Fabrication of high-expansion microcellular PLA foams based on pre-isothermal cold crystallization and supercritical CO2 foaming

Poly(lactic acid) (PLA) is a kind of bio-based and biodegradable polymers. PLA foams with high expansion ratio have great application potential in heat insulation, adsorption and other areas. However, due to its low melt strength and slow crystallization rate, linear PLA has poor foaming ability, so it is hard to prepare PLA foams with high expansion ratio. Although chain branching, blending, and other methods have been utilized to improve PLA's melt strength and foaming ability, they easily destroy PLA's biodegradability, cause chemical pollution, and raise production costs. In this study, a new supercritical fluid foaming process, based on pre-isothermal cold crystallization, was proposed to prepare PLA foams with high expansion ratio. To improve PLA's melt strength and foaming ability, a pre-isothermal treatment was applied to induce sufficient cold crystallization. According to the SEM results, the cold crystallization of PLA becomes more sufficient and the crystal morphology becomes more perfect with the pre-isothermal treating. The DSC and WAXD results confirm that, the pre-isothermal treatment remarkably promotes the PLA's cold crystallization, and endows the PLA sample higher crystallinity and more perfect crystalline structure. Moreover, the high-pressure rheology testing results indicate that the pre-isothermal treatment improves the PLA's melt viscoelasticity significantly. Finally, the foaming results show that the pre-isothermal treatment significantly enhances the PLA's foaming ability. With the pre-isothermal treatment, the PLA's maximum expansion ratio increases from 6.40-fold to 17.7-fold, and the uniformity of cellular structure is also improved obviously. The new process provides a green, flexible, and low-cost way to prepare fully biodegradable PLA foams with high expansion ratio.