Silicone (VMQ) O-Rings

Silicone is the odd one out. Every other elastomer in the materials lineup is built on a carbon backbone. Silicone is synthesized from siloxane monomers, and its backbone alternates oxygen and silicon atoms instead. That one structural difference explains nearly everything silicone does.

The good is a service span almost nothing else touches: high thermal stability and good low-temperature flexibility in one compound, plus very good resistance to oxygen, ozone, and high-energy radiation. Its temperature dependency is also low compared to other elastomers, so the seal behaves about the same at -60°F as at +350°F.

The bad is mechanical. Tear and abrasion resistance are only moderate, and gas permeability is significantly higher than other elastomers. Silicone is a static sealing material, full stop. Ask it to move, rub, or hold a vacuum and it will disappoint you.

Quick specs

Common names

Silicone shows up on drawings under several designations, all the same family:

• VMQ, the ASTM D1418 designation for vinyl-methyl-polysiloxane, the standard grade
• MVQ or SI, older and European shorthand for the same material
• PVMQ, phenyl-vinyl-methyl-polysiloxane, the cold-flexible variant
• FVMQ, fluorosilicone, the fuel-resistant variant
• Silicone rubber, the shop-floor name

Mechanical properties

• High thermal stability combined with good low-temperature flexibility. Almost no other elastomer offers both in one compound. This is silicone’s entire reason for existing.
• Low temperature dependency of properties. It behaves consistently across its range instead of stiffening at one end and going soft at the other.
• Very good resistance to oxygen, ozone, and high-energy radiation, which is why it lives outdoors and in sterilization service without cracking.
• Highly electrically insulating and flame-retardant.
• Physiologically inert and odorless, the property behind every food and medical grade.
• Tear and abrasion resistance are moderate. Not poor, but not enough for moving service.
• Gas permeability is significantly higher than other elastomers.

Limitations

The honest list. Silicone is a specialist, and most silicone failures come from treating it as a general-purpose seal:

• Not a dynamic seal material. Moderate tear and abrasion resistance means it wears out in reciprocating, rotating, or rubbing service. This is the single most common silicone mistake we see. For moving seals use Nitrile, FKM (Viton®), or cast urethane.
• High gas permeability makes it a poor vacuum and gas seal. Use Butyl, which has the lowest permeability of the common elastomers, or FKM.
• Poor resistance to fuels, to oils at high temperature, and to aromatic hydrocarbons. For silicone’s temperature span plus fuel resistance, the material is Fluorosilicone (FVMQ).
• Not for steam or hot water. Use EPDM.
• Low tensile strength. Silicone tears over sharp edges, threads, and ports far more easily than nitrile does. See Installation and installation damage.
• Depolymerizes at high temperature in the absence of oxygen. In confined, oxygen-starved hot service the polymer chains come apart and the seal reverts. Hot open air is fine. A hot closed cavity is not.

Chemical compatibility

Handles well:

• Oxygen, ozone, sunlight, weather, and high-energy radiation
• Hot air and dry heat, in open or vented service
• Water at moderate temperature, dilute salt solutions, dilute acids and bases
• Alcohols, glycols, and many polar fluids
• Animal and vegetable oils and fats
• Food, beverage, and pharmaceutical media, where inertness matters as much as resistance

Attacks it:

• Fuels, gasoline, and hydrocarbon solvents
• Petroleum oils at high temperature (cool oil is tolerable, hot oil is not)
• Aromatic and chlorinated hydrocarbons
• Steam and hot water
• Concentrated acids and bases
• High-pressure gas, which permeates it rather than attacking it, though the seal fails just the same

Silicone’s chemical personality is nearly the inverse of nitrile: it shrugs off the weather and heat that kill nitrile and folds on the petroleum fluids nitrile handles all day. Confirm your exact media on the Chemical Compatibility page or in the interactive Chemical Compatibility Chart.

A word on temperature ranges

Our published range is -75°F to +400°F (-59°C to +204°C), the widest span of any common elastomer we stock. That span is silicone’s superpower: not the hot end alone and not the cold end alone, but both in one compound, with properties that stay consistent in between. Around that center:

• Freudenberg publishes -58°F to +392°F for the material, and notes that VMQ and PVMQ parts operate roughly -67°F to +347°F. Molded parts are always more conservative than the raw polymer.
• PVMQ, the phenyl-modified grade, retains elastic properties down to about -130°F, the coldest working elastomer most people will ever specify.
• The hot end depends on air. Silicone is degraded by depolymerization at high temperature in the absence of oxygen. In a vented oven the top number holds. In a sealed, oxygen-free hot cavity it does not, and no chart will warn you.

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 instead of guessing from a chart.

Shop Silicone O-Rings

Every Silicone variation we stock, by grade and size:

Inch (AS568):

• Silicone (VMQ) 70 Duro FDA
• Silicone (VMQ) 50 Duro (Brick Red)

Metric:

• Silicone Metric (Food, Medical & High-Temp)

Need PVMQ, a USP Class VI compound, a mil-spec callout, or a durometer that is not listed? Ask us. Much of the silicone world is made to order, and we quote it every week.

Variations and grades explained

Silicone is a family, not a single recipe. The grade decides the cold end, the approvals, and the paperwork.

By polymer

• VMQ (vinyl-methyl-polysiloxane): standard silicone, the general-purpose grade, and what most O-Rings are molded from.
• PVMQ (phenyl-vinyl-methyl-polysiloxane): phenyl groups added for cold flexibility, retaining elastic properties to roughly -130°F. Specified for aerospace and arctic static seals.
• FVMQ (fluorosilicone): fluorine added for the fuel and oil resistance standard silicone lacks, at some cost in heat life and price.

By hardness (durometer)

• 50 Shore A: soft, for low closing force, light flanges, or irregular sealing surfaces.
• 70 Shore A: the standard for most applications, and the better choice under any real pressure. Silicone is already prone to extrusion and nibbling, so tighten the gland gap before reaching for a softer durometer.

By approval and grade

• FDA food-grade: compounds meeting FDA 21 CFR 177.2600 for repeated food contact, used throughout food, beverage, and dairy equipment.
• Medical: high-purity compounds tested to USP Class VI biological reactivity requirements.
• 3-A sanitary: for dairy and sanitary process equipment.

Military and federal callouts

Unlike HNBR, silicone has a deep spec family of its own: MS9068 and AMS3304 for general-purpose parts, AMS7267 for hot air, and ZZ-R-765, the classic federal silicone specification, now largely superseded by A-A-59588 for commercial procurement. The mil-spec reference sorts out which number belongs to which duty.

Standards and specifications

Silicone O-Rings are supplied across the full AS568 inch size range and common metric sizes. Useful callouts:

• ASTM D1418: designates the polymers VMQ, PVMQ, and FVMQ.
• ASTM D2000 / SAE J200: the line-callout system most commercial silicone is bought to.
• MS9068, AMS3304, AMS7267, ZZ-R-765 / A-A-59588: the silicone military and federal family.
• FDA 21 CFR 177.2600, USP Class VI, and 3-A: the compliance grades, covered on the FDA, USP Class VI, and 3-A pages.

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 Design & Engineering for the gland around the seal.

Is Silicone the right choice?

Three questions settle it:

1. Does the seal move? If yes, silicone is out. Its moderate tear and abrasion resistance will not survive the duty, no matter how good the temperature range looks.
2. Does it hold gas or a vacuum? If yes, silicone is out. Gas walks through it. Go to Butyl or FKM.
3. Is the fluid a fuel, a hot oil, or an aromatic solvent? If yes, silicone is out. That job belongs to Fluorosilicone, FKM, or Nitrile.

Clear all three and silicone is often unbeatable. A static joint that has to survive both deep cold and sustained dry heat, or be clean enough for a patient or a food line, or sit outdoors for years without an ozone crack, is exactly what silicone was made for. Ask it to do anything else and you are buying a beautiful seal for the wrong hole.

Not sure whether silicone covers your temperature, media, and duty? 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.

Siloxane polymer structure, the silicon-oxygen backbone, the property set, depolymerization without oxygen, the -58°F to +392°F material range, the -67°F to +347°F VMQ and PVMQ part range, and the -130°F PVMQ figure: Freudenberg FST 2025 Technical Manual, ch. 2 (Materials and Process Engineering), §3.4, p. 70. Our published -75°F to +400°F range and the grade listings reflect the wider commercial silicone market. The datasheet for your specific compound always governs; test parts in your actual service conditions before committing to a critical seal.