Research On The Foaming Process And Properties Of Polypropylene With Supercritical CO₂

Polypropylene (PP) foams have been widely used in many areas with excellent heat resistance, thermal insulation and environment-friendly properties, and have the trend of replacing other thermoplastic foams gradually.In this paper, isotactic polypropylene (IPP) was foamed with supercritical CO₂ in the autoclave, and the foaming technology was studied by designing orthogonal experiments to determine the optimal foaming process and discuss the impact of foaming temperature and other factors on the volume expansion ratio of the foamed IPP.The effect of foaming temperature on the volume expansion ratio of the foamed IPP was greatest of the three factors, and the effect of infiltration pressure greater than infiltration time by using direct analysis of orthogonal experiment. The volume expansion ratio showed the trend of first increasing and then decreasing with the increasing of foaming temperature and infiltration pressure. In addition, the impact of foaming temperature and infiltration pressure on the volume expansion ratio was prominent and the impact of infiltration time limited with using analysis of variance.IPP was modified by doping nano-MgO and nano-SiO₂ particles, respectively. Scanning electron microscope(SEM) results showed that when the dosage of nanoparticles was 2mass%, the two kinds of nanoparticles were well dispersed in the IPP. In contrast, the nano-SiO₂ particles dispersed in the IPP were smaller, whose sizes were only about 50nm and about the combination of two nano-SiO₂ particles. But when adding to 6mass%, the two kinds of nanoparticles were all badly dispersed in the IPP with clearly large aggregates.The results showed that doping nano-MgO particles in the IPP can not only reduce the bulk density of the foamed IPP or enhance the volume expansion ratio obviously, but also improve the cell structure significantly. The reason was probably that although the nano-MgO particles can act as the nucleus agents of CO2, but improve the crystallinity of IPP, and did not change the structure of the crystalline region. But doping nano-SiO₂ particles in the IPP can reduce bulk density of the foamed IPP to enhance the volume expansion ratio significantly. When adding 2mass% nano-SiO₂ particles, the effect was best that the bulk density was about 0.0511g/cm3 and the volume expansion ratio was about 18.03, which was 36.9% higher than 13.17 of the undoped IPP foams. At the same time,that also can improve the cell structure of the foamed IPP obviously to make it more symmetrical. The reason was that the nano-SiO₂ particles uniformly dispersed in the IPP had the structure of the crystalline region changed, and enhanced the melt strength of IPP by serving as physical crosslinking points to make it foamable.The comparison results of the IPP/nano-SiO₂ foams and the high melt strength PP (HMSPP) foams indicated that doping 2mass% nano-SiO₂ particles in the IPP improved the cell structure of the foamed IPP significantly, enhanced the volume expansion ratio, and met with similar results to the HMSPP foams.