The heat aging tensile strength reduction rate test is a fundamental durability evaluation for vulcanized rubber and thermoplastic elastomers, quantifying the percentage loss of breaking strength after exposure to elevated temperatures in an air oven. Performed in accordance with ISO 188, ASTM D573, and ASTM D412, or their GB, JIS, and IEC equivalents, this test is widely used to verify the thermal-oxidative aging resistance of seals, hoses, cables, tires, and other rubber components. This article explains the test principle, sample requirements, procedures, calculation methods, and typical industrial applications.
Technical Overview
The heat aging tensile strength reduction rate test measures the relative decrease in the breaking strength of rubber after the material has been exposed to a controlled elevated temperature in an air oven for a specified duration. Thermal-oxidative degradation of rubber proceeds through chain scission, additional crosslinking, and oxidation of the polymer network, all of which progressively reduce the material's ability to carry tensile load. By comparing the tensile strength of aged specimens against unaged control specimens, the test delivers a quantitative aging index that is directly correlated with product durability and expected service life.
The tensile strength reduction rate is calculated with the following expression:
Tensile Strength Reduction Rate (%) = (T0 − T1) / T0 × 100
In this formula, T0 is the median tensile strength of the unaged specimens and T1 is the median tensile strength of the aged specimens. Many specifications express the same result as a retention rate, which equals 100 minus the reduction rate. A negative reduction rate indicates that aging increased the strength, which can occur in under-cured compounds due to post-vulcanization crosslinking.
The test is governed by a family of internationally recognized standards, including:
- ISO 188 — Rubber, vulcanized or thermoplastic: accelerated ageing and heat resistance tests
- ASTM D573 — Standard Test Method for Rubber Deterioration in an Air Oven
- ISO 37, ASTM D412, GB/T 528, and JIS K 6251 — determination of tensile stress-strain properties
- GB/T 3512 and JIS K 6257 — hot air aging methods for vulcanized rubber
- IEC 60811 series — tensile strength and elongation tests for polymeric cable insulation and sheaths
Testing Samples
Standard Vulcanized Rubber Compounds
Dumbbell-shaped specimens (Type 1, 2, 3, or 4 as defined in ISO 37, or Die C specimens per ASTM D412) are die-cut from cured test sheets of approximately 2 mm thickness. A minimum of three specimens is required for the aged group and three for the unaged control group, although five specimens per group are recommended to improve statistical reliability.
Thermoplastic Elastomers (TPE, TPV, TPU)
Injection-molded or extruded plaques can be evaluated using the same dumbbell geometry. Testing of thermoplastic elastomers is valuable for grade screening, formulation comparison, and long-term heat resistance qualification of engineering materials.
Finished Rubber Products
Specimens may be prepared directly from manufactured articles such as O-rings, gaskets, hoses, anti-vibration mounts, diaphragms, and footwear soling. Where flat dumbbells cannot be obtained, ring specimens tested per ISO 37 Method B are an accepted alternative. Product-specific geometry and thickness must be documented, as they influence the measured values.
Polymeric Cable Insulation and Sheathing
Dumbbell or tubular specimens prepared from cable insulation and sheath compounds are aged in an air oven per the IEC 60811 series, after which tensile testing is performed and the result is reported as the variation of tensile strength relative to the unaged condition.
Testing Items
The core deliverable of the test is the tensile strength reduction rate, supported by the underlying measurement data. Typical reporting items include:
- Tensile strength of unaged specimens, in MPa
- Tensile strength of heat-aged specimens, in MPa
- Tensile strength reduction rate, retention rate, or change rate, in percent
- Elongation at break before and after aging, and its change rate, in percent
- Optionally: hardness change, modulus change, and mass change after aging
Testing Methods
Specimen Preparation and Conditioning
Specimens are die-cut with a sharp cutter, and their width and thickness are measured at multiple positions. Unless otherwise specified, all specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 16 hours before testing, in line with ISO 23529.
Baseline Tensile Testing
The unaged control specimens are tested to failure in a calibrated universal tensile testing machine per ISO 37, ASTM D412, GB/T 528, or JIS K 6251. A typical crosshead speed is 500 mm/min for standard dumbbells. The median tensile strength of the control group is recorded as T0.
Accelerated Heat Aging Exposure
The aged specimens are placed in a forced-ventilation air oven complying with ISO 188 or ASTM D573, with the air exchanged several times per hour. Common aging conditions include 70 °C, 85 °C, 100 °C, 125 °C, or 150 °C for durations of 24, 48, 72, 96, or 168 hours. The specific temperature and duration are selected based on the polymer type and the intended service environment, following the requirements of the applicable product standard.
Post-Aging Testing and Calculation
After aging, the specimens are removed from the oven and allowed to cool, then reconditioned at standard laboratory atmosphere for at least 16 hours and no more than 96 hours before tensile testing. The median tensile strength of the aged group is recorded as T1, and the reduction rate is computed as (T0 − T1) / T0 × 100. Results are compared against the acceptance limits defined in the product specification or purchase agreement.
Applications
The heat aging tensile strength reduction rate test is applied across the full rubber product lifecycle, from material development to batch release and failure analysis. Typical application areas include:
- Automotive components: cooling system hoses, fuel hoses, engine mounts, seals, and bushings that must retain strength after prolonged exposure to under-hood temperatures
- Wire and cable: qualification of insulation and sheathing compounds against thermal aging requirements in IEC-based product standards
- Industrial sealing products: O-rings, gaskets, and diaphragms for pipelines, valves, and hydraulic systems
- Footwear and consumer goods: evaluation of soling compounds and elastic components for heat durability
- Research and development: aging kinetics studies, Arrhenius-based lifetime prediction, and formulation optimization
- Quality control: incoming material inspection and batch conformity verification at accredited third-party laboratories
Manufacturers use the test data to select heat-resistant polymer systems, optimize antioxidant packages, validate cure systems, and demonstrate compliance with regulatory and customer specifications before products enter service.
FAQ
What is the difference between reduction rate and retention rate?
The reduction rate describes the percentage of tensile strength lost after aging, while the retention rate describes the percentage remaining. The two values are complementary and sum to 100. For example, a 25% reduction rate corresponds to a 75% retention rate, and many specifications simply state a minimum retention requirement.
What aging temperatures and durations are commonly specified?
Common conditions include 70 °C for general-purpose goods, 100 °C or 125 °C for automotive and industrial rubbers, and up to 150 °C or higher for specialty heat-resistant compounds such as EPDM, silicone, or fluoroelastomers. Durations of 24 to 168 hours are typical, with longer exposures used to simulate extended service.
What is a typical acceptance criterion?
Acceptance limits depend entirely on the product standard and end-use requirements. A common specification requires a tensile strength reduction of no more than 20% to 30% after the specified aging period, but critical sealing and cable applications may impose stricter limits.
Why can tensile strength sometimes increase after heat aging?
In under-cured compounds, additional crosslinking during oven exposure can raise the tensile strength, producing a negative reduction rate. While this may appear favorable, it often signals incomplete vulcanization, and elongation at break typically decreases at the same time.
How many specimens are needed and how should results be treated?
At least three specimens per group are required by the standards, and five are recommended. The median value of each group is used for the calculation, which reduces the influence of outliers caused by cutting defects or local inhomogeneity.
