Determination of Plastic Aging Degree: Testing Methods, Standards, and Evaluation Techniques

Plastic aging degree determination is a critical materials testing process that quantifies how polymers degrade under thermal, UV, oxidative, and humidity exposure over time. By combining accelerated weathering methods such as xenon arc and UV fluorescent testing with pre- and post-exposure property measurements, accredited laboratories can evaluate retention of mechanical strength, color stability, and oxidation resistance in accordance with ISO, ASTM, and GB standards. This article explains the sample types, testing items, test methods, and typical applications involved in assessing the aging degree of plastic materials.

Technical Overview

Plastic aging refers to the irreversible deterioration of a polymer's chemical structure and physical properties caused by environmental factors such as ultraviolet radiation, heat, oxygen, moisture, chemical agents, and mechanical stress over the service life of a material. The degradation process generally follows two competing pathways: chain scission, which reduces molecular weight and leads to embrittlement, and crosslinking, which increases stiffness and reduces ductility. In parallel, thermo-oxidative and photo-oxidative reactions generate carbonyl and hydroperoxide groups that manifest as discoloration, chalking, surface cracking, and loss of mechanical performance.

Determination of the aging degree involves exposing representative specimens to controlled aging conditions—either accelerated laboratory exposure or natural outdoor weathering—and then measuring the changes in key properties relative to the unaged baseline. The aging degree is typically expressed as a percentage retention or percentage change in properties such as tensile strength, elongation at break, impact strength, color, gloss, and oxidation induction time. This evaluation approach supports material selection, stabilizer formulation optimization, quality control, and service life prediction across industries including automotive, construction, wire and cable, packaging, and renewable energy.

Testing Samples

Standard Laboratory Specimens

Injection-molded multi-purpose test specimens, such as dumbbell-shaped tensile bars and plaques prepared in accordance with ISO 3167 or ASTM D638 Type I geometry, are the most common samples for aging studies. Using standardized specimens ensures that property changes measured after exposure can be attributed to material degradation rather than variations in part geometry or processing.

Films and Sheets

Polyethylene, polypropylene, EVA, PET, and multilayer packaging films, as well as rigid and flexible sheets, are tested in sheet form. Film samples allow direct measurement of elongation at break retention, light transmission loss, and yellowness development, which are sensitive indicators of photo-oxidative degradation.

Finished and Semi-Finished Products

Pipes and fittings, window profiles, cable insulation and jackets, automotive interior and exterior trim, geomembranes, greenhouse covers, and photovoltaic encapsulants can be tested as produced or as cut sections. Testing real products captures the influence of pigmentation, wall thickness, processing history, and stabilizer distribution on aging behavior.

Field-Retrieved and Failed Parts

Samples recovered after in-service exposure are analyzed to diagnose failure, quantify the extent of degradation, and validate lifetime models. Comparative testing between exposed and archived unexposed material from the same production batch provides the most reliable assessment of accumulated aging damage.

Testing Items

The aging degree of plastics is characterized through a combination of mechanical, optical, morphological, chemical, and thermal measurements taken before and after exposure:

• Tensile strength and elongation at break retention (ISO 527, ASTM D638)
• Flexural strength and flexural modulus retention (ISO 178, ASTM D790)
• Charpy and Izod impact strength retention (ISO 179, ISO 180, ASTM D256)
• Color difference (ΔE) and yellowness index change (ASTM E313, ISO 11664)
• Gloss retention and surface chalking assessment
• Surface morphology evaluation for cracking, blistering, and microcrack density by optical or scanning electron microscopy
• Melt mass-flow rate (MFR) or melt volume-flow rate change (ISO 1133, ASTM D1238)
• Carbonyl index and hydroxyl index determination by infrared spectroscopy (FTIR)
• Oxidation induction time (OIT) and oxidation onset temperature (OOT) by DSC (ISO 11357-6, ASTM D3895)
• Weight-average molecular weight and molecular weight distribution shift by gel permeation chromatography (GPC)
• Thermal stability characterization by thermogravimetric analysis (TGA)
• Hardness change (Shore hardness, ball indentation)
• Electrical property retention such as dielectric strength and volume resistivity for insulation materials (IEC 60243, IEC 62631)
• Visual rating of surface defects and degradation grading after defined exposure intervals

Testing Methods

Xenon Arc Weathering

Xenon arc lamps with filtered radiation provide the best available simulation of full-spectrum terrestrial sunlight, including ultraviolet, visible, and infrared energy. Specimens are exposed to controlled cycles of light, water spray, temperature, and humidity in accordance with ISO 4892-2, ASTM G155, or GB/T 16422.2. This method is widely used for automotive components, building products, and colored materials where lightfastness and color change are critical.

UV Fluorescent Weathering

Fluorescent UV lamps emitting primarily in the UVA or UVB range, combined with condensation humidity cycles, deliver aggressive and repeatable exposure per ISO 4892-3, ASTM G154, and GB/T 16422.3. This technique is particularly effective for screening the durability of coatings, films, and geomembranes where UV radiation is the dominant degradation driver.

Thermal Aging in Air Ovens

Forced-ventilation ovens expose specimens to elevated constant temperatures for extended periods following ISO 2578, ASTM D3045, and related practices. Multiple temperature points above and below the expected service temperature enable Arrhenius extrapolation and the determination of time–temperature limits, which form the basis of long-term thermal endurance ratings for materials such as cable insulation.

Natural Outdoor Weathering

Direct exposure at benchmark climates—for example, humid subtropical or hot arid desert sites—is conducted following ISO 877 and ASTM D1435. Specimens are mounted at defined angles facing the equator and retrieved at intervals for evaluation. Natural weathering provides the reference data used to correlate and validate accelerated test results.

Damp Heat and Combined Exposure

Steady-state and cyclic humidity exposure per IEC 60068-2-78, combined light–humidity–temperature cycles per ISO 4892-1, and immersion or chemical exposure protocols are applied when hydrolytic degradation or environmental stress cracking is a concern, particularly for polyesters, polyamides, and polycarbonate.

Property Evaluation and Data Interpretation

Following ISO 4582 and ASTM D5870, exposed specimens are conditioned and tested alongside unaged controls. The aging degree is calculated as the retention rate: Retention (%) = (property value after aging ÷ property value before aging) × 100. A common failure criterion is the time required for elongation at break or tensile strength to fall to 50 percent of its initial value (t50). Chemical indicators such as the carbonyl index, measured by FTIR as the ratio of the carbonyl absorption band to a reference band, and the reduction in OIT provide early-stage degradation markers before mechanical properties change significantly.

Applications

Aging degree determination is applied across a broad range of industries and product categories:

• Automotive exterior and interior plastic parts requiring validated heat and light durability
• PVC window profiles, pipes, and fittings for building product certification
• Wire and cable insulation and sheathing materials for long-term thermal endurance rating
• Agricultural and greenhouse films requiring defined service seasons
• Photovoltaic encapsulant films and backsheets for damp heat and UV durability qualification
• Geomembranes and pond liners for civil and environmental engineering projects
• Packaging films and consumer product housings for shelf life and lightfastness verification
• Stabilizer, masterbatch, and resin development programs comparing formulation performance
• Failure analysis of cracked, discolored, or embrittled parts returned from service

FAQ

What is the aging degree of a plastic material?

The aging degree is a quantitative measure of how far a plastic has degraded relative to its original condition, expressed through retention rates or percentage changes in mechanical, optical, chemical, or thermal properties after a defined exposure period.

How long does an accelerated aging test take?

Depending on the material and standard, accelerated weathering tests typically run from several hundred to several thousand hours. Common schedules include 1,000 to 4,000 hours of xenon arc or UV exposure, with intermediate evaluations at fixed intervals to track the degradation curve.

Can accelerated test results predict real service life?

Accelerated results support lifetime estimation when combined with multi-temperature Arrhenius analysis per ISO 2578 and validated against natural weathering data. However, predictions must account for differences between laboratory spectra and real climate, and the correlation should be established separately for each material family and end-use environment.

Why measure both mechanical properties and the carbonyl index?

Chemical indicators such as the carbonyl index and OIT reveal oxidation damage at a molecular level before macroscopic changes appear, while mechanical retention tests confirm whether that chemical degradation has translated into functional performance loss. Combining both provides a complete picture of the aging state.

What specimen preparation is required?

Specimens should be conditioned to a standard atmosphere before baseline testing, exposed in replicate sets, and reconditioned after aging before post-exposure measurement so that reversible moisture effects do not distort the comparison. Sufficient replicate samples are needed at each retrieval point for statistically valid results.