Analysis of PH value of offset printing paper

Excessively large or small pH values ​​of offset printing paper can adversely affect the quality of printed products. The surface of offset printing paper, especially coated paper, has a certain acidity and alkalinity. When the pH is less than 7, the surface of the paper is acidic; when the pH is greater than 7, the surface of the paper is alkaline. The ideal pH value of offset printing paper is preferably neutral, which will greatly improve the quality of printed products and significantly reduce the occurrence of common faults. Therefore, the quality of paper has a great influence on the quality of printed products. However, due to factors such as raw materials and paper processing, the ideal paper is almost non-existent.

Since offset printing is based on the principle of oil and water incompatibility, the graphic part of the printing plate is hydrophobic and water-repellent, and the blank part is hydrophilic and oil-repellent. Therefore, the fountain solution is indispensable in offset printing, and the fountain solution is generally weakly acidic.

1. The pH of the paper is too high

When the pH value of the paper is too high, the alkaline substances in the paper will continuously dissolve into the fountain solution, neutralizing part of the acid in the fountain solution, causing the pH of the fountain solution to rise and destroy its acid. Alkalinity reduces the surface tension of the fountain solution, thereby accelerating the ink emulsification, causing the ink separation speed to slow down and also affecting the normal transfer of the ink. If the ink emulsification is serious, it will cause the printed image to be dull in color and the outline will be unclear. Once the tinting power of the ink is reduced, the amount of ink will have to be increased, thus destroying the balance of the offset ink and printing, resulting in the printing network being graded, crushed, or even caused. Flower version, paste version and ink roller deinking phenomenon.

The high pH of the paper will also make the printing plate's resistance to printing drop, because the surface strength of the paper is essentially the binding force between the fiber, the filler, and the rubber. It is difficult to achieve p≥x+u+n+w during the printing process. , p is the binding force between the fiber and the filler and the rubber compound, x is the viscosity of the ink on the paper, u is the peeling tension, n is the viscosity of the blanket, and w is the mechanical friction, so the powder dropping and the hair loss phenomenon will inevitably occur. . The fibers thus removed will be transferred to the blanket. When the blanket cylinder is in contact with the plate cylinder, these alkaline substances will be transferred to the printing plate, resulting in a blank part of the printing plate, and the film forming agent in the graphic part will be eroded. , Make the printing plate resistant to printing force.

2. The pH of the paper is too low

Too low a pH of the paper will significantly reduce the pH of the fountain solution. When the acidity of fountain solution is increased, the drying speed of the ink will be affected and the printed product will be dried poorly because the higher concentration of H+ in the fountain solution will cause a substitution reaction with the metal salt in the dry oil, thus inhibiting the drying speed of the ink. . It has been proved that when the pH is too low, the printing on the acidic paper is slower than on the alkaline paper, thereby preventing and destroying the oxidative polymerization of the ink. In order to ensure the quality of printed matter, the pH of the paper must be measured. When the pH of the paper is too low, the pH of the fountain solution should be slightly increased. There are two methods:

(1) Overall pH Measurement of Paper

The paper sample was first soaked with distilled water, then the aqueous solution was withdrawn and its pH was measured with a pH meter. This method has a certain reference value for paper and cardboard other than coated paper.

(2) Paper surface pH measurement method

1 Electrical test method: The potential difference between the indicator electrode and the reference electrode in the solution is determined. The reference electrode potential is a constant value. The potential of the indicator electrode changes with the pH value of the test solution. At this time, the pH value is applied again. The pH of the solution is measured directly by the meter.
2 color ratio method: First we wet the paper surface with distilled water, and then use the acid-base indicator to drop on the surface to observe the color change and compare it with the color sample with known pH.
3 Titration: The pH of the paper is represented by the pH of the aqueous solution. The acidity of an acidic solution is expressed as a fraction of sulfuric acid, and the alkalinity of an alkaline solution is expressed as a fraction of sodium hydroxide.

Author: Wu Ying Cui

Reprinted from: Printing Technology