EP2076338B1 - Method for grading water-absorbent polymer particles - Google Patents

Description

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

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

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

To improve application properties, such as swelling in the swollen gel bed (SFC) in the diaper and absorption under pressure (AUL), water-absorbing polymer particles are generally postcrosslinked. As a result, only the degree of crosslinking of the particle surface increases, whereby the absorption under pressure (AUL) and the centrifuge retention capacity (CRC) can be at least partially decoupled. This postcrosslinking can be carried out in aqueous gel phase. Preferably, however, dried, ground and sieved polymer particles (base polymer) are coated on the surface with a postcrosslinker, postcrosslinked 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 used as pulverulent, granular product, preferably in the hygiene sector. For example, particle sizes between 200 and 850 .mu.m are used here, and the particulate polymer material is already classified to these particle sizes during the production process. Here, continuous sieving machines with two sieves are used, whereby sieves with the mesh sizes of 200 and 850 μm are used. Particles with a grain size of up to 200 μm fall through both screens and are collected at the bottom of the screening machine as undersize. Particles with a particle size of greater than 850 microns remain as oversize on the top sieve and are discharged. The product fraction with a particle size of greater than 200 to 850 microns is used as the middle grain between the two Seven removed from the screening machine. Depending on the screening quality, each particle size fraction still contains a proportion of particles with the wrong particle size as a so-called faulty discharge. For example, the oversize fraction may still contain a proportion of particles with a particle size of 850 microns or less.

Extracted undersize and oversize is usually attributed to production. The undersize can be added to the polymerization, for example. The oversize grain is usually crushed, which inevitably leads to a forced attack of further undersize.

In the conventional classifying operations, various problems arise when particulate polymers are classified. The most common problem is the clogging of the screen surface as well as the deterioration of the classification efficiency and the classifying ability. Another problem is the caking tendency of the product, which leads to undesirable agglomerates before, after and during sieving. Therefore, the screening step can not be carried out free of disturbances, often accompanied by unwanted stoppages during polymer production. Such problems are particularly problematic in the continuous production process. Overall, however, this results in an insufficient selectivity in the screening. This problem can be observed especially in the classification of nachvemetztem product.

A higher screening quality is usually achieved by adding to the product substances which serve to increase the flowability and / or the mechanical stability of the polymer powder. In general, a free-flowing product is achieved by adding to the polymer powder, usually after drying and / or as part of the post-crosslinking auxiliaries, for example surfactants, which prevent mutual sticking of the individual particles. In other cases, attempts are made to influence the caking tendencies by procedural measures.

In order to achieve higher separation levels without further product additions, improvements by alternative screening plants have been proposed. Thus, higher separating powers are achieved when Sieböffnungsflächen be driven spirally. This is the case, for example, with tumbler screening machines. However, if the throughput of such screening devices is increased, the above problems are aggravated, and it becomes less and less possible to maintain the high classification ability.

The addition of screening aids, such as screen balls, PVC friction rings, Teflon friction rings or rubber cube, on the sieve surface, helps only slightly to increase the selectivity. Especially with amorphous polymer material, such as water-absorbing polymer particles, this can lead to increased abrasion.

A general overview of the classification is, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 4th Edition, Volume 2, pages 43 to 56, Verlag Chemie, Weinheim, 1972 , to find.

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

DE 10 2005 001 789 A1 describes a classification process performed at reduced pressure.

JP 2003/320308 A describes a method in which agglomerates are avoided by the bottom of the sieve with warm air is flown.

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 method for producing water-absorbing polymer particles.

The object was achieved by a method for classifying water-absorbing polymer particles, wherein the polymer particles are separated into n particle size fractions and n is an integer greater than 1, characterized in that at least n sieves are used and decrease the mesh sizes of n sieves in the product flow direction.

Through a sieve, a particulate material is separated into two sieve fractions, the particles remaining on the sieve and the particles passing through the meshes of the sieve. By using additional sieves, each sieve fraction can be separated into a further two sieve fractions. When n sieves are used, (n + 1) sieve fractions are obtained, whereby each sieve fraction can be processed separately as a grain size fraction. On the other hand, an essential feature of the present invention is that at least two of these sieve fractions are combined to form a particle size fraction and further processed together. Compared with the hitherto conventional method for classifying water-absorbing polymer particles, the method according to the invention thus uses at least one sieve more.

By using the at least one additional screen, water-absorbing polymer particles having improved absorption under pressure (AUL) and improved fluid conduction in the swollen gel bed (SFC) are obtained.

The sieve fractions can be combined according to the inventive method in different ways to grain size fractions, for example in the Sequence (2,1), (3,1), (2,1,1), (1,2,1), (2,2,1), (3,1,1), (1,3, 1), (3,2,1), (2,3,1) or (3,3,1), where the number of numbers is in parenthesis for the number of grain size fractions, the grain size fractions in product stream sequence in parentheses of are arranged left to right and the numerical values themselves stand for the number of successive sieve fractions which are combined to the respective particle size fraction.

The number of particle 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 obtained in succession in the product flow direction are combined to form a particle size fraction, wherein the mesh sizes of the sieves on which these sieve fractions are obtained are usually usually at least 50 μm, preferably at least 100 μm, preferably around in each case at least 150 .mu.m, particularly preferably by at least 200 .mu.m, very particularly preferably by at least 250 microns, different.

In a further preferred embodiment of the present invention, the at least two sieve fractions initially obtained in the product flow direction are combined to form a particle size fraction, wherein the mesh sizes of the sieves on which these sieve fractions are obtained are preferably at least 500 .mu.m, preferably at least 1000 .mu.m, more preferably each differ by at least 1,500 microns, most preferably by at least 2,000 microns.

The water-absorbing polymer particles preferably have a temperature of from 40 to 120 ° C., more preferably from 45 to 100 ° C., very preferably from 50 to 80 ° C., during classification.

In a preferred embodiment of the present invention, the product is classified under reduced pressure. The pressure is preferably 100 mbar less than the ambient pressure.

Particularly advantageously, the classification method according to the invention is carried out continuously. The throughput of water-absorbing polymer is usually 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 which are suitable for the classification method according to the invention are not subject to any restrictions; plane sieve methods are preferred, tumble screening machines are very particularly preferred. The screening device is used to support the Classification typically shaken. This is preferably done so that the material to be classified is spirally guided over the sieve. 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 of 5 to 10 Hz.

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

Preferably, the water-absorbing resin is overflowed during the classifying with a gas stream, more preferably air. 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, the gas volume being measured under standard conditions (25 ° C and 1 bar). Particularly preferably, the gas stream is heated before entering the sieve, 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, more preferably to a Temperature of 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 condensing a corresponding amount of water from the gas stream having a higher water content 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 polymerization of monomer solutions comprising 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, 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 50 g / 100 g of water preferably at least one acid group each.

Suitable monomers a) are, for example, 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. Very particular preference is given to acrylic acid.

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

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), in particular acrylic acid, preferably contain up to 0.025 wt .-% of a Hydrochinonhalbethers. Preferred hydroquinone half ethers 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 as meaning compounds of the following formula

wherein 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 acceptable carboxylic acids. The carboxylic acids may be mono-, di- or tricarboxylic acids.

Preference is given to alpha-tocopherol with R ¹ = R ² = R ³ = methyl, in particular racemic alpha-tocopherol. R ¹ is more preferably hydrogen or acetyl. Especially preferred is RRR-alpha-tocopherol.

The monomer solution preferably contains 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 ppm by weight, hydroquinone, in each case based on Acrylic acid, wherein acrylic acid salts are taken into account as acrylic acid. For example, to prepare the monomer solution, an acrylic acid having a corresponding content of hydroquinone half-ether can be used.

Crosslinkers b) are compounds having at least two polymerizable groups which can be radically copolymerized into the polymer network. 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 in addition to acrylate groups further ethylenically unsaturated groups, as in DE 103 31 456 A1 and DE 103 55 401 A1 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 monocarboxylic 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 esters, polyallyl esters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allyl esters of phosphoric acid and vinylphosphonic acid derivatives, as described, for example, in US Pat EP 343 427 A2 are described. Further suitable crosslinkers b) are pentaerythritol di-, pentaerythritol tri- and pentaerythritol tetraallyl ethers, polyethylene glycol diallyl ether, ethylene glycol diallyl ether, glycerol and glycerol triallyl ethers, polyallyl ethers based on sorbitol, and ethoxylated variants thereof. Useful in the process according to the invention are di (meth) acrylates of polyethylene glycols, where the polyethylene glycol used has a molecular weight between 100 and 1000.

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

Very particularly preferred crosslinkers b) are the polyethoxylated and / or propoxylated glycerols which are esterified with acrylic acid or methacrylic acid to form di- or triacrylates, as are described, for example, in US Pat WO 2003/104301 A1 are described. 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 glycerin.

The amount of crosslinker b) is preferably 0.01 to 5 wt .-%, particularly preferably 0.05 to 2 wt .-%, most preferably 0.1 to 1 wt .-%, each based on the monomer solution.

As initiators c) it is possible to use all compounds which form radically 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 advantageous to use mixtures of 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 proportion.

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 0.001 to 5 wt .-%, preferably 0.01 to 1 wt .-%, based on the monomers a).

The preferred polymerization inhibitors require dissolved oxygen for optimum performance. Thus, the monomer solution may be polymerized prior to polymerization by inerting, i. H. Flow through with an inert gas, preferably nitrogen, to be freed of dissolved oxygen. Preferably, the oxygen content of the monomer solution before polymerization is reduced to less than 1 ppm by weight, more preferably less than 0.5 ppm by weight.

The preparation of a suitable polymer and further suitable hydrophilic ethylenically unsaturated monomers a) are described 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 polymerization of an aqueous monomer solution is comminuted continuously by, for example, counter-rotating stirring shafts, as in WO 2001/38402 A1 described. The polymerization on the belt is for example in DE 38 25 366 A1 and US 6,241,928 described. 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. After leaving the polymerization reactor, the hydrogel is advantageously stored even at a higher temperature, preferably at least 50 ° C., more preferably at least 70 ° C., very preferably at least 80 ° C., and preferably less than 100 ° C., for example in isolated containers. By storage, usually 2 to 12 hours, the monomer conversion is further increased.

At higher monomer conversions in the polymerization reactor, the storage can also be significantly shortened or omitted storage.

The acid groups of the hydrogels obtained are usually partially neutralized, preferably from 25 to 95 mol%, preferably from 50 to 80 mol%, 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 alkali metal hydrogencarbonates 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.

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, 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.

However, it is also possible to carry out the neutralization after the polymerization at the hydrogel stage. 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 only after the polymerization is adjusted at the level of the hydrogel. If the hydrogel is at least partially neutralized after the polymerization, the hydrogel is preferably comminuted mechanically, for example by means of a meat grinder, 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 further gewolfft for homogenization.

The hydrogel is then preferably dried with a belt dryer until the residual moisture content is preferably below 15% by weight, in particular below 10% by weight, the water content being determined in accordance with the test method No. 430.2- recommended by EDANA (European Disposables and Nonwovens Association). 02 "Moisture content" is determined. Optionally, for drying but also a fluidized bed dryer or a heated ploughshare mixer can be used. To obtain particularly white products, it is advantageous in the drying of this gel to ensure rapid removal of the evaporating water. For this purpose, the dryer temperature must be optimized, the air supply and removal must be controlled, and it is in any case to ensure adequate ventilation. Naturally, drying is all the easier and the product is 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. Particularly advantageous is the ventilation of the dryer with nitrogen or other non-oxidizing inert gas. Optionally, however, it is also possible simply to lower only the partial pressure of the oxygen during the drying in order to prevent oxidative yellowing processes.

The dried hydrogel is thereafter 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 μm, more preferably from 250 to 600 μm, very particularly from 300 to 500 μm. The mean particle size of the product fraction can be determined by means of the test method No. 420.2-02 "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 average particle size is determined graphically. The mean particle size here is the value of the mesh size, which results for accumulated 50 wt .-%.

The polymer particles can be postcrosslinked to further improve the properties. Suitable postcrosslinkers are compounds containing groups which can form 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 US 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-oxazolidinone, 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 Oxetane and cyclic ureas and in WO 2003/31482 A1 Morpholine-2,3-dione and its derivatives described as suitable Nachvernetzer.

It is also possible to use postcrosslinkers which contain additional polymerizable ethylenically unsaturated groups, as in DE 37 13 601 A1 described

The amount of postcrosslinker is preferably 0.01 to 1 wt .-%, particularly preferably 0.05 to 0.5 wt .-%, most preferably 0.1 to 0.2 wt .-%, each based on the polymer ,

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

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 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 the counterion, chloride, bromide, sulfate, hydrogensulfate, carbonate, hydrogencarbonate, nitrate, phosphate, hydrogenphosphate, dihydrogenphosphate 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 postcrosslinking is usually carried out so that a solution of the postcrosslinker is sprayed onto the hydrogel or the dry polymer particles. Subsequent to the spraying, it is thermally dried, whereby the postcrosslinking reaction can take place both before and during the drying.

The spraying of a solution of the crosslinker is preferably carried out in mixers with agitated mixing tools, such as screw mixers, paddle mixers, disk mixers, plowshare mixers and paddle mixers. Vertical mixers are particularly preferred, plowshare 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, more preferably paddle dryers, very particularly preferably disk dryers. Suitable dryers include Bepex dryers and Nara dryers. 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. 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.

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

Subsequently, the postcrosslinked polymer can be re-classified.

The average diameter of the polymer fraction separated as a product fraction is preferably at least 200 μm, more 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 .mu.m, more preferably from 150 to 700 .mu.m, most preferably from 200 to 600 .mu.m.

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, more preferably at least 30 g / g, most preferably at least 35 g / g. The centrifuge retention capacity (CRC) of the water-absorbing 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 the EDANA (European Disposables and Nonwovens Association).

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

methods:

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.

Permeability (SFC Saline Flow Conductivity)

The permeability of a swollen gel layer under compressive load of 0.3 psi (2070 Pa) becomes as in EP-A-0 640 330 described as gel-layer permeability of a swollen gel layer of superabsorbent polymer, wherein the apparatus described in the aforementioned patent application on page 19 and in Figure 8 has been modified so that the glass frit (40) is no longer used, the stamp (39 ) consists of the same plastic material as the cylinder (37) and now evenly distributed over the entire bearing surface contains 21 equal holes. The procedure and evaluation of the measurement remains unchanged EP-A-0 640 330 , The flow is automatically detected.

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.The permeability (SFC) is calculated as follows: $$\mathrm{SFC}\left[{\mathrm{cm}}^{\mathrm{2}}\mathrm{s}\mathrm{/}\mathrm{G}\right]\mathrm{=}\left(\mathrm{fg}\left(\mathrm{t}\mathrm{=}\mathrm{0}\right)\mathrm{x\; L}\mathrm{0}\right)\mathrm{/}\left(\mathrm{dxAxWP}\right)\mathrm{.}$$

where Fg (t = 0) is the flow rate of NaCl solution in g / s obtained from a linear regression analysis of data Fg (t) of flow determinations by extrapolation to t = 0, L0 is the thickness of the gel layer in cm, d the density of the NaCl solution in g / cm ³ , A the area of the gel layer in cm ² and WP the hydrostatic pressure over the gel layer in dyn / cm ² .

Examples Preparation of the water-absorbing polymer particles

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

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

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

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

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

Between the addition points for crosslinkers and initiators, the monomer solution was rendered inert with nitrogen.

At the end of the reactor, an additional 1000 kg / h of separated undersized particles having a particle size of less than 150 μm were added.

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

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

The dried hydrogel was ground and sieved. The fraction with the particle size 150 to 850 microns was postcrosslinked. The separated undersize (undersize A) was returned.

The postcrosslinker solution was sprayed onto the polymer particles in a Schugi mixer (Fa, Hosokawa-Micron B.V., Doetichem, NL). The postcrosslinker solution was a 2.7% by weight solution of ethylene glycol diglycidyl ether in propylene glycol / water weight ratio 1: 3).

The following quantities were dosed: 7.5 t / h water-absorbing polymer particles (base polymer) 308.25 kg / h postcrosslinker

The mixture was then dried for 60 minutes at 150 ° C. in a NARA paddle dryer (GMF Gouda, Waddinxveen, NL) and postcrosslinked.

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

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

Examples 1 to 12

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

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

Each 200 g of each mixture was separated for 30 and 60 seconds by means of a vibrating screening machine (AS 200 control, Retsch GmbH, Haan, DE) with a sieving tower with 2 or 3 sieves.

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

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

The test results are summarized in Table 1: Tab. 1: Sieve tests 1 example commitment Duration of sieving Number of sieves SFC [10 ^{-7.cm 3} s / g] 1 Mixture I 30 s 3 42 2 *) Mixture I 30 s 2 37 3 Mixture I 60 s 3 44 4 *) Mixture I 60 s 2 34 5 Mixture II 30 s 3 26 6 *) Mixture II 30 s 2 19 7 Mixture II 60 s 3 22 8th*) Mixture II 60 s 2 21 9 Mixture III 30 s 3 33 10 *) Mixture III 30 s 2 25 11 Mixture III 60 s 3 34 12 *) Mixture III 60 s 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 prepared (mixture IV).

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

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

Variant B: Sieves with mesh sizes of 850 μm, x μm and 150 μm (3 sieves) were used, 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 product fraction.

The test results are summarized in Table 2: Tab. 2: Screening tests 2 example commitment Number of sieves Mesh size of the middle sieve SFC [10 ^{-7.cm 3} s / g] 13 Mix IV 3 500 μm 30 14 Mix IV 3 600 μm 29 15 Mix IV 3 710 μm 25 16 *) Mix IV 2 deleted 25 *) Comparative Example

Claims (16)

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, which comprises using at least n screens with decreasing mesh sizes of the n screens in product flow direction.
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 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.
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.
6. The process according to any of claims 1 to 5, 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.
7. The process according to any of claims 1 to 6, wherein at least one screening machine with n screens is used.
8. The process according to any of claims 1 to 7, wherein the water-absorbing polymer beads, during the classification, have a temperature of at least 40°C.
9. The process according to any of claims 1 to 8, wherein classification is effected under reduced pressure.
10. The process according to any of claims 1 to 9, wherein the throughput per hour of water-absorbing polymer beads in the course of classification is at least 100 kg per m² of screen area.
11. The process according to any of claims 1 to 10, wherein the water-absorbing polymer beads are flowed over by a gas stream during the classification.
12. The process according to claim 11, wherein the gas stream has a temperature of from 40 to 120°C.
13. The process according to claim 11 or 12, wherein the gas stream has a steam content of less than 5 g/kg.
14. The process according to any of claims 1 to 13, wherein the water-absorbing polymer beads have been obtained by polymerization of an aqueous monomer solution.
15. The process according to any of claims 1 to 14, wherein the water-absorbing polymer beads comprise at least 50 mol% of at least partly neutralized polymerized acrylic acid
16. The process according to any of claims 1 to 15, wherein the water-absorbing polymer beads have a centrifuge retention capacity of at least 15 g/g.