Foaming of linear isotactic polypropylene based on its non-isothermal crystallization behaviors under compressed CO 2

Abstract

The non-isothermal crystallization behaviors of isotactic polypropylene (iPP) under ambient N 2 and compressed CO 2 (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 CO 2 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 CO 2 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 CO 2, respectively. Due to the acute depression of CO 2 solubility in the iPP matrix during the foaming process, the iPP foams with the bi-modal cell structure were fabricated.