WO2020151969A1

WO2020151969A1 - Method for producing superabsorbent particles

Info

Publication number: WO2020151969A1
Application number: PCT/EP2020/050625
Authority: WO
Inventors: Rene CALLOT; Karl Possemiers; Ruediger Funk; Marco Krueger; Matthias Weismantel; Juergen Schroeder
Original assignee: Basf Se
Priority date: 2019-01-23
Filing date: 2020-01-13
Publication date: 2020-07-30
Also published as: TW202035464A

Abstract

The invention relates to a method for producing superabsorbent particles by polymerisation of a monomer solution or suspension, comprising the steps of drying the obtained aqueous polymer gel in a circulating-air belt dryer, grinding, classifiying, and optionally thermal surface post-cross-linking, wherein the aqueous polymer gel is introduced into the circulating-air belt dryer by means of an oscillating conveyor belt, the conveyor belt being guided by a drive drum and a reversing drum, at least one of the two drums having openings in the running surface, the opening ratio being at least 5%.

Description

Process for the production of superabsorbent particles

description

The present invention relates to a process for the preparation of superabsorbent particles by polymerizing a monomer solution or suspension, comprising drying the aqueous polymer gel obtained in a circulating air belt dryer, grinding, classification, and optionally thermal surface postcrosslinking, the aqueous polymer gel using an oscillating conveyor belt in the circulating air belt dryer is introduced, the conveyor belt is guided over a drive drum and a deflection drum, at least one of the two drums has openings in the running surface and the opening ratio is at least 5%.

Superabsorbents are used for the production of diapers, tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in agricultural horticulture. The superabsorbents 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, pages 71 to 103.

To improve the application properties, such as, for example, gel bed permeability (GBP) and absorption under a pressure of 49.2 g / cm ² (AUL0.7psi), superabsorbent particles are generally post-crosslinked. 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 the aqueous gel phase. However, dried, ground and sieved polymer particles (base polymer) are preferably coated on the surface with a surface postcrosslinker and thermally crosslinked after the surface. Crosslinkers suitable for this purpose are compounds which can form covalent bonds with at least two carboxylate groups of the polymer particles.

WO 2008/087114 A1, WO 2010/139680 A2 and EP 2 700 667 A1 describe the loading of the conveyor belts of circulating air belt dryers with aqueous polymer gel by means of oscillating conveyor belts.

The object of the present invention was to provide an improved method for producing superabsorbents, in particular a more stable operation of the oscillating conveyor belt used.

The object was achieved by a process for the preparation of superabsorbers by polymerizing a monomer solution or suspension containing a) at least one ethylenically unsaturated, acid-bearing monomer which is at least partially neutralized,

b) at least one crosslinker and

c) at least one initiator, comprising drying of the aqueous polymer gel obtained in a circulating air belt dryer, grinding, classification, and optionally thermal surface postcrosslinking, characterized in that the aqueous polymer gel is introduced into the air belt dryer by means of an oscillating conveyor belt, the conveyor belt via a drive drum and To be guided around the steering drum, at least one of the two drums has openings in the tread and the opening ratio is at least 5%.

An oscillating conveyor belt is a conveyor belt that can be periodically pivoted on a vertical axis. The conveyor belt of the oscillating conveyor belt itself runs essentially at a constant speed. The conveyor belt of the oscillating conveyor belt has a significantly smaller width than the conveyor belt of the circulating air belt dryer. By periodic

Swiveling movement of the oscillating conveyor belt, the conveyor belt of the circulating air dryer is evenly coated with polymer gel over the entire width.

The running surface of the drum is the drum surface minus the end faces.

In a preferred embodiment of the present invention, the drive current mel has openings in the tread.

In a very preferred embodiment of the present invention, the drive and the deflection drum have openings in the tread.

The shape of the openings in the tread is not restricted. Suitable openings are, for example, grids, holes or columns.

In the following, holes in the tread are referred to as holes 1.

The opening ratio is preferably from 10 to 70%, particularly preferably from 15 to 60, very particularly preferably from 20 to 50%. The opening ratio is the quotient of the sum of the area of all openings in the tread and the total tread without taking the openings into account. The area of the openings is assumed to be planar for the calculation. In the case of openings with a variable area, for example conical holes, the smallest area is assumed for the calculation. The drum is assumed to be a cylinder for the calculation, the cylinder diameter being the maximum diameter of the drum. For a cylindrical drum with a diameter d of 200 mm and a length I of 1,000 mm, the total running surface without taking the openings into account is pdl = 0.6283 m ² .

For 640 holes 1 with a diameter d of 25 mm, the sum of the area of all openings in the running surface is 640 TT (d / 2) ² = 0.3142 m ² . This results in an opening ratio of 0.3142 m ² / 0.6283 m ² = 50% for this example.

For 15 bars with a minimum distance b of 20 mm and a length I of 1,000 mm, the total area of all openings in the running surface is 15 bl = 0.3000 m ² . This results in an opening ratio of 0.3000 m ² / 0.6283 m ² = 48% for this example.

Treads with holes 1 can be produced for example by drilling holes 1 with the desired diameter in the tread. The diameter of the holes 1 is preferably from 10 to 50 mm, particularly preferably from 15 to 40 mm, very particularly preferably from 20 to 30 mm. The holes 1 are usually evenly distributed over the tread.

Treads with gaps can be constructed, for example, from running wheels (1) and bars (2). Figures 1 and 2 show rod drums which are constructed from impellers (1) and rods (2). The minimum distance between the rods (2) is preferably from 4 to 30 mm, particularly preferably from 6 to 20 mm, very particularly preferably from 8 to 12 mm. The Stan gene (2) are usually evenly distributed on the wheels (1).

The drums have a diameter of preferably 50 to 500 mm, particularly preferably from 100 to 400 mm, very particularly preferably from 150 to 300 mm. The drums have a length of preferably 500 to 1,400 mm, particularly preferably from 600 to 1,200 mm, very particularly preferably from 700 to 1,000 mm.

The present invention is based on the finding that polymer gel can get under the conveyor belt. This polymer gel then gets onto the running surface of the drive or deflection drum. The polymer gel on the tread leads to belt misalignment, i.e. the revolving conveyor belt moves to the side on the drum. In addition, the polymer gel on the tread of the drive drum causes the drum to slip, i.e. to reduce the drive power. The polymer gel can get inside the drum through the openings in the tread and is thus removed from the tread. Belt misalignment and slipping of the drive current mel are thus prevented.

In a preferred embodiment of the present invention, the at least one drum has further openings on the end face. The shape of the openings on the front side is not subject to any restrictions. Suitable openings are, for example, recesses or holes. In the following, holes in the end face are referred to as holes 2.

A drum with holes 1 in the running surface is suitable, for example, the further openings on the end face being recesses adjacent to the drum surface. The end face of such drums can have the shape of a six-pointed star, for example.

For example, a rod drum consisting of impellers (1) and rods (2) is also suitable, the further openings on the end face being holes 2 in the impellers (1). Such a rod drum is shown in FIGS. 1 and 2. The diameter of the holes 2 is preferably from 10 to 50 mm, particularly preferably from 15 to 35 mm, very particularly preferably from 20 to 40 mm. The holes 2 are usually evenly distributed over the impeller (1).

Through the further openings in the end face, polymer gel located inside the drum can leave the inside of the drum from the side. This can be additionally supported by blowing out, for example using compressed air.

The diameter in the middle of at least one of the drums is preferably 2 to 8%, particularly preferably 2.5 to 6%, very particularly preferably 3 to 5%, larger than at the edges of the drum. The diameter is the outer diameter of the drum.

The central region of the at least one drum can have a constant diameter. The middle range preferably comprises from 20 to 80%, particularly preferably from 30 to 70%, very particularly preferably from 40 to 60%, of the total length of the drum.

For a drum with a length of 1,000 mm, the diameter of which increases from the respective end towards the center of the drum over a length of 250 mm and remains constant from there to the center of the drum, the middle range is 50% of the total length of the drum.

Figure 3 shows the longitudinal section of a drum with a larger diameter in the middle (3).

Drums with a larger diameter in the middle increase the service life of the conveyor belts.

The conveyor belt has a length of preferably 2 to 10 m, particularly preferably from 2.5 to 8 m, very particularly preferably from 3 to 6 m, the length of the conveyor belt being the distance of the pivot axis from the discharge end.

The conveyor belt has a width of preferably 0.5 to 1.5 m, particularly preferably 0.6 to 1.2 m, very particularly preferably 0.7 to 0.9 m. The conveyor belt speed is preferably from 0.2 to 2.0 m / s, particularly preferably from 0.3 to 1.5 m / s, very particularly preferably from 0.4 to 1.0 m / s.

The conveyor belts customary for this purpose can be used. The surface of the conveyor belts, i.e. the side coming into contact with the polymer gel should be water-repellent and at 23 ° C should have a contact angle with water of preferably at least 60 °, particularly preferably at least 80 °, very particularly preferably at least 100 °. The contact angle is a measure of the wetting behavior and is measured according to DIN 53900.

The water content of the polymer gel on the conveyor belt is preferably from 20 to 80% by weight, particularly preferably from 30 to 70% by weight, very particularly preferably from 40 to 60% by weight.

The temperature of the polymer gel on the conveyor belt is preferably from 60 to 105 ° C, particularly preferably from 70 to 100 ° C, very particularly preferably from 80 to 95 ° C.

The positioning of the superabsorbers 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, i.e. the solubility in water at 23 ° C. is typically at least 1 g / 100 g of water, preferably at least 5 g / 100 g of water, particularly preferably at least 25 g / 100 g of water, very particularly preferably at least 35 g / 100 g of water.

Suitable monomers a) are, for example, ethylenically unsaturated carboxylic acids, such as acrylic acid, methacrylic acid, and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Acrylic acid is very particularly preferred.

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

Suitable crosslinkers b) are compounds with 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). Polyvalent metal salts which can form coordinative bonds with at least two acid groups of the monomer a) are also suitable as crosslinkers b). Crosslinkers b) are preferably compounds having at least two polymerizable groups which 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 triacrylate EP 477, EP 477, EP 477, EP 477, and EP 475 477, as well as triacrylate, EP 477, EP 477, EP 477, and EP 475 477, as well as triacrylate 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 ethylenically unsaturated groups, as described in DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinker mixtures, for example in DE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO 02/032962 A2.

The amount of crosslinker b) is preferably 0.05 to 1.5% by weight, particularly preferably 0.1 to 1% by weight, very particularly preferably 0.3 to 0.6% by weight, in each case calculated on the total amount of monomer used a). With increasing crosslinker content, the centrifuge retention capacity (CRC) and the absorption under a pressure of 21.0 g / cm ² decrease

(AUL0.3psi) goes 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, what peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite and hydrogen peroxide / sodium bisulfite. Mixtures of thermal initiators and redox initiators, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid, are preferably used. The reducing component is preferably the disodium salt of 2-hydroxy-2-sulfonatoacetic acid or a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the dinate salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such mixtures are as Brüggolite® FF6 and Brüggolite® FF7 (Brüggemann Chemicals; Fleilbronn;

Germany) available.

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 have monomer suspensions, i.e. To use monomer solutions with the solubility-exceeding monomer a), for example sodium acrylate. With increasing water content, the energy consumption during the subsequent drying increases and with decreasing water content, the heat of polymerization can only be dissipated insufficiently.

The preferred polymerization inhibitors require dissolved oxygen for an optimal effect. Therefore, the monomer solution can be freed from dissolved oxygen by inerting, ie by flowing with an inert gas, preferably nitrogen or carbon dioxide, before the polymerization. The oxygen content of the monomer solution is preferably polymerization to less than 1 ppm by weight, particularly preferably to less than 0.5 ppm by weight, very particularly preferably to less than 0.1 ppm by weight.

Suitable reactors for the polymerization are, for example, kneading reactors or band reactors. In the kneader, the polymer gel resulting from the polymerization of an aqueous monomer solution or suspension is continuously comminuted by, for example, counter-rotating stirring shafts, as described in WO 2001/038402 A1. The polymerization on the 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 has to be comminuted, for example in an extruder or kneader.

To improve the drying properties, the crushed 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, before, 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 for the customary neutralizing agents to be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal hydrogen carbonates as well as 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 hydrogen carbonate 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 into the polymer gel and before its drying. The encapsulation is carried out by coating the surface with an insoluble or only slowly soluble material (for example by means of film-forming polymers, inert inorganic materials or meltable organic materials), which delays the solution and reaction of the solid carbonate or hydrogen carbonate in such a way that carbon dioxide only occurs during drying is released and the resulting superabsorbent has a high internal porosity.

The polymer gel is then usually dried with a forced 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, the residual moisture content determined according to Test Method No. WSP 230.2-05 "Mass Löss Upon Heating" recommended by EDANA. If the residual moisture is too high, the dried polymer gel has a glass transition temperature T _{g which} is too low and is difficult to process further. If the residual moisture is too low, the dried polymer gel is too brittle and undesirable large amounts of polymer particles with too small a particle size (“fine”) are produced in the subsequent comminution steps. The solids content of the polymer gel is preferably before drying from 25 and 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 roughly crushed.

The dried polymer gel is then usually ground and classified, it being possible to use single- or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibrating mills, for grinding.

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

The polymer particles can be post-crosslinked thermally to further improve the properties. Suitable surface postcrosslinkers are compounds which contain groups which 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 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.

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

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

The polyvalent cations which can be used in the process according to the invention are, for example, divalent cations, such as the cations of zinc, magnesium, calcium and strontium, trivalent cations, such as the cations of aluminum, iron, chromium, rare earths and manganese, four valent cations, such as the cations of Titanium and zirconium. As a 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 hydride, 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. each based on the polymer.

The surface postcrosslinking is usually carried out in such a way that a solution of the surface postcrosslinker is sprayed onto the dried polymer particles. Following spraying, the polymer particles coated with surface postcrosslinker are surface postcrosslinked and dried, the surface postcrosslinking reaction being able to take place both before and during drying.

The spraying of a solution of the surface postcrosslinker is preferably carried out in mixers with moving mixing tools, such as screw mixers, disk mixers and shovel 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 bearing of the mixing shaft, i.e. Horizontal mixers have a horizontal mixing shaft and vertical mixers have a vertical mixing shaft. Suitable mixers are, for example, Horizontal Ploughshare® Mixers (Gebr. Lödige Maschi nenbau GmbH; Paderborn; Germany), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Doetinchem; Netherlands), Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA) and Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; Netherlands). However, it is also possible to spray the surface postcrosslinker solution in a fluidized bed.

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

The surface postcrosslinking is preferably carried out in contact dryers, particularly preferably paddle dryers, very particularly preferably disc 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). Fluid bed dryers can also be used.

The surface post-crosslinking can take place in the mixer itself, by heating the jacket or by blowing in warm air. A downstream dryer, such as a tray dryer, a rotary kiln or a heated screw, is also suitable. Mixing in a fluidized bed dryer and thermal surface post-crosslinking are particularly advantageous.

Preferred reaction temperatures are in the range 100 to 250 ° C, preferably 1 10 to 220 ° C, particularly preferably 120 to 210 ° C, very particularly preferably 130 to 200 ° C. The preferred Residence time at this temperature is preferably at least 10 minutes, particularly preferably at least 20 minutes, very particularly preferably at least 30 minutes, and usually not more than 60 minutes.

The surface post-crosslinked 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 re-moistened to further improve the properties.

The rewetting is preferably carried out at 30 to 80 ° C., particularly preferably at 35 to 70 ° C., very particularly preferably at 40 to 60 ° C. If the temperature is too low, the polymer particles tend to clump together and at higher temperatures, water will evaporate noticeably. The amount of water used for rewetting 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 rewetting increases the mechanical stability of the polymer particles and reduces their tendency towards static charging. The post-moistening in the cooler is advantageously carried out after the thermal surface postcrosslinking.

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

Another object of the present invention are hygiene articles which contain superabsorbents produced according to the method according to the invention.

Methods:

The standard test methods described below, designated "WSP", are described in: "Standard Test Methods for the Nonwovens Industry", Edition 2005, jointly published by the "Worldwide Strategy Partners" EDANA (Herrmann-Debrouxlaan 46, 1 160 Oudergem, Belgium, www.edana.org) and INDA (1 100 Crescent Green, Suite 1 15, Cary, North Carolina 27518, USA, www.inda.org). This publication is available from both EDANA and INDA. Unless otherwise stated, the 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 are mixed well before the measurement.

Centrifuge Retention Capacity

The centrifuge retention capacity (CRC) is determined in accordance with the test method recommended by EDANA No. WSP 241 .2 (05) "Fluid Retention Capacity in Saline, After Centrifugation".

Extractable

The extractable constituents of the water-absorbing polymer particles are determined in accordance with test method No. WSP 270.2 (05) "Extractable" recommended by EDANA.

Examples Example 1

An acrylic acid / sodium acrylate solution was prepared by continuously mixing deionized water, 50% by weight sodium hydroxide solution and acrylic acid, so that the degree of neutralization corresponded to 71.3 mol%. The solids content of the monomer solution was 38.8% by weight.

Polyethylene glycol 400 diacrylate (diacylate starting from a polyethylene glycol with an average molecular weight of 400 g / mol) was used as the polyethylenically unsaturated crosslinker. The amount used was 2 kg of crosslinking agent per ton of monomer solution.

To initiate the radical polymerization, 1.03 kg of a 0.25% by weight aqueous hydrogen peroxide solution, 3.10 kg of a 15% by weight aqueous sodium peroxodisulfate solution and 1.05 kg of a 1% by weight aqueous ascorbic acid solution were used per ton of monomer solution used.

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 metered in the following quantities into a List Contikneter reactor with a volume of 6.3 m ³ (LIST AG, Arisdorf, Switzerland):

20 t / h monomer solution

40 kg / h polyethylene glycol 400 diacrylate 82.6 kg / h hydrogen peroxide solution / sodium peroxodisulfate solution

21 kg / h ascorbic acid solution

The monomer solution was inertized with nitrogen between the addition point for the crosslinking agent and the addition points for the initiators.

After about 50% of the residence time, there was also an addition of fine particles (1000 kg / h) from the production process by grinding and sieving into the reactor.

The residence time of the reaction mixture in the reactor was 15 minutes.

The aqueous polymer gel obtained was applied to the conveyor belt of a circulating air belt dryer by means of an oscillating conveyor belt.

The circulating air belt dryer had a length of 48 m. The conveyor belt of the circulating air belt dryer had an effective width of 4.4 m.

The oscillating conveyor belt was 5 m long. The conveyor belt had a width of 0.8 m and an effective width of 0.5 m. The angle of repose of the aqueous polymer gel on the conveyor belt was approximately 15 °. The cross section of the polymer gel bed on the conveyor belt was approximately 0.04 m ² . The speed of the conveyor belt was 0.5 m / s.

The oscillating conveyor belt was started from one end position over a first

Swivel angle ßi accelerated from 13 ° to an angular speed of 33 ° / s, decelerated by a second swivel angle ß2 from 20 ° to an angular speed of 17 ° / s and decelerated to the other end position by a third swivel angle ß3. The total swivel angle was 50 °. A double stroke (from the first end position to the other end position and back again) took approx. 7 s. The rotating conveyor belt had a surface made of polytetrafluoroethylene (PTFE).

The temperature of the aqueous polymer gel on the oscillating conveyor belt was 90 ° C.

Bar drums were used as the drive and deflection drum of the conveyor belt. The drums had a total length of 850 mm. The middle area of the drums had a diameter of 218.0 mm and a length of 426 mm. At the edges, the diameter of the drum decreased by 7.4 mm to 210.4 mm. The diameter of the drum was 3.5% larger in the middle of the drum than at the edges of the drum. The angle a was approx. 1 °. The rod drum had 15 rods (2) with a width of approximately 20 mm, a height of 25 mm and a minimum rod spacing of approximately 10 mm. The opening ratio was 22%. The impellers (1) each had six holes 2 with a diameter of 40 mm.

Polymer gel inside the drums was regularly blown out with compressed air. The rotating conveyor belt could be operated without any problems.

The aqueous polymer gel was continuously washed with a

Air / gas mixture flows around and dried. The residence time in the circulating air belt dryer was 37 minutes.

The dried polymer gel was ground and sieved to a particle size fraction of 150 to 850 pm.

The water-absorbing polymer particles obtained had a centrifuge retention capacity (CRC) of 34.9 g / g and an extractable content of 8.5% by weight.

Example 2 (comparative example)

The procedure was as in Example 1, but drums without openings in the tread were used as the drive and deflection drum (opening ratio of 0%). During the operation of the rotating conveyor belt, polymer gel particles often came between the conveyor belt and the drive or deflection drum. This resulted in belt misalignment and the drive drum slipping.

Claims

1. A process for the preparation of superabsorbent particles by polymerizing a monomer solution or suspension, comprising a) at least one ethylenically unsaturated, acid-bearing monomer which is at least partially neutralized,

b) at least one crosslinker and

c) at least one initiator, comprising drying of the aqueous polymer gel obtained in a circulating air dryer, grinding, classification, and optionally thermal surface postcrosslinking, characterized in that the aqueous polymer gel is introduced into the circulating air belt dryer by means of an oscillating conveyor belt, the conveyor belt via a Drive drum and a deflection drum is guided, at least one of the two drums has openings in the tread and the opening ratio is at least 5%.

2. The method according to claim 1, characterized in that the drive current mel has openings in the tread.

3. The method according to claim 1 or 2, characterized in that the drive current mel and the deflection drum have openings in the tread.

4. The method according to any one of claims 1 to 3, characterized in that the opening ratio is from 20 to 50%.

5. The method according to any one of claims 1 to 4, characterized in that the running surface has holes 1.

6. The method according to claim 5, characterized in that the holes 1 have a diameter of 20 to 30 mm.

7. The method according to any one of claims 1 to 4, characterized in that the

Drum is a rod drum consisting of impellers (1) and rods (2).

8. The method according to claim 7, characterized in that the minimum distance between the rods (2) is from 8 to 12 mm.

9. The method according to any one of claims 1 to 8, characterized in that the

Drums have a diameter of 150 to 300 mm.

10. The method according to any one of claims 1 to 9, characterized in that the drums have a length of 700 to 1,000 mm.

1 1. The method according to any one of claims 1 to 10, characterized in that the min least one drum has further openings on the end face.

12. The method according to claim 11, characterized in that the at least one drum is a drum with holes 1 in the tread and the further openings on the end face are recesses adjacent to the tread.

13. The method according to claim 11, characterized in that the at least one drum is a rod drum consisting of impellers (1) and rods (2) and the further openings on the end face are holes 2 in the impellers (1).

14. The method according to any one of claims 11 to 13, characterized in that polymer gel located in the interior of the drum is blown out.

15. Hygiene article containing superabsorbents produced according to a method of claims 1 to 14.