Research Status of Silanization Treatment on Metal Surface
Text / Li Xiaoyuan
Abstract: With the increasing emphasis on environmental protection, silane coupling agent metal surface treatment technology has gradually become the focus of research. This paper briefly introduces the structure and film formation mechanism of silane coupling agent, and summarizes the latest research progress of silanization treatment technology and prospects for its application.
Keywords: silane coupling agent; surface treatment; environmental protection
1 Introduction
At present, in the coating industry, the phosphating process is mostly used for painting, dusting or electrophoresis. The traditional phosphating temperature is mostly 30~50 °C, so the auxiliary heating equipment and heat source are needed to heat the phosphating tank; phosphating A large amount of phosphating slag produced after the treatment also needs a set of slag removing device; and the process contains heavy metal ions such as zinc, manganese and nickel. These all lead to heavy pollution and high cost of phosphating. With the country's environmental protection requirements and cost of use in the coating industry, the search for a new surface treatment technology that is environmentally friendly, energy-saving, low-emission, and cost-effective has become the focus of research.
The early theory of silane coupling agents used in metal surface treatment appeared in 1985, which is the "high-cost surface treatment without anodizing, silane coupling can prevent corrosion and improve metal adhesion". The silylation treatment is a process in which a metal or non-metal material is surface-treated with a silane coupling agent as a main raw material. Compared with the traditional phosphating, it has the following advantages; no harmful heavy metal ions, phosphating sediment, no heating, short processing time, less processing steps, can save the process of repeating the table, the bath can be reused, effectively improve the paint pair The adhesion of the substrate can be used to treat a variety of substrates such as iron plates, galvanized sheets, and aluminum sheets.
2. Structure and mechanism of silane coupling agent
Silane coupling agents are a class of organic compounds with special structures. Their greatest feature is the inclusion of organic and inorganic functional groups, which can simultaneously bond with cathodes and non-polar materials. The chemical formula of the silane coupling agent is YR-SiX3, wherein Y is a non-hydrolyzable organic functional group bonded to silicon through a carbon atom, and can react with the resin in the binder body to improve compatibility, such as an amino group, Vinyl, epoxy, fluorenyl, acryloxypropyl, etc.; R is a carbon chain having saturated and unsaturated bonds, linking Y and Si atoms; X is a hydrolyzable group such as a halogen or alkoxy group , isopropenyloxy and the like. The silanol formed by the hydrolysis of these groups can react with the oxide or hydrocarbon group on the surface of the metal to form a Si-O-Si three-dimensional network structure of silane on the metal surface to prevent corrosion of the metal. Commercially available silane coupling agents are all silanes in which the alkoxy group is a hydrolyzable group, and the main reason is that the alkoxy hydrolyzate is an alcohol, which is neutral and relatively stable. In short, it is precisely because of the existence of these two functional groups in the silane coupling agent molecule that the two materials with widely different properties can be connected through it, thereby improving the bonding force between the metal substrate and the coating.
The silane coupling agent is usually hydrolyzed prior to use. The formation of the silane film is generally the following steps;
(1) Hydrolysis of SiX groups into SiOH
(2) The SiOH group forms a hydrogen bond with the MeOH group of the metal surface (where Me = metal) to rapidly attach to the metal surface to form a Me-O-Si bond.
(3) Dehydration condensation between SiOH forms an SI-O-Si bond.
After the silanization reaction, a dense protective film with Me-O-Si and Si-O-Si features is formed on the surface of the metal, and the silane film is dried during the drying process and the subsequent electrophoretic paint or powder is passed through the organic film. The cross-linking reactions between the functional groups combine to form a strong chemical bond, which ultimately forms a more stable film structure, thereby increasing the corrosion resistance of the metal.
3. Development status of silanization treatment
The study of silane pretreatment in the United States and Europe began in the 1990s. Professor Van Ooij WJ of the University of Cincinnati took the lead in the hydrolysis of bisamine silane, dithio silane, vinyl triacetoxy silane and BTSE. The film-forming process of the aluminum alloy surface, the properties of the film and the anti-corrosion effect of the coating after bonding have been studied in detail, and a large number of research results and patents have been obtained. Now the silane surface treatment technology of Germany's Chemmit and American Yike has been widely used in Europe and the United States.
In recent years, Chinese technicians have also begun to study the surface technology of silanized metal.
On the one hand, the factors affecting the formation of silane film were studied. Zhao et al. studied the influence of metal matrix, solvent, solution pH, curing temperature and time on the silane film of silane species. It believed that the three-dimensional network structure of silane coupling agent can make The organic coating, the coating coating and the metal matrix are chemically bonded to greatly enhance the adhesion of the coating. Therefore, an ultrathin organosilane film having excellent coating and anticorrosive effects can be obtained by pretreating a metal surface with an appropriate silane coupling agent. After Gong Jianmin et al. conducted a detailed study on the hydrolysis of DTMS and γ-APS silanes and their application on aluminum alloy surfaces, it was found that when a mixed solvent of ethanol and deionized water was used as the hydrolysis medium for the silane coupling agent, the silane solution The stability decreases as the concentration of silane in the hydrolyzate increases and the hydrolysis time increases. Moreover, the optimal solution ratio is obtained when the two silanes are hydrolyzed; the higher the pH value, the more favorable the hydrolysis, and vice versa; the optimum temperature for hydrolysis is between 20 ° C and 40 ° C. Xu Yi et al. used reflection absorption infrared spectroscopy to study the silane film formation on the surface of metal substrate. When the treatment time is within 2 min, the silane molecules are continuously adsorbed to the metal surface by chemical bond=bonding (adsorption is basically instantaneous) The thickness of the silane film on the metal surface is also constantly increasing; however, after 2 minutes, the thickness of the formed film is no longer increased, indicating that the properties of the silane film are not related to the immersion time thereafter.
On the other hand, the properties of the silane film were investigated. Zhu Danqing et al. formulated the silane coupling agent, characterized the structure of the formed silane film, and tested the corrosion resistance of the treated substrate. The treated metal surface has excellent corrosion resistance and also good adhesion to the coating. An important basis for good protection of metal surfaces is the "interfacial layer" of the interfacial region of the silane. Yin Zhizheng used the tensile method to study the effect of introducing silane coupling agent on the bonding strength of stainless steel polymer coating and related optimization. Zeng et al. galvanized steel sheets were coated with silane after surface treatment, and electrochemical impedance polarization test box AC impedance test was carried out in 3.5% NaCl solution. The results show that the coating of silane film can significantly reduce the corrosion process of the chair and significantly reduce it. The corrosion rate of the galvanized sheet has a good corrosion resistance. Zhu et al. conducted neutral salt spray, copper accelerated acetate spray, galvanic corrosion atmospheric exposure and seawater immersion test on the metal surface of the metal surface silanized. The results show that the metal surface silanization process technology can replace Phosphating and chromium passivation before painting.
4 Conclusion
In recent years, people have gained a deeper understanding of silane coupling agents, especially in the application research of metal surface treatment. It has been found that silanization can be effectively used for the protection of metals and alloys; aluminum alloys, zinc and zinc alloys (including galvanized steel sheets), iron and iron alloys (including ordinary carbon steel and stainless steel), copper and copper alloys, and magnesium. And magnesium alloys. When energy conservation and emission reduction become the common concern of the world, green environmental protection processes will become increasingly important. At present, the companies that have adopted the silylation process include Haier, Gree, Hisense Kelon, etc. Although there are few examples of the practical application of silane coupling agents in engineering in China, we should believe that with the surface of the silane coupling agent The technology continues to mature, and in the near future, the technology will be widely used in the automotive, home appliances, aviation, electronic machinery and other industries, and will have a profound impact on cleaner production.
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