FKM is a partially fluorinated elastomer: a copolymer whose backbone still carries hydrogen on the vinylidene fluoride unit, and is classified generically under ASTM D1418 and ISO 1629, with trade names including Viton (now owned by Chemours after the 2015 DuPont spin-off), Dai-El, Tecnoflon, Dyneon, and Fluonox [S4][S1].
All FKM families contain VDF as the common monomer, with second and third fluorinated comonomers controlling fluorine content (typically 66–71 wt%) and therefore chemical resistance, low-temperature flexibility, and base resistance [S4][S3].
Backbone Chemistry: Why C-F Bonds Set FKM Apart
The carbon-fluorine bond is one of the strongest single bonds in organic chemistry, stronger than the C-H bonds in hydrocarbon rubbers and substantially stronger than the C-Cl bonds in neoprene, which gives FKM its thermal stability and chemical inertness [S3].
FKM's polymer backbone is composed predominantly of carbon-fluorine bonds; the high electronegativity of fluorine shields the chain from polar and non-polar solvents, acids, and oxidizing agents that attack other organic polymers [S3][S1]. Standard FKM grades maintain mechanical properties and sealing force at continuous service temperatures up to 400°F (204°C), with specialty grades reaching 500°F (260°C) for short-term exposure, roughly 150°F above nitrile and 100°F above neoprene [S3]. Density runs about 1.8–1.9 g/cm³, noticeably heavier than the 1.0–1.2 g/cm³ typical of hydrocarbon rubbers, a useful field identification clue [S3].
FKM Type 1 to Type 5: Monomer Compositions and Fluorine Content
FKM is not a single compound but a family split by ASTM/ISO nomenclature into five monomer-defined types, and the type drives both performance and price [S4].
Type-1 FKM is a VDF + hexafluoropropylene (HFP) copolymer with about 66 wt% fluorine, the general-purpose workhorse grade with a balanced property profile [S4]. Type-2 adds tetrafluoroethylene (TFE) to make a VDF/HFP/TFE terpolymer with 68–69 wt% fluorine, improving chemical and heat resistance at the cost of low-temperature flexibility and compression set [S4]. Type-3 substitutes perfluoro(methyl vinyl ether) (PMVE) for HFP, giving VDF/TFE/PMVE terpolymers at 62–68 wt% fluorine and substantially better low-temperature performance [S4]. Type-4 replaces HFP with propylene, producing a TFE/Pr/VdF structure at roughly 67 wt% fluorine with improved base resistance but worse hydrocarbon swelling [S4]. Type-5 (VDF/HFP/TFE/PMVE/ethylene) is engineered for base and high-temperature H₂S resistance in oilfield service [S4]. The Viton-style grade letters A, B, F, GF, and GLT are the practical shorthand for these type families in many OEM specifications [S3].
How Fluorine Content Drives the FKM Trade-off Matrix
Higher backbone fluorine content correlates directly with improved resistance to aggressive solvents, fuels, and oxidizing chemicals, but it also raises glass transition temperature and stiffens the compound at low temperature [S3][S5].
The relationship is roughly linear: Tg shifts from about -20°C at 64 wt% F to -10°C at 70 wt% F in VDF/HFP systems, the same fluorine-vs-flexibility trade-off that acrylonitrile content plays in nitrile rubber [S5]. Mooney viscosity ML(1+10) at 121°C typically lands between 20 and 80 for general-purpose FKM, and this molecular-weight window is what the processor sees on the spec sheet [S5]. Standard service temperature caps around 230°C for FKM, well below the 327°C continuous ceiling reported for fully fluorinated FFKM perfluoroelastomers built on TFE + perfluoromethylvinylether backbones [S5]. For a broader material-context view of where FKM sits among high-performance sealing polymers, see the fluororubber reference page.
FKM vs FFKM vs Fluorosilicone: Backbone Comparison
The clearest way to read the FKM backbone is against its two neighbors: perfluoroelastomer (FFKM), which is fully fluorinated with no backbone hydrogen, and fluorosilicone (FVMQ), which is a silicone backbone with fluorine side groups [S2][S1].
FFKM replaces every backbone hydrogen with fluorine, producing a tightly packed, nearly chemically inert network that handles concentrated acids, strong alkalis, amines, ketones, esters, and oxidizers that degrade FKM; base-polymer fluorine content runs about 72% in FFKM versus about 66% in FKM [S2]. FKM carries residual VDF hydrogen in the chain, which is exactly what makes it vulnerable to strong bases, amines, hot water, and steam via dehydrofluorination [S2][S4]. Fluorosilicone inverts the logic: a silicone backbone with fluorine additions gives excellent low-temperature flexibility down to about -80°F (-62°C) but only moderate chemical resistance compared to FKM, which is rated good against oils, fuels, and many aggressive chemicals but only fair against steam and hot water [S1]. On temperature, FKM caps near 400°F (200°C) continuous versus about 450°F (232°C) for fluorosilicone, a flip of the FKM-vs-FVMQ low-temperature ranking [S1].
Cross-linking Systems: How the Backbone Gets Cured
Three cross-linking chemistries are used to cure FKM, and the choice changes which fluids the finished part can tolerate [S4].
Diamine cross-linking uses a blocked diamine plus magnesium oxide acid acceptor; VDF undergoes dehydrofluorination in alkaline media to let the diamine add to the chain, giving the best rubber-to-metal bond but the worst hot-water and steam resistance because the diamine link itself hydrates [S4]. Ionic (dihydroxy) cross-linking, typically with bisphenol AF, is the dominant modern route and is what most general-purpose Type-1 and Type-2 compounds use; it offers the best compression set and broadest fluid resistance profile [S4]. Peroxide cross-linking is used for specialty grades and is the only viable route for fully fluorinated FFKM backbones, which are inert to ionic curing; organic peroxides such as 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane are dosed at 0.5–6 phr with co-agents like triallyl isocyanurate (TAIC) at 2–8 phr, raising tensile strength from about 8 MPa to 14 MPa and dropping compression set from 35% to 18% after 70 hours at 200°C [S5]. Scorch retarders such as o-phenylphenol at 0.1–1.0 phr extend scorch time from under 2 minutes to over 5 minutes at 170°C, which is what makes peroxide-cured FKM and FFKM injection-moldable [S5].
Specification, Standards, and Sourcing Notes
The correct way to call out FKM on a drawing or purchase order is by ASTM D1418 / ISO 1629 family, by FKM type or Viton-equivalent grade letter (A, B, F, GF, GLT), and by fluorine content window; trade names such as Viton are registered trademarks and should not be substituted for the generic designation [S4][S3].
FKM is roughly 5 to 15 times the cost of an equivalent nitrile or EPDM compound, a premium that pays back only where the application actually demands C-F backbone stability [S3]. Current FKM production spans Chemours, Daikin, Solvay (Tecnoflon), 3M (Dyneon), HaloPolymer (Elaftor), Gujarat Fluorochemicals (Fluonox), and several Chinese manufacturers, so multi-source qualification is realistic for most industrial buyers [S4]. For related background on advanced sealing and high-performance polymer categories, the advanced material reference and the PEEK polymer profile sit alongside FKM in the high-end material toolbox.
This topic is covered further in Dump Truck Models by Axle Count and Body Length: 2026 Spec Map.