Vapor diffusion thru Sifel sealants  

Thread by Ed on 06 Sep 2009 at 03:48:08 
I have subjected a sealed silicone oil filled pressure sensor to 300 hrs of 95% RH humidity testing. The temperature varies between 15C and 60C.

It appears based on sensor output at high temperatures (>100C) that moisture is entering into the sensor oil cavity.i

Several type of Shin-Etsu Sifel sealants are used within this sensors.
Sifel is a combination of perfluoropolyether backbone with a terminal silicone cross linking group.

This materail has a reported moisture permeability of 5 gm /m**2* 24 hr
How does this permeability compare to silicone adhesives?

I have no experiences with permeability rates.
Would water vapor pass thru long thin films of Sifel?

    Comment by Composite Agency on 07 Sep 2009 at 14:36:44  | |responses: 1|
    Dear Ed,

    Thanks for your interesting posting.

    SHIN-ETSU Sifel® is a rather specific sealant which stays flexible below - 50 degrees Celsius and is chemically resistant to Amine based oil additives.

    As a consequence of this low temperature applicability, the glass transition temperature is low - lower than -50 degrees Celsius - and therefore the free volume in the sealant will be considerable - at room temperature, and of course at higher temperatures. The Tg is -65 degrees Celsius. The available free volume determines the diffusion rate of water in the sealant.

    As such, the diffusion rate of water in Viton® or Viton® ETP-S is lower. And since the matrix is slightly less polar, the water permeation in these elastomers is less than in SHIN-ETSU Sifel® elastomer.

    Nevertheless, whether water vapor diffusion is an issue also depends on the shortest path length between outside environment and the cavity. Secondly, the units of the given permeation figure seems to be related to the path length, since a common SI unit would have been [ gram / m day] following from the Diffusion coefficient [m2/s] times Solubility [gram/m3]. If one divides this unit by the path length [m] one obtains [gram / m2 day].
    Hence, do you have any idea of the path length used, then we can calculate the actual permeation figure and compare this to other materials / get a better indication of the role of water diffusion in the oil filled pressure sensor.

    Kind Regards,
    Composite Agency

      Comment by Ed on 23 Sep 2009 at 12:11:37  | |responses: 0|
      Many thhanks for your assistance.