Rubber Hardness Aging Determination: Test Methods, Standards, and Applications

Rubber hardness aging determination measures the change in hardness of vulcanized or thermoplastic rubber after exposure to accelerated aging conditions such as elevated temperature, providing a reliable indicator of long-term material durability. This test is widely used to qualify seals, O-rings, hoses, tires, and other elastomeric components for demanding service environments. This article explains the relevant standards, sample requirements, testing procedures, calculation methods, and practical applications of hardness change after aging.

Technical Overview

Rubber hardness aging determination is a comparative test that evaluates how the hardness of an elastomer changes after it is subjected to an artificial aging environment, most commonly hot air at elevated temperature. During service, rubber materials undergo thermal oxidation, additional crosslinking, chain scission, and loss of plasticizers or other volatile additives. These molecular-level changes manifest as measurable shifts in hardness, making hardness change one of the simplest and most informative indicators of rubber aging resistance.

The core result of the test is the hardness change, calculated as the difference between the hardness measured after aging and the hardness measured on the unaged material. A small hardness change indicates good aging stability, while a large positive change (hardening) or negative change (softening) signals material degradation that may compromise sealing performance, flexibility, and mechanical strength.

The test is typically performed in two stages: accelerated aging of the specimen in an air oven, followed by hardness measurement using a durometer or dead-load hardness tester. The procedures are governed by internationally recognized standards organizations including ISO, ASTM, and the Chinese national standards body (GB).

Testing Samples

Standard Vulcanized Test Plaques

Flat cured rubber sheets are the preferred specimen form. For Shore A durometer testing per ASTM D2240 and ISO 7619-1, the preferred thickness is 6 mm or greater; for IRHD testing per ISO 48 and GB/T 531.1, a thickness of 8 to 10 mm is recommended. Thinner sheets may be stacked without bonding to reach the required thickness.

O-Rings and Sealing Elements

O-rings, gaskets, and other sealing products with small cross-sections are tested using micro-hardness methods. Micro-IRHD testing per ISO 48 is suitable for cross-sections as thin as approximately 1 mm, making it the standard approach for evaluating aged elastomeric seals.

Finished Rubber Products

Specimens may be cut from finished articles such as hoses, vibration isolators, bushings, tire components, and molded parts, provided a sufficiently flat and uniform test area with adequate thickness is available.

Rubber Compounds

Uncured or freshly mixed compounds are first vulcanized into standard slabs under controlled conditions, and the cured material is then divided into unaged and aged groups for comparison testing.

Testing Items

The typical scope of rubber hardness aging determination includes the following measured and calculated parameters:

• Initial hardness (Shore A, Shore D, or IRHD) of the unaged specimen
• Hardness after accelerated aging at specified temperature and duration
• Hardness change, expressed in Shore points or IRHD points, calculated as the aged value minus the initial value
• Optional companion properties often measured alongside hardness, including tensile strength change and elongation at break change per ISO 37, ASTM D412, or GB/T 528
• Visual assessment of the aged specimen for cracking, blistering, or surface degradation

Testing Methods

Accelerated Heat Aging in Air

The most common aging method uses a forced-air ventilation oven in accordance with ISO 188, ASTM D573, or GB/T 3512. Specimens are exposed at standardized temperatures such as 70 °C, 100 °C, 125 °C, or 150 °C for fixed periods, commonly 24 h, 72 h, 168 h, or 1000 h, selected to simulate the intended service life and operating temperature of the product.

Hardness Measurement

After aging, specimens are conditioned at standard laboratory atmosphere, typically 23 ± 2 °C and 50 ± 5 % relative humidity for at least 16 hours, before hardness is measured. Hardness is determined using a Shore A durometer per ASTM D2240, ISO 7619-1, or GB/T 531.1, or by the dead-load IRHD method per ISO 48. At least five readings are taken at different points on the specimen, spaced away from edges, and the median value is reported.

Calculation and Reporting

The hardness change is calculated as the aged hardness minus the initial hardness. A positive value indicates hardening, usually caused by continued crosslinking or oxidation, while a negative value indicates softening, often due to plasticizer loss, reversion, or chain scission. The test report includes the aging temperature and duration, the hardness method used, individual and median hardness values, and the calculated hardness change.

Alternative Aging Environments

Depending on the application, hardness aging determination may also be performed after exposure to other aggressive environments, including:

• Ozone aging per ISO 1431 or ASTM D1149 for elastomers exposed to atmospheric ozone
• Liquid immersion aging per ISO 1817 or ASTM D471 for fuel, oil, and chemical resistance
• Artificial weathering and UV exposure per ISO 4892 for outdoor applications
• Oxygen pressure aging per ISO 188 for highly accelerated oxidative aging

Applications

Rubber hardness aging determination is applied across many industries where elastomeric components must retain their properties over long service periods:

• Automotive: engine seals, coolant hoses, brake components, door and window seals, and anti-vibration mounts exposed to under-hood heat
• Aerospace: gaskets, O-rings, and seals qualified for wide temperature ranges
• Oil and gas: downhole sealing elements, blowout preventer seals, and chemically resistant linings
• Electronics and cable: insulation and jacketing compounds that must resist thermal embrittlement
• Medical devices: biocompatible elastomeric closures and tubing evaluated for shelf-life stability
• Footwear and consumer goods: outsole and midsole compounds assessed for durability
• Construction: bridge bearings, expansion joints, and waterproofing membranes

Manufacturers, designers, and accredited third-party laboratories use hardness aging data for material qualification, quality control, formulation development, and lifetime prediction of rubber products.

FAQ

What is rubber hardness aging determination?

It is a laboratory test that measures how much the hardness of a rubber material changes after exposure to accelerated aging conditions, most commonly hot air, compared with its initial unaged hardness.

Which standards apply to this test?

Aging is typically performed per ISO 188, ASTM D573, or GB/T 3512, while hardness is measured per ISO 48, ISO 7619-1, ASTM D2240, or GB/T 531.1. The specific standard combination depends on the product specification and target market.

What are typical acceptance criteria for hardness change?

Common specifications limit hardness change to within ±5 to ±10 Shore A points, though stricter or looser limits apply depending on the industry, application, and base polymer. For example, fluorocarbon and silicone elastomers generally show smaller changes than natural rubber compounds.

What does a positive hardness change indicate?

An increase in hardness usually indicates continued vulcanization, oxidative crosslinking, or loss of plasticizer, all of which reduce elasticity and can lead to cracking or seal leakage. A decrease indicates reversion or chemical breakdown that softens the material.

How long does the test take?

Typical turnaround ranges from a few days for short aging periods such as 72 h at 70 °C to several weeks for extended exposures such as 1000 h, plus conditioning and hardness measurement time.

Can finished products be tested instead of standard plaques?

Yes. Finished parts can be tested directly if a suitable flat area with adequate thickness exists, or by using micro-hardness methods for small cross-sections such as O-rings, with results interpreted in the context of the part geometry.