EP2076338B2 - Method for grading water-absorbent polymer particles - Google Patents

Description

The present invention relates to a method for classifying water-absorbing polymer particles, the polymer particles being separated into n grain size fractions by means of at least n sieves and n being an integer greater than 1.

The production of water-absorbing polymer particles is described in the monograph " Modern Superabsorbent Polymer Technology", FL Buchholz and AT Graham, Wiley-VCH, 1998, pages 71 to 103 , described.

Water-absorbing polymers are used as aqueous solution-absorbing products 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.

Water-absorbing polymer particles are generally post-crosslinked to improve the application properties, such as liquid conduction in the swollen gel bed (SFC) in the diaper and absorption under pressure (AUL). This only increases the degree of crosslinking of the particle surface, which means that the absorption under pressure (AUL) and the centrifuge retention capacity (CRC) can be at least partially decoupled. This post-crosslinking can be carried out in an aqueous gel phase. Preferably, however, dried, ground and sieved polymer particles (base polymer) are coated on the surface with a post-crosslinking agent, post-crosslinked thermally 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 hydrophilic polymer.

The water-absorbing polymers are preferably used in the hygiene sector as a powdery, granular product. Here, for example, particle sizes between 200 and 850 µm are used and the particulate polymer material is already classified to these grain sizes during the manufacturing process. Continuous screening machines with two screens are used here, with screens having mesh sizes of 200 and 850 µm. Particles with a grain size of up to 200 µm fall through both screens and are collected as undersize at the bottom of the screening machine. Particles with a grain size of more than 850 µm remain as oversize on the top sieve and are discharged. The product fraction with a grain size of more than 200 to 850 µm is removed as a middle grain between the two screens of the screening machine. Depending on the screening quality, each grain size fraction still contains a proportion of particles with the wrong grain size as so-called incorrect discharge. For example, the oversize fraction can still contain a proportion of particles with a grain size of 850 μm or less.

Discharged undersize and oversize is usually sent to the production returned. The undersize can be added to the polymerization, for example. The oversize is usually crushed, which inevitably leads to an inevitable accumulation of further undersize.

In the conventional classification operations, various problems are encountered when classifying particulate polymers. The most common problem is clogging of the screen surface and deterioration in classification efficiency and classability. Another problem is the product's tendency to cake, which leads to undesirable agglomerates before, after and during screening. The process step of screening can therefore not be carried out without disruptions, often accompanied by unwanted downtimes in polymer production. Such disturbances prove to be particularly problematic in the continuous production process. Overall, however, this results in insufficient selectivity during screening. This problem can be observed above all in the classification of post-crosslinked product.

A higher screening quality is usually achieved by adding substances to the product that serve to increase the pourability and/or the mechanical stability of the polymer powder. As a rule, a free-flowing product is obtained if auxiliaries, for example surfactants, which prevent the individual particles from sticking together, are added to the polymer powder, usually after drying and/or during post-crosslinking. In other cases, attempts are made to influence the caking tendencies by means of procedural measures.

In order to achieve higher selectivity without additional product additives, improvements using alternative screening systems were proposed. In this way, higher degrees of selectivity are achieved when the screen opening surfaces are driven in a spiral manner. This is the case, for example, with tumbler screening machines. However, as the throughput of such screening devices is increased, the above problems are exacerbated, and it becomes less and less possible to maintain the high classifying ability.

The addition of sieving aids, such as sieve balls, PVC friction rings, Teflon friction rings or rubber cubes, to the surface of the sieve only helps to increase the selectivity to an insignificant extent. This can lead to increased abrasion, especially in the case of amorphous polymer material, such as water-absorbing polymer particles.

A general overview of the classification is for example in Ullmann's encyclopedia of technical chemistry, 4th edition, volume 2, pages 43 to 56, Verlag Chemie, Weinheim, 1972 , to find.

EP 855 232 A2 describes a classification method for water-absorbing polymers. By using heated or thermally insulated screens, agglomerates below the screen are avoided, especially with small grain sizes.

DE 10 2005 001 789 A1 describes a classification process that is carried out at reduced pressure.

JP 2003/320308 A describes a process in which agglomerates are avoided by blowing warm air onto the underside of the screen.

WO 92/18171 A1 describes the addition of inorganic powders as screening aids.

The object of the present invention was to provide an improved classification process for producing water-absorbing polymer particles.

The object was achieved by a method for classifying water-absorbing polymer particles, the polymer particles being separated into n particle size fractions and n being an integer greater than 1, characterized in that at least n sieves are used, the mesh sizes of the n sieves decrease in the product flow direction by at least two in Screen fractions that occur one after the other in the product flow direction are combined to form a grain size fraction, with the mesh sizes of the screens on which these screen fractions occur differing by at least 50 µm, the grain size fractions in the sequence (1,2,1), (2,2,1) , (1,3,1), (3,2,1), (2,3,1) or (3,3,1) are summarized, where the number of numbers in brackets stands for the number of grain size fractions, the particle size fractions are arranged in the product flow sequence in the brackets from left to right and the numerical values themselves stand for the number of successive screening fractions that belong to the respective Gen grain size fraction are summarized, and the hourly throughput of water-absorbing polymer particles during classification is at least 100 kg per m ² screen surface.

A sieve separates a particulate material into two sieve fractions, the particles that remain on the sieve and the particles that pass through the mesh of the sieve. By using additional sieves, each sieve fraction can be separated into two further sieve fractions. When using n sieves, (n+1) sieve fractions are obtained, with each sieve fraction being able to be further processed separately as a grain size fraction. An essential feature of the present invention, on the other hand, is that at least two of these sieve fractions are combined into one grain size fraction and further processed together. Compared to the previously customary methods for classifying water-absorbing polymer particles, the method according to the invention therefore uses at least one more sieve.

Water-absorbing polymer particles with improved absorption under pressure (AUL) and improved liquid transfer in the swollen gel bed (SFC) are obtained by using the at least one additional sieve.

The number of grain size fractions is preferably at least 3. The number of sieves used is preferably at least (n+1).

In a preferred embodiment of the present invention, at least two sieve fractions occurring one after the other in the product flow direction are combined to form a grain size fraction, with the mesh sizes of the sieves on which these sieve fractions occur preferably increasing by at least 100 μm each, preferably by at least 150 μm each, particularly preferably by each at least 200 μm, very particularly preferably by at least 250 μm in each case.

In a further preferred embodiment of the present invention, the at least two sieve fractions occurring first in the product flow direction are combined to form a grain size fraction, with the mesh sizes of the sieves on which these sieve fractions occur preferably increasing by at least 500 μm each, preferably by at least 1,000 μm each, particularly preferably differ by at least 1,500 μm in each case, very particularly preferably by at least 2,000 μm in each case.

During classification, the water-absorbing polymer particles preferably have a temperature of 40 to 120°C, particularly preferably 45 to 100°C, very particularly preferably 50 to 80°C.

In a preferred embodiment of the present invention, classification is carried out at reduced pressure. The pressure is preferably 100 mbar less than the ambient pressure.

The classification method according to the invention is particularly advantageously carried out continuously. The throughput of water-absorbing polymer is at least 100 kg/m ² .h, preferably at least 150 kg/m ² .h, preferably at least 200 kg/m ² .h, particularly preferably at least 250 kg/m ² .h, very particularly preferably at least 300 kg/m ² .h.

The screening devices suitable for the classification process according to the invention are not subject to any restrictions; flat screening processes are preferred, and tumbling screening machines are very particularly preferred. The screening apparatus is typically vibrated to aid in classification. This is preferably done in such a way that the material to be classified is guided spirally over the screen. This forced vibration typically has an amplitude of 0.7 to 40 mm, preferably 1.5 to 25 mm, and a frequency of 1 to 100 Hz, preferably 5 to 10 Hz.

In a preferred embodiment of the present invention, at least one screening machine with n screens is used. It is advantageous if several screening machines are operated in parallel.

A stream of gas, particularly preferably air, preferably flows over the water-absorbing resin during classification. The amount of gas is typically from 0.1 to 10 m ³ /h per m ² screen area, preferably from 0.5 to 5 m ³ /h per m ² screen area, particularly preferably from 1 to 3 m ³ /h per m ² screen area, where the gas volume is measured under standard conditions (25 °C and 1 bar). The gas stream is particularly preferably heated before entering the screening device, typically to a temperature of 40 to 120° C., preferably to a temperature of 50 to 110° C., preferably to a temperature of 60 to 100° C., particularly preferably to Temperature from 65 to 90 °C, most preferably to a temperature of 70 to 80 °C. The water content of the gas stream is typically less than 5 g/kg, preferably less than 4.5 g/kg, preferably less than 4 g/kg, more preferably less than 3.5 g/kg, most preferably less than 3 g/kg . A gas stream with a low water content can be generated, for example, by a gas stream with higher Water content a corresponding amount of water is condensed by cooling.

The screening machines are usually electrically grounded.

The water-absorbing polymer particles to be used in the process according to the invention can be prepared by polymerizing monomer solutions containing at least one ethylenically unsaturated monomer a), optionally at least one crosslinker b), at least one initiator c) and water d).

The monomers a) are preferably water-soluble, ie the solubility in water at 23° C. is typically at least 1 g/100 g water, preferably at least 5 g/100 g water, particularly preferably at least 25 g/100 g water, very particularly preferably at least 50 g/100 g water, and preferably each have at least one acid group.

Examples of suitable monomers a) are ethylenically unsaturated carboxylic acids, such as acrylic acid, methacrylic acid, maleic acid, fumaric acid and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Acrylic acid is very particularly preferred.

The preferred monomers a) have at least one acid group, the acid groups preferably being at least partially neutralized.

The proportion of acrylic acid and/or its salts in the total amount of the monomers a) is preferably at least 50 mol %, particularly preferably at least 90 mol %, very particularly preferably at least 95 mol %.

The monomers a), in particular acrylic acid, preferably contain up to 0.025% by weight of a hydroquinone monoether. Preferred hydroquinone hemiethers are hydroquinone monomethyl ether (MEHQ) and/or tocopherols.

wobei R¹ Wasserstoff oder Methyl, R² Wasserstoff oder Methyl, R³ Wasserstoff oder Methyl und R⁴ Wasserstoff oder ein Säurerest mit 1 bis 20 Kohlenstoffatomen bedeutet.Tocopherol is understood to mean compounds of the following formula

where R ^{1 is} hydrogen or methyl, R ^{2 is} hydrogen or methyl, R ^{3 is} hydrogen or methyl and R ^{4 is} hydrogen or an acid radical having 1 to 20 carbon atoms.

Preferred radicals for R ⁴ are acetyl, ascorbyl, succinyl, nicotinyl and other physiologically tolerable carboxylic acids. The carboxylic acids can be mono-, di- or tri-carboxylic acids.

Alpha-tocopherol with R ¹ ═R ² ═R ³ ═methyl is preferred, in particular racemic alpha-tocopherol. R ¹ is particularly preferably hydrogen or acetyl. RRR-alpha-tocopherol is particularly preferred.

The monomer solution preferably contains at most 130 ppm by weight, especially preferably at most 70 ppm by weight, preferably at least 10 ppm by weight, particularly preferably at least 30 ppm by weight, in particular around 50 ppm by weight, of hydroquinone monoether, based in each case on acrylic acid, acrylic acid salts also being taken into account as acrylic acid. For example, an acrylic acid with a corresponding content of hydroquinone half ether can be used to prepare the monomer solution.

Crosslinkers b) are compounds having at least two polymerizable groups which can be polymerized into the polymer network by free radicals. Suitable crosslinkers b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallyloxyethane, as in EP 530 438 A1 described, 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 described, mixed acrylates containing other ethylenically unsaturated groups in addition to acrylate groups, as in DE 103 31 456 A1 and DE 103 55 401 A1 described, or crosslinker mixtures, such as in DE 195 43 368 A1 , DE 196 46 484 A1 , WO 90/15830 A1 and WO 2002/32962 A2 described.

Suitable crosslinkers b) are in particular N,N'-methylenebisacrylamide and N,N'-methylenebismethacrylamide, esters of unsaturated mono- or polycarboxylic acids of polyols such as diacrylate or triacrylate, for example butanediol or ethylene glycol diacrylate or methacrylate and trimethylolpropane triacrylate and allyl compounds such as allyl (Meth)acrylate, triallyl cyanurate, maleic acid diallyl ester, polyallyl ester, tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allyl ester of phosphoric acid and vinylphosphonic acid derivatives, as for example in EP 343 427 A2 are described. Other suitable crosslinkers b) are pentaerythritol di-, pentaerythritol tri- and pentaerythritol tetraallyl ether, polyethylene glycol diallyl ether, ethylene glycol diallyl ether, glycerol di- and glycerol triallyl ether, polyallyl ether based on sorbitol, and ethoxylated variants thereof. Di(meth)acrylates of polyethylene glycols can be used in the process according to the invention, the polyethylene glycol used having a molecular weight of between 100 and 1000.

However, particularly advantageous crosslinkers b) are di- and triacrylates of 3- to 20-fold ethoxylated glycerol, 3- to 20-fold ethoxylated trimethylolpropane, 3- to 20-fold ethoxylated trimethylolethane, in particular di- and triacrylates of 2- to 6-fold ethoxylated glycerol or trimethylolpropane, 3-fold propoxylated glycerol or trimethylolpropane, and 3-fold mixed ethoxylated or propoxylated glycerol or trimethylolpropane, 15-fold ethoxylated glycerol or trimethylolpropane, and at least 40-fold ethoxylated glycerol, trimethylolethane or trimethylolpropane.

Very particularly preferred crosslinkers b) are the multiply ethoxylated and/or propoxylated glycerols esterified with acrylic acid or methacrylic acid to form di- or triacrylates, such as those described in WO 2003/104301 A1 are described. Di- and/or triacrylates of 3- to 10-tuply ethoxylated glycerol are particularly advantageous. Very particular preference is given to di- or triacrylates of 1- to 5-tuply ethoxylated and/or propoxylated glycerol. The triacrylates of 3- to 5-tuply ethoxylated and/or propoxylated glycerol are most preferred.

The amount of crosslinker b) is preferably 0.01 to 5% by weight, particularly preferably from 0.05 to 2% by weight, very particularly preferably from 0.1 to 1% by weight, based in each case on the monomer solution.

All compounds which form free radicals under the polymerization conditions can be used as initiators c), for example peroxides, hydroperoxides, hydrogen peroxide, persulfates, azo compounds and the so-called redox initiators. The use of water-soluble initiators is preferred. In some cases it is advantageous to use mixtures of different initiators, for example mixtures of hydrogen peroxide and sodium or potassium peroxodisulphate. 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-imidazoline-2). -yl)propane] dihydrochloride, and photoinitiators such as 2-hydroxy-2-methylpropiophenone and 1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one, redox initiators , such as sodium persulfate/hydroxymethylsulfinic acid, ammonium peroxodisulfate/hydroxymethylsulfinic acid, hydrogen peroxide/hydroxymethylsulfinic acid, sodium persulfate/ascorbic acid, ammonium peroxodisulfate/ascorbic acid and hydrogen peroxide/ascorbic acid, photoinitiators such as 1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl -1-propan-1-one, and mixtures thereof.

The initiators are used in customary amounts, for example in amounts of from 0.001 to 5% by weight, preferably from 0.01 to 1% by weight, based on the monomers a).

The preferred polymerization inhibitors require dissolved oxygen for optimal activity. Therefore, the monomer solution can be rendered inert prior to polymerization, i. H. Flowing through with an inert gas, preferably nitrogen, are freed from dissolved oxygen. The oxygen content of the monomer solution is preferably reduced to less than 1 ppm by weight, particularly preferably to less than 0.5 ppm by weight, before the polymerization.

The preparation of a suitable polymer and other suitable hydrophilic ethylenically unsaturated monomers a) are in DE 199 41 423 A1 , EP 686 650 A1 , WO 2001/45758 A1 and WO 2003/104300 A1 described.

Suitable reactors are kneading reactors or belt reactors. In the kneader, the polymer gel formed during the polymerisation of an aqueous monomer solution is continuously comminuted, for example by counter-rotating stirrer shafts, as in WO 2001/38402 A1 described. Polymerization on the belt is used, for example, in DE 38 25 366 A1 and U.S. 6,241,928 described. Polymerization in a belt reactor produces a polymer gel that has to be comminuted in a further process step, for example in a meat grinder, extruder or kneader. After leaving the polymerization reactor, the hydrogel is advantageously stored at a higher temperature, preferably at least 50° C., particularly preferably at least 70° C., very particularly preferably at least 80° C. and preferably less than 100° C., for example in insulated containers. The monomer conversion is further increased by the storage, usually 2 to 12 hours.

In the case of higher monomer conversions in the polymerization reactor, storage can also be significantly reduced or storage can be dispensed with.

The acid groups of the resulting hydrogels are usually partially neutralized, preferably to an extent of 25 to 95 mol %, preferably 50 to 80 mol %, particularly preferably 60 to 75 mol %, it being possible to use the customary neutralizing agents, preferably alkali metal hydroxides, alkali metal oxides , Alkali metal carbonates or alkali metal hydrogen carbonates and mixtures thereof. Instead of alkali metal salts, ammonium salts can also be used. Sodium and potassium are particularly preferred as alkali metals, but sodium hydroxide, sodium carbonate or sodium bicarbonate and mixtures thereof are very particularly preferred.

The neutralization is preferably carried out at the monomer stage. This is usually done by mixing in the neutralizing agent as an aqueous solution, as a melt, or preferably also as a solid. For example, sodium hydroxide with a water content well below 50% by weight can be present as a waxy mass with a melting point above 23.degree. In this case, dosing as piece goods or melt at elevated temperature is possible.

However, it is also possible to carry out the neutralization after the polymerization at the hydrogel stage. It is also 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 part of the neutralizing agent to the monomer solution and the desired final degree of neutralization only afterwards of the polymerization is stopped at the stage of the hydrogel. If the hydrogel is at least partially neutralized after the polymerization, the hydrogel is preferably mechanically comminuted, for example using a meat grinder, in which case the neutralizing agent can be sprayed on, sprinkled over or poured on and then carefully mixed in. For this purpose, the gel mass obtained can be minced several times for homogenization.

The hydrogel is then preferably dried using a belt dryer until the residual moisture content is preferably below 15% by weight, in particular below 10% by weight, the water content being in accordance with test method no. 430.2- recommended by EDANA (European Disposables and Nonwovens Association). 02 "Moisture content" is determined. Alternatively, a fluidized bed dryer or a heated ploughshare mixer can also be used for drying. In order to obtain particularly white products, it is advantageous to ensure rapid removal of the evaporating water when drying this gel. To do this, the dryer temperature must be optimized, the air supply and exhaust must be controlled, and sufficient ventilation must be ensured in any case. Drying is naturally all the easier and the product all the whiter when the solids content of the gel is as high as possible. The solids content of the gel before drying is therefore preferably between 30 and 80% by weight. Venting the dryer with nitrogen or another non-oxidizing inert gas is particularly advantageous. Optionally, however, just the partial pressure of the oxygen be lowered during drying in order to prevent oxidative yellowing processes.

The dried hydrogel is then ground and classified, in which case one-stage or multi-stage roller mills, preferably two-stage or three-stage roller mills, pinned mills, hammer mills or vibratory mills can usually be used for grinding.

The mean particle size of the polymer particles separated off as the product fraction is preferably at least 200 μm, particularly preferably from 250 to 600 μm, very particularly from 300 to 500 μm. The average particle size of the product fraction can be determined using the test method no. 420.2-02 "Particle size distribution" recommended by EDANA (European Disposables and Nonwovens Association), whereby the mass fractions of the sieve fractions are applied cumulatively and the average particle size is determined graphically. The mean particle size here is the value of the mesh size that results for a cumulative 50% by weight.

The polymer particles can be post-crosslinked to further improve the properties. Suitable post-crosslinkers are compounds containing groups capable of forming covalent bonds with the at least two carboxylate groups of the hydrogel. Suitable compounds are, for example, alkoxysilyl compounds, polyaziridines, polyamines, polyamidoamines, di- or polyepoxides, as in EP 83 022 A2 , EP 543 303 A1 and EP 937 736 A2 described, di- or polyfunctional alcohols, as in DE 33 14 019 A1 , DE 35 23 617 A1 and EP 450 922 A2 described, or ß-hydroxyalkylamides, as in DE 102 04 938 A1 and U.S. 6,239,230 described.

Furthermore, in DE 40 20 780 C1 cyclic carbonates, in DE 198 07 502 A1 2-oxazolidone and its derivatives, such as 2-hydroxyethyl-2-oxazolidone, in DE 198 07 992 C1 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-oxazolidones, in DE 102 04 937 A1 cyclic ureas, in DE 103 34 584 A1 bicyclic amide acetals, in EP 1 199 327 A2 Oxetanes and cyclic ureas and in WO 2003/31482 A1 Morpholine-2,3-dione and its derivatives are described as suitable post-crosslinkers.

Furthermore, post-crosslinkers can also be used which contain additional polymerizable ethylenically unsaturated groups, as in DE 37 13 601 A1 described

The amount of post-crosslinker is preferably 0.01 to 1% by weight, particularly preferably 0.05 to 0.5% by weight, very particularly preferably 0.1 to 0.2% by weight, based in each case on the polymer .

In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the post-crosslinkers.

The polyvalent cations that can be used in the process according to the invention are, for example, divalent cations such as zinc, magnesium, calcium and strontium cations, trivalent cations such as aluminum cations, iron cations, Chromium, rare earths and manganese, tetravalent cations such as the cations of titanium and zirconium. As a counter ion, chloride, bromide, sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate and carboxylate such as acetate and lactate are possible. Aluminum sulfate is preferred. In addition to metal salts, polyamines can also be used as polyvalent cations.

The amount of polyvalent cation used is, for example, 0.001 to 0.5% by weight, preferably 0.005 to 0.2% by weight, particularly preferably 0.02 to 0.1% by weight. in each case based on the polymer.

The post-crosslinking is usually carried out by spraying a solution of the post-crosslinker onto the hydrogel or the dry polymer particles. Following the spraying, thermal drying takes place, with the post-crosslinking reaction being able to take place both before and during the drying.

A solution of the crosslinking agent is preferably sprayed on in mixers with moving mixing tools, such as screw mixers, paddle mixers, disc mixers, plowshare mixers and paddle mixers. Vertical mixers are particularly preferred, and ploughshare mixers and paddle mixers are very particularly preferred. Examples of suitable mixers are Lödige mixers, Bepex mixers, Nauta mixers, Processall mixers and Schugi mixers.

The thermal drying is preferably carried out in contact dryers, particularly preferably paddle dryers, very particularly preferably disc dryers. Suitable dryers are, for example, Bepex dryers and Nara dryers. In addition, fluidized bed dryers can also be used.

Drying can take place in the mixer itself, by heating the jacket or blowing in warm air. A downstream dryer, such as a tray dryer, a rotary kiln or a heatable screw, is also suitable. Mixing and drying are particularly advantageous in a fluidized bed dryer.

Preferred drying temperatures are in the range from 100 to 250.degree. C., preferably from 120 to 220.degree. C., and particularly preferably from 130 to 210.degree. 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.

The post-crosslinked polymer can then be classified again.

The mean diameter of the polymer particles separated off as the product fraction is preferably at least 200 μm, particularly preferably from 250 to 600 μm, very particularly from 300 to 500 μm. 90% of the polymer particles have a diameter of preferably 100 to 800 μm, particularly preferably 150 to 700 μm, very particularly preferably 200 to 600 μm.

The water-absorbing polymer particles have a centrifuge retention capacity (CRC) of typically at least 15 g/g, preferably at least 20 g/g, preferably at least 25 g/g, particularly preferably at least 30 g/g, very particularly preferably at least 35 g/g. The centrifuge retention capacity (CRC) of the water-absorbent polymer particles is usually less than 60 g/g, the centrifuge retention capacity (CRC) being determined according to the test method No. 441.2-02 "Centrifuge retention capacity" recommended by EDANA (European Disposables and Nonwovens Association).

The water-absorbing polymer particles are tested using the test methods described below.

Methods:

Unless otherwise specified, the measurements should be carried out at an ambient temperature of 23 ± 2 °C and a relative humidity of 50 ± 10 %. The water-absorbing polymer particles are thoroughly mixed before the measurement.

Permeability (SFC Saline Flow Conductivity)

The permeability of a swollen gel layer under a compressive load of 0.3 psi (2070 Pa) is determined as in EP-A-0 640 330 described, determined as the gel layer permeability of a swollen gel layer made of superabsorbent polymer, the apparatus described in the aforementioned patent application on page 19 and in Figure 8 being modified in such a way that the glass frit (40) is no longer used, the plunger (39 ) consists of the same plastic material as the cylinder (37) and now contains 21 holes of the same size distributed evenly over the entire contact surface. The procedure and evaluation of the measurement remains unchanged compared to EP-A-0 640 330 . The flow is automatically recorded.

wobei Fg(t=0) der Durchfluss an NaCl-Lösung in g/s ist, der anhand einer linearen Regressionsanalyse der Daten Fg(t) der Durchflussbestimmungen durch Extrapolation gegen t=0 erhalten wird, L0 die Dicke der Gelschicht in cm, d die Dichte der NaCl-Lösung in g/cm³, A die Fläche der Gelschicht in cm² und WP der hydrostatische Druck über der Gelschicht in dyn/cm² darstellt.Permeability (SFC) is calculated as follows: $$\mathrm{SFC}\left[{\mathrm{cm}}^{\mathrm{3}}\mathrm{s}/\mathrm{G}\right]=\left(\mathrm{fg}\left(\mathrm{t}=0\right)\mathrm{xL}0\right)/\left(\mathrm{dxAxWP}\right)$$

where Fg(t=0) is the flux of NaCl solution in g/s obtained from a linear regression analysis of the data Fg(t) of flux determinations by extrapolation against t=0, L0 is the thickness of the gel layer in cm, i.e is the density of the NaCl solution in g/cm ³ , A is the area of the gel layer in cm ² and WP is the hydrostatic pressure across the gel layer in dynes/cm ² .

examples Production of the water-absorbing polymer particles

A 38.8% by weight acrylic acid/sodium acrylate solution was prepared by continuously mixing water, 50% by weight sodium hydroxide solution and acrylic acid, so that the degree of neutralization was 71.3 mol%. The solids content of the monomer solution was 38.8% by weight. The monomer solution was continuously cooled by a heat exchanger after mixing the components.

Polyethylene glycol 400 diacrylate (diacrylate of a polyethylene glycol with an average molecular weight of 400 g/mol) is used as the polyethylenically unsaturated crosslinker. The amount used was 2 kg per t of monomer solution.

The following components were used to initiate the radical polymerization: hydrogen peroxide (1.03 kg (0.25% by weight) per t of monomer solution), sodium peroxodisulfate (3.10 kg (15% by weight) per t of monomer solution), and ascorbic acid (1.05 kg (1% by weight) per t of monomer solution).

The throughput of the monomer solution was 20 t/h.

The individual components are continuously metered into a List Contikneter with a ^{volume of 6.3m 3} (List, Arisdorf, Switzerland) in the following quantities: 20 t/h monomer solution 40 kg/hr Polyethylene glycol 400 diacrylate 82.6 kg/hr Hydrogen peroxide solution/sodium peroxodisulphate solution 21 kg/hr ascorbic acid solution

The monomer solution was rendered inert with nitrogen between the points at which the crosslinkers and initiators were added.

At the end of the reactor, an additional 1,000 kg/h of separated undersize with a particle size of less than 150 µm were metered in.

The reaction solution had a temperature of 23.5° C. at the inlet. The reactor was operated with a shaft speed of 38 rpm. The residence time of the reaction mixture in the reactor was 15 minutes.

After polymerization and gel comminution, the aqueous polymer gel was loaded onto a belt dryer. The residence time on the dryer belt was about 37 minutes.

The dried hydrogel was ground and sieved. The fraction with a particle size of 150 to 850 μm was post-crosslinked. The separated undersize (undersize A) was returned.

The postcrosslinker solution was sprayed onto the polymer particles in a Schugi mixer (Hosokawa-Micron BV, Doeticem, Netherlands). The post-crosslinker solution was a 2.7% strength by weight solution of ethylene glycol diglycidyl ether in propylene glycol/water (weight ratio 1:3).

The following amounts were dosed: 7.5 t/h water-absorbing polymer particles (base polymer) 308.25 kg/hr post-crosslinker solution

This was followed by drying and post-crosslinking at 150° C. in a NARA Paddle Dryer (GMF Gouda, Waddinxveen, NL) for 60 minutes.

The post-crosslinked polymer particles were cooled to 60° C. (mixture I) in a NARA paddle dryer (from GMF Gouda, Waddinxveen, NL).

The cooled polymer particles were screened to a particle size of 150 to 850 μm. The separated undersize (undersize B) was returned.

Examples 1 to 12

A homogeneous mixture of mixture I and undersize A in a weight ratio of 4:1 was produced (mixture II).

A homogeneous mixture of mixture I and undersize B in a weight ratio of 4:1 was produced (mixture III).

In each case 200 g of each mixture were separated for 30 or 60 seconds by means of a vibrating screening machine (AS 200 control; Retsch GmbH, Haan, DE) with a screening tower with 2 or 3 screens.

Variant A: Sieves with mesh sizes of 850 µm and 150 µm (2 sieves) were used. The sieve fraction on the sieve with a mesh size of 150 μm was analyzed as the product fraction.

Variant B: Sieves with mesh sizes of 850 µm, 500 µm and 150 µm (3 sieves) were used. The fractions on the 500 µm and 150 µm sieves were pooled, homogenized and analyzed as a product fraction.

The test results are summarized in Table 1: Tab. 1: Sieving tests 1 example mission duration of the seven number of sieves SFC [10 ^-7 cm ³ s/g] 1 Mixture I 30s 3 42 2*) Mixture I 30s 2 37 3 Mixture I 60s 3 44 4*) Mixture I 60s 2 34 5 mix II 30s 3 26 6*) mix II 30s 2 19 7 mix II 60s 3 22 8th*) mix II 60s 2 21 9 mixture III 30s 3 33 10*) mixture III 30s 2 25 11 mixture III 60s 3 34 12*) mixture III 60s 2 33 *) Comparative examples

Examples 13 to 16

A homogeneous mixture of mixture I and undersize (mixture of undersize A and undersize B) in a weight ratio of 2:1 was produced (mixture IV).

In each case 200 g of each mixture was separated for 60 seconds using a vibrating sieve machine (AS 200 control; Retsch GmbH, Haan, DE) with a sieve tower with 2 or 3 sieves.

Variant A: Sieves with mesh sizes of 850 µm and 150 µm (2 sieves) were used. The sieve fraction on the sieve with a mesh size of 150 μm was analyzed as the product fraction.

Variant B: Sieves with mesh sizes of 850 μm, x μm and 150 μm (3 sieves) were used, with the middle sieve having a mesh size of 500 μm, 600 μm or 710 μm. The fractions on the sieves with x μm and 150 μm were combined, homogenized and analyzed as a product fraction.

The test results are summarized in Table 2: Tab. 2: Sieving tests 2 example mission number of sieves Mesh size of the middle sieve SFC [10 ^-7 cm ³ s/g] 13 Mixture IV 3 500 microns 30 14 Mixture IV 3 600 microns 29 15 Mixture IV 3 710 µm 25 16*) Mixture IV 2 omitted 25 *) Comparative example

Claims (14)

1. A process for classifying water-absorbing polymer beads by separating the polymer beads into n particle size fractions, where n is an integer greater than 1, wherein at least n screens with decreasing mesh sizes of the n screens in product flow direction are used, at least two screen fractions obtained in succession in product flow direction are combined to give one particle size fraction, and the mesh sizes of the screens on which these screen fractions occur differ in each case by at least 50 µm, the particle size fractions are unified in the sequence (1,2,1), (2,2,1), (1,3,1), (3,2,1), (2,3,1) or (3,3,1), where the number of figures in one set of brackets represents the number of particle size fractions, the particle size fractions are arranged from left to right in the brackets in product flow sequence, and the numerical values themselves represent the number of successive screen fractions which are combined to give the particular particle size fraction, and the throughput per hour of water-absorbing polymer beads in the course of classification is at least 100 kg per m² of screen area.
2. The process according to claim 1, wherein n is greater than 2.
3. The process according to claim 1 or 2, wherein at least (n+1) screens are used.
4. The process according to any of claims 1 to 3, wherein the at least two screen fractions which occur first in product flow direction are combined to give one particle size fraction.
5. The process according to any of claims 1 to 4, wherein the at least two screen fractions which occur first in product flow direction are combined to give one particle size fraction, and the mesh sizes of the screens on which these screen fractions are obtained differ in each case by at least 500 µm.
6. The process according to any of claims 1 to 5, wherein at least one screening machine with n screens is used.
7. The process according to any of claims 1 to 6, wherein the water-absorbing polymer beads, during the classification, have a temperature of at least 40°C.
8. The process according to any of claims 1 to 7, wherein classification is effected under reduced pressure.
9. The process according to any of claims 1 to 8, wherein a gas stream flows over the the water-absorbing polymer beads during the classification.
10. The process according to claim 9, wherein the gas stream has a temperature of from 40 to 120°C.
11. The process according to claim 9 or 10, wherein the gas stream has a steam content of less than 5 g/kg.
12. The process according to any of claims 1 to 11, wherein the water-absorbing polymer beads have been obtained by polymerization of an aqueous monomer solution.
13. The process according to any of claims 1 to 12, wherein the water-absorbing polymer beads comprise at least 50 mol% of at least partly neutralized polymerized acrylic acid
14. The process according to any of claims 1 to 13, wherein the water-absorbing polymer beads have a centrifuge retention capacity of at least 15 g/g.