The pH value of the wetting liquid has a great influence on the transfer of the lithographic ink, so the pH value of the lithographic wetting liquid must be strictly controlled.
The base of the lithographic plate is aluminum or zinc. Both aluminum and zinc are very active two metals, and are very unstable in strongly acidic or alkaline media. For example, in a weakly acidic medium, the surface of the zinc plate is slightly corroded to form a Zn3 (PO4) 2 hydrophilic layer. If the acidity of the wetting fluid increases, the pH value drops too much, and the blank part of the printing plate will be deeply corroded Trachoma occurs, and a strong hydrophilic salt layer cannot be formed.
In order to study the degree of corrosion of the zinc plate by the acidity of the wetting fluid, the zinc plate samples can be immersed in a wetting fluid of different pH values ​​for a certain period of time, generally 24h, and then taken out for washing and drying; before and after water soaking The weights W and W1 are calculated, and the weight loss ΔW is calculated, and the relative weight loss after corrosion of the zinc plate is represented by (ΔW / W) × 1000. The results obtained are listed in Table 5-11. It can be seen from the table that when the wetting liquid is acidic, as the pH value decreases, the corrosion of the printing plate increases; the wetting liquid is close to neutral, and the corrosion of the printing plate also tends to be moderate; after the wetting liquid is alkaline, The corrosion of the printing plate has intensified again. The common aluminum plate base has a similar situation. What's more serious is that as the corrosion of the printing plate gradually intensifies, the combination of the graphic part of the printing plate and the metal base may be destroyed, causing the oleophilic layer on the graphic part of the printing plate to fall off, resulting in a "plate-off" phenomenon. This is a malfunction in lithography.
Table 5-11 The effect of the pH value of the dampening solution on the corrosion of the printing plate
pH 3 4 5 6 7 (△ W / W) × 1000 -3.1417 -2.3643 -0.2327 -0.1648 -2.2390A certain amount of drying agent (drying oil) is often added to the ink. The drying agent is generally a salt of metals such as lead, cobalt, and manganese, which has an accelerated drying effect on the dry vegetable oil binder. When the pH of the wetting fluid is too low, the wetting fluid will chemically react with the drier, making the drier ineffective. Practice has proved that when the pH value of the ordinary wetting liquid drops from 5.6 to 2.5, the drying time of the ink is extended from the original 6h to 24h; when the pH value of the non-ionic surfactant wetting liquid decreases from 6.5 to 4.0, the drying of the ink The time was extended from the original 3h to 40h. The delay of the ink drying time will aggravate the back of the printed matter and will also affect the effect of overprinting.
The photodegradable PS plate used in lithography, the diazo compound in the blank part of the plate is decomposed after being exposed to light, and is removed by the weak alkaline developer; the diazo compound that is not exposed to light remains on the plate surface, forming a lipophilic figure Text section. If the pH value of the wetting liquid is too high, the diazo compound in the graphic part of the printing plate will be dissolved by the wetting liquid during the printing process, making the graphic part incomplete and affecting the quality of the printed matter.
After long-term placement of the lithographic ink using dry vegetable oil as the binder, the chemical bonds of some molecules in the dry vegetable oil will break, and fatty acid RCOOH will be generated under the action of oxygen in the air. The molecular structure of fatty acids is similar to surfactants and consists of hydrophilic and lipophilic groups. However, the R-hydrocarbon chain of fatty acids is longer, and the number of carbon atoms in the general R-hydrocarbon chain is more than 17, and the ionization constant of RCOOH is very small, which is extremely difficult to ionize. Come out, so the orientation ability of fatty acids at the interface is very poor, often in a free state in the ink layer. However, when the pH value of the wetting liquid is high, the [OH-] in the wetting liquid increases, and the wetting liquid mixed in the ink will neutralize RCOOH to generate RCOO-, and the ion reaction formula is:
RCOOH + OH- = RCOO- + H2O
RCOO- is a typical anionic surfactant. At the water-ink interface, the hydrophilic base end faces the wetting fluid and the lipophilic base end faces the ink. The interfacial tension between the wetting fluid and the ink is due to the RCOO-on the ink-water interface. Oriented and dropped. The higher the pH value of the wetting liquid, the more RCOO- generated after the RCOOH is neutralized, the greater the amount of adsorbed on the interface, and the more the interface tension drops. Figure 5-17 shows the relationship between the interfacial tension of water and fatty acids and the pH of water. It can be seen that when the pH is less than 6, the interfacial tension is very stable. After the pH exceeds 6, the interfacial tension drops rapidly. Generally speaking, the lower the interfacial tension, the more serious the emulsification of the ink. For non-ionic surfactant wetting fluids with low surface tension, as the pH value increases, the emulsification of the ink becomes more serious, and controlling the pH value is particularly important. It can be seen that the pH value of the wetting liquid is too high or too low will bring various disadvantages to lithography.
The pH value of the wetting liquid is related to the amount of the wetting liquid stock solution. The amount of the stock solution is large, the pH value of the wetting liquid is low, and the acidity is enhanced. During the printing process, the actual printing conditions should be combined to control the pH value of the wetting fluid and increase or decrease the amount of wetting fluid stock solution.
It is generally believed that the PS plate has low corrosion resistance to acids and alkalis, and the pH value of the wetting liquid is preferably between 5 and 6; the graphic basis of the flat intaglio plate is relatively strong and has a certain acid resistance, and the pH value can be compared The PS version is lower and suitable between 4 and 6; the multi-layer metal version, the blank part is metal chromium with good hydrophilicity, and the wetting liquid is suitable between 5 and 7.
When the thickness of the ink layer increases, the ink in the graphic part will inevitably expand to the blank part, which is prone to "dirty version". Therefore, the amount of the wetting liquid stock solution should be increased to maintain the oleophobic and hydrophilic performance of the blank part.
When the amount of desiccant in the ink is increased, although the drying speed is accelerated, the viscosity of the ink increases, the particles become thicker, the adhesion to the blank part of the printing plate increases, and dirty plates are prone to occur, so the amount of the original liquid is increased to increase the moisture The pH value of the wet liquid drops slightly, which is beneficial to prevent the dirty version.
Use loose-texture non-coated paper for printing. If the viscosity of the ink is large, the powder will fall and the hair will be serious, and the printing plate will become dirty due to abrasion. Therefore, it is necessary to appropriately reduce the pH value of the wetting liquid. Using high-quality coated paper for printing, the pH of the wetting fluid can be increased appropriately.
Figure 5-17 Relationship between interfacial tension and pH
Using field printing, the pH of the wetting fluid can be lower. On the contrary, if the dot printing plate is used, the pH of the wetting fluid can be higher.
When the temperature in the printing workshop increases, the viscosity of the ink decreases and the fluidity increases. At the same time, the dry vegetable oil separates more free fatty acids, causing the plate surface to become dirty, so the pH value of the wetting liquid should be lower.
The reasons for the need to reduce or increase the pH value of the wetting fluid mentioned above do not exist independently during the printing process, and are often interlaced and constrained by each other. Therefore, when adjusting the pH value of the wetting fluid, it is necessary to carefully analyze and comprehensively consider, otherwise, it will cause the opposite effect.
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