Moisture Relationship
Physical properties of cellulose fibers exhibit different moisture content levels depending on whether they are absorbing or releasing water. The phenomenon of hysteresis equilibrium describes this divergence in moisture levels when paper reaches a stable state from different initial conditions. This behavior arises because the internal fiber structure changes during drying.
Structural Mechanics
During initial drying, hydrogen bonds form between cellulose chains as water is driven out of the fiber walls. When the dry fiber is exposed to high humidity, some of these bonding sites remain closed and cannot bind new water molecules. This chemical behavior means a paper sample dried to a specific relative humidity will hold less water than one that is moistened to that same level.
Under hysteresis equilibrium, the moisture content is always higher during the desorption phase than during adsorption. This difference can reach up to two percent moisture by weight under identical environmental conditions.
Converting Impact
Packaging converters must account for this discrepancy to prevent warping and dimensional distortion in corrugated board. When sheets are printed in a pressroom with fluctuating humidity, the fibers expand or contract along their cross direction. This dimensional movement occurs unevenly if some stacks arrived from a damp warehouse while others were stored in dry conditions.
By conditioning all paperboards to a uniform humidity history, manufacturers can minimize registration errors during multicolor print runs. This thermal and humidity conditioning stabilizes the fiber matrix before it undergoes high-speed folding or gluing.
