ASTM G90-23

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it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: G90 − 17 G90 − 23
Standard Practice for
Performing Accelerated Outdoor Weathering of Materials
Using Concentrated Natural Sunlight
This standard is issued under the fixed designation G90; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope Scope*
1.1 Fresnel-reflecting Linear Fresnel reflector concentrators using the sun as source are utilized in the accelerated outdoor
exposure testing of materials.
1.2 This practice covers a procedure for performing accelerated outdoor exposure testing of materials using a Fresnel-reflector
linear Fresnel reflector, accelerated outdoor weathering, test machine. The apparatus (see Fig. 1 and Fig. 2) and guidelines are
described herein to minimize the variables encountered during outdoor accelerated exposure testing.
1.3 This practice does not specify the exposure conditions best suited for the materials to be tested but is limited to the method
of obtaining, measuring, and controlling the procedures and certain conditions of the exposure. Sample preparation, test conditions,
and evaluation of results are covered in existing methods or specifications for specific materials.
1.4 The Fresnel-reflector linear Fresnel reflector accelerated outdoor exposure test machinesapparatus described may be suitable
for the determination of the relative durability of materials when these materials are exposed to concentrated sunlight, heat, and
moisture, provided the mechanisms of chemical or physical change, or both, which control the acceleration factors for the materials
do not differ significantly.moisture.
1.5 This practice establishes uniform sample mounting and in-test maintenance procedures. Also included in the practice are
standard provisions for maintenance of the machine and Fresnel-reflector linear Fresnel reflector mirrors to ensure cleanliness and
durability.
1.6 This practice shall apply to specimens whose size meets the dimensions of the target board as described in 8.2.
1.7 For test machines currently in use, this practice may not apply tois not recommended for specimens exceeding 13 mm ( ⁄2 in.)
in thickness because cooling may be questionable.of specimen cooling.
1.8 Values stated in SI units are to be regarded as the standard. The inch-pound units in parentheses are provided for information
only.
1.9 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
This practice is under the jurisdiction of ASTM Committee G03 on Weathering and Durabilityand is the direct responsibility of Subcommittee G03.02 on Natural and
Environmental Exposure Tests.
Current edition approved Dec. 1, 2017July 1, 2023. Published December 2017August 2023. Originally approved in 1985. Last previous edition approved in 20102017 as
G90 – 10.G90 – 17. DOI: 10.1520/G0090-17.10.1520/G0090-23.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G90 − 23
FIG. 1 Schematic of Fresnel-Reflecting Linear Fresnel Reflector Concentrator Accelerated Weathering Machine Single Axis Tracking
1.10 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D859 Test Method for Silica in Water
D1014 Practice for Conducting Exterior Exposure Tests of Paints and Coatings on Metal Substrates
D1435 Practice for Outdoor Weathering of Plastics
D1898 Practice for Sampling of Plastics (Withdrawn 1998)
D4141 Practice for Conducting Black Box and Solar Concentrating Exposures of Coatings
D4364 Practice for Performing Outdoor Accelerated Weathering Tests of Plastics Using Concentrated Sunlight
D4517 Test Method for Low-Level Total Silica in High-Purity Water by Flameless Atomic Absorption Spectroscopy
D5722 Practice for Performing Accelerated Outdoor Weathering of Factory-Coated Embossed Hardboard Using Concentrated
Natural Sunlight and a Soak-Freeze-Thaw Procedure
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
G90 − 23
FIG. 2 Dual Axis Tracking
E816 Test Method for Calibration of Pyrheliometers by Comparison to Reference Pyrheliometers
E824 Test Method for Transfer of Calibration From Reference to Field Radiometers
E903 Test Method for Solar Absorptance, Reflectance, and Transmittance of Materials Using Integrating Spheres
G7 Practice for Natural Weathering of Materials
G24 Practice for Conducting Exposures to Daylight Filtered Through Glass
G113 Terminology Relating to Natural and Artificial Weathering Tests of Nonmetallic Materials
G130 Test Method for Calibration of Narrow- and Broad-Band Ultraviolet Radiometers Using a Spectroradiometer
G167 Test Method for Calibration of a Pyranometer Using a Pyrheliometer
G169 Guide for Application of Basic Statistical Methods to Weathering Tests
G173 Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface
G179 Specification for Metal Black Panel and White Panel Temperature Devices for Natural Weathering Tests
2.2 Other Standards:
SAE J576 Plastic Materials for Use in Optical Parts Such as Lenses and Reflectors of Motor Vehicle Lighting Devices
SAE J1961 Accelerated Exposure of Automotive Exterior Materials Using A Solar Fresnel Reflector Apparatus
WMO Guide to Meteorological Instruments and Methods of Observation WMO No. 8, Fifth Edition
Available from SAE International (SAE), 400 Commonwealth Dr., Warrendale, PA 15096-0001, http://www.sae.org.
Available from World Meteorological Organization, Geneva, Switzerland.
G90 − 23
2.3 ISO Standards:
ISO 4892–1 Plastics— Methods of Exposure to Laboratory Light Sources—Part 1: General Guidance
ISO 9060 (2018) Specification and Classification of Instruments for Measuring Hemispherical Solar and Direct Solar Radiation
3. Terminology
3.1 DefinitionsDefinitions:—Definitions
3.1.1 Definitions of terms common to G03 durability standards can be found in Terminology G113. of terms common to G03
durability standards can be found in Terminology G113.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 linear Fresnel reflector, n—an array of long narrow segments of mirrors to concentrate sunlight onto a fixed receiver located
at a common focal point of the reflectors.
4. Significance and Use
4.1 Results obtained from this practice can be used to compare the relative durability of materials subjected to the specific test
cycle used. No accelerated test can be specified as a perfect simulation of natural or field exposures. Results obtained from this
practice can be considered as representative of natural weathering only when the degree of comparative performance has been
established for the specific materials being tested.a sufficient magnitude of mathematical correlation exists between exposures.
4.2 The acceleration factor relating the rate of degradation in this accelerated exposure to the rate of degradation in a natural
weathering exposure varies with the type and formulation of the material. Each material and formulation may respond differently
to the increased level of irradiance and differences in temperature and humidity. Thus an acceleration factor determined for one
material may not be applicable to other materials. For this reason, the use of a single acceleration factor is not recommended. Also,
a different acceleration factor may be obtained by using different mirror types and configurations. Because of variability in test
results for both accelerated and natural weathering exposures, results from a sufficient number of tests must be obtained to
determine an acceleration factor for a material. Further, the acceleration factor is applicable to only one exposure location because
results from natural weathering will vary due to seasonal or annual differences in climatic factors.
4.3 The relative durability of materials determined by this practice can be used to determine the relative durability of the materials
exposed under natural weathering conditions provided the materials have similar acceleration factors. However, even if results
from a specific accelerated test condition are found to be useful for comparing the durability of materials exposed in a particular
exterior location, it cannot be assumed that they will be useful for determining the relative durability for a different location. The
relative durability of materials in natural weathering exposure can be very different depending on the location of the exposure
because of differences in important climatic factors, such as sunlight, time of wetness, temperature, pollutants, etc.
4.4 Variations in results may be expected when operating conditions vary within the limits of this practice.
4.5 Variations in results may be expected when operating conditions vary within the limits of this practice. For example, there can
be large differences in the amount of degradation in a single material between separate, although supposedly identical, exposures
carried out for the same duration or number of exposure cycles. This practice is best used to compare the relative performance of
materials tested at the same time in the same fresnel linear Fresnel reflector device. Because of possible variability between the
same type of exposure device and variability in irradiance, temperature and moisture levels at different times, comparing the
amount of degradation in materials exposed for the same duration or radiant energy at separatedifferent times is not recommended.
4.6 This practice should not be used to establish a “pass/fail” approval of materials after a specific period of exposure unless
performance comparisons are made relative to a control material exposed simultaneously. It is strongly recommended that at least
one control test specimen be exposed with each test. It is preferable to use two control test specimens, one with relatively good
durability and one with relatively poor durability. Alternatively, the variability in the test can be defined so that statistically
significant pass/fail judgements can be made.
4.7 The use of at least three replicates of each control test specimen and each material being evaluated is recommended. Consult
Guide G169 for performing statistical analysis.
Available from International Organization for Standardization (ISO), ISO Central Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier, Geneva,
Switzerland, http://www.iso.org.
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5. Apparatus
5.1 Test Machines—Fresnel-reflector test machines used in Cycles 1, 2, and 3 ofLinear Fresnel reflector test machines are equipped
with a water delivery system to Table 1 are nearly identical. The only difference betweenspray test specimens using the cycles
specified in Table 1the machines is the addition of a water delivery system to the device used in Cycles 1 and 3. Use of the specific
cycle should relate to end use of the material and should be agreed upon by all interested parties. Document the spray cycle in
the test report.
5.1.1 The Fresnel-reflector linear Fresnel reflector test machine is a follow-the-sun“follow-the-sun” apparatus having flat mirrors
so positioned that the sun’s rays strike them at near-normal incident angles while in operation. The mirrors are arranged to simulate
tangents to a parabolic trough in order to reflect sunlight uniformly onto the specimens in the target area (see Fig. 1, Fig. 2, and
Fig. 3).
TABLE 1 Fresnel-Reflector Test Machine Typical Spray Cycles
Daytime Nighttime
Cycle
Spray Dry-Time Spray Dry-Time
Cycles/h Cycles/h
Duration Duration Duration Duration
1 8 min 52 min 1 8 min
water is sprayed on the
test specimens at:
9:00 p.m.
12:00 midnight
3:00 a.m.
2 no water spray used no water spray used
A
3 no water spray used 3 min 12 min 4 cycles per hour
(from 7PM to 5 AM)
TABLE 1 Typical Spray Cycles for ASTM G90
Daytime (during operation) Nighttime (7 pm to 5 am)
Cycle
Spray Dry-Time Cycles/ Spray Dry-Time
Cycles/hour
Duration Duration hour Duration Duration
1 8 min 52 min 1 8 min
cycle/ water is sprayed on
hour specimens at:
9 pm,
12 midnight, and
3 am
2 No water spray used No water spray used
A
3 No water spray used 3 min 12 min 4 cycles/hour
A
This is the cycle specified in Procedure C of Practice D4141.
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FIG. 3 Schematic of Optical System for a Linear Fresnel ReflectingReflector Concentrator Accelerated Weathering Machine
5.2 Mirrors—The Fresnel-reflector system mirrors of typical machines currently in use have a typical specular, spectral reflectance
curve such as that presented Typical spectral reflectance values for mirrors commonly in use as a function of wavelength are shown
in Fig. 4. Other If other mirror types and configurations may be used providing they are used, they shall meet the requirements
FIG. 4 Typical Specular Reflectance of Mirror Material
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of 6.2.
5.3 Photoreceptor Cells—Two photoreceptor cells, such as silicon solar cells, are installed near the top of the air tunnel on the side
facing the sun. A “T” shadow maker is mounted above the cells to illuminate equally one-half of each cell when the test machine
is in proper focus. As one cell receives more radiation than the other, the balance is disturbed and a signal is furnished through
an amplifier to a reversible motor which adjusts the machine to maintain focus.
5.4 Tracking System—The test machine shall be equipped with a system to keep the target area in focus throughout the day. Several
options are possible.
5.4.1 Single-axis tracking with manual altitude adjustment (Fig. 1). The test machine’s axis is oriented in the north/south direction,
with the north pole being altitude-adjustable to account for seasonable variations in solar altitude at zenith.
5.4.2 Dual axis tracking (Fig. 2). The test machine is equipped with two sets of photoreceptor cells, one to control the azimuth
rotation of the machine, the other to control the tilt elevation. The axis of the target area remains parallel to the ground. The
machine rotates about horizontal and vertical axes to keep the target area in focus.
5.5 Temperature—The test machine is equipped with a blower to cool the test specimens. The air is directed over the specimens
by an adjustable deflector along one side of the target area. For unbacked mounting, air is also directed under the specimens. This
limits the increase in surface temperatures of most specimens to 10°C above the maximum surface temperature that would be
reached when identically mounted specimens are exposed to direct sunlight at normal incidence at the same time and location
without concentration.
5.5.1 Temperature Control—Unless otherwise specified, if measurement of uninsulated black- or white-panel temperature is
required, the panels shall be constructed, calibrated, and maintained according to Specification G179 except that the size shall be
a minimum width of 50 mm by 125 mm. Unless otherwise specified, if measurement of insulated black- or white-panel temperature
is required, the panels shall be constructed and maintained according to ISO 4892-1.
NOTE 1—If an insulated black-panel temperature is used, the temperature indicated will be higher than that indicated by an uninsulated black-panel
thermometer under typical exposure conditions.
5.5.2 Typically, temperature of specimens is not controlled on solar concentrating test devices. Typical specimen temperatures
obtained are likely to be greater than the temperature that would be experienced if the specimen were exposed to direct solar
radiation at normal incidence (without concentration) at the same time. Temperature differences between exposures with and
without concentration depend on the specimen absorptance and total coefficient of heat transfer, thickness, backing, and
composition. During daytime exposures in solar concentrating test devices, variation in cloud cover may cause specimen
temperatures to vary.
5.5.3 If control of specimen temperature is required, the temperature of an uninsulated or insulated black or white panel, or a
specimen temperature may be monitored and used as an input to control specimen temperature by adjusting the speed of air
blowing across the specimens. Other methods of temperature control may be used if agreed upon between concerned parties.
5.5.4 Temperatures during night time are typically not controlled.
5.6 Nozzles—The test machine used in Cycles 1 and 3 of Table 1 is provided with a nozzle assembly for spraying water onto the
specimens during exposure. Fan spray nozzles which provide a uniform fine mist over the specimen area are recommended.
5.7 Spray Orientation—The apparatus shall be positioned so that specimens are sprayed at night either with specimens facing up
or down.
5.7.1 Specimens Face Down—The apparatus is oriented with the mirrors below the target specimen area such that nozzles spray
high purity water in an upward direction onto the specimens.
5.7.2 Specimens Face Up—The apparatus is oriented with the mirrors above the target specimen area such that nozzles spray high
purity water in a downward direction onto the specimens.
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NOTE 2—No data has been presented indicating that exposures performed using different spray orientations provide equivalent results, and as such, may
provide different test results.
5.8 Ultraviolet Radiometers—Instrumental means of measuring 295 to 385 nm ultraviolet radiant exposure shall consist of two
wavelength-band specific global irradiance radiometers, each connected to an integrating device to indicate the energy received
in the specified wavelength band over a given period. The spectral response of the ultraviolet radiometers shall be known and shall
be as flat as possible throughout the 295 to 385 nm spectral region utilized. Calibrations shall be performed using sunlight as the
source. TheEach ultraviolet radiometer shall be calibrated in accordance with Method E824 or G130 no less often than annually.
A black-painted permanent shading disk is positioned over one radiometer as shown in Fig. 6 and Figs. 7-9 to provide a
diffuse-only measurement (excluding 6° field of view).
5.9 A black-painted permanent shading disk is positioned over one radiometer to provide a diffuse-only measurement (excluding
6 6 0.5 degree field of view). The 6 6 0.5 degree field of view measurement is calculated as:
I 5 I 2 I (1)
6degfieldofviewUV globalUV diffuseonlyUV
where:
I = the UV irradiance in a 6 6 0.5 degree field of view around the sun,
6 deg field of view UV
I = the UV irradiance measured in a 180 degree field of view, and
global UV
I = the diffuse-only UV irradiance.
diffuse only UV
FIG. 5 Examples of Correctly and Incorrectly Mounted Specimens
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FIG. 6 Shading Disk In OperationGeometry
The shading disk and radiometers shall be mounted on a tracking device to follow the sun. The tracking accuracy shall be
sufficient such that at no time during operation does the shaded receiver receive direct radiation.
5.10 Construction of Shading Disk Apparatus:
5.10.1 To calculate the Field of View (FOV) of an existing shading disk, the equation is as follows:
FOV5 2*arctan D ⁄ 2 * h (2)
~ ~ !!
where:
D = the diameter of the shading disk, and
h = the distance between the diffuser or dome on the radiometer and the shading disk.
For ultraviolet radiometers which meet the requirements of 5.8, the shading disk shall be co
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