Overview
A note on names: Viton is a Chemours trade name for FKM, and Buna N is nitrile, NBR. Buna S is SBR, a different material entirely, so the term is worth pinning down before a quote goes out.
| Buna N (NBR) | Viton (FKM) | |
|---|---|---|
| Continuous service | −30 to 120 °C | −20 to 200 °C+ |
| Petroleum oil, fuel, grease | Excellent | Excellent |
| Aromatic and chlorinated solvents | Fair to poor | Excellent |
| Ozone, sunlight, weathering | Poor | Outstanding |
| Hot water and steam | Fair | Poor |
| Low temperature | Fair to good | Fair |
| Abrasion and tear | Good | Fair |
| Contains fluorine (PFAS) | No | Yes |
| Relative cost | Low | High |
The verdict
On oil alone, these two are close enough that price decides. Nitrile is the least expensive good oil resistance available, and it is why most sealing hardware in the world is nitrile. If a part sees petroleum oil at moderate temperature and nothing else, Viton is money spent on capability the part will never use.
Three things move the answer to FKM. Temperature is the first and clearest: nitrile is done around 120 °C, and FKM keeps going past 200 °C. Chemistry is the second: aromatic and chlorinated solvents, and aggressive process fluids, pass straight through nitrile. Outdoor and ozone exposure is the third, and it catches people out, because nitrile cracks in sunlight and ozone while FKM is essentially immune.
Nitrile keeps two real advantages beyond cost. It is tougher mechanically, with better abrasion and tear resistance, which matters for dynamic seals and rough installation. And it contains no fluorine, so it sits outside the PFAS restrictions now in progress.
Choose Buna N when
- The media is petroleum oil, fuel, grease, or hydraulic fluid
- Service stays under about 120 °C
- The part is indoors or otherwise protected from sunlight and ozone
- Cost per part matters, or volumes are high
- The seal is dynamic and takes abrasion
Choose Viton when
- Service runs above 120 °C, continuously or in excursions
- Aromatic solvents, chlorinated hydrocarbons, or aggressive chemistry are present
- The part is outdoors, or ozone exposure is unavoidable
- Very low compression set over long static life is required
- A customer specification or existing qualification names FKM
Between the two sits HNBR, which keeps nitrile's oil resistance and raises the ceiling to about 150 °C while adding steam resistance, without fluorine. It is worth pricing before jumping the full distance to a fluoroelastomer.