METHOD FOR THE PREPARATION OF AN AQUEOUS POLYMER DISPERSION OF VINYL AROMATIC COMPOUND AND CONJUGATED ALIPHATIC DIENE

Description

[0001] The invention relates to a process for producing an aqueous polymer dispersion, wherein a vinylaromatic compound and a conjugated aliphatic diene are copolymerized in an aqueous medium in the presence of a free-radical initiator. 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] Known binders for paper coating slips based on copolymers of vinylaromatic compounds and aliphatic dienes are not yet entirely satisfactory in every respect. In aqueous emulsion copolymers of vinylaromatic compounds and aliphatic dienes, side reactions of the dienes lead to the formation of strong-smelling Diels-Alder adducts.

[0003] EP 2 370 484 teaches a process for the preparation of low-coagulum styrene-butadiene / acrylonitrile dispersions in which the metering of acrylonitrile is not started at the start of the polymerization, but is delayed after one third of the feed time of the other monomers has elapsed.

[0004] WO 99 / 09251 discloses a process for the preparation of styrene-butadiene copolymers in which the polymerization is carried out in such a way that a portion of the monomer mixture and the radical initiator are first added to the aqueous solution or dispersion of the degraded starch heated to the polymerization temperature and, after the polymerization has started, the remaining monomer mixture is continuously metered in.

[0005] Furthermore, EP 1 408 059 teaches a process for producing a styrene-butadiene latex by metering styrene and butadiene along a discontinuous gradient by constantly reducing the amount of butadiene added and constantly increasing that of styrene. The polymerization is initiated with ammonium persulfate as the radical initiator, with the metering of all monomers starting simultaneously. The addition is continued several times with a stepwise increase or decrease. However, dispersions obtained in this way have only slightly reduced residual amounts of 4-phenylcyclohexene (4-PCH) compared to the process using a continuous gradient. However, all these processes using discontinuous addition of monomers have in common the requirement of sulfur regulators.

[0006] WO2010 / 094641 describes an arylcyclohexene-poor aqueous polymer dispersion and its process for preparation by radically initiated emulsion polymerization from the three monomer types: vinyl aromatic compound, conjugated aliphatic diene and ethylenically unsaturated acid.

[0007] The object of the invention is to provide a process for the preparation of aqueous polymer dispersions based on copolymers of vinylaromatics and conjugated aliphatic dienes, which have a lower proportion of 4-phenylcyclohexene. Furthermore, they should exhibit good performance properties when used in paper coating slips, in particular, cause minimal yellowing of the paper.

[0008] The object is achieved according to the invention by a process for the preparation of an aqueous polymer dispersion by radically initiated aqueous emulsion polymerization by reacting in an aqueous medium (a) 40 to 75 parts by weight at least one vinyl aromatic compound and (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 in the presence of a radical initiator, with the proviso that the vinylaromatic compound is metered in under polymerization conditions in a continuous stream over a period of at least 120 minutes, the metering of the conjugated aliphatic diene is started at a time at which at least 5% and at most 30% of the vinylaromatic compound have already been metered in in a continuous stream and no chain transfer agent selected from aliphatic and / or araliphatic halogen compounds, organic thio compounds and substituted thiols is used during the polymerization and optionally the polymerization is initiated in an aqueous polymerization mixture which contains up to 5% of the vinylaromatic compound and contains no aliphatic diene.

[0009] When 5% of the vinyl aromatic compound is mentioned in this application, this refers to the total amount of vinyl aromatic compound.

[0010] If a quantity is given below in parts by weight, this refers, unless otherwise stated, to 100 parts by weight of total monomers.

[0011] The following ethylenically unsaturated monomers (a), (b), (c) and (d) can be used to prepare the aqueous polymer dispersions.

[0012] Examples of vinylaromatic compounds (monomers of group (a)) include styrene, α Methylstyrene and / or vinyltoluene are suitable. Styrene is preferred from this group of monomers.

[0013] The total amount of monomers (a) is 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 d).

[0014] 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.

[0015] 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.

[0016] Examples of monomers containing acid groups (monomers (c)) are ethylenically unsaturated carboxylic acids, ethylenically unsaturated sulfonic acids, and vinylphosphonic acid. Preferably, ethylenically unsaturated carboxylic acids containing 3 to 6 carbon atoms in the molecule are used. α , βMonoethylenically unsaturated mono- and dicarboxylic acids are used. Examples include 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. These acids can be used either as sole components or in combination.

[0017] 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.

[0018] 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 of one or more acid group-containing monomers, based on 100 parts by weight of total monomers.

[0019] 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-dialkylaminoalkyl methacrylamides, N,N-dialkylaminoalkyl acrylates, N,N-dialkylaminoalkyl methacrylates, vinyl chloride and vinylidene chloride (monomers of group (d)).

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] Particularly preferred are (a) 60 to 70 parts by weight at least one vinyl aromatic compound (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.

[0025] 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 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.%.

[0026] 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.

[0027] 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.

[0028] 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'-dimethylenisobutyroamidine) dihydrochloride and 2,2'-azobis(amidinopropyl) dihydrochloride (AlBA, equivalent to V-50 from Wako Chemicals).

[0029] 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.

[0030] Preferred radical initiators are inorganic and organic peroxides, preferably ammonium or alkali metal salts of peroxosulfates or peroxodisulfates, as well as tert-butyl, p-menthol, 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.

[0031] 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.

[0032] Initiation of the polymerization reaction is understood to mean the start of the polymerization reaction of the monomers present in the polymerization vessel by decomposition of the radical initiator.

[0033] To polymerize the monomers, an aqueous solution is first prepared containing a protective colloid and / or an emulsifier in dissolved form 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 the polymerization temperature has been reached, the metered addition of monomers (a) and (c) and, optionally, (d) is begun. In this variant, no monomers are initially introduced. The polymerization starts when the polymerization mixture contains vinylaromatic compound and free-radical initiator and reaches a temperature in the range of ≥ 80°C to ≤ 95°C.

[0034] 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. According to this variant, monomers are initially introduced. The polymerization then starts when 0.1 to 5% of the vinylaromatic compound are initially charged, the reaction temperature is set to a temperature in the range from ≥ 80° C to ≤ 95° C and radical initiator, preferably 0.1 to 0.5 parts by weight of an inorganic peroxide, based on 100 parts by weight of total monomer, is initially charged.

[0035] The order in which the conditions are set is not crucial. Preferably, the vinylaromatic compound is initially introduced, and the inorganic peroxide is added after heating to the reaction temperature in the range of 80-95°C, thus initiating the polymerization.

[0036] According to a preferred process variant, a portion of inorganic peroxide, preferably an ammonium salt or an alkali metal salt of a peroxodisulfate, is selected to initiate the polymerization, and the remaining amount of radical initiator, preferably the inorganic peroxide and an organic peroxide, is subsequently metered in a continuous flow. The metering of the organic peroxide is preferably started at a point in time at which at least 5% and at most 20% of the vinylaromatic compound has already been metered in a continuous flow.

[0037] 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, and 0.1 to 2.0 parts by weight of inorganic peroxide are metered in directly thereafter, and the metering of 0.1 to 2.0 parts by weight, in each case based on 100 parts by weight of total monomer, of the organic peroxide is started from a point in time at which at least 5% and at most 20% of the vinyl aromatic compound have already been metered in, the metering of the inorganic peroxide, the organic peroxide and the vinyl aromatic compound each taking place in a continuous flow.

[0038] According to a preferred variant, the inorganic peroxide is continuously dosed following initiation throughout the polymerization.

[0039] Furthermore, a process is preferred according to which an organic peroxide is added from a point in time at which at least 5%, particularly preferably at least 8%, in particular at least 10% and at most 20% of the total amount of vinyl aromatic compound has already been added under polymerization conditions.

[0040] 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 on the type and amount of radical initiator used.

[0041] Preferred polymerization conditions are a temperature in the range of ≥ 80° C to ≤ 105° C, preferably ≥ 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.

[0042] The monomers are preferably added continuously, i.e., without interruption. The monomers are 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. According to a preferred embodiment, the monomer addition rate (increase in monomers) approximately corresponds to the polymerization rate of the monomers (decrease in monomers).

[0043] According to the process according to the invention, the conjugated aliphatic diene is only added from a point in time at which at least 5%, preferably at least 8, in particular at least 10% of the total amount of vinyl aromatic compound has already been added in a continuous flow.

[0044] 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.

[0045] The conjugated aliphatic diene is metered in in a continuous stream, 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 stream 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.

[0046] According to one embodiment of the invention, the polymerization is carried out in the presence of a degraded starch. In emulsion copolymerization, preferably 15 to 100 parts by weight of a degraded starch per 100 parts by weight of the monomers are used.

[0047] 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%.

[0048] 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.

[0049] Other suitable starches are cationically modified starches, i.e. starch compounds which contain amino groups or ammonium groups.

[0050] 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.

[0051] Particularly preferred are degraded native starches, especially native starches degraded to maltodextrin.

[0052] Preference is given to degraded starches with an intrinsic viscosity n i of ≤0.07 dl / g or ≤0.05 dl / g. The intrinsic viscosity ni of the degraded starches is preferably in the range of 0.02 to 0.06 dl / g. The intrinsic viscosity n i is determined according to DIN EN1628 at a temperature of 23° C.

[0053] To assist the emulsification of the monomers in the aqueous medium, commonly used protective colloids and / or emulsifiers can be used. A detailed description of suitable protective colloids can be found in Houben-Weyl, Methods of Organic Chemistry, Volume XIV / 1, Macromolecular Materials, Georg-Thieme-Verlag, Stuttgart, 1961, pages 411 to 420.

[0054] Suitable emulsifiers are surfactants whose number-average molecular weight is usually below 2000 g / mol or preferably below 1500 g / mol, while the number-average molecular weight of the protective colloids is above 2000 g / mol, for example from 2000 to 100000 g / mol, in particular from 5000 to 50000 g / mol.

[0055] Suitable emulsifiers include, for example, ethoxylated C8 to C36 fatty alcohols with a degree of ethoxylation of 3 to 50, ethoxylated mono-, di-, and tri-C4 to C12 alkylphenols with a degree of ethoxylation of 3 to 50, alkali metal salts of dialkyl esters of sulfosuccinic acid, alkali metal and ammonium salts of C8 to C12 alkyl sulfates, alkali metal and ammonium salts of C12 to C18 alkylsulfonic acids, and alkali metal and ammonium salts of C9 to C18 alkylarylsulfonic acids. Cationic emulsifiers include, for example, compounds with at least one amino or ammonium group and at least one C8-C22 alkyl group.

[0056] Compounds of the general formula I have also proven to be emulsifiers wherein R 1< and R 2< are C 4 - to C 24 -alkyl and one of the radicals R 1< or R 2< can also be hydrogen, and A and B can be alkali metal ions and / or ammonium ions. In the general formula I, R 1< and R 2< are preferably linear or branched alkyl radicals having 6 to 18 C atoms, in particular having 6, 12 and 16 C atoms or H atoms, where R 1< and R 2< are not both H atoms at the same time. A and B are preferably sodium, potassium or ammonium ions, with sodium ions being particularly preferred. Particularly advantageous compounds I are those in which A and B are sodium ions, R 1< is a branched alkyl radical having 12 C atoms and R 2< is an H atom or R 1<. Technical mixtures containing 50 to 90 wt. % of the monoalkylated product are frequently used, for example, Dowfax®< 2A1 (a trademark of the Dow Chemical Company). The compounds I are well known, e.g., from US Pat. No. 4,269,749, and are commercially available.

[0057] If emulsifiers and / or protective colloids are used as auxiliaries for dispersing the monomers, the amounts used are, for example, 0.1 to 5 parts by weight based on 100 parts by weight of monomers.

[0058] According to one process variant, the polymerization is carried out in the presence of a polystyrene seed, preferably in the presence of an aqueous dispersion of finely divided polystyrene with an average particle diameter of 20 to 40 nm (determined by ultracentrifuge).

[0059] 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.

[0060] The process according to the invention does 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.

[0061] 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 such as isopropanol in particular and phosphorus compounds such as sodium hypophosphite. If a chain transfer agent is 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 in offset relation to the monomers. A process in which no chain transfer agent is present during the polymerization is particularly preferred.

[0062] 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 adding all monomers. Typically, a conversion of around 95% is achieved at this point.

[0063] In order to increase the conversion even further, one can, for example, add additional radical initiator from the group of initiators mentioned above to the reaction mixture or extend the addition and carry out a so-called "post-polymerization", i.e. a polymerization to achieve conversions of >95% up to 99%.

[0064] 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.

[0065] 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.

[0066] Chemical deodorization can also be performed. If traces of residual monomers still need to be removed, this can also be done chemically using redox initiator systems, such as those listed in DE-A 44 35 423, DE-A 44 19 518, and DE-A 44 35 422. The above-mentioned organic and / or inorganic peroxides are particularly suitable as oxidizing agents. Suitable reducing agents are preferably sodium disulfite, sodium hydrogen sulfite, sodium dithionite, sodium hydroxymethanesulfinate, formamidine sulfinic acid, acetone bisulfite (= sodium hydrogen sulfite addition product with acetone), ascorbic acid or reducing sugar compounds, or water-soluble mercaptans such as mercaptoethanol.

[0067] Treatment with the redox initiator system is carried out in a 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.

[0068] 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.

[0069] The present invention also relates to the dispersions obtainable by the process according to the invention. These are 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. Furthermore, they have a uniform particle size and generally a solids content of approximately 50 wt. %, preferably in the range of 45 to 55 wt. The dispersions according to the invention have a low proportion of 4-phenylcyclohexene.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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 and 1 bar) are used per 100 parts by weight of pigments.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] Paper coating slips according to the invention may 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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

[0084] Unless otherwise indicated by the context, percentages always refer to percentages by weight. The stated content refers to the content in an aqueous solution or dispersion. The percentage pphm (parts per hundred monomers) refers to the weight percentage based on 100 parts by weight of monomer.

[0085] When water was used in the examples, demineralized water was used. Glass transition temperature TG

[0086] The glass transition temperature is determined according to DIN 53765 using a DSC820 instrument, series TA8000 from Mettler-Toledo Int. Inc. Determination of gel content

[0087] A polymer film is produced from the dispersion by drying in a silicone mold. Rectangles measuring approximately 2 x 1.5 cm are punched out of this film and weighed. The pieces are then placed in a Petri dish containing methyl ethyl ketone and left there for 48 hours at room temperature. The uncrosslinked portion of the polymer dissolves in the solvent. After this time, the polymer pieces are removed from the solvent, dried for one hour at 140 °C, and weighed again. The weight loss corresponds to the soluble portion of the polymer, and the insoluble portion is the so-called gel. This insoluble weight fraction, based on the weight of the dried total polymer, is the so-called gel content. Determination of the content of 4-PCH

[0088] The 4-phenylcyclohexene content is determined by gas chromatography (direct injection). The values are expressed in ppm based on the dispersion. Determination of the coagulum

[0089] The amount of coagulum in the dispersion refers to particles whose diameter is >45 µ m. It was determined by filtering the finished dispersion through a sieve with a known pore diameter. Determination of thermal yellowing

[0090] Thermal yellowing is determined on coated paper. The coating color to be tested is applied to one side of a coated paper base using a laboratory coating machine and dried using an IR lamp. The weight of the applied coating layer is 10 g / m². The CIE whiteness of the coated side of the paper produced in this way is determined. It is then stored in a circulating air drying cabinet at 120 °C for 48 hours. The CIE whiteness of the stored paper is then measured. The difference (Δ) between the two measurements determines the yellowing. A value of 0 means no yellowing. The lower the value, the lower the yellowing.

[0091] The following materials were used in the examples: Emulsifier A: Sodium lauryl sulfate in the form of a 15 wt.% solution (Disponil ®< SDS from BASF) Emulsifier B: Ethoxylated sodium lauryl ether sulfate in the form of a 28 wt.% solution (Texapon ®< NSO P from BASF) Complexing agents: EDTA in the form of a 2 wt.% solution (Trilon BX from BASF) Seed latex: Polystyrene seed in the form of a 29.7 wt.% dispersion with a particle size of approximately 30 nm (determined by analytical ultracentrifuge) Initiator A: 7 wt.% solution of sodium peroxodisulfate (NaPS) Initiator B: 10 wt.% solution of tert-butyl hydroperoxide Reducing agent: 13 wt.% solution of acetone bisulfite Mined starch: commercially available 72% w / w aqueous glucose syrup with a DE value (dextrose equivalent) of 28

[0092] In all examples, the feeds were metered in a uniform flow rate. Production of emulsion polymers

[0093] 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 Template:

[0094] 47,48 g Styrene (2.11 pphm) 192,86 g a 7 wt.% aqueous solution of itaconic acid (0.6 pphm) 4,5 g Acrylic acid (0.2 pphm) 75,76 g a 29.7 wt.% dispersion of a polystyrene latex with an average particle size of 30 nm (1.0 pphm) 18 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) 86,79 g a 7 wt.% solution of sodium peroxodisulfate (initiator A) (0.27 pphm) Inlet 1:

[0095] 1252,13 g Styrene (55.6 pphm) Inlet 2:

[0096] 90 g Acrylic acid (4.0 pphm) 36 g a 15 wt.% solution of sodium lauryl sulfate (0.24 pphm) 40,18 g a 28 wt.% solution of ethoxylated sodium lauryl ether sulfate (emulsifier B) (0.5 pphm) 37,5 g 15 wt.% sodium hydroxide solution (0.25 pphm) 598 ml Water Inlet 3:

[0097] 842,4 g Butadiene (37.44 pphm) Inlet 4:

[0098] 273,21 g a 7 wt.% solution of sodium peroxodisulfate (initiator A) (0.85 pphm) Inlet 5:

[0099] 231,7 g a 10 wt.% solution of tert-butyl hydroperoxide as (initiator B) (1.03 pphm) Inlet 6:

[0100] 66,98 g Acetone bisulfite (0.39 pphm)

[0101] The components of the initial reaction mixture and 360 ml of water were placed in a 6 l pressure reactor. The initial reaction mixture was mixed and heated to 90°C. The polymerization was then initiated by adding 0.27 pphm of Initiator A.

[0102] Immediately afterwards, feeds 1, 2 and 4 were started (time: 0 minutes), with feeds 1 and 2 taking place over 4 hours and feed 4 over a period of 4 hours and 15 minutes. The butadiene feed (feed 3) started 30 minutes after the start (time: 30 minutes) of feeds 1, 2 and 4 and took place over 3.5 hours. Feed 5 started at the same time as the butadiene (time: 30 minutes). An amount of 0.8 pphm was metered over 3 hours. The addition was then interrupted and resumed after 75 minutes. This second dosing stage took place over a period of 2 hours, parallel to the addition of feed 6. Immediately before the addition of feed 5 was resumed, 15 g of a 15 wt% sodium hydroxide solution (0.1 pphm) were added. Finally, the mixture was cooled to room temperature and neutralized with sodium hydroxide solution to a pH of 6-7.

[0103] The dosage of the various feeds in Example 1 can be shown in the following overview: Time [min] Inlet 1 Inlet 2 Inlet 3 Inlet 4 Inlet 5 Inlet 6 0 Template 90° C + Initiator A + Monomers of the template 0 started started started 30 started started 210 stopped (0.8 pphm) 240 stopped stopped stopped 255 stopped 285 Addition of caustic soda started started 405 stopped stopped Example 2 (comparative example, without delayed addition of butadiene)

[0104] Example 2 was carried out analogously to Example 1, with the difference that feeds 1 to 4 started at the same time (all at time 0). Feed 3 ended at time 240 min. Examples 3 and 4

[0105] Examples 3 and 4 were carried out analogously to Example 1, whereby the addition of butadiene was started 60 minutes (Example 3) and 90 minutes (Example 4) after the start of feeds 1, 2, and 4. The feed was stopped after 240 minutes for feeds 1 to 3. Example 5

[0106] Example 5 was carried out analogously to Example 1, with the difference that the amount of styrene was increased by 2 pphm (57.6 pphm styrene) and the amount of butadiene was decreased by 2 pphm (35.44 pphm butadiene). Example 6 (comparative example without delayed addition)

[0107] The emulsion polymerization was carried out as in Example 1, except that the addition of feed 3 started simultaneously with feeds 1, 2, and 4 and ended after 240 minutes. Another difference was that 53.6 pphm of styrene was added (reduced by 2 pphm) and the amount of butadiene was increased by 2 pphm (39.44 pphm butadiene). Example 7

[0108] Example 7 was carried out analogously to Example 1, increasing styrene by 8 pphm (53.6 pphm styrene) and decreasing the amount of butadiene by 8 pphm (29.44 pphm butadiene). Example 8 (Comparative example to Example 7, without delayed addition of butadiene)

[0109] The emulsion polymerization was carried out as in Example 1, with the difference that the addition of feed 3 started simultaneously with feeds 1, 2, and 4 and ended simultaneously with feeds 1 and 2 (after 240 min). Another difference was that the amount of styrene was increased by 8 pphm and the amount of butadiene was decreased by 8 pphm. Example 9 (Comparison example to Example 7)

[0110] The emulsion polymerization was carried out as in Example 1, except that the amount of styrene was increased by 10 pphm and the amount of butadiene was decreased by 10 pphm. Table 1: Overview of the reaction conditions of the various examples and the properties of the polymer dispersions Example Butadiene retardation [min] Butadiene [pphm] Styrene [pphm] Tg [°C] Gel content [%] PCH content [ppm] 1 30 37,44 55,6 4 91 10 2 new 0 37,44 55,6 9 90 40 3 60 37,44 55,6 -4 90 20 4 90 37,44 55,6 -9 92 15 5 30 35,44 57,6 10 88 15 6 new 0 39,44 53,6 3 93 35 7 30 29,44 63,6 21 88 10 8 new 0 29,44 63,6 26 87 35 9 30 27,44 65,6 26 84 15 not according to the invention

[0111] All dispersions obtained from Examples 1-9 showed small amounts of coagulum. Example 10 Template:

[0112] 93,96 g a 29.7 wt.% dispersion of a polystyrene latex with an average particle size of 30 nm (1.6 pphm) 6,25 g a 28 wt.% solution of ethoxylated sodium lauryl ether sulfate (emulsifier B) (0.1 pphm) 8,75 g a 2 wt.% solution of EDTA (complexing agent) (0.01 pphm) 743,75 g a 72 wt.% aqueous glucose syrup (DE value 28) (30 pphm) 25 g a 7 wt.% solution of sodium peroxodisulfate (initiator A) (0.1 pphm) Inlet 1:

[0113] 997,5 g Styrene (57 pphm) 52,5 g Acrylic acid (3 pphm) Inlet 2:

[0114] 250 g a 7 wt.% aqueous solution of itaconic acid (1.0 pphm) Inlet 3:

[0115] 682,5 g Butadiene (39 pphm) Inlet 4:

[0116] 175 g a 7 wt.% solution of sodium peroxodisulfate (initiator A) (0.7 pphm) Inlet 5:

[0117] 192,5 g a 10 wt.% solution of tert-butyl hydroperoxide (initiator B) (1.1 pphm) Inlet 6:

[0118] 45,42 g Acetone bisulfite (0.34 pphm)

[0119] In a 6 l pressure reactor, the components of the initial charge were placed in 442.08 g of water. This initial charge was heated to 90 °C. Then, 0.1 pphm of initiator A was added. Feeds 1, 2, 4, and 5 were then started simultaneously, with feed 2 being added over a period of 30 minutes and feeds 1 and 4 over a period of 2.5 hours. The addition of butadiene (feed 3) started 30 minutes after the start of feeds 1, 2, 4, and 5 and continued over 2 hours (ending after 150 minutes). Feed 5 was interrupted after 2.5 hours following the addition of 0.9 pphm. After 3 hours 30 minutes, 70 g of a 15 wt% sodium hydroxide solution (0.6 pphm) was added, followed by the remaining amount of feed 5 being added in parallel with feed 6 over a period of 2 hours. Finally, the mixture was cooled to room temperature and neutralized with sodium hydroxide solution to a pH of 6-7. Example 11 (Comparative example without delayed addition of the diene)

[0120] The emulsion polymerization was carried out as in Example 10, with the difference that the start of feeds 1, 2, 3, 4 and 5 occurs at the same time (all at time 0) and ends at the same time (time 150 min). Example 12 (according to the invention)

[0121] Example 12 was carried out analogously to Example 10, whereby the addition of butadiene was started 15 minutes after the start of feeds 1, 2, 4 and 5 and ended together with these. Table 2: Overview of the reaction conditions of Examples 10-12 and the properties of the polymer dispersions Example Butadiene retardation [min] Butadiene [pphm] Styrene [pphm] Tg [°C] Gel content [%] PCH content [ppm] 10 30 39 57 -3 95 <10 11 0 39 57 6 95 25 12 15 39 57 4 95 <10 TG glass transition temperature

[0122] All dispersions obtained according to Examples 1-12 contained small amounts of coagulum.

[0123] The particle sizes of the polymer particles resulted from the seed control and showed no significant deviations due to the delays in the diene feed.

[0124] Tables 1 and 2 clearly demonstrate the advantages of a procedure with a delayed diene feed. The amount of the odor-causing 4-PCH is significantly lower than in non-inventive examples 2, 6, 8, and 11. At the same time, a smaller amount of the diene can be used to achieve a desired glass transition temperature (see Experiments 1 and 6, 2 and 5, and 8 and 9). To produce a polymer with virtually the same glass transition temperature (TG), a 2 pphm reduction in the amount of butadiene used is sufficient when using the process according to the invention.

[0125] With the same proportion of diene, the gel content of the dispersions is not significantly changed by the delayed feed.

[0126] Preparation of the paper coating slips: The coating slip was prepared in a stirred unit, into which the individual components were added sequentially. The pigments were added in a predispersed form (slurry). The other components were added after the pigments, following the order specified in the coating slip recipe. The final solids content was adjusted by adding water.

[0127] The coating was applied to one side of a coating base paper using a laboratory coating machine and dried using an IR lamp. The weight of the applied coating layer was approximately 10 g / m². Production of a coating color

[0128] The quantities given refer to the solid content. 100 parts by weight precipitated calcium carbonate (Opacarb A 40 from Specialty Minerals) 9.5 parts by weight Emulsion polymer of the respective example 0.25 parts by weight Rheology aids (Sterocoll FS from BASF SE)

[0129] The coating color was prepared in a stirring unit into which the individual components were added one after the other.

[0130] The pigment (precipitated calcium carbonate) was added in predispersed form (slurry). The other components were added in the order above. The final solids content was adjusted by adding water. Coating color data:

[0131] Solids content 66 wt.% Viscosity (Brookfield RVT, spindle 4, 100 rpm): 1000-1400 mPas Table 3: Thermal yellowing of the coated paper d Paper Polymer dispersion of example Polymer TG [°C] Thermal yellowing Δ CIE after 48 h at 120° C P1 1 4 30 P2 no 2 9 30 P3 3 -4 31 P4 4 -9 30 P5 5 10 28 P6 no 6 3 33 P7 7 21 25 P8 ne 8 26 24 P9 9 26 20 P10 10 -3 31 P11 ne 11 6 32 P12 12 4 31 ne: not according to the invention

[0132] The examples show that papers coated with paper coating slips containing polymers of the same glass transition temperature exhibit less thermal yellowing when the polymers were produced by the process according to the invention. Example 13 (not according to the invention):

[0133] Polymerization was carried out analogously to Comparative Example 1 of EP 1408059 A1, with the difference that the individual feeds were added in stages at a constant dosing rate. Template:

[0134] 10,0 g a 2 wt.% solution of EDTA (complexing agent) (0.01 pphm) 59,07 g a 33 wt.% dispersion of a polystyrene latex with an average particle size of 30 nm (0.97 pphm) Inlet 1:

[0135] 40,4 g Acrylic acid (2.02 pphm) 8,0 g a 20 wt.% solution of alkylbenzenesulfonate (Disponil LDBS 20) (0.08 pphm) 649,9 g Water Inlet 2:

[0136] 353,6 g Styrene (17.68 pphm) 7,47 g tert-dodecyl mercaptan (0.37 pphm) Inlet 3:

[0137] 949,4 g Butadiene (47.47 pphm) Inlet 4:

[0138] 288,6 g a 7 wt.% solution of sodium peroxodisulfate (initiator) (1.01 pphm) 405 g Water Inlet 5:

[0139] 656,6 g Styrene (32.83 pphm) 3,64 g tert-dodecyl mercaptan (0.18 pphm)

[0140] The components of the initial mixture and 809 ml of water were placed in a 6 l pressure reactor. The initial mixture was mixed and heated to 78°C. Feed 4 was then started and added over a period of 8 hours.

[0141] 5 minutes after the start of inflow 4, inflows 1, 2 and 3 were started (time: 5 minutes), with inflow 1 taking a duration of 6 hours, inflow 2 a total of 3 hours and inflow 3 a total of 6 hours.

[0142] Inlets 1 and 4 were continuous.

[0143] Inlets 2 and 3 were dosed in the following stages: Inlet 2: 28g in 22 min Inlet 3: 181g in 45 min 32.5g in 22 min 163.2g in 45 min 37g in 22 min 145.4g in 45 min 41.5g in 22 min 127.6g in 45 min 46g in 22 min 109.8g in 45 min 50.5g in 22 min 92g in 45 min 55g in 22 minutes 74.2g in 45 min 70.7g in 26 min 56.2g in 45 min

[0144] After the end of feed 2, feed 5 was started and added over a total of 3 hours.

[0145] The addition took place in the following stages: Inlet 5: 56g in 22 min 62.9g in 22 min 69.7g in 22 min 76.6g in 22 min 83.5g in 22 min 90.4g in 22 min 97.3g in 22 min 123.8g in 26 min

[0146] After the end of feed 4, polymerization was continued for one hour, during which 138.4 g of 10 wt.% sodium hydroxide solution were continuously added. Example 14 (according to the invention)

[0147] The example was carried out analogously to Example 13, with the difference that feed 3 (butadiene) was started half an hour later (time 35 min) and feeds 2, 3 and 5 were added continuously.

[0148] The measured values of 4-PCH were: Example 13: 40 ppm PCH Example 14: 20 ppm PCH.

Claims

1. A process for producing an aqueous polymer dispersion by free-radically initiated aqueous emulsion polymerization, which comprises polymerizing, in an aqueous medium, (a) 40 to 75 parts by weight of at least one vinylaromatic compound and (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 comprising acid groups and (d) 0 to 20 parts by weight of at least one other monoethylenically unsaturated monomer, the amounts of the monomers (a) to (d) adding up to 100 parts by weight, in a monomer feed process in the presence of a free-radical initiator, with the proviso that - the vinylaromatic compound is metered in under polymerization conditions in a continuous mass flow over a period of at least 120 minutes, - the metering of the conjugated aliphatic diene is started at a time at which at least 5% and not more than 30% of the vinylaromatic compound has already been metered in in a continuous mass flow, - no chain transfer agent selected from aliphatic and / or araliphatic halogen compounds, organic thio compounds and substituted thiols is used during the polymerization, and - optionally, the polymerization is initiated in an aqueous polymerization mixture comprising up to 5% of the vinylaromatic compound and comprising no aliphatic diene.

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 free-radical initiator is selected from inorganic peroxides and organic peroxides.

4. The process according to any of claims 1 to 3, wherein a proportion of inorganic peroxide is selected for initiating the polymerization and then the remaining free-radical initiator amount is metered in in a continuous mass flow.

5. The process according to any of claims 1 to 4, wherein an organic peroxide is metered in starting from the time at which at least 5% and 20% of the vinylaromatic compound has already been metered in.

6. The process according to any of claims 1 to 5, wherein the conjugated aliphatic diene is metered in in a continuous mass flow over a period of at least 60 minutes.

7. The process according to any of claims 1 to 5, wherein 15 to 60 parts by weight of a degraded starch are used per 100 parts by weight of the monomers.

8. An aqueous polymer dispersion obtainable by free-radically initiated emulsion polymerization according to any of claims 1 to 7.

9. The use of the aqueous polymer dispersion according to claim 8 as a binder, adhesive, sizing agent for fibers, for the production of coatings or for the production of paper coating slips.

10. A paper coating slip comprising (i) inorganic pigments and (ii) an aqueous polymer dispersion according to claim 8.

11. The paper coating slip according to claim 10, wherein the polymers of the aqueous polymer dispersion are used in an amount of 1 to 50 parts by weight based on the total amount of pigments, and wherein the pigments are present in an amount of 80 to 95 parts by weight based on the total solids content and are selected from the group consisting of calcium sulfate, calcium aluminate sulfate, barium sulfate, magnesium carbonate, calcium carbonate, silicas, aluminum oxides, aluminum hydroxide, silicates, titanium dioxide, zinc oxide, kaolin, alumina, talc and silicon dioxide, and wherein the paper coating slip additionally comprises at least one auxiliary selected from the group consisting of thickeners, further polymeric binders, co-binders, optical brighteners, fillers, leveling agents, dispersants, surfactants, lubricants, neutralizing agents, defoamers, deaerators, preservatives and dyes.

12. A paper or board coated with a paper coating slip according to claim 10 or 11.

13. A process for coating paper or board, wherein - an aqueous polymer dispersion according to claim 8 is provided; and - a paper coating slip is produced using the aqueous 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 board.