Fluororubber (FKM)

What is Fluororubber (FKM)

Fluororubber is a synthetic elastomer in which fluorine atoms are bonded directly to a carbon backbone, replacing most of the hydrogen found in ordinary hydrocarbon rubbers such as nitrile or EPDM. The carbon-fluorine bond is one of the strongest single bonds in organic chemistry, and it is this bond that gives the material its defining property set: resistance to heat, to oxidation, to fuels and oils, and to a wide range of aggressive chemicals. The generic abbreviation FKM comes from ASTM D1418, the standard that classifies rubbers by their chemical composition; the identical material is called FPM under the European ISO 1629 and DIN nomenclature.

Structurally, every commercial FKM is a copolymer or terpolymer of vinylidene fluoride (VDF, sometimes written VF2) with one or more comonomers. The most common comonomer is hexafluoropropylene (HFP); higher-performance grades add tetrafluoroethylene (TFE), and specialty low-temperature grades substitute perfluoromethyl vinyl ether (PMVE). The relative proportions of these monomers set the polymer's fluorine content, which in turn governs almost every engineering property a buyer cares about. FKM is distinct from FFKM (perfluoroelastomer, fully fluorinated) and from FEPM (tetrafluoroethylene-propylene, sold as AFLAS), both of which are separately classified in ASTM D1418.

The commercial history is well documented. DuPont introduced the first fluoroelastomer under the Viton trademark in 1957, targeting jet-engine and high-temperature aerospace seals. Over the following decades the technology spread into automotive fuel systems, chemical processing, and oilfield service. Today the raw polymer is produced by Chemours (Viton), Daikin (Dai-El), 3M (Dyneon), Solvay (Tecnoflon), AGC, and Gujarat Fluorochemicals, while finished O-rings, lip seals, and gaskets are molded by Parker Hannifin, Trelleborg, Freudenberg, and a large base of regional compounders. The global fluoroelastomer market was valued in the mid-teens of billions of US dollars in the mid-2020s, with seals and gaskets representing roughly a third of consumption.

A key point of vocabulary clears up most purchasing confusion: Viton is a brand, FKM is the material class. Specifying "Viton" on a drawing is like specifying "Teflon" instead of PTFE. It communicates intent but not a verifiable property set, and it locks the buyer to one supplier's grade naming. The professional practice, covered in Chapter 6, is to specify the FKM type, fluorine content, cure system, and an ASTM D2000 property class so that several molders can quote an equivalent compound.

Four engineering attributes decide whether an FKM seal succeeds or fails in service: continuous and intermittent temperature rating, chemical and fuel compatibility, low-temperature sealing limit, and compression set retention. These four are not independent. Pushing fluorine content up to improve fuel resistance raises the low-temperature limit; switching to a peroxide cure for steam resistance lowers the maximum continuous temperature. Understanding these trade-offs is the core skill this guide teaches.

FKM Types and Classification

FKM is not one material but a graded family. The cleanest way to classify it is by monomer composition and the resulting fluorine content, because fluorine content is the single number that most reliably predicts chemical resistance and low-temperature behavior. Industry and the Chemours Viton naming system both organize grades into copolymers, terpolymers, and specialty types. The table below summarizes the mainstream families using widely published composition data.

Copolymers (Type A) are dipolymers of VDF and HFP with a fluorine content near 66 percent. They are the original and most economical FKM, easy to mold, and fully adequate for engine oil, mineral hydraulic fluids, and air at temperatures up to about +200 degrees Celsius. Type A is the default when a datasheet simply says "Viton A" or "FKM 66." Its limitation is reduced resistance to oxygenated fuels (gasoline blended with ethanol or methanol) and to aromatic solvents, where higher-fluorine grades perform measurably better.

Terpolymers (Type B and Type F) add TFE as a third monomer to raise fluorine content to roughly 68 and 70 percent respectively. The added fluorine tightens the polymer against fluid permeation and swelling. Type F, at about 70 percent fluorine, is the grade specified for flex-fuel and high-ethanol gasoline, where Type A would swell unacceptably. The cost of this chemical robustness is low-temperature stiffness: a Type F seal becomes hard and loses sealing force in the cold sooner than a Type A seal.

Specialty grades (GLT, GFLT, ETP) change the comonomer chemistry rather than just the ratio. GLT and GFLT replace HFP with perfluoromethyl vinyl ether, which lowers the glass transition temperature and extends static sealing to approximately -40 degrees Celsius while keeping fluorine content competitive. ETP-type grades (marketed as Viton Extreme) use an ethylene-TFE-PMVE chemistry that resists bases, amines, and a much broader chemical span than standard FKM, narrowing the gap to FFKM at a lower cost. These specialty families are always peroxide cured.

Two adjacent materials are often confused with FKM and deserve a clear boundary. FFKM (perfluoroelastomer, such as Kalrez or Chemraz) is fully fluorinated, rated to +260 degrees Celsius or higher, and resists nearly all chemicals, but costs many times more. FEPM (tetrafluoroethylene-propylene, sold as AFLAS by AGC) excels in hot amine, base, and sour-gas service where FKM is weak. Both are separate ASTM D1418 classes, not FKM grades, and a procurement engineer should not treat them as interchangeable substitutions.

Cure Systems and Grades

After the polymer family, the second decision that defines an FKM compound is its cure system, the chemistry that crosslinks the raw gum into a finished elastomer. Two cure systems dominate, and the choice between them is not cosmetic: it changes heat rating, compression set, and resistance to water, steam, acids, and bases. A third historical system, diamine cure, survives only in adhesion-critical and legacy applications. The table below compares the two mainstream systems on the parameters that drive selection.

Bisphenol cure, an ionic crosslinking reaction using bisphenol AF with an onium accelerator, is the most widespread FKM cure system. It produces the lowest compression set of any FKM cure, meaning the seal recovers its shape and sealing force best after long-term compression at temperature. This is exactly the property that matters for static O-rings and the rotating-shaft seals in mechanical seals running in hot engine oil, so bisphenol-cured Type A and Type B dominate automotive and industrial powertrain applications. Its weakness is the ether crosslink itself, which is vulnerable to hydrolysis: hot water, steam, strong acids, and bases gradually break the network and the seal hardens or softens.

Peroxide cure is a free-radical reaction that requires the polymer to contain a cure-site monomer (CSM), typically a brominated or iodinated site, so not every FKM gum can be peroxide cured. The resulting carbon-carbon crosslinks are far more hydrolytically stable than bisphenol ether links. Peroxide-cured FKM is therefore the correct specification for steam service, hot water, aqueous acids and bases, and clean-in-place / sterilize-in-place cycles in food and pharmaceutical equipment. Peroxide cure is mandatory for the GF high-fluorine and GLT low-temperature families. The trade-offs are marginally higher compression set and a slightly lower maximum continuous temperature than an equivalent bisphenol compound.

Diamine cure was the original FKM crosslinking method. It offers excellent metal-to-rubber adhesion, useful for bonded seals and rubber-to-metal parts, but it suffers from short scorch time that makes processing difficult and gives poorer heat and compression-set performance than bisphenol cure. For these reasons most modern copolymer FKM is bisphenol cured, and diamine cure persists mainly where its adhesion characteristics are specifically needed.

Two practical implications follow for a buyer. First, the cure system is not always printed on a generic "Viton O-ring" listing, so it must be requested explicitly when the application involves water, steam, acid, base, or amine. Second, post-cure matters: most FKM compounds require a secondary oven post-cure, typically 4 to 24 hours between +200 and +250 degrees Celsius, to develop their full compression-set resistance, chemical resistance, and rated mechanical properties. A seal that was not properly post-cured can show outgassing, dimensional drift, and degraded sealing in service even if the base polymer was correct.

Chemical and Thermal Compatibility

The reason engineers reach for FKM despite its cost is its compatibility envelope: it tolerates fluids and temperatures that destroy nitrile, EPDM, and most general-purpose grades of industrial rubber. But that envelope has hard edges. FKM is excellent against one set of media and outright incompatible with another, and a seal placed on the wrong side of that line fails fast. The first table maps the media classes where FKM excels and where it fails, the chemistry behind most field failures.

The thermal envelope is the second axis of compatibility. General-purpose bisphenol-cured FKM is rated for continuous service from roughly -20 degrees Celsius to +200 degrees Celsius, with intermittent excursions to +230 degrees and brief peaks approaching +250 degrees. The high-temperature end is genuinely strong: FKM keeps useful sealing properties at temperatures that embrittle nitrile and degrade EPDM. The practical ceiling is governed less by the polymer melting and more by long-term compression set and oxidation; running continuously at the top of the band shortens seal life.

The low-temperature end is where FKM disappoints engineers who expect a premium rubber to do everything. Standard copolymer FKM begins to lose sealing force and turns glassy below about -15 to -20 degrees Celsius. This is a function of the polymer's glass transition temperature, which rises with fluorine content. For cold-climate fuel systems, refrigeration, and outdoor equipment, the answer is a GLT or GFLT low-temperature grade, which uses perfluoromethyl vinyl ether to push static sealing to approximately -40 degrees Celsius. Below -40 degrees, FKM is the wrong material and silicone rubber, low-temperature FFKM, or a different elastomer family should be considered.

Three failure mechanisms account for most field returns. Swelling occurs when an incompatible solvent (ketone, ester, amine) penetrates the network, raising volume and dropping hardness until the seal extrudes. Hardening and cracking occurs when hydrolysis or amine attack breaks bisphenol crosslinks, raising hardness until the seal loses elasticity and fractures. Compression set is the slow, permanent loss of recovery after prolonged compression at temperature; an FKM with poor compression set leaves a flattened O-ring that no longer fills its gland. Specifying the right family and cure system for the actual media and temperature eliminates the first two; choosing a bisphenol cure and a proper post-cure minimizes the third.

For media that fall outside the FKM envelope, the substitution logic is straightforward. Hot amines, strong bases, and sour gas point to FEPM (AFLAS) or ETP-type Viton Extreme. Glycol-ether brake fluids, ketones, and hot water/steam at modest temperature point to EPDM. Universal chemical resistance with high temperature points to FFKM, accepting its cost. There is no single rubber that does everything, which is precisely why a media-driven selection table is the heart of any seal specification.

Key Specification Parameters

An FKM compound datasheet can list two dozen properties, but only a handful drive a selection decision. Reading them correctly, and knowing which are physical constants of the polymer versus which are tunable by the compounder, is the core skill of seal procurement. The parameters below are the ones that belong on every FKM purchase specification.

Fluorine content (weight percent) is the most predictive single number, as discussed in Chapter 2. It ranges from about 64 percent in low-temperature GLT grades to 70 percent in high-fluorine Type F. Higher fluorine means better fuel and chemical resistance but stiffer cold behavior. When a datasheet omits fluorine content, the grade name (A/B/F or 66/68/70) usually encodes it.

Hardness is reported in Shore A durometer. Commercial FKM seal compounds span roughly 55 to 90 Shore A, with 70 to 75 Shore A being the default for general O-rings. Harder compounds resist extrusion under high pressure but conform less readily to surface imperfections; softer compounds seal low-pressure and rough surfaces better but extrude sooner. Hardness is tuned by the compounder with fillers and is not a property of the base polymer.

Tensile strength and elongation describe mechanical robustness. Typical molded FKM shows tensile strength in the range of about 7 to 16 MPa (roughly 1,000 to 2,300 psi) and elongation at break commonly between 150 and 300 percent. These values are filler-dependent and matter most for dynamic seals and parts that flex during assembly. For static O-rings, compression set usually outranks tensile strength in importance.

Compression set is arguably the most important sealing property. It measures the permanent deformation remaining after a sample is held compressed at temperature for a fixed time, typically reported per ASTM D395 Method B (for example, 22 or 70 hours at +200 degrees Celsius). A good bisphenol-cured FKM achieves low compression set, often quoted under 15 to 20 percent in such tests; lower is better, because it means the seal recovers and keeps sealing force over time. Cure system, post-cure quality, and temperature all influence this number.

Temperature rating should be stated as a continuous range plus an intermittent peak, not a single number. A correct callout reads, for example, "-20 to +200 degrees Celsius continuous, +230 degrees intermittent." For cold service, the controlling figure is the low-temperature limit (the TR-10 or brittle point), which determines whether a standard or GLT/GFLT grade is required.

Standards and classification callouts tie the compound to a verifiable specification. The relevant designations include:

• ASTM D1418 (FKM): composition-based class designation for the rubber family.
• ISO 1629 (FPM): the European equivalent designation for the same material.
• ASTM D2000: line-callout system (for example, type H, class K for FKM) that ties hardness, heat resistance, and oil-swell limits into a single specification code molders can quote against.
• ASTM D395: the compression-set test method referenced on most seal datasheets.
• FDA 21 CFR 177.2600 / USP Class VI / 3-A / EHEDG: regulatory and sanitary approvals required for food, beverage, and pharmaceutical contact.

One caution unites these parameters: brand grade names are not interchangeable across raw-polymer suppliers. A "Type A" from one producer and a "66 percent copolymer" from another may behave similarly, but exact compression set, low-temperature limit, and processing window differ by compound. The defensible practice is to specify the measurable properties and a standard callout, then let qualified molders demonstrate compliance, rather than assuming brand equivalence.

Selection Decision Factors

To turn the preceding chapters into a specific compound and a defensible purchase order, follow the decision sequence below. Most FKM selection errors come not from a single wrong number but from deciding hardness or brand before media and temperature have been pinned down. These eight steps double as a fixed RFQ template.

1. Define the media and its concentration: List every fluid the seal contacts, including cleaning agents and lubricants used during assembly. Map each against the Chapter 4 compatibility table. If any medium is a ketone, ester, amine, glycol brake fluid, or hot caustic, FKM may be the wrong family before any other step matters.
2. Define the temperature window: State continuous low and high limits plus intermittent peaks. If the low limit is below about -20 degrees Celsius, you need a GLT or GFLT grade. If continuous service exceeds +200 degrees Celsius for long periods, plan for shorter seal life or step up to FFKM.
3. Choose the polymer family and fluorine content: Use Type A (66 percent) for oil and standard fuel, Type F (70 percent) for ethanol or methanol blends and aggressive solvents, and GLT/GFLT for cold service. Fluorine content is the lever that trades chemical resistance against cold flexibility.
4. Choose the cure system: Specify bisphenol cure for engine, powertrain, and general oil seals where compression set is paramount. Specify peroxide cure for steam, hot water, acids, bases, and CIP/SIP service, and remember it is mandatory for GF and GLT grades.
5. Set hardness and dimensions: Default to 70 to 75 Shore A for general O-rings; go harder (80 to 90) for high-pressure anti-extrusion duty and softer (55 to 65) for low-pressure or rough surfaces. Confirm the gland design, squeeze, and groove fill against the standard O-ring sizing tables.
6. Attach the standard callouts: Specify ASTM D1418 (FKM) or ISO 1629 (FPM), an ASTM D2000 line callout, and any sanitary or food-contact approvals (FDA, USP Class VI, 3-A, EHEDG) the application demands. This converts intent into a contract molders can quote against.
7. Verify pressure and dynamic conditions: Confirm system pressure, whether the seal is static or dynamic, surface speed and finish for dynamic seals, and the presence of rapid gas decompression (explosive decompression) risk in high-pressure gas service, which calls for a specially formulated RGD-resistant compound.
8. Total cost of ownership: Weigh purchase price against service life and downtime. FKM costs several times more than nitrile per seal, and FFKM costs many times more than FKM. The right material is the cheapest one that meets the media, temperature, and life requirement, not the cheapest per piece. A wrong-material seal that fails in service usually costs far more than the premium for the correct compound.

One dimension that buyers routinely overlook is compound traceability and serviceability: which raw polymer the molder uses, whether the compound is lot-traceable, whether food-contact or RGD certifications are on file, and whether equivalent spares can be sourced from more than one molder years later. A seal is a small part of equipment cost but a large part of unplanned downtime. Established polymer suppliers (Chemours, Daikin, 3M Dyneon, Solvay, AGC, Gujarat Fluorochemicals) and major molders (Parker Hannifin, Trelleborg, Freudenberg) maintain documented compound libraries and regional supply, which de-risks long-life and safety-critical applications. For commodity duties, qualified regional compounders can supply equivalent FKM at lower cost, provided the specification is written in measurable terms rather than a brand name.