Effects of Film Blowing Conditions on Molecular Orientation and Mechanical Properties of Polyethylene Films

Abstract

An experimental study of the effects of material characteristics and processing parameters on the orientation and mechanical properties of different polyethylene (PE) films is presented. Several counter-intuitive results were observed. For the linear low density PE (LLDPE) the machine direction (MD) and transverse direction (TD) modulus values decreased with increasing take-up-ratio (TUR). For the low density PE (LDPE) the moduli also decreased with increasing TUR at high blow-up-ratio (BUR). At low BUR, however, there was a considerable overall increase in the TD modulus as TUR increased while the MD modulus was not much affected as TUR increased. Upon increasing BUR, the TD modulus drastically decreased, while the MD modulus remained almost constant. The extrusion temperature and frost line height did not significantly influence the modulus of the films studied. For the LLDPE, it is shown that the modulus somewhat decreased with increasing polymer flow rate. Significant variations of birefringence along the TD and MD were observed in blown films. At low BUR, the in-plane birefringence of the LLDPE increased with TUR up to a maximum, before decreasing to negative values. No general correlation between the birefringence and tensile modulus data could be found. Based on our present and previous results, we believe that it is also unlikely that final film properties can be related to strain rates and stresses occurring in the film blowing zone by any simple correlation. Many processing variables and polymer structural factors affect the ultimate film properties. Any attempt to control film properties will have to take those into account.