Foaming of Poly(lactic acid) Using Supercritical Carbon Dioxide as Foaming Agent: Influence of Crystallinity and Spherulite Size on Cell Structure and Expansion Ratio

Abstract

Using supercritical carbon dioxide (Sc-CO2) as physical foaming agent, foamed poly(lactic acid) (PLA) samples were prepared in a batch process via constant- and varying-temperature modes (CTM and VTM). Their crystallinity, cellular structure, and expansion ratio were investigated. In the CTM, the samples foamed at low saturation temperatures present three regions (skin, inner, and core regions). The uniformity of cellular structure is much improved with increasing saturation temperature. In the VTM, saturation temperature exerts a significant impact on the size of spherulites formed in gas saturation stage. Large spherulites evolve into entities surrounded by elongated cells or submicro-sized cells in interlamellar regions after foaming at 140 °C and into small cells (mean diameter of 0.6 μm) at 160 °C, whereas small spherulites generally evolve into stamen-like cell structure at 140 °C. Interestingly, uniform cellular structure with high expansion ratio (49.8) or bimodal cellular structure can be obtained by tuning saturation and foaming temperatures.