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-----Transparent Powder Over Aluminum
We have been contracted to powder coat aluminum extrusions with a transpaernt polyester powder. The customer wants to maintain the appearance of the aluminum substrate because it shows through the powder. Adhesion is a real problem. We cannot grit blast the part because it changes the appearance. We have not found a pretreatment that preps the surface while allowing it to maintain its original appearance. We have been relegated to washing the part in a hot aqueous washer, but there are many adhesion failures. How can we clean and etch for maximum adhesion, while not disturbing the appearance of the part?
Paul Norman
- Odessa, Texas
1999
Dear Paul
From your description I see that you have some mistakes during trying to solve the problem. First of all cleaning and Etching , in most of the cases does not change the apperance of the aluminium . You won't succeed to paint the part without doing some pretreatment to the Aluminium parts. The process I suggest is as follow: A.Non etching Alkaline Cleaner , Hot dipping bath B.Rinse C.Deoxidize : Remove Oxides with some commercial Deoxidzier , acidic , based on Nitric Acid D.Rinse E.Conversion Coat :Clear conversion coating ,Chromate base. F.Rinse G.Dry H.Paint with clear paint. Note : You can even improve your Aluminium looking , there are some Hot chemical polishing solutions which will do your parts brighter then your original looking.
Yours,
Yehuda Blau
YB Plating Engineering and Quality - Haifa Israel
1999
Dear Paul,
Based on your information I believe there are at least two methods of solving your problem. Both methods are in practical industrial use here in Europe. First, we have to know wether the exposition of the coated parts will be in- or outdoors. For indoor use a pretreatment -either dip or spray- existing of an acid degreasing -rinsing one ore two stages followed by a light organic passivation is sufficient. This system will not alter the looks of the alu-surface (colour, gloss etc)It is important to have the parts coated immediately afterwards. On extruded alu-profiles and polyester powder we found adhesion GT-O and salt spray of over 800 hrs. For outdoor use you need a better pretreatment. Instead of chromating actually two systems are more and more commonly used: a) the system which uses zirkonium solutions in stead of chromate. The "disadvantage" for normal use is that the zirkonium layer is invisible. In your case this would be the advantage. b) the system where polymeric solutions are used to build the conversion layer. The intercoat adhesion between these systems and paint and/or powder is excellent. These systems are GSB and Qualicoat accepted. Regards,
Remmelt Bosklopper
- Enschede, The Netherlands
1999
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