Influence of pH of Fountain Solution on Plate Quality

Offset is the use of ink and fountain solution to form graphic and blank parts on the printing plate, using the oil and water incompatible principle to achieve graphic reproduction. Just as the mastering of water is equal to mastering offset printing, it is enough to prove the importance of water (flood solution) to offset printing. The pH of the fountain solution is a necessary condition for the formation of hydrophilic inorganic salts in the blank part of the printing plate, and it has a great effect on the transfer effect of the ink. The pH of the dampening solution is not suitable for normal offset printing because of its pH (<7) or partial alkali (pH>7). Therefore, in order to give full play to the role of the fountain solution in offset printing, it is necessary to strictly control the pH of the fountain solution. The pH value is generally tested with a test strip, an indicator, and an acidity meter. In this paper, we mainly discuss the influence of the pH of the fountain solution on the quality, drying speed, and interfacial tension of the printing plate.

The role of fountain solution pH can not be ignored

1. Affect plate quality

The printing plate base used in offset printing is metallic aluminum, which is unstable in both strong and weak acid media. The dampening solution can only produce mild corrosion on the surface of the aluminum plate when it is weakly acidic, resulting in a hydrophilic inorganic salt layer. If the pH of the fountain solution is too small (strongly acidic), the blank part of the printing plate will be subject to deep corrosion, resulting in coarse sanding, which will not easily form a strong hydrophilic layer, so that the blank part will affect the ink transfer due to the existence of more dampening solution. Even the corrosion of the graphic part and its combination with the plate base are destroyed, causing the diazonium compound layer of the oleophilic layer to fall off, resulting in damage to the network, resulting in "off plate" phenomenon.

At present, the offset printing plate is generally a photodecomposition type PS plate. The blank part of the printing plate is produced after the photochemical decomposition of the diazo compound is removed by a weakly alkaline developing solution. The graphic part is covered by a printing plate film and is not seen. Light diazo photosensitive layer. If the pH of the fountain solution is too high, the diazo compounds in the graphic part will be dissolved, resulting in incomplete graphics and influencing the printing quality. This is one of the reasons why the printing plate's resistance to printing has declined, but it is often overlooked.

2. Affect the ink drying speed

In order to shorten the ink drying time, a certain amount of driers are often added. When the pH of the fountain solution is too low, a chemical reaction occurs with the lead (Pb), cobalt (Co), manganese (Mn), and other metal salts in the driers to disable the driers and extend the drying time. Practice has proved that when the pH of general dampening solution drops from 5.6 to 2.5, the ink drying time will be extended from the original 6 hours to 24 hours; the pH of non-ionic surfactant dampening solution (“F” series wetting agent) When the value drops from 6.5 to 4.0, the ink drying time will be extended from 3 hours to 40 hours.

3. Influence the interfacial tension

The ink with the dry vegetable oil as the linking material can generate the fatty acid RCOOH under the action of oxygen in the air. The orientation ability of the RCOOH at the interface is poor and is in a free state. When the pH of the fountain solution is higher, the OH- in the fountain solution increases, and RCOO- is formed in the fountain solution. The orientation alignment of the RCOO- at the ink interface reduces the interfacial tension. The greater the pH, the more RCOO-produced, the more severe the interfacial tension drop. Figure 1 shows the relationship between the interfacial tension of water and fatty acids and the pH of the fountain solution. It can be seen that the interfacial tension is very stable when the pH is less than 6, and the interfacial tension drops rapidly when the pH is greater than 6. In general, the lower the interfacial tension is, the more serious the ink emulsification is.

It can be seen that the pH of the fountain solution cannot be a certain value but changes within a range. It is generally believed that the pH value of the fountain solution for the PS plate is between 5 and 6, the fountain solution for the gravure plate is between 4 and 6, and the plate fountain solution for the multi-layer metal plate is between 5 and 7.

Targeting fountain solution pH

1. Desiccant addition amount and pigment in ink

When the amount of desiccant is large, the drying speed is fast, the viscosity of the ink is increased, the ink particles are thickened, the sense fat of the blank part of the layout is increased, and the paste is easy to paste. Therefore, the pH of the fountain solution should be reduced (increase the amount of added liquid) and prevent the paste plate. But pay attention to dosage and avoid creating a vicious cycle.

Different inks have different requirements for the pH of the fountain solution due to the differences in properties of pigments, oiliness, viscosity, flowability, and acid resistance. The general rules are yellow> cyan> black> magenta, light-color ink> dark ink.

2. Surface strength and pH of the paper

When the printed texture is loose, due to the high viscosity of the ink, the paper is powdered off and the lint is severe, which increases the wear of the printing plate and tends to cause dirt. In this case, the pH of the fountain solution should be properly reduced to supplement the hydrophilic inorganic salt layer. And dirty cleaning oil. When printing high-strength papers, especially coated paper, increase the pH (reducing the amount of liquid added) to prevent yellowing and affect product quality. Alkaline fountain solution is only suitable for newsprint (acid paper) printing. It has been proposed that the pH value of the paper plus moisturizing solution is equal to 13.

3. Graphic structure and ink layer thickness

Plates based on field, lines, and text have a thick ink layer, a large amount of ink, and a serious spread. The pH of the fountain solution should be reduced. The graphic-based screen adjustment should improve the fountain solution. pH value. For the plates in the field, lines, characters, and dots, the pH setting should be conducive to the stability of blanks and graphic elements.

4. Workshop temperature

With high temperature, the viscosity of the ink is reduced, and the fluidity is high. At the same time, more free fatty acids are decomposed and the plate is easy to be dirty. Therefore, the pH of the fountain solution should be appropriately increased. The pH of the fountain solution should also be appropriately increased with the speed of the machine, the printing pressure, and the hardness of the blanket and pad.

The process of lowering or increasing the pH of the fountain solution does not occur independently in the printing, and often interlaces and pinches. Therefore, when adjusting the pH of the fountain solution, it is necessary to comprehensively analyze and carefully consider it, otherwise it will cause adverse effects.