UV-responsive single-microcapsule self-healing material with enhanced UV-shielding SiO2/ZnO hybrid shell for potential application in space coatings

UV-responsive single-microcapsule self-healing systems exhibit considerable potential to effectively repair the microcracks and then extend the service life and reliability of polymeric composite coatings on spacecraft because of abundant UV irradiation in space environment and the stable healing process based on an accurate stoichiometric ratio. Unfortunately, UV irradiation in space environment also could cause undesirable curing of healing agents in the microcapsules embedded into the coatings before the generation and UV-responsive self-healing of the microcracks in these coatings and therefore deactivation of healing agents. Herein, we propose a UV-responsive single-microcapsule self-healing system with epoxy resin core and enhanced UV-shielding SiO2/ZnO hybrid shell for polymer coatings through a layer-by-layer deposition strategy. Homogenous embedment of ultrasmall ZnO nanocrystals into the mesoporous SiO2 shell endows the microcapsules with excellent UV-shielding performance. Moreover, in contrast to conventional polymer shell, our stable inorganic SiO2 counterpart could avoid the degradation fate catalyzed by photo-responsive ZnO or TiO2 nanoparticles, thereby enhancing the stability of the microcapsule. As expected, when the SiO2/ZnO microcapsules after aging up to 21 days under UV radiation were embedded in the epoxy coating, the epoxy coating containing these aged microcapsules still displays good self-healing capability upon scratching and UV-induced self-healing, and then scratches are still filled and repaired, attributing to the shielding ability of ultrasmall ZnO nanocrystals highly dispersed into the SiO2 shell from undesirable UV-induced curing of epoxy resin core before self-healing behavior in the UV-sufficient space environment.