Technical points for analyzing graphene waterborne anticorrosive coatings

Graphene is the world's thinnest corrosion-resistant material for metal protection. A large number of research results show that graphene's large specific surface area, excellent barrier properties, high chemical stability and good electrical conductivity have strong enhancement effects on the overall performance of anti-corrosion coatings, such as reinforced coatings on substrates. The adhesion enhances the wear resistance and corrosion resistance of the coating, and is environmentally safe and has no secondary pollution.

In recent years, graphene-based anticorrosion application research has focused on pure graphene anticorrosive coatings and graphene composite anticorrosive coatings. However, the simple use of graphene anti-corrosion coating has many limitations: high quality requirements for graphene; high metal substrate restrictions; high equipment requirements; difficult to large-scale, large-area preparation, difficult to industrialize.

Compared with pure graphene anticorrosive coatings, graphene composite anticorrosive coatings can take into account the excellent chemical stability, rapid electrical conductivity, outstanding mechanical properties, strong adhesion of polymer resins, film forming properties, and synergistic improvement of coatings. Comprehensive performance.

In addition, the preparation method and coating process of the graphene composite anticorrosive coating can be established on the basis of the traditional coating production process, and exhibits good controllability and constructionability in industrial synthesis and industrial application. Therefore, graphene composite anticorrosive coatings will be a new force in the future of new anti-corrosion coating materials.

Graphene antiseptic mechanism

The unique structural properties of graphene itself have shown certain advantages in both physical and electrochemical corrosion protection.

(1) The lamellar structure can form a "labyrinth" structure, which can effectively improve the physical barrier property of the coating.

(2) Due to its small size effect, it can effectively fill coating defects, reduce porosity and enhance compactness.

(3) The sheet structure can divide the coating into a plurality of cells, which can effectively reduce the internal stress of the coating, consume the breaking energy, and improve the flexibility, impact resistance and wear resistance of the coating.

(4) The electron mobility is high and exhibits good electrical conductivity.

Application of Graphene in Waterborne Composite Anticorrosive Coatings

Water-based coatings have become a green and environmentally-friendly coating for the coatings industry due to its low pollution, easy purification and no irritation. However, the protective effect of water-based paints is still not comparable to its corresponding solvent-based paints, resulting in its application in heavy-duty anti-corrosion fields is still not high.

There are some technical problems aqueous coating: As the mechanism of film formation, as compared to solvent-borne coatings, water-based anti-corrosion coating composition is difficult to form highly uniform, high quality structural height of the complete coating, film forming property, abrasion performance is not Good; the water-based groups remaining in the water-based anti-corrosion coating make it poorly shielded against corrosive media such as water and oxygen; due to the large surface tension of water, it is difficult for the water-based coating to achieve high infiltration and dispersion of the pigment and filler, thus improving the water-based coating. The anti-corrosion has become the focus of the development of environmentally friendly coatings.

The unique properties of graphene bring new ways to improve the compactness, barrier properties, mechanical properties and corrosion resistance of waterborne coatings.

1.

Graphene waterborne polyurethane anticorrosive coating

Waterborne polyurethane (WPU) has the properties of solvent-based polyurethane and overcomes the environmental pollution caused by solvent evaporation. However, the thermal stability, solvent resistance and mechanical properties of WPU are poor, which affects its application range. Therefore, in order to provide the comprehensive properties of WPU, it is usually modified by crosslinking, epoxy resin modification and silicone modification. And modification of inorganic nanomaterials (SiO2, TiO2, CNTs).

As a new high-performance nano-reinforcement, graphene has improved the water resistance, thermal properties and mechanical properties of polyurethane to varying degrees.

2.

Graphene waterborne epoxy anticorrosive coating

After years of efforts by R&D workers, waterborne epoxy coatings have overcome the shortcomings of poor water resistance/corrosion resistance and are gradually applied to the heavy-duty anti-corrosion field involved in solvent-based coatings. To further improve its corrosion resistance, researchers have developed a new composite coating by combining graphene into waterborne epoxy coatings.

3.

Graphene waterborne acrylic anticorrosive coating

Water-based acrylic anticorrosive coatings are inexpensive, safe, environmentally friendly, excellent in aging resistance, good in alkali resistance, and simple in synthesis and processing. However, due to the residual of hydrophilic groups, their water resistance is poor and easy to flash. However, the water-based graphene coating prepared by adding graphene has outstanding water resistance and salt spray resistance, and its anticorrosive effect is obviously superior to that of other carbon-based materials.

4.

Graphene aqueous inorganic zinc-rich primer

The water-based inorganic zinc-rich primer is a silicate solution as an important film-forming substance with a high content of zinc powder. (In order to improve the film properties, some flake aluminum powder, sericite powder, ferrophosphorus powder, phosphorus may be blended in an appropriate amount. Iron-zinc-silicon powder, etc.) is an aqueous heavy-duty anti-corrosive primer for anti-corrosion pigments.

Due to the high zinc content, the zinc powder tends to be white in the air, which reduces the adhesion of the coating. The coating is easy to foam and dry during use, and the corrosion resistance is reduced. However, the addition of graphene can improve the resistance of the coating. Salt spray performance.

Corrosion protection workers at home and abroad have done a lot of work on the performance research of graphene waterborne composite anticorrosive coatings. The effect exhibited by graphene waterborne composite anticorrosive coatings shows that the performance of waterborne coatings after graphene modification is improved.

However, most of the research is laboratory results, the research content is fragmented, and the research focuses on how to prepare graphene composite protective coating and verify the corrosion resistance of graphene, ignoring the matching of graphene material and graphene waterborne composite coating. The research of the system, especially the structure-effect relationship between graphene and anti-corrosion performance of water-based coatings, as well as the dispersion of graphene and coating, interface problems, etc., are insufficient.

Difficulties in the application of graphene in the field of waterborne anticorrosion

1.

Solve the problem of selecting graphene and supporting waterborne coatings

Graphene is prepared by different methods, and its physical structure and chemical properties are also different. Although the structure of graphene oxide GO and reduced graphene oxide RGO is similar to that of graphene GNP, due to the influence of chemical modification, there are a large number of structural defects on the surface, resulting in excellent electrical conductivity, mechanical and mechanical properties without GNP.

In terms of hydrophilicity and hydrophobicity, GNP has poor wettability to water and exhibits good hydrophobicity. Compared with GNP, GO and RGO surfaces exhibit good performance due to the presence of large or small amounts of oxygen-containing organic functional groups. Hydrophilicity. When GNP and GO are added as fillers to the resin, 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 the corrosive medium to some extent.

In terms of dispersibility and compatibility, GO and RGO have some reactivity with some organic functional groups (carboxyl, carbonyl, epoxy) on the surface, and can react with some groups in the resin to form chemical bonds, showing a specific GNP. Better interfacial compatibility with the resin.

In terms of electrical conductivity, GNP exhibits excellent conductivity due to a good conjugated structure. Compared with GNP, the surface of GO and RGO destroys its original conjugated structure due to the presence of organic functional groups, and its conductivity is far less than that of GNP.

In addition, the thickness of the graphene, the chip size, the degree of curl of the sheet structure, the specific surface area and the like are also directly related to the protective properties of the coating. At present, there are hundreds of research institutes and manufacturers related to graphene in China. The preparation methods and production processes used are different. The properties of graphene products produced are different. When using graphene for anticorrosive coatings, the effect is inevitable. Different, so choosing which graphene to use is the primary consideration for researchers.

The coating is a complex supporting system, and the components play a protective role. At present, the research on graphene waterborne composite anticorrosive coatings tends to be diversified. Not only the choice of graphene is diverse, but also the choice of film-forming resin, pigment filler and auxiliary agent is diverse. Therefore, what kind of graphene and which are selected for different corrosive environments The formation of a complete supporting system for waterborne anticorrosive coatings is the focus of research.

In this regard, it is necessary to establish a comprehensive evaluation system of graphene and anti-corrosion coatings, and examine in detail the effects of graphene materials with different structures and physicochemical properties on the protective properties of different components of water-based coatings, and explore its mechanism of action in depth for subsequent water-based anti-corrosion coatings. The choice of dedicated graphene provides theoretical and experimental practice basis.

2.

Solve the problem of the amount of graphene in waterborne coatings

When no graphene filler is added, the pure resin is prone to cracks during the film formation process, and the coating is microporous, and the corrosive medium easily diffuses through the voids and cracks. When the ideal content is added, the sheet structure of graphene is superimposed and staggered up and down, and tens to hundreds of dense physical barrier layers can be formed in the coating layer, thereby greatly improving the permeability resistance of the coating.

When the addition amount of graphene filler is too large, on the one hand, due to its surface effect, graphene aggregates, a large amount of disordered accumulation occurs in the coating, and hard agglomerates become coating defects; on the other hand, graphene content is too high. The viscosity of the coating and the concentration of the pigment (PVC) are too high, which affects the film formation and adhesion of the coating, which causes the coating to generate a large number of cracks and defects and promote corrosion. In short, the graphene content is too low or too high to provide good protection. Therefore, it is necessary to investigate the influence of the amount of graphene on the microstructure, viscosity, adhesion and protective properties of the coating, and choose the ideal coating system. The amount of graphene added.

3.

Solve the problem of dispersibility and compatibility of graphene in waterborne coatings

The high surface area, strong van der Waals force and π-π action of graphene make it easy to agglomerate, and it can not form stable chemical bond with water, organic solvent and polymer, resulting in weak interfacial bonding force with resin and poor compatibility. It is prone to phase separation, which seriously affects the performance of the coating.

Graphene dispersion techniques, which are currently studied more, include chemical dispersion and physical dispersion, that is, functionalization of graphene by covalent bond and non-covalent bond modification. The fusion of graphene and coating resin is mainly through blending and polymerization. Law and so on.

3.1 blending method

The blending method is to directly disperse graphene in a coating, and the mixed form may be a solution or melt blending. Generally, a high-speed magnetic stirring process, a shear emulsification process, a ball milling process or a sanding dispersion process is employed, and the polymer chain is adsorbed into the graphene sheet by shear force, and the applied matrix mainly includes polyurethane (PU) and polystyrene ( PS), polymethyl methacrylate (PMMA), polycarbonate (PC), and polyethylene terephthalate (PET).

However, this method has certain drawbacks. On the one hand, graphene has a high surface free energy and is prone to self-agglomeration. On the other hand, there is no chemical bond between graphene and polymer, and the relative position is not strong. Therefore, graphite is inevitably present during the blending process. Aene aggregation.

In order to solve this problem, before blending, researchers use non-covalent bond modification methods to achieve modifiers (auxiliaries, stabilizers, etc.) on graphite through hydrogen bonding, electrostatic interaction, and π-π interaction. The olefin is pre-soaked in order to improve the solubility of graphene and its compatibility with the coating, and the method can maintain its excellent performance without destroying the conjugated structure of graphene.

For example, in the process of graphene reduction, a water-soluble small molecule or an aromatic polymer (such as pyridine acid, sulfonated polyaniline, sodium polypyrene sulfonate, polyvinylpyrrolidone, etc.) is added as a stabilizer. A dispersion-stable graphene nanosheet is prepared by a π-π interaction between a stabilizer and graphene.

3.2 polymerization method

In recent years, researchers have grafted active materials with specific functional groups to the surface of graphene by in-situ polymerization, emulsion polymerization or controlled radical polymerization, etc., to achieve the surface structure of graphene. The tailoring improves the reactivity and effectively improves the solubility, dispersibility and compatibility of the graphene inorganic nanofiller in the coating matrix.

The polymerization method can ensure that the polymer molecular chain is connected and entangled on the graphene surface, and there is a strong interfacial interaction between the two, which can effectively solve the problem of dispersibility and compatibility of graphene in the coating. However, the polymerization method has high requirements on the reaction, and it is difficult to achieve effective control of the position, ratio and graft ratio of the functional groups during the reaction, and is not suitable for large-scale applications.

Summary and outlook

After the water-based anti-corrosion coating is modified by graphene, the mechanical properties, chemical stability and anti-corrosion performance have been improved. There have been many related research work and patent publications in China, and the development momentum is good. However, the application of graphene in waterborne coatings is mostly laboratory results. The research is still in its infancy, and there are still many difficult scientific problems and technical problems. The application and development of graphene waterborne composite anticorrosive coatings continues to heat up, and its further development Can be expected.