ASTM G90

Test your Materials Outdoors with ASTM G90

ASTM G90 uses linear Fresnel reflectors to concentrate sunlight in clear sky conditions. The Q‑TRAC natural sunlight concentrators found at our Arizona site follow this standard to provide intense exposure to natural sunlight. Q-TRAC solar concentrators can also operate at controlled temperatures, with the ability to apply moisture in order to simulate Florida natural weathering exposures. This internationally-recognized test method delivers guidelines for setup, operation, specimen mounting and performance requirements to ensure Q‑TRAC exposures are consistent and reliable. The Q‑TRAC provides roughly eight times the solar radiation of a natural exposure which over a year, approximates five years of a Florida outdoor exposure. Furthermore, the design of the Q‑TRAC allows a variety of water spray and freeze/thaw cycles, providing meaningful results that directly reflect long-term real results in a fraction of time.

Key Test Parameters

Exposure Apparatus

ASTM G90 specifies a Q-TRAC natural sunlight concentrator, designed to concentrate outdoor sunlight uniformly on specimens.

Concentrator : An array of flat mirrors (typically 10) is arranged to focus sunlight onto a fixed target area.
Tracking system options:
- Single-axis: North-south orientation with manual seasonal altitude adjustments.
- Dual-axis: Controls azimuth and elevation to maximize solar exposure. Q-TRAC solar concentrators perform dual-axis tracking.
Mirrors: Flat and highly-reflective at UV and visible wavelengths, giving full-spectrum natural sunlight.
Target area: Specimens are mounted on a board facing the mirrors, limiting the dimensions.
Cooling system: A blower with an adjustable deflector, designed to direct air across specimen surfaces to prevent overheating. This can be controlled, limited, or uncontrolled.
Spray system: Fan nozzles deliver a uniform mist of highly purified water to specimens, providing either thermal shock to simulate rainstorms during the day, or overnight surface wetness to simulate dew formation in the evening.

Moisture and Spray Cycles

Moisture exposure is critical for simulating environmental degradation in some materials. The practice outlines two typical water spray cycles, and can also be performed with no water spray:

Typical Spray Schedules

Cycle	Daytime Spray	Nighttime Spray
1	8 min spray, 52 min dry (1 cycle/hour)	8 min spray at 9 PM, 12 AM, 3 AM
3	No spray	3 min spray, 12 min dry (4 cycles/hour)

Common Alternatives

Cycle	Daytime Spray	Nighttime Spray
Prefinish Hardboard	8 min spray, 52 min dry (1 cycle/hour)	Overnight soak, freeze, thaw cycle
Extended (4)	8 min spray, 52 min dry (1 cycle/hour)	3 min spray, 12 min dry (4 cycles/hour)

Temperature Control and Monitoring

Without cooling, the high amount of solar energy that is being concentrated on specimens can result in high surface temperatures. Maintaining appropriate test temperatures can be handled in a variety of ways.

Uncontrolled temperature: During the day, the Q-TRAC cooling system is running at maximum capacity. This provides seasonal variability, where summer and winter test temperatures fluctuate.
Limited temperature: The Q-TRAC cooling system is running at maximum capacity but, if the surface temperatures exceed a certain value, mirrors will fall out of focus to prevent specimen overheating.
Controlled temperature: A setpoint is chosen and the Q-TRAC cooling system varies to ensure the surface temperatures are maintained throughout the day. This provides a high degree of stability and repeatability, where summer and winter test temperatures remain the same.

Radiant Exposure Measurement

Q-TRAC testing is usually scheduled according to radiant dosage, not according to time on exposure. Testing is performed only under clear conditions. Guidelines are given for the irradiance in ASTM G90:

UV Exposure (295-385 nm): Use two calibrated radiometers - one shaded for diffuse-only (excluding 6° ±0.5° field); calculate direct beam as global minus diffuse.
Total Solar Exposure (300-3000 nm): Measured with a Class C pyranometer or WMO First Class pyrheliometer in a 5-5.7° field.
Clear Sky Definition: Diffuse ≤20% of total radiation.

Specimen Parameters

Specimens must be prepared and mounted to ensure consistent exposure. Q-TRAC test costs are determined based on the space utilized, in linear inches, by the specimens.

Size and Replicates: Length and width must fit target board; thickness recommended ≤13 mm and consistent for all specimens, for proper airflow. Replicates are encouraged.
Mounting types:
- Non-insulated: 5-6 mm off board for best airflow.
- Insulated: Backed by 13 mm plywood.
Clearance: 10-14 mm from air deflector to specimen surface is recommended - align edges to avoid airflow disruption.
Controls: Include at least one (ideally two: good and poor durability) exposed simultaneously.
For under-glass tests, use specified glass and adjust cooling.

Operational and Maintenance Guidelines

Test Duration/Endpoint: Can be based on preset UV/total exposure, control changes, or property change (such as gloss or color). Q-Lab recommends testing to failure.
Maintenance:
- Clean mirrors routinely; measure UV reflectance every 6 months
- Ensure full target illumination; calibrate instruments annually; check UV radiometers
  monthly.
Safety: Use UV/IR-blocking eye protection, protective clothing, and blower screens.
Evaluation: Compare against exposed control materials with known performance and use file specimens for visual evaluations.

Why Test to ASTM G90

ASTM G90 is an accelerated outdoor test, meaning it combines the realism of natural sunlight with the short exposure times of laboratory testing. Q-TRAC testing according to ASTM G90 is an excellent choice for extremely durable materials like Coil Coatings, Powder Coatings, Roofing and Building Materials, Automotive Paints and Parts, and highly temperature-resistant Plastics. Natural solar concentrator testing is a fast, affordable way to achieve fast results.

Pros and Cons of ASTM G90

Testing in real-world conditions
- By exposing test specimens to actual sunlight, moisture, and temperature fluctuations, ASTM G90 provides realistic conditions.
Concentrated sunlight
- ASTM G90 exposes materials to extreme sunlight & heat using a series of 10 mirrors, for fast and accurate results.
Cost-effective
- Natural outdoor testing can be a cost-effective way to understand your material's durability.

Extreme temperatures
- Linear fresnel reflectors can create extremely high temperatures, leading to potentially exaggerated degradation and limiting the types of specimens suitable for Q-TRAC testing.
Specimen sizing
- The geometry of natural solar concentrator testing according to ASTM G90 prevents testing of large components or three-dimensional specimens.

Q-Lab's Natural Outdoor Weathering Test Facilities

Q-Lab offers outdoor exposure testing programs at our internationally-recognized, ISO 17025-accredited benchmark locations in Florida and Arizona. Weathering exposures at these sites are not only realistic; they also could be considered a form of accelerated weathering testing (depending on where the product will be used outdoors) and are the best way to verify laboratory test results. Q-Lab also provides all users with climate data to enhance interpretation of data collected during outdoor exposures. See our Outdoor Testing brochure for more information on these sites and our Testing Services brochure to learn about our full range of weathering testing services.

Natural Outdoor Testing Services Resources

Standards

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