The role of substrate treatment is mainly divided into the following three aspects
1. Improve the adhesion between the coating and the surface of the material
According to the adsorption theory, the physical adsorption strength is inversely proportional to the sixth power of the distance, so the coating must have sufficient penetration with the substrate to form a good film adhesion. Wetting occurs on contacting surfaces and is related to the different binding energies and surface tensions between different objects. When the contact angle is small, the coating and the substrate can be fully saturated to produce good adhesion. In general, clean steel surfaces have a higher surface tension than any coating and are therefore well wetted by the coating. When some grease and grease are attached to the substrate, these pollutants will make the surface tension very low, so that the coating cannot fully wet the substrate, resulting in poor adhesion, which will cause the entire film to fall off or various appearance defects. Therefore, the pollutants on the surface of the substrate need to be completely removed before coating.
2. Improve the anti-corrosion ability of the coating on the metal substrate
After the steel is rusted, the rust product contains very unstable ferric acid (aFeOOH), which will still cause corrosion to expand and spread under the coating, resulting in rapid damage to the coating and loss of protection
function. If coating is carried out after rust removal, the reliability of coating protection will be greatly improved. In recent years, some rust coatings that do not require rust removal from the substrate have appeared, but so far, the long-term reliability of rust coatings is still lacking.
3. Improve the flatness of the substrate surface
Molding sand, welding slag, rust, etc. on the surface of the casting will seriously affect the appearance of the coating, which needs to be removed by sandblasting, grinding, etc.; after rough surface coating, the gloss of the coating will become dull due to the influence of surface roughness , so the painting objects with high decorative requirements need to be smoothed through surface treatment first.
