Moisture Content of Paper

Moisture Content of Paper

The moisture content of paper has a significant impact on printing. If the moisture level is too low, the paper becomes brittle and is prone to static electricity during printing. If the moisture content is too high, it will make ink drying difficult. Changes in moisture content also greatly affect various properties of paper. As moisture fluctuates, its basis weight, tensile strength, flexibility, and folding endurance will all change. The paper may also expand or contract in size, and may even exhibit curling, edge lifting, wrinkling, or waviness.

Each type of paper has an optimal moisture content at which its performance is at its best. For cast-coated paper and cast-coated white paperboard, the optimal moisture content is 7% ± 2%; for white card paper, it is 4%–7%; and for single-sided coated white paperboard, it is 8% ± 2%. These values refer to the moisture content at the time of delivery from the factory.

During storage in printing plants, paper, as a hygroscopic material, can absorb moisture from the air. The absorption rate depends on the relative humidity and temperature of the air. Paper can also release moisture into the air, and the rate of moisture loss depends on the paper's moisture content as well as environmental temperature and humidity. When the rate of moisture absorption equals the rate of moisture loss, the paper and the air reach equilibrium, and the moisture content of the paper remains constant. This condition is referred to as the equilibrium moisture content of the paper.

Relative humidity refers to the ratio of the amount of moisture in the air at a given temperature to the maximum amount of moisture the air can hold at that temperature when saturated. The equilibrium moisture content of the same type of paper varies under different relative humidity conditions.

Equilibrium Moisture Content

In addition, the variation of equilibrium moisture content has the following characteristics:

1. Influence of Paper Properties
At the same relative humidity, paper with higher hydrophilicity will have higher equilibrium moisture content, and vice versa. Paper without additives tends to have higher equilibrium moisture, while paper containing fillers, sizing, or coatings tends to have lower equilibrium moisture. For the same type of paper, thicker products usually have higher equilibrium moisture content than thinner ones, because they contain a higher proportion of base paper that is more absorbent.

2. Influence of Temperature
At the same relative humidity, a temperature change of about 15°C will result in a maximum equilibrium moisture variation of approximately 0.5%. However, in printing, especially for color registration, moisture variation must be controlled within ±0.1%, otherwise it will affect registration accuracy. Therefore, in color printing workshops, while controlling relative humidity, temperature variation should also be maintained within ±3°C.

3. Influence of the Path to Equilibrium (Absorption vs. Desorption)
When paper reaches equilibrium by absorbing moisture from a lower moisture level, its equilibrium moisture content is lower than when it reaches equilibrium by desorbing moisture from a higher moisture level under the same relative humidity. This phenomenon is known as the hysteresis effect of paper moisture absorption.

To restore paper to its original moisture content, a corrective approach must be taken. For example, if paper that has reached equilibrium at a certain relative humidity absorbs moisture in a higher humidity environment, to return to its original equilibrium moisture content, it must be placed in an environment with lower relative humidity than before, and vice versa.

Additionally, the rates of absorption and desorption differ, with desorption being slower. In both cases, the process is faster at the beginning and slows down as it approaches equilibrium. This process is also influenced by paper quality and air circulation. Under standard temperature and humidity conditions, a single sheet of cigarette paper can reach an equilibrium moisture content of 5.8% in about 35 minutes; printing papers generally require 2–4 hours to reach 5%–8%; packaging paperboard takes even longer. This lag between changes in relative humidity and paper moisture content can lead to delayed deformation effects.

4. Influence of Paper Directionality
Directionality refers to the difference in equilibrium moisture behavior between the machine direction (MD) and cross direction (CD) of paper. The expansion and contraction in the cross direction are much greater than in the machine direction, meaning transverse deformation is more significant. Tests show that a single fiber's transverse expansion can be about 20 times its longitudinal expansion.

However, since fibers within paper are arranged in various orientations (though more tend to align in the machine direction), the difference in expansion is less extreme in practice. For example, when relative humidity changes from 50% to 60%, the ratio of longitudinal to transverse expansion is approximately 3:7 (about 2.3 times). This ratio varies depending on fiber orientation-the stronger the orientation, the larger the ratio. Reducing irregular fiber alignment during papermaking is key to minimizing transverse expansion and preventing misregistration in printing.

5. Influence of Paper Two-Sidedness
Even a very flat sheet of paper will respond to environmental changes by absorbing or releasing moisture. During moisture absorption, the back side tends to expand more than the front side, causing the paper to curl toward the front. During moisture loss, the back side shrinks more, causing the paper to curl toward the back.

Regardless of the direction of curling, it typically occurs along the machine direction axis. This is due to differences in fiber orientation between the two sides of the paper. This effect is especially pronounced in paperboard.