Paper moisture content and equilibrium moisture(2)

Update:15 Mar 2019

In addition, the balance of moisture changes have the f […]

In addition, the balance of moisture changes have the following characteristics:
1 is affected by the nature of the paper. At the same relative humidity, the more hydrophilic the paper, the higher the equilibrium moisture; on the contrary, the lower. Paper with no excipients has a higher equilibrium moisture, while fillers, sized, coated papers have a lower equilibrium moisture. The same variety, its thick products have higher moisture balance than thin products, because the ratio of raw paper rich in water absorption in thick products is high.
2 is affected by temperature. Under the same relative humidity, the temperature changes by about 15 °C, and the paper balance moisture change is about 0.5% at the maximum. However, in printing, the overprinting changes the moisture content of the paper, and the control is required to be ±0.1%. Otherwise, the accuracy of the overprinting will be affected. Therefore, the color printing workshop should control the relative humidity while controlling the temperature change to ±3 °C.
3 by the stroke to reach equilibrium - moisture absorption or dehumidification. The moisture content of the paper at a certain relative humidity, which is absorbed by low moisture and reached equilibrium, is lower than the moisture content when it is dehumidified by high moisture at the same relative humidity, which is the hygroscopic hysteresis effect of the paper. In order to make the water content of the paper consistent with the original, it is necessary to adopt a "overcorrect" approach. That is, paper that has reached equilibrium at a relative humidity, if it is dehumidified after being absorbed in a high-humidity environment to reach the original equilibrium moisture, must be placed in an environment lower than the original relative humidity, and vice versa. In addition, the speed of moisture absorption and desorption is also different, and the dehumidification speed is much slower. But whether it is moisture or dehumidification, the starting speed is relatively fast, and the closer it is to the balance, the slower it is. This process is also related to paper and air mobility. Under standard wet and temperature conditions, a single cigarette paper can be processed for 35 minutes to achieve 5.8% equilibrium moisture; for various printing papers, it takes 2-4 hours to reach 5%-8% equilibrium moisture; packaging paperboard takes longer. . The hysteresis between this change in relative humidity and the change in water content of the paper causes a hysteresis between the deformation of the paper and the change in relative humidity.
4 is affected by the directionality of the paper. The so-called directionality means that the paper balance moisture is inconsistent in the longitudinal and transverse directions, and the lateral expansion ratio is much larger than the longitudinal expansion ratio. Therefore, the lateral expansion and contraction deformation of the paper is larger than the longitudinal direction. Tests have shown that the single fiber is stretched in the transverse direction by about 20 times in the longitudinal direction. However, the arrangement of the fibers inside the paper is various, but it is usually a larger number in the longitudinal direction. Therefore, the difference in the ratio of the longitudinal and lateral directions of the paper is not as large as that of the single fiber. Taking the relative humidity from 50% to 60% as an example, the ratio of the longitudinal and transverse expansion is about 3:7, that is, about 2.3 times. This ratio differs depending on the degree of orientation of the fiber arrangement, and the stronger the directivity, the greater the ratio. One of the key problems in reducing the lateral expansion ratio and avoiding misregistration is the paper with irregularly arranged fibers.
5 is affected by the two sides of the paper. A very flat sheet of paper that absorbs or dehumidifies in the air when its environment changes. If it is hygroscopic, the elongation on the reverse side is larger than that on the front side, and the paper will curl on the front side; if it is dehumidification, the reduction on the reverse side is larger than that on the front side, and the paper will curl toward the reverse side. Regardless of which direction is curled, it is always oriented in the longitudinal direction of the paper, which is caused by the difference in the orientation of the front and back sides of the paper. Especially cardboard, the degree of performance is even greater.