Method for preparing color-stable superabsorbent particles

Abstract

The invention relates to a method for producing color-stable superabsorbent particles, in which an aqueous monomer solution or suspension is polymerized into a polymer gel, the resulting polymer gel is optionally comminuted, the polymer gel is subsequently dried, the dried polymer gel is optionally ground and sorted, characterized in that the superabsorbent particles are dispersed in a water-soluble polymer, and the superabsorbent particles are dispersed in a water-soluble polymer. A pyrazole is added prior to drying and the aqueous monomer solution or suspension contains a different polymerization inhibitor than the pyrazole.

Description

[0001] The present invention relates to a method for preparing color-stable superabsorbent particles, in which an aqueous monomer solution or suspension is polymerized into a polymer gel, optionally the resulting polymer gel is comminuted, and subsequently the polymer gel is dried, optionally ground and classified, characterized in that pyrazole is added before drying, and the aqueous monomer solution or suspension contains a polymerization inhibitor different from the pyrazole.

[0002] Superabsorbents are used for manufacturing diapers, tampons, sanitary napkins, and other hygiene products, but also as water retention agents in agriculture and horticulture. Superabsorbents are also referred to as water-absorbing polymers.

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

[0004] To improve application properties, such as gel bed permeability (GBP) and absorption at a pressure of 49.2 g / cm 2 (AUL0.7 psi), the superabsorbent particles are usually surface post-crosslinked. This increases the degree of crosslinking on the particle surface, so that the absorption at a pressure of 49.2 g / cm 2 (AUL0.7 psi) and the centrifuge retention capacity (CRC) can be at least partially decoupled. This surface post-crosslinking can be carried out in the aqueous gel phase. Preferably, however, a surface post-crosslinking agent is applied to the surface of the dried, ground, and sieved polymer particles (basic polymer) and thermally post-surface crosslinked. Crosslinking agents suitable for this are compounds that can form covalent bonds with at least two carboxyl groups of the polymer particles.

[0005] The earlier PCT application with the case number PCT / EP2022 / 059572 discloses 4-hydroxypyrazole as a polymerization inhibitor.

[0006] The object of the present invention is to provide an improved method for preparing color-stable superabsorbent particles.

[0007] The solution of the present invention to achieve the above object is a method for preparing surface post-crosslinked superabsorbent particles by polymerizing an aqueous monomer solution or suspension, said aqueous monomer solution or suspension containing

[0008] a) at least one at least partially neutralized ethylenically unsaturated carboxylic acid,

[0009] b) at least one crosslinking agent, and

[0010] c) at least one initiator,

[0011] Among them, the aqueous monomer solution or suspension is polymerized into a polymer gel, optionally the resulting polymer gel is crushed, and then the polymer gel is dried, optionally the dried polymer gel is ground and classified, and then optionally the dried polymer gel is subjected to thermal surface post-crosslinking and cooling, characterized in that at least one pyrazole is added before the drying, and the aqueous monomer solution or suspension contains at least one inhibitor different from the pyrazole.

[0012] The pyrazole is usually monomeric pyrazole.

[0013] The pyrazole used in the method of the present invention is preferably a compound of the general formula (I)

[0014]

[0015] wherein

[0016] R 1 is C1 to C 20 alkyl or C6 to C 20 arylalkyl,

[0017] R 2 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl, and

[0018] R 3 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl;

[0019] or a compound of the general formula (II)

[0020]

[0021] wherein

[0022] R 4 is C1 to C 20 alkyl or C6 to C 20 arylalkyl,

[0023] R 5 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl, and

[0024] R 6 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl;

[0025] or a compound of the general formula (III)

[0026]

[0027] wherein

[0028] R 7 is C1 - C 20 alkyl or C6 - C 20 arylalkyl,

[0029] R 8 is H, C1 - C 20 alkyl or C6 - C 20 arylalkyl, and

[0030] R 9 is H, C1 - C 20 alkyl or C6 - C 20 arylalkyl.

[0031] The pyrazole used in the method of the present invention is particularly preferably a compound of the general formula (I)

[0032]

[0033] wherein

[0034] R 1 is C1 - C3 alkyl or C6 - C8 arylalkyl,

[0035] R 2 is C1 - C3 alkyl or C6 - C8 arylalkyl, and

[0036] R 3 is H, C1 - C3 alkyl or C6 - C8 arylalkyl;

[0037] or a compound of the general formula (II)

[0038]

[0039] wherein

[0040] R 4 is C1 - C3 alkyl or C6 - C8 arylalkyl,

[0041] R 5 is C1 - C3 alkyl or C6 - C8 arylalkyl, and

[0042] R 6 is C1 - C3 alkyl or C6 - C8 arylalkyl,

[0043] or a compound of the general formula (III)

[0044]

[0045] wherein

[0046] R 7 is a C1-C3 alkyl or a C6-C8 arylalkyl,

[0047] R 8 is H, a C1-C3 alkyl or a C6-C8 arylalkyl, and

[0048] R 9 is a C1-C3 alkyl or a C6-C8 arylalkyl.

[0049] The alkyl group can be straight-chain, branched and / or cyclic.

[0050] The compounds of general formula (I) exist in equilibrium with their keto form. For example, 1,3-dimethyl-5-pyrazolone is the keto form of 1,3-dimethyl-5-hydroxypyrazole.

[0051] The pyrazoles used in the process of the present invention are particularly 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.

[0052] Based on the ethylenically unsaturated carboxylic acid a), the monomer solution or suspension preferably contains 0.001 wt% to 1 wt%, particularly preferably 0.005 wt% to 0.2 wt%, and even more particularly preferably 0.01 wt% to 0.1 wt% of pyrazole.

[0053] The polymerization inhibitor is not subject to any restrictions. For example, all polymerization inhibitors suitable for inhibiting the polymerization of acrylic acid are suitable, preferably hydroquinone monomethyl ether.

[0054] Based on the ethylenically unsaturated carboxylic acid a), the monomer solution or suspension preferably contains 0.0001 wt% to 0.1 wt%, particularly preferably 0.0005 wt% to 0.02 wt%, and even more particularly preferably 0.001 wt% to 0.01 wt% of a polymerization inhibitor different from pyrazole.

[0055] The present invention is based on the discovery that pyrazole significantly improves the color stability of superabsorbents.

[0056] The preferred ethylenically unsaturated carboxylic acid is acrylic acid. The preferred initiator c) is a persulfate, particularly ammonium persulfate, sodium persulfate and / or potassium persulfate.

[0057] The preparation of the superabsorbent will be described in detail below.

[0058] The superabsorbent is prepared by polymerizing a monomer solution and is generally insoluble in water.

[0059] The ethylenically unsaturated monomer carrying an acid group preferably has water solubility, i.e., the solubility in water at 23 °C is generally at least 1 g / 100 g of water, preferably at least 5 g / 100 g of water, particularly preferably at least 25 g / 100 g of water, and further particularly preferably at least 35 g / 100 g of water.

[0060] 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. Further particularly preferably, acrylic acid is used.

[0061] The ethylenically unsaturated monomer carrying an acid group is usually partially neutralized. The neutralization is carried out in the monomer stage. This is generally achieved by mixing in a neutralizing agent in aqueous solution form or preferably in solid form. The degree of neutralization is preferably 40 mol% to 85 mol%, particularly preferably 50 mol% to 80 mol%, and further particularly preferably 60 mol% to 75 mol%. Common neutralizing agents can be used, and preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates, or alkali metal bicarbonates and their mixtures are used. Ammonium salts can also be used instead of alkali metal salts. Particularly preferred alkali metals are sodium and potassium, but further particularly preferably sodium hydroxide, sodium carbonate, or sodium bicarbonate and their mixtures, especially sodium hydroxide.

[0062] The monomer usually contains an inhibitor, preferably contains a hydroquinone semi-ether acting as a storage stabilizer.

[0063] Suitable crosslinking agents are compounds having at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups capable of free-radical polymerization into the polymer chain, and functional groups capable of forming covalent bonds with the acid groups of the monomer. In addition, polyvalent metal salts capable of forming coordination bonds with at least two acid groups of the monomer are also suitable as crosslinking agents.

[0064] Suitable crosslinking agents 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, diacrylates and triacrylates as described in EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93 / 21237A1, WO 03 / 104299A1, WO 03 / 104300A1, WO 03 / 104301A1 and DE 103 31450A1, mixed acrylates containing ethylenically unsaturated groups in addition to acrylate groups as described in DE 103 31 456A1 and DE 103 55401A1, or crosslinking agent mixtures as described, for example, in DE195 43 368A1, DE 196 46 484A1, WO 90 / 15830A1 and WO 02 / 032962A2.

[0065] Based on the total amount of monomers used, the amount of crosslinking agent is preferably from 0.05 wt% to 1.5 wt%, particularly preferably from 0.1 wt% to 1 wt%, and even more particularly preferably from 0.15 wt% to 0.6 wt%. As the crosslinking agent content increases, the centrifugal retention capacity (CRC) decreases, and the absorption rate (AUL0.3 psi) at a pressure of 21.0 g / cm 2 passes through a maximum.

[0066] All compounds that generate free radicals under polymerization conditions can be used as initiators, such as thermal initiators, redox initiators, photoinitiators. Suitable redox initiators are sodium persulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium persulfate / sodium bisulfite and hydrogen peroxide / sodium bisulfite. A mixture composed of a thermal initiator and a redox initiator is preferably used, such as sodium persulfate / hydrogen peroxide / ascorbic acid. Preferably, the disodium salt of 2-hydroxy-2-sulfinoacetic acid, or a mixture composed of the sodium salt of 2-hydroxy-2-sulfinoacetic acid, the disodium salt of 2-hydroxy-2-sulfinoacetic acid and sodium bisulfite is used as the reducing component. Commercially available mixtures of this type are FF6 and FF7 (Brüggemann Chemicals; Heilbronn; Germany).

[0067] The water content of the monomer solution is preferably 40 wt% to 75 wt%, particularly preferably 45 wt% to 70 wt%, and even more particularly preferably 50 wt% to 65 wt%. As the water content increases, the energy consumption in the subsequent drying process increases. When the water content decreases, it may not be possible to sufficiently dissipate the heat of polymerization.

[0068] The temperature of the monomer solution is preferably 10°C to 90°C, particularly preferably 20°C to 70°C, and even more particularly preferably 30°C to 50°C.

[0069] To achieve optimal results, the preferred polymerization inhibitor requires dissolved oxygen. Therefore, prior to polymerization, the dissolved oxygen in the monomer solution can be removed by inerting, i.e., flowing an inert gas, preferably nitrogen or carbon dioxide, through. Preferably, the oxygen content of the monomer solution is reduced to less than 1 wppm prior to polymerization, particularly preferably reduced to less than 0.5 wppm, and even more particularly preferably reduced to less than 0.1 wppm.

[0070] Reactors suitable for polymerization are, for example, kneading reactors or belt reactors. In a kneader, the polymer gel produced during the polymerization of an aqueous monomer solution or suspension is continuously comminuted, for example, by counter-rotating agitator shafts, as described in WO 2001 / 038402 A1. Polymerization on a belt reactor has been described, for example, in DE 38 25 366 A1 and US 6,241,928. The polymerization in the belt reactor produces a polymer gel that must be comminuted, for example, in an extruder or a kneader.

[0071] To improve the drying properties, the comminuted polymer gel obtained from the kneader can be additionally extruded.

[0072] Subsequently, the polymer gel is usually dried using a circulating air belt dryer until the water content is preferably 0.5 wt% to 10 wt%, particularly preferably 1 wt% to 7 wt%, and even more particularly preferably 2 wt% to 5 wt%. The remaining water content is determined in accordance with test method No. WSP 230.2-05 "Mass Loss Upon Heating" recommended by EDANA. When the remaining water content is too high, the dried polymer gel has too low a glass transition temperature T g, and it is difficult to further process. In the case of too low residual moisture, the dried polymer gel is too brittle and produces an undesirably large amount of polymer particles with too small particle sizes ("fines") during the subsequent comminution step. The solids content of the polymer gel before drying is preferably 25 wt% to 90 wt%, particularly preferably 35 wt% to 70 wt%, and further particularly preferably 40 wt% to 60 wt%. Subsequently, the dried polymer gel is broken up and optionally coarsely comminuted.

[0073] Subsequently, the dried polymer gel is generally ground and classified. Among them, generally, a single-stage or multi-stage roller mill (preferably a two-stage or three-stage roller mill), a pin mill, a hammer mill or a vibration mill can be used for grinding.

[0074] The average particle size of the polymer particles separated as the product part is preferably at least 150 μm to 850 μm, particularly preferably 250 μm to 600 μm, and further particularly preferably 300 μm to 500 μm. The average particle size of the product part can be determined by the test method No. WSP 220.2(05) "Partikel Size Distribution" recommended by EDANA. Among them, the mass ratio of sieving is cumulatively plotted, and the average particle size is graphically determined. Here, the average particle size is the mesh width value corresponding to the cumulative 50 wt%.

[0075] In order to further improve the properties, the polymer particles can be post-crosslinked thermally on the surface. Suitable surface post-crosslinking agents are compounds containing groups capable of forming covalent bonds with at least two carboxyl groups of the polymer particles. Suitable compounds are, for example, polyfunctional amines, polyfunctional aminoamines, polyfunctional epoxides as described in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, difunctional alcohols or polyfunctional alcohols as described in DE 33 14 019 A1, DE 35 23 617 A1 and EP 0 450 922 A2, or β-hydroxyalkylamides as described in DE 102 04 938 A1 and US 6,239,230.

[0076] Based on the polymer particles, the amount of the surface post-crosslinking agent is preferably 0.001 wt% to 2 wt%, particularly preferably 0.01 wt% to 1 wt%, and further particularly preferably 0.03 wt% to 0.7 wt%.

[0077] In a preferred embodiment of the present invention, in addition to the surface post-crosslinking agent, polyvalent cations are additionally applied to the particle surface.

[0078] The polyvalent cations applicable in the method of the present invention are, for example, divalent cations such as cations of zinc, magnesium, calcium, and strontium, trivalent cations such as cations of aluminum, iron, chromium, rare earths, and manganese, and tetravalent cations such as cations of titanium and zirconium. Possible counterions are chloride, bromide, hydroxide, sulfate, bisulfate, carbonate, bicarbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate, and carboxylates such as acetate and lactate. Aluminum hydroxide, aluminum sulfate, and aluminum lactate are preferably used.

[0079] Based on the polymer, the amount of the polyvalent cation is, for example, from 0.001 wt% to 1.5 wt%, preferably from 0.005 wt% to 1 wt%, and particularly preferably from 0.02 wt% to 0.8 wt%.

[0080] Surface post-crosslinking is generally carried out by spraying a solution of the surface post-crosslinking agent onto the dried polymer particles. After spraying, the polymer particles coated with the surface post-crosslinking agent are heat-treated.

[0081] Preferably, the solution of the surface post-crosslinking agent is sprayed in a mixer having a moving stirring tool, such as in a screw mixer, a disk mixer, and a paddle mixer. A horizontal mixer, such as a paddle mixer, is particularly preferably used, and a vertical mixer is further particularly preferably used. The difference between a horizontal mixer and a vertical mixer lies in the supporting manner of the stirring shaft body, that is, the horizontal mixer has a horizontally supported stirring shaft body, and the vertical mixer has a vertically supported stirring shaft body. Applicable mixers are, for example, Horizontale Mischer (Gebr. Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Deventer; Netherlands), Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA) and Schugi (Hosokawa Micron BV; Deventer; Netherlands). However, the surface post-crosslinking agent solution can also be sprayed in a fluidized bed.

[0082] The surface post-crosslinking agent is generally used as 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 or the total solvent amount.

[0083] The heat treatment is preferably carried out in a contact dryer, particularly preferably in a paddle dryer, and further preferably in a disk dryer. Applicable dryers are, for example, Hosokawa Horizontal Paddle Dryer (Hosokawa Micron GmbH; Reingarten; Germany), Hosokawa Disc Dryer (Hosokawa Micron GmbH; Reingarten; Germany), dryers (Metso Minerals Industries Inc.; Danville; USA) and Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany). In addition, a fluidized bed dryer can also be used.

[0084] Surface post-crosslinking can be carried out in the mixer itself by heating the jacket or injecting hot air. The same applies to downstream dryers such as shelf dryers, rotary furnaces or heatable worms. Particularly advantageously, mixing and thermal surface post-crosslinking are carried out in a fluidized bed dryer.

[0085] The preferred reaction temperature ranges from 100 °C to 250 °C, preferably from 110 °C to 220 °C, particularly preferably from 120 °C to 210 °C, and further particularly preferably from 130 °C to 200 °C. The preferred residence time at this temperature is preferably at least 10 minutes, particularly preferably at least 20 minutes, further particularly preferably at least 30 minutes, and generally at most 60 minutes.

[0086] Subsequently, the surface post-crosslinked polymer particles can be reclassified, wherein the too small and / or too large polymer particles are separated and recycled to the method.

[0087] In order to further improve the properties, the surface post-crosslinked polymer particles can be coated or re-wetted.

[0088] Re-wetting is preferably carried out at 30 °C to 80 °C, particularly preferably at 35 °C to 70 °C, and further particularly preferably at 40 °C to 60 °C. At too low a temperature, the polymer particles tend to agglomerate, and at higher temperatures, the water has significantly evaporated. The amount of water used for re-wetting is preferably 1 wt% to 10 wt%, particularly preferably 2 wt% to 8 wt%, and further particularly preferably 3 wt% to 5 wt%. Re-wetting enhances the mechanical stability of the polymer particles and reduces their tendency to become electrostatically charged. Preferably, re-wetting is carried out in a cooler after thermal surface post-crosslinking.

[0089] Coatings suitable for improving the swelling rate and 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. Coatings suitable for binding dust are, for example, polyols. Coatings suitable for counteracting the adverse caking tendency of the polymer particles are, for example, fumed silica (such as 200), precipitated silica (such as D17) and a surfactant (such as 20).

[0090] Another subject of the present invention is superabsorbent particles containing at least one pyrazole and at least one polymerization inhibitor different from pyrazole.

[0091] The pyrazole is usually monomeric pyrazole.

[0092] The pyrazole is preferably a compound of the general formula (I)

[0093]

[0094] wherein

[0095] R 1 is C1 to C 20 alkyl or C6 to C 20 arylalkyl,

[0096] R 2 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl, and

[0097] R 3 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl;

[0098] or a compound of the general formula (II)

[0099]

[0100] wherein

[0101] R 4 is C1 to C 20 alkyl or C6 to C 20 arylalkyl,

[0102] R 5 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl, and

[0103] R 6 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl;

[0104] or a compound of the general formula (III)

[0105]

[0106] wherein

[0107] R 7 is C1 to C 20 alkyl or C6 to C 20 arylalkyl,

[0108] R 8 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl, and

[0109] R 9 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl.

[0110] The pyrazole used in the method of the present invention is particularly preferably a compound of the general formula (I)

[0111]

[0112] wherein

[0113] R 1 is C1 to C3 alkyl or C6 to C8 arylalkyl,

[0114] R 2 is C1 to C3 alkyl or C6 to C8 arylalkyl, and

[0115] R 3 is H, C1 to C3 alkyl or C6 to C8 arylalkyl;

[0116] or a compound of the general formula (II)

[0117]

[0118] wherein

[0119] R 4 is C1 to C3 alkyl or C6 to C8 arylalkyl,

[0120] R 5 is C1 to C3 alkyl or C6 to C8 arylalkyl, and

[0121] R 6 is C1 to C3 alkyl or C6 to C8 arylalkyl,

[0122] or a compound of the general formula (III)

[0123]

[0124] wherein

[0125] R 7is a C1-C3 alkyl or a C6-C8 arylalkyl,

[0126] R 8 is H, a C1-C3 alkyl or a C6-C8 arylalkyl, and

[0127] R 9 is a C1-C3 alkyl or a C6-C8 arylalkyl.

[0128] The alkyl group can be straight-chain, branched and / or cyclic.

[0129] The compounds of general formula (I) exist in equilibrium with their keto form. For example, 1,3-dimethyl-5-pyrazolone is the keto form of 1,3-dimethyl-5-hydroxypyrazole.

[0130] The pyrazoles used in the process of the present invention are particularly 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.

[0131] Based on the polymer particles, the superabsorbent particles preferably contain 0.001 wt% to 1 wt%, particularly preferably 0.005 wt% to 0.2 wt%, and even more particularly preferably 0.01 wt% to 0.1 wt% of pyrazole.

[0132] The polymerization inhibitor is not subject to any restrictions. For example, all polymerization inhibitors suitable for inhibiting the polymerization of acrylic acid are suitable, preferably hydroquinone monomethyl ether.

[0133] Based on the polymer particles, the superabsorbent particles preferably contain 0.0001 wt% to 0.1 wt%, particularly preferably 0.0005 wt% to 0.02 wt%, and even more particularly preferably 0.001 wt% to 0.01 wt% of a polymerization inhibitor different from pyrazole.

[0134] The preferred ethylenically unsaturated carboxylic acid is acrylic acid. The preferred initiator c) is a persulfate, particularly ammonium persulfate, sodium persulfate and / or potassium persulfate.

[0135] Another subject of the present invention is a sanitary article containing the superabsorbent particles of the present invention.

[0136] Method:

[0137] Unless otherwise specified, the measurements are 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.

[0138] Color value (CIE color number [L, a, b])

[0139] The measurement of color values was carried out using a colorimeter of the model "LabScan XE Spectrometer" (HunterLab; Reston; USA) according to the CIELAB method (Hunterlab, Volume 8, 1996, Issue 7, pages 1 to 4). Color is described by the coordinates L, a, and b of a three-dimensional system. L represents luminance, where L = 0 is black and L = 100 is white. The values of a and b describe the position of the color on the red / green axis and the yellow / blue axis respectively, where a positive a value represents red, a negative a value represents green, a positive b value represents yellow, and a negative b value represents blue.

[0140] The Hunter 60 value (HC60) is an index for measuring the whiteness of a surface and is defined as L - 3b, that is, the lower the value, the darker and yellower the color.

[0141] The tests were carried out using tissue culture dishes (35 mm in diameter and 10 mm in height) and 0.5-inch port plate openings.

[0142] The measurement of color values was carried out according to the tristimulus method of DIN 5033-6.

[0143] Yellowness index (YI)

[0144] The yellowness index (YI) was measured according to ASTM D1925 or ASTM E313. The higher the value, the darker and yellower the color. Examples

[0145] Example 1

[0146] 0.39 g of triethoxylated glycerol triacrylate was added to a 500 ml polypropylene beaker, and 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. Then the monomer solution was inerted for 30 minutes with nitrogen at 200 l / h through a glass filter plate. The degree of neutralization was 71% and the solid content was 40 wt%.

[0147] For polymerization, 0.59 g of a 15 wt% aqueous sodium persulfate 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.

[0148] The obtained polymer gel was crushed and dried in a circulating air drying oven at 175 °C for 90 minutes. Then the dried polymer gel was ground and sieved to a particle size of 150 μm to 710 μm.

[0149] 20 g of the obtained superabsorbent was stored in a climate chamber at 70 °C and 80% relative humidity for 14 days.

[0150] Example 2

[0151] Operate as in Example 1. 0.02 wt% of pyrazole (based on acrylic acid) was added to the monomer solution each time.

[0152] Each time, 20 g of the superabsorbent in Example 1 was stored in a climatic chamber at 70 °C and 80% relative humidity for 14 days.

[0153] The results are summarized in Table 1:

[0154] Pyrazole added L a b HC 60 YID1925 YI E313 None *) 53.9 7.8 17.2 2.5 68.2 69.3 3-Carboxypyrazole 56.0 7.5 16.4 6.0 63.7 64.5 3,5-Dimethyl-1-hydroxymethylpyrazole 59.9 5.9 15.6 13.1 54.5 55.2 1,3-Dimethyl-5-pyrazolone 74.4 2.8 15.0 29.5 38.6 38.7 3-Acetyl-1,5-dimethyl-4-hydroxy-pyrazole 59.9 4.7 17.5 7.4 57.8 58.0 3-Ethyl-4-hydroxy-1-isopropyl-5-methylpyrazole 73.6 2.9 14.7 29.6 38.4 38.5 3-Hydroxy-1-methylpyrazole 66.1 3.8 17.3 14.2 50.8 51.0 1,5-Dimethyl-4-hydroxy-3-phenylpyrazole 73.8 2.0 14.9 29.1 37.9 38.1 1,5-Dimethyl-3-ethyl-4-hydroxy-pyrazole 76.6 2.0 13.1 37.2 32.5 32.6

[0155] *) Comparative example

[0156] Example 3

[0157] Operate as in Example 1. 1,5-Dimethyl-3-ethyl-4-hydroxypyrazole was added to the monomer solution.

[0158] Each time, 20 g of the superabsorbent in Example 1 was stored in a climatic chamber at 70 °C and 80% relative humidity for 14 days.

[0159] The results are summarized in Table 2:

[0160]

[0161]

[0162] *) Comparative example **) Based on acrylic acid.

Claims

1. A process for preparing surface post-crosslinked superabsorbent particles by polymerizing an aqueous monomer solution or suspension, said aqueous monomer solution or suspension containing a) at least one at least partially neutralized ethylenically unsaturated carboxylic acid, b) at least one crosslinking agent, and c) at least one initiator, Among them, polymerizing the aqueous monomer solution or suspension into a polymer gel, optionally comminuting the resulting polymer gel, subsequently drying the polymer gel, optionally grinding and classifying the dried polymer gel, and subsequently optionally subjecting the dried polymer gel to thermal surface post-crosslinking and cooling, characterized in that at least one pyrazole is added before the drying, and the aqueous monomer solution or suspension contains at least one inhibitor different from the pyrazole.

2. The method according to claim 1, characterized in that, The pyrazole is a compound of the general formula (I) wherein R 1 is C1 to C 20 alkyl, wherein C1 to C 20 alkyl includes straight-chain or branched C1 to C 20 alkyl groups or cyclic C3 to C 20 alkyl groups, or is C6 to C 20 arylalkyl, R 2 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl, and R 3 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl; or a compound of the general formula (II) wherein R 4 is C1 to C 20 alkyl or C6 to C 20 arylalkyl, R 5 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl, and R 6 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl; or a compound of the general formula (III) wherein R 7 is C1 to C 20 alkyl or C6 to C 20 arylalkyl, R 8 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl, and R 9 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl.

3. The method according to claim 1 or 2, characterized in that, Acrylic acid is used as the ethylenically unsaturated carboxylic acid.

4. The method according to any one of claims 1 to 3, characterized in that, Persulfates, in particular ammonium persulfate, sodium persulfate and / or potassium persulfate, are used as initiator c).

5. The method according to any one of claims 1 to 4, characterized in that, 0.001 wt% to 1 wt% of pyrazole, based on the ethylenically unsaturated carboxylic acid a), is added to the monomer solution or suspension.

6. The method according to any one of claims 1 to 5, characterized in that, 1,3-Dimethyl-5-pyrazolidinone is used as the compound of the general formula (I), or 3-ethyl-4-hydroxy-1-isopropyl-pyrazole, 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).

7. The method according to any one of claims 1 to 6, characterized in that 0.0001 wt% to 0.1 wt% of the inhibitor different from the pyrazole, based on the ethylenically unsaturated carboxylic acid a), is added to the monomer solution or suspension.

8. The method according to any one of claims 1 to 7, characterized in that, Hydroquinone monomethyl ether is used as the inhibitor different from the pyrazole.

9. A superabsorbent particle obtained by the process according to claims 1 to 8, wherein the superabsorbent particle contains at least one pyrazole and at least one inhibitor different from the pyrazole.

10. The superabsorbent particle according to claim 9, wherein the pyrazole is a compound of the general formula (I) wherein R 1 is C1 to C 20 alkyl or C6 to C 20 arylalkyl, R 2 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl, and R 3 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl; or a compound of the general formula (II) wherein R 4 is C1 to C 20 alkyl or C6 to C 20 arylalkyl, R 5 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl, and R 6 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl; or a compound of the general formula (III) wherein R 7 is C1 to C 20 alkyl or C6 to C 20 arylalkyl, R 8 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl, and R 9 is H, C1 to C 20 alkyl or C6 to C 20 arylalkyl.

11. The superabsorbent particle according to claim 9 or 10, wherein the superabsorbent particle contains 0.001 wt% to 1 wt% of pyrazole, based on the polymer particles.

12. The superabsorbent particle according to claim 10 or 11, wherein 1,3-dimethyl-5-pyrazolidinone is the compound of the general formula (I), or 3-ethyl-4-hydroxy-1-isopropyl-pyrazole, 1,5-dimethyl-3-ethyl-4-hydroxypyrazole or 1,5-dimethyl-4-hydroxy-3-phenylpyrazole is the compound of the general formula (II).

13. The superabsorbent particle according to any one of claims 9 to 12, wherein the superabsorbent contains 0.001 wt% to 1 wt% of the inhibitor different from the pyrazole.

14. The superabsorbent particles according to any one of claims 9 to 13, wherein the inhibitor different from the pyrazole is hydroquinone monomethyl ether.

15. A sanitary article containing the superabsorbent particles according to any one of claims 9 to 14.

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