Anodic Oxidation Dyeing Technology

1 Oxidation principle

As we all know, the anodic oxide film is composed of a large number of hexagonal cells perpendicular to the metal surface, each film cell has a film hole in the center, and has a strong adsorption force, when the oxidized aluminum product is immersed in the dye solution, the dye The molecules enter the pores of the oxide film by diffusion, and at the same time form covalent bonds and ionic bonds that are difficult to separate from the oxide film. This bond is reversible and desorption occurs under certain conditions. Therefore, after the dyeing process, the pores must be sealed and the dye should be fixed in the film pores to increase the corrosion resistance and wear resistance of the oxide film.

2 Effect of Anodizing Process on Dyeing

In the entire oxidative dyeing process, poor dyeing due to oxidation processes is common. The film thickness and the uniform pore size of the oxide film are the premise and basis for obtaining a uniform color when dyeing. In order to obtain a uniform and uniform oxide film, it is important to ensure sufficient circulation quantity, cooling capacity, and good electrical conductivity. In addition, oxidation is important. The stability of the process.

Sulfuric acid concentration is controlled at 180-200 g/l. Slightly higher concentration of sulfuric acid can accelerate the dissolution reaction of the oxide film, facilitate the expansion of the pores, and make it easier to dye;

Aluminum ion concentration, controlled at 5-15 g/l. Aluminum ions less than 5g/l, the resulting oxide film has reduced adsorption capacity, affecting the speed of color, aluminum ions greater than 15

At g/l, the uniformity of the oxide film is affected, and irregular film layers tend to occur.

Oxidation temperature is controlled at about 20°C. The effect of oxidation bath temperature on the dyeing is very significant. Too low temperature causes dense film holes in the oxide film, and the dyeing speed is significantly slowed down. The temperature is too high, the oxide film is loose, and the powder is easily pulverized. , is not conducive to the control of dyeing, oxidation tank temperature difference should be within 2 °C is appropriate.

The current density is controlled at 120-180a/m2. If the current density is too large, the electrolysis time of the aluminum product in the bath must be shortened accordingly in the case of a certain film thickness, so that the dissolution of the oxide film in the solution is reduced, the film pores are dense, and the dyeing time is prolonged. At the same time, the film is easily pulverized.

Film thickness, dyeing requirements Oxide film thickness is generally 10µm above the solution. If the film thickness is too low, the dyeing tends to occur unevenly. At the same time, when a dark color (for example, black) is required, the film thickness is insufficient, resulting in a limited amount of dye deposition and the desired color depth cannot be achieved.

In summary, anodization is the pre-process of dyeing and is the basis of dyeing. The problem of anodization is difficult to see or even impossible to see before dyeing. Once dyed, we can clearly see phenomena such as uneven color. At this time, production workers often attribute the cause of the problem to abnormal dyeing and ignore the reason for finding the oxidation process. I often make these mistakes when I am exposed to oxidative staining.

3 Staining

3.1 Washing before dyeing

After the anodic oxidation, the sulfuric acid solution remains in the film pores of the oxide film. Therefore, the aluminum product must be thoroughly cleaned before dyeing. Avoid impurity ions in the dyeing tank, especially phosphate ions, fluoride ions, etc. Pure water cleaning is required before the dyeing tank, and it is necessary to monitor the water quality.

3.2 Preparation of the dyeing tank

Most of the dyes used for dyeing are organic dyes, and organic dyes are easy to mold. In order to effectively prevent moldy liquid in the bath, the tank body can be disinfected with bleaching powder, phenol and the like before the preparation of the bath liquid. Adding anti-mould agent can effectively extend the use time of dyeing liquid when preparing bath solution. After the bath is well-prepared, it must be stored for several hours before it can be put into use. In order to ensure the stability of the pH value, acetic acid, sodium acetate, may be added.

3.3 Dyeing Process Control

(1) Temperature

During the dyeing process, the dyeing rate increases with increasing temperature. Therefore, the time required to dye a certain depth of color shortens as the temperature increases. At the same time, the temperature of the bath rises, and the simultaneous sealing will also be accelerated. If the temperature is too high, the simultaneous sealing will be too fast. In the dye molecules, there will not be enough amount adsorbed in the pores of the membrane. The accumulation of dye will be caused by the pores of the oxide film. Closed and suspended, can not reach the required depth, but the dyeing at relatively low temperatures can dye deeper colors, but the corresponding time is longer. Therefore, according to different color requirements, the dyeing temperature can be properly adjusted to avoid the dyeing time Too long or too short.

(2) Dye concentration

According to the law of adsorption, under certain working conditions, the amount of dye adsorbed on the anodic oxidation film increases with the increase of dye concentration. However, this rule applies only when the oxide film itself also has an adsorption capability. For different depths of color, the dye concentration should also be adjusted accordingly. In the initial preparation of the bath solution, the lower concentration solution should be prepared as much as possible. As the production progresses, the dye is continuously consumed, and the consumed part is continuously replenished. It should be a small number of times. If the concentration of the dye is determined, the effect of impurity ions must be taken into consideration. The actual effective concentration may be quite different from the detection. Therefore, the actual staining force of the dyeing tank should be regularly compared and tested.

In order to ensure a stable dyeing force, after a period of production, it is possible to partially replace the bath solution.

(3) Time

Like the electrolytic coloring, when other conditions are constant, the color gradually deepens with the extension of time. In general, when the oxidation conditions are determined, the concentration of dyeing solution, temperature, etc. are determined. Only by adjusting the dyeing time to obtain the color depth requested by the customer, if the dyeing time is too short to obtain the required color, there are two drawbacks, first, the color is too fast, it is not easy to get a uniform color; It is colored too fast and the obtained color weather resistance is not enough. The dyeing time is too long, or the desired color depth cannot be obtained no matter how long the dyeing is. At this time, we must consider whether the oxide film is too thin or the dye concentration is too low.

(4) Ph value

General requirements ph value is 5 ~ 6, stable ph value is very important for dyeing, especially for mixed dyes, different pH value, may have different colors, in order to enhance the stability of ph value, adding in the preparation of the bath Buffer solution is a viable method, while at the same time to enhance the washing before dyeing, to avoid the acidic material.

3.4 Washed after dyeing

After dyeing, the aluminum product should be washed with water to remove the floating color attached to the surface of the aluminum product. At this time, attention should be paid to the quality of the water washing tank because the binding of the dye molecules to the oxide film is reversible when there are more impurity ions in the water. It will cause the dye molecules to separate from the oxide film into the water. At this time, it will show fade. This kind of discoloration is often non-uniform and eventually leads to color difference on the same material.

4 sealed

Sealing treatment is an indispensable part of anodic oxidation. After oxidative dyeing, only the sealing treatment is performed to ensure the original color of the dyed film. There are many kinds of sealing techniques. Steam sealing, hot water sealing, and medium temperature sealing Holes are a good choice, some dyes can also choose cold sealing process. After the hole is sealed, the color may be slightly lighter than before the hole is sealed because of fading, but only a little attention is required.

Dyeing, as a branch of the anodizing post-treatment, is believed to be applied energetically due to its pleasing color and is increasingly favored by people.