Water transport during the drying of cardboard studied by NMR imaging and diffusion techniques

Abstract

An understanding of the distribution of water in wood pulp during drying is of fundamental importance to the paper industry. In this study nuclear magnetic resonance (NMR) imaging is used to measure quantitative water profiles during the drying of liquid packaging board with initial water ratio ( water : dry mass ) of approximately 1.5. The drying was carried out in situ within the magnet, under a range of air temperatures (20–60°C), flow rates (135– 400 l h −1 ) and humidity conditions. One dimensional (1D) slice selective profiles were acquired along the thickness direction of the cardboard with a spatial resolution of 15 μm pixel −1 at regular time points during the drying process. To give further insight into the distribution of water within the cellulose fibre matrix pulsed gradient spin echo (PGSE) diffusion measurements were acquired on samples equilibrated to water ratios between 0.2 and 2.0. The water profiles show that the initial water concentration varies between the three layers in the composite cardboard. At faster drying conditions the initial water loss is predominantly from the two outer layers, while with slower drying conditions the water loss is more homogeneous throughout all layers. The diffusion coefficient of the fibre water was found to decrease significantly with water content and was slower across the cardboard thickness (thus across the fibres) than parallel to the cardboard surface.