In order to treat the porosity properties of paper from a common viewpoint, its porous structure is described by means of the pore size distribution . Two methods for its determination are discussed in detail. A general result, supported by statistical considerations, is that the frequency distribution of the pore radii can be approximated by a logarithmic Gauss distribution . Over a wide range of porosity, simple linear relationships between the characteristic parameters of the distributions were found.
Considering the various possible permeation phenomena, the permeation of inert gases (such as air) is the simplest case. It is governed by the Poiseuille law at normal pressures. At low pressures, the Knudsen and the slip effect come into play. Permeation of water vapour is shown to be a surface diffusion process, additionally controlled by the adsorption of water vapour on cellulose . This process, including its temperature dependence, can be reduced to air permeability and water vapour adsorption. The penetration of inert liquids follows the Poiseuille law, but is affected additionally by the interface tension between the liquid and cellulose. The quantitative treatment reveals a relationship between the leak time of such liquids (as grease) and the air permeability, which has been proved by experiments.