The crystallization behavior of solid and microcellular injection molded polypropylene and its calcium carbonate composites using carbon dioxide as a physical blowing agent

This work is aimed at investigating the crystallization behavior of microcellular injection molded polypropylene (PP)/nano calcium carbonate (nano-CaCO₃) composites. The effects of processing conditions - such as injection speed, mold temperature, and CO₂ concentration, as well as the filler concentration - on the crystal form, crystal orientation, and crystallinity were studied using 2D-wide-angle X-ray diffractometry (2D-WXRD) and differential scanning calorimetry (DSC). The ß-form crystal was found in the surface layer of the injection molded samples under high injection and mold temperatures, due to high shear stress. The addition of the nano-CaCO ₃ filler promoted the formation of ß-form crystals while the foaming process inhibited it. The orientation degree calculated from the XRD images by the Hermans function was high in the surface layer and decreased as the distance from the surface increased. Meanwhile, the orientation degree decreased with foaming and the addition of fillers, as both the bubbles and fillers disturbed the orientation of the molecular chains. The degree of crystallinity of the samples was greater in the surface layer and increased with foaming. However, the degree of crystallinity decreased with the addition of fillers and had no relationship with the injection speed or mold temperature.