Influence of injection rate on crystallization of injection-molded β-nucleated isotactic polypropylene

It is widely believed that β-nucleating agent is beneficial for effectively toughening isotactic polypropylene (iPP). However, for the injection molding process, the shearing and thermo-mechanical conditions make the nucleation and crystallization process complicated. In this paper, the effects of injection rate on crystallization of β-nucleated iPP were studied by scanning electron microscope (SEM), two-dimensional wide-angle X-ray diffraction and differential scanning calorimetry (DSC). It is observed that with increasing injection rate, the content of β-crystals exhibits different tendencies in the skin, intermediate layers, and core zone. Specifically, for the intermediate layer, the β-crystals content first increases with increasing injection rate to 85 cm·s−1, and begins to decrease afterward. By simulating the injection process, the most likely explanation for the β-crystal change is the comparatively high shear rate and low shearing time that the melt experienced. Variations in β-form content are mainly responsible for the mechanical properties of β-nucleated iPP. The results of this study provide a valuable way to control the iPP toughness in the injection molding process. POLYM. ENG. SCI., 57:172–182, 2017. © 2016 Society of Plastics Engineers