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WO2025040439A1 - Process for thermal surface postcrosslinking of superabsorbents - Google Patents

Process for thermal surface postcrosslinking of superabsorbents Download PDF

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Publication number
WO2025040439A1
WO2025040439A1 PCT/EP2024/072317 EP2024072317W WO2025040439A1 WO 2025040439 A1 WO2025040439 A1 WO 2025040439A1 EP 2024072317 W EP2024072317 W EP 2024072317W WO 2025040439 A1 WO2025040439 A1 WO 2025040439A1
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WO
WIPO (PCT)
Prior art keywords
contact dryer
superabsorbent particles
process according
dryer
post
Prior art date
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PCT/EP2024/072317
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German (de)
French (fr)
Inventor
Karl Possemiers
Ann Becker
Ronny De Kaey
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BASF SE
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BASF SE
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Classifications

    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J3/00—Processes of treating or compounding macromolecular substances
    • C08J3/24—Crosslinking, e.g. vulcanising, of macromolecules
    • C08J3/245—Differential crosslinking of one polymer with one crosslinking type, e.g. surface crosslinking
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
    • A61L15/00—Chemical aspects of, or use of materials for, bandages, dressings or absorbent pads
    • A61L15/16—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons
    • A61L15/42—Use of materials characterised by their function or physical properties
    • A61L15/60—Liquid-swellable gel-forming materials, e.g. super-absorbents
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F120/00—Homopolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical or a salt, anhydride, ester, amide, imide or nitrile thereof
    • C08F120/02—Monocarboxylic acids having less than ten carbon atoms; Derivatives thereof
    • C08F120/04—Acids; Metal salts or ammonium salts thereof
    • C08F120/06—Acrylic acid; Methacrylic acid; Metal salts or ammonium salts thereof
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2333/00—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers
    • C08J2333/02—Homopolymers or copolymers of acids; Metal or ammonium salts thereof

Definitions

  • the present invention relates to a method for the 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 surface-post-crosslinked in a contact dryer 1, the superabsorbent particles leaving the contact dryer 1 are transferred to a contact dryer 2 via a funnel and a metering device, the fill level in the funnel is regulated by means of the metering device, the fill level in the funnel is from 10 to 100% and the thermally surface-post-crosslinked superabsorbent particles are cooled in the contact dryer 2.
  • Superabsorbents are used to make diapers, tampons, sanitary napkins and other hygiene products, but also as water-retaining agents in agricultural horticulture. Superabsorbents are also known as water-absorbing polymers.
  • superabsorbent particles are generally surface-crosslinked. This increases the degree of crosslinking of the particle surface, whereby the absorption under a pressure of 49.2 g/cm 2 (AUHL) and the centrifuge retention capacity (CRC - Centrifuge Retention Capacity) can be at least partially decoupled.
  • This surface-crosslinking can be carried out in an aqueous gel phase.
  • dried, ground and sieved polymer particles are coated on the surface with a surface-crosslinker and thermally surface-crosslinked.
  • Suitable crosslinkers for this are compounds that can form covalent bonds with at least two carboxylate groups of the polymer particles.
  • the object of the present invention was to provide an improved process for the thermal surface post-crosslinking of superabsorbent particles, in particular a higher liquid conductivity (SFC) of the superabsorbent particles produced.
  • SFC liquid conductivity
  • the object was achieved by a method for the 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 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 to the contact dryer 2 via a funnel and a metering device, the fill level in the funnel is regulated by means of the metering device and the fill level in the funnel is from 10 to 100%.
  • Contact dryers suitable for the continuous process according to the invention are, for example, paddle dryers and disk dryers. In contact dryers, the goods to be dried are guided along a heated surface using a dynamic tool and are then rearranged. Contact dryers can also be used for cooling.
  • Discharge aids can be used to empty the hopper more evenly. There are no restrictions on the discharge aids. Suitable discharge aids include vibrators, knockers and pulsators. Vibrators cause the hopper wall to vibrate. Knockers cause the hopper wall to vibrate by means of targeted impacts. In both cases, the superabsorbent in the hopper is loosened and any bridges collapse. Pulsators loosen the superabsorbent in the hopper by means of short pressure pulses. However, ultrasonic discharge aids are preferred. The hopper is set into barely audible vibrations using ultrasound.
  • the discharge aids are usually evenly distributed in the lower third of the hopper.
  • the use of three discharge aids is sufficient in most cases.
  • a suitable dosing device is, for example, a screw conveyor or a rotary valve.
  • a rotary valve consists of a rotor that rotates in a precisely fitting housing. There are a certain number of rotor blades on the rotor, which create individual rotor cells. Each rotor cell takes in material to be conveyed under the inlet opening and the material falls out at the outlet. This creates a volumetrically continuous conveying process. The conveying capacity is determined by the volume of the rotor cells and the speed of the rotor.
  • the fill level in the funnel is 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 funnel.
  • 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 , very particularly preferably from 0.1 to 0.2 m 3 , in each case per m 3 internal volume of the contact dryer 1 .
  • the present invention is based on the finding that the filling level in the funnel between the contact dryer 1 and the contact dryer 2 is important for the properties of the superabsorbents produced, in particular a high liquid transfer factor (SFC).
  • SFC liquid transfer factor
  • the average droplet diameter when spraying the surface postcrosslinker solution is, for example, 200 to 4,500 pim, preferably 300 to 3,500 pim, particularly preferably 400 to 2,500 pim, very particularly preferably 500 to 1,500 pim.
  • the average droplet diameter can be determined by light scattering.
  • the temperature of the superabsorbent particles when spraying on 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.
  • the surface postcrosslinker solution preferably contains from 0.001 to 2% by weight, particularly preferably from 0.01 to 1% by weight, very particularly 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 also preferably contains from 0.5 to 5% by weight, particularly preferably from 1.0 to 4% by weight, very particularly preferably from 1.5 to 3% by weight, of water, based in each case on the superabsorbent particles.
  • the superabsorbent particles are heated in the contact dryer 1 to a temperature of preferably 110 to 220°C, particularly preferably 120 to 210°C, very particularly preferably 130 to 200°C.
  • the residence time of the superabsorbent particles in the contact dryer 1 is preferably from 10 to 60 minutes, particularly preferably from 15 to 50 minutes, very particularly preferably from 20 to 40 minutes.
  • the contact dryer 1 and the connection to the contact dryer 2 can be trace heated and/or thermally insulated.
  • the superabsorbent particles are cooled in the contact dryer 2 to a temperature of preferably 30 to 80°C, particularly preferably 35 to 70°C, very particularly preferably 40 to 60°C.
  • the residence time of the superabsorbent particles in the contact dryer 2 is preferably from 10 to 60 minutes, particularly preferably from 15 to 50 minutes, very particularly preferably from 20 to 40 minutes.
  • a gas stream with an oxygen content of less than 10 vol. % is passed 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, very particularly preferably from 20 to 30 Nm 3 /h, in each case 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 led out of 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, particularly preferably less than 2 m/s, very particularly preferably less than 1 m/s.
  • the superabsorbents are produced by polymerization of 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, particularly preferably at least 25 g/100 g water, very particularly preferably at least 35 g/100 g water.
  • Suitable monomers are, 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 particularly preferred.
  • the ethylenically unsaturated monomers carrying acid groups are usually partially neutralized.
  • the neutralization is carried out at the monomer stage. This is usually done by mixing in the neutralizing agent as an aqueous solution or preferably also as a solid.
  • the degree of neutralization is preferably from 40 to 85 mol%, particularly preferably from 50 to 80 mol%, very particularly preferably from 60 to 75 mol%, it being possible to use the usual neutralizing agents, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal hydrogen carbonates 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 particularly preferred are sodium hydroxide, sodium carbonate or sodium hydrogen carbonate 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. Such groups are, for example, ethylenically unsaturated groups that can be radically polymerized into the polymer chain and functional groups that can form covalent bonds with the acid groups of the monomer. Furthermore, 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 contain other ethylenically unsaturated groups in addition to acrylate groups, as in DE 103 31 456 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
  • the amount of crosslinker is preferably 0.05 to 1.5% by weight, particularly preferably 0.1 to 1% by weight, very particularly preferably 0.15 to 0.6% by weight, in each case calculated on the total amount of monomer used.
  • CRC centrifuge retention capacity
  • AUL absorption under a pressure of 21.0 g/cm 2
  • 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.
  • Mixtures of thermal initiators and redox initiators are preferably used, such as sodium peroxodisulfate/hydrogen peroxide/ascorbic acid.
  • 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% by weight, particularly preferably from 45 to 70% by weight, very particularly preferably from 50 to 65% by weight. As the water content increases, the energy required for 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, most preferably from 30 to 50°C.
  • the monomer solution can be freed of dissolved oxygen before polymerization by inerting, i.e. by passing an inert gas, preferably nitrogen or carbon dioxide, through it.
  • an inert gas preferably nitrogen or carbon dioxide
  • the oxygen content of the monomer solution before polymerization is reduced to less than 1 ppm by weight, particularly preferably to less than 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.
  • Suitable reactors for polymerization are, for example, kneader reactors or belt reactors.
  • the polymer gel formed during the polymerization of an aqueous monomer solution or suspension is continuously comminuted by, for example, counter-rotating stirring shafts, as described in WO 2001/038402 A1.
  • Polymerization on the belt is described, for example, in DE 3825 366 A1 and US 6,241,928.
  • Polymerization in a belt reactor produces a polymer gel that must be comminuted, for example in an extruder or kneader.
  • the comminuted polymer gel obtained by means of a kneader can be additionally extruded.
  • the polymer gel is then usually dried using a circulating air belt dryer until the residual moisture content is preferably 0.5 to 10% by weight, particularly preferably 1 to 7% by weight, very particularly preferably 2 to 5% by weight, the residual moisture content being determined according to test method no. WSP 230.2-05 "Mass Loss Upon Heating" recommended by EDANA. If the residual moisture is too high, the dried polymer gel has a glass transition temperature T g that is too low and is difficult to process further. If the residual moisture 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% by weight, particularly preferably between 35 and 70% by weight, very particularly preferably between 40 and 60% by weight. The dried polymer gel is then broken and optionally coarsely crushed.
  • 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 usually be used for grinding.
  • single- or multi-stage roller mills preferably two- or three-stage roller mills, pin mills, hammer mills or vibrating mills, can usually be used for grinding.
  • the average particle size of the polymer particles separated as a product fraction is preferably from 150 to 850 pim, particularly preferably from 250 to 600 pim, most particularly from 300 to 500 pim.
  • 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 EDANA, whereby the mass fractions of the sieve fractions are plotted cumulatively and the average particle size is determined graphically.
  • the average particle size is the value of the mesh size that results for a cumulative 50 wt.%.
  • the polymer particles are thermally surface-crosslinked to further improve their properties.
  • Suitable surface-crosslinkers are compounds that contain groups that can form covalent bonds with at least two carboxylate 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, 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 6,239,230.
  • polyvalent cations are applied to the particle surface in addition to the surface postcrosslinkers.
  • the polyvalent cations that can be used in the process according to the invention are, 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, tetravalent cations, such as the cations of titanium and zirconium.
  • Chloride, bromide, hydroxide, sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate and carboxylate, such as acetate and lactate, are possible as counterions.
  • Aluminum hydroxide, aluminum sulfate and aluminum lactate are preferred.
  • the amount of polyvalent cation used is, for example, 0.001 to 1.5% by weight, preferably 0.005 to 1% by weight, particularly preferably 0.02 to 0.8% by weight, in each case based on the polymer.
  • the surface post-crosslinking is carried out 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.
  • the spraying of a solution of the surface post-crosslinker is preferably carried out in mixers with moving mixing tools, such as screw mixers, disk mixers and paddle mixers.
  • Moving mixing tools such as screw mixers, disk mixers and paddle mixers.
  • Horizontal mixers such as paddle mixers, are particularly preferred, and vertical mixers are very particularly 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 are, for example, Horizontal Pflugschar® mixers (Gebr.
  • the surface post-crosslinkers are typically used as an aqueous solution.
  • the penetration depth of the surface post-crosslinker into the polymer particles can be adjusted via the content of non-aqueous solvent or the total amount of solvent.
  • the thermal surface post-crosslinking is carried out in contact dryers, particularly preferably paddle dryers, most preferably disc dryers.
  • Suitable dryers are, for example, Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Leingart; Germany), Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Leingart; Germany), Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; USA) and Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany).
  • the surface-crosslinked polymer particles can then be classified again, with polymer particles that are too small and/or too large being separated and returned to the process.
  • the surface-crosslinked polymer particles can be coated or moistened to further improve their properties.
  • the remoistening is preferably carried out at 30 to 80°C, particularly preferably at 35 to 70°C, very particularly 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 remoistening is preferably from 1 to 10% by weight, particularly preferably from 2 to 8% by weight, very particularly preferably from 3 to 5% by weight.
  • the remoistening increases the mechanical stability of the polymer particles and reduces their tendency to become statically charged.
  • the remoistening is advantageously carried out in the cooler after the thermal surface post-crosslinking.
  • Suitable coatings for improving the swelling rate and gel bed permeability include inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers and divalent or multivalent metal cations.
  • Suitable coatings for binding dust include polyols.
  • Suitable coatings to prevent the polymer particles from caking include pyrogenic silica such as Aerosil® 200, precipitated silica such as Sipernat® D17 and surfactants such as Span® 20.
  • measurements should be carried out at an ambient temperature of 23 ⁇ 2°C and a relative humidity of 50 ⁇ 10%.
  • the superabsorbent particles are mixed well before measurement.
  • 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.
  • a monomer solution was prepared by continuously mixing deionized water, 50 wt.% sodium hydroxide solution and acrylic acid so that the degree of neutralization corresponded to 71.0 mol%.
  • the water content of the monomer solution was 58.25 wt.%.
  • Triple ethoxylated glycerol triacrylate (approx. 85% by weight) was used as crosslinker. The amount used was 1.04 kg per t of monomer solution.
  • Citric acid was used as a complexing agent.
  • the amount used was 0.07 kg per ton of monomer solution.
  • the monomer solution was dosed 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.
  • the monomer solution was rendered inert with nitrogen. Ascorbic acid was dosed directly into the reactor.
  • the polymer gel obtained 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.
  • the conveyor belt of the circulating air belt dryer had an effective width of 4.4 m.
  • the aqueous polymer gel was continuously surrounded by an air/gas mixture (approx. 175°C) and dried.
  • the residence time in the circulating air belt dryer was approx. 37 minutes.
  • the dried polymer gel was crushed 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 of greater than 850 pim were returned to the crushing process. Polymer particles with a particle size in the range of 150 to 850 pim were thermally surface-crosslinked.
  • the polymer particles were coated with a surface post-crosslinking 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 3 .
  • the surface post-crosslinked superabsorbent particles fell over a weir into a funnel. At the lower end of the funnel was a rotary valve.
  • the funnel had a round outlet with a diameter of 25 cm at the bottom and expanded to a height of 99.8 cm to 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 approx. 3 m 3 .
  • the fill level in the funnel was at 75 to 80% held.
  • the hopper was equipped with ultrasonic discharge aids consisting of three converters of type C35-HP1 and a generator of type DGS35-200-T-EU (adams & öztas oHG, Gernsbach, Germany).
  • the converters were located on the sides and the back of the hopper approximately 60 cm above the hopper outlet.
  • the surface post-crosslinker 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).
  • 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.
  • the superabsorbent particles obtained were filled into flexible intermediate bulk containers (FIBC) and analyzed.
  • the liquid flow transmission rate (SFC) was approximately 50 x 10 7 cm 3 s/g.
  • Example 2 The procedure is as in Example 1.
  • the filling level in the funnel was 0%, ie the surface-crosslinked superabsorbent particles were immediately conveyed further.
  • the obtained superabsorbent particles were filled into Flexible Intermediate Bulk Containers (FIBC) and analyzed.
  • FIBC Flexible Intermediate Bulk Containers
  • the liquid conductance (SFC) was approximately 36 x 10 7 cm 3 s/g.

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Abstract

The present invention relates to a process for continuous thermal surface postcrosslinking of superabsorbents, wherein superabsorbent particles are coated by spray application of 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 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 fill level in the funnel is from 10% to 100%, and the thermally surface postcrosslinked superabsorbent particles are cooled in the contact dryer (2).

Description

Verfahren zur thermischen Oberflächennachvernetzung von Superabsorbern Process for thermal surface post-crosslinking of superabsorbents

Die vorliegende Erfindung betrifft ein Verfahren zur kontinuierlichen thermischen Oberflächennachvernetzung von Superabsorbern, wobei Superabsorberpartikel durch Aufsprühen einer Oberflächennachvernetzer-Lösung beschichtet werden, die beschichteten Superabsorberpartikel in einem Kontakttrockner 1 thermisch oberflächennachvernetzt werden, die den Kontakttrockner 1 verlassenden Superabsorberpartikel über einen Trichter und eine Dosiereinrichtung in einen Kontakttrockner 2 überführt werden, der Füllstand im Trichter mittels der Dosiereinrichtung geregelt wird, der Füllstand im Trichter von 10 bis 100% beträgt und die thermisch oberflächennachvernetzten Superabsorberpartikel im Kontakttrockner 2 gekühlt werden. The present invention relates to a method for the 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 surface-post-crosslinked in a contact dryer 1, the superabsorbent particles leaving the contact dryer 1 are transferred to a contact dryer 2 via a funnel and a metering device, the fill level in the funnel is regulated by means of the metering device, the fill level in the funnel is from 10 to 100% and the thermally surface-post-crosslinked superabsorbent particles are cooled in the contact dryer 2.

Superabsorber werden zur Herstellung von Windeln, Tampons, Damenbinden und anderen Hygieneartikeln, aber auch ais wasserzurückhaltende Mittel im landwirtschaftlichen Gartenbau verwendet. Die Superabsorber werden auch als wasserabsorbierende Polymere bezeichnet. Superabsorbents are used to make diapers, tampons, sanitary napkins and other hygiene products, but also as water-retaining agents in agricultural horticulture. Superabsorbents are also known as water-absorbing polymers.

Die Herstellung von Superabsorbern wird in der Monographie ''Modern Superabsorbent Polymer Technology”, F.L. Buchholz und A.T. Graham, Wiley-VCH, 1998, Seiten 71 bis 103, beschrieben. The production 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.

Zur Verbesserung der Anwendungseigenschaften, wie beispielsweise Flüssigkeitsweiterleitung (SFC - Saline Flow Conductivity) und Absorption unter einem Druck von 49,2 g/cm2 (AUHL- Absorption Under High Load), werden Superabsorberpartikel im allgemeinen oberflächennachvernetzt. Dadurch steigt der Vernetzungsgrad der Partikeloberfläche, wodurch die Absorption unter einem Druck von 49,2 g/cm2 (AUHL) und die Zentrifugenretentionskapazität (CRC - Centrifuge Retention Capacity) zumindest teilweise entkoppelt werden können. Diese Oberflächennachvernetzung kann in wässriger Gelphase durchgeführt werden. Vorzugsweise werden aber getrocknete, gemahlene und abgesiebte Polymerpartikel (Grundpolymer) an der Oberfläche mit einem Oberflächennachvernetzer beschichtet und thermisch oberflächennachvernetzt. Dazu geeignete Vernetzer sind Verbindungen, die mit mindestens zwei Car- boxylatgruppen der Polymerpartikel kovalente Bindungen bilden können. To improve the application properties, such as liquid conductivity (SFC - Saline Flow Conductivity) and absorption under a pressure of 49.2 g/cm 2 (AUHL - Absorption Under High Load), superabsorbent particles are generally surface-crosslinked. This increases the degree of crosslinking of the particle surface, whereby the absorption under a pressure of 49.2 g/cm 2 (AUHL) and the centrifuge retention capacity (CRC - Centrifuge Retention Capacity) can be at least partially decoupled. This surface-crosslinking can be carried out in an 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 are compounds that can form covalent bonds with at least two carboxylate groups of the polymer particles.

Aufgabe der vorliegenden Erfindung war die Bereitstellung eines verbesserten Verfahrens zur thermischen Oberflächennachvernetzung von Superabsorberpartikeln, insbesondere eine höhere Flüssigkeitsweiterleitung (SFC) der hergestellten Superabsorberpartikel. The object of the present invention was to provide an improved process for the thermal surface post-crosslinking of superabsorbent particles, in particular a higher liquid conductivity (SFC) of the superabsorbent particles produced.

Gelöst wurde die Aufgabe durch ein Verfahren zur kontinuierlichen thermischen Oberflächennachvernetzung von Superabsorbern, wobei Superabsorberpartikel durch Aufsprühen einer Oberflächennachvernetzer-Lösung beschichtet werden, die beschichteten Superabsorberpartikel in einem Kontakttrockner 1 thermisch oberflächennachvernetzt werden und die thermisch oberflächennachvernetzten Superabsorberpartikel in einem Kontakttrockner 2 gekühlt werden, dadurch gekennzeichnet, dass die den Kontakttrockner 1 verlassenden Superabsorberpartikel über einen Trichter und eine Dosiereinrichtung in den Kontakttrockner 2 überführt werden, der Füllstand im Trichter mittels der Dosiereinrichtung geregelt wird und der Füllstand im Trichter von 10 bis 100% beträgt. Für das erfindungsgemäße kontinuierliche Verfahren geeignete Kontakttrockner sind beispielsweise Schaufeltrock- ner und Scheibentrockner. Beim Kontakttrockner werden die zu trocknenden Güter an einer beheizten Oberfläche mittels eines dynamischen Werkzeuges entlanggeführt und umgeschichtet. Kontakttrockner können auch zum Kühlen eingesetzt werden. The object was achieved by a method for the 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 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 to the contact dryer 2 via a funnel and a metering device, the fill level in the funnel is regulated by means of the metering device and the fill level in the funnel is from 10 to 100%. Contact dryers suitable for the continuous process according to the invention are, for example, paddle dryers and disk dryers. In contact dryers, the goods to be dried are guided along a heated surface using a dynamic tool and are then rearranged. Contact dryers can also be used for cooling.

Zur gleichmäßigeren Entleerung des Trichters können Austraghilfen eingesetzt werden. Die Austragshilfen unterliegen keiner Beschränkung. Geeignete Austragshilfen sind beispielsweise Vibratoren, Klopfer und Pulsatoren. Vibratoren versetzen die Trichterwand in Schwingungen. Klopfer versetzen die Trichterwand durch gezielte Stöße in Schwingungen. In beiden Fällen wird der Superabsorber im Trichter dadurch aufgelockert und etwaige Brücken stürzen ein. Pulsatoren lockern die Superabsorber im Trichter durch kurze Druckstöße. Bevorzugt werden aber Ultra- schall-Austragshilfen verwendet. Dabei wird der Trichter mittels Ultraschalls in kaum hörbare Schwingungen versetzt. Discharge aids can be used to empty the hopper more evenly. There are no restrictions on the discharge aids. Suitable discharge aids include vibrators, knockers and pulsators. Vibrators cause the hopper wall to vibrate. Knockers cause the hopper wall to vibrate by means of targeted impacts. In both cases, the superabsorbent in the hopper is loosened and any bridges collapse. Pulsators loosen the superabsorbent in the hopper by means of short pressure pulses. However, ultrasonic discharge aids are preferred. The hopper is set into barely audible vibrations using ultrasound.

Die Austragshilfen befinden sich üblicherweise gleichmäßig verteilt im unteren Drittel des Trichters. Die Verwendung von drei Austragshilfen ist in den meisten Fällen ausreichend. The discharge aids are usually evenly distributed in the lower third of the hopper. The use of three discharge aids is sufficient in most cases.

Eine geeignete Dosiereinrichtung ist beispielsweise eine Förderschnecke oder eine Zellradschleuse. Eine Zellradschleuse besteht aus einem Rotor, der sich in einem genau passenden Gehäuse dreht. Auf dem Rotor befindet sich eine bestimmte Anzahl von Rotorblättern, wodurch einzelne Rotorzellen entstehen. Jede Rotorzelle nimmt unter der Einlassöffnung Fördergut auf und am Austritt fällt das Fördergut heraus. So entsteht eine volumetrisch kontinuierliche Förderung. Die Förderleistung wird durch das Volumen der Rotorzellen und die Drehzahl des Rotors bestimmt. A suitable dosing device is, for example, a screw conveyor or a rotary valve. A rotary valve consists of a rotor that rotates in a precisely fitting housing. There are a certain number of rotor blades on the rotor, which create individual rotor cells. Each rotor cell takes in material to be conveyed under the inlet opening and the material falls out at the outlet. This creates a volumetrically continuous conveying process. The conveying capacity is determined by the volume of the rotor cells and the speed of the rotor.

Der Füllstand im Trichter beträgt vorzugsweise von 20 bis 95%, bevorzugt von 30 bis 90%, besonders bevorzugt von 40 bis 85%, ganz besonders bevorzugt von 50 bis 80%. Der Füllstand bezieht sich auf das Innenvolumen des Trichters. The fill level in the funnel is 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 funnel.

Das Innenvolumen des Trichters beträgt vorzugsweise von 0,01 bis 0,5 m3, bevorzugt von 0,02 bis 0,4 m3, besonders bevorzugt von 0,05 bis 0,3 m3, ganz besonders bevorzugt von 0,1 bis 0,2 m3, jeweils pro m3 Innenvolumen des Kontakttrockners 1 . 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 , very particularly preferably from 0.1 to 0.2 m 3 , in each case per m 3 internal volume of the contact dryer 1 .

Der vorliegenden Erfindung liegt die Erkenntnis zugrunde, dass der Füllstand im Trichter zwischen dem Kontakttrockner 1 und dem Kontakttrockner 2 für die Eigenschaften der hergestellten Superabsorber, insbesondere eine hohe Flüssigkeitsweiterleitung (SFC), wichtig ist. The present invention is based on the finding that the filling level in the funnel between the contact dryer 1 and the contact dryer 2 is important for the properties of the superabsorbents produced, in particular a high liquid transfer factor (SFC).

Der mittlere Tropfendurchmesser beim Aufsprühen der Oberflächennachvernetzer-Lösung beträgt beispielsweise 200 bis 4.500 pim, vorzugsweise 300 bis 3.500 pim, besonders bevorzugt von 400 bis 2.500 pim, ganz besonders bevorzugt von 500 bis 1.500 pim. Der mittlere Tropfendurchmesser kann durch Lichtstreuung bestimmt werden. Die Temperatur der Superabsorberpartikel beim Aufsprühen der Oberflächennachvernetzer-Lösung beträgt vorzugsweise von 30 bis 80°C, besonders bevorzugt von 35 bis 75°C, ganz besonders bevorzugt von 40 bis 70°C. The average droplet diameter when spraying the surface postcrosslinker solution is, for example, 200 to 4,500 pim, preferably 300 to 3,500 pim, particularly preferably 400 to 2,500 pim, very particularly preferably 500 to 1,500 pim. The average droplet diameter can be determined by light scattering. The temperature of the superabsorbent particles when spraying on 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.

Die Oberflächennachvernetzer-Lösung enthält vorzugsweise von 0,001 bis 2 Gew.-%, besonders bevorzugt von 0,01 bis 1 Gew.-%, ganz besonders bevorzugt von 0,03 bis 0,7 Gew.-%, eines Oberflächennachvernetzers, jeweils bezogen auf die Superabsorberpartikel. Die Oberflächennachvernetzer-Lösung enthält weiterhin vorzugsweise von 0,5 bis 5 Gew.-%. Besonders bevorzugt von 1 ,0 bis 4 Gew.-%, ganz besonders bevorzugt von 1 ,5 bis 3 Gew.-%, Wasser, jeweils bezogen auf die Superabsorberpartikel. The surface postcrosslinker solution preferably contains from 0.001 to 2% by weight, particularly preferably from 0.01 to 1% by weight, very particularly 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 also preferably contains from 0.5 to 5% by weight, particularly preferably from 1.0 to 4% by weight, very particularly preferably from 1.5 to 3% by weight, of water, based in each case on the superabsorbent particles.

Die Superabsorberpartikel werden im Kontakttrockner 1 auf eine Temperatur von vorzugsweise 110 bis 220°C, besonders bevorzugt von 120 bis 210°C, ganz besonders bevorzugt von 130 bis 200°C, erwärmt. Die Verweilzeit der Superabsorberpartikel im Kontakttrockner 1 beträgt vorzugsweise von 10 bis 60 Minuten, besonders bevorzugt von 15 bis 50 Minuten, ganz besonders bevorzugt von 20 bis 40 Minuten. The superabsorbent particles are heated in the contact dryer 1 to a temperature of preferably 110 to 220°C, particularly preferably 120 to 210°C, very particularly preferably 130 to 200°C. The residence time of the superabsorbent particles in the contact dryer 1 is preferably from 10 to 60 minutes, particularly preferably from 15 to 50 minutes, very particularly preferably from 20 to 40 minutes.

Der Kontakttrockner 1 und die Verbindung zum Kontakttrockner 2 können begleitbeheizt und/oder thermisch isoliert werden. The contact dryer 1 and the connection to the contact dryer 2 can be trace heated and/or thermally insulated.

Die Superabsorberpartikel werden im Kontakttrockner 2 auf eine Temperatur von vorzugsweise 30 bis 80°C, besonders bevorzugt 35 bis 70°C, ganz besonders bevorzugt 40 bis 60°C, gekühlt. Die Verweilzeit der Superabsorberpartikel im Kontakttrockner 2 beträgt vorzugsweise von 10 bis 60 Minuten, besonders bevorzugt von 15 bis 50 Minuten, ganz besonders bevorzugt von 20 bis 40 Minuten. The superabsorbent particles are cooled in the contact dryer 2 to a temperature of preferably 30 to 80°C, particularly preferably 35 to 70°C, very particularly preferably 40 to 60°C. The residence time of the superabsorbent particles in the contact dryer 2 is preferably from 10 to 60 minutes, particularly preferably from 15 to 50 minutes, very particularly preferably from 20 to 40 minutes.

In einer bevorzugten Ausführungsform der vorliegenden Erfindung wird ein Gasstrom mit einem Sauerstoffanteil von weniger als 10 Vol.-% in den Kontakttrockner 1 geleitet. Der Gasstrom beträgt beispielsweise von 5 bis 60 Nm3/h, vorzugsweise von 10 bis 50 Nm3/h, besonders bevorzugt von 15 bis 40 Nm3/h, ganz besonders bevorzugt von 20 bis 30 Nm3/h, jeweils pro m3 Innenvolumen des Kontakttrockners 1. Ein Nm3 entspricht einem Gasvolumen von 1 m3 bei 273, 15 K und 1.013,25 hPa. Ein Abgasstrom wird aus den Kontakttrockner 1 herausgeführt. Der Abgasstrom wird im Kontakttrockner 1 um mindestens 75° von der horizontalen Produktstromrichtung nach oben abgelenkt. Die Gasgeschwindigkeit des Abgasstroms direkt nach der Umlenkung beträgt vorzugsweise weniger als 5 m/s, besonders bevorzugt weniger als 2 m/s, ganz besonders bevorzugt weniger als 1 m/s. In a preferred embodiment of the present invention, a gas stream with an oxygen content of less than 10 vol. % is passed 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, very particularly preferably from 20 to 30 Nm 3 /h, in each case 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 led out of 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, particularly preferably less than 2 m/s, very particularly preferably less than 1 m/s.

Im Folgenden wird die Herstellung der Superabsorber näher erläutert: The production of superabsorbents is explained in more detail below:

Die Superabsorber werden durch Polymerisation einer Monomerlösung hergestellt und sind üblicherweise wasserunlöslich. Die ethylenisch ungesättigten, säuregruppentragenden Monomere sind vorzugsweise wasserlöslich, d.h. die Löslichkeit in Wasser bei 23°C beträgt typischerweise mindestens 1 g/100 g Wasser, vorzugsweise mindestens 5 g/100 g Wasser, besonders bevorzugt mindestens 25 g/100 g Wasser, ganz besonders bevorzugt mindestens 35 g/100 g Wasser. The superabsorbents are produced by polymerization of 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, particularly preferably at least 25 g/100 g water, very particularly preferably at least 35 g/100 g water.

Geeignete Monomere sind beispielsweise ethylenisch ungesättigte Carbonsäuren, wie Acrylsäure, Methacrylsäure, und Itaconsäure. Besonders bevorzugte Monomere sind Acrylsäure und Methacrylsäure. Ganz besonders bevorzugt ist Acrylsäure. Suitable monomers are, 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 particularly preferred.

Die ethylenisch ungesättigten, säuregruppentragenden Monomere sind üblicherweise teilweise neutralisiert. Die Neutralisation wird auf der Stufe der Monomeren durchgeführt. Dies geschieht üblicherweise durch Einmischung des Neutralisationsmittels als wässrige Lösung oder bevorzugt auch als Feststoff. Der Neutralisationsgrad beträgt vorzugsweise von 40 bis 85 mol-%, besonders bevorzugt von 50 bis 80 mol-%, ganz besonders bevorzugt von 60 bis 75 mol-%, wobei die üblichen Neutralisationsmittel verwendet werden können, vorzugsweise Alkalimetallhydroxide, Alkalimetalloxide, Alkalimetallkarbonate oder Alkalimetallhydrogenkarbonate sowie deren Mischungen. Statt Alkalimetallsalzen können auch Ammoniumsalze verwendet werden. Natrium und Kalium sind als Alkalimetalle besonders bevorzugt, ganz besonders bevorzugt sind jedoch Natriumhydroxid, Natriumkarbonat oder Natriumhydrogenkarbonat sowie deren Mischungen, insbesondere Natriumhydroxid. The ethylenically unsaturated monomers carrying acid groups are usually partially neutralized. The neutralization is carried out at the monomer stage. This is usually done by mixing in the neutralizing agent as an aqueous solution or preferably also as a solid. The degree of neutralization is preferably from 40 to 85 mol%, particularly preferably from 50 to 80 mol%, very particularly preferably from 60 to 75 mol%, it being possible to use the usual neutralizing agents, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal hydrogen carbonates 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 particularly preferred are sodium hydroxide, sodium carbonate or sodium hydrogen carbonate and mixtures thereof, in particular sodium hydroxide.

Die Monomere enthalten üblicherweise Polymerisationsinhibitoren, vorzugsweise Hydrochinonhalbether, als Lagerstabilisator. The monomers usually contain polymerization inhibitors, preferably hydroquinone hemiether, as storage stabilizers.

Geeignete Vernetzer sind Verbindungen mit mindestens zwei zur Vernetzung geeigneten Gruppen. Derartige Gruppen sind beispielsweise ethylenisch ungesättigte Gruppen, die in die Polymerkette radikalisch einpolymerisiert werden können, und funktionelle Gruppen, die mit den Säuregruppen des Monomers kovalente Bindungen ausbilden können. Weiterhin sind auch polyvalente Metallsalze, die mit mindestens zwei Säuregruppen des Monomeren koordinative Bindungen ausbilden können, als Vernetzer geeignet. Suitable crosslinkers are compounds with at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups that can be radically polymerized into the polymer chain and functional groups that can form covalent bonds with the acid groups of the monomer. Furthermore, polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer are also suitable as crosslinkers.

Geeignete Vernetzer sind beispielsweise Ethylenglykoldimethacrylat, Diethylenglykoldiacrylat, Polyethylenglykoldiac- rylat, Allylmethacrylat, Trimethylolpropantriacrylat, Triallylamin, Tetraallylammoniumchlorid, Tetraallyloxyethan, wie in EP 0 530 438 A1 beschrieben, Di- und Triacrylate, wie 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 und DE 103 31 450 A1 beschrieben, gemischte Acrylate, die neben Acrylatgruppen weitere ethylenisch ungesättigte Gruppen enthalten, wie in DE 103 31 456 A1 und DE 10355 401 A1 beschrieben, oder Vernetzermischungen, wie beispielsweise in DE 195 43368 A1, DE 196 46 484 A1, WO 90/15830 A1 und WO 02/032962 A2 beschrieben. Die Menge an Vernetzer beträgt vorzugsweise 0,05 bis 1,5 Gew.-%, besonders bevorzugt 0,1 bis 1 Gew.-%, ganz besonders bevorzugt 0,15 bis 0,6 Gew.-%, jeweils berechnet auf die Gesamtmenge an eingesetztem Monomer. Mit steigendem Vernetzergehalt sinkt die Zentrifugenretentionskapazität (CRC) und die Absorption unter einem Druck von 21,0 g/cm2 (AUL) durchläuft ein Maximum. 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 contain other ethylenically unsaturated groups in addition to acrylate groups, as in DE 103 31 456 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. The amount of crosslinker is preferably 0.05 to 1.5% by weight, particularly preferably 0.1 to 1% by weight, very particularly preferably 0.15 to 0.6% by weight, in each case calculated on the total amount of monomer used. As the crosslinker content increases, the centrifuge retention capacity (CRC) decreases and the absorption under a pressure of 21.0 g/cm 2 (AUL) passes through a maximum.

Als Initiatoren können sämtliche unter den Polymerisationsbedingungen Radikale erzeugende Verbindungen eingesetzt werden, beispielsweise thermische Initiatoren, Redox-Initiatoren, Photoinitiatoren. Geeignete Redox-Initiatoren sind Natriumperoxodisulfat/Ascorbinsäure, Wasserstoffperoxid/Ascorbinsäure, Natriumperoxodisulfat/Natriumbisulfit und Wasserstoffperoxid/Natriumbisulfit. Vorzugsweise werden Mischungen aus thermischen Initiatoren und Redox- Initiatoren eingesetzt, wie Natriumperoxodisulfat/Wasserstoffperoxid/Ascorbinsäure. Als reduzierende Komponente wird vorzugsweise das Dinatriumsalz der 2-Hydroxy-2-sulfonatoessig-säure oder ein Gemisch aus dem Natriumsalz der 2-Hydroxy-2-sulfinatoessigsäure, dem Dinatriumsalz der 2-Hydroxy-2-sulfonatoessigsäure und Natriumbisulfit eingesetzt. Derartige Gemische sind als Brüggolite® FF6 und Brüggolite® FF7 (Brüggemann Chemicals; Heilbronn; Deutschland) erhältlich. All compounds that generate 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. Mixtures of thermal initiators and redox initiators are preferably used, such as sodium peroxodisulfate/hydrogen peroxide/ascorbic acid. 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).

Der Wassergehalt der Monomerlösung beträgt vorzugsweise von 40 bis 75 Gew.-%, besonders bevorzugt von 45 bis 70 Gew.-%, ganz besonders bevorzugt von 50 bis 65 Gew.-%. Mit steigendem Wassergehalt steigt der Energieaufwand bei der anschließenden Trocknung und mit sinkendem Wassergehalt kann die Polymerisationswärme nur noch ungenügend abgeführt werden. The water content of the monomer solution is preferably from 40 to 75% by weight, particularly preferably from 45 to 70% by weight, very particularly preferably from 50 to 65% by weight. As the water content increases, the energy required for the subsequent drying increases, and as the water content decreases, the heat of polymerization can only be dissipated insufficiently.

Die Temperatur der Monomerlösung beträgt vorzugsweise von 10 bis 90°C, besonders bevorzugt von 20 bis 70°C, ganz besonders bevorzugt von 30 bis 50°C. The temperature of the monomer solution is preferably from 10 to 90°C, particularly preferably from 20 to 70°C, most preferably from 30 to 50°C.

Die bevorzugten Polymerisationsinhibitoren benötigen für eine optimale Wirkung gelösten Sauerstoff. Daher kann die Monomerlösung vor der Polymerisation durch Inertisierung, d.h. Durchströmen mit einem inerten Gas, vorzugsweise Stickstoff oder Kohlendioxid, von gelöstem Sauerstoff befreit werden. Vorzugsweise wird der Sauerstoffgehalt der Monomerlösung vor der Polymerisation auf weniger als 1 Gew.-ppm, besonders bevorzugt auf weniger als 0,5 Gew.- ppm, ganz besonders bevorzugt auf weniger als 0,1 Gew.-ppm, gesenkt. The preferred polymerization inhibitors require dissolved oxygen for optimal effect. Therefore, the monomer solution can be freed of dissolved oxygen before polymerization by inerting, i.e. by passing an inert gas, preferably nitrogen or carbon dioxide, through it. Preferably, the oxygen content of the monomer solution before polymerization is reduced to less than 1 ppm by weight, particularly preferably to less than 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.

Geeignete Reaktoren für die Polymerisation sind beispielsweise Knetreaktoren oder Bandreaktoren. Im Kneter wird das bei der Polymerisation einer wässrigen Monomerlösung oder -suspension entstehende Polymergel durch beispielsweise gegenläufige Rührwellen kontinuierlich zerkleinert, wie in WO 2001/038402 A1 beschrieben. Die Polymerisation auf dem Band wird beispielsweise in DE 3825 366 A1 und US 6,241,928 beschrieben. Bei der Polymerisation in einem Bandreaktor entsteht ein Polymergel, das zerkleinert werden muss, beispielsweise in einem Extruder oder Kneter. Zur Verbesserung der Trocknungseigenschaften kann das mittels eines Kneters erhaltene zerkleinerte Polymergel zusätzlich extrudiert werden. Suitable reactors for polymerization are, for example, 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 stirring shafts, as described in WO 2001/038402 A1. Polymerization on the belt is described, for example, in DE 3825 366 A1 and US 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 comminuted polymer gel obtained by means of a kneader can be additionally extruded.

Das Polymergel wird dann üblicherweise mit einem Umluftbandtrockner getrocknet bis der Restfeuchtegehalt vorzugsweise 0,5 bis 10 Gew.-%, besonders bevorzugt 1 bis 7 Gew.-%, ganz besonders bevorzugt 2 bis 5 Gew.-%, beträgt, wobei der Restfeuchtegehalt gemäß der von der EDANA empfohlenen Testmethode Nr. WSP 230.2-05 "Mass Loss Upon Heating" bestimmt wird. Bei einer zu hohen Restfeuchte weist das getrocknete Polymergel eine zu niedrige Glasübergangstemperatur Tg auf und ist nur schwierig weiter zu verarbeiten. Bei einer zu niedrigen Restfeuchte ist das getrocknete Polymergel zu spröde und in den anschließenden Zerkleinerungsschritten fallen unerwünscht große Mengen an Polymerpartikeln mit zu niedriger Partikelgröße („fines") an. Der Feststoffgehalt des Polymergels beträgt vor der Trocknung vorzugsweise von 25 und 90 Gew.-%, besonders bevorzugt von 35 bis 70 Gew.-%, ganz besonders bevorzugt von 40 bis 60 Gew.-%. Anschließend wird das getrocknete Polymergel gebrochen und optional grob zerkleinert. The polymer gel is then usually dried using a circulating air belt dryer until the residual moisture content is preferably 0.5 to 10% by weight, particularly preferably 1 to 7% by weight, very particularly preferably 2 to 5% by weight, the residual moisture content being determined according to test method no. WSP 230.2-05 "Mass Loss Upon Heating" recommended by EDANA. If the residual moisture is too high, the dried polymer gel has a glass transition temperature T g that is too low and is difficult to process further. If the residual moisture 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% by weight, particularly preferably between 35 and 70% by weight, very particularly preferably between 40 and 60% by weight. The dried polymer gel is then broken and optionally coarsely crushed.

Das getrocknete Polymergel wird hiernach üblicherweise gemahlen und klassiert, wobei zur Mahlung üblicherweise ein- oder mehrstufige Walzenstühle, bevorzugt zwei- oder dreistufige Walzenstühle, Stiftmühlen, Hammermühlen oder Schwingmühlen, eingesetzt werden können. 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 usually be used for grinding.

Die mittlere Partikelgröße der als Produktfraktion abgetrennten Polymerpartikel beträgt vorzugsweise von 150 bis 850 pim, besonders bevorzugt von 250 bis 600 pim, ganz besonders von 300 bis 500 pim. Die mittlere Partikelgröße der Produktfraktion kann mittels der von der EDANA empfohlenen Testmethode Nr. WSP 220.2 (05) "Partikel Size Distribution" ermittelt werden, wobei die Massenanteile der Siebfraktionen kumuliert aufgetragen werden und die mittlere Partikelgröße graphisch bestimmt wird. Die mittlere Partikelgröße ist hierbei der Wert der Maschenweite, der sich für kumulierte 50 Gew.-% ergibt. The average particle size of the polymer particles separated as a product fraction is preferably from 150 to 850 pim, particularly preferably from 250 to 600 pim, most particularly from 300 to 500 pim. 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 EDANA, whereby the mass fractions of the sieve fractions are plotted cumulatively and the average particle size is determined graphically. The average particle size is the value of the mesh size that results for a cumulative 50 wt.%.

Die Polymerpartikel werden zur weiteren Verbesserung der Eigenschaften thermisch oberflächennachvernetzt. Geeignete Oberflächennachvernetzer sind Verbindungen, die Gruppen enthalten, die mit mindestens zwei Carboxylat- gruppen der Polymerpartikel kovalente Bindungen bilden können. Geeignete Verbindungen sind beispielsweise polyfunktionelle Amine, polyfunktionelle Amidoamine, polyfunktionelle Epoxide, wie in EP 0 083 022 A2, EP 0 543 303 A1 und EP 0 937 736 A2 beschrieben, di- oder polyfunktionelle Alkohole, wie in DE 33 14 019 A1, DE 35 23 617 A1 und EP 0 450 922 A2 beschrieben, oder ß-Hydroxyalkylamide, wie in DE 102 04 938 A1 und US 6,239,230 beschrieben. The polymer particles are thermally surface-crosslinked to further improve their properties. Suitable surface-crosslinkers are compounds that contain groups that can form covalent bonds with at least two carboxylate 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, 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 6,239,230.

In einer bevorzugten Ausführungsform der vorliegenden Erfindung werden zusätzlich zu den Oberflächennachvernetzern polyvalente Kationen auf die Partikeloberfläche aufgebracht. Die im erfindungsgemäßen Verfahren einsetzbaren polyvalenten Kationen sind beispielsweise zweiwertige Kationen, wie die Kationen von Zink, Magnesium, Kalzium und Strontium, dreiwertige Kationen, wie die Kationen von Aluminium, Eisen, Chrom, Seltenerden und Mangan, vierwertige Kationen, wie die Kationen von Titan und Zirkonium. Als Gegenion sind Chlorid, Bromid, Hydroxid, Sulfat, Hydrogensulfat, Carbonat, Hydrogencarbonat, Nitrat, Phosphat, Hydrogenphosphat, Dihydrogenphosphat und Carboxylat, wie Acetat und Lactat, möglich. Aluminiumhydroxid, Aluminiumsulfat und Aluminiumlaktat sind bevorzugt. 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 that can be used in the process according to the invention are, 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, tetravalent cations, such as the cations of titanium and zirconium. Chloride, bromide, hydroxide, sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate and carboxylate, such as acetate and lactate, are possible as counterions. Aluminum hydroxide, aluminum sulfate and aluminum lactate are preferred.

Die Einsatzmenge an polyvalentem Kation beträgt beispielsweise 0,001 bis 1 ,5 Gew.-%, vorzugsweise 0,005 bis 1 Gew.-%, besonders bevorzugt 0,02 bis 0,8 Gew.-%. jeweils bezogen auf das Polymer. The amount of polyvalent cation used is, for example, 0.001 to 1.5% by weight, preferably 0.005 to 1% by weight, particularly preferably 0.02 to 0.8% by weight, in each case based on the polymer.

Die Oberflächennachvernetzung wird so durchgeführt, dass eine Lösung des Oberflächennachvernetzers auf die getrockneten Polymerpartikel aufgesprüht wird. Im Anschluss an das Aufsprühen werden die mit Oberflächennachvernetzer beschichteten Polymerpartikel thermisch behandelt. The surface post-crosslinking is carried out 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.

Das Aufsprühen einer Lösung des Oberflächennachvernetzers wird vorzugsweise in Mischern mit bewegten Mischwerkzeugen, wie Schneckenmischer, Scheibenmischer und Schaufelmischer, durchgeführt. Besonders bevorzugt sind Horizontalmischer, wie Schaufelmischer, ganz besonders bevorzugt sind Vertikalmischer. Die Unterscheidung in Horizontalmischer und Vertikalmischer erfolgt über die Lagerung der Mischwelle, d.h. Horizontalmischer haben eine horizontal gelagerte Mischwelle und Vertikalmischer haben eine vertikal gelagerte Mischwelle. Geeignete Mischer sind beispielsweise Horizontale Pflugschar® Mischer (Gebr. Lödige Maschinenbau GmbH; Paderborn; Deutschland), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Doetinchem; Niederlande), Processall Mixmill Mixer (Proces- sall Incorporated; Cincinnati; USA) und Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; Niederlande). Es ist aber auch möglich die Oberflächennachvernetzerlösung in einem Wirbelbett aufzusprühen. The spraying of a solution of the surface post-crosslinker is preferably carried out in mixers with moving mixing tools, such as screw mixers, disk mixers and paddle mixers. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are very particularly 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 are, 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). However, it is also possible to spray the surface post-crosslinker solution in a fluidized bed.

Die Oberflächennachvernetzer werden typischerweise als wässrige Lösung eingesetzt. Über den Gehalt an nichtwässrigem Lösungsmittel bzw. Gesamtlösungsmittelmenge kann die Eindringtiefe des Oberflächennachvernetzers in die Polymerpartikel eingestellt werden. The surface post-crosslinkers are typically used as an aqueous solution. The penetration depth of the surface post-crosslinker into the polymer particles can be adjusted via the content of non-aqueous solvent or the total amount of solvent.

Die thermische Oberflächennachvernetzung wird in Kontakttrocknern, besonders bevorzugt Schaufeltrocknern, ganz besonders bevorzugt Scheibentrocknern, durchgeführt. Geeignete Trockner sind beispielsweise Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Leingarten; Deutschland), Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Leingarten; Deutschland), Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; USA) und Nara Paddle Dryer (NARA Machinery Europe; Frechen; Deutschland). The thermal surface post-crosslinking is carried out in contact dryers, particularly preferably paddle dryers, most preferably disc dryers. Suitable dryers are, 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).

Anschließend können die oberflächennachvernetzten Polymerpartikel erneut klassiert werden, wobei zu kleine und/oder zu große Polymerpartikel abgetrennt und in das Verfahren rückgeführt werden. Die oberflächennachvernetzten Polymerpartikel können zur weiteren Verbesserung der Eigenschaften beschichtet oder nachbefeuchtet werden. The surface-crosslinked polymer particles can then be classified again, with polymer particles that are too small and/or too large being separated and returned to the process. The surface-crosslinked polymer particles can be coated or moistened to further improve their properties.

Die Nachbefeuchtung wird vorzugsweise bei 30 bis 80°C, besonders bevorzugt bei 35 bis 70°C, ganz besonders bevorzugt bei 40 bis 60°C, durchgeführt. Bei zu niedrigen Temperaturen neigen die Polymerpartikel zum Verklumpen und bei höheren Temperaturen verdampft bereits merklich Wasser. Die zur Nachbefeuchtung eingesetzte Wassermenge beträgt vorzugsweise von 1 bis 10 Gew.-%, besonders bevorzugt von 2 bis 8 Gew.-%, ganz besonders bevorzugt von 3 bis 5 Gew.-%. Durch die Nachbefeuchtung wird die mechanische Stabilität der Polymerpartikel erhöht und deren Neigung zur statischen Aufladung vermindert. Vorteilhaft wird die Nachbefeuchtung im Kühler nach der thermischen Oberflächennachvernetzung durchgeführt. The remoistening is preferably carried out at 30 to 80°C, particularly preferably at 35 to 70°C, very particularly 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 remoistening is preferably from 1 to 10% by weight, particularly preferably from 2 to 8% by weight, very particularly preferably from 3 to 5% by weight. The remoistening increases the mechanical stability of the polymer particles and reduces their tendency to become statically charged. The remoistening is advantageously carried out in the cooler after the thermal surface post-crosslinking.

Geeignete Beschichtungen zur Verbesserung der Quellgeschwindigkeit sowie der Gelbettpermeabilität (GBP) sind beispielsweise anorganische inerte Substanzen, wie wasserunlösliche Metallsalze, organische Polymere, kationische Polymere sowie zwei- oder mehrwertige Metallkationen. Geeignete Beschichtungen zur Staubbindung sind beispielsweise Polyole. Geeignete Beschichtungen gegen die unerwünschte Verbackungsneigung der Polymerpartikel sind beispielsweise pyrogene Kieselsäure, wie Aerosil® 200, Fällungskieselsäure, wie Sipernat® D17, und Tenside, wie Span® 20. 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 binding dust include polyols. Suitable coatings to prevent the polymer particles from caking include pyrogenic silica such as Aerosil® 200, precipitated silica such as Sipernat® D17 and surfactants such as Span® 20.

Methoden: Methods:

Die Messungen sollten, wenn nicht anders angegeben, bei einer Umgebungstemperatur von 23 ± 2°C und einer relativen Luftfeuchte von 50 ± 10% durchgeführt werden. Die Superabsorberpartikel werden vor der Messung gut durchmischt.

Figure imgf000009_0001
Unless otherwise stated, measurements should be carried out at an ambient temperature of 23 ± 2°C and a relative humidity of 50 ± 10%. The superabsorbent particles are mixed well before measurement.
Figure imgf000009_0001

Die Flüssigkeitsweiterleitung (SFC) wird gemäß der in der EP 2 535 698 A1 auf den Seiten 19 bis 22 beschrieben Testmethode „Urine Permeability Measurement (UPM) Test method" bestimmt. Beispiele 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

Beispiel 1 (erfindungsgemäß) Example 1 (according to the invention)

Durch kontinuierliches Mischen von entionisiertem Wasser, 50 gew.-%iger Natronlauge und Acrylsäure wurde eine Monomerlösung hergestellt, so dass der Neutralisationsgrad 71,0 mol-% entsprach. Der Wassergehalt der Monomerlösung betrug 58,25 Gew.-%. A monomer solution was prepared by continuously mixing deionized water, 50 wt.% sodium hydroxide solution and acrylic acid so that the degree of neutralization corresponded to 71.0 mol%. The water content of the monomer solution was 58.25 wt.%.

Als Vernetzer wurde 3-fach ethoxiliertes Glyzerintriacryl at (ca. 85 gew.-%ig) verwendet. Die Einsatzmenge betrug 1,04 kg pro t Monomerlösung. Triple ethoxylated glycerol triacrylate (approx. 85% by weight) was used as crosslinker. The amount used was 1.04 kg per t of monomer solution.

Als Komplexbildner wurde Zitronensäure eingesetzt. Die Einsatzmenge betrug 0,07 kg pro t Monomerlösung. Citric acid was used as a complexing agent. The amount used was 0.07 kg per ton of monomer solution.

Zur Initiierung der radikalischen Polymerisation wurden pro t Monomerlösung 1 ,38 kg einer 0,25gew.-%igen wässriger Wasserstoffperoxid-Lösung, 3,22 kg einer 15gew.-%igen wässrigen Natriumperoxodisulfat-Lösung und 1,04 kg einer 1gew.-%igen wässrigen Ascorbinsäure-Lösung eingesetzt. 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.

Die Monomerlösung wurde in einen Reaktor vom Typ List Contikneter mit einem Volumen 6,3m3 (LIST AG, Arisdorf, Schweiz) dosiert. Der Durchsatz der Monomerlösung betrug ca. 22,5 t/h. Die Reaktionslösung hatte am Zulauf eine Temperatur von 30°C. The monomer solution was dosed 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.

Zwischen dem Zugabepunkt für den Vernetzer und den Zugabestellen für die Wasserstoffperoxid- und Natriumper- oxodisulfatlösungen wurde die Monomerlösung mit Stickstoff inertisiert. Ascorbinsäure wurde direkt in den Reaktor dosiert. Between the addition point for the crosslinker and the addition points for the hydrogen peroxide and sodium peroxodisulfate solutions, the monomer solution was rendered inert with nitrogen. Ascorbic acid was dosed directly into the reactor.

Nach ca. 50% der Verweilzeit wurden zusätzlich ca. 1.000 kg/h durch Zerkleinerung und Klassierung im Herstellungsprozess anfallende Polymerpartikel mit einer Partikelgröße von weniger als 150 pim in den Reaktor dosiert. Die Verweilzeit der Reaktionsmischung im Reaktor betrug ca. 15 Minuten. After about 50% of the residence time, an additional 1,000 kg/h of polymer particles with a particle size of less than 150 pim that had been produced during the production process by crushing and classifying were metered into the reactor. The residence time of the reaction mixture in the reactor was about 15 minutes.

Das erhaltene Polymergel wurde mittels eines oszillierenden Förderbandes auf das Förderband eines Umluftbandtrockners aufgegeben. Der Umluftbandtrockner hatte eine Länge von 48 m. Das Förderband des Umluftbandtrockners hatte eine effektive Breite von 4,4 m. Auf dem Umluftbandtrockner wurde das wässrige Polymergel kontinuierlich mit Luft/Gasgemisch (ca. 175°C) umströmt und getrocknet. Die Verweilzeit im Umluftbandtrockner betrug ca. 37 Minuten. The polymer gel obtained 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. 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 surrounded by an air/gas mixture (approx. 175°C) and dried. The residence time in the circulating air belt dryer was approx. 37 minutes.

Das getrocknete Polymergel wurde mittels eines dreistufigen Walzenstuhls zerkleinert und auf eine Partikelgröße von 150 bis 850 pim abgesiebt. Polymerpartikel mit einer Partikelgröße von weniger als 150 pim wurden abgetrennt. Polymerpartikel mit einer Partikelgröße von größer 850 pim wurden in die Zerkleinerung zurückgeführt. Polymerpartikel mit einer Partikelgröße im Bereich von 150 bis 850 pim wurden thermisch oberflächennachvernetzt. The dried polymer gel was crushed 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 of greater than 850 pim were returned to the crushing process. Polymer particles with a particle size in the range of 150 to 850 pim were thermally surface-crosslinked.

Die Polymerpartikel wurden in einem Schugi Flexomix® (Hosokawa Micron B.V., Doetinchem, Niederlande mit einer Oberflächennachvernetzerlösung beschichtet und anschließend in einem NARA Paddle Dryer (GMF Gouda, Wad- dinxveen, Niederlande) 45 Minuten bei 186,5°C getrocknet. Der NARA Paddle Dryer hatte ein Innenvolumen von 18,8 m3. Die oberflächennachvernetzten Superabsorberpartikel fielen über ein Wehr in einen Trichter. Am unteren Ende des Trichters war eine Zellradschleuse. Der Trichter hatte unten einen runden Auslass mit einem Durchmesser von 25 cm und erweiterte sich bis zu einer Höhe von 99,8 cm auf einen rechteckigen Querschnitt von 200 cm x 85 cm. Der Trichter endete nach weiteren 122 cm mit konstantem Querschnitt von 200 cm x 85 cm. Der Trichter hatte ein Innenvolumen von ca. 3 m3. Der Füllstand im Trichter wurde bei 75 bis 80% gehalten. The polymer particles were coated with a surface post-crosslinking 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 3 . The surface post-crosslinked superabsorbent particles fell over a weir into a funnel. At the lower end of the funnel was a rotary valve. The funnel had a round outlet with a diameter of 25 cm at the bottom and expanded to a height of 99.8 cm to 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 approx. 3 m 3 . The fill level in the funnel was at 75 to 80% held.

Der Trichter verfügte über Ultraschall-Austragshilfen bestehend aus drei Convertern vom Typ C35-HP1 und einen Generator vom Typ DGS35-200-T-EU (adams & öztas oHG, Gernsbach, Deutschland). Die Converter befanden sich jeweils an den Seiten und der Rückseite des Trichters ca. 60 cm oberhalb des Trichterausgangs. The hopper was equipped with ultrasonic discharge aids consisting of three converters of type C35-HP1 and a generator of type DGS35-200-T-EU (adams & öztas oHG, Gernsbach, Germany). The converters were located on the sides and the back of the hopper approximately 60 cm above the hopper outlet.

Es wurden folgende Mengen in den Schugi Flexomix® dosiert: The following quantities were dosed into the Schugi Flexomix®:

9,5 t/h Polymerpartikel 9.5 t/h polymer particles

366,7 kg/h Oberflächennachvernetzerlösung 366.7 kg/h surface post-crosslinker solution

Die Oberflächennachvernetzerlösung enthielt 1,55 Gew.-% 2-Hydroxyethyl-2 oxazolidon, 1,55 Gew.-% 1,3-Propan- diol, 12,95 Gew.-% 1 ,2-Propandiol, 10,88 Gew.-% Aluminiumlaktat, 50,70 Gew.-% Wasser, 22,28 Gew.-% Isopropanol und 0,08 Gew.-% Sorbitanmonolaurat (Span®20). The surface post-crosslinker 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).

Die oberflächennachvernetzten Polymerpartikel wurden mit der Zellradschleuse in einem NARA Paddle-Cooler (GMF Gouda, Waddinxveen, Niederlande) überführt und auf ca. 60°C abgekühlt. Dabei wurden die oberflächennachvernetzten Polymerpartikel mit 118,75 kg/h Wasser beschichtet. 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.

Die erhaltenen Superabsorberpartikel wurden in Flexible Intermediate Bulk Container (FIBC) abgefüllt und analysiert. Die Flüssigkeitsweiterleitung (SFC) betrug ca. 50 x 107 cm3s/g. The superabsorbent particles obtained were filled into flexible intermediate bulk containers (FIBC) and analyzed. The liquid flow transmission rate (SFC) was approximately 50 x 10 7 cm 3 s/g.

Beispiel 2 (nicht erfindungsgemäß) Example 2 (not according to the invention)

Es wird wie in Beispiel 1 verfahren. Der Füllstand im Trichter betrug 0%, d.h. die oberflächennachvernetzten Superabsorberpartikel wurden sofort weiter gefördert. Die erhaltenen Superabsorberpartikel wurden in Flexible Intermediate Bulk Container (FIBC) abgefüllt und analysiert.The procedure is as in Example 1. The filling level in the funnel was 0%, ie the surface-crosslinked superabsorbent particles were immediately conveyed further. The obtained superabsorbent particles were filled into Flexible Intermediate Bulk Containers (FIBC) and analyzed.

Die Flüssigkeitsweiterleitung (SFC) betrug ca. 36 x 107 cm3s/g. The liquid conductance (SFC) was approximately 36 x 10 7 cm 3 s/g.

Claims

Patentansprüche patent claims 1. Verfahren zur kontinuierlichen thermischen Oberflächennachvernetzung von Superabsorbern, wobei Superabsorberpartikel durch Aufsprühen einer Oberflächennachvernetzer-Lösung beschichtet werden, die beschichteten Superabsorberpartikel in einem Kontakttrockner 1 thermisch oberflächennachvernetzt werden und die thermisch oberflächennachvernetzten Superabsorberpartikel in einem Kontakttrockner 2 gekühlt werden, dadurch gekennzeichnet, dass die den Kontakttrockner 1 verlassenden Superabsorberpartikel über einen Trichter und eine Dosiereinrichtung in den Kontakttrockner 2 überführt werden, der Füllstand im Trichter mittels der Dosiereinrichtung geregelt wird und der Füllstand im Trichter von 10 bis 100% beträgt. 1. Process for the 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 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 to the contact dryer 2 via a funnel and a metering device, the fill level in the funnel is regulated by means of the metering device and the fill level in the funnel is from 10 to 100%. 2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass das Innenvolumen des Trichters von 0,01 bis 0,5 m3 pro m3 Innenvolumen des Kontakttrockners 1 beträgt. 2. Process according to claim 1, characterized in that the internal volume of the hopper is from 0.01 to 0.5 m 3 per m 3 internal volume of the contact dryer 1. 3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, dass teilneutralisierte, vernetzte Polyacrylsäure als Superabsorber eingesetzt werden. 3. Process according to claim 1 or 2, characterized in that partially neutralized, cross-linked polyacrylic acid is used as superabsorbent. 4. Verfahren nach einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, dass der Oberflächennachvernetzer mit dem Superabsorber kovalente Bindungen bilden kann. 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. Verfahren nach einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, dass der mittlere Tropfendurchmesser beim Aufsprühen der Oberflächennachvernetzer-Lösung von 200 bis 4.500 pim beträgt. 5. Process according to one of claims 1 to 4, characterized in that the average droplet diameter when spraying the surface post-crosslinker solution is from 200 to 4,500 pim. 6. Verfahren nach einem der Ansprüche 1 bis 5, dadurch gekennzeichnet, dass die Temperatur der Superabsorberpartikel beim Aufsprühen der Oberflächennachvernetzer-Lösung von 30 bis 80°C beträgt. 6. Process according to one of claims 1 to 5, characterized in that the temperature of the superabsorbent particles when spraying on the surface post-crosslinker solution is from 30 to 80°C. 7. Verfahren nach einem der Ansprüche 1 bis 6, dadurch gekennzeichnet, dass die Oberflächennachvernetzer- Lösung von 0,001 bis 2 Gew.-% eines Oberflächennachvernetzers enthält, bezogen auf die Superabsorberpartikel. 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. Verfahren nach einem der Ansprüche 1 bis 7, dadurch gekennzeichnet, dass die Oberflächennachvernetzer- Lösung von 0,5 bis 5 Gew.-% Wasser enthält, bezogen auf die Superabsorberpartikel. 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. Verfahren nach einem der Ansprüche 1 bis 8, dadurch gekennzeichnet, dass die Superabsorberpartikel im Kontakttrockner 1 auf eine Temperatur von 110 bis 220°C erwärmt werden. 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. Verfahren nach einem der Ansprüche 1 bis 9, dadurch gekennzeichnet, dass die Verweilzeit der Superabsorberpartikel im Kontakttrockner 1 von 10 bis 60 Minuten beträgt. 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. Verfahren nach einem der Ansprüche 1 bis 10, dadurch gekennzeichnet, dass der Kontakttrockner 1 und die Verbindung zum Kontakttrockner 2 einschließlich Trichter und Dosiereinrichtung begleitbeheizt und/oder thermisch isoliert werden. 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 heated and/or thermally insulated. 12. Verfahren nach einem der Ansprüche 1 bis 11 , dadurch gekennzeichnet, dass die Superabsorberpartikel im Kontakttrockner 2 auf eine Temperatur von 30 bis 80°C gekühlt werden. 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. Verfahren nach einem der Ansprüche 1 bis 12, dadurch gekennzeichnet, dass die Verweilzeit der Superabsorberpartikel im Kontakttrockner 2 von 10 bis 60 Minuten beträgt. 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. Verfahren nach einem der Ansprüche 1 bis 13, dadurch gekennzeichnet, dass ein Gasstrom mit einem Sauerstoffanteil von weniger als 10 Vol.-% Sauerstoff durch den Kontakttrockner 1 geleitet wird. 14. The method according to any one of claims 1 to 13, characterized in that a gas stream having an oxygen content of less than 10 vol.% oxygen is passed through the contact dryer 1. 15. Verfahren nach Anspruch 14, dadurch gekennzeichnet, dass ein Gas in den Kontakttrockner 1 geleitet wird, die Gesamtgasmenge des eingeleiteten Gasstromes von 5 bis 60 Nm3/h pro m3 Innenvolumen des Kontakttrockners 1 beträgt, ein Abgasstrom aus den Kontakttrockner 1 herausgeführt wird, der Abgasstrom im Kontakttrockner 1 um mindestens 75° von der horizontalen Produktstromrichtung nach oben abgelenkt wird und die Gasgeschwindigkeit des Abgasstroms direkt nach der Umlenkung weniger als 5 m/s beträgt. 15. Process according to claim 14, characterized in that a gas is passed into the contact dryer 1, the total gas quantity of the introduced gas stream is from 5 to 60 Nm 3 /h per m 3 internal volume of the contact dryer 1, 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.
PCT/EP2024/072317 2023-08-18 2024-08-07 Process for thermal surface postcrosslinking of superabsorbents Pending WO2025040439A1 (en)

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