Suppression of the Skin-Core Structure of Injection-Molded Polypropylene Part: Role of Balance Effect Caused by the Incorporation of Glass Fiber

Abstract

The skin-core hierarchy structure of isotactic polypropylene (iPP) injection-molded parts was successfully suppressed by the introduction of glass fibers (GFs) as a result of the “balance effect.” The pure iPP presents a large fraction of spherulitic core layer, while the thickness of the core layer of the iPP/GF composites was greatly thinner. For pure iPP, the morphology can be divided into three regions along the thickness direction: skin layer, shear layer, and core layer. However, the morphology of the sample with 7 wt% GF was so homogenized that it could not be roughly divided into the three regions. Furthermore, the area of the shear layer becomes larger with increasing GF content. It was full of shish-kebab-like cylindrite structures. These results indicated that GF can homogenize the gradient of shear stress perpendicular to the flow direction. It was confirmed that the GF could be used to stabilize the shear-induced nuclei, especially in the core region, and resulted in the enhanced crystallinity of the β-form. Based on our investigation, a schematic model was proposed to interpret the “balance effect” of GF on suppression of the skin-core structure.