Process for thermal surface-post-crosslinking of superabsorbents

Abstract

The present invention relates to a process for continuous thermal surface-post-crosslinking of superabsorbents, wherein superabsorbent particles are coated by spraying-on a surface-post-crosslinking solution, the coated superabsorbent particles are thermally post treated in a contact dryer (1), and the thermally post-treated superabsorbent particles are cooled in a contact dryer (2), characterised in that, immediately before spraying-on a surface-post-crosslinking solution, the superabsorbent particles have a temperature of 40 to 85°C and a moisture content of 1.5 to 6.0 wt.%, and the surface-post-crosslinking solution contains 1.5 to 4.5 wt.% water, relative to the superabsorbent particles.

Description

Process for thermal surface post-crosslinking of superabsorbents The present invention relates to a process for the continuous thermal surface postcrosslinking of superabsorbents, wherein superabsorbent particles are coated by spraying on a surface postcrosslinker solution, the superabsorbent particles have a temperature of 40 to 85°C and a moisture content of 1.5 to 6.0% by weight immediately before spraying on a surface postcrosslinker solution, the surface postcrosslinker solution contains from 1.5 to 4.5% by weight of water, based on the superabsorbent particles, the coated superabsorbent particles are thermally posttreated in a contact dryer 1 and the thermally posttreated superabsorbent particles are cooled in a contact dryer 2. 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. 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. To improve application properties such as gel bed permeability (GBP) and absorption under a pressure of 49.2 g / cm 2 (AUL0.7 psi), superabsorbent particles are generally surface-crosslinked. This increases the degree of crosslinking of the particle surface, which increases absorption under a pressure of 49.2 g / cm 2(AUL 0.7 psi) and the centrifuge retention capacity (CRC) can be at least partially decoupled. This surface crosslinking can be carried out in the aqueous gel phase. Preferably, however, dried, ground, and sieved superabsorbent particles (base polymer) are coated on the surface with a surface crosslinker and thermally surface crosslinked. Suitable crosslinkers for this purpose are compounds that can form covalent bonds with at least two carboxylate groups of the superabsorbent particles. Surface post-crosslinking is described, for example, in EP 1 130 045 A2, US 2007 / 0106013 A1, US 2019 / 0329219 A1, US 2019 / 0329220 A1 and US 2021 / 0269606 A1. The object of the present invention was to provide an improved process for the thermal surface post-crosslinking of superabsorbent particles, in particular an improved absorption capacity (CRC) and an improved permeability (GBP). The object was achieved by a process for the continuous thermal surface post-crosslinking of superabsorbents, wherein superabsorbent particles are coated by spraying on a surface post-crosslinker solution, the coated superabsorbent particles are thermally post-treated in a contact dryer 1 and the thermally post-treated superabsorbent particles are cooled in a contact dryer 2, characterized in that the superabsorbent particles have a temperature of 40 to 85°C and a moisture content of 1.5 to 6.0% by weight immediately before spraying on a surface post-crosslinker solution and the surface post-crosslinker solution contains from 1.5 to 4.5% by weight of water. The temperature of the superabsorbent particles when spraying the surface postcrosslinker solution is 40 to 85°C, preferably from 45 to 82°C, particularly preferably from 50 to 78°C, most particularly preferably from 55 to 75°C. The moisture content of the superabsorbent particles when spraying the surface postcrosslinker solution is from 1.5 to 6.0 wt.%, preferably from 2.0 to 5.5 wt.%, particularly preferably from 2.5 to 5.0 wt.%, and most preferably from 3.0 to 4.5 wt.%. The moisture content of the water-absorbing superabsorbent particles is determined according to EDANA-recommended test method No. WSP 230.2 (05) "Moisture Content - Weight Loss Upon Heating." The surface postcrosslinker solution contains 1.5 to 4.5 wt.%, preferably 1.6 to 4.2 wt.%, particularly preferably 1.7 to 3.9 wt.%, very particularly preferably 1.8 to 3.6 wt.%, especially preferably 1.9 to 3.3 wt.%, of water, in each case based on the superabsorbent particles. The surface postcrosslinker solution according to the invention can be sprayed on all at once, i.e. as a mixture containing all components. However, it is also conceivable that the surface postcrosslinker solution or the components of the surface postcrosslinker solution are divided into at least two partial solutions (a first surface postcrosslinker solution and a second surface postcrosslinker solution), which are sprayed on one after the other. The temperature of the superabsorbent particles according to the invention when spraying on the surface postcrosslinker solution refers to the temperature of the superabsorbent particles when spraying on the first surface postcrosslinker solution. The moisture content of the superabsorbent particles according to the invention when spraying on the surface postcrosslinker solution refers to the moisture content of the superabsorbent particles when spraying on the first surface postcrosslinker solution. The water content of the Surface post-crosslinker solution refers to the water content of the surface post-crosslinker solution as a whole, i.e. the sum of all partial solutions (water content of the first surface post-crosslinker solution and the second surface post-crosslinker solution). Contact dryers suitable for the continuous process according to the invention include paddle dryers and disc dryers. In contact dryers, the materials to be dried are guided along a heated surface by means of a dynamic tool and are then layered. Contact dryers can also be used for cooling. The present invention is based on the finding that both the temperature and humidity of the superabsorbent particles, as well as the amount of water used, have a decisive influence on surface post-crosslinking. The process according to the invention enables the production of superabsorbents with high absorption capacity, for example, high centrifuge retention capacity (CRC), and simultaneously with high permeability, for example, high gel bed permeability (GBP). The average droplet diameter when spraying the surface postcrosslinker solution can be from 100 to 4,500 pm, preferably from 150 to 3,500 pm, more preferably from 200 to 2,500 pm, most preferably from 250 to 1,500 pm, especially preferably from 300 to 1,250 pm, and especially preferably from 350 to 1,000 pm. The average droplet diameter can be determined by light scattering. The average droplet diameter refers to the mean volumetric diameter (MVD), also referred to as Dvo.s or mean mass diameter (MMD). In a spray nozzle, droplet size and flow rate are directly related, meaning that as the flow rate increases, the droplet size decreases. The droplet size also decreases when the nozzle pressure is increased or the spray angle is increased. There are no further restrictions on spraying the surface post-crosslinker solution. Hydraulic nozzles and two-component nozzles, for example, can be used. The surface postcrosslinker solution preferably contains from 0.001 to 2 wt.%, particularly preferably from 0.01 to 1 wt.%, and most preferably from 0.03 to 0.7 wt.%, of a surface postcrosslinker, based in each case on the superabsorbent particles. The surface postcrosslinker preferably forms covalent bonds on the surface of the superabsorbent particles. The superabsorbent particles are heated in the contact dryer 1 to a temperature of preferably 110 to 220°C, more preferably 120 to 210°C, most preferably 130 to 200°C. The residence time of the superabsorbent particles in the contact dryer 1 is preferably 10 to 60 minutes, more preferably 15 to 55 minutes, most preferably 20 to 50 minutes. The contact dryer 1 and the connection to the contact dryer 2 can be trace heated. The superabsorbent particles are cooled in the contact dryer 2 to a temperature of preferably 30 to 80°C, more preferably 35 to 70°C, most preferably 40 to 60°C. The residence time of the superabsorbent particles in the contact dryer 2 is preferably from 5 to 50 minutes, more preferably from 10 to 40 minutes, most preferably from 15 to 30 minutes. A gas stream with an oxygen content of less than 10 vol.% can be passed through the contact dryer 1. The total gas flow rate is, for example, 5 to 60 Nm 3 / h, preferably from 10 to 50 Nm 3 / h, particularly preferably from 15 to 40 Nm 3 / h, most preferably from 20 to 30 Nm 3 / h, each per m 3 Internal volume of the contact dryer 1. One Nm 3 corresponds to a gas volume of 1 m 3at 273.15 K and 1,013.25 hPa. Preferably, an exhaust gas stream is discharged from the contact dryer 1. The exhaust gas stream is preferably deflected upwards in the contact dryer 1 by at least 75° from the horizontal product flow direction. The gas velocity of the exhaust gas stream directly after the deflection is preferably less than 5 m / s, more preferably less than 2 m / s, and most preferably less than 1 m / s. The production of superabsorbents is explained in more detail below: The superabsorbents are produced by polymerizing a monomer solution and are usually water-insoluble. The ethylenically unsaturated, acid group-bearing monomers 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, more preferably at least 25 g / 100 g water, most preferably at least 35 g / 100 g water. Suitable monomers 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. The ethylenically unsaturated, acid-containing monomers are usually partially neutralized. Neutralization is 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 very particular preference is given to sodium hydroxide, sodium carbonate, or sodium bicarbonate, and mixtures thereof, in particular sodium hydroxide. The monomers usually contain polymerization inhibitors, preferably hydroquinone hemiether, as storage stabilizers. Suitable crosslinkers are compounds with at least two groups suitable for crosslinking. Examples of such groups include 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 the monomer. Polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer are also suitable as crosslinkers. Suitable crosslinkers 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. The amount of crosslinker is preferably 0.05 to 1.5 wt.%, particularly preferably 0.1 to 1 wt.%, most preferably 0.15 to 0.6 wt.%, in each case calculated on the total amount of monomer used. With increasing crosslinker content, the centrifuge retention capacity (CRC) and the absorption under a pressure of 21.0 g / cm decrease. 2 (AUL) passes through a maximum. Any compounds that generate radicals under the polymerization conditions can be used as initiators, 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). The water content of the monomer solution is preferably from 40 to 75 wt.%, particularly preferably from 45 to 70 wt.%, and most preferably from 50 to 65 wt.%. 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. The temperature of the monomer solution is preferably from 10 to 90°C, more preferably from 20 to 70°C, most preferably from 30 to 50°C. 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. 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. To improve the drying properties, the crushed polymer gel obtained by means of a kneader can be additionally extruded. The solids content of the polymer gel before drying is preferably between 25 and 90 wt.%, particularly preferably between 35 and 70 wt.%, most preferably between 40 and 60 wt.%. The polymer gel is then typically dried using a circulating air belt dryer until the residual moisture content is preferably 0.5 to 10 wt.%, more preferably 1 to 7 wt.%, most preferably 1.5 to 6 wt.%, with the residual moisture content being determined according to EDANA-recommended test method No. WSP 230.2 (05) "Moisture Content - Weight Loss Upon Heating." The dried polymer gel is then crushed and optionally coarsely ground. 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. The average particle size of the superabsorbent particles separated as the product fraction is preferably from 150 to 850 pm, more preferably from 250 to 600 pm, and most preferably from 300 to 500 pm. The average particle size of the product fraction can be determined using the E-DANA 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.%. The superabsorbent particles are thermally surface-crosslinked to further improve their properties. Suitable surface crosslinkers are compounds containing groups that form covalent bonds with at least two carboxylate groups of the superabsorbent particles. can form bonds. 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 0 450 922 A2, or ß-hydroxyalkylamides, as described in DE 102 04 938 A1 and US 6,239,230. 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 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. 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. Surface crosslinking is performed by spraying a solution of the surface crosslinker onto the dried superabsorbent particles. Following spraying, the superabsorbent particles coated with the surface crosslinker are thermally treated. 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. Surface postcrosslinkers are typically used as an aqueous solution. The penetration depth of the surface postcrosslinker into the superabsorbent particles can be adjusted by varying the non-aqueous solvent content or the total solvent quantity. Thermal surface post-crosslinking is carried out in contact dryers, particularly preferably paddle dryers, and most preferably disc dryers. Suitable dryers include, 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). The surface-crosslinked superabsorbent particles can then be classified again, with superabsorbent particles that are too small and / or too large being separated and returned to the process. The surface-crosslinked superabsorbent particles can be coated or remoistened to further improve their properties. Remoistening 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 superabsorbent particles tend to clump, and at higher temperatures, water evaporates noticeably. The amount of water used for remoistening is preferably from 1 to 10 wt.%, particularly preferably from 2 to 8 wt.%, and most preferably from 3 to 5 wt. Remoistening increases the mechanical stability of the superabsorbent particles and reduces their tendency to static charge. Remoistening is advantageously carried out in the cooler after thermal surface postcrosslinking. 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 binding include polyols. Suitable coatings for counteracting the undesirable caking tendency of superabsorbent particles include fumed silica, such as Aerosil® 200, precipitated silica, such as Sipernat® D17, and surfactants, such as Span® 20. Methods: Unless otherwise specified, measurements should be performed at an ambient temperature of 23 ± 2°C and a relative humidity of 50 ± 10%. The superabsorbent particles are thoroughly mixed before measurement. Moisture content The moisture content of the water-absorbing superabsorbent particles is determined according to EDANA recommended test method No. WSP 230.2 (05) "Moisture Content - Weight Loss Upon Heating". The centrifuge retention capacity (CRC) is determined according to the EDANA recommended test method No. WSP 241.2 (05) "Gravimetric Determination of Fluid Retention Capacity in Saline Solution After Centrifugation". under The absorption under a pressure of 21 .0 g / cm 2 (AUL0.3psi) is determined according to the EDANA recommended test method WSP 242.2 (05) "Gravimetric Determination of Absorption Under Pressure". Extractable The content of extractable polymers (Extractables 16h) is determined according to the test method WSP 270.2 (05) “Determination of Extractable Polymer Content by Potentiometry Titration” recommended by EDANA. The gel bed permeability (GBP) of a swollen gel layer under pressure of 0.3 psi (2070 Pa) is determined as described in US 2005 / 0256757 (paragraphs

[0061] and

[0075] ), as gel-bed permeability of a swollen gel layer of water-absorbing superabsorbent particles The sample weight of superabsorbent particles according to US 2005 / 0256757 was adjusted from 0.9 g to 2.0 g. Examples Production of superabsorbent particles (= SP) By continuously mixing deionized water, 50 wt.% sodium hydroxide solution, and acrylic acid, a monomer solution was prepared so that the degree of neutralization corresponded to 71.0 mol%. The water content of the monomer solution was 60.5 wt.%. Triple-ethoxylated glycerol triacrylate (approx. 85 wt.%) was used as the crosslinker. The amount used was 1.42 kg per t of monomer solution. Polyethylene glycol with an average molecular weight of 4,000 g / mol was also added. The amount used was 1.75 kg per t of monomer solution. To initiate the radical polymerization, 0.91 kg of a 0.25 wt.% aqueous hydrogen peroxide solution, 4.30 kg of a 15 wt.% aqueous sodium peroxodisulfate solution and 0.84 kg of a 1 wt.% aqueous ascorbic acid solution were used per t of monomer solution. The monomer solution was transferred into a List Contikneter reactor with a volume of 6.3m 3 (LIST AG, Arisdorf, Switzerland). The throughput of the monomer solution was approximately 20 t / h. The reaction solution had a temperature of 30°C at the inlet. Between the addition point for the crosslinker and the addition points for the hydrogen peroxide and sodium peroxodisulfate solutions, the monomer solution was inertized with nitrogen. Ascorbic acid was dosed directly into the reactor. After approximately 50% of the residence time, an additional 1,000 kg / h of superabsorbent particles with a particle size of less than 180 pm, which had been generated during the crushing and classification process during the production process, were added to the reactor. The residence time of the reaction mixture in the reactor was approximately 15 minutes. For better drying, the resulting polymer gel was extruded. The resulting extruded polymer gel was fed onto the conveyor belt of a circulating air belt dryer using an oscillating conveyor belt. The circulating air belt dryer was 48 m long and the conveyor belt of the circulating air belt dryer had an effective width of 4.4 m. On the circulating air belt dryer, the aqueous polymer gel was continuously circulated with an air / gas mixture and dried. The temperature of the air / gas mixture and the residence time in the circulating air belt dryer for drying the polymer gel were each adjusted so that the moisture content of the obtained superabsorbent particles was 0.9 wt%, 1.7 wt%, 4.1 wt% and 12.4 wt%, respectively. The dried polymer gel was ground using a three-stage roller mill and sieved to a particle size of 180 to 700 pm. Superabsorbent particles with a particle size of less than 180 pm were separated. Superabsorbent particles with a particle size greater than 700 pm were returned to the grinding process. The superabsorbent particles with a particle size in the range of 180 to 700 pm were thermally surface crosslinked. The properties of the superabsorbent particles used before surface post-crosslinking are summarized in Table 1. Table 1 : Thermal surface post-crosslinking 190 g of superabsorbent particles were placed in a mixing container of type MP-LB (Somakon Verfahrenstechnik UG, Lünen, Germany) and heated to 30°C, 50°C, 70°C, 80°C and 90°C, respectively. Subsequently, a first surface postcrosslinker solution was sprayed on using a two-component nozzle at a rotational speed of 500 llpm. The solution contained 1.2 wt.% isopropanol (IPA), 0.5 wt.% ethylene carbonate (EC), 0.14 wt.% N-(2-hydroxyethyl)-2-oxazolidone:1,3-propanediol (50:50 mixture; HEONON) or 0.05 wt.% ethylene glycol diglycidyl ether (EGDGE), and 0 wt.%, 0.9 wt.%, 2.0 wt.%, or 5.1 wt.% water, each based on the superabsorbent particles. A second surface postcrosslinker solution, an aqueous aluminum sulfate solution, was then sprayed on. The solution contained 0.37 wt.% aluminum sulfate and 1.0 wt.% water, again based on the superabsorbent particles. Mixing continued for another 5 minutes. The conditions for surface post-crosslinking are summarized in Table 2. Table 2: *) Comparison example **) Water = total water from first and second surface post-crosslinker solution EC ethylene carbonate EGDGE ethylene glycol diglycidyl ether HEONON 50:50 mixture N-(2-Hydroxyethyl)-2-oxazolidone:1,3-Propanediol After applying the surface-crosslinked solution, the superabsorbent particles were evenly distributed on a drying tray (40 x 25 cm) and thermally surface-crosslinked at 175°C in a circulating air drying cabinet. After 20, 30, 40, 50, 60, and 70 minutes, a sample was taken, and the agglomerates larger than 850 μm were separated using a sieve. The remaining surface-crosslinked superabsorbent particles were analyzed. The results are summarized in Table 3. Table 3: Table 3 (continued): *) Comparison example The measurement results are plotted graphically in Figures 1 to 3. Figure 1 shows the dependence of CRC and GBP at different temperatures of the superabsorbent particles immediately before thermal surface post-crosslinking. The worst CRC and GBP ratios were found at 30°C and 90°C. The optimum temperature lies between these temperatures. Figure 2 shows the dependence of CRC and GBP at different moisture contents of the superabsorbent particles immediately before thermal surface crosslinking. A moisture optimum between 1.5 wt.% and 6.0 wt.% can be seen. Figure 3 shows the dependence of CRC and GBP at different water quantities in the surface crosslinked solution during thermal surface crosslinking. An optimum can also be seen between a water quantity of 1.5 wt% and 4.5 wt%.

Claims

Patent claims 1. A process for the continuous thermal surface postcrosslinking of superabsorbents, wherein superabsorbent particles are coated by spraying on a surface postcrosslinker solution, the coated superabsorbent particles are thermally posttreated in a contact dryer 1 and the thermally posttreated superabsorbent particles are cooled in a contact dryer 2, characterized in that the superabsorbent particles have a temperature of 40 to 85°C and a moisture content of 1.5 to 6.0% by weight immediately before spraying on a surface postcrosslinker solution and the surface postcrosslinker solution contains from 1.5 to 4.5% by weight of water, based on the superabsorbent particles.

2. Process according to claim 1, characterized in that the superabsorbent particles have a temperature of 55 to 75°C immediately before spraying on a surface post-crosslinker solution.

3. Process according to claim 1 or 2, characterized in that the superabsorbent particles have a moisture content of 3.0 to 4.5 wt.% immediately before spraying on a surface post-crosslinker solution 4. Process according to one of claims 1 to 3, characterized in that the surface post-crosslinker solution contains, based on the superabsorbent particles, from 1.9 to 3.3 wt.% water.

5. Process according to one of claims 1 to 4, characterized in that partially neutralized, crosslinked polyacrylic acid is used as superabsorbent.

6. Process according to one of claims 1 to 5, characterized in that the surface post-crosslinker forms covalent bonds on the surface of the superabsorbent particles.

7. Process according to one of claims 1 to 6, characterized in that the average drop diameter when spraying the surface post-crosslinker solution is from 250 to 1500 pm.

8. The method according to any one of claims 1 to 7, characterized in that the surface postcrosslinker solution contains from 0.001 to 2% by weight of a surface postcrosslinker, based on the superabsorbent particles.

9. Process according to one of claims 1 to 8, characterized in that the superabsorbent particles are heated in the contact dryer 1 to a temperature of 110 to 220°C.

10. Process according to one of claims 1 to 9, characterized in that the residence time of the superabsorbent particles in the contact dryer 1 is from 10 to 60 minutes.

11. Method according to one of claims 1 to 10, characterized in that the contact dryer 1 and the connection to the contact dryer 2 including hopper and dosing device are trace heated.

12. Process according to one of claims 1 to 11, characterized in that the superabsorbent particles are cooled in the contact dryer 2 to a temperature of 30 to 80°C.

13. Process according to one of claims 1 to 12, characterized in that the residence time of the superabsorbent particles in the contact dryer 2 is from 5 to 50 minutes.

14. The method according to any one of claims 1 to 13, characterized in that a gas stream with an oxygen content of less than 10 vol.% is passed through the contact dryer 1.

15. A method according to claim 14, characterized in that the total gas quantity of the gas stream is from 10 to 50 Nm 3 / h per m 3 Internal volume of the contact dryer 1 and / or an exhaust gas stream is led out of the contact dryer 1, the exhaust gas stream in the contact dryer 1 is deflected upwards by at least 75° from the horizontal product flow direction and the gas velocity of the exhaust gas stream directly after the deflection is less than 5 m / s.

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