Non-Fickian behaviour of moisture diffusion in paper

Moisture diffuses through paper by many pathways. The transient response of paper sheets to step changes in external humidity can be used to analyze the relative contribution of these pathways to overall sorption. We report experimental moisture uptake by thick handsheets of bleached kraft softwood pulp subjected to rapid humidity changes. Predictions were made of the moisture uptake based on Fickian and non-Fickian models for moisture transport by earlier workers. We found that moisture transport exhibited significant non-Fickian behaviour principally due to: (a) resistance due to external surface boundary layers, (b) the rate of humidity changes and (c) local relaxation of the moisture content of the fibres. Moisture transport in paper remains non-Fickian even when (a) and (b) are eliminated. Hence, generalized moisture diffusion models should incorporate the ability to describe non-Fickian behaviour.