Effect of processing conditions on the orientation and properties of polyethylene blown film

Abstract

The amorphous and crystallite orientation in a number of films produced from characterised grades of high density polyethylene has been examined and some tensile properties measured. The films were produced under commercial production conditions with various blow ratios, freeze line heights and thicknesses. Orientation was assessed using optical and X-ray methods. The relative contributions of the crystallite and amorphous components were calculated for a limited number of the films produced. Amongst the properties measured the ultimate tensile strength and elongation at break appeared to be the most variable. Birefringence was more affected by variation in blow ratio and film thickness than by freeze height changes. Attempts have been made to correlate these variations with crystallite and amorphous orientation as well as the individual behaviour of the polyethylene grades. In carrying out this correlation it became clear that the films could not generally be described by established low or high stress models of crystallite orientation. For these films produced on full-scale commercial equipment it appears that transcrystallisation behaviour is frequently observed, with a tendency towards the low stress model.