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The mass of the fluid permeating through the polymer at equilibrium conditions is given
by [7]
where
m - mass of the fluid permeating (g)
p = permeability [g/(m • s • Pa)]
t = time of diffusion (s)
A = area of the film or membrane (m 2 )
Pv Pi = Partial pressures on the side 1 and 2 of the film (Pa)
s = thickness of the film (mm)
(3.75)
In addition to its dependence on temperature, the permeability is influenced by the
difference in partial pressures of the fluid and thickness of the film. Other factors
influencing permeability are the structure of the polymer film, such as crystallinity, and
the type of fluid.
3.8.2 Absorption and Desorption
The process by which the fluid is absorbed or desorbed by a plastic material is timedependent,
governed by its solubility and by the diffusion coefficient [7]. The period
until equilibrium value is reached can be very long. Its magnitude can be estimated by
the half-life of the process given by [7]
where
t05 = half life of the process
s = thickness of the polymer assumed to be penetrated by one side
D = diffusion coefficient
The value of tQ 5 for moisture in polymethyl methacrylate (PMMA) for
and s = 3 mm
(3.76)
is 17.1 days when the sheet is wetted from one side only [7]. However, the equilibrium
absorption takes much longer, as the absorption rate decreases with saturation.