Water Vapor Permeability of Hydrophilic Films

The modified procedure for water vapor permeability (WVP) is a modification to the established ASTM E96 method for measuring the WVP of films. The E96 method works by putting water in cups and measuring the mass transfer rate of water vapor through films that are secured as lids to the cups. The WVP is calculated from a formula including this mass transfer rate as well as estimated partial water vapor pressure under the film lid at the testing constant temperature. Using the E96 method, the partial water vapor pressure under the film lid is assumed to be the same as the saturated water vapor pressure at the surface of the water. This assumption is only true for hydrophobic films, which is why the partial water vapor pressure under the film lid must be calculated in this modified procedure when measuring the WVP of hydrophilic films. Here, we provide a detailed account of the foundation for this correction and the procedure to reliably use it to measure the permeability of water vapor through hydrophilic films.