Biobased, catalyst-free, non-isocyanate polyurethane network foams with enhanced circularity through extrusion and foam-to-foam recycling

Traditional thermoset polyurethane (PU) foams raise concerns around safety and sustainability due to isocyanate toxicity, dominance of fossil-fuel precursors, and limited recyclability prospects caused by the sluggish urethane exchange. Here, we developed reprocessable, re-foamable, biobased, catalyst-free, non-isocyanate polythiourethane (NIPTU) network foams crosslinked via the auto-oxidation of pendant thiol groups into disulfides. We prepared our foams from monomers derived from biowaste, e.g., cashew nutshell liquid, rice husk, and biodiesel waste. We demonstrated tunability of morphological and physical properties through judicious choice of physical blowing agents or incorporation of a small amount of trifunctional crosslinker. We leveraged the rapid and catalyst-free disulfide dynamic exchange to achieve reprocessability through extrusion of spent NIPTU materials. Extending our promising extrusion findings, we showed, for the first time, an approach to foam-to-foam recycling of non-isocyanate polyurethanes. By mixing a small amount of sodium bicarbonate blowing agent into spent foams, CO₂ gas was generated during high-temperature extrusion, which led to a cellular structure post-extrusion. In summary, this work highlights the inherent circularity advantages of emergent NIPTU foams: the catalyst-free, rapid synthesis of foams with excellent property tunability, and their amenability towards a family of industrially relevant reprocessing and upcycling technologies.