Method for producing color-stable superabsorbent particles - Patent Application 20070122997

Abstract

The present invention relates to a process for producing color-stable superabsorbent particles, in which an aqueous monomer solution or suspension is polymerized to form a polymer gel, the resulting polymer gel is optionally milled, the polymer gel is then dried, the dried polymer gel is optionally ground and classified, a pyrazole is added before drying, and the aqueous monomer solution or suspension contains a polymerization inhibitor other than pyrazole.

Description

[Technical Field]

[0001] The present invention relates to a process for producing color-stable superabsorbent particles, in which an aqueous monomer solution or suspension is polymerized to form a polymer gel, the resulting polymer gel is optionally milled, the polymer gel is then dried, the dried polymer gel is optionally ground and classified, a pyrazole is added before drying, and the aqueous monomer solution or suspension contains a polymerization inhibitor other than pyrazole. [Background technology]

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

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

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

[0005] A previous PCT application having the reference number PCT / EP2022 / 059572 discloses 4-hydroxypyrazoles as polymerization inhibitors. Summary of the Invention [Problem to be solved by the invention]

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

[0007] The purpose of this is to a) at least one ethylenically unsaturated carboxylic acid that is at least partially neutralized; b) at least one cross-linking agent; c) at least one initiator; wherein the aqueous monomer solution or suspension is polymerized to form a polymer gel, the resulting polymer gel is optionally pulverized, the polymer gel is then dried, the dried polymer gel is optionally ground and classified, the dried polymer gel is then optionally thermally surface postcrosslinked and cooled, at least one pyrazole is added before drying, and the aqueous monomer solution or suspension contains at least one polymerization inhibitor other than pyrazole. DETAILED DESCRIPTION OF THE INVENTION

[0008] The pyrazole is typically a monomeric pyrazole.

[0009] The pyrazoles used in the process of the present invention preferably have the general formula (I) [ka] (In the formula, R1 is C1~C 20 -Alkyl or C6-C 20 -arylalkyl, R 2 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl, and R 3 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl) or a compound of general formula (II) [ka] (In the formula, R 4 is C1~C 20 -Alkyl or C6-C 20 -arylalkyl, R 5 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl, and R 6 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl) or a compound of general formula (III) [ka] (In the formula, R 7 is C1~C 20 -Alkyl or C6-C 20 -arylalkyl, R 8 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl, and R 9 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl) is a compound of

[0010] The pyrazoles used in the process of the present invention are more preferably of the general formula (I) [ka] (In the formula, R 1 is C1-C3-alkyl or C6-C8-arylalkyl, R 2 is C1-C3-alkyl or C6-C8-arylalkyl, and R 3 is H, C1-C3-alkyl or C6-C8-arylalkyl or a compound of general formula (II) [ka] (In the formula, R 4 is C1-C3-alkyl or C6-C8-arylalkyl, R 5 is C1-C3-alkyl or C6-C8-arylalkyl, and R 6 is C1-C3-alkyl or C6-C8-arylalkyl) or a compound of general formula (III) [ka] (In the formula, R 7 is C1-C3-alkyl or C6-C8-arylalkyl, R 8 is H, C1-C3-alkyl or C6-C8-arylalkyl, and R 9 is C1-C3-alkyl or C6-C8-arylalkyl) is a compound of

[0011] The alkyl group can be linear, branched and / or cyclic.

[0012] Compounds of general formula (I) exist in equilibrium with their keto forms, for example 1,3-dimethyl-5-pyrazolone is the keto form of 1,3-dimethyl-5-hydroxypyrazole.

[0013] The pyrazole used in the process of the present invention is most preferably 1,3-dimethyl-5-pyrazolone, 3-ethyl-4-hydroxy-1-isopropylpyrazole, 1,5-dimethyl-3-ethyl-4-hydroxypyrazole or 1,5-dimethyl-4-hydroxy-3-phenylpyrazole.

[0014] The monomer solution or suspension preferably contains 0.001% to 1% by weight, more preferably 0.005% to 0.2% by weight, most preferably 0.01% to 0.1% by weight of pyrazole, in each case based on the ethylenically unsaturated carboxylic acid a).

[0015] The polymerization inhibitors are not subject to any restrictions. Suitable examples are all polymerization inhibitors that are suitable for inhibiting the polymerization of acrylic acid, preferably hydroquinone monomethyl ether.

[0016] The monomer solution or suspension preferably contains 0.0001% to 0.1% by weight, more preferably 0.0005% to 0.02% by weight, most preferably 0.001% to 0.01% by weight of a polymerization inhibitor other than a pyrazole, in each case based on the ethylenically unsaturated carboxylic acid a).

[0017] The present invention is based on the finding that pyrazoles significantly improve the color stability of superabsorbents.

[0018] Acrylic acid is the preferred ethylenically unsaturated carboxylic acid. Peroxodisulfates, in particular ammonium peroxodisulfate, sodium peroxodisulfate and / or potassium peroxodisulfate, are preferred initiators c).

[0019] The manufacture of superabsorbents is described in detail below.

[0020] Superabsorbents are made by polymerizing a monomer solution and are typically water-insoluble.

[0021] The ethylenically unsaturated monomers having acid groups are preferably water-soluble, i.e., their solubility in water at 23°C is typically at least 1 g / 100 g water, preferably at least 5 g / 100 g water, more preferably at least 25 g / 100 g water and most preferably at least 35 g / 100 g water.

[0022] Suitable monomers are, for example, ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Very particularly preferred is acrylic acid.

[0023] Ethylenically unsaturated monomers having acid groups are typically partially neutralized. Neutralization is carried out at the monomer stage. Neutralization is typically carried out by mixing a neutralizing agent as an aqueous solution or, preferably, as a solid. The degree of neutralization is preferably 40 to 85 mol %, more preferably 50 to 80 mol %, and most preferably 60 to 75 mol %. Customary neutralizing agents, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal bicarbonates, and mixtures thereof, can be used. Instead of alkali metal salts, ammonium salts can also be used. Particularly preferred alkali metals are sodium and potassium, but sodium hydroxide, sodium carbonate or sodium bicarbonate, and mixtures thereof, especially sodium hydroxide, are very particularly preferred.

[0024] The monomer typically contains a polymerization inhibitor, preferably a hydroquinone monoether, as a storage stabilizer.

[0025] Suitable crosslinking agents are compounds having at least two groups suitable for crosslinking. Such groups are, for example, a functional group that can form a covalent bond with an ethylenically unsaturated group capable of free radical polymerization in the polymer chain and an acid group of the monomer. In addition, polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer are also suitable as crosslinking agents.

[0026] Suitable crosslinkers are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 0 530 438 A1, EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93 / 21237 A1, WO 03 / 104299 A1, WO 03 / 104300 A1, WO 03 / 104301 A1 and DE 103 31 450 A1. A1, mixed acrylates which contain further ethylenically unsaturated groups as well as acrylate groups, such as those described in DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinker mixtures, such as those described in DE 195 43 368 A1, DE 196 46 484 A1, WO 90 / 15830 A1 and WO 02 / 032962 A2.

[0027] The amount of crosslinker is preferably 0.05% to 1.5% by weight, more preferably 0.1% to 1% by weight, and most preferably 0.15% to 0.6% by weight, calculated in each case based on the total amount of monomers used. As the crosslinker content increases, the centrifuge retention capacity (CRC) decreases, reaching 21.0 g / cm. 2 The absorbency under pressure (AUL 0.3 psi) exceeds the maximum value.

[0028] The initiator used can be any compound that generates free radicals under polymerization conditions, such as a thermal initiator, a redox initiator, or a photoinitiator. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. It is preferable to use a mixture of a thermal initiator and a redox initiator, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid. The reducing component used is preferably the disodium salt of 2-hydroxy-2-sulfonatoacetic acid or the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, a mixture of the disodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such mixtures are known as Brueggolite® FF6 and Brueggolite® FF6. ) It is available as FF7 (Brueggemann Chemicals; Heilbronn; Germany).

[0029] The water content of the monomer solution is preferably 40% to 75% by weight, more preferably 45% to 70% by weight, and most preferably 50% to 65% by weight. If the water content is high, the energy consumption in the subsequent drying will increase, and if the water content is low, the heat of polymerization may simply not be sufficiently removed.

[0030] The temperature of the monomer solution is preferably 10 to 90°C, particularly preferably 20 to 70°C, very particularly preferably 30 to 50°C.

[0031] To function optimally, preferred polymerization inhibitors require dissolved oxygen. Therefore, the monomer solution may be inerted, i.e., flushed with an inert gas, preferably nitrogen or carbon dioxide, to remove dissolved oxygen before polymerization. Preferably, the oxygen content of the monomer solution is reduced to less than 1 ppm by weight, more preferably less than 0.5 ppm by weight, and most preferably less than 0.1 ppm by weight before polymerization.

[0032] Suitable reactors for polymerization are, for example, kneader reactors or belt reactors. In kneaders, the polymer gel formed during the polymerization of aqueous monomer solutions or suspensions is continuously comminuted, for example, by counter-rotating stirrer shafts, as described in WO 2001 / 038402 A1. Belt polymerization is described, for example, in DE 38 25 366 A1 and U.S. Pat. No. 6,241,928. Polymerization in a belt reactor forms a polymer gel, which must be comminuted, for example, in an extruder or kneader.

[0033] To improve the drying properties, the ground polymer gel obtained by the kneader can be further extruded.

[0034] The polymer gel is then dried, typically by an air-circulating belt dryer, until the residual moisture content is preferably 0.5-10 wt%, more preferably 1-7 wt%, and most preferably 2-5 wt%, as measured by EDANA recommended test method No. WSP 230.2-05 "Mass Loss on Heating." If the residual moisture content is too high, the glass transition temperature T of the dried polymer gel may be increased. gIf the residual moisture content is too low, the dried polymer gel may become too brittle, and the subsequent grinding step may result in an unnecessarily large amount of excessively small polymer particles ("fines"). The solids content of the polymer gel before drying is preferably 25% to 90% by weight, more preferably 35% to 70% by weight, and most preferably 40% to 60% by weight. The dried polymer gel is then ground, and optionally coarsely ground.

[0035] The dried polymer gel is then typically milled and classified, and the equipment used for milling can typically be a single or multi-roll mill, preferably a two or three roll mill, a pin mill, a hammer mill, or a vibratory mill.

[0036] The average particle size of the polymer particles removed as the product fraction is preferably 150 to 850 μm, more preferably 250 to 600 μm, and most particularly 300 to 500 μm. The average particle size of the product fraction can be measured by EDANA recommended test method No. WSP 220.2(05) "Particle Size Distribution," in which the mass proportions of the selected fractions are plotted in cumulative form and the average particle size is determined graphically. The average particle size in this specification is the mesh size value resulting from 50% cumulative weight.

[0037] To further improve the properties, the polymer particles can be thermally crosslinked on the surface.Suitable surface crosslinking agents are compounds that contain a group that can form a covalent bond with at least two carboxylate groups of the polymer particles.Suitable compounds are, for example, multifunctional amines, multifunctional amidoamines, multifunctional epoxides as described in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, difunctional or multifunctional alcohols as described in DE 33 14 019 A1, DE 35 23 617 A1 and EP 0 450 922 A2, or β-hydroxyalkylamides as described in DE 102 04 938 A1 and U.S. Pat. No. 6,239,230.

[0038] The amount of surface postcrosslinker is preferably 0.001% to 2% by weight, more preferably 0.01% to 1% by weight and most preferably 0.03% to 0.7% by weight, in each case based on the polymer particles.

[0039] In a preferred embodiment of the present invention, in addition to the surface postcrosslinker, a multivalent cation is applied to the particle surface.

[0040] Polyvalent cations that can be used in the process of the present invention include, for example, divalent cations such as those of zinc, magnesium, calcium, and strontium; trivalent cations such as those of aluminum, iron, chromium, rare earths, and manganese; and tetravalent cations such as those of titanium and zirconium. Possible counterions are chloride, bromide, hydroxide, sulfate, hydrogensulfate, carbonate, bicarbonate, nitrate, phosphate, hydrogenphosphate, dihydrogenphosphate, and carboxylates, such as acetate and lactate. Aluminum hydroxide, aluminum sulfate, and aluminum lactate are preferred.

[0041] The amount of polyvalent cation used is for example 0.001% to 1.5% by weight, preferably 0.005% to 1% by weight and more preferably 0.02% to 0.8% by weight, in each case based on the polymer.

[0042] Surface postcrosslinking is typically carried out by spraying a solution of the surface postcrosslinker onto dried polymer particles. After spray application, the polymer particles coated with the surface postcrosslinker are subjected to a heat treatment.

[0043] The spray application of the solution of surface postcrosslinker is preferably carried out using a mixer with moving mixing implements, such as a screw mixer, a disk mixer and a paddle mixer.Horizontal mixers, such as a paddle mixer, are particularly preferred, and vertical mixers are particularly preferred.Horizontal mixers and vertical mixers are distinguished by the position of the mixing shaft, that is, horizontal mixers have horizontally mounted mixing shafts, and vertical mixers have vertically mounted mixing shafts.Suitable mixers are, for example, the horizontal Pflugschar® plowshare mixer (Gebr. Loedige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta continuous mixer (Hosokawa Micron BV; Doetinchem; Netherlands), Processall Mixmill mixer (Processall Incorporated; Cincinnati; USA) and Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; Netherlands).However, it is also possible to spray the solution of surface postcrosslinker in a fluidized bed.

[0044] The surface post-crosslinking agent is typically used in the form of an aqueous solution. The penetration depth of the surface post-crosslinking agent into the polymer particles can be adjusted by the content of the non-aqueous solvent and the total amount of the solvent.

[0045] The heat treatment is preferably carried out in a contact dryer, more preferably a paddle dryer, and most preferably a disk dryer. Suitable dryers include, for example, the Hosokawa Bepex® horizontal paddle dryer (Hosokawa Micron GmbH; Leingarten; Germany), the Hosokawa Bepex® disk dryer (Hosokawa Micron GmbH; Leingarten; Germany), the Holo-Flite® dryer (Metso Minerals Industries Inc.; Danville; USA), and the Nara paddle dryer (NARA Machinery Europe; Frechen; Germany). Furthermore, a fluidized bed dryer can also be used.

[0046] Surface after crosslinking can be carried out in the mixer body by heating the jacket or blowing hot air into it.Also suitable is downstream dryer, such as tray dryer, rotary tube oven or heatable screw.It is particularly advantageous to mix in fluidized bed dryer and carry out thermal surface after crosslinking.

[0047] Preferred reaction temperatures are in the range of 100 to 250° C., preferably 110 to 220° C., more preferably 120 to 210° C., and most preferably 130 to 200° C. Preferred residence times at this temperature are preferably at least 10 minutes, more preferably at least 20 minutes, most preferably at least 30 minutes, and typically up to 60 minutes.

[0048] The surface postcrosslinked polymer particles can then be classified again to remove overly small and / or overly large polymer particles and reuse them in the process.

[0049] To further improve properties, the surface postcrosslinked polymer particles can be coated or rewetted.

[0050] Remoistening is preferably carried out at 30 to 80°C, more preferably 35 to 70°C, and most preferably 40 to 60°C. At excessively low temperatures, the polymer particles tend to form agglomerates, while at higher temperatures, water evaporates quickly and noticeably. The amount of water used for remoistening is preferably 1 to 10% by weight, more preferably 2 to 8% by weight, and most preferably 3 to 5% by weight. Remoistening increases the mechanical stability of the polymer particles and reduces their tendency to become electrostatically charged. Remoistening is advantageously carried out in a cooler after thermal surface postcrosslinking.

[0051] Suitable coatings for improving swelling rate and gel bed permeability (GBP) include, for example, inorganic inert materials such as water-insoluble metal salts, organic polymers, cationic polymers, and divalent or polyvalent metal cations. Suitable coatings for dust adsorption include, for example, polyols. Suitable coatings for combating the undesirable tendency of polymer particles to caking include, for example, fumed silica such as Aerosil® 200, precipitated silica such as Sipernat® D17, and surfactants such as Span® 20.

[0052] The present invention further provides superabsorbent particles comprising at least one pyrazole and at least one polymerization inhibitor other than a pyrazole.

[0053] The pyrazole is typically a monomeric pyrazole.

[0054] The pyrazole preferably has the general formula (I) [ka] (In the formula, R 1 is C1~C 20 -Alkyl or C6-C 20 -arylalkyl, R 2 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl, and R3 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl) or a compound of general formula (II) [ka] (In the formula, R 4 is C1~C 20 -Alkyl or C6-C 20 -arylalkyl, R 5 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl, and R 6 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl) or a compound of general formula (III) [ka] (In the formula, R 7 is C1~C 20 -Alkyl or C6-C 20 -arylalkyl, R 8 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl, and R 9 H, C1~C 20 -Alkyl or C6-C 20 -arylalkyl) is a compound of

[0055] The pyrazoles used in the process of the present invention are more preferably of the general formula (I) [ka] (In the formula, R 1is C1-C3-alkyl or C6-C8-arylalkyl, R 2 is C1-C3-alkyl or C6-C8-arylalkyl, and R 3 is H, C1-C3-alkyl or C6-C8-arylalkyl or a compound of general formula (II) [ka] (In the formula, R 4 is C1-C3-alkyl or C6-C8-arylalkyl, R 5 is C1-C3-alkyl or C6-C8-arylalkyl, and R 6 is C1-C3-alkyl or C6-C8-arylalkyl) or a compound of general formula (III) [ka] (In the formula, R 7 is C1-C3-alkyl or C6-C8-arylalkyl, R 8 is H, C1-C3-alkyl or C6-C8-arylalkyl, and R 9 is C1-C3-alkyl or C6-C8-arylalkyl) is a compound of

[0056] The alkyl group can be linear, branched and / or cyclic.

[0057] Compounds of general formula (I) exist in equilibrium with their keto forms, for example 1,3-dimethyl-5-pyrazolone is the keto form of 1,3-dimethyl-5-hydroxypyrazole.

[0058] The pyrazole used in the process of the present invention is most preferably 1,3-dimethyl-5-pyrazolone, 3-ethyl-4-hydroxy-1-isopropylpyrazole, 1,5-dimethyl-3-ethyl-4-hydroxypyrazole or 1,5-dimethyl-4-hydroxy-3-phenylpyrazole.

[0059] The superabsorbent particles preferably contain 0.001% to 1% by weight, more preferably 0.005% to 0.2% by weight, most preferably 0.01% to 0.1% by weight of pyrazole, based on the polymer particles.

[0060] The polymerization inhibitors are not subject to any restrictions. Suitable examples are all polymerization inhibitors that are suitable for inhibiting the polymerization of acrylic acid, preferably hydroquinone monomethyl ether.

[0061] The superabsorbent particles preferably contain from 0.0001% to 0.1% by weight, more preferably from 0.0005% to 0.02% by weight, most preferably from 0.001% to 0.01% by weight of a polymerization inhibitor other than a pyrazole, in each case based on the polymer particles.

[0062] Acrylic acid is the preferred ethylenically unsaturated carboxylic acid. Peroxodisulfates, in particular ammonium peroxodisulfate, sodium peroxodisulfate and / or potassium peroxodisulfate, are preferred initiators c).

[0063] The present invention further provides hygiene articles comprising the superabsorbent particles of the present invention.

[0064] method: Unless otherwise stated, measurements should be carried out at an ambient temperature of 23±2° C. and a relative air humidity of 50±10%. The superabsorbent particles are thoroughly mixed before the measurement.

[0065] Color value (CIE color number [L, a, b]) Color values ​​are measured by the CIELAB method (Hunterlab, volume 8, 1996, edition 7, pages 1 to 4) using a "LabScan XE Spectrometer" colorimeter (HunterLab; Reston; USA). Color is described via the coordinates L, a, and b in a three-dimensional system. L characterizes the luminance, L=0 is black, and L=100 is white. The a and b values ​​represent the color position on the red / green and yellow / blue color axes, respectively, with positive a values ​​representing red, negative a values ​​representing green, positive b values ​​representing yellow, and negative b values ​​representing blue.

[0066] The Hunter 60 value (HC60) is a measure of the whiteness of a surface and is defined as L-3b, meaning that the lower the value, the darker and more yellow the color.

[0067] Testing was performed using tissue culture dishes (35 mm diameter and 10 mm height) and a 0.5 inch port plate opening.

[0068] The colour index is determined according to the tristimulus method according to DIN 5033-6.

[0069] Yellowness index (YI) Yellowness Index (YI) is measured according to ASTM D1925 or ASTM E313. The higher the value, the darker and more yellow the color. [Example]

[0070] Example 1 To an initial charge of 0.39 g of 3-tuply ethoxylated glycerol triacrylate in a 500 ml polypropylene beaker, 24.41 g of acrylic acid (stabilized with 0.02 wt % hydroquinone monomethyl ether) was added. Then, 245.05 g of a 37.3 wt % aqueous sodium acrylate solution and 24.18 g of water were added. The monomer solution was then inerted with 200 L / h of nitrogen using a glass frit for 30 minutes. The neutralization level was 71% and the solids content was 40 wt %.

[0071] For the polymerization, 0.59 g of a 15 wt % aqueous sodium peroxodisulfate solution, 0.20 g of a 1 wt % hydrogen peroxide solution and 0.99 g of a 0.5 wt % ascorbic acid solution were added successively.

[0072] The resulting polymer gel was crushed and dried for 90 minutes in an air-circulating drying cabinet at 175° C. It was then ground and sieved to a particle size of 150-710 μm.

[0073] 20 g of the resulting superabsorbent were stored for 14 days in a temperature and humidity controlled cabinet at 70° C. and 80% relative humidity.

[0074] Example 2 The procedure was as in Example 1. In each case 0.02% by weight of pyrazole, based in each case on acrylic acid, was added to the monomer solution.

[0075] 20 g of each case of superabsorbent from Example 1 were stored for 14 days in a temperature and humidity controlled cabinet at 70° C. and 80% relative humidity.

[0076] The results are summarized in Table 1.

[0077] [Table 1]

[0078] Example 3 The procedure was as in Example 1. 1,5-dimethyl-3-ethyl-4-hydroxypyrazole was added to the monomer solution.

[0079] 20 g of each case of superabsorbent from Example 1 were stored for 14 days in a temperature and humidity controlled cabinet at 70° C. and 80% relative humidity.

[0080] The results are summarized in Table 2.

[0081] Table 2

Claims

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

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

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

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

5. 5. The process of any one of claims 1 to 4, wherein 0.001% to 1% by weight of the pyrazole, based on the ethylenically unsaturated carboxylic acid a), is added to the monomer solution or suspension.

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

7. 7. The process of any one of claims 1 to 6, wherein the monomer solution or suspension comprises 0.0001% to 0.1% by weight, based on the ethylenically unsaturated carboxylic acid a), of the polymerization inhibitor other than the pyrazole.

8. 8. The process of claim 1, wherein hydroquinone monomethyl ether is used as the polymerization inhibitor other than the pyrazole.

9. Superabsorbent particles obtainable by the process according to any one of claims 1 to 8, comprising at least one pyrazole and at least one polymerization inhibitor other than said pyrazole.

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

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

12. 12. Superabsorbent particles according to claim 10 or 11, wherein 1,3-dimethyl-5-pyrazolone is the compound of general formula (I) or 3-ethyl-4-hydroxy-1-isopropylpyrazole, 1,5-dimethyl-3-ethyl-4-hydroxypyrazole or 1,5-dimethyl-4-hydroxy-3-phenylpyrazole is the compound of general formula (II).

13. Superabsorbent particles according to any one of claims 9 to 12, comprising from 0.001% to 1% by weight, based on the polymer particles, of said polymerization inhibitor other than said pyrazole.

14. Superabsorbent particles according to any one of claims 9 to 13, wherein said polymerization inhibitor other than said pyrazole is hydroquinone monomethyl ether.

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

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