Supercritical CO2 utilization for development of graded cellular structures in semicrystalline polymers

Abstract

Carbon dioxide (CO2) in a supercritical condition is utilized to develop hierarchical cellular structures in injection molding. Polypropylene is used as sample semicrystalline polymer. First, a single-phase CO2-charged melt was produced under pressure. Then, a gradient cell structure was induced onto the polymer melt by pressure reduction and crystal-induced shrinkage. A visualization mold was employed to monitor real-time nucleation of CO2 cells within the mold cavity. Distinctive cell nucleation behaviour was visualized for the first time in which a myriad of ultra-fine cells nucleated rapidly inside the system in an abnormal manner. This type of cell nucleation was different from that occurring due to a pressure drop during foam processing. This mode of cell nucleation was likely driven by the crystallization of the polypropylene chains in the presence of the supercritical CO2. The crystallization behaviour of polypropylene was then investigated within the mold cavity and in the presence of dissolved CO2 by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The presence of supercritical CO2 affected the size of α and β crystals. Furthermore, γ crystals were detected in the core regions of samples. The formed crystals served as effective nucleating sites which promoted the nucleation of large number of fine cells and produced structurally graded foams. The effects of CO2 content, melt packing, and melt temperature on the cell structure development was studied. It was concluded that these parameters can be regulated to design cellular structures with desired cell size and cell morphologies.