Chemical recycling of polyurethanes: sustainable processes to increase circularity

Plastic recycling is of primary importance in these years considering the major issues of climate crisis and environmental pollution. Polyurethane is one of the most produced plastics worldwide, and its recycling is particularly challenging because of its thermosetting nature and the different applications it has. As classical thermal recycling methods are not suitable for the recovery of polyurethane waste products, landfilling has always been the most common end-of-life scenario. However, phasing out of landfilling is now inevitable as it is in contrast to the established and growing concept of circularity. In this perspective, chemical recycling allows us to consider polyurethane waste as a resource to get the most out of to produce new valuable materials called secondary raw materials. The aim of this study was to develop a portfolio of sustainable processes to recycle a broad variety of polyurethanes considering the use of alternative energy sources to reduce the energy consumption and strategies to minimise the presence of by-products. Particular care was also given to the use of recycled products in the production of new polyurethane foams, enabling the closure of the recycling loop. Three systems were considered, a recycling process to recover a rigid polyurethane foam and produce a new rigid foam with similar if not better properties, the recycling of a viscoelastic foam to produce a polyisocyanurate foam (a niche polyurethane foam application), and a process to recycle a viscoelastic foam waste and produce the original viscoelastic foam with similar if not better properties. One of the major problems of the chemical recycling of polyurethane is the occurring of secondary reactions due to the high temperature of the processes and to the humidity of the reaction environments. These two conditions lead to the production of free aromatic carcinogen amines, namely methylenedianiline (MDA). Strategies to eliminate free aromatic amines were developed considering optimal reaction conditions and the use of deaminating agents, adding a further step to the process. Furthermore, since these recycling processes generally last long, it was tried to employ microwave heating as an alternative energy source, with the aim of decreasing the overall energy expenditure. All these aims are in agreement with the Principles of Green Chemistry and Green Engineering.