EP2076337A1

EP2076337A1 - Method for the classification of water absorbent polymer particles

Info

Publication number: EP2076337A1
Application number: EP07820479A
Authority: EP
Inventors: Uwe Stueven; Rüdiger Funk; Matthias Weismantel; Meinhard Nitschke; Filip Mees; Leo Van Miert
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2006-09-25
Filing date: 2007-09-24
Publication date: 2009-07-08
Also published as: US9751110B2; CN101516529A; CN101516529B; JP2010504210A; US20090261023A1; WO2008037672A1

Abstract

The invention relates to a method for the continuous classification of water absorbent polymer particles by means of at least one screen, wherein at least one screen comprises a conductive device on the screen surface.

Description

Method for classifying water-absorbing polymer particles

description

The present invention relates to a method for continuously classifying water-absorbing polymer particles by means of at least one sieve, wherein at least one sieve on the sieve surface has at least one guide device.

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

Water-absorbing polymers are used as aqueous solution-absorbing products for the production of diapers, tampons, sanitary napkins and other non-sanitary, 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 greater than 200 to 850 μm is used as middle grain between the two taken from the sieves of 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 postcrosslinked 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 can be found, for example, in Ullmann's Encyklopadie der technischen Chemie, 4th Edition, Volume 2, pages 43 to 56, Verlag Chemie, Weinheim, 1972.

EP 855 232 A2 describes a classification method 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 method which is carried out under reduced pressure.

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

WO 92/18171 A1 describes the addition of inorganic powders as screen assistants.

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 continuously classifying water-absorbing polymer particles by means of at least one sieve, characterized in that at least one sieve on the sieve surface has at least one guide device.

As a result of the guide device according to the invention, the water-absorbing polymer particles moving on the screen are at least partially deflected from their original direction of movement. The guide devices according to the invention improve the selectivity in classifying water-absorbing polymer particles.

The guiding devices according to the invention are, of course, different from the lateral sieve restriction, which prevents unwanted dropping of the polymer particles beyond the lateral edge of the sieve.

The number of sieves in the process according to the invention is preferably at least 2, particularly preferably at least 3, very particularly preferably at least 4.

The number of guide devices per screen in the process according to the invention is preferably at least 2, particularly preferably at least 3, very particularly preferably at least 4.

The guide devices preferably have a height of from 1 to 10 cm, particularly preferably from 3 to 8 cm, very particularly preferably from 3 to 8 cm in relation to the sieve surface 4 to 6 cm, on. If the height is too low, the efficiency of the guiding devices will decrease because they can be overcome by some of the water-absorbing polymer particles. On the other hand, an excessive height unnecessarily increases the mechanical stress on the screen by the guide device itself.

In a preferred embodiment of the present invention, at least one guide device terminates flush with the edge of the screen.

The type of screening device for carrying out the following process according to the invention is not restricted further. Any screening device known to those skilled in the art can be used. To achieve a higher selectivity from the outset, tumble screening devices are preferably used.

The tumble screening machines suitable for the classification method according to the invention are not subject to any restriction. The screening device is typically shaken to aid classification. 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 from 0.7 to 40 mm, preferably from 1.5 to 25 mm, and a frequency of from 1 to 100 Hz, preferably from 5 to 10 Hz.

The tumble screening machines which can be used in the process according to the invention preferably have at least 2, more preferably at least 3, very preferably at least 4, sieves. Advantageously, the polymer particles falling down from the upper sieve are deflected in the direction of the center of the sieve by a preferably funnel-shaped device.

The guide devices according to the invention preferably direct the water-absorbing polymer particles in the direction of the middle of the sieve or in a spiral shape to the discharge opening of the sieve. Advantageously, the sieves in the process according to the invention have guide devices of both types. The discharge opening of the sieve is located on Siebrand. About the discharge the polymer particles are removed, which do not pass through the mesh of the screen.

The figure shows an example of a sieve according to the invention with guide devices. Here, the reference numerals have the following meanings:

A: edge of the screen

B: Guide device that directs the polymer particles towards the center of the screen

C: guide device which spirals the polymer particles to the discharge opening D: discharge opening at the edge of the screen The guide devices which direct the polymer particles in the direction of the screen center are preferably 1 to 20%, more preferably 3 to 15%, most preferably 5 to 10%, in each case based on the sieve radius in the direction of the screen center, the value calculated from the difference of maximum to minimum distance of the guide device to the screen center.

The directors that direct the polymer particles toward the center of the screen are preferably straight.

The guide devices which direct the polymer particles in the direction of the center of the sieve have a length of preferably 5 to 40%, particularly preferably 10 to 30%, very particularly preferably 15 to 25%, based on the sieve radius.

The guide devices, which guide the polymer particles in a spiral to the discharge opening of the screen, preferably contain a concentric circular segment lying around the screen center point of 180 to 330 °. The circle segment is particularly preferably 270 to 300 °.

The diameter of the circular segment is preferably 50 to 85%, particularly preferably 60 to 75%, very particularly preferably 65 to 70%, based on the sieve diameter.

Structure and material of the guide devices is not further limited. All materials known to the person skilled in the art can be used. Preference is given to using plastics. Examples for this are:

Thermoplastics such as, for example, polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET) and polystyrene (PS), acrylonitrile-butadiene-styrene copolymer (ABS), polyacetal (POM), polyamide (PA), polybutylene terephthalate (PBT ), Polyethersulfone (PES), polycarbonate (PC), polyphenylene sulfide (PPS), polytetrafluoroethylene (PTFE), polyetheretherketone (PEEK), polyimide (PI) or polyvinyl chloride (PVC). These can be used as such; however, they are usually modified with additives in the properties (hard or soft). In order to create new, previously non-existent properties, two or more thermoplastics can be mixed.

Thermosets, such as bakelite, synthetic resins, epoxy resins. Thermosets are usually hard and brittle.

- Elastomers, such as crosslinked rubber. The crosslinking takes place, for example, by vulcanization with sulfur by means of peroxides, metal oxides or irradiation. Examples of elastomers are natural rubber (NR), acrylonitrile butadiene Rubber (NBR), styrene-butadiene rubber (SBR), chloroprene rubber (CR), butadiene rubber (BR), ethylene-propylene-diene rubber (EPDM), silicone elastomers (silicone rubber) and silicone rubber.

More preferably, silicones are used, such as silicone rubber, silicone rubber and silicone resin. Generally, silicone rubbers and silicone rubber have a density of 1, 1 to 1, 3 g / cm ³ and are from -60 ⁰ C to 200 ⁰ C (special types from -90 ⁰ C to 250 ⁰ C) elastic. Silicone rubbers are rubber-elasticizable compositions containing polydiorganosiloxanes which have groups accessible to crosslinking reactions. As such, predominantly hydrogen atoms, hydroxyl groups and vinyl groups come into question, which are located at the chain ends, but can also be incorporated into the chain. In this system fillers are incorporated as an amplifier whose type and amount significantly affect the mechanical and chemical behavior of the vulcanizates. Silicone rubber and silicone rubber can be colored by inorganic pigments. A distinction is made between hot and cold vulcanizing silicone rubbers (high / room temperature vulcanizing = HTV / RTV). The HTV silicone rubbers are usually plastically deformable, just still flowable materials containing highly dispersed silicic acid and crosslinking catalysts organic peroxides and after vulcanization at temperatures greater than 100 ⁰ C heat-resistant, between -100 ⁰ C and +250 ⁰ C elastic

Silicone elastomers (silicone rubber) give the z. B. as sealing, damping, electrical roisoliermaterialien, cable sheathing and the like. Silicone resins are crosslinked polymethylsiloxanes or polymethylphenylsiloxanes whose elasticity and heat resistance increase with the content of phenyl groups. Pure methyl silicone resins are relatively brittle and moderately heat resistant.

The processing and manufacture of the guide means made of plastic materials is not further limited. All processing methods known to those skilled in the art can be used. Examples include extrusion, injection molding, calendering, foaming, bonding with and without solvents, vulcanization.

Screening systems in combination with the guide devices according to the invention provide the highest quality screening in the fine and ultra-fine screen area. For sensitive products no grain destruction is observed. Through the use of the guide devices, a higher specific Siebbeladung is possible compared to conventional vibrating sieves. In addition, the guide allows a stable Siebbewegung under full load. By controlling the tumbling movement, the particle residence time on the screen can be changed and the performance and quality of the screen matched.

The sieves are cleaned using product-specific sieve cleaning systems. A simple access enables a quick change of the sieve inserts with guide obligations. Through high throughputs, easy maintenance and low operating costs optimum economy is achieved when using the screens according to the invention.

The mesh size of the sieves is preferably in the range from 100 to 1000 .mu.m, particularly preferably in the range from 125 to 900 .mu.m, very particularly preferably in the range from 150 to 850 .mu.m.

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 the 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, more preferably at least 250 kg / m ² h, very particularly preferably at least 300 kg / m ² h.

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 of} 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 Sieve surface, wherein the gas volume is measured under standard conditions (25 ⁰ C and 1 bar). Particularly preferably, the gas stream is heated before entering the sieve, typically at a temperature of 40 to 120 ⁰ C, preferably at a temperature of 50 to 1 10 ⁰ C, preferably at a temperature of 60 to 100 ⁰ C, more preferably a temperature of 65 to 90 ^° C., very particularly 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.

In a preferred embodiment of the present invention, several screening machines are operated in parallel.

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, ie 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, and preferably have 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. Especially preferred is 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.

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 can 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 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.

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 EP 343 427 A2. Further suitable crosslinkers b) are pentaerythritol di-, pentaerythritol tri- and pentaerythritol tetraallyl ethers, polyethylene glycol diallyl ether, ethylene glycol diallyl ether, glycerol di- and glycerol triallyl ether, 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, and the 3-fold mixed ethoxylated or propoxylated glycerol or trimethylolpropane, the 15-fold 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 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. 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. 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 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 initiato 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. 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. Preferably, the oxygen content of the monomer solution before the polymerization to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, lowered.

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.

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. 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.

After leaving the polymerization reactor, the hydrogel is advantageously still stored at elevated 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, it is also possible to use ammonium salts. 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 having a melting point above 23 ⁰ 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 up to 40 mol%, preferably 10 to neutralize 30 to 25 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 setting the desired final degree of neutralization only after the polymerization at the hydrogel stage. 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. Alternatively, a fluidized bed dryer or a heated ploughshare mixer can be used for drying. 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 at least two groups which can form covalent bonds with the carboxylate groups of the hydrogel. Suitable compounds are, for example, alkoxysilyl compounds, polyaziridines, polyamines, polyamidoamines, di- or polyepoxides, as described in EP 83 022 A2, EP 543 303 A1 and EP 937 736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1 DE 35 23 617 A1 and EP 450 922 A2, or β-hydroxyalkylamides, as described in DE 102 04 938 A1 and US Pat. No. 6,239,230.

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, EP 1 199 327 A2 describes oxetanes and cyclic ureas and, in WO 2003/31482 A1, morpholine-2,3-dione and its derivatives as suitable postcrosslinkers.

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

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 a counterion, chloride, bromide, sulfate, hydrogen sulfate, carbonate, bicarbonate, 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 postcrosslinking is usually carried out by spraying a solution of the postcrosslinker 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 display 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-T rockner and Nara-T rockner. 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 more 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, completely more 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, on. 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 method described by EDANA (European Dispo). Sables and Nonwovens Association). The recommended test method no. 441.2-02 "centrifuge retention capacity" is determined.

Examples

Comparative Example:

By continuously mixing water, 50% strength by weight sodium hydroxide solution and acrylic acid, a 38.8% strength by weight acrylic acid / sodium acrylate solution was prepared so that the degree of neutralization was 71.3 mol%. 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 mole of monomer solution ), as well as ascorbic acid (1, 05 kg (1 wt .-%) 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 6.3m ³ volume (List, Arisdorf, Switzerland) in the following quantities:

20 t / h monomer solution

40 kg / h of polyethylene glycol 400 diacrylate

82.6 kg / h hydrogen peroxide solution / sodium peroxodisulfate solution 21 kg / h ascorbic acid solution

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 at the inlet, a temperature of 23.5 ⁰ C. The reactor was operated at a rotational speed of the shafts 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 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 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 of water-absorbing polymer particles (base polymer)

308.25 kg / h postcrosslinker solution

It 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 in a NARA paddle dryer (Fa. GMF Gouda, Waddinxveen, NL) at 60 ⁰ C.

The cooled polymer particles were continuously sieved in a tumble sieve machine (Allgaier Werke GmbH, Uhingen, DE) with three sieve decks. The sieves had a diameter of 260 cm and had, from bottom to top, a mesh size of 150 microns, 500 microns and 850 microns on.

The two sieve fractions in the range from 150 to 850 μm were combined and analyzed. The particle size distribution was determined photo-optically using a PartAn Particle Analyzer (AnaTec, Duisburg, DE). The measurement results are summarized in the table.

Example 1 :

The procedure was as in the comparative example.

The bottom sieve had guides as shown in Figure 1. The three linear guides at the edge of the sieve were 30 cm long and the maximum distance from the edge of the sieve was 10 cm. The circular portion of the spiral guide was a circle segment of about 300 ° and had a radius of about 175 cm. The height of the guide devices was 5 cm. Example 2

The procedure was as in Example 1. All sieves of the tumble screening machine had guide devices such as the bottom sieve in Example 1.

Tab .: Particle size distribution

Claims

claims

1. A method for continuously classifying water-absorbing polymer particles by means of at least one sieve, characterized in that at least one sieve on the sieve surface has at least one guide device.

2. The method according to claim 1, characterized in that the guide device with respect to the sieve surface has a height of 1 to 10 cm.

3. The method according to claim 1 or 2, characterized in that at least one guide device terminates flush with the edge of the screen at one end.

4. The method according to any one of claims 1 to 3, characterized in that a tumble screening machine is used.

5. The method according to claim 4, characterized in that the tumble screening machine has at least two sieves.

6. The method according to claim 5, characterized in that the Taumelsieb- machine between at least two screens has a device for deflecting the falling of the upper sieve polymer particles in the direction of Siebmitte the lower screen.

7. The method according to any one of claims 4 to 6, characterized in that at least one guide device is directed to the screen center.

8. The method according to claim 7, characterized in that the directed to the screen center guide device is directed by at least 1%, based on the sieve radius, to the screen center.

9. The method according to claim 7 or 8, characterized in that the directed towards the screen center guide is straight.

10. The method according to any one of claims 7 to 9, characterized in that the directed towards the screen center guide device has a length of 5 to 20%, based on the sieve radius.

1 1. A method according to any one of claims 4 to 10, characterized in that at least one guide device spirally directs the polymer particles to the Austragsöff- opening of the screen.

12. The method according to claim 1 1, characterized in that the polymer particles which guide spirally to the discharge opening of the screen guide means a concentric about the Siebmittelpunkt lying circular segment of at least 180 "contains.

13. The method according to claim 12, characterized in that the concentric lying around the screen center circle segment has a diameter of at least 50%, based on the screen diameter.

14. The method according to any one of claims 1 to 13, characterized in that all sieves have at least one guide device.

15. The method according to any one of claims 1 to 14, characterized in that at least one sieve has a mesh size in the range of 100 to 1,000 microns up.

16. The method according to any one of claims 1 to 15, characterized in that the water-absorbing polymer particles have a temperature of at least 40 ° C during classification.

17. The method according to any one of claims 1 to 16, characterized in that is classified under reduced pressure.

18. The method according to any one of claims 1 to 17, characterized in that the hourly throughput of water-absorbing polymer particles in classifying at least 100 kg per m ² screen area.

19. The method according to any one of claims 1 to 18, characterized in that the water-absorbing polymer particles are flowed over during classification of a gas stream.

20. The method according to claim 19, characterized in that the gas stream has a temperature of 40 to 120 ⁰ C.

21. The method according to claim 19 or 20, characterized in that the gas stream has a water vapor content of less than 5 g / kg.

22. The method according to any one of claims 1 to 21, characterized in that the water-absorbing polymer particles contain at least 50 mol% at least partially neutralized polymerized acrylic acid.

23. The method according to any one of claims 1 to 22, characterized in that the water-absorbing polymer particles have a Zentrifugeretentionskapazität of at least 15 g / g.