Study of the Influence of the Pressure Drop Rate on the Foaming Behavior and Dynamic-Mechanical Properties of CO2 Dissolution Microcellular Polypropylene Foams

This article presents the preparation of microcellular polypropylene foams produced by a CO2 batch-foaming process and their characterization regarding the influence of the pressure drop rate on the foaming behavior and dynamic-mechanical properties. A polypropylene-based material was prepared by melt-mixing in a twin-screw extruder, cooled, and pelletized and later compression-molded in a hot-plate press to solid discs. These discs were finally foamed inside a high pressure vessel by dissolving CO2 and carefully controlling its sudden decompression drop. The dynamic-mechanical properties of the different expansion ratio-produced PP foams were studied, analyzing the influence of the pressure drop rate and residual pressures on the cellular structure and subsequent dynamic-mechanical behavior of the foams. With increasing the sudden pressure drop by reducing the residual pressure value, higher expansion ratio PP foams were obtained, reaching a maximum value of 3. Only slight differences were observed between foams regarding the cell size (maximum cell size approximate to 100 mm), the foams presenting slightly lower specific storage moduli than that of the solid material, indicating the efficiency of this process in nucleating and generating relatively high expansion ratio foams with a closed-cell type of structure and cell sizes in the micrometer range.