Choose the right groove geometry and the seal will not leak because of pressure or tolerance. Choose the right elastomer and the seal will not fail because of temperature, chemistry, or time. This post is the materials companion to the geometry we have been building through the series — a side-by-side comparison of the eight commonly used O-ring compounds, plus the practical rules of thumb for picking one.
| Code | Meaning |
|---|---|
| 1 | Little or no effect — Volume swell < 10 % |
| 2 | Possible loss of physical properties — Volume swell 10–20 % |
| 3 | Noticeable change — Volume swell 20–40 % |
| 4 | Excessive change — Volume swell > 40 %. Not suitable for service. |
| 0 | Insufficient information |
| Common Name | ASTM D1418 | Low Temp | High Temp | Headline Strength | Headline Weakness |
|---|---|---|---|---|---|
| Simriz® Perfluoroelastomer | FFKM | −20 °C (−4 °F) | 300 °C (572 °F) | Best all-around chemical resistance; excellent vacuum outgassing | Cost; some helium permeability |
| Highly Fluorinated Fluoroelastomer (Super FKM) | ETP | −20 °C (−4 °F) | 200 °C (392 °F) | Better than FKM in many aggressive chemicals | Strong bases, amines |
| Tetrafluoroethylene/Propylene (Aflas®) | TFE/P | −10 °C (14 °F) | 200 °C (400 °F) | Strong acids, bases, water, amines; popular in oil fields | Polar solvents, aromatic fuels; compression set |
| Standard Fluoroelastomer (Viton®) | FKM | −40 °C (−40 °F) | 250 °C (482 °F) | Industry default for petroleum and high temp | Polar solvents, amines, hydrazine |
| Ethylene-Propylene | EP / EPDM | −55 °C (−67 °F) | 150 °C (302 °F) | Hot water, steam, polar solvents, brake fluids | Petroleum oils, aromatic fuels |
| Nitrile (NBR) | NBR / HNBR | −50 °C (−58 °F) | 120 °C (248 °F) | Lowest cost; great with petroleum oils and fuels | Polar solvents, ozone, sunlight |
| Silicone | VMQ | −75 °C (−103 °F) | 230 °C (446 °F) | Extreme temperatures; flex life; UV/ozone resistance | Poor physical strength, abrasion; gas-permeable |
| Fluorosilicone | FVMQ | −65 °C (−85 °F) | 180 °C (356 °F) | Cold flexibility + improved chemical resistance vs silicone | Hydrocarbon fluids, phosphate ester brake fluids |
A glance at an installed O-ring can tell you what you're dealing with.
| Compound | Typical Colour(s) |
|---|---|
| FFKM (Simriz) | Black, White, Clear |
| ETP (Super FKM) | Black, White |
| TFE/P (Aflas) | Black |
| FKM (Viton) | Black, White, Brown, Green |
| EPDM | Black, Purple |
| NBR | Black |
| VMQ (Silicone) | Red, White |
| FVMQ (Fluorosilicone) | Blue |
Caveat. Colour is not a substitute for a part number. Compounds with similar appearance can vary widely — always rely on the documented material code.
| Compound | Low Temp | High Temp |
|---|---|---|
| FFKM (Simriz) | −20 °C / −4 °F | 300 °C / 572 °F |
| ETP | −20 °C / −4 °F | 200 °C / 392 °F |
| TFE/P (Aflas) | −10 °C / 14 °F | 200 °C / 400 °F |
| FKM | −40 °C / −40 °F | 250 °C / 482 °F |
| EPDM | −55 °C / −67 °F | 150 °C / 302 °F |
| NBR | −50 °C / −58 °F | 120 °C / 248 °F |
| VMQ (Silicone) | −75 °C / −103 °F | 230 °C / 446 °F |
| FVMQ (Fluorosilicone) | −65 °C / −85 °F | 180 °C / 356 °F |
Practical interpretation. If your system runs hot (above 120 °C / 250 °F steadily), you are usually in FKM, TFE/P, or FFKM territory. If your system runs cold (sub-zero, or cryogenic transient), silicone-based compounds or EPDM dominate.
| Compound | Abrasion Resistance | Permeation Resistance | Compression Set Resistance | Tear Resistance |
|---|---|---|---|---|
| FFKM | 3 | 2 | 2 | 3 |
| ETP | 2 | 2 | 2 | 3 |
| TFE/P | 2 | 2 | 2 | 3 |
| FKM | 2 | 1 | 1 | 3 |
| EPDM | 1 | 2 | 2 | 1 |
| NBR | 2 | 2 | 1 | 2 |
| VMQ | 4 | 4 | 1 | 4 |
| FVMQ | 4 | 4 | 1 | 4 |
1 is best, 4 is worst. Note that silicone scores poorly on abrasion and tear — it is excellent for static seals at temperature extremes, but it is not the right pick for dynamic or high-stress sealing surfaces.
The full compatibility table is reproduced below. Anything rated 3 or 4 in your service fluid is a no-go.
| Chemical Family | FFKM | ETP | TFE/P | FKM | EPDM | NBR | VMQ | FVMQ |
|---|---|---|---|---|---|---|---|---|
| Inorganic Acids | 1 | 1 | 1 | 1 | 1 | 2 | 3 | 2 |
| Bases | 1 | 2 | 1 | 4 | 1 | 2 | 3 | 3 |
| Organic Acids | 1 | 1 | 1 | 1 | 1 | 2 | 3 | 2 |
| Alcohols | 1 | 1 | 1 | 3 | 1 | 1 | 1 | 1 |
| Aldehydes | 1 | 2 | 4 | 3 | 1 | 3 | 2 | 4 |
| Amines | 2 | 2 | 1 | 4 | 1 | 4 | 2 | 4 |
| Aromatic Hydrocarbons | 1 | 1 | 4 | 1 | 4 | 3 | 4 | 1 |
| Ether | 1 | 3 | 4 | 4 | 3 | 4 | 4 | 3 |
| Halogenides | 2 | 1 | 4 | 1 | 4 | 4 | 4 | 4 |
| Ketone | 1 | 3 | 4 | 4 | 1 | 4 | 4 | 4 |
| Water | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Steam (< 149 °C / 300 °F) | 1 | 2 | 2 | 2 | 1 | 4 | 3 | 4 |
| Steam (> 149 °C / 300 °F) | 2 | 3 | 3 | 4 | 4 | 4 | 4 | 4 |
Here is the field guide version:
Trademark notes. Viton® is a registered trademark of DuPont. Aflas® is a registered trademark of Asahi Glass Co.
SIMRIZ® Perfluoroelastomer (FFKM). Excellent resistance to all chemicals. Excellent outgassing in vacuum. Avoid low-molecular-weight fully halogenated fluids and molten alkali metals; strong oxidizing acids may swell. Compression set is not as good as some alternatives. Specific Simriz compounds extend low-temperature performance and amine resistance. Operating range −20 to 300 °C (−4 to 572 °F).
Highly Fluorinated Fluoroelastomer (ETP, Viton® class). Excellent resistance to most chemicals; performance greater than traditional FKM. Contamination performance somewhat less than FFKM. Avoid strong bases and amines. Operating range −20 to 200 °C (−4 to 392 °F).
Tetrafluoroethylene/Propylene (TFE/P, Aflas®). Fluorocarbon elastomer with exceptional thermal and chemical resistance. Excellent in acids, bases, water, and amines. Widely used in oil fields. Avoid polar solvents and aromatic fuels. Compression set may be too high for some applications. Operating range −10 to 200 °C (14 to 400 °F).
Standard Fluoroelastomer (FKM). Excellent resistance to petroleum products and solvents. Very good high-temperature performance. The most widely used seals in semiconductor industry. Avoid polar solvents, amines, anhydrous ammonia, Skydrol, hydrazine, and hot acids. Operating range −40 to 200 °C (−40 to 392 °F).
Ethylene-Propylene (EP, EPDM). Excellent resistance to water, steam, and polar solvents, plus ozone and sunlight. Also resistant to alcohols, glycol engine coolants, and Skydrol (phosphate-ester hydraulic fluid). Peroxide-cured grades carry higher temperature and lower compression set. Avoid non-polar solvents, petroleum oils, and aromatic fuels. Operating range −55 to 150 °C (−67 to 302 °F).
Nitrile (NBR, XNBR, HNBR). Very common for O-rings due to good mechanical properties and low cost. Standard Nitrile is also known as Buna-N. Excellent resistance to petroleum-based oils and fuels, water, and alcohols; good with acids and bases that are not strongly oxidizing. HNBR improves heat, ozone, and aging resistance. Avoid highly polar solvents (acetone, MEK) and direct exposure to ozone and sunlight. Operating range −55 to 120 °C (−65 to 248 °F).
Silicone (MQ, PMQ, VMQ, PVMQ). Excellent for static service at extreme temperatures (hot or cold). Outstanding flex and fatigue life. Very good for ozone/UV, fungal, and biological attack. Avoid chlorinated solvents, aliphatic and aromatic hydrocarbons, and petroleum oils. Permeable to gases. Poor physical strength and abrasion resistance. Operating range −115 to 232 °C (−175 to 450 °F).
Fluorosilicone (FVMQ). Combines excellent low-temperature performance of silicone with improved chemical resistance. Very good resistance to military and aerospace fuels. Excellent in oxygen plasma environments. Avoid polar solvents, hydrocarbon fluids, phosphate-ester brake fluids. Susceptible to hydrolysis by acids and bases. Limited abrasion resistance. Operating range −60 to 180 °C (−76 to 356 °F).
The seal design chain is now closed. To recap, the right O-ring is the intersection of three filters:
If you have more than one or two candidates left after applying those filters, default to FKM. It is the highest-performance generalist available before stepping up to FFKM or down to NBR. EPDM wins wherever the chemistry is polar or aqueous. HNBR is the cost-effective NBR-with-better-heat option.
Our seventh and final post zooms out from the seal itself to the process capability of the manufacturer: what Six Sigma Quality actually means in the O-ring industry, why process capability matters more than nominal compound choice, and how to read a vendor's quality claim.
Continue with Part 7 — Six Sigma Quality and Process Capability.