Reprocessable, Bio-Based, Self-Blowing Non-Isocyanate Polyurethane Network Foams from Cashew Nutshell Liquid

Growing environmental concerns and the goal of a circular economy for polymers necessitate the development of biowaste-based materials and efficient recycling of polymer materials. Here, we developed a series of self-blowing network polyhydroxyurethane (PHU) foams by leveraging the aminolysis and decarboxylation of cashew nutshell liquid (CNSL)-based cyclic carbonate with thiols to release CO2 as a blowing agent; these foams contain up to 80 wt % bio-based content. By systematically varying the blowing agent concentrations, we demonstrated the tunability of the morphologies and mechanical properties of CNSL-based PHU foams. Using dynamic mechanical analysis (DMA), compression testing, and hysteresis testing, we showed that these foams fall into the category of flexible foams with potential as memory foams or resiliency foams. To address the recyclability challenges of thermoset foams, we repurposed these CNSL-based PHU foams into bulk materials and reprocessed them by exploiting the dynamic chemistries of the hydroxyurethane linkages. Notably, the reprocessed bulk networks exhibited full property retention. Moreover, the systematic inclusion of permanent linkages to substitute dynamic cross-links presents an avenue to study the interplay of permanent linkages and cross-link density toward the dynamic characteristics. We showed that average relaxation times and activation energies increase with increasing levels of permanent linkages in the system, demonstrating highly tunable dynamic behaviors in PHU network materials.