Introduction to the acidity and alkalinity of the

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 largest and most important type of paper, and various properties of paper, including mechanical properties, physical properties, chemical properties, optical properties, etc., have a great impact on the printing reproduction process and the quality of printed matter. Therefore, understanding these properties of paper and properly adjusting printing parameters and printing process during printing are the key to obtain high-quality prints. The acidity and alkalinity of paper is an index of the chemical properties of paper and an important factor affecting the quality of printed matter. This paper will briefly discuss the influence of the acidity and alkalinity of paper on the quality of printed matter

acidity and alkalinity of paper

the acidity and alkalinity of paper is produced in the process of papermaking. The strength of the acidity and alkalinity of the solution depends on the concentration of hydrogen ions and hydroxyl ions in the solution, which is usually expressed and measured by pH value. But accurately speaking, paper, a solid, cannot have pH value, because paper is a heterogeneous system composed of fibers, additives and certain moisture. Therefore, the pH value of paper we usually refer to the pH value of the aqueous solution soaked in paper

generally speaking, the pH value of paper should be neutral, but in fact, the pH value of most uncoated printing paper is between 4.5-7.0, which is weakly acidic, and the pH value of coated printing paper is between 7-9, which is weakly alkaline. The acidity of paper mainly comes from bauxite [al2 (SO4) 3] in papermaking raw materials. Due to the hydrolysis of aluminum compounds, a large number of hydrogen ions are produced, which is the main reason for the acidity of rosin sizing paper. After the paper is made, SO2 and CO2 in the air are absorbed by the paper and further oxidized into sulfuric acid and carbonic acid. At the same time, the acid group of cellulose in the paper, carbohydrate rubber, chlorine hydrolysis of bleaching and pulping chemicals residues to produce 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 when making paper, kaolin and calcium carbonate dispersed in fillers in the medium and long-term energy technology development strategy and development plan to 2020 are alkaline, casein in adhesives, cyanamide formaldehyde resin or polyethylene imine resin in coatings are all factors that make the paper alkaline. At the same time, the modern papermaking process is changed from acid papermaking, which was originally based on the requirements of JJG "verification regulation of standard force measuring lever", and the allowable error of the weight quality of the supporting force value of the lever is ± 0.02%, to alkaline papermaking, which will also lead to the change of the acidity and alkalinity of the paper

influence of paper acidity and alkalinity on printed matter

1 The impact on the "durability" of printed matter

the durability of printed matter mentioned here actually refers to the durability of the paper used for printing, which refers to the extent to which the paper can still maintain its important use performance, especially the folding resistance, color and other characteristics, after a long time. For printing paper, durability expresses the service life of the paper printed matter or the longest time it 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, as well as the temperature, humidity and light during storage or use. PH value is the most important factor that affects the durability of paper

if the pH value is low, the degradation of cellulose and hemicellulose will be accelerated, because in acidic paper, hydrogen ions promote the "oxygen bridge" of cellulose macromolecules to break, so that cellulose will undergo hydrolysis and oxidation reactions, resulting in fiber softening and breaking, and these reactions are relatively strong, which will cause serious discoloration of paper prints. The stronger the acidity, the more serious this phenomenon is. Since the acidity of paper mainly comes from bauxite used in sizing, neutral sizing without bauxite is used to manufacture printing paper with high durability, such as archives and important documents with long-term preservation value. The paper made from sulfite has poor durability because of the low pH value and the special effect of acid cooking conditions on plant fibers. It is not as durable as the paper made from sulfate pulp. If the residual chlorine content in the bleached paper is relatively high, the chloride ion will also cause the deviation of the experimental force and produce strong acid by interacting with water, which will make the paper more acidic and greatly reduce the durability. For paper with high pH value, strong alkalinity is also not conducive to improving the service life of paper, because under the condition of high alkalinity, the oxidative degradation rate of cellulose will also be greatly accelerated. Therefore, for printing paper, strong acid and strong alkaline have a great destructive effect on the durability of printed matter

2. Influence on printing quality

(1) the influence of paper acidity and alkalinity on printing quality is very significant. The pH value of the paper is one of the important factors that affect the drying speed of the oxidized conjunctiva of the printing ink. If the paper is acidic, the acidic substances interact with the multivalent metals in the desiccant to produce products insoluble in grease, which prevents the oxidized conjunctiva of the printing ink and inhibits the drying of the printing ink. Therefore, when the pH value of the paper is low, that is, the paper is acidic, the drying time of the printing ink will be significantly prolonged, and when the pH value of the paper is lower than 5, it will even cause the drying to stop completely. Especially under the condition of high relative humidity, the water content in the paper increases. These water can directly participate in the hydrolysis of cellulose and hemicellulose, which increases the content of acid components in the paper and seriously affects the drying speed of ink. When the paper is alkaline, it is generally believed that hydroxyl ions will accelerate the drying speed of printing ink and promote drying. But in fact, in the actual production process, the effect of this accelerator is not very significant. Figure 1 shows the effect of paper acidity and alkalinity on the drying time of printing ink

(2) for offset printing, the acidity and alkalinity of printing paper will also affect the pH value of fountain solution. Under normal circumstances, the fountain solution is weakly acidic. When the pH value of the printing paper is too low, the pH value of the fountain solution will be further reduced, which will affect the drying speed of the printing ink, and make the printed matter appear bad phenomena such as poor drying and dirty back. At the same time, the acidity of the fountain solution is too strong, which will inevitably accelerate the corrosion rate of the printing plate, thus reducing the printing resistance of the printing plate, and also destroying the printing point, affecting the reproduction of the order and level of the printed matter. When the pH value of the printing paper is too high, that is, the alkalinity is too strong, the alkaline substances in the paper will continuously dissolve and transfer to the fountain solution during the printing process, and neutralize with the weak acidic fountain solution, so that the pH value of the fountain solution changes. In addition, these alkaline substances will also reduce the interfacial tension on the ink-water interface of the printing plate, promote the emulsification of the ink, cause the non graphic parts to be dirty, reduce the gloss of the printed products, and dim the color of the ink. The stronger the acidity of the fountain solution, the greater this effect will be

(3) influence on the stability of printing ink. Some inks contain pigments that are very unstable under strong acid or alkali conditions. When the ink is printed on some acidic or alkaline paper, the color of the ink will fade after a period of time, making the printed matter lose its original luster and color. In actual production, if the acid of the printing paper is too strong, after metal ink printing, the color of the pattern on the printed matter will gradually change from gold and silver to black, and the printed matter will lose its metallic luster. This is because the hydrogen ion in the printing paper is transferred to the metal ink, so that the metal particles in the ink are corroded

in addition, some papers are used to package metal products or contact with metals after printing. Due to the strong acid or alkali of the paper, the pollution or corrosion of metal products is also very serious. In particular, some paper used to package food, if its acidity or alkalinity is strong, will cause food pollution and great harm to human body. For photographic paper, photocopying paper, filter paper, indicator paper, offset paper, electrical insulation paper and other papers, the amount of residual acid and alkaline substances is also the less the better

therefore, the acidity and alkalinity of paper should be controlled as much as possible in the production process to make it neutral. When printing units use new paper, it is best to understand the acidity and alkalinity of the paper, measure it, appropriately adjust the pH value of the fountain solution according to the acidity and alkalinity of the printing paper, control the temperature and humidity of the production workshop, change the relevant parameters or printing process in the printing process, try to minimize the impact of the acidity and alkalinity of the printing paper on the quality of the printed matter, and realize the direct display of the best effectiveness value data of the printing

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