Resorcinol-formaldehyde resin with donor–acceptor couples to enhance the long-term service stability of a single UV-triggered self-healing SiO2@RF microcapsule for potential application in a UV-sufficient space environment

Microcapsules containing a liquid photoinitiated core exhibit a promising self-healing ability for the microcracks in polymeric spacecraft coatings by utilizing plentiful UV radiation in space. However, their wide applications in UV-sufficient environment are always hindered by UV radiation penetrating into the weak UV-shielding shell to trigger undesirable photocuring of the liquid core before self-healing. Herein, through an in-situ intra-mesopore thermal deposition strategy, we successfully developed the novel SiO2@resorcinol–formaldehyde (RF) microcapsules containing a UV-responsive self-healing core and strong UV-blocking SiO2@RF shell, thereby enhancing their long-term service stability against UV radiation. It was confirmed that the unique UV-absorbing ability of RF resin into mesoporous SiO2 shell is due to photo-induced charge transfer from the inherent benzenoid resorcinol units as electron donors (D) to partially oxidized resorcinol, quinoid units, as electron acceptors (A). As expected, the epoxy coating embedded with UV-aged SiO2@RF microcapsules for up to 48 days preserves good self-healing ability. Moreover, phenolic resins prepared from 3-aminophenol and hydroquinone also exhibit good UV-absorbing ability because their D-A coupled features were similar to that of the RF resin. The deposition of phenolic resins with D-A couples onto UV-triggered self-healing microcapsules is an efficient strategy to improve their long-term service stability in spacecraft coatings.