Performance and use of reel newsprint
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Reel newsprint is a special paper for newspaper printing, commonly known as white paper. It is mainly used for newspapers, periodicals and large-scale printing of general-purpose prints in letterpress printing. Because it is re-rolled into a cylindrical shape after being copied, it is mostly used in rotary offset printing machines, which greatly improves the printing efficiency.
The composition of the reel newsprint is mainly about mechanical wood pulp, about 90%, and about 10% of bleached chemical wood pulp, but in the case of insufficient wood pulp raw materials, supplemented with a certain proportion of mechanical straw pulp, Chemical grass pulp, chemical pulp, etc. are used instead of mechanical wood pulp. Due to the large amount of mechanical wood pulp used in newsprint, the content of impurities is large, the pulp contains a lot of short fibers, and many other unfibrated impurity spots are bonded to the fibers, making the news paper appear to be low in whiteness. With the improvement of modern papermaking technology, waste paper deinking is used to recycle scraps, and the chemicals such as inks in paper prints are decomposed and eliminated by high-tech technology, and the secondary newsprint pulp is restored. These pulps can be added again. The copying of newsprint can reduce the cost of raw materials and reuse paper materials. The production of newsprint is made by pressing and multi-drying cylinders, which are pressed by ordinary calenders. Due to the papermaking process of the reel newsprint and the characteristics of the original excipients, it is determined that the finished product and its use have unique requirements.
One. The structure and composition of newsprint
Newsprint is a blend of plant fibers, fillers, compounds and colorants. The plant fiber is composed of cellulose, hemicellulose, lignin, etc., and the paper structure is a porous pore structure. The pore structure is the basis of the paper. Add between paper fibers
Filling in the filler changes the physical properties of the paper.
Cellulose is a polysaccharide natural polymer compound composed of three elements of carbon, hydrogen and oxygen. Cellulose is a typical hydrophilic substance with stable chemical properties. It is insoluble in water and neutral inorganic solvents.
Hemicellulose is a carbohydrate composed of some monosaccharides that are accompanied by the presence of cellulose in the cell walls of plants. It has a branched structure, a small degree of polymerization, good hydrophilicity and adsorption, it is insoluble in water, but it is very easy to absorb water and swell.
Lignin is a compound composed of phenylpropane monomer which has aromatic characteristics. Its chemical nature is very unstable. Under the action of sunlight and air, the paper texture will be brittle, the mechanical strength will decrease, the paper color will be darkened, and the lignin will be yellowish brown.
The whiteness of the paper, etc.
In the paper fiber structure, due to the action of hydroxyl groups and the overlap between fibers, interlaced into a network of pores, making paper a strong hydrophilic substance, which only adsorbs water and is insoluble in water, and through the fiber The hydrolytic effect produces hydrogen bond association, which enhances the physical strength between the papers.
Adding filler to paper means adding a certain amount of finely dispersed minerals to the paper to change the opacity, whiteness and smoothness of the paper surface.
Applying a compound means adding a certain amount of rubber material to the paper to make the paper have certain water resistance and water resistance, hinder the penetration of moisture, and improve the surface strength of the paper. The sizing is divided into two types: internal sizing and paper sizing.
The colorant refers to the material added to the bleaching of the pulp. In newsprint benzene, the blue-purple material is often added to the bleached pulp.
Two. Web characteristics
Newsprint is also a printing paper. Therefore, it first has the commonality of paper. At the same time, it is mainly used for newspapers and other printing crystals with a short period of time, so it has its own personality.
1. Strong ink absorption. Since the reel newsprint continuously prints ink under the high-speed operation of the rotary machine, and the ink adhesion effect is good in a short time, the ink absorption property is strong. In general reel newsprint, its best ink absorbing effect is 1/3 of the thickness of the paper. In addition, the news paper is very particular about the control of raw wood, chemical wood pulp and accessories during the papermaking. The proportion of raw materials directly affects the ink absorption of newsprint.
2. The paper is soft, has a high void ratio, and has good compressibility and elasticity. Since newsprint paper is used in the production of a large amount of mechanical wood pulp and straw pulp, the content of impurities is large, so that the paper fibers are soft, the fibers are filled with fillers, and a lot of paper ash is often present during printing. The water resistance is poor, the filler is pressed by the ordinary calender in the fiber gap, and the surface void is relatively large, and at the same time, in order to satisfy the better ink absorption, the news paper has compressibility and elasticity, and the paper is under high speed operation. After the paper is pressed by the blanket, the better compressibility and elasticity can solve the fast absorption of the ink, and achieve good copying and transfer of the graphic and text.
3. The whiteness of the paper is low, the opacity is high, the fiber interlacing strength is low, and the mechanical strength of the paper is not high. Newsprint contains a large amount of wood pulp fiber, wood pulp contains a lot of lignin, lignin is very easy to oxidize and discolor in the air, generally good newsprint, its whiteness is about 60 degrees. The opacity depends on the light scattering ability of the paper, the amount of light scattering interface inside the paper, and the difference in refractive index of the scattering interface. The more the scattering interface, the larger the refractive index of the scattering interface, the higher the opacity. In the process of papermaking, because of the addition of fillers, the gap between the fibers is filled with the filler to increase the scattering interface of the light, and the scattering rate between the filler and the air is large, thereby increasing the opacity of the newsprint. Newsprint uses a large amount of wood pulp. In the process of pulping, the fiber is short and thick, and the content of impurities is large. The interweaving and bonding force between the fibers will inevitably decrease, and the mechanical strength will decrease accordingly.
Three, the nature of the reel news paper
The nature of reel newsprint generally refers to its suitability for printing, including some of its performance characteristics during transportation and storage. The printability refers to the comprehensive physical and chemical reaction of copying a certain quality of printed matter under the necessary conditions. The nature of the reel newsprint must reflect the best print quality and color effects. During the operation of the drum, the ink adsorbed on the printing plate can be smoothly transferred to the surface to obtain a complete, full, clear and rich graphic imprint.
The printability of the roll newsprint includes the pore structure and ink absorbency of the paper, the smoothness and compressibility of the paper, the surface strength of the paper, the acidity and alkalinity of the paper, and the moisture content and stretchability of the paper.
1. The pore structure and ink absorption of paper. Paper is a kind of network structure in which various plant fibers are intertwined. In this network structure, some fillers, rubber materials and color materials are added to the pores between the fibers to satisfy the paper itself. Has the characteristics. Adding filler is to improve the whiteness, smoothness and opacity of the paper; adding the rubber is to make the paper have certain water resistance, not easily infiltrated by water or other liquid; adding the coloring material to make the paper have higher whiteness. The paper's L-gap structure is the basis for the paper to absorb ink, and is one of the key factors for the penetration and drying of newspaper printing crystals. Therefore, to understand the ink absorption of paper, you must first understand the pore structure of the paper. Because the pore structure of the paper is closely related to the amount of ink penetration, penetration speed, printing smoothness, penetration and solidification of the ink, and print quality.
The pore structure of the paper mainly includes the void ratio of the paper. The void ratio of the paper indicates the ratio of the volume of air in the paper to the volume of the paper. The void ratio of the paper can be obtained by calculation and experimental measurement.
Paper void ratio = volume of air in the paper (unit area) / volume of paper (unit area) x 100%
It can be seen from the above equation that the larger the volume of air in the paper, the higher the void ratio of the paper, which also confirms the condition in the pore structure of the paper. The less the paper fibers, the looser the interlacing, the more the interfiber gap Large, the poorer the paper pore structure. On the contrary, the more paper fibers, the denser the interlacing, the smaller the interfiber space, and the better the paper pore structure.
The solidification and drying of the ink on the paper is accelerated as the void ratio increases, and the ink absorbency also increases, but the ink gloss on the paper surface decreases as the void ratio increases.
The paper has a pore structure, and the porous structure of the paper determines the paper's breathability, hygroscopicity, absorbency and compressibility, which is the basis for the paper to absorb water, ink and the like. In order to ensure better printing quality and printing effect, printing paper is required to have better ink absorbing properties. The ink absorbing property of the paper refers to the property that the ink penetrates into the paper and is absorbed after the paper comes into contact with the ink and the ink is transferred onto the surface of the paper. The larger the paper void ratio, the looser the structure, the more ink it absorbs, and vice versa. This is why we often say that the paper absorbs poorly.
2. Paper smoothness and compressibility. The smoothness of the paper refers to the degree to which the surface of the paper is even and smooth. The smoothness of the paper depends on the processing method used during the processing of the paper, the distribution of the fibers in the paper, and the application of the filler.
Paper smoothness includes paper smoothness and print smoothness. The smoothness of the paper surface refers to the time required for a certain volume of air to pass through the gap between the paper surface and the glass under a certain pressure under a certain degree of vacuum, expressed in seconds. Paper smoothness is generally concentrated between 50 and 100 seconds, and as the smoothness increases, the printed solid density increases significantly. Printing smoothness refers to the ratio of the surface of the paper to the surface of the smoothing cylinder at a pressure equivalent to the printing pressure and printing speed. The measurement of printing smoothness requires special advanced instruments, and the actual operability is not strong. Currently, paper smoothness is generally used to indicate the smoothness of the paper.
The reproduction of printed images is expressed by dots and layers. The carrier of the graphic is a blanket. The contact between the blanket and the paper directly affects the transfer of the graphic dots. Under the same pressure and blanket, the smoothness of the paper. It is the precondition of the transfer of graphics and texts. The smoother the paper, the more the paper dots are attached to the paper surface, the closer the ink speed of the paper surface absorption dots is, the closer the image dot size and the ink absorption are, the overall reduction of the image and text. More realistic. On the contrary, the picture restoration is even worse.
The influence of the compressibility of the paper on the printing is reflected in the thickness of the paper during the imprinting, and the surface of the paper is flattened, which facilitates the good contact between the surface of the paper and the surface of the blanket, and provides better conditions for the transfer of the ink. . Because newsprint is flexible and easily compressed under the influence of printing pressure, the more complete contact between the paper surface and the surface of the blanket is ensured. If printing on a firm paper surface, due to the thickness of the paper itself and the unevenness of the surface, when the blanket is in contact with the paper surface, the force on the various parts of the paper is uneven, and the blanket is transferred to the surface of the paper. There are distortions such as blurring or dot deformation and loss.
3. The surface strength of the paper. The surface strength of the paper is a property of the bonding strength between the fiber, the filler and the rubber on the surface of the paper. This property has a direct impact on the physical properties of the paper surface. The fiber pore structure of newsprint paper is relatively inferior, especially some short wood pulp fibers and impurities affect the pore structure of the fiber. In the papermaking, in order to increase the whiteness, opacity and smoothness of the newsprint, increase the water resistance of the paper. , a certain amount of fillers and compounds are added more or less. The amount of filler and rubber added directly affects the surface strength of the paper. In addition, the calendering of the calender directly affects the surface strength of the paper. In the actual printing process, the ability of the paper to resist powder removal and pulling is collectively referred to as the surface strength of the paper.
At present, the measurement of the surface strength of the paper has been expressed by the value of the pulling speed. The so-called paper pulling speed refers to the printing speed of the paper when the outward tension of the ink on the paper surface is greater than the bonding force of the paper as the printing speed increases, causing the paper surface to start to be powdered. It is the pulling speed of the paper. From this value of the pulling speed, the correlation of the surface strength of the paper surface can be known. Under the conditions of constant printing pressure and constant ink, the faster the printing speed, the greater the reverse adhesion of the ink to the paper, and the higher the pulling speed of the paper. In order to improve the surface strength of the paper, in addition to increasing the bonding strength of the paper surface, reducing the viscosity of the ink is also an effective method.
4. The acidity and alkalinity of the paper. Any paper itself has a certain pH. The pH is usually expressed in terms of pH.
The acidity and alkalinity of the paper itself is because the paper is in the process of making paper. Because of the different pulping methods, some use acid pulping, some use alkali pulping, and then add the coating ingredients, and finally make the paper finished. It has a certain acidity and alkalinity. The acidity and alkalinity of the paper itself has an effect on the drying speed of the ink during printing, especially the oxidized conjunctiva type drying ink, because the drying method is completed by the oxidation form, and the acidity and alkalinity of the paper itself is specified within a certain pH range. The strength of the acidity and alkalinity of the paper directly affects the degree of drying of the ink. In addition, the printing ink adheres to the paper surface, and the vividness of the ink color is also affected by the acidity of the paper.
The acidity and alkalinity of paper have certain control requirements, as well as newsprint. In the printing process of newspapers, it is generally used to add a weak acid wetting powder to the water to form a dampening solution, so that the pH of the dampening solution is controlled between 5-6, which facilitates the formation of ink and water balance on the printing plate, and the acidity and alkali of the paper Sex is best controlled between weak acid and weak base, so as not to destroy the acid-base balance on the printing plate.
5. The moisture content and stretchability of the paper. The water content of the paper is a physical quantity indicating the ratio of the weight of the paper reduced to the weight of the base paper when the paper is dried to a constant value at 100 ° C to 105 ° C, expressed as a percentage. In fact, the paper has always contained moisture after being copied into the product. It is always in a balance, and its moisture content changes with the change of the ambient temperature and humidity. The paper is placed in the air under a certain temperature and humidity, and it is in the fiber itself. The water content and the water content in the air have been undergoing dynamic moisture exchange, maintaining a relative balance at all times.
The water content of the paper is inversely proportional to the temperature, that is, the water content of the paper is increased when the temperature is raised. On the contrary, the temperature is lowered, the water content of the paper is increased, and the water content of the paper is changed by +5 ° C, and the water content of the paper is changed by 0.15%, which is inversely proportional. alternative relation. Therefore, the temperature control in the printing workshop should preferably not exceed ±3 degrees C. During the overprinting process, the water content of the paper does not change by more than ±0.1%, otherwise the accuracy of overprinting will be affected.
Relative humidity has an effect on the water content of the paper. Due to the change of climate, the relative humidity in the air is not fixed. The paper absorbs water and drains with the change of humidity, so the paper has different water content under different humidity. The relative humidity in the air has a certain relationship with the change in the moisture content of the paper. The water content of the paper increases as the relative humidity of the air increases, and conversely, decreases as the relative humidity decreases. When the relative humidity in the air does not exceed 80%, the relative humidity changes by 100%, and the water content of the paper has a corresponding change of 1%. However, when the relative humidity exceeds 80%, the amount of change in the water content increases. Generally increased to 1.5% to 2%.
The stretchability of the paper is directly related to the moisture content of the paper. Due to the uneven water content, the paper fibers are often expanded and stretched, which causes the paper size to increase or decrease, resulting in inaccurate overprinting.
Paper stretchability refers to the relative change of the paper size after the paper is immersed in a certain temperature and air-dried and then air-dried. The ratio of the increase or decrease of the paper size to the size of the base paper is expressed as a percentage.
The paper fiber swells after it absorbs water, and the expansion of the fiber diameter is much larger than the longitudinal expansion, usually 2-8 times. The expansion and contraction of the paper causes many quality failures in printing. For example, the water content on both sides of the paper is higher than the middle, and the two sides will form a "crease", and along the circumferential direction will form a "pleat" of the tight edge, which will produce wrinkles and creases once printed on the machine. In severe cases, it will affect the normal operation of the tape, and it will tear directly. When it comes to the color printing, its color overprinting will not be possible. Therefore, we must carefully grasp the change law of paper water content and strictly control the stretching and deformation of paper. One of the raw materials, the quality of paper storage will directly affect the quality and use of the paper. At the time of purchase and storage, it is necessary to pay attention to the inspection and acceptance of the roll paper. In particular, the outer packaging should be strictly required. The storage site is very particular. In addition to ventilation, moisture and light, it should also be protected against fire, electricity and water. And other security risks. In the management of newsprint, the following management requirements should be noted:
1. Storage: The stacking of the web should not be too high. If it is conditional, it can be put up and set up to ensure the roundness of the rolled paper, and at the same time prevent the paper from being corrugated and the paper core tube deformed.
2. Moisture-proof: The paper has strong hygroscopicity and is sensitive to the influence of temperature and humidity on the environment. The paper is easily deformed after moisture absorption, which causes great trouble for use. In particular, if the wrapping paper is removed or the wrapping paper is partially broken, etc., attention should be paid to its moisture-proof requirements. The storage location should be ventilated, the ground should be protected from moisture, and the walls should be dry. It is best to make a professional paper warehouse for long-term mobile storage and use of newsprint. The relative humidity of the storage environment is preferably controlled at 50-55%.
3. Sun protection: The place where the roll paper is stored should be sun-protected to prevent the paper roll from being exposed to sunlight or exposed to sunlight. Because the paper itself has a certain amount of moisture, the moisture in the roll paper package is relatively balanced. Once exposed, the paper roll warms up, the paper water balance is destroyed, the paper will release water to the air, the paper shrinks and the paper fiber becomes brittle. Loss of softness and normal printing.
4. Heat protection: In the environment where the paper is placed, the surface temperature is not as sensitive as humidity, and the water content changes rapidly. However, when the temperature exceeds certain requirements, the mechanical strength will decrease significantly. The storage temperature is generally controlled at 18 °C to 25 °C.
5. Transportation, loading and unloading: Due to the large volume and large volume of the paper, the transportation process must pay attention to its packaging requirements, and at the same time achieve the above several prevention requirements. Especially in the case of rain, it is necessary to prevent rain and moisture, otherwise, the paper surface I will be directly threatened by quality. When loading and unloading, all use professional forklift or car, the only requirement is not to damage the outer packaging of the roll paper during the loading and unloading process, otherwise it will cause the loss of the roll paper and threaten the storage and use of the roll paper.
6. Use: Newsprint should strictly control the temperature and humidity of the printing workshop when using it to avoid the moisture absorption of the paper. The tensile strength of the paper will decrease after moisture absorption, and the paper breakage will occur during the printing process. In addition, changes in temperature and humidity can cause the paper to stretch and affect the accuracy of printing. The ink of the newsprint is very strong. After printing, the ink can be quickly absorbed and cured, which is suitable for high-speed printing. In printing, the requirements for the permeability and viscosity of the ink are required to suit the characteristics of the newsprint. The newsprint contains a large amount of wood. It is easy to oxidize and turn yellow and become brittle after seeing light. Therefore, it is not suitable for long-term storage, and it is not suitable for printing long-term publications.
As a kind of commonly used printing paper, as long as we have a good grasp of its characteristics and performance, we can control its printing suitability and properly store and use it, we will be able to maximize its function and increase its printing speed. And quality, bringing greater economic benefits.