Why do rubber seals swell?

I know the shock when you pull out a seal from a test rig and see it swollen, soft, and completely out of tolerance.

Rubber seals swell when liquid, gas, or additives penetrate the rubber, attack the polymer network, or extract ingredients, so the material volume, hardness, and dimensions change beyond design limits.

When I investigate swelling problems for customers, I never blame “bad rubber” alone. I always check the medium, temperature, material, and test method together. Only then does the real root cause become clear.

What makes rubber seals swell in the first place?

Many people think swelling is only “rubber absorbing oil”, but the mechanism is more complex and more interesting.

Rubber seals swell because molecules from the fluid move into the rubber network, push polymer chains apart, and sometimes remove ingredients, so the compound changes volume and mechanical properties.

When I explain this to buyers, I say: “Your seal is not a solid stone. It is a sponge at the molecular level. The wrong fluid walks inside and rearranges the whole structure.”

How the swelling mechanism really works

Rubber is a crosslinked sponge, not a rock

Rubber is a network of polymer chains¹ that are crosslinked. The crosslinks² keep the material elastic. Between the chains, there are spaces. When a compatible liquid touches the seal, small molecules can diffuse into these spaces. They push chains apart and the part grows.

The degree of swelling³ depends on:

• The type of polymer (NBR, EPDM, FKM, silicone, etc.)
• The structure and polarity of the fluid
• The crosslink density
• The type and amount of plasticizer⁴ and filler⁵

If the fluid likes the polymer (similar polarity or solubility parameter), it will enter more easily. If the network is loose, it will expand more.

Swelling vs simple softening

Swelling is not always just “bigger size”. It often comes with:

• Hardness drop
• Tensile strength loss
• Tear strength loss
• Permanent deformation after drying

Sometimes a seal looks almost the same size, but it becomes very soft and weak. In practice, I always measure hardness and volume change together, not just OD or ID.

Why some swelling is acceptable and some is not

Many standards allow small volume change after immersion, for example ±5% or ±10% depending on the application. A little swelling can even improve sealing in some static applications.

The problem starts when:

• Volume increases too much and the seal cannot fit in the groove
• Edges mushroom or squeeze out of the gland
• Hardness drops so much that extrusion or nibbling happens
• The seal cracks after drying because the structure was damaged

I use a simple internal table like this when I review test results:

Volume change after immersion	My typical judgement in projects
0–5%	Usually safe for many applications
5–15%	Needs careful review, maybe acceptable
>15%	High risk, I look for a better compound

When you see swelling in your seals, it is the visible symptom. The real story is the balance between polymer, fluid, and crosslink network behind it.

How do media and material compatibility affect swelling?

Most swelling complaints I see come from one simple problem: wrong rubber for the fluid. The drawing just says “NBR” or “EPDM”, but the fluid has changed over time.

Different rubbers react very differently to oils, fuels, water, solvents, and coolants. Swelling becomes serious when the chosen polymer is not compatible with the actual media and additives⁶ in the system.

I once helped a customer who switched to a new biodegradable hydraulic oil⁷ without telling their seal supplier. Within weeks, NBR seals swelled and softened. The oil was “green”, but for NBR it was a disaster.

Typical media vs material behaviour

How common rubber families react

Each general rubber family has its own “comfort zone”. A very rough guide looks like this:

Rubber	Good with…	Risk of serious swelling with…
NBR	Mineral oils, many fuels	Strong polar solvents, some synthetic esters
HNBR	Hot oils, some fuels	Strong polar solvents, some esters
EPDM	Water, steam, many coolants	Mineral oils, fuels, many greases
FKM (Viton)	Many oils, fuels, chemicals	Some amines, some brake fluids
Silicone (VMQ)	Dry heat, many inert fluids	Many oils, fuels, concentrated solvents
Neoprene (CR)	Moderate oils, weather, some refrigerants	Specific oils and modern fuel blends

This table is not a full compatibility chart⁸, but it shows why EPDM seals swell badly in oil and why NBR seals swell badly in hot water and coolants.

Role of additives and real blends

Real fluids are not pure chemicals. They contain:

• Additives for anti-wear, corrosion, or oxidation
• Detergents and dispersants
• Biocides or dyes

These additives⁶ can attack rubber or change how the base oil interacts with it. A seal that worked fine in one oil can start to swell when the formulation changes, even if the product name stays the same.

That is why I always ask customers for the exact fluid specification, not just “hydraulic oil⁹” or “coolant”.

Water, glycol, and brake fluids

Water and glycol-based fluids behave very differently from oils:

• EPDM is usually strong in water, steam, and many coolants
• NBR and ACM often swell or crack in long-term hot water
• Silicone can show high swelling or softening in some coolants

Brake fluids based on glycol ethers are especially aggressive for many common rubbers. That is why brake systems use very specific materials.

Simple compatibility thinking that I use

When I see swelling, I ask very direct questions:

1. Did the fluid change, even slightly?
2. Is the seal material really selected for this type of fluid?
3. Are there cleaning chemicals or flushing fluids that contact the seal?

Often the answer to question one or three reveals the true cause. A system cleaner or new “eco” fluid might be the hidden culprit.

If you treat the seal and the fluid as one system, not as separate parts, you will avoid many swelling surprises.

Can temperature, pressure, and time make swelling worse?

Even with a decent material choice, seals can still swell more than expected when the system runs hotter, longer, or under high pressure.

High temperature, long exposure time, and pressure all speed up diffusion and chemical attack, so swelling progresses faster and often combines with softening, cracking, and permanent deformation.

I sometimes see buyers approve a seal after a 24-hour lab test at room temperature¹⁰, then feel surprised when field parts fail after six months at 120 °C. The test was not wrong; it was simply too gentle.

How operating conditions change the picture

Temperature as a swelling accelerator

Temperature increases molecular movement. When you raise temperature:

• Fluids penetrate the rubber faster
• Reactions between additives⁶ and polymer speed up
• Crosslinks age faster and network tightens or breaks

A system that is stable at 60 °C may show strong swelling and property loss at 120 °C, even in the same oil. That is why immersion tests¹¹ for hot-oil seals usually run at elevated temperature, often at or above the real operating temperature.

Pressure and squeeze

High pressure can:

• Force fluid deeper into micro-cracks and interfaces
• Increase local mechanical stress where the seal contacts metal
• Combine with softening and swelling to cause extrusion and nibbling

I see this especially in hydraulic systems. A slightly swollen seal might be okay at low pressure. At higher pressure, it extrudes into gaps and tears.

Time and duty cycle

Time is also a key factor:

• Short exposure may only cause reversible swelling
• Long exposure allows irreversible chemical changes
• Cycles of swelling and drying can crack the material

I sometimes run long-term soak tests in stages: 24 h, 72 h, 168 h, and longer. The trend is often more important than a single number.

Putting conditions into a simple view

I like to use a simple matrix when I explain this:

Condition level	Effect on swelling and seal life
Low temp, short time	Small, often reversible swelling
High temp, short time	Faster swelling, can still be reversible
High temp, long time	Swelling plus aging, hardness change, permanent damage
High temp + high pressure + long time	Maximum risk: swelling plus extrusion, cracks

So when you ask “Why did my seal swell so much?”, you must include operating temperature, system pressure¹², and exposure time in your answer. The material alone does not tell the whole story.

How can I prevent rubber seals from swelling in my projects?

Most swelling problems are avoidable. You just need a structured way to choose materials, design glands¹³, and test before mass production.

You prevent rubber seals from swelling by choosing compatible materials, keeping a safety margin in media and temperature, designing glands for controlled compression, and validating seals with realistic immersion and endurance tests.

When I support a new project, I always try to “design out” swelling risk before we cut a mold. This saves a lot of money and many painful field returns.

Practical steps I use with customers

1. Start with the real media list

I ask for a full list of fluids:

• Process fluids (oils, fuels, water, coolants)
• Cleaning agents and disinfectants
• Possible “accidental” contacts (greases, sprays, solvents)

Then I pick candidate elastomers based on known compatibility. I usually shortlist two or three families first, not ten.

2. Consider temperature and pressure margins

I ask for:

• Normal operating temperature and peaks
• Minimum start-up temperature
• Maximum system pressure and duty cycle

I avoid designs that run seals right at the edge of the material’s capability. I prefer a comfortable margin, especially on the hot side.

3. Design the gland to tolerate some change

Even with good compatibility, small volume change may still happen. I design glands¹³ so that:

• The seal has room to swell a few percent without jamming
• The squeeze stays within a safe range over time
• There are no sharp corners where swelling could cause cutting

For O-rings and cords, I pay special attention to groove fill at maximum swelling. Overfilled grooves can generate very high internal stress.

4. Run realistic immersion and functional tests

I always recommend:

• Immersion tests at real or slightly higher temperature
• Measurement of volume, hardness, and weight change
• Functional tests in the real hardware when possible

If a seal shows, for example, 12% volume increase but still passes all functional tests¹⁴, the application might be safe. If it shows 5% increase but hardness drops a lot and leaks appear, the risk is high. Numbers must be read together with behavior.

5. Lock down fluid and material specifications

Once we find a combination that works, I ask both sides to:

• Freeze the fluid specification (or at least document changes)
• Freeze the rubber compound recipe and supplier
• Record test results as a baseline for future audits

This prevents “silent” changes, like a new oil additive or an unapproved material swap, which often re-open swelling problems years later.

Simple checklist you can reuse

Here is a short list you can adapt inside your own company:

Step	Key question
Media definition	Do we know all fluids and cleaners?
Material selection	Is the rubber rated for these fluids and temps?
Gland design	Can the seal tolerate small volume changes?
Testing	Have we tested at realistic temperature and time?
Change control	Are fluid and compound changes controlled?

At Julong Rubber, I use this approach with buyers from HVAC, machinery, and automotive-related industries. When we follow these steps together, swelling becomes a topic we manage, not a surprise we suffer.

If you want to review a specific swelling issue, you can send me the fluid name, temperature, and some photos through info@rubberandseal.com or via www.rubberandseal.com. I am happy to share more practical ideas based on your real case.

Conclusion

Rubber seals swell when the fluid, temperature, and time exceed the material’s comfort zone; with honest media data, smart material choice, and realistic testing, you can prevent swelling and keep your sealing system stable.

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