Research progress of self-healing coating materials

　　Self-repairing materials are a branch of smart materials that mimic the mechanism of self-repairing damage in organisms to repair damage to materials during use. Among numerous self-repairing materials, the research and development of self-repairing coatings that can protect the substrate and impart special properties to the substrate has become a hot topic in the scientific community. It has wide applications in conductive coatings, anticorrosion coatings, scratch-resistant coatings, etc., especially in some high-end fields with harsh conditions and difficult maintenance and repair, such as special adhesive coatings used in aerospace and military marine, anticorrosion coatings for offshore drilling platforms and underground oil pipelines, etc., there is an urgent need.

　　Currently, self-healing coatings are mainly divided into external self-healing coatings and intrinsic self-healing coatings according to the repair type. External self-healing coatings refer to the realization of self-healing function by introducing external components such as microcapsules containing a repair agent system, carbon nanotubes, micro-vessels, glass fibers, or nanoparticles into the coating matrix. This method requires pre-embedding various repair agent systems and then adding them to the matrix. When the material is damaged, the repair agent in the damaged area is released under external stimuli (force, pH value, temperature, etc.), thus achieving self-repair. Intrinsic self-healing does not require an external repair system, but the coating material itself contains special chemical bonds or other physicochemical properties such as reversible covalent bonds, non-covalent bonds, molecular diffusion, etc., to achieve self-healing function. This method does not rely on repair agents, eliminating complex steps such as pre-embedding repair agents, and has little effect on the substrate performance, but the molecular structure design of the coating matrix material is the biggest challenge facing this method, and it has become a research focus.

　　This article summarizes the latest research progress in the field of self-healing coatings in recent years, focusing on the types, mechanisms, and applications of external and intrinsic self-healing coating systems, and prospects the application prospects of self-healing coatings.

 

　　1. External self-healing coatings

　　1.1 Micro/nanocapsule-filled self-healing coatings

　　The microcapsule self-healing method is currently the most widely used method in the field of self-healing coatings. Since White et al. first reported the microcapsule self-healing mechanism in 2001, it has recently received extensive attention from scientific researchers. The self-healing mechanism of micro/nanocapsule-filled self-healing coatings is shown in Fig. 1. Micro/nanocapsules containing a repair agent are pre-embedded in the polymer matrix or coating. When the matrix or coating material is damaged (initiated by light, heat, pressure, pH change, etc.), the capsules rupture and release the repair agent. When the repair agent encounters a catalyst in the matrix or coating, a crosslinking curing reaction occurs, repairing the crack surface and achieving self-repair of the damaged area. This method has been widely used in the field of coating materials.

 

 

　　1.1.1 Encapsulated corrosion inhibitor system: Using microencapsulated corrosion inhibitors as self-healing coatings, mainly used in the field of metal anticorrosion coatings. This method avoids the disadvantages of high toxicity of corrosion inhibitors and the destruction of coating stability, which are not suitable for direct addition to the coating. Kumar et al., Mehta et al. prepared microcapsules containing different types of corrosion inhibitors, discussed the influence of the particle size of microcapsules on the stability of several different coating systems, studied the release ability of corrosion inhibitors when the microcapsules ruptured, and applied the coating containing microcapsules of corrosion inhibitors to steel plates, showing good anticorrosion effects. Zheludkevich et al. reported an environmentally friendly microcapsule with chitosan as the wall material and green corrosion inhibitor cerium ions as the core material. The pH change in the corrosive environment leads to the release of cerium ions to achieve the anticorrosion performance of the coating. Koh et al. prepared microcapsules of polyurethane-encapsulated isosorbide derivative corrosion inhibitors. Experiments showed that the coating containing microcapsules has good anticorrosion and self-healing functions. Sauvant et al. proposed an inorganic film-forming corrosion inhibitor self-healing mechanism. Microcapsules with a particle size of 10-240 μm were prepared using MgSO4 as the core material, embedded in the coating material, and coated on the steel surface. When corrosion occurs, the microcapsules rupture, and the released Mg2+ automatically migrates to the crack under the anodic effect and deposits to form Mg(OH)2 under a certain pH effect, sealing the crack.

　　1.1.2 Encapsulated drying oil system: Using drying oil as a repair agent to prepare self-healing coatings is also a major current research trend. The mechanism is that the drying oil released after the microcapsule rupture is oxidized by oxygen after contact with air to form a self-healing film layer. Currently, the more commonly used drying oils are linseed oil and tung oil. Suryanarayana et al., Behzadnasab et al., Karan et al., Szabó et al., and Majdeh et al. prepared micro-sized (20-150 μm) urea-formaldehyde resin microcapsules containing linseed oil and linseed oil/CeO2. They investigated the effects of preparation process parameters such as stirring rate and reaction time on capsule formation, and investigated the effect of the amount of microcapsules added on the mechanical properties of the coating. The experimental results show that the microcapsules have sufficient strength and can withstand a certain shear force without being damaged during the preparation and spraying of the adhesive coating; the surface of the microcapsules is rough, which is conducive to good bonding between the adhesive coating and the substrate interface; when cracks occur in the coating, the microcapsules rupture and release the repair agent, exhibiting good self-healing and anticorrosion properties.

　　Masoumeh et al. added micro/nanocapsules containing linseed oil to epoxy resin coating materials. The smallest particle size of the capsules was 450 nm, and the largest was 6 μm. The study pointed out that at room temperature, the addition of microcapsules caused a slight decrease in the bonding strength and flexibility of the coating material, while at high temperatures, the flexibility decreased significantly, and the coating showed good self-healing performance on metals. Eshaghi et al. prepared microcapsules of silane coupling agent-modified vinyl cellulose-coated linseed oil with a particle size of 5-35 μm. They focused on the grafting efficiency of silane coupling agents and vinyl cellulose. The presence of silane coupling agents makes the microcapsules have good interfacial bonding performance with the water-based acrylic resin coating matrix. Zhao Peng et al. prepared microcapsules with a particle size of 1-50 μm using tung oil as the core material, applied them to a 150 μm thick coating, and applied them to the surface of tinplate. The self-healing and anticorrosion performance of the coating was observed through a dispersed red indicator.

　　1.1.3 Encapsulated Reactive Repair Agent System: Repair agents such as dicyclopentadiene (DCPD), epoxy resin, organosilicon series reagents, and reagents with special functional groups are encapsulated in microcapsules. These reagents have certain reactivity and, after being released from the microcapsules, will undergo a polymerization reaction to form a cross-linked structure to bond the cracks upon contact with a catalyst or under the initiation of ultraviolet light, heat, or oxygen, thus achieving self-healing. Epoxy resin is frequently reported as a self-healing agent. For example, Liu et al. added microcapsules with epoxy resin as the repair agent to an epoxy coating. The coating uses an amide-based curing agent, which on the one hand cures the coating resin, and on the other hand, the excess amide can polymerize with the epoxy resin repair agent released from the ruptured microcapsules to achieve self-healing. This coating material has good self-healing properties and good corrosion resistance to carbon steel. Liao et al. prepared an epoxy resin self-healing coating using a urea-formaldehyde resin-coated E-51 epoxy resin microcapsule as the repair system, which also showed good self-healing effects. Self-healing coatings containing organosilicon series repair agent microcapsules have also been reported. Utilizing the reactivity of the vinyl group on the repair agent molecule chain, some photosensitizers are added. When the microcapsules are broken under external force, the repair agent overflows, and under ultraviolet radiation, the repair agent can react rapidly to achieve coating self-healing. Song et al. prepared microcapsules containing polydimethylsiloxane repair agents with functional ends. This system can initiate polymerization under ultraviolet or sunlight irradiation to achieve self-healing, is environmentally friendly, and can achieve multiple self-healing through photoinitiation. This is the first reported capsule-type repeatable self-healing system. Huang et al. [19, 20] prepared microcapsules using perfluorooctyltriethoxysilane as the repair agent, with a capsule size of 40-400 μm. Electrochemical experiments confirmed that this type of repair agent has good self-healing properties for coating materials and good corrosion resistance to steel. Its self-healing mechanism is achieved through the formation of a network structure after hydrolysis of the repair agent. In addition, they also prepared microcapsules of polyurethane (PU) coated hexamethylene diisocyanate and discussed the influence of microcapsule size and content on the self-healing performance of the coating. They concluded that the coating only has good self-healing and corrosion resistance when the microcapsule size is no less than 100 μm and the mass fraction of microcapsules is no less than 5%.

　　1.2 Micro/Nano Container Filling Self-Healing Coatings

　　There are many reports on the application of hollow micro/nanospheres or mesoporous microspheres as micro/nano containers to load corrosion inhibitors in self-healing anticorrosion coatings.

　　For example, using a layer-by-layer assembly method, nano-active units are prepared using nano-SiO2, kaolin, or porous nano-TiO2 particles as the core and depositing multi-layered polyelectrolytes containing the corrosion inhibitor benzotriazole (BTA) on the outer layer to prepare metal anticorrosion coatings. When corrosion occurs, the change in pH (most chemical corrosion processes are accompanied by changes in pH) causes changes in the structure and permeability of the active unit polyelectrolyte layer, releasing the corrosion inhibitor and forming an adsorption layer on the metal surface, passivating the metal surface, and effectively preventing metal corrosion. Fu et al. prepared SiO2 microspheres loaded with the corrosion inhibitor caffeine molecules and modified their surface with ferrocene-based cucurbituril with pH sensitivity to achieve controllable release of the corrosion inhibitor under different acid-base conditions. This was applied to the anticorrosion coating on the surface of aluminum alloy, showing good self-healing effects. Zhao et al. prepared hollow raspberry-like polystyrene submicron spheres with open pores on the surface. The microspheres were loaded with the corrosion inhibitor BTA. The surface pores of the microcapsules open under acidic and alkaline conditions and close under neutral conditions, thus achieving controllable release of BTA. The application of this submicron capsule to polyurethane anticorrosion coatings on copper metal surfaces showed good anticorrosion performance. Li et al. prepared silicon/polymer double-walled hybrid nanotube containers, with porous silicon as the inner wall and the polymer layer as the outer wall. Different polymer outer layers can be selected to achieve controllable release of the core material. They prepared silicon/polymer double-walled nanocontainers with pH sensitivity, temperature sensitivity, and redox responsiveness, respectively. Loading the corrosion inhibitor benzotriazole in the nanotube containers, self-healing coatings with good self-healing performance were prepared. Rahimi et al. [26] prepared organosilicon nanocontainers containing a mixture of two corrosion inhibitors, 2-mercaptobenzothiazole (MBT) or 2-mercaptobenzimidazole (MBI), and α-cyclodextrin (α-CD). When encountering a humid environment, MBT or MBI and α-CD can form hydrogen bonds, thus playing a self-healing role. The application of this nanocontainer to aluminum surface coatings showed significant anticorrosion and self-healing performance.

　　Borisova et al. used mesoporous silica as a container to load corrosion inhibitors and investigated the influence of nanocontainer size on the self-healing performance of the coating. Recently, Chen et al. reported a mesoporous silica nanocontainer with ultraviolet light-controlled release. The container is filled with the corrosion inhibitor benzotriazole. The mesoporous structure on the surface of silica can be achieved by introducing azobenzene functional groups. This type of functional group can change its chemical structure under ultraviolet light irradiation, thus achieving the opening and closing of the mesopores. In this way, not only the release amount of the anticorrosion agent can be controlled, but also multiple self-healing of the coating can be achieved.

　　1.3 Shape Memory Fiber/Polymer Self-Healing Coatings

　　Shape memory fibers are metal alloys or polymers with shape memory effects. After deformation under external force, the material can recover its original shape when heated to a certain temperature. For example, if shape memory polymer fibers and thermoplastic particles are embedded in an epoxy resin material, the shape memory fibers serve as the skeleton structure of the self-healing system, and the thermoplastic resin serves as the repair agent. When cracks occur in the material, the damaged area is heated to above the glass transition temperature of the shape memory fiber. The pre-stretched fibers will contract due to the shape memory effect, pulling the matrix material to close the cracks under the contraction force. At the same time, the thermoplastic resin particles are heated to the melting temperature and begin to flow, filling the cracks, ultimately achieving self-healing. The Leng Jingsong research group at Harbin Institute of Technology has also studied a large number of shape memory polymers. Using shape memory epoxy polymer (SMEP) as the matrix and thermoplastic polycaprolactone (PCL) as the repair agent, a shape memory polymer with self-healing function was prepared. This type of polymer can achieve 3 cycles of repair at the damaged area, with a maximum repair efficiency of 67.87%, which has great application value.

 

　　2. Intrinsic Self-Healing Coatings

　　Intrinsic self-healing coatings refer to coating materials that contain special chemical bonds or functional groups. After damage occurs, they self-heal through chemical bond reorganization, functional group reactions, or physical interactions. Compared to extrinsic self-healing coatings, this method does not require external substances such as microcapsules or microcontainers, thus having minimal impact on the mechanical properties of the coating substrate. However, because it involves modification of the coating substrate material, its preparation is more difficult than extrinsic self-healing systems.

　　2.1 UV-light-initiated self-healing coatings

　　Ghosh et al. prepared a polyurethane coating with self-healing capabilities, with the self-healing mechanism shown in Fig. 2. The polyurethane network structure in the coating contains chitosan and oxetane structures. When the coating surface is scratched, the cyclic structure of the oxetane breaks, exposing two reactive ends. Under UV irradiation, the chitosan in the coating and the exposed ends of the oxetane attract and bind together to repair the cyclic structure, thus achieving self-healing of the coating damage.

 

 

　　Supramolecular polymers are materials that can achieve self-healing functionality under UV-light initiation. Coulibaly et al. prepared a supramolecular polymer composed of short-chain polymers with telechelic structures and metal ligands (zinc or lanthanum) through chelation. The metal ligands and low-molecular-weight polymers are connected by non-covalent bonds (ionic bonds). Under UV irradiation, the energy absorbed by the metal ligand is converted into heat, breaking the non-covalent bonds, and the metal ligand temporarily detaches from the polymer. The molecular weight of the polymer decreases, the viscosity decreases, and it becomes a flowable state. When cracks or damage occur in the material, after UV irradiation in the damaged area, the flowable molecules can fill the damaged area to achieve self-healing. In the experiment, a 200μm deep scratch was made on a 400μm thick plastic coating. After two UV irradiations, each for 30s, the scratch was well repaired, with a repair efficiency of 100% ± 36%.

　　Wang et al. developed a polydimethylsiloxane-polyurethane (PDMS-PUR) and polyethylene glycol-polyurethane (PEG-PUR) network structure with UV-light-initiated self-healing capabilities using CuCl2 as a catalyst. Self-healing is achieved through UV irradiation-initiated reorganization of supramolecular or covalent bonds and conformational changes.

　　2.2 Thermally reversible crosslinking self-healing coatings

　　Thermally reversible crosslinking self-healing coatings mainly rely on the presence of characteristic functional groups in the coating matrix that can undergo reversible Diels-Alder crosslinking reactions. Self-healing is achieved through reversible D-A reactions, the mechanism of which is shown in Fig. 3. Wouters et al. prepared a copolymer of furfuryl methacrylate (FMA) and butyl methacrylate (BMA) using free radical copolymerization. The functionality (hardness, elastic modulus, crosslinking density) and glass transition temperature of the copolymer can be adjusted by adjusting the ratio of FMA to BMA. This copolymer was polymerized with bismaleimide to prepare a powder, which was used to prepare a self-healing powder coating on an aluminum substrate. The powder coating was heated to 175℃ to form a polymer film, cooled to room temperature for use. When the coating is scratched and damaged, the polymer film is reheated to 175℃, the film reflows, and the damaged area is repaired in 30s. Multiple repairs can be achieved without affecting the substrate properties. Pratama et al. prepared a self-healing thermosetting resin coating based on the D-A reaction. They microencapsulated maleimide monomers capable of undergoing D-A reactions and introduced another monomer capable of undergoing D-A reactions, difuran, into the coating matrix to prepare a furan-functionalized epoxy resin coating. Experimental results show that the self-healing efficiency of the coating with 10% microcapsules with a particle size of 185μm can reach 71%. Postiglione et al. prepared a self-healing coating system of trifunctional and difunctional furfuryl resins and bismaleimide. This system undergoes a D-A reaction at 50℃ and a reverse D-A reaction at 120℃. The self-healing performance was improved by adding the plasticizer benzyl alcohol to the coating matrix. Experimental results show that this coating system can achieve 48% recovery of mechanical strength.

 

 

　　2.3 Layer-by-layer assembled self-healing polymer films

　　Layer-by-layer assembled self-healing polymer films are based on the action of intermolecular non-covalent forces. Composite coatings are constructed through reciprocal interfacial assembly, and various types of functional groups are introduced into the coating to regulate the mechanical and self-healing properties of the coating. Andreeva et al. prepared a layer-by-layer assembled self-healing film containing a repair agent. They assembled the corrosion inhibitor 8-hydroxyquinoline into the polymer film layer. When the coating is damaged, self-healing of the layer-by-layer assembled polymer film is achieved through the movement of polymer segments and the exudation of the corrosion inhibitor, and it exhibits good corrosion resistance. Sun Junqi's research group at Jilin University used an exponentially increasing layer-by-layer self-assembly method to construct a branched polyethylenimine (bPEI)/polyacrylic acid (PAA) polyelectrolyte coating. This coating can self-heal scratches with a width of 50μm within 10s. Self-healing can be achieved simply by immersing the coating in water or spraying water on the scratched surface. At the same time, (bPEI/PAA)*30 film can achieve multiple scratch self-healing at the same location. The self-healing mechanism is that during the film preparation process, the interpenetration of polymer chains in the bPEI/PAA film can be controlled. The prepared film is stable in air, but in water or humid environments, the polymer chains can flow or swell, thus repairing the damaged area. Hu Xiaoxia et al. prepared a polyurethane/sodium carboxymethyl cellulose (PU/CMC) multilayer film using layer-by-layer assembly technology, which has self-healing ability. They also introduced a third polyelectrolyte, polydimethyldiallylammonium chloride (PDDA), into the film structure. The prepared PDDA(CMC/PU)n film showed enhanced self-healing effect. Scratches with a width of 20-30μm can be self-healed within seconds of immersion in physiological saline.

 

　　3. Conclusion

　　Research in the field of self-healing coatings has made rapid progress in the past 10 years. Current and future research focuses on the optimization of existing self-healing systems, the discovery of new self-healing mechanisms, the design of recyclable self-healing materials, and the construction and application of self-healing coating materials. This field of research involves interdisciplinary crossovers of chemistry, materials science, and mechanics, requiring more research enthusiasts to participate. It is believed that self-healing technology will have broad application prospects.