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

Abstract

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 substantially with decreasing RH. 1 Current addres: Mustang Ind. Inc., Richmond, BC, Canada. . The water vapor resistance of textiles can be measured by a variety of techniques, but these rarely give the same results. This may be in part because of inaccuracies inherent in any such method, but it is probably largely due to the fact that the different methods actually measure different things.The concept of water vapor resistance is derived from an expression such as Equation 1:Here M is the rate of diffusion of the mass of water vapor per unit area across the sample ( kg J m 2s ), C, and C2 are the concentrations of water vapor in the air on either side of the sample ( kg/ m 3 ), and R is the resistance of the sample ( s / m ) . To obtain a physical feel for the quantity, it is often convenient to express the water vapor resistance as the thickness of a layer of still air that would present the same resistance to the diffusion of vapor as does the sample ( d~ ). This equivalent thickness is related to the actual resistance through the diffusion constant D for water vapor into air, D is about 2.5 X 10-5 m 2 / s at room temperature.Typi~lly, uncbated textiles have a resistance of the order of 100 s/m, equivalent to a few millimeters of still air.The value of R in Equation 1 is the resistance of everything between the two planes where C, and C2 are determined. Often, in application as well as in measurement, this includes air layers on one or both sides of the textile layer, as well as the textile itself. Since these air layers are usually several millimeters thick, their influence can be significant, especially in apparatus originally developed to measure low permeability materials. Different sorts of apparatus with air layers of different thicknesses can give different apparent resistances unless those air layers are carefully controlled and their resistance subtracted from the total.Another possible source of discrepancy is that the textile fabric can experience different conditions of temperature and humidity in the various sorts of apparatus, and thus the actual resistance of the sample may genuinely be different in the different measurements. This is particularly likely for textiles coated with vapor-permeable hydrophilic films. The water vapor resistance of films from cellophane [ 2 ] , keratin [ 3 ] , and nylon [ 6 is known to increase with decreasing RH, whereas the diffusion rate in polyethylene [6] ] decreases with decreasing RH. Lotens [...

Effect of Temperature on Water Vapor Transport Through Polymer Membrane Laminates

“…The analogy is to the water vapor sorption isotherm, which is the equilibrium water content of a hygroscopic polymer as a function of relative humidity. The mean relative humidity has proven to be useful in showing the mass transport behavior in terms of concentration dependence of the permeability coefficient [4,7,8]. Figure 4 shows the measured water vapor diffusion resistance of the three materials using the setpoints of Table 1.…”

“…The diffusion resistance given in Figure 4 is the sum of the intrinsic material resistance (/?.) plus the boundary layer resistance (R b ), as defined in equation (4). In this report we present the combined resistances as a single value, but procedures exist for separately determining the intrinsic material mass transfer and boundary layer resistances [7,8].…”

“…The resistances given in Figure 6 include the boundary layer resistance due to the air flow over the surface of the sample, as defined by Equation (4). The boundary layer resistance at a given temperature may be found from the microporous PTFE sample, which has been found to have an intrinsic diffusion resistance of approximately 7 s/m at 20°C [7,8].…”

“…Test methods which evaluate concentration-dependent permeability need to be capable of independently varying the relative humidity of the environment on the two sides of the material. Concentration-dependent permeability has been studied and reported on extensively [1][2][3][4][5]. Reported work on measured temperature-dependent water vapor transport properties of clothing membrane laminates is less common [6].…”

Moisture accumulation in sleeping bags at − 7°C and − 20°C in relation to cover material and method of use

“…The behaviour of these materials has been studied under various circumstances, as e.g. in different ambient humidities (Farnworth et al 1990), with condensation at its surface (Osczevski 1993), at various atmospheric pressures (Fukazawa et al 1999), and in various ambient temperatures (Finn et al 2000). Studies on the behaviour of such materials at low temperature, however, have shown that the vapour resistance of some of these materials increases dramatically when temperatures fall below zero degrees Celsius (Osczevski 1993(Osczevski , 1996.…”