Hot Air Aging Testing of Plastics: Standards, Methods, and Applications

Hot air aging testing is an accelerated thermal aging method used to evaluate the long-term heat resistance and durability of plastic materials by exposing specimens to elevated temperatures in a forced-air-circulation oven. By measuring changes in mechanical, physical, and electrical properties before and after exposure, accredited third-party laboratories can determine property retention rates, temperature indices, and time-temperature limits in accordance with standards such as ISO 188, ISO 2578, ASTM D3045, IEC 60216, and UL 746B. This article explains the principles, sample requirements, test items, procedures, and industrial applications of hot air aging testing for plastics.

Technical Overview

Hot air aging testing, also known as air oven aging or accelerated thermal aging, is a standardized laboratory method in which plastic specimens are exposed to elevated temperatures inside a forced-ventilation oven for defined periods. The test accelerates the thermo-oxidative degradation that plastics undergo during long-term service, allowing engineers to evaluate heat resistance, thermal endurance, and durability in weeks or months rather than years.

During exposure, heat and oxygen act together on the polymer matrix. Depending on the material, this drives oxidative chain scission, post-crosslinking, hydroperoxide formation, migration and evaporation of stabilizers and plasticizers, discoloration, and progressive embrittlement. These changes are quantified by comparing the properties of aged specimens against unaged control specimens, with results typically expressed as a percentage retention of the initial value.

Because chemical degradation rates increase predictably with temperature according to the Arrhenius relationship, exposing materials at several elevated temperatures allows extrapolation of time-temperature limits and prediction of service life at actual operating temperatures. Hot air aging is therefore a cornerstone of material qualification for wire and cable, electronics, automotive, and construction applications, and is routinely performed by accredited third-party laboratories under ISO/IEC 17025 quality systems.

The most commonly applied standards include:

• ISO 188 and GB/T 3512 — accelerated ageing and heat resistance tests for vulcanized and thermoplastic rubbers and elastomers
• ISO 2578 and GB/T 7142 — determination of time-temperature limits of plastics after prolonged action of heat
• GB/T 7141 — test method for thermal aging of plastics in a hot air oven
• ASTM D3045 — standard practice for heat aging of plastics without load
• ASTM D573 — deterioration of rubber in an air oven
• IEC 60216 series and GB/T 11026 — thermal endurance evaluation of electrical insulating materials, including the temperature index (TI)
• UL 746B — long-term property evaluation of polymeric materials and assignment of the Relative Temperature Index (RTI)
• JIS K 6257 — heat aging resistance tests for rubber and thermoplastic elastomers

Testing Samples

Molded Test Specimens

Most test programs use multipurpose dumbbell specimens conforming to ISO 3167 or ASTM D638 Type I for tensile evaluation, together with bars for flexural testing (ISO 178 / ASTM D790) and impact testing (ISO 179-1, ISO 180 / ASTM D256). Specimens should be injection or compression molded under controlled conditions, free of flash, voids, and excessive residual stress, since processing history significantly influences aging behavior.

Films and Sheets

Flexible packaging films, electrical insulation films, geomembranes, and calendered sheets are tested either as strips for tensile testing or as flat coupons for mass, dimensional, and appearance assessment. Thin specimens age faster than thick sections because oxygen diffusion is not a limiting factor.

Product Sections and Finished Components

When the behavior of an actual product is of interest, specimens can be taken directly from cable insulation and sheathing, pipes and fittings, window profiles, housings, gaskets, and connectors. Testing real product geometry captures the influence of wall thickness, molecular orientation, crystallinity, and post-curing, all of which strongly affect the aging rate.

Sample planning is critical: for every temperature and every exposure interval, a statistically sufficient number of specimens, commonly at least five per property, must be withdrawn and tested, along with unaged reference specimens. All specimens should be conditioned before and after aging at 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 or ASTM D618.

Testing Items

Mechanical Properties

• Tensile strength, yield strength, and elongation at break (ISO 527 / ASTM D638)
• Flexural strength and flexural modulus (ISO 178 / ASTM D790)
• Charpy and Izod impact strength, notched and unnotched (ISO 179-1, ISO 180 / ASTM D256)
• Shore hardness and ball indentation hardness (ISO 868 / ASTM D2240)

Physical and Appearance Characteristics

• Mass change and loss of volatile components
• Dimensional stability, shrinkage, and warpage
• Color difference and gloss retention
• Surface evaluation: cracking, crazing, chalking, embrittlement, and additive migration
• Change in melt flow rate (ISO 1133 / ASTM D1238), indicating chain scission or crosslinking
• Density change

Electrical Properties

• Dielectric strength (IEC 60243-1 / ASTM D149)
• Volume and surface resistivity (IEC 62631 series / ASTM D257)
• Dissipation factor and relative permittivity (IEC 60250 / ASTM D150)

The failure criterion is normally defined as the exposure time or temperature at which a critical property, most often elongation at break or impact strength, falls to 50 % of its initial value. Project-specific criteria such as 25 % or 70 % retention are also frequently specified, particularly for safety-critical applications.

Testing Methods

Equipment Requirements

Testing is performed in a forced-air-circulation aging oven meeting the requirements of ISO 188 or IEC 60216. Key equipment parameters include:

• Temperature uniformity within the working volume, typically within ±1 °C to ±2 °C depending on the oven class
• A controlled air exchange rate, usually 3 to 10 oven volumes per hour, ensuring constant oxygen supply and removal of volatile degradation products
• No direct radiant heating of specimens and no contamination from heating elements
• Continuous temperature recording with calibration against traceable reference sensors

Specimens are mounted on clean, inert racks or grid carriers with free air circulation around each specimen and without mutual contact. Direct contact with metals that could catalyze oxidation is avoided.

Test Procedure

• Determine the initial properties of conditioned, unaged specimens.
• Select at least three, preferably four or five, exposure temperatures spaced approximately 10 °C apart. The highest temperature should bring the material to the endpoint criterion within days to a few weeks, while the lowest requires several weeks to months.
• Place the specimen sets in the oven once the temperature has stabilized, and record the start time.
• Withdraw specimen sets at predefined intervals, for example 24, 72, 168, 336, 500, 1000 hours, and longer as required.
• Recondition the withdrawn specimens to standard atmosphere before testing to eliminate transient moisture and temperature effects.
• Perform the agreed property tests and calculate percentage retention relative to the unaged values.
• Repeat exposure and testing until the endpoint criterion is reached at every temperature.

Data Evaluation and Lifetime Prediction

For each temperature, the retention of the critical property is plotted against exposure time to determine the time to endpoint. The logarithm of this time is then plotted against the reciprocal of absolute temperature in an Arrhenius diagram. If the relationship is linear, the regression line can be extrapolated to derive:

• The temperature index (TI) — the temperature at which the material reaches the endpoint after a defined time, typically 20,000 hours per IEC 60216
• The Relative Temperature Index (RTI) per UL 746B, based on an extrapolated service life of 100,000 hours
• Time-temperature limits per ISO 2578 for a target service life at the intended operating temperature
• The halving interval, describing the temperature dependence of the time required for a property to fall to half its initial value

Extrapolation must remain within a range where the degradation mechanism is unchanged. As a rule of thumb, the extrapolated temperature should not lie more than about 20 K to 30 K below the lowest tested temperature, and curvature in the Arrhenius plot is a warning sign of mechanism changes or competing physical aging effects.

Applications

Hot air aging data support design, material selection, quality control, and product certification across a wide range of industries:

• Wire and cable: thermal class ratings of insulation and sheathing compounds, RTI ratings, and comparison of stabilizer systems
• Electrical and electronics: connectors, terminal blocks, coil formers, encapsulants, and housings requiring verified long-term heat endurance per IEC 60216 and UL 746B
• Automotive: under-hood components, interior trim, lamp housings, and cable channels exposed to sustained elevated temperatures
• Building and construction: PVC pipes and fittings, window profiles, roofing membranes, cable trunking, and geosynthetics
• Household appliances: enclosures, internal supports, and insulating parts located near heat sources
• Medical devices and packaging: validation of polymer performance after long-term storage and heat-related sterilization exposure
• Material development: screening of antioxidant and stabilizer packages, formulation optimization, and batch-to-batch quality control

FAQ

What temperatures are used for hot air aging of plastics?

Typical programs use three to five temperatures between 70 °C and 180 °C or above, depending on the polymer. Polyethylene is often aged between 80 °C and 120 °C, polypropylene between 100 °C and 150 °C, PVC between 100 °C and 130 °C, and engineering plastics such as polyamide and polycarbonate at 150 °C and above. The exact choice follows the expected service temperature and the guidance of the applied standard.

How long does a hot air aging test take?

A single-point quality-control test, such as 168 hours or 720 hours at one temperature with a pass/fail retention requirement, can be completed in one to four weeks. A full thermal endurance evaluation used to derive a TI or RTI value requires aging at several temperatures and may run from several months to more than a year when lower, more realistic temperatures are included.

Can hot air aging results predict actual service life?

Yes, within limits. When degradation is controlled by a single thermally activated mechanism, Arrhenius extrapolation provides reliable rankings and time-temperature limits. Predictions become less certain when materials undergo physical aging, crystallinity changes, additive migration, or different degradation mechanisms at high and low temperatures, which is why extrapolation is restricted to a moderate range below the lowest test temperature.

What is the difference between hot air aging and weathering tests?

Hot air aging evaluates the isolated effect of heat and oxygen, while xenon-arc or UV weathering tests combine light, heat, and moisture to simulate outdoor exposure. The two methods are complementary: hot air aging underpins thermal class and RTI ratings, whereas weathering addresses photodegradation and colorfastness.

Why is the air exchange rate of the oven important?

The air exchange rate controls oxygen availability and the removal of volatile degradation products from the oven atmosphere. An insufficient rate can suppress oxidation and produce optimistic results, while an excessive rate can accelerate the loss of volatile stabilizers. Standards therefore specify a controlled rate, generally 3 to 10 air changes per hour, and require that the actual rate be reported together with the results.

How many specimens are required for a complete test?

A minimum of five specimens per property per exposure interval is customary, multiplied by the number of temperatures and intervals, plus unaged controls. Brittle or highly variable materials require larger sample sets. A complete TI or RTI program frequently consumes several hundred specimens.