Studies On The Crystal Structure Regulations And Heat Resistance Of Poly(Lactic Acid) Microcellular Foams

Polylactic acid (PLA) is one of the bio-plastics derived from natural resourses such as starch, potatoes and might take the place of the petroleum-based materials. PLA has two enantiomers, poly(L-lactie) (PLLA) and poly(D-lactide) (PDLA). PLA can form a stereocomplex crystallite which has a high melting point and heat resistance by the melt blending of PLLA and PDLA. In recent years, due to the excellent advantages, supercritical CO2 assistanted microcellular foams are drawing more and more attention. PLA microcellular foams prepared by using supercritical CO2 foaming methods could improve the matrix toughness and heat resistance properties, and further broaden its applications. In this study, combining the regulation of homocrystal and stereocomplex structures, a solid-state foaming process was carried out to foam PLLA and PLLA/PDLA blends to investigate the crystal structure evolutions and the influences of crystallites on the cell morphology and heat resistance properties of PLA foams.Firstly, we studied the solubility of CO2 inPLLA matrix and the evolutions of crystal structures of PLLA and PLLA/PDLA blends under CO2 saturation and foaming process. It turned out that the solubility of CO2 in PLA matrix increased with increasing the CO2 saturation pressures in a certain range. What’s more, the CO2 saturation process could induce a mesomorphic structure of PLA which is an intermediate ordering state between the crystalline state and the totally amorphous state, while the CO2 saturation process has few effects on the PLA SC crystallites. At last, the foaming process further increased the perfection of the mesomorphic structure and it turned into the more stable a structure.Then, the effects of PLA crystallites and the CO2 saturation pressures on the cell morphology and heat resisting properties of PLA foams were further studied. The results showed that, with introducing the PDLA contents, the expansion ratios and average cell size decreased while the cell density increased. What’s more, the addition of PDLA did improve the heat resistance of PLLA/PDLA foams.At last, for further improving the expansion ratios of PLLA/PDLA foams, the effects of in-situ induced PLA crystallites during the foaming procedure on the cell morphology and heat resistance properties were also discussed. It turned out that the foaming process could in-situ induce PLA SC and a crystallites and that the foaming temperature had a key effect on the amount of the induced PLA SC and a crystallites, while the foaming time had few effects on them. In addition, the as-prepared PLLA/PDLA foams did have a better heat resistance than that of the pure PLLA foams.Through the associate studies on PLA crystal structures, foaming behaviors and heat resistance, we achieved the goal of improving the foams’s cell morphology and heat resisting properties by regulating the crystal structures. So, this paper could provide a feasible method to fabricate PLA microcellular foams with good heat resisting properties.