The effect of acidity and alkalinity of printing paper on the quality of printed products
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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 scientific culture. Among them, printing paper is the most important and most important type of paper, and the various properties of paper, including mechanical properties, physical properties, chemical properties, optical properties, etc., have a greater quality in the printing and copying process and printed matter. Impact. Therefore, understanding these properties of paper, and properly adjusting the 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, which is an important factor affecting the quality of printed matter. This paper will briefly discuss the influence of acidity and alkalinity of paper on the quality of printed matter.
Acidity and alkalinity of paper
The acidity and alkalinity of the paper is produced during the papermaking process. The acidity and alkalinity of the solution depends on the concentration of hydrogen ions and hydroxide ions in the solution, which is usually expressed and measured by pH. But to be precise, it is impossible for paper solids to have a pH because paper is a multiphase system composed of fibers, additives and certain moisture. Therefore, what we usually call the pH of the paper refers to the pH of the aqueous solution soaked in the paper.
In general, the pH of the paper should be neutral, but in fact most of the non-coated printing paper has a pH between 4.5 and 7.0, which is weakly acidic. The pH of the coated printing paper is 7-9. Between, it is weakly alkaline. The acidity of paper is mainly derived from bauxite [Al2(SO4)3] in papermaking raw materials. The hydrolysis of aluminum compounds produces a large amount of hydrogen ions, which is the main reason for the acidity of rosin sized paper. 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 acidity of the cellulose, the carbohydrate compound, the bleaching and the residual chlorine of the pulping chemicals in the paper are hydrolyzed to form hydrochloric acid. And 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 papermaking, the casein in the adhesive, the cyanamide-formaldehyde resin or the polyethyleneimine resin in the coating. They are all factors that make the paper alkaline. At the same time, the modern papermaking process changed from the original acid papermaking to the alkaline papermaking, which also led to changes in the acidity and alkalinity of the paper.
The effect of paper acidity and alkalinity on printed matter
1. Influence on the durability of printed products
The durability of the printed matter referred to herein actually refers to the durability of the paper used for printing, and means that the paper can maintain its important performance properties, particularly the degree of folding resistance and color, after a long period of time. For printing paper, durability expresses the useful life of paper prints or the longest time that can be preserved. 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, and the temperature, humidity, and light during storage or use. pH is the most important factor affecting paper durability. the elements of.
If the pH is low, it will accelerate the degradation of cellulose and hemicellulose, because in acidic paper, hydrogen ions cause the "oxygen bridge" of cellulose macromolecules to break, which causes hydrolysis and oxidation of cellulose, resulting in fiber softening. And the fracture, and these reactions are relatively strong, which will cause the paper print to be seriously discolored, and the stronger the acidity, the more serious this phenomenon. Since the acidity of the paper is mainly derived from the bauxite used in the sizing, the printing paper with high durability, such as archival documents and important document papers with long-term preservation value, is used in the neutral soil without bauxite. Glue method. Paper made from sulfite has a poor durability due to its low pH and the special effect of acidic cooking conditions on plant fibers, and is inferior to paper copied from kraft pulp. If the residual chlorine content in the paper after bleaching is relatively high, the chloride ions will react with water to produce a strong acid, which makes the paper more acidic, resulting in greatly reduced durability. For paper with a higher pH value, strong alkalinity is also not conducive to improving the service life of the paper, because the oxidative degradation rate of cellulose is greatly accelerated under the condition of high alkalinity. Therefore, for printing paper, strong acid and strong alkali have a great destructive effect on the durability of printed matter.
2. Influence on print quality
(1) The influence 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 speed of the oxidized conjunct film 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 the oil and prevents the oxidation of the ink. The drying of the ink is suppressed. Therefore, when the pH of the paper is low, that is, the paper is acidic, the drying time of the printing ink is significantly prolonged. When the pH of the paper is lower than 5, the drying may be completely stopped. Especially in the case of relatively high relative humidity, the water content in the paper increases, and the water can directly participate in the hydrolysis of cellulose and hemicellulose, so that the content of acidic components in the paper increases, which seriously affects the drying speed of the ink. When the paper is alkaline, it is generally believed that the hydroxide ions will accelerate the drying speed of the printing ink and promote drying. But in fact, in the actual production process, the role of this accelerator is not very significant. Figure 1 shows the effect of acidity and alkalinity of the paper on the drying time of the printing ink.
(2) For offset printing, the acidity and alkalinity of the printing paper also affects the pH of the fountain solution. Under normal circumstances, the dampening solution is weakly acidic. When the pH of the printing paper is too low, the pH of the dampening solution is further reduced, which affects the drying speed of the printing ink, making the printed matter dry and the back is dirty. unpleasant sight. At the same time, the acidity of the dampening solution is too strong, which will inevitably accelerate the corrosion rate of the printing plate, thereby reducing the printing durability of the printing plate, and also destroying the printing plate dot, affecting the reproduction of the tone and level of the printed matter. When the pH of the printing paper is too high, that is, the alkalinity is too strong, during the printing process, the alkaline substance in the paper is continuously dissolved and transferred to the fountain solution, and neutralized with the weakly acidic fountain solution. The pH of the dampening solution is changed. In addition, these alkaline substances also reduce the interfacial tension at the ink interface of the printing plate, promote the emulsification of the ink, cause the non-image portion to be dirty, the gloss of the printed product is lowered, and the color of the ink is dim. The stronger the acidity of the fountain solution, the greater the effect.
(3) The influence on the stability of printing ink. Some of the pigments contained in the ink are very unstable under strong acid or alkali conditions. When the ink is printed on some acid or alkaline paper, the color of the ink will fade after a period of time, causing the print to lose its original color. Some shine and color. In actual production, if the acidity of the printing paper is too strong, after printing the metal ink, the color of the pattern on the printed matter will gradually change from gold to silver to black, and the printed product loses metallic luster. This is a result of the transfer of hydrogen ions in the printing paper to the metallic ink, causing the metal particles in the ink to be corroded.
In addition, some papers are used to package metal products or contact with metals. Due to the strong acidity or alkalinity of the paper, the pollution or corrosion of metal products is also serious. In particular, some papers used to package foods, if they are acidic or alkaline, cause food contamination and are very harmful to the human body. For a variety of 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 alkaline substances is preferably as small as possible.
Therefore, the paper should be controlled as much as possible in the process of production to make it neutral. When printing a new paper, it is best to know the acidity and alkalinity of the paper, measure it, adjust the pH of the dampening solution according to the acidity and alkalinity of the printing paper, control the temperature and humidity of the production workshop, and change the printing. Relevant parameters or printing processes in the process, as far as possible, minimize the influence of acidity and alkalinity of printing paper on the quality of printed matter, and achieve the best effect of printing.