ASK JOE POWDER
The “Ask an Expert” Column Kevin Biller
kbiller@chemquest.com
Hello Dear Joe Powder,
Dear Joe Powder,
I hope you are well. I have a question, which size of sieve is
I have just started a powder coating operation in Bangladesh
normal and usual for production of high-gloss and smooth
and could use your advice. What type of weatherability can I
powder coating?
expect out of powder coating technology? Many of my customers
Mohammed Seddighian
are in the architectural business.
Tehran
Abdur Rahim Dhaka
Hello Dear Mohammed, It’s nice to hear from you. High-gloss and smooth powder coatings
Hello Abdur Rahim,
require a fairly narrow particle size distribution. The top end “scalp”
Congratulations on your new business venture. Powder coating is a
is critical to ensure the desired smoothness. Important also are good
great technology in many ways and is a good fit for industries in your
milling conditions to make sure that there is minimal waste, so it is wise
developing nation.
to manage the grinding process first. The rotor speed and classifier
A variety of powder coating technologies are commercially available
wheel settings need to be optimized to minimize both the fines (< 10
to meet the demands of the architectural industry. Most people
microns) and coarse fractions (> 100 microns).
differentiate weatherable coatings by their performance in South Florida
As for sieve size, I would start with a 125-micron screen and then check
exposure. The primary criteria are retention of gloss and resistance
particle size distribution and, most important, film smoothness. If this
to color change. Powder coatings generally have excellent corrosion
does not produce the desired smoothness, then move to a 100-micron
resistance when applied to properly prepared substrates.
screen. This should yield the desired smoothness; however, you may
Polyester-TGIC, polyester-Primid®, and polyester-urethane powders,
have a large quantity of “overs” (> 100 microns). This fraction can be
if suitably formulated, will endure 18 to 24 months in Florida before
reintroduced to the mill to manage the yield.
displaying cosmetic signs of degradation. “Superdurable” forms of these
If the 100-micron screen doesn’t produce the desired smoothness, then
three powders have maintained appearance and coating integrity after
you need to work on reducing the viscosity of the resin system. You may
being subjected to 5 years’ exposure in South Florida.
also have to slow down the cure rate of the resin/crosslinker as well.
Acrylic-diacid powders are considered automotive quality. Tests
I hope that this helps. Please let me know if you have any more
have demonstrated that these products withstand up to 9 years of
questions.
Florida exposure before showing signs of deterioration. Silicone-
Kind regards,
containing powder technologies have emerged in the past few years.
Joe Powder
These are almost exclusively targeted at the high-heat market, which
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N. 85 - JANUARY/FEBRUARY 2024 - international PAINT&COATING magazine