Method for producing superabsorbent polymer particles

Abstract

The present invention relates to a method for producing superabsorbent polymer particles, comprising the steps of polymerizing a monomer solution, drying the formed polymer gel, grinding the dried polymer gel, classifying and thermally surface postcrosslinking the polymer particles, wherein the monomer solution contains a chelating agent and an aluminum salt.

Description

Technical Field

[0001] The present invention relates to a method for producing superabsorbent polymer particles, which includes steps of polymerizing a monomer solution, drying the formed polymer gel, pulverizing the dried polymer gel, classifying the polymer particles and thermally post-crosslinking the surface, and the monomer solution contains a chelating agent and an aluminum cation.

Background Art

[0002] Superabsorbent polymer particles are also used for manufacturing diapers, tampons, sanitary napkins, and other hygiene products, as well as water retention agents in horticulture. Superabsorbent polymer particles are often referred to as "absorbent resin", "superabsorbent", "superabsorbent polymer", "absorbent polymer", "absorbent gelling material", "hydrophilic polymer", or "hydrogel".

[0003] The production of superabsorbent polymer particles is described in the monograph "Modern Superabsorbent Polymer Technology", F.L. Buchholz and A.T. Graham, Wiley-VCH, 1998, pages 71 to 103.

[0004] WO2008 / 009599A1 discloses the use of an aluminum salt as a permeability promoter, and EP3264485A1 discloses the use of an aluminum salt as a foam promoter.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0006] [Non-Patent Document 1] "Modern Superabsorbent Polymer Technology", FL Buchholz and AT Graham, Wiley-VCH, 1998, pp. 71-103 [Overview of the project]

[0007] The object of the present invention was to provide a method for producing superabsorbent polymer particles having improved properties, particularly a high absorption rate.

[0008] The above objective is a method for producing superabsorbent polymer particles, a) At least one ethylenically unsaturated monomer having an acid group and which may be at least partially neutralized, b) at least one crosslinking agent, c) at least one initiator, d) Depending on the case, one or more ethylenically unsaturated monomers copolymerizable with the monomers described in a), and e) Depending on the case, one or more water-soluble polymers This was achieved by a method comprising the steps of polymerizing a monomer solution containing a chelating agent, drying the formed polymer gel, pulverizing the dried polymer gel, and separating the polymer particles and thermally post-crosslinking the surface, wherein the monomer solution further contains 0.001 to 1.00 mol%, preferably 0.005 to 0.75 mol%, more preferably 0.010 to 0.50 mol%, most preferably 0.050 to 0.25 mol%, of monomer a) as a chelating agent and 0.0001 to 0.100 mol%, preferably 0.0005 to 0.075 mol%, more preferably 0.001 to 0.050 mol%, most preferably 0.005 to 0.025 mol%, of aluminum cations, respectively. [Modes for carrying out the invention]

[0009] Suitable chelating agents are aminocarboxylic acids, such as methyliminodiacetic acid, methylglycinediacetic acid, nitrilotriacetic acid, diethylenetriaminepentaacetic acid, (hydroxyethyl)-ethylenediaminetriacetic acid, and ethylenediaminetetraacetic acid, or salts thereof.

[0010] All water-soluble aluminum salts, such as aluminum sulfate, aluminum chloride, aluminum monoacetate, and aluminum trilactate, can be used as aluminum cation sources in monomer solutions. Aluminum trilactate is preferred.

[0011] This invention is based on the discovery that the absorption rate can be improved by adding a chelating agent and an aluminum cation to a monomer solution. Adding a chelating agent alone or an aluminum cation alone has no effect on the absorption rate or has the opposite effect.

[0012] Based on the polymer particles, aluminum hydroxide is preferably added to the polymer particles in an amount of 0.01 to 2.0% by weight, more preferably 0.05 to 1.2% by weight, and most preferably 0.15 to 0.6% by weight, before, during, or after thermal post-crosslinking of the surface. X-ray amorphous aluminum hydroxide as described in WO2019 / 197194A1 is preferred.

[0013] The aluminum hydroxide on the surface of the superabsorbent polymer particles improves permeability.

[0014] Amounting aluminum cations in an aqueous solution are added to the polymer particles, preferably at a concentration of 0.001 to 0.15 mol / kg, more preferably 0.003 to 0.10 mol / kg, and most preferably 0.005 to 0.05 mol / kg, based on the polymer particles, before, during, or after thermal post-crosslinking of the surface.

[0015] All water-soluble aluminum salts, such as aluminum sulfate, aluminum chloride, aluminum monoacetate, and aluminum trilactate, can be used as aluminum cation sources added to polymer particles. Aluminum trilactate is preferred.

[0016] The present invention further provides superabsorbent polymer particles obtained by the method of the present invention.

[0017] The production of superabsorbent polymer particles is described in detail below.

[0018] Superabsorbent polymer particles are typically water-insoluble but swellable.

[0019] Monomer a) is preferably water-soluble, i.e., typically having a solubility in water at 23 °C of at least 1 g / 100 g of water, preferably at least 5 g / 100 g of water, more preferably at least 25 g / 100 g of water, and most preferably at least 35 g / 100 g of water.

[0020] Suitable monomers a) are, for example, ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid, and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Most particularly preferred is acrylic acid.

[0021] More suitable monomers a) are, for example, ethylenically unsaturated sulfonic acids such as vinylsulfonic acid, styrenesulfonic acid, and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

[0022] Impurities can strongly affect the polymerization. Particularly purified monomer a) is preferred. Useful purification methods are disclosed in WO2002 / 055469A1, WO2003 / 078378A1, and WO2004 / 035514A1. A suitable monomer a) is a purified acrylic acid having 99.8460% by weight of acrylic acid, 0.0950% by weight of acetic acid, 0.03% by weight of water, 0.0203% by weight of propionic acid, 0.0001% by weight of furfural, 0.0001% by weight of maleic anhydride, 0.0003% by weight of diacrylic acid, and 0.0050% by weight of hydroquinone monomethyl ether according to WO2004 / 035514A1.

[0023] The content of acrylic acid and / or its salts in the total amount of monomer a) is preferably at least 50 mol%, more preferably at least 90 mol%, and most preferably at least 95 mol%.

[0024] Monomer a) typically contains a polymerization inhibitor, preferably hydroquinone monoether, as an inhibitor for storage.

[0025] The monomer solution preferably contains hydroquinone monoether at most 250 weight ppm, preferably at most 130 weight ppm, more preferably at most 70 weight ppm, preferably at least 10 weight ppm, more preferably at least 30 weight ppm, and particularly around 50 weight ppm, based on non-neutralized acrylic acid in each case. For example, the monomer solution can be prepared by using acrylic acid together with an appropriate content of hydroquinone monoether.

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

[0027] The crosslinking agent b) is preferably a compound having at least two polymerizable groups that can be polymerized into a polymer network by free radicals. Suitable crosslinking agents b) include, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, EP0547847A1, EP0559476A1, EP0632068A1, WO93 / 21237A1, and WO2003 / 104299A. 1. Di- and triacrylates as described in WO2003 / 104300A1, WO2003 / 104301A1, and DE10331450A1, mixed acrylates containing further ethylenically unsaturated groups along with acrylate groups as described in DE10331456A1 and DE10355401A1, or mixtures of crosslinking agents as described, for example, DE19543368A1, DE19646484A1, WO90 / 15830A1, and WO2002 / 032962A2.

[0028] Preferred crosslinking agents b) are pentaerythrityl trialyl ether, tetraallyloxyethane, methylene bismethacrylamide, 15 (tuply) ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate, and triallylamine.

[0029] A very particularly preferred crosslinking agent b) is polyethoxylated and / or polypropoxylated glycerol, esterified with acrylic acid or methacrylic acid to give a di- or triacrylate, as described, for example, in WO2003 / 104301A1. Di- and / or triacrylates of 3 to 10 ethoxylated glycerols are particularly advantageous. Di- or triacrylates of 1 to 5 ethoxylated and / or propoxylated glycerols are very particularly preferred. Most preferred are triacrylates of 3 to 5 ethoxylated and / or propoxylated glycerols, especially triacrylates of 3 ethoxylated glycerols.

[0030] The amount of crosslinking agent b) is preferably 0.05 to 1.5% by weight, more preferably 0.1 to 1% by weight, and most preferably 0.3 to 0.6% by weight, based on the amount of unneutralized acrylic acid in each case. As the crosslinking agent content increases, the centrifugal retention capacity (CRC) decreases to 21.0 g / cm³. 2 The pressure absorption capacity passes through the maximum value.

[0031] The initiator c) used may be any compound that generates free radicals under polymerization conditions, such as a thermal initiator, a redox initiator, or a photoinitiator. Preferred redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. A mixture of a thermal initiator and a redox initiator, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid, is preferred. However, the reducing component used is preferably disodium 2-hydroxy-2-sulfonatoacetate, or a mixture of disodium 2-hydroxy-2-sulfinatoacetate, disodium 2-hydroxy-2-sulfonatoacetate, and sodium bisulfite. Such mixtures are available as Brueggolite® FF6 and Brueggolite® FF7 (Brueggemann Chemicals (Heilbronn; Germany)).

[0032] Examples of ethylenically unsaturated monomers copolymerizable with acrylic acid include acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacrylate, and diethylaminoethyl methacrylate.

[0033] The water-soluble polymer used (e) may be polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, modified cellulose, such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycol, or polyacrylic acid, and preferably starch, starch derivatives, and modified cellulose.

[0034] Typically, an aqueous monomer solution is used. The water content of the monomer solution is preferably 40-75% by weight, more preferably 45-70% by weight, and most preferably 50-65% by weight. A monomer suspension, i.e., a monomer solution containing excess sodium acrylate, can also be used. As the water content increases, the energy requirement for subsequent drying increases, and as the water content decreases, the heat of polymerization may not be removed sufficiently.

[0035] For optimal action, preferred polymerization inhibitors require dissolved oxygen. Therefore, the monomer solution may have its dissolved oxygen removed before polymerization by inactivation, i.e., by flowing an inert gas, preferably nitrogen or carbon dioxide. The oxygen content of the monomer solution is preferably reduced to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, and most preferably to less than 0.1 ppm by weight, before polymerization.

[0036] Polymerization of the monomer solution is performed. Suitable reactors are, for example, a kneading reactor or a belt reactor. In a kneading reactor, the polymer gel formed in the polymerization of the aqueous monomer solution or suspension is continuously pulverized by an inverting stirring shaft, for example, as described in WO2001 / 038402A1. Polymerization on a belt is described, for example, in DE3825366A1 and U.S. Patent No. 6,241,928. Polymerization in a belt reactor forms a polymer gel, which must be pulverized in a further process step, for example, in an extruder or kneading reactor.

[0037] To improve drying properties, the pulverized polymer gel obtained by the kneader can be further extruded.

[0038] The acidic groups of the resulting polymer gel are typically partially neutralized. Neutralization is preferably carried out at the monomer stage. This is typically achieved by mixing with a neutralizing agent, either as an aqueous solution or, preferably, as a solid. The degree of neutralization is preferably 50-85 mol%, more preferably 60-80 mol%, and most preferably 65-75 mol%, and conventional neutralizing agents, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates, or alkali metal bicarbonates, and mixtures thereof, can also be used for neutralization. Ammonium salts can also be used instead of alkali metal salts. Particularly preferred alkali metals are sodium and potassium, but sodium hydroxide, potassium hydroxide, and mixtures thereof are also very particularly preferred.

[0039] The resulting polymer gel is dried. The drying oven is not limited in any way. However, drying of the polymer gel is preferably carried out by a belt dryer until the residual moisture content is preferably 0.5 to 10% by weight, more preferably 1 to 7% by weight, and most preferably 2 to 5% by weight, and the residual moisture content is determined by EDANA recommended test method No. WSP230.2(05) "Mass Loss Upon Heating". If the residual moisture content is too high, the dried polymer gel will have a glass transition temperature T that is too low.g It may have further processing, but this is only difficult. If the residual moisture content is too low, the dried polymer gel becomes too brittle, and in the subsequent grinding step, an undesirable large amount of polymer particles (fine powder) with an excessively small particle size is obtained. The solid content of the gel before drying is preferably 25-90% by weight, more preferably 35-70% by weight, and most preferably 40-60% by weight. However, a fluidized bed dryer or a paddle dryer may also be used for drying purposes if necessary.

[0040] Next, the dried polymer gel is pulverized and sorted. The equipment used for pulverization is typically a single-stage or multi-stage roll mill, preferably a two-stage or three-stage roll mill, a pin mill, a hammer mill, or a vibratory mill.

[0041] The average particle size of the polymer particles extracted as the product fraction is preferably at least 200 μm, more preferably 250-600 μm, and very specifically 300-500 μm. The average particle size of the product fraction can be determined by EDANA recommended test method No. WSP220.2(05) "Particle Size Distribution," but the average particle size was determined by plotting the percentage by mass of the sieved fraction in cumulative form. The average particle size here is the mesh size value that produces a cumulative 50% by weight.

[0042] The proportion of particles having a particle size of at least 150 μm is preferably at least 90% by weight, more preferably at least 95% by weight, and most preferably at least 98% by weight.

[0043] Polymer particles with excessively small particle sizes reduce saline flow conductivity (SFC). Therefore, the proportion of excessively small polymer particles ("fine powders") should be reduced.

[0044] Therefore, excessively small polymer particles are typically removed and reused in the process. This is preferably done before, during, or immediately after polymerization, i.e., before the polymer gel dries. Excessively small polymer particles can be moistened with water and / or an aqueous surfactant before or during reuse.

[0045] In a later process step, for example, after a post-crosslinking surface or another coating step, the excessively small polymer particles can be removed. In this case, the reused excessively small polymer particles are post-crosslinked or coated by another method, for example, fumed silica.

[0046] When using a kneading reactor for polymerization, excessively small polymer particles are preferably added to the last third of the polymerization.

[0047] If excessively small polymer particles are added to the monomer solution at a very early stage, for example, the centrifugal retention capacity (CRC) of the resulting superabsorbent polymer particles decreases. However, this can be compensated for, for example, by adjusting the amount of crosslinking agent b) used.

[0048] If excessively small polymer particles are added at a very late stage, for example, not until they reach an instrument connected downstream of the polymerization reactor, such as an extruder, then while the excessively small polymer particles can be incorporated into the resulting polymer gel, it is only difficult. However, insufficiently incorporated excessively small polymer particles will detach again from the dried polymer gel during grinding, thus increasing the amount of excessively small polymer particles that can be removed and reused during the sorting process.

[0049] The proportion of particles having a maximum particle size of 850 μm is preferably at least 90% by weight, more preferably at least 95% by weight, and most preferably at least 98% by weight.

[0050] The proportion of particles having a maximum particle size of 600 μm is preferably at least 90% by weight, more preferably at least 95% by weight, and most preferably at least 98% by weight.

[0051] Excessively large polymer particles reduce the free swelling rate (FSR). Therefore, the proportion of excessively large polymer particles should also be kept low.

[0052] Therefore, excessively large polymer particles are typically removed and reused for grinding the dried polymer gel.

[0053] To improve the properties, the polymer particles are subsequently thermally post-crosslinked. Suitable post-crosslinking agents are compounds containing groups that can form covalent bonds with at least two acid groups of the polymer particles. Suitable compounds include, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides described in EP0083022A2, EP0543303A1, and EP0937736A2, difunctional or polyfunctional alcohols described in DE3314019A1, DE3523617A1, and EP0450922A2, or β-hydroxyalkylamides described in DE10204938A1 and U.S. Patent No. 6,239,230.

[0054] The following are also listed as suitable post-crosslinking agents: cyclic carbonates in DE4020780C1, 2-oxazolidinones and their derivatives in DE19807502A1, such as 2-hydroxyethyl-2-oxazolidinone, bis- and poly-2-oxazolidinones in DE19807992C1, 2-oxotetrahydro-1,3-oxazines and their derivatives in DE19854573A1, N-acyl-2-oxazolidinone in DE19854574A1, cyclic ureas in DE10204937A1, bicyclic amide acetals in DE10334584A1, oxetanes and cyclic ureas in EP1199327A2, and morpholine-2,3-diones and their derivatives in WO2003 / 031482A1.

[0055] Preferred post-crosslinking agents include ethylene carbonate, ethylene glycol diglycidyl ether, reaction products of polyamide and epichlorohydrin, and mixtures of propylene glycol and 1,4-butanediol.

[0056] Particularly preferred post-crosslinking agents are 2-hydroxyethyl oxazolidine-2-one, oxazolidine-2-one, and 1,3-propanediol.

[0057] In addition, a post-crosslinking agent containing additional polymerizable ethylenically unsaturated groups, as described in DE3713601A1, can also be used.

[0058] The amount of post-crosslinking agent is preferably 0.001 to 2% by weight, more preferably 0.02 to 1% by weight, and most preferably 0.05 to 0.2% by weight, based on the polymer particles in each case.

[0059] In a preferred embodiment of the present invention, the aluminum cation is applied to the particle surface in addition to the post-crosslinking agent, either before, during, or after post-crosslinking.

[0060] Post-surface crosslinking is typically carried out by spraying a solution of the post-surface crosslinking agent onto dry polymer particles. After spraying, the polymer particles coated with the post-surface crosslinking agent are dried by heat, but the post-surface crosslinking reaction can occur either before or during drying.

[0061] The spray application of the post-crosslinking agent solution is preferably carried out using a mixer with a movable mixing device, such as a screw mixer, a disc mixer, and a paddle mixer. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are very particularly preferred. The distinction between horizontal and vertical mixers is made by the position of the mixing shaft, i.e., horizontal mixers have a horizontally mounted mixing shaft, and vertical mixers have a vertically mounted mixing shaft. Suitable mixers include, for example, the horizontal Pflugschar® mixer (Gebr. Loedige Maschinenbau GmbH; Paderborn; Germany), the Vrieco-Nauta continuous mixer (Hosokawa Micron BV; Dootinghem; Netherlands), the Processall Mixmill mixer (Processall Incorporated; Cincinnati; USA), and the Schugi Flexomix® mixer (Hosokawa Micron BV; Dootinghem; Netherlands). However, it is also possible to spray a post-crosslinking agent solution onto a fluidized bed.

[0062] Post-crosslinking agents are typically used in aqueous solution form. The penetration depth of the post-crosslinking agent into polymer particles can be adjusted by the content and total amount of the non-aqueous solvent.

[0063] When water alone is used as the solvent, it is advantageous to add a surfactant. This improves the wetting behavior and reduces the tendency to form clumps. However, it is preferable to use a solvent mixture, such as isopropanol / water, 1,3-propanediol / water, and propylene glycol / water, with a preferred mass mixing ratio of 20:80 to 40:60.

[0064] Thermal post-crosslinking of the surface is preferably carried out in a contact dryer, more preferably a paddle dryer, and most preferably a disc dryer. Suitable dryers include, for example, the Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Rheingarten; Germany), the Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Rheingarten; Germany), and the Nara Paddle Dryer (NARA Machinery Europe; Frächen; Germany). Furthermore, a fluidized bed dryer may also be used.

[0065] Thermal post-crosslinking can be performed within the mixer itself by heating the jacket or blowing in warm air. Downflow dryers, such as rack dryers, rotary tubular furnaces, or heatable screws, are equally suitable. Mixing and drying in a fluidized bed dryer is particularly advantageous.

[0066] The preferred post-surface crosslinking temperature is in the range of 100 to 250°C, preferably 120 to 220°C, more preferably 130 to 210°C, and most preferably 150 to 200°C. In the reaction mixer or dryer, the preferred residence time at this temperature is preferably at least 10 minutes, more preferably at least 20 minutes, most preferably at least 30 minutes, and typically up to 60 minutes.

[0067] Next, the surface-crosslinked polymer particles can be sorted again, and excessively small and / or excessively large polymer particles are removed and reused in the process.

[0068] To further improve the properties, the surface-crosslinked polymer particles can be coated or re-humidified.

[0069] Re-humidification is preferably carried out at 30-80°C, more preferably at 35-70°C, and most preferably at 40-60°C. At excessively low temperatures, the superabsorbent polymer particles tend to form clumps, and at high temperatures, the water has already visibly evaporated. The amount of water used for re-humidification is preferably 1-10% by weight, more preferably 2-8% by weight, and most preferably 3-5% by weight. Re-humidification increases the mechanical stability of the polymer particles and reduces their tendency to become electrostatically charged.

[0070] Suitable coatings for improving free swelling rate and saline flow induction (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 for suppressing the undesirable caking tendency of polymer particles include, for example, fumed silica such as Aerosil® 200, and surfactants such as Span® 20.

[0071] method Unless otherwise specified, measurements shall be performed at an ambient temperature of 23±2°C and a relative atmospheric humidity of 50±10%. Superabsorbent polymer particles shall be thoroughly mixed before measurement.

[0072] Centrifuge retention capacity (CRC) The centrifugal retention capacity of superabsorbent polymer particles is determined by the EDANA recommended test method No. WSP241.2(05) "Gravimetric Determination of Fluid Retention Capacity in Saline Solution After Centrifugation," but for large centrifugal retention capacities, a large tea bag must be used.

[0073] Vortex Weigh 1.00 g of dry superabsorbent polymer particles into a 25 ml glass beaker and spread them evenly on the bottom of the beaker. Next, dispense 20 ml of 0.9 wt% sodium chloride solution into a second glass beaker, quickly add the contents of this beaker to the first beaker, and start the stopwatch. Stop the stopwatch as soon as you confirm that the last drop of salt solution has been absorbed by the disappearance of reflection on the liquid surface.

[0074] Gel bed permeability (GBP) The gel bed permeability (GBP) of the swollen gel layer under a pressure of 0.3 psi (2070 Pa) is defined in US2005 / 0256757 (paragraph

[0061] and

[0075] As described in ( ), it was measured as the gel bed permeability of the swollen gel layer of superabsorbent polymer particles. [Examples]

[0075] Example 1 First, 284.46 g of 50 wt% aqueous sodium hydroxide and 558.44 g of deionized water were added to a 2 L stainless steel container. The mixture was cooled to 15°C using a cooling bath. Next, 251.21 g of the first part of acrylic acid was added while stirring. The rate of addition was adjusted so that the temperature would not exceed 30°C. The mixture was kept at approximately 30°C for 2 minutes after the addition.

[0076] Subsequently, the mixture was kept at a temperature below 30°C and, while stirring, an additional 90.43 g of the second part of acrylic acid was added. The degree of neutralization was 75 mol%. After the addition of the second part of acrylic acid, the mixture was cooled to 20°C and, while stirring, 0.55 g of 17.5 ethoxylated trimethylolpropane triacrylate was added. Then, while stirring, 0.036 g of 2-hydroxy-2-methylpropiophenone and 0.012 g of 2,2-dimethoxy-1,2-diphenylethane-1-one were added and the mixture was cooled to 18°C. Oxygen was removed from the mixture by passing nitrogen through glass frit while the mixture was cooled to 5°C.

[0077] Next, 5.12 g of aqueous disodium 2-hydroxy-2-sulfonatoacetate (as a 5 wt% aqueous solution), 0.68 g of aqueous aluminum lactate (as a 22 wt% aqueous solution), 3.42 g of aqueous trisodium methylglycine diacetate (available as a 40 wt% solution under Trilon® M Liquid (BASF SE, Ludwigshafen, Germany)), and 5.64 g of aqueous sodium persulfate (as a 10 wt% aqueous solution) were sequentially added to the monomer solution. The monomer solution was transferred to a glass dish. The dimensions of the glass dish were such that a 5 cm thick layer of monomer solution would be constructed.

[0078] A glass dish containing a monomer solution is placed under a UV lamp (UV intensity = 25 mW / cm²). 2 The mixture was polymerized by placing it under a UV lamp for 11.5 minutes, followed by 4.5 minutes with the UV lamp turned off (total reaction time 16 minutes). The resulting polymer gel was pulverized using a commercially available meat grinder with a 6 mm perforated disc. 3.42 g of aqueous sodium metabisulfite (as a 5 wt% aqueous solution) was sprayed onto the pulverized gel, and the gel was passed through the meat grinder two more times. The resulting polymer gel was dried in a laboratory drying cabinet at 180°C for 60 minutes. The product was then pulverized using an ultracentrifuge mill (Retsch Model ZM100 with a 12-tooth rotor and a 1.5 mm annular sieve, at a speed of 14000 rpm, manufactured by Retsch GmbH (Hahn, Germany)), and the milled product was sieved to obtain sieve fractions of 150–710 μm.

[0079] 1200 g of polymer particles were mixed with 3.6 aluminum hydroxide (as powder) for 2 minutes in a laboratory Plowscher mixer (Model MR5, manufactured by Gebruder Lodige Maschinenbau GmbH (Paderborn, Germany)).

[0080] Next, solution A was prepared containing 0.36 g of ethylene glycol diglycidyl ether, 24 g of 1,2-propanediol, and 36 g of deionized water, and solution B was prepared containing 24 g of aqueous aluminum lactate (as a 22 wt% aqueous solution). The aqueous solutions (solutions A and B) were sprayed onto the polymer particles using a spray nozzle at a mixer speed of 200 rpm within 1 minute. The coated polymer particles were heated at 163°C for 120 minutes. The polymer particles were then cooled to 70°C. 32.5 g of deionized water was added to the polymer particles. The polymer particles were mixed for 5 minutes. The polymer particles were then removed from the mixer and finally sieved (150-710 μm fraction).

[0081] Example 2 Example 1 was repeated without adding aluminum trihydrate to the polymer particles.

[0082] Example 3 (Comparison) Example 2 was repeated without adding aluminum lactate to the monomer solution.

[0083] Example 4 (Comparison) Example 2 was repeated without adding trisodium methylglycine diacetate to the monomer solution.

[0084] Example 5 (Comparison) Example 2 was repeated without adding aluminum lactate to the monomer solution and without adding trisodium methylglycine diacetate to the monomer solution.

[0085] [Table 1]

Claims

1. A method for producing superabsorbent polymer particles, a) At least one ethylenically unsaturated monomer having an acid group and which may be at least partially neutralized, b) at least one crosslinking agent having at least two polymerizable groups, and c) At least one initiator A method comprising the steps of polymerizing a monomer solution containing a), drying the formed polymer gel, pulverizing the dried polymer gel, and separating the polymer particles and thermally post-crosslinking the surface, wherein the monomer solution further contains 0.001 to 1.00 mol% of an aminocarboxylic acid or a salt thereof and 0.0001 to 0.100 mol% of an aluminum cation, based on monomer a).

2. The method according to claim 1, wherein the monomer solution contains 0.050 to 0.25 mol% of an aminocarboxylic acid or a salt thereof based on monomer a).

3. The method according to claim 1 or 2, wherein the monomer solution contains 0.005 to 0.025 mol% of aluminum cations based on monomer a).

4. The method according to any one of claims 1 to 3, wherein the aminocarboxylic acid or salt thereof in the monomer solution is trisodium methylglycine diacetate.

5. The method according to any one of claims 1 to 4, wherein the aluminum cation source in the monomer solution is aluminum trilactate.

6. The method according to any one of claims 1 to 5, wherein 0.01 to 2.0% by weight of aluminum hydroxide, based on the polymer particles, is added to the polymer particles before, during, or after thermal post-crosslinking of the surface.

7. The method according to any one of claims 1 to 6, wherein 0.15 to 0.6% by weight of aluminum hydroxide, based on the polymer particles, is added to the polymer particles before, during, or after thermal post-crosslinking of the surface.

8. The method according to any one of claims 1 to 7, wherein 0.001 to 0.15 mol / kg of aluminum cations, based on polymer particles, is added to the polymer particles as an aqueous solution before, during, or after thermal post-crosslinking of the surface.

9. The method according to any one of claims 1 to 8, wherein 0.005 to 0.05 mol / kg of aluminum cations, based on polymer particles, is added to the polymer particles as an aqueous solution before, during, or after thermal post-crosslinking of the surface.

10. The method according to claim 8 or 9, wherein the aluminum cation source added to the polymer particles before, during, or after thermal post-crosslinking of the surface is aluminum trilactate.

11. Superabsorbent polymer particles containing an aminocarboxylic acid or a salt thereof and an aluminum salt, obtained according to the method described in any one of claims 1 to 10.

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