3.2.2 Sealing materials
18.104.22.168 Elastomer seals
Elastomer seals are basically gas permeable. The overall process of gas penetration is called gas permeation and is dependent on the material, the type of gas and the ambient conditions - especially the temperature. In addition, elastomers are subject to outgassing. In vacuum-compatible elastomers, the outgassing rate decreases and after a sufficiently long evacuation time, the permeation dominates and with constant ambient conditions, a constant gas rate penetrates the seal. Permeation and outgassing are dependent on the diffusion. A high gas tightness causes slow outgassing, leading to a long period of time until a steady permeation gas flow sets in. The time period can be as long as several hundred hours, which can be greatly accelerated by the baking out method. If the vacuum system has no major sources of gas, for example, due to desorption or leaks, elastomeric seals can significantly determine the final pressure. Example: the material FKM (fluoroelastomer) has a low gas permeability for air. For a seal with a nominal diameter of DN 500 ISO-K, the permeation of atmospheric air with 60 % humidity is approximately 4 · 10-7 Pa · m3/s. In vacuum systems with FKM seals, an operating pressure greater than 1 · 10-8 hPa is therefore rarely achieved.
Elastomer seals can be reused several times, if properly used. They require a contact force of several N/mm2, which is already exceeded by sufficiently large O-ring seal diameters due to the atmospheric air pressure acting on the flange.
Elastomer seals can change their properties due to poor storage conditions or improper handling. Factors that must be avoided are: UV radiation, oxygen, ozone, heat, moisture, solvents or excessively large deformations. To preserve their properties, we recommend the following ambient conditions during storage:
During use, they lose their elasticity partially due to cyclic loads, long service life in the deformed state, high temperature or aging. If they are used for too long they can even become brittle. Therefore, elastomer seals have to be replaced regularly. Since the operating conditions are very diverse, no general statement can be made regarding the durability. If there is no empirical data available for the elastomers used under the respective process conditions, their service life may need to be determined by experiments.
|Brief description||Base elastomer||Temperature range [°C]||Properties1) 2)|
|FKM||Fluoroelastomer||-15 to 200||
|NBR||Nitrile butadiene rubber||-25 to 120||
|CR||Chloroprene rubber||-5 to 120||
|EPDM||Ethylene propylene-diene-monomer rubber||-50 to 130||
|VMQ||Silicone rubber||-55 to 200||
1) This information has only an indicative value and is intended for general information. They should facilitate the selection of elastomer, but do not constitute a guarantee, as they are not readily applicable to the actual operating conditions.
2) The operating pressures shown are based on experience, as they can be achieved in a well-designed vacuum system. In some cases they can be exceeded and also go below.
Table 3.4: Elastomer properties