Determination of hardness change after hot air aging is a fundamental accelerated aging test used to evaluate the thermal-oxidative resistance and long-term durability of vulcanized rubber materials. By measuring hardness before and after exposure to elevated temperatures in an air oven, this test provides essential data for material qualification, formulation optimization, and product lifetime prediction. This article explains the test principle, applicable international standards, sample requirements, testing procedures, calculation methods, and typical industrial applications.
Technical Overview
Hot air aging, also known as air oven aging, is one of the most widely used accelerated aging methods for evaluating the heat resistance of rubber materials. During service, rubber products are continuously exposed to oxygen and elevated temperatures, which trigger oxidation reactions including additional crosslinking and chain scission of polymer macromolecules. These structural changes manifest as measurable alterations in physical properties, with hardness change being one of the most sensitive and convenient indicators.
In this test, rubber specimens are exposed to a controlled elevated temperature in a ventilated air oven for a specified duration. The hardness of the specimens is measured before and after aging using either the Shore durometer method or the International Rubber Hardness Degree (IRHD) method. The hardness change is calculated as the difference between the post-aging and initial hardness values, and it is typically expressed in Shore A units or IRHD units.
A positive hardness change (hardening) generally indicates dominant oxidative crosslinking or the loss of plasticizers and volatile components, while a negative hardness change (softening) suggests dominant chain scission or degradation of the network structure. The principal standards governing this test include ISO 188, ASTM D573, GB/T 3512, and JIS K 6257 for the aging procedure, and ISO 48, ISO 7619-1, ASTM D2240, and GB/T 531.1 for the hardness measurement.
Testing Samples
Standard Vulcanized Rubber Test Sheets
The preferred specimens are flat vulcanized rubber sheets with a smooth, flat surface free of defects, bubbles, or contamination. For Shore A hardness testing, the recommended thickness is 6 mm for a single layer, although thinner sheets may be plied to reach the required thickness. For IRHD testing, specimens of approximately 8 to 10 mm thickness and at least 4 mm in diameter are typically required. A minimum of three and preferably five specimens should be tested for each condition.
Finished Rubber Products
Cut samples or whole articles such as O-rings, seals, gaskets, hoses, cable sheaths, and molded parts may also be evaluated. When testing finished products, the measurement area should be flat and sufficiently thick, and the sampling location should be representative of the functional surface of the product.
Sample Conditioning
Before the initial hardness measurement, specimens should be conditioned in a controlled atmosphere, typically 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 23529 or GB/T 2941, for a minimum of 16 hours after vulcanization. After removal from the aging oven, specimens must be conditioned again at standard laboratory temperature for at least 16 hours before the final hardness measurement is performed.
Testing Items
The core testing items for hot air aging hardness change determination include:
- Initial hardness of the specimen before aging, measured in Shore A or IRHD units
- Hardness of the specimen after aging at the specified temperature and duration
- Hardness change value, calculated as the post-aging hardness minus the initial hardness
- Optional complementary properties determined on the same aged specimens, such as tensile strength change, elongation at break change, and mass change, in accordance with ISO 37, ASTM D412, or GB/T 528
Testing Methods
Test Apparatus
The aging test is performed in a forced-convection air oven complying with ISO 188, ASTM D573, or GB/T 3512 requirements. The oven must maintain uniform temperature within a tolerance of ±1 °C at 100 °C or below, and ±2 °C at temperatures above 100 °C, with a continuous air exchange rate of 3 to 10 changes per hour or an air velocity of 0.5 to 1.5 m/s, depending on the standard applied. A calibrated ventilated aging oven with programmable temperature control and an independent temperature recorder is recommended.
Test Conditions
The test temperature and duration are selected according to the intended service conditions of the material and the requirements of the relevant product specification. Commonly used conditions include:
- 70 °C, 100 °C, or 125 °C for general-purpose rubbers such as NR, SBR, and BR
- 125 °C to 150 °C for heat-resistant rubbers such as EPDM, NBR, and CR
- 150 °C to 300 °C for specialty elastomers such as silicone (VMQ), fluorocarbon (FKM), and fluorosilicone rubbers
- Common aging durations of 24 h, 48 h, 72 h, 96 h, 168 h, or longer, depending on the specification requirements
Test Procedure
The standard test procedure consists of the following steps:
- Condition the specimens at standard laboratory temperature and humidity, then measure and record the initial hardness of each specimen in accordance with ISO 48, ISO 7619-1, ASTM D2240, or GB/T 531.1
- Place the specimens in the aging oven, freely suspended or placed on racks coated with inert material, ensuring they do not contact each other and receive uniform air exposure
- Maintain the oven at the specified temperature for the specified duration, recording the actual temperature throughout the exposure period
- Remove the specimens and allow them to cool and recondition at standard laboratory conditions for at least 16 hours
- Measure the post-aging hardness at the same locations and under the same conditions as the initial measurement
Calculation and Expression of Results
The hardness change is calculated using the formula ΔH = H1 − H0, where H0 is the median initial hardness and H1 is the median hardness after aging. The result is reported as a positive value when the material hardens and a negative value when it softens, together with the test temperature, aging duration, hardness method used, and specimen description. Typical acceptance criteria in product specifications range from −5 to +10 Shore A units, although exact limits depend on the material type and application requirements.
Applications
Hardness change determination after hot air aging is applied extensively across the rubber and elastomer industries for material qualification, quality control, and research and development:
- Automotive industry: evaluating sealing rings, gaskets, fuel hoses, brake components, vibration isolators, and tire compounds for thermal durability
- Wire and cable industry: assessing the aging resistance of insulation and sheathing compounds, often in conjunction with the test methods specified in the IEC 60811 series
- Industrial rubber products: conveyor belts, rollers, diaphragms, and rubber linings exposed to elevated service temperatures
- Footwear and consumer goods: verifying the heat resistance of outsoles and elastic components
- Medical and pharmaceutical devices: qualifying elastomeric closures, tubing, and seals for storage stability
- Material research: comparing antioxidant packages, comparing base polymers, rank-ordering formulations, and generating input data for lifetime prediction models based on Arrhenius analysis
Manufacturers, certification bodies, and accredited third-party laboratories rely on this test to demonstrate compliance with product standards, to support supplier approvals, and to investigate field failures caused by thermal-oxidative degradation.
FAQ
Why does rubber hardness change during hot air aging?
Exposure to heat and oxygen causes two competing reactions in rubber: additional crosslinking, which increases hardness, and oxidative chain scission, which decreases hardness. The loss of plasticizers and other volatile ingredients can also increase hardness. The measured hardness change reflects the net effect of these mechanisms on the rubber network.
Is a positive hardness change always a sign of poor quality?
Not necessarily. A small, uniform hardness increase is normal for many rubbers and remains acceptable if it stays within the limits of the applicable product specification. Excessive hardening or softening, however, indicates inadequate heat resistance, improper curing, or a deficient protective additive system.
What is the typical turnaround time for this test?
The total time depends on the specified aging duration plus conditioning and measurement time. A standard 72-hour or 96-hour aging test, combined with pre- and post-aging conditioning of at least 16 hours each, generally requires four to six working days from sample receipt to reporting.
How are Shore A hardness and IRHD results compared?
Shore A durometer hardness and IRHD values are numerically similar for most medium-hardness compounds, but they are not strictly interchangeable because the measurement principles differ. The same method used for the initial measurement must be used for the post-aging measurement so that the hardness change is valid.
Can hardness change testing replace tensile aging tests?
Hardness change is a convenient screening indicator, but product specifications commonly require the simultaneous determination of tensile strength and elongation at break changes after aging, since different degradation mechanisms can affect these properties to different degrees. Running the full property set provides a more complete assessment of aging resistance.
