FKM, also sold as FPM or under the Viton trade name, is a fluorinated synthetic elastomer whose carbon-fluorine backbone gives it the highest oil, fuel and heat resistance of any common O-ring rubber [S2]. Continuous service sits at roughly -25 °C to +200 °C for general-purpose grades, with short excursions to 230 °C and hard-to-reach upper limits near 325 °C in peroxide-cured specialty compounds [S2].
For O-ring, gasket and seal work above about 150 °C, or wherever ASTM D2000 "HK" material cannot be met by HNBR, EPDM or silicone, FKM is the default specification - typically 70 to 90 Shore A hardness and a specific gravity near 1.80 to 1.85 g/cm³ [S2]. Below 0 °C, or in steam, ketones or amines, FKM is the wrong material; FFKM perfluoroelastomer is the next step up, and silicone or EPDM cover the lower-cost cold or polar-fluid envelopes.
FKM material definition, FKM vs FPM naming
FKM is the ASTM D1418 designation, FPM is the ISO 1629 / DIN abbreviation, and Viton is a Chemours (formerly DuPont Performance Elastomers) trade name widely used in Europe and China for the same HFP-VDF copolymer family [S2]. Chemically the chain carries vinylidene fluoride (VDF), hexafluoropropylene (HFP), and often tetrafluoroethylene (TFE) or perfluoromethylvinyl ether (PMVE) comonomers, with fluorine contents typically 66 to 70 wt% [S2].
Higher fluorine content (Type GFLT, fluorocarbon-terpolymer) trades off low-temperature flexibility for better chemical resistance; PMVE-modified "low-temperature FKM" grades push TR-10 down to roughly -30 °C while keeping the same upper service ceiling [S2]. When a datasheet lists "FKM" without a suffix, assume Type A (HFP-VDF, 66% F) as the baseline; type B, G, GF, GFLT and GLT all differ in chemistry, fluid resistance, and cold performance.
Temperature window, hardness, mechanical baseline
General-purpose FKM (Type A) is rated -15 °C to +200 °C continuous, 230 °C short-term, and survives dry-heat spikes to roughly 325 °C; specialty peroxide-cured Viton GLT, GFLT and ETP types push the lower end to about -30 °C while keeping the +200 °C ceiling [S2]. Hardness is sold in 70, 75, 80 and 90 Shore A as the most common grades, with tensile strength typically 7 to 15 MPa, elongation 150 to 300%, and compression set 15 to 25% after 70 h at 200 °C [S2].
Specific gravity runs 1.80 to 1.85 g/cm³ for filled compounds (the FKM base polymer is ~1.85), and the material is non-flammable in air (LOI typically above 60%) with a service-proven track record in aerospace, missile and rocket propulsion systems where standard elastomers fail [S2]. Compared with HNBR, silicone or EPDM at the same Shore A hardness, FKM costs roughly 5 to 10x as much per kilogram, so it is specified at the seal and not at the hose-jacketing level.
Chemical compatibility: hydrocarbons, acids, steam
FKM resists aliphatic and aromatic hydrocarbons, petroleum oils, fuels (including oxygenated gasoline with up to 20% ethanol), mineral acids (HCl, H₂SO₄, HNO₃ up to ~70%), and chlorinated solvents; this is the reason it dominates fuel-injector O-rings, chemical-plant agitator seals, and pressure transmitter process diaphragms exposed to hydrocarbon media [S2].
FKM does NOT handle polar solvents well: ketones (acetone, MEK), esters (ethyl acetate), low-molecular-weight ethers, amines, hot water and steam above ~100 °C cause rapid swelling or degradation. For steam service above 120 °C, for hot amines (e.g. oilfield packer fluids under sour service per NACE MR0175), or for aerospace hydraulic Skydrol LD-4 phosphate-ester fluid, switch to EPDM, FFKM, or an FKM/FFKM blend - FKM as a single elastomer will fail.
Compound families: A, B, G, GF, GFLT, GLT, FEPM, FFKM
Type A (HFP-VDF, 66% F) is the workhorse - the cheap, general-purpose grade most Chinese compounders and eBay watch-band extruders ship by default. Type B (HFP-VDF-TFE, 68% F) adds better acid and solvent resistance. Type G and GF (HFP-VDF-TFE with cure-site modifiers) push fluid resistance further; GFLT (G + PMVE) gives -29 °C TR-10 with broadside chemical resistance for aerospace fuel systems; GLT is a low-temperature-permeation grade for high-pressure fuel rails [S2].
Above FKM sits FEPM (Aflas, tetrafluoroethylene-propylene copolymer) for hot amines, steam and high-pH chemicals where standard FKM fails, and at the top sits FFKM (perfluoroelastomer, Kalrez, Chemraz, Perlast) - the only elastomer with usable service to roughly +325 °C continuous and the broadest chemical envelope. For typical [industrial valve](/encyclopedia/industrial-valve.html] and pressure sensor sealing, A/B/G grades cover 80%+ of applications; FFKM is reserved for semiconductor wet-etch, pharma, and any seal exposed to plasma or hot concentrated oxidizers.
Standards, datasheet checks and quality gates
For O-ring specification, FKM compounds should be called out to ASTM D2000 line callouts beginning with "HK" (Heat-resistance + K = FKM), for example HK 715 Z1 (70 Shore A, 15% swell limit, heat-aged 70 h/200 °C). Compression-set limits are governed by ASTM D395 Method B, fluid aging by ASTM D471, low-temperature by ASTM D2137 (TR-10) or D1053 (Gehman T10), and hardness by ASTM D2240 [S2].
For oil-and-gas packer fluids and valve stem seals under sour service, require NACE MR0175 / ISO 15156 compliance; for food, water and pharma contact, specify FDA 21 CFR 177.2600, USP Class VI, or 3-A Sanitary Standard; for automotive fuel rails, OEM specifications GM 6109, Ford WSA-M2D401-A, and Chrysler MS-AR30 govern. Aerospace fuel-system FKM usually needs Mil-R-83485 or AMS-R-83485 qualification, while general industrial seals often only need an ASTM D2000 line-call-out datasheet plus a batch cure date and lot number.
Common FKM sealing formats and sourcing levers
O-rings, AS568 standard inch sizes and ISO 6149 metric O-rings, dominate FKM consumption; rotary shaft seals (TC/TCV oil seals) and V-rings are next, then lathe-cut gaskets, rubber-bonded-metal flange gaskets, and injection-molded flow meter diaphragms. For static seals in chemical-plant flanges, specify FKM with a hardness of 75 to 80 Shore A and a compression set below 20% after 70 h at 200 °C; for dynamic reciprocating seals, drop to 70 Shore A and bias toward 15% glass-filled or carbon-filled compounds to prevent extrusion at high pressure [S2].