Why Should The Surface Treatment Process Be Carried Out before Painting And Its Function

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What is spraying? Spraying is a coating method in which the paint is applied to the surface of the object being coated by means of a spray gun or disc atomizer, which is dispersed into uniform and fine droplets with the help of pressure or centrifugal force.

Why is surface treatment necessary before spraying? The primary purpose of surface treatment before spraying is to enhance the adhesion of the coating and improve the corrosion resistance of the substrate. Some manufacturers fail to perform or inadequately perform pre-treatment due to cost-cutting measures and inadequate technology, which directly affects the performance of the product after spraying.

The coating world insists on complete and professional pretreatment process for each product to ensure that the products received by users are all fine.

What is the function of surface treatment processes? 1. Removing dirt from the substrate surface If the substrate surface has oils, dust, rust, old paint films, oxide layers, and other forms of dirt, it will be difficult to form a continuous and smooth coating. Even if a complete coating is formed, the adhesion to the substrate will significantly decrease. Since intermolecular forces are inversely proportional to the sixth power of the distance between molecules, only when the coating adheres tightly to the substrate with very small molecular distances can strong bonding be achieved. The presence of various types of dirt inevitably increases the distance between the coating and the substrate, greatly reducing the adhesion of the coating to the substrate surface. If the coating does not firmly adhere to the substrate, it can easily peel off in large sections, leading to various appearance defects. Therefore, it is essential to remove dirt thoroughly before painting. This not only enhances the adhesion of the coating to the substrate surface but also ensures that the coating has a good appearance and excellent performance.

2. Adjust the flatness of the substrate surface

To ensure good adhesion of the coating to the substrate surface, it is generally required that the substrate surface has an appropriate level of flatness (or roughness). An overly rough surface hinders the adhesion and film formation of the coating, resulting in a dull finish. Compared to smooth surfaces, using the same amount of paint yields much thinner and less uniform coatings. The thinner areas are often damaged first, and excessive roughness can trap air during application, causing the coating to bubble and peel off. Therefore, various processing methods are needed to remove surface defects and improve flatness. When the surface of a metal casting is too rough, sandblasting or grinding is typically used for leveling. Generally, the material's surface roughness should reach 4-6 grades. At this point, the substrate has an appropriate shape of protrusions and depressions, which enhances mechanical bonding between the substrate and the coating, greatly benefiting the adhesion of the coating.

But surfaces smoother than grade 6 are too smooth. As the surface area of the substrate decreases, the contact area between the coating and the substrate also reduces, leading to a decrease in adhesion. The coating becomes less likely to adhere, so at this point, roughening treatment through sanding is required. For instance, plastic surfaces that are too smooth and have low surface activity often rely on sandpaper for grinding to enhance the adhesion of the coating. The requirements for surface flatness vary depending on the type of substrate. For high-end products like car bodies, the requirement for material surface flatness is very strict.

3. Various chemical treatments are applied to the surface

Chemical treatment of surfaces has different objectives for various substrates. For metals like steel, it can enhance both their corrosion resistance and the adhesion of coatings. Steel and some non-ferrous metals are highly reactive chemically; they often lose electrons and oxidize when exposed to air and water, a process known as metal corrosion. According to its mechanism, direct oxidation reactions between metals and chemicals are called chemical corrosion, while the formation of a chemical cell where the metal acts as the anode is referred to as electrochemical corrosion.

Electrochemical corrosion occurs faster and is more harmful. To prevent metal corrosion, various protective treatments are often applied to the metal surface, aiming to isolate it from external chemical substances; painting is a commonly used method. However, if necessary chemical treatments are not performed before painting, the protective effect of the coating cannot be fully realized. This is because micro-pores still exist on the surface of the coating, allowing water to slowly penetrate and cause electrochemical corrosion.

At this point, the part of the metal surface that contains the coating acts as the cathode of the chemical battery, while the part without the coating serves as the anode. When carbon dioxide and various inorganic salts dissolve in the aqueous solution of the chemical battery, the rate of electrochemical corrosion increases. Therefore, relying solely on painting cannot effectively prevent metal corrosion. Instead, it is necessary to perform chemical treatments such as phosphating, oxidation, and passivation to form an inert, dense protective layer on the metal surface. This not only better prevents metal corrosion but also facilitates the coordination with the coating, enhancing the overall protective performance of the substrate and thus improving the durability of the coating.

The surface treated by chemical treatment can form a rough structure similar to sandblasting effect, which expands the contact area and adhesion between the coating and the substrate surface. If the substrate surface such as plastic is chemically treated with organic solvent, the main purpose is to carry out roughening treatment and improve the adhesion of the coating to the substrate.