Determination of Elongation at Break of Rubber After Hot Air Aging – Testing Methods and Standards

Hot air aging elongation at break testing is a critical accelerated aging evaluation used to assess the thermal durability and service life of rubber materials under elevated temperature conditions. This article provides a comprehensive technical overview of the test principle, applicable standards, sample requirements, testing procedures, and typical industrial applications for measuring the change in elongation at break of vulcanized and thermoplastic rubber after hot air exposure.

Technical Overview

Elongation at break is one of the most sensitive indicators of rubber degradation, and it typically declines more rapidly than tensile strength when elastomers are exposed to heat, oxygen, and oxidative crosslinking over time. Determination of elongation at break after hot air aging, also referred to as air oven aging or accelerated thermal aging, is therefore a fundamental method for evaluating the heat resistance and long-term aging performance of rubber compounds.

The test involves exposing standard tensile specimens to a controlled elevated temperature in a ventilated air oven for a specified duration, followed by tensile testing to measure the retained elongation at break. The result is expressed either as the absolute aged value or, more commonly, as the percentage change (degradation rate) or retention rate relative to the unaged value. This method is widely specified in material qualifications, product certifications, and quality control programs across the automotive, aerospace, wire and cable, sealing, and consumer goods industries.

The primary reference standards include ISO 188 (Rubber, vulcanized or thermoplastic — Accelerated ageing and heat resistance tests), ASTM D573 (Standard Test Method for Rubber Deterioration in an Air Oven), GB/T 3512 (the Chinese national standard for hot air aging of vulcanized rubber or thermoplastic rubber), and ISO 37 or ASTM D412 for the tensile determination itself. These standards define oven requirements, temperature tolerances, aging durations, and specimen conditioning to ensure reproducible and comparable results.

Testing Samples

Vulcanized Rubber Sheets

Standard cured rubber sheets with a thickness of 2.0 mm ± 0.2 mm are the most common sample form. Dumbbell-shaped specimens are die-cut from the sheets along the grain direction in accordance with ISO 37 or GB/T 528 requirements.

Dumbbell Specimens

Type 1, Type 2, Type 3, or Type 4 dumbbell specimens may be used depending on the available material and the specified standard. Type 1 (with a 6 mm narrow section width and 25 mm benchmark length) is the preferred geometry for most aging evaluations. Specimens must be free from surface defects, bubbles, or die-cut damage that could cause premature failure.

Finished Product Cut Samples

For finished goods such as hoses, seals, belts, cable sheaths, and gaskets, specimens may be cut directly from the product provided that a uniform test section can be obtained. The sampling location, direction, and specimen type must be documented.

Sample Quantity Requirements

A sufficient number of specimens is required for both the unaged control group and each aging condition. A minimum of three valid specimens per condition is standard practice, with five specimens recommended to improve statistical confidence.

Testing Items

The hot air aging test program typically includes the following measurement items:

• Elongation at break before aging (initial reference value)
• Elongation at break after hot air aging at specified temperature and duration
• Percentage change in elongation at break (aging degradation rate)
• Performance retention rate of elongation at break
• Supplementary properties measured on parallel specimens: tensile strength, 100 percent, 200 percent, and 300 percent tensile stress (modulus), and hardness change
• Visual assessment of surface condition after aging, such as cracking, blooming, or stickiness

The change in elongation at break is calculated as the difference between the aged value and the unaged value, divided by the unaged value, expressed as a percentage. A negative value indicates degradation, while the retention rate is the ratio of the aged value to the unaged value expressed as a percentage.

Testing Methods

Test Apparatus

The test requires a forced-ventilation air aging oven capable of maintaining the specified temperature within a tolerance of ±1 °C at 100 °C or below, and within approximately ±2 °C at higher temperatures. The oven must provide continuous air circulation and fresh air exchange as specified in ISO 188 or ASTM D573. A universal tensile testing machine with an appropriate load cell capacity, pneumatic or mechanical grips, and an extensometer or benchmark tracking system is used for the tensile measurement.

Test Conditions

Common aging temperatures include 70 °C, 85 °C, 100 °C, 125 °C, 150 °C, and 175 °C, selected according to the material grade and intended service environment. Typical aging durations are 24 h, 48 h, 72 h, 96 h, 168 h (7 days), and longer intervals up to 1000 h or more for extended life assessment. The conditions are normally specified by the relevant product standard or agreed between the parties.

Test Procedure

• Step 1: Die-cut and mark the dumbbell specimens, measure their thickness at three points within the narrow section, and record the median value.
• Step 2: Determine the initial elongation at break of the control specimens using a tensile machine at a crosshead speed of 500 mm/min for Type 1 specimens, in accordance with ISO 37 or GB/T 528.
• Step 3: Suspend or suspend-rack the aging specimens in the air oven, ensuring free circulation of air on all sides and no contact between specimens or with metal surfaces that could cause localized overheating.
• Step 4: Maintain the oven at the specified aging temperature for the specified duration, starting the timing once the specimens are inserted and the oven recovers to the set temperature.
• Step 5: After aging, remove the specimens and condition them at standard laboratory atmosphere (23 °C ± 2 °C, 50 percent ± 5 percent relative humidity) for not less than 16 hours and not more than 96 hours before tensile testing.
• Step 6: Perform the tensile test on the aged specimens using the same crosshead speed as the control test, and record the elongation at break.
• Step 7: Calculate the median elongation at break for each group, then compute the percentage change and retention rate relative to the unaged control value.

Result Expression

The final report states the aging temperature and duration, the specimen type, the unaged and aged median elongation at break values, the percentage change, and any observed surface defects. Results of specimens that fail outside the narrow parallel section are treated as invalid unless the recorded value meets or exceeds the specification limit, in which case the result may be retained.

Applications

Hot air aging elongation at break testing is applied extensively across industries where rubber components are exposed to elevated temperatures during service:

• Automotive industry: engine mounts, radiator hoses, brake system seals, weatherstrips, and under-hood components requiring validation against OEM material specifications.
• Wire and cable industry: insulation and sheathing compounds evaluated for thermal endurance classes, including aging tests at 100 °C to 150 °C and above in accordance with IEC 60811 test methods.
• Sealing and gasket products: O-rings, door and window seals, and pipe gaskets assessed for compression resistance and long-term elasticity retention.
• Footwear and consumer goods: sole compounds, elastic components, and sporting goods verified for durability.
• Aerospace and railway applications: specialty elastomers qualified against stringent material specifications involving multi-temperature aging profiles.
• Material research and development: formulation screening, antioxidant evaluation, and shelf-life or service-life prediction using time-temperature extrapolation techniques such as Arrhenius analysis.

FAQ

Why is elongation at break more sensitive to aging than tensile strength?

Thermal oxidative aging typically increases the crosslink density or introduces chain scission in the rubber network. Both mechanisms restrict molecular chain extensibility, so elongation at break decreases earlier and more sharply than tensile strength, making it a sensitive early indicator of degradation.

What is the difference between percentage change and retention rate?

Percentage change compares the difference between aged and unaged values relative to the unaged value, where a negative result indicates deterioration. Retention rate is the aged value divided by the unaged value expressed as a percentage, where a result of 80 percent or higher is a common acceptance criterion in many product specifications.

How are aging temperature and duration selected?

They are usually dictated by the governing product standard, the intended service temperature of the component, or agreement between supplier and customer. A common rule of thumb is to age at temperatures 20 °C to 50 °C above the maximum service temperature for durations of 24 h to 168 h to accelerate degradation while avoiding non-representative failure mechanisms.

Can specimens be tested immediately after removal from the oven?

No. Specimens must be conditioned at standard laboratory atmosphere for at least 16 hours and no more than 96 hours after aging before tensile testing, as specified in ISO 188 and GB/T 3512, to allow physical relaxation and temperature equilibration.

What causes scatter or invalid results in this test?

Common causes include poor die-cut quality, thickness variation, oven temperature non-uniformity, excessive air exchange rates, specimens touching during aging, grips slippage, and failure occurring outside the gauge length. Careful specimen preparation, oven calibration, and verification of airflow are essential for reliable data.