Impact testing of plastics after thermal aging evaluates how prolonged exposure to elevated temperatures affects the toughness, ductility, and long-term reliability of polymer materials. By combining heat-aging treatments in forced-ventilation ovens with standardized Charpy and Izod impact tests (ISO 179, ISO 180, ASTM D256), engineers can quantify embrittlement trends and predict service life in heat-stressed applications. This article explains the testing principles, sample requirements, procedures, and typical applications of post-aging impact evaluation for plastics.
Technical Overview
Thermal aging, also known as heat aging or heat exposure aging, is an accelerated conditioning process in which plastic specimens are held at elevated temperatures in an air-circulating oven for defined periods. During this exposure, polymers undergo oxidative degradation, molecular chain scission, post-crystallization, crosslinking, and loss of low-molecular-weight additives such as plasticizers and stabilizers. These structural changes typically reduce ductility and increase brittleness, making impact performance one of the most sensitive indicators of thermal degradation.
Impact testing after thermal aging therefore measures the residual toughness of a material by comparing the impact strength of aged specimens against unaged reference specimens. The key output is the impact strength retention rate, calculated as the ratio of aged to initial impact strength expressed as a percentage. A significant drop in retention rate, or a shift from ductile to brittle failure mode, signals that the material may no longer be suitable for the intended service temperature and lifetime. This approach is widely used in material qualification, quality control, and lifetime prediction studies, and it supports requirements found in standards such as ISO 2578, ASTM D3045, and UL 746B for long-term thermal endurance.
Testing Samples
Common Thermoplastic Materials
Typical materials submitted for post-aging impact testing include polypropylene (PP), polyethylene (PE), polystyrene (PS and HIPS), ABS, polycarbonate (PC), PC/ABS blends, polyamide 6 and 66 (PA6, PA66), polyoxymethylene (POM), polyvinyl chloride (PVC), PMMA, PBT, PET, and various glass-fiber-reinforced grades. Thermosetting materials such as phenolics and epoxies can also be evaluated.
Standard Specimen Geometry
- Charpy impact bars per ISO 179-1: 80 mm × 10 mm × 4 mm, either unnotched or with a type A single notch (notch tip radius 0.25 mm).
- Izod impact bars per ISO 180 and ASTM D256: 63.5 mm × 12.7 mm cross-section, with thickness typically between 3.2 mm and 12.7 mm, notched or unnotched.
- Specimens are preferably injection-molded in accordance with ISO 294 or ASTM D638-adjacent molding practices, with notches machined immediately before testing to ensure consistent notch quality.
Finished Parts and Cut Sections
In addition to molded bars, samples may be taken from finished components such as pipe sections, sheet material, appliance housings, connectors, or cable sheathing. When parts are sectioned, specimens must be prepared carefully to avoid introducing additional stresses, sharp edges, or heat damage that could distort the impact results.
Testing Items
The evaluation typically covers the following measurable parameters:
- Charpy impact strength, notched and unnotched, expressed in kJ/m² (ISO 179-1, GB/T 1043).
- Izod impact strength, notched and unnotched, expressed in kJ/m² (ISO 180, GB/T 1843) or J/m and ft·lb/in (ASTM D256, ASTM D6110).
- Impact strength retention rate after each aging condition, reported as a percentage of the unaged value.
- Failure mode classification per ISO 179-1, including complete break, partial break, hinge break, and non-break, used to track the ductile-to-brittle transition.
- Time-to-embrittlement or time-to-criterion values derived from aging series at multiple temperatures.
- Optional supplementary data such as tensile properties, color change, and surface condition after aging.
Testing Methods
Thermal Aging Exposure
Specimens are placed in a forced-ventilation air-aging oven compliant with the requirements of ISO 2578 or ASTM D3045. Aging temperatures are selected based on the expected service environment, commonly in the range of 70 °C to 180 °C, and are often set 10 °C to 30 °C above the maximum service temperature to accelerate degradation. Standard exposure durations include 24 h, 168 h (1 week), 500 h, 1000 h, 2000 h, and longer, with multiple time points used to construct degradation curves. Specimens must be positioned with adequate air circulation on all sides and must not touch each other.
Post-Aging Conditioning
After removal from the oven, specimens are allowed to cool to room temperature and then conditioned in a standard atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 or ASTM D618. A conditioning period of at least 24 hours is typical before impact testing, and notches are preferably machined after aging unless the standard being followed requires notched specimens to be aged in the notched state.
Impact Test Execution
Charpy tests are performed with a pendulum impact tester using a 40 mm support span and a striking edge velocity of approximately 3.5 m/s, with hammer energy selected so that the absorbed energy falls within the valid range defined by ISO 179-1 and ISO 13802. Izod tests clamp the specimen as a vertical cantilever and strike it at a fixed distance from the notch. At least five valid specimens are tested for each aging condition, and ten specimens are recommended when results show high scatter. The corrected absorbed energy is used to calculate impact strength, and individual values, mean values, and failure modes are recorded.
Data Evaluation and Lifetime Assessment
Impact strength retention is plotted against aging time for each temperature. When data from at least three aging temperatures are available, an Arrhenius-type extrapolation per ISO 2578 can estimate the time required to reach a defined end-point criterion, such as 50 % retention of initial impact strength, at the actual service temperature. This information is frequently used to support relative temperature index (RTI) evaluations under UL 746B and comparable long-term thermal endurance claims.
Applications
Post-aging impact testing is essential across many industries where plastics are exposed to sustained heat during service:
- Automotive interior and under-hood components such as dashboards, trim panels, air ducts, battery housings, and radiator fans, where cabin and engine-bay temperatures accelerate embrittlement.
- Electrical and electronic products including enclosures, switches, circuit breaker housings, and connectors that must maintain mechanical integrity after years of operating heat.
- Household appliances such as kettle bodies, oven handles, microwave parts, and hair dryer components.
- Pipes, fittings, and valves for hot-water distribution and heating systems that require long-term hydrostatic and mechanical reliability.
- Wire and cable insulation and sheathing materials that combine thermal aging resistance with mechanical robustness.
- Outdoor equipment and construction products where heat exposure combines with UV radiation, and where material datasheets must be validated by accredited third-party laboratories.
FAQ
Why does impact strength decrease after thermal aging?
Heat exposure promotes oxidative degradation and chain scission in many polymers, while semi-crystalline materials may undergo additional crystallization and stabilizer or plasticizer loss. These changes reduce the material's ability to absorb energy through plastic deformation, causing a shift from ductile to brittle failure and a measurable drop in impact strength.
How are aging temperature and duration selected?
Temperatures are chosen to be high enough to accelerate degradation without causing unrealistic failure mechanisms such as melting or severe distortion, usually 10 °C to 30 °C above the maximum service temperature. Durations are set as a series of time points so that degradation trends can be plotted, and ISO 2578 recommends multiple temperatures for Arrhenius-based extrapolation.
Should Charpy or Izod testing be used?
Charpy impact per ISO 179-1 is predominant in ISO and European markets, while Izod impact per ASTM D256 or ISO 180 is more common in North American specifications. The two methods are not directly interchangeable because of different specimen clamping, notch orientation, and energy calculations, so the method should match the applicable product standard or customer specification.
What is the difference between notched and unnotched impact testing after aging?
Notched specimens measure resistance to crack propagation from a stress concentrator and reveal notch sensitivity, which often increases sharply as a material embrittles. Unnotched specimens reflect the overall toughness of the aged material and are useful for detecting global embrittlement, particularly in materials such as polyamide that become highly notch-sensitive after heat exposure and moisture loss.
How is the impact strength retention rate calculated?
Retention rate is the mean impact strength of aged specimens divided by the mean impact strength of unaged reference specimens, multiplied by 100. For example, if a PP grade retains 2.8 kJ/m² notched Charpy strength after 1000 h at 120 °C compared with 6.0 kJ/m² initially, the retention rate is approximately 47 %.
Can the results be used for lifetime prediction?
Yes, when aging is performed at several temperatures and a clear end-point criterion is defined, the data can be analyzed using the Arrhenius method described in ISO 2578 to estimate time-to-failure at the real service temperature. These predictions are commonly incorporated into material qualification programs and support long-term thermal endurance ratings.