Classifications

• • 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
  • C08F6/00—Post-polymerisation treatments
  • C08F6/008—Treatment of solid polymer wetted by water or organic solvents, e.g. coagulum, filter cakes
• • 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
  • A61L15/22—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons containing macromolecular materials
  • A61L15/24—Macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds; Derivatives thereof
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J3/00—Processes of treating or compounding macromolecular substances
  • C08J3/12—Powdering or granulating
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J2300/00—Characterised by the use of unspecified polymers
  • C08J2300/14—Water soluble or water swellable polymers, e.g. aqueous gels

Definitions

• the present invention relates to a process for the continuous production of water-absorbing polymer particles, comprising polymerization, drying, milling, classification and at least partial recycling of the undersize resulting from the classification, wherein the polymer gel obtained by the polymerization is removed from the polymerization reactor and mixed with the recirculated undersize.
• Water-absorbent polymers are used in the manufacture of diapers, tampons, sanitary napkins and other sanitary articles, but also as water-retaining agents in agricultural horticulture.
• the properties of the water-absorbing polymers can be adjusted via the degree of crosslinking. As the degree of cross-linking increases, the gel strength increases and the centrifuge retention capacity (CRC) decreases.
• CRC centrifuge retention capacity
• water-absorbing polymer particles are generally surface-postcrosslinked.
• the degree of crosslinking of the particle surface increases, whereby the absorption under a pressure of 49.2 g / cm 2 and the centrifuge retention capacity (CRC) can be at least partially decoupled.
• This surface postcrosslinking can be carried out in aqueous gel phase.
• dried, ground and sieved polymer particles (base polymer) are coated on the surface with a surface postcrosslinker, thermally surface-postcrosslinked and dried.
• Crosslinkers suitable for this purpose are compounds which contain at least two groups which can form covalent bonds with the carboxylate groups of the water-absorbing polymers.
• the water-absorbing polymers are used as pulverulent, granular product, preferably in the hygiene sector.
• particle sizes between 150 and 850 microns are used and the particulate polymer material is already classified in the manufacturing process to these particle sizes.
• continuous lending screening machines are used with at least two sieves, with sieves are used with the mesh sizes of 150 and 850 microns. Particles with a grain size of up to 150 ⁇ m fall through both screens and are collected at the bottom of the screening machine as undersize. Particles with a particle size of larger 850 microns remain as oversize grain on the top sieve and are discharged, ground again and recycled.
• the product fraction with a particle size of greater than 150 to 850 microns is taken as a medium grain between the two sieves of the screening machine.
• the undersize and oversize grain resulting from the classification process is usually returned to the production process.
• the recirculation of the undersize is described, for example, in EP 0 463 388 A1, EP 0 496 594 A2, EP 0 785 224 A2, EP 1 878 761 A1 and US Pat. No. 5,064,582.
• EP 0 463 388 A1 describes that pumpable polymer gels having a high solids content can be obtained by adding a small amount of undersize.
• EP 0 496 594 A2 teaches the recirculation of the undersize into the polymerization reactor.
• EP 0 785 224 A2 describes the recirculation of the undersize into the polymer gel formed during the polymerization, surfactants being added.
• EP 1 878 761 A1 discloses a process for recycling an undersize coated with water-soluble polyvalent metal salts.
• the undersize can be mixed for example by means of a kneader in the polymer gel.
• US 5,064,582 discloses a process for recirculating undersize wherein the undersize is mixed with water prior to recycle.
• the object of the present invention was to provide an improved process for recycling the undersize resulting from the production of water-absorbing polymer particles.
• the object was achieved by a process for the continuous production of water-absorbing polymer particles by polymerization of a monomer solution or suspension containing
• a thick or viscous mass can be forced under high pressure through at least one shaping orifice.
• a very high mixing energy can be entered.
• an undersized grain is a grain size fraction obtained during the classification which has a smaller mean grain size than the grain size fraction of the target product.
• a sieve with a mesh size of up to 300 ⁇ m is usually used.
• the mesh size of the screen is preferably at least 100 ⁇ m, particularly preferably at least 150 ⁇ m, very particularly preferably at least 200 ⁇ m.
• the water content of the polymer gel removed from the polymerization reactor is preferably from 40 to 75% by weight, particularly preferably from 50 to 70% by weight, very particularly preferably from 55 to 65% by weight.
• the water content of the recirculated undersize is preferably less than 8% by weight, more preferably less than 6% by weight, most preferably less than 5% by weight.
• the low water content of the recirculated undersize has the consequence that the undersize is metered as a free-flowing powder in the mixing device.
• the surface tension of the aqueous extract of the water-absorbing polymer particles is preferably at least 0.063 N / m, more preferably at least 0.066 N / m, most preferably at least 0.068 N / m.
• the water content of the mixture obtained by mixing the polymer gel taken from the polymerization reactor with the recirculated undersize is preferably from 40 to 68% by weight, more preferably from 50 to 65% by weight, most preferably from 55 to 62% by weight.
• the polymer gel taken from the polymerization reactor is preferably mixed with the recirculated undersize from 1 to 180 minutes, more preferably from 2 to 60 minutes, most preferably from 5 to 20 minutes.
• the withdrawn from the polymerization reactor polymer gel is mixed at a temperature of preferably 40 to 80 0 C, particularly preferably from 45 up to 75 ° C, very particularly preferably from 50 to 70 0 C, with the recycled undersize.
• the ratio of polymer gel to recirculated undersize is preferably from 5 to 50, particularly preferably from 10 to 40, very particularly from 12 to 30.
• the polymerization reactors which can be used for the process according to the invention are subject to no restriction.
• the inventive method is particularly advantageous in a static polymerization, for example, in the polymerization on a continuously circulating belt.
• the process according to the invention preferably comprises at least one surface postcrosslinking.
• both before and after the surface postcrosslinking are classified.
• the present invention is based on the finding that the addition of undersize from the ongoing process leads to a significant drop in centrifuge retention capacity. This undesirable effect can be largely suppressed if the undersize is added only after the polymerization reactor.
• the present invention is further based on the finding that the mixing result can be significantly improved if the water content during mixing in the extruder is kept low. Only thereby is it possible a sufficiently large Entering mixing energy and break up intermediately occurring Unterkornagglomerate.
• the prior art known from the prior art pasting the undersize with water or the addition of surfactants is no longer necessary.
• the water-absorbing polymer particles are prepared by polymerization of a monomer solution or suspension and are usually water-insoluble.
• 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, 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
• 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 of furfurals, 0.0001% by weight of maleic 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 monomers a) usually contain polymerization inhibitors, preferably hydroquinone half ethers, as a 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, more preferably at least 30 ppm by weight, in particular by 50% by weight .-ppm, hydroquinone, in each case based on the unneutralized monomer a).
• an ethylenically unsaturated compound may be used to prepare the monomer solution. saturated acid-bearing monomer having a corresponding hydroquinone half-ether content.
• 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).
• 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 530 438 A1, di- and triacrylates, as in EP 547 847 A1, EP 559 476 A1, EP 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 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 2002/32962 A2.
• Preferred crosslinkers b) are pentaerythritol triallyl ether, tetraalloxyethane, methylenebis-methacrylamide, 15-times ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate and triallylamine.
• Very particularly preferred crosslinkers b) are the polyethoxylated and / or propoxylated glycerols esterified with acrylic acid or methacrylic acid to form di- or 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 0.05 to 1, 5 wt .-%, particularly preferably 0.1 to 1 wt .-%, most preferably 0.3 to 0.6 wt .-%, in each case attracted to monomer a).
• the centrifuge retention capacity decreases and the absorption under a pressure of 21.0 g / cm 2 passes through a maximum.
• initiators c) it is possible to use all compounds which generate free radicals under the polymerization conditions, for example thermal initiators, redox initiators, photoinitiators.
• Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite and hydrogen peroxide / sodium bisulfite.
• Preference is given to using mixtures of thermal initiators and redox initiators, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid.
• a reducing component but preferably a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite is used.
• Such mixtures are available as Brüggolite® FF6 and Brüggolite® FF7 (Brüggemann Chemicals, Heilbronn, DE).
• Examples of ethylenically unsaturated monomers d) which can be copolymerized with the ethylenically unsaturated monomers having acid groups are acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate.
• 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.
• an aqueous monomer solution is used.
• the water content of the monomer solution is preferably from 40 to 75 wt .-%, particularly preferably from 45 to 70 wt .-%, most preferably from 50 to 65 wt .-%.
• monomer suspensions i. Monomer solutions with excess monomer a), for example sodium acrylate, use. With increasing water content, the energy expenditure increases during the subsequent drying and with decreasing water content, the heat of polymerization can only be dissipated insufficiently.
• the preferred polymerization inhibitors require dissolved oxygen for optimum performance. Therefore, the monomer solution can be freed of dissolved oxygen prior to the polymerization by inertization, ie by flowing through with an inert gas, preferably nitrogen or carbon dioxide.
• the oxygen content of the monomer solution before the polymerization is preferably reduced to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.
• Suitable reactors are, for example, kneading reactors or belt reactors.
• the polymer gel formed in the polymerization of an aqueous monomer solution or suspension is comminuted continuously by, for example, counter-rotating stirring shafts, as described in WO 2001/38402 A1.
• the polymerization on the belt is described, for example, in DE 38 25 366 A1 and US Pat. No. 6,241,928.
• Polymerization in a belt reactor produces a polymer gel which must be comminuted in a further process step, for example in a meat grinder, extruder or kneader.
• the acid groups of the polymer gels obtained are usually partially neutralized.
• the neutralization is preferably carried out at the stage of the monomers. This is usually done by mixing the neutralizing agent as an aqueous solution or preferably as a solid.
• the degree of neutralization is preferably from 25 to 95 mol%, particularly preferably from 50 to 80 mol%, very particularly preferably from 60 to 75 mol%, the usual neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or Alkalimetallhydrogenkarbonate and mixtures thereof.
• 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.
• the polymer gel is also possible to carry out the neutralization after the polymerization at the stage of the polymer gel formed during the polymerization. Furthermore, it is possible to neutralize up to 40 mol%, preferably 10 to 30 mol%, particularly preferably 15 to 25 mol%, of the acid groups before the polymerization by adding a part of the neutralizing agent already to the monomer solution and the desired final degree of neutralization is adjusted only after the polymerization at the level of the polymer gel. If the polymer gel is at least partially neutralized after the polymerization, the polymer gel is preferably comminuted mechanically, for example by means of an extruder, wherein the neutralizing agent can be sprayed, sprinkled or poured on and then thoroughly mixed in. For this purpose, the gel mass obtained can be extruded several times for homogenization.
• the polymer gel is then preferably dried with a belt dryer until the residual moisture content is preferably 0.5 to 15 wt .-%, particularly preferably 1 to 10 wt .-%, most preferably 2 to 8 wt .-%, wherein the residual moisture content according to the test method No. WSP 230.2-05 "Moisture content" recommended by EDANA (European Disposables and Nonwovens Association). If the residual moisture content is too high, the dried polymer gel has too low a glass transition temperature T 9 and is difficult to process further. At a too low residual moisture, the dried polymer gel is too brittle and in the subsequent crushing steps fall undesirable large amounts of polymer particles with too small particle size (undersize).
• the solids content of the gel before drying is preferably from 25 to 90% by weight, more preferably from 35 to 70% by weight, most preferably from 40 to 60% by weight.
• a fluidized bed dryer or a heated ploughshare mixer can be used for drying.
• the dried polymer gel is then ground and classified, wherein for grinding usually one- or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibratory mills can be used.
• the mean particle size of the polymer fraction separated as a product fraction is preferably at least 200 ⁇ m, more preferably from 250 to 600 ⁇ m, very particularly from 300 to 500 ⁇ m.
• the mean particle size of the product fraction can be determined by means of the test method no. WSP 220.2-05 "particle size distribution" recommended by the EDANA (European Disposables and Nonwovens Association), in which the mass fractions of the sieve fractions are cumulatively applied and the mean particle size is determined graphically becomes.
• the mean particle size here is the value of the mesh size, which results for accumulated 50 wt .-%.
• the proportion of particles having a particle size of at least 150 .mu.m is preferably at least 90 wt .-%, more preferably at least 95 wt .-%, most preferably at least 98 wt .-%.
• Too small polymer particles are therefore usually separated and recycled to the process.
• the proportion of particles having a particle size of at most 850 microns is preferably at least 90 wt .-%, more preferably at least 95 wt .-%, most preferably at least 98 wt .-%.
• Suitable surface postcrosslinkers are compounds containing groups that can form covalent bonds with at least two carboxylate groups of the polymer particles. Suitable compounds are, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, as described in EP 83 022 A2, EP 543 303 A1 and EP 937 736 A2, di- or polyfunctional alcohols, as in DE 33 14 019 A1, DE 35 23 617 A1 and EP 450 922 A2, or ⁇ -hydroxyalkylamides, as described in DE 102 04 938 A1 and US Pat. No. 6,239,230.
• 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-hydroxyethyloxazolidin-2-one, oxazolidin-2-one and 1,3-propanediol.
• the amount of surface postcrosslinker is preferably 0.001 to 2 wt .-%, more preferably 0.02 to 1 wt .-%, most preferably 0.05 to 0.2 wt .-%, each based on the polymer particles.
• polyvalent cations are applied to the particle surface before, during or after the surface postcrosslinking in addition to the surface postcrosslinkers.
• the amount of polyvalent cation used is, for example, 0.001 to 1.5% by weight, preferably 0.005 to 1% by weight, particularly preferably 0.02 to 0.8% by weight. in each case based on the polymer 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 polymer particles coated with the surface postcrosslinker are thermally dried, whereby 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 agitated mixing tools, such as screw mixers, disk mixers, plowshare mixers and paddle mixers.
• agitated mixing tools such as screw mixers, disk mixers, plowshare mixers and paddle mixers.
• horizontal mixers such as plowshare mixers and paddle mixers
• vertical mixers very particularly preferred are vertical mixers.
• 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 Lödige mixers, Bepex mixers, Nauta mixers, Processall mixers and Schugi mixers.
• the thermal drying is preferably carried out in contact dryers, more preferably paddle dryers, very particularly preferably disk dryers.
• Suitable dryers include Bepex-T rockner and Nara-T rockner.
• 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.
• the surface-postcrosslinked polymer particles can be coated or post-moistened for further improvement of the properties.
• 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.
• the water-absorbing polymer particles produced by the process according to the invention have an absorption under a pressure of 49.2 g / cm 2 of typically at least 15 g / g, preferably at least 20 g / g, preferably at least 22 g / g, particularly preferably at least 24 g / g, most preferably at least 26 g / g, on.
• the absorption under a pressure of 49.2 g / cm 2 of the water-absorbing polymer particles is usually less than 35 g / g.
• the absorption under a pressure of 49.2 g / cm 2 is determined analogously to the recommended by the EDANA (European Disposables and Nonwovens Association) Test Method No. WSP 242.2-05 "absorption under pressure", instead of a pressure of 21, 0 g / cm 2 a pressure of 49.2 g / cm 2 is set.
• the water-absorbing polymer particles are tested by the test methods described below.
• the measurements should be taken at an ambient temperature of 23 ⁇ 2 0 C and a relative humidity of 50 ⁇ 10% unless otherwise specified.
• the water-absorbing polymer particles are thoroughly mixed before the measurement.
• the residual monomers are determined according to the test method no. WSP 210.2-05 "Residual Monomers” recommended by the EDANA (European Disposables and Nonwovens Association).
• the centrifuge retention capacity (CRC) is determined according to the test method no. WSP 241.2- 05 "Centrifuge Retention Capacity" recommended by the EDANA (European Disposables and Nonwovens Association).
• the EDANA test methods are available, for example, from the European Disposables and Nonwovens Association, Avenue Eugene Plasky 157, B-1030 Brussels, Associates.
• the monomer solution was transferred by means of a funnel into a glass dish with a
• the glass dish was covered with a plastic film and also inertized with 150 l / h of nitrogen.
• the monomer solution in the glass dish was stirred by means of a magnetic stir bar.
• 5.88 g of a 1% strength by weight aqueous solution of Bruggolite® FF6 (disodium salt of 2-hydroxy-2-sulfinatoacetic acid) were metered into the monomer solution using a disposable syringe. After the start of the reaction, the magnetic stirrer was switched off.
• the polymer gel was removed and crushed with a perforated plate extruder (6 mm hole diameter), sprayed with 17.6 g of a 1% by weight aqueous solution of sodium bisulfite, and extruded twice.
• the gel was spread over four sheets and dried for one hour at 16O 0 C in a convection oven.
• the loading of the sheets with polymer gel was 0.59 g / cm 2 . It was then pre-shredded with a roller mill with a gap width of 1,000 ⁇ m and homogenized using a roller mixer.
• a partial amount of about 100 g was comminuted with a two-stage roller mill with a gap width of 600 microns and 400 microns and sieved to 150 to 850 microns (base polymer A). The remaining amount was comminuted by means of a rotor mill (Retsch® ZM200) to a particle size of less than 150 ⁇ m (undersize A).
• the monomer solution was transferred by means of a funnel into a glass dish with a
• the glass dish was covered with a plastic film and also inertized with 150 l / h of nitrogen.
• the monomer solution in the glass dish was stirred by means of a magnetic stir bar.
• 5.88 g of a 1% strength by weight aqueous solution of Bruggolit® FF6 (disodium salt of 2-hydroxy-2-sulfinatoacetic acid) were metered into the monomer solution by means of a disposable syringe. After the start of the reaction, the magnetic stirrer was switched off.
• the polymer gel was removed and crushed with a perforated plate extruder (6 mm hole diameter), sprayed with 17.6 g of a 1% by weight aqueous solution of sodium bisulfite, and re-extruded. Subsequently, a total of 84 g of undersize A from Example 1 in two portions powdered by means of a 180 ⁇ m sieve and a spoon and extruded a third time.
• the gel was spread over four sheets and dried for one hour at 16O 0 C in a convection oven.
• the loading of the sheets with polymer gel was 0.59 g / cm 2 . It was then pre-shredded with a roller mill with a gap width of 1,000 ⁇ m, comminuted with a two-stage roller mill with a gap width of 600 ⁇ m and 400 ⁇ m and sieved to 150 to 850 ⁇ m.
• Example 8 The procedure was as in Example 8. Together with the dosage of the undersize, 0.0100 or 0.0166% by weight of sorbitan monococoate in 1% by weight of water, in each case based on the polymer gel, was sprayed onto the polymer gel.

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