Evaporative Rate
Physical laws determine how quickly a porous material releases its stored water content into the surrounding environment. The study of moisture desorption kinetics involves measuring the speed at which paperboard dries out when exposed to lower humidity. This is not a linear process, it starts rapidly at the surface and slows down as the water must move from the center of the board to the outside.
The rate is influenced by the temperature, the air flow and the density of the fiber network.
Equilibrium Shift
Substrates strive to reach a state of balance with the air around them. As the moisture desorption kinetics progress, the board loses weight and its physical dimensions shrink. This movement can cause significant problems in multi pass printing where the board might change size between the first and second colors.
Understanding these rates allows converters to predict how long a stack of board can be left in the pressroom before it starts to warp.
Structural Stability
Different coatings and treatments can slow the release of moisture to help maintain a stable product. A heavily coated board will show slower kinetics than a raw, uncoated kraft paper. This delay is useful for maintaining the flatness of the sheet during the converting process.
If the water leaves too quickly, the sudden contraction of fibers can lead to surface cracking.
