Design of fully reprocessable composite non-isocyanate polyurethane (NIPU) foams from sustainable blends of cyclic carbonates

In response to the growing need to reduce reliance on petrochemical feedstock and eliminate toxic isocyanates in polyurethane production, this study presents the synthesis of composite non-isocyanate polyurethane (NIPU) foams starting from a novel blend of bio-based cyclic carbonates. The resulting flexible foams are fully reprocessable and have a bio-based content ranging from 92% to 99%. They were prepared through a two-step procedure where aminolysis was first carried out between 1,4-butane diamine (BDA) and blends of carbonated soybean oil (CSBO) and bio-based butanediol bis-cyclic carbonate (BCC). A blowing reaction was induced via S-alkylation using a dithiol while diatomite was incorporated as a renewable nanoporous filler providing nucleating and reinforcing properties. The influence of the CSBO/BCC ratio on foam properties was systematically investigated through chemical, physical, thermal, and morphological analyses. A structure–property relationship was established using an adapted Gibson–Ashby model. The resulting foams exhibited open-cell morphology with uniform cell sizes (400–600 µm) and apparent densities between 200 and 250 kg m−3. Notably, the foams were successfully re-shaped into flexible films via temperature-controlled compression molding, confirming their potential for recyclability and reuse. The stress-relaxation behavior of the re-processed NIPU presented a decreasing trend by increasing the relaxation time and was described by the Kohlrausch–Williams–Watts function. The activation energy was calculated according to the Arrhenius equation and was found to be 91 ± 8 kJ mol−1 indicating a relatively strong temperature dependence of the relaxation mechanisms. This work unlocks a new design strategy for the sustainable synthesis of fully recyclable NIPU foams, opening up new directions in green polyurethane chemistry.