UV-triggered self-healing
of single microcapsules has been a good candidate to enhance the life
of polymer-based aerospace coatings because of its rapid healing process
and healing chemistry based on an accurate stoichiometric ratio. However,
free radical photoinitiators used in single microcapsules commonly
suffer from possible deactivation due to the presence of oxygen in
the space environment. Moreover, entrapment of polymeric microcapsules
into coatings often involves elevated temperature or a strong solvent,
probably leading to swelling or degradation of polymer shell, and
ultimately the loss of active healing species into the host matrix.
We herein describe the first single robust SiO2 microcapsule
self-healing system based on UV-triggered cationic polymerization
for potential application in aerospace coatings. On the basis of the
similarity of solubility parameters of the active healing species
and the SiO2 precursor, the epoxy resin and cationic photoinitiator
are successfully encapsulated into a single SiO2 microcapsule
via a combined interfacial/in situ polymerization. The single SiO2 microcapsule shows solvent resistance and thermal stability,
especially a strong resistance for thermal cycling in a simulated
space environment. In addition, the up to 89% curing efficiency of
the epoxy resin in 30 min, and the obvious filling of scratches in
the epoxy matrix demonstrate the excellent UV-induced healing performance
of SiO2 microcapsules, attributed to a high load of healing
species within the capsule (up to 87 wt %) and healing chemistry based
on an accurate stoichiometric ratio of the photoinitiator and epoxy
resin at 9/100. More importantly, healing chemistry based on a UV-triggered
cationic polymerization mechanism is not sensitive to oxygen, extremely
facilitating future embedment of this single SiO2 microcapsule
in spacecraft coatings to achieve self-healing in a space environment
with abundant UV radiation and oxygen.