Meaning
Diffusion process where the mass flux is proportional to the concentration gradient across a polymer boundary. Fickian transport describes how molecules move through a solid plastic matrix from areas of high concentration to areas of low concentration. The model remains valid as long as the polymer does not undergo significant structural changes or chemical reactions with the diffusing substance.
Diffusion Mechanism
Movement of the penetrant relies on the presence of free volume between the polymer chains. Fickian transport assumes that the rate of diffusion is much slower than the rate of polymer relaxation. This means the plastic matrix reaches equilibrium instantly, allowing the molecules to hop between gaps in the molecular structure.
Concentration Gradient
Driving force for the migration is the difference in chemical potential between the plastic and the surrounding medium. Fickian transport predicts that the amount of material leaving the plastic will increase with the square root of time. This relationship allows engineers to estimate how much additive will migrate into food over a specific shelf life.
Barrier Performance
Efficiency of a packaging layer depends on the diffusion coefficient which measures how easily molecules pass through the material. Fickian transport is used to compare the permeability of different resins like polyethylene and polyester. A lower coefficient results in a better barrier, reducing the loss of flavors or the entry of oxygen.
The model is the standard mathematical tool for evaluating food contact safety in rigid plastics.
