Analysis of the influence of paper characteristics on printing performance Author: You Congge (Fujian Putian Product Quality Supervision and Inspection Institute 35 1 100) Abstract: This paper analyzes the influence of paper smoothness, ink absorption, surface strength, moisture content, pH value, whiteness and opacity on printing performance, so that printers can accurately grasp the suitability of printing technology and ensure the quality of printed products. Keywords: paper characteristics; Printing performance; Paper is an important basic material used in printing industry. The quality of printed matter is not only related to the quality, theme and content of the manuscript, but also related to factors such as paper, ink, printing equipment and printing conditions [1]. Among them, the influence of paper on printing is more important, and the quality of paper directly affects the quality of printed matter. The paper with excellent performance can completely transfer the ink, so that the pictures and texts can be reproduced clearly and completely on the paper, thus obtaining satisfactory reproduction effect. However, due to the limitation of paper, technology and other materials, most color continuous tone images cannot be faithfully reproduced, and only the printed images can be as close as possible to the original [2]. Different types and batches of paper have different properties. Therefore, a correct understanding of some characteristics of paper is of great significance for accurately grasping the suitability of printing technology and ensuring the quality of printed products [3]. This paper briefly discusses the effects of paper smoothness, ink absorption, surface strength, water content, pH value, whiteness and opacity on printing performance. 1 paper smoothness is an index to evaluate the unevenness of paper surface. It represents the smoothness of the paper surface. It is an important printing performance of paper, and printing smoothness is the first condition to obtain printed matter faithful to the original on paper. It determines the degree of contact between the paper surface and the inked printing plate or blanket surface at the moment of stamping, and is an important factor affecting whether the ink transfer is comprehensive and the picture and text are clear. For paper with high smoothness, using less printing pressure will make the printing plate contact with the paper surface more closely. In this way, clear handwriting and images can be printed, and the printed matter with uniform dark color can also save ink consumption [3]. On the other hand, when printing paper with low smoothness, from the microscopic point of view, the contact between the paper surface and the printing plate is uneven, so the pressure is uneven everywhere, and the transfer amount of ink is affected, which is reflected in the inconsistent ink film thickness on the printing plate, and the fine dots on the printing plate are copied because of the subsidence of the paper, that is, the brightness level of the image is lost. If it is the real part of the image, the color uniformity of the ink decreases and the expected printing density cannot be achieved. In addition, the smoothness of paper also affects the gloss of printed matter. High printing smoothness is beneficial to form a uniform and smooth ink film on paper, thus improving the gloss of printed matter and making the printed matter bright and beautiful [4]. Therefore, only very smooth paper can completely reflect the original tone level and dot pattern, so that the tone of the replica is ideal and bright. 2 paper ink absorption The so-called paper ink absorption refers to the ability of paper to absorb ink. The ink absorption of paper is a comprehensive reflection of the properties of fibers, the interweaving between fibers, the properties and contents of fillers and rubber compounds, and its essence is the adsorption of ink binders by the gaps between fibers [5]. The ink absorption of paper is an important printability index that affects the quality of printed matter. The ink absorption of paper is too strong, and the adhesive is excessively absorbed into the paper feeding layer or coating, resulting in the lack of adhesive in the ink layer on the paper, so that pigment particles are suspended on the paper. After the conjunctiva is dry, the printed matter is obsolete or loses its luster, and even some printed matter will flake off when it is wiped, resulting in pulverization. The paper has a strong absorption of ink, and the adhesive permeates into the pores of the paper, replacing part of the air in the pores of the paper, resulting in air-fiber and air-filler. The scattering interface between air and paint is reduced, which reduces the light scattering ability of the paper and increases the transparency, which may lead to the occurrence of through printing. If the ink absorption of paper is too small, it will slow down the drying speed of ink on the surface of printed matter, and the printing speed will not be improved, and at the same time, it will cause the back of printed matter to be dirty, especially the relief printing and high-speed printing relying on osmotic drying. In addition, in printing, the paper with poor ink absorption performance is poor after inking, such as the first color group BK in four-color printing. However, BK ink will be torn off the paper after being impressed by the rubber blanket of three color groups, C, M and Y, or it is difficult for the later color to be superimposed on the previous ink when overprinting in three or even four colors [3]. It can be seen that the ink absorption of paper has an important influence on the printing performance, and attention should be paid to the ink absorption of paper in the printing process. 3 paper surface strength surface strength refers to the bonding strength between paper surface fibers, sizing materials, fillers or paper surface coating particles and between coating and paper base. It shows the ability of paper to resist ink splitting in the printing process. In the printing process, when the outward strength of ink applied to the surface of paper and paperboard is greater than the cohesion of paper and paperboard, the visible damage on the surface of paper such as pilling, powder dropping, peeling, bulging and delamination is called paper or paperboard napping. The outward tension exerted by ink on the surface of paper and paperboard increases with the increase of printing speed. When the printing speed is increased to the extent that the outward tension of ink is greater than the cohesion of paper or paperboard and the surface of paper begins to fluff, the printing speed is the fuzzing speed of paper or paperboard [6]. Fuzzing speed is an important index to measure the surface strength of paper. In the modern printing process, the printing speed is generally improved, and the force required to peel off ink from blanket and transfer it to paper is also increased. Conversely, the peeling force of the blanket on the paper surface through ink is also great [7]. If the printed paper has a bad surface, it will stick to the paper during printing, resulting in powder loss or hair loss, and in serious cases, it will lead to paper breakage. On the surface of the glued paper, white spots were formed where there should be ink. However, loose coating, filler particles and stuck fibers will have adverse consequences, which mainly affect printing in two aspects: first, it will cause graphic pollution, which is mainly reflected in the white spots caused by the peeling of paper particles and the pollution caused by the nap of paper powder or fine coating; Secondly, the cleaning times of blanket and ink roller in offset printing should be increased. Due to the poor surface of paper, it is easy to drop materials and powder, and the cleaning times of blanket increase obviously, which greatly affects printing production [8]. 4 Water content and pH value of paper The water content of paper is expressed by the weight of water in paper as a percentage of the weight of paper. In general, the water content of printing paper should be controlled between 7% 1%, and the error of water content between paper center and paper periphery should not exceed 1% [9]. When the water content of paper is too high, the binding force between fibers, fillers and sizing materials in paper is easy to decrease, which affects the surface strength of paper and leads to the bad situation of powder removal and paper scraps in the printing process, thus reducing the hardness and stiffness of the paper edge, and further easily leading to unstable positioning and overprinter. If the water content of the paper is too low, the elasticity of the paper will be poor, and the imprint will easily expand when printing dots, which means that the dot expansion rate is large. In addition, static electricity and moisture absorption deformation are easy to occur in the printing process, which makes the product overprint inaccurate [2]. In general, the pH value of brush paper should be neutral or weakly alkaline. Improper pH of paper will discolor some non-acid-base inks and affect the drying of imprinting. Pigments contained in some inks are very unstable under strong acid or alkali conditions. When the ink is printed on some acidic or alkaline paper, after a period of time, the color of the ink will fade, making the printed matter lose its original luster and color. In the process of offset printing, if the paper is too alkaline, the alkaline substances in the paper will be continuously transferred to the barrel, so that part of the solution in the barrel will be neutralized, which will directly affect the pH value of the barrel liquid (the pH value of the barrel liquid is generally between 5 and 6). In addition, these alkaline substances will also reduce the interfacial tension of printing plate ink interface, promote excessive emulsification of ink, and lead to dirt in non-graphic parts [4]. If the paper is acidic, acidic substances will react with multivalent metals in the desiccant to produce oil-insoluble products, thus preventing the ink from oxidizing the conjunctiva and inhibiting the ink from drying. The drying time of ink will be obviously prolonged, or even completely stopped. In addition, the acidity and alkalinity of paper also affect the durability of printed matter and the service life of printing plate. 5 paper whiteness and opacity paper whiteness refers to the ability of paper to fully reflect after being irradiated by light, also called paper brightness, expressed in percentage. The higher the whiteness of paper, the more accurately the characteristics of ink color can be displayed on its surface. White paper can truly and objectively reflect all the colors of printed color pictures, improve the contrast between words and ink, and make replicas rich in colors, levels and contrasts, thus achieving the effect of illustrations [7]. The higher the whiteness of paper, the more obvious this effect is. But not all printed matter requires that the higher the whiteness of the paper, the better. For example, the whiteness of printed books, newspapers and periodicals should not be too high, otherwise the contrast between paper and pictures and texts is too strong, and it is too dazzling when reading, which is easy to cause eye fatigue [10]. Paper with low whiteness only absorbs part of the color light, which can neither faithfully show the contrast between light and dark parts, nor easily lead to color cast. If some are yellow, some are red, some are green, some are blue and so on. When the paper itself has color cast, the color printed on the paper is the comprehensive color effect of ink and paper, which will inevitably produce some color cast. Based on this situation, it is necessary to analyze the whiteness and color deviation of paper with reference to the original when printing, and take corresponding measures to correct it. Opacity refers to the opacity of paper, which is another important optical performance index of printing paper. The opacity of paper is very important for printing paper. Only when the paper has a certain opacity can it prevent the paper from appearing through printing and ensure the printing quality. Opacity is an important physical characteristic of printing paper, securities paper and book paper. For writing paper, it is also required to have a certain opacity to facilitate writing on both sides of the paper. For some special paper (such as opaque glossy paper), opacity is also very important. Summary In the printing process, there are many paper characteristics indexes that affect the printing performance of paper, such as sizing degree, basis weight, thickness, bursting resistance, tearing degree, folding endurance, color, gloss, transparency, longitudinal and transverse direction, elongation, Z-direction elasticity, tensile strength, etc., which are not suitable for detailed description due to space limitations. In a word, the quality of paper directly affects the quality of printed matter. Because of the different characteristics of the paper used, its printability is obviously different. Before paper printing, it is necessary to test and analyze the relevant characteristics of paper in order to better adapt to printing and avoid affecting the quality of printed matter because of the quality of printed paper. Reference: Chen. Influence of paper evenness on printing performance [J]. International Paper Making, 2004,23 (5): 31-35. [2] Zhai Ru. On the influence of paper characteristics on printing quality [J]. Printing today, 2004. Analysis of the influence of paper properties on the quality of printed matter [J]. Shanghai Paper, 36 (5): 22-27. [4] Liao Zhihao, Li Zhigao. Influence of paper properties on printing quality [J]. Paper Science and Technology, 2003,22 (3): 47-51. [(10): 54-55.[6] Liu. Discussion on improving the printing quality of paper [J]. Tianjin Paper Industry, 1998, (4): 3 1-34. [7] Liu Keqian. Performance requirements of printing paper [J]..( 1): 1-2.[8] Gu Jianmin. Understanding the influence of paper on printing [J]. Guangdong Printing, 2004, (5): 37. [9] Kang Qilai. The Influence of Paper Properties on Printing Quality [J]. China Printing, 20065438. (1 1): 79-80. Qi Cheng. The influence of paper quality on printed matter quality [J]. Printing Today, 2003, (4): 63-65. Ink drying and its influence on printing quality are called ink drying. Short drying time, fast ink drying speed, long drying time and slow ink drying speed. The drying speed of ink has a great influence on the quality of printed matter. If drying is too fast, the ink will crust and paste on the surface of the printing plate, which will make the printed ink accumulate, have poor gloss and appear "ink spots"; If drying is too slow, the back of printed matter may be sticky and transfer, which is not conducive to the next process. The drying of ink is a complicated process, involving many aspects of knowledge such as physics, chemistry, fluid mechanics and surface treatment. At the same time, it is necessary to discuss it here because of the influence of substrate, ink composition, printing conditions, printing methods, printing machinery, printing environment and other factors. The key point is to apply the knowledge of physics and chemistry to better control the drying performance of ink from the aspects of ink composition, substrate, printing environment and printing conditions.

First, the drying methods of ink can be roughly divided into three types: osmotic drying, volatile drying and oxidation conjunctival drying. Generally speaking, the drying of ink is mainly based on a certain drying method, supplemented by other drying forms. Osmotic drying type means that ink permeates the substrate, and the substrate also absorbs the connecting substances in the ink. Generally speaking, paper is a porous material interwoven with fibers, and its pore size is very small. When the ink is transferred to the paper, it begins to absorb the adhesive in the ink due to the capillary action of the fiber gap in the paper. With the decrease of binder, the liquid content also decreases, the cohesion between pigment particles gradually increases, and the ink gradually loses its fluid properties and presents a solid form. It can be seen that the process of paper absorbing the adhesive in ink (that is, the process of ink infiltrating into paper) is the drying process of ink on paper. On the other hand, there are tiny gaps between ink particles, which constitute countless capillaries. When the ink penetrates into the paper, the capillary action between the ink pigment particles prevents the penetration. When the capillary action between paper fibers and pigment particles reaches a balance, the adhesive in the ink stops penetrating into the paper. Volatilization drying refers to the process that solvent molecules with high energy in ink overcome the mutual attraction between molecules in ink and escape from the ink surface to the air, so that the residual resin and pigment form a solid film and are fixed on the substrate surface. This drying form mainly exists in solvent-based gravure liquid ink. Dry-type oxidized conjunctiva refers to the process that ink with dry vegetable oil as binder absorbs oxygen in the air, and then undergoes oxidative polymerization to form a network structure and dry-fix it on the surface of the substrate. This dry form can form high-gloss, friction-resistant and firm ink films, such as offset printing ink, iron printing ink, screen printing ink and so on.

Secondly, from the substrate to see the drying performance of ink. As for the influence of substrate on the drying performance of ink, as mentioned above, we should focus on paper, because paper has a great influence on the drying performance of osmotic drying ink. The other drying methods of ink, the substrate itself does not absorb ink or the absorption amount is very small, which can be ignored. Generally speaking, paper with rough surface and loose structure is beneficial to ink drying; At the same time, the affinity of paper for ink also affects the penetration. The greater the affinity of paper for ink (the smaller the contact angle), the faster the seepage velocity of ink, and obviously the faster the drying speed of ink at this time. The pH of paper also has a great influence on the drying of ink. Acidic paper will hinder the drying of ink, while neutral or alkaline paper is beneficial to the drying of ink. It is generally believed that the inorganic acid existing on the surface of paper reacts with the drier in ink through the medium of water or steam, thus weakening the activity of the drier. The test data in table 1 can strengthen the understanding in this respect. It is worth mentioning that the water content of paper has a certain influence on the penetration and oxidation drying of ink. Assuming that the water content of paper is high, the fiber or coating on the paper surface is in a loose state, the attraction between molecules is reduced, and the pores are blocked by water film, which not only reduces the absorption of ink binder, but also reduces the absorption of oxygen, which obviously slows down the drying speed of ink. Third, from the composition of ink to see the drying performance of ink, we know that ink is mainly composed of the following four parts: binder (or resin), solvent (or water), pigment and additive. Among them, solvents play a direct and obvious role in the drying performance of ink, and different resins, pigments and additives also have certain effects on the drying performance. As far as a single solvent is concerned, the lower the boiling point, the faster the volatilization speed.

But there are exceptions, for example, the boiling points of ethyl acetate and ethanol are similar, but the evaporation rate of ethyl acetate is almost twice that of ethanol. The boiling point of water is not high, but the evaporation rate is very slow. Generally, medium, low and high solvents are used together to meet the drying requirements of ink. The evaporation rate of mixed solvents is restricted by various solvent molecules, which is not a simple arithmetic average. Table 2 lists the boiling points and volatilities of different solvents for reference. After the resin is dissolved in the organic solvent, the evaporation rate of the solvent decreases, and different resins slow down the evaporation rate of the solvent to varying degrees. The greater the solubility of the resin, the more difficult it is for the solvent to escape from it, and the lower the volatilization speed, that is, the worse the resin releases the solvent. When preparing volatile dry ink, the selected resin should have a certain release to the solvent, otherwise poor solvent extraction will easily lead to dirty back. However, in oxidized conjunctival drying ink, the more double bonds in unsaturated fatty acid molecules, that is, the higher the unsaturation, the slower the drying speed. For water-based inks, the higher the content of emulsion resin, the faster the drying speed. When pigment is added to make ink, the volatilization speed is greatly reduced. The smaller the pigment particles, the larger the surface area and the faster the drying, and vice versa. Moreover, the greater the proportion of pigment in ink, the lower the evaporation rate of solvent and the slower the drying. At the same time, different kinds of pigments have different dissolution properties, such as black and blue, which have poor dissolution performance and slow volatilization speed. Table 3 lists the influence of pigment particle size on solvent evaporation rate. Except for the special drier, other auxiliaries have little influence on the drying performance of ink because of their small dosage and different functions. Fourthly, the influence of printing environment and printing conditions on ink drying. The increase of printing environment temperature is beneficial to the drying of ink on printed matter. This can be explained by the increase of temperature and the acceleration of molecular movement of substances, thus promoting penetration, volatilization and oxidative polymerization. Similarly, the increase of printing drying temperature is also beneficial to ink drying. At the same time, slowing down the printing speed can make the drying sufficient. The humidity of printing environment also has a great influence on ink drying. The higher the relative humidity, the slower the drying speed of ink. For example, under normal conditions, when the air relative humidity is 65%, the ink drying time is 6.7 hours. When the humidity increases to 75%, it takes 23.3 hours for the same ink to dry completely, which delays the drying time of ink by more than 3 times. V. Conclusion To sum up, the drying of ink on the substrate is a complicated problem. To obtain a suitable drying speed, ink manufacturers and printing manufacturers need to make unremitting efforts.