The invention of paper is an important symbol of human progress and an important material condition for human civilization and scientific culture to be recorded, accumulated, spread and developed. Among them, printing paper is the largest and most important product in paper, and the various properties of paper, including mechanical properties, physical properties, chemical properties, optical properties, etc., have a greater impact on the printing process and the quality of the printed matter. Impact. Therefore, to understand these properties of paper and adjust printing parameters and printing process properly during printing is the key to obtaining high-quality printed products. The acidity and alkalinity of paper is an indicator of the chemical properties of paper and an important factor affecting the quality of printing. This article will briefly discuss the effect of acidity and alkalinity of paper on the quality of printing.
Acidity and alkalinity of paper
The acidity and alkalinity of paper are produced during the papermaking process. The strength of the solution's acidity and alkalinity 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 as a solid to have a pH value, because paper is a multi-phase system composed of fibers, additives and a certain amount of moisture. Therefore, what we usually call the pH value of paper refers to the pH value of the aqueous solution soaked in paper.
Generally speaking, the pH value of paper should be neutral, but in fact, most non-coated printing papers have a pH between 4.5 and 7.0, which is weakly acidic, and the pH of coated printing papers is between 7 and 9. Between, it was weakly alkaline. The acidity of paper mainly comes from the alumina [Al2 (SO4) 3] in the papermaking raw materials. The hydrolysis of aluminum compounds generates 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 to sulfuric acid and carbonic acid. At the same time, the residual acid chloride of cellulose, carbohydrate rubber, bleaching and pulping chemicals in the paper are hydrolyzed to generate hydrochloric acid Hypochlorous acid and the acidic components of some paints 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, casein in the adhesive, cyanamide-formaldehyde resin or polyethyleneimine resin in the coating Both are factors that make the paper alkaline. At the same time, the modern papermaking process from the original acid paper to alkaline paper, will also lead to changes in paper acidity and alkalinity.
Effect of paper acidity and alkalinity on printed matter
1. The impact on the "durability" of printed products
The durability of printed products mentioned here actually refers to the durability of the paper used for printing, which means that after a long time, the paper can still maintain its important performance, especially the degree of characteristics such as folding resistance and color. For printing paper, durability expresses the service life of the paper print or the longest time it can be kept. There are many factors that affect the durability of paper, including the type of pulp used in papermaking, the content of sizing materials, paints, acids and metal compounds, and the temperature, humidity, light, etc. during storage or use, and pH is the main factor affecting the durability of paper. the elements of.
If the pH value is lower, the degradation of cellulose and hemicellulose will be accelerated, because in acid paper, hydrogen ions promote the "oxygen bridge" of cellulose macromolecules to break, thereby causing hydrolysis and oxidation of cellulose, causing fiber softening And break, and these reactions are relatively strong, will make paper prints seriously fade, the stronger the acidity, the more serious this phenomenon. Since the acidity of the paper is mainly derived from the alumina used in the sizing, the printing paper with high durability, such as archival literature and important document paper with long-term preservation value, uses a neutral application without alumina.胶 方法。 Glue method. Paper made with sulfite, because of the special effect of low pH and acidic cooking conditions on plant fibers, so the durability is relatively poor, not as good as paper made with sulfate pulp. If the residual chlorine content in the paper after bleaching is relatively high, the chloride ions will react with water to generate strong acids, making the paper more acidic, resulting in a great decline in durability. For paper with a high pH value, strong alkalinity is also not conducive to improving the service life of the paper, because the oxidative degradation rate of cellulose will be greatly accelerated under the condition of higher alkalinity. Therefore, for printing paper, strong acid and strong alkali have a great destructive effect on the durability of the printed matter.
2. Impact on printing quality
(1) The effect 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 substance interacts with the polyvalent metal in the desiccant to form a product that is insoluble in grease and prevents the oxidized conjunctiva of the ink. Suppresses the drying of ink. Therefore, when the pH of the paper is low, that is, the acidity of the paper is strong, the drying time of the printing ink will be significantly extended. When the pH of the paper is lower than 5, the drying will even stop completely. Especially under the condition of relatively high relative humidity, the water content in the paper increases. These waters can directly participate in the hydrolysis of cellulose and hemicellulose, which increases the content of acidic components in the paper, which seriously affects the drying speed of the ink. When the paper is alkaline, it is generally believed that the hydroxide ion 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 paper acidity and alkalinity on the drying time of printing ink.
(2) For offset printing, the acidity and alkalinity of the printing paper will also affect the pH of the 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 lowered, which will affect the drying speed of the printing ink, causing the printing to be poorly dried and the back side to be dirty unpleasant sight. At the same time, the acidity of the fountain solution is too strong, which will inevitably accelerate the corrosion rate of the printing plate, thereby reducing the printing plate's endurance, and also destroying the printing plate dots, affecting the reproduction of the tone and level of the printed product. When the pH value of the printing paper is too high, that is, the alkalinity is too strong, during the printing process, the alkaline substances in the paper will continue to dissolve and transfer to the fountain solution, and neutralize the weak acidic fountain solution, Changes the pH of the fountain solution. In addition, these alkaline substances will also reduce the interfacial tension on the water-ink interface of the printing plate, promote the emulsification of the ink, cause the non-graphic parts to become dirty, reduce the gloss of the printed product, and make the ink dull. 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 acid or alkali paper, the color of the ink will fade after a period of time, causing the printed matter to lose its originality. Some gloss and color. In actual production, if the acidity of the printing paper is too strong, after printing with metallic ink, the color of the pattern on the printed product will gradually change from gold and silver to black, and the printed product loses its metallic luster. This is due to the transfer of hydrogen ions in the printing paper to the metal ink, which causes the metal particles in the ink to be corroded.
In addition, some paper is used to package metal products or contact with metal after printing. Due to the strong acid or alkali of paper, the pollution or corrosion of metal products is also very serious. In particular, some paper used for packaging food, if its acidity or alkalinity is strong, will cause pollution to the food and also cause great harm to the human body. For photographic paper, copy paper, filter paper, indicator paper, offset paper, electrical insulation paper and other papers, the amount of residual acidic and alkaline substances is also as small as possible.
Therefore, during the production process, the paper should try to control its acidity and alkalinity to make it neutral. When the printing unit uses new paper, it is best to understand the acidity and alkalinity of the paper, measure it, and 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, and change the printing. Relevant parameters in the process or printing process, etc., try to minimize the effect of the acidity and alkalinity of the printing paper on the printing quality, and achieve the best effect of printing.
We are manufacturer and supplier of Stainless Steel Pineapple Slicer,Pineapple Cutter,Pineapple Peeler,Pineapple Corer,Pineapple Core Remover. And we located in Jiangmen, Guangdong, China. If any interested, please contact us for free.
Pineapple Slicer,Pineapple Cutter,Pineapple Peeler,Pineapple Corer,Pineapple Core Remover
Jiangmen Xinhui Zhancha Metal Products Co,. Ltd. , https://www.zckitchenwares.com