[China Aluminum Industry Network] Causes of electrophoretic aluminum yellowing summarizes the following points: electrophoretic painting itself; poor oxidative conductivity; water washing before electrophoresis is not thorough; over-cured; oxide bath contamination by nitric acid:
1. The yellowing phenomenon caused by the poor conduct of aluminum in the oxidation of aluminum causes poor contact between the aluminum profile and the conductive rod. The resistance at the contact point will increase greatly. The tip of the profile will be heated, the oxide film will be generated too quickly and accompanied by cauterization, and even an oxide film may be formed. Powdering. At this time, the oxide film is somewhat turbid and the color appears yellow. If electrophoretic production is performed again, there will be a very obvious yellowing phenomenon. This kind of yellowing phenomenon generally has only a few branches in the next row, and it is basically at the end of the profile. Therefore, measures must be taken to ensure good contact between the aluminum profile and the conductive rod.
2. The phenomenon of yellowing caused by incomplete washing before electrophoresis is an oxide film, and the porous structure determines that sulfuric acid remains in the pores of the oxide film. It is well known that if the profile used for electrophoresis is not thoroughly washed, yellowing is likely to occur. For this yellowing phenomenon, it is generally considered 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, the 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 remains in the pores of the oxide film, the oxide film will react with the sulfate at high temperatures, making the clear, transparent oxide film hazy and transparent. Drop; At the same time coupled with the high transparency of the electrophoretic paint film, high reflectivity of light, so that this defect is further amplified, the formation of the yellow. Therefore, several rinses before electrophoresis are critical, not only to ensure water quality, but also to ensure water washing temperature and washing time.
3. Yellowing caused by oxidation of the oxidation bath solution by nitric acid To achieve better ash removal effect, it is understandable to add a certain percentage of nitric acid in the neutralization tank, but if the water washing control is not good after neutralization, nitric acid will be brought to the Oxidation tank, when the nitrate in the oxidation tank reaches a certain concentration, it will have a certain impact on the oxidation, and even cause yellowing of the electrophoretic profile. During the oxidation process, the nitrates that enter the oxide film pores will etch the oxide film, erode the oxide film barrier layer, deepen the oxide film pores, and 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 thin, and the adhesion with the aluminum matrix becomes poor, which in turn causes the adhesion of the oxide film to decrease. 2. Under normal washing conditions, it is difficult to remove the sulfate in the membrane pores. The electrophoretic profiles produced under these conditions also exhibit yellowing. How to avoid this yellowing phenomenon? Neutralizer is added to the pure water tank before the hot tank, and the PH value is adjusted to 8 to 9.5. The water is washed for 2 to 3 minutes. The amine radical is used to neutralize the sulfate in the oxide film hole, and then the electrophoretic production is performed. Changed.
4. Yellowing caused by over-curing phenomenon Electrophoretic paints currently used in the market are basically baked at 180°C for 30 minutes. Under normal conditions, the paint film does not substantially yellow. But some manufacturers curing furnace temperature is very uneven, the local temperature is even more than 30 °C; some manufacturers curing furnace temperature control system is poor, the actual temperature and the display temperature is too different, the quality of poor electrophoretic paint in this condition The phenomenon of yellowing is very obvious, even if it is colored. The electrophoretic paint with good quality has relatively strong ability to withstand such extreme conditions. Some electrophoretic paints will not suffer from yellowing even when 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. It is also necessary to use a good quality electrophoretic paint.
5. Yellowing caused by the electrophoretic paint itself The anode electrophoretic paint is mainly composed of acrylic resin and amine-based resin. During the baking process of the electrophoresis 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 range of curing due to the immature production process or the use of poor quality chemical raw materials to reduce costs. The baking is slightly insufficient, and the hardness of the film is not enough; baking will cause the yellowing of the film to occur, which will bring certain difficulties to production management. Therefore, it is recommended that you choose a paint supplier with stable product quality and a certain reputation.
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