 Re: barrier thickness is a factor?

Posted by Diffusion in Polymer on March 28, 2004 at 11:21:22:

In Reply to: barrier thickness is a factor? posted by Thomas LeBlanc on March 26, 2004 at 21:06:00:

Dear Thomas LeBlanc,

General:

This diffusion problem follow Fick's first and second law, which means that we have a constant diffusion
coefficient: D.

Specific:

D has the units m^2 / s and is independent of the distance of diffusion.  However, to obtain the RATE [m/s]
of diffusion, you have to divide D by the distance. So the RATE = D / distance [m/s].

So bottom line the RATE decreases in function of distance. This is due to the fact that the molecules move
more slowly in time. (see Brownian Motion in "Theory Section")

Example:

The diffusion coefficient D of water through epoxy resin is 1 E-12 m2/s.
If the thickness of the barrier is 1 mm. the average RATE would be 1 x E-9 m/s.
If the thickness of the barrier is 1 m. the average RATE would be 1 x E-12 m/s.

Mind that the TOTAL AMOUNT of water (GRAM/S) that diffuses through the polymer is -
as well in a not steady as steady state -  dependent on more parameters than only (1) the RATE,
like (2) humidity on both sides, (3) the diffusion surface, (4) the density of water,
(5) the maximum volume fraction of water in epoxy (1.2 vol% at 100% relative humidity).

See LIQUID & GAS Diffusion Section for a complete example of complex diffusion in plastics cases.

If you need more information do not hesitate to use the forum.
Good luck!

kind regards,
Diffusion in Polymers

: Hello,

: I'm new to investigating water vapor diffusion in polymers, and I'm hoping someone can answer for me what I think is a basic question...but unfortunately I haven't been able to find a definitive answer! Here is a detailed scenario of the question:

: Assume I have a cube that is hermetically sealed and contains nitrogen at atmospheric pressure. The exception is a small "window" on one side of the cube. This "window" is a film of UV-cured epoxy adhesive. (This may sound like a strange setup, but it's the best way for me to describe the potential problem I need to solve)

: I'm interested in determining if, at some "steady state" condition, the rate of water vapor molecules which will desorb from the inner surface of the "window" (i.e., can get into the "inner sanctum" of the cube) is at all affected by the THICKNESS of the adhesive film. The surface area of the window would not change, just the thickness of the film. I understand that time lag is affected by the film thickness, but what about at a "steady state" condition?

: Thank you very much in advance for any response!