Sealing element anti-aging testing evaluates the resistance of O-rings, gaskets, oil seals, and other polymer sealing components to thermal, ozone, UV, and fluid-induced degradation over time. By exposing seal materials to accelerated aging conditions in accordance with ISO, ASTM, EN, and GB standards, accredited third-party laboratories can quantify changes in hardness, tensile strength, elongation, and compression set to verify material qualification and predict long-term service life. This technical guide explains the sample types, test items, testing methods, acceptance criteria, and typical applications for elastomeric and thermoplastic sealing products.
Technical Overview
Sealing elements, including O-rings, gaskets, rotary shaft seals, diaphragms, and custom-molded profiles, are engineered to maintain leak-tight performance across years of continuous service. Because most of these components are manufactured from elastomers such as NBR, FKM, EPDM, silicone (VMQ), and AEM, or from thermoplastics such as PTFE, they are inherently vulnerable to aging: a slow, irreversible degradation driven by heat, oxygen, ozone, ultraviolet radiation, contact fluids, and cyclic mechanical stress.
Anti-aging testing, also referred to as accelerated aging or weathering resistance testing, reproduces these degradation mechanisms within a compressed timeframe inside controlled laboratory chambers. By comparing the physical properties of specimens before and after exposure, engineers can verify material qualification, benchmark suppliers, validate shelf life, and support service-life predictions. Typical degradation indicators include hardness drift, loss of tensile strength and elongation at break, permanent compression set, surface ozone cracking, extraction or blooming of plasticizers, and volume or mass change after fluid immersion.
Accredited third-party laboratories perform these evaluations under internationally recognized frameworks such as ISO, ASTM, EN, JIS, and GB (Chinese national) standards, ensuring that results are reproducible and accepted across global supply chains.
Testing Samples
Anti-aging evaluations may be performed on finished sealing components or on standardized material coupons prepared from the same compound. The following sample categories are routinely accepted for testing:
O-Rings and Toroidal Seals
Torus-shaped seals in standard ISO 3601 or AS568 size series, tested as whole parts under ASTM D1414 so that the actual production cure state, parting-line quality, and molding effects are captured rather than only the bulk compound behavior.
Gaskets and Flat Seals
Flange gaskets, valve plate seals, die-cut sheets, and bonded seals, aged either as finished articles or in sheet form, with dumbbell specimens die-cut after exposure for tensile and hardness measurement.
Rotary Shaft Seals and Lip Seals
Oil seals with elastomeric lips, evaluated for lip hardness change, radial force retention, and heat-aging resistance at temperatures relevant to engine, gearbox, and electric-drive unit applications.
Custom Molded and Extruded Seals
Diaphragms, boots, bellows, extruded profiles, and rubber-to-metal bonded parts, tested either as finished articles or as witness slabs molded from the same production batch of compound.
Material Coupons and Test Slabs
Standard tensile slabs and hardness buttons prepared from the raw compound, allowing direct comparison of aged properties against the material datasheet limits and against future incoming lots.
Testing Items
A comprehensive anti-aging assessment for sealing elements typically includes the following test items:
- Air-heat aging (thermal oxidative aging in a convection oven)
- Ozone-aging resistance under static or dynamic strain
- Artificial weathering using xenon-arc or fluorescent UV light sources
- Fluid immersion aging in oils, fuels, coolants, water, and process chemicals
- Compression set after thermal or fluid aging
- Compression stress relaxation over extended durations
- Tensile strength and elongation at break retention after aging
- Hardness change (Shore A durometer or IRHD microhardness)
- Volume change and mass change after fluid exposure
- Low-temperature flexibility retention (TR-10 retraction, Gehman torsion, brittleness point)
- Visual rating of surface cracking, blistering, blooming, and permanent deformation
- Functional leak or pressure-hold verification of aged seals
Testing Methods
Thermal Air-Aging (Heat Aging)
Specimens are exposed in multi-cell ventilated air ovens in accordance with ISO 188, ASTM D573, JIS K 6257, or GB/T 3512. Test temperatures are selected according to the polymer class, for example approximately 100°C for NBR, 125–150°C for EPDM, 150–175°C for AEM, and 200–250°C for FKM and silicone. Common durations include 24, 48, 72, 168, 336, 504, and 1008 hours. After exposure, hardness, tensile strength, and elongation at break are remeasured and compared with unaged control values.
Ozone-Aging Resistance
Ozone testing is performed in chambers conforming to ISO 1431-1, ASTM D1149 for static strain, ASTM D3395 for dynamic strain, and GB/T 7762. Elongated specimens, typically at 20% static elongation, are exposed to ozone concentrations such as 50 pphm at 40°C for 72 hours. Surfaces are then examined at 2x and 7x magnification and graded for crack initiation and severity. Diene rubbers such as NR, SBR, and NBR are most susceptible, whereas saturated polymers such as EPDM and FKM are intrinsically ozone resistant.
Artificial Weathering
For seals exposed to sunlight and moisture, such as EPDM window and door gaskets, xenon-arc exposure per ISO 4892-2 or ASTM G155, and fluorescent UV condensation exposure per ISO 4892-3 or ASTM G154, are applied with programmed light, dark, and water-spray cycles. Natural weathering benchmarks can be referenced through outdoor exposure practices such as ASTM D1435 or ISO 4607.
Compression Set After Aging
Compression set is measured per ASTM D395 Method B or ISO 815-1, with the specimen compressed to a constant deflection, typically 25%, and aged at the specified temperature for a defined period. Whole O-rings are tested per ASTM D1414. The permanent set expresses the fraction of deflection not recovered and directly reflects the ability of a seal to maintain contact pressure over its service life.
Fluid-Resistance Aging
Immersion aging is conducted per ASTM D471, ISO 1817, or GB/T 1690 using standard reference media such as IRM 901 and IRM 903 oils, ASTM reference fuels, engine oils, brake fluids, and glycol-based coolants. Volume swell, mass change, and retention of mechanical properties are measured to confirm compatibility with the intended service environment.
Post-Aging Measurement and Acceptance Criteria
Tensile properties are measured per ASTM D412 or ISO 37, hardness per ASTM D2240, ISO 7619, or ISO 48, and compression stress relaxation per ISO 3384. Typical acceptance limits commonly specified for elastomeric seals include:
- Hardness change within plus or minus 10 Shore A or IRHD points
- Tensile strength retention of at least 75–85% of the original value
- Elongation at break retention of at least 50–70% of the original value
- Compression set not exceeding 20–25% for static sealing applications
- No visible ozone cracking at 7x magnification after the specified exposure
- Volume change within the material-specific window, for example 0% to +30% for fuel-contact seals
Service-Life Prediction
Where qualification requires an estimated lifetime, multi-temperature aging data can be extrapolated using the Arrhenius model and time–temperature superposition. The logarithm of the time required to reach a defined failure criterion, such as a 50% loss of elongation, is plotted against reciprocal absolute temperature to extrapolate performance at the intended service temperature.
Applications
Anti-aging testing of sealing elements supports material qualification and quality control across a wide range of industries:
- Automotive and electric vehicles: engine, transmission, cooling circuit, brake system, fuel system, and battery pack sealing
- Aerospace: hydraulic actuators, fuel system seals, door and access panel gaskets requiring long-term reliability
- Oil, gas, and chemical processing: valve seats, flange gaskets, and downhole tool seals exposed to heat and aggressive media
- Industrial hydraulics and pneumatics: cylinder rod and piston seals, fittings, and manifold gaskets
- Building and plumbing: EPDM window and curtain-wall gaskets, potable water seals, and HVAC joints
- Electronics and energy: enclosure IP-rating gaskets, connector seals, solar module edge sealing, and battery enclosures
- Medical and food equipment: sterilizable silicone seals requiring repeated thermal and fluid aging resistance
Frequently Asked Questions (FAQ)
How is accelerated aging used to estimate the service life of a seal?
Specimens are aged at several elevated temperatures until a defined property limit, such as a 50% reduction in elongation at break, is reached. Applying the Arrhenius relationship to these time-to-failure data allows extrapolation to the lower actual service temperature, producing a statistically supported lifetime estimate.
What is the difference between heat aging and ozone aging?
Heat aging evaluates thermo-oxidative degradation, including chain scission and post-crosslinking, in a circulating air oven. Ozone aging specifically evaluates the attack of ozone on unsaturated polymer chains in strained rubber, which produces characteristic surface cracks. A material can pass heat aging yet fail ozone testing, so the two methods complement each other.
How are test temperatures selected for different elastomers?
Temperatures are usually chosen relative to the maximum continuous service temperature of the polymer, for example approximately 100°C for NBR, 125–150°C for EPDM, 150–175°C for AEM, and 200–250°C for FKM and silicone. Product specifications and material datasheets typically define the exact aging points.
How many specimens are required for an aging study?
As a general practice, at least five tensile dumbbells per condition per time point, together with unaged controls and sufficient whole seals for compression set and hardness testing, are required. Multi-temperature lifetime studies naturally require larger sample quantities.
What does a typical pass or fail result look like?
A common specification might require that after 168 hours at 125°C, hardness changes by no more than 10 points, tensile strength and elongation retain at least 80% and 70% of their initial values respectively, and compression set does not exceed 25%, with no visible cracks after ozone exposure. Failure of any single criterion generally triggers material review.