Process for producing color-stable superabsorbent particles

Abstract

The present invention relates to a process for producing color-stable superabsorbent particles, in which an aqueous monomer solution or suspension is polymerized to form a polymer gel, the resulting polymer gel is optionally pulverized, the polymer gel is then dried, the dried polymer gel is optionally ground and classified, and the dried polymer gel is then thermally surface crosslinked and cooled, the process comprising coating with a pyrazole after polymerization.

Description

[Technical Field]

[0001] The present invention relates to a process for producing color-stable superabsorbent particles, in which an aqueous monomer solution or suspension is polymerized to form a polymer gel, the resulting polymer gel is optionally pulverized, the polymer gel is then dried, the dried polymer gel is optionally ground and classified, and the dried polymer gel is then thermally surface crosslinked and cooled, the process comprising coating with a pyrazole after polymerization. [Background technology]

[0002] Superabsorbents are used to make diapers, tampons, sanitary napkins and other hygiene articles, but are also used as moisture-retaining agents in commercial horticulture. Superabsorbents are also called water-absorbing polymers.

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

[0004] Performance characteristics such as gel bed permeability (GBP) and 49.2 g / cm 2 It is common to surface postcrosslink superabsorbent particles to improve their absorbency under a pressure of 49.2 g / cm (AUL 0.7 psi). This increases the level of crosslinking on the particle surface, resulting in a 2 The absorbency at a pressure of 0.7 psi (AUL) and the centrifuge retention capacity (CRC) can be at least partially separated. This surface postcrosslinking can be carried out in an aqueous gel phase. However, it is preferred to surface-coat dried, ground, and sieved polymer particles (base polymer) with a surface postcrosslinking agent and then thermally postcrosslink them. Suitable crosslinking agents for this purpose are compounds capable of forming covalent bonds with at least two carboxylate groups of the polymer particles.

[0005] WO 2021 / 105038 A1 and the earlier PCT application with reference number PCT / EP2022 / 059572 disclose pyrazoles as polymerization inhibitors. Summary of the Invention [Problem to be solved by the invention]

[0006] It was an object of the present invention to provide an improved process for producing color-stable superabsorbent particles. [Means for solving the problem]

[0007] The purpose of this is to a) at least one ethylenically unsaturated monomer having an acid group and which is at least partially neutralized; b) at least one cross-linking agent; c) at least one initiator; wherein the aqueous monomer solution or suspension is polymerized to form a polymer gel, the resulting polymer gel is optionally pulverized, the polymer gel is then dried, the dried polymer gel is optionally ground and classified, and the dried polymer gel is then thermally surface postcrosslinked and cooled, the process being achieved by coating the resulting polymer particles with at least one pyrazole before, during or after the thermal surface postcrosslinking. DETAILED DESCRIPTION OF THE INVENTION

[0008] The pyrazole is typically a monomeric pyrazole.

[0009] The pyrazoles used in the process of the present invention preferably have the general formula (I) [ka] (In the formula, R 1 is C1 or C2-alkyl, R 2 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl, and R 3 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl) or a compound of general formula (II) [ka] (In the formula, R 4 is C1 or C2-alkyl, R 5 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl, and R 6 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl) or a compound of general formula (III) [ka] (In the formula, R 7 is C1 or C2-alkyl, R 8 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl, and R 9 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl) is a compound of

[0010] The alkyl group can be linear, branched and / or cyclic.

[0011] The pyrazoles used in the process of the present invention are more preferably of the general formula (I) [ka] (In the formula, R 1 is C1-alkyl, R 2 is C1-C3-alkyl or C6-C8-arylalkyl, and R 3 is H, C1-C3-alkyl or C6-C8-arylalkyl or a compound of general formula (II) [ka] (In the formula, R 4 is C1-alkyl, R 5 is C1-C3-alkyl or C6-C8-arylalkyl, and R 6 is H, C1-C3-alkyl or C6-C8-arylalkyl or a compound of general formula (III) [ka] (In the formula, R 7 is C1-alkyl, R 8 is H, C1-C3-alkyl or C6-C8-arylalkyl, and R 9 is H, C1-C3-alkyl or C6-C8-arylalkyl is a compound of

[0012] The alkyl group can be linear, branched and / or cyclic.

[0013] Compounds of general formula (I) exist in equilibrium with their keto forms, for example 1,3-dimethyl-5-pyrazolone is the keto form of 1,3-dimethyl-5-hydroxypyrazole.

[0014] The pyrazole used in the process of the present invention is most preferably 1,3-dimethyl-5-pyrazolone, 1,5-dimethyl-3-ethyl-4-hydroxypyrazole or 1,5-dimethyl-4-hydroxy-3-phenylpyrazole.

[0015] The polymer particles are preferably coated with 0.001% to 1% by weight, more preferably 0.005% to 0.2% by weight, most preferably 0.01% to 0.1% by weight of pyrazole, in each case based on the polymer particle.

[0016] The present invention is based on the finding that pyrazoles significantly improve the color stability of superabsorbents.

[0017] Acrylic acid is the preferred ethylenically unsaturated carboxylic acid. Peroxodisulfates, in particular ammonium peroxodisulfate, sodium peroxodisulfate and / or potassium peroxodisulfate, are preferred initiators c).

[0018] The manufacture of superabsorbents is described in detail below.

[0019] Superabsorbents are made by polymerizing a monomer solution and are typically water-insoluble.

[0020] The ethylenically unsaturated monomers a) having acid groups are preferably water-soluble, i.e. their solubility in water at 23°C is typically at least 1 g / 100 g water, preferably at least 5 g / 100 g water, more preferably at least 25 g / 100 g water and most preferably at least 35 g / 100 g water.

[0021] Suitable monomers are, for example, ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Very particularly preferred is acrylic acid.

[0022] The ethylenically unsaturated monomer a) having an acid group is typically partially neutralized. Neutralization is carried out at the monomer stage. Neutralization is typically carried out by mixing a neutralizing agent as an aqueous solution or, preferably, as a solid. The degree of neutralization is preferably 40 to 85 mol %, more preferably 50 to 80 mol %, and most preferably 60 to 75 mol %. Customary neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal bicarbonates, and also mixtures thereof. Instead of alkali metal salts, it is also possible to use ammonium salts. Particularly preferred alkali metals are sodium and potassium, but sodium hydroxide, sodium carbonate or sodium bicarbonate, and also mixtures thereof, in particular sodium hydroxide, are very particularly preferred.

[0023] The monomer typically contains a polymerization inhibitor, preferably a hydroquinone monoether, as a storage stabilizer.

[0024] Suitable crosslinking agent b) is a compound having at least two groups suitable for crosslinking.Such groups are, for example, a functional group that can form a covalent bond with an ethylenically unsaturated group capable of free radical polymerization in the polymer chain and the acid group of the monomer.In addition, polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer are also suitable as crosslinking agents.

[0025] Suitable crosslinkers b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described, for example, in EP 0 530 438 A1, EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93 / 21237 A1, WO 03 / 104299 A1, WO 03 / 104300 A1, WO 03 / 104301 A1 and DE 103 31 450 A1. A1, mixed acrylates which contain further ethylenically unsaturated groups as well as acrylate groups, such as those described in DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinker mixtures, such as those described in DE 195 43 368 A1, DE 196 46 484 A1, WO 90 / 15830 A1 and WO 02 / 032962 A2.

[0026] The amount of crosslinker b) is preferably 0.05% to 1.5% by weight, more preferably 0.1% to 1% by weight, and most preferably 0.15% to 0.6% by weight, calculated in each case based on the total amount of monomers used. As the crosslinker content increases, the centrifuge retention capacity (CRC) decreases, reaching 21.0 g / cm. 2 The absorbency under pressure (AUL 0.3 psi) exceeds the maximum value.

[0027] The initiator c) used can be any compound that generates free radicals under polymerization conditions, such as a thermal initiator, a redox initiator, or a photoinitiator. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. Mixtures of thermal and redox initiators, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid, are preferably used. The reducing component used is preferably the disodium salt of 2-hydroxy-2-sulfonatoacetic acid or a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid, and sodium bisulfite. Such mixtures are available as Brueggolite® FF6 and Brueggolite® FF7 (Brueggemann Chemicals; Heilbronn; Germany).

[0028] The water content of the monomer solution is preferably 40% to 75% by weight, more preferably 45% to 70% by weight, and most preferably 50% to 65% by weight. If the water content is high, the energy consumption in the subsequent drying will increase, and if the water content is low, the heat of polymerization may simply not be sufficiently removed.

[0029] The temperature of the monomer solution is preferably 10 to 90°C, particularly preferably 20 to 70°C, very particularly preferably 30 to 50°C.

[0030] To function optimally, preferred polymerization inhibitors require dissolved oxygen. Therefore, the monomer solution may be inerted, i.e., flushed with an inert gas, preferably nitrogen or carbon dioxide, to remove dissolved oxygen before polymerization. Preferably, the oxygen content of the monomer solution is reduced to less than 1 ppm by weight, more preferably less than 0.5 ppm by weight, and most preferably less than 0.1 ppm by weight before polymerization.

[0031] Suitable reactors for polymerization are, for example, kneader reactors or belt reactors. In kneaders, the polymer gel formed during the polymerization of aqueous monomer solutions or suspensions is continuously comminuted, for example, by counter-rotating stirrer shafts, as described in WO 2001 / 038402 A1. Belt polymerization is described, for example, in DE 38 25 366 A1 and U.S. Pat. No. 6,241,928. Polymerization in a belt reactor forms a polymer gel, which must be comminuted, for example, in an extruder or kneader.

[0032] To improve the drying properties, the ground polymer gel obtained by the kneader can be further extruded.

[0033] The polymer gel is then dried, typically by an air-circulating belt dryer, until the residual moisture content is preferably 0.5-10 wt%, more preferably 1-7 wt%, and most preferably 2-5 wt%, as measured by EDANA recommended test method No. WSP 230.2-05 "Mass Loss on Heating." If the residual moisture content is too high, the glass transition temperature T of the dried polymer gel may be increased. g If the residual moisture content is too low, the dried polymer gel may become too brittle, and the subsequent grinding step may result in an unnecessarily large amount of excessively small polymer particles ("fines"). The solids content of the polymer gel before drying is preferably 25% to 90% by weight, more preferably 35% to 70% by weight, and most preferably 40% to 60% by weight. The dried polymer gel is then ground, and optionally coarsely ground.

[0034] The dried polymer gel is then typically milled and classified, and the equipment used for milling can typically be a single or multi-roll mill, preferably a two or three roll mill, a pin mill, a hammer mill, or a vibratory mill.

[0035] The average particle size of the polymer particles removed as the product fraction is preferably 150 to 850 μm, more preferably 250 to 600 μm, and most particularly 300 to 500 μm. The average particle size of the product fraction can be measured by EDANA recommended test method No. WSP 220.2(05) "Particle Size Distribution," in which the mass proportions of the selected fractions are plotted in cumulative form and the average particle size is determined graphically. The average particle size in this specification is the mesh size value resulting from 50% cumulative weight.

[0036] To further improve the properties, the polymer particles are thermally crosslinked on the surface.Suitable surface crosslinking agents are compounds that contain a group that can form a covalent bond with at least two carboxylate groups of the polymer particles.Suitable compounds are, 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, difunctional 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 U.S. Pat. No. 6,239,230.

[0037] The amount of surface postcrosslinker is preferably 0.001% to 2% by weight, more preferably 0.01% to 1% by weight and most preferably 0.03% to 0.7% by weight, in each case based on the polymer particles.

[0038] In a preferred embodiment of the present invention, in addition to the surface postcrosslinker, a multivalent cation is applied to the particle surface.

[0039] Polyvalent cations that can be used in the process of the present invention include, for example, divalent cations such as those of zinc, magnesium, calcium, and strontium; trivalent cations such as those of aluminum, iron, chromium, rare earths, and manganese; and tetravalent cations such as those of titanium and zirconium. Possible counterions are chloride, bromide, hydroxide, sulfate, hydrogensulfate, carbonate, bicarbonate, nitrate, phosphate, hydrogenphosphate, dihydrogenphosphate, and carboxylates, such as acetate and lactate. Aluminum hydroxide, aluminum sulfate, and aluminum lactate are preferred.

[0040] The amount of polyvalent cation used is for example 0.001% to 1.5% by weight, preferably 0.005% to 1% by weight and more preferably 0.02% to 0.8% by weight, in each case based on the polymer.

[0041] Surface postcrosslinking is typically carried out by spraying a solution of the surface postcrosslinker onto dried polymer particles. After spray application, the polymer particles coated with the surface postcrosslinker are subjected to a heat treatment.

[0042] The spray application of the solution of surface postcrosslinker is preferably carried out using a mixer with moving mixing implements, such as a screw mixer, a disk mixer and a paddle mixer.Horizontal mixers, such as a paddle mixer, are particularly preferred, and vertical mixers are particularly preferred.Horizontal mixers and vertical mixers are distinguished by the position of the mixing shaft, that is, horizontal mixers have horizontally mounted mixing shafts, and vertical mixers have vertically mounted mixing shafts.Suitable mixers are, for example, the horizontal Pflugschar® plowshare mixer (Gebr. Loedige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta continuous mixer (Hosokawa Micron BV; Doetinchem; Netherlands), Processall Mixmill mixer (Processall Incorporated; Cincinnati; USA) and Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; Netherlands).However, it is also possible to spray the solution of surface postcrosslinker in a fluidized bed.

[0043] The surface post-crosslinking agent is typically used in the form of an aqueous solution. The penetration depth of the surface post-crosslinking agent into the polymer particles can be adjusted by the content of the non-aqueous solvent and the total amount of the solvent.

[0044] The heat treatment is preferably carried out in a contact dryer, more preferably a paddle dryer, and most preferably a disk dryer. Suitable dryers include, for example, the Hosokawa Bepex® horizontal paddle dryer (Hosokawa Micron GmbH; Leingarten; Germany), the Hosokawa Bepex® disk dryer (Hosokawa Micron GmbH; Leingarten; Germany), the Holo-Flite® dryer (Metso Minerals Industries Inc.; Danville; USA), and the Nara paddle dryer (NARA Machinery Europe; Frechen; Germany). Furthermore, a fluidized bed dryer can also be used.

[0045] Surface after crosslinking can be carried out in the mixer body by heating the jacket or blowing hot air into it.Also suitable is downstream dryer, such as tray dryer, rotary tube oven or heatable screw.It is particularly advantageous to mix in fluidized bed dryer and carry out thermal surface after crosslinking.

[0046] Preferred reaction temperatures are in the range of 100 to 250° C., preferably 110 to 220° C., more preferably 120 to 210° C., and most preferably 130 to 200° C. Preferred residence times at this temperature are preferably at least 10 minutes, more preferably at least 20 minutes, most preferably at least 30 minutes, and typically up to 60 minutes.

[0047] The surface postcrosslinked polymer particles can then be classified again to remove overly small and / or overly large polymer particles and reuse them in the process.

[0048] To further improve properties, the surface postcrosslinked polymer particles can be coated or rewetted.

[0049] Remoistening is preferably carried out at 30 to 80°C, more preferably 35 to 70°C, and most preferably 40 to 60°C. At excessively low temperatures, the polymer particles tend to form agglomerates, while at higher temperatures, water evaporates quickly and noticeably. The amount of water used for remoistening is preferably 1 to 10% by weight, more preferably 2 to 8% by weight, and most preferably 3 to 5% by weight. Remoistening increases the mechanical stability of the polymer particles and reduces their tendency to become electrostatically charged. Remoistening is advantageously carried out in a cooler after thermal surface postcrosslinking.

[0050] Suitable coatings for improving swelling rate and gel bed permeability (GBP) include, for example, inorganic inert materials such as water-insoluble metal salts, organic polymers, cationic polymers, and divalent or polyvalent metal cations. Suitable coatings for dust adsorption include, for example, polyols. Suitable coatings for combating the undesirable tendency of polymer particles to caking include, for example, fumed silica such as Aerosil® 200, precipitated silica such as Sipernat® D17, and surfactants such as Span® 20.

[0051] The present invention further provides superabsorbent particles coated with at least one pyrazole.

[0052] The pyrazole is typically a monomeric pyrazole.

[0053] The pyrazole preferably has the general formula (I) [ka] (In the formula, R 1 is C1 or C2-alkyl, R 2 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl, and R 3 H, C1~C 20 -Alkyl or C6-C 20-arylalkyl) or a compound of general formula (II) [ka] (In the formula, R 4 is C1 or C2-alkyl, R 5 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl, and R 6 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl) or a compound of general formula (III) [ka] (In the formula, R 7 is C1 or C2-alkyl, R 8 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl, and R 9 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl) is a compound of

[0054] The pyrazole is more preferably of the general formula (I) [ka] (In the formula, R 1 is C1-alkyl, R 2 is C1-C3-alkyl or C6-C8-arylalkyl, and R 3 is H, C1-C3-alkyl or C6-C8-arylalkyl or a compound of general formula (II) [ka] (In the formula, R 4 is C1-alkyl, R 5 is C1-C3-alkyl or C6-C8-arylalkyl, and R 6 is H, C1-C3-alkyl or C6-C8-arylalkyl or a compound of general formula (III) [ka] (In the formula, R 7 is C1-alkyl, R 8 is C1-C3-alkyl or C6-C8-arylalkyl, and R 9 is H, C1-C3-alkyl or C6-C8-arylalkyl is a compound of

[0055] The alkyl group can be straight, branched or cyclic.

[0056] Compounds of general formula (I) exist in equilibrium with their keto forms, for example 1,3-dimethyl-5-pyrazolone is the keto form of 1,3-dimethyl-5-hydroxypyrazole.

[0057] The pyrazole is most preferably 1,3-dimethyl-5-pyrazolone, 1,5-dimethyl-3-ethyl-4-hydroxypyrazole or 1,5-dimethyl-4-hydroxy-3-phenylpyrazole.

[0058] The superabsorbent particles are preferably coated with 0.001% to 1% by weight, more preferably 0.005% to 0.2% by weight, most preferably 0.01% to 0.1% by weight of pyrazole, in each case based on the polymer particles.

[0059] Acrylic acid is the preferred ethylenically unsaturated carboxylic acid. Peroxodisulfates, in particular ammonium peroxodisulfate, sodium peroxodisulfate and / or potassium peroxodisulfate, are preferred initiators c).

[0060] The present invention further provides hygiene articles comprising the superabsorbent particles of the present invention.

[0061] method: Unless otherwise stated, measurements should be carried out at an ambient temperature of 23±2° C. and a relative air humidity of 50±10%. The superabsorbent particles are thoroughly mixed before the measurement.

[0062] Color value (CIE color number [L, a, b]) Color values ​​are measured by the CIELAB method (Hunterlab, volume 8, 1996, edition 7, pages 1 to 4) using a "LabScan XE Spectrometer" colorimeter (HunterLab; Reston; USA). Color is described via the coordinates L, a, and b in a three-dimensional system. L characterizes the luminance, L=0 is black, and L=100 is white. The a and b values ​​represent the color position on the red / green and yellow / blue color axes, respectively, with positive a values ​​representing red, negative a values ​​representing green, positive b values ​​representing yellow, and negative b values ​​representing blue.

[0063] The Hunter 60 value (HC60) is a measure of the whiteness of a surface and is defined as L-3b, meaning that the lower the value, the darker and more yellow the color.

[0064] Testing was performed using tissue culture dishes (35 mm diameter and 10 mm height) and a 0.5 inch port plate opening.

[0065] The colour index is determined according to the tristimulus method according to DIN 5033-6.

[0066] Yellowness index (YI) Yellowness Index (YI) is measured according to ASTM D1925 or ASTM E313. The higher the value, the darker and more yellow the color. [Example]

[0067] Example 1 A monomer solution was prepared by successively mixing deionized water, a 50% by weight sodium hydroxide solution, and acrylic acid to a degree of neutralization corresponding to 74.0 mol %. The water content of the monomer solution was 59.0% by weight.

[0068] The crosslinking agent used was triple ethoxylated glyceryl triacrylate (purity: approximately 85% by weight), and the amount used was 1.34 kg per ton of monomer solution.

[0069] Free radical polymerization was initiated using 2.14 kg of a 0.25 wt % aqueous hydrogen peroxide solution, 2.91 kg of a 15 wt % aqueous sodium peroxodisulfate solution, and 1.97 kg of a 1 wt % aqueous ascorbic acid solution per ton of monomer solution.

[0070] 6.3 ml of monomer solution 3 The monomer solution was introduced into a List Contikneter continuous kneading reactor (LIST AG, Arisdorf, Switzerland) with a capacity of 1000 t / h. The throughput of the monomer solution was approximately 20 t / h. The feed temperature of the reaction solution was 23.5°C.

[0071] The monomer solution was inerted with nitrogen between the addition of the crosslinker and the addition of the hydrogen peroxide and sodium peroxodisulfate solutions. Ascorbic acid was metered directly into the reactor.

[0072] After about 50% of the residence time had elapsed, about 1000 kg / hour of polymer particles having a particle size of less than 150 μm and obtained by a manufacturing process by grinding and classification were additionally metered into the reactor. The residence time of the reaction mixture in the reactor was about 15 minutes.

[0073] The resulting polymer gel was applied to the conveyor belt of an air-circulating belt dryer by vibrating the conveyor belt. The total length of the air-circulating belt dryer was 48 m. The conveyor belt of the air-circulating belt dryer had an effective width of 4.4 m. On this air-circulating belt dryer, an air / gas mixture (approximately 175°C) was continuously flowed around the aqueous polymer gel to dry it. The residence time in the air-circulating belt dryer was 37 minutes.

[0074] The dried polymer gel was crushed using a three-roll mill and sieved to a particle size of 150-700 μm. Polymer particles with a particle size of less than 150 μm were separated. Polymer particles with a particle size of more than 700 μm were recycled for crushing. Polymer particles with a particle size in the range of 150-700 μm were thermally surface crosslinked.

[0075] The polymer particles were coated with the surface postcrosslinker solution in a Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, Netherlands) and then dried at 175°C for 45 minutes in a NARA paddle dryer (GMF Gouda, Waddinxveen, Netherlands).

[0076] The following amounts were weighed into a Schugi Flexomix®: 7.5t / hour of polymer particles 334.50 kg / hour of surface postcrosslinker solution

[0077] The surface postcrosslinker solution contained 1.35 wt% ethylene glycol diglycidyl ether, 44.84 wt% propane-1,2-diol and 53.81 wt% water.

[0078] After drying, the surface-crosslinked polymer particles were cooled to about 60° C. in a NARA paddle cooler (GMF Gouda, Waddinxveen, Netherlands). Simultaneously, the surface-crosslinked polymer particles were coated with 7.5 kg / h of a 50 wt. % aqueous polyethylene glycol solution (polyethylene glycol having an average molar mass of 400 g / mol), 375 kg / h of water, 22.5 kg / h of aluminum trihydroxide ("Dry Aluminum Hydroxide Gel", product number 511066100, Dr. Paul Lohmann GmbH KG, Hauptstrasse 2, 31860 Emmerthal, Germany), and 18.75 kg / h of a 1 wt. % aqueous sorbitan monolaurate solution.

[0079] Example 2 20 g in each case of superabsorbent from Example 1 were mixed with 200 ppm by weight of pyrazole in each case and stored for 14 days in a temperature- and humidity-controlled cabinet at 70° C. and 80% relative humidity.

[0080] The results are summarized in Table 1.

[0081] [Table 1]

[0082] Example 3 20 g in each case of the superabsorbent from Example 1 were mixed with 1,5-dimethyl-3-ethyl-4-hydroxypyrazole and stored for 14 days in a temperature- and humidity-controlled cabinet at 70° C. and 80% relative humidity.

[0083] The results are summarized in Table 2.

[0084] [Table 2]

Claims

1. a) at least one ethylenically unsaturated carboxylic acid that is at least partially neutralized; b) at least one cross-linking agent; and c) at least one initiator; 1. A process for producing surface postcrosslinked superabsorbent particles by polymerizing an aqueous monomer solution or suspension comprising:

2. The pyrazole is represented by the general formula (I): 【Chemistry 1】 (In the formula, R １ is C １ or C ２ - alkyl, R ２ is H, C １ ~C ２０ -Alkyl or C ６ ~C ２０ -arylalkyl, and R ３ is H, C １ ~C ２０ -Alkyl or C ６ ~C ２０ -arylalkyl) or a compound of general formula (II) 【Chemistry 2】 (In the formula, R ４ is C １ or C ２ - alkyl, R ５ is H, C １ ~C ２０ -Alkyl or C ６ ~C ２０ -arylalkyl, and R ６ is H, C １ ~C ２０ -Alkyl or C ６ ~C ２０ -arylalkyl) or a compound of general formula (III) 【Transformation 3】 (In the formula, R ７ is C １ or C ２ - alkyl, R ８ is H, C １ ~C ２０ -Alkyl or C ６ ~C ２０ -arylalkyl, and R ９ is H, C １ ~C ２０ -Alkyl or C ６ ~C ２０ -arylalkyl) 2. The process of claim 1, wherein the compound is

3. 3. The process according to claim 1 or 2, wherein acrylic acid is used as the ethylenically unsaturated carboxylic acid.

4. 4. The process according to claim 1, wherein peroxodisulfates, in particular ammonium peroxodisulfate, sodium peroxodisulfate and / or potassium peroxodisulfate, are used as initiator c).

5. The process of any one of claims 1 to 4, wherein the polymer particles are coated with 0.001% to 1% by weight of the pyrazole based on the polymer particles.

6. The process of any one of claims 1 to 5, wherein the polymer particles are coated with 0.01 to 0.1% by weight of the pyrazole, based on the polymer particles.

7. In the general formula (I), R １ is C １ -alkyl, and R ２ is C １ ~C ３ -Alkyl or C ６ ~C ８ -arylalkyl, and R ３ is H, C １ ~C ３ -Alkyl or C ６ ~C ８ -arylalkyl, or in the general formula (II), R ４ is C １ -alkyl, and R ５ is C １ ~C ３ -Alkyl or C ６ ~C ８ -arylalkyl, and R ６ is C １ ~C ３ -Alkyl or C ６ ~C ８ -arylalkyl, or in the general formula (III), R ７ is C １ -alkyl, and R ８ is H, C １ ~C ３ -Alkyl or C ６ ~C ８ -arylalkyl, and R ９ is C １ ~C ３ -Alkyl or C ６ ~C ８ The process of any one of claims 2 to 6, wherein the aryl is -arylalkyl.

8. 8. The process according to any one of claims 2 to 7, wherein 1,3-dimethyl-5-pyrazolone is used as the compound of general formula (I), or 1,5-dimethyl-3-ethyl-4-hydroxypyrazole or 1,5-dimethyl-4-hydroxy-3-phenylpyrazole is used as the compound of general formula (II).

9. Superabsorbent particles obtainable by the process according to any one of claims 1 to 8, wherein the superabsorbent particles are coated with at least one pyrazole.

10. The pyrazole is represented by the general formula (I): 【Chemistry 4】 (In the formula, R １ is C １ or C ２ - alkyl, R ２ is H, C １ ~C ２０ -Alkyl or C ６ ~C ２０ -arylalkyl, and R ３ is H, C １ ~C ２０ -Alkyl or C ６ ~C ２０ -arylalkyl) or a compound of general formula (II) 【Transformation 5】 (In the formula, R ４ is C １ or C ２ - alkyl, R ５ is H, C １ ~C ２０ -Alkyl or C ６ ~C ２０ -arylalkyl, and R ６ is H, C １ ~C ２０ -Alkyl or C ６ ~C ２０ -arylalkyl) or a compound of general formula (III) 【Transformation 6】 (In the formula, R ７ is C １ or C ２ - alkyl, R ８ is H, C １ ~C ２０ -Alkyl or C ６ ~C ２０ -arylalkyl, and R ９ is H, C １ ~C ２０ -Alkyl or C ６ ~C ２０ -arylalkyl) 10. The superabsorbent particle of claim 9, which is a compound of the formula:

11. Superabsorbent particles according to claim 9 or 10, coated with 0.001% to 1% by weight of said pyrazole, based on said polymer particles.

12. Superabsorbent particles according to any one of claims 9 to 11, coated with 0.01 to 0.1% by weight of said pyrazole, based on said polymer particles.

13. In the general formula (I), R １ is C １ -alkyl, and R ２ is C １ ~C ３ -Alkyl or C ６ ~C ８ -arylalkyl, and R ３ is H, C １ ~C ３ -Alkyl or C ６ ~C ８ -arylalkyl, or in the general formula (II), R ４ is C １ -alkyl, and R ５ is C １ ~C ３ -Alkyl or C ６ ~C ８ -arylalkyl, and R ６ is C １ ~C ３ -Alkyl or C ６ ~C ８ -arylalkyl, or in the general formula (III), R ７ is C １ -alkyl, and R ８ is H, C １ ~C ３ -Alkyl or C ６ ~C ８ -arylalkyl, and R ９ is C １ ~C ３ -Alkyl or C ６ ~C ８ Superabsorbent particles according to any one of claims 10 to 12, wherein the aryl group is -arylalkyl.

14. Superabsorbent particles according to any one of claims 10 to 13, wherein the compound of general formula (I) is 1,3-dimethyl-5-pyrazolone or the compound of general formula (II) is 1,5-dimethyl-3-ethyl-4-hydroxypyrazole or 1,5-dimethyl-4-hydroxy-3-phenylpyrazole.

15. Hygiene article comprising superabsorbent particles according to any one of claims 9 to 14.

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