An FKM compound designation such as "70 FKM 576" is a three-field shorthand: the leading number is the nominal hardness in Shore A, the middle token is the ASTM D1418 family code for fluoroelastomer, and the trailing number is a vendor-specific material grade identifier [S1][S2].
The 70 in the leading field means 70 Shore A (ISO 868 IRHD within ±5° on production parts), the FKM token tells you the base elastomer is a saturated fluoroelastomer covered by ASTM D1418, and the 576 is a recipe code the compounder uses to tie hardness, cure system, filler loading, and post-cure to a single datasheet [S1][S2]. ERIKS' FKM 70-compound 514625 is a worked example: 70 IRHD, copolymer, 66% fluorine, specific gravity 1.86, tensile 15 MPa, elongation 170%, and a -20 to +200°C service envelope [S1].
ASTM D1418 families: A, B, F, and the specialty grades
The FKM family is not one polymer but a ladder of monomer combinations, and the A/B/F subtype is what drives fluid and low-temperature behaviour far more than the trailing recipe number [S2][S3]. Type A (FKM Type 1) is a dipolymer of vinylidene fluoride (VF2) and hexafluoropropylene (HFP) with bisphenol cure and about 66% fluorine; Type B and Type F (both FKM Type 2) are terpolymers of VF2, HFP, and tetrafluoroethylene (TFE) at roughly 68% and 70% fluorine respectively [S2][S3]. The quick field test is 168 h / 23°C methanol immersion: A swells about 90%, B about 40%, F about 5%, which is a useful way to verify what a compounder actually shipped [S2].
Specialty grades sit on top of this ladder: GLT and GFLT use a fluorinated vinyl ether monomer (PMVE) for low-temperature flexibility (TR-10 around -24 to -30°C), and the GFLT/EPT-S family pushes peroxide cure and higher fluorine for amine and ketone resistance that bisphenol-cured A/B/F cannot deliver [S2]. Chemours (Viton), Daikin (Dai-El), Solvay (Tecnoflon), and formerly 3M Dyneon are the main base-polymer suppliers; Dyneon is scheduled to discontinue FKM production in 2025, so spec sheets and equal-grade cross-references written before that point need re-checking [S2].
What the trailing recipe number actually controls
Once the polymer family and fluorine level are fixed by the A/B/F/GLT/GFLT/ETP field, the trailing compound code sets the secondary parameters: filler type and loading (carbon black, clay, barium sulfate at 5-60 PHR), acid acceptor (MgO, Ca(OH)2, ZnO at 0-6 PHR), processing aid at 0-2 PHR, and the cure package at 1-8 PHR [S2]. The acid acceptor choice is what makes a 70 FKM compound DVGW EN 549 E1 certified for gas at 0 to +150°C, the way the 514625 example is, versus a generic industrial grade [S1].
Compression set, low-temperature TR-10, and post-cure schedule are also recipe-level decisions. ERIKS' 514625 hits 15% compression set after 70 h at 200°C on slab (ISO 815), TR-10 of -16°C, and -9° hardness change plus -9% elongation change after 70 h at 250°C air ageing (ASTM D 573) [S1]. A second 70 FKM from a different compounder can land in the same hardness window but show a noticeably different compression set or TR-10, which is why procurement specs need both the polymer family and the recipe code, not just "FKM 70" [S1][S2].
Selection criteria mapped to the three code fields
Read the code in this order. First, the Shore A number sets the seal mechanics: 70-75 is the workhorse for O-rings and pressure transmitter process diaphragms, 80-90 takes over for higher extrusion resistance in hydraulic industrial valve service, and 60-65 is used where low closing force or better conformability is needed [S3]. Second, the FKM subtype sets the fluid envelope: A for aliphatic and aromatic hydrocarbons at moderate cost, F for fuels with methanol or ethanol content and for low molecular weight oxygenated solvents, GLT/GFLT for sub-zero TR-10, and ETP for amines, ketones, and strong bases where A/B/F grade as "NR" (not recommended) [S2].
Third, the trailing recipe code carries certifications, cure system, and post-cure. Bisphenol cure gives faster cycle times and lower compression set for general sealing; peroxide cure is specified for hot water, steam, and acidic environments that would attack bisphenol-cured parts [S2]. For a sealed flow meter in chemical dosing, or a pressure sensor wetted by amine-based corrosion inhibitors, the right call is usually a peroxide-cured GFLT or ETP-S grade at 75-80 Shore A, not a generic 70 FKM [S2][S3].
Service envelopes by FKM subtype
The headline numbers for FKM sit between -10°F (-23°C) for general-purpose grades and +400°F to +600°F (+204°C to +315°C) for continuous service, with the upper end held by the specialty peroxide-cured types and the lower end set by TR-10 of the chosen grade [S3]. For comparison, fluororubber compounds at 66% fluorine (Type A) tolerate hydrocarbon service well but swell badly in methanol, while 70% fluorine (Type F) cuts that methanol swell to roughly 5% over 168 h at 23°C [S2][S3].
Against other elastomer choices covered in adjacent selection guides, the trade is straightforward: FKM beats NBR and EPDM on heat and aliphatic/aromatic fluids, but loses to silicone on dry cold, to EPDM on hot water and steam, and to ETP-S or Kalrez perfluoroelastomer on amines, ketones, and strong oxidizers [S2][S3]. Within the FKM family, the jump from A to F to GFLT/ETP-S is paid for in compounding cost and cycle time, and only when the fluid or temperature window forces it [S2].
Limits, failure modes, and what the code does not tell you
Three failure modes are not visible in the "70 FKM 576" string. First, low-temperature flexibility: a 70 FKM Type A typically has TR-10 around -17°C and is not a cold-climate seal, even though the datasheet's static cold-range number may look generous [S2]. Second, steam and hot water: bisphenol-cured A/B/F grades are rated "3 - Moderate to Severe Effect" in 212°F steam and are usually the wrong pick for boiler or sterilizer service, where peroxide-cured GFLT or ETP-S is the documented option [S2]. Third, regulatory approvals: a 70 FKM can be food-grade WRAS, USP Class VI, DVGW EN 549, NACE MR0175, or none of these, and the recipe code, not the family token, carries the certification [S1].
Procurement specs that read only "FKM 70" invite the cheapest A-type with 66% fluorine and no approvals; specs that pin the subtype (A, B, F, GLT, GFLT, ETP-S), the cure system, and the standard callouts (EN 549 E1 for gas, USP Class VI for pharmaceutical, NACE MR0175 for sour service) lock the recipe to the right part [S1][S2]. Cross-checks for incoming material are the 168 h methanol swell test for A/B/F verification and a TR-10 measurement for cold-rated grades [S2].
Verification signals to track
Two signals are worth tracking over the next procurement cycle. First, the 3M Dyneon FKM exit announced for 2025 will reshape equal-grade cross-reference tables through 2026, especially for GLT and GFLT equivalents that were originally Dyneon products; expect Chemours, Daikin, and Solvay to publish updated cross-reference documents [S2]. Second, peroxide-cured specialty grades (GFLT, ETP-S) are the fastest-growing FKM segment for amine-rich process streams and biofuel service, and the recipe-code lists from each compounder will be the most useful artifact for engineers re-specifying seals in those services [S2][S3].