Polyurethane (PU/AU) seals — abrasion resistance for hydraulics

Polyurethane is the odd one out among sealing materials: not a classic soft rubber but a tough elastomer with abrasion resistance no other elastomer can match. Catalogues list it as PU or PUR, chemical compatibility charts often use the code AU (the polyester type under ISO/ASTM nomenclature), and you will also meet it under the trade names Vulkollan and Adiprene. You usually recognise it by its typical amber colour. If you are sealing a hydraulic cylinder rod, a high-cycle pneumatic actuator or a machine working in sand and slurry, polyurethane is the first candidate to consider. This article sums up where PU beats standard nitrile, where its hard limits lie — hydrolysis and temperature — and how to decide between PU and NBR for a dynamic seal.

Why polyurethane is different from ordinary rubber

Polyurethane seal compounds are produced in hardnesses from 70 to 95 Shore A. The upper half of that range is territory ordinary elastomers barely reach — a standard nitrile O-ring is 70 Shore A and even its hard variants stop at 90. On top of the hardness comes high strength and toughness: polyurethane does not wear away under friction as quickly as soft rubber, so in the same dynamic application it lasts several times longer. Among elastomers it simply has the best abrasion resistance — which is why it is the first choice for dynamic, high-cycle applications.

Its second strength is extrusion resistance. Under high pressure a soft seal gets squeezed into the clearance gap between the rod and its guide; on the low-pressure side you then find the typical nibbled edge — the first sign that the material cannot take the pressure. Hard polyurethane at 90 to 95 Shore A keeps its shape where a softer elastomer would need a PTFE back-up ring. That is exactly why rod and piston seals in high-pressure hydraulics are so often made of PU.

Where abrasion resistance actually pays off

On a static seal that never moves, abrasion resistance is almost worthless — there chemistry and temperature decide, and polyurethane has nothing special to offer. Its advantage grows with the cycle count. Typical applications: rod seals and wipers on hydraulic cylinders, high-cycle pneumatics (cylinders switching thousands of times per shift), mining and construction equipment, and any abrasive environment where slurry, sand or dust moves in the medium or around the seal. Every stroke means friction — and the gap between a material that wears and a material that resists multiplies with every cycle.

What polyurethane tolerates and what it does not

PU media resistance suits hydraulics well: mineral oils, greases and cold water are no problem. Price-wise it comes to roughly twice the cost of NBR — with a service life several times longer in dynamic duty, that is a good trade.

The list of forbidden media is short but strict: hot water and steam, strong acids, polar solvents and fire-resistant HFD hydraulic fluids. Standard polyurethane compounds also come without food-grade certifications, so for a food or pharmaceutical line pick a different material. For any less common medium, check the pairing in the chemical compatibility chart — it is faster than a warranty claim.

Hydrolysis: polyurethane's most underrated limit

Polyurethane has one Achilles heel people forget about during selection: hydrolysis. Hot water and steam attack the urethane bonds in the polymer — the material gradually softens, swells and loses strength until the seal falls apart. That is why cold water is fine while hot water and steam sit on the incompatible list. Watch out for permanently humid, hot environments too: a seal that never touches liquid water but works in a hot, moist atmosphere degrades just the same. If the application has even occasional contact with hot water, rule polyurethane out at the very start of the selection.

The second hard limit is temperature. Polyurethane runs continuously from -40 °C to +80 °C and tolerates +100 °C short-term. That is a 20 °C lower continuous ceiling than plain NBR — and temperature is the most common reason PU drops out of the running. Above +80 °C continuous, go back to NBR (up to +100 °C); for higher temperatures reach for HNBR (up to +150 °C) or FKM (up to +200 °C).

PU vs NBR: deciding for a dynamic seal

NBR is the default choice in hydraulics — the most widely used elastomer with the best price-to-performance ratio for mineral oils, available in 40 to 90 Shore A with a working range of -30 °C to +100 °C. Polyurethane costs roughly twice as much. So the question is not which one is better, but when the premium pays back. In practice these rules work:

• Static seal or slow, occasional motion → NBR. Polyurethane's abrasion resistance has nothing to earn here.
• High-cycle rod or piston seal → PU. Longer life and fewer replacements pay back the price difference with the first avoided downtime.
• Abrasive medium (slurry, sand) on a tight budget → XNBR, carboxylated nitrile. At roughly 1.5 times the price of NBR it offers better abrasion resistance than standard nitrile, though it does not match polyurethane.
• Continuous temperature above +80 °C → polyurethane is out. NBR serves up to +100 °C, HNBR up to +150 °C at roughly three times the price of NBR.
• Fire-resistant fluids: polyurethane cannot handle the HFD type. For HFA, HFB and HFC fluids the safe choice is HNBR.

Practical notes for ordering

Amber colour is a strong hint you are holding polyurethane, but do not rely on it — PU is also made in green, red, blue and black. Reliable identification is dimensions plus material: measure the inner diameter and cord thickness, and for inch sizes from overseas machines the size converter helps. Then enter the dimensions into the configurator and pick the material by medium and temperature. For hardness a simple rule applies: 70 Shore A for ordinary pressures, 90 and above where extrusion threatens.

Is polyurethane suitable for hot water or steam?

No. Hot water and steam break polyurethane down by hydrolysis — the seal softens, swells and falls apart. Cold water is fine. For hot water and steam the right choice is EPDM.

When is it worth paying extra for PU instead of NBR?

When the seal actually works for a living: high-cycle dynamic duty, abrasive media, or pressures where a softer material risks extrusion. Twice the price of a seal is negligible against the cost of downtime and replacement. For static seals, stay with NBR.

What temperature can a polyurethane seal withstand?

Continuously from -40 °C to +80 °C, short-term up to +100 °C. Above that ceiling reach for NBR up to +100 °C, HNBR up to +150 °C or FKM up to +200 °C.

Most polyurethane decisions you can make on your own: check dimensions and availability in the configurator, verify the material and medium pairing in the chemical compatibility chart, and if an old seal has failed, the failure analysis guide will tell you why from the nibbled edge and other traces. If you are still unsure after that — say with an exotic fluid or a combination of high pressure and borderline temperature — write to Filip.