Evaluating the foamability of polypropylene with nitrogen as the blowing agent

The foamability of polypropylene (PP) was carefully evaluated with nitrogen (N2) as the blowing agent. Four PP samples with different molecular architectures, namely isotactic polypropylene (iPP), ethylene propylene block copolymer (E-b-P), ethylene propylene random copolymer (E-r-P) and branched PP (high melt strength PP, HMSPP), were selected to observe the relationship between the molecular structure and polymer foamability. Two measuring parameters, the foaming temperature window and the correlation factor of foam density with foaming temperature, were used to characterize the foamability. The results showed that the foaming temperature window of HMSPP was more than 34 °C, whereas that of linear PPs was as narrow as 2-4 °C. In addition, unlike linear PPs, the foam density of HMSPP was insensitive to foaming temperature. HMSPP was indeed the best of all PP resins at present available for PP industrial foam production.