Plastic thermal aging tests evaluate how polymeric materials degrade under prolonged exposure to elevated temperatures, providing essential data for material selection, product design, and long-term service life prediction. This guide explains the major international and national standards—including ISO 188, ASTM D3045, IEC 60216, UL 746B, and GB/T 7141—along with sample requirements, test procedures, evaluation criteria, and typical industry applications.
Technical Overview
Thermal aging is the progressive deterioration of plastic materials caused by sustained exposure to elevated temperatures. During this process, polymer chains undergo oxidative degradation, chain scission, crosslinking, post-curing, plasticizer or additive migration, and physical aging. These molecular-level changes manifest as reduced tensile strength, decreased elongation at break, embrittlement, discoloration, increased dielectric loss, and dimensional instability.
Thermal aging tests accelerate these degradation mechanisms in laboratory ovens so that engineers can extrapolate long-term performance from short-duration experiments. The core scientific basis is the Arrhenius model, which relates the rate of chemical degradation to absolute temperature, allowing test data obtained at several elevated temperatures to be extrapolated to lower service temperatures. Accredited third-party laboratories routinely conduct these tests for certification, qualification, and research purposes.
The principal standards governing plastic thermal aging testing include:
- ISO 188 — Accelerated ageing and heat resistance tests for vulcanized and thermoplastic rubber and plastics.
- ASTM D3045 — Standard Practice for Heat Aging of Plastics Without Load.
- IEC 60216 series — Determination of thermal endurance properties of electrical insulating materials, including the Temperature Index (TI).
- UL 746B — Polymeric Materials: Long Term Property Evaluation, which establishes the Relative Thermal Index (RTI).
- ISO 2578 — Determination of time-temperature limits after prolonged exposure to heat.
- GB/T 7141 — Chinese national standard practice for thermal aging of plastics in air.
- GB/T 11026 series — Chinese adoption of IEC 60216 for electrical insulating materials.
Testing Samples
The type, quantity, and preparation of specimens directly affect the validity of thermal aging results. Specimens must be representative of the material or product in its final formulation and processing state.
Multipurpose Test Specimens
For mechanical property evaluation, multipurpose specimens conforming to ISO 3167 or dumbbell-shaped tensile bars per ISO 527-2 and ASTM D638 are the most common choices. Typically, at least five specimens are required for each aging temperature and each sampling interval, with a sufficient number of intervals (usually five or more) to define the degradation curve down to the endpoint criterion.
Electrical Insulation Specimens
For IEC 60216 and UL 746B programs, specimens are prepared according to the specific test method used as the diagnostic property—such as plaques for dielectric strength, narrow strips for tensile elongation, or molded parts for flexural strength. Specimen geometry must remain consistent across all aging temperatures to ensure data comparability.
Finished Products and Components
Complete parts—connectors, enclosures, cable samples, gaskets, or appliance components—may be aged directly when the objective is qualification of an end product. In such cases, aging is often followed by functional testing such as impact resistance, insulation resistance, or pressure testing.
Films, Sheets, and Pipes
Thin films are tested per ISO 2578 or related film standards, while pipes and fittings are typically aged in air ovens or water immersion systems according to ISO 9080 methodology for lifetime prediction of pressure pipe grades.
Testing Items
Thermal aging testing tracks one or more diagnostic properties over time. The choice of property depends on the intended application and the failure mode of concern.
- Tensile strength and elongation at break — the most widely used diagnostic properties; elongation at break is especially sensitive to embrittlement.
- Flexural strength and modulus — relevant for rigid structural components.
- Izod and Charpy impact strength — indicates the onset of embrittlement.
- Dielectric strength and volume resistivity — critical for electrical insulation materials.
- Mass loss — reflects evaporation of plasticizers, stabilizers, or low-molecular-weight species.
- Color change and visual appearance — yellowing, cracking, warpage, and surface oxidation.
- Hardness and dimensional stability — secondary indicators of post-curing or physical aging.
- Melt flow rate — reveals molecular weight changes due to chain scission or crosslinking.
The endpoint criterion is typically defined as retention of 50% of the initial (unaged) property value, although stricter criteria (70% retention) are sometimes specified for critical applications such as electrical insulation.
Testing Methods
All major standards follow a common experimental framework: expose specimens at several elevated temperatures, measure the diagnostic property at planned intervals, determine the time to reach the endpoint at each temperature, and extrapolate to the service temperature.
Air Oven Aging
Specimens are placed in forced-ventilation air ovens with temperature uniformity typically controlled within ±2 °C (±1 °C near the specimen for precision work per IEC 60216). Aging temperatures are selected above the anticipated service temperature but below the polymer's melting point or glass transition region where degradation mechanisms change—commonly three to five temperatures spaced 10–20 °C apart. Sampling intervals are spaced logarithmically to capture the full degradation curve.
Long-Term Thermal Aging for Temperature Index Determination
For IEC 60216 and UL 746B, specimens are aged at multiple temperatures until the endpoint criterion is reached at each temperature. The resulting time-to-endpoint data are plotted using the Arrhenius relationship (logarithm of time versus reciprocal absolute temperature). Extrapolation to a specified time—commonly 5,000 hours for a confirmed index or 60,000 hours for some applications—yields the Temperature Index (TI) or Relative Thermal Index (RTI). UL 746B additionally assigns RTI values with and without impact (RTI Mech w/ Imp and RTI Mech w/o Imp) and an RTI for electrical properties.
Comparative and Screening Aging
For shorter engineering studies, specimens may be aged at a single elevated temperature (for example, 1,000 hours at 120 °C) and compared against retention requirements specified in product standards. This approach, common under GB/T 7141 and ASTM D3045, provides a pass/fail or comparative ranking rather than a lifetime extrapolation.
Data Analysis and Reporting
- Measure the initial diagnostic property on unaged reference specimens.
- Remove aged specimens at each interval and condition them at standard laboratory atmosphere before testing.
- Plot property retention versus aging time for each temperature.
- Determine time-to-endpoint graphically or by regression.
- Construct the Arrhenius plot and extrapolate to calculate the temperature index or estimated service life.
- Report aging temperatures, exposure durations, property retention curves, calculated indices, and any anomalies such as non-Arrhenius behavior.
Applications
Thermal aging test data are fundamental to safety certification, warranty setting, and material qualification across many industries.
- Wire and cable — insulation and jacket compounds are qualified for 90 °C, 105 °C, or 125 °C ratings through long-term aging programs under IEC 60216 and UL 746B.
- Electrical and electronic equipment — enclosures, connectors, coil bobbins, and circuit board substrates require verified thermal endurance for safety certification under IEC and UL schemes.
- Automotive — under-hood components, cable harnesses, and interior trim are aged to confirm performance over the vehicle design life, often combined with heat-humidity cycling.
- Building and construction — PVC profiles, geosynthetics, and pipe materials rely on thermal endurance data for long-term durability claims.
- Appliances and consumer products — plastic parts adjacent to heat sources are qualified for continuous and intermittent temperature exposure.
- Aerospace and rail — engineering polymers and composites are evaluated for thermal endurance in demanding service environments.
FAQ
What is the difference between TI, RTI, and the temperature rating of a plastic?
The Temperature Index (TI, per IEC 60216) and the Relative Thermal Index (RTI, per UL 746B) are both derived from long-term thermal aging data. They represent the temperature at which the material retains at least 50% of a critical property after a specified exposure period. The nominal "temperature rating" of a plastic is not a measured property by itself—it must be supported by such aging data.
Why are several aging temperatures used instead of one?
A single elevated temperature cannot be reliably extrapolated to service conditions because degradation kinetics may shift. Multiple temperatures allow construction of an Arrhenius plot, which validates the degradation model and enables statistically defensible extrapolation.
How long does a thermal aging test take?
Screening tests at a single temperature may be completed in several hundred to a few thousand hours. Full temperature index programs under IEC 60216 or UL 746B typically require 5,000–10,000 hours or more, sometimes exceeding one year, because the lowest aging temperature must run until the endpoint is reached.
What does the 50% retention criterion mean?
It means the material is considered to have reached its thermal endurance limit when the diagnostic property (for example, elongation at break or dielectric strength) falls to half of its original unaged value. This criterion balances safety margins with practical test duration.
How do ISO 188, ASTM D3045, and GB/T 7141 differ?
All three describe oven aging of plastics in air without applied load. ISO 188 originated for rubber and elastomers but is widely applied to thermoplastics; ASTM D3045 is the corresponding American practice; GB/T 7141 aligns closely with these international practices and is commonly referenced in the Chinese market. Differences concern oven specifications, specimen handling, and reporting details rather than fundamental methodology.
Can thermal aging results predict real service life?
They provide a scientifically grounded estimate when degradation follows Arrhenius kinetics and the dominant failure mechanism at high and low temperatures is the same. Deviations caused by physical aging, diffusion-limited oxidation in thick sections, or synergistic effects with humidity, UV, or mechanical stress should be assessed, and combined environmental testing may be required for a complete lifetime assessment.