NBR and FKM are the two elastomers most often specified for oil seals, O-rings and many other sealing products. Both perform well when used within their capability, so the useful question is not which material is better but which one suits the duty. This comparison is deliberately qualitative: actual ratings depend on the compound, the seal design and the application, and should be confirmed against the supplier's data for the specific product.
The two materials in brief
NBR
NBR (nitrile butadiene rubber, often simply called nitrile) is a widely used general-purpose sealing material for mineral oils and greases. It offers good mechanical properties and wear resistance, is economical, and is readily available in a very wide range of standard seal sizes.
Its main limitations are heat, ozone and weathering. Prolonged exposure to temperatures beyond its capability hardens the material, and outdoor exposure to sunlight and ozone can cause surface cracking unless the compound is formulated to resist it. Some fuels, aggressive chemicals and certain lubricant additives are also outside its scope.
FKM
FKM is a fluoroelastomer (often known by the trade name Viton™). It generally offers better heat and chemical resistance than NBR, and it resists ozone and weathering well. That makes it a common upgrade where NBR seals are hardening early or where the fluid is more aggressive.
It is not a universal answer, though. FKM costs more, standard grades are generally less flexible at low temperatures than NBR, and certain media, such as some ketones, amine-containing fluids and hot water or steam, can be unsuitable for standard grades. As with NBR, there are many FKM compounds and they do not all behave alike.
Side-by-side comparison
| Aspect | NBR | FKM |
|---|---|---|
| Mineral oils and greases | Widely used general-purpose choice | Also suitable; usually chosen when heat or chemistry demands it |
| Heat resistance | Moderate | Generally better than NBR |
| Chemical and fuel resistance | Adequate for many common lubricants | Generally broader, with some exceptions |
| Low-temperature flexibility | Generally good in suitable grades | Generally more limited in standard grades |
| Ozone and weathering | Limited unless specially compounded | Generally good |
| Cost and availability | Economical; very wide size range | Higher cost; common sizes widely available |
Every row carries the same caveat: it depends on the compound, the seal design and the application.
When NBR is the sensible choice
If the seal runs in mineral oil or grease at moderate temperature, in equipment such as general-purpose gearboxes, motors, pumps and hydraulic systems, NBR is usually the starting point. Specifying FKM where NBR is comfortably within its capability adds cost without a matching benefit. NBR can also be the better option where low-temperature flexibility matters more than heat resistance.
When FKM is worth the extra cost
- The seal runs hot, whether from the process, the surroundings or frictional heat at high shaft speed.
- NBR seals removed from the same position are hard, glazed or cracked.
- The fluid includes fuels, synthetic lubricants or chemicals that attack NBR.
- Access is difficult, and the cost of a seal change far outweighs the cost of the seal itself.
Where the duty sits between the two, HNBR is worth considering. It generally improves on NBR for heat, ozone and wear resistance. Our guide to choosing the right seal material covers the wider range of options.
Questions to ask before deciding
- What exactly is the fluid, including the grade and any additives, if known?
- What temperature does the seal see continuously, and at peaks?
- What is the lowest temperature at start-up?
- Is the seal exposed to sunlight, ozone or chemicals on the outside?
- How did the previous seal fail, and what did it look like?
Changing the material does not fix a problem caused by shaft damage, misalignment or poor installation, so examine the failed seal before assuming the compound is at fault.
Talk to us about your application
We supply oil seals and other sealing products in NBR, FKM and a range of other seal materials. If you are unsure which suits your duty, send the shaft, housing and width dimensions, the media, temperature, pressure and speed, and a drawing or sample if available, to our sales engineers through the enquiry form. They will help you select a material and confirm it against the supplier's data.
