Polyurethane (PU) as a sealing material

What is polyurethane (PU)?

Polyurethane, usually abbreviated to PU, is a polymer made up of organic units linked by urethane bonds. It is one of the most versatile materials in modern industry and is known for its flexibility, durability and resilience.

PU was first developed in the late 1930s. The material was originally created as a substitute for rubber, and today it is used across a wide range of industries.

Polyurethane is manufactured from two main components: polyols and diisocyanates. The polyols are organic compounds with several hydroxyl groups and largely determine the flexibility of the end product. The diisocyanates react with these hydroxyl groups to form long polymer chains and define the strength of the material. The properties can be adjusted from soft to very hard via the mixing ratio.

For sealing technology, it is above all the PU elastomer that is relevant. It is used to make wheels, rollers and seals, including O-rings for demanding mechanical loads.

Properties of polyurethane

The properties of PU are remarkably diverse. The material can be tailored precisely to the respective application via its formulation.

• Flexibility: PU can be adjusted from extremely soft to very hard.
• Durability: High longevity and resistance to abrasion, cracking and UV radiation.
• Insulation properties: Very good thermal and acoustic insulation.
• Chemical resistance: Resistant to oils, solvents and many chemicals.

This combination makes PU a preferred material in many areas, from furniture and mattresses through insulation materials to footwear and automotive components.

Polyurethane is available in several basic forms. Flexible foam is found in furniture and mattresses, rigid foam in insulation boards and construction applications. PU elastomers are used in wheels, rollers and seals, alongside coatings, adhesives and sealants.

High abrasion and wear resistance

The most important strength of PU in sealing applications is its excellent abrasion resistance. Seals made of polyurethane withstand mechanical stress over long service lives and wear down considerably more slowly than many classic elastomers.

Added to this is a high resistance to cracking and tearing. Combined with its good mechanical strength, the result is a material that excels above all where seals must operate reliably over the long term under pressure and movement.

PU retains its shape and elasticity across a wide temperature range, which allows it to be used in a variety of environments. This wear resistance is also one reason why polyurethane is often chosen where abrasive media or particles act on the seal. If you want to look into the topic of wear in more depth, you will find further information in our article on the NBR material as a widely used alternative in the hydraulics field.

Mechanical strength

Compared with many other plastics and elastomers, PU offers outstanding mechanical strength while remaining low in weight.

• Versatility: Can be produced in various hardness grades, from soft foam to rigid plastic.
• Durability: Resistant to abrasion, cracking and various chemicals.
• Insulation properties: Very good thermal and acoustic insulation.
• Light weight: Low weight compared with metals, combined with high strength.

As with other elastomers, the hardness of PU is stated in Shore and covers a very wide range. The following overview assigns the common PU variants to their typical hardness ranges.

Compared with PVC, PU is more flexible and more durable, but more expensive to manufacture. Compared with silicone, PU is less heat-resistant but offers better mechanical properties.

PU in hydraulics

Hydraulics and pneumatics are classic fields of application for seals made of polyurethane. What matters there is the combination of mechanical strength, abrasion resistance and resistance to oils and greases that PU offers to a particular degree.

PU resists many oils, greases and some solvents, making it suitable for chemically demanding situations in the hydraulic circuit. In hydraulic and pneumatic systems, PU elements seal components effectively and prevent leakage, even under high pressure.

As a rod and piston seal in particular, polyurethane makes the most of its wear resistance, because here the seal is exposed to constant movement. You can read more about designing dynamic sealing points in our article on the piston seal.

Limits: heat and hydrolysis

As mechanically capable as PU is, its limits with heat and in combination with water are just as clear. Compared with silicone, polyurethane is less heat-resistant, so the material reaches its limits at permanently high temperatures.

Added to this is its sensitivity to hydrolysis. In hot water, steam or a warm, humid environment, PU can degrade over time, which impairs the sealing effect. For such conditions, other materials are the better choice.

Which material suits your medium cannot be answered in general terms. You will find an overview of how various materials behave towards specific media in our resistance table.

PU O-rings and further applications

PU O-rings are an essential part of sealing technology in many industrial and technical applications. They combine excellent abrasion resistance with good flexibility across a wide temperature range and resistance to many oils, greases and some solvents.

This combination of durability, flexibility and resistance makes PU O-rings a suitable solution for demanding sealing tasks where conventional materials reach their limits.

Typical fields of application for PU O-rings are:

• Hydraulics and pneumatics: effective sealing of components against leakage.
• Automotive industry: used in fuel, oil and cooling systems.
• Mechanical engineering: protection against dust and contaminants in machines and equipment.
• Aerospace: in demand for its resilience against extreme conditions.
• Household appliances: as a sealing element in washing machines and dishwashers.

Beyond sealing technology, polyurethane is ubiquitous in everyday life: in seat cushions and mattresses in the furniture industry, in seat padding and vehicle insulation, in sportswear and water-repellent fabrics in the textile industry, and in building insulation and floor coverings in the construction industry. The material is also used in electronics as a protective coating and in medical technology.

Innovation and sustainability

The manufacture of PU is energy-intensive, and the industry is working towards more environmentally friendly methods. Recycling and reuse of PU materials are central priorities here. PU products can be recycled, even though the process is complex. The methods include mechanical shredding and chemical breakdown into the basic constituents.

New developments such as bio-based polyurethanes offer more sustainable alternatives and are obtained from renewable raw materials, which reduces the environmental impact. Further fields of research are biocompatible materials for medical technology, smart textiles, and lighter and stronger PU forms for aerospace.

Overall, the focus on sustainability and environmentally friendly production is growing, driven by industries such as electromobility and renewable energies.

Frequently asked questions