Fundamental Influences of Induced Crystallization and Phase Separation on the Foaming Behavior of Poly(lactic acid)/Polyethylene Glycol Blends Blown with Compressed CO2

A sinterable poly(lactic acid) (PLA) foam is critical to the creation of strong interface bonding for the production of large-size PLA bead foam products. To fabricate such a foam, we blended a polyethylene glycol (PEG) that has a low melting temperature and high CO2 solubility with PLA. Solid-state foaming technology was used to foam the PLA/PEG blends, and CO2 was the blowing agent. The PEG was miscible with the PLA and enhanced the crystallization of PLA at a loading of lower than 10 wt % under a CO2 saturation process. Consequently, the PLA/PEG blends did not foam well because of the matrix's improved stiffness. The CO2 saturation process induced the PEG's phase separation because its content was higher than 20 wt %. This weakened the PEG's plasticization effect on the PLA, but it significantly improved the PLA's foaming behavior. This was characterized by both the foam expansion ratio, which increased from 4.6 to 11.8-24.1, and the decreased cell size. We discuss the effects of crystallization and the separated PEG phase on cell nucleation and growth in order to explain the possible operational mechanisms. Both the PEG's self-diffusion behavior and the PLA/PEG's improved foaming behavior enabled us to fabricate large-size PLA bead foam products.