The film of electrophoretic aluminum profile has a good decorative performance, but in the production process, there will sometimes be yellowing in the film, affecting using aesthetics. As following, 5 key factors that will cause the yellowing are presented for the aluminum profile manufactures:
1. The yellowing phenomenon caused by the electrophoretic paint itself
Anodic electrophoretic paint is mainly composed of acrylic resin and amine resin. In the baking process of electrophoresis profiles, the resin has the cross-linking reaction, producing a smooth and transparent coating. But, some electrophoretic paint manufacturers still do not obtain immature production techniques, or use poor quality chemical raw materials in order to reduce the cost. As a result, the electrophoretic paint a relatively narrow curing range, so that if it is baked slightly less, the film is not hard enough, and if it is baked slightly over the head, the film will be yellowing.
2. The yellowing phenomenon caused by poor electrical conductivity during oxidation
If the contact between thealuminum profile and the conductive rod is poor, the resistance at the contact will be greatly increased, and the end of the profile will be hot. Then, the oxide film will be produced too fast and accompanied by burning phenomenon, and even worse, the oxide film will be chalking. Under this situation, some the oxide film will be turbid and appear yellowing, and the further electrophoresis production will appear very obvious yellowing phenomenon. This yellowing phenomenon in general happens to only several aluminum profiles at the bottom row, basically at the end of the profile.
3. The yellowing phenomenon caused by excessive curing
Currently, the curing of electrophoretic paint using on the market are basically under the backing conditions of the 180 ℃X 30min. Under normal conditions, the film is basically no yellowing. However, the curing furnaces of some manufacturers equip with uneven temperature, and the local temperature has even a difference of 30 ℃or more; the curing furnaces of some manufacturers has poor temperature control system, so the actual temperature is different far from the display temperature. So, the poor electrophoretic paint under these conditions is prone to yellowing phenomenon obviously, even like being colored.
4. The yellowing phenomenon caused by incomplete washing before electrophoresis
The oxide film is honeycomb, so the porous structure determines the sulfur will be left in the oxide film pores. As we all know, if the aluminum profiles used for electrophoresis has washed completely, it is likely to appear yellowing phenomenon. For this yellowing phenomenon, it is generally considered that it is the acid left in the oxide film hole that reacts with the electrophoretic paint to cause yellowing. Actually, the yellowing is not caused by the reaction, but caused by the oxide film. The normal oxide film is clear and transparent, and if the oxide film pores remain excessive sulfate, under the high temperature, the oxide film will react with the sulfate, so that the clear transparent oxide film becomes turbid, and the transparency is reduced. At the same time, the high transparency of the electrophoretic film has high reflectivity of the light enlarged the defect of yellowing.
5. The yellowing phenomenon caused by oxidation tank polluted by nitric acid
In order to achieve a better effect of ash removal, a certain proportion of nitric acid is added in the neutralization tank, but if after the neutralization, the washing control is incomplete, the nitric acid in the oxidation tank will affect the oxidation and even cause the yellowing of electrophoretic aluminum profile. In the oxidation process, the nitrate radical in the oxide film pores will corrode the oxide film in the barrier layer, so that the oxide film pore becomes darker and changes the structure. The corrosion have two effects on the oxide film: the barrier layer of the oxide film becomes thinner and the tightness of bonding with the aluminum substrate deteriorates, resulting in lower adhesion of the oxide film.
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