Difusão de umidade no papel

Study of Anisotropic Moisture Diffusion in Paper Material

The influence of moisture on the final use of paper has been a critical issue for papermakers. In this thesis, we studied transient and steady state moisture diffusion in paper under different humidity conditions and its relationship to sheet structure. Moisture transport through this medium occurs by a number of mechanisms, which the most important are: diffusion through the pores, diffusion of condensed water through the cell-wall of the fibers, surface diffusion and capillary transport. In the first part of this work, we investigated the sorption of moisture by paper sheets exposed to rapid changes in the external humidity. We found that transient moisture transported in paper material is Non-Fickian, most likely being caused by two sequential diffusion steps: the first being a rapid diffusion through the pores followed by a slow diffusion through the fibers/cell wall material. External boundary layers cause further departures from Fickian sorption.
The porous structure of paper significantly impacts its diffusion characteristics. At low to moderate moisture contents, it is the pore space that conducts water vapor by diffusion: transport is therefore proportional to the sheet porosity and is inversely proportional to the tortuosity. Pulp refining reduces the porosity and increases tortuosity, decreasing the moisture diffusivity. Since the pore structure is strongly anisotropic, reflecting the layered structure of paper, diffusion is also anisotropic and is usually greater in the lateral (in-plane or XY) dimensions as compared to the transverse (through plane or ZD) dimension. In machine made paper, there could be a weak dependence on the in-plane fiber orientation giving rise to higher diffusion in the machine direction (MD) as compared to the cross machine direction, (CD). Parameters describing the moisture diffusivity in paper are necessary for calculating transport rates and moisture profiles.