Printability control of dampening solution
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Offset printing is based on the ink and dampening solution on the printing plate to form graphics and blanks, using the principle of oil and water incompatibility to achieve graphic reproduction. The so-called mastery of water is equivalent to mastering offset printing, which is enough to prove the importance of water (dampening solution) for offset printing. The pH of the dampening solution is a necessary condition for the formation of a hydrophilic inorganic salt in the blank portion of the printing plate and keeping it clean, which has a great influence on the transfer effect of the ink. The acidity of the dampening solution (pH<7) or alkalinity (pH>7) is not conducive to the normal operation of the offset printing. Therefore, in order to fully exert the role of dampening solution in offset printing, it is necessary to strictly control the pH value of the dampening solution, and the pH value is generally tested by a test paper, an indicator, and a pH meter. In this paper, we mainly discuss the influence of pH value of dampening solution on plate quality, drying speed and interfacial tension.
The effect of pH value of dampening solution can not be ignored
1. Affect the quality of the printing plate: the PH value is too high, resulting in the lack of graphic and text, and the printing resistance is reduced; the PH value is too low, the dot is damaged, and the "off-print" occurs.
The printing plate used in offset printing is metal aluminum, which is unstable in strong acid and weak acid media. When the dampening solution is only weakly acidic, it can cause mild corrosion on the surface of the aluminum plate to form a hydrophilic inorganic salt layer. If the pH of the dampening solution is too small (strongly acidic), the blank part of the printing plate will be deeply corroded, coarse grit will appear, and it will not form a strong hydrophilic layer, which will affect the transfer of ink due to the presence of more dampening solution in the blank part. Even the image portion is corroded and the bond with the plate base is destroyed, so that the diazonium compound layer of the oleophilic layer is detached, causing damage to the dot and causing a "disappearance" phenomenon.
The plate in offset printing is generally a photo-decomposable PS plate. The blank portion of the plate is produced after the photo-decomposition of the diazo compound is removed by the weakly alkaline developer. The image is covered by the film. Light diazo photosensitive layer. When the pH value of the dampening solution is too high, the diazonium compound in the graphic part will be dissolved, resulting in incomplete image and image, which will affect the printing quality. This is one of the reasons for the decline in printing durability, but it is often overlooked.
2. Affect the drying speed of the ink: the pH value is too low, and the drying speed of the ink is prolonged.
In order to shorten the drying time of the ink, a certain amount of drier is often added. When the pH of the dampening solution is too low, it will chemically react with lead salts (Pb), cobalt (Co), manganese (Mn) and other metal salts in the drier to invalidate the drier and prolong the drying time. Practice has proved that when the pH of ordinary dampening solution drops from 5.6 to 2.5, the drying time of ink will be extended from 6 hours to 24 hours; the pH of nonionic surfactant dampening solution ("F" series wetting agent) When the value drops from 6.5 to 4.0, the ink drying time is extended from 3 hours to 40 hours.
3. Affect the interfacial tension: the PH value is large, the ink is emulsified seriously
The ink with the dry vegetable oil as the binder can form the fatty acid RCOOH under the action of oxygen in the air, and the RCOOH has poor orientation ability at the interface and is in a free state. When the pH of the dampening solution is too high, the OH- in the dampening solution increases, and the RCOOH in the dampening solution generates RCOO-, and the orientation of the RCOO- at the ink interface reduces the interfacial tension. The larger the pH, the more RCOO- is produced and the more the interfacial tension drops. When the pH is less than 6, the interfacial tension is very stable. When the pH is greater than 6, the interfacial tension drops rapidly. In general, the lower the interfacial tension, the more severe the ink emulsification phenomenon.
It can be seen that the pH of the dampening solution cannot be a certain value, but varies within a range. It is generally considered that the pH value of the dampening solution for the PS plate is between 5 and 6, the dampening solution for the flat plate is between 4 and 6, and the dampening solution for the multi-layer metal plate is between 5 and 7.
Targeted control of dampening solution pH
1. The amount of desiccant added to the ink and the pigment
When the amount of the desiccant is large, the drying speed is fast, the viscosity of the ink is increased, the ink particles are coarsened, the lipotoxicity of the blank portion of the plate is increased, and the paste is easy to paste. Therefore, the pH of the dampening solution should be lowered (increasing the amount of the stock solution added) to prevent the paste from being applied. But pay attention to the amount to avoid the formation of a vicious circle.
Different inks have different pH requirements for dampening solution due to different properties such as pigment, oiliness, viscosity, fluidity and acid resistance. The general rule is yellow > cyan > black > magenta, light ink > dark ink.
2. Paper surface strength and pH
When printing loose paper, the viscosity of the ink is high, the paper is dusty and the hair is severely damaged, which increases the wear of the printing plate and is easy to get dirty. In this case, the pH of the dampening solution should be appropriately lowered to supplement the hydrophilic inorganic salt layer. And clean the oil dirty. When printing paper with high surface strength, especially when printing coated paper, increase the pH value (reducing the amount of stock solution added) to prevent yellowing and affect product quality. Alkaline fountain solution is only suitable for printing on newsprint (acid paper). It has been suggested that the pH of the paper plus the pH of the fountain solution is preferably equal to 13.
3. Graphic structure and ink layer thickness
The printing plate with the main body, lines and characters is thick, the ink layer is thick, the ink load is large, and the spreading is serious. The pH value of the dampening solution should be lowered; the graphic-based mesh adjustment should improve the dampening solution. pH value. For printing plates in the field, lines, texts, and dots, the pH value should be set to facilitate the stability of the blank and the graphic part.
4. Workshop temperature
The temperature is high, the viscosity of the ink is lowered, the fluidity is large, and at the same time, more free fatty acids are decomposed, and the plate is easy to be smeared. Therefore, the pH of the dampening solution should be appropriately increased. The pH of the dampening solution should be appropriately increased when the machine speed is high, the printing pressure is high, and the hardness of the blanket and the pad is high.
The process of lowering or increasing the pH of the dampening solution does not occur independently in printing, and is often staggered and pinned. Therefore, when adjusting the pH value of the dampening solution, it is necessary to comprehensively analyze and carefully consider it, otherwise it will cause adverse effects.