Foaming of linear isotactic polypropylene based on its non-isothermal crystallization behaviors under compressed CO2

The non-isothermal crystallization behaviors of isotactic polypropylene (iPP) under ambient N2 and compressed CO2 (5–50 bar) at cooling rates of 0.2–5.0 °C/min were carefully studied using high-pressure differential scanning calorimeter. The presence of compressed CO2 had strong plasticization effect on the iPP matrix and retarded the formation of critical size nuclei, which effectively postponed the crystallization peak to lower temperature region. On the basis of these findings, a new foaming strategy was utilized to fabricate iPP foams using the ordinary unmodified linear iPP with supercritical CO2 as the foaming agent. The foaming temperature range of this strategy was determined to be as wide as 40 °C and the upper and lower temperature limits were 155 and 105 °C, which were determined by the melt strength and crystallization temperature of the iPP specimen under supercritical CO2, respectively. Due to the acute depression of CO2 solubility in the iPP matrix during the foaming process, the iPP foams with the bi-modal cell structure were fabricated.