WATERBORNE TOP COATINGS FOR ALUMINUM HEAT EXCHANGERS
Disclosed are methods for coating aluminum alloy heat exchangers that include the use of a waterborne top coating. By using a waterborne top coating, relative to a solvent borne, useful advantages can be achieved, such as improved penetration depth of the top coating. An example method includes cleaning a surface of a heat exchanger; contacting the surface of the heat exchanger with a first mixture to provide a passivation layer on the surface of the heat exchanger; contacting the passivation layer with a second mixture in a bath and applying a positive charge to the bath to provide an electro-coating on a surface of the passivation layer; and contacting the electro-coating with a third mixture, the third mixture including water and a polymer resin having anti-corrosion and ultraviolet protection properties, to provide a top coating including the polymer resin on a surface of the electro-coating.
1 . A method for coating an aluminum alloy heat exchanger, the method comprising:
cleaning a surface of a heat exchanger, the heat exchanger comprising an aluminum alloy;
contacting the surface of the heat exchanger with a first mixture, the first mixture comprising a trivalent chromium salt, to provide a passivation layer derived from the trivalent chromium salt on the surface of the heat exchanger;
contacting the passivation layer with a second mixture in a bath, the second mixture comprising a paste and a first resin, and applying a positive charge to the bath to provide an electro-coating derived from the paste and the first resin on a surface of the passivation layer; and
contacting the electro-coating with a third mixture, the third mixture comprising water and a second resin having anti-corrosion and ultraviolet protection properties, to provide a top coating comprising the second resin on a surface of the electro-coating.
2 . The method of claim 1 , wherein the cleaning comprises contacting the surface with an alkaline mixture.
3 . The method of claim 1 , wherein the first mixture further comprises a salt of hexafluorozirconic acid.
4 . The method of claim 1 , wherein the trivalent chromium salt comprises trivalent chromium sulfate.
5 . The method of claim 1 , wherein the passivation layer has a weight of at least 0.05 g/m 2 .
6 . The method of claim 1 , wherein the surface of the heat exchanger is contacted with the first mixture for less than or equal to 12 minutes.
7 . The method of claim 1 , wherein the heat exchanger acts as a cathode when the positive charge is being applied.
8 . The method of claim 1 , wherein the electro-coating has a thickness of about 5 μm to about 50 μm.
9 . The method of claim 1 , wherein the heat exchanger is washed, heated, or both after applying the electro-coating.
10 . The method of claim 1 , wherein the third mixture comprises a solids content of less than 35% by weight of the third mixture.
11 . The method of claim 1 , wherein the second resin of the third mixture comprises an acrylic resin, a polyurethane resin, or a combination thereof.
12 . The method of claim 1 , wherein the first mixture, the third mixture, or both further comprise an adhesion promoter.
13 . The method of claim 1 , wherein the top coating penetrates the heat exchanger at least 0.5 mm.
14 . The method of claim 1 , wherein the third mixture is sprayed onto the electro-coating.
15 . The method of claim 1 , wherein the third mixture comprises less than or equal to 75 g/L of volatile organic compounds.
16 . The method of claim 1 , wherein the top coating has a thickness of about 10 μm to about 100 μm.
17 . The method of claim 1 , wherein the method further comprises coloring the top coating.
18 . The method of claim 17 , wherein coloring comprises adding a pigment dispersion to the top coating.
19 . The method of claim 1 , wherein the top coating does not require a heat or chemical cure.