Orientation determination and morphological study of high density polyethylene (HDPE) extruded tubular films: effect of processing variables and molecular weight distribution

Abstract

The structure—property behaviour of high density polyethylene (HDPE) extruded tubular films having a blow up ratio of unity was investigated. Two different HDPE resins with identical M n (14 600 g/mol) values but different distributions ( M w M n = 10.3, 15.1 ) were utilized in this extrusion process. Systematic changes were also made in the process variables—these being the melt temperature at the die exit, the quench height, which is the distance from the exit of the die to the cooling ring, the flow rate of the air through the cooling ring, and the film line speed. The morphological features and state of orientation of the extruded tubular films were examined by transmission electron microscopy (TEM), high resolution scanning electron microscopy (HSEM), birefringence, wide and small angle X-ray scattering studies (WAXS, SAXS), and linear infra-red (i.r.) dichroism. A stacked lamellar or, in some cases, a fibril nucleated morphology was observed in the melt-extruded films by TEM. This technique clearly showed that the broader molecular weight samples possessed fibril nuclei that were not evident in the narrower molecular weight distribution films. The existence of the fibril nuclei was further confirmed from WAXS by the intense sharp spots superimposed on the equatorial (110), (200), and (020) reflections. The origin of this pronounced fibril nucleation was clearly believed to be due to the longer relaxation time behaviour of the broader distribution resin. The amorphous phase orientation and form birefringence for all films were found to be very small, and the observed birefringence arose almost exclusively from the crystalline phase. It was also found that the morphology and the orientation of the chain axis in the crystalline phase were particularly sensitive to the quench height and melt temperature for the narrower distribution HDPE films, but much less so for the broader distribution HDPE samples. Little dependence of structure/orientation was noted for either resin for the variables of the flow rate of the air through the cooling ring and the film line speed. These findings show the sensitivity of the molecular weight distribution to be a very important parameter, as are the specific processing variables of quench height and melt temperature, in determining the final solid state structure of the HDPE extruded tubular films.