Effect of Paper Acidity and Alkali on Print Quality

The invention of paper is an important symbol of human progress and an important material condition for the recording, accumulation, dissemination and development of human civilization and science and culture. Among them, printing paper is the most used and most important type of product in paper, and various properties of paper, including mechanical properties, physical properties, chemical properties, optical properties, etc., are of great importance to the quality of the print reproduction process and prints. Impact. Therefore, to understand these properties of paper, proper adjustment of printing parameters and printing process during printing is the key to obtaining high quality prints. The acidity and alkalinity of paper is an indicator of the chemical properties of paper, and it is an important factor affecting the quality of printed matter. This paper will briefly discuss the influence of paper acidity and alkalinity on the quality of printed matter.

Acidity of paper

The acidity of the paper is produced during the papermaking process. The strength of the solution is determined by the concentration of hydrogen and hydroxide ions in the solution, which is usually expressed and measured by pH. But precisely, it is impossible for papers such solids to have a pH because paper is a multiphase system composed of fibers, additives and certain moisture. Therefore, the pH value of paper, which we usually refer to, refers to the pH of an aqueous solution that has been soaked in paper.

In general, the paper pH should be neutral, but in fact most non-coated printing papers have a pH between 4.5 and 7.0 and are slightly acidic. The pH of the coated printing paper is between 7 and 9 Between, weak alkaline. The acidity of the paper is mainly due to the alumina [Al2(SO4)3] in the papermaking raw material, which is the main reason for the acidity of the rosin sizing paper due to the large amount of hydrogen ions generated by the hydrolysis of the aluminum compound. After the paper is made, the SO2 and CO2 gases in the air are absorbed by the paper and further oxidized into sulfuric acid and carbonic acid. At the same time, the acid residues of the cellulose in the paper, the carbohydrate compound, and the residual chlorine of the bleaching and pulping chemicals are hydrolyzed to produce hydrochloric acid. Hypochlorous acid, as well as the acidic components of some coatings, are also factors that make the paper acidic. The paper is alkaline, mainly because the kaolin and calcium carbonate dispersed in the filler are alkaline during the papermaking process. The presence of casein in the adhesive, cyanamide-formaldehyde resin or polyethyleneimine resin in the coating, etc. Are factors that make the paper alkaline. At the same time, the modern papermaking process changed from the original acid paper to alkaline papermaking, which will also lead to changes in the acidity and alkalinity of the paper.

Effect of paper acidity and alkalinity on printed matter

1. Effect on the "durability" of printed products

The durability of printed products mentioned here actually refers to the durability of the paper used for printing. It means that after a long time, the paper can still maintain its important performance, especially the degree of resistance to folding and color. For printing paper, durability expresses the life of the paper print or the longest time it can hold. There are many factors that affect the durability of paper, including the type of pulp used in papermaking, the content of rubber, paint, acid and metal compounds, temperature, humidity, and light when stored or used, and pH is the most important factor affecting the durability of paper. the elements of.

If the pH is low, the degradation of cellulose and hemicellulose will be accelerated because in the acidic paper, hydrogen ions promote the breaking of the “oxygen bridge” of the cellulose macromolecules, thereby causing hydrolysis and oxidation reaction of the cellulose, resulting in fiber softening. And the breakage, and these reactions are relatively strong, will make the paper prints serious discoloration, the stronger the acidity, the more serious this phenomenon. Since the acidity of paper is mainly derived from the alumina used in sizing, printing papers with high durability, such as archives and important papers with long-term preservation value, are made of neutral, non-aluminum applications. The Glue method. Papers made with sulfite have poor durability due to the low pH and acidic cooking conditions on the plant fibers, and are inferior to the kraft pulp paper. If the residual chlorine content in the paper after bleaching is relatively high, the chloride ions will react with water to produce strong acid, making the paper more acidic, leading to a significant decrease in durability. For high-pH papers, strong alkalinity is also not conducive to improving the service life of the paper, because cellulose will accelerate its oxidative degradation at a higher alkaline condition. Therefore, for printing paper, strong acid and strong alkaline have a great destructive effect on the durability of the printed matter.

2. Impact on print quality

(1) The effect of acidity and alkalinity of paper on printing quality is very significant. The pH value of the paper is one of the important factors affecting the drying rate of the oxidized conjunctiva of the printing ink. If the paper is acidic, the acidic substance acts on the polyvalent metal in the desiccant to form a product that is insoluble in oil and prevents the oxidized conjunctiva of the ink. Inhibition of ink drying. Therefore, when the pH value of the paper is low, that is, the acidity of the paper is strong, the drying time of the printing ink is significantly prolonged. When the pH of the paper is lower than 5, drying may even stop completely. Especially when the relative humidity is relatively high, the water content in the paper increases, and these waters can directly participate in the hydrolysis of cellulose and hemicellulose, which increases the content of acidic components in the paper and seriously affects the drying speed of the ink. When the paper is alkaline, hydroxide ions are generally considered to accelerate the drying speed of the printing ink and promote drying. However, in fact, the effect of this accelerator is not very significant in the actual production process. Figure 1 shows the effect of pH on the drying time of printing ink.

(2) For offset printing, the pH of the printing paper also affects the pH of the fountain solution. Under normal circumstances, the dampening solution is weakly acidic. When the pH value of the printing paper is too low, the pH of the dampening solution will be further lowered, thus affecting the drying speed of the printing ink, causing the prints to be poorly dried and the back side to become dirty. unpleasant sight. At the same time, the strong acidity of the fountain solution will inevitably accelerate the corrosion rate of the printing plate, thereby reducing the printing plate's resistance to printing, and will also destroy the printing plate outlets and affect the reproduction of the printed media and the level. When the pH value of the printing paper is too high, ie, the alkalinity is too strong, the alkaline substances in the paper will be continuously dissolved and transferred to the fountain solution during the printing process, resulting in a neutralization reaction with the weakly acidic fountain solution. Change the pH of the fountain solution. In addition, these alkaline substances will also reduce the interfacial tension at the ink interface of the printing plate, promoting the emulsification of the ink, resulting in the non-graphic part of the dirty, so that the gloss of the printed product is reduced, the ink color is dim and so on. The stronger the acidity of the fountain solution, the greater the effect.

(3) The effect on the stability of the printing ink. Some inks contain pigments that are very unstable under strong acid or alkali conditions. When the ink is printed on some acid or alkaline paper, the ink color will fade over time, causing the print to lose its original value. Some shine and color. In actual production, if the acidity of the printing paper is too strong, after the metal ink is printed, the color of the pattern on the print will gradually change from gold to silver to black, and the print will lose its metallic luster. This is because the hydrogen ions in the printing paper are transferred to the metal ink and the metal particles in the ink are corroded.

In addition, some papers are used to package metal products or come into contact with metals after printing. Due to the strong acid or strong alkalinity of paper, the pollution or corrosion of metal products is also very serious. In particular, some of the papers used to package foods, if they are acidic or alkaline, will cause food contamination and damage to the human body. For papers such as photographic paper and copy paper, filter paper, indicator paper, offset paper, and electrical insulating paper, the amount of residual acidic substances and basic substances is preferably as small as possible.

Therefore, during the production process, the paper should try its best to control its acidity and alkalinity, making it neutral. When the printing unit uses new paper, it is better to understand the acidity and alkalinity of the paper, measure it, adjust the pH of the fountain solution according to the acidity and alkalinity of the printing paper, control the temperature and humidity of the production workshop, and change the printing. In the process of the relevant parameters or printing processes, try to minimize the impact of acidity and alkalinity of printing paper on the quality of the printed matter, and achieve the best effect of printing.


Source: Ke Yin Media

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