Biaxial orientation of polypropylene by hydrostatic solid state extrusion. Part III: Mechanical properties and deformation mechanisms

The deformation of biaxially oriented polypropylene sheet produced by the BeXor process has been examined in terms of the hierarchical solid state structure described previously. The mechanical properties have been examined over a range of temperatures and strain rates. The anisotropic nature of uniaxial tensile deformation was analyzed from simultaneous measurements of longitudinal extension and lateral contraction in the width and thickness directions, by determining the shape of uniaxially deformed spherulites, and by examining fracture surfaces in the scanning electron microscope (SEM). It was determined that deformation proceeds by elastic extension of the amorphous network and plastic shear deformation of the crystalline regions in the plane of the sheet, and in the thickness dimension, by voiding with induced fibrillation. It is suggested that the property improvements, particularly at low temperatures and high strain rates, which are achieved by the BeXor process, result from changes in the hierarchical structure at the size scale of the crystalline lamellae. Specifically it appears that the small crystalline blocks formed by breakup and recrystallization of lamellae are more easily deformed than the original large coherent lamellae.