The main purpose of surface treatment before spraying is to improve the adhesion of the coating and the anti-corrosion ability of the substrate, generally including the following three aspects of work.
1. Remove the dirt on the surface of the substrate
If there are different forms of dirt such as grease, dust, rust, scale, old paint film, etc. on the surface of the substrate, it will be difficult to form a continuous and smooth coating. Even if a complete coating is formed, the adhesion to the substrate will be greatly improved. reduce. Since the intermolecular force is inversely proportional to the sixth power of the distance between molecules, the coating film and the substrate can only form a strong binding force when the distance between the molecules is very small, and the presence of various dirt will inevitably increase. The distance between the coating film and the substrate greatly reduces the adhesion of the coating to the substrate surface. If the coating film cannot be firmly attached to the substrate, the coating film is prone to serious consequences such as the entire piece falling off, and various appearance defects are also prone to occur. Therefore, the dirt must be removed before painting. In this way, on the one hand, the adhesion of the coating to the surface of the substrate can be improved, and on the other hand, the coating film can have a good appearance and excellent performance.
2. Adjust the flatness of the substrate surface
In order to make the surface of the substrate have good adhesion to the paint, it is generally required that the surface of the substrate has a suitable flatness (or roughness). Too rough surface is not conducive to the adhesion and film formation of paint, and the formed coating will also be dull. Compared with a smooth surface, using the same amount of paint, the thickness of the coating obtained is much thinner and uneven, and the thinner coating is often preferentially damaged, the roughness is too large, and air will be trapped during coating, causing Coating blisters off. Therefore, it is necessary to remove all kinds of defects on the surface by various processing methods to improve its flatness. When the surface of the metal casting is too rough, it usually needs to be smoothed by means of sandblasting and grinding. Generally, the surface roughness of the material is required to reach grade 4~6. The mechanical run-in between the coating films is very beneficial to the coating adhesion. However, the surface of more than grade 6 is too smooth. As the surface area of the substrate decreases, the contact area between the coating film and the substrate decreases, the adhesion of the coating film will also decrease, and the coating film is not easy to adhere. For example, the general plastic surface is too smooth and has low surface activity, so it is often sanded to improve the adhesion of the coating film to it. The requirements for the surface flatness of various substrates are different. For high-end products such as automobile bodies, the requirements for the surface flatness of materials are very strict.
3. Various chemical treatments on the surface
The chemical treatment of the surface has different purposes for different substrates. For metals such as steel, it can not only improve its anti-corrosion ability, but also improve the adhesion of the coating film to it. Because steel and some non-ferrous metals are chemically active metals, they tend to lose electrons and be oxidized when they are in contact with air and water in the environment. This process is called metal corrosion. According to its mechanism, the direct oxidation reaction between metal and chemical substances is called chemical corrosion, while the corrosion of metal as an anode when forming a chemical battery is called electrochemical corrosion. Electrochemical corrosion is faster and more harmful. In order to prevent the occurrence of metal corrosion, various protective treatments are often carried out on the metal surface, the purpose is to isolate the metal surface from external chemical substances, and painting is one of the commonly used methods. But often without necessary chemical treatment before painting, the protective effect of the coating film cannot be well exerted. This is because there are still micropores on the surface of the coating, and water can slowly pass through the coating film and electrochemical corrosion occurs. At this time, the part of the metal surface containing the coating film is the cathode of the chemical battery, and the part without the coating film is the anode of the chemical battery. When carbon dioxide and various inorganic salts are dissolved in the aqueous solution forming the chemical battery, the speed of electrochemical corrosion will increase. Therefore, the method of painting alone cannot avoid the occurrence of metal corrosion well, but chemical treatments such as phosphating, oxidation, passivation, etc. are required to form an inert and dense protective layer on the metal surface in order to be more durable. It can prevent the metal from being corroded well, and at the same time, it is also beneficial to coordinate with the coating to improve the overall protective performance of the substrate, thereby also improving the durability of the coating film. Moreover, the chemically treated surface can form a rough structure similar to sandblasting, which expands the contact surface and adhesion between the coating and the substrate surface. For example, the surface of substrates such as plastics is chemically treated with organic solvents, the main purpose is to roughen the surface and improve the adhesion of the coating to the substrate.
