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 1. In order to ensure adhesion
of powder to galvanized substrate sweep blast is mandatory. Some specs
ask for profile .8 to 3 mil. 3 mil profile over galvanizing that is only
3.5 to 4 mil thick is effectively removing the galvanizing from the steel
substrate. Even with the best efforts it is difficult to be consistent
hour after hour and day after day. Over-blasting
is a real possibility. Even if over-blasting may be avoided de-lamination
is another significant problem which may not be detected by the naked
eye. De-lamination will become apparent only after a certain amount of
time sometimes years, by causing accelerated corrosion and consequently
system failure. Rough surface is desirable in powder applications to create
mechanical bond. To the best of my knowledge there is no powder specifically
formulated for galvanized substrate in order to create a chemical bond.
In comparison there are many wet coating primers specifically designed
for galvanized, edge primer and water born primers etc. These primers
still demand oxide free substrate but they offer chemical bond in addition
to mechanical bond. Being of the lower viscosity wet coatings will tend
to develop better mechanical bond than powder by its ability to "wet
the surface" better and "soaks" into all surface imperfection
removing air bubble more effectively. Conclusion: Given the same substrate
conditions there is no benefit to use powder with respect to adhesion.
If sweep blast is insisted upon the risks greatly outweigh the benefits
of this process. Corrosion protection is the main purpose of coatings
aesthetics come second. Chemical cleaning is much safer and will not compromise
corrosion protection provided by galvanizing.
2. Color gloss and film retention:
Without going into many different grades of powder that could be used
the most commonly used is some type of polyester. That type of powder
offers 18 months to 3 years (maybe 5 at low UV exposed sights) protection
against chalk. After this time chalk
is sure to accrue. Consequently color
loss is imminent. Without looking too far you will find wet coatings
offering 5 to 10 year life expectancy. For a premium there are wet coatings
available that offer 25 years plus life expectancy. Conclusion: Today
wet coatings greatly out perform powders. They have been used successfully
for many years on monumental buildings. Powders offer greater film thickness
(better hiding properties) but the benefit is quickly outweighed by faster
film lost and degradation.
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3. There are many practical
problems with powder process: Out gassing - Very unpredictable, vary from
job to job and from pole to pole. In order to minimize this phenomena
specifications require up to one hour preheat time. This may in effect
minimize the problem but in my view is creating a much more severe problem.
Mainly if not all ovens presently used are direct fire type (no heat exchanger,
no condensers). By burning natural gas or propane large amounts of water
is generated as by product. This water effectively condensates on the
poles until the metal reaches 180 degrees F or so. This moisture in addition
to elevated temperature (450 F) is promoting very rapid oxidation
of the surface. This is precisely the condition we are trying
to avoid and great efforts were spent to remove all the oxides from the
surface. Wet coatings eliminate this condition entirely. Priming the Pole
- With epoxy primer as required by spec is causing set of problems by
itself. As previously mentioned preheating the pole is requested in order
to minimize out gassing. Primer is applied to the hot pole (450 F or so).
At this temperature primer will cure in 5 to 7 minutes. Topcoat
will not adhere to fully cured primer. By letting the pole cool
down the out gassing problem starts to persist. Not mentioning it takes
a long time to heat the pole, cool it down and heat it again. For 2000
pound pole the process may take up to 3 hours. Distribution of steel also
causes a problem. You have ¼ inch plate welded to a 2inch plate
some areas get over cured and some under cured. In addition energy consumption
is astronomical comparing to different products. At the extreme you could
bake 2000 square feet of metal per hour (in a form of 4X8 foot sheets)
comparing to 100 to 200 square feet of surface in a form of poles. Giving
the same temperatures and oven size. Giving the large amount of energy
required per square foot of finished surface the benefit of using VOC
free powder may be quickly outweighed by a large amount of CO2 in burning
natural gas (Fossil fuel). This comparison should be made by architects
or spec writers. The fact that today's wet coatings are 50 to 75% of solids
and electrostatic application offers 75 to 80% transfer efficiency given
their relatively small surface area of the pole the VOC issue may not
be that significant.
4. Wet coatings offer also
more practical properties. Touch up and repairs are much easier. Some
paints offer excellent anti-graffiti properties. Color availability is
unlimited and instant.
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Over viewing the entire process I cannot find any significant benefits
in choosing powder coating for the product in question. Giving many technical
uncertainties, difficulty in maintaining workmanship consistency even
with great effort you may control the process 95% successfully the question
remains: who will deal with remaining 5% of system failure that in my
opinion is imminent. To underline again over blasting, de-lamination,
out gassing, oxide build up in oven, over bake of primer, under or over
cure of top coat, large energy consumption.
At this time I would to like thank you for reading this report. I tried
to be as objective as possible and concentrate on the technical aspect
of the process and ignoring other reasons that some manufacturers or coating
applicators may have. Please do not hesitate to contact me if I may be
of further assistance or elaborate further on the issue being raised.
Yours truly
Greg Palamarz, Eng., R & D
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Powder
Coating