| HNBR | FKM (Viton) | |
|---|---|---|
| Continuous service | −30 to 150 °C | −20 to 200 °C+ |
| Petroleum oil and fuel | Excellent | Excellent |
| Hot water and steam | Good, its main advantage | Poor, it degrades |
| Aromatic solvents | Poor | Excellent |
| Ketones, esters, amines | Poor | Poor |
| Low temperature | Fair to good | Fair, brittle early |
| Compression set | Good | Excellent |
| Contains fluorine (PFAS) | No | Yes |
| Relative cost | Moderate | High |
The verdict
This is rarely a performance question. FKM is the broader material: wider chemical resistance, continuous service to 200 °C and beyond, and the lowest compression set of anything in general use. If your part genuinely lives in that envelope, specify FKM and stop reading.
The problem is that most parts do not live there. A large share of FKM specified in the field is running at 120 to 140 °C in oil, which HNBR handles comfortably at a fraction of the price. Where HNBR wins outright rather than on cost is hot water and steam, which attacks FKM, and where the two are closest is exactly where the price difference is hardest to justify.
Cold service tilts toward HNBR in most compounds, though both can be formulated down. Neither survives ketones, esters, or amines, so if that is your media the answer is AFLAS or a perfluoroelastomer, not either of these.
One factor that did not exist a few years ago now decides some programs on its own: FKM contains fluorine and falls inside the PFAS restrictions in progress, and HNBR does not. For teams asked to remove fluorine from an assembly, HNBR is the strongest non-fluorinated candidate for hot-wet sealing.
Choose HNBR when
- Service tops out around 150 °C rather than 200 °C
- Hot water, steam, or a water-glycol mix is in the picture
- Cost per part is under real pressure and the duty allows it
- The program needs fluorine out of the assembly
- The part is dynamic in hot oil, where HNBR's abrasion resistance helps
Choose FKM when
- Continuous service is above 150 °C
- Aromatic solvents, fuels with high aromatic content, or aggressive process chemistry are present
- Compression set over long static life is the governing requirement
- An existing qualification or customer specification names it
If your duty sits on the line, that is what testing is for. Both families are compounded in-house and aged in your media at your temperature in our accredited lab, which resolves the question with data rather than a datasheet.