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Optical indicators of paper properties include. Geometric properties of paper

Paper during printing experiences a variety of mechanical effects: compression, bending, stretching.

In the process of using a printed product, in addition to mechanical influences, the paper is exposed to light, changing humidity, etc. All these tests the paper must pass without destruction and remain unchanged for a long time without changing its properties.

Paper Properties, which ensure the normal conduct of the technological process (printing, stitching, finishing printed products) are called technological. These include:

Evenness and smoothness of the surface, ensuring the contact of the paper with the printing plate;

Softness, i.e., the ability of paper to smooth out under pressure;

absorbency, which determines the perception and fixation of ink on the print;

Mechanical properties (strength and deformation), due to which the paper withstands various influences in the technological process;

Optical characteristics: whiteness, opacity, gloss, which determine the contrast and correct color reproduction of the image.

are of great practical importance consumer properties, i.e. those that determine the appearance of printed products and ensure their durability. These include:

Light fastness, i.e. stability of paper properties under prolonged exposure to light;

Resilience to change atmospheric conditions (temperature, humidity);

Mechanical and optical properties, which are both technological and consumer.

Paper dimensions

The most important dimensional indicators of paper - thickness and weight of paper with an area of 1 m². The thickness of the paper depends on the amount of paper pulp fed to the paper machine mesh, its concentration and the speed of the mesh. The paper is not uniform in thickness and when measured, an average value is obtained.

Thickness affects many paper properties. As the thickness increases, paper strength, opacity, and compression strain increase.

The thinner the paper, the denser, more compact the book block. The quality of printing is affected by the uniformity of paper thickness in a sheet, roll, bale. Deviations in thickness lead to non-printing on the print.

Paper for printing produced weighing 1 m ² from 30 to 300 g. Material weighing more than 300 g/m ² called cardboard.

Dimensional indicators ( thickness and weight of paper with an area of 1 m²) are the main ones in calculating the required amount of paper for the publication.

paper structure

The most important characteristics of the structure are density and porosity paper.

Density is determined by the ratio of the mass of a sheet of paper to its volume and expressed in. g/cm3.

For various types of paper, the density ranges from 0.5 g/cm3 - for loose, porous and up to 1.2 g/cm3 - for highly compacted types of paper.

The density of paper depends on the type and degree of grinding of the fiber, the amount of filler, the degree of paper calendering, etc.

Porosity(presence of interfiber spaces) is indirectly related to density. The thicker the paper, the lower its porosity.

The high porosity of the paper provides good absorbency and, therefore, affects the speed of fixing the ink, but at the same time, as a result of the strong absorption of the ink, the prints are less contrasting, less saturated.

Thicker, less porous paper achieves better image clarity.

Heterogeneity of paper structure

Paper is a capillary-porous heterogeneous material. The heterogeneity of paper is explained by the multicomponent nature of its composition and the peculiarities of the manufacturing technology. In the process of defibration of wood and grinding of cellulose, fibers of different sizes are obtained. The fibers themselves are also distributed unevenly in the thickness of the sheet, forming more or less dense areas that are clearly visible when the paper is viewed through the light. The distribution of filler particles in the thickness of the sheet is also uneven. The filler content on the mesh side is 15-18% less than on the top side.

The heterogeneity of the paper structure affects many of its properties. Thus, paper has uneven thickness, different smoothness and absorbency on the upper and net sides of the sheet, different strength in the machine and transverse directions, etc. The heterogeneity of paper properties worsens its quality and causes great difficulties in working with it.

Paper surface characteristic

Smoothness- the main property of paper, characterizing its surface. Smooth paper provides full contact with the surface of a rigid printing plate, against which the paper is pressed under a certain pressure. The accuracy of reproduction of image elements depends on the completeness of contact of the paper with the form. Thus, the smoother the surface of the paper, the more it resolution, i.e., the ability to reproduce the smallest details of the image on it, and therefore, the print quality is higher. On a very smooth surface, full coverage of all image elements can be achieved with minimal pressure.

It is impossible to obtain paper with a completely smooth surface. The paper surface is always microroughness, formed during its manufacture as a result of the interlacing of fibers and the presence of filler particles on its surface. Poor grinding and accumulation of fibers, as well as occasional coarse inclusions form macro-roughness. In addition, traces of the net are left on the paper machine wire side of the sheet, which increases the roughness of the wire side of the paper.

The smoothness of the paper increases significantly with the introduction of a filler and especially when applied to its surface during the coating process. a cover pigment layer that covers the irregularities of the base paper. Only on high smooth coated paper small printing elements can be reproduced.

Good print quality can also be achieved on paper that is not very smooth, but under a certain pressure, when the paper is compressed and its surface is leveled during the printing process. Smoothing the surface of the paper at the moment of its contact with the printing form increases the resolution of the paper, improves the fidelity of the original, increases the transfer of ink from the form to the paper.

Mechanical properties of paper

The mechanical properties of paper combine two groups of properties:

Strength properties characterizing the resistance of a material to destruction under mechanical stress,

Deformation properties characterizing the deformation of the material without destruction.

Strength properties

Paper strength, i.e. its resistance to destruction under mechanical stress is an important characteristic that determines the possibility of using paper in printing and other machines and ensures the safety and durability of finished printed products.

For example, due to insufficient strength of the paper, the paper web may break. Thus, insufficient paper strength leads to unproductive downtime of printing machines. In addition, low paper porosity causes the printing speed to slow down.

The strength properties of paper depend on its composition and structure. Under mechanical action on the paper, the destruction of the fibers themselves is possible, but the bonds between them are more likely to be broken. Therefore, the strength of paper is determined not by the strength of the fiber itself, but by the strength of the bonds between the fibers.

Splitting, fibrillation of fibers during their grinding increases the number of bonds between the fibers and, consequently, increases the strength of the paper. The filler particles, located between the fibers, serve as a kind of "spacers" and weaken these bonds. Different sizing agents affect strength differently. Hydrophobic (for example, rosin), form fragile, easily collapsing bonds. Hydrophilic sizing agents, similar in molecular nature to the fiber (for example, starch), increase the strength of the paper. Moistening paper leads to a sharp decrease in its strength.

The strength of paper depends on its structure and therefore is not uniform in different directions of the sheet. The tensile strength in the machine direction can be several times higher than the tensile strength in the transverse direction. The strength depends on the thickness of the paper: when comparing paper of the same composition, but of different thicknesses, the thicker paper is more durable .;

Paper breaking strength plays an important role in the process of using printing products. This indicator is especially important for cartographic, cover, flyleaf, document paper, intended for the manufacture of large-format inserts, tabs, which, when used, are subjected to repeated folds when folded.

tear strength of paper is important for roll paper, especially when printing newspapers on high-speed rotary presses, when the paper web often breaks due to insufficient edge tear resistance.

Paper surface abrasion resistance important when printing on high speed web presses. When rubbing against the metal parts of the paper-carrying system, small fibers and filler particles| can separate from the surface of the paper, forming paper dust, which contaminates the printing form, the inking unit and lowers the quality of printed products.

The abrasion resistance of paper increases with surface sizing.

If the surface of the paper is not strong enough, then "plucking" of the fibers may occur, i.e. destruction of the surface layer of paper when printing with sticky inks. "Plucking" is also observed in coated paper as a result of the breakdown of the cover layer.

Deformation properties

Deformation properties are the most important characteristic of printed paper, which determines its behavior during printing and other technological operations. The deformation properties of paper are manifested at all stages of the technological process; the destruction of the paper occurs after deformation.

On paper, may appear resilient and elastic deformations. Elasticity- the ability to reversible deformations that occur under the action of a load and instantly disappear when the load is removed. Elasticity- is the ability to large reversible deformations under the action of small loads.

In offset printing, the use of stiffer paper is allowed, since the uneven surface of the hard paper comes into good contact with the easily deformed rubber-fabric plate and printing on the hard paper is achieved due to the deformation of the plate. Therefore, offset printing can be done on various hard surfaces: metal, plastic, wood.

In stitching and binding processes, when folding prints, crimping a block, etc., it is necessary that residual deformation appear in the paper, for greater stability of the fold.

Paper absorbency

Paper perception of ink depends on the ability of its surface to be wetted with paint and on the absorbency of the paper. As a rule, all printing inks wet the paper surface well. The practical absorbency of paper depends primarily on its porosity. The greater the porosity of the paper, the more intense the absorption process.

The speed and depth of ink absorption depend on the number and size of the nop, as well as on the composition and properties of the printing ink.

Paper with large pores, such as newsprint, absorbs ink well.

This ensures that it is quickly fixed on paper. However, excessive wicking reduces the strength of prints and may cause ink to bleed through to the back of the paper, i.e. to "piercing" the print.

Obtaining intensive prints on large-pored paper will require a significant increase in the thickness of the ink layer, which will lead to set-off and overspending of the ink.

Paper to moisture ratio. The composition of printed paper includes plant fibers, which, due to their chemical structure, the presence of a large number of hydroxyl groups, are hygroscopic. Therefore, paper easily absorbs and releases moisture. If dry paper is placed in a room with high humidity, then moisture from the air is absorbed by the paper, and, conversely, in a dry room, moisture evaporates from wet paper.

Fluctuations in paper moisture change many of its working properties and cause problems in the printing process.

When paper is dried, its dimensions decrease, rigidity increases, and electrical conductivity decreases. Over-dried paper is very hard, easily torn in the printing press. When printing on dry paper, more pressure is required, which reduces the runtime of the printing plate. The decrease in electrical conductivity at low paper moisture makes the paper seem to be magnetized, which causes paper sheets to stick together, which complicates the operation of the printing machine.

Changes in paper moisture with fluctuations in atmospheric humidity in the pressroom cause paper waviness, curl, wrinkling on the printout, and ink misregistration in multi-color printing. Therefore, in the workshops it is necessary to maintain constant temperature and humidity.

To prevent deformation of the paper when moistened, its acclimatization is provided. The paper received from the warehouse is kept in the workshop for some time so that it acquires the humidity indicators of the temperature of the printing workshop.

Optical properties of paper

The quality of printed products largely depends on the optical properties of printed paper: whiteness, gloss (gloss) and transparency.

The light incident from the source can either be reflected from the material or penetrate the material. Whiteness and gloss are determined by the nature and amount of the reflected light flux, and transparency is determined by the past.

White- the ability of paper to reflect light diffusely and evenly over the entire spectrum. This means, firstly, that white surfaces reflect light diffusely in different directions; secondly, the spectral composition of light incident on a white surface does not change upon reflection. Therefore, when illuminated by the sun or an artificial source, the light reflected from a white surface will be uncolored, achromatic.

Real bodies never reflect or absorb the entire incident light stream. In nature, there are neither absolutely white nor absolutely black bodies.

The whiteness of the paper affects the gradation in the light areas of reproductions.

The whiteness of the paper is especially important when printing color illustrations. On insufficiently white paper with a yellowish tint, the colors on the print are distorted compared to the original.

The whiteness of the paper depends on the whiteness of the original fibrous materials, the whiteness and the amount of fillers and tinting introduced.

Blue-violet tint enhances whiteness, eliminates the yellow tint characteristic of the fibers. Optical brightener eliminates yellowness, increases the amount of reflected light. The application of the integumentary pigment layer is very effective.

Gloss The polished surface mirrors a significant part of the rays falling on it. Gloss is defined as the ratio of the amount of light reflected specularly to the total reflected light. Coated paper reflects 40 - 70%, and machine smoothness - 10% of the incident light.

The surface of the paper acquires gloss, gloss when finishing on supercalenders. In order to increase the gloss, special additives are introduced into the composition of the coating layer of coated paper: wax, latex, methylcellulose.

When choosing paper for printing reproduction of various originals, it should be borne in mind that black and color photographs, as well as reproductions from oil paintings, are well reproduced on high-gloss paper with a high gloss. To reproduce text in books and magazines, paper with a low surface gloss should be taken. Reading text printed on paper with a glossy surface quickly tires the eyesight. It is impossible to equate gloss, gloss with smoothness.

Opacity. If light rays, passing through a layer of a substance or some material, exit it in parallel, then this layer appears transparent. Examples of transparent bodies are glass, tracing paper, if light is completely absorbed by a substance, then it is opaque.

It is desirable that the printed paper be opaque. The main condition for the opacity of paper is the absorption of light, which is caused by the repeated refraction of rays inside the material. In order to reduce the transparency of paper, fillers are introduced into its composition, and their effect is the more effective, the more the refractive index of the filler differs from the refractive index of the fibers.

Sizing agents with a refractive index close to that of cellulose have virtually no effect on paper transparency.

The properties of paper determine its appearance, quality and purpose. These include - structural, geometric, mechanical, optical, chemical, electrical and properties determined using a microscope.

The structural and geometric properties of paper include parameters such as weight, thickness, smoothness, bulk, lumen and porosity.

The mechanical properties of paper can be divided into strength and deformation properties. Deformation properties are manifested when external forces act on the material and are characterized by a temporary or permanent change in the shape or volume of the body. During the main technological operations of printing, paper is subjected to significant deformation of the paper, for example: stretching, compression, bending.

The main indicators of optical properties are: whiteness, opacity, transparency (opacity), gloss and color.

The chemical properties of paper are determined mainly by the type of wood used, the method and degree of pulping and bleaching, as well as the type and amount of non-fibrous components added, and are important because they determine the physical, electrical and optical properties.

Structural and geometric properties. According to GOST R53636-2009 “Pulp, paper, cardboard. Terms and Definitions"

Mass or weight.

The mass (or weight) of one square meter of paper is the most common indicator, since most papers are sold by weight of 1m 2. The mass of paper is more often referred to as a unit of area than to a unit of volume (as is done with respect to other materials), because paper is used in the form of a sheet and area in this case plays a more important role than volume. According to the accepted classification, the mass of 1 m 2 of printed paper can be from 40 to 250 g. Papers weighing more than 250g/m2 are classified as boards.

Thickness

The thickness of the paper, measured in microns (µm), determines both the permeability of the paper in the printing press and the consumer properties - primarily strength - of the finished product.

Smoothness

Smoothness characterizes the state of the surface of the paper, due to mechanical finishing, and determines the appearance of the paper - rough paper, as a rule, is unattractive in appearance. Smoothness is important for writing papers, for printing papers, and also when gluing paper.

The opposite of smoothness is roughness, which is measured in microns (µm). It directly characterizes the microrelief of the paper surface. One of these two values is always present in paper specifications.

Bulk

Bulkness is measured in cubic centimeters per gram (cc/g). The bulk of printed papers ranges on average from 2 cm 3 /g (for loose, porous) to 0.73 cm 3 /g (for high-density calendered papers). In practice, this means that if you take a thicker paper of a smaller gram, then with equal opacity there will be more sheets in a ton of paper.

Clearance

The clearance of paper characterizes the degree of uniformity of its structure, that is, the degree of uniformity of the distribution of fibers in it. The lumen of the paper is judged by observation in transmitted light. Paper with a highly cloudy gap is extremely inhomogeneous. Its thin spots are also the least durable and easily pass water, ink, printing ink. Due to the uneven perception of the printing ink by the paper, printing on cloudy paper is of poor quality.

Paper with a cloudy gap is difficult to color, multi-tone clouds are formed. Thick sections of the paper web are colored more intensively and less intensively thin ones.

Porosity

Porosity directly affects the absorbency of the paper, that is, its ability to accept printing ink, and may well serve as a characteristic of the structure of the paper. Paper is a porous capillary material; at the same time, macro- and microporosity are distinguished. Macropores, or simply pores, are spaces between fibers filled with air and moisture. Micropores, or capillaries, are the smallest spaces of indefinite shape penetrating the coating layer of coated papers, as well as spaces formed between filler particles or between them and the walls of cellulose fibers in uncoated papers. There are also capillaries inside cellulose fibers. All uncoated, not overly compacted papers, such as newsprint, are macroporous. The total pore volume in such papers reaches 60% or more, and the average pore radius is about 0.160.18 µm. Such papers absorb paint well due to the loose structure, that is, the highly developed inner surface.

Mechanical properties

Mechanical strength.

The tensile strength of paper does not depend on the strength of individual components, but on the strength of the paper structure itself, which is formed during the paper production process. This property is usually characterized by breaking length in meters or breaking force in Newtons. So, for softer printing papers, the breaking length is at least 2500 m, and for hard offset papers this value increases to 3500 m and more.

Break resistance.

The fracture resistance index depends on the length of the fibers from which the paper is formed, on their strength, flexibility and on the bonding forces between the fibers. Therefore, the highest fracture resistance is characterized by paper, consisting of long, strong, flexible and tightly bonded fibers.

Burst resistance.

This indicator is of great importance for wrapping papers. It is related to the breaking load of paper and its elongation at break.

Extensibility.

Paper elongation to break, or its extensibility, characterizes the ability of paper to stretch. This property is especially important for packaging paper, sack paper and cardboard, for the production of stamped products, for the base of waxed paper used for automatic wrapping.

Softness.

The softness of paper is related to its structure, that is, its density and porosity. Thus, large-pore newsprint can deform under compression up to 28%, and for dense coated paper, the compression deformation does not exceed 68%.

Linear deformation when moistened.

The increase in the dimensions of a wetted sheet of paper in its width and length, expressed as a percentage of the original dimensions of a dry sheet, is called linear deformation when wetted. The values of paper deformation when wet and permanent deformation are important indicators for many types of paper (for offset, chart, cartographic, for the base of the photo substrate, for paper with watermarks). High values of these indicators lead to misalignment of ink contours during printing and, as a result, to low-quality printing. However, it should be noted that in GOST 12057-81 “Paper and cardboard. Methods for determining linear deformation.» very stringent test conditions are laid down (wetting a calibrated strip of paper for a certain time), the use of which is impractical for most printed types of paper. European standards suggest the use of the term "moisture expansion", which determines the change in the linear dimensions of a strip of paper when the air humidity changes from 30 to 80%. High humidity dramatically reduces the mechanical tensile strength of the paper.

Optical properties

optical brightness.

Optical brightness is the ability of paper to reflect light diffusely and evenly in all directions.

White.

The true whiteness of a paper is related to its luminosity or absolute reflectivity, i.e. visual efficiency. Whiteness is based on the measurement of light reflection by white or almost white papers with one wavelength (GOST 30113-94 "Paper and cardboard. Method for determining whiteness." Provides 457 millimicrons, that is, in the visible spectrum) and is defined as the ratio of the amounts of incident and distributed reflected light (%).

Yellowing.

Paper yellowing is a term that conventionally refers to the decrease in its whiteness due to exposure to light rays or elevated temperature. Paper can be protected from light damage by storing it in a room without windows or with windows covered with thick curtains.

Opacity, or opacity.

Opacity is the ability of paper to transmit light rays. The opacity property of paper is determined by the total amount of light transmitted (diffuse and non-diffuse). Opacity is usually determined by the degree of penetration of the image into the material under test, placed directly opposite the object in question.

The term "paper opacity" is more commonly used - the ratio of the amount of light reflected from a sheet lying on a black substrate to the light reflected by an opaque stack of this paper.

Transparency

Transparency is related in some way to opacity, but differs from it in that it is determined by the amount of light that passes through without scattering. The transparency ratio is a better estimate of highly transparent materials (cripples), while the opacity measurement is more suitable for relatively opaque papers.

Gloss or gloss.

Gloss (gloss) is a property of paper that expresses the degree of glossiness, gloss, or the ability of a surface to reflect light falling on it. This indicator can be thought of as the property of the surface of the paper to reflect light at a given angle. Thus, gloss (gloss) can be characterized as the ratio of the amount of light reflected in the specular direction to the amount of incident light.

Chemical properties.

Wet strength.

The wet strength of paper is judged by the degree to which it retains its original strength in the wet state, that is, by the strength that it had before wetting, being in the air-dry state.

Humidity.

Ash content.

The ash content of paper depends on the quantitative content of fillers in its composition. High-strength paper should have a low ash content, as minerals reduce paper strength.

Paper weight is measured by the weight of one square meter (g/m2) and varies depending on the destination from 40 to 250 grams. When printing newspapers or magazines on sheet-fed offset machines, it is recommended to use thicker grades of paper (at least 80 g/m2), it is advisable to use thin grades of paper in role-rotating machines: newspapers - about 50 g/m2, books - 60-80 g/m2 . One of the most important properties of paper is smoothness. The higher this indicator, the tighter the contact of the paper web with the printing form and the ability to reproduce thin strokes without distortion. The smoothness of paper is determined on a special device and is characterized by the time of expiration of a specified volume of air between a paper sample and a smooth plate tightly pressed against it; measured in seconds. Newsprint cannot be smooth, as it contains a lot of wood pulp, and therefore it is porous. Intaglio printing (300-500 sec) has the highest requirements for paper smoothness, while offset paper has an average level of smoothness of 80-150 sec).

The degree of compaction of paper affects its bulk (thickness). The higher this indicator, the higher the degree of opacity. As a rule, the largest bulkiness is 2 cm3/g, the smallest is 0.7 cm3/g.

The porosity index indicates the degree of absorption of the printing ink by the paper. Macro- and micropores are formed between the fibers, therefore loose grades of paper, such as newsprint, are called macroporous (the pore radius can vary from 0.16 microns to 0.18 microns), and compressed coated papers are called microporous (with a pore size of about 0.03 microns). This indicator is important to take into account already at the prepress stage of preparing the graphic material, since it most of all affects the value of the dot gain. If you want to get saturated colors, you need to choose paper with the lowest porosity.

For legibility, it is necessary to create a brightness difference between the black ink and the color of the unprinted areas of the paper. Therefore, the higher the whiteness value, the greater the contrast can be achieved. Cellulose fibers have a yellow tint, which is sometimes eliminated by adding a blue dye of the opposite color. The whiteness of newsprint is about 60%, offset - about 70%, and coated - more than 80%.

One of the main properties of paper for printing is opacity. For an optimal level of opacity, a combination of a mixture of unmilled pulps from different tree species is necessary. It has been established that paper samples consisting of 30% unmilled softwood pulp and 70% hardwood pulp have a more uniform gap, the length of the main part of the fibers of these samples is from 0.4 mm to 1.0 mm. In samples with poor lumen, about 10% of fibers longer than 1 mm are present. Coated papers have an opacity level of more than 90%, newsprint - from 50%.

The paper softness setting is important for choosing the printing method. For example, under high pressure from embossed letterpress printing plates, the paper should provide the greatest contact with the printing plate, that is, be soft and quickly recover from deformation. Embossing paper should have completely opposite indicators.

Paper intended for offset printing has increased moisture resistance; for this, special hydrophobic substances are introduced into its composition. Otherwise, when the printing plate is moistened and moisturizing solutions get on the printed material, the paper web will deform, which will lead to loss of strength and the effect of ink misalignment in full-color printing.

The paper has two sides: the side adjacent to the paper machine mesh and the side adjacent to the felt. The mesh side is almost always rougher due to the diamond-shaped markings of the mesh along which the still-cured paper web travels during manufacture. The difference in smoothness and porosity of both sides of the paper is called double-sidedness.

The paper has a certain structure due to the greater orientation of the fibers in the direction of movement of the paper machine mesh and the greater tension experienced by the paper in this direction, known as the machine. Transverse is the direction of the paper at right angles to the direction of movement of the paper machine mesh.

Structural and mechanical properties

Mass (weight) is the most common indicator, because. most papers are sold by weight of 1 m 2. The mass of paper is more often referred to as a unit of area than to a unit of volume, as is done with respect to other materials, since. paper is used in the form of a sheet, and therefore the area in this case plays a more important role than the volume. According to the mass of one square meter of coated sheet, paper is divided into light (up to 60 g / m 2), medium density (70-150 g / m 2) and high density (more than 150 g / m 2). The use of the word "density" in
In this case, it is not entirely correct, but it sounds more harmonious than the term "grammage", which is often used in a professional environment to denote the mass of one square meter of paper.

The thickness of paper (µm) is an important factor in the characteristics of many other types of paper and determines both the permeability of paper in a printing press and consumer properties (primarily strength) of the finished product.

Mechanical strength is one of the main and important properties of most types of paper and cardboard. Standards for printed types of paper provide for certain requirements for mechanical tensile strength. These requirements are determined by the possibility of producing printed types of paper without breaks on modern high-speed machines, followed by passing it through high-speed rewinders and then on printing machines.

Sufficient mechanical strength of the paper should ensure the non-stop operation of printing machines in printing companies. In the paper industry, it is customary to characterize the resistance of paper to rupture by the indicators of the breaking weight or the breaking length of the paper. Plain paper made on a paper machine (PM) is different
various indicators of strength in the machine and transverse direction of the sheet. It is larger in the machine direction because the fibers in the finished paper are oriented in the machine direction.

The resistance index of paper (cardboard) to fracture is one of the essential indicators characterizing the mechanical strength of paper. It depends on the length of the fibers from which the paper is formed, their strength, flexibility and bonding forces between the fibers. Therefore, the highest fracture resistance is characterized by paper, consisting of long, strong, flexible and tightly bonded fibers. For printed types of paper, the most significant indicator in the process of bookbinding and bookbinding in printing production.

The quality indicator - bursting resistance - cannot be classified as the main one. It provides for current standards for a relatively limited number of types of paper. This indicator is important for packaging and wrapping types of paper. This indicator is somewhat related to the indicators of the breaking load of paper and its elongation at break.

For some types of paper and cardboard, the resistance of the surface of these materials to abrasion is one of the criteria that determine the consumer properties of the material. This applies to drawing and drawing and cartographic types of paper. These papers allow, without undue damage to the surface, the possibility of removing written, drawn or
printed by erasing with an eraser, razor blade or knife.
At the same time, such paper with good surface abrasion resistance should retain a satisfactory appearance after re-applying text or a pattern on an erased area.

Wet strength, or wet strength, is an important factor in most papers, especially those made on high-speed paper machines, since the PM must operate smoothly when the paper web passes from one section of the machine to another. The wet strength of paper is judged by the degree to which it remains wet.
its initial strength, i.e. the strength that it had before wetting, being in an air-dry state.

The elongation of the paper before breaking, or its extensibility, characterizes the ability of the paper to stretch; especially important for packaging paper, sack paper, paper and cardboard for the production of stamped products (paper cups), waxed paper base for automatic candy wrapping (so-called caramel paper).

The increase in the dimensions of a wet sheet of paper in its width and length, expressed as a percentage in relation to the original dimensions of a dry sheet, is called linear deformation during moistening. The values of paper deformation when wet and residual are important indicators for many types of paper (offset, chart, cartographic, photo substrate, paper with watermarks). High paper warp values lead to misalignment of ink contours during printing and, as a result, poor-quality printing. However, it should be noted that GOST assumes very stringent test conditions (wetting a calibrated strip of paper for a certain time), the use of which is impractical for most printed types of paper. European standards suggest the use of the term "moisture expansion", which determines the change in the linear dimensions of a strip of paper when the air humidity changes from 30 to 80%.

Smoothness characterizes the state of the surface of the paper, due to mechanical finishing. Smoothness characterizes the appearance of the paper; rough paper is usually unattractive in appearance. Smoothness is important for writing papers, for printing papers, and also when gluing paper.

Paper clearance characterizes the degree of homogeneity of its structure, i.e. the degree of uniformity of the distribution of fibers in it. The lumen of the paper is judged by observation in transmitted light. Paper with a highly cloudy gap is extremely inhomogeneous. Its thin spots are also the least durable. They offer less resistance to the passage of water, ink, printing ink. As a result, printing on cloudy paper is of poor quality due to the uneven perception of the printing ink by the paper.

The paper is uneven in lumen and, consequently, in thickness, is characterized by an increased tendency to warp the surface. The application of coatings to the surface of such paper (chalking, varnishing, waxing) is associated with production difficulties and entails the appearance of defects. Cloud gap paper calendering is also associated with increased scrap formation; polished spots appear on the surface.

Paper with a cloudy gap is difficult to color, multi-tone clouds are formed. Thick sections of the paper web are colored more intensively and less intensively thin ones.

Optical properties

The optical properties of paper are no less important than the structural-mechanical ones. For some types of paper (such as printing, transparent packaging, drawing, photographic, writing), optical properties are of paramount importance. Important indicators of optical properties are: whiteness, opacity, transparency (opacity), gloss and color.

The true whiteness of a paper is related to its brightness or absolute reflectivity, i.e. visual efficiency. Whiteness is based on the measurement of light reflection by white or almost white papers with one wavelength (GOST provides for 457 millimicrons, i.e. in the visible spectrum).
Whiteness is defined as the ratio of the amounts of "fallen" and distributed reflected light (%).

Opacity - the ability of paper to transmit light rays. The opacity property of paper is determined by the total amount of light transmitted (diffuse and non-diffuse). Opacity is usually determined by the degree of "penetration" of the image into the material being tested, placed directly against the object in question.

The term paper opacity is more commonly used - the ratio of the amount of light reflected from a sheet lying on a black substrate to the light reflected by an opaque stack of this paper.

Transparency is related in some way to opacity, but differs from opacity in that it is determined by the amount of light that passes through without scattering. The transparency ratio is a better estimate of highly transparent materials (cripples), while the opacity measurement is more suitable for relatively opaque papers.

Gloss (gloss) is a property of paper that expresses the degree of glossiness, gloss, or the ability of a surface to reflect images. Gloss can be thought of as the property of a paper surface to reflect light at a given angle of reflection more than diffuse reflection of light at the same angle. Thus, gloss (gloss) is the relative amount of light,
reflected in the specular direction to the amount of incident light.

Chemical properties

The chemical properties of paper are mainly determined by the type of wood used, the method and degree of pulping and bleaching, as well as the type and amount of non-fibrous components added. These paper properties are important as they affect its physical, electrical and optical properties.

For some types of paper, chemical properties are as important as physical properties, and in some cases even more important. An example is anti-corrosion paper used to wrap silver and polished steel products. This paper must be free of sulfur and sulfides, as well as free acids, chlorine and strong alkalis that cause tarnishing or etching of the metal surface. The best grades of anti-corrosion paper are made from well-cleaned and bleached rags.
or from sulfide pulp, which are thoroughly washed several times to remove bleach residues. In the same way paper should be made for printing ink with metallic type or for covering with gold foil, since the metal in the ink or the foil will tarnish when in contact with paper containing even two parts per million of reducible sulfur on the paper. Some anti-corrosion papers used for packaging silver products are impregnated with salts (for example, copper acetate, lead acetate, or zinc acetate), which react with hydrogen sulfide contained in a certain amount in the atmosphere, and thus eliminate the contact of gas with silver.

Chemical properties are of great importance for the following types of paper:

photographic (for reproduction);
safe (against fakes);
for paper that requires a high degree of immutability, electric paper intended for resin impregnation, and paper for
food packaging.

These papers must not contain toxic substances; acidity and fillers in the paper must be appropriate for its purpose.

Humidity. The cellulose/water ratio is the most important factor in paper chemistry. The amount of water contained in individual fibers affects their strength, elasticity and paper-forming properties. The moisture content of paper affects its weight, strength, immutability, dimensional stability, and electrical properties; it is very important in calendering, printing, coating and impregnation. When testing paper, it is usually conditioned in order to create a constant, predetermined humidity during testing during testing.

Ash content of paper depends mainly on the quantitative content of fillers in its composition. High strength paper should have a low ash content, as minerals reduce the strength of the paper. High ash content is undesirable in papers such as photographic, electrically insulating, and filter papers.

Microscopic analyzes

In addition to the commonly used chemical, physical and optical tests of paper, important information about its properties can be obtained by examination under a microscope. Important practical applications of the microscope include determination of fiber length and type, fiber composition, analysis of dirt, stains, determination of the degree of fiber processing, the study of resin and starch sizing, and the study of paper in relation to fillers.

The printing properties that define it can be combined into the following groups:

Geometric: smoothness, thickness and weight of 1 m 2, density and porosity;
Optical: optical brightness, opacity, gloss;
Mechanical (strength and deformation): surface pluck strength, breaking length or tear strength, fracture strength, tear resistance, delamination resistance, stiffness, compressive resilience, etc.
Sorption: wet strength, hydrophobicity, ability to absorb printing ink solvents.

All these indicators are closely related to each other. The degree of their influence on the assessment of the printing properties of paper is different for different printing methods.

Paper is often classified by the degree of surface finish. It can be unfinished paper - matte, machine smooth paper and glazed (otherwise calendered) paper, which was additionally processed in supercalenders to give it a high density and smoothness.

Geometric Properties paper

(In a practical application, this means that if you take a thicker paper of a smaller gram, then with equal opacity, there will be more sheets in a ton of paper)

Porosity directly affects the absorbency of paper, that is, its ability to accept printing ink, and may well serve as a characteristic of the structure of the paper. Paper is a porous-capillary material, while macro- and microporosity are distinguished. Macropores, or simply pores, are spaces between fibers filled with air and moisture. Micropores, or capillaries, are the smallest spaces of indefinite shape penetrating the cover layer of coated papers, and also formed between filler particles or between them and the walls of cellulose fibers in uncoated paper. There are also capillaries inside cellulose fibers. All uncoated, not overly compacted papers, such as newsprint, are macroporous. The total pore volume in such papers reaches 60% or more, and the average pore radius is about 0.16-0.18 µm. Such papers absorb paint well, due to their loose structure, that is, a highly developed inner surface.

A special place in the structure of the printing properties of paper is occupied by optical properties, that is, whiteness, opacity, gloss (gloss).

Optical brightness is the ability of paper to reflect light diffusely and evenly in all directions. High optical brightness for printed papers is highly desirable, since the clarity, readability of the publication depends on the contrast of the printed and blank areas of the print.

With multi-color printing, the color accuracy of the image, its correspondence to the original is possible only when printing on sufficiently white paper. To increase the optical brightness, so-called optical brighteners - phosphors, as well as blue and violet dyes, which eliminate the yellowish tint inherent in cellulose fibers, are added to expensive high-quality papers. This technique is called highlighting. Thus, coated papers without optical brightener have an optical brightness of at least 76%, and with optical brightener - at least 84%. Printed papers containing wood pulp should have an optical brightness of at least 72%, but newsprint may not be white enough. Its optical brightness averages 65%.

Another important practical property of printed paper is its opacity. Opacity is especially important for duplex printing. To increase the opacity, a composition of fibrous materials is selected, their degree of grinding is combined, and fillers are introduced.

The next group of printing properties is the mechanical properties of paper, which can be divided into strength and deformation. Deformation properties are manifested when external forces act on the material and are characterized by a temporary or permanent change in the shape or volume of the body. The main technological operations of printing are accompanied by significant paper deformation, for example: stretching, compression, bending. The normal (uninterrupted) course of technological processes of printing and subsequent processing of printed products depends on how paper behaves under these influences. So, when printing with a high method from rigid forms at high pressures, the paper should be soft, that is, it should be easy to compress, level out under pressure, providing the most complete contact with the printing plate.

Sorption properties paper

Finally, we come close to one of the most important properties of printed paper - its absorbency. A correct assessment of absorbency means that the conditions for the timely and complete fixing of the ink are met and, as a result, a high-quality print is obtained.

absorbency paper, primarily depends on its structure, since the processes of interaction between paper and printing ink are fundamentally different. Before talking about the features of this interaction in certain cases, it is necessary to recall once again the main types of structures of modern printed papers. If we depict the structure of paper in the form of a scale, then macroporous papers consisting entirely of wood pulp, for example, newsprint, will be placed at one of its ends. The other end of the scale, respectively, will be occupied by pure cellulose microporous papers, for example, coated papers. A little to the left will be pure cellulose uncoated paper, also microporous. And everyone else will take the rest of the gap.

Macroporous papers take ink well, absorbing it as a whole. The colors are thin here. Liquid paint quickly fills large pores, absorbing to a sufficiently large depth. Moreover, its excessive absorption can even cause the “breakthrough” of the print, that is, the image becomes visible from the defense side of the sheet. Increased macroporosity of paper is undesirable, for example, in illustrative printing, when excessive absorbency leads to loss of saturation and glossiness of the ink. Microporous (capillary) papers are characterized by the so-called “selective absorption” mechanism, when, under the action of capillary pressure forces, mainly a low-viscosity paint component (solvent) is absorbed into the micropores of the surface layer of the paper, while the pigment and film former remain on the surface of the paper. This is exactly what is required to obtain a clear image. Since the mechanism of paper-ink interaction in these cases is different, for coated and uncoated papers, various paints are prepared.

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