Swelling

Definition and Distinction

Swelling is the volume increase of a sealing material made of elastomers or plastics, because a medium (e.g., oil, fuel, water-glycol) penetrates the material and is absorbed there. The central mechanism is diffusion: small molecules migrate into the polymer. At the same time, sorption acts (uptake at or into the material), with the result that the distance between the polymer chains increases and the component becomes measurably “thicker”.

In sealing technology, what matters is when and how strongly swelling occurs, because it changes geometry, contact pressure, and friction. Swelling is often partly reversible when the medium is removed again. It can, however, be accompanied by permanent damage — for example when the medium leaches out constituents or triggers chemical aging.

Swelling is frequently confused with other effects. The following distinction helps to assign causes correctly:

Swelling, Shrinkage, and Extraction (Net Effect)

In practice, not only “more volume” counts. Media can cause extraction — that is, the leaching of additives (e.g., plasticizers, processing aids) from the elastomer. As a result, shrinkage can occur. Frequently, swelling and extraction run simultaneously, so that the measured net volume change describes the material state only partially.

Therefore, hardness (e.g., Shore A) and strength are often considered as accompanying parameters. A seal can become noticeably softer despite low net swelling and thus lose its load capacity.

What Happens in the Material and Which Factors Determine Swelling

In the polymer network, the absorbed molecules act like an “insertion substance”. They increase chain spacing and often reduce cohesion (internal cohesion). With elastomers, cross-linking density also plays a role: a more densely cross-linked network usually limits swelling more strongly, because less “free space” is available for expansion.

Decisive is which medium acts, at which temperature, for how long, and under which operating mode (static or dynamic) the seal is used. Standstill periods are also relevant, because diffusion takes time and swelling can “accumulate” during long rest phases.

Medium and Polarity as Drivers of Media Compatibility

A practice-oriented principle is: chemically similar substances tend to be compatible. In materials science, this is often described via solubility parameters. Simplified, the following applies: polar (e.g., water, many glycol mixtures) and non-polar (e.g., many mineral oils, hydrocarbons) tend to “mix” with similar polymers.

For seal selection, this means: a combination may appear unremarkable in the laboratory yet swell strongly in the field when media composition, additives, or temperature differ. Cleaning chemicals and fuel additives are typical “drivers” of unexpected swelling.

Temperature, Time, and Operating Mode (Static vs. Dynamic)

Swelling is time- and temperature-dependent. Higher temperature accelerates diffusion and often increases the absorbable amount, so that test values at room temperature can be transferred to hot operation only to a limited extent. The question “does the system run continuously or is it often stopped?” is also important, because swelling can continue during standstill phases, while temperature and media changes during operation generate additional effects.

With dynamic seals, swelling is usually more critical than with static ones. Even moderate volume increase can raise friction, promote stick-slip (jerky sliding due to alternating static and sliding friction), and accelerate wear.

Why Swelling Changes the Sealing Function (Risks and Possible Effects)

Seals work via contact stress: the seal is deformed in such a way that it lies against the mating surface with sufficient pressure. Swelling changes this pressure directly, because the seal geometry — and frequently also the material stiffness — change.

A moderate swelling can even help in the short term, because contact pressure rises and leakage drops. In many applications, however, this effect flips at over-swelling. Then strength often drops, the seal becomes softer, and the risk that material is pushed into gaps rises.

Typical consequences in sealing technology are:

• Friction rise and higher drive power under motion.

• Extrusion / gap pressing: material is pushed into the sealing gap, particularly at high pressure and large gap size.

• Accelerated wear, up to cracks or break-outs.

• Change in leakage behavior: at first better, later worse, when material degrades or geometry becomes unstable.

Typical Classification via Volume and Hardness Change (Guideline Values)

In practice, swelling is frequently stated as volume change in %. Complementary, hardness change is considered, because it co-determines load capacity and extrusion tendency. Rigid limit values, however, are rarely reliable, because geometry, preload, pressure, gap size, and motion strongly contribute.

Testing and Assessment in Practice (Standards, Measurement Parameters, Transferability)

Customary are immersion tests. In these, test specimens or sealing parts are placed in a defined medium and stored at a defined temperature for a defined time. After this, volume, mass, and mechanical parameters are measured. For elastomers, ISO 1817 and ASTM D471 are widely used test bases.

Transferability depends on whether the test realistically reflects the application. Decisive are media composition (including additives), temperature profile, exposure time, alternating loads, and pretreatment. A test “72 h at X °C” can be sensible but does not automatically answer the question of what happens after months or years in the field.

Which Questions Should Be Answered Before Material Selection

• Which medium actually applies (including additives, cleaning residues, mixed media)?

• Which temperatures occur in operation and during standstill, and how long does the medium act in each case?

• Is the sealing point static or dynamic, and which friction and wear requirements result?

• Which volume, mass, and hardness change is measured, and does the test method match the real loading?

• How large are pressure and gap size, and is there a relevant risk of extrusion?

A brief note in closing: when media, temperatures, and load profiles are complex, specialized material and application consultation is often sensible, because swelling is strongly application-dependent.