Development of Polyurethane Sealing

Thomas Schwarz, Managing Director, SKF Economos GmbH, Judenburg, Austria

he history of polyurethane began 75 years ago. Otto Bayer, a German industrial chemist at IG Farben and his team discovered the isocyanate polyaddition reaction. It was the basis for one of the most versatile and most successful polymeric materials. During the 1940's and 50's, elastic, solid polymeric materials like hot cast elastomers, thermoplastic elastomers and polyurethane rubber were developed. By 1970, the global consumption was still significantly below 10.000 t/per annum.

 

test_failure07cFig. 1: Polyurethane seal destroyed by chemical degradation

 

This was followed by a rapid growth in polyurethane elastomers. Nevertheless today, elastomers make up less than 10% of the overall polyurethane consumption. Of this 10 %, only approx. 1% is used for the production of seals, which means we are talking here about a niche market that sometimes requires the materials to operate in extremely difficult working conditions.

Polyurethane elastomers are segmented block copolymers with unique properties, based on the phase separation of physically cross linked hard and soft segments. This provides the necessary elongation characteristics and compared to the chemically cross linked elastomers, also the benefits of thermoplastic processability.

For the synthesis of polyurethane elastomers a multitude of raw materials is available, from the diisocyanate, polyol compound group, with ester and ether bonding, as well as chain extenders with glycol or amine structure. Depending on the composition, a large number of elastomers with varying degrees of hardness and characteristics can be synthesized.

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Fig. 2: Immersion test of hydrolysis resistant polyurethane (H-ECOPUR) compared to a standard polyurethane
sealing grade in hot seawater

 

The history of polyurethane elastomers in sealing technology goes back to the late 1960's and the early 70's and was subject to considerable problems with functionality in the early years. On the one hand this was due to the fact, that the new material was confronted with heavy duty hydraulic applications without being able to draw on previous experience. On the other hand, specialized raw materials with the relevant properties were still missing and synthesis and manufacturing processes were not yet optimized. Of particular concern was the insufficient hydrolysis and chemical resistance of the polyurethane materials of that time, which caused chemical damage and premature seal failure, especially in the warmer earth climates. This often happened already during seal storage. (Fig. 1 shows a damaged polyurethane seal).

Furthermore the insufficient compression set of these early materials caused the loss of the seal preload and loss of the sealing force, resulting in extensive leakage and the reduced life time of the seal. The consequences were that many seal manufacturers and seal end users resorted to using the tried and trusted rubber based sealing systems, even though the advantages of the new material category were obvious: excellent sealing properties, outstanding pressure resistance, superior wear resistance and the processing of the materials using conventional thermoplastic methods.

The successful breakthrough for polyurethanes in the sealing industry was only briefly delayed and during the 1980's polyurethanes became the leading sealing materials, especially for the hydraulics and pneumatic industry. Several different factors made this possible. Through intensive research the knowledge of material characteristics increased and with the development of new basic raw materials and adequate operating processes, it was possible to significantly improve the quality of the products.

Furthermore, several leading sealing manufacturers realized that, in order to optimize sealing properties, they needed tailor-made polyurethane materials. Manufacturers began to invest in research and development departments and in their own lines of synthesis, thus speeding up material development and production. The quality of polyurethane used by seal manufacturers today features only the basic chemistry from the earliest dawn of the polyurethane history. They have been considerably improved, focusing on the essential properties for all dynamic seals like mechanical strength, pressure-, temperature- and wear resistance, resilience, permanent material deformation and especially hydrolytic stability and chemical resistance.

Today, polyurethanes are suitable for applications with hydraulic pressures up to several hundred bar and temperatures above 100°C. When using certain special raw materials, even applications in hot sea water are now possible and manageable.

Fig. 2 shows the immersion of a hydrolysis resistant special polyurethane (H-ECOPUR) in hot sea water compared to a polyurethane sealing grade, normally used in mineral oil based hydraulic applications. It is hard to imagine today's different market segments without polyurethane seals. It has gained a leading position in the mobile and stationary hydraulics and pneumatic industry, in special applications in the food and beverage sector and in oil and gas exploration. Due to efficient processing methods, e.g. the hot runner technology, polyurethane was able to enter even price sensitive applications like in the automotive industry.

The development of the machining- and mould free manufacturing process of polyurethane seals in the late 1980's and early 90's led to a high availability of seals for the service and repair business, as well as for rapid prototyping and customized solutions. This process became well established, particularly for seals market with diameters ranging up to several metres. By combining the appropriate sealing material with an optimized processing technology, applications in hydro power stations, wind power plants, large sized hydraulic presses and tunnel boring equipment are successfully served (please see Fig. 3).

Due to the development of a special welding process, the production of seals above 10 metres in diameter with complex dimensions and high-quality joints is possible. Seals for Kaplan turbines are just one example for these large diameter seals. This technology also allows the fast and economical repair of large industrial plants between routine maintenance and service operations.
The future development of sealing materials and systems will be massively influenced and driven by the following topics: rapid growth of renewable energy generation, increased efforts in raw material extraction and production to minimize environmental impact and maximize conservation of natural resources as well as the ongoing optimization of production processes relating to energy consumption and sustainability.
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Fig. 3: Large diameter polyurethane seal, produced using the SKF SEAL JET machined sealing technology 

 

In that context especially developments for alternative, ecological rod coatings based on metallic and ceramic materials, the increased power density of mobile hydraulics and the extension of renewable energy sources under harshest conditions like wind- and wave power plants, are drivers. Particular attention is paid to the optimization of the tribological properties of sealing materials, e.g. the reduction of seal friction, material abrasion and wear resistance.

Polyurethanes and their primary properties, like the capability to selectively adapt the matrix systems with functional fillers or nano particles, offer tremendous opportunities to change the materials according to application requirements. The thermal limits offer further development opportunities. This group of materials is a ray of hope carrying the expectations that it can successfully master the challenges of the sealing industry in the future.

Foto-Biographie-TSBiography: Thomas Schwarz has studied „Polymer engineering and science "at the University of Leoben and finished his doctorate (academic degree Dr.) in 1993. Thomas Schwarz is Managing Director of the SKF Economos GmbH in Judenburg, Austria, and responsible for Product Development and Research for Fluid System Seals within the SKF Sealing Solutions.