The formation of interfacial morphologies of iPP derived from transverse flow during multi-penetration in secondary melt flow

Abstract

A growth mechanism of various oriented structures of isotactic polypropylene (iPP) has been investigated in multiple penetrated multi-melt multi-injection molding (MPM3IM), in which multiple high density polyethylene (HDPE) melts as the second injection melts to rapidly penetrate the first iPP melt. The crystal forms of iPP skin layer at different positions were characterized by differential scanning calorimetry (DSC) and 2D-WAXD. And the evolution of hierarchical crystal morphologies in MPM3IM samples from MN1 to MN2 were characterized using polar light microscopy (PLM) and scanning electron microscopy (SEM), respectively. The MN1was the position at which the weld line between two core melts situated and the MN2 was the position of the maximum diameter of single core melt. From the results, on account of the temperature field and complex flow field consisted of penetration flow along the machine direction (MD) and transverse flow (TF) along the transverse direction (TD), the oriented structure, shish-kebab-like structure which was adjacent to interface in iPP skin layer was relaxed to turn into banded fibrillary row nuclei and confined β-cylindrites, at the same time the banded row nuclei receded from interface with its width decreased from MN2 to MN1. Nevertheless, there was only one simplex highly oriented structure formed near the interface at cross section in skin layer of single penetrated multi-melt multi-injection molding (SPM3IM) sample, for instance shish-kebab, which can be attributed to the absence of TF.