Diffusion Gradient
Moisture migration through cellulose networks proceeds according to fickian kinetics when concentration changes establish steady state flux across the barrier layer. Sorption and desorption rates depend directly on concentration gradients and temperature within the converting environment. High density polyethylene extrusion coatings applied to liquid packaging board alter the internal diffusion coefficient significantly.
Permeation testing measures mass transfer through the substrate over a fixed duration under controlled climate conditions.
Permeation Velocity
Barrier films retard moisture ingress only until equilibrium saturation is reached inside the paperboard core. Temperature fluctuations accelerate molecular mobility and shorten the time required for vapor to breach the foil laminate. Converting lines monitor nip pressure during lamination to prevent microchannel formation that would bypass standard diffusion pathways.
Laboratory chambers record water vapor transmission rates to verify compliance with shelf life specifications for dry food packaging.
Saturation Threshold
Equilibrium moisture content marks the physical boundary where concentration gradients cease driving net mass transfer. Desiccant pouch sizing formulas rely on the diffusion coefficient to predict ingress rates inside sealed secondary containers. Overvarnishing reduces porosity on folding boxboard and extends the period before interior humidity reaches critical levels.
Final packaging performance depends entirely on maintaining seal integrity around the perimeter of the barrier layer.
