Investigation of extensional rheological behaviors of polypropylene for foaming

This study investigated the relationship between the extensional rheological behaviors and the foamability of polypropylenes (PPs). Both the shear and extensional rheological properties of four types of PPs with different molecular structures (linear PP, linear PP with high-molecular component, long-chain-branched PP, and cross-linked PP) were tested by rheometers and the PPs were foamed by batch process with supercritical carbon dioxide as foam agent. It was found that when the PP showed strain-hardening behavior at certain strain rates (on the order of 1s(-1)), the obtained foams had high expansion ratio and fine cell structures. Otherwise, the strain-hardening behavior of PP had limited effects on the foamability. In addition, a theoretical analysis suggested that the strain-hardening behavior provided higher melt strength and simultaneously changed the rupture behavior of the melt, thus the gas bubble could be stable during expansion step and the cellular structure was generated. The foamability of PPs can be predicted from the extensional properties without considering their molecular structures.