Variation of Water Vapor Resistance of Microporous and Hydrophilic Films with Relative Humidity

A method is described for measuring the water vapor resistance of textiles under variable conditions of relative humidity (RH). It consists basically of varying the position of the sample in an air gap between a wet and a dry surface while keeping all other conditions constant. The resistance is determined by the rate of water loss from the apparatus and the temperature of the water. The results for microporous PTFE and polyurethane films show little variation with RH, but fabrics and films with hydrophilic coatings added show strong variations, the resistance increasing substan tially with decreasing RH.