Reprocessable, Bio-Based,\nSelf-Blowing Non-Isocyanate\nPolyurethane Network Foams from Cashew Nutshell Liquid

Abstract

Growing environmental concerns and the goal of a circular\neconomy\nfor polymers necessitate the development of biowaste-based materials\nand efficient recycling of polymer materials. Here, we developed a\nseries of self-blowing network polyhydroxyurethane (PHU) foams by\nleveraging the aminolysis and decarboxylation of cashew nutshell liquid\n(CNSL)-based cyclic carbonate with thiols to release CO₂ as a blowing agent; these foams contain up to 80 wt % bio-based\ncontent. By systematically varying the blowing agent concentrations,\nwe demonstrated the tunability of the morphologies and mechanical\nproperties of CNSL-based PHU foams. Using dynamic mechanical analysis\n(DMA), compression testing, and hysteresis testing, we showed that\nthese foams fall into the category of flexible foams with potential\nas memory foams or resiliency foams. To address the recyclability\nchallenges of thermoset foams, we repurposed these CNSL-based PHU\nfoams into bulk materials and reprocessed them by exploiting the dynamic\nchemistries of the hydroxyurethane linkages. Notably, the reprocessed\nbulk networks exhibited full property retention. Moreover, the systematic\ninclusion of permanent linkages to substitute dynamic cross-links\npresents an avenue to study the interplay of permanent linkages and\ncross-link density toward the dynamic characteristics. We showed that\naverage relaxation times and activation energies increase with increasing\nlevels of permanent linkages in the system, demonstrating highly tunable\ndynamic behaviors in PHU network materials.