Under ASTM D1418, FKM Type 3 is the low-temperature branch of the fluoroelastomer family, built from vinylidene fluoride (VDF), tetrafluoroethylene (TFE), and perfluoromethylvinylether (PMVE), with fluorine content typically landing between 62 and 68 weight percent [S1][S2]. The PMVE monomer is what gives this grade its edge: it disrupts crystallinity and pushes the glass transition down, while TFE and VDF keep the heat and fluid-resistance profile close to the rest of the FKM family [S1].
ASTM D1418 is the classification backbone for the entire FKM family, paired with ISO 1629, which uses the FPM designation for the same polymer class [S1][S3]. Anyone reading a datasheet or a purchase order will see FKM/FPM used interchangeably, and the Type 1 through Type 5 numbering stays the same across both standards, which is why a fluororubber callout on a drawing almost always carries a D1418 type suffix.
Monomer Package, Fluorine Range, and What 62-68 wt% Actually Buys You
Type 3 is a terpolymer: VDF provides the reactive sites for crosslinking, TFE lifts the fluorine content, and PMVE delivers the low-temperature flexibility, with the reported fluorine window sitting at 62-68 wt% versus 66 wt% for a baseline Type 1 dipolymer and 68-69 wt% for a Type 2 terpolymer [S1]. The 4-7 wt% drop in fluorine versus Type 2 is the deliberate trade: engineers accept a small loss in hydrocarbon and oxidative resistance in exchange for a TR-10 around -24 to -30 °C, depending on the specific polymer [S2].
In a methanol volume-swell comparison at 168 h / 23 °C, an FKM Type 3 polymer cured and compounded to a similar chemical-resistance grade as a Chemours A- or F-type Viton will land in the same swell range, while a Type 1 A-grade will swell close to 90% and a Type 2 F-grade around 5% [S2]. That single data point is why the standard calls Type 3 "low temperature FKM" rather than "high chemical resistance FKM": the monomer mix tilts toward flexibility, not fluid resistance.
Why the Curing System Has to Change for PMVE-Containing Polymers
PMVE is the reason Type 3 FKM is almost always peroxide-cured rather than diamine- or bisphenol-cured. Diamine and bisphenolic crosslinking systems cause backbone cleavage in polymers containing PMVE, so peroxide, a free-radical mechanism, is the original cure developed for this type [S1]. Peroxide cures also help with hot aqueous, amine, and acid exposure, which is why peroxide is the default for any low-temperature FKM that will see modern biofuel or engine-oil service.
Bisphenol (ionic) curing is the most common across the broader FKM market because it gives the best compression set and hydrolytic stability, but it is a poor match for PMVE-containing polymers [S1]. For Type 3, picking a bisphenol-cured compound to chase a "standard" FKM look and feel is one of the most common ways specifiers lose the low-temperature benefit the polymer was bought for in the first place.
Type 3 vs Type 1 vs Type 2 on the Decision Criteria That Matter
On a side-by-side, the three most relevant ASTM D1418 FKM grades sort out roughly as follows: [S2]
Low-temperature flexibility (TR-10): Type 1 around -17 °C, Type 2 around -14 to -7 °C, Type 3 around -24 to -30 °C, with specialty Type 5 ETP-S variants going colder still [S2]. For any seal in a cold-start environment, industrial valve trim in arctic piping, or a chiller-side pressure transmitter isolation block, that 10-15 °C TR-10 shift is the reason Type 3 exists.
Fluid resistance: Type 2 (VDF/HFP/TFE, 68-69 wt% F) and Type 5 lead in aggressive fuels and aromatic hydrocarbons, while Type 1 (VDF/HFP, ~66 wt% F) is the general-purpose baseline [S1][S2]. Type 3's chemical resistance is "similar to A or F type" depending on formulation, not a guaranteed upgrade [S2].
Steam, amine, and base resistance: None of the standard FKM types are recommended for strong base, high pH amines, or low-molecular-weight carbonyls like MEK and MTBE, and a Type 3 compound follows the same rule unless reformulated [S2]. Specifiers chasing amine resistance need to look at FEPM (Aflas) or FFKM, not just a different FKM type.
Where Type 3 FKM Is Specified, and Where It Is the Wrong Choice
Type 3 is the right call when seal flexibility below -20 °C matters more than maximum fluid resistance: aerospace O-rings and exhaust-housing seals that have to flex on cold soak, low-outgassing seals for vacuum and space hardware, and automotive seals exposed to flex fuels and cold-start thermal cycling [S2]. It is also a common choice for hydraulic U-cup and V-packing rings, pump seals, and check-valve balls where low-temperature sealing force is the limiting factor [S2].
Type 3 is the wrong call when the service is high-temperature steam, hot amines, glycol-based brake fluid, or strong acids, which attack the VDF backbone regardless of PMVE content, and when maximum hydrocarbon resistance is required at ambient or elevated temperature, where a Type 2 or Type 5 will outperform it on volume swell and permeation [S2][S4]. It is also not a free upgrade over a standard FKM in a flow meter or seal kit that lives indoors at 20-80 °C, where the low-temperature benefit is never used and the slight loss in baseline chemical resistance is real.
Standards, Testing, and How to Read a Type 3 Datasheet
ASTM D1418 supplies the classification code (FKM Type 3), while ASTM D2000 is typically called out on automotive drawings to specify the actual compound property requirements, hardness, tensile, elongation, heat aging, and fluid swell for the part [S5]. ISO 1817 is the standard test method referenced for fluid-resistance data, and ISO 1629 carries the equivalent FPM designation [S1][S5]. The Viton equivalents from Chemours are the GLT and GFLT families, which are Type 3 chemistry sold under trade name rather than ASTM type [S2][S4].
A useful trackable signal: 3M Dyneon, listed in 2023 as an FKM polymer producer, is reported to be discontinuing FKM production in 2025, so Type 3 buyers should confirm the supply chain against Chemours (Viton), Solvay (Tecnoflon), Daikin (Dai-El), and Gujarat Fluorochemicals before locking a long-term qualification [S2]. The same logic explains why measurement and test protocols on incoming Type 3 lots should include a TR-10 or low-temperature flexibility check, not just hardness and tensile, since the low-temperature claim is the entire reason the type exists.
See also our earlier report, Arc vs TIG welding for pipe root pass: process selection, prep geometry, and code fit.