Pneumatic cylinder seal materials: NBR vs FKM (Viton) vs PU
The seal decides what fluid and what temperature a cylinder tolerates — and there is one trap that catches buyers before the chemistry even matters.
Read the cylinder’s rated temperature band, not the seal material’s maximum. They are different numbers. The cylinder’s band is never widerthan the seal material’s, and in practice it is almost always narrower — because the assembled cylinder is also limited by its grease, its magnets and switches, and its tube surface treatment, not just the rubber. SMC’s C96, for example, uses NBR seals: the rubber itself tolerates around 100 °C, but the cylinder is rated to about 70 °C.
After that, the choice is straightforward: NBR (nitrile) is the standard for compressed air including oiled air; FKM (Viton) is for higher temperature and aggressive chemicals; PU (polyurethane) gives the longest wear life on clean, dry air.
The trap: material rating is not cylinder rating
This is the mistake worth avoiding, because it is easy to make from a datasheet and it fails months later in service. A seal-material chart will tell you FKM handles about 200 °C. That figure describes the elastomer (the rubber itself) on its own — not the cylinder you are buying.
The assembled cylinder has other temperature-limited parts: the grease on the bore and rod — conventional pneumatic lubricants are typically approved only to around 80 °C — plus the magnet and any position switches, and the tube’s surface treatment. Whichever part fails first sets the rating, and it is often not the seal. At the cold end the limit is usually different again: condensate inside the cylinder must not freeze. So the published cylinder band comes out inside the raw elastomer numbers:
| Rating | Typical published range |
|---|---|
| NBR as a material | up to about 100 °C |
| NBR pneumatic seal (component) | about −30 to +100 °C |
| SMC C96 cylinder, NBR seals (assembled) | about −20 to +70 °C |
Practical rule: specify from the cylinder datasheet. Use the seal-material chart only to understand why a cylinder is rated the way it is, or to choose between builds the maker offers.
Which compound, in one table
| Condition | NBR (nitrile) | FKM (Viton) | PU / TPU |
|---|---|---|---|
| Dry compressed air | good — the default | good | excellent, even oil-free |
| Oiled / lubricated air | excellent | excellent | good |
| Ozone, weather, UV | poor — cracks | excellent | excellent |
| Water | good to about 70 °C | good | limited — about 50 °C, or 80 °C for a hydrolysis-stabilised grade |
| Steam | poor | poor — hardens | poor |
| High temperature | to about 100 °C | to about 200 °C | about 80 °C standard; some grades to about 110 °C |
| Wear life (moving parts) | good | good | excellent |
These are material-level properties — always cross-check against the assembled cylinder’s own rated band, per the section above. Note that none of the three is a steam material: if you genuinely have steam, EPDM is the usual choice, and that is a different cylinder build, not a seal swap.
Hot room, clean air — the case people miss
A common real situation: the compressed air is clean, dry and cool, but the cylinder sits beside a shrink tunnel, an oven or a heater and stays hot for the whole work shift. People check the air, find it fine, and conclude the standard build is fine too.
What matters is the temperature the cylinder body actually reaches. Measure at the mounting position rather than assuming the room average, and compare that against the cylinder’s rated surrounding temperature — not the seal material’s. If it is over, move to a high-temperature build (typically an FKM/Viton seal set with matching grease), and accept a smaller low-temperature range in return: FKM stiffens in the cold, with a dynamic limit around −15 °C.
What to specify when you order
On most China-manufactured cylinders the seal compound is a character in the ordering code, so you choose it rather than accepting a default. On AirTAC cylinders the seal option sits in the part number — the standard build is TPU (thermoplastic polyurethane), with NBR and Viton offered as options — so an equivalent can be ordered with the same compound as the part you are replacing.
- Match the original. If the part you are replacing runs NBR, specify NBR. Do not let a supplier change the seal compound without telling you — along with the port thread and the pressure range, the seal is one of the three things that decide whether a substitution actually works.
- Confirm the code for your series. Ordering codes vary by series and change between catalog revisions. Check the current code with the supplier before you order.
- Ask what the cylinder is rated at, not what the seal is. If a supplier answers a temperature question with the rubber’s maximum, that is the wrong number.
Where each one fails
Useful when you are diagnosing a failure rather than choosing a part.
- NBR — heat ageing hardens it and increases compression set (the seal stays squashed and no longer springs back), so it stops sealing. It is not resistant to ozone and weathering, which causes surface cracking. It also stiffens in the cold, though that effect reverses as it warms up again.
- FKM (Viton) — poor elasticity in the cold, so it stiffens and leaks below its dynamic limit; and it hardens and takes a permanent set in hot water or steam.
- PU / TPU — attacked by hot water and steam. Standard polyurethane is ester-based, which breaks down in hot water; if your application is warm and wet, ask for a hydrolysis-stabilised grade, which roughly raises the water limit from about 50 °C to about 80 °C. Above its temperature limit polyurethane also softens, and can be squeezed out into the clearance gap — the small space between the piston and the tube wall — instead of sealing.
A note on lubrication
Seals need a lubricating film to survive sliding contact — but that does notmean adding oil to your air. Most modern pneumatic cylinders are greased for life at the factory and specified “non-lube”: they need no lubrication in service.
The trap runs in both directions. Adding air-line oil to a non-lube cylinder washes out the factory grease — and once you start, you cannot stop, because the original lubricant is gone. Equally, if a cylinder was designed for lubricated air, running it dry is a wear problem. Check which the datasheet specifies, and stay with it.
Frequently asked
Can I use a Viton (FKM) cylinder at 150 °C because Viton handles 200 °C?
No — not unless the cylinder itself is rated for it. The seal compound is only one part. The assembled cylinder is also limited by its grease, its magnets and switches, and its tube surface treatment, so the cylinder's rated temperature band is never wider than the seal material's, and in practice it is almost always narrower. Read the cylinder datasheet's rated range, not the rubber's maximum.
Which seal material should I specify for normal shop air?
NBR (nitrile) is the standard choice for compressed air, including oiled air — it has excellent oil resistance and good wear properties. Polyurethane (PU/TPU) is the other common default and gives longer wear life on clean, dry air. Both are fine for ordinary duty; match whichever the original part used if you are substituting.
When do I need FKM (Viton) instead of NBR?
When the temperature is higher than your cylinder's standard band allows, or when the fluid is chemically aggressive — FKM resists high temperature, ozone, and many solvents far better than NBR. The trade-offs: FKM is poor in the cold, with a dynamic limit around −15 °C, it hardens in hot water and steam, and it costs more.
How do I know what seal a China-manufactured equivalent uses?
It is usually a character in the part number. On AirTAC cylinders the seal option is part of the ordering code — the standard build is TPU, with NBR and Viton available as options — so you specify the compound when you order rather than accepting whatever ships. Confirm the exact code for your series with the supplier before ordering.
My air is clean and dry but the machine sits in a hot room. Does that change the seal?
It can. What matters is the temperature the cylinder body actually reaches, not the temperature of the air inside it. A cylinder next to an oven, a shrink tunnel or a heater can stay above its rated surrounding temperature for the whole work shift even though the compressed air is cool and clean. Measure at the mounting position and compare it against the cylinder's rated range, then move to a high-temperature build if it is over.
Sources
- Parker O-Ring Handbook (ORD 5700) — elastomer properties, temperature ranges, chemical compatibility, and failure modes.
- Trelleborg Sealing Solutions, pneumatic catalogue — pneumatic seal compound temperature ranges and lubricant limits.
- SMC and AirTAC cylinder catalogs and installation manuals — assembled-cylinder rated temperature bands, lubrication requirements, and AirTAC seal ordering codes.
Not sure which seal your application needs?
Send us the part you run today, with your fluid and temperature. We confirm the seal, the rated band, and the rest of the spec — with the sources — before you commit.