Ask Joe Powder May-Jun-17
Posted on Wednesday, May 31, 2017
It’s Snow Joke!
One of my customers in Thailand is facing
an unknown problem when they use the
The coated pieces have an aluminum
profile and their application is vertical. The
unknown compound is similar to snow and
mostly found inside the baking oven and is
the cause of defects on the coated surface. It
seems to happen from my powder and is the
usual condition at the customer’s line oven,
in my opinion. Can you advise from your
experience what it is and how it happens?
Thank you in advice,
Thank you for your question. I have seen
this problem before. It can be summed up in
one word: benzoin. Nearly all powder coating
formulas use a degassing agent to assist in film
formation of the coating while it is melting in
the oven. Degassing is needed because as the
particles coalesce, air pockets form in between
the particles (I like to call it interstitial air).
Incorporating benzoin allows the bubbles to
escape during this melt phase. Sounds great
so far. Part of the mechanism of degassing
involves the volatilization of the benzoin as its
melting point is 270°F (132°C) and it tends to
sublime above this temperature. How do we know
this? I have taken samples of the snowflake-like
residue found in powder cure ovens and had our
analytical lab characterize it. What they found was
that the majority of the residue is benzoin.
So, how do you minimize/stop this problem?
My experience with finding high concentrations
of benzoin in a cure oven usually indicated a very
“tight” oven. By that, I mean an oven with very
little exhaust. This is a common issue as some oven
designers think that since powder coatings are
promoted as having no volatile organic compounds
(VOCs), they do not have any volatiles. This is
not the case. VOCs refer to regulated organic
compounds that present a deleterious effect on the
atmosphere. Powder volatiles do not, but they still
exist. Hence the solution to your problem is to
- thoroughly clean/vacuum your oven and
- provide more exhaust to the oven. This should
eliminate the problem
Oh, and one other thing, have a trusted
analytical laboratory analyze the residue with
infrared spectroscopy. Benzoin absorption bands
Best regards and let me know if you have further
- Joe Powder
Quest for a Cure
Good Day Joe,
Please assist if you can. In South Africa it is
very difficult and very expensive to get piped gas
into my plant. We are starting up a powder coating
section for architectural extrusions. I would like
to get some input as to whether I should select my
ovens to be gas/diesel or infrared (IR). Is there
a strengths, weaknesses, opportunities and
threats (SWOT) analysis that has been done to
determine the best options?
Regards and thank you in anticipation.
Johannesburg, South Africa
Good day, Belinda,
This is a very common problem in the African
continent. We ran into a similar problem in
Nairobi. The coating operation there could either
pay for gas lines to be laid to their plant (cost
prohibitive and would take an indeterminate
amount of time for installation) or install a large
gas storage tank and buy a year’s worth of gas
(also cost-prohibitive). They elected to use electric
resistance heating for their finishing line. I felt this
was the lowest cost option for capital expenditure,
however operating costs are quite high.
For your situation, I recommend a
combination of IR and electric resistance heat.
The IR will have to be electric as opposed to
gas-fired or gas-catalytic IR for the reasons you
mention above. I think it will be best to use a
medium- to long-wavelength IR system as it
provides better heat-up consistency and is less
sensitive to coating color differences as compared
to short wavelength types. It is important to orient
the elements so they do not cause “striping” from
localized focal points of IR energy. IR energy is
relatively “line-of-sight” and will preferentially heat
up the surfaces it “sees” (lots of quotations today).
Because of this localized delivery of heat, it is wise to
follow the IR zones with convection heat, in your case
resistance electric. Convection heat will even out the
overall temperature of your parts and is “color-blind,”
as opposed to IR. This way you can be assured of
thorough, even cure across your parts.
I would avoid diesel-fueled ovens. They are rather
inexpensive to purchase and operate, however the
combustion is not as clean as that of natural gas and
can cause inconsistencies in color. If your target
market was less critical than architectural, you could
contemplate using a diesel fueled oven, but I don’t
think this is an option for you.
I hope that this helps you in your quest for a curing
system for your powder coating shop. Good luck and
let me know if you have any further queries.
- Joe Powder
Joe Powder is our technical editor, Kevin Biller. Please send your
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Editor’s Note: Letters to and responses from Joe Powder have been edited for space and style.