METHOD FOR THE PRODUCTION OF SUPERABSORBENT PARTICLES

Description

[0001] The present invention relates to a process for producing superabsorbent particles by polymerizing a monomer solution or suspension, comprising drying the resulting aqueous polymer gel in a circulating air belt dryer, grinding, classification, and optionally thermal surface post-crosslinking, wherein the aqueous polymer gel is introduced into the circulating air belt dryer by means of an oscillating conveyor belt and that guide devices are located at the edges of the conveyor belt.

[0002] Superabsorbents are used in the manufacture of diapers, tampons, sanitary pads, and other hygiene products, as well as as water-retaining agents in agricultural horticulture. Superabsorbents 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 gel bed permeability (GBP) and absorption under a pressure of 49.2 g / cm² (AUL <0.7 psi), superabsorbent particles are generally surface-crosslinked. This increases the degree of crosslinking of the particle surface, allowing absorption under a pressure of 49.2 g / cm² (AUL <0.7 psi) and centrifuge retention capacity (CRC) to be at least partially decoupled. This surface-crosslinking can be carried out in the aqueous gel phase. Preferably, however, dried, ground, 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] 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.

[0006] The object of the present invention was to provide an improved process for the production of superabsorbents, in particular a more stable operation of the oscillating conveyor belt used.

[0007] The problem was solved by a process for producing superabsorbents by polymerizing a monomer solution or suspension containing a) at least one ethylenically unsaturated, acid group-bearing monomer which is at least partially neutralized, b) at least one crosslinker and c) at least one initiator, comprising drying the resulting aqueous polymer gel in a circulating air belt dryer, grinding, classification, and optionally thermal surface post-crosslinking, characterized in that the aqueous polymer gel is introduced into the circulating air belt dryer by means of an oscillating conveyor belt and that guide devices (1) are located at the edges of the conveyor belt (2).

[0008] An oscillating conveyor belt is a conveyor belt that periodically pivots on a vertical axis. The conveyor belt itself essentially runs at a constant speed. The conveyor belt of the oscillating conveyor belt is significantly narrower than the conveyor belt of a circulating air belt dryer. Due to the periodic pivoting movement of the oscillating conveyor belt, the conveyor belt of the circulating air belt dryer is evenly coated with polymer gel across its entire width.

[0009] Figure 1shows an embodiment of the present invention with guide devices (1), conveyor belt (2) and drive or deflection drum (3).

[0010] In a preferred embodiment of the present invention, the guide devices (1) are located at the discharge end of the conveyor belt, wherein the length of the guide devices (1) is preferably from 5 to 70%, particularly preferably from 10 to 50%, very particularly preferably from 15 to 30% of the length of the conveyor belt, wherein the length of the conveyor belt is the distance of the pivot axis from the discharge end.

[0011] The height of the guide devices (1) is preferably from 5 to 30 cm, particularly preferably from 8 to 25 cm, most preferably from 10 to 20 cm.

[0012] The length and height of the guide devices (1) are, for example, 5 to 70% and 5 to 30 cm, 5 to 70% and 8 to 25 cm, 5 to 70% and 10 to 20 cm, 10 to 50% and 5 to 30 cm, 10 to 50% and 8 to 25 cm, 10 to 50% and 10 to 20 cm, 15 to 30% and 5 to 30 cm, 15 to 30% and 8 to 25 cm or 15 to 30% and 10 to 20 cm.

[0013] The guide devices (1) can be rotated by the angle perpendicular to the running direction of the conveyor belt to the center of the conveyor belt in the running direction, wherein the guide devices (1) are preferably rotated by 3 to 15°, particularly preferably by 6 to 12°, most particularly preferably by 8 to 10°.

[0014] The part of the guide devices (1) in contact with the conveyor belt should not be too hard. Suitable materials are natural or synthetic rubbers, such as silicone rubber, polyetheretherketone (PEEK), halogenated polyolefins such as polyvinyl chloride (PVC) or polytetrafluoroethylene (PTFE), polyamide (PA), polypropylene (PP), or polyethylene (PE). Silicone rubber is preferred.

[0015] Preferably, the guide devices (1) follow the curvature of the conveyor belt. The preferred guide devices are positioned as close as possible to the conveyor belt. Too much distance results in polymer gel escaping from between the guide device (1) and the conveyor belt. Too little distance results in friction, i.e., undesirable mechanical stress, between the guide device (1) and the conveyor belt.

[0016] If the material of the guide devices (1) in contact with the conveyor belt is sufficiently flexible, the guide devices (1) can also rest directly on the conveyor belt. It is even possible for the flexible material of the guide devices (1) resting on the conveyor belt, for example, silicone rubber, to be bent inward, ie, toward the transported polymer gel.

[0017] The guide devices (1) preferably extend beyond the discharge end of the conveyor belt. The preferred guide devices (1) follow the downward curve of the deflection pulley (3). The areas of the guide devices (1) that extend beyond the discharge end of the conveyor belt are preferably parallel to the running direction of the conveyor belt.

[0018] The present invention is based on the discovery that polymer gel can get under the conveyor belt. This polymer gel then reaches the drum surfaces of the drive or return pulley. The polymer gel on the drum surfaces leads to belt misalignment, i.e., the circulating conveyor belt moves sideways on the drum. Furthermore, the polymer gel on the surface of the drive drum causes the drum to slip, i.e., to a reduction in drive power. The guide devices (1) prevent polymer gel from getting under the conveyor belt. Belt misalignment and slippage of the drive drum are thus prevented.

[0019] In a preferred embodiment of the present invention, the underside of the returning conveyor belt is additionally cleaned of adhering polymer gel by means of at least one scraping device.

[0020] The underside of the returning conveyor belt is the outer side of the conveyor belt, onto which polymer gel is again dispensed after redirection. The top side of the returning conveyor belt is the inner side of the conveyor belt, which should not come into contact with polymer gel.

[0021] The distance of the stripping device from the discharge end of the conveyor belt is preferably less than 20% of the length of the conveyor belt, particularly preferably less than 10% of the length of the conveyor belt, most preferably less than 5% of the length of the conveyor belt, wherein the length of the conveyor belt is the distance of the pivot axis from the discharge end.

[0022] The scraping device is not subject to any restrictions. Brushes arranged transversely to the direction of travel are suitable, for example. It is also possible to use a scraper. A scraper is a scraping device made of a non-flexible material and arranged transversely to the conveying direction. A suitable non-flexible material is, for example, polytetrafluoroethylene. To avoid damage to the conveyor belt, the scraper should have as little or no direct contact with the conveyor belt as possible. The scraper should be inclined relative to the direction of travel of the returning conveyor belt. This promotes the peeling off of the adhering polymer gel and prevents a jam between the conveyor belt and the scraper. The scraped-off polymer gel usually falls onto the conveyor belt of the circulating air belt dryer.

[0023] The scraper is preferably inclined at an angle of 5 to 45°, particularly preferably 10 to 35°, and most preferably 15 to 25°, relative to the vertical, against the running direction of the conveyor belt. The distance of the scraper from the underside of the returning conveyor belt is preferably 0.1 to 5 mm, particularly preferably 0.2 to 2 mm, and most preferably 0.5 to 1.5 mm.

[0024] The conveyor belt has a length of preferably 2 to 10 m, particularly preferably 2.5 to 8 m, most preferably 3 to 6 m, wherein the length of the conveyor belt is the distance of the pivot axis from the discharge end.

[0025] The conveyor belt has a width of preferably 0.5 to 1.5 m, particularly preferably 0.6 to 1.2 m, most preferably 0.7 to 0.9 m.

[0026] The conveyor belt speed is preferably from 0.2 to 2.0 m / s, particularly preferably from 0.3 to 1.5 m / s, most preferably from 0.4 to 1.0 m / s.

[0027] Conveyor belts commonly used for this purpose can be used. The surface of the conveyor belts, i.e., the side that comes into contact with the polymer gel, should be water-repellent and, at 23°C, have a contact angle with water of preferably at least 60°, more preferably at least 80°, and most preferably at least 100°. The contact angle is a measure of wetting behavior and is measured according to DIN 53900.

[0028] The water content of the polymer gel on the conveyor belt is preferably from 20 to 80 wt%, more preferably from 30 to 70 wt%, most preferably from 40 to 60 wt%.

[0029] The temperature of the polymer gel on the conveyor belt is preferably from 60 to 105°C, more preferably from 70 to 100°C, most preferably from 80 to 95°C.

[0030] The production of superabsorbents is explained in more detail below: Superabsorbents are produced by polymerizing a monomer solution or suspension and are usually water-insoluble.

[0031] 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, most particularly preferably at least 35 g / 100 g water.

[0032] Suitable monomers a) include, 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 most preferred.

[0033] The monomers a) usually contain polymerization inhibitors, preferably hydroquinone hemiether, as storage stabilizers.

[0034] Suitable crosslinkers b) are compounds with at least two groups suitable for crosslinking. Such groups include, for example, ethylenically unsaturated groups that can be radically polymerized into the polymer chain and functional groups that can form covalent bonds with the acid groups of monomer a). Furthermore, polyvalent metal salts that can form coordinate bonds with at least two acid groups of monomer a) are also suitable as crosslinkers b).

[0035] Crosslinkers b) are preferably compounds with at least two polymerizable groups that can be radically polymerized into the polymer network. 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 0 530 438 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 ethylenically unsaturated groups, as in DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinker 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.

[0036] 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.%, each calculated based 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 / cm 2 < (AUL 0.3 psi) reaches a maximum.

[0037] As initiators c), any compounds that generate 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. Mixtures of thermal initiators and redox initiators, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid, are preferably used. 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 Brüggolite ®< FF6 and Brüggolite ®< FF7 (Brüggemann Chemicals; Heilbronn; Germany).

[0038] Typically, an aqueous monomer solution is used. The water content of the monomer solution is preferably from 40 to 75 wt.%, particularly preferably from 45 to 70 wt.%, most preferably from 50 to 65 wt.%. It is also possible to use monomer suspensions, i.e., monomer solutions with monomer a) that exceeds the solubility, for example, sodium acrylate. As the water content increases, the energy required for subsequent drying increases, and as the water content decreases, the heat of polymerization can only be dissipated insufficiently.

[0039] The preferred polymerization inhibitors require dissolved oxygen for optimal effectiveness. Therefore, the monomer solution can be freed of dissolved oxygen before polymerization by inerting, i.e., by flowing an inert gas, preferably nitrogen or carbon dioxide. The oxygen content of the monomer solution before polymerization 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.

[0040] Suitable reactors for polymerization include kneader reactors or belt reactors. In the kneader, the polymer gel formed during the polymerization of an aqueous monomer solution or suspension is continuously comminuted by, for example, counter-rotating agitator shafts, as described in WO 2001 / 038402 A1. Belt polymerization is described, for example, in DE 38 25 366 A1 and US Pat. No. 6,241,928. Polymerization in a belt reactor produces a polymer gel that must be comminuted, for example, in an extruder or kneader.

[0041] To improve the drying properties, the crushed polymer gel obtained by means of a kneader can be additionally extruded.

[0042] The acid groups of the resulting polymer gels are usually partially neutralized. 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, as a solid. The degree of neutralization is preferably from 40 to 85 mol%, more preferably from 50 to 80 mol%, most preferably from 60 to 75 mol%, and the usual neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates, or alkali metal bicarbonates, and mixtures thereof. Ammonium salts can also be used instead of alkali metal salts. 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 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 slowly soluble material (e.g., film-forming polymers, inert inorganic materials, or meltable 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.

[0043] The polymer gel is then typically dried using a circulating air belt dryer until the residual moisture content is preferably between 0.5 and 10 wt. %, particularly preferably between 1 and 7 wt. %, and very particularly preferably between 2 and 5 wt. %, whereby the residual moisture content is 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 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 undesirably large amounts of polymer particles with excessively small particle sizes ("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. %, very particularly preferably between 40 and 60 wt. %.The dried polymer gel is then broken and optionally coarsely crushed.

[0044] 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 be used for grinding.

[0045] The average particle size of the polymer particles separated as the product fraction is preferably from 150 to 850 µm, more preferably from 250 to 600 µm, and most preferably from 300 to 500 µm. The average particle size of the product fraction can be determined using the EDANA-recommended test method No. WSP 220.2 (05) "Particle Size Distribution," in which the mass fractions of the sieve fractions are plotted cumulatively and the average particle size is determined graphically. The average particle size is the mesh size value resulting for a cumulative 50 wt.%.

[0046] The polymer particles can be thermally surface-crosslinked to further improve their properties. Suitable surface-crosslinkers are compounds containing groups that can form covalent bonds with at least two carboxylate groups of the 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 Pat. No. 6,239,230.

[0047] The amount of surface postcrosslinker is preferably 0.001 to 2 wt.%, particularly preferably 0.02 to 1 wt.%, very particularly preferably 0.05 to 0.2 wt.%, in each case based on the polymer particles.

[0048] In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the surface postcrosslinkers.

[0049] The polyvalent cations usable in the process according to the invention include, 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; and tetravalent cations, such as the cations of 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.

[0050] 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.

[0051] Surface postcrosslinking is typically performed by spraying a solution of the surface postcrosslinker onto the dried polymer particles. Following spraying, the polymer particles coated with the surface postcrosslinker are surface postcrosslinked and dried. The surface postcrosslinking reaction can occur either before or during drying.

[0052] The spraying of a solution of the surface post-crosslinker 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 most preferred. The distinction between horizontal mixers and vertical mixers is made by the bearing 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 include, for example, Horizontale Pflugschar ®< mixers (Gebr. Lödige Maschinenbau 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 post-crosslinker solution in a fluidized bed.

[0053] Surface postcrosslinkers are typically used as an aqueous solution. The penetration depth of the surface postcrosslinker into the polymer particles can be adjusted by varying the non-aqueous solvent content or the total solvent quantity.

[0054] Surface post-crosslinking is preferably carried out in contact dryers, particularly preferably paddle dryers, and most preferably disc dryers. Suitable dryers include, for example, Hosokawa Bepex® Horizontal Paddle Dryers (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Bepex® Disc Dryers (Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; USA), and Nara Paddle Dryers (NARA Machinery Europe; Frechen; Germany). Fluidized-bed dryers can also be used.

[0055] Surface post-crosslinking can occur 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. Mixing and thermal surface post-crosslinking are particularly advantageous in a fluidized-bed dryer.

[0056] 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, particularly preferably at least 20 minutes, very particularly preferably at least 30 minutes, and usually at most 60 minutes.

[0057] The surface-crosslinked polymer particles can then be classified again, with polymer particles that are too small and / or too large being separated and returned to the process.

[0058] The surface-crosslinked polymer particles can be coated or re-moistened to further improve their properties.

[0059] Re-moistening 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 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 re-moistening is preferably from 1 to 10 wt.%, particularly preferably from 2 to 8 wt.%, and most preferably from 3 to 5 wt.%. Re-moistening increases the mechanical stability of the polymer particles and reduces their tendency to static charge. . Advantageously, the re-humidification in the cooler is carried out after the thermal surface post-crosslinking.

[0060] Suitable coatings for improving the swelling rate and gel bed permeability (GBP) include inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers, and divalent or multivalent metal cations. Suitable coatings for dust control include polyols. Suitable coatings for counteracting the undesirable caking tendency of polymer particles include fumed silica, such as Aerosil®<200, precipitated silica, such as Sipernat®<D17, and surfactants, such as Span®<20. Methods:

[0061] 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.

[0062] Unless otherwise specified, measurements should be performed at an ambient temperature of 23 ± 2 °C and a relative humidity of 50 ± 10%. The water-absorbing polymer particles are thoroughly mixed before measurement. Centrifuge Retention Capacity

[0063] The centrifuge retention capacity (CRC) is determined according to EDANA recommended test method No. WSP 241.2 (05) "Fluid Retention Capacity in Saline, After Centrifugation". Extractable

[0064] The extractable content of the water-absorbing polymer particles is determined according to EDANA recommended test method No. WSP 270.2 (05) "Extractable". Example

[0065] By continuously mixing deionized water, 50 wt% sodium hydroxide solution, and acrylic acid, an acrylic acid / sodium acrylate solution was prepared, resulting in a degree of neutralization of 71.3 mol%. The solids content of the monomer solution was 38.8 wt%.

[0066] Polyethylene glycol 400 diacrylate (a diacrylate derived 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 crosslinker per t of monomer solution.

[0067] To initiate the radical polymerization, 1.03 kg of a 0.25 wt.% aqueous hydrogen peroxide solution, 3.10 kg of a 15 wt.% aqueous sodium peroxodisulfate solution and 1.05 kg of a 1 wt.% aqueous ascorbic acid solution were used per t of monomer solution.

[0068] The throughput of the monomer solution was 20 t / h. The reaction solution had a temperature of 23.5°C at the inlet.

[0069] The individual components were continuously dosed into a List Contikneter reactor with a volume of 6.3m 3 (LIST AG, Arisdorf, Switzerland) in the following quantities: 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

[0070] The monomer solution was inerted with nitrogen between the addition point for the crosslinker and the addition points for the initiators.

[0071] After approximately 50% of the residence time, an additional dose of fine grain (1000 kg / h) resulting from grinding and sieving during the production process was added to the reactor. The residence time of the reaction mixture in the reactor was 15 minutes.

[0072] The resulting aqueous polymer gel was applied to the conveyor belt of a circulating air belt dryer using an oscillating conveyor belt.

[0073] 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.

[0074] 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 slope angle 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 m². The conveyor belt speed was 0.5 m / s.

[0075] The oscillating conveyor belt was accelerated from one end position through a first swivel angle ß1 of 13° to an angular velocity of 33° / s, decelerated through a second swivel angle ß2 of 20° to an angular velocity of 17° / s, and decelerated through a third swivel angle ß3 to the other end position. The total swivel angle was 50°. A double stroke (from the first end position to the other end position and back again) lasted approximately 7 s. The circulating conveyor belt had a surface made of polytetrafluoroethylene (PTFE).

[0076] The temperature of the aqueous polymer gel on the oscillating conveyor belt was 90°C.

[0077] At the discharge end of the conveyor belt there was a guide device (1) at each edge of the conveyor belt, as shown in Figure 1 shown. The guide devices (1) were each approximately 1,200 mm long and protruded approximately 100 mm beyond the discharge end of the conveyor belt. The part of the guide devices (1) that was in contact with the conveyor belt was made of silicone rubber and had a thickness of approximately 5 mm. The guide devices (1) were each rotated by approximately 8.5° perpendicular to the direction of travel of the conveyor belt towards the center of the conveyor belt in the direction of travel. The part of the guide devices that extended beyond the discharge end of the conveyor belt was again parallel to the direction of travel of the conveyor belt. The guide devices (1) followed the curvature of the deflection pulley (3) up to the vertical and from there were extended straight downwards for approximately 165 mm.

[0078] A scraper device was located on the underside of the oscillating conveyor belt. The scraper device was an elongated scraper mounted transversely to the direction of travel of the returning conveyor belt. The scraper was inclined at 20° against the direction of travel of the returning conveyor belt. The distance of the scraper device from the discharge end was approximately 5 cm, meaning the scraper device was located in the area of ​​the deflection pulley. The distance of the scraper device from the returning conveyor belt was 1 mm. The scraper device scrapes off the aqueous polymer gel adhering to the outside of the returning conveyor belt.

[0079] The recirculating conveyor belt operated without problems. The guide devices (1) prevented polymer gel from falling onto the top of the returning conveyor belt and preventing polymer gel from getting between the drum and the conveyor belt.

[0080] On the circulating air belt dryer, the aqueous polymer gel was continuously circulated with an air / gas mixture and dried. The residence time in the circulating air belt dryer was 37 minutes.

[0081] The dried polymer gel was ground and sieved to a particle size fraction of 150 to 850 µm.

[0082] The obtained water-absorbing polymer particles had a centrifuge retention capacity (CRC) of 34.9 g / g and an extractable content of 8.5 wt%.

Claims

1. A process for producing superabsorbent particles by polymerizing a monomer solution or suspension 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, comprising drying of the resultant aqueous polymer gel in an air circulation belt dryer, grinding, classifying, and optionally thermal surface postcrosslinking, wherein the aqueous polymer gel is introduced into the air circulation belt dryer by means of an oscillating conveyor belt, and wherein guide devices (1) are located at the edges of the conveyor belt (2), where an oscillating conveyor belt is a conveyor belt pivotable periodically at a vertical axis and the guide devices (1) prevent polymer gel from getting under the conveyor belt.

2. The process according to claim 1, wherein the guide devices (1) are located at the discharge end of the conveyor belt and the length of the guide devices (1) is from 15% to 30% of the length of the conveyor belt, the length of the conveyor belt being the distance of the pivot axis from the discharge end, and / or wherein the height of the guide devices (1) is from 10 to 20 cm.

3. The process according to claim 1 or 2, wherein the guide devices (1) are rotated by the angle α at right angles to the running direction of the conveyor belt to the middle of the conveyor belt in running direction.

4. The process according to claim 3, wherein the guide devices (1) are rotated by 8° to 10°.

5. The process according to any of claims 1 to 4, wherein the part of the guide devices (1) that is in contact with the conveyor belt is made of silicone rubber.

6. The process according to any of claims 1 to 5, wherein the guide devices (1) follow the curvature of the conveyor belt.

7. The process according to claim 6, wherein the guide devices (1) protrude beyond the discharge end of the conveyor belt.

8. The process according to any of claims 1 to 7, wherein the underside of the returning conveyor belt is cleaned of adhering polymer gel by means of at least one stripping device.

9. The process according to claim 8, wherein the distance of the stripping device from the discharge end of the conveyor belt is less than 5% of the length of the conveyor belt, the length of the conveyor belt being the distance of the pivot axis from the discharge end.

10. The process according to claim 8 or 9, wherein the stripper device mounted on the underside of the returning conveyor belt is a scraper.

11. The process according to claim 10, wherein the scraper is inclined at 15° to 25° relative to the horizontal counter to the running direction of the conveyor belt.

12. The process according to claim 10 or 11, wherein the distance of the scraper from the underside of the returning conveyor belt is from 0.1 to 5 mm.

13. The process according to any of claims 1 to 12, wherein the conveyor belt has a length of 2 to 10 m, the length of the conveyor belt being the distance of the pivot axis from the discharge end.

14. The process according to any of claims 1 to 13, wherein the surface of the conveyor belt at 23°C has a contact angle with respect to water of at least 60°, where the contact angle is measured in accordance with DIN 53900.