FKM O-Rings (Viton / FPM Fluoroelastomer)
Premium fluorocarbon sealing for high-temperature, aggressive chemical and fuel system applications where standard elastomers fail.
Overview
FKM (Fluorocarbon Rubber), also known as FPM in ISO nomenclature and widely known by the DuPont trade name Viton, is the premium fluoroelastomer for demanding sealing environments. FKM, FPM and fluoroelastomer are three names for the same polymer family — FKM is the ASTM designation, FPM is the ISO/DIN designation, and fluoroelastomer is the generic chemical term. Its outstanding resistance to high temperatures, aggressive chemicals, fuels and oils makes it the material of choice in aerospace, chemical processing, oil and gas, and automotive applications where NBR would rapidly degrade.
FKM O-rings — also searched as FPM O-rings, fluoroelastomer O-rings, and Viton 75 O-rings — maintain sealing integrity up to +200°C in continuous service, approximately 80°C higher than standard NBR, and retain flexibility down to -20°C. Fluorine content of 65–70% by weight provides exceptional resistance to petroleum hydrocarbons, aromatic fuels, chlorinated solvents, and most industrial chemicals.
FKM is available in four distinct grades, each optimised for different service conditions: - **Standard FKM (66% fluorine, Type 1/Type 2):** General chemical and hydrocarbon service to +200°C. The most cost-effective FKM grade and the default for most applications. - **Low-Temperature FKM (GF grade, 68% fluorine):** Service to -40°C without loss of flexibility. Required for outdoor and cold-climate equipment. - **Base-Resistant FKM (GBL grade, 66–68% fluorine):** Improved resistance to amines, caustic soda, and steam versus standard FKM. Not a substitute for AFLAS in severe alkaline or steam service, but extends FKM's range. - **High-Fluorine FKM (70–71% fluorine, Type 3/GFLT):** Maximum chemical resistance including ketones and concentrated acids; reduced low-temperature performance.
Compression set (ASTM D395 Method B, 70 h/200°C) is ≤ 20%, ensuring long-term sealing force in static applications. Lead time: 7–15 days standard; 3–5 days for stocked compounds. MOQ: 1 piece. ISO 9001 certified.
The chemical stability of FKM originates from its molecular architecture: a copolymer or terpolymer of vinylidene fluoride (VDF), hexafluoropropylene (HFP), and optionally tetrafluoroethylene (TFE). The high electronegativity of fluorine atoms creates a dense, protective electron cloud around the carbon backbone. The carbon-fluorine bond energy is approximately 485 kJ/mol, compared to 410 kJ/mol for C-H bonds in NBR — this 18% higher bond energy directly translates to thermal and chemical stability. Each 1% increase in fluorine content raises the resistance to aromatic hydrocarbons by approximately 5–8°C but simultaneously increases the glass transition temperature (Tg) by roughly 2°C, degrading low-temperature flexibility. Standard Type 1 FKM (66% F) uses VDF-HFP dipolymerization; Type 2 (68% F) adds TFE; Type 3/GFLT (70–71% F) employs a terpolymer with higher TFE ratio. Curing is typically achieved with bisphenol-AF (for Type 1/2) or peroxides (for GFLT), producing crosslinks that resist thermal scission up to 220°C.
Quantified performance differentials versus NBR explain FKM's value proposition in elevated-temperature service. At 150°C in IRM 903 oil (ASTM D471, 70 h), standard FKM shows volume swell of 3–8% versus NBR's 25–35% — a 3–4× improvement in dimensional stability. Compression set at 200°C (ASTM D395 Method B, 70 h) for FKM is ≤ 20%, while NBR at the same temperature would exceed 60% and fail within days. Gas permeability data is equally compelling: FKM's permeability to oxygen is approximately 10–30× lower than NBR, and to automotive fuel vapors it is 50–100× lower — this is why FKM is mandatory for evaporative emissions control seals in modern vehicles. In dynamic seal testing (DIN 3761), FKM rotary shaft seals operate at surface speeds up to 25 m/s at 150°C, whereas NBR is limited to 12–15 m/s at 100°C before thermal degradation accelerates wear.
A practical grade-selection decision tree: if your application is a general hydraulic or fuel system operating between -20°C and +150°C with petroleum oils or non-aromatic fuels, standard Type 1 or Type 2 FKM (66–68% F, 75 Shore A) is the correct default. If your application must start reliably at -40°C in cold-climate automotive or aerospace service, specify GF low-temperature grade (68% F) — verify that the TR10 of -40°C meets your cold-start compression load requirements. If your application involves amine-based corrosion inhibitors, caustic soda washdown, or mild steam up to 160°C, specify GBL base-resistant grade. If your application involves concentrated acids, some ketone exposure, or the most aggressive mixed chemical environments, specify high-fluorine Type 3/GFLT (70–71% F) and accept the degraded low-temperature limit of approximately -10°C. For continuous steam above 170°C or strong amines, step up to AFLAS or FFKM regardless of FKM grade.
Storage of FKM O-rings requires attention to the cure system. Bisphenol-cured FKM (standard grades) is shelf-stable for 7–10 years when stored below 25°C in opaque packaging away from amines and strong bases, which can catalyze premature crosslink degradation. Peroxide-cured GFLT grades have a shorter optimal shelf life of 5–7 years because residual peroxide radicals can slowly advance the cure state, increasing hardness by 3–5 Shore A over time. FKM is not susceptible to ozone cracking due to its highly fluorinated, saturated backbone — a significant storage advantage over NBR. However, prolonged exposure to UV light causes surface oxidation and slight brown discoloration without significantly affecting mechanical properties. A common error is storing FKM O-rings in the same inventory bin as NBR seals; while not chemically reactive, mixing inventory increases the risk of material misidentification — FKM and NBR are visually similar in black 70 Shore A, but specific gravity (1.80–1.95 for FKM vs 1.00–1.25 for NBR) provides an immediate field distinction.
Material Properties
| Temperature Range | -20°C to +200°C (-4°F to +392°F); GF grade to -40°C |
| Hardness Range | 60–90 Shore A (ASTM D2240) |
| Tensile Strength | 10–16 MPa (ASTM D412) |
| Elongation at Break | 150–250% (ASTM D412) |
| Compression Set (70 h/200°C) | ≤ 20% (ASTM D395 Method B) |
| Fluorine Content | 65–71% by weight (grade dependent) |
| Specific Gravity | 1.80–1.95 |
| Color (standard) | Black or brown |
| Standards | ASTM D2000 HK, SAE AMS7276, AMS3276 |
Typical Applications
Chemical Processing
Pump and valve seals in hydrocarbon solvents, aromatic chemical service, chlorinated solvent handling, and reactor vessel seals above +120°C.
Oil & Gas
Downhole tool seals, wellhead and Christmas tree seals in sour gas service with dry H₂S. For high-amine environments, AFLAS should be considered.
Aerospace
Hydraulic system seals, fuel system O-rings in aromatic jet fuel, and engine component sealing to AMS3276/AMS7276 specifications.
Automotive
Fuel injector O-rings, turbocharger seals, and high-temperature engine seals where operating temperatures exceed +120°C.
Industrial
High-temperature autoclave seals (dry cycle), heat exchanger seals, and continuous service above +150°C in aggressive chemical environments.
Chemical Compatibility Summary
Compatible With
- - Mineral oils and greases
- - Synthetic hydraulic fluids
- - Gasoline and diesel fuels
- - Aromatic hydrocarbons
- - Chlorinated hydrocarbons
Incompatible With
- - Hot water and steam
- - Polar solvents (ketones, esters)
- - Low molecular weight esters
- - Amines
- - Skydrol (phosphate ester hydraulic fluids)
Compare Nearby Materials
Further Reading
FKM vs FFKM
The cost-performance boundary between mainstream fluorocarbon and premium perfluoroelastomer.
Read articleKey Advantages
Superior High-Temperature Performance
Continuous service to +200°C with short-term excursions to +230°C. FKM retains sealing force and elasticity at temperatures that cause NBR and EPDM to harden, crack, and fail. Compression set ≤ 20% at 200°C ensures long-term clamping force.
Broad Chemical Resistance
65–70% fluorine content provides exceptional resistance to petroleum oils, aromatic and aliphatic fuels, hydraulic fluids, chlorinated solvents, and most industrial chemicals where NBR fails within hours.
Low Gas Permeability
FKM has exceptionally low gas and vapor permeability — 10–100× lower than NBR — making it the material of choice for containment sealing in chemical and gas systems where diffusion through the seal material is a concern.
Long Service Life
Excellent ozone, UV, and weathering resistance combined with low compression set help FKM seals maintain sealing force for 5–10 year service intervals in static chemical applications.
Quantifiably Superior Oil Stability at Elevated Temperature
ASTM D471 immersion in IRM 903 oil at 150°C for 70 hours shows FKM volume swell of only 3–8%, compared to 25–35% for NBR and 15–25% for HNBR. At 200°C, FKM compression set remains ≤ 20%, while NBR exceeds 60% within days. This dimensional stability eliminates the leakage progression common in hot oil service with non-fluorinated elastomers.
Aerospace-Grade Material Traceability and Certification
FKM compounds are available with full aerospace material certification including SAE AMS3216, AMS-R-83485, and AMS7276 compliance. Each production lot is traceable to the polymer batch, with certificates of conformance documenting hardness, tensile strength, elongation, compression set, and fluid immersion data. NADCAP-accredited testing is available for flight-critical applications.
Frequently Asked Questions - FKM
What is the difference between FKM and Viton?
Viton is a registered trademark of Chemours (formerly DuPont Performance Elastomers) for a range of fluorocarbon elastomer compounds. FKM is the ISO 1629 and ASTM D1418 generic material designation for the polymer family. All Viton products are FKM, but FKM O-rings from other manufacturers (Daikin, 3M Dyneon, Solvay) are not Viton-branded — they are chemically equivalent fluorocarbon elastomers. When specifying FKM, define compound by fluorine content and grade (standard, GF, GBL) rather than brand name to ensure cross-supplier compatibility.
Can FKM O-rings be used with steam?
No, FKM O-rings are not recommended for steam service. FKM has limited steam resistance due to dehydrofluorination — high-temperature steam and hot water attack the fluorocarbon polymer, causing hardening, cracking, and loss of sealing force. The practical steam limit for standard FKM is approximately 120–150°C for short-term exposure only. For continuous steam service up to 200°C, EPDM or AFLAS are the correct materials. For steam above 200°C, FFKM or PTFE seals are required.
What chemicals attack FKM O-rings?
FKM has four critical chemical weaknesses: (1) Ketones — acetone, MEK, MIBK cause severe swelling and softening even at room temperature; (2) Low-molecular-weight esters — ethyl acetate, methyl acetate; (3) Amines and strong alkalis — cause dehydrofluorination, the same mechanism as steam attack; (4) Hot concentrated nitric acid (>70%) and fuming sulfuric acid. For ketone service, FFKM is required. For amine/alkali/steam environments, AFLAS is the more cost-effective solution than FFKM.
What are the different FKM grades and when do I need them?
Standard FKM (66% fluorine): adequate for most hydrocarbon, fuel, and chemical service to +200°C — use this as the default. Low-temperature GF grade (68% fluorine): specify when cold-start sealing below -20°C is required (outdoor equipment, cold-climate installation); extends flexibility to -40°C. Base-resistant GBL grade: improved performance in amines, caustic, and steam versus standard FKM — use when FKM is preferred over AFLAS for mild alkaline conditions. High-fluorine Type 3 (70–71%): maximum chemical resistance, including some ketone resistance improvement, but worse low-temperature performance — specify for the most aggressive mixed chemical environments. Contact us with your specific media and temperature for compound recommendation.
Is FKM available in low-temperature grades?
Yes. Standard FKM has a low-temperature flexibility limit (TR10, ASTM D1329) of approximately -20°C to -25°C. Low-temperature FKM (GF grade) extends this to -40°C, suitable for outdoor equipment in cold climates. Note that even GF-grade FKM is stiffer at -40°C than EPDM or VMQ at the same temperature — cold-start compression force must be verified for dynamic applications. For sealing below -40°C, VMQ (to -60°C) or FVMQ (to -60°C) are required.
Why are FKM O-rings more expensive than NBR?
FKM compounds are synthesised from fluorinated monomers — vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene — which cost 10–30× more than the butadiene and acrylonitrile monomers used in NBR. The total FKM O-ring cost is typically 3–8× the equivalent NBR price depending on size and compound. The business case for FKM is lifecycle cost: if an NBR seal requires replacement every 6 months in a hot chemical environment and an FKM seal lasts 5+ years, the labor and downtime savings far exceed the material cost premium.
When should I choose AFLAS or HNBR instead of FKM?
Choose AFLAS instead of FKM when the service involves steam above 150°C, amines (MEA, DEA, MDEA), or concentrated caustic soda — media that attack FKM by dehydrofluorination. AFLAS is typically comparable in cost to FKM and significantly outperforms it in these media. Choose HNBR instead of FKM when the application is petroleum oil or fuel service at temperatures between +120°C and +150°C and budget is constrained — HNBR costs 30–50% less than FKM in this temperature range. Use FKM when: temperature exceeds +150°C in petroleum service, aromatic fuel content is high (>10%), or the chemical mix includes both hydrocarbons and moderate acid exposure.
Do your FKM compounds carry automotive IATF 16949 or aerospace AMS certifications?
Yes. Our standard FKM compounds are manufactured under ISO 9001, and selected grades are produced at IATF 16949-certified facilities for automotive applications. For aerospace, we supply FKM O-rings certified to SAE AMS3216 (standard FKM, 75 Shore A), AMS-R-83485 (low-temperature FKM), and AMS7276 (high-temperature fuel system FKM). Each aerospace lot includes a Certificate of Conformance citing the AMS specification, batch test data (hardness, tensile, elongation, compression set, fluid immersion per the AMS test protocol), and full material traceability. PPAP submission (Levels 1–5) is available for automotive OEM programs.
How do you ensure batch consistency for FKM compounds?
FKM batch consistency is ensured through four control stages: (1) Monomer and polymer feedstock testing — every fluoropolymer lot is tested for Mooney viscosity (ML 1+10 at 121°C), fluorine content by combustion ion chromatography (target ±0.5%), and molecular weight distribution. (2) Compounding control — carbon black and curative batches are weighed to ±0.1% on validated scales; mixed compound is tested for Mooney viscosity, cure rheometry (MDR tS2, t90, MH-ML), and specific gravity. (3) Press-cured slab testing — every production lot is tested for hardness (±3 Shore A), tensile strength (±10%), elongation (±15%), compression set at 200°C (≤ 20%), and ASTM D471 fluid immersion in IRM 903 at 150°C (volume change 3–8%). (4) Statistical process control — hardness and compression set are tracked on control charts with Cpk targets ≥ 1.33. Out-of-spec batches are rejected before O-ring moulding.
Is FKM from different suppliers interchangeable in the same groove?
Dimensionally, yes — all AS568 and ISO 3601 FKM O-rings from reputable suppliers are physically interchangeable in the same groove. Material equivalence requires closer scrutiny: specify the same fluorine content range (66%, 68%, or 70–71%), the same cure system (bisphenol vs peroxide), and the same hardness (±5 Shore A). Bisphenol-cured standard FKM from Chemours (Viton A), Daikin (Dai-El), and Solvay (Tecnoflon) are broadly interchangeable for general service. However, low-temperature GF grades vary more significantly in TR10 and low-temperature compression set — switching between suppliers for GF-grade aerospace or automotive seals requires side-by-side ASTM D1329 and D395 low-temperature data comparison. Always request the supplier's compound data sheet and verify that the listed test values fall within your application's acceptable range.
What color conventions are used for FKM O-rings, and can I order custom colors?
Standard FKM O-rings are black or brown. Brown is the traditional color for Viton-branded compounds and is sometimes used to distinguish FKM from NBR in mixed inventory. Black FKM is visually indistinguishable from black NBR — specific gravity measurement (FKM 1.80–1.95 vs NBR 1.00–1.25) is the fastest field identification method. Custom colors including green, blue, red, and orange are available for FKM at a 15–25% price premium and 1–2 week additional lead time. Color coding is commonly used in chemical plants and refineries to prevent material mix-up: for example, brown = standard FKM, green = GF low-temp grade, blue = GBL base-resistant grade. Custom color requests should specify the Pantone or RAL reference and the minimum order quantity, which is typically 500 pieces for custom-pigmented compounds.
What is the environmental and sustainability profile of FKM?
FKM production involves fluorinated monomers and curing agents that require careful environmental management. Our manufacturing partners operate with zero liquid discharge for fluorochemical waste and full VOC capture systems. FKM compounds are REACH and RoHS compliant and do not contain intentionally added PFAS below regulatory thresholds for articles. From a lifecycle perspective, FKM's extended service life (5–10 years in harsh chemical environments versus 6–18 months for NBR) significantly reduces total material consumption and replacement waste. FKM is a thermoset elastomer and cannot be remelted, but post-industrial flash and trim can be reclaimed as filler in lower-performance compounds. We are actively evaluating bio-based and recycled-content FKM alternatives as they become commercially viable, and we provide environmental product declarations (EPDs) on request for customers with sustainability reporting requirements.