Basic knowledge of paper
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Paper is one of the four inventions in ancient China. It is a sheet-like fiber product used for writing, printing, painting or packaging. It is usually composed of an aqueous suspension of pulp-treated plant fibers, which are interlaced on the net and initially dehydrated. It is pressed and dried.
1. The composition of the paper
The paper is composed of plant fibers, fillers, rubber compounds, colorants and the like.
(1) Plant fiber
Plant fiber is a basic component of paper. Plant fiber as a raw material for papermaking must have easy dissociation during pulping, high cellulose content in plant fiber, low wood cord content; desirable strength, length and width; Flexibility and interweaving ability; rich source and low cost, adapt to mass production and other conditions.
The papermaking plant fibers commonly used in China are: straw, wheat straw, reed, bamboo, wood, hemp, cotton, etc. Waste cotton, waste cloth, waste linen, waste paper, etc. are also the main raw materials for papermaking.
In the process of making pulp, the plant fiber manager processes and removes other components such as lignin, pectin, resin, fat, etc. contained in the plant fiber, and only retains active ingredients such as cellulose and hemicellulose.
The nature of the paper, under certain conditions, depends on the physicochemical properties of the plant fiber chosen, as well as the pulping process.
(2) Filler
Paper made of interwoven fibers has many voids. It is necessary to add filler to fill, increase flexibility, reduce paper transparency and flexibility, and make the surface uniform and adapt to the requirements of use.
Commonly used fillers include kaolin, talc, gypsum powder, carbonic acid and barium sulfate. Generally, talc is used for printing paper, and kaolin and barium sulfate are used for advanced printing paper.
The amount of filler generally accounts for about 20%. Excessive fillers will affect the quality of the paper, reduce the tensile strength and toughness, hinder the absorption of the ink, and easily fall off the powder during printing.
(3) rubber compound
The rubber compound is added to fill the gap on the surface of the paper, reduce the capillary action in the paper, improve the water resistance of the paper, and improve the gloss, strength and paper fluffing of the paper after sizing.
Commonly used rubber compounds are rosin, aluminum sulfate, alum, starch, water glass, cheese acid and the like.
According to the different use requirements of various papers, there are various sizing methods, such as paper sizing, surface sizing, heavy sizing and light sizing. The amount of sizing varies greatly, ranging from 0.25 to 9% of the weight of the slurry. Excessive sizing also affects the ink absorbing performance of the paper.
(4) Colorant
Plant fiber has a certain color. It is still not pure white after bleaching, but it is slightly yellowish or light green. It can't meet the requirements of making white paper. Therefore, it is necessary to add coloring material for toning and whitening.
The commonly used color materials for making white paper are blue, ultramarine, etc., and high-grade paper should be added with a certain fluorescent whitening agent.
In the production of colored paper, it is also necessary to use a coloring material, and most of them use an inorganic pigment or an organic dye.
2. Types of paper
There are many types of paper, and our Ministry of Light Industry is divided into 17 categories according to the use of various papers (paper 11 and cardboard 6).
The paper 11 categories are: printing paper, writing paper, drawing, drawing paper, electric insulating paper, cigarette paper, paper suction, meter paper, photosensitive paper, transfer paper (base paper), industrial technical paper, wrapping paper.
The 6 types of cardboard are: binding cardboard, box cardboard, insulating cardboard, industrial technical cardboard, architectural cardboard, and shoeboard.
3. Paper specifications
Paper specifications are generally calibrated by type, size and weight.
(1) Type: The type of paper is divided into flat paper and web in printing paper. Web is used in high-speed rotary presses, and most of the printing uses flat paper.
(2) Dimensions: According to the original national standard GB147-59, the size of the base paper for printing, writing and drawing is: the width of the original paper of the reel is set to 1575 (ie 2×787), 1092, 880, 787 mm; flat base paper The dimensions are 880×1230, 850×1168, 880×1092, 787×1092, 787×960 and 690×960 mm.
The national standard GB786-87 stipulates that the standard of book and magazine and its size will be printed on 880×l230, 900×280 and 1000×1400 mm uncut single paper size to adapt to international cultural exchange. Adopting international standards, and due to equipment, paper supply, etc., the original 787×1092 mm paper was used as a non-standard copy, ie paper production, which can still be used during the transition period (to AD 2000). Dimensional specifications will transition to new national standards.
(3) Weight: The weight of the paper is expressed by theorem and weight. Usually expressed in quantitative terms.
Quantitative weight, also known as gram weight, is the specified weight per square meter of paper. The standard is expressed in grams, ie grams per square meter (g/m2), such as: 60 g/m2, .... 200 g/m2 or less (including 200 g/m2) is called paper, and 200 g/m2 or more is called cardboard.
Let the weight represent the total weight of 500 sheets of paper. For example, the weight of 52 grams of paper of 880 × l230 mm is more than 28 kg.
4. Printing paper
In the printing paper category, there are papers with different properties and characteristics, such as newsprint, letterpress printing paper, offset printing paper, offset printing coated paper, dictionary paper, map paper, sea drawings, gravure printing paper, weekly newspaper, Draw newspapers, white paper, written paper, and more.
(1) Newsprint
It is mainly used for printing newspapers and letterpress books. Its paper is soft, it has good elastoplasticity, and its ink absorption performance is strong. The ink can be fixed on the paper surface quickly. The paper surface is pressed by the calender, and the two sides are smooth and difficult. Fleece, both sides of the printing are relatively clear and real, have a certain mechanical strength, can be suitable for high-speed rotary printing, opacity is very good. Since the raw materials used are mainly mechanical wood pulp, containing essential substances and impurities, the paper should not be stored for a long time, the paper is prone to yellowing and brittleness, the paper is highly absorbent, the water resistance is poor, and the paper is easily damaged.
(2) Letterpress printing paper
Mainly used for printing selection of letterpress printing machines, classic works, general books, textbooks, magazines, etc. Its characteristics are similar to newsprint, the quality is worse than newsprint, the smoothness, water resistance and whiteness of paper are better than newsprint, and the ink absorption ability is not as good as newsprint, but the ink absorption is uniform.
(3) Offset printing paper
Mainly for lithographic color pictorials, albums, posters, color labels and some advanced publications.
There are single-sided offset paper and double-sided offset paper in offset paper. Single-sided stencil paper is often used for color posters, cigarette cases, trademarks, etc. Double-sided offset paper is used to print pictures, illustrations, maps, etc.
Offset paper should have small stretchability and strong water resistance, so as to prevent paper deformation caused by multi-color overprinting, improper overprinting, and should not be pulled, powdered, and tightly structured to prevent adhesion of ink in multiple printing. Under the action, it causes the phenomenon of pulling and removing powder, so that the imprint has white spots, which affects the quality of the printed product; the ink absorption should not be too high, it can hinder the penetration of the connecting material in the ink, so that the ink is dried on the paper surface to form a film layer, and the printing is ensured. The gloss of the product; the surface is smooth, the imprint is solid, and the paper can not have sand and other debris to prevent the blanket and the plate from being damaged.
(4) Offset printing paper
Also known as coated paper. It is made on the paper surface of the coating base paper, coated with a layer of inorganic paint, and then made by super calendering. The surface of the paper is smooth and the whiteness is high. It is specially used for offset printing of monochromatic or multi-color art pictures, illustrations and pictorials. , albums, commodity trademarks, product samples, calendars, etc.
Coated paper has a high degree of smoothness, and can obtain extremely fine and smooth dots during printing, which can better reproduce the layering and temperament of the original. The paper should not be powdered and layered, and the ink absorption of the paper should not be too fast. If the ink is absorbed too quickly, it will cause the print to be dull, and in severe cases, the print will be powdered.
(5) Dictionary paper
Mainly for letterpress printing dictionary, pocket manual, reference books, scientific and technical materials, high-grade printed matter, etc., quantitative and light, there are four kinds of 25 ~ 40 g / square meter.
The dictionary paper is light and booked, requiring good opacity (preventing print-through), uniform fiber structure, flat paper surface, and uniform thickness. The dictionary paper is soft and the paper edge is easy to curl.
(6) Map paper, sea drawings
The map paper is suitable for offset multi-color topographic maps, maps and atlases. It is divided into two types: special number and number one. The special number is used to print topographic maps, and the first number is used to print maps and atlases. Sea drawings are suitable for paper on offset multicolor charts.
The map paper and the sea drawing are white and dusty, especially the black dust with a length of more than 1 mm (avoiding confusion with the map symbol), the two sides are smooth, the structure is tight and tough, the flexibility is small, the elasticity is flexible, and the precision can be printed. The graphics, the folding resistance is much higher than the offset paper, and the cracking length of the sea drawings is also very high.
(7) Gravure printing paper
It is mainly used in papers for monochrome and color gravure printing, art pictures, illustrations, etc.
Gravure paper requires paper white and firm, has good smoothness and water resistance, can not have obvious powder drop during printing, and has a print-through phenomenon.
The original gravure printing paper includes printing paper, stamps and other high-quality and not easy to copy printed paper, and has separately separated banknote paper, stamp paper and the like.
(8) Whiteboard paper
It can be used for printing various kinds of goods packaging cartons and commodity lining papers, and can also print various educational pictures.
The white paper has a relatively uniform fiber structure, the surface layer has filler and rubber composition, and the surface is coated with a layer of paint. After multi-roll calendering, the paperboard texture is tight, the paper surface is generally whiter and smoother, and has a more uniform ink absorption. The surface has less powder and hair loss, and the paper has better folding resistance.
(9) Synthetic paper
Mainly used for printing high-quality art, maps and dictionary books.
Synthetic paper is made of chemical raw materials. It is made from hydrocarbons as the main raw material and added with some additives. It has soft texture, strong pulling force, good water resistance, light and heat resistance, no mold and good stability. And other characteristics, and resistant to chemical corrosion. It can be used as various printing papers in the temperature range of -60 to 60 °C.
Synthetic paper is not only suitable for printing, but also because it is non-toxic, non-polluting and breathable, it is also an ideal packaging material. It is clean and dust-free, and it does not drop paper powder. It is an ideal paper for information industry. It is now replacing plain paper into office paper and computer paper for ultra-clean indoors.
5. Paper printing performance
The printing performance of the paper determines whether it can be printed smoothly during the printing process, and whether high-quality printed matter can be obtained. Of course, it is closely related to printing conditions and ink performance in printing.
The quality of printing paper is generally as follows: the paper color should be as white as possible, and each paper in the same batch of paper should have the same texture, the dust of the paper should not exceed the allowable range; the minimum light transmittance and the same gloss; The mechanical strength, paper thickness, tightness, structure and other properties should be the same in the same batch, if the difference is very large. It will increase the difficulty in the printing process, reduce the quality of the printed matter, the water content is between 6 and 8%, the edge of the flat paper should be a right angle, and the slope error should not exceed ±3 mm.
The printing performance of paper mainly includes:
(1) Smoothness
Smoothness is the most important printing performance of paper. No matter which type of printing, the printed matter with smooth surface paper, the outline of the writing and the image are clear, because the smooth paper can be embossed. Better contact with the plate surface, so that the ink layer on the printing plate can be uniformly transferred to the paper surface; paper with low smoothness, due to uneven surface during printing, contact with the printing plate surface during imprinting It must be uneven, and the transfer of the ink layer is affected, so that the ink layer is transferred to the paper surface in an uneven state. In lithographic printing, the use of a flexible blanket for indirect transfer of the ink layer is not very demanding on smoothness, but in the case of printing advanced products, in order to ensure that the reproduction of the layer is not lost, the smoothness is also selected. High paper to print.
Paper with a low smoothness can appropriately compensate for the phenomenon that the print is faint due to unevenness when the printing pressure is appropriately increased during printing.
The sheen of the paper also depends on the smoothness of the paper surface, the paper with a very smooth surface is shiny, and the paper with a rough surface is dim.
(2) Paper absorbency
The ink absorbency of the paper is the extent to which the paper absorbs the ink. The more the paper absorbs the ink, the ink absorption of the paper is strong, and vice versa, the ink absorption is mainly reflected by the penetration of the binder in the ink.
The absorption of ink by paper mainly depends on the size of the gap between the paper fibers, that is, the tightness of the paper. When the gap between the paper fibers is small, the amount of tweeted fibers is also small, which affects the capillary action of the paper. Ink absorption is poor. If the gap is too large, not only the absorption of the binder is increased, but also the pigment is absorbed together, resulting in a print-through phenomenon.
The ink absorbency of the paper is related to the structure of the paper itself. It is also related to the viscosity of the ink, and is related to the printing pressure and the length of the imprinting time.
(3) Paper elasticity and plasticity
The paper undergoes various deformations during the printing process. For example, in letterpress printing, the printing plate is pressed against the paper to make the paper deform and not flat.
Under the action of the machine, due to the different pressures, the paper will produce sensitive elastic deformation, elastic deformation and plastic deformation. Under the action of external force, the paper will instantly change its shape and size. When the external force stops, the paper will immediately return to its original shape and size, which is called sensitive elastic deformation; the paper is affected by external force and changes within a certain time interval. Your own shape and size. When the external force stops, the paper gradually returns to its original shape and size, which is called hysteretic deformation. When the external force is canceled, the paper still maintains the deformation caused by the external force, which is called plastic deformation. Sensitive elastic deformation and hysteretic deformation are reversible deformations, and plastic deformation is irreversible deformation.
The deformation of the paper depends on the humidity, calendering and compaction of the paper itself. The fibers are elastic and the sensitivity of the fibers is advantageous for the printing process.
(4) Surface strength of paper
The surface strength of the paper is particularly important for the wear resistance, powder drop and lint of the surface during printing. To obtain a clear dot, it is necessary to use a higher viscosity ink. If the paper strength is insufficient, it is easy to produce powder. , lint phenomenon, and adhere to the surface of the plate, if the ink viscosity is low, in the lithography, the ink and dampening solution emulsified, the blank part of the plate will be dirty.
(5) Water content of paper
The weight of the water contained in the paper as a percentage of the weight of the paper is called the water content of the paper.
The water content of the paper affects the printing quality. If the water content of the paper is too much, the strength of the paper is reduced. Under the action of external force, the fiber will be pulled out, the plasticity will be enhanced, and the drying speed of the blot will be affected; if the water content of the paper is too small The paper is brittle, which is easy to cause damage and static electricity.
The fiber of the paper is hygroscopic. The temperature and humidity of the surrounding environment determine the water content of the paper. The water content of the paper is balanced with the temperature and humidity of the environment. The humidity changes with temperature. The relative humidity in the air changes by 10%. Change 1%, therefore, to control the water content in the paper, we must control the temperature and humidity of the workshop, the general temperature is controlled at 18 ° ~ 24 ° C, the relative humidity is controlled at 60 ~ 65%, in order to maintain the balance of water content.