Moisture sorption and transport processes in paper materials

The moisture content of paper materials is an important variable which sign ificantly effects their physical properties. At higher moisture contents, p aper's stiffness, tensile strength and compression strength are all affecte d adversely. Similarly, the electrical properties also are strong functions of sheet moisture content. In this chapter, some rei:ent studies on moisture transport processes in pa per materials are reviewed. The equilibrium aspect of moisture interaction with paper shows significant hysteresis which can be estimated by an applic ation of Everett's theory of independent domain complexions. Thus, when a p aper sheet is subjected to arbitrary cycles of humidity all the while allow ed to reach equilibrium at each state, the sheet's moisture content evoluti on may be predicted by an analysis of the sorption isotherms and the interi or of the sorption hysteresis loop. It is shown that the theoretical predic tions of equilibrium moisture content are in good agreement with experiment ally determined values. Transient moisture sorption under ramp changes in external humidity is anal yzed. A general model describing the dynamics of moisture sorption is deriv ed. The paper sheet is considered as a composite structure of fibers and vo ids through which moisture is transported by diffusion. The mathematical de scription of moisture transport embodies two suitably averaged concentratio n fields, c and q. Two unsteady state diffusion equations describe the time and spatial evolution of these fields. The average moisture content of the sheet and the moisture flux at the surface are evaluated. A set of limiting cases of transport is developed, comprising of situations where diffusion through the void space, fiber space and external boundary layers, each contribute significantly to transport. By means of a scaling a nalysis, the conditions under which each limiting case is valid are identif ied. Finally, a comparison of the model predictions with experimental data indicates that the model is capable of describing transient sorption dynami cs quite well.