Moisture Regain between Mill Conditioning and an Unheated Warehouse
Unheated storage drives moisture ingress along sheet edges via psychrometric gradients, requiring sealed barrier wraps and strict thermal acclimation before press run.
Z-Direction Diffusion
Moisture Penetration
Transport phenomena of this class describe the movement of water vapor or liquid through the thickness of a paperboard sheet from one surface to the other. The z-direction diffusion determines how quickly moisture can penetrate the board and affect the barrier properties of the packaging. It is an important factor in the shelf life of food and beverage cartons that are exposed to high humidity.
Diffusion Transport
Process analysis involves measuring the rate of moisture transfer across a known thickness of paperboard under a controlled humidity gradient. The water vapor molecules travel through the air spaces between the cellulose fibers and along the fibers themselves. This movement is driven by the difference in relative humidity between the inside and outside of the package.
Applying a continuous plastic or wax barrier to the board surface slows down this transport and protects the package contents.
Board Structure
Achieving a low diffusion rate requires a high density board with a tight fiber network and effective internal sizing. This parameter does not apply to metal foils or continuous glass barriers that have zero permeability to water vapor. Moisture diffusion can cause the paperboard to lose its stiffness and stackability in humid warehouses.
Regular barrier testing ensures that the packaging maintains its protective performance throughout its intended shelf life.