Towards circularity: optimization of acidolysis for the chemical recycling of viscoelastic polyurethane waste

The growing environmental impact of plastic waste has made the transition toward a circular economy increasingly urgent, aiming to reduce waste generation and promote a more sustainable use of raw materials. In this context, this work focuses on the chemical recycling of polyurethane (PU), currently the sixth most produced plastic worldwide. A major challenge in PU recycling lies in its structural variability, as the final properties of PU materials strongly depend on the nature and proportion of the starting reagents. Also, recycling polyurethane foams waste is difficult as the recycled product loses some of its chemical and mechanical properties, making the production of foams similar to the original more difficult. The research investigates acidolysis using succinic acid, identified as one of the most effective reagents in terms of the properties of the resulting recycled polyol. The study focuses on the influence of key process parameters, including nitrogen atmosphere, temperature and acid concentration. Particular attention is given to the stepwise addition of the acid, with the aim of reducing the formation of solid by-products arising from secondary reactions, which can negatively affect the quality of the final product. The recycled polyol is then characterized through viscosity measurements, 4,4'-methylenedianiline (MDA) content, acid value, hydroxyl number, colour and presence of solid residues. Additionally, acidolysis is briefly compared with aminolysis as an alternative depolymerization route. Model compounds are also employed to investigate the nature of solid by-products and to better understand the mechanisms of depolymerization and deamination involved in both acidolysis and aminolysis processes. Finally, the study includes the production of new polyurethane foams from the recycled polyols and their subsequent characterization. Overall, the results of this study can propose some interesting solutions for chemical recycling by acidolysis of viscoelastic PU foams supporting the development of more sustainable and circular strategies for PU-based materials.