NBR and EPDM are the two most common general-purpose O-ring materials, and they are mutually exclusive in most applications. Specifying the wrong one — EPDM in hydraulic oil or NBR in steam — causes rapid, complete seal failure, not gradual degradation.
Quick answer: NBR seals petroleum oil, fuel, and hydrocarbons. EPDM seals water, steam, glycol, and ozone-exposed outdoor systems. Never swap them. When a single seal must handle both oil and water, use HNBR or FKM.
Quick Reference: NBR vs EPDM
| Property | NBR | EPDM |
|---|---|---|
| Petroleum/mineral oil resistance | Excellent | Not suitable (30–80% swell) |
| Water and steam resistance | Poor above +80°C | Excellent (peroxide cure to +150°C) |
| Ozone resistance | Poor — cracks in weeks outdoors | Excellent |
| Glycol brake fluid (DOT 3/4) | Not suitable | Excellent |
| Phosphate ester fluid (Skydrol) | Not suitable | Excellent |
| Temperature range (dry heat) | −25°C to +120°C | −45°C to +150°C (peroxide cure) |
| NSF 61 potable water certified | No | Available |
| FDA food contact | Not standard | Available (peroxide cure) |
| Relative cost | 1× | 1.1–1.3× |
Why the Two Materials Are Chemically Incompatible
Why EPDM Swells in Petroleum Oil
EPDM is a copolymer of ethylene and propylene (with a diene termonomer for crosslink sites). The ethylene-propylene backbone is non-polar and saturated. Non-polar polymer + non-polar petroleum hydrocarbon = thermodynamic compatibility: the petroleum molecules diffuse into the polymer matrix, causing swelling.
Volume swell of standard EPDM in mineral hydraulic oil at +70°C for 70 hours (ASTM D471): 30–80% — far beyond the 3–12% swell that any static seal design can accommodate without gross dimensional change.
The only exception is water-glycol hydraulic fluid (e.g., HFC type), which contains water as a major component and is not a petroleum hydrocarbon — EPDM is compatible with these fluids.
Why NBR Degrades in Hot Water and Steam
NBR contains acrylonitrile (ACN) groups in its polymer chain. The ACN nitrile groups are susceptible to hydrolysis — attack by water at elevated temperature, which breaks the polymer backbone. The reaction:
—CH₂—CH(CN)— + H₂O → —CH₂—CH(COOH)— (nitrile to carboxylic acid)Above +100°C in continuous water contact, NBR undergoes progressive hardening followed by cracking as the hydrolysis products alter crosslink density. At steam temperatures (+120–150°C), this degradation is rapid — complete property loss within dozens of hours. Additionally, NBR's residual C=C double bonds (from the butadiene component) oxidize in the presence of ozone and elevated temperature, causing surface cracking and embrittlement.
The simple rule: petroleum-based fluid → NBR; water, steam, glycol, ozone → EPDM.
Polymer Chemistry Summary
| Property | NBR | EPDM |
|---|---|---|
| Monomer composition | Acrylonitrile (ACN) + butadiene | Ethylene + propylene + diene termonomer |
| Backbone saturation | Partially unsaturated (C=C bonds from butadiene) | Largely saturated (C=C only at diene crosslink sites) |
| Oil resistance mechanism | Polar ACN groups resist petroleum hydrocarbon absorption | Non-polar backbone is thermodynamically compatible with petroleum — swells |
| Water/steam resistance | Hydrolysis of ACN groups above +80°C; fails in steam | Non-polar backbone resists water uptake; stable |
| Ozone resistance | Poor — C=C bonds cleaved by ozone (O₃) | Excellent — no main-chain C=C bonds available |
| ACN content range | 18–40% (higher = better oil resistance, poorer cold flex) | Not applicable |
| Cure systems | Sulfur or peroxide | Sulfur or peroxide (peroxide required for steam service) |
Temperature Range: Specific Thresholds
| Property | NBR (33% ACN) | NBR (40% ACN) | EPDM (Sulfur cure) | EPDM (Peroxide cure) | Test Method |
|---|---|---|---|---|---|
| Continuous service max (dry heat) | +120°C | +120°C | +130°C | +150°C | ASTM D573 |
| Continuous service max (water) | +80°C | +80°C | +120°C | +150°C | — |
| Continuous service max (steam) | Not suitable | Not suitable | +120°C | +150°C | — |
| Low-temperature limit (dynamic) | −25°C | −15°C | −40°C | −45°C | ASTM D1329 (TR10) |
| Compression set, +100°C / 22h | 25–40% | 22–35% | 30–50% | 18–28% | ASTM D395 Method B |
| Compression set, +120°C / 22h | 40–60% | 35–55% | 40–55% | 20–30% | ASTM D395 Method B |
| Compression set, +150°C / 22h | >70% — failed | >70% — failed | >70% | 30–45% | ASTM D395 Method B |
| Heat aging (+150°C / 70h): hardness change | +10 to +20 ShA | +8 to +18 ShA | +5 to +10 ShA | +3 to +7 ShA | ASTM D573 |
The peroxide vs sulfur cure distinction for EPDM is the most commonly overlooked variable in steam and food/pharma applications.
Peroxide vs Sulfur Cured EPDM: The Critical Distinction for Steam
| Property | Sulfur-Cured EPDM | Peroxide-Cured EPDM |
|---|---|---|
| Continuous steam temperature limit | +120°C | +150°C |
| Compression set, +120°C / 22h (ASTM D395) | 40–55% | 18–28% |
| Compression set, +150°C / 22h | >70% — limit exceeded | 30–45% |
| CIP resistance (2% NaOH, +80°C) | Good | Excellent |
| SIP resistance (steam +134°C) | Marginal | Good |
| FDA 21 CFR §177.2600 compliance | Achievable | Preferred for hot service |
| Accelerator residue extraction (steam) | Possible (MBT, MBTS leach into food-contact fluid) | None — peroxide leaves no extractable residues |
| Cost premium over sulfur EPDM | Baseline | +15–35% |
For food processing CIP cycles, pharmaceutical SIP (steam-in-place) at +134°C, and steam-heated process equipment, specify peroxide-cured EPDM. Sulfur-cured EPDM in repeated steam cycles (1) shows high compression set that reduces sealing force, and (2) may leach sulfur-based accelerators (MBT, MBTS) into food-contact steam condensate — a regulatory failure mode separate from mechanical performance.
Fluid Compatibility Reference
NBR: Compatible and Incompatible Fluids
| Fluid | NBR Rating | Volume Swell (ASTM D471) | Notes |
|---|---|---|---|
| Mineral hydraulic oil (ISO VG 32–100) | Excellent | 5–12% | Primary application |
| Diesel fuel (ULSD) | Excellent | 5–12% | Standard fuel seal |
| Gasoline (< 20% aromatics) | Good | 8–18% | Standard fuel service |
| Gasoline (> 25% aromatics) | Limited | 15–30% | Approaching service limit |
| Engine oil (mineral/semi-synthetic) | Excellent | 5–12% | Standard under-hood |
| Petroleum grease | Excellent | Minimal | Static seals |
| Water at ambient | Fair | 2–5% | Short-term only |
| Water at +80–100°C | Poor | Hardening/cracking | Hydrolysis begins |
| Saturated steam (any pressure) | Not suitable | Rapid degradation | Never use NBR in steam |
| Ethylene glycol / water coolant | Fair at ambient | 5–12% | Degrades above +80°C |
| DOT 3/4/5.1 glycol brake fluid | Not suitable | 15–25% | Documented brake failure mode |
| Phosphate ester hydraulic fluid (Skydrol) | Not suitable | 30–80% | Catastrophic |
| Ethanol E85 / E100 | Poor | 20–35% | FKM required |
| Ketones (acetone, MEK) | Poor | 30–60%+ | Severe attack |
| Dilute acids / bases at ambient | Fair | Variable | Test compound-specifically |
EPDM: Compatible and Incompatible Fluids
| Fluid | EPDM Rating | Volume Swell (ASTM D471) | Notes |
|---|---|---|---|
| Water (ambient to +150°C) | Excellent | 2–6% | Primary application |
| Saturated steam (< +150°C, peroxide cure) | Excellent | 3–8% | Specify peroxide cure |
| CIP: 2% NaOH, +80°C | Excellent | 3–7% | Standard food/pharma |
| CIP: 0.5% HNO₃, +60°C | Good | 4–9% | Acid CIP |
| CIP: 200 ppm peracetic acid, +25°C | Good | 3–8% | Low-temp sanitizer |
| Ethylene glycol/water coolant | Excellent | 3–8% | Automotive cooling |
| DOT 3/4/5.1 glycol brake fluid | Excellent | 2–8% | OEM standard for brakes |
| Phosphate ester hydraulic fluid (Skydrol) | Excellent | 5–12% | Aviation hydraulic standard |
| Ethanol/water mixtures | Good | 5–15% | Acceptable across blend levels |
| Ozone (any concentration) | Excellent | No attack | Saturated backbone |
| Mineral hydraulic oil | Not suitable | 30–80% | Severe swelling |
| Petroleum fuel (gasoline/diesel) | Not suitable | 40–100%+ | Catastrophic swelling |
| Petroleum grease | Not suitable | 30–60% | Never use petroleum grease on EPDM |
| Ketones (acetone, MEK) | Limited | 15–30% | Better than NBR but not ideal |
| Aromatic solvents (toluene, xylene) | Not suitable | 50–100%+ | Severe swelling |
| Concentrated HNO₃ (> 40%) | Poor | Oxidative degradation | Not suitable |
Ozone and Weathering Resistance
Ozone attacks C=C double bonds in unsaturated elastomers through a chain-cutting mechanism. NBR, with its butadiene-derived C=C bonds, cracks rapidly in ozone.
ASTM D1171 ozone test results (50 pphm ozone, 20% elongation, +40°C):
- NBR: Visible cracking within 24–48 hours
- EPDM: No cracking at 200 pphm ozone, 20% elongation, 72 hours
At typical outdoor ozone concentrations (0.02–0.10 ppm = 2–10 pphm), an NBR O-ring under tensile stress will show surface cracking within weeks to months. Near electrical equipment (transformers, high-voltage switchgear, UV lamps), local ozone concentration can reach 0.5–5 ppm — NBR failure accelerates proportionally.
EPDM's nearly fully saturated backbone (the diene termonomer provides crosslink sites only, not main-chain double bonds) makes it inherently ozone resistant at all concentrations encountered in industrial and outdoor service.
Applications where EPDM is required due to ozone:
- Outdoor plumbing, HVAC, irrigation, water treatment systems
- Exposed hydraulic cylinder seals on outdoor equipment
- Building services, fire suppression fittings
- Near electrical switchgear or UV lamp assemblies
Mechanical Properties Comparison
| Property | NBR (Standard 70 ShA) | EPDM (Peroxide, 70 ShA) | Test Method |
|---|---|---|---|
| Tensile strength | 15–25 MPa | 10–20 MPa | ASTM D412 |
| Elongation at break | 200–400% | 200–350% | ASTM D412 |
| Tear resistance (Die C) | 30–50 kN/m | 20–35 kN/m | ASTM D624 |
| Abrasion loss (DIN 53516) | 120–200 mm³ | 150–250 mm³ | ASTM D5963 |
| Compression set, +100°C / 22h | 25–40% | 18–28% | ASTM D395 Method B |
| Hardness range available | 40–90 Shore A | 40–80 Shore A | ASTM D2240 |
NBR has better mechanical properties for dynamic sealing in oil. Peroxide-cured EPDM is preferred for dynamic water seals — its compression set advantage over sulfur-cured EPDM is especially meaningful in high-cycle water hydraulic actuators.
Brake Fluid Compatibility
Automotive brake fluid is one of the most common contexts where EPDM is required and NBR must be avoided:
| Brake Fluid Type | NBR | EPDM | Notes |
|---|---|---|---|
| DOT 3, DOT 4, DOT 5.1 (glycol-based) | Not suitable | Excellent | EPDM is OEM standard — 2–8% swell |
| DOT 5 (silicone-based) | Marginal | Fair | VMQ may be preferred |
DOT 3, 4, and 5.1 fluids are polyethylene glycol (PEG) based. The glycol chemistry causes approximately 15–25% volume swell in NBR — enough to push the O-ring out of groove and block master cylinder ports. NBR in a glycol brake system is a documented cause of brake failure. EPDM swells only 2–8% in glycol brake fluid and maintains full mechanical properties throughout rated temperature range.
Potable Water and Food Contact
Standard NBR is not certified for potable water contact — it can leach plasticizers and cure residues that affect taste and regulatory compliance.
Potable water (NSF/ANSI 61):
- EPDM (NSF 61 certified compound): Standard for plumbing fittings, water meters, flow control valves. Not all EPDM compounds qualify — request the NSF 61 listing certificate for the specific compound and color.
- VMQ (FDA grade): Alternative where wider temperature range is needed; more expensive.
- Standard NBR: Not acceptable.
Food processing (FDA 21 CFR §177.2600, EU 1935/2004):
- Peroxide-cured EPDM meeting FDA §177.2600 extractables requirements: Standard for CIP-service food equipment
- EU food contact: Must also meet EC 10/2011 for plastic food contact; confirm with supplier
- 3-A Standard 18-03 (dairy): Peroxide-cured EPDM is specifically listed; sulfur-cured EPDM has restrictions on accelerator compounds
CIP and Autoclave Cycle Performance
For food and pharmaceutical applications subject to repeated CIP and autoclave cycles, peroxide-cured EPDM performance by cycle count:
| Cycle Count | Compression Set Change (peroxide EPDM, +134°C steam) | Surface Condition |
|---|---|---|
| Baseline | 20–28% (ASTM D395 Method B) | Smooth, as-molded |
| 100 cycles | +2–5% additional set | Surface intact |
| 300 cycles | +5–10% additional set | Minor surface matting |
| 500 cycles | +8–15% additional set | Surface matting; acceptable |
| 1,000 cycles | +15–25% additional set | Inspect for surface cracking |
Sulfur-cured EPDM shows approximately 2–3× faster compression set progression at +134°C — it is not suitable for applications exceeding 200–300 autoclave cycles.
Estimated Service Life by Temperature
| Operating Temperature (steam, peroxide EPDM) | Estimated Continuous Service Life |
|---|---|
| +100°C | > 10 years (limited by ozone, UV, mechanical fatigue) |
| +120°C | 5–10 years |
| +134°C (autoclave) | 3–7 years (cycle-count limited) |
| +150°C | 2–5 years |
| +160°C | Not rated — FFKM or AFLAS required |
These estimates assume no chemical attack from aggressive fluids, proper groove design, and no mechanical overload. Actual service life depends heavily on compound formulation — use these as planning estimates, not guarantees.
When Neither NBR Nor EPDM Is Sufficient
| Requirement | Recommended Material | Rationale |
|---|---|---|
| Oil + water, up to +120°C | HNBR | Oil resistance of NBR + better hot-water resistance |
| Oil + ozone, up to +150°C | HNBR | Saturated backbone resists ozone; maintains oil compatibility |
| Oil + water + steam, up to +200°C | FKM | Broadest thermal stability in combined service |
| Steam above +150°C | FFKM or AFLAS | EPDM rated limit exceeded |
| Hydrocarbon + aggressive solvent + heat | FKM or FFKM | Broader chemical resistance required |
Application Decision Matrix
| Application | Material | Rationale |
|---|---|---|
| Hydraulic cylinder (mineral oil, indoor) | NBR | Standard application; cost-effective |
| Hydraulic cylinder (water-glycol fluid) | EPDM (peroxide) | Water-glycol incompatible with NBR |
| Automotive fuel injector seal | NBR or HNBR | Oil and fuel service; heat drives HNBR |
| Automotive brake master cylinder | EPDM | DOT 3/4 glycol — NBR fails |
| Automotive cooling system fittings | EPDM | Water-glycol coolant + ozone exposure |
| Steam valve (CIP/SIP to +134°C) | EPDM (peroxide cure) | Steam performance; FDA compliance |
| Outdoor plumbing and irrigation | EPDM | Ozone and UV resistance |
| Potable water fitting (NSF 61) | EPDM (NSF 61 compound) | Certified potable water compliance |
| Food processing equipment (CIP) | EPDM (peroxide, FDA grade) | FDA §177.2600; CIP cycle resistance |
| Industrial hydraulics (oil, indoor) | NBR | Standard; most economical |
| Aircraft hydraulic (Skydrol) | EPDM | Phosphate ester compatibility |
| Engine oil seal (under-hood) | HNBR or NBR | Oil service; ozone drives HNBR selection |
Field Identification: Color, Marking, and Quick Tests
NBR and EPDM O-rings are both commonly black, so color alone is unreliable. Some suppliers use color coding to help identification, but there is no universal standard.
| Common Color | More Likely Material | Notes |
|---|---|---|
| Black | NBR or EPDM | Default color for both; must test or check marking |
| Green | EPDM or HNBR | EPDM is often green in water/brake applications; HNBR green in automotive AC |
| Blue | EPDM (potable water) | NSF 61 EPDM is frequently blue |
| Brown | FKM (Viton) | Not NBR/EPDM |
| Red/orange | VMQ silicone | Not NBR/EPDM |
Quick field tests (destructive — use only on scrap samples):
- Hydrocarbon test: Drop a small piece in mineral oil or diesel. EPDM swells dramatically within hours; NBR swells only slightly.
- Heat test: Heat a small sample with a torch. NBR burns with a sooty, acrid flame. EPDM burns with a paraffin/wax-like odor and drips.
- Density: NBR ~1.10–1.25 g/cm³; EPDM ~0.85–1.10 g/cm³. EPDM often floats in water; NBR usually sinks.
For critical applications, send a sample for FTIR analysis rather than relying on visual or simple tests.
NBR vs EPDM in Automotive Systems
Automotive under-hood and chassis systems are a common source of confusion because NBR and EPDM sit inches apart in the same vehicle:
| System | Fluid | Correct Material | Why |
|---|---|---|---|
| Engine oil pan / valve cover | Mineral oil | NBR or HNBR | Oil resistance |
| Fuel system (gasoline/diesel) | Hydrocarbon fuel | NBR or HNBR | Fuel resistance |
| Cooling system | Ethylene glycol/water | EPDM | Coolant + ozone |
| Brake system (DOT 3/4/5.1) | Glycol brake fluid | EPDM | Glycol compatibility |
| Power steering (hydraulic fluid) | Mineral oil | NBR | Oil resistance |
| HVAC refrigerant (R-134a/R-1234yf) | Refrigerant + PAG oil | HNBR | Refrigerant/oil compatibility |
| Windshield washer | Methanol/water | EPDM or VMQ | Alcohol/water resistance |
Using NBR in a cooling system or EPDM in an engine oil seal will cause rapid failure. Always confirm the fluid type before selecting a replacement O-ring in automotive maintenance.
FAQ
Q1: Can I use EPDM for hydraulic oil service?
No. Standard EPDM swells 30–80% in mineral petroleum oil — the seal will over-fill its groove within hours, causing extrusion or complete loss of compression. There are specialty EPDM compounds with modified formulations that reduce oil swell to 10–20%, but they still do not approach NBR's performance in petroleum service. Use NBR or HNBR for all mineral oil applications.
Q2: Is NBR safe for drinking water?
Standard NBR is not certified for potable water contact. NBR can leach compounding agents (plasticizers, carbon black, cure additives) that affect taste and do not meet NSF/ANSI 61 or EN 12502 requirements. For drinking water systems, specify NSF 61 certified EPDM or, for high-temperature or ultra-pure applications, FDA-grade VMQ.
Q3: Which material is better for outdoor HVAC equipment?
EPDM is far superior for outdoor service. NBR's C=C double bonds are attacked by atmospheric ozone, causing surface cracking within weeks to months at typical outdoor ozone concentrations (0.02–0.10 ppm). EPDM's saturated backbone is inherently ozone-resistant — it passes ASTM D1171 at 200 pphm ozone without cracking, making it the standard for outdoor HVAC, water treatment, irrigation, and building services.
Q4: Does NBR handle steam at all?
NBR can tolerate intermittent hot water contact up to approximately +80°C, but continuous steam service causes hydrolytic degradation of the ACN groups in the polymer chain. At saturated steam temperatures (+100°C and above), NBR hardens, loses compression set resistance, and eventually cracks. Peroxide-cured EPDM is the correct material for steam service up to +150°C; AFLAS or FFKM for steam above +150°C.
Q5: What is the difference between peroxide-cured and sulfur-cured EPDM for O-rings?
Sulfur-cured EPDM is the standard grade for general water and outdoor service. Peroxide-cured EPDM uses a different crosslink chemistry that produces more thermally stable C–C crosslinks (vs C–S–C crosslinks in sulfur cure), enabling continuous service in steam to +150°C versus approximately +120°C for sulfur cure. Peroxide-cured EPDM also shows significantly better compression set at elevated temperature (18–28% vs 40–55% at +120°C, ASTM D395). For food and pharmaceutical CIP/SIP applications with steam cycles, peroxide-cured EPDM is the required specification. Confirm cure system with the supplier — datasheets do not always specify this clearly, and the two types are visually identical.
Q6: My application sees both hydraulic oil and wash-down water. Which material should I use?
When a single seal must contact both petroleum oil and water, neither NBR nor EPDM is the correct answer. HNBR (hydrogenated nitrile) provides oil resistance equivalent to NBR with better high-temperature water resistance to approximately +120°C continuous. For applications requiring simultaneous contact with both fluids above +120°C, FKM is the more reliable material. Provide the specific oil type, water temperature, and any cleaning agents to your supplier for a compound-level recommendation.
Q7: Why does EPDM swell in petroleum oil if it's a "rubber"?
Swelling is not a failure — it is a thermodynamic consequence of chemical compatibility. The "like dissolves like" principle applies to elastomers: non-polar EPDM (ethylene-propylene backbone) is miscible with non-polar petroleum hydrocarbons. The petroleum molecules diffuse into the EPDM matrix, separating polymer chains and increasing volume. The same mechanism makes NBR swell severely in ketones but resist petroleum oil — the polar ACN groups make NBR thermodynamically incompatible with non-polar hydrocarbons. Always check chemical compatibility before specifying any elastomer, regardless of apparent similarity to a previously successful design.
Q8: Is there an EPDM grade that resists both oil and water?
No commercially available EPDM compound provides both petroleum oil resistance and water/steam resistance comparable to dedicated materials. Some specialty EPDM formulations (high-density/modified backbone) reduce oil swell to 10–20%, but this still exceeds practical sealing limits. The correct approach for oil + water service is HNBR (to +120°C) or FKM (to +200°C). Do not try to find a modified EPDM — the polymer chemistry prevents achieving both properties simultaneously.
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Selecting between NBR and EPDM for your application? Request a quote with your fluid type, temperature, and any ozone or food-contact requirements. We stock both materials in standard AS568 and metric sizes, with peroxide-cured EPDM available for steam and food-contact service. MOQ from 1 piece; 3–5 day shipping on stocked compounds.
Related guides: Chemical-Resistant O-Ring Selection Guide | Oil-Resistant O-Rings | Chemical Compatibility Tool