Method for surface thermal post-crosslinking of superabsorbent

Abstract

The invention relates to a method for continuous thermal surface post-crosslinking of superabsorbents, in which superabsorbent particles are coated by spraying a surface post-crosslinking agent solution, the coated superabsorbent particles are subjected to thermal surface post-crosslinking in a contact dryer (1), the superabsorbent particles leaving the contact dryer (1) are transferred into the contact dryer (2) via a hopper and a metering device, the liquid level in the hopper is adjusted by the metering device and the liquid level in the hopper is between 10% and 100%, and the hot surface post-crosslinked superabsorbent particles are cooled in the contact dryer (2).

Description

[0001] The present invention relates to a method for the continuous thermal surface postcrosslinking of superabsorbents, wherein superabsorbent particles are coated by spraying a surface postcrosslinker solution, the coated superabsorbent particles are thermally surface postcrosslinked in a contact dryer 1, the superabsorbent particles leaving the contact dryer 1 are transferred via a hopper and a metering device into a contact dryer 2, the level in the hopper is adjusted by the metering device and the level in the hopper is between 10% and 100%, and the thermally surface postcrosslinked superabsorbent particles are cooled in the contact dryer 2.

[0002] Superabsorbents are used for the manufacture of diapers, sanitary tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in agriculture and horticulture. Superabsorbents are also known as water-absorbing polymers.

[0003] The preparation 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.

[0004] In order to improve the application properties, such as the liquid flow conductivity (SFC - Saline Flow Conductivity) and the absorption under high load (AUHL - Absorption Under High Load) at 49.2 g / cm2pressure, superabsorbent particles are usually subjected to a surface postcrosslinking treatment. By this, the degree of crosslinking on the surface of the particles is increased, so that the absorption under high load (AUHL) and the centrifuge retention capacity (CRC - Centrifuge Retention Capacity) can be at least partially decoupled. This surface postcrosslinking can be carried out in an aqueous gel phase. Preferably, however, the surface postcrosslinker is applied to the surface of the dried, ground and sieved polymer particles (base polymer) and the surface thermal postcrosslinking is carried out. Suitable crosslinkers are compounds which are capable of forming covalent bonds with at least two carboxyl groups of the polymer particles.

[0005] It is an object of the present invention to provide an improved method for the thermal surface postcrosslinking of superabsorbent particles, in particular with a higher liquid transfer (SFC) of the superabsorbent particles produced.

[0006] This object is achieved by a method for the continuous thermal surface-postcrosslinking of superabsorbents, wherein the superabsorbent particles are coated by spraying a surface-postcrosslinker solution, the coated superabsorbent particles are thermally surface-postcrosslinked in a contact dryer 1 and the thermally surface-postcrosslinked superabsorbent particles are cooled in a contact dryer 2, characterized in that the superabsorbent particles leaving the contact dryer 1 are transferred into the contact dryer 2 via a hopper and a metering device, the level in the hopper is adjusted by the metering device and the level in the hopper is between 10% and 100%.

[0007] Contact dryers suitable for the continuous process according to the application are, for example, paddle dryers and disc dryers. In contact dryers, the material to be dried is transported along the heated surface and turned over by a power tool. Contact dryers can also be used for cooling.

[0008] In order to empty the hopper more evenly, a discharge aid can be used. The discharge aid is not limited. Suitable discharge aids are, for example, vibrators, rappers and pulsators. Vibrators vibrate the hopper walls. Rappers vibrate the hopper walls by targeted impacts. In both cases, the superabsorbent in the hopper is thus loosened and any bridges collapse. Pulsators loosen the superabsorbent in the hopper by short pressure pulses. However, the use of an ultrasonic discharge aid is preferred. In this case, the hopper is placed in almost inaudible vibrations by ultrasound.

[0009] The discharge aids are usually distributed evenly over the lower third of the hopper. In most cases, three discharge aids are sufficient.

[0010] Suitable metering devices are, for example, screw conveyors or cell wheel locks. A cell wheel lock consists of a rotor that rotates in a precisely fitting housing. The rotor has a certain number of rotor blades, thus forming individual rotor cells. Each rotor cell receives the material to be conveyed below the inlet opening, which is dropped at the outlet. This results in a continuous conveying in terms of volume. The conveying capacity is determined by the volume of the rotor cells and the rotational speed of the rotor.

[0011] The level in the hopper is preferably between 20% and 95%, preferably between 30% and 90%, particularly preferably between 40% and 85% and very particularly preferably between 50% and 80%. The level refers to the internal volume of the hopper.

[0012] The internal volume of the hopper is preferably between 0.01 m3 and 0.5 m3, preferably between 0.02 m3 and 0.4 m3, particularly preferably between 0.05 m3 and 0.3 m3 and very particularly preferably between 0.1 m3 and 0.2 m3, in each case per m3 of the internal volume of the contact dryer 1.

[0013] The present application is based on the insight that the liquid level in the hopper between contact dryer 1 and contact dryer 2 is important for the properties of the produced superabsorbent, in particular the high liquid transfer (SFC).

[0014] The average droplet diameter at the spraying of the post-surface crosslinker solution is for example 200 μm to 4,500 μm, preferably 300 μm to 3,500 μm, particularly preferably 400 μm to 2,500 μm, further particularly preferably 500 μm to 1,500 μm. The average droplet diameter can be determined by light scattering.

[0015] The temperature of the superabsorbent particles at the spraying of the post-surface crosslinker solution is preferably 30°C to 80°C, particularly preferably 35°C to 75°C, further particularly preferably 40°C to 70°C.

[0016] The post-surface crosslinker solution contains, based on the superabsorbent particles, preferably 0.001 wt% to 2 wt%, particularly preferably 0.01 wt% to 1 wt%, further particularly preferably 0.03 wt% to 0.7 wt% of the post-surface crosslinker. The post-surface crosslinker solution also preferably contains 0.5 wt% to 5 wt%. Particularly preferably 1.0 wt% to 4 wt%, very particularly preferably 1.5 wt% to 3 wt% of water in each case based on the superabsorbent particles.

[0017] The superabsorbent particles are heated in the contact dryer 1 to a temperature of preferably 110°C to 220°C, particularly preferably 120°C to 210°C, further particularly preferably 130°C to 200°C. The residence time of the superabsorbent particles in the contact dryer 1 is preferably 10 minutes to 60 minutes, particularly preferably 15 minutes to 50 minutes, further particularly preferably 20 minutes to 40 minutes.

[0018] The contact dryer 1 and the connection to the contact dryer 2 can be assisted by heating and / or thermally insulated.

[0019] The superabsorbent particles are cooled in the contact dryer 2 to a temperature of preferably 30°C to 80°C, particularly preferably 35°C to 70°C, further particularly preferably 40°C to 60°C. The residence time of the superabsorbent particles in the contact dryer 2 is preferably 10 minutes to 60 minutes, particularly preferably 15 minutes to 50 minutes, further particularly preferably 20 minutes to 40 minutes.

[0020] In a preferred embodiment of the application, a gas stream having an oxygen content of less than 10% by volume is introduced into the contact dryer 1. The gas flow per m3 of the internal volume of the contact dryer 1 is, for example, from 5 Nm3 / h to 60 Nm3 / h, preferably from 10 Nm3 / h to 50 Nm3 / h, further preferably from 15 Nm3 / h to 40 Nm3 / h, particularly preferably from 20 Nm3 / h to 30 Nm3 / h. 1 Nm3corresponds to a gas volume of 1 m3under the conditions of 273.15 K and 1,013.25 hPa. A waste gas stream is drawn off from the contact dryer 1. The waste gas stream is deflected in the contact dryer 1 by at least 75° from the horizontal product flow direction upwards. Immediately after the deflection, the velocity of the waste gas stream is preferably less than 5 m / s, particularly preferably less than 2 m / s, further particularly preferably less than 1 m / s.

[0021] The preparation of superabsorbents is explained in detail below:

[0022] Superabsorbents are prepared by polymerization of monomer solutions and are generally water-insoluble.

[0023] The acid group-carrying olefinically unsaturated monomers preferably have water solubility, i.e. a solubility in water at 23°C of generally 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, further particularly preferably at least 35 g / 100 g of water.

[0024] Suitable monomers are, for example, olefinically unsaturated carboxylic acids, such as acrylic acid, methacrylic acid and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Further particularly preferred is the use of acrylic acid.

[0025] The acid group-carrying olefinically unsaturated monomers are generally partially neutralized. The neutralization is carried out in the monomer stage. This is generally effected by mixing in a neutralizing agent in the form of an aqueous solution or, preferably, in solid form. The degree of neutralization is preferably from 40 mol% to 85 mol%, particularly preferably from 50 mol% to 80 mol%, further particularly preferably from 60 mol% to 75 mol%, wherein customary neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal hydrogencarbonates and mixtures thereof. Ammonium salts can also be used instead of alkali metal salts. Particularly preferred alkali metals are sodium and potassium, but further particularly preferred is the use of sodium hydroxide, sodium carbonate or sodium hydrogencarbonate and mixtures thereof, in particular sodium hydroxide.

[0026] The monomers generally contain a polymerization inhibitor, preferably hydroquinone semiketal, which acts as a storage stabilizer.

[0027] Suitable crosslinking agents are compounds having at least two groups suitable for crosslinking. Such groups are, for example, olefinically unsaturated groups which are capable of being radically polymerized into the polymer chain, and functional groups which are capable of forming covalent bonds with the acid groups of the monomers. Furthermore, polyvalent metal salts which are capable of forming coordination bonds with at least two acid groups of the monomers are also suitable for use as crosslinking agents.

[0028] Suitable crosslinking agents are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallyl ammonium chloride, tetraallyloxyethane, as described in EP 0 530 438 A1, diacrylates 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 containing, in addition to acrylate groups, olefinically unsaturated groups, as described in DE 103 31 456 A1 and DE 103 55 401 A1, or mixtures of crosslinking agents, for example, as described in DE 195 43 368 A1, DE 196 46 484 A1, WO 90 / 15830 A1 and WO 02 / 032962 A2.

[0029] The amount of crosslinking agent is preferably from 0.05 to 1.5 wt%, particularly preferably from 0.1 to 1 wt%, further particularly preferably from 0.15 to 0.6 wt%, based on the total amount of monomers used. With increasing crosslinking agent content, the centrifuge retention capacity (CRC) decreases and the absorbency under load (AUL) at 21.0 g / cm2passes through a maximum.

[0030] All compounds which generate free radicals under the polymerization conditions can be used as initiators, for example thermal initiators, redox initiators, photoinitiators. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite and hydrogen peroxide / sodium bisulfite. Preference is given to using mixtures consisting of a thermal initiator and a redox initiator, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid. Preferably, the disodium salt of 2-hydroxy-2-sulfinatoacetic acid, or a mixture consisting of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfinatoacetic acid and sodium bisulfite, is used as reducing component. A commercially available mixture of this type is Bruggolite® FF6 and Bruggolite® ® FF6 and Bruggolite ®FF7 (Bruggemann Chemicals; Heilbronn; Germany).

[0031] The monomer solution preferably has a water content of from 40 to 75 wt%, particularly preferably from 45 to 70 wt%, further particularly preferably from 50 to 65 wt%. With increasing water content, the energy consumption in the subsequent drying process increases, and at a reduced water content, the polymerization heat can not be sufficiently dissipated.

[0032] The monomer solution preferably has a temperature of from 10 to 90°C, particularly preferably from 20 to 70°C, further particularly preferably from 30 to 50°C.

[0033] In order to achieve optimum results, the preferred polymerization inhibitors require dissolved oxygen. Therefore, prior to the polymerization, the dissolved oxygen in the monomer solution can be removed by inertization, i.e. by flowing an inert gas, preferably nitrogen or carbon dioxide, through the monomer solution. Preferably, the oxygen content of the monomer solution is reduced to less than 1 wppm, particularly preferably to less than 0.5 wppm, further particularly preferably to less than 0.1 wppm, prior to the polymerization.

[0034] Suitable reactors for the polymerization are, for example, kneader reactors or belt reactors. In a kneader, the polymer gel produced in the polymerization of the aqueous monomer solution or suspension is continuously comminuted by, for example, counter-rotating stirring shafts, as described in WO 2001 / 038402 Al. The polymerization in a belt reactor is described, for example, in DE 38 25 366 Al and US 6,241,928. The polymerization in a belt reactor produces a polymer gel which has to be comminuted, for example, in an extruder or a kneader.

[0035] In order to improve the drying properties, the comminuted polymer gel obtained by the kneader can additionally be extruded.

[0036] Subsequently, the polymer gel is dried, typically by means of a circulating air belt dryer, until the residual moisture content is preferably from 0.5 to 10 wt%, particularly preferably from 1 to 7 wt%, further particularly preferably from 2 to 5 wt%, wherein the residual moisture content is determined in accordance with EDANA recommended test method No. WSP 230.2-05 "Mass Loss Upon Heating". At too high residual moisture, the dried polymer gel has too low a glass transition temperature Tg, and at too low residual moisture, the dried polymer gel has too high a glass transition temperature Tg. gand further processing is difficult. At too low residual moisture, the dried polymer gel is too brittle and produces undesirably large amounts of polymer particles of too small particle size ("fines") in the subsequent comminution step. The solids content of the polymer gel prior to drying is preferably from 25 to 90 wt.%, particularly preferably from 35 to 70 wt.%, further particularly preferably from 40 to 60 wt.%. The dried polymer gel is then broken up and optionally coarsely comminuted.

[0037] The dried polymer gel is then usually ground and classified, wherein grinding can usually be carried out using a single- or multi-stage roll mill, preferably a two- or three-stage roll mill, a pin mill, a hammer mill or a vibration mill.

[0038] The average particle size of the polymer particles isolated as product fraction is preferably at least 150 to 850 pm, particularly preferably 250 to 600 pm, further particularly preferably 300 to 500 pm. The average particle size of the product fraction can be determined using the test method No. WSP 220.2 (05) "Partikel Size Distribution" recommended by EDANA, wherein the mass proportions of the sieve fractions are plotted cumulatively and the average particle size is determined graphically. Here, the average particle size is the mesh width value corresponding to 50 wt.% of the cumulative plot.

[0039] For further improvement of the properties, the polymer particles are surface postcrosslinked. Suitable surface postcrosslinking agents are compounds containing groups which are capable of forming covalent bonds with at least two carboxyl 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, difunctional or polyfunctional alcohols as described in DE 33 14 019 A1, DE 35 23 617 A1 and EP 0 450 922 A2, or ß-hydroxyalkylamides as described in DE 102 04 938 A1 and US 6,239,230.

[0040] In a preferred embodiment of the application, in addition to the surface postcrosslinking agent, polyvalent cations are additionally applied to the particle surface.

[0041] The polyvalent cations applicable to the methods of this invention are, for example, divalent cations such as those of zinc, magnesium, calcium, and strontium; trivalent cations such as those of aluminum, iron, chromium, rare earth elements, and manganese; and tetravalent cations such as those of titanium and zirconium. Possible counterions include chlorides, bromides, hydroxides, sulfates, bisulfates, carbonates, bicarbonates, nitrates, phosphates, hydrogen phosphates, dihydrogen phosphates, and carboxylates such as acetates and lactates. Aluminum hydroxide, aluminum sulfate, and aluminum lactate are preferred.

[0042] Based on the polymer, the amount of polyvalent cation used is, for example, 0.001 wt% to 1.5 wt%, preferably 0.005 wt% to 1 wt%, and particularly preferably 0.02 wt% to 0.8 wt%.

[0043] Post-crosslinking of the surface is performed by spraying a solution of the post-crosslinking agent onto dried polymer particles. After spraying, the polymer particles coated with the post-crosslinking agent are subjected to heat treatment.

[0044] The surface crosslinking agent solution is preferably sprayed in a mixer with a moving mixing tool, such as a spiral mixer, disc mixer, or paddle mixer. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are even more preferred. The difference between horizontal and vertical mixers lies in the support method of the mixing shaft; that is, horizontal mixers have a horizontally supported mixing shaft, while vertical mixers have a vertically supported mixing shaft. Suitable mixers include, for example, Horizontal Pflugschar. ® Mischer (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Duttingheim; Netherlands), Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA), and SchugiFlexomix ® (Hosokawa Micron BV; Duttingheim; Netherlands). However, a surface crosslinking agent solution can also be sprayed in a fluidized bed.

[0045] Post-crosslinking agents are typically used as aqueous solutions. The penetration depth of the post-crosslinking agent into the polymer particles can be adjusted by utilizing the content of non-aqueous solvents or the total solvent volume.

[0046] The surface thermal post-crosslinking is carried out in a contact dryer, particularly preferably in a paddle dryer, and even more preferably in a disc dryer. Suitable dryers include, for example, the 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; Freising; Germany).

[0047] The surface-postcrosslinked polymer particles can subsequently be reclassified, wherein too small and / or too large polymer particles are separated and fed back into the process.

[0048] For further improving the properties, the surface-postcrosslinked polymer particles can be coated or re-wetted.

[0049] The re-wetting is preferably carried out at 30°C to 80°C, particularly preferably at 35°C to 70°C, further particularly preferably at 40°C to 60°C. At too low temperatures, the polymer particles tend to agglomerate, at higher temperatures, the water has evaporated significantly. The amount of water used for re-wetting is preferably 1 wt% to 10 wt%, particularly preferably 2 wt% to 8 wt%, further particularly preferably 3 wt% to 5 wt%. The mechanical stability of the polymer particles is enhanced by re-wetting and their tendency to charge is reduced. Preferably, the re-wetting is carried out in a cooler after the surface-thermal postcrosslinking.

[0050] 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 multivalent metal cations. Suitable coatings for binding dust are, for example, polyols. Suitable coatings for counteracting the tendency of the polymer particles to agglomerate are, for example, fumed silicas (such as Aerosil ® 200), precipitated silicas (such as Sipernat ® D17) and surfactants (such as Span ® 20).

[0051] Methods :

[0052] The measurements are carried out at an ambient temperature of 23 ± 2°C and a relative air humidity of 50 ± 10%, unless stated otherwise. The superabsorbent particles are thoroughly mixed before the measurements.

[0053] Saline flow conductivity

[0054] The liquid flow conductivity (SFC) was determined according to the test method "Urine Permeability Measurement (UPM) Test method" described in EP 2 535 698 A1 on pages 19 to 22. Example

[0055] Example 1 (according to the invention)

[0056] A monomer solution was prepared by continuously mixing deionized water, a 50 wt% sodium hydroxide solution and acrylic acid such that the degree of neutralization was equal to 71.0 mol%. The water content of the monomer solution was 58.25 wt%.

[0057] 3 Ethoxylated glyceryl triacrylate (about 85 wt%) was used as crosslinker. The amount used was 1.04 kg per ton of monomer solution.

[0058] Citric acid was used as complexing agent. The amount used was 0.07 kg per ton of monomer solution.

[0059] For initiating the radical polymerization, 1.38 kg of a 0.25 wt% aqueous hydrogen peroxide solution, 3.22 kg of a 15 wt% aqueous sodium peroxodisulfate solution and 1.04 kg of a 1 wt% aqueous ascorbic acid solution were added per ton of monomer solution.

[0060] The monomer solution was fed into a reactor (LIST AG, Arisdorf, Switzerland) of the type List Contikneter with a volume of 6.3 m3. The throughput of the monomer solution was about 22.5 t / h. The temperature of the reaction solution at the inlet was 30°C.

[0061] Between the point of addition of the crosslinker and the points of addition of the hydrogen peroxide and sodium peroxodisulfate solutions, the monomer solution was inertized with nitrogen. The ascorbic acid was fed directly into the reactor.

[0062] After about 50% of the residence time, additionally about 1,000 kg / h of polymer particles with a particle size of less than 150 pm, which were produced during the comminution and classification, were fed into the reactor. The residence time of the reaction mixture in the reactor was about 15 minutes.

[0063] The resulting polymer gel was conveyed by means of a vibrating conveyor belt onto the conveyor belt of a circulating air belt dryer. 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 aqueous polymer gel was continuously circulated and dried on the circulating air belt dryer using an air / gas mixture (about 175°C). The residence time in the circulating air belt dryer was about 37 minutes.

[0064] The dried polymer gel was comminuted by means of a three-stage roll mill and sieved to a particle size of 150 pm to 850 pm. The polymer particles having a particle size of less than 150 pm were separated. The polymer particles having a particle size of more than 850 pm were fed back for comminution. The polymer particles having a particle size in the range of 150 pm to 850 pm were subjected to surface postcrosslinking.

[0065] The polymer particles were coated in a Schugi Flexomix ® (Hosokawa Micron B.V., Duiven, The Netherlands) with the surface postcrosslinking agent solution and subsequently dried in a NARA Paddle Dryer (GMF Gouda, Waardinge, The Netherlands) at 186.5 °C for 45 minutes. The NARA Paddle Dryer has an internal volume of 18.8 m3. The surface postcrosslinked superabsorbent particles fell over a weir into a hopper. The hopper bottom is a vane rotary valve. The hopper bottom has a circular outlet with a diameter of 25 cm and extends to a height of 99.8 cm, forming a rectangular cross section of 200 cm x 85 cm. The hopper ends after 122 cm with a constant cross section of 200 cm x 85 cm. The internal volume of the hopper is approximately 3 m3. The liquid level in the hopper was kept at 75-80%.

[0066] The hopper has an ultrasonic discharge aid consisting of three C35-HP1 transducers and one DGS35-200-T-EU generator (adams & öztas oHG, Gernsbach, Germany). The transducers are located at the side and at the back of the hopper, approximately 60 cm above the hopper outlet.

[0067] The following amounts of materials were fed into the Schugi Flexomix ® :

[0068] 9.5 t / h of polymer particles

[0069] 366.7 kg / h of surface postcrosslinking agent solution

[0070] The surface postcrosslinking agent solution contains 1.55 wt% of 2-hydroxyethyl-2-oxazolidinone, 1.55 wt% of 1,3-propanediol, 12.95 wt% of 1,2-propanediol, 10.88 wt% of aluminum lactate, 50.70 wt% of water, 22.28 wt% of isopropyl alcohol and 0.08 wt% of sorbitan monolaurate (Span ® 20).

[0071] The surface-postcrosslinked polymer particles were transported using an impeller rotating valve to a NARA Paddle-Cooler (GMF Gouda, Waardingerd, The Netherlands) and cooled to about 60°C. Here, the surface-postcrosslinked polymer particles were coated with 118.75 kg / h of water.

[0072] The obtained superabsorbent particles were filled into a flexible intermediate bulk container (FIBC) and analyzed. The liquid flux (SFC) was about 50 x 10 -7 cm3s / g.

[0073] Example 2 (not according to the invention)

[0074] The operation was carried out as in Example 1. The level in the hopper was 0%, i.e. the surface-postcrosslinked superabsorbent particles were immediately further transported.

[0075] The obtained superabsorbent particles were filled into a flexible intermediate bulk container (FIBC) and analyzed. The liquid flux (SFC) was about 36 x 10 -7 cm3s / g.

Claims

1. A method for continuous thermal surface crosslinking of a superabsorbent, wherein, Superabsorbent particles are coated with a surface-crosslinking agent solution by spraying, the coated superabsorbent particles are hot-surface-crosslinked in contact dryer 1, and the hot-surface-crosslinked superabsorbent particles are cooled in contact dryer 2. The superabsorbent particles leaving the contact dryer 1 are transferred to the contact dryer 2 via a hopper and a metering device, the liquid level in the hopper is regulated by the metering device, and the liquid level in the hopper is 10% to 100%.

2. The method according to claim 1, characterized in that, The internal volume of the hopper is 0.01 cubic meters to 0.5 cubic meters per cubic meter of the internal volume of the contact dryer 1.

3. The method according to claims 1 to 2, characterized in that, Partially neutralized, cross-linked polyacrylic acid is used as a superabsorbent.

4. The method according to any one of claims 1 to 3, characterized in that, The surface post-crosslinking agent can form covalent bonds with the superabsorbent.

5. The method according to any one of claims 1 to 4, characterized in that, The average droplet diameter during the spraying of the crosslinking agent solution on the surface is between 200 µm and 4,500 µm.

6. The method according to any one of claims 1 to 5, characterized in that, The temperature during the spraying of the superabsorbent particles onto the surface of the crosslinking agent solution is between 30°C and 80°C.

7. The method according to any one of claims 1 to 6, characterized in that, Based on the superabsorbent particles, the surface post-crosslinking agent solution contains 0.001 wt% to 2 wt% of surface post-crosslinking agent.

8. The method according to any one of claims 1 to 7, characterized in that, Based on the superabsorbent particles, the surface post-crosslinking agent solution contains 0.5 wt% to 5 wt% water.

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

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

11. The method according to any one of claims 1 to 10, characterized in that, The contact dryer 1 and its connection to the contact dryer 2, along with the hopper and feeding equipment, can be subjected to accompanying heating and / or thermal isolation.

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

13. The method according to any one of claims 1 to 12, characterized in that, The residence time of the superabsorbent particles in the contact dryer 2 is 10 to 60 minutes.

14. The method according to any one of claims 1 to 13, characterized in that, An airflow with an oxygen content of less than 10% by volume is guided through the contact dryer 1.

15. The method according to claim 14, characterized in that, The gas is guided into the contact dryer 1, and the total amount of gas introduced is 5 Nm³ / h to 60 Nm³ / h per cubic meter of the internal volume of the contact dryer 1. The waste gas is guided out of the contact dryer 1, and the waste gas in the contact dryer 1 is deflected upward at least 75° from the horizontal product flow direction, and the gas velocity of the waste gas after deflection is less than 5 m / s.