Anomalous Permeation
Moisture movement in polymeric and cellulosic materials often deviates from standard concentration-dependent gradients due to the structural relaxation of the substrate. This non-fickian transport occurs when the polymer chains or cellulose fibers rearrange at a rate comparable to or slower than the penetration of the solvent molecules. The process defines the limit where classical diffusion models fail to predict the absorption behavior of coated boards exposed to liquid water.
Swelling Interaction
In classical diffusion, the penetrant moves through static pores, but in hydrophilic networks, the water molecules interact with and alter the solid matrix. As the moisture enters, the glass transition temperature of the cellulose or barrier polymer drops, triggering a phase change that relaxes the molecular structure. This relaxation opens new sites for moisture binding, causing a rapid, step-like increase in the local absorption rate.
Consequently, non-fickian transport displays a linear mass uptake over time rather than the square-root-of-time dependency typical of Fickian behavior.
Packaging Impact
Predicting the barrier properties of food packages during liquid filling or high-humidity exposure requires sophisticated transport models. If non-fickian transport is present, the protective barrier may degrade much faster than expected, leading to premature softening of the package walls. Paperboard producers minimize these molecular shifts by selecting highly cross-linked polymer coatings that resist swelling.
