FKM O-rings: the proven standard for demanding requirements

What is FKM?

FKM is the abbreviation for fluoroelastomer, also known as fluororubber. It denotes a partially fluorinated synthetic rubber that ranks among the most important high-performance sealing materials. The abbreviation FKM is standardized in accordance with ASTM D1418; in Europe, the designation FPM was formerly also common.

The key is the 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, the FKM molecule still carries hydrogen. These sites are the point of attack where certain media and very high temperatures push FKM to its limits.

How to read the graphic: The grey carbon chain (C) is the backbone. In FKM, most sites carry fluorine (F), but some still carry hydrogen (H, yellow). These hydrogen atoms are the weak points, attacked by hot water, steam, amines and alkalis.

This is why FKM offers very good resistance to oils, fuels and many chemicals and withstands continuous use up to +200 °C, but reaches its limits with hot water, steam and amines. Anyone sealing under those conditions turns to FFKM, which no longer has any hydrogen weak points.

The most important properties

• Very good media resistance: resistant to mineral oils, greases, fuels, many acids, ozone, oxygen and weathering. Check specific media in the media resistance tool.
• Temperature resistance: continuous −25 to +200 °C, or −40 to +225 °C with the Extreme type FKM EX40.
• Fully elastic: unlike PTFE or FEP-encapsulated O-rings, FKM remains elastic and is suitable as a flange, piston and rod seal.
• Hardness: 70 to 75 Shore A depending on the compound.
• Approvals: available as an FDA variant for food contact.
• Storage life: 10 years with proper storage in accordance with DIN 7716.

FKM thus offers a very good balance of resistance, temperature range and price and covers a large share of industrial applications. Where the requirements for temperature or medium go beyond this, FFKM is the next step.

Temperature ranges compared

FKM covers a wide temperature range: the standard operates continuously up to +200 °C, the Extreme type FKM EX40 up to +225 °C and down to −40 °C. The graphic places FKM between the common elastomers and the high-performance materials.

Continuous service temperatures of typical sealing materials as a standard compound, NH materials in coral. Values are guide figures depending on the compound; the relevant data sheet is authoritative.

Price level compared

FKM offers its resistance at a moderate price. The graphic shows the relative price level of the sealing materials, based on NBR as the cheapest reference equal to 1. At around four times that, FKM remains clearly in the economical range. PTFE and FEP cost several times as much, and FFKM, as the highest-performing material, many times as much.

Cost factor per material as a guide figure, based on NBR equal to 1, NH high-performance materials in coral. The values are for rough orientation only. The actual price depends on dimension, compound and quantity.

What is FKM resistant to, and where are the limits?

FKM owes its resistance to its high fluorine content. Against most industrial media, the material is exceptionally well equipped:

• Mineral oils, lubricating greases and hydraulic oils
• Fuels and propellants
• Aliphatic and aromatic hydrocarbons
• Many acids as well as ozone, oxygen and weathering

Unlike FFKM, however, FKM is not universally resistant. With these media you should check carefully or choose a different material:

1. Hot water and steam (with the standard types)
2. Glycol-based brake fluids
3. Ketones, esters and ethers
4. Amines and low-molecular-weight organic acids

For many of these borderline cases there are peroxide cross-linked FKM types with improved resistance to water, steam and acids. Where that is not enough, FFKM is the near-universally resistant alternative.

FKM types: copolymer, terpolymer and low-temperature

Not all FKM is the same. Fluorine content and cross-linking allow resistance, cold flexibility and processing to be tuned specifically. The usual classification is by the structure of the polymer:

Added to this is the cross-linking system. Bisphenol cross-linked types are the economical standard with good compression set. Peroxide cross-linked types are more resistant to hot water, steam and acids.

FKM vs. FFKM: a direct comparison

FKM and FFKM sound similar, but worlds separate them in resistance, temperature and price. Which material fits depends on how far the medium and temperature go.

For the majority of industrial applications, FKM is the economical choice. FFKM is used where FKM fails on temperature or medium. More on this in the article FFKM O-rings.

FKM, Viton and brands

FKM is the material designation in accordance with ASTM D1418. In the market, one brand name in particular is common:

• Viton® is the best-known FKM brand, originally from DuPont, now Chemours
• Tecnoflon® comes from Solvay
• Dyneon™ comes from 3M

Viton is therefore not a material in its own right, but a brand for FKM. We supply FKM O-rings in tested quality, from stock and with no minimum order quantity, as a standard, as an FDA variant and as the Extreme type EX40.

Viton®, Tecnoflon® and Dyneon™ are trademarks of their respective owners. NH O-RING has no business relationship with these companies and names these designations solely to classify the material.

Our FKM compounds

Our FKM range covers the common requirement profiles, from the economical standard to the Extreme type with an extended temperature range:

If you need FKM at 80 Shore A, we supply FKM 75 as a substitute and point this out transparently. You will find all test values in the data sheets.

Technical properties overview

The following values come from the FKM data sheets and are tested in accordance with ASTM. The hardness is given as a rounded nominal value, and the compression set applies to 22 h at 200 °C.

DVR = compression set (22 h / 200 °C). Tensile strength and elongation at break in accordance with ASTM D412, compression set in accordance with ASTM D395 B1, TR-10 in accordance with ASTM D1329, hardness in accordance with ASTM D2240. Density: FKM 75 1.85 g/cm³, FKM 75 FDA 1.84 g/cm³, FKM EX40 on request. Production-related variations possible; the relevant data sheet is authoritative.

What does an FKM compound consist of?

An FKM compound is more than the base polymer. Only the interplay of several components determines resistance, temperature range, hardness and compression set:

• Base polymer (fluoroelastomer): the partially fluorinated rubber. Its fluorine content determines the chemical resistance; a higher fluorine proportion improves the media resistance.
• Cross-linking system: bisphenol for the economical standard with good compression set, peroxide for greater resistance to water, steam and acids.
• Fillers: reinforce the mechanical properties, usually carbon black in the black standard types, light-colored food-compliant fillers in the FDA variant.
• Pigments: determine the color. FKM is classically black; for food and visual inspection, light-colored compounds are possible.

We are happy to clarify directly with you which compound and cross-linking suit your application.

Typical guide values for a black, bisphenol cross-linked standard FKM compound. phr stands for parts per hundred rubber, i.e. parts by weight per 100 parts of rubber. Specific formulations are manufacturer-specific.

Peroxide cross-linking for FDA and EX40: Alongside the standard bisphenol cross-linking there is peroxide cross-linking, in which a peroxide cross-links with the coagent TAIC instead of bisphenol. It forms more hydrolysis-stable carbon-carbon bridges and is cleaner in the final product. We use it in the FDA variant and in FKM EX40 because it is considerably more resistant to hot water, steam and bases than the bisphenol variant.

Typical applications

Design and installation

You design FKM O-rings like other elastomers. Determine groove dimensions and tolerances conveniently with the O-ring groove calculator and the O-ring tolerances. FKM swells only slightly in oils and fuels, and the compression is within the usual range. Mineral oils and most lubricating greases are suitable as installation grease, since FKM is resistant to oils and greases. During installation, cover sharp edges and threads so that the O-ring is not damaged.

Are there alternatives to FKM?

The choice of material depends on medium, temperature and budget. Where the requirements go beyond the limits of FKM, for example with aggressive media, hot water or temperatures above +225 °C, FFKM (ECOLAST) is the next step. If, on the other hand, water, hot water or polar media are the focus and oil and fuel resistance are not required, EPDM or NBR can be more economical. Where universal media resistance matters most and elasticity less so, FEP-encapsulated O-rings and PTFE O-rings are an option, though considerably more rigid.

Frequently asked questions