Effect of moisture on paper properties
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1 meaning
Moisture refers to the water content in the paper, that is, the ratio of the weight (mass) reduced in the paper to the weight (mass) at the time of sampling after drying according to the prescribed method, generally expressed as a percentage (%), and the water content of the printing paper. Usually between 4% and 7%.
2 Testing instruments and test methods
Generally, an oven is used for moisture detection. The test method is: weigh the quality before drying, and then dry the sample to a constant weight at a temperature of 103-107 degrees Celsius, weigh the mass again, the paper moisture = (the quality before drying - after drying) Quality) / quality before drying * 100%.
3 The effect of paper moisture on printing
Paper is mainly composed of cellulose, while fiber bundles are highly hydrophilic materials, while other papermaking materials, while other papermaking materials are mostly hydrophilic substances, paper is a material that absorbs easily. The change in the moisture content of the paper causes the following problems.
3.1 Generate ruffle/tight edge problem
When the paper fibers swell and swell, the size of the fiber in the cross-sectional direction changes greatly, while the length direction hardly changes. Therefore, the stretch ratio of the paper thread in the transverse direction during printing is larger than that in the longitudinal direction. Since the edge of the paper is in contact with the air during the storage process, the middle contact is less. When the ambient humidity is lower than the paper humidity, the paper absorbs moisture to produce a "lobe edge" phenomenon; when the ambient humidity is lower than the paper humidity, the paper is dampened. Tight edge phenomenon. If the paper has "flank edge" or "tight edge" phenomenon before printing, the paper will be subjected to different pressure deformation during printing, which will cause the printing and printing to be inaccurate. In severe cases, the paper is prone to printing wrinkles.
3.2 The problem of producing a single copper curl
Changes in water content can also cause problems with paper curling. Since the one side of the coated paper is coated on one side and the other side is a highly absorbent fiber, the coating does not substantially expand and contract, and the degree of fiber expansion is large, so the problem of curling of the single-sided coated paper is more prominent than that of the double-sided coated paper. In a relatively high humidity environment, the single copper paper absorbs moisture and tends to curl toward the coated surface; in a relatively low humidity environment, the single copper paper is easily wetted to the fiber surface. At the same time, in the printing process, if the water content changes (excessive drying or excessive wetting), the water balance of the fiber and the coating is destroyed, and the water will be curled to the uncoated surface, and the moisture will be absorbed. The coated surface is curled. If the paper has curling problems before printing, it will affect the smoothness of printing and printing efficiency; if the paper has curling problems during printing or after printing, it will have a serious impact on the subsequent processing, packaging and other operations.
3.3 Impact on other aspects of printing
The change in the moisture content of the paper not only causes long deformation, but also causes other changes in the paper properties, directly affecting the printability of the paper. If the water content of the paper is too large, the paper's stretch capacity and surface strength will be reduced, the plasticity will be enhanced, the drying speed of the blot will be affected, and the back of the paper will be easily smeared. If the water content of the paper is too small, the paper will be brittle and hard during the printing process. In the middle, a large amount of static electricity is generated due to friction, resulting in poor paper feeding or misfeeding, which wastes paper and affects printing efficiency.
4 Related instructions
Since the paper is greatly affected by the environment, it is recommended that the outer packaging of the paper should not be removed too early.