Morphological Distribution in Micro-Injected Polypropylene Parts in the Presence of β-Nucleating Agent

Abstract

Micro-injection molding is attracting much attention nowadays. Characterization of the morphological distribution in parts prepared by micro-injection molding is thus of growing importance. The morphological features of micro-parts may strongly differ from those of the macro-parts prepared by conventional injection molding, resulting in specific physical properties. In the present study, β-nucleated isotactic polypropylene micro-parts (μPPB) with 200 μm thickness, as well as macro-parts (PPB) with 2000 μm thickness, were prepared. Polarized light microscopy (PLM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD) were used to investigate their morphological features. The results show that the morphology distribution in μPPB had many differences from that of the PPB. The one-dimensional WAXD and DSC analysis showed similar results; the degree of crystallinity of the μPPB was higher than that of the PPB. However, the content of β-crystals of μPPB was lower than that of the PPB. This can be explained by the restraining effect for the formation of β-crystals in β-nucleated isotactic polypropylene (iPP) under the strong shear field. The through-the thickness-morphology of both μPPB and PPB exhibited a “skin-core” structure from PLM observations, but the former had a large fraction of shear layer in comparison to the latter implied. The SEM observations showed that the shear layer of μPPB consisted of a highly oriented shish-kebab structure, while that of the core layer consisted of deformed spherulites structure. The two-dimensional WAXD pattern of the core layer of PPB, showing full Debye rings, indicated an overall random orientation of the iPP chains, while the arcing indicated a pronounced orientation in the shear layer. The more pronounced arcing of the μPPB indicated a more pronounced orientation.