Paper is mainly composed of cellulose, which is a highly hydrophilic substance. Most of the other raw materials of paper are also highly hydrophilic substances, so paper is a material that easily absorbs water, and it easily absorbs moisture from the atmosphere. The amount of absorption depends on the amount of water vapor contained in the atmosphere, that is, the relative humidity, and also depends on the composition of the paper, the beating situation and some other process conditions during papermaking, that is, under different atmospheric conditions Different papers will have different moisture content when stored, and the following table lists the moisture content of two kinds of paper under different relative humidity conditions.
| Relative humidity | 43 | 65 | 82 |
| litmus paper | 7.46 | 9.44 | 12.3 |
| cellophane | 8.76 | 10.5 | 13.5 |
Table 1. Moisture content of two papers at different relative humidity
The moisture in the paper is combined with the paper in three forms: one is chemically combined with the fibers in the paper, which cannot remove this part of the moisture during dehydration and will not decompose in the subsequent process; It is combined in the form of physical and chemical combination, which is manifested as adsorption and osmotic combination; the other is combined in the form of physical combination, which appears as capillary water and infiltration water.
The amount of moisture in the paper will affect all other properties, some properties will be more affected and some properties will be less affected. From the perspective of the printing industry, its impact on paper size is the most troublesome phenomenon, because the change in size will cause the problem of misregistration during color printing. Changes in moisture content will also bring other problems to printing and post-processing, such as excessive moisture will slow down the drying speed of ink, reduce the tensile strength of paper, reduce the surface strength of paper, and change the folding performance of carton board; when moisture is low, the paper will be brittle Hard, easy to generate static electricity, so that double sheets or empty sheets are prone to appear when printing dramas.
