Permeation can be defined as the passage of gases and liquids through a second material such as a solid. It is a significant consideration in the selection of plastic material for the construction of chemical pro- cessing equipment because process fluids may travel across the thickness of the polymer by permeation. Permeated species in sufficient quantities could cause.

Permeation concerns the movement of a species through the molecules of another species, e.g., a gas through a polymer. It does not take into account the transport of material through cracks, voids, and, in general, physical flaws in the structure of the second species such as the polymer.

To be sure, both phenomenon result in the migration of the structure. This means that after an appropriate plastic material has been selected to meet the permeation requirements of a process, the equipment must be fabricated carefully to avoid flaws in the polymer structure. Several factors affect the permeation rate of the polymer.

Temperature increase raises the permeation rate for two reasons. First, the solubility of the permeant increases in the polymer at higher temperatures. Second, polymer chain movements are more abundant which allows easier diffusion of the permeant. The permeation rate of gases increases at higher partial pressures.

For liquids, permeation rates rise with an increase in the concentration of the permeant. Un- the permeant species are highly soluble in the polymer, the permeation rate increases linearly with pressure, concentration, and the area of permeation. The permeation rate decreases at higher thicknesses. The effect of thickness is usually nonlinear.

The permeation rate is very high at a low thickness and rapidly decreases with an increase in the thickness. After a critical thickness is reached, the effect of thickness is diminished and the permeation rate reaches a plateau.