Sealing Strip Ozone Aging Testing: Standards, Methods, and Evaluation Criteria

Ozone aging testing is a critical durability assessment for sealing strips, evaluating how rubber and thermoplastic elastomer materials resist ozone-induced cracking under controlled concentrations and strain conditions. This accelerated test simulates years of atmospheric exposure in just 72 to 168 hours, helping manufacturers verify product quality for automotive, architectural, and industrial sealing applications. This article explains the testing principles, applicable standards such as GB/T 7762, ISO 1431-1, and ASTM D1149, sample requirements, procedures, and acceptance criteria.

Technical Overview

Sealing strips are flexible elastomeric profiles widely used in vehicles, buildings, and industrial equipment to block dust, water, air, and noise. Most sealing strips are manufactured from materials such as EPDM, CR (chloroprene rubber), NR (natural rubber), SBR, NBR, silicone rubber, PVC, and TPE. Among these, diene rubbers containing carbon-carbon double bonds in their molecular backbone are inherently vulnerable to ozone attack.

Ozone aging testing (also called ozone resistance testing or ozone cracking testing) exposes strained sealing strip samples to an atmosphere containing a controlled, elevated ozone concentration. Ozone reacts with the double bonds in unsaturated rubber chains through ozonolysis, causing chain scission. When the rubber is simultaneously subjected to tensile strain, this chemical degradation manifests as characteristic cracks perpendicular to the direction of stress. In actual service, sealing strips are constantly under compression, bending, or stretching, so ozone cracks typically initiate on exposed surfaces and progressively destroy both appearance and sealing performance.

Because atmospheric ozone concentrations are extremely low, natural weathering would take years to produce observable cracking. Laboratory ozone aging tests accelerate this process by using ozone concentrations of 50 to 200 pphm (parts per hundred million) at elevated temperatures, allowing engineers to assess long-term crack resistance within a few days. Accredited third-party laboratories routinely perform this test as part of material qualification, incoming material inspection, and product certification for sealing strip manufacturers.

Testing Samples

Standard Tensile Specimens

Dumbbell-shaped test pieces are die-cut from flat sheets of the same compound as the sealing strip, following the dimensions specified in GB/T 528 or ISO 37. Typical specimens have a thickness of 2 ± 0.2 mm and are mounted on clamping fixtures that stretch them to a defined elongation, most commonly 20%, before ozone exposure begins.

Finished Sealing Strip Sections

Sections cut directly from production strips, usually 100 to 250 mm in length, can be tested in their finished form, including flocked coatings, surface treatments, corners, and complex profile geometry. This verifies the ozone resistance of the actual product surface, skin layer, and protective coatings rather than only the base compound.

Sampling and Conditioning Requirements

Samples should be taken from properly vulcanized or extruded material, preferably at least 24 hours after production. Before testing, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 16 hours, and typically 3 days for referee testing, in accordance with GB/T 2941 or ISO 23529.

Testing Items

A complete ozone aging test on sealing strips evaluates the following parameters:

• Ozone concentration: typically 50 ± 5 pphm, with higher severity levels of 100 ± 10 pphm or 200 ± 20 pphm for demanding applications.
• Test temperature: commonly 40 ± 2 °C, with 23 °C or 50 °C also specified depending on the standard and application.
• Relative humidity: usually controlled at approximately 65% or as dry air where specified, since humidity influences crack growth rates.
• Static or dynamic elongation: 20% elongation is the most common static strain; dynamic testing applies cyclic strain to simulate flexible seals in service.
• Exposure duration: standard cycles of 72 h, 96 h, 120 h, or 168 h, with intermediate inspections.
• Crack evaluation: assessment of crack initiation time, crack quantity, crack length, and overall crack severity rating on the stretched surface.
• Appearance changes: surface dulling, blooming of additives, or permanent deformation after release.

Testing Methods

Reference Standards

The ozone aging test for sealing strips is conducted in accordance with internationally and nationally recognized standards, including:

• GB/T 7762 — Rubber, vulcanized or thermoplastic: determination of resistance to ozone cracking under static strain exposure.
• GB/T 13642 — Determination of resistance to ozone cracking under dynamic strain conditions.
• ISO 1431-1 — Resistance to ozone cracking, Part 1: static and dynamic strain testing in laboratory.
• ISO 1431-3 — Reference and alternative methods for determining the ozone concentration in laboratory test atmospheres.
• ASTM D1149 — Test methods for rubber deterioration: cracking in an ozone controlled environment.
• JIS K 6259 — Rubber testing methods for ozone resistance.

Static Strain Test Procedure

The specimen is mounted on a stretching fixture and elongated to the specified level, commonly 20%, and allowed to relax for a short conditioning period. It is then placed in an ozone aging chamber that has been pre-equilibrated at the target ozone concentration and temperature. The sample remains under constant strain for the specified duration, and the stretched surface is inspected at defined intervals using a magnifier of 2x or 7x magnification. Specimens must not be removed from the chamber for more than the permitted short inspection time to avoid concentration disturbances and moisture condensation.

Dynamic Strain Test Procedure

For dynamic testing per GB/T 13642 or ISO 1431-3, the specimen is subjected to cyclic elongation, for example oscillating between 0% and 25% at a frequency of 0.5 Hz, inside the ozone chamber. This method better represents door seals and movable glazing seals that experience repeated flexing, which accelerates crack initiation and growth compared with static exposure.

Evaluation and Acceptance Criteria

After exposure, the sample surface is examined under magnification. The most common pass criterion in sealing strip specifications is that no visible cracks appear under 2x magnification after 72 h or 96 h at 50 pphm ozone, 40 °C, and 20% elongation. Where cracks are present, severity is graded according to crack count and size scales defined in the standards, and the results are reported together with all test conditions, inspection intervals, and magnification used.

Applications

Ozone aging testing is essential wherever sealing strips are exposed to atmospheric ozone under mechanical stress:

• Automotive industry: door seals, window channels, sunroof seals, trunk and hood seals, where ozone resistance is a mandatory requirement in OEM material specifications.
• Architectural applications: gaskets for aluminum and UPVC windows and doors, curtain wall sealing profiles, and structural glazing seals exposed to outdoor environments.
• Rail transit and marine: door and window seals on trains, metros, and ships that face high ozone levels from electrical traction equipment and prolonged outdoor exposure.
• Industrial equipment: enclosure gaskets, electrical cabinet seals, container seals, and machinery weatherstrips requiring long-term environmental reliability.
• Material development: comparative evaluation of antiozonant packages, EPDM grades, and surface coating formulations in research and quality control laboratories.

FAQ

What are the most common test conditions for sealing strip ozone aging testing?

The most widely specified condition is 50 ± 5 pphm ozone concentration, 40 ± 2 °C temperature, 20% static elongation, and a duration of 72 h to 96 h, with the acceptance criterion of no visible cracks under 2x magnification.

Why is the sample stretched during the test?

Ozone only produces cracks in rubber when the material is under strain. Unstretched rubber reacts with ozone but forms a non-cracking oxidized surface layer, whereas stretched rubber develops the characteristic transverse cracks that this test is designed to detect.

Which sealing strip materials need ozone aging testing?

Diene rubbers such as NR, SBR, CR, and NBR are highly ozone-sensitive and always require testing. EPDM and silicone have saturated backbones and excellent inherent resistance, but finished EPDM sealing strips are still routinely tested to verify compound quality and protective additives.

What is the difference between static and dynamic ozone testing?

Static testing holds the specimen at a constant elongation throughout exposure, suitable for fixed seals. Dynamic testing applies cyclic strain, simulating flexible and frequently moved seals, and typically produces crack initiation sooner than static exposure at the same concentration.

How long does the test take?

A standard single-condition test requires 72 h to 168 h of chamber exposure plus sample conditioning time, so the complete process, including report issuance by an accredited third-party laboratory, generally takes five to eight working days.