# Advanced Micro/Nanocapsules for Self-Healing Coatings
## 3 Self-Healing Coatings Based on Additives Including Healing Agents
In a similar study, also by [PERSON] and coworkers, emulsion polymerization followed by interfacial polymerization was used for the fabrication of chemically and thermally stable isophorone diisocyanate microcapsules [30]. The obtained microcapsules were embedded into an epoxy coating for carbon steel corrosion protection. Electrochemical characterization results showed an increase over time of the barrier properties of the modified coatings. Moreover, the produced coatings demonstrated self-healing properties that were attributed to the ability of the capsules to heal damaged areas in the coating and the formation of a protective polymeric barrier layer. The healing mechanism encompassed several reactions involving amine group formation due to partial conversion of isocyanate groups in the presence of water, the formation of polyurea due to an amine reaction with isocyanates, and finally the formation of a polyurethane layer. In another study, [PERSON] et al. reported the preparation of a smart coating with autonomous self-responding and self-healing functions without external intervention based on a simple integration microcapsule composed of one component into the matrix [31]. For this purpose, microcapsules were fabricated, including a hexamethylene diisocyanate solution, under photopolymerization. The formatted coatings were evaluated with respect to their morphology and thermal, corrosion, and mechanical resistance. The microcapsule-embedded coatings demonstrated improved self-repairing ability. Additionally, in the work of [PERSON] et al., healing agent-filled capsules with mean diameters as small as 450 nm and as large as 6 \(\upmu\)m were fabricated by in situ polymerization [32]. The produced core-shell micro/nanocapsules consisted of urea-formaldehyde as the shell and drying linseed oil as the core. Then, the micro/nanocapsules were embedded in a polymer in a content range of 91-94%. The obtained coatings were assessed with respect to their mechanical and corrosion behavior properties and showed improved performance in self-healing applications. The outcome of the experiments demonstrated a release of the healing materials from the micro/nanocapsules in paint films when cracks were formed, resulting in efficient crack healing with satisfactory protective properties. In situ polymerization of aniline onto CeO\({}_{2}\) nanoparticles was used for the fabrication of polyaniline/cerium dioxide nanocomposites by [PERSON] and coworkers [33]. The prepared nanocomposites were embedded into epoxy coatings and applied to carbon steel substrates, revealing excellent corrosion resistance. The enhancement of corrosion protection performance of the modified epoxy coatings was correlated to the synergetic protection and enhancement of the protective barrier and the hindrance of the diffusion of aggressive ions of the nanoparticles. Moreover, the self-healing protection improvement was associated with the redox behavior of polyaniline. Thus, the hybrid polyaniline/CeO\({}_{2}\) nanoparticles were considered the best route to enhance the protection performance of epoxy coatings on carbon steel. A very interesting concept was developed by [PERSON] et al. in order for microcapsules with high encapsulating capacity to be obtained [34]. These microcapsules had enhanced self-healing efficiency. For this purpose, graphene oxide (GO)-modified double-walled polyurea microcapsules were fabricated using 1,6-diaminohexane as the inner core and a prepolymer based on isophorone diisocyanate as the outer core. The synthetic process included as a first step the preparation of GO-modified prepolymer emulsion; then, the emulsion of 1,6-diaminohexane was prepared; after that, initial single-walled microcapsules were fabricated through interfacial polymerization between the two aforementioned emulsions; and finally, double-walled microcapsules and coating of the second core material were developed.