FKM and FPM refer to the identical fluoroelastomer polymer family; FKM is the abbreviation assigned by ASTM D1418, FPM is the abbreviation assigned by ISO 1629 and the corresponding DIN nomenclature, with no chemical or performance difference between the two letters [S1][S2][S3].
Viton is the registered trademark held by Chemours (the former DuPont Performance Elastomers business) for its FKM range, and competing equivalent products are sold as Tecnoflon (Syensqo/Solvay), Dai-El (Daikin), Dyneon (3M), Fluorel and Noxtite, all of which are FKM polymers built on vinylidene fluoride (VDF) and hexafluoropropylene (HFP) chemistry [S1][S2][S5]. Specifying a seal simply as "FKM" or "FPM" defines a family, not a specific compound, and procurement documents that fail to add a type letter invite in-service mismatches.
Why two letters exist: the ASTM D1418 vs ISO 1629 split
The split is institutional, not technical. ASTM D1418 is the US standard for rubber nomenclature and assigns the code "FKM" to the saturated-backbone fluoroelastomer family, where F denotes a fluoro-containing rubber, K is the historical code for the German Kohlenstoff backbone and M marks a saturated main chain [S5][S6].
ISO 1629, applied across Europe as DIN ISO 1629, uses the same letter logic but emits "FPM" for the same family, so the European purchasing document, the European moulded-part drawing and the European MSDS will say FPM while the equivalent US document will say FKM for the same bag of polymer [S1][S4][S7]. On a multinational project the practical rule is: FKM on ASTM-traceable certificates, FPM on ISO-traceable certificates, never mix the two on a single BOM line and assume the supplier will harmonise them, because the FKM vs FPM issue is purely a standard-vs-standard naming convention, not a material grade difference [S3][S8][S9].
Five FKM types engineers actually have to choose between
Once the FKM/FPM family is fixed, real selection happens inside the family, and ASTM D1418 plus the historical Chemours grade letters are the simplest decision grid: Type 1 (VDF/HFP copolymer, ~66 wt% fluorine, general purpose), Type 2 (VDF/HFP/TFE terpolymer, 68–69 wt% fluorine, better chemical resistance at the cost of low-temperature flexibility), Type 3 (VDF/TFE/PMVE, 62–68 wt% fluorine, best low-temperature flexibility), Type 4 (propylene/TFE/VDF, ~67 wt% fluorine, better base resistance, worse hydrocarbon swelling) and Type 5 (VDF/HFP/TFE/PMVE/ethylene, formulated for H2S and base-rich service) [S2][S5].
The decision matrix in practice is short: aromatic or chlorinated solvents above 80 °C point to a Type 2 (or higher) compound; sub-zero service below about -20 °C points to a Type 3 (GLT/GFLT-style); sour hydrocarbon or amine-rich service points to a Type 5; routine hot-oil and fuel service on a pump or industrial valve at 150–200 °C is fine with a standard Type 1 at 66 wt% fluorine [S1][S2]. Stating only "FKM" or "Viton" on the drawing collapses all five into one and shifts the technical risk from the specifier to the receiving warehouse.
FKM vs FFKM vs FEPM: where the FKM/FPM line stops
ASTM D1418 lists FKM as the designation for about 80% of fluoroelastomers; the remaining fluorinated elastomers sit in adjacent letter codes that engineers routinely confuse with FKM and FPM, so the line has to be drawn explicitly [S2].
FFKM (perfluoroelastomer) is a separate ASTM code with a fully perfluorinated backbone, continuous service ratings commonly in the 250–325 °C window and aggressive-chemical resistance that exceeds FKM, at a price point typically 10–100x higher; FEPM (tetrafluoroethylene/propylene, the Asahi Glass Aflas type) shares some of FKM's chemical profile but with different base and amine resistance and a different temperature ceiling, so FEPM is not a drop-in for FKM even though both are "fluorinated" [S2].
Cure system matters as much as the FKM/FPM label
The label on the bag does not capture the cure chemistry, and the cure chemistry drives steam, acid and base performance, which is why two "FKM" O-rings from different compounders can behave very differently in a CIP/SIP or amine service loop. [S1]
Bisphenol (ionic / dihydroxy) cure is the dominant system today, gives the best compression set and is the default in most Type 1 and Type 2 compounds; peroxide cure is preferred where the seal sees steam, hot acids or hot bases, including food, pharmaceutical and bioprocessing lines with repeated sterilisation cycles; diamine cure is largely historical but still appears in some rubber-to-metal bonded parts because of its adhesion behaviour [S1][S2]. The practical consequence for a seal drawing is that the spec line must carry all three: the family (FKM per ASTM D1418 / FPM per ISO 1629), the type (1–5) and the cure system, otherwise the same "FKM 75 Shore A" callout can be met by a compound that fails steam in 72 hours and one that survives.
Procurement and traceability: when to write FKM, when to write FPM
For a project shipped to a European end-user, the BOM, the certificate of conformity and the REACH/RoHS paperwork are written against ISO 1629, so the correct line item is "FPM" plus the ISO hardness and the ISO 3601-1 size code, not "FKM" [S1][S7][S8].
For a project delivered into North America with ASME B16.20 gasket callouts and ASME Section VIII divider traceable documentation, the correct line item is "FKM" per ASTM D1418 plus the relevant ASTM D2000 line callout (for example "FKM 75 Shore A, H2 service per NACE MR0175" for sour hydrocarbon seals), and the same polymer bag will arrive with FKM on the certificate [S3][S9]. Multinational projects should fix one of the two conventions project-wide and convert at the document interface, because mixing FKM and FPM on the same drawing is the single most common root cause of "wrong material" non-conformances at goods-in inspection, even when the polymer in the box is correct.
Selection rules for a sealing engineer in 2026
Use FKM/FPM as a family code only, never as a complete material callout: add the type (1–5), the fluorine content range (typical 66, 68–69 or 62–68 wt%), the cure system (bisphenol, peroxide, or diamine), the hardness class (usually 70–80 Shore A) and any mandatory compliance line (NACE MR0175 for sour service, FDA 21 CFR 177.2600 or USP Class VI for food/pharma, EN 549 for gas) [S1][S2][S3].
Write Viton only when the engineering case requires a specific Chemours polymer family (for example Viton ETP-S for strong amine or mixed-acid service, or Viton GLT for low-temperature flexibility) and the supply chain is qualified to deliver it, because locking the drawing to a single brand removes the second-source option and typically extends lead time by 2–6 weeks versus a generic FKM/FPM compound [S1]. For most hydrocarbon, hot-oil and general chemical service on static O-rings in pumps, pressure sensor housings and PLC panel enclosures, a generic FKM/FPM Type 1 or Type 2 bisphenol-cured compound at 75 Shore A is the cost-defensible default; step up to FFKM, FEPM or a peroxide-cured FKM only when the media and temperature list rules out the default. For a deeper look at how the FKM backbone composition maps to those trade-offs, see FKM Polymer Backbone: Composition, Monomer Types, and Material Trade-offs; for the temperature ceiling question that always comes next, FKM Continuous Service vs Peak Excursion: Where the +200°C Line Actually Sits and FKM vs NBR O-rings: hot oil and fuel resistance decision guide are the natural follow-ups.