Artificial Ozone Aging Test: Principles, Methods, Standards, and Applications

The artificial ozone aging test is a laboratory accelerated aging procedure that evaluates the ozone crack resistance of rubber and elastomeric materials under controlled ozone concentration, temperature, humidity, and strain conditions. By simulating years of atmospheric exposure in just a few days, this test helps manufacturers of seals, tires, hoses, cables, and other rubber products verify long-term durability and material performance before products reach the market.

Technical Overview

The artificial ozone aging test is a standardized accelerated weathering method used to assess the resistance of vulcanized rubber and thermoplastic elastomers to ozone-induced degradation. Although ozone exists in the normal atmosphere at very low concentrations, typically below 5 pphm (parts per hundred million), it is one of the most aggressive agents responsible for the surface cracking of rubber products exposed to outdoor environments.

The degradation mechanism is well understood: ozone attacks the carbon-carbon double bonds (C=C) present in unsaturated rubber chains, such as natural rubber (NR), styrene-butadiene rubber (SBR), butadiene rubber (BR), and nitrile rubber (NBR). The reaction forms ozonide structures that cleave the polymer backbone. When the rubber is simultaneously subjected to tensile stress, the cleaved chains separate and characteristic cracks appear perpendicular to the direction of strain. These cracks grow rapidly and can lead to catastrophic product failure, even though the bulk mechanical properties of the rubber remain largely unaffected.

In an artificial ozone aging test, specimens are stretched to a specified elongation and exposed inside a sealed chamber containing an elevated, precisely controlled ozone concentration, usually 10 to 100 times higher than ambient levels. This acceleration allows engineers to evaluate crack initiation and growth behavior that would otherwise require years of natural exposure. Saturated polymers such as EPDM, silicone rubber (VMQ), and fluoroelastomers (FKM) are inherently ozone-resistant and are frequently selected based on the results of this test.

Testing Samples

Standard Test Pieces

The most common specimens are tensile dumbbell pieces or rectangular strips cut from vulcanized rubber sheets with a thickness of approximately 2 mm. These specimens are mounted on elongation fixtures and stretched to a defined strain, typically 20 percent, before being placed in the ozone chamber. Specimens must be free of surface defects, since scratches or molding marks can act as crack initiation sites and distort the results.

Finished and Semi-Finished Products

In addition to standard test pieces, actual components can be evaluated directly, including O-rings, oil seals, gaskets, hydraulic and automotive hoses, transmission belts, tire sidewall sections, cable sheathing, window weatherstrips, and rubber diaphragms. Testing real products is valuable because geometry, surface finish, and processing conditions all influence ozone cracking behavior in service.

Sample Preparation Requirements

Rubber specimens should be vulcanized and conditioned for at least 16 hours, and preferably 24 to 72 hours, after molding before testing begins. Surfaces must not be touched, polished, or contaminated, as ozone attack is strictly a surface phenomenon. Any accidental stretching or bending of the specimens prior to mounting must be avoided.

Testing Items

Depending on the applicable standard and the purpose of the evaluation, the artificial ozone aging test can include the following measurement items:

• Ozone crack resistance rating based on visual inspection of the specimen surface after defined exposure intervals
• Time to first crack initiation, recorded by periodic examination at 2-hour or 4-hour intervals
• Crack severity classification, including crack number, length, and depth categories
• Change in tensile strength before and after ozone exposure
• Change in elongation at break before and after ozone exposure
• Retention rate of mechanical properties expressed as a percentage of original values
• Critical strain determination, which identifies the minimum elongation at which cracking begins

Testing Methods

Static Strain Method

In the static method, specimens are elongated to a fixed strain, commonly 20 percent, and held at that elongation throughout the entire exposure period. Typical test conditions include an ozone concentration of 50 pphm (with 100, 200, and 500 pphm used for more severe evaluation), a chamber temperature of 40 degrees Celsius, and exposure durations of 24, 48, 72, 96, or 168 hours. The specimen surface is inspected at regular intervals under magnification, and cracking is graded according to the rating scale defined in the standard.

Dynamic Strain Method

In the dynamic method, specimens are cyclically stretched between two strain limits, for example between 0 and 25 percent elongation, at a defined frequency. Dynamic testing better simulates service conditions for components such as belts, tires, and flexible hoses, where repeated deformation accelerates crack growth. Because cracks open and close during cycling, ozone continuously attacks fresh surfaces, making this method more severe than static exposure.

Referenced Standards

The artificial ozone aging test is governed by several internationally recognized standards, and the most frequently applied include:

• ISO 1431-1: Rubber, vulcanized or thermoplastic - Resistance to ozone cracking - Static and dynamic strain testing
• ASTM D1149: Standard Test Methods for Rubber Deterioration - Cracking in an Ozone Controlled Environment
• GB/T 7762: Rubber, vulcanized or thermoplastic - Resistance to ozone cracking - Static strain test method
• DIN 53509-1: Testing of rubber - Determination of the resistance to ozone cracking
• JIS K 6259: Rubber, vulcanized or thermoplastic - Determination of ozone resistance

Equipment and Chamber Control

The test is performed in an ozone aging chamber equipped with an ozone generator, which produces ozone either by ultraviolet irradiation or by silent electrical discharge (corona discharge) of dry air or oxygen. An automatic ozone analyzer continuously monitors and feedback-controls the concentration inside the chamber. Temperature uniformity, air velocity, and relative humidity are regulated, since all three parameters influence the rate of ozone attack and the reliability of the results.

Applications

The artificial ozone aging test is widely applied across industries where rubber components are exposed to atmospheric ozone during storage or service:

• Automotive industry: door and window seals, engine mounts, coolant hoses, air intake ducts, tire sidewalls, and wiper blades
• Aerospace industry: sealing systems, gaskets, and elastomeric components exposed to high-altitude ozone concentrations
• Wire and cable industry: outdoor cable sheathing and insulation materials requiring long-term weather resistance
• Construction industry: expansion joint seals, curtain wall gaskets, roofing membranes, and glazing seals
• Rail transit: vehicle weatherstrips, vibration dampers, and pneumatic hose assemblies
• Research and quality control: antiozonant formulation development, incoming material inspection, and supplier qualification

Test results guide material selection, antiozonant and wax protection package optimization, and product lifetime predictions. Components located near electrical equipment, such as motors, switchgear, and high-voltage lines, deserve particular attention, because corona discharge in these environments generates locally elevated ozone levels.

FAQ

What is the difference between ozone aging and thermal aging?

Thermal aging evaluates the combined effects of heat and oxygen on the bulk properties of rubber, including changes in tensile strength and hardness. Ozone aging specifically evaluates surface cracking caused by ozone attack under tensile strain. The two mechanisms are chemically distinct, and a compound can perform well in thermal aging yet fail rapidly in ozone aging if it lacks ozone protection.

Why do only stretched rubber samples crack during ozone exposure?

Ozone reacts with double bonds on the rubber surface regardless of strain, but in an unstressed state the damaged layer remains intact and protects the underlying material. When the rubber is stretched, the cleaved polymer chains separate and form cracks that propagate into the interior. This is why a minimum critical strain must be exceeded for visible ozone cracking to develop.

What ozone concentration and temperature are typically used?

The most common conditions are 50 pphm ozone at 40 degrees Celsius with 20 percent static elongation. More severe screening may use 100 to 500 pphm, while qualification testing of highly resistant materials such as EPDM may extend to several hundred hours of exposure without any cracking.

Which rubbers need ozone testing?

Unsaturated rubbers, including NR, SBR, BR, NBR, and CR, are susceptible to ozone cracking and should always be tested. Saturated or effectively saturated polymers such as EPDM, silicone, and FKM are intrinsically resistant, and testing is used to confirm this resistance for critical applications.

How are ozone cracking results reported?

Results are reported as a crack rating, typically from grade 0 (no visible cracks) to grade 4 or 5 (severe cracking with numerous deep cracks), together with the exposure conditions and the time at which cracking was first observed. Property retention values are reported when mechanical testing is performed after exposure.

Can the test replace outdoor weathering trials?

The artificial ozone aging test is an accelerated screening tool that provides rapid and reproducible comparisons between materials under identical conditions. It cannot fully replicate the combined effects of sunlight, rain, temperature cycling, and pollutants encountered outdoors, so critical applications often combine chamber testing with long-term natural exposure programs.