What fluorosilicone is
Fluorosilicone is a silicone where some of the methyl groups on the polymer chain are replaced with trifluoropropyl groups. The siloxane backbone is unchanged, so the material keeps silicone's temperature range and flexibility. The fluorine side groups change what the material resists.
Standard silicone swells 80 to 120 percent in hydrocarbon fuel and essentially dissolves in some solvents. Fluorosilicone with 40 to 50 percent fluorine swells less than 20 percent in the same fuel, which is the difference between a seal that works and a seal that fails.
The price is three to five times standard silicone, a loss of low-temperature flexibility, and a much narrower range of available compounds. It is a material specified for a specific problem, not a general upgrade.
| Własność | Standard silicone | Fluorosilicone | FKM |
|---|---|---|---|
| Service range | −60 to 200 °C | −40 to 200 °C | −20 to 230 °C |
| Fuel swell | 80 – 120% | 10 – 20% | 5 – 15% |
| Tensile strength | 6 – 12 MPa | 7 – 11 MPa | 10 – 18 MPa |
| Tear strength | 15 – 40 kN/m | 10 – 25 kN/m | 8 – 20 kN/m |
| Cost index | 100 | 300 – 500 | 250 – 400 |
Table 1 — Fluorosilicone against the two materials it competes with. It wins on the combination of fuel resistance and cold flexibility, not on either alone.
Fuel and oil resistance, in numbers
Volume swell is the standard measure. A seal is immersed in a reference oil or fuel at a set temperature for a set time, and the volume change is measured. Above about 20 percent swell, the seal has absorbed enough liquid that its mechanical properties are compromised and its dimensions have changed.
Fluorosilicone's swell falls roughly linearly with fluorine content, about two percentage points of swell per point of fluorine. At 50 percent fluorine the material is at 20 percent swell in ASTM reference oil; at 60 percent it would be lower but the polymer becomes difficult to process and the cold flexibility is gone.
The resistance is specific to hydrocarbons. Fluorosilicone is poor in ketones, esters, and some phosphate esters, where standard silicone may actually do better. A seal specification must name the fluid, not just say 'oil resistant'.
The three things you give up
Low-temperature flexibility goes first. Standard silicone is flexible to −60 °C; fluorosilicone stops at −40 and some grades at −30. For an aerospace seal that sees −55 °C on the ground, that is decisive.
Tear strength drops. Fluorosilicone tears at 10 to 25 kN/m against 15 to 40 for standard. A thin lip or a sharp edge that survives in standard silicone will tear in fluorosilicone.
Compression set is worse at high temperature. Fluorosilicone holds up to 200 °C but takes a set faster than standard silicone at the same temperature, which matters for a static seal that must hold force for years.
Processing: mostly the same, with two differences
Fluorosilicone compounds are processed on the same equipment as standard HCR — two-roll mill, compression or transfer press, extrusion. The differences are in cure and in cost discipline.
Cure is slower. The fluorine groups reduce the mobility of the curative and the compound needs a longer cure or a higher temperature. A compound that cures in 3 minutes at 170 °C in standard silicone may need 5 minutes in fluorosilicone.
Post-cure is more important. Fluorosilicone contains more volatiles from the filler treatment and the cure by-products, and a part that is not post-cured will show compression set drift and surface bloom. Four hours at 200 °C is the standard.
Cost discipline matters because the scrap value is high. A fluorosilicone runner or flash is worth recovering, and a shop that treats it like standard silicone scrap is throwing away real money. Keep the scrap segregated and clean.
Where it is actually used
- Aerospace fuel system seals. The combination of fuel resistance and −40 °C flexibility is what keeps it in this application despite FKM being cheaper.
- Automotive fuel and vapour lines. O-rings and gaskets in the fuel system where a standard silicone would swell shut.
- Industrial solvent handling. Diaphragms and seals in equipment that sees hydrocarbon solvents at temperature.
- Military and marine. Fuel and hydraulic systems where the temperature range is wider than FKM can handle.
The common thread is a hydrocarbon fluid plus a temperature range that extends below what FKM tolerates. Where the fluid is not a hydrocarbon, or the temperature does not go below −20 °C, FKM is usually the better and cheaper answer.
Selecting between silicone, fluorosilicone and FKM
| Condition | Choice | Reason |
|---|---|---|
| No hydrocarbon contact | Standard silicone | Cheapest, toughest, widest range |
| Fuel contact, down to −40 °C | Fluorosilicone | Only material that covers both |
| Fuel contact, above −20 °C | FKM | Cheaper, better compression set |
| Ketone or ester contact | Neither fluorosilicone nor FKM | Check a compatibility table |
| Steam or hot water | EPDM | Silicone hydrolyses over time |
Table 2 — A selection shortcut. Always confirm with an immersion test in the actual fluid at the actual temperature.
The immersion test is the only reliable answer. Twenty-eight days in the real fluid at the real temperature, then measure volume swell, hardness change, tensile retention and elongation retention. A material that swells 15 percent and loses 40 percent of its elongation has failed even though the swell number looks acceptable.
What is fluorosilicone rubber used for?
Seals and diaphragms that see hydrocarbon fuel or oil at temperatures down to −40 °C. Aerospace fuel systems are the classic application, where FKM cannot handle the cold end.
How much does fluorosilicone swell in fuel?
About 10 to 20 percent by volume in ASTM reference fuel, against 80 to 120 percent for standard silicone. The swell falls roughly two points for every point of fluorine content.
Is fluorosilicone better than FKM?
It is better below −20 °C and comparable on fuel resistance. FKM is cheaper, has better compression set and better tear strength. The choice depends on the cold-end requirement.
Why is fluorosilicone so expensive?
The fluorine-containing monomer is expensive to make and the compounding is more demanding. Expect three to five times the price of standard silicone per kilogram.
Does fluorosilicone need post-curing?
Yes, four hours at 200 °C is standard. Without it the part shows compression set drift and surface bloom, and the volatiles affect any downstream bonding or printing.
Can fluorosilicone be recycled?
Scrap can be reground and added to a virgin compound at low ratio, but cured fluorosilicone cannot be reprocessed like a thermoplastic. Keep scrap segregated because of its value.
Sourcing fuel-resistant silicone seals?
Tell us the fluid, the temperature range and the seal geometry. We will tell you whether fluorosilicone is necessary or whether FKM or standard silicone will do the job.
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