Foam-to-Elastomer Recycling of Polyurethane Materials through Incorporation of Dynamic Covalent TAD–Indole Linkages

Polyurethane (PU) foams are a large volume commodity product and as such pose a considerable recycling challenge for the polymer manufacturing industry. The incorporation of dynamic covalent bonds within a PU network chemistry is a possible strategy to improve the sustainable development of PU foams. Herein, we report the outcome of a research program aimed at the incorporation of thermoreversible triazolinedione (TAD)-indole linkages within an industrial standard PU foam formulation. A scalable synthesis of the required TAD–indole building blocks was developed, aiming at maximizing their physicochemical compatibility with standard polyol and isocyanate PU foam ingredients. A pilot scale synthesis of a TAD-based cross-linker was developed, affording more than 50 kg of an IPDI-derived bis-urazole building block. Propoxylation of the indole fragments proved to be a key technology enabling the kilogram scale production of TAD–indole based polyols. The innovative dynamic covalent polyols were successfully used to produce a range of flexible PU foams and elastomers in a solvent-free foam formulation. Owing to the thermoreversible nature of the TAD–indole linkers, the PU foams could be processed into PU elastomers through thermal compression molding, which could be further recycled up to 7 times in the same manner. We studied the effect of network compositions on the foaming process and on the recycling efficiency. The thermal and mechanical properties of the materials have been studied with thermal analysis, tensile measurements, and rheology. This work demonstrates that TAD–indole chemistry is a viable strategy to improve polyurethane recycling but also points out some critical aspects and obstacles related to the design of such dynamic covalent PU foams.