Supercritical CO2 Foaming for Poly(3-hydroxybutyrate-co-4-hydroxybutyrate)/Poly(lactic acid) Blends

This study focused on the foaming characteristics of poly(3-hydroxybutyrate-co-4-hydroxybutyrate)/polylactic acid(PHA/PLA) bio-blends under supercritical CO2 conditions. The favorable foaming characteristics, similar to PLA, were obtained for the PHA/PLA(3/7) blend at 130 degrees C and 90 bar. These conditions are a favorable for both crystalline PHA which has 10% of 4HB content, and PLA. However, the foaming was not possible for other blend compositions, due to the decrease in melt strength resulting from an increase in PHA composition in PHA/PLA blends. In the case of amorphous PHA50, which was not capable of CO2 foaming alone, forming was possible by blending it with PLA, although the expansion ratio was lower than PLA. The improvement of melt strength in PHA and PLA was understood to be a result of chain branching caused by the modification of the chain through transesterification between -OH and -COOH end-groups. It was found that the foaming characteristics of the PHA/PLA blend were effectively controlled by foaming pressure rather than foaming temperature.