Foaming of Poly(lactic acid) Based on Its Nonisothermal Crystallization Behavior under Compressed Carbon Dioxide

The nonisothermal crystallization behaviors of poly(lactic acid) (PLA) under ambient N2 and compressed CO2 (5−50 bar) at cooling rates of 0.2−2.0 °C/min were carefully studied using high-pressure differential scanning calorimetry. The presence of compressed CO2 postponed the crystallization peak to a lower temperature region while effectively reducing the half-crystallization time and enhancing the crystallinity of the PLA specimen. On the basis of these findings, a new foaming strategy was proposed and utilized to fabricate PLA foams using the ordinary unmodified PLA. The upper and lower temperature limits of this foaming strategy were 105 and 90 °C, which were determined by the melt strength and crystallization behavior of the unmodified PLA specimen, respectively. In this temperature range, PLA foams with interconnected structures, porosity of 67.9−91.4%, and expansion ratio of 15−30 times are controllably produced. The obtained PLA foams have widely distributed average bubble size of 80−270 μm and CO2 enhanced crystallinity of 32−38%.