Moisture-triggered self-healing of a polyurethane coating based on isocyanate–oxazolidine-loaded microcapsules synthesized via thiol-ene photopolymerization without CO2 release

Abstract

The self-healing properties of the material synthesized based on conventional isocyanate microcapsules as a healing agent are most likely degraded by the release of CO2 during self-healing. Herein, we report the self-healing capability in a moisture environment of a particle-encapsulated isocyanate that is activated through the curing process involving an isocyanate agent. A novel isocyanate–oxazolidine compound loaded-spherical microcapsule, with a shell thickness of 10 μm and a median diameter of 151.4 μm, is first prepared via interfacial photoinitiated thiol-ene click chemistry in a stabilized aqueous emulsion. The encapsulation of the synthesized oxazolidine enables a significant improvement in the thermal stability of the microcapsules. Furthermore, the evaluation and analysis of the self-healing properties of the polyurethane coatings based on the isocyanate–oxazolidine-loaded microcapsules provide the evidence to elucidate the self-healing pathway on the molecular level. The oxazolidine preferentially undergoes a ring-opening reaction, yielding amino and hydroxyl compounds that further react with isocyanate to achieve autonomous self-healing in a moisture environment. This study opens up a route to develop isocyanate-encapsulated self-healing materials for practical applications.