There is no best polymer. Each has a service window where it wins, and the work is to find which window your part sits in. The table summarises general tendencies. Individual grades can sit outside these tendencies, so confirm with data on the grade and test your conditions.
| Service condition | HNBR | FKM (bisphenol) | FKM (peroxide) | FEPM |
|---|---|---|---|---|
| Hot oil to 150 °C | Very good | Very good | Very good | Good |
| Hot oil above 175 °C | Limited | Very good | Very good | Very good |
| Aromatic fuels | Limited | Very good | Very good | Fair |
| Steam, hot water | Good | Weak | Good | Very good |
| Amines and strong bases | Fair to good | Weak | Fair | Very good |
| Acids | Fair | Good | Very good | Very good |
| Hydrogen sulfide (sour) | Good (peroxide) | Weak when amines present | Good | Very good |
| Low-temperature flexibility | Good (low ACN) | Fair | Good (low-temperature grades) | Weak |
| Compression set at high temperature | Good | Very good | Very good | Fair |
| Tear, wear and tensile strength | Very good | Good | Good | Good |
| Relative cost | Lower | Higher | Higher | Highest |
A simple selection path
- Write down the fluids, not only the application. Include oil type, water cut, amines, inhibitors, aromatics, CO2 and H2S, and how often the mix changes.
- Write down the temperature range, including peaks and cold starts. Use the real minimum, not the room temperature.
- Is there steam or hot water? If yes, rule out bisphenol-cured FKM. Choose HNBR, peroxide FKM or FEPM, depending on temperature.
- Are amines or strong bases present? If yes, rule out standard FKM. Consider HNBR, FEPM or a base-resistant FKM.
- Are aromatic hydrocarbons present at high temperature? If yes, FKM terpolymer is likely the safest start.
- Does the pressure drop quickly in gas service? Consider rapid gas decompression resistance, and see the report on RGD.
- Is the part dynamic? HNBR often has the edge for wear and tear. Check friction and extrusion.
- Is there a drinking water, food or gas-appliance approval? Check whether the approval is held by the specific formulation and manufacturer.
Where choices overlap
- HNBR versus FKM for hot oil. Below about 150 °C and without aromatics, HNBR often costs less and wears better. Above 175 °C, or with aromatic fuels, FKM is usually needed.
- FKM peroxide versus FEPM for steam. Peroxide-cured FKM is stronger in cold and compression set. FEPM is stronger with amines and bases.
- Cost versus risk. A seal failure in a downhole tool often costs far more than the compound. Do not choose on price alone.
What to do before you commit
Choose two or three candidates and test them in the actual fluids, temperatures and pressure cycles of the application. Compare compression set, swell, property change after ageing and, if gas is involved, damage after decompression. A test plan costs less than a field failure.
Sources and further reading
- Manufacturer selection guides for FKM, HNBR and FEPM, from the polymer producers and compound suppliers.
- P. A. Ciullo and N. Hewitt, The Rubber Formulary, 1999.
About this guide. It is an original summary written from public sources and standards. It is not a copy of any standard or book, and it is not engineering advice. Confirm values against the current edition of the standard and test in your own application.