Effect of pH value of offset printing paper on print quality
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1. The pH of the paper is too high
When the pH of the paper is too high, the alkaline substance in the paper will continuously dissolve into the dampening solution, neutralizing a part of the acid in the dampening solution, and raising the pH of the dampening solution to destroy the acid. Alkaline, reducing the surface tension of the dampening solution, thereby accelerating the emulsification of the ink, causing the ink separation speed to slow down, and also affecting the normal transfer of the ink. If the emulsification of the ink is serious, the color of the printed product will be dim and the outline will be unclear. Once the tinting strength of the ink is reduced, the amount of ink has to be increased, thereby destroying the balance of the offset ink, which causes the printed dots to be leveled, pasted, and even caused. Flower plate, paste plate and ink roller deinking phenomenon.
If the pH of the paper is too high, the printing durability of the printing plate will be reduced, because the surface strength of the paper is essentially the bonding force between the fiber, the filler and the rubber compound, and it is difficult to achieve p≥x+u+n+w in the printing process. , p is the bonding force between the fiber and the filler and the rubber, x is the adhesion of the ink to the paper, u is the peeling tension, n is the viscosity of the blanket, and w is the mechanical friction, so the phenomenon of falling powder and falling hair is inevitable . The fibers and the like which are detached are transferred to the blanket. When the blanket cylinder is in contact with the plate cylinder, the alkaline substance is transferred to the printing plate, causing the blank portion of the printing plate to be dirty, and the film forming agent in the graphic portion is eroded. To reduce the printing durability of the printing plate.
2. The pH of the paper is too low
Too low a paper pH will cause a significant drop in the pH of the fountain solution. When the acidity of the dampening solution is enhanced, it will affect the drying speed of the ink, resulting in poor drying of the printed product, because the higher concentration of H+ in the dampening solution and the metal salt in the dry oil are displaced, thereby inhibiting the drying speed of the ink. . Practice has shown that when the pH is too low, the blot will be slower on acid paper than on alkaline paper, thereby preventing and destroying the oxidative polymerization of the ink. In order to ensure the quality of the printed product, the pH of the paper must be measured. When the pH of the paper is too low, the pH of the dampening solution should be slightly increased. There are two methods:
(1) Paper overall pH measurement method
The paper sample was first soaked in distilled water, then the aqueous solution was withdrawn, and the pH was measured with a pH meter. This method has certain reference value for paper and paperboard other than coated paper.
(2) Paper surface pH measurement method
1Electrical measurement method: measuring the potential difference between the indicator electrode and the reference electrode in the solution, the potential of the reference electrode is a constant value, and the potential of the indicator electrode changes according to the pH value of the measurement solution, and then the pH value is applied again. The pH of the solution was measured directly by the instrument.
2-color ratio method: Firstly, the surface of the paper is wetted with distilled water, and then the surface of the sample is dropped with an acid-base indicator, and the color change is observed and compared with a color sample of known pH.
3 titration method: the pH value of the paper is expressed by the pH of the aqueous solution of the sample. The acidity of the acidic solution is expressed as the fraction of sulfuric acid, and the alkalinity of the alkaline solution is expressed as the fraction of sodium hydroxide.