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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 


Hi Joe:
I'm selling powder coating in South America. So now I have many questions regard to powder coatings. But the most important is this: How long can resist the powder coating after the due date, when it still is at storage?
Thanks
Claudia, Colombia

Hello Claudia,
Thanks for the question. I have traveled and worked in Colombia and have very fond memories of my time spent there. I will be very happy to answer your question.

The stability of a powder coating is influenced by many factors. First I must differentiate two distinct types of stability. Physical stability involves the requirement for the individual particles to resist clumping or sticking together. Chemical stability refers to a powder coating's resistance to premature polymerization during storage and handling.

Physical stability is influenced by the melt point or more specifically the Tg (glass transition temperature) of the powder coating and the storage conditions. A powder coating with a low melt point will sinter and clump more readily than one with a higher melt point. Accordingly powders stored in high temperature environments will clump more readily than those stored in cooler areas. Additionally powders exposed to high humidity environments tend to absorb moisture which will also cause clumping. (Please note that most powders are supplied in moisture impervious containers and the exposure to moisture occurs after the package has been opened.) Clumping obviously makes a powder difficult to fluidize, transport and spray in an application system.

On the other hand, chemical instability causes a powder coating to chemically advance or polymerize during storage. Fast reacting and low temperature curing powders can start to polymerize in high temperature storage conditions. This premature chemical advancement reduces the melt flow of a powder coating causing a more textured finish.

Both physical and chemical instability can be present in a powder coating. Typically fast cure (or low temperature cure) powders also have low melt points to help facilitate better melt flow at low cure temperatures. Consequently these types of powders are most susceptible to clumping and chemical advancement and should be stored and applied in a climate controlled environment. I recommend less
than 27 deg C and 50 to 70% relative humidity.

Now to answer your question (are you asleep yet?). Standard curing powder coatings (175 -200 deg C) that have been stored in a reasonable environment can remain usable long past their "due date". Low cure powders and those that have been stored in environments exceeding 27 deg C for long periods of time can suffer from clumping and chemical advancement. I would inspect (for clumps,etc) then spray and bake a sample of any powder that is past it's due date and make the determination yourself. If the finish is still within your requirement for smoothness and appearance (no blistering, dirt, etc) then I would continue to use it.

Claudia, I hope that this helps you. Someday I hope to return to Colombia to enjoy the beautiful weather, excellent food and wonderful people.

Peace,

Joe

Hey Joe,
We have a dedicated/automated powder line. The parts go through the wash stages and are powdered in an enclosed area. Powder guns are oriented from both the top and bottom. We have a problem where powder collects on the top inside of the booth and eventually falls in clumps on the product. We have a regular cleaning schedule for the booth but it is cumbersome and taking too much time. Do you know of any way to prevent the powder from collecting in the first place or any other suggestions that would make this more efficient? I would appreciate any time you could spare to consider this.

Thanks,

Jeff

Dear Jeff,

Sounds like a very aggravating problem. I think that you need to explore a combination of design, process and materials for tackle the problem. Here are a few areas that I would investigate:

Design - I would consider replacing flat surfaces (if you have them) with curved ones. The curved surface will allow for better air flow and a potentially cleaner surface. Check also the air flow of your system near the top of your booth. Is there a dead space? Can your airflow be redirected to pull air from this area?

Process - Do you have an adequate ground for your parts? Powder is electrostatically attracted to the closest and best ground. If your booth ceiling is conductive and well-grounded then the powder will be preferentially attracted to this surface. Is your spray gun orientation and pattern too wide? Are you overshooting the top parts on your hangers and thereby spraying powder too high? Do your guns trigger too soon before seeing parts and stay on too long after the hanger has passed? Again, powder
will look for a ground which will most probably be your booth walls and ceiling.

Materials - Is the top of your booth comprised of metal or a lesser conductive plastic? Metal will attract powder much better than polypropylene or HDPE. A quick fix may involve installing an intermediate plastic baffle between your parts conveyor and the booth ceiling.

I hope that these ideas will help you in your quest to eliminate this problem. Please let me know how you progress.

Best Regards,

Joe