The short answer

NBR (nitrile, Buna-N) is the low-cost default for petroleum oils, fuels and hydraulic fluid at moderate temperatures. FKM (fluoroelastomer, often called Viton™) costs more but handles far higher temperatures and a much wider range of aggressive chemicals. If your seal runs hot, sees fuel vapour, or contacts solvents that swell nitrile, FKM is usually the safer choice. If it is a general-purpose oil or hydraulic seal near room temperature, NBR is hard to beat on price.

Temperature range

Temperature is the first filter, because a material that hardens or softens outside its useful band will leak regardless of how well the groove is designed.

Property NBR (nitrile) FKM (fluoroelastomer)
Typical continuous range −35 °C to +120 °C −20 °C to +200 °C
Low-temperature limit Good; special grades to −55 °C Poor; special grades to −40 °C
High-temperature limit Hardens and takes a set above ~120 °C Excellent to 200 °C, short excursions higher

NBR’s weakness is heat: above roughly 120 °C it embrittles and its compression set climbs quickly. FKM’s weakness is cold: standard grades stiffen well before −20 °C, so a cold-start or refrigeration seal in FKM needs a low-temperature grade.

Chemical and fluid resistance

This is where the two materials diverge the most.

NBR resists well: mineral and petroleum-based hydraulic oil, diesel and gasoline, greases, water and many dilute acids and bases. It is the workhorse of hydraulics and fuel systems.

NBR struggles with: ozone, weathering and sunlight (it cracks), ketones, esters, strong acids, brake fluid, and high-aromatic fuels.

FKM resists well: a very broad set — mineral oils, fuels (including high-aromatic and oxygenated blends), many solvents, strong acids, and hot air. It is the standard choice for engine, fuel-system and chemical-plant sealing.

FKM struggles with: hot water and steam, low-molecular-weight ketones and esters, amines, and glycol-based brake fluid. For steam or hot water, EPDM is usually the correct answer instead of either material.

Always confirm the specific fluid against a compatibility chart, because a single incompatible medium can swell or embrittle an otherwise ideal elastomer.

Cost and availability

NBR is one of the least expensive sealing elastomers and is stocked in almost every catalogue size. FKM typically costs several times more per ring and is heavier, but the price gap is usually small next to the cost of an unplanned seal failure. For high-volume, low-duty applications the NBR saving matters; for critical or hot service the FKM premium is easily justified.

A quick selection guide

  • Hydraulic cylinder, mineral oil, ≤100 °C → NBR.
  • Engine or transmission oil, >120 °C → FKM.
  • Fuel system, modern gasoline or diesel → FKM (or a specific fuel-grade NBR for older, low-aromatic fuel).
  • Ozone, outdoor or weathering exposure → FKM or EPDM, not standard NBR.
  • Hot water, steam or brake fluid → neither; use EPDM.
  • Aggressive chemicals or high heat combined → FKM, and check FFKM if FKM is still marginal.

How hardness and squeeze still apply

Choosing the polymer is only half of the decision. Whichever material you pick, the ring still has to be sized for the right squeeze and gland fill so it seals across the full tolerance stack and temperature range. Use the radial static seal calculator to check compression and fill for your chosen cord size, then confirm the material temperature band covers your duty.

Frequently asked questions

Is Viton the same as FKM? Viton™ is a Chemours brand name for a family of FKM fluoroelastomers. “FKM” is the generic ASTM designation, so the terms are often used interchangeably, though not every FKM is Viton-branded.

Can FKM replace NBR everywhere? Almost, on chemical and heat resistance — but not in the cold, and not in hot water, steam or brake fluid. Check low-temperature limits and the specific media before substituting.

Which lasts longer? In hot or chemically aggressive service, FKM lasts far longer because it resists hardening and compression set. In mild oil service near room temperature, both can last for years, so NBR’s lower cost wins.

Explore the FKM and NBR material pages for temperature bands and Parker compound letters, and use the calculators to size the groove once the material is chosen.