Fickian Diffusion Kinetics

Meaning

Mathematical laws describe how small molecules move through a solid medium according to concentration gradients. Under standard conditions, fickian diffusion kinetics assume that the flux of a substance into or out of a polymer depends linearly on how much of it is present. This model holds until the plastic swells or undergoes a chemical reaction that changes its internal structure.

Linear Gradient

Predictable movement occurs from areas of high saturation to areas where the molecule is scarce. When fickian diffusion kinetics are used in migration models, the math suggests that the initial transfer happens rapidly and then slows as equilibrium is approached. Designers use this to estimate the longevity of package contents.

Molecular Constraint

Tightly packed chains restrict the movement of larger additives differently than small ones. Although fickian diffusion kinetics simplify complex internal interactions, the model remains the most common way to calculate risk for food contact certification. Success requires knowing the diffusion coefficient for that specific polymer.

Accuracy Limitation

Environmental stressors sometimes force a plastic out of the linear behavior described by the equations. If the resin experiences significant temperature spikes or humidity changes, fickian diffusion kinetics might underpredict the actual speed of chemical leaching. Moulders apply safety factors to account for these risks.

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