Spraying and paint requirements for pre-treatment

Spraying and paint requirements for pre-treatment

Compared with the paint film, the powder coating film has more advantages, such as less environmental pollution, hard film, good toughness, strong adhesion to the substrate, and high corrosion resistance. However, the pre-treatment requirements are higher, so the requirements of the pre-treatment process and the dusting system are mentioned for the workpieces requiring dusting, thereby restricting the conditions for using the powder.

Pretreatment of steel or zinc, aluminum and their alloys is necessary in order to convert the metal surface into a surface that enhances the solid powder binding and enhances the barrier properties.

The adhesion of powder coatings to sandblasted steel sheets and oil-free galvanized steel sheets that are not pretreated is quite good. However, 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 bonding force and protection of the plastic film are also affected by the quality of the processing system and the powder itself.

Although the protective effect in the powder film is better than that of most wet paint films, and it is harder and thicker than the wet paint film, it is still a semi-permeable membrane, and corrosive substances such as oxygen moisture and sulfur dioxide can still penetrate. Once they enter the film, they contact the base metal and begin to corrode, causing the loss of the bonding force between the organic film and the substrate, especially the phenomenon of zinc and aluminum.

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

Attention must be paid to several important requirements of powder film for pre-painting:

1. In order to ensure the quality of the powder, the metal surface must be completely degreased because the residue of the powder on the grease is unbearable. For paints, the thinner is a solvent, which usually dissolves a small amount of oil into the paint film, reducing the adverse effects of its binding to the substrate. The powder does not have this property, and often leaves during the baking process. Burst holes or blisters, or complete loss of bonding.

2. The presence of rust, scale and ash is another important cause of defects in dry paint films. When these dirt are present on the surface of the workpiece, the combination of the powder film and them is very reluctant. Once the mechanical collision or deformation occurs, the powder film will immediately break down. This phenomenon is the most common. The various powders left by the pretreatment are also important causes of the decrease in binding force.

3, strong acid pickling, especially for long-term strong acid pickling of hot-rolled steel parts, used for powder spraying is not suitable. In the strong acid pickling process, a large amount of hydrogen is permeated, and during the high temperature (180-200 degrees Celsius) baking process, hydrogen is released, so that the coating film forms a large amount of sand holes and holes. Therefore, it must be washed with strong acid, and it should be dehydrogenated after pickling or phosphating.

4. In order to form a beautiful and beautiful appearance of the powdered aluminum parts, it is necessary to separate the cleaning (deoiling) and corrosion (except natural oxide) processes. It is not believed that the oil surface and the thickness of the metal surface contaminated by the oil are uniform. When the cleaning/corrosion two-in-one process is adopted, the corrosion does not occur before the oil is removed, but the oil stain is different due to the different degrees of the metal surface. In the light part, the oil is first removed and corroded, and the corrosion of the highly contaminated surface is delayed, resulting in unbalanced corrosion, which may result in a visible damage to the metal surface. This damaged surface shape is produced. In the final stage, the use of a powder coating film cannot be masked.

In principle, various pretreatments for wet lacquers can be used for powder coating, except that the pretreatment requirements are higher, the conversion film is required to be finer, and the film weight is desirably below 3.0 g/m2, and is better. Thermal stability.