Emulsion polymerization with in-situ hydrolyzed solketal (METH)acrylate

Abstract

A process for preparing an aqueous polymer latex by polymerizing, optionally in the presence of a seed latex, a monomer composition containing solketal (meth)acrylate by radical emulsion polymerization in an aqueous polymerization medium in the presence of a free-radical polymerization initiator, wherein the monomer composition comprises, based on the total amount of monomers, a) 0.1 to 45.0 % by weight of solketal (meth)acrylate and b) 55.0 to 99.9 % by weight of one or more further ethylenically unsaturated monomers, and wherein a pH value of 4 or less is maintained in the aqueous polymerization medium during the radical emulsion polymerization reaction by presence of an acidic reacting free-radical polymerization initiator, a buffer system and / or a further acidic compound. The invention is further directed to an aqueous polymer latex and the use thereof.

Description

[0001] Emulsion polymerization with in-situ hydrolyzed solketal (meth)acrylate

[0002] Description

[0003] The invention concerns a process for preparing an aqueous polymer latex containing glyceryl (meth)acrylate as a monomer by radical emulsion polymerization, as well as the aqueous polymer latex obtained thereby and its use. The aqueous polymer latex is prepared from solketal (meth)acrylate monomer as starting material.

[0004] Glyceryl (meth)acrylate is also named 2,3-dihydroxypropyl (meth)acrylate. Glyceryl acrylate is not available in a commercial scale.

[0005] Solketal (meth)acrylate, also named (2,2-dimethyl-1 ,3-dioxolan-4-yl)methyl (meth)acrylate, IPG(M)A, is known as a reactive diluent in curable compositions, such as printing inks, in particular inkjet printing inks. Solketal acrylate can be prepared by transesterification of ethyl acrylate with solketal, as described in WO 2018 / 146258 A1.

[0006] In an emulsion polymerization process conducted at an acidic pH value, solketal (meth)acrylate is hydrolyzed in-situ to glyceryl (meth)acrylate (Formula 1) and glyceryl (meth)acrylate is part of the formed polymer.

[0007] Formula 1

[0008] Solketal (meth)acrylate has become increasingly interesting as a monomer for preparing aqueous polymer latices by radical emulsion polymerization for various applications, such as waterborne coatings, films and paints and diverse adhesives. Further application fields of the polymer latices are e.g. pigment printing, road marking, flooring or oil drilling fluids.

[0009] Glyceryl (meth)acrylate is an interesting functional monomer for preparing aqueous polymer latices by emulsion polymerization. The vicinal hydroxy-groups in glyceryl (meth)acrylate can e.g. strongly promote inter- and intramolecular interactions as well as interactions to diverse surfaces such as metal oxide surfaces. Aqueous polymer latices with glyceryl (meth)acrylate as

[0010] SUBSTITUTE SHEET (RULE 26) functional monomer can be applied for i.a. diverse adhesives, coatings, films and paints as well as printing, road marking, flooring or drilling fluids.

[0011] Zhen et al., Atom Transfer Radical Polymerization of Solketal Acrylate Using Cyclohexanone as the Solvent, Macromolecular Chemistry and Physics 206 (2005), pp. 607 to 612, describe the homo-polymerization of solketal acrylate by atom transfer radical polymerization (ATRP) using CuBr / pentamethyldiethylenetriamine as catalyst and cyclohexanone as solvent. The prepared bromine terminated homopolymers were used as macroinitiators to initiate polymerization of tert-butyl acrylate under similar ATRP conditions to produce diblock copolymers with controlled molecular weights and low polydispersities.

[0012] Pham et al., Glycerol-based co-oligomers by free-radical chain transfer polymerization: Towards reactive polymers bearing acetal and / or carbonate groups with enhanced properties, European Polymer Journal 95 (2017), pp. 491 to 502, report the synthesis of solvent-soluble oligomers from glycerin carbonate acrylate and solketal acrylate by free-radical chain transfer polymerization using 2-mercaptoethanol as chain transfer agent.

[0013] Yu et al., Two-Step Chemical Transformation of Polystyrene-blockpoly(solketal acrylate) Copolymers for Increasing x, Macromolecules 52 (2019), pp. 6458 to 6466, disclose the preparation of block copolymers for applications in semiconductors, storage devices and porous membranes. Poly(styrene-b-sol ketal acrylate) (PS-b-PSA) copolymer is synthesized from poly(styrene) homopolymer. After completion of the copolymerization PS-b-PSA copolymer is subsequently contacted with an acid-catalyst to convert the PSA block into poly(glycerol acrylate) (PGA).

[0014] Kipping et al., Synthesis and characterization of particles consisting of a biodegradable poly(i_- lactide) core and a functional hydrophilic shell, European Polymer Journal 46 (2010), pp. 313 to 323, suggest the synthesis of block copolymers consisting of poly(solketal acrylate) (PSKA) and poly(L-lactide) (PLLA) by combination of atom transfer radical polymerization and ring opening polymerization technique for applications including catalysis, electro optics and carriers for drug delivery. After synthesis of PSKA-b-PLLA block copolymers and change of solvent, deprotection of hydroxyl groups of the solketal acrylate segments was effectuated by acidic hydrolysis in tetrahydrofuran (THF). Atom transfer radical polymerization of 2,3-dihydroxypropyl acrylate is described as being not possible because of undesirable side reactions due to the functionalities in this monomer. Tang et al., Double hydrophilic block copolymers PEO-b-PGA: Synthesis, application as potential drug carrier and drug release via pH-sensitive linkage, Journal of Biomedical Materials Research - Part A, 86(2) (2008), pp. 428 to 438, is directed to poly(ethylene oxide) (PEO)-block- poly(glycerol monoacrylate) (PGA) for use as drug carriers, which have been synthesized via atom transfer radical polymerization of solketal acrylate (SA) and subsequent hydrolysis of the acetal protecting group in THF.

[0015] According to Zhen et al., Pham et al., Yu et al, Kipping et al. and Tang et al. organic solvents are applied. Neither emulsion polymerization nor in-situ hydrolysis is addressed.

[0016] Emulsion polymerization for preparation of aqueous coating compositions comprising pigments is described in various publications. WO 2017 / 191167 A1 discloses an aqueous polymer latex that is used as binder in waterborne coating compositions containing a titanium dioxide pigment. The aqueous polymer latex is obtained by polymerizing a monomer composition M by radical emulsion polymerization using a specific feed method, where the monomer composition M consists of i) 80 to 99.95 % by weight, based on the total weight of the monomer composition M, of ethylenically unsaturated monomers M1 , which are selected from mixtures of at least one monomer M 1a, selected from Ci-C2o-alkyl esters of acrylic acid and Cs-C2o-alkylesters of methacrylic acid; and at least one monomer M1 b, selected from vinyl aromatic monomers and Ci-C4-alkyl esters of methacrylic acid; ii) 0.05 to 5 % by weight, based on the total weight of the monomer composition M, of one or more monoethylenically unsaturated monomers M2, which are selected from monoethylenically unsaturated monocarboxylic acids having 3 to 6 carbon atoms and monoethylenically unsaturated dicarboxylic acids having 4 to 6 carbon atoms; iii) 0 to 20 % by weight of non-ionic monomers M3, which are different from monomers M1 , such as hydroxy-C2-C4-alkylesters of acrylic acid and methacrylic acid.

[0017] The aqueous polymer latex is used as a binder in an aqueous coating composition containing a titanium dioxide pigment.

[0018] The free-radically initiated aqueous emulsion polymerization is triggered by means of a free- radical polymerization initiator (free-radical initiator). These are in principle peroxides such as persulfates, azo compounds and redox initiator systems. The peroxides can be inorganic peroxides or organic peroxides. Some of the free-radical polymerization initiators, especially inorganic persulfates, react acidic when they are contacted with water: They produce an acidic pH value of below 6 when dissolved in an aqueous reaction medium, especially under elevated temperatures.

[0019] It is an object of the present invention to provide a process for the preparation of an aqueous polymer latex containing glyceryl (meth)acrylate as a monomer by radical emulsion polymerization, wherein solketal (meth)acrylate is used as starting material and thus indirectly used as glyceryl (meth)acrylate. Further, a coagulate free and stable dispersion is desired.

[0020] The problem is solved by a process for preparing an aqueous polymer latex by polymerizing, optionally in the presence of a seed latex, a monomer composition containing solketal (meth)acrylate by radical emulsion polymerization in an aqueous polymerization medium in the presence of a free-radical polymerization initiator, wherein the monomer composition comprises, based on the total amount of monomers, a) 0.1 to 45.0 % by weight of solketal (meth)acrylate and b) 55.0 to 99.9 % by weight of one or more further ethylenically unsaturated monomers, and wherein a pH value of 4 or less, in particular in a range from 2 to 4, more preferably from 2.0 to 3.5, is maintained in the aqueous polymerization medium during the radical emulsion polymerization reaction by presence of an acidic reacting free-radical polymerization initiator, a buffer system and / or a further acidic compound.

[0021] With the process of the present invention coagulate free dispersions can be obtained.

[0022] By the acidic pH value in the range of 4 or less in the aqueous polymerization medium, solketal (meth)acrylate is hydrolyzed in-situ to glyceryl (meth)acrylate. Preferably, the solketal (meth)acrylate is solketal acrylate. Preferably, the glyceryl (meth)acrylate is glyceryl acrylate. In particular, the aqueous polymer latex comprises polymerized glyceryl (meth)acrylate, preferably polymerized glyceryl acrylate. The polymerized glyceryl (meth)acrylate comprised in the aqueous polymer latex is in particular polymerized glyceryl acrylate.

[0023] Preferably, the conversion from solketal (meth)acrylate to glyceryl (meth)acrylate during in-situ hydrolysis accounts to at least 90 % by weight, more preferably at least 98 % by weight. Solketal (meth)acrylate is an abbreviation for solketal acrylate and / or solketal methacrylate.

[0024] Solketal acrylate is preferred.

[0025] Glyceryl (meth)acrylate is an abbreviation for glyceryl acrylate and / or glyceryl methacrylate. Glyceryl acrylate is preferred.

[0026] The further ethylenically unsaturated monomers may be selected from a group b1) consisting of Ci-C2o-alkyl esters of acrylic acid or methacrylic acid.

[0027] The term C1-C20 alkyl denominates a group of linear, branched or cyclic saturated hydrocarbon radicals having from 1 to 20 carbon atoms. Examples of alkyl include but are not limited to methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, pentyl, 1 -methylbutyl, 2- methylbutyl, 3-methylbutyl, 2,2-dimethylpropyl, 1 -ethylpropyl, hexyl, 1 , 1-dimethylpropyl, 1 ,2- dimethylpropyl, 1 -methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1 ,1- dimethylbutyl, 1 ,2-dimethylbutyl, 1 ,3-dimethylbutyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3- dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 1 , 1 ,2-trimethylpropyl, 1 ,2,2-trimethylpropyl, 1-ethyl-1- methylpropyl, 1-ethyl-2-methylpropyl, n-heptyl, 2-heptyl, n-octyl, 2-octyl, isooctyl, 2-ethylhexyl, 2-propylheptyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl, heneicosyl, docosyl and in case of nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl, heneicosyl, docosyl their isomers, cyclopentyl, cyclohexyl, methylcyclohexyl, dimethylcyclohexyl, cycloheptyl, cyclooctyl, cyclododecyl, cyclohexadecyl, norbornyl (= bicyclo[2.2.1]heptyl) and isobornyl (= 1 ,7,7-trimethyl-bicyclo[2.2.1]heptyl).

[0028] In particular tert-butyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, 2-propyl- heptyl (meth)acrylate, cyclohexyl (meth)acrylate, n-octyl (meth)acrylate, 2-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, methyl (meth)acrylate are used as monomers b1). (Meth)acrylate is an abbreviation for acrylate and / or methacrylate.

[0029] The further ethylenically unsaturated monomers may be selected from a group b2) consisting of vinyl esters, in particular vinyl acetate, vinyl propionate, vinyl laurate, vinyl neodecanoate, vinyl esters of versatic acid and vinyl ester of 2-ethyl hexanoic acid.

[0030] The further ethylenically unsaturated monomers may be selected from a group b3) consisting of vinyl aromatic monomers, in particular styrene.

[0031] The further ethylenically unsaturated monomers may be selected from a group b4) consisting of monoethylenically unsaturated monocarboxylic acids having 3 to 6 carbon atoms, such as acrylic acid, methacrylic acid, crotonic acid, 2-ethylpropenoic acid, 2-propylpropenoic acid, 2-acryloxyacetic acid and 2-methacryloxyacetic acid; monoethylenically unsaturated dicarboxylic acids having 4 to 6 carbon atoms, such as itaconic acid, citraconic acid, maleic acid and fumaric acid; semi-esters of monoethylenically unsaturated dicarboxylic acids having 4 to 6 carbon atoms, with C1-C4 alkanols, such as methanol or ethanol, such as semi-esters of itaconic acid, citraconic acid, maleic acid or fumaric acid with methanol or ethanol; monoethylenically unsaturated sulfonic acids, such as vinylsulfonic acid, allylsulfonic acid, styrenesulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid; monoethylenically unsaturated phosphonic acids such as vinylphosphonic acid, allyl- phosphonic acid, styrenephosphonic acid and 2-acrylamido-2-methylpropane phosphonic acid; and monoethylenically unsaturated phosphoric acids such as monophosphates of hydroxyalkyl acrylates, monophosphates of hydroxyalkyl methacrylates, monophosphates of alkoxylated hydroxyalkyl acrylates and monophosphates of alkoxylated hydroxyalkyl methacrylates, in particular monophosphates of hydroxyethyl acrylate, hydroxypropyl acrylate or hydroxybutyl acrylate, monophosphates of hydroxyethyl methacrylate, hydroxypropyl methacrylate or hydroxybutyl methacrylate, monophosphates of ethoxylated hydroxy-C2-C4-alkyl acrylates, monophosphates of propoxylated hydroxy-C2- C4-alkyl acrylates, monophosphates of ethoxylated hydroxy-C2-C4-alkyl methacrylates and monophosphates of propoxylated hydroxy-C2-C4-alkyl methacrylates.

[0032] Preferred monomers b4) are acrylic acid, methacrylic acid, maleic acid, half methyl ester of maleic acid, half ethyl ester of maleic acid, citraconic acid, half methyl ester of citraconic acid, itaconic acid, half methyl ester of itaconic acid, fumaric acid, half methyl ester of fumaric acid, and half ethyl ester of fumaric acid.

[0033] The further ethylenically unsaturated monomers may be selected from a group b5) consisting of primary amides of monoethylenically unsaturated monocarboxylic acids having 3 to 8 carbon atoms such as acrylamide and methacrylamide;

[0034] N-C1-C10 alkyl amides of monoethylenically unsaturated monocarboxylic acids having 3 to 6 carbon atoms, in particular N-C1-C10 alkyl amides of acrylic acid or methacrylic acid, such as N-methyl acrylamide, N-ethyl acrylamide, N-propyl acrylamide, N-isopropyl acrylamide, N-butyl acrylamide, N-methyl methacrylamide, N-ethyl methacrylamide, N- propyl methacrylamide, N-isopropyl methacrylamide and N-butyl methacrylamide; N-hydroxy-Ci-C alkyl amides of monoethylenically unsaturated monocarboxylic acids having 3 to 6 carbon atoms, in particular N-hydroxy-Ci-Cw alkyl amides of acrylic acid or methacrylic acid, such as N-hydroxymethyl acrylamide, monoethylenically unsaturated monomers bearing urea or keto groups, such as 2-(2-oxo-imidazolidin-1-yl)ethyl (meth)acrylate, 2-ureido (meth)acrylate, N-[2-(2-oxooxazolidin-3-yl)ethyl] methacrylate, acetoacetoxyethyl acrylate, acetoacetoxypropyl methacrylate, acetoacetoxybutyl methacrylate, 2- (acetoacetoxy) ethyl methacrylate, diacetone acrylamide (DAAM) and diacetone methacrylamide; hydroxyalkyl esters of monoethylenically unsaturated C3-C6 monocarboxylic acids, especially hydroxyalkyl esters of acrylic acid and hydroxyalkyl esters of methacrylic acid, also referred to hereinafter as hydroxyalkyl (meth)acrylates, in particular hydroxy-C2-C4- alkylesters of acrylic acid and hydroxy-C2-C4-alkylesters of methacrylic acid, such as 2- hydroxyethyl acrylate, 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl methacrylate and 4-hydroxybutyl methacrylate; and monoethylenically unsaturated monomers which bear at least one tri-Ci-C4-alkoxysilane group such as vinyl trimethoxysilane, vinyl triethoxysilane, methacryloxyethyl trimethoxysilane, methacryloxyethyl triethoxysilane.

[0035] Preferred monomers b5) are acrylamide, methacrylamide, N-hydroxymethyl acrylamide, 2- hydroxyethyl acrylate, ureido methacrylate and 4-hydroxybutyl acrylate.

[0036] The further ethylenically unsaturated monomers may be selected from a group b6) consisting of diesters of monoethylenically unsaturated C3-C6 monocarboxylic acids with saturated aliphatic or cycloaliphatic diols, in particular diesters of acrylic acid or methacrylic acid, such as the diacrylates and the dimethacrylates of ethylene glycol (1 ,2-ethanediol), propylene glycol (1 ,2-propanediol), 1 ,2-butanediol, 1 ,3-butanediol, 1 ,4-butanediol, neopentyl glycol (2,2-dimethyl-1 ,3-propanediol) and 1 ,2-cyclohexanediol; monoesters of monoethylenically unsaturated C3-C6 monocarboxylic acids with monoethylenically unsaturated aliphatic or cycloaliphatic monohydroxy compounds, such as the acrylates and the methacrylates of vinyl alcohol (ethenol), allyl alcohol (2-propen-1-ol), 2- cyclohexen-1-ol or norbornenol; and divinyl aromatic compounds, such as 1 ,3-divinyl benzene and 1 ,4-divinyl benzene.

[0037] A limited and reduced amount of solketal (meth)acrylate in the monomer composition promotes the formation of a stable dispersion and prevents coagulation.

[0038] Preferably, the monomer composition comprises, based on the total amount of monomers, a) 0.2 to 30.0 % by weight of solketal (meth)acrylate and b) 70.0 to 99.8 % by weight of one or more further ethylenically unsaturated monomers.

[0039] In some particular embodiments, the monomer composition comprises, based on the total amount of monomers, a) 0.3 to 20.0 % by weight of solketal (meth)acrylate and b) 80.0 to 99.7 % by weight of one or more further ethylenically unsaturated monomers.

[0040] In some particular embodiments, the monomer composition comprises, based on the total amount of monomers, a) 0.5 to 10.0 % by weight of solketal (meth)acrylate and b) 90.0 to 98.5 % by weight of one or more further ethylenically unsaturated monomers.

[0041] In some particular embodiments, the monomer composition comprises, based on the total amount of monomers, a) 0.5 to 7.0 % by weight of solketal (meth)acrylate and b) 93.0 to 98.5 % by weight of one or more further ethylenically unsaturated monomers.

[0042] In some particular embodiments, the further ethylenically unsaturated monomers are selected from one or more of groups b1), b2) b3), b4), b5) and b6).

[0043] The process for the preparation of the aqueous polymer latex is performed according to the well-known processes of radical emulsion polymerisation technology. The conditions required for the performance of the free-radical emulsion polymerization of the monomers are sufficiently familiar to those skilled in the art, for example from the prior art cited at the outset and from "Emulsions polymerisation" [Emulsion Polymerization] in Encyclopedia of Polymer Science and Engineering, vol. 8, pages 659 ff. (1987); D. C. Blackley, in High Polymer Latices, vol. 1 , pages 35 ff. (1966); H. Warson, The Applications of Synthetic Resin Emulsions, chapter 5, pages 246 ff. (1972); D. Diederich, Chemie in unserer Zeit, 24, pages 135 to 142 (1990); Emulsion Polymerisation, Inter-science Publishers, New York (1965); DE-A 40 03422 and Dispersionen synthetischer Hochpolymere [Dispersions of Synthetic High Polymers], F. Holscher, Springer- Verlag, Berlin (1969). However, according to the present invention, it is essential that the radical emulsion polymerization is carried out by maintaining in the aqueous polymerization medium a pH value of 4 or less, preferably in the range from 2 to 4, more preferably from 2.0 to 3.5, in the course of the polymerization reaction, in particular by presence of the acidic reacting free-radical polymerization initiator, the buffer system and / or the further acidic compound in the aqueous polymerization medium. The pH value can be measured according to DIN 19263:2007-05.

[0044] The free-radically initiated aqueous emulsion polymerization is initiated by means of the free- radical polymerization initiator (free-radical initiator). Free-radical polymerization initiators encompass peroxides such as persulfates, azo compounds and redox initiator systems. Advantageously, the type and amount of the free-radical initiator, polymerization temperature and polymerization pressure are selected such that a sufficient amount of initiating radicals is always present to initiate or to maintain the polymerization reaction.

[0045] The free-radical polymerization initiator according to the invention can be chosen from peroxides, in particular persulfates, azo compounds and redox initiator systems. The free- radical polymerization initiator is preferably an acidic reacting free-radical polymerization initiator. Preferably, the aqueous polymerization medium contains the acidic reacting free- radical polymerization initiator, which is used as the free-radical polymerization initiator. Thus, the acidic reacting free-radical polymerization initiator is preferably the free-radical polymerization initiator. The acidic reacting free-radical polymerization initiator in general causes an acidic pH value of less than 6, preferably of 4 or less, for example from 2 to 4, when dissolved in water, in particular at elevated temperatures.

[0046] The aqueous polymerization medium can contain the acidic reacting free-radical polymerization initiator and additionally the buffer system and / or the further acidic compound.

[0047] In another embodiment the aqueous polymerization medium contains a free-radical polymerization initiator other than an acidic reacting free-radical polymerization initiator, for example an azo compound, and a buffer system and / or a further acidic compound to maintain the pH value of 4 or less.

[0048] Preferably, the acidic reacting free-radical polymerization initiator is a persulfate. More preferably, the acidic reacting free-radical polymerization initiator is sodium persulfate, potassium persulfate and / or ammonium persulfate. More preferably, the free-radical polymerization initiator is a persulfate. Even more preferably, the free-radical polymerization initiator is sodium persulfate, potassium persulfate and / or ammonium persulfate. Persulfates are inexpensive and commonly used in industrial processes.

[0049] Preferably, the further acidic compound is an organic acid such as citric acid or an inorganic acid such as sulfuric acid. The further acidic compound is particularly added in aqueous solution, the aqueous solution preferably having a content of the further acidic compound in a range from 5% to 20% by weight, referring to the total aqueous solution. The buffer system preferably comprises citrate buffer.

[0050] The pH value of 4 or less of the aqueous polymerization medium is maintained by the presence of the acidic reacting free-radical polymerization initiator, the buffer system and / or the further acidic compound. Preferably, the pH value of 4 or less of the aqueous polymerization medium is maintained by the presence of the acidic reacting free-radical polymerization initiator. In some embodiments of the inventive process, part or the entire amount of the acidic reacting free- radical polymerization initiator, the buffer system and / or the further acidic compound is initially charged with the aqueous polymerization medium to a reaction vessel. Preferably, the aqueous reaction medium contains the acidic reacting free-radical polymerization initiator, the buffer system and / or the further acidic compound before the solketal (meth)acrylate and optionally one or more further ethylenically unsaturated monomers is added.

[0051] Preferably, the aqueous polymerization medium is neutralized to an essentially neutral pH value in the range from 6 to 8 after the polymerization reaction, in particular after the polymerization reaction is accomplished to a conversion of at least 99 % by weight, by adding a basic compound.

[0052] Preferably, the basic compound is selected from water-soluble hydroxides, oxides, carbonates, hydrogen carbonates (bicarbonates), acetates, citrates, borates, phosphates, hydrogen phosphates of alkali or alkaline earth metals, ammonium or mixtures thereof. Other basic compounds are also possible, like ammonia or water-soluble organic amines such as lower aliphatic amines.

[0053] Preferred basic compounds are the hydroxides of sodium, potassium and ammonium, the carbonates of sodium and potassium and hydrogen carbonates (bicarbonates) of sodium and potassium. The basic compound is more preferably ammonium, in particular aqueous ammonium. The basic compound is preferably dissolved in water. Preferably at least a portion of the acidic reacting free-radical polymerization initiator, the further acidic compound and / or the buffer system is initially charged in the reaction vessel to achieve the pH value of 4 or less, preferably in the range from 2 to 4, more preferably from 2.0 to 3.5, in the aqueous reaction mixture.

[0054] In an embodiment, the amount of the acidic reacting free-radical polymerization initiator for the emulsion polymerization is initially charged in a reaction vessel completely. However, it is preferred to initially charge at least a portion of the acidic reacting free-radical polymerization initiator, e.g. at least 5% by weight, especially at least 10% by weight, based on the total amount of the acidic reacting free-radical polymerization initiator required in the aqueous polymerization medium. Then, under polymerization conditions, during the free-radical emulsion polymerization of the monomers, the remaining residual amount of the acidic reacting free- radical polymerization initiator can be added batchwise in one or more portions or continuously with constant or varying flow rates, in order to maintain the pH value of 4 or less, preferably in the range from 2 to 4, more preferably from 2.0 to 3.5, in the aqueous polymerization medium.

[0055] In a preferred embodiment, the radical emulsion polymerization of the invention is conducted at temperatures in the range from 0°C to 170°C, more preferably in the range from 50°C to 100°C, most preferably in the range from 80°C to 95°C and in particularly the radical emulsion polymerization of the invention is conducted at temperatures in the range from 80°C to 90°C. The radical emulsion polymerization can be conducted at a pressure of less than, equal to or greater than 1 atm (101325 Pa).

[0056] Preferably, after addition of the monomer composition and the free-radical polymerization initiator to the aqueous polymerization medium, the reaction is post-polymerized for up to 2 hours, more preferably for a time range from 0.5 hours to 1.5 hours, in particular 1 hour. The term post-polymerization denotes the continued reaction after all feeds, in particular the monomer composition and the free-radical polymerization initiator, are added to the reaction mixture. During post-polymerization the reaction mixture is preferably stirred at an elevated temperature, in particular at a post-polymerizing temperature in the range from 0°C to 170°C, more preferably in the range from 50°C to 100°C, most preferably in the range from 80°C to 95°C.

[0057] In certain embodiments, the polymerization is conducted in the presence of a chain transfer agent. The chain transfer agent is preferably selected from the group consisting of aliphatic and / or araliphatic halogen compounds, for example n-butyl chloride, n-butyl bromide, n-butyl iodide, methylene chloride, ethylene dichloride, chloroform, bromoform, bromotrichloromethane, dibromodichloromethane, carbon tetrachloride, carbon tetrabromide, benzyl chloride, benzyl bromide; organic thio compounds such as primary, secondary or tertiary aliphatic thiols, for example ethanethiol, n-propanethiol, 2-propanethiol, n-butanethiol, 2-butanethiol, 2-methyl-2- propanethiol, n-pentanethiol, 2-pentanethiol, 3-pentanethiol, 2-methyl-2-butanethiol, 3-methyl-2- butanethiol, n-hexanethiol, 2-hexanethiol, 3-hexanethiol, 2-methyl-2-pentanethiol, 3-methyl-2- pentanethiol, 4-methyl-2-pentanethiol, 2-methyl-3-pentanethiol, 3-methyl-3-pentanethiol, 2- ethylbutanethiol, 2-ethyl-2-butanethiol, n-heptanethiol and the isomeric compounds thereof, n- octanethiol and the isomeric compounds thereof, n-nonanethiol and the isomeric compounds thereof, n-decanethiol and the isomeric compounds thereof, n-undecanethiol and the isomeric compounds thereof, n-dodecanethiol and the isomeric compounds thereof, n-tridecanethiol and isomeric compounds thereof, substituted thiols, for example 2-hydroxyethanethiol, aromatic thiols such as benzenethiol, ortho-, meta- or para-methylbenzenethiol, alkylesters of mercaptoacetic acid (thioglycolic acid) such as 2-ethylhexyl thioglycolate, alkylesters of mercaptopropionic acid such as octyl mercapto propionate, and also further sulfur compounds described in the Polymer Handbook, 3rd edition, 1989, J. Brandrup and E.H. Immergut, John Wiley & Sons, section II, pages 133 to 141 , and aliphatic and / or aromatic aldehydes such as acetaldehyde, propionaldehyde and / or benzaldehyde, unsaturated fatty acids such as oleic acid, dienes having nonconjugated double bonds, such as divinylmethane or vinylcyclohexane, or hydrocarbons having readily abstractable hydrogen atoms, for example toluene.

[0058] In general, the total amount of chain transfer agents, if present, does not exceed 1 % by weight, based on the total amount of monomers.

[0059] Often, the radical emulsion polymerization is conducted in presence of a surfactant. The surfactant can be selected from emulsifiers and protective colloids. The protective colloids, as opposed to emulsifiers, are understood to mean polymeric compounds having molecular weights above 2000 Daltons, whereas emulsifiers typically have lower molecular weights. The surfactants may be anionic or nonionic or mixtures of non-ionic and anionic surfactants.

[0060] The anionic surfactants usually bear at least one anionic group, which is selected from phosphate, phosphonate, sulfate and sulfonate groups. The anionic surfactants, which bear at least one anionic group, are typically used in the form of their alkali metal salts, especially of their sodium salts or in the form of their ammonium salts.

[0061] In preferred embodiments, the anionic surfactants are anionic emulsifiers, which bear in particular at least one sulfate or sulfonate group. Likewise, anionic emulsifiers, which bear at least one phosphate or phosphonate group may be used, either as sole anionic emulsifiers or in combination with one or more anionic emulsifiers, which bear at least one sulfate or sulfonate group. Examples of anionic emulsifiers, which bear at least one sulfate or sulfonate group, are, for example, the salts, especially the alkali metal and ammonium salts, of alkyl sulfates, especially of Cs-C22-alkyl sulfates, the salts, especially the alkali metal and ammonium salts, of sulfuric monoesters of ethoxylated alkanols, especially of sulfuric monoesters of ethoxylated Cs- C22- alkanols, preferably having an ethoxylation level (EO level) in the range from 2 to 40, the salts, especially the alkali metal and ammonium salts, of sulfuric monoesters of ethoxylated alkylphenols, especially of sulfuric monoesters of ethoxylated C4-Ci8-alkylphenols (EO level preferably 3 to 40), the salts, especially the alkali metal and ammonium salts, of alkylsulfonic acids, especially of C8-C22-alkylsulfonic acids, the salts, especially the alkali metal and ammonium salts, of dialkyl esters, especially di-Ci-Cis-alkyl esters of sulfosuccinic acid, the salts, especially the alkali metal and ammonium salts, of alkylbenzenesulfonic acids, especially of C4-C22-alkylbenzenesulfonic acids, and the salts, especially the alkali metal and ammonium salts, of mono- or disulfonated, alkyl-substituted diphenyl ethers, for example of bis(phenylsulfonic acid) ethers bearing a C4-C24-alkyl group on one or both aromatic rings. The examples are described in IIS-A 4,269,749, and Dowfax® 2A1 (Dow Chemical Company).

[0062] In particularly preferred embodiments, the anionic surfactants are anionic emulsifiers, which are selected from the following groups: the salts, especially the alkali metal and ammonium salts, of alkyl sulfates, especially of Cs-C22-alkyl sulfates, the salts, especially the alkali metal salts, of sulfuric monoesters of ethoxylated alkanols, especially of sulfuric monoesters of ethoxylated Cs-C22-alkanols, preferably having an ethoxylation level (EO level) in the range from 2 to 40, sulfuric monoesters of ethoxylated alkylphenols, especially of sulfuric monoesters of ethoxylated C4-Ci8-alkylphenols (EO level preferably 3 to 40), of alkylbenzenesulfonic acids, especially of C4-C22-alkylbenzenesulfonic acids, and mono- or disulfonated, alkyl-substituted diphenyl ethers, for example of bis(phenylsulfonic acid) ethers bearing a C4-C24-alkyl group on one or both aromatic rings.

[0063] Examples of anionic emulsifies, which bear a phosphate or phosphonate group, include, but are not limited to the following salts are selected from the following groups: the salts, especially the alkali metal and ammonium salts, of mono- and dialkyl phosphates, especially Cs-C22-alkyl phosphates, the salts, especially the alkali metal and ammonium salts, of phosphoric monoesters of C2-C3-alkoxylated alkanols, preferably having an alkoxylation level in the range from 2 to 40, especially in the range from 3 to 30, for example phosphoric monoesters of ethoxylated Cs-C22-alkanols, preferably having an ethoxylation level (EO level) in the range from 2 to 40, phosphoric monoesters of propoxylated Cs-C22-alkanols, preferably having a propoxylation level (PO level) in the range from 2 to 40, and phosphoric monoesters of ethoxylated-co- propoxylated Cs-C22-alkanols, preferably having an ethoxylation level (EO level) in the range from 1 to 20 and a propoxylation level of 1 to 20, the salts, especially the alkali metal and ammonium salts, of phosphoric monoesters of ethoxylated alkylphenols, especially phosphoric monoesters of ethoxylated C4-C18- alkylphenols (EO level preferably 3 to 40), the salts, especially the alkali metal and ammonium salts, of alkylphosphonic acids, especially C8-C22-alkylphosphonic acids, and the salts, especially the alkali metal and ammonium salts, of alkylbenzenephosphonic acids, especially C4-C22-alkylbenzenephosphonic acids.

[0064] Further suitable anionic surfactants can be found in Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry], volume XIV / 1 , Makromolekulare Stoffe [Macromolecular Substances], Georg-Thieme-Verlag, Stuttgart, 1961, pp. 192 to 208.

[0065] In other preferred embodiments, the surfactant comprises at least one anionic emulsifier, which bears at least one sulfate or sulfonate group. The at least one anionic emulsifier, which bears at least one sulfate or sulfonate group, may be the sole type of anionic emulsifiers. However, mixtures of at least one anionic emulsifier, which bears at least one sulfate or sulfonate group, and at least one anionic emulsifier, which bears at least one phosphate or phosphonate group, may also be used. In such mixtures, the amount of the at least one anionic emulsifier, which bears at least one sulfate or sulfonate group, is preferably at least 50% by weight, based on the total weight of anionic surfactants used in the process of the present invention. In particular, the amounts of anionic emulsifiers, which bear at least one phosphate or phosphonate group do not exceed 20% by weight, based on the total weight of anionic surfactants used in the process of the present invention. In other preferred embodiments, the surfactant may also comprise one or more nonionic surface-active substances, which are especially selected from nonionic emulsifiers. Suitable nonionic emulsifiers are e.g. araliphatic or aliphatic nonionic emulsifiers, for example ethoxylated mono-, di- and trialkylphenols (EO level 3 to 50, alkylchain: C4-C10), ethoxylates of long-chain alcohols (EO level: 3 to 100, alkyl chain: Cs-Cae), and polyethylene oxide / polypropylene oxide homo- and copolymers. These may comprise the alkylene oxide units copolymerized in random distribution or in the form of blocks. Very suitable examples are the EO / PO block copolymers. Preference is given to ethoxylates of long-chain alkanols (alkyl chain C1-C30, mean ethoxylation level 5 to 100) and, among these, particular preference is given to those having a C12-C20 alkyl chain and a mean ethoxylation level of 5 to 20, and also to ethoxylated mono-alkylphenols. Preferably, the surfactants used in the process of the present invention comprise less than 60% by weight, especially not more than 50% by weight, of nonionic surfactants, based on the total amount of surfactants used in the process of the present invention.

[0066] In other embodiments, the surfactants used in the process of the present invention comprise at least one anionic surfactant and at least one nonionic surfactant, the ratio of anionic surfactants to non-ionic surfactants being usually in the range from 0.5: 1 to 10: 1 , in particular from 1 : 1 to 5:1.

[0067] In other preferred embodiments, the surfactant / surfactants will be used in such an amount that the amounts of surfactant / surfactants are in the range from 0.2% to 5.0% by weight, especially in the range from 0.5% to 3.0% by weight, based on the monomers to be polymerized.

[0068] The aqueous reaction medium in polymerization may in principle also comprise minor amounts (< 5 % by weight) of water-soluble organic solvents, for example methanol, ethanol, isopropanol, butanols, pentanols, but also acetone, propylene glycol etc. Preferably, however, the process of the invention is conducted in the absence of such solvents.

[0069] In general, the aqueous polymer dispersions obtained have polymer solid contents in the range from 10 % to 70 % by weight, preferably from 20 % to 65 % by weight, more preferably from 30 % to 60 % by weight, and most preferably from 40 % to 60 % by weight, based in each case on the total weight of the aqueous polymer dispersion.

[0070] It has been found advantageous to perform the radical emulsion polymerization in the presence of a seed latex. A seed latex is a polymer latex which is present in the aqueous polymerization medium before the metering of the solketal (meth)acrylate and the further ethylenically unsaturated monomers is started. The seed latex may help to better adjust the particle size of the final aqueous polymer latex obtained in the free-radical emulsion polymerization of the invention.

[0071] Principally every polymer latex may serve as a seed latex. For the purpose of the invention, preference is given to seed latices, where the particle size of the polymer particles is comparatively small. The Z average particle diameter of the polymer particles of the seed latex, as determined by dynamic light scattering at 20°C is preferably in the range from 10 nm to 80 nm, in particular from 10 nm to 50 nm. Preferably, the polymer particles of the seed latex are made of ethylenically unsaturated monomers, which comprise at least 95 % by weight, based on the total weight of the monomers forming the seed latex, of one or more monomers selected from C2-C -alkyl esters of acrylic acid, in particular ethyl acrylate, n-butyl acrylate, 2-ethyl-hexyl acrylate, Ci-C4-alkyl esters of methacrylic acid, in particular methyl methacrylate, and vinylaromatic monomers, in particular styrene.

[0072] For this, the seed latex is usually charged into the reaction vessel before the metering of solketal (meth)acrylate and optionally the further ethylenically unsaturated monomers is started. In particular, the seed latex is charged into the polymerisation vessel followed by establishing the polymerization conditions and charging at least a portion of the free-radical polymerization initiator into the reaction vessel before the metering of solketal (meth)acrylate and optionally the further ethylenically unsaturated monomers is started.

[0073] The amount of seed latex, calculated as solids, may frequently be in the range from 0.1 % to 10.0 % by weight, in particular from 0.5 % to 5.0 % by weight, based on the total weight of the monomers to be polymerized.

[0074] In general, none, part or the entire amount of the free-radical polymerization initiator, in particular of the acidic reacting free-radical polymerization initiator, optionally part or the entire amount of the surfactant, and none, part or the entire amount of solketal (meth)acrylate and the one or more further ethylenically unsaturated monomers, are initially charged to the reaction vessel. Preferably, part or the entire amount of the acidic reacting free-radical polymerization initiator, the buffer system and / or the further acidic compound is initially charged to the reaction vessel.

[0075] In preferred embodiments, the radical emulsion polymerization is carried out by adding at least two feeds to the reaction vessel containing the aqueous reaction medium, wherein a first feed contains the acidic reacting free-radical polymerization initiator and a second feed contains solketal (meth)acrylate, optionally one or more further ethylenically unsaturated monomers and optionally a surfactant. The first feed and the second feed can be started at the same time. Preferably, the first feed is started before the second feed. In particular, the first feed enters the reaction vessel before the second feed. In particular, the first feed is free of solketal (meth)acrylate and the second feed is free of the acidic reacting free-radical polymerization initiator.

[0076] In more preferred embodiments, at least part of the acidic reacting free-radical polymerization initiator, the buffer system and / or the further acidic compound is initially charged to the reaction vessel.

[0077] Accordingly, in more preferred embodiments, the radical emulsion polymerization is carried out by initially charging at least part of the acidic reacting free-radical polymerization initiator, the buffer system and / or the further acidic compound and adding at least two feeds to the reaction vessel containing the aqueous reaction medium, wherein a first feed contains the acidic reacting free-radical polymerization initiator and a second feed contains solketal (meth)acrylate, optionally one or more further ethylenically unsaturated monomers and optionally a surfactant.

[0078] The present invention also concerns the aqueous polymer latices obtained by the process of the invention as well as their use. Preferred uses of the aqueous polymer latices are the use in coatings, films, paints and / or diverse adhesives. Further application fields of the polymer latices are e.g. pigment printing, road marking, flooring and / or oil drilling fluids.

[0079] The invention is explained in more detail by the following examples and comparative examples.

[0080] Examples and comparative examples

[0081] Example 1

[0082] Offset feed supply, pH value between 2.0 and 3.5

[0083] A reactor equipped with stirrer, temperature control, nitrogen inlet and several injection possibilities was charged with 560.0 g deionized water, 11.0 g sodium lauryl sulfate and 0.4 g potassium persulfate. The reaction mixture was purged with nitrogen and heated to 85°C. At 85°C a feed 1 was added in 90 min. 5 min after the start of feed 1 , a feed 2 was added in 80 min. Feed 1 : 3.0 g potassium persulfate in 60.0 g deionized water. Feed 2: 350.0 g deionized water, 5.0 g sodium lauryl sulfate, 372.9 g methyl methacrylate, 372.9 g butyl acrylate and 39.3 g solketal acrylate. The reaction mixture was post-polymerized at 85°C for 60 min. The pH value was between 2.0 and 3.5 during the entire reaction. Then the reaction mixture was cooled down to ambient temperature and neutralized with 5 wt.-% aqueous ammonia.

[0084] No coagulate was observed.

[0085] The dispersion was dialyzed in benzoylated dialysis tubings to remove the surfactants. The dialyzed dispersion was dried in a petri dish at room temperature. With13C NMR and1H NMR glyceryl acrylate but no solketal acrylate was detected in the copolymer.

[0086] Example 2

[0087] Simultaneous feed supply, pH value between 2.0 and 3.5

[0088] The emulsion polymerization of example 2 was prepared by analogy to the protocol of example

[0089] 1. In Example 2 the initial charge of the reactor contained 0.6 g potassium persulfate. Feeds 1 and 2 were started at the same time.

[0090] No coagulate was observed.

[0091] The dispersion was dialyzed in benzoylated dialysis tubings to remove the surfactants. The dialyzed dispersion was dried in a petri dish at room temperature. With13C NMR and1H NMR glyceryl acrylate but no solketal acrylate was detected in the copolymer.

[0092] Example 3

[0093] Simultaneous feed supply, pH value adjusted to 3.0

[0094] The emulsion polymerization of example 3 was prepared by analogy to the protocol of example

[0095] 2. In Example 3 the initial charge of the reactor contained 0.4 g potassium persulfate. The pH value of the reaction mixture, the reactor was initially charged with, was adjusted to 3.0 with 10 wt.-% aqueous sulfuric acid.

[0096] No coagulate was observed. The dispersion was dialyzed in benzoylated dialysis tubings to remove the surfactants. The dialyzed dispersion was dried in a petri dish at room temperature. With13C NMR and1H NMR glyceryl acrylate but no solketal acrylate was detected in the copolymer.

[0097] Example 4

[0098] Simultaneous feed supply, pH value adjusted to 3.0

[0099] The emulsion polymerization of example 4 was prepared by analogy to the protocol of example 3. The pH value of the reaction mixture, the reactor was initially charged with, was adjusted to pH value 3.0 with 10 wt.-% aqueous citric acid.

[0100] No coagulate was observed.

[0101] The dispersion was dialyzed in benzoylated dialysis tubings to remove the surfactants. The dialyzed dispersion was dried in a petri dish at room temperature. With13C NMR and1H NMR glyceryl acrylate but no solketal acrylate was detected in the copolymer.

[0102] Example 5

[0103] Simultaneous feed supply, pH value adjusted to 3.0

[0104] The emulsion polymerization of example 5 was prepared by analogy to the protocol of example 3. Feed 1 was added in 160 min, feed 2 was added in 150 min.

[0105] No coagulate was observed.

[0106] The dispersion was dialyzed in benzoylated dialysis tubings to remove the surfactants. The dialyzed dispersion was dried in a petri dish at room temperature. With13C NMR and1H NMR glyceryl acrylate but no solketal acrylate was detected in the copolymer.

[0107] Example 6

[0108] Offset feed supply, pH value between 2.0 and 3.5 The emulsion polymerization of example 6 was prepared by analogy to the protocol of example 1. The amount of sodium lauryl sulfate charged in the reactor was only 1.1 g and only 0.5 g in feed 2.

[0109] No coagulate was observed.

[0110] The dispersion was dialyzed in benzoylated dialysis tubings to remove the surfactants. The dialyzed dispersion was dried in a petri dish at room temperature. With13C NMR and1H NMR glyceryl acrylate but no solketal acrylate was detected in the copolymer.

[0111] Example 7

[0112] Seeded polymerization, pH value between 2.0 and 3.5

[0113] A reactor equipped with stirrer, temperature control, nitrogen inlet and several injection possibilities was charged with 244.3 g deionized water and 27.3 g polystyrene seed dispersion (33 % by weight polystyrene in water, particle diameter: 30 nm). The reaction mixture was purged with nitrogen and heated to 85°C. At 85°C 10.0 g of feed 1 is added. After 5 min, feed 2 was added in 80 min and feed 1 was added in 90 min. Feed 1 : 39.6 g aqueous potassium persulfate solution (7 wt.-%). Feed 2: 400.5 g deionized water, 18.5 g Dowfax 2A1 , 4.6 g Lutensol TO 89, 34.9 g solketal acrylate, 331.5 g butyl acrylate and 331.5 g methyl methacrylate. The reaction mixture was post-polymerized at 85°C for 60 min. The pH value was between 2.0 and 3.5 during the entire reaction. Then the reaction mixture was cooled down to ambient temperature and neutralized with 5 wt.-% aqueous ammonia.

[0114] No coagulate was observed.

[0115] The dispersion was dialyzed in benzoylated dialysis tubings to remove the surfactants. The dialyzed dispersion was dried in a petri dish at room temperature. With13C NMR and1H NMR glyceryl acrylate but no solketal acrylate was detected in the copolymer.

[0116] Comparative example 1

[0117] High solketal acrylate content, pH value between 2.0 and 3.5

[0118] A reactor equipped with stirrer, temperature control, nitrogen inlet and several injection possibilities was charged with 500.0 g deionized water, 11.0 g sodium lauryl sulfate and 0.4 g potassium persulfate. The reaction mixture was purged with nitrogen and heated to 85°C. At 85°C a feed 1 was added in 90 min and a feed 2 was added in 80 min. Feed 1 : 3.0 g potassium persulfate in 60.0 g deionized water. Feed 2: 350.0 g deionized water, 5.0 g sodium lauryl sulfate, 196.25 g methyl methacrylate, 196.25 g butyl acrylate and 392.5 g solketal acrylate. The reaction mixture was post-polymerized at 85°C for 60 min. The pH value was between 2 and 3.5 during the entire reaction. Then the reaction mixture was cooled down to ambient temperature.

[0119] Complete coagulation was observed. No dispersion was formed.

[0120] Comparative example 2

[0121] Simultaneous feed supply, pH value between 6.5 and 7.5

[0122] A reactor equipped with stirrer, temperature control, nitrogen inlet and several injection possibilities was charged with 500.0 g deionized water, 11.0 g sodium lauryl sulfate and 0.4 g potassium persulfate. The reaction mixture was purged with nitrogen, heated to 85°C and adjusted with a feed 3 to a pH value of 7.0. At 85°C a feed 1 was added in 90 min and a feed 2 was added in 80 min. The pH value was kept between 6.5 and 7.5 by continuously adding droplets of feed 3 in required amounts. Feed 1 : 3.0 g potassium persulfate in 60.0 g deionized water. Feed 2: 350.0 g deionized water, 5.0 g sodium lauryl sulfate, 353.3 g methyl methacrylate, 353.3 g butyl acrylate and 78.5 g solketal acrylate. Feed 3: 10.0 g sodium bicarbonate in 70.0 g deionized water. The reaction mixture was post-polymerized at 85°C for 60 min. Then the reaction mixture was cooled down to ambient temperature.

[0123] No coagulate was observed.

[0124] The dispersion was dialyzed in benzoylated dialysis tubings to remove the surfactants. The dialyzed dispersion was dried in a petri dish at room temperature. With13C NMR and1H NMR solketal acrylate but no glyceryl acrylate was detected in the copolymer.

Claims

Claims1. A process for preparing an aqueous polymer latex by polymerizing, optionally in the presence of a seed latex, a monomer composition containing solketal (meth)acrylate by radical emulsion polymerization in an aqueous polymerization medium in the presence of a free-radical polymerization initiator, wherein the monomer composition comprises, based on the total amount of monomers, a) 0.1 to 45.0 % by weight of solketal (meth)acrylate and b) 55.0 to 99.9 % by weight of one or more further ethylenically unsaturated monomers, and wherein a pH value of 4 or less is maintained in the aqueous polymerization medium during the radical emulsion polymerization reaction by presence of an acidic reacting free- radical polymerization initiator, a buffer system and / or a further acidic compound.

2. The process of claim 1 , wherein the monomer composition comprises, based on the total amount of monomers, a) 0.2 to 30.0 % by weight of solketal (meth)acrylate and b) 70.0 to 99.8 % by weight of one or more further ethylenically unsaturated monomers.

3. The process of claim 1 or 2, wherein the aqueous polymerization medium contains the acidic reacting free-radical polymerization initiator, which is used as the free-radical polymerization initiator.

4. The process of any one of claims 1 to 3, wherein the pH value in the aqueous polymerization medium is maintained by adding the acidic reacting free-radical polymerization initiator to the aqueous polymerization medium during the polymerization reaction.

5. The process of any one of claims 1 to 4, wherein the acidic reacting free-radical polymerization initiator is a persulfate.

6. The process of any one of claims 1 to 5, wherein the further acidic compound is an organic acid such as citric acid or an inorganic acid such as sulfuric acid.

7. The process of any one of claims 1 to 6, wherein the aqueous polymerization medium is neutralized to an essentially neutral pH value in the range from 6 to 8 after the polymerization reaction by adding a basic compound.

8. The process of claim 7, wherein the basic compound is selected from the group consisting of water-soluble hydroxides, oxides, carbonates, hydrogen carbonates (bicarbonates), acetates, citrates, borates, phosphates, hydrogen phosphates of alkali or alkaline earth metals or ammonium, ammonia or water-soluble organic amines.

9. The process of any one of claims 1 to 8, wherein the radical emulsion polymerization is carried out in the presence of a surfactant.

10. The process of any one of claims 1 to 9, wherein the aqueous reaction medium contains the acidic reacting free-radical polymerization initiator, the buffer system and / or the further acidic compound before the solketal (meth)acrylate and optionally one or more further ethylenically unsaturated monomers is added.

11. The process of any one of claims 1 to 10, wherein the radical emulsion polymerization is carried out by adding at least two feeds to a reaction vessel containing the aqueous reaction medium, wherein a first feed contains the acidic reacting free-radical polymerization initiator and a second feed contains solketal (meth)acrylate, optionally one or more further ethylenically unsaturated monomers and optionally a surfactant.

12. The process of any one of claims 1 to 11 , wherein at least part of the acidic reacting free- radical polymerization initiator, the buffer system and / or the further acidic compound is initially charged to the reaction vessel.

13. The process of any one of claims 1 to 12, wherein the aqueous polymer latex comprises polymerized glyceryl (meth)acrylate.

14. An aqueous polymer latex obtainable by the process of any one of claims 1 to 13.

15. The use of the aqueous polymer latex of claim 14 in coatings, films, paints, adhesives and / or for pigment printing, road marking, flooring or oil drilling.

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