Aging of Sealing Materials

Definition and Relevance for the Sealing Function

Aging describes the time-dependent change in the properties of an elastomer caused by influences from operation and environment. Elastomers are rubber-like materials that achieve their sealing effect mainly through elastic recovery and sufficient surface pressure (contact pressure between the seal and the mating surface). For that reason, aging is not a side topic in sealing technology — it is directly tied to leakage and failure.

In practice, aging typically shows up as hardening or embrittlement, loss of recovery force, crack formation, and swelling or shrinkage. These effects change the geometry and the force level at the sealing point. When the recovery force drops, the surface pressure at the sealing gap falls as well, and micro-leakages can develop. If a dimensional change is added on top, the sealing gap can widen locally, or the seal can be pushed into unfavorable areas.

This is particularly critical in hydraulics and pneumatics, because continuous compression, changing pressures, temperature peaks, and media contact often combine there. A static O-ring seal, installed for years under defined pre-deformation, can lose its sealing force through aging — even with a correctly designed groove. In dynamic applications, friction and heat build-up additionally accelerate the process.

Main Mechanisms and Drivers of Aging

Aging rarely arises from a single process. In many cases, chemical reactions, physical transport processes, and continuous mechanical loading overlap. Which mechanism dominates depends on where the seal sits (air side vs. media side), which temperature is present, which medium acts on it, and how high the pre-deformation and pressure load are.

Chemically, oxidation by oxygen is the dominant driver, often accelerated by heat. Physically, diffusion (molecules entering the material) and extraction (additives being leached out) matter. Mechanically, relaxation and permanent deformation under sustained load play a central role. The damage pattern at disassembly often gives clues: fine surface cracks point more toward ozone/UV, strong dimensional change toward media influence, and a “smooth, hard” seal with low spring-back often toward thermo-oxidative aging or post-curing.

Thermo-Oxidative Aging (Heat + Oxygen)

Thermo-oxidative aging describes reactions between elastomer and oxygen that proceed much faster at higher temperatures. Polymer chains can be split (chain scission), or additional crosslinks can form (post-curing). Both paths shift the mechanical parameters, but in different directions.

In sealing technology, this is critical because hardness, elasticity, and fracture behavior change. Post-curing often leads to hardening and therefore reduced ability to conform to surfaces. Chain scission can lower strength and make the seal more susceptible to cracks. Where oxygen is available also matters: on the air side or in gas-permeable environments, the material often ages more strongly than in fully oil-wetted zones.

Media Influence (Swelling, Extraction, Property Changes)

With media influence, components of the fluid enter the elastomer. This diffusion can trigger swelling — that is, an increase in volume. Conversely, media can extract additives such as plasticizers or stabilizers, which can cause the elastomer to shrink or embrittle. Which direction prevails depends strongly on the material-fluid pair, including oil type, additive package, and temperature.

For the sealing point, the decisive factor is that dimensional changes alter groove fill and the local contact pressure. Swelling can initially raise contact pressure, yet later lead to mechanical overload, extrusion, or higher friction. Extraction and shrinkage, by contrast, often lower the contact pressure and raise the leakage risk. Therefore, media effects are always system-dependent and should be tested with the real operating fluids.

Ozone/UV and Surface Damage

Ozone and UV radiation attack many elastomers primarily at the surface. Ozone cracks are typically fine, often parallel cracks that form especially when the surface is simultaneously under strain. In seals, that strain can come from installation pre-deformation, assembly offset, or local geometric peaks.

UV radiation likewise damages the surface and can promote crack formation, especially in outdoor applications or under transparent covers. For assessment, it is important to distinguish surface cracks from mechanical cuts or installation damage, because the corrective actions then differ.

Measurement Values, Tests, and Typical Misunderstandings (Compression Set, CSR, Media Testing)

For functional assessment, parameters that aim to represent aging are commonly used in practice. What matters is what a value actually measures and how well it represents the real sealing point. Test results depend heavily on temperature, time, sample geometry, and oxygen access. In addition, the test type (air oven, oil bath, pressure medium) influences which mechanism dominates in the test.

For media resistance, volume change (swelling/shrinkage) is often considered together with changes in hardness or strength. A pure swell value without mechanical parameters can be misleading, however, because a moderately swelling seal can still remain sufficiently strong, while a low-swelling sample can lose substantial strength through extraction.

Compression Set: What It Says and What It Doesn’t

The compression set describes how much permanent deformation remains in a sample after defined compression for a defined time. A high compression set means the material springs back only slightly. In a sealing point, this is often a sign of dropping surface pressure and therefore rising leakage risk.

At the same time, compression set reflects the remaining sealing force in service only indirectly. It is a geometric recovery measure after unloading, while in the application the seal usually remains permanently deformed. Therefore, the value works well as a screening parameter, but less so as a sole functional prediction.

CSR (Compression Stress Relaxation): Sealing-Force Loss as a Functional Criterion

CSR (compression stress relaxation) measures how much the contact force of a sample drops over time under constant deformation. For static sealing points, this is often closer to reality, because the seal in the installed state is not unloaded but instead “relaxes” its force.

CSR reacts sensitively to temperature, time, medium, and installation deformation. In design work, the CSR curve is particularly relevant when tightness must be ensured over long service lives without re-adjustment of bolt force or without pressure support of the seal.

Service-Life Estimation and Practical Measures (Hydraulics/Pneumatics)

Service life is often estimated through accelerated aging, usually with elevated temperatures. For extrapolation, Arrhenius approaches are common — they assume a temperature-dependent reaction rate — as is time-temperature superposition (TTS), where curves are shifted along the time axis. This works reasonably well as long as the aging mechanism remains comparable across the temperature range.

Limits emerge when several mechanisms act in parallel, or when a different mechanism takes over at higher temperatures than in the field. In addition, media changes, pressure cycles, and dry-running phases shift the real load spectrum. In hydraulics and pneumatics, the combination of continuous compression, temperature peaks, air/oxygen contact (for example on the rod side), and the additive chemistry of the fluid is often decisive for the actual aging.

Therefore, practical levers lie not only in the material but also in design and operation. Frequently effective measures are:

• Reduction of mean and peak temperature through cooling or lower friction.

• Correct pre-deformation and groove geometry, so that surface pressure remains sufficient over time without overloading the material.

• Minimization of ozone and UV exposure in outdoor applications through shielding or suitable material choice.

• Avoidance of dry running and unfavorable lubrication, because additional heating and surface damage can accelerate aging.

• Matching of elastomer and medium/additives against real operating fluids and temperatures.

Diffusion-Limited Oxidation (DLO): Why Real Seals Age Differently Than Test Specimens

Diffusion-limited oxidation (DLO) means that oxygen cannot diffuse into the component interior quickly enough, while oxidation reactions at the surface are already proceeding rapidly. As a result, the outer zone ages strongly, while the core remains softer for longer. This gradient behavior occurs especially with thicker cross-sections or tightly installed seals, because oxygen access is restricted.

For testing, this has a direct consequence: small specimens in an oven can oxidize more uniformly than real seals with larger cross-sections. As a result, transferring service-life data without reference to geometry and installation can lead to significant deviations. In practice, it is often sensible to use component-level tests, or at least geometry-near specimens, when oxidation is suspected as the dominant mechanism.

In the end, aging remains a systemic topic. When requirements are critical, specialized material and sealing-point assessment is advisable.