New catalytic system for the chemical recycling of rigid polyurethane foams

Polyurethane is the fifth most widely produced plastic worldwide, and its increasing consumption generates considerable amounts of post-industrial and post-consumer waste. Since rigid polyurethane and polyisocyanurate foams are thermosetting materials that cannot be melted and reprocessed, chemical recycling represents a suitable strategy for breaking down their crosslinked structure and recovering valuable raw materials. Therefore, this work investigates and optimizes the glycolysis of different rigid foam wastes, with particular attention to the influence of their chemical composition, especially the content of isocyanurate rings and flame-retardant additives, on the degradation process and on the properties of the recovered polyols. In the first part of the work, different rigid polyurethane foam wastes were glycolyzed by varying the main process parameters, including reaction time, temperature, glycol-to-foam ratio and catalyst concentration. A novel catalytic system was investigated with the aim of promoting polyurethane degradation while limiting the formation of undesirable secondary products. The progress of the reaction and the quality of the resulting glycolyzates were evaluated by measuring their viscosity, hydroxyl number and acid number. The concentration of 4,4′-methylenedianiline (MDA) was also monitored: MDA is a carcinogenic aromatic amine that can be generated during polyurethane glycolysis because of the hydrolysis and degradation of structures derived from 4,4′-methylene diphenyl diisocyanate and should therefore be minimized to ensure the safe reuse of the recovered products. The results showed that the degradation behaviour was strongly dependent on the nature of the processed waste. Polyurethane-rich foams were more readily degraded, whereas wastes characterized by a higher concentration of isocyanurate structures required more severe reaction conditions because of their greater thermal and chemical stability. The presence and concentration of flame-retardant additives also affected the catalyst activity, the evolution of viscosity and the chemical composition of the recovered products. During the optimization phase, the effect of catalyst concentration and reaction time was investigated for each waste stream in order to improve foam degradation, control the viscosity of the final product and limit the concentration of free aromatic amines. Particular attention was also given to the possible degradation or transformation of the flame-retardant additives and to their influence on the chemical reactions occurring during glycolysis. Finally, the optimized recycled polyols were used as partial replacements for virgin polyol in the production of new rigid polyurethane foams. Increasing amounts of recycled material were introduced into the formulations, and the resulting foams were characterized in terms of density, cellular morphology, thermal conductivity and compressive properties. The results demonstrated that recycled polyols obtained from differently formulated wastes can be successfully reused for the production of new rigid foams, although the maximum suitable replacement percentage and the final foam properties depend on both the composition of the starting waste and the adopted glycolysis conditions. Overall, this work highlights the importance of adapting the chemical recycling process to the specific characteristics of each polyurethane waste stream, contributing to the development of a more flexible and effective recycling strategy for complex rigid polyurethane and polyisocyanurate foams.