FKM (fluoroelastomer, ASTM D1418 designation) is a carbon-fluorine backbone elastomer that delivers the highest oil, fuel and heat resistance of any common O-ring rubber, with general-purpose grades rated -15 to +200 °C continuous and short peaks to 230 °C [S2].
Selection in 2026 is a four-axis problem (type, temperature, chemistry, cure), and the practical job is matching the family and cure system to the actual service envelope rather than picking a generic "Viton" [S4].
What FKM Actually Is and Why It Costs 5-10x NBR
FKM is a copolymer or terpolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), with optional tetrafluoroethylene (TFE) and perfluoromethyl vinyl ether (PMVE) comonomers, and the C-F bond dissociation energy of roughly 485 kJ/mol is what gives it thermal and chemical stability no hydrocarbon rubber can match [S3]. FKM costs approximately 10-20 times more than NBR and 3-5 times more than HNBR per kilogram, so specifiers route it to the seal face and not the hose jacket [S3].
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, with fluorine content typically 66 to 70 wt% [S2]. When a datasheet lists "FKM" without a suffix, assume Type A (HFP-VDF, 66% F) as the baseline; types B, G, GF, GFLT, GLT and ETP all differ in chemistry, fluid resistance, and cold performance [S2].
Family Comparison: A, B, F, GLT, GFLT, ETP
The six FKM sub-types line up against four decision criteria (low-T flexibility, upper-T ceiling, chemical resistance, cost band), and the practical rule of thumb is: A-type for general O-rings at moderate temperature, B-type or F-type for aggressive fuels, GLT/GFLT for low-temperature sealing, and ETP for oilfield amine/H2S service [S3].
The headline spec values per ASTM D2240 / D412 / D395 / D2137 / D1329 are: A-type (66% F, TR-10 -17 °C, tensile 8-17 MPa, compression set 15-35% at 70 h/200 °C), B-type (68% F, TR-10 -14 °C, tensile 10-18 MPa), F-type (70% F, TR-10 -10 °C, tensile 12-20 MPa, the highest swell resistance), and GLT (64-67% F, TR-10 -30 to -32 °C, low-temperature brittle point -40 to -50 °C) [S3]. Higher fluorine content (F-type at 70%, GFLT-class) trades off low-temperature flexibility for better chemical resistance; PMVE-modified low-temperature grades push TR-10 down to roughly -30 °C while keeping the +200 °C ceiling [S2].
Peroxide-cured FKM (peroxide / bisphenol AF dual-cure) addresses a chronic weakness in amine-stabilised, sulfur-containing or strong-acid media that attack conventional bisphenol-cured VDF/HFP, and switching the cure system is often a cheaper fix than moving to FFKM [S4]. FFKM perfluoroether (Chemours ETP and equivalents) is the upper tier: continuous service to roughly +260 °C with short-term peaks approaching +325 °C, costing 50-100 times more than FKM [S3][S4].
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]. A standard FKM terpolymer rated +230 °C continuous and +250 °C short-peak is the realistic ceiling for non-perfluoro grades, and above those bands the rubber hardens, compression set degrades and the seal will not recover when the housing cools [S4].
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 approximately 1.85), and the material is non-flammable in air with a Limiting Oxygen Index typically above 60%, with a service-proven track record in aerospace, missile and rocket propulsion systems where standard elastomers fail [S2].
For static seal service the practical temperature band is -26 to +232 °C, up to 275 °C for short times, while dynamic seals are limited to -15 to +200 °C; GLT class opens the static window to -45 to +225 °C and VPL extends it further to -55 to +225 °C [S5]. Compression set must be sized at the actual hot-side temperature, not at 25 °C, because the recovery penalty compounds quickly above 200 °C [S4].
Chemical Compatibility: Hydrocarbons Win, Polar Solvents Lose
FKM resists aliphatic and aromatic hydrocarbons, petroleum oils, fuels (including oxygenated gasoline with up to 20% ethanol), mineral acids (HCl, H2SO4, HNO3 up to roughly 70%), and chlorinated solvents, which is why 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, ammonia, hot water and steam above roughly 100-130 °C cause rapid swelling or degradation, plus brake fluid DOT 3/4 on older vehicles [S2][S4]. For steam service above 120 °C, for hot amines 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 within hours [S2]. The same C-F chemistry that shrugs off petroleum oils is the reason it cannot handle these polar or hot aqueous media, and that trade-off is structural rather than a cure-system problem.
Cure System and Compound Selection by Application
Bisphenol-bridged FKM is the default standard grade, while peroxide-bridged FKM offers better resistance to acid solutions, lubricants with added organic amides or amines, and aggressive chemical media [S5]. A real shop-floor decision tree: FKM-A (bisphenol, 66% F, TR-10 around -17 °C) is the most common standard grade for general-purpose O-rings; FKM-B (bisphenol, 69% F) suits National VI ethanol gasoline; FKM-F (bisphenol, 70% F) handles National VI methanol gasoline; FKM-GBL and FKM-GF (peroxide-cured B and F) push chemical resistance higher; FKM-GLT, FKM-GBLT and FKM-GFLT (peroxide-cured) progressively extend the low-temperature envelope to TR-10 -30 °C; and FKM-TP (peroxide, 57% F) handles alkali [S5].
AFLAS (TFE/propylene copolymer) sits next to FKM in the spec catalog and is the better choice where alkali, steam, amines, amides and certain bases dominate, even though its fluorine content is lower [S5]. For static and dynamic industrial valve sealing in chemical and pharmaceutical service, material decisions usually cascade from media compatibility first, temperature second, and pressure third, with the cure system chosen last to fix the residual chemical weak spot.
Who FKM Is For, and Who Should Walk Away
FKM is the right call for O-rings, gaskets and seal work above about 150 °C, or wherever ASTM D2000 "HK" material cannot be met by HNBR, EPDM or silicone, typically 70 to 90 Shore A and specific gravity near 1.80 to 1.85 g/cm³ [S2]. It is the default for fuel-system O-rings, chemical-plant agitator seals, aerospace hydraulic seals, semiconductor wet-etch, and pharmaceutical CIP loops where a wrong compound can swell, embrittle or leak in a single shift [S4].
FKM is the wrong material below 0 °C in standard grades, in steam, ketones, esters, amines, ammonia, hot water above about 100-130 °C, or where standard bisphenol-cured VDF/HFP hits amine-stabilised or strong-acid media; FFKM perfluoroelastomer is the next step up, and silicone or EPDM cover the lower-cost cold or polar-fluid envelopes [S2][S3]. 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 face and not at the hose-jacketing level [S2]. When a single flow meter or pressure sensor elastomer change-out is contemplated, the cost of the seal is rarely the dominant line item; the cost of an unscheduled shutdown almost always is.
Verification, Sourcing and Standards
Specifiers should require material certifications tied to ASTM D2000 line callouts (HK family for FKM), ASTM D1418 type designation, and cure-system disclosure on every certificate of conformance, with PPAP documentation available for automotive Tier-1 work [S1]. Independent ISO 9001 quality systems with material traceability are the baseline; for UL-listed service, FKM materials can be submitted for Underwriters Laboratories approval per UL157, and that file number should appear on the datasheet [S5].
For sour service, NACE MR0175 compliance is the binding constraint and rules out standard bisphenol-cured FKM in hot amine packer fluids, which is where Viton ETP, AFLAS or FFKM take over [S2][S5]. The next trackable signal to watch is the FFKM cost curve: as semiconductor wet-etch and pharmaceutical reactor demand pulls volume through Chemours ETP and equivalents, FFKM unit price typically softens 10-20% per calendar year, which pulls more CPI pump-seal specs upward into FFKM territory [S3].
See also our earlier report, Insulation Board Selection for Industrial Facilities: 2026 Spec Map.