Determination of Hardness After Oxygen Aging

Determination of hardness after oxygen aging is an accelerated aging test that evaluates the oxidation resistance of rubber and elastomeric materials by measuring hardness before and after exposure to pressurized oxygen at elevated temperature. Performed in accordance with recognized standards such as ASTM D572 and ISO 188, the method quantifies the hardening or softening caused by thermo-oxidative degradation and is widely used for quality control, material qualification, and service life prediction. This article explains the test principle, sample requirements, testing procedures, result evaluation, and typical industrial applications of this technique.

Technical Overview

Thermo-oxidative degradation is one of the most important deterioration mechanisms for vulcanized rubber and elastomeric materials. During service, oxygen diffuses into the polymer network and, under the combined action of heat, reacts with the macromolecular chains. This reaction typically produces two competing effects: additional crosslinking, which makes the material harder and stiffer, and chain scission, which softens the material and reduces its strength. Because hardness responds sensitively to both mechanisms, measuring the hardness change of a specimen before and after oxygen exposure is a practical and reliable way to assess its aging resistance.

In a typical oxygen aging hardness determination, standard specimens are placed inside a sealed pressure vessel, often referred to as an oxygen bomb, where they are exposed to pure oxygen at elevated temperature and defined pressure. ASTM D572 specifies a common condition of 70 °C with an oxygen pressure of approximately 2.1 MPa (300 psi), using test durations such as 24 hours or 96 hours. ISO 188 also describes accelerated ageing procedures involving oxygen at elevated pressure as an alternative to conventional hot-air oven exposure. After the aging cycle, the specimens are cooled, conditioned, and remeasured, and the hardness change is calculated and compared with specification limits.

• Initial hardness of the unaged specimen (Shore A, Shore D, or IRHD)
• Final hardness after the oxygen aging cycle
• Hardness change expressed in hardness points or as a percentage
• Comparison against product specifications or material qualification criteria

Testing Samples

Standard Test Plaques and Sheets

Flat cured sheets or molded plaques of uniform thickness are the preferred specimens for this test. For Shore A hardness in accordance with ASTM D2240 or ISO 7619-1, a thickness of at least 6 mm is generally required, while ISO 48 IRHD measurements typically use test pieces of 8 mm to 10 mm in thickness. Where thin-walled products must be evaluated, micro-hardness methods based on ISO 48 can be applied to thinner sections. Specimens should be flat, free from surface defects, and fully cured before testing.

Molded and Extruded Rubber Products

Finished or semi-finished components such as O-rings, gaskets, seals, diaphragms, hoses, vibration isolators, and cable sheaths can be tested directly or by cutting representative sections from the product. When products are tested, the report should identify the sampling location and the hardness method used, since curved or thin surfaces may require specialized procedures or micro-hardness techniques.

Elastomeric Compounds for Material Qualification

Development and qualification samples of specific rubber compounds, including natural rubber, SBR, NBR, EPDM, CR, silicone rubber, fluoroelastomers, and thermoplastic elastomers, are frequently evaluated using this method. The test provides comparative data on antioxidant package effectiveness, cure system stability, and expected long-term performance of the compound in oxidative environments.

Testing Items

The core deliverable of this test is the quantified change in hardness caused by oxygen aging. Typical testing items include the following:

• Initial hardness of the specimen prior to aging (Shore A, Shore D, or IRHD as appropriate)
• Hardness of the specimen after the specified oxygen-pressure aging cycle
• Absolute hardness change in hardness points (final hardness minus initial hardness)
• Relative hardness change expressed as a percentage
• Assessment of compliance with contractual or specification limits, commonly within plus or minus 5 to plus or minus 10 Shore A points depending on the product class

Testing Methods

Standards Basis

The exposure procedure is primarily governed by ASTM D572, which covers the deterioration of rubber by heat and oxygen in a pressurized vessel, and by the accelerated ageing methods of ISO 188, which include oxygen-pressure exposure. Hardness determination is carried out using ASTM D2240 or ISO 7619-1/7619-2 for Shore hardness, or ISO 48 for IRHD hardness. The corresponding Chinese national standards, such as GB/T 531.1 and GB/T 6031, are harmonized with these international methods and may be specified for domestic projects. Testing is conducted by accredited third-party laboratories using calibrated pressure vessels, temperature-controlled chambers, and certified durometers or dead-load IRHD testers.

Test Procedure

• Step 1: Condition the specimens at standard laboratory atmosphere, typically 23 °C and 50% relative humidity, and measure the initial hardness at multiple points on each specimen.
• Step 2: Place the specimens in a clean stainless-steel oxygen pressure vessel, ensuring they do not touch one another and are supported to allow full exposure to the oxygen atmosphere.
• Step 3: Purge the vessel with oxygen, pressurize it to approximately 2.1 MPa (300 psi), and maintain the vessel at the specified temperature, commonly 70 °C, for the selected duration such as 24 hours or 96 hours.
• Step 4: After the cycle, release the pressure gradually, remove the specimens, and allow them to recover and condition at standard laboratory temperature for the period defined by the governing standard.
• Step 5: Measure the final hardness at locations corresponding to the initial measurements and record the values.
• Step 6: Calculate the hardness change and compare it with the acceptance criteria specified by the client or the product standard.

Result Evaluation

A positive hardness change generally indicates dominant oxidative crosslinking, which is accompanied by increased modulus and reduced elongation, while a negative change indicates dominant chain scission and softening. Results are reported as the mean hardness change of the tested specimens together with the test conditions, including temperature, oxygen pressure, exposure duration, and hardness method. Because pressure-vessel aging intensifies oxidation compared with ambient-pressure air oven aging, results from the two methods should not be compared directly without careful interpretation.

Applications

Oxygen aging hardness determination is applied across many industries in which elastomeric components must retain their mechanical properties over long service lives in the presence of air and heat:

• Automotive engineering: aging verification of engine bay seals, cooling and fuel hose compounds, gaskets, bushings, and vibration isolators
• Wire and cable: evaluation of insulation and sheathing compounds for thermal-oxidative stability in line with cable material specifications
• Aerospace: qualification of seals, grommets, and elastomeric parts exposed to elevated temperatures and varying oxygen concentrations
• Oil and gas: assessment of downhole sealing elements and chemically resistant elastomers operating in hot, oxygen-limited or oxygen-rich environments
• Medical and pharmaceutical: stability screening of gaskets, closures, and elastomeric components in devices and packaging
• Research and development: comparison of antioxidant systems, cure packages, and polymer blends, and generation of input data for accelerated aging lifetime models
• Incoming quality control: verification that supplied compounds and finished parts meet contractual aging resistance requirements

FAQ

What is the difference between oxygen aging and hot air aging?

Hot air aging, for example per ASTM D573 or the air oven methods of ISO 188, exposes specimens to circulating air at atmospheric pressure, whereas oxygen aging exposes them to pure oxygen at elevated pressure in a sealed vessel. The pressurized oxygen atmosphere accelerates oxidation significantly, so oxygen aging produces a more severe and faster deterioration and is particularly useful for discriminating between compounds with different oxidation resistance.

Why does hardness increase or decrease after oxygen aging?

Hardness change reflects the balance between two competing degradation reactions. Oxidative crosslinking creates additional network junctions, making the rubber harder, while oxidative chain scission breaks the polymer backbone, making it softer. Depending on the polymer type, cure system, and antioxidant formulation, either mechanism may dominate, and some compounds first harden and later soften as aging progresses.

Which hardness method should be chosen for this test?

Shore A per ASTM D2240 or ISO 7619-1 is the most common choice for soft vulcanized rubbers, Shore D is used for harder materials, and IRHD per ISO 48 is preferred when higher measurement precision is required or when thin or small specimens must be tested using micro IRHD techniques. The method must be identical before and after aging so that the change values are directly comparable.

What are the typical exposure conditions and durations?

ASTM D572 specifies exposure at 70 °C under approximately 2.1 MPa (300 psi) of oxygen, with 24 hours and 96 hours as standard cycles. Other temperatures, pressures, and durations may be agreed between the client and the laboratory to simulate specific service conditions or to accelerate testing for quality control purposes.

How should specimens be prepared and handled?

Specimens should be flat, uniformly cured, free from surface contamination, and thick enough for the selected hardness method. Initial and final measurements should be taken at corresponding positions under standard laboratory conditions, and the recovery interval between removal from the vessel and final measurement should follow the governing standard to ensure reproducible and comparable results.