METHOD FOR THE PRODUCTION OF WATER-ABSORBING POLYMER PARTICLES BY SUSPENSION POLYMERIZATION

Description

[0001] The present invention relates to a process for producing water-absorbing polymer particles by suspension polymerization and thermal surface crosslinking, wherein the agglomerated base polymer obtained by suspension polymerization has a centrifuge retention capacity of at least 37 g / g and the thermal surface crosslinking is carried out at 100 to 190° C.

[0002] The production of water-absorbing polymer particles is described in the monograph "Modern Superabsorbent Polymer Technology", FL Buchholz and AT Graham, Wiley-VCH, 1998, pages 69 to 117. The water-absorbing polymer particles are usually produced by solution polymerization or suspension polymerization.

[0003] Water-absorbing polymers are used as aqueous solution-absorbing products for the manufacture of diapers, tampons, sanitary napkins and other hygiene products, but also as water-retaining agents in agricultural horticulture.

[0004] The properties of water-absorbing polymers can be adjusted via the degree of cross-linking. With increasing degree of cross-linking, the gel strength increases and the absorption capacity decreases.

[0005] To improve application properties, such as permeability in the swollen gel bed of the diaper and absorption under pressure, water-absorbing polymer particles are generally surface-crosslinked. This increases only the degree of crosslinking of the particle surface, which at least partially decouples absorption under pressure and centrifugal retention capacity.

[0006] JP S63-218702 describes a continuous process for the production of water-absorbing polymer particles by suspension polymerization.

[0007] WO 2006 / 014031 A1 describes a process for producing water-absorbing polymer particles by suspension polymerization. At the significantly higher temperatures during thermal post-crosslinking, the hydrophobic solvent is driven off. The resulting superabsorbents exhibit a centrifuge retention capacity (CRC) of significantly less than 37 g / g.

[0008] WO 2008 / 068208 A1 also relates to a process for the production of water-absorbing polymer particles with a low proportion of hydrophobic solvents by suspension polymerization.

[0009] US 2011 / 0238026 A1, US 2011 / 0059329 A1 and WO 2015 / 110321 A1 each disclose processes for spray polymerization. Agglomeration in a hydrophobic solvent is not disclosed.

[0010] WO 2015 / 062883 A2 discloses a process for reverse suspension polymerization. It is not agglomerated in a hydrophobic solvent.

[0011] WO 2008 / 026783 A1 discloses a process for the production of superabsorbents with a high pressurized void average radius index. The superabsorbents can also be produced by reverse suspension polymerization. The resulting superabsorbents exhibit a centrifuge retention capacity (CRC) of significantly less than 37 g / g.

[0012] EP 1 433 526 A2 concerns superabsorbent mixtures consisting of a superabsorbent (R1) and a superabsorbent (R2). The superabsorbent (R1) is produced by solution polymerization and the superabsorbent (R2) by reverse suspension polymerization. It is not agglomerated in a hydrophobic solvent, and the centrifugal retention capacity (CRC) of at least 37 g / g or the absorption at a pressure < 49.2 g / cm² (AUHL) of at least 14 g / g is not achieved.

[0013] The object of the present invention was to provide an improved process for the production of water-absorbing polymer particles by suspension polymerization, wherein the water-absorbing polymer particles should have a high centrifugal retention capacity (CRC), a high absorption at a pressure of 49.2 g / cm² < (AUHL), a high sum of centrifugal retention capacity (CRC) and absorption at a pressure of 49.2 g / cm² < (AUHL), and low extractability.

[0014] The problem was solved by a process for producing water-absorbing polymer particles through polymerization of a monomer solution containing a) at least one ethylene-unsaturated, acid-group-bearing monomer, which may be at least partially neutralized, b) optionally one or more crosslinkers, c) at least one initiator, d) optionally one or more ethylene-unsaturated monomers copolymerizable with the monomers mentioned under a), and e) optionally one or more water-soluble polymers. wherein the monomer solution is suspended in a hydrophobic organic solvent during polymerization, the hydrophobic organic solvent having a solubility in water of less than 5 g / 100 g at 23°C, during or after polymerization agglomeration in a hydrophobic organic solvent and thermal surface crosslinking of the obtained agglomerated polymer particles by means of an organic surface crosslinker, characterized in that the amount of crosslinker b) is selected such that the agglomerated polymer particles have a centrifugal retention capacity of at least 37 g / g before surface crosslinking and the thermal surface crosslinking is carried out at 100 to 190° C.

[0015] In a preferred embodiment of the present invention, the amount of crosslinker b) is selected such that the agglomerated polymer particles have a centrifuge retention capacity of at least 38 g / g before surface post-crosslinking, and the thermal surface post-crosslinking is carried out at 105 to 180° C.

[0016] In a particularly preferred embodiment of the present invention, the amount of crosslinker b) is selected such that the agglomerated polymer particles have a centrifuge retention capacity of at least 39 g / g before surface post-crosslinking, and the thermal surface post-crosslinking is carried out at 110 to 175° C.

[0017] In a particularly preferred embodiment of the present invention, the amount of crosslinker b) is selected such that the agglomerated polymer particles have a centrifuge retention capacity of at least 40 g / g before surface post-crosslinking, and the thermal surface post-crosslinking is carried out at 120 to 170° C.

[0018] The monomers a) are preferably water-soluble, i.e. the solubility in water at 23°C is typically at least 1 g / 100 g water, preferably at least 5 g / 100 g water, particularly preferably at least 25 g / 100 g water, most preferably at least 35 g / 100 g water.

[0019] Suitable monomers a) are, for example, ethylene-unsaturated carboxylic acids such as acrylic acid, methacrylic acid, and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Acrylic acid is most preferred.

[0020] Other suitable monomers a) are, for example, ethylene unsaturated sulfonic acids, such as styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

[0021] Impurities can have a significant impact on polymerization. Therefore, the raw materials used should be of the highest possible purity. It is therefore often advantageous to purify the monomers (a) specifically. Suitable purification methods are described, for example, in WO 2002 / 055469 A1, WO 2003 / 078378 A1, and WO 2004 / 035514 A1. A suitable monomer (a) is, for example, acrylic acid purified according to WO 2004 / 035514 A1 with 99.8460 wt% acrylic acid, 0.0950 wt% acetic acid, 0.0332 wt% water, 0.0203 wt% propionic acid, 0.0001 wt% furfurals, 0.0001 wt% maleic anhydride, 0.0003 wt% diacrylic acid, and 0.0050 wt% hydroquinone monomethyl ether.

[0022] The proportion of acrylic acid and / or its salts in the total amount of monomers a) is preferably at least 50 mol-%, particularly preferably at least 90 mol-%, most preferably at least 95 mol-%.

[0023] The acid groups of the monomers a) may be partially neutralized. Neutralization is carried out at the monomer stage. This is usually done by mixing in the neutralizing agent as an aqueous solution or, more preferably, as a solid. The degree of neutralization is preferably 25 to 95 mol%, particularly preferably 30 to 80 mol%, and most preferably 40 to 75 mol%, wherein the usual neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates, or alkali metal hydrogen carbonates, as well as mixtures thereof. Ammonium salts can also be used instead of alkali metal salts. Sodium and potassium are particularly preferred alkali metals, but sodium hydroxide, sodium carbonate, or sodium hydrogen carbonate, as well as mixtures thereof, are most preferred.

[0024] The monomers a) usually contain polymerization inhibitors, preferably hydroquinone semi-ethers, as a storage stabilizer.

[0025] The monomer solution preferably contains up to 250 ppm by weight, more preferably at most 130 ppm by weight, more preferably at most 70 ppm by weight, more preferably at least 10 ppm by weight, more preferably at least 30 ppm by weight, and particularly around 50 ppm by weight, hydroquinone semi-ether, in each case based on the unneutralized monomer a). For example, an ethylene-unsaturated, acid-group-bearing monomer with a corresponding hydroquinone semi-ether content can be used to prepare the monomer solution.

[0026] Preferred hydroquinone semi-ethers are hydroquinone monomethyl ether (MEHQ) and / or alpha-tocopherol (vitamin E).

[0027] Suitable crosslinking agents b) are compounds with at least two groups suitable for crosslinking. Such groups include, for example, ethylene-unsaturated groups that can be radically polymerized into the polymer chain, and functional groups that can form covalent bonds with the acid groups of monomer a). Furthermore, polyvalent metal salts that can form coordinate bonds with at least two acid groups of monomer a) are also suitable as crosslinking agents b).

[0028] Crosslinkers b) are preferably compounds with at least two polymerizable groups that can be radically polymerized into the polymer network. Suitable crosslinking agents b) are, for example, methylene bisacrylamide, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 0 530 438 A1, di- and triacrylates, as described in EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93 / 21237 A1, WO 2003 / 104299 A1, WO 2003 / 104300 A1, WO 2003 / 104301 A1 and DE 103 31 450 A1, mixed acrylates which, in addition to acrylate groups, contain further ethylene unsaturated groups, as described in DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinking mixtures, as described for example in DE 195 43 368 A1, DE 196 46 484 A1, WO 90 / 15830 A1 and WO 2002 / 032962 A2.

[0029] Preferred crosslinkers b) are pentaerythritol triallyl ether, tetraallyloxyethane, methylenebismethacrylamide, 15-fold ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate and triallylamine.

[0030] Particularly preferred crosslinking agents (b) are methylenebisacrylamide and the multiply ethoxylated and / or propoxylated glycerols esterified with acrylic acid or methacrylic acid to form di- or triacrylates, as described, for example, in WO 2003 / 104301 A1. Methylenebisacrylamide and di- and / or triacrylates of 3- to 10-fold ethoxylated glycerol are particularly advantageous. Methylenebisacrylamide and di- or triacrylates of 1- to 5-fold ethoxylated and / or propoxylated glycerol are especially preferred. Methylenebisacrylamide and the triacrylates of 3- to 5-fold ethoxylated and / or propoxylated glycerol are most preferred, in particular methylenebisacrylamide and the triacrylate of 3-fold ethoxylated glycerol.

[0031] The amount of crosslinker in the monomer solution is selected such that the water-absorbing polymer particles, after polymerization and before thermal surface crosslinking (base polymer), exhibit a centrifugal retention capacity (CRC) of at least 37 g / g, preferably at least 38 g / g, particularly preferably at least 39 g / g, and most preferably at least 40 g / g. The centrifugal retention capacity (CRC) should not exceed 75 g / g. If the centrifugal retention capacity (CRC) of the base polymer is too high, sufficient absorption cannot be achieved during the subsequent thermal surface crosslinking under a pressure of 49.2 g / cm² (AUHL).

[0032] As initiators c), all compounds that generate radicals under the polymerization conditions can be used, for example thermal initiators, redox initiators, photoinitiators.

[0033] Suitable redox initiators include potassium or sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, potassium or sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. Preferably, mixtures of thermal and redox initiators are used, such as potassium or sodium peroxodisulfate / hydrogen peroxide / ascorbic acid. However, a mixture of the sodium salt of 2-hydroxy-2-sulfonate acetic acid, the disodium salt of 2-hydroxy-2-sulfonate acetic acid, and sodium bisulfite is preferably used as the reducing component. Such mixtures are available as Brüggolite® < FF6 and Brüggolite® < FF7 (Brüggemann Chemicals; Heilbronn; Germany).

[0034] Suitable thermal initiators include, in particular, azo initiators such as 2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride and 2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis(2-amidinopropane)dihydrochloride, 4,4'-azobis(4-cyano-pentanoic acid), 4,4' and their sodium salts, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide] and 2,2'-azobis(-imino-1-pyrrolidino-2-ethylpropane)dihydrochloride.

[0035] Suitable photoinitiators include, for example, 2-hydroxy-2-methylpropiophenone and 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one.

[0036] Examples of ethylene unsaturated monomers that can be copolymerized with the ethylene-unsaturated, acid-group-bearing monomers a) and d) are acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacrylate, and diethylaminoethyl methacrylate.

[0037] Water-soluble polymers e) can include polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, modified cellulose such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycols or polyacrylic acids, preferably starch, starch derivatives and modified cellulose.

[0038] Optionally, one or more chelating agents can be added to the monomer solution or its starting materials to mask metal ions, such as iron, for stabilization purposes. Suitable chelating agents include, for example, alkali citrates, citric acid, alkali tartrates, pentasodium triphosphate, ethylenediaminetetraacetate, nitrilotriacetic acid, and all chelating agents known under the name Trilon®, such as Trilon® C (pentasodium diethylenetriaminepentaacetate), Trilon® D (trisodium (hydroxyethyl)ethylenediaminetriacetic acid), and Trilon® M (methylglycine diacetic acid).

[0039] The preferred polymerization inhibitors require dissolved oxygen for optimal effectiveness. Therefore, the monomer solution can be freed of dissolved oxygen prior to polymerization by inerting, i.e., by passing an inert gas, preferably nitrogen or carbon dioxide, through it.

[0040] If polymerization is carried out under sufficient reflux, inerting can be omitted. In this process, the dissolved oxygen is removed from the polymerization reactor along with the evaporating solvent.

[0041] For polymerization, the monomer solution is suspended or emulsified in a hydrophobic solvent.

[0042] All solvents known to those skilled in the art for use in suspension polymerization can be employed as hydrophobic solvents. Aliphatic hydrocarbons, such as n-hexane, n-heptane, n-octane, n-nonane, n-decane, cyclohexane, or mixtures thereof, are preferred. Hydrophobic solvents exhibit a solubility in water at 23°C of less than 5 g / 100 g, preferably less than 1 g / 100 g, and particularly preferably less than 0.5 g / 100 g.

[0043] The hydrophobic solvent boils in the range of preferably 50 to 150°C, particularly preferably 60 to 120°C, and most preferably 70 to 90°C.

[0044] The ratio between hydrophobic solvent and monomer solution is 0.2 to 3.0, preferably 0.3 to 2.7, and most preferably 0.4 to 2.4.

[0045] Dispersing agents can be added to disperse the aqueous monomer solution in the hydrophobic solvent or to disperse the resulting water-absorbing polymer particles. These agents can be anionic, cationic, nonionic, or amphoteric surfactants, or natural, semi-synthetic, or synthetic polymers.

[0046] Anionic surfactants include sodium polyoxyethylene endodecyl ether sulfate and sodium dodecyl ether sulfate. A cationic surfactant is, for example, trimethylstearylammonium chloride. An amphoteric surfactant is, for example, carboxymethyldimethylcetylammonium. Nonionic surfactants include, for example, sucrose fatty acid esters such as sucrose monostearate and sucrose dilaurate, sorbitan esters such as sorbitan monostearate, trehalose fatty acid esters such as trehalose stearic acid esters, and polyoxyalkylene compounds based on sorbitan esters such as polyoxyethylene sorbitan monostearate.

[0047] Suitable polymers include, for example, cellulose derivatives such as hydroxyethyl cellulose, methylhydroxyethyl cellulose, methylhydroxypropyl cellulose, methyl cellulose and carboxymethyl cellulose, polyvinylpyrrolidone, copolymers of vinylpyrrolidone, gelatin, gum arabic, xanthan gum, casein, polyglycerols, polyglycerol fatty acid esters, polyethylene glycols, modified polyethylene glycol such as polyethylene glycol stearate or polyethylene glycol stearyl ether stearate, polyvinyl alcohol, partially hydrolyzed polyvinyl acetates and modified polyethylene such as maleic acid-modified polyethylene.

[0048] Inorganic particles can also be used as dispersing aids, so-called Pickering systems. Such a Pickering system can consist of the solid particles alone or additionally of additives that improve the dispersibility of the particles in water or their wettability by the hydrophobic solvent. Their mode of action and use are described in WO 99 / 24525 A1 and EP 1 321 182 A1.

[0049] The inorganic solid particles can be metal salts, such as salts, oxides, and hydroxides of calcium, magnesium, iron, zinc, nickel, titanium, aluminum, silicon, barium, and manganese. Examples include magnesium hydroxide, magnesium carbonate, magnesium oxide, calcium oxalate, calcium carbonate, barium carbonate, barium sulfate, titanium dioxide, aluminum oxide, aluminum hydroxide, and zinc sulfide. Silicates, bentonite, hydroxyapatite, and hydrotalcite should also be mentioned. SiO₂-based silicas, magnesium pyrophosphate, and tricalcium phosphate are particularly preferred.

[0050] Suitable SiO₂-based dispersing agents are highly dispersed silicas. They can be dispersed in water as fine, solid particles. However, it is also possible to use so-called colloidal dispersions of silica in water. Such colloidal dispersions are alkaline aqueous mixtures of silica. In the alkaline pH range, the particles swell and are stable in water. Preferred colloidal dispersions of silica have a specific surface area in the range of 20 to 90 m² / g at pH 9.3.

[0051] Furthermore, any mixture of dispersing agents can be used.

[0052] The dispersing agent is usually dissolved or dispersed in the hydrophobic solvent. The dispersing agent is used in amounts between 0.01 and 10 wt.%, preferably between 0.2 and 5 wt.%, and particularly preferably between 0.5 and 2 wt.%, based on the monomer solution. The diameter of the monomer solution droplets can be adjusted by the type and amount of the dispersing agent.

[0053] The diameter of the monomer solution droplets can be adjusted via the applied stirring energy and by using suitable dispersing aids.

[0054] The agglomeration process is known to those skilled in the art and is not subject to any restrictions. Polymerization and agglomeration can be carried out simultaneously (single-stage dosing) or sequentially (two-stage dosing).

[0055] In one-step dosing, the monomer solution is dosed into the hydrophobic solvent and the monomer solution droplets agglomerate during polymerization.

[0056] In the two-stage dosing process, a first monomer solution is added to the hydrophobic solvent, and the monomer solution droplets are polymerized. A second monomer solution is then added to the resulting dispersed polymer particles and polymerized again. The polymer particles only agglomerate during the second polymerization. The first and second monomer solutions can be identical or different in composition.

[0057] With each additional addition of monomer to already formed agglomerates, regardless of whether these were produced by single-stage or two-stage dosing, the agglomerates can be further agglomerated into larger agglomerates.

[0058] Cooling steps may occur between the monomer doses. Some of the dispersing agent may precipitate out during this process.

[0059] Whether the monomer solution droplets agglomerate during polymerization can be controlled by the type and amount of dispersing agent. A sufficient amount of dispersing agent prevents agglomeration during polymerization. The required amount depends on the type of dispersing agent.

[0060] The two-stage dosing method, i.e., agglomeration after polymerization of the monomer solution droplets, is preferred.

[0061] For polymerization, it is advantageous to connect several stirred reactors in series. The subsequent reaction in further stirred reactors can increase the monomer conversion and reduce backmixing. It is also advantageous if the first stirred reactor is not too large. As the size of the stirred reactor increases, the size distribution of the dispersed monomer solution droplets inevitably broadens. A smaller first reactor therefore enables the production of water-absorbing polymer particles with a particularly narrow particle size distribution.

[0062] The reaction is preferably carried out under reduced pressure, for example at a pressure of 800 mbar. The boiling point of the reaction mixture can be adjusted to the desired reaction temperature by changing the pressure.

[0063] In a preferred embodiment of the present invention, the polymerization is carried out in the presence of a chain transfer reagent that is usually water-soluble.

[0064] Chain transfer reagents interfere with polymerization kinetics and regulate the molecular weight. Suitable chain transfer reagents include thiols, thiolic acids, secondary alcohols, phosphorus compounds, lactic acid, amino acids, etc.

[0065] The chain transfer reagent is used in an amount of preferably 0.00001 to 0.1 mol / mol, particularly preferably 0.00015 to 0.08 mol / mol, most preferably 0.0002 to 0.06 mol / mol, in each case based on monomer a).

[0066] The resulting water-absorbing polymer particles undergo thermal surface crosslinking. This thermal surface crosslinking can be carried out within the polymer dispersion or with the water-absorbing polymer particles separated and dried from the polymer dispersion.

[0067] The addition of the monomer solution can also be above the boiling point of water or the solvent or the solvent-water azeotrope, so that solvent or a solvent-water azeotrope is continuously distilled off during the monomer addition.

[0068] In a preferred embodiment of the present invention, the water-absorbing polymer particles in the polymer dispersion are azeotropically dehydrated, filtered from the polymer dispersion, the filtered water-absorbing polymer particles are dried to remove the adhering residual hydrophobic solvent and thermally surface crosslinked.

[0069] Suitable surface crosslinkers are compounds containing groups that can form covalent bonds with at least two carboxylate groups of the polymer particles. Suitable compounds include, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, as described in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1, DE 35 23 617 A1 and EP 0 450 922 A2, or β-hydroxyalkylamides, as described in DE 102 04 938 A1 and US 6,239,230.

[0070] Furthermore, alkylene carbonates are listed in DE 40 20 780 C1, 2-oxazolidinone and its derivatives, such as 2-hydroxyethyl-2-oxazolidinone, in DE 198 07 502 A1, bis- and poly-2-oxazolidinones and its derivatives in DE 198 07 992 C1, 2-oxotetrahydro-1,3-oxazine and its derivatives in DE 198 54 573 A1, N-acyl-2-oxazolidinones in DE 198 54 574 A1, cyclic ureas in DE 102 04 937 A1, bicyclic amidoacetals in DE 103 34 584 A1, oxetanes and cyclic ureas in EP 1 199 327 A2, and in WO 2003 / 031482 A1 Morpholine-2,3-dione and its derivatives are described as suitable surface crosslinking agents.

[0071] Furthermore, surface crosslinking agents containing additional polymerizable ethylene unsaturated groups, as described in DE 37 13 601 A1, can also be used.

[0072] Furthermore, any mixture of suitable surface crosslinking agents can be used.

[0073] Preferred surface crosslinking agents are alkylene carbonates, 2-oxazolidinones, bis- and poly-2-oxazolidinones, 2-oxotetrahydro-1,3-oxazines, N-acyl-2-oxazolidinones, cyclic ureas, bicyclic amidoacetals, oxetanes, bisoxetanes and morpholine-2,3-diones.

[0074] Particularly preferred surface crosslinking agents are ethylene carbonate (1,3-dioxolan-2-one), trimethylene carbonate (1,3-dioxan-2-one), 3-methyl-3-oxethanmethanol, 2-hydroxyethyl-2-oxazolidinone, 2-oxazolidinone and methyl-2-oxazolidinone.

[0075] Ethylene carbonate is particularly preferred.

[0076] The amount of surface crosslinking agent is preferably 0.1 to 10 wt.%, particularly preferably 0.5 to 7.5 wt.%, most preferably 1 to 5 wt.%, in each case based on the polymer particles.

[0077] Surface crosslinking agents are typically used as an aqueous solution. The amount of solvent is preferably 0.001 to 8 wt.%, particularly preferably 2 to 7 wt.%, most preferably 3 to 6 wt.%, and particularly 4 to 5 wt.%, in each case based on the polymer particles. The penetration depth of the surface crosslinking agent into the polymer particles can be adjusted by the content of non-aqueous solvent or the total amount of solvent.

[0078] If water is used exclusively as the solvent, a surfactant is advantageously added. This improves the wetting properties and reduces the tendency to clump. Preferably, however, solvent mixtures are used, for example isopropanol / water, 1,3-propanediol / water, and propylene glycol / water, with the mixture mass ratio preferably ranging from 10:90 to 60:40.

[0079] In a preferred embodiment of the present invention, cations, in particular polyvalent cations, are applied to the particle surface before, during or after thermal surface crosslinking in addition to the surface crosslinking agents.

[0080] The polyvalent cations that can be used in the process according to the invention are, for example, divalent cations such as zinc, magnesium, calcium, iron, and strontium; trivalent cations such as aluminum, iron, chromium, rare earth elements, and manganese; and tetravalent cations such as titanium and zirconium. Possible counterions include hydroxide, chloride, bromide, sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate, and carboxylates such as acetate, citrate, and lactate. Salts with different counterions are also possible, for example, basic aluminum salts such as aluminum monoacetate or aluminum monolactate. Aluminum sulfate, aluminum monoacetate, and aluminum lactate are preferred. In addition to metal salts, polyamines can also be used as polyvalent cations.

[0081] The amount of polyvalent cation used is, for example, 0.001 to 1.5 wt.%, preferably 0.005 to 1 wt.%, particularly preferably 0.02 to 0.8 wt.%, in each case based on the polymer particles.

[0082] In a further preferred embodiment of the present invention, hydrophilizing agents are additionally applied before, during or after the thermal surface post-crosslinking, for example sugar alcohols such as sorbitol, mannitol and xylitol, water-soluble polymers or copolymers such as cellulose, polyethylene glycols, polyvinyl alcohols, polyvinylpyrrolidones and polyacrylamides.

[0083] Surface recrosslinking is typically carried out by spraying a solution of the surface recrosslinker onto the dried polymer particles. Following spraying, the polymer particles coated with the surface recrosslinker are thermally surface recrosslinked.

[0084] Spraying a solution of the surface re-curing agent is preferably carried out in mixers with moving mixing tools, such as screw mixers, disc mixers, and paddle mixers. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are especially preferred. The distinction between horizontal and vertical mixers is made by the orientation of the mixing shaft; that is, horizontal mixers have a horizontally mounted mixing shaft, and vertical mixers have a vertically mounted mixing shaft. Suitable mixers include, for example, the Horizontal Ploughshare® mixer (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany), the Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Doetinchem; Netherlands), the Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA), and the Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; Netherlands). However, it is also possible to spray the surface re-curing agent solution in a fluidized bed.

[0085] Thermal surface recurding is preferably carried out in contact dryers, particularly paddle dryers, and most preferably disc dryers. Suitable dryers include, for example, the Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Leingarten; Germany), the Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Leingarten; Germany), the Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; USA), and the Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany). Fluidized bed dryers can also be used.

[0086] Thermal surface re-crosslinking can take place within the mixer itself, by heating the jacket or blowing in warm air. A downstream dryer, such as a tray dryer, a rotary kiln, or a heated screw dryer, is equally suitable. Mixing and thermal surface re-crosslinking in a fluidized bed dryer is particularly advantageous.

[0087] For thermal surface re-crosslinking, it may be advantageous to carry this out under negative pressure or using drying gases, such as dried air and nitrogen, to ensure the most complete possible removal of solvents.

[0088] Subsequently, the surface-crosslinked polymer particles can be classified, whereby polymer particles that are too small and / or too large are separated and recycled back into the process.

[0089] Surface post-crosslinking can also be carried out in the polymer dispersion. For this purpose, the solution of the surface post-crosslinker is added to the polymer dispersion. It can be advantageous to perform the thermal surface post-crosslinking under pressure, for example, when using hydrophobic organic solvents with a boiling point of 1013 mbar below the desired temperature for thermal surface post-crosslinking. After thermal surface post-crosslinking in the polymer dispersion, the water-absorbing polymer particles are azeotropically dehydrated, separated from the polymer dispersion, and dried to remove any remaining hydrophobic solvent.

[0090] The surface post-curing temperatures are in the range of 100 to 190°C, preferably in the range of 105 to 180°C, particularly preferably in the range of 110 to 175°C, and most preferably in the range of 120 to 170°C. The preferred residence time at this temperature is preferably at least 10 minutes, particularly preferably at least 20 minutes, most preferably at least 30 minutes, and usually at most 120 minutes.

[0091] In a preferred embodiment of the present invention, the water-absorbing polymer particles are cooled in a contact dryer after thermal surface post-curing. Cooling is preferably carried out in contact coolers, particularly preferably paddle coolers, and most preferably disc coolers. Suitable coolers include, for example, the Hosokawa Bepex® Horizontal Paddle Cooler (Hosokawa Micron GmbH; Leingarten; Germany), the Hosokawa Bepex® Disc Cooler (Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® coolers (Metso Minerals Industries Inc.; Danville; USA), and the Nara Paddle Cooler (NARA Machinery Europe; Frechen; Germany). Fluidized bed coolers can also be used.

[0092] In the cooler, the water-absorbing polymer particles are cooled to 20 to 150°C, preferably 30 to 120°C, particularly preferably 40 to 100°C, and most preferably 50 to 80°C.

[0093] The polymer particles, which are thermally surface-crosslinked in the contact dryer, can be coated or re-moistened to further improve their properties.

[0094] Post-moistening is preferably carried out at 30 to 80°C, particularly preferably at 35 to 70°C, and most preferably at 40 to 60°C. At excessively low temperatures, the water-absorbing polymer particles tend to clump together, and at higher temperatures, a significant amount of water evaporates. The amount of water used for post-moistening is preferably 1 to 10 wt.%, particularly preferably 2 to 8 wt.%, and most preferably 3 to 5 wt.%. Post-moistening increases the mechanical stability of the polymer particles and reduces their tendency to accumulate static electricity.

[0095] Suitable coatings for improving swelling rate and permeability (SFC) include, for example, inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers, and divalent or polyvalent metal cations. Suitable coatings for dust binding include, for example, polyols. Suitable coatings to prevent the undesirable tendency of polymer particles to clump together include, for example, fumed silica, such as Aerosil® < 200, and surfactants, such as Span® < 20 and Plantacare 818 UP, and surfactant mixtures.

[0096] Another object of the present invention is the water-absorbing polymer particles obtainable according to the inventive method.

[0097] The water-absorbing polymer particles obtainable according to the inventive process have a centrifugal retention capacity (CRC) of at least 37 g / g, an absorption at a pressure of 21.0 g / cm²< of at least 30 g / g, an absorption at a pressure of 49.2 g / cm²< (AUHL) of at least 14 g / g, a sum of centrifugal retention capacity and absorption at a pressure of 21.0 g / cm²< (CRC+AUL) of at least 69 g / g, a sum of centrifugal retention capacity and absorption at a pressure of 49.2 g / cm²< (CRC+AUHL) of at least 54 g / g, and less than 20 wt.% extractables.

[0098] The water-absorbing polymer particles according to the invention have a centrifugal retention capacity (CRC) of preferably at least 38 g / g, particularly preferably at least 40 g / g, and most preferably at least 41 g / g. The centrifugal retention capacity (CRC) of water-absorbing polymer particles is typically less than 75 g / g.

[0099] The water-absorbing polymer particles according to the invention exhibit an absorption at a pressure of 21.0 g / cm² (AUL) of preferably at least 32 g / g, particularly preferably at least 33 g / g, and most preferably at least 34 g / g. The absorption at a pressure of 21.0 g / cm² (AUL) of the water-absorbing polymer particles is typically less than 50 g / g.

[0100] The water-absorbing polymer particles according to the invention exhibit an absorption at a pressure of 49.2 g / cm² (AUHL) of preferably at least 16 g / g, particularly preferably at least 18 g / g, and most preferably at least 20 g / g. The absorption at a pressure of 49.2 g / cm² (AUHL) of the water-absorbing polymer particles is typically less than 35 g / g.

[0101] The sum of centrifugal retention capacity (CRC) and absorption under a pressure of 21.0 g / cm² (AUL) of the water-absorbing polymer particles according to the invention is preferably at least 71 g / g, particularly preferably at least 73 g / g, and most preferably at least 74 g / g.

[0102] The sum of centrifugal retention capacity (CRC) and absorption under a pressure of 49.2 g / cm² (AUHL) of the water-absorbing polymer particles according to the invention is preferably at least 56 g / g, particularly preferably at least 58 g / g, and most preferably at least 59 g / g.

[0103] The water-absorbing polymer particles according to the invention preferably contain less than 17 wt.%, particularly preferably less than 15 wt.%, and most preferably less than 14 wt.%, of extractables.

[0104] The water-absorbing polymer particles according to the invention have a proportion of particles with a particle size of 300 to 600 µm of preferably at least 30 wt.%, particularly preferably at least 40 wt.%, most preferably at least 50 wt.%.

[0105] Another object of the present invention is hygiene articles, comprising (A) an upper liquid-permeable layer, (B) a lower liquid-impermeable layer, (C) a liquid-absorbing storage layer between layer (A) and layer (B), containing 0 to 30 wt.% of a fiber material and 70 to 100 wt.% of water-absorbing polymer particles obtainable according to the inventive process, (D) optionally an absorption and distribution layer between layer (A) and layer (C), containing 80 to 100 wt.% of a fiber material and 0 to 20 wt.% of water-absorbing polymer particles obtainable according to the inventive process, (E) optionally a fabric layer directly above and / or below layer (C) and (F) further optional components.

[0106] The proportion of water-absorbing polymer particles obtainable according to the inventive method in the liquid-absorbing storage layer (C) is preferably at least 75 wt.%, particularly preferably at least 80 wt.%, most preferably at least 90 wt.%.

[0107] The mean sphericity of the water-absorbing polymer particles obtainable according to the inventive method in the liquid-absorbing storage layer (C) is preferably less than 0.84, particularly preferably less than 0.82, most preferably less than 0.80.

[0108] Water-absorbing polymer particles with relatively low sphericity are obtained by suspension polymerization if the polymer particles are agglomerated during or after polymerization. Agglomerated water-absorbing polymer particles are used in the hygiene articles according to the invention.

[0109] The water-absorbing polymer particles are tested using the test methods described below. Methods:

[0110] Unless otherwise specified, measurements should be carried out at an ambient temperature of 23 ± 2 °C and a relative humidity of 50 ± 10%. The water-absorbing polymers should be thoroughly mixed before measurement. Residual monomer

[0111] The residual monomer content of the water-absorbing polymer particles is determined according to the EDANA recommended test method WSP No. 210.2-05 "Residual Monomers". moisture content

[0112] The moisture content of the water-absorbing polymer particles is determined according to the test method recommended by EDANA No. WSP 230.3 (11) "Mass Loss Upon Heating". Centrifuge retention capacity

[0113] The centrifuge retention capacity (CRC) is determined according to the EDANA recommended test method No. WSP 241.3 (11) "Fluid Retention Capacity in Saline, After Centrifugation". Absorption at a pressure of 0.0 g / cm²<

[0114] The absorption under a pressure of 0.0 g / cm 2< (AUNL) is determined analogously to the EDANA recommended test method No. WSP 242.3 (11) "Gravimetric Determination of Absorption Under Pressure", except that instead of a pressure of 21.0 g / cm 2< (AUL0.3psi) a pressure of 0.0 g / cm 2< (AUL0.0psi) is used. Absorption at a pressure of 21.0 g / cm²<

[0115] The absorption under a pressure of 21.0 g / cm 2< (AUL) of the water-absorbing polymer particles is determined according to the EDANA recommended test method No. WSP 242.3 (11) "Gravimetric Determination of Absorption Under Pressure". Absorption at a pressure of 49.2 g / cm²<

[0116] The absorption under a pressure of 49.2 g / cm² (AUHL) is determined analogously to the EDANA recommended test method No. WSP 242.3 (11) "Gravimetric Determination of Absorption Under Pressure", except that instead of a pressure of 21.0 g / cm² (AUL0.3psi) a pressure of 49.2 g / cm² (AUL0.7psi) is used. bulk density

[0117] The bulk density is determined according to the test method recommended by EDANA No. WSP 250.3 (11) "Gravimetric Determination of Density". Extractable

[0118] The extractable content of the water-absorbing polymer particles is determined according to the EDANA recommended test method No. WSP 270.3 (11) "Extractable". The extraction time is 16 hours. Free Swell Rate

[0119] To determine the swelling rate (FSR), 1.00 g (= W1) of the water-absorbing polymer particles are weighed into a 25 ml beaker and evenly distributed on the bottom. Then, 20 ml of a 0.9 wt% saline solution are dispensed into a second beaker using a dispenser, and the contents of this beaker are quickly added to the first. A stopwatch is then started. As soon as the last drop of saline solution is absorbed, which is indicated by the disappearance of the reflection on the liquid surface, the stopwatch is stopped. The exact amount of liquid poured from the second beaker and absorbed by the polymer in the first beaker is precisely determined by back-weighing the second beaker (= W2). The time interval required for absorption, measured with the stopwatch, is denoted as t. The time at which the last drop of liquid disappears from the surface is recorded as time t.

[0120] The source velocity (FSR) is calculated as follows: FSR g / g s = W 2 / W 1 xt

[0121] However, if the moisture content of the water-absorbing polymer particles is more than 3 wt.%, the weight W1 must be corrected by this moisture content. Vortex test

[0122] Into a 100 ml beaker containing a 30 mm x 6 mm magnetic stir bar, 50.0 ml ± 1.0 ml of a 0.9 wt% aqueous saline solution are added. The saline solution is stirred at 600 rpm using a magnetic stirrer. Then, 2.000 g ± 0.010 g of water-absorbing polymer particles are added as quickly as possible, and the time is measured until the stir cluster disappears due to the absorption of the saline solution by the water-absorbing polymer particles. During this time, the entire contents of the beaker may still rotate as a uniform gel mass, but the surface of the gelled saline solution must no longer exhibit any individual turbulence. The time required is reported as the vortex. Examples: Production of the base polymer: Example 1

[0123] In a 2 L ground glass vessel equipped with an impeller stirrer and reflux condenser, 340.00 g of heptane and 0.92 g of sucrose stearate (Ryoto ®< Sugar Ester S-370, Mitsubishi Chemical Europe GmbH, Düsseldorf, Germany) were placed and heated to 70° C until the sucrose stearate was completely dissolved.

[0124] A monomer solution (first dose), prepared from 73.40 g (1.019 mol) acrylic acid, 61.20 g (0.765 mol) 50 wt% aqueous sodium hydroxide solution, 109.5 g water, and 0.11 g (0.407 mmol) potassium peroxodisulfate, was then placed in a feed vessel and purged with air. Immediately before adding the monomer solution dropwise, at a stirrer speed of 300 rpm, the solution was inerted by introducing nitrogen, and an oil bath temperature of 55°C was established.

[0125] After the initial dose, the mixture was stirred for one hour at 70°C. The reaction solution was then cooled to approximately 25°C, and an ice-cold monomer solution (second dose), prepared from 95.90 g (1.331 mol) acrylic acid, 79.30 g (0.991 mol) 50 wt% aqueous sodium hydroxide solution, 143.10 g water, and 0.14 g (0.518 mmol) potassium peroxodisulfate, was added to a feed vessel and purged with air. Immediately before adding the monomer solution dropwise, at a stirrer speed of 300 rpm, the solution was inerted by introducing nitrogen. The monomer solution was added dropwise over 15 minutes.

[0126] After the oil started flowing in, an oil bath temperature of 70°C was set. 120 minutes after the heating began, the reflux condenser was replaced with a water circulator and water was circulated.

[0127] The suspension was cooled to 60°C and the resulting polymer particles were collected via a Büchner funnel with a paper filter. Further drying was carried out at 45°C in a forced-air drying oven and, if necessary, in a vacuum drying oven at 800 mbar until a residual moisture content of less than 15 wt% was achieved.

[0128] The properties of the polymer particles obtained are summarized in Table 2. Examples 2 and 3

[0129] The base polymer was produced analogously to Example 1 using the quantities specified in Table 1.

[0130] The properties of the polymer particles obtained are summarized in Table 2. Example 4

[0131] The preparation of the base polymer was carried out analogously to Example 1 with the quantities specified in Table 1, wherein the first monomer solution (first dose) additionally contained 3.0 g of 2-propanol (isopropanol).

[0132] The properties of the polymer particles obtained are summarized in Table 2. Example 5

[0133] In a 2 L ground glass vessel equipped with an impeller stirrer and reflux condenser, 896.00 g of cyclohexane, 2.00 g of Span ®< 20 (sorbitan monolaurate), 3.20 g of Tixoge| ®< VZ (organophilic bentonite) and 20.0 g of a 0.015% aqueous ascorbic acid solution were placed and heated to an internal temperature of 75°C with stirring and introduction of nitrogen.

[0134] A monomer solution, prepared from 150.00 g (2.082 mol) acrylic acid, 125.10 g (1.613 mol) 50 wt% aqueous sodium hydroxide solution, 138 g water, 0.0375 g (0.243 mmol) N,N'-methylenebisacrylamide (MBA), and 0.5 g (1.850 mmol) potassium peroxide disulfate, was then placed in a feed vessel and purged with air. Immediately before adding the monomer solution dropwise, at a stirrer speed of 300 rpm, the solution was inerted by introducing nitrogen. Reflux conditions were maintained throughout the entire monomer dosing process. The monomer solution was added dropwise over 60 minutes.

[0135] After the water flowed in, a post-reaction period of 60 minutes followed. The reflux condenser was then replaced with a water circulator, and water was circulated.

[0136] The suspension was cooled to 60°C and the resulting polymer particles were collected via a Büchner funnel with a paper filter. Further drying was carried out at 45°C in a forced-air drying oven and, if necessary, in a vacuum drying oven at 800 mbar until a residual moisture content of less than 15 wt% was achieved.

[0137] The properties of the polymer particles obtained are summarized in Table 2. Example 6

[0138] In a 2 L ground glass vessel equipped with an impeller stirrer and reflux condenser, 896.00 g of cyclohexane, 2.00 g of Span ®< 20 (sorbitan monolaurate), 3.20 g of Tixogel ®< VZ (organophilic bentonite) and 20.0 g of a 0.015% aqueous ascorbic acid solution were placed and heated to an internal temperature of 75° C under stirring and nitrogen introduction.

[0139] A monomer solution, prepared from 150.00 g (2.082 mol) acrylic acid, 118.0 g (1.475 mol) 50 wt% aqueous sodium hydroxide solution, 136.8 g water, 0.075 g (0.194 mmol) of the triacrylate of triply ethoxylated glycerol (Gly-(EO-AA)₃), and 0.5 g (1.850 mmol) potassium peroxide disulfate, was then placed in a feed vessel and purged with air. Immediately before adding the monomer solution dropwise, at a stirrer speed of 300 rpm, the solution was inerted by introducing nitrogen. Reflux conditions were maintained throughout the entire monomer dosing process. The monomer solution was added dropwise over 60 minutes.

[0140] After the water flowed in, a post-reaction period of 60 minutes followed. The reflux condenser was then replaced with a water circulator, and water was circulated.

[0141] The suspension was cooled to 60°C and the resulting polymer particles were collected via a Büchner funnel with a paper filter. Further drying was carried out at 45°C in a forced-air drying oven and, if necessary, in a vacuum drying oven at 800 mbar until a residual moisture content of less than 15 wt% was achieved.

[0142] The properties of the polymer particles obtained are summarized in Table 2. Table 1: Amounts of crosslinker used b) Example. Step / Approach Networker b) g mmol ppm boaa mmol% boaa 1 1 MBA 0 0 0 0 2 MBA 0 0 0 0 2 1 MBA 0,009 0,06 125 6 2 MBA 0,012 0,08 125 6 3 1 MBA 0,018 0,12 250 12 2 MBA 0,023 0,15 250 12 4 1 MBA*) 0,018 0,12 250 12 2 MBA*) 0,023 0,15 250 12 5 - MBA**) 0,0375 0,24 250 12 6 - Gly-(EO-AA) 3 **) 0,075 0,19 500 9 *) additionally isopropanol as chain transfer reagent **) one-step dosage boaa: based on unneutralized acrylic acid MBA: Methylenebisacrylamide Gly-(EO-AA) 3 Triacrylate of 3-fold ethoxylated glycerol Table 2: Properties of the water-absorbing polymer particles (base polymer) Example. CRC g / g AUNL g / g AUL g / g AUHL g / g Bulk density g / 100 ml Moisture content % Extractable % 1 55,0 44,3 7,2 6,6 89 2,2 34 2 46,1 54,1 7,3 6,7 78 4,5 21 3 42,8 52,0 7,4 6,5 79 4,4 21 4 48,4 54,1 7,8 7,2 83 4,0 20 5 40,7 39,9 7,5 6,6 70 3,5 26 6 64,0 9,7 6,9 6,1 91 2,0 29 Thermal surface recrosslinking: Examples 1-1 and 1-2

[0143] 20 g of the base polymer from Example 1 were placed in a Waring® Blender 32BL80 (8011) mixer. The mixer was then switched on to speed setting 1. Immediately afterward, 1.5 g of an aqueous solution consisting of 0.5 g of ethylene carbonate and 1.0 g of water, as specified in Table 3, was drawn into a syringe and injected into the mixer within 2 seconds. After 3 seconds, the mixer was switched off, and the resulting polymer particles were evenly distributed in a 20 cm diameter glass dish. For thermal surface curing, the glass dish containing the polymer particles was annealed in a circulating air drying oven at 160°C for 60 or 120 minutes. The polymer particles were then transferred to a cold glass dish. Finally, the larger particles were removed using a sieve with a mesh size of 850 µm.

[0144] The properties of the polymer particles are summarized in Table 4. Examples 2-1 and 2-2

[0145] Thermal surface curing was carried out analogously to Examples 1-1 and 1-2, but using the base polymer from Example 2. The temperature in the forced-air drying oven was 160°C. The curing time was 60 or 120 minutes. The conditions are summarized in Table 3.

[0146] The properties of the polymer particles are summarized in Table 4. Examples 3-1 and 3-2

[0147] Thermal surface curing was carried out analogously to Examples 1-1 and 1-2, but using the base polymer from Example 3. The temperature in the forced-air drying oven was 160°C. The curing time was 60 or 120 minutes. The conditions are summarized in Table 3.

[0148] The properties of the polymer particles are summarized in Table 4. Examples 3-3 and 3-4

[0149] Thermal surface post-curing was carried out analogously to Example 1-1, but using the base polymer from Example 3 and N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine (Primid® < XL 552) as the surface post-curing agent. The temperature in the forced-air drying oven was 160°C. The curing time was 60 minutes. The conditions are summarized in Table 3.

[0150] The properties of the polymer particles are summarized in Table 4. Examples 3-5 and 3-6

[0151] Thermal surface curing was carried out analogously to Example 1-1, but using the base polymer from Example 3. The temperature in the forced-air drying oven was 90°C or 200°C. The curing time was 60 minutes. The conditions are summarized in Table 3.

[0152] The properties of the polymer particles are summarized in Table 4. Examples 4-1 and 4-2

[0153] Thermal surface curing was carried out analogously to Examples 1-1 and 1-2, but using the base polymer from Example 4. The temperature in the forced-air drying oven was 160°C. The curing time was 60 or 120 minutes. The conditions are summarized in Table 3.

[0154] The properties of the polymer particles are summarized in Table 4. Examples 5-1 and 5-2

[0155] Thermal surface curing was carried out analogously to Examples 1-1 and 1-2, but using the base polymer from Example 5. The temperature in the forced-air drying oven was 160°C. The curing time was 60 or 120 minutes. The conditions are summarized in Table 3.

[0156] The properties of the polymer particles are summarized in Table 4. Examples 6-1 and 6-2

[0157] Thermal surface curing was carried out analogously to Examples 1-1 and 1-2, but using the base polymer from Example 6. The temperature in the forced-air drying oven was 160°C. The curing time was 60 or 120 minutes. The conditions are summarized in Table 3.

[0158] The properties of the polymer particles are summarized in Table 4. Example 7

[0159] Example 1 from US 8,003,210 has been reworked.

[0160] The properties of the polymer particles are summarized in Table 4. Table 3: Thermal surface re-crosslinking in the Waring® blender - conditions Example. Networker b) Temperature °C Time min Ethylene carbonate wt.% bop Ethylene carbonate mmol per 20 g of polymer Water wt.% bop Primid® < XL 552 wt% bop Primid ®< XL 552 mmol per 20 g polymer 1 0 ppm MBA - - - - - - - 1-1 160 60 2,5 5,68 5 - - 1-2 160 120 2,5 5,68 5 - - 2 125 ppm MBA - - - - - - - 2-1 160 60 2,5 5,68 5 - - 2-2 160 120 2,5 5,68 5 - - 3 250 ppm MBA - - - - - - - 3-1 160 60 2,5 5,68 5 - - 3-2 160 120 2,5 5,68 5 - - 3-3 160 60 - - 5 4,55 2,84 3-4 160 60 - - 5 9,05 5,68 3-5 90 60 2,5 5,68 5 3-6 200 60 2,5 5,68 5 4 250 ppm MBA*) - - - - - - - 4-1 160 60 2,5 5,68 5 - - 4-2 160 120 2,5 5,68 5 - - 5 250 ppm MBA**) - - - - - - - 5-1 160 60 2,5 5,68 5 - - 5-2 160 120 2,5 5,68 5 - - 6 500 ppm Gly-(EO-AA) 3 **) - - - - - - 6-1 160 60 2,5 5,68 5 - - 6-2 160 120 2,5 5,68 5 - - bop: based on polymer *) additionally isopropanol as chain transfer reagent **) single-stage agglomerated Table 4: Thermal surface post-crosslinking in the Waring® blender - Properties of the polymer particles Example. CRC g / g AUNL g / g AUL g / g AUHL g / g Moisture content wt.% Extractable wt.% Vortex s FSR g / gs CRC+AUL g / g CRC+AUHL g / g 1 55,0 44,3 7,2 6,6 2,2 34 200 0,06 62,2 61,6 1-1 46,0 59,0 26,0 9,3 1,2 5 194 0,08 72,0 55,3 1-2 43,8 55,5 34,0 19,7 1,1 13 192 0,12 77,8 63,5 2 46,1 54,1 7,32 6,7 4,5 21 96 0,27 53,4 52,79 2-1 37,5 63,8 32,9 10,7 0,6 6 100 0,19 70,4 48,2 2-2 37,5 60,4 38,7 24,3 0,8 4 95 0,15 76,2 61,8 3 42,8 52,0 7,4 6,5 4,4 21 70 0,28 50,2 49,3 3-1 38,8 61,1 36,4 17,8 1,2 17 85 0,12 75,2 56,6 3-2 34,0 61,9 32,9 18,1 1,1 8 91 0,15 66,9 52,1 3-3 30,4 50,0 22,5 15,1 0,8 18 50 0,34 52,9 45,5 3-4 29,1 42,6 22,4 14,9 1,2 21 75 0,30 51,5 43,9 3-5*) 41,9 52,6 7,6 7,1 2,9 22 71 0,26 49,5 49,0 3-6*) 29,0 49,2 26,2 14,2 1,3 21 76 0,33 55,2 43,2 4 48,4 54,1 7,8 7,2 4,0 20 130 0,13 56,2 55,6 4-1 45,0 62,3 37,5 14,4 1,7 3 120 0,07 82,5 59,4 4-2 42,8 60,2 39,3 26,2 1,2 3 127 0,10 82,1 69,0 5 40,7 39,3 7,5 6,6 3,5 26 60 0,09 48,2 47,3 5-1 40,1 53,7 18,8 7,2 2,4 16,1 50 0,14 58,9 47,3 5-2 37,8 55,3 33,0 16,3 0,9 11,2 49 0,18 70,8 54,1 6 64,0 9,7 6,9 6,1 2 29 92 0,18 70,9 70,1 6-1 45,2 53,0 26,9 12,1 1,1 18,3 90 0,13 72,1 57,3 6-2 40,5 56,6 30,3 16,3 0,9 12,0 90 0,11 70,8 56,8 7*) 44,3 63,9 12,1 7,1 4,0 22 62 0,33 56,4 51,4 *) Comparative example

Claims

1. A process for producing water-absorbing polymer particles by polymerizing a monomer solution comprising a) at least one ethylenically unsaturated monomer which bears acid groups and may have been at least partly neutralized, b) optionally one or more crosslinkers, c) at least one initiator, d) optionally one or more ethylenically unsaturated monomers copolymerizable with the monomers mentioned under a) and e) optionally one or more water-soluble polymers, the monomer solution suspended in a hydrophobic organic solvent having a solubility in water at 23°C of less than 5 g / 100 g during the polymerization being agglomerated during or after the polymerization in the hydrophobic organic solvent, and thermally surface postcrosslinking the resultant agglomerated polymer particles by means of an organic surface postcrosslinker, wherein the amount of crosslinker b) is selected such that the agglomerated polymer particles before the surface postcrosslinking have a centrifuge retention capacity of at least 37 g / g and the thermal surface postcrosslinking is conducted at 100 to 190°C.

2. The process according to claim 1, wherein agglomeration is effected in the hydrophobic organic solvent after the polymerization.

3. The process according to claim 1 or 2, wherein the polymerization is conducted in the presence of a chain transfer reagent.

4. The process according to any of claims 1 to 3, wherein the amount of crosslinker b) is selected such that the polymer particles before the surface postcrosslinking have a centrifuge retention capacity of at least 40 g / g.

5. The process according to any of claims 1 to 5, wherein the thermal surface postcrosslinking is conducted at 120 to 170°C.

6. The process according to any of claims 1 to 5, wherein the organic surface postcrosslinker is selected from alkylene carbonates, 2-oxazolidinones, bis- and poly-2-oxazolidinones, 2-oxotetrahydro-1,3-oxazines, N-acyl-2-oxazolidinones, cyclic ureas, bicyclic amido acetals, oxetanes and morpholine-2,3-diones.

7. The process according to any of claims 1 to 6, wherein from 1 to 5% by weight of organic surface postcrosslinker is used, based on the resultant polymer particles.

8. The process according to any of claims 1 to 7, wherein at least one dispersing aid is used in the polymerization.

9. The process according to any of claims 1 to 8, wherein the resultant polymer particles are at least partly dewatered azeotropically after the polymerization.

10. The process according to claim 9, wherein the resultant polymer particles are filtered and dried after the azeotropic dewatering.

11. The process according to any of claims 1 to 10, wherein the thermal surface postcrosslinking is conducted in a mixer with moving mixing tools.

12. A water-absorbing polymer particle obtainable by a process of claims 1 to 11, having a centrifuge retention capacity of at least 37 g / g, an absorption under a pressure of 21.0 g / cm2 of at least 30 g / g, an absorption under a pressure of 49.2 g / cm2 of at least 14 g / g, a sum total of centrifuge retention capacity and absorption under a pressure of 21.0 g / cm2 of at least 69 g / g, a sum total of centrifuge retention capacity and absorption under a pressure of 49.2 g / cm2 of at least 54 g / g, and less than 20% by weight of extractables.

13. The water-absorbing polymer particle according to claim 12, having a centrifuge retention capacity of at least 41 g / g.

14. The water-absorbing polymer particle according to claim 12 or 13, having an absorption under a pressure of 21.0 g / cm2 of at least 34 g / g.

15. The water-absorbing polymer particle according to any of claims 12 to 14, having an absorption under a pressure of 49.2 g / cm2 of at least 20 g / g.

16. The water-absorbing polymer particle according to any of claims 12 to 15, having a sum total of centrifuge retention capacity and absorption under a pressure of 21.0 g / cm2 of at least 74 g / g.

17. The water-absorbing polymer particle according to any of claims 12 to 16, having a sum total of centrifuge retention capacity and absorption under a pressure of 49.2 g / cm2 of at least 59 g / g.

18. The water-absorbing polymer particle according to any of claims 12 to 17, having less than 14% by weight of extractables.

19. The water-absorbing polymer particle according to any of claims 12 to 18, having a bulk density of less than 1.0 g / cm3.

20. The water-absorbing polymer particle according to any of claims 12 to 19, wherein the proportion of particles having a particle size of 300 to 600 µm is at least 30% by weight.

21. A hygiene article comprising (A) an upper liquid-permeable layer, (B) a lower liquid-impermeable layer, (C) a liquid-absorbing storage layer between layer (A) and layer (B), comprising from 0% to 30% by weight of a fibrous material and from 70 to 100% by weight of water-absorbing polymer particles according to any of claims 12 to 20, (D) optionally an acquisition and distribution layer between layer (A) and layer (C), comprising from 80% to 100% by weight of a fibrous material and from 0 to 20% by weight of water-absorbing polymer particles according to any of claims 12 to 20, (E) optionally a fabric layer directly above and / or beneath layer (C) and (F) further optional components.