Highly oriented microstructures and surface mechanical properties of polypropylene (PP) molded by ultra-high shear rate

Raman spectroscopy and wide-angle X-ray diffraction (WAXD) were used to evaluate microstructures, i.e. molecular orientation, crystalline orientation and the relative amount of β-phase crystal (K), near the surface of injection molded polypropylene (PP). It was found that highly ordered structure near the surface (0–30 μm) was formed when ultra-high injection speed (1000 mm/s or γ˙ = 1670s−1) was utilized. Subsequently, increase of relative amount of β-phase crystal near the surface was achieved. The relationship between the microstructures and surface mechanical properties measured by micro-cutting method and progressive load scratch tests was investigated. The highly oriented PP showed higher surface mechanical properties due to the increase in degree of molecular and crystalline orientation. The increased scratch properties were considered to be also affected by higher β-phase crystal content. [Display omitted] •Highly oriented ordered structure of polypropylene was formed near the surface.•The highly oriented structure included molecular and crystalline orientation.•Surface mechanical properties were enhanced by the highly oriented structure.