FKM

The term fluorine rubber ( FKM abbreviation according to ISO 1629 [ formerly FPM ] and FKM according to ASTM D 1418 ) refers to a whole group of rubbers which the common feature of vinylidene ( di ) fluoride (VDF ) have as one of its monomers. Fluorine rubbers were developed in the 1950s by DuPont (Viton), also from Lanxess ( Levatherm F), Solvay Solexis ( Tecnoflon ), Dyneon ( Dyneon ™ fluoroelastomer ) and Daikin Chemical ( Dai- El) are offered today but.

In addition to the fluoroelastomer, there are other groups of fluorinated elastomers, such as perfluoroelastomer (FFKM ), tetrafluoro ethylene / propylene rubbers ( FEPM ), and fluorinated silicone rubber.

The two main types of fluorine rubbers are

• Copolymers of vinylidene fluoride (VDF ) and hexafluoropropylene ( HFP), and
• Terpolymers of VDF, HFP and tetrafluoroethylene (TFE)

By additionally introduced TFE terpolymers have a higher fluorine content than the copolymers (about 68-69 % compared to 66 % for the copolymers ), which is reflected in better resistance to chemicals and high temperatures. Copolymers, however, have advantages in the compression set and low-temperature flexibility.

In addition, there are also

• Polymers of VDF, HFP, TFE and perfluoromethylvinylether ( PMVE )
• Polymers of VDF, TFE and propylene, as well as
• Polymers of VDF, HFP, TFE, PMVE and ethene

Perfluoromethylvinylether ( PMVE ) is used in the rubber to improve the low temperature flexibility. The polymers with propene are more resistant to alkalis than other FPM types, but sources for much more in hydrocarbons. Polymers with ethylene show even at high temperatures or resistance to hydrogen sulfide.

Networking mechanisms

Fluororubbers can not be networked, such as natural rubber with sulfur, because the polymer chains are saturated (ie, they lack the data required for sulfur vulcanization double bonds). Other mechanisms have therefore been developed to link the fluoropolymer to the elastic network.

The oldest method for the vulcanization of fluoroelastomers is the diaminic networking. Here blocked diamines are used as crosslinkers. In a basic environment, the vinylidene fluoride hydrogen fluoride (HF ) cleaving, whereby addition of the amine to be delivered to the polymer chain. The resulting hydrofluoric acid ( HF) is trapped in the control by magnesium oxide which is thus transformed into magnesium fluoride. The diaminic networking is still often used because of the good adhesion between rubber and metal, which is achieved with diaminically crosslinked elastomers. In aqueous media, the diaminic link shows, however, susceptible to hydrogenation.

Modern contrast, the bisphenolic mechanism than the diaminic method (also dihydroxy- mechanism ), and a quaternary phosphonium AF are used as cross-linker components in the bisphenol. It is a nucleophilic substitution mechanism. In comparison with the cross-linking diaminic better resistance to hydrolysis and higher temperatures can be achieved and an improvement in compression set.

Furthermore, fluorine rubbers can also peroxide (also called triazine method ), ie crosslink by free radicals. Particularly important is the peroxide crosslinking when rubbers are used, the perfluoromethylvinylether ( PMVE ), since the two ionic mechanisms can lead by attacking the PMVE to the destruction of the polymer chains. In aqueous and non-aqueous electrolytes peroxide crosslinking of fluoroelastomers are superior to the products of other crosslinking mechanisms. When the temperature resistance, they rank just behind the bisphenol- crosslinked materials.