Self-Healing Coatings

Coatings allow the preservation and enhancement of a material's properties. In this context, they can also provide protection against environmental influences. For this reason, when damage typically occurs in the form of micro-cracks, environmental elements such as water and oxygen can penetrate through the coating and cause material damage or degradation. Micro-cracks in coatings can lead to mechanical deterioration or delamination of the coating, and in fiber-reinforced composites and microelectronics, to electrical failure. Despite their very small scale, repair of such damage is generally difficult and costly. For this reason, a coating that can automatically heal itself (self-healing coating), with automatic recovery properties (such as mechanical, electrical and aesthetic properties), can be very beneficial in extending the service life of the coating and thus the material as well. In the literature, most approaches described in relation to self-healing materials are viewed as the addition of microcapsules and reversible physical bonds such as hydrogen bonding, ionomers and chemical bonds. Microencapsulation is the most common method for developing self-healing coatings. In self-healing epoxy polymers, the capsule approach using microencapsulated dicyclopentadiene (DCPD) monomer and Grubbs catalyst was later adapted to epoxy adhesive films widely used in the aerospace and automotive industries for bonding metallic and composite substrates. Recently, microencapsulated metal or carbon black liquid suspensions have been used to recover electrical conductivity in multilayer microelectronic devices and battery electrodes. However, the use of microencapsulation for restoration of electrical properties in coatings is limited. In addition, liquid metal micro-droplets have been suspended in silicone elastomer to create stretchable electrical conductors that mimic the elasticity of soft biological tissue and maintain electrical conductivity when damaged. The most common application of this technique has been demonstrated in polymer coatings for corrosion protection. Protection of metallic materials against corrosion is of great importance on economic and ecological scales. Corrosion protection in polymer coatings using microcapsules has been proven effective by researchers who have encapsulated a range of materials. These materials include monomers such as isocyanates, DCPD GMA epoxy resin, linseed oil and tung oil. As mentioned above, shell materials such as phenol formaldehyde, urea formaldehyde, dendritic or PAMAM, melamine formaldehyde, etc. have been used for encapsulation of the core. Each shell material has its own characteristics. Microencapsulation has been proven to effectively protect metal against corrosion and extend the service life of a coating. Self-healing coatings repair both surface scratches and mesoscopic damage (e.g. micro-cracks and cavitation). Self-healing is a two-stage process: void closure followed by healing. Self-healing functionality provides high-degree scratch repair and gloss recovery even in repeatedly damaged areas. For example, self-healing epoxies can be incorporated into metals to prevent corrosion. In one test, a substrate metal exhibited significant deterioration and rust formation after 72 hours of exposure, but after being coated with self-healing epoxy, no visible damage was observed after the same 72-hour exposure. These coatings can automatically correct damage, reduce inefficiency caused by degradation over time, and also prevent costs resulting from material failure. The ability of self-healing coatings to autonomously repair localized damage caused by external factors is an important factor contributing to the attractiveness of this subject. Self-healing coatings include polymer layers, silica-organic layers, conversion layers, metallic layers and ceramic layers. These coatings respond intelligently to mechanical or chemical damage caused by the external environment and reproduce their original properties, including adhesion to the substrate and integrity. The self-healing capability is a particularly vital property for coatings designed to protect a structure's material against corrosion. Self-healing corrosion-preventive coatings are produced using macromolecular compounds such as ceramics, metals and composites. The properties of such coatings are activated by suitable stimuli such as temperature, radiation, pH, pressure changes and/or mechanical motion. Prepared by: B. Serhat Cengiz

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