FKM grade selection drives seal life: Type A (66% F), Type B (68% F), and Type F (70% F) form the standard Viton™ ladder, while GF and GLT extend the envelope for aggressive fluids and low-temperature service respectively [S2][S3][S4].
ASTM D1418 splits these into Type 1 (dipolymer A), Type 2 (terpolymer B and F), Type 3 (low-temperature with fluorinated vinyl ether), and Type 5 (ETP, perfluoroether architecture for harshest media) [S4]. Engineers who write "Viton" on a print without naming the family accept whichever polymer a compounder reaches for first, which is a documented source of field failures in fuels, amines, and MTBE service [S3].
How the A, B, F, GF, GLT families map to ASTM D1418
Viton™ A is a dipolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), classed as ASTM D1418 FKM Type 1, with about 66% fluorine and the best compression set in the family but the weakest resistance to low-molecular-weight oxygenated solvents [S4][S6]. Viton™ B and Viton™ F are both HFP/VDF/TFE terpolymers (FKM Type 2); F carries roughly 70% fluorine versus about 68% in B, and the extra TFE drives volume swell in methanol down to single digits, a defining test result that reliably distinguishes the two [S4].
GF is the high-fluorine (about 70%) variant with peroxide cure for better steam and acid resistance, while GLT and GFLT are Type 3 polymers that swap part of the HFP for perfluoromethyl vinyl ether (PMVE) to push low-temperature TR-10 down to roughly -30°C in GLT-S, with dynamic seal serviceability rated to about -31°C and static to about -45°C per Chemours [S1][S2]. GFLT combines that low-temperature backbone with high fluorine content, so it is the grade to specify when a seal sees both arctic starts and aggressive oxygenated fuels [S3][S8]. For projects that need to compare broader fluorocarbon sealing behavior, see the reference entry on FKM and Viton fluoroelastomers.
Fluorine content and the methanol swell test
Immersing an FKM sample in methanol for 168 hours at 23°C produces roughly 90% volume swell for Type A, about 40% for Type B, and only about 5% for Type F, which is the single most informative bench test for fingerprinting an unknown compound [S4]. That ordering is the same one engineers see in real service: Type A is fine for mineral oils, aliphatic hydrocarbons, and silicone fluids, but it balloons in MTBE, MEK, and gasoline with more than 5% alcohol, where Type F and GF hold dimensional stability [S3][S4].
Type A also loses ground quickly in hot water, steam, and engine oils meeting API SG/SH or higher, where its rating drops to "moderate to severe effect" in the Chemours resistance matrix, against "minor to moderate" for B and F [S4]. In methanol-rich flex-fuel service the practical gap is even wider: Type A and GLT are not recommended at 100% alcohol, while B, F, GF, GFLT, and ETP all clear the same duty cycle with only minor swelling [S3][S4]. When a service envelope includes aromatic hydrocarbons plus alcohols plus hot oil, the safest default is a high-fluorine terpolymer (F, GF, or GFLT) or a peroxide-curable APA grade rather than the cheapest A-type on the shelf [S3][S4].
Cure system: bisphenol vs peroxide
FKM compounds cure by either bisphenol (ionic) or peroxide routes, and the choice is made at the compounder, not by the polymer grade letter [S3][S4]. Bisphenol-cured A and B compounds dominate general industry because they deliver excellent compression set at moderate cost, but they hydrolyze in amines, hot water above about 100°C, and strong bases, where ratings collapse to "not recommended" in the Chemours matrix [S4].
Peroxide cure adds about 1 to 8 PHR of organic peroxide plus co-agent and pushes hot-water, steam, and acid performance up a tier, which is why the same polymer family behaves very differently in a food/pharmaceutical CIP/SIP seal than in a static O-ring in hot lube oil [S3][S4]. The trade-off is processing: peroxide cures typically need longer press times, post-cures, and tighter control of filler acidity, and they often sacrifice a few points of compression set versus bisphenol on the same polymer backbone [S3]. For equipment builders sourcing industrial sealing components that share a parts list with pumps and valves, asking the supplier for the cure system in addition to the grade letter is non-negotiable on hot-water or amine duty.
Low-temperature behavior: GLT, GFLT, and the TR-10 ladder
TR-10 (temperature of retraction at 10%) is the standard proxy for low-temperature sealing limit, and Chemours publishes roughly -17°C for A, -14°C for B, -7°C for F, -6°C for GF, -30°C for GLT, and -24°C for GFLT [S4]. GLT and GFLT achieve those numbers by incorporating PMVE, which disrupts crystallinity in the VDF backbone and keeps the compound flexible at temperatures that turn a Type A O-ring glassy and leak-prone [S2][S8].
For dynamic seals, Chemours rates GLT-S down to about -31°C service; for static seals, the same polymer extends to about -45°C, both numbers being noticeably better than any A or B grade can reach even with plasticizer tricks [S2]. Real-world cold-soak data in fuel-handling equipment, aerospace actuators, and outdoor chemical processing all point to GFLT as the grade of choice when the seal must survive both -30°C starts and modern ethanol-rich fuels, which is exactly the dual-envelope that defeats ordinary F [S3][S8]. When the operating window goes deeper still, FFKM perfluoroelastomers extend the high end to roughly 325°C continuous and push low-temperature behavior back toward the -15°C zone, so FKM and FFKM overlap only in a narrow band around -15 to 205°C [S7].
Application-fit comparison: which grade to specify
For general industrial O-rings in mineral oil, hydraulic fluid, and air at up to about 200°C, a 66% fluorine Type A compound with bisphenol cure remains the lowest-risk, lowest-cost choice, and it is also the formulation most likely to come back from a distributor as "Viton" with no further qualifier [S3][S4]. For automotive fuel systems, chemical processing with aromatic solvents, and any service with more than 5% alcohol in the fuel, step to F, GF, or GFLT; the methanol swell test is the cheapest way to confirm you actually received a Type 2 terpolymer [S3][S4].
Use GLT or GFLT when the seal sees sub-zero cold starts, particularly in aerospace, outdoor chemical skids, and cold-climate oil & gas, and pick a peroxide-cured grade whenever the medium is hot water above 100°C, steam, organic acids, or amines [S2][S4]. For the most aggressive envelope, including strong bases, low-molecular-weight carbonyls such as MTBE and MIBK, and oxidizing acids, even GFLT is rated "not recommended"; only the ETP (Type 5, perfluoroether) family or full FFKM compounds clear those fluids [S3][S4]. On a print, the safest call-out is "FKM, Viton™ F (or GF/GFLT/GLT-S as applicable), 68-70% fluorine, bisphenol or peroxide cure per media, hardness 70-90 Shore A, post-cured 24h at 200°C" rather than the bare word "Viton" [S2][S3]. When this kind of seal ends up inside industrial process machinery, the same naming discipline should propagate to the BOM so the compounder cannot quietly substitute Type A for Type F.
Procurement pitfalls and traceability
Viton™ is a Chemours trademark covering more than 25 distinct polymers, so a purchase order that only says "Viton" leaves the door open for substitution between A, B, F, and the special families without violating the spec [S1][S3]. Other FKM producers use their own trade names: Daikin DAI-EL, Solvay Tecnoflon, 3M Dyneon (production scheduled to wind down in 2025), and various regional brands such as Noxtite and Fluorel, all of which can meet a generic FKM call-out but differ in polymer architecture, cure chemistry, and additive package [S3][S4].
For incoming inspection, the methanol swell test, a Fourier-transform infrared spectroscopy check for HFP/TFE/PMVE ratios, and a simple density or fluorine-content measurement (target 66% for A, 68% for B, 70% for F/GF/GFLT) are the three quickest ways to verify that the polymer in the bag matches the one on the certificate [S3][S4]. On long lead-time equipment such as process instrumentation and lighting assemblies where seals are embedded deep in the BOM, a one-line grade call-out (Viton™ F or GFLT-S) plus a fluorine-content range is far cheaper than a field failure in a hot oil or amine service.
Track these signals over the next procurement cycle: the 3M Dyneon FKM exit in 2025 has already tightened supply of certain terpolymer grades, and several compounders are extending lead times on Type 3 GLT/GFLT polymers while keeping Type 1 A inventory in stock, which makes a generic "Viton" call-out more likely to land on a Type A substitute by default [S3][S4].
Related analysis: Monopile Welding Automation and NDT Throughput: Specs That Matter.