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)
Seal Science is structured as an engineer support organization, providing application and manufacturing solutions to a diverse industry group. SSI incorporates design engineering, material science, and mechanical and physical property testing in its total engineering methodology. Seal Science utilizes 3-dimensional solid modeling in its design practice, as well as advanced mechanical application analysis and finite element analysis.
Click here to see a list of Seal Science’s most widely used elastomers.
Temperature
Temperature, though seemingly elementary, is often the most misunderstood and exaggerated of all sealing parameters; hence, it is all too often over-specified.
Low Temperature
Low temperature changes in the elastomer properties are generally physical in nature. As the temperature decreases below acceptable limits, the elastomeric properties are lost and the material becomes hard and brittle. Duration of the effects of low temperature exposure is not significant and the original properties are regained upon resumption of moderate temperatures; not before seal failure has most likely occurred.
High Temperature
High Temperatures also affect the properties of elastomers in the opposite way as the low temperatures. As the temperature begins to rise, the elastomers will soften, lowering its extrusion resistance. Tensile strength and modulus also decrease under high temperatures, and elongation is increased. But these initial changes reverse if the exposure to high temperatures is brief. However, exposure to high temperatures for extended periods of time will impact the service life of the gasket by causing the elastomer to fracture or crack along the surfaces experiencing the higher stresses or loads. Changes due to the prolonged exposure are chemical in nature, rather than physical, and are not reversible.
Seal Service Life
The expected life of a seal may involve only a few seconds (in the case of highly specialized seals used in solid propellant rocket casings) to as much as 10 to 20 years and beyond in the case of seals used in deep-space missions. Service life is often a balance between the other environmental conditions that the seal must be exposed to.
Depending on specific applications, it is often necessary to choose a material that will increase service life in one category and decrease life in another. For example, Hypalon® can be compounded with different amounts of chlorine. Higher chlorine levels lead to an improved oil resistance at the expense of heat resistance and low temperature flexibility. Vice-versa, you can achieve higher heat resistance and low temperature flex with lower levels of chlorine by sacrificing the elastomers total oil resistance. These choices must be carefully weighed, and often times tested, to achieve an acceptable seal life.
Pressure/Vacuum
Pressure has a bearing on the choice of material and hardness. Low durometer materials are used for low pressure applications, whereas high pressure may require a combination and specialized design. Outgassing and/or sublimation in a high vacuum system can cause shrinkage (loss of volume), resulting in a possible los of sealing ability. When properly designed and confined, an O-ring, molded shape, or a molded plate seals can provide adequate environmental sealing, as well as EMI shielding if necessary, for vacuum (to 1 x 10-6 Torr) applications.
Fluid Compatibility
Consideration must be given to the basic compatibility between the elastomer seal element and any fluids with which it may come in contact. Rapid deterioration can occur when an elastomer is exposed to a specific elastomer. There are many different elastomers to choose from, and many have been tested against a wide range of chemicals that they would be exposed to in a sealing environment. Note that any proposed material and design should be thoroughly tested by the user under all possible conditions prior to production. For conductive seals, the complex chemistry involved in the combination of the polymer and any metallic fillers in conductive elastomers makes it imperative that tests be conducted to determine suitability for use with any given fluid.
Click here for a compatibility chart with our most widely used elastomers.
Galvanic Compatibility (Conductive Materials)
Compatibility between the gasket and the mating flanges is another area which must be given proper attention when designing a gasket for sealing/shielding. This problem can be minimized by various means, the simplest and most effective of which is proper gasket and flange design. This must be coupled with the judicious selection of a gasket material compatible with the mating surfaces. A large difference in corrosion potential between the mating surface and the conductive elastomer and the presence of a conductive electrolyte, such as salt water or a humid environment, will accelerate galvanic corrosion. Under dry conditions, such as the typical office environment, there will be little danger of galvanic corrosion. However, when the gasket is exposed to high humidity or salt–water environments, galvanic corrosion will occur between dissimilar metals. The likelihood of galvanic corrosion increases as the potential difference between the mating surface and the elastomer increases. In addition, the less permeable elastomers, such as EPDM and Fluorosilicone, limit galvanic corrosion by restricting the access of the electrolyte to the conductive fillers in the gasket.