High-temperature aging testing of sealing materials is a critical accelerated durability test used to evaluate how elastomeric and polymer seals perform when exposed to elevated temperatures over extended periods. Conducted in accordance with international standards such as ISO 188 and ASTM D573, this testing quantifies changes in hardness, tensile strength, elongation, and compression set to verify long-term sealing reliability. This article explains the sample types, test items, methodologies, acceptance criteria, and industrial applications of thermal aging tests for sealing materials.
Technical Overview
Sealing materials—including elastomers such as NBR, EPDM, silicone rubber (VMQ), fluorocarbon rubber (FKM, FFPM), and thermoplastics such as PTFE—are permanently deformed, chemically attacked, and structurally degraded when operated at sustained high temperatures. Thermal exposure triggers oxidation, chain scission, additional crosslinking, and loss of plasticizers or volatile additives, all of which reduce the ability of a seal to maintain contact stress and prevent leakage.
High-temperature aging testing accelerates these degradation mechanisms under controlled laboratory conditions. Specimens are exposed to elevated temperatures in ventilated air ovens or oxygen-pressure vessels for defined durations, after which their physical and mechanical properties are measured and compared with unaged baseline values. The resulting data allow engineers to qualify materials for specific service temperatures, compare candidate compounds, and support lifetime estimations using Arrhenius-type extrapolation models.
Primary reference standards include ISO 188 and ASTM D573 for air oven aging, ASTM D865 for oxygen pressure aging, ISO 37 and ASTM D412 for tensile properties, ISO 48 and ASTM D2240 for hardness, and ASTM D395 and ISO 815 for compression set. Testing is typically performed by accredited third-party laboratories or in-house material laboratories equipped with calibrated forced-convection aging ovens.
Testing Samples
Standard Dumbbell Specimens from Vulcanized Sheets
For compound qualification, cured rubber sheets of 2 mm nominal thickness are die-cut into dumbbell-shaped tensile specimens (ISO 37 Type 1 or Type 2, or ASTM D412 Die C). Parallel sets of specimens are prepared: one set retained as unaged control, and additional sets for each aging condition. This configuration provides the most statistically reliable data on tensile strength and elongation retention.
Finished Seal Components
Actual products such as O-rings, flat gaskets, oil seals, valve stem seals, grommets, and door seals can be aged as received. Testing finished parts captures the influence of production processing, geometry, and post-cure state on aging behavior. Hardness, mass, dimensions, compression set, and visual condition are typically evaluated on these components.
Compression Set Specimens
Cylindrical buttons (Type 1) or stacked O-ring segments (Type 2) per ASTM D395 are compressed between polished plates at a fixed deflection and aged in the compressed state. This specimen format directly measures the permanent set that determines whether a seal will maintain sealing force after long-term thermal cycling.
Specialty Sealing Materials
Non-elastomeric materials such as PTFE compounds, flexible graphite sheets, mica-based gaskets, compressed fiber sheet, and cork-rubber composites are also subjected to thermal aging. Evaluation of these materials emphasizes mass loss, dimensional stability, compressibility and recovery, and structural integrity rather than tensile elongation.
Testing Items
After thermal exposure, specimens are conditioned at standard laboratory temperature and evaluated against unaged reference values. The core measured parameters include:
- Hardness change (Shore A or IRHD), typically required to remain within plus or minus 10 degrees
- Tensile strength change, commonly limited to a maximum reduction of 20 to 30 percent
- Elongation at break change, often the most sensitive indicator of thermal degradation
- Modulus change (stress at 100 percent elongation), reflecting crosslink density shifts
- Compression set under heat, with acceptance limits such as 20 to 35 percent depending on material class
- Mass change and dimensional change after aging
- Visual inspection for cracking, blistering, chalking, sticky or brittle surfaces, and permanent distortion
- Functional verification such as leak rate or seal force retention, where required by the product specification
Results are expressed as percentage change or percentage retention relative to the original values, following the calculation conventions of the governing material specification.
Testing Methods
Air Oven Aging (ISO 188 / ASTM D573)
The most widely applied method places free-hanging specimens in a forced-ventilation air oven maintained at a controlled temperature with tolerance typically within plus or minus 2 degrees Celsius in the critical range. Air circulation and specimen spacing are controlled to ensure uniform oxygen supply and to prevent localized cold zones. After the exposure period, specimens are removed, conditioned for 16 to 96 hours, and tested.
Oxygen Pressure Aging (ASTM D865)
For accelerated oxidation screening, specimens are exposed to heated oxygen under elevated pressure in a sealed vessel. This method intensifies oxidative degradation and is useful for comparing compound formulations, though results are not directly translatable to service life without correlation data.
Selection of Temperature and Duration
Test temperatures are selected based on the material class and the intended service condition. Typical guideline ranges are:
- NBR compounds: 70 to 125 degrees Celsius
- EPDM compounds: 100 to 150 degrees Celsius
- VMQ silicone: 200 to 250 degrees Celsius
- FKM fluoroelastomers: 200 to 275 degrees Celsius
- FFPM perfluoroelastomers: 275 to 325 degrees Celsius
- PTFE and graphite-based materials: 250 to 450 degrees Celsius depending on filler content
Standard exposure durations include 24 hours, 70 hours, 168 hours, 336 hours, and 1000 hours, with longer exposures such as 3000 hours used for extended qualification programs. A common qualification strategy tests at increments above the maximum service temperature to demonstrate an adequate thermal margin.
Evaluation and Data Interpretation
Measured property changes are compared with the acceptance limits defined in the applicable material specification or customer drawing. Statistical treatment of replicate specimens is required, and anomalous results caused by specimen defects or oven non-uniformity must be identified and reinvestigated. For service life prediction, multiple aging temperatures can be combined in an Arrhenius analysis to extrapolate the time required for a critical property, usually elongation at break, to reach a defined failure threshold at the actual operating temperature.
Applications
High-temperature aging testing of sealing materials supports material selection, quality control, and certification across many industries:
- Automotive: engine cylinder head gaskets, valve stem seals, exhaust and turbocharger seals, transmission and braking system components
- Aerospace: engine and auxiliary power unit seals, fuel system O-rings, thermal insulation barriers requiring qualification to aerospace material specifications
- Oil and gas: downhole packer seals, wellhead gaskets, and refinery flange sealing elements exposed to sustained heat
- Chemical processing: reactor, pump, and valve sealing resistant to combined thermal and chemical exposure
- Electronics and electrical equipment: enclosure gaskets, connector seals, and potting materials verified for elevated operating temperatures
- HVAC, pipelines, pressure vessels, and household appliances: long-life gaskets qualified for continuous heat exposure
- Nuclear and power generation: seals evaluated for thermal aging as part of equipment qualification programs
FAQ
How is the aging test temperature determined?
The temperature is normally set at or above the maximum continuous service temperature of the seal, often with a margin of 25 to 50 degrees Celsius to demonstrate durability. Material class limits, specification requirements, and prior formulation data all guide the selection.
What changes are typically considered a failure?
Common rejection criteria include hardness change exceeding plus or minus 10 Shore A points, tensile strength loss greater than 30 percent, elongation loss greater than 50 percent, or compression set exceeding the specification limit. Visible cracking or embrittlement at any magnification is also cause for rejection in most specifications.
Can oven aging results predict actual service life?
Yes, when multi-temperature aging data are analyzed with an Arrhenius model, an estimated thermal life can be extrapolated. The prediction must, however, account for compression state, fluid contact, cyclic conditions, and mechanical stress, which is why aging data are usually complemented by application-specific testing.
What is the difference between air oven aging and oxygen pressure aging?
Air oven aging uses atmospheric air with forced circulation and approximates real service conditions more closely. Oxygen pressure aging uses pure oxygen at elevated pressure to accelerate oxidation and is primarily a comparative formulation screening tool rather than a service simulation.
How many specimens are required?
At least three tensile specimens per condition per time point are recommended, together with a matched unaged control set. For full qualification programs covering multiple temperatures and durations, specimen planning typically involves several dozen samples to ensure statistically valid conclusions.