Selecting the right O-ring material is the most critical decision in seal design. The wrong material causes premature failure through chemical attack, thermal degradation, or compression set — often within hours of installation. This guide provides a systematic comparison of all major O-ring materials, a one-sentence positioning for each, and decision trees for temperature, chemical, and hardness-based selection.
Quick answer: For petroleum oil below +120°C, NBR is the default. For temperatures above +120°C or aggressive chemicals, FKM is the standard upgrade. For steam or hot water, EPDM is required. For food or medical, VMQ silicone is the common choice. For universal chemical resistance, PTFE or FFKM are the options. Use the tables and decision trees below to narrow to the correct material in under 60 seconds.
O-Ring Material Overview Table
| Material | Type | Temp Range | Best For | Avoid | Relative Cost |
|---|---|---|---|---|---|
| NBR | Nitrile elastomer | -40°C to +120°C | Mineral oil, hydraulic fluid, fuel | Polar solvents, ozone, glycol | 1× |
| HNBR | Hydrogenated nitrile | -40°C to +150°C | Bio-oils, sour gas, automotive AC | Polar solvents, steam | 3× |
| FKM | Fluorocarbon (Viton/FPM) | -20°C to +200°C | High temp, chemicals, fuel, acids | Hot water, steam, amines, ketones | 5× |
| FFKM | Perfluoroelastomer | -15°C to +325°C | Universal chemical resistance | Cost-sensitive applications | 50× |
| EPDM | Ethylene propylene | -50°C to +150°C | Steam, hot water, brake fluid, polar solvents | Mineral oil, fuel, hydrocarbons | 2× |
| VMQ | Silicone | -60°C to +230°C | Food, medical, extreme temperature | Petroleum oils, dynamic pressure | 4× |
| FVMQ | Fluorosilicone | -60°C to +175°C | Fuel + low temperature combined | Acids, bases, high pressure | 8× |
| CR | Chloroprene (Neoprene) | -40°C to +120°C | Refrigerants, weather, marine | Aromatic fuels, strong oxidizers | 2× |
| PU | Polyurethane (Urethane/TPU) | -35°C to +80°C | High abrasion, dynamic hydraulics | Hot water, steam, glycol, acids | 3× |
| AFLAS | Tetrafluoroethylene propylene | -5°C to +230°C | Steam, caustic, sour gas, amines | Aromatic hydrocarbons, ketones | 8× |
| PTFE | Fluoropolymer (Teflon) | -200°C to +260°C | Universal chemical resistance (static) | Dynamic service (without energizer) | 5× |
| Butyl | Isobutylene isoprene (IIR) | -40°C to +120°C | Vacuum, gas-tight, brake fluid | Mineral oil, fuel, hydrocarbons | 2× |
| FEP Encapsulated | FEP/PTFE shell + elastomer core | -60°C to +260°C | Aggressive chemicals + elasticity | Dynamic service | 8× |
Selection by Operating Temperature
Below -40°C (Cryogenic)
Only VMQ silicone (to -60°C) and PTFE (to -200°C) function reliably. FVMQ extends to -60°C with fuel resistance. For liquid nitrogen (-196°C), PTFE is the only practical choice.
-40°C to +120°C (General Industrial)
NBR is the default for oil and fuel. EPDM for water and steam. CR for refrigerants and outdoor. Butyl for vacuum and brake fluid. PU for dynamic abrasion. Most industrial applications fall in this band.
+120°C to +200°C (High Temperature)
HNBR bridges NBR's limit to +150°C. FKM is the standard to +200°C. AFLAS handles steam and caustic to +230°C. VMQ tolerates dry heat to +230°C but not oil.
Above +200°C (Extreme Heat)
FFKM is the only elastomer above +250°C. PTFE handles static seals to +260°C. AFLAS manages steam to +230°C. For +300°C+, FFKM or metal seals are required.
Field Identification: Color, Density, and Flame Test
When you inherit a machine with unmarked O-rings, color and a few quick tests can narrow the material family before you send a sample to the lab. Color is not definitive — carbon-black-filled NBR and FKM both look black — but it is a useful first filter.
| Visual / Test | NBR | FKM | EPDM | VMQ | PTFE |
|---|---|---|---|---|---|
| Common color | Black, sometimes green/brown | Brown, black, green | Black, blue | Translucent/white/red | White |
| Density (g/cm³) | 1.10–1.25 | 1.80–1.90 | 1.10–1.20 | 1.15–1.25 | 2.14–2.20 |
| Flame behavior | Burns with sooty flame, acrid smoke | Self-extinguishing, HF odor | Burns with paraffin/wax odor | White ash, no soot | Does not burn |
| Acetone swell (24 h) | Minimal | 5–15% | Severe swell | Severe swell | None |
For a reliable identification, cut a small sample and measure specific gravity with a density kit, or send it for FTIR analysis. For a deeper guide to color conventions, see our O-ring color coding guide.
Selection by Fluid / Chemical Environment
Petroleum Oils and Hydraulic Fluids
NBR 70 Shore A is the standard. Upgrade to HNBR for bio-oils or temperatures above +100°C. Use FKM for synthetic fluids or continuous operation above +120°C.
Fuels (Gasoline, Diesel, Jet Fuel)
NBR for standard fuels below +100°C. FKM for aromatic fuels and temperatures above +100°C. FVMQ when low temperature below -20°C is also required. FFKM for the most aggressive aviation fuels.
Water, Steam, and Hot Water
EPDM is the standard — superior hydrolysis resistance and steam tolerance. AFLAS for steam above +150°C. Never use NBR or FKM in continuous steam service.
Brake Fluid and Glycol Systems
EPDM for glycol-ether DOT 3/4/5.1 brake fluid. Butyl also compatible with excellent gas barrier. Never use NBR or PU in glycol fluids — rapid degradation.
Acids and Strong Chemicals
FKM for concentrated acids and most industrial chemicals. PTFE for the most aggressive acids and solvents. FFKM when elasticity is required in acid service. AFLAS for caustic and base environments.
Ketones, Esters, and Amines
FFKM is the only elastomer for ketones and concentrated amines. PTFE for static seals in ketone service. Standard FKM is attacked by ketones and amines — do not use.
Food and Pharmaceutical Contact
VMQ silicone (FDA, USP Class VI). EPDM (FDA grades for water/steam). PTFE (FDA, inert). FEP Encapsulated (FDA shell, chemical resistance).
Selection by Hardness / Mechanical Requirement
| Hardness | Typical Use | Materials Available |
|---|---|---|
| 40–50 Shore A | Low friction, large clearance conformability | NBR, EPDM, VMQ, CR, Butyl |
| 60–70 Shore A | General-purpose static and dynamic | All materials |
| 75–80 Shore A | Dynamic seals, improved extrusion resistance | NBR, FKM, EPDM, FVMQ, CR |
| 90 Shore A | High pressure, small clearance gaps | NBR, HNBR, FKM, PU |
| 55–65 Shore D | Extreme extrusion resistance, static only | PTFE |
Material Selection Decision Tree
Step 1: What is the maximum operating temperature?
- Below +80°C → All elastomers are candidates; proceed to Step 2
- +80°C to +120°C → Eliminate PU; proceed to Step 2
- +120°C to +150°C → Eliminate NBR, CR, Butyl; consider HNBR, FKM, EPDM, VMQ
- +150°C to +200°C → Eliminate NBR, HNBR (marginal), CR, Butyl; FKM, AFLAS, VMQ remain
- +200°C to +260°C → FKM (short-term), AFLAS, VMQ, PTFE, FFKM
- Above +260°C → FFKM or PTFE only
Step 2: What is the primary fluid?
- Petroleum oil / mineral hydraulic fluid → NBR (default), HNBR (high temp), FKM (extreme)
- Fuel / gasoline / diesel → NBR (cold), FKM (hot), FVMQ (cold + fuel)
- Water / steam / hot water → EPDM (standard), AFLAS (high temp steam)
- Brake fluid / glycol → EPDM, Butyl
- Acid / aggressive chemical → FKM (most), PTFE (all), FFKM (elastic + all)
- Ketone / ester / amine → FFKM only (elastomer); PTFE (static)
- Food / pharma / medical → VMQ, EPDM (FDA), PTFE, FEP Encapsulated
Step 3: Is the seal dynamic or static?
- Dynamic reciprocating (rod/piston) → PU (best), NBR (standard), HNBR, FKM
- Dynamic rotary (shaft) → NBR, HNBR, FKM with proper design
- Static flange/face → All materials; PTFE acceptable
- Static vacuum → Butyl (low permeability), FKM (low outgassing), FFKM (UHV)
Step 4: Are there regulatory requirements?
- FDA food contact → VMQ, EPDM, PTFE
- USP Class VI medical → VMQ (platinum cure), PTFE
- Aerospace / MIL-SPEC → FKM, FVMQ, FFKM with specific grade certification
- Automotive IATF → NBR, HNBR, FKM, VMQ with OEM approval
- Oil & gas NACE → HNBR, FFKM, AFLAS with sour gas grades
Specialty and Niche Materials
FVMQ (Fluorosilicone)
The hybrid material: silicone's -60°C low-temperature flexibility plus fuel resistance. Used exclusively in aerospace fuel systems and military vehicles where both arctic cold-start and jet fuel contact are required. Not a general-purpose material — specify only when FKM's low-temperature limit is the blocking issue.
AFLAS (TFE/P)
The base-resistant fluorocarbon. Handles steam, caustic soda, and amines that destroy FKM. Temperature range to +230°C. Incompatible with aromatic fuels and ketones. The correct choice for oilfield steam injection, geothermal wells, and chemical plants with alkaline process streams.
Butyl (IIR)
The gas barrier specialist. Lowest permeability of any elastomer — 10× better than EPDM for helium. Used in vacuum seals, brake systems, and pneumatic accumulators. Completely incompatible with petroleum oils and fuels.
FEP Encapsulated
The chemical resistance + elasticity compromise. An FEP or PFA fluoropolymer shell provides PTFE-level chemical resistance; an elastomer core (VMQ or FKM) provides compressibility and elastic recovery. Static service only — the shell cracks in dynamic motion.
Comparison: FKM vs FVMQ vs VMQ
| Property | VMQ (Silicone) | FVMQ (Fluorosilicone) | FKM (Viton) |
|---|---|---|---|
| Temperature | -60°C to +230°C | -60°C to +175°C | -20°C to +200°C |
| Fuel resistance | Poor (100–200% swell) | Good (10–25% swell) | Excellent (<5% swell) |
| Low temp | Excellent | Excellent | Moderate |
| Oil resistance | Poor | Good | Excellent |
| Acid/base | Moderate | Moderate | Good (acid) |
| Cost | 4× NBR | 8× NBR | 5× NBR |
| Typical use | Food, medical, dry heat | Aerospace fuel, military | Chemical, high temp, fuel |
FAQ
Q1: What is the most versatile all-purpose O-ring material?
NBR 70 Shore A is the most versatile general-purpose material — it handles the majority of hydraulic, fuel, and oil applications at the lowest cost. However, "versatile" depends on environment: for temperature extremes, VMQ is widest; for chemical resistance, PTFE is universal; for dynamic service, PU is strongest. There is no single best material — only the best material for your specific temperature, fluid, and mechanical requirements.
Q2: What is the difference between FKM and FPM?
FKM and FPM are the same material. FKM is the ASTM (American) designation; FPM is the ISO/DIN (European) designation. Both refer to fluorocarbon rubber, commonly known by the DuPont trade name Viton. FKM O-rings and FPM O-rings are dimensionally and chemically identical. The same applies to fluoroelastomer — it is the generic chemical term for the polymer family.
Q3: Can I use the same O-ring material for all seals in my system?
Usually no. Most fluid power systems contain multiple fluid types: hydraulic oil (NBR), brake fluid (EPDM), fuel (FKM or NBR), coolant (EPDM). Using NBR everywhere fails the brake fluid circuit; using EPDM everywhere fails the hydraulic circuit. Multi-fluid systems require material-specific seal selection for each circuit. Our material selector tool helps identify the correct material for each fluid.
Q4: What does Shore A hardness mean for O-rings?
Shore A hardness measures the resistance of the elastomer to indentation — a proxy for stiffness. 70 Shore A is the standard for most O-rings, balancing conformability and sealing force. Softer materials (40–50A) conform to rougher surfaces but extrude more easily. Harder materials (90A) resist extrusion at high pressure but require smoother surfaces and tighter tolerances. For PTFE, hardness is measured on the Shore D scale (55–65D) because it is a rigid plastic, not an elastomer.
Q5: When should I upgrade from NBR to FKM?
Upgrade from NBR to FKM when: (1) continuous operating temperature exceeds +100–120°C; (2) the fluid contains aromatics, acids, or chlorinated solvents that attack NBR; (3) compression set failure is occurring in NBR within the service interval; or (4) the application requires longer service life and the 5× cost premium is justified by reduced downtime. Do not upgrade to FKM for hot water or steam — FKM degrades in these environments; use EPDM or AFLAS instead.
Q6: What is the difference between an elastomer and a fluoropolymer O-ring?
Elastomers (NBR, FKM, EPDM, VMQ, etc.) are rubber materials that stretch and spring back elastically. Fluoropolymers (PTFE, FEP, PFA) are plastics with no elastic recovery. PTFE O-rings seal by mechanical compression and cold flow into the groove — they do not spring back when pressure is removed. This means PTFE is for static service only, while elastomers handle both static and dynamic applications. FEP encapsulated O-rings combine a fluoropolymer shell (chemical resistance) with an elastomer core (elasticity) for aggressive chemical static seals.
Q7: How do I verify that an O-ring material is compatible with my fluid?
The definitive test is ASTM D471 immersion testing: immerse the O-ring material in your specific fluid at maximum operating temperature for 70–168 hours, then measure volume change, hardness change, tensile strength retention, and elongation retention. Acceptable limits vary by application, but generally: volume swell <25%, hardness change <10 points, tensile retention >75%. For quick screening, use our chemical compatibility tool or contact our engineering team with your fluid specification.
Q8: What are the most common O-ring material mistakes?
- Using NBR in hot water or steam → hydrolysis and compression set failure. Use EPDM.\n2. Using EPDM in oil or fuel → severe swell and extrusion. Use NBR or FKM.\n3. Using FKM in steam or amines → dehydrofluorination and cracking. Use EPDM or AFLAS.\n4. Using VMQ in petroleum oil → 100%+ swell. Use NBR or FKM.\n5. Using PTFE in dynamic service without energizer → cold flow and leakage. Use spring-energized PTFE or elastomer.\n6. Reusing O-rings after disassembly → compression set prevents reliable resealing. Always replace.\n
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Need help selecting the right O-ring material? Contact our engineering team with your operating temperature, fluid type, pressure, and application. We supply all materials discussed in this guide — NBR, HNBR, FKM, FFKM, EPDM, VMQ, FVMQ, CR, PU, AFLAS, PTFE, Butyl, and FEP Encapsulated. Custom compounds and sizes from MOQ 1 piece.