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

An understanding of the distribution of water in wood pulp during drying is of fundamental importance to the paper industry. In this study nuclear mag netic resonance (NMR) imaging is used to measure quantitative water profile s during the drying of liquid packaging board with initial water ratio (wat er: dry mass) of approximately 1.5. The drying was carried out in situ with in the magnet, under a range of air temperatures (20-60 degreesC), flow rat es (135-400 1 h(-1)) and humidity conditions. One dimensional (1D) slice se lective profiles were acquired along the thickness direction of the cardboa rd with a spatial resolution of 15 mum pixel(-1) at regular time points dur ing the drying process. To give further insight into the distribution of wa ter within the cellulose fibre matrix pulsed gradient spin echo (PGSE) diff usion measurements were acquired on samples equilibrated to water ratios be tween 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 t he initial water loss is predominantly from the two outer layers, while wit h slower drying conditions the water loss is more homogeneous throughout al l layers. The diffusion coefficient of the fibre water was found to decreas e significantly with water content and was slower across the cardboard thic kness (thus across the fibres) than parallel to the cardboard surface. (C) 2001 Elsevier Science Ltd. All rights reserved.