Use of stereocomplex crystallites for fully-biobased microcellular low-density poly(lactic acid) foams for green packaging

Success in the typical extrusion foaming process is always achieved by adding inorganic/organic fillers, which inevitably sacrifice poly(lactic acid) (PLA)’s green nature. Herein, we successfully fabricated well-defined cell structures of fully bio-based PLA foams using a series of asymmetric biodegradable poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) blends with small amounts of PDLA. We obtained large-scale PLLA/PDLA blends using a continuous melt compounding technique. The crystallization kinetics in the presence of dissolved carbon dioxide (CO2) under high pressures was investigated by using regular and high-pressure DSC. The PLA’s crystallization rates were notably accelerated due to the significant synergy of the dissolved CO2 and the reserved SC crystallites. The continuous extrusion process, which could be easily scaled-up to industry, was used to fabricate the PLLA/PDLA blended foams. Using the neat PLLA foam, we successfully demonstrated how to make fully bio-based PLA foams with three orders of magnitude increased cell density, an enhanced expansion ratio over ten-fold, and increased crystallinity by 15%. Such a high cell density, a high expansion ratio and a high crystallinity are unprecedented in the continuous processing of fully green PLA foams. They possess great potential for use as a sustainable product in thermoformed food packaging applications.