Thermodynamic Gradient
Thermodynamic pressure difference describes the gradient in water vapor concentration between a porous substrate and the surrounding atmosphere. This vapor pressure differential acts as the driving force that causes paperboard sheets to absorb or lose moisture until they reach equilibrium. When the vapor pressure of the moisture trapped within the paperboard fibers exceeds the partial pressure of water vapor in the surrounding air, the sheet will shed moisture to the environment.
This continuous exchange of water molecules occurs until the two pressures balance.
Moisture Transfer
The rate and direction of moisture movement depend on both the temperature of the paperboard and the relative humidity of the room. Warm paperboard has a higher internal vapor pressure than cold paperboard, which accelerates evaporation when exposed to dry air. Conversely, a cold pallet placed in a warm room creates a negative differential that draws water vapor out of the air and onto the cold fibers.
To slow this process and prevent rapid, localized moisture shifts, handlers utilize plastic barrier wraps that isolate the board from the room air until the temperatures have equalized.
Dimensional Shift
Rapid water exchange alters the physical dimensions of the cellulose fibers, which swell as they absorb water and shrink as they dry. This volumetric change causes warping, wavy edges, or curl, which can disrupt the alignment on printing presses. Controlling the ambient humidity on the production floor keeps the pressure differential low and ensures sheet stability.
