Diffusion Characteristic
Moisture transport in hydrophilic polymer matrices diverges from standard concentration-driven predictions when the polymer structure relaxes slowly in response to water penetration. Material scientists refer to non-fickian relaxation when the rate of paper swelling is controlled by the structural restructuring of the cellulose fibres rather than by simple concentration gradients. This behaviour occurs because the glass transition of the polymer is triggered by the incoming moisture.
It alters the mechanical properties of the sheet.
Physical Process
Water absorption creates localized swelling which forces the dense crystalline regions of the cellulose to reorganize. As non-fickian relaxation occurs, the material develops a distinct boundary between the swollen gel state and the dry glassy state. This boundary moves through the board at a constant velocity.
It causes internal stresses that can trigger delamination of coatings.
Barrier Engineering
Packaging barrier design must account for this complex moisture absorption behavior to ensure that food cartons maintain their rigidity in chilled environments. High-performance coatings are applied to block water vapor before it can trigger the structural relaxation process in the underlying board, which prevents the box from becoming soft and collapsing during storage. These coatings must be flexible enough to withstand the initial expansion of the paper fibers without cracking, which keeps the protective barrier intact even under fluctuating moisture conditions.
