Programming degradation and closed-loop recycling of ultra-stable bio-based thermosetting polyurethane relied on double-locked covalent adaptable networks

Castor oil (CO) derived thermoset polyurethanes are usually limited by their poor mechanical properties and the difficulty in removing or recycling because of the aliphatic or alicyclic backbone and their permanent cross-linking structures. Although covalent adaptable networks (CANs) based on dynamic bonds have been proposed, it is still highly desired to fabricate polymer CANs with high performance, stimulus resistance, stepwise degradation and closed-loop recycling from renewable CO for sustainable development. In this study, CO-based poly(urethane urea) (PU) thermosets relied on double-locked covalent adaptable networks (DLCANs) were proposed to achieve programmable degradation and closed-loop recycling. From a CO-derived isocyanate monomers and two dynamic curing agents of aminophenyl disulfide (AFDS) and aminophenyl diimine (AFDI), bio-based thermosets of PU DLCANs were prepared. The bio-based PU DLCANs were ultra-stable and still kept cross-linked upon disadvantageous chemical stimuli instead of dissociating like the conventional CANs. Moreover, the bio-based PU DLCANs became degradable only by applying programmed stimuli, when the two types of dynamic cross-links in parallel connection were both broken step by step. Meanwhile, the broken cross-links could be reformed when dynamic bonds regenerated upon heating, leading to the closed-loop recycling of the bio-based PU DLCANs. Besides, being applied as coatings, the bio-based PU DLCANs exhibited UV shielding, self-healing properties and long-term stability as well as programming removable properties. The bio-based thermosets relied on DLCANs have the potential to guarantee the reliability of polymeric CANs and maintain the closed-loop recycling by applying programmable stimuli, which is quite meaningful and practical for life-time environmental-friendly coatings.