Physical mechanisms of moisture diffusion into hygroscopic fabrics during humidity transients

A mathematical simulation carried out to study the physical mechanisms of m oisture diffusion into hygroscopic fabrics during humidity transients is re ported. On the basis of a mathematical model developed to describe the coup led heat and moisture transfer in wool fabric, the moisture-sorption mechan isms are investigated for fabrics made from fibers with different degrees o f hygroscopicity. Theoretical predictions on the moisture uptake and temper ature changes under humidity transients are compared with those measured pr eviously in a sorption-cell experiment for fabrics made from wool, cotton, acrylic fiber, and polypropylene fiber. It is concluded that the physical m echanism of moisture diffusion into highly hygroscopic fibers such as wool and cotton can be described by a two-stage moisture-diffusion process: a fa st Fickian diffusion,vith a concentration-dependent diffusion coefficient a nd a slow diffusion with a time-dependent diffusion coefficient. For weakly hygroscopic fibers such as polypropylene fiber, the moisture-sorption proc ess can be described by a single Fickian diffusion with a constant diffusio n coefficient. Through theoretical calculations of the distributions of moisture concentra tion in the air of fabric void space, fiber moisture content, and temperatu re through fabric thickness, we show that moisture diffusion into a fabric through air is a fast process for all the fabrics studied. Meanwhile, the m oisture diffusion into fibers is coupled,vith the heat-transfer process, wh ich is much slower and is dependent on the ability of fibers to absorb mois ture. The strength of the coupling effect is a function of a number of fibe r properties, such as the moisture-sorption isotherms, water-diffusion coef ficient, fiber diameter, and heat of sorption.