the thickness. The predicted average crystallinity is in good agreement with experimental measurements for a range of process conditions.