PROPERTIES OF UNIAXIALLY STRETCHED POLYPROPYLENE FILMS: EFFECT OF DRAWING TEMPERATURE AND RANDOM COPOLYMER CONTENT

Cast films of two linear polypropylenes (PP) having different molecular weights and their blends with 20wt% random copolymer were prepared by extrusion. The produced cast films were then uniaxially hot drawn at T=110 and 140 degrees 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 degrees C. The films drawn at 140 degrees C revealed coexisting lamellae and fibrils whereas at 110 degrees C mainly a fibrillar structure was observed. Tensile measurements showed a greater strength for the sample stretched at lower temperature (i.e., 110 degrees C). The effect of drawing temperature and copolymer content on tear resistance was also explored. The medium-molecular weight PP film stretched at 110 degrees C showed greater tear resistance than at 140 degrees 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.