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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: firstname.lastname@example.org
The Powder Coating Research Group
15 W. Cherry Street, 3rd Floor
Columbus, OH 43215
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 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
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
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.
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.
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
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.