Chemical Attack

Definition and Context

Chemical attack is a damaging interaction between a medium and a sealing or plastic material. It arises when the medium penetrates the material or reacts with it. In sealing technology, this is particularly relevant because the damage often does not stay at the surface but changes the entire sealing volume. As a result, geometry, preload, and friction behavior change — and ultimately, often the sealing function as well.

Which media show up in practice? Common ones are oils, fuels, water and hot water, steam, cleaning chemistry (alkaline or oxidizing), coolants, solvents, and media mixtures with additives. Additives are substances added to improve properties such as lubricity, corrosion protection, or cleaning performance — yet they can shift material compatibility significantly.

Typical consequences are swelling (volume increase), shrinkage (volume decrease), embrittlement, crack formation, and blistering. One key driver of volume effects is permeation: the medium diffuses into the material as small molecules, distributes itself inside, and thereby changes the properties.

Distinction: Chemical Attack vs. Thermal Aging vs. Mechanical Wear

In failure analysis, this distinction is decisive because the countermeasures differ. Thermal aging arises primarily from temperature and oxygen. It can occur even without a problematic medium and often shows up as general hardening, loss of elasticity, or crack formation due to long-term heat exposure.

Chemical attack, by contrast, requires a media incompatibility. It typically shows up most strongly where contact with the medium exists, or where the medium is driven into the material under pressure. Mechanical wear, on the other hand, is caused by motion, particles, or unfavorable surfaces. It tends to leave abrasion marks, score marks, or edge wear, without swelling or shrinkage being central.

Mechanisms: How Media Damage Sealing Materials

Three main mechanisms occur in seals. They can act individually but often overlap in many applications. What matters is whether the change is primarily physical (absorption/extraction) or chemical (chain breakdown, reaction). For the sealing function, both are critical, because seals depend strongly on defined dimensions and an appropriate hardness.

1. Swelling through absorption of small molecules: The medium diffuses into the material, volume increases, and the material often becomes softer. In dynamic seals, friction therefore frequently rises, the break-away torque can increase, and the tendency toward extrusion grows because the material is more easily pushed into gaps.

2. Shrinkage through extraction: The medium leaches components out of the material — for example, plasticizers or other formulation ingredients. Plasticizers are additives that keep elastomers flexible. If too much is removed, the seal shrinks, becomes harder and more brittle, and can pull back from the sealing surface. Leakage then often occurs despite an “intact”-looking geometry.

3. Chemical reaction or degradation: Here, polymer chains or crosslink sites are attacked — for example, by oxidation or hydrolysis. Oxidation refers to reaction with oxygen or oxidizing media; hydrolysis is breakdown by water, often accelerated by heat. Such damage is usually irreversible and shows up as significant loss of strength, crack formation, chalking, or flaking.

Permeation and Decompression as a Special Case (Blistering)

Blistering often appears “sudden” in practice, yet it follows a clear sequence. Under pressure, a gas or volatile liquid enters the material. With rapid decompression, the medium inside can outgas because it cannot diffuse out fast enough. As a result, bubbles and cavities form that can later lead to cracks.

Such damage occurs especially with the combination of pressure, temperature, and repeated pressure cycles — for example, in hydraulic power packs or gas seals. The core of the seal often looks intact while the outer zones are heavily damaged. Therefore, assessment becomes complex and frequently requires a cross-section examination and additional testing.

Influencing Factors and Proof: Why Temperature, Time, and Test Conditions Are Decisive

Whether a material is “resistant” depends strongly on the operating conditions. A medium can barely act at room temperature but trigger significant swelling or degradation within a short time at 100 °C. Time plays a dual role: diffusion and reactions continue over time, and many damage processes are cumulative. In addition, concentration, pressure, media mixtures, and additives can substantially shift the effect.

For verification, sealing specimens are often immersed in defined media and measured afterwards. It is important that test reports state the conditions — for example, “70 h at 23 °C” or “168 h at 100 °C”. In sealing technology, standards such as ISO 1817 and ASTM D471 are widely used; they describe immersion and evaluation of elastomers in fluids.

Which Measurement Values Indicate Chemical Attack

Chemical attack can usually be detected through a few well-interpretable parameters. These should always be considered together, because a single value tends to invite misinterpretation. Particularly telling are volume or mass change, hardness change, and changes in tensile strength and elongation at break.

In addition, visual observations matter: cracks, crazing (fine network cracks), blisters, or a sticky or dried-out surface. Functionally, chemical effects show up as altered friction, stick-slip, a changed break-away torque, and leakage. Together, these produce a characteristic picture that can be matched against typical media reactions.

Why Compatibility Lists Are Only Orientation

Compatibility lists are useful for pre-selection, yet they rarely deliver a robust decision. Sealing materials are formulation-dependent, because fillers, plasticizers, curing agents, and additives noticeably change behavior toward media. In addition, real media are often mixtures — for example, oils with additives or cleaning chemistry at varying concentrations.

In the application, pressure, motion, and gap dimensions are added on top, which can quickly turn a “just barely acceptable” swelling into an extrusion problem. Therefore, lists should serve as a starting point, not as a final release. Critical applications almost always benefit from application-near testing with original media and realistic temperatures.

Practice in Hydraulics/Pneumatics: Typical Damage Patterns and Countermeasures

In hydraulics and pneumatics, medium, pressure, temperature, and motion act simultaneously. Even moderate incompatibilities then show up early through friction and tightness. Common media are hydraulic oil, compressed air with condensate, water-glycol mixtures, fuels, and cleaning or maintenance chemicals. With CIP/SIP processes (Cleaning/Sterilization in Place), alkaline cleaners, oxidizing agents, and steam are added, often with high temperature peaks.

Typical damage patterns in O-rings, rod and piston seals, or wipers include swelling and softening with rising friction and extrusion tendency; shrinkage and hardening with pull-back from the sealing surface; and crack formation or crazing after contact with aggressive media. Blistering occurs primarily after rapid decompression. For material selection, broad directions help — yet they should always be verified; the specific formulation and the real medium decide actual resistance.

From a design and operational perspective, the following measures lower the risk: control gap dimensions and add back-up rings against extrusion if needed, limit temperature peaks, and align cleaning chemistry with seal design. In dynamic applications, a closer look at surfaces, lubrication, and pressure-cycle profiles also pays off, because they functionally amplify chemical effects. The result is a holistic concept that ties material, design, and process management together.

Checklist for Design and Failure Analysis

A robust diagnosis usually only succeeds when operating conditions are cleanly documented. Particularly important are specifications for medium, temperature, pressure, time, and any additional loads such as cleaning or sterilization. Equally relevant is information about the installation space — for example, gap dimensions, guidance quality, and existing extrusion protection.

In the end, a trade-off between material, design, and process management often remains. When the medium, temperature, or cleaning chemistry is not clear, specialized material and sealing consultation is advisable, because small details can strongly influence service life.