Hydrothermal decomposition of virgin and waste polylactic acid with subcritical water under N2 and air atmospheres

Bio-based polymers are an important step towards solving environmental problems, but there is a need to consider and develop procedures for dealing with these materials at the end of their life and to ensure that effective disposal methods are available. Recently, polylactic acid (PLA) has replaced many plastics based on non-renewable resources. In this work, subcritical water was used for the chemical decomposition of virgin and waste PLA under different atmospheres. The main degradation product was lactic acid with very high yields. The highest yield of lactic acid was obtained under N₂ atmosphere, where the optimum conditions were 200 °C and 60 min and the yield was 88.96 ± 1.5 % for virgin PLA and 54.58 ± 1.3 % for waste PLA. In an air atmosphere, the maximum yields of lactic acid were obtained at 250 °C and 30 min and were slightly lower than in the N₂ atmosphere, i.e. 87.20 ± 0.9 % for virgin PLA and 49.29 ± 0.6 % for waste PLA. The lower yield of lactic acid from waste PLA is due to the impurities and additives in waste PLA. Other carboxylic acids were also formed in the aqueous phase, while the gas phase mainly contained CO₂, N₂/CO and C₁-C₅ hydrocarbons. The hydrothermal degradation pathway was presented and the electricity costs for lab-scale PLA recycling were estimated. With this sustainable technology, PLA could be successfully recycled down to its monomer, providing a secondary raw material for the re-synthesis of the polymer, closing the loop and reducing the impact on the environment.