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Reasons and Solutions for yellowing of electrophoretic aluminium profiles

September 11, 2019

Abstract: 5 reasons for yellowing of electrophoresis aluminum profiles.

The reasons for the yellowing phenomenon of electrophoresis aluminum profiles are summarized as follows:

  • 1) Electrophoretic paint itself;
  • 2) Poor oxidation and conduction;
  • 3) The water wash before electrophoresis is not thorough;
  • 4) Over-curing; 5) The oxidizing tank fluid is contaminated with nitric acid.

1. yellowing phenomenon caused by poor conductivity of aluminium profile during anodising

If the contact between aluminum profile and conductive rod is not good, the resistance at the contact will increase greatly, the end of the profile will be heated, the oxidation film will be formed too fast and accompanied by burning phenomenon, and even the oxide film will be powdered. At this time, the oxide film is somewhat turbid, the color appears yellow, if electrophoresis production will appear very obvious yellowing phenomenon. This yellowing phenomenon usually has only a few branches in a row, and basically occurs at the end of the profile. Therefore, measures must be taken to ensure good contact between aluminum profiles and conductive poles.

2. yellowing caused by incomplete washing before electrophoresis

The oxide film is honeycomb-like, and its porous structure determines the sulfuric acid that remains in the oxide film hole. As we all know, the profile used for electrophoresis, if the washing is not thorough, there is a good chance of yellowing phenomenon. For this phenomenon of yellowing, it is generally believed that the acid roots in the oxide film hole and electrophoresis paint reaction to the electrophoresis paint film yellowing, in fact, this yellowing is not the yellowing of the paint film, but the yellowing of the oxide film. Normal oxide film is clear, transparent, if the oxide film hole left more sulfate, high temperature, the oxide film will react with sulfate, so that the clear, transparent oxide film becomes cloudy, transparent decline; Thus, this defect is further amplified to form what is called a yellowing change. Therefore, a few washing before electrophoresis is very critical, not only to ensure the water quality, but also to ensure the water washing temperature and water washing time.

3. yellowing caused by nitric acid contamination of oxidation tank liquor

In order to achieve a better ash removal effect, in the middle and groove to add a certain proportion of nitric acid this is not justified, but if the water washing control is not good, nitric acid will be taken to the oxidation tank, oxidation tank nitric acid root to reach a certain concentration, will cause a certain impact on oxidation, and even cause the yellowing of electrophoresis profile. During the oxidation process, the nitric acid root entering the oxide film hole will have an etching effect on the oxide film, and the blocking layer of the oxide film will be corroded, so that the oxide film hole becomes deep, thus changing the structure of the membrane hole. This corrosion has two effects on the oxide film: 1, the oxide film's blocking layer thins, and the tight ness of the bonding with the aluminum substrate is poor, which in turn results in a decrease in the adhesion of the oxide film. 2, in normal washing conditions, it is difficult to remove sulfuric acid in membrane holes. The electrophoretic profiles produced under this condition will also have yellowing. How to avoid this yellowing phenomenon? Add a mixture in the pure sink before the hot wash tank, adjust the PH value of 8 to 9.5, wash 2 to 3 minutes, with the amine root and oxidation membrane hole in the sulphate root, and then electrophoresis production, there will be no yellowing phenomenon.

4. yellowing caused by excessive curing

At present, the electrophoretic paints used in the market are basically baked and cured at 180℃ for X 30 min. Under normal conditions, the paint film will basically not yellowing. However, some manufacturers curing furnace temperature is very uneven, local temperature is even more than 30 ℃; some manufacturers curing furnace temperature control system is poor, the actual temperature and display temperature is too different, poor quality electrophoretic paint in this condition yellowing phenomenon is very obvious, even like coloring. Good quality electrophoretic paints have strong tolerance to such extreme conditions. Some electrophoretic paints do not yellowing even when they are baked at 230℃. In order to prevent yellowing, uniformity of furnace temperature and sensitivity of temperature control system are necessary. It is also necessary to use high quality electrophoretic paint.

5. yellowing caused by electrophoretic paint itself

Anode electrophoresis are mainly made up of acrylic and amine-based resins. During the baking process, the resin is cross-linked, resulting in a flat, transparent coating. However, some electrophoresis paint manufacturers due to the immaturity of the production process, or in order to reduce costs to reduce the use of poor quality chemical raw materials, resulting in a narrow scope of curing. Baking slightly insufficient, paint film hardness is not enough, baking slightly too much, paint film will occur yellowing, to the production management brought some difficulties. Therefore, it is recommended that we still choose product quality is stable, there is a certain degree of visibility of paint suppliers.


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