Is there a known problem with powder coatings being contaminated, resulting in white flecks on our finished product?
The most common cause that I have encountered is a contaminated oven. Powder coatings emit anywhere from 0.5 to 3.0% volatiles (although not considered volatile organic compounds in the U.S.) during the baking cycle. These volatiles condense on the interior of ovens and can descend on coated parts as they pass through the oven.
Another source of white specks is ash from recently cleaned racks and hooks. Some shops use a burn-off process to clean these items, and this process can leave a fugitive residue that becomes airborne in the oven.
To diagnose the problem, isolate a coated part or panel as it travels through your oven. I have used a clean, empty paint can to accomplish this. Place a coated test panel or small part in the can and compare its appearance to that of an adjacent part traveling through the oven simultaneously. If the isolated piece is clean and the other part has white specks, then the oven is producing the contamination. If both pieces are contaminated, then either the powder is dirty or the application area is causing the problem.
I have heard that TGIC powder coatings cannot be used in Europe. Is this true? If so, why?
Basically powder coatings containing TGIC (triglycidyl isocyanurate) can be used in Europe. However, TGIC powders sold in European Union countries must have a skull and crossbones label and certain hazard warning labels depending on the concentration of TGIC. (A link to a downloadable PDF from the European Council of the Paint, Printing Ink and Artists‘ Colours Industry [CEPE] can be found in this column at www.finishingtodaymag.com.) Because of this labeling, many finishers have found alternatives to TGIC-based powders.
The crux of the matter is that TGIC is classified by the EU as a potential mutagen based on exposure test results involving laboratory animals.
However, TGIC historically has not caused any observed mutagenic response in humans. Most people who continue to handle TGIC containing powders simply use the necessary personal protection equipment recommended by the TGIC suppliers and don’t seem to incur any deleterious health issues.
The most important point is always to respect chemicals and chemical-containing materials. This means thoroughly reading the material safety data sheets (MSDSs) and abiding by the handling instructions provided by the supplier.
I run a powder coat line for a custom sheet metal house. Lately we have been having application issues due to extreme variations in humidity levels. Currently the RH has been around the 40% range, and I have my guns set at 60 KV. I know that the tuning of the KV setting is a “feel” and the desired outcome is in the results of the correct setting, but do you know of any guidelines for various RH conditions versus KV settings?
I asked one of my old friends, Ron Srsa, what he thought. He’s been around powder application systems for over 25 years. He says, “Forty percent RH is a little too low. A low RH creates a static electricity (SE) environment, in which it is hard to control KV to parts being sprayed. Remember, there is a lot of air moving in and around the booth. In any new powder system, we always recommend 45 to 60% RH. If there was a question about being able to maintain that RH, we would install humidifiers in the powder coating room.
“The quality of coating depends on the gun KV, an acceptable ground and the coating environment. We always talk about KV and ground but often forget about the environment.”
So I would install humidifiers to increase your RH above 40% and set your KVs between 65 and 80. If 80 doesn’t allow you to penetrate tight inside corners I would keep lowering the KVs until you get good penetration. And remember to check that you’re getting a good ground with your hangers.
Ask Joe Powder is a regular feature of Finishing Today magazine. Please send your questions to email@example.com.
FINISHING ANSWERS: Ask Joe Powder
October 1, 2007