Water-based ink paper gravure printing process (1)

With the arrival of the green printing era, the replacement of solvent-based inks with water-based inks is an inevitable change in the printing industry and ink manufacturing industry, and it is also an inevitable trend of social development. The use of water-based ink has greatly improved the surrounding environment and the printing work environment. However, in actual use, since the main solvent of the water-based ink is water, the high-quality gravure printing on the paper product will cause some corresponding problems, such as moisture absorption and deformation of the paper, misprinting, and gloss smoothness. Problems of printability change such as ink diffusibility. The author analyzes and analyzes the problem law and its influencing factors from three aspects, so as to obtain a feasible improvement method or point out the selection requirements for the printing material plate making equipment.

For fine prints, the print quality is directly related to the printability of the paper used. However, the printability of paper is not a fixed property, but whether the characteristics of paper should be suitable for printing in different printing conditions. For example, under the condition of gravure printing with water-based ink, the printability of the paper is not only related to the uniformity of the smoothness of the paper, but also the property of not easily deforming after moisture absorption. It is well known that when paper products are printed with a water-based ink containing a large amount of a water base, the paper may be deformed due to a change in water content, resulting in problems such as overprinting, paper feed failures, and the like. Therefore, it is one of the main topics of water-based ink sheet gravure to study the deformation characteristics and hygroscopicity law of paper in high humidity environment [1~3].

1 Water-based ink paper gravure paper deformation surface smoothness and ink film pH change principle

1.1 Water-based ink paper gravure paper deformation surface smoothness

Gravure printing is a type of contact printing in which the ink is transferred from the recesses to the paper. The width of the pits of the gravure roll is in the range of about 15 to 250 μm. Therefore, the appropriate gravure printing adaptability largely depends on the smoothness and compressibility of the substrate. Since the printing plate undergoes the squeegee function of the squeegee prior to imprinting on the paper and the plate cylinder, the ink surface in the concave hole is concave in the shape of the lunar surface, making ink transfer more complicated than offset printing. Therefore, the defects such as the concave and the rough surface of the paper all affect the normal transfer of the ink at this position, resulting in dot distortion or deinking (white spots).

In the gravure printing process of the water-based ink, the water in the water-based ink transferred to the paper penetrates the paper and molecules diffuse, so that the fibers absorb water and expand to generate displacement between the fibers, causing the paper size to expand and change. At the same time, it leads to an increase in surface roughness. Moisture absorption deformation and surface roughness increase will have a certain influence on the gravure printing quality of water-based ink paper. Therefore, it is of significance to fully grasp the paper's moisture absorption deformation characteristics and moisture absorption law and to reduce the failure caused by this characteristic.

The main factors affecting the paper deformation are the degree of pulp sizing and the type and ratio of pulp raw materials. For example, grass pulps have shorter fiber lengths, higher pentose contents, more heterocells, looser fiber structures, and a lower dimensional stability than wood pulp. In addition to the influence of the paper stock, a reasonable degree of beating also affects it. Appropriately increase the hardness of the slurry, reduce the hemicellulose content, use appropriate bleaching techniques, enhance slurry washing, reduce the content of heterocells, etc., and control the expansion and contraction rate of the paper. In addition, the deformation of the paper is related to the thickness of the fiber wall and the heating of the paper.

1.2 Acid-base reactions (pH changes) when the water-based ink comes in contact with the surface of the paper

During the printing process, changes in the pH of the water-based ink phase not only affect the release of moisture and viscosity changes in the ink film transferred to the paper surface, but also affect the drying and printing stability of the ink film. And the greater the difference between the pH value of the paper and the water-based ink in the water-based ink gravure printing, the greater the change.

Based on the water-based ink and the composition of the paper [3], it is easy to experimentally verify that when a very thin layer of water-based ink is applied to the surface of the paper, the pH value of the ink film is within one second of contacting the paper surface. Change has occurred. The reason for the change in the pH value is the acid-base reaction that occurs when the ink sheet is touched. The acid-base reaction has a great influence on the diffusion of the ink on the surface of the paper. Further, whether or not the pH of the ink can be dissolved/dispersed is stable. The necessary conditions for obtaining good prints. When using a water-based ink, the pH is generally controlled between 8.5 and 9.5. If it exceeds this range, the printability will be affected. During the printing process, pH stabilizers (water-based ink additives) can be used to adjust the printing inks to maintain their printability and pH stability.

1.2.1 Acid-base reaction and pH stability curve

As with any printing process, the key to print gravure printing is the physical and chemical reaction between the paper and the ink. Wet ink diffusion The ink adjustment and drying are directly affected by these reactions. When printing with water-based inks, water reacts very easily with all the components of the paper, including cellulosic binders and fillers. The most obvious effect of these reactions is the wrinkles (surface and volume) of the paper. There are also chemical effects, the most notable of which is the pH change (acid-base reaction) in the aqueous phase. Since water-based inks containing polymers (linking aids, etc.) are generally very sensitive to pH, any change in pH in the aqueous phase can have a significant effect on the diffusion and stability of the ink, leading to polymer/coloration precipitation. The change of water content changes the viscosity and other problems. The acid-base reaction process is very fast. It is controlled by the speed of water permeation into the interior of the paper and the acid ion diffusion of the ink toward the ink. The relationship between ink film pH and ink printability stability is shown in Figure 1. When the ink is transferred to acid paper, the pH value of the ink film on the paper surface drops below the stability limit as the ink spreads on the paper and the moisture is released, resulting in instability of the ink printability.

1.2.2 pH Change and Influencing Factors

The equilibrium state of the pH in the ink layer printed on the paper is shown in Fig. 2 [3]. Two water-based inks were used in the experiment, one with a high acid number and a low pH, and the other with a low acid number and a high pH. The paper used was Type A Supercalendered Paper (SCA). As can be seen from Figure 2, the pH in the ink layer changes when it comes into contact with the surface of the paper, and after about 100 ms, the rate of change in pH significantly slows. In theory, the moment the water-based ink comes into contact with the paper, the liquid portion of the ink penetrates into the paper. Acid-base reactions occur between the aqueous phase and the components of the paper, resulting in a change in pH. Specifically, the release of hydrogen ions during the diffusion of the water-based ink lowers the pH value, changes the viscosity of the ink, and the like, so that the speed of ink penetration into the paper becomes slower, and this results in the dynamic characteristics of the pH change.

If the diffusion coefficient of the inorganic acid contained in the system is known, the time required for the hydrogen ions to penetrate into the ink layer can be estimated. The thickness of the ink layer is determined by the number of bars (May Bar). It is very difficult to make precise determination of the diffusion coefficient. Similarly, it is difficult to accurately determine the time for infiltration and diffusion. However, the time taken for the change in pH in the ink layer is substantially consistent with the time of diffusion. A rough estimate can be made on the order of 10-5 cm2/s cited in [4]. Calculate the number of acid groups permeating the 1 μm ink layer within 1 s and the time required for hydrogen ions to diffuse into the ink layer. The result is approximately a few milliseconds. In other words, the rate of pH change is determined by the number and timing of acid ion diffusions, and the rate of acid ion diffusion depends, in turn, on the rate at which the water phase penetrates into the paper and the speed at which the ink interacts with the components of the paper. In short, the diffusion of acid ions in the ink basically controls the entire process of pH change. Looking at the relationship between pH and time, it can be seen that the pH values ​​shown by the high acid and low acid water based inks are significantly different from the time curves. The curve shows that the higher the acid value of the polymer in the ink, the greater its buffering capacity and resistance to changes in pH, and it is easier to maintain a stable pH, which is very favorable for paper water-based ink gravure .