The most popular water-based ink paper gravure pri

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Gravure printing process of water-based ink paper (II)

1.2.3 effect of acid-base reaction (pH change) on Printability

obviously, within the stability range of water-based ink, a drop in pH will have an impact on the expansion characteristics of ink on paper. Or even if there is research and development, it is also extensive, that is, to meet the low needs of customers with simple roughness. In order to observe the influence of pH value on ink diffusion and stability, literature [3] has studied two kinds of polymers: one is modified polyester which is sensitive to changes in pH value within the range of 5~7, and the other is aqueous solution which is sensitive in the whole pH range. The research shows that acrylic resin is sensitive to the change of pH value within 6~8, while polyester is not sensitive to the change of pH value in a wide range. This shows that these polymers show very different behaviors when it comes to the absorption of paper. Moreover, the penetration rate of the polymer sensitive to pH change into the paper is much slower than that of the polymer insensitive to pH change. Therefore, at present, water-soluble acrylic copolymer resin is mostly used as binder in water-based ink

The dynamic relationship of acid-base reaction between ink layer and paper is very important for the diffusion of ink in paper products and the stability of ink. When the paper and the ink just touch, only a limited amount of acid substances on the paper surface react with the ink layer, and more acid substances penetrate into the paper in liquid phase. This infiltration was slow at first and accelerated with time. This phenomenon is called "wetting hysteresis" or "permeability hysteresis". This "hysteresis" slows down the time of acid-base interaction (pH change). The amount of acid required to completely neutralize the ink layer depends on the thickness of the ink layer, just as it depends on the buffer capacity of the buffer. For example, high acid polymers will consume more paper acidity. As an offset for water-based ink printing, the paper must have the appropriate pH, so that the drying time of the ink is appropriate, and at the same time, it can properly prevent the paper from absorbing the ink too quickly, thus damaging the gloss of the ink layer. In addition, a buffer can be added to the ink formula to change the diffusion performance of the ink

it can be seen from the above experiments that the acid-base reaction will reduce the pH value of the water-based ink printed on the paper. The decrease of pH value has a great impact on the paper expansion and water release of the ink. Moreover, the decrease of pH value occurs very quickly, and its rate is controlled by the number and time of acid ions diffusing from the paper to the ink layer; When in contact with the ink layer, the production rate and quantity of acid ions in the paper depend on the rate of the water phase penetrating into the paper and the speed of the interaction between the ink and the paper components, and the acid-base reaction depends on the reanalysis process of the cross pair data curve of the water in the ink penetrating into the paper base; In the process of ink diffusion, the influence of acid-base reaction is second only to that of surfactants mixed in ink. The greater the pH difference between ink and paper, the more meaningful the acid-base reaction is to promote ink diffusion. Moreover, the smaller the ink particle, the faster the diffusion of ink on the paper surface according to the random investigation of the effective stroke of the electronic tensile machine sold on the market at present; The higher the acid value in the polymer, the greater its buffer capacity and resistance to pH change

2 treatment process of paper deformation surface smoothness and pH change of ink film. In order to keep the moisture content of the paper consistent on the whole paper surface, adapt to the relative humidity of printing, reduce the sensitivity of the paper to external ambient temperature and humidity, and improve the stability of paper size. Take advantage of the paper moisture absorption hysteresis effect [1] and adopt the humidity control treatment method to solve the problem. Due to the high wet method, the paper fiber is first allowed to absorb more moisture, so that the paper can be fully extended, and then the humidity is reduced, so that the paper can obtain the same humidity as the workshop after humidity adjustment. In this way, the moisture absorption hysteresis effect of the paper can be more fully displayed, so that in the gravure printing of water-based ink paper, the paper will no longer absorb too much moisture and stretch, maintaining relative stability (see Figure 3). Therefore, in sheet fed water-based ink gravure printing, the paper that has been treated with high wet humidity regulation is used for printing, which can reduce the printing failure caused by the water absorption deformation of the paper to a certain extent

a in the figure indicates that the moisture content of the paper is 3 Therefore, the moisture content of the printed paper changes by 8%; B refers to the change of water content of the paper after printing when the paper is adjusted to 5.2%; C refers to the change of paper moisture content after the paper is adjusted to 5.7%; D refers to the change of paper moisture content after printing when the paper moisture content is adjusted to 6.2%

the moisture content of paper is related to the ambient temperature, which decreases with the increase of temperature, and the relationship is approximately linear. Therefore, in addition to the above humidity control treatment, the water content of the paper should not change by more than ± 0.1% during the overprint process, otherwise the accuracy of overprint will be affected, and the change of workshop temperature should not exceed ± 3 ℃

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