The moisture content of the paper has a great influence on printing. If the moisture content is too low, the paper will be brittle and easy to produce static electricity in printing. If the moisture content is too high, the ink will be difficult to dry. With the change of moisture content, the quantitative, tensile strength, flexibility, and folding resistance of the paper will change, and the paper size will also shrink, even curl, edge warping, wrinkling, ruffle, and other phenomena will occur. All kinds of paper have an optimum moisture content. Under this value, all kinds of paper properties are in a good state. The optimum moisture content of cast coated paper and cast coated whiteboard is 7% ± 2%, that of white cardboard is 4% - 7%, and that of the single coated whiteboard is 8% ± 2%. This is the moisture content of the paper when it leaves the factory.
During the storage period in the printing plant, as a kind of hygroscopic material, paper can absorb moisture from the air, and the absorption rate depends on the relative humidity and temperature of the air; it can also transfer moisture to the air and lose moisture, and the rate depends on the moisture content of the paper and the ambient temperature and humidity. When the moisture absorption rate and the water loss rate are the same, the paper and the air are in balance, and the moisture content in the paper remains unchanged. In this case, the moisture content of the paper is called equilibrium moisture.
The so-called relative humidity refers to the ratio of the moisture content in the air at a certain temperature to the moisture content that the air can hold when it reaches saturation at that temperature. The same kind of paper has different equilibrium moisture under different relative humidity.
Balance moisture
In addition, the change of equilibrium moisture has the following characteristics
① Affected by the nature of the paper. At the same relative humidity, the more hydrophilic the paper is, the higher the equilibrium moisture is; on the contrary, the lower the equilibrium moisture is. The balance moisture of the paper without auxiliary material is higher than that of the paper with filler, sizing, and coating. For the same variety, the balanced moisture of thick products is higher than that of thin products, because the ratio of absorbent base paper in thick products is higher.
② Affected by temperature. Under the same relative humidity, when the temperature changes about 15 ℃, the maximum change of paper equilibrium moisture are about 0.5%. However, the change of moisture content of the paper should be controlled within ± 0.1%, otherwise, the accuracy of overprint will be affected. Therefore, the change of temperature should be controlled within ± 3 ℃ while the relative humidity is controlled in the color printing workshop.
③ It is affected by the stroke to reach equilibrium - moisture absorption or dehumidification. The moisture content of the paper in equilibrium by low moisture absorption under certain relative humidity is lower than that by high moisture dehumidification under the same relative humidity, which is the hysteresis effect of paper moisture absorption. In order to make the moisture content of paper consistent with the original, we must take the way of "overcorrection". That is to say, if the paper reaches equilibrium at a certain relative humidity after moisture absorption in a high humidity environment, it must be placed in an environment lower than the original relative humidity, and vice versa. In addition, the speed of moisture absorption and dehumidification is also different, and the dehumidification speed is much slower. But no matter moisture absorption or dehumidification, the starting speed is relatively fast, and the closer to equilibrium, the slower. This process is also related to paper and air mobility. Under the standard humidity and temperature, the balanced moisture content of cigarette paper can reach 5.8% after 35 min of single cigarette paper treatment; the balanced moisture content of various printing papers can reach 5% - 8% after 2-4 h of treatment; the packaging paperboard takes a longer time. This lag phenomenon between the change of relative humidity and the change of paper moisture content will lead to the lag effect between the paper deformation and the change of relative humidity.
④ Affected by the direction of the paper. The so-called directionality refers to the paper balance moisture in the longitudinal and transverse is inconsistent, the transverse expansion rate is far greater than the longitudinal expansion rate, therefore, the transverse expansion deformation of the paper is greater than the longitudinal. The test shows that the expansion and contraction of a single fiber in the transverse direction is about 20 times that in the longitudinal direction. But the arrangement direction of the fiber inside the paper is various, but usually along with the longitudinal arrangement of the number of more just. Therefore, the paper vertical and horizontal expansion ratio of the gap is not as large as a single fiber vertical and horizontal expansion. Taking the relative humidity from 50% to 60% as an example, the ratio of vertical and horizontal expansion is about 3:7, which is about 2.3 times. This ratio varies according to the degree of orientation of fiber arrangement. The stronger the directivity is, the greater the ratio is. It is one of the key problems to reduce the lateral stretch and avoid the overprint to make the paper with scattered and irregular fiber arrangement.
⑤ Affected by the two sides of the paper. A very flat paper, when its environment changes, the paper will be moisture absorption or dehumidification in the air. If it is hygroscopic, the elongation of the reverse side is larger than that of the front side, and the paper will curl to the front side; if it is dehumidified, the shortening of the reverse side is larger than that of the front side, and the paper will curl to the backside. No matter which direction it curls, it always takes the longitudinal direction of the paper as the axis, which is caused by the different degrees of the directional arrangement of the front and back sides of the paper. Paperboard, in particular, performs even better in this respect.