ASTM D1418 splits FKM fluoroelastomers into chemistry families, and the three families engineers actually specify, Type 1, Type 2, and Type 3, sit in narrow but consequential fluorine-content windows that drive chemical resistance, compression set, and low-temperature flexibility [S1][S2][S4].
The numbers themselves are not disputed across the literature: Type 1 around 66 wt% F, Type 2 at 68–69 wt% F, Type 3 at 62–68 wt% F depending on PMVE loading [S2][S4]. For context on the broader fluorocarbon family, the fluororubber reference page maps the same chemistry against FFKM and FEPM neighbours.
Type 1 (VDF/HFP copolymer): the 66 wt% baseline
Type 1 FKMs are dipolymers of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), with fluorine content landing at approximately 66 weight percent [S2][S4]. This is the "general purpose" FKM, equivalent to Chemours' Viton A family, and it is the polymer most engineers actually receive when a drawing says "Viton" with no further qualification [S3].
Volume swell in methanol at 23 °C for 168 hours sits close to 90% for Type 1, against roughly 40% for Type 2 and about 5% for Type 5-class F polymers, which is the classical differentiation test [S1]. Service temperature window is conventionally -26 °C to +205 °C continuous, with peaks to +230 °C on bisphenol-cured A-type compounds, and the polymer carries an "1" rating (little to minor effect) against aliphatic hydrocarbons, aromatics, and non-alcohol automotive fuels on the standard Apple Rubber chemical-resistance matrix [S1].
Engineers should reach for Type 1 when the media set is conventional: petroleum oils, diesel, mineral hydraulic fluid, and the upper end of aliphatic solvent exposure. Where the methanol, MTBE, MEK, MIBK, or strong-amine content of the service stream is non-trivial, Type 1 reads "NR" (not recommended) on the same matrix, which is the cue to step up the family, not the cure system [S1].
Type 2 (VDF/HFP/TFE terpolymer): 68–69 wt% F, more chemical headroom, less low-temperature margin
Type 2 adds tetrafluoroethylene (TFE) to the VDF/HFP backbone, pushing fluorine content into the 68–69 weight percent band and tightening the polymer's resistance to aromatic hydrocarbons, chlorinated solvents, and concentrated acids [S2][S4]. The trade is mechanical: higher TFE loading raises compression set at room temperature and pulls the TR-10 (temperature of retraction at 10%) upward, narrowing the low-temperature flexibility window relative to Type 3 [S1][S2].
This is the family that maps onto Chemours' Viton B and F grades, and it is the workhorse for engine lubricating oils rated SG–SH, automotive fuels containing 5–15% alcohol, and most chemical-process seals where aromatics show up in the fluid stream [S1]. On the same Apple Rubber matrix, Type 2 drops the rating on automotive fuels with up to 100% alcohol to a "2" (minor to moderate effect) rather than the "NR" that Type 1 carries, and the hot-water/steam rating improves from "3" down to "2" [S1].
For hot aggressive service the comparison lines up cleanly: Type 1 at ~66 wt% F and a TR-10 near -17 °C, Type 2 at 68–69 wt% F and a TR-10 near -14 °C, and Type 3 at 62–68 wt% F with a TR-10 down to -30 °C in GLT/GFLT compounds [S1][S2][S4]. Engineers specifying above the +200 °C continuous line should pair this article with the FKM continuous-service vs peak-excursion reference before locking the compound.
Type 3 (VDF/TFE/PMVE): lower fluorine ceiling, best low-temperature flexibility
Type 3 swaps the HFP for perfluoromethyl vinyl ether (PMVE), which puts the fluorine content window at 62–68 weight percent, lower than Type 2 in many formulations, but buys the best cold-flexibility behaviour in the FKM family [S2][S4]. The PMVE monomer also changes the cure-system menu: peroxide curing is the established route for Type 3 because diamine and bisphenolic ionic mechanisms can cleave the PMVE-bearing polymer backbone [S2].
Low-temperature TR-10 values of -30 °C (GLT) and -24 °C (GFLT) on the Apple Rubber matrix are the headline numbers; GFLT is the Type 3 variant that retains the higher-fluorine terpolymer backbone for chemical resistance while gaining PMVE-driven cold flexibility [S1]. This is the family specified for aerospace low-temperature seals, cold-climate hydraulic systems, and any FKM service where the TR-10 of a Type 1 or Type 2 compound would put the seal below its glass transition in the operating envelope [S1][S3].
The catch is cost and chemical headroom: a Type 3 GLT/GFLT compound costs more than a Type 1 A-type at the same hardness, and the lower fluorine ceiling caps resistance to aggressive aromatics and chlorinated solvents compared to a Type 2 terpolymer at 68–69 wt% F [S3][S5].
Selection matrix: which Type to call out on the drawing
Across the three families, the decision reduces to four criteria: dominant chemistry in the service fluid, continuous and peak temperature, lowest expected operating temperature (TR-10 ceiling), and cost per kg of compound. [S3]
On those four axes the matrix reads: Type 1 for petroleum oils, diesel, and mineral hydraulics at -26 °C to +205 °C continuous, lowest cost, ~66 wt% F [S1][S2]. Type 2 for automotive fuels with alcohol content, aromatic and chlorinated solvents, and SG–SH engine oils at 68–69 wt% F, with TR-10 around -14 °C and a mid-range cost premium [S1][S2][S4]. Type 3 for low-temperature aerospace and cold-climate hydraulics, TR-10 down to -30 °C, fluorine content 62–68 wt% F, and a cost step above Type 2 that the trade press puts at roughly 1.5–2× the price of a Type 1 baseline [S5].
A practical decision rule: if the fluid stream contains methanol, MTBE, MEK, MIBK, or strong amines at any meaningful concentration, the Apple Rubber matrix marks Type 1 and Type 2 as "NR" or "2", which is the point at which engineers should be looking at Type 5 (VDF/HFP/TFE/PMVE/ethylene) or stepping up to FFKM rather than re-specifying within Type 1–3 [S1][S7]. Where the question is mineral-oil compatibility specifically, the FKM vs EPDM in mineral oil decision guide gives the side-by-side elastomer comparison.
Cure system and standards: what the Type number does not tell you
The Type 1/2/3 classification is a chemistry-family call, not a finished-compound call. Within any Type, the cure system, bisphenol (ionic), peroxide, or the legacy diamine route, moves the steam, acid, and base resistance significantly, and it also controls compression set and rubber-to-metal bond strength [S2][S3]. Bisphenol is the most common and gives the best general compression set; peroxide cure is preferred for steam, acids, bases, and any application exposed to CIP/SIP cycles in food or pharmaceutical service, and it is effectively mandatory for Type 3 polymers containing PMVE [S2][S3].
Standard references to write on the drawing or purchase order: ASTM D1418 for the Type designation, ISO 1629 (FPM) for the European equivalent, and a callout of the Viton A/B/F/GLT/GFLT/ETP family if the spec must be Chemours-specific, or the equivalent Tecnoflon, DAI-EL, or Dyneon grade for non-Chemours supply [S1][S3][S4]. Note that 3M has communicated intent to discontinue Dyneon FKM production in 2025, which tightens the supplier count for Type 1 and Type 2 compounds and is worth flagging in any second-source qualification [S1].
FKM materials in instrument and process skids are typically validated against the same chemical-exposure matrices used for elastomer selection in electronic test and measurement housings, and the fluid lists in pressure transmitter datasheets are a useful cross-check on what real-world media an FKM wetted seal will actually see.
Failure modes and what to watch in service
The dominant in-service failure for under-specified FKM is volume swell, not thermal breakdown, and the methanol-swell test (168 h at 23 °C) is the single most discriminating field check: 90% swell flags Type 1, 40% flags Type 2, and 5% flags the high-fluorine F/GF/GFLT family [S1]. For Type 3, the failure mode inverts toward compression set and low-temperature hardening if the polymer is pushed above its TR-10, with seal leakage on cold start as the typical symptom [S1][S2].
Steam and hot-water service is the second failure cluster: Type 1 ratings of "3" (moderate to severe) drop to "2" for Type 2 and improve further with peroxide cure, which is why amine-based and steam-exposed chemical-plant seals are almost always either Type 2 with peroxide cure or a Type 5/ETP compound [S1][S3]. When the seal sees FFKM-class chemistry on a competitive tender, the FFKM vs FKM comparison (and the dedicated FFKM reference at [S7]) is the next step up the performance ladder, with FFKM typically running 5–10× the cost of a Type 1 FKM and operating continuously to +300 °C or beyond.
Trackable signals to watch over the next sourcing cycle: 3M Dyneon FKM exit status (production was reported to discontinue in 2025 per [S1]), any Chemours Viton Extreme ETP-S grade extensions that blur the FKM/FFKM line, and the steady downward drift of Type 3 GLT compound pricing as PMVE monomer volumes scale.