FI126571B - Use of the coating composition as a paint - Google Patents

Description

FIELD OF THE INVENTION Field of the Invention

The present invention relates to a coating using a composite structure having a carbonate precipitated polysaccharide. In particular, the invention relates to the use of such a carbonate-polysaccharide structure in the coating of paper or board or in the coating of paints or in paint-like materials (for example, varnishes). This design improves the smoothness, thickness, mottling, strength, opacity and brightness of the coating or paint coating on the finished paper or paperboard, and the optical density, stiffness, and gluing of the coated paper or paperboard ink.

Description of the Prior Art

The most common coating pigments used in paper and board production are kaolins, talc, gypsum, powdered calcium carbonate (GCC) and precipitated calcium carbonate (PCC). In addition, there are more expensive specialty pigments, such as precipitated aluminum silicates, satin white and titanium dioxide. The use of coating pigments in paper and board production is desirable because they improve the optical properties of the paper and board as well as the ink deposition (printability) and enable the basis weight of the base paper, board or coating to be reduced, especially if the coating has good coverage. Good opacity is seen by the human eye as mottling. Coating and the pigments, binders, and other additives used therein are described, for example, in "Papermaking Science and Technology," Part 11, "Pigment Coating and Surface Sizing of Paper" (published by Fapet Oy, 2000, ed. Esa Lehtinen).

The same coating pigments mentioned above are used in the paints. However, titanium is used much more than in paper or cardboard coating. Another major difference in the coating of paper and board is that latex in particular is used as a binder for binding pigments much more in latex paints. The purpose of the paint is to give the surface to be painted a film that is as durable as possible and with good opacity. In addition, the paint uses various film-forming additives to soften latex particles and help form a uniform paint film.

In both coating applications, i.e. paper and board coatings and paints, the purpose of binders is to adhere pigments to the substrate and coating being coated. Inks, a uniform coating surface is formed while the coating of paper and cardboard with binders aims to ensure that the pigments of the coating and the coating itself remain adhered to the paper or board, particularly during printing. Paper and board coatings do not have a uniform film surface due to the lower amount of binder. The porous surface is intended to achieve, among other things, an appropriate ink settling rate for each printing method (for example, offset, ink, flexo, inkjet).

The aim is to make the surface of the base paper or board as smooth as possible in order to achieve the best possible coverage with as little coating as possible. Typically, the base paper or board is therefore precalibrated prior to coating. The machine calender has metal rollers on both sides of the paper or cardboard to keep the thickness constant, but the density varies. In a soft calender, paper or cardboard passes between a hard and a rubber-coated roll. This in turn has the advantage that the density variation is small. In both types of fish knives, one metal roll is usually heated. In addition, there are supercalenders and multi-roll calenders with about ten rolls between which paper or board passes. Pre-calendering often has two rolls and finishing calendars (super- and multi-roll calendering) have about ten rolls. In calendering, the upper and lower surfaces of paper or board are mechanically pressed against each other. Finishing coatings are performed, inter alia, to provide a better pre-coating surface or a printable surface coating. The major disadvantage of calendering is that it reduces the thickness of the coated paper or board, i.e. increases the density of the product. Other disadvantages include loss of stiffness, strength, and optical properties. Poor smoothness of coated paper or board is reflected by poor coverage and a greater need for coating to fill the "wells" of the surface to be coated. When printing on a coated surface, poor coverage of the coating is often manifested in the form of unevenness of the print, or so-called mottling. Poor smoothness of the coating or consequent poor coverage may also cause glossy finish, i.e., glossy unevenness, on glossy coated paper and board grades. There is also talk of opacity and brightness of the coating. Among the blemishes, the following blemishes can be distinguished: backtracking, wet rejection, monochrome, density, and dry blemishes. The spotting is believed to be due to differences in microporous porosity of the coating structure or uneven distribution of binder in the coating. The patchiness of the print is caused by the fact that more or less of the ink is absorbed into the patchy area of the paper or cardboard than to the surrounding area.

In addition to calendering, attempts have been made to address the roughness of the rough base paper or board by adding plastic pigments to the paste. These have been used, among other things, to increase the ability of the coating to react to unevenness of the substrate in order to maintain the lightweight structure of the coated paper or board in calendering with smaller calender loads. The lightweight structure also usually shows better opacity, brightness and optical density of the print. Ideally, after calendering, the coating would be highly opaque, with minimal patching, lightweight, high thickness, good smoothness, and other paper or board quality properties such as gloss on target and mottling. The nip pressure of the calendering can be reduced if one of the coatings achieves the desired level of smoothness more easily than the other coat. This results in a lighter, stiffer and stronger coating, which however has the desired level of smoothness and is closer to the above ideal case.

Paints are not calendered, but with this smoother coating, this coating application also provides better coverage of the surface to be painted. This means that a smaller amount of paint can achieve sufficient coverage, which saves especially expensive titanium.

Picking of paper and cardboard coatings, that is to say the removal of fibers, pigments or the surface of the coating itself, is a particularly harmful quality defect, especially in printing. With printing machines, the material released from the coating collects especially on the printing surfaces of the first printing unit (multicolor printing) and the wetting water. On printed paper or board, this appears as blemishes, missing dots, peeling off of the coating, and other quality problems. Peeling of the coating surface most commonly causes the individual fibers and bundles of fibers to peel off - at worst, it can be de-lamination of the coating portion. Flaking in printing results from the fact that the force perpendicular to the surface of the coating at the printing nip when the ink is cracked exceeds the local strength of the coating surface at the outlet of the printing nip. Binders are used to prevent these quality problems. An example of this is starch. The binding strength of the starch coating is due to its large number of hydroxyl groups which increase the ability to form hydrogen bonds. The starch is present in plants in the form of small (2-150 μηπ) granules which are separated during the manufacturing process. All plants contain starch. However, commercially, starch is made from potatoes, tapioca, barley, wheat, rice and corn, and to some extent from tuber, pea, fruit, and hybrids. Starch is usually insoluble in cold water. This is because the starch polymers are in a well-organized form bound by hydrogen bonds in the starch granules. Upon heating of the aqueous starch solution, the starch granules first swell, after which the individual starch polymers are released from each other. The starch is also often modified to make the starch cationic, to increase the stability of the solution and / or to improve theological properties at higher solids contents. Chemical modification products for starch are generally ethers or esters because starch is a polyol. These processing options include hydroxyalkylation, cationization, carboxymethylation, acetylation, thermomechanical treatment, enzyme treatment, treatment with hydrogen peroxide, treatment with sodium hypochlorite, and acid treatment. Modification of the starch also produces starches soluble in cold water, or so-called cold-soluble starches.

Native (unmodified) starch is anionic and therefore has poor adhesion to fiber in paper and board making without cationic modification. Today, canonical starches are ethers which are prepared using a quaternary ammonium-containing epoxy chemical. Of the most processed starches, cationized starch is used. Typically, the starch polymers inside the starch granules must be released either by adding to hot water or by boiling the aqueous starch solution. An exception to this is the above-mentioned cold-soluble starches. Typically, the coating paste used for coating paper or cardboard is aimed at the highest possible dry solids content. This usually results in better water retention, runnability and quality. For this reason, when using starch, it is sought to minimize the amount of water needed to make the starch. In addition to starch, other strength enhancing additives have been used. Guar gum is the most commonly used vegetable gum. Guar gum and locust gum are seed gums and consist of galactomannan. Karayakumi has a more complex structure; it is a branched polysaccharide. The use of vegetable gums has been limited by the higher price of starch. They have also been more difficult to modify than starch.

Cellulose and nanocellulose also belong to polysaccharides. Nanocellulose or microfibrillated cellulose can be made from any cellulose-containing raw material, such as wood. Nanocellulose is significantly smaller in structure than the conventional cellulose polymer and thus contains a much larger amount of hydroxyl groups forming hydrogen bonds. Nanocellulose is also referred to as microfibrillated cellulose, nanofibrillated cellulose, nanofibres and microfibres.

Thus, there is a need for a light coating of paper or board or a paint coating that provides good coverage while improving essential quality properties. The lightweight, smooth coating gives the paper or board manufacturer the ability to achieve a smooth surface that delivers superior printing results and reduces the cost of coating material needed. A well-covering coating can also slightly reduce the basis weight of the base paper or board. In addition, in the pigmentation of paper and board in particular, an improved pre-coating can be achieved compared to known solutions.

Brief Description of the Invention

It is an object of the present invention to provide a novel coating pigment suitable for both paper and board products and paints, and to improve their smoothness, strength, blemish, opacity, opacity, brightness, optical density and adhesion.

In particular, it is an object of the invention to provide a coating pigment comprising a carbonate-polysaccharide composite in a coating structure.

This coating pigment is intended to achieve better smoothness and thickness while maintaining the brightness and opacity of paper, board and paint in particular. By increasing the thickness, a lighter coating structure is achieved.

The present invention thus relates to a coating composition for coating, pigmenting or surface sizing a paper or board product containing said composite structure. This composition is suitable, inter alia, for use as a coating pigment for the above applications. More specifically, the use of a coating composition according to the present invention as a paint is characterized in what is set forth in claim 1.

The present invention is multifunctional and improves many different features. The composite structure according to the invention, among other things, enables the smoothness properties and the patchiness of the products of different applications and the uniformity of the coating to be improved.

In the case of fiber products, their important properties such as brightness, opacity and printability are also maintained at a good level, while some of the coating materials can be replaced by the coating pigment according to the invention due to its lighter structure. The lightweight structure and smoothness resulting from the use of the composite according to the invention make it possible to reduce the calender pressures on the Hip, thereby reducing the compaction of paper or board.

Brief Description of the Drawings

Fig. 1 (Fig. 1) shows SEM images of coatings with the carbonate starch composite of the invention (Fig. 1A) and a reference coating using GCC (Fig. 1B) on the surface of LWC base paper.

Detailed Description of Preferred Embodiments of the Invention

The present invention relates to a coating composition comprising a composite structure having a carbonate precipitated polysaccharide.

The invention also relates to a coated paper or board article on which one or both surfaces are applied in one or more layers.

Another object of the invention is the use of paints which are of a kind of coatings and which may similarly consist of or contain a coating composition according to the invention.

Preferred polysaccharides are starch, carboxymethylcellulose, guar gum, pectin, chitosan, dextrins, galactomannan, alginates or nanocellulose or a mixture of the above, more preferably starch or nanocellulose, most preferably starch or cold starch. In the case of starch, its particles can be treated prior to use in said composite structure such that the particle is swollen, or even partially or completely broken. This is generally done by heating, which in the case of the present invention can be done, for example, in a solution containing carbonate ions and other forms of carbonate at ambient pH (i.e., a so-called carbonate ion solution).

Calcium and / or magnesium carbonate is precipitated from the carbonate ion solution in cold-soluble starch, nanocellulose or a mixture thereof by vacuum, heating, mixing, addition of a base-containing additive, or some combination thereof.

Preferably, the polysaccharide precipitated carbonate contained in the composite structure is calcium or magnesium carbonate or a combination thereof, wherein the carbonate ion solution also contains calcium or magnesium ions or a combination thereof.

The precipitated calcium carbonates typically have a crystalline structure of calcite, aragonite or vaterite. Calcite typically occurs as a lamellar and cubic crystalline form. The scalenohedral, rhombohedric, needle-like, spherical and prismatic crystalline forms can also be obtained by varying precipitation conditions such as the concentration of slaked lime and the reaction temperature.

The manufacture of a composite structure according to the invention is described in patent application 20125569 and the present invention relates to the use of the composite structure according to the above application in coating paper or board or in paints. "Carbonate ion solution" means a carbonate system which may contain carbonate ions (CO32 '), bicarbonate ions (HCO3'), carbonic acid (H2CO3) and even carbon dioxide (CO2), but this content and the relationship between these states vary according to the pH of the solution. .

By carbonate system is thus meant the change of different forms of carbonate according to pH. The main states are as follows:

At acidic pH, soluble carbon dioxide (CO2) and to a lesser extent carbonic acid (H2CO3) are the major forms of carbonate. In the neutral (on both sides of pH 7) and basic range, bicarbonate, or bicarbonate (HCO3 '), is the predominant form of carbonate up to about pH 10. In the very basic range (pH> 10), carbonate (CO32 ') is the predominant form. By alkali titer of carbonates is meant the amount of strong acid which is used to tear the aqueous solution to the end point of phenolphthalein. In this case, all CO32 'is converted to the HCO3' ionic form. This occurs at about pH 8.3. At the most important pH range of paper and board production, pH 6-8, bicarbonate (HC03) is the predominant form. In addition, moderately dissolved carbon dioxide and low carbon dioxide and colloidal carbonate have entered the pH range from the acidic range. Acidic solution in the pH range is referred to herein as an acidic carbonate ion solution.

Raising the pH or temperature causes the bicarbonate ions to be converted from a carbonate ion solution to carbonate particles. Similarly, when the temperature is raised, carbon dioxide is released and the bicarbonate reacts with free calcium or magnesium ions according to the following reaction equation:

This above reaction also works by bringing a sufficiently warm carbonate ion solution under reduced pressure or centrifugal force such that carbon dioxide is released from the solution into the air. By raising the pH with a base such as NaOH or Ca (OH) 2, the carbonate particles can be precipitated according to the following reaction equations:

The bicarbonate in the acidic carbonate ion solution is believed to help the carbonate filler to attach to the polysaccharide polymer. The bicarbonate ion itself has the potential to form hydrogen bonds, which is important for, among other things, the attachment of polysaccharides to the fiber. The bicarbonate can be thought to direct the attachment to the hydroxyl groups of the polysaccharides before it precipitates into the carbonate.

When using a cationic modified polysaccharide in the invention, it is believed that the cationic groups help to attach the resulting carbonate polysaccharide structure to other components of the coating. For these reasons, the composite pigment is able to add strength binders of the coating binders to the pigments used.

As mentioned above, for example, the polysaccharide may be starch, carboxymethyl cellulose, guar gum or nanocellulose or a mixture of any of the above, more preferably starch or nanocellulose, most preferably starch.

Amphoteric starches operate at a wider pH range than cationic starches. Thus, these are preferred polysaccharides for use in the present invention. Other modes of processing starch include hydroxyalkylation, cationization, carboxymethylation, acetylation, thermomechanical treatment, enzyme treatment, hydrogen peroxide treatment, sodium hypochlorite treatment and acid treatment.

Vegetable gums are more difficult to modify than starches. Similarly, polyacrylamide (PAM), nanocellulose, and carboxymethylcellulose (CMC) do not need to be modified, since PAMs are either cationic or anionic, and others contain a sufficient amount of hydrogen bond forming hydroxyl groups. Therefore, these are suitable as unmodified well utilized polysaccharides in the invention.

Applications of the above-mentioned composite structure according to the invention may include paper coating, cardboard coating and paints. The composite structure then generally acts as a coating pigment in the final product.

In paper or board coatings or paint coatings, the composite structure may be used in combination with other coating pigments or as the sole coating pigment.

Preferably, the composite structure is metered into the paint or coating paste at the highest solids content possible.

The fibrous product to be manufactured may be coated, surface-glued or pigmented printing paper, packaging board, kraft paper or other paper grades using mechanical pulp, cellulose fiber, or both.

Patent Application 20125569 shows that by raising the pH of an acidic carbonate ion solution to a neutral range while the polysaccharide is present in an acidic carbonate ion solution (pH> 6.5), heating, dosing with an alkaline additive, vacuum, mixing or any combination of the above by means of which the carbonate is precipitated into a polysaccharide. In addition, it is possible to use known additives, such as acids, which affect the crystalline form and structure of the precipitated carbonate. The process for precipitating the carbonate in the polysaccharide described in that patent application, and in particular in the claims, can thus also be carried out in connection with the formation of the coating composition of the present invention. This composite structure of the precipitated carbonate and polysaccharide gives the surface-glued, pigmented or coated paper or board final product or latex paints not only opacity, brightness and printability (improved ink absorption properties), but also a mottling (mottling).

Typically, the carbonate-polysaccharide composite of the invention is used in paper or board coating, surface sizing or pigmentation and paints according to the following alternatives (A-D). The coating, surface sizing or pigmentation may be done on one or both sides and one or more times alternatively using one or more on-machine or off-machine coating units. On multiple applications, there may be drying between the coating layers, or a new coating layer may be applied directly over the wet coating. Coating layer herein refers to a coating layer obtained by coating, pigmentation or surface sizing on a base paper or board. The base paper or paperboard may be pre - calendered or non - calendered. A) The coating paste is mixed with the other pigments, binders, and other additives used in the paste to provide a dry solids content of at least 55%. Paper or cardboard is coated with this paste at least once. The dried and coated paper or board can then be calendered to the desired smoothness and thickness target. Other possible coating layers, or pigmentation layers, do not necessarily contain the composite of the invention in the paste. When coating paper or board with more than one coating layer, the prior coating layer may be dried and / or calendered before the new coating layer is applied over the former. Alternatively, the new coating layer may also be applied directly over the previous layer without drying and / or calendering. B) The pigment paste is mixed with the other pigments, binders and other additives used in the paste in a composition having a dry matter content of at least 20%. The paper or board is pigmented at least once with this paste. The dried, pigmented paper or board can then be calendered to the desired smoothness and thickness target. The once or more frequently pigmented base paper or board may be coated with one or more coating layers according to Option A, or one or more coating layers not containing said composite. When coating paper or board with more than one coating layer, the prior coating layer may be dried and / or calendered before the new coating layer is applied over the former. Alternatively, the new coating layer may also be applied directly over the previous layer without drying and / or calendering. C) The surface sizing uses the composite of the invention to control the surface linting problem and provide a coating that enhances the brightness, opacity and optical density of the print. When using the composite of the invention in surface sizing, other pigments, binders and additives may also be used in the surface sizing paste. The surface sizing may be used as the first coating on the base paper or board prior to pigmentation B and / or A coating, or to apply non-composite pigmentation or coating layers. When coating paper or board with more than one coating layer, the previous coating layer may be dried and / or calendered before the new coating layer is applied over the former. Alternatively, the new coating layer may also be applied directly over the previous layer without drying and / or calendering. Thus, one or more pigmentation layers, or one or more coating layers, or one or more surface sizing layers, contain the composite of the invention, while other layers may or may not contain said composite. D) The use of the composite according to the invention in paints, in particular to reduce the amount of paint required to achieve better coverage, without reducing the surface strength, opacity or wear resistance of the paint.

The other additives mentioned above include other pigments, binders and additives such as latexes, polyvinyl alcohol, CMC, starches, proteins, caseins or dispersants, antifoams, anti-foaming agents, pH adjusting agents, hardeners, water retention agents, and theological adjustment, lubricants, optical brighteners, dyes and antimicrobial agents.

Latex is the most commonly used binder in both paper or board applications and paint applications. Typically, latexes are combinations of styrene butadiene, polyvinyl acetate, acrylate latex, or various copolymers thereof. Both of the above applications aim to reduce the proportion of latex in the coating paste or paint for cost reasons.

The amount of latex will vary according to the printing process. In offset printing, the surface of the paper is dampened with water, which is why surface strength (especially wet strength) is important and generally requires 10 to 15 parts of latex. Gravity printing does not use wetting water, but an important feature is the smoothness of the paper. In this case, the amount of latex is usually 4-5 parts of latex.

The most common of the other binders are starch, carboxymethylcellulose (CMC), polyvinyl alcohol, protein and casein. Polyvinyl alcohol and CMC give the best bond strength, but because they are more expensive than latex, their use is quite limited.

Other additives used in coating pastes include dispersants, antifoams, defoamers, pH adjusters, hardeners, water retention and Theology regulators, synthetic thickeners, lubricants, optical brighteners, colorants and antimicrobial agents.

In particular, the base paper or board coated with the aforementioned composite structure is one which contains cellulose and / or wood fiber and / or synthetic fiber. In the present invention, the fibers may be any natural and / or synthetic fibers. In particular, the fibers may be chemical pulp or mechanical pulp or a mixture thereof. For example, sulfate and sulphite cellulose fibers, solution cellulose, nanocellulose, chemimechanical (CTMP), thermomechanical (TMP), pressure grit (PGW), grit, recycled fiber or deinked pulp fibers can be solids. Typically, chemical pulps are called sulphate and sulphite pulp, and mechanical pulps are thermomechanical pulp, pressure grit and grit. In addition to coated, surface-glued or pigmented printing and packaging materials, various specialty papers (such as wallpaper and kraft paper) as well as different paints are good applications because the smoothness and patchiness of the coating layer are very important in these products. These applications are particularly concerned with surface sizing and pigmentation.

The present invention also relates to a process for making a coated paper or board product, wherein the above coating composition is applied to one or both sides of one or both sides of the uncoated paper or board product. As a coating method, that is, as a method of applying the composition, paper or board contact coating methods or non-contact coating methods may be used. The most prominent of the non-contact methods is curtain and spray coating. Blade, rod, air brush and film transfer coating are the most important touch coating methods. The blade coating can be further subdivided into brush roll, short-stay and nozzle coating. In these aforementioned processes, the coating paste is prepared in an aqueous solution which, after coating, is dried. In paper and board coating, drying is usually done with hot air or infrared lamps. Paints usually dry by air drying. In addition, there are anhydrous or powder coating methods in which dry pigments are prepared to adhere to the surface to be coated. A binder may also be used. The coating can be applied directly to the surface of the manufactured paper or cardboard without separate on-roll coating (or on-machine coating) or after off-coating on a separate coating station (off machine coating). Both sides of the paper or board can be coated simultaneously or in different stages. The coating can be done one or more times, whereby there may be drying between different coating applications. Typically, the coating pastes have a solids content of about 60% or more and a coating composition of 10-30 g / m / half. In addition, there are surface sizing and pigmentation where the composition of the pastes is simpler, the solids content of the pastes and the desired coating amounts are lower.

Surface sizing improves the surface strength and stiffness of paper or paperboard, and partially binds loose fibers and other components to the surface. It also improves smoothness, porosity, gloss and ink absorption. Surface sizing may also be a pre-treatment for the paper and board to be coated. The starch as such may be used in surface paste pastes at a dry solids content of about 10%. In surface sizing, the amount of surface sizing on the surface of the base paper or board 2 is usually 0.5 ... 3 g / m / side. A pond glue press or sym-sizer fil film applicator is usually used for surface sizing.

Light coating, or pigmentation, improves the surface properties and printability of the paper / board. The paste often contains pigment added to the starch solution, often calcium carbonate, and possible additives. In pigmentation, the dry solids content is about 20-55%. Coating amounts are 1 ... 10 2 g / m / half, depending on the paper and board grade and the intended use. In terms of coating coverage, blade coating gives a poorer coating surface than coarse coating paper, such as air brush coating, film transfer coating and non - contact coating methods. This is due to the inability of the blade coating to follow the unevenness of the surface to be coated.

The following non-limiting examples illustrate the invention and its advantages.

eXAMPLES

Example 1

The carbonate ion solution was prepared by mixing 170 grams of burnt lime (CaO) with 1000 grams of 55 ° C water. The resulting Ca (OH) 2 slurry was reacted with carbon dioxide introduced thereto to a final pH of 6.2. This solution was allowed to sediment for 48 hours, after which the clear solution was separated from the sedimented material. The solids content of this solution was 42%. The carbonate ion solution thus prepared was used as a raw material in the coating starch cooking in the experiments described below.

Example 2

The carbonate ion solution prepared according to the previous example is used in this example as cooking water for starch. In test point A, 20 parts of starch have been cooked (95 ° C, 30 minutes, mixing) together with the carbonate ion solution of Example 1 so that the final solids content of the pasta is 50%. The starch used was potato starch (coating grade) from Chemigate Oy. In test point B, the same starch as in test point A was used. In test point B, the starch was first boiled (95 ° C, 30 minutes, stirring) and mixed with powdered calcium carbonate (Hydrocarb 90, Omya) with 20 parts and finally dry. content of 50%.

Three different rods were coated with uncoated 80 g / m2 copy paper in three different coatings. Coating was done with a laboratory coater (RK K303 multi-coater) using a speed of 6. Thereafter, the sheets were dried in an air-conditioning room for 48 hours at 23 ° C and 50% relative humidity. They are then checked for coating quantities and soft calendered under the following conditions:

• 50 ° C • two nipples • 200 kN / m pressure

The following properties have been determined from calendered samples: • ISO brightness (Minolta Spectrophotometer 361 Od), ISO 2470 • Opacity (Minolta Spectrophotometer 361 Od), ISO 2471 • Thickness (L & W Thickness Tester SE51), ISO 534 • Wax pick test ), T 459 OM-93 • Sileys (Parker print surface), ISO 8791-4

The results are shown in Table 1. All results are normalized to a coating volume of 8 g / m 2.

Table 1. Results normalized to a coating amount of 8 g / m 2.

Table 1 shows that when the coating structure can be kept light (high thickness) and the surface of the coating is smooth, excellent lightness and opacity properties are also obtained. If the coated samples of these two test points had been calendered to the same level of smoothness, the properties of the coating of the invention (test point A) would have been even better since the calendering could have been done at a lower nip pressure.

Example 3

The carbonate ion solutions prepared according to Example 1 are used in this example as cooking water for starch cooking. The potato starch used in the experiments was from Chemigate Oy (Raisamyl).

In test point A, 35 parts of starch have been cooked (95 ° C, 30 minutes, mixing) together with the carbonate ion solution of Example 1 so that the final solids content of the pasta is 40%. In test point B, the same starch as in test point A was used. In test point B, the starch was first boiled (95 ° C, 30 minutes, stirring) and with gentle stirring, ground calcium carbonate (Hydrocarb 90, Omya) was added to 35 parts starch. content is 40%.

Three different coatings have been run on a CLC Coating Machine (CLC 6000) on 40 g / m2 LWC base paper. The blade coating unit and a speed of 800 m / min have been used for the coating. The target coatings ranged from 3 to 9 g / m2. The sheets were then conditioned for 48 hours at 23 ° C and 50% relative humidity prior to measurement. They were then checked for coating quantities and calendered under the same conditions as in Example 2 above. The following properties were determined from the calendered samples: • ISO Brightness (Minolta Spectrophotometer 361 Od), • Opacity (Minolta Spectrophotometer 361 Od), ISO 2471 • Thickness ( L & W Thickness Tester SE51), ISO 534 • IGT pick (AIC2-5), 4 m / s, medium viscosity oil, ISO 3783 • Smoothness (Parker print surface), ISO 8791-4

The mottling was evaluated as follows. The samples are printed with a 4-color Heatset offset with a 60 l / cm line density and round dots. The samples were measured on both sides of the "Mottling" areas of the C70 and B70 panes, with a small amount of paper around them included in the image. Here, a cluster method is used in which patches (0.5 to 10 mm) are searched by thresholding and grouped by size and contrast. The instrument used was a Hewlett-Packard ScanJet 7400C desktop scanner with Hewlett-Packard Precision Scan Pro version 3.02. The firmware settings for the measurement were as shown in Table 2.

Table 2. Precision Scan pro program settings for specifying patches.

The results are shown in Table 3. All results are normalized to a coating volume of 8 g / m 2.

Table 3. Results normalized to a coating amount of 8 g / m 2.

Table 3 shows that when the texture of the coating can be kept light (thickness is high) and the surface of the coating is smooth, the patch index is also improved (lower value). The patch index is an index that measures the size and contrast of patches. If the coated samples of these two test points had been calendered to the same level of smoothness, the properties of the coating of the invention (test point A) would have been even better since the calendering could have been done at a lower nip pressure.

The choice of base paper has clearly affected the light levels. Otherwise, the brightness result would also be better.

From the SEM images of Fig. 1 (Fig. 1), it is observed that the composite (test point A) of the invention gives a surface formed by small granules (Fig. 1A). The powdered potassium carbonate-containing coating (test point B) forms a much denser and coarser surface (Figure 1B) than test point A (Figure 1A) of the invention. The magnification of the images is significantly lower than the resolution of the human eye. The smaller grain gives better coverage to the human eye due to homogeneous and small granules.

Claims (5)

Use of a coating composition as a target paint, the coating composition containing a composite structure, the body of which consists of swollen or partially or fully dissolved polysaccharide in which carbonate is precipitated. The use according to claim 1, characterized in that the polysaccharide is starch, cold-soluble starch, carboxymethyl cellulose, guar gum or nanocellulose or a mixture of the aforementioned, preferably starch or nanocellulose, most preferably cold-soluble starch. The use according to claim 1 or 2, characterized in that the precipitated carbonate is calcium or magnesium carbonate or a combination thereof, and at least part of the carbonate is preferably precipitated from the solution directly in the polysaccharide body. The use according to claim 3, characterized in that a portion of the carbonate is composed of added carbonate, which is preferably calcium carbonate, such as precipitated calcium carbonate (PCC) or ground calcium carbonate (GCC) or a combination thereof. The use according to any one of the preceding claims, characterized in that the composition further comprises other pigments, binders and additives, such as e.g. latex, polyvinyl alcohol, CMC, starch, kaolin, calcined kaolin, talc, titanium dioxide, composite pigments, granular pigments, protein, casein, dispersants, anti-foaming agents, foam-removing agents, pH modifiers, hardeners, water retention and rheology modifiers, synthetic modifiers, rheology modifiers brighteners, dyes and / or microbial agents.