Unpublished

Dry paper is composed of ribbon-shaped paper fibers randomly oriented in the plane of the sheet. Paper fiber cross-sections transform from elliptical in the pulp slurry to ribbon-like on drying due to capillary pressure of water contained in the lumen of the paper fiber. On contacting a dry paper with water or simply exposing the paper to high humidity, paper fibers tend to regain their initial elliptical cross-sectional shape due to breakage of hydrogen bonds and relaxation of drying stresses. Fiber expansion is the resultant of swelling of the fiber wall and opening of the collapsed lumen. A single-fiber model is presented to predict the fiber cross-sectional shapes at various stages during drying and/or moisturization. Swelling of paper fibers affects the physical and transport properties of paper. Predicted cross-sectional shapes of paper fibers are mapped onto an idealized unit cell model of paper structure to predict the effective diffusion coefficient and permeability of paper as function of fiber swelling. The model predicts that the effective diffusion coefficient drops as relative humidity decreases and that there is a sudden drop in effective diffusion coefficient as the fibers collapse from elliptical to ribbon-like cross-sections. Single Fiber Deformation Model On contacting dry paper with water or simply exposing paper to high humidity, fibers within the paper expand (Forseth 1998). Fiber expansion is the resultant of the swelling of the fiber wall and opening of the collapsed lumen (Figure 1). Contact with water breaks hydrogen bonds and releases drying and calendaring stresses (Forseth and Helle 1997). Figure 1: Schematic of paper fiber cross-sectional shape at various stages of drying and/or moisturization. The cross-sectional shape of paper fiber changes from (a) elliptical in the pulp slurry to (b) bi-lobed when partially dried to (c) ribbon-like shape in completely dry paper