Plastic Thermal Aging Tensile Strength Reduction Rate Testing: Principles, Standards, and Procedures

Plastic thermal aging tensile strength reduction rate testing measures the percentage loss of a plastic material's breaking strength after prolonged exposure to elevated temperatures, providing a quantitative indicator of thermal stability and expected service life. This test is essential for qualifying wire and cable compounds, elastomeric seals, and engineering plastics used in heat-sensitive applications. In this article, we explain the underlying principles, applicable international standards, specimen requirements, step-by-step test procedures, calculation methods, and typical industry applications.

Technical Overview

Thermal aging tensile strength reduction rate testing quantifies how much of a plastic material's breaking strength is lost after it is exposed to a controlled elevated temperature for a defined period. During thermal aging, polymer chains undergo oxidative degradation, chain scission, additional crosslinking, post-crystallization, and plasticizer migration or volatilization. These molecular-level changes reduce the material's load-bearing capacity, and the extent of degradation is expressed as a percentage relative to the strength of unaged control specimens.

The central result is calculated with the following formula: Tensile Strength Reduction Rate (%) = (σ0 − σ1) / σ0 × 100, where σ0 is the median tensile strength of the unaged specimens and σ1 is the median tensile strength measured after thermal aging. A positive value indicates degradation, while a negative value indicates that the material gained strength through post-curing or further crosslinking. Many product specifications define a permissible absolute variation, commonly within ±20% to ±30% depending on the material grade and end use.

This test is widely applied in material qualification, incoming inspection, formulation development, and long-term performance assessment. By comparing aged and unaged specimens, engineers can verify compliance with heat-resistance requirements in product standards, benchmark competing formulations, and support lifetime extrapolation using Arrhenius analysis or time–temperature superposition methods.

Testing Samples

Accredited third-party laboratories typically accept the following categories of plastic and elastomeric materials for thermal aging tensile strength reduction rate evaluation.

Wire and Cable Insulation and Sheath Compounds

PVC insulation and sheath compounds, cross-linked polyethylene (XLPE), polyethylene (PE), halogen-free flame-retardant compounds, and ethylene-propylene rubber (EPR) compounds are the most frequently tested materials. Cable product standards such as IEC 60227, IEC 60245, GB/T 5023, GB/T 5013, and GB/T 8815 impose strict limits on strength variation after air-oven aging, making this test a mandatory part of cable certification.

Rubber and Thermoplastic Elastomer Products

Seals, gaskets, hoses, O-rings, and molded TPE or TPV parts are evaluated using methods aligned with ISO 188, ASTM D573, and GB/T 3512 to confirm that heat exposure during service does not excessively compromise mechanical integrity.

Engineering Thermoplastics

Polyamide (PA), polycarbonate (PC), polybutylene terephthalate (PBT), polyoxymethylene (POM), and modified polypropylene grades used in automotive and electrical assemblies are assessed by combining heat-exposure protocols from ISO 2578 or GB/T 7141 with tensile testing per ISO 527 or ASTM D638.

Plastic Films and Sheets

Packaging films, protective membranes, and electrical insulation tapes are tested to evaluate the embrittlement and strength loss that occur during prolonged thermal exposure in processing or service environments.

Testing Items

A complete thermal aging evaluation package generally includes the following measured and calculated parameters:

• Original tensile strength of unaged control specimens (MPa)
• Tensile strength after thermal aging (MPa)
• Tensile strength reduction rate, or aging variation, expressed as a percentage
• Tensile strength retention rate (σ1 / σ0 × 100%) as an alternative expression
• Elongation at break before and after aging, together with its percentage change
• Visual assessment of aged specimens for cracking, warpage, discoloration, or surface embrittlement

Testing Methods

Specimen Preparation and Conditioning

Dumbbell-shaped or parallel-strip specimens are die-cut from sheets or directly injection molded, following the geometry defined in ISO 527-2, ASTM D638, or IEC 60811-501. At least five valid specimens are prepared for each condition, meaning one group for the unaged control and one group for the aged evaluation. Specimen thickness is measured at three points along the test section to determine the cross-sectional area accurately. Before testing, all specimens are conditioned in a standard atmosphere of 23°C ± 2°C and 50% ± 10% relative humidity in accordance with ISO 291, typically for a minimum of 24 hours.

Thermal Aging Exposure

Specimens are freely suspended in a forced-ventilation air oven complying with IEC 60811-401, ISO 188, or GB/T 3512, with an air exchange rate of approximately 8 to 20 complete changes per hour and temperature uniformity maintained within about ±2°C of the set point. Common aging conditions include 80°C, 100°C, 120°C, or 135°C for 168 hours, with heat-resistant grades often evaluated at 150°C or higher for extended durations such as 240, 336, or 1000 hours. The specific temperature and duration are selected according to the material class and the requirements of the governing product standard. After aging, specimens are removed from the oven and conditioned again at ambient laboratory conditions for at least 16 hours before tensile testing.

Tensile Testing Procedure

Tensile testing is performed on a calibrated universal testing machine. Reference marks are applied to the narrow test section to measure elongation. The crosshead speed is specified by the applicable standard, for example 250 mm/min for soft PVC cable compounds per IEC 60811-501, or speeds between 1 and 500 mm/min for rigid plastics per ISO 527 depending on modulus. The maximum force and elongation at break are recorded for each specimen, and tensile strength is calculated as the maximum force divided by the original minimum cross-sectional area. The median of the valid results is reported as the characteristic value, which suppresses the influence of occasional outliers.

Calculation and Evaluation of Results

Using the median strengths, the reduction rate is computed as (σ0 − σ1) / σ0 × 100%, and the retention rate as σ1 / σ0 × 100%. The result is then compared with the acceptance criteria of the relevant specification. For example, many cable standards require that the absolute variation of tensile strength after aging not exceed 25%, which corresponds to a minimum retention of 75%. If the aged strength falls below the allowed retention level, or if specimens exhibit thermal cracking or severe embrittlement, the material is judged to have failed the heat-aging requirement for that temperature rating.

Applications

Thermal aging tensile strength reduction rate testing supports quality and safety decisions across multiple industries:

• Wire and cable certification and routine factory quality control, verifying the temperature ratings of insulation and sheath compounds required by IEC, EN, GB, UL, and JIS cable standards
• Automotive engineering, where under-hood plastics and elastomers must retain strength after long-term exposure to engine bay heat
• Household appliances and electrical equipment, confirming that plastic components withstand continuous operating temperatures
• Formulation research and development, comparing heat stabilizers, antioxidants, and filler systems in candidate compounds
• Lifetime prediction studies, using multi-temperature aging data to establish time–temperature limits and Arrhenius-based service-life extrapolations

FAQ

What is the difference between reduction rate and retention rate?

Both describe the same aging effect from opposite directions. The reduction rate expresses the percentage of strength lost, calculated as (σ0 − σ1) / σ0 × 100%, while the retention rate expresses the percentage of strength remaining, calculated as σ1 / σ0 × 100%. A reduction rate of 20% equals a retention rate of 80%.

How are the aging temperature and duration selected?

They are prescribed by the product standard governing the material. Typical regimes for cable compounds include 80°C for 168 hours for ordinary PVC, 100°C to 135°C for 168 hours for heat-resistant grades, and 135°C or 150°C for XLPE and EPR compounds. Higher temperatures or longer durations are used when evaluating long-life or high-temperature insulation classes.

How many specimens are required for a reliable result?

A minimum of five valid specimens per condition is standard practice, providing one unaged control group and one aged group. Using median values, as required by IEC 60811-501 and related standards, improves reproducibility by limiting the influence of individual specimen defects.

What causes an excessive reduction in tensile strength?

Common causes include oxidative chain scission, depletion of antioxidants or heat stabilizers, plasticizer volatilization from PVC, insufficient crosslinking in cured systems, or physical aging such as post-crystallization. A poorly formulated or contaminated compound will typically show a rapid strength drop at the rated temperature.

Can the test be applied to thermosetting materials?

Yes. Cross-linked compounds such as XLPE and cured rubber are regularly tested. In some thermosets, initial aging may produce a slight strength increase from additional crosslinking, so the evaluation criterion is usually expressed as an absolute variation limit rather than a one-directional loss limit.

How long does the complete test take?

The total turnaround depends on the aging duration specified by the standard. For the common 168-hour regime, the overall test typically requires about two to three weeks, covering specimen preparation, conditioning, oven exposure, post-aging stabilization, tensile testing, and report issuance.