Mechanism-enabled sustainability: Dynamic retro-Diels-Alder polyurethane network for reprocessable flexible foams

The irreversible nature of thermoset polyurethanes, PU, poses a challenge in recycling, prompting the application of Covalent Adaptable Networks, CANs. CANs combine structural stability while providing reprocessability through reversible bond exchange. Specifically, retro-Diels-Alder, rDA, chemistry offers potential due to its thermoreversible behavior and mild reprocessing conditions. However, its application in PU foams is unexplored. In this study, open-cell PU foams are synthesized selecting sustainable 1,5-pentane-diisocyanate, PDI, opportunely modified via rDA adducts to incorporate thermoreversible bonds into the PU matrices. A two-step procedure is designated: i) synthesis of precursors based on isocyanate prepolymers at different PDI: rDA molar ratios, ii) incorporation of the prepolymers into PU formulation, and subsequent foaming. Foam reprocessing and subsequent bulk to bulk reprocessing are performed without the use of additional catalyst via compression molding. The mechanical/thermal and self-healing properties of the reprocessed bulk materials are evaluated. Stress-relaxation behavior is analyzed to determine characteristic relaxation times and activation energy of network rearrangement. PU foams are successfully reprocessed into bulk elastomeric materials with enhanced self-healing characteristics. Stress-relaxation experiments indicated Arrhenius-type behavior with activation energies of 50–85 kJ mol−1, confirming thermally activated macroscopic network rearrangement. This study offers insights into the application of rDA adducts in PU foams, enhancing their reprocessability and contributing to a closed-loop lifecycle.