Orientation studies of high density polyethylene extruded films

Blown films from five grades of high density polyethylene have been produced to study the effect upon orientation of the three major machine parameters: blow ratio, freeze height and 'film thickness. To study the effect of the structural variations between the polyethylenes, the polymers have been characterised in terms of MFI, density, molecular weight distribution, degree of crystallinity, degree of branching and rheological behaviour. The orientation in the films has been measured by two major techniques, polarised light microscopy and x-ray pole figure analysis. The results show that four grades of polyethylene follow the expected curves of increasing birefringence with decreasing blow ratio and decreasing film thickness, whilst the fifth grade shows an opposing trend. The results for increasing freeze height show three of the polyethylene grades to follow the expected trends. The use of the pole figure analysis has enabled the complete measurements of the a- and b-axis distributions of the unit cell. In general, the b-axis distribution has been found to lie in the sheet normaltrarisverse direction plane, and the a-axis to be distributed in the sheet normal - machine direction plane. Orientation models have been proposed for these polyethylene grades from results derived by the two techniques, and the effect of the machine parameters upon these models has been evaluated. Mechanical testing of the films has been carried out both in the machine and transverse directions, and correlations have been drawn between the proposed orientation models and the'mechanical properties. Finally, a Small Angle light'Scattering technique has been studied as a possible on-line method of. recording_orientation during manufacture, but the results show this technique to be unsatisfactory due,to large amounts of scattering from the film surface.