Water-based ink paper gravure printing process (3)

2.2 Paper sizing processing

The most common problems with water-based ink gravure printing on paper products are paper creases caused by the high water content of water-based inks and poor printability due to changes in ink and paper composition, resulting in smooth paper gloss, for example. Degree of compressibility Roughness Print Flatness Changes in print density, etc. For example, in the gravure printing process of water-based ink, the surface roughness of the paper rises due to moisture swelling of the paper fiber. This phenomenon is caused by the infiltration of water in the ink into the paper and the diffusion of water molecules, and the acceleration of the two processes leads to stress relaxation and fiber expansion of the paper product, which is related to the size of the paper's hygroscopicity. of. The hygroscopicity of paper is related to the method of pulping and papermaking. It is known from experiments that paper produced by chemical pulping has less hygroscopicity than paper made by mechanical pulping. At the same time, if the wood pulp and chemical wood pulp fibers are treated to reduce the degree of fiber swelling, the hygroscopicity thereof can also be reduced. The use of paper sizing to increase the hydrophobicity of the paper is another effective way to reduce the moisture absorption of the paper.

The internal sizing is to add the sizing agent with lower surface free energy to the pulp and mix it. After the sizing agent is precipitated, it is evenly distributed and adheres to the surface of the fiber to form a non-polar surface and exposes it to the atmosphere. The surface of the fiber is reduced to improve the performance of the liquid. This is effective by reducing fiber adsorption to reduce capillary penetration and in-plane diffusion rates to make it more hydrophobic. However, the degree of sizing should be appropriate. Excessively high sizing can cause excessive hydrophobicity, which can cause ink transfer problems.

Several appearance criteria, such as gloss roughness flatness printing density, are commonly used to quantitatively measure printability changes resulting from internal sizing levels.

Uncoated paper refers to a paper made by a paper machine under controlled conditions with different degrees of paper sizing. It has the characteristics of super calendered paper. In the gravure printing with water-based inks, all papers were free of groundwood except for the addition of ground wood pulp to make specific paper. Practice has shown that papers containing no wood pulp have only slight changes in their physical properties as the degree of sizing increases (hydrophobicity increases). Both the smoothness and the compressibility were not changed. Although the surface smoothness increased as the degree of sizing increased, the printing density continued to increase until the colloid content reached a considerable level. After this critical point, it begins to decline as the gum continues to rise. For example, the paper made from chemical kraft pulp has a slight less than 10% drop in surface flatness as the degree of sizing increases. Although the physical properties of the paper only show minor changes, the printability of the paper is quite different. The surface smoothness of paper without wood pulp increases as the degree of sizing increases, but when the gum content reaches a certain level, the smoothness of the paper surface decreases. In addition, with the increase of the degree of sizing, the printing flatness (the deinking area of ​​the field printing) has also greatly increased. When the hydrophobicity of the paper continues to rise beyond a certain critical point, the wettability of the ink and the paper deteriorates due to the high print flatness, making it impossible for the continuous absorption of moisture on the paper surface and the regular diffusion of the ink. More cavities (two times the number of voids of wood-free paper is twice as much as paper containing wood pulp). It is difficult for liquid ink to flow in and form a solid ink layer in a small void area. The "rhizard pattern" appears (so that once the "rhizard pattern" appears, the spread of the ink on the paper surface has been in a poor state at this time), so that the printing density is significantly reduced. Obviously, when the paper becomes extremely hydrophobic, that is, when the degree of sizing in the paper exceeds a certain suitable range, the distribution of the water-based ink on the surface of the paper will be very uneven, resulting in extremely poor image quality.

Through careful observation of the actual area of ​​the print, the following conclusions can be drawn qualitatively: When the degree of sizing in the paper is high, not only is there a defect in the screening area, but also the deinking area is also deformed, indicating that the surface of the paper is glial Coverage is uneven. The cause of this defect comes from the poor wettability of the paper and the uneven spreading of the water-based ink on the highly hydrophobic paper surface. This problem can be adjusted by changing the contact angle of the ink on the paper. Wood pulp containing paper has the following properties: significant fiber expansion, low print density.

2.3 ink film pH change processing

The acid-base reaction when the water-based ink is in contact with the surface of the paper should be treated as follows:

1) Select paper and water based inks with similar pH;

2) Select a water-based ink that contains a polymer with a higher acid number because its buffer capacity and resistance to pH changes are lower than the acid value;

3) Select inks that contain pH-sensitive polymers because they penetrate the paper much more slowly than pH-insensitive polymers.

4) In water-based ink gravure, the acidity and alkalinity have a certain influence on the coloring of the pigment or the ink, so the acid-base and alkali-resistant ink is generally selected.

5) In order to increase the drying speed, the depth of the printing plate is required to be shallower than that of the solvent-based plate at the time of plate making, and the design accuracy is higher.

3 conclusions

In summary, in order to obtain good printing characteristics and printing quality, the paper ink characteristics and printing conditions must be coordinated. If they can achieve their respective optimal conditions, the printing quality of water-based ink gravure on paper can be compared with that of solvent-based ink gravure. From the above research and analysis, we can get the following points of the process of gravure printing on a sheet of water-based ink: First of all, from the paper point of view, we must use non-polished wood pulp paper, and use the internal sizing paper to achieve printing. Paper has a degree of hydrophobicity. However, when the hydrophobicity exceeds a certain value, the degradation of the printing quality will be very fast, ie, the ink diffusion will severely reduce the image distorted printing density, etc. Therefore, it must be controlled, and this critical value may not be exceeded. Control the change of temperature and humidity in the printing environment. The relative humidity is maintained at 55%-60% and 18-20°C, and the temperature changes within ±3°C. Prior to printing, humidity was adjusted using a high humidity method to reduce the change in moisture content of the paper during the printing process. Secondly, the acid-base reaction between paper and ink causes a large amount of water to infiltrate into the paper base from the viewpoint of the acid-base reaction that occurs when the paper comes into contact with the ink. In order to reduce the paper deformation, it is allowed during the printing process. In the range, try to make the pH value of the paper and the ink close to each other. When printing with alkaline paper, the pH value of the ink is mostly controlled at about 9. At the same time use small particles of ink, so that the ink on the paper diffusion beneficial. In addition, in order to increase the drying speed, the depth of the printing plate is required to be shallower than the solvent-based plate at the time of plate making, and the design accuracy is higher.




Author: Zhang Yi Zhu Lan Zhao new