With the increasing emphasis on environmental protection by the country, water-based coatings have become one of the green development directions of the coating industry due to their environmental advantages. However, there is still a significant gap in the performance of water-based coatings compared to solvent based coatings, and the poor water resistance and corrosion resistance of water-based coatings hinder their widespread promotion and application. With the increasing emphasis on environmental protection by the country, water-based coatings have become one of the green development directions in the coatings industry due to their environmental advantages. However, there is still a significant gap in the performance of water-based coatings compared to solvent based coatings, and the poor water resistance and corrosion resistance of water-based coatings hinder their widespread promotion and application. By surface treatment of metal substrate materials, the overall protection of metal/coating systems can be improved, which is beneficial for the use and promotion of water-based coatings on metal substrates. Traditional surface treatment methods for metal substrates, such as electroplating and chemical plating, can easily cause environmental pollution. Plasma treatment, as a new surface treatment method, has received widespread attention from researchers. Plasma treatment not only has obvious effects, but also is environmentally friendly. The modification of the substrate surface can not cause significant changes to the properties of the substrate surface. In addition, it has the advantages of high safety and easy operation, and is expected to be vigorously promoted by the industry.

Plasma is an ionized gaseous substance composed of atoms and atomic clusters that have been partially electron deprived and ionized to produce positive and negative ions. It is often regarded as the fourth state of substances other than solids, liquids, and gases. Plasma treatment can connect functional functional groups such as hydroxyl and carbonyl groups to the surface of the sample to be treated. After plasma treatment, the hydrophilicity of the material surface can be improved, the spreading performance of the surface can be improved, and the organic coating can better wet the surface of the body, reduce the generation of pores, and thus better bond the substrate surface with the coating, delaying its failure process.

SEM characterization of substrate and coating after plasma treatment

Air plasma treatment can add functional groups such as hydroxyl and carbonyl groups to the surface of the substrate. These functional groups can react with unsaturated carbon carbon double bonds, carboxyl groups, hydroxyl groups, ester groups, and other functional groups in alkyd resins to form chemical bonds, thereby improving the adhesion between the coating and the substrate. Additionally, surface roughness can be increased to enhance adhesion.

The SEM morphology of the substrate and coating before and after plasma treatment was compared experimentally, as shown in Figure 1.

SEM image of the cross-section of the varnish coating

From Figure 2, it can be seen that when the substrate is not treated with plasma, the varnish coating and the metal substrate only adhere to each other, while after the substrate is treated with plasma, there is a clear transition layer between the varnish coating and the metal substrate. The coating and substrate are connected during the adhesion process to form a new film layer, namely the plasma layer.

Three dimensional confocal map of surface defects on substrate without plasma treatment and failure of varnish coating

Due to the plasma treatment, many active groups are attached to the surface of the metal substrate, resulting in partial chemical bonding between the plasma layer and the substrate metal, making the coating and metal integrated. Therefore, the appearance of the plasma layer enhances the wet adhesion between the coating and the metal substrate, significantly improves the anti stripping performance of the coating, and creates a layer of resistance for water transport, thereby improving the service life and failure time of the coating.