Gas Permeability
Gas Permeability
the property of solids that allows the passage of a gas through the body in the presence of a pressure differential. A distinction is made, depending on the structure of the solid and the magnitude of the pressure differential, among three main types of gas permeability: diffusion flow, molecular effusion, and laminar flow.
Diffusion flow defines gas permeability in the absence of pores in the solid (for example, the gas permeability of polymeric films or coatings). In this case gas permeability includes the dissolution of the gas in the boundary layer of the body, its diffusion through the body, and its emergence on the other side of the body.
Gas permeability through a system of pores whose diameter is small in comparison with the mean free path length X of the gas molecules at a pressure of 10-3 to 10-4 mm of mercury (1 mm of mercury = 133.322 newtons per sq m, or N/m2) molecular effusion.
The laminar flow of a gas through a solid body takes place in the presence of pores whose diameter greatly exceeds λ. As the diameter of the pores further increases, in the transition to coarsely porous bodies (such as fabrics), the gas permeability is determined by the laws governing the escape of the gas from the apertures.
The gas permeability of substances is defined in terms of the penetrability effect P, expressed in units of m4/(sec·N), or cm2/(sec·atm), where 1 cm2/(sec·atm) = 1.02 x 10-9 m4/(sec·N), and by the volume of gas that passes through a unit area (perpendicular to the gas flow) in the body in 1 sec with a pressure differential of 1. The coefficient P depends on the nature of the gas, and therefore the gas permeability of substances is usually compared on the basis of their hydrogen permeability. The values of P, in cm2/(sec·atm), of several materials at 20° C are presented in Table 1.
| Table 1. Values of the penetrability effect (P) (20° C) | |
|---|---|
| Metals ............... | 10-18 to 10-12 |
| Glass ............... | 10-15 to 10-10 |
| Polymers (films) ............... | 10-12 to 10-5 |
| Liquids ............... | 10-7 to 10-5 |
| Paper, skin ............... | 10-5 to 10 |
The polymers widely used in all sectors of production occupy an intermediate position with respect to gas permeability between inorganic solids and liquids. The values of P for polymers, in units of 108 cm2/(sec·atm), are given in Table 2.
| Table 2. Values of the permeability effect (P) for polymers [in 108 cm2/(sec atm)] | |
|---|---|
| Silicone rubber ............... | 390 |
| Natural rubber ............... | 30 |
| Polystyrene ............... | 6.9 |
| Low-density polyethylene ............... | 5.9 |
| Nylon ............... | 0.7 |
| Polyethyleneterephthalate (lavsan) ............... | 0.5 |
Amorphous polymers with very flexible molecular chains in a highly elastic state (such as rubber) have the highest gas permeability. Crystalline polymers—for example, polyethylene—have much lower gas permeability. Macro-molecular glasslike polymers with rigid chains have very low gas permeability. This is due to the fact that more flexible chains are easily displaced, thus passing the molecules of the diffused gas.