diffusion of ethanol / propellant through EPDM  


Posting by russell walker on May 06, 2008 at 17:54:54.

Hi, I have an aerosol application where EPDM (Ethylene Propylene Diene Monomer) rubber is being used as a seal. The contents consist of a propellant (HFA-227) and Ethanol. We have conducted some experiments and the container appears to be losing ethanol at a much faster rate than the propellant. Elevated temperature accelerates the loss. The two contents are not coming out at proportional rates. The ethanol is coming out at ~10x rate.

The molecular weight of Ethanol is smaller than the propellant (46 vs. 170) but its boiling point is higher and its partial pressure and concentration (almost 100x more propellant) in the container are lower. Does this sound like Ethanol is diffusing through the EPDM rubber or does another mechanism explain what I am seeing?

Thank you,
Russell

          follow up posts
    On 05/07/2008 kate ryan posts: Hi All:

    Let me try to do a suggestion.

    As you describe, the activity of ethanol (defined as partial pressure of ethanol divided by vapour pressure of ethanol) in this system is smaller than the propellant. For solubility of ethanol in Ethylene Propylene Diene Monomer (EPDM), not the actual concentration is of interest, this may be a 100 times smaller, but the activity of ethanol in this system. As stated the activity depends on the partial pressure of ethanol relative to the vapour pressure. When the mixture is far non ideal, the partial pressure may be equal to the vapour pressure even at very low concentrations, as such giving an activity of 1 with regard to the EPDM (maximum solubility). This would give a high permeability for ethanol, even at low concentrations, relative to the propellant when the solubility of ethanol in Ethylene propylene diene monomer (EPDM) is higher than the propellant. So we should assess whether the mixture is ideal or non ideal.

    Looking forward to your comments!

    Keep up the good work,
    Kate
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