FKM O-rings: the proven standard for demanding applications
FKM O-rings are among the most widely used high-performance seals: they offer excellent resistance to oils, fuels, and many acids, and can be used continuously from −25 to +200 °C; the FKM EX40 grade can be used from −40 to +225 °C. Where NBR or EPDM fail due to temperature or the medium, FKM is the cost-effective solution.
- FKM (fluorinated rubber) offers excellent resistance to mineral oils, fuels, many acids, and ozone: It is listed in our tool for 250 out of 430 media.
- Continuous operation from −25 to +200 °C; with FKM EX40, from −40 to +225 °C.
- Fully elastic; can be used as a flange, piston, or rod seal.
- Limitations apply to hot water, steam above 120 °C, glycol-based brake fluid, ketones, esters, and amines.
- The following NH O-RINGs are in stock and available without a minimum order quantity: FKM 75, FKM 75 FDA, and FKM EX40.
What is FKM?
FKM is the abbreviation for fluorocarbon rubber, also known as fluoroelastomer, and is standardized according to ASTM D1418. In Europe, the designation FPM was also commonly used in accordance with ISO 1629. FKM seals, such as O-rings, are among the most important high-performance seals in industry, automotive manufacturing, and the chemical industry.
The key lies in fluorine. The carbon-fluorine bond is one of the most stable bonds in organic chemistry and makes FKM resistant to heat, oils, fuels, and many aggressive media. Unlike fully fluorinated FFKM, however, FKM still contains hydrogen in its chain. Hot water, steam, amines, and hot alkalis attack the material at these points. The diagram at the top shows the structure; for more information on fully fluorinated FFKM, see the article “FFKM O-Rings.”
Key Features
Media outlets have rated our FKM tool as "excellent," while 98 have rated it as "not recommended."
With FKM 75, FKM 75 FDA, and FKM EX40, from −40 to +225 °C.
Unlike PTFE or FEP, it is therefore used as a flange, piston, and rod seal.
Depending on the mixture, 80 Shore A is available upon request. Learn more about Shore hardness.
With FKM 75, FDA-approved, EC 1935/2004 for all mixtures.
You can test specific media using the Media Compatibility Tool. FKM offers an excellent balance of resistance, temperature, and price, and covers a wide range of industrial applications. If the temperature or medium exceeds these limits, FFKM is the next step.
Comparison of Temperature Ranges
FKM covers a wide temperature range: FKM 75 and FKM 75 FDA operate continuously from −25 to +200 °C, while FKM EX40 operates from −40 to +225 °C. The chart places FKM between common elastomers and high-performance materials:
The article “O-Ring Materials” provides a direct comparison of all materials; for more information on heat resistance, see “Heat-Resistant O-Rings.”
Price Levels in Comparison
FKM offers durability at a moderate price. The levels provide a rough indication of how the materials compare:
FKM outperforms NBR and EPDM by a wide margin, but falls significantly short of PTFE, FEP-coated O-rings, and FFKM. Therefore, FKM is the best choice in applications where NBR or EPDM are no longer sufficient, but FFKM is not yet necessary.
What is FKM resistant to, and what are its limitations?
FKM owes its durability to its high fluorine content. Our tool uses ten typical media to illustrate where FKM excels and where it does not:
Our data sheets summarize it as follows:
- mineral oils and greases
- Fuels, including those containing aromatics
- aliphatic, aromatic, and chlorinated hydrocarbons
- many acids
- Ozone, weathering, and UV
- Ketones, esters, and ethers
- Amines
- Hot water
- Water vapor above 120 °C; use EPDM here
- Glycol-based brake fluids
- Hot alkalis
Hot Water and Steam
In hot water and steam, the temperature limit for FKM is significantly lower than in air. For steam, EPDM (peroxide-cured) is generally the better choice; for higher temperatures, FFKM is recommended:
| Material | Hot water | Steam |
|---|---|---|
| FKM | Not recommended | up to 120 °C |
| Peroxide-cured EPDM, Comparison | up to 170 °C | up to 170 °C |
| FEP-coated | up to 205 °C | up to 205 °C |
| ECOLAST NH5755HTFFKM | up to 270 °C | up to 270 °C |
Based on NH O-RING's experience; no liability assumed; test in individual cases. Comparison of all materials: O-rings for hot water.
FKM Types: Copolymer, Terpolymer, and Low-Temperature
Not all FKM is the same. Durability, low-temperature flexibility, and processability can be specifically adjusted by varying the fluorine content and cross-linking. It is common to classify FKM based on the polymer’s structure:
| Type | Structure | Characteristic |
|---|---|---|
| Copolymer Type A | two monomers, approx. 66% fluorine | the proven standard with a good overall profile |
| Type B terpolymer | three monomers, approx. 68% fluorine | Improved resistance to media and temperature |
| High-Fluoride Type F | Approx. 70% fluorine | Excellent resistance to fuels and methanol |
| Cryogenic GLT, GFLT | with perfluoroether | flexible down to approx. −40 °C |
Then there is the cross-linking system. Bisphenol-cross-linked grades are the cost-effective standard with good compression set. Peroxide-cured types are more resistant to water, steam, and acids. Based on our experience, however, the following applies to our FKM compounds: hot water is not recommended; steam up to 120 °C. For low temperatures down to −40 °C, we offer FKM EX40.
FKM vs. FFKM: A Direct Comparison
FKM and FFKM sound similar, but they are worlds apart in terms of durability, temperature resistance, and price:
| Characteristic | FKM | FFKM |
|---|---|---|
| Fluorination | Partial, with hydrogen | completely |
| Media:excellentTool, 430 media items | 250 (58%) | 407 (95%) |
| Not recommended | 98 media | 1 medium, elemental fluorine |
| Air Temperature | −25/+200 °C, FKM EX40 −40/+225 °C | −40/+340 °C depending on the type |
| Hot Water and Steam | Hot water not recommended; steam up to 120 °C | up to 210–270 °C depending on the model |
| Price Level | medium | very high |
| Typical Applications | Industry, automotive manufacturing, oils, fuels | Chemicals, pharmaceuticals, semiconductors, steam |
For most industrial applications, FKM is the cost-effective choice. FFKM is used in situations where FKM cannot withstand the temperature or the medium. For more information, see the article “FFKM O-Rings.”
FKM, Viton, and other brands
FKM is the material designation according to ASTM D1418. In addition, brand names used by raw material manufacturers are commonly found on the market:
Viton is therefore not a material in its own right, but rather a brand name for FKM. We supply FKM O-rings in certified quality, from stock, and with no minimum order quantity.
Our FKM Compounds
Our FKM product line covers all common application requirements, from cost-effective standard grades to "Extreme-Type" grades with an extended temperature range. Select your requirement:
FKM with a Shore A hardness of 80 is available upon request. All test results are listed in the data sheets; you can find an overview under " O-Ring Data Sheets."
Technical Specifications at a Glance
The values are taken from the FKM data sheets and have been tested in accordance with ASTM. The hardness is given as a rounded nominal value:
| Compound | Shore A hardness | Tensile Strength MPa | Elongation at Break % | DVR % | TR-10 °C | Density g/cm³ |
|---|---|---|---|---|---|---|
| FKM 75 | 75 | 14.9 | 183 | 22 | −17 | 1.85 |
| FKM 75 FDA | 75 | 16.5 | 300 | 15 | −14 | 1.84 |
| FKM EX40 | 70 | 13.8 | 200 | 10 | −30 | 2.10 |
DVR = Indentation Set according to ASTM D395 Method B, 22 h at 200 °C. Tensile strength and elongation at break per ASTM D412, TR-10 per ASTM D1329, hardness per ASTM D2240, density per ASTM D1817. Manufacturing-related variations are possible; the respective data sheet is authoritative. For FFKM, our data sheets specify the compression set after 70 h at 200 °C; please compare only values tested under the same conditions.
What is an FKM compound made of?
An FKM compound is more than just the base polymer. It is the interaction of several components that determines its durability, temperature range, hardness, and compression set:
- Base polymer (fluorinated rubber): partially fluorinated rubber. A higher fluorine content improves resistance to various media.
- Curing system: bisphenol-based for standard commercial applications with good compression set; peroxide-based for greater resistance to water, steam, and acids.
- Fillers: enhance mechanical properties; carbon black is typically used in black formulations, while light-colored fillers are used in light-colored formulations.
- Acid scavengers: Metal oxides and hydroxides bind the decomposition products formed during bisphenolic cross-linking.
| Component | Function | Proportion |
|---|---|---|
| FKM basepolymer: fluorocarbon rubber | Partially fluorinated backbone, which determines the chemical resistance | 100phr, approx. 70% |
| Carbon black, e.g., N990, 15–45 phr depending on hardness | Reinforcement; adjusts hardness and mechanical properties | 30phr (approx. 21%) |
| Acid scavengers: calcium hydroxideand magnesium oxide | bind the cross-linking byproducts and stabilize the network | 9phrca. 6% |
| Bisphenol AF cross-linking agent with accelerator | forms the network and determines the compression set | 2.5phr, approx. 2% |
| Processing Aids: Release Agents | facilitate mixing and demolding | 0.5–1 phrless than 1% |
Typical reference values for a standard black, bisphenol-crosslinked FKM compound. The content is given in phr (parts by weight per 100 parts of rubber); the approximate percentage by weight is shown below. Specific formulations vary by manufacturer.
In peroxide curing, a peroxide combined with a coagent such as TAIC replaces bisphenol. The carbon-carbon bonds are more resistant to hydrolysis, and acid scavengers are largely eliminated. We would be happy to discuss directly with you which mixture is best suited for your application.
Typical applications
Design and Installation
FKM O-rings are installed in the same way as other elastomers. The guidelines for the groove are as follows:
static, dynamic 6–20%, as with other elastomers.
so that the ring has room for thermal expansion and slight swelling.
FKM is resistant to both.
FKM swells only slightly in mineral oils and fuels; therefore, compression remains within the normal range. FKM becomes stiffer in cold temperatures: If your operating temperature is below −25 °C, use FKM EX40. During installation, cover any sharp edges and threads to prevent damage to the O-ring. Determine groove dimensions and tolerances using the O-ring groove calculation and O-ring tolerances; all guidelines are provided in the article “O-Ring Design.”
Are there any alternatives to FKM?
The choice of material depends on the medium, temperature, and budget. We do not carry EPDM and NBR in our standard product line; they are listed here for reference:
Frequently Asked Questions
What does FKM stand for?
Is FKM the same as Viton?
What temperature range do FKM O-rings cover?
What is FKM not resistant to?
What is the difference between FKM and FFKM?
Is there any FKM that is approved for food contact?
How long can FKM O-rings be stored?
Is FKM suitable for hot water and steam?
