Dynamics of Water Vapor Transmission through Fabric Barriers

Abstract

An experimental apparatus has been developed to permit the simultaneous measurement of moisture sorption by a fabric and moisture flux through a fabric during the transient period after the fabric is exposed to a humidity gradient. By suitable manipulation of the experimental conditions, the method demonstrates that moisture flux occurs predominantly through the air spaces of the fabric; the role of the fibers is to act as a moisture source or sink. Two mathematical descriptions of the transient period have been developed, in which the interaction between moisture sorption by the fibers and moisture flux through the air spaces of a fabric is modeled. In the first model, the dominant mass transfer resistance for the sorption process is considered to be diffusion of water molecules through the air to the fiber surface. Diffusion within the fiber is considered to be so rapid that the fiber moisture content is always in sorptive equilibrium with the air at the fiber surface. This simple model correctly predicts the qualitative trends exhibited by experimental data collected on fabrics of widely varying sorptive capacity, but it under-predicts the duration of the transient period for a large proportion of these fabrics. A more realistic and complex model is also presented, in which the dominant mass transfer resistance is molecular diffusion of water molecules within the fiber interior. In this case the fiber moisture content lags behind the changes in the moisture content of the air at the fiber surface. The added complexity of this model provides a more accurate fit to the experimental data, although the correction provided by accounting for intrafiber diffusional resistance is small compared to the effect of sorptive capacity.