Influence of pH value change of the hottest offset

2022-08-24
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Influence of pH value change of offset paper on print quality

too large or too small pH value of offset paper will have an adverse impact on print quality. The surface of offset paper, especially coated paper, has certain acidity and basicity. When the pH value is less than 7, the surface of the paper is acidic; When the pH value is greater than 7, the paper surface is alkaline. The pH value of ideal offset paper should be neutral, which will greatly improve the quality of printing products and significantly reduce the probability of common faults. Therefore, the quality of paper has a great impact on the quality of printing. However, due to the influence of raw materials and paper processing technology, ideal paper almost does not exist

because offset printing uses the principle of oil-water incompatibility, the graphic part of the printing plate is lipophilic and water repellent, and the blank part is hydrophilic and oil repellent, so the fountain solution is essential in offset printing, and the fountain solution is generally weakly acidic

1. The pH value of the paper is too high

when the pH value of the paper is too high, the alkaline substances in the paper will continue to dissolve into the fountain solution, neutralize part of the acid in the fountain solution, and make the pH value of the fountain solution rise, destroy its acidity and alkalinity, reduce the surface tension of the fountain solution, so as to speed up the emulsification of the ink, slow down the speed of ink separation, and also affect the normal transfer of the ink. If the ink emulsification is serious, it will cause the color and luster of the printed matter to be dim and the outline to be unclear. Once the ink coloring power decreases, it has to increase the amount of ink loaded with more things, thus destroying the balance of offset ink and water, resulting in the point merging and pasting of the printed matter, and even the deinking of the stencil, the pasted version and the ink roller

too high pH value of the paper will also reduce the printing resistance of the printing plate, because the surface strength of the paper is essentially the binding force between fiber, filler and rubber. Because it is difficult to achieve P ≥ x+u+n+w in the printing process, where p is the binding force between fiber and filler and rubber, X is the adhesion of ink to the paper, u is the peeling tension, n is the viscosity of rubber blanket, W is the mechanical friction, so the phenomenon of powder and wool falling is inevitable. In this way, the fallen fibers will be transferred to the blanket. When the rubber cylinder contacts with the printing plate cylinder, these alkaline substances will be transferred to the printing plate, causing the blank part of the printing plate to be dirty, and the film-forming agent of the graphic part to be eroded, reducing the printing resistance of the printing plate

2. the pH value of the paper is too low

paper PH3. Power switch: check whether the power indicator light of the switch is on. If the value is too low, the pH value of the fountain solution will decrease significantly. When the acidity of the fountain solution increases, it will affect the drying speed of the ink, causing poor printing drying, because the high concentration of h+ in the fountain solution has a displacement reaction with the metal salt in the drying oil, which inhibits the drying speed of the ink. Practice has proved that when the pH value is too low, the drying speed of imprinting on acidic paper will be slower than that on alkaline paper, thus preventing and destroying the oxidative polymerization of ink. In order to ensure the print quality, the pH value of the paper must be measured. When the pH value of the paper is too low, the pH value of the fountain solution should be slightly increased. There are two methods:

(1) the overall pH value measurement method of the paper

first soak the paper sample with distilled water, then extract the aqueous solution, and measure its pH value with a high-value pH meter with transmission accuracy. This method has certain reference value for paper and paperboard other than coated paper

(2) determination method of pH value of paper surface material in the stretching process

① electric measurement method: measure 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 with the pH value of the measured solution. At this time, the pH value of the solution is directly measured by the pH value tester again

② colorimetry: first, wet the surface of the paper sample with distilled water, then drop the acid-base indicator on the surface, observe the color change, and compare it with the color sample with known pH

③ titration: the pH value of the paper is expressed by the pH value of the sample aqueous solution. The acidity of acidic solution is expressed in the fraction of sulfuric acid, and the alkalinity of alkaline solution is expressed in the fraction of sodium hydroxide

reprinted from: printing industry channel

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