Fluoroelastomer Specialists
Elastomer O-ring seals are commonly used to provide demountable vacuum seals. The proper choice of material, installation, and handling are key to a successful vacuum design.
The sealing integrity of vacuum systems is, undeniably, a crucial aspect of vacuum technology, and demountable seals are an important segment. A demountable seal can be considered to be any seal that is capable of being opened or closed as opposed to permanent seals such as welded or brazed joints, which includes important components such as flanges and valves.
Any discussion and consideration of these components leads us directly and inexorably to the subject of gaskets and gasket materials, which can be readily broken down into two main groups: metal and elastomer. Metal seals are inherently cleaner and more reliable, but their use has the overall disadvantage of being more expensive and more time and labor intensive than elastomer gaskets. Consequently, elastomer seals are used whenever possible in most applications, and they are usually applied in the form of O-rings.
DEFINING MATERIALS
Elastomers, in general, are organic polymers that exhibit an elastic resilience that allows them to deform slightly when under compression but with enough elasticity to prevent permanently deforming or flowing. In an O-ring seal, the material can form a seal between two smooth surfaces, such as flanges, by flowing slightly into microscopic imperfections in the flange and still retain an internally stored force to maintain the seal when opened and closed time after time. It is helpful to picture a “rubbery” material as opposed to materials such as Teflon that will flow into a different shape upon compression and retain that shape when the compressive force is removed.
O-ring materials are difficult to characterize since they are not clearly defined chemical compounds. To add to the confusion, there are a number of possible variations that are encountered within a single material category.
What this means is that they are “compounded” of a number of component materials such as the basic resin, fillers to promote resilience, plasticizers, and curing agents. Within a particular generic category, the actual compounding and the molding processes will have a number of variations and many of these variations will be considered proprietary. For example, the filler might be carbon black or it might be diatomaceous earth. What this tells us is that any properties of the materials will have wide variations, and that it’s necessary to take this into consideration when using them or designing them into vacuum systems and processes.
The most commonly applied generic groups of O-ring materials are nitrile such as buna-N, butyl, fluoroelastomers such as Viton and Fluorel, and perfluoroelastomers such as Chemraz and Kalrez. Each of these groups has widely different vacuum related properties that make it impossible to make a generalized statement of which is the best material. Instead, it is necessary to try to find the best material for a particular application where the specific properties of the elastomer will make it a better choice.
Read more: USE AND MISUSE OF O-RINGS
