Biobased, catalyst-free non-isocyanate polythiourethane foams: Highly dynamic nature affords fast reprocessability, extrudability and refoamability

We have developed a series of reprocessable, re-foamable, biobased, catalyst-free non-isocyanate polythiourethane (NIPTU) network foams crosslinked via the auto-oxidation of the pendant thiol groups into disulfides. Capitalizing on the interplay of fast ring-opening of cyclic thiocabonate to create linear backbones and slightly slower thiol auto-oxidation to create disulfide crosslinks, the gelling reaction synchronized well with the vaporization of the physical blowing agent. Different physical blowing agents were used to achieve facile tunability of morphological and physical properties. In addition, incorporating a small amount of trifunctional crosslinker significantly enhanced the compressive mechanical properties of the foam. Moreover, we leveraged the rapid and catalyst-free disulfide dynamic exchange to achieve both reprocessability and extrudability of the NIPTU foams. We demonstrated that the foams are intrinsically self-healable and reprocessable by compression molding. We observed rapid stress relaxation at temperatures above 160 °C, prompting us to explore continuous processing techniques like extrusion and pseudo-injection molding. Spent foams can be extruded into bulk films at 180 °C with excellent property retention. Additionally, with our NIPTU system we demonstrated, for the first time, foam-to-foam recycling of non-isocyanate polyurethanes. By adding a small amount of sodium bicarbonate blowing agent into the spent foams prior to extrusion, CO₂ gas was generated during extrusion, leading to a cellular structure. This work highlights the advantages of NIPTU foams: the catalyst-free, rapid synthesis of foams and property tunability, self-healing capability, and amenability towards a family of reprocessing techniques including compression molding, extrusion into bulk films, and foam-to-foam extrusion.