Thermal analysis of polylactic acid under high CO2 pressure applied in supercritical impregnation and foaming process design

Abstract

Abstract Thermal properties of extrusion and injection grade polylactic acid (PLA) were analysed using high pressure differential scanning calorimetry (HP-DSC) under CO2 pressures of up to 50\u2009MPa. The greatest depression of melting point and degree of crystallinity of the samples occurred at 20–30\u2009MPa (∼97–115\u2009°C). Batch and semi-continuous processes for supercritical foaming and impregnation of PLA with thymol or thyme extract were performed at 30\u2009MPa and 100–110\u2009°C to prevent thymol degradation, decrease heating requirements and ease polymer processing. At these conditions, PLA foams containing 5.6% or 1.1% of thymol and 0.7% of thyme extract were obtained using static or dynamic batch impregnation and semi-continuous extraction-impregnation process for 7\u2009h, respectively. DSC and HP-DSC analyses revealed more pronounced effect of scCO2 plasticizing than foaming on PLA crystallinity. Neat and impregnated PLA foams with pores size of 15-200\u2009μm have potential for food packaging, biomedical and insulation applications.