Ozone / UV Attack

Ozone / UV Attack (Aging of Elastomers in Seals)

Definition and Practical Relevance

Ozone / UV attack describes the damage to elastomers (rubber-elastic materials) caused by ozone (O₃) in the air and by ultraviolet radiation (UV) from sunlight or artificial sources. In sealing technology, this matters because seals are often in air contact for long periods and are easily pre-stretched or bent in that state. Under these conditions, cracks can develop that at first only appear superficial but lead to leakage over time.

Particularly affected are seals that see ambient air or run in ventilated areas — for example O-rings, lip seals, diaphragms, and elastic molded parts. It becomes critical when the seal is already under strain in installation — for example through preload, tight radii, or edges. Then the risk of crack formation rises noticeably.

Ozone and UV resistance is a form of environmental resistance. It differs from media resistance — that is, the question of how well an elastomer holds up against oil, fuel, solvents, or coolant. A material can be very stable in the medium and still age quickly in air under ozone or UV.

Mechanisms: Ozone Cracking and Photo-Oxidation

Ozone primarily attacks elastomers when they contain double bonds in the polymer structure. These double bonds react preferentially with ozone. The reaction first takes place at the surface, because ozone from the air is available there. Chain scission (breakdown of polymer chains) occurs, with the result that the surface loses strength and cracks initiate more easily.

A central point is the dependence on strain: under tensile stress, microcracks open more quickly. In practice, this means that it is not the mere presence of ozone that decides, but the combination of ozone, time, and mechanical loading.

UV radiation acts differently. It supplies energy that generates free radicals within the material (highly reactive intermediates). In the presence of oxygen, a photo-oxidation follows — that is, oxidation triggered by light. Depending on the recipe, this leads either to chain breaking (the material becomes softer and weaker) or to post-cross-linking (the material becomes harder and more brittle). Frequently, a near-surface, hardened zone forms that in operation reacts like a hard skin and can tear open during motion.

In real weathering, ozone and UV often occur together. Then the effects overlay each other, and the seal ages faster than either factor alone would suggest.

Why Strain and Bending Promote Ozone Cracks

Ozone cracking is strongly driven by tensile strain. A seal in service is rarely stress-free: O-rings are squeezed for the sealing function and locally stretched in the process, and lip seals are also under stress through assembly and contact pressure. Particularly high local strain arises at edges, at small radii, or with unfavorable groove geometry. Cracks preferentially initiate there and grow from the surface into the bulk.

Loosely stored elastomer parts can also age, but they often show less pronounced ozone cracking, because the driving tensile stress is absent or smaller. For practice, this means: the installation situation co-decides when a material appears ozone-sensitive.

Typical Damage Patterns and Diagnosis in Practice

Ozone cracks are usually visible as many fine surface cracks that frequently appear parallel to each other. Characteristic is the orientation: under tensile strain, the cracks typically lie perpendicular to the direction of strain. First signs are often found at edges, on the outer radius, or at locations that are permanently under tension in service. As damage progresses, elasticity and tear strength drop until the sealing line tears open and leakage arises.

UV damage often first appears as discoloration, chalking (powdery, matte surface), or a noticeably harder surface. In dynamic applications, this more brittle layer can break open, producing cracks that propagate further with motion.

A rough distinction from other failure causes helps in diagnosis:

For failure analysis, it is therefore important where the damage starts: ozone/UV usually starts on the surface facing air and light, while medium attack is often more dominant where the fluid is present.

Influencing Factors, Material Selection, Testing, and Prevention

How quickly ozone and UV damage occurs depends on several factors. For ozone, primarily ozone concentration, temperature, strain, and exposure time are decisive. For UV, light intensity, spectrum, oxygen, and often also humidity are added, because oxidation processes are influenced by them.

For material selection, rules of thumb help, but recipe, fillers, and protective additives can change the behavior strongly. As orientation, the following applies in many applications: EPDM is frequently used as a very weather- and ozone-resistant material, NBR shows higher ozone sensitivity in many recipes, and FKM is often rated as relatively resistant but also remains condition-dependent. This classification does not replace application-near validation, because installation strain, geometry, and real environmental conditions are decisive.

For testing, established standards are used. ISO 1431-1 typically evaluates ozone resistance under a defined ozone atmosphere and defined strain, because precisely this combination is crack-critical in practice. UV and weathering effects are frequently simulated with fluorescent UV lamps — for example according to ASTM G154 or ISO 4892-3. Such tests are useful for comparisons and approvals, but the translation to field years usually remains only approximate.

Prevention is often a mix of design, storage, and material:

• Store seals in a stress-free state — that is, not permanently stretched or bent.
• Consider ozone sources nearby, such as electrical spark formation or certain electrical equipment in storage areas.
• Shield against UV, for example through covers, suitable installation positions, or opaque packaging during storage.
• Design geometry so that sharp edges and very small radii are avoided, because local strain peaks arise there.
• When outdoor weathering is expected, plan application-near validation that reflects strain and real air/light conditions.

Practical Minimum Information for Material Selection

A reliable selection only succeeds when, alongside the medium, the environmental conditions are also known. The inquiry should therefore include at least the following points:

When it is unclear which environmental loads actually dominate, a brief, specialized material and application clarification is often sensible before a sealing material is finally fixed.