Correlation of Micro- and Macrostructural Attributes with the Foamability of Modified Polypropylene Using Supercritical CO2

This study covers the micro- and macrostructural changes that take place prior to and after foaming in polypropylene (PP). Four industrial grades of PP with varying molecular weights and amorphous contents were considered. Batch foaming of the samples was carried out using supercritical CO2 at three different temperatures, keeping the saturation pressure constant. A straight correlation between the cell size and the density of foams was drawn. Differential scanning calorimetry measurements in coherence with wide-angle X-ray analysis gave unique insights into the crystallinity of foams. Furthermore, dynamic mechanical analysis (DMA) showed shifts in the transition peaks of foamed PPs as compared with their corresponding unfoamed states. The tensile properties of foams of impact copolymers, foamed at low temperature, possessed superior tensile toughness to that of the foams obtained from homopolymers and random copolymers. This tensile characteristic of foams was in coherence with the DMA analysis.