Vapor/liquid permeation dependence on pressure differential? 1

[nodalDOF]

Hi,

I am new to vacuum technology and recently confronted with a problem of polymer seal design for a vacuum application. I am calculating the permeation rates of various gases through polymers.
After going through some handbooks for permeability coefficient values of gases for polymers I am confused about the units used. Could someone please clarify the theory behind these two units...

The units of Gas permeability, P = (cmm³ mm)/(cm² day atm) or (gm mm)/(cm² day atm)


where as, the units of Vapor permeability, P = (gm mm)/(cm² day)


When it comes to vapor, why the pressure unit in the denominator disappeared?

Doesn't the vapor/liquid permeation depend on pressure differential between the surfaces (in and out) of polymer membrane?


Thanks,
Nodal

 

[btrueblood]

Water vapor permeability is tested at a standard temperature and RH level (there's an ASTM standard for the test somewhere). Fixing temperature and RH fixes the partial pressure of the vapor on the wet side of the membrane. You would probably want to adjust the value for different temperatures and humidity levels, by the ratio of the new partial pressure to the test partial pressure.

 

[btrueblood]

pretty sure the standard you want is this one:

http://www.astm.org/Standards/F1249.htm

Do read the scope, where this appears:

"Note 1—Values for water vapor permeance and water vapor permeability must be used with caution. The inverse relationship of WVTR to thickness and the direct relationship of WVTR to the partial pressure differential of water vapor may not always apply."

 

[nodalDOF]

Thanks much bruteblood. The note makes sense. May be that's the reason why some times the permeability coefficient for H₂O is given for certain materials in Polymer Handbook and the Vapor Transmission rate other times.

On the the other hand, I am still not clear when "the inverse relationship of WVTR to thickness and the direct relationship of WVTR to the partial pressure differential of water vapor" applies. I have been thinking these relations always apply for gases that do not interact with the material they are diffusing through..any ideas?

In my calculations i am assuming air (either dry or humid) as an ideal gas and caclulating the leak rates based on the partial pressures of each in the atmosphere. Now when vapor transmission rate of H₂O is given instead of permeability, should I just ignore the pressure differential? I mean, according to the equation for transmission rate,P in my post

H₂O Leak amount (gm)= P x Transmission Area x time/Thickness

assuming P value and my claculations are at STP and same RH.

Thanks,
Nodal

 

[btrueblood]

"On the the other hand, I am still not clear when "the inverse relationship of WVTR to thickness and the direct relationship of WVTR to the partial pressure differential of water vapor" applies. I have been thinking these relations always apply for gases that do not interact with the material they are diffusing through..any ideas?"

Um, I think the caution is two-fold. First, water vapor does interact with some materials (nylon for instance), and might cause slight swelling of the material as it absorbs water, which might then change the permeability. Second thing is that temperature may have more to do with permeability of some membranes to water, and since partial pressure of H2O and temperature are hard to seperate, the permeability may be hard to interpret.

Past those cautions, I'd say permeability rates really depend more on the particular method of manufacture of the parts, i.e. molded PVC will have a different permeability than sheet extruded form, which may differ from a blown film; and one factory's formulation may have a markedly different rate than another's. It really does pay, when vapor permeability is critical, to have an independent test conducted.

I've done seal permeability testing on my own before, but I had the advantage that I was trying to seal a thermoelectric device whose cold face was running below 0 deg. C, so I could simply weigh the sealed unit before and after operating for a fixed time in a humid environment (the cold face would just collect ice over time, and hold the cold side humidity very constant). It did make a very nifty test setup, and wouldn't be very hard to rig...

 

[nodalDOF]

Sounds like an interesting setup. For now, I am just into calculations. Thanks for clarifying.