METHOD FOR THE PRODUCTION OF SUPERABSORBENT PARTICLES

Description

[0001] The present invention relates to a process for the production of superabsorbent particles, comprising polymerization of a monomer preparation, drying of the resulting aqueous polymer gel, milling, classification and thermal surface crosslinking, wherein the monomer preparation is produced by mixing an aqueous monomer solution and a foamed aqueous surfactant solution.

[0002] 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.

[0003] 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.

[0004] To improve application properties, such as permeability (SFC) and absorption at a pressure < 49.2 g / cm² (AUHL), superabsorbent particles are generally surface-crosslinked. This increases the degree of crosslinking of the particle surface, thereby at least partially decoupling absorption at a pressure < 49.2 g / cm² (AUHL) and centrifugal retention capacity (CRC). This surface 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 crosslinker and thermally surface-crosslinked. Suitable crosslinkers are compounds that can form covalent bonds with at least two carboxylate groups of the polymer particles.

[0005] EP 2 518 092 A1 describes the production of porous superabsorbent particles by foaming the monomer solution before polymerization.

[0006] DE10 2005 011165 A1 describes the production of superabsorbent polymer foam containing wood pulp or waste paper.

[0007] The object of the present invention was to provide improved methods for the production of superabsorbent particles, in particular for superabsorbent particles with a higher liquid absorption capacity.

[0008] The problem was solved by a process for the production of superabsorbent particles, comprising polymerization of a monomer preparation, drying of the resulting aqueous polymer gel, milling, classification and thermal surface post-crosslinking, characterized in that the monomer preparation is produced by mixing an aqueous monomer solution containing a) at least one ethylene-unsaturated, acid-group-bearing monomer that is at least partially neutralized, b) at least one crosslinker, and c) at least one initiator. and is produced with a foamed aqueous surfactant solution.

[0009] In a preferred embodiment of the present invention, the polymerization of the monomer preparation is carried out in a kneading reactor. The aqueous monomer solution and the foamed aqueous surfactant solution are mixed in the kneading reactor. The kneading reactor can be operated continuously or batchwise. A continuous kneading reactor is preferred.

[0010] Redox initiators are particularly suitable as initiators (c). A redox initiator consists of an oxidizing component, for example sodium peroxodisulfate and / or hydrogen peroxide, and a reducing component, for example ascorbic acid. The reducing component is preferably added only in the kneading reactor.

[0011] The aqueous monomer solution preferably contains 30 to 60 wt.%, particularly preferably 35 to 65 wt.%, and most preferably 40 to 50 wt.% of monomer a). Monomer a) is preferably neutralized to 40 to 90 mol.%, particularly preferably 50 to 85 mol.%, and most preferably 60 to 80 mol.%. The preferred monomer a) is partially neutralized acrylic acid.

[0012] The foamed aqueous surfactant solution preferably contains 0.1 to 10 wt.%, particularly preferably 0.5 to 5 wt.%, and most preferably 1 to 3 wt.%, at least one surfactant. The preferred surfactants are non-ionic surfactants, for example, ethoxylated C14-C20 alcohols.

[0013] The surfactant solution is first foamed, and only the already foamed surfactant solution is mixed with the monomer solution.

[0014] The weight ratio of foamed aqueous surfactant solution to aqueous monomer solution in the monomer preparation is preferably from 0.01 to 0.30, particularly preferably from 0.02 to 0.20, and most preferably from 0.03 to 0.10.

[0015] The present invention is based on the understanding that the sequence of process steps has a decisive influence on the properties of the resulting superabsorbent particles. The sequence of process steps according to the invention, i.e., mixing an already foamed surfactant solution with a monomer solution, leads to a higher maximum temperature during polymerization. The centrifugal retention capacity (CRC) and the extractables of the base polymer are increased. After thermal surface crosslinking, the superabsorbent particles produced according to the inventive process exhibit a significantly improved volumetric liquid absorption at 0.3 psi (2.07 kPa) pressure (VAUL), a slightly improved absorption at a pressure of < 49.2 g / cm² (AUHL), and a comparable centrifugal retention capacity (CRC).

[0016] The foamed aqueous surfactant solution may additionally contain a water-soluble polymer, preferably in a concentration of 0.5 to 20 wt.%, particularly preferably in a concentration of 2 to 15 wt.%, and most preferably in a concentration of 5 to 10 wt.%. The preferred water-soluble polymers are polyethylene glycols.

[0017] The following explains in more detail the production of the superabsorbent particles: The superabsorbent particles are produced by polymerization of a monomer solution and are usually insoluble in water.

[0018] The monomers a) 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, most preferably at least 35 g / 100 g water.

[0019] Suitable monomers a) are, for example, ethylene-unsaturated carboxylic acids such as acrylic acid, methacrylic acid, and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Acrylic acid is especially preferred.

[0020] The monomers a) usually contain polymerization inhibitors, preferably hydroquinone semi-ethers, as a storage stabilizer.

[0021] Suitable crosslinking agents b) 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 a). Furthermore, polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer a) are also suitable as crosslinking agents b).

[0022] Crosslinkers b) are preferably compounds with at least two polymerizable groups that can be radically polymerized into the polymer network. Suitable crosslinking agents b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 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, mixed acrylates which, in addition to acrylate groups, contain further ethylene-unsaturated groups, as described in DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinking mixtures, as described for example in DE 195 43 368 A1, DE 196 46 484 A1, WO 90 / 15830 A1 and WO 02 / 032962 A2.

[0023] The amount of crosslinker b) is preferably 0.05 to 1.5 wt.%, particularly preferably 0.1 to 1 wt.%, most preferably 0.3 to 0.6 wt.%, in each case calculated on the total amount of monomer a) used. With increasing crosslinker content, the centrifuge retention capacity (CRC) decreases and the absorption under a pressure of 21.0 g / cm² (AUL) reaches a maximum.

[0024] As initiators (c), any compound that generates radicals under the polymerization conditions can be used, for example, thermal initiators, redox initiators, and photoinitiators. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. Preferably, mixtures of thermal 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).

[0025] An aqueous monomer solution is typically used. The water content of the monomer solution is preferably 40 to 70 wt.%, particularly preferably 45 to 65 wt.%, and most preferably 50 to 60 wt.%. It is also possible to use monomer suspensions, i.e., monomer solutions with a monomer (a) exceeding its solubility, for example, sodium acrylate. 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.

[0026] 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.

[0027] The foamed surfactant solution contains at least one surfactant. This at least one surfactant can be an anionic, cationic, and / or non-ionic surfactant. Non-ionic surfactants are preferred, particularly those with an HLB value of 10 to 25. The HLB value is a measure of the water or oil solubility of predominantly non-ionic surfactants and can be determined using standard methods.

[0028] A surfactant consists of at least one nonpolar and at least one polar group. Preferred surfactants have large nonpolar and / or polar groups. Large groups are groups with a molecular weight of at least 130 g / mol, preferably at least 250 g / mol, and particularly preferably at least 500 g / mol.

[0029] Suitable surfactants include, for example, sorbitan esters such as sorbitan monostearate, sorbitan monooleate, sorbitan monopalmitate and sorbitan monolaurate, as well as glycerol esters whose acid component is derived from C 14 to C 20 carboxylic acids.

[0030] Preferred surfactants are alkoxilated, preferably ethoxylated, alcohols, wherein the alcohols may optionally be branched and / or unsaturated, as well as alkoxilated, preferably ethoxylated, sorbitan monoesters, such as sorbitan monostearate and sorbitan monooleate. Particularly preferred surfactants are ethoxylated C14-C20 alcohols.

[0031] The at least one surfactant preferably has a viscosity of over 20 mPas, particularly preferably over 25 mPas, most preferably over 30 mPas (measured at 23°C according to EN12092).

[0032] The foamed surfactant solution may additionally contain water-soluble polymers. Suitable water-soluble polymers include polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, modified cellulose such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycols or polyacrylic acids, preferably polyglycols such as polyethylene glycol.

[0033] The surfactant solution is foamed. All known foaming methods are suitable for this purpose. The surfactant solution can, for example, be intensively mixed with an inert gas such as nitrogen or carbon dioxide.

[0034] The foamed surfactant solution is then mixed with the monomer solution and polymerized. Since the foam has already been generated beforehand, intensive mixing, such as rapid stirring, is no longer necessary.

[0035] 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 preparation 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 38 25 366 A1 and US 6,241,928. Polymerization in a belt reactor produces a polymer gel that must be comminuted, for example, in an extruder or kneading reactor.

[0036] To improve the drying properties, the crushed polymer gel obtained using a kneading reactor can be additionally extruded.

[0037] The acid groups of the resulting polymer gels are usually partially neutralized. Neutralization is preferably carried out at the monomer level. This is typically done by mixing in the neutralizing agent as an aqueous solution or, more preferably, as a solid. The degree of neutralization is preferably 40 to 90 mol%, particularly preferably 50 to 85 mol%, and most preferably 60 to 80 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, are most preferred.Solid carbonates and bicarbonates can also be used in encapsulated form, preferably in the monomer solution directly before polymerization, during or after polymerization into the polymer gel, and before drying. Encapsulation is achieved by coating the surface with an insoluble or only slowly soluble material (for example, film-forming polymers, inert inorganic materials, or fusible organic materials), which delays the dissolution and reaction of the solid carbonate or bicarbonate so that carbon dioxide is only released during drying, and the resulting superabsorbent exhibits high internal porosity.

[0038] 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 2 to 5 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 has a glass transition temperature Tg that is too low and is difficult to process further. If the residual moisture content is too low, the dried polymer gel is too brittle, and undesirably large quantities of polymer particles with an excessively small particle size ("fines") are produced in the subsequent comminution steps. The solids content of the polymer gel before drying is preferably between 25 and 90 wt.%, particularly preferably between 35 and 70 wt.%, and most preferably between 40 and 60 wt.%.The dried polymer gel is then broken up and optionally coarsely crushed.

[0039] 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.

[0040] The mean particle size of the polymer particles separated as the product fraction is preferably from 150 to 850 µm, particularly preferably from 250 to 600 µm, and most preferably from 300 to 500 µm. 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", wherein 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%.

[0041] The polymer particles undergo thermal surface crosslinking 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 polymer particles. Suitable compounds include, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, as described in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, di- 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.

[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 0.2 wt.%, in each case based on the polymer 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 polymer particles. Following spraying, the polymer particles coated with the surface recrosslinker are surface recrosslinked and dried, whereby the surface recrosslinking reaction can take place both before and during drying.

[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 polymer particles can be adjusted by varying the content of non-aqueous solvent or the total amount of solvent.

[0049] Surface recrosslinking is preferably carried out in contact dryers, particularly paddle dryers, and most preferably disc dryers. Suitable dryers include, for example, the Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Leingarten; Germany), the Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Leingarten; Germany), the Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; USA), and the Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany). Fluidized bed dryers can also be used.

[0050] 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.

[0051] 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 20 minutes, most preferably at least 30 minutes, and usually at most 60 minutes.

[0052] Subsequently, the surface-crosslinked polymer particles can be reclassified, whereby polymer particles that are too small and / or too large are separated and recycled back into the process.

[0053] The surface-crosslinked polymer particles can be coated or re-moistened to further improve their properties.

[0054] 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 polymer 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 polymer particles and reduces their tendency to accumulate static electricity. Advantageously, post-humidification is carried out in the cooler after thermal surface crosslinking.

[0055] 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 polymer 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. Methods:

[0056] The standard test methods described below, designated "WSP", are described in: "Standard Test Methods for the Nonwovens Industry", 2005 edition, jointly published by the "Worldwide Strategic Partners" EDANA (Herrmann-Debrouxlaan 46, 1160 Oudergem, Belgium, www.edana.org) and INDA (1100 Crescent Green, Suite 115, Cary, North Carolina 27518, USA, www.inda.org). This publication is available from both EDANA and INDA.

[0057] Unless otherwise specified, measurements should be carried out at an ambient temperature of 23 ± 2 °C and a relative humidity of 50 ± 10%. The water-absorbing polymer particles should be thoroughly mixed before measurement. moisture content

[0058] The moisture content is determined according to the EDANA recommended test method no. WSP 230.2 (05) "Mass Loss Upon Heating". Centrifuge retention capacity

[0059] The centrifuge retention capacity (CRC) is determined according to the EDANA recommended test method No. WSP 241.2 (05) "Fluid Retention Capacity in Saline, After Centrifugation". Absorption at a pressure of 21.0 g / cm²< (Absorption under Load)

[0060] The absorption under a pressure of 21.0 g / cm² (AUL) is determined according to the EDANA recommended test method No. WSP 242.2 (05) "Absorption Under Pressure, Gravimetric Determination". Absorption at a pressure of 49.2 g / cm²< (Absorption under High Load)

[0061] The absorption under a pressure of 49.2 g / cm² (AUHL) is determined analogously to the EDANA recommended test method No. WSP 242.2 (05) "Absorption Under Pressure, Gravimetric Determination", except that a pressure of 49.2 g / cm² (0.7 psi) is used instead of a pressure of 21.0 g / cm² (0.3 psi). Extractable

[0062] The extractable content of the superabsorbent particles is determined according to the EDANA recommended test method No. WSP 270.2 (05) "Extractable". Surface tension of the aqueous extract

[0063] To determine the surface tension of the aqueous extract (OFS), 0.50 g of the superabsorbent particles are weighed into a small beaker and mixed with 40 ml of a 0.9 wt% salt solution. The contents of the beaker are stirred for 3 minutes at 500 rpm using a magnetic stir bar, then allowed to settle for 2 minutes. Finally, the surface tension of the supernatant aqueous phase is measured using a K10-ST digital tensiometer or a comparable instrument with a platinum plate (Krüss GmbH, Hamburg, Germany). The measurement is performed at a temperature of 23°C. Free Swell Rate

[0064] To determine the swelling rate (FSR), 1.00 g (= W1) of the superabsorbent particles are weighed into a 25 ml beaker and evenly distributed on the bottom. Then, 20 ml of a 0.9 wt% saline solution are dispensed into a second beaker using a dispenser, and the contents of this beaker are quickly added to the first. A stopwatch is then started. As soon as the last drop of saline solution is absorbed, which is indicated by the disappearance of the reflection on the liquid surface, the stopwatch is stopped. The exact amount of liquid poured from the second beaker and absorbed by the polymer in the first beaker is precisely determined by back-weighing the second beaker (= W2). The time interval required for absorption, measured with the stopwatch, is denoted as t. The disappearance of the last drop of liquid from the surface is recorded as time t.

[0065] The source velocity (FSR) is calculated as follows: FSR g / gs = W 2 / W 1 xt

[0066] However, if the moisture content of the superabsorbent particles is more than 3 wt%, the weight W1 must be corrected by this moisture content. Permeability (Saline Flow Conductivity)

[0067] The swollen gel layer's permeability (SFC) under a pressure of 0.3 psi (2070 Pa) is determined, as described in EP 2 535 698 A1, using a sample of 1.5 g of superabsorbent particles as a urine permeability measurement (UPM) of a swollen gel layer. The flow rate is automatically recorded.

[0068] The permeability (SFC) is calculated as follows: SFC cm 3 s / g = Fg t = 0 × L 0 / d × A × WP , where Fg(t=0) is the flow rate of NaCl solution in g / s, obtained by extrapolating the flow rate data Fg(t) to t=0 using linear regression analysis, L 0 is the thickness of the gel layer in cm, d is the density of the NaCl solution in g / cm 3< , A is the area of ​​the gel layer in cm 2< and WP is the hydrostatic pressure over the gel layer in dyne / cm 2< . Volumetric fluid intake at 0.3 psi (2.07 kPa) pressure (VAUL)

[0069] For volumetric fluid absorption under 0.3 psi (2.07 kPa) pressure (VAUL), the τ-value is determined according to the test method "Volumetric Absorbency Under Load (VAUL)" described on pages 39 and 40 of WO 2014 / 079694 A1. The τ-value is referred to there as "characteristic swelling time". Examples Production of the base polymer Example 1

[0070] A twin-shaft kneading reactor of type LUK 8.0K2 (Coperion Werner & Pfleiderer GmbH & Co. KG, Stuttgart, Germany) was inerted by purging with nitrogen. The kneading reactor shafts were operated at 96 rpm and 48 rpm, respectively. The reactor jacket could be heated using a heat transfer fluid.

[0071] 4901 g of a 37.3 wt% aqueous sodium acrylate solution and 571.9 g of acrylic acid were mixed, purged with nitrogen to remove oxygen, and placed in the kneading reactor. Subsequently, a mixture of 7.9 g of triple ethoxylated glycerol triacrylate (approx. 85 wt%) and 100 g of acrylic acid, 11.89 g of aqueous sodium peroxodisulfate solution (approx. 15 wt%), and 132 g of aqueous hydrogen peroxide solution (approx. 3 wt%) were successively dosed into the kneading reactor.

[0072] A surfactant solution consisting of 250 g water, 29.6 g aqueous polyethylene glycol-4000 (approx. 50 wt%), and 9.9 g 80-fold ethoxylated C16 / C18 fatty alcohol (Lutensol® < AT80) was foamed with nitrogen in a static mixer. Polyethylene glycol-4000 is a polyethylene glycol with an average molecular weight of approximately 4,000 g / mol. The resulting stable foam was then dosed into the kneading reactor. The line was subsequently flushed with approximately 200 g of water.

[0073] Subsequently, 19.7 g of aqueous ascorbic acid solution (approx. 0.5 wt%) was dosed into the kneading reactor, and the reactor jacket was heated using a heat transfer fluid (80°C). The temperature inside the kneading reactor rose from 22°C to 102°C. Once the temperature increase was complete, the heating was switched off, the resulting polymer gel was kneaded for another 13 minutes, cooled to 63°C, and discharged from the kneading reactor.

[0074] The resulting polymer gel was homogeneously distributed in portions of approximately 1,080 g onto wire mesh trays and dried in a circulating air drying oven for 90 minutes at 175°C. The dried polymer gel was then milled in three stages (1,000 µm, 600 µm, and 400 µm) using an LRC 250 roller mill (Bauermeister Zerkleinerungstechnik GmbH, Norderstedt, Germany) and sieved to a particle size of 150 to 710 µm.

[0075] The obtained superabsorbent particles were analyzed. The results are summarized in Table 1. Example 2

[0076] The procedure was as in Example 1, wherein the surfactant solution used was a solution of 250 g water, 14.7 g aqueous polyethylene glycol-4000 (approx. 50 wt%) and 4.9 g 80-fold ethoxylated C 16 / C 18 fatty alcohol (Lutensol ®< AT80).

[0077] The obtained superabsorbent particles were analyzed. The results are summarized in Table 1. Example 3 (comparative example)

[0078] The procedure was the same as in Example 1, except that no foaming was used.

[0079] The obtained superabsorbent particles were analyzed. The results are summarized in Table 1. Example 4 (comparative example)

[0080] The procedure was the same as in Example 1, except that instead of the surfactant solution, a solution of 250 g water and 29.6 g aqueous polyethylene glycol-4000 (approx. 50 wt%) was used and no foaming was performed.

[0081] The obtained superabsorbent particles were analyzed. The results are summarized in Table 1. Example 5 (comparative example)

[0082] The procedure was the same as in Example 1, except that instead of the surfactant solution, a surfactant solution consisting of 250 g water and 9.9 g 80-fold ethoxylated C 16 / C 18 fatty alcohol (Lutensol ®< AT80) was used and no foaming was performed.

[0083] The obtained superabsorbent particles were analyzed. The results are summarized in Table 1. Example 6 (comparative example)

[0084] The procedure was the same as in Example 1, whereby the surfactant solution was dosed into the kneading reactor along with the monomer solution, and only then was the mixture of monomer solution and surfactant solution in the kneading reactor foamed with nitrogen (1 bar, 1166 ml / min) for two minutes using a cylindrical sintered filter element of type SIK-R 15 AX (GKN Sinter Metal Filters GmbH, Radevormwald, Germany).

[0085] The obtained superabsorbent particles were analyzed. The results are summarized in Table 1. Table 1: Properties of the base polymers Example. CRC [g / g] AUL [g / g] FSR [g / gs] Ext. [wt.%] OFS [mN / m] T max [° C] 1 38,4 11,9 0,33 14,5 56,7 102 2 37,4 15,2 0,31 11,4 55,1 95 3*) 35,9 18,7 0,31 7,9 54,1 89 4*) 35,9 20,5 0,31 8,7 63,7 91 5*) 34,5 23,2 0,34 8,6 54,7 93 6*) 35,3 22,8 0,33 8,2 53,8 88 *) Comparative example Surface recrosslinking Example 7

[0086] 1,200 g of base polymer from Example 1 was coated in a plowshare mixer type M5 with heating jacket (Gebr. Lödige Maschinenbau GmbH; Paderborn, Germany) at 23° C and a shaft speed of 200 revolutions per minute using a two-component spray nozzle with 54.4 g of a mixture of 1.5 wt% N-hydroxyethyl-2-oxazolidinone, 1.5 wt% 1,3-propanediol, 26.7 wt% isopropanol, 11.0 wt% aluminium lactate and 59.3 wt% water.

[0087] After spraying, the shaft speed was reduced to 50 revolutions per minute and the product temperature increased to 185°C. The reaction mixture was then held at this temperature and shaft speed for 40 minutes. The resulting product was cooled to ambient temperature and re-classified using a 710 µm sieve.

[0088] The obtained superabsorbent particles were analyzed. The results are summarized in Table 2. Example 8

[0089] The procedure was the same as in Example 7, using the base polymer from Example 2. The resulting superabsorbent particles were analyzed. The results are summarized in Table 2. Example 9 (comparative example)

[0090] The procedure was as described in the example. 7, The base polymer from Example 3 was used. The resulting superabsorbent particles were analyzed. The results are summarized in Table 2. Example 10 (comparative example)

[0091] The procedure was as described in the example. 7, The base polymer from Example 4 was used. The resulting superabsorbent particles were analyzed. The results are summarized in Table 2. Example 11 (comparative example)

[0092] The procedure was the same as in Example 7, using the base polymer from Example 5. The resulting superabsorbent particles were analyzed. The results are summarized in Table 2. Example 12 (comparative example)

[0093] The procedure was the same as in Example 7, using the base polymer from Example 6. The resulting superabsorbent particles were analyzed. The results are summarized in Table 2. Table 2: Properties after surface re-crosslinking Example. SFC [10 -7< cm 3< s / g] CRC [g / g] AUHL [g / g] VAUL [s] 7 35 30,0 27,4 162 8 33 31,0 27,6 196 9*) 41 29,6 26,0 240 10*) 39 29,1 26,2 232 11*) 49 28,4 26,3 221 12*) 47 30,0 26,5 254 *) Comparative example

[0094] The comparison of examples 7 and 12 shows that the foaming of the surfactant solution in the absence of the monomer solution (example 7) leads to a significantly different property profile than the foaming in the presence of the monomer solution (example 12).

Claims

1. A process for producing superabsorbent particles, comprising polymerization of a monomer preparation, drying of the resultant aqueous polymer gel, grinding, classifying and thermal surface postcrosslinking, which comprises producing the monomer preparation by mixing an aqueous monomer solution comprising a) at least one ethylenically unsaturated monomer which bears acid groups and is at least partly neutralized, b) at least one crosslinker and c) at least one initiator, and a foamed aqueous surfactant solution.

2. The process according to claim 1, wherein the polymerization of the monomer preparation is conducted in a kneading reactor and the aqueous monomer solution and the foamed aqueous surfactant solution are mixed in the kneading reactor.

3. The process according to claim 1 or 2, wherein the polymerization is conducted in a continuous kneading reactor.

4. The process according to any of claims 1 to 3, wherein the aqueous monomer solution comprises from 40% to 50% by weight of monomer a).

5. The process according to any of claims 1 to 4, wherein monomer a) has been neutralized to an extent of 60 to 80 mol%.

6. The process according to any of claims 1 to 5, wherein monomer a) is partly neutralized acrylic acid.

7. The process according to any of claims 1 to 6, wherein the foamed aqueous surfactant solution comprises from 1% to 3% by weight of at least one surfactant.

8. The process according to any of claims 1 to 7, wherein the foamed aqueous surfactant solution comprises at least one nonionic surfactant.

9. The process according to any of claims 1 to 8, wherein the weight ratio of foamed aqueous surfactant solution to aqueous monomer solution in the monomer preparation is from 0.03 to 0.10.

10. The process according to any of claims 1 to 9, wherein the foamed aqueous surfactant solution additionally comprises a water-soluble polymer.

11. The process according to claim 10, wherein the foamed aqueous surfactant solution comprises from 5% to 10% by weight of the additional water-soluble polymer.

12. The process according to claim 10 or 11, wherein the additional water-soluble polymer is polyethylene glycol.

13. The process according to any of claims 1 to 12, wherein the at least one initiator c) is a redox initiator.