Method for the thermal surface post-crosslinking of superabsorbent polymers

Abstract

The present invention relates to a method for the continuous thermal surface post-crosslinking of superabsorbent polymers, wherein superabsorbent-polymer particles are coated by the spraying of a surface post-crosslinking agent solution, the coated superabsorbent-polymer particles are thermally surface post-crosslinked in a contact dryer (1), the superabsorbent-polymer particles leaving the contact dryer (1) are transferred, by means of a funnel and a metering device, into a contact dryer (2), the fill level in the funnel is controlled by means of the metering device, the funnel has a discharge aid, and the thermally surface post-crosslinked superabsorbent-polymer particles are cooled in the contact dryer (2).

Description

[0001] Process for thermal surface post-crosslinking of superabsorbents

[0002] The present invention relates to a process for the continuous thermal surface post-crosslinking of superabsorbents, wherein superabsorbent particles are coated by spraying a surface post-crosslinker solution, the coated superabsorbent particles are thermally surface post-crosslinked 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 fill level in the hopper is regulated by means of the metering device, the hopper has a discharge aid and the thermally surface-post-crosslinked superabsorbent particles are cooled in the contact dryer 2.

[0003] 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.

[0004] 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.

[0005] To improve application properties such as Saline Flow Conductivity (SFC) and absorption under a pressure of 49.2 g / cm 2 (AU HL - Absorption Under High Load), superabsorbent particles are generally surface-crosslinked. This increases the degree of crosslinking of the particle surface, thereby increasing absorption under a pressure of 49.2 g / cm 2(AU HL) 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 polymer 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 polymer particles.

[0006] The object of the present invention was to provide an improved process for the thermal surface post-crosslinking of superabsorbent particles, in particular to achieve lower deviations in the liquid flow conduction (SFC) of the produced superabsorbent particles.

[0007] 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 surface-post-crosslinked in a contact dryer 1 and the thermally surface-post-crosslinked 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 fill level in the hopper is regulated by means of the metering device and the hopper has a discharge aid.

[0008] 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 and rearranged by means of a dynamic tool. Contact dryers can also be used for cooling.

[0009] Discharge aids are used to ensure more even emptying of the hopper. The discharge aids to be used according to the invention are not subject to any restrictions. Suitable discharge aids include vibrators, impactors, and pulsators. Vibrators cause the hopper wall to vibrate. Impactors cause the hopper wall to vibrate through targeted impacts. In both cases, the superabsorbent in the hopper is loosened, causing any bridges to collapse. Pulsators loosen the superabsorbent in the hopper through short pressure pulses. However, ultrasonic discharge aids are preferred. In this case, the hopper is set into barely audible vibrations using ultrasound.

[0010] The discharge aids are usually evenly distributed in the lower third of the hopper. Three discharge aids are sufficient in most cases.

[0011] A suitable dosing device could be a screw conveyor or a rotary valve. A rotary valve consists of a rotor that rotates in a precisely fitting housing. A specific number of rotor blades are mounted on the rotor, forming individual rotor cells. Each rotor cell receives material below the inlet opening, and the material falls out at the outlet. This creates a volumetrically continuous feed. The feed rate is determined by the volume of the rotor cells and the rotor speed.

[0012] The fill level in the hopper is, for example, from 10 to 100%, preferably from 20 to 95%, more preferably from 30 to 90%, particularly preferably from 40 to 85%, most preferably from 50 to 80%. The fill level refers to the internal volume of the hopper.

[0013] The internal volume of the funnel is preferably from 0.01 to 0.5 m 3 , preferably from 0.02 to 0.4 m 3, particularly preferably from 0.05 to 0.3 m 3 , most preferably from 0.1 to 0.2 m 3 , each per m 3 Internal volume of the contact dryer 1 .

[0014] The present invention is based on the finding that the dispensing aids according to the invention are important for a constant fill level and thus for surface post-crosslinking with consistent product properties, in particular consistent liquid flow (SFC). The average droplet diameter when spraying the surface post-crosslinker solution is, for example, 200 to 4,500 pim, preferably 300 to 3,500 pim, particularly preferably 400 to 2,500 pim, and most preferably 500 to 1,500 pim. The average droplet diameter can be determined by light scattering.

[0015] The temperature of the superabsorbent particles when spraying the surface postcrosslinker solution is preferably from 30 to 80°C, particularly preferably from 35 to 75°C, most preferably from 40 to 70°C.

[0016] The surface postcrosslinker solution preferably contains from 0.001 to 2 wt. %, particularly preferably from 0.01 to 1 wt. %, very particularly preferably from 0.03 to 0.7 wt. %, of a surface postcrosslinker, in each case based on the superabsorbent particles. The surface postcrosslinker solution further preferably contains from 0.5 to 5 wt. %, particularly preferably from 1.0 to 4 wt. %, very particularly preferably from 1.5 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 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 50 minutes, most preferably 20 to 40 minutes.

[0018] The contact dryer 1 and the connection to the contact dryer 2 can be trace heated and / or thermally insulated.

[0019] 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 10 to 60 minutes, more preferably from 15 to 50 minutes, most preferably from 20 to 40 minutes.

[0020] In a preferred embodiment of the present invention, a gas stream with an oxygen content of less than 10 vol.% is fed into the contact dryer 1. The gas flow is, for example, from 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 3 at 273.15 K and 1,013.25 hPa. An exhaust gas stream is discharged from the contact dryer 1. The exhaust gas stream is 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 the superabsorbents is explained in more detail below:

[0021] The superabsorbents are produced by polymerizing a monomer solution and are usually water-insoluble.

[0022] 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, particularly preferably at least 25 g / 100 g water, most particularly preferably at least 35 g / 100 g water.

[0023] 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.

[0024] 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.

[0025] The monomers usually contain polymerization inhibitors, preferably hydroquinone hemiether, as storage stabilizers.

[0026] 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.

[0027] 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 10331 456 A1 and DE 10355 401 A1, or crosslinker mixtures as described, for example, in DE 195 43368 A1, DE 196 46 484 A1, WO 90 / 15830 A1 and WO 02 / 032962 A2.

[0028] 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. %, each calculated on the total amount of monomer used. As the crosslinker content increases, the centrifuge retention capacity (CRC) and the absorption under a pressure of 21.0 g / cm decrease. 2 (AUL) passes through a maximum.

[0029] Any compound that generates 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).

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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 3825 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.

[0034] To improve the drying properties, the crushed polymer gel obtained by means of a kneader can be additionally extruded.

[0035] 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.%, particularly preferably 1 to 7 wt.%, most preferably 2 to 5 wt.%, wherein 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 will have a glass transition temperature T too low. gand 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 too small a particle size ("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. %, and most preferably between 40 and 60 wt. %. The dried polymer gel is then broken and optionally coarsely crushed.

[0036] 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.

[0037] The average particle size of the polymer particles separated as the product fraction is preferably from 150 to 850 pim, more preferably from 250 to 600 pim, and most preferably from 300 to 500 pim. 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.%.

[0038] The polymer particles are 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 ß-hydroxyalkylamides, as described in DE 102 04 938 A1 and US Pat. No. 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.

[0039] 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.

[0040] 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.

[0041] Surface post-crosslinking is performed by spraying a solution of the surface post-crosslinker onto the dried polymer particles. Following spraying, the polymer particles coated with the surface post-crosslinker are thermally treated.

[0042] 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 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.

[0043] 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.

[0044] 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-post-crosslinked polymer particles can then be reclassified, with overly small and / or overly large polymer particles being separated and returned to the process.

[0045] The surface-crosslinked polymer particles can be coated or re-moistened to further improve their properties.

[0046] 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. Re-moistening is advantageously carried out in the cooler after thermal surface post-crosslinking.

[0047] 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.

[0048] Methods:

[0049] 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.

[0050] Saline Flow Conductivity

[0051] The liquid conductance (SFC) is determined according to the test method "Urine Permeability Measurement (UPM) Test method" described in EP 2 535 698 A1 on pages 19 to 22. Examples

[0052] Example 1 (according to the invention)

[0053] 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 58.25 wt%.

[0054] Triple-ethoxylated glycerol triacrylate (approx. 85 wt.%) was used as the crosslinker. The amount used was 1.04 kg per t of monomer solution.

[0055] Citric acid was used as a complexing agent. The amount used was 0.07 kg per ton of monomer solution.

[0056] To initiate 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 used per t of monomer solution.

[0057] 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 22.5 t / h. The reaction solution had a temperature of 30°C at the inlet.

[0058] 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.

[0059] After approximately 50% of the residence time, an additional 1,000 kg / h of polymer particles with a particle size of less than 150 μm, which had been generated during the production process through comminution and classification, were added to the reactor. The residence time of the reaction mixture in the reactor was approximately 15 minutes.

[0060] The resulting 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 its conveyor belt had an effective width of 4.4 m. On the circulating air belt dryer, the aqueous polymer gel was continuously exposed to an air / gas mixture (approx. 175°C) and dried. The residence time in the circulating air belt dryer was approximately 37 minutes.

[0061] The dried polymer gel was ground using a three-stage roller mill and sieved to a particle size of 150 to 850 pim. Polymer particles with a particle size of less than 150 pim were separated. Polymer particles with a particle size greater than 850 pim were returned to the grinding process. Polymer particles with a particle size in the range of 150 to 850 pim were thermally surface-crosslinked.

[0062] The polymer particles were coated with a surface post-crosslinker solution in a Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, Netherlands) and then dried in a NARA Paddle Dryer (GMF Gouda, Waddinxveen, Netherlands) for 45 minutes at 186.5°C. The NARA Paddle Dryer had an internal volume of 18.8 m 3The surface-crosslinked superabsorbent particles fell over a weir into a funnel. A rotary valve was located at the bottom of the funnel. The funnel had a round outlet with a diameter of 25 cm and widened to a height of 99.8 cm, forming a rectangular cross-section of 200 cm x 85 cm. The funnel ended after a further 122 cm with a constant cross-section of 200 cm x 85 cm. The funnel had an internal volume of approximately 3 m 3 The fill level in the hopper was maintained at 75 to 80%.

[0063] The hopper was equipped with ultrasonic discharge aids consisting of three C35-HP1 converters and one DGS35-200-T-EU generator (adams & öztas oHG, Gernsbach, Germany). The converters were located on the sides and rear of the hopper, approximately 60 cm above the hopper outlet.

[0064] The following quantities were dosed into the Schugi Flexomix®:

[0065] 9.5 t / h polymer particles

[0066] 366.7 kg / h surface post-crosslinker solution

[0067] The surface postcrosslinker solution contained 1.55 wt% 2-hydroxyethyl-2 oxazolidone, 1.55 wt% 1,3-propanediol, 12.95 wt% 1,2-propanediol, 10.88 wt% aluminum lactate, 50.70 wt% water, 22.28 wt% isopropanol and 0.08 wt% sorbitan monolaurate (Span®20).

[0068] The surface-crosslinked polymer particles were transferred to a NARA paddle cooler (GMF Gouda, Waddinxveen, Netherlands) using a rotary valve and cooled to approximately 60°C. The surface-crosslinked polymer particles were coated with 118.75 kg / h of water.

[0069] The fill level in the hopper was maintained within the desired range. The resulting superabsorbent particles were filled into flexible intermediate bulk containers (FIBCs) and analyzed. The liquid flow rate (SFC) was approximately 50 x 10 7 cm 3 b / w

[0070] Example 2 (not according to the invention)

[0071] The procedure is as in Example 1. No ultrasonic discharge aids were used. Fluctuations in the liquid flow conductance (SFC) of up to ±10 x 10 7 cm 3 b / w

Claims

Patent claims 1. 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 surface-post-crosslinked in a contact dryer 1 and the thermally surface-post-crosslinked 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 fill level in the hopper is regulated by means of the metering device and the hopper has a discharge aid.

2. Method according to claim 1, characterized in that the internal volume of the funnel is from 0.01 to 0.5 m 3 per m 3 Internal volume of the contact dryer 1 is.

3. Process according to claim 1 or 2, characterized in that partially neutralized, cross-linked polyacrylic acid is used as superabsorbent.

4. Process according to one of claims 1 to 3, characterized in that the surface post-crosslinker can form covalent bonds with the superabsorbent.

5. Process according to one of claims 1 to 4, characterized in that the average drop diameter when spraying the surface post-crosslinker solution is from 200 to 4,500 pm.

6. Process according to one of claims 1 to 5, characterized in that the temperature of the superabsorbent particles when spraying the surface post-crosslinker solution is from 30 to 80°C.

7. Process according to one of claims 1 to 6, characterized in that the surface postcrosslinker solution contains from 0.001 to 2% by weight of a surface postcrosslinker, based on the superabsorbent particles.

8. Process according to one of claims 1 to 7, characterized in that the surface post-crosslinker solution contains from 0.5 to 5 wt.% water, 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 and / or thermally insulated.

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 10 to 60 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.% oxygen is passed through the contact dryer 1.

15. The method according to claim 14, characterized in that a gas is introduced into the contact dryer 1, the total gas quantity of the introduced gas stream of 5 to 60 Nm 3 / h per m 3 Internal volume of the contact dryer 1, an exhaust gas flow is led out of the contact dryer 1, the exhaust gas flow 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 flow directly after the deflection is less than 5 m / s.