AAA Dry Battery.Technical difficulties in graphene water-based anticorrosive coatings

　　Graphene is the world's thinnest anti-corrosion material and can be used to protect metals. A large number of research results show that graphene's large specific surface area, excellent barrier properties, high chemical stability and good conductivity have a strong effect on improving the overall performance of anti-corrosion coatings, such as enhancing the coating's effect on the substrate. It has excellent adhesion, improves the wear resistance and anti-corrosion of the coating, and has the characteristics of environmental protection, safety and no secondary pollution. In recent years, research on anti-corrosion applications based on graphene has mainly focused on pure graphene anti-corrosion coatings and graphene composite anti-corrosion coatings. However, simply using graphene anti-corrosion coatings has many limitations: high quality requirements for graphene; many restrictions on metal substrates; high equipment requirements; it is difficult to prepare on a large scale and in a large area, and it is difficult to industrialize. Compared with pure graphene anti-corrosion coatings, graphene composite anti-corrosion coatings can take into account the excellent chemical stability, rapid conductivity, outstanding mechanical properties of graphene and the strong adhesion and film-forming properties of polymer resin, which can synergistically improve the coating's performance. Overall performance. In addition, the preparation method and coating process of graphene composite anti-corrosion coatings can be based on the traditional coating production process, showing good controllability and constructability in industrial synthesis and industrial applications. Therefore, graphene composite anti-corrosion coatings will be a new force in new anti-corrosion coating materials in the future. Graphene anti-corrosion mechanism The unique structural properties of graphene itself give it certain advantages in both physical anti-corrosion and electrochemical anti-corrosion. (1) The lamellar structure can form a "maze-like" structure, which can effectively improve the physical barrier properties of the coating. (2) Due to its small size effect, it can effectively fill coating defects, reduce porosity and enhance density. (3) The lamellar structure can divide the coating into many small sections, which can effectively reduce the internal stress of the coating, consume fracture energy, and improve the flexibility, impact resistance and wear resistance of the coating. (4) High electron mobility and good conductivity. Application of graphene in water-based composite anti-corrosion coatings Water-based coatings have become a green and environmentally friendly coating vigorously developed by the coating industry due to their low pollution, easy purification, and non-irritation characteristics. Combined with the unique properties of graphene, it can bring new ways to improve the density, barrier properties, mechanical properties and anti-corrosion properties of water-based coatings. Graphene water-based polyurethane anti-corrosion coating Water-based polyurethane (WPU) has the performance of solvent-based polyurethane, but also overcomes the environmental pollution caused by solvent volatilization. However, WPU has poor thermal stability, solvent resistance and mechanical properties, which affects its application range. Therefore, in order to provide WPU with comprehensive properties, it is usually cross-linked, epoxy resin modified, and silicone modified. And modification of inorganic nanomaterials (SiO2, TiO2, CNTs), etc. Graphene, as a new high-performance nano-reinforcement, improves the water resistance, thermal properties, and mechanical properties of polyurethane to varying degrees. Graphene water-based epoxy anti-corrosion coating After years of efforts by R&D workers, water-based epoxy coating has overcome the shortcomings of poor water resistance/corrosion resistance and has gradually been applied to heavy anti-corrosion fields involving solvent-based coatings. To further improve its anti-corrosion properties, researchers have developed a new composite coating by compounding graphene into water-based epoxy coatings. Graphene water-based acrylic anti-corrosion coating Water-based acrylic anti-corrosion coating is cheap, safe and environmentally friendly, has excellent aging resistance, good alkali resistance, and simple synthesis and processing. However, due to the residue of hydrophilic groups, its water resistance is poor and easy to Flash erosion. However, the water-based graphene coating produced by adding graphene has outstanding water resistance and salt spray resistance, and its anti-corrosion effect is significantly better than water-based coatings filled with other carbon-based materials. Graphene water-based inorganic zinc-rich primer. The water-based inorganic zinc-rich primer uses silicate solution as the main film-forming substance and a high content of zinc powder (in order to improve the coating performance, some flaky aluminum powder and silk can be mixed in an appropriate amount. Mica powder, phosphorus iron powder, phosphorus iron zinc silicon powder, etc.) are water-based heavy-duty anti-corrosion primers with anti-corrosion pigments. Due to its high zinc content, zinc powder easily turns white in the air, which reduces the adhesion of the coating. The coating is prone to blistering and cracking during use, and the anti-corrosion performance is reduced. However, adding graphene can improve the resistance of the coating film. Salt spray performance. Corrosion protection workers at home and abroad have done a lot of work on the performance research of graphene water-based composite anti-corrosion coatings. The effects of graphene water-based composite anti-corrosion coatings indicate that the performance of water-based coatings has been improved after being modified by graphene. However, most of the research is laboratory results, and the research content is fragmented. The research focus is on how to prepare graphene composite protective coatings and verify the anti-corrosion performance of graphene, ignoring the selection of graphene materials and the matching of graphene water-based composite coatings. Research on the system, especially the lack of understanding of the structure-activity relationship between graphene and the anti-corrosion properties of water-based coatings, as well as the dispersion and interface issues between graphene and coatings, is insufficient. Difficulties in the application of graphene in the field of water-based anti-corrosion solve the problems of material selection and matching with water-based coatings. The preparation methods of graphene are different, and their physical structures and chemical properties are also different. Although the structures of graphene oxide GO and reduced graphene oxide RGO are similar to graphene GNP, due to the influence of chemical modification, there are a large number of structural defects on their surface, resulting in that their conductive, mechanical, mechanical and other properties are not as good as GNP. In terms of hydrophilicity and hydrophobicity, due to the surface effect, GNP has poor wettability to water and shows good hydrophobicity. Compared with GNP, the surface of GO and RGO contains a large or small amount of oxygen-containing organic functional groups, which shows good hydrophobicity. of hydrophilicity. When GNP and GO are added to the resin as fillers, the hydrophobic GNP will prevent or delay the penetration of corrosive media such as water and oxygen, while the hydrophilic GO will promote the penetration of corrosive media to a certain extent. In terms of dispersion and compatibility, GO and RGO have certain reactivity due to some organic functional groups (carboxyl groups, carbonyl groups, epoxy groups) contained on the surface, and can react with some groups in the resin to form chemical bonds, showing better performance than GNP. Better interfacial compatibility with resin. In terms of electrical conductivity, GNP shows excellent electrical conductivity due to its good conjugated structure. Compared with GNP, the presence of organic functional groups on the surface of GO and RGO destroys its original conjugated structure, and its electrical conductivity is far inferior to GNP. In addition, properties such as graphene's thickness, sheet diameter, degree of curling of the sheet structure, and specific surface area are also directly related to the coating's protective performance. At present, there are hundreds of graphene-related research institutions and manufacturers in China. The preparation methods and production processes used are different. The graphene products produced have different properties. When graphene is used in anti-corrosion coatings, the effect is inevitable. Different, so choosing which graphene to use is the primary consideration for researchers. Coating is a complex supporting system, and the various components work together to exert a protective effect. At present, the research on graphene water-based composite anti-corrosion coatings tends to be diversified. Not only are the choices of graphene diverse, but also the choices of film-forming resins, pigments, fillers, and additives. Therefore, what kind of graphene and graphene should be selected for different corrosive environments? The formation of a complete supporting system for water-based anti-corrosion coatings is the focus of research. In this regard, it is necessary to establish a comprehensive evaluation system for graphene and anti-corrosion coatings, to examine in detail the impact of graphene materials with different structures and physical and chemical properties on the protective properties of water-based coatings with different components, and to deeply explore their mechanism of action to provide guidance for subsequent water-based anti-corrosion coatings. The selection of specialized graphene provides theoretical and experimental practical basis. Solve the problem of graphene usage in water-based coatings. When no graphene filler is added, pure resin is prone to cracks during the film formation process. The coating is microscopically porous, and corrosive media can easily spread through the gaps and cracks. When the ideal content is added, the lamellar structure of graphene is stacked layer by layer and arranged staggered up and down, forming dozens to hundreds of dense physical barrier layers in the coating, which greatly improves the anti-permeability of the coating. When the amount of graphene filler added is too large, on the one hand, due to its surface effect, graphene aggregates, and a large amount of disordered accumulation occurs in the coating, forming hard agglomerates that become coating defects; on the other hand, the graphene content is too high The viscosity and pigment volume concentration (PVC) of the coating are too high, which affects the film-forming property and adhesion of the coating, causing a large number of cracks and defects in the coating and promoting corrosion. In short, graphene content that is too low or too high cannot provide good protective properties. Therefore, it is necessary to examine the impact of graphene dosage on the microstructure, viscosity, adhesion and protective properties of the coating, and select the ideal coating system for a specific The amount of graphene added. Solve the problem of dispersion and compatibility in water-based coatings. Graphene's high surface area, strong van der Waals force and π-π interaction make it prone to agglomeration, and it cannot form stable chemical bonds with water, organic solvents and polymers, resulting in The interface bonding force between it and the resin is weak, the compatibility is poor, and phase separation is easy to occur, seriously affecting the performance of the coating. Currently, the most studied graphene dispersion technologies include chemical dispersion and physical dispersion, that is, functionalization of graphene through covalent bond and non-covalent bond modification. The fusion of graphene and coating resin is mainly through blending and polymerization. Law etc. 3.1 Blending method The blending method is to directly disperse graphene in the coating. The mixing form can be solution or melt blending. Generally, high-speed magnetic stirring process, shear emulsification process, ball milling method or sand grinding dispersion process are used to adsorb and insert the polymer chains into the graphene sheets using shear force. The main substrates used are polyurethane (PU), polystyrene ( PS), polymethyl methacrylate (PMMA), polycarbonate (PC) and polyethylene terephthalate (PET), etc. However, this method has certain drawbacks. On the one hand, graphene has a high surface free energy and is prone to self-aggregation; on the other hand, there is no chemical bond between graphene and the polymer, and the relative position is not strong. Therefore, graphene inevitably appears during the blending process. ene aggregation. In order to solve this problem, before blending, researchers often use non-covalent bond modification methods to achieve the effect of modifiers (auxiliaries, stabilizers, etc.) on graphite through hydrogen bonding, electrostatic interactions, and π-π interactions. The graphene is pre-soaked to improve the solubility of graphene and its compatibility with coatings. Moreover, this method does not destroy the conjugated structure of graphene and can maintain its excellent performance. For example, during the reduction process of graphene, water-soluble small molecules or aromatic polymers (such as pyridine acid, sulfonated polyaniline, polysodium p-styrene sulfonate, polyvinylpyrrolidone, etc.) are added as stabilizers. , through the π-π interaction between the stabilizer and graphene, dispersed and stable graphene nanosheets are prepared. 3.2 Polymerization method In recent years, researchers have grafted active substances with specific functional groups onto the surface of graphene in a covalent manner through synthesis methods such as in-situ polymerization, emulsion polymerization or controlled radical polymerization, achieving the goal of controlling graphene. Tailoring the surface structure of graphene increases its reactivity and effectively improves the solubility, dispersion and compatibility of graphene inorganic nanofillers in the coating matrix. The polymerization method can ensure that the polymer molecular chains are connected and wound to the surface of graphene, and there is a strong interfacial interaction between the two, which can effectively solve the dispersion and compatibility problems of graphene in coatings. However, the polymerization method has high requirements for the reaction. It is difficult to effectively control the position, proportion and grafting rate of functional groups during the reaction process, and is not suitable for large-scale applications.

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