Self-repairing material is a branch of smart material that simulates the mechanism of self-repair of biological damage and self-repairs the damage caused by the material during use. Among many self-healing materials, the research and development of self-healing coatings that can protect the substrate and impart special properties to the substrate have become a hot topic in the scientific community. It is in the fields of conductive coatings, anti-corrosion coatings, scratch-resistant coatings, etc. It has a wide range of applications, especially in high-tech fields with harsh conditions, difficult to maintain, such as special adhesive coatings for aerospace and military applications, marine rigs and anti-corrosion coatings for underground oil pipelines. Urgent needs.
At present, the self-repair coatings are mainly divided into foreign-type self-healing coatings and intrinsic self-healing coatings. The foreign aid self-healing coating refers to self-repairing function in the coating matrix by introducing additional components such as microcapsules, carbon nanotubes, micro-vessels, glass fibers or nanoparticles containing a repairing system. The various repair agent systems are pre-embedded and then added to the matrix. When the material is damaged, the repair agent in the damaged area is released under external stimulation （force, pH, temperature, etc.）, thereby achieving self-repair. Intrinsic self-repair does not require an external repair system, but the coating material itself contains special chemical bonds or other physical and chemical properties such as reversible covalent bonds, non-covalent bonds, molecular diffusion, etc. to achieve self-repair function. The method does not depend on the repair agent, and the complicated steps such as the pre-repairing agent embedding technology are omitted, and the influence on the matrix performance is small, but the molecular structure design of the coating base material is the biggest challenge of the method, and has become a research focus. .
This paper summarizes the latest research progress in the field of self-repair coatings in recent years, focusing on the types, mechanisms and applications of self-repair coatings and intrinsic self-healing coating systems, and prospects for the application of self-healing coatings.
1. Foreign aid self-healing coating
1.1 micro/nano capsule filled self-healing coating
Microcapsule self-repairing method is currently the most widely used method in the field of self-repair coating. In 2001, White et al. reported the micro-capsule self-repair mechanism for the first time, and recently received extensive attention from researchers. The self-repairing mechanism of the micro/nano-capsule-filled self-healing coating is as shown in Fig. 1. The micro/nano capsule containing the repairing agent is preliminarily embedded in the polymer matrix or coating, and the substrate or coating material is damaged. At the time （light, heat, pressure, pH change, etc.）, the capsule ruptures and releases the repair agent. When the repair agent encounters the catalyst in the substrate or coating, the cross-linking curing reaction occurs, repairing the crack surface, and realizing the self-repair of the damaged part. . This method has been widely used in the field of coating materials.
1.1.1 Encapsulation Corrosion Inhibitor System: The microcapsuleization of corrosion inhibitors is used as a self-healing coating. It is mainly used in the field of metal anti-corrosion coatings. This method avoids toxicity and corrosion-resistant coatings. It is not suitable to directly add to the disadvantages of the coating. Kumar et al., Mehta et al. prepared microcapsules containing different types of corrosion inhibitors, discussed the effect of particle size of microcapsules on the stability of several different coating systems, and studied the release ability of corrosion inhibitors when microcapsules rupture. The coating containing the corrosion inhibitor microcapsules was applied to the steel sheet and showed good corrosion resistance. Zheludkevich et al. reported an environmentally friendly microcapsule with chitosan as the wall material and green corrosion inhibitor cesium ion as the core material. The pH change in the corrosive environment leads to the release of cerium ions to achieve the corrosion resistance of the coating. Koh et al. prepared microcapsules of polyurethane-coated isosorbide derivative corrosion inhibitors. The tests showed that the microcapsule-containing coatings have good antiseptic and self-repairing functions. Sauvant et al. proposed a self-repairing mechanism for inorganic film-forming corrosion inhibitors. Microcapsules with a particle size of 10 to 240 μm were prepared using MgSO4 as a core material, which was embedded in the coating material and applied to the surface of the steel. When corrosion occurred. When the microcapsules are broken, the released Mg2+ will automatically migrate to the crack under the action of the anode, and form Mg（OH）2 under a certain pH to seal the crack.
1.1.2 Encapsulated dry oil system: The self-repair coating prepared by using dry oil as a repairing agent is also the main trend of current research. The mechanism is that the dry oil released after the microcapsule is broken is contacted with air and then oxidized by oxygen. Self-healing membranes, the currently used dry oils are linseed oil and tung oil. Suryanarayana et al., Behzadnasab et al., Karan et al., Szabó et al., Majdeh et al. prepared micronized （20-150 μm） urea-formaldehyde resin microcapsules containing linseed oil and linseed oil/CeO2, respectively, and their preparation parameters such as stirring rate, The effect of reaction time on the formation of capsules was investigated. The effect of the amount of microcapsules on the mechanical properties of the coatings was investigated. The experimental results show that the microcapsules have sufficient strength to withstand certain shear during the preparation and spraying of the bond coat. The force is not destroyed; the surface of the microcapsule is rough, which is good for bonding with the bonding coating and the interface of the substrate; the microcapsule breaks when the coating is cracked and releases the repairing agent, and has good self-repairing and anti-corrosion properties.
Masoumeh et al. added micro/nano capsules containing linseed oil to the epoxy resin coating material. The smallest capsule size is 450 nm and the maximum is 6 μm. The study indicates that the addition of microcapsules at room temperature makes the coating material bond strength. There is a slight decrease in flexibility and flexibility, and the flexibility is greatly reduced at high temperatures, and the coating exhibits good self-healing properties to the metal. Eshaghi et al. prepared microcapsules of silane coupling agent modified vinyl cellulose coated linseed oil with a particle size of 5～35μm. The grafting efficiency of silane coupling agent and vinyl cellulose was discussed. The presence of the crosslinking agent provides good interfacial adhesion between the microcapsules and the water-based acrylic resin coated substrate. Zhao Peng et al. prepared microcapsules with a particle size of 1-50 μm using tung oil as the core material, applied it to a 150 μm thick coating and applied it to the surface of tinplate. The coating was observed to have good self-healing by dispersing the red indicator. Corrosion resistance.
1.1.3 Encapsulation Reactive Repair System: The repair agent such as dicyclopentadiene （DCPD）, epoxy resin, silicone series reagents and reagents with special functional groups are coated in the microcapsules. The reactivity is released from the microcapsules and contacted with the catalyst or initiated by ultraviolet light, heat, oxygen, etc., and polymerization occurs to form a crosslinked structure to bond the cracks to achieve self-repair. Among them, epoxy resin is used as a self-healing agent. For example, Liu et al. added microcapsules with an epoxy resin as an epoxy resin coating, and the coating uses an amide curing agent to cure the coating resin. On the other hand, the excess amide can be polymerized with the epoxy resin repairing agent released from the broken microcapsule to realize self-repairing function, and the coating material has good self-repairing property and good anticorrosive effect on carbon steel. Liao et al. used urea-formaldehyde resin coated E-51 epoxy resin microcapsules as a repair system to prepare epoxy resin self-healing coatings, which also showed good self-healing effect. Self-healing coatings containing silicone-based repairing agent microcapsules have also been reported. The photosensitizer is added to the vinyl group on the molecular chain of the repairing agent, and some photosensitizers are added. Under the action of external force, the repairing agent overflows when the microcapsules are destroyed, under the action of ultraviolet radiation. The repair agent reacts quickly to achieve self-healing of the coating. Song et al. prepared microcapsules containing functional dimethicone repair agents. The system can initiate self-healing under ultraviolet or sunlight illumination, is environmentally friendly, and can be photo-initiated multiple times. Self-repairing, this is the first report of the capsule-type repeatable self-healing system. Huang et al [19,20] prepared microcapsules with perfluorooctyltriethoxysilane as a repairing agent. The capsules have a particle size of 40-400 μm. Electrochemical experiments confirmed that these repairing agents have good self-coating materials. It has good repair performance and good corrosion resistance to steel. Its self-repair mechanism is realized by hydrolyzing the repair agent to form a network structure. In addition, they also prepared polyurethane （PU） coated hexamethylene diisocyanate microcapsules, discussed the effect of microcapsule size and content on the self-healing properties of the coating, they found that the microcapsule particle size is not less than 100μm When the mass fraction of microcapsules is not less than 5%, the coating has good self-repairing and anti-corrosion effects.
1.2 micro/nano container filled self-healing coating
There are many reports on the application of micro-nano-container-loaded corrosion inhibitors such as hollow micro-nanospheres or mesoporous microspheres in the field of self-repairing anti-corrosion coatings.
If a layer-by-layer assembly method is adopted, nano-SiO2, kaolin or porous nano-TiO2 particles are used as a core, and a nano-active unit of a multilayer polyelectrolyte containing a corrosion inhibitor benzotriazole （BTA） is deposited on the outer layer to prepare a metal anticorrosive coating. When corrosion occurs, the change in pH （the chemical corrosion process is mostly accompanied by a change in pH） causes a change in the structure and permeability of the polyelectrolyte layer of the active unit, releasing a corrosion inhibitor, forming an adsorption layer on the metal surface, and passivating the metal surface. , effectively preventing the corrosion of metals. Fu et al. prepared SiO2 microspheres loaded with preservative caffeine molecules and modified pH-sensitive ferrocyanuric acid cucurbituril on the surface to achieve controlled release of corrosive agents under different acid-base conditions. The aluminum alloy surface anti-corrosion coating has a good self-repairing effect. Zhao et al. prepared a hollow raspberry-type polystyrene submicron sphere with open pores on the surface, and the microspheres were loaded with a corrosion inhibitor BTA. The surface pores of the microcapsules were opened under acid and alkali conditions and closed under neutral conditions. A controlled release of the BTA is achieved. The submicron capsule is applied to a polyurethane anticorrosive coating and applied to a copper metal surface to exhibit a good anticorrosive function. Li et al. prepared a silicon/polymer double-walled mixed nanotube container. Porous silicon was used as the inner wall of the container, and the polymer layer was used as the outer wall. Different polymer outer layers could be selected to achieve controlled release of the core material. Value-sensitive, temperature-sensitive, and redox-responsive silicon/polymer double-walled nanocontainers, loaded with a corrosion inhibitor benzotriazole in a nanotube container, provide a self-healing coating that exhibits good self-healing capabilities. Rahimi et al.  prepared a silicone nanocontainer containing a mixture of 2-mercaptobenzothiazole （MBT） or 2-mercaptobenzimidazole （MBI） and α-cyclodextrin （α-CD）. , MBT or MBI and α-CD can form hydrogen bonds when encountering a humid environment, thereby playing a self-repairing effect. The nano-container is applied to the aluminum surface coating to study its anti-corrosion and self-repairing properties. With.
Borisova et al. used mesoporous silica as a container and loaded a corrosion inhibitor in the container. The effect of the size of the nano-container on the self-healing properties of the coating was investigated. Recently, Chen et al. reported a UV-controlled release mesoporous silica nano-container filled with a corrosion inhibitor benzotriazole. The mesoporous structure of the silica surface can be introduced by introducing an azobenzene functional group. The chemical structure of the mesopores can be changed under the irradiation of ultraviolet light to realize the opening and closing of the mesopores, and in this way, not only the release amount of the preservative can be controlled, but also the self-repair of the coating can be achieved.
1.3 shape memory fiber / polymer self-healing coating
The shape memory fiber is a metal alloy or polymer having a shape memory effect, and after being deformed by an external force, the material is heated to a certain temperature to restore the original shape. For example, the shape memory polymer fiber is embedded in the epoxy resin material together with the thermoplastic particles, wherein the shape memory fiber serves as a skeleton structure of the self-healing system, and the thermoplastic resin acts as a repairing agent. When the material is cracked, the damage is heated to above the glass transition temperature of the shape memory fiber, and the previously stretched fiber filament shrinks due to the shape memory effect, and the base material is pulled down by the action of the contraction force to close the crack. The thermoplastic resin particles are heated to the melting temperature to start to flow, and the cracks are filled to finally achieve self-repair. The research group of Harbin Institute of Technology has also studied a large number of shape memory polymers, including shape memory epoxy polymer （SMEP） based on thermoplastic polycaprolactone （PCL） as a repair agent. Functional shape memory polymer, which can achieve 3 cycles of repair at the damage, and the repair efficiency is up to 67.87%, which has great application value.
2. Intrinsic self-healing coating
Intrinsic self-healing coating means that the coating material itself contains special chemical bonds or functional groups, which can be self-repaired by chemical bond recombination, functional group reaction or physical action after the destruction. Compared with the foreign aid self-healing coating, this method does not have a great influence on the mechanical properties of the coating material matrix because there are no additional substances such as microcapsules, micro-containers, etc., but it involves the coating of the matrix material. Modification, etc., so the preparation difficulty is higher than the foreign aid self-repair system.
2.1 UV-induced self-healing coating
Ghosh et al. prepared a polyurethane coating with self-healing function, and the self-repair mechanism is shown in Fig. 2. The polyurethane network structure in the coating contains chitosan and oxetane structures. When the surface of the coating is scratched, the ring structure of oxetane is broken, and two ends which can generate chemical reactions are exposed. When exposed to ultraviolet light, the exposed ends of chitosan and oxetane in the coating attract and combine to repair the ring structure, thereby achieving self-repair of the damage of the coating.
Supramolecular polymers are materials that perform self-healing under UV light. Coulibaly et al. prepared a supramolecular polymer which was chelated by a short-chain polymer with a telechelic structure and a metal ligand （zinc or ruthenium）. The metal ligand was oligomerized with low relative molecular mass. The substances are connected by non-covalent bonds （ion bonds）. When ultraviolet light is irradiated, the energy absorbed by the metal ligand is converted into heat, the non-covalent bond is broken, and the metal ligand is temporarily separated from the polymer, so that the relative molecular mass of the polymer is lowered, the viscosity is lowered, and the flowable state is obtained. When the material is cracked or damaged, after the ultraviolet light is irradiated in the damaged area, the molecules in the flowable state can fill the damaged area and achieve self-repair. In the experiment, a 200 μm deep scratch was drawn on a 400 μm thick plastic coating. After 2 times of irradiation under ultraviolet light, the scratches can be repaired well after 30s, and the repair efficiency can reach 100%±36%.
Wang et al. developed a polydimethylsiloxane-polyurethane （PDMS-PUR） and polyethylene glycol-polyurethane （PEG-PUR） network structure using CuCl2 as a catalyst and UV-induced self-healing. Self-repair is achieved by the reorganization and conformational changes of supramolecular or covalent bonds initiated by ultraviolet light irradiation.
2.2 Thermally reversible cross-linking self-healing coating
The thermoreversible cross-linking self-healing coating mainly relies on the characteristic functional group material in the coating matrix which can undergo the Diels-Alder reversible cross-linking reaction, and the self-repairing of the coating is achieved by the DA reversible reaction. The reversible reaction mechanism of DA is shown in Fig. 3. . Wouters et al. prepared a copolymer of furan methacrylate （FMA） and butyl methacrylate （BMA） by free radical copolymerization. The functionality of the copolymer （hardness, modulus of elasticity） can be adjusted by adjusting the ratio of FMA to BMA. , crosslink density） and glass transition temperature, using this copolymer and bismimide to prepare a powder, the powder is applied to the surface of aluminum to prepare a self-repairing powder coating, and the powder coating is heated to 175 ° C The polymer film layer is formed and cooled to room temperature. When the coating is scratched to cause damage, the polymer film is reheated to 175 ° C, and the film reflows, and the damaged area can be repaired in 30 seconds. Multiple repairs are possible and have no effect on matrix performance. Pratama et al. prepared a self-healing thermosetting resin coating based on DA reaction. They microencapsulated the monomeric maleimide that can undergo DA reaction, and introduced another monomeric difuran capable of DA reaction into the coating. A furan-functionalized epoxy resin coating is prepared in the matrix. The experimental results show that the self-repairing efficiency of the coating of 10% microcapsules with a particle size of 185 μm can reach 71%. Postiglione et al. prepared a trifunctional and difunctional furan resin and a bismaleimide self-healing coating system. The system undergoes a DA reaction at 50 ° C, a reverse DA reaction occurs at 120 ° C, and passes through the coating matrix. The plasticizer benzyl alcohol was added to improve the self-repairing performance. The experimental results show that the coating system can achieve 48% mechanical strength recovery.
2.3 layer assembly self-repairing polymer film
The layer-by-layer self-repairing polymer film is based on the intermolecular non-covalent force. The composite coating film is assembled through a reciprocating interface, and various types of functional groups are introduced into the coating film to control the mechanical properties and self-repairing properties of the coating film. Andreeva et al. prepared a layer-assembled self-healing film containing a repairing agent. They assembled the anti-corrosion agent 8-hydroxyquinoline into the polymer film layer. After the coating was damaged, it passed the movement and resistance of the polymer segment. The overflow of the etchant achieves self-healing of the layer-assembled polymer film and has good corrosion resistance. The Sun Junqi team of Jilin University constructed a branched polyethyleneimine （bPEI）/polyacrylic acid （PAA） polyelectrolyte coating using an exponentially increasing layer self-assembly method. The coating can be scratched at a width of 50 μm. Self-repair is completed within 10s. Self-repair can be achieved by immersing the coating in water or spraying water on the scratched surface. At the same time, （bPEI/PAA）*30 film can achieve multiple scratches and self-repair at the same position. The self-repair mechanism is that during the preparation of the film layer, the intercalation of the polymer chain in the bPEI/PAA film can be regulated, and the prepared film layer is stable in the air, and the polymer chain can be in water or in a humid environment. Flow or swelling occurs to repair the damaged area. Hu Xiaoxia et al. prepared a polyurethane/carboxymethylcellulose sodium （PU/CMC） multilayer film by layer-by-layer assembly technology, which has self-repairing ability. At the same time, they introduced a third polyelectrolyte polydimethyldiallylammonium chloride （PDDA） into the membrane structure. The prepared PDDA（CMC/PU）n film showed enhanced self-healing effect in physiological Self-repairing of scratches with a width of 20 to 30 μm can be performed within a few seconds of soaking in salt water.
The research in the field of self-healing coatings has developed rapidly in the past 10 years. The current and future research focuses on the optimization of the original self-repairing system, the discovery of new self-repairing mechanisms, and the design of recyclable self-repairing materials. And the construction and application of self-healing coating materials. The research in this field involves the intersection of chemistry, materials science and mechanics, and more research enthusiasts are needed. It is believed that self-repair technology will have broad application prospects.