Plastic Thermal Aging Test Methods: Standards, Procedures and Evaluation

Plastic thermal aging test methods are essential laboratory procedures used to evaluate how polymeric materials degrade when exposed to elevated temperatures over extended periods. By combining accelerated air-oven aging with standardized measurement of mechanical, thermal, and electrical properties, manufacturers and accredited third-party laboratories can predict long-term service performance, compare material formulations, and demonstrate compliance with international standards such as ISO 188, ISO 2578, ASTM D3045, IEC 60216, and UL 746B. This article provides a comprehensive technical overview of the test samples, test items, test methods, and typical applications of thermal aging evaluation for plastics.

Technical Overview

Thermal aging is one of the most common and practically important degradation mechanisms affecting plastic materials. When a polymer is exposed to heat in the presence of oxygen over time, it undergoes chemical and physical changes including oxidative degradation, chain scission, crosslinking, post-polymerization, migration and evaporation of plasticizers and stabilizers, and relaxation of molded-in stresses. The macroscopic consequences are loss of ductility, embrittlement, discoloration, surface cracking, dimensional change, and reduced dielectric performance.

Thermal aging tests are designed to accelerate these degradation processes under controlled laboratory conditions so that the long-term behavior of a material at its intended service temperature can be estimated within a practical time frame. Specimens are exposed to elevated temperatures in forced-convection air ovens for defined periods, and selected properties are measured at intermediate aging intervals. By testing at several temperatures and applying the Arrhenius relationship between degradation rate and temperature, a time-temperature limit or temperature index can be established for the material.

The principal international and national standards governing plastic thermal aging tests include:

• ISO 188 — Rubber, vulcanized or thermoplastic: accelerated ageing and heat resistance tests (widely referenced for elastomeric and plasticized polymers)
• 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 (temperature index TI and thermal endurance profile)
• UL 746B — Polymeric materials: long-term property evaluation (Relative Temperature Index, RTI)
• GB/T 7141 — Plastics: test method of exposure to dry heat (forced-convection oven aging), the Chinese national standard aligned with the ISO methodology
• JIS K 7212 — Japanese industrial standard for thermal aging of plastics by oven exposure

These standards share a common framework: define an end-of-life criterion, age specimens at multiple temperatures, measure the time required to reach the criterion at each temperature, and extrapolate the results to predict performance at lower, service-relevant temperatures.

Testing Samples

Thermoplastic Resins and Compounds

Thermal aging evaluation is routinely performed on commodity and engineering thermoplastics, including polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polyamide (PA), polyoxymethylene (POM), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and high-temperature specialty polymers such as polyphenylene sulfide (PPS), polyetherimide (PEI), and polyetheretherketone (PEEK).

Thermosetting Materials

Crosslinked systems such as epoxy resins, phenolic molding compounds, unsaturated polyester laminates, and polyurethane foams are also tested. For thermosets, aging may involve further post-curing, oxidation-driven embrittlement, or degradation at excessively high exposure temperatures, so temperature selection requires particular care.

Specimen Forms

The following specimen types are commonly used:

• Multipurpose test specimens per ISO 3167 or ASTM D638 Type I tensile dumbbells, which can be machined or directly used for tensile, flexural, and impact testing after aging
• Impact bars for Izod (ASTM D256 / ISO 180) and Charpy (ISO 179) testing
• Flat plaques for color, gloss, and surface defect evaluation
• Miniaturized electrical specimens for dielectric strength, volume resistivity, and comparative tracking index measurements
• Finished components such as connectors, housings, grommets, and cable insulation samples for qualification-level aging studies

Sufficient replicate specimens must be prepared for each aging temperature and each withdrawal time, including unaged reference specimens, since destructive mechanical testing is performed at every interval.

Testing Items

The properties monitored before, during, and after thermal aging are selected based on the intended function of the material. Typical test items include:

• Tensile strength, tensile modulus, and elongation at break (ISO 527, ASTM D638), with retention percentages calculated against unaged values
• Flexural strength and modulus (ISO 178, ASTM D790)
• Izod and Charpy impact strength (ISO 180, ISO 179, ASTM D256), a sensitive indicator of embrittlement
• Shore or Rockwell hardness (ISO 868, ASTM D2240)
• Mass change and dimensional stability, including shrinkage and warpage after exposure
• Color change and yellowness index (ASTM E313, ISO 11664) for appearance-critical parts
• Visual assessment of surface cracking, crazing, blistering, chalking, and gloss loss
• Melt flow rate (ISO 1133, ASTM D1238) to detect molecular weight changes in thermoplastics
• Dielectric strength, volume resistivity, and surface resistivity (IEC 60243, IEC 62631) for electrical insulation materials
• Oxidation induction time (OIT) by differential scanning calorimetry (ISO 11357-6) as a rapid screening measure of antioxidant effectiveness

In most aging programs, one or two critical properties are designated as the diagnostic properties, and the aging end point is defined as the time at which that property falls to a specified fraction of its initial value, commonly 50 percent retention.

Testing Methods

Air Oven Aging Without Load

The core method, described in ISO 188, ASTM D3045, and GB/T 7141, uses a forced-convection air oven with uniform temperature distribution, typically controlled within ±2 °C of the set point. Specimens are suspended or placed on racks with free air circulation on all surfaces, avoiding contact between different materials that could cause cross-contamination by volatile additives. Specimens are withdrawn at predetermined intervals, conditioned at 23 ± 2 °C and 50 ± 5 percent relative humidity, and tested for the selected diagnostic properties.

Selection of Aging Temperatures and Times

Aging temperatures are chosen so that degradation at the highest temperature does not proceed through a different chemical mechanism than at the service temperature. Common practice involves:

• Selecting at least three to five exposure temperatures spaced approximately 10 to 20 °C apart
• Setting the lowest temperature so that the end point is reached in a time long enough to be meaningful, often several thousand hours
• Setting the highest temperature so the end point is reached within days to weeks, enabling rapid data generation
• Assigning at least five withdrawal times per temperature, spaced to define the descending property curve clearly

Determination of Time-Temperature Limits (ISO 2578)

For each aging temperature, the time to reach the end-of-life criterion is determined and plotted on a logarithmic scale against the reciprocal of absolute temperature (Arrhenius plot). The resulting straight line is extrapolated to the intended service temperature to obtain the time-temperature limit: the maximum temperature at which the material retains the specified property level for the required service life.

Thermal Endurance of Insulating Materials (IEC 60216)

Electrical insulation materials are evaluated according to the IEC 60216 series, which specifies procedures for determining the temperature index (TI) and the half-reliability temperature. Aging is conducted at multiple temperatures, and a diagnostic property such as dielectric strength or tensile strength is monitored to a defined end point, typically 50 percent of the initial value. The resulting thermal endurance profile supports insulation system classifications.

Relative Temperature Index (UL 746B)

For polymeric materials used in electrical equipment, long-term aging programs per UL 746B establish the Relative Temperature Index, defined as the temperature at which critical properties are retained for approximately 60,000 hours. Candidate materials are aged in parallel with a reference material of known RTI, and comparative property retention is verified at intermediate points such as 2,000, 5,000, and 10,000 hours, continuing until the long-term index can be justified.

Supplementary Short-Term Heat Resistance Tests

Although they are not aging tests, short-term heat resistance methods are frequently reported together with thermal aging data to fully characterize a plastic:

• Heat deflection temperature (HDT) under load per ISO 75 or ASTM D648
• Vicat softening temperature per ISO 306 or ASTM D1525
• Dimensional stability and oven sag tests at elevated temperature

Short-term tests measure softening behavior, whereas aging tests measure irreversible chemical degradation over time; both are needed for material selection and qualification.

Data Evaluation and Reporting

A complete test report should specify the material and formulation, specimen geometry and molding conditions, aging temperatures and durations, the diagnostic properties and end point criterion, the conditioning atmosphere, individual and mean measured values with retention percentages, and the derived time-temperature limit, temperature index, or RTI as applicable. Statistical treatment of the failure times, such as least-squares regression of the Arrhenius plot with confidence intervals, strengthens the reliability of extrapolations.

Applications

Thermal aging test data support a wide range of engineering and regulatory decisions across industries:

• Automotive: qualification of under-hood components, connectors, sensors, ducting, and interior trim required to survive thousands of hours at continuous operating temperatures
• Electrical and electronics: temperature ratings for enclosures, switchgear components, printed circuit board substrates, coil bobbins, and terminal blocks based on IEC 60216 and UL 746B evaluations
• Wire and cable: aging resistance of PVC, polyethylene, and crosslinked insulation compounds used in building wire and power cables
• Household appliances: verification that plastic housings, gears, and handles withstand sustained heat from motors and heating elements
• Building and construction: service life prediction for hot-water pipes, profiles, roofing membranes, and geosynthetics exposed to elevated temperatures
• Medical and packaging: stability of sterilizable devices, trays, and containers subjected to repeated steam or dry-heat exposure
• Material development: comparison of stabilizer packages, antioxidant systems, and base resin grades during formulation optimization

Manufacturers, designers, and accredited third-party laboratories rely on these test results to set use temperature limits, validate design margins, demonstrate compliance with product safety standards, and reduce the risk of in-service brittleness or electrical failure.

FAQ

What is the difference between thermal aging testing and heat deflection testing?

Heat deflection temperature and Vicat softening tests measure the short-term softening behavior of a plastic under load, usually completed within minutes. Thermal aging tests expose the material to heat for hours to months and measure irreversible chemical degradation such as oxidation and embrittlement. A material can have a high softening point yet still age poorly, so the two test types answer different questions and are typically both required for qualification.

How are aging temperatures selected?

Temperatures are chosen based on the intended service temperature and the material's degradation behavior. The lowest temperature must produce degradation slowly enough to represent real service, while the highest temperature must not change the degradation mechanism. Preliminary screening or supplier data is often used to bracket the range, and at least three temperatures are required for a valid Arrhenius extrapolation.

How long does a thermal aging test take?

Screening comparisons can be completed in a few days to several weeks. A full determination of a temperature index per IEC 60216 generally requires several thousand hours of exposure across multiple temperatures, and a complete UL 746B RTI program with 60,000-hour confirmation may extend beyond one year of continuous oven aging.

What end-of-life criterion is commonly used?

The most common criterion is 50 percent retention of a critical property, such as elongation at break, tensile strength, or dielectric strength, measured against unaged control specimens. Standards also permit other fractions or absolute values when justified by the application, and appearance criteria such as cracking may be used for surface-critical parts.

Can thermal aging tests predict actual service life?

Yes, within defined limitations. Arrhenius extrapolation from multi-temperature aging data provides a scientifically based estimate of service life at lower temperatures, provided the degradation mechanism remains unchanged and humidity, mechanical stress, and light exposure are not dominant factors in service. For products exposed to combined stresses, thermal aging results should be complemented by humidity aging, weathering, and creep testing for a complete durability assessment.

Which standards should be specified on a purchase or test request?

For general plastic aging without load, specify ASTM D3045 or GB/T 7141; for time-temperature limits, specify ISO 2578; for electrical insulation endurance, specify the IEC 60216 series; and for long-term polymeric material ratings in electrical equipment, specify UL 746B. Elastomeric or plasticized materials referenced to ISO 188 should also state the exposure temperature and duration required by the product specification.