Fluorosilicone (FVMQ) O-Rings

Fluorosilicone exists to solve one problem, and it solves it better than anything else on the shelf: fuel resistance at very low temperature.

Consider the two obvious candidates and where each one quits. Silicone (VMQ) has the temperature span, sealing well below anything a hydrocarbon rubber can manage, but put it in fuel and it swells, softens, and gives up. FKM (Viton®) has the fuel resistance in abundance, but it stiffens around -20°F, and at altitude cold it is a plastic ring in a groove. Neither covers a fuel line on an aircraft.

Fluorosilicone is the overlap. It is still a silicone: the backbone is the same alternating chain of silicon and oxygen atoms, which is where the cold flexibility and heat stability come from. The difference is the side groups hanging off that backbone, which carry fluorine. Freudenberg puts the result plainly: elastomers made from FVMQ are considerably more resistant in fuels and in mineral, synthetic, and aromatic oils than those made from VMQ or PVMQ.

Silicone’s cold, real fuel resistance. That is why FVMQ is an aerospace fuel-system staple rather than a general-purpose compound.

Quick specs

Common names

The same material shows up on drawings under several names. These all mean fluorosilicone:

• FVMQ, the ASTM D1418 designation
• Fluorosilicone, fluoro silicone, or fluorinated silicone
• FSR or fluorosilicone rubber
• M25988 callouts, which specify fluorosilicone by spec rather than by polymer name

Note the letters. VMQ is silicone. FVMQ is silicone with fluorine on the side groups. One letter is the whole difference between a compound that dissolves in fuel and one that does not.

Mechanical properties

FVMQ inherits the silicone family’s character. The fluorine changes the chemistry, not the physics:

• High thermal stability and good low-temperature flexibility, the silicone hallmark
• Very good resistance to oxygen, ozone, and high-energy radiation
• Low temperature-dependency of properties: it behaves much the same across its range instead of stiffening as it cools
• Moderate tear and abrasion resistance, which is the honest way of saying it is not a strong rubber
• High gas permeability: gases move through it readily
• Electrically insulating
• Physiologically inert

Read that list twice before designing with it. The top lines are why you want it. The plain ones (moderate tear, moderate abrasion, high permeability) are why it belongs in static grooves and nowhere else.

Limitations

The honest list. FVMQ is a specialist, and the failures we see almost always come from using it as if it were a general-purpose seal:

• Not a dynamic seal material. Like every silicone, its tear and abrasion resistance are moderate. Static service only. For rod, piston, or rotary duty use FKM (Viton®) or Nitrile (NBR).
• High gas permeability, straight from the silicone backbone. It makes a poor vacuum or gas-retention seal. For that, Butyl is the material.
• Low mechanical strength. It tears on sharp edges, threads, and ports during assembly. Ease the corners, lubricate, and go slow. See Installation and installation damage.
• Not for brake fluids, ketones, or hot concentrated acids.
• Not for high pressure. It has no strength to hold back extrusion.
• Expensive, considerably more than nitrile or standard FKM. Buy it for the cold-plus-fuel combination, not as a general upgrade.
• If you do not need the cold, you probably do not need FVMQ. FKM handles fuel better and costs less. The cold end is what you are paying for.

Chemical compatibility

Handles well:

• Petroleum fuels, including aviation fuels and gasoline
• Mineral, synthetic, and aromatic oils, considerably better than plain silicone
• Aromatic hydrocarbons
• Engine and gear lubricants
• Natural gas
• Oxygen, ozone, weather, sunlight, and high-energy radiation

Attacks it:

• Glycol brake fluids (DOT 3, 4, 5.1). Use EPDM.
• Ketones. Use EPDM or FFKM.
• Hot concentrated acids
• High-pressure steam
• Some chlorinated hydrocarbons at temperature

“Fuel” covers a lot of ground, and additive packages sometimes matter more than the base stock. Confirm your exact fluid on the Chemical Compatibility page or in the interactive Chemical Compatibility Chart.

A word on temperature ranges

Our published range is -76°F to +392°F (-60°C to +200°C), and the two ends of that range are doing very different jobs.

The cold end is the reason to buy the material. A seal that stays flexible near -76°F while sitting in jet fuel is not something any other common compound offers, and it is why the aerospace world writes FVMQ into fuel-system drawings.

The hot end is inherited, not engineered. It comes from the silicone backbone: Freudenberg publishes roughly -67°F to +347°F for VMQ and PVMQ parts, and about -58°F to +392°F for silicone material generally. FVMQ sits in the same span.

The practical caveat: in fuel service, the continuous limit is lower than the dry-heat limit. A number derived from hot air does not survive contact with a hot aggressive fluid. Treat the top of the range as a short-excursion figure and design continuous fuel service well under it.

Every published range assumes the compound, the media, the pressure, the hardware, and the duty cycle all cooperate. 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 Fluorosilicone O-Rings

Every Fluorosilicone variation we stock, by grade:

Inch (AS568):

• Fluorosilicone (FVMQ) 70 Duro (M25988/1)
• Fluorosilicone (FVMQ) 75 Duro

Metric fluorosilicone is available on request, contact us.

Variations and grades explained

Fluorosilicone comes in a narrower range of grades than nitrile or silicone. Three things decide it: the base chemistry, the durometer, and the spec callout.

FVMQ versus VMQ

This is the fork in the road. Silicone (VMQ) and fluorosilicone share a backbone, a temperature span, ozone resistance, and a set of mechanical weaknesses. The fluorinated side groups on FVMQ are the only real difference, and they buy resistance to fuels and to mineral, synthetic, and aromatic oils. If the seal never touches fuel or oil, plain silicone does the same job for much less money. If it does, VMQ is not an option and FVMQ is.

By hardness (durometer)

• 70 Shore A: the standard for fuel-system and general static sealing, and what most M25988 work calls out.
• 75 Shore A: slightly firmer, for glands that want a bit more resistance to extrusion or a firmer seal.

Given FVMQ’s low strength, gland design carries more of the load here than durometer does. Get the groove right and keep the clearance gap tight. See O-Ring Sizing.

The M25988 family

M25988 is the fluorosilicone standard, and in aerospace or defense it is probably how the material arrives on your drawing. The spec has traveled under three designations, all the same family: MIL-R-25988, then MIL-DTL-25988, and AMS-R-25988 on the SAE side. The slash sheets break it out by class and grade:

We stock M25988/1, the Class 1 Grade 70 blue material, which covers the majority of aircraft fuel and oil system callouts. The blue is not decoration, it is a spec requirement, and it is how a stockroom tells fluorosilicone from everything else in the drawer. See the mil-spec reference for how the M-number system works and which other compounds carry one.

Standards and specifications

Fluorosilicone O-Rings are supplied in the full AS568 inch size range, with metric available on request. Useful callouts:

• ASTM D1418: designates the polymer as FVMQ.
• ASTM D2000 / SAE J200: the line-callout system for commercial fluorosilicone, covering its heat and fluid resistance by type and class.
• M25988 family (MIL-R-25988, MIL-DTL-25988, AMS-R-25988): the aerospace specification for fluorosilicone rubber in aircraft fuel and oil systems. Details in the mil-spec reference.

If your drawing calls out a specific spec or compound, 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 Materials to compare the compound family side by side.

Is Fluorosilicone the right choice?

Two questions settle it, and they have to both be yes:

1. Does the seal touch fuel or oil? If not, silicone is the same temperature range for a fraction of the price.
2. Does it also have to work in serious cold, below what FKM tolerates? If not, FKM (Viton®) handles the fuel better and costs less.

Only when both are true does fluorosilicone win, and when both are true nothing else comes close. Then confirm the third: the seal is static. If it moves, FVMQ is the wrong material no matter how the first two answers came out.

That narrow window is not a knock. It is why FVMQ has a mil-spec family of its own and a permanent place in aerospace fuel systems.

Not sure whether fluorosilicone is the answer or an expensive detour? Send us the conditions: the fluid, the temperature at both ends, the pressure, and whether the seal moves. We will spec the compound and the size with you, and we will tell you when plain silicone or FKM does the job for less. We make getting O-Rings easy.

Polymer chemistry, the fluorinated side-group structure, comparative fuel and oil resistance versus VMQ and PVMQ, the silicone-family property set, the -67°F to +347°F and -58°F to +392°F ranges, and the application list: Freudenberg FST 2025 Technical Manual, ch. 2 (Materials and Process Engineering), §3.4, p. 70. Commercial ranges and grade details reflect the wider fluorosilicone market. The datasheet for your specific compound always governs; test parts in your actual service conditions before committing to a critical seal.