Method for producing superabsorbent particles having a low residual monomer content

Abstract

The present invention relates to a method for producing superabsorbent particles having a low residual monomer content, wherein an aqueous monomer solution or monomer suspension is polymerised to form a polymer gel, the resulting polymer gel is optionally comminuted, the polymer gel is subsequently dried, and the dried polymer gel is optionally ground and classified, characterised in that phytic acid or a salt thereof is added prior to polymerisation.

Description

[0001] 241033W001

[0002] 1

[0003] Process for the production of superabsorbent particles with low residual monomer content

[0004] The present invention relates to a process for producing superabsorbent particles with a low residual monomer content, wherein an aqueous monomer solution or suspension is polymerized to form a polymer gel, the resulting polymer gel is optionally comminuted, the polymer gel is subsequently dried, and the dried polymer gel is optionally ground and classified, characterized in that phytic acid or its salt is added prior to polymerization.

[0005] Superabsorbent polymers are used in the production of diapers, tampons, sanitary napkins, and other hygiene products, as well as water-retaining agents in horticulture. They are also known as water-absorbing polymers.

[0006] 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.

[0007] To improve application properties, such as gel bed permeability (GBP) and absorption under a pressure of 49.2 g / cm² 2 (AUL 0.7 psi), superabsorbent particles generally undergo surface cross-linking. This increases the degree of cross-linking of the particle surface, thereby improving absorption at a pressure of 49.2 g / cm². 2(AUL 0.7 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, dried, milled, and sieved polymer particles (base polymer) are coated on the surface with a surface post-crosslinker and thermally surface post-crosslinked. Suitable crosslinkers for this purpose are compounds that can form covalent bonds with at least two carboxylate groups of the polymer particles.

[0008] The object of the present invention was to provide an improved process for the production of superabsorbent particles with a low residual monomer content.

[0009] The problem was solved by a process for producing surface-crosslinked superabsorbent particles by polymerization of an aqueous monomer solution or suspension, comprising a) at least one ethylene-unsaturated carboxylic acid that 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 a polymer gel, the resulting polymer gel is optionally comminuted, the polymer gel is subsequently dried, the dried polymer gel is optionally milled and classified, and the dried polymer gel is subsequently optionally thermally surface-crosslinked. 241033W001

[0010] 2 and is cooled, characterized in that phytic acid or its salt is added before polymerization, preferably before the addition of the at least one initiator c).

[0011] Preferably, 0.001 to 0.3 wt.%, more preferably 0.005 to 0.2 wt.%, further preferably 0.0075 to 0.125 wt.%, and particularly preferably 0.01 to 0.1 wt.%, based on the ethylene-unsaturated carboxylic acid a), phytic acid or its salt is added to the monomer solution or suspension in amounts based on the ethylene-unsaturated carboxylic acid a). The quantity is calculated based on phytic acid.

[0012] Preferably, 0.01 to 2.0 wt.%, preferably 0.1 to 1.0 wt.%, and more preferably 0.2 to 0.8 wt.%, respectively, based on the ethylene-unsaturated carboxylic acid a), of the crosslinking agent c) is added to the monomer solution or suspension.

[0013] Preferably, at least one complexing agent other than phytic acid is added to the aqueous monomer solution or suspension. The choice of complexing agent is not subject to any restrictions.Suitable examples include all complexing agents suitable for the complexation of metal ions, such as ethylenediaminetetraacetic acid (EDTA), 2,3-disulfanyl succinic acid (DMSA), 2,3-dimercapto-1-propanesulfonic acid (DMPS), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), nitrilotriacetic acid (NTA), methylglycine diacetic acid (MGDA), glutamic acid-N,N-diacetic acid (GLDA), diethylenetriaminepentaacetic acid (DTPA), aminotrimethylenephosphonic acid (ATMP), 1-hydroxyethane-(1,1-diphosphonic acid) (HEDP), diethylenetriaminepenta(methylenephosphonic acid) (DTPMP), ethylenediaminetetra(methylenephosphonic acid) (EDTMP), and phosphonobutanetricarboxylic acid (PBTC). Bis(hexamethylenetriamine-penta(methylenephosphonic acid)) (BHMTMP), Hexaethylenediamine-tetramethylenephosphonic acid (HDTMP), Hexa-methylenediamine-tetra(methylenephosphonic acid) (HEMPA) or their (multivalent) salts.

[0014] Preferably, the superabsorbent particles obtained have a residual monomer content measured according to EDANA WSP No. 210.2-05 "Residual Monomers" of at most 1500 ppm, preferably at most 1250 ppm, more preferably at most 1000 ppm.

[0015] Acrylic acid is the preferred ethylene-unsaturated carboxylic acid. Peroxodisulfate, in particular ammonium peroxodisulfate, sodium peroxodisulfate and / or potassium peroxodisulfate, is the preferred initiator c).

[0016] The present invention is based on the finding that phytic acid can significantly reduce the residual monomer content in superabsorbent particles.

[0017] The following section explains the production of superabsorbent polymers in more detail: 241033W001

[0018] 3

[0019] Superabsorbents are produced by polymerization of a monomer solution and are usually insoluble in water.

[0020] The ethylene unsaturated, acid-group-bearing monomers are preferably water-soluble, i.e., the solubility in water at 23°C is typically at least 1 g / 100 g water, preferably at least 5 g / 100 g water, particularly preferably at least 25 g / 100 g water, and most preferably at least 35 g / 100 g water.

[0021] Suitable monomers include, for example, ethylene-unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid, fumaric acid, and itaconic acid. Acrylic acid and methacrylic acid are particularly preferred monomers. Acrylic acid is especially preferred.

[0022] The ethylene-unsaturated, acid-group-bearing monomers are usually partially neutralized. Neutralization is carried out at the monomer stage (ethylene-unsaturated carboxylic acids). This is typically done by mixing in the neutralizing agent (the base) as an aqueous solution or, more preferably, as a solid. The degree of neutralization is preferably 40 to 85 mol%, particularly preferably 50 to 80 mol%, and most preferably 60 to 75 mol%, whereby the usual neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates, or alkali metal hydrogen carbonates, as well as mixtures thereof. Ammonium salts can also be used instead of alkali metal salts. Sodium and potassium are particularly preferred alkali metals, but sodium hydroxide, sodium carbonate, or sodium hydrogen carbonate, as well as mixtures thereof, especially sodium hydroxide, are particularly preferred.The sodium hydroxide content in the sodium hydroxide solution is preferably at least 10 wt.%, particularly preferably at least 25 wt.%, and most preferably at least 40 wt.%.

[0023] The monomers typically contain polymerization inhibitors, preferably hydroquinone semi-ethers, as a storage stabilizer.

[0024] Suitable crosslinking agents are compounds with at least two groups suitable for crosslinking. Such groups include, for example, ethylene-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.

[0025] Suitable crosslinking agents include, 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, di- and triacrylates, as described in 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, 241033W001

[0026] 4 mixed acrylates containing, in addition to acrylate groups, further ethylene unsaturated groups, as described in DE 10331 456 A1 and DE 10355 401 A1, or crosslinking mixtures, as described, for example, in DE 195 43368 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 wt.%, particularly preferably 0.1 to 1 wt.%, and most preferably 0.15 to 0.6 wt.%, in each case calculated on the total amount of monomer used. With increasing crosslinker content, the centrifuge retention capacity (CRC) and the absorption at a pressure of 21.0 g / cm³ decrease. 2 (AUL0.3psi) passes through a maximum.

[0028] All compounds that generate radicals under the polymerization conditions can be used as initiators, for example, thermal initiators, redox initiators, and photoinitiators. Suitable redox initiators include sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. Preferably, mixtures of thermal 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-sulfonatoacetic 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).

[0029] The water content of the monomer solution M is preferably 40 to 75 wt.%, particularly preferably 45 to 70 wt.%, and most preferably 50 to 65 wt.%. With increasing water content, the energy required for subsequent drying increases, and with decreasing water content, the heat of polymerization can only be dissipated insufficiently.

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

[0031] The preferred polymerization inhibitors require dissolved oxygen for optimal effectiveness. Therefore, the monomer solution can be purified of dissolved oxygen prior to polymerization by inerting, i.e., by passing an inert gas, preferably nitrogen or carbon dioxide, through it. Preferably, the oxygen content of the monomer solution is reduced to less than 1 ppm by weight prior to polymerization, particularly preferably to less than 0.5 ppm by weight, and most preferably to less than 0.1 ppm by weight.

[0032] Suitable reactors for polymerization include kneading reactors and belt reactors. In a kneading reactor, 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 a belt is described, for example, in DE 3825 366 A1 and US 6,241,928. (See 241033W001.)

[0033] 5

[0034] Polymerization in a belt reactor produces a polymer gel that must be broken down, for example in an extruder or kneader.

[0035] To improve the drying properties, the crushed polymer gel obtained using a kneader can be additionally extruded.

[0036] The solids content of the polymer gel before drying is preferably between 25 and 90 wt.%, particularly preferably between 35 and 70 wt.%, and most preferably between 40 and 60 wt.%.

[0037] The polymer gel is then typically dried using a circulating air belt dryer until the residual moisture content is preferably 0.5 to 10 wt.%, particularly preferably 1 to 7 wt.%, and most preferably 1.5 to 6 wt.%, the residual moisture content being determined according to the EDANA recommended test method No. WSP 230.2-05 "Mass Loss Upon Heating". If the residual moisture content is too high, the dried polymer gel will have a glass transition temperature T that is too low. g It is difficult to process further. If the residual moisture content is too low, the dried polymer gel becomes too brittle, and the subsequent comminution steps result in undesirably large quantities of superabsorbent particles with an excessively small particle size ("fines"). The dried polymer gel is then broken up and optionally coarsely ground.

[0038] 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.

[0039] The mean particle size of the superabsorbent particles separated as the product fraction is preferably from 150 to 850 pim, particularly preferably from 250 to 600 pim, and most particularly from 300 to 500 pim. The mean particle size of the product fraction can be determined using the EDANA recommended test method No. WSP 220.2 (05) "Particle Size Distribution", whereby the mass fractions of the sieve fractions are plotted cumulatively and the mean particle size is determined graphically. The mean particle size is the mesh size value obtained for a cumulative 50 wt%.

[0040] The superabsorbent particles can be thermally surface-crosslinked to further improve their properties. Suitable surface crosslinkers are compounds containing groups that can form covalent bonds with at least two carboxylate groups of the superabsorbent particles. Suitable compounds include, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, as described in EP 0 083022 A2, EP 0 543303 A1 and EP 0 937 736 A2; di- or polyfunctional alcohols, as described in DE 33 14 019 A1, DE 3523 617 A1 and EP 0 450 922 A2; cyclic carbonates, e.g., ethylene carbonate, propylene carbonate or glycerol carbonate, as described in EP 4289 888 A1, KR10-2021-0038252 A and EP 3 424988 A1; or β-hydroxyalkylamides, as described in DE 102 04938 A1 and US 6,239,230. 241033W001

[0041] 6

[0042] The amount of surface crosslinking agent is preferably 0.001 to 2 wt.%, particularly preferably 0.02 to 1 wt.%, most preferably 0.05 to 0.7 wt.%, in each case based on the superabsorbent particles.

[0043] In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the surface crosslinking agents.

[0044] The polyvalent cations that can be used in the process according to the invention are, for example, divalent cations such as zinc, magnesium, calcium, and strontium; trivalent cations such as aluminum, iron, chromium, rare earth elements, and manganese; and tetravalent cations such as titanium and zirconium. Possible counterions include chloride, bromide, hydroxide, sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate, and carboxylates such as acetate and lactate. Aluminum hydroxide, aluminum sulfate, and aluminum lactate are preferred.

[0045] The amount of polyvalent cation used is, for example, 0.001 to 1.5 wt.%, preferably 0.005 to 1 wt.%, particularly preferably 0.02 to 0.8 wt.%, in each case based on the polymer.

[0046] Surface recrosslinking is typically carried out by spraying a solution of the surface recrosslinker onto the dried superabsorbent particles. Following spraying, the superabsorbent particles coated with the surface recrosslinker are thermally treated.

[0047] Spraying a solution of the surface re-curing agent is preferably carried out in mixers with moving mixing tools, such as screw mixers, disc mixers, and paddle mixers. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are especially preferred. The distinction between horizontal and vertical mixers is made by the orientation of the mixing shaft; that is, horizontal mixers have a horizontally mounted mixing shaft, and vertical mixers have a vertically mounted mixing shaft. Suitable mixers include, for example, the Horizontal Ploughshare® Mixer (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany), the Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Doetinchem; Netherlands), the Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA), and the Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; Netherlands). However, it is also possible to spray the surface re-curing agent solution in a fluidized bed.

[0048] Surface crosslinking agents are typically used as aqueous solutions. The penetration depth of the surface crosslinking agent into the superabsorbent particles can be adjusted by varying the content of non-aqueous solvent or the total amount of solvent.

[0049] The thermal treatment is preferably carried out in contact dryers, particularly preferably paddle dryers, and most preferably disc dryers. Suitable dryers include, for example, the Hosokawa Bepex® 241033W001.

[0050] 7

[0051] Horizontal paddle dryers (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.

[0052] Surface post-crosslinking can occur within 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 heated screw dryer, is equally suitable. Mixing and thermal surface post-crosslinking in a fluidized bed dryer is particularly advantageous.

[0053] Preferred reaction temperatures are in the range of 100 to 250°C, preferably 110 to 220°C, particularly preferably 120 to 210°C, and most preferably 130 to 200°C. The preferred residence time at this temperature is preferably at least 10 minutes, particularly preferably at least 15 minutes, most preferably at least 20 minutes, and usually at most 60 minutes.

[0054] Subsequently, the surface-crosslinked superabsorbent particles can be reclassified, whereby superabsorbent particles that are too small and / or too large are separated and recycled back into the process.

[0055] The surface-crosslinked superabsorbent particles can be coated or re-moistened to further improve their properties.

[0056] Post-humidification is preferably carried out at 30 to 80°C, particularly preferably at 35 to 70°C, and most preferably at 40 to 60°C. At excessively low temperatures, the superabsorbent particles tend to clump together, and at higher temperatures, a significant amount of water evaporates. The amount of water used for post-humidification is preferably 1 to 10 wt.%, particularly preferably 2 to 8 wt.%, and most preferably 3 to 5 wt.%. Post-humidification increases the mechanical stability of the superabsorbent particles and reduces their tendency to accumulate static electricity. Advantageously, post-humidification is carried out in the cooler after thermal surface crosslinking.

[0057] Suitable coatings for improving swelling rate and gel bed permeability (GBP) include, for example, inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers, and divalent or polyvalent metal cations. Suitable coatings for dust binding include, for example, polyols. Suitable coatings to prevent the undesirable tendency of the superabsorbent particles to clump together include, for example, fumed silica, such as Aerosil® 200, precipitated silica, such as Sipernat® D17, and surfactants, such as Span® 20. 241033W001

[0058] 8

[0059] Another object of the present invention is superabsorbent particles containing phytic acid or its salt. The superabsorbent particles according to the invention can preferably be produced by the process according to the invention.

[0060] Preferably, the superabsorbent particles contain 0.005 to 0.5 wt.%, more preferably 0.01 to 0.3 wt.%, and further preferably 0.015 to 0.175 wt.%, in each case based on the ethylene-unsaturated carboxylic acid a), phytic acid or its salt. The amount is always calculated based on phytic acid.

[0061] Preferably, the superabsorbent particles contain at least one complexing agent other than phytic acid. The complexing agent is not subject to any restrictions.Suitable examples include all complexing agents suitable for the complexation of metal ions, such as ethylenediaminetetraacetic acid (EDTA), 2,3-disulfanyl succinic acid (DMSA), 2,3-dimercapto-1-propanesulfonic acid (DMPS), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), nitrilotriacetic acid (NTA), methylglycine diacetic acid (MGDA), glutamic acid N,N-diacetic acid (GLDA), diethylenetriaminepentaacetic acid (DTPA), aminotrimethylenephosphonic acid (ATMP), 1-hydroxyethane-(1,1-diphosphonic acid) (HEDP), diethylenetriaminepenta(methylenephosphonic acid) (DTPMP), ethylenediaminetetra(methylenephosphonic acid) (EDTMP), and phosphonobutane tricarboxylic acid (PBTC). Bis(hexamethylenetriaminepenta(methylenephosphonic acid)) (BHMTMP), hexaethylenediamine-tetramethylenephosphonic acid (HDTMP), hexamethylenediamine-tetra(methylenephosphonic acid) (HEMPA) or their (multivalent) salts.

[0062] Acrylic acid is the preferred ethylene-unsaturated carboxylic acid. Peroxodisulfate, in particular ammonium peroxodisulfate, sodium peroxodisulfate and / or potassium peroxodisulfate, is the preferred initiator c).

[0063] Another aspect of the present invention is hygiene articles containing superabsorbent particles according to the invention.

[0064] Methods:

[0065] Unless otherwise specified, measurements should be carried out at an ambient temperature of 23 ± 2°C and a relative humidity of 50 ± 10%. The superabsorbent particles should be thoroughly mixed before measurement.

[0066] Residual monomer

[0067] The residual monomer content of the water-absorbing superabsorbent particles is determined according to the EDANA-recommended test method WSP No. 210.2-05 "Residual Monomers". 241033W001

[0068] 9

[0069] Examples

[0070] Example 1

[0071] In a 1000 ml polypropylene beaker, 0.37 g of triple ethoxylated glyceryl triacrylate (Laromer® PO 9044V; BASF SE, Ludwigshafen, Germany) was placed as a core crosslinker (CXL), and 38.56 g of acrylic acid (stabilized with 0.02 wt% hydroquinone monomethyl ether) was added. Subsequently, 347.03 g of 37.3 wt% aqueous sodium acrylate solution, 2.07 g of polyethylene glycol-4000, and 9.20 g of water were added. The monomer solution was then inerted for 30 minutes using a glass frit with nitrogen at a rate of 300 l / h. The degree of neutralization was 72 mol%, and the solids content was 42 wt%.

[0072] For polymerization, 1.84 g of a 15 wt% aqueous sodium peroxodisulfate solution, 0.11 g of a 1 wt% hydrogen peroxide solution, and 0.83 g of a 0.5 wt% ascorbic acid solution were added successively while stirring. After approximately 10 seconds, stirring was stopped, and the mixture was allowed to stand for 30 minutes to cool after reaching a peak temperature of approximately 110 °C.

[0073] The resulting polymer gel was comminuted and dried for 90 minutes at 175°C in a circulating air drying oven. It was then milled using a roller mill (Gebrüder Baumeister LRC-125; gap settings: 2 mm, 1 mm, 0.6 mm and 0.4 mm) and sieved to a particle size of 150 to 710 µm.

[0074] The residual monomer content of the superabsorbent particles obtained in this way was determined. The results are summarized in Table 1.

[0075] Examples 2-4

[0076] The procedure was the same as in Example 1. Phytic acid was added to the monomer solution in each case.

[0077] The residual monomer content of the superabsorbent particles obtained in this way was determined. The results are summarized in Table 1. 241033W001

[0078] Table 1: Summary with 0.27 wt% CXL based on acrylic acid

[0079] Comparative example

[0080] Example 5

[0081] In a 1000 ml polypropylene beaker, 1.03 g of triple ethoxylated glyceryl triacrylate (Laromer® PO 9044V; BASF SE, Ludwigshafen, Germany) was placed as a core crosslinker (CXL), and 38.56 g of acrylic acid (stabilized with 0.02 wt% hydroquinone monomethyl ether) was added. Subsequently, 347.03 g of 37.3 wt% aqueous sodium acrylate solution, 2.07 g of polyethylene glycol-4000, and 8.54 g of water were added. The monomer solution was then inerted for 30 minutes using a glass frit with nitrogen at a rate of 300 l / h. The degree of neutralization was 72 mol%, and the solids content was 42 wt%.

[0082] For polymerization, 1.84 g of a 15 wt% aqueous sodium peroxodisulfate solution, 0.11 g of a 1 wt% hydrogen peroxide solution, and 0.83 g of a 0.5 wt% ascorbic acid solution were added successively while stirring. After approximately 10 seconds, stirring was stopped, and the mixture was allowed to stand for 30 minutes to cool after reaching a peak temperature of approximately 110 °C.

[0083] The resulting polymer gel was comminuted and dried for 90 minutes at 175°C in a circulating air drying oven. It was then milled using a roller mill (Gebrüder Baumeister LRC-125; gap settings: 2 mm, 1 mm, 0.6 mm and 0.4 mm) and sieved to a particle size of 150 to 710 µm.

[0084] The residual monomer content of the superabsorbent particles obtained in this way was determined. The results are summarized in Table 2. 241033W001

[0085] 11

[0086] Examples 6-9

[0087] The procedure was the same as in Example 5. Phytic acid was added to the monomer solution in each case. The residual monomer content of the resulting superabsorbent particles was determined. The results are summarized in Table 2.

[0088] Table 2: Summary with 0.75 wt% CXL based on acrylic acid

[0089] Comparative example

Claims

241033W001 12 Patent claims 1. A process for producing surface-crosslinked superabsorbent particles by polymerization of an aqueous monomer solution or suspension, comprising a) at least one ethylene-unsaturated carboxylic acid that 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 a polymer gel, the polymer gel obtained is optionally comminuted, the polymer gel is subsequently dried, the dried polymer gel is optionally milled and classified, the dried polymer gel is subsequently optionally thermally surface-crosslinked and cooled, characterized in that phytic acid or its salt is added before polymerization.

2. The method according to claim 1, characterized in that acrylic acid is used as an ethylene-unsaturated carboxylic acid.

3. Method according to claim 1 or 2, characterized in that a peroxodisulfate, in particular ammonium peroxodisulfate, sodium peroxodisulfate and / or potassium peroxodisulfate, is used as initiator c).

4. Method according to one of claims 1 to 3, characterized in that 0.001 to 0.3 wt.% phytic acid or its salt is added to the monomer solution or suspension, based on the ethylene unsaturated carboxylic acid a).

5. Method according to one of claims 1 to 3, characterized in that 0.0075 to 0.125 wt.% phytic acid or its salt is added to the monomer solution or suspension, based on the ethylene unsaturated carboxylic acid a).

6. Method according to one of claims 1 to 5, characterized in that the phytic acid or its salt is added before the at least one initiator c).

7. Method according to any one of claims 1 to 6, characterized in that 0.01 to 2.0 wt.% of the crosslinking agent c) is added to the monomer solution or suspension, based on the ethylene unsaturated carboxylic acid a). 241033W001 13 8. Method according to any one of claims 1 to 6, characterized in that 0.1 to 1.0 wt.% of the crosslinking agent c) is added to the monomer solution or suspension, based on the ethylene unsaturated carboxylic acid a).

9. Method according to one of claims 1 to 8, characterized in that at least one complexing agent other than phytic acid is added to the aqueous monomer solution or suspension.

10. Method according to any one of claims 1 to 9, characterized in that the superabsorbent particles obtained have a residual monomer content measured according to EDANA WSP No. 210.2-05 "Residual Monomers" of at most 1500 ppm.

11. Method according to any one of claims 1 to 9, characterized in that the superabsorbent particles obtained have a residual monomer content measured according to EDANA WSP No. 210.2-05 "Residual Monomers" of at most 1000 ppm.

12. Superabsorbent particles containing phytic acid or its salt.

13. Superabsorbent particles according to claim 12, wherein the superabsorbent particles contain phytic acid or its salt in 0.005 to 0.5 wt.%, based on the superabsorbent particles.

14. Superabsorbent particles according to claim 12 or 13, wherein the superabsorbent particles contain at least one complexing agent other than phytic acid.

15. Hygiene articles containing superabsorbent particles according to one of claims 12 to 14.

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