Natural Weathering Testing of Plastics: Standards, Exposure Methods, and Property Evaluation

Natural weathering testing of plastics is the most reliable method for evaluating how polymeric materials degrade under real-world outdoor conditions such as solar radiation, temperature, humidity, and rainfall. Conducted in accordance with internationally recognized standards including ISO 877, ISO 4607, ASTM D1435, and ASTM G7, this testing provides essential durability data for material selection, product certification, and quality assurance. This article explains the test principles, sample requirements, evaluation items, exposure methods, and typical applications of natural climate aging experiments for plastics.

Technical Overview

Natural weathering, also referred to as natural climate aging or outdoor exposure testing, is a long-term durability test in which plastic specimens are exposed to actual outdoor environmental conditions at dedicated exposure sites. Unlike laboratory accelerated weathering, which uses xenon arc lamps or fluorescent UV lamps to simulate sunlight, natural weathering subjects materials to the full spectrum of real climatic stresses, including solar ultraviolet radiation, temperature cycling, humidity, rainfall, atmospheric oxygen, and airborne pollutants.

The degradation of plastics under outdoor conditions is driven primarily by photo-oxidation. Ultraviolet photons in the wavelength range of approximately 290 nm to 400 nm carry sufficient energy to break polymer chemical bonds, generating free radicals that react with oxygen to form carbonyl and hydroperoxide groups. This chain reaction leads to discoloration, surface cracking, chalking, embrittlement, and loss of mechanical strength. Because natural weathering reproduces the exact spectral distribution, cyclic moisture, and thermal history of real service environments, it is widely regarded as the reference method against which all accelerated tests are ultimately validated.

The principal standards governing natural weathering of plastics include ISO 877 (Plastics — Methods of exposure to solar radiation), ISO 4607 (Plastics — Methods of exposure to natural weathering), ASTM D1435 (Standard Practice for Outdoor Weathering of Plastics), ASTM G7 (Standard Practice for Atmospheric Environmental Exposure Testing of Nonmetallic Materials), ASTM G24 (exposure behind glass), ASTM D4364 (concentrated natural sunlight exposure), and GB/T 3681 for natural climate exposure of plastics. Property changes after exposure are evaluated according to ISO 4582 and ASTM D5870.

Testing Samples

The type, form, and preparation of test specimens significantly influence the relevance and reproducibility of weathering results. Specimens should be produced from the same formulation, processing conditions, and thickness as the intended commercial product.

Injection-Molded Specimens and Plaques

Standardized dumbbell-shaped tensile bars and flat plaques are injection molded directly from the compound under evaluation. These specimens allow direct measurement of mechanical property retention, color change, and surface morphology before and after exposure. Typical plaque dimensions are 100 mm × 100 mm or 75 mm × 150 mm with product-representative thicknesses.

Filils and Sheets

Thin polymer films such as greenhouse films, mulch films, and packaging laminates are mounted on backing frames with defined tension. Because films have a high surface-to-volume ratio, they generally show faster and more pronounced degradation responses than thick molded parts.

Pipes, Profiles, and Finished Products

Sections of pipes, window profiles, cable jackets, automotive trim, and other finished articles may be exposed in their as-manufactured form. Testing actual products captures the influence of processing-induced residual stress, skin-core morphology, pigmentation, and surface texture on weathering performance.

Coated and Multi-Layer Materials

Painted plastics, coated profiles, laminates, and coextruded multilayer structures are tested to assess the weathering behavior of the complete system, including adhesion retention, gloss loss, and interlayer compatibility.

Testing Items

The scope of evaluation typically covers appearance, mechanical, chemical, and physical property changes, all compared against unexposed control specimens stored under dark, controlled conditions.

• Color change and yellowness index, quantified as color difference values in accordance with ISO 4582 and ASTM D2244 using spectrophotometers
• Gloss retention and surface appearance attributes such as chalking, cracking, blistering, flaking, crazing, and dirt pickup, rated using standardized photographic reference scales
• Tensile strength, yield strength, and elongation at break measured according to ISO 527 or ASTM D638
• Flexural strength and flexural modulus measured according to ISO 178 or ASTM D790
• Impact resistance, including Izod and Charpy impact strength, measured according to ISO 179, ISO 180, ASTM D256, or instrumented dart impact for films
• Carbonyl index and oxidation depth determined by infrared spectroscopy to characterize the chemical extent of photo-oxidation
• Hardness, mass change, dimensional stability, and surface roughness variations
• Molecular weight distribution changes measured by gel permeation chromatography for research-grade degradation studies

Results are commonly reported as percentage property retention, which expresses the measured property of the exposed specimen as a percentage of the corresponding value for the unexposed control.

Testing Methods

Natural weathering experiments are conducted by mounting specimens on open racks at accredited third-party exposure sites or manufacturer-operated exposure fields representing distinct climate zones. Common reference climates include hot-humid subtropical coastal environments, hot-dry desert environments with high solar irradiance, temperate industrial climates, and high-altitude sites with intense UV flux.

Direct Outdoor Exposure

In the standard configuration described by ISO 877-1, ISO 4607, and ASTM D1435, specimens are mounted on racks tilted at a defined angle, most commonly 45 degrees facing the equator in the northern hemisphere, or laid flat at 0 degrees for maximum rainfall contact. Unbacked or plywood-backed mounting is selected according to the end-use scenario. Exposure continues for defined intervals such as 3, 6, 12, 24, 36, or 60 months, with specimens periodically retrieved for testing.

Exposure Behind Glass

For materials used indoors near windows or inside vehicles, specimens are exposed under glass in accordance with ISO 877-2, ASTM G24, and related practices. The glass filters out short-wavelength UV radiation and rain contact while transmitting visible and near-UV light, simulating in-service conditions for automotive interiors and indoor applications.

Concentrated Natural Sunlight Exposure

Fresnel-mirror concentrating arrays described in ASTM D4364 and ISO 877-3 track the sun and multiply the incident irradiance on specimens by a factor of roughly eight, providing accelerated degradation under genuine natural sunlight. This method maintains the authentic solar spectrum while shortening exposure periods considerably.

Radiation Dose Measurement and Control

Accumulated solar radiant exposure is continuously monitored with pyranometers and narrow-band UV radiometers and reported in megajoules per square meter, both as total solar radiation and as UV radiation between 290 nm and 400 nm. Climatological data including temperature, relative humidity, rainfall, and hours of sunshine are recorded in parallel, allowing degradation to be correlated with environmental dose rather than calendar time alone.

Applications

Natural weathering data support material qualification, formulation development, warranty substantiation, and compliance demonstration across many industries.

• Automotive: exterior trim, bumpers, mirror housings, body panels, interior components evaluated for colorfastness and gloss retention
• Construction: window and door profiles, roofing membranes, siding, decking, geosynthetics, and geomembranes requiring multi-decade service life
• Agriculture: greenhouse covering films, mulch films, irrigation pipes, and netting exposed to intense sunlight for full growing seasons
• Wire and cable: jacketing and insulation compounds for outdoor and buried installations
• Photovoltaics: backsheet films, encapsulants, junction boxes, and connector housings for long-term outdoor reliability
• Consumer and marine products: garden furniture, playground equipment, boats, buoys, and signage
• Packaging and coatings: outdoor-transport packaging, paints, powder coatings, and surface treatments applied to plastic substrates

Regulatory bodies, certifiers, and purchasing specifications in many sectors require documented natural weathering performance before products may be placed on the market or labeled for extended outdoor use.

FAQ

How long does a natural weathering test take? Exposure periods typically range from several months to several years, depending on the material, the climate zone, and the performance criterion. Degradation is often tracked as a function of accumulated UV dose rather than elapsed time.

What is the difference between natural weathering and accelerated weathering? Natural weathering uses real sunlight and climate, offering the highest real-world relevance but slow turnaround, while accelerated weathering uses laboratory light sources such as xenon arc or UV fluorescent lamps to compress the timeline. The two methods are complementary, and correlations between them are validated through documented natural exposure data.

How many specimens are required? Replicate specimens are needed for each withdrawal interval together with unexposed controls, so the total quantity depends on the planned number of inspection points and the number of properties to be measured.

Which exposure site should be chosen? Site selection should mirror the intended service climate. Hot-humid subtropical sites emphasize combined UV, heat, and moisture stress; desert sites maximize solar irradiance and thermal loading; industrial sites add pollutant and acid-rain effects.

Which standards apply? Commonly applied standards include ISO 877-1, ISO 877-2, ISO 877-3, ISO 4607, ISO 4582, ASTM D1435, ASTM G7, ASTM G24, ASTM D4364, ASTM D5870, and the corresponding GB/T series for the Chinese national framework.

Can finished products be tested instead of molded specimens? Yes, testing actual articles is recommended whenever possible because processing history, geometry, and surface finish all influence weathering behavior.