Hot Air Aging Elongation at Break Testing for Rubber Materials

Hot air aging elongation at break testing is an accelerated thermal-oxidative aging evaluation that measures how elevated temperature exposure in air degrades the extensibility of vulcanized rubber and thermoplastic elastomers. By comparing the elongation at break of aged specimens against unaged baseline values, manufacturers can assess material durability, verify formulation quality, and demonstrate compliance with international standards such as ISO 188, ASTM D573, and GB/T 3512. This article explains the test principle, sample requirements, methodology, result calculation, and typical industrial applications.

Technical Overview

Elongation at break is the tensile strain a rubber specimen sustains at the moment of rupture, expressed as a percentage increase over its original gauge length. It is one of the most sensitive indicators of rubber degradation because even minor molecular-level changes dramatically affect the ultimate extensibility of the material.

During real-world service, rubber products such as seals, hoses, gaskets, and cable sheaths are continuously exposed to oxygen at elevated temperatures. This triggers oxidative chain scission, additional crosslinking, and changes in the rubber network structure, which manifest macroscopically as hardening, embrittlement, and a significant reduction in elongation at break. Hot air aging testing accelerates this degradation process in the laboratory by exposing standardized test pieces to controlled hot air at a specified temperature and duration, followed by tensile measurement.

The core deliverables of the test are:

• Elongation at break after aging at a specified temperature and time
• The percentage change or retention rate of elongation at break relative to unaged specimens
• An objective basis for material qualification, formulation comparison, and service life estimation

The aging portion of the test is governed by ISO 188, ASTM D573, and GB/T 3512, while the tensile measurement is performed in accordance with ISO 37, ASTM D412, and GB/T 528.

Testing Samples

Standard Dumbbell Test Pieces

The preferred specimen is a Type 1 or Type 2 dumbbell (ISO 37) or a Die C dumbbell (ASTM D412) die-cut from a flat vulcanized sheet of 2 mm nominal thickness. Dumbbell specimens provide a well-defined gauge region that concentrates failure within the narrow section, ensuring repeatable and comparable elongation results.

Specimens Cut from Finished Products

When the actual material of a finished product must be evaluated, dumbbells or parallel strips can be die-cut directly from items such as molded seals, hose walls, belts, or sheet goods, provided the thickness and surface condition permit uniform specimen preparation. For products with closed geometries such as O-rings, whole ring specimens may be tested in accordance with the tensile testing standards.

Thermoplastic Elastomers and Specialty Compounds

The test also applies to thermoplastic elastomers (TPE, TPV) and specialty vulcanizates including natural rubber (NR), SBR, NBR, EPDM, silicone rubber (VMQ), and fluoroelastomer (FKM). Specimens are typically prepared from compression-molded, injection-molded, or extruded plaques representing the production cure conditions.

Testing Items

The primary and associated test items include:

• Original (unaged) elongation at break, serving as the reference baseline
• Elongation at break after hot air aging at specified conditions, for example 70 °C × 168 h, 100 °C × 72 h, 125 °C × 24 h, or 150 °C × 48 h
• Percentage change in elongation at break, or elongation retention rate, calculated by comparing aged and unaged results
• Optional companion parameters: tensile strength change, 100 percent and 300 percent modulus change, hardness change, and the comprehensive aging coefficient

Testing Methods

Reference Standards

• ISO 188 and GB/T 3512 for accelerated hot air aging of vulcanized or thermoplastic rubber
• ASTM D573 for rubber deterioration in an air oven
• ISO 37, ASTM D412, and GB/T 528 for determination of tensile stress-strain properties
• ISO 23529 and ASTM D1349 for specimen preparation and standard conditioning atmospheres

Aging Procedure

Test pieces are suspended freely in a forced-ventilation aging oven so that air circulates uniformly around all surfaces. Specimens are positioned apart from one another and away from the oven walls to avoid localized temperature deviations and mutual contamination by volatile constituents. The oven maintains the specified aging temperature within a tight tolerance, typically ±1 °C for temperatures up to 100 °C, with an air exchange rate on the order of 3 to 10 volume changes per hour as required by ISO 188.

Common aging conditions are selected according to the intended service environment and product specification, with temperatures ranging from 70 °C for ambient-temperature applications up to 150 °C or higher for heat-resistant compounds such as EPDM, silicone, and FKM. After the aging period, specimens are removed and conditioned at 23 ± 2 °C and 50 ± 5 percent relative humidity for not less than 16 hours and preferably no more than 96 hours before tensile testing.

Tensile Testing and Elongation Measurement

Bench marks are applied at the standard gauge length, for example 25 mm for a Type 1 dumbbell. The specimen is clamped in a universal tensile testing machine and extended at the standard crosshead speed, typically 500 mm/min for ISO dumbbells, until rupture occurs in the narrow section. The elongation at break is calculated as the increase in distance between bench marks at rupture divided by the original gauge length, multiplied by 100. At least three, and preferably five, valid specimens are tested for each condition, and the median value is reported. Specimens that fail outside the gauge section are discarded and retested.

Calculation and Expression of Results

The percentage change in elongation at break is calculated by subtracting the unaged value from the aged value, dividing by the unaged value, and multiplying by 100. A negative result indicates degradation. Alternatively, the retention rate is expressed as the aged value divided by the unaged value multiplied by 100 percent. Typical acceptance criteria in automotive, cable, and industrial specifications limit the reduction in elongation at break to between 30 and 50 percent relative to the unaged baseline, depending on the severity of the intended service conditions.

Applications

Hot air aging elongation at break testing is widely applied across industries where rubber components must retain flexibility after prolonged thermal exposure:

• Automotive engineering: engine bay seals, radiator hoses, fuel system components, suspension bushings, and weatherstrips
• Wire and cable: heat resistance qualification of insulation and sheathing compounds
• Tire and tube manufacturing: evaluation of sidewall and inner liner compounds
• Industrial rubber goods: conveyor belts, synchronous and V-belts, diaphragms, and flange gaskets
• Footwear: durability assessment of outsole and midsole compounds
• Medical and pharmaceutical devices: stability evaluation of stoppers, seals, and tubing
• Research and development: antioxidant efficiency screening, formulation benchmarking, and Arrhenius-based service life prediction
• Quality control and supplier verification against buyer and regulatory specifications

FAQ

Why is elongation at break so sensitive to hot air aging?

Thermal-oxidative aging alters the rubber network through chain scission and secondary crosslinking. Elongation at break depends directly on the extensibility of the molecular network, so even small structural changes cause a disproportionately large drop in this property, often well before tensile strength or hardness show significant movement.

How are the aging temperature and duration selected?

Conditions are chosen to reflect the intended service environment and are usually dictated by the relevant product standard or customer specification. Mildly loaded components may be evaluated at 70 °C for 168 hours, while heat-resistant compounds for engine or cable applications are commonly tested at 125 °C to 150 °C for 24 to 96 hours.

What change in elongation at break is considered acceptable?

Acceptance limits vary by specification. Many automotive and cable standards require the retention rate to remain at or above 50 to 70 percent of the original value after the specified aging period, meaning the reduction must not exceed 30 to 50 percent.

Can finished products be tested instead of molded sheets?

Yes. Specimens can be die-cut directly from finished items whenever geometry and thickness allow, and ring-shaped products such as O-rings can be tested as complete rings. Testing actual product material captures the influence of production curing and processing on aging performance.

What is the typical turnaround time for this test?

The total duration consists of the aging period, post-aging conditioning of at least 16 hours, and the tensile test itself. A standard 70 °C × 168 h evaluation is typically completed within about ten working days, while shorter aging conditions can be finished within three to five days, excluding specimen preparation time.