Overview:
FKM or fluoroelastomers are a class of elastomers that have resistance to a wide range of chemicals and very good high temperature performance. Special types with different monomer compositions and fluorine content are tailored for superior fluid resistance, low and high temperature performance, and combinations thereof.
Features:
▲ Excellent temperature resistance
▲ Good weather, aging and oxygen stability
▲ High chemical stability
▲ Excellent resistance in mineral oils and fats
▲ Very good resistance in non-polar media
▲ Temperature range from -55℃ to +250℃
▲ Low gas permeability
FPM is the international abbreviation according to DIN/ISO, and FKM is the abbreviation for the fluororubber category according to the American standard ASTM. Viton® is a registered trademark of Chemours, Tecnoflon® is a registered trademark of Solvay Specialty Polymers, Dyneon™ is a registered trademark of 3M Dyneon, and DAI-EL™ is a registered trademark of polymers supplied by Daikin.
Chemical Resistance and Stability:
The chemical resistance and high temperature stability are due to the bulky fluorine atoms, which protect the polymer backbone and carbon-fluorine bonds from attack, and the high bond energy of the carbon-fluorine bonds, as shown in the figure below.
Figure 1. Bonding energy (kJ/mol)
Figure 2. Bindungsenergie (kJ/mol)
FKM can typically be used for extended periods of time at temperatures up to 200 °C. In automotive specifications, a service life of at least 5000 hours is usually defined.
FKM monomer:
In order to exhibit elastic behavior, polymers must be flexible and able to recover from significant deformation. This requires the polymer to be substantially amorphous. Often the polymers are crosslinked to form a three-dimensional network. The driving force for deformation recovery is the tendency of chain segments to revert to a disordered state. In general, fluorocarbon chains are relatively stiff compared to hydrocarbons and thus exhibit rather slow relaxation and recovery from strain. Fluoroelastomers are composed of two or more different monomer units. Chains with monomers such as VF2 (or VDF), TFE and ethylene tend to crystallize if long enough. Therefore, monomers containing bulky pendant groups such as HFP, PMVE and propylene are incorporated to produce amorphous polymers. Figure 3 outlines commonly used monomers.