EP0542159A1 - Process for treating an aqueous pigment suspension with an aqueous binder agent - Google Patents

Abstract

An aqueous binder, consisting of an alkaline solution of an amphoteric high molecular-weight polymer, is added to an aqueous pigment suspension for the paper industry and gradually neutralised with an acidifying agent, whereby the binder is precipitated on the pigment suspended in water.

Description

The invention relates to a method for treating a pigment suspended in water for the paper industry with an aqueous binder. The invention further relates to a method for producing pigment-containing paper with increased tear strength or with an 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 referred to as art paper, art paper or chromo paper and in the highest quality level as 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 usually compacted by satin and brought to shine. It enables the reproduction of the finest screen dots.

Coating is a costly 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 coating 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 manufacture, 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, e.g. 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. Part of the binder used is therefore found in the circulating water and in the waste water of the paper mills, 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 stage of the process, 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 e.g. be formed from an aqueous solution of a neutralized acrylic acid polymer 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.

Task and solution

The aim of the invention is a process for the treatment of a pigment suspended in water for the paper industry with a binder and subsequent fixing of 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 so firmly bound to the pigment that it is does not come off again and that the aqueous phase of the suspension contains less than 5% by weight of the binder and that the content of the binder in the aqueous phase does not increase even after intensive shear treatment.

It has been found that this goal is achieved if an alkaline solution of an amphoteric high molecular weight polymer is used as the binder and the pigment suspension mixed with the binder is gradually neutralized with an acidifying agent.

The amphoteric polymer apparently reaches a solubility minimum in the neutral range, ie at a pH of about 5.5 to 7.5, preferably from 6 to 7. It is assumed that the polymer molecules are strongly solvated in the alkaline and acidic range and are therefore readily soluble. This state appears due to the clarity and relatively high viscosity of the pure polymer solution. In the neutral range, the solvation decreases, so that the polymer molecules clump together, which can be seen from a slight clouding of the solution and a decrease in its viscosity. The binding capacity of the amphoteric polymer is highest in this state. It is able to attach to the pigment particles and, if necessary, to the fibers, so that it is almost completely retained when the leaves form. If the neutral range is exceeded, the solubility of the polymer increases again. In the acidic range, there is at most a partial bond to the pigment.

The attachment of the binder to the pigment or fiber surface seems to be brought about by a synergism of electrokinetic and coacervative interactions. Solubility effects are likely to play an important role, so that the deposition of the binder can be regarded as coacervation. This means (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 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 pigment particles and changes into an insoluble state with increasing drainage.

If the treated pigment suspension is sedimented after the coacervation, the supernatant water is completely clear and shows no Tyndall effect. Therefore, in the aqueous phase of the pigment suspension, practically no more of the binder is found after the coacervation is complete. In any case, the aqueous phase contains less than 5% by weight, usually even less than 1% by weight, of the binder originally used. Common detection methods, such as the COD measurement, can often no longer find organic substance levels 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 fibrous material for the production of a pigment-containing paper.

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 prolonged period, 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.

A complete solvation of the polymer is not absolutely necessary at the beginning of the process. A limited solvation is often sufficient, which allows at least one colloidal solution state. When the binder solution is neutralized, the carboxylate groups originally present are converted into non-neutralized carboxyl groups or a betaine structure is formed in which the carboxylate groups are countered by the cationic groups of the amphoteric polymer. The insoluble coacervate, which has deposited on the surface of the pigment particles, 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 if possible during neutralization. In any case, the pigment suspension should not be added to the solution of the acidifying agent. The acidifying agent is preferably 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 making alkaline, the coacervate can be solvated again or even brought into solution. This is important for the processing of waste paper.

Application of the treated pigment suspension

The pigment suspension treated according to the invention is suitable for the production of papers with a high pigment content on the paper machine. The highest strength values are achieved when the treated 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 and the coacervation is effected by adding the acidifying agent to this mixture. You can also work the binder into the alkaline fiber, then add the pigment and then perform the coacervation. Thereafter, the sheet is formed 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 be increased even further; 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 binder

Amphoteric polymers suitable for the process of the invention can be available as clear to colloidal solutions. The amphoteric character is based on the presence of anionic and cationic monomer units of the polymer molecules. The equivalence ratio of anionic to cationic groups is preferably in the range from 5: 1 to 1: 5. The additional presence of nonionic, in particular hydrophobic, comonomer units has proven to be advantageous.

The anionic monomer units are preferably derived from ethylenically unsaturated, free-radically polymerizable mono- or dicarboxylic acids, such as, for example, acrylic, methacrylic, itaconic or / and maleic acid. Suitable cationic monomer units contain an ethylenically unsaturated, free-radically polymerizable group and an amino or ammonium group covalently linked to it. Preferred amphoteric polymers are 6 to 80, preferably 10 to 80% by weight of units of unsaturated carboxylic acids and 10 to 90% by weight, preferably 20 to 50% by weight of units of a nitrogen-containing cationic monomer and optionally up to 84% by weight made up of nonionic monomer units. Typical examples of the cationic monomers are:
2-dimethylaminoethyl acrylate and methacrylate
3- (N, N-dimethylamino) propyl acrylate and methacrylate,
4- (N, N-dimethylamino) butyl acrylate and methacrylate,
3- (N, N-dimethylamino) propyl acrylamide and methacrylamide, triethanolamine monoacrylate and monomethacrylate,
2- (dimethylaminoethyloxy) ethyl acrylate and methacrylate,
2-imidazolyl ethyl acrylate and methacrylate,
2-piperazinyl ethyl acrylate and methacrylate,
2-piperazinyl-ethyl-acrylamide and -methacrylamide,
N, N-dimethylamino-neopentyl acrylate and methacrylate,
N, N-dimethylamino-neopentyl acrylamide and methacrylamide,
(1,2,2,6,6-pentamethyl-piperidyl-4) acrylate and methacrylate
3-morpholino-propyl-acrylamide and methacrylamide,
2-morpholino-ethyl acrylate and methacrylate,
2- (N, N-dibutylamino) ethyl acrylate and methacrylate,
4-diethylamino-1-methyl-butyl-acrylamide and methacrylamide,
The amphoteric polymer preferably contains units of cationic monomers with quaternary ammonium groups. The corresponding monomers are derived from the aminoalkyl monomers listed above by reaction with quaternizing agents, such as methyl chloride or dimethyl sulfate.

The amphoteric polymer definitely contains a sufficient proportion of acidic monomer units to be soluble at least in the alkaline range (pH ≧ 8).

The carboxylate content required for sufficient solvation depends on the hydrophilicity of the entire polymer. It is largely determined by the units of the nitrogen-containing cationic monomers. The solvation can be further enhanced by the incorporation of nonionic, highly hydrophilic monomer units, such as acrylamide and methacrylamide or hydroxyalkyl esters of acrylic and / or methacrylic acid, but must not be so high that the Content of such groups at the neutral point is sufficient for complete solvation. This would make the deposition of the polymer more difficult. The content of nonionic hydrophilic monomer units is therefore preferably not more than 60% by weight.

As other comonomers, slightly or poorly water-soluble ethylenically unsaturated, free-radically polymerizable monomers can be involved in the structure of the polymer. Styrene, ethylene and alkyl esters of acrylic and / or methacrylic acid have an advantageous effect, in particular with 1 to 4 carbon atoms in the alkyl radical. 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. Acrylonitrile or vinyl acetate. 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.

In principle, any base containing monovalent cations is suitable for the alkaline setting of the binder solution. Aqueous alkali, especially sodium hydroxide solution, is preferred for economic reasons. The binder solution preferably has a pH of 8 to 12, in particular 9 to 11.

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 1 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 achieved by very high molecular weight binders at a concentration of around 30 g / l.

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 a pure, non-neutralized polymer.

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 their mixtures. Kaolin and talc are particularly preferred. Usually the particle size is from at least 50% by weight of the pigment particles between 0.1 and 10, preferably between 0.3 and 5 micrometers. Most of the pigments in aqueous slurries have a negative zeta potential, ie they are in the anionic state.

The acidulant

These are understood to mean 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 5.5 to 7.5. 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 acidifier is critical to achieve the minimum solubility at which the desired state of coacervation is achieved. The optimal pH of the treated suspension depends on the polymer composition. If a mineral acid is used as the acidifying agent, the amount of acid equivalent used is less than the equivalent amount of the carboxylate groups of the polymer. When using aluminum sulfate, which reacts acidly due to hydrolysis, a stoichiometric calculation of the need for acidifying agents is hardly possible.

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 fiber before or after coacervation. All of the fibrous materials commonly used in papermaking can be used, such as ground wood pulp, cellulose, semi-cellulose, high-yield materials, waste paper. When the pigment suspension is mixed in, the fibrous material 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 sheet formation. The mixture is expediently prepared directly in the central unit of a paper machine. Common additives, such as defoamers, dispersants, thickeners, retention aids, 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 formed into a sheet in the usual way and can then be satinized.

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 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. It consists of a copolymer of approximately 50% by weight of trimethylammoniumethyl methacrylate hydrochloride, 30% by weight of styrene and 20% by weight of acrylic acid. So much aluminum sulfate is added until a pH of 5.5 is reached. This mixture is mixed in the pulp center of a paper machine with the fibrous material, consisting of spruce pulp and wood pulp 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 and then satinized. The tear length is measured on the finished paper. Results table Pigment-fiber ratios Binder insert G% related pigment Weight g / m² Tear length m Retention Filler content% Fiber% Ash% 50/50 0 166 1050 66.4 37.4 20.2 1 168 1090 73.4 52.8 36.0 4th 166 1520 76.1 53.4 35.1 60/40 1 170 852 60.3 49.9 43.8 4th 165 1273 67.0 47.2 42.3 68/32 0 168 655 51.7 30.7 40.2 1 178 590 62.1 46.0 50.1 4th 177 1130 64.5 47.9 50.2

Claims (10)

Process for treating a pigment suspended in water for the paper industry with an aqueous binder by precipitation of the binder on the pigment
characterized,
that an alkaline solution of an amphoteric high molecular weight polymer is used as the binder and that the pigment suspension mixed with the binder is gradually neutralized with an acidifying agent. Process for the production of filler-containing paper by sheet formation from an aqueous paper stock, characterized in that it contains a pigment treated according to claim 1. A method according to claim 1 or 2, characterized in that kaolin, calcium sulfate, talc or titanium dioxide is used as the inorganic filler. Process according to Claim 1 or 2, characterized in that an amphoteric polymer having a molecular weight M _w > 20,000, preferably> 50,000, is used as the binder. A method according to claim 4, characterized in that an amphoteric polymer is used as the binder, the 6 to 80, preferably 10 to 80 wt .-% units of acrylic, methacrylic or / and maleic acid and 10 to 90 wt .-%, preferably contains 20 to 80 wt .-% units of a nitrogen-containing cationic monomer and optionally up to 84 wt .-% units of a nonionic monomer. A method according to claim 5, characterized in that the cationic monomer contains a quaternary ammonium group. Method according to one or more of claims 1 to 6, characterized in that an acidic salt of a polyvalent metal cation is used as the acidifying agent. A method according to claim 7, characterized in that aluminum sulfate is used. Aqueous suspension according to claim 8, characterized in that the aqueous phase contains less than 5% by weight of the binder. Aqueous suspension according to Claim 9, characterized in that the content of the binder in the aqueous phase after an intensive shear treatment of 3 min using a high-speed stirrer at 4000 rpm is not more than 10% by weight, based on the total binder content of the suspension.