FKM (by ASTM D1418 standard) (equivalent to FPM by ISO/DIN 1629 standard) is the designation for about 80% of fluoroelastomers as defined in ASTM D1418. Other fluorinated elastomers are perfluoro-elastomers ( FFKM) and tetrafluoro ethylene/propylene rubbers ( FEPM). All FKMs contain vinylidene fluoride as a monomer. Originally developed by DuPont ( Viton), FKMs are today also produced by Daikin Chemical (Dai-El), 3M's Dyneon (Dyneon Fluoroelastomers), Solvay Specialty Polymers (Tecnoflon) and HaloPolymer (Elaftor). Fluoroelastomers are more expensive than neoprene or nitrile rubber elastomers. They provide additional heat and chemical resistance. FKMs can be divided into different classes on the basis of either their chemical composition, their fluorine content or their crosslinking mechanism.