If you have any needs pls contact me-
Whatsapp number of Ivy: +852 57463641 (My Wechat +86 18933510459)
Email me: 01@songhongpaper.com
The moisture content of paper and its equilibrium moisture characteristics play a critical role in printing processes. Insufficient moisture content can render paper brittle and prone to static electricity, while excessive moisture hinders ink drying. Variations in moisture content also significantly affect key paper properties, including basis weight, tensile strength, flexibility, folding endurance, and dimensional stability. Dimensional changes may lead to defects such as curling, edge lifting, wrinkling, and ruffled edges.
Each paper type has an optimal moisture content range that ensures balanced physical performance. For example, cast-coated paper and cast-coated white board exhibit optimal performance at 7%±2%, white card paper at 4%–7%, and single-sided coated white board at 8%±2%. These values typically represent the moisture content at the time of manufacturing.
During storage in printing facilities, paper-being hygroscopic-exchanges moisture with the surrounding air. The rate of moisture absorption depends on ambient temperature and relative humidity, while moisture loss is governed by the paper's current moisture level and environmental conditions. When the rates of absorption and desorption equalize, the paper reaches equilibrium with the environment, and its moisture content stabilizes. This stabilized moisture level is referred to as equilibrium moisture.
Relative humidity (RH) is defined as the ratio of the actual water vapor content in the air at a given temperature to the maximum water vapor capacity at saturation under the same temperature. The equilibrium moisture content of a given paper type varies with changes in RH.
Equilibrium Moisture Characteristics:
1. Influence of Paper Composition
Under identical RH conditions, more hydrophilic papers exhibit higher equilibrium moisture content. Uncoated paper without additives retains more moisture, whereas fillers, sizing agents, and coatings reduce hygroscopicity. For the same paper grade, thicker products generally have higher equilibrium moisture due to a greater proportion of absorbent base material.
2. Influence of Temperature
At constant RH, a temperature fluctuation of approximately 15°C results in a maximum change of about 0.5% in equilibrium moisture content. However, for precise color registration in multicolor printing, moisture variation must be controlled within ±0.1%. Therefore, in color printing environments, both RH and temperature must be tightly regulated, with temperature fluctuations maintained within ±3°C.
3. Hysteresis Effect and Path Dependency
The equilibrium moisture content achieved through moisture absorption from a dry state is lower than that reached via desorption from a wet state under the same RH-a phenomenon known as moisture sorption hysteresis. To restore original moisture levels after exposure to high humidity, a process of "overcorrection" is required: paper must be conditioned in an environment with lower RH than the target, and vice versa.
Additionally, desorption occurs more slowly than absorption. Both processes are rapid initially but decelerate as equilibrium approaches. The kinetics depend on paper structure and air circulation. Under standard conditions, cigarette paper may reach equilibrium (5.8%) within 35 minutes, while printing papers typically require 2–4 hours to stabilize between 5% and 8%. Packaging board requires even longer. This time lag between ambient RH changes and paper moisture response contributes to delayed dimensional deformation, affecting print registration accuracy.
4. Anisotropy (Directional Effects)
Paper exhibits directional differences in moisture expansion due to fiber alignment. Transverse expansion is significantly greater than longitudinal expansion. Single cellulose fibers expand transversely up to 20 times more than longitudinally. Although fibers in paper are randomly oriented, a preferential alignment along the machine direction reduces the overall anisotropy. For instance, when RH increases from 50% to 60%, the transverse-to-longitudinal expansion ratio is approximately 3:7 (about 2.3:1). This ratio increases with stronger fiber orientation. Minimizing transverse expansion and improving print registration can be achieved by optimizing fiber dispersion during manufacturing.
5. Dual-Sided Asymmetry
Flat paper exposed to humidity changes may curl due to differential moisture movement between surfaces. During moisture absorption, the reverse side expands more than the coated or printed side, causing curl toward the front. Conversely, during drying, greater shrinkage on the reverse side induces curl toward the back. Curling predominantly follows the longitudinal direction, resulting from asymmetric fiber orientation and structural differences between the two sides. This effect is particularly pronounced in cardboard and other multi-ply materials.
