Investigation of crystallization behavior of solid and microcellular injection molded polypropylene/nano-calcium carbonate composites using carbon dioxide as a blowing agent

This work is aimed at investigating the crystallization behavior of solid and microcellular injection molded polypropylene/nano-calcium carbonate composites. The effects of processing conditions, such as injection speed, mold temperature, and carbon dioxide concentration (used in microcellular injection molding), as well as the filler concentration on the crystal form, crystal orientation, and crystallinity were studied using 2D-wide-angle X-ray diffraction and differential scanning calorimetry. β-form crystals found in the surface layer of injection molded samples under high injection and mold temperature due to stronger shear effect. The orientation degree calculated from the X-ray diffraction images by the Hermans function was high in the surface layer and decreased as the distance from the mold surface increased. The addition of the nano-calcium carbonate filler promoted the formation of β-form crystals but reduced the orientation degree and crystallinity as the nanoparticles disturbed the orientation of the molecular chains. On the other hand, when using the foaming process, the formation of β-form crystals was inhibited and the orientation degree was reduced, but the crystallinity of the samples increased, likely due to enhanced molecular chain mobility from the supercritical carbon dioxide which acted as a plasticizer. The crystallinity of the samples was greater in the surface layer but showed no dependence on the injection speed or mold temperature.