# Advanced Micro/Nanocapsules for Self-Healing Coatings
## 3 Self-Healing Coatings Based on Additives Including Healing Agents  
One effective approach to obtaining coatings with self-healing properties is the use of additives, including healing agents. Healing agents refer to substances, compounds, or factors that initiate and encourage the healing process in the damaged coatings. In the study of [PERSON] and his team, a self-healing carbon/epoxy composite including healing agent loaded into core-shell nanofibers between carbon fiber fabric layers was fabricated [26]. A coaxial electrospinning method was used for healing agents based on low-viscosity epoxy resin together with its amine-precursor curing agent to be encapsulated into nanofibers, which consisted of copolymers of acrylonitrile and styrene. The encapsulation yield reached up to 97%. Mechanical studies confirmed that the core-shell nanofiber incorporation into the epoxy coating did not reduce the overall composite mechanical properties. On the other hand, the embodiment of the aforementioned nanofibers between carbon layers imparted a self-healing ability, resulting in the self-repairing and restoring of the composite mechanical properties. Additionally, [PERSON] and coworkers examined [27] the efficient and facile encapsulation of a lubricant oil that acts as a core material into microcapsules of SiO\({}_{2}\) shell wrapped with polystyrene. The fabrication of the loaded microcapsules included Triton X-100 together with 3-isocyana-topropyl triethoxysilane and fumed SiO\({}_{2}\) nanoparticles to be used as emulsifiers for the formation of oil/water Pickering emulsions via the Pickering polymerization technique. The produced microcapsules were spheres with an average diameter of 3.3 \(\upmu\)m and a shell thickness of roughly 900 nm. Moreover, the lubricant oil demonstrated improved thermal stability with a decomposition temperature of 250 \({}^{\circ}\)C. The obtained core-shell microcapsules were embedded into an epoxy coating, and the final self-lubricating coating was applied onto aluminum substrates. The produced coatings composed of SiO\({}_{2}\)-wrapped PS microcapsules/epoxy composites demonstrated improved self-lubricating properties, which were ascribed to the synergetic effect between lubricant oil and SiO\({}_{2}\) nanoparticles, making a positive contribution to enhancing the tribological properties of the polymer coatings. A very interesting concept was developed by [PERSON] and his team [28]. Solid wastes, such as fluorinated silica gel waste and polystyrene (PS) foam waste, were recycled and reused for the fabrication of a hydrophobic coating with self-healing properties. First, silica gel waste and PS foam waste were used as raw materials for the superhydrophobic coating fabrication. Second, near-infrared-responsive microcapsules were synthesized using fluorinated carbon nanoparticles via the method of electrostatic adsorption onto the surface of microcapsules. Then, the obtained particles were used as fillers with hydrophobic properties together with the polystyrene foam waste that acted as a coating binder. The produced superhydrophobic coating was applied onto several different substrates via a doctor blade and presented robust durability and enhanced self-healing properties. Further studies have evaluated the mechanism of the microcapsules that are applied to the interlaminar interface during the synthesis of carbon fiber composites [29]. Microcapsules with enhanced thermal and mechanical properties composed of a high-boiling-point organic solvent and a diglycidyl ether bisphenol A-based epoxy resin were fabricated via in situ polymerization of formaldehyde and urea and applied to carbon fiber composite structural materials in order to improve their self-healing performance. The results showed enhanced self-healing properties when the aforementioned microcapsules were used together with Lewis acidic catalysts due to the encapsulated epoxy monomer polymerization.