ou might think that conducting paint-weathering tests is about as exciting as watching paint dry, only a lot slower. Admittedly, not too much has changed in weather-testing coat- ings by placing test specimens on “test fences” at original equipment manufacturer (OEM) or commercial “test farms.” In fact, even those colloquial terms, test fences and test farms, are throwbacks to the first doc- umented paint exposure tests, begun in 1906 at the Government Agricultural Experiment Station in Fargo, N.D. About the only significant changes in “sta- tic weathering” since then are that the expo- sure racks now face toward or away from the equator for mildew tests, rather than east-west; and that the severe climate of South Florida has become the standard benchmark reference instead of Fargo. Of course, the variety of architectural, protective and functional coatings, as well as the range of substrate materials, has evolved and expanded tremendously since those days. And while the majority of test specimens are still coatings on metal panels or lumber “trade panels,” a walk through a test-farm reveals no end of specimen types, from beams and pipes running through con- crete and masonry walls, to products such as outdoor furniture. 42 D+D OCTOBER 2014 Y Coatings A to Zielnik How Paint Weathers Paints and coatings weather in many ways, depending on their formulation chemistry, the substrate and the climate. Even the definition of “failure” varies, often subjectively. For some coatings, such as architectural and au- tomotive, appearance properties such as color and gloss retention are paramount. For oth- ers, protecting the substrate is the main criterion. In some cases, both are required. The Florida Department of Transportation, for ex- ample, has much higher standards for bridge- coating appearance than do most other states, owing to Florida’s dependence on tourism. After all, they have to keep up with Disney. So what causes paint to weather? For most coatings, solar radiation, and particularly its ultraviolet (UV) component, degrades the paint film along with temperature and moisture. These stresses are interconnected and also vary with the natural daily and seasonal cy- cles. Film-forming resins can photo-degrade, resulting in surface erosion which leads to chalking and loss of gloss, or loss of adhesion to the substrate. In the case of wood coatings, sunlight can degrade the wood’s lignin and cellulose, caus- ing adhesion loss. UV and visible sunlight can also fade pigments, resulting in color loss. Architects often ask me if I know a high- chroma red that won’t fade to pink. Yellowing of the binder or loss of a pigment can cause hue-shift. High temperatures can accelerate some of the degradation chemistry, with darker colors usually growing hotter than lighter colors. Temperature cycles affect coating moisture levels and cause thermo-mechanical stress on the coating and adhesion boundary, especially as the substrate expands and contracts. Temperature cycles also help drive coating moisture cycles. These, in combination with sunlight, can cause coatings to micro-crack, blister and lose adhesion. It’s that aspect — high levels of solar radia- tion and warm temperatures combined with plenty of moisture — that makes the subtropical southern tip of Florida so harsh on coatings — and other organics and polymers such as plas- tics. Arizona’s Sonoran Desert, another weath- ering benchmark climate, is much hotter in summer and colder in winter than Miami, and has about 19 percent higher annual UV totals. But with an average of only about 9 percent of Miami’s coating “wet time” (measureable surface moisture), we see only a fraction of South Florida’s weathering in the dry desert. By Allen Zielnik, Atlas Material Testing Technology LLC Outdoor Testing Never Lies Laboratory weather testing of coatings has come a long way, but exposure to the elements is still king.
8
Embed
Outdoor Testing Never Lies - PaintSquare042-49... · 2016-10-24 · paring outdoor test results from different time periods difficult, especially if tests began in different seasons.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
ou might think that conducting
paint-weathering tests is about as
exciting as watching paint dry, only
a lot slower.
Admittedly, not too much has
changed in weather-testing coat-
ings by placing test specimens on “test
fences” at original equipment manufacturer
(OEM) or commercial “test farms.” In fact,
even those colloquial terms, test fences and
test farms, are throwbacks to the first doc-
umented paint exposure tests, begun in 1906
at the Government Agricultural Experiment
Station in Fargo, N.D.
About the only significant changes in “sta-
tic weathering” since then are that the expo-
sure racks now face toward or away from
the equator for mildew tests, rather than
east-west; and that the severe climate of
South Florida has become the standard
benchmark reference instead of Fargo.
Of course, the variety of architectural,
protective and functional coatings, as well as
the range of substrate materials, has
evolved and expanded tremendously since
those days. And while the majority of test
specimens are still coatings on metal panels
or lumber “trade panels,” a walk through a
test-farm reveals no end of specimen types,
from beams and pipes running through con-
crete and masonry walls, to products such
as outdoor furniture.
42 D+D OCTOBER 2014
Y
Coatings A to Zielnik
How Paint WeathersPaints and coatings weather in many ways,
depending on their formulation chemistry, the
substrate and the climate. Even the definition
of “failure” varies, often subjectively. For
some coatings, such as architectural and au-
tomotive, appearance properties such as color
and gloss retention are paramount. For oth-
ers, protecting the substrate is the main
criterion.
In some cases, both are required. The
Florida Department of Transportation, for ex-
ample, has much higher standards for bridge-
coating appearance than do most other
states, owing to Florida’s dependence on
tourism. After all, they have to keep up with
Disney.
So what causes paint to weather? For most
coatings, solar radiation, and particularly its
ultraviolet (UV) component, degrades the paint
film along with temperature and moisture.
These stresses are interconnected and also
vary with the natural daily and seasonal cy-
cles. Film-forming resins can photo-degrade,
resulting in surface erosion which leads to
chalking and loss of gloss, or loss of adhesion
to the substrate.
In the case of wood coatings, sunlight can
degrade the wood’s lignin and cellulose, caus-
ing adhesion loss. UV and visible sunlight can
also fade pigments, resulting in color loss.
Architects often ask me if I know a high-
chroma red that won’t fade to pink.
Yellowing of the binder or loss of a pigment
can cause hue-shift.
High temperatures can accelerate some of
the degradation chemistry, with darker colors
usually growing hotter than lighter colors.
Temperature cycles affect coating moisture
levels and cause thermo-mechanical stress on
the coating and adhesion boundary, especially
as the substrate expands and contracts.
Temperature cycles also help drive coating
moisture cycles. These, in combination with
sunlight, can cause coatings to micro-crack,
blister and lose adhesion.
It’s that aspect — high levels of solar radia-
tion and warm temperatures combined with
plenty of moisture — that makes the subtropical
southern tip of Florida so harsh on coatings —
and other organics and polymers such as plas-
tics. Arizona’s Sonoran Desert, another weath-
ering benchmark climate, is much hotter in
summer and colder in winter than Miami, and
has about 19 percent higher annual UV totals.
But with an average of only about 9 percent of
Miami’s coating “wet time” (measureable
surface moisture), we see only a fraction of
South Florida’s weathering in the dry desert.
By Allen Zielnik, Atlas Material Testing
Technology LLC
Outdoor Testing Never Lies Laboratory weather testing
of coatings has come a long way, but exposure to the elements is still king.
Coatings A to Zielnik43
This points out how important moisture is,
in balance with the other stresses of heat
and solar radiation. So is the South Florida
climate unique in the world? Clearly not; in
fact, some locations have harsher condi-
tions. But due to its long history, historical
data and easy logistics, South Florida has
become the de facto global benchmark loca-
tion for coating testing. In addition to Atlas’
South Florida Test Service, established in
1931, the location hosts several coating
OEM sites, including those of PPG, DuPont
and Valspar.
Hurrying It Up The balance of the main weathering factors
and their natural cycles presents a dilemma in
weathering testing. As Warren Ketola, a well-
known researcher at 3M’s Weathering Re-
source Center, was fond of saying, “Outdoor
testing never lies.” The problem is that it
takes a long time to generate data, especially
in coating formulation development. Re-
searchers have tried various ways of acceler-
ating natural weathering.
For instance, the EMMA (Equatorial Mount
with Mirrors for Acceleration) Fresnel solar
concentrator was originally developed in 1958
by DSET Laboratories in New River, Ariz. It is
a solar tracker with 10 special mirrors to con-
centrate direct-beam solar radiation onto the
test specimens. It requires the dry desert air
to operate — high humidity or clouds scatter
UV in the atmosphere.
Equatorial Mount with Mirrors for Accelera-
tion with Water, or EMMAQUA, simulates the
rain and nighttime condensation of South
Florida, with custom-programmed sprays of
deionized water during the day, and at night
when the device inverts so the test specimens
Countless coatings samples face a harsh environment for prolonged times at a South Florida test site in this photo taken with a fish eye lens.Photos courtesy of Atlas Materials Testing Inc.
Weather-Ometer was an enhancement of
the textile lightfastness color Fade-Ometer
instruments used since 1919. The original
devices used one of two types of electrically
burning carbon arc lamp technologies. These
represented the best simulation of sunlight at
the time.
However, they had serious spectral mis-
matches in the UV and visible wavelengths
which could distort the underlying photochem-
ical processes and, thus, the weathering test
results. Nonetheless, the technique of artifi-
cial weathering for coatings was established,
especially for comparative purposes, though
less useful as a lifetime predictor.
The quest for a better sunlight simulator
made a major leap forward when the xenon
arc gas discharge lamp debuted in Atlas’
Xenotest instrument in 1955. With proper
optical filtering, the xenon arc lamp provides
the closest full-spectrum sunlight simulation.
The spectral match in the UV and visible por-
tions is critical for reproducing the photo-
chemical processes that occur outdoors for
the coating binder, colorants and performance
additives. The near infrared (IR) match is also
important so that specimen temperature,
which varies with IR absorptivity (e.g., coating
color), and its effect on coating moisture con-
point skyward. Special temperature controls
can provide a variety of test conditions. The
technique is subject to the varying sunlight
and weather of the seasons, but does apply
about four times Miami’s annual total ultravio-
let solar radiation on a typical 45-degree in-
clined south-facing exposure rack in one year.
A UA (ultra-accelerated)-EMMAQUA
debuted in 2014. This advanced version uses
20 specialized “cool” mirrors that reflect
solar radiation in the UV and near-visible
(<500 nm) region to increase the UV intensi-
fication factor 10 to 12 times while simulta-
neously eliminating excess heat from the
near-infrared portion of sunlight.
The U.S. Department of Energy National
Renewable Energy Laboratory and Atlas
co-developed this specialized mirror technol-
ogy. It was first used in the Ultra Accelerated
Weathering System (UAWS), which intensifies
annual Miami UV 63 times.
44 D+D OCTOBER 2014
Bringing the Outdoors InOne problematic aspect
of both natural and
accelerated outdoor
testing is that “climate
is what you expect;
weather is what you
get.” This makes com-
paring outdoor test
results from different
time periods difficult,
especially if tests
began in different
seasons.
For a more consis-
tent and reproducible
test environment,
researchers have artificially weathered sam-
ples in the laboratory since 1927. That’s when
the Weather-Ometer was introduced. The
This photo shows the inside of an artificial laboratory weather instrument or “Weather-Ometer.”The machine reproduces and accelerates the effects of full-spectrum solar radiation, temperature,humidity and rain/condensation on the racks of colored samples, but at an accelerated pace.
At a South Florida test facility, samples are angled toward the sun for maxi-mum solar radiation exposure, and are attached to a backing, increasingspecimen temperature, similar to conditions on a building.
ASTM D7869
It is being called “the most thoroughly researched weathering test procedure ever developed.”
Attend a free webinar and get your free copy, compliments of Q-Lab:
for automotive, railway, aerospace, and
other transportation coatings
www.q-lab.com/newASTM
It is being called “the
It is being called “the weathering test procedure ever developed.”
It is being called “the oughly researched most thorweathering test procedure ever developed.”
oughly researched weathering test procedure ever developed.”
oughly researched
ASTM D7869for automotive, railway
other transpor
ASTM D7869, aeros otive, railway y, aerospace, and
tation coatings
ASTM D7869, aerospace, and
tation coatings
Attend a
Attend a free webinar and get your
other transpor
webinar and get your free copy
tation coatingsother transpor
, compliments of Q opy y, compliments of Q-Lab:
tation coatings
, compliments of Q-Lab:
.q-lab.comwwww.q-lab.com/newASTM
.q-lab.com/newASTM
Click our Reader e-Card at durabilityanddesign.com/ric
46 D+D OCTOBER 2014
tent and secondary chemical reactions are
also correct.
Xenon arc artificial weathering combines
the stresses of full spectrum solar radiation,
relative humidity, water sprays and elevated
temperatures under either steady-state or
cyclic conditions, depending on the test proto-
col used. It offers the ability to alter the indi-
vidual parameters to probe the coating for
individual or combined stress sensitivities.
With the proper choice of conditions, it can,
within limits, simulate various climates.
Researchers use Xenon arc artificial weather-
ing for relative comparisons under prescribed
conditions, or to determine failure modes and
estimate the service lifetime of coatings,
although the latter is complex and only in
its infancy.
UV emission at 313 nm. The other is the UVA-
340 which peaks at 340 nm. Both lack the
higher visible and near infrared wavelengths
of sunlight.
The UVB lamp has wavelengths well below
that of natural sunlight and was once popular,
since some researchers believed that the
extra UV would increase test acceleration.
However, given the possibility of radically
altering the natural photo-degradation path-
ways, and producing different but not neces-
sarily faster results, its use is now widely not
advised in the governing standards.
Although the UVA lamps’ lack of higher UV
and longer wavelengths can result in photo-
degradation differences compared to sunlight
for some coating chemistries, the overall
coating industry experience with it has been
good. However, it is rarely used to test high-
performance coatings such as automotive
paint systems. The UV test generally isn’t
considered a true weathering test, but rather
a useful screening tool.
A New Model for Artificial Weathering TestsLaboratory artificial weathering instruments
have various adjustable parameters which
control the spectral and irradiance charac-
teristics of the solar simulation, mode and
quantity of moisture delivery, temperatures
and their cycles. One criticism of artificial
weathering is that correlation to field expo-
sure, such as South Florida, can be lacking,
either universally or for specific coating test
specimens because many of the test cycles
which are used in these devices are inade-
quate. For example, many of the most com-
mon test cycles go back to the carbon arc
heydays of the 1930s and 1940s and are still
used today with xenon, even though they
were never designed to match South Florida.
The situation is gradually improving, however.
A consortium of companies has worked on
developing a xenon arc test method with
greatly improved South Florida correlation
for high-performance automotive and aero-
How to Test the BinderIn coating formulation development, the binder
performance is of primary interest. The binder
fundamentally determines coating perform-
ance and lifetime. This includes mechanical
and other physical properties such as hard-
ness, erosion- and embrittlement-resistance,
and adhesion.
As lower wavelength UV and moisture expo-
sure most commonly affect binder properties,
another laboratory technique alternating expo-
sure to UV fluorescent lamps and moisture
condensation was developed in the 1950s. The
QUV and UVTest instruments are widely used
for high-throughput, low-cost comparative
screening of coating resin formulations.
The test uses one of two types of UV fluo-
rescent lamps. One is the UVB-313 with peak
The Equatorial Mount with Mirrors for Acceleration with Water or EMMAQUA focuses sunlight onto a rackof samples with 10 first-surface reflective mirrors as it tracks the sun across the sky. Water spraysduring the day provide thermal shock.
Over a Decade of Quality Painting Leads
COMMERCIAL PAINTINGLEADS ON DEMAND
Scan this barcode with your smartphonefor a FREE trial
Find work faster with Paint BidTracker, the only lead servicedesigned for the coatings industry. Try it for FREE today, visit www.paintbidtracker.com/trial to claim your 5-day trial.
• Targeted painting leads sent right to your inbox • Searchable database • Plans and specifications
P
48 D+D OCTOBER 2014
space coatings. Their efforts focused on get-
ting an even better sunlight spectral match,
and cycling irradiance levels and specimen
temperatures consistent with Miami daylight.
Further, they based moisture delivery and
timing on actual coating behavior (rates and
cycles of moisture uptake and desorption)
in Miami.
The new test cycle, published in 2013 after
a decade of work, is ASTM D7869 Standard
Practice for Xenon Arc Exposure Test with
Enhanced Light and Water Exposure for
Transportation Coatings. The test serves as
a model of how to develop weathering test
conditions.
In addition to greatly improved South
Florida correlation, the test is 40 percent
more accelerated than the previously
employed test. Although not yet validated for
other coating systems, the standard is being
Hundreds of thousands of paint samples face the weather at a South Florida test site. Some are rela-tively short one- to two-year exposures for evaluating formulations in development. Others are longerfive- to 10-year tests to establish warranty and service life. Some specimens have weathered for morethan 30 years and haven’t yet failed.
Click our Reader e-Card at durabilityanddesign.com/ric
he specializes in the
weather durability testing
of materials and products.
A frequent speaker at
various worldwide techni-
cal symposia, he is the
author of more than 120
Coatings A to Zielnik49
Scan this QR code withyour smartphone to
learn more.
WWW.DURABILITYANDDESIGN.COM
Building Performance & AestheticsOnline • In Print • Daily eNewsletter
A Technology Publishing Co. Product
With the Durability + Design Digital Edition• Content optimized for all smartphones and tablets
• Fully downloadable to Mac or PC desktops and laptops for offline viewing
• Fully searchable and printable
• Articles shareable via email, Facebook, Twitter & Google+
DURABILITY + DESIGN GOES WHERE YOU GO
evaluated by many coatings researchers to
speed product development. It is under the
jurisdiction of ASTM subcommittee D01.27 on
Accelerated Testing.
The Outdoor Test is Always RightThe reality of weather durability testing is
that the outdoor test is always right. Any
accelerated test invariably alters the natural
ratio and cycles of the key weathering
stresses, which can introduce error. Labora-
tory artificial weathering can minimize the
variability of outdoor exposures, but test con-
ditions must be appropriate for both the coat-
ing chemistry and the intent of the test, such
as a match to South Florida results. With the
proper selection of testing technique and
parameters, coatings manufacturers can
increase their confidence in and reliance on
accelerated weathering to make decisions
about coating formulations. Often this
requires a break from past methods. Ulti-
mately, the results of any laboratory test
must be validated with real-world, real-time
exposures.
To help facilitate this, researchers created
the Worldwide Exposure Network to include
major global climates and geographies.
As coatings become more durable and long-
lived, the need for reliable accelerated testing
increases, and new tools and techniques have
already emerged. It’s an exciting time in dura-
bility testing, certainly better than sitting
around watching paint panels slowly weather
on a test fence.
NotesEMMA, EMMAQUA, UA-EMMAQUA, Xenotest,
Weather-Ometer, Fade-Ometer and UVTest
are trademarks of Atlas Material Testing
Technology. QUV is a trademark of Q-Lab
Corporation.
About the Author Allen Zielnik is senior consultant-weathering
science in Atlas Material Testing Technology
LLC’s global Consulting Solutions group, where
publications and conference presentations. He
has degrees in electronics engineering and ana-
lytical chemistry, and since 1994, he has been
involved with natural and accelerated weather-
ing technology and solar simulation with Atlas
Material Testing Technology. D+D
Click our Reader e-Card at durabilityanddesign.com/ric