Near-surface deformation under scratches in polypropylene blends Part I Microscopic characterization of deformation

Abstract

A microstructural characterization approach has been developed to study the mechanisms of near-surface deformation under surface scratches in injection-molded polypropylene blends with over 20% rubber modifier (thermoplastic polyolefin or TPO). The near-surface microstructure of the material before and after scratching was characterized with different techniques such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), optical microscopy and X-ray diffraction. It was observed that the TPO material plastically deformed by forming shear band structure under surface scratches. Materials inside shear band dilated and the extent of dilation could be measured from the characteristic angles between the shearband boundary and rubber particles. At a higher applied normal load (>200 g for the test in this study), evidence for surface fracture was observed. At even higher loads (>400 g), significant amounts of sub-surface voiding were observed, due to the delamination between the rubber phases and the polypropylene matrix. The observation of both the dilation of materials inside shearbands and the subsurface voiding at high normal loads advanced the understanding of scratching whitening mechanism in this kind of important materials. It was observed that the talc additives had no obvious influence on shear band nucleation and propagation. Results obtained in this study suggest that a strong interfacial adhesion between rubber phase and PP matrix is crucial to improving the scratching resistance of rubber modified polypropylene blends.