ON SURFACE-MORPHOLOGY AND DRAWING OF POLYPROPYLENE FILMS

Two nominally equivalent grades of polypropylene designed for similar applications were extruded into sheets, with one surface chilled rapidly, giving a smooth texture; and the other being rougher, with a structure of spherulites separated by crevices formed by retraction of the last traces of molten polymer. Differences in the two grades gave rise to differences in spherulitic morphology, in terms of both spatial distribution of spherulites among die lines and the proportions of alpha- and beta-spherulites. The sheets were subjected to three kinds of deformation: unrestricted monoaxial, restricted monoaxial (plane strain), and biaxial. The effects of surface variations persisted right through the drawing process. The morphology of the rough surface in particular was observed (a) to dominate the process of deformation, especially necking as it initiated at surface crevices between spherulites; and (b) to affect the optical quality of the final product, most significantly in the structure and number of haze rings. Legacies of beta-spherulites in fully drawn films can be observed at all levels, from the unaided eye down to lamellar structures observed in the transmission electron microscope.