Spraying and paint requirements for pretreatment

Compared with paint film, powder coating film has more advantages, such as small environmental pollution, hard film, good toughness, strong bonding with the substrate, and high corrosion resistance. However, the requirements for pretreatment are higher. Therefore, the requirements for the pretreatment process and the dusting system are all mentioned for the parts required to be powdered, so that the conditions for using the powder are restricted.

In order to convert the surface of the metal into a surface that enhances the bond strength of the solid powder and improves the protection, both steel and zinc, aluminum and its alloys are necessary for the pretreatment.

The binding of powder coatings to sandblasted steel plates and oil-free galvanized steel plates without pretreatment is quite good. However, even in a corrosive environment, even milder corrosion conditions can cause separation of the protective film from the substrate in a short period of time. In addition, the binding and protection properties of plastic films are also affected by the quality of the processing system and the powder itself.

Although the protection within the powder film is better than most wet paint films, and it is harder and thicker than the wet paint film, it is still a semi-permeable film. Corrosive substances such as oxygen gas and sulfur dioxide can still penetrate. Once they enter the membrane, they come into contact with the base metal and begin to corrode, resulting in the loss of the bonding force between the organic membrane and the substrate, especially the phenomenon of zinc and aluminum is more serious.

In order to enhance the binding force between the powder film and the substrate and improve the protection, different treatment methods may be adopted depending on the type of metal.

Attention must be paid to several important requirements for powder coating before painting:

1. In order to ensure the quality of the powder, the metal surface must be thoroughly deoiled because the powder is unbearable to the grease residue. For paints, the diluent is a solvent. It can usually dissolve a small amount of oil into the paint film to reduce the adverse effect of its adhesion to the substrate. The powder does not have this property, and it often stays in the baking process. Burst holes or trachoma, or completely lose their strength.

2. The presence of rust, oxide scale, and dust is another important cause of dry film defects. The presence of these contaminants on the surface of the workpiece, the powder film and their combination is very reluctant, once the mechanical collision or deformation occurs, the powder film will immediately break, this phenomenon is the most common. The various powders left by the pretreatment are also important reasons for the reduction of binding force.

3, strong acid pickling, especially for long-time hot pickling steel acid pickling, used for powder coating is not suitable. In the strong acid pickling process, a large amount of hydrogen will be infiltrated, and hydrogen is released during baking at a high temperature (180-200 degrees Celsius), so that a large number of blisters and holes are formed in the coating film. Therefore, it must be acid-washed with strong acid, and hydrogen removal should be performed after pickling or phosphating.

4. In order to form a beautiful and beautiful appearance for the powdered aluminum parts, it is necessary to separate the cleaning (degreasing) and corrosion (except natural oxides) processes. It cannot be believed that the oil stain and thickness of the metal surface contaminated by the oil stain is uniform. When the cleaning/corrosion two-in-one process is adopted, corrosion does not occur until the oil stain is removed, but due to the different degree of oil stain on the metal surface, the oil stain The light parts of the oil are first cleaned and corrosion begins, and the corrosion of highly contaminated surfaces will be delayed, resulting in unbalanced corrosion, resulting in the metal surface may produce a visible damage shape, this type of damaged surface profile produced In the final stage, the use of powder coatings cannot be masked.

In principle, various pretreatments for wet paints can be used for powder coating. The difference is that the pretreatment requirements are higher, the conversion film requirements are more fine, the film weight is expected to be less than 3.0g/m2, and better Thermal stability.