HANSER Hanser Publishers, Munich • Hanser Gardner Publications ...
HANSER Hanser Publishers, Munich • Hanser Gardner Publications ...
HANSER Hanser Publishers, Munich • Hanser Gardner Publications ...
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3.8 Fick's Law of Diffusion<br />
Analogous to Fourier's law of heat conduction (Equation 3.1) and the equation for shear<br />
stress in shear flow, the diffusion rate in mass transfer is given by Fick's law. This can be<br />
written as [16]<br />
where<br />
mA = mass flux per unit time<br />
A - Area<br />
D^B = diffusion coefficient of the constituent A in constituent B<br />
cA = mass concentration of component A per unit volume<br />
x = distance.<br />
The governing expression for the transient rate of diffusion is [2]<br />
where<br />
t = time<br />
x = distance<br />
(3.72)<br />
(3.73)<br />
The desorption of volatile or gaseous components from a molten polymer in an extruder<br />
can be calculated from [17] using Equation 3.73<br />
where<br />
^1 = rate of desorption (g/s)<br />
Ac = area of desorption (cm 2 )<br />
C0 = initial concentration of the volatile component (g/cm 3 ) in the polymer<br />
D = diffusion coefficient (cm 2 /s)<br />
t - time of exposure (s) of the polymer to the surrounding atmosphere<br />
3.8.1 Permeability<br />
(3.74)<br />
Plastics are permeable by gases, vapors and liquids to a certain extent. The dififiisional<br />
characteristics of polymers can be described in terms of a quantity known as permeability.