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I have considered using latex paint on my Pietenpol rebuild,
but wanted to be certain that it would, in-fact protect
the fabric from UV degradation. There are builder that have
had latex covered fabric for over 10 years without incident,
but better safe than sorry. One should not assume that because
visible light is being blocked that UV light is also being
blocked.
Methodology:
I contacted a friend of mine, Dr. Tom Varberg, a Professor
of Chemistry at Macalester College in St Paul, MN and
asked him about testing the paints. Tom agreed to help me
in the testing by using a Beckman DU7400 Spectrometer.
The spectrometer can record Transmittance, the amount of
visible and ultraviolet light lost (or conversely, absorbed) by
a material. Transmittance is the ratio of radiant power (p/o )
that makes it through the substance (paint) to the radiant
power (P ) sent into the substance.
Transmittance can then be converted to Absorption (A) with
the formula.
Tom also supplied me with a small disk of quartz to use as
a base for testing the paint at an appropriate thickness -
similar to that expected when covering fabric. Quartz does
not absorb or restrict UV light in the wavelengths we were
PO
P
T = PO
P
A = −logT
Ultraviolet Absorption of Latex Paints by Kirk Huizengatesting (400-200nm) and
therefore would not introduce
error in the absorption readings.
I applied 4 different paints
1. Kilz Latex White
2. Glidden
Exterior Latex-Wooland Green
3. Behr Exterior Flat
– Black
4. Poly-Fiber Poly-Brush) to the quartz with a
small brush.
After shooting a “blank” to calibrate the spectrometer,
one of the painted areas on the disk was placed between
the light source and the sensor. The light source flashes
on (2 seconds for our test) and the sensor picks-up any
energy that makes it through the paint.
The DU7400 gives a graphic representation of Absorbance
at each wavelength (in nanometers). In the charts
below, I averaged the Absorption of every block of 10
nanometer since there were 600 data points for each
sample (from 800 to 200 nanometers).
As a curiosity, I also prepared some lightweight Poly-
Fiber® fabric generously given to me by Gil Leiter of St
Paul, MN and tested it. I heat shrunk the fabric and
tested it with and without paint and Poly-Brush®
(generously given by Pietenpoler, Robert Haines).
The
results are shown in Chart #2.
Limitations:
The Beckman DU7400 Spectrometer is limited to an absorption
of 4.5 (but we will consider that to be sufficient
at T=.0031%)
The DU7400 is likely to show erroneous data or noise
when at either extreme of its measurement (0 or 4.5).
Some data for specific wavelengths may not be accurate,
but a trend is obvious
The testing we did does not account for reflection. Any
light reflected by the paint would show up as being absorbed.
This doesn’t really change the applicability of
the test results, but does raise a question of what color
paint is the best to use on sealing and protecting the fabric.
This is only a test of the UV blocking ability of latex
(Continued on page 6)
F or many years there has been discussions on the
Pietenpol mail list as to the suitability of exterior
latex paint as a “system” for covering an aircraft.
A number
of builders have completed their projects and have used
latex paint to cover the fabric with claimed success.
One issue that had not been tested is the ability of latex
paint to protect Dacron fabric from damaging UV light. In
standard systems, there is a “barrier” layer of paint applied
prior to color coats. In the Poly-fiber system this is called
Poly-spray. In the past, builders that have used latex paint
to cover their fabric have sealed the fabric with black latex
paint with the idea that black paint would absorb the most
visible light and, hopefully, UV light. This, in turn, would
protect the fabric from degradation.
I have considered using latex paint on my Pietenpol rebuild,
but wanted to be certain that it would, in-fact protect
the fabric from UV degradation.
There are builder that have
had latex covered fabric for over 10 years without incident,
but better safe than sorry. One should not assume that because
visible light is being blocked that UV light is also being
blocked.
Methodology:
I contacted a friend of mine, Dr. Tom Varberg, a Professor
of Chemistry at Macalester College in St Paul, MN and
asked him about testing the paints. Tom agreed to help me
in the testing by using a Beckman DU7400 Spectrometer.
The spectrometer can record Transmittance, the amount of visible and
ultraviolet light lost (or conversely, absorbed) by a material.
Transmittance is the ratio of radiant power that makes it through
the substance (paint) to the radiant power sent into the substance.
Transmittance can then be converted to Absorption with the
formula. Tom also supplied me with a small disk of quartz to use as
a base for testing the paint at an appropriate thickness - similar
to that expected when covering fabric. Quartz does not absorb or
restrict UV light in the wavelengths we were
Ultraviolet paints.
It does not deal with any other issues of using
non-certified methods of covering and painting one’s experimental
aircraft like longevity, ability to seal the
weave, adhesion, or flexibility/brittleness of latex paint.
The Poly-Spray was tested with two layers – one on
the front of the disk and one on the back. We
found that this introduced some errors. Some of
the energy that makes it through the first layer of
paint gets bounced around between the two layers
and gives odd readings.
Conclusions
Latex paint can sufficiently blocks UV radiation and
therefore protect Dacron fabric.
Color does not seem to matter as far as level of absorption
differences are extremely minor (in the range of thousands
of a percent)
This is a matter open for discussion, but the practice of
using black paint as the base/sealing coat on fabric to
block UV light may not be the best practice.
White, in theory,
would be a better paint for that. White paint has a
high amount of Titanium Dioxide, which is highly
reflective.
Black paint, on the other hand, gets its “color”
from Carbon Black primarily. Black paints have much less
reflectivity and more absorption of light.
Now, as I mentioned
above, color does not seem to matter much in
terms of protecting the fabric, but paint that is more reflective
should last longer than paint that is more absorbing
of light.
It is not an issue of black or white being better at protecting the
fabric, but rather the longevity and protection of the paint itself
from breakdown. Brand of paint shouldn’t make much difference in
terms of UV protection, but could make a difference in durability
and longevity of your paint job.
The above courtesy of http://www.eaa54.org
54NewsletterMarch2003.pdf |
