WO2024115160A1 - Method for the production of colour-stable superabsorber particles - Google Patents

Method for the production of colour-stable superabsorber particles Download PDF

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Publication number
WO2024115160A1
WO2024115160A1 PCT/EP2023/082340 EP2023082340W WO2024115160A1 WO 2024115160 A1 WO2024115160 A1 WO 2024115160A1 EP 2023082340 W EP2023082340 W EP 2023082340W WO 2024115160 A1 WO2024115160 A1 WO 2024115160A1
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Prior art keywords
alkyl
arylalkyl
general formula
compound
pyrazole
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PCT/EP2023/082340
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German (de)
French (fr)
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Christian HILS
Christophe Bauduin
Jan Niclas GORGES
Matthias Weismantel
Ruediger Funk
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Basf Se
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Publication of WO2024115160A1 publication Critical patent/WO2024115160A1/en

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    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J3/00—Processes of treating or compounding macromolecular substances
    • C08J3/24—Crosslinking, e.g. vulcanising, of macromolecules
    • C08J3/245—Differential crosslinking of one polymer with one crosslinking type, e.g. surface crosslinking
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/22—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising organic material
    • B01J20/26—Synthetic macromolecular compounds
    • B01J20/265—Synthetic macromolecular compounds modified or post-treated polymers
    • B01J20/267—Cross-linked polymers
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J3/00—Processes of treating or compounding macromolecular substances
    • C08J3/02—Making solutions, dispersions, lattices or gels by other methods than by solution, emulsion or suspension polymerisation techniques
    • C08J3/03—Making solutions, dispersions, lattices or gels by other methods than by solution, emulsion or suspension polymerisation techniques in aqueous media
    • C08J3/075—Macromolecular gels
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J3/00—Processes of treating or compounding macromolecular substances
    • C08J3/12—Powdering or granulating
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
    • A61L15/00—Chemical aspects of, or use of materials for, bandages, dressings or absorbent pads
    • A61L15/16—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons
    • A61L15/42—Use of materials characterised by their function or physical properties
    • A61L15/60—Liquid-swellable gel-forming materials, e.g. super-absorbents
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2333/00—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers
    • C08J2333/02—Homopolymers or copolymers of acids; Metal or ammonium salts thereof
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
    • C08K5/00—Use of organic ingredients
    • C08K5/16—Nitrogen-containing compounds
    • C08K5/34—Heterocyclic compounds having nitrogen in the ring
    • C08K5/3442—Heterocyclic compounds having nitrogen in the ring having two nitrogen atoms in the ring
    • C08K5/3445—Five-membered rings

Definitions

  • the present invention relates to a process for producing color-stable superabsorbent particles, wherein an aqueous monomer solution or suspension is polymerized to form a polymer gel, the polymer gel obtained is optionally comminuted, the polymer gel is then dried, the dried polymer gel is optionally ground and classified, the dried polymer gel is then thermally surface-crosslinked and cooled, characterized in that after polymerization it is coated with a pyrazole.
  • Superabsorbents are used to produce diapers, tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in agricultural horticulture.
  • the superabsorbents are also referred to as water-absorbing polymers.
  • the production of superabsorbents is described in the monograph "Modern Superabsorbent Polymer Technology", FL Buchholz and AT Graham, Wiley-VCH, 1998, pages 71 to 103.
  • GBP gel bed permeability
  • AUL0.7psi absorption under a pressure of 49.2 g/cm2
  • superabsorbent particles are generally surface-crosslinked. This increases the degree of crosslinking of the particle surface, which means that the absorption under a pressure of 49.2 g/cm2 (AUL0.7psi) and the centrifuge retention capacity (CRC) can be at least partially decoupled.
  • This surface-crosslinking can be carried out in an aqueous gel phase.
  • dried, ground and sieved polymer particles base polymer
  • a surface-crosslinker for this purpose are compounds that can form covalent bonds with at least two carboxylate groups of the polymer particles.
  • WO 2021/105038 A1 and the older PCT application with the file number PCT/EP2022/059572 disclose pyrazoles as polymerization inhibitors.
  • the object of the present invention was to provide an improved process for producing color-stable superabsorbent particles.
  • the object was achieved by a process for producing surface-postcrosslinked superabsorbent particles by polymerizing an aqueous monomer solution or suspension containing 221049 2 a) at least one ethylenically unsaturated, acid group-bearing monomer which is at least partially neutralized, b) at least one crosslinker and c) at least one initiator, wherein the aqueous monomer solution or suspension is polymerized to form a polymer gel, the polymer gel obtained is optionally comminuted, the polymer gel is then dried, the dried polymer gel is optionally ground and classified, the dried polymer gel is then thermally surface-postcrosslinked and cooled, characterized in that the polymer particles obtained are coated with at least one pyrazole before, during or after the thermal surface-postcrosslinking.
  • the pyrazole is usually a monomeric pyrazole.
  • the pyrazole used in the process according to the invention is preferably a compound of the general formula (I) where R 1 is C 1 or C 2 alkyl, R 2 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 3 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl, or a compound of the general formula (II) 221049 3 where R 4 is C 1 or C 2 alkyl, R 5 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 6 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl, or a compound of the general formula (III) where R 7 is C 1 or C 2 alkyl, R 8 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 9 is H, C 1 to C 20 alkyl or
  • the alkyl groups can be straight, branched and/or cyclic.
  • the pyrazole used in the process according to the invention is particularly preferably a compound of the general formula (I) 221049 4 where R 1 is C 1 alkyl, R 2 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 3 is H, C 1 to C 3 alkyl or C 6 to C 8 arylalkyl, or a compound of the general formula (II) where R 4 is C 1 alkyl, R 5 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 6 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl, or a compound of the general formula (III) 221049 5 where R 7 is C 1 alkyl, R 8 is H, C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 9 is C 1 to C 3 alkyl or C 6 to C 8 arylal
  • the alkyl groups can be straight, branched and/or cyclic.
  • the compounds of the general formula (I) are in equilibrium with their keto form.
  • 1,3-dimethyl-5-pyrazolone is the keto form of 1,3-dimethyl-5-hydroxypyrazole.
  • the pyrazole used in the process according to the invention is very particularly preferably 1,3-dimethyl-5-pyrazolone, 1,5-dimethyl-3-ethyl-4-hydroxypyrazole or 1,5-dimethyl-4-hydroxy-3-phenylpyrazole.
  • the polymer particles are coated with preferably 0.001 to 1% by weight, particularly preferably 0.005 to 0.2% by weight, very particularly preferably 0.01 to 0.1% by weight of the pyrazole, in each case based on the polymer particles.
  • the present invention is based on the finding that pyrazoles significantly improve the color stability of superabsorbents.
  • Acrylic acid is the preferred ethylenically unsaturated carboxylic acid.
  • Peroxodisulfate particularly ammonium peroxodisulfate, sodium peroxodisulfate and/or potassium peroxodisulfate, is the preferred initiator c).
  • the superabsorbents are produced by polymerizing a monomer solution and are usually insoluble in water.
  • the ethylenically unsaturated, acid group-bearing monomers a) are preferably water-soluble, ie the solubility in water at 23°C is typically at least 1 g/100 g. 221049 6 Water, preferably at least 5 g/100 g water, particularly preferably at least 25 g/100 g water, very particularly preferably at least 35 g/100 g water.
  • 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. Acrylic acid is very particularly preferred.
  • the ethylenically unsaturated, acid group-bearing monomers a) are usually partially neutralized. The neutralization is carried out at the monomer stage. This is usually done by mixing in the neutralizing agent as an aqueous solution or preferably also as a solid.
  • the degree of neutralization is preferably from 40 to 85 mol%, particularly preferably from 50 to 80 mol%, very particularly preferably from 60 to 75 mol%, it being possible to use the usual neutralizing agents, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal hydrogen carbonates and mixtures thereof.
  • Ammonium salts can also be used instead of alkali metal salts.
  • Sodium and potassium are particularly preferred as alkali metals, but sodium hydroxide, sodium carbonate or sodium hydrogen carbonate and mixtures thereof, especially sodium hydroxide, are very particularly preferred.
  • the monomers usually contain polymerization inhibitors, preferably hydroquinone half ethers, as storage stabilizers.
  • Suitable crosslinkers b) are compounds with at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups that can be radically polymerized into the polymer chain and functional groups that can form covalent bonds with the acid groups of the monomer.
  • polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer are also suitable as crosslinking agents.
  • 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 in EP 0530438 A1, di- and triacrylates, as described in EP 0547847 A1, EP 0559476 A1, EP 0632068 A1, WO 93/21237 A1, WO 03/104299 A1, WO 03/104300 A1, WO 03/104301 A1 and DE 10331450 A1, mixed acrylates which, in addition to acrylate groups, contain further ethylenically unsaturated groups, as described in DE 10331456 A1 and DE 10355401 A1, or crosslinker mixtures, 221049 7 as described for example in DE 19543368 A1, DE 19646484 A1, WO 90/15830 A1 and WO
  • the amount of crosslinker b) is preferably 0.05 to 1.5 wt. %, particularly preferably 0.1 to 1 wt. %, very particularly preferably 0.15 to 0.6 wt. %, in each case calculated on the total amount of monomer used.
  • the centrifuge retention capacity (CRC) decreases and the absorption under a pressure of 21.0 g/cm2 (AUL0.3 psi) passes through a maximum.
  • All compounds which generate radicals under the polymerization conditions can be used as initiators c), for example thermal initiators, redox initiators, photoinitiators.
  • Suitable redox initiators are sodium peroxodisulfate/ascorbic acid, hydrogen peroxide/ascorbic acid, sodium peroxodisulfate/sodium bisulfite and hydrogen peroxide/sodium bisulfite.
  • mixtures of thermal initiators and redox initiators are used, such as sodium peroxodisulfate/hydrogen peroxide/ascorbic acid.
  • 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 is preferably used as the reducing component.
  • the water content of the monomer solution is preferably from 40 to 75% by weight, particularly preferably from 45 to 70% by weight, very particularly preferably from 50 to 65% by weight. As the water content increases, the energy required for the subsequent drying increases, and as the water content decreases, the heat of polymerization can only be dissipated insufficiently.
  • the temperature of the monomer solution is preferably from 10 to 90°C, particularly preferably from 20 to 70°C, very particularly preferably from 30 to 50°C.
  • the preferred polymerization inhibitors require dissolved oxygen for optimal effect.
  • the monomer solution can therefore be freed of dissolved oxygen before polymerization by inerting, ie by flowing an inert gas, preferably nitrogen or carbon dioxide, through it.
  • an inert gas preferably nitrogen or carbon dioxide
  • the oxygen content of the monomer solution before polymerization is reduced to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight. 221049 8
  • Suitable reactors for polymerization are, for example, kneading reactors or belt reactors.
  • the polymer gel formed during the polymerization of an aqueous monomer solution or suspension is continuously comminuted by, for example, counter-rotating agitator shafts, as described in WO 2001/038402 A1.
  • Polymerization on the belt is described, for example, in DE 3825366 A1 and US 6,241,928.
  • Polymerization in a belt reactor produces a polymer gel that must be comminuted, for example in an extruder or kneader.
  • the comminuted polymer gel obtained by means of a kneader can also be extruded.
  • the polymer gel is then usually dried using a circulating air belt dryer until the residual moisture content is preferably 0.5 to 10% by weight, particularly preferably 1 to 7% by weight, very particularly preferably 2 to 5% by weight, the residual moisture content being determined according to test method no. WSP 230.2-05 "Mass Loss Upon Heating" recommended by EDANA. If the residual moisture is too high, the dried polymer gel has a glass transition temperature T g that is too low and is difficult to process further. If the residual moisture is too low, the dried polymer gel is too brittle and undesirably large amounts of polymer particles with too small a particle size ("fines") are produced in the subsequent comminution steps.
  • the solids content of the polymer gel before drying is preferably between 25 and 90% by weight, particularly preferably between 35 and 70% by weight, very particularly preferably between 40 and 60% by weight.
  • the dried polymer gel is then broken and optionally coarsely crushed.
  • the dried polymer gel is then usually ground and classified, whereby single or multi-stage roller mills, preferably two or three-stage roller mills, pin mills, hammer mills or vibrating mills can usually be used for grinding.
  • the average particle size of the polymer particles separated as a product fraction is preferably from 150 to 850 ⁇ m, particularly preferably from 250 to 600 ⁇ m, very particularly from 300 to 500 ⁇ m.
  • the average particle size of the product fraction can be determined using the test method No.
  • WSP 220.2 (05) "Particle Size Distribution” recommended by EDANA, whereby the mass fractions of the sieve fractions are plotted cumulatively and the average particle size is determined graphically.
  • the average particle size is the value of the mesh size that results for a cumulative 50 wt.%. 221049 9
  • the polymer particles are thermally surface-crosslinked to further improve their properties. Suitable surface-crosslinkers are compounds that contain groups that can form covalent bonds 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 0083022 A2, EP 0543303 A1 and EP 0937736 A2, di- or polyfunctional alcohols, as described in DE 3314019 A1, DE 35 23617 A1 and EP 0450922 A2, or ß-hydroxyalkylamides, as described in DE 10204938 A1 and US 6,239,230.
  • the amount of surface post-crosslinker is preferably 0.001 to 2% by weight, particularly preferably 0.01 to 1% by weight, very particularly preferably 0.03 to 0.7% by weight, based in each case on the polymer particles.
  • polyvalent cations are applied to the particle surface in addition to the surface post-crosslinkers.
  • the polyvalent cations that can be used in the process according to the invention are, for example, divalent cations, such as the cations of zinc, magnesium, calcium and strontium, trivalent cations, such as the cations of aluminum, iron, chromium, rare earths and manganese, tetravalent cations, such as the cations of titanium and zirconium.
  • Chloride, bromide, hydroxide, sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate and carboxylate, such as acetate and lactate, are possible as counterions.
  • Aluminum hydroxide, aluminum sulfate and aluminum lactate are preferred.
  • the amount of polyvalent cation used is, for example, 0.001 to 1.5% by weight, preferably 0.005 to 1% by weight, particularly preferably 0.02 to 0.8% by weight, based in each case on the polymer.
  • the surface post-crosslinking is usually carried out by spraying a solution of the surface post-crosslinker onto the dried polymer particles.
  • the polymer particles coated with surface post-crosslinker are thermally treated.
  • the spraying of a solution of the surface post-crosslinker is preferably carried out in mixers with moving mixing tools, such as screw mixers, disk mixers and paddle mixers.
  • Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are very particularly preferred.
  • the distinction between horizontal mixers and Vertical mixers are based on the bearing 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 are, for example, horizontal Pflugschar® mixers (Gebr.
  • Suitable dryers include Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Leingart; Germany), Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Leingart; Germany), Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; USA) and Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany). Fluidized bed dryers can also be used. Surface post-crosslinking can take place in 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 heatable screw, is also suitable. Mixing and thermal surface post-crosslinking are particularly advantageous in a fluidized bed dryer.
  • Preferred reaction temperatures are in the range 100 to 250°C, preferably 110 to 220°C, particularly preferably 120 to 210°C, very particularly preferably 130 to 200°C.
  • the preferred residence time at this temperature is preferably at least 10 minutes, particularly preferably at least 20 minutes, very particularly preferably at least 30 minutes, and usually at most 60 minutes.
  • the surface-crosslinked polymer particles can then be classified again, with polymer particles that are too small and/or too large being separated off and returned to the process.
  • the surface-crosslinked polymer particles can be coated or remoistened to further improve their properties. 11
  • the remoistening is preferably carried out at 30 to 80°C, particularly preferably at 35 to 70°C, very particularly preferably at 40 to 60°C.
  • the amount of water used for remoistening is preferably from 1 to 10% by weight, particularly preferably from 2 to 8% by weight, very particularly preferably from 3 to 5% by weight.
  • the remoistening increases the mechanical stability of the polymer particles and reduces their tendency to become statically charged.
  • the remoistening is advantageously carried out in the cooler after the thermal surface post-crosslinking.
  • Suitable coatings for improving the swelling rate and the gel bed permeability (GBP) are, for example, inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers and divalent or polyvalent metal cations.
  • Suitable coatings for binding dust are, for example, polyols.
  • Suitable coatings against the undesirable tendency of the polymer particles to cake are, for example, pyrogenic silica, such as Aerosil® 200, precipitated silica, such as Sipernat® D17, and surfactants, such as Span® 20.
  • the present invention also relates to superabsorbent particles coated with at least one pyrazole.
  • the pyrazole is usually a monomeric pyrazole.
  • the pyrazole is preferably a compound of the general formula (I) where R 1 is C 1 or C 2 alkyl, R 2 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 3 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl, 221049 12 or a compound of general formula (II) where R 4 is C 1 or C 2 alkyl, R 5 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 6 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl, or a compound of the general formula (III) where R 7 is C 1 or C 2 alkyl, R 8 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 9 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl.
  • R 1 is C 1 or C 2 alkyl
  • the pyrazole is particularly preferably a compound of the general formula (I) 221049 13 where R 1 is C 1 alkyl, R 2 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 3 is H, C 1 to C 3 alkyl or C 6 to C 8 arylalkyl, or a compound of the general formula (II) where R 4 is C 1 alkyl, R 5 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 6 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl, or a compound of the general formula (III) 221049 14 where R 7 is C 1 alkyl, R 8 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 9 is H, C 1 to C 3 alkyl or C 6 to C 8 arylalkyl.
  • the alkyl groups can be straight, branched and/or cyclic.
  • the compounds of the general formula (I) are in equilibrium with their keto form.
  • 1,3-dimethyl-5-pyrazolone is the keto form of 1,3-dimethyl-5-hydroxypyrazole.
  • the pyrazole is very particularly preferably 1,3-dimethyl-5-pyrazolone, 1,5-dimethyl-3-ethyl-4-hydroxypyrazole or 1,5-dimethyl-4-hydroxy-3-phenylpyrazole.
  • the superabsorbent particles were coated with preferably 0.001 to 1% by weight, particularly preferably 0.005 to 0.2% by weight, very particularly preferably 0.01 to 0.1% by weight of the pyrazole, based in each case on the polymer particles.
  • Acrylic acid is the preferred ethylenically unsaturated carboxylic acid.
  • Peroxodisulfate particularly ammonium peroxodisulfate, sodium peroxodisulfate and/or potassium peroxodisulfate, is the preferred initiator c).
  • the present invention further relates to hygiene articles containing superabsorbent particles according to the invention. Methods: Unless otherwise stated, the measurements should be carried out at an ambient temperature of 23 ⁇ 2°C and a relative humidity of 50 ⁇ 10%. The superabsorbent particles are mixed well before the measurement.
  • the Hunter 60 value is a measure of the whiteness of surfaces and is defined as L-3b, i.e. the lower the value, the darker and yellower the color.
  • the test was carried out using a tissue culture dish (diameter of 35 mm and height of 10 mm) and a port plate opening of 0.5 inch. The color value is measured in accordance with the tristimulus method according to DIN 5033-6.
  • Yellowness Index YI
  • the Yellowness Index (YI) is measured according to ASTM D1925 or according to ASTM E313. The higher the value, the darker and yellower the color.
  • Example 1 A monomer solution was prepared by continuously mixing deionized water, 50 wt.% sodium hydroxide solution and acrylic acid so that the degree of neutralization was 74.0 mol%. The water content of the monomer solution was 59.0 wt.%. Triple ethoxylated glycerol triacrylate (approx. 85% by weight) was used as crosslinker. The amount used was 1.34 kg per t of monomer solution. 221049 16 To initiate the radical polymerization, 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 were used per t of monomer solution.
  • the monomer solution was dosed into a List Contikneter reactor with a volume of 6.3 m3 (LIST AG, Arisdorf, Switzerland). The throughput of the monomer solution was approximately 20 t/h.
  • the reaction solution had a temperature of 23.5°C at the inlet.
  • the monomer solution was rendered inert with nitrogen between the addition point for the crosslinker and the addition points for the hydrogen peroxide and sodium peroxodisulfate solutions.
  • Ascorbic acid was dosed directly into the reactor.
  • the residence time of the reaction mixture in the reactor was about 15 minutes.
  • the polymer gel obtained was fed onto the conveyor belt of a circulating air belt dryer using an oscillating conveyor belt.
  • the circulating air belt dryer was 48 m long.
  • the conveyor belt of the circulating air belt dryer had an effective width of 4.4 m.
  • the aqueous polymer gel was continuously blown around with an air/gas mixture (about 175°C) and dried.
  • the residence time in the circulating air belt dryer was 37 minutes.
  • the dried polymer gel was crushed using a three-stage roller mill and sieved to a particle size of 150 to 700 ⁇ m. Polymer particles with a particle size of less than 150 ⁇ m were separated. Polymer particles with a particle size of greater than 700 ⁇ m were returned to the shredding process. Polymer particles with a particle size in the range of 150 to 700 ⁇ m were thermally surface-crosslinked.
  • the polymer particles were coated with a surface-crosslinking solution in a Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, Netherlands) and then dried in a NARA Paddle Dryer (GMF Gouda, Waddinxveen, Netherlands) for 45 minutes at 175°C.
  • the surface crosslinker solution contained 1.35 wt.% ethylene glycol diglycidyl ether, 44.84 wt.% 1,2-propanediol and 53.81 wt.% water. After drying, the surface crosslinked polymer particles were cooled to approx. 60°C in a NARA paddle cooler (GMF Gouda, Waddinxveen, Netherlands).
  • the surface-crosslinked polymer particles were coated with 7.5 kg/h of a 50 wt.% aqueous polyethylene glycol solution (polyethylene glycol with an average molecular weight of 400 g/mol), 375 kg/h of water, 22.5 kg/h of aluminum trihydroxide ("aluminum hydroxide dry gel", article number 511066100, Dr. Paul Lohmann GmbH KG, Haupt No 2, 31860 Emmerthal, Germany) and 18.75 kg/h of a 1 wt.% aqueous solution of sorbitan monolaurate.
  • Example 2 20 g of superabsorbent from Example 1 were mixed with 200 ppm by weight of a pyrazole and stored for 14 days in a climate cabinet at 70°C and 80% relative humidity.

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Abstract

The present invention relates to a method for the production of colour-stable superabsorber particles, wherein an aqueous monomer solution or monomer suspension is polymerised to form a polymer gel, the obtained polymer gel optionally is optionally comminuted, the polymer gel is subsequently dried, the dried polymer gel is optionally ground and classified, the dried polymer gel is subsequently thermally surface-crosslinked and cooled, characterised in that coating is carried out with a pyrazol after polymerisation.

Description

221049 1 Verfahren zur Herstellung von farbstabilen Superabsorberpartikeln Die vorliegende Erfindung betrifft ein Verfahren zur Herstellung von farbstabilen Superabsorber- partikeln, wobei eine wässrige Monomerlösung oder -suspension zu einem Polymergel polyme- risiert wird, das erhaltene Polymergel optional zerkleinert wird, das Polymergel anschließend getrocknet wird, das getrocknete Polymergel optional gemahlen und klassiert wird, das getrock- nete Polymergel anschließend thermisch oberflächennachvernetzt und gekühlt wird, dadurch gekennzeichnet, dass nach der Polymerisation mit einem Pyrazol beschichtet wird. Superabsorber werden zur Herstellung von Windeln, Tampons, Damenbinden und anderen Hy- gieneartikeln, aber auch als wasserzurückhaltende Mittel im landwirtschaftlichen Gartenbau ver- wendet. Die Superabsorber werden auch als wasserabsorbierende Polymere bezeichnet. Die Herstellung von Superabsorbern wird in der Monographie ”Modern Superabsorbent Poly- mer Technology”, F.L. Buchholz und A.T. Graham, Wiley-VCH, 1998, Seiten 71 bis 103, be- schrieben. Zur Verbesserung der Anwendungseigenschaften, wie beispielsweise Gelbettpermeabilität (GBP) und Absorption unter einem Druck von 49,2 g/cm² (AUL0.7psi), werden Superabsorber- partikel im allgemeinen oberflächennachvernetzt. Dadurch steigt der Vernetzungsgrad der Parti- keloberfläche, wodurch die Absorption unter einem Druck von 49,2 g/cm² (AUL0.7psi) und die Zentrifugenretentionskapazität (CRC) zumindest teilweise entkoppelt werden können. Diese Oberflächennachvernetzung kann in wässriger Gelphase durchgeführt werden. Vorzugsweise werden aber getrocknete, gemahlene und abgesiebte Polymerpartikel (Grundpolymer) an der Oberfläche mit einem Oberflächennachvernetzer beschichtet und thermisch oberflächennach- vernetzt. Dazu geeignete Vernetzer sind Verbindungen, die mit mindestens zwei Carboxylat- gruppen der Polymerpartikel kovalente Bindungen bilden können. WO 2021/105038 A1 und die ältere PCT-Anmeldung mit dem Aktenzeichen PCT/EP2022/059572 offenbaren Pyrazole als Polymerisationsinhibitoren. Aufgabe der vorliegenden Erfindung war die Bereitstellung eines verbesserten Verfahrens zur Herstellung von farbstabilen Superabsorberpartikeln. Gelöst wurde die Aufgabe durch ein Verfahren zur Herstellung von oberflächennachvernetzten Superabsorberpartikeln durch Polymerisation einer wässrigen Monomerlösung oder -suspen- sion, enthaltend 221049 2 a) mindestens ein ethylenisch ungesättigtes, säuregruppentragendes Monomer, das zumin- dest teilweise neutralisiert ist, b) mindestens einen Vernetzer und c) mindestens einen Initiator, wobei die wässrige Monomerlösung oder -suspension zu einem Polymergel polymerisiert wird, das erhaltene Polymergel optional zerkleinert wird, das Polymergel anschließend getrocknet wird, das getrocknete Polymergel optional gemahlen und klassiert wird, das getrocknete Poly- mergel anschließend thermisch oberflächennachvernetzt und gekühlt wird, dadurch gekenn- zeichnet, dass die erhaltenen Polymerpartikel vor, während oder nach der thermischen Oberflä- chennachvernetzung mit mindestens einem Pyrazol beschichtet werden. Das Pyrazol ist üblicherweise ein monomeres Pyrazol. Das im erfindungsgemäßen Verfahren eingesetzte Pyrazol ist vorzugsweise eine Verbindung der allgemeinen Formel (I)
Figure imgf000003_0001
wobei R1 C1- oder C2-Alkyl ist, R2 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist und R3 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist, oder eine Verbindung der allgemeinen Formel (II) 221049 3
Figure imgf000004_0001
wobei R4 C1- oder C2-Alkyl ist, R5 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist und R6 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist, oder eine Verbindung der allgemeinen Formel (III)
Figure imgf000004_0002
wobei R7 C1- oder C2-Alkyl ist, R8 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist und R9 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist, ist. Die Alkylgruppen können gerade, verzweigt und/oder zyklisch sein. Das im erfindungsgemäßen Verfahren eingesetzte Pyrazol ist besonders bevorzugt eine Ver- bindung der allgemeinen Formel (I) 221049 4
Figure imgf000005_0001
wobei R1 C1-Alkyl ist, R2 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist und R3 H, C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist, oder eine Verbindung der allgemeinen Formel (II)
Figure imgf000005_0002
wobei R4 C1-Alkyl ist, R5 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist und R6 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist, oder eine Verbindung der allgemeinen Formel (III)
Figure imgf000005_0003
221049 5 wobei R7 C1-Alkyl ist, R8 H, C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist und R9 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist, ist. Die Alkylgruppen können gerade, verzweigt und/oder zyklisch sein. Die Verbindungen der allgemeinen Formel (I) liegen im Gleichgewicht mit ihrer Keto-Form vor. Beispielsweise ist 1,3-Dimethyl-5-pyrazolon die Keto-Form von 1,3-Dimethyl-5-hydroxy-pyrazol. Das im erfindungsgemäßen Verfahren eingesetzte Pyrazol ist ganz besonders bevorzugt 1,3-Dimethyl-5-pyrazolon, 1,5-Dimethyl-3-ethyl-4-hydroxypyrazol oder 1,5-Dimethyl-4-hydroxy- 3-phenylpyrazol. Die Polymerpartikel werden mit vorzugsweise 0,001 bis 1 Gew.-%, besonders bevorzugt 0,005 bis 0,2 Gew.-%, ganz besonders bevorzugt 0,01 bis 0,1 Gew.-%, des Pyrazols beschichtet, je- weils bezogen auf die Polymerpartikel. Der vorliegenden Erfindung liegt die Erkenntnis zugrunde, dass Pyrazole die Farbstabilität von Superabsorbern deutlich verbessern. Acrylsäure ist die bevorzugte ethylenisch ungesättigte Carbonsäure. Peroxodisulfat, insbeson- dere Ammoniumperoxodisulfat, Natriumperoxodisulfat und/oder Kaliumperoxodisulfat, ist der bevorzugte Initiator c). Im Folgenden wird die Herstellung der Superabsorber näher erläutert: Die Superabsorber werden durch Polymerisation einer Monomerlösung hergestellt und sind üb- licherweise wasserunlöslich. Die ethylenisch ungesättigten, säuregruppentragenden Monomere a) sind vorzugsweise was- serlöslich, d.h. die Löslichkeit in Wasser bei 23°C beträgt typischerweise mindestens 1 g/100 g 221049 6 Wasser, vorzugsweise mindestens 5 g/100 g Wasser, besonders bevorzugt mindestens 25 g/100 g Wasser, ganz besonders bevorzugt mindestens 35 g/100 g Wasser. Geeignete Monomere sind beispielsweise ethylenisch ungesättigte Carbonsäuren, wie Acryl- säure, Methacrylsäure, und Itaconsäure. Besonders bevorzugte Monomere sind Acrylsäure und Methacrylsäure. Ganz besonders bevorzugt ist Acrylsäure. Die ethylenisch ungesättigten, säuregruppentragenden Monomere a) sind üblicherweise teil- weise neutralisiert. Die Neutralisation wird auf der Stufe der Monomeren durchgeführt. Dies ge- schieht üblicherweise durch Einmischung des Neutralisationsmittels als wässrige Lösung oder bevorzugt auch als Feststoff. Der Neutralisationsgrad beträgt vorzugsweise von 40 bis 85 mol- %, besonders bevorzugt von 50 bis 80 mol-%, ganz besonders bevorzugt von 60 bis 75 mol-%, wobei die üblichen Neutralisationsmittel verwendet werden können, vorzugsweise Alkalimetall- hydroxide, Alkalimetalloxide, Alkalimetallkarbonate oder Alkalimetallhydrogenkarbonate sowie deren Mischungen. Statt Alkalimetallsalzen können auch Ammoniumsalze verwendet werden. Natrium und Kalium sind als Alkalimetalle besonders bevorzugt, ganz besonders bevorzugt sind jedoch Natriumhydroxid, Natriumkarbonat oder Natriumhydrogenkarbonat sowie deren Mi- schungen, insbesondere Natriumhydroxid. Die Monomere enthalten üblicherweise Polymerisationsinhibitoren, vorzugsweise Hydrochinon- halbether, als Lagerstabilisator. Geeignete Vernetzer b) sind Verbindungen mit mindestens zwei zur Vernetzung geeigneten Gruppen. Derartige Gruppen sind beispielsweise ethylenisch ungesättigte Gruppen, die in die Polymerkette radikalisch einpolymerisiert werden können, und funktionelle Gruppen, die mit den Säuregruppen des Monomers kovalente Bindungen ausbilden können. Weiterhin sind auch po- lyvalente Metallsalze, die mit mindestens zwei Säuregruppen des Monomeren koordinative Bin- dungen ausbilden können, als Vernetzer geeignet. Geeignete Vernetzer b) sind beispielsweise Ethylenglykoldimethacrylat, Diethylenglykoldiac- rylat, Polyethylenglykoldiacrylat, Allylmethacrylat, Trimethylolpropantriacrylat, Triallylamin, Tet- raallylammoniumchlorid, Tetraallyloxyethan, wie in EP 0530438 A1 beschrieben, Di- und Triac- rylate, wie in EP 0547847 A1, EP 0559476 A1, EP 0632068 A1, WO 93/21237 A1, WO 03/104299 A1, WO 03/104300 A1, WO 03/104301 A1 und DE 10331450 A1 beschrieben, ge- mischte Acrylate, die neben Acrylatgruppen weitere ethylenisch ungesättigte Gruppen enthal- ten, wie in DE 10331456 A1 und DE 10355401 A1 beschrieben, oder Vernetzermischungen, 221049 7 wie beispielsweise in DE 19543368 A1, DE 19646484 A1, WO 90/15830 A1 und WO 02/032962 A2 beschrieben. Die Menge an Vernetzer b) beträgt vorzugsweise 0,05 bis 1,5 Gew.-%, besonders bevorzugt 0,1 bis 1 Gew.-%, ganz besonders bevorzugt 0,15 bis 0,6 Gew.-%, jeweils berechnet auf die Gesamtmenge an eingesetztem Monomer. Mit steigendem Vernetzergehalt sinkt die Zentrifu- genretentionskapazität (CRC) und die Absorption unter einem Druck von 21,0 g/cm² (AUL0.3psi) durchläuft ein Maximum. Als Initiatoren c) können sämtliche unter den Polymerisationsbedingungen Radikale erzeu- gende Verbindungen eingesetzt werden, beispielsweise thermische Initiatoren, Redox-Initiato- ren, Photoinitiatoren. Geeignete Redox-Initiatoren sind Natriumperoxodisulfat/Ascorbinsäure, Wasserstoffperoxid/Ascorbinsäure, Natriumperoxodisulfat/Natriumbisulfit und Wasserstoffper- oxid/Natriumbisulfit. Vorzugsweise werden Mischungen aus thermischen Initiatoren und Redox- Initiatoren eingesetzt, wie Natriumperoxodisulfat/Wasserstoffperoxid/Ascorbinsäure. Als redu- zierende Komponente wird vorzugsweise das Dinatriumsalz der 2-Hydroxy-2-sulfonatoessig- säure oder ein Gemisch aus dem Natriumsalz der 2-Hydroxy-2-sulfinatoessigsäure, dem Dinat- riumsalz der 2-Hydroxy-2-sulfonatoessigsäure und Natriumbisulfit eingesetzt. Derartige Gemi- sche sind als Brüggolite® FF6 und Brüggolite® FF7 (Brüggemann Chemicals; Heilbronn; Deutschland) erhältlich. Der Wassergehalt der Monomerlösung beträgt vorzugsweise von 40 bis 75 Gew.-%, besonders bevorzugt von 45 bis 70 Gew.-%, ganz besonders bevorzugt von 50 bis 65 Gew.-%. Mit stei- gendem Wassergehalt steigt der Energieaufwand bei der anschließenden Trocknung und mit sinkendem Wassergehalt kann die Polymerisationswärme nur noch ungenügend abgeführt wer- den. Die Temperatur der Monomerlösung beträgt vorzugsweise von 10 bis 90°C, besonders bevor- zugt von 20 bis 70°C, ganz besonders bevorzugt von 30 bis 50°C. Die bevorzugten Polymerisationsinhibitoren benötigen für eine optimale Wirkung gelösten Sau- erstoff. Daher kann die Monomerlösung vor der Polymerisation durch Inertisierung, d.h. Durch- strömen mit einem inerten Gas, vorzugsweise Stickstoff oder Kohlendioxid, von gelöstem Sau- erstoff befreit werden. Vorzugsweise wird der Sauerstoffgehalt der Monomerlösung vor der Po- lymerisation auf weniger als 1 Gew.-ppm, besonders bevorzugt auf weniger als 0,5 Gew.-ppm, ganz besonders bevorzugt auf weniger als 0,1 Gew.-ppm, gesenkt. 221049 8 Geeignete Reaktoren für die Polymerisation sind beispielsweise Knetreaktoren oder Bandreak- toren. Im Kneter wird das bei der Polymerisation einer wässrigen Monomerlösung oder -sus- pension entstehende Polymergel durch beispielsweise gegenläufige Rührwellen kontinuierlich zerkleinert, wie in WO 2001/038402 A1 beschrieben. Die Polymerisation auf dem Band wird beispielsweise in DE 3825366 A1 und US 6,241,928 beschrieben. Bei der Polymerisation in einem Bandreaktor entsteht ein Polymergel, das zerkleinert werden muss, beispielsweise in ei- nem Extruder oder Kneter. Zur Verbesserung der Trocknungseigenschaften kann das mittels eines Kneters erhaltene zer- kleinerte Polymergel zusätzlich extrudiert werden. Das Polymergel wird dann üblicherweise mit einem Umluftbandtrockner getrocknet bis der Rest- feuchtegehalt vorzugsweise 0,5 bis 10 Gew.-%, besonders bevorzugt 1 bis 7 Gew.-%, ganz be- sonders bevorzugt 2 bis 5 Gew.-%, beträgt, wobei der Restfeuchtegehalt gemäß der von der EDANA empfohlenen Testmethode Nr. WSP 230.2-05 "Mass Loss Upon Heating" bestimmt wird. Bei einer zu hohen Restfeuchte weist das getrocknete Polymergel eine zu niedrige Glas- übergangstemperatur Tg auf und ist nur schwierig weiter zu verarbeiten. Bei einer zu niedrigen Restfeuchte ist das getrocknete Polymergel zu spröde und in den anschließenden Zerkleine- rungsschritten fallen unerwünscht große Mengen an Polymerpartikeln mit zu niedriger Partikel- größe („fines“) an. Der Feststoffgehalt des Polymergels beträgt vor der Trocknung vorzugsweise von 25 und 90 Gew.-%, besonders bevorzugt von 35 bis 70 Gew.-%, ganz besonders bevorzugt von 40 bis 60 Gew.-%. Anschließend wird das getrocknete Polymergel gebrochen und optional grob zerkleinert. Das getrocknete Polymergel wird hiernach üblicherweise gemahlen und klassiert, wobei zur Mahlung üblicherweise ein- oder mehrstufige Walzenstühle, bevorzugt zwei- oder dreistufige Walzenstühle, Stiftmühlen, Hammermühlen oder Schwingmühlen, eingesetzt werden können. Die mittlere Partikelgröße der als Produktfraktion abgetrennten Polymerpartikel beträgt vorzugs- weise von 150 bis 850 µm, besonders bevorzugt von 250 bis 600 µm, ganz besonders von 300 bis 500 µm. Die mittlere Partikelgröße der Produktfraktion kann mittels der von der EDANA empfohlenen Testmethode Nr. WSP 220.2 (05) "Partikel Size Distribution" ermittelt werden, wo- bei die Massenanteile der Siebfraktionen kumuliert aufgetragen werden und die mittlere Parti- kelgröße graphisch bestimmt wird. Die mittlere Partikelgröße ist hierbei der Wert der Maschen- weite, der sich für kumulierte 50 Gew.-% ergibt. 221049 9 Die Polymerpartikel werden zur weiteren Verbesserung der Eigenschaften thermisch oberflä- chennachvernetzt. Geeignete Oberflächennachvernetzer sind Verbindungen, die Gruppen ent- halten, die mit mindestens zwei Carboxylatgruppen der Polymerpartikel kovalente Bindungen bilden können. Geeignete Verbindungen sind beispielsweise polyfunktionelle Amine, polyfunkti- onelle Amidoamine, polyfunktionelle Epoxide, wie in EP 0083022 A2, EP 0543303 A1 und EP 0937736 A2 beschrieben, di- oder polyfunktionelle Alkohole, wie in DE 3314019 A1, DE 35 23617 A1 und EP 0450922 A2 beschrieben, oder ß-Hydroxyalkylamide, wie in DE 10204938 A1 und US 6,239,230 beschrieben. Die Menge an Oberflächennachvernetzer beträgt vorzugsweise 0,001 bis 2 Gew.-%, besonders bevorzugt 0,01 bis 1 Gew.-%, ganz besonders bevorzugt 0,03 bis 0,7 Gew.-%, jeweils bezogen auf die Polymerpartikel. In einer bevorzugten Ausführungsform der vorliegenden Erfindung werden zusätzlich zu den Oberflächennachvernetzern polyvalente Kationen auf die Partikeloberfläche aufgebracht. Die im erfindungsgemäßen Verfahren einsetzbaren polyvalenten Kationen sind beispielsweise zweiwertige Kationen, wie die Kationen von Zink, Magnesium, Kalzium und Strontium, dreiwer- tige Kationen, wie die Kationen von Aluminium, Eisen, Chrom, Seltenerden und Mangan, vier- wertige Kationen, wie die Kationen von Titan und Zirkonium. Als Gegenion sind Chlorid, Bromid, Hydroxid, Sulfat, Hydrogensulfat, Carbonat, Hydrogencarbonat, Nitrat, Phosphat, Hydrogen- phosphat, Dihydrogenphosphat und Carboxylat, wie Acetat und Lactat, möglich. Aluminiumhyd- roxid, Aluminiumsulfat und Aluminiumlaktat sind bevorzugt. Die Einsatzmenge an polyvalentem Kation beträgt beispielsweise 0,001 bis 1,5 Gew.-%, vor- zugsweise 0,005 bis 1 Gew.-%, besonders bevorzugt 0,02 bis 0,8 Gew.-%. jeweils bezogen auf das Polymer. Die Oberflächennachvernetzung wird üblicherweise so durchgeführt, dass eine Lösung des Oberflächennachvernetzers auf die getrockneten Polymerpartikel aufgesprüht wird. Im An- schluss an das Aufsprühen werden die mit Oberflächennachvernetzer beschichteten Polymer- partikel thermisch behandelt. Das Aufsprühen einer Lösung des Oberflächennachvernetzers wird vorzugsweise in Mischern mit bewegten Mischwerkzeugen, wie Schneckenmischer, Scheibenmischer und Schaufelmi- scher, durchgeführt. Besonders bevorzugt sind Horizontalmischer, wie Schaufelmischer, ganz besonders bevorzugt sind Vertikalmischer. Die Unterscheidung in Horizontalmischer und Vertikalmischer erfolgt über die Lagerung der Mischwelle, d.h. Horizontalmischer haben eine horizontal gelagerte Mischwelle und Vertikalmischer haben eine vertikal gelagerte Mischwelle. Geeignete Mischer sind beispielsweise Horizontale Pflugschar® Mischer (Gebr. Lödige Maschi- nenbau GmbH; Paderborn; Deutschland), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Doetinchem; Niederlande), Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA) und Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; Niederlande). Es ist aber auch möglich die Oberflächennachvernetzerlösung in einem Wirbelbett aufzusprühen. Die Oberflächennachvernetzer werden typischerweise als wässrige Lösung eingesetzt. Über den Gehalt an nichtwässrigem Lösungsmittel bzw. Gesamtlösungsmittelmenge kann die Ein- dringtiefe des Oberflächennachvernetzers in die Polymerpartikel eingestellt werden. Die thermische Behandlung wird vorzugsweise in Kontakttrocknern, besonders bevorzugt Schaufeltrocknern, ganz besonders bevorzugt Scheibentrocknern, durchgeführt. Geeignete Trockner sind beispielsweise Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Leingarten; Deutschland), Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Leingarten; Deutschland), Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; USA) und Nara Paddle Dryer (NARA Machinery Europe; Frechen; Deutschland). Überdies können auch Wirbelschichttrockner eingesetzt werden. Die Oberflächennachvernetzung kann im Mischer selbst erfolgen, durch Beheizung des Mantels oder Einblasen von Warmluft. Ebenso geeignet ist ein nachgeschalteter Trockner, wie beispiels- weise ein Hordentrockner, ein Drehrohrofen oder eine beheizbare Schnecke. Besonders vorteil- haft wird in einem Wirbelschichttrockner gemischt und thermisch oberflächennachvernetzt. Bevorzugte Reaktionstemperaturen liegen im Bereich 100 bis 250°C, bevorzugt 110 bis 220°C, besonders bevorzugt 120 bis 210°C, ganz besonders bevorzugt 130 bis 200°C. Die bevorzugte Verweilzeit bei dieser Temperatur beträgt vorzugsweise mindestens 10 Minuten, besonders be- vorzugt mindestens 20 Minuten, ganz besonders bevorzugt mindestens 30 Minuten, und übli- cherweise höchstens 60 Minuten. Anschließend können die oberflächennachvernetzten Polymerpartikel erneut klassiert werden, wobei zu kleine und/oder zu große Polymerpartikel abgetrennt und in das Verfahren rückgeführt werden. Die oberflächennachvernetzten Polymerpartikel können zur weiteren Verbesserung der Eigen- schaften beschichtet oder nachbefeuchtet werden. 11 Die Nachbefeuchtung wird vorzugsweise bei 30 bis 80°C, besonders bevorzugt bei 35 bis 70°C, ganz besonders bevorzugt bei 40 bis 60°C, durchgeführt. Bei zu niedrigen Temperaturen nei- gen die Polymerpartikel zum Verklumpen und bei höheren Temperaturen verdampft bereits merklich Wasser. Die zur Nachbefeuchtung eingesetzte Wassermenge beträgt vorzugsweise von 1 bis 10 Gew.-%, besonders bevorzugt von 2 bis 8 Gew.-%, ganz besonders bevorzugt von 3 bis 5 Gew.-%. Durch die Nachbefeuchtung wird die mechanische Stabilität der Polymerparti- kel erhöht und deren Neigung zur statischen Aufladung vermindert. Vorteilhaft wird die Nachbe- feuchtung im Kühler nach der thermischen Oberflächennachvernetzung durchgeführt. Geeignete Beschichtungen zur Verbesserung der Quellgeschwindigkeit sowie der Gelbettper- meabilität (GBP) sind beispielsweise anorganische inerte Substanzen, wie wasserunlösliche Metallsalze, organische Polymere, kationische Polymere sowie zwei- oder mehrwertige Metall- kationen. Geeignete Beschichtungen zur Staubbindung sind beispielsweise Polyole. Geeignete Beschichtungen gegen die unerwünschte Verbackungsneigung der Polymerpartikel sind bei- spielsweise pyrogene Kieselsäure, wie Aerosil® 200, Fällungskieselsäure, wie Sipernat® D17, und Tenside, wie Span® 20. Ein weiterer Gegenstand der vorliegenden Erfindung sind mit mindestens einem Pyrazol be- schichtete Superabsorberpartikel. Das Pyrazol ist üblicherweise ein monomeres Pyrazol. Das Pyrazol ist vorzugsweise eine Verbindung der allgemeinen Formel (I)
Figure imgf000012_0001
wobei R1 C1- oder C2-Alkyl ist, R2 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist und R3 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist, 221049 12 oder eine Verbindung der allgemeinen Formel (II)
Figure imgf000013_0001
wobei R4 C1- oder C2-Alkyl ist, R5 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist und R6 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist, oder eine Verbindung der allgemeinen Formel (III)
Figure imgf000013_0002
wobei R7 C1- oder C2-Alkyl ist, R8 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist und R9 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist, Das Pyrazol ist besonders bevorzugt eine Verbindung der allgemeinen Formel (I) 221049 13
Figure imgf000014_0001
wobei R1 C1-Alkyl ist, R2 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist und R3 H, C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist, oder eine Verbindung der allgemeinen Formel (II)
Figure imgf000014_0002
wobei R4 C1-Alkyl ist, R5 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist und R6 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist, oder eine Verbindung der allgemeinen Formel (III)
Figure imgf000014_0003
221049 14 wobei R7 C1- Alkyl ist, R8 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist und R9 H, C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist, ist. Die Alkylgruppen können gerade, verzweigt und/oder zyklisch sein. Die Verbindungen der allgemeinen Formel (I) liegen im Gleichgewicht mit ihrer Keto-Form vor. Beispielsweise ist 1,3-Dimethyl-5-pyrazolon die Keto-Form von 1,3-Dimethyl-5-hydroxy-pyrazol. Das Pyrazol ist ganz besonders bevorzugt 1,3-Dimethyl-5-pyrazolon, 1,5-Dimethyl-3-ethyl-4- hydroxypyrazol oder 1,5-Dimethyl-4-hydroxy-3-phenylpyrazol. Die Superabsorberpartikel wurden mit vorzugsweise 0,001 bis 1 Gew.-%, besonders bevorzugt 0,005 bis 0,2 Gew.-%, ganz besonders bevorzugt 0,01 bis 0,1 Gew.-%, des Pyrazols beschich- tet, jeweils bezogen auf die Polymerpartikel. Acrylsäure ist die bevorzugte ethylenisch ungesättigte Carbonsäure. Peroxodisulfat, insbeson- dere Ammoniumperoxodisulfat, Natriumperoxodisulfat und/oder Kaliumperoxodisulfat, ist der bevorzugte Initiator c). Ein weiterer Gegenstand der vorliegenden Erfindung sind erfindungsgemäße Superabsorber- partikel enthaltende Hygieneartikel. Methoden: Die Messungen sollten, wenn nicht anders angegeben, bei einer Umgebungstemperatur von 23 ± 2°C und einer relativen Luftfeuchte von 50 ± 10% durchgeführt werden. Die Superabsor- berpartikel werden vor der Messung gut durchmischt. 221049 15 Farbwert (CIE-Farbnummern [L, a, b]) Die Messung des Farbwertes erfolgt mittels eines Kolorimetermodells "LabScan XE Spectrome- ter" (HunterLab; Reston; USA) nach dem CIELAB-Verfahren (Hunterlab, Band 8, 1996, Heft 7, Seiten 1 bis 4). Farben werden durch die Koordinaten L, a und b eines dreidimensionalen Sys- tems beschrieben. L charakterisiert die Helligkeit, wobei L = 0 schwarz und L = 100 weiß ist. Die Werte für a und b beschreiben die Position der Farbe auf der Farbachse Rot/Grün bzw. Gelb/Blau, wobei positive a-Werte für rote Farben, negative a-Werte für grüne Farben, positive b-Werte für gelbe Farben und negative b-Werte für blaue Farben stehen. Der Hunter 60-Wert (HC60) ist ein Maß für den Weißgrad von Oberflächen und wird als L-3b definiert, d.h. je niedriger der Wert, desto dunkler und gelber ist die Farbe. Der Test wurde mit einer Tissue Culture Dish (Durchmesser von 35 mm und Höhe von 10 mm) und einer Port Plate Opening von 0,5 Zoll durchgeführt. Die Messung des Farbwertes erfolgt in Übereinstimmung mit der Tristimulus-Methode nach DIN 5033-6. Yellowness Index (YI) Der Yellowness Index (YI) wird gemäß ASTM D1925 oder gemäß ASTM E313 gemessen. Je höher der Wert, desto dunkler und gelber ist die Farbe. Beispiele Beispiel 1 Durch kontinuierliches Mischen von entionisiertem Wasser, 50 gew.-%iger Natronlauge und Ac- rylsäure wurde eine Monomerlösung hergestellt, so dass der Neutralisationsgrad 74,0 mol-% entsprach. Der Wassergehalt der Monomerlösung betrug 59,0 Gew.-%. Als Vernetzer wurde 3-fach ethoxiliertes Glyzerintriacrylat (ca.85 gew.-%ig) verwendet. Die Ein- satzmenge betrug 1,34 kg pro t Monomerlösung. 221049 16 Zur Initiierung der radikalischen Polymerisation wurden pro t Monomerlösung 2,14 kg einer 0,25gew.-%igen wässriger Wasserstoffperoxid-Lösung, 2,91 kg einer 15gew.-%igen wässrigen Natriumperoxodisulfat-Lösung und 1,97 kg einer 1gew.-%igen wässrigen Ascorbinsäure-Lösung eingesetzt. Die Monomerlösung wurde in einen Reaktor vom Typ List Contikneter mit einem Volumen 6,3m³ (LIST AG, Arisdorf, Schweiz) dosiert. Der Durchsatz der Monomerlösung betrug ca.20 t/h. Die Reaktionslösung hatte am Zulauf eine Temperatur von 23,5°C. Zwischen dem Zugabepunkt für den Vernetzer und den Zugabestellen für die Wasserstoffper- oxid- und Natriumperoxodisulfatlösungen wurde die Monomerlösung mit Stickstoff inertisiert. Ascorbinsäure wurde direkt in den Reaktor dosiert. Nach ca.50% der Verweilzeit wurden zusätzlich ca.1.000 kg/h durch Zerkleinerung und Klas- sierung im Herstellungsprozess anfallende Polymerpartikel mit einer Partikelgröße von weniger als 150 µm in den Reaktor dosiert. Die Verweilzeit der Reaktionsmischung im Reaktor betrug ca.15 Minuten. Das erhaltene Polymergel wurde mittels eines oszillierenden Förderbandes auf das Förderband eines Umluftbandtrockners aufgegeben. Der Umluftbandtrockner hatte eine Länge von 48 m. Das Förderband des Umluftbandtrockners hatte eine effektive Breite von 4,4 m. Auf dem Um- luftbandtrockner wurde das wässrige Polymergel kontinuierlich mit Luft/Gasgemisch (ca.175°C) umströmt und getrocknet. Die Verweilzeit im Umluftbandtrockner betrug 37 Minuten. Das getrocknete Polymergel wurde mittels eines dreistufigen Walzenstuhls zerkleinert und auf eine Partikelgröße von 150 bis 700 µm abgesiebt. Polymerpartikel mit einer Partikelgröße von weniger als 150 µm wurden abgetrennt. Polymerpartikel mit einer Partikelgröße von größer 700 µm wurden in die Zerkleinerung zurückgeführt. Polymerpartikel mit einer Partikelgröße im Be- reich von 150 bis 700 µm wurden thermisch oberflächennachvernetzt. Die Polymerpartikel wurden in einem Schugi Flexomix® (Hosokawa Micron B.V., Doetinchem, Niederlande mit einer Oberflächennachvernetzerlösung beschichtet und anschließend in einem NARA Paddle Dryer (GMF Gouda, Waddinxveen, Niederlande) 45 Minuten bei 175°C getrock- net. Es wurden folgende Mengen in den Schugi Flexomix® dosiert: 221049 17 7,5 t/h Polymerpartikel 334,50 kg/h Oberflächennachvernetzerlösung Die Oberflächennachvernetzerlösung enthielt 1,35 Gew.-% Ethylenglykoldiglycidylether, 44,84 Gew.-% 1,2-Propandiol und 53,81 Gew.-% Wasser. Nach dem Trocken wurden die oberflächennachvernetzten Polymerpartikel in einem NARA Paddle-Cooler (GMF Gouda, Waddinxveen, Niederlande) auf ca.60°C abgekühlt. Dabei wur- den die oberflächennachvernetzten Polymerpartikel mit 7,5 kg/h einer 50 gew.-%igen wässrigen Polyethylenglykol-Lösung (Polyethylenglykol mit einer mittleren Molmasse von 400 g/mol), 375 kg/h Wasser, 22,5 kg/h Aluminiumtrihydroxid („Aluminiumhydroxid Trockengel“, Artikelnummer 511066100, Dr. Paul Lohmann GmbH KG, Hauptstraße 2, 31860 Emmerthal, Deutschland) und 18,75 kg/h einer 1 gew.-%igen wässrigen Lösung von Sorbitanmonolaurat beschichtet. Beispiel 2 Jeweils 20 g Superabsorber aus Beispiel 1 wurden mit jeweils 200 Gew-ppm eines Pyrazols ge- mischt und 14 Tage im Klimaschrank bei 70°C und 80% relativer Feuchte gelagert. Die Ergebnisse sind in Tabelle 1 zusammengefasst:
Figure imgf000018_0001
*) Vergleichsbeispiel 221049 18 Beispiel 3 Jeweils 20 g Superabsorber aus Beispiel 1 wurden mit 1,5-Dimethyl-3-ethyl-4-hydroxypyrazol gemischt und 14 Tage im Klimaschrank bei 70°C und 80% relativer Feuchte gelagert. Die Ergebnisse sind in Tabelle 2 zusammengefasst:
Figure imgf000019_0001
*) Vergleichsbeispiel 221049 1 Process for producing color-stable superabsorbent particles The present invention relates to a process for producing color-stable superabsorbent particles, wherein an aqueous monomer solution or suspension is polymerized to form a polymer gel, the polymer gel obtained is optionally comminuted, the polymer gel is then dried, the dried polymer gel is optionally ground and classified, the dried polymer gel is then thermally surface-crosslinked and cooled, characterized in that after polymerization it is coated with a pyrazole. Superabsorbents are used to produce diapers, tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in agricultural horticulture. The superabsorbents are also referred to as water-absorbing polymers. The production of superabsorbents is described in the monograph "Modern Superabsorbent Polymer Technology", FL Buchholz and AT Graham, Wiley-VCH, 1998, pages 71 to 103. To improve the application properties, such as gel bed permeability (GBP) and absorption under a pressure of 49.2 g/cm² (AUL0.7psi), superabsorbent particles are generally surface-crosslinked. This increases the degree of crosslinking of the particle surface, which means that the absorption under a pressure of 49.2 g/cm² (AUL0.7psi) and the centrifuge retention capacity (CRC) can be at least partially decoupled. This surface-crosslinking can be carried out in an aqueous gel phase. Preferably, however, dried, ground and sieved polymer particles (base polymer) are coated on the surface with a surface-crosslinker and thermally surface-crosslinked. Suitable crosslinkers for this purpose are compounds that can form covalent bonds with at least two carboxylate groups of the polymer particles. WO 2021/105038 A1 and the older PCT application with the file number PCT/EP2022/059572 disclose pyrazoles as polymerization inhibitors. The object of the present invention was to provide an improved process for producing color-stable superabsorbent particles. The object was achieved by a process for producing surface-postcrosslinked superabsorbent particles by polymerizing an aqueous monomer solution or suspension containing 221049 2 a) at least one ethylenically unsaturated, acid group-bearing monomer which is at least partially neutralized, b) at least one crosslinker and c) at least one initiator, wherein the aqueous monomer solution or suspension is polymerized to form a polymer gel, the polymer gel obtained is optionally comminuted, the polymer gel is then dried, the dried polymer gel is optionally ground and classified, the dried polymer gel is then thermally surface-postcrosslinked and cooled, characterized in that the polymer particles obtained are coated with at least one pyrazole before, during or after the thermal surface-postcrosslinking. The pyrazole is usually a monomeric pyrazole. The pyrazole used in the process according to the invention is preferably a compound of the general formula (I)
Figure imgf000003_0001
where R 1 is C 1 or C 2 alkyl, R 2 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 3 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl, or a compound of the general formula (II) 221049 3
Figure imgf000004_0001
where R 4 is C 1 or C 2 alkyl, R 5 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 6 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl, or a compound of the general formula (III)
Figure imgf000004_0002
where R 7 is C 1 or C 2 alkyl, R 8 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 9 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl. The alkyl groups can be straight, branched and/or cyclic. The pyrazole used in the process according to the invention is particularly preferably a compound of the general formula (I) 221049 4
Figure imgf000005_0001
where R 1 is C 1 alkyl, R 2 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 3 is H, C 1 to C 3 alkyl or C 6 to C 8 arylalkyl, or a compound of the general formula (II)
Figure imgf000005_0002
where R 4 is C 1 alkyl, R 5 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 6 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl, or a compound of the general formula (III)
Figure imgf000005_0003
221049 5 where R 7 is C 1 alkyl, R 8 is H, C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 9 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl. The alkyl groups can be straight, branched and/or cyclic. The compounds of the general formula (I) are in equilibrium with their keto form. For example, 1,3-dimethyl-5-pyrazolone is the keto form of 1,3-dimethyl-5-hydroxypyrazole. The pyrazole used in the process according to the invention is very particularly preferably 1,3-dimethyl-5-pyrazolone, 1,5-dimethyl-3-ethyl-4-hydroxypyrazole or 1,5-dimethyl-4-hydroxy-3-phenylpyrazole. The polymer particles are coated with preferably 0.001 to 1% by weight, particularly preferably 0.005 to 0.2% by weight, very particularly preferably 0.01 to 0.1% by weight of the pyrazole, in each case based on the polymer particles. The present invention is based on the finding that pyrazoles significantly improve the color stability of superabsorbents. Acrylic acid is the preferred ethylenically unsaturated carboxylic acid. Peroxodisulfate, particularly ammonium peroxodisulfate, sodium peroxodisulfate and/or potassium peroxodisulfate, is the preferred initiator c). The production of the superabsorbents is explained in more detail below: The superabsorbents are produced by polymerizing a monomer solution and are usually insoluble in water. The ethylenically unsaturated, acid group-bearing monomers a) are preferably water-soluble, ie the solubility in water at 23°C is typically at least 1 g/100 g. 221049 6 Water, preferably at least 5 g/100 g water, particularly preferably at least 25 g/100 g water, very particularly preferably at least 35 g/100 g water. 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. Acrylic acid is very particularly preferred. The ethylenically unsaturated, acid group-bearing monomers a) are usually partially neutralized. The neutralization is carried out at the monomer stage. This is usually done by mixing in the neutralizing agent as an aqueous solution or preferably also as a solid. The degree of neutralization is preferably from 40 to 85 mol%, particularly preferably from 50 to 80 mol%, very particularly preferably from 60 to 75 mol%, it being possible to use the usual neutralizing agents, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal hydrogen carbonates and mixtures thereof. Ammonium salts can also be used instead of alkali metal salts. Sodium and potassium are particularly preferred as alkali metals, but sodium hydroxide, sodium carbonate or sodium hydrogen carbonate and mixtures thereof, especially sodium hydroxide, are very particularly preferred. The monomers usually contain polymerization inhibitors, preferably hydroquinone half ethers, as storage stabilizers. Suitable crosslinkers b) are compounds with at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups that can be radically polymerized into the polymer chain and functional groups that can form covalent bonds with the acid groups of the monomer. Furthermore, polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer are also suitable as crosslinking agents. 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 in EP 0530438 A1, di- and triacrylates, as described in EP 0547847 A1, EP 0559476 A1, EP 0632068 A1, WO 93/21237 A1, WO 03/104299 A1, WO 03/104300 A1, WO 03/104301 A1 and DE 10331450 A1, mixed acrylates which, in addition to acrylate groups, contain further ethylenically unsaturated groups, as described in DE 10331456 A1 and DE 10355401 A1, or crosslinker mixtures, 221049 7 as described for example in DE 19543368 A1, DE 19646484 A1, WO 90/15830 A1 and WO 02/032962 A2. The amount of crosslinker b) is preferably 0.05 to 1.5 wt. %, particularly preferably 0.1 to 1 wt. %, very particularly preferably 0.15 to 0.6 wt. %, in each case calculated on the total amount of monomer used. As the crosslinker content increases, the centrifuge retention capacity (CRC) decreases and the absorption under a pressure of 21.0 g/cm² (AUL0.3 psi) passes through a maximum. All compounds which generate radicals under the polymerization conditions can be used as initiators c), for example thermal initiators, redox initiators, photoinitiators. Suitable redox initiators are sodium peroxodisulfate/ascorbic acid, hydrogen peroxide/ascorbic acid, sodium peroxodisulfate/sodium bisulfite and hydrogen peroxide/sodium bisulfite. Preferably, mixtures of thermal initiators and redox initiators are used, such as sodium peroxodisulfate/hydrogen peroxide/ascorbic acid. 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 is preferably used as the reducing component. Such mixtures are available as Brüggolite® FF6 and Brüggolite® FF7 (Brüggemann Chemicals; Heilbronn; Germany). The water content of the monomer solution is preferably from 40 to 75% by weight, particularly preferably from 45 to 70% by weight, very particularly preferably from 50 to 65% by weight. As the water content increases, the energy required for the subsequent drying increases, and as the water content decreases, the heat of polymerization can only be dissipated insufficiently. The temperature of the monomer solution is preferably from 10 to 90°C, particularly preferably from 20 to 70°C, very particularly preferably from 30 to 50°C. The preferred polymerization inhibitors require dissolved oxygen for optimal effect. The monomer solution can therefore be freed of dissolved oxygen before polymerization by inerting, ie by flowing an inert gas, preferably nitrogen or carbon dioxide, through it. Preferably, the oxygen content of the monomer solution before polymerization is reduced to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight. 221049 8 Suitable reactors for polymerization are, for example, kneading reactors or belt reactors. In the kneader, the polymer gel formed during the polymerization of an aqueous monomer solution or suspension is continuously comminuted by, for example, counter-rotating agitator shafts, as described in WO 2001/038402 A1. Polymerization on the belt is described, for example, in DE 3825366 A1 and US 6,241,928. Polymerization in a belt reactor produces a polymer gel that must be comminuted, for example in an extruder or kneader. To improve the drying properties, the comminuted polymer gel obtained by means of a kneader can also be extruded. The polymer gel is then usually dried using a circulating air belt dryer until the residual moisture content is preferably 0.5 to 10% by weight, particularly preferably 1 to 7% by weight, very particularly preferably 2 to 5% by weight, the residual moisture content being determined according to test method no. WSP 230.2-05 "Mass Loss Upon Heating" recommended by EDANA. If the residual moisture is too high, the dried polymer gel has a glass transition temperature T g that is too low and is difficult to process further. If the residual moisture is too low, the dried polymer gel is too brittle and undesirably large amounts of polymer particles with too small a particle size ("fines") are produced in the subsequent comminution steps. The solids content of the polymer gel before drying is preferably between 25 and 90% by weight, particularly preferably between 35 and 70% by weight, very particularly preferably between 40 and 60% by weight. The dried polymer gel is then broken and optionally coarsely crushed. The dried polymer gel is then usually ground and classified, whereby single or multi-stage roller mills, preferably two or three-stage roller mills, pin mills, hammer mills or vibrating mills can usually be used for grinding. The average particle size of the polymer particles separated as a product fraction is preferably from 150 to 850 µm, particularly preferably from 250 to 600 µm, very particularly from 300 to 500 µm. The average particle size of the product fraction can be determined using the test method No. WSP 220.2 (05) "Particle Size Distribution" recommended by EDANA, whereby the mass fractions of the sieve fractions are plotted cumulatively and the average particle size is determined graphically. The average particle size is the value of the mesh size that results for a cumulative 50 wt.%. 221049 9 The polymer particles are thermally surface-crosslinked to further improve their properties. Suitable surface-crosslinkers are compounds that contain groups that can form covalent bonds 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 0083022 A2, EP 0543303 A1 and EP 0937736 A2, di- or polyfunctional alcohols, as described in DE 3314019 A1, DE 35 23617 A1 and EP 0450922 A2, or ß-hydroxyalkylamides, as described in DE 10204938 A1 and US 6,239,230. The amount of surface post-crosslinker is preferably 0.001 to 2% by weight, particularly preferably 0.01 to 1% by weight, very particularly preferably 0.03 to 0.7% by weight, based in each case on the polymer particles. In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the surface post-crosslinkers. The polyvalent cations that can be used in the process according to the invention are, for example, divalent cations, such as the cations of zinc, magnesium, calcium and strontium, trivalent cations, such as the cations of aluminum, iron, chromium, rare earths and manganese, tetravalent cations, such as the cations of titanium and zirconium. Chloride, bromide, hydroxide, sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate and carboxylate, such as acetate and lactate, are possible as counterions. Aluminum hydroxide, aluminum sulfate and aluminum lactate are preferred. The amount of polyvalent cation used is, for example, 0.001 to 1.5% by weight, preferably 0.005 to 1% by weight, particularly preferably 0.02 to 0.8% by weight, based in each case on the polymer. The surface post-crosslinking is usually carried out by spraying a solution of the surface post-crosslinker onto the dried polymer particles. Following spraying, the polymer particles coated with surface post-crosslinker are thermally treated. The spraying of a solution of the surface post-crosslinker is preferably carried out in mixers with moving mixing tools, such as screw mixers, disk mixers and paddle mixers. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are very particularly preferred. The distinction between horizontal mixers and Vertical mixers are based on the bearing 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 are, for example, horizontal Pflugschar® mixers (Gebr. Lödige 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 surface postcrosslinker solution in a fluidized bed. The surface postcrosslinkers are typically used as an aqueous solution. The penetration depth of the surface postcrosslinker into the polymer particles can be adjusted via the content of non-aqueous solvent or the total amount of solvent. The thermal treatment is preferably carried out in contact dryers, particularly preferably paddle dryers, most preferably disk dryers. Suitable dryers include Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; USA) and Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany). Fluidized bed dryers can also be used. Surface post-crosslinking can take place in 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 heatable screw, is also suitable. Mixing and thermal surface post-crosslinking are particularly advantageous in a fluidized bed dryer. Preferred reaction temperatures are in the range 100 to 250°C, preferably 110 to 220°C, particularly preferably 120 to 210°C, very particularly preferably 130 to 200°C. The preferred residence time at this temperature is preferably at least 10 minutes, particularly preferably at least 20 minutes, very particularly preferably at least 30 minutes, and usually at most 60 minutes. The surface-crosslinked polymer particles can then be classified again, with polymer particles that are too small and/or too large being separated off and returned to the process. The surface-crosslinked polymer particles can be coated or remoistened to further improve their properties. 11 The remoistening is preferably carried out at 30 to 80°C, particularly preferably at 35 to 70°C, very particularly preferably at 40 to 60°C. At temperatures that are too low, the polymer particles tend to clump together, and at higher temperatures, water evaporates noticeably. The amount of water used for remoistening is preferably from 1 to 10% by weight, particularly preferably from 2 to 8% by weight, very particularly preferably from 3 to 5% by weight. The remoistening increases the mechanical stability of the polymer particles and reduces their tendency to become statically charged. The remoistening is advantageously carried out in the cooler after the thermal surface post-crosslinking. Suitable coatings for improving the swelling rate and the gel bed permeability (GBP) are, for example, inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers and divalent or polyvalent metal cations. Suitable coatings for binding dust are, for example, polyols. Suitable coatings against the undesirable tendency of the polymer particles to cake are, for example, pyrogenic silica, such as Aerosil® 200, precipitated silica, such as Sipernat® D17, and surfactants, such as Span® 20. The present invention also relates to superabsorbent particles coated with at least one pyrazole. The pyrazole is usually a monomeric pyrazole. The pyrazole is preferably a compound of the general formula (I)
Figure imgf000012_0001
where R 1 is C 1 or C 2 alkyl, R 2 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 3 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl, 221049 12 or a compound of general formula (II)
Figure imgf000013_0001
where R 4 is C 1 or C 2 alkyl, R 5 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 6 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl, or a compound of the general formula (III)
Figure imgf000013_0002
where R 7 is C 1 or C 2 alkyl, R 8 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 9 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl. The pyrazole is particularly preferably a compound of the general formula (I) 221049 13
Figure imgf000014_0001
where R 1 is C 1 alkyl, R 2 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 3 is H, C 1 to C 3 alkyl or C 6 to C 8 arylalkyl, or a compound of the general formula (II)
Figure imgf000014_0002
where R 4 is C 1 alkyl, R 5 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 6 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl, or a compound of the general formula (III)
Figure imgf000014_0003
221049 14 where R 7 is C 1 alkyl, R 8 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 9 is H, C 1 to C 3 alkyl or C 6 to C 8 arylalkyl. The alkyl groups can be straight, branched and/or cyclic. The compounds of the general formula (I) are in equilibrium with their keto form. For example, 1,3-dimethyl-5-pyrazolone is the keto form of 1,3-dimethyl-5-hydroxypyrazole. The pyrazole is very particularly preferably 1,3-dimethyl-5-pyrazolone, 1,5-dimethyl-3-ethyl-4-hydroxypyrazole or 1,5-dimethyl-4-hydroxy-3-phenylpyrazole. The superabsorbent particles were coated with preferably 0.001 to 1% by weight, particularly preferably 0.005 to 0.2% by weight, very particularly preferably 0.01 to 0.1% by weight of the pyrazole, based in each case on the polymer particles. Acrylic acid is the preferred ethylenically unsaturated carboxylic acid. Peroxodisulfate, particularly ammonium peroxodisulfate, sodium peroxodisulfate and/or potassium peroxodisulfate, is the preferred initiator c). The present invention further relates to hygiene articles containing superabsorbent particles according to the invention. Methods: Unless otherwise stated, the measurements should be carried out at an ambient temperature of 23 ± 2°C and a relative humidity of 50 ± 10%. The superabsorbent particles are mixed well before the measurement. 221049 15 Color value (CIE color numbers [L, a, b]) The color value is measured using a colorimeter model "LabScan XE Spectrometer"(HunterLab;Reston; USA) according to the CIELAB method (Hunterlab, Volume 8, 1996, Issue 7, pages 1 to 4). Colors are described by the coordinates L, a and b of a three-dimensional system. L characterizes the brightness, where L = 0 is black and L = 100 is white. The values for a and b describe the position of the color on the red/green or yellow/blue color axis, where positive a values represent red colors, negative a values represent green colors, positive b values represent yellow colors and negative b values represent blue colors. The Hunter 60 value (HC60) is a measure of the whiteness of surfaces and is defined as L-3b, i.e. the lower the value, the darker and yellower the color. The test was carried out using a tissue culture dish (diameter of 35 mm and height of 10 mm) and a port plate opening of 0.5 inch. The color value is measured in accordance with the tristimulus method according to DIN 5033-6. Yellowness Index (YI) The Yellowness Index (YI) is measured according to ASTM D1925 or according to ASTM E313. The higher the value, the darker and yellower the color. Examples Example 1 A monomer solution was prepared by continuously mixing deionized water, 50 wt.% sodium hydroxide solution and acrylic acid so that the degree of neutralization was 74.0 mol%. The water content of the monomer solution was 59.0 wt.%. Triple ethoxylated glycerol triacrylate (approx. 85% by weight) was used as crosslinker. The amount used was 1.34 kg per t of monomer solution. 221049 16 To initiate the radical polymerization, 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 were used per t of monomer solution. The monomer solution was dosed into a List Contikneter reactor with a volume of 6.3 m³ (LIST AG, Arisdorf, Switzerland). The throughput of the monomer solution was approximately 20 t/h. The reaction solution had a temperature of 23.5°C at the inlet. The monomer solution was rendered inert with nitrogen between the addition point for the crosslinker and the addition points for the hydrogen peroxide and sodium peroxodisulfate solutions. Ascorbic acid was dosed directly into the reactor. After about 50% of the residence time, an additional 1,000 kg/h of polymer particles with a particle size of less than 150 µm that were produced by crushing and classifying in the production process were dosed into the reactor. The residence time of the reaction mixture in the reactor was about 15 minutes. The polymer gel obtained was fed onto the conveyor belt of a circulating air belt dryer using an oscillating conveyor belt. The circulating air belt dryer was 48 m long. The conveyor belt of the circulating air belt dryer had an effective width of 4.4 m. On the circulating air belt dryer, the aqueous polymer gel was continuously blown around with an air/gas mixture (about 175°C) and dried. The residence time in the circulating air belt dryer was 37 minutes. The dried polymer gel was crushed using a three-stage roller mill and sieved to a particle size of 150 to 700 µm. Polymer particles with a particle size of less than 150 µm were separated. Polymer particles with a particle size of greater than 700 µm were returned to the shredding process. Polymer particles with a particle size in the range of 150 to 700 µm were thermally surface-crosslinked. The polymer particles were coated with a surface-crosslinking solution in a Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, Netherlands) and then dried in a NARA Paddle Dryer (GMF Gouda, Waddinxveen, Netherlands) for 45 minutes at 175°C. The following quantities were dosed into the Schugi Flexomix®: 221049 17 7.5 t/h polymer particles 334.50 kg/h surface crosslinker solution The surface crosslinker solution contained 1.35 wt.% ethylene glycol diglycidyl ether, 44.84 wt.% 1,2-propanediol and 53.81 wt.% water. After drying, the surface crosslinked polymer particles were cooled to approx. 60°C in a NARA paddle cooler (GMF Gouda, Waddinxveen, Netherlands). The surface-crosslinked polymer particles were coated with 7.5 kg/h of a 50 wt.% aqueous polyethylene glycol solution (polyethylene glycol with an average molecular weight of 400 g/mol), 375 kg/h of water, 22.5 kg/h of aluminum trihydroxide ("aluminum hydroxide dry gel", article number 511066100, Dr. Paul Lohmann GmbH KG, Hauptstraße 2, 31860 Emmerthal, Germany) and 18.75 kg/h of a 1 wt.% aqueous solution of sorbitan monolaurate. Example 2 20 g of superabsorbent from Example 1 were mixed with 200 ppm by weight of a pyrazole and stored for 14 days in a climate cabinet at 70°C and 80% relative humidity. The results are summarized in Table 1:
Figure imgf000018_0001
*) Comparison example 221049 18 Example 3 20 g of superabsorbent from Example 1 were mixed with 1,5-dimethyl-3-ethyl-4-hydroxypyrazole and stored for 14 days in a climate chamber at 70°C and 80% relative humidity. The results are summarized in Table 2:
Figure imgf000019_0001
*) Comparison example

Claims

221049 19 Patentansprüche 1. Verfahren zur Herstellung von oberflächennachvernetzen Superabsorberpartikeln durch Po- lymerisation einer wässrigen Monomerlösung oder -suspension, enthaltend a) mindestens eine ethylenisch ungesättigte Carbonsäure, die zumindest teilweise neutralisiert ist, b) mindestens einen Vernetzer und c) mindestens einen Initiator, wobei die wässrige Monomerlösung oder -suspension zu einem Polymergel polymerisiert wird, das erhaltene Polymergel optional zerkleinert wird, das Polymergel anschließend ge- trocknet wird, das getrocknete Polymergel optional gemahlen und klassiert wird, das ge- trocknete Polymergel anschließend thermisch oberflächennachvernetzt und gekühlt wird, dadurch gekennzeichnet, dass die erhaltenen Polymerpartikel vor, während oder nach der thermischen Oberflächennachvernetzung mit mindestens einem Pyrazol beschichtet wer- den. 221049 19 claims 1. Process for producing surface-postcrosslinked superabsorbent particles by polymerizing an aqueous monomer solution or suspension containing a) at least one ethylenically unsaturated carboxylic acid which is at least partially neutralized, b) at least one crosslinker and c) at least one initiator, wherein the aqueous monomer solution or suspension is polymerized to form a polymer gel, the polymer gel obtained is optionally comminuted, the polymer gel is then dried, the dried polymer gel is optionally ground and classified, the dried polymer gel is then thermally surface-postcrosslinked and cooled, characterized in that the polymer particles obtained are coated with at least one pyrazole before, during or after the thermal surface postcrosslinking.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass das Pyrazol eine Verbindung der allgemeinen Formel (I)
Figure imgf000020_0001
wobei R1 C1- oder C2-Alkyl ist, R2 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist und R3 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist, oder eine Verbindung der allgemeinen Formel (II) 221049 20
Figure imgf000021_0001
wobei R4 C1- oder C2-Alkyl ist, R5 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist und R6 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist, oder eine Verbindung der allgemeinen Formel (III)
Figure imgf000021_0002
wobei R7 C1- oder C2-Alkyl ist, R8 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist und R9 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist, ist. 2. Process according to claim 1, characterized in that the pyrazole is a compound of the general formula (I)
Figure imgf000020_0001
where R 1 is C 1 or C 2 alkyl, R 2 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 3 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl, or a compound of the general formula (II) 221049 20
Figure imgf000021_0001
where R 4 is C 1 or C 2 alkyl, R 5 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 6 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl, or a compound of the general formula (III)
Figure imgf000021_0002
where R 7 is C 1 or C 2 alkyl, R 8 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 9 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl.
3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, dass Acrylsäure als ethyle- nisch ungesättigte Carbonsäure eingesetzt wird. 3. Process according to claim 1 or 2, characterized in that acrylic acid is used as ethylenically unsaturated carboxylic acid.
4. Verfahren nach einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, dass ein Peroxo- disulfat, insbesondere Ammoniumperoxodisulfat, Natriumperoxodisulfat und/oder Kalium- peroxodisulfat, als Initiator c) eingesetzt wird. 4. Process according to one of claims 1 to 3, characterized in that a peroxodisulfate, in particular ammonium peroxodisulfate, sodium peroxodisulfate and/or potassium peroxodisulfate, is used as initiator c).
5. Verfahren nach einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, dass die Polymer- partikel mit 0,001 bis 1 Gew.-%, bezogen auf die Polymerpartikel, des Pyrazols beschichtet werden. 5. Process according to one of claims 1 to 4, characterized in that the polymer particles are coated with 0.001 to 1 wt.%, based on the polymer particles, of the pyrazole.
6. Verfahren nach einem der Ansprüche 1 bis 5, dadurch gekennzeichnet, dass die Polymer- partikel mit 0,01 bis 0,1 Gew.-%, bezogen auf die Polymerpartikel, des Pyrazols beschichtet werden. 6. Process according to one of claims 1 to 5, characterized in that the polymer particles are coated with 0.01 to 0.1% by weight, based on the polymer particles, of the pyrazole.
7. Verfahren nach einem der Ansprüche 2 bis 6, dadurch gekennzeichnet, dass in der allge- meinen Formel (I) R1 C1-Alkyl ist, R2 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist und R3 H, C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist oder in der allgemeinen Formel (II) R4 C1-Alkyl ist, R5 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist und R6 C1- bis C3-Alkyl oder C6- bis C8- Arylalkyl ist oder in der allgemeinen Formel (III) R7 C1-Alkyl ist, R8 H, C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist und R9 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist. 7. Process according to one of claims 2 to 6, characterized in that in the general formula (I) R 1 is C 1 alkyl, R 2 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 3 is H, C 1 to C 3 alkyl or C 6 to C 8 arylalkyl or in the general formula (II) R 4 is C 1 alkyl, R 5 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 6 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl or in the general formula (III) R 7 is C 1 alkyl, R 8 is H, C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 9 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl.
8. Verfahren nach einem der Ansprüche 2 bis 7, dadurch gekennzeichnet, dass 1,3-Dimethyl- 5-pyrazolon als Verbindung der allgemeinen Formel (I) eingesetzt wird oder 1,5-Dimethyl-3- ethyl-4-hydroxypyrazol oder 1,5-Dimethyl-4-hydroxy-3-phenylpyrazol als Verbindung der all- gemeinen Formel (II) eingesetzt wird. 8. Process according to one of claims 2 to 7, characterized in that 1,3-dimethyl-5-pyrazolone is used as the compound of the 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 the general formula (II).
9. Superabsorberpartikel, erhältlich nach einem Verfahren der Ansprüche 1 bis 8, wobei die Superabsorberpartikel mit mindestens einem Pyrazol beschichtet wurden. 9. Superabsorbent particles obtainable by a process of claims 1 to 8, wherein the superabsorbent particles have been coated with at least one pyrazole.
10. Superabsorberpartikel nach Anspruch 9, wobei das Pyrazol eine Verbindung der allgemei- nen Formel (I)
Figure imgf000022_0001
wobei R1 C1- oder C2-Alkyl ist, R2 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist und R3 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist, oder eine Verbindung der allgemeinen Formel (II)
Figure imgf000023_0001
wobei R4 C1- oder C2-Alkyl ist, R5 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist und R6 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist, oder eine Verbindung der allgemeinen Formel (III)
Figure imgf000023_0002
wobei R7 C1- oder C2-Alkyl ist, R8 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist und R9 H, C1- bis C20-Alkyl oder C6- bis C20-Arylalkyl ist, 10. Superabsorbent particles according to claim 9, wherein the pyrazole is a compound of the general formula (I)
Figure imgf000022_0001
where R 1 is C 1 or C 2 alkyl, R 2 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 3 is H, C 1 - to C 20 -alkyl or C 6 - to C 20 -arylalkyl, or a compound of the general formula (II)
Figure imgf000023_0001
where R 4 is C 1 or C 2 alkyl, R 5 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 6 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl, or a compound of the general formula (III)
Figure imgf000023_0002
where R 7 is C 1 or C 2 alkyl, R 8 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl and R 9 is H, C 1 to C 20 alkyl or C 6 to C 20 arylalkyl,
11. Superabsorberpartikel nach Anspruch 9 oder 10, wobei die Superabsorberpartikel mit 0,001 bis 1 Gew.-%, bezogen auf die Polymerpartikel, des Pyrazols beschichtet wurden. 23 12. Superabsorberpartikel nach einem der Ansprüche 9 bis 11, wobei die Superabsorberpartikel mit 0,01 bis 0,1 Gew.-%, bezogen auf die Polymerpartikel, des Pyrazols beschichtet wur- den. 13. Superabsorberpartikel nach einem der Ansprüche 10 bis 12, wobei in der allgemeinen For- mel (I) R1 C1-Alkyl ist, R2 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist und R3 H, C1- bis C3- Alkyl oder C6- bis C8-Arylalkyl ist oder in der allgemeinen Formel (II) R4 C1-Alkyl ist, R5 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist und R6 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist oder in der allgemeinen Formel (III) R7 C1-Alkyl ist, R8 H, C1- bis C3-Alkyl oder C6- bis C8-A- rylalkyl ist und R9 C1- bis C3-Alkyl oder C6- bis C8-Arylalkyl ist. 14. Superabsorberpartikel nach einem der Ansprüche 10 bis 13, wobei die Verbindung der all- gemeinen Formel (I) 1,3-Dimethyl-5-pyrazolon ist oder die Verbindung der allgemeinen For- mel (II) 1,5-Dimethyl-3-ethyl-4-hydroxypyrazol oder 1,5-Dimethyl-4-hydroxy-3-phenylpyrazol ist. 15. Hygieneartikel, enthaltend eine Superabsorberpartikel nach einem der Ansprüche 9 bis 14. 11. Superabsorbent particles according to claim 9 or 10, wherein the superabsorbent particles have been coated with 0.001 to 1 wt.%, based on the polymer particles, of the pyrazole. 23 12. Superabsorbent particles according to one of claims 9 to 11, wherein the superabsorbent particles were coated with 0.01 to 0.1 wt. %, based on the polymer particles, of the pyrazole. 13. Superabsorbent particles according to one of claims 10 to 12, wherein in the general formula (I) R 1 is C 1 alkyl, R 2 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 3 is H, C 1 to C 3 alkyl or C 6 to C 8 arylalkyl or in the general formula (II) R 4 is C 1 alkyl, R 5 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 6 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl or in the general formula (III) R 7 is C 1 alkyl, R 8 is H, C 1 to C 3 alkyl or C 6 to C 8 arylalkyl and R 9 is C 1 to C 3 alkyl or C 6 to C 8 arylalkyl. 14. Superabsorbent particles according to one of claims 10 to 13, wherein the compound of the general formula (I) is 1,3-dimethyl-5-pyrazolone or the compound of the general formula (II) is 1,5-dimethyl-3-ethyl-4-hydroxypyrazole or 1,5-dimethyl-4-hydroxy-3-phenylpyrazole. 15. Hygiene article containing a superabsorbent particle according to one of claims 9 to 14.
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Citations (32)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0083022A2 (en) 1981-12-30 1983-07-06 Seitetsu Kagaku Co., Ltd. Water-absorbent resin having improved water-absorbency and improved water-dispersibility and process for producing same
DE3314019A1 (en) 1982-04-19 1984-01-12 Nippon Shokubai Kagaku Kogyo Co. Ltd., Osaka ABSORBENT OBJECT
DE3523617A1 (en) 1984-07-02 1986-01-23 Nippon Shokubai Kagaku Kogyo Co. Ltd., Osaka WATER ABSORBING AGENT
DE3825366A1 (en) 1987-07-28 1989-02-09 Dai Ichi Kogyo Seiyaku Co Ltd METHOD FOR CONTINUOUS PRODUCTION OF AN ACRYLIC POLYMER GEL
WO1990015830A1 (en) 1989-06-12 1990-12-27 Weyerhaeuser Company Hydrocolloid polymer
EP0450922A2 (en) 1990-04-02 1991-10-09 Nippon Shokubai Kagaku Kogyo Co. Ltd. Method for production of fluid stable aggregate
EP0530438A1 (en) 1991-09-03 1993-03-10 Hoechst Celanese Corporation A superabsorbent polymer having improved absorbency properties
EP0543303A1 (en) 1991-11-22 1993-05-26 Hoechst Aktiengesellschaft Hydrophilic hydrogels having a high swelling capacity
EP0547847A1 (en) 1991-12-18 1993-06-23 Nippon Shokubai Co., Ltd. Process for producing water-absorbent resin
EP0559476A1 (en) 1992-03-05 1993-09-08 Nippon Shokubai Co., Ltd. Method for the production of absorbent resin
WO1993021237A1 (en) 1992-04-16 1993-10-28 The Dow Chemical Company Crosslinked hydrophilic resins and method of preparation
EP0632068A1 (en) 1993-06-18 1995-01-04 Nippon Shokubai Co., Ltd. Process for preparing absorbent resin
DE19543368A1 (en) 1995-11-21 1997-05-22 Stockhausen Chem Fab Gmbh Water-absorbing polymers with improved properties, process for their preparation and their use
DE19646484A1 (en) 1995-11-21 1997-05-22 Stockhausen Chem Fab Gmbh Liquid-absorbing polymers, process for their preparation and their use
EP0937736A2 (en) 1998-02-24 1999-08-25 Nippon Shokubai Co., Ltd. Crosslinking a water-absorbing agent
US6239230B1 (en) 1999-09-07 2001-05-29 Bask Aktiengesellschaft Surface-treated superabsorbent polymer particles
WO2001038402A1 (en) 1999-11-20 2001-05-31 Basf Aktiengesellschaft Method for continuously producing cross-linked fine-particle geleous polymerizates
US6241928B1 (en) 1998-04-28 2001-06-05 Nippon Shokubai Co., Ltd. Method for production of shaped hydrogel of absorbent resin
EP1110999A2 (en) * 1999-11-23 2001-06-27 Great Lakes Chemical (Europe) GmbH Stabilizing mixtures for organic polymers
WO2002032962A2 (en) 2000-10-20 2002-04-25 Millennium Pharmaceuticals, Inc. Compositions of human proteins and method of use thereof
DE10204938A1 (en) 2002-02-07 2003-08-21 Stockhausen Chem Fab Gmbh Process for post-crosslinking of a water absorbing polymer surface with a cyclic urea useful in foams, fibers, films, cables, especially sealing materials, liquid absorbing hygiene articles, packaging materials, and soil additives
WO2003104299A1 (en) 2002-06-11 2003-12-18 Basf Aktiengesellschaft Method for the production of esters of polyalcohols
WO2003104301A1 (en) 2002-06-11 2003-12-18 Basf Aktiengesellschaft (meth)acrylic esters of polyalkoxylated glycerine
WO2003104300A1 (en) 2002-06-01 2003-12-18 Basf Aktiengesellschaft (meth)acrylic esters of polyalkoxylated trimethylolpropane
DE10331450A1 (en) 2003-07-10 2005-01-27 Basf Ag (Meth) acrylic esters of monoalkoxylated polyols and their preparation
DE10331456A1 (en) 2003-07-10 2005-02-24 Basf Ag (Meth) acrylic esters of alkoxylated unsaturated polyol ethers and their preparation
DE10355401A1 (en) 2003-11-25 2005-06-30 Basf Ag (Meth) acrylic esters of unsaturated amino alcohols and their preparation
WO2011157656A2 (en) * 2010-06-14 2011-12-22 Basf Se Water-absorbing polymer particles with improved colour stability
US20130260988A1 (en) * 2012-03-30 2013-10-03 Basf Se Color-Stable Superabsorbent
WO2019011793A1 (en) * 2017-07-12 2019-01-17 Basf Se Process for producing superabsorbent polymer particles
EP3828159A1 (en) * 2019-11-28 2021-06-02 Basf Se Storage and transport stabilizers for polymerizable compounds
WO2022223336A1 (en) * 2021-04-20 2022-10-27 Basf Se Polymerization inhibitors

Patent Citations (33)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0083022A2 (en) 1981-12-30 1983-07-06 Seitetsu Kagaku Co., Ltd. Water-absorbent resin having improved water-absorbency and improved water-dispersibility and process for producing same
DE3314019A1 (en) 1982-04-19 1984-01-12 Nippon Shokubai Kagaku Kogyo Co. Ltd., Osaka ABSORBENT OBJECT
DE3523617A1 (en) 1984-07-02 1986-01-23 Nippon Shokubai Kagaku Kogyo Co. Ltd., Osaka WATER ABSORBING AGENT
DE3825366A1 (en) 1987-07-28 1989-02-09 Dai Ichi Kogyo Seiyaku Co Ltd METHOD FOR CONTINUOUS PRODUCTION OF AN ACRYLIC POLYMER GEL
WO1990015830A1 (en) 1989-06-12 1990-12-27 Weyerhaeuser Company Hydrocolloid polymer
EP0450922A2 (en) 1990-04-02 1991-10-09 Nippon Shokubai Kagaku Kogyo Co. Ltd. Method for production of fluid stable aggregate
EP0530438A1 (en) 1991-09-03 1993-03-10 Hoechst Celanese Corporation A superabsorbent polymer having improved absorbency properties
EP0543303A1 (en) 1991-11-22 1993-05-26 Hoechst Aktiengesellschaft Hydrophilic hydrogels having a high swelling capacity
EP0547847A1 (en) 1991-12-18 1993-06-23 Nippon Shokubai Co., Ltd. Process for producing water-absorbent resin
EP0559476A1 (en) 1992-03-05 1993-09-08 Nippon Shokubai Co., Ltd. Method for the production of absorbent resin
WO1993021237A1 (en) 1992-04-16 1993-10-28 The Dow Chemical Company Crosslinked hydrophilic resins and method of preparation
EP0632068A1 (en) 1993-06-18 1995-01-04 Nippon Shokubai Co., Ltd. Process for preparing absorbent resin
DE19543368A1 (en) 1995-11-21 1997-05-22 Stockhausen Chem Fab Gmbh Water-absorbing polymers with improved properties, process for their preparation and their use
DE19646484A1 (en) 1995-11-21 1997-05-22 Stockhausen Chem Fab Gmbh Liquid-absorbing polymers, process for their preparation and their use
EP0937736A2 (en) 1998-02-24 1999-08-25 Nippon Shokubai Co., Ltd. Crosslinking a water-absorbing agent
US6241928B1 (en) 1998-04-28 2001-06-05 Nippon Shokubai Co., Ltd. Method for production of shaped hydrogel of absorbent resin
US6239230B1 (en) 1999-09-07 2001-05-29 Bask Aktiengesellschaft Surface-treated superabsorbent polymer particles
WO2001038402A1 (en) 1999-11-20 2001-05-31 Basf Aktiengesellschaft Method for continuously producing cross-linked fine-particle geleous polymerizates
EP1110999A2 (en) * 1999-11-23 2001-06-27 Great Lakes Chemical (Europe) GmbH Stabilizing mixtures for organic polymers
WO2002032962A2 (en) 2000-10-20 2002-04-25 Millennium Pharmaceuticals, Inc. Compositions of human proteins and method of use thereof
DE10204938A1 (en) 2002-02-07 2003-08-21 Stockhausen Chem Fab Gmbh Process for post-crosslinking of a water absorbing polymer surface with a cyclic urea useful in foams, fibers, films, cables, especially sealing materials, liquid absorbing hygiene articles, packaging materials, and soil additives
WO2003104300A1 (en) 2002-06-01 2003-12-18 Basf Aktiengesellschaft (meth)acrylic esters of polyalkoxylated trimethylolpropane
WO2003104299A1 (en) 2002-06-11 2003-12-18 Basf Aktiengesellschaft Method for the production of esters of polyalcohols
WO2003104301A1 (en) 2002-06-11 2003-12-18 Basf Aktiengesellschaft (meth)acrylic esters of polyalkoxylated glycerine
DE10331450A1 (en) 2003-07-10 2005-01-27 Basf Ag (Meth) acrylic esters of monoalkoxylated polyols and their preparation
DE10331456A1 (en) 2003-07-10 2005-02-24 Basf Ag (Meth) acrylic esters of alkoxylated unsaturated polyol ethers and their preparation
DE10355401A1 (en) 2003-11-25 2005-06-30 Basf Ag (Meth) acrylic esters of unsaturated amino alcohols and their preparation
WO2011157656A2 (en) * 2010-06-14 2011-12-22 Basf Se Water-absorbing polymer particles with improved colour stability
US20130260988A1 (en) * 2012-03-30 2013-10-03 Basf Se Color-Stable Superabsorbent
WO2019011793A1 (en) * 2017-07-12 2019-01-17 Basf Se Process for producing superabsorbent polymer particles
EP3828159A1 (en) * 2019-11-28 2021-06-02 Basf Se Storage and transport stabilizers for polymerizable compounds
WO2021105038A1 (en) 2019-11-28 2021-06-03 Basf Se Shelf life- and transport stabilizers for polymerizable compounds
WO2022223336A1 (en) * 2021-04-20 2022-10-27 Basf Se Polymerization inhibitors

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
F.L. BUCHHOLZA.T. GRAHAM: "Monographie", 1998, WILEY-VCH, article "Modern Superabsorbent Polymer Technology", pages: 71 - 103

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