Chemical Resistance

Definition and Distinction

Chemical resistance describes how well a material retains its function and key properties when in contact with a medium. For seals, this means above all: the seal must remain tight over the planned service time, while keeping geometry, hardness, strength, and recovery (the ability to press back after deformation) within an acceptable range. Whether a material is “resistant” can therefore only be answered with reference to a specific medium, a specific temperature, and a defined contact time.

In practice, chemical resistance depends heavily on the material formulation. With elastomers (rubber-like materials), polymer base, curing system, and fillers all influence behavior. With thermoplastics and PTFE, crystallinity, fillers, and processing play a role. As a result, the same material name in a data sheet can show noticeably different results depending on the compound. “Resistant” does not mean “unchangeable” — it means changes within acceptable limits. A certain volume or hardness change can be tolerable as long as the sealing function stays stable.

Chemical resistance is often confused with other requirements. The following table helps with classification:

What Counts as a Medium and Why Context Matters

Anything that wets the seal or can penetrate it counts as a medium. This includes hydraulic oils, fuels, water-glycol fluids, cutting fluids, cleaning media, acids and alkalis, as well as gases and steam. What matters is which substances are actually present, because industrial media are often mixtures. Even additive packages in oils (for example, anti-aging agents, detergents) or small amounts of water and cleaning agents can change the behavior significantly.

For a meaningful statement, the context is therefore always required: which medium exactly, at which temperature, for how long, and under which operating conditions (pressure, motion, cycles)? Only then can it be assessed whether an observed change is still within an acceptable range.

Mechanisms and Typical Damage Patterns at Seals

Several processes can run inside sealing materials during media contact, and they often overlap. A common one is swelling — the absorption of medium into the material. The component grows larger and often softer. In the short term, this can even improve the sealing effect, because the seal is pressed more firmly. At the same time, mechanical strength typically decreases, and the risk of extrusion rises. Extrusion means that material is pushed into a gap under pressure, where it can shear or tear out.

The counterpart is shrinkage through extraction. In this case, the medium leaches components out of the material — for example, plasticizers or low-molecular constituents. The seal can become smaller, harder, and more brittle. As a result, contact force drops, and leakage occurs more readily, especially with tolerances, settling effects, or temperature changes.

In addition, the polymer structure itself can change. Chain scission (breakdown of molecular chains) frequently leads to softening and loss of strength. Additional crosslinking (post-curing) often leads to hardening and embrittlement. Both reduce functional reserve, because seals — especially under dynamic load — need elasticity and sufficient tensile strength.

Permeation vs. Visible Material Attack

Beyond visible changes, there is permeation. Permeation is the migration of molecules through the material, mostly via diffusion. This is particularly relevant with gases, vapors, and thin sealing zones, such as sealing lips or membranes. A seal can appear externally intact while still allowing substances to pass through. Therefore, in applications with emission requirements, pressure retention, or safety regulations, permeation is treated as its own design case — not just as “chemical attack” in the classical sense.

Practice: Selection Criteria, Orientation Tables, and Typical Materials

In development, compatibility tables are often used to pre-screen materials. Such tables are helpful because they consolidate experience. Yet they remain coarse, because they can only reflect the actual combination of temperature profile, contact time, pressure, motion, and formulation to a limited extent. A “good” rating in a table is therefore usually a starting point, not a proof.

For pre-selection, a rough classification of common sealing materials is useful:

When classifying, the question always comes down to which failure mode is critical. In a static flange, moderate swelling can be tolerable. In a dynamic rod seal, the same swelling can quickly lead to failure through extrusion and abrasion. In sealing technology, chemical resistance is therefore always coupled with the sealing concept.

Quick Checklist for Material Selection

For a robust material decision, a few points are clarified very precisely in practice:

• Which medium is actually present, including mixture and additives?
• Which temperature applies continuously, and which peaks occur?
• How long and how often is the seal in contact (continuous operation, cycles, standstill)?
• Is pressure applied, and is the seal point static or dynamic?
• Which changes are acceptable (volume, hardness, strength) before the function fails?
• Is permeation relevant — for example, with gases, steam, or emission requirements?
• How will assurance be established (immersion testing, functional testing, acceptance criteria)?

Mini-Example Hydraulics: Switching from Oil to Water-Glycol

A medium change is a typical trigger for sealing problems. A seal that works reliably in mineral-oil-based hydraulic fluid can swell significantly more, or lose mechanical key values, when the system is switched to water-glycol. The result shows up as leakage, extrusion damage, or early crack formation. The decisive point is that the previous practical experience was tied to a specific medium. After the switch, material selection and testing must be set up anew.

Testing and Assurance: Immersion Tests and Documentation of Conditions

In practice, chemical resistance is often assessed through immersion tests. Specimens of a material are placed in a defined test medium, and after a defined time and temperature they are measured. Common references are the standard families ISO 1817 and ASTM D471. The results are comparison values under defined conditions. They can only be interpreted meaningfully if medium, temperature, and duration are fully documented.

A statement like “resistant to X” remains incomplete without an addition. Only specifications such as “70 h at 100 °C in medium Y” make the assessment technically sound. In sealing applications, real component geometry and load (pressure, motion, gap) often reveal functional loss faster than a pure specimen test. Therefore, immersion-test results are frequently complemented with functional tests on the actual component.

Which Measurement Values Are Truly Meaningful

Few key values are typically captured in immersion tests, and they connect well with the sealing function. Volume or mass change indicates swelling or shrinkage. Hardness change points to softening or post-curing. Changes in strength (such as tensile strength) show whether mechanical reserve is decreasing.

These values matter because they correlate with typical sealing risks: strong swelling can promote extrusion, strong shrinkage can reduce contact force and tightness, and a clear loss of strength raises the risk of cracks or pit-outs under dynamic load.

A short closing note: with new media, limit temperatures, or safety-relevant applications, specialized material and testing consultation is often advisable, because formulation differences and real-world media mixtures can only be partially captured in tables.