Hysteresis Loops

Hydrogen Binding Kinetics

Adsorption and desorption paths diverge because of the molecular accessibility of hydroxyl groups within the cellulose and hemicellulose amorphous regions. In dry paper, neighboring cellulose chains form close mutual hydrogen bonds that satisfy internal energy states. As ambient humidity rises, incoming water molecules must break these inter-chain bonds to create sorption sites, which delays initial water uptake along the adsorption curve.

During subsequent drying from high relative humidity, the swollen fiber matrix leaves water molecules bonded to surface hydroxyl groups. Moisture desorption requires surplus thermal energy to sever these bonds, leaving surplus water molecules trapped inside narrowing capillary pores through capillary condensation. The structural rearrangement leaves higher residual water contents at every intermediate relative humidity step during desorption than during adsorption.

Repeated climatic cycling causes internal stresses to relax, altering the loop geometry over successive sorption cycles.

Converting Stability

Paperboard converting operations encounter register shift, edge curl and flat crush variations caused by this path-dependent moisture hysteresis. A carton stock reaching seven percent moisture via drying behaves differently during die-cutting and creasing than the same board reaching that moisture through humidification. Creasing channels shear cleanly when fiber plasticity corresponds to true moisture content, whereas unexpected brittleness arises when the substrate sits on the lower adsorption arm of the loop.

Flexographic press rooms maintain uniform environmental humidity to prevent sheet stocks from drifting across opposing sorption paths during multi-pass printing. Packaging specifications specify whether target moisture contents refer to adsorption or desorption equilibrium to ensure consistent structural box performance.