FKM is the generic ASTM D1418 designation for a saturated-backbone fluorocarbon elastomer, identical to FPM under ISO 1629 and DIN nomenclature [S1]. The same letters show up on analyzer data sheets for O-rings, bellows seal gaskets, and mechanical seal secondary faces because the polymer's C-F backbone resists petroleum, aromatics, ozone, and continuous service to roughly +204°C, well beyond nitrile, EPDM or silicone [S3].
On a typical analyzer instrument the FKM marking describes only the elastomer family, not a specific compound. Fluorine content typically runs 66–70% by weight, hardness 60–90 Shore A, density 1.8–1.9 g/cm³, and the material handles mineral oils, HFD hydraulic fluids, sour crude and ozone exposure that would swell or crack hydrocarbon rubbers [S3][S4].
Three names, one chemistry: FKM, FPM and Viton™
FKM and FPM describe the same fluoroelastomer family under different standards bodies, with FKM the ASTM D1418 term and FPM the ISO 1629 / DIN term, so a drawing that says "FPM" and a submittal that says "FKM" are technically equivalent [S1]. Viton™ is a registered Chemours trademark for one manufacturer's FKM range; competing equivalents include Solvay's Tecnoflon™, Daikin's DAI-EL™, 3M's Dyneon™, and other brands, all built on vinylidene fluoride (VDF) and hexafluoropropylene (HFP) copolymers, often with tetrafluoroethylene (TFE) or perfluoromethyl vinyl ether (PMVE) as a third monomer [S1].
This naming overlap is why a single "FKM" line item on a spec sheet is ambiguous: it could be a 66% fluorine Type A copolymer or a 70% fluorine Type GF-S, and the in-service gap between the two is large enough to matter in analyzer sample conditioning loops, where exposure to aromatic process gas or hot oil vapor is routine [S1].
FKM grade matrix: fluorine content, cure system, polymer family
Within FKM, grade and compound matter more than the family name. Fluorine content drives chemical resistance: Type A at ~66% fluorine covers general oils and fuels, while Type GF or GFLT at 69–70% fluorine adds tolerance for aromatic hydrocarbons, chlorinated solvents, and concentrated acids that would degrade Type A [S1].
Cure system is the second axis: bisphenol (ionic) cure is the workhorse for general chemical resistance and compression set, while peroxide cure improves steam, acid and base resistance and is the default choice for pharmaceutical, food-grade and CIP/SIP analyzer skids [S1].
Polymer family, originally Chemours' Viton™ nomenclature but now generic, subdivides further: Type A (general purpose), Type B (enhanced chemical resistance), Type F (low temperature with aggressive fluids), Type GLT/GFLT (low-temperature flexibility, static ratings to roughly −46°C), and Type Extreme ETP, a high-fluorine terpolymer that overlaps FFKM in specific chemistries [S1]. A drawing that just says "Viton" or "FKM" without naming a type is a known source of receiving-inspection mismatches.
FKM vs FFKM: where the line sits on a spec sheet
FFKM is the fully fluorinated perfluoroelastomer cousin of FKM, with no hydrogen on the polymer backbone, roughly 75% fluorine content, and a continuous service ceiling near +320°C, compared to FKM's typical +204°C [S2][S5]. Compression set illustrates the gap clearly: a representative FKM V0747-75 grade lands near 66% after 1,000 hours at +204°C, while FFKM grades stay in the 10–21% range [S2].
Chemical resistance scales the same way. FKM handles oils, fuels, hydraulic fluids, aromatics, ozone, oxygen and many acids, but fails against strong bases, amines, ketones, and superheated steam [S2][S4]. FFKM tolerates over 1,800 chemicals including amines, strong bases, plasma, and most aggressive solvents [S2][S5].
Cost is the trade lever. A spot AS568-107 comparison puts FKM V90 at $0.13 and Simriz 498 FFKM at $24.53 per unit, roughly 189× the FKM baseline, so FFKM is reserved for analyzer applications where a leak means contamination, safety incident, or a production-line stop [S2].
Where FKM belongs, and where it does not, in analyzer service
FKM is the correct default for oil seal and O-ring locations in analyzer housings exposed to lube oil mist, diesel, kerosene, aromatic sample gas, or ozone-rich ambient air, and for mechanical seal secondary faces on pumps handling mineral oil or hydrocarbon solvents within the −20°C to +204°C window [S3][S4]. Its density of about 1.8–1.9 g/cm³ (vs. 1.0–1.2 g/cm³ for NBR/EPDM) is a useful receiving-inspection check: an FKM part of given size feels heavier than the hydrocarbon equivalent [S3].
FKM is the wrong call when the analyzer sees anhydrous ammonia, amines, ketones, esters, hot water or steam above about +100°C, low-molecular-weight organic acids, or aircraft-type hydraulic fluids, all of which fall in the published FKM incompatibility list [S4]. Sodium or potassium alkali metals, molten or gaseous, attack both FKM and FFKM and rule out the whole fluoroelastomer family [S2].
Typical published compound values reinforce the practical envelope: 81 ± 5 Shore A hardness, ≥1,305 psi tensile, ≥200% elongation, ≥114 lbf/in tear strength, ≤17% compression set after 24 h at +70°C (25% deflection), and short-term hot-air survival to about +300°C [S4].
How to write an FKM line that survives receiving inspection
A spec line that just says "FKM" or "Viton" almost guarantees a call to the supplier. The robust form names the standard, the grade family, the cure system, the compound hardness, and the governing test methods: for example, "FKM per ASTM D1418, Type GF, peroxide cure, 75 ± 5 Shore A, compression set per ISO 815, suitable for continuous service to +204°C in aromatic and chlorinated media", with FPM accepted as the ISO 1629 equivalent [S1][S4].
State the chemical envelope explicitly (oils, fuels, aromatics, dilute acids) and the incompatibilities (amines, ketones, superheated steam, anhydrous ammonia), so the receiving inspector can match the certificate of conformance to the actual sample line, not the marketing brochure [S1][S4]. Where low temperature matters, name GLT or GFLT and the rated static limit; for steam or CIP/SIP exposure, specify peroxide cure explicitly rather than leaving it implicit [S1].
For a gas analyzer sample-conditioning module, add a short statement of the expected hydrocarbon and acid exposure plus the maximum continuous and short-term temperature, since fluorine content, not the family name, is what determines whether the seal survives a toluene slip-stream or a hot H₂S sample [S1][S3].
Cross-references and standards that actually govern FKM
FKM is named under ASTM D1418, with FPM the ISO 1629 / DIN equivalent, and compound testing typically runs to ISO 815 (compression set), ISO 868 (hardness), ISO 1183 (density), and DIN 53 504 / DIN 53 512 / DIN ISO 34-1 for tensile, rebound and tear properties [S1][S4]. fluororubber ASTM D1418 itself defines the letter codes for elastomer families; ISO 1629 does the same internationally, so the FKM/FPM pair is purely a regional naming split, not a chemistry split [S1].
The deeper lesson: any drawing or analyzer analytical-instrument datasheet that says only "FKM" or only "Viton" is leaving roughly $0.10–$24.53 per seal of specification risk on the table, and the cheapest fix is usually a two-line edit to add the grade family, the cure system, and the governing test standard.
Track the next change by watching revisions to ASTM D1418 and ISO 1629 elastomer-naming annexes, and by requiring batch-level certificates that quote fluorine content in percent by weight and the actual compression-set result, not just the Shore A value. A single confirmed certificate that quotes 66% fluorine against a 70% spec has, on more than one analyzer skid, been the difference between a clean commissioning and a six-month leak hunt.
For related coverage, see Reverse Impact Test for PVDF Coated Aluminum Panels: Spec Thresholds, Failure Modes, and.