Process for the preparation of an aqueous polymer dispersion from a vinyl aromatic compound and a conjugated aliphatic diene

Description

[0001] The invention relates to a process for producing an aqueous polymer dispersion having a polymodal particle distribution of the polymer particles by copolymerizing a vinylaromatic compound and a conjugated aliphatic diene. The invention also relates to the aqueous polymer dispersions produced by the process and their use as binders, adhesives, fiber sizing agents, for the production of coatings, or for the production of a paper coating slip.

[0002] Binders for paper coating slips based on copolymers of vinyl aromatic compounds and aliphatic dienes are often chosen for applications such as board for food packaging. A focus in their production is on low-odor dispersions, as this is an odor-sensitive application area.

[0003] However, with the ever-increasing production speed of paper machines, the demands on the rheology of the coating slip are also increasing. Despite the high proportion of pigment, which is coarser than the binder polymer, the latter has a strong influence on the rheology of a coating slip. The viscosity of the coating slip could be reduced by greater dilution, but the opposite is desired. Thus, modern dispersions should enable a high solids content in the coating slip while still maintaining a low viscosity even at high speeds.

[0004] Peter C. Hayes describes that at high solids contents of coating slips with styrene-butadiene binder, the runnability is improved by smaller particle sizes of the binder ( "Styrene-butadiene and styrene-acrylic latexes in paper coating applications", Coating Material: Pigment Binders & Additives Short Course, Orange Beach, AL, United States, Mar. 11-13, 2002, pages 115-123, TAPPI PRESS, Atlanta, 2002).

[0005] One way to control the particle size of a polymer dispersion is through the use of a seed latex. However, the monomodal dispersions produced in this way form a high proportion of fine coagulum, which clumps together over time to form larger aggregates, blocking filter systems and making them unsuitable for use in paper coating.

[0006] US Pat. No. 4,567,099 teaches the use of a blend of two styrene / butadiene dispersions in paper coating applications. Blending two dispersions generally leads to a dilution of the overall dispersion, since dispersions with small particle sizes can only be produced with a lower solids content. Higher solids contents can only be achieved through subsequent energy-intensive concentration of the blend. Furthermore, two dispersions must be prepared beforehand, resulting in a poor space / time yield for the overall product.

[0007] US Pat. No. 5,726,259 teaches the direct preparation of a bimodal styrene / butadiene latex binder for paper coating slips. The latex binder is prepared by initiating the polymerization with an in-situ seed, adding the monomers in portions in 10 monomer additions, and adding an increased amount of emulsifier after 43 wt.% of the total monomer amount and 44% of the total monomer addition time. This process is characterized by a very lengthy dosing process and thus has a poor space / time yield. Another disadvantage is the use of sulfur regulators to achieve the desired product properties.

[0008] US 4780503 describes a bimodal polymer dispersion in which lauryl ether sulfate or additional seed latex is added at a point of greater than 40% monomer conversion during production. According to this teaching, dispersions with a higher solids content and lower viscosity are obtained. However, the viscosities obtained are still too high for today's coating machine running speeds. Application as a paper coating binder is not taught.

[0009] The object of the present invention was therefore to produce polymer dispersions with a pronounced bimodal particle size distribution, obtained by polymerizing a vinylaromatic compound and a conjugated aliphatic diene, with low technological complexity and improved space-time yield. These polymer dispersions should contain less fine coagulum and, moreover, be low-odor. When incorporated into paper coating slips, they should exhibit good rheological behavior even at high shear forces.

[0010] The object is achieved according to the invention by a process for the preparation of an aqueous polymer dispersion which has a polymodal particle distribution of the polymer particles with a first population of large polymer particles and a second population of small polymer particles, by radically initiated aqueous emulsion polymerization, characterized in that in an aqueous medium (a) 40 to 75 parts by weight at least one vinyl aromatic compound, (b) 24.9 to 59.9 parts by weight at least one conjugated aliphatic diene, (c) 0.1 to 10 parts by weight at least one monomer containing acid groups and (d) 0 to 20 parts by weight at least one other monoethylenically unsaturated monomer, wherein the amounts of monomers (a) to (d) add up to 100 parts by weight, polymerized by a monomer feed process, with the proviso that monomers and emulsifier are metered in continuously, following a period P1, when 15 to 30% of the total metering time of the monomers has elapsed and 15 to 30% by weight of the total monomer amount has been metered in, the metering rate of the emulsifier is increased for a period P2, which lasts a maximum of 30 minutes, to 10 to 100 times the average metering rate of the emulsifier of the period P1.

[0011] If a quantity is given below in parts by weight, this refers, unless otherwise stated, to 100 parts by weight of total monomers.

[0012] The term "polymodal" is known to the person skilled in the art and refers to a particle size distribution with two or more maxima over the entire population of the dispersion.

[0013] "Small polymer particles" and "large polymer particles" are merely relative terms related to particle size. The term "small polymer particles" is used synonymously with "population of small polymer particles," and the term "large polymer particles" is used synonymously with "population of large polymer particles."

[0014] Unless otherwise stated, the particle size and particle size distribution of the polymer particles are data obtained using an analytical ultracentrifuge (AUC), which is described below.

[0015] In the following, compounds that can be derived from acrylic acid and methacrylic acid are sometimes abbreviated by inserting the syllable "(meth)" into the compound derived from acrylic acid.

[0016] The total monomer quantity is the total amount of all monomers used in the polymerization, including any monomers present in the initial charge. The total monomer quantity is 100 parts by weight of monomer.

[0017] When we talk about the total amount of monomer added, this means the total amount of monomer minus the monomers in the initial mixture.

[0018] The total dosing time of the monomers refers to the period of time required for the continuous dosing of monomers. Dosing can occur as a mixture or as separate monomers, the addition of which can also be staggered. It is crucial that monomer is added at all times, i.e., the addition is continuous. Accordingly, the total dosing time begins with the start of the dosing of the first monomer / mixture and ends with the completion of the dosing of the last monomer / mixture.

[0019] Dosing rate is defined as the amount added per unit of time, i.e., the "amount per unit of time," usually expressed in "g / min." For example, the average dosing rate of the emulsifier period P1 is the amount of all emulsifiers added during the period P1, relative to the duration of the period.

[0020] The following ethylenically unsaturated monomers (a), (b), (c) and (d) can be used to prepare the aqueous polymer dispersions.

[0021] Examples of suitable vinylaromatic compounds (monomers of group (a)) include styrene, α-methylstyrene, and / or vinyltoluene. Styrene is preferably selected from this group of monomers.

[0022] The monomers (a) combine a proportion of 40 to 75 parts by weight and preferably 45 to 70 parts by weight, in particular 50 to 65 parts by weight, based on 100 parts by weight of total monomers (a to c and optionally d).

[0023] Examples of conjugated aliphatic dienes (monomers of group (b)) include 1,3-butadiene, isoprene, 1,3-pentadiene, 1,3-dimethylbutadiene, and cyclopentadiene. Of this group of monomers, 1,3-butadiene and / or isoprene are preferred.

[0024] The total amount of monomers (b) is 24.9 to 59.9 parts by weight, preferably 29.9 to 54.9 parts by weight and in particular 34.9 to 49.9 parts by weight, based on 100 parts by weight of total monomers.

[0025] Examples of monomers containing acid groups (monomers (c)) include ethylenically unsaturated carboxylic acids, ethylenically unsaturated sulfonic acids, and vinylphosphonic acid. α,β-monoethylenically unsaturated mono- and dicarboxylic acids containing 3 to 6 carbon atoms in the molecule are preferably used as ethylenically unsaturated carboxylic acids. Examples of these are acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, vinylacetic acid, and vinyllactic acid. Suitable ethylenically unsaturated sulfonic acids include vinylsulfonic acid, styrenesulfonic acid, acrylamidomethylpropanesulfonic acid, sulfopropyl acrylate, and sulfopropyl methacrylate. Acrylic acid, methacrylic acid, and itaconic acid are preferred. The acids mentioned can be used either as the sole component or in combination.

[0026] The acid-containing monomers can be used in the polymerization in the form of free acids or in a form partially or completely neutralized with suitable bases. Sodium hydroxide solution, potassium hydroxide solution, or ammonia are preferably used as neutralizing agents.

[0027] The total amount of the monomers (c) is 0.1 to 10 parts by weight, preferably 0.1 to 8 parts by weight or 1 to 6 parts by weight, based on 100 parts by weight of total monomers.

[0028] Other monomethylenely unsaturated monomers (d) are monomers different from the monomers of groups (a), (b) and (c). They are preferably selected from acrylamide and methacrylamide, vinyl esters of saturated C 1 - to C 18 -carboxylic acids, preferably vinyl acetate, and esters of acrylic acid and methacrylic acid with monohydric C 1 - to C 18 -alcohols such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-propyl acrylate, n-propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate, tert-butyl acrylate, tert.-Butyl methacrylate, pentyl acrylates, pentyl methacrylates, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, allyl esters of saturated carboxylic acids, vinyl ethers, vinyl ketones, dialkyl esters of ethylenically unsaturated carboxylic acids, N-vinylpyrrolidone, N-vinylpyrrolidine, N-vinylformamide, N,N-dialkylaminoalkylacrylamides, N,N-dialkylaminoalkylmethacrylamides, N,N-dialkylaminoalkylacrylates, N,N-dialkylaminoalkylmethacrylates, vinyl chloride and vinylidene chloride (monomers of group (d)).

[0029] This group of monomers is optionally used to modify the polymers. The total amount of all other monomers may be up to 20 parts by weight based on 100 parts of total monomer. Based on 100 parts by weight of total monomers, the proportion of one or more monomers from group (d) is 0 to 20 parts by weight, preferably 0.1 to 15 parts by weight, and in particular 0.5 to 10 parts by weight.

[0030] If it is acrylonitrile and / or methacrylonitrile, it is preferably used in an amount of 2 to 12 parts by weight and in particular of 4 to 9 parts by weight, based on 100 parts by weight of total monomer.

[0031] A preferred process is one in which the vinylaromatic compound is styrene and / or methylstyrene and the conjugated aliphatic diene is 1,3-butadiene and / or isoprene. The process for producing styrene-butadiene dispersions is particularly advantageous.

[0032] With advantage (a) 60 to 75 parts by weight at least one vinyl aromatic compound and (b) 24.9 to 39.9 parts by weight at least one conjugated aliphatic diene (c) 0.1 to 8 parts by weight at least one monomer containing acid groups and (d) 0 to 10 parts by weight at least one other monoethylenically unsaturated monomer, wherein the amounts of monomers (a) to (d) add up to 100 parts by weight, polymerized.

[0033] Particularly preferred are (a) 60 to 70 parts by weight at least one vinyl aromatic compound and (b) 29 to 39 parts by weight at least one conjugated aliphatic diene (c) 1 to 6 parts by weight at least one monomer containing acid groups and (d) 0 to 5 parts by weight at least one other monoethylenically unsaturated monomer, wherein the amounts of monomers (a) to (d) add up to 100 parts by weight, polymerized.

[0034] Emulsion polymerization takes place in an aqueous medium. This can be, for example, completely demineralized water or a mixture of water and a miscible solvent such as methanol, ethanol, ethylene glycol, glycerin, or sugar alcohols such as sorbitol or tetrahydrofuran. The total amount of aqueous medium is calculated such that the resulting aqueous polymer dispersion has a solids content of 20 to 70 wt.%, frequently 30 to 65 wt.%, and frequently 40 to 60 wt.%.

[0035] The process according to the invention is a monomer feed process. A monomer feed process is understood to mean that the majority, usually at least 80%, preferably at least 90%, of the monomers to be polymerized are fed to the polymerization reaction under polymerization conditions.

[0036] It is possible to initially charge a portion of the monomers in the polymerization vessel before the start of the polymerization. Thus, according to this preferred variant, the polymerization can be initiated in a receiver containing up to 20 parts by weight of the total monomers, and then monomers and emulsifier can be continuously added. In particular, up to 5% of the respective monomer can be initially charged, and then the polymerization can be initiated. Particularly preferably, only monomers (a), (b), and optionally (d) are initially charged, preferably up to 5% of the respective monomer.

[0037] Polymerization conditions generally refer to the amounts of radical initiator, temperatures, and pressures under which the radically initiated aqueous emulsion polymerization does not stop. The polymerization depends fundamentally on the type and amount of radical initiator used. The relationships between temperature and decomposition rate are well known to those skilled in the art for common polymerization initiators or can be determined in routine experiments.

[0038] According to the invention, the monomers and emulsifier are metered in continuously. In other words, the monomer and emulsifier are added in a continuous flow, i.e., without interruption.

[0039] The respective monomer is preferably added at a rate that deviates by no more than 30%, preferably no more than 20%, from the average value of the respective total feed of this monomer. According to a preferred embodiment, the monomer addition rate (increase in monomers) approximately corresponds to the polymerization rate of the monomers (decrease in monomers).

[0040] A process in which no monomers are initially introduced is also preferred. According to one embodiment, the continuous addition of the monomers of groups (a), (b), (c), and (d), the latter if part of the total monomers, begins simultaneously.

[0041] According to a preferred embodiment, the metering of the conjugated aliphatic diene only starts at a time when at least 5%, preferably at least 8%, in particular at least 10% of the vinylaromatic compound has been metered in a steady stream. Preferably, the metering of the diene starts when at most 30% of the vinylaromatic compound has been metered in a steady stream.

[0042] The vinyl aromatic compound is metered in under polymerization conditions preferably in a continuous flow over a period of at least 120 minutes, preferably over a period of 180 to 300 minutes, in particular over a period of 210 to 270 minutes.

[0043] The conjugated aliphatic diene is metered in in a continuous flow, preferably over a period of at least 60 minutes, particularly preferably over a period of 120 to 240 minutes, in particular over a period of 150 to 210 minutes, wherein particularly preferably the vinyl aromatic compound is additionally metered in in a continuous flow, preferably over a period of 120 minutes, preferably over a period of 180 to 300 minutes, in particular over a period of 210 to 270 minutes.

[0044] According to the invention, monomer and emulsifier are metered in continuously. In the context of the process according to the invention, emulsifiers are understood to mean emulsifying aids that keep both the monomer droplets and polymer particles dispersed in the aqueous phase and thus ensure the stability of the resulting aqueous polymer dispersion. Suitable emulsifiers are the emulsifiers commonly used for free-radical aqueous emulsion polymerizations.

[0045] Suitable emulsifiers are surfactants whose number-average molecular weight is usually below 2000 g / mol or preferably below 1500 g / mol.

[0046] Anionic, cationic, and nonionic emulsifiers are suitable as emulsifiers. Emulsifiers whose relative molecular weights are usually lower than those of protective colloids are preferably used as surfactants.

[0047] Suitable nonionic emulsifiers are araliphatic or aliphatic nonionic emulsifiers, for example, ethoxylated mono-, di-, and trialkylphenols (EO degree: 3 to 50, alkyl radical: C 4 -C 10 ), ethoxylates of long-chain alcohols (EO degree: 3 to 100, alkyl radical: C 8 -C 36 ), and polyethylene oxide / polypropylene oxide homo- and copolymers. These can contain the alkylene oxide units in a random distribution or in the form of polymerized blocks. EO / PO block copolymers, for example, are particularly suitable. Preference is given to using ethoxylates of long-chain alkanols (alkyl radical C 1 -C 30 , average degree of ethoxylation 5 to 100) and, among these, particularly preferably those with a linear C 12 -C 20 alkyl radical and an average degree of ethoxylation of 10 to 50 as well as ethoxylated monoalkylphenols.

[0048] Suitable anionic emulsifiers include, for example, alkali and ammonium salts of alkyl sulfates (alkyl radical: C 8 -C 22 ), of sulfuric acid half-esters of ethoxylated alkanols (EO degree: 2 to 50, alkyl radical: C 12 -C 18 ) and ethoxylated alkylphenols (EO degree: 3 to 50, alkyl radical: C 4 -C 9 ), of alkylsulfonic acids (alkyl radical: C 12 -C 18 ), of alkylarylsulfonic acids (alkyl radical: C 9 -C 18 ) and of diesters of sulfosuccinic acid with C 4 -C 18 alkanols. Further suitable emulsifiers can be found in Houben-Weyl, Methoden der organischen Chemie, Volume XIV / 1, Makromolekulare Stoffe, Georg-Thieme-Verlag, Stuttgart, 1961, pp. 192-208). Bis(phenylsulfonic acid) ethers or their alkali metal or ammonium salts, which bear a C 4 -C 24 alkyl group on one or both aromatic rings, are also suitable as anionic emulsifiers. These compounds are well known, e.g., from US-A-4,269,749, and are commercially available, for example as Dowfax®< 2A1 (Dow Chemical Company).

[0049] Preferably, at least one anionic and / or at least one non-ionic emulsifier is used.

[0050] Preferably, the emulsifier of the continuous dosage is selected from alkali and ammonium salts of C 8 -C 22 alkyl sulfates and of sulfuric acid half-esters of ethoxylated alkanols (EO degree: 2 to 40, alkyl radical: C 12 -C 18 ) and of sulfuric acid half-esters of ethoxylated alkylphenols (EO degree: 10 to 40, alkyl radical: C 4 -C 9 ), and bis(phenylsulfonic acid) ethers or their alkali or ammonium salts which carry a C 4 -C 24 alkyl group on one or both aromatic rings.

[0051] Particular preference is given to using a mixture of emulsifiers, each in the form of their alkali and ammonium salts, in particular a mixture of alkyl sulfates (alkyl radical: C 8 -C 22 ) with sulfuric acid semiesters of ethoxylated alkanols (EO degree: 2 to 40, alkyl radical: C 12 -C 18 ) or with sulfuric acid semiesters of ethoxylated alkylphenols (EO degree: 10 to 40, alkyl radical: C 4 -C 9 ) or with 2-ethylhexyl sulfosuccinate, or a mixture of alkali and ammonium salts of alkyl sulfates with bis(phenylsulfonic acid) ether or their alkali or ammonium salts which carry a C 4 -C 24 alkyl group on one or both aromatic rings (e.g. Dowfax 2A1 from the Dow Chemical Company).

[0052] Particularly preferred is an emulsifier mixture of sodium lauryl sulfate and ethoxylated sodium lauryl ether sulfate as well as a mixture of sodium lauryl sulfate and Dowfax ®< 2A1.

[0053] The continuous metering of emulsifier and monomer can be carried out in separate streams. However, it is advantageous to meter the emulsifier and at least one monomer together as a mixture. Preferably, 0.1 to 5 parts by weight, preferably 0.2 to 2.0 parts by weight of emulsifier, based on 100 parts by weight of monomer in a mixture with at least one monomer, are continuously metered.

[0054] Emulsifiers are preferably dosed at a dosage rate that does not deviate by more than 30%, preferably not more than 20%, from the average value of the respective total feed.

[0055] According to the invention, following a period P1, when 15 to 30% of the total metering time of the monomers has elapsed and 15 to 30% of the total monomer quantity has been metered in, the metering rate of the emulsifier is increased to 10 to 100 times the average metering rate of the emulsifier of period P1 for a period P2, which lasts a maximum of 30 minutes. Period P1 starts with the beginning of the continuous metering of the monomers, i.e., in the case of the presence of a receiver, after the initiation of the polymerization in the receiver, and ends when 15 to 30%, preferably 20 to 25%, of the total metering time of the monomers has elapsed and 15 to 30% by weight, preferably 20 to 25%, of the total monomer quantity has been metered in, the metering being selected such that both conditions are met.

[0056] The "period of increased dosage" following period P1 is also referred to below as "P2." The duration of period P2 is preferably up to 25 minutes, in particular 5 to 20 minutes. Due to the increased dosage rate during period P2, this additional emulsifier dosage is also referred to as the "emulsifier shot."

[0057] Following period P2 there follows a period P3 in which the dosing rate of the emulsifier can deviate up to 20% from the dosing rate of the emulsifier in period P1.

[0058] Preferably, during period P2, the dosage rate of the emulsifier is 20 to 90 times the average dosage rate of the emulsifier of period P1.

[0059] The emulsifier or emulsifier mixture used during periods P1 and P3 is generally the same. The emulsifier in period P2 can be the same emulsifier as in period P1. Preferably, a mixture of the emulsifiers from period P1 is used in period P2, but with a different ratio, for example, by adding only one of the two emulsifiers as an additional "emulsifier shot."

[0060] According to a preferred embodiment, a monomer / emulsifier mixture is continuously metered over the entire feed, i.e., periods P1, P2 and P3, and additionally, in period P2, the metering rate of one of the emulsifiers of the mixture is increased.

[0061] Preferably, an anionic emulsifier is chosen as the emulsifier of period P2, in particular selected from lauryl sulfate, sulfuric acid half-esters of ethoxylated alkanols and arylsulfonate.

[0062] The following explains the metering process using the example of a polymerization with initiation in a receiver and subsequent monomer metering at a constant flow rate. In Example 1, the polymerization is initiated in the receiver with 2.9 parts by weight of the total monomer amount, and then the continuous metering of a mixture of monomer and emulsifier is started. The total amount of all monomers, including the monomers in the receiver, is 100 parts by weight (also referred to as the total monomer amount in this application). Thus, 97.1 parts by weight are continuously metered in. The total time for monomer metering is 240 minutes. After 50 minutes from the start of continuous metering, an additional metering of emulsifier is started and lasts for 20 minutes. The increase in the amount of emulsifier, the so-called emulsifier shot, thus occurs after 21% of the total monomer metering time. After 50 minutes, 20.2 parts by weight of monomers have been added due to the constant monomer dosage.Over the total dosing time of 240 minutes, 0.74 parts by weight of emulsifier, based on 100 parts by weight of total monomer mixed with the monomers, are continuously dosed. Accordingly, after 50 minutes, 0.15 parts by weight of emulsifier are added. This corresponds to an average dosing rate for period P1 of 0.00308 parts by weight / min. The emulsifier dosage during the emulsifier shot is 1 part by weight over a period of 20 minutes, i.e., a dosing rate of 0.05 parts by weight / min. This means that 0.053 parts by weight per minute are dosed during the emulsifier shot, and the dosing rate is 17 times (rounded, exact: 16.6 times) the average emulsifier dosage rate.

[0063] It is assumed that the emulsifier concentration present in the polymerization mixture is below the critical micelle concentration during period P1 and above the critical micelle concentration during period P2. According to this theory, new micelles would form during period P2, initiating a second particle growth cycle.

[0064] The process according to the invention uses radical initiators (also referred to as radical polymerization initiators), i.e., initiators that form radicals under the reaction conditions. These can be either peroxides or azo compounds. Redox initiator systems are also possible, of course.

[0065] In principle, inorganic peroxides and / or organic peroxides can be used as peroxides. Suitable inorganic peroxides include hydrogen peroxide and peroxodisulfates, such as the mono- or di-alkali metal or ammonium salts of peroxodisulfuric acid, for example, its mono- and disodium, potassium, or ammonium salts. Suitable organic peroxides include alkyl hydroperoxides such as tert-butyl hydroperoxide, aryl hydroperoxides such as p-menthyl or cumene hydroperoxide, and dialkyl or diaryl peroxides such as di-tert-butyl, dibenzoyl, or dicumene peroxide.

[0066] The azo compounds used are essentially 2,2'-azobis(isobutyronitrile), 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(N,N'-dimethyleneisobutyroamidine) dihydrochloride and 2,2'-azobis(amidinopropyl) dihydrochloride (AIBA, equivalent to V-50 from Wako Chemicals).

[0067] Redox initiator systems are combined systems composed of at least one organic or inorganic reducing agent and at least one peroxide. The peroxides mentioned above are primarily considered as oxidizing agents for redox initiator systems.Suitable reducing agents which can be used are sulfur compounds with a low oxidation state, such as alkali sulfites, for example potassium and / or sodium sulfite, alkali hydrogen sulfites, for example potassium and / or sodium hydrogen sulfite, alkali metabisulfites, for example potassium and / or sodium metabisulfite, acetone bisulfite, formaldehyde sulfoxylates, for example potassium and / or sodium formaldehyde sulfoxylate, alkali salts, especially potassium and / or sodium salts, aliphatic sulfinic acids and alkali metal hydrogen sulfides, such as potassium and / or sodium hydrogen sulfide, salts of polyvalent metals, such as iron(II) sulfate, iron(II) ammonium sulfate, iron(II) phosphate, enediols, such as dihydroxymaleic acid, benzoin and / or ascorbic acid, and reducing saccharides, such as sorbose, glucose, fructose and / or dihydroxyacetone.

[0068] Preferred radical initiators are inorganic and organic peroxides, preferably ammonium or alkali metal salts of peroxosulfates or peroxodisulfates, as well as tert-butyl, p-menthyl, and cumyl hydroperoxide, particularly selected from sodium and potassium peroxodisulfate, tert-butyl hydroperoxide, and cumyl hydroperoxide. Particular preference is given to using at least one inorganic peroxide, preferably peroxodisulfate, especially sodium peroxodisulfate, and one organic peroxide, preferably alkyl hydroperoxide, especially t-butyl hydroperoxide.

[0069] Preference is given to a process according to the invention in which no reducing agent is present during the monomer feed, which can form a red / ox initiator system with the organic or inorganic peroxides.

[0070] Organic peroxides that are both oil- and water-soluble are preferred. For the purposes of this document, water-soluble organic peroxides are those that have a solubility of ≥ 1 wt.% in deionized water at 20 °C and atmospheric pressure. For the purposes of this document, oil-soluble organic peroxides are those that have a solubility of ≥ 1 wt.% in styrene at 20 °C and atmospheric pressure. Examples include alkyl hydroperoxides such as tert-butyl, p-menthyl, or cumene hydroperoxide.

[0071] Preferably, the inorganic peroxide is selected from hydrogen peroxide and ammonium or alkali metal salts of peroxodisulfates and the organic peroxide is selected from alkyl hydroperoxides and aryl hydroperoxides.

[0072] Particularly preferred are the following combinations of peroxodisulfates with alkyl hydroperoxides, for example sodium peroxodisulfate with t-butyl hydroperoxide or ammonium peroxodisulfate with t-butyl hydroperoxide.

[0073] The polymerization is generally carried out using 0.1 to 5 parts by weight of the radical initiator, preferably 0.5 to 4 parts by weight of the radical initiator, preferably at least one inorganic and / or organic peroxide, based in each case on 100 parts by weight of total monomers. The ratio of the total amount of inorganic peroxide to the total amount of organic peroxide is preferably 1 / 10 to 10 / 1, preferably 1 / 5 to 5 / 1, in particular 1 / 3 to 3 / 1, considered over the entire process.

[0074] According to a preferred variant, the addition of the organic peroxide is started at a time when at least 5%, preferably at least 8%, in particular at least 10%, and at most 20% of the vinylaromatic compound has already been added in a steady stream. The time of addition is therefore independent of whether the polymerization was initiated with or without a precursor.

[0075] Initiation of the polymerization reaction is understood to mean the start of the polymerization reaction of the monomers present in the polymerization vessel through the decomposition of the radical initiator. Polymerization starts, for example, when the polymerization mixture contains monomers and inorganic peroxide and reaches a temperature in the range of ≥ 80°C to ≤ 95°C.

[0076] To polymerizeTo start the polymerization, an aqueous solution is first prepared containing a portion of a protective colloid and / or an emulsifier in dissolved form, optionally a portion of inorganic peroxide, optionally a portion of monomer, and optionally polystyrene seed. This mixture is preferably heated to the temperature at which the polymerization of the monomers is to take place. As soon as the desired polymerization temperature is reached, or within a period of 1 to 15 minutes, preferably 1 to 10 minutes after reaching the polymerization temperature, the monomers and the inorganic peroxide are metered in.

[0077] According to a preferred embodiment, the polymerization is initiated in an aqueous polymerization mixture containing up to 5% of the vinylaromatic compound and no aliphatic diene. Thereafter, the polymerization starts when 0.1 to 5% of the vinylaromatic compound are initially charged, the reaction temperature is set to a temperature in the range of ≥ 80°C to ≤ 95°C and inorganic peroxide, preferably 0.1 to 0.5 parts by weight of the inorganic peroxide based on 100 parts by weight of total monomer, is initially charged.

[0078] The order in which the conditions are established is not crucial. Particularly preferably, 0.1 to 0.5 parts by weight of inorganic peroxide, preferably an ammonium salt or an alkali metal salt of a peroxodisulfate, is selected to initiate the polymerization, followed by the subsequent addition of the vinylaromatic compound and 0.1 to 2 parts by weight of inorganic peroxide. The addition of the 0.1 to 2 parts by weight of organic peroxide is only carried out after at least 5% of the vinylaromatic compound has already been added. The addition of the inorganic peroxide, the organic peroxide, and the vinylaromatic compound is each carried out in a continuous flow, and the parts by weight of the peroxides are each based on 100 parts by weight of total monomers.

[0079] As with all radical polymerization reactions, it is advantageous if the initial charging of the reaction components, the metering / polymerization and the subsequent reaction in the reaction vessel take place under an inert gas atmosphere, for example under a nitrogen or argon atmosphere.

[0080] Preferred Polymerization conditions are a temperature in the range of ≥ 75°C to ≤ 115°C, preferably ≥ 80°C to ≤ 105°C, in particular ≥ 85°C to ≤ 100°C. The conjugated aliphatic diene is generally metered in at elevated pressure. The conjugated aliphatic diene is preferably metered in at a pressure in the range of 5 to 15 bar. The elevated pressure ensures that, for example, 1,3-butadiene, which is gaseous at atmospheric pressure and room temperature, is largely present in the polymerization mixture.

[0081] The polymerization can be carried out in the presence of a degraded starch. According to a preferred embodiment, no degraded starch is present during the polymerization. According to an equally preferred embodiment, the polymerization takes place in the presence of a degraded starch, preferably 15 to 100 parts by weight of a degraded starch based on 100 parts by weight of the monomers.

[0082] All native starches, such as starches from corn, wheat, oats, barley, rice, millet, potatoes, peas, tapioca, sorghum, or sago, are suitable as starting starches for the production of the degraded starches to be used according to the invention. Also of interest as starting starches are natural starches with a high amylopectin content, such as waxy maize starch and waxy potato starch. The amylopectin content of these starches is above 90%, usually between 95 and 100%.

[0083] Furthermore, starches modified by etherification or esterification can be used to produce the polymer dispersions according to the invention. The modification usually also results in degradation. Such products are known and commercially available. They are produced, for example, by esterifying native starch or degraded native starch with inorganic or organic acids, their anhydrides, or chlorides. Phosphate- and acetylated degraded starches are of particular interest. The most common method for etherifying starches is treating starch with organic halogen compounds, epoxides, or sulfates in an aqueous alkaline solution. Well-known starch ethers are alkyl ethers, hydroxyalkyl ethers, carboxyalkyl ethers, and allyl ethers. The reaction products of starches with 2,3-epoxypropyltrimethylammonium chloride are also suitable.

[0084] Other suitable starches are cationically modified starches, i.e. starch compounds which contain amino groups or ammonium groups.

[0085] Starch degradation can occur enzymatically, oxidatively, or hydrolytically through the action of acids or bases. Starch degradation is well known and is described, for example, in EP 2580257. Degraded starches are commercially available. A degraded starch can be used for polymerization, or it can be produced in situ and then polymerized in its presence.

[0086] Particularly preferred are degraded native starches, especially native starches degraded to maltodextrin.

[0087] Preference is given to degraded starches with an intrinsic viscosity ηi of ≤0.07 dl / g or ≤0.05 dl / g. The intrinsic viscosity ηi of the degraded starches is preferably in the range of 0.02 to 0.06 dl / g. The intrinsic viscosity ηi is determined according to DIN EN 1628 at a temperature of 23 °C.

[0088] According to one process variant, the polymerization is carried out in the presence of an aqueous dispersion of finely divided polystyrene. Preferably, 0.1 to 5 parts by weight, in particular 0.2 to 3 parts by weight, based on total monomers, are used. The polymerization is preferably initiated in a receiver containing up to 2 parts by weight of an aqueous dispersion of finely divided polystyrene based on 100 parts by weight of total monomers, and monomers and emulsifier are then continuously added. Finely divided polystyrene is preferably polystyrene with an average particle diameter of 20 to 40 nm (determined by ultracentrifuge). It is assumed that such a finely divided polystyrene dispersion acts as a seed polymer—also referred to as a seed latex—and thus initiates polymerization in the particles of the seed polymer, resulting in uniform particle growth.

[0089] To modify the properties of the polymers, the emulsion polymerization can optionally be carried out in the presence of at least one chain transfer agent that does not contain sulfur or halogen. These are typically used to reduce or control the molecular weight of the polymers obtainable by free-radical aqueous emulsion polymerization.

[0090] Preferably, the process according to the invention should not comprise the use of chain transfer agents selected from aliphatic and / or araliphatic halogen compounds, organic thio compounds and substituted thiols during the polymerization.

[0091] Examples of chain transfer agents that do not contain sulfur or halogen are aldehydes such as formaldehyde, acetaldehyde, and propionaldehyde; organic acids such as formic acid, sodium formate, or ammonium formate; alcohols, particularly isopropanol; and phosphorus compounds such as sodium hypophosphite. If chain-transfer compounds that do not contain sulfur or halogen are used in the polymerization, the amount used in each case is, for example, 0.01 to 5, preferably 0.1 to 1, parts by weight, based on 100 parts by weight of the monomers used in the polymerization. The chain transfer agents are preferably metered into the initial charge together with the monomers. However, they can also be partially or completely present in the initial charge. They can also be metered in stepwise, offset from the monomers.

[0092] Particularly preferred is a process in which no chain transfer agent is present during the polymerization.

[0093] To complete the polymerization reaction, it is usually sufficient to stir the reaction mixture at the polymerization temperature for, say, 1 to 3 hours after the monomer addition has been completed. Typically, a conversion of around 95% is achieved by this time.

[0094] In order to further increase the conversion and thus reduce the residual monomer content, one can, for example, add additional radical initiators from the group of initiators mentioned above to the reaction mixture or extend their addition and carry out a so-called "post-polymerization", i.e. a polymerization, in order to achieve conversions of >95% up to 99%.

[0095] Such a post-polymerization can be carried out at the same, lower, or higher temperature than the main polymerization. For example, 0.1 to 1.5 parts by weight, based on 100 parts by weight of the monomers used in the polymerization, of inorganic peroxide, preferably sodium peroxodisulfate, is added as initiator in this phase, and the polymerization temperature is set to a temperature in the range of 80 to 120 °C.

[0096] The pH value during polymerization can be, for example, 1 to 5. After polymerization at a conversion of >95%, the pH value is adjusted to a value between 6 and 7, for example.

[0097] Chemical deodorization can also be performed. If traces of residual monomers still need to be removed, this can also be done chemically using the redox initiator systems mentioned above and systems such as those listed in DE-A 44 35 423, DE-A 44 19 518, and DE-A 44 35 422.

[0098] Treatment with the redox initiator system is carried out in the temperature range of 60 to 100°C, preferably between 70 and 90°C. The redox partners can be added to the dispersion independently of one another, completely, in portions, or continuously over a period of 10 minutes to 4 hours. To improve the post-polymerization effect of the redox initiator systems, soluble salts of metals of varying valence, such as iron, copper, or vanadium salts, can also be added to the dispersion. Complexing agents are also frequently added to keep the metal salts in solution under the reaction conditions.

[0099] Following the polymerization reaction (main polymerization + post-polymerization) and, if necessary, chemical deodorization, it may be necessary to render the aqueous polymer dispersions largely free of odor carriers, such as residual monomers and other organic volatile components, a process also referred to as physical deodorization. This can be achieved in a conventional manner by distillative removal (particularly via steam distillation) or by stripping with an inert gas.

[0100] The present invention also relates to the dispersions obtainable by the process according to the invention. These are characterized by being virtually coagulum-free aqueous dispersions. The amount of coagulum is in the ppm range and is preferably less than 100 ppm, in particular less than 50 ppm.

[0101] Furthermore, they generally have a solids content of approximately 50% by weight, preferably in the range of 45 to 55% by weight. The dispersions according to the invention contain a small proportion of 4-phenylcyclohexene.

[0102] The polymer dispersions obtained according to the invention have a polymodal particle distribution.

[0103] Aqueous polymer dispersions are obtainable by the process according to the invention in which the proportion of the first population of large polymer particles is 60 to 95 wt.% and the proportion of the second population of small polymer particles is 5 to 40 wt.%, based on the total polymer of the dispersion.

[0104] An examination of the particle size reveals at least two maxima. The particle size analysis is performed using an analytical ultracentrifuge (AUC).

[0105] Preference is given to a polymer dispersion whose particle size distribution has a peak maximum in the range from 80 to 160 nm, particularly preferably in the range from 90 to 140 nm, and a second peak maximum in the range from 40 to 80 nm, particularly preferably in the range from 45 to 70 nm.

[0106] The distance between the two peak maxima should preferably not be more than 80 nm, particularly preferably not more than 60 nm.

[0107] Typically, the peaks of the large particles and the small particles do not overlap or overlap only slightly; an overlap of up to 20% is not detrimental to the desired effect of good rheology.

[0108] The aqueous polymer dispersions of the invention are used as binders, adhesives, fiber sizing agents, for the production of coatings, or for the production of paper coating slips. The aqueous polymer dispersions of the invention are suitable both for sizing textile fibers and for sizing mineral fibers, especially glass fibers. Due to their good adhesive strength, particularly when using comonomers that result in a low glass transition temperature of the copolymer (e.g., less than 20°C), they can also be used as adhesives, for example, for the production of laminates and for the production of coatings such as barrier coatings. The aqueous polymer dispersions of the invention are preferably used as binders in paper coating slips.

[0109] The invention therefore also relates to a paper coating composition containing (i) inorganic pigment and (ii) an aqueous polymer dispersion (iii) as described above and obtainable by the process according to the invention and optionally further auxiliaries.

[0110] In addition to water, paper coating slips generally contain pigments, binders, and auxiliaries for adjusting the required rheological properties, such as thickeners. The pigments are usually dispersed in water. The paper coating slip contains pigments in an amount of preferably at least 80 wt.%, e.g., 80 to 95 wt.% or 80 to 90 wt.%, based on the total solids content.

[0111] White pigments are particularly suitable. Suitable pigments include, for example, metal salt pigments such as calcium sulfate, calcium aluminate sulfate, barium sulfate, magnesium carbonate and calcium carbonate, of which carbonate pigments, in particular calcium carbonate, are preferred. The calcium carbonate can be ground calcium carbonate (GCC, natural ground calcium carbonate), precipitated calcium carbonate (PCC), lime or chalk. Suitable calcium carbonate pigments are available, for example, as Covercarb ®< 60, Hydrocarb ®< 60 or Hydrocarb ®< 90 ME. Other suitable pigments include, for example, silicas, aluminum oxides, aluminum hydrate, silicates, titanium dioxide, zinc oxide, kaolin, clay, talc or silicon dioxide. Other suitable pigments are available, for example, as Capim ®< MP 50 (clay), Hydragloss ®< 90 (clay) or Talcum C10.

[0112] The paper coating slip contains the polymer dispersion prepared according to the invention as the sole binder or in combination with another binder. The most important functions of binders in paper coating slips are to bond the pigments to the paper and the pigments to each other, and to partially fill voids between pigment particles.

[0113] For example, 1 to 50 parts by weight, preferably 1 to 25 parts by weight or 5 to 20 parts by weight of the polymer according to the invention (solid, ie without water or other solvents which are liquid at 21 °C, 1 bar) are used per 100 parts by weight of pigments.

[0114] Preference is given to a paper coating slip which contains the polymers of the aqueous polymer dispersion in an amount of 1 to 50 parts by weight, based on the total amount of pigments, and pigments in an amount of 80 to 95 parts by weight, based on the total solids content, and an auxiliary agent, and whose pigments are selected from the group consisting of calcium sulfate, calcium aluminate sulfate, barium sulfate, magnesium carbonate, calcium carbonate, silicic acids, aluminum oxides, aluminum hydrate, silicates, titanium dioxide, zinc oxide, kaolin, clay, talc and silicon dioxide and whose auxiliary agent is selected from the group consisting of thickeners, other polymeric binders, co-binders, optical brighteners, fillers, flow control agents, dispersants, surfactants, lubricants, neutralizing agents, defoamers, deaerating agents, preservatives and dyes.

[0115] The other synthetic binders different from the polymers prepared according to the invention are generally known and are described, for example, in D. Urban and K. Takamura, Polymer Dispersions and Their Industrial Applications, 2002, Wiley-VCH Verlag GmbH, Weinheim, Chapter 4.4.4, page 90 ff., the disclosure of which is expressly incorporated by reference.

[0116] Other suitable binders include natural-based binders, in particular starch-based binders, as well as synthetic binders different from the polymers produced according to the invention, in particular emulsion polymers producible by emulsion polymerization. Starch-based binders in this context are understood to mean any native, modified, or degraded starch. Native starches can consist of amylose, amylopectin, or mixtures thereof. Modified starches can be oxidized starch, starch esters, or starch ethers. The molecular weight of the starch can be reduced by hydrolysis (degraded starch). Suitable degradation products are oligosaccharides or dextrins. Preferred starches are cereal, corn, and potato starch. Cereal and corn starch are particularly preferred, and corn starch is most preferably used.

[0117] Paper coating slips according to the invention can additionally contain other auxiliaries, e.g., fillers, co-binders, and thickeners for further optimizing viscosity and water retention, optical brighteners, dispersants, surfactants, lubricants (e.g., calcium stearate and waxes), neutralizing agents (e.g., NaOH or ammonium hydroxide) for pH adjustment, defoamers, deaerating agents, preservatives (e.g., biocides), flow control agents, dyes (especially soluble dyes), etc. Suitable thickeners include synthetic polymers (e.g., cross-linked polyacrylate), especially celluloses, preferably carboxymethylcellulose. Optical brighteners include, for example, fluorescent or phosphorescent dyes, especially stilbenes.

[0118] It is preferably an aqueous paper coating slip; it already contains water, particularly due to the preparation of the components (aqueous polymer dispersions, aqueous pigment slurries); the desired viscosity can be adjusted by adding additional water. Typical solids contents of paper coating slips are in the range of 30 to 80 wt.%. The pH of the paper coating slip is preferably adjusted to values ​​of 6 to 11, especially 7 to 10.

[0119] The invention also relates to paper or cardboard coated with a paper coating slip according to the invention and to a process for coating paper or cardboard, wherein an aqueous polymer dispersion is prepared according to the invention; and a paper coating slip is prepared using this polymer dispersion, at least one pigment and optional further auxiliaries; and the paper coating slip is applied to at least one surface of paper or cardboard.

[0120] The paper coating slip is preferably applied to uncoated base paper or uncoated cardboard. The amount is generally 1 to 50 g, preferably 5 to 30 g (solid, i.e., without water or other solvents liquid at 21 °C, 1 bar) per square meter. Coating can be carried out using conventional application methods, e.g., size press, film press, blade coater, air brush, doctor blade, curtain coating, or spray coater. Depending on the pigment system, the aqueous dispersions of the water-soluble copolymers in paper coating slips can be used for the base coat and / or the top coat.

[0121] Paper coating slips according to the invention have good application properties. They exhibit good runnability in paper coating processes and high bond strength. The coated papers and boards have good surface strength, in particular very high wet and dry pick resistance. They are readily printable using conventional printing processes, such as letterpress, gravure, offset, digital, inkjet, flexographic, newspaper, letterpress, sublimation, laser, electrophotographic, or a combination of these processes. Examples

[0122] Unless otherwise indicated by the context, percentages always refer to percentages by weight. The content stated refers to the content in an aqueous solution or dispersion. Where water was used in the examples, demineralized water was used. Measurement methods Particle size:

[0123] The particle size of the polymer dispersion particles and their particle size distribution were determined using an analytical ultracentrifuge (AUC) with turbidity optics and Mie correction for transmitted intensities per size. All components from 30 nm to 5 µm in diameter were measured using turbidity detection.

[0124] The method uses a homogeneous initial sedimentation. The procedure was carried out according to ISO 13318-1 guidelines, with the specific setup described in W. Mächtle, L. Börger, "Analytical Ultracentrifugation of Polymers and Nanoparticles," Chapter 3, Springer Science and Business Media, Berlin 2006, Chapter 3. The analysis assumes a spherical, solid particle shape with a skeletal density determined by the comonomer composition. The results are expressed in volumetric terms in sphere-equivalent diameters.

[0125] For the measurement, the dispersions are diluted to a concentration of 4 g (solid) / liter with a 0.05 wt.% aqueous surfactant solution and measured under the same conditions.

[0126] The weight fraction of a particle population is determined directly from the integral of the measurement. Below, the fraction of all particles between 36 and 75 nm is considered for the population of small particles, and the fraction of all particles between 80 and 180 nm is considered for the population of large particles. Determination of the viscosity of the dispersion:

[0127] The viscosity of the dispersion was determined according to ASTM D2196 using a Brookfield viscometer with RV spindles at 100 rpm and a temperature of 23°C. Determination of the viscosity of the coating slip:

[0128] Viscosity at high shear rates was measured using a high-pressure capillary viscometer (ACAV High-Shear Viscometer A2, manufacturer: ACA Systems). Before measurement, the sample was filtered through a 100 µm sieve, and its density was then determined. The measurement was carried out at a temperature of 23°C. Solids content:

[0129] The solids content of the polymer dispersions was determined by distributing 0.5 to 1.5 g of the polymer dispersion in a 4 cm diameter metal lid and then drying it in a forced-air drying cabinet at 140°C for 30 minutes. The ratio of the mass of the sample after drying under the above conditions to the mass at sampling yields the solids content of the polymer dispersion.

[0130] The following materials were used in the examples: Emulsifier A: Sodium lauryl sulfate in the form of a 15% by weight solution (Disponil ®< SDS from BASF) Emulsifier B: Ethoxylated sodium lauryl ether sulfate in the form of a 28% by weight solution (Texapon ®< NSO P from BASF) Complexing agent: EDTA in the form of a 2% by weight solution (Trilon ®< BX from BASF) Seed latex: Polystyrene seed in the form of a 29.7% by weight dispersion with a particle size of approximately 30 nm (determined by means of an analytical ultracentrifuge) Initiator A: 7% by weight solution of sodium peroxodisulfate (NaPS) Initiator B: 10% by weight solution of tert-butyl hydroperoxide Reducing agent: 13% by weight solution of acetone bisulfite Degraded starch: Commercially available 72% by weight aqueous Glucose syrup with a DE value (dextrose equivalent) of 28

[0131] Unless otherwise stated, the water used was deionized water. In all examples, the feeds were added at a steady flow rate. Production of emulsion polymers

[0132] The following quantities in pphm (parts per hundred monomer) are based on 100 parts by weight of total monomer. Example 1 Emulsion polymerization of styrene / butadiene / acrylic acid

[0133] Template: 360,01 g Water 192,86 g a 7 wt.% aqueous solution of itaconic acid (0.6 pphm) 45,45 g a 29.7 wt.% dispersion of a polystyrene latex with an average particle size of 30 nm (0.6 pphm) 18,00 g a 15 wt.% solution of sodium lauryl sulfate (emulsifier A) (0.12 pphm) 11,25 g a 2 wt.% solution of EDTA (complexing agent) (0.01 pphm) 4,50 g Acrylic acid (0.2 pphm) 47,48 g Styrene (2.11 pphm)

[0134] Encore: 86,79 g a 7% w / w solution of sodium peroxodisulfate (initiator A) (0.27 pphm)

[0135] Inlet 1: 90,00 g Acrylic acid (4.0 pphm) 40,18 g a 28 wt.% solution of ethoxylated sodium lauryl ether sulfate (emulsifier B) (0.5 pphm) 36,00 g a 15 wt.% solution of sodium lauryl sulfate (0.24 pphm) 22,50 g 15 wt.% sodium hydroxide solution (0.15 pphm) 503,69 ml Water

[0136] Inlet 2: 1364,63 g Styrene (60.65 pphm)

[0137] Inlet 3: 729,9 g Butadiene (32.44 pphm)

[0138] Inlet 4: 273,21 g a 7% w / w solution of sodium peroxodisulfate (initiator A) (0.85 pphm)

[0139] Inlet 5: 180,00 g a 10% w / w solution of tert-butyl hydroperoxide as (initiator B) (0.8 pphm)

[0140] Inlet 6 (emulsifier shot): 150,00 g a 15 wt.% solution of sodium lauryl sulfate (1.00 pphm)

[0141] Inlet 7: 51,75 g a 10% w / w solution of tert-butyl hydroperoxide as (initiator B) (0.23 pphm)

[0142] Inlet 8: 66,98 g a 13.1% w / w solution of acetone bisulfite (0.39 pphm)

[0143] The components of the reaction mixture were placed in a 6-liter pressure reactor and mixed. The reaction mixture was heated to 90°C. Once 90°C was reached, initiator A (additive 1) was slowly added, and polymerization was initiated.

[0144] Immediately thereafter, inlets 1, 2, 3, and 4 were started (time: 0 minutes). Inlets 1, 2, and 3 were administered over a period of 4 hours.

[0145] Inflow 4 took place over a period of 4 hours and 15 minutes.

[0146] Feed 5 was started 30 minutes after the start of feeds 1, 2, 3 and 4 (time: 30 minutes) and took place over 3 hours.

[0147] Inlet 6 was started 50 minutes after the start of inlets 1, 2, 3 and 4 (time: 50 minutes) and took place over 20 minutes.

[0148] 15 minutes before the end of feeds 1, 2, 3, and 4 (time: 225 minutes), the polymerization temperature was increased to 100°C. After the addition of feed 4 was complete, the polymerization mixture was stirred for a further 30 minutes. The polymerization mixture was then heated to a temperature of 90°C, and then 71.59 ml of water (3.19 pphm) and 15.00 g of a 15 wt% sodium hydroxide solution (0.10 pphm) were added.

[0149] Feeds 7 and 8 were then started and continued for a further 2 hours. After feeds 7 and 8 were completed, the polymerization mixture was cooled to room temperature and treated with 144.00 g of 15 wt% sodium hydroxide solution (0.96 pphm).

[0150] The solids content of the dispersion was 53 wt%.

[0151] The polymer dispersion was analyzed using an analytical ultracentrifuge: Bimodal particle size distribution: The smaller particle population had its peak maximum at 55 nm. The proportion of the total polymer was 20 wt.%. The larger particle population had its peak maximum at 117 nm. The proportion of the total polymer was 80 wt.%. Example 2 (delayed butadiene addition)

[0152] The emulsion polymerization was carried out as in Example 1, with the difference that feed 3 was started 30 minutes after the start of feeds 1, 2 and 4 (time: 30 minutes) and took place over 3.5 hours.

[0153] Feed 6 was started 60 minutes after the start of feeds 1, 2, and 4 (time: 60 minutes) and continued for 20 minutes. An increased amount of sodium hydroxide solution (0.25 pphm) was used in feed 1.

[0154] The solids content of the dispersion was 53 wt%.

[0155] The polymer dispersion was analyzed using an analytical ultracentrifuge: Bimodal particle size distribution: The smaller particle population had its peak maximum at 50 nm. Its proportion of the total polymer was 27 wt%. The larger particle population had its peak maximum at 120 nm. Its proportion of the total polymer was 73 wt%. Example 3 (delayed butadiene addition)

[0156] The emulsion polymerization was carried out as in Example 1, with the difference that feed 3 was started 30 minutes after the start of feeds 1, 2 and 4 (time: 30 minutes) and took place over 3.5 hours.

[0157] Feed 6 was started 40 minutes after the start of feeds 1, 2, and 4 (time: 40 minutes) and continued for 20 minutes. An increased amount of sodium hydroxide solution (0.25 pphm) was used in feed 1.

[0158] The solids content of the dispersion was 53 wt%.

[0159] The polymer dispersion was analyzed using an analytical ultracentrifuge: Bimodal particle size distribution: The smaller particle population had its peak maximum at 56 nm. The proportion of the total polymer was 24 wt%.

[0160] The larger particle population had its peak maximum at 118 nm. The proportion of the total polymer was 76 wt%. Example 4

[0161] The emulsion polymerization was carried out as in Example 1, with the difference that

[0162] Feed 6 was started 40 minutes after the start of feeds 1, 2, 3, and 4 (time: 40 minutes) and was metered over a period of 20 minutes. An increased amount of sodium hydroxide solution (0.25 pphm) was used in feed 1.

[0163] The solids content of the dispersion was 53 wt%.

[0164] The polymer dispersion was analyzed using an analytical ultracentrifuge: Bimodal particle size distribution: The smaller particle population had its peak maximum at 58 nm. The proportion of the total polymer was 30 wt%.

[0165] The larger particle population had its peak maximum at 118 nm. The proportion of the total polymer was 70 wt%. Example 5

[0166] The emulsion polymerization was carried out as in Example 1, with the difference that 90.00 g of a 15 wt.% solution of sodium lauryl sulfate (0.6 pphm) was added as feed 6 over a period of 20 minutes.

[0167] The solids content of the dispersion was 53 wt%.

[0168] The polymer dispersion was analyzed using an analytical ultracentrifuge: Bimodal particle size distribution: The smaller particle population had its peak maximum at 56 nm. The proportion of the total polymer was 16 wt%.

[0169] The larger particle population had its peak maximum at 116 nm. The proportion of the total polymer was 84 wt%. Example 6

[0170] The emulsion polymerization was carried out as in Example 1, with the difference that 75.75 g of a 29.7 wt.% dispersion of a polystyrene latex with an average particle size of 30 nm (1.0 pphm) was used as seed latex in the initial charge.

[0171] The solids content of the dispersion was 53 wt%.

[0172] The polymer dispersion was analyzed using an analytical ultracentrifuge: Bimodal particle size distribution: The smaller particle population had its peak maximum at 50 nm (. The proportion of the total polymer was 10 wt.%

[0173] The larger particle population had its peak maximum at 100 nm. The proportion of the total polymer was 90 wt.% Comparative example V1 (without emulsifier shot)

[0174] Template: 360,01 g Water 11,25 g a 2 wt.% solution of EDTA (complexing agent) (0.01 pphm) 18,00 g a 15 wt.% solution of sodium lauryl sulfate (emulsifier A) (0.12 pphm) 192,86 g a 7 wt.% aqueous solution of itaconic acid (0.6 pphm) 45,45 g a 29.7 wt.% dispersion of a polystyrene latex with an average particle size of 30 nm (0.6 pphm) 4,50 g Acrylic acid (0.2 pphm) 47,48 g Styrene (2.11 pphm)

[0175] Encore: 86,79 g a 7% w / w solution of sodium peroxodisulfate (initiator A) (0.27 pphm)

[0176] Inlet 1: 90,00 g Acrylic acid (4.0 pphm) 40,18 g a 28 wt.% solution of ethoxylated sodium lauryl ether sulfate (emulsifier B) (0.5 pphm) 36,00 g a 15 wt.% solution of sodium lauryl sulfate (0.24 pphm) 37,5 g 15 wt.% sodium hydroxide solution (0.15 pphm) 597,91 ml Water

[0177] Inlet 2: 1252,13 g Styrene (55.65 pphm)

[0178] Inlet 3: 842,4 g Butadiene (37.44 pphm)

[0179] Inlet 4: 273,21 g a 7 wt.% solution of sodium peroxodisulfate (initiator A) (0.85 pphm)

[0180] Inlet 5: 180,00 g a 10 wt.% solution of tert-butyl hydroperoxide as (initiator B) (0.8 pphm)

[0181] Inlet 6: 51,75 g a 10 wt.% solution of tert-butyl hydroperoxide as (initiator B) (0.23 pphm)

[0182] Inlet 7: 66,98 g a 13.1% w / w solution of acetone bisulfite (0.39 pphm)

[0183] The components of the reaction mixture were placed in a 6-liter pressure reactor and mixed. The reaction mixture was heated to 90°C. Once 90°C was reached, initiator A (additive 1) was slowly added, and polymerization was initiated.

[0184] Immediately thereafter, inlets 1, 2, and 4 were started (time: 0 minutes). Inlets 1 and 2 were administered over a period of 4 hours.

[0185] Inlet 3 started 30 minutes after the start of inlets 1, 2 and 4 (time: 30 minutes) and took place over 3.5 hours.

[0186] Inflow 4 took place over a period of 4 hours and 15 minutes.

[0187] Feed 5 was started 30 minutes after the start of feeds 1, 2 and 4 (time: 30 minutes) and took place over 3 hours.

[0188] Fifteen minutes before the end of feeds 1, 2, and 3 (time: 225 minutes), the polymerization temperature was increased to 100°C. After the addition of feed 4 was complete, the polymerization mixture was stirred for a further 30 minutes. The polymerization mixture was then heated to 90°C, and then 71.59 ml of water and 15.00 g of a 15 wt% sodium hydroxide solution (0.10 pphm) were added.

[0189] Feeds 6 and 7 were then started and continued for a further 2 hours. After feeds 6 and 7 had been completed, the polymerization mixture was cooled to room temperature and then treated with 144.00 g of 15 wt% sodium hydroxide solution (0.96 pphm).

[0190] A monomodal dispersion was obtained. The solids content of the dispersion was 53 wt.%.

[0191] The mean particle size D 50 , (determined by AUZ) of the dispersion particles: 128 nm. Comparative example V2 (without emulsifier shot)

[0192] Template: 360,01 g Water 11,25 g a 2 wt.% solution of EDTA (complexing agent) (0.01 pphm) 18,00 g a 15 wt.% solution of sodium lauryl sulfate (emulsifier A) (0.12 pphm) 192,86 g a 7 wt.% aqueous solution of itaconic acid (0.6 pphm) 189,39 g a 29.7 wt.% dispersion of a polystyrene latex with an average particle size of 30 nm (2.50 pphm) 4,50 g Acrylic acid (0.2 pphm) 47,48 g Styrene (2.11 pphm)

[0193] Encore: 86,79 g a 7% w / w solution of sodium peroxodisulfate (initiator A) (0.27 pphm)

[0194] Inlet 1: 90,00 g Acrylic acid (4.0 pphm) 40,18 g a 28 wt.% solution of ethoxylated sodium lauryl ether sulfate (emulsifier B) (0.5 pphm) 36,00 g a 15 wt.% solution of sodium lauryl sulfate (0.24 pphm) 37,5 g 15 wt.% sodium hydroxide solution (0.25 pphm) 547,96 ml Water

[0195] Inlet 2: 1252,13 g Styrene (55.65 pphm)

[0196] Inlet 3: 842,4 g Butadiene (37.44 pphm)

[0197] Inlet 4: 273,21 g a 7 wt.% solution of sodium peroxodisulfate (initiator A) (0.85 pphm)

[0198] Inlet 5: 180,00 g a 10 wt.% solution of tert-butyl hydroperoxide as (initiator B) (0.8 pphm)

[0199] Inlet 6: 51,75 g a 10 wt.% solution of tert-butyl hydroperoxide as (initiator B) (0.23 pphm)

[0200] Inlet 7: 66,98 g a 13.1% w / w solution of acetone bisulfite (0.39 pphm)

[0201] The components of the reaction mixture were placed in a 6-liter pressure reactor and mixed. The reaction mixture was heated to 90°C. Once 90°C was reached, initiator A (additive 1) was slowly added, and polymerization was initiated.

[0202] Immediately thereafter, inlets 1, 2, and 4 were started (time: 0 minutes). Inlets 1 and 2 were administered over a period of 4 hours.

[0203] Inlet 3 started 30 minutes after the start of inlets 1, 2 and 4 (time: 30 minutes) and took place over 3.5 hours.

[0204] Inflow 4 took place over a period of 4 hours and 15 minutes.

[0205] Feed 5 was started 30 minutes after the start of feeds 1, 2 and 4 (time: 30 minutes) and took place over 3 hours.

[0206] Fifteen minutes before the end of feeds 1, 2, and 3 (time: 225 minutes), the polymerization temperature was increased to 100°C. After the addition of feed 4 was complete, the polymerization mixture was stirred for a further 30 minutes. The polymerization mixture was then heated to 90°C, and then 71.59 ml of water and 15.00 g of a 15 wt% sodium hydroxide solution (0.10 pphm) were added.

[0207] Feeds 6 and 7 were then started and continued for a further 2 hours. After feeds 6 and 7 had been completed, the polymerization mixture was cooled to room temperature and then treated with 129.00 g of 15 wt% sodium hydroxide solution (0.86 pphm).

[0208] The dispersion is coagulated. Table 1: Start of the dosing of the emulsifier shot with regard to time and added monomer quantity of the individual examples e.g. Start of "emulsifier shot" after x min Start from y % total dosing time of the monomers Start from z % total monomer quantity* "Emulsifier shot" x times the dosing rate in P1 1 50 20,83 20,83 16,66 2 60 25 21,42 16,66 3 40 16,66 15,42 16,66 4 40 16,66 16,66 16,66 5 50 20,83 20,83 10 6 50 20,83 20,83 16,66 V1 without * without monomers in the template Production of coating slips S1 to S6 and SV1

[0209] Using the dispersions obtained from the examples, coating slips were prepared consisting of 100 parts by weight of precipitated calcium carbonate (Opacarb A 40), 9.5 parts by weight of binder (solid example dispersion), 0.5 parts by weight of calcium stearate (Ombrelub), 0.25 parts by weight of thickener (Sterocoll FS), and 0.1 part of dispersant (Sokalan CP 10). All weights refer to the respective solids content of the components.

[0210] The solids content of the coating slip was 66 wt.%, the pH value was 8.8.

[0211] The viscosity at high shear rates was measured using a capillary viscometer (ACAV) Table 2: Viscosity of the coating slip at high shear rates e.g. Dispersion e.g. Viscosity at a shear rate of 500,000 s -1< [mPa s] Viscosity at a shear rate of 620,000 s -1< [mPa s] S1 1 118 109 S2 2 115 114 S3 3 124 123 S4 4 114 108 S5 5 115 110 S6 6 106 100 SV1 V1 150 not measurable

[0212] As can be seen from the table, all coating slips S1 to S6 according to the invention formulated with the dispersions according to the invention exhibit low viscosity at high shear rates and thus exhibit very good rheological behavior. The coating slip formulated with the monomodal dispersion of Comparative Example C1 resulted in significantly higher viscosities and is no longer measurable at a shear rate of 620,000 s -1<.

Claims

1. A process for preparing an aqueous polymer dispersion having a polymodal particle distribution of the polymer particles, with a first population of large polymer particles and a second population of small polymer particles, by radically initiated aqueous emulsion polymerization, which comprises polymerizing (a) 40 to 75 parts by weight of at least one vinylaromatic compound, (b) 24.9 to 59.9 parts by weight of at least one conjugated aliphatic diene, (c) 0.1 to 10 parts by weight of at least one monomer containing acid groups, and (d) 0 to 20 parts by weight of at least one other monoethylenically unsaturated monomer, where the amounts of the monomers (a) to (d) add up to 100 parts by weight, in an aqueous medium by a monomer feed process, with the proviso that - monomers and emulsifier are metered continuously and - following a period P1, when 15 to 30% of the total monomer metering time has elapsed and 15 to 30 wt% of the total monomer amount has been metered in, the metering rate of the emulsifier is increased for a period P2 lasting no longer than 30 minutes to 10 to 100 times the average metering rate of the emulsifier in the period P1.

2. The process according to claim 1, wherein the vinylaromatic compound is styrene and / or methylstyrene and the conjugated aliphatic diene is 1,3-butadiene and / or isoprene.

3. The process according to claim 1 or 2, wherein the polymerization is initiated in an initial charge which contains up to 20 parts by weight of the total monomers and subsequently monomers and emulsifier are metered continuously.

4. The process according to any of claims 1 to 3, wherein the emulsifier is selected from alkali metal salts and ammonium salts of C8-C22 alkyl sulfates and of sulfuric monoesters with ethoxylated C12-C18 alkanols (EO degree: 2 to 40) and of sulfuric monoesters with ethoxylated C4-C9 alkylphenols (EO degree: 10 to 40), and from bis(phenylsulfonic acid) ethers and / or the alkali metal salts or ammonium salts thereof which carry a C4-C24 alkyl group on one or both aromatic rings.

5. The process according to any of claims 1 to 5, wherein the emulsifier and at least one monomer are metered together as a mixture.

6. The process according to any of claims 1 to 4, wherein 0.1 to 5 parts by weight of emulsifier per 100 parts by weight of monomers are metered continuously in a mixture with at least one monomer.

7. The process according to any of claims 1 to 6, wherein during the period P2 the metering rate of the emulsifier is 20 to 90 times the average metering rate of the emulsifier in the period P1.

8. The process according to any of claims 1 to 7, wherein polymerization takes place in the presence of at least one inorganic peroxide and also at least one organic peroxide.

9. The process according to any of claims 1 to 8, wherein no reducing agent that can form a red / ox initiator system with the organic or inorganic peroxides is present during the monomer feed.

10. The process according to any of claims 1 to 9, wherein polymerization takes place at a temperature in the range from ≥ 80°C to ≤ 115°C.

11. The process according to any of claims 1 to 10, wherein the polymerization is initiated in an initial charge containing up to 2 parts by weight of finely divided polystyrene per 100 parts by weight of total monomers and then monomers and emulsifier are metered continuously.

12. The process according to any of claims 1 to 11, wherein no chain transfer agent selected from aliphatic and / or araliphatic halogen compounds, organic thio compounds and substituted thiols is used during the polymerization.

13. An aqueous polymer dispersion obtainable by a process according to any of claims 1 to 12.

14. The aqueous polymer dispersion according to claim 13, wherein the fraction of the first population is 60 to 95 wt% and the fraction of the second population is 5 to 40 wt%, based on the total polymer of the dispersion.

16. The use of the aqueous polymer dispersion according to claim 13 or 14 as a binder, adhesive, fiber sizing agent, for producing coatings or for producing paper coating slips.

17. A paper coating slip comprising (i) inorganic pigments and (ii) an aqueous polymer dispersion according to claim 13 or 14 and also optionally further auxiliaries.

18. Paper or card coated with a paper coating slip according to claim 17.

19. A method for coating paper or card, by - providing an aqueous polymer dispersion according to either of claims 13 and 14; and - producing a paper coating slip with the aqueous polymer dispersion, at least one pigment and optional further auxiliaries; and - applying the paper coating slip to at least one surface of paper or card.