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Best practices in weathering
Manufacturers need to know how well their products
will perform in the environment in which they are used.
Testing for weathering and light stability is essential.
Thus, outdoor exposure testing and accelerated testing
in the laboratory are widely used for research and
product development, quality control, or material
certification. Is one more reliable than the other? Can
they be correlated?
Outdoor and accelerated testing compared
Michael J. Crewdson*
Warren D. Ketola 1)
Most weathering damage is caused by light (especially
short-wavelength UV), high temperature and moisture. Any
one of these factors may cause deterioration. They may
work also together to cause even more damage.
In the laboratory, these natural elements can be reproduced
and accelerated in test devices such as the fluorescent
UV ("QUV"), or xenon-arc test chambers. Such equipment
can provide fast and reproducible results. Accelerated
weathering is a useful tool for product research and it
is a requirement of many international specifications.Time
pressures in product development may drive manufacturers
to choose only accelerated testing and not outdoor
exposures. However, variables exist which, without
comparative data, may lead to incorrect results or faulty
conclusions.
A well-planned weathering programme, incorporating both
outdoor exposures and accelerated weathering, allows
measurement of the rank correlation between laboratory
and real-world results. Rank correlation is the amount of
agreement in how two sets of data rank performance. An
example of the rank correlation statistic is given later in this
paper.
Researchers must be cautious about trying to find an
"acceleration factor" that will magically calculate how
many hours in a weathering test device equal how much
outdoor exposure for all materials. There is too much
inherent variability in weathering testing and the response of
materials to the effects of light, heat, and moisture for such
a factor to be reliable.
Too many variables
A major problem in comparing outdoor and laboratory
tests is that the weathering-chamber environment is well
controlled, whilst nature is not. Outdoor exposure tests are
subject to a number of variables. They include:
››› Daily light/dark cycle and weather changes
››› Latitude of the exposure site (more UV closer to the
equator)
››› Altitude (higher means more UV)
››› Local conditions (e.g. a constant wind dries the test
samples)
››› Random year-to-year variations in the weather
(degradation can vary significantly in successive years at
the same location)
››› Seasonal variations (winter exposure may be less severe
than summer exposure) (Figure 1)››› Orientation of the
sample (many orientations can be used, ranging from 5°
facing the equator to vertical facing away from equator)
››› Sample insulation (outdoor samples with insulated
backing often degrade 50 % faster than non-insulated
samples)
››› Variable properties of test materials
Accelerated testing has fewer variables. These may be
controlled by using correct operating and maintenance
procedures:››› Laboratory test equipment operating cycle
(light/dark/wet cycles are usually specified by a standard)
››› Temperature used in the laboratory test (hotter is faster)
››› Variability in the test equipment light source (spectralpower distribution, manual or automatic irradiance control,
lamp and filter aging)
››› Variable properties of test materials
No matter how carefully the conditions are set, laboratory
and outdoor exposures are different one is controlled, the
other is unpredictable. Not all of the factors of weathering
can be re-created in the equipment used for accelerated
tests. There are cyclic conditions in nature that laboratory
tests can closely simulate, but not match. The best practice
is to always use outdoor weathering to verify accelerated
testing. Without this verification, results and predictions
from accelerated testing are ultimately just "guesses".
Start testing outdoors
It is important to begin testing outdoors as soon as
possible. With a correct plan and execution, an outdoor
test can produce meaningful data in a relatively short
time. A successful outdoor weathering programme starts
with good experimental design, use of control materials
and test repetition. An outdoor location should be chosen
that has the most intense weather for the required test
conditions. Southern Florida provides a hot, wet and highUV environment (Figure 2). Desert locations in Arizona
are very hot and very dry, with higher annual total UV
than Florida. Other outdoor climate selections are available,
depending on how the product under test will be used.
Any material can be tested outdoors. Choose an
appropriate test for the material’s expected end-use
environment. Natural, direct exposures may be combined
with additional factors such as salt spray for corrosion
testing, or freeze/thaw cycles to produce thermal shock.
Under-glass exposures permit testing of interior materials.
Concentrator systems use mirrors to increase the intensity
of solar radiation by 5 to 7.5 times for accelerated
outdoor testing (Figure 3).It is important to consider critical
formulation variables when setting up the weathering
experiment and to make sure that critical variables are
included in each test. An equal number of specimens
should be exposed in each test and the exposure periods
should be similar, taking into account the time of year.
Mounting, handling and analysis methods must be repeated
consistently to enable test comparisons to be reliable.
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Evaluation intervals must be scheduled so that rapidlyoccurring changes are not overlooked.
The greater the number of specimens, the better the
data analysis. The use of three or more replicates of
each material under test allows mean and standard
deviation calculations and permits greater confidence in the
conclusions derived from the results.
The use of a control is very important. This is material which
reacts to weathering in a known way. It should not be a
standard reference material such as the polystyrene chip,
but a similar material to the one being tested for which the
data is reliable and repeatable. It is best to use control
materials with good and bad durability. The control can be
used to compare different tests and different exposures and
to normalize experimental results.
Test repetition is vital to prove the reliability of the test
method. The first step in writing a standard test is to prove
that it is repeatable. The additional variable factors that can
appear when testing need to be determined and measured.
Laboratory accelerated testing
Accelerated weathering testing is used for R&D or
product development and enables confidence in the
selection between various material or process options when
developing a product, or between various product options
for an application.
To determine which accelerated-weathering test conditions
to use, the type of degradation or failure that is most
critical for your customer must be considered. For colour
stability, a xenon-arc exposure device is normally used,
because it provides the full wavelength spectrum of UV and
visible radiation known to cause fade and colour change
(Figure 4). If surface integrity (such as gloss retention),
or the retention of mechanical properties (such as impact,
tensile, or flexural strength) are important, a fluorescent UV
chamber with UVA340 lamps is generally used. It provides
the best possible simulation of short-wave UV, a radiation
which is known to cause changes in physical properties.
A wide variety of exposure conditions may be reproduced
in either type of device. Here are some general principles
that should help when selecting test conditions:
››› High irradiance will produce faster results, but will
probably increase test equipment operation costs, because
lamps or filters will require replacement more frequently.
››› A test temperature that is high, but not so high that it will
cause unrealistic thermal degradation, should be used (70
°C black-panel temperature is a reasonable starting point
for many materials).
››› The highest possible moisture stress is needed (water
spray, relative humidity, condensation time). In most cases,
product failures occur faster in climates where dew causes
materials to become wet for longer periods. Fluorescent UV/
condensation exposure is the only accelerated weathering
test that actually produces dew on the surface of materials
during exposure.
When comparing materials, a control provides a benchmark
for evaluating the performance of new materials and
assessing their durability for use in a product. Comparisons
of test materials must not be made with a material of
known performance that was exposed at a different time.
Test variability makes such comparisons extremely risky.
A control material must be exposed side-by-side with test
materials.
Replicate specimens of all the test materials and of the
control must be used. Using only one specimen of each
material does not permit statistical analysis of the results;
so one cannot be confident about differences between the
materials. Two replicates is an acceptable number, but
three or more is better.
It is best to run the test until the materials fail. Another
approach is to stop once there is a statistically significant
difference between the test and control material. The best
method is to run the test until all the materials fail.
Comparing exposure results
In order to develop confidence in accelerated tests, it must
be possible to compare their results with those from outdoor
exposures hence, the critical need to start outdoor exposure
testing first. Ideally, there would be outdoor results for
materials or products with a range of performance from
good to poor. When accelerated weathering tests are run
on these materials, rank correlation is used to determine
whether the test conditions chosen will be useful for the
application.
Control materials should be exposed in each test and
the type and rate of degradation compared with the test
material. This will ensure that the accelerated test is
providing the correct results. Table 1 shows an example of
a balanced weathering test programme
The degradation mode must be the same in all laboratory
and outdoor tests. Tests that produce different failure
modes will have poor correlation to outdoor tests, since they
are not affecting the same variable. All known degradation
modes should be evaluated.
Non-destructive observations such as visual appearance,
gloss and colour measurement can be measured
repeatedly on the same sample. However, to obtain
information such as chemical and internal properties for
the samples, destructive methods are needed and several
samples should be used. Tensile strength, hardness and
abrasion are also evaluated using destructive test methods.
Statistical methods may be employed to assess the
reliability of test results. Spearman rank-order correlation
can indicate whether an accelerated laboratory test is a
good predictor of real-time, outdoor weathering results.
Spearman rank correlation
This is carried out by ranking the order of performance for
the materials in each test and determining the differences
between the rank data by subtraction.
The differences are squared and added up to give The
following equation is used to calculate the Spearman rank
correlation coefficient rs (N is the number of materials
compared).Rank order sorts the results in order of
performance. For example, if ten materials are being tested,
each material is assigned a rank number from 1 to 10
in order of performance. Materials that are the same in
performance are assigned the same rank.
The time to achieve 50 % loss of initial gloss is an example
of a measurable target for rank order. In this case, ranking
should be done at the end of the test. If measuring a
property or change in a property for ranking, this can take
place at any time. The highest rank correlations occur when
the difference in performance between the materials is the
largest.
Acceleration factor
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Accelerated (laboratory) and natural (outdoor) test results
are often compared by looking at the respective exposure
time needed to produce the same degree of degradation.
Ranking is used to verify the observation. Alternatively, the
mean values of the experimental data sets are compared.
By correlating laboratory and outdoor test results, it is
possible to calculate an acceleration factor (AF). This is the
outdoor exposure time divided by the laboratory exposure
time needed to produce the same amount of change in the
material.
An acceleration factor for one material is often wrongly
assumed to apply to other materials and may be incorrectly
used to develop warranties for product lifetime. The huge
range of material responses to the effects of light, heat, and
moisture means that an acceleration factor for one material
will not apply to others. Thus, using one acceleration factor
for many different materials can lead to very large errors in
the estimates of actual times to fail in the real environment.
Michael J. Crewdson
Q-Lab Weathering
Research Service
Tel. +1 305 245 5600
mcrewdson@q-lab.com
Results at a glance
»› Accelerated weathering must be verified using outdoor
weathering. »› Laboratory testing is a useful tool for product
research and development, but without benchmark of
outdoor data, there are a number of variables that may
lead to a wrong conclusion.»› Statistical comparison of
accelerated and outdoor weathering tests is essential.»›
Reference specimens, a large sample size and repeated
testing are the key to confidence in the results.»›
Weathering analysis costs money. Not doing it costs
even more.»› A single conversion factor for accelerated
v. outdoor exposure does not exist. Weathering data is
comparative, not absolute.
Tips for successful testing
Accelerated weathering must be verified by outdoor
weathering. Laboratory testing is a useful tool for product
research and development. Without benchmark outdoor
data, there are a number of variables that may lead to a
wrong conclusion.
It is important to use as many replicates as possible for each
material being evaluated in a weathering programme. The
specimens under test should be examined and evaluated
often and regularly. Control or reference specimens are
useful for comparison with historical data. Tests should be
repeated to gain confidence that the observed weathering
effect is understood.
Relative costs should be considered in planning the test
programme. Outdoor weather testing provides inexpensive,
realistic data. The cost to test a useful set of samples can be
as low as $500-$1000 USD a year. This small expense can
result in very useful benchmark data. Accelerated testing
entails higher costs, but results are available much faster.
The cost of not testing at all is potentially the highest. It can
result in recalls and replacements and damage a company’s
reputation for producing reliable products.
Fast and easy formulas do not exist. A single conversion
factor between hours of accelerated weathering and months
of outdoor exposure would desirable, but is theoretically
impossible. The former is measured under constant
conditions, whereas the conditions of the latter are variable.
The search for a reliable conversion factor requires pushing
the data beyond the limits of its validity. Weathering data is
comparative, not absolute. í
REFERENCES
[1] M. Crewdson, Outdoor Weathering Must Verify
Accelerated Testing (technical paper presented at The
Waterborne Symposium, 2008, New Orleans, Louisiana) [2]
D. Grossman, Correlation Questions & Answers, Q-Panel
Lab Products Technical Bulletin LU-0833, 2002. [3] W.
Ketola, Make the Most of Accelerated Laboratory Testing
and Don’t Trust Acceleration Factors to Compare Lab &
Outdoor Testing, Q-Lab Corporation LabNotes, 2008-2009.
[4] ASTM G141, Standard Guide for Addressing Variability
in Exposure Testing of Nonmetallic Materials. [5] ASTM
G169, Standard Guide for Application of Basic Statistical
Methods to Weathering Tests.
* Corresponding Author:
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