EP0542125B1 - Process for separating a binder agent dissolved in water - Google Patents

Abstract

Binders dissolved in water, which contain a mixture of a high molecular-weight polymer, solvated by carboxylate groups, of an ethylenically unsaturated carboxylic acid polymerisable by a free-radical mechanism and of dissolved native starch, are almost completely precipitated if an acidifying agent is gradually added until coacervation and precipitation of the binder occur. In the manufacture of highly filled papers, the pigment can be bound in this way from an aqueous suspension.

Description

The invention relates to a method for separating a binder dissolved in water. Aqueous binder solutions or glues play a role in many technical processes in which finely divided substrates such as pigments or fibers are bound to one another or to a solid surface. As a rule, the binding process occurs when a film of the binder solution dries up. However, there are also methods in which a separation of the dissolved binder from the water phase and its precipitation on the dispersed substrate are aimed for.

Such processes offer advantages in papermaking, for example in the binding of pigments for the paper industry. The invention therefore also relates to the treatment of a pigment suspended in water for the paper industry with an aqueous binder or the pigment thus treated. The invention further relates to a method for producing pigment-containing paper with increased tear strength or with increased pigment content.

State of the art

In the manufacture of printing papers, it has been common for over a century to paint the surface in order to obtain a good printed image. The coated papers are also used as art paper, art paper or chromo paper and in the highest quality level Designated enamel paper. The purpose of the stroke is to form a layer for printing, which consists exclusively of pigments and a binder. This layer is mostly compacted by satin and brought to shine. It enables the reproduction of the finest halftone dots.

Coating is an expensive process, which is usually carried out in a separate coating system behind the paper machine. Since printing on pigments or pigment layers leads to significantly better printing results than printing on a pure nonwoven fabric, efforts have been made for decades to introduce more pigments into the paper directly on the paper machine without reducing its tear strength. This could avoid the complex painting process.

Wood-containing, highly filled and highly satinized gravure papers with pigment contents between 17 and 30% by weight are widely used. They are called SC papers (super calendered). During their production, the pigments, usually kaolin or talc, are bound in the nonwoven fabric by adsorption and filtration.

To better bind the pigment, binders have also been used, for example modified starch, carboxymethyl cellulose, alginates, mannogalactans (meyproid), gelatin and skin glue. They are used as colloidal solutions in the material input and are adsorptively bound to the pigment and the fiber by electrokinetic forces. This bond is never complete. One finds therefore in the circulation water and in the wastewater Paper mills part of the binder used, which is lost as a result and makes a waste water treatment necessary.

EP-A 50 316 describes a paper production process in which, in a first process step, an aqueous suspension of an inorganic pigment is mixed with a classic organic paper binder, such as dextrin, starch, carboxymethyl cellulose, polyvinyl alcohol or plastic dispersions, and the binder precipitates using a cationic flocculant. Possible flocculants are polycationic compounds, such as polyethyleneimine, cationically modified polyacrylamides, polyaluminium chloride and cationic starch. The pigment suspension used can optionally contain conventional dispersants, such as polyphosphates or sodium polyacrylate; such dispersants do not act as binders.

In the second process stage, the pigment pretreated in this way is added to an aqueous paper stock and the sheet formation is then carried out. Excellent pigment retention is achieved during sheet formation and paper with improved tear strength is obtained.

In one of the processes described in DE-A 2 115 409, mineral fillers for the paper industry, especially calcium carbonate, are provided with a coating of an organic polymer, which is primarily intended to suppress the decomposition of calcium carbonate in the acidic range. The coating can be, for example, from an aqueous solution neutralized acrylic acid polymer can be formed by precipitation using aluminum sulfate. The aluminum ions have the effect of imparting a positive charge to the filler or pigment and thereby increasing their affinity for the cellulose fibers.

Although dissolved starch is a very good and cheap binder, it is practically not used in the mass sizing of paper. It is known that unmodified cooking starch or nonionically modified starch is only retained in the paper stock as a binder to about 40%, while the majority of the starch remains in the circulating water. For this reason alone, despite its high binding capacity and low price, the cooking starch has not been able to establish itself in the paper industry. In practice, mainly cationically modified starch is used in connection with retention aids, whereby retention values of up to over 80% can be achieved; see. C.Palm et al. J.-L. Hemmes, Wochenblatt für Papierfabrikation 5/1991, pp.149-154.

WO-A 8201020 (EP-A 60 291) describes a process for the production of papers with high tear strength and high pigment content using cationically modified starch. In addition to the cationic starch, carboxymethyl cellulose and an inorganic polymer are added to the pigment-containing substance, which causes these binders to coacervate. Instead of carboxymethyl cellulose, polyacrylic acid can also be used. Partially hydrolyzed alum can be used as the inorganic polymer under certain circumstances.

Cationic starch is now widely used in papermaking to improve strength, tear length, retention and drainage, and to increase filler and waste paper content. Since the cationic state of charge is low, cationic retention aids are mostly used. The effect is based on the electrokinetic fixation of the starch to the cellulose fibers and the fillers.

Electrokinetic effects, which are based on the anionic charges of cellulose fibers, fillers and anionic binders on the one hand and the cationic charges of modified starch, retention aids or cationic surfactants on the other hand, are used in numerous processes in paper production and finishing. Typical examples are the processes according to EP-A 263 519, 279 313 and 234 513. In the latter process, a mixture of cationic starch, an anionic binder polymer and finely dispersed silica is incorporated into a paper stock. As a result of the electrokinetic interactions, the previously dissolved binder precipitates, which is falsely referred to as coacervation. The separated binder improves dewatering and filler retention during sheet formation. The strength of the paper is also increased.

The inventors have found that binders precipitated by electrokinetic effects are not bound in a shear-stable manner, so that binders always get into the circulating water during the subsequent sheet formation.

Task and solution

The aim of the invention is to make native starch usable as a binder in aqueous systems, from which it has so far not been possible to separate it using simple means. In a narrower sense, the aim of the invention is a process for the complete separation of a starch-containing binder dissolved in water in the form of a film on the surface of a solid substrate without flocculation of the binder.

Another object of the invention is a method for treating a pigment suspended in water for the paper industry with a starch-containing binder and then fixing the binder to form a pigment suspension which is suitable for the production of high-pigment paper by sheet formation from an aqueous paper stock. The binder should be bound as firmly as possible to the pigment so that it does not detach from it even after intensive shear treatment.

It has been found that these objectives are achieved if a mixture of a high molecular weight polymer of an ethylenically unsaturated, free-radically polymerizable carboxylic acid and dissolved native starch and solvated by carboxylate groups is used as the binder and an acidifying agent is gradually added to the dissolved binder until coacervation and separation of the Enter binder. If the coacervation occurs in the presence of a fine particle, in the Binder solution dispersed substrate, the coacervate is deposited on the surface. The amount of acidifying agent should be limited so that the substrate maintains a negative surface charge.

The reaction of the binder which has been solvated by carboxylate groups with the acidifying agent has the character of a coacervation, without thereby fixing the invention to any particular theory. One understands this (according to Römpps Lexikon der Chemie, 9th edition, p.2770) the transition of the binder originally present as a dissolved colloid from the sol state into a solid precipitate. An intermediate state is run through in which the previously evenly distributed polymer separates out in a separate, still liquid, water-containing phase. Apparently, this phase connects to the surface of the substrate and changes into a completely insoluble state with increasing drainage.

If the starch has been gelatinized intensively, preferably under pressure, and not too much starch, based on the polymer content, is used, the starch can be completely bound. If the treated pigment suspension is allowed to sediment after the coacervation, the supernatant water is completely clear and shows no Tyndall effect. In the aqueous phase of the pigment suspension, practically no more of the binder is found after the coacervation is complete. As a rule, the aqueous phase contains less than 5% by weight, usually even less than 1% by weight, of the binder originally used. Often With conventional detection methods, such as the COD measurement or the iodine starch test, it is no longer possible to find organic substance contents in the supernatant aqueous phase that go beyond zero. This applies even more to the white water of sheet formation when the pigment suspension treated according to the invention is added to a fiber material for the production of a pigment-containing paper.

The incorporation of the starch portion during the coacervation must be regarded as surprising, especially since starch alone is not accessible to the coacervation process according to the invention. It is apparently entrained by the coacervation of the anionic binder, so that the term "co-coacervation" is appropriate in relation to the starch content.

Surprisingly, the adhesion of the binder to the pigment proves to be shear stable. Even if the pigments treated according to the invention are exposed to high shear forces for a long time, the binder is not detached from the pigment particles again and the aqueous phase remains free of the binder used. As a rule, the content of the binder in the aqueous phase in a shear treatment with an intensive mixer according to Prof. Wilms ("Ultraturrax" ^(R) , manufactured by Janke & Kunkel) increases to not more than 5 within 3 minutes at 4000 rpm % By weight, based on the total binder content of the suspension.

For the coacervation process characterizing the invention it is essential that the dispersed substrate particles, e.g. Pigment particles, in anionic form, as they normally exist, are used and are not cationically transferred during coacervation. Deposition of the binder with agglomeration and flocculation of the pigment based on electrokinetic attractive forces would be undesirable in papermaking. The electrical charge state of the particles, which is also referred to as zeta potential, can be recognized from their migration behavior in the electrical field. Charged particles with negative zeta potential migrate to the anode during electrophoresis.

It is important that the binder is not completely drained through the gradual process of coacervation. A solvation state is sought which lies between the complete solvation of the solution state and the desolvated state of a hard and solid precipitate. This condition is brought about by gradually lowering the zeta potential without reaching or exceeding the isoelectric point of the anionic polymer. Maintaining sufficient solvation, which has a plasticizing and elasticizing effect on the polymer mixture, is important for its binding ability.

By adding acidifying agents, the degree of neutralization and thus also the solvation decrease. The polymer containing carboxylate groups is becoming less and less soluble and begins to separate - together with the starch - as a water-containing phase from the surrounding aqueous phase. That is the beginning of coacervation. It is continued until a solvation state is reached in which the insoluble coacervate, including the starch fraction, has deposited on the surface of the substrate particles, but still contains enough water to develop a high binding force. Only when the sheet formed dries does the binder change to a solid state and develop its binding and strengthening effect.

Local acidification should be avoided as much as possible during coacervation. It would lead to heavily drained portions of low binding power. In any case, the pigment suspension should not be added to the solution of the acidifying agent, because this would then be temporarily in excess. The acidifying agent is added with stirring, with as uniform a distribution as possible, at a rate which keeps pace with the reaction with the polymer. In order to avoid uneconomically long coacervation times, stirring as intensely as possible is advantageous.

By re-neutralization, the coacervate from the synthetic and the native binder can be solvated again or even brought into solution. This is important for the processing of waste paper.

Application of the deposition process

The method according to the invention is not only important for paper production. It can be done with everyone Use methods in which the surface of a solid substrate is coated with a film of a binder previously dissolved in water, without the relative charge state of this surface being important. The coated substrate usually does not bond firmly until it is separated from the water phase or deposited on the surface of a larger solid body. The coacervate film then establishes the connection between the substrate particles as well as to the coated body surface. The full binding power develops when drying.

Pigment suspensions treated according to the invention are particularly suitable for the production of papers with a high pigment content on the paper machine. The highest strength values are achieved when the treated pigment suspension is incorporated into the fiber. If appropriate, the procedure can also be such that the pigment, the binder and the fibrous material are mixed in the central unit of the paper machine, made alkaline and the coacervation is effected by adding the acidifying agent to this mixture. You can also incorporate the binder into the alkaline fiber, then add the pigment and then perform the coacervation. Thereafter, sheet formation takes place on the screen in accordance with customary methods. The paper is then preferably satinized.

In this way, papers with a total pigment content of up to 45% by weight, preferably 17 to 35% by weight, are obtained. In extreme cases, the pigment content can still be further increase; even contents of 60% by weight can be achieved. In relation to the high pigment content, the tear length of the paper - as a characteristic measure of its strength - is surprisingly high. The invention thus makes it possible to produce papers with the usual high pigment contents and increased tear length or papers with the usual tear length and significantly increased pigment content. The latter means a reduction in costs, since the pigments are generally cheaper than the fibers, and at the same time an improvement in the quality of the printing properties due to the high pigment content.

The pigment suspension treated according to the invention can optionally also be used to coat papers.

The polymer containing carboxylate groups

The process of the invention can also be carried out without the starch component of the binder using only the anionic polymer. In this embodiment, it forms the subject of the international patent application PCT / DE 91/00376 (currently unpublished). Since native starch is an inexpensive tool widely used in the paper industry, the use of a binder composed of starch and the synthetic anionic polymer is of great economic importance.

The carboxylate-containing polymers suitable for the process of the invention can be available as water- or alkali-soluble solid products, as colloidal solutions or aqueous dispersions, such as homopolymers and copolymers based on vinyl acetate and crotonic acid or partially saponified poly (meth) acrylates. Homopolymers and copolymers of acrylic and / or methacrylic acid in the form of their sodium salts are preferred.

In the pure acid form, the polymer is not water-soluble and must be brought into a solvation state suitable for coacervation. For this purpose, a sufficient part of the carboxyl groups must be in the form of carboxylate groups. They bring about the solvation of the polymer with water, so that it is in the truly dissolved or at least in the colloidally dissolved state. Real solutions are largely clear. Colloidal solutions are characterized by a more or less clear turbidity. If the polymer contains non-neutralized carboxyl groups, a colloidal, slightly cloudy solution can be converted into a real solution by further neutralization.

The required state of solvation is achieved by a sufficient content of carboxylate groups in the polymer. In the case of polymers with a high carboxyl group, partial neutralization of the carboxyl groups to give carboxylate groups is sometimes sufficient, whereas in the case of copolymers with a low carboxyl group content, complete neutralization is usually necessary.

If the carboxyl group content is too low, sufficient solvation cannot be achieved even with complete neutralization.

The carboxylate content required for sufficient solvation depends on the hydrophilicity of the entire polymer. As a rule, it is in the range from 3 to 10% by weight, calculated as COO⁻ and based on the weight of the non-neutralized polymer. If the polymer is composed entirely or predominantly of units of an ethylenically unsaturated, free-radically polymerizable carboxylic acid, complete neutralization is admittedly advantageous, but not essential. The pH of the binder solution is in the range of about 8 to 11, depending on the degree of neutralization.

In principle, any base which contains monovalent cations is suitable for neutralizing the carboxyl to carboxylate groups. Aqueous alkali, especially sodium hydroxide solution, is preferred for economic reasons.

The proportion of the ethylenically unsaturated, free-radically polymerizable carboxylic acid should generally be not less than 6 and not more than 80% by weight, preferably 10 to 80% by weight, in particular 20 to 80% by weight. Acrylic and / or methacrylic acid and maleic acid are preferred; Fumaric, itaconic or crotonic acid are also suitable.

Non-ionic, slightly or poorly water-soluble ethylenically unsaturated, free-radically polymerizable monomers can be involved as comonomers in the structure of the polymer. Ethylene and alkyl esters of acrylic and / or methacrylic acid, in particular with 1 to 4 carbon atoms in the alkyl radical, have an advantageous effect.

Their proportion is preferably 20 to 90% by weight, particularly preferably 20 to 80% by weight. Other comonomers that can be used are e.g. Styrene, acrylonitrile or vinyl acetate. More hydrophilic or water-soluble comonomers, such as acrylic and / or methacrylamide or hydroxyalkyl esters of acrylic and / or methacrylic acid, can be used, for example, in proportions up to a total of about 30% by weight, preferably up to 10% by weight.

Finally, even small amounts of crosslinking comonomers with two or more ethylenically unsaturated, free-radically polymerizable groups in the molecule, such as ethylene glycol diacrylate and dimethacrylate, allyl acrylate and methacrylate, can be involved in the synthesis of the polymer. However, their proportion must be low enough to permit adequate solvation, for example up to 3, preferably up to 1, in particular up to 0.1% by weight.

A satisfactory effect as a binder requires a sufficient molecular weight of the polymer. It should generally be at least 20,000, preferably 50,000 to 2 million, each determined as the weight average. Even higher molecular weights lead to high viscosities which make it difficult to use them on the paper machine without being beneficial for the binder effect. Preferred binders in the form of an aqueous solution adjusted to pH 9 with sodium hydroxide solution at a concentration of 200 g / l and 20 ° C. have a viscosity of more than 100, in particular more than 1000 mPa s. This viscosity is already achieved by very high molecular weight binders at a concentration of around 30 g / l.

The native strength

Completely untreated starch, such as potato, wheat, corn or tapioca starch, are best suited for the process of the invention. Mixtures of starches of different vegetable origin can also be brought together in solution and used in the method of the invention. Any pretreatment of the native starch increases its price, often many times over, and is therefore unnecessary.

In order to ensure the coacervation of the starch with the polymer containing carboxylate groups, both binder components must be in a sufficiently solvated, colloidally dissolved state. A mixture that is as homogeneous as possible is important for the effectiveness of the binder mixture. The starch should therefore be gelatinized as intensively as possible. Good starch qualities dissolve sufficiently when cooking. However, it is usually more advantageous to gelatinize at a temperature above 100 ° C, e.g. at 105 to 150 ° C, especially 110 to 130 ° C. These temperatures are reached in pressure cookers or continuous jet cookers at steam pressures of 2 to 7 bar. It is particularly advantageous to digest the native starch in an autoclave at a temperature above 100 ° C. in the presence of the polymer containing carboxylate groups.

Native starch has the advantage over the cationic starch which is predominantly used today that it contains no constituents which increase the AOX value (absorbable organic halides) of the circulating water. The use of AOX-free binders is becoming increasingly important as pulping with low AOX pollution is increasingly successful.

Amount of binder

Based on the weight of the dry pigment, the binder is expediently used in an amount of 1 to 11, preferably 2 to 5,% by weight, calculated as the total weight of the pure, unneutralized polymer and the dry starch. If the binder content is high, there is an increased risk that it will not be completely bound to the pigment.

In the interest of saving costs, the proportion of starch is chosen as high as possible. On the other hand, the higher the proportion of the polymer component, the better the incorporation of the starch in the co-coacervation. In practice, therefore, a compromise has to be made between the binder costs and the still acceptable losses of starch. With starch contents below 10% by weight, based on the weight of the binder, the cost advantage is hardly noticeable. With proportions of 20 to 50% by weight, in particular 25 to 40% by weight, the most favorable ratio of low costs and high effectiveness is achieved. If the starch content is further increased, the risk of incomplete binding to the pigment increases.

The pigment

The process of the invention can be carried out with all pigments customary in the paper industry. The term "pigment" includes all fillers commonly used in the paper industry. Inorganic, especially acid-resistant, pigments are preferred. These include kaolin, talc, calcium carbonate, Calcium sulfate, silica, barium sulfate, titanium dioxide, and mixtures thereof. Kaolin and talc are particularly preferred. As a rule, the particle size of at least 50% by weight of the pigment particles is between 0.1 and 10, preferably between 0.3 and 5 micrometers. The majority of the pigments in aqueous slurry have a negative zeta potential, that is, they are in the anionic state.

The acidulant

These are to be understood as meaning all agents of sufficient acidic action with which the pH of the binder solution can be reduced from the initial value of 8 to 11 to values of about 4 to 8. As a rule, they are low molecular weight, in particular inorganic, acidic compounds. These include mineral acids such as Sulfuric acid. It is preferred to use acidic salts, such as alkali hydrogen sulfates or, in particular, aluminum sulfate, which is mostly referred to as alum in the paper industry.

The amount of acidifying agent is critical in order to achieve the desired coacervation state and to avoid electrical charge reversal of the pigment. The pH of the treated suspension depends on the type of polymer. Polymers with a high carboxyl group content achieve the optimal coacervation state at lower pH values, namely about pH 5-6, than polymers with a low carboxyl group content which achieve their best binding capacity at about pH 7-8. If a mineral acid is used as an acidifying agent, then the acid equivalent amount used below the equivalent amount of the carboxylate groups of the polymer. When using aluminum sulfate, which reacts acidically due to hydrolysis, a stoichiometric calculation of the need for acidifying agents is hardly possible.

In the preferred poly (meth) acrylates, the coacervation takes place in such a way that the binder solution, which has a pH in the alkaline range, is acidified, preferably with aluminum sulfate, whereby the colloidal system is destroyed at a certain pH and the binder fails.

Preferred way of working

The inorganic pigment is suspended in water in a concentration of 2 to 30% by weight, preferably 2 to 20% by weight. Common dispersants, such as polyphosphates, can be used as long as they do not interfere with coacervation. The pH of the suspension is adjusted to the pH of the binder solution. With stirring, the binder is stirred into the suspension in the form of an aqueous solution and distributed evenly. Then an aqueous solution of the acidifying agent is gradually stirred in, avoiding local acidification, which triggers the coacervation.

The suspension is added to the fibrous material before or after coacervation. All of the fibrous materials commonly used in paper production can be used, such as wood pulp, cellulose, semi-cellulose, high-yield materials, waste paper. The pulp has when mixing Pigment suspension preferably has a solids content of 3 to 4% by weight and is diluted to 0.1 to 1% by weight with circulating water before the sheet formation. The mixture is expediently prepared directly in the central unit of a paper machine. Common additives, such as defoamers, dispersants, thickeners, retention agents, optical brighteners, dyes, fungicides, bactericides, lubricants, can also be used in customary amounts. All of the process steps mentioned can be carried out at the temperatures customary in paper production. The mass is then shaped into a sheet in the usual way and can then be satined.

When using acid-sensitive pigments, such as calcium carbonate, it may be advantageous to initiate the coacervation in the absence of the pigment, to finely emulsify the resulting coacervate, if necessary with gentle heating, and only then to mix in the pigment and the fiber.

Papers with a basis weight of 32 to 170 g / m 2 are preferably produced. They have the quality of known SC papers or even surpass them. They are particularly suitable as printing papers.

EXAMPLES a) General mode of operation

A 5% suspension of kaolin in water is adjusted to pH 11 with sodium hydroxide solution. Then an alkaline solution of the binder is added with stirring. A pH of 5.5 is set by gradually adding aluminum sulfate. Measuring the zeta potential ensures that the pigment has a negative surface charge. This mixture is mixed in the central unit of a paper machine with the fibrous material consisting of spruce pulp and ground wood in a ratio of 1: 1, so that a solids content of 0.5% by weight results. The mass is then shaped into a sheet in the usual way. The tear length is measured on the finished paper.

b) Binder used

Rohagit S mV (trade name of Röhm GmbH, Darmstadt):
Powdery alkali-soluble acrylic resin with an acid number of 405 - 440 mg KOH / g. A 3% aqueous solution adjusted to pH 9 with NaOH has a viscosity of about 4000 mPa s.

Starch: Oxidized, hot water-soluble potato starch with about 16 to 20 mg carboxyl groups per 100 g (commercial product "Perfectamyl ^R PH 255 SH, AVEBE GmbH, Düsseldorf)

c) Test series and results: see Tables 1 and 2.

Experiments V1 to V13 were carried out as blank tests without the addition of a binder or with the use of starch or the synthetic polymer alone, in order to have a comparison basis for the tear length with the same pigment content, but without the binder according to the invention.

In experiments 7 to 10 and comparative experiment V13, the pigment / fiber ratio was adjusted so that a filler content of the paper of 35 to 38% by weight resulted.

Results Table 1 : Examples Nos. 1 to 6

The binder components are in% by weight, based on. specified on filler weight. RS = Rohagit S; St = strength. V1 - V12 = comparison tests Example No. binder Pigment / fiber ratio Tear length m Fill i.Pap. % By weight % RS % St V1 0 0 50: 50 1058 28.2 V2 1 0 50: 50 1082 40.5 V3 4th 0 50: 50 1311 40.9 V4 0 2nd 50: 50 1684 21.5 1 1 2nd 50: 50 1301 39.6 2nd 4th 2nd 50: 50 1383 38.3 V5 0 0 60: 40 881 32.1 V6 1 0 60: 40 934 49.5 V7 4th 0 60: 40 1028 49.6 V8 0 2nd 60: 40 1320 26.9 3rd 1 2nd 60: 40 1032 48.9 4th 4th 2nd 60: 40 1040 49.3 V9 0 0 70:30 670 38.9 V10 1 0 70:30 633 59.3 V11 4th 0 70:30 756 58.0 V12 0 2nd 70:30 1228 29.8 5 1 2nd 70:30 848 56.6 6 4th 2nd 70:30 862 55.2

Results table 2 : Examples Nos. 7 to 10

The binder components are in% by weight, based on. specified on filler weight. RS = Rohagit S; St = strength. V13 = comparison test Example No. binder Tear length m Fill i.Pap. % By weight % RS % St V13 0 0 914 36.3 7 2nd 2nd 1571 37.7 8th 2.4 1.6 1807 35.2 9 2.8 1.2 1764 35.9 10th 3.2 0.8 1723 35.9

Claims (18)

1. A process for separating a binding agent dissolved in water, characterised in that a mixture of a high molecular weight polymer of an ethylenically unsaturated, radically polymerisable carboxylic acid and a native starch solution is used as binding agent, the polymer being solvated due to its carboxylate groups, and that an acidifying agent is gradually added until coacervation and separation of the binding agent occurs.
2. A process according to claim 1, characterised in that native starch is used which has been dissolved at a temperature above 100°C.
3. A process according to claim 1 or 2, characterised in that the coacervation is carried out in the presence of a finely divided substrate distributed in the binding agent solution.
4. A process according to claim 3, characterised in that the amount of acidifying agent is limited in such a way that the finely divided substrate retains a negative surface charge.
5. A process according to one or more of claims 1 to 4, characterised in that the binding agent consists, in an amount of 50 to 80 wt.%, of the high molecular weight polymer solvated due to its carboxylate groups and, in an amount of 20 to 50 wt.%, of the soluble starch.
6. A process according to one or more of claims 1 to 5, characterised in that a pigment is used as the finely divided substrate.
7. A process according to claim 6, characterised in that an inorganic pigment for the paper industry is used.
8. A process according to one or more of claims 1 to 7, characterised in that paper and/or textile fibres are used as finely divided substrates.
9. A process for preparing filler-containing paper, in which leaves are formed from an aqueous paper material, characterised in that this material contains a pigment treated according to claim 6 or 7.
10. A process according to claim 7 or 9, characterised in that kaolin, calcium sulphate, talc or titanium dioxide are used as inorganic pigment.
11. A process according to one or more of claims 1 to 10, characterised in that a polymer with a molecular weight Mw >20,000, preferably >50,000 is used as the polymer part of the binding agent.
12. A process according to claim 11, characterised in that a polymer is used which contains 6 to 80, preferably 10 to 80 wt.% of units of acrylic, methacrylic and/or maleic acid.
13. A process according to one or more of claims 1 to 12, characterised in that an acidically reacting salt of a multi-valent metal cation is used as acidifying agent.
14. A process according to claim 13, characterised in that aluminium sulphate is used.
15. A process according one or more of claims 1 to 14, characterised in that native starch is used which has been dissolved at a temperature above 100°C, in the presence of the high molecular weight polymer solvated due to its carboxylate groups.
16. An aqueous suspension of a pigment for the paper industry, characterised in that the treated pigment has a negative surface charge and contains a coacervate of a high molecular weight polymer of an ethylenically unsaturated, radically polymerisable carboxylic acid and soluble native starch, the polymer being solvated due to its carboxylate groups.
17. An aqueous suspension according to claim 15, characterised in that less than 5 wt.% of the binding agent is contained in the aqueous phase.
18. An aqueous suspension according to claim 16, characterised in that the content of binding agent in the aqueous phase is not more than 10 wt.%, based on the total binding agent content of the suspension, after an intensive shearing treatment of 3 min, using a high-speed stirrer at 4000 rpm.