Recycling and rheology of poly(lactic acid) (PLA) to make foams usingsupercritical fluid

Biodegradable plastics are thought to be the possible directions in managing plasticpollutions. Unfortunately, they are not recycled in most countries since they are designedto decompose even though recycling is a more pragmatic method than landfill orincineration. Thus, it is more constructive to develop methods to recycle biodegradableplastics or to develop biodegradable yet recyclable plastics. In this study, we usedcutlery with a composite of poly(lactic acid) (PLA) and talc. The possibility to recycleit to make foams was studied even though it will have lowered mechanical strength from therecycling process as it is less significant for this product. Tensile properties of solidPLA and foams showed no significant decrease in the strength up to three processes ofcompression molding and foaming. We performed shear rheometry to determine the thermalstability and dependences of the complex viscosity on frequency and temperature. Themagnitude of the complex viscosity dramatically increased with decreasing frequency andsuch an upturn increased with temperature, but time-temperature superposition was valid athigh temperatures. The extensional rheometry showed no strain hardening, but physicalfoaming using supercritical carbon dioxide (CO2) could still occur, and theoperating conditions to obtain various foamed structures were determined. We also comparedthe effects of one-directional against three-dimensional expansion. Overall, theconcentration of CO2 in PLA and crystallinity of the foams are the two keyvariables to describe the bulkiness of foams. Surprisingly, the lower the CO2concentration, the bulkier the foams at any sorption temperature and pressure.