AFLAS vs FKM: Two Fluoroelastomers That Fail in Opposite Places

The verdict

These two are not really competing on grade, they are competing on chemistry, and the split is unusually clean. FKM is the better material in fuels, aromatic solvents, and hydrocarbons generally. AFLAS is the better material in steam, hot water, amines, and the strong bases and aggressive additive packages that break FKM down. Get the media right and the choice makes itself.

The failure we see most is an FKM seal specified on temperature alone, put into steam or an amine-inhibited system, which then fails in weeks. AFLAS was developed for exactly that duty, and it is the reason it shows up in oilfield service, downhole tools, and amine gas treating.

AFLAS's real penalty is cold. Its glass transition is high, and low-temperature sealing is where it gives up the most against FKM, so a part that has to seal below about −10 °C needs a hard look. It also has better electrical properties than FKM, which occasionally decides the specification on its own.

Both contain fluorine, so neither is a PFAS answer. If removing fluorine is the requirement, this comparison is the wrong one, and the question becomes how much of the envelope you actually need.

Choose AFLAS when

• Steam, hot water, or a water-based process fluid is present at temperature
• Amines, strong bases, or aggressive additive packages are in the system
• Hot lubricating oils with high additive loads are the media
• Electrical properties matter
• Cold service is not part of the duty cycle

Choose FKM when

• Fuels, aromatic solvents, or hydrocarbons dominate the media
• Sealing below about −10 °C is required
• Compression set over long static life governs
• A broader chemical envelope is needed and steam is not in the picture

Both are difficult compounds to mold well and both are expensive to get wrong, which is a good argument for testing a candidate in your actual fluid before committing tooling.

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