June 13, 2006
Film ThicknessHow can I tell if the film build is thick enough before I cure the powder coating?
A gun or guns operating at a specific output and position will provide coating thickness within a certain range. If the equipment is used the same each time and is properly maintained, the film build can be predicted within an acceptable range for many coaters. Some coaters use more sophisticated methods, such as film gages, to meter the output of powder and better control the film build. Although these gages are not inexpensive, they can be extremely valuable for controlling film thickness. In installations where a lot of powder is used, a savings in film build of 1 mil can provide an excellent payback.
Aluminum AdhesionWe build aluminum bench seating for outdoor stadiums and also process steel for the support structure. We use a five-stage washer with an alkaline cleaner, rinse, iron phosphate with a fluoride additive, water rinse and seal rinse. We have had some adhesion failure on the aluminum, where the powder coating comes off in large pieces. How can we control the pretreatment process to achieve good adhesion on the aluminum, and how can we test the aluminum before it is shipped so that we know it is not going to fail?
Extruded aluminum has a tight oxide layer that must be removed prior to powder coating, and the surface must be passive when the coating is applied. The aluminum oxidation process can begin very quickly after any etching treatment, so it is important to use a conversion coating when the product will be exposed to outdoor conditions. Historically, chromate has been the preferred method for treating aluminum, but dry-in-place non-chrome seals can also provide excellent stability and adhesion before powder coating. These treatments are typically not compatible with steel, so you might need to treat the two metals on separate lines.
Another factor to keep in mind is that you need a continuous film at least 3 mils thick for outdoor applications. If any breaks exist in the film, moisture can penetrate and creep under the film. If your coating is not thick enough, it might not take the punishment of an outdoor environment.
For testing, you should use a wet adhesion test as described in the American Architectural Manufacturers Association (AAMA) 2605-02 specifications. The test panel is scribed and placed in boiling, de-mineralized water for 20 minutes. The cross hatch pattern is then dried, and the tape pull test is performed. This test is similar to the dry cross hatch test outlined in ASTM 3359, but the use of boiling water is much more aggressive than a dry test. If the coating passes the AAMA test, you can have a reasonable amount of confidence in the pretreatment process.
Finally, be aware that under-curing the powder can also contribute to adhesion loss. Check your cure cycle with a Datapaq or similar temperature recording device and perform cure testing on the finished film (hardness, chemical rub, etc.) to confirm full cure.