In offset printing, the pH of paper and fountain solution will directly affect the water-ink balance, ink drying performance and ink layer quality of the printing process. Therefore, when controlling and adjusting the water-ink balance, it is necessary to understand the acidity and alkalinity of the paper and fountain solution, and understand its impact on the printing process.

1. Paper pH

Due to the increasingly higher requirements for printing quality in the market, and the advancement of printing technology, the impact of paper characteristics and printability on printing quality has begun to attract attention, and paper pH is a factor that should be controlled.

If the pH value of the printing paper is neutral, the printing quality can be greatly improved. However, due to the influence of raw materials and processing technology in paper production, absolutely neutral paper with a pH value of 7 is not available. The printed paper surface or coating layer often has a certain acidity or alkalinity. When the pH value is 7, the paper The surface is alkaline. At present, China's papermaking mainly adopts alkaline pulping and uses NaOH as a cooking agent, so most papers are slightly alkaline. If the alkalinity is too large, the acidity of the fountain solution will obviously change, and the acidity of the hydrophilic glue layer of the non-graphic part of the printing plate will be weakened. If it cannot be adjusted in time, it will inevitably cause the hydrophilicity of the non-graphic part to decrease, and the ink and ink will be unbalanced. , The coloring power of the ink is reduced, resulting in the dullness of the printed graphics and text, and even causing the plate, paste and ink roller deinking and other phenomena. At the same time, the weak alkaline fibers that fall off the paper coating will be transferred to the blanket, and then transferred to the printing plate through the blanket, resulting in dirty printing plate and reduced durability.

When the pH of the paper is too low, the acidity of the fountain solution will be significantly enhanced, thereby inhibiting the oxidative polymerization reaction and drying speed of the ink, resulting in poor drying of the printed matter.

2. pH value of fountain solution

Traditional offset printing materials rely on the principle of oil-water repulsion to achieve ink image transfer, fountain solution is indispensable, and its acidity and alkalinity have a significant impact on the printing plate and water-ink balance. The aluminum oxide film layer formed during the offset printing plate base treatment is an amphoteric oxide. If the fountain solution is too acidic, it will increase the corrosion of the printing plate sand, destroy the blank part of the printing plate, and cause the printing plate to become dirty; If it is too strong, it will not protect the blank part, and it will corrode the ink and paper. Therefore, the offset fountain solution is generally weakly acidic, and the ideal pH value is between 5 and 6, which can ensure that the printing dots are clear, and the hydrophilicity of the non-graphic parts of the printing plate is stable, which helps extend the life of the printing plate and can also be suppressed The fountain solution is attacked by bacteria and other impurities to better maintain the balance of water and ink.

The components of the fountain solution are mainly buffers, preservatives, printing plate protection agents, desiccant algae inhibitors, etc., of which the buffering agent has the greatest influence on the pH value of the fountain solution. The function of the buffer is to stabilize the acidity of the fountain solution because the tap water is usually neutral. In order to reach the pH value (5 ~ 6) required for normal printing, it is necessary to add acid to the fountain solution to maintain the pH value of the fountain solution It is weakly acidic, and the carbonate in the water will react with the added acid to increase the pH. At this time, a buffering agent must be added to maintain the pH of the fountain solution at a stable level. At the same time, the buffering agent also has a significant effect on alleviating the influence of cleaning agents and other factors on the fountain solution. However, as the amount of buffer added changes, the pH of the fountain solution will fluctuate greatly. Inappropriate amount of addition will also affect the service life of the printing machinery and will also hinder the drying of ink, especially inks containing metal pigments.

Water also has a great influence on the pH of fountain solution. The quality of tap water is soft and hard. Water with a high content of metal cations is hard water, and water with a high content of anions is soft water. The high hardness of water will increase the pH of the fountain solution. When preparing fountain solution, the softness and hardness of water should be adjusted according to actual needs, preferably 10 ° dH. It is also important to note that changes in water temperature will affect the concentration of ions in the water. Therefore, it is necessary to control the hardness and temperature of the water to achieve the best effect of the fountain solution.

In order to ensure that the pH of the fountain solution is relatively stable, an advanced method is to monitor the conductivity of the fountain solution while controlling the amount of buffer added and the hardness of the water.

Because with the increase in the concentration of fountain solution, the hardness of water and the increase of water temperature, the conductivity of fountain solution will increase and the pH value will decrease; on the contrary, if alcohol is added to fountain solution, the conductivity of fountain solution The rate decreases and the pH increases. The conductivity of fountain solution should generally be controlled at (1400 ± 200) μs / cm, but due to the influence of dust, ink and other mixed substances, the conductivity is also allowed within 800μs / cm ~ 2000μs / cm, but when dampened When the fluid appears to be obviously contaminated, it must be reconstituted.

In summary, before printing production, we can take more precise methods, such as measuring the surface pH of the paper with a surface acidity meter, measuring the pH value and conductivity of the fountain solution with a liquid pH meter and a conductivity meter, and comparing it with Control within a relatively reasonable range, effectively avoid the occurrence of pH-related problems in the printing process, improve the quality of printed matter, and reduce printing consumables. This is very important for printing houses.

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