Solidification Process
The phase transition from a dissolved state to a highly ordered solid structure occurs when a solute concentration exceeds its equilibrium solubility in a solvent. The process of supersaturation crystallization is used to control the formation of pigment particles, wax coatings and polymer crystallinity. This phase change is driven by the thermodynamic instability of the highly concentrated solution.
Kinetic Rate
Industrial processes use rapid cooling or evaporation to force a liquid coating beyond its saturation point. In supersaturation crystallization, the rate of nucleating new crystals versus the growth of existing crystals is highly sensitive to the cooling speed. If cooling is slow, a small number of large crystals will form, which can create a brittle and uneven surface texture.
When cooling is rapid, a large number of very fine crystals form, yielding a smooth and highly uniform coating layer on paper and paperboard substrates.
Barrier Quality
Performance limits of protective coatings on food cartons depend on the structural density of the crystallized layer. By managing supersaturation crystallization, manufacturers of wax and polymer barriers can create highly crystalline structures that are highly resistant to moisture and grease. A high level of crystallinity creates a tortuous path for migrating molecules, which significantly improves the shelf life of the packaged food.
If the crystallization process is uncontrolled, amorphous regions will dominate, allowing water vapor to pass through the barrier layer with ease.
