Choosing the Right O-Ring Material
The o-ring cross-section and groove are important, but the material determines whether your seal actually works in your application. Temperature, chemical exposure, pressure, and dynamic vs. static service all influence material choice.
Material Comparison Table
| Property | Nitrile (NBR) | Viton (FKM) | EPDM | Silicone | PTFE (Teflon) | Neoprene (CR) |
|---|---|---|---|---|---|---|
| Temp Range (°F) | -40 to 250 | -15 to 400 | -65 to 300 | -75 to 400 | -100 to 500 | -40 to 250 |
| Petroleum Oils | Excellent | Excellent | Poor | Poor | Excellent | Fair |
| Water/Steam | Good | Fair | Excellent | Good | Excellent | Good |
| Fuel Resistance | Good | Excellent | Poor | Poor | Excellent | Fair |
| Ozone/UV | Poor | Excellent | Excellent | Good | Excellent | Good |
| Acids | Fair | Good | Good | Fair | Excellent | Fair |
| Dynamic Sealing | Good | Good | Fair | Poor | Excellent | Good |
| Compression Set | Good | Excellent | Good | Fair | Excellent | Good |
| Cost | $ | $$$$ | $$ | $$ | $$$$$ | $$ |
Decision Tree
- Petroleum oil or hydraulic fluid? → NBR (standard) or FKM (high temp)
- Water, steam, or antifreeze? → EPDM
- Fuel (gasoline, diesel, jet fuel)? → FKM (Viton)
- Wide temperature range needed? → Silicone (-75°F to 400°F) or PTFE (-100°F to 500°F)
- Aggressive chemicals? → PTFE (resists almost everything) or check specific compatibility charts
- Outdoor/UV exposure? → EPDM or FKM (NBR degrades rapidly in UV/ozone)
- FDA food contact? → Silicone, EPDM, or specific FDA-grade compounds
- Budget-conscious, general purpose? → NBR (covers 80% of applications)
Durometer (Hardness) Selection
| Durometer (Shore A) | Application |
|---|---|
| 40–50 | Low-pressure face seals, vacuum seals — conforms easily to surface irregularities |
| 60–70 | General static seals, most o-ring applications — the standard |
| 70–80 | Dynamic seals, higher pressure — resists extrusion better |
| 80–90 | High-pressure seals, backup rings — maximum extrusion resistance |
Common Mistakes
- Using NBR with EPDM brake fluid: NBR is destroyed by DOT 3/4/5.1 brake fluid (glycol-based). Use EPDM for brake systems.
- Using EPDM with petroleum oil: EPDM swells and fails in petroleum-based fluids. Use NBR or FKM.
- Using silicone for dynamic seals: Silicone has poor tear strength and abrasion resistance. It works for static seals in temperature extremes but fails quickly in reciprocating or rotary applications.
- Ignoring compression set: After long periods compressed in a groove, o-rings take a permanent “set” and lose sealing force. FKM has the best compression set resistance for long-service seals.
Related: O-Ring Groove Calculator | Static O-Ring Groove Design | AS568 O-Ring Sizes