Moisture transport in paper under steady and unsteady conditions investigated with a parallel diffusion concept

A parallel diffusion model was formulated for moisture transport in paper under steadystate conditions. The model considered external mass transport resistances, water-vapor diffusion in the inter-fiber pore space and bound-water diffusion in the fiber phase. The effective water-vapor and bound-water diffusion coefficients (D_p and D_q) of a bleached kraft paperboard (BKP) were estimated from steady-state moisture flux measurements in a diffusion-cup apparatus. The model was further validated by comparing its predictions with measurements of the steady-state moisture transmission rate, average moisture content, relative humidity (RH) and moisture profiles in a BKP stack under a variety of experimental conditions. Another model was developed for moisture transport in paper under unsteady-state (ramp) conditions of the ambient RH. This model used the moisture diffusion coefficients estimated from the steady-state investigation. The model could predict the unsteady-state weight gain of single and multiple BKP sheets in a stack as well as the RH profile within stacks of such sheets.