Foaming process of polylactide and toughened polylactide blends using supercritical CO₂

The microcellular foams of polylactic acid(PLA) and its binary blend with polybutylene succinate(PBS) or butylene glycol-adipic acid -ternary terephthalate polyester(PBAT) were prepared by bath foaming process with supercritical carbon dioxide (SC-CO₂) as physical foaming agent. The crystallization behavior and morphology of PLA blends were studied, and the adsorption and desorption of CO₂ in PLA or PLA blends were further investigated. The effects of foaming temperature and foaming time on the final morphology of microcellular foam of different PLA systems were also discussed. With 10% PBS or PBAT added into the PLA matrix, the "sea-island" structure indicates that both PBS and PBAT are incompatible with PLA, and DSC studies show that PLA-10% PBS has the highest crystallinity. The saturated conditions are fixed to be 12 MPa at 45℃, and lasting for 8 h. The adsorption capacity of three PLA systems is around 20% with slight difference, while that of PLA-10% PBS blend is the maximum and that of PLA-10% PBAT blend is the minimum. The diffusion process of CO₂ in those PLA systems shows interesting three-steps diffusion mode with the first stage as skin diffusion having the highest diffusion rate. The second stage shows lower diffusion rate due to dominated CO₂ from amorphous area in the PLA systems. The final stage has the lowest rate and the diffusion happens in crystallization area. The second stage is chosen as foaming stage for the final microcellular PLA. The SEM pictures indicate that the microcellular of pure PLA shows a typical bimodal distribution of cell structure. However, PLA-10%PBS system shows a high cell density and smaller average cell diameter compared to PLA-10% PBAT, which is attributed to a higher interfacial interaction of PLA with PBS.