Although nitrile butadiene rubber (NBR) has been the dominant wind turbine seal material for decades, advances in polyurethane seal formulation, processing and design are rapidly eroding NBR’s position in the industry. It turns out that the most useful properties include wear resistance, fluid compatibility, ozone resistance, mechanical strength, and the ability to maintain all these properties at low temperatures.
Polyurethane has become an ideal material for sealing main bearings/generators, longitudinal and transverse bearings. However, simply replacing materials in an existing structure is often not enough. Seals must be designed with polyurethane in mind.
One way to evaluate the wear resistance of polyurethanes is through a standardized drum wear test such as ASTM D5963. This method is typically used to evaluate rubber, but it also applies to polyurethane, especially when comparing wear rates. Below are wear index values for various materials tested by System Seals in Cleveland. Note that NBR and HNBR have an ARI of about 1.5, while polyurethanes have an ARI of 4 to 8. This is an improvement of up to six times.
Polyurethane maintains its ARI values over time and after exposure to various fluids, especially oil-based fluids. One way to determine this is to age ASTM D5963 wear samples in fluids at 100°C for 90 days (80°C for water-based fluids) and repeat the test every 30 days. Below are typical results, but confirmation is recommended for each liquid.
Figure 3. Retention of ARI in NBR and hydrolysis-resistant polyurethane after aging in distilled mineral oil at 100°C.
Although specifications indicate compatibility with finished fluids, accelerated aging testing (or years of service) should determine the long-term performance and stability of materials exposed to specific fluids. System Seals tests for fluid compatibility for 90 days rather than the standard 168 hour test because System Seals consistently detects significant changes in key material properties after 168 hours of fluid exposure.
Customized polyurethane exhibits improved fluid resistance compared to NBR in the most common lubricants in the wind energy industry. Below is a compatibility table for these popular lubricants.
NBR is known to be susceptible to ozonolysis, which is when ozone molecules break the chemical bonds in unsaturated NBR. Ozone cracking is common when nitrile butadiene rubber (NBR) is subjected to even the slightest deformation. One solution is to inject wax into the NBR, creating an anti-ozone barrier that protects the NBR. Unfortunately, wax does not change the chemical bond of NBR. If NBR is exposed to environmental conditions that remove wax, it becomes susceptible to degradation again. Some specialty polyurethanes used in wind energy seals are naturally ozone resistant.
The elastic modulus, strength and elongation of polyurethane are two to three times higher than most NBR. As a result, polyurethane seals are able to withstand greater mechanical deformations and withstand higher mechanical loads.
A typical NBR has an elastic modulus of 10-15 MPa and a tensile strength of 20 MPa. Most polyurethanes have an elastic modulus of 45-60 MPa and a tensile strength of 50-60 MPa. This means that the material is less hard than NBR, which means better shape retention and greater resistance to pressure loads.
In wind turbines, high temperatures are usually not a problem. However, depending on location and altitude, minimum temperatures of -40°C are not uncommon. Standard NBR has a minimum operating temperature of -20°C, and dynamic mechanical analysis has shown that many wind energy polyurethanes are unaffected by temperatures down to -40°C.
Polyurethane is a natural choice for wind power seals due to its superior mechanical properties, better ozone resistance, lower wear rates and lower operating temperatures. Below are two families of applications for which polyurethane is well suited. The image on the left shows the simulated deformation and contact characteristics of a polyurethane bearing seal. The image at right shows the System Seals swirl seal, a main bearing seal that continuously pumps lubricant back into the reservoir as the bearing rotates.
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Post time: Dec-24-2023