Recently, people have tried to use phenyl silicone rubber to solve the problem that the O-ring installed on the siphon of liquid nitrogen dewar bottle is not tightly sealed. The O-ring is not in direct contact with liquid nitrogen, but the neck temperature of Dewar bottle is very low. Therefore, when using Dewar bottle, the current O-ring will still become very fragile and lead to poor sealing effect.


Liquid nitrogen dewar


It is well known that the boiling point of liquid nitrogen is very low, which is - 196 ° C (or - 321 ° F）。 At present, the minimum temperature that thermosetting rubber can withstand is about - 104 ℃ ° C. Such as phenyl silicone rubber. Of course, sometimes the most difficult part of choosing a rubber material is to understand the exact temperature at which the rubber needs to be sealed and work.


Because the temperature of liquid nitrogen is too low, and the temperature loss from convection to environment or conduction to equipment, the temperature will drop sharply when liquid nitrogen is gasified. Therefore, only when the temperature of liquid nitrogen is within the tolerance range of the rubber material, the rubber material can seal it.


Sealing performance is directly related to rubber deformation and sealing ability. At low temperature, the modulus of rubber will increase when it reaches the glass transition temperature. As the rubber hardens, the ability to deform and seal may be reduced enough to allow leakage. Table 1 shows the low temperature test for different types of silicone materials（ Note: when testing rubber at low temperature, there will be some deviation according to the application.)


People have tried to seal liquid nitrogen with metal or PTFE. When using metal, large clamping force is needed to prevent leakage. When PTFE seals are used, the material may flow cold, or the material may creep and deform after compression. When the material does not return to the original shape, there are also the results of leakage and seal failure.


When using phenyl silicone rubber, we found that it can provide us with the lowest possible sealing conditions at very low temperature. As long as it is not directly exposed to nitrogen for a long time, the sealing performance will remain good, because phenylsiloxane can maintain elasticity at lower temperature, and slightly increasing the cross section of O-ring can provide higher compression force - which may be more conducive to sealing.