Mechanistic Insights into Acid-Mediated Polyurethane Foam Recycling

ConspectusImproved plastic waste management strategies are required to combat the rapid accumulation of postconsumer waste in landfills and the environment. Unfortunately, mechanical recycling currently accounts for only 10% of plastic recovery and is limited by product downcycling, highlighting the need for more effective chemical alternatives. Ironically, the robust mechanochemical properties that make plastics so useful also make them difficult to process at the end of their lifetime. Unlike polyolefins, made up of C-C bonds, polyurethanes contain reactive C-O and C-N bonds that make them amenable to chemical degradation. Chemical recycling of polyurethanes toward their molecular building units can be driven using a variety of chemical reagents, including acids, amines, glycols, and water. Acidolysis has shown particular promise as an effective and sustainable method for polyurethane closed-loop recycling. However, the mechanistic and fundamental chemical principles governing this process remain elusive.In this Account, we detail our mechanistic and kinetic studies of polyurethane foam acidolysis. By systematically varying the carboxylic acid reagent, we highlight the impact of the acid transport phase, acid structure, and acid electronics on the rate, product distribution, and rate-limiting regimes of polyurethane depolymerization. We suggest design rules that can be used to predict and explain how new reagents will impact the rate and products of polyurethane acidolysis. From these design rules, we suggest a promising choice of acid for the efficient and facile recovery of polyol, the major building block used in polyurethane synthesis. We conclude by recognizing the remaining challenges related to polyurethane chemical recycling, largely related to making this process more economically feasible.