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