PU O-Rings (Polyurethane / Urethane)

Polyurethane (PU) O-rings — also called urethane O-rings, TPU O-rings, or poly O-rings — are designated AU (polyester urethane) and EU (polyether urethane) under ISO 1629. They deliver the highest abrasion resistance, tensile strength and tear resistance of any elastomeric seal material. Tensile strengths of 25–55 MPa and abrasion resistance 5–10× that of NBR make polyurethane / urethane O-rings the standard choice for hydraulic rod seals, piston seals, and pneumatic cylinders in demanding reciprocating service.

With hardness options from 60 to 95 Shore A, PU O-rings resist extrusion into clearance gaps far better than softer elastomers. 80–95 Shore A PU seals operate reliably in hydraulic systems exceeding 40 MPa without requiring backup rings in many groove configurations. Low compression set and excellent rebound characteristics contribute to long service intervals in continuous-duty equipment.

Polyester-based (AU) PU offers higher mechanical strength and lower cost but is susceptible to hydrolysis in wet environments. Polyether-based (EU) PU provides superior hydrolysis resistance for water-based hydraulic fluids and wet environments — the correct choice for mobile hydraulic equipment operating outdoors or in humid conditions.

PU is not suitable for hot water above 50°C, steam, glycol brake fluids, or polar solvents (ketones, chlorinated solvents), where NBR, EPDM, or FKM should be used instead.

Lead time: 7–15 days standard; 3–5 days for stocked sizes. MOQ: 1 piece. ISO 9001 certified.

The molecular structure of polyurethane explains its exceptional mechanical properties: PU is not a single polymer type but a class of materials formed by the reaction of a diisocyanate (MDI, TDI, or H12MDI) with a long-chain polyol (polyester or polyether) and a short-chain chain extender (typically a glycol or diamine). The result is a microphase-separated structure with hard segments (diisocyanate + chain extender, high glass transition temperature) and soft segments (polyol, low Tg). The hard segments act as physical crosslinks and filler particles, providing hardness, modulus, and tear strength; the soft segments provide elasticity and low-temperature flexibility. The ratio of hard to soft segments determines the final properties: a 40% hard-segment content yields approximately 90 Shore A, while 25% hard-segment content yields approximately 70 Shore A. Polyester soft segments (AU) contain ester linkages (-CO-O-) that are vulnerable to hydrolysis by water, especially at elevated temperatures and in the presence of acids or bases. Polyether soft segments (EU) contain ether linkages (-C-O-C-) that are far more resistant to hydrolysis, making EU PU the correct choice for wet or humid environments.

Quantified performance comparisons versus other elastomers demonstrate why PU dominates dynamic hydraulic sealing. In tensile strength (ASTM D412), PU achieves 25–55 MPa versus NBR's 10–20 MPa — a 150–175% advantage that directly translates to better extrusion resistance and resistance to installation damage. In tear strength (ASTM D624, Die C), PU measures 40–100 kN/m versus NBR's 15–30 kN/m — a 2–3× improvement that prevents nick propagation in contaminated hydraulic fluid. In Taber abrasion testing (ASTM D4060, H-18 wheel, 1000 g, 1000 cycles), PU wear loss is 50–150 mg versus NBR's 200–400 mg — a 3–5× improvement that extends seal life in abrasive environments. In compression set (ASTM D395 Method B, 22 h/70°C), PU achieves 15–30%, comparable to or slightly better than NBR at the same temperature. However, PU's upper temperature limit of +80°C is a critical constraint: at 100°C, PU's compression set degrades to >40% and its tensile strength drops by 30–50% due to soft-segment mobility. Dynamic friction coefficient of PU (0.3–0.6 against steel) is lower than NBR's (0.5–1.0), reducing stick-slip and heat generation in reciprocating seals.

A practical selection decision tree: if your application is an indoor industrial hydraulic cylinder using mineral oil at 15–35 MPa in a clean, dry environment, specify AU (polyester) PU at 85–90 Shore A — it provides the highest mechanical strength at the lowest cost. If your application is a mobile excavator hydraulic cylinder operating outdoors with rain, condensation, and potential water contamination in the hydraulic fluid, specify EU (polyether) PU at 85–90 Shore A — the hydrolysis resistance prevents embrittlement from moisture ingress. If your application is a pneumatic cylinder at ≤ 10 bar requiring low friction and long maintenance intervals, specify 70 Shore A EU PU. If your application involves water-glycol hydraulic fluid (HFC), specify EU PU and verify compatibility with the specific glycol formulation — some HFC fluids require specialized EU compounds. If your application is a high-pressure injection moulding press at >40 MPa, specify 95 Shore A PU or add PTFE backup rings to 90 Shore A PU. If temperature exceeds +80°C or involves steam, hot water above 50°C, or glycol brake fluid, do not use PU — specify NBR, HNBR, or EPDM.

Storage of PU requires attention to its sensitivity to humidity and heat. Polyester (AU) PU is particularly vulnerable to hydrolysis during storage: at 70% relative humidity and 30°C, AU PU can lose 10–15% of its tensile strength within 12 months due to moisture-induced chain scission. Store all PU O-rings in airtight packaging with desiccant, below 25°C, and away from direct sunlight. EU PU is more tolerant of humidity but should still be stored under the same conditions. The recommended shelf life is 3–5 years for AU PU and 5–7 years for EU PU when stored properly. PU is not susceptible to ozone cracking. A critical error is installing AU PU in a water-glycol hydraulic system without verifying hydrolysis resistance — within 3–6 months, the seal will embrittle, crack, and fail, often causing catastrophic hydraulic fluid loss. Another common mistake is using PU in brake systems: glycol-based brake fluid (DOT 3, DOT 4) causes immediate and severe swelling of PU, rendering it completely unsuitable.