ASTM G141-96
(Guide)Standard Guide for Addressing Variability in Exposure Testing on Nonmetallic Materials
Standard Guide for Addressing Variability in Exposure Testing on Nonmetallic Materials
SCOPE
1.1 This guide covers information on sources of variability and strategies for its reduction in exposure testing, and for taking variability into consideration in the design, execution, and data analysis of both exterior and laboratory accelerated exposure tests.
1.2 The values stated in either inch-pound or SI units are to be regarded separately as the standard. The values given in parentheses are for information only.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
General Information
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Standards Content (Sample)
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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Designation: G 141 – 96
Standard Guide for
Addressing Variability in Exposure Testing on Nonmetallic
Materials
This standard is issued under the fixed designation G 141; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
No experimental procedure is exactly repeatable or reproducible. Exposure testing is susceptible to
poor test reproducibility because of many contributing factors. These include the type of material and
its homogeneity, the complexity and variability of the outdoor environment, difficulty in precisely
controlling the laboratory testing environment, and the variability in the measurement of performance.
It is extremely difficult to compare “absolute data”, that is, color shift, gloss, tensile, and elongation,
and so forth, from different exposure tests. This is true for natural and accelerated exposures conducted
outdoors or for accelerated exposure tests conducted at different times in one laboratory or comparing
results between laboratories. The purpose of this guide is to provide the user with background
information on test variability and guidance to conduct an exposure test that will provide valid and
useful durability information.
1. Scope G 7 Practice for Atmospheric Environmental Exposure
Testing of Nonmetallic Materials
1.1 This guide covers information on sources of variability
G 23 Practice for Operating Light-Exposure Apparatus
and strategies for its reduction in exposure testing, and for
(Carbon-Arc Type) With and Without Water for Exposure
taking variability into consideration in the design, execution,
of Nonmetallic Materials
and data analysis of both exterior and laboratory accelerated
G 24 Practice for Conducting Exposures to Daylight Fil-
exposure tests.
tered Through Glass
1.2 The values stated in either inch-pound or SI units are to
G 26 Practice for Operating Light-Exposure Apparatus
be regarded separately as the standard. The values given in
(Xenon-Arc Type) With and Without Water for Exposure
parentheses are for information only.
of Nonmetallic Materials
1.3 This standard does not purport to address all of the
G 53 Practice for Operating Light- and Water-Exposure
safety concerns, if any, associated with its use. It is the
Apparatus (Fluorescent UV-Condensation Type) for Expo-
responsibility of the user of this standard to establish appro-
sure of Nonmetallic Materials
priate safety and health practices and determine the applica-
G 90 Practice for Performing Accelerated Outdoor Weath-
bility of regulatory limitations prior to use.
ering of Nonmetallic Materials Using Concentrated Natu-
2. Referenced Documents
ral Sunlight
G 147 Practice for Conditioning and Handling of Nonme-
2.1 ASTM Standards:
tallic Materials for Natural and Artificial Weathering Tests
D 4853 Guide for Reducing Test Variability
E 177 Practice for Use of the Terms Precision and Bias in
3. Terminology
ASTM Test Methods
3.1 Definitions:
E 691 Practice for Conducting an Interlaboratory Study to
3.1.1 Terminology G 113 is generally applicable to this
Determine the Precision of a Test Method
guide.
4. Significance and Use
This guide is under the jurisdiction of ASTM Committee G-3 on Durability of
Nonmetallic Materials and is the direct responsibility of Subcommittee G03.93 on
4.1 Many standards and specifications reference exposure
Statistics.
tests performed according to standards that are the responsi-
Current edition approved July 10, 1996. Published December 1996.
2 bility of Committee G-3 on Durability of Nonmetallic Mate-
Annual Book of ASTM Standards, Vol 07.01.
Annual Book of ASTM Standards, Vol 14.02. rials. In many cases, use of the data generated in these tests
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NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
G 141
fails to consider the ramifications of variability in the exposure the same solar-ultraviolet radiation dose may not give the same
test practices. This variability can have a profound effect on the degree of change unless the heat and moisture levels are also
interpretation of results from the exposure tests, and if not identical.
taken into consideration in test design and data analysis, can 5.2.1 Another problem related to timing exposures by
lead to erroneous or misleading conclusions. This guide lists broad-band radiation measurements is that solar radiation in
some of the sources for test variability and recommends the 290 to 310-nm band pass exhibits the most seasonal
strategies for executing successful weathering studies. Not all variability. Many polymer systems are extremely sensitive to
sources of variability in weathering testing are addressed in this radiation in this band pass. Variations in radiance in this critical
guide. Specific materials, sampling procedures, specimen region (because of their relatively small magnitude) are not
preparation, specimen conditioning, and material property adequately reflected in total solar radiation or broad-band solar
measurements can contribute significantly to variability in ultraviolet (UV) measurements.
weathering test results. Many of these concerns are addressed 5.2.2 The time of year (season) that an exposure test is
in Guide G 147. To reduce the contribution of an instrumental initiated has, in certain instances, led to different failure rates
method to test variability, it is essential to follow appropriate for identical materials (1).
calibration procedures and ASTM standards associated with 5.3 Variability Due to Year-to-Year Climatological
the particular property measurement. Additional sources of Variations—Even the comparison of test results of full-year
variability in test results are listed in Guide D 4853, along with exposure increments can be difficult. Average temperature,
methods for identifying probable causes. hours of sunshine, and precipitation can vary considerably
from year to year at any given location. The microclimate for
the test specimens can be affected by yearly differences in
5. Variability in Outdoor Exposure Tests
pollution levels, airborne particulates, mold, and mildew.
5.1 Variability Due to Climate—Climate at the test site
These differences can impact material failure rates. Results
location can significantly affect the material failure rates and
from a single-exposure test cannot be used to predict the
modes. Typical climatological categories are; arctic, temperate,
absolute rate at which a polymer degrades. Several years of
subtropical, and tropical (that are primarily functions of lati-
repeat exposures are needed to get an “average” test result for
tude). Subcategories may be of more importance as being
any given test site.
dictated by geographic, meteorological, terrain, ecological, and
5.4 Variability Due to Test Design—Every exposure test has
land-use factors, and include such categories as desert, for-
some variability inherent in its structure and design. Specimen
ested, (numerous classifications), open, marine, industrial, and
placement on an exposure rack (2), and type or color of
so forth. Because different climates, or even different locations
adjacent specimens can also affect specimen temperature and
or orientation in the same climate, produce different rates of
time of wetness. Sample backing or insulation as well as rack
degradation or different degradation mechanisms, it is ex-
location in an exposure site field can affect specimen tempera-
tremely important to know the characteristics of the exposure
ture and time of wetness.
sites used and to evaluate materials at a range of sites that 5.5 Variability in Glass-filtered Daylight Exposures—Glass-
represent the full spectrum of anticipated service conditions. filtered daylight exposures as described by Practice G 24 are
Exposure sites in one climate (even those in close proximity) subject to many of the test variables previously described.
Recent studies conducted by ASTM Subcommittee G03.02 on
can cause significantly different results, depending on material.
Natural Environmental Testing has demonstrated that the glass
5.2 Variability Due to Time of Year—Solar-ultraviolet radia-
used in these exposures can be highly variable in its light
tion, temperature, and time of wetness vary considerably with
transmission characteristics between 300 and 400 nm that can
time of year. This can cause significant differences in the rate
significantly impact exposure results (3). In addition, solariza-
of degradation in many polymers. Therefore, comparison of
tion processes can alter these transmission characteristics
results between short-term exposure studies (less than one or
during the first few months of exposure. Specimen temperature
more full years) is extremely risky. If exposures of less than a
can also vary depending on location within an under glass test
full year are required, consider using times when climatologi-
rack (4).
cal stress is maximized so a worst case test result is obtained.
It may also be valuable to make several exposure tests with
6. Variability in Accelerated Outdoor Exposures Using
varying start dates in order to provide more representative data.
Concentrated Sunlight
This is especially true when the material’s response to the
6.1 Accelerated outdoor exposures using Fresnel concentra-
environment cannot be predetermined, or when materials with
tors are described in Practice G 90. Test results are subject to
different environmental responses are to be compared. Often
normal climatological and seasonal variations. Exposure peri-
exposure periods are timed by total solar or solar-ultraviolet
ods are described by a radiant energy dose, most often in the
dose, or both. This approach may reduce variability in certain
UV region of sunlight. The UV content of the concentrated
instances. However, an inherent limitation in solar-radiation
sunlight is significantly reduced during winter exposures and is
measurements is that they do not reflect the effects of variation
also subject to normal year-to-year variations. As mentioned in
in temperature and moisture, which are often as important as
5.2, current radiant energy band passes, both total solar and
solar radiation. Temperature and time of wetness are highly
dependent on time of year, especially in temperate climates.
With materials that are sensitive to heat or moisture, or both, Annual Book of ASTM Standards, Vol 06.01.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
G 141
broad-band UV, used in reporting solar dose do not adequately 7.2.2 Irradiance and specimen temperatures can vary sig-
reflect variations in the critical 290 to 310-nm range. Because nificantly throughout the allowed specimen exposure area,
of the time of year differences in the amount of available especially in older test equipment.
ultraviolet, timing exposures based on accumulated ultraviolet 7.2.3 Water contaminants or impurities and poor spray
dose can improve test-to-test variability, but may not account quality, that is, clogged spray nozzles, can cause specimen
for the substantial specimen temperature differences that exist spotting that will give misleading durability results by impact-
between summer and winter. ing visual observations, reducing specular gloss values, caus-
ing unnatural color shifts, or by impacting other optical
6.2 Significant variability in test results can occur between
properties.
laboratories conducting accelerated outdoor exposures using
7.2.4 Ambient temperature and humidity conditions in the
concentrated sunlight (4). Identical materials exposed for the
testing laboratory can affect test chamber conditions and device
same time period, but at different sites within close proximity
operation. In fluorescent UV condensation devices, high am-
to each other, had significantly different failure rates.
bient temperatures can reduce the amount of condensate that
6.3 When test conditions specify water spray, water quality
forms on the test specimens. If the device does not have an
is extremely critical. Water contaminants or impurities can
irradiance control system, ambient temperature can also affect
cause specimen spotting that will give misleading durability
irradiance at the specimen plane.
results.
8. Addressing Variability in All Exposure Tests
7. Variability in Laboratory Exposure Tests
8.1 Extreme caution must be used when comparing test
7.1 A round-robin evaluation of Practice G 23, G 26, and
results between different laboratories or from different time
G 53 exposure tests was performed between 1985 and 1992
periods. This applies equally to laboratory accelerated tests,
comparing the gloss retention of various vinyl tapes (6).
outdoor exposure tests, and outdoor accelerated tests. The
Although the variability reported is specific to the materials
safest approach is to treat each exposure test as a separate
tested and the participating laboratories, these referenced
entity and make durability comparisons for materials exposed
round-robin studies serve as a warning to users of durability
at the same time in the same device or at the same outdoor
test standards that high levels of variability may be possible
exposure site.
with any test or material.
8.2 The proper use of experimental design and data analysis
7.1.1 Repeatability—In general, test precision within labo-
techniques can cope with the variability inherent to weathering
ratories (a single test period in a test device) will always be
testing. Examples of suitable statistical methods for analyzing
better than precision between laboratories. By testing replicate
weathering results are described in a guide currently under
specimens, statistically significant performance differences
development in Subcommittee G03.93.
among materials can be readily established during a specific
8.3 General Considerations:
exposure period in an individual test device.
8.3.1 Round-robin studies (6) conducted by Committee G-3
7.1.2 Reproducibility—The G03.03 round-robin studies indicate that nominally similar tests can cause significantly
found that between laboratory comparisons of absolute gloss differing failure rates, but rank performance for a series of
values after a fixed exposure time is, in a practical sense, materials is quite reproduc
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