Water-based ink paper gravure printing process (2)

1.2.3 Effect of acid-base reaction (pH change) on printability

Obviously, within a range of water-based ink stability, a drop in the pH value will have an effect on the characteristics of the ink spread on the paper. In order to observe the effect of pH on ink diffusion and stability, the literature [3] is for two polymers: one is a modified polyester that is sensitive to changes in the pH range of 5 to 7, and the other is in the whole pH. A range of sensitive aqueous solutions were studied. Studies have shown that acrylic resins are sensitive to changes in pH within 6 to 8, whereas polyesters are insensitive to wide changes in pH. This shows that these polymers show very different behaviors when it comes to the absorption of paper bases. Moreover, the pH-sensitive polymer penetrates the paper much more slowly than the pH-insensitive polymer. Therefore, water-based acrylic copolymer resins are currently used as the binder in water-based inks.

The dynamic relationship between the acid-base reaction between the ink layer and the paper is important for the diffusion of the ink in the paper product and the stability of the ink. Immediately after the paper comes into contact with the ink, only a limited amount of acid material reacts with the ink layer on the surface of the paper, and a greater amount of acid material penetrates into the paper in the liquid phase. This penetration started slowly and accelerated with time. This phenomenon is called "wet hysteresis" or "osmotic hysteresis." This "hysteresis" slows down the time of the acid-base effect (pH change). The amount of acid needed to completely neutralize the ink layer depends on the thickness of the ink layer, just like the buffering capacity of the buffer. For example, a high acid value polymer will consume more paper acidity. As a water-based ink printing offset - the paper must have a suitable pH, so that the ink drying time is appropriate, while at the same time can properly prevent the paper from absorbing ink too quickly, thereby destroying the gloss of the ink layer. In addition, a buffer can be added to the ink formulation to change the ink diffusion properties.

From the above experiments, it can be known that the acid-base reaction will cause the pH value of the water-based ink printed on the paper surface to decrease, and the drop in the pH value has a great influence on the paper surface expansion and water release of the ink. Moreover, the decrease in pH occurs very quickly. The rate is controlled by the number and time of acid ions diffused from the paper to the ink layer. When contacting the ink layer, the rate and amount of acid ions in the paper depends on the penetration of the aqueous phase. The speed of the paper and the speed at which the ink interacts with the constituents of the paper, and the acid-base reaction depends on the process by which the water in the ink penetrates into the paper base; during the ink diffusion process, the effect of the acid-base reaction is second only to the ink. The effect of surfactants. The greater the difference between the pH of the ink and the paper, the more meaningful it is for the acid-base reaction to promote ink diffusion. Moreover, the smaller the ink particles, the faster the ink spreads over the surface of the paper; the higher the acid value in the polymer, the greater its buffering capacity and resistance to pH changes.

2 paper deformation surface smoothness and ink film pH change treatment process

2.1 Sheet-fed gravure treatment

Deformation of paper is affected by many factors, and its essence lies in the change in water content. In order to make the paper's moisture content consistent throughout the paper surface, and to adapt to the relative humidity of the printing, but also to reduce the sensitivity of the paper to the external temperature and humidity, improve the stability of the paper size. Using hygroscopic hysteresis effect [1], the use of humidity treatment method to solve. Because the high-wet method uses the moisture content of the paper fiber to absorb more moisture, the paper is fully stretched, and then the humidity is lowered, so that the humidity of the paper is the same as that of the workshop after humidity adjustment, so that the hygroscopicity of the paper can be delayed. The effect is more fully manifested, so that in water-based ink paper gravure, the paper no longer absorbs excess moisture and elongates, maintaining relative stability (see Figure 3). Therefore, in the water-based ink gravure printing of sheet-fed ink, the use of high-moisture wet-processed paper for printing, to a certain extent, reduce the printing failure caused by paper absorbing deformation.

A in the figure indicates that the paper is moistened to a moisture content of 3.8%, and the moisture content of the printed paper changes; B indicates that the paper is moistened to 5.2%, and the moisture content of the printed paper changes; C indicates that the paper is moisturized to 5.7%, printing The change in the moisture content of the post-paper; D is the moisture content of the paper adjusted to 6.2%, and the moisture content of the paper after printing.

The moisture content of the paper is related to the ambient temperature, which decreases with increasing temperature and approximates a straight line relationship. Therefore, in addition to the humidity conditioning process described above, during the overprinting process, the moisture content of the paper should be changed by no more than ±0.1%. Otherwise, the accuracy of overprinting is affected. The change in the shop temperature is preferably not more than ±3°C.