Determination of Ozone Aging Cracking in Rubber: Test Methods, Standards, and Evaluation

Ozone aging cracking determination is a critical rubber performance test that evaluates the resistance of vulcanized or thermoplastic elastomers to crack initiation and growth when exposed to ozone under static or dynamic strain. Performed in accordance with ISO 1431, ASTM D1149, and GB/T 7762, this testing method is essential for qualifying sealing materials, weatherstrips, hoses, and tire components used in ozone-rich service environments. This article explains the test principle, sample requirements, testing procedures, evaluation criteria, and typical industrial applications of ozone cracking determination.

Technical Overview

Ozone cracking is one of the most characteristic degradation mechanisms of unsaturated rubbers. Ozone, even at trace concentrations of only a few parts per hundred million (pphm) in ambient air, reacts rapidly with the carbon–carbon double bonds in the polymer backbones of diene elastomers such as natural rubber (NR), styrene-butadiene rubber (SBR), butadiene rubber (BR), and nitrile rubber (NBR). The reaction, known as ozonolysis, causes chain scission at the rubber surface. When the material is simultaneously subjected to tensile strain, the scission concentrates into fine surface cracks that grow perpendicular to the direction of strain.

A key feature of ozone cracking is the existence of a critical strain: below a threshold elongation, cracks do not initiate or propagate, while above it, crack growth accelerates with increasing strain. Laboratory ozone aging tests accelerate this process by exposing strained specimens to controlled, elevated ozone concentrations inside a closed chamber, allowing prediction of long-term outdoor and in-service durability within days or weeks.

The principal standards governing ozone cracking determination include ISO 1431-1 (static and dynamic strain testing), ISO 1431-3 (determination of ozone concentration), ASTM D1149 (cracking in ozone under controlled exposure), ASTM D1171 (outdoor surface ozone cracking), GB/T 7762 (static strain method), GB/T 13642 (dynamic strain method), and JIS K 6259. Accredited third-party laboratories routinely perform these tests for material qualification, quality control, and research purposes.

Testing Samples

Standard Laboratory Specimens

Preferred test pieces are elongated strip or dumbbell specimens cut from cured sheets, typically with a width of 10 mm and a thickness of approximately 2 mm, with the working section free of surface defects. Specimens must not be touched or contaminated on the test surface, and any talc or surface bloom should be minimally disturbed prior to exposure.

Finished and Semi-Finished Products

• O-rings, oil seals, and other radial lip seals
• Automotive weatherstrips, door and window gaskets
• Hose covers, liners, and tube sections
• Tire sidewall and white sidewall specimens
• Cable sheathing and insulation compounds
• Molded parts, diaphragms, vibration isolators, and conveyor belting samples

Specimen Preparation and Conditioning

Vulcanized specimens should be tested after a maturation period of between 16 hours and 4 weeks after curing. Before mounting, specimens are conditioned at a standard laboratory atmosphere, typically 23 ± 2 °C and 50 ± 5% relative humidity, for a minimum of 3 hours. Gauge marks are applied so that the required elongation can be set accurately on the stretching fixture or mandrel.

Testing Items

Typical parameters measured and reported in an ozone cracking determination include:

• Time to first visible crack (crack initiation time) at a specified elongation
• Crack severity rating after a defined exposure period
• Crack length and crack growth rate under dynamic strain conditions
• Critical strain or threshold elongation below which no cracking occurs
• Comparison of ozone resistance between unprotected and protected compounds
• Verification of antiozonant and wax protection systems in the compound formulation
• Influence of test temperature, ozone concentration, and humidity on cracking behavior

Testing Methods

Static Strain Ozone Exposure Test

In the static method per ISO 1431-1, ASTM D1149, and GB/T 7762, specimens are elongated to a specified strain, commonly 20% (or a series such as 10%, 20%, 30%, and 50% for critical strain studies), and mounted in an ozone chamber. Typical test conditions are an ozone concentration of 50 ± 5 pphm (approximately 1.0 ppm by volume in some specifications), a temperature of 40 ± 2 °C, and exposure durations of 24, 48, 72, or 96 hours, or longer as agreed. Higher concentrations such as 100 or 200 pphm may be used for accelerated screening. At defined intervals, specimens are removed and inspected without bending or additional straining of the surface.

Dynamic Strain Ozone Test

The dynamic method per ISO 1431-1 and GB/T 13642 subjects specimens to cyclic elongation between specified limits, for example between 0% and 10% or between 5% and 25%, at a frequency of about 0.5 Hz. Because cyclic straining continuously exposes fresh, unprotected surface to ozone, dynamic testing is more severe than static testing and better simulates flexible components such as tire sidewalls and flexible hoses in service.

Outdoor Exposure Reference Method

ASTM D1171 describes a reference procedure using triangular cross-section specimens wrapped around mandrels and exposed outdoors, typically mounted facing south at a 45-degree angle. This method is widely used to evaluate the performance of antiozonant wax and chemical antiozonant systems under natural atmospheric ozone and sunlight conditions.

Ozone Concentration Determination and Chamber Control

Reliable results depend on accurate control and verification of the ozone concentration inside the test chamber. ISO 1431-3 specifies methods including ultraviolet absorption photometry with calibrated instruments and wet-chemical absorption in neutral buffered potassium iodide solution. Ozone is generated in the chamber by a corona discharge or UV ozone generator, and the atmosphere is circulated to ensure uniformity. Reference rubber specimens with known cracking behavior may also be exposed alongside test specimens to verify chamber performance.

Crack Evaluation and Rating

After exposure, specimen surfaces are examined visually and with a 2x magnifier, with a 7x magnifier used to confirm questionable observations. Results are reported on descriptive rating scales, for example: no cracks; a few scattered fine cracks; several distinct small cracks; numerous cracks of significant length; and heavy, deeply cracked surfaces. The test report must state the standard used, ozone concentration, temperature, elongation, exposure duration, inspection intervals, and the crack rating at each interval.

Applications

Ozone cracking determination is applied across industries where rubber components operate under strain in air:

• Automotive: weatherstrips, door and glass run channel seals, radiator and air hoses, engine mounts, and tire sidewall compounds
• Wire and cable: sheathing and jacketing compounds for outdoor and underground installations
• Construction: window and curtain wall gaskets, expansion joint seals, and roofing membranes
• Industrial equipment: conveyor belts, rollers, seals, gaskets, and vibration isolators
• Aerospace and rail: sealing systems exposed to high-altitude ozone concentrations and variable strain
• Research and development: screening of antiozonant packages, protective waxes, and ozone-resistant polymer selections such as EPDM, chloroprene, and halobutyl rubber

FAQ

Why do ozone cracks only appear when rubber is stretched?

Ozone attacks the rubber surface regardless of strain, but cracks propagate only when tensile strain concentrates the chain scission into oriented fracture paths. Below the critical strain for a given compound, any micro-cracks that form remain stationary because the damaged surface layer self-protects and prevents further growth.

Which elastomers have poor ozone resistance, and which resist it naturally?

Unsaturated diene rubbers such as NR, SBR, BR, and NBR are highly susceptible and require wax and chemical antiozonants. Saturated or low-unsaturation elastomers such as EPDM, silicone rubber, fluorocarbon rubber (FKM), and ethylene-acrylate rubber exhibit inherently excellent ozone resistance.

What are the most commonly specified test conditions?

The most widely used condition is 20% static elongation, 50 ± 5 pphm ozone, and 40 °C for 72 or 96 hours, per ISO 1431-1, ASTM D1149, or GB/T 7762. Product specifications may define different concentrations, temperatures, and durations depending on the intended service environment.

How can manufacturers improve ozone resistance of rubber products?

Effective measures include incorporating chemical antiozonants and protective microcrystalline waxes, minimizing service strain below the critical elongation, avoiding high permanent set designs, and selecting inherently ozone-resistant polymers such as EPDM for outdoor sealing applications.

Is ozone cracking testing relevant for indoor products?

Yes. Indoor air can contain elevated ozone from office equipment, electric motors, and corona discharge sources, and products such as cable jackets, printer rollers, and equipment gaskets can suffer ozone cracking even without outdoor exposure, making the test relevant for many indoor applications.