Paper moisture content and equilibrium moisture(1)

The moisture content of the paper has a great influence on the printing. The moisture is too low, the paper is brittle, and static electricity is easily generated during printing. If the content is too high, the ink will be difficult to dry. Moisture changes have a great influence on various characteristics of paper. As the moisture changes, its quantitative, tensile strength, flexibility, and folding resistance will change. The paper size will also expand and contract, and even curl and warp. Edges, wrinkles, ruffles, etc. All kinds of papers have an optimal moisture content value. Under this value, the various properties of the paper are in a good state. The optimum moisture content of cast coated paper and cast white paperboard is 7%±2%, white cardboard. It is 4%-7%, and the single-sided coated white paperboard is 8%±2%. This is the moisture content of the paper when it leaves the factory.

During storage in a printing house, paper acts as a hygroscopic material that absorbs moisture from the air. The rate of absorption depends on the relative humidity and temperature of the air. It also transfers moisture to the air and loses moisture. The rate depends on the moisture content of the paper. And 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 water content in the paper is constant. In this case, the moisture content of the paper is called balance water.

The relative humidity refers to the ratio of the moisture content in the air at a certain temperature to the moisture content that can be accommodated when the air is saturated at that temperature. The same paper balances moisture at different relative humidity levels.