EP0016465B2 - Process for producing coated paper and cardboard and coating composition to carry out the process - Google Patents

Abstract

A method for the coating of a raw paper or cardboard substrate wherein the substrate is prepared having a pH of less than about 6.5, and a coating is applied thereto composed of an aqueous dispersion of a plastic and an inorganic pigment, the coating having a pH of more than about 6.5 wherein the particles of the plastic in the dispersion have a non-cation-active charge and wherein the coating also contains a material which does not affect the preparation or storage stability of the coating, but which in the pH range below about 6.5 becomes highly cation-active, and then drying the thus applied coating. Using this method, the coating solidifies at the time of contact of the coating and the paper surface thereby minimizing penetration of the coating into the paper. As a result, improved coated papers are obtained.

Description

The present invention relates to a method for producing coated paper and cardboard, in which an aqueous coating slip with particles of at least one aqueous plastic dispersion and inorganic pigments is applied to coating base paper or cardboard and dried, and a coating slip for carrying out the method.

The high quality requirements of the printing papers are opposed to the constant demands to bring about a cost reduction. Of particular urgency is the solution to this problem with the so-called LWC (light weight coated) papers used for mail order catalogs, illustrated magazines and the like. Ä. are used. LWC papers are now manufactured with extremely low basis weights, since the possibilities of using more cost-effective raw materials without reducing quality at the same time have been largely exhausted. The coating base papers are between 36 and 40 g / m ² , the coating is 8-12 g / m ² and page. These weights form a technological limit that, despite the greatest efforts, can no longer be fallen below. Rather, working in this border area already leads to enormous difficulties during the coating process. As a result of the low weight per unit area, the coating mass "strikes through". The coating slip penetrates the substrate and builds up on the coater roller. This initially results in line errors and, if the paper web is heavily deposited, tears. The coating system must be turned off and cleaned. The downtimes and the resulting scrap lead to increased production costs.

Striking the coating slips also has another disadvantage. The proportion of the coating slip penetrating into the paper can no longer contribute to improving the quality of the paper surface. It is therefore generally desirable that only a certain proportion of the coating slip penetrates into the substrate and causes the surface coating to be anchored there. The larger part, on the other hand, should remain on the surface and improve printability. Coating compounds that penetrate too deep therefore cause loss of quality even in those papers and cardboard types in which strikethrough of the coating compound does not lead to the operational disruptions described above due to its higher basis weight.

In the past there has been no lack of efforts to prevent the coating slips from penetrating too deeply, and in particular to prevent the coating slips from striking through on LWC papers. The known proposals to influence the penetration behavior of the coating slips by increasing the solids content or the viscosity, but have not led to satisfactory results. With a coating slip, the increase in the solids content is limited by the water required to disperse the pigments used. As a rule, coating slips with a solids content of more than 60% cannot be produced or at least cannot be processed under practical conditions. Strikethrough of the coating slip is only avoided to a small extent in the case of coating slips which have an even higher solids content. An additional difficulty with LWC papers is that coating weights in the area of interest below 12 g / m ² can hardly be achieved with coating slips that have a solids content of over 60%. Only coating compositions which are increased in viscosity likewise show no lower penetration behavior. The addition of electrolytes and cationic substances to the coating slip has also been proposed. In most cases, however, the coating rheology is so adversely affected that these suggestions have not found their way into practice either.

The object of the present invention is to eliminate these known difficulties and to provide a method for producing coated paper and cardboard in which the coating slip penetrates the substrates only to a small extent. In particular, LWC papers are designed to prevent the coating compound from streaking through and to ensure a smooth and trouble-free coating process.

Another important task is the production of LWC papers with a reduced basis weight, whereby a smaller amount of valuable raw materials and less energy is to be used to produce a flat amount of paper with unchanged quality level.

The object of the invention is also to provide a coating slip which can be produced using conventional coating color components, in particular conventional plastic dispersions and pigments, and which can be produced and processed with the usual equipment, but penetrates to a substantially lesser extent into the substrates and when used to coat LWC papers, strikethrough is largely avoided.

This problem is solved by a process for the production of coated paper or cardboard, which is characterized in that a coating slip is applied to a coating base paper or a coating base cardboard with a pH value <6.5, the pH value of which is <6, 5, in which the particles of the plastic dispersion carry a non-cationic charge and which contains one or more water-soluble, weakly cation-active monomeric substances from the group of the tertiary and quaternary, which does not impair their stability during the preparation and storage of the coating slip, in pH Area below 6.5 however becomes strongly cationic and that upon contact with the acidified paper or cardboard the coating slip solidifies at the interface with the base paper or cardboard. If the printout of the substance added to the coating slip is subsequently used, this means an amino compound.

For the process according to the invention to be carried out successfully, it is of crucial importance that all process conditions are observed. The use of a coating base paper or cardboard with a pH value below 6.5 and the setting of the coating slip to a pH value above 6.5 initially allow working in the pH range of the usual coating slips. Only when the coating slip comes into contact with the substrate does the substance added to the coating slip become cation-active. The selection and possible uses of the substances that become more cationic and the plastic dispersion must be carefully coordinated. The possible uses of the plastic dispersions that can be used are not linked to the presence of a specific polymer. Rather, it is possible to use all plastic dispersions which have hitherto been used for the production of coating slips, such as, for. B. butadiene styrene copolymers, acrylic acid ester copolymers with z. B. polyvinyl acetate or the like, provided that they meet the demand for a non-cationically charged state of their plastic particles.

According to the present application, the expression plastic particles with a non-cationic charge should be understood to mean those plastic dispersions which have an anionic emulsifier system or are nonionically stabilized. Many plastic dispersions whose emulsifier system consists of both anionic and nonionic substances also fall under the term mentioned. However, the selected formulation also includes dispersions in which the ionic charge state is determined by certain groups of the polymer itself.

Not suitable for the purposes of the present invention are the plastic dispersions, the particles of which carry a cation-active charge and also which are not so stable in their stability when added to a coating slip under the influence of a weakly cation-active substance that problem-free manufacture, storage and Further processing of the coating slip is no longer possible. In order to speak of a sensitive disturbance of the degree of stabilization, there is no need for coagulation of the coating slip, rather an influence on the degree of stabilization is often already noticeable by an increase in the biscuitity which is above the usual measure.

Plastic dispersions which have such a high degree of stabilization are also unsuitable that when a weakly cation-active substance is added to a coating slip under the influence of an acidic paper surface, their degree of stabilization is not influenced or is influenced only to such an extent that the coating slip solidifies the points of contact between the line and paper did not occur.

The substances which are added to the coating slip and only become strongly cation-active in the pH range below 6.5 generally have a weakly cation-active character even at higher pH values, and are therefore referred to below as weakly cationic substances. However, the cationic effect they emanate is so low that it has no significant influence on the stability of the coating slips. Their characteristic effect only occurs at an acidic pH. When it comes into contact with an acid-produced paper, its national effect increases and causes the coating slip to solidify at the points where there is contact between the coating slip and acidic paper.

By solidifying the coating slip at the interface area mentioned, further penetration of the coating slip into the interior of the coating base paper is prevented and stripping through is prevented in the case of LWC papers. However, the paper coating slip can also fulfill its intended purpose, which consists in improving the surface, far better than such a coating slip, which has penetrated the paper to a considerable extent.

Even if it has not yet been completely clarified how the coating slip solidifies in the interface area in individual cases, the mode of action can probably be explained as follows: The substances added to the coating slip impair the stability of the coating slip adjusted to a pH value of <6.5 initially not. The production and processing of the coating slips are therefore not subject to any restrictions and can be carried out in the usual way. If, however, the coating slip is applied to the surface of a coating base paper or coating base cardboard, the surface of which has an acidic pH, the substances become strongly cationic and impair the degree of stabilization of the plastic binder used. The reduced degree of stabilization of the plastic dispersion can be due both to complete coagulation in the interface area of the substrate and to the coating slip, with water being usually eliminated, but it can also be a matter of converting the plastic dispersion into a non-reversible, gel-like state. In any case, the coating compound will solidify due to the reduced stability of the plastic dispersion in the interface area. In this interface area, the solidified coating slip acts as a quasi-barrier layer and prevents further penetration of the coating slip portion that does not coincide with the surface of the substrate Has come into contact as a result, has an unchanged pH value and its stability is not impaired.

To carry out the process according to the invention, the use of a coating base paper or cardboard with a pH value between 4.5 and 6.0 has proven to be particularly advantageous. Coating base papers and cardboards are often produced in this pH range and can therefore be used in the process according to the invention without additional measures. However, it is also particularly advantageous that the use of such substrates results in very rapid and complete destabilization in the interface area.

In a particularly preferred embodiment of the process according to the invention, such base papers or cardboards are used in which the pH is adjusted to a value of 4.5 to 6.0 by adding a sulfuric acid compound of aluminum to the pulp pulp. Both aluminum sulfate and potassium aluminum sulfate have proven to be suitable. If an even lower pH value setting is desired, this is expediently also carried out using sulfuric acid.

Aqueous coating slips are mostly produced and processed in the alkaline range. In order to carry out the process according to the invention, the setting of the coating slip to a pH value between 7.5 and 11 has proven to be expedient. With coating slips in this pH range, sufficient stability during production and processing as well as safe solidification of the coating slip in the interface area is guaranteed when the coating slip is applied to the substrate.

If plastic dispersions whose particles have a nonionic charge state are also suitable for carrying out the process according to the invention, plastic dispersions in which the particles are anionically charged have proven to be particularly useful. With these dispersions, destabilization can be achieved particularly easily by the substance which becomes more cation-active in the acidic pH range.

In an expedient embodiment of the method according to the invention, plastic dispersions are used in which the anion-active charge state of the particles is brought about by anionic groups of the plastic molecule.

However, plastic dispersions in which the anion-active charge state of the particles is due to an anion-active emulsifier or stabilizer system have proven to be very particularly advantageous.

In order to carry out the process effectively, it is important that the substance added to the coating slip is reinforced by its contact with the acidic surface of the substrate. Substances which are only weakly cation-active in the coating slip are particularly suitable in this regard. Such substances can be changed particularly well in their charge state by a shift in their pH.

The cationic substances to be added to the coating slip must have the property of becoming strongly cationic at a pH <6.5. Monomeric, tertiary and quaternary amino compounds which are soluble in water and have alkyl, aryl or aralkyl radicals on the nitrogen atom have proven suitable. The quaternary amino compounds are preferably suitable. The destabilization of the coating slip in the interface area with the substrate depends on the type of substance which becomes strongly cationic in a pH range below 6.5, on the stability of the binder used and on the amount in which this substance is added to the coating slip. The addition is advantageously carried out only in such an amount that it does not impair the stability of the coating slip.

An addition amount of 2 to 20% by weight, based on the dry parts of the plastic dispersion, in the coating slip has proven to be particularly expedient.

In an expedient embodiment, the method according to the invention is characterized in that a coating slip with a solids content of 30 to 35% by weight is applied on one side to an airbrush coating device on a coating base paper or cardboard in such an amount that the weight of the dried coating 10-12 g / m ² . It shows that the coating slip penetrates less deeply into the substrate and that a larger amount of the coating remains on the surface and contributes to improving the quality.

The process according to the invention is particularly suitable for producing LWC papers. Its application avoids the feared streaking of the coating slip and enables a trouble-free coating process. The elimination of costly machine downtimes and the much better utilization of the machines and aggregates, which are costly to invest, creates a considerable economic advantage if the process is used while maintaining the previous basis weights of the base paper or the coat weight.

The use of the method according to the invention offers further significant advantages while consistently taking advantage of the advantages that result from the lower penetration of the coating slips.

LWC papers that were produced by the coating process according to the invention are considerably better printable using the gravure printing process. The improved quality is noticeable by the greatly reduced number of «missing dots» (tiny, unprinted places), which are in the Compared to the previously common coating methods. In many cases, missing dots do not even occur if the coating weight is simultaneously reduced when using the coating method according to the invention in comparison with the methods previously used. This is a clear indication that the coating slip penetrates less deeply into the coating base paper and remains on the surface to a considerably greater extent than before.

Another important advantage that is made possible by the invention is the reduction in the basis weight of the coating base paper to a basis weight of up to 32 g / m ² . So far, the lower basis weight limit for LWC base papers was 36 g / m ² . Falling below this limit led to increased streaking of the coating slips. The use of a lighter coating base paper is of particular economic interest because the paper is the expensive raw material component compared to the coating slip.

The present invention makes it possible to shift the ratio of coating base paper and line coat weight beyond the previously existing limits and to better adapt the products to the respective requirements. Another advantage is that additional raw material costs can be saved by reducing the coat weight.

Whereas previously a coat weight of 8 g / m ^{2 was} considered the lower limit, coat weights of 6 to 7 g / m ² and side can now be achieved. In order to achieve these low coat weights, the solids content of the coating slip is reduced in a simple manner. The lowering of the solids content is in direct contrast to the previously known proposals to prevent the coating slips from streaking through by increasing the solids content. The previously mentioned lower limit of 8 g / m ² in the coat weight per side was due to the fact that with the high solids content and the low coat weight it was no longer possible to achieve a uniform application. A further reduction in the coat weight would only be possible with coating slips with an even lower solids content, which would then lead to the coating slips breaking through. However, by using the method according to the invention and the resulting solidification of the coating slip at the interface with the coating base paper, there is the possibility of further reducing the coating weight and the solids content of the coating slip.

In a very particularly advantageous embodiment for producing lightweight coated paper, a coating slip with a solids content of 45 to 62% by weight is used. This coating slip is applied to a wood-free coating base paper with a weight per unit area of 32-38 g / m ² , preferably of 34-36 g / m ² , with a knife coater on both sides, the weight of the dry coating 6 to 7 g / m ² and side is.

The measure popular in the practice of paper coating, in the production of a coating slip not only using a plastic dispersion, but several, if appropriate also binders based on starch or protein, does not have to be abandoned when producing the coating slips according to the invention. In order to achieve the object of the invention, it is not always necessary for all the binders used to coagulate or be converted into a gel-like state. Rather, it is usually sufficient if one of the plastic dispersions used - as a rule that is the one whose proportion in the coating slip is the largest - is impaired in its stability in such a way that the coating slip solidifies as much as possible. Partial solidification in the interface area also already fulfills the task of preventing further penetration or penetration of the coating slip into the coating base paper.

• 3-12 Gew.-Teile einer anionaktiven Kunststoffdispersion, gerechnet als Trockengewichtsteile, bis 95 Gew.-Teile anorganischer Pigmente, 0,1-5 Gew.-Teile einer oder mehrerer wasserlöslicher schwach kationaktiver monomerer Substanzen aus der Gruppe der tertiären und quaternären Aminoverbindungen, gerechnet als Trokkenteile und
  bis 3 Gew.-Teile sonstige Hilfsstoffe, wie Dispergiermittel, Entschäumer und Verdickungsmittel,

wobei sich alle Gewichtsteile auf 100 Gewichtsteile ergänzen.
Zwischen pH-Wert, Feststoffgehalt und Menge bzw. Art der schwach kationaktiven Substanz besteht dabei ein Zusammenhang. Werden in einer Streichmasse alle übrigen Komponenten in ihrer Art und Menge unverändert gelassen, kann bei gleichzeitiger Absenkung des pH-Wertes der Anteil der schwach kationaktiven Substanz verringert werden. Eine Reduzierung kann ebenfalls erfolgen, wenn der Feststoffgehalt der Streichmasse erhöht wird. Im Einzelfall sind alle Streichmassekomponenten aufeinander abzustimmen. Für Streichmassen von 55-62% Feststoffgehalt haben sich pH-Werte von 9-10 als besonders gut geeignet herausgestellt. Sie verfügen dabei über ein Fliessverhalten, das sich nicht von den bisher üblichen Streichmassen unterscheidet.A coating slip has proven to be particularly suitable for carrying out the process according to the invention, which is characterized in that it is adjusted to a pH of 8.1 to 10.1 and, with a solids content of 45-62% by weight, the following composition having:

• 3-12 parts by weight of an anionic plastic dispersion, calculated as parts by dry weight, up to 95 parts by weight of inorganic pigments, 0.1-5 parts by weight of one or more water-soluble, weakly cationic monomeric substances from the group of tertiary and quaternary amino compounds, calculated as dry parts and
  up to 3 parts by weight of other auxiliaries, such as dispersants, defoamers and thickeners,

where all parts by weight add up to 100 parts by weight.
There is a connection between pH value, solids content and the amount or type of weakly cation-active substance. If all of the other components in a coating slip are left unchanged in terms of their type and amount, the proportion of the weakly cation-active substance can be reduced while the pH is lowered. A reduction can also occur if the solids content of the coating slip is increased. In individual cases, all coating slip components must be coordinated. For coating slips with a solids content of 55-62%, pH values of 9-10 have proven to be particularly suitable. They have a flow behavior that does not differ from the usual coating slips.

The preparation of the coating slip is not subject to any particular restrictions, but it has proven to be expedient after the pigments have been dispersed in the alkaline range and the plastic or plastics has been added dispersions first add the cationic substance and then correct the pH.

The following examples illustrate the invention without restricting it. Examples 1 to 5 are coating compositions according to the invention which are preferably suitable for the production of lightweight papers which are very suitable for rotogravure printing. Comparative Example 1 describes a coating slip composition without the added substances, which become strongly cationic at pH <6.5.

Examples 6 to 8 describe coating slip compositions according to the invention which are preferably suitable for the production of lightweight papers which are very suitable for web offset printing. Comparative Example 2 in turn describes corresponding coating slip compositions without the added substances, which become strongly cationic at pH <6.5.

Example 9 gives a typical example of the production of coated paper using the coating slips of Examples 1 to 8 and Comparative Examples 1 and 2.

example 1

An aqueous slurry of 10444 parts by weight of china clay, prepared with the addition of a dispersant, which was adjusted to a pH of 8.8 with sodium hydroxide solution, was mixed with 80 parts by weight of a 50% calcium stearate solution and 1106 parts by weight of a anionic plastic dispersion based on a copolymer containing acrylic acid ester and having a solids content of 50% by weight.
857 parts by weight of a 2.3% aqueous solution of a tertiary dimethyl hexylammonium base are introduced into the mixture of pigment and plastic dispersion. The coating slip is diluted to a solids content of 50% by weight and adjusted to a pH of 8.7.

Example 2

1716 parts by weight of the solution used in Example 1 of tertiary dimethyl hexylammonium base are added to a mixture of pigment and plastic dispersion corresponding to Example 1. The coating slip is diluted to 50% solids content and a pH of 8.4 is set.

Example 3

A coating slip is prepared in accordance with Example 1, the solution used in Example 1 being replaced by 857 parts by weight of a 2.3% strength aqueous solution of a quaternary trimethylbenzylammonium compound. The coating slip is adjusted to a solids content of 50% by weight and a pH of 8.6.

Example 4

A coating slip is prepared in accordance with Example 1, 1287 parts by weight of the solution used in Example being added. The coating slip is adjusted to a solids content of 50% by weight and a pH of 8.6.

Example 5

According to Example 4, a coating slip is produced, the solids content of which is adjusted to 46% by weight and which has a pH of 8.2.

Comparative Example 1

A coating slip is prepared in accordance with Example 1 and adjusted to a solids content of 50% and a pH of 8.5. In contrast to Examples 1 to 5, however, no solution of a cation-active substance is added to this coating slip.

Example 6

With the addition of 9 parts by weight of a dispersant, 20 parts by weight of 33% sodium hydroxide solution and 15 parts by weight of sodium carbonate, an aqueous slurry is prepared from 9300 parts by weight of china clay.

A solution is prepared from 800 parts by weight of water and 200 parts by weight of casein by adding 19 parts by weight of 33% sodium hydroxide solution and 12 parts by weight of 25% ammonia, which solution is added to the pigment slurry.

1440 parts by weight of an anionic plastic dispersion based on an acrylic acid ester and styrene copolymer with a solids content of 50% by weight and 600 parts by weight of an anionic plastic dispersion based on a polyvinyl acetate copolymer with a solids content of 45% by weight are added in succession and with the mixture well mixed from pigment and casein.

Finally, 1515 parts by weight of a 12.5% strength solution of a trimethylbenzylammonium compound are mixed in and the coating slip is adjusted to a solids content of 50% and a pH of 8.4.

Example 7

A coating slip is prepared in accordance with Example 6, but to which 1516 parts by weight of a 15.1% strength solution of the same substance as in Example 6 are added instead of the solution added in Example 6. The coating slip is adjusted to a solids content of 50% and a pH of 8.2.

Example 8

A coating slip is prepared in accordance with Example 6, but to which 1312 parts by weight of a 12.5% strength solution of the same substance as in Example 6 are added instead of the solution added in Example 6. The coating slip is adjusted to a solids content of 60% and a pH of 9.6.

Comparative Example 2

According to the recipes of Examples 6 to 8, a coating slip is produced, each but no solution of a cation-active substance is added. The coating slip is adjusted to a pH of 10 and a solids content of 50%.

Example 9

On a test coater at a machine speed of 85 m / min with a doctor coater, the coating slips according to Examples 1 to 4, 6 to 8 and Comparative Examples 1 and 2 are applied to a lightweight, wood-containing coating base paper with a basis weight of 39 g / m2 and a surface pH of 4.8 spread on one side and dried as usual. The weight of the dried coating is 11 g / m ² .

Under the same test conditions on the test coating system, a coating slip according to the example is applied to a wood-containing coating base paper with a basis weight of 35 g / m ² and a surface pH of 4.6 and dried, the weight of the dried coating 6.5 g / m ² and side is. With the coating slips according to Examples 1 to 8, no streaking through of the coating slip was observed over a two-hour test period, while in the tests with the coating slips according to Comparative Examples 1 and 2, streaking through and spreading of the coating rolls occurred.

In order to enable an exact assessment and classification of the penetration behavior of the coating slips described according to Examples 1 to 8 and Comparative Examples 1 and 2, a new test method and a suitable test device were developed especially for this purpose and thus the individual coating slips were examined. Essentially, suitable substrates are coated with the coating slips under shear and pressure and the penetration behavior is made visible. The test device used for this is shown in FIGS. 1-3. 1 and 2 show schematic side views of a part of the device. Fig. 3 is an overall view of the device in perspective.

Fig. 1 illustrates the principle in a simple manner. A paint sump 17 is formed from the coating slip to be examined: the pressure of the squeeze rollers 4, 5 pushes the coating slip through the test track 12 and thereby produces more or less strong markings on a contrast material 16. The larger and more numerous the markings, the more the coating slip tends to strike through. A paper that differs in color from the coating slip can be used as contrast material.

An improved test method used in the present case can be seen in FIGS. 2 and 3. The squeeze roller 5 is covered with a moving film web 11 made of polyethylene. The film web prevents the test web 12 from sticking to the squeeze roller 5. An LWC base paper of 38 g / m ^{2 is} used as the test web. An initial marking 18 is placed on the test track 12, which lies directly in front of the line of contact of the two squeeze rollers 4, 5 and above the start of the contrast material 16a. A black paper is used as the contrast material 16a, which is glued to the nip roller 4 with its beginning. The length of the contrast material 16a corresponds to the circumference of the squeeze roller 4. The length of the test track 12 is selected such that it corresponds to 6 times the circumference of the squeeze roller 4 and thus 6 times the length of the contrast material 16a. This measure ensures that the markings on the contrast paper are reproduced in 6 times. The end of the test web 12 is connected by an adhesive seam 14, which also represents the end marking, to a material web 13 made of polyethylene film that is impenetrable for the coating slip. This makes it possible to guide the test track past the end of the adhesive seam 14 through the ink sump without more markings appearing on the contrast material than correspond to the measured test length. The width of the contrast material 16a is approximately 3 cm less than the width of the squeeze rollers 4, 5.

In order to achieve perfect web guidance, the film web 11 and the impenetrable material web 13 are unwound from unwinding rolls 2 and 3. The unwinding rolls 2 are mounted in bearing blocks 15 attached to frame 1 and are braked by band brakes 6. The width of the film web 11 corresponds to the width of the contrast material 16a. The steering roller 9 for guiding the film web 11 is arranged below the squeeze roller 5. The collecting trough 10 is attached below the squeeze rollers 4, 5 to accommodate dripping coating slip. The squeeze rollers 4, 5 are driven by hand crank 7 onto the permanently mounted squeeze roller 5. The squeeze roller 4 can be pressed against the squeeze roller 5, the contact pressure being able to be set via the rotary knob 8 and being readable on a scale (not shown). The squeeze roller 4 is driven by a squeeze roller 5 via a gear pair, not shown.

Test execution and evaluation

After roller 4, as described, is wrapped with contrast paper, the test web 12 connected to the material web 13 and the film web 11 are introduced and the squeeze rollers 4, 5 are pressed together. A paint sump 17 is then formed from the coating slip. By actuating the hand crank 7, the test track 12 is guided through the ink sump in the measured length. The contrast material 16a is then removed and compared with an evaluation scale which consists of eight different breakdown levels.

With level 1 there is very little through impact behavior and marked a very strong penetration behavior with level 8.

The evaluation of the

The informative value of this test result is additionally supported when samples of the finished papers are examined using the Heliotest method. The Heliotest process is a process commonly used in the paper and printing industry and was developed by the Center Technique de Industrie des Papiers, Cartons et Celluloses, Grenoble, France.

The heliotest is used to determine the number of missing dots. Compared to the previously common coating methods, the missing dots are reduced by between 30 and 40%.

Claims (13)

1. Process for the manufacture of coated paper or cardboard, in which an aqueous coating composition having particles of at least one aqueous latex and an inorganic pigment, such as chalk, titanium dioxide, talcum or kaolin, is applied to a raw paper or cardboard for coating and then dried, characterised in that a coating composition having a pH of more than 6.5 is applied to a raw paper or cardboard for coating having a pH of less than 6.5, the particles of the latex carry a non-cation-active charge and the coating composition contains one or more water-soluble weakly cation-active monomeric substances, selected from the group consisting of tertiary and quaternary amino compounds, that do not impair the stability of the coating composition during the manufacture and storage thereof, but that become highly cation-active in the pH range below 6.5, and, on contact with the acidically adjusted paper or cardboard, the coating composition solidifies at the boundary area of the raw paper or cardboard for coating.

2. Process according to claim 1, characterised in that there is used a raw paper or cardboard for coating having a pH of between 4.5 and 6.0.

3. Process according to claim 1 or claim 2, characterised in that there is used a raw paper or cardboard for coating of which the pH is adjusted to from 4.5 to 6.0 by the addition to the fibrous material pulp of a sulphuric acid compound of aluminium.

4. Process according to any one of claims 1 to 3, characterised in that there is used a raw paper or cardboard for coating of which the pH is adjusted by the addition of sulphuric acid.

5. Process according to any one of claims 1 to 4, characterised in that there is used a coating composition having a pH of between 7.5 and 11.

6. Process according to any one of claims 1 to 5, characterised in that there is used a coating composition containing a latex of which the particles carry an anion-active charge.

7. Process according to claim 6, characterised in that there is used a coating composition in which the anion-active charge of the latex is produced by anionic groups of the macromolecule itself.

8. Process according to claim 6, characterised in that there is used a coating composition in which the anion-active charge of the latex is produced by stabilisers and/or emulsifiers.

9. Process according to claim 8, characterised in that there is used a coating composition that contains the weakly cation-active substance in an amount of from 2 to 20 parts by weight of solids, calculated on 100 parts by weight of dry substance of the latex.

10. Process for the manufacture of coated paper and cardboard according to any one of claims 1 to 9, characterised in that a coating composition having a solids content of from 30 to 35 % by weight is applied with an airknife coating device to one side of a raw paper or cardboard for coating, and dried, in such an amount that the weight of the dried coating is from 10 to 12 g/m².

11. Process for the manufacture of coated paper according to any one of claims 1 to 9, characterised in that a coating composition that has a solids content of from 45 to 62 % by weight is applied with a blade coating device to both sides of a mechanical-wood-pulp-containing raw paper for coating having a weight per unit area of from 32 to 38 g/m², and is dried, and the weight of the dried coating is from 6 to 7 g/m² per side.

12. Process according to claim 11, characterised in that there is used a mechanical-wood-pulp-containing raw paper for coating having a weight per unit area of from 34 to 36 g/m².

13. Coating composition for carrying out a process according to any one of claims 1 to 9 and 11 to 12, characterised in that the coating composition is adjusted to a pH of from 8.1 to 10.1 and at a solids content of from 45 to 62 % by weight has the following composition: 3-12 parts by weight of an anion-active latex, calculated as dry weight parts; up to 95 parts by weight of inorganic pigment, 0.1-5 parts by weight of one or more water-soluble, weakly cation-active monomeric substances selected from the group consisting of tertiary and quaternary amino compounds, and up to 3 parts of other auxiliary substances such as dispersion agents, defoaming agents and thickening agents, the specified amounts adding up to 100 parts by weight of dry material.

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