Natural weathering exposure testing is the most reliable method for evaluating the long-term durability of plastic materials under real outdoor conditions such as solar UV radiation, temperature cycling, humidity, and rainfall. Performed in accordance with international standards including ISO 877, ASTM D1435, and GB/T 3681, this test provides authentic aging data that supports material qualification, product warranty claims, and service-life prediction for plastics used in outdoor applications.
Technical Overview
Natural weathering exposure testing, also known as outdoor exposure aging testing, is a long-term durability evaluation method in which plastic specimens are mounted on exposure racks and subjected to actual climatic conditions at outdoor test sites. Unlike laboratory accelerated weathering, which uses xenon arc lamps or fluorescent UV lamps to simulate sunlight, natural exposure delivers the full, unmodified spectrum of environmental stress factors: solar ultraviolet radiation, infrared heating, diurnal temperature cycling, humidity, rainfall, atmospheric oxygen, and airborne pollutants.
Because photo-oxidative degradation of polymers is driven by a complex interaction of these factors, natural weathering produces the most authentic degradation profile available, including effects that artificial light sources cannot fully reproduce, such as seasonal variations, mildew growth, acid rain, and salt deposition in coastal environments. For this reason, outdoor exposure data are widely regarded as the reference standard against which accelerated weathering methods are validated and correlated.
The test is governed by a mature framework of international and national standards. ISO 877 (Parts 1 and 2) and ISO 4607 define exposure to direct solar radiation, glass-filtered solar radiation, and concentrated solar radiation using Fresnel mirror arrays. ASTM D1435 and ASTM G7 cover outdoor weathering practice in the North American system, while GB/T 3681 specifies the equivalent procedures in the Chinese national standard system. Changes in properties after exposure are assessed according to ISO 4582 and ASTM D5870. Accredited third-party laboratories operate exposure stations across globally recognized benchmark climates, including hot-humid subtropical coastal zones, hot-dry desert zones, and temperate industrial zones, allowing manufacturers to select the environment most relevant to their product's end use.
Testing Samples
Injection-Molded Mechanical Test Specimens
The most common sample type is the injection-molded dumbbell-shaped tensile bar, prepared in accordance with ISO 3167 multi-purpose specimens or ASTM D638 Type I geometry. These specimens allow direct measurement of tensile strength and elongation at break retention after defined exposure periods. Impact specimens such as Izod and Charpy bars per ISO 179 and ISO 180, or ASTM D256, are also frequently exposed in parallel.
Flat Panels and Sheets
Square or rectangular panels, typically 100 mm × 100 mm or 75 mm × 150 mm, are used for appearance evaluation. These panels permit color measurement, gloss measurement, visual defect rating, and surface spectroscopy on a single specimen. Panels may be tested as unbacked (open-backed) specimens, allowing full air circulation and rain washing on both surfaces, or mounted on a solid backing such as a black box configuration that elevates specimen temperature.
Films and Flexible Materials
Agricultural mulch films, greenhouse films, packaging films, and geomembranes are exposed in coupon form with appropriate edge retention. Thin films are particularly sensitive to UV degradation and often require reinforced mounting to prevent tearing during long exposure periods.
Finished Products and Components
Complete or sectioned end-use articles, such as automotive trim pieces, pipe segments, cable jackets, and housing enclosures, can be exposed to evaluate the combined effect of material formulation, geometry, color, processing history, and surface texture on weathering performance.
Control Specimens
A sufficient quantity of unexposed control specimens from the same production batch must be retained in dark, climate-controlled storage. All property changes are calculated by comparing exposed specimens against these controls, which isolates the effect of weathering from batch-to-batch variation.
Testing Items
The scope of evaluation typically combines appearance characterization with mechanical and chemical property measurement:
- Color change: instrumental color difference (Delta E), yellowness index, and gray-scale ratings per ISO 4582, ISO 105-A02, and ASTM D2244
- Gloss retention: specular gloss at 20, 60, and 85 degrees per ISO 2813 and ASTM D523
- Visual defect assessment: chalking, cracking, crazing, blistering, flaking, warping, staining, and biological growth, rated using photographic reference scales such as ASTM D660, D661, and ISO 4628 series methods adapted for plastics
- Tensile properties: retention of tensile strength, yield strength, and elongation at break per ISO 527 or ASTM D638
- Flexural properties: retention of flexural strength and modulus per ISO 178 or ASTM D790
- Impact properties: retention of Izod and Charpy impact strength per ISO 179, ISO 180, ASTM D256
- Chemical microstructure: carbonyl index and hydroxyl index by FTIR spectroscopy, indicating the extent of photo-oxidation
- Thermal properties: oxidation induction time (OIT) by DSC per ISO 11357-6, and melting behavior changes
- Molecular weight: changes in average molecular weight by gel permeation chromatography for engineering polymers
- Dimensional stability: warp, shrinkage, and distortion measurements after exposure
Testing Methods
Direct Outdoor Exposure
Specimens are mounted on racks fabricated from corrosion-resistant material and secured with stainless steel fasteners that avoid contact stress points. The most common configurations are racks tilted 45 degrees facing the equator, racks tilted at the local latitude angle for maximum annual radiant exposure, and horizontal (0 degree) or near-horizontal (5 degree) mounting. Direct exposure per ISO 877-1 Method A, ASTM D1435, and GB/T 3681 subjects specimens to full solar radiation, rainfall, and dew formation, and is the default method for most durability programs.
Glass-Filtered Exposure
ISO 877-1 Method B places specimens behind a glass cover that transmits visible light while cutting off most radiation below roughly 320 to 400 nm depending on glass type. This method simulates the indoor environment of materials located behind window glass, such as automotive interiors and glazing components, where degradation proceeds more slowly but still occurs.
Concentrated Natural Sunlight Exposure
ISO 877-2, ASTM G90, and ASTM D4364 describe Fresnel mirror reflector arrays that track the sun and concentrate natural sunlight onto specimens by a factor of approximately 8, while cooling with forced air and spraying with water. This technique accelerates degradation using genuine solar radiation rather than an artificial source, bridging the gap between outdoor and laboratory testing.
Dosimetry and Exposure Documentation
Exposure is quantified not only by calendar time but by accumulated radiant energy. Total solar radiation and UV radiation (typically 295 to 385 nm) are continuously monitored with pyranometers and radiometers mounted at the specimen plane, and cumulative dose is reported in MJ/m² or kJ/m². Temperature and relative humidity at the site, total rainfall, and wet-time are logged so that degradation can be correlated with environmental dose rather than elapsed time alone. The start-of-exposure season is documented because initial exposure season measurably affects results, particularly in temperate climates.
Inspection Intervals and Evaluation
Typical retrieval schedules are 3, 6, 12, 18, 24, 36, and 48 months, adjusted to the expected durability of the material. Retrieved specimens are conditioned in a standard laboratory atmosphere, typically 23 degrees C and 50 percent relative humidity per ISO 291, before testing. Property change is expressed as percentage retention relative to unexposed controls, following ISO 4582 and ASTM D5870. A useful practice is to plot property retention against radiant exposure dose, which allows data from different sites and seasons to be compared and enables extrapolation toward end-of-life criteria.
Selection of Exposure Climate
Climate selection is a critical engineering decision. Hot-humid subtropical coastal climates provide high UV dose combined with humidity, condensation, and salt aerosols, and are appropriate for materials sensitive to hydrolysis and biological attack. Hot-dry desert climates provide the highest UV intensity and surface temperatures, suitable for evaluating thermal-oxidative stability and pigment fading. Temperate climates with industrial pollution replicate acid rain and pollutant attack. Where the end-use environment is known, exposure in a matching climate provides the most predictive data.
Applications
Natural weathering exposure testing is applied across every industry that places polymeric materials outdoors or behind glazing:
- Automotive: exterior body panels, bumpers, trim, mirror housings, window seals, and interior components exposed through glazing, supporting validation of light stabilizer packages and warranty targets
- Building and construction: PVC profiles, pipes and fittings, roofing membranes, siding, sealants, glazing sheets, and geosynthetics requiring decades of demonstrated service life
- Agriculture: greenhouse films, mulch films, irrigation pipes, and netting, where UV stability directly determines replacement cycles
- Renewable energy: photovoltaic encapsulant films, backsheets, junction box housings, and cable systems qualified for 25-year field service
- Coatings and finishes: paints and coatings on plastic substrates, evaluated for gloss retention, chalking, and color fastness
- Consumer and marine products: outdoor furniture, playground equipment, boat hulls, and buoyancy components exposed to sun and salt water
- Materials R&D: comparative performance of new resin grades, UV absorbers, hindered amine light stabilizers, pigments, and filler systems
- Regulatory compliance and certification: generating the outdoor durability evidence required by certain product standards and procurement specifications
Beyond pass/fail qualification, long-term exposure databases support service-life prediction models, accelerate-by-landscape comparisons between climates, and the correlation ratios needed to set scientifically justified test durations for laboratory accelerated weathering per ISO 4892 and ASTM G151 series methods.
FAQ
How long does a natural weathering test take?
Program durations are driven by the target service life of the product. Screening studies may run 6 to 12 months, while qualification programs commonly extend 24 to 36 months or longer. Because results are dose-dependent, reporting accumulated solar and UV radiation alongside elapsed time is essential for meaningful interpretation.
What is the difference between natural weathering and accelerated laboratory weathering?
Natural exposure uses genuine sunlight and uncontrolled real climate, giving the highest realism at the cost of long duration and seasonal variability. Accelerated laboratory testing using xenon arc or UV fluorescent lamps per ISO 4892 or ASTM G154/G155 compresses the timeline but requires correlation with natural exposure data to establish validity. Best practice combines both: accelerated screening followed by outdoor confirmation.
Which standard should I specify: ISO 877, ASTM D1435, or GB/T 3681?
The three standards are technically aligned in principle. ISO 877 is preferred for international supply chains, ASTM D1435 is common in North American specifications, and GB/T 3681 is required for Chinese national conformity contexts. Accredited third-party laboratories can typically run a single exposure program that satisfies several of these standards simultaneously when specimen mounting and dosimetry are harmonized.
What specimen quantities and sizes are required?
For each retrieval interval, a statistically meaningful set of specimens is needed, typically a minimum of five replicate tensile bars plus dedicated panels for nondestructive color and gloss measurement, along with unexposed controls. Exact dimensions follow the downstream test method, such as ISO 527 or ASTM D638.
How are results reported?
Standard reporting includes exposure site and climate description, rack configuration and tilt angle, exposure dates and duration, cumulative total and UV radiant dose, and the measured property retentions, color differences, and visual ratings with photographic records. Failure criteria, such as 50 percent elongation retention or a defined color change limit, are agreed with the client before the program begins.
Can finished products be tested instead of test plaques?
Yes. Exposing actual components captures the influence of processing conditions, wall thickness, color, surface texture, and assembly details, and is encouraged for final qualification. Test plaques remain valuable for controlled material comparison and for destructive mechanical testing.