Properties of uniaxially stretched polypropylene films: Effect of drawing temperature and random copolymer content

Abstract

Cast films of two linear polypropylenes (PP) having different molecular weights and their blends with 20 wt% random copolymer were prepared by extrusion. The produced cast films were then uniaxially hot drawn at T = 110 and 140°C under a draw ratio (DR) of 6 using a machine direction orientation (MDO) unit. The effects of drawing temperature and addition of random copolymer on the properties of the films were investigated. The type of crystals (spherulites or fibrils) was studied using differential scanning calorimetry (DSC). The drawing process generated a highly oriented fibrillar crystalline structure, resulting in an increase in the melting point of the films by about 10°C. The films drawn at 140°C revealed coexisting lamellae and fibrils whereas at 110°C mainly a fibrillar structure was observed. Tensile measurements showed a greater strength for the sample stretched at lower temperature (i.e., 110°C). The effect of drawing temperature and copolymer content on tear resistance was also explored. The medium-molecular weight PP film stretched at 110°C showed greater tear resistance than at 140°C. The addition of the random copolymer into the medium-molecular weight PP reduced the tear resistance significantly whereas the impact for the high-molecular weight was not noticeable. Adding the random copolymer significantly decreased the haze for the films and hence drastically improved the clarity. © 2010 Canadian Society for Chemical Engineering.