Advances in the study of microcapsules filled self-healing coating materials

coating protection is the most widely used corrosion protection in construction facilities, aerospace, automobile manufacturing, petroleum and petrochemical fields. However, the coating material is always affected by external forces, and its internal structure and properties are easy to change, resulting in micro-cracks of different sizes. The continuous generation and collection of micro cracks inside the coating will eventually lead to cracking or breakage at the macro level, which will cause a great deal of damage to the technical properties and protective effects of the coating, and these internal micro cracks will be difficult to repair from the outside by existing means. In order to solve this problem, self-healing coating materials have come into being. Self-healing coatings can automatically detect, identify and repair damaged areas of materials, thus restoring their physical properties, corrosion resistance and appearance to a certain extent.
self-healing coating material was proposed in the 1970s, the current self-healing coating material is divided into two main categories: the self-healing coating and the foreign aid self-healing coating. Among them, the repair of the self-healing coating is mainly achieved by the molecular structure of the polymer material itself with reversible chemical reactions or the diffusion of large molecules. According to the different types of reversible chemical reactions, the self-healing coating of this type is divided into two categories: reversible non-co-priced bond self-healing (physical type) and reversible co-priced bond self-healing (chemical type). However, the self-healing coating of this type has high requirements on the material itself, so its application range has certain limitations. Foreign aid self-healing coating is mainly the use of certain technical means to encapsulate the repair material, and added to the polymer material, when the polymer damage, the repair material spread to the damage to repair it. Compared with the self-healing coating, the development of foreign aid self-healing coating is becoming more and more mature, and the research work of self-healing method is mainly focused on this kind of method.
past ten years, the research of foreign aid self-healing technology mainly includes microcapsules technology, hollow fiber technology, microvascular network technology, and the latest meso-hole hollow micro-ball technology. Among the above-mentioned self-healing coating techniques, the microcapsules-filled self-healing coating is the packaging of the repair agent in the microcapsules and the combination of the microcapsules with catalysts (or curing agents) that polymerize the repair agents in the polymer material. When the polymer coating is damaged to produce micro-cracks, the microcapsules are cracked by the action of cracks, wherein the covered repair agent flows out under siphon action and fills the crack inside, and then reacts with the catalyst (or curing agent) in the substation material to trigger polymerization, so as to repair the cracks, so that the performance of the coating is restored to a certain extent. Compared with other foreign aid self-healing technology, microcapsules technology and microcapsules composite polymer technology have been relatively mature, and can meet the needs of industrial production, in addition to the crack response speed, the difficulty of repair material packaging, composite coating preparation process, etc. , the application value of microcapsules filled self-healing coating is higher.
Since 2010, microcapsules filled self-healing coating materials have gradually become the focus and focus of scientific research, and play an increasingly important role in the production and application of various fields, such as concrete coatings, bonding coatings, decorative coatings, pavement coatings and other fields. This paper will focus on the current common microcapsules self-healing system, microcapsules preparation methods, microcapsules self-healing coating performance evaluation methods and application status in various fields, and make a look forward to the application of microcapsules filled self-healing coatings and future development trends.。 1. Common microcapsules filled self-healing coating system
In 2001, White et al. first used microcapsules technology to carry out research on self-healing coating materials, using in-place polymerization to prepare a diaper resin (DCPD) microcapsules (DCPD-UF), and then Brown, Keller and others applied DCPD-UF microcapsules to the composite polymer system based on epoxy resin (EP). Depending on the repair agent, there are several common microcapsules filled self-healing systems.1.1. Double-ring diethylene-Grubbs curing agent system
double-ring diethylene (DCPD) has good polymerization activity and fluidity, can easily polymerize with the polymer substate, and fast curing speed, so it is the first repair agent used in the self-healing coating system. The dual-ring diethylene repair agent and Grubbs catalyst self-healing system are the addition of microcapsules containing DCPD to a polymer substation pre-buried with Grubbs catalysts. When micro-cracks occur inside the coating, the microcapsules in the coating break under the action of micro-cracks, releasing the internal DCPD repair agent, DCPD filling cracks and curing in contact with the catalyst, thus bonding and filling micro-cracks, so that the internal damage to the material can be repaired.
microcapsules coated with DCPD repair agents generally use polyformaldehyde as a wall material, so that microcapsules can have good storage stability and thermal stability, and make microcapsules in the composite material processing process has a good structural integrity. In this system, the addition of microcapsules has some effect on the performance of polymer sociology. Brown et al. studied the effect of the addition of covered DCPD microcapsules on the fracture performance of polymer substituts, and the results showed that the addition of microcapsules changed the fracture surface appearance from a flat plastic fracture to a step-like toughness fracture. At the same time, the particle size and addition of microcapsules also have an effect on the toughness of composite materials. Brown et al. found that the smaller the particle size, the more obvious its toughening effect, but after adding small-size microcapsules, the repair efficiency of the coating will be reduced to a certain extent, combined with experimental data, the optimal diameter of microcapsules is generally 180 m. In addition, with the decrease of microcapsules content, the toughening effect is more significant.
in the dual-ring diethylene-Grubbs catalyst system, not only do microcapsules affect the performance of the self-healing coating, but the content and nature of the catalyst also affect the performance of the self-healing coating. In view of the self-healing system, Kessler and others studied the reaction dynamics of DCPD open-loop polymerization reaction under the condition of different mass fraction Grubbs catalysts, and the results showed that DCPD's open-loop polymerization reaction effect was optimal when the mass fraction of the catalyst in the substation was 5%. The study also shows that the thermal stability of Grubbs catalysts is not ideal, and decomposition occurs when the temperature reaches 120 degrees C, thus limiting the curing process in the preparation of composite materials to a certain extent. In addition, DCPD shrinks to a certain extent after polymerization, which reduces its interface interaction with polymer substation materials, thereby reducing the basic electrodynamic properties of the coating after repair.
, the research of this system mainly focuses on the effect of DCPD microcapsules and catalysts on self-healing properties. For the former, the addition of DCPD microcapsules has no in-depth theoretical explanation of the mechanism of the effect of coating self-healing performance, resynical performance and other properties, and for the latter, Grubbs catalyst itself has some application defects, so it is urgent to find catalysts with more stable performance and wider application.1.2. Epoxy Resin-Curing Agent System
DCPD has good polymerization activity, but is volatile and flammable and toxic, making it difficult to be widely used in actual production. Epoxy resin is stable, non-toxic, good fluidity, and can be polymerized with polymer substates, has gradually replaced DCPD as a commonly used self-healing coating repair agent. Using the characteristics of epoxy resin, Yuan et al. used in-place polymerization method to prepare polyformaldehyde microcapsules coated with epoxy resin, thus creating a self-healing system of epoxy resin. Yuan et al. further studied the effect of microcapsule preparation process on its physical properties, and analyzed and studied the properties of polyformaldehyde microcapsules, storage stability, thermal stability and structural composition. On the basis of Yuan et al., Tao et al. developed a microcapsules-curing coating system with epoxy resin as a repair agent. The polyamide curing agent in the system can be stored steadily in the polymer coating system for more than 60 days, and the polyamide will only cross-link with the coating resin when the temperature of the microcapsules reaches the temperature that triggers the open-loop reaction of the curing agent.
Yuan et al. developed a new self-healing system on the basis of Tao et al., the binary system of epoxy resin/polythionol microcapsules, and studied the effect of reaction conditions (catalyst concentration, reaction temperature, reaction time, cystic/cystic wall mass ratio, emulsifying agent content, stirring speed, etc.) on the preparation rate of microcapsules. The system can achieve the purpose of coating crack self-healing with fewer repair agents, and because there are fewer substances introduced into the outside world, the microcapsules content has a weak influence on the mechanical properties of the polymer itself, which can balance the strength and toughness of the material well. At present, the research of this system mainly focuses on the preparation of microcapsules and the performance of self-healing coating, and the structure of epoxy resin repair cracks is not deep. And because of the limitations of the system's own characteristics, it often takes binary and above repair systems to meet the requirements of self-healing, which to some extent sacrifices the basic aesthetic properties of the coating. Therefore, how to improve the self-healing performance of polymer coating as much as possible on the basis of maintaining its self-healing performance will become one of the hot research directions of the self-healing coating system.1.3. Other Systems
For microcapsules filled self-healing coating systems, there are currently polar solvent-epoxy self-healing systems, polydymethane (PDMS) self-healing systems, etc. For the polar solvent-epoxy resin self-healing system, Caruso et al. selected chlorobenzene and xylene as microcapsules core, and studied the repair properties of these two polar solvent self-healing systems. In addition, Caruso et al. prepared three microcapsules that were coated with different solvents mixed with resin, and studied the effect of stirring rate on the particle size during the mixture of solvent and epoxy resin. For the polymethylsiloxane (PDMS) self-healing system, Cho et al. used polyformaldehyde to wrap butyl tin-glycerate catalysts to prepare microcapsules and pre-bury them in vinyl resin base polymers, using hydroxymethyl polymethylsiloxane and similar silicane derivatives as repair agents. Since the repair agent is insoluble on the vinyl resin substate, the vinyl resin after mixing the two will cover the repair agent, and the vinyl resin cures to form a self-healing material. Through scratch corrosion test and electrochemical test, it is proved that the coating of this system not only has anti-corrosion function, but also has the function of self-healing.
With the development of science and technology, more and more microcapsules systems are used in self-healing coatings, but due to the limitations of coating substrates and existing technologies, microcapsules systems that can be applied to industrial production are still epoxy resins and their curing agent systems.。 2. The research on the preparation method of microcapsules for self-healing coating
microcapsules preparation technology began around the 1930s and made a major breakthrough in the 1950s. Wurster first invented air suspension for synthetic solid particulate capsules. Green et al. of NCR in the United States used phase separation composite coagulation to synthesize microcapsules containing gelatin, and used this technology to produce carbon-free copy paper, which is the first time that liquid materials have been microcapsules, creating a new field of physicochemical synthesis of microcapsules. At present, microcapsules preparation technology mainly includes physical methods, phase separation methods and chemical methods, and the preparation of self-healing coatings with microcapsules is mainly using chemical methods, of which in-place polyornation and interface poly-legal applications are the most extensive.2.1. In-place polymerization is legal
in-place polymerization is the simultaneous addition of reaction raw materials (or soluble prepolymers) and catalysts to the reaction medium (continuous phase), because raw materials (or prepolymers) are soluble in the medium, and their polymers are insoluble throughout the system, so polymerization occurs at the interface between dispersed phase and continuous phase. As the reaction continues, the reaction materials gradually converg, and when the reaction product reacts to a certain volume, it will be gradually deposited on the surface of the core material, the formation principle shown in Figure 1. The particle size and wall thickness are easy to control by in-place poly-legal preparation, the preparation process is simple, the raw material and operation cost is low, the industrial production is easy, but the overall reaction time is longer, and the catalyst needs to be added during the reaction process. The core materials commonly used for in-place polymerization reactions are ethylene, paraffin, epoxy resin, methyl acrylic and so on. In order to ensure the tightness of microcapsules, the wall material commonly used in this method is urea-formaldehyde or honeyamine-formaldehyde pre-ester. All catalysts need to be added to these reactions, and the overall reaction takes a long time, where the control of the resulting polymer deposition process on the surface of the cyst core is the key to the success or failure of the preparation of microcapsules. Compared with other preparation methods, in-place poly-legal industrial production applications are less.2.2. Interface convergence is legal
the aggregation reaction on the interface (or the organic side of the interface) that dissolves the solution of two monomers is called interface aggregation. It is characterized by: the formation of the wall material of the two monoliths are located in the continuous phase and dispersion phase, the two raw materials in the liquid-liquid interface contact and polymerization reaction, the reaction product is insoluble in the two phases, so gradually deposited at the two-phase interface, thus generating the final microcapsules, the principle shown in Figure 2. The use of interface polymerization legal microcapsules, its process is convenient, simple, do not need expensive and complex equipment, can be carried out at room temperature, the reaction speed is fast, the reaction monosome purity requirements are not high, and the two reaction monosome raw material ratio requirements are not strict. Solvents commonly used in in-place polymerization reactions are dichloromethane, chloromethane, carbon tetrachloride, 1,1,1-troploroethylene, tetrachloroformethane, benzene, toluene, xylene, carbon disulfide, pyrethroids, mineral oils, etc., or a mixture of the above solvents. Commonly used wall materials are vinyl glycol glyceroid ether, pyramine-formaldehyde pre-polymer and so on. This method is simple, but the coating wall material requirements are high, its reaction monomer must have a high reaction activity, can be adjudication reaction. The product will be mixed with some unresponsive monoliths, the formation of a wall film permeability is high, not suitable for cladding requires sealing repair agent.。 3. Performance evaluation of microcapsules filled self-healing coating
Repair efficiency is an important index of self-healing coating performance evaluation, which refers to the ability of the coating to heal when it produces damage, which can be represented by a mechanical performance of the coating (e.g. fracture toughness, binding strength, etc.) or corrosion resistance ratio after repair and before repair. Where mechanical properties charactertage the coating's ability to resist external damage, it can be evaluated by fracture toughness and binding strength. Corrosion-resistant performance-indicating coatings prevent the diffusion of corrosive particles