Moisture permeability, diffusion and sorption in organic film‐forming materials

Abstract Experimental results which have been obtained in the course of comprehensive investigations of the water‐vapour permeability, diffusion and sorption of organic film‐forming materials are summarized. These data are used for elucidating the mechanisms and inter‐relationships of the processes. The analysis is based on thermodynamic principles and the multimolecular theory of adsorption. It is shown that the moisture sorption/relative humidity curves agree with the B.E.T. (Brunauer, Emmett and Teller) adsorption equation up to about 90% R.H. The heat of sorption is found to be the same as the heat of liquefaction of free water. It is concluded that the moisture sorption is a result of hydrogen‐bond formation with hydrophilic radicals in the molecules of the solid plus van der Waals' forces. The number of adsorption sites per g. of solid is calculated. The entropies of sorption for the various materials are estimated to show the degree of order of the sorbed moisture as compared with ice. Diffusion constants are derived from sorption‐ and desorption‐rate curves; an anomaly in some of the sorption‐rate curves is discussed. The activation energies of diffusion indicate that, with one exception, diffusion is a process of repeated vaporization and sorption; the energy required for ‘hole’ formation is relatively low except with polyvinyl chloride (PVC). Fick's law of diffusion is not applicable at medium and high R.H.s; it is probable that the rate of permeation depends directly on the vapour‐pressure difference and not on the concentration gradient, but it appears that quantitative calculation of sorption and desorption rates by the classical diffusion equation is not very sensitive to appreciable deviations from Fick's law. The derivations of the B.E.T. equations for multimolecular adsorption and for the free energy of sorption are given as Appendices.