EP4263630A1

EP4263630A1 - Process for producing superabsorbent particles

Info

Publication number: EP4263630A1
Application number: EP21823321.1A
Authority: EP
Inventors: Ruediger Funk; Matthias Weismantel; Marcus MAEMECKE
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2020-12-16
Filing date: 2021-12-07
Publication date: 2023-10-25
Also published as: CN116568732A; JP2024503203A; US20240100506A1; WO2022128619A1

Abstract

A process for producing thermally surface postcrosslinked superabsorbent particles, comprising polymerization of a monomer solution or suspension, static drying of the resultant aqueous polymer gel, comminution of the dried polymer gel, classification of the resultant polymer particles, with removal of excessively small polymer particles as undersize, mixing of the removed undersize with an aqueous solution, wherein the aqueous solution comprises a crosslinker, and recycling of the polymer gel obtained from the undersize into the static drying.

Description

Process for the production of superabsorbent particles

description

The present invention relates to a method for producing thermally surface-postcrosslinked superabsorbent particles, comprising polymerization of a monomer solution or suspension, static drying of the aqueous polymer gel obtained, comminution of the dried polymer gel, classification of the polymer particles obtained, with polymer particles that are too small being separated off as undersize, mixing the separated off Undersize with an aqueous solution, wherein the aqueous solution contains a crosslinker, and recycling of the polymer gel obtained from the undersize into the static drying.

Superabsorbents are used in the manufacture of diapers, tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in agricultural horticulture. The superabsorbers are also referred to as water-absorbing polymers.

The production of superabsorbents is described in the monograph ''Modern Superabsorbent Polymer Technology'', F.L. Buchholz and A.T. Graham, Wiley-VCH, 1998, pp. 71-103.

In order to improve the application properties, such as gel bed permeability (GBP) and absorption under a pressure of 49.2 g/cm ² (AUL0.7psi), superabsorbent particles are generally surface postcrosslinked. This increases the degree of crosslinking of the particle surface, whereby the absorption under a pressure of 49.2 g/cm ² (AUL0.7psi) and the centrifuge retention capacity (CRC) can be at least partially decoupled. This surface post-crosslinking can be carried out in an aqueous gel phase. Preferably, however, dried, ground and sieved polymer particles (base polymer) are surface-coated with a surface post-crosslinking agent and surface post-crosslinked thermally. Crosslinkers suitable for this purpose are compounds which can form covalent bonds with at least two carboxylate groups of the polymer particles.

EP 0 789 047 A1 describes a process for producing superabsorbents, in which polymer particles are agglomerated with an aqueous solution and the aqueous solution contains a crosslinker. WO 2019/221235 A1 and WO 2019/221236 A1 describe processes for producing superabsorbent particles, polymer particles being agglomerated with water and the polymer gel obtained being recycled.

The object of the present invention was to provide an improved process for producing superabsorbent particles, in particular for producing superabsorbent particles with a high absorption rate.

The object was achieved by a process for producing superabsorbent particles by polymerizing a monomer solution or suspension containing a) at least one ethylenically unsaturated, acid-group-carrying monomer which is at least partially neutralized, b) at least one crosslinker 1 and c) at least one initiator, comprising the steps i) polymerization of the monomer solution or suspension and optional extrusion of the resulting polymer gel 1, ii) drying of the polymer gel, iii) comminution of the dried polymer gel, iv) classification of the polymer particles obtained in step iii), with too small polymer particles separated as undersize 1 and optionally oversized polymer particles are recycled in step iii), and the remaining polymer particles are thermally surface post-crosslinked in a further step, v) mixing the separated undersize with an aqueous solution and optionally extrusion of the resulting polymer gel 2 and vi) recycling of the polymer rgels 2 in step ii), characterized in that the polymer gel is statically dried in step ii), the moisture content of the polymer gel 2 obtained in step v) is from 20 to 80% by weight, the aqueous solution in step v) at least one Crosslinker 2 contains and the crosslinker 2 can form covalent or ionic bonds with at least two carboxylate groups of the polymer particles. The present invention is based on the finding that agglomerates produced from separated undersize increase the rate of absorption. It is important here that agglomeration takes place in the presence of a crosslinking agent and that the polymer gel 2 thus obtained is dried together with the rest of the polymer gel, with the two polymer gels not being mixed.

If the polymer gels are mixed, for example extruded together, the crosslinking agent is distributed uniformly over the entire polymer gel and the concentration of crosslinking agent in polymer gel 2 is reduced. This leads to more unstable agglomerates. At the same time, the polymer gel 1 is additionally crosslinked. This leads to a lower absorption capacity.

In a preferred embodiment of the present invention, the separated undersize in step v) is first mixed with water and optionally extruded and then mixed with the aqueous solution and optionally extruded. As a result, the separated undersize is pre-swollen before the crosslinking agent 2 is actually added.

If the separated undersize is pre-swollen with water before the addition of the aqueous solution, preferably at least 50% by weight, particularly preferably at least 70% by weight, very particularly preferably at least 90% by weight, of the total amount of in step v) added water used for pre-soaking. The total amount of water added in step v) is the amount of water added and the water content of the aqueous solution added.

This pre-swelling of the undersize causes the crosslinking agent applied with the aqueous solution to penetrate less deeply into the undersize. This leads to better cross-linking of the undersize particles and thus to more stable agglomerates when wet.

In a further preferred embodiment of the present invention, the separated undersize in step v) is first mixed with an aqueous base and optionally extruded and then mixed with the aqueous solution and optionally extruded. As a result, the separated undersize is pretreated before the crosslinking agent 2 is actually added.

If the separated undersize is pretreated with an aqueous base before the addition of the aqueous solution, preferably from 0.1 to 12% by weight of base, based on the undersize, is used. Suitable bases are sodium hydroxide, sodium carbonate and sodium hydroxide roe carbonate. For example, a 50% strength by weight sodium hydroxide solution can be used as the aqueous base.

The pretreatment of the undersize causes crosslinks in the undersize to be hydrolyzed and the centrifuge retention capacity (CRC) of the undersize to increase.

The temperature in step v) is preferably from 20 to 90°C, particularly preferably from 25 to 75°C, very particularly preferably from 30 to 60°C.

The aqueous solution in step v) preferably contains from 0.01 to 1.0% by weight, particularly preferably from 0.02 to 0.5% by weight, very particularly preferably from 0.05 to 0.2% by weight. -%, of the crosslinker 2, in each case based on the amount of separated undersize,

Suitable crosslinkers 2 which can be used in step v) are compounds which can form covalent bonds with at least two carboxylate groups of the polymer particles, for example ethylene carbonate and ethylene glycol diglycidyl ether. Such compounds are also known as surface post-crosslinkers and are described there in this document.

Other suitable crosslinkers 2 are able to form ionic bonds with at least two carboxylate groups of the polymer particles. Such compounds are also used as salts of polyvalent cations in surface post-crosslinking and are described there in this document.

Particularly suitable crosslinkers 2 which can be used in step v) are compounds which contain at least two epoxide groups, for example ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether and polyglycerol polyglycidyl ether.

The moisture content of the polymer gel obtained in step v) is preferably from 30 to 70% by weight, particularly preferably from 35 to 65% by weight, very particularly preferably from 40 to 60% by weight, the moisture content being analogous to that of the EDANA recommended test method no. WSP 230.2-05 "Mass Loss Upon Heating". 90% by weight of the undersize separated in step iv) has a particle size of preferably not more than 250 μm, particularly preferably not more than 200 μm, very particularly preferably 150 μm.

The amount of polymer gel 2 recycled in step vi) is preferably from 1 to 50% by weight, more preferably from 10 to 40% by weight, most preferably from 20 to 30% by weight, based in each case on the total amount of polymer gels to be dried in step ii).

The crosslinker 2 can hydrolyze, especially when it comes to compounds that contain at least two epoxide groups. Therefore the residence time of the polymer gel 2 between steps v) and ii) should not be too long. The residence time of the polymer gel 2 between steps v) and ii) is therefore preferably at most 15 minutes, particularly preferably at most 10 minutes, very particularly preferably at most 5 minutes.

The crosslinking reaction of crosslinker 2 should take place during drying in step ii). The residence time during drying in step ii) is preferably at least 120°C, particularly preferably at least 150°C, very particularly preferably at least 170°C. The residence time during drying in step ii) is preferably at least 10 minutes, particularly preferably at least 20 minutes, very particularly preferably at least 30 minutes.

It is customary to classify thermally surface-postcrosslinked polymer particles, with polymer particles that are too small being separated off as undersize 2 and the separated undersize 2 likewise being returned to step v). In this case, the mass of undersize 2 in relation to the total mass of undersize is preferably at most 10% by weight, particularly preferably at most 5% by weight, very particularly preferably at most 2% by weight.

The process according to the invention is preferably carried out continuously.

The production of the superabsorbents is explained in more detail below:

The superabsorbents are produced by polymerizing a monomer solution or suspension and are usually water-insoluble. The monomers a) are preferably water-soluble, ie 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, very particularly preferably at least 35g/100g water.

Examples of suitable monomers a) are ethylenically unsaturated carboxylic acids, such as acrylic acid, methacrylic acid and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Acrylic acid is very particularly preferred.

The monomers a) usually contain polymerization inhibitors, preferably hydroquinone monoethers, as storage stabilizers.

Suitable crosslinkers b) are compounds having at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups which can be radically polymerized into the polymer chain, and functional groups which can form covalent bonds with the acid groups of the monomer a). Furthermore, polyvalent metal salts which can form coordinate bonds with at least two acid groups of the monomer a) are also suitable as crosslinkers b).

Suitable crosslinkers b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 0 530438 A1, di- and triacrylates, as described in EP 0 547 847 A1, EP 0 559476 A1, EP 0632 068 A1, WO 93/21237 A1, WO 03/104299 A1, WO 03/104300 A1, WO 03/104301 A1 and DE 103 31 450 A1 describe mixed acrylates which contain further ethylenically unsaturated groups in addition to acrylate groups, as described in DE 103 31 456 A1 and DE 103 55401 A1, or crosslinker mixtures as described, for example, in DE 19543 368 A1, DE 196 46484 A1, WO 90/15830 A1 and WO 02/032962 A2.

The amount of crosslinker b) is preferably from 0.05 to 1.5% by weight, particularly preferably from 0.1 to 1% by weight, very particularly preferably from 0.3 to 0.6% by weight, calculated in each case 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/cm ² (AUL0.3psi) passes through a maximum. All compounds which generate free radicals under the polymerization conditions can be used as initiators c), for example thermal initiators, redox initiators, photoinitiators. Suitable redox initiators are sodium peroxodisulfate/ascorbic acid, hydrogen peroxide/ascorbic acid, sodium peroxodisulfate/sodium bisulfite and hydrogen peroxide/sodium bisulfite. Mixtures of thermal initiators and redox initiators are preferably used, such as sodium peroxodisulfate/hydrogen peroxide/ascorbic acid. The disodium salt of 2-hydroxy-2-sulfonatoacetic acid or a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite is preferably used as the reducing component. Such mixtures are available as Bruggolite® FF6 and Bruggolite® FF7 (Bruggemann Chemicals; Heilbronn; Germany).

An aqueous monomer solution is usually used. The water content of the monomer solution is preferably from 40 to 75% by weight, particularly preferably from 45 to 70% by weight, very particularly preferably from 50 to 65% by weight. It is also possible to use monomer suspensions, i.e. monomer solutions with monomer a) exceeding the solubility, for example sodium acrylate. As the water content increases, the energy required for the subsequent drying increases, and as the water content decreases, the heat of polymerization can only be dissipated insufficiently.

The preferred polymerization inhibitors require dissolved oxygen for optimal activity. The monomer solution can therefore be freed from dissolved oxygen before the polymerization by rendering it inert, i.e. flowing through it with an inert gas, preferably nitrogen or carbon dioxide. The oxygen content of the monomer solution is preferably reduced to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight, before the polymerization.

Suitable reactors for the polymerization are, for example, kneading reactors or belt reactors. In the kneader, the polymer gel formed during the polymerization of an aqueous monomer solution or suspension is continuously comminuted, for example by counter-rotating stirrer shafts, as described in WO 2001/038402 A1. Polymerization on the belt is described, for example, in DE 3825 366 A1 and US Pat. No. 6,241,928. Polymerization in a belt reactor produces a polymer gel that has to be comminuted, for example in an extruder or kneader. In order to improve the drying properties, the comminuted polymer gel obtained by means of a kneader can additionally be extruded.

The acid groups of the polymer gels obtained are usually partially neutralized. The neutralization is preferably carried out at the monomer stage. This is usually done by mixing in the neutralizing agent as an aqueous solution or preferably also as a solid. The degree of neutralization is preferably from 40 to 85 mol%, particularly preferably from 50 to 80 mol%, very particularly preferably from 60 to 75 mol%, it being possible to use the customary neutralizing agents, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal hydrogen carbonates as well their mixtures. Instead of alkali metal salts, ammonium salts can also be used. Sodium and potassium are particularly preferred as alkali metals, but sodium hydroxide, sodium carbonate or sodium bicarbonate and mixtures thereof are very particularly preferred. Solid carbonates and hydrogen carbonates can also be used here in encapsulated form, preferably in the monomer solution directly before the polymerization, during or after the polymerization in the polymer gel and before it is dried. The encapsulation is carried out by coating the surface with an insoluble or only slowly soluble material (e.g. using film-forming polymers, inert inorganic materials or fusible organic materials), which delays the solution and reaction of the solid carbonate or bicarbonate so that carbon dioxide is only released during drying is set and the resulting superabsorbent has a high internal porosity.

The polymer gel is then usually dried with a circulating air belt dryer until the residual moisture content is preferably 0.5 to 10% by weight, particularly preferably 1 to 7% by weight, very particularly preferably 2 to 5% by weight, with the residual moisture content according to the test method no. WSP 230.2-05 "Mass Loss Upon Heating" recommended by EDANA. If the residual moisture is too high, the dried polymer gel has a glass transition temperature T _g that is too low and is difficult to process further. If the residual moisture content is too low, the dried polymer gel is too brittle and the subsequent comminution steps result in undesirably large amounts of polymer particles with too small a particle size (“fines”). Before drying, the solids content of the polymer gel is preferably from 25 to 90% by weight, particularly preferably from 35 to 70% by weight, very particularly preferably from 40 to 60% by weight. The dried polymer gel is then broken up and optionally coarsely comminuted. The dried polymer gel is then usually ground and classified, it being possible to use single-stage or multi-stage roller mills, preferably two-stage or three-stage roller mills, pinned mills, hammer mills or vibratory mills for the grinding.

The mean particle size of the polymer particles separated off as the product fraction is preferably from 150 to 850 μm, particularly preferably from 250 to 600 μm, very particularly from 300 to 500 μm. The average particle size of the product fraction can be determined using the test method no. WSP 220.2 (05) "Particle Size Distribution" recommended by EDANA, whereby the mass fractions of the sieve fractions are applied cumulatively and the average particle size is determined graphically. The mean particle size here is the value of the mesh size that results for a cumulative 50% by weight.

To further improve the properties, the polymer particles can be thermally surface post-crosslinked. Suitable surface postcrosslinkers are compounds that contain groups that can form covalent bonds with at least two carboxylate groups of the polymer particles. Suitable compounds are, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, as described in EP 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 3523617 A1 and EP 0450 922 A2, or β-hydroxyalkylamides, as described in DE 102 04 938 A1 and US Pat. No. 6,239,230.

The amount of surface postcrosslinker is preferably from 0.001 to 2% by weight, particularly preferably from 0.02 to 1% by weight, very particularly preferably from 0.05 to 0.2% by weight, based in each case on the polymer particles.

In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the surface postcrosslinkers.

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 cations, trivalent cations such as aluminum, iron, chromium, rare earth and manganese cations, tetravalent cations such as titanium cations and Zirconium. As the counter ion, chloride, bromide, hydroxide, sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate and carboxylate such as acetate and lactate are possible. Aluminum hydroxide, aluminum sulfate and aluminum lactate are preferred. The amount of polyvalent cation used is, for example, 0.001 to 1.5% by weight, preferably 0.005 to 1% by weight, particularly preferably 0.02 to 0.8% by weight. in each case based on the polymer.

The surface postcrosslinking is usually carried out by spraying a solution of the surface postcrosslinker onto the dried polymer particles. After spraying, the polymer particles coated with surface post-crosslinking agent are thermally treated.

A solution of the surface postcrosslinker is preferably sprayed on in mixers with moving mixing tools, such as screw mixers, disk mixers and paddle mixers. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are very particularly preferred. The distinction between horizontal mixers and vertical mixers is based on the mounting of the mixing shaft, i.e. horizontal mixers have a horizontally mounted mixing shaft and vertical mixers have a vertically mounted mixing shaft. Suitable mixers are, for example, Horizontal Ploughshare® Mixer (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Doetinchem; The Netherlands), Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA) and Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; The Netherlands). However, it is also possible to spray on the surface postcrosslinker solution in a fluidized bed.

The surface post-crosslinkers are typically used as an aqueous solution. The depth of penetration of the surface postcrosslinker into the polymer particles can be adjusted via the content of nonaqueous solvent or the total amount of solvent.

The thermal treatment is preferably carried out in contact dryers, particularly preferably paddle dryers, very particularly preferably disk dryers. Suitable dryers are, for example, Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; USA ) and Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany). In addition, fluidized bed dryers can also be used.

The surface post-crosslinking can take place in the mixer itself, by heating the jacket or blowing in warm air. A downstream dryer, such as a tray dryer, a rotary kiln or a heatable screw, is also suitable. Especially Advantageously, the mixture is mixed in a fluidized bed dryer and the surface post-crosslinked thermally.

Preferred reaction temperatures are in the range from 100 to 250°C, preferably from 110 to 220°C, particularly preferably from 120 to 210°C, very particularly preferably from 130 to 200°C. The preferred residence time at this temperature is preferably at least 10 minutes, more preferably at least 20 minutes, most preferably at least 30 minutes, and usually at most 60 minutes.

The surface-postcrosslinked polymer particles can then be reclassified, with polymer particles that are too small and/or too large being separated off and returned to the process.

The surface post-crosslinked polymer particles can be coated or post-moistened to further improve their properties.

The subsequent moistening is preferably carried out at 30 to 80.degree. C., particularly preferably at 35 to 70.degree. C., very particularly preferably at 40 to 60.degree. At temperatures that are too low, the polymer particles tend to clump together and at higher temperatures, water evaporates noticeably. The amount of water used for post-wetting is preferably from 1 to 10% by weight, particularly preferably from 2 to 8% by weight, very particularly preferably from 3 to 5% by weight. The remoistening increases the mechanical stability of the polymer particles and reduces their tendency to static charging. Advantageously, the post-wetting is carried out in the cooler after the thermal surface post-crosslinking.

Examples of suitable coatings for improving the swelling rate and the gel bed permeability (GBP) are inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers and divalent or polyvalent metal cations. Suitable coatings for dust binding are, for example, polyols. Suitable coatings to counteract the undesirable tendency of the polymer particles to cake are, for example, pyrogenic silica such as Aerosil® 200, precipitated silica such as Sipernat® D17, and surfactants such as Span® 20. Methods:

The Standard Test Methods described below, labeled "WSP", are described in: "Standard Test Methods for the Nonwovens Industry", Edition 2005, published jointly by "Worldwide Strategie 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 release is available from both EDANA and INDA.

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

Centrifuge Retention Capacity

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 under a pressure of 21.0 g/cm ² (absorption under load)

The absorption under a pressure of 21.0 g/cm ² (AUL) is determined according to the test method no. WSP 242.2 (05) "Absorption Under Pressure, Gravimetric Determination" recommended by EDANA.

Absorption under a pressure of 49.2 g/cm ² (Absorption under High Load)

The absorption under a pressure of 49.2 g/cm ² (AU HL) is determined analogously to the test method no. WSP 242.2 (05) "Absorption Under Pressure, Gravimetric Determination" recommended by EDANA, whereby instead of a pressure of 21.0 g/cm ² (0.3psi) a pressure of 49.2 g/cm ² (0.7psi) is set.

vortex test

50.0 ml±1.0 ml of a 0.9% strength by weight aqueous sodium chloride solution are placed in a 100 ml beaker containing a magnetic stirring rod measuring 30 mm×6 mm. With help Using a magnetic stirrer, the saline solution is stirred at 600 ± 10 llpm. 2.000 g ± 0.010 g of superabsorbent particles are then added as quickly as possible, and the time that elapses before the cluster disappears due to the absorption of the saline solution by the superabsorbent particles is measured. The entire content of the beaker can still rotate as a uniform mass of gel, but the surface of the gelled saline solution must no longer show any individual turbulence. The time taken is reported as a vortex. The average of two measurements is sufficient for a sample if the measured values do not deviate from each other by more than 5%.

examples

Examples 1 to 4

Production of the super absorber particles:

An acrylic acid/sodium acrylate solution was prepared by continuously mixing deionized water, 50% strength by weight sodium hydroxide solution and acrylic acid, so that the degree of neutralization corresponded to 72.0 mol %. The solids content of the monomer solution was 42.5% by weight.

The crosslinker 1 used was 3-tuply ethoxylated glycerol triacrylate (about 85% strength by weight). The amount used was 1.2 kg per t of monomer solution.

To initiate the radical polymerization, 1.39 kg of a 0.25% by weight aqueous hydrogen peroxide solution, 3.58 kg of a 15% by weight aqueous sodium peroxodisulfate solution and 1.28 kg of a 1% by weight solution were used per t of monomer solution. own aqueous ascorbic acid solution.

The throughput of the monomer solution was 20 t/h. The reaction solution had a temperature of 23.5° C. at the inlet.

The individual components were continuously in a reactor from the following amounts

Type List Contikneter with a volume of 6.3m ³ (LIST AG, Arisdorf, Switzerland) metered:

20 t/h monomer solution

24 kg/h triple ethoxylated glycerol triacrylate (approx. 85% by weight) 99.4 kg/h hydrogen peroxide solution/sodium peroxodisulphate solution

25.6 kg/h ascorbic acid solution

The monomer solution was rendered inert with nitrogen between the point at which the crosslinker was added and the points at which the initiators were added.

After about 50% of the dwell time, polymer particles with a particle size of less than 150 μm (1000 kg/h) obtained from the manufacturing process by comminution and classification were additionally metered into the reactor. The residence time of the reaction mixture in the reactor was 15 minutes.

The polymer gel obtained (polymer gel A) was placed on the conveyor belt of a circulating air belt dryer by means of an oscillating conveyor belt. The circulating air belt dryer was 48 m long. The conveyor belt of the circulating air belt dryer had an effective width of 4.4 m. On the circulating air belt dryer, air/gas mixture (approx. 175° C.) flowed continuously around the aqueous polymer gel and dried. The residence time in the circulating air belt dryer was 37 minutes.

The dried polymer gel was comminuted using a three-stage roller mill and sieved off to a particle size of 150 to 850 μm. Polymer particles with a particle size of less than 150 μm were separated (polymer particles B). Polymer particles with a particle size greater than 850 μm were returned to the comminution. Polymer particles with a particle size in the range from 150 to 850 μm (polymer particles A) were thermally surface post-crosslinked.

The polymer particles were coated with a surface postcrosslinker solution in a Schugi Flexomix® (Hosokawa Micron B.V., Doetinchem, Netherlands) and then dried in a NARA Paddle Dryer (GMF Gouda, Waddinxveen, Netherlands) at 176° C. for 45 minutes.

The following quantities were dosed into the Schugi Flexomix®:

7.5 t/h polymer particles

348.75 kg/h surface post-crosslinking solution The surface postcrosslinker solution contained 2.2% by weight of 2-hydroxyethyl-2-oxazolidone, 2.2% by weight of 1,3-propanediol, 29.0% by weight of 1,2-propanediol, 3.2% by weight aluminum sulfate, 56.9% by weight water and 6.5% by weight isopropanol.

After drying, the surface post-crosslinked polymer particles were cooled to about 60° C. in a NARA paddle cooler (GMF Gouda, Waddinxveen, Netherlands). The surface post-crosslinked polymer particles were coated with 124.5 kg of a 2.4% strength by weight aqueous polyethylene glycol solution (polyethylene glycol with an average molar mass of 400 g/mol).

Agglomeration of the separated undersize:

207.0 g of water were added to 180.0 g of polymer particles B and the mixture was ground in an X70 meat grinder (Scharfen Slicing Machines GmbH, Witten, Germany). The polymer gel obtained was transferred to a polyethylene tub, sprayed with a mixture of 0.18 g of ethylene glycol diglycidyl ether (crosslinker 2) and 13.0 g of water and ground twice.

The polymer gel obtained in this way (polymer gel B) was immediately dried together with the polymer gel A in a circulating air drying cabinet at 170° C. for 60 minutes. For this purpose, polymer gel A was distributed on a drying tray and then polymer gel B was added. A total of 700 g of polymer gel were dried.

The dried polymer gel was crushed using a roller mill and sieved to a particle size of 300 to 600 μm. The polymer particles were then thermally surface post-crosslinked. For this purpose, the polymer particles were oxazolidone in a food processor with a mixture of 0.088 g of 2-hydroxyethyl-2, 0.088 g of 1,3-propanediol, 1.8 g of 1,2-propanediol, 0.88 g of a 26.8 wt Sprayed aluminum sulfate solution and 3.5 g of water and stirred for one minute.

It was then dried at 180° C. for 40 minutes and screened again to a particle size of 300 to 600 μm. The superabsorbent particles obtained were analyzed. Tab. 1: Mixing with pre-swelling

*) Comparative example

The examples show a significant improvement in absorption speed (vortex) with increasing proportion of polymer gel B.

Examples 5 to 8

The procedure was as in Examples 1 to 4, except that, to produce the polymer gel B, 180.0 g of polymer particles B were sprayed with a mixture of 0.18 g of ethylene glycol diglycidyl ether (crosslinker 2) and 220.0 g of water and ground three times.

Tab. 2: Test results

*) Comparative example

The examples show an improvement in absorption speed (vortex) with increasing proportion of polymer gel B. Examples 9 to 12

The procedure was as in Examples 1 to 4, with polymer gel A and polymer gel B being minced twice together.

Tab. 3: Test results

*) Comparative example

Overall, the examples show a significant drop in centrifuge retention capacity (CRC), absorption under a pressure of 49.2 g/cm ² (AlIHL) and absorption under a pressure of 21.0 g/cm ² (AUL). The reason for this is probably the additional extrusion of polymer gel A in Examples 9 to 12.

Claims

patent claims

1 . Process for the production of surface-postcrosslinked superabsorbent particles by polymerization of a monomer solution or suspension containing a) at least one ethylenically unsaturated, acid-group-carrying monomer which is at least partially neutralized, b) at least one crosslinker 1 and c) at least one initiator, comprising the steps i) polymerization the monomer solution or suspension and optional extrusion of the resulting polymer gel 1, ii) drying of the polymer gel, iii) comminution of the dried polymer gel, iv) classification of the polymer particles obtained in step iii), with too small polymer particles separated as undersize 1 and optionally too large polymer particles be recycled in step iii), and the remaining polymer particles are thermally surface post-crosslinked in a further step, v) mixing the separated undersize with an aqueous solution and optional extrusion of the resulting polymer gel 2 and vi) recycling of the polymer gel 2 in step ii), characterized in that the polymer gel is statically dried in step ii), the moisture content of the polymer gel 2 obtained in step v) is from 20 to 80% by weight, the aqueous solution in step v) contains at least one crosslinker 2 contains and the crosslinker 2 can form covalent or ionic bonds with at least two carboxylate groups of the polymer particles.

2. The method according to claim 1, characterized in that in step v) the separated undersize is first mixed with water and/or an aqueous base and optionally extruded and then mixed with the aqueous solution and optionally extruded.

3. The method according to claim 1 or 2, characterized in that the aqueous solution in step v) contains from 0.01 to 1.0% by weight of the crosslinking agent 2, based on the amount of separated undersize.

4. The method according to any one of claims 1 to 3, characterized in that the aqueous solution in step v) from 0.05 to 0.2 wt .-% of the crosslinker 2, based on the amount of separated undersize contains.

5. The method according to any one of claims 1 to 4, characterized in that the crosslinking agent 2 used in step v) can form covalent bonds with at least two carboxylate groups of the polymer particles.

6. The method according to claim 5, characterized in that the crosslinker 2 used in step v) contains at least two epoxide groups.

7. The method according to any one of claims 1 to 6, characterized in that the moisture content of the polymer gel obtained in step v) is from 40 to 60% by weight.

8. The method as claimed in any of claims 1 to 7, characterized in that 90% by weight of the undersize 1 separated off in step iv) has a particle size of at most 250 μm.

9. The method as claimed in any of claims 1 to 8, characterized in that 90% by weight of the undersize 1 separated off in step iv) has a particle size of at most 150 μm.

10. The method according to any one of claims 1 to 9, characterized in that the amount of polymer gel recycled in step vi) is from 1 to 50% by weight, based on the total amount of polymer gel to be dried in step ii).

11. The method according to any one of claims 1 to 10, characterized in that the amount of polymer gel recycled in step vi) is from 20 to 30% by weight, based on the total amount of polymer gel to be dried in step ii).

12. The method according to any one of claims 1 to 11, characterized in that the residence time of the polymer gel 2 between steps v) and ii) is at most 5 minutes.

13. The method according to any one of claims 1 to 12, characterized in that the

Temperature during drying in step ii) is at least 120°C.

14. The method according to any one of claims 1 to 13, characterized in that the residence time during drying in step ii) is at least 10 minutes.

15. The method according to any one of claims 1 to 14, characterized in that the surface post-crosslinked polymer particles are classified, too small polymer particles are separated as undersize 2, the separated undersize 2 is recycled in step v), the mass of undersize 2 at the Total mass of undersize is at most 10% by weight.