Method for the production of superabsorbents

Abstract

The present invention relates to a method for continuous production of superabsorbents, in which superabsorbent particles are coated by spray application of a surface postcrosslinker solution, the coated superabsorbent particles undergo thermal surface-postcrosslinking in a contact dryer 1, the thermally surface-postcrosslinked superabsorbent particles are cooled in a contact dryer 2, and a particulate solid is metered into the product stream between contact dryer 1 and contact dryer 2.

Description

[0001] Process for producing superabsorbents

[0002] The present invention relates to a process for the continuous production 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 thermally surface postcrosslinked superabsorbent particles are cooled in a contact dryer 2 and a particulate solid is metered into the product stream between contact dryer 1 and 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 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.

[0006] The object of the present invention was to provide an improved process for coating surface-crosslinked superabsorbent particles with particulate solids.

[0007] The object was achieved by a process for the continuous production of superabsorbents, wherein superabsorbent particles are coated by spraying on 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 surface postcrosslinked superabsorbent particles are additionally coated with a particulate solid, the particulate solid is metered into the product stream between contact dryer 1 and contact dryer 2, the contact dryer 2 has two horizontal shafts with mixing tools and the speed of the mixing tools corresponds to a Froude number of 0.005 to 0.25.

[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 by means of a dynamic tool and are then layered. Contact dryers can also be used for cooling.

[0009] The speed of the mixing tools corresponds to a Froude number of preferably 0.01 to 0.21, particularly preferably 0.02 to 0.18, most particularly preferably 0.04 to 0.15.

[0010] For mixers with horizontally mounted mixing tools, the Froude number is defined as follows: ar 2r Fr = -

[0011] 9 with r: radius of the mixing tool a> : angular frequency g: acceleration due to gravity

[0012] The particulate solid can be dispersed in a gas stream and dosed into the product stream.

[0013] The present invention is based on the finding that particulate solids, especially aluminum trihydroxide, are difficult to dose smoothly in the cooler (contact dryer 2). Smooth mixing is achieved when the particulate solids are added to the product stream entering the cooler.

[0014] The temperature of the superabsorbent particles when spraying on the surface postcrosslinker solution is preferably from 30 to 80°C, more preferably from 35 to 75°C, most preferably from 40 to 70°C. The surface postcrosslinker solution preferably contains from 0.001 to 2% by weight, more preferably from 0.01 to 1% by weight, most preferably from 0.03 to 0.7% by weight, of a surface postcrosslinker, based in each case on the superabsorbent particles. The surface postcrosslinker solution further preferably contains from 0.5 to 5% by weight, more preferably from 1.0 to 4% by weight, most preferably from 1.5 to 3% by weight, of water, based in each case on the superabsorbent particles.

[0015] 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.

[0016] The contact dryer 1 and the connection to the contact dryer 2 can be trace heated and / or thermally insulated.

[0017] The amount of particulate solid used is preferably from 0.001 to 2.0 wt. %, particularly preferably from 0.01 to 1.0 wt. %, very particularly preferably from 0.1 to 0.5 wt. %, based in each case on the superabsorbent particles. The average particle size of the particulate solid is preferably from 0.1 to 100 μm, particularly preferably from 0.5 to 50 μm, very particularly preferably from 1 to 25 μm. The average particle size is the volume-average particle size and can be determined by light scattering. A suitable particulate solid is aluminum trihydroxide.

[0018] The temperature of the superabsorbent particles during coating with the particulate solid is preferably less than 180°C, particularly preferably less than 160°C, most particularly preferably less than 140°C.

[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] The mixing tools of the contact dryer 2 have a diameter of preferably 0.2 to 2 m, particularly preferably 0.4 to 1.2 m, very particularly preferably 0.6 to 1.2 m. The rotational speed of the mixing tools is preferably less than 25, particularly preferably less than 20, very particularly preferably less than 10, revolutions per minute.

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

[0022] The superabsorbents are produced by polymerizing a monomer solution and are usually water-insoluble.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] The monomers usually contain polymerization inhibitors, preferably hydroquinone hemiether, as storage stabilizers.

[0027] 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.

[0028] 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.

[0029] 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 (AUL0.3psi) passes through a maximum.

[0030] 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).

[0031] The water content of the monomer solution is preferably from 40 to 75 wt.%, particularly preferably from 45 to 70 wt.%, very particularly preferably from 50 to 65 wt.%. As the water content increases, the energy consumption during the 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, particularly preferably from 20 to 70°C, very particularly 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 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.

[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 superabsorbent 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. %, more preferably between 35 and 70 wt. %, and most preferably between 40 and 60 wt. %. The dried polymer gel is then crushed and optionally coarsely crushed.

[0036] The dried polymer gel is then typically ground and classified, with single- or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills, or vibratory mills being 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 especially from 300 to 500 pm. The average particle size of the product fraction can be determined using the test method No. WSP 220.2 (05) "Particle Size Distribution" recommended by E-DANA, 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. %.

[0037] The superabsorbent 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 superabsorbent 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 3523617 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.

[0038] 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 carried out by spraying a solution of the surface post-crosslinker onto the dried superabsorbent particles. Following spraying, the superabsorbent particles coated with surface post-crosslinker are thermally surface post-crosslinked. 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, with vertical mixers being even more 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, Horizontal 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). It is also possible to spray the surface post-crosslinker solution in a fluidized bed.

[0042] 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.

[0043] 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).

[0044] 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.

[0045] The surface-crosslinked superabsorbent particles can be coated or remoistened to further improve their properties.

[0046] 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 together, 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.

[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 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.

[0048] Examples

[0049] Example 1 (according to the invention)

[0050] 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.%.

[0051] Triple-ethoxylated glycerol triacrylate (approx. 85 wt.%) was used as the crosslinker. The amount used was 1.42 kg per t of monomer solution.

[0052] 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.

[0053] 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 t / h. The reaction solution had a temperature of 23.5°C at the inlet.

[0054] 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 approximately 1,000 kg / h of superabsorbent particles with a particle size of less than 150 pm, which had been generated during the comminution and classification process during the production process, were dosed into the reactor. The residence time of the reaction mixture in the reactor was approximately 15 minutes.

[0055] 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 circulated with an air / gas mixture (approx. 175°C) and dried. The residence time in the circulating air belt dryer was 37 minutes.

[0056] The dried polymer gel was ground using a three-stage roller mill and sieved to a particle size of 150 to 850 pm. Superabsorbent particles with a particle size of less than 150 pm were separated. Superabsorbent particles with a particle size greater than 850 pm were returned to the grinding process. Superabsorbent particles with a particle size in the range of 150 to 850 pm were thermally surface-crosslinked.

[0057] The superabsorbent particles were coated with a surface post-crosslinker solution in a Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, Netherlands) and then thermally surface post-crosslinked in a NARA Paddle Dryer (Contact Dryer 1, GMF Gouda, Waddinxveen, Netherlands) for 45 minutes at 120°C.

[0058] The following quantities were dosed into the Schugi Flexomix®:

[0059] 9.5 t / h superabsorbent particles

[0060] 530.10 kg / h surface post-crosslinker solution

[0061] The surface postcrosslinker solution contained 1.43 wt% ethylene glycol diglycidyl ether, 44.8 wt% 1,2-propanediol and 53.77 wt% water.

[0062] The surface-crosslinked superabsorbent particles were transferred to a NARA paddle cooler (Contact Dryer 2, GMF Gouda, Waddinxveen, Netherlands) using a rotary valve and cooled to approximately 60°C. The surface-crosslinked superabsorbent particles were coated with a mixture of approximately 285 kg / h of water and 23.75 kg / h of a 1 wt% aqueous solution of sorbitan monolaurate (Span®20). The mixture was injected into the product bed from below in the mixer trough. The distance between the metering device and the end wall was approximately 200 cm. The blades had a diameter of approximately 0.9 m and rotated at approximately 1000 rpm.

[0063] 10 revolutions per minute. The dwell time was approximately 20 minutes.

[0064] The product stream flowing from the rotary valve into the NARA Paddle Cooler had a temperature of approximately 120°C. The connecting pipe between the rotary valve and the NARA Paddle Cooler was extended into the NARA Paddle Cooler by means of an insert pipe. The insert pipe had a diameter of approximately 20 cm and a total length of approximately 50 cm, with approximately 30 cm extending into the NARA Paddle Cooler. The insert pipe should not extend too far to avoid collision with the mixing tools of the NARA Paddle Cooler. At the same time, the insert pipe within the NARA Paddle Cooler should not be too short to prevent some of the aluminum trihydroxide from being drawn off via the gas space with the exhaust air, thus preventing the entire amount of aluminum trihydroxide from remaining in the product or on the product surface. A mixture of approx. was added to the product stream falling through the insert tube, approx. 20 cm before the lower end of the insert tube.33.25 kg / h of aluminum trihydroxide (aluminum hydroxide dry gel, Dr. Paul Lohmann GmbH KG, Emmerthal, Germany) and 45 kg / h of air were added. The aluminum trihydroxide had an average particle size of approximately 20 pm.

[0065] The coating with aluminum trihydroxide was carried out without any problems.

[0066] Example 2 (not according to the invention)

[0067] The procedure was as in Example 1. The mixture of aluminum trihydroxide and air was injected into the product bed from below in the mixer trough. The distance between the metering device and the end wall was approximately 180 cm.

[0068] There were blockages in the supply line for the aluminum trihydroxide in the NARA paddle cooler.

Claims

Patent claims 1 . A process for the continuous production of superabsorbents, wherein superabsorbent particles are coated by spraying on 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 surface postcrosslinked superabsorbent particles are additionally coated with a particulate solid, the particulate solid is metered into the product stream between contact dryer 1 and contact dryer 2, the contact dryer 2 has two horizontal shafts with mixing tools and the speed of the mixing tools corresponds to a Froude number of 0.005 to 0.

25.

2. Process according to claim 1, characterized in that the speed of the mixing tools corresponds to a Froude number of 0.04 to 0.

15.

3. Process according to claim 1 or 2, characterized in that the residence time of the superabsorbent particles in the contact dryer 2 is from 10 to 60 minutes.

4. Process according to one of claims 1 to 3, characterized in that the particulate solid has an average particle size of 1 to 25 pm.

5. Process according to one of claims 1 to 4, characterized in that the amount of particulate solid used is from 0.001 to 2.0% by weight, based on the superabsorbent particles.

6. Method according to one of claims 1 to 5, characterized in that the mixing tools have a diameter of 0.2 to 2 m.

7. Method according to one of claims 1 to 6, characterized in that the rotational speed of the mixing tools is less than 25 revolutions per minute.

8. Process according to one of claims 1 to 7, characterized in that the temperature of the superabsorbent particles during coating with the particulate solid is less than 180°C.

9. Process according to one of claims 1 to 8, characterized in that the superabsorbent particles are cooled in the contact dryer 2 to a temperature of 30 to 80°C.

10. Process according to one of claims 1 to 9, characterized in that aluminum trihydroxide is used as a particulate solid.

11. Process according to one of claims 1 to 10, characterized in that partially neutralized, crosslinked polyacrylic acid is used as superabsorbent.

12. Process according to one of claims 1 to 11, characterized in that the surface post-crosslinker can form covalent bonds with the superabsorbent.

13. Process according to one of claims 1 to 12, characterized in that the temperature of the superabsorbent particles when spraying the surface post-crosslinker solution is from 30 to 80°C.

14. Process according to one of claims 1 to 13, characterized in that the superabsorbent particles are heated in the contact dryer 1 to a temperature of 110 to 220°C.

15. Process according to one of claims 1 to 14, characterized in that the residence time of the superabsorbent particles in the contact dryer 1 is from 10 to 60 minutes.