PU

Definition and Distinction: What Does “PU” Mean in Sealing Technology?

PU stands for polyurethane. In sealing technology, “PU” almost always refers to a polyurethane elastomer — that is, a rubber-like plastic that deforms elastically and then returns to its original shape. The term is deliberately broad, because polyurethanes are a material family with strongly differing recipes (compounds). Therefore, “PU” alone is not a complete material specification.

Important is the distinction from TPU. TPU stands for thermoplastic polyurethane. Thermoplastic means: the material can be softened and processed again by heating. Alongside this, there is cast PU (frequently non-thermoplastic, often called “cast elastomer”). These variants are typically cast and cross-link in the process, which influences mechanical properties and processing.

PU, TPU, Cast PU: Practical Classification

In practice, with “PU”, the first question is: which PU type is meant and how is it processed? This matters because typical property profiles follow from it — for example regarding abrasion, recovery behavior, or temperature use.

AU vs. EU: Meaning of the Standard Abbreviations

In specifications, AU and EU often appear. These abbreviations come from the standard nomenclature (e.g., ISO 1629) and designate polyurethane rubbers by their chemical base:

• AU: polyester-based urethane elastomer

• EU: polyether-based urethane elastomer

For sealing technology, this is a first hint when it comes to water and hydrolysis. Hydrolysis means that water can attack chemical bonds in the polymer, which leads to embrittlement, cracks, or loss of strength. Polyether-based grades (EU) are frequently considered more hydrolysis-resistant, while polyester-based grades (AU) often deliver very good mechanical parameters but can be more critical under moist-hot loading. This becomes binding only with data for the specific compound and matching tests.

Property Profile and Advantages for Dynamic Seals

PU is in many cases used where seals work dynamically — that is, seal under motion. Typical examples are piston and rod seals in cylinders as well as wipers, which keep dirt away from the rod. The reason usually lies in the combination of high abrasion resistance and high mechanical load capacity.

A central topic is extrusion resistance. Extrusion here means: under pressure, a softer material is “pushed” into a sealing gap (gap extrusion). PU can frequently limit this effect well, particularly at higher hardness and with suitable geometry. As a result, service life rises in applications with pressure peaks or unfavorable gap sizes.

Compared with NBR (nitrile rubber) and FKM (fluoroelastomer), PU is frequently chosen when wear and gap extrusion dominate. NBR and FKM can have advantages depending on medium and temperature, but PU is often mechanically “tougher” under dynamic loading. Compared with PTFE (polytetrafluoroethylene), PU is usually more elastic and thereby often achieves a good sealing effect under deformation, while PTFE is frequently stronger on chemical resistance and high-temperature topics.

Why PU Is Frequently Chosen for Hydraulic/Pneumatic Cylinders

In hydraulic and pneumatic cylinders, reciprocating motion, load changes, and dirt ingress often occur. Precisely here, PU shows its strengths, because the sealing edge frequently remains stable for a long time under friction and particles. Service life depends noticeably on lubrication, sliding speed, and the surface quality of the mating surface, because friction generates heat and can accelerate aging.

Limits, Risks, and Typical Parameters (Temperature, Hardness, Media Resistance)

PU is not an “all-round” material. Limits depend strongly on the recipe, but there are typical orientation values. Frequently cited application ranges lie at approximately $-30\hspace{0.17em}°C$ to $+100\hspace{0.17em}°C\dots +115\hspace{0.17em}°C$. Special formulations exist for different temperature windows, but without a data sheet every number remains an approximation.

The hardness is usually expressed in Shore A. It influences sealing effect, extrusion resistance, and friction behavior. Harder compounds support better against gap extrusion but can react more sensitively to mating surface defects. Media resistance is also not generally applicable: certain oils work well, while some additives, solvents, or aggressive chemicals can age PU noticeably faster.

Hydrolysis and Water: AU/EU Selection and Hydrolysis-Resistant Variants

As soon as water or water-based hydraulic fluids are involved, hydrolysis becomes the leading question: how long does the material remain stable under moist-hot loading? In many cases, polyether-based PU types (EU) are then preferred or hydrolysis-resistant compounds are explicitly specified. Nevertheless, in the end it is not the abbreviation that decides, but the tested recipe in the specific medium and temperature window.

Design and Validation: What to Consider for Material Selection and Testing

A PU seal works reliably when mechanics, medium, and thermal conditions fit together. Mechanically, pressure, gap size, and the risk of extrusion are decisive in particular. Thermally, friction and speed act: higher sliding speeds raise frictional heat, especially with poor lubrication. As a result, this can age PU more quickly, even when the ambient temperature is moderate.

For material approval, table-based resistance is often not sufficient. Compatibility tests in the real fluid are sensible, because additive packages, water content, and temperature cycles strongly influence aging. This applies in particular when the fluid is not clearly specified or when varying operating states occur.

Brief Checklist for the Specification of a PU Seal

For a reliable inquiry or drawing specification, the following points should be named so that selection does not get stuck on “PU” as an umbrella term:

• Medium including additives and water content (if relevant).

• Temperature profile: continuous operation, peaks, cold start.

• Pressure level and gap size (extrusion risk).

• Type of motion: static, reciprocating, rotating; and speed.

• Lubrication and expected dirt ingress.

• Material specification as specific as possible: AU/EU, TPU/cast PU, hardness (Shore A), and ideally compound/grade designation.

• Validation: media storage, function test rig, or approval test under real conditions.

In the end, PU is a very capable sealing material when the recipe fits the application case. With water content, unknown media, or high friction power, a brief material-specific check or specialist consultation is usually sensible.