FKM

FKMs or fluoroelastomers, are a family of elastomers with resistance to a broad range of chemicals combined with very good high temperature properties.

Specialty types with differing monomer compositions and fluorine content are tailored for superior fluid resistance, low and high temperature performance, and combinations thereof.

Key features

  • Excellent temperature resistance
  • High chemical stability
  • Good weather, aging and oxygen stability
  • Excellent resistance in mineral oils and fats
  • Low gas permeability
  • Very good resistance in non-polar media
  • Temperature range from –55 °C to +250 °C

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Introduction FKM

The original fluoroelastomer was developed by the DuPont Company in 1957 in response to high performance sealing needs in the aerospace industry. To provide even greater thermal stability and solvent resistance, tetrafluoroethylene (TFE) containing fluoroelastomer terpolymers were introduced in 1959. The latest FKM polymers have a much broader fluids resistance profile than standard fluoroelastomers, and are able to withstand strong bases and ketones as well as aromatic hydrocarbons, oils, acids, and steam. In 1975 a new fluoroelastomer family was introduced by Asahi Glass Co., FEPM, based on an alternating copolymer of TFE and propylene. It is marketed under the trade name AFLAS®.

FPM is the international abbreviation according to DIN/ISO, whereas FKM is the short form for the fluoroelastomer category according to the American standard ASTM. Viton® is the registered trade mark of Chemours, as is Tecnoflon® for Solvay Specialty Polymers, Dyneon™ for 3M Dyneon and DAI-EL™ for polymers supplied by Daikin. On this website we will use the abbreviation FKM.

Chemical Resistance and Stability

The chemical resistance and high temperature stability is due to the bulkiness of the fluorine atoms, shielding the polymer backbone and carbon-fluor bond from attack, and the high bonding energy of the carbon-fluor bond, as illustrated in the following figures. FKMs can normally be used at temperature up to 200 °C for prolonged periods of time. In automotive specifications often lifetimes of at least 5000 h are defined. See further “Heat resistance and thermal stability”.

Figure 1. Bonding energy (kJ/mol).

Figure 2. Bindungsenergie (kJ/mol).

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Monomers for FKM

To exhibit elastomeric behaviour, a polymer must be flexible and recover from substantial deformation. This requires the polymer to be substantially amorphous. Normally the polymer is cross-linked to form a three-dimensional network. The driving force for recovery upon deformation is the tendency of chain segments to return to their disordered state. Generally, fluorocarbon chains are relatively stiff compared to hydrocarbons, and therefore show rather slow relaxation and recovery from strain. Fluoroelastomers are made up of two or more different monomer units. Chains with monomers as VF2 (or VDF), TFE, and ethylene tend to crystallize if long enough. Therefore monomers with bulky side groups like HFP, PMVE and propylene are incorporated to produce an amorphous polymer. Figure 3 gives an overview of monomers typically used.

Figure 3. Monomers for FKM production.

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