Seals Material Selection
There are a wide range of choices when considering materials while designing a seal. The material selection should begin with a careful analysis of the following environmental conditions:
- Temperature
- Seal Service Life
- Pressure/Vacuum
- Fluid and Chemical
- Static or Dynamic Surfaces
- Galvanic Compatibility (Conductive Applications)
The role of temperature in seal design cannot be overemphasized. While it is seemingly basic, it is also one of the most misunderstood and over-specified parameters. Temperature changes can significantly affect the properties of elastomers, and the resulting impact on seals can either be physical or chemical in nature, depending on the duration of exposure.
At low temperatures, the elastomeric properties of seals become hard and brittle, leading to seal failure, especially when the exposure is prolonged. However, the original properties can be regained once moderate temperatures are resumed. In contrast, high temperatures cause elastomers to soften and decrease in tensile strength and modulus, and increased elongation. Short-term exposure to high temperatures will only cause initial changes that reverse quickly. However, long-term exposure will lead to chemical changes that are not reversible, causing the elastomer to fracture or crack along the surfaces under higher stresses or loads.
The service life of a seal is dependent on the specific application and can range from a few seconds to more than 10 or 20 years, depending on the environmental conditions it is exposed to. Achieving a balance between different environmental conditions often requires selecting a material that increases service life in one area while decreasing it in another. For example, Hypalon® can be compounded with different amounts of chlorine to improve oil resistance or heat resistance and low-temperature flexibility.
Pressure and vacuum also play a crucial role in seal design. Low durometer materials are suitable for low-pressure applications, while high-pressure applications require a combination of materials and specialized design. Outgassing and sublimation in high vacuum systems can cause shrinkage, resulting in a loss of sealing ability.
The compatibility of the elastomer seal element with fluids it comes in contact with must be considered. Rapid deterioration can occur when an elastomer is exposed to certain chemicals. Therefore, selecting an appropriate elastomer that is compatible with the sealing environment is crucial. Conductive seals require extra care as the complex chemistry involved in the combination of the polymer and metallic fillers in conductive elastomers makes it imperative to conduct tests to determine their suitability for use with any given fluid.
Finally, the compatibility of the gasket with mating flanges is also an essential consideration when designing a gasket for sealing or shielding. Proper gasket and flange design, coupled with selecting a gasket material compatible with the mating surfaces, can help minimize this problem. A significant difference in corrosion potential between the mating surface and the conductive elastomer, and the presence of a conductive electrolyte like saltwater or humid environments, can accelerate galvanic corrosion. However, the likelihood of galvanic corrosion can be limited by using less permeable elastomers like EPDM and Fluorosilicone.
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