Embrittlement / Aging

Definition and Classification: Aging vs. Embrittlement

In elastomers, aging refers to a time-dependent change in properties that is triggered by environmental and operating conditions. In sealing technology, aging matters because seals fulfill their function through contact force and conformability to the sealing surface. When these properties decline over weeks, months or years, the risk of leakage, crack formation or permanent deformation rises.

Embrittlement describes a frequent outcome of this aging. The material loses toughness (resistance to crack propagation) and extensibility. At the same time, stiffness increases. In practice, an embrittled seal feels “glass-hard”, is harder to install and tends to crack, particularly at edges or in notches. As a result, the sealing reserve under pressure and temperature changes often drops as well.

Distinction: True Embrittlement vs. Cold Stiffening and Swelling

Misdiagnoses often arise when a seal failure is judged solely by “hard or soft”. The decisive question is whether the change is reversible or based on permanent structural change.

Swelling can occur alongside aging, but it is not automatically embrittlement. A swollen seal may even feel softer, while the elongation at break has already dropped significantly.

Causes and Mechanisms of Aging (Chemical and Mechanical)

Elastomers in seals are crosslinked polymers. The crosslinking provides elastic recovery. Over time, however, both the chains and the crosslink points change. Two basic mechanisms are particularly important: chain scission (molecular chains shorten) and post-curing (additional crosslinks form). Both processes shift the balance between elasticity and strength. As a result, the material often becomes harder, less stretchable and therefore more crack-prone.

In service, these mechanisms rarely act alone. In hydraulics and pneumatics, temperature, oxygen, media contact and mechanical stress all combine. Consequently, seals usually age faster than they would in a clean storage environment.

Thermo-Oxidative Aging as the Most Common Driver in Service

Thermo-oxidative aging is the dominant driver in many applications. Heat accelerates chemical reactions, while oxygen enables oxidation. This combination changes the crosslinking state and, with it, the mechanical properties. At the sealing interface, the effect typically appears as rising hardness, declining elongation and an increasing compression set (permanent deformation after compression). As a result, the available contact force decreases, and the seal loses sealing reserve.

Ozone, UV/Light and Media Contact: Typical Accelerators

Ozone primarily attacks certain unsaturated rubbers and produces near-surface ozone cracks, often perpendicular to the direction of strain. These cracks may begin as fine fissures, but they quickly become critical under tension and micro-movement. UV/light, by contrast, can trigger photo-oxidative reactions, particularly under unfavorable storage conditions or in outdoor applications.

Media contact acts in two ways: it can extract substances from the elastomer (for example plasticizers) or react chemically. Either way, the material properties shift. In hydraulic and pneumatic systems, this concerns not only base oils and water but also additives, cleaning agents and condensates. Mechanical influences such as friction, pressure cycling and micro-movements accelerate the damage further, because they activate crack initiation sites and locally raise the temperature.

Detection in Service: Symptoms, Damage Patterns and Relevant Indicators

Embrittlement is often spotted first via the surface and the installation behavior. An aged seal typically shows fine cracks, edge chipping or brittle bending behavior. In service, it becomes noticeable through rising leakage, because it conforms less well to surface roughness and shape deviations. After disassembly in static applications, the seal often fails to return to its original shape once the load is removed.

These changes can be quantified through indicators that describe the loss of elastic function. For seals, the relevant indicators are those that reflect contact force and recovery — not just Shore hardness on its own.

Key Indicator: Compression Set (CS)

The compression set (CS) describes how much permanent deformation remains after a defined compression and time. A high CS means: the seal “takes a set” and delivers less restoring force. In many sealing concepts, this restoring force is precisely what maintains the contact pressure. When it is missing, the leakage risk rises, particularly under temperature changes or with small relative movements.

Stress Relaxation, Hardness Change and Elongation Change as Supplementary Indicators

Stress relaxation describes the drop in contact force at constant deformation. It is highly relevant for sealing positions in practice, because the geometry is usually fixed while the force decreases over time. A rise in Shore hardness is a frequent indicator, but on its own it is not sufficient. Particularly meaningful for embrittlement is the falling elongation at break (the elongation up to fracture), because it directly reveals the loss of toughness and stretch reserve.

Testing, Material Selection and Prevention (Concise and Practical)

Aging and embrittlement are usually evaluated through accelerated tests. Such tests do not replace field conditions, but they enable comparison and qualification. Frequently used standards families include ISO 815-1 (compression set), ISO 188 (heat aging) and ISO 1431 (ozone resistance). In practice, it matters that the test conditions match the actual application — temperature, medium and installation strain in particular.

Test Methods in Sealing Technology (Overview)

Practical Levers: Material, Operating Conditions, Storage

Material selection is the strongest lever, because elastomer families age very differently. NBR, EPDM and FKM are often mentioned, since their media and temperature resistance differ clearly. In sealing technology, what counts is therefore not a “good” material but a suitable material for the temperature, medium, pressure and motion profile.

Prevention typically starts at simple points: lower the temperature where possible, and reduce exposure to oxygen, ozone and UV. Media compatibility should be checked beforehand, because extraction and chemical reactions can sharply accelerate aging. Mechanical loads can often be limited through suitable lubrication, appropriate surfaces and reduced micro-movements.

For storage, the rule in many cases is: store seals cool, dry, protected from light and stress-free, because pre-aging often begins on the shelf. When assessing condition, CS and stress relaxation are usually more informative than a pure hardness measurement.

One closing note: in safety-critical or cost-intensive failures, specialised material and application consulting can be valuable, because aging is usually multi-causal.