Polyurethane (PU, also designated AU or EU) and nitrile rubber (NBR, Buna-N) are both common choices for hydraulic and pneumatic O-rings, but they fail by different mechanisms and excel in different operating conditions. Choosing between them requires matching the seal material to the dominant failure mode in the application rather than defaulting to the lower-cost option.
Quick answer: Use NBR for general-purpose hydraulic and pneumatic service from −40°C to +120°C where oil compatibility, compression set resistance, and low cost are the primary requirements. Use PU when the application demands superior abrasion resistance, tear strength, and extrusion resistance in dynamic cylinders — particularly reciprocating rods and pistons in mobile hydraulics — but limit continuous service temperature to approximately +80°C (+90°C short-term) and avoid hot water, steam, or strong acids/bases. PU typically costs 2–4× more than NBR, but the cost is justified when abrasive media or high pressure would cause NBR to wear or extrude prematurely.
Polymer Structure: Why Performance Differs
NBR (Nitrile Butadiene Rubber)
NBR is an emulsion copolymer of acrylonitrile (ACN) and butadiene. The ACN content controls oil resistance and low-temperature flexibility, while the butadiene segments provide elasticity and relatively low modulus. NBR is a thermoset elastomer with a flexible backbone and moderate mechanical strength.
| ACN Content | Oil/Fuel Resistance | Low-Temp TR10 | Typical Use |
|---|---|---|---|
| 18–24% | Moderate | −50 to −45°C | Arctic/low-temp pneumatics |
| 33–36% | Very good | −35 to −30°C | General hydraulics — most common |
| 38–45% | Excellent | −20 to −15°C | Fuel systems, aromatic fluids |
The butadiene units introduce carbon–carbon double bonds (C=C) that make NBR vulnerable to ozone, oxidative hardening above +100°C, and UV cracking. These limitations are irrelevant in many enclosed hydraulic systems but matter in outdoor or high-temperature service.
PU (Polyurethane)
Polyurethane O-rings are based on segmented block copolymers containing soft polyol segments and hard urethane/urea segments. The hard segments form physical crosslinks and hydrogen-bonded domains that give PU its exceptional mechanical strength, abrasion resistance, and high modulus. Two common elastomeric polyurethane families are:
| Designation | Chemistry | Notes |
|---|---|---|
| AU | Polyester urethane | Higher mechanical strength; better oil resistance; more sensitive to hydrolysis in hot/humid conditions |
| EU | Polyether urethane | Better hydrolysis and microbial resistance; slightly lower tensile strength than polyester grades |
The high cohesive energy of the urethane hard segments produces tensile strength 2–3× higher than NBR and tear resistance 2–4× higher. This is the origin of PU's advantage in abrasive and high-pressure dynamic sealing. However, the same stiff structure limits high-temperature performance and produces higher compression set than NBR.
Temperature Range
| Condition | NBR 70A | PU 70A (Standard) | Correct Choice |
|---|---|---|---|
| Continuous service maximum | +120°C | +80°C | NBR above +80°C |
| Short-term peak | +135°C | +90 to +100°C | NBR if peaks exceed +90°C |
| Low-temperature static limit | −40°C (standard) | −30 to −35°C | NBR for cold-start flexibility |
| Low-temperature dynamic limit | −25°C | −20 to −25°C | NBR for cold dynamic start |
| TR10 (elastic recovery to 10%) | −30 to −35°C (33% ACN) | −25 to −30°C | NBR slightly better |
ASTM D573 heat aging comparison (70 h at test temperature):
| Property | NBR 70A (+100°C) | PU 70A (+80°C) |
|---|---|---|
| Tensile retention | 80–90% | 70–85% |
| Elongation retention | 70–85% | 60–75% |
| Hardness change | +3 to +8 Shore A | +5 to +10 Shore A |
The practical temperature ceiling for PU is approximately +80°C continuous. Above this temperature, the urethane hard segments begin to dissociate, compression set rises rapidly, and the material softens. NBR tolerates +100°C continuous service in standard mineral oil and remains usable to +120°C for shorter intervals or less critical static seals.
Abrasion & Wear Resistance
PU dominates this category. Its high hardness combined with high tensile strength produces exceptional resistance to abrasive wear from contaminated hydraulic fluid, metal particles, dust, and reciprocating motion against metal surfaces.
| Property | NBR 70A | PU 70A | Test Method |
|---|---|---|---|
| Tensile strength | 10–20 MPa | 25–45 MPa | ASTM D412 |
| Elongation at break | 250–500% | 350–650% | ASTM D412 |
| Abrasion loss (DIN 53516 / ASTM D5963) | 120–250 mm³ | 20–60 mm³ | ASTM D5963 |
| Taber abrasion index | Moderate | Excellent | — |
In contaminated hydraulic systems — construction equipment, mining machinery, agricultural implements — PU O-rings and U-cups routinely outlast NBR by 3–5× in rod seal applications. The same abrasive contamination that scuffs and erodes NBR surfaces has little effect on PU because of its higher tear strength and modulus.
Extrusion Resistance
Extrusion occurs when pressure forces the O-ring into the clearance gap between mating metal surfaces. Resistance depends on modulus, hardness, and tear strength — all areas where PU outperforms NBR.
| Property | NBR 70A | PU 70A | Significance |
|---|---|---|---|
| Modulus at 100% elongation (M100) | 1.5–3.0 MPa | 4–8 MPa | Higher modulus resists gap entry |
| Tear resistance (Die C) | 25–45 kN/m | 50–90 kN/m | Higher tear strength resists nibbling |
| Recommended max pressure, 70A, no backup ring | ~100 bar | ~150–200 bar | PU tolerates higher pressure |
| Recommended max pressure, 90A, no backup ring | ~200 bar | ~300–400 bar | High-hardness PU resists extrusion strongly |
For hydraulic cylinders operating at > 150 bar without backup rings, PU is the safer choice. NBR at 70 Shore A will extrude into typical rod seal clearances at pressures above 100–130 bar unless the clearance is tightly controlled or a PTFE backup ring is used. For pressure-specific guidance, see our O-Ring Extrusion & Backup Rings Guide.
Tear & Dynamic Performance
PU is preferred for dynamic reciprocating seals because it combines high tear strength with low friction characteristics in lubricated systems. The material resists the surface initiation and propagation of cracks that terminate NBR seal life in high-cycle applications.
| Application Factor | NBR | PU | Recommendation |
|---|---|---|---|
| Reciprocating rod seals | Good | Excellent | PU for long stroke/high cycle |
| Piston seals (dynamic) | Good | Excellent | PU preferred above 50 bar |
| Rotary seals | Fair | Fair to good | Neither is ideal; consider FKM or PTFE |
| Oscillating seals | Good | Excellent | PU for abrasive duty |
| Installation damage sensitivity | Low | Moderate | PU requires chamfered grooves |
The higher modulus of PU means it is slightly less forgiving during installation than NBR. Sharp groove edges, inadequate chamfers, or twisting during assembly can nick PU and initiate dynamic cracks. Proper groove design eliminates this risk, and PU's longer service life more than compensates for careful installation.
Oil, Fuel & Chemical Resistance
NBR has broader compatibility with petroleum-based fluids and is the default choice for mineral hydraulic oils, diesel, gasoline, and many greases. PU resists many mineral oils well but is degraded by hot water, steam, and polar solvents.
Volume swell in common fluids (ASTM D471, 70 h)
| Fluid | NBR 34% ACN | PU (AU) | Recommendation |
|---|---|---|---|
| Mineral hydraulic oil ISO VG 46 at +100°C | +3 to +8% | +2 to +6% | Both suitable; NBR more cost-effective |
| IRM 903 oil at +100°C | +8 to +20% | +3 to +10% | Both acceptable |
| ASTM Fuel B (aromatic fuel) | +25 to +40% | +15 to +30% | Neither ideal; FKM preferred |
| Diesel fuel at +60°C | +3 to +8% | +2 to +6% | Both suitable |
| Water at +23°C | +1 to +3% | +1 to +2% | Both acceptable |
| Water at +80°C | +2 to +5% | +5 to +15% (hydrolysis risk) | NBR preferred |
| Steam at +100°C | Degrades | Severe hydrolysis | Neither; use EPDM |
| Dilute acids (pH 3–5) | Fair to good | Poor to fair | NBR preferred |
| Dilute bases (pH 9–11) | Fair | Poor | NBR preferred |
| Brake fluid DOT 3/4 | +20 to +40% | +10 to +25% | Neither ideal; EPDM standard |
For any fluid uncertainty, verify compatibility with the chemical compatibility tool before selecting either material.
Compression Set & Permanent Deformation
Compression set resistance is a traditional weakness of PU compared with NBR. After long-term compression at moderate temperatures, PU retains less elastic recovery than NBR, making it less suitable for long-interval static seals.
| Test Condition | NBR 70A Compression Set | PU 70A Compression Set | Test Method |
|---|---|---|---|
| +70°C / 22 h | 15–25% | 20–35% | ASTM D395 Method B |
| +100°C / 22 h | 25–40% | 40–60% | ASTM D395 Method B |
| +120°C / 22 h | 40–60% | Not serviceable | ASTM D395 Method B |
Implication: For static face seals, cover seals, or any application where the seal is compressed continuously and not expected to be replaced frequently, NBR is the safer material. For dynamic reciprocating seals that are inspected and replaced on normal maintenance intervals, PU's compression set disadvantage is outweighed by its wear and extrusion advantages.
Cost Comparison
| Factor | NBR | PU |
|---|---|---|
| Unit cost (relative) | 1× | 2–4× |
| Standard AS568 sizes | Broad | Moderate |
| Custom molded sizes | Readily available | Available with tooling cost |
| Typical MRO inventory approach | Full range | Carry for high-wear/HP applications |
The cost comparison should be made on total lifecycle cost, not unit price. A PU rod seal in an abrasive hydraulic cylinder may last 2–3× as long as an NBR seal, eliminating downtime and labor for replacement. When seal replacement requires cylinder disassembly and lost production, the PU premium is quickly justified. Conversely, specifying PU for a low-pressure, clean-fluid, ambient-temperature static seal adds cost without functional benefit.
Application Selection Matrix
| Application | Recommended Material | Rationale |
|---|---|---|
| General hydraulic cylinder (< 80°C, clean mineral oil, < 100 bar) | NBR 70A | Lowest cost; adequate life |
| Mobile hydraulic rod seal (contaminated fluid, abrasive) | PU 90A | Abrasion and extrusion resistance |
| High-pressure hydraulic cylinder (> 150 bar, no backup ring) | PU 80–90A | Extrusion resistance |
| Pneumatic cylinder (dry air, ambient) | NBR 70A | Low cost; adequate performance |
| Pneumatic cylinder with abrasive dust ingestion | PU 70–80A | Better wear resistance |
| Injection molding machine tie-bar seals | PU 90A | High load; abrasive contamination |
| Press brake / stamping cylinder | PU 80A | Dynamic load; contamination |
| Food/pharmaceutical water service | Neither standard | Use FDA-grade EPDM or VMQ |
| Hot water / steam (> 80°C) | NBR (short-term) or EPDM | PU hydrolyzes |
| Outdoor ozone exposure | Neither ideal | Use FKM, HNBR, or EPDM |
Procurement Checklist
Before selecting PU or NBR, confirm the service conditions that actually determine material life:
| RFQ Detail | Why It Matters | Practical Rule |
|---|---|---|
| Continuous and peak temperature | PU degrades above +80°C continuous | Use NBR above +80°C; specify PU only if temperature verified |
| Pressure and clearance gap | High pressure extrudes soft materials | PU > 100 bar; NBR requires backup rings above ~100 bar |
| Fluid contamination level | Abrasive particles accelerate NBR wear | PU for contaminated systems |
| Static vs. dynamic sealing | PU compression set limits static life | NBR preferred for long-interval static seals |
| Fluid chemistry | PU hydrolyzes in hot water; NBR handles petroleum better | Use chemical compatibility tool |
| Replacement cost / downtime | High labor cost favors longer-life PU | Calculate lifecycle cost, not unit cost |
| Hardness requirement | Higher hardness improves extrusion resistance | Specify 80–90 Shore A for high-pressure PU |
When requesting a quote, provide the operating temperature range, fluid type, maximum pressure, dynamic vs. static duty, and estimated annual cycle count. This allows the supplier to recommend the correct compound rather than a generic material family.
FAQ
Q1: When should I choose PU instead of NBR for a hydraulic O-ring?
Choose PU when the dominant failure modes are abrasive wear, extrusion into clearance gaps, or dynamic tearing — typical in mobile hydraulics, construction equipment, mining machinery, and high-pressure cylinders. NBR is the better choice for general-purpose hydraulic service below +80°C where the fluid is clean mineral oil and replacement intervals are acceptable. PU's 2–4× cost premium is justified only when its mechanical advantages reduce downtime or extend service life.
Q2: Can PU O-rings handle the same temperature range as NBR?
No. Standard cast or injection-molded PU O-rings are limited to approximately −30°C to +80°C continuous, with short-term peaks to +90°C or +100°C depending on compound. Above +80°C, compression set rises rapidly and hydrolysis accelerates in the presence of moisture. NBR operates from −40°C to +120°C and is the safer choice for elevated-temperature hydraulic service.
Q3: Is PU compatible with mineral hydraulic oil?
Yes. PU has excellent compatibility with petroleum-based mineral oils, hydraulic fluids ISO VG 32–68, and most lubricating oils. Volume swell in mineral oil is typically low (2–6%) and mechanical properties remain stable. The limitation is temperature: PU in mineral oil above +80°C will harden and take a set faster than NBR. For hot oil systems, NBR, HNBR, or FKM is more appropriate.
Q4: Why does PU fail in water or steam service?
Polyurethanes are susceptible to hydrolysis — the reaction of the urethane linkages with water, accelerated by temperature. In cold water, PU may perform acceptably for limited durations, but in hot water above +60–70°C or steam, hydrolysis breaks polymer chains, causing softening, loss of tensile strength, and eventual disintegration. Polyester-based PU (AU) is more hydrolysis-sensitive than polyether-based PU (EU), but neither should be used in continuous hot-water service.
Q5: Does PU require different groove design than NBR?
Standard AS568 and ISO 3601 groove dimensions apply to both PU and NBR O-rings when hardness is matched. However, because PU has higher modulus and lower elastic recovery after deformation, installation damage is more likely. Groove edges should be chamfered (typically 15–20°, 0.2–0.3 mm land width), surface finish should be Ra ≤ 0.8 µm dynamic, and the seal should be lubricated during assembly. For high-pressure PU seals, tighter diametral clearance reduces extrusion risk and may allow elimination of backup rings.
Q6: How does compression set compare between PU and NBR?
NBR has lower compression set than PU across the entire useful temperature range. At +70°C, NBR 70A typically shows 15–25% compression set versus 20–35% for PU 70A. At +100°C, NBR is 25–40% while PU is 40–60% and climbing. This makes NBR the better choice for static seals with long service intervals. PU's higher compression set is acceptable in dynamic seals that are replaced on normal maintenance cycles.
Q7: Is PU better than NBR for pneumatic seals?
For clean, dry air at ambient temperature, NBR is the economical choice and performs adequately. Choose PU for pneumatic systems where abrasive dust ingress, high cycle rates, or fluctuating pressure create wear and nibbling conditions. Pneumatic cylinders in dusty environments (foundries, cement plants, agriculture) often benefit from PU piston seals because the material resists the particle embedding and surface cutting that degrades NBR.
Q8: What hardness should I specify for PU vs NBR in high-pressure hydraulics?
For hydraulic service above 100 bar, specify 80–90 Shore A for both materials. NBR at 90 Shore A resists extrusion better than 70A but still benefits from a PTFE backup ring above 150–200 bar. PU at 90 Shore A offers very high extrusion resistance and can often operate at 250–400 bar without backup rings, depending on clearance. Do not drop below 70 Shore A for high-pressure dynamic seals in either material.
---
Need help selecting PU or NBR O-rings for your hydraulic or pneumatic application? Compare our urethane O-rings (PU) and NBR O-rings specifications, contact our engineering team with your fluid type, temperature, pressure, and duty cycle, or request a quote for standard AS568 and metric sizes. We stock NBR 70A and PU 70–90A in common hydraulic sizes with short lead times and can recommend the right compound for abrasive, high-pressure, or temperature-critical service.