What is the Difference Between FKM and NBR Seals?
FKM (Fluorocarbon Elastomer) and NBR (Nitrile Butadiene Rubber) seals are widely used in industrial and automotive applications, but they differ significantly in material properties, performance, and suitability for specific environments. This comparison highlights their key distinctions to guide engineers and designers in selecting the optimal sealing solution.
1. Material Composition
FKM (Fluoroelastomer / Viton®)
- Chemical Structure: A synthetic rubber with a fluorine-carbon backbone, where fluorine atoms replace hydrogen atoms. This structure provides exceptional thermal stability and chemical resistance.
- Standards:
- ASTM designation: FKM (Fluorocarbon Elastomer).
- ISO equivalent: FPM (Fluorinated Polymer).
- Trade Names: Viton®, Fluorel™, Dai-El™.
NBR (Nitrile Rubber / Buna-N)
- Chemical Structure: A copolymer of acrylonitrile and butadiene, offering strong oil resistance and mechanical flexibility.
- Standards:
- ASTM designation: NBR.
- ISO equivalent: N (Nitrile).
2. Temperature Resistance
| Property | FKM | NBR |
| Continuous Range | -26°C to +205°C | -30°C to +110°C |
| Short-Term Peak | Up to +230°C (specialized grades) | Up to +130°C |
| Low-Temperature Behavior | Stiffens below -20°C (unless modified) | Retains flexibility down to -35°C |
| Thermal Aging | Retains >70% tensile strength at 200°C/1,000h | Rapid degradation above +110°C |
3. Chemical Compatibility
| Exposure | FKM | NBR |
| Hydrocarbons | Resists gasoline, diesel, biodiesel, and synthetic oils | Resists mineral oils; swells in biodiesel and ethanol blends |
| Acids & Solvents | Good resistance to sulfuric/nitric acids and aromatic solvents | Poor resistance to acids, ketones, and chlorinated solvents |
| Polar Fluids | Incompatible with ketones, amines, and esters | Better compatibility with glycol-based brake fluids |
| Ozone/UV Resistance | Immune to ozone and UV degradation | Susceptible to ozone cracking and weathering |
4. Mechanical Properties
| Property | FKM | NBR |
| Hardness (Shore A) | 55–95 (typically ~75) | 40–90 (typically ~70) |
| Tensile Strength | ~12–20 MPa | ~10–30 MPa |
| Compression Set | ~11% at 175°C (22h) | ~25% at 100°C (22h) |
| Abrasion Resistance | Moderate; suitable for dynamic applications | Good; widely used in rotary shaft seals |
5. Cost and Availability
- FKM:
- Cost: ~3.9× more expensive than NBR due to complex fluorine chemistry.
- Processing: Requires high-temperature molding and tight quality controls.
- NBR:
- Cost: Economical (~1× baseline cost).
- Processing: Easier to mold and extrude; widely available.
6. Applications
| Industry | FKM Applications | NBR Applications |
| Automotive | Turbocharger seals, fuel injectors, exhaust systems | Engine gaskets, coolant hoses, axle seals |
| Aerospace | Jet engine seals, hydraulic systems | Non-fuel hydraulic systems |
| Industrial | Chemical pumps, high-temperature valves | Conveyor belts, general-purpose oil seals |
| Food/Pharma | FDA-compliant grades for steam sterilization | Rarely used due to limited chemical resistance |
7. Key Advantages and Limitations
FKM Advantages:
- Extreme heat resistance (up to +230°C).
- Universal resistance to fuels, oils, acids, and ozone.
- Low compression set for long-term sealing.
FKM Limitations:
- High cost.
- Poor low-temperature flexibility without specialized grades.
- Incompatible with polar solvents (e.g., ketones).
NBR Advantages:
- Cost-effective for non-extreme environments.
- Excellent low-temperature flexibility (down to -55°C).
- Strong abrasion resistance.
NBR Limitations:
- Degrades rapidly above +110°C.
- Swells in biodiesel and ethanol blends.
- Susceptible to ozone and UV damage.
8. Head-to-Head Comparison Table
| Factor | FKM | NBR |
| Cost | High (~3.9× NBR) | Low (Baseline) |
| Max Temperature | +205°C (continuous), +230°C (peak) | +110°C (continuous), +130°C (peak) |
| Min Temperature | -20°C (standard), -40°C (special grades) | -35°C (standard), -55°C (special grades) |
| Fuel Resistance | Excellent (gasoline, biodiesel) | Poor (swells in biofuels) |
| Compression Set | ~11% (175°C/22h) | ~25% (100°C/22h) |
| Lifespan in Harsh Conditions | >5,000 hours at 200°C | Rapid degradation under heat/chemicals |
When to Choose FKM?
- High-Temperature Environments: Turbochargers, exhaust systems, or industrial ovens.
- Aggressive Chemicals: Exposure to fuels, acids, or aromatic solvents.
- Critical Systems: Aerospace hydraulics or chemical processing equipment requiring long-term reliability.
When to Choose NBR?
- Budget Constraints: Non-critical applications with moderate temperatures.
- Low-Temperature Flexibility: Cold climates or refrigeration systems.
- General-Purpose Sealing: Hydraulic cylinders, engine oil seals, or water-glycol systems.
Conclusion
FKM and NBR seals serve distinct roles based on operational demands. FKM excels in extreme thermal and chemical environments but comes at a premium cost. NBR is a versatile, cost-effective solution for moderate conditions but falters under high heat or aggressive media. Engineers must prioritize temperature ranges, chemical exposure, and budget to select the optimal material. By understanding these differences, industries can enhance equipment reliability, reduce downtime, and optimize lifecycle costs.
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