Rubber ozone aging testing to Chinese national standards (GB/T 7762 for static strain and GB/T 13642 for dynamic strain) is the primary method for evaluating the ozone cracking resistance of vulcanized and thermoplastic rubbers. By exposing strained specimens to a controlled ozone atmosphere, these tests reveal crack initiation and growth behavior that directly determines the service life of seals, tires, hoses, and automotive rubber components. This guide explains the test principles, sample requirements, standard test conditions, evaluation criteria, and practical answers to frequently asked questions.
Technical Overview
Ozone, even at trace concentrations in the atmosphere, attacks the carbon–carbon double bonds present in unsaturated diene rubbers such as natural rubber (NR), styrene-butadiene rubber (SBR), butadiene rubber (BR), nitrile rubber (NBR), and to a lesser degree polychloroprene (CR). When a rubber article is simultaneously subjected to tensile strain, ozone attack is highly localized and produces characteristic surface cracks that grow perpendicular to the direction of strain. These cracks can propagate rapidly and cause premature failure of seals, weatherstrips, tires, and flexible joints long before thermal aging alone would be significant. Ozone aging testing therefore simulates this degradation mechanism under accelerated, reproducible laboratory conditions.
Why Ozone Aging Testing Matters
Untested rubber compounds may appear perfectly elastic yet fail within weeks of outdoor or near-electrical-equipment service simply because the protective antiozonant package (chemical antiozonants such as p-phenylenediamine derivatives and protective waxes) is inadequate. Ozone aging tests verify whether a compound's protection system is effective, compare candidate formulations, qualify incoming materials, and support failure analysis of cracked parts returned from the field. Saturated elastomers such as EPDM, silicone rubber, and fluoroelastomers are inherently ozone resistant, but blends and contaminated or aged compounds still require verification.
The Chinese National Standard Framework
The Chinese national standards (GB) for ozone aging are technically aligned with the ISO 1431 series, allowing laboratories and manufacturers to report results that are comparable internationally. The relevant standards are:
- GB/T 7762-2014 — Rubber, vulcanized or thermoplastic; determination of resistance to ozone cracking under static strain (corresponding to ISO 1431-1).
- GB/T 13642-2015 — Rubber, vulcanized or thermoplastic; determination of resistance to ozone cracking under dynamic strain (corresponding to ISO 1431-2).
- ISO 1431-3 — Reference method for measuring the ozone concentration in the test chamber, supporting calibration and verification.
- Comparable international methods include ASTM D1149 for ozone cracking in a controlled chamber and JIS K 6259 in the Japanese standard system.
Testing Samples
Standard Strip and Dumbbell Specimens
The preferred test piece for ozone aging is a rectangular strip, typically approximately 100 mm long and 10 mm wide, with a preferred thickness of 2 mm ± 0.2 mm, prepared from flat cured sheets. Dumbbell specimens cut according to tensile-test practice may also be used, with the test gauge placed in the ozone atmosphere. A minimum of three specimens per test condition is required, and specimens must be free of surface defects, since ozone cracks initiate at stress-concentrating flaws.
Specimens Taken from Finished Products
For product verification, strips may be cut directly from finished articles such as extruded profiles, molded seals, hose covers, or tire components, provided the cut surfaces are smooth and the original product surface is retained where possible. Complete articles may also be mounted in the chamber by agreement between supplier and purchaser, for example inflated or stretched boots, bellows, and gaskets, to evaluate real geometry and assembled stress states.
Conditioning Requirements
The time between vulcanization (or production) and testing should be at least 16 hours, and for laboratory-prepared sheets it is recommended that testing be completed within four weeks. Before ozone exposure, specimens are clamped at the specified elongation and equilibrated in ozone-free air at the test temperature for the conditioning period defined in the standard, which stabilizes the stress state and the surface wax/antiozonant layer before ozone is introduced. Specimens must not touch each other or the chamber walls during exposure.
Testing Items
Depending on the specification and the agreement between the parties, an ozone aging test program may include the following measurable items:
- Ozone concentration and its uniformity throughout the chamber (typically 50, 100, or 200 pphm, i.e. 0.5, 1.0, or 2.0 ppm by volume).
- Test temperature (standard condition 40 °C ± 2 °C, with other temperatures by agreement).
- Static elongation of the specimens (commonly 20%, with a series of 5% to 50% used for critical-strain determination).
- Dynamic strain amplitude and cyclic frequency for GB/T 13642 testing.
- Exposure duration (commonly 72 h, with 24 h, 48 h, 96 h, or 168 h alternatives).
- Time to first visible crack at defined inspection intervals.
- Crack severity assessment, including crack density, length, and growth behavior.
- Critical elongation, the lowest strain at which ozone cracks appear.
- Optional retention of tensile strength and elongation at break after ozone exposure, measured per GB/T 528.
Testing Methods
Static Strain Test — GB/T 7762
In the static method, specimens are clamped in suitable holders and stretched to a fixed elongation, most commonly 20%, and conditioned in ozone-free air at the test temperature. Ozone is then admitted and maintained at the specified concentration, with the most widely applied standard condition being 50 pphm ± 5 pphm at 40 °C ± 2 °C for 72 hours. At agreed inspection intervals, specimens are removed briefly or observed through the chamber window and examined for cracks. Higher severities (100 or 200 pphm) or longer durations are frequently specified by automotive and tire manufacturers to screen compounds more aggressively. For critical-strain evaluation, parallel sets of specimens are held at stepped elongations, and the lowest elongation showing cracks identifies the threshold below which the compound will not crack in service.
Dynamic Strain Test — GB/T 13642
The dynamic method reproduces service conditions in which the rubber experiences cyclic deformation, such as drive belts, bellows, flexible couplings, and door seals on moving vehicles. Specimens are mounted on a reciprocating mechanism that cycles the elongation, typically at a frequency of about 0.5 Hz (approximately 30 cycles per minute), between agreed strain limits, for example from a relaxed state up to 10% elongation. Both continuous cyclic exposure and intermittent cyclic exposure (alternating dynamic and static periods) can be applied. Because cyclic strain continuously opens new surface to ozone attack, dynamic testing is generally more severe and more discriminating than static testing for flexing applications.
Ozone Generation and Chamber Control
Ozone is generated inside a closed environmental chamber, most commonly by a silent electrical discharge ozonizer fed with clean dry air or oxygen, or alternatively by ultraviolet lamps. The chamber atmosphere must be continuously circulated for uniformity, and the ozone concentration is monitored in real time, typically by ultraviolet photometric analyzers calibrated against reference chemical methods consistent with ISO 1431-3. Temperature is regulated within ±2 °C, and the chamber atmosphere is periodically replenished to prevent ozone depletion effects. Because ozone is toxic, accredited third-party laboratories and in-house facilities must interlock the chamber, monitor the room air, and destroy excess ozone in the exhaust using a catalytic destructor before release.
Evaluation and Rating of Results
Specimens are inspected under 2× magnification, with 7× magnification used to confirm the presence of fine cracks. The standard reporting outcomes are: no visible cracks at the end of the exposure period; the time at which the first crack appears; and a description of crack severity in terms of number and size of cracks on the strained surface. For most commercial specifications, the acceptance criterion is simply "no cracks at 2× magnification after the specified duration," although automotive OEM specifications often add severity or crack-growth limits. All test parameters, including concentration, temperature, strain, duration, and inspection results, must be documented in the test report for traceability.
Applications
Ozone aging testing under GB/T 7762 and GB/T 13642 is applied across every major rubber-consuming industry where components are exposed to atmospheric ozone, indoor corona discharge, or polluted urban environments. Typical applications include:
- Automotive weatherstrips, door and window seals, glass run channels, and sunroof seals.
- Tire sidewalls, innerliners, white sidewall compounds, and tread base materials.
- Rubber boots, bellows, and constant-velocity joint covers subject to flexing.
- Hoses, tubes, and flexible connectors for fluid and air systems.
- Cable sheathing and insulation exposed to ozone generated by electrical equipment.
- Industrial gaskets, O-rings, diaphragms, anti-vibration mounts, and bushings.
- Building profiles, curtain-wall gaskets, and waterproofing membranes.
- Research and development of antiozonant and wax protection systems, incoming material qualification, supplier comparisons, and failure analysis of cracked field returns.
Manufacturers routinely engage accredited third-party laboratories to perform GB/T ozone aging tests when preparing for product certification, supplier approvals, export documentation, or independent verification of compound quality.
FAQ
What are the default test conditions for a GB ozone aging test?
The most common specification is 50 ± 5 pphm (0.5 ppm) ozone at 40 °C ± 2 °C with specimens held at 20% static elongation for 72 hours, judged as "no cracks" at 2× magnification. However, concentrations of 100 or 200 pphm, longer durations, and different elongations are frequently specified, so the controlling technical specification should always be confirmed before testing begins.
What is the difference between GB/T 7762 and GB/T 13642?
GB/T 7762 holds specimens at a constant (static) elongation throughout the exposure, which suits seals and profiles under fixed installation strain. GB/T 13642 applies cyclic (dynamic) strain, typically at about 0.5 Hz, which better represents flexing parts such as boots and bellows and is generally the more severe condition.
How many specimens are required and what dimensions?
A minimum of three specimens per test condition is standard. The preferred specimen is a strip of approximately 100 mm × 10 mm with 2 mm thickness; dumbbell tensile pieces or product-cut strips are also acceptable when agreed. More specimens are needed for critical-strain studies because several elongation levels must be tested in parallel.
Are GB/T results comparable with ISO and ASTM tests?
Yes. GB/T 7762 and GB/T 13642 are technically aligned with ISO 1431-1 and ISO 1431-2 respectively, and the conditions overlap substantially with ASTM D1149 practice (50 pphm, 40 °C). Minor procedural differences exist, so the report should always state the exact standard and conditions used.
Can finished products be tested without cutting specimens?
Yes. Complete articles such as seals, boots, or gaskets can be mounted in the chamber in their assembled or pre-strained state by agreement, which evaluates real geometry, mold parting lines, and installed stress simultaneously. Many OEM specifications explicitly combine strip tests with whole-part tests.
How long does a typical ozone aging test take?
In addition to the exposure period itself (commonly 72 hours), the schedule should include specimen conditioning, chamber temperature stabilization, and the time needed to stabilize and verify the ozone concentration. In practice, a standard 72-hour test is normally completed and reported within three to five working days.
What does a "no cracks" result actually mean?
It means that at each inspection interval through the end of the specified exposure, no ozone cracks were visible on the strained surface when examined under 2× magnification. Cracks detectable only at higher magnification are treated according to the reporting rules of the standard, which is why the inspection method should be stated in the report.