Aflas® (FEPM / TFE-P) O-Rings

Aflas® is the trade name for FEPM, a class of rubbers synthesized from tetrafluoroethylene (TFE) and propylene. It sits in the fluoroelastomer family right alongside FKM (Viton®), and almost everybody who meets it for the first time assumes that means it is a fancier, higher-grade FKM. It is not. It is a different chemistry that happens to share a shelf.

The number that explains the whole material is the fluorine content: FEPM runs 57 percent fluorine, while FKM runs 65 to 71 percent. Aflas® has less fluorine than FKM, not more. That is not a downgrade, it is the entire point. The propylene in the backbone is what buys the resistance to hot water, steam, and basic media like amines, and those are exactly the media that take standard FKM apart. The trade is honest and it goes both ways: Aflas® gives back some oil resistance to get it.

So the choice between Aflas® and FKM is never about which one is the better rubber. It is about what is in the pipe.

Quick specs

Common names

The same material shows up on drawings and datasheets under several names. These all mean the same thing:

• FEPM, the ASTM D1418 designation and the correct generic name
• TFE/P or TFE-propylene, describing the two monomers
• Aflas®, a registered trademark of AGC (Asahi Glass), and the name almost everyone in the field actually uses
• Tetrafluoroethylene-propylene rubber, the long form

Write FEPM on the drawing if you want the material. Write Aflas® if you want that specific manufacturer’s polymer.

Mechanical properties

• Outstanding resistance to hot water and high-pressure steam, the best of any common sealing elastomer short of FFKM
• Excellent resistance to basic media, including amines and amine-containing liquids
• Excellent heat resistance, usable to +450°F in the right service
• Very good electrical insulating properties, unusual for a fluoroelastomer
• Good resistance to sour gas (hydrogen sulfide) and to rapid gas decompression in qualified grades
• Good compression set at temperature, though FKM is generally better here
• Only medium resistance to mineral oils, which is the price of everything above

Limitations

The honest list, and it is a short one because Aflas® is a specialist:

• Poor low-temperature flexibility. Do not use it in the cold. This is the main limitation, full stop. At -25°F it is at its published floor, which is worse than nitrile, worse than HNBR, and worse than most FKM. Cold service rules it out.
• Only medium resistance to mineral oils and petroleum fuels. Nitrile, HNBR, and FKM are all better in oil. If oil is the fluid, Aflas® is the wrong answer.
• Not for aromatic fuels or many hydrocarbons. Aromatics, chlorinated solvents, and hydrocarbon fuels belong to FKM.
• Costs more than FKM. You are paying a premium for a narrow, valuable ability.
• Availability is thinner. Far fewer sizes and durometers sit on a shelf than for nitrile or FKM, and much of the Aflas® world is quoted to order in a mold that already exists or one that has to be built.

Here is the positioning stated plainly, because it saves people money: if the fluid is oil or fuel, do not buy Aflas®. Buy it for bases, amines, steam, sour gas, and hot water. Those five words are the entire reason this material exists.

Chemical compatibility

Handles well:

• Hot water and steam, including high-pressure steam service
• Basic media: amines, amine-containing liquids, amine-based corrosion inhibitors, caustic and strong bases
• Sour gas (H2S) and downhole production fluids
• Many acids, including hot mineral acids in the right grade
• Engine and transmission oils carrying amine-based additive packages, which are the quiet killer of FKM seals
• Brake fluids, phosphate-ester hydraulic fluids, and many polar solvents
• Ozone, weather, and sunlight

Attacks it:

• Aromatic fuels and aromatic hydrocarbons
• Chlorinated hydrocarbons and many hydrocarbon solvents
• Nothing “attacks” it in mineral oil, but resistance is only medium: expect swell and softening that FKM would shrug off

Now put the two side by side, because this is the comparison that matters:

Read that table in both directions. Neither column wins. Where one dies, the other thrives, and the fluid decides. Confirm your exact media on the Chemical Compatibility page or in the interactive Chemical Compatibility Chart, and if it is a blend rather than a pure chemical, send us the blend.

A word on temperature ranges

Our published range for Aflas® is -25°F to +450°F (-32°C to +232°C), and both ends of it are worth talking about because they are lopsided.

The top end is genuinely high. Among the compounds people actually buy, +450°F is beaten only by FFKM and PTFE. It clears FKM at about +400°F, and it clears HNBR at +300°F by a wide margin. In steam service that heat rating and the water resistance work together, which is the combination almost nothing else offers.

The bottom end is the material’s real weakness. -25°F is a poor floor. Standard nitrile goes to -40°F. Most FKM does better than -25°F. Silicone is not even in the same conversation. If your seal sees winter, sees a chiller, or sees any kind of cold start, Aflas® is the wrong compound and no grade selection fixes it.

As always, a published range assumes the compound, the media, the pressure, the hardware, and the duty cycle all cooperate. Heat plus an aggressive fluid plus a dynamic gland shortens life faster than any one of them alone. If your application sits near an edge, send us the conditions and we will pull the datasheet for the actual compound rather than guess from a chart.

Shop Aflas® O-Rings

Inch (AS568):

• Aflas® FEPM (TFE/P) O-Rings

For metric sizes, metric Aflas® is available on request, contact us.

Need a durometer, size, or a sour-gas or RGD-qualified grade that is not listed? Ask us. Most of the Aflas® world is made to order, and we quote it regularly.

Variations and grades explained

Aflas® is not a family of blends the way nitrile is. The base chemistry is fixed: TFE and propylene, peroxide cured. The levers you actually pull are hardness and qualification.

Base chemistry and cure

• FEPM (TFE/P): the TFE-and-propylene chemistry, at 57 percent fluorine, that produces the steam, base, and amine resistance. There is no low-fluorine or high-fluorine version of Aflas® the way there is with FKM.
• Peroxide cure: Aflas® is peroxide cured, which is part of why it survives hot water, steam, and aggressive chemical media. Bisphenol cures, common in FKM, are what amines and bases attack first, and Aflas® simply does not use them.
• Trademark and generic naming: Aflas® is a registered trademark of AGC (Asahi Glass). FEPM is the generic ASTM D1418 designation. They are not synonyms in a legal sense, only in a practical one.

By hardness (durometer)

• 75 Shore A: the general-purpose standard and what most Aflas® service uses.
• 90 Shore A: for higher pressure and to resist extrusion through the gland gap. Downhole work leans on 90 duro for exactly this reason.
• Softer and harder grades exist but are quoted to order.

Oilfield and sour-gas grades

• Explosive decompression / RGD grades: formulated and qualified to survive rapid gas decompression when a high-pressure gas well is bled down. The gas dissolved in the rubber has to get out without tearing the seal apart from the inside.
• Sour-gas qualified grades: tested in H2S at temperature and pressure. This is the service where Aflas® has quietly become a default.

Standards and specifications

Aflas® O-Rings are supplied in the AS568 inch size range, with metric available on request. The callouts you will see:

• ASTM D1418: designates the polymer as FEPM.
• ASTM D2000 / SAE J200: the line-callout system most commercial FEPM is bought to.
• AS568: the standard inch size dimensions.

There is no dedicated military O-Ring spec for FEPM. Unlike nitrile, FKM, fluorosilicone, silicone, and EPR, Aflas® has no M-number part standard of its own, so it gets called out through ASTM D2000 or on a customer drawing. See the mil-spec reference for the compounds that do have one.

In oilfield service, NORSOK and customer sour-gas qualification are far more common than any government spec, and those qualifications are compound-specific rather than material-specific. If your drawing calls out a qualification, tell us and we will match it, with a Certificate of Conformance and lot traceability under our ISO 9001:2015 system.

Typical applications by industry

See O-Rings by Industry for the wider picture, or browse all O-Ring materials for the compounds around this one.

Is Aflas® the right choice?

Three questions settle it, and they are all about the fluid:

1. Is there an amine, a strong base, caustic, or steam in the system? If yes, Aflas® is very likely your answer, and FKM is very likely not. Amines are destructive to FKM. This is the single most common reason a good FKM seal fails and nobody can explain why.
2. Is it sour gas or hot water? Same answer.
3. Is the fluid oil, fuel, or an aromatic solvent, or does the seal ever see cold? Then Aflas® is the wrong call. Go to FKM (Viton®) for oil and aromatics, HNBR for hot oil and refrigerant, or nitrile for ordinary oil at ordinary temperature.

If the answer to all three is “some of each,” and you truly need oil resistance and base resistance and cold, you are shopping for FFKM and you should know that going in. We will tell you that rather than sell you the wrong specialist.

Not sure whether the amine in your additive package is the reason your FKM seals keep failing? Send us the conditions: the media, the temperature, the pressure, and whether the seal moves. We will spec the compound and the size with you. We make getting O-Rings easy.

FEPM polymer chemistry, the 57 percent fluorine content, the FKM comparison at 65 to 71 percent, the hot water, steam, and basic-media resistance, the destructive effect of amines on FKM, and the medium resistance to mineral oils: Freudenberg FST 2025 Technical Manual, ch. 2 (Materials and Process Engineering), §3.4, p. 66. Temperature range and grade detail reflect the wider commercial Aflas® market. The datasheet for your specific compound always governs; test parts in your actual service conditions before committing to a critical seal.