Impact Strength Testing of Aged Plastics: Methods, Standards, and Applications

Plastic aging impact strength testing measures how thermal, UV, and environmental degradation affect the impact resistance of polymer materials by comparing impact performance before and after accelerated aging. This testing is essential for predicting long-term durability, validating stabilizer formulations, and demonstrating compliance with international standards such as ISO 179, ISO 180, ASTM D256, and the ISO 4892 weathering series. This guide covers degradation mechanisms, specimen preparation, testing procedures, data evaluation, and typical industrial applications.

Technical Overview

Thermoplastic and thermoset polymers in service are continuously exposed to heat, ultraviolet radiation, oxygen, moisture, and mechanical stress. Over time, these factors alter the molecular structure of the material and reduce its ability to absorb impact energy. Among all mechanical properties, impact strength is one of the most sensitive indicators of polymer degradation, because embrittlement caused by chain scission and oxidation typically appears earlier and more dramatically in impact testing than in tensile or flexural testing. For this reason, aged-impact evaluation is a core requirement in material qualification, product certification, and failure analysis programs.

Plastic aging relevant to impact performance occurs through several mechanisms:

• Photo-oxidative degradation driven by UV radiation, which generates free radicals and causes surface embrittlement in polyolefins, styrenics, and many engineering resins
• Thermal-oxidative aging, where sustained elevated temperatures accelerate oxidation throughout the material cross-section
• Hydrolytic degradation, which reduces molecular weight in moisture-sensitive polymers such as polyesters, polyamides, and polycarbonate
• Physical aging and post-crystallization, where amorphous and semi-crystalline polymers slowly densify or increase crystallinity, raising stiffness while lowering toughness
• Plasticizer migration or loss, which is a dominant embrittlement mechanism in flexible PVC compounds
• Crosslinking reactions that produce a hard but brittle network structure

The central quantitative output of this testing is the impact strength retention rate, calculated as the ratio of impact strength measured after aging to the initial (unaged) impact strength, multiplied by 100 and expressed as a percentage. A retention value significantly below 100 percent signals progressive embrittlement, and many product specifications define minimum acceptable retention thresholds, commonly in the range of 50 to 80 percent depending on the application and safety margin required.

Testing Samples

Standard Molded Bars

For pendulum impact testing, specimens are typically injection molded or machined from compression-molded plaques. Charpy specimens per ISO 179-1 and GB/T 1043.1 are usually 80 mm × 10 mm × 4 mm bars, while Izod specimens per ASTM D256 are commonly 63.5 mm × 12.7 mm × 3.2 mm and ISO 180 permits 80 mm × 10 mm × 4 mm bars. Specimens must be molded under controlled conditions, since molding parameters, gate location, and flow direction can significantly influence both initial and aged impact results.

Notched Specimens

Notched specimens concentrate stress at a machined root and are used to evaluate notch sensitivity and the ductile-to-brittle transition. The A-type notch with a 0.25 mm radius is the most widely referenced geometry. Notches are machined in accordance with ISO 2818. The test protocol must clearly define whether notching is performed before or after aging, since pre-notched specimens expose the notch root to degradation and often show larger strength reductions than specimens notched after exposure.

Flat Plaques and Sheet Materials

Rigid sheets and molded plaques intended for multiaxial impact testing are cut into squares of approximately 60 mm × 60 mm or larger, or into discs of at least 100 mm diameter, for clamping in instrumented falling-dart fixtures per ISO 6603-2 and ASTM D3763. Thickness is measured at multiple points because puncture energy scales strongly with specimen thickness.

Finished Parts and Cut Samples

Pipes, fittings, window profiles, electrical enclosures, and other components can be tested either as whole parts in component-level impact rigs or as specimens machined from the part. When specimens are cut from finished products, the cutting direction relative to extrusion or molding flow must be recorded. Accredited third-party laboratories routinely handle both standard specimens and components taken directly from production or field service.

Films and Flexible Materials

Thin films used in packaging and agriculture are evaluated with free-falling dart methods such as ASTM D1709 and GB/T 8809, or with tensile impact testing per ISO 8256, using larger sample populations because film impact data typically exhibit wide scatter.

Testing Items

A comprehensive aged-impact evaluation program typically includes the following items, each tied to recognized international or national standards:

• Charpy impact strength, notched and unnotched, per ISO 179-1, ASTM D6110, GB/T 1043.1, and JIS K 7110
• Izod impact strength, notched and unnotched, per ISO 180, ASTM D256, and GB/T 1843
• Tensile impact strength per ISO 8256 and GB/T 13525
• Instrumented high-speed puncture impact per ISO 6603-2 and ASTM D3763
• Free-falling dart impact of plastic films per ASTM D1709 and GB/T 8809
• Impact strength retention after air-oven thermal aging per ISO 188, ASTM D573, GB/T 3512, and GB/T 7141
• Impact strength retention after artificial weathering per ISO 4892-2 (xenon arc), ISO 4892-3 (fluorescent UV), ISO 4892-4 (carbon arc), ASTM G155, ASTM G154, and the GB/T 16422 series
• Break type classification and ductile-to-brittle transition assessment based on the failure mode categories defined in the pendulum impact standards
• Instrumented force-deflection analysis, including peak force, deflection at peak force, energy to peak force, and total absorbed energy

Testing Methods

The standard workflow for plastic aging impact strength testing consists of five controlled stages, and the integrity of the conclusion depends on consistent execution of each stage.

Step 1: Baseline Characterization

Representative specimens are conditioned at 23 °C and 50 percent relative humidity per ISO 291 or GB/T 2918 and then impact tested to establish the initial strength value. ISO 179-1 recommends testing at least 10 specimens per condition, and larger populations are advisable for materials with high data scatter.

Step 2: Accelerated Aging Exposure

Sister specimens are exposed under one or more accelerated aging regimes selected to simulate the intended service environment. Common protocols include air-oven thermal aging at temperatures spanning the expected service range, for example 80 °C to 150 °C for polyolefins, with multiple exposure durations and multiple temperatures to support time-temperature extrapolation. Xenon arc weathering per ISO 4892-2 typically uses an irradiance of 0.35 W/(m²·nm) at 340 nm with a black panel temperature near 65 °C and cyclic water spray, such as 102 minutes of light followed by 18 minutes of light plus spray. Fluorescent UV condensation exposure per ISO 4892-3 and ASTM G154 commonly applies UVA-340 lamps with an 8-hour UV step at 60 °C and a 4-hour condensation step at 50 °C. Exposure duration is documented in total radiant energy, supported by radiometry per ISO 9370 where applicable.

Step 3: Post-Aging Reconditioning

After exposure, specimens are returned to the standard atmosphere per ISO 291 for a defined reconditioning period, typically 24 to 96 hours, so that transient moisture effects are separated from true structural degradation. The reconditioning time must be held identical across all test points.

Step 4: Impact Testing

Charpy testing is performed on a calibrated pendulum impact machine using a support span of 40 mm for standard 80 mm × 10 mm × 4 mm bars, with hammer energies selected so that the absorbed energy falls within 10 to 80 percent of the capacity. Izod testing clamps the specimen as a vertical cantilever and strikes it above the clamp. Instrumented puncture testing drives a 20 mm diameter hemispherical striker through clamped plaques at standardized velocities, delivering full force-deflection curves that reveal whether the material shifts from ductile, high-energy penetration to brittle, low-energy fracture after aging.

Step 5: Data Evaluation and Interpretation

Impact strength retention is calculated for each exposure condition, together with mean values, standard deviations, and break type distributions. A shift from complete ductile tearing toward complete brittle fracture is often a more sensitive degradation indicator than the numeric strength loss itself. Where multi-temperature oven aging data are available, Arrhenius analysis or time-shift factors can be used to estimate the time required for the impact retention to reach the specification limit at the actual service temperature, forming the basis of long-term thermal endurance ratings such as those developed under UL 746B relative temperature index programs.

Applications

Aged-impact strength testing is applied across virtually every industry in which plastics carry structural, protective, or safety functions:

• Automotive: bumper fascias, exterior trim, battery housings, and interior components must retain impact performance after prolonged heat and solar exposure, and OEM material specifications routinely specify minimum impact retention after defined aging schedules
• Building and construction: PVC window and door profiles, pipes and fittings, roofing membranes, and siding products are qualified with impact testing following accelerated and natural weathering, with product standards such as GB/T 8814 for profiles defining explicit post-aging impact requirements
• Electrical and electronics: enclosures, connector bodies, and insulating parts are evaluated for embrittlement after thermal aging, supporting long-term temperature ratings and enclosure impact classification schemes
• Packaging: films, containers, and closures are tested for dart impact retention after sterilization, hot filling, or extended storage
• Agriculture and outdoor goods: greenhouse films, geomembranes, irrigation components, and outdoor furniture are assessed after UV and moisture exposure
• Renewable energy: photovoltaic backsheets, junction box housings, and cable accessories undergo combined damp-heat and UV aging followed by mechanical integrity verification
• Recycled and regrind materials: quality control programs use aged-impact testing to quantify how repeated processing and stabilizer depletion accelerate embrittlement

FAQ

Why does impact strength decrease after plastic aging?

Aging introduces chain scission, oxidation, hydrolysis, or post-crystallization, all of which reduce the polymer's ability to undergo plastic deformation. As the material becomes stiffer and less ductile, less energy is absorbed before fracture, which appears directly as reduced impact strength and a shift toward brittle break types.

Should notched or unnotched specimens be used?

Unnotched specimens characterize the overall toughness of the material, while notched specimens reveal notch sensitivity and are far more responsive to surface-oxidative and hydrolytic degradation. Most aging specifications rely on notched data because they detect early-stage embrittlement that unnotched testing can miss.

How many specimens are required for each test point?

A minimum of 10 specimens per condition is recommended by ISO 179-1, and larger populations of 15 to 20 specimens are advisable for highly scattered materials such as films, filled compounds, and parts machined from finished products.

How long should the aging exposure be?

Exposure duration depends on the material, the service life being simulated, and the product specification. Programs typically include several exposure intervals, for example 500, 1000, and 2000 hours, so that a retention-versus-time curve can be constructed and the time to reach the specification limit can be estimated by extrapolation.

Which artificial weathering method best simulates outdoor exposure?

Xenon arc weathering per ISO 4892-2 and ASTM G155 provides the closest match to full-spectrum sunlight, while fluorescent UV condensation per ISO 4892-3 and ASTM G154 reproduces the short-wavelength UV and condensation moisture cycles responsible for surface embrittlement. The most defensible studies combine both methods or correlate artificial exposure with natural outdoor exposure.

Can finished products be tested instead of molded specimens?

Yes. Accredited third-party laboratories regularly test whole components, cut sections from parts, or machine specimens from pipes, profiles, and housings. Component-level testing captures the influence of processing history, wall thickness variation, and design features on aged impact performance.

What retention value is considered acceptable?

There is no universal threshold. Many specifications require retention of at least 50 percent of initial impact strength, while safety-critical applications may demand higher values. The acceptance criterion should always be taken from the governing product standard or agreed upon between manufacturer and end user before testing begins.