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 MPa. The greatest depression of melting point and degree of crystallinity of the samples occurred at 20–30 MPa (∼97–115 °C). Batch and semi-continuous processes for supercritical foaming and impregnation of PLA with thymol or thyme extract were performed at 30 MPa and 100–110 °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 h, 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 μm have potential for food packaging, biomedical and insulation applications.
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