Green Catalysts for Reprocessing Thermoset Polyurethanes

Polyurethane (PU) thermosets are usually landfilled at the end of their service lifetimes and cannot be recycled through conventional means. Although dibutyltin dilaurate (DBTDL) is an effective catalyst for reprocessing PU thermosets, organotins are immunotoxic and teratogenic, which is concerning for catalysts embedded within covalent adaptable networks (CANs). Here, we identify Zr(acac)(4 )and Zr(tmhd)(4) as catalysts to reprocess PU CANs more sustainably. Thermoset PU foams containing Zr-based catalysts exhibit characteristic stress relaxation times between 6 and 200 s, similar or faster than DBTDL. These catalysts are capable of reprocessing a PU foam for four or five cycles. Dynamic mechanical thermal analysis indicates that zirconium-based catalysts preserve glass transition temperature and crosslink density between cycles and are promising catalysts for bulk reprocessing of thermoset PUs. Furthermore, a solvent-free reprocessing method was developed by co-milling the PU foam and the Zr catalyst. Ultimately, these reprocessing methods using Zr-based catalysts will impart greater sustainability and circularity to PU plastics.