A plastic heat aging test (thermal aging test) is an accelerated laboratory procedure that evaluates how plastic materials retain their mechanical, physical, and electrical properties after prolonged exposure to elevated temperatures. Performed in accordance with international standards such as ISO 2578, ASTM D3045, IEC 60216, UL 746B, and GB/T 7141, the test is essential for material qualification, relative temperature index (RTI) determination, and service lifetime prediction. This article explains the test principle, sample requirements, test items, methods, and typical applications of thermal aging evaluation for plastics.
Technical Overview
Plastic heat aging testing is an accelerated durability evaluation in which polymer specimens are exposed to controlled elevated temperatures for extended periods, after which changes in key properties are measured. The objective is to simulate, in weeks or months, the degradation that a material would experience over years of service at normal operating temperatures, allowing engineers to compare formulations, verify material suitability, and estimate long-term service life.
At elevated temperatures, plastics undergo thermo-oxidative degradation driven by several simultaneous mechanisms: oxidative chain scission, post-crosslinking, hydrolysis of ester or amide groups, volatilization and migration of plasticizers, and depletion of antioxidants and stabilizers. The macroscopic consequences include embrittlement, discoloration or yellowing, surface cracking, loss of impact toughness, reduced elongation at break, mass loss, dimensional change, and deterioration of electrical insulation properties.
The scientific basis of the test is the Arrhenius model, which describes the temperature dependence of chemical reaction rates. By aging materials at several elevated temperatures, measuring the time required to reach a defined end-of-life criterion (commonly 50% retention of a critical property), and plotting the logarithm of failure time against reciprocal absolute temperature, a linear relationship is obtained that can be extrapolated to predict behavior at lower service temperatures. This approach underpins the determination of the temperature index (TI) per IEC 60216 and the relative temperature index (RTI) per UL 746B.
The principal standards governing plastic heat aging evaluation include:
- ISO 2578 — Plastics: Determination of time-temperature limits after prolonged exposure to heat
- ASTM D3045 — Standard Practice for Heat Aging of Plastics Without Load
- IEC 60216 series — Electrical insulating materials: Determination of thermal endurance properties (TI, TEP, HIC)
- UL 746B — Polymeric materials: Long-term property evaluation (RTI)
- GB/T 7141 — Plastics: Heat aging test methods
- GB/T 11026 — Determination of thermal endurance properties of electrical insulating materials (Chinese adoption of IEC 60216)
- ISO 291 / ASTM D618 — Conditioning of test specimens prior to testing
Testing Samples
Molded Test Specimens
Standard specimens are the preferred sample type, typically multipurpose test specimens per ISO 3167 or dumbbell-shaped tensile bars per ASTM D638 (Type I) or ISO 527-2. Specimens should be injection molded or compression molded under conditions representative of actual production, since molding-induced stresses and thermal history significantly influence aging behavior. For each aging temperature and each aging interval, a statistically sufficient number of specimens (typically at least five per property) must be prepared, along with an unaged control group.
Films and Sheets
Plastic films, membranes, and sheets are cut into strips suitable for tensile testing per ISO 527-3 or ASTM D882. Thin samples present higher surface-to-volume ratios, making them particularly sensitive to oxidative degradation and loss of volatile additives.
Pipes, Profiles, and Finished Products
Sections cut from actual manufactured goods, such as pipe rings, cable insulation, connector housings, or appliance components, may be aged directly and subsequently tested as complete parts or as prepared coupons. This approach captures the influence of processing, wall thickness, and part geometry on aging performance.
Compounds and Pellets
Raw granules are first molded into standardized test specimens and then aged. This is common during formulation development, stabilizer package screening, and incoming material qualification, where the aging response of different resin batches or additive systems is compared.
Testing Items
Depending on the material and end use, one or more of the following properties are measured before and after aging, and the percentage retention is calculated:
- Tensile strength and elongation at break (most common end-of-life criterion)
- Flexural strength and flexural modulus
- Izod and Charpy impact strength (notched and unnotched)
- Hardness (Shore, ball indentation)
- Color change (Delta E) and yellowing index
- Mass loss and dimensional change (shrinkage, warpage)
- Melt flow rate (MFR/MVR) shift, indicating molecular weight change
- Surface condition: cracking, crazing, blistering, chalking, gloss loss
- Dielectric strength, volume and surface resistivity, insulation resistance (for electrical applications)
- Oxidation induction time (OIT) as a complementary stabilizer-depletion indicator
- Thermal endurance indices: temperature index (TI), halving interval (HIC), relative temperature index (RTI)
Testing Methods
Oven Aging Procedure
Samples are suspended or placed on racks in a forced-convection aging oven with controlled temperature uniformity (typically within plus or minus 1 to 2 degrees Celsius) and a defined air exchange rate, ensuring consistent oxygen supply to the specimens. The oven must not expose specimens to direct radiant heating. After each aging interval, specimens are removed, conditioned to standard atmosphere (23 degrees Celsius, 50% relative humidity, per ISO 291 or ASTM D618), and tested for the selected properties. Unaged reference specimens are tested under identical conditions for comparison.
Selection of Aging Temperatures and Durations
For simple comparative aging, a single temperature and duration may be specified, for example 500 or 1000 hours at the intended service temperature plus a defined margin. Common comparative conditions include 70, 85, 100, 113, 125, 135, 150, and 175 degrees Celsius, with durations of 168, 336, 500, 1000, 2000, or 3000 hours. For thermal endurance evaluation per IEC 60216 or UL 746B, at least three and preferably four or more aging temperatures are used, spaced approximately 10 to 20 degrees Celsius apart. The lowest temperature should produce a time-to-failure of several thousand hours, while the highest must remain below any transition that would change the degradation mechanism, such as the melting point of semi-crystalline polymers.
Retention Calculation and End-of-Life Criterion
The property retention rate is calculated as the measured property value after aging divided by the initial unaged value, multiplied by 100. A retention of 50% of tensile strength or elongation at break is the most widely used end-of-life criterion, although critical applications may specify stricter thresholds such as 25% elongation at break for safety-related parts. The aging time at each temperature required to reach the criterion is recorded as the failure time.
Arrhenius Extrapolation and Index Determination
The logarithm of failure time is plotted against the reciprocal of absolute temperature (in kelvin). The resulting straight line, validated for linearity across the tested range, is extrapolated to determine the temperature at which the material reaches the end-of-life criterion after the specified reference time. IEC 60216 defines the temperature index (TI) as the temperature corresponding to a 20,000-hour endurance, accompanied by the halving interval (HIC) describing slope steepness, while UL 746B establishes the relative temperature index (RTI) based on a 100,000-hour, 50% retention criterion. These indices are the fundamental parameters for selecting plastics in thermally demanding electrical and automotive designs.
Applications
Plastic heat aging testing is applied throughout the polymer value chain, from resin formulation to end-product certification. Typical application areas include:
- Automotive: under-hood components, air ducts, sensors, connectors, battery module parts, and interior trim near heat sources
- Electrical and electronics: connectors, switch housings, coil bobbins, circuit breaker parts, and enclosures requiring RTI ratings
- Wire and cable: insulation and sheathing compounds rated for continuous operating temperature
- Household appliances: components adjacent to heating elements, motors, and power supplies
- Industrial and construction: hot-water pipes, valves, fittings, and engineering components exposed to sustained heat
- Research and development: antioxidant and stabilizer screening, competitive material benchmarking, and lifetime modeling
Manufacturers use the resulting data to demonstrate compliance with customer specifications and regulatory requirements, to support certifications issued by accredited third-party laboratories, and to establish derating curves that guide safe operating envelopes in product design.
FAQ
What is a plastic heat aging test?
It is an accelerated test in which plastic specimens are held at elevated temperatures in ventilated ovens for defined periods, and their mechanical, physical, or electrical properties are compared before and after exposure. The test quantifies thermo-oxidative degradation and enables long-term performance prediction.
What is the difference between heat aging and heat deflection temperature (HDT)?
HDT and Vicat softening point are short-term mechanical measurements of dimensional stability under load at increasing temperature, typically completed within hours. Heat aging is a long-duration chemical durability test assessing irreversible property degradation caused by oxidation and additive loss over hundreds to thousands of hours.
Which temperatures and durations are typically selected?
Selection depends on the material and its service environment. Comparative screening often uses a single condition such as 1000 hours at a temperature 20 to 40 degrees Celsius above the intended service temperature. Thermal endurance programs per IEC 60216 or UL 746B use at least three temperatures spaced 10 to 20 degrees Celsius apart, with total programs lasting from several months to over a year.
What does a 50% retention criterion mean?
It means the material is considered to have reached the end of its useful life when a critical property, usually tensile strength or elongation at break, falls to half of its initial value. This criterion anchors the failure times used in Arrhenius extrapolation for TI and RTI determination.
How many samples are required?
For each combination of aging temperature and withdrawal interval, typically a minimum of five specimens per measured property are required, plus an unaged control set. Multi-temperature endurance programs therefore require careful planning of specimen quantities, often exceeding one hundred specimens per property.
Can results be extrapolated to real service life?
Yes, provided the Arrhenius plot remains linear across the tested range and the degradation mechanism does not change between test and service temperatures. Caution is required when extrapolating across phase transitions, and critical applications should supplement oven aging with application-specific validation such as OIT measurements and actual product testing in accredited third-party laboratories.