Reasons and solutions for the yellowing of electrophoresis elevators

The reasons for the yellowing of electrophoretic elevators are summarized as follows: electrophoretic paint itself; poor oxidation conductivity; incomplete washing before electrophoresis; excessive curing; oxidation tank liquid is polluted by nitric acid:

1. Yellowing phenomenon caused by poor electrical conductivity of elevators during oxidation

Poor contact between the elevator and the conductive rod will greatly increase the resistance at the contact point, the end of the profile will heat up, the oxide film will be formed too quickly and accompanied by burning, and even the oxide film will be powdered. At this time, the oxide film is somewhat turbid, and the color turns yellow. If electrophoresis is carried out again, there will be a very obvious yellowing phenomenon. This kind of yellowing phenomenon is generally only a few in a row, and basically it appears at the end of the profile. Therefore, measures must be taken to ensure good contact between the aluminum profile and the conductive rod.

2. Yellowing phenomenon caused by incomplete washing before electrophoresis

The oxide film is honeycomb, and its porous structure determines that sulfuric acid will remain in the pores of the oxide film. As we all know, if the profiles used for electrophoresis are not washed thoroughly, yellowing is likely to occur. For this yellowing phenomenon, it is generally believed that the acid radicals in the pores of the oxide film react with the electrophoretic paint to cause the yellowing of the electrophoretic paint film. In fact, this yellowing is not the yellowing of the paint film, but the yellowing of the oxide film. Change. The normal oxide film is clear and transparent. If more sulfate radicals remain in the pores of the oxide film, the oxide film will react with sulfate radicals under high temperature conditions, thus making the clear and transparent oxide film turbid and transparent. At the same time, coupled with the high transparency of the electrophoretic paint film and the high reflectivity to light, this defect is further amplified, forming the so-called yellowing. Therefore, several washings before electrophoresis are very critical, not only to ensure the water quality of the washing, but also to ensure the washing temperature and washing time.

3. Yellowing phenomenon caused by the contamination of nitric acid in the oxidation bath

In order to achieve a better ash removal effect, it is understandable to add a certain proportion of nitric acid in the neutralization tank, but if the water washing control after neutralization is not well controlled, the nitric acid will be brought to the oxidation tank, and the nitrate in the oxidation tank will reach a certain concentration. It will have a certain impact on the oxidation, and even cause the yellowing of the electrophoretic profiles. During the oxidation process, the nitrates entering the pores of the oxide film will etch the oxide film, corrode the barrier layer of the oxide film, make the oxide film pores deeper, and then change the structure of the film pores. This corrosion has two effects on the oxide film: 1. The barrier layer of the oxide film becomes thinner, and the tightness of the bonding with the aluminum substrate becomes worse, which in turn reduces the adhesion of the oxide film. 2. Under normal water washing conditions, it is difficult to remove the sulfate radicals in the membrane pores. The electrophoretic profiles produced under such conditions also have yellowing phenomenon. How to avoid this yellowing phenomenon? Add neutralizer to the pure water tank before the scalding tank, adjust the pH value to 8~9.5, wash with water for 2~3 minutes, neutralize the sulfate group in the pores of the oxide film with amine, and then carry out electrophoresis production, there will be no yellowing. phenomenon has changed.

4. Yellowing phenomenon caused by over-curing

At present, the electrophoretic paints used in the market are basically baked and cured under the conditions of 180℃X30min. Under normal conditions, the paint film basically does not yellow. However, the temperature of the curing furnace of some manufacturers is very uneven, and the local temperature even differs by more than 30 °C; the temperature control system of the curing furnace of some manufacturers is poor, the actual temperature is too different from the displayed temperature, and the electrophoretic paint of poor quality is under such conditions. The yellowing phenomenon is very obvious, even like coloring. Good-quality electrophoretic paints are more resistant to such extreme conditions, and some electrophoretic paints will not turn yellow even if they are baked at 230°C. In order to prevent the occurrence of yellowing, the uniformity of the furnace temperature and the sensitivity of the temperature control system are necessary, and it is also necessary to use a good quality electrophoretic paint.

5. Yellowing phenomenon caused by electrophoretic paint itself

Anodized electrophoretic paint is mainly composed of acrylic resin and amine-based resin. During the baking process of the electrophoretic profile, the resin undergoes a cross-linking reaction to form a smooth and transparent coating film. However, some electrophoretic paint manufacturers have a relatively narrow curing range due to the immature production process or the use of poor-quality chemical raw materials to reduce costs. If the baking is not enough, the hardness of the paint film is not enough; if the baking is too much, the paint film will turn yellow, which will bring certain difficulties to the production management. Therefore, it is recommended that you choose a paint supplier with stable product quality and a certain reputation.