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The moisture content and equilibrium moisture of paper significantly impact printing. If the moisture content is too low, the paper becomes brittle, and static electricity may occur during printing. If the moisture content is too high, it will hinder ink drying. Variations in moisture content also affect various properties of paper, including basis weight, tensile strength, flexibility, and folding endurance. The size of the paper may expand or contract, and phenomena such as curling, curling edges, wrinkling, and ruffles may occur. Each type of paper has an optimal moisture content value at which its properties are in their best state. For cast-coated paper and cast-coated white board, the optimal moisture content is 7%±2%; for white card paper, it is 4%-7%; and for single-sided coated white board, it is 8%±2%. This represents the moisture content of the paper upon leaving the factory.
During storage in the printing factory, paper, as a hygroscopic material, can absorb moisture from the air. The absorption rate depends on the relative humidity and temperature of the air. Conversely, paper can lose moisture to the air, with the rate depending on the paper's moisture content and the environmental temperature and humidity. When the rates of moisture absorption and loss are equal, the paper and air reach equilibrium, and the moisture content in the paper remains constant. Under these conditions, the moisture contained in the paper is referred to as equilibrium moisture.
Relative humidity refers to the ratio of the moisture content in the air at a given temperature to the moisture content when the air reaches saturation at that temperature. The equilibrium moisture content of the same type of paper varies under different relative humidity levels.
In addition, changes in equilibrium moisture exhibit the following characteristics:
① Affected by the nature of the paper. At the same relative humidity, the more hydrophilic the paper is, the higher its equilibrium moisture content will be. Conversely, it will be lower. Paper without auxiliary materials added has a higher equilibrium moisture content, while paper with fillers, sizing, and coatings has a lower equilibrium moisture content. For the same variety, the equilibrium moisture content of thicker products is higher than that of thinner ones due to the higher proportion of absorbent base paper in thicker products.
② Affected by temperature. Under the same relative humidity, when the temperature changes by approximately 15°C, the maximum change in the equilibrium moisture content of the paper is about 0.5%. However, during the printing process, the change in paper moisture content due to overprinting should be controlled within ±0.1%; otherwise, it will affect the accuracy of overprinting. Therefore, in the color printing workshop, while controlling the relative humidity, the temperature change should also be controlled within ±3°C.
③ Affected by the path to equilibrium-whether it is moisture absorption or dehumidification. The moisture content of paper when it reaches equilibrium by absorbing moisture from a low-moisture environment under a certain relative humidity is lower than that when it reaches equilibrium by dehumidifying from a high-moisture environment under the same relative humidity. This is the moisture absorption lag effect of paper. To make the moisture content of the paper consistent with the original, the approach of "overcorrection" must be adopted. That is, for paper that has reached equilibrium at a certain relative humidity, if it needs to be dehumidified to reach the original equilibrium moisture content after absorbing moisture in a high-humidity environment, it must be placed in an environment with a lower relative humidity than the original, and vice versa. Additionally, the speed of moisture absorption and dehumidification differs; dehumidification is much slower. Whether it is moisture absorption or dehumidification, the initial speed is relatively fast, and the closer it gets to equilibrium, the slower it becomes. This process is also related to the paper quality and air fluidity. Under standard humidity and temperature conditions, a single sheet of cigarette paper can achieve an equilibrium moisture content of 5.8% after being treated for 35 minutes. It takes 2 to 4 hours for various printing papers to reach an equilibrium moisture content of 5% to 8%. Packaging cardboard requires even longer time. The lag phenomenon existing between the change in relative humidity and the change in paper moisture content will bring about a lag effect between paper deformation and the change in relative humidity.
④ Affected by the directionality of the paper. Directionality refers to the inconsistency of equilibrium moisture content in both the longitudinal and transverse directions of the paper. The transverse expansion rate is much greater than the longitudinal expansion rate. Therefore, the transverse expansion deformation of paper is greater than that in the longitudinal direction. Tests show that the stretching and contraction of a single fiber in the transverse direction is approximately 20 times that in the longitudinal direction. However, the arrangement directions of fibers inside the paper are diverse, with more fibers usually arranged along the longitudinal direction. Thus, the difference in the longitudinal and transverse expansion ratio of paper is not as large as that of a single fiber. Taking the change of relative humidity from 50% to 60% as an example, the ratio of vertical to horizontal expansion is approximately 3:7, which is about 2.3 times. This ratio varies depending on the degree of orientation of the fiber arrangement. The stronger the directionality, the greater the ratio. Copying paper with scattered and irregular fiber arrangement directions is one of the key issues in reducing the lateral expansion rate and avoiding misregistration.
⑤ Affected by the dual nature of paper. A very flat piece of paper will absorb or dehumidify in the air when its environment changes. If it is hygroscopic, the elongation on the reverse side is greater than that on the front side, causing the paper to curl towards the front side. If it is dehumidification, the shrinkage on the reverse side is greater than that on the front side, causing the paper to curl towards the reverse side. Regardless of the direction of curling, it always takes the longitudinal direction of the paper as the axis. This situation is caused by the different degrees of directional arrangement on the front and back sides of the paper. Especially for cardboard, this characteristic is even more pronounced.
