Re: barrier thickness is a factor? |

Posted by Diffusion in Polymer on

In Reply to: barrier
thickness is a factor? posted by Thomas LeBlanc on

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!