X‐ray diffraction orientation studies on blown polyethylene films. III. High‐stress crystallization orientation

Abstract

AbstractX‐Ray diffraction orientation measurements have been made on a wide range of films blown from three high‐density polyethylenes, to determine more precisely the conditions which lead to the high‐stress crystallization type of orientation. The most extensive measurements relate to films from Hostalen GM 9955F; the results show that there is a very wide range of orientational behavior. Under very low‐stress conditions there is almost pure a‐axis orientation; with very high stress there is substantial c‐axis orientation, both with reference to the machine direction. Commercial blowing conditions give rather high stress and the a axis is inclined at 60° to 70° to the machine direction in the sheet‐normal—machine‐direction plane. Calcium stearate, used to improve the surface finish, increases the stress for a given set of machine conditions and, of these, a high draw ratio and a low extrusion temperature are most effective in promoting high‐stress crystallization. The less extensive results for an experimental Rigidex grade and Shell LPPE 040 fit into this overall pattern; for a given set of blowing conditions they have lower stress than the Hostalen polymer. Commercial blowing conditions give an a‐axis inclination of about 45°.