Polyurethanes have been around for decades as a seal material for high pressure reciprocating mobile hydraulic applications. Original equipment manufacturers select it as their “go to” seal material for mobile equipment due to fluid compatibility, cost effectiveness, long service life, and reliable sealing. These capabilities are attributable to the molecular chemistry of polyurethane that produces desirable performance characteristics such as:
Resistance to compression set
Resiliency in retaining sealing ability
Seal manufacturers are closely dialed in to the need for equipment manufacturers to extend the overall useful life of mobile equipment. In addition, there are global expectations and owner/operator use trends pushing the envelope and driving the need for polyurethane materials capable of sealing higher temperatures and higher pressures, including:
Shared ownership and equipment leasing trends in response to high costs of ownership that keep agricultural, construction, forestry and material handling equipment in operation for extended periods of time.
Expectations of longer operating times before required maintenance or overhaul.
Expectations of increased durability.
Expectations of the overall lengthened useful life of mobile equipment.
Given the trends mentioned above, how will prolonged exposure at elevated temperatures – still within range – affect seal performance and ultimately, seal life? Seals are assigned a temperature rating by manufacturers, but how much is too much when it comes to heat exposure?
What do temperature ratings mean?
Manufacturers like Parker quantify thermal capabilities of their engineered sealing materials by assigning them “Temperature Ratings” – from X to Y (min to max), but what does that really mean for equipment designers?
Temperature ratings for sealing materials are generally based upon the typical physical characteristics of the material alone. A material's suitability for a specific application, however, is dependent on actual use conditions which take into account wide-ranging variables which include but are not limited to: hardware attributes and configuration, seal profile geometry, fluid compatibility, and expected duration and frequency of service exposure at pressure, temperature, and speed (i.e. ambient, continuous operating, intermittent, excursion).
Assuming one has taken into account actual use conditions, specified a profile geometry, and selected a compatible material, let's next consider whether one can reasonably expect that seal performance will be constant at all temperatures within the material's broad stated range.
Supported by exhaustive mechanical test lab data representing millions of cycles and decades of experience designing sealing systems, our application engineers know that best sealing performance with polyurethane can be expected when an application’s continuous operating temperature falls well within the maximum and minimum temperature limits for a compound. This is illustrated in Figures 1 and 2.1
As a practical example, the four scenarios in Figure 1 represent expected seal performance of Parker’s high-performance Resilon® 4300 polyurethane material along its stated thermal range of -65°F to 275°F as configured in the application types and profiles shown (i.e., rod/piston, rod wiper, static O-ring/head seal, bumper/damper). Figure 2 is the key showing expected performance at each color-coded interval.
A closer look at what this means
The purple range represents conditions where performance is compromised due to compound stiffness. In this extremely low-temperature range, the polyurethane material is hardening and is approaching or surpassed its glass transition temperature while remaining above its brittle point temperature.
The blue range represents values where performance is compromised due to stiffness and compound rigidity. Polyurethane lip seals may require a low-temperature energizer to offset compromised resiliency.
The green range represents the recommended continuous operating temperature range for best performance.
The yellow range represents extended or continuous exposure under system pressure in temperatures roughly spanning 225 to 240°F. In this scenario, continuous dynamic cycling and increased frictional heat buildup compromises three critical performance characteristics of polyurethane: tensile – most closely associated with wear resistance; modulus – most closely associated with extrusion resistance; and compression set – most closely associated with resiliency (sometimes referred to as sealing force or the ability of sealing lips to “bounce-back” after being compressed).
In thermal conditions represented in the red area, the reference material Resilon 4300, is capable only for short duration before tensile, modulus, and compression set are irreversibly compromised.
In summary, temperature ratings of polyurethane seal materials are primarily based on laboratory and service tests and should be used as a guide only. They do not take into account all of the variables that may be encountered in actual use. Seal designers will have more certainty and better judgment of the fit of the seal material to the application when all variables are considered and then matched to expected performance.
1It is always advisable to test material under actual service conditions before specifying.
This article was contributed by Shannon Johnson, marketing communications manager, Engineered Polymer Systems Division.