Volume 17, Issue 5 (September 1989)
Effect of Relative Humidity on Permeance of Coatings
The water vapor permeance is defined as a ratio of the water vapor transmission of a body between two specified parallel surfaces, to the water vapor pressure difference between the two surfaces. Since water vapor pressure differential across a film is the principal driving force in water vapor diffusion through the film, the transmission rate through any particular film should have a linear relationship to the difference in relative humidity across the film for any fixed temperature condition. This linear relationship can only be true if the coating has no interaction with the water molecules passing through the film, and the film remains unchanged by relative humidity conditions.
Test data are presented showing that for various coating compositions there is an interaction with moisture passing through the film, which is dependent upon each film's particular water sensitivity and absorption. This interaction is shown to result in a non-ideal behavior, with the permeance not being independent of relative humidity conditions. Rather the permeance was found to increase with an increasing relative humidity differential across the film.