BIAXIAL ORIENTATION OF POLYPROPYLENE BY HYDROSTATIC SOLID-STATE EXTRUSION .2. MORPHOLOGY AND PROPERTIES

Changes in the lamellar structure of polypropylene during biaxial orientation by hydrostatic extrusion have been examined. Examination of etched surfaces in the transmission electron microscope has revealed that up to a draw ratio of about 1. 5 multiplied by 1. 5 the primary mechanism is lamellar rotation into the plane of orientation. At higher draw ratios breakup of the lamellae is observed with formation of a granular structure. Thermal measurements suggest that localized melting and recrystallization may accompany the process of lamellar breakup. The structural change at a draw ratio of about 1. 5 multiplied by 1. 5 is accompanied by a large increase in the tensile impact strength.