Surface Migration
Soluble active agents move outward toward lower concentration zones within paper and board matrices during drying stages. A chemical potential gradient drives surfactant and sizing distribution across caliper boundaries when moisture evaporates from external faces faster than internal capillary replenishment occurs. Wet end additions migrate alongside water molecules toward the warmest evaporating surface, which shifts surface energy and alters ink receptivity profiles.
High drying temperatures steepen the profile and pull hydrophobic sizing molecules away from internal plies, leaving the core under-bonded and the outer plies excessively densified.
Boundary Equilibrium
Concentration differentials eventually stabilize when local activities equalize throughout multi-ply structures during conditioning phases. Equilibrium prevents further mass transfer once the driving force reaches zero, provided ambient relative humidity remains constant. Converters specify maximum allowable residual variance across sheets to avoid curl formation during offset lithography runs.
Lamination adhesives rely on uniform local activities to prevent interfacial delamination caused by localized chemical gradients pulling moisture toward the bond line.
Rate Control
Drying kinetics dictate the velocity of molecular displacement within porous substrates under thermal influence. Evaporation rates dictate the steepness of the profile because rapid removal of moisture concentrates solutes near the boundary before diffusion can flatten the concentration curve. Mill operators regulate dryer cylinder temperatures to manage this velocity and prevent localized spotting on coated fine papers.
Diffusion coefficients inside dense cellulose networks establish the maximum allowable drying speed without inducing surface skinning or picking defects during subsequent printing operations.
