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Ask Joe Powder

"Ask Joe Powder"
"Ask Joe Powder" is a question and answer column authored by Kevin Biller of the Powder Coating Research Group. Mr. Biller has over 30 years experience formulating and manufacturing powder coatings. He welcomes your questions regarding powder coating technology. Please write to: askjoepowder@yahoo.com 

Kevin Biller
The Powder Coating Research Group
15 W. Cherry Street, 3rd Floor
Columbus, OH 43215
614-354-11988
www.powdercoatingresearch.com 


Dear Mr. Joe
We are in a business to produce diesel generators with acoustic enclosures. We apply pure polyester powder coating on these enclosure parts.
Recently we have been asked to install our generator at a place which is surrounded by heavy chlorine gas throughout the life of all installations over there. Please suggest suitable surface treatment and painting system for our enclosure parts to resist such an atmosphere, and also guide us life expectancy of such coating system and if required frequency of recoating on site to sustain durability of coating.

Matter urgent and your guide line may help others also having similar requirement.

Regards

Sunil Thaker
Super Nova Engineers ltd.
Ahmedabad

Dear Sunil,

You are wise to consider using a more durable powder technology than polyester chemistry for this hostile chemical environment. First let’s talk about metal pretreatment. The steel must be completely cleaned and devoid of any oxidation. The best cleaning technique would involve a high quality alkaline cleaner followed by a water rinse. Next stage should be a high quality iron phosphate conversion coating followed by two rinses, clean water then a deionized water rinse. The final stage should be a seal that can be either chromate based or an organic alternative.

The parts should then be thoroughly forced air dried in an oven and cooled before the powder coating application. Your choice of powder will depend upon whether it is in an indoor or outdoor environment. I would assume that this is an indoor application from your explanation.

For indoor environments I recommend that you use a functional grade epoxy-based powder coating. These can be either phenolic cured or amide cured products. Either will work well. The powder coating should be applied to a thickness of at least 75 microns, but preferably 100 to 150 microns and thoroughly cured per the manufacturer’s recommendation. The color stability of epoxy powders is only marginal so you can expect to see a variation of color for lighter shades.

For exterior environments you have two choices – one is using an epoxy based primer followed by
a polyurethane topcoat. Alternately you can consider using a single coat of polyurethane. The two
coat system will provide the best overall durability, however you will need to take measures to ensure acceptable adhesion of the polyurethane to the epoxy. The best process is to slightly undercure the epoxy primer then apply the polyurethane and fully baked the two-coat system.

Both the epoxy and polyurethane can resist chlorine gas if formulated and applied properly. Recoating should not necessary for a long time. I strongly recommend that you test any coating system thoroughly before committing to a commercial process. It would make sense to coat a number of test panels wirth the systems described above and expose them to the service environment targeted to determine if the products will withstand the possible chemical attack.

Good luck.

Joe

Please forward any questions or comments to askjoepowder@yahoo.com .