The uptake of moisture by paper sheets was analyzed as an adsorption process occurring in a porous medium. Water vapor was assumed to diffuse into the pore space and was subsequently adsorbed onto the surfaces of the fibers constituting the paper sheet. The response of the sheets to variations in relative humidity was investigated.
Since the transient moisture profiles inside paper sheets depend upon the moisture sorption equilibria for paper, the equilibrium behavior was investigated with special emphasis on a description of sorption hysteresis. It is necessary to follow equilibrium trajectories inside the hysteresis loop for paper sheets. Sorption equilibria inside the hysteresis loop for paper sheets (bleached kraft linerboard, 290 gsm) were evaluated experimentally. Complete sets of desorption and adsorption scanning trajectories were obtained. Further higher order loops were obtained experimentally. The theory of independent domain complexions was applied to the hysteresis loop. By constructing a so called moisture distribution function for the hysteresis, arbitrary trajectories representing equilibrium sorption behavior under cyclic humidity changes could be predicted.
An investigation of transient sorption was also undertaken. A model for moisture uptake based upon diffusion inside the pore space and the fibers in the sheet was set up. Experimental data on transient moisture uptake was obtained under ramped changes in humidity. It was found that the model for moisture uptake incorporating a linearized isotherm could describe the sorption response of paper sheets to ramped changes in humidity adequately. From the experimental data, a value for the intro-fiber mass transfer coefficient representing moisture diffusion through fibers was determined.