Application of silane coupling agents in pre-treatment for painting

The workpiece should be degreased, rusted, chemically formed and passivated before painting in order to improve the corrosion resistance of the workpiece and the quality of the coating. Chemical film formation is mainly blackening and phosphating, creating a layer of metal oxide and phosphating film on the surface of the substrate respectively. These two film-forming substances can protect the substrate for a short time and improve the adhesion of the substrate to the coating. As the chemical conversion film is mostly inorganic salt film, the film is thin and poorly dense and needs to be sealed between processes for long-term preservation. In the traditional saponification sealing process, it is found that the soap attached to the workpiece is not easy to clean and seriously affects the adhesion of the coating. With silane coupling agent instead of saponification passivation blackened workpiece, the corrosion resistance of the workpiece has been greatly improved, the adhesion of the coating to 0 level, a good solution to the problem.

The coupling mechanism of silane coupling agent

Silane coupling agent is a compound with a special structure. Its molecules have both reactive groups that can be chemically combined with inorganic materials and groups that can be combined with organic materials. It can be represented by the general formula YR(CH2)nSiX3. y is an organic functional group that binds reactively to organic materials; x is an alkoxy group that on hydrolysis produces the silanol RSi.
By using a silane coupling agent, a "molecular bridge" is built between the interface of inorganic and organic materials, linking two materials with different properties, thus increasing adhesion and improving material properties. When the blackened workpiece is immersed in the silane coupling agent sealing solution, the silane coupling agent adsorbed on the surface of the workpiece enters the void of the blackened film skeleton and undergoes hydrolysis to produce silanol groups, which combine with the hydroxyl groups on the blackened film to produce -SiO-M (M is the inorganic surface), while some organic functional groups combine with the coating organic matter in the void. . The silanol groups in the silane coupling agent molecules condense with each other and oligomerise to form a reticular film covering the surface of the blackened film. This tight structure with hydrophobic properties improves the corrosion resistance of the workpiece.

1. Research on sealing technology

The formation of oxide films on workpieces, the film-forming mechanism of The oxidation treatment of steel, is also called high-temperature blackening. After oxidation, a thin oxide film is formed on the surface of the workpiece.
Film-forming process organic solvent degreasing chemical degreasing hot water washing flowing cold water washing acid washing flowing cold water washing chemical oxidation recovery tank immersion flowing cold water washing sealing treatment.
The nature of the film The oxide film on steel consists of divalent iron oxides, some of which become hydrated. The oxide film is typically 0.6 ~ 0.8 microns thick and is black or blue-black in colour. Typically, alloyed and mild steels have a red iron oxide hanging ash sandwiched between the oxide film. Because the oxide film is very thin, the storage time in the atmosphere is very short, so it must be sealed.

2. Selection of sealing process

After saponification of the workpiece into a film, soak it for 3 ~ 5 minutes in a soap solution with a concentration of 5% and a temperature of 80 ~ 90°C, then rinse it with hot water and then soak it in oil. Soap is corrosive due to the hard water quality and tends to make white spots appear on the oxide film, while the coating adhesion is reduced due to incomplete cleaning.
After passivation with chromic anhydride to form a film, the workpiece is passivated in a solution containing 2% ~ 5% chromic anhydride at 60 ~ 70°C for 0.5 ~ 1 minute, washed with hot water and then dipped in oil. Chromic anhydride is a toxic carcinogen and the discharge of chromic anhydride waste solution tends to cause environmental pollution.
The use of silane coupling agent to seal the oxidised workpiece solves the problems of poor coating adhesion and environmental pollution caused by saponification and chromic anhydride passivation, and can significantly improve the corrosion resistance of the workpiece. The construction process is simple, operable, energy-saving and environmentally friendly, and suitable for engineering applications in the surface treatment industry.

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