Gasket Materials Column – Vol. 1: HPU

Properties, temperatures and areas of use of hydraulic polyurethane

In our guide to choosing the right material for industrial gaskets We presented HPU as the reference material when the main problem to be solved is intense abrasion, extreme dynamics and resistance to high sliding speeds. 

With this materials section, we begin a series of dedicated in-depth analyses. The first is HPU, the material furthest removed from traditional elastomers, yet the one that offers the most significant advantages when used in the right context.

What is HPU: the engineering polymer

The HPU (Hydrolysis-resistant Polyurethane, or polyurethane hydrolytic) is a polyurethane elastomer in the TPU family (Thermoplastic Polyurethane), or thermoplastic polyurethane elastomers. This clarification is important for understanding how the material behaves under load and how it is processed.

Conventional elastomers such as NBR, FKM, and EPDM are thermosetting materials: once vulcanized, they do not change state. HPU, being a thermoplastic, can be melted, molded, and solidifies again upon cooling. This makes it machinable by turning, a feature that allows for customized and precise sealing profiles with tolerances unattainable with traditional elastomers.

Chemically, HPU is obtained from the reaction between a long-chain polyol and a diisocyanate, with the addition of a chain extender. The ratio between rigid (hard) segments and flexible segments determines the final mechanical properties: hardness, elasticity, and abrasion resistance. Polyether-based formulations (those of HPU) hydrolytic) ensure better low-temperature properties and hydrolysis stability, distinguishing them from standard polyurethanes.

Technical properties of the HPU: the strong points

Mechanical resistance and abrasion resistance

This is where HPU is unrivaled. Its abrasion and cut resistance is significantly superior to any conventional elastomer, with wear values ​​5-10 times lower than natural rubber. Added to this are:

•       High tear resistance: It does not tear even in the presence of sharp edges, metal particles in the fluid or irregularities in the mounting locations.
•       High breaking strength and good elongation: absorbs stresses without structurally giving way.
•       Low permanent deformation: maintains its original shape and preload on the sealing surfaces even under prolonged loads.
•       High hardness: around 95 Shore A in high pressure formulations, which reduces the risk of material extrusion.

Pressure resistance

The HPU is designed for the most severe hydraulic systems. 

Here's how it behaves at different pressure levels:

Pressure regime	HPU Behavior	Operational notes
Low pressure (< 160 bar)	Work in complete safety	Perfect seal even at values ​​close to zero
High pressure (160-400 bar)	Ideal field of HPU	Shore A hardness 95 prevents extrusion
Very high pressure (> 400 bar or peaks)	Excellent with support	Add anti-extrusion ring in POM or PTFE

 

Operating temperature range

The standard operating range of HPU is -30 degrees C to +110 degrees C, with special compounds extending the range to -50 degrees C and +130 degrees C. This is superior to standard NBR and suitable for the vast majority of hydraulic and industrial applications.

One aspect that deserves specific attention is its behavior in the presence of water at high temperatures. Here, HPU stands out significantly from standard polyurethanes:

HPU and hydrolysis: what's different compared to standard PUR?

Standard polyurethanes – particularly polyester-based ones – undergo hydrolysis when exposed to water: water penetrates the molecular chains and breaks them, leading to a rapid loss of mechanical properties until the component disintegrates.

HPU counteracts this phenomenon thanks to a special polyether-based formulation with specific anti-hydrolysis additives that prevent water molecules from attacking the polymer bonds. The result:

•       Hot water up to +90 degrees C: stability guaranteed in continuous operation.
•       Seawater and HFA/HFB fluids: excellent long-term performance, even in marine environments.
•       Microbial resistance: HPU resists attack by fungi, bacteria and microorganisms that degrade standard polyurethanes in hot and humid environments.
•       Storage in tropical climates: It does not suffer from aging from storage in very humid environments, where standard components degrade even before installation.

Chemical compatibility

HPU offers good resistance to mineral oils, hydrocarbons, and greases, as well as fire-resistant HFA and HFB hydraulic fluids and biodegradable hydraulic fluids (vegetable oils and synthetic esters). Chemical compatibility is the most important consideration during the selection process.

Here are some of the main cases: 

Fluid / Agent	HPU Compatibility	Note
Mineral hydraulic oils (ISO VG 32-68)	Excellent	Environment of choice for HPU
HFA and HFB fluids (water-based)	Good (up to 60 degrees C)	Check with supplier for T > 60 degrees C
HETG fluids (vegetable oils)	Good	Check long-term compatibility
HEES fluids (synthetic esters)	Good	Beware of high concentrations > 60 degrees C
Aliphatic hydrocarbons	Good	Gasoline, kerosene
Cold water (< 40 degrees C)	Good	/
Hot water (up to +90 degrees C)	Good (hydrolytically stable HPU)	Not valid for standard PUR
Water > 90 degrees C / steam	Not recommended	Beyond the limits of HPU: prefer EPDM or FKM
Strong polar solvents (acetone, MEK)	Incompatible	Accelerated degradation
Ozone / prolonged UV rays	poor	Progressive surface aging

Important: Some HPU formulations are certified for food contact according to EU Regulation 1935/2004, EU 10/2011, 3A Sanitary standards, and FDA directives. This also opens HPU to sectors such as food processing and pharmaceuticals – although silicone and PTFE remain the preferred materials for most static food applications.

 

HPU and Movement Type: Where It Really Works

As we emphasized in the general guide, the type of motorcycle is one of the key factors to consider when choosing the right material. This is particularly important for the HPU.

Reciprocating movement: hydraulic cylinders

This is the context of HPU excellence. Its extremely low dynamic friction and extraordinary resistance to sliding abrasion make it the highest-performing material for rod and piston seals in linear motion applications. HPU withstands linear speeds up to 0,5 m/s while maintaining dimensional integrity and preload over time.

The stresses typical of this context (e.g. high-frequency cycles, potentially particulate-contaminated oil, pressure fluctuations) are those for which HPU offers the greatest competitive advantage over NBR.

Static stress

The HPU also works very well as a static seal, provided that the load limits that would cause long-term compression set are not exceeded. The low permanent deformation of the material ensures that the preload is maintained.

High-speed rotary or oscillating motion

Polyurethane is susceptible to heat buildup from friction in applications involving continuous, high-speed rotation. If the shaft rotates at high speeds, the material risks overheating and degrading. In these cases, PTFE or FKM (Viton) are preferable. HPU, however, remains adequate for low-frequency oscillating motions.

 

Main areas of use of HPU gaskets

Hydraulics and industrial hydraulic systems

Hydraulics is the preferred sector for HPUs. Linear hydraulic cylinders, piston pumps, distributors, actuators, and linear guides find HPU seals the most effective solution to the need for dynamic sealing at high pressures and with long life.  

Construction and earthmoving industry

Excavators, mechanical shovels, cranes, compactors: the actuation cylinders of these machines operate in environments with intense dirt, vibrations, temperature fluctuations, and frequently contaminated oil. HPU is the material that best tolerates these conditions. Furthermore, its resistance to hydrolysis makes it suitable for machinery constantly exposed to rain, mud, and high-pressure washdowns—conditions that rapidly degrade standard polyurethanes.

Food and pharmaceutical industry: certified formulations

In EU 1935/2004 and FDA-certified formulations, HPU is suitable for machinery used in the processing, bottling, transportation, and packaging of food and pharmaceutical products. A specific advantage in this sector is its resistance to CIP (Clean-in-Place) cycles: HPU withstands aggressive cleaning with hot water and chemical detergents without swelling or loss of hardness, a critical limitation of silicone and many standard elastomers in intensive sanitization environments.

Press and stamping industry

Hydraulic presses for metalworking and stamping operate at high pressures and in close cycles. HPU's ability to maintain its mechanical properties after a high number of cycles, without failure due to elastic fatigue, makes it the preferred material for main piston sealing systems.

Metallurgical industry and rolling mills

Rolling mills use hydraulic cylinders in environments with rolling emulsions and microscopic metal flakes. HPU, thanks to its resistance to tearing and abrasion from hard particles, maintains a seal in conditions that would quickly compromise gaskets made of softer materials.

Marine and outdoor applications

Hydrolysis stability and resistance to seawater make HPU suitable for hydraulic systems on agricultural machinery, port equipment, and any system exposed to continuous atmospheric humidity. It is one of the few gasket materials that does not suffer degradation from storage in tropical or very humid warehouses.

 

HPU comparison: when to choose one

As we saw in the general guide to materials, every choice begins with an analysis of the main problem to be solved. HPU excels in a specific area, and other contexts where another material is preferable.

Scenario / Main problem	Correct choice	Motivation
Intense abrasion + dynamism + mineral oils	HPU	Ideal condition: no elastomer competes
High pressure (160-400 bar) with reciprocating motion	HPU	Extrusion resistance + cycle life
Humid environment, outdoor, water up to 90 degrees C	HPU (hydrolytically stable)	Higher hydrolysis resistance than standard PUR
High temperature > 150 degrees C + aggressive chemicals	FK extension	HPU exceeds thermal limit
Steam, water > 90 degrees C	EPDM	HPU not suitable beyond hydrolytic limits
Universal chemical resistance, static application	PTFE	HPU does not withstand strong polar solvents
Mineral oils, low cost, moderate pressures	NBR	HPU does not justify the additional cost
Food / pharmaceutical (static application)	Silicone or PTFE	Preferred for ease of certification
Continuous rotation at high speed	PTFE or FKM	HPU suffers heat buildup from friction

 

How to recognize the deterioration of an HPU gasket

Identifying signs of wear or material incompatibility is the first step to intervening before a failure occurs. For HPUs, the signs are clearly distinguished by their cause:

•       Sticky surface or swelling: Indicates contact with a chemically incompatible fluid such as polar solvents, highly concentrated esters, or off-spec fluids. The material absorbs the fluid, loses cohesion, and permanently deforms.
•       Softening and loss of shape: Hydrolysis occurs when water, exceeding the thermal limits of the HPU, or steam comes into contact with the material. The material chemically degrades and irreversibly loses its mechanical properties.
•       Surface hardening and cracking: Prolonged exposure to excessive temperatures, ozone, or UV rays. The gasket loses elasticity and the seal progressively deteriorates.
•       Edge wear and tear: mechanical abrasion caused by excessive roughness of the contact surfaces, hard particles in the fluid or sliding speeds outside the design range.

 If any of these phenomena occur before the expected end of life, the problem isn't the gasket itself, but the operating conditions. The Universalflex technical team is available to analyze the situation and propose the most suitable solution.

 

The material in its application context

The choice of material is never disconnected from other design parameters. This is particularly true for the HPU: 

•       Gasket geometry: HPU is available in specific profiles for hydraulic applications (U-cups/lip seals, wipers, O-rings) and is often machined to create custom profiles. The geometry influences contact pressure distribution and extrusion resistance.
•       Seat tolerances: Having less compressibility than NBR, the HPU requires more precise seat tolerances to ensure the correct preload. A seat sized for NBR may not be optimal for the HPU.
•       Surface roughness: Excessive roughness also accelerates wear on the HPU, eliminating the durability advantage. Seat surface finish is a critical parameter for all dynamic seals.
•       Anti-extrusion ring: For very high pressure applications (> 400 bar or with water hammer), the HPU requires the combination with a virgin/filled POM or PTFE anti-extrusion ring to protect the seal heel.

At Universalflex we always support customers even in the selection phase, integrating knowledge of materials with design experience in specific industrial applications.

 

FAQ – Frequently Asked Questions about HPU Gaskets

Q: What's the difference between HPU and standard PUR? Can I use any polyurethane for my hydraulic seals?

A: No. HPU is a specific formulation of polyether-based thermoplastic polyurethane (TPU) designed for hydraulic sealing: its hardness, resistance to extrusion pressure, hydrolysis stability, and compatibility with mineral oils are optimized for this purpose. Standard polyurethanes, often polyester-based, degrade rapidly in contact with hot water and cannot withstand the conditions of hydraulic systems. When specifying seals for hydraulic cylinders, it is always necessary to explicitly specify HPU or a grade certified for hydraulic use.

Q: Can I replace an NBR gasket with an HPU one of the same size?

A: In most cases, yes, but with a preliminary check of the seat tolerances. HPU has a lower compressibility than NBR: if the seat was sized with wide tolerances for NBR, it may not guarantee the correct preload for the HPU. Before replacement, it is always advisable to check the seat dimensions with the supplier or a trusted technical partner.

Q: Is the HPU resistant to hot water?

A: The answer depends on the type of HPU. Standard polyester-based polyurethanes degrade rapidly in water above 40-50°C. Hydrolytically stable HPU (polyether-based with anti-hydrolysis additives) guarantees continuous stability up to +90°C in hot water and good performance even in salt water and HFA/HFB fluids. This is the version used by Universalflex for seals in hydraulic and outdoor applications.

Q: Is the HPU suitable for HEES and HETG biodegradable hydraulic fluids?

A: Generally, yes, with appropriate testing. HPU offers good compatibility with vegetable oils (HETG) and synthetic esters (HEES) under standard operating conditions. However, for temperatures above 60°C with HEES fluids, a specific compatibility check with the supplier is required, as some formulations may exhibit swelling over time.

Q: What are the signs that my HPU gasket is made of the wrong material?

A: The most common signs of material incompatibility are swelling when in contact with the fluid, loss of hardness and abnormal softening, surface cracking and hardening, and accelerated edge wear. If any of these phenomena occur before the expected end of life, the problem is likely in the operating conditions, not the seal itself.

Q: Is the HPU food grade certified?

A: Some HPU formulations are certified for food contact pursuant to EU Regulation 1935/2004, EU Regulation 10/2011, 3A Sanitary standards, and FDA directives. Certification depends on the specific compound used, not the material in general: it is always necessary to request documentation of compliance for each food application. For static applications in the food sector, silicone and PTFE remain the reference materials. Certified HPU is particularly useful where aggressive CIP cycles or mechanical resistance superior to that of silicone are required.

Q: How long does an HPU gasket last compared to an NBR one?

A: Under optimal operating conditions, an HPU seal can last 3 to 10 times longer than an equivalent NBR seal. This advantage is further enhanced in environments with contaminated oil or high-frequency cycling, which are the most critical conditions for NBR. In particularly demanding environments such as earthmoving or rolling mills, the difference in life can be even more significant.

 

Do you need technical support in choosing gaskets?

Universalflex supports its customers in the selection and supply of seals for hydraulic and industrial applications. Need to evaluate whether HPU is the right material for your application or identify the most suitable alternative? Our technical team is available.

Contact us for specialized advice.