In order to make the luxurious purple copper-colored aluminum profiles suitable for building doors and windows, drawing on the successful experience that the golden color obtained by electrolytic coloring with potassium permanganate has significantly improved weather resistance after electrophoretic treatment, people have been continuously searching for a suitable process for electrophoretic treatment of purple copper-colored aluminum profiles in recent years. Unfortunately, it has been found that purple copper color is more difficult to be subjected to electrophoretic treatment than the golden color obtained by electrolytic coloring with potassium permanganate. The reason is that the purple copper color obtained by depositing copper in the pores of the oxide film is prone to anodic dissolution during electrophoretic treatment, resulting in complete fading of the purple copper color.
To prevent or minimize the fading of the purple copper color obtained by electrolytic coloring during subsequent electrophoretic treatment and baking processes, appropriate protective measures can be taken for the copper deposited in the oxide film to prevent its tendency to undergo anodic dissolution.
The following protective measures can effectively prevent the fading of purple copper color:
1.Perform semi-sealing on the purple copper-colored anodic oxide film, which can be achieved by cold sealing or medium-temperature sealing. However, complete sealing of the purple copper-colored anodic oxide film should be avoided; otherwise, while inhibiting the fading of the purple copper color during electrophoresis, it will seriously affect the electrophoresis process, hinder the formation of the electrophoretic paint film, and the thermoplasticity of the completely sealed purple copper-colored anodic oxide film will be reduced too much, inevitably leading to oxide film cracking during the baking of the electrophoretic film.
2.Conduct passivation treatment on the purple copper-colored anodic oxide film to form a passivation film on the surface of the copper deposited in the pores of the oxide film, so as to prevent the anodic dissolution of copper during the electrophoresis process.
3.Adopt the above two preventive measures simultaneously; the anti-fading effect is more stable than using either one alone.
4.Subject the purple copper-colored anodic oxide film to high-temperature hot water treatment at 80～85℃. This is equivalent to the protective measures in (1) and (2), but it will affect the subsequent electrophoresis process and deteriorate the performance of the surface electrophoretic film.
5.On the basis of (1) or (2), control the temperature of hot water washing before electrophoresis at 70～75℃ and the hot water washing time at 7～10min (take the upper limit at higher temperatures and the lower limit at lower temperatures); the anti-fading effect is also more stable than using a single measure alone.
6.Carry out secondary electrolytic coloring on the purple copper-colored anodic oxide film (i.e., after purple copper color coloring and water washing, perform a second electrolytic coloring treatment). Either a single tin salt or a tin-nickel mixed salt coloring bath solution can be used to protect the surface of the copper deposited in the pores of the oxide film with a small amount of tin and nickel.
This preventive measure makes it relatively easy to obtain a stable purple copper color and has no negative impact on inhibiting baking film cracking. A small amount of copper sulfate carried into the tin salt or tin-nickel mixed salt bath generally does not cause pollution, because copper sulfate is usually reduced to elemental copper by the reducing agent present in the tin salt or tin-nickel mixed salt bath and precipitates at the bottom of the bath.
