Grease Migration
Chemical transport through a fibrous substrate occurs when lipids from a food product move into the pore structure of the packaging material. The mechanism of fatty acid diffusion involves the gradual movement of oils that can cause staining, structural softening, scent transfer, or reduced adhesion. This process depends on the temperature of the contents and the density of the paper web.
Barrier layers such as aqueous coatings or speciality polymers are used to block the path of these molecules.
Permeation Rate
Pore size and the surface energy of the fibres govern the velocity at which oils penetrate the board. In high-fat applications, fatty acid diffusion accelerates when the paper surface is porous or lacks a continuous protective film. Testing involves exposing the material to specific oils at elevated temperatures for set durations to measure the depth of penetration.
A failure in this barrier leads to visible darkening of the outer package and may result in grease transfer to the consumer. The resistance of the paper is often measured using the kit test or more advanced grease resistance methods.
Fibrous Integrity
Fibrous integrity declines as lipids saturate the cellulose network. When fatty acid diffusion reaches the adhesive layers, it can cause delamination or weaken the bond of the side seams. Hydrophobic treatments must remain intact across folds and creases to prevent localised entry points for the oils.
