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WO2016050397A1 - Method for producing water-absorbing polymer particles - Google Patents

Method for producing water-absorbing polymer particles Download PDF

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Publication number
WO2016050397A1
WO2016050397A1 PCT/EP2015/068126 EP2015068126W WO2016050397A1 WO 2016050397 A1 WO2016050397 A1 WO 2016050397A1 EP 2015068126 W EP2015068126 W EP 2015068126W WO 2016050397 A1 WO2016050397 A1 WO 2016050397A1
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Prior art keywords
polymer particles
monomer
water
ethylenically unsaturated
initiator
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PCT/EP2015/068126
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German (de)
French (fr)
Inventor
Thomas Pfeiffer
Rüdiger Funk
Matthias Weismantel
Marco BISCHOFF
Oskar Stephan
Karl Possemiers
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BASF SE
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BASF SE
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    • 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
    • C08F2/00—Processes of polymerisation
    • C08F2/01—Processes of polymerisation characterised by special features of the polymerisation apparatus used
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
    • B01J19/18—Stationary reactors having moving elements inside
    • B01J19/22—Stationary reactors having moving elements inside in the form of endless belts
    • 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
    • C08F220/00—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 a salt, anhydride ester, amide, imide or nitrile thereof
    • C08F220/02—Monocarboxylic acids having less than ten carbon atoms; Derivatives thereof
    • C08F220/04—Acids; Metal salts or ammonium salts thereof
    • C08F220/06—Acrylic acid; Methacrylic acid; Metal salts or ammonium salts thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2219/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
    • B01J2219/00002—Chemical plants
    • B01J2219/00004—Scale aspects
    • B01J2219/00006—Large-scale industrial plants

Definitions

  • the present invention relates to a process for producing water-absorbing polymer particles by thermal polymerization of discrete droplets of a monomer solution on a hydrophobic surface.
  • the preparation of water-absorbing polymer particles is described in the monograph "Modern Superabsorbent Polymer Technology", F.L. Buchholz and AT. Graham, Wiley-VCH, 1998, pages 71-103.
  • the Korean patent application KR 2013-01 15810 describes a UV polymerization of discrete drops of a monomer solution.
  • Water-absorbent polymer particles are used as aqueous solution-absorbing products for the production of diapers, tampons, sanitary towels and other sanitary articles, but also as water-retaining agents in agricultural horticulture. Water-absorbing polymer particles are also referred to as "superabsorbent polymers" or “superabsorbents”.
  • the object of the present invention was to provide an improved process for the preparation of polymer particles.
  • the object has been achieved by a process for the polymerization of an aqueous monomer solution comprising at least one ethylenically unsaturated monomer a), optionally a crosslinker b), at least one initiator c) and water, wherein in a first step i) drops are produced and the droplets in a second step ii) are polymerized on a hydrophobic surface, characterized in that the at least one initiator c) is a thermal initiator and the polymerization in step ii) is carried out thermally.
  • thermal initiators are, for example, azo initiators, peroxides, hydroperoxides, hydrogen peroxide and persulfates.
  • the monomer solution contains substantially no photoinitiator and the polymerization is carried out substantially in the absence of UV radiation.
  • the temperature during the thermal polymerization in step ii) is preferably from 100 to 250.degree. C., more preferably from 120 to 200.degree. C., most preferably from 150 to 180.degree.
  • the droplets have an average diameter of preferably 50 to 2000 ⁇ , more preferably from 100 to 1000 ⁇ , most preferably from 300 to 600 ⁇ , wherein the average diameter of the volume-average diameter and can be measured by light scattering.
  • the aqueous monomer solution usually comprises a) at least one ethylenically unsaturated, acid group-carrying monomer which may be at least partially neutralized,
  • the monomers a) are preferably water-soluble, i. the solubility in water at 23 ° C. is typically at least 1 g / 100 g of water, preferably at least 5 g / 100 g of water, more preferably at least 25 g / 100 g of water, most preferably at least 35 g / 100 g of water.
  • Suitable monomers a) are, for example, ethylenically unsaturated carboxylic acids, such as acrylic acid, methacrylic acid, maleic acid and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Very particular preference is given to acrylic acid.
  • Suitable monomers a) are, for example, ethylenically unsaturated sulfonic acids, such as styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).
  • sulfonic acids such as styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).
  • AMPS 2-acrylamido-2-methylpropanesulfonic acid
  • a suitable monomer a) is, for example, an acrylic acid purified according to WO 2004/035514 A1 with 99.8460% by weight of acrylic acid, 0.0950% by weight of acetic acid, 0.0332% by weight of water, 0.0203% by weight.
  • Propionic acid 0.0001% by weight furfurale, 0.0001 linoleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% by weight of hydroquinone monomethyl ether.
  • the proportion of acrylic acid and / or salts thereof in the total amount of monomers a) is preferably at least 50 mol%, particularly preferably at least 90 mol%, very particularly preferably at least 95 mol%.
  • the acid groups of the monomers a) are usually partially neutralized, preferably at least 25 mol%, preferably from 50 to 85 mol%, more preferably from 60 to 75 mol%, very preferably from 65 to 72 mol%, using the customary neutralizing agents may be, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal bicarbonates and mixtures thereof. Instead of alkali metal salts and ammonium salts can be used. Sodium and potassium are particularly preferred as alkali metals, but most preferred are sodium hydroxide, sodium carbonate or sodium bicarbonate and mixtures thereof. Usually, the neutralization is achieved by mixing the neutralizing agent as an aqueous solution, as a melt, or preferably as a solid.
  • sodium hydroxide with a water content well below 50 wt .-% may be present as a waxy mass with a melting point above 23 ° C.
  • a dosage as general cargo or melt at elevated temperature is possible.
  • one or more chelating agents can be added to the monomer solution or its starting materials to mask metal ions, such as iron, for stabilization.
  • chelating agents are alkali citrates, citric acid, alkali tartrates, pentasodium triphosphate, ethylenediamine tetraacetate, nitrilotriacetic acid and all chelating agents known by the name Trilon®, for example Trilon® C (pentasodium diethylenetriamine pentaacetate), Trilon® D (trisodium (hydroxyethyl) ethylene-diaminotriazetate ), as well as Trilon® M (methylglycinediacetic acid).
  • Trilon® C penentasodium diethylenetriamine pentaacetate
  • Trilon® D trisodium (hydroxyethyl) ethylene-diaminotriazetate
  • Trilon® M methylglycinediacetic acid
  • the monomers a) usually contain polymerization inhibitors, preferably hydroquinone half ethers, as storage stabilizer.
  • the monomer solution preferably contains up to 250 ppm by weight, preferably at most 130 ppm by weight, more preferably at most 70 ppm by weight, preferably at least 10 ppm by weight, particularly preferably at least 30 ppm by weight, in particular by 50% by weight .
  • ppm hydroquinone half ether, in each case based on the unneutralized monomer a).
  • an ethylenically unsaturated, acid group-carrying monomer having a corresponding content of hydroquinone half-ether can be used to prepare the monomer solution.
  • the hydroquinone half ethers can also be removed from the monomer solution by absorption, for example on activated charcoal.
  • Preferred hydroquinone half ethers are hydroquinone monomethyl ether (MEHQ) and / or alpha tocopherol (vitamin E).
  • Suitable crosslinkers b) are compounds having at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups which can be radically copolymerized into the polymer chain, and functional groups which can form covalent bonds with the acid groups of the monomer a).
  • polyvalent metal salts which can form coordinative bonds with at least two acid groups of the monomer a) are also suitable as crosslinking agents b).
  • Crosslinkers b) are preferably compounds having at least two polymerizable groups which can be incorporated in the polymer network in free-radically polymerized form.
  • Suitable crosslinkers b) 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 in EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93/21237 A1, WO 2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 A1 and DE 103 31 450 A1, mixed acrylates which, in addition to acrylate groups, contain further ethylenically unsaturated Groups, as described in DE 103 31 456
  • Preferred crosslinkers b) are pentaerythritol triallyl ether, tetraalloxyethane, methylenebismethacrylamide, 15 to 30 times ethoxylated glycerol triacrylate, 15 to 30 times ethoxylated trimethylolpropane triacrylate, 15 to 20 times ethoxylated trimethylolethane triacrylate, 15 to 20 times ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate and triallylamine ,
  • Very particularly preferred crosslinkers b) are the polyethyleneglyoxylated and / or propoxylated glycerols esterified with acrylic acid or methacrylic acid to form diioder triacrylates, as described, for example, in WO 2003/104301 A1.
  • Particularly advantageous are di- and / or triacrylates of 3- to 10-fold ethoxylated glycerol.
  • diacrylates or triacrylates of 1 to 5 times ethoxylated and / or propoxylated glycerol.
  • Most preferred are the triacrylates of 3 to 5 times ethoxylated and / or propoxylated glycerol, in particular the triacrylate of 3-times ethoxylated glycerol.
  • the amount of crosslinker b) is preferably from 0.01 to 1, 5 wt .-%, particularly preferably 0.05 to 1 wt .-%, most preferably 0.1 to 0.6 wt .-%, each based on the unneutralized monomer a).
  • the centrifuge retention capacity (CRC) decreases and the absorption under a pressure of 21.0 g / cm 2 (AUL 0.3 psi) passes through a maximum.
  • initiators c) it is possible to use all compounds which decompose into free radicals under the polymerization conditions, for example peroxides, hydroperoxides, hydrogen peroxide, persulfates, azo compounds and the so-called redox initiators.
  • it is beneficial to have misohnnn p n ⁇ p r different initiators for example mixtures of hydrogen peroxide and sodium or potassium peroxodisulfate. Mixtures of hydrogen peroxide and sodium peroxodisulfate can be used in any ratio.
  • Particularly preferred initiators c) are azo initiators, such as 2,2'-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride and 2,2'-azobis [2- (5-methyl-2-) imidazolin-2-yl) propane] dihydrochloride, hydrogen peroxide, persulfates such as sodium peroxodisulfate and ammonium peroxodisulfate, and redox initiators such as sodium peroxodisulfate / ascorbic acid, ammonium peroxodisulfate / ascorbic acid and hydrogen peroxide / ascorbic acid, and mixtures thereof.
  • azo initiators such as 2,2'-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride and 2,2'-azobis [2- (5-methyl-2-) imidazolin-2-yl) propane] dihydrochloride, hydrogen peroxide, persulfates such as sodium peroxodis
  • the initiators are used in customary amounts, for example in amounts of 0.001 to 5 wt .-%, preferably 0.01 to 2 wt .-%, based on the unneutralized monomer a).
  • acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate are ethylenically unsaturated monomers d) which are copolymerizable with the ethylenically unsaturated acid group-carrying monomers a).
  • water-soluble polymers e it is possible to use polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, modified cellulose, such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycols or polyacrylic acids, preferably starch, starch derivatives and modified cellulose.
  • the water content of the monomer solution is preferably less than 65 wt .-%, preferably less than 62 wt .-%, more preferably less than 60 wt .-%, most preferably less than 58 wt .-%.
  • the monomer solution at 20 ° C has a density of preferably 1 to 1, 3 g / cm 3, more preferably 1, 05-1, 25 g / cm 3, more preferably 1, 1 to 1, 2 g / cm 3.
  • the aqueous monomer solution is metered onto the hydrophobic surface to form discrete droplets.
  • the hydrophobic surface has a contact angle with respect to water of preferably at least 60 °, more preferably at least 80 °, most preferably at least 100 °.
  • the contact angle is a measure of the wetting behavior and is measured according to DIN 53900. The higher the contact angle, the more hydrophobic a surface is.
  • the drops can be generated by means of pneumatic drawing nozzles, rotation, cutting a jet or quickly activatable micro-valve nozzles.
  • a liquid jet is accelerated along with a gas flow through a baffle. About the amount of gas, the diameter of the liquid jet and thus the droplet diameter can be influenced.
  • the liquid passes through the openings of a rotating disk. Due to the centrifugal force acting on the liquid, drops of a defined size are torn off.
  • Preferred devices for Rotationsvertropfung be described for example in DE 43 08 842 A1.
  • the exiting liquid jet can also be cut into defined segments by means of a rotating knife. Each segment then forms a drop.
  • micro-valve nozzles When using micro-valve nozzles directly drops are generated with a defined volume of liquid.
  • the monomer solution is metered (dripped) by means of at least one bore to form droplets.
  • the drop diameter is usually 1, 9 times the diameter of the hole. It is important that the liquid does not pass through the hole too quickly or that the pressure loss through the hole is not too great. Otherwise, the liquid is not dripped, but the liquid jet is torn due to the high kinetic energy (sprayed).
  • the Reynolds number based on the throughput per bore and the bore diameter is preferably less than 2,000, preferably less than 1,600, more preferably less than 1, 400, most preferably less than 1200.
  • the temperature of the monomer solution when passing through the bore is preferably 10 to 60 ° C, more preferably 15 to 50 ° C, most preferably 20 to 40 ° C.
  • the reaction can be carried out in overpressure or under reduced pressure, a negative pressure of up to 100 mbar relative to the ambient pressure is preferred.
  • the water-absorbing polymer particles can be surface-postcrosslinked to further improve the properties.
  • Suitable surface postcrosslinkers are compounds which contain groups which 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 ⁇ -hydroxyalkylamides, as described in DE 102 04 938 A1 and US Pat. No. 6,239,230.
  • 2-Oxazolidinone and its derivatives such as 2-hydroxyethyl-2-oxazolidinone, in DE 1 QR ⁇ 7 QQ9 CA Bis- and poly-2-oxazolidinones, in DE 198 54 573 A1 2-oxotetrahydro-1,3-oxazine and its derivatives, in DE 198 54 574 A1 N-acyl-2-oxazolidinones, in DE 102 04 937 A1 cyclic ureas in DE 103 34 584 A1 bicyclic amidoacetals, in EP 1 199 327 A2 oxetanes and cyclic ureas and in WO 2003/031482 A1 morpholine-2,3-dione and its derivatives as suitable surface postcrosslinkers.
  • Preferred surface postcrosslinkers are ethylene carbonate, ethylene glycol diglycidyl ether, reaction products of polyamides with epichlorohydrin and mixtures of propylene glycol and 1,4-butanediol.
  • Very particularly preferred surface postcrosslinkers are 2-hydroxyethyl-2-oxazolidinone, 2-oxazolidinone and 1, 3-propanediol.
  • the amount of surface postcrosslinker is preferably 0.001 to 5 wt .-%, more preferably 0.02 to 2 wt .-%, most preferably 0.05 to 1 wt .-%, each based on the polymer particles.
  • polyvalent cations are applied to the particle surface in addition to the surface postcrosslinkers before, during or after the surface postcrosslinking.
  • the polyvalent cations which can be used in the process according to the invention are, for example, divalent cations, such as the cations of zinc, magnesium, calcium, iron 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.
  • salts with different counterions for example basic aluminum salts, such as aluminum monoacetate or aluminum monolactate. Aluminum sulfate, aluminum monoacetate and aluminum lactate are preferred.
  • polyamines can also be used as polyvalent cations.
  • the amount of polyvalent cation used is, for example, from 0.001 to 1% by weight, preferably from 0.005 to 0.5% by weight, more preferably from 0.02 to 0.2% by weight. in each case based on the polymer particles.
  • the surface postcrosslinking is usually carried out so that a solution of the surface postcrosslinker is sprayed onto the dried polymer particles. Subsequent to the spraying, the surface postcrosslinker-coated polymer Particles thermally dried, wherein the surface postcrosslinking reaction can take place both before and during drying.
  • the spraying of a solution of the surface postcrosslinker 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
  • vertical mixers very particularly preferred are vertical mixers.
  • the distinction between horizontal mixer and vertical mixer is made by the storage of the mixing shaft, i.
  • 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 postcrosslinkers are typically used as an aqueous solution.
  • the penetration depth of the surface postcrosslinker into the polymer particles can be adjusted by the content of nonaqueous solvent or total solvent amount. If only water is used as the solvent, it is advantageous to add a surfactant. As a result, the wetting behavior is improved and the tendency to clog is reduced.
  • solvent mixtures for example isopropanol / water, 1,3-propanediol / water and propylene glycol / water, the mixing mass ratio preferably being from 20:80 to 40:60.
  • the thermal drying is preferably carried out in contact dryers, more preferably paddle dryers, very particularly preferably disk dryers.
  • Suitable dryers include 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). Moreover, fluidized bed dryers can also be used.
  • the drying can take place in the mixer itself, by heating the jacket or blowing hot air.
  • a downstream dryer such as a hopper dryer, a rotary kiln or a heatable screw. Particularly advantageous is mixed and dried in a fluidized bed dryer.
  • Preferred drying temperatures are in the range 100 to 250 ° C, preferably 120 to 220 ° C, more preferably 130 to 210 ° C, most preferably 150 to 200 ° C.
  • the preferred residence time at this temperature in the reaction mixer or dryer is preferably at least 10 minutes, more preferably at least 20 minutes, most preferably at least 30 minutes, and usually at most 60 minutes.
  • the water-absorbing polymer particles are cooled after the thermal drying.
  • the cooling is preferably carried out in contact coolers, particularly preferably blade coolers, very particularly preferably disk coolers.
  • Suitable coolers are, for example, Hosokawa Bepex® Horizontal Paddle Coolers (Hosokawa Micron GmbH, Leingart, Germany), Hosokawa Bepex® Disc Coolers (Hosokawa Micron GmbH, Leingart, Germany), Holo-Flite® coolers (Metso Minerals Industries, Inc., Danville, USA ) and Nara Paddle Cooler (NARA Machinery Europe, Frechen, Germany). Moreover, fluidized bed coolers can also be used.
  • the water-absorbing polymer particles to 20 to 150 ° C, preferably 30 to 120 ° C, more preferably 40 to 100 ° C, most preferably 50 to 80 ° C, cooled.
  • the surface-postcrosslinked polymer particles can be coated or post-moistened for further improvement of the properties.
  • the post-wetting is preferably carried out at 30 to 80 ° C, more preferably at 35 to 70 ° C, most preferably at 40 to 60 ° C. If the temperatures are too low, the water-absorbing polymer particles tend to clump together, and water evaporates appreciably at higher temperatures.
  • the amount of water used for the rewetting is preferably from 1 to 10 wt .-%, particularly preferably from 2 to 8 wt .-%, most preferably from 3 to 5 wt .-%, each based on the water-absorbing polymer particles.
  • the post-humidification is carried out in the cooler after the thermal drying.
  • Suitable coatings for improving the swelling rate and the permeability are, for example, inorganic inert substances, such as water-insoluble metal salts, organic polymers, cationic polymers and di- or polyvalent metal cations.
  • Suitable coatings for dust binding are, for example, polyols.
  • Suitable coatings against the unwanted caking tendency of the polymer particles are, for example, fumed silica, such as Aerosil® 200, and surfactants, such as Span® 20.
  • Another object of the present invention are obtainable by the process according to the invention polymer particles.
  • the polymer particles according to the invention have a proportion of particles with a particle size of 300 to 600 ⁇ m, preferably of at least 30% by weight, more preferably at least 50% by weight, very particularly preferably at least 70% by weight.
  • the polymer particles of the invention have a moisture content of preferably 0.1 to 20 wt .-%, particularly preferably from 1 to 15 wt .-%, most preferably from 5 to 10 wt .-%, on.
  • the moisture content is determined according to the EDANA recommended test method No. WSP 230.2-05 "Mass Loss Upon Heating".
  • the polymer particles according to the invention have a centrifuge retention capacity (CRC) of typically at least 25 g / g, preferably at least 30 g / g, preferably at least 32 g / g, more preferably at least 34 g / g, most preferably at least 36 g / g.
  • the centrifuge retention capacity (CRC) of the polymer particles is usually less than 60 g / g.
  • Centrifuge retention capacity (CRC) is determined according to EDANA recommended test method no. WSP 241.2-05 "Fluid Retention Capacity in Saline, After Centrifugation".
  • the polymer particles according to the invention have a content of residual monomers of typically less than 2% by weight, preferably less than 1.5% by weight, preferably less than 1% by weight, more preferably less than 0.6% by weight. %, most preferably less than 0.4 wt .-%, on.
  • the content of residual monomers of the polymer particles is determined according to the EDANA recommended test method no. WSP 210.2-05 "Residual Monomers".
  • Further articles of the present invention are hygiene articles which contain the water-absorbing polymer particles according to the invention.
  • the water-absorbing polymer particles are tested by the test methods described below.
  • Measurements should be taken at an ambient temperature of 23 ⁇ 2 ° C and a relative humidity of 50 ⁇ 10%, unless otherwise specified.
  • the water-absorbing polymer particles are thoroughly mixed before the measurement.
  • Centrifuge Retention Capacity is determined analogously to the EDANA recommended Test Method No. WSP 241.2-05 "Fluid Retention Capacity in Saline, After Centrifugation", using 0.1 g of water-absorbing polymer particles instead of 0.2 g. residual monomer
  • the residual monomer content of the water-absorbing polymer particles is determined according to the EDANA-recommended test method WSP No. 210.2-05 "Residual Monomers”.
  • the EDANA test methods are available, for example, from EDANA, Avenue Eugene Plasky 157, B-1030 Brussels, Belgium.
  • Example 1 (Comparative Example) A monomer solution was sprayed on a silicone rubber-coated pad.
  • the monomer solution contained partially neutralized acrylic acid with caustic soda.
  • the monomer solution additionally contained 0.05% by weight of Sartomer® SR454 (15-times ethoxylated trimethylolpropane triacylate), 0.1% by weight of Wako® VA-044 (2,2'-azobis [2- (2-imidazoline -2-yl) propane] dihydrochloride), 0.008% by weight of Büggolit® FF7 (disodium salt of 2-hydroxy-2-sulfinatoacetic acid), 0.15% by weight of sodium peroxodisulfate and 0.19% by weight of Darocur® 1 173 ( 2-hydroxy-2-methyl-1-phenylpropane-1-one), each based on unneutralized acrylic acid.
  • the solids content of the monomer solution was 40% by weight.
  • the drops were irradiated for 5 minutes by means of a UV radiator of the type MH radiator UV 400 F / 2 (Dr. Höhnle AG, Gräfelfing, Germany) and then stored for 30 minutes at 160 ° C in a drying oven.
  • the resulting polymer particles were screened to 150 to 850 ⁇ and analyzed. The results are summarized in Table 1.
  • Example 3 The procedure was as in Example 1. Deviating from Example 1, the drops were irradiated for 66 minutes by means of a UV radiator of the MH-UV 400 F / 2 type and then not stored in a drying oven. Example 3
  • a monomer solution was sprayed onto a silicone rubber coated pad.
  • the monomer solution contained partially neutralized acrylic acid with caustic soda.
  • the monomer solution additionally contained 0.05% by weight of Sartomer® SR344 (polyethylene glycol 400-diacrylate), 0.1% by weight of Wako® VA-044 (2,2'-azobis [2- (2-imidazoline-2 -yl) propane] dihydrochloride), 0.008 wt%
  • Büggolit® FF7 (disodium salt of 2-hydroxy-2-sulfinatoacetic acid) 0.15% by weight of sodium peroxodisulfate and 0.042% by weight of Darocur® TPO (diphenyl- (2,4,6-trimethylbenzoyl) -phosphine oxide), in each case based on unneutralized acrylic acid.
  • the solids content of the monomer solution was 40% by weight.
  • the drops were irradiated for 5 minutes by means of a UV radiator of the type MH radiator UV 400 F / 2 and then stored for 30 minutes at 160 ° C in a drying oven.
  • the resulting polymer particles were screened to 150 to 850 ⁇ and analyzed. The results are summarized in Table 2.

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Abstract

The invention relates to a method for producing water-absorbing polymer particles by thermal polymerization of discrete droplets of a monomer solution on a hydrophobic surface.

Description

Verfahren zur Herstellung wasserabsorbierender Polymerpartikel  Process for the preparation of water-absorbing polymer particles

Beschreibung description

Die vorliegende Erfindung betrifft ein Verfahren zur Herstellung wasserabsorbierender Polymerpartikel durch thermische Polymerisation diskreter Tropfen einer Monomerlösung auf einer hydrophoben Oberfläche. Die Herstellung wasserabsorbierender Polymerpartikel wird in der Monographie "Modern Su- perabsorbent Polymer Technology", F.L. Buchholz und AT. Graham, Wiley-VCH, 1998, Seiten 71 bis 103, beschrieben. The present invention relates to a process for producing water-absorbing polymer particles by thermal polymerization of discrete droplets of a monomer solution on a hydrophobic surface. The preparation of water-absorbing polymer particles is described in the monograph "Modern Superabsorbent Polymer Technology", F.L. Buchholz and AT. Graham, Wiley-VCH, 1998, pages 71-103.

Die koreanische Patentanmeldung KR 2013-01 15810 beschreibt eine UV-Polymerisation dis- kreter Tropfen einer Monomerlösung. The Korean patent application KR 2013-01 15810 describes a UV polymerization of discrete drops of a monomer solution.

Wasserabsorbierende Polymerpartikel werden als wässrige Lösungen absorbierende Produkte zur Herstellung von Windeln, Tampons, Damenbinden und anderen Hygieneartikeln, aber auch als wasserzurückhaltende Mittel im landwirtschaftlichen Gartenbau verwendet. Wasserabsor- bierende Polymerpartikel werden auch„superabsorbent polymers" bzw.„Superabsorber" bezeichnet. Water-absorbent polymer particles are used as aqueous solution-absorbing products for the production of diapers, tampons, sanitary towels and other sanitary articles, but also as water-retaining agents in agricultural horticulture. Water-absorbing polymer particles are also referred to as "superabsorbent polymers" or "superabsorbents".

Aufgabe der vorliegenden Erfindung war die Bereitstellung eines verbesserten Verfahrens zur Herstellung von Polymerpartikeln. The object of the present invention was to provide an improved process for the preparation of polymer particles.

Gelöst wurde die Aufgabe durch ein Verfahren zur Polymerisation einer wässrigen Monomerlösung, enthaltend mindestens ein ethylenisch ungesättigtes Monomer a), optional einen Vernetzer b), mindestens einen Initiator c) und Wasser, wobei in einem ersten Schritt i) Tropfen erzeugt werden und die Tropfen in einem zweiten Schritt ii) auf einer hydrophoben Oberfläche polymerisiert werden, dadurch gekennzeichnet, dass der mindestens eine Initiator c) ein thermischer Initiator ist und die Polymerisation in Schritt ii) thermisch durchgeführt wird. The object has been achieved by a process for the polymerization of an aqueous monomer solution comprising at least one ethylenically unsaturated monomer a), optionally a crosslinker b), at least one initiator c) and water, wherein in a first step i) drops are produced and the droplets in a second step ii) are polymerized on a hydrophobic surface, characterized in that the at least one initiator c) is a thermal initiator and the polymerization in step ii) is carried out thermally.

Es ist wesentlich, dass die Polymerisation in Schritt ii) thermisch durchgeführt wird, d.h., dass die Startradikale durch den Zerfall thermischer Initiatoren erzeugt werden und die Polymerisati- on durch die so erzeugten Startradikale initiiert wird. Thermische Initiatoren sind beispielsweise Azoinitiatoren, Peroxide, Hydroperoxide, Wasserstoffperoxid und Persulfate. It is essential that the polymerization in step ii) be carried out thermally, that is, that the starting radicals are generated by the decomposition of thermal initiators and that the polymerization is initiated by the starting radicals thus produced. Thermal initiators are, for example, azo initiators, peroxides, hydroperoxides, hydrogen peroxide and persulfates.

In einer bevorzugten Ausführungsform der vorliegenden Erfindung enthält die Monomerlösung im Wesentlichen keinen Photoinitiator und die Polymerisation wird im Wesentlichen in Abwe- senheit von UV-Strahlen durchgeführt. Die Temperatur beträgt bei der thermischen Polymerisation in Schritt ii) vorzugsweise von 100 bis 250°C, besonders bevorzugt von 120 bis 200°C, ganz besonders bevorzugt von 150 bis 180°C. Die Tropfen weisen einen mittleren Durchmesser von vorzugsweise 50 bis 2000 μηη, besonders bevorzugt von 100 bis 1000 μηη, ganz besonders bevorzugt von 300 bis 600 μηη auf, wobei der mittlere Durchmesser der volumengemittelte Durchmesser ist und durch Lichtstreuung gemessen werden kann. In a preferred embodiment of the present invention, the monomer solution contains substantially no photoinitiator and the polymerization is carried out substantially in the absence of UV radiation. The temperature during the thermal polymerization in step ii) is preferably from 100 to 250.degree. C., more preferably from 120 to 200.degree. C., most preferably from 150 to 180.degree. The droplets have an average diameter of preferably 50 to 2000 μηη, more preferably from 100 to 1000 μηη, most preferably from 300 to 600 μηη, wherein the average diameter of the volume-average diameter and can be measured by light scattering.

Im Folgenden wird die Herstellung wasserabsorbierender, üblicherweise wasserunlöslicher, Polymerpartikel näher erläutert. The production of water-absorbing, usually water-insoluble, polymer particles is explained in more detail below.

Zur Herstellung wasserabsorbierender Polymerpartikel enthält die wässrige Monomerlösung üblicherweise a) mindestens ein ethylenisch ungesättigtes, säuregruppentragendes Monomer, das zumindest teilweise neutralisiert sein kann, For the preparation of water-absorbing polymer particles, the aqueous monomer solution usually comprises a) at least one ethylenically unsaturated, acid group-carrying monomer which may be at least partially neutralized,

b) mindestens einen Vernetzer, b) at least one crosslinker,

c) mindestens einen Initiator c) at least one initiator

d) optional ein oder mehrere mit den unter a) genannten Monomeren copolymerisierbare ethylenisch ungesättigte Monomere und d) optionally one or more copolymerizable with the monomers mentioned under a) ethylenically unsaturated monomers and

e) optional ein oder mehrere wasserlösliche Polymere. Die Monomeren a) 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. Geeignete Monomere a) sind beispielsweise ethylenisch ungesättigte Carbonsäuren, wie Acryl- säure, Methacrylsäure, Maleinsäure und Itaconsäure. Besonders bevorzugte Monomere sind Acrylsäure und Methacrylsäure. Ganz besonders bevorzugt ist Acrylsäure. e) optionally one or more water-soluble polymers. The monomers a) are preferably water-soluble, i. the solubility in water at 23 ° C. is typically at least 1 g / 100 g of water, preferably at least 5 g / 100 g of water, more preferably at least 25 g / 100 g of water, most preferably at least 35 g / 100 g of water. Suitable monomers a) are, for example, ethylenically unsaturated carboxylic acids, such as acrylic acid, methacrylic acid, maleic acid and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Very particular preference is given to acrylic acid.

Weitere geeignete Monomere a) sind beispielsweise ethylenisch ungesättigte Sulfonsäuren, wie Styrolsulfonsäure und 2-Acrylamido-2-methylpropansulfonsäure (AMPS). Further suitable monomers a) are, for example, ethylenically unsaturated sulfonic acids, such as styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

Verunreinigungen können einen erheblichen Einfluss auf die Polymerisation haben. Daher sollten die eingesetzten Rohstoffe eine möglichst hohe Reinheit aufweisen. Es ist daher oft vorteilhaft die Monomeren a) speziell zu reinigen. Geeignete Reinigungsverfahren werden beispiels- weise in der WO 2002/055469 A1 , der WO 2003/078378 A1 und der WO 2004/035514 A1 beschrieben. Ein geeignetes Monomer a) ist beispielsweise eine gemäß WO 2004/035514 A1 gereinigte Acrylsäure mit 99,8460 Gew. % Acrylsäure, 0,0950 Gew.-% Essigsäure, 0,0332 Gew.-% Wasser, 0,0203 Gew.-% Propionsäure, 0,0001 Gew.-% Furfurale, 0,0001 Ma- leinsäureanhydrid, 0,0003 Gew.-% Diacrylsäure und 0,0050 Gew.-% Hydrochinonmonomethyl- ether. Impurities can have a significant influence on the polymerization. Therefore, the raw materials used should have the highest possible purity. It is therefore often advantageous to purify the monomers a) specifically. Suitable purification processes are described, for example, in WO 2002/055469 A1, WO 2003/078378 A1 and WO 2004/035514 A1. A suitable monomer a) is, for example, an acrylic acid purified according to WO 2004/035514 A1 with 99.8460% by weight of acrylic acid, 0.0950% by weight of acetic acid, 0.0332% by weight of water, 0.0203% by weight. Propionic acid, 0.0001% by weight furfurale, 0.0001 linoleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% by weight of hydroquinone monomethyl ether.

Der Anteil an Acrylsäure und/oder deren Salzen an der Gesamtmenge der Monomeren a) be- trägt vorzugsweise mindestens 50 mol-%, besonders bevorzugt mindestens 90 mol-%, ganz besonders bevorzugt mindestens 95 mol-%. The proportion of acrylic acid and / or salts thereof in the total amount of monomers a) is preferably at least 50 mol%, particularly preferably at least 90 mol%, very particularly preferably at least 95 mol%.

Die Säuregruppen der Monomere a) sind üblicherweise teilweise neutralisiert, vorzugsweise zu mindestens 25 mol-%, bevorzugt zu 50 bis 85 mol-%, besonders bevorzugt 60 bis 75 mol %, ganz besonders bevorzugt 65 bis 72 mol%, wobei die üblichen Neutralisationsmittel verwendet werden können, vorzugsweise Alkalimetallhydroxide, Alkalimetalloxide, Alkalimetallcarbonate oder Alkalimetallhydrogencarbonate sowie deren Mischungen. Statt Alkalimetallsalzen können auch Ammoniumsalze verwendet werden. Natrium und Kalium sind als Alkalimetalle besonders bevorzugt, ganz besonders bevorzugt sind jedoch Natriumhydroxid, Natriumcarbonat oder Nat- riumhydrogencarbonat sowie deren Mischungen. Üblicherweise wird die Neutralisation durch Einmischung des Neutralisationsmittels als wässrige Lösung, als Schmelze, oder bevorzugt auch als Feststoff erreicht. Beispielsweise kann Natriumhydroxid mit einem Wasseranteil deutlich unter 50 Gew.-% als wachsartige Masse mit einem Schmelzpunkt oberhalb 23°C vorliegen. In diesem Fall ist eine Dosierung als Stückgut oder Schmelze bei erhöhter Temperatur möglich. Optional können der Monomerlösung oder ihren Ausgangsstoffen ein oder mehrere Chelatbild- ner zur Maskierung von Metallionen, wie beispielsweise Eisen, zwecks Stabilisierung zugesetzt werden. Geeignete Chelatbildner sind beispielsweise Alkalicitrate, Zitronensäure, Alkalitartrate, Pentanatriumtriphosphat, Ethylendiamintetraazetat, Nitrilotriessigsäure, sowie alle unter dem Namen Trilon® bekannten Chelatbildner, wie beispielsweise Trilon® C (Pentanatriumdiethylen- triaminpentaazetat), Trilon® D (Trinatrium-(hydroxyethyl)-ethylen-diamintriazetat), sowie Trilon® M (Methylglycindiessigsäure). The acid groups of the monomers a) are usually partially neutralized, preferably at least 25 mol%, preferably from 50 to 85 mol%, more preferably from 60 to 75 mol%, very preferably from 65 to 72 mol%, using the customary neutralizing agents may be, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal bicarbonates and mixtures thereof. Instead of alkali metal salts and ammonium salts can be used. Sodium and potassium are particularly preferred as alkali metals, but most preferred are sodium hydroxide, sodium carbonate or sodium bicarbonate and mixtures thereof. Usually, the neutralization is achieved by mixing the neutralizing agent as an aqueous solution, as a melt, or preferably as a solid. For example, sodium hydroxide with a water content well below 50 wt .-% may be present as a waxy mass with a melting point above 23 ° C. In this case, a dosage as general cargo or melt at elevated temperature is possible. Optionally, one or more chelating agents can be added to the monomer solution or its starting materials to mask metal ions, such as iron, for stabilization. Examples of suitable chelating agents are alkali citrates, citric acid, alkali tartrates, pentasodium triphosphate, ethylenediamine tetraacetate, nitrilotriacetic acid and all chelating agents known by the name Trilon®, for example Trilon® C (pentasodium diethylenetriamine pentaacetate), Trilon® D (trisodium (hydroxyethyl) ethylene-diaminotriazetate ), as well as Trilon® M (methylglycinediacetic acid).

Die Monomere a) enthalten üblicherweise Polymerisationsinhibitoren, vorzugsweise Hydrochi- nonhalbether, als Lagerstabilisator. The monomers a) usually contain polymerization inhibitors, preferably hydroquinone half ethers, as storage stabilizer.

Die Monomerlösung enthält vorzugsweise bis zu 250 Gew.-ppm, bevorzugt höchstens 130 Gew.-ppm, besonders bevorzugt höchstens 70 Gew.-ppm, bevorzugt mindestens 10 Gew.- ppm, besonders bevorzugt mindestens 30 Gew.-ppm, insbesondere um 50 Gew.-ppm, Hydro- chinonhalbether, jeweils bezogen auf das unneutralisierte Monomer a). Beispielsweise kann zur Herstellung der Monomerlösung ein ethylenisch ungesättigtes, säuregruppentragendes Monomer mit einem entsprechenden Gehalt an Hydrochinonhalbether verwendet werden. Die Hydro- chinonhalbether können aber auch durch Absorption, beispielsweise an Aktivkohle, aus der Monomerlösung entfernt werden. Bevorzugte Hydrochinonhalbether sind Hydrochinonmonomethylether (MEHQ) und/oder alpha- Tocopherol (Vitamin E). Geeignete Vernetzer b) 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 Monomeren a) kovalente Bindungen ausbilden können. Weiterhin sind auch polyvalente Metallsalze, die mit mindestens zwei Säuregruppen des Monomeren a) koordinative Bindungen ausbilden können, als Vernetzer b) geeignet. The monomer solution preferably contains up to 250 ppm by weight, preferably at most 130 ppm by weight, more preferably at most 70 ppm by weight, preferably at least 10 ppm by weight, particularly preferably at least 30 ppm by weight, in particular by 50% by weight . ppm, hydroquinone half ether, in each case based on the unneutralized monomer a). For example, an ethylenically unsaturated, acid group-carrying monomer having a corresponding content of hydroquinone half-ether can be used to prepare the monomer solution. However, the hydroquinone half ethers can also be removed from the monomer solution by absorption, for example on activated charcoal. Preferred hydroquinone half ethers are hydroquinone monomethyl ether (MEHQ) and / or alpha tocopherol (vitamin E). Suitable crosslinkers b) are compounds having at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups which can be radically copolymerized into the polymer chain, and functional groups which can form covalent bonds with the acid groups of the monomer a). Furthermore, polyvalent metal salts which can form coordinative bonds with at least two acid groups of the monomer a) are also suitable as crosslinking agents b).

Vernetzer b) sind vorzugsweise Verbindungen mit mindestens zwei polymerisierbaren Gruppen, die in das Polymernetzwerk radikalisch einpolymerisiert werden können. Geeignete Vernetzer b) sind beispielsweise Ethylenglykoldimethacrylat, Diethylenglykoldiacrylat, Polyethylenglykoldi- acrylat, 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 2003/104299 A1 , WO 2003/104300 A1 , WO 2003/104301 A1 und DE 103 31 450 A1 beschrieben, gemischte Acryla- te, die neben Acrylatgruppen weitere ethylenisch ungesättigte Gruppen enthalten, wie in DE 103 31 456 A1 und DE 103 55 401 A1 beschrieben, oder Vernetzermischungen, wie beispielsweise in DE 195 43 368 A1 , DE 196 46 484 A1 , WO 90/15830 A1 und WO 2002/032962 A2 beschrieben. Crosslinkers b) are preferably compounds having at least two polymerizable groups which can be incorporated in the polymer network in free-radically polymerized form. Suitable crosslinkers b) 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 in EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93/21237 A1, WO 2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 A1 and DE 103 31 450 A1, mixed acrylates which, in addition to acrylate groups, contain further ethylenically unsaturated Groups, as described 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 2002/032962 A2.

Bevorzugte Vernetzer b) sind Pentaerythrittriallylether, Tetraalloxyethan, Methylenbismethac- rylamid, 15 bis 30-fach ethoxiliertes Glyzerintriacrylat, 15 bis 30-fach ethoxiliertes Trimethylolpropantriacrylat, 15 bis 20-fach ethoxiliertes Trimethylolethantriacrylat, 15 bis 20-fach ethoxiliertes Trimethylolpropantriacrylat, Polyethylenglykoldiacrylat, Trimethylolpropantriacrylat und Triallylamin. Preferred crosslinkers b) are pentaerythritol triallyl ether, tetraalloxyethane, methylenebismethacrylamide, 15 to 30 times ethoxylated glycerol triacrylate, 15 to 30 times ethoxylated trimethylolpropane triacrylate, 15 to 20 times ethoxylated trimethylolethane triacrylate, 15 to 20 times ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate and triallylamine ,

Ganz besonders bevorzugte Vernetzer b) sind die mit Acrylsäure oder Methacrylsäure zu Dioder Triacrylaten veresterten mehrfach ethoxylierten und/oder propoxylierten Glyzerine, wie sie beispielsweise in WO 2003/104301 A1 beschrieben sind. Besonders vorteilhaft sind Di- und/oder Triacrylate des 3- bis 10-fach ethoxylierten Glyzerins. Ganz besonders bevorzugt sind Di- oder Triacrylate des 1 - bis 5-fach ethoxylierten und/oder propoxylierten Glyzerins. Am meisten bevorzugt sind die Triacrylate des 3- bis 5-fach ethoxylierten und/oder propoxylierten Glyzerins, insbesondere das Triacrylat des 3-fach ethoxylierten Glyzerins. Very particularly preferred crosslinkers b) are the polyethyleneglyoxylated and / or propoxylated glycerols esterified with acrylic acid or methacrylic acid to form diioder triacrylates, as described, for example, in WO 2003/104301 A1. Particularly advantageous are di- and / or triacrylates of 3- to 10-fold ethoxylated glycerol. Very particular preference is given to diacrylates or triacrylates of 1 to 5 times ethoxylated and / or propoxylated glycerol. Most preferred are the triacrylates of 3 to 5 times ethoxylated and / or propoxylated glycerol, in particular the triacrylate of 3-times ethoxylated glycerol.

Die Menge an Vernetzer b) beträgt vorzugsweise 0,01 bis 1 ,5 Gew.-%, besonders bevorzugt 0,05 bis 1 Gew.-%, ganz besonders bevorzugt 0,1 bis 0,6 Gew.-%, jeweils bezogen auf das unneutralisierte Monomer a). Mit steigendem Vernetzergehalt sinkt die Zentrifugenretentionska- pazität (CRC) und die Absorption unter einem Druck von 21 ,0 g/cm2 (AUL0.3psi) durchläuft ein Maximum. Als Initiatoren c) können sämtliche unter den Polymerisationsbedingungen in Radikale zerfallende Verbindungen eingesetzt werden, beispielsweise Peroxide, Hydroperoxide, Wasserstoffperoxid, Persulfate, Azoverbindungen und die so genannten Redoxinitiatoren. Bevorzugt ist der Einsatz von wasserlöslichen Initiatoren. In manchen Fällen ist es vorteilhaft, Misohnnnpn \ pr- schiedener Initiatoren zu verwenden, beispielsweise Mischungen aus Wasserstoffperoxid und Natrium- oder Kaliumperoxodisulfat. Mischungen aus Wasserstoffperoxid und Natriumperoxo- disulfat können in jedem beliebigen Verhältnis verwendet werden. Besonders bevorzugte Initiatoren c) sind Azoinitiatoren, wie 2,2'-Azobis-[2-(2-imidazolin-2- yl)propan]-dihydrochlorid und 2,2'-Azobis-[2-(5-methyl-2-imidazolin-2-yl)propan]-dihydrochlorid, Wasserstoffperoxid, Persulfate, wie Natriumperoxodisulfat und Ammoniumperoxodisulfat, und Redoxinitiatoren, wie Natriumperoxodisulfat/Ascorbinsäure, Ammoniumperoxodisul- fat/Ascorbinsäure und Wasserstoffperoxid/Ascorbinsäure, sowie deren Mischungen. The amount of crosslinker b) is preferably from 0.01 to 1, 5 wt .-%, particularly preferably 0.05 to 1 wt .-%, most preferably 0.1 to 0.6 wt .-%, each based on the unneutralized monomer a). As the crosslinker content increases, the centrifuge retention capacity (CRC) decreases and the absorption under a pressure of 21.0 g / cm 2 (AUL 0.3 psi) passes through a maximum. As initiators c) it is possible to use all compounds which decompose into free radicals under the polymerization conditions, for example peroxides, hydroperoxides, hydrogen peroxide, persulfates, azo compounds and the so-called redox initiators. Preference is given to the use of water-soluble initiators. In some cases, it is beneficial to have misohnnn p n \ p r different initiators, for example mixtures of hydrogen peroxide and sodium or potassium peroxodisulfate. Mixtures of hydrogen peroxide and sodium peroxodisulfate can be used in any ratio. Particularly preferred initiators c) are azo initiators, such as 2,2'-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride and 2,2'-azobis [2- (5-methyl-2-) imidazolin-2-yl) propane] dihydrochloride, hydrogen peroxide, persulfates such as sodium peroxodisulfate and ammonium peroxodisulfate, and redox initiators such as sodium peroxodisulfate / ascorbic acid, ammonium peroxodisulfate / ascorbic acid and hydrogen peroxide / ascorbic acid, and mixtures thereof.

Die Initiatoren werden in üblichen Mengen eingesetzt, beispielsweise in Mengen von 0,001 bis 5 Gew.-%, vorzugsweise 0,01 bis 2 Gew.-%, bezogen auf das unneutralisierte Monomer a). The initiators are used in customary amounts, for example in amounts of 0.001 to 5 wt .-%, preferably 0.01 to 2 wt .-%, based on the unneutralized monomer a).

Mit den ethylenisch ungesättigten, säuregruppentragenden Monomeren a) copolymerisierbare ethylenisch ungesättigte Monomere d) sind beispielsweise Acrylamid, Methacrylamid, Hydro- xyethylacrylat, Hydroxyethylmethacrylat, Dimethylaminoethylmethacrylat, Dimethylaminoethyl- acrylat, Dimethylaminopropylacrylat, Diethylaminopropylacrylat, Dimethylaminoethylmethacrylat, Diethylaminoethylmethacrylat. Als wasserlösliche Polymere e) können Polyvinylalkohol, Polyvinylpyrrolidon, Stärke, Stärkederivate, modifizierte Cellulose, wie Methylcellulose oder Hydroxyethylcellulose, Gelatine, Polygly- kole oder Polyacrylsäuren, vorzugsweise Stärke, Stärkederivate und modifizierte Cellulose, eingesetzt werden. Der Wassergehalt der Monomerlösung beträgt vorzugsweise weniger als 65 Gew.-%, bevorzugt weniger als 62 Gew.-%, besonders bevorzugt weniger als 60 Gew.-%, ganz besonders bevorzugt weniger als 58 Gew.-%. For example, acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, are ethylenically unsaturated monomers d) which are copolymerizable with the ethylenically unsaturated acid group-carrying monomers a). As water-soluble polymers e) it is possible to use polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, modified cellulose, such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycols or polyacrylic acids, preferably starch, starch derivatives and modified cellulose. The water content of the monomer solution is preferably less than 65 wt .-%, preferably less than 62 wt .-%, more preferably less than 60 wt .-%, most preferably less than 58 wt .-%.

Die Monomerlösung hat bei 20°C eine Dichte von vorzugsweise 1 bis 1 ,3 g/cm3, besonders bevorzugt 1 ,05 bis 1 ,25 g/cm3, besonders bevorzugt 1 ,1 bis 1 ,2 g/cm3. The monomer solution at 20 ° C has a density of preferably 1 to 1, 3 g / cm 3, more preferably 1, 05-1, 25 g / cm 3, more preferably 1, 1 to 1, 2 g / cm 3.

Die wässrige Monomerlösung wird unter Ausbildung diskreter Tropfen auf die hydrophobe Oberfläche dosiert. Die hydrophobe Oberfläche weist gegenüber Wasser einen Randwinkel von vorzugsweise mindestens 60°, besonders bevorzugt mindestens 80°, ganz besonders bevor- zugt mindestens 100°, auf. Der Randwinkel ist ein Maß für das Benetzungsverhalten und wird gemäß DIN 53900 gemessen. Je höher der Randwinkel ist, desto hydrophober ist eine Oberfläche. The aqueous monomer solution is metered onto the hydrophobic surface to form discrete droplets. The hydrophobic surface has a contact angle with respect to water of preferably at least 60 °, more preferably at least 80 °, most preferably at least 100 °. The contact angle is a measure of the wetting behavior and is measured according to DIN 53900. The higher the contact angle, the more hydrophobic a surface is.

Die Tropfen können mittels pneumatischer Ziehdüsen, Rotation, Zerschneiden eines Strahls oder schnell ansteuerbarer Mikroventildüsen erzeugt werden. In einer pneumatischen Ziehdüse wird ein Flüssigkeitsstrahl zusammen mit einem Gasstrom durch eine Blende beschleunigt. Über die Gasmenge kann der Durchmesser des Flüssigkeitsstrahls und damit der Tropfendurchmesser beeinflusst werden. Bei der Tropfenerzeugung durch Rotation tritt die Flüssigkeit durch die Öffnungen einer rotierenden Scheibe. Durch die auf die Flüssigkeit wirkende Fliehkraft werden Tropfen definierter Größe abgerissen. Bevorzugte Vorrichtungen zur Rotationsvertropfung werden beispielsweise in DE 43 08 842 A1 beschrieben. Der austretende Flüssigkeitsstrahl kann aber auch mittels eines rotierenden Messers in definierte Segmente zerschnitten werden. Jedes Segment bildet anschließend einen Tropfen. The drops can be generated by means of pneumatic drawing nozzles, rotation, cutting a jet or quickly activatable micro-valve nozzles. In a pneumatic die, a liquid jet is accelerated along with a gas flow through a baffle. About the amount of gas, the diameter of the liquid jet and thus the droplet diameter can be influenced. In the case of droplet generation by rotation, the liquid passes through the openings of a rotating disk. Due to the centrifugal force acting on the liquid, drops of a defined size are torn off. Preferred devices for Rotationsvertropfung be described for example in DE 43 08 842 A1. However, the exiting liquid jet can also be cut into defined segments by means of a rotating knife. Each segment then forms a drop.

Bei Verwendung von Mikroventildüsen werden direkt Tropfen mit definiertem Flüssigkeitsvolumen erzeugt. When using micro-valve nozzles directly drops are generated with a defined volume of liquid.

In einer bevorzugten Ausführungsform der vorliegenden Erfindung wird die Monomerlösung mittels mindestens einer Bohrung unter Ausbildung von Tropfen dosiert (vertropft). Der Tropfendurchmesser beträgt dabei üblicherweise das 1 ,9-fache des Durchmessers der Bohrung. Wichtig ist hierbei, dass die Flüssigkeit nicht zu schnell durch die Bohrung tritt bzw. der Druck- verlust über die Bohrung nicht zu groß ist. Ansonsten wird die Flüssigkeit nicht vertropft, sondern der Flüssigkeitsstrahl wird infolge der hohen kinetischen Energie zerrissen (versprüht). Die Reynoldszahl bezogen auf den Durchsatz pro Bohrung und den Bohrungsdurchmesser ist vorzugsweise kleiner als 2.000, bevorzugt kleiner 1.600, besonders bevorzugt kleiner 1 .400, ganz besonders bevorzugt kleiner 1.200. In a preferred embodiment of the present invention, the monomer solution is metered (dripped) by means of at least one bore to form droplets. The drop diameter is usually 1, 9 times the diameter of the hole. It is important that the liquid does not pass through the hole too quickly or that the pressure loss through the hole is not too great. Otherwise, the liquid is not dripped, but the liquid jet is torn due to the high kinetic energy (sprayed). The Reynolds number based on the throughput per bore and the bore diameter is preferably less than 2,000, preferably less than 1,600, more preferably less than 1, 400, most preferably less than 1200.

Die Temperatur der Monomerlösung beim Durchtritt durch die Bohrung beträgt vorzugsweise 10 bis 60°C, besonders bevorzugt 15 bis 50°C, ganz besonders bevorzugt 20 bis 40°C. The temperature of the monomer solution when passing through the bore is preferably 10 to 60 ° C, more preferably 15 to 50 ° C, most preferably 20 to 40 ° C.

Die Reaktion kann im Überdruck oder im Unterdruck durchgeführt werden, ein Unterdruck von bis zu 100 mbar gegenüber dem Umgebungsdruck ist bevorzugt. The reaction can be carried out in overpressure or under reduced pressure, a negative pressure of up to 100 mbar relative to the ambient pressure is preferred.

Die wasserabsorbierenden Polymerpartikel können zur weiteren Verbesserung der Eigenschaften oberflächennachvernetzt werden. Geeignete Oberflächennachvernetzer sind Verbindungen, die Gruppen enthalten, die mit mindestens zwei Carboxylatgruppen der Polymerpartikel kova- lente 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 water-absorbing polymer particles can be surface-postcrosslinked to further improve the properties. Suitable surface postcrosslinkers are compounds which contain groups which 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 β-hydroxyalkylamides, as described in DE 102 04 938 A1 and US Pat. No. 6,239,230.

Des Weiteren sind in DE 40 20 780 C1 zyklische Karbonate, in DE 198 07 502 A1 Furthermore, in DE 40 20 780 C1 cyclic carbonates, in DE 198 07 502 A1

2-Oxazolidinon und dessen Derivate, wie 2-Hydroxyethyl-2-oxazolidinon, in DE 1 QR Π7 QQ9 CA Bis- und Poly-2-oxazolidinone, in DE 198 54 573 A1 2-Oxotetrahydro-1 ,3-oxazin und dessen Derivate, in DE 198 54 574 A1 N-Acyl-2-Oxazolidinone, in DE 102 04 937 A1 zyklische Harnstoffe, in DE 103 34 584 A1 bizyklische Amidoacetale, in EP 1 199 327 A2 Oxetane und zyklische Harnstoffe und in WO 2003/031482 A1 Morpholin-2,3-dion und dessen Derivate als ge- eignete Oberflächennachvernetzer beschrieben. 2-Oxazolidinone and its derivatives, such as 2-hydroxyethyl-2-oxazolidinone, in DE 1 QR Π7 QQ9 CA Bis- and poly-2-oxazolidinones, in DE 198 54 573 A1 2-oxotetrahydro-1,3-oxazine and its derivatives, in DE 198 54 574 A1 N-acyl-2-oxazolidinones, in DE 102 04 937 A1 cyclic ureas in DE 103 34 584 A1 bicyclic amidoacetals, in EP 1 199 327 A2 oxetanes and cyclic ureas and in WO 2003/031482 A1 morpholine-2,3-dione and its derivatives as suitable surface postcrosslinkers.

Bevorzugte Oberflächennachvernetzer sind Ethylenkarbonat, Ethylenglykoldiglycidylether, Umsetzungsprodukte von Polyamiden mit Epichlorhydrin und Gemische aus Propylenglykol und 1 ,4-Butandiol. Preferred surface postcrosslinkers are ethylene carbonate, ethylene glycol diglycidyl ether, reaction products of polyamides with epichlorohydrin and mixtures of propylene glycol and 1,4-butanediol.

Ganz besonders bevorzugte Oberflächennachvernetzer sind 2-Hydroxyethyl-2-oxazolidinon, 2- Oxazolidinon und 1 ,3-Propandiol. Very particularly preferred surface postcrosslinkers are 2-hydroxyethyl-2-oxazolidinone, 2-oxazolidinone and 1, 3-propanediol.

Weiterhin können auch Oberflächennachvernetzer eingesetzt werden, die zusätzliche polymeri- sierbare ethylenisch ungesättigte Gruppen enthalten, wie in DE 37 13 601 A1 beschrieben Furthermore, it is also possible to use surface postcrosslinkers which contain additional polymerizable ethylenically unsaturated groups, as described in DE 37 13 601 A1

Die Menge an Oberflächennachvernetzer beträgt vorzugsweise 0,001 bis 5 Gew.-%, besonders bevorzugt 0,02 bis 2 Gew.-%, ganz besonders bevorzugt 0,05 bis 1 Gew.-%, jeweils bezogen auf die Polymerpartikel. The amount of surface postcrosslinker is preferably 0.001 to 5 wt .-%, more preferably 0.02 to 2 wt .-%, most preferably 0.05 to 1 wt .-%, each based on the polymer particles.

In einer bevorzugten Ausführungsform der vorliegenden Erfindung werden vor, während oder nach der Oberflächennachvernetzung 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, Eisen 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 Hydroxid, Chlorid, Bromid, Sulfat, Hydrogensulfat, Carbonat, Hydrogencarbonat, Nitrat, Phosphat, Hydrogenphosphat, Dihydrogenphosphat und Carboxylat, wie Acetat, Citrat und Lactat, möglich. Es sind auch Salze mit unterschiedlichen Gegenionen möglich, beispielsweise basische Aluminiumsalze, wie Aluminiummonoacetat oder Aluminiummonolaktat. Aluminiumsulfat, Alu- miniummonoacetat und Aluminiumlaktat sind bevorzugt. Außer Metallsalzen können auch Poly- amine als polyvalente Kationen eingesetzt werden. In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the surface postcrosslinkers before, during or after the surface postcrosslinking. The polyvalent cations which can be used in the process according to the invention are, for example, divalent cations, such as the cations of zinc, magnesium, calcium, iron 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. As a counterion hydroxide, chloride, bromide, sulfate, hydrogen sulfate, carbonate, bicarbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate and carboxylate, such as acetate, citrate and lactate, are possible. It is also possible to use salts with different counterions, for example basic aluminum salts, such as aluminum monoacetate or aluminum monolactate. Aluminum sulfate, aluminum monoacetate and aluminum lactate are preferred. Apart from metal salts, polyamines can also be used as polyvalent cations.

Die Einsatzmenge an polyvalentem Kation beträgt beispielsweise 0,001 bis 1 Gew.-%, vorzugsweise 0,005 bis 0,5 Gew.-%, besonders bevorzugt 0,02 bis 0,2 Gew.-%. jeweils bezogen auf die Polymerpartikel. Die Oberflächennachvernetzung wird üblicherweise so durchgeführt, dass eine Lösung des Oberflächennachvernetzers auf die getrockneten Polymerpartikel aufgesprüht wird. Im An- schluss an das Aufsprühen werden die mit Oberflächennachvernetzer beschichteten Polymer- Partikel thermisch getrocknet, wobei die Oberflächennachvernetzungsreaktion sowohl vor als auch während der Trocknung stattfinden kann. The amount of polyvalent cation used is, for example, from 0.001 to 1% by weight, preferably from 0.005 to 0.5% by weight, more preferably from 0.02 to 0.2% by weight. in each case based on the polymer particles. The surface postcrosslinking is usually carried out so that a solution of the surface postcrosslinker is sprayed onto the dried polymer particles. Subsequent to the spraying, the surface postcrosslinker-coated polymer Particles thermally dried, wherein the surface postcrosslinking reaction can take place both before and during drying.

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. Ge- eignete Mischer sind beispielsweise Horizontale Pflugschar® Mischer (Gebr. Lödige Maschinenbau GmbH; Paderborn; Deutschland), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Doetinchem; Niederlande), Processall Mixmill Mixer (Processall 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 postcrosslinker is preferably carried out in mixers with moving mixing tools, such as screw mixers, disk mixers and paddle mixers. Particularly preferred are horizontal mixers, such as paddle mixers, very particularly preferred are vertical mixers. The distinction between horizontal mixer and vertical mixer is made by the storage of the mixing shaft, i. 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, The Netherlands), Processall Mixmill Mixer (Processall Incorporated, Cincinnati, USA) and Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, The Netherlands). However, it is also possible to spray the surface postcrosslinker 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. Wird ausschließlich Wasser als Lösungsmittel verwendet, so wird vorteilhaft ein Tensid zugesetzt. Dadurch wird das Benetzungsverhalten verbessert und die Verklumpungsneigung vermindert. Vorzugsweise werden aber Lösungsmittelgemische eingesetzt, beispielsweise Isopro- panol/Wasser, 1 ,3-Propandiol/Wasser und Propylenglykol/Wasser, wobei das Mischungsmassenverhältnis vorzugsweise von 20:80 bis 40:60 beträgt. The surface postcrosslinkers are typically used as an aqueous solution. The penetration depth of the surface postcrosslinker into the polymer particles can be adjusted by the content of nonaqueous solvent or total solvent amount. If only water is used as the solvent, it is advantageous to add a surfactant. As a result, the wetting behavior is improved and the tendency to clog is reduced. However, preference is given to using solvent mixtures, for example isopropanol / water, 1,3-propanediol / water and propylene glycol / water, the mixing mass ratio preferably being from 20:80 to 40:60.

Die thermische Trocknung wird vorzugsweise 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 NaraThe thermal drying is preferably carried out in contact dryers, more preferably paddle dryers, very particularly preferably disk dryers. Suitable dryers include 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; Deutschland). Überdies können auch Wirbelschichttrockner eingesetzt werden. Paddle Dryer (NARA Machinery Europe, Frechen, Germany). Moreover, fluidized bed dryers can also be used.

Die Trocknung kann im Mischer selbst erfolgen, durch Beheizung des Mantels oder Einblasen von Warmluft. Ebenso geeignet ist ein nachgeschalteter Trockner, wie beispielsweise ein Hordentrockner, ein Drehrohrofen oder eine beheizbare Schnecke. Besonders vorteilhaft wird in einem Wirbelschichttrockner gemischt und getrocknet. The drying can take place in the mixer itself, by heating the jacket or blowing hot air. Also suitable is a downstream dryer, such as a hopper dryer, a rotary kiln or a heatable screw. Particularly advantageous is mixed and dried in a fluidized bed dryer.

Bevorzugte Trocknungstemperaturen liegen im Bereich 100 bis 250°C, bevorzugt 120 bis 220°C, besonders bevorzugt 130 bis 210°C, ganz besonders bevorzugt 150 bis 200°C. Die bevorzugte Verweilzeit bei dieser Temperatur im Reaktionsmischer oder Trockner beträgt vorzugsweise mindestens 10 Minuten, besonders bevorzugt mindestens 20 Minuten, ganz besonders bevorzugt mindestens 30 Minuten, und üblicherweise höchstens 60 Minuten In einer bevorzugten Ausführungsform der vorliegenden Erfindung werden die wasserabsorbierenden Polymerpartikel nach der thermischen Trocknung gekühlt. Die Kühlung wird vorzugsweise in Kontaktkühlern, besonders bevorzugt Schaufelkühlern, ganz besonders bevorzugt Scheibenkühlern, durchgeführt. Geeignete Kühler sind beispielsweise Hosokawa Bepex® Horizontal Paddle Cooler (Hosokawa Micron GmbH; Leingarten; Deutschland), Hosokawa Bepex® Disc Cooler (Hosokawa Micron GmbH; Leingarten; Deutschland), Holo-Flite® coolers (Metso Minerals Industries Inc.; Danville; USA) und Nara Paddle Cooler (NARA Machinery Europe; Frechen; Deutschland). Überdies können auch Wirbelschichtkühler eingesetzt werden. Preferred drying temperatures are in the range 100 to 250 ° C, preferably 120 to 220 ° C, more preferably 130 to 210 ° C, most preferably 150 to 200 ° C. The preferred residence time at this temperature in the reaction mixer or dryer is preferably at least 10 minutes, more preferably at least 20 minutes, most preferably at least 30 minutes, and usually at most 60 minutes In a preferred embodiment of the present invention, the water-absorbing polymer particles are cooled after the thermal drying. The cooling is preferably carried out in contact coolers, particularly preferably blade coolers, very particularly preferably disk coolers. Suitable coolers are, for example, Hosokawa Bepex® Horizontal Paddle Coolers (Hosokawa Micron GmbH, Leingarten, Germany), Hosokawa Bepex® Disc Coolers (Hosokawa Micron GmbH, Leingarten, Germany), Holo-Flite® coolers (Metso Minerals Industries, Inc., Danville, USA ) and Nara Paddle Cooler (NARA Machinery Europe, Frechen, Germany). Moreover, fluidized bed coolers can also be used.

Im Kühler werden die wasserabsorbierenden Polymerpartikel auf 20 bis 150°C, vorzugsweise 30 bis 120°C, besonders bevorzugt 40 bis 100°C, ganz besonders bevorzugt 50 bis 80°C, abgekühlt. Die oberflächennachvernetzten Polymerpartikel können zur weiteren Verbesserung der Eigenschaften beschichtet oder nachbefeuchtet werden. In the cooler, the water-absorbing polymer particles to 20 to 150 ° C, preferably 30 to 120 ° C, more preferably 40 to 100 ° C, most preferably 50 to 80 ° C, cooled. The surface-postcrosslinked polymer particles can be coated or post-moistened for further improvement of the 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 nei- gen die wasserabsorbierenden 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.-%, jeweils bezogen auf die wasserabsorbierenden Polymerpartikel. 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 Trocknung durchgeführt. The post-wetting is preferably carried out at 30 to 80 ° C, more preferably at 35 to 70 ° C, most preferably at 40 to 60 ° C. If the temperatures are too low, the water-absorbing polymer particles tend to clump together, and water evaporates appreciably at higher temperatures. The amount of water used for the rewetting is preferably from 1 to 10 wt .-%, particularly preferably from 2 to 8 wt .-%, most preferably from 3 to 5 wt .-%, each based on the water-absorbing polymer particles. By rewetting the mechanical stability of the polymer particles is increased and their tendency to static charge reduced. Advantageously, the post-humidification is carried out in the cooler after the thermal drying.

Geeignete Beschichtungen zur Verbesserung der Anquellgeschwindigkeit sowie der Permeabilität (SFC) sind beispielsweise anorganische inerte Substanzen, wie wasserunlösliche Metallsal- ze, 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, und Tenside, wie Span® 20. Suitable coatings for improving the swelling rate and the permeability (SFC) are, for example, inorganic inert substances, such as water-insoluble metal salts, organic polymers, cationic polymers and di- or polyvalent metal cations. Suitable coatings for dust binding are, for example, polyols. Suitable coatings against the unwanted caking tendency of the polymer particles are, for example, fumed silica, such as Aerosil® 200, and surfactants, such as Span® 20.

Ein weiterer Gegenstand der vorliegenden Erfindung sind gemäß dem erfindungsgemäßen Verfahren erhältliche Polymerpartikel. Another object of the present invention are obtainable by the process according to the invention polymer particles.

Die erfindungsgemäßen Polymerpartikel haben einen Anteil an Partikeln mit einer Partikelgröße von 300 bis 600 μηη von vorzugsweise mindestens 30 Gew.-%, besonders bevorzugt mindesten 50 Gew.-%, ganz besonders bevorzugt mindestens 70 Gew.-%. Die erfindungsgemäßen Polymerpartikel weisen einen Feuchtegehalt von vorzugsweise 0,1 bis 20 Gew.-%, besonders bevorzugt von 1 bis 15 Gew.-%, ganz besonders bevorzugt von 5 bis 10 Gew.-%, auf. Der Feuchtegehalt wird gemäß der von der EDANA empfohlenen Testmethode Nr. WSP 230.2-05 "Mass Loss Upon Heating" bestimmt. The polymer particles according to the invention have a proportion of particles with a particle size of 300 to 600 μm, preferably of at least 30% by weight, more preferably at least 50% by weight, very particularly preferably at least 70% by weight. The polymer particles of the invention have a moisture content of preferably 0.1 to 20 wt .-%, particularly preferably from 1 to 15 wt .-%, most preferably from 5 to 10 wt .-%, on. The moisture content is determined according to the EDANA recommended test method No. WSP 230.2-05 "Mass Loss Upon Heating".

Die erfindungsgemäßen Polymerpartikel weisen eine Zentrifugenretentionskapazität (CRC) von typischerweise mindestens 25 g/g, vorzugsweise mindestens 30 g/g, bevorzugt mindestens 32 g/g, besonders bevorzugt mindestens 34 g/g, ganz besonders bevorzugt mindestens 36 g/g, auf. Die Zentrifugenretentionskapazität (CRC) der Polymerpartikel beträgt üblicherweise weni- ger als 60 g/g. Die Zentrifugenretentionskapazität (CRC) wird gemäß der von der EDANA empfohlenen Testmethode Nr. WSP 241.2-05 "Fluid Retention Capacity in Saline, After Centrifuga- tion" bestimmt. The polymer particles according to the invention have a centrifuge retention capacity (CRC) of typically at least 25 g / g, preferably at least 30 g / g, preferably at least 32 g / g, more preferably at least 34 g / g, most preferably at least 36 g / g. The centrifuge retention capacity (CRC) of the polymer particles is usually less than 60 g / g. Centrifuge retention capacity (CRC) is determined according to EDANA recommended test method no. WSP 241.2-05 "Fluid Retention Capacity in Saline, After Centrifugation".

Die erfindungsgemäßen Polymerpartikel weisen einen Gehalt an Restmonomeren von typi- scherweise weniger als 2 Gew.-%, vorzugsweise weniger als 1 ,5 Gew.-%, bevorzugt weniger als 1 Gew.-%, besonders bevorzugt weniger als 0,6 Gew.-%, ganz besonders bevorzugt weniger als 0,4 Gew.-%, auf. Der Gehalt an Restmonomeren der Polymerpartikel wird gemäß der von der EDANA empfohlenen Testmethode Nr. WSP 210.2-05 " Residual Monomers " bestimmt. The polymer particles according to the invention have a content of residual monomers of typically less than 2% by weight, preferably less than 1.5% by weight, preferably less than 1% by weight, more preferably less than 0.6% by weight. %, most preferably less than 0.4 wt .-%, on. The content of residual monomers of the polymer particles is determined according to the EDANA recommended test method no. WSP 210.2-05 "Residual Monomers".

Weitere Gegenstände der vorliegenden Erfindung sind Hygieneartikel, welche die erfindungsgemäßen wasserabsorbierenden Polymerpartikel enthalten. Further articles of the present invention are hygiene articles which contain the water-absorbing polymer particles according to the invention.

Die wasserabsorbierenden Polymerpartikel werden mittels der nachfolgend beschriebenen Testmethoden geprüft. The water-absorbing polymer particles are tested by the test methods described below.

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 wasserabsorbierenden Polymerpartikel werden vor der Messung gut durchmischt. Measurements should be taken at an ambient temperature of 23 ± 2 ° C and a relative humidity of 50 ± 10%, unless otherwise specified. The water-absorbing polymer particles are thoroughly mixed before the measurement.

Zentrifugenretentionskapazität (Centrifuge Retention Capacity) Centrifuge Retention Capacity

Die Zentrifugenretentionskapazität (CRC) wird analog der von der EDANA empfohlenen Testmethode Nr. WSP 241.2-05 "Fluid Retention Capacity in Saline, After Centrifugation" bestimmt, wobei 0,1 g wasserabsorbierende Polymerpartikel statt 0,2 g eingewogen werden. Restmonomer Centrifuge Retention Capacity (CRC) is determined analogously to the EDANA recommended Test Method No. WSP 241.2-05 "Fluid Retention Capacity in Saline, After Centrifugation", using 0.1 g of water-absorbing polymer particles instead of 0.2 g. residual monomer

Der Gehalt an Restmonomer der wasserabsorbierenden Polymerpartikel wird gemäß der von der EDANA empfohlenen Testmethode WSP Nr. 210.2-05 "Residual Monomers" bestimmt. The residual monomer content of the water-absorbing polymer particles is determined according to the EDANA-recommended test method WSP No. 210.2-05 "Residual Monomers".

Die EDANA-Testmethoden sind beispielsweise erhältlich bei der EDANA, Avenue Eugene Plasky 157, B-1030 Brüssel, Belgien. The EDANA test methods are available, for example, from EDANA, Avenue Eugene Plasky 157, B-1030 Brussels, Belgium.

Beispiele: Examples:

Beispiel 1 (Vergleichsbeispiel) Eine Monomerlösung wurde auf eine mit Silikonkautschuk beschichteten Unterlage versprüht. Die Monomerlösung enthielt mit Natronlauge teilneutralisierte Acrylsäure. Die Monomerlösung enthielt zusätzlich 0,05 Gew.-% Sartomer® SR454 (15-fach ethoxiliertes Trimethylolpropantriac- rylat), 0,1 Gew.-% Wako® VA-044 (2,2'-Azobis[2-(2-imidazolin-2-yl)propan]dihydrochlorid), 0,008 Gew.-% Büggolit® FF7 (Dinatriumsalz der 2-Hydroxy-2-sulfinatoessigsäure) 0,15 Gew.-% Natriumperoxodisulfat und 0,19 Gew.-% Darocur® 1 173 (2-Hydroxy-2-methyl-1 -phenylpropan- 1 -on), jeweils bezogen auf unneutralisierte Acrylsäure. Der Feststoffgehalt der Monomerlösung betrug 40 Gew.-%. Example 1 (Comparative Example) A monomer solution was sprayed on a silicone rubber-coated pad. The monomer solution contained partially neutralized acrylic acid with caustic soda. The monomer solution additionally contained 0.05% by weight of Sartomer® SR454 (15-times ethoxylated trimethylolpropane triacylate), 0.1% by weight of Wako® VA-044 (2,2'-azobis [2- (2-imidazoline -2-yl) propane] dihydrochloride), 0.008% by weight of Büggolit® FF7 (disodium salt of 2-hydroxy-2-sulfinatoacetic acid), 0.15% by weight of sodium peroxodisulfate and 0.19% by weight of Darocur® 1 173 ( 2-hydroxy-2-methyl-1-phenylpropane-1-one), each based on unneutralized acrylic acid. The solids content of the monomer solution was 40% by weight.

Die Tropfen wurden 5 Minuten mittels eines UV-Strahlers vom Typ MH-Strahler UV 400 F/2 (Dr. Höhnle AG, Gräfelfing, Deutschland) bestrahlt und anschließend 30 Minuten bei 160°C im Trockenschrank gelagert. Die erhaltenen Polymerpartikel wurden auf 150 bis 850 μηη abgesiebt und analysiert. Die Ergebnisse sind in Tabelle 1 zusammengefasst. The drops were irradiated for 5 minutes by means of a UV radiator of the type MH radiator UV 400 F / 2 (Dr. Höhnle AG, Gräfelfing, Germany) and then stored for 30 minutes at 160 ° C in a drying oven. The resulting polymer particles were screened to 150 to 850 μηη and analyzed. The results are summarized in Table 1.

Beispiel 2 (Vergleichsbeispiel) Example 2 (comparative example)

Es wurde verfahren wie unter Beispiel 1. Abweichend von Beispiel 1 wurden die Tropfen 66 Minuten mittels eines UV-Strahlers vom Typ MH-Strahler UV 400 F/2 bestrahlt und anschließend nicht im Trockenschrank gelagert. Beispiel 3 The procedure was as in Example 1. Deviating from Example 1, the drops were irradiated for 66 minutes by means of a UV radiator of the MH-UV 400 F / 2 type and then not stored in a drying oven. Example 3

Es wurde verfahren wie unter Beispiel 1. Abweichend von Beispiel 1 wurden die Tropfen nicht bestrahlt, sondern nur für 30 Minuten bei 160°C im Trockenschrank gelagert. Tab. 1 : Ergebnisse der Polymerisation The procedure was as in Example 1. Deviating from Example 1, the drops were not irradiated, but stored only for 30 minutes at 160 ° C in a drying oven. Tab. 1: Results of the polymerization

Figure imgf000013_0001
Figure imgf000013_0001

*) Vergleichsbeispiel * ) Comparative Example

Die Beispiele zeigen, dass nur bei der thermischen Polymerisation ein Polymer mit hoher Zent- rifugenretentionskapazität (CRC) und wenig Restmonomer erhalten wird. The examples show that only in thermal polymerization is a polymer with high center retention capacity (CRC) and little residual monomer obtained.

Beispiel 4 (Vergleichsbeispiel) Example 4 (Comparative Example)

Eine Monomerlösung wurde auf eine mit Silikonkautschuk beschichteten Unterlage versprüht. Die Monomerlösung enthielt mit Natronlauge teilneutralisierte Acrylsaure. Die Monomerlösung enthielt zusätzlich 0,05 Gew.-% Sartomer® SR344 (Polyethylenglykol-400-diacrylat), 0,1 Gew.- % Wako® VA-044 (2,2'-Azobis[2-(2-imidazolin-2-yl)propan]dihydrochlorid), 0,008 Gew.-%A monomer solution was sprayed onto a silicone rubber coated pad. The monomer solution contained partially neutralized acrylic acid with caustic soda. The monomer solution additionally contained 0.05% by weight of Sartomer® SR344 (polyethylene glycol 400-diacrylate), 0.1% by weight of Wako® VA-044 (2,2'-azobis [2- (2-imidazoline-2 -yl) propane] dihydrochloride), 0.008 wt%

Büggolit® FF7 (Dinatriumsalz der 2-Hydroxy-2-sulfinatoessigsäure) 0,15 Gew.-% Natriumper- oxodisulfat und 0,042 Gew.-% Darocur® TPO (Diphenyl-(2,4,6-trimethylbenzoyl)-phosphinoxid), jeweils bezogen auf unneutralisierte Acrylsaure. Der Feststoffgehalt der Monomerlösung betrug 40 Gew.-%. Büggolit® FF7 (disodium salt of 2-hydroxy-2-sulfinatoacetic acid) 0.15% by weight of sodium peroxodisulfate and 0.042% by weight of Darocur® TPO (diphenyl- (2,4,6-trimethylbenzoyl) -phosphine oxide), in each case based on unneutralized acrylic acid. The solids content of the monomer solution was 40% by weight.

Die Tropfen wurden 5 Minuten mittels eines UV-Strahlers vom Typ MH-Strahler UV 400 F/2 bestrahlt und anschließend 30 Minuten bei 160°C im Trockenschrank gelagert. Die erhaltenen Polymerpartikel wurden auf 150 bis 850 μηη abgesiebt und analysiert. Die Ergebnisse sind in Tabelle 2 zusammengefasst. The drops were irradiated for 5 minutes by means of a UV radiator of the type MH radiator UV 400 F / 2 and then stored for 30 minutes at 160 ° C in a drying oven. The resulting polymer particles were screened to 150 to 850 μηη and analyzed. The results are summarized in Table 2.

Beispiel 5 Example 5

Es wurde verfahren wie unter Beispiel 4. Abweichend von Beispiel 5 wurde kein Darocur® TPO verwendet und die Tropfen wurden nicht bestrahlt, sondern nur für 30 Minuten bei 160°C im Trockenschrank gelagert. Tab. 2: Ergebnisse der Polymerisation The procedure was as in Example 4. Deviating from Example 5, no Darocur® TPO was used and the drops were not irradiated, but stored only for 30 minutes at 160 ° C in a drying oven. Tab. 2: Results of the polymerization

Figure imgf000014_0001
Figure imgf000014_0001

*) Vergleichsbeispiel * ) Comparative Example

Die Beispiele zeigen wieder, dass nur bei der thermischen Polymerisation ein Polymer her Zentrifugenretentionskapazitat (CRC) und wenig Restmonomer erhalten wird. The examples again show that only in the thermal polymerization is a polymer obtained centrifuge retention capacity (CRC) and little residual monomer.

Claims

Patentansprüche claims Verfahren zur Polymerisation einer wässrigen Monomerlösung, enthaltend mindestens ein ethylenisch ungesättigtes Monomer a), optional einen Vernetzer b), mindestens einen Initiator c) und Wasser, wobei in einem ersten Schritt i) Tropfen erzeugt werden und die Tropfen in einem zweiten Schritt ii) auf einer hydrophoben Oberfläche polymerisiert werden, dadurch gekennzeichnet, dass der mindestens eine Initiator c) ein thermischer Initiator ist und die Polymerisation in Schritt ii) thermisch durchgeführt wird. A process for the polymerization of an aqueous monomer solution comprising at least one ethylenically unsaturated monomer a), optionally a crosslinker b), at least one initiator c) and water, wherein in a first step i) drops are produced and the droplets in a second step ii) a hydrophobic surface are polymerized, characterized in that the at least one initiator c) is a thermal initiator and the polymerization in step ii) is carried out thermally. Verfahren gemäß Anspruch 1 , dadurch gekennzeichnet, dass die Monomerlösung im Wesentlichen keinen Photoinitiator enthält und die Polymerisation im Wesentlichen in Abwesenheit von UV-Strahlen durchgeführt wird. A method according to claim 1, characterized in that the monomer solution contains substantially no photoinitiator and the polymerization is carried out substantially in the absence of UV rays. Verfahren gemäß Anspruch 1 oder 2, dadurch gekennzeichnet, dass die Temperatur bei der thermischen Polymerisation in Schritt ii) von 100 bis 250°C beträgt. A method according to claim 1 or 2, characterized in that the temperature in the thermal polymerization in step ii) is from 100 to 250 ° C. Verfahren gemäß einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, dass der Initiator c) ein Azoinitiator, ein Peroxid, ein Hydroperoxid, Wasserstoffperoxid oder ein Persulfat ist. A method according to any one of claims 1 to 3, characterized in that the initiator c) is an azo initiator, a peroxide, a hydroperoxide, hydrogen peroxide or a persulfate. Verfahren gemäß einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, dass die Monomerlösung mindestens ein ethylenisch ungesättigtes, säuregruppentragendes Monomer, mindestens einen Vernetzer und mindestens einen Initiator enthält. Method according to one of claims 1 to 4, characterized in that the monomer solution contains at least one ethylenically unsaturated, acid group-carrying monomer, at least one crosslinker and at least one initiator. Verfahren gemäß Anspruch 5, dadurch gekennzeichnet, dass die Säuregruppen des ethylenisch ungesättigten, säuregruppentragenden Monomers zu 50 bis 85 mol-% neutralisiert sind. A method according to claim 5, characterized in that the acid groups of the ethylenically unsaturated, acid group-carrying monomer are neutralized to 50 to 85 mol%. Verfahren gemäß Anspruch 5 oder 6, dadurch gekennzeichnet, dass das ethylenisch ungesättigte, säuregruppentragende Monomer eine ethylenisch ungesättigte Carbonsäure ist. A method according to claim 5 or 6, characterized in that the ethylenically unsaturated, acid group-carrying monomer is an ethylenically unsaturated carboxylic acid. Polymerpartikel, erhältlich gemäß einem Verfahren der Ansprüche 1 bis 7. Polymer particles obtainable according to a process of claims 1 to 7. Polymerpartikel gemäß Anspruch 8, wobei die Polymerpartikel eine Zentrifugenretenti- onskapazität von mindestens 25 g/g aufweisen. Polymer particles according to claim 8, wherein the polymer particles have a centrifuge retention capacity of at least 25 g / g. Polymerpartikel gemäß Anspruch 8 oder 9, wobei die Polymerpartikel weniger als Polymer particles according to claim 8 or 9, wherein the polymer particles are less than 0,4 Gew.-% Restmonomer enthalten.  0.4 wt .-% residual monomer.
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