ASTM D1418 is the nomenclature backbone used across the sealing industry to classify elastomers, and within that system FKM is the designator for the fluoroelastomer family, equivalent to FPM under ISO 1629 [S1][S3][S5]. All FKM grades share one common monomer, vinylidene fluoride (VDF), and are differentiated by the additional co-monomers built around it [S3].
The standard currently splits the family into five polymer types, numbered 1 through 5, each defined by a specific monomer set and a characteristic fluorine content range that directly drives chemical and thermal performance [S2][S3][S5]. Engineers should treat the FKM label as a class, not a material, and always confirm the sub-type before specifying a seal.
FKM Type 1: the HFP-VDF copolymer baseline
Type 1 FKM is a dipolymer of vinylidene fluoride and hexafluoropropylene (HFP), and is the original FKM chemistry developed for sealing [S2][S3][S5]. Fluorine content sits around 66 weight percent, which gives a balanced profile of heat aging, mechanical strength, and compression-set resistance at moderate cost [S3][S5].
The reference lab signal for a Type 1 grade is high volume swell in low-molecular-weight oxygenated solvents, around 90% after 168 h at 23°C in methanol, which is a quick disambiguation test against Types 2 and 3 [S2]. In service, Type 1 is the go-to material for aliphatic hydrocarbon oils, fuels without alcohol, and general-purpose fluororubber seals where fluid chemistry is mild [S2][S5].
FKM Type 2: terpolymers for higher fluorine and broader chemical resistance
Type 2 FKM adds tetrafluoroethylene (TFE) to the HFP and VDF base, producing a terpolymer with 68 to 69 weight percent fluorine and noticeably better resistance to aggressive fuels and aromatic hydrocarbons [S2][S3][S5]. The trade-off is reduced low-temperature flexibility and higher compression set compared with Type 1, so design engineers should not select Type 2 purely on chemical data without checking the TR-10 number [S3].
Methanol swell after 168 h at 23°C drops to about 40% for B-type and roughly 5% for F-type, which is a practical way to sort Type 2 grades in incoming inspection [S2]. Compound families such as Seals Eastern 5722, 5723, and 5724 (BPAF cure) and 7235, 7245, 7248 (peroxide cure) are commercial examples of Type 2 stocks at 60 to 92 Shore A durometer [S5].
FKM Type 3: PMVE-modified grades for low-temperature sealing
Type 3 FKM introduces perfluoromethyl vinyl ether (PMVE) into a VDF and TFE backbone, with fluorine content typically between 62 and 68 weight percent depending on grade [S2][S3]. The PMVE monomer disrupts polymer crystallinity, which is what delivers a TR-10 as low as approximately -30°C in GLT and -24°C in GFLT variants, the best low-temperature performance in the FKM family [S2][S3].
Chemical resistance depends on which base the PMVE is dropped into: PMVE-modified A-type behaves like a Type 1 in fluid, while PMVE-modified F-type behaves like a Type 2, so it is critical to confirm the underlying polymer, not just the Type 3 label [S2]. This is the standard choice for cold-climate aerospace and automotive flex-fuel sealing where the alternative would be a much more expensive perfluoroelastomer [S2][S5].
FKM Type 4 and Type 5: specialty chemistries for bases and H2S
Type 4 FKM is a terpolymer of propylene, TFE, and VDF with around 67 weight percent fluorine, and is selected specifically for improved base resistance because VDF content is reduced [S3][S5]. The penalty is worse swelling in hydrocarbons, so it is almost never used as a general-purpose seal and is reserved for amine- or base-rich service [S3].
Type 5 FKM is the most heavily modified chemistry, a pentapolymer of VDF, HFP, TFE, ethylene, and a fluorinated vinyl ether, with elevated base and hydrogen sulfide resistance suited to oilfield and refinery downhole tooling [S3][S5]. In oil and gas specifications it commonly sits next to FEPM, TFE/P, and ETP-S in the same material class for sour service, and should be cross-checked against NACE MR0175 limits for the specific partial pressure of H2S rather than substituted blindly for FFKM [S2][S3].
ASTM D1418 FKM Type 1 vs Type 2 vs Type 3 vs Type 4 vs Type 5: a side-by-side
Selection pivots on four criteria, and the table below lines the five ASTM D1418 sub-types up against them using the data published by Apple Rubber, Eagle Elastomer, and Seals Eastern [S2][S3][S5].
Type 1 vs Type 2 vs Type 3 vs Type 4 vs Type 5 on fluorine content, low-temperature TR-10, typical fluid service, and methanol swell after 168 h at 23°C: Type 1 has around 66 wt% F, TR-10 around -17°C, suited to aliphatic oils and non-alcohol fuels, with around 90% methanol swell [S2][S3][S5]. Type 2 has 68 to 69 wt% F, TR-10 around -14°C, suited to aromatic hydrocarbons and alcohol fuels, with 5 to 40% methanol swell depending on B vs F cure [S2][S3][S5]. Type 3 has 62 to 68 wt% F, TR-10 as low as -30°C, suited to cold-climate flex-fuel and aerospace, with base chemistry inherited from its A or F parent [S2][S3]. Type 4 has around 67 wt% F, low-temperature flexibility limited, suited to amines and bases, with hydrocarbon swelling significantly worse than Type 1 [S3][S5]. Type 5 has a high-fluorine, ethylene-modified pentapolymer backbone, suited to H2S and refinery service, with chemical profile closer to FEPM than to Type 1 [S3][S5].
Cure system and compound reality: bisphenol, peroxide, and diamine
The ASTM D1418 type describes polymer chemistry, not cure chemistry, and the two interact strongly. Ionic bisphenol cure is the most common, gives good heat resistance and compression set, and is the workhorse for Type 1 and Type 2 [S3]. Peroxide cure is required for any grade containing PMVE, including all of Type 3 GLT and GFLT, because diamine and bisphenol routes cleave the PMVE-containing backbone [S2][S3].
Diamine cure is largely historical today, but still shows up in legacy formulations and is preferred where rubber-to-metal bonding is the dominant requirement, even though the resulting crosslink is vulnerable in hot water and steam [S3]. From a spec standpoint, an O-ring data sheet that lists FKM without identifying cure chemistry is incomplete, because peroxide-cured Type 2 compounds such as 7248 can carry certifications like ISO 23936-2 for explosive decompression service that bisphenol-cured equivalents cannot [S5].
Limits, failure modes, and what FKM is not
FKM is the right call for hot hydrocarbons, many acids, and most engine oils, with continuous upper service temperatures that the manufacturer literature quotes around 200°C and 400°F, and a lower limit that ranges from about -10°C for Type 1 to -30°C for Type 3 GLT [S5][S6]. Continuous low-temperature performance in real assemblies is more demanding than TR-10 implies, so the dynamic seal limit is usually 5 to 10°C warmer than the published TR-10 [S2].
FKM is not a fit for low-molecular-weight carbonyls such as MEK, MIBK, and MTBE in the base grade, with only Type 5 and FEPM showing usable ratings, and is not recommended for strong amines, brake fluids based on glycol ethers, or hot steam above roughly 150°C without a specialty formulation [S2][S3]. Where the fluid set includes amines, ketones, or hot water and steam, the correct escalation is FEPM (TFE/P, AFLAS) or FFKM perfluoroelastomer, not a higher-numbered FKM [S3][S5].
Procurement and traceability: what to ask the supplier
For traceability, the part drawing, the purchase order, and the certificate of conformance should all carry the same ASTM D1418 sub-type, ISO 1629 FPM or FKM label, the cure system, the fluorine content window in weight percent, and the published TR-10 value [S1][S5]. On the shop floor a 168 h, 23°C methanol swell test is the lowest-cost confirmation that the supplied compound actually matches the FKM Type 1, 2, or 3 on the drawing, because it returns roughly 90%, 40%, or 5% swell on those three types respectively [S2].
For a fresh build in 2026, the practical sequence is: define the fluid set and peak temperature, pick the lowest-numbered ASTM D1418 type that survives, then verify that the chosen sub-type holds the relevant industry standard, for example NACE MR0175 for sour hydrocarbon service, ISO 23936-2 for rapid gas decompression, or the appropriate FDA and USP class for pharmaceutical use. Cross-referencing the polyurethane elastomer family is useful only when a part number turns out to need higher abrasion resistance than any FKM can deliver, in which case the next material up is usually an AU or EU polyurethane rather than another FKM sub-type. Watch the next 12 months for further supplier consolidation, given that 3M Dyneon flagged the discontinuation of its FKM polymer line for 2025 [S2], which is likely to shift share among Chemours (Viton), Daikin (Dai-El), and Solvay (Tecnoflon) on Type 1 and Type 2 grades.
For component-level specifications, see construction machinery and equipment.
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